| 1 | \input texinfo |
| 2 | @setfilename ld.info |
| 3 | @c Copyright 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, |
| 4 | @c 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 |
| 5 | @c Free Software Foundation, Inc. |
| 6 | @syncodeindex ky cp |
| 7 | @c man begin INCLUDE |
| 8 | @include configdoc.texi |
| 9 | @c (configdoc.texi is generated by the Makefile) |
| 10 | @include bfdver.texi |
| 11 | @c man end |
| 12 | |
| 13 | @c @smallbook |
| 14 | |
| 15 | @macro gcctabopt{body} |
| 16 | @code{\body\} |
| 17 | @end macro |
| 18 | |
| 19 | @c man begin NAME |
| 20 | @ifset man |
| 21 | @c Configure for the generation of man pages |
| 22 | @set UsesEnvVars |
| 23 | @set GENERIC |
| 24 | @set ARM |
| 25 | @set C6X |
| 26 | @set H8300 |
| 27 | @set HPPA |
| 28 | @set I960 |
| 29 | @set M68HC11 |
| 30 | @set M68K |
| 31 | @set MMIX |
| 32 | @set MSP430 |
| 33 | @set POWERPC |
| 34 | @set POWERPC64 |
| 35 | @set Renesas |
| 36 | @set SPU |
| 37 | @set TICOFF |
| 38 | @set WIN32 |
| 39 | @set XTENSA |
| 40 | @end ifset |
| 41 | @c man end |
| 42 | |
| 43 | @ifnottex |
| 44 | @dircategory Software development |
| 45 | @direntry |
| 46 | * Ld: (ld). The GNU linker. |
| 47 | @end direntry |
| 48 | @end ifnottex |
| 49 | |
| 50 | @copying |
| 51 | This file documents the @sc{gnu} linker LD |
| 52 | @ifset VERSION_PACKAGE |
| 53 | @value{VERSION_PACKAGE} |
| 54 | @end ifset |
| 55 | version @value{VERSION}. |
| 56 | |
| 57 | Copyright @copyright{} 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, |
| 58 | 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc. |
| 59 | |
| 60 | Permission is granted to copy, distribute and/or modify this document |
| 61 | under the terms of the GNU Free Documentation License, Version 1.3 |
| 62 | or any later version published by the Free Software Foundation; |
| 63 | with no Invariant Sections, with no Front-Cover Texts, and with no |
| 64 | Back-Cover Texts. A copy of the license is included in the |
| 65 | section entitled ``GNU Free Documentation License''. |
| 66 | @end copying |
| 67 | @iftex |
| 68 | @finalout |
| 69 | @setchapternewpage odd |
| 70 | @settitle The GNU linker |
| 71 | @titlepage |
| 72 | @title The GNU linker |
| 73 | @sp 1 |
| 74 | @subtitle @code{ld} |
| 75 | @ifset VERSION_PACKAGE |
| 76 | @subtitle @value{VERSION_PACKAGE} |
| 77 | @end ifset |
| 78 | @subtitle Version @value{VERSION} |
| 79 | @author Steve Chamberlain |
| 80 | @author Ian Lance Taylor |
| 81 | @page |
| 82 | |
| 83 | @tex |
| 84 | {\parskip=0pt |
| 85 | \hfill Red Hat Inc\par |
| 86 | \hfill nickc\@credhat.com, doc\@redhat.com\par |
| 87 | \hfill {\it The GNU linker}\par |
| 88 | \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par |
| 89 | } |
| 90 | \global\parindent=0pt % Steve likes it this way. |
| 91 | @end tex |
| 92 | |
| 93 | @vskip 0pt plus 1filll |
| 94 | @c man begin COPYRIGHT |
| 95 | Copyright @copyright{} 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 96 | 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free |
| 97 | Software Foundation, Inc. |
| 98 | |
| 99 | Permission is granted to copy, distribute and/or modify this document |
| 100 | under the terms of the GNU Free Documentation License, Version 1.3 |
| 101 | or any later version published by the Free Software Foundation; |
| 102 | with no Invariant Sections, with no Front-Cover Texts, and with no |
| 103 | Back-Cover Texts. A copy of the license is included in the |
| 104 | section entitled ``GNU Free Documentation License''. |
| 105 | @c man end |
| 106 | |
| 107 | @end titlepage |
| 108 | @end iftex |
| 109 | @contents |
| 110 | @c FIXME: Talk about importance of *order* of args, cmds to linker! |
| 111 | |
| 112 | @ifnottex |
| 113 | @node Top |
| 114 | @top LD |
| 115 | This file documents the @sc{gnu} linker ld |
| 116 | @ifset VERSION_PACKAGE |
| 117 | @value{VERSION_PACKAGE} |
| 118 | @end ifset |
| 119 | version @value{VERSION}. |
| 120 | |
| 121 | This document is distributed under the terms of the GNU Free |
| 122 | Documentation License version 1.3. A copy of the license is included |
| 123 | in the section entitled ``GNU Free Documentation License''. |
| 124 | |
| 125 | @menu |
| 126 | * Overview:: Overview |
| 127 | * Invocation:: Invocation |
| 128 | * Scripts:: Linker Scripts |
| 129 | @ifset GENERIC |
| 130 | * Machine Dependent:: Machine Dependent Features |
| 131 | @end ifset |
| 132 | @ifclear GENERIC |
| 133 | @ifset H8300 |
| 134 | * H8/300:: ld and the H8/300 |
| 135 | @end ifset |
| 136 | @ifset Renesas |
| 137 | * Renesas:: ld and other Renesas micros |
| 138 | @end ifset |
| 139 | @ifset I960 |
| 140 | * i960:: ld and the Intel 960 family |
| 141 | @end ifset |
| 142 | @ifset ARM |
| 143 | * ARM:: ld and the ARM family |
| 144 | @end ifset |
| 145 | @ifset HPPA |
| 146 | * HPPA ELF32:: ld and HPPA 32-bit ELF |
| 147 | @end ifset |
| 148 | @ifset M68HC11 |
| 149 | * M68HC11/68HC12:: ld and the Motorola 68HC11 and 68HC12 families |
| 150 | @end ifset |
| 151 | @ifset M68K |
| 152 | * M68K:: ld and Motorola 68K family |
| 153 | @end ifset |
| 154 | @ifset POWERPC |
| 155 | * PowerPC ELF32:: ld and PowerPC 32-bit ELF Support |
| 156 | @end ifset |
| 157 | @ifset POWERPC64 |
| 158 | * PowerPC64 ELF64:: ld and PowerPC64 64-bit ELF Support |
| 159 | @end ifset |
| 160 | @ifset SPU |
| 161 | * SPU ELF:: ld and SPU ELF Support |
| 162 | @end ifset |
| 163 | @ifset TICOFF |
| 164 | * TI COFF:: ld and the TI COFF |
| 165 | @end ifset |
| 166 | @ifset WIN32 |
| 167 | * Win32:: ld and WIN32 (cygwin/mingw) |
| 168 | @end ifset |
| 169 | @ifset XTENSA |
| 170 | * Xtensa:: ld and Xtensa Processors |
| 171 | @end ifset |
| 172 | @end ifclear |
| 173 | @ifclear SingleFormat |
| 174 | * BFD:: BFD |
| 175 | @end ifclear |
| 176 | @c Following blank line required for remaining bug in makeinfo conds/menus |
| 177 | |
| 178 | * Reporting Bugs:: Reporting Bugs |
| 179 | * MRI:: MRI Compatible Script Files |
| 180 | * GNU Free Documentation License:: GNU Free Documentation License |
| 181 | * LD Index:: LD Index |
| 182 | @end menu |
| 183 | @end ifnottex |
| 184 | |
| 185 | @node Overview |
| 186 | @chapter Overview |
| 187 | |
| 188 | @cindex @sc{gnu} linker |
| 189 | @cindex what is this? |
| 190 | |
| 191 | @ifset man |
| 192 | @c man begin SYNOPSIS |
| 193 | ld [@b{options}] @var{objfile} @dots{} |
| 194 | @c man end |
| 195 | |
| 196 | @c man begin SEEALSO |
| 197 | ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and |
| 198 | the Info entries for @file{binutils} and |
| 199 | @file{ld}. |
| 200 | @c man end |
| 201 | @end ifset |
| 202 | |
| 203 | @c man begin DESCRIPTION |
| 204 | |
| 205 | @command{ld} combines a number of object and archive files, relocates |
| 206 | their data and ties up symbol references. Usually the last step in |
| 207 | compiling a program is to run @command{ld}. |
| 208 | |
| 209 | @command{ld} accepts Linker Command Language files written in |
| 210 | a superset of AT&T's Link Editor Command Language syntax, |
| 211 | to provide explicit and total control over the linking process. |
| 212 | |
| 213 | @ifset man |
| 214 | @c For the man only |
| 215 | This man page does not describe the command language; see the |
| 216 | @command{ld} entry in @code{info} for full details on the command |
| 217 | language and on other aspects of the GNU linker. |
| 218 | @end ifset |
| 219 | |
| 220 | @ifclear SingleFormat |
| 221 | This version of @command{ld} uses the general purpose BFD libraries |
| 222 | to operate on object files. This allows @command{ld} to read, combine, and |
| 223 | write object files in many different formats---for example, COFF or |
| 224 | @code{a.out}. Different formats may be linked together to produce any |
| 225 | available kind of object file. @xref{BFD}, for more information. |
| 226 | @end ifclear |
| 227 | |
| 228 | Aside from its flexibility, the @sc{gnu} linker is more helpful than other |
| 229 | linkers in providing diagnostic information. Many linkers abandon |
| 230 | execution immediately upon encountering an error; whenever possible, |
| 231 | @command{ld} continues executing, allowing you to identify other errors |
| 232 | (or, in some cases, to get an output file in spite of the error). |
| 233 | |
| 234 | @c man end |
| 235 | |
| 236 | @node Invocation |
| 237 | @chapter Invocation |
| 238 | |
| 239 | @c man begin DESCRIPTION |
| 240 | |
| 241 | The @sc{gnu} linker @command{ld} is meant to cover a broad range of situations, |
| 242 | and to be as compatible as possible with other linkers. As a result, |
| 243 | you have many choices to control its behavior. |
| 244 | |
| 245 | @c man end |
| 246 | |
| 247 | @ifset UsesEnvVars |
| 248 | @menu |
| 249 | * Options:: Command Line Options |
| 250 | * Environment:: Environment Variables |
| 251 | @end menu |
| 252 | |
| 253 | @node Options |
| 254 | @section Command Line Options |
| 255 | @end ifset |
| 256 | |
| 257 | @cindex command line |
| 258 | @cindex options |
| 259 | |
| 260 | @c man begin OPTIONS |
| 261 | |
| 262 | The linker supports a plethora of command-line options, but in actual |
| 263 | practice few of them are used in any particular context. |
| 264 | @cindex standard Unix system |
| 265 | For instance, a frequent use of @command{ld} is to link standard Unix |
| 266 | object files on a standard, supported Unix system. On such a system, to |
| 267 | link a file @code{hello.o}: |
| 268 | |
| 269 | @smallexample |
| 270 | ld -o @var{output} /lib/crt0.o hello.o -lc |
| 271 | @end smallexample |
| 272 | |
| 273 | This tells @command{ld} to produce a file called @var{output} as the |
| 274 | result of linking the file @code{/lib/crt0.o} with @code{hello.o} and |
| 275 | the library @code{libc.a}, which will come from the standard search |
| 276 | directories. (See the discussion of the @samp{-l} option below.) |
| 277 | |
| 278 | Some of the command-line options to @command{ld} may be specified at any |
| 279 | point in the command line. However, options which refer to files, such |
| 280 | as @samp{-l} or @samp{-T}, cause the file to be read at the point at |
| 281 | which the option appears in the command line, relative to the object |
| 282 | files and other file options. Repeating non-file options with a |
| 283 | different argument will either have no further effect, or override prior |
| 284 | occurrences (those further to the left on the command line) of that |
| 285 | option. Options which may be meaningfully specified more than once are |
| 286 | noted in the descriptions below. |
| 287 | |
| 288 | @cindex object files |
| 289 | Non-option arguments are object files or archives which are to be linked |
| 290 | together. They may follow, precede, or be mixed in with command-line |
| 291 | options, except that an object file argument may not be placed between |
| 292 | an option and its argument. |
| 293 | |
| 294 | Usually the linker is invoked with at least one object file, but you can |
| 295 | specify other forms of binary input files using @samp{-l}, @samp{-R}, |
| 296 | and the script command language. If @emph{no} binary input files at all |
| 297 | are specified, the linker does not produce any output, and issues the |
| 298 | message @samp{No input files}. |
| 299 | |
| 300 | If the linker cannot recognize the format of an object file, it will |
| 301 | assume that it is a linker script. A script specified in this way |
| 302 | augments the main linker script used for the link (either the default |
| 303 | linker script or the one specified by using @samp{-T}). This feature |
| 304 | permits the linker to link against a file which appears to be an object |
| 305 | or an archive, but actually merely defines some symbol values, or uses |
| 306 | @code{INPUT} or @code{GROUP} to load other objects. Specifying a |
| 307 | script in this way merely augments the main linker script, with the |
| 308 | extra commands placed after the main script; use the @samp{-T} option |
| 309 | to replace the default linker script entirely, but note the effect of |
| 310 | the @code{INSERT} command. @xref{Scripts}. |
| 311 | |
| 312 | For options whose names are a single letter, |
| 313 | option arguments must either follow the option letter without intervening |
| 314 | whitespace, or be given as separate arguments immediately following the |
| 315 | option that requires them. |
| 316 | |
| 317 | For options whose names are multiple letters, either one dash or two can |
| 318 | precede the option name; for example, @samp{-trace-symbol} and |
| 319 | @samp{--trace-symbol} are equivalent. Note---there is one exception to |
| 320 | this rule. Multiple letter options that start with a lower case 'o' can |
| 321 | only be preceded by two dashes. This is to reduce confusion with the |
| 322 | @samp{-o} option. So for example @samp{-omagic} sets the output file |
| 323 | name to @samp{magic} whereas @samp{--omagic} sets the NMAGIC flag on the |
| 324 | output. |
| 325 | |
| 326 | Arguments to multiple-letter options must either be separated from the |
| 327 | option name by an equals sign, or be given as separate arguments |
| 328 | immediately following the option that requires them. For example, |
| 329 | @samp{--trace-symbol foo} and @samp{--trace-symbol=foo} are equivalent. |
| 330 | Unique abbreviations of the names of multiple-letter options are |
| 331 | accepted. |
| 332 | |
| 333 | Note---if the linker is being invoked indirectly, via a compiler driver |
| 334 | (e.g. @samp{gcc}) then all the linker command line options should be |
| 335 | prefixed by @samp{-Wl,} (or whatever is appropriate for the particular |
| 336 | compiler driver) like this: |
| 337 | |
| 338 | @smallexample |
| 339 | gcc -Wl,--start-group foo.o bar.o -Wl,--end-group |
| 340 | @end smallexample |
| 341 | |
| 342 | This is important, because otherwise the compiler driver program may |
| 343 | silently drop the linker options, resulting in a bad link. Confusion |
| 344 | may also arise when passing options that require values through a |
| 345 | driver, as the use of a space between option and argument acts as |
| 346 | a separator, and causes the driver to pass only the option to the linker |
| 347 | and the argument to the compiler. In this case, it is simplest to use |
| 348 | the joined forms of both single- and multiple-letter options, such as: |
| 349 | |
| 350 | @smallexample |
| 351 | gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map |
| 352 | @end smallexample |
| 353 | |
| 354 | Here is a table of the generic command line switches accepted by the GNU |
| 355 | linker: |
| 356 | |
| 357 | @table @gcctabopt |
| 358 | @include at-file.texi |
| 359 | |
| 360 | @kindex -a @var{keyword} |
| 361 | @item -a @var{keyword} |
| 362 | This option is supported for HP/UX compatibility. The @var{keyword} |
| 363 | argument must be one of the strings @samp{archive}, @samp{shared}, or |
| 364 | @samp{default}. @samp{-aarchive} is functionally equivalent to |
| 365 | @samp{-Bstatic}, and the other two keywords are functionally equivalent |
| 366 | to @samp{-Bdynamic}. This option may be used any number of times. |
| 367 | |
| 368 | @kindex --audit @var{AUDITLIB} |
| 369 | @item --audit @var{AUDITLIB} |
| 370 | Adds @var{AUDITLIB} to the @code{DT_AUDIT} entry of the dynamic section. |
| 371 | @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME |
| 372 | specified in the library. If specified multiple times @code{DT_AUDIT} |
| 373 | will contain a colon separated list of audit interfaces to use. If the linker |
| 374 | finds an object with an audit entry while searching for shared libraries, |
| 375 | it will add a corresponding @code{DT_DEPAUDIT} entry in the output file. |
| 376 | This option is only meaningful on ELF platforms supporting the rtld-audit |
| 377 | interface. |
| 378 | |
| 379 | @ifset I960 |
| 380 | @cindex architectures |
| 381 | @kindex -A @var{arch} |
| 382 | @item -A @var{architecture} |
| 383 | @kindex --architecture=@var{arch} |
| 384 | @itemx --architecture=@var{architecture} |
| 385 | In the current release of @command{ld}, this option is useful only for the |
| 386 | Intel 960 family of architectures. In that @command{ld} configuration, the |
| 387 | @var{architecture} argument identifies the particular architecture in |
| 388 | the 960 family, enabling some safeguards and modifying the |
| 389 | archive-library search path. @xref{i960,,@command{ld} and the Intel 960 |
| 390 | family}, for details. |
| 391 | |
| 392 | Future releases of @command{ld} may support similar functionality for |
| 393 | other architecture families. |
| 394 | @end ifset |
| 395 | |
| 396 | @ifclear SingleFormat |
| 397 | @cindex binary input format |
| 398 | @kindex -b @var{format} |
| 399 | @kindex --format=@var{format} |
| 400 | @cindex input format |
| 401 | @cindex input format |
| 402 | @item -b @var{input-format} |
| 403 | @itemx --format=@var{input-format} |
| 404 | @command{ld} may be configured to support more than one kind of object |
| 405 | file. If your @command{ld} is configured this way, you can use the |
| 406 | @samp{-b} option to specify the binary format for input object files |
| 407 | that follow this option on the command line. Even when @command{ld} is |
| 408 | configured to support alternative object formats, you don't usually need |
| 409 | to specify this, as @command{ld} should be configured to expect as a |
| 410 | default input format the most usual format on each machine. |
| 411 | @var{input-format} is a text string, the name of a particular format |
| 412 | supported by the BFD libraries. (You can list the available binary |
| 413 | formats with @samp{objdump -i}.) |
| 414 | @xref{BFD}. |
| 415 | |
| 416 | You may want to use this option if you are linking files with an unusual |
| 417 | binary format. You can also use @samp{-b} to switch formats explicitly (when |
| 418 | linking object files of different formats), by including |
| 419 | @samp{-b @var{input-format}} before each group of object files in a |
| 420 | particular format. |
| 421 | |
| 422 | The default format is taken from the environment variable |
| 423 | @code{GNUTARGET}. |
| 424 | @ifset UsesEnvVars |
| 425 | @xref{Environment}. |
| 426 | @end ifset |
| 427 | You can also define the input format from a script, using the command |
| 428 | @code{TARGET}; |
| 429 | @ifclear man |
| 430 | see @ref{Format Commands}. |
| 431 | @end ifclear |
| 432 | @end ifclear |
| 433 | |
| 434 | @kindex -c @var{MRI-cmdfile} |
| 435 | @kindex --mri-script=@var{MRI-cmdfile} |
| 436 | @cindex compatibility, MRI |
| 437 | @item -c @var{MRI-commandfile} |
| 438 | @itemx --mri-script=@var{MRI-commandfile} |
| 439 | For compatibility with linkers produced by MRI, @command{ld} accepts script |
| 440 | files written in an alternate, restricted command language, described in |
| 441 | @ifclear man |
| 442 | @ref{MRI,,MRI Compatible Script Files}. |
| 443 | @end ifclear |
| 444 | @ifset man |
| 445 | the MRI Compatible Script Files section of GNU ld documentation. |
| 446 | @end ifset |
| 447 | Introduce MRI script files with |
| 448 | the option @samp{-c}; use the @samp{-T} option to run linker |
| 449 | scripts written in the general-purpose @command{ld} scripting language. |
| 450 | If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories |
| 451 | specified by any @samp{-L} options. |
| 452 | |
| 453 | @cindex common allocation |
| 454 | @kindex -d |
| 455 | @kindex -dc |
| 456 | @kindex -dp |
| 457 | @item -d |
| 458 | @itemx -dc |
| 459 | @itemx -dp |
| 460 | These three options are equivalent; multiple forms are supported for |
| 461 | compatibility with other linkers. They assign space to common symbols |
| 462 | even if a relocatable output file is specified (with @samp{-r}). The |
| 463 | script command @code{FORCE_COMMON_ALLOCATION} has the same effect. |
| 464 | @xref{Miscellaneous Commands}. |
| 465 | |
| 466 | @kindex --depaudit @var{AUDITLIB} |
| 467 | @kindex -P @var{AUDITLIB} |
| 468 | @item --depaudit @var{AUDITLIB} |
| 469 | @itemx -P @var{AUDITLIB} |
| 470 | Adds @var{AUDITLIB} to the @code{DT_DEPAUDIT} entry of the dynamic section. |
| 471 | @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME |
| 472 | specified in the library. If specified multiple times @code{DT_DEPAUDIT} |
| 473 | will contain a colon separated list of audit interfaces to use. This |
| 474 | option is only meaningful on ELF platforms supporting the rtld-audit interface. |
| 475 | The -P option is provided for Solaris compatibility. |
| 476 | |
| 477 | @cindex entry point, from command line |
| 478 | @kindex -e @var{entry} |
| 479 | @kindex --entry=@var{entry} |
| 480 | @item -e @var{entry} |
| 481 | @itemx --entry=@var{entry} |
| 482 | Use @var{entry} as the explicit symbol for beginning execution of your |
| 483 | program, rather than the default entry point. If there is no symbol |
| 484 | named @var{entry}, the linker will try to parse @var{entry} as a number, |
| 485 | and use that as the entry address (the number will be interpreted in |
| 486 | base 10; you may use a leading @samp{0x} for base 16, or a leading |
| 487 | @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults |
| 488 | and other ways of specifying the entry point. |
| 489 | |
| 490 | @kindex --exclude-libs |
| 491 | @item --exclude-libs @var{lib},@var{lib},... |
| 492 | Specifies a list of archive libraries from which symbols should not be automatically |
| 493 | exported. The library names may be delimited by commas or colons. Specifying |
| 494 | @code{--exclude-libs ALL} excludes symbols in all archive libraries from |
| 495 | automatic export. This option is available only for the i386 PE targeted |
| 496 | port of the linker and for ELF targeted ports. For i386 PE, symbols |
| 497 | explicitly listed in a .def file are still exported, regardless of this |
| 498 | option. For ELF targeted ports, symbols affected by this option will |
| 499 | be treated as hidden. |
| 500 | |
| 501 | @kindex --exclude-modules-for-implib |
| 502 | @item --exclude-modules-for-implib @var{module},@var{module},... |
| 503 | Specifies a list of object files or archive members, from which symbols |
| 504 | should not be automatically exported, but which should be copied wholesale |
| 505 | into the import library being generated during the link. The module names |
| 506 | may be delimited by commas or colons, and must match exactly the filenames |
| 507 | used by @command{ld} to open the files; for archive members, this is simply |
| 508 | the member name, but for object files the name listed must include and |
| 509 | match precisely any path used to specify the input file on the linker's |
| 510 | command-line. This option is available only for the i386 PE targeted port |
| 511 | of the linker. Symbols explicitly listed in a .def file are still exported, |
| 512 | regardless of this option. |
| 513 | |
| 514 | @cindex dynamic symbol table |
| 515 | @kindex -E |
| 516 | @kindex --export-dynamic |
| 517 | @kindex --no-export-dynamic |
| 518 | @item -E |
| 519 | @itemx --export-dynamic |
| 520 | @itemx --no-export-dynamic |
| 521 | When creating a dynamically linked executable, using the @option{-E} |
| 522 | option or the @option{--export-dynamic} option causes the linker to add |
| 523 | all symbols to the dynamic symbol table. The dynamic symbol table is the |
| 524 | set of symbols which are visible from dynamic objects at run time. |
| 525 | |
| 526 | If you do not use either of these options (or use the |
| 527 | @option{--no-export-dynamic} option to restore the default behavior), the |
| 528 | dynamic symbol table will normally contain only those symbols which are |
| 529 | referenced by some dynamic object mentioned in the link. |
| 530 | |
| 531 | If you use @code{dlopen} to load a dynamic object which needs to refer |
| 532 | back to the symbols defined by the program, rather than some other |
| 533 | dynamic object, then you will probably need to use this option when |
| 534 | linking the program itself. |
| 535 | |
| 536 | You can also use the dynamic list to control what symbols should |
| 537 | be added to the dynamic symbol table if the output format supports it. |
| 538 | See the description of @samp{--dynamic-list}. |
| 539 | |
| 540 | Note that this option is specific to ELF targeted ports. PE targets |
| 541 | support a similar function to export all symbols from a DLL or EXE; see |
| 542 | the description of @samp{--export-all-symbols} below. |
| 543 | |
| 544 | @ifclear SingleFormat |
| 545 | @cindex big-endian objects |
| 546 | @cindex endianness |
| 547 | @kindex -EB |
| 548 | @item -EB |
| 549 | Link big-endian objects. This affects the default output format. |
| 550 | |
| 551 | @cindex little-endian objects |
| 552 | @kindex -EL |
| 553 | @item -EL |
| 554 | Link little-endian objects. This affects the default output format. |
| 555 | @end ifclear |
| 556 | |
| 557 | @kindex -f @var{name} |
| 558 | @kindex --auxiliary=@var{name} |
| 559 | @item -f @var{name} |
| 560 | @itemx --auxiliary=@var{name} |
| 561 | When creating an ELF shared object, set the internal DT_AUXILIARY field |
| 562 | to the specified name. This tells the dynamic linker that the symbol |
| 563 | table of the shared object should be used as an auxiliary filter on the |
| 564 | symbol table of the shared object @var{name}. |
| 565 | |
| 566 | If you later link a program against this filter object, then, when you |
| 567 | run the program, the dynamic linker will see the DT_AUXILIARY field. If |
| 568 | the dynamic linker resolves any symbols from the filter object, it will |
| 569 | first check whether there is a definition in the shared object |
| 570 | @var{name}. If there is one, it will be used instead of the definition |
| 571 | in the filter object. The shared object @var{name} need not exist. |
| 572 | Thus the shared object @var{name} may be used to provide an alternative |
| 573 | implementation of certain functions, perhaps for debugging or for |
| 574 | machine specific performance. |
| 575 | |
| 576 | This option may be specified more than once. The DT_AUXILIARY entries |
| 577 | will be created in the order in which they appear on the command line. |
| 578 | |
| 579 | @kindex -F @var{name} |
| 580 | @kindex --filter=@var{name} |
| 581 | @item -F @var{name} |
| 582 | @itemx --filter=@var{name} |
| 583 | When creating an ELF shared object, set the internal DT_FILTER field to |
| 584 | the specified name. This tells the dynamic linker that the symbol table |
| 585 | of the shared object which is being created should be used as a filter |
| 586 | on the symbol table of the shared object @var{name}. |
| 587 | |
| 588 | If you later link a program against this filter object, then, when you |
| 589 | run the program, the dynamic linker will see the DT_FILTER field. The |
| 590 | dynamic linker will resolve symbols according to the symbol table of the |
| 591 | filter object as usual, but it will actually link to the definitions |
| 592 | found in the shared object @var{name}. Thus the filter object can be |
| 593 | used to select a subset of the symbols provided by the object |
| 594 | @var{name}. |
| 595 | |
| 596 | Some older linkers used the @option{-F} option throughout a compilation |
| 597 | toolchain for specifying object-file format for both input and output |
| 598 | object files. |
| 599 | @ifclear SingleFormat |
| 600 | The @sc{gnu} linker uses other mechanisms for this purpose: the |
| 601 | @option{-b}, @option{--format}, @option{--oformat} options, the |
| 602 | @code{TARGET} command in linker scripts, and the @code{GNUTARGET} |
| 603 | environment variable. |
| 604 | @end ifclear |
| 605 | The @sc{gnu} linker will ignore the @option{-F} option when not |
| 606 | creating an ELF shared object. |
| 607 | |
| 608 | @cindex finalization function |
| 609 | @kindex -fini=@var{name} |
| 610 | @item -fini=@var{name} |
| 611 | When creating an ELF executable or shared object, call NAME when the |
| 612 | executable or shared object is unloaded, by setting DT_FINI to the |
| 613 | address of the function. By default, the linker uses @code{_fini} as |
| 614 | the function to call. |
| 615 | |
| 616 | @kindex -g |
| 617 | @item -g |
| 618 | Ignored. Provided for compatibility with other tools. |
| 619 | |
| 620 | @kindex -G @var{value} |
| 621 | @kindex --gpsize=@var{value} |
| 622 | @cindex object size |
| 623 | @item -G @var{value} |
| 624 | @itemx --gpsize=@var{value} |
| 625 | Set the maximum size of objects to be optimized using the GP register to |
| 626 | @var{size}. This is only meaningful for object file formats such as |
| 627 | MIPS ECOFF which supports putting large and small objects into different |
| 628 | sections. This is ignored for other object file formats. |
| 629 | |
| 630 | @cindex runtime library name |
| 631 | @kindex -h @var{name} |
| 632 | @kindex -soname=@var{name} |
| 633 | @item -h @var{name} |
| 634 | @itemx -soname=@var{name} |
| 635 | When creating an ELF shared object, set the internal DT_SONAME field to |
| 636 | the specified name. When an executable is linked with a shared object |
| 637 | which has a DT_SONAME field, then when the executable is run the dynamic |
| 638 | linker will attempt to load the shared object specified by the DT_SONAME |
| 639 | field rather than the using the file name given to the linker. |
| 640 | |
| 641 | @kindex -i |
| 642 | @cindex incremental link |
| 643 | @item -i |
| 644 | Perform an incremental link (same as option @samp{-r}). |
| 645 | |
| 646 | @cindex initialization function |
| 647 | @kindex -init=@var{name} |
| 648 | @item -init=@var{name} |
| 649 | When creating an ELF executable or shared object, call NAME when the |
| 650 | executable or shared object is loaded, by setting DT_INIT to the address |
| 651 | of the function. By default, the linker uses @code{_init} as the |
| 652 | function to call. |
| 653 | |
| 654 | @cindex archive files, from cmd line |
| 655 | @kindex -l @var{namespec} |
| 656 | @kindex --library=@var{namespec} |
| 657 | @item -l @var{namespec} |
| 658 | @itemx --library=@var{namespec} |
| 659 | Add the archive or object file specified by @var{namespec} to the |
| 660 | list of files to link. This option may be used any number of times. |
| 661 | If @var{namespec} is of the form @file{:@var{filename}}, @command{ld} |
| 662 | will search the library path for a file called @var{filename}, otherwise it |
| 663 | will search the library path for a file called @file{lib@var{namespec}.a}. |
| 664 | |
| 665 | On systems which support shared libraries, @command{ld} may also search for |
| 666 | files other than @file{lib@var{namespec}.a}. Specifically, on ELF |
| 667 | and SunOS systems, @command{ld} will search a directory for a library |
| 668 | called @file{lib@var{namespec}.so} before searching for one called |
| 669 | @file{lib@var{namespec}.a}. (By convention, a @code{.so} extension |
| 670 | indicates a shared library.) Note that this behavior does not apply |
| 671 | to @file{:@var{filename}}, which always specifies a file called |
| 672 | @var{filename}. |
| 673 | |
| 674 | The linker will search an archive only once, at the location where it is |
| 675 | specified on the command line. If the archive defines a symbol which |
| 676 | was undefined in some object which appeared before the archive on the |
| 677 | command line, the linker will include the appropriate file(s) from the |
| 678 | archive. However, an undefined symbol in an object appearing later on |
| 679 | the command line will not cause the linker to search the archive again. |
| 680 | |
| 681 | See the @option{-(} option for a way to force the linker to search |
| 682 | archives multiple times. |
| 683 | |
| 684 | You may list the same archive multiple times on the command line. |
| 685 | |
| 686 | @ifset GENERIC |
| 687 | This type of archive searching is standard for Unix linkers. However, |
| 688 | if you are using @command{ld} on AIX, note that it is different from the |
| 689 | behaviour of the AIX linker. |
| 690 | @end ifset |
| 691 | |
| 692 | @cindex search directory, from cmd line |
| 693 | @kindex -L @var{dir} |
| 694 | @kindex --library-path=@var{dir} |
| 695 | @item -L @var{searchdir} |
| 696 | @itemx --library-path=@var{searchdir} |
| 697 | Add path @var{searchdir} to the list of paths that @command{ld} will search |
| 698 | for archive libraries and @command{ld} control scripts. You may use this |
| 699 | option any number of times. The directories are searched in the order |
| 700 | in which they are specified on the command line. Directories specified |
| 701 | on the command line are searched before the default directories. All |
| 702 | @option{-L} options apply to all @option{-l} options, regardless of the |
| 703 | order in which the options appear. @option{-L} options do not affect |
| 704 | how @command{ld} searches for a linker script unless @option{-T} |
| 705 | option is specified. |
| 706 | |
| 707 | If @var{searchdir} begins with @code{=}, then the @code{=} will be replaced |
| 708 | by the @dfn{sysroot prefix}, a path specified when the linker is configured. |
| 709 | |
| 710 | @ifset UsesEnvVars |
| 711 | The default set of paths searched (without being specified with |
| 712 | @samp{-L}) depends on which emulation mode @command{ld} is using, and in |
| 713 | some cases also on how it was configured. @xref{Environment}. |
| 714 | @end ifset |
| 715 | |
| 716 | The paths can also be specified in a link script with the |
| 717 | @code{SEARCH_DIR} command. Directories specified this way are searched |
| 718 | at the point in which the linker script appears in the command line. |
| 719 | |
| 720 | @cindex emulation |
| 721 | @kindex -m @var{emulation} |
| 722 | @item -m @var{emulation} |
| 723 | Emulate the @var{emulation} linker. You can list the available |
| 724 | emulations with the @samp{--verbose} or @samp{-V} options. |
| 725 | |
| 726 | If the @samp{-m} option is not used, the emulation is taken from the |
| 727 | @code{LDEMULATION} environment variable, if that is defined. |
| 728 | |
| 729 | Otherwise, the default emulation depends upon how the linker was |
| 730 | configured. |
| 731 | |
| 732 | @cindex link map |
| 733 | @kindex -M |
| 734 | @kindex --print-map |
| 735 | @item -M |
| 736 | @itemx --print-map |
| 737 | Print a link map to the standard output. A link map provides |
| 738 | information about the link, including the following: |
| 739 | |
| 740 | @itemize @bullet |
| 741 | @item |
| 742 | Where object files are mapped into memory. |
| 743 | @item |
| 744 | How common symbols are allocated. |
| 745 | @item |
| 746 | All archive members included in the link, with a mention of the symbol |
| 747 | which caused the archive member to be brought in. |
| 748 | @item |
| 749 | The values assigned to symbols. |
| 750 | |
| 751 | Note - symbols whose values are computed by an expression which |
| 752 | involves a reference to a previous value of the same symbol may not |
| 753 | have correct result displayed in the link map. This is because the |
| 754 | linker discards intermediate results and only retains the final value |
| 755 | of an expression. Under such circumstances the linker will display |
| 756 | the final value enclosed by square brackets. Thus for example a |
| 757 | linker script containing: |
| 758 | |
| 759 | @smallexample |
| 760 | foo = 1 |
| 761 | foo = foo * 4 |
| 762 | foo = foo + 8 |
| 763 | @end smallexample |
| 764 | |
| 765 | will produce the following output in the link map if the @option{-M} |
| 766 | option is used: |
| 767 | |
| 768 | @smallexample |
| 769 | 0x00000001 foo = 0x1 |
| 770 | [0x0000000c] foo = (foo * 0x4) |
| 771 | [0x0000000c] foo = (foo + 0x8) |
| 772 | @end smallexample |
| 773 | |
| 774 | See @ref{Expressions} for more information about expressions in linker |
| 775 | scripts. |
| 776 | @end itemize |
| 777 | |
| 778 | @kindex -n |
| 779 | @cindex read-only text |
| 780 | @cindex NMAGIC |
| 781 | @kindex --nmagic |
| 782 | @item -n |
| 783 | @itemx --nmagic |
| 784 | Turn off page alignment of sections, and disable linking against shared |
| 785 | libraries. If the output format supports Unix style magic numbers, |
| 786 | mark the output as @code{NMAGIC}. |
| 787 | |
| 788 | @kindex -N |
| 789 | @kindex --omagic |
| 790 | @cindex read/write from cmd line |
| 791 | @cindex OMAGIC |
| 792 | @item -N |
| 793 | @itemx --omagic |
| 794 | Set the text and data sections to be readable and writable. Also, do |
| 795 | not page-align the data segment, and disable linking against shared |
| 796 | libraries. If the output format supports Unix style magic numbers, |
| 797 | mark the output as @code{OMAGIC}. Note: Although a writable text section |
| 798 | is allowed for PE-COFF targets, it does not conform to the format |
| 799 | specification published by Microsoft. |
| 800 | |
| 801 | @kindex --no-omagic |
| 802 | @cindex OMAGIC |
| 803 | @item --no-omagic |
| 804 | This option negates most of the effects of the @option{-N} option. It |
| 805 | sets the text section to be read-only, and forces the data segment to |
| 806 | be page-aligned. Note - this option does not enable linking against |
| 807 | shared libraries. Use @option{-Bdynamic} for this. |
| 808 | |
| 809 | @kindex -o @var{output} |
| 810 | @kindex --output=@var{output} |
| 811 | @cindex naming the output file |
| 812 | @item -o @var{output} |
| 813 | @itemx --output=@var{output} |
| 814 | Use @var{output} as the name for the program produced by @command{ld}; if this |
| 815 | option is not specified, the name @file{a.out} is used by default. The |
| 816 | script command @code{OUTPUT} can also specify the output file name. |
| 817 | |
| 818 | @kindex -O @var{level} |
| 819 | @cindex generating optimized output |
| 820 | @item -O @var{level} |
| 821 | If @var{level} is a numeric values greater than zero @command{ld} optimizes |
| 822 | the output. This might take significantly longer and therefore probably |
| 823 | should only be enabled for the final binary. At the moment this |
| 824 | option only affects ELF shared library generation. Future releases of |
| 825 | the linker may make more use of this option. Also currently there is |
| 826 | no difference in the linker's behaviour for different non-zero values |
| 827 | of this option. Again this may change with future releases. |
| 828 | |
| 829 | @kindex -q |
| 830 | @kindex --emit-relocs |
| 831 | @cindex retain relocations in final executable |
| 832 | @item -q |
| 833 | @itemx --emit-relocs |
| 834 | Leave relocation sections and contents in fully linked executables. |
| 835 | Post link analysis and optimization tools may need this information in |
| 836 | order to perform correct modifications of executables. This results |
| 837 | in larger executables. |
| 838 | |
| 839 | This option is currently only supported on ELF platforms. |
| 840 | |
| 841 | @kindex --force-dynamic |
| 842 | @cindex forcing the creation of dynamic sections |
| 843 | @item --force-dynamic |
| 844 | Force the output file to have dynamic sections. This option is specific |
| 845 | to VxWorks targets. |
| 846 | |
| 847 | @cindex partial link |
| 848 | @cindex relocatable output |
| 849 | @kindex -r |
| 850 | @kindex --relocatable |
| 851 | @item -r |
| 852 | @itemx --relocatable |
| 853 | Generate relocatable output---i.e., generate an output file that can in |
| 854 | turn serve as input to @command{ld}. This is often called @dfn{partial |
| 855 | linking}. As a side effect, in environments that support standard Unix |
| 856 | magic numbers, this option also sets the output file's magic number to |
| 857 | @code{OMAGIC}. |
| 858 | @c ; see @option{-N}. |
| 859 | If this option is not specified, an absolute file is produced. When |
| 860 | linking C++ programs, this option @emph{will not} resolve references to |
| 861 | constructors; to do that, use @samp{-Ur}. |
| 862 | |
| 863 | When an input file does not have the same format as the output file, |
| 864 | partial linking is only supported if that input file does not contain any |
| 865 | relocations. Different output formats can have further restrictions; for |
| 866 | example some @code{a.out}-based formats do not support partial linking |
| 867 | with input files in other formats at all. |
| 868 | |
| 869 | This option does the same thing as @samp{-i}. |
| 870 | |
| 871 | @kindex -R @var{file} |
| 872 | @kindex --just-symbols=@var{file} |
| 873 | @cindex symbol-only input |
| 874 | @item -R @var{filename} |
| 875 | @itemx --just-symbols=@var{filename} |
| 876 | Read symbol names and their addresses from @var{filename}, but do not |
| 877 | relocate it or include it in the output. This allows your output file |
| 878 | to refer symbolically to absolute locations of memory defined in other |
| 879 | programs. You may use this option more than once. |
| 880 | |
| 881 | For compatibility with other ELF linkers, if the @option{-R} option is |
| 882 | followed by a directory name, rather than a file name, it is treated as |
| 883 | the @option{-rpath} option. |
| 884 | |
| 885 | @kindex -s |
| 886 | @kindex --strip-all |
| 887 | @cindex strip all symbols |
| 888 | @item -s |
| 889 | @itemx --strip-all |
| 890 | Omit all symbol information from the output file. |
| 891 | |
| 892 | @kindex -S |
| 893 | @kindex --strip-debug |
| 894 | @cindex strip debugger symbols |
| 895 | @item -S |
| 896 | @itemx --strip-debug |
| 897 | Omit debugger symbol information (but not all symbols) from the output file. |
| 898 | |
| 899 | @kindex -t |
| 900 | @kindex --trace |
| 901 | @cindex input files, displaying |
| 902 | @item -t |
| 903 | @itemx --trace |
| 904 | Print the names of the input files as @command{ld} processes them. |
| 905 | |
| 906 | @kindex -T @var{script} |
| 907 | @kindex --script=@var{script} |
| 908 | @cindex script files |
| 909 | @item -T @var{scriptfile} |
| 910 | @itemx --script=@var{scriptfile} |
| 911 | Use @var{scriptfile} as the linker script. This script replaces |
| 912 | @command{ld}'s default linker script (rather than adding to it), so |
| 913 | @var{commandfile} must specify everything necessary to describe the |
| 914 | output file. @xref{Scripts}. If @var{scriptfile} does not exist in |
| 915 | the current directory, @code{ld} looks for it in the directories |
| 916 | specified by any preceding @samp{-L} options. Multiple @samp{-T} |
| 917 | options accumulate. |
| 918 | |
| 919 | @kindex -dT @var{script} |
| 920 | @kindex --default-script=@var{script} |
| 921 | @cindex script files |
| 922 | @item -dT @var{scriptfile} |
| 923 | @itemx --default-script=@var{scriptfile} |
| 924 | Use @var{scriptfile} as the default linker script. @xref{Scripts}. |
| 925 | |
| 926 | This option is similar to the @option{--script} option except that |
| 927 | processing of the script is delayed until after the rest of the |
| 928 | command line has been processed. This allows options placed after the |
| 929 | @option{--default-script} option on the command line to affect the |
| 930 | behaviour of the linker script, which can be important when the linker |
| 931 | command line cannot be directly controlled by the user. (eg because |
| 932 | the command line is being constructed by another tool, such as |
| 933 | @samp{gcc}). |
| 934 | |
| 935 | @kindex -u @var{symbol} |
| 936 | @kindex --undefined=@var{symbol} |
| 937 | @cindex undefined symbol |
| 938 | @item -u @var{symbol} |
| 939 | @itemx --undefined=@var{symbol} |
| 940 | Force @var{symbol} to be entered in the output file as an undefined |
| 941 | symbol. Doing this may, for example, trigger linking of additional |
| 942 | modules from standard libraries. @samp{-u} may be repeated with |
| 943 | different option arguments to enter additional undefined symbols. This |
| 944 | option is equivalent to the @code{EXTERN} linker script command. |
| 945 | |
| 946 | @kindex -Ur |
| 947 | @cindex constructors |
| 948 | @item -Ur |
| 949 | For anything other than C++ programs, this option is equivalent to |
| 950 | @samp{-r}: it generates relocatable output---i.e., an output file that can in |
| 951 | turn serve as input to @command{ld}. When linking C++ programs, @samp{-Ur} |
| 952 | @emph{does} resolve references to constructors, unlike @samp{-r}. |
| 953 | It does not work to use @samp{-Ur} on files that were themselves linked |
| 954 | with @samp{-Ur}; once the constructor table has been built, it cannot |
| 955 | be added to. Use @samp{-Ur} only for the last partial link, and |
| 956 | @samp{-r} for the others. |
| 957 | |
| 958 | @kindex --unique[=@var{SECTION}] |
| 959 | @item --unique[=@var{SECTION}] |
| 960 | Creates a separate output section for every input section matching |
| 961 | @var{SECTION}, or if the optional wildcard @var{SECTION} argument is |
| 962 | missing, for every orphan input section. An orphan section is one not |
| 963 | specifically mentioned in a linker script. You may use this option |
| 964 | multiple times on the command line; It prevents the normal merging of |
| 965 | input sections with the same name, overriding output section assignments |
| 966 | in a linker script. |
| 967 | |
| 968 | @kindex -v |
| 969 | @kindex -V |
| 970 | @kindex --version |
| 971 | @cindex version |
| 972 | @item -v |
| 973 | @itemx --version |
| 974 | @itemx -V |
| 975 | Display the version number for @command{ld}. The @option{-V} option also |
| 976 | lists the supported emulations. |
| 977 | |
| 978 | @kindex -x |
| 979 | @kindex --discard-all |
| 980 | @cindex deleting local symbols |
| 981 | @item -x |
| 982 | @itemx --discard-all |
| 983 | Delete all local symbols. |
| 984 | |
| 985 | @kindex -X |
| 986 | @kindex --discard-locals |
| 987 | @cindex local symbols, deleting |
| 988 | @item -X |
| 989 | @itemx --discard-locals |
| 990 | Delete all temporary local symbols. (These symbols start with |
| 991 | system-specific local label prefixes, typically @samp{.L} for ELF systems |
| 992 | or @samp{L} for traditional a.out systems.) |
| 993 | |
| 994 | @kindex -y @var{symbol} |
| 995 | @kindex --trace-symbol=@var{symbol} |
| 996 | @cindex symbol tracing |
| 997 | @item -y @var{symbol} |
| 998 | @itemx --trace-symbol=@var{symbol} |
| 999 | Print the name of each linked file in which @var{symbol} appears. This |
| 1000 | option may be given any number of times. On many systems it is necessary |
| 1001 | to prepend an underscore. |
| 1002 | |
| 1003 | This option is useful when you have an undefined symbol in your link but |
| 1004 | don't know where the reference is coming from. |
| 1005 | |
| 1006 | @kindex -Y @var{path} |
| 1007 | @item -Y @var{path} |
| 1008 | Add @var{path} to the default library search path. This option exists |
| 1009 | for Solaris compatibility. |
| 1010 | |
| 1011 | @kindex -z @var{keyword} |
| 1012 | @item -z @var{keyword} |
| 1013 | The recognized keywords are: |
| 1014 | @table @samp |
| 1015 | |
| 1016 | @item combreloc |
| 1017 | Combines multiple reloc sections and sorts them to make dynamic symbol |
| 1018 | lookup caching possible. |
| 1019 | |
| 1020 | @item defs |
| 1021 | Disallows undefined symbols in object files. Undefined symbols in |
| 1022 | shared libraries are still allowed. |
| 1023 | |
| 1024 | @item execstack |
| 1025 | Marks the object as requiring executable stack. |
| 1026 | |
| 1027 | @item global |
| 1028 | This option is only meaningful when building a shared object. It makes |
| 1029 | the symbols defined by this shared object available for symbol resolution |
| 1030 | of subsequently loaded libraries. |
| 1031 | |
| 1032 | @item initfirst |
| 1033 | This option is only meaningful when building a shared object. |
| 1034 | It marks the object so that its runtime initialization will occur |
| 1035 | before the runtime initialization of any other objects brought into |
| 1036 | the process at the same time. Similarly the runtime finalization of |
| 1037 | the object will occur after the runtime finalization of any other |
| 1038 | objects. |
| 1039 | |
| 1040 | @item interpose |
| 1041 | Marks the object that its symbol table interposes before all symbols |
| 1042 | but the primary executable. |
| 1043 | |
| 1044 | @item lazy |
| 1045 | When generating an executable or shared library, mark it to tell the |
| 1046 | dynamic linker to defer function call resolution to the point when |
| 1047 | the function is called (lazy binding), rather than at load time. |
| 1048 | Lazy binding is the default. |
| 1049 | |
| 1050 | @item loadfltr |
| 1051 | Marks the object that its filters be processed immediately at |
| 1052 | runtime. |
| 1053 | |
| 1054 | @item muldefs |
| 1055 | Allows multiple definitions. |
| 1056 | |
| 1057 | @item nocombreloc |
| 1058 | Disables multiple reloc sections combining. |
| 1059 | |
| 1060 | @item nocopyreloc |
| 1061 | Disables production of copy relocs. |
| 1062 | |
| 1063 | @item nodefaultlib |
| 1064 | Marks the object that the search for dependencies of this object will |
| 1065 | ignore any default library search paths. |
| 1066 | |
| 1067 | @item nodelete |
| 1068 | Marks the object shouldn't be unloaded at runtime. |
| 1069 | |
| 1070 | @item nodlopen |
| 1071 | Marks the object not available to @code{dlopen}. |
| 1072 | |
| 1073 | @item nodump |
| 1074 | Marks the object can not be dumped by @code{dldump}. |
| 1075 | |
| 1076 | @item noexecstack |
| 1077 | Marks the object as not requiring executable stack. |
| 1078 | |
| 1079 | @item norelro |
| 1080 | Don't create an ELF @code{PT_GNU_RELRO} segment header in the object. |
| 1081 | |
| 1082 | @item now |
| 1083 | When generating an executable or shared library, mark it to tell the |
| 1084 | dynamic linker to resolve all symbols when the program is started, or |
| 1085 | when the shared library is linked to using dlopen, instead of |
| 1086 | deferring function call resolution to the point when the function is |
| 1087 | first called. |
| 1088 | |
| 1089 | @item origin |
| 1090 | Marks the object may contain $ORIGIN. |
| 1091 | |
| 1092 | @item relro |
| 1093 | Create an ELF @code{PT_GNU_RELRO} segment header in the object. |
| 1094 | |
| 1095 | @item max-page-size=@var{value} |
| 1096 | Set the emulation maximum page size to @var{value}. |
| 1097 | |
| 1098 | @item common-page-size=@var{value} |
| 1099 | Set the emulation common page size to @var{value}. |
| 1100 | |
| 1101 | @item stack-size=@var{value} |
| 1102 | Specify a stack size for in an ELF @code{PT_GNU_STACK} segment. |
| 1103 | Specifying zero will override any default non-zero sized |
| 1104 | @code{PT_GNU_STACK} segment creation. |
| 1105 | |
| 1106 | @end table |
| 1107 | |
| 1108 | Other keywords are ignored for Solaris compatibility. |
| 1109 | |
| 1110 | @kindex -( |
| 1111 | @cindex groups of archives |
| 1112 | @item -( @var{archives} -) |
| 1113 | @itemx --start-group @var{archives} --end-group |
| 1114 | The @var{archives} should be a list of archive files. They may be |
| 1115 | either explicit file names, or @samp{-l} options. |
| 1116 | |
| 1117 | The specified archives are searched repeatedly until no new undefined |
| 1118 | references are created. Normally, an archive is searched only once in |
| 1119 | the order that it is specified on the command line. If a symbol in that |
| 1120 | archive is needed to resolve an undefined symbol referred to by an |
| 1121 | object in an archive that appears later on the command line, the linker |
| 1122 | would not be able to resolve that reference. By grouping the archives, |
| 1123 | they all be searched repeatedly until all possible references are |
| 1124 | resolved. |
| 1125 | |
| 1126 | Using this option has a significant performance cost. It is best to use |
| 1127 | it only when there are unavoidable circular references between two or |
| 1128 | more archives. |
| 1129 | |
| 1130 | @kindex --accept-unknown-input-arch |
| 1131 | @kindex --no-accept-unknown-input-arch |
| 1132 | @item --accept-unknown-input-arch |
| 1133 | @itemx --no-accept-unknown-input-arch |
| 1134 | Tells the linker to accept input files whose architecture cannot be |
| 1135 | recognised. The assumption is that the user knows what they are doing |
| 1136 | and deliberately wants to link in these unknown input files. This was |
| 1137 | the default behaviour of the linker, before release 2.14. The default |
| 1138 | behaviour from release 2.14 onwards is to reject such input files, and |
| 1139 | so the @samp{--accept-unknown-input-arch} option has been added to |
| 1140 | restore the old behaviour. |
| 1141 | |
| 1142 | @kindex --as-needed |
| 1143 | @kindex --no-as-needed |
| 1144 | @item --as-needed |
| 1145 | @itemx --no-as-needed |
| 1146 | This option affects ELF DT_NEEDED tags for dynamic libraries mentioned |
| 1147 | on the command line after the @option{--as-needed} option. Normally |
| 1148 | the linker will add a DT_NEEDED tag for each dynamic library mentioned |
| 1149 | on the command line, regardless of whether the library is actually |
| 1150 | needed or not. @option{--as-needed} causes a DT_NEEDED tag to only be |
| 1151 | emitted for a library that satisfies an undefined symbol reference |
| 1152 | from a regular object file or, if the library is not found in the |
| 1153 | DT_NEEDED lists of other libraries linked up to that point, an |
| 1154 | undefined symbol reference from another dynamic library. |
| 1155 | @option{--no-as-needed} restores the default behaviour. |
| 1156 | |
| 1157 | @kindex --add-needed |
| 1158 | @kindex --no-add-needed |
| 1159 | @item --add-needed |
| 1160 | @itemx --no-add-needed |
| 1161 | These two options have been deprecated because of the similarity of |
| 1162 | their names to the @option{--as-needed} and @option{--no-as-needed} |
| 1163 | options. They have been replaced by @option{--copy-dt-needed-entries} |
| 1164 | and @option{--no-copy-dt-needed-entries}. |
| 1165 | |
| 1166 | @kindex -assert @var{keyword} |
| 1167 | @item -assert @var{keyword} |
| 1168 | This option is ignored for SunOS compatibility. |
| 1169 | |
| 1170 | @kindex -Bdynamic |
| 1171 | @kindex -dy |
| 1172 | @kindex -call_shared |
| 1173 | @item -Bdynamic |
| 1174 | @itemx -dy |
| 1175 | @itemx -call_shared |
| 1176 | Link against dynamic libraries. This is only meaningful on platforms |
| 1177 | for which shared libraries are supported. This option is normally the |
| 1178 | default on such platforms. The different variants of this option are |
| 1179 | for compatibility with various systems. You may use this option |
| 1180 | multiple times on the command line: it affects library searching for |
| 1181 | @option{-l} options which follow it. |
| 1182 | |
| 1183 | @kindex -Bgroup |
| 1184 | @item -Bgroup |
| 1185 | Set the @code{DF_1_GROUP} flag in the @code{DT_FLAGS_1} entry in the dynamic |
| 1186 | section. This causes the runtime linker to handle lookups in this |
| 1187 | object and its dependencies to be performed only inside the group. |
| 1188 | @option{--unresolved-symbols=report-all} is implied. This option is |
| 1189 | only meaningful on ELF platforms which support shared libraries. |
| 1190 | |
| 1191 | @kindex -Bstatic |
| 1192 | @kindex -dn |
| 1193 | @kindex -non_shared |
| 1194 | @kindex -static |
| 1195 | @item -Bstatic |
| 1196 | @itemx -dn |
| 1197 | @itemx -non_shared |
| 1198 | @itemx -static |
| 1199 | Do not link against shared libraries. This is only meaningful on |
| 1200 | platforms for which shared libraries are supported. The different |
| 1201 | variants of this option are for compatibility with various systems. You |
| 1202 | may use this option multiple times on the command line: it affects |
| 1203 | library searching for @option{-l} options which follow it. This |
| 1204 | option also implies @option{--unresolved-symbols=report-all}. This |
| 1205 | option can be used with @option{-shared}. Doing so means that a |
| 1206 | shared library is being created but that all of the library's external |
| 1207 | references must be resolved by pulling in entries from static |
| 1208 | libraries. |
| 1209 | |
| 1210 | @kindex -Bsymbolic |
| 1211 | @item -Bsymbolic |
| 1212 | When creating a shared library, bind references to global symbols to the |
| 1213 | definition within the shared library, if any. Normally, it is possible |
| 1214 | for a program linked against a shared library to override the definition |
| 1215 | within the shared library. This option is only meaningful on ELF |
| 1216 | platforms which support shared libraries. |
| 1217 | |
| 1218 | @kindex -Bsymbolic-functions |
| 1219 | @item -Bsymbolic-functions |
| 1220 | When creating a shared library, bind references to global function |
| 1221 | symbols to the definition within the shared library, if any. |
| 1222 | This option is only meaningful on ELF platforms which support shared |
| 1223 | libraries. |
| 1224 | |
| 1225 | @kindex --dynamic-list=@var{dynamic-list-file} |
| 1226 | @item --dynamic-list=@var{dynamic-list-file} |
| 1227 | Specify the name of a dynamic list file to the linker. This is |
| 1228 | typically used when creating shared libraries to specify a list of |
| 1229 | global symbols whose references shouldn't be bound to the definition |
| 1230 | within the shared library, or creating dynamically linked executables |
| 1231 | to specify a list of symbols which should be added to the symbol table |
| 1232 | in the executable. This option is only meaningful on ELF platforms |
| 1233 | which support shared libraries. |
| 1234 | |
| 1235 | The format of the dynamic list is the same as the version node without |
| 1236 | scope and node name. See @ref{VERSION} for more information. |
| 1237 | |
| 1238 | @kindex --dynamic-list-data |
| 1239 | @item --dynamic-list-data |
| 1240 | Include all global data symbols to the dynamic list. |
| 1241 | |
| 1242 | @kindex --dynamic-list-cpp-new |
| 1243 | @item --dynamic-list-cpp-new |
| 1244 | Provide the builtin dynamic list for C++ operator new and delete. It |
| 1245 | is mainly useful for building shared libstdc++. |
| 1246 | |
| 1247 | @kindex --dynamic-list-cpp-typeinfo |
| 1248 | @item --dynamic-list-cpp-typeinfo |
| 1249 | Provide the builtin dynamic list for C++ runtime type identification. |
| 1250 | |
| 1251 | @kindex --check-sections |
| 1252 | @kindex --no-check-sections |
| 1253 | @item --check-sections |
| 1254 | @itemx --no-check-sections |
| 1255 | Asks the linker @emph{not} to check section addresses after they have |
| 1256 | been assigned to see if there are any overlaps. Normally the linker will |
| 1257 | perform this check, and if it finds any overlaps it will produce |
| 1258 | suitable error messages. The linker does know about, and does make |
| 1259 | allowances for sections in overlays. The default behaviour can be |
| 1260 | restored by using the command line switch @option{--check-sections}. |
| 1261 | Section overlap is not usually checked for relocatable links. You can |
| 1262 | force checking in that case by using the @option{--check-sections} |
| 1263 | option. |
| 1264 | |
| 1265 | @kindex --copy-dt-needed-entries |
| 1266 | @kindex --no-copy-dt-needed-entries |
| 1267 | @item --copy-dt-needed-entries |
| 1268 | @itemx --no-copy-dt-needed-entries |
| 1269 | This option affects the treatment of dynamic libraries referred to |
| 1270 | by DT_NEEDED tags @emph{inside} ELF dynamic libraries mentioned on the |
| 1271 | command line. Normally the linker won't add a DT_NEEDED tag to the |
| 1272 | output binary for each library mentioned in a DT_NEEDED tag in an |
| 1273 | input dynamic library. With @option{--copy-dt-needed-entries} |
| 1274 | specified on the command line however any dynamic libraries that |
| 1275 | follow it will have their DT_NEEDED entries added. The default |
| 1276 | behaviour can be restored with @option{--no-copy-dt-needed-entries}. |
| 1277 | |
| 1278 | This option also has an effect on the resolution of symbols in dynamic |
| 1279 | libraries. With @option{--copy-dt-needed-entries} dynamic libraries |
| 1280 | mentioned on the command line will be recursively searched, following |
| 1281 | their DT_NEEDED tags to other libraries, in order to resolve symbols |
| 1282 | required by the output binary. With the default setting however |
| 1283 | the searching of dynamic libraries that follow it will stop with the |
| 1284 | dynamic library itself. No DT_NEEDED links will be traversed to resolve |
| 1285 | symbols. |
| 1286 | |
| 1287 | @cindex cross reference table |
| 1288 | @kindex --cref |
| 1289 | @item --cref |
| 1290 | Output a cross reference table. If a linker map file is being |
| 1291 | generated, the cross reference table is printed to the map file. |
| 1292 | Otherwise, it is printed on the standard output. |
| 1293 | |
| 1294 | The format of the table is intentionally simple, so that it may be |
| 1295 | easily processed by a script if necessary. The symbols are printed out, |
| 1296 | sorted by name. For each symbol, a list of file names is given. If the |
| 1297 | symbol is defined, the first file listed is the location of the |
| 1298 | definition. The remaining files contain references to the symbol. |
| 1299 | |
| 1300 | @cindex common allocation |
| 1301 | @kindex --no-define-common |
| 1302 | @item --no-define-common |
| 1303 | This option inhibits the assignment of addresses to common symbols. |
| 1304 | The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect. |
| 1305 | @xref{Miscellaneous Commands}. |
| 1306 | |
| 1307 | The @samp{--no-define-common} option allows decoupling |
| 1308 | the decision to assign addresses to Common symbols from the choice |
| 1309 | of the output file type; otherwise a non-Relocatable output type |
| 1310 | forces assigning addresses to Common symbols. |
| 1311 | Using @samp{--no-define-common} allows Common symbols that are referenced |
| 1312 | from a shared library to be assigned addresses only in the main program. |
| 1313 | This eliminates the unused duplicate space in the shared library, |
| 1314 | and also prevents any possible confusion over resolving to the wrong |
| 1315 | duplicate when there are many dynamic modules with specialized search |
| 1316 | paths for runtime symbol resolution. |
| 1317 | |
| 1318 | @cindex symbols, from command line |
| 1319 | @kindex --defsym=@var{symbol}=@var{exp} |
| 1320 | @item --defsym=@var{symbol}=@var{expression} |
| 1321 | Create a global symbol in the output file, containing the absolute |
| 1322 | address given by @var{expression}. You may use this option as many |
| 1323 | times as necessary to define multiple symbols in the command line. A |
| 1324 | limited form of arithmetic is supported for the @var{expression} in this |
| 1325 | context: you may give a hexadecimal constant or the name of an existing |
| 1326 | symbol, or use @code{+} and @code{-} to add or subtract hexadecimal |
| 1327 | constants or symbols. If you need more elaborate expressions, consider |
| 1328 | using the linker command language from a script (@pxref{Assignments,, |
| 1329 | Assignment: Symbol Definitions}). @emph{Note:} there should be no white |
| 1330 | space between @var{symbol}, the equals sign (``@key{=}''), and |
| 1331 | @var{expression}. |
| 1332 | |
| 1333 | @cindex demangling, from command line |
| 1334 | @kindex --demangle[=@var{style}] |
| 1335 | @kindex --no-demangle |
| 1336 | @item --demangle[=@var{style}] |
| 1337 | @itemx --no-demangle |
| 1338 | These options control whether to demangle symbol names in error messages |
| 1339 | and other output. When the linker is told to demangle, it tries to |
| 1340 | present symbol names in a readable fashion: it strips leading |
| 1341 | underscores if they are used by the object file format, and converts C++ |
| 1342 | mangled symbol names into user readable names. Different compilers have |
| 1343 | different mangling styles. The optional demangling style argument can be used |
| 1344 | to choose an appropriate demangling style for your compiler. The linker will |
| 1345 | demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE} |
| 1346 | is set. These options may be used to override the default. |
| 1347 | |
| 1348 | @cindex dynamic linker, from command line |
| 1349 | @kindex -I@var{file} |
| 1350 | @kindex --dynamic-linker=@var{file} |
| 1351 | @item -I@var{file} |
| 1352 | @itemx --dynamic-linker=@var{file} |
| 1353 | Set the name of the dynamic linker. This is only meaningful when |
| 1354 | generating dynamically linked ELF executables. The default dynamic |
| 1355 | linker is normally correct; don't use this unless you know what you are |
| 1356 | doing. |
| 1357 | |
| 1358 | @kindex --fatal-warnings |
| 1359 | @kindex --no-fatal-warnings |
| 1360 | @item --fatal-warnings |
| 1361 | @itemx --no-fatal-warnings |
| 1362 | Treat all warnings as errors. The default behaviour can be restored |
| 1363 | with the option @option{--no-fatal-warnings}. |
| 1364 | |
| 1365 | @kindex --force-exe-suffix |
| 1366 | @item --force-exe-suffix |
| 1367 | Make sure that an output file has a .exe suffix. |
| 1368 | |
| 1369 | If a successfully built fully linked output file does not have a |
| 1370 | @code{.exe} or @code{.dll} suffix, this option forces the linker to copy |
| 1371 | the output file to one of the same name with a @code{.exe} suffix. This |
| 1372 | option is useful when using unmodified Unix makefiles on a Microsoft |
| 1373 | Windows host, since some versions of Windows won't run an image unless |
| 1374 | it ends in a @code{.exe} suffix. |
| 1375 | |
| 1376 | @kindex --gc-sections |
| 1377 | @kindex --no-gc-sections |
| 1378 | @cindex garbage collection |
| 1379 | @item --gc-sections |
| 1380 | @itemx --no-gc-sections |
| 1381 | Enable garbage collection of unused input sections. It is ignored on |
| 1382 | targets that do not support this option. The default behaviour (of not |
| 1383 | performing this garbage collection) can be restored by specifying |
| 1384 | @samp{--no-gc-sections} on the command line. |
| 1385 | |
| 1386 | @samp{--gc-sections} decides which input sections are used by |
| 1387 | examining symbols and relocations. The section containing the entry |
| 1388 | symbol and all sections containing symbols undefined on the |
| 1389 | command-line will be kept, as will sections containing symbols |
| 1390 | referenced by dynamic objects. Note that when building shared |
| 1391 | libraries, the linker must assume that any visible symbol is |
| 1392 | referenced. Once this initial set of sections has been determined, |
| 1393 | the linker recursively marks as used any section referenced by their |
| 1394 | relocations. See @samp{--entry} and @samp{--undefined}. |
| 1395 | |
| 1396 | This option can be set when doing a partial link (enabled with option |
| 1397 | @samp{-r}). In this case the root of symbols kept must be explicitly |
| 1398 | specified either by an @samp{--entry} or @samp{--undefined} option or by |
| 1399 | a @code{ENTRY} command in the linker script. |
| 1400 | |
| 1401 | @kindex --print-gc-sections |
| 1402 | @kindex --no-print-gc-sections |
| 1403 | @cindex garbage collection |
| 1404 | @item --print-gc-sections |
| 1405 | @itemx --no-print-gc-sections |
| 1406 | List all sections removed by garbage collection. The listing is |
| 1407 | printed on stderr. This option is only effective if garbage |
| 1408 | collection has been enabled via the @samp{--gc-sections}) option. The |
| 1409 | default behaviour (of not listing the sections that are removed) can |
| 1410 | be restored by specifying @samp{--no-print-gc-sections} on the command |
| 1411 | line. |
| 1412 | |
| 1413 | @kindex --print-output-format |
| 1414 | @cindex output format |
| 1415 | @item --print-output-format |
| 1416 | Print the name of the default output format (perhaps influenced by |
| 1417 | other command-line options). This is the string that would appear |
| 1418 | in an @code{OUTPUT_FORMAT} linker script command (@pxref{File Commands}). |
| 1419 | |
| 1420 | @cindex help |
| 1421 | @cindex usage |
| 1422 | @kindex --help |
| 1423 | @item --help |
| 1424 | Print a summary of the command-line options on the standard output and exit. |
| 1425 | |
| 1426 | @kindex --target-help |
| 1427 | @item --target-help |
| 1428 | Print a summary of all target specific options on the standard output and exit. |
| 1429 | |
| 1430 | @kindex -Map=@var{mapfile} |
| 1431 | @item -Map=@var{mapfile} |
| 1432 | Print a link map to the file @var{mapfile}. See the description of the |
| 1433 | @option{-M} option, above. |
| 1434 | |
| 1435 | @cindex memory usage |
| 1436 | @kindex --no-keep-memory |
| 1437 | @item --no-keep-memory |
| 1438 | @command{ld} normally optimizes for speed over memory usage by caching the |
| 1439 | symbol tables of input files in memory. This option tells @command{ld} to |
| 1440 | instead optimize for memory usage, by rereading the symbol tables as |
| 1441 | necessary. This may be required if @command{ld} runs out of memory space |
| 1442 | while linking a large executable. |
| 1443 | |
| 1444 | @kindex --no-undefined |
| 1445 | @kindex -z defs |
| 1446 | @item --no-undefined |
| 1447 | @itemx -z defs |
| 1448 | Report unresolved symbol references from regular object files. This |
| 1449 | is done even if the linker is creating a non-symbolic shared library. |
| 1450 | The switch @option{--[no-]allow-shlib-undefined} controls the |
| 1451 | behaviour for reporting unresolved references found in shared |
| 1452 | libraries being linked in. |
| 1453 | |
| 1454 | @kindex --allow-multiple-definition |
| 1455 | @kindex -z muldefs |
| 1456 | @item --allow-multiple-definition |
| 1457 | @itemx -z muldefs |
| 1458 | Normally when a symbol is defined multiple times, the linker will |
| 1459 | report a fatal error. These options allow multiple definitions and the |
| 1460 | first definition will be used. |
| 1461 | |
| 1462 | @kindex --allow-shlib-undefined |
| 1463 | @kindex --no-allow-shlib-undefined |
| 1464 | @item --allow-shlib-undefined |
| 1465 | @itemx --no-allow-shlib-undefined |
| 1466 | Allows or disallows undefined symbols in shared libraries. |
| 1467 | This switch is similar to @option{--no-undefined} except that it |
| 1468 | determines the behaviour when the undefined symbols are in a |
| 1469 | shared library rather than a regular object file. It does not affect |
| 1470 | how undefined symbols in regular object files are handled. |
| 1471 | |
| 1472 | The default behaviour is to report errors for any undefined symbols |
| 1473 | referenced in shared libraries if the linker is being used to create |
| 1474 | an executable, but to allow them if the linker is being used to create |
| 1475 | a shared library. |
| 1476 | |
| 1477 | The reasons for allowing undefined symbol references in shared |
| 1478 | libraries specified at link time are that: |
| 1479 | |
| 1480 | @itemize @bullet |
| 1481 | @item |
| 1482 | A shared library specified at link time may not be the same as the one |
| 1483 | that is available at load time, so the symbol might actually be |
| 1484 | resolvable at load time. |
| 1485 | @item |
| 1486 | There are some operating systems, eg BeOS and HPPA, where undefined |
| 1487 | symbols in shared libraries are normal. |
| 1488 | |
| 1489 | The BeOS kernel for example patches shared libraries at load time to |
| 1490 | select whichever function is most appropriate for the current |
| 1491 | architecture. This is used, for example, to dynamically select an |
| 1492 | appropriate memset function. |
| 1493 | @end itemize |
| 1494 | |
| 1495 | @kindex --no-undefined-version |
| 1496 | @item --no-undefined-version |
| 1497 | Normally when a symbol has an undefined version, the linker will ignore |
| 1498 | it. This option disallows symbols with undefined version and a fatal error |
| 1499 | will be issued instead. |
| 1500 | |
| 1501 | @kindex --default-symver |
| 1502 | @item --default-symver |
| 1503 | Create and use a default symbol version (the soname) for unversioned |
| 1504 | exported symbols. |
| 1505 | |
| 1506 | @kindex --default-imported-symver |
| 1507 | @item --default-imported-symver |
| 1508 | Create and use a default symbol version (the soname) for unversioned |
| 1509 | imported symbols. |
| 1510 | |
| 1511 | @kindex --no-warn-mismatch |
| 1512 | @item --no-warn-mismatch |
| 1513 | Normally @command{ld} will give an error if you try to link together input |
| 1514 | files that are mismatched for some reason, perhaps because they have |
| 1515 | been compiled for different processors or for different endiannesses. |
| 1516 | This option tells @command{ld} that it should silently permit such possible |
| 1517 | errors. This option should only be used with care, in cases when you |
| 1518 | have taken some special action that ensures that the linker errors are |
| 1519 | inappropriate. |
| 1520 | |
| 1521 | @kindex --no-warn-search-mismatch |
| 1522 | @item --no-warn-search-mismatch |
| 1523 | Normally @command{ld} will give a warning if it finds an incompatible |
| 1524 | library during a library search. This option silences the warning. |
| 1525 | |
| 1526 | @kindex --no-whole-archive |
| 1527 | @item --no-whole-archive |
| 1528 | Turn off the effect of the @option{--whole-archive} option for subsequent |
| 1529 | archive files. |
| 1530 | |
| 1531 | @cindex output file after errors |
| 1532 | @kindex --noinhibit-exec |
| 1533 | @item --noinhibit-exec |
| 1534 | Retain the executable output file whenever it is still usable. |
| 1535 | Normally, the linker will not produce an output file if it encounters |
| 1536 | errors during the link process; it exits without writing an output file |
| 1537 | when it issues any error whatsoever. |
| 1538 | |
| 1539 | @kindex -nostdlib |
| 1540 | @item -nostdlib |
| 1541 | Only search library directories explicitly specified on the |
| 1542 | command line. Library directories specified in linker scripts |
| 1543 | (including linker scripts specified on the command line) are ignored. |
| 1544 | |
| 1545 | @ifclear SingleFormat |
| 1546 | @kindex --oformat=@var{output-format} |
| 1547 | @item --oformat=@var{output-format} |
| 1548 | @command{ld} may be configured to support more than one kind of object |
| 1549 | file. If your @command{ld} is configured this way, you can use the |
| 1550 | @samp{--oformat} option to specify the binary format for the output |
| 1551 | object file. Even when @command{ld} is configured to support alternative |
| 1552 | object formats, you don't usually need to specify this, as @command{ld} |
| 1553 | should be configured to produce as a default output format the most |
| 1554 | usual format on each machine. @var{output-format} is a text string, the |
| 1555 | name of a particular format supported by the BFD libraries. (You can |
| 1556 | list the available binary formats with @samp{objdump -i}.) The script |
| 1557 | command @code{OUTPUT_FORMAT} can also specify the output format, but |
| 1558 | this option overrides it. @xref{BFD}. |
| 1559 | @end ifclear |
| 1560 | |
| 1561 | @kindex -pie |
| 1562 | @kindex --pic-executable |
| 1563 | @item -pie |
| 1564 | @itemx --pic-executable |
| 1565 | @cindex position independent executables |
| 1566 | Create a position independent executable. This is currently only supported on |
| 1567 | ELF platforms. Position independent executables are similar to shared |
| 1568 | libraries in that they are relocated by the dynamic linker to the virtual |
| 1569 | address the OS chooses for them (which can vary between invocations). Like |
| 1570 | normal dynamically linked executables they can be executed and symbols |
| 1571 | defined in the executable cannot be overridden by shared libraries. |
| 1572 | |
| 1573 | @kindex -qmagic |
| 1574 | @item -qmagic |
| 1575 | This option is ignored for Linux compatibility. |
| 1576 | |
| 1577 | @kindex -Qy |
| 1578 | @item -Qy |
| 1579 | This option is ignored for SVR4 compatibility. |
| 1580 | |
| 1581 | @kindex --relax |
| 1582 | @cindex synthesizing linker |
| 1583 | @cindex relaxing addressing modes |
| 1584 | @cindex --no-relax |
| 1585 | @item --relax |
| 1586 | @itemx --no-relax |
| 1587 | An option with machine dependent effects. |
| 1588 | @ifset GENERIC |
| 1589 | This option is only supported on a few targets. |
| 1590 | @end ifset |
| 1591 | @ifset H8300 |
| 1592 | @xref{H8/300,,@command{ld} and the H8/300}. |
| 1593 | @end ifset |
| 1594 | @ifset I960 |
| 1595 | @xref{i960,, @command{ld} and the Intel 960 family}. |
| 1596 | @end ifset |
| 1597 | @ifset XTENSA |
| 1598 | @xref{Xtensa,, @command{ld} and Xtensa Processors}. |
| 1599 | @end ifset |
| 1600 | @ifset M68HC11 |
| 1601 | @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}. |
| 1602 | @end ifset |
| 1603 | @ifset POWERPC |
| 1604 | @xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}. |
| 1605 | @end ifset |
| 1606 | |
| 1607 | On some platforms the @samp{--relax} option performs target specific, |
| 1608 | global optimizations that become possible when the linker resolves |
| 1609 | addressing in the program, such as relaxing address modes, |
| 1610 | synthesizing new instructions, selecting shorter version of current |
| 1611 | instructions, and combinig constant values. |
| 1612 | |
| 1613 | On some platforms these link time global optimizations may make symbolic |
| 1614 | debugging of the resulting executable impossible. |
| 1615 | @ifset GENERIC |
| 1616 | This is known to be the case for the Matsushita MN10200 and MN10300 |
| 1617 | family of processors. |
| 1618 | @end ifset |
| 1619 | |
| 1620 | @ifset GENERIC |
| 1621 | On platforms where this is not supported, @samp{--relax} is accepted, |
| 1622 | but ignored. |
| 1623 | @end ifset |
| 1624 | |
| 1625 | On platforms where @samp{--relax} is accepted the option |
| 1626 | @samp{--no-relax} can be used to disable the feature. |
| 1627 | |
| 1628 | @cindex retaining specified symbols |
| 1629 | @cindex stripping all but some symbols |
| 1630 | @cindex symbols, retaining selectively |
| 1631 | @kindex --retain-symbols-file=@var{filename} |
| 1632 | @item --retain-symbols-file=@var{filename} |
| 1633 | Retain @emph{only} the symbols listed in the file @var{filename}, |
| 1634 | discarding all others. @var{filename} is simply a flat file, with one |
| 1635 | symbol name per line. This option is especially useful in environments |
| 1636 | @ifset GENERIC |
| 1637 | (such as VxWorks) |
| 1638 | @end ifset |
| 1639 | where a large global symbol table is accumulated gradually, to conserve |
| 1640 | run-time memory. |
| 1641 | |
| 1642 | @samp{--retain-symbols-file} does @emph{not} discard undefined symbols, |
| 1643 | or symbols needed for relocations. |
| 1644 | |
| 1645 | You may only specify @samp{--retain-symbols-file} once in the command |
| 1646 | line. It overrides @samp{-s} and @samp{-S}. |
| 1647 | |
| 1648 | @ifset GENERIC |
| 1649 | @item -rpath=@var{dir} |
| 1650 | @cindex runtime library search path |
| 1651 | @kindex -rpath=@var{dir} |
| 1652 | Add a directory to the runtime library search path. This is used when |
| 1653 | linking an ELF executable with shared objects. All @option{-rpath} |
| 1654 | arguments are concatenated and passed to the runtime linker, which uses |
| 1655 | them to locate shared objects at runtime. The @option{-rpath} option is |
| 1656 | also used when locating shared objects which are needed by shared |
| 1657 | objects explicitly included in the link; see the description of the |
| 1658 | @option{-rpath-link} option. If @option{-rpath} is not used when linking an |
| 1659 | ELF executable, the contents of the environment variable |
| 1660 | @code{LD_RUN_PATH} will be used if it is defined. |
| 1661 | |
| 1662 | The @option{-rpath} option may also be used on SunOS. By default, on |
| 1663 | SunOS, the linker will form a runtime search patch out of all the |
| 1664 | @option{-L} options it is given. If a @option{-rpath} option is used, the |
| 1665 | runtime search path will be formed exclusively using the @option{-rpath} |
| 1666 | options, ignoring the @option{-L} options. This can be useful when using |
| 1667 | gcc, which adds many @option{-L} options which may be on NFS mounted |
| 1668 | file systems. |
| 1669 | |
| 1670 | For compatibility with other ELF linkers, if the @option{-R} option is |
| 1671 | followed by a directory name, rather than a file name, it is treated as |
| 1672 | the @option{-rpath} option. |
| 1673 | @end ifset |
| 1674 | |
| 1675 | @ifset GENERIC |
| 1676 | @cindex link-time runtime library search path |
| 1677 | @kindex -rpath-link=@var{dir} |
| 1678 | @item -rpath-link=@var{dir} |
| 1679 | When using ELF or SunOS, one shared library may require another. This |
| 1680 | happens when an @code{ld -shared} link includes a shared library as one |
| 1681 | of the input files. |
| 1682 | |
| 1683 | When the linker encounters such a dependency when doing a non-shared, |
| 1684 | non-relocatable link, it will automatically try to locate the required |
| 1685 | shared library and include it in the link, if it is not included |
| 1686 | explicitly. In such a case, the @option{-rpath-link} option |
| 1687 | specifies the first set of directories to search. The |
| 1688 | @option{-rpath-link} option may specify a sequence of directory names |
| 1689 | either by specifying a list of names separated by colons, or by |
| 1690 | appearing multiple times. |
| 1691 | |
| 1692 | This option should be used with caution as it overrides the search path |
| 1693 | that may have been hard compiled into a shared library. In such a case it |
| 1694 | is possible to use unintentionally a different search path than the |
| 1695 | runtime linker would do. |
| 1696 | |
| 1697 | The linker uses the following search paths to locate required shared |
| 1698 | libraries: |
| 1699 | @enumerate |
| 1700 | @item |
| 1701 | Any directories specified by @option{-rpath-link} options. |
| 1702 | @item |
| 1703 | Any directories specified by @option{-rpath} options. The difference |
| 1704 | between @option{-rpath} and @option{-rpath-link} is that directories |
| 1705 | specified by @option{-rpath} options are included in the executable and |
| 1706 | used at runtime, whereas the @option{-rpath-link} option is only effective |
| 1707 | at link time. Searching @option{-rpath} in this way is only supported |
| 1708 | by native linkers and cross linkers which have been configured with |
| 1709 | the @option{--with-sysroot} option. |
| 1710 | @item |
| 1711 | On an ELF system, for native linkers, if the @option{-rpath} and |
| 1712 | @option{-rpath-link} options were not used, search the contents of the |
| 1713 | environment variable @code{LD_RUN_PATH}. |
| 1714 | @item |
| 1715 | On SunOS, if the @option{-rpath} option was not used, search any |
| 1716 | directories specified using @option{-L} options. |
| 1717 | @item |
| 1718 | For a native linker, the search the contents of the environment |
| 1719 | variable @code{LD_LIBRARY_PATH}. |
| 1720 | @item |
| 1721 | For a native ELF linker, the directories in @code{DT_RUNPATH} or |
| 1722 | @code{DT_RPATH} of a shared library are searched for shared |
| 1723 | libraries needed by it. The @code{DT_RPATH} entries are ignored if |
| 1724 | @code{DT_RUNPATH} entries exist. |
| 1725 | @item |
| 1726 | The default directories, normally @file{/lib} and @file{/usr/lib}. |
| 1727 | @item |
| 1728 | For a native linker on an ELF system, if the file @file{/etc/ld.so.conf} |
| 1729 | exists, the list of directories found in that file. |
| 1730 | @end enumerate |
| 1731 | |
| 1732 | If the required shared library is not found, the linker will issue a |
| 1733 | warning and continue with the link. |
| 1734 | @end ifset |
| 1735 | |
| 1736 | @kindex -shared |
| 1737 | @kindex -Bshareable |
| 1738 | @item -shared |
| 1739 | @itemx -Bshareable |
| 1740 | @cindex shared libraries |
| 1741 | Create a shared library. This is currently only supported on ELF, XCOFF |
| 1742 | and SunOS platforms. On SunOS, the linker will automatically create a |
| 1743 | shared library if the @option{-e} option is not used and there are |
| 1744 | undefined symbols in the link. |
| 1745 | |
| 1746 | @kindex --sort-common |
| 1747 | @item --sort-common |
| 1748 | @itemx --sort-common=ascending |
| 1749 | @itemx --sort-common=descending |
| 1750 | This option tells @command{ld} to sort the common symbols by alignment in |
| 1751 | ascending or descending order when it places them in the appropriate output |
| 1752 | sections. The symbol alignments considered are sixteen-byte or larger, |
| 1753 | eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps |
| 1754 | between symbols due to alignment constraints. If no sorting order is |
| 1755 | specified, then descending order is assumed. |
| 1756 | |
| 1757 | @kindex --sort-section=name |
| 1758 | @item --sort-section=name |
| 1759 | This option will apply @code{SORT_BY_NAME} to all wildcard section |
| 1760 | patterns in the linker script. |
| 1761 | |
| 1762 | @kindex --sort-section=alignment |
| 1763 | @item --sort-section=alignment |
| 1764 | This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section |
| 1765 | patterns in the linker script. |
| 1766 | |
| 1767 | @kindex --split-by-file |
| 1768 | @item --split-by-file[=@var{size}] |
| 1769 | Similar to @option{--split-by-reloc} but creates a new output section for |
| 1770 | each input file when @var{size} is reached. @var{size} defaults to a |
| 1771 | size of 1 if not given. |
| 1772 | |
| 1773 | @kindex --split-by-reloc |
| 1774 | @item --split-by-reloc[=@var{count}] |
| 1775 | Tries to creates extra sections in the output file so that no single |
| 1776 | output section in the file contains more than @var{count} relocations. |
| 1777 | This is useful when generating huge relocatable files for downloading into |
| 1778 | certain real time kernels with the COFF object file format; since COFF |
| 1779 | cannot represent more than 65535 relocations in a single section. Note |
| 1780 | that this will fail to work with object file formats which do not |
| 1781 | support arbitrary sections. The linker will not split up individual |
| 1782 | input sections for redistribution, so if a single input section contains |
| 1783 | more than @var{count} relocations one output section will contain that |
| 1784 | many relocations. @var{count} defaults to a value of 32768. |
| 1785 | |
| 1786 | @kindex --stats |
| 1787 | @item --stats |
| 1788 | Compute and display statistics about the operation of the linker, such |
| 1789 | as execution time and memory usage. |
| 1790 | |
| 1791 | @kindex --sysroot=@var{directory} |
| 1792 | @item --sysroot=@var{directory} |
| 1793 | Use @var{directory} as the location of the sysroot, overriding the |
| 1794 | configure-time default. This option is only supported by linkers |
| 1795 | that were configured using @option{--with-sysroot}. |
| 1796 | |
| 1797 | @kindex --traditional-format |
| 1798 | @cindex traditional format |
| 1799 | @item --traditional-format |
| 1800 | For some targets, the output of @command{ld} is different in some ways from |
| 1801 | the output of some existing linker. This switch requests @command{ld} to |
| 1802 | use the traditional format instead. |
| 1803 | |
| 1804 | @cindex dbx |
| 1805 | For example, on SunOS, @command{ld} combines duplicate entries in the |
| 1806 | symbol string table. This can reduce the size of an output file with |
| 1807 | full debugging information by over 30 percent. Unfortunately, the SunOS |
| 1808 | @code{dbx} program can not read the resulting program (@code{gdb} has no |
| 1809 | trouble). The @samp{--traditional-format} switch tells @command{ld} to not |
| 1810 | combine duplicate entries. |
| 1811 | |
| 1812 | @kindex --section-start=@var{sectionname}=@var{org} |
| 1813 | @item --section-start=@var{sectionname}=@var{org} |
| 1814 | Locate a section in the output file at the absolute |
| 1815 | address given by @var{org}. You may use this option as many |
| 1816 | times as necessary to locate multiple sections in the command |
| 1817 | line. |
| 1818 | @var{org} must be a single hexadecimal integer; |
| 1819 | for compatibility with other linkers, you may omit the leading |
| 1820 | @samp{0x} usually associated with hexadecimal values. @emph{Note:} there |
| 1821 | should be no white space between @var{sectionname}, the equals |
| 1822 | sign (``@key{=}''), and @var{org}. |
| 1823 | |
| 1824 | @kindex -Tbss=@var{org} |
| 1825 | @kindex -Tdata=@var{org} |
| 1826 | @kindex -Ttext=@var{org} |
| 1827 | @cindex segment origins, cmd line |
| 1828 | @item -Tbss=@var{org} |
| 1829 | @itemx -Tdata=@var{org} |
| 1830 | @itemx -Ttext=@var{org} |
| 1831 | Same as @option{--section-start}, with @code{.bss}, @code{.data} or |
| 1832 | @code{.text} as the @var{sectionname}. |
| 1833 | |
| 1834 | @kindex -Ttext-segment=@var{org} |
| 1835 | @item -Ttext-segment=@var{org} |
| 1836 | @cindex text segment origin, cmd line |
| 1837 | When creating an ELF executable or shared object, it will set the address |
| 1838 | of the first byte of the text segment. |
| 1839 | |
| 1840 | @kindex --unresolved-symbols |
| 1841 | @item --unresolved-symbols=@var{method} |
| 1842 | Determine how to handle unresolved symbols. There are four possible |
| 1843 | values for @samp{method}: |
| 1844 | |
| 1845 | @table @samp |
| 1846 | @item ignore-all |
| 1847 | Do not report any unresolved symbols. |
| 1848 | |
| 1849 | @item report-all |
| 1850 | Report all unresolved symbols. This is the default. |
| 1851 | |
| 1852 | @item ignore-in-object-files |
| 1853 | Report unresolved symbols that are contained in shared libraries, but |
| 1854 | ignore them if they come from regular object files. |
| 1855 | |
| 1856 | @item ignore-in-shared-libs |
| 1857 | Report unresolved symbols that come from regular object files, but |
| 1858 | ignore them if they come from shared libraries. This can be useful |
| 1859 | when creating a dynamic binary and it is known that all the shared |
| 1860 | libraries that it should be referencing are included on the linker's |
| 1861 | command line. |
| 1862 | @end table |
| 1863 | |
| 1864 | The behaviour for shared libraries on their own can also be controlled |
| 1865 | by the @option{--[no-]allow-shlib-undefined} option. |
| 1866 | |
| 1867 | Normally the linker will generate an error message for each reported |
| 1868 | unresolved symbol but the option @option{--warn-unresolved-symbols} |
| 1869 | can change this to a warning. |
| 1870 | |
| 1871 | @kindex --verbose[=@var{NUMBER}] |
| 1872 | @cindex verbose[=@var{NUMBER}] |
| 1873 | @item --dll-verbose |
| 1874 | @itemx --verbose[=@var{NUMBER}] |
| 1875 | Display the version number for @command{ld} and list the linker emulations |
| 1876 | supported. Display which input files can and cannot be opened. Display |
| 1877 | the linker script being used by the linker. If the optional @var{NUMBER} |
| 1878 | argument > 1, plugin symbol status will also be displayed. |
| 1879 | |
| 1880 | @kindex --version-script=@var{version-scriptfile} |
| 1881 | @cindex version script, symbol versions |
| 1882 | @item --version-script=@var{version-scriptfile} |
| 1883 | Specify the name of a version script to the linker. This is typically |
| 1884 | used when creating shared libraries to specify additional information |
| 1885 | about the version hierarchy for the library being created. This option |
| 1886 | is only fully supported on ELF platforms which support shared libraries; |
| 1887 | see @ref{VERSION}. It is partially supported on PE platforms, which can |
| 1888 | use version scripts to filter symbol visibility in auto-export mode: any |
| 1889 | symbols marked @samp{local} in the version script will not be exported. |
| 1890 | @xref{WIN32}. |
| 1891 | |
| 1892 | @kindex --warn-common |
| 1893 | @cindex warnings, on combining symbols |
| 1894 | @cindex combining symbols, warnings on |
| 1895 | @item --warn-common |
| 1896 | Warn when a common symbol is combined with another common symbol or with |
| 1897 | a symbol definition. Unix linkers allow this somewhat sloppy practise, |
| 1898 | but linkers on some other operating systems do not. This option allows |
| 1899 | you to find potential problems from combining global symbols. |
| 1900 | Unfortunately, some C libraries use this practise, so you may get some |
| 1901 | warnings about symbols in the libraries as well as in your programs. |
| 1902 | |
| 1903 | There are three kinds of global symbols, illustrated here by C examples: |
| 1904 | |
| 1905 | @table @samp |
| 1906 | @item int i = 1; |
| 1907 | A definition, which goes in the initialized data section of the output |
| 1908 | file. |
| 1909 | |
| 1910 | @item extern int i; |
| 1911 | An undefined reference, which does not allocate space. |
| 1912 | There must be either a definition or a common symbol for the |
| 1913 | variable somewhere. |
| 1914 | |
| 1915 | @item int i; |
| 1916 | A common symbol. If there are only (one or more) common symbols for a |
| 1917 | variable, it goes in the uninitialized data area of the output file. |
| 1918 | The linker merges multiple common symbols for the same variable into a |
| 1919 | single symbol. If they are of different sizes, it picks the largest |
| 1920 | size. The linker turns a common symbol into a declaration, if there is |
| 1921 | a definition of the same variable. |
| 1922 | @end table |
| 1923 | |
| 1924 | The @samp{--warn-common} option can produce five kinds of warnings. |
| 1925 | Each warning consists of a pair of lines: the first describes the symbol |
| 1926 | just encountered, and the second describes the previous symbol |
| 1927 | encountered with the same name. One or both of the two symbols will be |
| 1928 | a common symbol. |
| 1929 | |
| 1930 | @enumerate |
| 1931 | @item |
| 1932 | Turning a common symbol into a reference, because there is already a |
| 1933 | definition for the symbol. |
| 1934 | @smallexample |
| 1935 | @var{file}(@var{section}): warning: common of `@var{symbol}' |
| 1936 | overridden by definition |
| 1937 | @var{file}(@var{section}): warning: defined here |
| 1938 | @end smallexample |
| 1939 | |
| 1940 | @item |
| 1941 | Turning a common symbol into a reference, because a later definition for |
| 1942 | the symbol is encountered. This is the same as the previous case, |
| 1943 | except that the symbols are encountered in a different order. |
| 1944 | @smallexample |
| 1945 | @var{file}(@var{section}): warning: definition of `@var{symbol}' |
| 1946 | overriding common |
| 1947 | @var{file}(@var{section}): warning: common is here |
| 1948 | @end smallexample |
| 1949 | |
| 1950 | @item |
| 1951 | Merging a common symbol with a previous same-sized common symbol. |
| 1952 | @smallexample |
| 1953 | @var{file}(@var{section}): warning: multiple common |
| 1954 | of `@var{symbol}' |
| 1955 | @var{file}(@var{section}): warning: previous common is here |
| 1956 | @end smallexample |
| 1957 | |
| 1958 | @item |
| 1959 | Merging a common symbol with a previous larger common symbol. |
| 1960 | @smallexample |
| 1961 | @var{file}(@var{section}): warning: common of `@var{symbol}' |
| 1962 | overridden by larger common |
| 1963 | @var{file}(@var{section}): warning: larger common is here |
| 1964 | @end smallexample |
| 1965 | |
| 1966 | @item |
| 1967 | Merging a common symbol with a previous smaller common symbol. This is |
| 1968 | the same as the previous case, except that the symbols are |
| 1969 | encountered in a different order. |
| 1970 | @smallexample |
| 1971 | @var{file}(@var{section}): warning: common of `@var{symbol}' |
| 1972 | overriding smaller common |
| 1973 | @var{file}(@var{section}): warning: smaller common is here |
| 1974 | @end smallexample |
| 1975 | @end enumerate |
| 1976 | |
| 1977 | @kindex --warn-constructors |
| 1978 | @item --warn-constructors |
| 1979 | Warn if any global constructors are used. This is only useful for a few |
| 1980 | object file formats. For formats like COFF or ELF, the linker can not |
| 1981 | detect the use of global constructors. |
| 1982 | |
| 1983 | @kindex --warn-multiple-gp |
| 1984 | @item --warn-multiple-gp |
| 1985 | Warn if multiple global pointer values are required in the output file. |
| 1986 | This is only meaningful for certain processors, such as the Alpha. |
| 1987 | Specifically, some processors put large-valued constants in a special |
| 1988 | section. A special register (the global pointer) points into the middle |
| 1989 | of this section, so that constants can be loaded efficiently via a |
| 1990 | base-register relative addressing mode. Since the offset in |
| 1991 | base-register relative mode is fixed and relatively small (e.g., 16 |
| 1992 | bits), this limits the maximum size of the constant pool. Thus, in |
| 1993 | large programs, it is often necessary to use multiple global pointer |
| 1994 | values in order to be able to address all possible constants. This |
| 1995 | option causes a warning to be issued whenever this case occurs. |
| 1996 | |
| 1997 | @kindex --warn-once |
| 1998 | @cindex warnings, on undefined symbols |
| 1999 | @cindex undefined symbols, warnings on |
| 2000 | @item --warn-once |
| 2001 | Only warn once for each undefined symbol, rather than once per module |
| 2002 | which refers to it. |
| 2003 | |
| 2004 | @kindex --warn-section-align |
| 2005 | @cindex warnings, on section alignment |
| 2006 | @cindex section alignment, warnings on |
| 2007 | @item --warn-section-align |
| 2008 | Warn if the address of an output section is changed because of |
| 2009 | alignment. Typically, the alignment will be set by an input section. |
| 2010 | The address will only be changed if it not explicitly specified; that |
| 2011 | is, if the @code{SECTIONS} command does not specify a start address for |
| 2012 | the section (@pxref{SECTIONS}). |
| 2013 | |
| 2014 | @kindex --warn-shared-textrel |
| 2015 | @item --warn-shared-textrel |
| 2016 | Warn if the linker adds a DT_TEXTREL to a shared object. |
| 2017 | |
| 2018 | @kindex --warn-alternate-em |
| 2019 | @item --warn-alternate-em |
| 2020 | Warn if an object has alternate ELF machine code. |
| 2021 | |
| 2022 | @kindex --warn-unresolved-symbols |
| 2023 | @item --warn-unresolved-symbols |
| 2024 | If the linker is going to report an unresolved symbol (see the option |
| 2025 | @option{--unresolved-symbols}) it will normally generate an error. |
| 2026 | This option makes it generate a warning instead. |
| 2027 | |
| 2028 | @kindex --error-unresolved-symbols |
| 2029 | @item --error-unresolved-symbols |
| 2030 | This restores the linker's default behaviour of generating errors when |
| 2031 | it is reporting unresolved symbols. |
| 2032 | |
| 2033 | @kindex --whole-archive |
| 2034 | @cindex including an entire archive |
| 2035 | @item --whole-archive |
| 2036 | For each archive mentioned on the command line after the |
| 2037 | @option{--whole-archive} option, include every object file in the archive |
| 2038 | in the link, rather than searching the archive for the required object |
| 2039 | files. This is normally used to turn an archive file into a shared |
| 2040 | library, forcing every object to be included in the resulting shared |
| 2041 | library. This option may be used more than once. |
| 2042 | |
| 2043 | Two notes when using this option from gcc: First, gcc doesn't know |
| 2044 | about this option, so you have to use @option{-Wl,-whole-archive}. |
| 2045 | Second, don't forget to use @option{-Wl,-no-whole-archive} after your |
| 2046 | list of archives, because gcc will add its own list of archives to |
| 2047 | your link and you may not want this flag to affect those as well. |
| 2048 | |
| 2049 | @kindex --wrap=@var{symbol} |
| 2050 | @item --wrap=@var{symbol} |
| 2051 | Use a wrapper function for @var{symbol}. Any undefined reference to |
| 2052 | @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any |
| 2053 | undefined reference to @code{__real_@var{symbol}} will be resolved to |
| 2054 | @var{symbol}. |
| 2055 | |
| 2056 | This can be used to provide a wrapper for a system function. The |
| 2057 | wrapper function should be called @code{__wrap_@var{symbol}}. If it |
| 2058 | wishes to call the system function, it should call |
| 2059 | @code{__real_@var{symbol}}. |
| 2060 | |
| 2061 | Here is a trivial example: |
| 2062 | |
| 2063 | @smallexample |
| 2064 | void * |
| 2065 | __wrap_malloc (size_t c) |
| 2066 | @{ |
| 2067 | printf ("malloc called with %zu\n", c); |
| 2068 | return __real_malloc (c); |
| 2069 | @} |
| 2070 | @end smallexample |
| 2071 | |
| 2072 | If you link other code with this file using @option{--wrap malloc}, then |
| 2073 | all calls to @code{malloc} will call the function @code{__wrap_malloc} |
| 2074 | instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will |
| 2075 | call the real @code{malloc} function. |
| 2076 | |
| 2077 | You may wish to provide a @code{__real_malloc} function as well, so that |
| 2078 | links without the @option{--wrap} option will succeed. If you do this, |
| 2079 | you should not put the definition of @code{__real_malloc} in the same |
| 2080 | file as @code{__wrap_malloc}; if you do, the assembler may resolve the |
| 2081 | call before the linker has a chance to wrap it to @code{malloc}. |
| 2082 | |
| 2083 | @kindex --eh-frame-hdr |
| 2084 | @item --eh-frame-hdr |
| 2085 | Request creation of @code{.eh_frame_hdr} section and ELF |
| 2086 | @code{PT_GNU_EH_FRAME} segment header. |
| 2087 | |
| 2088 | @kindex --ld-generated-unwind-info |
| 2089 | @item --no-ld-generated-unwind-info |
| 2090 | Request creation of @code{.eh_frame} unwind info for linker |
| 2091 | generated code sections like PLT. This option is on by default |
| 2092 | if linker generated unwind info is supported. |
| 2093 | |
| 2094 | @kindex --enable-new-dtags |
| 2095 | @kindex --disable-new-dtags |
| 2096 | @item --enable-new-dtags |
| 2097 | @itemx --disable-new-dtags |
| 2098 | This linker can create the new dynamic tags in ELF. But the older ELF |
| 2099 | systems may not understand them. If you specify |
| 2100 | @option{--enable-new-dtags}, the dynamic tags will be created as needed. |
| 2101 | If you specify @option{--disable-new-dtags}, no new dynamic tags will be |
| 2102 | created. By default, the new dynamic tags are not created. Note that |
| 2103 | those options are only available for ELF systems. |
| 2104 | |
| 2105 | @kindex --hash-size=@var{number} |
| 2106 | @item --hash-size=@var{number} |
| 2107 | Set the default size of the linker's hash tables to a prime number |
| 2108 | close to @var{number}. Increasing this value can reduce the length of |
| 2109 | time it takes the linker to perform its tasks, at the expense of |
| 2110 | increasing the linker's memory requirements. Similarly reducing this |
| 2111 | value can reduce the memory requirements at the expense of speed. |
| 2112 | |
| 2113 | @kindex --hash-style=@var{style} |
| 2114 | @item --hash-style=@var{style} |
| 2115 | Set the type of linker's hash table(s). @var{style} can be either |
| 2116 | @code{sysv} for classic ELF @code{.hash} section, @code{gnu} for |
| 2117 | new style GNU @code{.gnu.hash} section or @code{both} for both |
| 2118 | the classic ELF @code{.hash} and new style GNU @code{.gnu.hash} |
| 2119 | hash tables. The default is @code{sysv}. |
| 2120 | |
| 2121 | @kindex --reduce-memory-overheads |
| 2122 | @item --reduce-memory-overheads |
| 2123 | This option reduces memory requirements at ld runtime, at the expense of |
| 2124 | linking speed. This was introduced to select the old O(n^2) algorithm |
| 2125 | for link map file generation, rather than the new O(n) algorithm which uses |
| 2126 | about 40% more memory for symbol storage. |
| 2127 | |
| 2128 | Another effect of the switch is to set the default hash table size to |
| 2129 | 1021, which again saves memory at the cost of lengthening the linker's |
| 2130 | run time. This is not done however if the @option{--hash-size} switch |
| 2131 | has been used. |
| 2132 | |
| 2133 | The @option{--reduce-memory-overheads} switch may be also be used to |
| 2134 | enable other tradeoffs in future versions of the linker. |
| 2135 | |
| 2136 | @kindex --build-id |
| 2137 | @kindex --build-id=@var{style} |
| 2138 | @item --build-id |
| 2139 | @itemx --build-id=@var{style} |
| 2140 | Request creation of @code{.note.gnu.build-id} ELF note section. |
| 2141 | The contents of the note are unique bits identifying this linked |
| 2142 | file. @var{style} can be @code{uuid} to use 128 random bits, |
| 2143 | @code{sha1} to use a 160-bit @sc{SHA1} hash on the normative |
| 2144 | parts of the output contents, @code{md5} to use a 128-bit |
| 2145 | @sc{MD5} hash on the normative parts of the output contents, or |
| 2146 | @code{0x@var{hexstring}} to use a chosen bit string specified as |
| 2147 | an even number of hexadecimal digits (@code{-} and @code{:} |
| 2148 | characters between digit pairs are ignored). If @var{style} is |
| 2149 | omitted, @code{sha1} is used. |
| 2150 | |
| 2151 | The @code{md5} and @code{sha1} styles produces an identifier |
| 2152 | that is always the same in an identical output file, but will be |
| 2153 | unique among all nonidentical output files. It is not intended |
| 2154 | to be compared as a checksum for the file's contents. A linked |
| 2155 | file may be changed later by other tools, but the build ID bit |
| 2156 | string identifying the original linked file does not change. |
| 2157 | |
| 2158 | Passing @code{none} for @var{style} disables the setting from any |
| 2159 | @code{--build-id} options earlier on the command line. |
| 2160 | @end table |
| 2161 | |
| 2162 | @c man end |
| 2163 | |
| 2164 | @subsection Options Specific to i386 PE Targets |
| 2165 | |
| 2166 | @c man begin OPTIONS |
| 2167 | |
| 2168 | The i386 PE linker supports the @option{-shared} option, which causes |
| 2169 | the output to be a dynamically linked library (DLL) instead of a |
| 2170 | normal executable. You should name the output @code{*.dll} when you |
| 2171 | use this option. In addition, the linker fully supports the standard |
| 2172 | @code{*.def} files, which may be specified on the linker command line |
| 2173 | like an object file (in fact, it should precede archives it exports |
| 2174 | symbols from, to ensure that they get linked in, just like a normal |
| 2175 | object file). |
| 2176 | |
| 2177 | In addition to the options common to all targets, the i386 PE linker |
| 2178 | support additional command line options that are specific to the i386 |
| 2179 | PE target. Options that take values may be separated from their |
| 2180 | values by either a space or an equals sign. |
| 2181 | |
| 2182 | @table @gcctabopt |
| 2183 | |
| 2184 | @kindex --add-stdcall-alias |
| 2185 | @item --add-stdcall-alias |
| 2186 | If given, symbols with a stdcall suffix (@@@var{nn}) will be exported |
| 2187 | as-is and also with the suffix stripped. |
| 2188 | [This option is specific to the i386 PE targeted port of the linker] |
| 2189 | |
| 2190 | @kindex --base-file |
| 2191 | @item --base-file @var{file} |
| 2192 | Use @var{file} as the name of a file in which to save the base |
| 2193 | addresses of all the relocations needed for generating DLLs with |
| 2194 | @file{dlltool}. |
| 2195 | [This is an i386 PE specific option] |
| 2196 | |
| 2197 | @kindex --dll |
| 2198 | @item --dll |
| 2199 | Create a DLL instead of a regular executable. You may also use |
| 2200 | @option{-shared} or specify a @code{LIBRARY} in a given @code{.def} |
| 2201 | file. |
| 2202 | [This option is specific to the i386 PE targeted port of the linker] |
| 2203 | |
| 2204 | @kindex --enable-long-section-names |
| 2205 | @kindex --disable-long-section-names |
| 2206 | @item --enable-long-section-names |
| 2207 | @itemx --disable-long-section-names |
| 2208 | The PE variants of the Coff object format add an extension that permits |
| 2209 | the use of section names longer than eight characters, the normal limit |
| 2210 | for Coff. By default, these names are only allowed in object files, as |
| 2211 | fully-linked executable images do not carry the Coff string table required |
| 2212 | to support the longer names. As a GNU extension, it is possible to |
| 2213 | allow their use in executable images as well, or to (probably pointlessly!) |
| 2214 | disallow it in object files, by using these two options. Executable images |
| 2215 | generated with these long section names are slightly non-standard, carrying |
| 2216 | as they do a string table, and may generate confusing output when examined |
| 2217 | with non-GNU PE-aware tools, such as file viewers and dumpers. However, |
| 2218 | GDB relies on the use of PE long section names to find Dwarf-2 debug |
| 2219 | information sections in an executable image at runtime, and so if neither |
| 2220 | option is specified on the command-line, @command{ld} will enable long |
| 2221 | section names, overriding the default and technically correct behaviour, |
| 2222 | when it finds the presence of debug information while linking an executable |
| 2223 | image and not stripping symbols. |
| 2224 | [This option is valid for all PE targeted ports of the linker] |
| 2225 | |
| 2226 | @kindex --enable-stdcall-fixup |
| 2227 | @kindex --disable-stdcall-fixup |
| 2228 | @item --enable-stdcall-fixup |
| 2229 | @itemx --disable-stdcall-fixup |
| 2230 | If the link finds a symbol that it cannot resolve, it will attempt to |
| 2231 | do ``fuzzy linking'' by looking for another defined symbol that differs |
| 2232 | only in the format of the symbol name (cdecl vs stdcall) and will |
| 2233 | resolve that symbol by linking to the match. For example, the |
| 2234 | undefined symbol @code{_foo} might be linked to the function |
| 2235 | @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked |
| 2236 | to the function @code{_bar}. When the linker does this, it prints a |
| 2237 | warning, since it normally should have failed to link, but sometimes |
| 2238 | import libraries generated from third-party dlls may need this feature |
| 2239 | to be usable. If you specify @option{--enable-stdcall-fixup}, this |
| 2240 | feature is fully enabled and warnings are not printed. If you specify |
| 2241 | @option{--disable-stdcall-fixup}, this feature is disabled and such |
| 2242 | mismatches are considered to be errors. |
| 2243 | [This option is specific to the i386 PE targeted port of the linker] |
| 2244 | |
| 2245 | @kindex --leading-underscore |
| 2246 | @kindex --no-leading-underscore |
| 2247 | @item --leading-underscore |
| 2248 | @itemx --no-leading-underscore |
| 2249 | For most targets default symbol-prefix is an underscore and is defined |
| 2250 | in target's description. By this option it is possible to |
| 2251 | disable/enable the default underscore symbol-prefix. |
| 2252 | |
| 2253 | @cindex DLLs, creating |
| 2254 | @kindex --export-all-symbols |
| 2255 | @item --export-all-symbols |
| 2256 | If given, all global symbols in the objects used to build a DLL will |
| 2257 | be exported by the DLL. Note that this is the default if there |
| 2258 | otherwise wouldn't be any exported symbols. When symbols are |
| 2259 | explicitly exported via DEF files or implicitly exported via function |
| 2260 | attributes, the default is to not export anything else unless this |
| 2261 | option is given. Note that the symbols @code{DllMain@@12}, |
| 2262 | @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and |
| 2263 | @code{impure_ptr} will not be automatically |
| 2264 | exported. Also, symbols imported from other DLLs will not be |
| 2265 | re-exported, nor will symbols specifying the DLL's internal layout |
| 2266 | such as those beginning with @code{_head_} or ending with |
| 2267 | @code{_iname}. In addition, no symbols from @code{libgcc}, |
| 2268 | @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported. |
| 2269 | Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will |
| 2270 | not be exported, to help with C++ DLLs. Finally, there is an |
| 2271 | extensive list of cygwin-private symbols that are not exported |
| 2272 | (obviously, this applies on when building DLLs for cygwin targets). |
| 2273 | These cygwin-excludes are: @code{_cygwin_dll_entry@@12}, |
| 2274 | @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12}, |
| 2275 | @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll}, |
| 2276 | @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2}, |
| 2277 | @code{cygwin_premain3}, and @code{environ}. |
| 2278 | [This option is specific to the i386 PE targeted port of the linker] |
| 2279 | |
| 2280 | @kindex --exclude-symbols |
| 2281 | @item --exclude-symbols @var{symbol},@var{symbol},... |
| 2282 | Specifies a list of symbols which should not be automatically |
| 2283 | exported. The symbol names may be delimited by commas or colons. |
| 2284 | [This option is specific to the i386 PE targeted port of the linker] |
| 2285 | |
| 2286 | @kindex --exclude-all-symbols |
| 2287 | @item --exclude-all-symbols |
| 2288 | Specifies no symbols should be automatically exported. |
| 2289 | [This option is specific to the i386 PE targeted port of the linker] |
| 2290 | |
| 2291 | @kindex --file-alignment |
| 2292 | @item --file-alignment |
| 2293 | Specify the file alignment. Sections in the file will always begin at |
| 2294 | file offsets which are multiples of this number. This defaults to |
| 2295 | 512. |
| 2296 | [This option is specific to the i386 PE targeted port of the linker] |
| 2297 | |
| 2298 | @cindex heap size |
| 2299 | @kindex --heap |
| 2300 | @item --heap @var{reserve} |
| 2301 | @itemx --heap @var{reserve},@var{commit} |
| 2302 | Specify the number of bytes of memory to reserve (and optionally commit) |
| 2303 | to be used as heap for this program. The default is 1Mb reserved, 4K |
| 2304 | committed. |
| 2305 | [This option is specific to the i386 PE targeted port of the linker] |
| 2306 | |
| 2307 | @cindex image base |
| 2308 | @kindex --image-base |
| 2309 | @item --image-base @var{value} |
| 2310 | Use @var{value} as the base address of your program or dll. This is |
| 2311 | the lowest memory location that will be used when your program or dll |
| 2312 | is loaded. To reduce the need to relocate and improve performance of |
| 2313 | your dlls, each should have a unique base address and not overlap any |
| 2314 | other dlls. The default is 0x400000 for executables, and 0x10000000 |
| 2315 | for dlls. |
| 2316 | [This option is specific to the i386 PE targeted port of the linker] |
| 2317 | |
| 2318 | @kindex --kill-at |
| 2319 | @item --kill-at |
| 2320 | If given, the stdcall suffixes (@@@var{nn}) will be stripped from |
| 2321 | symbols before they are exported. |
| 2322 | [This option is specific to the i386 PE targeted port of the linker] |
| 2323 | |
| 2324 | @kindex --large-address-aware |
| 2325 | @item --large-address-aware |
| 2326 | If given, the appropriate bit in the ``Characteristics'' field of the COFF |
| 2327 | header is set to indicate that this executable supports virtual addresses |
| 2328 | greater than 2 gigabytes. This should be used in conjunction with the /3GB |
| 2329 | or /USERVA=@var{value} megabytes switch in the ``[operating systems]'' |
| 2330 | section of the BOOT.INI. Otherwise, this bit has no effect. |
| 2331 | [This option is specific to PE targeted ports of the linker] |
| 2332 | |
| 2333 | @kindex --major-image-version |
| 2334 | @item --major-image-version @var{value} |
| 2335 | Sets the major number of the ``image version''. Defaults to 1. |
| 2336 | [This option is specific to the i386 PE targeted port of the linker] |
| 2337 | |
| 2338 | @kindex --major-os-version |
| 2339 | @item --major-os-version @var{value} |
| 2340 | Sets the major number of the ``os version''. Defaults to 4. |
| 2341 | [This option is specific to the i386 PE targeted port of the linker] |
| 2342 | |
| 2343 | @kindex --major-subsystem-version |
| 2344 | @item --major-subsystem-version @var{value} |
| 2345 | Sets the major number of the ``subsystem version''. Defaults to 4. |
| 2346 | [This option is specific to the i386 PE targeted port of the linker] |
| 2347 | |
| 2348 | @kindex --minor-image-version |
| 2349 | @item --minor-image-version @var{value} |
| 2350 | Sets the minor number of the ``image version''. Defaults to 0. |
| 2351 | [This option is specific to the i386 PE targeted port of the linker] |
| 2352 | |
| 2353 | @kindex --minor-os-version |
| 2354 | @item --minor-os-version @var{value} |
| 2355 | Sets the minor number of the ``os version''. Defaults to 0. |
| 2356 | [This option is specific to the i386 PE targeted port of the linker] |
| 2357 | |
| 2358 | @kindex --minor-subsystem-version |
| 2359 | @item --minor-subsystem-version @var{value} |
| 2360 | Sets the minor number of the ``subsystem version''. Defaults to 0. |
| 2361 | [This option is specific to the i386 PE targeted port of the linker] |
| 2362 | |
| 2363 | @cindex DEF files, creating |
| 2364 | @cindex DLLs, creating |
| 2365 | @kindex --output-def |
| 2366 | @item --output-def @var{file} |
| 2367 | The linker will create the file @var{file} which will contain a DEF |
| 2368 | file corresponding to the DLL the linker is generating. This DEF file |
| 2369 | (which should be called @code{*.def}) may be used to create an import |
| 2370 | library with @code{dlltool} or may be used as a reference to |
| 2371 | automatically or implicitly exported symbols. |
| 2372 | [This option is specific to the i386 PE targeted port of the linker] |
| 2373 | |
| 2374 | @cindex DLLs, creating |
| 2375 | @kindex --out-implib |
| 2376 | @item --out-implib @var{file} |
| 2377 | The linker will create the file @var{file} which will contain an |
| 2378 | import lib corresponding to the DLL the linker is generating. This |
| 2379 | import lib (which should be called @code{*.dll.a} or @code{*.a} |
| 2380 | may be used to link clients against the generated DLL; this behaviour |
| 2381 | makes it possible to skip a separate @code{dlltool} import library |
| 2382 | creation step. |
| 2383 | [This option is specific to the i386 PE targeted port of the linker] |
| 2384 | |
| 2385 | @kindex --enable-auto-image-base |
| 2386 | @item --enable-auto-image-base |
| 2387 | Automatically choose the image base for DLLs, unless one is specified |
| 2388 | using the @code{--image-base} argument. By using a hash generated |
| 2389 | from the dllname to create unique image bases for each DLL, in-memory |
| 2390 | collisions and relocations which can delay program execution are |
| 2391 | avoided. |
| 2392 | [This option is specific to the i386 PE targeted port of the linker] |
| 2393 | |
| 2394 | @kindex --disable-auto-image-base |
| 2395 | @item --disable-auto-image-base |
| 2396 | Do not automatically generate a unique image base. If there is no |
| 2397 | user-specified image base (@code{--image-base}) then use the platform |
| 2398 | default. |
| 2399 | [This option is specific to the i386 PE targeted port of the linker] |
| 2400 | |
| 2401 | @cindex DLLs, linking to |
| 2402 | @kindex --dll-search-prefix |
| 2403 | @item --dll-search-prefix @var{string} |
| 2404 | When linking dynamically to a dll without an import library, |
| 2405 | search for @code{<string><basename>.dll} in preference to |
| 2406 | @code{lib<basename>.dll}. This behaviour allows easy distinction |
| 2407 | between DLLs built for the various "subplatforms": native, cygwin, |
| 2408 | uwin, pw, etc. For instance, cygwin DLLs typically use |
| 2409 | @code{--dll-search-prefix=cyg}. |
| 2410 | [This option is specific to the i386 PE targeted port of the linker] |
| 2411 | |
| 2412 | @kindex --enable-auto-import |
| 2413 | @item --enable-auto-import |
| 2414 | Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for |
| 2415 | DATA imports from DLLs, and create the necessary thunking symbols when |
| 2416 | building the import libraries with those DATA exports. Note: Use of the |
| 2417 | 'auto-import' extension will cause the text section of the image file |
| 2418 | to be made writable. This does not conform to the PE-COFF format |
| 2419 | specification published by Microsoft. |
| 2420 | |
| 2421 | Note - use of the 'auto-import' extension will also cause read only |
| 2422 | data which would normally be placed into the .rdata section to be |
| 2423 | placed into the .data section instead. This is in order to work |
| 2424 | around a problem with consts that is described here: |
| 2425 | http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html |
| 2426 | |
| 2427 | Using 'auto-import' generally will 'just work' -- but sometimes you may |
| 2428 | see this message: |
| 2429 | |
| 2430 | "variable '<var>' can't be auto-imported. Please read the |
| 2431 | documentation for ld's @code{--enable-auto-import} for details." |
| 2432 | |
| 2433 | This message occurs when some (sub)expression accesses an address |
| 2434 | ultimately given by the sum of two constants (Win32 import tables only |
| 2435 | allow one). Instances where this may occur include accesses to member |
| 2436 | fields of struct variables imported from a DLL, as well as using a |
| 2437 | constant index into an array variable imported from a DLL. Any |
| 2438 | multiword variable (arrays, structs, long long, etc) may trigger |
| 2439 | this error condition. However, regardless of the exact data type |
| 2440 | of the offending exported variable, ld will always detect it, issue |
| 2441 | the warning, and exit. |
| 2442 | |
| 2443 | There are several ways to address this difficulty, regardless of the |
| 2444 | data type of the exported variable: |
| 2445 | |
| 2446 | One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task |
| 2447 | of adjusting references in your client code for runtime environment, so |
| 2448 | this method works only when runtime environment supports this feature. |
| 2449 | |
| 2450 | A second solution is to force one of the 'constants' to be a variable -- |
| 2451 | that is, unknown and un-optimizable at compile time. For arrays, |
| 2452 | there are two possibilities: a) make the indexee (the array's address) |
| 2453 | a variable, or b) make the 'constant' index a variable. Thus: |
| 2454 | |
| 2455 | @example |
| 2456 | extern type extern_array[]; |
| 2457 | extern_array[1] --> |
| 2458 | @{ volatile type *t=extern_array; t[1] @} |
| 2459 | @end example |
| 2460 | |
| 2461 | or |
| 2462 | |
| 2463 | @example |
| 2464 | extern type extern_array[]; |
| 2465 | extern_array[1] --> |
| 2466 | @{ volatile int t=1; extern_array[t] @} |
| 2467 | @end example |
| 2468 | |
| 2469 | For structs (and most other multiword data types) the only option |
| 2470 | is to make the struct itself (or the long long, or the ...) variable: |
| 2471 | |
| 2472 | @example |
| 2473 | extern struct s extern_struct; |
| 2474 | extern_struct.field --> |
| 2475 | @{ volatile struct s *t=&extern_struct; t->field @} |
| 2476 | @end example |
| 2477 | |
| 2478 | or |
| 2479 | |
| 2480 | @example |
| 2481 | extern long long extern_ll; |
| 2482 | extern_ll --> |
| 2483 | @{ volatile long long * local_ll=&extern_ll; *local_ll @} |
| 2484 | @end example |
| 2485 | |
| 2486 | A third method of dealing with this difficulty is to abandon |
| 2487 | 'auto-import' for the offending symbol and mark it with |
| 2488 | @code{__declspec(dllimport)}. However, in practise that |
| 2489 | requires using compile-time #defines to indicate whether you are |
| 2490 | building a DLL, building client code that will link to the DLL, or |
| 2491 | merely building/linking to a static library. In making the choice |
| 2492 | between the various methods of resolving the 'direct address with |
| 2493 | constant offset' problem, you should consider typical real-world usage: |
| 2494 | |
| 2495 | Original: |
| 2496 | @example |
| 2497 | --foo.h |
| 2498 | extern int arr[]; |
| 2499 | --foo.c |
| 2500 | #include "foo.h" |
| 2501 | void main(int argc, char **argv)@{ |
| 2502 | printf("%d\n",arr[1]); |
| 2503 | @} |
| 2504 | @end example |
| 2505 | |
| 2506 | Solution 1: |
| 2507 | @example |
| 2508 | --foo.h |
| 2509 | extern int arr[]; |
| 2510 | --foo.c |
| 2511 | #include "foo.h" |
| 2512 | void main(int argc, char **argv)@{ |
| 2513 | /* This workaround is for win32 and cygwin; do not "optimize" */ |
| 2514 | volatile int *parr = arr; |
| 2515 | printf("%d\n",parr[1]); |
| 2516 | @} |
| 2517 | @end example |
| 2518 | |
| 2519 | Solution 2: |
| 2520 | @example |
| 2521 | --foo.h |
| 2522 | /* Note: auto-export is assumed (no __declspec(dllexport)) */ |
| 2523 | #if (defined(_WIN32) || defined(__CYGWIN__)) && \ |
| 2524 | !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC)) |
| 2525 | #define FOO_IMPORT __declspec(dllimport) |
| 2526 | #else |
| 2527 | #define FOO_IMPORT |
| 2528 | #endif |
| 2529 | extern FOO_IMPORT int arr[]; |
| 2530 | --foo.c |
| 2531 | #include "foo.h" |
| 2532 | void main(int argc, char **argv)@{ |
| 2533 | printf("%d\n",arr[1]); |
| 2534 | @} |
| 2535 | @end example |
| 2536 | |
| 2537 | A fourth way to avoid this problem is to re-code your |
| 2538 | library to use a functional interface rather than a data interface |
| 2539 | for the offending variables (e.g. set_foo() and get_foo() accessor |
| 2540 | functions). |
| 2541 | [This option is specific to the i386 PE targeted port of the linker] |
| 2542 | |
| 2543 | @kindex --disable-auto-import |
| 2544 | @item --disable-auto-import |
| 2545 | Do not attempt to do sophisticated linking of @code{_symbol} to |
| 2546 | @code{__imp__symbol} for DATA imports from DLLs. |
| 2547 | [This option is specific to the i386 PE targeted port of the linker] |
| 2548 | |
| 2549 | @kindex --enable-runtime-pseudo-reloc |
| 2550 | @item --enable-runtime-pseudo-reloc |
| 2551 | If your code contains expressions described in --enable-auto-import section, |
| 2552 | that is, DATA imports from DLL with non-zero offset, this switch will create |
| 2553 | a vector of 'runtime pseudo relocations' which can be used by runtime |
| 2554 | environment to adjust references to such data in your client code. |
| 2555 | [This option is specific to the i386 PE targeted port of the linker] |
| 2556 | |
| 2557 | @kindex --disable-runtime-pseudo-reloc |
| 2558 | @item --disable-runtime-pseudo-reloc |
| 2559 | Do not create pseudo relocations for non-zero offset DATA imports from |
| 2560 | DLLs. This is the default. |
| 2561 | [This option is specific to the i386 PE targeted port of the linker] |
| 2562 | |
| 2563 | @kindex --enable-extra-pe-debug |
| 2564 | @item --enable-extra-pe-debug |
| 2565 | Show additional debug info related to auto-import symbol thunking. |
| 2566 | [This option is specific to the i386 PE targeted port of the linker] |
| 2567 | |
| 2568 | @kindex --section-alignment |
| 2569 | @item --section-alignment |
| 2570 | Sets the section alignment. Sections in memory will always begin at |
| 2571 | addresses which are a multiple of this number. Defaults to 0x1000. |
| 2572 | [This option is specific to the i386 PE targeted port of the linker] |
| 2573 | |
| 2574 | @cindex stack size |
| 2575 | @kindex --stack |
| 2576 | @item --stack @var{reserve} |
| 2577 | @itemx --stack @var{reserve},@var{commit} |
| 2578 | Specify the number of bytes of memory to reserve (and optionally commit) |
| 2579 | to be used as stack for this program. The default is 2Mb reserved, 4K |
| 2580 | committed. |
| 2581 | [This option is specific to the i386 PE targeted port of the linker] |
| 2582 | |
| 2583 | @kindex --subsystem |
| 2584 | @item --subsystem @var{which} |
| 2585 | @itemx --subsystem @var{which}:@var{major} |
| 2586 | @itemx --subsystem @var{which}:@var{major}.@var{minor} |
| 2587 | Specifies the subsystem under which your program will execute. The |
| 2588 | legal values for @var{which} are @code{native}, @code{windows}, |
| 2589 | @code{console}, @code{posix}, and @code{xbox}. You may optionally set |
| 2590 | the subsystem version also. Numeric values are also accepted for |
| 2591 | @var{which}. |
| 2592 | [This option is specific to the i386 PE targeted port of the linker] |
| 2593 | |
| 2594 | The following options set flags in the @code{DllCharacteristics} field |
| 2595 | of the PE file header: |
| 2596 | [These options are specific to PE targeted ports of the linker] |
| 2597 | |
| 2598 | @kindex --dynamicbase |
| 2599 | @item --dynamicbase |
| 2600 | The image base address may be relocated using address space layout |
| 2601 | randomization (ASLR). This feature was introduced with MS Windows |
| 2602 | Vista for i386 PE targets. |
| 2603 | |
| 2604 | @kindex --forceinteg |
| 2605 | @item --forceinteg |
| 2606 | Code integrity checks are enforced. |
| 2607 | |
| 2608 | @kindex --nxcompat |
| 2609 | @item --nxcompat |
| 2610 | The image is compatible with the Data Execution Prevention. |
| 2611 | This feature was introduced with MS Windows XP SP2 for i386 PE targets. |
| 2612 | |
| 2613 | @kindex --no-isolation |
| 2614 | @item --no-isolation |
| 2615 | Although the image understands isolation, do not isolate the image. |
| 2616 | |
| 2617 | @kindex --no-seh |
| 2618 | @item --no-seh |
| 2619 | The image does not use SEH. No SE handler may be called from |
| 2620 | this image. |
| 2621 | |
| 2622 | @kindex --no-bind |
| 2623 | @item --no-bind |
| 2624 | Do not bind this image. |
| 2625 | |
| 2626 | @kindex --wdmdriver |
| 2627 | @item --wdmdriver |
| 2628 | The driver uses the MS Windows Driver Model. |
| 2629 | |
| 2630 | @kindex --tsaware |
| 2631 | @item --tsaware |
| 2632 | The image is Terminal Server aware. |
| 2633 | |
| 2634 | @end table |
| 2635 | |
| 2636 | @c man end |
| 2637 | |
| 2638 | @ifset C6X |
| 2639 | @subsection Options specific to C6X uClinux targets |
| 2640 | |
| 2641 | @c man begin OPTIONS |
| 2642 | |
| 2643 | The C6X uClinux target uses a binary format called DSBT to support shared |
| 2644 | libraries. Each shared library in the system needs to have a unique index; |
| 2645 | all executables use an index of 0. |
| 2646 | |
| 2647 | @table @gcctabopt |
| 2648 | |
| 2649 | @kindex --dsbt-size |
| 2650 | @item --dsbt-size @var{size} |
| 2651 | This option sets the number of entires in the DSBT of the current executable |
| 2652 | or shared library to @var{size}. The default is to create a table with 64 |
| 2653 | entries. |
| 2654 | |
| 2655 | @kindex --dsbt-index |
| 2656 | @item --dsbt-index @var{index} |
| 2657 | This option sets the DSBT index of the current executable or shared library |
| 2658 | to @var{index}. The default is 0, which is appropriate for generating |
| 2659 | executables. If a shared library is generated with a DSBT index of 0, the |
| 2660 | @code{R_C6000_DSBT_INDEX} relocs are copied into the output file. |
| 2661 | |
| 2662 | @kindex --no-merge-exidx-entries |
| 2663 | The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent |
| 2664 | exidx entries in frame unwind info. |
| 2665 | |
| 2666 | @end table |
| 2667 | |
| 2668 | @c man end |
| 2669 | @end ifset |
| 2670 | |
| 2671 | @ifset M68HC11 |
| 2672 | @subsection Options specific to Motorola 68HC11 and 68HC12 targets |
| 2673 | |
| 2674 | @c man begin OPTIONS |
| 2675 | |
| 2676 | The 68HC11 and 68HC12 linkers support specific options to control the |
| 2677 | memory bank switching mapping and trampoline code generation. |
| 2678 | |
| 2679 | @table @gcctabopt |
| 2680 | |
| 2681 | @kindex --no-trampoline |
| 2682 | @item --no-trampoline |
| 2683 | This option disables the generation of trampoline. By default a trampoline |
| 2684 | is generated for each far function which is called using a @code{jsr} |
| 2685 | instruction (this happens when a pointer to a far function is taken). |
| 2686 | |
| 2687 | @kindex --bank-window |
| 2688 | @item --bank-window @var{name} |
| 2689 | This option indicates to the linker the name of the memory region in |
| 2690 | the @samp{MEMORY} specification that describes the memory bank window. |
| 2691 | The definition of such region is then used by the linker to compute |
| 2692 | paging and addresses within the memory window. |
| 2693 | |
| 2694 | @end table |
| 2695 | |
| 2696 | @c man end |
| 2697 | @end ifset |
| 2698 | |
| 2699 | @ifset M68K |
| 2700 | @subsection Options specific to Motorola 68K target |
| 2701 | |
| 2702 | @c man begin OPTIONS |
| 2703 | |
| 2704 | The following options are supported to control handling of GOT generation |
| 2705 | when linking for 68K targets. |
| 2706 | |
| 2707 | @table @gcctabopt |
| 2708 | |
| 2709 | @kindex --got |
| 2710 | @item --got=@var{type} |
| 2711 | This option tells the linker which GOT generation scheme to use. |
| 2712 | @var{type} should be one of @samp{single}, @samp{negative}, |
| 2713 | @samp{multigot} or @samp{target}. For more information refer to the |
| 2714 | Info entry for @file{ld}. |
| 2715 | |
| 2716 | @end table |
| 2717 | |
| 2718 | @c man end |
| 2719 | @end ifset |
| 2720 | |
| 2721 | @ifset UsesEnvVars |
| 2722 | @node Environment |
| 2723 | @section Environment Variables |
| 2724 | |
| 2725 | @c man begin ENVIRONMENT |
| 2726 | |
| 2727 | You can change the behaviour of @command{ld} with the environment variables |
| 2728 | @ifclear SingleFormat |
| 2729 | @code{GNUTARGET}, |
| 2730 | @end ifclear |
| 2731 | @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}. |
| 2732 | |
| 2733 | @ifclear SingleFormat |
| 2734 | @kindex GNUTARGET |
| 2735 | @cindex default input format |
| 2736 | @code{GNUTARGET} determines the input-file object format if you don't |
| 2737 | use @samp{-b} (or its synonym @samp{--format}). Its value should be one |
| 2738 | of the BFD names for an input format (@pxref{BFD}). If there is no |
| 2739 | @code{GNUTARGET} in the environment, @command{ld} uses the natural format |
| 2740 | of the target. If @code{GNUTARGET} is set to @code{default} then BFD |
| 2741 | attempts to discover the input format by examining binary input files; |
| 2742 | this method often succeeds, but there are potential ambiguities, since |
| 2743 | there is no method of ensuring that the magic number used to specify |
| 2744 | object-file formats is unique. However, the configuration procedure for |
| 2745 | BFD on each system places the conventional format for that system first |
| 2746 | in the search-list, so ambiguities are resolved in favor of convention. |
| 2747 | @end ifclear |
| 2748 | |
| 2749 | @kindex LDEMULATION |
| 2750 | @cindex default emulation |
| 2751 | @cindex emulation, default |
| 2752 | @code{LDEMULATION} determines the default emulation if you don't use the |
| 2753 | @samp{-m} option. The emulation can affect various aspects of linker |
| 2754 | behaviour, particularly the default linker script. You can list the |
| 2755 | available emulations with the @samp{--verbose} or @samp{-V} options. If |
| 2756 | the @samp{-m} option is not used, and the @code{LDEMULATION} environment |
| 2757 | variable is not defined, the default emulation depends upon how the |
| 2758 | linker was configured. |
| 2759 | |
| 2760 | @kindex COLLECT_NO_DEMANGLE |
| 2761 | @cindex demangling, default |
| 2762 | Normally, the linker will default to demangling symbols. However, if |
| 2763 | @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will |
| 2764 | default to not demangling symbols. This environment variable is used in |
| 2765 | a similar fashion by the @code{gcc} linker wrapper program. The default |
| 2766 | may be overridden by the @samp{--demangle} and @samp{--no-demangle} |
| 2767 | options. |
| 2768 | |
| 2769 | @c man end |
| 2770 | @end ifset |
| 2771 | |
| 2772 | @node Scripts |
| 2773 | @chapter Linker Scripts |
| 2774 | |
| 2775 | @cindex scripts |
| 2776 | @cindex linker scripts |
| 2777 | @cindex command files |
| 2778 | Every link is controlled by a @dfn{linker script}. This script is |
| 2779 | written in the linker command language. |
| 2780 | |
| 2781 | The main purpose of the linker script is to describe how the sections in |
| 2782 | the input files should be mapped into the output file, and to control |
| 2783 | the memory layout of the output file. Most linker scripts do nothing |
| 2784 | more than this. However, when necessary, the linker script can also |
| 2785 | direct the linker to perform many other operations, using the commands |
| 2786 | described below. |
| 2787 | |
| 2788 | The linker always uses a linker script. If you do not supply one |
| 2789 | yourself, the linker will use a default script that is compiled into the |
| 2790 | linker executable. You can use the @samp{--verbose} command line option |
| 2791 | to display the default linker script. Certain command line options, |
| 2792 | such as @samp{-r} or @samp{-N}, will affect the default linker script. |
| 2793 | |
| 2794 | You may supply your own linker script by using the @samp{-T} command |
| 2795 | line option. When you do this, your linker script will replace the |
| 2796 | default linker script. |
| 2797 | |
| 2798 | You may also use linker scripts implicitly by naming them as input files |
| 2799 | to the linker, as though they were files to be linked. @xref{Implicit |
| 2800 | Linker Scripts}. |
| 2801 | |
| 2802 | @menu |
| 2803 | * Basic Script Concepts:: Basic Linker Script Concepts |
| 2804 | * Script Format:: Linker Script Format |
| 2805 | * Simple Example:: Simple Linker Script Example |
| 2806 | * Simple Commands:: Simple Linker Script Commands |
| 2807 | * Assignments:: Assigning Values to Symbols |
| 2808 | * SECTIONS:: SECTIONS Command |
| 2809 | * MEMORY:: MEMORY Command |
| 2810 | * PHDRS:: PHDRS Command |
| 2811 | * VERSION:: VERSION Command |
| 2812 | * Expressions:: Expressions in Linker Scripts |
| 2813 | * Implicit Linker Scripts:: Implicit Linker Scripts |
| 2814 | @end menu |
| 2815 | |
| 2816 | @node Basic Script Concepts |
| 2817 | @section Basic Linker Script Concepts |
| 2818 | @cindex linker script concepts |
| 2819 | We need to define some basic concepts and vocabulary in order to |
| 2820 | describe the linker script language. |
| 2821 | |
| 2822 | The linker combines input files into a single output file. The output |
| 2823 | file and each input file are in a special data format known as an |
| 2824 | @dfn{object file format}. Each file is called an @dfn{object file}. |
| 2825 | The output file is often called an @dfn{executable}, but for our |
| 2826 | purposes we will also call it an object file. Each object file has, |
| 2827 | among other things, a list of @dfn{sections}. We sometimes refer to a |
| 2828 | section in an input file as an @dfn{input section}; similarly, a section |
| 2829 | in the output file is an @dfn{output section}. |
| 2830 | |
| 2831 | Each section in an object file has a name and a size. Most sections |
| 2832 | also have an associated block of data, known as the @dfn{section |
| 2833 | contents}. A section may be marked as @dfn{loadable}, which mean that |
| 2834 | the contents should be loaded into memory when the output file is run. |
| 2835 | A section with no contents may be @dfn{allocatable}, which means that an |
| 2836 | area in memory should be set aside, but nothing in particular should be |
| 2837 | loaded there (in some cases this memory must be zeroed out). A section |
| 2838 | which is neither loadable nor allocatable typically contains some sort |
| 2839 | of debugging information. |
| 2840 | |
| 2841 | Every loadable or allocatable output section has two addresses. The |
| 2842 | first is the @dfn{VMA}, or virtual memory address. This is the address |
| 2843 | the section will have when the output file is run. The second is the |
| 2844 | @dfn{LMA}, or load memory address. This is the address at which the |
| 2845 | section will be loaded. In most cases the two addresses will be the |
| 2846 | same. An example of when they might be different is when a data section |
| 2847 | is loaded into ROM, and then copied into RAM when the program starts up |
| 2848 | (this technique is often used to initialize global variables in a ROM |
| 2849 | based system). In this case the ROM address would be the LMA, and the |
| 2850 | RAM address would be the VMA. |
| 2851 | |
| 2852 | You can see the sections in an object file by using the @code{objdump} |
| 2853 | program with the @samp{-h} option. |
| 2854 | |
| 2855 | Every object file also has a list of @dfn{symbols}, known as the |
| 2856 | @dfn{symbol table}. A symbol may be defined or undefined. Each symbol |
| 2857 | has a name, and each defined symbol has an address, among other |
| 2858 | information. If you compile a C or C++ program into an object file, you |
| 2859 | will get a defined symbol for every defined function and global or |
| 2860 | static variable. Every undefined function or global variable which is |
| 2861 | referenced in the input file will become an undefined symbol. |
| 2862 | |
| 2863 | You can see the symbols in an object file by using the @code{nm} |
| 2864 | program, or by using the @code{objdump} program with the @samp{-t} |
| 2865 | option. |
| 2866 | |
| 2867 | @node Script Format |
| 2868 | @section Linker Script Format |
| 2869 | @cindex linker script format |
| 2870 | Linker scripts are text files. |
| 2871 | |
| 2872 | You write a linker script as a series of commands. Each command is |
| 2873 | either a keyword, possibly followed by arguments, or an assignment to a |
| 2874 | symbol. You may separate commands using semicolons. Whitespace is |
| 2875 | generally ignored. |
| 2876 | |
| 2877 | Strings such as file or format names can normally be entered directly. |
| 2878 | If the file name contains a character such as a comma which would |
| 2879 | otherwise serve to separate file names, you may put the file name in |
| 2880 | double quotes. There is no way to use a double quote character in a |
| 2881 | file name. |
| 2882 | |
| 2883 | You may include comments in linker scripts just as in C, delimited by |
| 2884 | @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent |
| 2885 | to whitespace. |
| 2886 | |
| 2887 | @node Simple Example |
| 2888 | @section Simple Linker Script Example |
| 2889 | @cindex linker script example |
| 2890 | @cindex example of linker script |
| 2891 | Many linker scripts are fairly simple. |
| 2892 | |
| 2893 | The simplest possible linker script has just one command: |
| 2894 | @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the |
| 2895 | memory layout of the output file. |
| 2896 | |
| 2897 | The @samp{SECTIONS} command is a powerful command. Here we will |
| 2898 | describe a simple use of it. Let's assume your program consists only of |
| 2899 | code, initialized data, and uninitialized data. These will be in the |
| 2900 | @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively. |
| 2901 | Let's assume further that these are the only sections which appear in |
| 2902 | your input files. |
| 2903 | |
| 2904 | For this example, let's say that the code should be loaded at address |
| 2905 | 0x10000, and that the data should start at address 0x8000000. Here is a |
| 2906 | linker script which will do that: |
| 2907 | @smallexample |
| 2908 | SECTIONS |
| 2909 | @{ |
| 2910 | . = 0x10000; |
| 2911 | .text : @{ *(.text) @} |
| 2912 | . = 0x8000000; |
| 2913 | .data : @{ *(.data) @} |
| 2914 | .bss : @{ *(.bss) @} |
| 2915 | @} |
| 2916 | @end smallexample |
| 2917 | |
| 2918 | You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS}, |
| 2919 | followed by a series of symbol assignments and output section |
| 2920 | descriptions enclosed in curly braces. |
| 2921 | |
| 2922 | The first line inside the @samp{SECTIONS} command of the above example |
| 2923 | sets the value of the special symbol @samp{.}, which is the location |
| 2924 | counter. If you do not specify the address of an output section in some |
| 2925 | other way (other ways are described later), the address is set from the |
| 2926 | current value of the location counter. The location counter is then |
| 2927 | incremented by the size of the output section. At the start of the |
| 2928 | @samp{SECTIONS} command, the location counter has the value @samp{0}. |
| 2929 | |
| 2930 | The second line defines an output section, @samp{.text}. The colon is |
| 2931 | required syntax which may be ignored for now. Within the curly braces |
| 2932 | after the output section name, you list the names of the input sections |
| 2933 | which should be placed into this output section. The @samp{*} is a |
| 2934 | wildcard which matches any file name. The expression @samp{*(.text)} |
| 2935 | means all @samp{.text} input sections in all input files. |
| 2936 | |
| 2937 | Since the location counter is @samp{0x10000} when the output section |
| 2938 | @samp{.text} is defined, the linker will set the address of the |
| 2939 | @samp{.text} section in the output file to be @samp{0x10000}. |
| 2940 | |
| 2941 | The remaining lines define the @samp{.data} and @samp{.bss} sections in |
| 2942 | the output file. The linker will place the @samp{.data} output section |
| 2943 | at address @samp{0x8000000}. After the linker places the @samp{.data} |
| 2944 | output section, the value of the location counter will be |
| 2945 | @samp{0x8000000} plus the size of the @samp{.data} output section. The |
| 2946 | effect is that the linker will place the @samp{.bss} output section |
| 2947 | immediately after the @samp{.data} output section in memory. |
| 2948 | |
| 2949 | The linker will ensure that each output section has the required |
| 2950 | alignment, by increasing the location counter if necessary. In this |
| 2951 | example, the specified addresses for the @samp{.text} and @samp{.data} |
| 2952 | sections will probably satisfy any alignment constraints, but the linker |
| 2953 | may have to create a small gap between the @samp{.data} and @samp{.bss} |
| 2954 | sections. |
| 2955 | |
| 2956 | That's it! That's a simple and complete linker script. |
| 2957 | |
| 2958 | @node Simple Commands |
| 2959 | @section Simple Linker Script Commands |
| 2960 | @cindex linker script simple commands |
| 2961 | In this section we describe the simple linker script commands. |
| 2962 | |
| 2963 | @menu |
| 2964 | * Entry Point:: Setting the entry point |
| 2965 | * File Commands:: Commands dealing with files |
| 2966 | @ifclear SingleFormat |
| 2967 | * Format Commands:: Commands dealing with object file formats |
| 2968 | @end ifclear |
| 2969 | |
| 2970 | * REGION_ALIAS:: Assign alias names to memory regions |
| 2971 | * Miscellaneous Commands:: Other linker script commands |
| 2972 | @end menu |
| 2973 | |
| 2974 | @node Entry Point |
| 2975 | @subsection Setting the Entry Point |
| 2976 | @kindex ENTRY(@var{symbol}) |
| 2977 | @cindex start of execution |
| 2978 | @cindex first instruction |
| 2979 | @cindex entry point |
| 2980 | The first instruction to execute in a program is called the @dfn{entry |
| 2981 | point}. You can use the @code{ENTRY} linker script command to set the |
| 2982 | entry point. The argument is a symbol name: |
| 2983 | @smallexample |
| 2984 | ENTRY(@var{symbol}) |
| 2985 | @end smallexample |
| 2986 | |
| 2987 | There are several ways to set the entry point. The linker will set the |
| 2988 | entry point by trying each of the following methods in order, and |
| 2989 | stopping when one of them succeeds: |
| 2990 | @itemize @bullet |
| 2991 | @item |
| 2992 | the @samp{-e} @var{entry} command-line option; |
| 2993 | @item |
| 2994 | the @code{ENTRY(@var{symbol})} command in a linker script; |
| 2995 | @item |
| 2996 | the value of a target specific symbol, if it is defined; For many |
| 2997 | targets this is @code{start}, but PE and BeOS based systems for example |
| 2998 | check a list of possible entry symbols, matching the first one found. |
| 2999 | @item |
| 3000 | the address of the first byte of the @samp{.text} section, if present; |
| 3001 | @item |
| 3002 | The address @code{0}. |
| 3003 | @end itemize |
| 3004 | |
| 3005 | @node File Commands |
| 3006 | @subsection Commands Dealing with Files |
| 3007 | @cindex linker script file commands |
| 3008 | Several linker script commands deal with files. |
| 3009 | |
| 3010 | @table @code |
| 3011 | @item INCLUDE @var{filename} |
| 3012 | @kindex INCLUDE @var{filename} |
| 3013 | @cindex including a linker script |
| 3014 | Include the linker script @var{filename} at this point. The file will |
| 3015 | be searched for in the current directory, and in any directory specified |
| 3016 | with the @option{-L} option. You can nest calls to @code{INCLUDE} up to |
| 3017 | 10 levels deep. |
| 3018 | |
| 3019 | You can place @code{INCLUDE} directives at the top level, in @code{MEMORY} or |
| 3020 | @code{SECTIONS} commands, or in output section descriptions. |
| 3021 | |
| 3022 | @item INPUT(@var{file}, @var{file}, @dots{}) |
| 3023 | @itemx INPUT(@var{file} @var{file} @dots{}) |
| 3024 | @kindex INPUT(@var{files}) |
| 3025 | @cindex input files in linker scripts |
| 3026 | @cindex input object files in linker scripts |
| 3027 | @cindex linker script input object files |
| 3028 | The @code{INPUT} command directs the linker to include the named files |
| 3029 | in the link, as though they were named on the command line. |
| 3030 | |
| 3031 | For example, if you always want to include @file{subr.o} any time you do |
| 3032 | a link, but you can't be bothered to put it on every link command line, |
| 3033 | then you can put @samp{INPUT (subr.o)} in your linker script. |
| 3034 | |
| 3035 | In fact, if you like, you can list all of your input files in the linker |
| 3036 | script, and then invoke the linker with nothing but a @samp{-T} option. |
| 3037 | |
| 3038 | In case a @dfn{sysroot prefix} is configured, and the filename starts |
| 3039 | with the @samp{/} character, and the script being processed was |
| 3040 | located inside the @dfn{sysroot prefix}, the filename will be looked |
| 3041 | for in the @dfn{sysroot prefix}. Otherwise, the linker will try to |
| 3042 | open the file in the current directory. If it is not found, the |
| 3043 | linker will search through the archive library search path. See the |
| 3044 | description of @samp{-L} in @ref{Options,,Command Line Options}. |
| 3045 | |
| 3046 | If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the |
| 3047 | name to @code{lib@var{file}.a}, as with the command line argument |
| 3048 | @samp{-l}. |
| 3049 | |
| 3050 | When you use the @code{INPUT} command in an implicit linker script, the |
| 3051 | files will be included in the link at the point at which the linker |
| 3052 | script file is included. This can affect archive searching. |
| 3053 | |
| 3054 | @item GROUP(@var{file}, @var{file}, @dots{}) |
| 3055 | @itemx GROUP(@var{file} @var{file} @dots{}) |
| 3056 | @kindex GROUP(@var{files}) |
| 3057 | @cindex grouping input files |
| 3058 | The @code{GROUP} command is like @code{INPUT}, except that the named |
| 3059 | files should all be archives, and they are searched repeatedly until no |
| 3060 | new undefined references are created. See the description of @samp{-(} |
| 3061 | in @ref{Options,,Command Line Options}. |
| 3062 | |
| 3063 | @item AS_NEEDED(@var{file}, @var{file}, @dots{}) |
| 3064 | @itemx AS_NEEDED(@var{file} @var{file} @dots{}) |
| 3065 | @kindex AS_NEEDED(@var{files}) |
| 3066 | This construct can appear only inside of the @code{INPUT} or @code{GROUP} |
| 3067 | commands, among other filenames. The files listed will be handled |
| 3068 | as if they appear directly in the @code{INPUT} or @code{GROUP} commands, |
| 3069 | with the exception of ELF shared libraries, that will be added only |
| 3070 | when they are actually needed. This construct essentially enables |
| 3071 | @option{--as-needed} option for all the files listed inside of it |
| 3072 | and restores previous @option{--as-needed} resp. @option{--no-as-needed} |
| 3073 | setting afterwards. |
| 3074 | |
| 3075 | @item OUTPUT(@var{filename}) |
| 3076 | @kindex OUTPUT(@var{filename}) |
| 3077 | @cindex output file name in linker script |
| 3078 | The @code{OUTPUT} command names the output file. Using |
| 3079 | @code{OUTPUT(@var{filename})} in the linker script is exactly like using |
| 3080 | @samp{-o @var{filename}} on the command line (@pxref{Options,,Command |
| 3081 | Line Options}). If both are used, the command line option takes |
| 3082 | precedence. |
| 3083 | |
| 3084 | You can use the @code{OUTPUT} command to define a default name for the |
| 3085 | output file other than the usual default of @file{a.out}. |
| 3086 | |
| 3087 | @item SEARCH_DIR(@var{path}) |
| 3088 | @kindex SEARCH_DIR(@var{path}) |
| 3089 | @cindex library search path in linker script |
| 3090 | @cindex archive search path in linker script |
| 3091 | @cindex search path in linker script |
| 3092 | The @code{SEARCH_DIR} command adds @var{path} to the list of paths where |
| 3093 | @command{ld} looks for archive libraries. Using |
| 3094 | @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}} |
| 3095 | on the command line (@pxref{Options,,Command Line Options}). If both |
| 3096 | are used, then the linker will search both paths. Paths specified using |
| 3097 | the command line option are searched first. |
| 3098 | |
| 3099 | @item STARTUP(@var{filename}) |
| 3100 | @kindex STARTUP(@var{filename}) |
| 3101 | @cindex first input file |
| 3102 | The @code{STARTUP} command is just like the @code{INPUT} command, except |
| 3103 | that @var{filename} will become the first input file to be linked, as |
| 3104 | though it were specified first on the command line. This may be useful |
| 3105 | when using a system in which the entry point is always the start of the |
| 3106 | first file. |
| 3107 | @end table |
| 3108 | |
| 3109 | @ifclear SingleFormat |
| 3110 | @node Format Commands |
| 3111 | @subsection Commands Dealing with Object File Formats |
| 3112 | A couple of linker script commands deal with object file formats. |
| 3113 | |
| 3114 | @table @code |
| 3115 | @item OUTPUT_FORMAT(@var{bfdname}) |
| 3116 | @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little}) |
| 3117 | @kindex OUTPUT_FORMAT(@var{bfdname}) |
| 3118 | @cindex output file format in linker script |
| 3119 | The @code{OUTPUT_FORMAT} command names the BFD format to use for the |
| 3120 | output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is |
| 3121 | exactly like using @samp{--oformat @var{bfdname}} on the command line |
| 3122 | (@pxref{Options,,Command Line Options}). If both are used, the command |
| 3123 | line option takes precedence. |
| 3124 | |
| 3125 | You can use @code{OUTPUT_FORMAT} with three arguments to use different |
| 3126 | formats based on the @samp{-EB} and @samp{-EL} command line options. |
| 3127 | This permits the linker script to set the output format based on the |
| 3128 | desired endianness. |
| 3129 | |
| 3130 | If neither @samp{-EB} nor @samp{-EL} are used, then the output format |
| 3131 | will be the first argument, @var{default}. If @samp{-EB} is used, the |
| 3132 | output format will be the second argument, @var{big}. If @samp{-EL} is |
| 3133 | used, the output format will be the third argument, @var{little}. |
| 3134 | |
| 3135 | For example, the default linker script for the MIPS ELF target uses this |
| 3136 | command: |
| 3137 | @smallexample |
| 3138 | OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips) |
| 3139 | @end smallexample |
| 3140 | This says that the default format for the output file is |
| 3141 | @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line |
| 3142 | option, the output file will be created in the @samp{elf32-littlemips} |
| 3143 | format. |
| 3144 | |
| 3145 | @item TARGET(@var{bfdname}) |
| 3146 | @kindex TARGET(@var{bfdname}) |
| 3147 | @cindex input file format in linker script |
| 3148 | The @code{TARGET} command names the BFD format to use when reading input |
| 3149 | files. It affects subsequent @code{INPUT} and @code{GROUP} commands. |
| 3150 | This command is like using @samp{-b @var{bfdname}} on the command line |
| 3151 | (@pxref{Options,,Command Line Options}). If the @code{TARGET} command |
| 3152 | is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET} |
| 3153 | command is also used to set the format for the output file. @xref{BFD}. |
| 3154 | @end table |
| 3155 | @end ifclear |
| 3156 | |
| 3157 | @node REGION_ALIAS |
| 3158 | @subsection Assign alias names to memory regions |
| 3159 | @kindex REGION_ALIAS(@var{alias}, @var{region}) |
| 3160 | @cindex region alias |
| 3161 | @cindex region names |
| 3162 | |
| 3163 | Alias names can be added to existing memory regions created with the |
| 3164 | @ref{MEMORY} command. Each name corresponds to at most one memory region. |
| 3165 | |
| 3166 | @smallexample |
| 3167 | REGION_ALIAS(@var{alias}, @var{region}) |
| 3168 | @end smallexample |
| 3169 | |
| 3170 | The @code{REGION_ALIAS} function creates an alias name @var{alias} for the |
| 3171 | memory region @var{region}. This allows a flexible mapping of output sections |
| 3172 | to memory regions. An example follows. |
| 3173 | |
| 3174 | Suppose we have an application for embedded systems which come with various |
| 3175 | memory storage devices. All have a general purpose, volatile memory @code{RAM} |
| 3176 | that allows code execution or data storage. Some may have a read-only, |
| 3177 | non-volatile memory @code{ROM} that allows code execution and read-only data |
| 3178 | access. The last variant is a read-only, non-volatile memory @code{ROM2} with |
| 3179 | read-only data access and no code execution capability. We have four output |
| 3180 | sections: |
| 3181 | |
| 3182 | @itemize @bullet |
| 3183 | @item |
| 3184 | @code{.text} program code; |
| 3185 | @item |
| 3186 | @code{.rodata} read-only data; |
| 3187 | @item |
| 3188 | @code{.data} read-write initialized data; |
| 3189 | @item |
| 3190 | @code{.bss} read-write zero initialized data. |
| 3191 | @end itemize |
| 3192 | |
| 3193 | The goal is to provide a linker command file that contains a system independent |
| 3194 | part defining the output sections and a system dependent part mapping the |
| 3195 | output sections to the memory regions available on the system. Our embedded |
| 3196 | systems come with three different memory setups @code{A}, @code{B} and |
| 3197 | @code{C}: |
| 3198 | @multitable @columnfractions .25 .25 .25 .25 |
| 3199 | @item Section @tab Variant A @tab Variant B @tab Variant C |
| 3200 | @item .text @tab RAM @tab ROM @tab ROM |
| 3201 | @item .rodata @tab RAM @tab ROM @tab ROM2 |
| 3202 | @item .data @tab RAM @tab RAM/ROM @tab RAM/ROM2 |
| 3203 | @item .bss @tab RAM @tab RAM @tab RAM |
| 3204 | @end multitable |
| 3205 | The notation @code{RAM/ROM} or @code{RAM/ROM2} means that this section is |
| 3206 | loaded into region @code{ROM} or @code{ROM2} respectively. Please note that |
| 3207 | the load address of the @code{.data} section starts in all three variants at |
| 3208 | the end of the @code{.rodata} section. |
| 3209 | |
| 3210 | The base linker script that deals with the output sections follows. It |
| 3211 | includes the system dependent @code{linkcmds.memory} file that describes the |
| 3212 | memory layout: |
| 3213 | @smallexample |
| 3214 | INCLUDE linkcmds.memory |
| 3215 | |
| 3216 | SECTIONS |
| 3217 | @{ |
| 3218 | .text : |
| 3219 | @{ |
| 3220 | *(.text) |
| 3221 | @} > REGION_TEXT |
| 3222 | .rodata : |
| 3223 | @{ |
| 3224 | *(.rodata) |
| 3225 | rodata_end = .; |
| 3226 | @} > REGION_RODATA |
| 3227 | .data : AT (rodata_end) |
| 3228 | @{ |
| 3229 | data_start = .; |
| 3230 | *(.data) |
| 3231 | @} > REGION_DATA |
| 3232 | data_size = SIZEOF(.data); |
| 3233 | data_load_start = LOADADDR(.data); |
| 3234 | .bss : |
| 3235 | @{ |
| 3236 | *(.bss) |
| 3237 | @} > REGION_BSS |
| 3238 | @} |
| 3239 | @end smallexample |
| 3240 | |
| 3241 | Now we need three different @code{linkcmds.memory} files to define memory |
| 3242 | regions and alias names. The content of @code{linkcmds.memory} for the three |
| 3243 | variants @code{A}, @code{B} and @code{C}: |
| 3244 | @table @code |
| 3245 | @item A |
| 3246 | Here everything goes into the @code{RAM}. |
| 3247 | @smallexample |
| 3248 | MEMORY |
| 3249 | @{ |
| 3250 | RAM : ORIGIN = 0, LENGTH = 4M |
| 3251 | @} |
| 3252 | |
| 3253 | REGION_ALIAS("REGION_TEXT", RAM); |
| 3254 | REGION_ALIAS("REGION_RODATA", RAM); |
| 3255 | REGION_ALIAS("REGION_DATA", RAM); |
| 3256 | REGION_ALIAS("REGION_BSS", RAM); |
| 3257 | @end smallexample |
| 3258 | @item B |
| 3259 | Program code and read-only data go into the @code{ROM}. Read-write data goes |
| 3260 | into the @code{RAM}. An image of the initialized data is loaded into the |
| 3261 | @code{ROM} and will be copied during system start into the @code{RAM}. |
| 3262 | @smallexample |
| 3263 | MEMORY |
| 3264 | @{ |
| 3265 | ROM : ORIGIN = 0, LENGTH = 3M |
| 3266 | RAM : ORIGIN = 0x10000000, LENGTH = 1M |
| 3267 | @} |
| 3268 | |
| 3269 | REGION_ALIAS("REGION_TEXT", ROM); |
| 3270 | REGION_ALIAS("REGION_RODATA", ROM); |
| 3271 | REGION_ALIAS("REGION_DATA", RAM); |
| 3272 | REGION_ALIAS("REGION_BSS", RAM); |
| 3273 | @end smallexample |
| 3274 | @item C |
| 3275 | Program code goes into the @code{ROM}. Read-only data goes into the |
| 3276 | @code{ROM2}. Read-write data goes into the @code{RAM}. An image of the |
| 3277 | initialized data is loaded into the @code{ROM2} and will be copied during |
| 3278 | system start into the @code{RAM}. |
| 3279 | @smallexample |
| 3280 | MEMORY |
| 3281 | @{ |
| 3282 | ROM : ORIGIN = 0, LENGTH = 2M |
| 3283 | ROM2 : ORIGIN = 0x10000000, LENGTH = 1M |
| 3284 | RAM : ORIGIN = 0x20000000, LENGTH = 1M |
| 3285 | @} |
| 3286 | |
| 3287 | REGION_ALIAS("REGION_TEXT", ROM); |
| 3288 | REGION_ALIAS("REGION_RODATA", ROM2); |
| 3289 | REGION_ALIAS("REGION_DATA", RAM); |
| 3290 | REGION_ALIAS("REGION_BSS", RAM); |
| 3291 | @end smallexample |
| 3292 | @end table |
| 3293 | |
| 3294 | It is possible to write a common system initialization routine to copy the |
| 3295 | @code{.data} section from @code{ROM} or @code{ROM2} into the @code{RAM} if |
| 3296 | necessary: |
| 3297 | @smallexample |
| 3298 | #include <string.h> |
| 3299 | |
| 3300 | extern char data_start []; |
| 3301 | extern char data_size []; |
| 3302 | extern char data_load_start []; |
| 3303 | |
| 3304 | void copy_data(void) |
| 3305 | @{ |
| 3306 | if (data_start != data_load_start) |
| 3307 | @{ |
| 3308 | memcpy(data_start, data_load_start, (size_t) data_size); |
| 3309 | @} |
| 3310 | @} |
| 3311 | @end smallexample |
| 3312 | |
| 3313 | @node Miscellaneous Commands |
| 3314 | @subsection Other Linker Script Commands |
| 3315 | There are a few other linker scripts commands. |
| 3316 | |
| 3317 | @table @code |
| 3318 | @item ASSERT(@var{exp}, @var{message}) |
| 3319 | @kindex ASSERT |
| 3320 | @cindex assertion in linker script |
| 3321 | Ensure that @var{exp} is non-zero. If it is zero, then exit the linker |
| 3322 | with an error code, and print @var{message}. |
| 3323 | |
| 3324 | @item EXTERN(@var{symbol} @var{symbol} @dots{}) |
| 3325 | @kindex EXTERN |
| 3326 | @cindex undefined symbol in linker script |
| 3327 | Force @var{symbol} to be entered in the output file as an undefined |
| 3328 | symbol. Doing this may, for example, trigger linking of additional |
| 3329 | modules from standard libraries. You may list several @var{symbol}s for |
| 3330 | each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This |
| 3331 | command has the same effect as the @samp{-u} command-line option. |
| 3332 | |
| 3333 | @item FORCE_COMMON_ALLOCATION |
| 3334 | @kindex FORCE_COMMON_ALLOCATION |
| 3335 | @cindex common allocation in linker script |
| 3336 | This command has the same effect as the @samp{-d} command-line option: |
| 3337 | to make @command{ld} assign space to common symbols even if a relocatable |
| 3338 | output file is specified (@samp{-r}). |
| 3339 | |
| 3340 | @item INHIBIT_COMMON_ALLOCATION |
| 3341 | @kindex INHIBIT_COMMON_ALLOCATION |
| 3342 | @cindex common allocation in linker script |
| 3343 | This command has the same effect as the @samp{--no-define-common} |
| 3344 | command-line option: to make @code{ld} omit the assignment of addresses |
| 3345 | to common symbols even for a non-relocatable output file. |
| 3346 | |
| 3347 | @item INSERT [ AFTER | BEFORE ] @var{output_section} |
| 3348 | @kindex INSERT |
| 3349 | @cindex insert user script into default script |
| 3350 | This command is typically used in a script specified by @samp{-T} to |
| 3351 | augment the default @code{SECTIONS} with, for example, overlays. It |
| 3352 | inserts all prior linker script statements after (or before) |
| 3353 | @var{output_section}, and also causes @samp{-T} to not override the |
| 3354 | default linker script. The exact insertion point is as for orphan |
| 3355 | sections. @xref{Location Counter}. The insertion happens after the |
| 3356 | linker has mapped input sections to output sections. Prior to the |
| 3357 | insertion, since @samp{-T} scripts are parsed before the default |
| 3358 | linker script, statements in the @samp{-T} script occur before the |
| 3359 | default linker script statements in the internal linker representation |
| 3360 | of the script. In particular, input section assignments will be made |
| 3361 | to @samp{-T} output sections before those in the default script. Here |
| 3362 | is an example of how a @samp{-T} script using @code{INSERT} might look: |
| 3363 | |
| 3364 | @smallexample |
| 3365 | SECTIONS |
| 3366 | @{ |
| 3367 | OVERLAY : |
| 3368 | @{ |
| 3369 | .ov1 @{ ov1*(.text) @} |
| 3370 | .ov2 @{ ov2*(.text) @} |
| 3371 | @} |
| 3372 | @} |
| 3373 | INSERT AFTER .text; |
| 3374 | @end smallexample |
| 3375 | |
| 3376 | @item NOCROSSREFS(@var{section} @var{section} @dots{}) |
| 3377 | @kindex NOCROSSREFS(@var{sections}) |
| 3378 | @cindex cross references |
| 3379 | This command may be used to tell @command{ld} to issue an error about any |
| 3380 | references among certain output sections. |
| 3381 | |
| 3382 | In certain types of programs, particularly on embedded systems when |
| 3383 | using overlays, when one section is loaded into memory, another section |
| 3384 | will not be. Any direct references between the two sections would be |
| 3385 | errors. For example, it would be an error if code in one section called |
| 3386 | a function defined in the other section. |
| 3387 | |
| 3388 | The @code{NOCROSSREFS} command takes a list of output section names. If |
| 3389 | @command{ld} detects any cross references between the sections, it reports |
| 3390 | an error and returns a non-zero exit status. Note that the |
| 3391 | @code{NOCROSSREFS} command uses output section names, not input section |
| 3392 | names. |
| 3393 | |
| 3394 | @ifclear SingleFormat |
| 3395 | @item OUTPUT_ARCH(@var{bfdarch}) |
| 3396 | @kindex OUTPUT_ARCH(@var{bfdarch}) |
| 3397 | @cindex machine architecture |
| 3398 | @cindex architecture |
| 3399 | Specify a particular output machine architecture. The argument is one |
| 3400 | of the names used by the BFD library (@pxref{BFD}). You can see the |
| 3401 | architecture of an object file by using the @code{objdump} program with |
| 3402 | the @samp{-f} option. |
| 3403 | @end ifclear |
| 3404 | |
| 3405 | @item LD_FEATURE(@var{string}) |
| 3406 | @kindex LD_FEATURE(@var{string}) |
| 3407 | This command may be used to modify @command{ld} behavior. If |
| 3408 | @var{string} is @code{"SANE_EXPR"} then absolute symbols and numbers |
| 3409 | in a script are simply treated as numbers everywhere. |
| 3410 | @xref{Expression Section}. |
| 3411 | @end table |
| 3412 | |
| 3413 | @node Assignments |
| 3414 | @section Assigning Values to Symbols |
| 3415 | @cindex assignment in scripts |
| 3416 | @cindex symbol definition, scripts |
| 3417 | @cindex variables, defining |
| 3418 | You may assign a value to a symbol in a linker script. This will define |
| 3419 | the symbol and place it into the symbol table with a global scope. |
| 3420 | |
| 3421 | @menu |
| 3422 | * Simple Assignments:: Simple Assignments |
| 3423 | * HIDDEN:: HIDDEN |
| 3424 | * PROVIDE:: PROVIDE |
| 3425 | * PROVIDE_HIDDEN:: PROVIDE_HIDDEN |
| 3426 | * Source Code Reference:: How to use a linker script defined symbol in source code |
| 3427 | @end menu |
| 3428 | |
| 3429 | @node Simple Assignments |
| 3430 | @subsection Simple Assignments |
| 3431 | |
| 3432 | You may assign to a symbol using any of the C assignment operators: |
| 3433 | |
| 3434 | @table @code |
| 3435 | @item @var{symbol} = @var{expression} ; |
| 3436 | @itemx @var{symbol} += @var{expression} ; |
| 3437 | @itemx @var{symbol} -= @var{expression} ; |
| 3438 | @itemx @var{symbol} *= @var{expression} ; |
| 3439 | @itemx @var{symbol} /= @var{expression} ; |
| 3440 | @itemx @var{symbol} <<= @var{expression} ; |
| 3441 | @itemx @var{symbol} >>= @var{expression} ; |
| 3442 | @itemx @var{symbol} &= @var{expression} ; |
| 3443 | @itemx @var{symbol} |= @var{expression} ; |
| 3444 | @end table |
| 3445 | |
| 3446 | The first case will define @var{symbol} to the value of |
| 3447 | @var{expression}. In the other cases, @var{symbol} must already be |
| 3448 | defined, and the value will be adjusted accordingly. |
| 3449 | |
| 3450 | The special symbol name @samp{.} indicates the location counter. You |
| 3451 | may only use this within a @code{SECTIONS} command. @xref{Location Counter}. |
| 3452 | |
| 3453 | The semicolon after @var{expression} is required. |
| 3454 | |
| 3455 | Expressions are defined below; see @ref{Expressions}. |
| 3456 | |
| 3457 | You may write symbol assignments as commands in their own right, or as |
| 3458 | statements within a @code{SECTIONS} command, or as part of an output |
| 3459 | section description in a @code{SECTIONS} command. |
| 3460 | |
| 3461 | The section of the symbol will be set from the section of the |
| 3462 | expression; for more information, see @ref{Expression Section}. |
| 3463 | |
| 3464 | Here is an example showing the three different places that symbol |
| 3465 | assignments may be used: |
| 3466 | |
| 3467 | @smallexample |
| 3468 | floating_point = 0; |
| 3469 | SECTIONS |
| 3470 | @{ |
| 3471 | .text : |
| 3472 | @{ |
| 3473 | *(.text) |
| 3474 | _etext = .; |
| 3475 | @} |
| 3476 | _bdata = (. + 3) & ~ 3; |
| 3477 | .data : @{ *(.data) @} |
| 3478 | @} |
| 3479 | @end smallexample |
| 3480 | @noindent |
| 3481 | In this example, the symbol @samp{floating_point} will be defined as |
| 3482 | zero. The symbol @samp{_etext} will be defined as the address following |
| 3483 | the last @samp{.text} input section. The symbol @samp{_bdata} will be |
| 3484 | defined as the address following the @samp{.text} output section aligned |
| 3485 | upward to a 4 byte boundary. |
| 3486 | |
| 3487 | @node HIDDEN |
| 3488 | @subsection HIDDEN |
| 3489 | @cindex HIDDEN |
| 3490 | For ELF targeted ports, define a symbol that will be hidden and won't be |
| 3491 | exported. The syntax is @code{HIDDEN(@var{symbol} = @var{expression})}. |
| 3492 | |
| 3493 | Here is the example from @ref{Simple Assignments}, rewritten to use |
| 3494 | @code{HIDDEN}: |
| 3495 | |
| 3496 | @smallexample |
| 3497 | HIDDEN(floating_point = 0); |
| 3498 | SECTIONS |
| 3499 | @{ |
| 3500 | .text : |
| 3501 | @{ |
| 3502 | *(.text) |
| 3503 | HIDDEN(_etext = .); |
| 3504 | @} |
| 3505 | HIDDEN(_bdata = (. + 3) & ~ 3); |
| 3506 | .data : @{ *(.data) @} |
| 3507 | @} |
| 3508 | @end smallexample |
| 3509 | @noindent |
| 3510 | In this case none of the three symbols will be visible outside this module. |
| 3511 | |
| 3512 | @node PROVIDE |
| 3513 | @subsection PROVIDE |
| 3514 | @cindex PROVIDE |
| 3515 | In some cases, it is desirable for a linker script to define a symbol |
| 3516 | only if it is referenced and is not defined by any object included in |
| 3517 | the link. For example, traditional linkers defined the symbol |
| 3518 | @samp{etext}. However, ANSI C requires that the user be able to use |
| 3519 | @samp{etext} as a function name without encountering an error. The |
| 3520 | @code{PROVIDE} keyword may be used to define a symbol, such as |
| 3521 | @samp{etext}, only if it is referenced but not defined. The syntax is |
| 3522 | @code{PROVIDE(@var{symbol} = @var{expression})}. |
| 3523 | |
| 3524 | Here is an example of using @code{PROVIDE} to define @samp{etext}: |
| 3525 | @smallexample |
| 3526 | SECTIONS |
| 3527 | @{ |
| 3528 | .text : |
| 3529 | @{ |
| 3530 | *(.text) |
| 3531 | _etext = .; |
| 3532 | PROVIDE(etext = .); |
| 3533 | @} |
| 3534 | @} |
| 3535 | @end smallexample |
| 3536 | |
| 3537 | In this example, if the program defines @samp{_etext} (with a leading |
| 3538 | underscore), the linker will give a multiple definition error. If, on |
| 3539 | the other hand, the program defines @samp{etext} (with no leading |
| 3540 | underscore), the linker will silently use the definition in the program. |
| 3541 | If the program references @samp{etext} but does not define it, the |
| 3542 | linker will use the definition in the linker script. |
| 3543 | |
| 3544 | @node PROVIDE_HIDDEN |
| 3545 | @subsection PROVIDE_HIDDEN |
| 3546 | @cindex PROVIDE_HIDDEN |
| 3547 | Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be |
| 3548 | hidden and won't be exported. |
| 3549 | |
| 3550 | @node Source Code Reference |
| 3551 | @subsection Source Code Reference |
| 3552 | |
| 3553 | Accessing a linker script defined variable from source code is not |
| 3554 | intuitive. In particular a linker script symbol is not equivalent to |
| 3555 | a variable declaration in a high level language, it is instead a |
| 3556 | symbol that does not have a value. |
| 3557 | |
| 3558 | Before going further, it is important to note that compilers often |
| 3559 | transform names in the source code into different names when they are |
| 3560 | stored in the symbol table. For example, Fortran compilers commonly |
| 3561 | prepend or append an underscore, and C++ performs extensive @samp{name |
| 3562 | mangling}. Therefore there might be a discrepancy between the name |
| 3563 | of a variable as it is used in source code and the name of the same |
| 3564 | variable as it is defined in a linker script. For example in C a |
| 3565 | linker script variable might be referred to as: |
| 3566 | |
| 3567 | @smallexample |
| 3568 | extern int foo; |
| 3569 | @end smallexample |
| 3570 | |
| 3571 | But in the linker script it might be defined as: |
| 3572 | |
| 3573 | @smallexample |
| 3574 | _foo = 1000; |
| 3575 | @end smallexample |
| 3576 | |
| 3577 | In the remaining examples however it is assumed that no name |
| 3578 | transformation has taken place. |
| 3579 | |
| 3580 | When a symbol is declared in a high level language such as C, two |
| 3581 | things happen. The first is that the compiler reserves enough space |
| 3582 | in the program's memory to hold the @emph{value} of the symbol. The |
| 3583 | second is that the compiler creates an entry in the program's symbol |
| 3584 | table which holds the symbol's @emph{address}. ie the symbol table |
| 3585 | contains the address of the block of memory holding the symbol's |
| 3586 | value. So for example the following C declaration, at file scope: |
| 3587 | |
| 3588 | @smallexample |
| 3589 | int foo = 1000; |
| 3590 | @end smallexample |
| 3591 | |
| 3592 | creates a entry called @samp{foo} in the symbol table. This entry |
| 3593 | holds the address of an @samp{int} sized block of memory where the |
| 3594 | number 1000 is initially stored. |
| 3595 | |
| 3596 | When a program references a symbol the compiler generates code that |
| 3597 | first accesses the symbol table to find the address of the symbol's |
| 3598 | memory block and then code to read the value from that memory block. |
| 3599 | So: |
| 3600 | |
| 3601 | @smallexample |
| 3602 | foo = 1; |
| 3603 | @end smallexample |
| 3604 | |
| 3605 | looks up the symbol @samp{foo} in the symbol table, gets the address |
| 3606 | associated with this symbol and then writes the value 1 into that |
| 3607 | address. Whereas: |
| 3608 | |
| 3609 | @smallexample |
| 3610 | int * a = & foo; |
| 3611 | @end smallexample |
| 3612 | |
| 3613 | looks up the symbol @samp{foo} in the symbol table, gets it address |
| 3614 | and then copies this address into the block of memory associated with |
| 3615 | the variable @samp{a}. |
| 3616 | |
| 3617 | Linker scripts symbol declarations, by contrast, create an entry in |
| 3618 | the symbol table but do not assign any memory to them. Thus they are |
| 3619 | an address without a value. So for example the linker script definition: |
| 3620 | |
| 3621 | @smallexample |
| 3622 | foo = 1000; |
| 3623 | @end smallexample |
| 3624 | |
| 3625 | creates an entry in the symbol table called @samp{foo} which holds |
| 3626 | the address of memory location 1000, but nothing special is stored at |
| 3627 | address 1000. This means that you cannot access the @emph{value} of a |
| 3628 | linker script defined symbol - it has no value - all you can do is |
| 3629 | access the @emph{address} of a linker script defined symbol. |
| 3630 | |
| 3631 | Hence when you are using a linker script defined symbol in source code |
| 3632 | you should always take the address of the symbol, and never attempt to |
| 3633 | use its value. For example suppose you want to copy the contents of a |
| 3634 | section of memory called .ROM into a section called .FLASH and the |
| 3635 | linker script contains these declarations: |
| 3636 | |
| 3637 | @smallexample |
| 3638 | @group |
| 3639 | start_of_ROM = .ROM; |
| 3640 | end_of_ROM = .ROM + sizeof (.ROM) - 1; |
| 3641 | start_of_FLASH = .FLASH; |
| 3642 | @end group |
| 3643 | @end smallexample |
| 3644 | |
| 3645 | Then the C source code to perform the copy would be: |
| 3646 | |
| 3647 | @smallexample |
| 3648 | @group |
| 3649 | extern char start_of_ROM, end_of_ROM, start_of_FLASH; |
| 3650 | |
| 3651 | memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM); |
| 3652 | @end group |
| 3653 | @end smallexample |
| 3654 | |
| 3655 | Note the use of the @samp{&} operators. These are correct. |
| 3656 | |
| 3657 | @node SECTIONS |
| 3658 | @section SECTIONS Command |
| 3659 | @kindex SECTIONS |
| 3660 | The @code{SECTIONS} command tells the linker how to map input sections |
| 3661 | into output sections, and how to place the output sections in memory. |
| 3662 | |
| 3663 | The format of the @code{SECTIONS} command is: |
| 3664 | @smallexample |
| 3665 | SECTIONS |
| 3666 | @{ |
| 3667 | @var{sections-command} |
| 3668 | @var{sections-command} |
| 3669 | @dots{} |
| 3670 | @} |
| 3671 | @end smallexample |
| 3672 | |
| 3673 | Each @var{sections-command} may of be one of the following: |
| 3674 | |
| 3675 | @itemize @bullet |
| 3676 | @item |
| 3677 | an @code{ENTRY} command (@pxref{Entry Point,,Entry command}) |
| 3678 | @item |
| 3679 | a symbol assignment (@pxref{Assignments}) |
| 3680 | @item |
| 3681 | an output section description |
| 3682 | @item |
| 3683 | an overlay description |
| 3684 | @end itemize |
| 3685 | |
| 3686 | The @code{ENTRY} command and symbol assignments are permitted inside the |
| 3687 | @code{SECTIONS} command for convenience in using the location counter in |
| 3688 | those commands. This can also make the linker script easier to |
| 3689 | understand because you can use those commands at meaningful points in |
| 3690 | the layout of the output file. |
| 3691 | |
| 3692 | Output section descriptions and overlay descriptions are described |
| 3693 | below. |
| 3694 | |
| 3695 | If you do not use a @code{SECTIONS} command in your linker script, the |
| 3696 | linker will place each input section into an identically named output |
| 3697 | section in the order that the sections are first encountered in the |
| 3698 | input files. If all input sections are present in the first file, for |
| 3699 | example, the order of sections in the output file will match the order |
| 3700 | in the first input file. The first section will be at address zero. |
| 3701 | |
| 3702 | @menu |
| 3703 | * Output Section Description:: Output section description |
| 3704 | * Output Section Name:: Output section name |
| 3705 | * Output Section Address:: Output section address |
| 3706 | * Input Section:: Input section description |
| 3707 | * Output Section Data:: Output section data |
| 3708 | * Output Section Keywords:: Output section keywords |
| 3709 | * Output Section Discarding:: Output section discarding |
| 3710 | * Output Section Attributes:: Output section attributes |
| 3711 | * Overlay Description:: Overlay description |
| 3712 | @end menu |
| 3713 | |
| 3714 | @node Output Section Description |
| 3715 | @subsection Output Section Description |
| 3716 | The full description of an output section looks like this: |
| 3717 | @smallexample |
| 3718 | @group |
| 3719 | @var{section} [@var{address}] [(@var{type})] : |
| 3720 | [AT(@var{lma})] |
| 3721 | [ALIGN(@var{section_align})] |
| 3722 | [SUBALIGN(@var{subsection_align})] |
| 3723 | [@var{constraint}] |
| 3724 | @{ |
| 3725 | @var{output-section-command} |
| 3726 | @var{output-section-command} |
| 3727 | @dots{} |
| 3728 | @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] |
| 3729 | @end group |
| 3730 | @end smallexample |
| 3731 | |
| 3732 | Most output sections do not use most of the optional section attributes. |
| 3733 | |
| 3734 | The whitespace around @var{section} is required, so that the section |
| 3735 | name is unambiguous. The colon and the curly braces are also required. |
| 3736 | The line breaks and other white space are optional. |
| 3737 | |
| 3738 | Each @var{output-section-command} may be one of the following: |
| 3739 | |
| 3740 | @itemize @bullet |
| 3741 | @item |
| 3742 | a symbol assignment (@pxref{Assignments}) |
| 3743 | @item |
| 3744 | an input section description (@pxref{Input Section}) |
| 3745 | @item |
| 3746 | data values to include directly (@pxref{Output Section Data}) |
| 3747 | @item |
| 3748 | a special output section keyword (@pxref{Output Section Keywords}) |
| 3749 | @end itemize |
| 3750 | |
| 3751 | @node Output Section Name |
| 3752 | @subsection Output Section Name |
| 3753 | @cindex name, section |
| 3754 | @cindex section name |
| 3755 | The name of the output section is @var{section}. @var{section} must |
| 3756 | meet the constraints of your output format. In formats which only |
| 3757 | support a limited number of sections, such as @code{a.out}, the name |
| 3758 | must be one of the names supported by the format (@code{a.out}, for |
| 3759 | example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the |
| 3760 | output format supports any number of sections, but with numbers and not |
| 3761 | names (as is the case for Oasys), the name should be supplied as a |
| 3762 | quoted numeric string. A section name may consist of any sequence of |
| 3763 | characters, but a name which contains any unusual characters such as |
| 3764 | commas must be quoted. |
| 3765 | |
| 3766 | The output section name @samp{/DISCARD/} is special; @ref{Output Section |
| 3767 | Discarding}. |
| 3768 | |
| 3769 | @node Output Section Address |
| 3770 | @subsection Output Section Address |
| 3771 | @cindex address, section |
| 3772 | @cindex section address |
| 3773 | The @var{address} is an expression for the VMA (the virtual memory |
| 3774 | address) of the output section. This address is optional, but if it |
| 3775 | is provided then the output address will be set exactly as specified. |
| 3776 | |
| 3777 | If the output address is not specified then one will be chosen for the |
| 3778 | section, based on the heuristic below. This address will be adjusted |
| 3779 | to fit the alignment requirement of the output section. The |
| 3780 | alignment requirement is the strictest alignment of any input section |
| 3781 | contained within the output section. |
| 3782 | |
| 3783 | The output section address heuristic is as follows: |
| 3784 | |
| 3785 | @itemize @bullet |
| 3786 | @item |
| 3787 | If an output memory @var{region} is set for the section then it |
| 3788 | is added to this region and its address will be the next free address |
| 3789 | in that region. |
| 3790 | |
| 3791 | @item |
| 3792 | If the MEMORY command has been used to create a list of memory |
| 3793 | regions then the first region which has attributes compatible with the |
| 3794 | section is selected to contain it. The section's output address will |
| 3795 | be the next free address in that region; @ref{MEMORY}. |
| 3796 | |
| 3797 | @item |
| 3798 | If no memory regions were specified, or none match the section then |
| 3799 | the output address will be based on the current value of the location |
| 3800 | counter. |
| 3801 | @end itemize |
| 3802 | |
| 3803 | @noindent |
| 3804 | For example: |
| 3805 | |
| 3806 | @smallexample |
| 3807 | .text . : @{ *(.text) @} |
| 3808 | @end smallexample |
| 3809 | |
| 3810 | @noindent |
| 3811 | and |
| 3812 | |
| 3813 | @smallexample |
| 3814 | .text : @{ *(.text) @} |
| 3815 | @end smallexample |
| 3816 | |
| 3817 | @noindent |
| 3818 | are subtly different. The first will set the address of the |
| 3819 | @samp{.text} output section to the current value of the location |
| 3820 | counter. The second will set it to the current value of the location |
| 3821 | counter aligned to the strictest alignment of any of the @samp{.text} |
| 3822 | input sections. |
| 3823 | |
| 3824 | The @var{address} may be an arbitrary expression; @ref{Expressions}. |
| 3825 | For example, if you want to align the section on a 0x10 byte boundary, |
| 3826 | so that the lowest four bits of the section address are zero, you could |
| 3827 | do something like this: |
| 3828 | @smallexample |
| 3829 | .text ALIGN(0x10) : @{ *(.text) @} |
| 3830 | @end smallexample |
| 3831 | @noindent |
| 3832 | This works because @code{ALIGN} returns the current location counter |
| 3833 | aligned upward to the specified value. |
| 3834 | |
| 3835 | Specifying @var{address} for a section will change the value of the |
| 3836 | location counter, provided that the section is non-empty. (Empty |
| 3837 | sections are ignored). |
| 3838 | |
| 3839 | @node Input Section |
| 3840 | @subsection Input Section Description |
| 3841 | @cindex input sections |
| 3842 | @cindex mapping input sections to output sections |
| 3843 | The most common output section command is an input section description. |
| 3844 | |
| 3845 | The input section description is the most basic linker script operation. |
| 3846 | You use output sections to tell the linker how to lay out your program |
| 3847 | in memory. You use input section descriptions to tell the linker how to |
| 3848 | map the input files into your memory layout. |
| 3849 | |
| 3850 | @menu |
| 3851 | * Input Section Basics:: Input section basics |
| 3852 | * Input Section Wildcards:: Input section wildcard patterns |
| 3853 | * Input Section Common:: Input section for common symbols |
| 3854 | * Input Section Keep:: Input section and garbage collection |
| 3855 | * Input Section Example:: Input section example |
| 3856 | @end menu |
| 3857 | |
| 3858 | @node Input Section Basics |
| 3859 | @subsubsection Input Section Basics |
| 3860 | @cindex input section basics |
| 3861 | An input section description consists of a file name optionally followed |
| 3862 | by a list of section names in parentheses. |
| 3863 | |
| 3864 | The file name and the section name may be wildcard patterns, which we |
| 3865 | describe further below (@pxref{Input Section Wildcards}). |
| 3866 | |
| 3867 | The most common input section description is to include all input |
| 3868 | sections with a particular name in the output section. For example, to |
| 3869 | include all input @samp{.text} sections, you would write: |
| 3870 | @smallexample |
| 3871 | *(.text) |
| 3872 | @end smallexample |
| 3873 | @noindent |
| 3874 | Here the @samp{*} is a wildcard which matches any file name. To exclude a list |
| 3875 | of files from matching the file name wildcard, EXCLUDE_FILE may be used to |
| 3876 | match all files except the ones specified in the EXCLUDE_FILE list. For |
| 3877 | example: |
| 3878 | @smallexample |
| 3879 | *(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors) |
| 3880 | @end smallexample |
| 3881 | will cause all .ctors sections from all files except @file{crtend.o} and |
| 3882 | @file{otherfile.o} to be included. |
| 3883 | |
| 3884 | There are two ways to include more than one section: |
| 3885 | @smallexample |
| 3886 | *(.text .rdata) |
| 3887 | *(.text) *(.rdata) |
| 3888 | @end smallexample |
| 3889 | @noindent |
| 3890 | The difference between these is the order in which the @samp{.text} and |
| 3891 | @samp{.rdata} input sections will appear in the output section. In the |
| 3892 | first example, they will be intermingled, appearing in the same order as |
| 3893 | they are found in the linker input. In the second example, all |
| 3894 | @samp{.text} input sections will appear first, followed by all |
| 3895 | @samp{.rdata} input sections. |
| 3896 | |
| 3897 | You can specify a file name to include sections from a particular file. |
| 3898 | You would do this if one or more of your files contain special data that |
| 3899 | needs to be at a particular location in memory. For example: |
| 3900 | @smallexample |
| 3901 | data.o(.data) |
| 3902 | @end smallexample |
| 3903 | |
| 3904 | To refine the sections that are included based on the section flags |
| 3905 | of an input section, INPUT_SECTION_FLAGS may be used. |
| 3906 | |
| 3907 | Here is a simple example for using Section header flags for ELF sections: |
| 3908 | |
| 3909 | @smallexample |
| 3910 | @group |
| 3911 | SECTIONS @{ |
| 3912 | .text : @{ INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) @} |
| 3913 | .text2 : @{ INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) @} |
| 3914 | @} |
| 3915 | @end group |
| 3916 | @end smallexample |
| 3917 | |
| 3918 | In this example, the output section @samp{.text} will be comprised of any |
| 3919 | input section matching the name *(.text) whose section header flags |
| 3920 | @code{SHF_MERGE} and @code{SHF_STRINGS} are set. The output section |
| 3921 | @samp{.text2} will be comprised of any input section matching the name *(.text) |
| 3922 | whose section header flag @code{SHF_WRITE} is clear. |
| 3923 | |
| 3924 | You can also specify files within archives by writing a pattern |
| 3925 | matching the archive, a colon, then the pattern matching the file, |
| 3926 | with no whitespace around the colon. |
| 3927 | |
| 3928 | @table @samp |
| 3929 | @item archive:file |
| 3930 | matches file within archive |
| 3931 | @item archive: |
| 3932 | matches the whole archive |
| 3933 | @item :file |
| 3934 | matches file but not one in an archive |
| 3935 | @end table |
| 3936 | |
| 3937 | Either one or both of @samp{archive} and @samp{file} can contain shell |
| 3938 | wildcards. On DOS based file systems, the linker will assume that a |
| 3939 | single letter followed by a colon is a drive specifier, so |
| 3940 | @samp{c:myfile.o} is a simple file specification, not @samp{myfile.o} |
| 3941 | within an archive called @samp{c}. @samp{archive:file} filespecs may |
| 3942 | also be used within an @code{EXCLUDE_FILE} list, but may not appear in |
| 3943 | other linker script contexts. For instance, you cannot extract a file |
| 3944 | from an archive by using @samp{archive:file} in an @code{INPUT} |
| 3945 | command. |
| 3946 | |
| 3947 | If you use a file name without a list of sections, then all sections in |
| 3948 | the input file will be included in the output section. This is not |
| 3949 | commonly done, but it may by useful on occasion. For example: |
| 3950 | @smallexample |
| 3951 | data.o |
| 3952 | @end smallexample |
| 3953 | |
| 3954 | When you use a file name which is not an @samp{archive:file} specifier |
| 3955 | and does not contain any wild card |
| 3956 | characters, the linker will first see if you also specified the file |
| 3957 | name on the linker command line or in an @code{INPUT} command. If you |
| 3958 | did not, the linker will attempt to open the file as an input file, as |
| 3959 | though it appeared on the command line. Note that this differs from an |
| 3960 | @code{INPUT} command, because the linker will not search for the file in |
| 3961 | the archive search path. |
| 3962 | |
| 3963 | @node Input Section Wildcards |
| 3964 | @subsubsection Input Section Wildcard Patterns |
| 3965 | @cindex input section wildcards |
| 3966 | @cindex wildcard file name patterns |
| 3967 | @cindex file name wildcard patterns |
| 3968 | @cindex section name wildcard patterns |
| 3969 | In an input section description, either the file name or the section |
| 3970 | name or both may be wildcard patterns. |
| 3971 | |
| 3972 | The file name of @samp{*} seen in many examples is a simple wildcard |
| 3973 | pattern for the file name. |
| 3974 | |
| 3975 | The wildcard patterns are like those used by the Unix shell. |
| 3976 | |
| 3977 | @table @samp |
| 3978 | @item * |
| 3979 | matches any number of characters |
| 3980 | @item ? |
| 3981 | matches any single character |
| 3982 | @item [@var{chars}] |
| 3983 | matches a single instance of any of the @var{chars}; the @samp{-} |
| 3984 | character may be used to specify a range of characters, as in |
| 3985 | @samp{[a-z]} to match any lower case letter |
| 3986 | @item \ |
| 3987 | quotes the following character |
| 3988 | @end table |
| 3989 | |
| 3990 | When a file name is matched with a wildcard, the wildcard characters |
| 3991 | will not match a @samp{/} character (used to separate directory names on |
| 3992 | Unix). A pattern consisting of a single @samp{*} character is an |
| 3993 | exception; it will always match any file name, whether it contains a |
| 3994 | @samp{/} or not. In a section name, the wildcard characters will match |
| 3995 | a @samp{/} character. |
| 3996 | |
| 3997 | File name wildcard patterns only match files which are explicitly |
| 3998 | specified on the command line or in an @code{INPUT} command. The linker |
| 3999 | does not search directories to expand wildcards. |
| 4000 | |
| 4001 | If a file name matches more than one wildcard pattern, or if a file name |
| 4002 | appears explicitly and is also matched by a wildcard pattern, the linker |
| 4003 | will use the first match in the linker script. For example, this |
| 4004 | sequence of input section descriptions is probably in error, because the |
| 4005 | @file{data.o} rule will not be used: |
| 4006 | @smallexample |
| 4007 | .data : @{ *(.data) @} |
| 4008 | .data1 : @{ data.o(.data) @} |
| 4009 | @end smallexample |
| 4010 | |
| 4011 | @cindex SORT_BY_NAME |
| 4012 | Normally, the linker will place files and sections matched by wildcards |
| 4013 | in the order in which they are seen during the link. You can change |
| 4014 | this by using the @code{SORT_BY_NAME} keyword, which appears before a wildcard |
| 4015 | pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the |
| 4016 | @code{SORT_BY_NAME} keyword is used, the linker will sort the files or sections |
| 4017 | into ascending order by name before placing them in the output file. |
| 4018 | |
| 4019 | @cindex SORT_BY_ALIGNMENT |
| 4020 | @code{SORT_BY_ALIGNMENT} is very similar to @code{SORT_BY_NAME}. The |
| 4021 | difference is @code{SORT_BY_ALIGNMENT} will sort sections into |
| 4022 | ascending order by alignment before placing them in the output file. |
| 4023 | |
| 4024 | @cindex SORT_BY_INIT_PRIORITY |
| 4025 | @code{SORT_BY_INIT_PRIORITY} is very similar to @code{SORT_BY_NAME}. The |
| 4026 | difference is @code{SORT_BY_INIT_PRIORITY} will sort sections into |
| 4027 | ascending order by numerical value of the GCC init_priority attribute |
| 4028 | encoded in the section name before placing them in the output file. |
| 4029 | |
| 4030 | @cindex SORT |
| 4031 | @code{SORT} is an alias for @code{SORT_BY_NAME}. |
| 4032 | |
| 4033 | When there are nested section sorting commands in linker script, there |
| 4034 | can be at most 1 level of nesting for section sorting commands. |
| 4035 | |
| 4036 | @enumerate |
| 4037 | @item |
| 4038 | @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)). |
| 4039 | It will sort the input sections by name first, then by alignment if 2 |
| 4040 | sections have the same name. |
| 4041 | @item |
| 4042 | @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)). |
| 4043 | It will sort the input sections by alignment first, then by name if 2 |
| 4044 | sections have the same alignment. |
| 4045 | @item |
| 4046 | @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is |
| 4047 | treated the same as @code{SORT_BY_NAME} (wildcard section pattern). |
| 4048 | @item |
| 4049 | @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)) |
| 4050 | is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern). |
| 4051 | @item |
| 4052 | All other nested section sorting commands are invalid. |
| 4053 | @end enumerate |
| 4054 | |
| 4055 | When both command line section sorting option and linker script |
| 4056 | section sorting command are used, section sorting command always |
| 4057 | takes precedence over the command line option. |
| 4058 | |
| 4059 | If the section sorting command in linker script isn't nested, the |
| 4060 | command line option will make the section sorting command to be |
| 4061 | treated as nested sorting command. |
| 4062 | |
| 4063 | @enumerate |
| 4064 | @item |
| 4065 | @code{SORT_BY_NAME} (wildcard section pattern ) with |
| 4066 | @option{--sort-sections alignment} is equivalent to |
| 4067 | @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)). |
| 4068 | @item |
| 4069 | @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with |
| 4070 | @option{--sort-section name} is equivalent to |
| 4071 | @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)). |
| 4072 | @end enumerate |
| 4073 | |
| 4074 | If the section sorting command in linker script is nested, the |
| 4075 | command line option will be ignored. |
| 4076 | |
| 4077 | @cindex SORT_NONE |
| 4078 | @code{SORT_NONE} disables section sorting by ignoring the command line |
| 4079 | section sorting option. |
| 4080 | |
| 4081 | If you ever get confused about where input sections are going, use the |
| 4082 | @samp{-M} linker option to generate a map file. The map file shows |
| 4083 | precisely how input sections are mapped to output sections. |
| 4084 | |
| 4085 | This example shows how wildcard patterns might be used to partition |
| 4086 | files. This linker script directs the linker to place all @samp{.text} |
| 4087 | sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}. |
| 4088 | The linker will place the @samp{.data} section from all files beginning |
| 4089 | with an upper case character in @samp{.DATA}; for all other files, the |
| 4090 | linker will place the @samp{.data} section in @samp{.data}. |
| 4091 | @smallexample |
| 4092 | @group |
| 4093 | SECTIONS @{ |
| 4094 | .text : @{ *(.text) @} |
| 4095 | .DATA : @{ [A-Z]*(.data) @} |
| 4096 | .data : @{ *(.data) @} |
| 4097 | .bss : @{ *(.bss) @} |
| 4098 | @} |
| 4099 | @end group |
| 4100 | @end smallexample |
| 4101 | |
| 4102 | @node Input Section Common |
| 4103 | @subsubsection Input Section for Common Symbols |
| 4104 | @cindex common symbol placement |
| 4105 | @cindex uninitialized data placement |
| 4106 | A special notation is needed for common symbols, because in many object |
| 4107 | file formats common symbols do not have a particular input section. The |
| 4108 | linker treats common symbols as though they are in an input section |
| 4109 | named @samp{COMMON}. |
| 4110 | |
| 4111 | You may use file names with the @samp{COMMON} section just as with any |
| 4112 | other input sections. You can use this to place common symbols from a |
| 4113 | particular input file in one section while common symbols from other |
| 4114 | input files are placed in another section. |
| 4115 | |
| 4116 | In most cases, common symbols in input files will be placed in the |
| 4117 | @samp{.bss} section in the output file. For example: |
| 4118 | @smallexample |
| 4119 | .bss @{ *(.bss) *(COMMON) @} |
| 4120 | @end smallexample |
| 4121 | |
| 4122 | @cindex scommon section |
| 4123 | @cindex small common symbols |
| 4124 | Some object file formats have more than one type of common symbol. For |
| 4125 | example, the MIPS ELF object file format distinguishes standard common |
| 4126 | symbols and small common symbols. In this case, the linker will use a |
| 4127 | different special section name for other types of common symbols. In |
| 4128 | the case of MIPS ELF, the linker uses @samp{COMMON} for standard common |
| 4129 | symbols and @samp{.scommon} for small common symbols. This permits you |
| 4130 | to map the different types of common symbols into memory at different |
| 4131 | locations. |
| 4132 | |
| 4133 | @cindex [COMMON] |
| 4134 | You will sometimes see @samp{[COMMON]} in old linker scripts. This |
| 4135 | notation is now considered obsolete. It is equivalent to |
| 4136 | @samp{*(COMMON)}. |
| 4137 | |
| 4138 | @node Input Section Keep |
| 4139 | @subsubsection Input Section and Garbage Collection |
| 4140 | @cindex KEEP |
| 4141 | @cindex garbage collection |
| 4142 | When link-time garbage collection is in use (@samp{--gc-sections}), |
| 4143 | it is often useful to mark sections that should not be eliminated. |
| 4144 | This is accomplished by surrounding an input section's wildcard entry |
| 4145 | with @code{KEEP()}, as in @code{KEEP(*(.init))} or |
| 4146 | @code{KEEP(SORT_BY_NAME(*)(.ctors))}. |
| 4147 | |
| 4148 | @node Input Section Example |
| 4149 | @subsubsection Input Section Example |
| 4150 | The following example is a complete linker script. It tells the linker |
| 4151 | to read all of the sections from file @file{all.o} and place them at the |
| 4152 | start of output section @samp{outputa} which starts at location |
| 4153 | @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o} |
| 4154 | follows immediately, in the same output section. All of section |
| 4155 | @samp{.input2} from @file{foo.o} goes into output section |
| 4156 | @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}. |
| 4157 | All of the remaining @samp{.input1} and @samp{.input2} sections from any |
| 4158 | files are written to output section @samp{outputc}. |
| 4159 | |
| 4160 | @smallexample |
| 4161 | @group |
| 4162 | SECTIONS @{ |
| 4163 | outputa 0x10000 : |
| 4164 | @{ |
| 4165 | all.o |
| 4166 | foo.o (.input1) |
| 4167 | @} |
| 4168 | @end group |
| 4169 | @group |
| 4170 | outputb : |
| 4171 | @{ |
| 4172 | foo.o (.input2) |
| 4173 | foo1.o (.input1) |
| 4174 | @} |
| 4175 | @end group |
| 4176 | @group |
| 4177 | outputc : |
| 4178 | @{ |
| 4179 | *(.input1) |
| 4180 | *(.input2) |
| 4181 | @} |
| 4182 | @} |
| 4183 | @end group |
| 4184 | @end smallexample |
| 4185 | |
| 4186 | @node Output Section Data |
| 4187 | @subsection Output Section Data |
| 4188 | @cindex data |
| 4189 | @cindex section data |
| 4190 | @cindex output section data |
| 4191 | @kindex BYTE(@var{expression}) |
| 4192 | @kindex SHORT(@var{expression}) |
| 4193 | @kindex LONG(@var{expression}) |
| 4194 | @kindex QUAD(@var{expression}) |
| 4195 | @kindex SQUAD(@var{expression}) |
| 4196 | You can include explicit bytes of data in an output section by using |
| 4197 | @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as |
| 4198 | an output section command. Each keyword is followed by an expression in |
| 4199 | parentheses providing the value to store (@pxref{Expressions}). The |
| 4200 | value of the expression is stored at the current value of the location |
| 4201 | counter. |
| 4202 | |
| 4203 | The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands |
| 4204 | store one, two, four, and eight bytes (respectively). After storing the |
| 4205 | bytes, the location counter is incremented by the number of bytes |
| 4206 | stored. |
| 4207 | |
| 4208 | For example, this will store the byte 1 followed by the four byte value |
| 4209 | of the symbol @samp{addr}: |
| 4210 | @smallexample |
| 4211 | BYTE(1) |
| 4212 | LONG(addr) |
| 4213 | @end smallexample |
| 4214 | |
| 4215 | When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the |
| 4216 | same; they both store an 8 byte, or 64 bit, value. When both host and |
| 4217 | target are 32 bits, an expression is computed as 32 bits. In this case |
| 4218 | @code{QUAD} stores a 32 bit value zero extended to 64 bits, and |
| 4219 | @code{SQUAD} stores a 32 bit value sign extended to 64 bits. |
| 4220 | |
| 4221 | If the object file format of the output file has an explicit endianness, |
| 4222 | which is the normal case, the value will be stored in that endianness. |
| 4223 | When the object file format does not have an explicit endianness, as is |
| 4224 | true of, for example, S-records, the value will be stored in the |
| 4225 | endianness of the first input object file. |
| 4226 | |
| 4227 | Note---these commands only work inside a section description and not |
| 4228 | between them, so the following will produce an error from the linker: |
| 4229 | @smallexample |
| 4230 | SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@ |
| 4231 | @end smallexample |
| 4232 | whereas this will work: |
| 4233 | @smallexample |
| 4234 | SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@ |
| 4235 | @end smallexample |
| 4236 | |
| 4237 | @kindex FILL(@var{expression}) |
| 4238 | @cindex holes, filling |
| 4239 | @cindex unspecified memory |
| 4240 | You may use the @code{FILL} command to set the fill pattern for the |
| 4241 | current section. It is followed by an expression in parentheses. Any |
| 4242 | otherwise unspecified regions of memory within the section (for example, |
| 4243 | gaps left due to the required alignment of input sections) are filled |
| 4244 | with the value of the expression, repeated as |
| 4245 | necessary. A @code{FILL} statement covers memory locations after the |
| 4246 | point at which it occurs in the section definition; by including more |
| 4247 | than one @code{FILL} statement, you can have different fill patterns in |
| 4248 | different parts of an output section. |
| 4249 | |
| 4250 | This example shows how to fill unspecified regions of memory with the |
| 4251 | value @samp{0x90}: |
| 4252 | @smallexample |
| 4253 | FILL(0x90909090) |
| 4254 | @end smallexample |
| 4255 | |
| 4256 | The @code{FILL} command is similar to the @samp{=@var{fillexp}} output |
| 4257 | section attribute, but it only affects the |
| 4258 | part of the section following the @code{FILL} command, rather than the |
| 4259 | entire section. If both are used, the @code{FILL} command takes |
| 4260 | precedence. @xref{Output Section Fill}, for details on the fill |
| 4261 | expression. |
| 4262 | |
| 4263 | @node Output Section Keywords |
| 4264 | @subsection Output Section Keywords |
| 4265 | There are a couple of keywords which can appear as output section |
| 4266 | commands. |
| 4267 | |
| 4268 | @table @code |
| 4269 | @kindex CREATE_OBJECT_SYMBOLS |
| 4270 | @cindex input filename symbols |
| 4271 | @cindex filename symbols |
| 4272 | @item CREATE_OBJECT_SYMBOLS |
| 4273 | The command tells the linker to create a symbol for each input file. |
| 4274 | The name of each symbol will be the name of the corresponding input |
| 4275 | file. The section of each symbol will be the output section in which |
| 4276 | the @code{CREATE_OBJECT_SYMBOLS} command appears. |
| 4277 | |
| 4278 | This is conventional for the a.out object file format. It is not |
| 4279 | normally used for any other object file format. |
| 4280 | |
| 4281 | @kindex CONSTRUCTORS |
| 4282 | @cindex C++ constructors, arranging in link |
| 4283 | @cindex constructors, arranging in link |
| 4284 | @item CONSTRUCTORS |
| 4285 | When linking using the a.out object file format, the linker uses an |
| 4286 | unusual set construct to support C++ global constructors and |
| 4287 | destructors. When linking object file formats which do not support |
| 4288 | arbitrary sections, such as ECOFF and XCOFF, the linker will |
| 4289 | automatically recognize C++ global constructors and destructors by name. |
| 4290 | For these object file formats, the @code{CONSTRUCTORS} command tells the |
| 4291 | linker to place constructor information in the output section where the |
| 4292 | @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is |
| 4293 | ignored for other object file formats. |
| 4294 | |
| 4295 | The symbol @w{@code{__CTOR_LIST__}} marks the start of the global |
| 4296 | constructors, and the symbol @w{@code{__CTOR_END__}} marks the end. |
| 4297 | Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_END__}} mark |
| 4298 | the start and end of the global destructors. The |
| 4299 | first word in the list is the number of entries, followed by the address |
| 4300 | of each constructor or destructor, followed by a zero word. The |
| 4301 | compiler must arrange to actually run the code. For these object file |
| 4302 | formats @sc{gnu} C++ normally calls constructors from a subroutine |
| 4303 | @code{__main}; a call to @code{__main} is automatically inserted into |
| 4304 | the startup code for @code{main}. @sc{gnu} C++ normally runs |
| 4305 | destructors either by using @code{atexit}, or directly from the function |
| 4306 | @code{exit}. |
| 4307 | |
| 4308 | For object file formats such as @code{COFF} or @code{ELF} which support |
| 4309 | arbitrary section names, @sc{gnu} C++ will normally arrange to put the |
| 4310 | addresses of global constructors and destructors into the @code{.ctors} |
| 4311 | and @code{.dtors} sections. Placing the following sequence into your |
| 4312 | linker script will build the sort of table which the @sc{gnu} C++ |
| 4313 | runtime code expects to see. |
| 4314 | |
| 4315 | @smallexample |
| 4316 | __CTOR_LIST__ = .; |
| 4317 | LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2) |
| 4318 | *(.ctors) |
| 4319 | LONG(0) |
| 4320 | __CTOR_END__ = .; |
| 4321 | __DTOR_LIST__ = .; |
| 4322 | LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2) |
| 4323 | *(.dtors) |
| 4324 | LONG(0) |
| 4325 | __DTOR_END__ = .; |
| 4326 | @end smallexample |
| 4327 | |
| 4328 | If you are using the @sc{gnu} C++ support for initialization priority, |
| 4329 | which provides some control over the order in which global constructors |
| 4330 | are run, you must sort the constructors at link time to ensure that they |
| 4331 | are executed in the correct order. When using the @code{CONSTRUCTORS} |
| 4332 | command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the |
| 4333 | @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and |
| 4334 | @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and |
| 4335 | @samp{*(.dtors)}. |
| 4336 | |
| 4337 | Normally the compiler and linker will handle these issues automatically, |
| 4338 | and you will not need to concern yourself with them. However, you may |
| 4339 | need to consider this if you are using C++ and writing your own linker |
| 4340 | scripts. |
| 4341 | |
| 4342 | @end table |
| 4343 | |
| 4344 | @node Output Section Discarding |
| 4345 | @subsection Output Section Discarding |
| 4346 | @cindex discarding sections |
| 4347 | @cindex sections, discarding |
| 4348 | @cindex removing sections |
| 4349 | The linker will not create output sections with no contents. This is |
| 4350 | for convenience when referring to input sections that may or may not |
| 4351 | be present in any of the input files. For example: |
| 4352 | @smallexample |
| 4353 | .foo : @{ *(.foo) @} |
| 4354 | @end smallexample |
| 4355 | @noindent |
| 4356 | will only create a @samp{.foo} section in the output file if there is a |
| 4357 | @samp{.foo} section in at least one input file, and if the input |
| 4358 | sections are not all empty. Other link script directives that allocate |
| 4359 | space in an output section will also create the output section. |
| 4360 | |
| 4361 | The linker will ignore address assignments (@pxref{Output Section Address}) |
| 4362 | on discarded output sections, except when the linker script defines |
| 4363 | symbols in the output section. In that case the linker will obey |
| 4364 | the address assignments, possibly advancing dot even though the |
| 4365 | section is discarded. |
| 4366 | |
| 4367 | @cindex /DISCARD/ |
| 4368 | The special output section name @samp{/DISCARD/} may be used to discard |
| 4369 | input sections. Any input sections which are assigned to an output |
| 4370 | section named @samp{/DISCARD/} are not included in the output file. |
| 4371 | |
| 4372 | @node Output Section Attributes |
| 4373 | @subsection Output Section Attributes |
| 4374 | @cindex output section attributes |
| 4375 | We showed above that the full description of an output section looked |
| 4376 | like this: |
| 4377 | |
| 4378 | @smallexample |
| 4379 | @group |
| 4380 | @var{section} [@var{address}] [(@var{type})] : |
| 4381 | [AT(@var{lma})] |
| 4382 | [ALIGN(@var{section_align})] |
| 4383 | [SUBALIGN(@var{subsection_align})] |
| 4384 | [@var{constraint}] |
| 4385 | @{ |
| 4386 | @var{output-section-command} |
| 4387 | @var{output-section-command} |
| 4388 | @dots{} |
| 4389 | @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] |
| 4390 | @end group |
| 4391 | @end smallexample |
| 4392 | |
| 4393 | We've already described @var{section}, @var{address}, and |
| 4394 | @var{output-section-command}. In this section we will describe the |
| 4395 | remaining section attributes. |
| 4396 | |
| 4397 | @menu |
| 4398 | * Output Section Type:: Output section type |
| 4399 | * Output Section LMA:: Output section LMA |
| 4400 | * Forced Output Alignment:: Forced Output Alignment |
| 4401 | * Forced Input Alignment:: Forced Input Alignment |
| 4402 | * Output Section Constraint:: Output section constraint |
| 4403 | * Output Section Region:: Output section region |
| 4404 | * Output Section Phdr:: Output section phdr |
| 4405 | * Output Section Fill:: Output section fill |
| 4406 | @end menu |
| 4407 | |
| 4408 | @node Output Section Type |
| 4409 | @subsubsection Output Section Type |
| 4410 | Each output section may have a type. The type is a keyword in |
| 4411 | parentheses. The following types are defined: |
| 4412 | |
| 4413 | @table @code |
| 4414 | @item NOLOAD |
| 4415 | The section should be marked as not loadable, so that it will not be |
| 4416 | loaded into memory when the program is run. |
| 4417 | @item DSECT |
| 4418 | @itemx COPY |
| 4419 | @itemx INFO |
| 4420 | @itemx OVERLAY |
| 4421 | These type names are supported for backward compatibility, and are |
| 4422 | rarely used. They all have the same effect: the section should be |
| 4423 | marked as not allocatable, so that no memory is allocated for the |
| 4424 | section when the program is run. |
| 4425 | @end table |
| 4426 | |
| 4427 | @kindex NOLOAD |
| 4428 | @cindex prevent unnecessary loading |
| 4429 | @cindex loading, preventing |
| 4430 | The linker normally sets the attributes of an output section based on |
| 4431 | the input sections which map into it. You can override this by using |
| 4432 | the section type. For example, in the script sample below, the |
| 4433 | @samp{ROM} section is addressed at memory location @samp{0} and does not |
| 4434 | need to be loaded when the program is run. |
| 4435 | @smallexample |
| 4436 | @group |
| 4437 | SECTIONS @{ |
| 4438 | ROM 0 (NOLOAD) : @{ @dots{} @} |
| 4439 | @dots{} |
| 4440 | @} |
| 4441 | @end group |
| 4442 | @end smallexample |
| 4443 | |
| 4444 | @node Output Section LMA |
| 4445 | @subsubsection Output Section LMA |
| 4446 | @kindex AT>@var{lma_region} |
| 4447 | @kindex AT(@var{lma}) |
| 4448 | @cindex load address |
| 4449 | @cindex section load address |
| 4450 | Every section has a virtual address (VMA) and a load address (LMA); see |
| 4451 | @ref{Basic Script Concepts}. The virtual address is specified by the |
| 4452 | @pxref{Output Section Address} described earlier. The load address is |
| 4453 | specified by the @code{AT} or @code{AT>} keywords. Specifying a load |
| 4454 | address is optional. |
| 4455 | |
| 4456 | The @code{AT} keyword takes an expression as an argument. This |
| 4457 | specifies the exact load address of the section. The @code{AT>} keyword |
| 4458 | takes the name of a memory region as an argument. @xref{MEMORY}. The |
| 4459 | load address of the section is set to the next free address in the |
| 4460 | region, aligned to the section's alignment requirements. |
| 4461 | |
| 4462 | If neither @code{AT} nor @code{AT>} is specified for an allocatable |
| 4463 | section, the linker will use the following heuristic to determine the |
| 4464 | load address: |
| 4465 | |
| 4466 | @itemize @bullet |
| 4467 | @item |
| 4468 | If the section has a specific VMA address, then this is used as |
| 4469 | the LMA address as well. |
| 4470 | |
| 4471 | @item |
| 4472 | If the section is not allocatable then its LMA is set to its VMA. |
| 4473 | |
| 4474 | @item |
| 4475 | Otherwise if a memory region can be found that is compatible |
| 4476 | with the current section, and this region contains at least one |
| 4477 | section, then the LMA is set so the difference between the |
| 4478 | VMA and LMA is the same as the difference between the VMA and LMA of |
| 4479 | the last section in the located region. |
| 4480 | |
| 4481 | @item |
| 4482 | If no memory regions have been declared then a default region |
| 4483 | that covers the entire address space is used in the previous step. |
| 4484 | |
| 4485 | @item |
| 4486 | If no suitable region could be found, or there was no previous |
| 4487 | section then the LMA is set equal to the VMA. |
| 4488 | @end itemize |
| 4489 | |
| 4490 | @cindex ROM initialized data |
| 4491 | @cindex initialized data in ROM |
| 4492 | This feature is designed to make it easy to build a ROM image. For |
| 4493 | example, the following linker script creates three output sections: one |
| 4494 | called @samp{.text}, which starts at @code{0x1000}, one called |
| 4495 | @samp{.mdata}, which is loaded at the end of the @samp{.text} section |
| 4496 | even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold |
| 4497 | uninitialized data at address @code{0x3000}. The symbol @code{_data} is |
| 4498 | defined with the value @code{0x2000}, which shows that the location |
| 4499 | counter holds the VMA value, not the LMA value. |
| 4500 | |
| 4501 | @smallexample |
| 4502 | @group |
| 4503 | SECTIONS |
| 4504 | @{ |
| 4505 | .text 0x1000 : @{ *(.text) _etext = . ; @} |
| 4506 | .mdata 0x2000 : |
| 4507 | AT ( ADDR (.text) + SIZEOF (.text) ) |
| 4508 | @{ _data = . ; *(.data); _edata = . ; @} |
| 4509 | .bss 0x3000 : |
| 4510 | @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@} |
| 4511 | @} |
| 4512 | @end group |
| 4513 | @end smallexample |
| 4514 | |
| 4515 | The run-time initialization code for use with a program generated with |
| 4516 | this linker script would include something like the following, to copy |
| 4517 | the initialized data from the ROM image to its runtime address. Notice |
| 4518 | how this code takes advantage of the symbols defined by the linker |
| 4519 | script. |
| 4520 | |
| 4521 | @smallexample |
| 4522 | @group |
| 4523 | extern char _etext, _data, _edata, _bstart, _bend; |
| 4524 | char *src = &_etext; |
| 4525 | char *dst = &_data; |
| 4526 | |
| 4527 | /* ROM has data at end of text; copy it. */ |
| 4528 | while (dst < &_edata) |
| 4529 | *dst++ = *src++; |
| 4530 | |
| 4531 | /* Zero bss. */ |
| 4532 | for (dst = &_bstart; dst< &_bend; dst++) |
| 4533 | *dst = 0; |
| 4534 | @end group |
| 4535 | @end smallexample |
| 4536 | |
| 4537 | @node Forced Output Alignment |
| 4538 | @subsubsection Forced Output Alignment |
| 4539 | @kindex ALIGN(@var{section_align}) |
| 4540 | @cindex forcing output section alignment |
| 4541 | @cindex output section alignment |
| 4542 | You can increase an output section's alignment by using ALIGN. |
| 4543 | |
| 4544 | @node Forced Input Alignment |
| 4545 | @subsubsection Forced Input Alignment |
| 4546 | @kindex SUBALIGN(@var{subsection_align}) |
| 4547 | @cindex forcing input section alignment |
| 4548 | @cindex input section alignment |
| 4549 | You can force input section alignment within an output section by using |
| 4550 | SUBALIGN. The value specified overrides any alignment given by input |
| 4551 | sections, whether larger or smaller. |
| 4552 | |
| 4553 | @node Output Section Constraint |
| 4554 | @subsubsection Output Section Constraint |
| 4555 | @kindex ONLY_IF_RO |
| 4556 | @kindex ONLY_IF_RW |
| 4557 | @cindex constraints on output sections |
| 4558 | You can specify that an output section should only be created if all |
| 4559 | of its input sections are read-only or all of its input sections are |
| 4560 | read-write by using the keyword @code{ONLY_IF_RO} and |
| 4561 | @code{ONLY_IF_RW} respectively. |
| 4562 | |
| 4563 | @node Output Section Region |
| 4564 | @subsubsection Output Section Region |
| 4565 | @kindex >@var{region} |
| 4566 | @cindex section, assigning to memory region |
| 4567 | @cindex memory regions and sections |
| 4568 | You can assign a section to a previously defined region of memory by |
| 4569 | using @samp{>@var{region}}. @xref{MEMORY}. |
| 4570 | |
| 4571 | Here is a simple example: |
| 4572 | @smallexample |
| 4573 | @group |
| 4574 | MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @} |
| 4575 | SECTIONS @{ ROM : @{ *(.text) @} >rom @} |
| 4576 | @end group |
| 4577 | @end smallexample |
| 4578 | |
| 4579 | @node Output Section Phdr |
| 4580 | @subsubsection Output Section Phdr |
| 4581 | @kindex :@var{phdr} |
| 4582 | @cindex section, assigning to program header |
| 4583 | @cindex program headers and sections |
| 4584 | You can assign a section to a previously defined program segment by |
| 4585 | using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to |
| 4586 | one or more segments, then all subsequent allocated sections will be |
| 4587 | assigned to those segments as well, unless they use an explicitly |
| 4588 | @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the |
| 4589 | linker to not put the section in any segment at all. |
| 4590 | |
| 4591 | Here is a simple example: |
| 4592 | @smallexample |
| 4593 | @group |
| 4594 | PHDRS @{ text PT_LOAD ; @} |
| 4595 | SECTIONS @{ .text : @{ *(.text) @} :text @} |
| 4596 | @end group |
| 4597 | @end smallexample |
| 4598 | |
| 4599 | @node Output Section Fill |
| 4600 | @subsubsection Output Section Fill |
| 4601 | @kindex =@var{fillexp} |
| 4602 | @cindex section fill pattern |
| 4603 | @cindex fill pattern, entire section |
| 4604 | You can set the fill pattern for an entire section by using |
| 4605 | @samp{=@var{fillexp}}. @var{fillexp} is an expression |
| 4606 | (@pxref{Expressions}). Any otherwise unspecified regions of memory |
| 4607 | within the output section (for example, gaps left due to the required |
| 4608 | alignment of input sections) will be filled with the value, repeated as |
| 4609 | necessary. If the fill expression is a simple hex number, ie. a string |
| 4610 | of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then |
| 4611 | an arbitrarily long sequence of hex digits can be used to specify the |
| 4612 | fill pattern; Leading zeros become part of the pattern too. For all |
| 4613 | other cases, including extra parentheses or a unary @code{+}, the fill |
| 4614 | pattern is the four least significant bytes of the value of the |
| 4615 | expression. In all cases, the number is big-endian. |
| 4616 | |
| 4617 | You can also change the fill value with a @code{FILL} command in the |
| 4618 | output section commands; (@pxref{Output Section Data}). |
| 4619 | |
| 4620 | Here is a simple example: |
| 4621 | @smallexample |
| 4622 | @group |
| 4623 | SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @} |
| 4624 | @end group |
| 4625 | @end smallexample |
| 4626 | |
| 4627 | @node Overlay Description |
| 4628 | @subsection Overlay Description |
| 4629 | @kindex OVERLAY |
| 4630 | @cindex overlays |
| 4631 | An overlay description provides an easy way to describe sections which |
| 4632 | are to be loaded as part of a single memory image but are to be run at |
| 4633 | the same memory address. At run time, some sort of overlay manager will |
| 4634 | copy the overlaid sections in and out of the runtime memory address as |
| 4635 | required, perhaps by simply manipulating addressing bits. This approach |
| 4636 | can be useful, for example, when a certain region of memory is faster |
| 4637 | than another. |
| 4638 | |
| 4639 | Overlays are described using the @code{OVERLAY} command. The |
| 4640 | @code{OVERLAY} command is used within a @code{SECTIONS} command, like an |
| 4641 | output section description. The full syntax of the @code{OVERLAY} |
| 4642 | command is as follows: |
| 4643 | @smallexample |
| 4644 | @group |
| 4645 | OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )] |
| 4646 | @{ |
| 4647 | @var{secname1} |
| 4648 | @{ |
| 4649 | @var{output-section-command} |
| 4650 | @var{output-section-command} |
| 4651 | @dots{} |
| 4652 | @} [:@var{phdr}@dots{}] [=@var{fill}] |
| 4653 | @var{secname2} |
| 4654 | @{ |
| 4655 | @var{output-section-command} |
| 4656 | @var{output-section-command} |
| 4657 | @dots{} |
| 4658 | @} [:@var{phdr}@dots{}] [=@var{fill}] |
| 4659 | @dots{} |
| 4660 | @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] |
| 4661 | @end group |
| 4662 | @end smallexample |
| 4663 | |
| 4664 | Everything is optional except @code{OVERLAY} (a keyword), and each |
| 4665 | section must have a name (@var{secname1} and @var{secname2} above). The |
| 4666 | section definitions within the @code{OVERLAY} construct are identical to |
| 4667 | those within the general @code{SECTIONS} contruct (@pxref{SECTIONS}), |
| 4668 | except that no addresses and no memory regions may be defined for |
| 4669 | sections within an @code{OVERLAY}. |
| 4670 | |
| 4671 | The sections are all defined with the same starting address. The load |
| 4672 | addresses of the sections are arranged such that they are consecutive in |
| 4673 | memory starting at the load address used for the @code{OVERLAY} as a |
| 4674 | whole (as with normal section definitions, the load address is optional, |
| 4675 | and defaults to the start address; the start address is also optional, |
| 4676 | and defaults to the current value of the location counter). |
| 4677 | |
| 4678 | If the @code{NOCROSSREFS} keyword is used, and there any references |
| 4679 | among the sections, the linker will report an error. Since the sections |
| 4680 | all run at the same address, it normally does not make sense for one |
| 4681 | section to refer directly to another. @xref{Miscellaneous Commands, |
| 4682 | NOCROSSREFS}. |
| 4683 | |
| 4684 | For each section within the @code{OVERLAY}, the linker automatically |
| 4685 | provides two symbols. The symbol @code{__load_start_@var{secname}} is |
| 4686 | defined as the starting load address of the section. The symbol |
| 4687 | @code{__load_stop_@var{secname}} is defined as the final load address of |
| 4688 | the section. Any characters within @var{secname} which are not legal |
| 4689 | within C identifiers are removed. C (or assembler) code may use these |
| 4690 | symbols to move the overlaid sections around as necessary. |
| 4691 | |
| 4692 | At the end of the overlay, the value of the location counter is set to |
| 4693 | the start address of the overlay plus the size of the largest section. |
| 4694 | |
| 4695 | Here is an example. Remember that this would appear inside a |
| 4696 | @code{SECTIONS} construct. |
| 4697 | @smallexample |
| 4698 | @group |
| 4699 | OVERLAY 0x1000 : AT (0x4000) |
| 4700 | @{ |
| 4701 | .text0 @{ o1/*.o(.text) @} |
| 4702 | .text1 @{ o2/*.o(.text) @} |
| 4703 | @} |
| 4704 | @end group |
| 4705 | @end smallexample |
| 4706 | @noindent |
| 4707 | This will define both @samp{.text0} and @samp{.text1} to start at |
| 4708 | address 0x1000. @samp{.text0} will be loaded at address 0x4000, and |
| 4709 | @samp{.text1} will be loaded immediately after @samp{.text0}. The |
| 4710 | following symbols will be defined if referenced: @code{__load_start_text0}, |
| 4711 | @code{__load_stop_text0}, @code{__load_start_text1}, |
| 4712 | @code{__load_stop_text1}. |
| 4713 | |
| 4714 | C code to copy overlay @code{.text1} into the overlay area might look |
| 4715 | like the following. |
| 4716 | |
| 4717 | @smallexample |
| 4718 | @group |
| 4719 | extern char __load_start_text1, __load_stop_text1; |
| 4720 | memcpy ((char *) 0x1000, &__load_start_text1, |
| 4721 | &__load_stop_text1 - &__load_start_text1); |
| 4722 | @end group |
| 4723 | @end smallexample |
| 4724 | |
| 4725 | Note that the @code{OVERLAY} command is just syntactic sugar, since |
| 4726 | everything it does can be done using the more basic commands. The above |
| 4727 | example could have been written identically as follows. |
| 4728 | |
| 4729 | @smallexample |
| 4730 | @group |
| 4731 | .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @} |
| 4732 | PROVIDE (__load_start_text0 = LOADADDR (.text0)); |
| 4733 | PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0)); |
| 4734 | .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @} |
| 4735 | PROVIDE (__load_start_text1 = LOADADDR (.text1)); |
| 4736 | PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1)); |
| 4737 | . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1)); |
| 4738 | @end group |
| 4739 | @end smallexample |
| 4740 | |
| 4741 | @node MEMORY |
| 4742 | @section MEMORY Command |
| 4743 | @kindex MEMORY |
| 4744 | @cindex memory regions |
| 4745 | @cindex regions of memory |
| 4746 | @cindex allocating memory |
| 4747 | @cindex discontinuous memory |
| 4748 | The linker's default configuration permits allocation of all available |
| 4749 | memory. You can override this by using the @code{MEMORY} command. |
| 4750 | |
| 4751 | The @code{MEMORY} command describes the location and size of blocks of |
| 4752 | memory in the target. You can use it to describe which memory regions |
| 4753 | may be used by the linker, and which memory regions it must avoid. You |
| 4754 | can then assign sections to particular memory regions. The linker will |
| 4755 | set section addresses based on the memory regions, and will warn about |
| 4756 | regions that become too full. The linker will not shuffle sections |
| 4757 | around to fit into the available regions. |
| 4758 | |
| 4759 | A linker script may contain at most one use of the @code{MEMORY} |
| 4760 | command. However, you can define as many blocks of memory within it as |
| 4761 | you wish. The syntax is: |
| 4762 | @smallexample |
| 4763 | @group |
| 4764 | MEMORY |
| 4765 | @{ |
| 4766 | @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len} |
| 4767 | @dots{} |
| 4768 | @} |
| 4769 | @end group |
| 4770 | @end smallexample |
| 4771 | |
| 4772 | The @var{name} is a name used in the linker script to refer to the |
| 4773 | region. The region name has no meaning outside of the linker script. |
| 4774 | Region names are stored in a separate name space, and will not conflict |
| 4775 | with symbol names, file names, or section names. Each memory region |
| 4776 | must have a distinct name within the @code{MEMORY} command. However you can |
| 4777 | add later alias names to existing memory regions with the @ref{REGION_ALIAS} |
| 4778 | command. |
| 4779 | |
| 4780 | @cindex memory region attributes |
| 4781 | The @var{attr} string is an optional list of attributes that specify |
| 4782 | whether to use a particular memory region for an input section which is |
| 4783 | not explicitly mapped in the linker script. As described in |
| 4784 | @ref{SECTIONS}, if you do not specify an output section for some input |
| 4785 | section, the linker will create an output section with the same name as |
| 4786 | the input section. If you define region attributes, the linker will use |
| 4787 | them to select the memory region for the output section that it creates. |
| 4788 | |
| 4789 | The @var{attr} string must consist only of the following characters: |
| 4790 | @table @samp |
| 4791 | @item R |
| 4792 | Read-only section |
| 4793 | @item W |
| 4794 | Read/write section |
| 4795 | @item X |
| 4796 | Executable section |
| 4797 | @item A |
| 4798 | Allocatable section |
| 4799 | @item I |
| 4800 | Initialized section |
| 4801 | @item L |
| 4802 | Same as @samp{I} |
| 4803 | @item ! |
| 4804 | Invert the sense of any of the attributes that follow |
| 4805 | @end table |
| 4806 | |
| 4807 | If a unmapped section matches any of the listed attributes other than |
| 4808 | @samp{!}, it will be placed in the memory region. The @samp{!} |
| 4809 | attribute reverses this test, so that an unmapped section will be placed |
| 4810 | in the memory region only if it does not match any of the listed |
| 4811 | attributes. |
| 4812 | |
| 4813 | @kindex ORIGIN = |
| 4814 | @kindex o = |
| 4815 | @kindex org = |
| 4816 | The @var{origin} is an numerical expression for the start address of |
| 4817 | the memory region. The expression must evaluate to a constant and it |
| 4818 | cannot involve any symbols. The keyword @code{ORIGIN} may be |
| 4819 | abbreviated to @code{org} or @code{o} (but not, for example, |
| 4820 | @code{ORG}). |
| 4821 | |
| 4822 | @kindex LENGTH = |
| 4823 | @kindex len = |
| 4824 | @kindex l = |
| 4825 | The @var{len} is an expression for the size in bytes of the memory |
| 4826 | region. As with the @var{origin} expression, the expression must |
| 4827 | be numerical only and must evaluate to a constant. The keyword |
| 4828 | @code{LENGTH} may be abbreviated to @code{len} or @code{l}. |
| 4829 | |
| 4830 | In the following example, we specify that there are two memory regions |
| 4831 | available for allocation: one starting at @samp{0} for 256 kilobytes, |
| 4832 | and the other starting at @samp{0x40000000} for four megabytes. The |
| 4833 | linker will place into the @samp{rom} memory region every section which |
| 4834 | is not explicitly mapped into a memory region, and is either read-only |
| 4835 | or executable. The linker will place other sections which are not |
| 4836 | explicitly mapped into a memory region into the @samp{ram} memory |
| 4837 | region. |
| 4838 | |
| 4839 | @smallexample |
| 4840 | @group |
| 4841 | MEMORY |
| 4842 | @{ |
| 4843 | rom (rx) : ORIGIN = 0, LENGTH = 256K |
| 4844 | ram (!rx) : org = 0x40000000, l = 4M |
| 4845 | @} |
| 4846 | @end group |
| 4847 | @end smallexample |
| 4848 | |
| 4849 | Once you define a memory region, you can direct the linker to place |
| 4850 | specific output sections into that memory region by using the |
| 4851 | @samp{>@var{region}} output section attribute. For example, if you have |
| 4852 | a memory region named @samp{mem}, you would use @samp{>mem} in the |
| 4853 | output section definition. @xref{Output Section Region}. If no address |
| 4854 | was specified for the output section, the linker will set the address to |
| 4855 | the next available address within the memory region. If the combined |
| 4856 | output sections directed to a memory region are too large for the |
| 4857 | region, the linker will issue an error message. |
| 4858 | |
| 4859 | It is possible to access the origin and length of a memory in an |
| 4860 | expression via the @code{ORIGIN(@var{memory})} and |
| 4861 | @code{LENGTH(@var{memory})} functions: |
| 4862 | |
| 4863 | @smallexample |
| 4864 | @group |
| 4865 | _fstack = ORIGIN(ram) + LENGTH(ram) - 4; |
| 4866 | @end group |
| 4867 | @end smallexample |
| 4868 | |
| 4869 | @node PHDRS |
| 4870 | @section PHDRS Command |
| 4871 | @kindex PHDRS |
| 4872 | @cindex program headers |
| 4873 | @cindex ELF program headers |
| 4874 | @cindex program segments |
| 4875 | @cindex segments, ELF |
| 4876 | The ELF object file format uses @dfn{program headers}, also knows as |
| 4877 | @dfn{segments}. The program headers describe how the program should be |
| 4878 | loaded into memory. You can print them out by using the @code{objdump} |
| 4879 | program with the @samp{-p} option. |
| 4880 | |
| 4881 | When you run an ELF program on a native ELF system, the system loader |
| 4882 | reads the program headers in order to figure out how to load the |
| 4883 | program. This will only work if the program headers are set correctly. |
| 4884 | This manual does not describe the details of how the system loader |
| 4885 | interprets program headers; for more information, see the ELF ABI. |
| 4886 | |
| 4887 | The linker will create reasonable program headers by default. However, |
| 4888 | in some cases, you may need to specify the program headers more |
| 4889 | precisely. You may use the @code{PHDRS} command for this purpose. When |
| 4890 | the linker sees the @code{PHDRS} command in the linker script, it will |
| 4891 | not create any program headers other than the ones specified. |
| 4892 | |
| 4893 | The linker only pays attention to the @code{PHDRS} command when |
| 4894 | generating an ELF output file. In other cases, the linker will simply |
| 4895 | ignore @code{PHDRS}. |
| 4896 | |
| 4897 | This is the syntax of the @code{PHDRS} command. The words @code{PHDRS}, |
| 4898 | @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords. |
| 4899 | |
| 4900 | @smallexample |
| 4901 | @group |
| 4902 | PHDRS |
| 4903 | @{ |
| 4904 | @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ] |
| 4905 | [ FLAGS ( @var{flags} ) ] ; |
| 4906 | @} |
| 4907 | @end group |
| 4908 | @end smallexample |
| 4909 | |
| 4910 | The @var{name} is used only for reference in the @code{SECTIONS} command |
| 4911 | of the linker script. It is not put into the output file. Program |
| 4912 | header names are stored in a separate name space, and will not conflict |
| 4913 | with symbol names, file names, or section names. Each program header |
| 4914 | must have a distinct name. The headers are processed in order and it |
| 4915 | is usual for them to map to sections in ascending load address order. |
| 4916 | |
| 4917 | Certain program header types describe segments of memory which the |
| 4918 | system loader will load from the file. In the linker script, you |
| 4919 | specify the contents of these segments by placing allocatable output |
| 4920 | sections in the segments. You use the @samp{:@var{phdr}} output section |
| 4921 | attribute to place a section in a particular segment. @xref{Output |
| 4922 | Section Phdr}. |
| 4923 | |
| 4924 | It is normal to put certain sections in more than one segment. This |
| 4925 | merely implies that one segment of memory contains another. You may |
| 4926 | repeat @samp{:@var{phdr}}, using it once for each segment which should |
| 4927 | contain the section. |
| 4928 | |
| 4929 | If you place a section in one or more segments using @samp{:@var{phdr}}, |
| 4930 | then the linker will place all subsequent allocatable sections which do |
| 4931 | not specify @samp{:@var{phdr}} in the same segments. This is for |
| 4932 | convenience, since generally a whole set of contiguous sections will be |
| 4933 | placed in a single segment. You can use @code{:NONE} to override the |
| 4934 | default segment and tell the linker to not put the section in any |
| 4935 | segment at all. |
| 4936 | |
| 4937 | @kindex FILEHDR |
| 4938 | @kindex PHDRS |
| 4939 | You may use the @code{FILEHDR} and @code{PHDRS} keywords after |
| 4940 | the program header type to further describe the contents of the segment. |
| 4941 | The @code{FILEHDR} keyword means that the segment should include the ELF |
| 4942 | file header. The @code{PHDRS} keyword means that the segment should |
| 4943 | include the ELF program headers themselves. If applied to a loadable |
| 4944 | segment (@code{PT_LOAD}), all prior loadable segments must have one of |
| 4945 | these keywords. |
| 4946 | |
| 4947 | The @var{type} may be one of the following. The numbers indicate the |
| 4948 | value of the keyword. |
| 4949 | |
| 4950 | @table @asis |
| 4951 | @item @code{PT_NULL} (0) |
| 4952 | Indicates an unused program header. |
| 4953 | |
| 4954 | @item @code{PT_LOAD} (1) |
| 4955 | Indicates that this program header describes a segment to be loaded from |
| 4956 | the file. |
| 4957 | |
| 4958 | @item @code{PT_DYNAMIC} (2) |
| 4959 | Indicates a segment where dynamic linking information can be found. |
| 4960 | |
| 4961 | @item @code{PT_INTERP} (3) |
| 4962 | Indicates a segment where the name of the program interpreter may be |
| 4963 | found. |
| 4964 | |
| 4965 | @item @code{PT_NOTE} (4) |
| 4966 | Indicates a segment holding note information. |
| 4967 | |
| 4968 | @item @code{PT_SHLIB} (5) |
| 4969 | A reserved program header type, defined but not specified by the ELF |
| 4970 | ABI. |
| 4971 | |
| 4972 | @item @code{PT_PHDR} (6) |
| 4973 | Indicates a segment where the program headers may be found. |
| 4974 | |
| 4975 | @item @var{expression} |
| 4976 | An expression giving the numeric type of the program header. This may |
| 4977 | be used for types not defined above. |
| 4978 | @end table |
| 4979 | |
| 4980 | You can specify that a segment should be loaded at a particular address |
| 4981 | in memory by using an @code{AT} expression. This is identical to the |
| 4982 | @code{AT} command used as an output section attribute (@pxref{Output |
| 4983 | Section LMA}). The @code{AT} command for a program header overrides the |
| 4984 | output section attribute. |
| 4985 | |
| 4986 | The linker will normally set the segment flags based on the sections |
| 4987 | which comprise the segment. You may use the @code{FLAGS} keyword to |
| 4988 | explicitly specify the segment flags. The value of @var{flags} must be |
| 4989 | an integer. It is used to set the @code{p_flags} field of the program |
| 4990 | header. |
| 4991 | |
| 4992 | Here is an example of @code{PHDRS}. This shows a typical set of program |
| 4993 | headers used on a native ELF system. |
| 4994 | |
| 4995 | @example |
| 4996 | @group |
| 4997 | PHDRS |
| 4998 | @{ |
| 4999 | headers PT_PHDR PHDRS ; |
| 5000 | interp PT_INTERP ; |
| 5001 | text PT_LOAD FILEHDR PHDRS ; |
| 5002 | data PT_LOAD ; |
| 5003 | dynamic PT_DYNAMIC ; |
| 5004 | @} |
| 5005 | |
| 5006 | SECTIONS |
| 5007 | @{ |
| 5008 | . = SIZEOF_HEADERS; |
| 5009 | .interp : @{ *(.interp) @} :text :interp |
| 5010 | .text : @{ *(.text) @} :text |
| 5011 | .rodata : @{ *(.rodata) @} /* defaults to :text */ |
| 5012 | @dots{} |
| 5013 | . = . + 0x1000; /* move to a new page in memory */ |
| 5014 | .data : @{ *(.data) @} :data |
| 5015 | .dynamic : @{ *(.dynamic) @} :data :dynamic |
| 5016 | @dots{} |
| 5017 | @} |
| 5018 | @end group |
| 5019 | @end example |
| 5020 | |
| 5021 | @node VERSION |
| 5022 | @section VERSION Command |
| 5023 | @kindex VERSION @{script text@} |
| 5024 | @cindex symbol versions |
| 5025 | @cindex version script |
| 5026 | @cindex versions of symbols |
| 5027 | The linker supports symbol versions when using ELF. Symbol versions are |
| 5028 | only useful when using shared libraries. The dynamic linker can use |
| 5029 | symbol versions to select a specific version of a function when it runs |
| 5030 | a program that may have been linked against an earlier version of the |
| 5031 | shared library. |
| 5032 | |
| 5033 | You can include a version script directly in the main linker script, or |
| 5034 | you can supply the version script as an implicit linker script. You can |
| 5035 | also use the @samp{--version-script} linker option. |
| 5036 | |
| 5037 | The syntax of the @code{VERSION} command is simply |
| 5038 | @smallexample |
| 5039 | VERSION @{ version-script-commands @} |
| 5040 | @end smallexample |
| 5041 | |
| 5042 | The format of the version script commands is identical to that used by |
| 5043 | Sun's linker in Solaris 2.5. The version script defines a tree of |
| 5044 | version nodes. You specify the node names and interdependencies in the |
| 5045 | version script. You can specify which symbols are bound to which |
| 5046 | version nodes, and you can reduce a specified set of symbols to local |
| 5047 | scope so that they are not globally visible outside of the shared |
| 5048 | library. |
| 5049 | |
| 5050 | The easiest way to demonstrate the version script language is with a few |
| 5051 | examples. |
| 5052 | |
| 5053 | @smallexample |
| 5054 | VERS_1.1 @{ |
| 5055 | global: |
| 5056 | foo1; |
| 5057 | local: |
| 5058 | old*; |
| 5059 | original*; |
| 5060 | new*; |
| 5061 | @}; |
| 5062 | |
| 5063 | VERS_1.2 @{ |
| 5064 | foo2; |
| 5065 | @} VERS_1.1; |
| 5066 | |
| 5067 | VERS_2.0 @{ |
| 5068 | bar1; bar2; |
| 5069 | extern "C++" @{ |
| 5070 | ns::*; |
| 5071 | "f(int, double)"; |
| 5072 | @}; |
| 5073 | @} VERS_1.2; |
| 5074 | @end smallexample |
| 5075 | |
| 5076 | This example version script defines three version nodes. The first |
| 5077 | version node defined is @samp{VERS_1.1}; it has no other dependencies. |
| 5078 | The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces |
| 5079 | a number of symbols to local scope so that they are not visible outside |
| 5080 | of the shared library; this is done using wildcard patterns, so that any |
| 5081 | symbol whose name begins with @samp{old}, @samp{original}, or @samp{new} |
| 5082 | is matched. The wildcard patterns available are the same as those used |
| 5083 | in the shell when matching filenames (also known as ``globbing''). |
| 5084 | However, if you specify the symbol name inside double quotes, then the |
| 5085 | name is treated as literal, rather than as a glob pattern. |
| 5086 | |
| 5087 | Next, the version script defines node @samp{VERS_1.2}. This node |
| 5088 | depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2} |
| 5089 | to the version node @samp{VERS_1.2}. |
| 5090 | |
| 5091 | Finally, the version script defines node @samp{VERS_2.0}. This node |
| 5092 | depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1} |
| 5093 | and @samp{bar2} are bound to the version node @samp{VERS_2.0}. |
| 5094 | |
| 5095 | When the linker finds a symbol defined in a library which is not |
| 5096 | specifically bound to a version node, it will effectively bind it to an |
| 5097 | unspecified base version of the library. You can bind all otherwise |
| 5098 | unspecified symbols to a given version node by using @samp{global: *;} |
| 5099 | somewhere in the version script. Note that it's slightly crazy to use |
| 5100 | wildcards in a global spec except on the last version node. Global |
| 5101 | wildcards elsewhere run the risk of accidentally adding symbols to the |
| 5102 | set exported for an old version. That's wrong since older versions |
| 5103 | ought to have a fixed set of symbols. |
| 5104 | |
| 5105 | The names of the version nodes have no specific meaning other than what |
| 5106 | they might suggest to the person reading them. The @samp{2.0} version |
| 5107 | could just as well have appeared in between @samp{1.1} and @samp{1.2}. |
| 5108 | However, this would be a confusing way to write a version script. |
| 5109 | |
| 5110 | Node name can be omitted, provided it is the only version node |
| 5111 | in the version script. Such version script doesn't assign any versions to |
| 5112 | symbols, only selects which symbols will be globally visible out and which |
| 5113 | won't. |
| 5114 | |
| 5115 | @smallexample |
| 5116 | @{ global: foo; bar; local: *; @}; |
| 5117 | @end smallexample |
| 5118 | |
| 5119 | When you link an application against a shared library that has versioned |
| 5120 | symbols, the application itself knows which version of each symbol it |
| 5121 | requires, and it also knows which version nodes it needs from each |
| 5122 | shared library it is linked against. Thus at runtime, the dynamic |
| 5123 | loader can make a quick check to make sure that the libraries you have |
| 5124 | linked against do in fact supply all of the version nodes that the |
| 5125 | application will need to resolve all of the dynamic symbols. In this |
| 5126 | way it is possible for the dynamic linker to know with certainty that |
| 5127 | all external symbols that it needs will be resolvable without having to |
| 5128 | search for each symbol reference. |
| 5129 | |
| 5130 | The symbol versioning is in effect a much more sophisticated way of |
| 5131 | doing minor version checking that SunOS does. The fundamental problem |
| 5132 | that is being addressed here is that typically references to external |
| 5133 | functions are bound on an as-needed basis, and are not all bound when |
| 5134 | the application starts up. If a shared library is out of date, a |
| 5135 | required interface may be missing; when the application tries to use |
| 5136 | that interface, it may suddenly and unexpectedly fail. With symbol |
| 5137 | versioning, the user will get a warning when they start their program if |
| 5138 | the libraries being used with the application are too old. |
| 5139 | |
| 5140 | There are several GNU extensions to Sun's versioning approach. The |
| 5141 | first of these is the ability to bind a symbol to a version node in the |
| 5142 | source file where the symbol is defined instead of in the versioning |
| 5143 | script. This was done mainly to reduce the burden on the library |
| 5144 | maintainer. You can do this by putting something like: |
| 5145 | @smallexample |
| 5146 | __asm__(".symver original_foo,foo@@VERS_1.1"); |
| 5147 | @end smallexample |
| 5148 | @noindent |
| 5149 | in the C source file. This renames the function @samp{original_foo} to |
| 5150 | be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}. |
| 5151 | The @samp{local:} directive can be used to prevent the symbol |
| 5152 | @samp{original_foo} from being exported. A @samp{.symver} directive |
| 5153 | takes precedence over a version script. |
| 5154 | |
| 5155 | The second GNU extension is to allow multiple versions of the same |
| 5156 | function to appear in a given shared library. In this way you can make |
| 5157 | an incompatible change to an interface without increasing the major |
| 5158 | version number of the shared library, while still allowing applications |
| 5159 | linked against the old interface to continue to function. |
| 5160 | |
| 5161 | To do this, you must use multiple @samp{.symver} directives in the |
| 5162 | source file. Here is an example: |
| 5163 | |
| 5164 | @smallexample |
| 5165 | __asm__(".symver original_foo,foo@@"); |
| 5166 | __asm__(".symver old_foo,foo@@VERS_1.1"); |
| 5167 | __asm__(".symver old_foo1,foo@@VERS_1.2"); |
| 5168 | __asm__(".symver new_foo,foo@@@@VERS_2.0"); |
| 5169 | @end smallexample |
| 5170 | |
| 5171 | In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the |
| 5172 | unspecified base version of the symbol. The source file that contains this |
| 5173 | example would define 4 C functions: @samp{original_foo}, @samp{old_foo}, |
| 5174 | @samp{old_foo1}, and @samp{new_foo}. |
| 5175 | |
| 5176 | When you have multiple definitions of a given symbol, there needs to be |
| 5177 | some way to specify a default version to which external references to |
| 5178 | this symbol will be bound. You can do this with the |
| 5179 | @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only |
| 5180 | declare one version of a symbol as the default in this manner; otherwise |
| 5181 | you would effectively have multiple definitions of the same symbol. |
| 5182 | |
| 5183 | If you wish to bind a reference to a specific version of the symbol |
| 5184 | within the shared library, you can use the aliases of convenience |
| 5185 | (i.e., @samp{old_foo}), or you can use the @samp{.symver} directive to |
| 5186 | specifically bind to an external version of the function in question. |
| 5187 | |
| 5188 | You can also specify the language in the version script: |
| 5189 | |
| 5190 | @smallexample |
| 5191 | VERSION extern "lang" @{ version-script-commands @} |
| 5192 | @end smallexample |
| 5193 | |
| 5194 | The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}. |
| 5195 | The linker will iterate over the list of symbols at the link time and |
| 5196 | demangle them according to @samp{lang} before matching them to the |
| 5197 | patterns specified in @samp{version-script-commands}. The default |
| 5198 | @samp{lang} is @samp{C}. |
| 5199 | |
| 5200 | Demangled names may contains spaces and other special characters. As |
| 5201 | described above, you can use a glob pattern to match demangled names, |
| 5202 | or you can use a double-quoted string to match the string exactly. In |
| 5203 | the latter case, be aware that minor differences (such as differing |
| 5204 | whitespace) between the version script and the demangler output will |
| 5205 | cause a mismatch. As the exact string generated by the demangler |
| 5206 | might change in the future, even if the mangled name does not, you |
| 5207 | should check that all of your version directives are behaving as you |
| 5208 | expect when you upgrade. |
| 5209 | |
| 5210 | @node Expressions |
| 5211 | @section Expressions in Linker Scripts |
| 5212 | @cindex expressions |
| 5213 | @cindex arithmetic |
| 5214 | The syntax for expressions in the linker script language is identical to |
| 5215 | that of C expressions. All expressions are evaluated as integers. All |
| 5216 | expressions are evaluated in the same size, which is 32 bits if both the |
| 5217 | host and target are 32 bits, and is otherwise 64 bits. |
| 5218 | |
| 5219 | You can use and set symbol values in expressions. |
| 5220 | |
| 5221 | The linker defines several special purpose builtin functions for use in |
| 5222 | expressions. |
| 5223 | |
| 5224 | @menu |
| 5225 | * Constants:: Constants |
| 5226 | * Symbolic Constants:: Symbolic constants |
| 5227 | * Symbols:: Symbol Names |
| 5228 | * Orphan Sections:: Orphan Sections |
| 5229 | * Location Counter:: The Location Counter |
| 5230 | * Operators:: Operators |
| 5231 | * Evaluation:: Evaluation |
| 5232 | * Expression Section:: The Section of an Expression |
| 5233 | * Builtin Functions:: Builtin Functions |
| 5234 | @end menu |
| 5235 | |
| 5236 | @node Constants |
| 5237 | @subsection Constants |
| 5238 | @cindex integer notation |
| 5239 | @cindex constants in linker scripts |
| 5240 | All constants are integers. |
| 5241 | |
| 5242 | As in C, the linker considers an integer beginning with @samp{0} to be |
| 5243 | octal, and an integer beginning with @samp{0x} or @samp{0X} to be |
| 5244 | hexadecimal. Alternatively the linker accepts suffixes of @samp{h} or |
| 5245 | @samp{H} for hexadeciaml, @samp{o} or @samp{O} for octal, @samp{b} or |
| 5246 | @samp{B} for binary and @samp{d} or @samp{D} for decimal. Any integer |
| 5247 | value without a prefix or a suffix is considered to be decimal. |
| 5248 | |
| 5249 | @cindex scaled integers |
| 5250 | @cindex K and M integer suffixes |
| 5251 | @cindex M and K integer suffixes |
| 5252 | @cindex suffixes for integers |
| 5253 | @cindex integer suffixes |
| 5254 | In addition, you can use the suffixes @code{K} and @code{M} to scale a |
| 5255 | constant by |
| 5256 | @c TEXI2ROFF-KILL |
| 5257 | @ifnottex |
| 5258 | @c END TEXI2ROFF-KILL |
| 5259 | @code{1024} or @code{1024*1024} |
| 5260 | @c TEXI2ROFF-KILL |
| 5261 | @end ifnottex |
| 5262 | @tex |
| 5263 | ${\rm 1024}$ or ${\rm 1024}^2$ |
| 5264 | @end tex |
| 5265 | @c END TEXI2ROFF-KILL |
| 5266 | respectively. For example, the following |
| 5267 | all refer to the same quantity: |
| 5268 | |
| 5269 | @smallexample |
| 5270 | _fourk_1 = 4K; |
| 5271 | _fourk_2 = 4096; |
| 5272 | _fourk_3 = 0x1000; |
| 5273 | _fourk_4 = 10000o; |
| 5274 | @end smallexample |
| 5275 | |
| 5276 | Note - the @code{K} and @code{M} suffixes cannot be used in |
| 5277 | conjunction with the base suffixes mentioned above. |
| 5278 | |
| 5279 | @node Symbolic Constants |
| 5280 | @subsection Symbolic Constants |
| 5281 | @cindex symbolic constants |
| 5282 | @kindex CONSTANT |
| 5283 | It is possible to refer to target specific constants via the use of |
| 5284 | the @code{CONSTANT(@var{name})} operator, where @var{name} is one of: |
| 5285 | |
| 5286 | @table @code |
| 5287 | @item MAXPAGESIZE |
| 5288 | @kindex MAXPAGESIZE |
| 5289 | The target's maximum page size. |
| 5290 | |
| 5291 | @item COMMONPAGESIZE |
| 5292 | @kindex COMMONPAGESIZE |
| 5293 | The target's default page size. |
| 5294 | @end table |
| 5295 | |
| 5296 | So for example: |
| 5297 | |
| 5298 | @smallexample |
| 5299 | .text ALIGN (CONSTANT (MAXPAGESIZE)) : @{ *(.text) @} |
| 5300 | @end smallexample |
| 5301 | |
| 5302 | will create a text section aligned to the largest page boundary |
| 5303 | supported by the target. |
| 5304 | |
| 5305 | @node Symbols |
| 5306 | @subsection Symbol Names |
| 5307 | @cindex symbol names |
| 5308 | @cindex names |
| 5309 | @cindex quoted symbol names |
| 5310 | @kindex " |
| 5311 | Unless quoted, symbol names start with a letter, underscore, or period |
| 5312 | and may include letters, digits, underscores, periods, and hyphens. |
| 5313 | Unquoted symbol names must not conflict with any keywords. You can |
| 5314 | specify a symbol which contains odd characters or has the same name as a |
| 5315 | keyword by surrounding the symbol name in double quotes: |
| 5316 | @smallexample |
| 5317 | "SECTION" = 9; |
| 5318 | "with a space" = "also with a space" + 10; |
| 5319 | @end smallexample |
| 5320 | |
| 5321 | Since symbols can contain many non-alphabetic characters, it is safest |
| 5322 | to delimit symbols with spaces. For example, @samp{A-B} is one symbol, |
| 5323 | whereas @samp{A - B} is an expression involving subtraction. |
| 5324 | |
| 5325 | @node Orphan Sections |
| 5326 | @subsection Orphan Sections |
| 5327 | @cindex orphan |
| 5328 | Orphan sections are sections present in the input files which |
| 5329 | are not explicitly placed into the output file by the linker |
| 5330 | script. The linker will still copy these sections into the |
| 5331 | output file, but it has to guess as to where they should be |
| 5332 | placed. The linker uses a simple heuristic to do this. It |
| 5333 | attempts to place orphan sections after non-orphan sections of the |
| 5334 | same attribute, such as code vs data, loadable vs non-loadable, etc. |
| 5335 | If there is not enough room to do this then it places |
| 5336 | at the end of the file. |
| 5337 | |
| 5338 | For ELF targets, the attribute of the section includes section type as |
| 5339 | well as section flag. |
| 5340 | |
| 5341 | If an orphaned section's name is representable as a C identifier then |
| 5342 | the linker will automatically @pxref{PROVIDE} two symbols: |
| 5343 | __start_SECNAME and __stop_SECNAME, where SECNAME is the name of the |
| 5344 | section. These indicate the start address and end address of the |
| 5345 | orphaned section respectively. Note: most section names are not |
| 5346 | representable as C identifiers because they contain a @samp{.} |
| 5347 | character. |
| 5348 | |
| 5349 | @node Location Counter |
| 5350 | @subsection The Location Counter |
| 5351 | @kindex . |
| 5352 | @cindex dot |
| 5353 | @cindex location counter |
| 5354 | @cindex current output location |
| 5355 | The special linker variable @dfn{dot} @samp{.} always contains the |
| 5356 | current output location counter. Since the @code{.} always refers to a |
| 5357 | location in an output section, it may only appear in an expression |
| 5358 | within a @code{SECTIONS} command. The @code{.} symbol may appear |
| 5359 | anywhere that an ordinary symbol is allowed in an expression. |
| 5360 | |
| 5361 | @cindex holes |
| 5362 | Assigning a value to @code{.} will cause the location counter to be |
| 5363 | moved. This may be used to create holes in the output section. The |
| 5364 | location counter may not be moved backwards inside an output section, |
| 5365 | and may not be moved backwards outside of an output section if so |
| 5366 | doing creates areas with overlapping LMAs. |
| 5367 | |
| 5368 | @smallexample |
| 5369 | SECTIONS |
| 5370 | @{ |
| 5371 | output : |
| 5372 | @{ |
| 5373 | file1(.text) |
| 5374 | . = . + 1000; |
| 5375 | file2(.text) |
| 5376 | . += 1000; |
| 5377 | file3(.text) |
| 5378 | @} = 0x12345678; |
| 5379 | @} |
| 5380 | @end smallexample |
| 5381 | @noindent |
| 5382 | In the previous example, the @samp{.text} section from @file{file1} is |
| 5383 | located at the beginning of the output section @samp{output}. It is |
| 5384 | followed by a 1000 byte gap. Then the @samp{.text} section from |
| 5385 | @file{file2} appears, also with a 1000 byte gap following before the |
| 5386 | @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678} |
| 5387 | specifies what data to write in the gaps (@pxref{Output Section Fill}). |
| 5388 | |
| 5389 | @cindex dot inside sections |
| 5390 | Note: @code{.} actually refers to the byte offset from the start of the |
| 5391 | current containing object. Normally this is the @code{SECTIONS} |
| 5392 | statement, whose start address is 0, hence @code{.} can be used as an |
| 5393 | absolute address. If @code{.} is used inside a section description |
| 5394 | however, it refers to the byte offset from the start of that section, |
| 5395 | not an absolute address. Thus in a script like this: |
| 5396 | |
| 5397 | @smallexample |
| 5398 | SECTIONS |
| 5399 | @{ |
| 5400 | . = 0x100 |
| 5401 | .text: @{ |
| 5402 | *(.text) |
| 5403 | . = 0x200 |
| 5404 | @} |
| 5405 | . = 0x500 |
| 5406 | .data: @{ |
| 5407 | *(.data) |
| 5408 | . += 0x600 |
| 5409 | @} |
| 5410 | @} |
| 5411 | @end smallexample |
| 5412 | |
| 5413 | The @samp{.text} section will be assigned a starting address of 0x100 |
| 5414 | and a size of exactly 0x200 bytes, even if there is not enough data in |
| 5415 | the @samp{.text} input sections to fill this area. (If there is too |
| 5416 | much data, an error will be produced because this would be an attempt to |
| 5417 | move @code{.} backwards). The @samp{.data} section will start at 0x500 |
| 5418 | and it will have an extra 0x600 bytes worth of space after the end of |
| 5419 | the values from the @samp{.data} input sections and before the end of |
| 5420 | the @samp{.data} output section itself. |
| 5421 | |
| 5422 | @cindex dot outside sections |
| 5423 | Setting symbols to the value of the location counter outside of an |
| 5424 | output section statement can result in unexpected values if the linker |
| 5425 | needs to place orphan sections. For example, given the following: |
| 5426 | |
| 5427 | @smallexample |
| 5428 | SECTIONS |
| 5429 | @{ |
| 5430 | start_of_text = . ; |
| 5431 | .text: @{ *(.text) @} |
| 5432 | end_of_text = . ; |
| 5433 | |
| 5434 | start_of_data = . ; |
| 5435 | .data: @{ *(.data) @} |
| 5436 | end_of_data = . ; |
| 5437 | @} |
| 5438 | @end smallexample |
| 5439 | |
| 5440 | If the linker needs to place some input section, e.g. @code{.rodata}, |
| 5441 | not mentioned in the script, it might choose to place that section |
| 5442 | between @code{.text} and @code{.data}. You might think the linker |
| 5443 | should place @code{.rodata} on the blank line in the above script, but |
| 5444 | blank lines are of no particular significance to the linker. As well, |
| 5445 | the linker doesn't associate the above symbol names with their |
| 5446 | sections. Instead, it assumes that all assignments or other |
| 5447 | statements belong to the previous output section, except for the |
| 5448 | special case of an assignment to @code{.}. I.e., the linker will |
| 5449 | place the orphan @code{.rodata} section as if the script was written |
| 5450 | as follows: |
| 5451 | |
| 5452 | @smallexample |
| 5453 | SECTIONS |
| 5454 | @{ |
| 5455 | start_of_text = . ; |
| 5456 | .text: @{ *(.text) @} |
| 5457 | end_of_text = . ; |
| 5458 | |
| 5459 | start_of_data = . ; |
| 5460 | .rodata: @{ *(.rodata) @} |
| 5461 | .data: @{ *(.data) @} |
| 5462 | end_of_data = . ; |
| 5463 | @} |
| 5464 | @end smallexample |
| 5465 | |
| 5466 | This may or may not be the script author's intention for the value of |
| 5467 | @code{start_of_data}. One way to influence the orphan section |
| 5468 | placement is to assign the location counter to itself, as the linker |
| 5469 | assumes that an assignment to @code{.} is setting the start address of |
| 5470 | a following output section and thus should be grouped with that |
| 5471 | section. So you could write: |
| 5472 | |
| 5473 | @smallexample |
| 5474 | SECTIONS |
| 5475 | @{ |
| 5476 | start_of_text = . ; |
| 5477 | .text: @{ *(.text) @} |
| 5478 | end_of_text = . ; |
| 5479 | |
| 5480 | . = . ; |
| 5481 | start_of_data = . ; |
| 5482 | .data: @{ *(.data) @} |
| 5483 | end_of_data = . ; |
| 5484 | @} |
| 5485 | @end smallexample |
| 5486 | |
| 5487 | Now, the orphan @code{.rodata} section will be placed between |
| 5488 | @code{end_of_text} and @code{start_of_data}. |
| 5489 | |
| 5490 | @need 2000 |
| 5491 | @node Operators |
| 5492 | @subsection Operators |
| 5493 | @cindex operators for arithmetic |
| 5494 | @cindex arithmetic operators |
| 5495 | @cindex precedence in expressions |
| 5496 | The linker recognizes the standard C set of arithmetic operators, with |
| 5497 | the standard bindings and precedence levels: |
| 5498 | @c TEXI2ROFF-KILL |
| 5499 | @ifnottex |
| 5500 | @c END TEXI2ROFF-KILL |
| 5501 | @smallexample |
| 5502 | precedence associativity Operators Notes |
| 5503 | (highest) |
| 5504 | 1 left ! - ~ (1) |
| 5505 | 2 left * / % |
| 5506 | 3 left + - |
| 5507 | 4 left >> << |
| 5508 | 5 left == != > < <= >= |
| 5509 | 6 left & |
| 5510 | 7 left | |
| 5511 | 8 left && |
| 5512 | 9 left || |
| 5513 | 10 right ? : |
| 5514 | 11 right &= += -= *= /= (2) |
| 5515 | (lowest) |
| 5516 | @end smallexample |
| 5517 | Notes: |
| 5518 | (1) Prefix operators |
| 5519 | (2) @xref{Assignments}. |
| 5520 | @c TEXI2ROFF-KILL |
| 5521 | @end ifnottex |
| 5522 | @tex |
| 5523 | \vskip \baselineskip |
| 5524 | %"lispnarrowing" is the extra indent used generally for smallexample |
| 5525 | \hskip\lispnarrowing\vbox{\offinterlineskip |
| 5526 | \hrule |
| 5527 | \halign |
| 5528 | {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr |
| 5529 | height2pt&\omit&&\omit&&\omit&\cr |
| 5530 | &Precedence&& Associativity &&{\rm Operators}&\cr |
| 5531 | height2pt&\omit&&\omit&&\omit&\cr |
| 5532 | \noalign{\hrule} |
| 5533 | height2pt&\omit&&\omit&&\omit&\cr |
| 5534 | &highest&&&&&\cr |
| 5535 | % '176 is tilde, '~' in tt font |
| 5536 | &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr |
| 5537 | &2&&left&&* / \%&\cr |
| 5538 | &3&&left&&+ -&\cr |
| 5539 | &4&&left&&>> <<&\cr |
| 5540 | &5&&left&&== != > < <= >=&\cr |
| 5541 | &6&&left&&\&&\cr |
| 5542 | &7&&left&&|&\cr |
| 5543 | &8&&left&&{\&\&}&\cr |
| 5544 | &9&&left&&||&\cr |
| 5545 | &10&&right&&? :&\cr |
| 5546 | &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr |
| 5547 | &lowest&&&&&\cr |
| 5548 | height2pt&\omit&&\omit&&\omit&\cr} |
| 5549 | \hrule} |
| 5550 | @end tex |
| 5551 | @iftex |
| 5552 | { |
| 5553 | @obeylines@parskip=0pt@parindent=0pt |
| 5554 | @dag@quad Prefix operators. |
| 5555 | @ddag@quad @xref{Assignments}. |
| 5556 | } |
| 5557 | @end iftex |
| 5558 | @c END TEXI2ROFF-KILL |
| 5559 | |
| 5560 | @node Evaluation |
| 5561 | @subsection Evaluation |
| 5562 | @cindex lazy evaluation |
| 5563 | @cindex expression evaluation order |
| 5564 | The linker evaluates expressions lazily. It only computes the value of |
| 5565 | an expression when absolutely necessary. |
| 5566 | |
| 5567 | The linker needs some information, such as the value of the start |
| 5568 | address of the first section, and the origins and lengths of memory |
| 5569 | regions, in order to do any linking at all. These values are computed |
| 5570 | as soon as possible when the linker reads in the linker script. |
| 5571 | |
| 5572 | However, other values (such as symbol values) are not known or needed |
| 5573 | until after storage allocation. Such values are evaluated later, when |
| 5574 | other information (such as the sizes of output sections) is available |
| 5575 | for use in the symbol assignment expression. |
| 5576 | |
| 5577 | The sizes of sections cannot be known until after allocation, so |
| 5578 | assignments dependent upon these are not performed until after |
| 5579 | allocation. |
| 5580 | |
| 5581 | Some expressions, such as those depending upon the location counter |
| 5582 | @samp{.}, must be evaluated during section allocation. |
| 5583 | |
| 5584 | If the result of an expression is required, but the value is not |
| 5585 | available, then an error results. For example, a script like the |
| 5586 | following |
| 5587 | @smallexample |
| 5588 | @group |
| 5589 | SECTIONS |
| 5590 | @{ |
| 5591 | .text 9+this_isnt_constant : |
| 5592 | @{ *(.text) @} |
| 5593 | @} |
| 5594 | @end group |
| 5595 | @end smallexample |
| 5596 | @noindent |
| 5597 | will cause the error message @samp{non constant expression for initial |
| 5598 | address}. |
| 5599 | |
| 5600 | @node Expression Section |
| 5601 | @subsection The Section of an Expression |
| 5602 | @cindex expression sections |
| 5603 | @cindex absolute expressions |
| 5604 | @cindex relative expressions |
| 5605 | @cindex absolute and relocatable symbols |
| 5606 | @cindex relocatable and absolute symbols |
| 5607 | @cindex symbols, relocatable and absolute |
| 5608 | Addresses and symbols may be section relative, or absolute. A section |
| 5609 | relative symbol is relocatable. If you request relocatable output |
| 5610 | using the @samp{-r} option, a further link operation may change the |
| 5611 | value of a section relative symbol. On the other hand, an absolute |
| 5612 | symbol will retain the same value throughout any further link |
| 5613 | operations. |
| 5614 | |
| 5615 | Some terms in linker expressions are addresses. This is true of |
| 5616 | section relative symbols and for builtin functions that return an |
| 5617 | address, such as @code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and |
| 5618 | @code{SEGMENT_START}. Other terms are simply numbers, or are builtin |
| 5619 | functions that return a non-address value, such as @code{LENGTH}. |
| 5620 | One complication is that unless you set @code{LD_FEATURE ("SANE_EXPR")} |
| 5621 | (@pxref{Miscellaneous Commands}), numbers and absolute symbols are treated |
| 5622 | differently depending on their location, for compatibility with older |
| 5623 | versions of @code{ld}. Expressions appearing outside an output |
| 5624 | section definition treat all numbers as absolute addresses. |
| 5625 | Expressions appearing inside an output section definition treat |
| 5626 | absolute symbols as numbers. If @code{LD_FEATURE ("SANE_EXPR")} is |
| 5627 | given, then absolute symbols and numbers are simply treated as numbers |
| 5628 | everywhere. |
| 5629 | |
| 5630 | In the following simple example, |
| 5631 | |
| 5632 | @smallexample |
| 5633 | @group |
| 5634 | SECTIONS |
| 5635 | @{ |
| 5636 | . = 0x100; |
| 5637 | __executable_start = 0x100; |
| 5638 | .data : |
| 5639 | @{ |
| 5640 | . = 0x10; |
| 5641 | __data_start = 0x10; |
| 5642 | *(.data) |
| 5643 | @} |
| 5644 | @dots{} |
| 5645 | @} |
| 5646 | @end group |
| 5647 | @end smallexample |
| 5648 | |
| 5649 | both @code{.} and @code{__executable_start} are set to the absolute |
| 5650 | address 0x100 in the first two assignments, then both @code{.} and |
| 5651 | @code{__data_start} are set to 0x10 relative to the @code{.data} |
| 5652 | section in the second two assignments. |
| 5653 | |
| 5654 | For expressions involving numbers, relative addresses and absolute |
| 5655 | addresses, ld follows these rules to evaluate terms: |
| 5656 | |
| 5657 | @itemize @bullet |
| 5658 | @item |
| 5659 | Unary operations on an absolute address or number, and binary |
| 5660 | operations on two absolute addresses or two numbers, or between one |
| 5661 | absolute address and a number, apply the operator to the value(s). |
| 5662 | @item |
| 5663 | Unary operations on a relative address, and binary operations on two |
| 5664 | relative addresses in the same section or between one relative address |
| 5665 | and a number, apply the operator to the offset part of the address(es). |
| 5666 | @item |
| 5667 | Other binary operations, that is, between two relative addresses not |
| 5668 | in the same section, or between a relative address and an absolute |
| 5669 | address, first convert any non-absolute term to an absolute address |
| 5670 | before applying the operator. |
| 5671 | @end itemize |
| 5672 | |
| 5673 | The result section of each sub-expression is as follows: |
| 5674 | |
| 5675 | @itemize @bullet |
| 5676 | @item |
| 5677 | An operation involving only numbers results in a number. |
| 5678 | @item |
| 5679 | The result of comparisons, @samp{&&} and @samp{||} is also a number. |
| 5680 | @item |
| 5681 | The result of other binary arithmetic and logical operations on two |
| 5682 | relative addresses in the same section or two absolute addresess |
| 5683 | (after above conversions) is also a number. |
| 5684 | @item |
| 5685 | The result of other operations on relative addresses or one |
| 5686 | relative address and a number, is a relative address in the same |
| 5687 | section as the relative operand(s). |
| 5688 | @item |
| 5689 | The result of other operations on absolute addresses (after above |
| 5690 | conversions) is an absolute address. |
| 5691 | @end itemize |
| 5692 | |
| 5693 | You can use the builtin function @code{ABSOLUTE} to force an expression |
| 5694 | to be absolute when it would otherwise be relative. For example, to |
| 5695 | create an absolute symbol set to the address of the end of the output |
| 5696 | section @samp{.data}: |
| 5697 | @smallexample |
| 5698 | SECTIONS |
| 5699 | @{ |
| 5700 | .data : @{ *(.data) _edata = ABSOLUTE(.); @} |
| 5701 | @} |
| 5702 | @end smallexample |
| 5703 | @noindent |
| 5704 | If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the |
| 5705 | @samp{.data} section. |
| 5706 | |
| 5707 | Using @code{LOADADDR} also forces an expression absolute, since this |
| 5708 | particular builtin function returns an absolute address. |
| 5709 | |
| 5710 | @node Builtin Functions |
| 5711 | @subsection Builtin Functions |
| 5712 | @cindex functions in expressions |
| 5713 | The linker script language includes a number of builtin functions for |
| 5714 | use in linker script expressions. |
| 5715 | |
| 5716 | @table @code |
| 5717 | @item ABSOLUTE(@var{exp}) |
| 5718 | @kindex ABSOLUTE(@var{exp}) |
| 5719 | @cindex expression, absolute |
| 5720 | Return the absolute (non-relocatable, as opposed to non-negative) value |
| 5721 | of the expression @var{exp}. Primarily useful to assign an absolute |
| 5722 | value to a symbol within a section definition, where symbol values are |
| 5723 | normally section relative. @xref{Expression Section}. |
| 5724 | |
| 5725 | @item ADDR(@var{section}) |
| 5726 | @kindex ADDR(@var{section}) |
| 5727 | @cindex section address in expression |
| 5728 | Return the address (VMA) of the named @var{section}. Your |
| 5729 | script must previously have defined the location of that section. In |
| 5730 | the following example, @code{start_of_output_1}, @code{symbol_1} and |
| 5731 | @code{symbol_2} are assigned equivalent values, except that |
| 5732 | @code{symbol_1} will be relative to the @code{.output1} section while |
| 5733 | the other two will be absolute: |
| 5734 | @smallexample |
| 5735 | @group |
| 5736 | SECTIONS @{ @dots{} |
| 5737 | .output1 : |
| 5738 | @{ |
| 5739 | start_of_output_1 = ABSOLUTE(.); |
| 5740 | @dots{} |
| 5741 | @} |
| 5742 | .output : |
| 5743 | @{ |
| 5744 | symbol_1 = ADDR(.output1); |
| 5745 | symbol_2 = start_of_output_1; |
| 5746 | @} |
| 5747 | @dots{} @} |
| 5748 | @end group |
| 5749 | @end smallexample |
| 5750 | |
| 5751 | @item ALIGN(@var{align}) |
| 5752 | @itemx ALIGN(@var{exp},@var{align}) |
| 5753 | @kindex ALIGN(@var{align}) |
| 5754 | @kindex ALIGN(@var{exp},@var{align}) |
| 5755 | @cindex round up location counter |
| 5756 | @cindex align location counter |
| 5757 | @cindex round up expression |
| 5758 | @cindex align expression |
| 5759 | Return the location counter (@code{.}) or arbitrary expression aligned |
| 5760 | to the next @var{align} boundary. The single operand @code{ALIGN} |
| 5761 | doesn't change the value of the location counter---it just does |
| 5762 | arithmetic on it. The two operand @code{ALIGN} allows an arbitrary |
| 5763 | expression to be aligned upwards (@code{ALIGN(@var{align})} is |
| 5764 | equivalent to @code{ALIGN(., @var{align})}). |
| 5765 | |
| 5766 | Here is an example which aligns the output @code{.data} section to the |
| 5767 | next @code{0x2000} byte boundary after the preceding section and sets a |
| 5768 | variable within the section to the next @code{0x8000} boundary after the |
| 5769 | input sections: |
| 5770 | @smallexample |
| 5771 | @group |
| 5772 | SECTIONS @{ @dots{} |
| 5773 | .data ALIGN(0x2000): @{ |
| 5774 | *(.data) |
| 5775 | variable = ALIGN(0x8000); |
| 5776 | @} |
| 5777 | @dots{} @} |
| 5778 | @end group |
| 5779 | @end smallexample |
| 5780 | @noindent |
| 5781 | The first use of @code{ALIGN} in this example specifies the location of |
| 5782 | a section because it is used as the optional @var{address} attribute of |
| 5783 | a section definition (@pxref{Output Section Address}). The second use |
| 5784 | of @code{ALIGN} is used to defines the value of a symbol. |
| 5785 | |
| 5786 | The builtin function @code{NEXT} is closely related to @code{ALIGN}. |
| 5787 | |
| 5788 | @item ALIGNOF(@var{section}) |
| 5789 | @kindex ALIGNOF(@var{section}) |
| 5790 | @cindex section alignment |
| 5791 | Return the alignment in bytes of the named @var{section}, if that section has |
| 5792 | been allocated. If the section has not been allocated when this is |
| 5793 | evaluated, the linker will report an error. In the following example, |
| 5794 | the alignment of the @code{.output} section is stored as the first |
| 5795 | value in that section. |
| 5796 | @smallexample |
| 5797 | @group |
| 5798 | SECTIONS@{ @dots{} |
| 5799 | .output @{ |
| 5800 | LONG (ALIGNOF (.output)) |
| 5801 | @dots{} |
| 5802 | @} |
| 5803 | @dots{} @} |
| 5804 | @end group |
| 5805 | @end smallexample |
| 5806 | |
| 5807 | @item BLOCK(@var{exp}) |
| 5808 | @kindex BLOCK(@var{exp}) |
| 5809 | This is a synonym for @code{ALIGN}, for compatibility with older linker |
| 5810 | scripts. It is most often seen when setting the address of an output |
| 5811 | section. |
| 5812 | |
| 5813 | @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize}) |
| 5814 | @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize}) |
| 5815 | This is equivalent to either |
| 5816 | @smallexample |
| 5817 | (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1))) |
| 5818 | @end smallexample |
| 5819 | or |
| 5820 | @smallexample |
| 5821 | (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - @var{commonpagesize}))) |
| 5822 | @end smallexample |
| 5823 | @noindent |
| 5824 | depending on whether the latter uses fewer @var{commonpagesize} sized pages |
| 5825 | for the data segment (area between the result of this expression and |
| 5826 | @code{DATA_SEGMENT_END}) than the former or not. |
| 5827 | If the latter form is used, it means @var{commonpagesize} bytes of runtime |
| 5828 | memory will be saved at the expense of up to @var{commonpagesize} wasted |
| 5829 | bytes in the on-disk file. |
| 5830 | |
| 5831 | This expression can only be used directly in @code{SECTIONS} commands, not in |
| 5832 | any output section descriptions and only once in the linker script. |
| 5833 | @var{commonpagesize} should be less or equal to @var{maxpagesize} and should |
| 5834 | be the system page size the object wants to be optimized for (while still |
| 5835 | working on system page sizes up to @var{maxpagesize}). |
| 5836 | |
| 5837 | @noindent |
| 5838 | Example: |
| 5839 | @smallexample |
| 5840 | . = DATA_SEGMENT_ALIGN(0x10000, 0x2000); |
| 5841 | @end smallexample |
| 5842 | |
| 5843 | @item DATA_SEGMENT_END(@var{exp}) |
| 5844 | @kindex DATA_SEGMENT_END(@var{exp}) |
| 5845 | This defines the end of data segment for @code{DATA_SEGMENT_ALIGN} |
| 5846 | evaluation purposes. |
| 5847 | |
| 5848 | @smallexample |
| 5849 | . = DATA_SEGMENT_END(.); |
| 5850 | @end smallexample |
| 5851 | |
| 5852 | @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp}) |
| 5853 | @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp}) |
| 5854 | This defines the end of the @code{PT_GNU_RELRO} segment when |
| 5855 | @samp{-z relro} option is used. Second argument is returned. |
| 5856 | When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END} |
| 5857 | does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that |
| 5858 | @var{exp} + @var{offset} is aligned to the most commonly used page |
| 5859 | boundary for particular target. If present in the linker script, |
| 5860 | it must always come in between @code{DATA_SEGMENT_ALIGN} and |
| 5861 | @code{DATA_SEGMENT_END}. |
| 5862 | |
| 5863 | @smallexample |
| 5864 | . = DATA_SEGMENT_RELRO_END(24, .); |
| 5865 | @end smallexample |
| 5866 | |
| 5867 | @item DEFINED(@var{symbol}) |
| 5868 | @kindex DEFINED(@var{symbol}) |
| 5869 | @cindex symbol defaults |
| 5870 | Return 1 if @var{symbol} is in the linker global symbol table and is |
| 5871 | defined before the statement using DEFINED in the script, otherwise |
| 5872 | return 0. You can use this function to provide |
| 5873 | default values for symbols. For example, the following script fragment |
| 5874 | shows how to set a global symbol @samp{begin} to the first location in |
| 5875 | the @samp{.text} section---but if a symbol called @samp{begin} already |
| 5876 | existed, its value is preserved: |
| 5877 | |
| 5878 | @smallexample |
| 5879 | @group |
| 5880 | SECTIONS @{ @dots{} |
| 5881 | .text : @{ |
| 5882 | begin = DEFINED(begin) ? begin : . ; |
| 5883 | @dots{} |
| 5884 | @} |
| 5885 | @dots{} |
| 5886 | @} |
| 5887 | @end group |
| 5888 | @end smallexample |
| 5889 | |
| 5890 | @item LENGTH(@var{memory}) |
| 5891 | @kindex LENGTH(@var{memory}) |
| 5892 | Return the length of the memory region named @var{memory}. |
| 5893 | |
| 5894 | @item LOADADDR(@var{section}) |
| 5895 | @kindex LOADADDR(@var{section}) |
| 5896 | @cindex section load address in expression |
| 5897 | Return the absolute LMA of the named @var{section}. (@pxref{Output |
| 5898 | Section LMA}). |
| 5899 | |
| 5900 | @kindex MAX |
| 5901 | @item MAX(@var{exp1}, @var{exp2}) |
| 5902 | Returns the maximum of @var{exp1} and @var{exp2}. |
| 5903 | |
| 5904 | @kindex MIN |
| 5905 | @item MIN(@var{exp1}, @var{exp2}) |
| 5906 | Returns the minimum of @var{exp1} and @var{exp2}. |
| 5907 | |
| 5908 | @item NEXT(@var{exp}) |
| 5909 | @kindex NEXT(@var{exp}) |
| 5910 | @cindex unallocated address, next |
| 5911 | Return the next unallocated address that is a multiple of @var{exp}. |
| 5912 | This function is closely related to @code{ALIGN(@var{exp})}; unless you |
| 5913 | use the @code{MEMORY} command to define discontinuous memory for the |
| 5914 | output file, the two functions are equivalent. |
| 5915 | |
| 5916 | @item ORIGIN(@var{memory}) |
| 5917 | @kindex ORIGIN(@var{memory}) |
| 5918 | Return the origin of the memory region named @var{memory}. |
| 5919 | |
| 5920 | @item SEGMENT_START(@var{segment}, @var{default}) |
| 5921 | @kindex SEGMENT_START(@var{segment}, @var{default}) |
| 5922 | Return the base address of the named @var{segment}. If an explicit |
| 5923 | value has been given for this segment (with a command-line @samp{-T} |
| 5924 | option) that value will be returned; otherwise the value will be |
| 5925 | @var{default}. At present, the @samp{-T} command-line option can only |
| 5926 | be used to set the base address for the ``text'', ``data'', and |
| 5927 | ``bss'' sections, but you can use @code{SEGMENT_START} with any segment |
| 5928 | name. |
| 5929 | |
| 5930 | @item SIZEOF(@var{section}) |
| 5931 | @kindex SIZEOF(@var{section}) |
| 5932 | @cindex section size |
| 5933 | Return the size in bytes of the named @var{section}, if that section has |
| 5934 | been allocated. If the section has not been allocated when this is |
| 5935 | evaluated, the linker will report an error. In the following example, |
| 5936 | @code{symbol_1} and @code{symbol_2} are assigned identical values: |
| 5937 | @smallexample |
| 5938 | @group |
| 5939 | SECTIONS@{ @dots{} |
| 5940 | .output @{ |
| 5941 | .start = . ; |
| 5942 | @dots{} |
| 5943 | .end = . ; |
| 5944 | @} |
| 5945 | symbol_1 = .end - .start ; |
| 5946 | symbol_2 = SIZEOF(.output); |
| 5947 | @dots{} @} |
| 5948 | @end group |
| 5949 | @end smallexample |
| 5950 | |
| 5951 | @item SIZEOF_HEADERS |
| 5952 | @itemx sizeof_headers |
| 5953 | @kindex SIZEOF_HEADERS |
| 5954 | @cindex header size |
| 5955 | Return the size in bytes of the output file's headers. This is |
| 5956 | information which appears at the start of the output file. You can use |
| 5957 | this number when setting the start address of the first section, if you |
| 5958 | choose, to facilitate paging. |
| 5959 | |
| 5960 | @cindex not enough room for program headers |
| 5961 | @cindex program headers, not enough room |
| 5962 | When producing an ELF output file, if the linker script uses the |
| 5963 | @code{SIZEOF_HEADERS} builtin function, the linker must compute the |
| 5964 | number of program headers before it has determined all the section |
| 5965 | addresses and sizes. If the linker later discovers that it needs |
| 5966 | additional program headers, it will report an error @samp{not enough |
| 5967 | room for program headers}. To avoid this error, you must avoid using |
| 5968 | the @code{SIZEOF_HEADERS} function, or you must rework your linker |
| 5969 | script to avoid forcing the linker to use additional program headers, or |
| 5970 | you must define the program headers yourself using the @code{PHDRS} |
| 5971 | command (@pxref{PHDRS}). |
| 5972 | @end table |
| 5973 | |
| 5974 | @node Implicit Linker Scripts |
| 5975 | @section Implicit Linker Scripts |
| 5976 | @cindex implicit linker scripts |
| 5977 | If you specify a linker input file which the linker can not recognize as |
| 5978 | an object file or an archive file, it will try to read the file as a |
| 5979 | linker script. If the file can not be parsed as a linker script, the |
| 5980 | linker will report an error. |
| 5981 | |
| 5982 | An implicit linker script will not replace the default linker script. |
| 5983 | |
| 5984 | Typically an implicit linker script would contain only symbol |
| 5985 | assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION} |
| 5986 | commands. |
| 5987 | |
| 5988 | Any input files read because of an implicit linker script will be read |
| 5989 | at the position in the command line where the implicit linker script was |
| 5990 | read. This can affect archive searching. |
| 5991 | |
| 5992 | @ifset GENERIC |
| 5993 | @node Machine Dependent |
| 5994 | @chapter Machine Dependent Features |
| 5995 | |
| 5996 | @cindex machine dependencies |
| 5997 | @command{ld} has additional features on some platforms; the following |
| 5998 | sections describe them. Machines where @command{ld} has no additional |
| 5999 | functionality are not listed. |
| 6000 | |
| 6001 | @menu |
| 6002 | @ifset H8300 |
| 6003 | * H8/300:: @command{ld} and the H8/300 |
| 6004 | @end ifset |
| 6005 | @ifset I960 |
| 6006 | * i960:: @command{ld} and the Intel 960 family |
| 6007 | @end ifset |
| 6008 | @ifset ARM |
| 6009 | * ARM:: @command{ld} and the ARM family |
| 6010 | @end ifset |
| 6011 | @ifset HPPA |
| 6012 | * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF |
| 6013 | @end ifset |
| 6014 | @ifset M68K |
| 6015 | * M68K:: @command{ld} and the Motorola 68K family |
| 6016 | @end ifset |
| 6017 | @ifset MMIX |
| 6018 | * MMIX:: @command{ld} and MMIX |
| 6019 | @end ifset |
| 6020 | @ifset MSP430 |
| 6021 | * MSP430:: @command{ld} and MSP430 |
| 6022 | @end ifset |
| 6023 | @ifset M68HC11 |
| 6024 | * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families |
| 6025 | @end ifset |
| 6026 | @ifset POWERPC |
| 6027 | * PowerPC ELF32:: @command{ld} and PowerPC 32-bit ELF Support |
| 6028 | @end ifset |
| 6029 | @ifset POWERPC64 |
| 6030 | * PowerPC64 ELF64:: @command{ld} and PowerPC64 64-bit ELF Support |
| 6031 | @end ifset |
| 6032 | @ifset SPU |
| 6033 | * SPU ELF:: @command{ld} and SPU ELF Support |
| 6034 | @end ifset |
| 6035 | @ifset TICOFF |
| 6036 | * TI COFF:: @command{ld} and TI COFF |
| 6037 | @end ifset |
| 6038 | @ifset WIN32 |
| 6039 | * WIN32:: @command{ld} and WIN32 (cygwin/mingw) |
| 6040 | @end ifset |
| 6041 | @ifset XTENSA |
| 6042 | * Xtensa:: @command{ld} and Xtensa Processors |
| 6043 | @end ifset |
| 6044 | @end menu |
| 6045 | @end ifset |
| 6046 | |
| 6047 | @ifset H8300 |
| 6048 | @ifclear GENERIC |
| 6049 | @raisesections |
| 6050 | @end ifclear |
| 6051 | |
| 6052 | @node H8/300 |
| 6053 | @section @command{ld} and the H8/300 |
| 6054 | |
| 6055 | @cindex H8/300 support |
| 6056 | For the H8/300, @command{ld} can perform these global optimizations when |
| 6057 | you specify the @samp{--relax} command-line option. |
| 6058 | |
| 6059 | @table @emph |
| 6060 | @cindex relaxing on H8/300 |
| 6061 | @item relaxing address modes |
| 6062 | @command{ld} finds all @code{jsr} and @code{jmp} instructions whose |
| 6063 | targets are within eight bits, and turns them into eight-bit |
| 6064 | program-counter relative @code{bsr} and @code{bra} instructions, |
| 6065 | respectively. |
| 6066 | |
| 6067 | @cindex synthesizing on H8/300 |
| 6068 | @item synthesizing instructions |
| 6069 | @c FIXME: specifically mov.b, or any mov instructions really? |
| 6070 | @command{ld} finds all @code{mov.b} instructions which use the |
| 6071 | sixteen-bit absolute address form, but refer to the top |
| 6072 | page of memory, and changes them to use the eight-bit address form. |
| 6073 | (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into |
| 6074 | @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the |
| 6075 | top page of memory). |
| 6076 | |
| 6077 | @item bit manipulation instructions |
| 6078 | @command{ld} finds all bit manipulation instructions like @code{band, bclr, |
| 6079 | biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor} |
| 6080 | which use 32 bit and 16 bit absolute address form, but refer to the top |
| 6081 | page of memory, and changes them to use the 8 bit address form. |
| 6082 | (That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into |
| 6083 | @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in |
| 6084 | the top page of memory). |
| 6085 | |
| 6086 | @item system control instructions |
| 6087 | @command{ld} finds all @code{ldc.w, stc.w} instructions which use the |
| 6088 | 32 bit absolute address form, but refer to the top page of memory, and |
| 6089 | changes them to use 16 bit address form. |
| 6090 | (That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into |
| 6091 | @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in |
| 6092 | the top page of memory). |
| 6093 | @end table |
| 6094 | |
| 6095 | @ifclear GENERIC |
| 6096 | @lowersections |
| 6097 | @end ifclear |
| 6098 | @end ifset |
| 6099 | |
| 6100 | @ifclear GENERIC |
| 6101 | @ifset Renesas |
| 6102 | @c This stuff is pointless to say unless you're especially concerned |
| 6103 | @c with Renesas chips; don't enable it for generic case, please. |
| 6104 | @node Renesas |
| 6105 | @chapter @command{ld} and Other Renesas Chips |
| 6106 | |
| 6107 | @command{ld} also supports the Renesas (formerly Hitachi) H8/300H, |
| 6108 | H8/500, and SH chips. No special features, commands, or command-line |
| 6109 | options are required for these chips. |
| 6110 | @end ifset |
| 6111 | @end ifclear |
| 6112 | |
| 6113 | @ifset I960 |
| 6114 | @ifclear GENERIC |
| 6115 | @raisesections |
| 6116 | @end ifclear |
| 6117 | |
| 6118 | @node i960 |
| 6119 | @section @command{ld} and the Intel 960 Family |
| 6120 | |
| 6121 | @cindex i960 support |
| 6122 | |
| 6123 | You can use the @samp{-A@var{architecture}} command line option to |
| 6124 | specify one of the two-letter names identifying members of the 960 |
| 6125 | family; the option specifies the desired output target, and warns of any |
| 6126 | incompatible instructions in the input files. It also modifies the |
| 6127 | linker's search strategy for archive libraries, to support the use of |
| 6128 | libraries specific to each particular architecture, by including in the |
| 6129 | search loop names suffixed with the string identifying the architecture. |
| 6130 | |
| 6131 | For example, if your @command{ld} command line included @w{@samp{-ACA}} as |
| 6132 | well as @w{@samp{-ltry}}, the linker would look (in its built-in search |
| 6133 | paths, and in any paths you specify with @samp{-L}) for a library with |
| 6134 | the names |
| 6135 | |
| 6136 | @smallexample |
| 6137 | @group |
| 6138 | try |
| 6139 | libtry.a |
| 6140 | tryca |
| 6141 | libtryca.a |
| 6142 | @end group |
| 6143 | @end smallexample |
| 6144 | |
| 6145 | @noindent |
| 6146 | The first two possibilities would be considered in any event; the last |
| 6147 | two are due to the use of @w{@samp{-ACA}}. |
| 6148 | |
| 6149 | You can meaningfully use @samp{-A} more than once on a command line, since |
| 6150 | the 960 architecture family allows combination of target architectures; each |
| 6151 | use will add another pair of name variants to search for when @w{@samp{-l}} |
| 6152 | specifies a library. |
| 6153 | |
| 6154 | @cindex @option{--relax} on i960 |
| 6155 | @cindex relaxing on i960 |
| 6156 | @command{ld} supports the @samp{--relax} option for the i960 family. If |
| 6157 | you specify @samp{--relax}, @command{ld} finds all @code{balx} and |
| 6158 | @code{calx} instructions whose targets are within 24 bits, and turns |
| 6159 | them into 24-bit program-counter relative @code{bal} and @code{cal} |
| 6160 | instructions, respectively. @command{ld} also turns @code{cal} |
| 6161 | instructions into @code{bal} instructions when it determines that the |
| 6162 | target subroutine is a leaf routine (that is, the target subroutine does |
| 6163 | not itself call any subroutines). |
| 6164 | |
| 6165 | @cindex Cortex-A8 erratum workaround |
| 6166 | @kindex --fix-cortex-a8 |
| 6167 | @kindex --no-fix-cortex-a8 |
| 6168 | The @samp{--fix-cortex-a8} switch enables a link-time workaround for an erratum in certain Cortex-A8 processors. The workaround is enabled by default if you are targeting the ARM v7-A architecture profile. It can be enabled otherwise by specifying @samp{--fix-cortex-a8}, or disabled unconditionally by specifying @samp{--no-fix-cortex-a8}. |
| 6169 | |
| 6170 | The erratum only affects Thumb-2 code. Please contact ARM for further details. |
| 6171 | |
| 6172 | @kindex --merge-exidx-entries |
| 6173 | @kindex --no-merge-exidx-entries |
| 6174 | The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent exidx entries in debuginfo. |
| 6175 | |
| 6176 | @ifclear GENERIC |
| 6177 | @lowersections |
| 6178 | @end ifclear |
| 6179 | @end ifset |
| 6180 | |
| 6181 | @ifset ARM |
| 6182 | @ifclear GENERIC |
| 6183 | @raisesections |
| 6184 | @end ifclear |
| 6185 | |
| 6186 | @ifset M68HC11 |
| 6187 | @ifclear GENERIC |
| 6188 | @raisesections |
| 6189 | @end ifclear |
| 6190 | |
| 6191 | @node M68HC11/68HC12 |
| 6192 | @section @command{ld} and the Motorola 68HC11 and 68HC12 families |
| 6193 | |
| 6194 | @cindex M68HC11 and 68HC12 support |
| 6195 | |
| 6196 | @subsection Linker Relaxation |
| 6197 | |
| 6198 | For the Motorola 68HC11, @command{ld} can perform these global |
| 6199 | optimizations when you specify the @samp{--relax} command-line option. |
| 6200 | |
| 6201 | @table @emph |
| 6202 | @cindex relaxing on M68HC11 |
| 6203 | @item relaxing address modes |
| 6204 | @command{ld} finds all @code{jsr} and @code{jmp} instructions whose |
| 6205 | targets are within eight bits, and turns them into eight-bit |
| 6206 | program-counter relative @code{bsr} and @code{bra} instructions, |
| 6207 | respectively. |
| 6208 | |
| 6209 | @command{ld} also looks at all 16-bit extended addressing modes and |
| 6210 | transforms them in a direct addressing mode when the address is in |
| 6211 | page 0 (between 0 and 0x0ff). |
| 6212 | |
| 6213 | @item relaxing gcc instruction group |
| 6214 | When @command{gcc} is called with @option{-mrelax}, it can emit group |
| 6215 | of instructions that the linker can optimize to use a 68HC11 direct |
| 6216 | addressing mode. These instructions consists of @code{bclr} or |
| 6217 | @code{bset} instructions. |
| 6218 | |
| 6219 | @end table |
| 6220 | |
| 6221 | @subsection Trampoline Generation |
| 6222 | |
| 6223 | @cindex trampoline generation on M68HC11 |
| 6224 | @cindex trampoline generation on M68HC12 |
| 6225 | For 68HC11 and 68HC12, @command{ld} can generate trampoline code to |
| 6226 | call a far function using a normal @code{jsr} instruction. The linker |
| 6227 | will also change the relocation to some far function to use the |
| 6228 | trampoline address instead of the function address. This is typically the |
| 6229 | case when a pointer to a function is taken. The pointer will in fact |
| 6230 | point to the function trampoline. |
| 6231 | |
| 6232 | @ifclear GENERIC |
| 6233 | @lowersections |
| 6234 | @end ifclear |
| 6235 | @end ifset |
| 6236 | |
| 6237 | @node ARM |
| 6238 | @section @command{ld} and the ARM family |
| 6239 | |
| 6240 | @cindex ARM interworking support |
| 6241 | @kindex --support-old-code |
| 6242 | For the ARM, @command{ld} will generate code stubs to allow functions calls |
| 6243 | between ARM and Thumb code. These stubs only work with code that has |
| 6244 | been compiled and assembled with the @samp{-mthumb-interwork} command |
| 6245 | line option. If it is necessary to link with old ARM object files or |
| 6246 | libraries, which have not been compiled with the -mthumb-interwork |
| 6247 | option then the @samp{--support-old-code} command line switch should be |
| 6248 | given to the linker. This will make it generate larger stub functions |
| 6249 | which will work with non-interworking aware ARM code. Note, however, |
| 6250 | the linker does not support generating stubs for function calls to |
| 6251 | non-interworking aware Thumb code. |
| 6252 | |
| 6253 | @cindex thumb entry point |
| 6254 | @cindex entry point, thumb |
| 6255 | @kindex --thumb-entry=@var{entry} |
| 6256 | The @samp{--thumb-entry} switch is a duplicate of the generic |
| 6257 | @samp{--entry} switch, in that it sets the program's starting address. |
| 6258 | But it also sets the bottom bit of the address, so that it can be |
| 6259 | branched to using a BX instruction, and the program will start |
| 6260 | executing in Thumb mode straight away. |
| 6261 | |
| 6262 | @cindex PE import table prefixing |
| 6263 | @kindex --use-nul-prefixed-import-tables |
| 6264 | The @samp{--use-nul-prefixed-import-tables} switch is specifying, that |
| 6265 | the import tables idata4 and idata5 have to be generated with a zero |
| 6266 | elememt prefix for import libraries. This is the old style to generate |
| 6267 | import tables. By default this option is turned off. |
| 6268 | |
| 6269 | @cindex BE8 |
| 6270 | @kindex --be8 |
| 6271 | The @samp{--be8} switch instructs @command{ld} to generate BE8 format |
| 6272 | executables. This option is only valid when linking big-endian objects. |
| 6273 | The resulting image will contain big-endian data and little-endian code. |
| 6274 | |
| 6275 | @cindex TARGET1 |
| 6276 | @kindex --target1-rel |
| 6277 | @kindex --target1-abs |
| 6278 | The @samp{R_ARM_TARGET1} relocation is typically used for entries in the |
| 6279 | @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32} |
| 6280 | or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel} |
| 6281 | and @samp{--target1-abs} switches override the default. |
| 6282 | |
| 6283 | @cindex TARGET2 |
| 6284 | @kindex --target2=@var{type} |
| 6285 | The @samp{--target2=type} switch overrides the default definition of the |
| 6286 | @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their |
| 6287 | meanings, and target defaults are as follows: |
| 6288 | @table @samp |
| 6289 | @item rel |
| 6290 | @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi) |
| 6291 | @item abs |
| 6292 | @samp{R_ARM_ABS32} (arm*-*-symbianelf) |
| 6293 | @item got-rel |
| 6294 | @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd) |
| 6295 | @end table |
| 6296 | |
| 6297 | @cindex FIX_V4BX |
| 6298 | @kindex --fix-v4bx |
| 6299 | The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF |
| 6300 | specification) enables objects compiled for the ARMv4 architecture to be |
| 6301 | interworking-safe when linked with other objects compiled for ARMv4t, but |
| 6302 | also allows pure ARMv4 binaries to be built from the same ARMv4 objects. |
| 6303 | |
| 6304 | In the latter case, the switch @option{--fix-v4bx} must be passed to the |
| 6305 | linker, which causes v4t @code{BX rM} instructions to be rewritten as |
| 6306 | @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction. |
| 6307 | |
| 6308 | In the former case, the switch should not be used, and @samp{R_ARM_V4BX} |
| 6309 | relocations are ignored. |
| 6310 | |
| 6311 | @cindex FIX_V4BX_INTERWORKING |
| 6312 | @kindex --fix-v4bx-interworking |
| 6313 | Replace @code{BX rM} instructions identified by @samp{R_ARM_V4BX} |
| 6314 | relocations with a branch to the following veneer: |
| 6315 | |
| 6316 | @smallexample |
| 6317 | TST rM, #1 |
| 6318 | MOVEQ PC, rM |
| 6319 | BX Rn |
| 6320 | @end smallexample |
| 6321 | |
| 6322 | This allows generation of libraries/applications that work on ARMv4 cores |
| 6323 | and are still interworking safe. Note that the above veneer clobbers the |
| 6324 | condition flags, so may cause incorrect progrm behavior in rare cases. |
| 6325 | |
| 6326 | @cindex USE_BLX |
| 6327 | @kindex --use-blx |
| 6328 | The @samp{--use-blx} switch enables the linker to use ARM/Thumb |
| 6329 | BLX instructions (available on ARMv5t and above) in various |
| 6330 | situations. Currently it is used to perform calls via the PLT from Thumb |
| 6331 | code using BLX rather than using BX and a mode-switching stub before |
| 6332 | each PLT entry. This should lead to such calls executing slightly faster. |
| 6333 | |
| 6334 | This option is enabled implicitly for SymbianOS, so there is no need to |
| 6335 | specify it if you are using that target. |
| 6336 | |
| 6337 | @cindex VFP11_DENORM_FIX |
| 6338 | @kindex --vfp11-denorm-fix |
| 6339 | The @samp{--vfp11-denorm-fix} switch enables a link-time workaround for a |
| 6340 | bug in certain VFP11 coprocessor hardware, which sometimes allows |
| 6341 | instructions with denorm operands (which must be handled by support code) |
| 6342 | to have those operands overwritten by subsequent instructions before |
| 6343 | the support code can read the intended values. |
| 6344 | |
| 6345 | The bug may be avoided in scalar mode if you allow at least one |
| 6346 | intervening instruction between a VFP11 instruction which uses a register |
| 6347 | and another instruction which writes to the same register, or at least two |
| 6348 | intervening instructions if vector mode is in use. The bug only affects |
| 6349 | full-compliance floating-point mode: you do not need this workaround if |
| 6350 | you are using "runfast" mode. Please contact ARM for further details. |
| 6351 | |
| 6352 | If you know you are using buggy VFP11 hardware, you can |
| 6353 | enable this workaround by specifying the linker option |
| 6354 | @samp{--vfp-denorm-fix=scalar} if you are using the VFP11 scalar |
| 6355 | mode only, or @samp{--vfp-denorm-fix=vector} if you are using |
| 6356 | vector mode (the latter also works for scalar code). The default is |
| 6357 | @samp{--vfp-denorm-fix=none}. |
| 6358 | |
| 6359 | If the workaround is enabled, instructions are scanned for |
| 6360 | potentially-troublesome sequences, and a veneer is created for each |
| 6361 | such sequence which may trigger the erratum. The veneer consists of the |
| 6362 | first instruction of the sequence and a branch back to the subsequent |
| 6363 | instruction. The original instruction is then replaced with a branch to |
| 6364 | the veneer. The extra cycles required to call and return from the veneer |
| 6365 | are sufficient to avoid the erratum in both the scalar and vector cases. |
| 6366 | |
| 6367 | @cindex ARM1176 erratum workaround |
| 6368 | @kindex --fix-arm1176 |
| 6369 | @kindex --no-fix-arm1176 |
| 6370 | The @samp{--fix-arm1176} switch enables a link-time workaround for an erratum |
| 6371 | in certain ARM1176 processors. The workaround is enabled by default if you |
| 6372 | are targetting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled |
| 6373 | unconditionally by specifying @samp{--no-fix-arm1176}. |
| 6374 | |
| 6375 | Further information is available in the ``ARM1176JZ-S and ARM1176JZF-S |
| 6376 | Programmer Advice Notice'' available on the ARM documentaion website at: |
| 6377 | http://infocenter.arm.com/. |
| 6378 | |
| 6379 | @cindex NO_ENUM_SIZE_WARNING |
| 6380 | @kindex --no-enum-size-warning |
| 6381 | The @option{--no-enum-size-warning} switch prevents the linker from |
| 6382 | warning when linking object files that specify incompatible EABI |
| 6383 | enumeration size attributes. For example, with this switch enabled, |
| 6384 | linking of an object file using 32-bit enumeration values with another |
| 6385 | using enumeration values fitted into the smallest possible space will |
| 6386 | not be diagnosed. |
| 6387 | |
| 6388 | @cindex NO_WCHAR_SIZE_WARNING |
| 6389 | @kindex --no-wchar-size-warning |
| 6390 | The @option{--no-wchar-size-warning} switch prevents the linker from |
| 6391 | warning when linking object files that specify incompatible EABI |
| 6392 | @code{wchar_t} size attributes. For example, with this switch enabled, |
| 6393 | linking of an object file using 32-bit @code{wchar_t} values with another |
| 6394 | using 16-bit @code{wchar_t} values will not be diagnosed. |
| 6395 | |
| 6396 | @cindex PIC_VENEER |
| 6397 | @kindex --pic-veneer |
| 6398 | The @samp{--pic-veneer} switch makes the linker use PIC sequences for |
| 6399 | ARM/Thumb interworking veneers, even if the rest of the binary |
| 6400 | is not PIC. This avoids problems on uClinux targets where |
| 6401 | @samp{--emit-relocs} is used to generate relocatable binaries. |
| 6402 | |
| 6403 | @cindex STUB_GROUP_SIZE |
| 6404 | @kindex --stub-group-size=@var{N} |
| 6405 | The linker will automatically generate and insert small sequences of |
| 6406 | code into a linked ARM ELF executable whenever an attempt is made to |
| 6407 | perform a function call to a symbol that is too far away. The |
| 6408 | placement of these sequences of instructions - called stubs - is |
| 6409 | controlled by the command line option @option{--stub-group-size=N}. |
| 6410 | The placement is important because a poor choice can create a need for |
| 6411 | duplicate stubs, increasing the code sizw. The linker will try to |
| 6412 | group stubs together in order to reduce interruptions to the flow of |
| 6413 | code, but it needs guidance as to how big these groups should be and |
| 6414 | where they should be placed. |
| 6415 | |
| 6416 | The value of @samp{N}, the parameter to the |
| 6417 | @option{--stub-group-size=} option controls where the stub groups are |
| 6418 | placed. If it is negative then all stubs are placed after the first |
| 6419 | branch that needs them. If it is positive then the stubs can be |
| 6420 | placed either before or after the branches that need them. If the |
| 6421 | value of @samp{N} is 1 (either +1 or -1) then the linker will choose |
| 6422 | exactly where to place groups of stubs, using its built in heuristics. |
| 6423 | A value of @samp{N} greater than 1 (or smaller than -1) tells the |
| 6424 | linker that a single group of stubs can service at most @samp{N} bytes |
| 6425 | from the input sections. |
| 6426 | |
| 6427 | The default, if @option{--stub-group-size=} is not specified, is |
| 6428 | @samp{N = +1}. |
| 6429 | |
| 6430 | Farcalls stubs insertion is fully supported for the ARM-EABI target |
| 6431 | only, because it relies on object files properties not present |
| 6432 | otherwise. |
| 6433 | |
| 6434 | @ifclear GENERIC |
| 6435 | @lowersections |
| 6436 | @end ifclear |
| 6437 | @end ifset |
| 6438 | |
| 6439 | @ifset HPPA |
| 6440 | @ifclear GENERIC |
| 6441 | @raisesections |
| 6442 | @end ifclear |
| 6443 | |
| 6444 | @node HPPA ELF32 |
| 6445 | @section @command{ld} and HPPA 32-bit ELF Support |
| 6446 | @cindex HPPA multiple sub-space stubs |
| 6447 | @kindex --multi-subspace |
| 6448 | When generating a shared library, @command{ld} will by default generate |
| 6449 | import stubs suitable for use with a single sub-space application. |
| 6450 | The @samp{--multi-subspace} switch causes @command{ld} to generate export |
| 6451 | stubs, and different (larger) import stubs suitable for use with |
| 6452 | multiple sub-spaces. |
| 6453 | |
| 6454 | @cindex HPPA stub grouping |
| 6455 | @kindex --stub-group-size=@var{N} |
| 6456 | Long branch stubs and import/export stubs are placed by @command{ld} in |
| 6457 | stub sections located between groups of input sections. |
| 6458 | @samp{--stub-group-size} specifies the maximum size of a group of input |
| 6459 | sections handled by one stub section. Since branch offsets are signed, |
| 6460 | a stub section may serve two groups of input sections, one group before |
| 6461 | the stub section, and one group after it. However, when using |
| 6462 | conditional branches that require stubs, it may be better (for branch |
| 6463 | prediction) that stub sections only serve one group of input sections. |
| 6464 | A negative value for @samp{N} chooses this scheme, ensuring that |
| 6465 | branches to stubs always use a negative offset. Two special values of |
| 6466 | @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct |
| 6467 | @command{ld} to automatically size input section groups for the branch types |
| 6468 | detected, with the same behaviour regarding stub placement as other |
| 6469 | positive or negative values of @samp{N} respectively. |
| 6470 | |
| 6471 | Note that @samp{--stub-group-size} does not split input sections. A |
| 6472 | single input section larger than the group size specified will of course |
| 6473 | create a larger group (of one section). If input sections are too |
| 6474 | large, it may not be possible for a branch to reach its stub. |
| 6475 | |
| 6476 | @ifclear GENERIC |
| 6477 | @lowersections |
| 6478 | @end ifclear |
| 6479 | @end ifset |
| 6480 | |
| 6481 | @ifset M68K |
| 6482 | @ifclear GENERIC |
| 6483 | @raisesections |
| 6484 | @end ifclear |
| 6485 | |
| 6486 | @node M68K |
| 6487 | @section @command{ld} and the Motorola 68K family |
| 6488 | |
| 6489 | @cindex Motorola 68K GOT generation |
| 6490 | @kindex --got=@var{type} |
| 6491 | The @samp{--got=@var{type}} option lets you choose the GOT generation scheme. |
| 6492 | The choices are @samp{single}, @samp{negative}, @samp{multigot} and |
| 6493 | @samp{target}. When @samp{target} is selected the linker chooses |
| 6494 | the default GOT generation scheme for the current target. |
| 6495 | @samp{single} tells the linker to generate a single GOT with |
| 6496 | entries only at non-negative offsets. |
| 6497 | @samp{negative} instructs the linker to generate a single GOT with |
| 6498 | entries at both negative and positive offsets. Not all environments |
| 6499 | support such GOTs. |
| 6500 | @samp{multigot} allows the linker to generate several GOTs in the |
| 6501 | output file. All GOT references from a single input object |
| 6502 | file access the same GOT, but references from different input object |
| 6503 | files might access different GOTs. Not all environments support such GOTs. |
| 6504 | |
| 6505 | @ifclear GENERIC |
| 6506 | @lowersections |
| 6507 | @end ifclear |
| 6508 | @end ifset |
| 6509 | |
| 6510 | @ifset MMIX |
| 6511 | @ifclear GENERIC |
| 6512 | @raisesections |
| 6513 | @end ifclear |
| 6514 | |
| 6515 | @node MMIX |
| 6516 | @section @code{ld} and MMIX |
| 6517 | For MMIX, there is a choice of generating @code{ELF} object files or |
| 6518 | @code{mmo} object files when linking. The simulator @code{mmix} |
| 6519 | understands the @code{mmo} format. The binutils @code{objcopy} utility |
| 6520 | can translate between the two formats. |
| 6521 | |
| 6522 | There is one special section, the @samp{.MMIX.reg_contents} section. |
| 6523 | Contents in this section is assumed to correspond to that of global |
| 6524 | registers, and symbols referring to it are translated to special symbols, |
| 6525 | equal to registers. In a final link, the start address of the |
| 6526 | @samp{.MMIX.reg_contents} section corresponds to the first allocated |
| 6527 | global register multiplied by 8. Register @code{$255} is not included in |
| 6528 | this section; it is always set to the program entry, which is at the |
| 6529 | symbol @code{Main} for @code{mmo} files. |
| 6530 | |
| 6531 | Global symbols with the prefix @code{__.MMIX.start.}, for example |
| 6532 | @code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special. |
| 6533 | The default linker script uses these to set the default start address |
| 6534 | of a section. |
| 6535 | |
| 6536 | Initial and trailing multiples of zero-valued 32-bit words in a section, |
| 6537 | are left out from an mmo file. |
| 6538 | |
| 6539 | @ifclear GENERIC |
| 6540 | @lowersections |
| 6541 | @end ifclear |
| 6542 | @end ifset |
| 6543 | |
| 6544 | @ifset MSP430 |
| 6545 | @ifclear GENERIC |
| 6546 | @raisesections |
| 6547 | @end ifclear |
| 6548 | |
| 6549 | @node MSP430 |
| 6550 | @section @code{ld} and MSP430 |
| 6551 | For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]} |
| 6552 | will select an appropriate linker script for selected MPU type. (To get a list of known MPUs |
| 6553 | just pass @samp{-m help} option to the linker). |
| 6554 | |
| 6555 | @cindex MSP430 extra sections |
| 6556 | The linker will recognize some extra sections which are MSP430 specific: |
| 6557 | |
| 6558 | @table @code |
| 6559 | @item @samp{.vectors} |
| 6560 | Defines a portion of ROM where interrupt vectors located. |
| 6561 | |
| 6562 | @item @samp{.bootloader} |
| 6563 | Defines the bootloader portion of the ROM (if applicable). Any code |
| 6564 | in this section will be uploaded to the MPU. |
| 6565 | |
| 6566 | @item @samp{.infomem} |
| 6567 | Defines an information memory section (if applicable). Any code in |
| 6568 | this section will be uploaded to the MPU. |
| 6569 | |
| 6570 | @item @samp{.infomemnobits} |
| 6571 | This is the same as the @samp{.infomem} section except that any code |
| 6572 | in this section will not be uploaded to the MPU. |
| 6573 | |
| 6574 | @item @samp{.noinit} |
| 6575 | Denotes a portion of RAM located above @samp{.bss} section. |
| 6576 | |
| 6577 | The last two sections are used by gcc. |
| 6578 | @end table |
| 6579 | |
| 6580 | @ifclear GENERIC |
| 6581 | @lowersections |
| 6582 | @end ifclear |
| 6583 | @end ifset |
| 6584 | |
| 6585 | @ifset POWERPC |
| 6586 | @ifclear GENERIC |
| 6587 | @raisesections |
| 6588 | @end ifclear |
| 6589 | |
| 6590 | @node PowerPC ELF32 |
| 6591 | @section @command{ld} and PowerPC 32-bit ELF Support |
| 6592 | @cindex PowerPC long branches |
| 6593 | @kindex --relax on PowerPC |
| 6594 | Branches on PowerPC processors are limited to a signed 26-bit |
| 6595 | displacement, which may result in @command{ld} giving |
| 6596 | @samp{relocation truncated to fit} errors with very large programs. |
| 6597 | @samp{--relax} enables the generation of trampolines that can access |
| 6598 | the entire 32-bit address space. These trampolines are inserted at |
| 6599 | section boundaries, so may not themselves be reachable if an input |
| 6600 | section exceeds 33M in size. You may combine @samp{-r} and |
| 6601 | @samp{--relax} to add trampolines in a partial link. In that case |
| 6602 | both branches to undefined symbols and inter-section branches are also |
| 6603 | considered potentially out of range, and trampolines inserted. |
| 6604 | |
| 6605 | @cindex PowerPC ELF32 options |
| 6606 | @table @option |
| 6607 | @cindex PowerPC PLT |
| 6608 | @kindex --bss-plt |
| 6609 | @item --bss-plt |
| 6610 | Current PowerPC GCC accepts a @samp{-msecure-plt} option that |
| 6611 | generates code capable of using a newer PLT and GOT layout that has |
| 6612 | the security advantage of no executable section ever needing to be |
| 6613 | writable and no writable section ever being executable. PowerPC |
| 6614 | @command{ld} will generate this layout, including stubs to access the |
| 6615 | PLT, if all input files (including startup and static libraries) were |
| 6616 | compiled with @samp{-msecure-plt}. @samp{--bss-plt} forces the old |
| 6617 | BSS PLT (and GOT layout) which can give slightly better performance. |
| 6618 | |
| 6619 | @kindex --secure-plt |
| 6620 | @item --secure-plt |
| 6621 | @command{ld} will use the new PLT and GOT layout if it is linking new |
| 6622 | @samp{-fpic} or @samp{-fPIC} code, but does not do so automatically |
| 6623 | when linking non-PIC code. This option requests the new PLT and GOT |
| 6624 | layout. A warning will be given if some object file requires the old |
| 6625 | style BSS PLT. |
| 6626 | |
| 6627 | @cindex PowerPC GOT |
| 6628 | @kindex --sdata-got |
| 6629 | @item --sdata-got |
| 6630 | The new secure PLT and GOT are placed differently relative to other |
| 6631 | sections compared to older BSS PLT and GOT placement. The location of |
| 6632 | @code{.plt} must change because the new secure PLT is an initialized |
| 6633 | section while the old PLT is uninitialized. The reason for the |
| 6634 | @code{.got} change is more subtle: The new placement allows |
| 6635 | @code{.got} to be read-only in applications linked with |
| 6636 | @samp{-z relro -z now}. However, this placement means that |
| 6637 | @code{.sdata} cannot always be used in shared libraries, because the |
| 6638 | PowerPC ABI accesses @code{.sdata} in shared libraries from the GOT |
| 6639 | pointer. @samp{--sdata-got} forces the old GOT placement. PowerPC |
| 6640 | GCC doesn't use @code{.sdata} in shared libraries, so this option is |
| 6641 | really only useful for other compilers that may do so. |
| 6642 | |
| 6643 | @cindex PowerPC stub symbols |
| 6644 | @kindex --emit-stub-syms |
| 6645 | @item --emit-stub-syms |
| 6646 | This option causes @command{ld} to label linker stubs with a local |
| 6647 | symbol that encodes the stub type and destination. |
| 6648 | |
| 6649 | @cindex PowerPC TLS optimization |
| 6650 | @kindex --no-tls-optimize |
| 6651 | @item --no-tls-optimize |
| 6652 | PowerPC @command{ld} normally performs some optimization of code |
| 6653 | sequences used to access Thread-Local Storage. Use this option to |
| 6654 | disable the optimization. |
| 6655 | @end table |
| 6656 | |
| 6657 | @ifclear GENERIC |
| 6658 | @lowersections |
| 6659 | @end ifclear |
| 6660 | @end ifset |
| 6661 | |
| 6662 | @ifset POWERPC64 |
| 6663 | @ifclear GENERIC |
| 6664 | @raisesections |
| 6665 | @end ifclear |
| 6666 | |
| 6667 | @node PowerPC64 ELF64 |
| 6668 | @section @command{ld} and PowerPC64 64-bit ELF Support |
| 6669 | |
| 6670 | @cindex PowerPC64 ELF64 options |
| 6671 | @table @option |
| 6672 | @cindex PowerPC64 stub grouping |
| 6673 | @kindex --stub-group-size |
| 6674 | @item --stub-group-size |
| 6675 | Long branch stubs, PLT call stubs and TOC adjusting stubs are placed |
| 6676 | by @command{ld} in stub sections located between groups of input sections. |
| 6677 | @samp{--stub-group-size} specifies the maximum size of a group of input |
| 6678 | sections handled by one stub section. Since branch offsets are signed, |
| 6679 | a stub section may serve two groups of input sections, one group before |
| 6680 | the stub section, and one group after it. However, when using |
| 6681 | conditional branches that require stubs, it may be better (for branch |
| 6682 | prediction) that stub sections only serve one group of input sections. |
| 6683 | A negative value for @samp{N} chooses this scheme, ensuring that |
| 6684 | branches to stubs always use a negative offset. Two special values of |
| 6685 | @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct |
| 6686 | @command{ld} to automatically size input section groups for the branch types |
| 6687 | detected, with the same behaviour regarding stub placement as other |
| 6688 | positive or negative values of @samp{N} respectively. |
| 6689 | |
| 6690 | Note that @samp{--stub-group-size} does not split input sections. A |
| 6691 | single input section larger than the group size specified will of course |
| 6692 | create a larger group (of one section). If input sections are too |
| 6693 | large, it may not be possible for a branch to reach its stub. |
| 6694 | |
| 6695 | @cindex PowerPC64 stub symbols |
| 6696 | @kindex --emit-stub-syms |
| 6697 | @item --emit-stub-syms |
| 6698 | This option causes @command{ld} to label linker stubs with a local |
| 6699 | symbol that encodes the stub type and destination. |
| 6700 | |
| 6701 | @cindex PowerPC64 dot symbols |
| 6702 | @kindex --dotsyms |
| 6703 | @kindex --no-dotsyms |
| 6704 | @item --dotsyms, --no-dotsyms |
| 6705 | These two options control how @command{ld} interprets version patterns |
| 6706 | in a version script. Older PowerPC64 compilers emitted both a |
| 6707 | function descriptor symbol with the same name as the function, and a |
| 6708 | code entry symbol with the name prefixed by a dot (@samp{.}). To |
| 6709 | properly version a function @samp{foo}, the version script thus needs |
| 6710 | to control both @samp{foo} and @samp{.foo}. The option |
| 6711 | @samp{--dotsyms}, on by default, automatically adds the required |
| 6712 | dot-prefixed patterns. Use @samp{--no-dotsyms} to disable this |
| 6713 | feature. |
| 6714 | |
| 6715 | @cindex PowerPC64 TLS optimization |
| 6716 | @kindex --no-tls-optimize |
| 6717 | @item --no-tls-optimize |
| 6718 | PowerPC64 @command{ld} normally performs some optimization of code |
| 6719 | sequences used to access Thread-Local Storage. Use this option to |
| 6720 | disable the optimization. |
| 6721 | |
| 6722 | @cindex PowerPC64 OPD optimization |
| 6723 | @kindex --no-opd-optimize |
| 6724 | @item --no-opd-optimize |
| 6725 | PowerPC64 @command{ld} normally removes @code{.opd} section entries |
| 6726 | corresponding to deleted link-once functions, or functions removed by |
| 6727 | the action of @samp{--gc-sections} or linker script @code{/DISCARD/}. |
| 6728 | Use this option to disable @code{.opd} optimization. |
| 6729 | |
| 6730 | @cindex PowerPC64 OPD spacing |
| 6731 | @kindex --non-overlapping-opd |
| 6732 | @item --non-overlapping-opd |
| 6733 | Some PowerPC64 compilers have an option to generate compressed |
| 6734 | @code{.opd} entries spaced 16 bytes apart, overlapping the third word, |
| 6735 | the static chain pointer (unused in C) with the first word of the next |
| 6736 | entry. This option expands such entries to the full 24 bytes. |
| 6737 | |
| 6738 | @cindex PowerPC64 TOC optimization |
| 6739 | @kindex --no-toc-optimize |
| 6740 | @item --no-toc-optimize |
| 6741 | PowerPC64 @command{ld} normally removes unused @code{.toc} section |
| 6742 | entries. Such entries are detected by examining relocations that |
| 6743 | reference the TOC in code sections. A reloc in a deleted code section |
| 6744 | marks a TOC word as unneeded, while a reloc in a kept code section |
| 6745 | marks a TOC word as needed. Since the TOC may reference itself, TOC |
| 6746 | relocs are also examined. TOC words marked as both needed and |
| 6747 | unneeded will of course be kept. TOC words without any referencing |
| 6748 | reloc are assumed to be part of a multi-word entry, and are kept or |
| 6749 | discarded as per the nearest marked preceding word. This works |
| 6750 | reliably for compiler generated code, but may be incorrect if assembly |
| 6751 | code is used to insert TOC entries. Use this option to disable the |
| 6752 | optimization. |
| 6753 | |
| 6754 | @cindex PowerPC64 multi-TOC |
| 6755 | @kindex --no-multi-toc |
| 6756 | @item --no-multi-toc |
| 6757 | If given any toc option besides @code{-mcmodel=medium} or |
| 6758 | @code{-mcmodel=large}, PowerPC64 GCC generates code for a TOC model |
| 6759 | where TOC |
| 6760 | entries are accessed with a 16-bit offset from r2. This limits the |
| 6761 | total TOC size to 64K. PowerPC64 @command{ld} extends this limit by |
| 6762 | grouping code sections such that each group uses less than 64K for its |
| 6763 | TOC entries, then inserts r2 adjusting stubs between inter-group |
| 6764 | calls. @command{ld} does not split apart input sections, so cannot |
| 6765 | help if a single input file has a @code{.toc} section that exceeds |
| 6766 | 64K, most likely from linking multiple files with @command{ld -r}. |
| 6767 | Use this option to turn off this feature. |
| 6768 | |
| 6769 | @cindex PowerPC64 TOC sorting |
| 6770 | @kindex --no-toc-sort |
| 6771 | @item --no-toc-sort |
| 6772 | By default, @command{ld} sorts TOC sections so that those whose file |
| 6773 | happens to have a section called @code{.init} or @code{.fini} are |
| 6774 | placed first, followed by TOC sections referenced by code generated |
| 6775 | with PowerPC64 gcc's @code{-mcmodel=small}, and lastly TOC sections |
| 6776 | referenced only by code generated with PowerPC64 gcc's |
| 6777 | @code{-mcmodel=medium} or @code{-mcmodel=large} options. Doing this |
| 6778 | results in better TOC grouping for multi-TOC. Use this option to turn |
| 6779 | off this feature. |
| 6780 | |
| 6781 | @cindex PowerPC64 PLT stub alignment |
| 6782 | @kindex --plt-align |
| 6783 | @kindex --no-plt-align |
| 6784 | @item --plt-align |
| 6785 | @itemx --no-plt-align |
| 6786 | Use these options to control whether individual PLT call stubs are |
| 6787 | aligned to a 32-byte boundary, or to the specified power of two |
| 6788 | boundary when using @code{--plt-align=}. By default PLT call stubs |
| 6789 | are packed tightly. |
| 6790 | |
| 6791 | @cindex PowerPC64 PLT call stub static chain |
| 6792 | @kindex --plt-static-chain |
| 6793 | @kindex --no-plt-static-chain |
| 6794 | @item --plt-static-chain |
| 6795 | @itemx --no-plt-static-chain |
| 6796 | Use these options to control whether PLT call stubs load the static |
| 6797 | chain pointer (r11). @code{ld} defaults to not loading the static |
| 6798 | chain since there is never any need to do so on a PLT call. |
| 6799 | |
| 6800 | @cindex PowerPC64 PLT call stub thread safety |
| 6801 | @kindex --plt-thread-safe |
| 6802 | @kindex --no-plt-thread-safe |
| 6803 | @item --plt-thread-safe |
| 6804 | @itemx --no-thread-safe |
| 6805 | With power7's weakly ordered memory model, it is possible when using |
| 6806 | lazy binding for ld.so to update a plt entry in one thread and have |
| 6807 | another thread see the individual plt entry words update in the wrong |
| 6808 | order, despite ld.so carefully writing in the correct order and using |
| 6809 | memory write barriers. To avoid this we need some sort of read |
| 6810 | barrier in the call stub, or use LD_BIND_NOW=1. By default, @code{ld} |
| 6811 | looks for calls to commonly used functions that create threads, and if |
| 6812 | seen, adds the necessary barriers. Use these options to change the |
| 6813 | default behaviour. |
| 6814 | @end table |
| 6815 | |
| 6816 | @ifclear GENERIC |
| 6817 | @lowersections |
| 6818 | @end ifclear |
| 6819 | @end ifset |
| 6820 | |
| 6821 | @ifset SPU |
| 6822 | @ifclear GENERIC |
| 6823 | @raisesections |
| 6824 | @end ifclear |
| 6825 | |
| 6826 | @node SPU ELF |
| 6827 | @section @command{ld} and SPU ELF Support |
| 6828 | |
| 6829 | @cindex SPU ELF options |
| 6830 | @table @option |
| 6831 | |
| 6832 | @cindex SPU plugins |
| 6833 | @kindex --plugin |
| 6834 | @item --plugin |
| 6835 | This option marks an executable as a PIC plugin module. |
| 6836 | |
| 6837 | @cindex SPU overlays |
| 6838 | @kindex --no-overlays |
| 6839 | @item --no-overlays |
| 6840 | Normally, @command{ld} recognizes calls to functions within overlay |
| 6841 | regions, and redirects such calls to an overlay manager via a stub. |
| 6842 | @command{ld} also provides a built-in overlay manager. This option |
| 6843 | turns off all this special overlay handling. |
| 6844 | |
| 6845 | @cindex SPU overlay stub symbols |
| 6846 | @kindex --emit-stub-syms |
| 6847 | @item --emit-stub-syms |
| 6848 | This option causes @command{ld} to label overlay stubs with a local |
| 6849 | symbol that encodes the stub type and destination. |
| 6850 | |
| 6851 | @cindex SPU extra overlay stubs |
| 6852 | @kindex --extra-overlay-stubs |
| 6853 | @item --extra-overlay-stubs |
| 6854 | This option causes @command{ld} to add overlay call stubs on all |
| 6855 | function calls out of overlay regions. Normally stubs are not added |
| 6856 | on calls to non-overlay regions. |
| 6857 | |
| 6858 | @cindex SPU local store size |
| 6859 | @kindex --local-store=lo:hi |
| 6860 | @item --local-store=lo:hi |
| 6861 | @command{ld} usually checks that a final executable for SPU fits in |
| 6862 | the address range 0 to 256k. This option may be used to change the |
| 6863 | range. Disable the check entirely with @option{--local-store=0:0}. |
| 6864 | |
| 6865 | @cindex SPU |
| 6866 | @kindex --stack-analysis |
| 6867 | @item --stack-analysis |
| 6868 | SPU local store space is limited. Over-allocation of stack space |
| 6869 | unnecessarily limits space available for code and data, while |
| 6870 | under-allocation results in runtime failures. If given this option, |
| 6871 | @command{ld} will provide an estimate of maximum stack usage. |
| 6872 | @command{ld} does this by examining symbols in code sections to |
| 6873 | determine the extents of functions, and looking at function prologues |
| 6874 | for stack adjusting instructions. A call-graph is created by looking |
| 6875 | for relocations on branch instructions. The graph is then searched |
| 6876 | for the maximum stack usage path. Note that this analysis does not |
| 6877 | find calls made via function pointers, and does not handle recursion |
| 6878 | and other cycles in the call graph. Stack usage may be |
| 6879 | under-estimated if your code makes such calls. Also, stack usage for |
| 6880 | dynamic allocation, e.g. alloca, will not be detected. If a link map |
| 6881 | is requested, detailed information about each function's stack usage |
| 6882 | and calls will be given. |
| 6883 | |
| 6884 | @cindex SPU |
| 6885 | @kindex --emit-stack-syms |
| 6886 | @item --emit-stack-syms |
| 6887 | This option, if given along with @option{--stack-analysis} will result |
| 6888 | in @command{ld} emitting stack sizing symbols for each function. |
| 6889 | These take the form @code{__stack_<function_name>} for global |
| 6890 | functions, and @code{__stack_<number>_<function_name>} for static |
| 6891 | functions. @code{<number>} is the section id in hex. The value of |
| 6892 | such symbols is the stack requirement for the corresponding function. |
| 6893 | The symbol size will be zero, type @code{STT_NOTYPE}, binding |
| 6894 | @code{STB_LOCAL}, and section @code{SHN_ABS}. |
| 6895 | @end table |
| 6896 | |
| 6897 | @ifclear GENERIC |
| 6898 | @lowersections |
| 6899 | @end ifclear |
| 6900 | @end ifset |
| 6901 | |
| 6902 | @ifset TICOFF |
| 6903 | @ifclear GENERIC |
| 6904 | @raisesections |
| 6905 | @end ifclear |
| 6906 | |
| 6907 | @node TI COFF |
| 6908 | @section @command{ld}'s Support for Various TI COFF Versions |
| 6909 | @cindex TI COFF versions |
| 6910 | @kindex --format=@var{version} |
| 6911 | The @samp{--format} switch allows selection of one of the various |
| 6912 | TI COFF versions. The latest of this writing is 2; versions 0 and 1 are |
| 6913 | also supported. The TI COFF versions also vary in header byte-order |
| 6914 | format; @command{ld} will read any version or byte order, but the output |
| 6915 | header format depends on the default specified by the specific target. |
| 6916 | |
| 6917 | @ifclear GENERIC |
| 6918 | @lowersections |
| 6919 | @end ifclear |
| 6920 | @end ifset |
| 6921 | |
| 6922 | @ifset WIN32 |
| 6923 | @ifclear GENERIC |
| 6924 | @raisesections |
| 6925 | @end ifclear |
| 6926 | |
| 6927 | @node WIN32 |
| 6928 | @section @command{ld} and WIN32 (cygwin/mingw) |
| 6929 | |
| 6930 | This section describes some of the win32 specific @command{ld} issues. |
| 6931 | See @ref{Options,,Command Line Options} for detailed description of the |
| 6932 | command line options mentioned here. |
| 6933 | |
| 6934 | @table @emph |
| 6935 | @cindex import libraries |
| 6936 | @item import libraries |
| 6937 | The standard Windows linker creates and uses so-called import |
| 6938 | libraries, which contains information for linking to dll's. They are |
| 6939 | regular static archives and are handled as any other static |
| 6940 | archive. The cygwin and mingw ports of @command{ld} have specific |
| 6941 | support for creating such libraries provided with the |
| 6942 | @samp{--out-implib} command line option. |
| 6943 | |
| 6944 | @item exporting DLL symbols |
| 6945 | @cindex exporting DLL symbols |
| 6946 | The cygwin/mingw @command{ld} has several ways to export symbols for dll's. |
| 6947 | |
| 6948 | @table @emph |
| 6949 | @item using auto-export functionality |
| 6950 | @cindex using auto-export functionality |
| 6951 | By default @command{ld} exports symbols with the auto-export functionality, |
| 6952 | which is controlled by the following command line options: |
| 6953 | |
| 6954 | @itemize |
| 6955 | @item --export-all-symbols [This is the default] |
| 6956 | @item --exclude-symbols |
| 6957 | @item --exclude-libs |
| 6958 | @item --exclude-modules-for-implib |
| 6959 | @item --version-script |
| 6960 | @end itemize |
| 6961 | |
| 6962 | When auto-export is in operation, @command{ld} will export all the non-local |
| 6963 | (global and common) symbols it finds in a DLL, with the exception of a few |
| 6964 | symbols known to belong to the system's runtime and libraries. As it will |
| 6965 | often not be desirable to export all of a DLL's symbols, which may include |
| 6966 | private functions that are not part of any public interface, the command-line |
| 6967 | options listed above may be used to filter symbols out from the list for |
| 6968 | exporting. The @samp{--output-def} option can be used in order to see the |
| 6969 | final list of exported symbols with all exclusions taken into effect. |
| 6970 | |
| 6971 | If @samp{--export-all-symbols} is not given explicitly on the |
| 6972 | command line, then the default auto-export behavior will be @emph{disabled} |
| 6973 | if either of the following are true: |
| 6974 | |
| 6975 | @itemize |
| 6976 | @item A DEF file is used. |
| 6977 | @item Any symbol in any object file was marked with the __declspec(dllexport) attribute. |
| 6978 | @end itemize |
| 6979 | |
| 6980 | @item using a DEF file |
| 6981 | @cindex using a DEF file |
| 6982 | Another way of exporting symbols is using a DEF file. A DEF file is |
| 6983 | an ASCII file containing definitions of symbols which should be |
| 6984 | exported when a dll is created. Usually it is named @samp{<dll |
| 6985 | name>.def} and is added as any other object file to the linker's |
| 6986 | command line. The file's name must end in @samp{.def} or @samp{.DEF}. |
| 6987 | |
| 6988 | @example |
| 6989 | gcc -o <output> <objectfiles> <dll name>.def |
| 6990 | @end example |
| 6991 | |
| 6992 | Using a DEF file turns off the normal auto-export behavior, unless the |
| 6993 | @samp{--export-all-symbols} option is also used. |
| 6994 | |
| 6995 | Here is an example of a DEF file for a shared library called @samp{xyz.dll}: |
| 6996 | |
| 6997 | @example |
| 6998 | LIBRARY "xyz.dll" BASE=0x20000000 |
| 6999 | |
| 7000 | EXPORTS |
| 7001 | foo |
| 7002 | bar |
| 7003 | _bar = bar |
| 7004 | another_foo = abc.dll.afoo |
| 7005 | var1 DATA |
| 7006 | doo = foo == foo2 |
| 7007 | eoo DATA == var1 |
| 7008 | @end example |
| 7009 | |
| 7010 | This example defines a DLL with a non-default base address and seven |
| 7011 | symbols in the export table. The third exported symbol @code{_bar} is an |
| 7012 | alias for the second. The fourth symbol, @code{another_foo} is resolved |
| 7013 | by "forwarding" to another module and treating it as an alias for |
| 7014 | @code{afoo} exported from the DLL @samp{abc.dll}. The final symbol |
| 7015 | @code{var1} is declared to be a data object. The @samp{doo} symbol in |
| 7016 | export library is an alias of @samp{foo}, which gets the string name |
| 7017 | in export table @samp{foo2}. The @samp{eoo} symbol is an data export |
| 7018 | symbol, which gets in export table the name @samp{var1}. |
| 7019 | |
| 7020 | The optional @code{LIBRARY <name>} command indicates the @emph{internal} |
| 7021 | name of the output DLL. If @samp{<name>} does not include a suffix, |
| 7022 | the default library suffix, @samp{.DLL} is appended. |
| 7023 | |
| 7024 | When the .DEF file is used to build an application, rather than a |
| 7025 | library, the @code{NAME <name>} command should be used instead of |
| 7026 | @code{LIBRARY}. If @samp{<name>} does not include a suffix, the default |
| 7027 | executable suffix, @samp{.EXE} is appended. |
| 7028 | |
| 7029 | With either @code{LIBRARY <name>} or @code{NAME <name>} the optional |
| 7030 | specification @code{BASE = <number>} may be used to specify a |
| 7031 | non-default base address for the image. |
| 7032 | |
| 7033 | If neither @code{LIBRARY <name>} nor @code{NAME <name>} is specified, |
| 7034 | or they specify an empty string, the internal name is the same as the |
| 7035 | filename specified on the command line. |
| 7036 | |
| 7037 | The complete specification of an export symbol is: |
| 7038 | |
| 7039 | @example |
| 7040 | EXPORTS |
| 7041 | ( ( ( <name1> [ = <name2> ] ) |
| 7042 | | ( <name1> = <module-name> . <external-name>)) |
| 7043 | [ @@ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) * |
| 7044 | @end example |
| 7045 | |
| 7046 | Declares @samp{<name1>} as an exported symbol from the DLL, or declares |
| 7047 | @samp{<name1>} as an exported alias for @samp{<name2>}; or declares |
| 7048 | @samp{<name1>} as a "forward" alias for the symbol |
| 7049 | @samp{<external-name>} in the DLL @samp{<module-name>}. |
| 7050 | Optionally, the symbol may be exported by the specified ordinal |
| 7051 | @samp{<integer>} alias. The optional @samp{<name3>} is the to be used |
| 7052 | string in import/export table for the symbol. |
| 7053 | |
| 7054 | The optional keywords that follow the declaration indicate: |
| 7055 | |
| 7056 | @code{NONAME}: Do not put the symbol name in the DLL's export table. It |
| 7057 | will still be exported by its ordinal alias (either the value specified |
| 7058 | by the .def specification or, otherwise, the value assigned by the |
| 7059 | linker). The symbol name, however, does remain visible in the import |
| 7060 | library (if any), unless @code{PRIVATE} is also specified. |
| 7061 | |
| 7062 | @code{DATA}: The symbol is a variable or object, rather than a function. |
| 7063 | The import lib will export only an indirect reference to @code{foo} as |
| 7064 | the symbol @code{_imp__foo} (ie, @code{foo} must be resolved as |
| 7065 | @code{*_imp__foo}). |
| 7066 | |
| 7067 | @code{CONSTANT}: Like @code{DATA}, but put the undecorated @code{foo} as |
| 7068 | well as @code{_imp__foo} into the import library. Both refer to the |
| 7069 | read-only import address table's pointer to the variable, not to the |
| 7070 | variable itself. This can be dangerous. If the user code fails to add |
| 7071 | the @code{dllimport} attribute and also fails to explicitly add the |
| 7072 | extra indirection that the use of the attribute enforces, the |
| 7073 | application will behave unexpectedly. |
| 7074 | |
| 7075 | @code{PRIVATE}: Put the symbol in the DLL's export table, but do not put |
| 7076 | it into the static import library used to resolve imports at link time. The |
| 7077 | symbol can still be imported using the @code{LoadLibrary/GetProcAddress} |
| 7078 | API at runtime or by by using the GNU ld extension of linking directly to |
| 7079 | the DLL without an import library. |
| 7080 | |
| 7081 | See ld/deffilep.y in the binutils sources for the full specification of |
| 7082 | other DEF file statements |
| 7083 | |
| 7084 | @cindex creating a DEF file |
| 7085 | While linking a shared dll, @command{ld} is able to create a DEF file |
| 7086 | with the @samp{--output-def <file>} command line option. |
| 7087 | |
| 7088 | @item Using decorations |
| 7089 | @cindex Using decorations |
| 7090 | Another way of marking symbols for export is to modify the source code |
| 7091 | itself, so that when building the DLL each symbol to be exported is |
| 7092 | declared as: |
| 7093 | |
| 7094 | @example |
| 7095 | __declspec(dllexport) int a_variable |
| 7096 | __declspec(dllexport) void a_function(int with_args) |
| 7097 | @end example |
| 7098 | |
| 7099 | All such symbols will be exported from the DLL. If, however, |
| 7100 | any of the object files in the DLL contain symbols decorated in |
| 7101 | this way, then the normal auto-export behavior is disabled, unless |
| 7102 | the @samp{--export-all-symbols} option is also used. |
| 7103 | |
| 7104 | Note that object files that wish to access these symbols must @emph{not} |
| 7105 | decorate them with dllexport. Instead, they should use dllimport, |
| 7106 | instead: |
| 7107 | |
| 7108 | @example |
| 7109 | __declspec(dllimport) int a_variable |
| 7110 | __declspec(dllimport) void a_function(int with_args) |
| 7111 | @end example |
| 7112 | |
| 7113 | This complicates the structure of library header files, because |
| 7114 | when included by the library itself the header must declare the |
| 7115 | variables and functions as dllexport, but when included by client |
| 7116 | code the header must declare them as dllimport. There are a number |
| 7117 | of idioms that are typically used to do this; often client code can |
| 7118 | omit the __declspec() declaration completely. See |
| 7119 | @samp{--enable-auto-import} and @samp{automatic data imports} for more |
| 7120 | information. |
| 7121 | @end table |
| 7122 | |
| 7123 | @cindex automatic data imports |
| 7124 | @item automatic data imports |
| 7125 | The standard Windows dll format supports data imports from dlls only |
| 7126 | by adding special decorations (dllimport/dllexport), which let the |
| 7127 | compiler produce specific assembler instructions to deal with this |
| 7128 | issue. This increases the effort necessary to port existing Un*x |
| 7129 | code to these platforms, especially for large |
| 7130 | c++ libraries and applications. The auto-import feature, which was |
| 7131 | initially provided by Paul Sokolovsky, allows one to omit the |
| 7132 | decorations to achieve a behavior that conforms to that on POSIX/Un*x |
| 7133 | platforms. This feature is enabled with the @samp{--enable-auto-import} |
| 7134 | command-line option, although it is enabled by default on cygwin/mingw. |
| 7135 | The @samp{--enable-auto-import} option itself now serves mainly to |
| 7136 | suppress any warnings that are ordinarily emitted when linked objects |
| 7137 | trigger the feature's use. |
| 7138 | |
| 7139 | auto-import of variables does not always work flawlessly without |
| 7140 | additional assistance. Sometimes, you will see this message |
| 7141 | |
| 7142 | "variable '<var>' can't be auto-imported. Please read the |
| 7143 | documentation for ld's @code{--enable-auto-import} for details." |
| 7144 | |
| 7145 | The @samp{--enable-auto-import} documentation explains why this error |
| 7146 | occurs, and several methods that can be used to overcome this difficulty. |
| 7147 | One of these methods is the @emph{runtime pseudo-relocs} feature, described |
| 7148 | below. |
| 7149 | |
| 7150 | @cindex runtime pseudo-relocation |
| 7151 | For complex variables imported from DLLs (such as structs or classes), |
| 7152 | object files typically contain a base address for the variable and an |
| 7153 | offset (@emph{addend}) within the variable--to specify a particular |
| 7154 | field or public member, for instance. Unfortunately, the runtime loader used |
| 7155 | in win32 environments is incapable of fixing these references at runtime |
| 7156 | without the additional information supplied by dllimport/dllexport decorations. |
| 7157 | The standard auto-import feature described above is unable to resolve these |
| 7158 | references. |
| 7159 | |
| 7160 | The @samp{--enable-runtime-pseudo-relocs} switch allows these references to |
| 7161 | be resolved without error, while leaving the task of adjusting the references |
| 7162 | themselves (with their non-zero addends) to specialized code provided by the |
| 7163 | runtime environment. Recent versions of the cygwin and mingw environments and |
| 7164 | compilers provide this runtime support; older versions do not. However, the |
| 7165 | support is only necessary on the developer's platform; the compiled result will |
| 7166 | run without error on an older system. |
| 7167 | |
| 7168 | @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly |
| 7169 | enabled as needed. |
| 7170 | |
| 7171 | @cindex direct linking to a dll |
| 7172 | @item direct linking to a dll |
| 7173 | The cygwin/mingw ports of @command{ld} support the direct linking, |
| 7174 | including data symbols, to a dll without the usage of any import |
| 7175 | libraries. This is much faster and uses much less memory than does the |
| 7176 | traditional import library method, especially when linking large |
| 7177 | libraries or applications. When @command{ld} creates an import lib, each |
| 7178 | function or variable exported from the dll is stored in its own bfd, even |
| 7179 | though a single bfd could contain many exports. The overhead involved in |
| 7180 | storing, loading, and processing so many bfd's is quite large, and explains the |
| 7181 | tremendous time, memory, and storage needed to link against particularly |
| 7182 | large or complex libraries when using import libs. |
| 7183 | |
| 7184 | Linking directly to a dll uses no extra command-line switches other than |
| 7185 | @samp{-L} and @samp{-l}, because @command{ld} already searches for a number |
| 7186 | of names to match each library. All that is needed from the developer's |
| 7187 | perspective is an understanding of this search, in order to force ld to |
| 7188 | select the dll instead of an import library. |
| 7189 | |
| 7190 | |
| 7191 | For instance, when ld is called with the argument @samp{-lxxx} it will attempt |
| 7192 | to find, in the first directory of its search path, |
| 7193 | |
| 7194 | @example |
| 7195 | libxxx.dll.a |
| 7196 | xxx.dll.a |
| 7197 | libxxx.a |
| 7198 | xxx.lib |
| 7199 | cygxxx.dll (*) |
| 7200 | libxxx.dll |
| 7201 | xxx.dll |
| 7202 | @end example |
| 7203 | |
| 7204 | before moving on to the next directory in the search path. |
| 7205 | |
| 7206 | (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll}, |
| 7207 | where @samp{<prefix>} is set by the @command{ld} option |
| 7208 | @samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec |
| 7209 | file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for |
| 7210 | @samp{cygxxx.dll}. |
| 7211 | |
| 7212 | Other win32-based unix environments, such as mingw or pw32, may use other |
| 7213 | @samp{<prefix>}es, although at present only cygwin makes use of this feature. It |
| 7214 | was originally intended to help avoid name conflicts among dll's built for the |
| 7215 | various win32/un*x environments, so that (for example) two versions of a zlib dll |
| 7216 | could coexist on the same machine. |
| 7217 | |
| 7218 | The generic cygwin/mingw path layout uses a @samp{bin} directory for |
| 7219 | applications and dll's and a @samp{lib} directory for the import |
| 7220 | libraries (using cygwin nomenclature): |
| 7221 | |
| 7222 | @example |
| 7223 | bin/ |
| 7224 | cygxxx.dll |
| 7225 | lib/ |
| 7226 | libxxx.dll.a (in case of dll's) |
| 7227 | libxxx.a (in case of static archive) |
| 7228 | @end example |
| 7229 | |
| 7230 | Linking directly to a dll without using the import library can be |
| 7231 | done two ways: |
| 7232 | |
| 7233 | 1. Use the dll directly by adding the @samp{bin} path to the link line |
| 7234 | @example |
| 7235 | gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx |
| 7236 | @end example |
| 7237 | |
| 7238 | However, as the dll's often have version numbers appended to their names |
| 7239 | (@samp{cygncurses-5.dll}) this will often fail, unless one specifies |
| 7240 | @samp{-L../bin -lncurses-5} to include the version. Import libs are generally |
| 7241 | not versioned, and do not have this difficulty. |
| 7242 | |
| 7243 | 2. Create a symbolic link from the dll to a file in the @samp{lib} |
| 7244 | directory according to the above mentioned search pattern. This |
| 7245 | should be used to avoid unwanted changes in the tools needed for |
| 7246 | making the app/dll. |
| 7247 | |
| 7248 | @example |
| 7249 | ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a] |
| 7250 | @end example |
| 7251 | |
| 7252 | Then you can link without any make environment changes. |
| 7253 | |
| 7254 | @example |
| 7255 | gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx |
| 7256 | @end example |
| 7257 | |
| 7258 | This technique also avoids the version number problems, because the following is |
| 7259 | perfectly legal |
| 7260 | |
| 7261 | @example |
| 7262 | bin/ |
| 7263 | cygxxx-5.dll |
| 7264 | lib/ |
| 7265 | libxxx.dll.a -> ../bin/cygxxx-5.dll |
| 7266 | @end example |
| 7267 | |
| 7268 | Linking directly to a dll without using an import lib will work |
| 7269 | even when auto-import features are exercised, and even when |
| 7270 | @samp{--enable-runtime-pseudo-relocs} is used. |
| 7271 | |
| 7272 | Given the improvements in speed and memory usage, one might justifiably |
| 7273 | wonder why import libraries are used at all. There are three reasons: |
| 7274 | |
| 7275 | 1. Until recently, the link-directly-to-dll functionality did @emph{not} |
| 7276 | work with auto-imported data. |
| 7277 | |
| 7278 | 2. Sometimes it is necessary to include pure static objects within the |
| 7279 | import library (which otherwise contains only bfd's for indirection |
| 7280 | symbols that point to the exports of a dll). Again, the import lib |
| 7281 | for the cygwin kernel makes use of this ability, and it is not |
| 7282 | possible to do this without an import lib. |
| 7283 | |
| 7284 | 3. Symbol aliases can only be resolved using an import lib. This is |
| 7285 | critical when linking against OS-supplied dll's (eg, the win32 API) |
| 7286 | in which symbols are usually exported as undecorated aliases of their |
| 7287 | stdcall-decorated assembly names. |
| 7288 | |
| 7289 | So, import libs are not going away. But the ability to replace |
| 7290 | true import libs with a simple symbolic link to (or a copy of) |
| 7291 | a dll, in many cases, is a useful addition to the suite of tools |
| 7292 | binutils makes available to the win32 developer. Given the |
| 7293 | massive improvements in memory requirements during linking, storage |
| 7294 | requirements, and linking speed, we expect that many developers |
| 7295 | will soon begin to use this feature whenever possible. |
| 7296 | |
| 7297 | @item symbol aliasing |
| 7298 | @table @emph |
| 7299 | @item adding additional names |
| 7300 | Sometimes, it is useful to export symbols with additional names. |
| 7301 | A symbol @samp{foo} will be exported as @samp{foo}, but it can also be |
| 7302 | exported as @samp{_foo} by using special directives in the DEF file |
| 7303 | when creating the dll. This will affect also the optional created |
| 7304 | import library. Consider the following DEF file: |
| 7305 | |
| 7306 | @example |
| 7307 | LIBRARY "xyz.dll" BASE=0x61000000 |
| 7308 | |
| 7309 | EXPORTS |
| 7310 | foo |
| 7311 | _foo = foo |
| 7312 | @end example |
| 7313 | |
| 7314 | The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}. |
| 7315 | |
| 7316 | Another method for creating a symbol alias is to create it in the |
| 7317 | source code using the "weak" attribute: |
| 7318 | |
| 7319 | @example |
| 7320 | void foo () @{ /* Do something. */; @} |
| 7321 | void _foo () __attribute__ ((weak, alias ("foo"))); |
| 7322 | @end example |
| 7323 | |
| 7324 | See the gcc manual for more information about attributes and weak |
| 7325 | symbols. |
| 7326 | |
| 7327 | @item renaming symbols |
| 7328 | Sometimes it is useful to rename exports. For instance, the cygwin |
| 7329 | kernel does this regularly. A symbol @samp{_foo} can be exported as |
| 7330 | @samp{foo} but not as @samp{_foo} by using special directives in the |
| 7331 | DEF file. (This will also affect the import library, if it is |
| 7332 | created). In the following example: |
| 7333 | |
| 7334 | @example |
| 7335 | LIBRARY "xyz.dll" BASE=0x61000000 |
| 7336 | |
| 7337 | EXPORTS |
| 7338 | _foo = foo |
| 7339 | @end example |
| 7340 | |
| 7341 | The line @samp{_foo = foo} maps the exported symbol @samp{foo} to |
| 7342 | @samp{_foo}. |
| 7343 | @end table |
| 7344 | |
| 7345 | Note: using a DEF file disables the default auto-export behavior, |
| 7346 | unless the @samp{--export-all-symbols} command line option is used. |
| 7347 | If, however, you are trying to rename symbols, then you should list |
| 7348 | @emph{all} desired exports in the DEF file, including the symbols |
| 7349 | that are not being renamed, and do @emph{not} use the |
| 7350 | @samp{--export-all-symbols} option. If you list only the |
| 7351 | renamed symbols in the DEF file, and use @samp{--export-all-symbols} |
| 7352 | to handle the other symbols, then the both the new names @emph{and} |
| 7353 | the original names for the renamed symbols will be exported. |
| 7354 | In effect, you'd be aliasing those symbols, not renaming them, |
| 7355 | which is probably not what you wanted. |
| 7356 | |
| 7357 | @cindex weak externals |
| 7358 | @item weak externals |
| 7359 | The Windows object format, PE, specifies a form of weak symbols called |
| 7360 | weak externals. When a weak symbol is linked and the symbol is not |
| 7361 | defined, the weak symbol becomes an alias for some other symbol. There |
| 7362 | are three variants of weak externals: |
| 7363 | @itemize |
| 7364 | @item Definition is searched for in objects and libraries, historically |
| 7365 | called lazy externals. |
| 7366 | @item Definition is searched for only in other objects, not in libraries. |
| 7367 | This form is not presently implemented. |
| 7368 | @item No search; the symbol is an alias. This form is not presently |
| 7369 | implemented. |
| 7370 | @end itemize |
| 7371 | As a GNU extension, weak symbols that do not specify an alternate symbol |
| 7372 | are supported. If the symbol is undefined when linking, the symbol |
| 7373 | uses a default value. |
| 7374 | |
| 7375 | @cindex aligned common symbols |
| 7376 | @item aligned common symbols |
| 7377 | As a GNU extension to the PE file format, it is possible to specify the |
| 7378 | desired alignment for a common symbol. This information is conveyed from |
| 7379 | the assembler or compiler to the linker by means of GNU-specific commands |
| 7380 | carried in the object file's @samp{.drectve} section, which are recognized |
| 7381 | by @command{ld} and respected when laying out the common symbols. Native |
| 7382 | tools will be able to process object files employing this GNU extension, |
| 7383 | but will fail to respect the alignment instructions, and may issue noisy |
| 7384 | warnings about unknown linker directives. |
| 7385 | @end table |
| 7386 | |
| 7387 | @ifclear GENERIC |
| 7388 | @lowersections |
| 7389 | @end ifclear |
| 7390 | @end ifset |
| 7391 | |
| 7392 | @ifset XTENSA |
| 7393 | @ifclear GENERIC |
| 7394 | @raisesections |
| 7395 | @end ifclear |
| 7396 | |
| 7397 | @node Xtensa |
| 7398 | @section @code{ld} and Xtensa Processors |
| 7399 | |
| 7400 | @cindex Xtensa processors |
| 7401 | The default @command{ld} behavior for Xtensa processors is to interpret |
| 7402 | @code{SECTIONS} commands so that lists of explicitly named sections in a |
| 7403 | specification with a wildcard file will be interleaved when necessary to |
| 7404 | keep literal pools within the range of PC-relative load offsets. For |
| 7405 | example, with the command: |
| 7406 | |
| 7407 | @smallexample |
| 7408 | SECTIONS |
| 7409 | @{ |
| 7410 | .text : @{ |
| 7411 | *(.literal .text) |
| 7412 | @} |
| 7413 | @} |
| 7414 | @end smallexample |
| 7415 | |
| 7416 | @noindent |
| 7417 | @command{ld} may interleave some of the @code{.literal} |
| 7418 | and @code{.text} sections from different object files to ensure that the |
| 7419 | literal pools are within the range of PC-relative load offsets. A valid |
| 7420 | interleaving might place the @code{.literal} sections from an initial |
| 7421 | group of files followed by the @code{.text} sections of that group of |
| 7422 | files. Then, the @code{.literal} sections from the rest of the files |
| 7423 | and the @code{.text} sections from the rest of the files would follow. |
| 7424 | |
| 7425 | @cindex @option{--relax} on Xtensa |
| 7426 | @cindex relaxing on Xtensa |
| 7427 | Relaxation is enabled by default for the Xtensa version of @command{ld} and |
| 7428 | provides two important link-time optimizations. The first optimization |
| 7429 | is to combine identical literal values to reduce code size. A redundant |
| 7430 | literal will be removed and all the @code{L32R} instructions that use it |
| 7431 | will be changed to reference an identical literal, as long as the |
| 7432 | location of the replacement literal is within the offset range of all |
| 7433 | the @code{L32R} instructions. The second optimization is to remove |
| 7434 | unnecessary overhead from assembler-generated ``longcall'' sequences of |
| 7435 | @code{L32R}/@code{CALLX@var{n}} when the target functions are within |
| 7436 | range of direct @code{CALL@var{n}} instructions. |
| 7437 | |
| 7438 | For each of these cases where an indirect call sequence can be optimized |
| 7439 | to a direct call, the linker will change the @code{CALLX@var{n}} |
| 7440 | instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R} |
| 7441 | instruction, and remove the literal referenced by the @code{L32R} |
| 7442 | instruction if it is not used for anything else. Removing the |
| 7443 | @code{L32R} instruction always reduces code size but can potentially |
| 7444 | hurt performance by changing the alignment of subsequent branch targets. |
| 7445 | By default, the linker will always preserve alignments, either by |
| 7446 | switching some instructions between 24-bit encodings and the equivalent |
| 7447 | density instructions or by inserting a no-op in place of the @code{L32R} |
| 7448 | instruction that was removed. If code size is more important than |
| 7449 | performance, the @option{--size-opt} option can be used to prevent the |
| 7450 | linker from widening density instructions or inserting no-ops, except in |
| 7451 | a few cases where no-ops are required for correctness. |
| 7452 | |
| 7453 | The following Xtensa-specific command-line options can be used to |
| 7454 | control the linker: |
| 7455 | |
| 7456 | @cindex Xtensa options |
| 7457 | @table @option |
| 7458 | @item --size-opt |
| 7459 | When optimizing indirect calls to direct calls, optimize for code size |
| 7460 | more than performance. With this option, the linker will not insert |
| 7461 | no-ops or widen density instructions to preserve branch target |
| 7462 | alignment. There may still be some cases where no-ops are required to |
| 7463 | preserve the correctness of the code. |
| 7464 | @end table |
| 7465 | |
| 7466 | @ifclear GENERIC |
| 7467 | @lowersections |
| 7468 | @end ifclear |
| 7469 | @end ifset |
| 7470 | |
| 7471 | @ifclear SingleFormat |
| 7472 | @node BFD |
| 7473 | @chapter BFD |
| 7474 | |
| 7475 | @cindex back end |
| 7476 | @cindex object file management |
| 7477 | @cindex object formats available |
| 7478 | @kindex objdump -i |
| 7479 | The linker accesses object and archive files using the BFD libraries. |
| 7480 | These libraries allow the linker to use the same routines to operate on |
| 7481 | object files whatever the object file format. A different object file |
| 7482 | format can be supported simply by creating a new BFD back end and adding |
| 7483 | it to the library. To conserve runtime memory, however, the linker and |
| 7484 | associated tools are usually configured to support only a subset of the |
| 7485 | object file formats available. You can use @code{objdump -i} |
| 7486 | (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to |
| 7487 | list all the formats available for your configuration. |
| 7488 | |
| 7489 | @cindex BFD requirements |
| 7490 | @cindex requirements for BFD |
| 7491 | As with most implementations, BFD is a compromise between |
| 7492 | several conflicting requirements. The major factor influencing |
| 7493 | BFD design was efficiency: any time used converting between |
| 7494 | formats is time which would not have been spent had BFD not |
| 7495 | been involved. This is partly offset by abstraction payback; since |
| 7496 | BFD simplifies applications and back ends, more time and care |
| 7497 | may be spent optimizing algorithms for a greater speed. |
| 7498 | |
| 7499 | One minor artifact of the BFD solution which you should bear in |
| 7500 | mind is the potential for information loss. There are two places where |
| 7501 | useful information can be lost using the BFD mechanism: during |
| 7502 | conversion and during output. @xref{BFD information loss}. |
| 7503 | |
| 7504 | @menu |
| 7505 | * BFD outline:: How it works: an outline of BFD |
| 7506 | @end menu |
| 7507 | |
| 7508 | @node BFD outline |
| 7509 | @section How It Works: An Outline of BFD |
| 7510 | @cindex opening object files |
| 7511 | @include bfdsumm.texi |
| 7512 | @end ifclear |
| 7513 | |
| 7514 | @node Reporting Bugs |
| 7515 | @chapter Reporting Bugs |
| 7516 | @cindex bugs in @command{ld} |
| 7517 | @cindex reporting bugs in @command{ld} |
| 7518 | |
| 7519 | Your bug reports play an essential role in making @command{ld} reliable. |
| 7520 | |
| 7521 | Reporting a bug may help you by bringing a solution to your problem, or |
| 7522 | it may not. But in any case the principal function of a bug report is |
| 7523 | to help the entire community by making the next version of @command{ld} |
| 7524 | work better. Bug reports are your contribution to the maintenance of |
| 7525 | @command{ld}. |
| 7526 | |
| 7527 | In order for a bug report to serve its purpose, you must include the |
| 7528 | information that enables us to fix the bug. |
| 7529 | |
| 7530 | @menu |
| 7531 | * Bug Criteria:: Have you found a bug? |
| 7532 | * Bug Reporting:: How to report bugs |
| 7533 | @end menu |
| 7534 | |
| 7535 | @node Bug Criteria |
| 7536 | @section Have You Found a Bug? |
| 7537 | @cindex bug criteria |
| 7538 | |
| 7539 | If you are not sure whether you have found a bug, here are some guidelines: |
| 7540 | |
| 7541 | @itemize @bullet |
| 7542 | @cindex fatal signal |
| 7543 | @cindex linker crash |
| 7544 | @cindex crash of linker |
| 7545 | @item |
| 7546 | If the linker gets a fatal signal, for any input whatever, that is a |
| 7547 | @command{ld} bug. Reliable linkers never crash. |
| 7548 | |
| 7549 | @cindex error on valid input |
| 7550 | @item |
| 7551 | If @command{ld} produces an error message for valid input, that is a bug. |
| 7552 | |
| 7553 | @cindex invalid input |
| 7554 | @item |
| 7555 | If @command{ld} does not produce an error message for invalid input, that |
| 7556 | may be a bug. In the general case, the linker can not verify that |
| 7557 | object files are correct. |
| 7558 | |
| 7559 | @item |
| 7560 | If you are an experienced user of linkers, your suggestions for |
| 7561 | improvement of @command{ld} are welcome in any case. |
| 7562 | @end itemize |
| 7563 | |
| 7564 | @node Bug Reporting |
| 7565 | @section How to Report Bugs |
| 7566 | @cindex bug reports |
| 7567 | @cindex @command{ld} bugs, reporting |
| 7568 | |
| 7569 | A number of companies and individuals offer support for @sc{gnu} |
| 7570 | products. If you obtained @command{ld} from a support organization, we |
| 7571 | recommend you contact that organization first. |
| 7572 | |
| 7573 | You can find contact information for many support companies and |
| 7574 | individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs |
| 7575 | distribution. |
| 7576 | |
| 7577 | @ifset BUGURL |
| 7578 | Otherwise, send bug reports for @command{ld} to |
| 7579 | @value{BUGURL}. |
| 7580 | @end ifset |
| 7581 | |
| 7582 | The fundamental principle of reporting bugs usefully is this: |
| 7583 | @strong{report all the facts}. If you are not sure whether to state a |
| 7584 | fact or leave it out, state it! |
| 7585 | |
| 7586 | Often people omit facts because they think they know what causes the |
| 7587 | problem and assume that some details do not matter. Thus, you might |
| 7588 | assume that the name of a symbol you use in an example does not |
| 7589 | matter. Well, probably it does not, but one cannot be sure. Perhaps |
| 7590 | the bug is a stray memory reference which happens to fetch from the |
| 7591 | location where that name is stored in memory; perhaps, if the name |
| 7592 | were different, the contents of that location would fool the linker |
| 7593 | into doing the right thing despite the bug. Play it safe and give a |
| 7594 | specific, complete example. That is the easiest thing for you to do, |
| 7595 | and the most helpful. |
| 7596 | |
| 7597 | Keep in mind that the purpose of a bug report is to enable us to fix |
| 7598 | the bug if it is new to us. Therefore, always write your bug reports |
| 7599 | on the assumption that the bug has not been reported previously. |
| 7600 | |
| 7601 | Sometimes people give a few sketchy facts and ask, ``Does this ring a |
| 7602 | bell?'' This cannot help us fix a bug, so it is basically useless. We |
| 7603 | respond by asking for enough details to enable us to investigate. |
| 7604 | You might as well expedite matters by sending them to begin with. |
| 7605 | |
| 7606 | To enable us to fix the bug, you should include all these things: |
| 7607 | |
| 7608 | @itemize @bullet |
| 7609 | @item |
| 7610 | The version of @command{ld}. @command{ld} announces it if you start it with |
| 7611 | the @samp{--version} argument. |
| 7612 | |
| 7613 | Without this, we will not know whether there is any point in looking for |
| 7614 | the bug in the current version of @command{ld}. |
| 7615 | |
| 7616 | @item |
| 7617 | Any patches you may have applied to the @command{ld} source, including any |
| 7618 | patches made to the @code{BFD} library. |
| 7619 | |
| 7620 | @item |
| 7621 | The type of machine you are using, and the operating system name and |
| 7622 | version number. |
| 7623 | |
| 7624 | @item |
| 7625 | What compiler (and its version) was used to compile @command{ld}---e.g. |
| 7626 | ``@code{gcc-2.7}''. |
| 7627 | |
| 7628 | @item |
| 7629 | The command arguments you gave the linker to link your example and |
| 7630 | observe the bug. To guarantee you will not omit something important, |
| 7631 | list them all. A copy of the Makefile (or the output from make) is |
| 7632 | sufficient. |
| 7633 | |
| 7634 | If we were to try to guess the arguments, we would probably guess wrong |
| 7635 | and then we might not encounter the bug. |
| 7636 | |
| 7637 | @item |
| 7638 | A complete input file, or set of input files, that will reproduce the |
| 7639 | bug. It is generally most helpful to send the actual object files |
| 7640 | provided that they are reasonably small. Say no more than 10K. For |
| 7641 | bigger files you can either make them available by FTP or HTTP or else |
| 7642 | state that you are willing to send the object file(s) to whomever |
| 7643 | requests them. (Note - your email will be going to a mailing list, so |
| 7644 | we do not want to clog it up with large attachments). But small |
| 7645 | attachments are best. |
| 7646 | |
| 7647 | If the source files were assembled using @code{gas} or compiled using |
| 7648 | @code{gcc}, then it may be OK to send the source files rather than the |
| 7649 | object files. In this case, be sure to say exactly what version of |
| 7650 | @code{gas} or @code{gcc} was used to produce the object files. Also say |
| 7651 | how @code{gas} or @code{gcc} were configured. |
| 7652 | |
| 7653 | @item |
| 7654 | A description of what behavior you observe that you believe is |
| 7655 | incorrect. For example, ``It gets a fatal signal.'' |
| 7656 | |
| 7657 | Of course, if the bug is that @command{ld} gets a fatal signal, then we |
| 7658 | will certainly notice it. But if the bug is incorrect output, we might |
| 7659 | not notice unless it is glaringly wrong. You might as well not give us |
| 7660 | a chance to make a mistake. |
| 7661 | |
| 7662 | Even if the problem you experience is a fatal signal, you should still |
| 7663 | say so explicitly. Suppose something strange is going on, such as, your |
| 7664 | copy of @command{ld} is out of sync, or you have encountered a bug in the |
| 7665 | C library on your system. (This has happened!) Your copy might crash |
| 7666 | and ours would not. If you told us to expect a crash, then when ours |
| 7667 | fails to crash, we would know that the bug was not happening for us. If |
| 7668 | you had not told us to expect a crash, then we would not be able to draw |
| 7669 | any conclusion from our observations. |
| 7670 | |
| 7671 | @item |
| 7672 | If you wish to suggest changes to the @command{ld} source, send us context |
| 7673 | diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or |
| 7674 | @samp{-p} option. Always send diffs from the old file to the new file. |
| 7675 | If you even discuss something in the @command{ld} source, refer to it by |
| 7676 | context, not by line number. |
| 7677 | |
| 7678 | The line numbers in our development sources will not match those in your |
| 7679 | sources. Your line numbers would convey no useful information to us. |
| 7680 | @end itemize |
| 7681 | |
| 7682 | Here are some things that are not necessary: |
| 7683 | |
| 7684 | @itemize @bullet |
| 7685 | @item |
| 7686 | A description of the envelope of the bug. |
| 7687 | |
| 7688 | Often people who encounter a bug spend a lot of time investigating |
| 7689 | which changes to the input file will make the bug go away and which |
| 7690 | changes will not affect it. |
| 7691 | |
| 7692 | This is often time consuming and not very useful, because the way we |
| 7693 | will find the bug is by running a single example under the debugger |
| 7694 | with breakpoints, not by pure deduction from a series of examples. |
| 7695 | We recommend that you save your time for something else. |
| 7696 | |
| 7697 | Of course, if you can find a simpler example to report @emph{instead} |
| 7698 | of the original one, that is a convenience for us. Errors in the |
| 7699 | output will be easier to spot, running under the debugger will take |
| 7700 | less time, and so on. |
| 7701 | |
| 7702 | However, simplification is not vital; if you do not want to do this, |
| 7703 | report the bug anyway and send us the entire test case you used. |
| 7704 | |
| 7705 | @item |
| 7706 | A patch for the bug. |
| 7707 | |
| 7708 | A patch for the bug does help us if it is a good one. But do not omit |
| 7709 | the necessary information, such as the test case, on the assumption that |
| 7710 | a patch is all we need. We might see problems with your patch and decide |
| 7711 | to fix the problem another way, or we might not understand it at all. |
| 7712 | |
| 7713 | Sometimes with a program as complicated as @command{ld} it is very hard to |
| 7714 | construct an example that will make the program follow a certain path |
| 7715 | through the code. If you do not send us the example, we will not be |
| 7716 | able to construct one, so we will not be able to verify that the bug is |
| 7717 | fixed. |
| 7718 | |
| 7719 | And if we cannot understand what bug you are trying to fix, or why your |
| 7720 | patch should be an improvement, we will not install it. A test case will |
| 7721 | help us to understand. |
| 7722 | |
| 7723 | @item |
| 7724 | A guess about what the bug is or what it depends on. |
| 7725 | |
| 7726 | Such guesses are usually wrong. Even we cannot guess right about such |
| 7727 | things without first using the debugger to find the facts. |
| 7728 | @end itemize |
| 7729 | |
| 7730 | @node MRI |
| 7731 | @appendix MRI Compatible Script Files |
| 7732 | @cindex MRI compatibility |
| 7733 | To aid users making the transition to @sc{gnu} @command{ld} from the MRI |
| 7734 | linker, @command{ld} can use MRI compatible linker scripts as an |
| 7735 | alternative to the more general-purpose linker scripting language |
| 7736 | described in @ref{Scripts}. MRI compatible linker scripts have a much |
| 7737 | simpler command set than the scripting language otherwise used with |
| 7738 | @command{ld}. @sc{gnu} @command{ld} supports the most commonly used MRI |
| 7739 | linker commands; these commands are described here. |
| 7740 | |
| 7741 | In general, MRI scripts aren't of much use with the @code{a.out} object |
| 7742 | file format, since it only has three sections and MRI scripts lack some |
| 7743 | features to make use of them. |
| 7744 | |
| 7745 | You can specify a file containing an MRI-compatible script using the |
| 7746 | @samp{-c} command-line option. |
| 7747 | |
| 7748 | Each command in an MRI-compatible script occupies its own line; each |
| 7749 | command line starts with the keyword that identifies the command (though |
| 7750 | blank lines are also allowed for punctuation). If a line of an |
| 7751 | MRI-compatible script begins with an unrecognized keyword, @command{ld} |
| 7752 | issues a warning message, but continues processing the script. |
| 7753 | |
| 7754 | Lines beginning with @samp{*} are comments. |
| 7755 | |
| 7756 | You can write these commands using all upper-case letters, or all |
| 7757 | lower case; for example, @samp{chip} is the same as @samp{CHIP}. |
| 7758 | The following list shows only the upper-case form of each command. |
| 7759 | |
| 7760 | @table @code |
| 7761 | @cindex @code{ABSOLUTE} (MRI) |
| 7762 | @item ABSOLUTE @var{secname} |
| 7763 | @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname} |
| 7764 | Normally, @command{ld} includes in the output file all sections from all |
| 7765 | the input files. However, in an MRI-compatible script, you can use the |
| 7766 | @code{ABSOLUTE} command to restrict the sections that will be present in |
| 7767 | your output program. If the @code{ABSOLUTE} command is used at all in a |
| 7768 | script, then only the sections named explicitly in @code{ABSOLUTE} |
| 7769 | commands will appear in the linker output. You can still use other |
| 7770 | input sections (whatever you select on the command line, or using |
| 7771 | @code{LOAD}) to resolve addresses in the output file. |
| 7772 | |
| 7773 | @cindex @code{ALIAS} (MRI) |
| 7774 | @item ALIAS @var{out-secname}, @var{in-secname} |
| 7775 | Use this command to place the data from input section @var{in-secname} |
| 7776 | in a section called @var{out-secname} in the linker output file. |
| 7777 | |
| 7778 | @var{in-secname} may be an integer. |
| 7779 | |
| 7780 | @cindex @code{ALIGN} (MRI) |
| 7781 | @item ALIGN @var{secname} = @var{expression} |
| 7782 | Align the section called @var{secname} to @var{expression}. The |
| 7783 | @var{expression} should be a power of two. |
| 7784 | |
| 7785 | @cindex @code{BASE} (MRI) |
| 7786 | @item BASE @var{expression} |
| 7787 | Use the value of @var{expression} as the lowest address (other than |
| 7788 | absolute addresses) in the output file. |
| 7789 | |
| 7790 | @cindex @code{CHIP} (MRI) |
| 7791 | @item CHIP @var{expression} |
| 7792 | @itemx CHIP @var{expression}, @var{expression} |
| 7793 | This command does nothing; it is accepted only for compatibility. |
| 7794 | |
| 7795 | @cindex @code{END} (MRI) |
| 7796 | @item END |
| 7797 | This command does nothing whatever; it's only accepted for compatibility. |
| 7798 | |
| 7799 | @cindex @code{FORMAT} (MRI) |
| 7800 | @item FORMAT @var{output-format} |
| 7801 | Similar to the @code{OUTPUT_FORMAT} command in the more general linker |
| 7802 | language, but restricted to one of these output formats: |
| 7803 | |
| 7804 | @enumerate |
| 7805 | @item |
| 7806 | S-records, if @var{output-format} is @samp{S} |
| 7807 | |
| 7808 | @item |
| 7809 | IEEE, if @var{output-format} is @samp{IEEE} |
| 7810 | |
| 7811 | @item |
| 7812 | COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is |
| 7813 | @samp{COFF} |
| 7814 | @end enumerate |
| 7815 | |
| 7816 | @cindex @code{LIST} (MRI) |
| 7817 | @item LIST @var{anything}@dots{} |
| 7818 | Print (to the standard output file) a link map, as produced by the |
| 7819 | @command{ld} command-line option @samp{-M}. |
| 7820 | |
| 7821 | The keyword @code{LIST} may be followed by anything on the |
| 7822 | same line, with no change in its effect. |
| 7823 | |
| 7824 | @cindex @code{LOAD} (MRI) |
| 7825 | @item LOAD @var{filename} |
| 7826 | @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename} |
| 7827 | Include one or more object file @var{filename} in the link; this has the |
| 7828 | same effect as specifying @var{filename} directly on the @command{ld} |
| 7829 | command line. |
| 7830 | |
| 7831 | @cindex @code{NAME} (MRI) |
| 7832 | @item NAME @var{output-name} |
| 7833 | @var{output-name} is the name for the program produced by @command{ld}; the |
| 7834 | MRI-compatible command @code{NAME} is equivalent to the command-line |
| 7835 | option @samp{-o} or the general script language command @code{OUTPUT}. |
| 7836 | |
| 7837 | @cindex @code{ORDER} (MRI) |
| 7838 | @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname} |
| 7839 | @itemx ORDER @var{secname} @var{secname} @var{secname} |
| 7840 | Normally, @command{ld} orders the sections in its output file in the |
| 7841 | order in which they first appear in the input files. In an MRI-compatible |
| 7842 | script, you can override this ordering with the @code{ORDER} command. The |
| 7843 | sections you list with @code{ORDER} will appear first in your output |
| 7844 | file, in the order specified. |
| 7845 | |
| 7846 | @cindex @code{PUBLIC} (MRI) |
| 7847 | @item PUBLIC @var{name}=@var{expression} |
| 7848 | @itemx PUBLIC @var{name},@var{expression} |
| 7849 | @itemx PUBLIC @var{name} @var{expression} |
| 7850 | Supply a value (@var{expression}) for external symbol |
| 7851 | @var{name} used in the linker input files. |
| 7852 | |
| 7853 | @cindex @code{SECT} (MRI) |
| 7854 | @item SECT @var{secname}, @var{expression} |
| 7855 | @itemx SECT @var{secname}=@var{expression} |
| 7856 | @itemx SECT @var{secname} @var{expression} |
| 7857 | You can use any of these three forms of the @code{SECT} command to |
| 7858 | specify the start address (@var{expression}) for section @var{secname}. |
| 7859 | If you have more than one @code{SECT} statement for the same |
| 7860 | @var{secname}, only the @emph{first} sets the start address. |
| 7861 | @end table |
| 7862 | |
| 7863 | @node GNU Free Documentation License |
| 7864 | @appendix GNU Free Documentation License |
| 7865 | @include fdl.texi |
| 7866 | |
| 7867 | @node LD Index |
| 7868 | @unnumbered LD Index |
| 7869 | |
| 7870 | @printindex cp |
| 7871 | |
| 7872 | @tex |
| 7873 | % I think something like @colophon should be in texinfo. In the |
| 7874 | % meantime: |
| 7875 | \long\def\colophon{\hbox to0pt{}\vfill |
| 7876 | \centerline{The body of this manual is set in} |
| 7877 | \centerline{\fontname\tenrm,} |
| 7878 | \centerline{with headings in {\bf\fontname\tenbf}} |
| 7879 | \centerline{and examples in {\tt\fontname\tentt}.} |
| 7880 | \centerline{{\it\fontname\tenit\/} and} |
| 7881 | \centerline{{\sl\fontname\tensl\/}} |
| 7882 | \centerline{are used for emphasis.}\vfill} |
| 7883 | \page\colophon |
| 7884 | % Blame: doc@cygnus.com, 28mar91. |
| 7885 | @end tex |
| 7886 | |
| 7887 | @bye |