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1 | \input texinfo |
2 | @setfilename ld.info | |
a2b64bed | 3 | @c Copyright 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, |
b717d30e | 4 | @c 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc. |
252b5132 | 5 | @syncodeindex ky cp |
dff70155 | 6 | @c man begin INCLUDE |
252b5132 RH |
7 | @include configdoc.texi |
8 | @c (configdoc.texi is generated by the Makefile) | |
9 | @include ldver.texi | |
dff70155 | 10 | @c man end |
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11 | |
12 | @c @smallbook | |
13 | ||
ff5dcc92 SC |
14 | @macro gcctabopt{body} |
15 | @code{\body\} | |
16 | @end macro | |
17 | ||
0285c67d NC |
18 | @c man begin NAME |
19 | @ifset man | |
20 | @c Configure for the generation of man pages | |
21 | @set UsesEnvVars | |
22 | @set GENERIC | |
0285c67d NC |
23 | @set ARC |
24 | @set ARM | |
25 | @set D10V | |
26 | @set D30V | |
27 | @set H8/300 | |
28 | @set H8/500 | |
29 | @set HPPA | |
30 | @set I370 | |
31 | @set I80386 | |
32 | @set I860 | |
33 | @set I960 | |
34 | @set M32R | |
35 | @set M68HC11 | |
36 | @set M680X0 | |
37 | @set MCORE | |
38 | @set MIPS | |
3c3bdf30 | 39 | @set MMIX |
2469cfa2 | 40 | @set MSP430 |
0285c67d NC |
41 | @set PDP11 |
42 | @set PJ | |
43 | @set SH | |
44 | @set SPARC | |
9418ab9c | 45 | @set TIC54X |
0285c67d NC |
46 | @set V850 |
47 | @set VAX | |
2ca22b03 | 48 | @set WIN32 |
e0001a05 | 49 | @set XTENSA |
0285c67d NC |
50 | @end ifset |
51 | @c man end | |
52 | ||
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53 | @ifinfo |
54 | @format | |
55 | START-INFO-DIR-ENTRY | |
56 | * Ld: (ld). The GNU linker. | |
57 | END-INFO-DIR-ENTRY | |
58 | @end format | |
59 | @end ifinfo | |
60 | ||
61 | @ifinfo | |
62 | This file documents the @sc{gnu} linker LD version @value{VERSION}. | |
63 | ||
62bf86b4 | 64 | Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, |
6bdafbeb | 65 | 2001, 2002, 2003, 2004 Free Software Foundation, Inc. |
252b5132 | 66 | |
252b5132 | 67 | @ignore |
cf055d54 NC |
68 | |
69 | Permission is granted to copy, distribute and/or modify this document | |
70 | under the terms of the GNU Free Documentation License, Version 1.1 | |
71 | or any later version published by the Free Software Foundation; | |
72 | with no Invariant Sections, with no Front-Cover Texts, and with no | |
73 | Back-Cover Texts. A copy of the license is included in the | |
36f63dca | 74 | section entitled ``GNU Free Documentation License''. |
cf055d54 | 75 | |
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76 | Permission is granted to process this file through Tex and print the |
77 | results, provided the printed document carries copying permission | |
78 | notice identical to this one except for the removal of this paragraph | |
79 | (this paragraph not being relevant to the printed manual). | |
80 | ||
81 | @end ignore | |
82 | @end ifinfo | |
83 | @iftex | |
84 | @finalout | |
85 | @setchapternewpage odd | |
86 | @settitle Using LD, the GNU linker | |
87 | @titlepage | |
88 | @title Using ld | |
89 | @subtitle The GNU linker | |
90 | @sp 1 | |
91 | @subtitle @code{ld} version 2 | |
92 | @subtitle Version @value{VERSION} | |
93 | @author Steve Chamberlain | |
94 | @author Ian Lance Taylor | |
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95 | @page |
96 | ||
97 | @tex | |
98 | {\parskip=0pt | |
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99 | \hfill Red Hat Inc\par |
100 | \hfill nickc\@credhat.com, doc\@redhat.com\par | |
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101 | \hfill {\it Using LD, the GNU linker}\par |
102 | \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par | |
103 | } | |
104 | \global\parindent=0pt % Steve likes it this way. | |
105 | @end tex | |
106 | ||
107 | @vskip 0pt plus 1filll | |
0285c67d | 108 | @c man begin COPYRIGHT |
9c8ebd6a | 109 | Copyright @copyright{} 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, |
6bdafbeb | 110 | 2002, 2003, 2004 Free Software Foundation, Inc. |
252b5132 | 111 | |
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112 | Permission is granted to copy, distribute and/or modify this document |
113 | under the terms of the GNU Free Documentation License, Version 1.1 | |
114 | or any later version published by the Free Software Foundation; | |
115 | with no Invariant Sections, with no Front-Cover Texts, and with no | |
116 | Back-Cover Texts. A copy of the license is included in the | |
36f63dca | 117 | section entitled ``GNU Free Documentation License''. |
0285c67d | 118 | @c man end |
252b5132 | 119 | |
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120 | @end titlepage |
121 | @end iftex | |
122 | @c FIXME: Talk about importance of *order* of args, cmds to linker! | |
123 | ||
84ec0e6d | 124 | @ifnottex |
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125 | @node Top |
126 | @top Using ld | |
127 | This file documents the @sc{gnu} linker ld version @value{VERSION}. | |
128 | ||
cf055d54 NC |
129 | This document is distributed under the terms of the GNU Free |
130 | Documentation License. A copy of the license is included in the | |
36f63dca | 131 | section entitled ``GNU Free Documentation License''. |
cf055d54 | 132 | |
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133 | @menu |
134 | * Overview:: Overview | |
135 | * Invocation:: Invocation | |
136 | * Scripts:: Linker Scripts | |
137 | @ifset GENERIC | |
138 | * Machine Dependent:: Machine Dependent Features | |
139 | @end ifset | |
140 | @ifclear GENERIC | |
141 | @ifset H8300 | |
142 | * H8/300:: ld and the H8/300 | |
143 | @end ifset | |
c2dcd04e NC |
144 | @ifset Renesas |
145 | * Renesas:: ld and other Renesas micros | |
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146 | @end ifset |
147 | @ifset I960 | |
148 | * i960:: ld and the Intel 960 family | |
149 | @end ifset | |
36f63dca NC |
150 | @ifset ARM |
151 | * ARM:: ld and the ARM family | |
152 | @end ifset | |
153 | @ifset HPPA | |
154 | * HPPA ELF32:: ld and HPPA 32-bit ELF | |
155 | @end ifset | |
93fd0973 SC |
156 | @ifset M68HC11 |
157 | * M68HC11/68HC12:: ld and the Motorola 68HC11 and 68HC12 families | |
158 | @end ifset | |
74459f0e TW |
159 | @ifset TICOFF |
160 | * TI COFF:: ld and the TI COFF | |
161 | @end ifset | |
2ca22b03 NC |
162 | @ifset WIN32 |
163 | * Win32:: ld and WIN32 (cygwin/mingw) | |
164 | @end ifset | |
e0001a05 NC |
165 | @ifset XTENSA |
166 | * Xtensa:: ld and Xtensa Processors | |
167 | @end ifset | |
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168 | @end ifclear |
169 | @ifclear SingleFormat | |
170 | * BFD:: BFD | |
171 | @end ifclear | |
172 | @c Following blank line required for remaining bug in makeinfo conds/menus | |
173 | ||
174 | * Reporting Bugs:: Reporting Bugs | |
175 | * MRI:: MRI Compatible Script Files | |
704c465c | 176 | * GNU Free Documentation License:: GNU Free Documentation License |
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177 | * Index:: Index |
178 | @end menu | |
84ec0e6d | 179 | @end ifnottex |
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180 | |
181 | @node Overview | |
182 | @chapter Overview | |
183 | ||
184 | @cindex @sc{gnu} linker | |
185 | @cindex what is this? | |
0285c67d | 186 | |
0879a67a | 187 | @ifset man |
0285c67d | 188 | @c man begin SYNOPSIS |
ff5dcc92 | 189 | ld [@b{options}] @var{objfile} @dots{} |
0285c67d NC |
190 | @c man end |
191 | ||
192 | @c man begin SEEALSO | |
193 | ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and | |
194 | the Info entries for @file{binutils} and | |
195 | @file{ld}. | |
196 | @c man end | |
197 | @end ifset | |
198 | ||
199 | @c man begin DESCRIPTION | |
200 | ||
ff5dcc92 | 201 | @command{ld} combines a number of object and archive files, relocates |
252b5132 | 202 | their data and ties up symbol references. Usually the last step in |
ff5dcc92 | 203 | compiling a program is to run @command{ld}. |
252b5132 | 204 | |
ff5dcc92 | 205 | @command{ld} accepts Linker Command Language files written in |
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206 | a superset of AT&T's Link Editor Command Language syntax, |
207 | to provide explicit and total control over the linking process. | |
208 | ||
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209 | @ifset man |
210 | @c For the man only | |
211 | This man page does not describe the command language; see the | |
ff5dcc92 | 212 | @command{ld} entry in @code{info}, or the manual |
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213 | ld: the GNU linker, for full details on the command language and |
214 | on other aspects of the GNU linker. | |
215 | @end ifset | |
216 | ||
252b5132 | 217 | @ifclear SingleFormat |
ff5dcc92 SC |
218 | This version of @command{ld} uses the general purpose BFD libraries |
219 | to operate on object files. This allows @command{ld} to read, combine, and | |
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220 | write object files in many different formats---for example, COFF or |
221 | @code{a.out}. Different formats may be linked together to produce any | |
222 | available kind of object file. @xref{BFD}, for more information. | |
223 | @end ifclear | |
224 | ||
225 | Aside from its flexibility, the @sc{gnu} linker is more helpful than other | |
226 | linkers in providing diagnostic information. Many linkers abandon | |
227 | execution immediately upon encountering an error; whenever possible, | |
ff5dcc92 | 228 | @command{ld} continues executing, allowing you to identify other errors |
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229 | (or, in some cases, to get an output file in spite of the error). |
230 | ||
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231 | @c man end |
232 | ||
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233 | @node Invocation |
234 | @chapter Invocation | |
235 | ||
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236 | @c man begin DESCRIPTION |
237 | ||
ff5dcc92 | 238 | The @sc{gnu} linker @command{ld} is meant to cover a broad range of situations, |
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239 | and to be as compatible as possible with other linkers. As a result, |
240 | you have many choices to control its behavior. | |
241 | ||
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242 | @c man end |
243 | ||
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244 | @ifset UsesEnvVars |
245 | @menu | |
246 | * Options:: Command Line Options | |
247 | * Environment:: Environment Variables | |
248 | @end menu | |
249 | ||
250 | @node Options | |
251 | @section Command Line Options | |
252 | @end ifset | |
253 | ||
254 | @cindex command line | |
255 | @cindex options | |
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256 | |
257 | @c man begin OPTIONS | |
258 | ||
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259 | The linker supports a plethora of command-line options, but in actual |
260 | practice few of them are used in any particular context. | |
261 | @cindex standard Unix system | |
ff5dcc92 | 262 | For instance, a frequent use of @command{ld} is to link standard Unix |
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263 | object files on a standard, supported Unix system. On such a system, to |
264 | link a file @code{hello.o}: | |
265 | ||
266 | @smallexample | |
267 | ld -o @var{output} /lib/crt0.o hello.o -lc | |
268 | @end smallexample | |
269 | ||
ff5dcc92 | 270 | This tells @command{ld} to produce a file called @var{output} as the |
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271 | result of linking the file @code{/lib/crt0.o} with @code{hello.o} and |
272 | the library @code{libc.a}, which will come from the standard search | |
273 | directories. (See the discussion of the @samp{-l} option below.) | |
274 | ||
ff5dcc92 | 275 | Some of the command-line options to @command{ld} may be specified at any |
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276 | point in the command line. However, options which refer to files, such |
277 | as @samp{-l} or @samp{-T}, cause the file to be read at the point at | |
278 | which the option appears in the command line, relative to the object | |
279 | files and other file options. Repeating non-file options with a | |
280 | different argument will either have no further effect, or override prior | |
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281 | occurrences (those further to the left on the command line) of that |
282 | option. Options which may be meaningfully specified more than once are | |
283 | noted in the descriptions below. | |
284 | ||
285 | @cindex object files | |
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286 | Non-option arguments are object files or archives which are to be linked |
287 | together. They may follow, precede, or be mixed in with command-line | |
288 | options, except that an object file argument may not be placed between | |
289 | an option and its argument. | |
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290 | |
291 | Usually the linker is invoked with at least one object file, but you can | |
292 | specify other forms of binary input files using @samp{-l}, @samp{-R}, | |
293 | and the script command language. If @emph{no} binary input files at all | |
294 | are specified, the linker does not produce any output, and issues the | |
295 | message @samp{No input files}. | |
296 | ||
36f63dca | 297 | If the linker cannot recognize the format of an object file, it will |
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298 | assume that it is a linker script. A script specified in this way |
299 | augments the main linker script used for the link (either the default | |
300 | linker script or the one specified by using @samp{-T}). This feature | |
301 | permits the linker to link against a file which appears to be an object | |
302 | or an archive, but actually merely defines some symbol values, or uses | |
303 | @code{INPUT} or @code{GROUP} to load other objects. Note that | |
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304 | specifying a script in this way merely augments the main linker script; |
305 | use the @samp{-T} option to replace the default linker script entirely. | |
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306 | @xref{Scripts}. |
307 | ||
308 | For options whose names are a single letter, | |
309 | option arguments must either follow the option letter without intervening | |
310 | whitespace, or be given as separate arguments immediately following the | |
311 | option that requires them. | |
312 | ||
313 | For options whose names are multiple letters, either one dash or two can | |
e4897a32 | 314 | precede the option name; for example, @samp{-trace-symbol} and |
36f63dca | 315 | @samp{--trace-symbol} are equivalent. Note---there is one exception to |
e4897a32 NC |
316 | this rule. Multiple letter options that start with a lower case 'o' can |
317 | only be preceeded by two dashes. This is to reduce confusion with the | |
318 | @samp{-o} option. So for example @samp{-omagic} sets the output file | |
319 | name to @samp{magic} whereas @samp{--omagic} sets the NMAGIC flag on the | |
320 | output. | |
321 | ||
322 | Arguments to multiple-letter options must either be separated from the | |
323 | option name by an equals sign, or be given as separate arguments | |
324 | immediately following the option that requires them. For example, | |
325 | @samp{--trace-symbol foo} and @samp{--trace-symbol=foo} are equivalent. | |
326 | Unique abbreviations of the names of multiple-letter options are | |
327 | accepted. | |
252b5132 | 328 | |
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329 | Note---if the linker is being invoked indirectly, via a compiler driver |
330 | (e.g. @samp{gcc}) then all the linker command line options should be | |
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331 | prefixed by @samp{-Wl,} (or whatever is appropriate for the particular |
332 | compiler driver) like this: | |
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333 | |
334 | @smallexample | |
335 | gcc -Wl,--startgroup foo.o bar.o -Wl,--endgroup | |
336 | @end smallexample | |
337 | ||
338 | This is important, because otherwise the compiler driver program may | |
339 | silently drop the linker options, resulting in a bad link. | |
340 | ||
341 | Here is a table of the generic command line switches accepted by the GNU | |
342 | linker: | |
343 | ||
ff5dcc92 | 344 | @table @gcctabopt |
dff70155 MM |
345 | @include @value{top_srcdir}/../libiberty/at-file.texi |
346 | ||
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347 | @kindex -a@var{keyword} |
348 | @item -a@var{keyword} | |
349 | This option is supported for HP/UX compatibility. The @var{keyword} | |
350 | argument must be one of the strings @samp{archive}, @samp{shared}, or | |
351 | @samp{default}. @samp{-aarchive} is functionally equivalent to | |
352 | @samp{-Bstatic}, and the other two keywords are functionally equivalent | |
353 | to @samp{-Bdynamic}. This option may be used any number of times. | |
354 | ||
355 | @ifset I960 | |
356 | @cindex architectures | |
357 | @kindex -A@var{arch} | |
358 | @item -A@var{architecture} | |
359 | @kindex --architecture=@var{arch} | |
360 | @itemx --architecture=@var{architecture} | |
ff5dcc92 SC |
361 | In the current release of @command{ld}, this option is useful only for the |
362 | Intel 960 family of architectures. In that @command{ld} configuration, the | |
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363 | @var{architecture} argument identifies the particular architecture in |
364 | the 960 family, enabling some safeguards and modifying the | |
ff5dcc92 | 365 | archive-library search path. @xref{i960,,@command{ld} and the Intel 960 |
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366 | family}, for details. |
367 | ||
ff5dcc92 | 368 | Future releases of @command{ld} may support similar functionality for |
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369 | other architecture families. |
370 | @end ifset | |
371 | ||
372 | @ifclear SingleFormat | |
373 | @cindex binary input format | |
374 | @kindex -b @var{format} | |
375 | @kindex --format=@var{format} | |
376 | @cindex input format | |
377 | @cindex input format | |
378 | @item -b @var{input-format} | |
379 | @itemx --format=@var{input-format} | |
ff5dcc92 SC |
380 | @command{ld} may be configured to support more than one kind of object |
381 | file. If your @command{ld} is configured this way, you can use the | |
252b5132 | 382 | @samp{-b} option to specify the binary format for input object files |
ff5dcc92 | 383 | that follow this option on the command line. Even when @command{ld} is |
252b5132 | 384 | configured to support alternative object formats, you don't usually need |
ff5dcc92 | 385 | to specify this, as @command{ld} should be configured to expect as a |
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386 | default input format the most usual format on each machine. |
387 | @var{input-format} is a text string, the name of a particular format | |
388 | supported by the BFD libraries. (You can list the available binary | |
389 | formats with @samp{objdump -i}.) | |
390 | @xref{BFD}. | |
391 | ||
392 | You may want to use this option if you are linking files with an unusual | |
393 | binary format. You can also use @samp{-b} to switch formats explicitly (when | |
394 | linking object files of different formats), by including | |
395 | @samp{-b @var{input-format}} before each group of object files in a | |
a1ab1d2a | 396 | particular format. |
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397 | |
398 | The default format is taken from the environment variable | |
399 | @code{GNUTARGET}. | |
400 | @ifset UsesEnvVars | |
401 | @xref{Environment}. | |
402 | @end ifset | |
403 | You can also define the input format from a script, using the command | |
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404 | @code{TARGET}; |
405 | @ifclear man | |
406 | see @ref{Format Commands}. | |
407 | @end ifclear | |
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408 | @end ifclear |
409 | ||
410 | @kindex -c @var{MRI-cmdfile} | |
411 | @kindex --mri-script=@var{MRI-cmdfile} | |
412 | @cindex compatibility, MRI | |
413 | @item -c @var{MRI-commandfile} | |
414 | @itemx --mri-script=@var{MRI-commandfile} | |
ff5dcc92 | 415 | For compatibility with linkers produced by MRI, @command{ld} accepts script |
252b5132 | 416 | files written in an alternate, restricted command language, described in |
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417 | @ifclear man |
418 | @ref{MRI,,MRI Compatible Script Files}. | |
419 | @end ifclear | |
420 | @ifset man | |
421 | the MRI Compatible Script Files section of GNU ld documentation. | |
422 | @end ifset | |
423 | Introduce MRI script files with | |
252b5132 | 424 | the option @samp{-c}; use the @samp{-T} option to run linker |
ff5dcc92 SC |
425 | scripts written in the general-purpose @command{ld} scripting language. |
426 | If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories | |
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427 | specified by any @samp{-L} options. |
428 | ||
429 | @cindex common allocation | |
430 | @kindex -d | |
431 | @kindex -dc | |
432 | @kindex -dp | |
a1ab1d2a | 433 | @item -d |
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434 | @itemx -dc |
435 | @itemx -dp | |
436 | These three options are equivalent; multiple forms are supported for | |
437 | compatibility with other linkers. They assign space to common symbols | |
438 | even if a relocatable output file is specified (with @samp{-r}). The | |
439 | script command @code{FORCE_COMMON_ALLOCATION} has the same effect. | |
440 | @xref{Miscellaneous Commands}. | |
441 | ||
442 | @cindex entry point, from command line | |
443 | @kindex -e @var{entry} | |
444 | @kindex --entry=@var{entry} | |
a1ab1d2a | 445 | @item -e @var{entry} |
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446 | @itemx --entry=@var{entry} |
447 | Use @var{entry} as the explicit symbol for beginning execution of your | |
448 | program, rather than the default entry point. If there is no symbol | |
449 | named @var{entry}, the linker will try to parse @var{entry} as a number, | |
450 | and use that as the entry address (the number will be interpreted in | |
451 | base 10; you may use a leading @samp{0x} for base 16, or a leading | |
452 | @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults | |
453 | and other ways of specifying the entry point. | |
454 | ||
b58f81ae DJ |
455 | @kindex --exclude-libs |
456 | @item --exclude-libs @var{lib},@var{lib},... | |
457 | Specifies a list of archive libraries from which symbols should not be automatically | |
458 | exported. The library names may be delimited by commas or colons. Specifying | |
459 | @code{--exclude-libs ALL} excludes symbols in all archive libraries from | |
460 | automatic export. This option is available only for the i386 PE targeted | |
461 | port of the linker and for ELF targeted ports. For i386 PE, symbols | |
462 | explicitly listed in a .def file are still exported, regardless of this | |
463 | option. For ELF targeted ports, symbols affected by this option will | |
464 | be treated as hidden. | |
465 | ||
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466 | @cindex dynamic symbol table |
467 | @kindex -E | |
468 | @kindex --export-dynamic | |
469 | @item -E | |
470 | @itemx --export-dynamic | |
471 | When creating a dynamically linked executable, add all symbols to the | |
472 | dynamic symbol table. The dynamic symbol table is the set of symbols | |
473 | which are visible from dynamic objects at run time. | |
474 | ||
475 | If you do not use this option, the dynamic symbol table will normally | |
476 | contain only those symbols which are referenced by some dynamic object | |
477 | mentioned in the link. | |
478 | ||
479 | If you use @code{dlopen} to load a dynamic object which needs to refer | |
480 | back to the symbols defined by the program, rather than some other | |
481 | dynamic object, then you will probably need to use this option when | |
482 | linking the program itself. | |
483 | ||
cb840a31 L |
484 | You can also use the version script to control what symbols should |
485 | be added to the dynamic symbol table if the output format supports it. | |
486 | See the description of @samp{--version-script} in @ref{VERSION}. | |
487 | ||
36f63dca | 488 | @ifclear SingleFormat |
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489 | @cindex big-endian objects |
490 | @cindex endianness | |
491 | @kindex -EB | |
492 | @item -EB | |
493 | Link big-endian objects. This affects the default output format. | |
494 | ||
495 | @cindex little-endian objects | |
496 | @kindex -EL | |
497 | @item -EL | |
498 | Link little-endian objects. This affects the default output format. | |
36f63dca | 499 | @end ifclear |
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500 | |
501 | @kindex -f | |
502 | @kindex --auxiliary | |
503 | @item -f | |
504 | @itemx --auxiliary @var{name} | |
505 | When creating an ELF shared object, set the internal DT_AUXILIARY field | |
506 | to the specified name. This tells the dynamic linker that the symbol | |
507 | table of the shared object should be used as an auxiliary filter on the | |
508 | symbol table of the shared object @var{name}. | |
509 | ||
510 | If you later link a program against this filter object, then, when you | |
511 | run the program, the dynamic linker will see the DT_AUXILIARY field. If | |
512 | the dynamic linker resolves any symbols from the filter object, it will | |
513 | first check whether there is a definition in the shared object | |
514 | @var{name}. If there is one, it will be used instead of the definition | |
515 | in the filter object. The shared object @var{name} need not exist. | |
516 | Thus the shared object @var{name} may be used to provide an alternative | |
517 | implementation of certain functions, perhaps for debugging or for | |
518 | machine specific performance. | |
519 | ||
520 | This option may be specified more than once. The DT_AUXILIARY entries | |
521 | will be created in the order in which they appear on the command line. | |
522 | ||
523 | @kindex -F | |
524 | @kindex --filter | |
525 | @item -F @var{name} | |
526 | @itemx --filter @var{name} | |
527 | When creating an ELF shared object, set the internal DT_FILTER field to | |
528 | the specified name. This tells the dynamic linker that the symbol table | |
529 | of the shared object which is being created should be used as a filter | |
530 | on the symbol table of the shared object @var{name}. | |
531 | ||
532 | If you later link a program against this filter object, then, when you | |
533 | run the program, the dynamic linker will see the DT_FILTER field. The | |
534 | dynamic linker will resolve symbols according to the symbol table of the | |
535 | filter object as usual, but it will actually link to the definitions | |
536 | found in the shared object @var{name}. Thus the filter object can be | |
537 | used to select a subset of the symbols provided by the object | |
538 | @var{name}. | |
539 | ||
ff5dcc92 | 540 | Some older linkers used the @option{-F} option throughout a compilation |
252b5132 | 541 | toolchain for specifying object-file format for both input and output |
36f63dca NC |
542 | object files. |
543 | @ifclear SingleFormat | |
544 | The @sc{gnu} linker uses other mechanisms for this purpose: the | |
545 | @option{-b}, @option{--format}, @option{--oformat} options, the | |
252b5132 | 546 | @code{TARGET} command in linker scripts, and the @code{GNUTARGET} |
36f63dca NC |
547 | environment variable. |
548 | @end ifclear | |
549 | The @sc{gnu} linker will ignore the @option{-F} option when not | |
550 | creating an ELF shared object. | |
252b5132 | 551 | |
3dbf70a2 MM |
552 | @cindex finalization function |
553 | @kindex -fini | |
554 | @item -fini @var{name} | |
555 | When creating an ELF executable or shared object, call NAME when the | |
556 | executable or shared object is unloaded, by setting DT_FINI to the | |
557 | address of the function. By default, the linker uses @code{_fini} as | |
558 | the function to call. | |
559 | ||
252b5132 RH |
560 | @kindex -g |
561 | @item -g | |
562 | Ignored. Provided for compatibility with other tools. | |
563 | ||
564 | @kindex -G | |
565 | @kindex --gpsize | |
566 | @cindex object size | |
567 | @item -G@var{value} | |
568 | @itemx --gpsize=@var{value} | |
569 | Set the maximum size of objects to be optimized using the GP register to | |
570 | @var{size}. This is only meaningful for object file formats such as | |
571 | MIPS ECOFF which supports putting large and small objects into different | |
572 | sections. This is ignored for other object file formats. | |
573 | ||
574 | @cindex runtime library name | |
575 | @kindex -h@var{name} | |
576 | @kindex -soname=@var{name} | |
577 | @item -h@var{name} | |
578 | @itemx -soname=@var{name} | |
579 | When creating an ELF shared object, set the internal DT_SONAME field to | |
580 | the specified name. When an executable is linked with a shared object | |
581 | which has a DT_SONAME field, then when the executable is run the dynamic | |
582 | linker will attempt to load the shared object specified by the DT_SONAME | |
583 | field rather than the using the file name given to the linker. | |
584 | ||
585 | @kindex -i | |
586 | @cindex incremental link | |
587 | @item -i | |
588 | Perform an incremental link (same as option @samp{-r}). | |
589 | ||
3dbf70a2 MM |
590 | @cindex initialization function |
591 | @kindex -init | |
592 | @item -init @var{name} | |
593 | When creating an ELF executable or shared object, call NAME when the | |
594 | executable or shared object is loaded, by setting DT_INIT to the address | |
595 | of the function. By default, the linker uses @code{_init} as the | |
596 | function to call. | |
597 | ||
252b5132 RH |
598 | @cindex archive files, from cmd line |
599 | @kindex -l@var{archive} | |
600 | @kindex --library=@var{archive} | |
601 | @item -l@var{archive} | |
602 | @itemx --library=@var{archive} | |
603 | Add archive file @var{archive} to the list of files to link. This | |
ff5dcc92 | 604 | option may be used any number of times. @command{ld} will search its |
252b5132 RH |
605 | path-list for occurrences of @code{lib@var{archive}.a} for every |
606 | @var{archive} specified. | |
607 | ||
ff5dcc92 | 608 | On systems which support shared libraries, @command{ld} may also search for |
252b5132 | 609 | libraries with extensions other than @code{.a}. Specifically, on ELF |
ff5dcc92 | 610 | and SunOS systems, @command{ld} will search a directory for a library with |
252b5132 RH |
611 | an extension of @code{.so} before searching for one with an extension of |
612 | @code{.a}. By convention, a @code{.so} extension indicates a shared | |
613 | library. | |
614 | ||
615 | The linker will search an archive only once, at the location where it is | |
616 | specified on the command line. If the archive defines a symbol which | |
617 | was undefined in some object which appeared before the archive on the | |
618 | command line, the linker will include the appropriate file(s) from the | |
619 | archive. However, an undefined symbol in an object appearing later on | |
620 | the command line will not cause the linker to search the archive again. | |
621 | ||
ff5dcc92 | 622 | See the @option{-(} option for a way to force the linker to search |
252b5132 RH |
623 | archives multiple times. |
624 | ||
625 | You may list the same archive multiple times on the command line. | |
626 | ||
627 | @ifset GENERIC | |
628 | This type of archive searching is standard for Unix linkers. However, | |
ff5dcc92 | 629 | if you are using @command{ld} on AIX, note that it is different from the |
252b5132 RH |
630 | behaviour of the AIX linker. |
631 | @end ifset | |
632 | ||
633 | @cindex search directory, from cmd line | |
634 | @kindex -L@var{dir} | |
635 | @kindex --library-path=@var{dir} | |
a1ab1d2a | 636 | @item -L@var{searchdir} |
252b5132 | 637 | @itemx --library-path=@var{searchdir} |
ff5dcc92 SC |
638 | Add path @var{searchdir} to the list of paths that @command{ld} will search |
639 | for archive libraries and @command{ld} control scripts. You may use this | |
252b5132 RH |
640 | option any number of times. The directories are searched in the order |
641 | in which they are specified on the command line. Directories specified | |
642 | on the command line are searched before the default directories. All | |
ff5dcc92 | 643 | @option{-L} options apply to all @option{-l} options, regardless of the |
252b5132 RH |
644 | order in which the options appear. |
645 | ||
9c8ebd6a DJ |
646 | If @var{searchdir} begins with @code{=}, then the @code{=} will be replaced |
647 | by the @dfn{sysroot prefix}, a path specified when the linker is configured. | |
648 | ||
252b5132 RH |
649 | @ifset UsesEnvVars |
650 | The default set of paths searched (without being specified with | |
ff5dcc92 | 651 | @samp{-L}) depends on which emulation mode @command{ld} is using, and in |
252b5132 RH |
652 | some cases also on how it was configured. @xref{Environment}. |
653 | @end ifset | |
654 | ||
655 | The paths can also be specified in a link script with the | |
656 | @code{SEARCH_DIR} command. Directories specified this way are searched | |
657 | at the point in which the linker script appears in the command line. | |
658 | ||
659 | @cindex emulation | |
660 | @kindex -m @var{emulation} | |
661 | @item -m@var{emulation} | |
662 | Emulate the @var{emulation} linker. You can list the available | |
663 | emulations with the @samp{--verbose} or @samp{-V} options. | |
664 | ||
665 | If the @samp{-m} option is not used, the emulation is taken from the | |
666 | @code{LDEMULATION} environment variable, if that is defined. | |
667 | ||
668 | Otherwise, the default emulation depends upon how the linker was | |
669 | configured. | |
670 | ||
671 | @cindex link map | |
672 | @kindex -M | |
673 | @kindex --print-map | |
674 | @item -M | |
675 | @itemx --print-map | |
676 | Print a link map to the standard output. A link map provides | |
677 | information about the link, including the following: | |
678 | ||
679 | @itemize @bullet | |
680 | @item | |
3b83e13a | 681 | Where object files are mapped into memory. |
252b5132 RH |
682 | @item |
683 | How common symbols are allocated. | |
684 | @item | |
685 | All archive members included in the link, with a mention of the symbol | |
686 | which caused the archive member to be brought in. | |
3b83e13a NC |
687 | @item |
688 | The values assigned to symbols. | |
689 | ||
690 | Note - symbols whose values are computed by an expression which | |
691 | involves a reference to a previous value of the same symbol may not | |
692 | have correct result displayed in the link map. This is because the | |
693 | linker discards intermediate results and only retains the final value | |
694 | of an expression. Under such circumstances the linker will display | |
695 | the final value enclosed by square brackets. Thus for example a | |
696 | linker script containing: | |
697 | ||
698 | @smallexample | |
699 | foo = 1 | |
700 | foo = foo * 4 | |
701 | foo = foo + 8 | |
702 | @end smallexample | |
703 | ||
704 | will produce the following output in the link map if the @option{-M} | |
705 | option is used: | |
706 | ||
707 | @smallexample | |
708 | 0x00000001 foo = 0x1 | |
709 | [0x0000000c] foo = (foo * 0x4) | |
710 | [0x0000000c] foo = (foo + 0x8) | |
711 | @end smallexample | |
712 | ||
713 | See @ref{Expressions} for more information about expressions in linker | |
714 | scripts. | |
252b5132 RH |
715 | @end itemize |
716 | ||
717 | @kindex -n | |
718 | @cindex read-only text | |
719 | @cindex NMAGIC | |
720 | @kindex --nmagic | |
721 | @item -n | |
722 | @itemx --nmagic | |
fa19fce0 | 723 | Turn off page alignment of sections, and mark the output as |
a1ab1d2a | 724 | @code{NMAGIC} if possible. |
252b5132 RH |
725 | |
726 | @kindex -N | |
727 | @kindex --omagic | |
728 | @cindex read/write from cmd line | |
729 | @cindex OMAGIC | |
a1ab1d2a | 730 | @item -N |
252b5132 RH |
731 | @itemx --omagic |
732 | Set the text and data sections to be readable and writable. Also, do | |
63fd3b82 NC |
733 | not page-align the data segment, and disable linking against shared |
734 | libraries. If the output format supports Unix style magic numbers, | |
4d8907ac DS |
735 | mark the output as @code{OMAGIC}. Note: Although a writable text section |
736 | is allowed for PE-COFF targets, it does not conform to the format | |
737 | specification published by Microsoft. | |
63fd3b82 NC |
738 | |
739 | @kindex --no-omagic | |
740 | @cindex OMAGIC | |
741 | @item --no-omagic | |
742 | This option negates most of the effects of the @option{-N} option. It | |
743 | sets the text section to be read-only, and forces the data segment to | |
744 | be page-aligned. Note - this option does not enable linking against | |
745 | shared libraries. Use @option{-Bdynamic} for this. | |
252b5132 RH |
746 | |
747 | @kindex -o @var{output} | |
748 | @kindex --output=@var{output} | |
749 | @cindex naming the output file | |
750 | @item -o @var{output} | |
751 | @itemx --output=@var{output} | |
ff5dcc92 | 752 | Use @var{output} as the name for the program produced by @command{ld}; if this |
252b5132 RH |
753 | option is not specified, the name @file{a.out} is used by default. The |
754 | script command @code{OUTPUT} can also specify the output file name. | |
755 | ||
756 | @kindex -O @var{level} | |
757 | @cindex generating optimized output | |
758 | @item -O @var{level} | |
ff5dcc92 | 759 | If @var{level} is a numeric values greater than zero @command{ld} optimizes |
252b5132 RH |
760 | the output. This might take significantly longer and therefore probably |
761 | should only be enabled for the final binary. | |
762 | ||
a712da20 NC |
763 | @kindex -q |
764 | @kindex --emit-relocs | |
765 | @cindex retain relocations in final executable | |
766 | @item -q | |
767 | @itemx --emit-relocs | |
768 | Leave relocation sections and contents in fully linked exececutables. | |
769 | Post link analysis and optimization tools may need this information in | |
770 | order to perform correct modifications of executables. This results | |
771 | in larger executables. | |
772 | ||
dbab7a7b NC |
773 | This option is currently only supported on ELF platforms. |
774 | ||
252b5132 RH |
775 | @cindex partial link |
776 | @cindex relocatable output | |
777 | @kindex -r | |
1049f94e | 778 | @kindex --relocatable |
252b5132 | 779 | @item -r |
1049f94e | 780 | @itemx --relocatable |
252b5132 | 781 | Generate relocatable output---i.e., generate an output file that can in |
ff5dcc92 | 782 | turn serve as input to @command{ld}. This is often called @dfn{partial |
252b5132 RH |
783 | linking}. As a side effect, in environments that support standard Unix |
784 | magic numbers, this option also sets the output file's magic number to | |
785 | @code{OMAGIC}. | |
ff5dcc92 | 786 | @c ; see @option{-N}. |
252b5132 RH |
787 | If this option is not specified, an absolute file is produced. When |
788 | linking C++ programs, this option @emph{will not} resolve references to | |
789 | constructors; to do that, use @samp{-Ur}. | |
790 | ||
62bf86b4 HPN |
791 | When an input file does not have the same format as the output file, |
792 | partial linking is only supported if that input file does not contain any | |
793 | relocations. Different output formats can have further restrictions; for | |
794 | example some @code{a.out}-based formats do not support partial linking | |
795 | with input files in other formats at all. | |
796 | ||
252b5132 RH |
797 | This option does the same thing as @samp{-i}. |
798 | ||
799 | @kindex -R @var{file} | |
800 | @kindex --just-symbols=@var{file} | |
801 | @cindex symbol-only input | |
802 | @item -R @var{filename} | |
803 | @itemx --just-symbols=@var{filename} | |
804 | Read symbol names and their addresses from @var{filename}, but do not | |
805 | relocate it or include it in the output. This allows your output file | |
806 | to refer symbolically to absolute locations of memory defined in other | |
807 | programs. You may use this option more than once. | |
808 | ||
ff5dcc92 | 809 | For compatibility with other ELF linkers, if the @option{-R} option is |
252b5132 | 810 | followed by a directory name, rather than a file name, it is treated as |
ff5dcc92 | 811 | the @option{-rpath} option. |
252b5132 RH |
812 | |
813 | @kindex -s | |
814 | @kindex --strip-all | |
815 | @cindex strip all symbols | |
a1ab1d2a | 816 | @item -s |
252b5132 RH |
817 | @itemx --strip-all |
818 | Omit all symbol information from the output file. | |
819 | ||
820 | @kindex -S | |
821 | @kindex --strip-debug | |
822 | @cindex strip debugger symbols | |
a1ab1d2a | 823 | @item -S |
252b5132 RH |
824 | @itemx --strip-debug |
825 | Omit debugger symbol information (but not all symbols) from the output file. | |
826 | ||
827 | @kindex -t | |
828 | @kindex --trace | |
829 | @cindex input files, displaying | |
a1ab1d2a | 830 | @item -t |
252b5132 | 831 | @itemx --trace |
ff5dcc92 | 832 | Print the names of the input files as @command{ld} processes them. |
252b5132 RH |
833 | |
834 | @kindex -T @var{script} | |
835 | @kindex --script=@var{script} | |
836 | @cindex script files | |
837 | @item -T @var{scriptfile} | |
838 | @itemx --script=@var{scriptfile} | |
839 | Use @var{scriptfile} as the linker script. This script replaces | |
ff5dcc92 | 840 | @command{ld}'s default linker script (rather than adding to it), so |
252b5132 | 841 | @var{commandfile} must specify everything necessary to describe the |
114283d8 NC |
842 | output file. @xref{Scripts}. If @var{scriptfile} does not exist in |
843 | the current directory, @code{ld} looks for it in the directories | |
844 | specified by any preceding @samp{-L} options. Multiple @samp{-T} | |
845 | options accumulate. | |
252b5132 RH |
846 | |
847 | @kindex -u @var{symbol} | |
848 | @kindex --undefined=@var{symbol} | |
849 | @cindex undefined symbol | |
850 | @item -u @var{symbol} | |
851 | @itemx --undefined=@var{symbol} | |
852 | Force @var{symbol} to be entered in the output file as an undefined | |
853 | symbol. Doing this may, for example, trigger linking of additional | |
854 | modules from standard libraries. @samp{-u} may be repeated with | |
855 | different option arguments to enter additional undefined symbols. This | |
856 | option is equivalent to the @code{EXTERN} linker script command. | |
857 | ||
858 | @kindex -Ur | |
859 | @cindex constructors | |
a1ab1d2a | 860 | @item -Ur |
252b5132 RH |
861 | For anything other than C++ programs, this option is equivalent to |
862 | @samp{-r}: it generates relocatable output---i.e., an output file that can in | |
ff5dcc92 | 863 | turn serve as input to @command{ld}. When linking C++ programs, @samp{-Ur} |
252b5132 RH |
864 | @emph{does} resolve references to constructors, unlike @samp{-r}. |
865 | It does not work to use @samp{-Ur} on files that were themselves linked | |
866 | with @samp{-Ur}; once the constructor table has been built, it cannot | |
867 | be added to. Use @samp{-Ur} only for the last partial link, and | |
868 | @samp{-r} for the others. | |
869 | ||
577a0623 AM |
870 | @kindex --unique[=@var{SECTION}] |
871 | @item --unique[=@var{SECTION}] | |
872 | Creates a separate output section for every input section matching | |
873 | @var{SECTION}, or if the optional wildcard @var{SECTION} argument is | |
874 | missing, for every orphan input section. An orphan section is one not | |
875 | specifically mentioned in a linker script. You may use this option | |
876 | multiple times on the command line; It prevents the normal merging of | |
877 | input sections with the same name, overriding output section assignments | |
878 | in a linker script. | |
a854a4a7 | 879 | |
252b5132 RH |
880 | @kindex -v |
881 | @kindex -V | |
882 | @kindex --version | |
883 | @cindex version | |
884 | @item -v | |
885 | @itemx --version | |
886 | @itemx -V | |
ff5dcc92 | 887 | Display the version number for @command{ld}. The @option{-V} option also |
252b5132 RH |
888 | lists the supported emulations. |
889 | ||
890 | @kindex -x | |
891 | @kindex --discard-all | |
892 | @cindex deleting local symbols | |
893 | @item -x | |
894 | @itemx --discard-all | |
895 | Delete all local symbols. | |
896 | ||
897 | @kindex -X | |
898 | @kindex --discard-locals | |
899 | @cindex local symbols, deleting | |
900 | @cindex L, deleting symbols beginning | |
a1ab1d2a | 901 | @item -X |
252b5132 RH |
902 | @itemx --discard-locals |
903 | Delete all temporary local symbols. For most targets, this is all local | |
904 | symbols whose names begin with @samp{L}. | |
905 | ||
906 | @kindex -y @var{symbol} | |
907 | @kindex --trace-symbol=@var{symbol} | |
908 | @cindex symbol tracing | |
909 | @item -y @var{symbol} | |
910 | @itemx --trace-symbol=@var{symbol} | |
911 | Print the name of each linked file in which @var{symbol} appears. This | |
912 | option may be given any number of times. On many systems it is necessary | |
913 | to prepend an underscore. | |
914 | ||
915 | This option is useful when you have an undefined symbol in your link but | |
916 | don't know where the reference is coming from. | |
917 | ||
918 | @kindex -Y @var{path} | |
919 | @item -Y @var{path} | |
920 | Add @var{path} to the default library search path. This option exists | |
921 | for Solaris compatibility. | |
922 | ||
923 | @kindex -z @var{keyword} | |
924 | @item -z @var{keyword} | |
cd6d6c15 NC |
925 | The recognized keywords are: |
926 | @table @samp | |
927 | ||
928 | @item combreloc | |
929 | Combines multiple reloc sections and sorts them to make dynamic symbol | |
930 | lookup caching possible. | |
931 | ||
932 | @item defs | |
560e09e9 | 933 | Disallows undefined symbols in object files. Undefined symbols in |
07f3b6ad | 934 | shared libraries are still allowed. |
cd6d6c15 | 935 | |
6aa29e7b JJ |
936 | @item execstack |
937 | Marks the object as requiring executable stack. | |
938 | ||
cd6d6c15 NC |
939 | @item initfirst |
940 | This option is only meaningful when building a shared object. | |
941 | It marks the object so that its runtime initialization will occur | |
942 | before the runtime initialization of any other objects brought into | |
943 | the process at the same time. Similarly the runtime finalization of | |
944 | the object will occur after the runtime finalization of any other | |
945 | objects. | |
946 | ||
947 | @item interpose | |
948 | Marks the object that its symbol table interposes before all symbols | |
949 | but the primary executable. | |
950 | ||
951 | @item loadfltr | |
952 | Marks the object that its filters be processed immediately at | |
953 | runtime. | |
954 | ||
955 | @item muldefs | |
956 | Allows multiple definitions. | |
957 | ||
958 | @item nocombreloc | |
959 | Disables multiple reloc sections combining. | |
960 | ||
961 | @item nocopyreloc | |
962 | Disables production of copy relocs. | |
963 | ||
964 | @item nodefaultlib | |
965 | Marks the object that the search for dependencies of this object will | |
966 | ignore any default library search paths. | |
967 | ||
968 | @item nodelete | |
969 | Marks the object shouldn't be unloaded at runtime. | |
970 | ||
971 | @item nodlopen | |
972 | Marks the object not available to @code{dlopen}. | |
973 | ||
974 | @item nodump | |
975 | Marks the object can not be dumped by @code{dldump}. | |
976 | ||
6aa29e7b JJ |
977 | @item noexecstack |
978 | Marks the object as not requiring executable stack. | |
979 | ||
980 | @item norelro | |
981 | Don't create an ELF @code{PT_GNU_RELRO} segment header in the object. | |
982 | ||
cd6d6c15 NC |
983 | @item now |
984 | When generating an executable or shared library, mark it to tell the | |
985 | dynamic linker to resolve all symbols when the program is started, or | |
986 | when the shared library is linked to using dlopen, instead of | |
987 | deferring function call resolution to the point when the function is | |
988 | first called. | |
989 | ||
990 | @item origin | |
991 | Marks the object may contain $ORIGIN. | |
992 | ||
6aa29e7b JJ |
993 | @item relro |
994 | Create an ELF @code{PT_GNU_RELRO} segment header in the object. | |
995 | ||
cd6d6c15 NC |
996 | @end table |
997 | ||
998 | Other keywords are ignored for Solaris compatibility. | |
252b5132 RH |
999 | |
1000 | @kindex -( | |
1001 | @cindex groups of archives | |
1002 | @item -( @var{archives} -) | |
1003 | @itemx --start-group @var{archives} --end-group | |
1004 | The @var{archives} should be a list of archive files. They may be | |
1005 | either explicit file names, or @samp{-l} options. | |
1006 | ||
1007 | The specified archives are searched repeatedly until no new undefined | |
1008 | references are created. Normally, an archive is searched only once in | |
1009 | the order that it is specified on the command line. If a symbol in that | |
1010 | archive is needed to resolve an undefined symbol referred to by an | |
1011 | object in an archive that appears later on the command line, the linker | |
1012 | would not be able to resolve that reference. By grouping the archives, | |
1013 | they all be searched repeatedly until all possible references are | |
1014 | resolved. | |
1015 | ||
1016 | Using this option has a significant performance cost. It is best to use | |
1017 | it only when there are unavoidable circular references between two or | |
1018 | more archives. | |
1019 | ||
69da35b5 NC |
1020 | @kindex --accept-unknown-input-arch |
1021 | @kindex --no-accept-unknown-input-arch | |
1022 | @item --accept-unknown-input-arch | |
1023 | @itemx --no-accept-unknown-input-arch | |
1024 | Tells the linker to accept input files whose architecture cannot be | |
2ca22b03 | 1025 | recognised. The assumption is that the user knows what they are doing |
69da35b5 NC |
1026 | and deliberately wants to link in these unknown input files. This was |
1027 | the default behaviour of the linker, before release 2.14. The default | |
1028 | behaviour from release 2.14 onwards is to reject such input files, and | |
1029 | so the @samp{--accept-unknown-input-arch} option has been added to | |
1030 | restore the old behaviour. | |
2ca22b03 | 1031 | |
4a43e768 AM |
1032 | @kindex --as-needed |
1033 | @kindex --no-as-needed | |
1034 | @item --as-needed | |
1035 | @itemx --no-as-needed | |
1036 | This option affects ELF DT_NEEDED tags for dynamic libraries mentioned | |
1037 | on the command line after the @option{--as-needed} option. Normally, | |
1038 | the linker will add a DT_NEEDED tag for each dynamic library mentioned | |
1039 | on the command line, regardless of whether the library is actually | |
77cfaee6 AM |
1040 | needed. @option{--as-needed} causes DT_NEEDED tags to only be emitted |
1041 | for libraries that satisfy some symbol reference from regular objects | |
1042 | which is undefined at the point that the library was linked. | |
4a43e768 AM |
1043 | @option{--no-as-needed} restores the default behaviour. |
1044 | ||
e56f61be L |
1045 | @kindex --add-needed |
1046 | @kindex --no-add-needed | |
1047 | @item --add-needed | |
1048 | @itemx --no-add-needed | |
1049 | This option affects the treatment of dynamic libraries from ELF | |
1050 | DT_NEEDED tags in dynamic libraries mentioned on the command line after | |
1051 | the @option{--no-add-needed} option. Normally, the linker will add | |
1052 | a DT_NEEDED tag for each dynamic library from DT_NEEDED tags. | |
1053 | @option{--no-add-needed} causes DT_NEEDED tags will never be emitted | |
1054 | for those libraries from DT_NEEDED tags. @option{--add-needed} restores | |
1055 | the default behaviour. | |
1056 | ||
252b5132 RH |
1057 | @kindex -assert @var{keyword} |
1058 | @item -assert @var{keyword} | |
1059 | This option is ignored for SunOS compatibility. | |
1060 | ||
1061 | @kindex -Bdynamic | |
1062 | @kindex -dy | |
1063 | @kindex -call_shared | |
1064 | @item -Bdynamic | |
1065 | @itemx -dy | |
1066 | @itemx -call_shared | |
1067 | Link against dynamic libraries. This is only meaningful on platforms | |
1068 | for which shared libraries are supported. This option is normally the | |
1069 | default on such platforms. The different variants of this option are | |
1070 | for compatibility with various systems. You may use this option | |
1071 | multiple times on the command line: it affects library searching for | |
da8bce14 | 1072 | @option{-l} options which follow it. |
252b5132 | 1073 | |
a1ab1d2a UD |
1074 | @kindex -Bgroup |
1075 | @item -Bgroup | |
1076 | Set the @code{DF_1_GROUP} flag in the @code{DT_FLAGS_1} entry in the dynamic | |
1077 | section. This causes the runtime linker to handle lookups in this | |
1078 | object and its dependencies to be performed only inside the group. | |
560e09e9 NC |
1079 | @option{--unresolved-symbols=report-all} is implied. This option is |
1080 | only meaningful on ELF platforms which support shared libraries. | |
a1ab1d2a | 1081 | |
252b5132 RH |
1082 | @kindex -Bstatic |
1083 | @kindex -dn | |
1084 | @kindex -non_shared | |
1085 | @kindex -static | |
a1ab1d2a | 1086 | @item -Bstatic |
252b5132 RH |
1087 | @itemx -dn |
1088 | @itemx -non_shared | |
1089 | @itemx -static | |
1090 | Do not link against shared libraries. This is only meaningful on | |
1091 | platforms for which shared libraries are supported. The different | |
1092 | variants of this option are for compatibility with various systems. You | |
1093 | may use this option multiple times on the command line: it affects | |
560e09e9 | 1094 | library searching for @option{-l} options which follow it. This |
e9156f74 NC |
1095 | option also implies @option{--unresolved-symbols=report-all}. This |
1096 | option can be used with @option{-shared}. Doing so means that a | |
1097 | shared library is being created but that all of the library's external | |
1098 | references must be resolved by pulling in entries from static | |
1099 | libraries. | |
252b5132 RH |
1100 | |
1101 | @kindex -Bsymbolic | |
1102 | @item -Bsymbolic | |
1103 | When creating a shared library, bind references to global symbols to the | |
1104 | definition within the shared library, if any. Normally, it is possible | |
1105 | for a program linked against a shared library to override the definition | |
1106 | within the shared library. This option is only meaningful on ELF | |
1107 | platforms which support shared libraries. | |
1108 | ||
1109 | @kindex --check-sections | |
1110 | @kindex --no-check-sections | |
1111 | @item --check-sections | |
308b1ffd | 1112 | @itemx --no-check-sections |
252b5132 RH |
1113 | Asks the linker @emph{not} to check section addresses after they have |
1114 | been assigned to see if there any overlaps. Normally the linker will | |
1115 | perform this check, and if it finds any overlaps it will produce | |
1116 | suitable error messages. The linker does know about, and does make | |
1117 | allowances for sections in overlays. The default behaviour can be | |
560e09e9 | 1118 | restored by using the command line switch @option{--check-sections}. |
252b5132 RH |
1119 | |
1120 | @cindex cross reference table | |
1121 | @kindex --cref | |
1122 | @item --cref | |
1123 | Output a cross reference table. If a linker map file is being | |
1124 | generated, the cross reference table is printed to the map file. | |
1125 | Otherwise, it is printed on the standard output. | |
1126 | ||
1127 | The format of the table is intentionally simple, so that it may be | |
1128 | easily processed by a script if necessary. The symbols are printed out, | |
1129 | sorted by name. For each symbol, a list of file names is given. If the | |
1130 | symbol is defined, the first file listed is the location of the | |
1131 | definition. The remaining files contain references to the symbol. | |
1132 | ||
4818e05f AM |
1133 | @cindex common allocation |
1134 | @kindex --no-define-common | |
1135 | @item --no-define-common | |
1136 | This option inhibits the assignment of addresses to common symbols. | |
1137 | The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect. | |
1138 | @xref{Miscellaneous Commands}. | |
1139 | ||
1140 | The @samp{--no-define-common} option allows decoupling | |
1141 | the decision to assign addresses to Common symbols from the choice | |
1142 | of the output file type; otherwise a non-Relocatable output type | |
1143 | forces assigning addresses to Common symbols. | |
1144 | Using @samp{--no-define-common} allows Common symbols that are referenced | |
1145 | from a shared library to be assigned addresses only in the main program. | |
1146 | This eliminates the unused duplicate space in the shared library, | |
1147 | and also prevents any possible confusion over resolving to the wrong | |
1148 | duplicate when there are many dynamic modules with specialized search | |
1149 | paths for runtime symbol resolution. | |
1150 | ||
252b5132 RH |
1151 | @cindex symbols, from command line |
1152 | @kindex --defsym @var{symbol}=@var{exp} | |
1153 | @item --defsym @var{symbol}=@var{expression} | |
1154 | Create a global symbol in the output file, containing the absolute | |
1155 | address given by @var{expression}. You may use this option as many | |
1156 | times as necessary to define multiple symbols in the command line. A | |
1157 | limited form of arithmetic is supported for the @var{expression} in this | |
1158 | context: you may give a hexadecimal constant or the name of an existing | |
1159 | symbol, or use @code{+} and @code{-} to add or subtract hexadecimal | |
1160 | constants or symbols. If you need more elaborate expressions, consider | |
1161 | using the linker command language from a script (@pxref{Assignments,, | |
1162 | Assignment: Symbol Definitions}). @emph{Note:} there should be no white | |
1163 | space between @var{symbol}, the equals sign (``@key{=}''), and | |
1164 | @var{expression}. | |
1165 | ||
1166 | @cindex demangling, from command line | |
28c309a2 | 1167 | @kindex --demangle[=@var{style}] |
252b5132 | 1168 | @kindex --no-demangle |
28c309a2 | 1169 | @item --demangle[=@var{style}] |
252b5132 RH |
1170 | @itemx --no-demangle |
1171 | These options control whether to demangle symbol names in error messages | |
1172 | and other output. When the linker is told to demangle, it tries to | |
1173 | present symbol names in a readable fashion: it strips leading | |
1174 | underscores if they are used by the object file format, and converts C++ | |
a1ab1d2a UD |
1175 | mangled symbol names into user readable names. Different compilers have |
1176 | different mangling styles. The optional demangling style argument can be used | |
1177 | to choose an appropriate demangling style for your compiler. The linker will | |
28c309a2 NC |
1178 | demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE} |
1179 | is set. These options may be used to override the default. | |
252b5132 RH |
1180 | |
1181 | @cindex dynamic linker, from command line | |
506eee22 | 1182 | @kindex -I@var{file} |
252b5132 RH |
1183 | @kindex --dynamic-linker @var{file} |
1184 | @item --dynamic-linker @var{file} | |
1185 | Set the name of the dynamic linker. This is only meaningful when | |
1186 | generating dynamically linked ELF executables. The default dynamic | |
1187 | linker is normally correct; don't use this unless you know what you are | |
1188 | doing. | |
1189 | ||
7ce691ae C |
1190 | |
1191 | @kindex --fatal-warnings | |
1192 | @item --fatal-warnings | |
1193 | Treat all warnings as errors. | |
1194 | ||
252b5132 RH |
1195 | @kindex --force-exe-suffix |
1196 | @item --force-exe-suffix | |
1197 | Make sure that an output file has a .exe suffix. | |
1198 | ||
1199 | If a successfully built fully linked output file does not have a | |
1200 | @code{.exe} or @code{.dll} suffix, this option forces the linker to copy | |
1201 | the output file to one of the same name with a @code{.exe} suffix. This | |
1202 | option is useful when using unmodified Unix makefiles on a Microsoft | |
1203 | Windows host, since some versions of Windows won't run an image unless | |
1204 | it ends in a @code{.exe} suffix. | |
1205 | ||
1206 | @kindex --gc-sections | |
1207 | @kindex --no-gc-sections | |
1208 | @cindex garbage collection | |
1209 | @item --no-gc-sections | |
1210 | @itemx --gc-sections | |
1211 | Enable garbage collection of unused input sections. It is ignored on | |
1212 | targets that do not support this option. This option is not compatible | |
755306be EB |
1213 | with @samp{-r}. The default behaviour (of not performing this garbage |
1214 | collection) can be restored by specifying @samp{--no-gc-sections} on | |
1215 | the command line. | |
252b5132 RH |
1216 | |
1217 | @cindex help | |
1218 | @cindex usage | |
1219 | @kindex --help | |
1220 | @item --help | |
1221 | Print a summary of the command-line options on the standard output and exit. | |
1222 | ||
ea20a7da CC |
1223 | @kindex --target-help |
1224 | @item --target-help | |
1225 | Print a summary of all target specific options on the standard output and exit. | |
1226 | ||
252b5132 RH |
1227 | @kindex -Map |
1228 | @item -Map @var{mapfile} | |
1229 | Print a link map to the file @var{mapfile}. See the description of the | |
560e09e9 | 1230 | @option{-M} option, above. |
252b5132 RH |
1231 | |
1232 | @cindex memory usage | |
1233 | @kindex --no-keep-memory | |
1234 | @item --no-keep-memory | |
ff5dcc92 SC |
1235 | @command{ld} normally optimizes for speed over memory usage by caching the |
1236 | symbol tables of input files in memory. This option tells @command{ld} to | |
252b5132 | 1237 | instead optimize for memory usage, by rereading the symbol tables as |
ff5dcc92 | 1238 | necessary. This may be required if @command{ld} runs out of memory space |
252b5132 RH |
1239 | while linking a large executable. |
1240 | ||
1241 | @kindex --no-undefined | |
a1ab1d2a | 1242 | @kindex -z defs |
252b5132 | 1243 | @item --no-undefined |
a1ab1d2a | 1244 | @itemx -z defs |
560e09e9 NC |
1245 | Report unresolved symbol references from regular object files. This |
1246 | is done even if the linker is creating a non-symbolic shared library. | |
1247 | The switch @option{--[no-]allow-shlib-undefined} controls the | |
1248 | behaviour for reporting unresolved references found in shared | |
1249 | libraries being linked in. | |
252b5132 | 1250 | |
aa713662 L |
1251 | @kindex --allow-multiple-definition |
1252 | @kindex -z muldefs | |
1253 | @item --allow-multiple-definition | |
1254 | @itemx -z muldefs | |
1255 | Normally when a symbol is defined multiple times, the linker will | |
1256 | report a fatal error. These options allow multiple definitions and the | |
1257 | first definition will be used. | |
1258 | ||
b79e8c78 | 1259 | @kindex --allow-shlib-undefined |
ae9a127f | 1260 | @kindex --no-allow-shlib-undefined |
b79e8c78 | 1261 | @item --allow-shlib-undefined |
ae9a127f | 1262 | @itemx --no-allow-shlib-undefined |
560e09e9 NC |
1263 | Allows (the default) or disallows undefined symbols in shared libraries. |
1264 | This switch is similar to @option{--no-undefined} except that it | |
1265 | determines the behaviour when the undefined symbols are in a | |
1266 | shared library rather than a regular object file. It does not affect | |
1267 | how undefined symbols in regular object files are handled. | |
1268 | ||
1269 | The reason that @option{--allow-shlib-undefined} is the default is that | |
1270 | the shared library being specified at link time may not be the same as | |
1271 | the one that is available at load time, so the symbols might actually be | |
ae9a127f | 1272 | resolvable at load time. Plus there are some systems, (eg BeOS) where |
560e09e9 NC |
1273 | undefined symbols in shared libraries is normal. (The kernel patches |
1274 | them at load time to select which function is most appropriate | |
1275 | for the current architecture. This is used for example to dynamically | |
1276 | select an appropriate memset function). Apparently it is also normal | |
1277 | for HPPA shared libraries to have undefined symbols. | |
b79e8c78 | 1278 | |
31941635 L |
1279 | @kindex --no-undefined-version |
1280 | @item --no-undefined-version | |
1281 | Normally when a symbol has an undefined version, the linker will ignore | |
1282 | it. This option disallows symbols with undefined version and a fatal error | |
1283 | will be issued instead. | |
1284 | ||
3e3b46e5 PB |
1285 | @kindex --default-symver |
1286 | @item --default-symver | |
1287 | Create and use a default symbol version (the soname) for unversioned | |
fc0e6df6 PB |
1288 | exported symbols. |
1289 | ||
1290 | @kindex --default-imported-symver | |
1291 | @item --default-imported-symver | |
1292 | Create and use a default symbol version (the soname) for unversioned | |
1293 | imported symbols. | |
3e3b46e5 | 1294 | |
252b5132 RH |
1295 | @kindex --no-warn-mismatch |
1296 | @item --no-warn-mismatch | |
ff5dcc92 | 1297 | Normally @command{ld} will give an error if you try to link together input |
252b5132 RH |
1298 | files that are mismatched for some reason, perhaps because they have |
1299 | been compiled for different processors or for different endiannesses. | |
ff5dcc92 | 1300 | This option tells @command{ld} that it should silently permit such possible |
252b5132 RH |
1301 | errors. This option should only be used with care, in cases when you |
1302 | have taken some special action that ensures that the linker errors are | |
1303 | inappropriate. | |
1304 | ||
1305 | @kindex --no-whole-archive | |
1306 | @item --no-whole-archive | |
ff5dcc92 | 1307 | Turn off the effect of the @option{--whole-archive} option for subsequent |
252b5132 RH |
1308 | archive files. |
1309 | ||
1310 | @cindex output file after errors | |
1311 | @kindex --noinhibit-exec | |
1312 | @item --noinhibit-exec | |
1313 | Retain the executable output file whenever it is still usable. | |
1314 | Normally, the linker will not produce an output file if it encounters | |
1315 | errors during the link process; it exits without writing an output file | |
1316 | when it issues any error whatsoever. | |
1317 | ||
0a9c1c8e CD |
1318 | @kindex -nostdlib |
1319 | @item -nostdlib | |
1320 | Only search library directories explicitly specified on the | |
1321 | command line. Library directories specified in linker scripts | |
1322 | (including linker scripts specified on the command line) are ignored. | |
1323 | ||
252b5132 RH |
1324 | @ifclear SingleFormat |
1325 | @kindex --oformat | |
1326 | @item --oformat @var{output-format} | |
ff5dcc92 SC |
1327 | @command{ld} may be configured to support more than one kind of object |
1328 | file. If your @command{ld} is configured this way, you can use the | |
252b5132 | 1329 | @samp{--oformat} option to specify the binary format for the output |
ff5dcc92 SC |
1330 | object file. Even when @command{ld} is configured to support alternative |
1331 | object formats, you don't usually need to specify this, as @command{ld} | |
252b5132 RH |
1332 | should be configured to produce as a default output format the most |
1333 | usual format on each machine. @var{output-format} is a text string, the | |
1334 | name of a particular format supported by the BFD libraries. (You can | |
1335 | list the available binary formats with @samp{objdump -i}.) The script | |
1336 | command @code{OUTPUT_FORMAT} can also specify the output format, but | |
1337 | this option overrides it. @xref{BFD}. | |
1338 | @end ifclear | |
1339 | ||
36af4a4e JJ |
1340 | @kindex -pie |
1341 | @kindex --pic-executable | |
1342 | @item -pie | |
1343 | @itemx --pic-executable | |
1344 | @cindex position independent executables | |
1345 | Create a position independent executable. This is currently only supported on | |
1346 | ELF platforms. Position independent executables are similar to shared | |
1347 | libraries in that they are relocated by the dynamic linker to the virtual | |
7e7d5768 | 1348 | address the OS chooses for them (which can vary between invocations). Like |
36af4a4e JJ |
1349 | normal dynamically linked executables they can be executed and symbols |
1350 | defined in the executable cannot be overridden by shared libraries. | |
1351 | ||
252b5132 RH |
1352 | @kindex -qmagic |
1353 | @item -qmagic | |
1354 | This option is ignored for Linux compatibility. | |
1355 | ||
1356 | @kindex -Qy | |
1357 | @item -Qy | |
1358 | This option is ignored for SVR4 compatibility. | |
1359 | ||
1360 | @kindex --relax | |
1361 | @cindex synthesizing linker | |
1362 | @cindex relaxing addressing modes | |
1363 | @item --relax | |
a1ab1d2a | 1364 | An option with machine dependent effects. |
252b5132 RH |
1365 | @ifset GENERIC |
1366 | This option is only supported on a few targets. | |
1367 | @end ifset | |
1368 | @ifset H8300 | |
ff5dcc92 | 1369 | @xref{H8/300,,@command{ld} and the H8/300}. |
252b5132 RH |
1370 | @end ifset |
1371 | @ifset I960 | |
ff5dcc92 | 1372 | @xref{i960,, @command{ld} and the Intel 960 family}. |
252b5132 | 1373 | @end ifset |
e0001a05 NC |
1374 | @ifset XTENSA |
1375 | @xref{Xtensa,, @command{ld} and Xtensa Processors}. | |
1376 | @end ifset | |
93fd0973 SC |
1377 | @ifset M68HC11 |
1378 | @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}. | |
1379 | @end ifset | |
252b5132 RH |
1380 | |
1381 | On some platforms, the @samp{--relax} option performs global | |
1382 | optimizations that become possible when the linker resolves addressing | |
1383 | in the program, such as relaxing address modes and synthesizing new | |
1384 | instructions in the output object file. | |
1385 | ||
1386 | On some platforms these link time global optimizations may make symbolic | |
1387 | debugging of the resulting executable impossible. | |
1388 | @ifset GENERIC | |
1389 | This is known to be | |
1390 | the case for the Matsushita MN10200 and MN10300 family of processors. | |
1391 | @end ifset | |
1392 | ||
1393 | @ifset GENERIC | |
1394 | On platforms where this is not supported, @samp{--relax} is accepted, | |
1395 | but ignored. | |
1396 | @end ifset | |
1397 | ||
1398 | @cindex retaining specified symbols | |
1399 | @cindex stripping all but some symbols | |
1400 | @cindex symbols, retaining selectively | |
1401 | @item --retain-symbols-file @var{filename} | |
1402 | Retain @emph{only} the symbols listed in the file @var{filename}, | |
1403 | discarding all others. @var{filename} is simply a flat file, with one | |
1404 | symbol name per line. This option is especially useful in environments | |
1405 | @ifset GENERIC | |
1406 | (such as VxWorks) | |
1407 | @end ifset | |
1408 | where a large global symbol table is accumulated gradually, to conserve | |
1409 | run-time memory. | |
1410 | ||
1411 | @samp{--retain-symbols-file} does @emph{not} discard undefined symbols, | |
1412 | or symbols needed for relocations. | |
1413 | ||
1414 | You may only specify @samp{--retain-symbols-file} once in the command | |
1415 | line. It overrides @samp{-s} and @samp{-S}. | |
1416 | ||
1417 | @ifset GENERIC | |
1418 | @item -rpath @var{dir} | |
1419 | @cindex runtime library search path | |
1420 | @kindex -rpath | |
1421 | Add a directory to the runtime library search path. This is used when | |
ff5dcc92 | 1422 | linking an ELF executable with shared objects. All @option{-rpath} |
252b5132 | 1423 | arguments are concatenated and passed to the runtime linker, which uses |
ff5dcc92 | 1424 | them to locate shared objects at runtime. The @option{-rpath} option is |
252b5132 RH |
1425 | also used when locating shared objects which are needed by shared |
1426 | objects explicitly included in the link; see the description of the | |
ff5dcc92 | 1427 | @option{-rpath-link} option. If @option{-rpath} is not used when linking an |
252b5132 RH |
1428 | ELF executable, the contents of the environment variable |
1429 | @code{LD_RUN_PATH} will be used if it is defined. | |
1430 | ||
ff5dcc92 | 1431 | The @option{-rpath} option may also be used on SunOS. By default, on |
252b5132 | 1432 | SunOS, the linker will form a runtime search patch out of all the |
ff5dcc92 SC |
1433 | @option{-L} options it is given. If a @option{-rpath} option is used, the |
1434 | runtime search path will be formed exclusively using the @option{-rpath} | |
1435 | options, ignoring the @option{-L} options. This can be useful when using | |
1436 | gcc, which adds many @option{-L} options which may be on NFS mounted | |
252b5132 RH |
1437 | filesystems. |
1438 | ||
ff5dcc92 | 1439 | For compatibility with other ELF linkers, if the @option{-R} option is |
252b5132 | 1440 | followed by a directory name, rather than a file name, it is treated as |
ff5dcc92 | 1441 | the @option{-rpath} option. |
252b5132 RH |
1442 | @end ifset |
1443 | ||
1444 | @ifset GENERIC | |
1445 | @cindex link-time runtime library search path | |
1446 | @kindex -rpath-link | |
1447 | @item -rpath-link @var{DIR} | |
1448 | When using ELF or SunOS, one shared library may require another. This | |
1449 | happens when an @code{ld -shared} link includes a shared library as one | |
1450 | of the input files. | |
1451 | ||
1452 | When the linker encounters such a dependency when doing a non-shared, | |
1453 | non-relocatable link, it will automatically try to locate the required | |
1454 | shared library and include it in the link, if it is not included | |
ff5dcc92 | 1455 | explicitly. In such a case, the @option{-rpath-link} option |
252b5132 | 1456 | specifies the first set of directories to search. The |
ff5dcc92 | 1457 | @option{-rpath-link} option may specify a sequence of directory names |
252b5132 RH |
1458 | either by specifying a list of names separated by colons, or by |
1459 | appearing multiple times. | |
1460 | ||
28c309a2 NC |
1461 | This option should be used with caution as it overrides the search path |
1462 | that may have been hard compiled into a shared library. In such a case it | |
1463 | is possible to use unintentionally a different search path than the | |
1464 | runtime linker would do. | |
1465 | ||
252b5132 RH |
1466 | The linker uses the following search paths to locate required shared |
1467 | libraries. | |
1468 | @enumerate | |
1469 | @item | |
ff5dcc92 | 1470 | Any directories specified by @option{-rpath-link} options. |
252b5132 | 1471 | @item |
ff5dcc92 SC |
1472 | Any directories specified by @option{-rpath} options. The difference |
1473 | between @option{-rpath} and @option{-rpath-link} is that directories | |
1474 | specified by @option{-rpath} options are included in the executable and | |
1475 | used at runtime, whereas the @option{-rpath-link} option is only effective | |
dcb0bd0e | 1476 | at link time. It is for the native linker only. |
252b5132 | 1477 | @item |
ff5dcc92 | 1478 | On an ELF system, if the @option{-rpath} and @code{rpath-link} options |
252b5132 | 1479 | were not used, search the contents of the environment variable |
dcb0bd0e | 1480 | @code{LD_RUN_PATH}. It is for the native linker only. |
252b5132 | 1481 | @item |
ff5dcc92 SC |
1482 | On SunOS, if the @option{-rpath} option was not used, search any |
1483 | directories specified using @option{-L} options. | |
252b5132 RH |
1484 | @item |
1485 | For a native linker, the contents of the environment variable | |
1486 | @code{LD_LIBRARY_PATH}. | |
1487 | @item | |
ec4eb78a L |
1488 | For a native ELF linker, the directories in @code{DT_RUNPATH} or |
1489 | @code{DT_RPATH} of a shared library are searched for shared | |
1490 | libraries needed by it. The @code{DT_RPATH} entries are ignored if | |
1491 | @code{DT_RUNPATH} entries exist. | |
1492 | @item | |
252b5132 RH |
1493 | The default directories, normally @file{/lib} and @file{/usr/lib}. |
1494 | @item | |
1495 | For a native linker on an ELF system, if the file @file{/etc/ld.so.conf} | |
1496 | exists, the list of directories found in that file. | |
1497 | @end enumerate | |
1498 | ||
1499 | If the required shared library is not found, the linker will issue a | |
1500 | warning and continue with the link. | |
1501 | @end ifset | |
1502 | ||
1503 | @kindex -shared | |
1504 | @kindex -Bshareable | |
1505 | @item -shared | |
1506 | @itemx -Bshareable | |
1507 | @cindex shared libraries | |
1508 | Create a shared library. This is currently only supported on ELF, XCOFF | |
1509 | and SunOS platforms. On SunOS, the linker will automatically create a | |
ff5dcc92 | 1510 | shared library if the @option{-e} option is not used and there are |
252b5132 RH |
1511 | undefined symbols in the link. |
1512 | ||
1513 | @item --sort-common | |
1514 | @kindex --sort-common | |
ff5dcc92 | 1515 | This option tells @command{ld} to sort the common symbols by size when it |
252b5132 | 1516 | places them in the appropriate output sections. First come all the one |
563e308f | 1517 | byte symbols, then all the two byte, then all the four byte, and then |
252b5132 RH |
1518 | everything else. This is to prevent gaps between symbols due to |
1519 | alignment constraints. | |
1520 | ||
bcaa7b3e L |
1521 | @kindex --sort-section name |
1522 | @item --sort-section name | |
1523 | This option will apply @code{SORT_BY_NAME} to all wildcard section | |
1524 | patterns in the linker script. | |
1525 | ||
1526 | @kindex --sort-section alignment | |
1527 | @item --sort-section alignment | |
1528 | This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section | |
1529 | patterns in the linker script. | |
1530 | ||
252b5132 | 1531 | @kindex --split-by-file |
a854a4a7 | 1532 | @item --split-by-file [@var{size}] |
ff5dcc92 | 1533 | Similar to @option{--split-by-reloc} but creates a new output section for |
a854a4a7 AM |
1534 | each input file when @var{size} is reached. @var{size} defaults to a |
1535 | size of 1 if not given. | |
252b5132 RH |
1536 | |
1537 | @kindex --split-by-reloc | |
a854a4a7 AM |
1538 | @item --split-by-reloc [@var{count}] |
1539 | Tries to creates extra sections in the output file so that no single | |
252b5132 | 1540 | output section in the file contains more than @var{count} relocations. |
a854a4a7 | 1541 | This is useful when generating huge relocatable files for downloading into |
252b5132 RH |
1542 | certain real time kernels with the COFF object file format; since COFF |
1543 | cannot represent more than 65535 relocations in a single section. Note | |
1544 | that this will fail to work with object file formats which do not | |
1545 | support arbitrary sections. The linker will not split up individual | |
1546 | input sections for redistribution, so if a single input section contains | |
1547 | more than @var{count} relocations one output section will contain that | |
a854a4a7 | 1548 | many relocations. @var{count} defaults to a value of 32768. |
252b5132 RH |
1549 | |
1550 | @kindex --stats | |
1551 | @item --stats | |
1552 | Compute and display statistics about the operation of the linker, such | |
1553 | as execution time and memory usage. | |
1554 | ||
e2243057 RS |
1555 | @kindex --sysroot |
1556 | @item --sysroot=@var{directory} | |
1557 | Use @var{directory} as the location of the sysroot, overriding the | |
1558 | configure-time default. This option is only supported by linkers | |
1559 | that were configured using @option{--with-sysroot}. | |
1560 | ||
252b5132 RH |
1561 | @kindex --traditional-format |
1562 | @cindex traditional format | |
1563 | @item --traditional-format | |
ff5dcc92 SC |
1564 | For some targets, the output of @command{ld} is different in some ways from |
1565 | the output of some existing linker. This switch requests @command{ld} to | |
252b5132 RH |
1566 | use the traditional format instead. |
1567 | ||
1568 | @cindex dbx | |
ff5dcc92 | 1569 | For example, on SunOS, @command{ld} combines duplicate entries in the |
252b5132 RH |
1570 | symbol string table. This can reduce the size of an output file with |
1571 | full debugging information by over 30 percent. Unfortunately, the SunOS | |
1572 | @code{dbx} program can not read the resulting program (@code{gdb} has no | |
ff5dcc92 | 1573 | trouble). The @samp{--traditional-format} switch tells @command{ld} to not |
252b5132 RH |
1574 | combine duplicate entries. |
1575 | ||
176355da NC |
1576 | @kindex --section-start @var{sectionname}=@var{org} |
1577 | @item --section-start @var{sectionname}=@var{org} | |
1578 | Locate a section in the output file at the absolute | |
1579 | address given by @var{org}. You may use this option as many | |
1580 | times as necessary to locate multiple sections in the command | |
1581 | line. | |
1582 | @var{org} must be a single hexadecimal integer; | |
1583 | for compatibility with other linkers, you may omit the leading | |
1584 | @samp{0x} usually associated with hexadecimal values. @emph{Note:} there | |
1585 | should be no white space between @var{sectionname}, the equals | |
1586 | sign (``@key{=}''), and @var{org}. | |
1587 | ||
252b5132 RH |
1588 | @kindex -Tbss @var{org} |
1589 | @kindex -Tdata @var{org} | |
1590 | @kindex -Ttext @var{org} | |
1591 | @cindex segment origins, cmd line | |
1592 | @item -Tbss @var{org} | |
1593 | @itemx -Tdata @var{org} | |
1594 | @itemx -Ttext @var{org} | |
a6e02871 AO |
1595 | Same as --section-start, with @code{.bss}, @code{.data} or |
1596 | @code{.text} as the @var{sectionname}. | |
252b5132 | 1597 | |
560e09e9 NC |
1598 | @kindex --unresolved-symbols |
1599 | @item --unresolved-symbols=@var{method} | |
1600 | Determine how to handle unresolved symbols. There are four possible | |
1601 | values for @samp{method}: | |
1602 | ||
1603 | @table @samp | |
1604 | @item ignore-all | |
da8bce14 | 1605 | Do not report any unresolved symbols. |
560e09e9 NC |
1606 | |
1607 | @item report-all | |
da8bce14 | 1608 | Report all unresolved symbols. This is the default. |
560e09e9 NC |
1609 | |
1610 | @item ignore-in-object-files | |
1611 | Report unresolved symbols that are contained in shared libraries, but | |
1612 | ignore them if they come from regular object files. | |
1613 | ||
1614 | @item ignore-in-shared-libs | |
1615 | Report unresolved symbols that come from regular object files, but | |
1616 | ignore them if they come from shared libraries. This can be useful | |
1617 | when creating a dynamic binary and it is known that all the shared | |
1618 | libraries that it should be referencing are included on the linker's | |
1619 | command line. | |
1620 | @end table | |
1621 | ||
1622 | The behaviour for shared libraries on their own can also be controlled | |
1623 | by the @option{--[no-]allow-shlib-undefined} option. | |
1624 | ||
1625 | Normally the linker will generate an error message for each reported | |
1626 | unresolved symbol but the option @option{--warn-unresolved-symbols} | |
1627 | can change this to a warning. | |
1628 | ||
252b5132 RH |
1629 | @kindex --verbose |
1630 | @cindex verbose | |
1631 | @item --dll-verbose | |
308b1ffd | 1632 | @itemx --verbose |
ff5dcc92 | 1633 | Display the version number for @command{ld} and list the linker emulations |
252b5132 | 1634 | supported. Display which input files can and cannot be opened. Display |
b9a8de1e | 1635 | the linker script being used by the linker. |
252b5132 RH |
1636 | |
1637 | @kindex --version-script=@var{version-scriptfile} | |
1638 | @cindex version script, symbol versions | |
1639 | @itemx --version-script=@var{version-scriptfile} | |
1640 | Specify the name of a version script to the linker. This is typically | |
1641 | used when creating shared libraries to specify additional information | |
36f63dca | 1642 | about the version hierarchy for the library being created. This option |
252b5132 RH |
1643 | is only meaningful on ELF platforms which support shared libraries. |
1644 | @xref{VERSION}. | |
1645 | ||
7ce691ae | 1646 | @kindex --warn-common |
252b5132 RH |
1647 | @cindex warnings, on combining symbols |
1648 | @cindex combining symbols, warnings on | |
1649 | @item --warn-common | |
1650 | Warn when a common symbol is combined with another common symbol or with | |
560e09e9 | 1651 | a symbol definition. Unix linkers allow this somewhat sloppy practise, |
252b5132 RH |
1652 | but linkers on some other operating systems do not. This option allows |
1653 | you to find potential problems from combining global symbols. | |
560e09e9 | 1654 | Unfortunately, some C libraries use this practise, so you may get some |
252b5132 RH |
1655 | warnings about symbols in the libraries as well as in your programs. |
1656 | ||
1657 | There are three kinds of global symbols, illustrated here by C examples: | |
1658 | ||
1659 | @table @samp | |
1660 | @item int i = 1; | |
1661 | A definition, which goes in the initialized data section of the output | |
1662 | file. | |
1663 | ||
1664 | @item extern int i; | |
1665 | An undefined reference, which does not allocate space. | |
1666 | There must be either a definition or a common symbol for the | |
1667 | variable somewhere. | |
1668 | ||
1669 | @item int i; | |
1670 | A common symbol. If there are only (one or more) common symbols for a | |
1671 | variable, it goes in the uninitialized data area of the output file. | |
1672 | The linker merges multiple common symbols for the same variable into a | |
1673 | single symbol. If they are of different sizes, it picks the largest | |
1674 | size. The linker turns a common symbol into a declaration, if there is | |
1675 | a definition of the same variable. | |
1676 | @end table | |
1677 | ||
1678 | The @samp{--warn-common} option can produce five kinds of warnings. | |
1679 | Each warning consists of a pair of lines: the first describes the symbol | |
1680 | just encountered, and the second describes the previous symbol | |
1681 | encountered with the same name. One or both of the two symbols will be | |
1682 | a common symbol. | |
1683 | ||
1684 | @enumerate | |
1685 | @item | |
1686 | Turning a common symbol into a reference, because there is already a | |
1687 | definition for the symbol. | |
1688 | @smallexample | |
1689 | @var{file}(@var{section}): warning: common of `@var{symbol}' | |
1690 | overridden by definition | |
1691 | @var{file}(@var{section}): warning: defined here | |
1692 | @end smallexample | |
1693 | ||
1694 | @item | |
1695 | Turning a common symbol into a reference, because a later definition for | |
1696 | the symbol is encountered. This is the same as the previous case, | |
1697 | except that the symbols are encountered in a different order. | |
1698 | @smallexample | |
1699 | @var{file}(@var{section}): warning: definition of `@var{symbol}' | |
1700 | overriding common | |
1701 | @var{file}(@var{section}): warning: common is here | |
1702 | @end smallexample | |
1703 | ||
1704 | @item | |
1705 | Merging a common symbol with a previous same-sized common symbol. | |
1706 | @smallexample | |
1707 | @var{file}(@var{section}): warning: multiple common | |
1708 | of `@var{symbol}' | |
1709 | @var{file}(@var{section}): warning: previous common is here | |
1710 | @end smallexample | |
1711 | ||
1712 | @item | |
1713 | Merging a common symbol with a previous larger common symbol. | |
1714 | @smallexample | |
1715 | @var{file}(@var{section}): warning: common of `@var{symbol}' | |
1716 | overridden by larger common | |
1717 | @var{file}(@var{section}): warning: larger common is here | |
1718 | @end smallexample | |
1719 | ||
1720 | @item | |
1721 | Merging a common symbol with a previous smaller common symbol. This is | |
1722 | the same as the previous case, except that the symbols are | |
1723 | encountered in a different order. | |
1724 | @smallexample | |
1725 | @var{file}(@var{section}): warning: common of `@var{symbol}' | |
1726 | overriding smaller common | |
1727 | @var{file}(@var{section}): warning: smaller common is here | |
1728 | @end smallexample | |
1729 | @end enumerate | |
1730 | ||
1731 | @kindex --warn-constructors | |
1732 | @item --warn-constructors | |
1733 | Warn if any global constructors are used. This is only useful for a few | |
1734 | object file formats. For formats like COFF or ELF, the linker can not | |
1735 | detect the use of global constructors. | |
1736 | ||
1737 | @kindex --warn-multiple-gp | |
1738 | @item --warn-multiple-gp | |
1739 | Warn if multiple global pointer values are required in the output file. | |
1740 | This is only meaningful for certain processors, such as the Alpha. | |
1741 | Specifically, some processors put large-valued constants in a special | |
1742 | section. A special register (the global pointer) points into the middle | |
1743 | of this section, so that constants can be loaded efficiently via a | |
1744 | base-register relative addressing mode. Since the offset in | |
1745 | base-register relative mode is fixed and relatively small (e.g., 16 | |
1746 | bits), this limits the maximum size of the constant pool. Thus, in | |
1747 | large programs, it is often necessary to use multiple global pointer | |
1748 | values in order to be able to address all possible constants. This | |
1749 | option causes a warning to be issued whenever this case occurs. | |
1750 | ||
1751 | @kindex --warn-once | |
1752 | @cindex warnings, on undefined symbols | |
1753 | @cindex undefined symbols, warnings on | |
1754 | @item --warn-once | |
1755 | Only warn once for each undefined symbol, rather than once per module | |
1756 | which refers to it. | |
1757 | ||
1758 | @kindex --warn-section-align | |
1759 | @cindex warnings, on section alignment | |
1760 | @cindex section alignment, warnings on | |
1761 | @item --warn-section-align | |
1762 | Warn if the address of an output section is changed because of | |
1763 | alignment. Typically, the alignment will be set by an input section. | |
1764 | The address will only be changed if it not explicitly specified; that | |
1765 | is, if the @code{SECTIONS} command does not specify a start address for | |
1766 | the section (@pxref{SECTIONS}). | |
1767 | ||
8fdd7217 NC |
1768 | @kindex --warn-shared-textrel |
1769 | @item --warn-shared-textrel | |
1770 | Warn if the linker adds a DT_TEXTREL to a shared object. | |
1771 | ||
560e09e9 NC |
1772 | @kindex --warn-unresolved-symbols |
1773 | @item --warn-unresolved-symbols | |
1774 | If the linker is going to report an unresolved symbol (see the option | |
1775 | @option{--unresolved-symbols}) it will normally generate an error. | |
1776 | This option makes it generate a warning instead. | |
1777 | ||
1778 | @kindex --error-unresolved-symbols | |
1779 | @item --error-unresolved-symbols | |
1780 | This restores the linker's default behaviour of generating errors when | |
1781 | it is reporting unresolved symbols. | |
1782 | ||
252b5132 RH |
1783 | @kindex --whole-archive |
1784 | @cindex including an entire archive | |
1785 | @item --whole-archive | |
1786 | For each archive mentioned on the command line after the | |
ff5dcc92 | 1787 | @option{--whole-archive} option, include every object file in the archive |
252b5132 RH |
1788 | in the link, rather than searching the archive for the required object |
1789 | files. This is normally used to turn an archive file into a shared | |
1790 | library, forcing every object to be included in the resulting shared | |
1791 | library. This option may be used more than once. | |
1792 | ||
7ec229ce | 1793 | Two notes when using this option from gcc: First, gcc doesn't know |
ff5dcc92 SC |
1794 | about this option, so you have to use @option{-Wl,-whole-archive}. |
1795 | Second, don't forget to use @option{-Wl,-no-whole-archive} after your | |
7ec229ce DD |
1796 | list of archives, because gcc will add its own list of archives to |
1797 | your link and you may not want this flag to affect those as well. | |
1798 | ||
252b5132 RH |
1799 | @kindex --wrap |
1800 | @item --wrap @var{symbol} | |
1801 | Use a wrapper function for @var{symbol}. Any undefined reference to | |
1802 | @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any | |
1803 | undefined reference to @code{__real_@var{symbol}} will be resolved to | |
1804 | @var{symbol}. | |
1805 | ||
1806 | This can be used to provide a wrapper for a system function. The | |
1807 | wrapper function should be called @code{__wrap_@var{symbol}}. If it | |
1808 | wishes to call the system function, it should call | |
1809 | @code{__real_@var{symbol}}. | |
1810 | ||
1811 | Here is a trivial example: | |
1812 | ||
1813 | @smallexample | |
1814 | void * | |
cc2f008e | 1815 | __wrap_malloc (size_t c) |
252b5132 | 1816 | @{ |
cc2f008e | 1817 | printf ("malloc called with %zu\n", c); |
252b5132 RH |
1818 | return __real_malloc (c); |
1819 | @} | |
1820 | @end smallexample | |
1821 | ||
ff5dcc92 | 1822 | If you link other code with this file using @option{--wrap malloc}, then |
252b5132 RH |
1823 | all calls to @code{malloc} will call the function @code{__wrap_malloc} |
1824 | instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will | |
1825 | call the real @code{malloc} function. | |
1826 | ||
1827 | You may wish to provide a @code{__real_malloc} function as well, so that | |
ff5dcc92 | 1828 | links without the @option{--wrap} option will succeed. If you do this, |
252b5132 RH |
1829 | you should not put the definition of @code{__real_malloc} in the same |
1830 | file as @code{__wrap_malloc}; if you do, the assembler may resolve the | |
1831 | call before the linker has a chance to wrap it to @code{malloc}. | |
1832 | ||
6aa29e7b JJ |
1833 | @kindex --eh-frame-hdr |
1834 | @item --eh-frame-hdr | |
1835 | Request creation of @code{.eh_frame_hdr} section and ELF | |
1836 | @code{PT_GNU_EH_FRAME} segment header. | |
1837 | ||
6c1439be L |
1838 | @kindex --enable-new-dtags |
1839 | @kindex --disable-new-dtags | |
1840 | @item --enable-new-dtags | |
1841 | @itemx --disable-new-dtags | |
1842 | This linker can create the new dynamic tags in ELF. But the older ELF | |
1843 | systems may not understand them. If you specify | |
ff5dcc92 SC |
1844 | @option{--enable-new-dtags}, the dynamic tags will be created as needed. |
1845 | If you specify @option{--disable-new-dtags}, no new dynamic tags will be | |
6c1439be L |
1846 | created. By default, the new dynamic tags are not created. Note that |
1847 | those options are only available for ELF systems. | |
1848 | ||
2d643429 | 1849 | @kindex --hash-size=@var{number} |
e185dd51 | 1850 | @item --hash-size=@var{number} |
2d643429 NC |
1851 | Set the default size of the linker's hash tables to a prime number |
1852 | close to @var{number}. Increasing this value can reduce the length of | |
1853 | time it takes the linker to perform its tasks, at the expense of | |
1854 | increasing the linker's memory requirements. Similarly reducing this | |
1855 | value can reduce the memory requirements at the expense of speed. | |
1856 | ||
35835446 JR |
1857 | @kindex --reduce-memory-overheads |
1858 | @item --reduce-memory-overheads | |
1859 | This option reduces memory requirements at ld runtime, at the expense of | |
1860 | linking speed. This was introduced to to select the old O(n^2) algorithm | |
1861 | for link map file generation, rather than the new O(n) algorithm which uses | |
2d643429 NC |
1862 | about 40% more memory for symbol storage. |
1863 | ||
4f9c04f7 | 1864 | Another effect of the switch is to set the default hash table size to |
2d643429 | 1865 | 1021, which again saves memory at the cost of lengthening the linker's |
a85785bc | 1866 | run time. This is not done however if the @option{--hash-size} switch |
2d643429 NC |
1867 | has been used. |
1868 | ||
1869 | The @option{--reduce-memory-overheads} switch may be also be used to | |
1870 | enable other tradeoffs in future versions of the linker. | |
35835446 | 1871 | |
252b5132 RH |
1872 | @end table |
1873 | ||
0285c67d NC |
1874 | @c man end |
1875 | ||
36f63dca | 1876 | @subsection Options Specific to i386 PE Targets |
252b5132 | 1877 | |
0285c67d NC |
1878 | @c man begin OPTIONS |
1879 | ||
ff5dcc92 | 1880 | The i386 PE linker supports the @option{-shared} option, which causes |
252b5132 RH |
1881 | the output to be a dynamically linked library (DLL) instead of a |
1882 | normal executable. You should name the output @code{*.dll} when you | |
1883 | use this option. In addition, the linker fully supports the standard | |
1884 | @code{*.def} files, which may be specified on the linker command line | |
1885 | like an object file (in fact, it should precede archives it exports | |
1886 | symbols from, to ensure that they get linked in, just like a normal | |
1887 | object file). | |
1888 | ||
1889 | In addition to the options common to all targets, the i386 PE linker | |
1890 | support additional command line options that are specific to the i386 | |
1891 | PE target. Options that take values may be separated from their | |
1892 | values by either a space or an equals sign. | |
1893 | ||
ff5dcc92 | 1894 | @table @gcctabopt |
252b5132 RH |
1895 | |
1896 | @kindex --add-stdcall-alias | |
1897 | @item --add-stdcall-alias | |
1898 | If given, symbols with a stdcall suffix (@@@var{nn}) will be exported | |
1899 | as-is and also with the suffix stripped. | |
bb10df36 | 1900 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
1901 | |
1902 | @kindex --base-file | |
1903 | @item --base-file @var{file} | |
1904 | Use @var{file} as the name of a file in which to save the base | |
1905 | addresses of all the relocations needed for generating DLLs with | |
1906 | @file{dlltool}. | |
bb10df36 | 1907 | [This is an i386 PE specific option] |
252b5132 RH |
1908 | |
1909 | @kindex --dll | |
1910 | @item --dll | |
1911 | Create a DLL instead of a regular executable. You may also use | |
ff5dcc92 | 1912 | @option{-shared} or specify a @code{LIBRARY} in a given @code{.def} |
252b5132 | 1913 | file. |
bb10df36 | 1914 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
1915 | |
1916 | @kindex --enable-stdcall-fixup | |
1917 | @kindex --disable-stdcall-fixup | |
1918 | @item --enable-stdcall-fixup | |
1919 | @itemx --disable-stdcall-fixup | |
1920 | If the link finds a symbol that it cannot resolve, it will attempt to | |
36f63dca | 1921 | do ``fuzzy linking'' by looking for another defined symbol that differs |
252b5132 RH |
1922 | only in the format of the symbol name (cdecl vs stdcall) and will |
1923 | resolve that symbol by linking to the match. For example, the | |
1924 | undefined symbol @code{_foo} might be linked to the function | |
1925 | @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked | |
1926 | to the function @code{_bar}. When the linker does this, it prints a | |
1927 | warning, since it normally should have failed to link, but sometimes | |
1928 | import libraries generated from third-party dlls may need this feature | |
ff5dcc92 | 1929 | to be usable. If you specify @option{--enable-stdcall-fixup}, this |
252b5132 | 1930 | feature is fully enabled and warnings are not printed. If you specify |
ff5dcc92 | 1931 | @option{--disable-stdcall-fixup}, this feature is disabled and such |
252b5132 | 1932 | mismatches are considered to be errors. |
bb10df36 | 1933 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
1934 | |
1935 | @cindex DLLs, creating | |
1936 | @kindex --export-all-symbols | |
1937 | @item --export-all-symbols | |
1938 | If given, all global symbols in the objects used to build a DLL will | |
1939 | be exported by the DLL. Note that this is the default if there | |
1940 | otherwise wouldn't be any exported symbols. When symbols are | |
1941 | explicitly exported via DEF files or implicitly exported via function | |
1942 | attributes, the default is to not export anything else unless this | |
1943 | option is given. Note that the symbols @code{DllMain@@12}, | |
b044cda1 CW |
1944 | @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and |
1945 | @code{impure_ptr} will not be automatically | |
1946 | exported. Also, symbols imported from other DLLs will not be | |
1947 | re-exported, nor will symbols specifying the DLL's internal layout | |
1948 | such as those beginning with @code{_head_} or ending with | |
1949 | @code{_iname}. In addition, no symbols from @code{libgcc}, | |
1950 | @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported. | |
1951 | Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will | |
1952 | not be exported, to help with C++ DLLs. Finally, there is an | |
1953 | extensive list of cygwin-private symbols that are not exported | |
1954 | (obviously, this applies on when building DLLs for cygwin targets). | |
1955 | These cygwin-excludes are: @code{_cygwin_dll_entry@@12}, | |
1956 | @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12}, | |
1957 | @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll}, | |
1958 | @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2}, | |
1959 | @code{cygwin_premain3}, and @code{environ}. | |
bb10df36 | 1960 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
1961 | |
1962 | @kindex --exclude-symbols | |
1d0a3c9c | 1963 | @item --exclude-symbols @var{symbol},@var{symbol},... |
252b5132 RH |
1964 | Specifies a list of symbols which should not be automatically |
1965 | exported. The symbol names may be delimited by commas or colons. | |
bb10df36 | 1966 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
1967 | |
1968 | @kindex --file-alignment | |
1969 | @item --file-alignment | |
1970 | Specify the file alignment. Sections in the file will always begin at | |
1971 | file offsets which are multiples of this number. This defaults to | |
1972 | 512. | |
bb10df36 | 1973 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
1974 | |
1975 | @cindex heap size | |
1976 | @kindex --heap | |
1977 | @item --heap @var{reserve} | |
1978 | @itemx --heap @var{reserve},@var{commit} | |
1979 | Specify the amount of memory to reserve (and optionally commit) to be | |
1980 | used as heap for this program. The default is 1Mb reserved, 4K | |
1981 | committed. | |
bb10df36 | 1982 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
1983 | |
1984 | @cindex image base | |
1985 | @kindex --image-base | |
1986 | @item --image-base @var{value} | |
1987 | Use @var{value} as the base address of your program or dll. This is | |
1988 | the lowest memory location that will be used when your program or dll | |
1989 | is loaded. To reduce the need to relocate and improve performance of | |
1990 | your dlls, each should have a unique base address and not overlap any | |
1991 | other dlls. The default is 0x400000 for executables, and 0x10000000 | |
1992 | for dlls. | |
bb10df36 | 1993 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
1994 | |
1995 | @kindex --kill-at | |
1996 | @item --kill-at | |
1997 | If given, the stdcall suffixes (@@@var{nn}) will be stripped from | |
1998 | symbols before they are exported. | |
bb10df36 | 1999 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 | 2000 | |
26d2d8a2 BF |
2001 | @kindex --large-address-aware |
2002 | @item --large-address-aware | |
2003 | If given, the appropriate bit in the ``Charateristics'' field of the COFF | |
2004 | header is set to indicate that this executable supports virtual addresses | |
2005 | greater than 2 gigabytes. This should be used in conjuction with the /3GB | |
2006 | or /USERVA=@var{value} megabytes switch in the ``[operating systems]'' | |
2007 | section of the BOOT.INI. Otherwise, this bit has no effect. | |
2008 | [This option is specific to PE targeted ports of the linker] | |
2009 | ||
252b5132 RH |
2010 | @kindex --major-image-version |
2011 | @item --major-image-version @var{value} | |
36f63dca | 2012 | Sets the major number of the ``image version''. Defaults to 1. |
bb10df36 | 2013 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2014 | |
2015 | @kindex --major-os-version | |
2016 | @item --major-os-version @var{value} | |
36f63dca | 2017 | Sets the major number of the ``os version''. Defaults to 4. |
bb10df36 | 2018 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2019 | |
2020 | @kindex --major-subsystem-version | |
2021 | @item --major-subsystem-version @var{value} | |
36f63dca | 2022 | Sets the major number of the ``subsystem version''. Defaults to 4. |
bb10df36 | 2023 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2024 | |
2025 | @kindex --minor-image-version | |
2026 | @item --minor-image-version @var{value} | |
36f63dca | 2027 | Sets the minor number of the ``image version''. Defaults to 0. |
bb10df36 | 2028 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2029 | |
2030 | @kindex --minor-os-version | |
2031 | @item --minor-os-version @var{value} | |
36f63dca | 2032 | Sets the minor number of the ``os version''. Defaults to 0. |
bb10df36 | 2033 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2034 | |
2035 | @kindex --minor-subsystem-version | |
2036 | @item --minor-subsystem-version @var{value} | |
36f63dca | 2037 | Sets the minor number of the ``subsystem version''. Defaults to 0. |
bb10df36 | 2038 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2039 | |
2040 | @cindex DEF files, creating | |
2041 | @cindex DLLs, creating | |
2042 | @kindex --output-def | |
2043 | @item --output-def @var{file} | |
2044 | The linker will create the file @var{file} which will contain a DEF | |
2045 | file corresponding to the DLL the linker is generating. This DEF file | |
2046 | (which should be called @code{*.def}) may be used to create an import | |
2047 | library with @code{dlltool} or may be used as a reference to | |
2048 | automatically or implicitly exported symbols. | |
bb10df36 | 2049 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 | 2050 | |
b044cda1 CW |
2051 | @cindex DLLs, creating |
2052 | @kindex --out-implib | |
2053 | @item --out-implib @var{file} | |
2054 | The linker will create the file @var{file} which will contain an | |
2055 | import lib corresponding to the DLL the linker is generating. This | |
2056 | import lib (which should be called @code{*.dll.a} or @code{*.a} | |
560e09e9 | 2057 | may be used to link clients against the generated DLL; this behaviour |
b044cda1 CW |
2058 | makes it possible to skip a separate @code{dlltool} import library |
2059 | creation step. | |
bb10df36 | 2060 | [This option is specific to the i386 PE targeted port of the linker] |
b044cda1 CW |
2061 | |
2062 | @kindex --enable-auto-image-base | |
2063 | @item --enable-auto-image-base | |
2064 | Automatically choose the image base for DLLs, unless one is specified | |
2065 | using the @code{--image-base} argument. By using a hash generated | |
2066 | from the dllname to create unique image bases for each DLL, in-memory | |
2067 | collisions and relocations which can delay program execution are | |
2068 | avoided. | |
bb10df36 | 2069 | [This option is specific to the i386 PE targeted port of the linker] |
b044cda1 CW |
2070 | |
2071 | @kindex --disable-auto-image-base | |
2072 | @item --disable-auto-image-base | |
2073 | Do not automatically generate a unique image base. If there is no | |
2074 | user-specified image base (@code{--image-base}) then use the platform | |
2075 | default. | |
bb10df36 | 2076 | [This option is specific to the i386 PE targeted port of the linker] |
b044cda1 CW |
2077 | |
2078 | @cindex DLLs, linking to | |
2079 | @kindex --dll-search-prefix | |
2080 | @item --dll-search-prefix @var{string} | |
489d0400 | 2081 | When linking dynamically to a dll without an import library, |
b044cda1 | 2082 | search for @code{<string><basename>.dll} in preference to |
560e09e9 | 2083 | @code{lib<basename>.dll}. This behaviour allows easy distinction |
b044cda1 CW |
2084 | between DLLs built for the various "subplatforms": native, cygwin, |
2085 | uwin, pw, etc. For instance, cygwin DLLs typically use | |
2086 | @code{--dll-search-prefix=cyg}. | |
bb10df36 | 2087 | [This option is specific to the i386 PE targeted port of the linker] |
b044cda1 CW |
2088 | |
2089 | @kindex --enable-auto-import | |
2090 | @item --enable-auto-import | |
0d888aac | 2091 | Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for |
b044cda1 | 2092 | DATA imports from DLLs, and create the necessary thunking symbols when |
4d8907ac DS |
2093 | building the import libraries with those DATA exports. Note: Use of the |
2094 | 'auto-import' extension will cause the text section of the image file | |
2095 | to be made writable. This does not conform to the PE-COFF format | |
2096 | specification published by Microsoft. | |
2097 | ||
2098 | Using 'auto-import' generally will 'just work' -- but sometimes you may | |
2099 | see this message: | |
0d888aac CW |
2100 | |
2101 | "variable '<var>' can't be auto-imported. Please read the | |
2102 | documentation for ld's @code{--enable-auto-import} for details." | |
2103 | ||
2104 | This message occurs when some (sub)expression accesses an address | |
2105 | ultimately given by the sum of two constants (Win32 import tables only | |
2106 | allow one). Instances where this may occur include accesses to member | |
2107 | fields of struct variables imported from a DLL, as well as using a | |
2f8d8971 NC |
2108 | constant index into an array variable imported from a DLL. Any |
2109 | multiword variable (arrays, structs, long long, etc) may trigger | |
2110 | this error condition. However, regardless of the exact data type | |
2111 | of the offending exported variable, ld will always detect it, issue | |
2112 | the warning, and exit. | |
2113 | ||
2114 | There are several ways to address this difficulty, regardless of the | |
2115 | data type of the exported variable: | |
0d888aac | 2116 | |
2fa9fc65 NC |
2117 | One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task |
2118 | of adjusting references in your client code for runtime environment, so | |
560e09e9 | 2119 | this method works only when runtime environment supports this feature. |
2fa9fc65 NC |
2120 | |
2121 | A second solution is to force one of the 'constants' to be a variable -- | |
0d888aac CW |
2122 | that is, unknown and un-optimizable at compile time. For arrays, |
2123 | there are two possibilities: a) make the indexee (the array's address) | |
2124 | a variable, or b) make the 'constant' index a variable. Thus: | |
2125 | ||
2126 | @example | |
2127 | extern type extern_array[]; | |
2128 | extern_array[1] --> | |
2129 | @{ volatile type *t=extern_array; t[1] @} | |
2130 | @end example | |
2131 | ||
2132 | or | |
2133 | ||
2134 | @example | |
2135 | extern type extern_array[]; | |
2136 | extern_array[1] --> | |
2137 | @{ volatile int t=1; extern_array[t] @} | |
2138 | @end example | |
2139 | ||
2f8d8971 NC |
2140 | For structs (and most other multiword data types) the only option |
2141 | is to make the struct itself (or the long long, or the ...) variable: | |
0d888aac CW |
2142 | |
2143 | @example | |
2144 | extern struct s extern_struct; | |
2145 | extern_struct.field --> | |
2146 | @{ volatile struct s *t=&extern_struct; t->field @} | |
2147 | @end example | |
2148 | ||
c406afaf NC |
2149 | or |
2150 | ||
2151 | @example | |
2152 | extern long long extern_ll; | |
2153 | extern_ll --> | |
2154 | @{ volatile long long * local_ll=&extern_ll; *local_ll @} | |
2155 | @end example | |
2156 | ||
2fa9fc65 | 2157 | A third method of dealing with this difficulty is to abandon |
0d888aac | 2158 | 'auto-import' for the offending symbol and mark it with |
560e09e9 | 2159 | @code{__declspec(dllimport)}. However, in practise that |
0d888aac CW |
2160 | requires using compile-time #defines to indicate whether you are |
2161 | building a DLL, building client code that will link to the DLL, or | |
2162 | merely building/linking to a static library. In making the choice | |
2163 | between the various methods of resolving the 'direct address with | |
2164 | constant offset' problem, you should consider typical real-world usage: | |
2165 | ||
2166 | Original: | |
2167 | @example | |
2168 | --foo.h | |
2169 | extern int arr[]; | |
2170 | --foo.c | |
2171 | #include "foo.h" | |
2172 | void main(int argc, char **argv)@{ | |
2173 | printf("%d\n",arr[1]); | |
2174 | @} | |
2175 | @end example | |
2176 | ||
2177 | Solution 1: | |
2178 | @example | |
2179 | --foo.h | |
2180 | extern int arr[]; | |
2181 | --foo.c | |
2182 | #include "foo.h" | |
2183 | void main(int argc, char **argv)@{ | |
2184 | /* This workaround is for win32 and cygwin; do not "optimize" */ | |
2185 | volatile int *parr = arr; | |
2186 | printf("%d\n",parr[1]); | |
2187 | @} | |
2188 | @end example | |
2189 | ||
2190 | Solution 2: | |
2191 | @example | |
2192 | --foo.h | |
2193 | /* Note: auto-export is assumed (no __declspec(dllexport)) */ | |
2194 | #if (defined(_WIN32) || defined(__CYGWIN__)) && \ | |
2195 | !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC)) | |
2196 | #define FOO_IMPORT __declspec(dllimport) | |
2197 | #else | |
2198 | #define FOO_IMPORT | |
2199 | #endif | |
2200 | extern FOO_IMPORT int arr[]; | |
2201 | --foo.c | |
2202 | #include "foo.h" | |
2203 | void main(int argc, char **argv)@{ | |
2204 | printf("%d\n",arr[1]); | |
2205 | @} | |
2206 | @end example | |
2207 | ||
2fa9fc65 | 2208 | A fourth way to avoid this problem is to re-code your |
0d888aac CW |
2209 | library to use a functional interface rather than a data interface |
2210 | for the offending variables (e.g. set_foo() and get_foo() accessor | |
2211 | functions). | |
bb10df36 | 2212 | [This option is specific to the i386 PE targeted port of the linker] |
b044cda1 CW |
2213 | |
2214 | @kindex --disable-auto-import | |
2215 | @item --disable-auto-import | |
560e09e9 | 2216 | Do not attempt to do sophisticated linking of @code{_symbol} to |
b044cda1 | 2217 | @code{__imp__symbol} for DATA imports from DLLs. |
bb10df36 | 2218 | [This option is specific to the i386 PE targeted port of the linker] |
b044cda1 | 2219 | |
2fa9fc65 NC |
2220 | @kindex --enable-runtime-pseudo-reloc |
2221 | @item --enable-runtime-pseudo-reloc | |
2222 | If your code contains expressions described in --enable-auto-import section, | |
2223 | that is, DATA imports from DLL with non-zero offset, this switch will create | |
2224 | a vector of 'runtime pseudo relocations' which can be used by runtime | |
2225 | environment to adjust references to such data in your client code. | |
bb10df36 | 2226 | [This option is specific to the i386 PE targeted port of the linker] |
2fa9fc65 NC |
2227 | |
2228 | @kindex --disable-runtime-pseudo-reloc | |
2229 | @item --disable-runtime-pseudo-reloc | |
2230 | Do not create pseudo relocations for non-zero offset DATA imports from | |
2231 | DLLs. This is the default. | |
bb10df36 | 2232 | [This option is specific to the i386 PE targeted port of the linker] |
2fa9fc65 | 2233 | |
b044cda1 CW |
2234 | @kindex --enable-extra-pe-debug |
2235 | @item --enable-extra-pe-debug | |
2236 | Show additional debug info related to auto-import symbol thunking. | |
bb10df36 | 2237 | [This option is specific to the i386 PE targeted port of the linker] |
b044cda1 | 2238 | |
252b5132 RH |
2239 | @kindex --section-alignment |
2240 | @item --section-alignment | |
2241 | Sets the section alignment. Sections in memory will always begin at | |
2242 | addresses which are a multiple of this number. Defaults to 0x1000. | |
bb10df36 | 2243 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2244 | |
2245 | @cindex stack size | |
2246 | @kindex --stack | |
2247 | @item --stack @var{reserve} | |
2248 | @itemx --stack @var{reserve},@var{commit} | |
2249 | Specify the amount of memory to reserve (and optionally commit) to be | |
559e4713 | 2250 | used as stack for this program. The default is 2Mb reserved, 4K |
252b5132 | 2251 | committed. |
bb10df36 | 2252 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2253 | |
2254 | @kindex --subsystem | |
2255 | @item --subsystem @var{which} | |
2256 | @itemx --subsystem @var{which}:@var{major} | |
2257 | @itemx --subsystem @var{which}:@var{major}.@var{minor} | |
2258 | Specifies the subsystem under which your program will execute. The | |
2259 | legal values for @var{which} are @code{native}, @code{windows}, | |
33f362e1 NC |
2260 | @code{console}, @code{posix}, and @code{xbox}. You may optionally set |
2261 | the subsystem version also. Numeric values are also accepted for | |
2262 | @var{which}. | |
bb10df36 | 2263 | [This option is specific to the i386 PE targeted port of the linker] |
252b5132 RH |
2264 | |
2265 | @end table | |
2266 | ||
0285c67d NC |
2267 | @c man end |
2268 | ||
93fd0973 SC |
2269 | @ifset M68HC11 |
2270 | @subsection Options specific to Motorola 68HC11 and 68HC12 targets | |
2271 | ||
2272 | @c man begin OPTIONS | |
2273 | ||
2274 | The 68HC11 and 68HC12 linkers support specific options to control the | |
2275 | memory bank switching mapping and trampoline code generation. | |
2276 | ||
2277 | @table @gcctabopt | |
2278 | ||
2279 | @kindex --no-trampoline | |
2280 | @item --no-trampoline | |
2281 | This option disables the generation of trampoline. By default a trampoline | |
2282 | is generated for each far function which is called using a @code{jsr} | |
2283 | instruction (this happens when a pointer to a far function is taken). | |
2284 | ||
2285 | @kindex --bank-window | |
2286 | @item --bank-window @var{name} | |
2287 | This option indicates to the linker the name of the memory region in | |
2288 | the @samp{MEMORY} specification that describes the memory bank window. | |
2289 | The definition of such region is then used by the linker to compute | |
2290 | paging and addresses within the memory window. | |
2291 | ||
2292 | @end table | |
2293 | ||
2294 | @c man end | |
2295 | @end ifset | |
2296 | ||
252b5132 RH |
2297 | @ifset UsesEnvVars |
2298 | @node Environment | |
2299 | @section Environment Variables | |
2300 | ||
0285c67d NC |
2301 | @c man begin ENVIRONMENT |
2302 | ||
560e09e9 | 2303 | You can change the behaviour of @command{ld} with the environment variables |
36f63dca NC |
2304 | @ifclear SingleFormat |
2305 | @code{GNUTARGET}, | |
2306 | @end ifclear | |
2307 | @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}. | |
252b5132 | 2308 | |
36f63dca | 2309 | @ifclear SingleFormat |
252b5132 RH |
2310 | @kindex GNUTARGET |
2311 | @cindex default input format | |
2312 | @code{GNUTARGET} determines the input-file object format if you don't | |
2313 | use @samp{-b} (or its synonym @samp{--format}). Its value should be one | |
2314 | of the BFD names for an input format (@pxref{BFD}). If there is no | |
ff5dcc92 | 2315 | @code{GNUTARGET} in the environment, @command{ld} uses the natural format |
252b5132 RH |
2316 | of the target. If @code{GNUTARGET} is set to @code{default} then BFD |
2317 | attempts to discover the input format by examining binary input files; | |
2318 | this method often succeeds, but there are potential ambiguities, since | |
2319 | there is no method of ensuring that the magic number used to specify | |
2320 | object-file formats is unique. However, the configuration procedure for | |
2321 | BFD on each system places the conventional format for that system first | |
2322 | in the search-list, so ambiguities are resolved in favor of convention. | |
36f63dca | 2323 | @end ifclear |
252b5132 RH |
2324 | |
2325 | @kindex LDEMULATION | |
2326 | @cindex default emulation | |
2327 | @cindex emulation, default | |
2328 | @code{LDEMULATION} determines the default emulation if you don't use the | |
2329 | @samp{-m} option. The emulation can affect various aspects of linker | |
2330 | behaviour, particularly the default linker script. You can list the | |
2331 | available emulations with the @samp{--verbose} or @samp{-V} options. If | |
2332 | the @samp{-m} option is not used, and the @code{LDEMULATION} environment | |
2333 | variable is not defined, the default emulation depends upon how the | |
2334 | linker was configured. | |
252b5132 RH |
2335 | |
2336 | @kindex COLLECT_NO_DEMANGLE | |
2337 | @cindex demangling, default | |
2338 | Normally, the linker will default to demangling symbols. However, if | |
2339 | @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will | |
2340 | default to not demangling symbols. This environment variable is used in | |
2341 | a similar fashion by the @code{gcc} linker wrapper program. The default | |
2342 | may be overridden by the @samp{--demangle} and @samp{--no-demangle} | |
2343 | options. | |
2344 | ||
0285c67d NC |
2345 | @c man end |
2346 | @end ifset | |
2347 | ||
252b5132 RH |
2348 | @node Scripts |
2349 | @chapter Linker Scripts | |
2350 | ||
2351 | @cindex scripts | |
2352 | @cindex linker scripts | |
2353 | @cindex command files | |
2354 | Every link is controlled by a @dfn{linker script}. This script is | |
2355 | written in the linker command language. | |
2356 | ||
2357 | The main purpose of the linker script is to describe how the sections in | |
2358 | the input files should be mapped into the output file, and to control | |
2359 | the memory layout of the output file. Most linker scripts do nothing | |
2360 | more than this. However, when necessary, the linker script can also | |
2361 | direct the linker to perform many other operations, using the commands | |
2362 | described below. | |
2363 | ||
2364 | The linker always uses a linker script. If you do not supply one | |
2365 | yourself, the linker will use a default script that is compiled into the | |
2366 | linker executable. You can use the @samp{--verbose} command line option | |
2367 | to display the default linker script. Certain command line options, | |
2368 | such as @samp{-r} or @samp{-N}, will affect the default linker script. | |
2369 | ||
2370 | You may supply your own linker script by using the @samp{-T} command | |
2371 | line option. When you do this, your linker script will replace the | |
2372 | default linker script. | |
2373 | ||
2374 | You may also use linker scripts implicitly by naming them as input files | |
2375 | to the linker, as though they were files to be linked. @xref{Implicit | |
2376 | Linker Scripts}. | |
2377 | ||
2378 | @menu | |
2379 | * Basic Script Concepts:: Basic Linker Script Concepts | |
2380 | * Script Format:: Linker Script Format | |
2381 | * Simple Example:: Simple Linker Script Example | |
2382 | * Simple Commands:: Simple Linker Script Commands | |
2383 | * Assignments:: Assigning Values to Symbols | |
2384 | * SECTIONS:: SECTIONS Command | |
2385 | * MEMORY:: MEMORY Command | |
2386 | * PHDRS:: PHDRS Command | |
2387 | * VERSION:: VERSION Command | |
2388 | * Expressions:: Expressions in Linker Scripts | |
2389 | * Implicit Linker Scripts:: Implicit Linker Scripts | |
2390 | @end menu | |
2391 | ||
2392 | @node Basic Script Concepts | |
2393 | @section Basic Linker Script Concepts | |
2394 | @cindex linker script concepts | |
2395 | We need to define some basic concepts and vocabulary in order to | |
2396 | describe the linker script language. | |
2397 | ||
2398 | The linker combines input files into a single output file. The output | |
2399 | file and each input file are in a special data format known as an | |
2400 | @dfn{object file format}. Each file is called an @dfn{object file}. | |
2401 | The output file is often called an @dfn{executable}, but for our | |
2402 | purposes we will also call it an object file. Each object file has, | |
2403 | among other things, a list of @dfn{sections}. We sometimes refer to a | |
2404 | section in an input file as an @dfn{input section}; similarly, a section | |
2405 | in the output file is an @dfn{output section}. | |
2406 | ||
2407 | Each section in an object file has a name and a size. Most sections | |
2408 | also have an associated block of data, known as the @dfn{section | |
2409 | contents}. A section may be marked as @dfn{loadable}, which mean that | |
2410 | the contents should be loaded into memory when the output file is run. | |
2411 | A section with no contents may be @dfn{allocatable}, which means that an | |
2412 | area in memory should be set aside, but nothing in particular should be | |
2413 | loaded there (in some cases this memory must be zeroed out). A section | |
2414 | which is neither loadable nor allocatable typically contains some sort | |
2415 | of debugging information. | |
2416 | ||
2417 | Every loadable or allocatable output section has two addresses. The | |
2418 | first is the @dfn{VMA}, or virtual memory address. This is the address | |
2419 | the section will have when the output file is run. The second is the | |
2420 | @dfn{LMA}, or load memory address. This is the address at which the | |
2421 | section will be loaded. In most cases the two addresses will be the | |
2422 | same. An example of when they might be different is when a data section | |
2423 | is loaded into ROM, and then copied into RAM when the program starts up | |
2424 | (this technique is often used to initialize global variables in a ROM | |
2425 | based system). In this case the ROM address would be the LMA, and the | |
2426 | RAM address would be the VMA. | |
2427 | ||
2428 | You can see the sections in an object file by using the @code{objdump} | |
2429 | program with the @samp{-h} option. | |
2430 | ||
2431 | Every object file also has a list of @dfn{symbols}, known as the | |
2432 | @dfn{symbol table}. A symbol may be defined or undefined. Each symbol | |
2433 | has a name, and each defined symbol has an address, among other | |
2434 | information. If you compile a C or C++ program into an object file, you | |
2435 | will get a defined symbol for every defined function and global or | |
2436 | static variable. Every undefined function or global variable which is | |
2437 | referenced in the input file will become an undefined symbol. | |
2438 | ||
2439 | You can see the symbols in an object file by using the @code{nm} | |
2440 | program, or by using the @code{objdump} program with the @samp{-t} | |
2441 | option. | |
2442 | ||
2443 | @node Script Format | |
2444 | @section Linker Script Format | |
2445 | @cindex linker script format | |
2446 | Linker scripts are text files. | |
2447 | ||
2448 | You write a linker script as a series of commands. Each command is | |
2449 | either a keyword, possibly followed by arguments, or an assignment to a | |
2450 | symbol. You may separate commands using semicolons. Whitespace is | |
2451 | generally ignored. | |
2452 | ||
2453 | Strings such as file or format names can normally be entered directly. | |
2454 | If the file name contains a character such as a comma which would | |
2455 | otherwise serve to separate file names, you may put the file name in | |
2456 | double quotes. There is no way to use a double quote character in a | |
2457 | file name. | |
2458 | ||
2459 | You may include comments in linker scripts just as in C, delimited by | |
2460 | @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent | |
2461 | to whitespace. | |
2462 | ||
2463 | @node Simple Example | |
2464 | @section Simple Linker Script Example | |
2465 | @cindex linker script example | |
2466 | @cindex example of linker script | |
2467 | Many linker scripts are fairly simple. | |
2468 | ||
2469 | The simplest possible linker script has just one command: | |
2470 | @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the | |
2471 | memory layout of the output file. | |
2472 | ||
2473 | The @samp{SECTIONS} command is a powerful command. Here we will | |
2474 | describe a simple use of it. Let's assume your program consists only of | |
2475 | code, initialized data, and uninitialized data. These will be in the | |
2476 | @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively. | |
2477 | Let's assume further that these are the only sections which appear in | |
2478 | your input files. | |
2479 | ||
2480 | For this example, let's say that the code should be loaded at address | |
2481 | 0x10000, and that the data should start at address 0x8000000. Here is a | |
2482 | linker script which will do that: | |
2483 | @smallexample | |
2484 | SECTIONS | |
2485 | @{ | |
2486 | . = 0x10000; | |
2487 | .text : @{ *(.text) @} | |
2488 | . = 0x8000000; | |
2489 | .data : @{ *(.data) @} | |
2490 | .bss : @{ *(.bss) @} | |
2491 | @} | |
2492 | @end smallexample | |
2493 | ||
2494 | You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS}, | |
2495 | followed by a series of symbol assignments and output section | |
2496 | descriptions enclosed in curly braces. | |
2497 | ||
252b5132 RH |
2498 | The first line inside the @samp{SECTIONS} command of the above example |
2499 | sets the value of the special symbol @samp{.}, which is the location | |
2500 | counter. If you do not specify the address of an output section in some | |
2501 | other way (other ways are described later), the address is set from the | |
2502 | current value of the location counter. The location counter is then | |
2503 | incremented by the size of the output section. At the start of the | |
2504 | @samp{SECTIONS} command, the location counter has the value @samp{0}. | |
2505 | ||
2506 | The second line defines an output section, @samp{.text}. The colon is | |
2507 | required syntax which may be ignored for now. Within the curly braces | |
2508 | after the output section name, you list the names of the input sections | |
2509 | which should be placed into this output section. The @samp{*} is a | |
2510 | wildcard which matches any file name. The expression @samp{*(.text)} | |
2511 | means all @samp{.text} input sections in all input files. | |
2512 | ||
2513 | Since the location counter is @samp{0x10000} when the output section | |
2514 | @samp{.text} is defined, the linker will set the address of the | |
2515 | @samp{.text} section in the output file to be @samp{0x10000}. | |
2516 | ||
2517 | The remaining lines define the @samp{.data} and @samp{.bss} sections in | |
2518 | the output file. The linker will place the @samp{.data} output section | |
2519 | at address @samp{0x8000000}. After the linker places the @samp{.data} | |
2520 | output section, the value of the location counter will be | |
2521 | @samp{0x8000000} plus the size of the @samp{.data} output section. The | |
2522 | effect is that the linker will place the @samp{.bss} output section | |
58434bc1 | 2523 | immediately after the @samp{.data} output section in memory. |
252b5132 RH |
2524 | |
2525 | The linker will ensure that each output section has the required | |
2526 | alignment, by increasing the location counter if necessary. In this | |
2527 | example, the specified addresses for the @samp{.text} and @samp{.data} | |
2528 | sections will probably satisfy any alignment constraints, but the linker | |
2529 | may have to create a small gap between the @samp{.data} and @samp{.bss} | |
2530 | sections. | |
2531 | ||
2532 | That's it! That's a simple and complete linker script. | |
2533 | ||
2534 | @node Simple Commands | |
2535 | @section Simple Linker Script Commands | |
2536 | @cindex linker script simple commands | |
2537 | In this section we describe the simple linker script commands. | |
2538 | ||
2539 | @menu | |
2540 | * Entry Point:: Setting the entry point | |
2541 | * File Commands:: Commands dealing with files | |
2542 | @ifclear SingleFormat | |
2543 | * Format Commands:: Commands dealing with object file formats | |
2544 | @end ifclear | |
2545 | ||
2546 | * Miscellaneous Commands:: Other linker script commands | |
2547 | @end menu | |
2548 | ||
2549 | @node Entry Point | |
36f63dca | 2550 | @subsection Setting the Entry Point |
252b5132 RH |
2551 | @kindex ENTRY(@var{symbol}) |
2552 | @cindex start of execution | |
2553 | @cindex first instruction | |
2554 | @cindex entry point | |
2555 | The first instruction to execute in a program is called the @dfn{entry | |
2556 | point}. You can use the @code{ENTRY} linker script command to set the | |
2557 | entry point. The argument is a symbol name: | |
2558 | @smallexample | |
2559 | ENTRY(@var{symbol}) | |
2560 | @end smallexample | |
2561 | ||
2562 | There are several ways to set the entry point. The linker will set the | |
2563 | entry point by trying each of the following methods in order, and | |
2564 | stopping when one of them succeeds: | |
2565 | @itemize @bullet | |
a1ab1d2a | 2566 | @item |
252b5132 | 2567 | the @samp{-e} @var{entry} command-line option; |
a1ab1d2a | 2568 | @item |
252b5132 | 2569 | the @code{ENTRY(@var{symbol})} command in a linker script; |
a1ab1d2a | 2570 | @item |
252b5132 | 2571 | the value of the symbol @code{start}, if defined; |
a1ab1d2a | 2572 | @item |
252b5132 | 2573 | the address of the first byte of the @samp{.text} section, if present; |
a1ab1d2a | 2574 | @item |
252b5132 RH |
2575 | The address @code{0}. |
2576 | @end itemize | |
2577 | ||
2578 | @node File Commands | |
36f63dca | 2579 | @subsection Commands Dealing with Files |
252b5132 RH |
2580 | @cindex linker script file commands |
2581 | Several linker script commands deal with files. | |
2582 | ||
2583 | @table @code | |
2584 | @item INCLUDE @var{filename} | |
2585 | @kindex INCLUDE @var{filename} | |
2586 | @cindex including a linker script | |
2587 | Include the linker script @var{filename} at this point. The file will | |
2588 | be searched for in the current directory, and in any directory specified | |
ff5dcc92 | 2589 | with the @option{-L} option. You can nest calls to @code{INCLUDE} up to |
252b5132 RH |
2590 | 10 levels deep. |
2591 | ||
2592 | @item INPUT(@var{file}, @var{file}, @dots{}) | |
2593 | @itemx INPUT(@var{file} @var{file} @dots{}) | |
2594 | @kindex INPUT(@var{files}) | |
2595 | @cindex input files in linker scripts | |
2596 | @cindex input object files in linker scripts | |
2597 | @cindex linker script input object files | |
2598 | The @code{INPUT} command directs the linker to include the named files | |
2599 | in the link, as though they were named on the command line. | |
2600 | ||
2601 | For example, if you always want to include @file{subr.o} any time you do | |
2602 | a link, but you can't be bothered to put it on every link command line, | |
2603 | then you can put @samp{INPUT (subr.o)} in your linker script. | |
2604 | ||
2605 | In fact, if you like, you can list all of your input files in the linker | |
2606 | script, and then invoke the linker with nothing but a @samp{-T} option. | |
2607 | ||
e3f2db7f AO |
2608 | In case a @dfn{sysroot prefix} is configured, and the filename starts |
2609 | with the @samp{/} character, and the script being processed was | |
2610 | located inside the @dfn{sysroot prefix}, the filename will be looked | |
2611 | for in the @dfn{sysroot prefix}. Otherwise, the linker will try to | |
2612 | open the file in the current directory. If it is not found, the | |
2613 | linker will search through the archive library search path. See the | |
2614 | description of @samp{-L} in @ref{Options,,Command Line Options}. | |
252b5132 | 2615 | |
ff5dcc92 | 2616 | If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the |
252b5132 RH |
2617 | name to @code{lib@var{file}.a}, as with the command line argument |
2618 | @samp{-l}. | |
2619 | ||
2620 | When you use the @code{INPUT} command in an implicit linker script, the | |
2621 | files will be included in the link at the point at which the linker | |
2622 | script file is included. This can affect archive searching. | |
2623 | ||
2624 | @item GROUP(@var{file}, @var{file}, @dots{}) | |
2625 | @itemx GROUP(@var{file} @var{file} @dots{}) | |
2626 | @kindex GROUP(@var{files}) | |
2627 | @cindex grouping input files | |
2628 | The @code{GROUP} command is like @code{INPUT}, except that the named | |
2629 | files should all be archives, and they are searched repeatedly until no | |
2630 | new undefined references are created. See the description of @samp{-(} | |
2631 | in @ref{Options,,Command Line Options}. | |
2632 | ||
b717d30e JJ |
2633 | @item AS_NEEDED(@var{file}, @var{file}, @dots{}) |
2634 | @itemx AS_NEEDED(@var{file} @var{file} @dots{}) | |
2635 | @kindex AS_NEEDED(@var{files}) | |
2636 | This construct can appear only inside of the @code{INPUT} or @code{GROUP} | |
2637 | commands, among other filenames. The files listed will be handled | |
2638 | as if they appear directly in the @code{INPUT} or @code{GROUP} commands, | |
2639 | with the exception of ELF shared libraries, that will be added only | |
2640 | when they are actually needed. This construct essentially enables | |
2641 | @option{--as-needed} option for all the files listed inside of it | |
2642 | and restores previous @option{--as-needed} resp. @option{--no-as-needed} | |
2643 | setting afterwards. | |
2644 | ||
252b5132 RH |
2645 | @item OUTPUT(@var{filename}) |
2646 | @kindex OUTPUT(@var{filename}) | |
2647 | @cindex output file name in linker scripot | |
2648 | The @code{OUTPUT} command names the output file. Using | |
2649 | @code{OUTPUT(@var{filename})} in the linker script is exactly like using | |
2650 | @samp{-o @var{filename}} on the command line (@pxref{Options,,Command | |
2651 | Line Options}). If both are used, the command line option takes | |
2652 | precedence. | |
2653 | ||
2654 | You can use the @code{OUTPUT} command to define a default name for the | |
2655 | output file other than the usual default of @file{a.out}. | |
2656 | ||
2657 | @item SEARCH_DIR(@var{path}) | |
2658 | @kindex SEARCH_DIR(@var{path}) | |
2659 | @cindex library search path in linker script | |
2660 | @cindex archive search path in linker script | |
2661 | @cindex search path in linker script | |
2662 | The @code{SEARCH_DIR} command adds @var{path} to the list of paths where | |
ff5dcc92 | 2663 | @command{ld} looks for archive libraries. Using |
252b5132 RH |
2664 | @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}} |
2665 | on the command line (@pxref{Options,,Command Line Options}). If both | |
2666 | are used, then the linker will search both paths. Paths specified using | |
2667 | the command line option are searched first. | |
2668 | ||
2669 | @item STARTUP(@var{filename}) | |
2670 | @kindex STARTUP(@var{filename}) | |
2671 | @cindex first input file | |
2672 | The @code{STARTUP} command is just like the @code{INPUT} command, except | |
2673 | that @var{filename} will become the first input file to be linked, as | |
2674 | though it were specified first on the command line. This may be useful | |
2675 | when using a system in which the entry point is always the start of the | |
2676 | first file. | |
2677 | @end table | |
2678 | ||
2679 | @ifclear SingleFormat | |
2680 | @node Format Commands | |
36f63dca | 2681 | @subsection Commands Dealing with Object File Formats |
252b5132 RH |
2682 | A couple of linker script commands deal with object file formats. |
2683 | ||
2684 | @table @code | |
2685 | @item OUTPUT_FORMAT(@var{bfdname}) | |
2686 | @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little}) | |
2687 | @kindex OUTPUT_FORMAT(@var{bfdname}) | |
2688 | @cindex output file format in linker script | |
2689 | The @code{OUTPUT_FORMAT} command names the BFD format to use for the | |
2690 | output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is | |
024531e2 | 2691 | exactly like using @samp{--oformat @var{bfdname}} on the command line |
252b5132 RH |
2692 | (@pxref{Options,,Command Line Options}). If both are used, the command |
2693 | line option takes precedence. | |
2694 | ||
2695 | You can use @code{OUTPUT_FORMAT} with three arguments to use different | |
2696 | formats based on the @samp{-EB} and @samp{-EL} command line options. | |
2697 | This permits the linker script to set the output format based on the | |
2698 | desired endianness. | |
2699 | ||
2700 | If neither @samp{-EB} nor @samp{-EL} are used, then the output format | |
2701 | will be the first argument, @var{default}. If @samp{-EB} is used, the | |
2702 | output format will be the second argument, @var{big}. If @samp{-EL} is | |
2703 | used, the output format will be the third argument, @var{little}. | |
2704 | ||
2705 | For example, the default linker script for the MIPS ELF target uses this | |
2706 | command: | |
2707 | @smallexample | |
2708 | OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips) | |
2709 | @end smallexample | |
2710 | This says that the default format for the output file is | |
2711 | @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line | |
2712 | option, the output file will be created in the @samp{elf32-littlemips} | |
2713 | format. | |
2714 | ||
2715 | @item TARGET(@var{bfdname}) | |
2716 | @kindex TARGET(@var{bfdname}) | |
2717 | @cindex input file format in linker script | |
2718 | The @code{TARGET} command names the BFD format to use when reading input | |
2719 | files. It affects subsequent @code{INPUT} and @code{GROUP} commands. | |
2720 | This command is like using @samp{-b @var{bfdname}} on the command line | |
2721 | (@pxref{Options,,Command Line Options}). If the @code{TARGET} command | |
2722 | is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET} | |
2723 | command is also used to set the format for the output file. @xref{BFD}. | |
2724 | @end table | |
2725 | @end ifclear | |
2726 | ||
2727 | @node Miscellaneous Commands | |
36f63dca | 2728 | @subsection Other Linker Script Commands |
252b5132 RH |
2729 | There are a few other linker scripts commands. |
2730 | ||
2731 | @table @code | |
2732 | @item ASSERT(@var{exp}, @var{message}) | |
2733 | @kindex ASSERT | |
2734 | @cindex assertion in linker script | |
2735 | Ensure that @var{exp} is non-zero. If it is zero, then exit the linker | |
2736 | with an error code, and print @var{message}. | |
2737 | ||
2738 | @item EXTERN(@var{symbol} @var{symbol} @dots{}) | |
2739 | @kindex EXTERN | |
2740 | @cindex undefined symbol in linker script | |
2741 | Force @var{symbol} to be entered in the output file as an undefined | |
2742 | symbol. Doing this may, for example, trigger linking of additional | |
2743 | modules from standard libraries. You may list several @var{symbol}s for | |
2744 | each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This | |
2745 | command has the same effect as the @samp{-u} command-line option. | |
2746 | ||
2747 | @item FORCE_COMMON_ALLOCATION | |
2748 | @kindex FORCE_COMMON_ALLOCATION | |
2749 | @cindex common allocation in linker script | |
2750 | This command has the same effect as the @samp{-d} command-line option: | |
ff5dcc92 | 2751 | to make @command{ld} assign space to common symbols even if a relocatable |
252b5132 RH |
2752 | output file is specified (@samp{-r}). |
2753 | ||
4818e05f AM |
2754 | @item INHIBIT_COMMON_ALLOCATION |
2755 | @kindex INHIBIT_COMMON_ALLOCATION | |
2756 | @cindex common allocation in linker script | |
2757 | This command has the same effect as the @samp{--no-define-common} | |
2758 | command-line option: to make @code{ld} omit the assignment of addresses | |
2759 | to common symbols even for a non-relocatable output file. | |
2760 | ||
252b5132 RH |
2761 | @item NOCROSSREFS(@var{section} @var{section} @dots{}) |
2762 | @kindex NOCROSSREFS(@var{sections}) | |
2763 | @cindex cross references | |
ff5dcc92 | 2764 | This command may be used to tell @command{ld} to issue an error about any |
252b5132 RH |
2765 | references among certain output sections. |
2766 | ||
2767 | In certain types of programs, particularly on embedded systems when | |
2768 | using overlays, when one section is loaded into memory, another section | |
2769 | will not be. Any direct references between the two sections would be | |
2770 | errors. For example, it would be an error if code in one section called | |
2771 | a function defined in the other section. | |
2772 | ||
2773 | The @code{NOCROSSREFS} command takes a list of output section names. If | |
ff5dcc92 | 2774 | @command{ld} detects any cross references between the sections, it reports |
252b5132 RH |
2775 | an error and returns a non-zero exit status. Note that the |
2776 | @code{NOCROSSREFS} command uses output section names, not input section | |
2777 | names. | |
2778 | ||
2779 | @ifclear SingleFormat | |
2780 | @item OUTPUT_ARCH(@var{bfdarch}) | |
2781 | @kindex OUTPUT_ARCH(@var{bfdarch}) | |
2782 | @cindex machine architecture | |
2783 | @cindex architecture | |
2784 | Specify a particular output machine architecture. The argument is one | |
2785 | of the names used by the BFD library (@pxref{BFD}). You can see the | |
2786 | architecture of an object file by using the @code{objdump} program with | |
2787 | the @samp{-f} option. | |
2788 | @end ifclear | |
2789 | @end table | |
2790 | ||
2791 | @node Assignments | |
2792 | @section Assigning Values to Symbols | |
2793 | @cindex assignment in scripts | |
2794 | @cindex symbol definition, scripts | |
2795 | @cindex variables, defining | |
2796 | You may assign a value to a symbol in a linker script. This will define | |
73ae6183 | 2797 | the symbol and place it into the symbol table with a global scope. |
252b5132 RH |
2798 | |
2799 | @menu | |
2800 | * Simple Assignments:: Simple Assignments | |
2801 | * PROVIDE:: PROVIDE | |
7af8e998 | 2802 | * PROVIDE_HIDDEN:: PROVIDE_HIDDEN |
73ae6183 | 2803 | * Source Code Reference:: How to use a linker script defined symbol in source code |
252b5132 RH |
2804 | @end menu |
2805 | ||
2806 | @node Simple Assignments | |
2807 | @subsection Simple Assignments | |
2808 | ||
2809 | You may assign to a symbol using any of the C assignment operators: | |
2810 | ||
2811 | @table @code | |
2812 | @item @var{symbol} = @var{expression} ; | |
2813 | @itemx @var{symbol} += @var{expression} ; | |
2814 | @itemx @var{symbol} -= @var{expression} ; | |
2815 | @itemx @var{symbol} *= @var{expression} ; | |
2816 | @itemx @var{symbol} /= @var{expression} ; | |
2817 | @itemx @var{symbol} <<= @var{expression} ; | |
2818 | @itemx @var{symbol} >>= @var{expression} ; | |
2819 | @itemx @var{symbol} &= @var{expression} ; | |
2820 | @itemx @var{symbol} |= @var{expression} ; | |
2821 | @end table | |
2822 | ||
2823 | The first case will define @var{symbol} to the value of | |
2824 | @var{expression}. In the other cases, @var{symbol} must already be | |
2825 | defined, and the value will be adjusted accordingly. | |
2826 | ||
2827 | The special symbol name @samp{.} indicates the location counter. You | |
b5666f2f | 2828 | may only use this within a @code{SECTIONS} command. @xref{Location Counter}. |
252b5132 RH |
2829 | |
2830 | The semicolon after @var{expression} is required. | |
2831 | ||
2832 | Expressions are defined below; see @ref{Expressions}. | |
2833 | ||
2834 | You may write symbol assignments as commands in their own right, or as | |
2835 | statements within a @code{SECTIONS} command, or as part of an output | |
2836 | section description in a @code{SECTIONS} command. | |
2837 | ||
2838 | The section of the symbol will be set from the section of the | |
2839 | expression; for more information, see @ref{Expression Section}. | |
2840 | ||
2841 | Here is an example showing the three different places that symbol | |
2842 | assignments may be used: | |
2843 | ||
2844 | @smallexample | |
2845 | floating_point = 0; | |
2846 | SECTIONS | |
2847 | @{ | |
2848 | .text : | |
2849 | @{ | |
2850 | *(.text) | |
2851 | _etext = .; | |
2852 | @} | |
156e34dd | 2853 | _bdata = (. + 3) & ~ 3; |
252b5132 RH |
2854 | .data : @{ *(.data) @} |
2855 | @} | |
2856 | @end smallexample | |
2857 | @noindent | |
2858 | In this example, the symbol @samp{floating_point} will be defined as | |
2859 | zero. The symbol @samp{_etext} will be defined as the address following | |
2860 | the last @samp{.text} input section. The symbol @samp{_bdata} will be | |
2861 | defined as the address following the @samp{.text} output section aligned | |
2862 | upward to a 4 byte boundary. | |
2863 | ||
2864 | @node PROVIDE | |
2865 | @subsection PROVIDE | |
2866 | @cindex PROVIDE | |
2867 | In some cases, it is desirable for a linker script to define a symbol | |
2868 | only if it is referenced and is not defined by any object included in | |
2869 | the link. For example, traditional linkers defined the symbol | |
2870 | @samp{etext}. However, ANSI C requires that the user be able to use | |
2871 | @samp{etext} as a function name without encountering an error. The | |
2872 | @code{PROVIDE} keyword may be used to define a symbol, such as | |
2873 | @samp{etext}, only if it is referenced but not defined. The syntax is | |
2874 | @code{PROVIDE(@var{symbol} = @var{expression})}. | |
2875 | ||
2876 | Here is an example of using @code{PROVIDE} to define @samp{etext}: | |
2877 | @smallexample | |
2878 | SECTIONS | |
2879 | @{ | |
2880 | .text : | |
2881 | @{ | |
2882 | *(.text) | |
2883 | _etext = .; | |
2884 | PROVIDE(etext = .); | |
2885 | @} | |
2886 | @} | |
2887 | @end smallexample | |
2888 | ||
2889 | In this example, if the program defines @samp{_etext} (with a leading | |
2890 | underscore), the linker will give a multiple definition error. If, on | |
2891 | the other hand, the program defines @samp{etext} (with no leading | |
2892 | underscore), the linker will silently use the definition in the program. | |
2893 | If the program references @samp{etext} but does not define it, the | |
2894 | linker will use the definition in the linker script. | |
2895 | ||
7af8e998 L |
2896 | @node PROVIDE_HIDDEN |
2897 | @subsection PROVIDE_HIDDEN | |
2898 | @cindex PROVIDE_HIDDEN | |
2899 | Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be | |
2900 | hidden and won't be exported. | |
2901 | ||
73ae6183 NC |
2902 | @node Source Code Reference |
2903 | @subsection Source Code Reference | |
2904 | ||
2905 | Accessing a linker script defined variable from source code is not | |
2906 | intuitive. In particular a linker script symbol is not equivalent to | |
2907 | a variable declaration in a high level language, it is instead a | |
2908 | symbol that does not have a value. | |
2909 | ||
2910 | Before going further, it is important to note that compilers often | |
2911 | transform names in the source code into different names when they are | |
2912 | stored in the symbol table. For example, Fortran compilers commonly | |
2913 | prepend or append an underscore, and C++ performs extensive @samp{name | |
2914 | mangling}. Therefore there might be a discrepancy between the name | |
2915 | of a variable as it is used in source code and the name of the same | |
2916 | variable as it is defined in a linker script. For example in C a | |
2917 | linker script variable might be referred to as: | |
2918 | ||
2919 | @smallexample | |
2920 | extern int foo; | |
2921 | @end smallexample | |
2922 | ||
2923 | But in the linker script it might be defined as: | |
2924 | ||
2925 | @smallexample | |
2926 | _foo = 1000; | |
2927 | @end smallexample | |
2928 | ||
2929 | In the remaining examples however it is assumed that no name | |
2930 | transformation has taken place. | |
2931 | ||
2932 | When a symbol is declared in a high level language such as C, two | |
2933 | things happen. The first is that the compiler reserves enough space | |
2934 | in the program's memory to hold the @emph{value} of the symbol. The | |
2935 | second is that the compiler creates an entry in the program's symbol | |
2936 | table which holds the symbol's @emph{address}. ie the symbol table | |
2937 | contains the address of the block of memory holding the symbol's | |
2938 | value. So for example the following C declaration, at file scope: | |
2939 | ||
2940 | @smallexample | |
2941 | int foo = 1000; | |
2942 | @end smallexample | |
2943 | ||
2944 | creates a entry called @samp{foo} in the symbol table. This entry | |
2945 | holds the address of an @samp{int} sized block of memory where the | |
2946 | number 1000 is initially stored. | |
2947 | ||
2948 | When a program references a symbol the compiler generates code that | |
2949 | first accesses the symbol table to find the address of the symbol's | |
2950 | memory block and then code to read the value from that memory block. | |
2951 | So: | |
2952 | ||
2953 | @smallexample | |
2954 | foo = 1; | |
2955 | @end smallexample | |
2956 | ||
2957 | looks up the symbol @samp{foo} in the symbol table, gets the address | |
2958 | associated with this symbol and then writes the value 1 into that | |
2959 | address. Whereas: | |
2960 | ||
2961 | @smallexample | |
2962 | int * a = & foo; | |
2963 | @end smallexample | |
2964 | ||
2965 | looks up the symbol @samp{foo} in the symbol table, gets it address | |
2966 | and then copies this address into the block of memory associated with | |
2967 | the variable @samp{a}. | |
2968 | ||
2969 | Linker scripts symbol declarations, by contrast, create an entry in | |
2970 | the symbol table but do not assign any memory to them. Thus they are | |
2971 | an address without a value. So for example the linker script definition: | |
2972 | ||
2973 | @smallexample | |
2974 | foo = 1000; | |
2975 | @end smallexample | |
2976 | ||
2977 | creates an entry in the symbol table called @samp{foo} which holds | |
2978 | the address of memory location 1000, but nothing special is stored at | |
2979 | address 1000. This means that you cannot access the @emph{value} of a | |
2980 | linker script defined symbol - it has no value - all you can do is | |
2981 | access the @emph{address} of a linker script defined symbol. | |
2982 | ||
2983 | Hence when you are using a linker script defined symbol in source code | |
2984 | you should always take the address of the symbol, and never attempt to | |
2985 | use its value. For example suppose you want to copy the contents of a | |
2986 | section of memory called .ROM into a section called .FLASH and the | |
2987 | linker script contains these declarations: | |
2988 | ||
2989 | @smallexample | |
2990 | @group | |
2991 | start_of_ROM = .ROM; | |
2992 | end_of_ROM = .ROM + sizeof (.ROM) - 1; | |
2993 | start_of_FLASH = .FLASH; | |
2994 | @end group | |
2995 | @end smallexample | |
2996 | ||
2997 | Then the C source code to perform the copy would be: | |
2998 | ||
2999 | @smallexample | |
3000 | @group | |
3001 | extern char start_of_ROM, end_of_ROM, start_of_FLASH; | |
3002 | ||
3003 | memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM); | |
3004 | @end group | |
3005 | @end smallexample | |
3006 | ||
3007 | Note the use of the @samp{&} operators. These are correct. | |
3008 | ||
252b5132 | 3009 | @node SECTIONS |
36f63dca | 3010 | @section SECTIONS Command |
252b5132 RH |
3011 | @kindex SECTIONS |
3012 | The @code{SECTIONS} command tells the linker how to map input sections | |
3013 | into output sections, and how to place the output sections in memory. | |
3014 | ||
3015 | The format of the @code{SECTIONS} command is: | |
3016 | @smallexample | |
3017 | SECTIONS | |
3018 | @{ | |
3019 | @var{sections-command} | |
3020 | @var{sections-command} | |
3021 | @dots{} | |
3022 | @} | |
3023 | @end smallexample | |
3024 | ||
3025 | Each @var{sections-command} may of be one of the following: | |
3026 | ||
3027 | @itemize @bullet | |
3028 | @item | |
3029 | an @code{ENTRY} command (@pxref{Entry Point,,Entry command}) | |
3030 | @item | |
3031 | a symbol assignment (@pxref{Assignments}) | |
3032 | @item | |
3033 | an output section description | |
3034 | @item | |
3035 | an overlay description | |
3036 | @end itemize | |
3037 | ||
3038 | The @code{ENTRY} command and symbol assignments are permitted inside the | |
3039 | @code{SECTIONS} command for convenience in using the location counter in | |
3040 | those commands. This can also make the linker script easier to | |
3041 | understand because you can use those commands at meaningful points in | |
3042 | the layout of the output file. | |
3043 | ||
3044 | Output section descriptions and overlay descriptions are described | |
3045 | below. | |
3046 | ||
3047 | If you do not use a @code{SECTIONS} command in your linker script, the | |
3048 | linker will place each input section into an identically named output | |
3049 | section in the order that the sections are first encountered in the | |
3050 | input files. If all input sections are present in the first file, for | |
3051 | example, the order of sections in the output file will match the order | |
3052 | in the first input file. The first section will be at address zero. | |
3053 | ||
3054 | @menu | |
3055 | * Output Section Description:: Output section description | |
3056 | * Output Section Name:: Output section name | |
3057 | * Output Section Address:: Output section address | |
3058 | * Input Section:: Input section description | |
3059 | * Output Section Data:: Output section data | |
3060 | * Output Section Keywords:: Output section keywords | |
3061 | * Output Section Discarding:: Output section discarding | |
3062 | * Output Section Attributes:: Output section attributes | |
3063 | * Overlay Description:: Overlay description | |
3064 | @end menu | |
3065 | ||
3066 | @node Output Section Description | |
36f63dca | 3067 | @subsection Output Section Description |
252b5132 RH |
3068 | The full description of an output section looks like this: |
3069 | @smallexample | |
a1ab1d2a | 3070 | @group |
7e7d5768 | 3071 | @var{section} [@var{address}] [(@var{type})] : |
bbf115d3 | 3072 | [AT(@var{lma})] [ALIGN(@var{section_align})] [SUBALIGN(@var{subsection_align})] |
252b5132 RH |
3073 | @{ |
3074 | @var{output-section-command} | |
3075 | @var{output-section-command} | |
3076 | @dots{} | |
562d3460 | 3077 | @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] |
252b5132 RH |
3078 | @end group |
3079 | @end smallexample | |
3080 | ||
3081 | Most output sections do not use most of the optional section attributes. | |
3082 | ||
3083 | The whitespace around @var{section} is required, so that the section | |
3084 | name is unambiguous. The colon and the curly braces are also required. | |
3085 | The line breaks and other white space are optional. | |
3086 | ||
3087 | Each @var{output-section-command} may be one of the following: | |
3088 | ||
3089 | @itemize @bullet | |
3090 | @item | |
3091 | a symbol assignment (@pxref{Assignments}) | |
3092 | @item | |
3093 | an input section description (@pxref{Input Section}) | |
3094 | @item | |
3095 | data values to include directly (@pxref{Output Section Data}) | |
3096 | @item | |
3097 | a special output section keyword (@pxref{Output Section Keywords}) | |
3098 | @end itemize | |
3099 | ||
3100 | @node Output Section Name | |
36f63dca | 3101 | @subsection Output Section Name |
252b5132 RH |
3102 | @cindex name, section |
3103 | @cindex section name | |
3104 | The name of the output section is @var{section}. @var{section} must | |
3105 | meet the constraints of your output format. In formats which only | |
3106 | support a limited number of sections, such as @code{a.out}, the name | |
3107 | must be one of the names supported by the format (@code{a.out}, for | |
3108 | example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the | |
3109 | output format supports any number of sections, but with numbers and not | |
3110 | names (as is the case for Oasys), the name should be supplied as a | |
3111 | quoted numeric string. A section name may consist of any sequence of | |
3112 | characters, but a name which contains any unusual characters such as | |
3113 | commas must be quoted. | |
3114 | ||
3115 | The output section name @samp{/DISCARD/} is special; @ref{Output Section | |
3116 | Discarding}. | |
3117 | ||
3118 | @node Output Section Address | |
2a16d82a | 3119 | @subsection Output Section Address |
252b5132 RH |
3120 | @cindex address, section |
3121 | @cindex section address | |
3122 | The @var{address} is an expression for the VMA (the virtual memory | |
3123 | address) of the output section. If you do not provide @var{address}, | |
3124 | the linker will set it based on @var{region} if present, or otherwise | |
3125 | based on the current value of the location counter. | |
3126 | ||
3127 | If you provide @var{address}, the address of the output section will be | |
3128 | set to precisely that. If you provide neither @var{address} nor | |
3129 | @var{region}, then the address of the output section will be set to the | |
3130 | current value of the location counter aligned to the alignment | |
3131 | requirements of the output section. The alignment requirement of the | |
3132 | output section is the strictest alignment of any input section contained | |
3133 | within the output section. | |
3134 | ||
3135 | For example, | |
3136 | @smallexample | |
3137 | .text . : @{ *(.text) @} | |
3138 | @end smallexample | |
3139 | @noindent | |
3140 | and | |
3141 | @smallexample | |
3142 | .text : @{ *(.text) @} | |
3143 | @end smallexample | |
3144 | @noindent | |
3145 | are subtly different. The first will set the address of the | |
3146 | @samp{.text} output section to the current value of the location | |
3147 | counter. The second will set it to the current value of the location | |
3148 | counter aligned to the strictest alignment of a @samp{.text} input | |
3149 | section. | |
3150 | ||
3151 | The @var{address} may be an arbitrary expression; @ref{Expressions}. | |
3152 | For example, if you want to align the section on a 0x10 byte boundary, | |
3153 | so that the lowest four bits of the section address are zero, you could | |
3154 | do something like this: | |
3155 | @smallexample | |
3156 | .text ALIGN(0x10) : @{ *(.text) @} | |
3157 | @end smallexample | |
3158 | @noindent | |
3159 | This works because @code{ALIGN} returns the current location counter | |
3160 | aligned upward to the specified value. | |
3161 | ||
3162 | Specifying @var{address} for a section will change the value of the | |
3163 | location counter. | |
3164 | ||
3165 | @node Input Section | |
36f63dca | 3166 | @subsection Input Section Description |
252b5132 RH |
3167 | @cindex input sections |
3168 | @cindex mapping input sections to output sections | |
3169 | The most common output section command is an input section description. | |
3170 | ||
3171 | The input section description is the most basic linker script operation. | |
3172 | You use output sections to tell the linker how to lay out your program | |
3173 | in memory. You use input section descriptions to tell the linker how to | |
3174 | map the input files into your memory layout. | |
3175 | ||
3176 | @menu | |
3177 | * Input Section Basics:: Input section basics | |
3178 | * Input Section Wildcards:: Input section wildcard patterns | |
3179 | * Input Section Common:: Input section for common symbols | |
3180 | * Input Section Keep:: Input section and garbage collection | |
3181 | * Input Section Example:: Input section example | |
3182 | @end menu | |
3183 | ||
3184 | @node Input Section Basics | |
36f63dca | 3185 | @subsubsection Input Section Basics |
252b5132 RH |
3186 | @cindex input section basics |
3187 | An input section description consists of a file name optionally followed | |
3188 | by a list of section names in parentheses. | |
3189 | ||
3190 | The file name and the section name may be wildcard patterns, which we | |
3191 | describe further below (@pxref{Input Section Wildcards}). | |
3192 | ||
3193 | The most common input section description is to include all input | |
3194 | sections with a particular name in the output section. For example, to | |
3195 | include all input @samp{.text} sections, you would write: | |
3196 | @smallexample | |
3197 | *(.text) | |
3198 | @end smallexample | |
3199 | @noindent | |
18625d54 CM |
3200 | Here the @samp{*} is a wildcard which matches any file name. To exclude a list |
3201 | of files from matching the file name wildcard, EXCLUDE_FILE may be used to | |
3202 | match all files except the ones specified in the EXCLUDE_FILE list. For | |
3203 | example: | |
252b5132 | 3204 | @smallexample |
765b7cbe | 3205 | (*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)) |
252b5132 | 3206 | @end smallexample |
765b7cbe JB |
3207 | will cause all .ctors sections from all files except @file{crtend.o} and |
3208 | @file{otherfile.o} to be included. | |
252b5132 RH |
3209 | |
3210 | There are two ways to include more than one section: | |
3211 | @smallexample | |
3212 | *(.text .rdata) | |
3213 | *(.text) *(.rdata) | |
3214 | @end smallexample | |
3215 | @noindent | |
3216 | The difference between these is the order in which the @samp{.text} and | |
3217 | @samp{.rdata} input sections will appear in the output section. In the | |
b6bf44ba AM |
3218 | first example, they will be intermingled, appearing in the same order as |
3219 | they are found in the linker input. In the second example, all | |
252b5132 RH |
3220 | @samp{.text} input sections will appear first, followed by all |
3221 | @samp{.rdata} input sections. | |
3222 | ||
3223 | You can specify a file name to include sections from a particular file. | |
3224 | You would do this if one or more of your files contain special data that | |
3225 | needs to be at a particular location in memory. For example: | |
3226 | @smallexample | |
3227 | data.o(.data) | |
3228 | @end smallexample | |
3229 | ||
3230 | If you use a file name without a list of sections, then all sections in | |
3231 | the input file will be included in the output section. This is not | |
3232 | commonly done, but it may by useful on occasion. For example: | |
3233 | @smallexample | |
3234 | data.o | |
3235 | @end smallexample | |
3236 | ||
3237 | When you use a file name which does not contain any wild card | |
3238 | characters, the linker will first see if you also specified the file | |
3239 | name on the linker command line or in an @code{INPUT} command. If you | |
3240 | did not, the linker will attempt to open the file as an input file, as | |
3241 | though it appeared on the command line. Note that this differs from an | |
3242 | @code{INPUT} command, because the linker will not search for the file in | |
3243 | the archive search path. | |
3244 | ||
3245 | @node Input Section Wildcards | |
36f63dca | 3246 | @subsubsection Input Section Wildcard Patterns |
252b5132 RH |
3247 | @cindex input section wildcards |
3248 | @cindex wildcard file name patterns | |
3249 | @cindex file name wildcard patterns | |
3250 | @cindex section name wildcard patterns | |
3251 | In an input section description, either the file name or the section | |
3252 | name or both may be wildcard patterns. | |
3253 | ||
3254 | The file name of @samp{*} seen in many examples is a simple wildcard | |
3255 | pattern for the file name. | |
3256 | ||
3257 | The wildcard patterns are like those used by the Unix shell. | |
3258 | ||
3259 | @table @samp | |
3260 | @item * | |
3261 | matches any number of characters | |
3262 | @item ? | |
3263 | matches any single character | |
3264 | @item [@var{chars}] | |
3265 | matches a single instance of any of the @var{chars}; the @samp{-} | |
3266 | character may be used to specify a range of characters, as in | |
3267 | @samp{[a-z]} to match any lower case letter | |
3268 | @item \ | |
3269 | quotes the following character | |
3270 | @end table | |
3271 | ||
3272 | When a file name is matched with a wildcard, the wildcard characters | |
3273 | will not match a @samp{/} character (used to separate directory names on | |
3274 | Unix). A pattern consisting of a single @samp{*} character is an | |
3275 | exception; it will always match any file name, whether it contains a | |
3276 | @samp{/} or not. In a section name, the wildcard characters will match | |
3277 | a @samp{/} character. | |
3278 | ||
3279 | File name wildcard patterns only match files which are explicitly | |
3280 | specified on the command line or in an @code{INPUT} command. The linker | |
3281 | does not search directories to expand wildcards. | |
3282 | ||
3283 | If a file name matches more than one wildcard pattern, or if a file name | |
3284 | appears explicitly and is also matched by a wildcard pattern, the linker | |
3285 | will use the first match in the linker script. For example, this | |
3286 | sequence of input section descriptions is probably in error, because the | |
3287 | @file{data.o} rule will not be used: | |
3288 | @smallexample | |
3289 | .data : @{ *(.data) @} | |
3290 | .data1 : @{ data.o(.data) @} | |
3291 | @end smallexample | |
3292 | ||
bcaa7b3e | 3293 | @cindex SORT_BY_NAME |
252b5132 RH |
3294 | Normally, the linker will place files and sections matched by wildcards |
3295 | in the order in which they are seen during the link. You can change | |
bcaa7b3e L |
3296 | this by using the @code{SORT_BY_NAME} keyword, which appears before a wildcard |
3297 | pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the | |
3298 | @code{SORT_BY_NAME} keyword is used, the linker will sort the files or sections | |
252b5132 RH |
3299 | into ascending order by name before placing them in the output file. |
3300 | ||
bcaa7b3e L |
3301 | @cindex SORT_BY_ALIGNMENT |
3302 | @code{SORT_BY_ALIGNMENT} is very similar to @code{SORT_BY_NAME}. The | |
3303 | difference is @code{SORT_BY_ALIGNMENT} will sort sections into | |
3304 | ascending order by alignment before placing them in the output file. | |
3305 | ||
3306 | @cindex SORT | |
3307 | @code{SORT} is an alias for @code{SORT_BY_NAME}. | |
3308 | ||
3309 | When there are nested section sorting commands in linker script, there | |
3310 | can be at most 1 level of nesting for section sorting commands. | |
3311 | ||
3312 | @enumerate | |
3313 | @item | |
3314 | @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)). | |
3315 | It will sort the input sections by name first, then by alignment if 2 | |
3316 | sections have the same name. | |
3317 | @item | |
3318 | @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)). | |
3319 | It will sort the input sections by alignment first, then by name if 2 | |
3320 | sections have the same alignment. | |
3321 | @item | |
3322 | @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is | |
3323 | treated the same as @code{SORT_BY_NAME} (wildcard section pattern). | |
3324 | @item | |
3325 | @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)) | |
3326 | is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern). | |
3327 | @item | |
3328 | All other nested section sorting commands are invalid. | |
3329 | @end enumerate | |
3330 | ||
3331 | When both command line section sorting option and linker script | |
3332 | section sorting command are used, section sorting command always | |
3333 | takes precedence over the command line option. | |
3334 | ||
3335 | If the section sorting command in linker script isn't nested, the | |
3336 | command line option will make the section sorting command to be | |
3337 | treated as nested sorting command. | |
3338 | ||
3339 | @enumerate | |
3340 | @item | |
3341 | @code{SORT_BY_NAME} (wildcard section pattern ) with | |
3342 | @option{--sort-sections alignment} is equivalent to | |
3343 | @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)). | |
3344 | @item | |
3345 | @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with | |
3346 | @option{--sort-section name} is equivalent to | |
3347 | @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)). | |
3348 | @end enumerate | |
3349 | ||
3350 | If the section sorting command in linker script is nested, the | |
3351 | command line option will be ignored. | |
3352 | ||
252b5132 RH |
3353 | If you ever get confused about where input sections are going, use the |
3354 | @samp{-M} linker option to generate a map file. The map file shows | |
3355 | precisely how input sections are mapped to output sections. | |
3356 | ||
3357 | This example shows how wildcard patterns might be used to partition | |
3358 | files. This linker script directs the linker to place all @samp{.text} | |
3359 | sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}. | |
3360 | The linker will place the @samp{.data} section from all files beginning | |
3361 | with an upper case character in @samp{.DATA}; for all other files, the | |
3362 | linker will place the @samp{.data} section in @samp{.data}. | |
3363 | @smallexample | |
3364 | @group | |
3365 | SECTIONS @{ | |
3366 | .text : @{ *(.text) @} | |
3367 | .DATA : @{ [A-Z]*(.data) @} | |
3368 | .data : @{ *(.data) @} | |
3369 | .bss : @{ *(.bss) @} | |
3370 | @} | |
3371 | @end group | |
3372 | @end smallexample | |
3373 | ||
3374 | @node Input Section Common | |
36f63dca | 3375 | @subsubsection Input Section for Common Symbols |
252b5132 RH |
3376 | @cindex common symbol placement |
3377 | @cindex uninitialized data placement | |
3378 | A special notation is needed for common symbols, because in many object | |
3379 | file formats common symbols do not have a particular input section. The | |
3380 | linker treats common symbols as though they are in an input section | |
3381 | named @samp{COMMON}. | |
3382 | ||
3383 | You may use file names with the @samp{COMMON} section just as with any | |
3384 | other input sections. You can use this to place common symbols from a | |
3385 | particular input file in one section while common symbols from other | |
3386 | input files are placed in another section. | |
3387 | ||
3388 | In most cases, common symbols in input files will be placed in the | |
3389 | @samp{.bss} section in the output file. For example: | |
3390 | @smallexample | |
3391 | .bss @{ *(.bss) *(COMMON) @} | |
3392 | @end smallexample | |
3393 | ||
3394 | @cindex scommon section | |
3395 | @cindex small common symbols | |
3396 | Some object file formats have more than one type of common symbol. For | |
3397 | example, the MIPS ELF object file format distinguishes standard common | |
3398 | symbols and small common symbols. In this case, the linker will use a | |
3399 | different special section name for other types of common symbols. In | |
3400 | the case of MIPS ELF, the linker uses @samp{COMMON} for standard common | |
3401 | symbols and @samp{.scommon} for small common symbols. This permits you | |
3402 | to map the different types of common symbols into memory at different | |
3403 | locations. | |
3404 | ||
3405 | @cindex [COMMON] | |
3406 | You will sometimes see @samp{[COMMON]} in old linker scripts. This | |
3407 | notation is now considered obsolete. It is equivalent to | |
3408 | @samp{*(COMMON)}. | |
3409 | ||
3410 | @node Input Section Keep | |
36f63dca | 3411 | @subsubsection Input Section and Garbage Collection |
252b5132 RH |
3412 | @cindex KEEP |
3413 | @cindex garbage collection | |
3414 | When link-time garbage collection is in use (@samp{--gc-sections}), | |
a1ab1d2a | 3415 | it is often useful to mark sections that should not be eliminated. |
252b5132 RH |
3416 | This is accomplished by surrounding an input section's wildcard entry |
3417 | with @code{KEEP()}, as in @code{KEEP(*(.init))} or | |
bcaa7b3e | 3418 | @code{KEEP(SORT_BY_NAME(*)(.ctors))}. |
252b5132 RH |
3419 | |
3420 | @node Input Section Example | |
36f63dca | 3421 | @subsubsection Input Section Example |
252b5132 RH |
3422 | The following example is a complete linker script. It tells the linker |
3423 | to read all of the sections from file @file{all.o} and place them at the | |
3424 | start of output section @samp{outputa} which starts at location | |
3425 | @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o} | |
3426 | follows immediately, in the same output section. All of section | |
3427 | @samp{.input2} from @file{foo.o} goes into output section | |
3428 | @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}. | |
3429 | All of the remaining @samp{.input1} and @samp{.input2} sections from any | |
3430 | files are written to output section @samp{outputc}. | |
3431 | ||
3432 | @smallexample | |
3433 | @group | |
3434 | SECTIONS @{ | |
3435 | outputa 0x10000 : | |
3436 | @{ | |
3437 | all.o | |
3438 | foo.o (.input1) | |
3439 | @} | |
36f63dca NC |
3440 | @end group |
3441 | @group | |
252b5132 RH |
3442 | outputb : |
3443 | @{ | |
3444 | foo.o (.input2) | |
3445 | foo1.o (.input1) | |
3446 | @} | |
36f63dca NC |
3447 | @end group |
3448 | @group | |
252b5132 RH |
3449 | outputc : |
3450 | @{ | |
3451 | *(.input1) | |
3452 | *(.input2) | |
3453 | @} | |
3454 | @} | |
3455 | @end group | |
a1ab1d2a | 3456 | @end smallexample |
252b5132 RH |
3457 | |
3458 | @node Output Section Data | |
36f63dca | 3459 | @subsection Output Section Data |
252b5132 RH |
3460 | @cindex data |
3461 | @cindex section data | |
3462 | @cindex output section data | |
3463 | @kindex BYTE(@var{expression}) | |
3464 | @kindex SHORT(@var{expression}) | |
3465 | @kindex LONG(@var{expression}) | |
3466 | @kindex QUAD(@var{expression}) | |
3467 | @kindex SQUAD(@var{expression}) | |
3468 | You can include explicit bytes of data in an output section by using | |
3469 | @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as | |
3470 | an output section command. Each keyword is followed by an expression in | |
3471 | parentheses providing the value to store (@pxref{Expressions}). The | |
3472 | value of the expression is stored at the current value of the location | |
3473 | counter. | |
3474 | ||
3475 | The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands | |
3476 | store one, two, four, and eight bytes (respectively). After storing the | |
3477 | bytes, the location counter is incremented by the number of bytes | |
3478 | stored. | |
3479 | ||
3480 | For example, this will store the byte 1 followed by the four byte value | |
3481 | of the symbol @samp{addr}: | |
3482 | @smallexample | |
3483 | BYTE(1) | |
3484 | LONG(addr) | |
3485 | @end smallexample | |
3486 | ||
3487 | When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the | |
3488 | same; they both store an 8 byte, or 64 bit, value. When both host and | |
3489 | target are 32 bits, an expression is computed as 32 bits. In this case | |
3490 | @code{QUAD} stores a 32 bit value zero extended to 64 bits, and | |
3491 | @code{SQUAD} stores a 32 bit value sign extended to 64 bits. | |
3492 | ||
3493 | If the object file format of the output file has an explicit endianness, | |
3494 | which is the normal case, the value will be stored in that endianness. | |
3495 | When the object file format does not have an explicit endianness, as is | |
3496 | true of, for example, S-records, the value will be stored in the | |
3497 | endianness of the first input object file. | |
3498 | ||
36f63dca | 3499 | Note---these commands only work inside a section description and not |
2b5fc1f5 NC |
3500 | between them, so the following will produce an error from the linker: |
3501 | @smallexample | |
3502 | SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@ | |
3503 | @end smallexample | |
3504 | whereas this will work: | |
3505 | @smallexample | |
3506 | SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@ | |
3507 | @end smallexample | |
3508 | ||
252b5132 RH |
3509 | @kindex FILL(@var{expression}) |
3510 | @cindex holes, filling | |
3511 | @cindex unspecified memory | |
3512 | You may use the @code{FILL} command to set the fill pattern for the | |
3513 | current section. It is followed by an expression in parentheses. Any | |
3514 | otherwise unspecified regions of memory within the section (for example, | |
3515 | gaps left due to the required alignment of input sections) are filled | |
a139d329 | 3516 | with the value of the expression, repeated as |
252b5132 RH |
3517 | necessary. A @code{FILL} statement covers memory locations after the |
3518 | point at which it occurs in the section definition; by including more | |
3519 | than one @code{FILL} statement, you can have different fill patterns in | |
3520 | different parts of an output section. | |
3521 | ||
3522 | This example shows how to fill unspecified regions of memory with the | |
563e308f | 3523 | value @samp{0x90}: |
252b5132 | 3524 | @smallexample |
563e308f | 3525 | FILL(0x90909090) |
252b5132 RH |
3526 | @end smallexample |
3527 | ||
3528 | The @code{FILL} command is similar to the @samp{=@var{fillexp}} output | |
9673c93c | 3529 | section attribute, but it only affects the |
252b5132 RH |
3530 | part of the section following the @code{FILL} command, rather than the |
3531 | entire section. If both are used, the @code{FILL} command takes | |
9673c93c | 3532 | precedence. @xref{Output Section Fill}, for details on the fill |
a139d329 | 3533 | expression. |
252b5132 RH |
3534 | |
3535 | @node Output Section Keywords | |
36f63dca | 3536 | @subsection Output Section Keywords |
252b5132 RH |
3537 | There are a couple of keywords which can appear as output section |
3538 | commands. | |
3539 | ||
3540 | @table @code | |
3541 | @kindex CREATE_OBJECT_SYMBOLS | |
3542 | @cindex input filename symbols | |
3543 | @cindex filename symbols | |
3544 | @item CREATE_OBJECT_SYMBOLS | |
3545 | The command tells the linker to create a symbol for each input file. | |
3546 | The name of each symbol will be the name of the corresponding input | |
3547 | file. The section of each symbol will be the output section in which | |
3548 | the @code{CREATE_OBJECT_SYMBOLS} command appears. | |
3549 | ||
3550 | This is conventional for the a.out object file format. It is not | |
3551 | normally used for any other object file format. | |
3552 | ||
3553 | @kindex CONSTRUCTORS | |
3554 | @cindex C++ constructors, arranging in link | |
3555 | @cindex constructors, arranging in link | |
3556 | @item CONSTRUCTORS | |
3557 | When linking using the a.out object file format, the linker uses an | |
3558 | unusual set construct to support C++ global constructors and | |
3559 | destructors. When linking object file formats which do not support | |
3560 | arbitrary sections, such as ECOFF and XCOFF, the linker will | |
3561 | automatically recognize C++ global constructors and destructors by name. | |
3562 | For these object file formats, the @code{CONSTRUCTORS} command tells the | |
3563 | linker to place constructor information in the output section where the | |
3564 | @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is | |
3565 | ignored for other object file formats. | |
3566 | ||
3567 | The symbol @w{@code{__CTOR_LIST__}} marks the start of the global | |
7e69709c AM |
3568 | constructors, and the symbol @w{@code{__CTOR_END__}} marks the end. |
3569 | Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_END__}} mark | |
3570 | the start and end of the global destructors. The | |
252b5132 RH |
3571 | first word in the list is the number of entries, followed by the address |
3572 | of each constructor or destructor, followed by a zero word. The | |
3573 | compiler must arrange to actually run the code. For these object file | |
3574 | formats @sc{gnu} C++ normally calls constructors from a subroutine | |
3575 | @code{__main}; a call to @code{__main} is automatically inserted into | |
3576 | the startup code for @code{main}. @sc{gnu} C++ normally runs | |
3577 | destructors either by using @code{atexit}, or directly from the function | |
3578 | @code{exit}. | |
3579 | ||
3580 | For object file formats such as @code{COFF} or @code{ELF} which support | |
3581 | arbitrary section names, @sc{gnu} C++ will normally arrange to put the | |
3582 | addresses of global constructors and destructors into the @code{.ctors} | |
3583 | and @code{.dtors} sections. Placing the following sequence into your | |
3584 | linker script will build the sort of table which the @sc{gnu} C++ | |
3585 | runtime code expects to see. | |
3586 | ||
3587 | @smallexample | |
3588 | __CTOR_LIST__ = .; | |
3589 | LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2) | |
3590 | *(.ctors) | |
3591 | LONG(0) | |
3592 | __CTOR_END__ = .; | |
3593 | __DTOR_LIST__ = .; | |
3594 | LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2) | |
3595 | *(.dtors) | |
3596 | LONG(0) | |
3597 | __DTOR_END__ = .; | |
3598 | @end smallexample | |
3599 | ||
3600 | If you are using the @sc{gnu} C++ support for initialization priority, | |
3601 | which provides some control over the order in which global constructors | |
3602 | are run, you must sort the constructors at link time to ensure that they | |
3603 | are executed in the correct order. When using the @code{CONSTRUCTORS} | |
bcaa7b3e L |
3604 | command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the |
3605 | @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and | |
3606 | @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and | |
252b5132 RH |
3607 | @samp{*(.dtors)}. |
3608 | ||
3609 | Normally the compiler and linker will handle these issues automatically, | |
3610 | and you will not need to concern yourself with them. However, you may | |
3611 | need to consider this if you are using C++ and writing your own linker | |
3612 | scripts. | |
3613 | ||
3614 | @end table | |
3615 | ||
3616 | @node Output Section Discarding | |
36f63dca | 3617 | @subsection Output Section Discarding |
252b5132 RH |
3618 | @cindex discarding sections |
3619 | @cindex sections, discarding | |
3620 | @cindex removing sections | |
3621 | The linker will not create output section which do not have any | |
3622 | contents. This is for convenience when referring to input sections that | |
3623 | may or may not be present in any of the input files. For example: | |
3624 | @smallexample | |
3625 | .foo @{ *(.foo) @} | |
3626 | @end smallexample | |
3627 | @noindent | |
3628 | will only create a @samp{.foo} section in the output file if there is a | |
3629 | @samp{.foo} section in at least one input file. | |
3630 | ||
3631 | If you use anything other than an input section description as an output | |
3632 | section command, such as a symbol assignment, then the output section | |
3633 | will always be created, even if there are no matching input sections. | |
3634 | ||
3635 | @cindex /DISCARD/ | |
3636 | The special output section name @samp{/DISCARD/} may be used to discard | |
3637 | input sections. Any input sections which are assigned to an output | |
3638 | section named @samp{/DISCARD/} are not included in the output file. | |
3639 | ||
3640 | @node Output Section Attributes | |
36f63dca | 3641 | @subsection Output Section Attributes |
252b5132 RH |
3642 | @cindex output section attributes |
3643 | We showed above that the full description of an output section looked | |
3644 | like this: | |
3645 | @smallexample | |
a1ab1d2a | 3646 | @group |
7e7d5768 | 3647 | @var{section} [@var{address}] [(@var{type})] : |
bbf115d3 | 3648 | [AT(@var{lma})] [ALIGN(@var{section_align})] [SUBALIGN(@var{subsection_align})] |
252b5132 RH |
3649 | @{ |
3650 | @var{output-section-command} | |
3651 | @var{output-section-command} | |
3652 | @dots{} | |
562d3460 | 3653 | @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] |
252b5132 RH |
3654 | @end group |
3655 | @end smallexample | |
3656 | We've already described @var{section}, @var{address}, and | |
3657 | @var{output-section-command}. In this section we will describe the | |
3658 | remaining section attributes. | |
3659 | ||
a1ab1d2a | 3660 | @menu |
252b5132 RH |
3661 | * Output Section Type:: Output section type |
3662 | * Output Section LMA:: Output section LMA | |
bbf115d3 | 3663 | * Forced Output Alignment:: Forced Output Alignment |
7e7d5768 | 3664 | * Forced Input Alignment:: Forced Input Alignment |
252b5132 RH |
3665 | * Output Section Region:: Output section region |
3666 | * Output Section Phdr:: Output section phdr | |
3667 | * Output Section Fill:: Output section fill | |
3668 | @end menu | |
3669 | ||
3670 | @node Output Section Type | |
36f63dca | 3671 | @subsubsection Output Section Type |
252b5132 RH |
3672 | Each output section may have a type. The type is a keyword in |
3673 | parentheses. The following types are defined: | |
3674 | ||
3675 | @table @code | |
3676 | @item NOLOAD | |
3677 | The section should be marked as not loadable, so that it will not be | |
3678 | loaded into memory when the program is run. | |
3679 | @item DSECT | |
3680 | @itemx COPY | |
3681 | @itemx INFO | |
3682 | @itemx OVERLAY | |
3683 | These type names are supported for backward compatibility, and are | |
3684 | rarely used. They all have the same effect: the section should be | |
3685 | marked as not allocatable, so that no memory is allocated for the | |
3686 | section when the program is run. | |
3687 | @end table | |
3688 | ||
3689 | @kindex NOLOAD | |
3690 | @cindex prevent unnecessary loading | |
3691 | @cindex loading, preventing | |
3692 | The linker normally sets the attributes of an output section based on | |
3693 | the input sections which map into it. You can override this by using | |
3694 | the section type. For example, in the script sample below, the | |
3695 | @samp{ROM} section is addressed at memory location @samp{0} and does not | |
3696 | need to be loaded when the program is run. The contents of the | |
3697 | @samp{ROM} section will appear in the linker output file as usual. | |
3698 | @smallexample | |
3699 | @group | |
3700 | SECTIONS @{ | |
3701 | ROM 0 (NOLOAD) : @{ @dots{} @} | |
3702 | @dots{} | |
3703 | @} | |
3704 | @end group | |
3705 | @end smallexample | |
3706 | ||
3707 | @node Output Section LMA | |
36f63dca | 3708 | @subsubsection Output Section LMA |
562d3460 | 3709 | @kindex AT>@var{lma_region} |
252b5132 RH |
3710 | @kindex AT(@var{lma}) |
3711 | @cindex load address | |
3712 | @cindex section load address | |
3713 | Every section has a virtual address (VMA) and a load address (LMA); see | |
3714 | @ref{Basic Script Concepts}. The address expression which may appear in | |
3715 | an output section description sets the VMA (@pxref{Output Section | |
3716 | Address}). | |
3717 | ||
3718 | The linker will normally set the LMA equal to the VMA. You can change | |
3719 | that by using the @code{AT} keyword. The expression @var{lma} that | |
562d3460 | 3720 | follows the @code{AT} keyword specifies the load address of the |
6bdafbeb NC |
3721 | section. |
3722 | ||
3723 | Alternatively, with @samp{AT>@var{lma_region}} expression, you may | |
3724 | specify a memory region for the section's load address. @xref{MEMORY}. | |
3725 | Note that if the section has not had a VMA assigned to it then the | |
3726 | linker will use the @var{lma_region} as the VMA region as well. | |
3727 | @xref{Output Section Region}. | |
252b5132 RH |
3728 | |
3729 | @cindex ROM initialized data | |
3730 | @cindex initialized data in ROM | |
3731 | This feature is designed to make it easy to build a ROM image. For | |
3732 | example, the following linker script creates three output sections: one | |
3733 | called @samp{.text}, which starts at @code{0x1000}, one called | |
3734 | @samp{.mdata}, which is loaded at the end of the @samp{.text} section | |
3735 | even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold | |
3736 | uninitialized data at address @code{0x3000}. The symbol @code{_data} is | |
3737 | defined with the value @code{0x2000}, which shows that the location | |
3738 | counter holds the VMA value, not the LMA value. | |
3739 | ||
3740 | @smallexample | |
3741 | @group | |
3742 | SECTIONS | |
3743 | @{ | |
3744 | .text 0x1000 : @{ *(.text) _etext = . ; @} | |
a1ab1d2a | 3745 | .mdata 0x2000 : |
252b5132 RH |
3746 | AT ( ADDR (.text) + SIZEOF (.text) ) |
3747 | @{ _data = . ; *(.data); _edata = . ; @} | |
3748 | .bss 0x3000 : | |
3749 | @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@} | |
3750 | @} | |
3751 | @end group | |
3752 | @end smallexample | |
3753 | ||
3754 | The run-time initialization code for use with a program generated with | |
3755 | this linker script would include something like the following, to copy | |
3756 | the initialized data from the ROM image to its runtime address. Notice | |
3757 | how this code takes advantage of the symbols defined by the linker | |
3758 | script. | |
3759 | ||
3760 | @smallexample | |
3761 | @group | |
3762 | extern char _etext, _data, _edata, _bstart, _bend; | |
3763 | char *src = &_etext; | |
3764 | char *dst = &_data; | |
3765 | ||
3766 | /* ROM has data at end of text; copy it. */ | |
3767 | while (dst < &_edata) @{ | |
3768 | *dst++ = *src++; | |
3769 | @} | |
3770 | ||
3771 | /* Zero bss */ | |
3772 | for (dst = &_bstart; dst< &_bend; dst++) | |
3773 | *dst = 0; | |
3774 | @end group | |
3775 | @end smallexample | |
3776 | ||
bbf115d3 L |
3777 | @node Forced Output Alignment |
3778 | @subsubsection Forced Output Alignment | |
3779 | @kindex ALIGN(@var{section_align}) | |
3780 | @cindex forcing output section alignment | |
3781 | @cindex output section alignment | |
7270c5ed | 3782 | You can increase an output section's alignment by using ALIGN. |
bbf115d3 | 3783 | |
7e7d5768 AM |
3784 | @node Forced Input Alignment |
3785 | @subsubsection Forced Input Alignment | |
3786 | @kindex SUBALIGN(@var{subsection_align}) | |
3787 | @cindex forcing input section alignment | |
3788 | @cindex input section alignment | |
3789 | You can force input section alignment within an output section by using | |
3790 | SUBALIGN. The value specified overrides any alignment given by input | |
3791 | sections, whether larger or smaller. | |
3792 | ||
252b5132 | 3793 | @node Output Section Region |
36f63dca | 3794 | @subsubsection Output Section Region |
252b5132 RH |
3795 | @kindex >@var{region} |
3796 | @cindex section, assigning to memory region | |
3797 | @cindex memory regions and sections | |
3798 | You can assign a section to a previously defined region of memory by | |
3799 | using @samp{>@var{region}}. @xref{MEMORY}. | |
3800 | ||
3801 | Here is a simple example: | |
3802 | @smallexample | |
3803 | @group | |
3804 | MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @} | |
3805 | SECTIONS @{ ROM : @{ *(.text) @} >rom @} | |
3806 | @end group | |
3807 | @end smallexample | |
3808 | ||
3809 | @node Output Section Phdr | |
36f63dca | 3810 | @subsubsection Output Section Phdr |
252b5132 RH |
3811 | @kindex :@var{phdr} |
3812 | @cindex section, assigning to program header | |
3813 | @cindex program headers and sections | |
3814 | You can assign a section to a previously defined program segment by | |
3815 | using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to | |
3816 | one or more segments, then all subsequent allocated sections will be | |
3817 | assigned to those segments as well, unless they use an explicitly | |
3818 | @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the | |
3819 | linker to not put the section in any segment at all. | |
3820 | ||
3821 | Here is a simple example: | |
3822 | @smallexample | |
3823 | @group | |
3824 | PHDRS @{ text PT_LOAD ; @} | |
3825 | SECTIONS @{ .text : @{ *(.text) @} :text @} | |
3826 | @end group | |
3827 | @end smallexample | |
3828 | ||
3829 | @node Output Section Fill | |
36f63dca | 3830 | @subsubsection Output Section Fill |
252b5132 RH |
3831 | @kindex =@var{fillexp} |
3832 | @cindex section fill pattern | |
3833 | @cindex fill pattern, entire section | |
3834 | You can set the fill pattern for an entire section by using | |
3835 | @samp{=@var{fillexp}}. @var{fillexp} is an expression | |
3836 | (@pxref{Expressions}). Any otherwise unspecified regions of memory | |
3837 | within the output section (for example, gaps left due to the required | |
a139d329 AM |
3838 | alignment of input sections) will be filled with the value, repeated as |
3839 | necessary. If the fill expression is a simple hex number, ie. a string | |
9673c93c | 3840 | of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then |
a139d329 AM |
3841 | an arbitrarily long sequence of hex digits can be used to specify the |
3842 | fill pattern; Leading zeros become part of the pattern too. For all | |
9673c93c | 3843 | other cases, including extra parentheses or a unary @code{+}, the fill |
a139d329 AM |
3844 | pattern is the four least significant bytes of the value of the |
3845 | expression. In all cases, the number is big-endian. | |
252b5132 RH |
3846 | |
3847 | You can also change the fill value with a @code{FILL} command in the | |
9673c93c | 3848 | output section commands; (@pxref{Output Section Data}). |
252b5132 RH |
3849 | |
3850 | Here is a simple example: | |
3851 | @smallexample | |
3852 | @group | |
563e308f | 3853 | SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @} |
252b5132 RH |
3854 | @end group |
3855 | @end smallexample | |
3856 | ||
3857 | @node Overlay Description | |
36f63dca | 3858 | @subsection Overlay Description |
252b5132 RH |
3859 | @kindex OVERLAY |
3860 | @cindex overlays | |
3861 | An overlay description provides an easy way to describe sections which | |
3862 | are to be loaded as part of a single memory image but are to be run at | |
3863 | the same memory address. At run time, some sort of overlay manager will | |
3864 | copy the overlaid sections in and out of the runtime memory address as | |
3865 | required, perhaps by simply manipulating addressing bits. This approach | |
3866 | can be useful, for example, when a certain region of memory is faster | |
3867 | than another. | |
3868 | ||
3869 | Overlays are described using the @code{OVERLAY} command. The | |
3870 | @code{OVERLAY} command is used within a @code{SECTIONS} command, like an | |
3871 | output section description. The full syntax of the @code{OVERLAY} | |
3872 | command is as follows: | |
3873 | @smallexample | |
3874 | @group | |
3875 | OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )] | |
3876 | @{ | |
3877 | @var{secname1} | |
3878 | @{ | |
3879 | @var{output-section-command} | |
3880 | @var{output-section-command} | |
3881 | @dots{} | |
3882 | @} [:@var{phdr}@dots{}] [=@var{fill}] | |
3883 | @var{secname2} | |
3884 | @{ | |
3885 | @var{output-section-command} | |
3886 | @var{output-section-command} | |
3887 | @dots{} | |
3888 | @} [:@var{phdr}@dots{}] [=@var{fill}] | |
3889 | @dots{} | |
3890 | @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] | |
3891 | @end group | |
3892 | @end smallexample | |
3893 | ||
3894 | Everything is optional except @code{OVERLAY} (a keyword), and each | |
3895 | section must have a name (@var{secname1} and @var{secname2} above). The | |
3896 | section definitions within the @code{OVERLAY} construct are identical to | |
3897 | those within the general @code{SECTIONS} contruct (@pxref{SECTIONS}), | |
3898 | except that no addresses and no memory regions may be defined for | |
3899 | sections within an @code{OVERLAY}. | |
3900 | ||
3901 | The sections are all defined with the same starting address. The load | |
3902 | addresses of the sections are arranged such that they are consecutive in | |
3903 | memory starting at the load address used for the @code{OVERLAY} as a | |
3904 | whole (as with normal section definitions, the load address is optional, | |
3905 | and defaults to the start address; the start address is also optional, | |
3906 | and defaults to the current value of the location counter). | |
3907 | ||
3908 | If the @code{NOCROSSREFS} keyword is used, and there any references | |
3909 | among the sections, the linker will report an error. Since the sections | |
3910 | all run at the same address, it normally does not make sense for one | |
3911 | section to refer directly to another. @xref{Miscellaneous Commands, | |
3912 | NOCROSSREFS}. | |
3913 | ||
3914 | For each section within the @code{OVERLAY}, the linker automatically | |
3915 | defines two symbols. The symbol @code{__load_start_@var{secname}} is | |
3916 | defined as the starting load address of the section. The symbol | |
3917 | @code{__load_stop_@var{secname}} is defined as the final load address of | |
3918 | the section. Any characters within @var{secname} which are not legal | |
3919 | within C identifiers are removed. C (or assembler) code may use these | |
3920 | symbols to move the overlaid sections around as necessary. | |
3921 | ||
3922 | At the end of the overlay, the value of the location counter is set to | |
3923 | the start address of the overlay plus the size of the largest section. | |
3924 | ||
3925 | Here is an example. Remember that this would appear inside a | |
3926 | @code{SECTIONS} construct. | |
3927 | @smallexample | |
3928 | @group | |
3929 | OVERLAY 0x1000 : AT (0x4000) | |
3930 | @{ | |
3931 | .text0 @{ o1/*.o(.text) @} | |
3932 | .text1 @{ o2/*.o(.text) @} | |
3933 | @} | |
3934 | @end group | |
3935 | @end smallexample | |
3936 | @noindent | |
3937 | This will define both @samp{.text0} and @samp{.text1} to start at | |
3938 | address 0x1000. @samp{.text0} will be loaded at address 0x4000, and | |
3939 | @samp{.text1} will be loaded immediately after @samp{.text0}. The | |
3940 | following symbols will be defined: @code{__load_start_text0}, | |
3941 | @code{__load_stop_text0}, @code{__load_start_text1}, | |
3942 | @code{__load_stop_text1}. | |
3943 | ||
3944 | C code to copy overlay @code{.text1} into the overlay area might look | |
3945 | like the following. | |
3946 | ||
3947 | @smallexample | |
3948 | @group | |
3949 | extern char __load_start_text1, __load_stop_text1; | |
3950 | memcpy ((char *) 0x1000, &__load_start_text1, | |
3951 | &__load_stop_text1 - &__load_start_text1); | |
3952 | @end group | |
3953 | @end smallexample | |
3954 | ||
3955 | Note that the @code{OVERLAY} command is just syntactic sugar, since | |
3956 | everything it does can be done using the more basic commands. The above | |
3957 | example could have been written identically as follows. | |
3958 | ||
3959 | @smallexample | |
3960 | @group | |
3961 | .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @} | |
3962 | __load_start_text0 = LOADADDR (.text0); | |
3963 | __load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0); | |
3964 | .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @} | |
3965 | __load_start_text1 = LOADADDR (.text1); | |
3966 | __load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1); | |
3967 | . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1)); | |
3968 | @end group | |
3969 | @end smallexample | |
3970 | ||
3971 | @node MEMORY | |
36f63dca | 3972 | @section MEMORY Command |
252b5132 RH |
3973 | @kindex MEMORY |
3974 | @cindex memory regions | |
3975 | @cindex regions of memory | |
3976 | @cindex allocating memory | |
3977 | @cindex discontinuous memory | |
3978 | The linker's default configuration permits allocation of all available | |
3979 | memory. You can override this by using the @code{MEMORY} command. | |
3980 | ||
3981 | The @code{MEMORY} command describes the location and size of blocks of | |
3982 | memory in the target. You can use it to describe which memory regions | |
3983 | may be used by the linker, and which memory regions it must avoid. You | |
3984 | can then assign sections to particular memory regions. The linker will | |
3985 | set section addresses based on the memory regions, and will warn about | |
3986 | regions that become too full. The linker will not shuffle sections | |
3987 | around to fit into the available regions. | |
3988 | ||
3989 | A linker script may contain at most one use of the @code{MEMORY} | |
3990 | command. However, you can define as many blocks of memory within it as | |
3991 | you wish. The syntax is: | |
3992 | @smallexample | |
3993 | @group | |
a1ab1d2a | 3994 | MEMORY |
252b5132 RH |
3995 | @{ |
3996 | @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len} | |
3997 | @dots{} | |
3998 | @} | |
3999 | @end group | |
4000 | @end smallexample | |
4001 | ||
4002 | The @var{name} is a name used in the linker script to refer to the | |
4003 | region. The region name has no meaning outside of the linker script. | |
4004 | Region names are stored in a separate name space, and will not conflict | |
4005 | with symbol names, file names, or section names. Each memory region | |
4006 | must have a distinct name. | |
4007 | ||
4008 | @cindex memory region attributes | |
4009 | The @var{attr} string is an optional list of attributes that specify | |
4010 | whether to use a particular memory region for an input section which is | |
4011 | not explicitly mapped in the linker script. As described in | |
4012 | @ref{SECTIONS}, if you do not specify an output section for some input | |
4013 | section, the linker will create an output section with the same name as | |
4014 | the input section. If you define region attributes, the linker will use | |
4015 | them to select the memory region for the output section that it creates. | |
4016 | ||
4017 | The @var{attr} string must consist only of the following characters: | |
4018 | @table @samp | |
4019 | @item R | |
4020 | Read-only section | |
4021 | @item W | |
4022 | Read/write section | |
4023 | @item X | |
4024 | Executable section | |
4025 | @item A | |
4026 | Allocatable section | |
4027 | @item I | |
4028 | Initialized section | |
4029 | @item L | |
4030 | Same as @samp{I} | |
4031 | @item ! | |
4032 | Invert the sense of any of the preceding attributes | |
4033 | @end table | |
4034 | ||
4035 | If a unmapped section matches any of the listed attributes other than | |
4036 | @samp{!}, it will be placed in the memory region. The @samp{!} | |
4037 | attribute reverses this test, so that an unmapped section will be placed | |
4038 | in the memory region only if it does not match any of the listed | |
4039 | attributes. | |
4040 | ||
4041 | @kindex ORIGIN = | |
4042 | @kindex o = | |
4043 | @kindex org = | |
9cd6d51a NC |
4044 | The @var{origin} is an numerical expression for the start address of |
4045 | the memory region. The expression must evaluate to a constant and it | |
4046 | cannot involve any symbols. The keyword @code{ORIGIN} may be | |
4047 | abbreviated to @code{org} or @code{o} (but not, for example, | |
4048 | @code{ORG}). | |
252b5132 RH |
4049 | |
4050 | @kindex LENGTH = | |
4051 | @kindex len = | |
4052 | @kindex l = | |
4053 | The @var{len} is an expression for the size in bytes of the memory | |
4054 | region. As with the @var{origin} expression, the expression must | |
9cd6d51a NC |
4055 | be numerical only and must evaluate to a constant. The keyword |
4056 | @code{LENGTH} may be abbreviated to @code{len} or @code{l}. | |
252b5132 RH |
4057 | |
4058 | In the following example, we specify that there are two memory regions | |
4059 | available for allocation: one starting at @samp{0} for 256 kilobytes, | |
4060 | and the other starting at @samp{0x40000000} for four megabytes. The | |
4061 | linker will place into the @samp{rom} memory region every section which | |
4062 | is not explicitly mapped into a memory region, and is either read-only | |
4063 | or executable. The linker will place other sections which are not | |
4064 | explicitly mapped into a memory region into the @samp{ram} memory | |
4065 | region. | |
4066 | ||
4067 | @smallexample | |
4068 | @group | |
a1ab1d2a | 4069 | MEMORY |
252b5132 RH |
4070 | @{ |
4071 | rom (rx) : ORIGIN = 0, LENGTH = 256K | |
4072 | ram (!rx) : org = 0x40000000, l = 4M | |
4073 | @} | |
4074 | @end group | |
4075 | @end smallexample | |
4076 | ||
4077 | Once you define a memory region, you can direct the linker to place | |
4078 | specific output sections into that memory region by using the | |
4079 | @samp{>@var{region}} output section attribute. For example, if you have | |
4080 | a memory region named @samp{mem}, you would use @samp{>mem} in the | |
4081 | output section definition. @xref{Output Section Region}. If no address | |
4082 | was specified for the output section, the linker will set the address to | |
4083 | the next available address within the memory region. If the combined | |
4084 | output sections directed to a memory region are too large for the | |
4085 | region, the linker will issue an error message. | |
4086 | ||
3ec57632 NC |
4087 | It is possible to access the origin and length of a memory in an |
4088 | expression via the @code{ORIGIN(@var{memory})} and | |
4089 | @code{LENGTH(@var{memory})} functions: | |
4090 | ||
4091 | @smallexample | |
4092 | @group | |
4093 | _fstack = ORIGIN(ram) + LENGTH(ram) - 4; | |
4094 | @end group | |
4095 | @end smallexample | |
4096 | ||
252b5132 RH |
4097 | @node PHDRS |
4098 | @section PHDRS Command | |
4099 | @kindex PHDRS | |
4100 | @cindex program headers | |
4101 | @cindex ELF program headers | |
4102 | @cindex program segments | |
4103 | @cindex segments, ELF | |
4104 | The ELF object file format uses @dfn{program headers}, also knows as | |
4105 | @dfn{segments}. The program headers describe how the program should be | |
4106 | loaded into memory. You can print them out by using the @code{objdump} | |
4107 | program with the @samp{-p} option. | |
4108 | ||
4109 | When you run an ELF program on a native ELF system, the system loader | |
4110 | reads the program headers in order to figure out how to load the | |
4111 | program. This will only work if the program headers are set correctly. | |
4112 | This manual does not describe the details of how the system loader | |
4113 | interprets program headers; for more information, see the ELF ABI. | |
4114 | ||
4115 | The linker will create reasonable program headers by default. However, | |
4116 | in some cases, you may need to specify the program headers more | |
4117 | precisely. You may use the @code{PHDRS} command for this purpose. When | |
4118 | the linker sees the @code{PHDRS} command in the linker script, it will | |
4119 | not create any program headers other than the ones specified. | |
4120 | ||
4121 | The linker only pays attention to the @code{PHDRS} command when | |
4122 | generating an ELF output file. In other cases, the linker will simply | |
4123 | ignore @code{PHDRS}. | |
4124 | ||
4125 | This is the syntax of the @code{PHDRS} command. The words @code{PHDRS}, | |
4126 | @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords. | |
4127 | ||
4128 | @smallexample | |
4129 | @group | |
4130 | PHDRS | |
4131 | @{ | |
4132 | @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ] | |
4133 | [ FLAGS ( @var{flags} ) ] ; | |
4134 | @} | |
4135 | @end group | |
4136 | @end smallexample | |
4137 | ||
4138 | The @var{name} is used only for reference in the @code{SECTIONS} command | |
4139 | of the linker script. It is not put into the output file. Program | |
4140 | header names are stored in a separate name space, and will not conflict | |
4141 | with symbol names, file names, or section names. Each program header | |
4142 | must have a distinct name. | |
4143 | ||
4144 | Certain program header types describe segments of memory which the | |
4145 | system loader will load from the file. In the linker script, you | |
4146 | specify the contents of these segments by placing allocatable output | |
4147 | sections in the segments. You use the @samp{:@var{phdr}} output section | |
4148 | attribute to place a section in a particular segment. @xref{Output | |
4149 | Section Phdr}. | |
4150 | ||
4151 | It is normal to put certain sections in more than one segment. This | |
4152 | merely implies that one segment of memory contains another. You may | |
4153 | repeat @samp{:@var{phdr}}, using it once for each segment which should | |
4154 | contain the section. | |
4155 | ||
4156 | If you place a section in one or more segments using @samp{:@var{phdr}}, | |
4157 | then the linker will place all subsequent allocatable sections which do | |
4158 | not specify @samp{:@var{phdr}} in the same segments. This is for | |
4159 | convenience, since generally a whole set of contiguous sections will be | |
4160 | placed in a single segment. You can use @code{:NONE} to override the | |
4161 | default segment and tell the linker to not put the section in any | |
4162 | segment at all. | |
4163 | ||
4164 | @kindex FILEHDR | |
4165 | @kindex PHDRS | |
4166 | You may use the @code{FILEHDR} and @code{PHDRS} keywords appear after | |
4167 | the program header type to further describe the contents of the segment. | |
4168 | The @code{FILEHDR} keyword means that the segment should include the ELF | |
4169 | file header. The @code{PHDRS} keyword means that the segment should | |
4170 | include the ELF program headers themselves. | |
4171 | ||
4172 | The @var{type} may be one of the following. The numbers indicate the | |
4173 | value of the keyword. | |
4174 | ||
4175 | @table @asis | |
4176 | @item @code{PT_NULL} (0) | |
4177 | Indicates an unused program header. | |
4178 | ||
4179 | @item @code{PT_LOAD} (1) | |
4180 | Indicates that this program header describes a segment to be loaded from | |
4181 | the file. | |
4182 | ||
4183 | @item @code{PT_DYNAMIC} (2) | |
4184 | Indicates a segment where dynamic linking information can be found. | |
4185 | ||
4186 | @item @code{PT_INTERP} (3) | |
4187 | Indicates a segment where the name of the program interpreter may be | |
4188 | found. | |
4189 | ||
4190 | @item @code{PT_NOTE} (4) | |
4191 | Indicates a segment holding note information. | |
4192 | ||
4193 | @item @code{PT_SHLIB} (5) | |
4194 | A reserved program header type, defined but not specified by the ELF | |
4195 | ABI. | |
4196 | ||
4197 | @item @code{PT_PHDR} (6) | |
4198 | Indicates a segment where the program headers may be found. | |
4199 | ||
4200 | @item @var{expression} | |
4201 | An expression giving the numeric type of the program header. This may | |
4202 | be used for types not defined above. | |
4203 | @end table | |
4204 | ||
4205 | You can specify that a segment should be loaded at a particular address | |
4206 | in memory by using an @code{AT} expression. This is identical to the | |
4207 | @code{AT} command used as an output section attribute (@pxref{Output | |
4208 | Section LMA}). The @code{AT} command for a program header overrides the | |
4209 | output section attribute. | |
4210 | ||
4211 | The linker will normally set the segment flags based on the sections | |
4212 | which comprise the segment. You may use the @code{FLAGS} keyword to | |
4213 | explicitly specify the segment flags. The value of @var{flags} must be | |
4214 | an integer. It is used to set the @code{p_flags} field of the program | |
4215 | header. | |
4216 | ||
4217 | Here is an example of @code{PHDRS}. This shows a typical set of program | |
4218 | headers used on a native ELF system. | |
4219 | ||
4220 | @example | |
4221 | @group | |
4222 | PHDRS | |
4223 | @{ | |
4224 | headers PT_PHDR PHDRS ; | |
4225 | interp PT_INTERP ; | |
4226 | text PT_LOAD FILEHDR PHDRS ; | |
4227 | data PT_LOAD ; | |
4228 | dynamic PT_DYNAMIC ; | |
4229 | @} | |
4230 | ||
4231 | SECTIONS | |
4232 | @{ | |
4233 | . = SIZEOF_HEADERS; | |
4234 | .interp : @{ *(.interp) @} :text :interp | |
4235 | .text : @{ *(.text) @} :text | |
4236 | .rodata : @{ *(.rodata) @} /* defaults to :text */ | |
4237 | @dots{} | |
4238 | . = . + 0x1000; /* move to a new page in memory */ | |
4239 | .data : @{ *(.data) @} :data | |
4240 | .dynamic : @{ *(.dynamic) @} :data :dynamic | |
4241 | @dots{} | |
4242 | @} | |
4243 | @end group | |
4244 | @end example | |
4245 | ||
4246 | @node VERSION | |
4247 | @section VERSION Command | |
4248 | @kindex VERSION @{script text@} | |
4249 | @cindex symbol versions | |
4250 | @cindex version script | |
4251 | @cindex versions of symbols | |
4252 | The linker supports symbol versions when using ELF. Symbol versions are | |
4253 | only useful when using shared libraries. The dynamic linker can use | |
4254 | symbol versions to select a specific version of a function when it runs | |
4255 | a program that may have been linked against an earlier version of the | |
4256 | shared library. | |
4257 | ||
4258 | You can include a version script directly in the main linker script, or | |
4259 | you can supply the version script as an implicit linker script. You can | |
4260 | also use the @samp{--version-script} linker option. | |
4261 | ||
4262 | The syntax of the @code{VERSION} command is simply | |
4263 | @smallexample | |
4264 | VERSION @{ version-script-commands @} | |
4265 | @end smallexample | |
4266 | ||
4267 | The format of the version script commands is identical to that used by | |
4268 | Sun's linker in Solaris 2.5. The version script defines a tree of | |
4269 | version nodes. You specify the node names and interdependencies in the | |
4270 | version script. You can specify which symbols are bound to which | |
4271 | version nodes, and you can reduce a specified set of symbols to local | |
4272 | scope so that they are not globally visible outside of the shared | |
4273 | library. | |
4274 | ||
4275 | The easiest way to demonstrate the version script language is with a few | |
4276 | examples. | |
4277 | ||
4278 | @smallexample | |
4279 | VERS_1.1 @{ | |
4280 | global: | |
4281 | foo1; | |
4282 | local: | |
a1ab1d2a UD |
4283 | old*; |
4284 | original*; | |
4285 | new*; | |
252b5132 RH |
4286 | @}; |
4287 | ||
4288 | VERS_1.2 @{ | |
4289 | foo2; | |
4290 | @} VERS_1.1; | |
4291 | ||
4292 | VERS_2.0 @{ | |
4293 | bar1; bar2; | |
86043bbb MM |
4294 | extern "C++" @{ |
4295 | ns::*; | |
4296 | "int f(int, double)"; | |
4297 | @} | |
252b5132 RH |
4298 | @} VERS_1.2; |
4299 | @end smallexample | |
4300 | ||
4301 | This example version script defines three version nodes. The first | |
4302 | version node defined is @samp{VERS_1.1}; it has no other dependencies. | |
4303 | The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces | |
4304 | a number of symbols to local scope so that they are not visible outside | |
313e35ee AM |
4305 | of the shared library; this is done using wildcard patterns, so that any |
4306 | symbol whose name begins with @samp{old}, @samp{original}, or @samp{new} | |
4307 | is matched. The wildcard patterns available are the same as those used | |
4308 | in the shell when matching filenames (also known as ``globbing''). | |
86043bbb MM |
4309 | However, if you specify the symbol name inside double quotes, then the |
4310 | name is treated as literal, rather than as a glob pattern. | |
252b5132 RH |
4311 | |
4312 | Next, the version script defines node @samp{VERS_1.2}. This node | |
4313 | depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2} | |
4314 | to the version node @samp{VERS_1.2}. | |
4315 | ||
4316 | Finally, the version script defines node @samp{VERS_2.0}. This node | |
4317 | depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1} | |
4318 | and @samp{bar2} are bound to the version node @samp{VERS_2.0}. | |
4319 | ||
4320 | When the linker finds a symbol defined in a library which is not | |
4321 | specifically bound to a version node, it will effectively bind it to an | |
4322 | unspecified base version of the library. You can bind all otherwise | |
a981ed6f | 4323 | unspecified symbols to a given version node by using @samp{global: *;} |
252b5132 RH |
4324 | somewhere in the version script. |
4325 | ||
4326 | The names of the version nodes have no specific meaning other than what | |
4327 | they might suggest to the person reading them. The @samp{2.0} version | |
4328 | could just as well have appeared in between @samp{1.1} and @samp{1.2}. | |
4329 | However, this would be a confusing way to write a version script. | |
4330 | ||
6b9b879a JJ |
4331 | Node name can be omited, provided it is the only version node |
4332 | in the version script. Such version script doesn't assign any versions to | |
4333 | symbols, only selects which symbols will be globally visible out and which | |
4334 | won't. | |
4335 | ||
4336 | @smallexample | |
7c9c73be | 4337 | @{ global: foo; bar; local: *; @}; |
9d201f2f | 4338 | @end smallexample |
6b9b879a | 4339 | |
252b5132 RH |
4340 | When you link an application against a shared library that has versioned |
4341 | symbols, the application itself knows which version of each symbol it | |
4342 | requires, and it also knows which version nodes it needs from each | |
4343 | shared library it is linked against. Thus at runtime, the dynamic | |
4344 | loader can make a quick check to make sure that the libraries you have | |
4345 | linked against do in fact supply all of the version nodes that the | |
4346 | application will need to resolve all of the dynamic symbols. In this | |
4347 | way it is possible for the dynamic linker to know with certainty that | |
4348 | all external symbols that it needs will be resolvable without having to | |
4349 | search for each symbol reference. | |
4350 | ||
4351 | The symbol versioning is in effect a much more sophisticated way of | |
4352 | doing minor version checking that SunOS does. The fundamental problem | |
4353 | that is being addressed here is that typically references to external | |
4354 | functions are bound on an as-needed basis, and are not all bound when | |
4355 | the application starts up. If a shared library is out of date, a | |
4356 | required interface may be missing; when the application tries to use | |
4357 | that interface, it may suddenly and unexpectedly fail. With symbol | |
4358 | versioning, the user will get a warning when they start their program if | |
4359 | the libraries being used with the application are too old. | |
4360 | ||
4361 | There are several GNU extensions to Sun's versioning approach. The | |
4362 | first of these is the ability to bind a symbol to a version node in the | |
4363 | source file where the symbol is defined instead of in the versioning | |
4364 | script. This was done mainly to reduce the burden on the library | |
4365 | maintainer. You can do this by putting something like: | |
4366 | @smallexample | |
4367 | __asm__(".symver original_foo,foo@@VERS_1.1"); | |
4368 | @end smallexample | |
4369 | @noindent | |
4370 | in the C source file. This renames the function @samp{original_foo} to | |
4371 | be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}. | |
4372 | The @samp{local:} directive can be used to prevent the symbol | |
96a94295 L |
4373 | @samp{original_foo} from being exported. A @samp{.symver} directive |
4374 | takes precedence over a version script. | |
252b5132 RH |
4375 | |
4376 | The second GNU extension is to allow multiple versions of the same | |
4377 | function to appear in a given shared library. In this way you can make | |
4378 | an incompatible change to an interface without increasing the major | |
4379 | version number of the shared library, while still allowing applications | |
4380 | linked against the old interface to continue to function. | |
4381 | ||
4382 | To do this, you must use multiple @samp{.symver} directives in the | |
4383 | source file. Here is an example: | |
4384 | ||
4385 | @smallexample | |
4386 | __asm__(".symver original_foo,foo@@"); | |
4387 | __asm__(".symver old_foo,foo@@VERS_1.1"); | |
4388 | __asm__(".symver old_foo1,foo@@VERS_1.2"); | |
4389 | __asm__(".symver new_foo,foo@@@@VERS_2.0"); | |
4390 | @end smallexample | |
4391 | ||
4392 | In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the | |
4393 | unspecified base version of the symbol. The source file that contains this | |
4394 | example would define 4 C functions: @samp{original_foo}, @samp{old_foo}, | |
4395 | @samp{old_foo1}, and @samp{new_foo}. | |
4396 | ||
4397 | When you have multiple definitions of a given symbol, there needs to be | |
4398 | some way to specify a default version to which external references to | |
4399 | this symbol will be bound. You can do this with the | |
4400 | @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only | |
4401 | declare one version of a symbol as the default in this manner; otherwise | |
4402 | you would effectively have multiple definitions of the same symbol. | |
4403 | ||
4404 | If you wish to bind a reference to a specific version of the symbol | |
4405 | within the shared library, you can use the aliases of convenience | |
36f63dca | 4406 | (i.e., @samp{old_foo}), or you can use the @samp{.symver} directive to |
252b5132 RH |
4407 | specifically bind to an external version of the function in question. |
4408 | ||
cb840a31 L |
4409 | You can also specify the language in the version script: |
4410 | ||
4411 | @smallexample | |
4412 | VERSION extern "lang" @{ version-script-commands @} | |
4413 | @end smallexample | |
4414 | ||
4415 | The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}. | |
4416 | The linker will iterate over the list of symbols at the link time and | |
4417 | demangle them according to @samp{lang} before matching them to the | |
4418 | patterns specified in @samp{version-script-commands}. | |
4419 | ||
86043bbb MM |
4420 | Demangled names may contains spaces and other special characters. As |
4421 | described above, you can use a glob pattern to match demangled names, | |
4422 | or you can use a double-quoted string to match the string exactly. In | |
4423 | the latter case, be aware that minor differences (such as differing | |
4424 | whitespace) between the version script and the demangler output will | |
4425 | cause a mismatch. As the exact string generated by the demangler | |
4426 | might change in the future, even if the mangled name does not, you | |
4427 | should check that all of your version directives are behaving as you | |
4428 | expect when you upgrade. | |
4429 | ||
252b5132 RH |
4430 | @node Expressions |
4431 | @section Expressions in Linker Scripts | |
4432 | @cindex expressions | |
4433 | @cindex arithmetic | |
4434 | The syntax for expressions in the linker script language is identical to | |
4435 | that of C expressions. All expressions are evaluated as integers. All | |
4436 | expressions are evaluated in the same size, which is 32 bits if both the | |
4437 | host and target are 32 bits, and is otherwise 64 bits. | |
4438 | ||
4439 | You can use and set symbol values in expressions. | |
4440 | ||
4441 | The linker defines several special purpose builtin functions for use in | |
4442 | expressions. | |
4443 | ||
4444 | @menu | |
4445 | * Constants:: Constants | |
4446 | * Symbols:: Symbol Names | |
ecca9871 | 4447 | * Orphan Sections:: Orphan Sections |
252b5132 RH |
4448 | * Location Counter:: The Location Counter |
4449 | * Operators:: Operators | |
4450 | * Evaluation:: Evaluation | |
4451 | * Expression Section:: The Section of an Expression | |
4452 | * Builtin Functions:: Builtin Functions | |
4453 | @end menu | |
4454 | ||
4455 | @node Constants | |
4456 | @subsection Constants | |
4457 | @cindex integer notation | |
4458 | @cindex constants in linker scripts | |
4459 | All constants are integers. | |
4460 | ||
4461 | As in C, the linker considers an integer beginning with @samp{0} to be | |
4462 | octal, and an integer beginning with @samp{0x} or @samp{0X} to be | |
4463 | hexadecimal. The linker considers other integers to be decimal. | |
4464 | ||
4465 | @cindex scaled integers | |
4466 | @cindex K and M integer suffixes | |
4467 | @cindex M and K integer suffixes | |
4468 | @cindex suffixes for integers | |
4469 | @cindex integer suffixes | |
4470 | In addition, you can use the suffixes @code{K} and @code{M} to scale a | |
4471 | constant by | |
4472 | @c TEXI2ROFF-KILL | |
36f63dca | 4473 | @ifnottex |
252b5132 RH |
4474 | @c END TEXI2ROFF-KILL |
4475 | @code{1024} or @code{1024*1024} | |
4476 | @c TEXI2ROFF-KILL | |
36f63dca | 4477 | @end ifnottex |
252b5132 RH |
4478 | @tex |
4479 | ${\rm 1024}$ or ${\rm 1024}^2$ | |
4480 | @end tex | |
4481 | @c END TEXI2ROFF-KILL | |
4482 | respectively. For example, the following all refer to the same quantity: | |
4483 | @smallexample | |
36f63dca NC |
4484 | _fourk_1 = 4K; |
4485 | _fourk_2 = 4096; | |
4486 | _fourk_3 = 0x1000; | |
252b5132 RH |
4487 | @end smallexample |
4488 | ||
4489 | @node Symbols | |
4490 | @subsection Symbol Names | |
4491 | @cindex symbol names | |
4492 | @cindex names | |
4493 | @cindex quoted symbol names | |
4494 | @kindex " | |
4495 | Unless quoted, symbol names start with a letter, underscore, or period | |
4496 | and may include letters, digits, underscores, periods, and hyphens. | |
4497 | Unquoted symbol names must not conflict with any keywords. You can | |
4498 | specify a symbol which contains odd characters or has the same name as a | |
4499 | keyword by surrounding the symbol name in double quotes: | |
4500 | @smallexample | |
36f63dca NC |
4501 | "SECTION" = 9; |
4502 | "with a space" = "also with a space" + 10; | |
252b5132 RH |
4503 | @end smallexample |
4504 | ||
4505 | Since symbols can contain many non-alphabetic characters, it is safest | |
4506 | to delimit symbols with spaces. For example, @samp{A-B} is one symbol, | |
4507 | whereas @samp{A - B} is an expression involving subtraction. | |
4508 | ||
ecca9871 L |
4509 | @node Orphan Sections |
4510 | @subsection Orphan Sections | |
4511 | @cindex orphan | |
4512 | Orphan sections are sections present in the input files which | |
4513 | are not explicitly placed into the output file by the linker | |
4514 | script. The linker will still copy these sections into the | |
4515 | output file, but it has to guess as to where they should be | |
4516 | placed. The linker uses a simple heuristic to do this. It | |
4517 | attempts to place orphan sections after non-orphan sections of the | |
4518 | same attribute, such as code vs data, loadable vs non-loadable, etc. | |
4519 | If there is not enough room to do this then it places | |
4520 | at the end of the file. | |
4521 | ||
4522 | For ELF targets, the attribute of the section includes section type as | |
4523 | well as section flag. | |
4524 | ||
252b5132 RH |
4525 | @node Location Counter |
4526 | @subsection The Location Counter | |
4527 | @kindex . | |
4528 | @cindex dot | |
4529 | @cindex location counter | |
4530 | @cindex current output location | |
4531 | The special linker variable @dfn{dot} @samp{.} always contains the | |
4532 | current output location counter. Since the @code{.} always refers to a | |
4533 | location in an output section, it may only appear in an expression | |
4534 | within a @code{SECTIONS} command. The @code{.} symbol may appear | |
4535 | anywhere that an ordinary symbol is allowed in an expression. | |
4536 | ||
4537 | @cindex holes | |
4538 | Assigning a value to @code{.} will cause the location counter to be | |
4539 | moved. This may be used to create holes in the output section. The | |
4540 | location counter may never be moved backwards. | |
4541 | ||
4542 | @smallexample | |
4543 | SECTIONS | |
4544 | @{ | |
4545 | output : | |
4546 | @{ | |
4547 | file1(.text) | |
4548 | . = . + 1000; | |
4549 | file2(.text) | |
4550 | . += 1000; | |
4551 | file3(.text) | |
563e308f | 4552 | @} = 0x12345678; |
252b5132 RH |
4553 | @} |
4554 | @end smallexample | |
4555 | @noindent | |
4556 | In the previous example, the @samp{.text} section from @file{file1} is | |
4557 | located at the beginning of the output section @samp{output}. It is | |
4558 | followed by a 1000 byte gap. Then the @samp{.text} section from | |
4559 | @file{file2} appears, also with a 1000 byte gap following before the | |
563e308f | 4560 | @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678} |
252b5132 RH |
4561 | specifies what data to write in the gaps (@pxref{Output Section Fill}). |
4562 | ||
5c6bbab8 NC |
4563 | @cindex dot inside sections |
4564 | Note: @code{.} actually refers to the byte offset from the start of the | |
4565 | current containing object. Normally this is the @code{SECTIONS} | |
69da35b5 | 4566 | statement, whose start address is 0, hence @code{.} can be used as an |
5c6bbab8 NC |
4567 | absolute address. If @code{.} is used inside a section description |
4568 | however, it refers to the byte offset from the start of that section, | |
4569 | not an absolute address. Thus in a script like this: | |
4570 | ||
4571 | @smallexample | |
4572 | SECTIONS | |
4573 | @{ | |
4574 | . = 0x100 | |
4575 | .text: @{ | |
4576 | *(.text) | |
4577 | . = 0x200 | |
4578 | @} | |
4579 | . = 0x500 | |
4580 | .data: @{ | |
4581 | *(.data) | |
4582 | . += 0x600 | |
4583 | @} | |
4584 | @} | |
4585 | @end smallexample | |
4586 | ||
4587 | The @samp{.text} section will be assigned a starting address of 0x100 | |
4588 | and a size of exactly 0x200 bytes, even if there is not enough data in | |
4589 | the @samp{.text} input sections to fill this area. (If there is too | |
4590 | much data, an error will be produced because this would be an attempt to | |
4591 | move @code{.} backwards). The @samp{.data} section will start at 0x500 | |
4592 | and it will have an extra 0x600 bytes worth of space after the end of | |
4593 | the values from the @samp{.data} input sections and before the end of | |
4594 | the @samp{.data} output section itself. | |
4595 | ||
b5666f2f AM |
4596 | @cindex dot outside sections |
4597 | Setting symbols to the value of the location counter outside of an | |
4598 | output section statement can result in unexpected values if the linker | |
4599 | needs to place orphan sections. For example, given the following: | |
4600 | ||
4601 | @smallexample | |
4602 | SECTIONS | |
4603 | @{ | |
4604 | start_of_text = . ; | |
4605 | .text: @{ *(.text) @} | |
4606 | end_of_text = . ; | |
4607 | ||
4608 | start_of_data = . ; | |
4609 | .data: @{ *(.data) @} | |
4610 | end_of_data = . ; | |
4611 | @} | |
4612 | @end smallexample | |
4613 | ||
4614 | If the linker needs to place some input section, e.g. @code{.rodata}, | |
4615 | not mentioned in the script, it might choose to place that section | |
4616 | between @code{.text} and @code{.data}. You might think the linker | |
4617 | should place @code{.rodata} on the blank line in the above script, but | |
4618 | blank lines are of no particular significance to the linker. As well, | |
4619 | the linker doesn't associate the above symbol names with their | |
4620 | sections. Instead, it assumes that all assignments or other | |
4621 | statements belong to the previous output section, except for the | |
4622 | special case of an assignment to @code{.}. I.e., the linker will | |
4623 | place the orphan @code{.rodata} section as if the script was written | |
4624 | as follows: | |
4625 | ||
4626 | @smallexample | |
4627 | SECTIONS | |
4628 | @{ | |
4629 | start_of_text = . ; | |
4630 | .text: @{ *(.text) @} | |
4631 | end_of_text = . ; | |
4632 | ||
4633 | start_of_data = . ; | |
4634 | .rodata: @{ *(.rodata) @} | |
4635 | .data: @{ *(.data) @} | |
4636 | end_of_data = . ; | |
4637 | @} | |
4638 | @end smallexample | |
4639 | ||
4640 | This may or may not be the script author's intention for the value of | |
4641 | @code{start_of_data}. One way to influence the orphan section | |
4642 | placement is to assign the location counter to itself, as the linker | |
4643 | assumes that an assignment to @code{.} is setting the start address of | |
4644 | a following output section and thus should be grouped with that | |
4645 | section. So you could write: | |
4646 | ||
4647 | @smallexample | |
4648 | SECTIONS | |
4649 | @{ | |
4650 | start_of_text = . ; | |
4651 | .text: @{ *(.text) @} | |
4652 | end_of_text = . ; | |
4653 | ||
4654 | . = . ; | |
4655 | start_of_data = . ; | |
4656 | .data: @{ *(.data) @} | |
4657 | end_of_data = . ; | |
4658 | @} | |
4659 | @end smallexample | |
4660 | ||
4661 | Now, the orphan @code{.rodata} section will be placed between | |
4662 | @code{end_of_text} and @code{start_of_data}. | |
4663 | ||
252b5132 RH |
4664 | @need 2000 |
4665 | @node Operators | |
4666 | @subsection Operators | |
4667 | @cindex operators for arithmetic | |
4668 | @cindex arithmetic operators | |
4669 | @cindex precedence in expressions | |
4670 | The linker recognizes the standard C set of arithmetic operators, with | |
4671 | the standard bindings and precedence levels: | |
4672 | @c TEXI2ROFF-KILL | |
36f63dca | 4673 | @ifnottex |
252b5132 RH |
4674 | @c END TEXI2ROFF-KILL |
4675 | @smallexample | |
4676 | precedence associativity Operators Notes | |
4677 | (highest) | |
4678 | 1 left ! - ~ (1) | |
4679 | 2 left * / % | |
4680 | 3 left + - | |
4681 | 4 left >> << | |
4682 | 5 left == != > < <= >= | |
4683 | 6 left & | |
4684 | 7 left | | |
4685 | 8 left && | |
4686 | 9 left || | |
4687 | 10 right ? : | |
4688 | 11 right &= += -= *= /= (2) | |
4689 | (lowest) | |
4690 | @end smallexample | |
4691 | Notes: | |
a1ab1d2a | 4692 | (1) Prefix operators |
252b5132 RH |
4693 | (2) @xref{Assignments}. |
4694 | @c TEXI2ROFF-KILL | |
36f63dca | 4695 | @end ifnottex |
252b5132 RH |
4696 | @tex |
4697 | \vskip \baselineskip | |
4698 | %"lispnarrowing" is the extra indent used generally for smallexample | |
4699 | \hskip\lispnarrowing\vbox{\offinterlineskip | |
4700 | \hrule | |
4701 | \halign | |
4702 | {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr | |
4703 | height2pt&\omit&&\omit&&\omit&\cr | |
4704 | &Precedence&& Associativity &&{\rm Operators}&\cr | |
4705 | height2pt&\omit&&\omit&&\omit&\cr | |
4706 | \noalign{\hrule} | |
4707 | height2pt&\omit&&\omit&&\omit&\cr | |
4708 | &highest&&&&&\cr | |
4709 | % '176 is tilde, '~' in tt font | |
a1ab1d2a | 4710 | &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr |
252b5132 RH |
4711 | &2&&left&&* / \%&\cr |
4712 | &3&&left&&+ -&\cr | |
4713 | &4&&left&&>> <<&\cr | |
4714 | &5&&left&&== != > < <= >=&\cr | |
4715 | &6&&left&&\&&\cr | |
4716 | &7&&left&&|&\cr | |
4717 | &8&&left&&{\&\&}&\cr | |
4718 | &9&&left&&||&\cr | |
4719 | &10&&right&&? :&\cr | |
4720 | &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr | |
4721 | &lowest&&&&&\cr | |
4722 | height2pt&\omit&&\omit&&\omit&\cr} | |
4723 | \hrule} | |
4724 | @end tex | |
4725 | @iftex | |
4726 | { | |
4727 | @obeylines@parskip=0pt@parindent=0pt | |
4728 | @dag@quad Prefix operators. | |
4729 | @ddag@quad @xref{Assignments}. | |
4730 | } | |
4731 | @end iftex | |
4732 | @c END TEXI2ROFF-KILL | |
4733 | ||
4734 | @node Evaluation | |
4735 | @subsection Evaluation | |
4736 | @cindex lazy evaluation | |
4737 | @cindex expression evaluation order | |
4738 | The linker evaluates expressions lazily. It only computes the value of | |
4739 | an expression when absolutely necessary. | |
4740 | ||
4741 | The linker needs some information, such as the value of the start | |
4742 | address of the first section, and the origins and lengths of memory | |
4743 | regions, in order to do any linking at all. These values are computed | |
4744 | as soon as possible when the linker reads in the linker script. | |
4745 | ||
4746 | However, other values (such as symbol values) are not known or needed | |
4747 | until after storage allocation. Such values are evaluated later, when | |
4748 | other information (such as the sizes of output sections) is available | |
4749 | for use in the symbol assignment expression. | |
4750 | ||
4751 | The sizes of sections cannot be known until after allocation, so | |
4752 | assignments dependent upon these are not performed until after | |
4753 | allocation. | |
4754 | ||
4755 | Some expressions, such as those depending upon the location counter | |
4756 | @samp{.}, must be evaluated during section allocation. | |
4757 | ||
4758 | If the result of an expression is required, but the value is not | |
4759 | available, then an error results. For example, a script like the | |
4760 | following | |
4761 | @smallexample | |
4762 | @group | |
4763 | SECTIONS | |
4764 | @{ | |
a1ab1d2a | 4765 | .text 9+this_isnt_constant : |
252b5132 RH |
4766 | @{ *(.text) @} |
4767 | @} | |
4768 | @end group | |
4769 | @end smallexample | |
4770 | @noindent | |
4771 | will cause the error message @samp{non constant expression for initial | |
4772 | address}. | |
4773 | ||
4774 | @node Expression Section | |
4775 | @subsection The Section of an Expression | |
4776 | @cindex expression sections | |
4777 | @cindex absolute expressions | |
4778 | @cindex relative expressions | |
4779 | @cindex absolute and relocatable symbols | |
4780 | @cindex relocatable and absolute symbols | |
4781 | @cindex symbols, relocatable and absolute | |
4782 | When the linker evaluates an expression, the result is either absolute | |
4783 | or relative to some section. A relative expression is expressed as a | |
4784 | fixed offset from the base of a section. | |
4785 | ||
4786 | The position of the expression within the linker script determines | |
4787 | whether it is absolute or relative. An expression which appears within | |
4788 | an output section definition is relative to the base of the output | |
4789 | section. An expression which appears elsewhere will be absolute. | |
4790 | ||
4791 | A symbol set to a relative expression will be relocatable if you request | |
4792 | relocatable output using the @samp{-r} option. That means that a | |
4793 | further link operation may change the value of the symbol. The symbol's | |
4794 | section will be the section of the relative expression. | |
4795 | ||
4796 | A symbol set to an absolute expression will retain the same value | |
4797 | through any further link operation. The symbol will be absolute, and | |
4798 | will not have any particular associated section. | |
4799 | ||
4800 | You can use the builtin function @code{ABSOLUTE} to force an expression | |
4801 | to be absolute when it would otherwise be relative. For example, to | |
4802 | create an absolute symbol set to the address of the end of the output | |
4803 | section @samp{.data}: | |
4804 | @smallexample | |
4805 | SECTIONS | |
4806 | @{ | |
4807 | .data : @{ *(.data) _edata = ABSOLUTE(.); @} | |
4808 | @} | |
4809 | @end smallexample | |
4810 | @noindent | |
4811 | If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the | |
4812 | @samp{.data} section. | |
4813 | ||
4814 | @node Builtin Functions | |
4815 | @subsection Builtin Functions | |
4816 | @cindex functions in expressions | |
4817 | The linker script language includes a number of builtin functions for | |
4818 | use in linker script expressions. | |
4819 | ||
4820 | @table @code | |
4821 | @item ABSOLUTE(@var{exp}) | |
4822 | @kindex ABSOLUTE(@var{exp}) | |
4823 | @cindex expression, absolute | |
4824 | Return the absolute (non-relocatable, as opposed to non-negative) value | |
4825 | of the expression @var{exp}. Primarily useful to assign an absolute | |
4826 | value to a symbol within a section definition, where symbol values are | |
4827 | normally section relative. @xref{Expression Section}. | |
4828 | ||
4829 | @item ADDR(@var{section}) | |
4830 | @kindex ADDR(@var{section}) | |
4831 | @cindex section address in expression | |
4832 | Return the absolute address (the VMA) of the named @var{section}. Your | |
4833 | script must previously have defined the location of that section. In | |
4834 | the following example, @code{symbol_1} and @code{symbol_2} are assigned | |
4835 | identical values: | |
4836 | @smallexample | |
4837 | @group | |
4838 | SECTIONS @{ @dots{} | |
4839 | .output1 : | |
a1ab1d2a | 4840 | @{ |
252b5132 RH |
4841 | start_of_output_1 = ABSOLUTE(.); |
4842 | @dots{} | |
4843 | @} | |
4844 | .output : | |
4845 | @{ | |
4846 | symbol_1 = ADDR(.output1); | |
4847 | symbol_2 = start_of_output_1; | |
4848 | @} | |
4849 | @dots{} @} | |
4850 | @end group | |
4851 | @end smallexample | |
4852 | ||
876f4090 NS |
4853 | @item ALIGN(@var{align}) |
4854 | @itemx ALIGN(@var{exp},@var{align}) | |
4855 | @kindex ALIGN(@var{align}) | |
4856 | @kindex ALIGN(@var{exp},@var{align}) | |
252b5132 RH |
4857 | @cindex round up location counter |
4858 | @cindex align location counter | |
876f4090 NS |
4859 | @cindex round up expression |
4860 | @cindex align expression | |
4861 | Return the location counter (@code{.}) or arbitrary expression aligned | |
4862 | to the next @var{align} boundary. The single operand @code{ALIGN} | |
4863 | doesn't change the value of the location counter---it just does | |
4864 | arithmetic on it. The two operand @code{ALIGN} allows an arbitrary | |
4865 | expression to be aligned upwards (@code{ALIGN(@var{align})} is | |
4866 | equivalent to @code{ALIGN(., @var{align})}). | |
4867 | ||
4868 | Here is an example which aligns the output @code{.data} section to the | |
4869 | next @code{0x2000} byte boundary after the preceding section and sets a | |
4870 | variable within the section to the next @code{0x8000} boundary after the | |
4871 | input sections: | |
252b5132 RH |
4872 | @smallexample |
4873 | @group | |
4874 | SECTIONS @{ @dots{} | |
4875 | .data ALIGN(0x2000): @{ | |
4876 | *(.data) | |
4877 | variable = ALIGN(0x8000); | |
4878 | @} | |
4879 | @dots{} @} | |
4880 | @end group | |
4881 | @end smallexample | |
4882 | @noindent | |
4883 | The first use of @code{ALIGN} in this example specifies the location of | |
4884 | a section because it is used as the optional @var{address} attribute of | |
4885 | a section definition (@pxref{Output Section Address}). The second use | |
4886 | of @code{ALIGN} is used to defines the value of a symbol. | |
4887 | ||
4888 | The builtin function @code{NEXT} is closely related to @code{ALIGN}. | |
4889 | ||
4890 | @item BLOCK(@var{exp}) | |
4891 | @kindex BLOCK(@var{exp}) | |
4892 | This is a synonym for @code{ALIGN}, for compatibility with older linker | |
4893 | scripts. It is most often seen when setting the address of an output | |
4894 | section. | |
4895 | ||
2d20f7bf JJ |
4896 | @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize}) |
4897 | @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize}) | |
4898 | This is equivalent to either | |
4899 | @smallexample | |
4900 | (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1))) | |
4901 | @end smallexample | |
4902 | or | |
4903 | @smallexample | |
4904 | (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - @var{commonpagesize}))) | |
4905 | @end smallexample | |
4906 | @noindent | |
4907 | depending on whether the latter uses fewer @var{commonpagesize} sized pages | |
4908 | for the data segment (area between the result of this expression and | |
4909 | @code{DATA_SEGMENT_END}) than the former or not. | |
4910 | If the latter form is used, it means @var{commonpagesize} bytes of runtime | |
4911 | memory will be saved at the expense of up to @var{commonpagesize} wasted | |
4912 | bytes in the on-disk file. | |
4913 | ||
4914 | This expression can only be used directly in @code{SECTIONS} commands, not in | |
4915 | any output section descriptions and only once in the linker script. | |
4916 | @var{commonpagesize} should be less or equal to @var{maxpagesize} and should | |
4917 | be the system page size the object wants to be optimized for (while still | |
4918 | working on system page sizes up to @var{maxpagesize}). | |
4919 | ||
4920 | @noindent | |
4921 | Example: | |
4922 | @smallexample | |
4923 | . = DATA_SEGMENT_ALIGN(0x10000, 0x2000); | |
4924 | @end smallexample | |
4925 | ||
4926 | @item DATA_SEGMENT_END(@var{exp}) | |
4927 | @kindex DATA_SEGMENT_END(@var{exp}) | |
4928 | This defines the end of data segment for @code{DATA_SEGMENT_ALIGN} | |
4929 | evaluation purposes. | |
4930 | ||
4931 | @smallexample | |
4932 | . = DATA_SEGMENT_END(.); | |
4933 | @end smallexample | |
4934 | ||
a4f5ad88 JJ |
4935 | @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp}) |
4936 | @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp}) | |
4937 | This defines the end of the @code{PT_GNU_RELRO} segment when | |
4938 | @samp{-z relro} option is used. Second argument is returned. | |
4939 | When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END} | |
4940 | does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that | |
4941 | @var{exp} + @var{offset} is aligned to the most commonly used page | |
4942 | boundary for particular target. If present in the linker script, | |
4943 | it must always come in between @code{DATA_SEGMENT_ALIGN} and | |
4944 | @code{DATA_SEGMENT_END}. | |
4945 | ||
4946 | @smallexample | |
4947 | . = DATA_SEGMENT_RELRO_END(24, .); | |
4948 | @end smallexample | |
4949 | ||
252b5132 RH |
4950 | @item DEFINED(@var{symbol}) |
4951 | @kindex DEFINED(@var{symbol}) | |
4952 | @cindex symbol defaults | |
4953 | Return 1 if @var{symbol} is in the linker global symbol table and is | |
420e579c HPN |
4954 | defined before the statement using DEFINED in the script, otherwise |
4955 | return 0. You can use this function to provide | |
252b5132 RH |
4956 | default values for symbols. For example, the following script fragment |
4957 | shows how to set a global symbol @samp{begin} to the first location in | |
4958 | the @samp{.text} section---but if a symbol called @samp{begin} already | |
4959 | existed, its value is preserved: | |
4960 | ||
4961 | @smallexample | |
4962 | @group | |
4963 | SECTIONS @{ @dots{} | |
4964 | .text : @{ | |
4965 | begin = DEFINED(begin) ? begin : . ; | |
4966 | @dots{} | |
4967 | @} | |
4968 | @dots{} | |
4969 | @} | |
4970 | @end group | |
4971 | @end smallexample | |
4972 | ||
3ec57632 NC |
4973 | @item LENGTH(@var{memory}) |
4974 | @kindex LENGTH(@var{memory}) | |
4975 | Return the length of the memory region named @var{memory}. | |
4976 | ||
252b5132 RH |
4977 | @item LOADADDR(@var{section}) |
4978 | @kindex LOADADDR(@var{section}) | |
4979 | @cindex section load address in expression | |
4980 | Return the absolute LMA of the named @var{section}. This is normally | |
4981 | the same as @code{ADDR}, but it may be different if the @code{AT} | |
4982 | attribute is used in the output section definition (@pxref{Output | |
4983 | Section LMA}). | |
4984 | ||
4985 | @kindex MAX | |
4986 | @item MAX(@var{exp1}, @var{exp2}) | |
4987 | Returns the maximum of @var{exp1} and @var{exp2}. | |
4988 | ||
4989 | @kindex MIN | |
4990 | @item MIN(@var{exp1}, @var{exp2}) | |
4991 | Returns the minimum of @var{exp1} and @var{exp2}. | |
4992 | ||
4993 | @item NEXT(@var{exp}) | |
4994 | @kindex NEXT(@var{exp}) | |
4995 | @cindex unallocated address, next | |
4996 | Return the next unallocated address that is a multiple of @var{exp}. | |
4997 | This function is closely related to @code{ALIGN(@var{exp})}; unless you | |
4998 | use the @code{MEMORY} command to define discontinuous memory for the | |
4999 | output file, the two functions are equivalent. | |
5000 | ||
3ec57632 NC |
5001 | @item ORIGIN(@var{memory}) |
5002 | @kindex ORIGIN(@var{memory}) | |
5003 | Return the origin of the memory region named @var{memory}. | |
5004 | ||
ba916c8a MM |
5005 | @item SEGMENT_START(@var{segment}, @var{default}) |
5006 | @kindex SEGMENT_START(@var{segment}, @var{default}) | |
5007 | Return the base address of the named @var{segment}. If an explicit | |
5008 | value has been given for this segment (with a command-line @samp{-T} | |
5009 | option) that value will be returned; otherwise the value will be | |
5010 | @var{default}. At present, the @samp{-T} command-line option can only | |
5011 | be used to set the base address for the ``text'', ``data'', and | |
5012 | ``bss'' sections, but you use @code{SEGMENT_START} with any segment | |
5013 | name. | |
5014 | ||
252b5132 RH |
5015 | @item SIZEOF(@var{section}) |
5016 | @kindex SIZEOF(@var{section}) | |
5017 | @cindex section size | |
5018 | Return the size in bytes of the named @var{section}, if that section has | |
5019 | been allocated. If the section has not been allocated when this is | |
5020 | evaluated, the linker will report an error. In the following example, | |
5021 | @code{symbol_1} and @code{symbol_2} are assigned identical values: | |
5022 | @smallexample | |
5023 | @group | |
5024 | SECTIONS@{ @dots{} | |
5025 | .output @{ | |
5026 | .start = . ; | |
5027 | @dots{} | |
5028 | .end = . ; | |
5029 | @} | |
5030 | symbol_1 = .end - .start ; | |
5031 | symbol_2 = SIZEOF(.output); | |
5032 | @dots{} @} | |
5033 | @end group | |
5034 | @end smallexample | |
5035 | ||
5036 | @item SIZEOF_HEADERS | |
5037 | @itemx sizeof_headers | |
5038 | @kindex SIZEOF_HEADERS | |
5039 | @cindex header size | |
5040 | Return the size in bytes of the output file's headers. This is | |
5041 | information which appears at the start of the output file. You can use | |
5042 | this number when setting the start address of the first section, if you | |
5043 | choose, to facilitate paging. | |
5044 | ||
5045 | @cindex not enough room for program headers | |
5046 | @cindex program headers, not enough room | |
5047 | When producing an ELF output file, if the linker script uses the | |
5048 | @code{SIZEOF_HEADERS} builtin function, the linker must compute the | |
5049 | number of program headers before it has determined all the section | |
5050 | addresses and sizes. If the linker later discovers that it needs | |
5051 | additional program headers, it will report an error @samp{not enough | |
5052 | room for program headers}. To avoid this error, you must avoid using | |
5053 | the @code{SIZEOF_HEADERS} function, or you must rework your linker | |
5054 | script to avoid forcing the linker to use additional program headers, or | |
5055 | you must define the program headers yourself using the @code{PHDRS} | |
5056 | command (@pxref{PHDRS}). | |
5057 | @end table | |
5058 | ||
5059 | @node Implicit Linker Scripts | |
5060 | @section Implicit Linker Scripts | |
5061 | @cindex implicit linker scripts | |
5062 | If you specify a linker input file which the linker can not recognize as | |
5063 | an object file or an archive file, it will try to read the file as a | |
5064 | linker script. If the file can not be parsed as a linker script, the | |
5065 | linker will report an error. | |
5066 | ||
5067 | An implicit linker script will not replace the default linker script. | |
5068 | ||
5069 | Typically an implicit linker script would contain only symbol | |
5070 | assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION} | |
5071 | commands. | |
5072 | ||
5073 | Any input files read because of an implicit linker script will be read | |
5074 | at the position in the command line where the implicit linker script was | |
5075 | read. This can affect archive searching. | |
5076 | ||
5077 | @ifset GENERIC | |
5078 | @node Machine Dependent | |
5079 | @chapter Machine Dependent Features | |
5080 | ||
5081 | @cindex machine dependencies | |
ff5dcc92 SC |
5082 | @command{ld} has additional features on some platforms; the following |
5083 | sections describe them. Machines where @command{ld} has no additional | |
252b5132 RH |
5084 | functionality are not listed. |
5085 | ||
5086 | @menu | |
36f63dca NC |
5087 | @ifset H8300 |
5088 | * H8/300:: @command{ld} and the H8/300 | |
5089 | @end ifset | |
5090 | @ifset I960 | |
5091 | * i960:: @command{ld} and the Intel 960 family | |
5092 | @end ifset | |
5093 | @ifset ARM | |
5094 | * ARM:: @command{ld} and the ARM family | |
5095 | @end ifset | |
5096 | @ifset HPPA | |
5097 | * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF | |
5098 | @end ifset | |
3c3bdf30 | 5099 | @ifset MMIX |
36f63dca | 5100 | * MMIX:: @command{ld} and MMIX |
3c3bdf30 | 5101 | @end ifset |
2469cfa2 | 5102 | @ifset MSP430 |
36f63dca | 5103 | * MSP430:: @command{ld} and MSP430 |
2469cfa2 | 5104 | @end ifset |
93fd0973 SC |
5105 | @ifset M68HC11 |
5106 | * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families | |
5107 | @end ifset | |
74459f0e | 5108 | @ifset TICOFF |
ff5dcc92 | 5109 | * TI COFF:: @command{ld} and TI COFF |
74459f0e | 5110 | @end ifset |
2ca22b03 NC |
5111 | @ifset WIN32 |
5112 | * WIN32:: @command{ld} and WIN32 (cygwin/mingw) | |
5113 | @end ifset | |
e0001a05 NC |
5114 | @ifset XTENSA |
5115 | * Xtensa:: @command{ld} and Xtensa Processors | |
5116 | @end ifset | |
252b5132 RH |
5117 | @end menu |
5118 | @end ifset | |
5119 | ||
252b5132 RH |
5120 | @ifset H8300 |
5121 | @ifclear GENERIC | |
5122 | @raisesections | |
5123 | @end ifclear | |
5124 | ||
5125 | @node H8/300 | |
ff5dcc92 | 5126 | @section @command{ld} and the H8/300 |
252b5132 RH |
5127 | |
5128 | @cindex H8/300 support | |
ff5dcc92 | 5129 | For the H8/300, @command{ld} can perform these global optimizations when |
252b5132 RH |
5130 | you specify the @samp{--relax} command-line option. |
5131 | ||
5132 | @table @emph | |
5133 | @cindex relaxing on H8/300 | |
5134 | @item relaxing address modes | |
ff5dcc92 | 5135 | @command{ld} finds all @code{jsr} and @code{jmp} instructions whose |
252b5132 RH |
5136 | targets are within eight bits, and turns them into eight-bit |
5137 | program-counter relative @code{bsr} and @code{bra} instructions, | |
5138 | respectively. | |
5139 | ||
5140 | @cindex synthesizing on H8/300 | |
5141 | @item synthesizing instructions | |
5142 | @c FIXME: specifically mov.b, or any mov instructions really? | |
ff5dcc92 | 5143 | @command{ld} finds all @code{mov.b} instructions which use the |
252b5132 RH |
5144 | sixteen-bit absolute address form, but refer to the top |
5145 | page of memory, and changes them to use the eight-bit address form. | |
5146 | (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into | |
5147 | @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the | |
5148 | top page of memory). | |
1502569c NC |
5149 | |
5150 | @item bit manipulation instructions | |
5151 | @command{ld} finds all bit manipulation instructions like @code{band, bclr, | |
5152 | biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor} | |
5153 | which use 32 bit and 16 bit absolute address form, but refer to the top | |
5154 | page of memory, and changes them to use the 8 bit address form. | |
5155 | (That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into | |
5156 | @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in | |
5157 | the top page of memory). | |
5158 | ||
5159 | @item system control instructions | |
5160 | @command{ld} finds all @code{ldc.w, stc.w} instrcutions which use the | |
5161 | 32 bit absolute address form, but refer to the top page of memory, and | |
5162 | changes them to use 16 bit address form. | |
5163 | (That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into | |
5164 | @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in | |
5165 | the top page of memory). | |
252b5132 RH |
5166 | @end table |
5167 | ||
5168 | @ifclear GENERIC | |
5169 | @lowersections | |
5170 | @end ifclear | |
5171 | @end ifset | |
5172 | ||
36f63dca | 5173 | @ifclear GENERIC |
c2dcd04e | 5174 | @ifset Renesas |
36f63dca | 5175 | @c This stuff is pointless to say unless you're especially concerned |
c2dcd04e NC |
5176 | @c with Renesas chips; don't enable it for generic case, please. |
5177 | @node Renesas | |
5178 | @chapter @command{ld} and Other Renesas Chips | |
36f63dca | 5179 | |
c2dcd04e NC |
5180 | @command{ld} also supports the Renesas (formerly Hitachi) H8/300H, |
5181 | H8/500, and SH chips. No special features, commands, or command-line | |
5182 | options are required for these chips. | |
36f63dca NC |
5183 | @end ifset |
5184 | @end ifclear | |
5185 | ||
5186 | @ifset I960 | |
5187 | @ifclear GENERIC | |
5188 | @raisesections | |
5189 | @end ifclear | |
5190 | ||
5191 | @node i960 | |
5192 | @section @command{ld} and the Intel 960 Family | |
5193 | ||
5194 | @cindex i960 support | |
5195 | ||
5196 | You can use the @samp{-A@var{architecture}} command line option to | |
5197 | specify one of the two-letter names identifying members of the 960 | |
5198 | family; the option specifies the desired output target, and warns of any | |
5199 | incompatible instructions in the input files. It also modifies the | |
5200 | linker's search strategy for archive libraries, to support the use of | |
5201 | libraries specific to each particular architecture, by including in the | |
5202 | search loop names suffixed with the string identifying the architecture. | |
5203 | ||
5204 | For example, if your @command{ld} command line included @w{@samp{-ACA}} as | |
5205 | well as @w{@samp{-ltry}}, the linker would look (in its built-in search | |
5206 | paths, and in any paths you specify with @samp{-L}) for a library with | |
5207 | the names | |
5208 | ||
5209 | @smallexample | |
5210 | @group | |
5211 | try | |
5212 | libtry.a | |
5213 | tryca | |
5214 | libtryca.a | |
5215 | @end group | |
5216 | @end smallexample | |
5217 | ||
5218 | @noindent | |
5219 | The first two possibilities would be considered in any event; the last | |
5220 | two are due to the use of @w{@samp{-ACA}}. | |
5221 | ||
5222 | You can meaningfully use @samp{-A} more than once on a command line, since | |
5223 | the 960 architecture family allows combination of target architectures; each | |
5224 | use will add another pair of name variants to search for when @w{@samp{-l}} | |
5225 | specifies a library. | |
5226 | ||
5227 | @cindex @option{--relax} on i960 | |
5228 | @cindex relaxing on i960 | |
5229 | @command{ld} supports the @samp{--relax} option for the i960 family. If | |
5230 | you specify @samp{--relax}, @command{ld} finds all @code{balx} and | |
5231 | @code{calx} instructions whose targets are within 24 bits, and turns | |
5232 | them into 24-bit program-counter relative @code{bal} and @code{cal} | |
5233 | instructions, respectively. @command{ld} also turns @code{cal} | |
5234 | instructions into @code{bal} instructions when it determines that the | |
5235 | target subroutine is a leaf routine (that is, the target subroutine does | |
5236 | not itself call any subroutines). | |
5237 | ||
5238 | @ifclear GENERIC | |
5239 | @lowersections | |
5240 | @end ifclear | |
5241 | @end ifset | |
5242 | ||
5243 | @ifset ARM | |
5244 | @ifclear GENERIC | |
5245 | @raisesections | |
5246 | @end ifclear | |
5247 | ||
93fd0973 SC |
5248 | @ifset M68HC11 |
5249 | @ifclear GENERIC | |
5250 | @raisesections | |
5251 | @end ifclear | |
5252 | ||
5253 | @node M68HC11/68HC12 | |
5254 | @section @command{ld} and the Motorola 68HC11 and 68HC12 families | |
5255 | ||
5256 | @cindex M68HC11 and 68HC12 support | |
5257 | ||
5258 | @subsection Linker Relaxation | |
5259 | ||
5260 | For the Motorola 68HC11, @command{ld} can perform these global | |
5261 | optimizations when you specify the @samp{--relax} command-line option. | |
5262 | ||
5263 | @table @emph | |
5264 | @cindex relaxing on M68HC11 | |
5265 | @item relaxing address modes | |
5266 | @command{ld} finds all @code{jsr} and @code{jmp} instructions whose | |
5267 | targets are within eight bits, and turns them into eight-bit | |
5268 | program-counter relative @code{bsr} and @code{bra} instructions, | |
5269 | respectively. | |
5270 | ||
5271 | @command{ld} also looks at all 16-bit extended addressing modes and | |
5272 | transforms them in a direct addressing mode when the address is in | |
5273 | page 0 (between 0 and 0x0ff). | |
5274 | ||
5275 | @item relaxing gcc instruction group | |
5276 | When @command{gcc} is called with @option{-mrelax}, it can emit group | |
5277 | of instructions that the linker can optimize to use a 68HC11 direct | |
5278 | addressing mode. These instructions consists of @code{bclr} or | |
5279 | @code{bset} instructions. | |
5280 | ||
5281 | @end table | |
5282 | ||
5283 | @subsection Trampoline Generation | |
5284 | ||
5285 | @cindex trampoline generation on M68HC11 | |
5286 | @cindex trampoline generation on M68HC12 | |
5287 | For 68HC11 and 68HC12, @command{ld} can generate trampoline code to | |
5288 | call a far function using a normal @code{jsr} instruction. The linker | |
5289 | will also change the relocation to some far function to use the | |
5290 | trampoline address instead of the function address. This is typically the | |
5291 | case when a pointer to a function is taken. The pointer will in fact | |
5292 | point to the function trampoline. | |
5293 | ||
5294 | @ifclear GENERIC | |
5295 | @lowersections | |
5296 | @end ifclear | |
5297 | @end ifset | |
5298 | ||
36f63dca | 5299 | @node ARM |
3674e28a | 5300 | @section @command{ld} and the ARM family |
36f63dca NC |
5301 | |
5302 | @cindex ARM interworking support | |
5303 | @kindex --support-old-code | |
5304 | For the ARM, @command{ld} will generate code stubs to allow functions calls | |
5305 | betweem ARM and Thumb code. These stubs only work with code that has | |
5306 | been compiled and assembled with the @samp{-mthumb-interwork} command | |
5307 | line option. If it is necessary to link with old ARM object files or | |
5308 | libraries, which have not been compiled with the -mthumb-interwork | |
5309 | option then the @samp{--support-old-code} command line switch should be | |
5310 | given to the linker. This will make it generate larger stub functions | |
5311 | which will work with non-interworking aware ARM code. Note, however, | |
5312 | the linker does not support generating stubs for function calls to | |
5313 | non-interworking aware Thumb code. | |
5314 | ||
5315 | @cindex thumb entry point | |
5316 | @cindex entry point, thumb | |
5317 | @kindex --thumb-entry=@var{entry} | |
5318 | The @samp{--thumb-entry} switch is a duplicate of the generic | |
5319 | @samp{--entry} switch, in that it sets the program's starting address. | |
5320 | But it also sets the bottom bit of the address, so that it can be | |
5321 | branched to using a BX instruction, and the program will start | |
5322 | executing in Thumb mode straight away. | |
5323 | ||
e489d0ae PB |
5324 | @cindex BE8 |
5325 | @kindex --be8 | |
5326 | The @samp{--be8} switch instructs @command{ld} to generate BE8 format | |
5327 | executables. This option is only valid when linking big-endian objects. | |
5328 | The resulting image will contain big-endian data and little-endian code. | |
5329 | ||
3674e28a PB |
5330 | @cindex TARGET1 |
5331 | @kindex --target1-rel | |
5332 | @kindex --target1-abs | |
5333 | The @samp{R_ARM_TARGET1} relocation is typically used for entries in the | |
5334 | @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32} | |
5335 | or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel} | |
5336 | and @samp{--target1-abs} switches override the default. | |
5337 | ||
5338 | @cindex TARGET2 | |
5339 | @kindex --target2=@var{type} | |
5340 | The @samp{--target2=type} switch overrides the default definition of the | |
5341 | @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their | |
5342 | meanings, and target defaults are as follows: | |
5343 | @table @samp | |
5344 | @item rel | |
eeac373a PB |
5345 | @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi) |
5346 | @item abs | |
5347 | @samp{R_ARM_ABS32} (arm*-*-symbianelf) | |
3674e28a PB |
5348 | @item got-rel |
5349 | @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd) | |
5350 | @end table | |
5351 | ||
319850b4 JB |
5352 | @cindex FIX_V4BX |
5353 | @kindex --fix-v4bx | |
5354 | The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF | |
5355 | specification) enables objects compiled for the ARMv4 architecture to be | |
5356 | interworking-safe when linked with other objects compiled for ARMv4t, but | |
5357 | also allows pure ARMv4 binaries to be built from the same ARMv4 objects. | |
5358 | ||
5359 | In the latter case, the switch @option{--fix-v4bx} must be passed to the | |
5360 | linker, which causes v4t @code{BX rM} instructions to be rewritten as | |
5361 | @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction. | |
5362 | ||
5363 | In the former case, the switch should not be used, and @samp{R_ARM_V4BX} | |
5364 | relocations are ignored. | |
5365 | ||
33bfe774 JB |
5366 | @cindex USE_BLX |
5367 | @kindex --use-blx | |
5368 | The @samp{--use-blx} switch enables the linker to use ARM/Thumb | |
5369 | BLX instructions (available on ARMv5t and above) in various | |
5370 | situations. Currently it is used to perform calls via the PLT from Thumb | |
5371 | code using BLX rather than using BX and a mode-switching stub before | |
5372 | each PLT entry. This should lead to such calls executing slightly faster. | |
5373 | ||
5374 | This option is enabled implicitly for SymbianOS, so there is no need to | |
5375 | specify it if you are using that target. | |
5376 | ||
36f63dca NC |
5377 | @ifclear GENERIC |
5378 | @lowersections | |
5379 | @end ifclear | |
5380 | @end ifset | |
5381 | ||
5382 | @ifset HPPA | |
5383 | @ifclear GENERIC | |
5384 | @raisesections | |
5385 | @end ifclear | |
5386 | ||
5387 | @node HPPA ELF32 | |
5388 | @section @command{ld} and HPPA 32-bit ELF Support | |
5389 | @cindex HPPA multiple sub-space stubs | |
5390 | @kindex --multi-subspace | |
5391 | When generating a shared library, @command{ld} will by default generate | |
5392 | import stubs suitable for use with a single sub-space application. | |
5393 | The @samp{--multi-subspace} switch causes @command{ld} to generate export | |
5394 | stubs, and different (larger) import stubs suitable for use with | |
5395 | multiple sub-spaces. | |
5396 | ||
5397 | @cindex HPPA stub grouping | |
5398 | @kindex --stub-group-size=@var{N} | |
5399 | Long branch stubs and import/export stubs are placed by @command{ld} in | |
5400 | stub sections located between groups of input sections. | |
5401 | @samp{--stub-group-size} specifies the maximum size of a group of input | |
5402 | sections handled by one stub section. Since branch offsets are signed, | |
5403 | a stub section may serve two groups of input sections, one group before | |
5404 | the stub section, and one group after it. However, when using | |
5405 | conditional branches that require stubs, it may be better (for branch | |
5406 | prediction) that stub sections only serve one group of input sections. | |
5407 | A negative value for @samp{N} chooses this scheme, ensuring that | |
5408 | branches to stubs always use a negative offset. Two special values of | |
5409 | @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct | |
5410 | @command{ld} to automatically size input section groups for the branch types | |
5411 | detected, with the same behaviour regarding stub placement as other | |
5412 | positive or negative values of @samp{N} respectively. | |
5413 | ||
5414 | Note that @samp{--stub-group-size} does not split input sections. A | |
5415 | single input section larger than the group size specified will of course | |
5416 | create a larger group (of one section). If input sections are too | |
5417 | large, it may not be possible for a branch to reach its stub. | |
5418 | ||
5419 | @ifclear GENERIC | |
5420 | @lowersections | |
5421 | @end ifclear | |
5422 | @end ifset | |
5423 | ||
5424 | @ifset MMIX | |
5425 | @ifclear GENERIC | |
5426 | @raisesections | |
5427 | @end ifclear | |
5428 | ||
5429 | @node MMIX | |
5430 | @section @code{ld} and MMIX | |
5431 | For MMIX, there is a choice of generating @code{ELF} object files or | |
5432 | @code{mmo} object files when linking. The simulator @code{mmix} | |
5433 | understands the @code{mmo} format. The binutils @code{objcopy} utility | |
5434 | can translate between the two formats. | |
5435 | ||
5436 | There is one special section, the @samp{.MMIX.reg_contents} section. | |
5437 | Contents in this section is assumed to correspond to that of global | |
5438 | registers, and symbols referring to it are translated to special symbols, | |
5439 | equal to registers. In a final link, the start address of the | |
5440 | @samp{.MMIX.reg_contents} section corresponds to the first allocated | |
5441 | global register multiplied by 8. Register @code{$255} is not included in | |
5442 | this section; it is always set to the program entry, which is at the | |
5443 | symbol @code{Main} for @code{mmo} files. | |
5444 | ||
5445 | Symbols with the prefix @code{__.MMIX.start.}, for example | |
5446 | @code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special; | |
5447 | there must be only one each, even if they are local. The default linker | |
5448 | script uses these to set the default start address of a section. | |
5449 | ||
5450 | Initial and trailing multiples of zero-valued 32-bit words in a section, | |
5451 | are left out from an mmo file. | |
5452 | ||
5453 | @ifclear GENERIC | |
5454 | @lowersections | |
5455 | @end ifclear | |
5456 | @end ifset | |
5457 | ||
5458 | @ifset MSP430 | |
5459 | @ifclear GENERIC | |
5460 | @raisesections | |
5461 | @end ifclear | |
5462 | ||
5463 | @node MSP430 | |
5464 | @section @code{ld} and MSP430 | |
5465 | For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]} | |
5466 | will select an appropriate linker script for selected MPU type. (To get a list of known MPUs | |
5467 | just pass @samp{-m help} option to the linker). | |
5468 | ||
5469 | @cindex MSP430 extra sections | |
5470 | The linker will recognize some extra sections which are MSP430 specific: | |
5471 | ||
5472 | @table @code | |
5473 | @item @samp{.vectors} | |
5474 | Defines a portion of ROM where interrupt vectors located. | |
5475 | ||
5476 | @item @samp{.bootloader} | |
5477 | Defines the bootloader portion of the ROM (if applicable). Any code | |
5478 | in this section will be uploaded to the MPU. | |
5479 | ||
5480 | @item @samp{.infomem} | |
5481 | Defines an information memory section (if applicable). Any code in | |
5482 | this section will be uploaded to the MPU. | |
5483 | ||
5484 | @item @samp{.infomemnobits} | |
5485 | This is the same as the @samp{.infomem} section except that any code | |
5486 | in this section will not be uploaded to the MPU. | |
5487 | ||
5488 | @item @samp{.noinit} | |
5489 | Denotes a portion of RAM located above @samp{.bss} section. | |
5490 | ||
5491 | The last two sections are used by gcc. | |
5492 | @end table | |
5493 | ||
5494 | @ifclear GENERIC | |
5495 | @lowersections | |
5496 | @end ifclear | |
5497 | @end ifset | |
5498 | ||
5499 | @ifset TICOFF | |
5500 | @ifclear GENERIC | |
5501 | @raisesections | |
5502 | @end ifclear | |
5503 | ||
5504 | @node TI COFF | |
5505 | @section @command{ld}'s Support for Various TI COFF Versions | |
5506 | @cindex TI COFF versions | |
5507 | @kindex --format=@var{version} | |
5508 | The @samp{--format} switch allows selection of one of the various | |
5509 | TI COFF versions. The latest of this writing is 2; versions 0 and 1 are | |
5510 | also supported. The TI COFF versions also vary in header byte-order | |
5511 | format; @command{ld} will read any version or byte order, but the output | |
5512 | header format depends on the default specified by the specific target. | |
5513 | ||
5514 | @ifclear GENERIC | |
5515 | @lowersections | |
5516 | @end ifclear | |
5517 | @end ifset | |
5518 | ||
2ca22b03 NC |
5519 | @ifset WIN32 |
5520 | @ifclear GENERIC | |
5521 | @raisesections | |
5522 | @end ifclear | |
5523 | ||
5524 | @node WIN32 | |
5525 | @section @command{ld} and WIN32 (cygwin/mingw) | |
5526 | ||
5527 | This section describes some of the win32 specific @command{ld} issues. | |
dc8465bf NC |
5528 | See @ref{Options,,Command Line Options} for detailed decription of the |
5529 | command line options mentioned here. | |
2ca22b03 NC |
5530 | |
5531 | @table @emph | |
5532 | @cindex import libraries | |
5533 | @item import libraries | |
69da35b5 | 5534 | The standard Windows linker creates and uses so-called import |
2ca22b03 | 5535 | libraries, which contains information for linking to dll's. They are |
69da35b5 NC |
5536 | regular static archives and are handled as any other static |
5537 | archive. The cygwin and mingw ports of @command{ld} have specific | |
2ca22b03 NC |
5538 | support for creating such libraries provided with the |
5539 | @samp{--out-implib} command line option. | |
5540 | ||
dc8465bf NC |
5541 | @item exporting DLL symbols |
5542 | @cindex exporting DLL symbols | |
5543 | The cygwin/mingw @command{ld} has several ways to export symbols for dll's. | |
5544 | ||
5545 | @table @emph | |
5546 | @item using auto-export functionality | |
5547 | @cindex using auto-export functionality | |
5548 | By default @command{ld} exports symbols with the auto-export functionality, | |
5549 | which is controlled by the following command line options: | |
5550 | ||
0a5d968e NC |
5551 | @itemize |
5552 | @item --export-all-symbols [This is the default] | |
5553 | @item --exclude-symbols | |
5554 | @item --exclude-libs | |
5555 | @end itemize | |
5556 | ||
5557 | If, however, @samp{--export-all-symbols} is not given explicitly on the | |
5558 | command line, then the default auto-export behavior will be @emph{disabled} | |
5559 | if either of the following are true: | |
5560 | ||
5561 | @itemize | |
5562 | @item A DEF file is used. | |
5563 | @item Any symbol in any object file was marked with the __declspec(dllexport) attribute. | |
5564 | @end itemize | |
dc8465bf NC |
5565 | |
5566 | @item using a DEF file | |
5567 | @cindex using a DEF file | |
5568 | Another way of exporting symbols is using a DEF file. A DEF file is | |
5569 | an ASCII file containing definitions of symbols which should be | |
5570 | exported when a dll is created. Usually it is named @samp{<dll | |
5571 | name>.def} and is added as any other object file to the linker's | |
0a5d968e | 5572 | command line. The file's name must end in @samp{.def} or @samp{.DEF}. |
dc8465bf NC |
5573 | |
5574 | @example | |
5575 | gcc -o <output> <objectfiles> <dll name>.def | |
5576 | @end example | |
5577 | ||
0a5d968e NC |
5578 | Using a DEF file turns off the normal auto-export behavior, unless the |
5579 | @samp{--export-all-symbols} option is also used. | |
5580 | ||
dc8465bf NC |
5581 | Here is an example of a DEF file for a shared library called @samp{xyz.dll}: |
5582 | ||
5583 | @example | |
5584 | LIBRARY "xyz.dll" BASE=0x10000000 | |
5585 | ||
5586 | EXPORTS | |
5587 | foo | |
5588 | bar | |
5589 | _bar = bar | |
5590 | @end example | |
5591 | ||
5592 | This example defines a base address and three symbols. The third | |
5593 | symbol is an alias for the second. For the complete format | |
5594 | specification see ld/deffilep.y in the binutils sources. | |
5595 | ||
5596 | @cindex creating a DEF file | |
5597 | While linking a shared dll, @command{ld} is able to create a DEF file | |
5598 | with the @samp{--output-def <file>} command line option. | |
0a5d968e NC |
5599 | |
5600 | @item Using decorations | |
5601 | @cindex Using decorations | |
5602 | Another way of marking symbols for export is to modify the source code | |
5603 | itself, so that when building the DLL each symbol to be exported is | |
5604 | declared as: | |
5605 | ||
5606 | @example | |
5607 | __declspec(dllexport) int a_variable | |
5608 | __declspec(dllexport) void a_function(int with_args) | |
5609 | @end example | |
5610 | ||
5611 | All such symbols will be exported from the DLL. If, however, | |
5612 | any of the object files in the DLL contain symbols decorated in | |
5613 | this way, then the normal auto-export behavior is disabled, unless | |
5614 | the @samp{--export-all-symbols} option is also used. | |
5615 | ||
5616 | Note that object files that wish to access these symbols must @emph{not} | |
5617 | decorate them with dllexport. Instead, they should use dllimport, | |
5618 | instead: | |
5619 | ||
5620 | @example | |
5621 | __declspec(dllimport) int a_variable | |
5622 | __declspec(dllimport) void a_function(int with_args) | |
5623 | @end example | |
5624 | ||
5625 | This complicates the structure of library header files, because | |
5626 | when included by the library itself the header must declare the | |
5627 | variables and functions as dllexport, but when included by client | |
5628 | code the header must declare them as dllimport. There are a number | |
5629 | of idioms that are typically used to do this; often client code can | |
5630 | omit the __declspec() declaration completely. See | |
5631 | @samp{--enable-auto-import} and @samp{automatic data imports} for more | |
5632 | imformation. | |
dc8465bf NC |
5633 | @end table |
5634 | ||
2ca22b03 NC |
5635 | @cindex automatic data imports |
5636 | @item automatic data imports | |
5637 | The standard Windows dll format supports data imports from dlls only | |
69da35b5 | 5638 | by adding special decorations (dllimport/dllexport), which let the |
2ca22b03 | 5639 | compiler produce specific assembler instructions to deal with this |
69da35b5 NC |
5640 | issue. This increases the effort necessary to port existing Un*x |
5641 | code to these platforms, especially for large | |
2ca22b03 | 5642 | c++ libraries and applications. The auto-import feature, which was |
69da35b5 NC |
5643 | initially provided by Paul Sokolovsky, allows one to omit the |
5644 | decorations to archieve a behavior that conforms to that on POSIX/Un*x | |
5645 | platforms. This feature is enabled with the @samp{--enable-auto-import} | |
5646 | command-line option, although it is enabled by default on cygwin/mingw. | |
5647 | The @samp{--enable-auto-import} option itself now serves mainly to | |
5648 | suppress any warnings that are ordinarily emitted when linked objects | |
5649 | trigger the feature's use. | |
5650 | ||
5651 | auto-import of variables does not always work flawlessly without | |
5652 | additional assistance. Sometimes, you will see this message | |
5653 | ||
5654 | "variable '<var>' can't be auto-imported. Please read the | |
5655 | documentation for ld's @code{--enable-auto-import} for details." | |
5656 | ||
5657 | The @samp{--enable-auto-import} documentation explains why this error | |
5658 | occurs, and several methods that can be used to overcome this difficulty. | |
5659 | One of these methods is the @emph{runtime pseudo-relocs} feature, described | |
5660 | below. | |
5661 | ||
5662 | @cindex runtime pseudo-relocation | |
5663 | For complex variables imported from DLLs (such as structs or classes), | |
5664 | object files typically contain a base address for the variable and an | |
5665 | offset (@emph{addend}) within the variable--to specify a particular | |
5666 | field or public member, for instance. Unfortunately, the runtime loader used | |
5667 | in win32 environments is incapable of fixing these references at runtime | |
5668 | without the additional information supplied by dllimport/dllexport decorations. | |
5669 | The standard auto-import feature described above is unable to resolve these | |
5670 | references. | |
5671 | ||
5672 | The @samp{--enable-runtime-pseudo-relocs} switch allows these references to | |
5673 | be resolved without error, while leaving the task of adjusting the references | |
5674 | themselves (with their non-zero addends) to specialized code provided by the | |
5675 | runtime environment. Recent versions of the cygwin and mingw environments and | |
5676 | compilers provide this runtime support; older versions do not. However, the | |
5677 | support is only necessary on the developer's platform; the compiled result will | |
5678 | run without error on an older system. | |
5679 | ||
5680 | @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly | |
5681 | enabled as needed. | |
2ca22b03 NC |
5682 | |
5683 | @cindex direct linking to a dll | |
5684 | @item direct linking to a dll | |
5685 | The cygwin/mingw ports of @command{ld} support the direct linking, | |
5686 | including data symbols, to a dll without the usage of any import | |
69da35b5 NC |
5687 | libraries. This is much faster and uses much less memory than does the |
5688 | traditional import library method, expecially when linking large | |
5689 | libraries or applications. When @command{ld} creates an import lib, each | |
5690 | function or variable exported from the dll is stored in its own bfd, even | |
5691 | though a single bfd could contain many exports. The overhead involved in | |
5692 | storing, loading, and processing so many bfd's is quite large, and explains the | |
5693 | tremendous time, memory, and storage needed to link against particularly | |
5694 | large or complex libraries when using import libs. | |
5695 | ||
5696 | Linking directly to a dll uses no extra command-line switches other than | |
5697 | @samp{-L} and @samp{-l}, because @command{ld} already searches for a number | |
5698 | of names to match each library. All that is needed from the developer's | |
5699 | perspective is an understanding of this search, in order to force ld to | |
5700 | select the dll instead of an import library. | |
5701 | ||
2ca22b03 | 5702 | |
69da35b5 NC |
5703 | For instance, when ld is called with the argument @samp{-lxxx} it will attempt |
5704 | to find, in the first directory of its search path, | |
2ca22b03 NC |
5705 | |
5706 | @example | |
5707 | libxxx.dll.a | |
5708 | xxx.dll.a | |
5709 | libxxx.a | |
69da35b5 | 5710 | cygxxx.dll (*) |
2ca22b03 NC |
5711 | libxxx.dll |
5712 | xxx.dll | |
5713 | @end example | |
5714 | ||
69da35b5 NC |
5715 | before moving on to the next directory in the search path. |
5716 | ||
5717 | (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll}, | |
5718 | where @samp{<prefix>} is set by the @command{ld} option | |
5719 | @samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec | |
5720 | file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for | |
5721 | @samp{cygxxx.dll}. | |
5722 | ||
5723 | Other win32-based unix environments, such as mingw or pw32, may use other | |
5724 | @samp{<prefix>}es, although at present only cygwin makes use of this feature. It | |
5725 | was originally intended to help avoid name conflicts among dll's built for the | |
5726 | various win32/un*x environments, so that (for example) two versions of a zlib dll | |
5727 | could coexist on the same machine. | |
5728 | ||
2ca22b03 NC |
5729 | The generic cygwin/mingw path layout uses a @samp{bin} directory for |
5730 | applications and dll's and a @samp{lib} directory for the import | |
69da35b5 | 5731 | libraries (using cygwin nomenclature): |
2ca22b03 NC |
5732 | |
5733 | @example | |
5734 | bin/ | |
5735 | cygxxx.dll | |
5736 | lib/ | |
5737 | libxxx.dll.a (in case of dll's) | |
5738 | libxxx.a (in case of static archive) | |
5739 | @end example | |
5740 | ||
69da35b5 NC |
5741 | Linking directly to a dll without using the import library can be |
5742 | done two ways: | |
2ca22b03 NC |
5743 | |
5744 | 1. Use the dll directly by adding the @samp{bin} path to the link line | |
5745 | @example | |
5746 | gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx | |
5747 | @end example | |
5748 | ||
69da35b5 NC |
5749 | However, as the dll's often have version numbers appended to their names |
5750 | (@samp{cygncurses-5.dll}) this will often fail, unless one specifies | |
5751 | @samp{-L../bin -lncurses-5} to include the version. Import libs are generally | |
5752 | not versioned, and do not have this difficulty. | |
5753 | ||
2ca22b03 NC |
5754 | 2. Create a symbolic link from the dll to a file in the @samp{lib} |
5755 | directory according to the above mentioned search pattern. This | |
5756 | should be used to avoid unwanted changes in the tools needed for | |
5757 | making the app/dll. | |
5758 | ||
5759 | @example | |
5760 | ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a] | |
5761 | @end example | |
5762 | ||
5763 | Then you can link without any make environment changes. | |
5764 | ||
5765 | @example | |
5766 | gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx | |
5767 | @end example | |
69da35b5 NC |
5768 | |
5769 | This technique also avoids the version number problems, because the following is | |
5770 | perfectly legal | |
5771 | ||
5772 | @example | |
5773 | bin/ | |
5774 | cygxxx-5.dll | |
5775 | lib/ | |
5776 | libxxx.dll.a -> ../bin/cygxxx-5.dll | |
5777 | @end example | |
5778 | ||
dc8465bf | 5779 | Linking directly to a dll without using an import lib will work |
69da35b5 NC |
5780 | even when auto-import features are exercised, and even when |
5781 | @samp{--enable-runtime-pseudo-relocs} is used. | |
5782 | ||
5783 | Given the improvements in speed and memory usage, one might justifiably | |
dc8465bf | 5784 | wonder why import libraries are used at all. There are two reasons: |
69da35b5 NC |
5785 | |
5786 | 1. Until recently, the link-directly-to-dll functionality did @emph{not} | |
5787 | work with auto-imported data. | |
5788 | ||
dc8465bf NC |
5789 | 2. Sometimes it is necessary to include pure static objects within the |
5790 | import library (which otherwise contains only bfd's for indirection | |
5791 | symbols that point to the exports of a dll). Again, the import lib | |
5792 | for the cygwin kernel makes use of this ability, and it is not | |
5793 | possible to do this without an import lib. | |
69da35b5 NC |
5794 | |
5795 | So, import libs are not going away. But the ability to replace | |
5796 | true import libs with a simple symbolic link to (or a copy of) | |
5797 | a dll, in most cases, is a useful addition to the suite of tools | |
5798 | binutils makes available to the win32 developer. Given the | |
5799 | massive improvements in memory requirements during linking, storage | |
5800 | requirements, and linking speed, we expect that many developers | |
5801 | will soon begin to use this feature whenever possible. | |
dc8465bf NC |
5802 | |
5803 | @item symbol aliasing | |
5804 | @table @emph | |
5805 | @item adding additional names | |
5806 | Sometimes, it is useful to export symbols with additional names. | |
5807 | A symbol @samp{foo} will be exported as @samp{foo}, but it can also be | |
5808 | exported as @samp{_foo} by using special directives in the DEF file | |
5809 | when creating the dll. This will affect also the optional created | |
5810 | import library. Consider the following DEF file: | |
5811 | ||
5812 | @example | |
5813 | LIBRARY "xyz.dll" BASE=0x61000000 | |
5814 | ||
5815 | EXPORTS | |
5816 | foo | |
5817 | _foo = foo | |
5818 | @end example | |
5819 | ||
5820 | The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}. | |
5821 | ||
5822 | Another method for creating a symbol alias is to create it in the | |
5823 | source code using the "weak" attribute: | |
5824 | ||
5825 | @example | |
5826 | void foo () @{ /* Do something. */; @} | |
5827 | void _foo () __attribute__ ((weak, alias ("foo"))); | |
5828 | @end example | |
5829 | ||
5830 | See the gcc manual for more information about attributes and weak | |
5831 | symbols. | |
5832 | ||
5833 | @item renaming symbols | |
5834 | Sometimes it is useful to rename exports. For instance, the cygwin | |
5835 | kernel does this regularly. A symbol @samp{_foo} can be exported as | |
5836 | @samp{foo} but not as @samp{_foo} by using special directives in the | |
5837 | DEF file. (This will also affect the import library, if it is | |
5838 | created). In the following example: | |
5839 | ||
5840 | @example | |
5841 | LIBRARY "xyz.dll" BASE=0x61000000 | |
5842 | ||
5843 | EXPORTS | |
5844 | _foo = foo | |
5845 | @end example | |
5846 | ||
5847 | The line @samp{_foo = foo} maps the exported symbol @samp{foo} to | |
5848 | @samp{_foo}. | |
5849 | @end table | |
5850 | ||
0a5d968e NC |
5851 | Note: using a DEF file disables the default auto-export behavior, |
5852 | unless the @samp{--export-all-symbols} command line option is used. | |
5853 | If, however, you are trying to rename symbols, then you should list | |
5854 | @emph{all} desired exports in the DEF file, including the symbols | |
5855 | that are not being renamed, and do @emph{not} use the | |
5856 | @samp{--export-all-symbols} option. If you list only the | |
5857 | renamed symbols in the DEF file, and use @samp{--export-all-symbols} | |
5858 | to handle the other symbols, then the both the new names @emph{and} | |
1be59579 | 5859 | the original names for the renamed symbols will be exported. |
0a5d968e NC |
5860 | In effect, you'd be aliasing those symbols, not renaming them, |
5861 | which is probably not what you wanted. | |
c87db184 CF |
5862 | |
5863 | @cindex weak externals | |
5864 | @item weak externals | |
5865 | The Windows object format, PE, specifies a form of weak symbols called | |
5866 | weak externals. When a weak symbol is linked and the symbol is not | |
5867 | defined, the weak symbol becomes an alias for some other symbol. There | |
5868 | are three variants of weak externals: | |
5869 | @itemize | |
5870 | @item Definition is searched for in objects and libraries, historically | |
5871 | called lazy externals. | |
5872 | @item Definition is searched for only in other objects, not in libraries. | |
5873 | This form is not presently implemented. | |
5874 | @item No search; the symbol is an alias. This form is not presently | |
5875 | implemented. | |
5876 | @end itemize | |
5877 | As a GNU extension, weak symbols that do not specify an alternate symbol | |
5878 | are supported. If the symbol is undefined when linking, the symbol | |
5879 | uses a default value. | |
2ca22b03 NC |
5880 | @end table |
5881 | ||
5882 | @ifclear GENERIC | |
5883 | @lowersections | |
5884 | @end ifclear | |
5885 | @end ifset | |
5886 | ||
e0001a05 NC |
5887 | @ifset XTENSA |
5888 | @ifclear GENERIC | |
5889 | @raisesections | |
5890 | @end ifclear | |
5891 | ||
5892 | @node Xtensa | |
5893 | @section @code{ld} and Xtensa Processors | |
5894 | ||
5895 | @cindex Xtensa processors | |
5896 | The default @command{ld} behavior for Xtensa processors is to interpret | |
5897 | @code{SECTIONS} commands so that lists of explicitly named sections in a | |
5898 | specification with a wildcard file will be interleaved when necessary to | |
5899 | keep literal pools within the range of PC-relative load offsets. For | |
5900 | example, with the command: | |
5901 | ||
5902 | @smallexample | |
5903 | SECTIONS | |
5904 | @{ | |
5905 | .text : @{ | |
5906 | *(.literal .text) | |
5907 | @} | |
5908 | @} | |
5909 | @end smallexample | |
5910 | ||
5911 | @noindent | |
5912 | @command{ld} may interleave some of the @code{.literal} | |
5913 | and @code{.text} sections from different object files to ensure that the | |
5914 | literal pools are within the range of PC-relative load offsets. A valid | |
5915 | interleaving might place the @code{.literal} sections from an initial | |
5916 | group of files followed by the @code{.text} sections of that group of | |
5917 | files. Then, the @code{.literal} sections from the rest of the files | |
5918 | and the @code{.text} sections from the rest of the files would follow. | |
e0001a05 | 5919 | |
43cd72b9 | 5920 | @cindex @option{--relax} on Xtensa |
e0001a05 | 5921 | @cindex relaxing on Xtensa |
43cd72b9 BW |
5922 | Relaxation is enabled by default for the Xtensa version of @command{ld} and |
5923 | provides two important link-time optimizations. The first optimization | |
5924 | is to combine identical literal values to reduce code size. A redundant | |
5925 | literal will be removed and all the @code{L32R} instructions that use it | |
5926 | will be changed to reference an identical literal, as long as the | |
5927 | location of the replacement literal is within the offset range of all | |
5928 | the @code{L32R} instructions. The second optimization is to remove | |
5929 | unnecessary overhead from assembler-generated ``longcall'' sequences of | |
5930 | @code{L32R}/@code{CALLX@var{n}} when the target functions are within | |
5931 | range of direct @code{CALL@var{n}} instructions. | |
5932 | ||
5933 | For each of these cases where an indirect call sequence can be optimized | |
5934 | to a direct call, the linker will change the @code{CALLX@var{n}} | |
5935 | instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R} | |
5936 | instruction, and remove the literal referenced by the @code{L32R} | |
5937 | instruction if it is not used for anything else. Removing the | |
5938 | @code{L32R} instruction always reduces code size but can potentially | |
5939 | hurt performance by changing the alignment of subsequent branch targets. | |
5940 | By default, the linker will always preserve alignments, either by | |
5941 | switching some instructions between 24-bit encodings and the equivalent | |
5942 | density instructions or by inserting a no-op in place of the @code{L32R} | |
5943 | instruction that was removed. If code size is more important than | |
5944 | performance, the @option{--size-opt} option can be used to prevent the | |
5945 | linker from widening density instructions or inserting no-ops, except in | |
5946 | a few cases where no-ops are required for correctness. | |
5947 | ||
5948 | The following Xtensa-specific command-line options can be used to | |
5949 | control the linker: | |
5950 | ||
5951 | @cindex Xtensa options | |
5952 | @table @option | |
e0001a05 | 5953 | @kindex --no-relax |
43cd72b9 BW |
5954 | @item --no-relax |
5955 | Since the Xtensa version of @code{ld} enables the @option{--relax} option | |
5956 | by default, the @option{--no-relax} option is provided to disable | |
5957 | relaxation. | |
5958 | ||
5959 | @item --size-opt | |
5960 | When optimizing indirect calls to direct calls, optimize for code size | |
5961 | more than performance. With this option, the linker will not insert | |
5962 | no-ops or widen density instructions to preserve branch target | |
5963 | alignment. There may still be some cases where no-ops are required to | |
5964 | preserve the correctness of the code. | |
5965 | @end table | |
e0001a05 NC |
5966 | |
5967 | @ifclear GENERIC | |
5968 | @lowersections | |
5969 | @end ifclear | |
5970 | @end ifset | |
5971 | ||
252b5132 RH |
5972 | @ifclear SingleFormat |
5973 | @node BFD | |
5974 | @chapter BFD | |
5975 | ||
5976 | @cindex back end | |
5977 | @cindex object file management | |
5978 | @cindex object formats available | |
5979 | @kindex objdump -i | |
5980 | The linker accesses object and archive files using the BFD libraries. | |
5981 | These libraries allow the linker to use the same routines to operate on | |
5982 | object files whatever the object file format. A different object file | |
5983 | format can be supported simply by creating a new BFD back end and adding | |
5984 | it to the library. To conserve runtime memory, however, the linker and | |
5985 | associated tools are usually configured to support only a subset of the | |
5986 | object file formats available. You can use @code{objdump -i} | |
5987 | (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to | |
5988 | list all the formats available for your configuration. | |
5989 | ||
5990 | @cindex BFD requirements | |
5991 | @cindex requirements for BFD | |
5992 | As with most implementations, BFD is a compromise between | |
5993 | several conflicting requirements. The major factor influencing | |
5994 | BFD design was efficiency: any time used converting between | |
5995 | formats is time which would not have been spent had BFD not | |
5996 | been involved. This is partly offset by abstraction payback; since | |
5997 | BFD simplifies applications and back ends, more time and care | |
5998 | may be spent optimizing algorithms for a greater speed. | |
5999 | ||
6000 | One minor artifact of the BFD solution which you should bear in | |
6001 | mind is the potential for information loss. There are two places where | |
6002 | useful information can be lost using the BFD mechanism: during | |
6003 | conversion and during output. @xref{BFD information loss}. | |
6004 | ||
6005 | @menu | |
6006 | * BFD outline:: How it works: an outline of BFD | |
6007 | @end menu | |
6008 | ||
6009 | @node BFD outline | |
36f63dca | 6010 | @section How It Works: An Outline of BFD |
252b5132 RH |
6011 | @cindex opening object files |
6012 | @include bfdsumm.texi | |
6013 | @end ifclear | |
6014 | ||
6015 | @node Reporting Bugs | |
6016 | @chapter Reporting Bugs | |
ff5dcc92 SC |
6017 | @cindex bugs in @command{ld} |
6018 | @cindex reporting bugs in @command{ld} | |
252b5132 | 6019 | |
ff5dcc92 | 6020 | Your bug reports play an essential role in making @command{ld} reliable. |
252b5132 RH |
6021 | |
6022 | Reporting a bug may help you by bringing a solution to your problem, or | |
6023 | it may not. But in any case the principal function of a bug report is | |
ff5dcc92 | 6024 | to help the entire community by making the next version of @command{ld} |
252b5132 | 6025 | work better. Bug reports are your contribution to the maintenance of |
ff5dcc92 | 6026 | @command{ld}. |
252b5132 RH |
6027 | |
6028 | In order for a bug report to serve its purpose, you must include the | |
6029 | information that enables us to fix the bug. | |
6030 | ||
6031 | @menu | |
6032 | * Bug Criteria:: Have you found a bug? | |
6033 | * Bug Reporting:: How to report bugs | |
6034 | @end menu | |
6035 | ||
6036 | @node Bug Criteria | |
36f63dca | 6037 | @section Have You Found a Bug? |
252b5132 RH |
6038 | @cindex bug criteria |
6039 | ||
6040 | If you are not sure whether you have found a bug, here are some guidelines: | |
6041 | ||
6042 | @itemize @bullet | |
6043 | @cindex fatal signal | |
6044 | @cindex linker crash | |
6045 | @cindex crash of linker | |
6046 | @item | |
6047 | If the linker gets a fatal signal, for any input whatever, that is a | |
ff5dcc92 | 6048 | @command{ld} bug. Reliable linkers never crash. |
252b5132 RH |
6049 | |
6050 | @cindex error on valid input | |
6051 | @item | |
ff5dcc92 | 6052 | If @command{ld} produces an error message for valid input, that is a bug. |
252b5132 RH |
6053 | |
6054 | @cindex invalid input | |
6055 | @item | |
ff5dcc92 | 6056 | If @command{ld} does not produce an error message for invalid input, that |
252b5132 RH |
6057 | may be a bug. In the general case, the linker can not verify that |
6058 | object files are correct. | |
6059 | ||
6060 | @item | |
6061 | If you are an experienced user of linkers, your suggestions for | |
ff5dcc92 | 6062 | improvement of @command{ld} are welcome in any case. |
252b5132 RH |
6063 | @end itemize |
6064 | ||
6065 | @node Bug Reporting | |
36f63dca | 6066 | @section How to Report Bugs |
252b5132 | 6067 | @cindex bug reports |
ff5dcc92 | 6068 | @cindex @command{ld} bugs, reporting |
252b5132 RH |
6069 | |
6070 | A number of companies and individuals offer support for @sc{gnu} | |
ff5dcc92 | 6071 | products. If you obtained @command{ld} from a support organization, we |
252b5132 RH |
6072 | recommend you contact that organization first. |
6073 | ||
6074 | You can find contact information for many support companies and | |
6075 | individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs | |
6076 | distribution. | |
6077 | ||
ff5dcc92 | 6078 | Otherwise, send bug reports for @command{ld} to |
d7ed7ca6 | 6079 | @samp{bug-binutils@@gnu.org}. |
252b5132 RH |
6080 | |
6081 | The fundamental principle of reporting bugs usefully is this: | |
6082 | @strong{report all the facts}. If you are not sure whether to state a | |
6083 | fact or leave it out, state it! | |
6084 | ||
6085 | Often people omit facts because they think they know what causes the | |
6086 | problem and assume that some details do not matter. Thus, you might | |
b553b183 NC |
6087 | assume that the name of a symbol you use in an example does not |
6088 | matter. Well, probably it does not, but one cannot be sure. Perhaps | |
6089 | the bug is a stray memory reference which happens to fetch from the | |
6090 | location where that name is stored in memory; perhaps, if the name | |
6091 | were different, the contents of that location would fool the linker | |
6092 | into doing the right thing despite the bug. Play it safe and give a | |
6093 | specific, complete example. That is the easiest thing for you to do, | |
6094 | and the most helpful. | |
6095 | ||
6096 | Keep in mind that the purpose of a bug report is to enable us to fix | |
6097 | the bug if it is new to us. Therefore, always write your bug reports | |
6098 | on the assumption that the bug has not been reported previously. | |
252b5132 RH |
6099 | |
6100 | Sometimes people give a few sketchy facts and ask, ``Does this ring a | |
36f63dca NC |
6101 | bell?'' This cannot help us fix a bug, so it is basically useless. We |
6102 | respond by asking for enough details to enable us to investigate. | |
6103 | You might as well expedite matters by sending them to begin with. | |
252b5132 RH |
6104 | |
6105 | To enable us to fix the bug, you should include all these things: | |
6106 | ||
6107 | @itemize @bullet | |
6108 | @item | |
ff5dcc92 | 6109 | The version of @command{ld}. @command{ld} announces it if you start it with |
252b5132 RH |
6110 | the @samp{--version} argument. |
6111 | ||
6112 | Without this, we will not know whether there is any point in looking for | |
ff5dcc92 | 6113 | the bug in the current version of @command{ld}. |
252b5132 RH |
6114 | |
6115 | @item | |
ff5dcc92 | 6116 | Any patches you may have applied to the @command{ld} source, including any |
252b5132 RH |
6117 | patches made to the @code{BFD} library. |
6118 | ||
6119 | @item | |
6120 | The type of machine you are using, and the operating system name and | |
6121 | version number. | |
6122 | ||
6123 | @item | |
ff5dcc92 | 6124 | What compiler (and its version) was used to compile @command{ld}---e.g. |
252b5132 RH |
6125 | ``@code{gcc-2.7}''. |
6126 | ||
6127 | @item | |
6128 | The command arguments you gave the linker to link your example and | |
6129 | observe the bug. To guarantee you will not omit something important, | |
6130 | list them all. A copy of the Makefile (or the output from make) is | |
6131 | sufficient. | |
6132 | ||
6133 | If we were to try to guess the arguments, we would probably guess wrong | |
6134 | and then we might not encounter the bug. | |
6135 | ||
6136 | @item | |
6137 | A complete input file, or set of input files, that will reproduce the | |
b553b183 NC |
6138 | bug. It is generally most helpful to send the actual object files |
6139 | provided that they are reasonably small. Say no more than 10K. For | |
6140 | bigger files you can either make them available by FTP or HTTP or else | |
6141 | state that you are willing to send the object file(s) to whomever | |
6142 | requests them. (Note - your email will be going to a mailing list, so | |
6143 | we do not want to clog it up with large attachments). But small | |
6144 | attachments are best. | |
252b5132 RH |
6145 | |
6146 | If the source files were assembled using @code{gas} or compiled using | |
6147 | @code{gcc}, then it may be OK to send the source files rather than the | |
6148 | object files. In this case, be sure to say exactly what version of | |
6149 | @code{gas} or @code{gcc} was used to produce the object files. Also say | |
6150 | how @code{gas} or @code{gcc} were configured. | |
6151 | ||
6152 | @item | |
6153 | A description of what behavior you observe that you believe is | |
6154 | incorrect. For example, ``It gets a fatal signal.'' | |
6155 | ||
ff5dcc92 | 6156 | Of course, if the bug is that @command{ld} gets a fatal signal, then we |
252b5132 RH |
6157 | will certainly notice it. But if the bug is incorrect output, we might |
6158 | not notice unless it is glaringly wrong. You might as well not give us | |
6159 | a chance to make a mistake. | |
6160 | ||
6161 | Even if the problem you experience is a fatal signal, you should still | |
6162 | say so explicitly. Suppose something strange is going on, such as, your | |
ff5dcc92 | 6163 | copy of @command{ld} is out of synch, or you have encountered a bug in the |
252b5132 RH |
6164 | C library on your system. (This has happened!) Your copy might crash |
6165 | and ours would not. If you told us to expect a crash, then when ours | |
6166 | fails to crash, we would know that the bug was not happening for us. If | |
6167 | you had not told us to expect a crash, then we would not be able to draw | |
6168 | any conclusion from our observations. | |
6169 | ||
6170 | @item | |
ff5dcc92 | 6171 | If you wish to suggest changes to the @command{ld} source, send us context |
252b5132 RH |
6172 | diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or |
6173 | @samp{-p} option. Always send diffs from the old file to the new file. | |
ff5dcc92 | 6174 | If you even discuss something in the @command{ld} source, refer to it by |
252b5132 RH |
6175 | context, not by line number. |
6176 | ||
6177 | The line numbers in our development sources will not match those in your | |
6178 | sources. Your line numbers would convey no useful information to us. | |
6179 | @end itemize | |
6180 | ||
6181 | Here are some things that are not necessary: | |
6182 | ||
6183 | @itemize @bullet | |
6184 | @item | |
6185 | A description of the envelope of the bug. | |
6186 | ||
6187 | Often people who encounter a bug spend a lot of time investigating | |
6188 | which changes to the input file will make the bug go away and which | |
6189 | changes will not affect it. | |
6190 | ||
6191 | This is often time consuming and not very useful, because the way we | |
6192 | will find the bug is by running a single example under the debugger | |
6193 | with breakpoints, not by pure deduction from a series of examples. | |
6194 | We recommend that you save your time for something else. | |
6195 | ||
6196 | Of course, if you can find a simpler example to report @emph{instead} | |
6197 | of the original one, that is a convenience for us. Errors in the | |
6198 | output will be easier to spot, running under the debugger will take | |
6199 | less time, and so on. | |
6200 | ||
6201 | However, simplification is not vital; if you do not want to do this, | |
6202 | report the bug anyway and send us the entire test case you used. | |
6203 | ||
6204 | @item | |
6205 | A patch for the bug. | |
6206 | ||
6207 | A patch for the bug does help us if it is a good one. But do not omit | |
6208 | the necessary information, such as the test case, on the assumption that | |
6209 | a patch is all we need. We might see problems with your patch and decide | |
6210 | to fix the problem another way, or we might not understand it at all. | |
6211 | ||
ff5dcc92 | 6212 | Sometimes with a program as complicated as @command{ld} it is very hard to |
252b5132 RH |
6213 | construct an example that will make the program follow a certain path |
6214 | through the code. If you do not send us the example, we will not be | |
6215 | able to construct one, so we will not be able to verify that the bug is | |
6216 | fixed. | |
6217 | ||
6218 | And if we cannot understand what bug you are trying to fix, or why your | |
6219 | patch should be an improvement, we will not install it. A test case will | |
6220 | help us to understand. | |
6221 | ||
6222 | @item | |
6223 | A guess about what the bug is or what it depends on. | |
6224 | ||
6225 | Such guesses are usually wrong. Even we cannot guess right about such | |
6226 | things without first using the debugger to find the facts. | |
6227 | @end itemize | |
6228 | ||
6229 | @node MRI | |
6230 | @appendix MRI Compatible Script Files | |
6231 | @cindex MRI compatibility | |
ff5dcc92 SC |
6232 | To aid users making the transition to @sc{gnu} @command{ld} from the MRI |
6233 | linker, @command{ld} can use MRI compatible linker scripts as an | |
252b5132 RH |
6234 | alternative to the more general-purpose linker scripting language |
6235 | described in @ref{Scripts}. MRI compatible linker scripts have a much | |
6236 | simpler command set than the scripting language otherwise used with | |
ff5dcc92 | 6237 | @command{ld}. @sc{gnu} @command{ld} supports the most commonly used MRI |
252b5132 RH |
6238 | linker commands; these commands are described here. |
6239 | ||
6240 | In general, MRI scripts aren't of much use with the @code{a.out} object | |
6241 | file format, since it only has three sections and MRI scripts lack some | |
6242 | features to make use of them. | |
6243 | ||
6244 | You can specify a file containing an MRI-compatible script using the | |
6245 | @samp{-c} command-line option. | |
6246 | ||
6247 | Each command in an MRI-compatible script occupies its own line; each | |
6248 | command line starts with the keyword that identifies the command (though | |
6249 | blank lines are also allowed for punctuation). If a line of an | |
ff5dcc92 | 6250 | MRI-compatible script begins with an unrecognized keyword, @command{ld} |
252b5132 RH |
6251 | issues a warning message, but continues processing the script. |
6252 | ||
6253 | Lines beginning with @samp{*} are comments. | |
6254 | ||
6255 | You can write these commands using all upper-case letters, or all | |
6256 | lower case; for example, @samp{chip} is the same as @samp{CHIP}. | |
6257 | The following list shows only the upper-case form of each command. | |
6258 | ||
6259 | @table @code | |
6260 | @cindex @code{ABSOLUTE} (MRI) | |
6261 | @item ABSOLUTE @var{secname} | |
6262 | @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname} | |
ff5dcc92 | 6263 | Normally, @command{ld} includes in the output file all sections from all |
252b5132 RH |
6264 | the input files. However, in an MRI-compatible script, you can use the |
6265 | @code{ABSOLUTE} command to restrict the sections that will be present in | |
6266 | your output program. If the @code{ABSOLUTE} command is used at all in a | |
6267 | script, then only the sections named explicitly in @code{ABSOLUTE} | |
6268 | commands will appear in the linker output. You can still use other | |
6269 | input sections (whatever you select on the command line, or using | |
6270 | @code{LOAD}) to resolve addresses in the output file. | |
6271 | ||
6272 | @cindex @code{ALIAS} (MRI) | |
6273 | @item ALIAS @var{out-secname}, @var{in-secname} | |
6274 | Use this command to place the data from input section @var{in-secname} | |
6275 | in a section called @var{out-secname} in the linker output file. | |
6276 | ||
6277 | @var{in-secname} may be an integer. | |
6278 | ||
6279 | @cindex @code{ALIGN} (MRI) | |
6280 | @item ALIGN @var{secname} = @var{expression} | |
6281 | Align the section called @var{secname} to @var{expression}. The | |
6282 | @var{expression} should be a power of two. | |
6283 | ||
6284 | @cindex @code{BASE} (MRI) | |
6285 | @item BASE @var{expression} | |
6286 | Use the value of @var{expression} as the lowest address (other than | |
6287 | absolute addresses) in the output file. | |
6288 | ||
6289 | @cindex @code{CHIP} (MRI) | |
6290 | @item CHIP @var{expression} | |
6291 | @itemx CHIP @var{expression}, @var{expression} | |
6292 | This command does nothing; it is accepted only for compatibility. | |
6293 | ||
6294 | @cindex @code{END} (MRI) | |
6295 | @item END | |
6296 | This command does nothing whatever; it's only accepted for compatibility. | |
6297 | ||
6298 | @cindex @code{FORMAT} (MRI) | |
6299 | @item FORMAT @var{output-format} | |
6300 | Similar to the @code{OUTPUT_FORMAT} command in the more general linker | |
a1ab1d2a | 6301 | language, but restricted to one of these output formats: |
252b5132 RH |
6302 | |
6303 | @enumerate | |
a1ab1d2a | 6304 | @item |
252b5132 RH |
6305 | S-records, if @var{output-format} is @samp{S} |
6306 | ||
6307 | @item | |
6308 | IEEE, if @var{output-format} is @samp{IEEE} | |
6309 | ||
6310 | @item | |
6311 | COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is | |
6312 | @samp{COFF} | |
6313 | @end enumerate | |
6314 | ||
6315 | @cindex @code{LIST} (MRI) | |
6316 | @item LIST @var{anything}@dots{} | |
6317 | Print (to the standard output file) a link map, as produced by the | |
ff5dcc92 | 6318 | @command{ld} command-line option @samp{-M}. |
252b5132 RH |
6319 | |
6320 | The keyword @code{LIST} may be followed by anything on the | |
6321 | same line, with no change in its effect. | |
6322 | ||
6323 | @cindex @code{LOAD} (MRI) | |
6324 | @item LOAD @var{filename} | |
6325 | @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename} | |
6326 | Include one or more object file @var{filename} in the link; this has the | |
ff5dcc92 | 6327 | same effect as specifying @var{filename} directly on the @command{ld} |
252b5132 RH |
6328 | command line. |
6329 | ||
6330 | @cindex @code{NAME} (MRI) | |
6331 | @item NAME @var{output-name} | |
ff5dcc92 | 6332 | @var{output-name} is the name for the program produced by @command{ld}; the |
252b5132 RH |
6333 | MRI-compatible command @code{NAME} is equivalent to the command-line |
6334 | option @samp{-o} or the general script language command @code{OUTPUT}. | |
6335 | ||
6336 | @cindex @code{ORDER} (MRI) | |
6337 | @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname} | |
6338 | @itemx ORDER @var{secname} @var{secname} @var{secname} | |
ff5dcc92 | 6339 | Normally, @command{ld} orders the sections in its output file in the |
252b5132 RH |
6340 | order in which they first appear in the input files. In an MRI-compatible |
6341 | script, you can override this ordering with the @code{ORDER} command. The | |
6342 | sections you list with @code{ORDER} will appear first in your output | |
6343 | file, in the order specified. | |
6344 | ||
6345 | @cindex @code{PUBLIC} (MRI) | |
6346 | @item PUBLIC @var{name}=@var{expression} | |
6347 | @itemx PUBLIC @var{name},@var{expression} | |
6348 | @itemx PUBLIC @var{name} @var{expression} | |
6349 | Supply a value (@var{expression}) for external symbol | |
6350 | @var{name} used in the linker input files. | |
6351 | ||
6352 | @cindex @code{SECT} (MRI) | |
6353 | @item SECT @var{secname}, @var{expression} | |
6354 | @itemx SECT @var{secname}=@var{expression} | |
6355 | @itemx SECT @var{secname} @var{expression} | |
6356 | You can use any of these three forms of the @code{SECT} command to | |
6357 | specify the start address (@var{expression}) for section @var{secname}. | |
6358 | If you have more than one @code{SECT} statement for the same | |
6359 | @var{secname}, only the @emph{first} sets the start address. | |
6360 | @end table | |
6361 | ||
36f63dca | 6362 | @include fdl.texi |
704c465c | 6363 | |
252b5132 RH |
6364 | @node Index |
6365 | @unnumbered Index | |
6366 | ||
6367 | @printindex cp | |
6368 | ||
6369 | @tex | |
6370 | % I think something like @colophon should be in texinfo. In the | |
6371 | % meantime: | |
6372 | \long\def\colophon{\hbox to0pt{}\vfill | |
6373 | \centerline{The body of this manual is set in} | |
6374 | \centerline{\fontname\tenrm,} | |
6375 | \centerline{with headings in {\bf\fontname\tenbf}} | |
6376 | \centerline{and examples in {\tt\fontname\tentt}.} | |
6377 | \centerline{{\it\fontname\tenit\/} and} | |
6378 | \centerline{{\sl\fontname\tensl\/}} | |
6379 | \centerline{are used for emphasis.}\vfill} | |
6380 | \page\colophon | |
6381 | % Blame: doc@cygnus.com, 28mar91. | |
6382 | @end tex | |
6383 | ||
6384 | ||
6385 | @contents | |
6386 | @bye |