3 @c Copyright 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
4 @c 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012
5 @c Free Software Foundation, Inc.
8 @include configdoc.texi
9 @c (configdoc.texi is generated by the Makefile)
15 @macro gcctabopt{body}
21 @c Configure for the generation of man pages
44 @dircategory Software development
46 * Ld: (ld). The GNU linker.
51 This file documents the @sc{gnu} linker LD
52 @ifset VERSION_PACKAGE
53 @value{VERSION_PACKAGE}
55 version @value{VERSION}.
57 Copyright @copyright{} 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
58 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2012 Free Software Foundation, Inc.
60 Permission is granted to copy, distribute and/or modify this document
61 under the terms of the GNU Free Documentation License, Version 1.3
62 or any later version published by the Free Software Foundation;
63 with no Invariant Sections, with no Front-Cover Texts, and with no
64 Back-Cover Texts. A copy of the license is included in the
65 section entitled ``GNU Free Documentation License''.
69 @setchapternewpage odd
70 @settitle The GNU linker
75 @ifset VERSION_PACKAGE
76 @subtitle @value{VERSION_PACKAGE}
78 @subtitle Version @value{VERSION}
79 @author Steve Chamberlain
80 @author Ian Lance Taylor
85 \hfill Red Hat Inc\par
86 \hfill nickc\@credhat.com, doc\@redhat.com\par
87 \hfill {\it The GNU linker}\par
88 \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par
90 \global\parindent=0pt % Steve likes it this way.
93 @vskip 0pt plus 1filll
94 @c man begin COPYRIGHT
95 Copyright @copyright{} 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
96 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2012 Free
97 Software Foundation, Inc.
99 Permission is granted to copy, distribute and/or modify this document
100 under the terms of the GNU Free Documentation License, Version 1.3
101 or any later version published by the Free Software Foundation;
102 with no Invariant Sections, with no Front-Cover Texts, and with no
103 Back-Cover Texts. A copy of the license is included in the
104 section entitled ``GNU Free Documentation License''.
110 @c FIXME: Talk about importance of *order* of args, cmds to linker!
115 This file documents the @sc{gnu} linker ld
116 @ifset VERSION_PACKAGE
117 @value{VERSION_PACKAGE}
119 version @value{VERSION}.
121 This document is distributed under the terms of the GNU Free
122 Documentation License version 1.3. A copy of the license is included
123 in the section entitled ``GNU Free Documentation License''.
126 * Overview:: Overview
127 * Invocation:: Invocation
128 * Scripts:: Linker Scripts
130 * Machine Dependent:: Machine Dependent Features
134 * H8/300:: ld and the H8/300
137 * Renesas:: ld and other Renesas micros
140 * i960:: ld and the Intel 960 family
143 * ARM:: ld and the ARM family
146 * HPPA ELF32:: ld and HPPA 32-bit ELF
149 * M68HC11/68HC12:: ld and the Motorola 68HC11 and 68HC12 families
152 * M68K:: ld and Motorola 68K family
155 * PowerPC ELF32:: ld and PowerPC 32-bit ELF Support
158 * PowerPC64 ELF64:: ld and PowerPC64 64-bit ELF Support
161 * SPU ELF:: ld and SPU ELF Support
164 * TI COFF:: ld and the TI COFF
167 * Win32:: ld and WIN32 (cygwin/mingw)
170 * Xtensa:: ld and Xtensa Processors
173 @ifclear SingleFormat
176 @c Following blank line required for remaining bug in makeinfo conds/menus
178 * Reporting Bugs:: Reporting Bugs
179 * MRI:: MRI Compatible Script Files
180 * GNU Free Documentation License:: GNU Free Documentation License
181 * LD Index:: LD Index
188 @cindex @sc{gnu} linker
189 @cindex what is this?
192 @c man begin SYNOPSIS
193 ld [@b{options}] @var{objfile} @dots{}
197 ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and
198 the Info entries for @file{binutils} and
203 @c man begin DESCRIPTION
205 @command{ld} combines a number of object and archive files, relocates
206 their data and ties up symbol references. Usually the last step in
207 compiling a program is to run @command{ld}.
209 @command{ld} accepts Linker Command Language files written in
210 a superset of AT&T's Link Editor Command Language syntax,
211 to provide explicit and total control over the linking process.
215 This man page does not describe the command language; see the
216 @command{ld} entry in @code{info} for full details on the command
217 language and on other aspects of the GNU linker.
220 @ifclear SingleFormat
221 This version of @command{ld} uses the general purpose BFD libraries
222 to operate on object files. This allows @command{ld} to read, combine, and
223 write object files in many different formats---for example, COFF or
224 @code{a.out}. Different formats may be linked together to produce any
225 available kind of object file. @xref{BFD}, for more information.
228 Aside from its flexibility, the @sc{gnu} linker is more helpful than other
229 linkers in providing diagnostic information. Many linkers abandon
230 execution immediately upon encountering an error; whenever possible,
231 @command{ld} continues executing, allowing you to identify other errors
232 (or, in some cases, to get an output file in spite of the error).
239 @c man begin DESCRIPTION
241 The @sc{gnu} linker @command{ld} is meant to cover a broad range of situations,
242 and to be as compatible as possible with other linkers. As a result,
243 you have many choices to control its behavior.
249 * Options:: Command Line Options
250 * Environment:: Environment Variables
254 @section Command Line Options
262 The linker supports a plethora of command-line options, but in actual
263 practice few of them are used in any particular context.
264 @cindex standard Unix system
265 For instance, a frequent use of @command{ld} is to link standard Unix
266 object files on a standard, supported Unix system. On such a system, to
267 link a file @code{hello.o}:
270 ld -o @var{output} /lib/crt0.o hello.o -lc
273 This tells @command{ld} to produce a file called @var{output} as the
274 result of linking the file @code{/lib/crt0.o} with @code{hello.o} and
275 the library @code{libc.a}, which will come from the standard search
276 directories. (See the discussion of the @samp{-l} option below.)
278 Some of the command-line options to @command{ld} may be specified at any
279 point in the command line. However, options which refer to files, such
280 as @samp{-l} or @samp{-T}, cause the file to be read at the point at
281 which the option appears in the command line, relative to the object
282 files and other file options. Repeating non-file options with a
283 different argument will either have no further effect, or override prior
284 occurrences (those further to the left on the command line) of that
285 option. Options which may be meaningfully specified more than once are
286 noted in the descriptions below.
289 Non-option arguments are object files or archives which are to be linked
290 together. They may follow, precede, or be mixed in with command-line
291 options, except that an object file argument may not be placed between
292 an option and its argument.
294 Usually the linker is invoked with at least one object file, but you can
295 specify other forms of binary input files using @samp{-l}, @samp{-R},
296 and the script command language. If @emph{no} binary input files at all
297 are specified, the linker does not produce any output, and issues the
298 message @samp{No input files}.
300 If the linker cannot recognize the format of an object file, it will
301 assume that it is a linker script. A script specified in this way
302 augments the main linker script used for the link (either the default
303 linker script or the one specified by using @samp{-T}). This feature
304 permits the linker to link against a file which appears to be an object
305 or an archive, but actually merely defines some symbol values, or uses
306 @code{INPUT} or @code{GROUP} to load other objects. Specifying a
307 script in this way merely augments the main linker script, with the
308 extra commands placed after the main script; use the @samp{-T} option
309 to replace the default linker script entirely, but note the effect of
310 the @code{INSERT} command. @xref{Scripts}.
312 For options whose names are a single letter,
313 option arguments must either follow the option letter without intervening
314 whitespace, or be given as separate arguments immediately following the
315 option that requires them.
317 For options whose names are multiple letters, either one dash or two can
318 precede the option name; for example, @samp{-trace-symbol} and
319 @samp{--trace-symbol} are equivalent. Note---there is one exception to
320 this rule. Multiple letter options that start with a lower case 'o' can
321 only be preceded by two dashes. This is to reduce confusion with the
322 @samp{-o} option. So for example @samp{-omagic} sets the output file
323 name to @samp{magic} whereas @samp{--omagic} sets the NMAGIC flag on the
326 Arguments to multiple-letter options must either be separated from the
327 option name by an equals sign, or be given as separate arguments
328 immediately following the option that requires them. For example,
329 @samp{--trace-symbol foo} and @samp{--trace-symbol=foo} are equivalent.
330 Unique abbreviations of the names of multiple-letter options are
333 Note---if the linker is being invoked indirectly, via a compiler driver
334 (e.g. @samp{gcc}) then all the linker command line options should be
335 prefixed by @samp{-Wl,} (or whatever is appropriate for the particular
336 compiler driver) like this:
339 gcc -Wl,--start-group foo.o bar.o -Wl,--end-group
342 This is important, because otherwise the compiler driver program may
343 silently drop the linker options, resulting in a bad link. Confusion
344 may also arise when passing options that require values through a
345 driver, as the use of a space between option and argument acts as
346 a separator, and causes the driver to pass only the option to the linker
347 and the argument to the compiler. In this case, it is simplest to use
348 the joined forms of both single- and multiple-letter options, such as:
351 gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map
354 Here is a table of the generic command line switches accepted by the GNU
358 @include at-file.texi
360 @kindex -a @var{keyword}
361 @item -a @var{keyword}
362 This option is supported for HP/UX compatibility. The @var{keyword}
363 argument must be one of the strings @samp{archive}, @samp{shared}, or
364 @samp{default}. @samp{-aarchive} is functionally equivalent to
365 @samp{-Bstatic}, and the other two keywords are functionally equivalent
366 to @samp{-Bdynamic}. This option may be used any number of times.
368 @kindex --audit @var{AUDITLIB}
369 @item --audit @var{AUDITLIB}
370 Adds @var{AUDITLIB} to the @code{DT_AUDIT} entry of the dynamic section.
371 @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME
372 specified in the library. If specified multiple times @code{DT_AUDIT}
373 will contain a colon separated list of audit interfaces to use. If the linker
374 finds an object with an audit entry while searching for shared libraries,
375 it will add a corresponding @code{DT_DEPAUDIT} entry in the output file.
376 This option is only meaningful on ELF platforms supporting the rtld-audit
380 @cindex architectures
381 @kindex -A @var{arch}
382 @item -A @var{architecture}
383 @kindex --architecture=@var{arch}
384 @itemx --architecture=@var{architecture}
385 In the current release of @command{ld}, this option is useful only for the
386 Intel 960 family of architectures. In that @command{ld} configuration, the
387 @var{architecture} argument identifies the particular architecture in
388 the 960 family, enabling some safeguards and modifying the
389 archive-library search path. @xref{i960,,@command{ld} and the Intel 960
390 family}, for details.
392 Future releases of @command{ld} may support similar functionality for
393 other architecture families.
396 @ifclear SingleFormat
397 @cindex binary input format
398 @kindex -b @var{format}
399 @kindex --format=@var{format}
402 @item -b @var{input-format}
403 @itemx --format=@var{input-format}
404 @command{ld} may be configured to support more than one kind of object
405 file. If your @command{ld} is configured this way, you can use the
406 @samp{-b} option to specify the binary format for input object files
407 that follow this option on the command line. Even when @command{ld} is
408 configured to support alternative object formats, you don't usually need
409 to specify this, as @command{ld} should be configured to expect as a
410 default input format the most usual format on each machine.
411 @var{input-format} is a text string, the name of a particular format
412 supported by the BFD libraries. (You can list the available binary
413 formats with @samp{objdump -i}.)
416 You may want to use this option if you are linking files with an unusual
417 binary format. You can also use @samp{-b} to switch formats explicitly (when
418 linking object files of different formats), by including
419 @samp{-b @var{input-format}} before each group of object files in a
422 The default format is taken from the environment variable
427 You can also define the input format from a script, using the command
430 see @ref{Format Commands}.
434 @kindex -c @var{MRI-cmdfile}
435 @kindex --mri-script=@var{MRI-cmdfile}
436 @cindex compatibility, MRI
437 @item -c @var{MRI-commandfile}
438 @itemx --mri-script=@var{MRI-commandfile}
439 For compatibility with linkers produced by MRI, @command{ld} accepts script
440 files written in an alternate, restricted command language, described in
442 @ref{MRI,,MRI Compatible Script Files}.
445 the MRI Compatible Script Files section of GNU ld documentation.
447 Introduce MRI script files with
448 the option @samp{-c}; use the @samp{-T} option to run linker
449 scripts written in the general-purpose @command{ld} scripting language.
450 If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories
451 specified by any @samp{-L} options.
453 @cindex common allocation
460 These three options are equivalent; multiple forms are supported for
461 compatibility with other linkers. They assign space to common symbols
462 even if a relocatable output file is specified (with @samp{-r}). The
463 script command @code{FORCE_COMMON_ALLOCATION} has the same effect.
464 @xref{Miscellaneous Commands}.
466 @kindex --depaudit @var{AUDITLIB}
467 @kindex -P @var{AUDITLIB}
468 @item --depaudit @var{AUDITLIB}
469 @itemx -P @var{AUDITLIB}
470 Adds @var{AUDITLIB} to the @code{DT_DEPAUDIT} entry of the dynamic section.
471 @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME
472 specified in the library. If specified multiple times @code{DT_DEPAUDIT}
473 will contain a colon separated list of audit interfaces to use. This
474 option is only meaningful on ELF platforms supporting the rtld-audit interface.
475 The -P option is provided for Solaris compatibility.
477 @cindex entry point, from command line
478 @kindex -e @var{entry}
479 @kindex --entry=@var{entry}
481 @itemx --entry=@var{entry}
482 Use @var{entry} as the explicit symbol for beginning execution of your
483 program, rather than the default entry point. If there is no symbol
484 named @var{entry}, the linker will try to parse @var{entry} as a number,
485 and use that as the entry address (the number will be interpreted in
486 base 10; you may use a leading @samp{0x} for base 16, or a leading
487 @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults
488 and other ways of specifying the entry point.
490 @kindex --exclude-libs
491 @item --exclude-libs @var{lib},@var{lib},...
492 Specifies a list of archive libraries from which symbols should not be automatically
493 exported. The library names may be delimited by commas or colons. Specifying
494 @code{--exclude-libs ALL} excludes symbols in all archive libraries from
495 automatic export. This option is available only for the i386 PE targeted
496 port of the linker and for ELF targeted ports. For i386 PE, symbols
497 explicitly listed in a .def file are still exported, regardless of this
498 option. For ELF targeted ports, symbols affected by this option will
499 be treated as hidden.
501 @kindex --exclude-modules-for-implib
502 @item --exclude-modules-for-implib @var{module},@var{module},...
503 Specifies a list of object files or archive members, from which symbols
504 should not be automatically exported, but which should be copied wholesale
505 into the import library being generated during the link. The module names
506 may be delimited by commas or colons, and must match exactly the filenames
507 used by @command{ld} to open the files; for archive members, this is simply
508 the member name, but for object files the name listed must include and
509 match precisely any path used to specify the input file on the linker's
510 command-line. This option is available only for the i386 PE targeted port
511 of the linker. Symbols explicitly listed in a .def file are still exported,
512 regardless of this option.
514 @cindex dynamic symbol table
516 @kindex --export-dynamic
517 @kindex --no-export-dynamic
519 @itemx --export-dynamic
520 @itemx --no-export-dynamic
521 When creating a dynamically linked executable, using the @option{-E}
522 option or the @option{--export-dynamic} option causes the linker to add
523 all symbols to the dynamic symbol table. The dynamic symbol table is the
524 set of symbols which are visible from dynamic objects at run time.
526 If you do not use either of these options (or use the
527 @option{--no-export-dynamic} option to restore the default behavior), the
528 dynamic symbol table will normally contain only those symbols which are
529 referenced by some dynamic object mentioned in the link.
531 If you use @code{dlopen} to load a dynamic object which needs to refer
532 back to the symbols defined by the program, rather than some other
533 dynamic object, then you will probably need to use this option when
534 linking the program itself.
536 You can also use the dynamic list to control what symbols should
537 be added to the dynamic symbol table if the output format supports it.
538 See the description of @samp{--dynamic-list}.
540 Note that this option is specific to ELF targeted ports. PE targets
541 support a similar function to export all symbols from a DLL or EXE; see
542 the description of @samp{--export-all-symbols} below.
544 @ifclear SingleFormat
545 @cindex big-endian objects
549 Link big-endian objects. This affects the default output format.
551 @cindex little-endian objects
554 Link little-endian objects. This affects the default output format.
557 @kindex -f @var{name}
558 @kindex --auxiliary=@var{name}
560 @itemx --auxiliary=@var{name}
561 When creating an ELF shared object, set the internal DT_AUXILIARY field
562 to the specified name. This tells the dynamic linker that the symbol
563 table of the shared object should be used as an auxiliary filter on the
564 symbol table of the shared object @var{name}.
566 If you later link a program against this filter object, then, when you
567 run the program, the dynamic linker will see the DT_AUXILIARY field. If
568 the dynamic linker resolves any symbols from the filter object, it will
569 first check whether there is a definition in the shared object
570 @var{name}. If there is one, it will be used instead of the definition
571 in the filter object. The shared object @var{name} need not exist.
572 Thus the shared object @var{name} may be used to provide an alternative
573 implementation of certain functions, perhaps for debugging or for
574 machine specific performance.
576 This option may be specified more than once. The DT_AUXILIARY entries
577 will be created in the order in which they appear on the command line.
579 @kindex -F @var{name}
580 @kindex --filter=@var{name}
582 @itemx --filter=@var{name}
583 When creating an ELF shared object, set the internal DT_FILTER field to
584 the specified name. This tells the dynamic linker that the symbol table
585 of the shared object which is being created should be used as a filter
586 on the symbol table of the shared object @var{name}.
588 If you later link a program against this filter object, then, when you
589 run the program, the dynamic linker will see the DT_FILTER field. The
590 dynamic linker will resolve symbols according to the symbol table of the
591 filter object as usual, but it will actually link to the definitions
592 found in the shared object @var{name}. Thus the filter object can be
593 used to select a subset of the symbols provided by the object
596 Some older linkers used the @option{-F} option throughout a compilation
597 toolchain for specifying object-file format for both input and output
599 @ifclear SingleFormat
600 The @sc{gnu} linker uses other mechanisms for this purpose: the
601 @option{-b}, @option{--format}, @option{--oformat} options, the
602 @code{TARGET} command in linker scripts, and the @code{GNUTARGET}
603 environment variable.
605 The @sc{gnu} linker will ignore the @option{-F} option when not
606 creating an ELF shared object.
608 @cindex finalization function
609 @kindex -fini=@var{name}
610 @item -fini=@var{name}
611 When creating an ELF executable or shared object, call NAME when the
612 executable or shared object is unloaded, by setting DT_FINI to the
613 address of the function. By default, the linker uses @code{_fini} as
614 the function to call.
618 Ignored. Provided for compatibility with other tools.
620 @kindex -G @var{value}
621 @kindex --gpsize=@var{value}
624 @itemx --gpsize=@var{value}
625 Set the maximum size of objects to be optimized using the GP register to
626 @var{size}. This is only meaningful for object file formats such as
627 MIPS ECOFF which supports putting large and small objects into different
628 sections. This is ignored for other object file formats.
630 @cindex runtime library name
631 @kindex -h @var{name}
632 @kindex -soname=@var{name}
634 @itemx -soname=@var{name}
635 When creating an ELF shared object, set the internal DT_SONAME field to
636 the specified name. When an executable is linked with a shared object
637 which has a DT_SONAME field, then when the executable is run the dynamic
638 linker will attempt to load the shared object specified by the DT_SONAME
639 field rather than the using the file name given to the linker.
642 @cindex incremental link
644 Perform an incremental link (same as option @samp{-r}).
646 @cindex initialization function
647 @kindex -init=@var{name}
648 @item -init=@var{name}
649 When creating an ELF executable or shared object, call NAME when the
650 executable or shared object is loaded, by setting DT_INIT to the address
651 of the function. By default, the linker uses @code{_init} as the
654 @cindex archive files, from cmd line
655 @kindex -l @var{namespec}
656 @kindex --library=@var{namespec}
657 @item -l @var{namespec}
658 @itemx --library=@var{namespec}
659 Add the archive or object file specified by @var{namespec} to the
660 list of files to link. This option may be used any number of times.
661 If @var{namespec} is of the form @file{:@var{filename}}, @command{ld}
662 will search the library path for a file called @var{filename}, otherwise it
663 will search the library path for a file called @file{lib@var{namespec}.a}.
665 On systems which support shared libraries, @command{ld} may also search for
666 files other than @file{lib@var{namespec}.a}. Specifically, on ELF
667 and SunOS systems, @command{ld} will search a directory for a library
668 called @file{lib@var{namespec}.so} before searching for one called
669 @file{lib@var{namespec}.a}. (By convention, a @code{.so} extension
670 indicates a shared library.) Note that this behavior does not apply
671 to @file{:@var{filename}}, which always specifies a file called
674 The linker will search an archive only once, at the location where it is
675 specified on the command line. If the archive defines a symbol which
676 was undefined in some object which appeared before the archive on the
677 command line, the linker will include the appropriate file(s) from the
678 archive. However, an undefined symbol in an object appearing later on
679 the command line will not cause the linker to search the archive again.
681 See the @option{-(} option for a way to force the linker to search
682 archives multiple times.
684 You may list the same archive multiple times on the command line.
687 This type of archive searching is standard for Unix linkers. However,
688 if you are using @command{ld} on AIX, note that it is different from the
689 behaviour of the AIX linker.
692 @cindex search directory, from cmd line
694 @kindex --library-path=@var{dir}
695 @item -L @var{searchdir}
696 @itemx --library-path=@var{searchdir}
697 Add path @var{searchdir} to the list of paths that @command{ld} will search
698 for archive libraries and @command{ld} control scripts. You may use this
699 option any number of times. The directories are searched in the order
700 in which they are specified on the command line. Directories specified
701 on the command line are searched before the default directories. All
702 @option{-L} options apply to all @option{-l} options, regardless of the
703 order in which the options appear. @option{-L} options do not affect
704 how @command{ld} searches for a linker script unless @option{-T}
707 If @var{searchdir} begins with @code{=}, then the @code{=} will be replaced
708 by the @dfn{sysroot prefix}, a path specified when the linker is configured.
711 The default set of paths searched (without being specified with
712 @samp{-L}) depends on which emulation mode @command{ld} is using, and in
713 some cases also on how it was configured. @xref{Environment}.
716 The paths can also be specified in a link script with the
717 @code{SEARCH_DIR} command. Directories specified this way are searched
718 at the point in which the linker script appears in the command line.
721 @kindex -m @var{emulation}
722 @item -m @var{emulation}
723 Emulate the @var{emulation} linker. You can list the available
724 emulations with the @samp{--verbose} or @samp{-V} options.
726 If the @samp{-m} option is not used, the emulation is taken from the
727 @code{LDEMULATION} environment variable, if that is defined.
729 Otherwise, the default emulation depends upon how the linker was
737 Print a link map to the standard output. A link map provides
738 information about the link, including the following:
742 Where object files are mapped into memory.
744 How common symbols are allocated.
746 All archive members included in the link, with a mention of the symbol
747 which caused the archive member to be brought in.
749 The values assigned to symbols.
751 Note - symbols whose values are computed by an expression which
752 involves a reference to a previous value of the same symbol may not
753 have correct result displayed in the link map. This is because the
754 linker discards intermediate results and only retains the final value
755 of an expression. Under such circumstances the linker will display
756 the final value enclosed by square brackets. Thus for example a
757 linker script containing:
765 will produce the following output in the link map if the @option{-M}
770 [0x0000000c] foo = (foo * 0x4)
771 [0x0000000c] foo = (foo + 0x8)
774 See @ref{Expressions} for more information about expressions in linker
779 @cindex read-only text
784 Turn off page alignment of sections, and disable linking against shared
785 libraries. If the output format supports Unix style magic numbers,
786 mark the output as @code{NMAGIC}.
790 @cindex read/write from cmd line
794 Set the text and data sections to be readable and writable. Also, do
795 not page-align the data segment, and disable linking against shared
796 libraries. If the output format supports Unix style magic numbers,
797 mark the output as @code{OMAGIC}. Note: Although a writable text section
798 is allowed for PE-COFF targets, it does not conform to the format
799 specification published by Microsoft.
804 This option negates most of the effects of the @option{-N} option. It
805 sets the text section to be read-only, and forces the data segment to
806 be page-aligned. Note - this option does not enable linking against
807 shared libraries. Use @option{-Bdynamic} for this.
809 @kindex -o @var{output}
810 @kindex --output=@var{output}
811 @cindex naming the output file
812 @item -o @var{output}
813 @itemx --output=@var{output}
814 Use @var{output} as the name for the program produced by @command{ld}; if this
815 option is not specified, the name @file{a.out} is used by default. The
816 script command @code{OUTPUT} can also specify the output file name.
818 @kindex -O @var{level}
819 @cindex generating optimized output
821 If @var{level} is a numeric values greater than zero @command{ld} optimizes
822 the output. This might take significantly longer and therefore probably
823 should only be enabled for the final binary. At the moment this
824 option only affects ELF shared library generation. Future releases of
825 the linker may make more use of this option. Also currently there is
826 no difference in the linker's behaviour for different non-zero values
827 of this option. Again this may change with future releases.
830 @kindex --emit-relocs
831 @cindex retain relocations in final executable
834 Leave relocation sections and contents in fully linked executables.
835 Post link analysis and optimization tools may need this information in
836 order to perform correct modifications of executables. This results
837 in larger executables.
839 This option is currently only supported on ELF platforms.
841 @kindex --force-dynamic
842 @cindex forcing the creation of dynamic sections
843 @item --force-dynamic
844 Force the output file to have dynamic sections. This option is specific
848 @cindex relocatable output
850 @kindex --relocatable
853 Generate relocatable output---i.e., generate an output file that can in
854 turn serve as input to @command{ld}. This is often called @dfn{partial
855 linking}. As a side effect, in environments that support standard Unix
856 magic numbers, this option also sets the output file's magic number to
858 @c ; see @option{-N}.
859 If this option is not specified, an absolute file is produced. When
860 linking C++ programs, this option @emph{will not} resolve references to
861 constructors; to do that, use @samp{-Ur}.
863 When an input file does not have the same format as the output file,
864 partial linking is only supported if that input file does not contain any
865 relocations. Different output formats can have further restrictions; for
866 example some @code{a.out}-based formats do not support partial linking
867 with input files in other formats at all.
869 This option does the same thing as @samp{-i}.
871 @kindex -R @var{file}
872 @kindex --just-symbols=@var{file}
873 @cindex symbol-only input
874 @item -R @var{filename}
875 @itemx --just-symbols=@var{filename}
876 Read symbol names and their addresses from @var{filename}, but do not
877 relocate it or include it in the output. This allows your output file
878 to refer symbolically to absolute locations of memory defined in other
879 programs. You may use this option more than once.
881 For compatibility with other ELF linkers, if the @option{-R} option is
882 followed by a directory name, rather than a file name, it is treated as
883 the @option{-rpath} option.
887 @cindex strip all symbols
890 Omit all symbol information from the output file.
893 @kindex --strip-debug
894 @cindex strip debugger symbols
897 Omit debugger symbol information (but not all symbols) from the output file.
901 @cindex input files, displaying
904 Print the names of the input files as @command{ld} processes them.
906 @kindex -T @var{script}
907 @kindex --script=@var{script}
909 @item -T @var{scriptfile}
910 @itemx --script=@var{scriptfile}
911 Use @var{scriptfile} as the linker script. This script replaces
912 @command{ld}'s default linker script (rather than adding to it), so
913 @var{commandfile} must specify everything necessary to describe the
914 output file. @xref{Scripts}. If @var{scriptfile} does not exist in
915 the current directory, @code{ld} looks for it in the directories
916 specified by any preceding @samp{-L} options. Multiple @samp{-T}
919 @kindex -dT @var{script}
920 @kindex --default-script=@var{script}
922 @item -dT @var{scriptfile}
923 @itemx --default-script=@var{scriptfile}
924 Use @var{scriptfile} as the default linker script. @xref{Scripts}.
926 This option is similar to the @option{--script} option except that
927 processing of the script is delayed until after the rest of the
928 command line has been processed. This allows options placed after the
929 @option{--default-script} option on the command line to affect the
930 behaviour of the linker script, which can be important when the linker
931 command line cannot be directly controlled by the user. (eg because
932 the command line is being constructed by another tool, such as
935 @kindex -u @var{symbol}
936 @kindex --undefined=@var{symbol}
937 @cindex undefined symbol
938 @item -u @var{symbol}
939 @itemx --undefined=@var{symbol}
940 Force @var{symbol} to be entered in the output file as an undefined
941 symbol. Doing this may, for example, trigger linking of additional
942 modules from standard libraries. @samp{-u} may be repeated with
943 different option arguments to enter additional undefined symbols. This
944 option is equivalent to the @code{EXTERN} linker script command.
949 For anything other than C++ programs, this option is equivalent to
950 @samp{-r}: it generates relocatable output---i.e., an output file that can in
951 turn serve as input to @command{ld}. When linking C++ programs, @samp{-Ur}
952 @emph{does} resolve references to constructors, unlike @samp{-r}.
953 It does not work to use @samp{-Ur} on files that were themselves linked
954 with @samp{-Ur}; once the constructor table has been built, it cannot
955 be added to. Use @samp{-Ur} only for the last partial link, and
956 @samp{-r} for the others.
958 @kindex --unique[=@var{SECTION}]
959 @item --unique[=@var{SECTION}]
960 Creates a separate output section for every input section matching
961 @var{SECTION}, or if the optional wildcard @var{SECTION} argument is
962 missing, for every orphan input section. An orphan section is one not
963 specifically mentioned in a linker script. You may use this option
964 multiple times on the command line; It prevents the normal merging of
965 input sections with the same name, overriding output section assignments
975 Display the version number for @command{ld}. The @option{-V} option also
976 lists the supported emulations.
979 @kindex --discard-all
980 @cindex deleting local symbols
983 Delete all local symbols.
986 @kindex --discard-locals
987 @cindex local symbols, deleting
989 @itemx --discard-locals
990 Delete all temporary local symbols. (These symbols start with
991 system-specific local label prefixes, typically @samp{.L} for ELF systems
992 or @samp{L} for traditional a.out systems.)
994 @kindex -y @var{symbol}
995 @kindex --trace-symbol=@var{symbol}
996 @cindex symbol tracing
997 @item -y @var{symbol}
998 @itemx --trace-symbol=@var{symbol}
999 Print the name of each linked file in which @var{symbol} appears. This
1000 option may be given any number of times. On many systems it is necessary
1001 to prepend an underscore.
1003 This option is useful when you have an undefined symbol in your link but
1004 don't know where the reference is coming from.
1006 @kindex -Y @var{path}
1008 Add @var{path} to the default library search path. This option exists
1009 for Solaris compatibility.
1011 @kindex -z @var{keyword}
1012 @item -z @var{keyword}
1013 The recognized keywords are:
1017 Combines multiple reloc sections and sorts them to make dynamic symbol
1018 lookup caching possible.
1021 Disallows undefined symbols in object files. Undefined symbols in
1022 shared libraries are still allowed.
1025 Marks the object as requiring executable stack.
1028 This option is only meaningful when building a shared object. It makes
1029 the symbols defined by this shared object available for symbol resolution
1030 of subsequently loaded libraries.
1033 This option is only meaningful when building a shared object.
1034 It marks the object so that its runtime initialization will occur
1035 before the runtime initialization of any other objects brought into
1036 the process at the same time. Similarly the runtime finalization of
1037 the object will occur after the runtime finalization of any other
1041 Marks the object that its symbol table interposes before all symbols
1042 but the primary executable.
1045 When generating an executable or shared library, mark it to tell the
1046 dynamic linker to defer function call resolution to the point when
1047 the function is called (lazy binding), rather than at load time.
1048 Lazy binding is the default.
1051 Marks the object that its filters be processed immediately at
1055 Allows multiple definitions.
1058 Disables multiple reloc sections combining.
1061 Disables production of copy relocs.
1064 Marks the object that the search for dependencies of this object will
1065 ignore any default library search paths.
1068 Marks the object shouldn't be unloaded at runtime.
1071 Marks the object not available to @code{dlopen}.
1074 Marks the object can not be dumped by @code{dldump}.
1077 Marks the object as not requiring executable stack.
1080 Don't create an ELF @code{PT_GNU_RELRO} segment header in the object.
1083 When generating an executable or shared library, mark it to tell the
1084 dynamic linker to resolve all symbols when the program is started, or
1085 when the shared library is linked to using dlopen, instead of
1086 deferring function call resolution to the point when the function is
1090 Marks the object may contain $ORIGIN.
1093 Create an ELF @code{PT_GNU_RELRO} segment header in the object.
1095 @item max-page-size=@var{value}
1096 Set the emulation maximum page size to @var{value}.
1098 @item common-page-size=@var{value}
1099 Set the emulation common page size to @var{value}.
1101 @item stack-size=@var{value}
1102 Specify a stack size for in an ELF @code{PT_GNU_STACK} segment.
1103 Specifying zero will override any default non-zero sized
1104 @code{PT_GNU_STACK} segment creation.
1108 Other keywords are ignored for Solaris compatibility.
1111 @cindex groups of archives
1112 @item -( @var{archives} -)
1113 @itemx --start-group @var{archives} --end-group
1114 The @var{archives} should be a list of archive files. They may be
1115 either explicit file names, or @samp{-l} options.
1117 The specified archives are searched repeatedly until no new undefined
1118 references are created. Normally, an archive is searched only once in
1119 the order that it is specified on the command line. If a symbol in that
1120 archive is needed to resolve an undefined symbol referred to by an
1121 object in an archive that appears later on the command line, the linker
1122 would not be able to resolve that reference. By grouping the archives,
1123 they all be searched repeatedly until all possible references are
1126 Using this option has a significant performance cost. It is best to use
1127 it only when there are unavoidable circular references between two or
1130 @kindex --accept-unknown-input-arch
1131 @kindex --no-accept-unknown-input-arch
1132 @item --accept-unknown-input-arch
1133 @itemx --no-accept-unknown-input-arch
1134 Tells the linker to accept input files whose architecture cannot be
1135 recognised. The assumption is that the user knows what they are doing
1136 and deliberately wants to link in these unknown input files. This was
1137 the default behaviour of the linker, before release 2.14. The default
1138 behaviour from release 2.14 onwards is to reject such input files, and
1139 so the @samp{--accept-unknown-input-arch} option has been added to
1140 restore the old behaviour.
1143 @kindex --no-as-needed
1145 @itemx --no-as-needed
1146 This option affects ELF DT_NEEDED tags for dynamic libraries mentioned
1147 on the command line after the @option{--as-needed} option. Normally
1148 the linker will add a DT_NEEDED tag for each dynamic library mentioned
1149 on the command line, regardless of whether the library is actually
1150 needed or not. @option{--as-needed} causes a DT_NEEDED tag to only be
1151 emitted for a library that satisfies an undefined symbol reference
1152 from a regular object file or, if the library is not found in the
1153 DT_NEEDED lists of other libraries linked up to that point, an
1154 undefined symbol reference from another dynamic library.
1155 @option{--no-as-needed} restores the default behaviour.
1157 @kindex --add-needed
1158 @kindex --no-add-needed
1160 @itemx --no-add-needed
1161 These two options have been deprecated because of the similarity of
1162 their names to the @option{--as-needed} and @option{--no-as-needed}
1163 options. They have been replaced by @option{--copy-dt-needed-entries}
1164 and @option{--no-copy-dt-needed-entries}.
1166 @kindex -assert @var{keyword}
1167 @item -assert @var{keyword}
1168 This option is ignored for SunOS compatibility.
1172 @kindex -call_shared
1176 Link against dynamic libraries. This is only meaningful on platforms
1177 for which shared libraries are supported. This option is normally the
1178 default on such platforms. The different variants of this option are
1179 for compatibility with various systems. You may use this option
1180 multiple times on the command line: it affects library searching for
1181 @option{-l} options which follow it.
1185 Set the @code{DF_1_GROUP} flag in the @code{DT_FLAGS_1} entry in the dynamic
1186 section. This causes the runtime linker to handle lookups in this
1187 object and its dependencies to be performed only inside the group.
1188 @option{--unresolved-symbols=report-all} is implied. This option is
1189 only meaningful on ELF platforms which support shared libraries.
1199 Do not link against shared libraries. This is only meaningful on
1200 platforms for which shared libraries are supported. The different
1201 variants of this option are for compatibility with various systems. You
1202 may use this option multiple times on the command line: it affects
1203 library searching for @option{-l} options which follow it. This
1204 option also implies @option{--unresolved-symbols=report-all}. This
1205 option can be used with @option{-shared}. Doing so means that a
1206 shared library is being created but that all of the library's external
1207 references must be resolved by pulling in entries from static
1212 When creating a shared library, bind references to global symbols to the
1213 definition within the shared library, if any. Normally, it is possible
1214 for a program linked against a shared library to override the definition
1215 within the shared library. This option is only meaningful on ELF
1216 platforms which support shared libraries.
1218 @kindex -Bsymbolic-functions
1219 @item -Bsymbolic-functions
1220 When creating a shared library, bind references to global function
1221 symbols to the definition within the shared library, if any.
1222 This option is only meaningful on ELF platforms which support shared
1225 @kindex --dynamic-list=@var{dynamic-list-file}
1226 @item --dynamic-list=@var{dynamic-list-file}
1227 Specify the name of a dynamic list file to the linker. This is
1228 typically used when creating shared libraries to specify a list of
1229 global symbols whose references shouldn't be bound to the definition
1230 within the shared library, or creating dynamically linked executables
1231 to specify a list of symbols which should be added to the symbol table
1232 in the executable. This option is only meaningful on ELF platforms
1233 which support shared libraries.
1235 The format of the dynamic list is the same as the version node without
1236 scope and node name. See @ref{VERSION} for more information.
1238 @kindex --dynamic-list-data
1239 @item --dynamic-list-data
1240 Include all global data symbols to the dynamic list.
1242 @kindex --dynamic-list-cpp-new
1243 @item --dynamic-list-cpp-new
1244 Provide the builtin dynamic list for C++ operator new and delete. It
1245 is mainly useful for building shared libstdc++.
1247 @kindex --dynamic-list-cpp-typeinfo
1248 @item --dynamic-list-cpp-typeinfo
1249 Provide the builtin dynamic list for C++ runtime type identification.
1251 @kindex --check-sections
1252 @kindex --no-check-sections
1253 @item --check-sections
1254 @itemx --no-check-sections
1255 Asks the linker @emph{not} to check section addresses after they have
1256 been assigned to see if there are any overlaps. Normally the linker will
1257 perform this check, and if it finds any overlaps it will produce
1258 suitable error messages. The linker does know about, and does make
1259 allowances for sections in overlays. The default behaviour can be
1260 restored by using the command line switch @option{--check-sections}.
1261 Section overlap is not usually checked for relocatable links. You can
1262 force checking in that case by using the @option{--check-sections}
1265 @kindex --copy-dt-needed-entries
1266 @kindex --no-copy-dt-needed-entries
1267 @item --copy-dt-needed-entries
1268 @itemx --no-copy-dt-needed-entries
1269 This option affects the treatment of dynamic libraries referred to
1270 by DT_NEEDED tags @emph{inside} ELF dynamic libraries mentioned on the
1271 command line. Normally the linker won't add a DT_NEEDED tag to the
1272 output binary for each library mentioned in a DT_NEEDED tag in an
1273 input dynamic library. With @option{--copy-dt-needed-entries}
1274 specified on the command line however any dynamic libraries that
1275 follow it will have their DT_NEEDED entries added. The default
1276 behaviour can be restored with @option{--no-copy-dt-needed-entries}.
1278 This option also has an effect on the resolution of symbols in dynamic
1279 libraries. With @option{--copy-dt-needed-entries} dynamic libraries
1280 mentioned on the command line will be recursively searched, following
1281 their DT_NEEDED tags to other libraries, in order to resolve symbols
1282 required by the output binary. With the default setting however
1283 the searching of dynamic libraries that follow it will stop with the
1284 dynamic library itself. No DT_NEEDED links will be traversed to resolve
1287 @cindex cross reference table
1290 Output a cross reference table. If a linker map file is being
1291 generated, the cross reference table is printed to the map file.
1292 Otherwise, it is printed on the standard output.
1294 The format of the table is intentionally simple, so that it may be
1295 easily processed by a script if necessary. The symbols are printed out,
1296 sorted by name. For each symbol, a list of file names is given. If the
1297 symbol is defined, the first file listed is the location of the
1298 definition. The remaining files contain references to the symbol.
1300 @cindex common allocation
1301 @kindex --no-define-common
1302 @item --no-define-common
1303 This option inhibits the assignment of addresses to common symbols.
1304 The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect.
1305 @xref{Miscellaneous Commands}.
1307 The @samp{--no-define-common} option allows decoupling
1308 the decision to assign addresses to Common symbols from the choice
1309 of the output file type; otherwise a non-Relocatable output type
1310 forces assigning addresses to Common symbols.
1311 Using @samp{--no-define-common} allows Common symbols that are referenced
1312 from a shared library to be assigned addresses only in the main program.
1313 This eliminates the unused duplicate space in the shared library,
1314 and also prevents any possible confusion over resolving to the wrong
1315 duplicate when there are many dynamic modules with specialized search
1316 paths for runtime symbol resolution.
1318 @cindex symbols, from command line
1319 @kindex --defsym=@var{symbol}=@var{exp}
1320 @item --defsym=@var{symbol}=@var{expression}
1321 Create a global symbol in the output file, containing the absolute
1322 address given by @var{expression}. You may use this option as many
1323 times as necessary to define multiple symbols in the command line. A
1324 limited form of arithmetic is supported for the @var{expression} in this
1325 context: you may give a hexadecimal constant or the name of an existing
1326 symbol, or use @code{+} and @code{-} to add or subtract hexadecimal
1327 constants or symbols. If you need more elaborate expressions, consider
1328 using the linker command language from a script (@pxref{Assignments,,
1329 Assignment: Symbol Definitions}). @emph{Note:} there should be no white
1330 space between @var{symbol}, the equals sign (``@key{=}''), and
1333 @cindex demangling, from command line
1334 @kindex --demangle[=@var{style}]
1335 @kindex --no-demangle
1336 @item --demangle[=@var{style}]
1337 @itemx --no-demangle
1338 These options control whether to demangle symbol names in error messages
1339 and other output. When the linker is told to demangle, it tries to
1340 present symbol names in a readable fashion: it strips leading
1341 underscores if they are used by the object file format, and converts C++
1342 mangled symbol names into user readable names. Different compilers have
1343 different mangling styles. The optional demangling style argument can be used
1344 to choose an appropriate demangling style for your compiler. The linker will
1345 demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE}
1346 is set. These options may be used to override the default.
1348 @cindex dynamic linker, from command line
1349 @kindex -I@var{file}
1350 @kindex --dynamic-linker=@var{file}
1352 @itemx --dynamic-linker=@var{file}
1353 Set the name of the dynamic linker. This is only meaningful when
1354 generating dynamically linked ELF executables. The default dynamic
1355 linker is normally correct; don't use this unless you know what you are
1358 @kindex --fatal-warnings
1359 @kindex --no-fatal-warnings
1360 @item --fatal-warnings
1361 @itemx --no-fatal-warnings
1362 Treat all warnings as errors. The default behaviour can be restored
1363 with the option @option{--no-fatal-warnings}.
1365 @kindex --force-exe-suffix
1366 @item --force-exe-suffix
1367 Make sure that an output file has a .exe suffix.
1369 If a successfully built fully linked output file does not have a
1370 @code{.exe} or @code{.dll} suffix, this option forces the linker to copy
1371 the output file to one of the same name with a @code{.exe} suffix. This
1372 option is useful when using unmodified Unix makefiles on a Microsoft
1373 Windows host, since some versions of Windows won't run an image unless
1374 it ends in a @code{.exe} suffix.
1376 @kindex --gc-sections
1377 @kindex --no-gc-sections
1378 @cindex garbage collection
1380 @itemx --no-gc-sections
1381 Enable garbage collection of unused input sections. It is ignored on
1382 targets that do not support this option. The default behaviour (of not
1383 performing this garbage collection) can be restored by specifying
1384 @samp{--no-gc-sections} on the command line.
1386 @samp{--gc-sections} decides which input sections are used by
1387 examining symbols and relocations. The section containing the entry
1388 symbol and all sections containing symbols undefined on the
1389 command-line will be kept, as will sections containing symbols
1390 referenced by dynamic objects. Note that when building shared
1391 libraries, the linker must assume that any visible symbol is
1392 referenced. Once this initial set of sections has been determined,
1393 the linker recursively marks as used any section referenced by their
1394 relocations. See @samp{--entry} and @samp{--undefined}.
1396 This option can be set when doing a partial link (enabled with option
1397 @samp{-r}). In this case the root of symbols kept must be explicitly
1398 specified either by an @samp{--entry} or @samp{--undefined} option or by
1399 a @code{ENTRY} command in the linker script.
1401 @kindex --print-gc-sections
1402 @kindex --no-print-gc-sections
1403 @cindex garbage collection
1404 @item --print-gc-sections
1405 @itemx --no-print-gc-sections
1406 List all sections removed by garbage collection. The listing is
1407 printed on stderr. This option is only effective if garbage
1408 collection has been enabled via the @samp{--gc-sections}) option. The
1409 default behaviour (of not listing the sections that are removed) can
1410 be restored by specifying @samp{--no-print-gc-sections} on the command
1413 @kindex --print-output-format
1414 @cindex output format
1415 @item --print-output-format
1416 Print the name of the default output format (perhaps influenced by
1417 other command-line options). This is the string that would appear
1418 in an @code{OUTPUT_FORMAT} linker script command (@pxref{File Commands}).
1424 Print a summary of the command-line options on the standard output and exit.
1426 @kindex --target-help
1428 Print a summary of all target specific options on the standard output and exit.
1430 @kindex -Map=@var{mapfile}
1431 @item -Map=@var{mapfile}
1432 Print a link map to the file @var{mapfile}. See the description of the
1433 @option{-M} option, above.
1435 @cindex memory usage
1436 @kindex --no-keep-memory
1437 @item --no-keep-memory
1438 @command{ld} normally optimizes for speed over memory usage by caching the
1439 symbol tables of input files in memory. This option tells @command{ld} to
1440 instead optimize for memory usage, by rereading the symbol tables as
1441 necessary. This may be required if @command{ld} runs out of memory space
1442 while linking a large executable.
1444 @kindex --no-undefined
1446 @item --no-undefined
1448 Report unresolved symbol references from regular object files. This
1449 is done even if the linker is creating a non-symbolic shared library.
1450 The switch @option{--[no-]allow-shlib-undefined} controls the
1451 behaviour for reporting unresolved references found in shared
1452 libraries being linked in.
1454 @kindex --allow-multiple-definition
1456 @item --allow-multiple-definition
1458 Normally when a symbol is defined multiple times, the linker will
1459 report a fatal error. These options allow multiple definitions and the
1460 first definition will be used.
1462 @kindex --allow-shlib-undefined
1463 @kindex --no-allow-shlib-undefined
1464 @item --allow-shlib-undefined
1465 @itemx --no-allow-shlib-undefined
1466 Allows or disallows undefined symbols in shared libraries.
1467 This switch is similar to @option{--no-undefined} except that it
1468 determines the behaviour when the undefined symbols are in a
1469 shared library rather than a regular object file. It does not affect
1470 how undefined symbols in regular object files are handled.
1472 The default behaviour is to report errors for any undefined symbols
1473 referenced in shared libraries if the linker is being used to create
1474 an executable, but to allow them if the linker is being used to create
1477 The reasons for allowing undefined symbol references in shared
1478 libraries specified at link time are that:
1482 A shared library specified at link time may not be the same as the one
1483 that is available at load time, so the symbol might actually be
1484 resolvable at load time.
1486 There are some operating systems, eg BeOS and HPPA, where undefined
1487 symbols in shared libraries are normal.
1489 The BeOS kernel for example patches shared libraries at load time to
1490 select whichever function is most appropriate for the current
1491 architecture. This is used, for example, to dynamically select an
1492 appropriate memset function.
1495 @kindex --no-undefined-version
1496 @item --no-undefined-version
1497 Normally when a symbol has an undefined version, the linker will ignore
1498 it. This option disallows symbols with undefined version and a fatal error
1499 will be issued instead.
1501 @kindex --default-symver
1502 @item --default-symver
1503 Create and use a default symbol version (the soname) for unversioned
1506 @kindex --default-imported-symver
1507 @item --default-imported-symver
1508 Create and use a default symbol version (the soname) for unversioned
1511 @kindex --no-warn-mismatch
1512 @item --no-warn-mismatch
1513 Normally @command{ld} will give an error if you try to link together input
1514 files that are mismatched for some reason, perhaps because they have
1515 been compiled for different processors or for different endiannesses.
1516 This option tells @command{ld} that it should silently permit such possible
1517 errors. This option should only be used with care, in cases when you
1518 have taken some special action that ensures that the linker errors are
1521 @kindex --no-warn-search-mismatch
1522 @item --no-warn-search-mismatch
1523 Normally @command{ld} will give a warning if it finds an incompatible
1524 library during a library search. This option silences the warning.
1526 @kindex --no-whole-archive
1527 @item --no-whole-archive
1528 Turn off the effect of the @option{--whole-archive} option for subsequent
1531 @cindex output file after errors
1532 @kindex --noinhibit-exec
1533 @item --noinhibit-exec
1534 Retain the executable output file whenever it is still usable.
1535 Normally, the linker will not produce an output file if it encounters
1536 errors during the link process; it exits without writing an output file
1537 when it issues any error whatsoever.
1541 Only search library directories explicitly specified on the
1542 command line. Library directories specified in linker scripts
1543 (including linker scripts specified on the command line) are ignored.
1545 @ifclear SingleFormat
1546 @kindex --oformat=@var{output-format}
1547 @item --oformat=@var{output-format}
1548 @command{ld} may be configured to support more than one kind of object
1549 file. If your @command{ld} is configured this way, you can use the
1550 @samp{--oformat} option to specify the binary format for the output
1551 object file. Even when @command{ld} is configured to support alternative
1552 object formats, you don't usually need to specify this, as @command{ld}
1553 should be configured to produce as a default output format the most
1554 usual format on each machine. @var{output-format} is a text string, the
1555 name of a particular format supported by the BFD libraries. (You can
1556 list the available binary formats with @samp{objdump -i}.) The script
1557 command @code{OUTPUT_FORMAT} can also specify the output format, but
1558 this option overrides it. @xref{BFD}.
1562 @kindex --pic-executable
1564 @itemx --pic-executable
1565 @cindex position independent executables
1566 Create a position independent executable. This is currently only supported on
1567 ELF platforms. Position independent executables are similar to shared
1568 libraries in that they are relocated by the dynamic linker to the virtual
1569 address the OS chooses for them (which can vary between invocations). Like
1570 normal dynamically linked executables they can be executed and symbols
1571 defined in the executable cannot be overridden by shared libraries.
1575 This option is ignored for Linux compatibility.
1579 This option is ignored for SVR4 compatibility.
1582 @cindex synthesizing linker
1583 @cindex relaxing addressing modes
1587 An option with machine dependent effects.
1589 This option is only supported on a few targets.
1592 @xref{H8/300,,@command{ld} and the H8/300}.
1595 @xref{i960,, @command{ld} and the Intel 960 family}.
1598 @xref{Xtensa,, @command{ld} and Xtensa Processors}.
1601 @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}.
1604 @xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}.
1607 On some platforms the @samp{--relax} option performs target specific,
1608 global optimizations that become possible when the linker resolves
1609 addressing in the program, such as relaxing address modes,
1610 synthesizing new instructions, selecting shorter version of current
1611 instructions, and combining constant values.
1613 On some platforms these link time global optimizations may make symbolic
1614 debugging of the resulting executable impossible.
1616 This is known to be the case for the Matsushita MN10200 and MN10300
1617 family of processors.
1621 On platforms where this is not supported, @samp{--relax} is accepted,
1625 On platforms where @samp{--relax} is accepted the option
1626 @samp{--no-relax} can be used to disable the feature.
1628 @cindex retaining specified symbols
1629 @cindex stripping all but some symbols
1630 @cindex symbols, retaining selectively
1631 @kindex --retain-symbols-file=@var{filename}
1632 @item --retain-symbols-file=@var{filename}
1633 Retain @emph{only} the symbols listed in the file @var{filename},
1634 discarding all others. @var{filename} is simply a flat file, with one
1635 symbol name per line. This option is especially useful in environments
1639 where a large global symbol table is accumulated gradually, to conserve
1642 @samp{--retain-symbols-file} does @emph{not} discard undefined symbols,
1643 or symbols needed for relocations.
1645 You may only specify @samp{--retain-symbols-file} once in the command
1646 line. It overrides @samp{-s} and @samp{-S}.
1649 @item -rpath=@var{dir}
1650 @cindex runtime library search path
1651 @kindex -rpath=@var{dir}
1652 Add a directory to the runtime library search path. This is used when
1653 linking an ELF executable with shared objects. All @option{-rpath}
1654 arguments are concatenated and passed to the runtime linker, which uses
1655 them to locate shared objects at runtime. The @option{-rpath} option is
1656 also used when locating shared objects which are needed by shared
1657 objects explicitly included in the link; see the description of the
1658 @option{-rpath-link} option. If @option{-rpath} is not used when linking an
1659 ELF executable, the contents of the environment variable
1660 @code{LD_RUN_PATH} will be used if it is defined.
1662 The @option{-rpath} option may also be used on SunOS. By default, on
1663 SunOS, the linker will form a runtime search patch out of all the
1664 @option{-L} options it is given. If a @option{-rpath} option is used, the
1665 runtime search path will be formed exclusively using the @option{-rpath}
1666 options, ignoring the @option{-L} options. This can be useful when using
1667 gcc, which adds many @option{-L} options which may be on NFS mounted
1670 For compatibility with other ELF linkers, if the @option{-R} option is
1671 followed by a directory name, rather than a file name, it is treated as
1672 the @option{-rpath} option.
1676 @cindex link-time runtime library search path
1677 @kindex -rpath-link=@var{dir}
1678 @item -rpath-link=@var{dir}
1679 When using ELF or SunOS, one shared library may require another. This
1680 happens when an @code{ld -shared} link includes a shared library as one
1683 When the linker encounters such a dependency when doing a non-shared,
1684 non-relocatable link, it will automatically try to locate the required
1685 shared library and include it in the link, if it is not included
1686 explicitly. In such a case, the @option{-rpath-link} option
1687 specifies the first set of directories to search. The
1688 @option{-rpath-link} option may specify a sequence of directory names
1689 either by specifying a list of names separated by colons, or by
1690 appearing multiple times.
1692 This option should be used with caution as it overrides the search path
1693 that may have been hard compiled into a shared library. In such a case it
1694 is possible to use unintentionally a different search path than the
1695 runtime linker would do.
1697 The linker uses the following search paths to locate required shared
1701 Any directories specified by @option{-rpath-link} options.
1703 Any directories specified by @option{-rpath} options. The difference
1704 between @option{-rpath} and @option{-rpath-link} is that directories
1705 specified by @option{-rpath} options are included in the executable and
1706 used at runtime, whereas the @option{-rpath-link} option is only effective
1707 at link time. Searching @option{-rpath} in this way is only supported
1708 by native linkers and cross linkers which have been configured with
1709 the @option{--with-sysroot} option.
1711 On an ELF system, for native linkers, if the @option{-rpath} and
1712 @option{-rpath-link} options were not used, search the contents of the
1713 environment variable @code{LD_RUN_PATH}.
1715 On SunOS, if the @option{-rpath} option was not used, search any
1716 directories specified using @option{-L} options.
1718 For a native linker, the search the contents of the environment
1719 variable @code{LD_LIBRARY_PATH}.
1721 For a native ELF linker, the directories in @code{DT_RUNPATH} or
1722 @code{DT_RPATH} of a shared library are searched for shared
1723 libraries needed by it. The @code{DT_RPATH} entries are ignored if
1724 @code{DT_RUNPATH} entries exist.
1726 The default directories, normally @file{/lib} and @file{/usr/lib}.
1728 For a native linker on an ELF system, if the file @file{/etc/ld.so.conf}
1729 exists, the list of directories found in that file.
1732 If the required shared library is not found, the linker will issue a
1733 warning and continue with the link.
1740 @cindex shared libraries
1741 Create a shared library. This is currently only supported on ELF, XCOFF
1742 and SunOS platforms. On SunOS, the linker will automatically create a
1743 shared library if the @option{-e} option is not used and there are
1744 undefined symbols in the link.
1746 @kindex --sort-common
1748 @itemx --sort-common=ascending
1749 @itemx --sort-common=descending
1750 This option tells @command{ld} to sort the common symbols by alignment in
1751 ascending or descending order when it places them in the appropriate output
1752 sections. The symbol alignments considered are sixteen-byte or larger,
1753 eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps
1754 between symbols due to alignment constraints. If no sorting order is
1755 specified, then descending order is assumed.
1757 @kindex --sort-section=name
1758 @item --sort-section=name
1759 This option will apply @code{SORT_BY_NAME} to all wildcard section
1760 patterns in the linker script.
1762 @kindex --sort-section=alignment
1763 @item --sort-section=alignment
1764 This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section
1765 patterns in the linker script.
1767 @kindex --split-by-file
1768 @item --split-by-file[=@var{size}]
1769 Similar to @option{--split-by-reloc} but creates a new output section for
1770 each input file when @var{size} is reached. @var{size} defaults to a
1771 size of 1 if not given.
1773 @kindex --split-by-reloc
1774 @item --split-by-reloc[=@var{count}]
1775 Tries to creates extra sections in the output file so that no single
1776 output section in the file contains more than @var{count} relocations.
1777 This is useful when generating huge relocatable files for downloading into
1778 certain real time kernels with the COFF object file format; since COFF
1779 cannot represent more than 65535 relocations in a single section. Note
1780 that this will fail to work with object file formats which do not
1781 support arbitrary sections. The linker will not split up individual
1782 input sections for redistribution, so if a single input section contains
1783 more than @var{count} relocations one output section will contain that
1784 many relocations. @var{count} defaults to a value of 32768.
1788 Compute and display statistics about the operation of the linker, such
1789 as execution time and memory usage.
1791 @kindex --sysroot=@var{directory}
1792 @item --sysroot=@var{directory}
1793 Use @var{directory} as the location of the sysroot, overriding the
1794 configure-time default. This option is only supported by linkers
1795 that were configured using @option{--with-sysroot}.
1797 @kindex --traditional-format
1798 @cindex traditional format
1799 @item --traditional-format
1800 For some targets, the output of @command{ld} is different in some ways from
1801 the output of some existing linker. This switch requests @command{ld} to
1802 use the traditional format instead.
1805 For example, on SunOS, @command{ld} combines duplicate entries in the
1806 symbol string table. This can reduce the size of an output file with
1807 full debugging information by over 30 percent. Unfortunately, the SunOS
1808 @code{dbx} program can not read the resulting program (@code{gdb} has no
1809 trouble). The @samp{--traditional-format} switch tells @command{ld} to not
1810 combine duplicate entries.
1812 @kindex --section-start=@var{sectionname}=@var{org}
1813 @item --section-start=@var{sectionname}=@var{org}
1814 Locate a section in the output file at the absolute
1815 address given by @var{org}. You may use this option as many
1816 times as necessary to locate multiple sections in the command
1818 @var{org} must be a single hexadecimal integer;
1819 for compatibility with other linkers, you may omit the leading
1820 @samp{0x} usually associated with hexadecimal values. @emph{Note:} there
1821 should be no white space between @var{sectionname}, the equals
1822 sign (``@key{=}''), and @var{org}.
1824 @kindex -Tbss=@var{org}
1825 @kindex -Tdata=@var{org}
1826 @kindex -Ttext=@var{org}
1827 @cindex segment origins, cmd line
1828 @item -Tbss=@var{org}
1829 @itemx -Tdata=@var{org}
1830 @itemx -Ttext=@var{org}
1831 Same as @option{--section-start}, with @code{.bss}, @code{.data} or
1832 @code{.text} as the @var{sectionname}.
1834 @kindex -Ttext-segment=@var{org}
1835 @item -Ttext-segment=@var{org}
1836 @cindex text segment origin, cmd line
1837 When creating an ELF executable or shared object, it will set the address
1838 of the first byte of the text segment.
1840 @kindex -Trodata-segment=@var{org}
1841 @item -Trodata-segment=@var{org}
1842 @cindex rodata segment origin, cmd line
1843 When creating an ELF executable or shared object for a target where
1844 the read-only data is in its own segment separate from the executable
1845 text, it will set the address of the first byte of the read-only data segment.
1847 @kindex --unresolved-symbols
1848 @item --unresolved-symbols=@var{method}
1849 Determine how to handle unresolved symbols. There are four possible
1850 values for @samp{method}:
1854 Do not report any unresolved symbols.
1857 Report all unresolved symbols. This is the default.
1859 @item ignore-in-object-files
1860 Report unresolved symbols that are contained in shared libraries, but
1861 ignore them if they come from regular object files.
1863 @item ignore-in-shared-libs
1864 Report unresolved symbols that come from regular object files, but
1865 ignore them if they come from shared libraries. This can be useful
1866 when creating a dynamic binary and it is known that all the shared
1867 libraries that it should be referencing are included on the linker's
1871 The behaviour for shared libraries on their own can also be controlled
1872 by the @option{--[no-]allow-shlib-undefined} option.
1874 Normally the linker will generate an error message for each reported
1875 unresolved symbol but the option @option{--warn-unresolved-symbols}
1876 can change this to a warning.
1878 @kindex --verbose[=@var{NUMBER}]
1879 @cindex verbose[=@var{NUMBER}]
1881 @itemx --verbose[=@var{NUMBER}]
1882 Display the version number for @command{ld} and list the linker emulations
1883 supported. Display which input files can and cannot be opened. Display
1884 the linker script being used by the linker. If the optional @var{NUMBER}
1885 argument > 1, plugin symbol status will also be displayed.
1887 @kindex --version-script=@var{version-scriptfile}
1888 @cindex version script, symbol versions
1889 @item --version-script=@var{version-scriptfile}
1890 Specify the name of a version script to the linker. This is typically
1891 used when creating shared libraries to specify additional information
1892 about the version hierarchy for the library being created. This option
1893 is only fully supported on ELF platforms which support shared libraries;
1894 see @ref{VERSION}. It is partially supported on PE platforms, which can
1895 use version scripts to filter symbol visibility in auto-export mode: any
1896 symbols marked @samp{local} in the version script will not be exported.
1899 @kindex --warn-common
1900 @cindex warnings, on combining symbols
1901 @cindex combining symbols, warnings on
1903 Warn when a common symbol is combined with another common symbol or with
1904 a symbol definition. Unix linkers allow this somewhat sloppy practice,
1905 but linkers on some other operating systems do not. This option allows
1906 you to find potential problems from combining global symbols.
1907 Unfortunately, some C libraries use this practice, so you may get some
1908 warnings about symbols in the libraries as well as in your programs.
1910 There are three kinds of global symbols, illustrated here by C examples:
1914 A definition, which goes in the initialized data section of the output
1918 An undefined reference, which does not allocate space.
1919 There must be either a definition or a common symbol for the
1923 A common symbol. If there are only (one or more) common symbols for a
1924 variable, it goes in the uninitialized data area of the output file.
1925 The linker merges multiple common symbols for the same variable into a
1926 single symbol. If they are of different sizes, it picks the largest
1927 size. The linker turns a common symbol into a declaration, if there is
1928 a definition of the same variable.
1931 The @samp{--warn-common} option can produce five kinds of warnings.
1932 Each warning consists of a pair of lines: the first describes the symbol
1933 just encountered, and the second describes the previous symbol
1934 encountered with the same name. One or both of the two symbols will be
1939 Turning a common symbol into a reference, because there is already a
1940 definition for the symbol.
1942 @var{file}(@var{section}): warning: common of `@var{symbol}'
1943 overridden by definition
1944 @var{file}(@var{section}): warning: defined here
1948 Turning a common symbol into a reference, because a later definition for
1949 the symbol is encountered. This is the same as the previous case,
1950 except that the symbols are encountered in a different order.
1952 @var{file}(@var{section}): warning: definition of `@var{symbol}'
1954 @var{file}(@var{section}): warning: common is here
1958 Merging a common symbol with a previous same-sized common symbol.
1960 @var{file}(@var{section}): warning: multiple common
1962 @var{file}(@var{section}): warning: previous common is here
1966 Merging a common symbol with a previous larger common symbol.
1968 @var{file}(@var{section}): warning: common of `@var{symbol}'
1969 overridden by larger common
1970 @var{file}(@var{section}): warning: larger common is here
1974 Merging a common symbol with a previous smaller common symbol. This is
1975 the same as the previous case, except that the symbols are
1976 encountered in a different order.
1978 @var{file}(@var{section}): warning: common of `@var{symbol}'
1979 overriding smaller common
1980 @var{file}(@var{section}): warning: smaller common is here
1984 @kindex --warn-constructors
1985 @item --warn-constructors
1986 Warn if any global constructors are used. This is only useful for a few
1987 object file formats. For formats like COFF or ELF, the linker can not
1988 detect the use of global constructors.
1990 @kindex --warn-multiple-gp
1991 @item --warn-multiple-gp
1992 Warn if multiple global pointer values are required in the output file.
1993 This is only meaningful for certain processors, such as the Alpha.
1994 Specifically, some processors put large-valued constants in a special
1995 section. A special register (the global pointer) points into the middle
1996 of this section, so that constants can be loaded efficiently via a
1997 base-register relative addressing mode. Since the offset in
1998 base-register relative mode is fixed and relatively small (e.g., 16
1999 bits), this limits the maximum size of the constant pool. Thus, in
2000 large programs, it is often necessary to use multiple global pointer
2001 values in order to be able to address all possible constants. This
2002 option causes a warning to be issued whenever this case occurs.
2005 @cindex warnings, on undefined symbols
2006 @cindex undefined symbols, warnings on
2008 Only warn once for each undefined symbol, rather than once per module
2011 @kindex --warn-section-align
2012 @cindex warnings, on section alignment
2013 @cindex section alignment, warnings on
2014 @item --warn-section-align
2015 Warn if the address of an output section is changed because of
2016 alignment. Typically, the alignment will be set by an input section.
2017 The address will only be changed if it not explicitly specified; that
2018 is, if the @code{SECTIONS} command does not specify a start address for
2019 the section (@pxref{SECTIONS}).
2021 @kindex --warn-shared-textrel
2022 @item --warn-shared-textrel
2023 Warn if the linker adds a DT_TEXTREL to a shared object.
2025 @kindex --warn-alternate-em
2026 @item --warn-alternate-em
2027 Warn if an object has alternate ELF machine code.
2029 @kindex --warn-unresolved-symbols
2030 @item --warn-unresolved-symbols
2031 If the linker is going to report an unresolved symbol (see the option
2032 @option{--unresolved-symbols}) it will normally generate an error.
2033 This option makes it generate a warning instead.
2035 @kindex --error-unresolved-symbols
2036 @item --error-unresolved-symbols
2037 This restores the linker's default behaviour of generating errors when
2038 it is reporting unresolved symbols.
2040 @kindex --whole-archive
2041 @cindex including an entire archive
2042 @item --whole-archive
2043 For each archive mentioned on the command line after the
2044 @option{--whole-archive} option, include every object file in the archive
2045 in the link, rather than searching the archive for the required object
2046 files. This is normally used to turn an archive file into a shared
2047 library, forcing every object to be included in the resulting shared
2048 library. This option may be used more than once.
2050 Two notes when using this option from gcc: First, gcc doesn't know
2051 about this option, so you have to use @option{-Wl,-whole-archive}.
2052 Second, don't forget to use @option{-Wl,-no-whole-archive} after your
2053 list of archives, because gcc will add its own list of archives to
2054 your link and you may not want this flag to affect those as well.
2056 @kindex --wrap=@var{symbol}
2057 @item --wrap=@var{symbol}
2058 Use a wrapper function for @var{symbol}. Any undefined reference to
2059 @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any
2060 undefined reference to @code{__real_@var{symbol}} will be resolved to
2063 This can be used to provide a wrapper for a system function. The
2064 wrapper function should be called @code{__wrap_@var{symbol}}. If it
2065 wishes to call the system function, it should call
2066 @code{__real_@var{symbol}}.
2068 Here is a trivial example:
2072 __wrap_malloc (size_t c)
2074 printf ("malloc called with %zu\n", c);
2075 return __real_malloc (c);
2079 If you link other code with this file using @option{--wrap malloc}, then
2080 all calls to @code{malloc} will call the function @code{__wrap_malloc}
2081 instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will
2082 call the real @code{malloc} function.
2084 You may wish to provide a @code{__real_malloc} function as well, so that
2085 links without the @option{--wrap} option will succeed. If you do this,
2086 you should not put the definition of @code{__real_malloc} in the same
2087 file as @code{__wrap_malloc}; if you do, the assembler may resolve the
2088 call before the linker has a chance to wrap it to @code{malloc}.
2090 @kindex --eh-frame-hdr
2091 @item --eh-frame-hdr
2092 Request creation of @code{.eh_frame_hdr} section and ELF
2093 @code{PT_GNU_EH_FRAME} segment header.
2095 @kindex --ld-generated-unwind-info
2096 @item --no-ld-generated-unwind-info
2097 Request creation of @code{.eh_frame} unwind info for linker
2098 generated code sections like PLT. This option is on by default
2099 if linker generated unwind info is supported.
2101 @kindex --enable-new-dtags
2102 @kindex --disable-new-dtags
2103 @item --enable-new-dtags
2104 @itemx --disable-new-dtags
2105 This linker can create the new dynamic tags in ELF. But the older ELF
2106 systems may not understand them. If you specify
2107 @option{--enable-new-dtags}, the dynamic tags will be created as needed.
2108 If you specify @option{--disable-new-dtags}, no new dynamic tags will be
2109 created. By default, the new dynamic tags are not created. Note that
2110 those options are only available for ELF systems.
2112 @kindex --hash-size=@var{number}
2113 @item --hash-size=@var{number}
2114 Set the default size of the linker's hash tables to a prime number
2115 close to @var{number}. Increasing this value can reduce the length of
2116 time it takes the linker to perform its tasks, at the expense of
2117 increasing the linker's memory requirements. Similarly reducing this
2118 value can reduce the memory requirements at the expense of speed.
2120 @kindex --hash-style=@var{style}
2121 @item --hash-style=@var{style}
2122 Set the type of linker's hash table(s). @var{style} can be either
2123 @code{sysv} for classic ELF @code{.hash} section, @code{gnu} for
2124 new style GNU @code{.gnu.hash} section or @code{both} for both
2125 the classic ELF @code{.hash} and new style GNU @code{.gnu.hash}
2126 hash tables. The default is @code{sysv}.
2128 @kindex --reduce-memory-overheads
2129 @item --reduce-memory-overheads
2130 This option reduces memory requirements at ld runtime, at the expense of
2131 linking speed. This was introduced to select the old O(n^2) algorithm
2132 for link map file generation, rather than the new O(n) algorithm which uses
2133 about 40% more memory for symbol storage.
2135 Another effect of the switch is to set the default hash table size to
2136 1021, which again saves memory at the cost of lengthening the linker's
2137 run time. This is not done however if the @option{--hash-size} switch
2140 The @option{--reduce-memory-overheads} switch may be also be used to
2141 enable other tradeoffs in future versions of the linker.
2144 @kindex --build-id=@var{style}
2146 @itemx --build-id=@var{style}
2147 Request creation of @code{.note.gnu.build-id} ELF note section.
2148 The contents of the note are unique bits identifying this linked
2149 file. @var{style} can be @code{uuid} to use 128 random bits,
2150 @code{sha1} to use a 160-bit @sc{SHA1} hash on the normative
2151 parts of the output contents, @code{md5} to use a 128-bit
2152 @sc{MD5} hash on the normative parts of the output contents, or
2153 @code{0x@var{hexstring}} to use a chosen bit string specified as
2154 an even number of hexadecimal digits (@code{-} and @code{:}
2155 characters between digit pairs are ignored). If @var{style} is
2156 omitted, @code{sha1} is used.
2158 The @code{md5} and @code{sha1} styles produces an identifier
2159 that is always the same in an identical output file, but will be
2160 unique among all nonidentical output files. It is not intended
2161 to be compared as a checksum for the file's contents. A linked
2162 file may be changed later by other tools, but the build ID bit
2163 string identifying the original linked file does not change.
2165 Passing @code{none} for @var{style} disables the setting from any
2166 @code{--build-id} options earlier on the command line.
2171 @subsection Options Specific to i386 PE Targets
2173 @c man begin OPTIONS
2175 The i386 PE linker supports the @option{-shared} option, which causes
2176 the output to be a dynamically linked library (DLL) instead of a
2177 normal executable. You should name the output @code{*.dll} when you
2178 use this option. In addition, the linker fully supports the standard
2179 @code{*.def} files, which may be specified on the linker command line
2180 like an object file (in fact, it should precede archives it exports
2181 symbols from, to ensure that they get linked in, just like a normal
2184 In addition to the options common to all targets, the i386 PE linker
2185 support additional command line options that are specific to the i386
2186 PE target. Options that take values may be separated from their
2187 values by either a space or an equals sign.
2191 @kindex --add-stdcall-alias
2192 @item --add-stdcall-alias
2193 If given, symbols with a stdcall suffix (@@@var{nn}) will be exported
2194 as-is and also with the suffix stripped.
2195 [This option is specific to the i386 PE targeted port of the linker]
2198 @item --base-file @var{file}
2199 Use @var{file} as the name of a file in which to save the base
2200 addresses of all the relocations needed for generating DLLs with
2202 [This is an i386 PE specific option]
2206 Create a DLL instead of a regular executable. You may also use
2207 @option{-shared} or specify a @code{LIBRARY} in a given @code{.def}
2209 [This option is specific to the i386 PE targeted port of the linker]
2211 @kindex --enable-long-section-names
2212 @kindex --disable-long-section-names
2213 @item --enable-long-section-names
2214 @itemx --disable-long-section-names
2215 The PE variants of the Coff object format add an extension that permits
2216 the use of section names longer than eight characters, the normal limit
2217 for Coff. By default, these names are only allowed in object files, as
2218 fully-linked executable images do not carry the Coff string table required
2219 to support the longer names. As a GNU extension, it is possible to
2220 allow their use in executable images as well, or to (probably pointlessly!)
2221 disallow it in object files, by using these two options. Executable images
2222 generated with these long section names are slightly non-standard, carrying
2223 as they do a string table, and may generate confusing output when examined
2224 with non-GNU PE-aware tools, such as file viewers and dumpers. However,
2225 GDB relies on the use of PE long section names to find Dwarf-2 debug
2226 information sections in an executable image at runtime, and so if neither
2227 option is specified on the command-line, @command{ld} will enable long
2228 section names, overriding the default and technically correct behaviour,
2229 when it finds the presence of debug information while linking an executable
2230 image and not stripping symbols.
2231 [This option is valid for all PE targeted ports of the linker]
2233 @kindex --enable-stdcall-fixup
2234 @kindex --disable-stdcall-fixup
2235 @item --enable-stdcall-fixup
2236 @itemx --disable-stdcall-fixup
2237 If the link finds a symbol that it cannot resolve, it will attempt to
2238 do ``fuzzy linking'' by looking for another defined symbol that differs
2239 only in the format of the symbol name (cdecl vs stdcall) and will
2240 resolve that symbol by linking to the match. For example, the
2241 undefined symbol @code{_foo} might be linked to the function
2242 @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked
2243 to the function @code{_bar}. When the linker does this, it prints a
2244 warning, since it normally should have failed to link, but sometimes
2245 import libraries generated from third-party dlls may need this feature
2246 to be usable. If you specify @option{--enable-stdcall-fixup}, this
2247 feature is fully enabled and warnings are not printed. If you specify
2248 @option{--disable-stdcall-fixup}, this feature is disabled and such
2249 mismatches are considered to be errors.
2250 [This option is specific to the i386 PE targeted port of the linker]
2252 @kindex --leading-underscore
2253 @kindex --no-leading-underscore
2254 @item --leading-underscore
2255 @itemx --no-leading-underscore
2256 For most targets default symbol-prefix is an underscore and is defined
2257 in target's description. By this option it is possible to
2258 disable/enable the default underscore symbol-prefix.
2260 @cindex DLLs, creating
2261 @kindex --export-all-symbols
2262 @item --export-all-symbols
2263 If given, all global symbols in the objects used to build a DLL will
2264 be exported by the DLL. Note that this is the default if there
2265 otherwise wouldn't be any exported symbols. When symbols are
2266 explicitly exported via DEF files or implicitly exported via function
2267 attributes, the default is to not export anything else unless this
2268 option is given. Note that the symbols @code{DllMain@@12},
2269 @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and
2270 @code{impure_ptr} will not be automatically
2271 exported. Also, symbols imported from other DLLs will not be
2272 re-exported, nor will symbols specifying the DLL's internal layout
2273 such as those beginning with @code{_head_} or ending with
2274 @code{_iname}. In addition, no symbols from @code{libgcc},
2275 @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported.
2276 Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will
2277 not be exported, to help with C++ DLLs. Finally, there is an
2278 extensive list of cygwin-private symbols that are not exported
2279 (obviously, this applies on when building DLLs for cygwin targets).
2280 These cygwin-excludes are: @code{_cygwin_dll_entry@@12},
2281 @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12},
2282 @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll},
2283 @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2},
2284 @code{cygwin_premain3}, and @code{environ}.
2285 [This option is specific to the i386 PE targeted port of the linker]
2287 @kindex --exclude-symbols
2288 @item --exclude-symbols @var{symbol},@var{symbol},...
2289 Specifies a list of symbols which should not be automatically
2290 exported. The symbol names may be delimited by commas or colons.
2291 [This option is specific to the i386 PE targeted port of the linker]
2293 @kindex --exclude-all-symbols
2294 @item --exclude-all-symbols
2295 Specifies no symbols should be automatically exported.
2296 [This option is specific to the i386 PE targeted port of the linker]
2298 @kindex --file-alignment
2299 @item --file-alignment
2300 Specify the file alignment. Sections in the file will always begin at
2301 file offsets which are multiples of this number. This defaults to
2303 [This option is specific to the i386 PE targeted port of the linker]
2307 @item --heap @var{reserve}
2308 @itemx --heap @var{reserve},@var{commit}
2309 Specify the number of bytes of memory to reserve (and optionally commit)
2310 to be used as heap for this program. The default is 1Mb reserved, 4K
2312 [This option is specific to the i386 PE targeted port of the linker]
2315 @kindex --image-base
2316 @item --image-base @var{value}
2317 Use @var{value} as the base address of your program or dll. This is
2318 the lowest memory location that will be used when your program or dll
2319 is loaded. To reduce the need to relocate and improve performance of
2320 your dlls, each should have a unique base address and not overlap any
2321 other dlls. The default is 0x400000 for executables, and 0x10000000
2323 [This option is specific to the i386 PE targeted port of the linker]
2327 If given, the stdcall suffixes (@@@var{nn}) will be stripped from
2328 symbols before they are exported.
2329 [This option is specific to the i386 PE targeted port of the linker]
2331 @kindex --large-address-aware
2332 @item --large-address-aware
2333 If given, the appropriate bit in the ``Characteristics'' field of the COFF
2334 header is set to indicate that this executable supports virtual addresses
2335 greater than 2 gigabytes. This should be used in conjunction with the /3GB
2336 or /USERVA=@var{value} megabytes switch in the ``[operating systems]''
2337 section of the BOOT.INI. Otherwise, this bit has no effect.
2338 [This option is specific to PE targeted ports of the linker]
2340 @kindex --major-image-version
2341 @item --major-image-version @var{value}
2342 Sets the major number of the ``image version''. Defaults to 1.
2343 [This option is specific to the i386 PE targeted port of the linker]
2345 @kindex --major-os-version
2346 @item --major-os-version @var{value}
2347 Sets the major number of the ``os version''. Defaults to 4.
2348 [This option is specific to the i386 PE targeted port of the linker]
2350 @kindex --major-subsystem-version
2351 @item --major-subsystem-version @var{value}
2352 Sets the major number of the ``subsystem version''. Defaults to 4.
2353 [This option is specific to the i386 PE targeted port of the linker]
2355 @kindex --minor-image-version
2356 @item --minor-image-version @var{value}
2357 Sets the minor number of the ``image version''. Defaults to 0.
2358 [This option is specific to the i386 PE targeted port of the linker]
2360 @kindex --minor-os-version
2361 @item --minor-os-version @var{value}
2362 Sets the minor number of the ``os version''. Defaults to 0.
2363 [This option is specific to the i386 PE targeted port of the linker]
2365 @kindex --minor-subsystem-version
2366 @item --minor-subsystem-version @var{value}
2367 Sets the minor number of the ``subsystem version''. Defaults to 0.
2368 [This option is specific to the i386 PE targeted port of the linker]
2370 @cindex DEF files, creating
2371 @cindex DLLs, creating
2372 @kindex --output-def
2373 @item --output-def @var{file}
2374 The linker will create the file @var{file} which will contain a DEF
2375 file corresponding to the DLL the linker is generating. This DEF file
2376 (which should be called @code{*.def}) may be used to create an import
2377 library with @code{dlltool} or may be used as a reference to
2378 automatically or implicitly exported symbols.
2379 [This option is specific to the i386 PE targeted port of the linker]
2381 @cindex DLLs, creating
2382 @kindex --out-implib
2383 @item --out-implib @var{file}
2384 The linker will create the file @var{file} which will contain an
2385 import lib corresponding to the DLL the linker is generating. This
2386 import lib (which should be called @code{*.dll.a} or @code{*.a}
2387 may be used to link clients against the generated DLL; this behaviour
2388 makes it possible to skip a separate @code{dlltool} import library
2390 [This option is specific to the i386 PE targeted port of the linker]
2392 @kindex --enable-auto-image-base
2393 @item --enable-auto-image-base
2394 Automatically choose the image base for DLLs, unless one is specified
2395 using the @code{--image-base} argument. By using a hash generated
2396 from the dllname to create unique image bases for each DLL, in-memory
2397 collisions and relocations which can delay program execution are
2399 [This option is specific to the i386 PE targeted port of the linker]
2401 @kindex --disable-auto-image-base
2402 @item --disable-auto-image-base
2403 Do not automatically generate a unique image base. If there is no
2404 user-specified image base (@code{--image-base}) then use the platform
2406 [This option is specific to the i386 PE targeted port of the linker]
2408 @cindex DLLs, linking to
2409 @kindex --dll-search-prefix
2410 @item --dll-search-prefix @var{string}
2411 When linking dynamically to a dll without an import library,
2412 search for @code{<string><basename>.dll} in preference to
2413 @code{lib<basename>.dll}. This behaviour allows easy distinction
2414 between DLLs built for the various "subplatforms": native, cygwin,
2415 uwin, pw, etc. For instance, cygwin DLLs typically use
2416 @code{--dll-search-prefix=cyg}.
2417 [This option is specific to the i386 PE targeted port of the linker]
2419 @kindex --enable-auto-import
2420 @item --enable-auto-import
2421 Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for
2422 DATA imports from DLLs, and create the necessary thunking symbols when
2423 building the import libraries with those DATA exports. Note: Use of the
2424 'auto-import' extension will cause the text section of the image file
2425 to be made writable. This does not conform to the PE-COFF format
2426 specification published by Microsoft.
2428 Note - use of the 'auto-import' extension will also cause read only
2429 data which would normally be placed into the .rdata section to be
2430 placed into the .data section instead. This is in order to work
2431 around a problem with consts that is described here:
2432 http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
2434 Using 'auto-import' generally will 'just work' -- but sometimes you may
2437 "variable '<var>' can't be auto-imported. Please read the
2438 documentation for ld's @code{--enable-auto-import} for details."
2440 This message occurs when some (sub)expression accesses an address
2441 ultimately given by the sum of two constants (Win32 import tables only
2442 allow one). Instances where this may occur include accesses to member
2443 fields of struct variables imported from a DLL, as well as using a
2444 constant index into an array variable imported from a DLL. Any
2445 multiword variable (arrays, structs, long long, etc) may trigger
2446 this error condition. However, regardless of the exact data type
2447 of the offending exported variable, ld will always detect it, issue
2448 the warning, and exit.
2450 There are several ways to address this difficulty, regardless of the
2451 data type of the exported variable:
2453 One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
2454 of adjusting references in your client code for runtime environment, so
2455 this method works only when runtime environment supports this feature.
2457 A second solution is to force one of the 'constants' to be a variable --
2458 that is, unknown and un-optimizable at compile time. For arrays,
2459 there are two possibilities: a) make the indexee (the array's address)
2460 a variable, or b) make the 'constant' index a variable. Thus:
2463 extern type extern_array[];
2465 @{ volatile type *t=extern_array; t[1] @}
2471 extern type extern_array[];
2473 @{ volatile int t=1; extern_array[t] @}
2476 For structs (and most other multiword data types) the only option
2477 is to make the struct itself (or the long long, or the ...) variable:
2480 extern struct s extern_struct;
2481 extern_struct.field -->
2482 @{ volatile struct s *t=&extern_struct; t->field @}
2488 extern long long extern_ll;
2490 @{ volatile long long * local_ll=&extern_ll; *local_ll @}
2493 A third method of dealing with this difficulty is to abandon
2494 'auto-import' for the offending symbol and mark it with
2495 @code{__declspec(dllimport)}. However, in practice that
2496 requires using compile-time #defines to indicate whether you are
2497 building a DLL, building client code that will link to the DLL, or
2498 merely building/linking to a static library. In making the choice
2499 between the various methods of resolving the 'direct address with
2500 constant offset' problem, you should consider typical real-world usage:
2508 void main(int argc, char **argv)@{
2509 printf("%d\n",arr[1]);
2519 void main(int argc, char **argv)@{
2520 /* This workaround is for win32 and cygwin; do not "optimize" */
2521 volatile int *parr = arr;
2522 printf("%d\n",parr[1]);
2529 /* Note: auto-export is assumed (no __declspec(dllexport)) */
2530 #if (defined(_WIN32) || defined(__CYGWIN__)) && \
2531 !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
2532 #define FOO_IMPORT __declspec(dllimport)
2536 extern FOO_IMPORT int arr[];
2539 void main(int argc, char **argv)@{
2540 printf("%d\n",arr[1]);
2544 A fourth way to avoid this problem is to re-code your
2545 library to use a functional interface rather than a data interface
2546 for the offending variables (e.g. set_foo() and get_foo() accessor
2548 [This option is specific to the i386 PE targeted port of the linker]
2550 @kindex --disable-auto-import
2551 @item --disable-auto-import
2552 Do not attempt to do sophisticated linking of @code{_symbol} to
2553 @code{__imp__symbol} for DATA imports from DLLs.
2554 [This option is specific to the i386 PE targeted port of the linker]
2556 @kindex --enable-runtime-pseudo-reloc
2557 @item --enable-runtime-pseudo-reloc
2558 If your code contains expressions described in --enable-auto-import section,
2559 that is, DATA imports from DLL with non-zero offset, this switch will create
2560 a vector of 'runtime pseudo relocations' which can be used by runtime
2561 environment to adjust references to such data in your client code.
2562 [This option is specific to the i386 PE targeted port of the linker]
2564 @kindex --disable-runtime-pseudo-reloc
2565 @item --disable-runtime-pseudo-reloc
2566 Do not create pseudo relocations for non-zero offset DATA imports from
2567 DLLs. This is the default.
2568 [This option is specific to the i386 PE targeted port of the linker]
2570 @kindex --enable-extra-pe-debug
2571 @item --enable-extra-pe-debug
2572 Show additional debug info related to auto-import symbol thunking.
2573 [This option is specific to the i386 PE targeted port of the linker]
2575 @kindex --section-alignment
2576 @item --section-alignment
2577 Sets the section alignment. Sections in memory will always begin at
2578 addresses which are a multiple of this number. Defaults to 0x1000.
2579 [This option is specific to the i386 PE targeted port of the linker]
2583 @item --stack @var{reserve}
2584 @itemx --stack @var{reserve},@var{commit}
2585 Specify the number of bytes of memory to reserve (and optionally commit)
2586 to be used as stack for this program. The default is 2Mb reserved, 4K
2588 [This option is specific to the i386 PE targeted port of the linker]
2591 @item --subsystem @var{which}
2592 @itemx --subsystem @var{which}:@var{major}
2593 @itemx --subsystem @var{which}:@var{major}.@var{minor}
2594 Specifies the subsystem under which your program will execute. The
2595 legal values for @var{which} are @code{native}, @code{windows},
2596 @code{console}, @code{posix}, and @code{xbox}. You may optionally set
2597 the subsystem version also. Numeric values are also accepted for
2599 [This option is specific to the i386 PE targeted port of the linker]
2601 The following options set flags in the @code{DllCharacteristics} field
2602 of the PE file header:
2603 [These options are specific to PE targeted ports of the linker]
2605 @kindex --dynamicbase
2607 The image base address may be relocated using address space layout
2608 randomization (ASLR). This feature was introduced with MS Windows
2609 Vista for i386 PE targets.
2611 @kindex --forceinteg
2613 Code integrity checks are enforced.
2617 The image is compatible with the Data Execution Prevention.
2618 This feature was introduced with MS Windows XP SP2 for i386 PE targets.
2620 @kindex --no-isolation
2621 @item --no-isolation
2622 Although the image understands isolation, do not isolate the image.
2626 The image does not use SEH. No SE handler may be called from
2631 Do not bind this image.
2635 The driver uses the MS Windows Driver Model.
2639 The image is Terminal Server aware.
2646 @subsection Options specific to C6X uClinux targets
2648 @c man begin OPTIONS
2650 The C6X uClinux target uses a binary format called DSBT to support shared
2651 libraries. Each shared library in the system needs to have a unique index;
2652 all executables use an index of 0.
2657 @item --dsbt-size @var{size}
2658 This option sets the number of entires in the DSBT of the current executable
2659 or shared library to @var{size}. The default is to create a table with 64
2662 @kindex --dsbt-index
2663 @item --dsbt-index @var{index}
2664 This option sets the DSBT index of the current executable or shared library
2665 to @var{index}. The default is 0, which is appropriate for generating
2666 executables. If a shared library is generated with a DSBT index of 0, the
2667 @code{R_C6000_DSBT_INDEX} relocs are copied into the output file.
2669 @kindex --no-merge-exidx-entries
2670 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent
2671 exidx entries in frame unwind info.
2679 @subsection Options specific to Motorola 68HC11 and 68HC12 targets
2681 @c man begin OPTIONS
2683 The 68HC11 and 68HC12 linkers support specific options to control the
2684 memory bank switching mapping and trampoline code generation.
2688 @kindex --no-trampoline
2689 @item --no-trampoline
2690 This option disables the generation of trampoline. By default a trampoline
2691 is generated for each far function which is called using a @code{jsr}
2692 instruction (this happens when a pointer to a far function is taken).
2694 @kindex --bank-window
2695 @item --bank-window @var{name}
2696 This option indicates to the linker the name of the memory region in
2697 the @samp{MEMORY} specification that describes the memory bank window.
2698 The definition of such region is then used by the linker to compute
2699 paging and addresses within the memory window.
2707 @subsection Options specific to Motorola 68K target
2709 @c man begin OPTIONS
2711 The following options are supported to control handling of GOT generation
2712 when linking for 68K targets.
2717 @item --got=@var{type}
2718 This option tells the linker which GOT generation scheme to use.
2719 @var{type} should be one of @samp{single}, @samp{negative},
2720 @samp{multigot} or @samp{target}. For more information refer to the
2721 Info entry for @file{ld}.
2730 @section Environment Variables
2732 @c man begin ENVIRONMENT
2734 You can change the behaviour of @command{ld} with the environment variables
2735 @ifclear SingleFormat
2738 @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}.
2740 @ifclear SingleFormat
2742 @cindex default input format
2743 @code{GNUTARGET} determines the input-file object format if you don't
2744 use @samp{-b} (or its synonym @samp{--format}). Its value should be one
2745 of the BFD names for an input format (@pxref{BFD}). If there is no
2746 @code{GNUTARGET} in the environment, @command{ld} uses the natural format
2747 of the target. If @code{GNUTARGET} is set to @code{default} then BFD
2748 attempts to discover the input format by examining binary input files;
2749 this method often succeeds, but there are potential ambiguities, since
2750 there is no method of ensuring that the magic number used to specify
2751 object-file formats is unique. However, the configuration procedure for
2752 BFD on each system places the conventional format for that system first
2753 in the search-list, so ambiguities are resolved in favor of convention.
2757 @cindex default emulation
2758 @cindex emulation, default
2759 @code{LDEMULATION} determines the default emulation if you don't use the
2760 @samp{-m} option. The emulation can affect various aspects of linker
2761 behaviour, particularly the default linker script. You can list the
2762 available emulations with the @samp{--verbose} or @samp{-V} options. If
2763 the @samp{-m} option is not used, and the @code{LDEMULATION} environment
2764 variable is not defined, the default emulation depends upon how the
2765 linker was configured.
2767 @kindex COLLECT_NO_DEMANGLE
2768 @cindex demangling, default
2769 Normally, the linker will default to demangling symbols. However, if
2770 @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will
2771 default to not demangling symbols. This environment variable is used in
2772 a similar fashion by the @code{gcc} linker wrapper program. The default
2773 may be overridden by the @samp{--demangle} and @samp{--no-demangle}
2780 @chapter Linker Scripts
2783 @cindex linker scripts
2784 @cindex command files
2785 Every link is controlled by a @dfn{linker script}. This script is
2786 written in the linker command language.
2788 The main purpose of the linker script is to describe how the sections in
2789 the input files should be mapped into the output file, and to control
2790 the memory layout of the output file. Most linker scripts do nothing
2791 more than this. However, when necessary, the linker script can also
2792 direct the linker to perform many other operations, using the commands
2795 The linker always uses a linker script. If you do not supply one
2796 yourself, the linker will use a default script that is compiled into the
2797 linker executable. You can use the @samp{--verbose} command line option
2798 to display the default linker script. Certain command line options,
2799 such as @samp{-r} or @samp{-N}, will affect the default linker script.
2801 You may supply your own linker script by using the @samp{-T} command
2802 line option. When you do this, your linker script will replace the
2803 default linker script.
2805 You may also use linker scripts implicitly by naming them as input files
2806 to the linker, as though they were files to be linked. @xref{Implicit
2810 * Basic Script Concepts:: Basic Linker Script Concepts
2811 * Script Format:: Linker Script Format
2812 * Simple Example:: Simple Linker Script Example
2813 * Simple Commands:: Simple Linker Script Commands
2814 * Assignments:: Assigning Values to Symbols
2815 * SECTIONS:: SECTIONS Command
2816 * MEMORY:: MEMORY Command
2817 * PHDRS:: PHDRS Command
2818 * VERSION:: VERSION Command
2819 * Expressions:: Expressions in Linker Scripts
2820 * Implicit Linker Scripts:: Implicit Linker Scripts
2823 @node Basic Script Concepts
2824 @section Basic Linker Script Concepts
2825 @cindex linker script concepts
2826 We need to define some basic concepts and vocabulary in order to
2827 describe the linker script language.
2829 The linker combines input files into a single output file. The output
2830 file and each input file are in a special data format known as an
2831 @dfn{object file format}. Each file is called an @dfn{object file}.
2832 The output file is often called an @dfn{executable}, but for our
2833 purposes we will also call it an object file. Each object file has,
2834 among other things, a list of @dfn{sections}. We sometimes refer to a
2835 section in an input file as an @dfn{input section}; similarly, a section
2836 in the output file is an @dfn{output section}.
2838 Each section in an object file has a name and a size. Most sections
2839 also have an associated block of data, known as the @dfn{section
2840 contents}. A section may be marked as @dfn{loadable}, which mean that
2841 the contents should be loaded into memory when the output file is run.
2842 A section with no contents may be @dfn{allocatable}, which means that an
2843 area in memory should be set aside, but nothing in particular should be
2844 loaded there (in some cases this memory must be zeroed out). A section
2845 which is neither loadable nor allocatable typically contains some sort
2846 of debugging information.
2848 Every loadable or allocatable output section has two addresses. The
2849 first is the @dfn{VMA}, or virtual memory address. This is the address
2850 the section will have when the output file is run. The second is the
2851 @dfn{LMA}, or load memory address. This is the address at which the
2852 section will be loaded. In most cases the two addresses will be the
2853 same. An example of when they might be different is when a data section
2854 is loaded into ROM, and then copied into RAM when the program starts up
2855 (this technique is often used to initialize global variables in a ROM
2856 based system). In this case the ROM address would be the LMA, and the
2857 RAM address would be the VMA.
2859 You can see the sections in an object file by using the @code{objdump}
2860 program with the @samp{-h} option.
2862 Every object file also has a list of @dfn{symbols}, known as the
2863 @dfn{symbol table}. A symbol may be defined or undefined. Each symbol
2864 has a name, and each defined symbol has an address, among other
2865 information. If you compile a C or C++ program into an object file, you
2866 will get a defined symbol for every defined function and global or
2867 static variable. Every undefined function or global variable which is
2868 referenced in the input file will become an undefined symbol.
2870 You can see the symbols in an object file by using the @code{nm}
2871 program, or by using the @code{objdump} program with the @samp{-t}
2875 @section Linker Script Format
2876 @cindex linker script format
2877 Linker scripts are text files.
2879 You write a linker script as a series of commands. Each command is
2880 either a keyword, possibly followed by arguments, or an assignment to a
2881 symbol. You may separate commands using semicolons. Whitespace is
2884 Strings such as file or format names can normally be entered directly.
2885 If the file name contains a character such as a comma which would
2886 otherwise serve to separate file names, you may put the file name in
2887 double quotes. There is no way to use a double quote character in a
2890 You may include comments in linker scripts just as in C, delimited by
2891 @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent
2894 @node Simple Example
2895 @section Simple Linker Script Example
2896 @cindex linker script example
2897 @cindex example of linker script
2898 Many linker scripts are fairly simple.
2900 The simplest possible linker script has just one command:
2901 @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the
2902 memory layout of the output file.
2904 The @samp{SECTIONS} command is a powerful command. Here we will
2905 describe a simple use of it. Let's assume your program consists only of
2906 code, initialized data, and uninitialized data. These will be in the
2907 @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively.
2908 Let's assume further that these are the only sections which appear in
2911 For this example, let's say that the code should be loaded at address
2912 0x10000, and that the data should start at address 0x8000000. Here is a
2913 linker script which will do that:
2918 .text : @{ *(.text) @}
2920 .data : @{ *(.data) @}
2921 .bss : @{ *(.bss) @}
2925 You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS},
2926 followed by a series of symbol assignments and output section
2927 descriptions enclosed in curly braces.
2929 The first line inside the @samp{SECTIONS} command of the above example
2930 sets the value of the special symbol @samp{.}, which is the location
2931 counter. If you do not specify the address of an output section in some
2932 other way (other ways are described later), the address is set from the
2933 current value of the location counter. The location counter is then
2934 incremented by the size of the output section. At the start of the
2935 @samp{SECTIONS} command, the location counter has the value @samp{0}.
2937 The second line defines an output section, @samp{.text}. The colon is
2938 required syntax which may be ignored for now. Within the curly braces
2939 after the output section name, you list the names of the input sections
2940 which should be placed into this output section. The @samp{*} is a
2941 wildcard which matches any file name. The expression @samp{*(.text)}
2942 means all @samp{.text} input sections in all input files.
2944 Since the location counter is @samp{0x10000} when the output section
2945 @samp{.text} is defined, the linker will set the address of the
2946 @samp{.text} section in the output file to be @samp{0x10000}.
2948 The remaining lines define the @samp{.data} and @samp{.bss} sections in
2949 the output file. The linker will place the @samp{.data} output section
2950 at address @samp{0x8000000}. After the linker places the @samp{.data}
2951 output section, the value of the location counter will be
2952 @samp{0x8000000} plus the size of the @samp{.data} output section. The
2953 effect is that the linker will place the @samp{.bss} output section
2954 immediately after the @samp{.data} output section in memory.
2956 The linker will ensure that each output section has the required
2957 alignment, by increasing the location counter if necessary. In this
2958 example, the specified addresses for the @samp{.text} and @samp{.data}
2959 sections will probably satisfy any alignment constraints, but the linker
2960 may have to create a small gap between the @samp{.data} and @samp{.bss}
2963 That's it! That's a simple and complete linker script.
2965 @node Simple Commands
2966 @section Simple Linker Script Commands
2967 @cindex linker script simple commands
2968 In this section we describe the simple linker script commands.
2971 * Entry Point:: Setting the entry point
2972 * File Commands:: Commands dealing with files
2973 @ifclear SingleFormat
2974 * Format Commands:: Commands dealing with object file formats
2977 * REGION_ALIAS:: Assign alias names to memory regions
2978 * Miscellaneous Commands:: Other linker script commands
2982 @subsection Setting the Entry Point
2983 @kindex ENTRY(@var{symbol})
2984 @cindex start of execution
2985 @cindex first instruction
2987 The first instruction to execute in a program is called the @dfn{entry
2988 point}. You can use the @code{ENTRY} linker script command to set the
2989 entry point. The argument is a symbol name:
2994 There are several ways to set the entry point. The linker will set the
2995 entry point by trying each of the following methods in order, and
2996 stopping when one of them succeeds:
2999 the @samp{-e} @var{entry} command-line option;
3001 the @code{ENTRY(@var{symbol})} command in a linker script;
3003 the value of a target specific symbol, if it is defined; For many
3004 targets this is @code{start}, but PE and BeOS based systems for example
3005 check a list of possible entry symbols, matching the first one found.
3007 the address of the first byte of the @samp{.text} section, if present;
3009 The address @code{0}.
3013 @subsection Commands Dealing with Files
3014 @cindex linker script file commands
3015 Several linker script commands deal with files.
3018 @item INCLUDE @var{filename}
3019 @kindex INCLUDE @var{filename}
3020 @cindex including a linker script
3021 Include the linker script @var{filename} at this point. The file will
3022 be searched for in the current directory, and in any directory specified
3023 with the @option{-L} option. You can nest calls to @code{INCLUDE} up to
3026 You can place @code{INCLUDE} directives at the top level, in @code{MEMORY} or
3027 @code{SECTIONS} commands, or in output section descriptions.
3029 @item INPUT(@var{file}, @var{file}, @dots{})
3030 @itemx INPUT(@var{file} @var{file} @dots{})
3031 @kindex INPUT(@var{files})
3032 @cindex input files in linker scripts
3033 @cindex input object files in linker scripts
3034 @cindex linker script input object files
3035 The @code{INPUT} command directs the linker to include the named files
3036 in the link, as though they were named on the command line.
3038 For example, if you always want to include @file{subr.o} any time you do
3039 a link, but you can't be bothered to put it on every link command line,
3040 then you can put @samp{INPUT (subr.o)} in your linker script.
3042 In fact, if you like, you can list all of your input files in the linker
3043 script, and then invoke the linker with nothing but a @samp{-T} option.
3045 In case a @dfn{sysroot prefix} is configured, and the filename starts
3046 with the @samp{/} character, and the script being processed was
3047 located inside the @dfn{sysroot prefix}, the filename will be looked
3048 for in the @dfn{sysroot prefix}. Otherwise, the linker will try to
3049 open the file in the current directory. If it is not found, the
3050 linker will search through the archive library search path. See the
3051 description of @samp{-L} in @ref{Options,,Command Line Options}.
3053 If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the
3054 name to @code{lib@var{file}.a}, as with the command line argument
3057 When you use the @code{INPUT} command in an implicit linker script, the
3058 files will be included in the link at the point at which the linker
3059 script file is included. This can affect archive searching.
3061 @item GROUP(@var{file}, @var{file}, @dots{})
3062 @itemx GROUP(@var{file} @var{file} @dots{})
3063 @kindex GROUP(@var{files})
3064 @cindex grouping input files
3065 The @code{GROUP} command is like @code{INPUT}, except that the named
3066 files should all be archives, and they are searched repeatedly until no
3067 new undefined references are created. See the description of @samp{-(}
3068 in @ref{Options,,Command Line Options}.
3070 @item AS_NEEDED(@var{file}, @var{file}, @dots{})
3071 @itemx AS_NEEDED(@var{file} @var{file} @dots{})
3072 @kindex AS_NEEDED(@var{files})
3073 This construct can appear only inside of the @code{INPUT} or @code{GROUP}
3074 commands, among other filenames. The files listed will be handled
3075 as if they appear directly in the @code{INPUT} or @code{GROUP} commands,
3076 with the exception of ELF shared libraries, that will be added only
3077 when they are actually needed. This construct essentially enables
3078 @option{--as-needed} option for all the files listed inside of it
3079 and restores previous @option{--as-needed} resp. @option{--no-as-needed}
3082 @item OUTPUT(@var{filename})
3083 @kindex OUTPUT(@var{filename})
3084 @cindex output file name in linker script
3085 The @code{OUTPUT} command names the output file. Using
3086 @code{OUTPUT(@var{filename})} in the linker script is exactly like using
3087 @samp{-o @var{filename}} on the command line (@pxref{Options,,Command
3088 Line Options}). If both are used, the command line option takes
3091 You can use the @code{OUTPUT} command to define a default name for the
3092 output file other than the usual default of @file{a.out}.
3094 @item SEARCH_DIR(@var{path})
3095 @kindex SEARCH_DIR(@var{path})
3096 @cindex library search path in linker script
3097 @cindex archive search path in linker script
3098 @cindex search path in linker script
3099 The @code{SEARCH_DIR} command adds @var{path} to the list of paths where
3100 @command{ld} looks for archive libraries. Using
3101 @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}}
3102 on the command line (@pxref{Options,,Command Line Options}). If both
3103 are used, then the linker will search both paths. Paths specified using
3104 the command line option are searched first.
3106 @item STARTUP(@var{filename})
3107 @kindex STARTUP(@var{filename})
3108 @cindex first input file
3109 The @code{STARTUP} command is just like the @code{INPUT} command, except
3110 that @var{filename} will become the first input file to be linked, as
3111 though it were specified first on the command line. This may be useful
3112 when using a system in which the entry point is always the start of the
3116 @ifclear SingleFormat
3117 @node Format Commands
3118 @subsection Commands Dealing with Object File Formats
3119 A couple of linker script commands deal with object file formats.
3122 @item OUTPUT_FORMAT(@var{bfdname})
3123 @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little})
3124 @kindex OUTPUT_FORMAT(@var{bfdname})
3125 @cindex output file format in linker script
3126 The @code{OUTPUT_FORMAT} command names the BFD format to use for the
3127 output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is
3128 exactly like using @samp{--oformat @var{bfdname}} on the command line
3129 (@pxref{Options,,Command Line Options}). If both are used, the command
3130 line option takes precedence.
3132 You can use @code{OUTPUT_FORMAT} with three arguments to use different
3133 formats based on the @samp{-EB} and @samp{-EL} command line options.
3134 This permits the linker script to set the output format based on the
3137 If neither @samp{-EB} nor @samp{-EL} are used, then the output format
3138 will be the first argument, @var{default}. If @samp{-EB} is used, the
3139 output format will be the second argument, @var{big}. If @samp{-EL} is
3140 used, the output format will be the third argument, @var{little}.
3142 For example, the default linker script for the MIPS ELF target uses this
3145 OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
3147 This says that the default format for the output file is
3148 @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line
3149 option, the output file will be created in the @samp{elf32-littlemips}
3152 @item TARGET(@var{bfdname})
3153 @kindex TARGET(@var{bfdname})
3154 @cindex input file format in linker script
3155 The @code{TARGET} command names the BFD format to use when reading input
3156 files. It affects subsequent @code{INPUT} and @code{GROUP} commands.
3157 This command is like using @samp{-b @var{bfdname}} on the command line
3158 (@pxref{Options,,Command Line Options}). If the @code{TARGET} command
3159 is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET}
3160 command is also used to set the format for the output file. @xref{BFD}.
3165 @subsection Assign alias names to memory regions
3166 @kindex REGION_ALIAS(@var{alias}, @var{region})
3167 @cindex region alias
3168 @cindex region names
3170 Alias names can be added to existing memory regions created with the
3171 @ref{MEMORY} command. Each name corresponds to at most one memory region.
3174 REGION_ALIAS(@var{alias}, @var{region})
3177 The @code{REGION_ALIAS} function creates an alias name @var{alias} for the
3178 memory region @var{region}. This allows a flexible mapping of output sections
3179 to memory regions. An example follows.
3181 Suppose we have an application for embedded systems which come with various
3182 memory storage devices. All have a general purpose, volatile memory @code{RAM}
3183 that allows code execution or data storage. Some may have a read-only,
3184 non-volatile memory @code{ROM} that allows code execution and read-only data
3185 access. The last variant is a read-only, non-volatile memory @code{ROM2} with
3186 read-only data access and no code execution capability. We have four output
3191 @code{.text} program code;
3193 @code{.rodata} read-only data;
3195 @code{.data} read-write initialized data;
3197 @code{.bss} read-write zero initialized data.
3200 The goal is to provide a linker command file that contains a system independent
3201 part defining the output sections and a system dependent part mapping the
3202 output sections to the memory regions available on the system. Our embedded
3203 systems come with three different memory setups @code{A}, @code{B} and
3205 @multitable @columnfractions .25 .25 .25 .25
3206 @item Section @tab Variant A @tab Variant B @tab Variant C
3207 @item .text @tab RAM @tab ROM @tab ROM
3208 @item .rodata @tab RAM @tab ROM @tab ROM2
3209 @item .data @tab RAM @tab RAM/ROM @tab RAM/ROM2
3210 @item .bss @tab RAM @tab RAM @tab RAM
3212 The notation @code{RAM/ROM} or @code{RAM/ROM2} means that this section is
3213 loaded into region @code{ROM} or @code{ROM2} respectively. Please note that
3214 the load address of the @code{.data} section starts in all three variants at
3215 the end of the @code{.rodata} section.
3217 The base linker script that deals with the output sections follows. It
3218 includes the system dependent @code{linkcmds.memory} file that describes the
3221 INCLUDE linkcmds.memory
3234 .data : AT (rodata_end)
3239 data_size = SIZEOF(.data);
3240 data_load_start = LOADADDR(.data);
3248 Now we need three different @code{linkcmds.memory} files to define memory
3249 regions and alias names. The content of @code{linkcmds.memory} for the three
3250 variants @code{A}, @code{B} and @code{C}:
3253 Here everything goes into the @code{RAM}.
3257 RAM : ORIGIN = 0, LENGTH = 4M
3260 REGION_ALIAS("REGION_TEXT", RAM);
3261 REGION_ALIAS("REGION_RODATA", RAM);
3262 REGION_ALIAS("REGION_DATA", RAM);
3263 REGION_ALIAS("REGION_BSS", RAM);
3266 Program code and read-only data go into the @code{ROM}. Read-write data goes
3267 into the @code{RAM}. An image of the initialized data is loaded into the
3268 @code{ROM} and will be copied during system start into the @code{RAM}.
3272 ROM : ORIGIN = 0, LENGTH = 3M
3273 RAM : ORIGIN = 0x10000000, LENGTH = 1M
3276 REGION_ALIAS("REGION_TEXT", ROM);
3277 REGION_ALIAS("REGION_RODATA", ROM);
3278 REGION_ALIAS("REGION_DATA", RAM);
3279 REGION_ALIAS("REGION_BSS", RAM);
3282 Program code goes into the @code{ROM}. Read-only data goes into the
3283 @code{ROM2}. Read-write data goes into the @code{RAM}. An image of the
3284 initialized data is loaded into the @code{ROM2} and will be copied during
3285 system start into the @code{RAM}.
3289 ROM : ORIGIN = 0, LENGTH = 2M
3290 ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
3291 RAM : ORIGIN = 0x20000000, LENGTH = 1M
3294 REGION_ALIAS("REGION_TEXT", ROM);
3295 REGION_ALIAS("REGION_RODATA", ROM2);
3296 REGION_ALIAS("REGION_DATA", RAM);
3297 REGION_ALIAS("REGION_BSS", RAM);
3301 It is possible to write a common system initialization routine to copy the
3302 @code{.data} section from @code{ROM} or @code{ROM2} into the @code{RAM} if
3307 extern char data_start [];
3308 extern char data_size [];
3309 extern char data_load_start [];
3311 void copy_data(void)
3313 if (data_start != data_load_start)
3315 memcpy(data_start, data_load_start, (size_t) data_size);
3320 @node Miscellaneous Commands
3321 @subsection Other Linker Script Commands
3322 There are a few other linker scripts commands.
3325 @item ASSERT(@var{exp}, @var{message})
3327 @cindex assertion in linker script
3328 Ensure that @var{exp} is non-zero. If it is zero, then exit the linker
3329 with an error code, and print @var{message}.
3331 @item EXTERN(@var{symbol} @var{symbol} @dots{})
3333 @cindex undefined symbol in linker script
3334 Force @var{symbol} to be entered in the output file as an undefined
3335 symbol. Doing this may, for example, trigger linking of additional
3336 modules from standard libraries. You may list several @var{symbol}s for
3337 each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This
3338 command has the same effect as the @samp{-u} command-line option.
3340 @item FORCE_COMMON_ALLOCATION
3341 @kindex FORCE_COMMON_ALLOCATION
3342 @cindex common allocation in linker script
3343 This command has the same effect as the @samp{-d} command-line option:
3344 to make @command{ld} assign space to common symbols even if a relocatable
3345 output file is specified (@samp{-r}).
3347 @item INHIBIT_COMMON_ALLOCATION
3348 @kindex INHIBIT_COMMON_ALLOCATION
3349 @cindex common allocation in linker script
3350 This command has the same effect as the @samp{--no-define-common}
3351 command-line option: to make @code{ld} omit the assignment of addresses
3352 to common symbols even for a non-relocatable output file.
3354 @item INSERT [ AFTER | BEFORE ] @var{output_section}
3356 @cindex insert user script into default script
3357 This command is typically used in a script specified by @samp{-T} to
3358 augment the default @code{SECTIONS} with, for example, overlays. It
3359 inserts all prior linker script statements after (or before)
3360 @var{output_section}, and also causes @samp{-T} to not override the
3361 default linker script. The exact insertion point is as for orphan
3362 sections. @xref{Location Counter}. The insertion happens after the
3363 linker has mapped input sections to output sections. Prior to the
3364 insertion, since @samp{-T} scripts are parsed before the default
3365 linker script, statements in the @samp{-T} script occur before the
3366 default linker script statements in the internal linker representation
3367 of the script. In particular, input section assignments will be made
3368 to @samp{-T} output sections before those in the default script. Here
3369 is an example of how a @samp{-T} script using @code{INSERT} might look:
3376 .ov1 @{ ov1*(.text) @}
3377 .ov2 @{ ov2*(.text) @}
3383 @item NOCROSSREFS(@var{section} @var{section} @dots{})
3384 @kindex NOCROSSREFS(@var{sections})
3385 @cindex cross references
3386 This command may be used to tell @command{ld} to issue an error about any
3387 references among certain output sections.
3389 In certain types of programs, particularly on embedded systems when
3390 using overlays, when one section is loaded into memory, another section
3391 will not be. Any direct references between the two sections would be
3392 errors. For example, it would be an error if code in one section called
3393 a function defined in the other section.
3395 The @code{NOCROSSREFS} command takes a list of output section names. If
3396 @command{ld} detects any cross references between the sections, it reports
3397 an error and returns a non-zero exit status. Note that the
3398 @code{NOCROSSREFS} command uses output section names, not input section
3401 @ifclear SingleFormat
3402 @item OUTPUT_ARCH(@var{bfdarch})
3403 @kindex OUTPUT_ARCH(@var{bfdarch})
3404 @cindex machine architecture
3405 @cindex architecture
3406 Specify a particular output machine architecture. The argument is one
3407 of the names used by the BFD library (@pxref{BFD}). You can see the
3408 architecture of an object file by using the @code{objdump} program with
3409 the @samp{-f} option.
3412 @item LD_FEATURE(@var{string})
3413 @kindex LD_FEATURE(@var{string})
3414 This command may be used to modify @command{ld} behavior. If
3415 @var{string} is @code{"SANE_EXPR"} then absolute symbols and numbers
3416 in a script are simply treated as numbers everywhere.
3417 @xref{Expression Section}.
3421 @section Assigning Values to Symbols
3422 @cindex assignment in scripts
3423 @cindex symbol definition, scripts
3424 @cindex variables, defining
3425 You may assign a value to a symbol in a linker script. This will define
3426 the symbol and place it into the symbol table with a global scope.
3429 * Simple Assignments:: Simple Assignments
3432 * PROVIDE_HIDDEN:: PROVIDE_HIDDEN
3433 * Source Code Reference:: How to use a linker script defined symbol in source code
3436 @node Simple Assignments
3437 @subsection Simple Assignments
3439 You may assign to a symbol using any of the C assignment operators:
3442 @item @var{symbol} = @var{expression} ;
3443 @itemx @var{symbol} += @var{expression} ;
3444 @itemx @var{symbol} -= @var{expression} ;
3445 @itemx @var{symbol} *= @var{expression} ;
3446 @itemx @var{symbol} /= @var{expression} ;
3447 @itemx @var{symbol} <<= @var{expression} ;
3448 @itemx @var{symbol} >>= @var{expression} ;
3449 @itemx @var{symbol} &= @var{expression} ;
3450 @itemx @var{symbol} |= @var{expression} ;
3453 The first case will define @var{symbol} to the value of
3454 @var{expression}. In the other cases, @var{symbol} must already be
3455 defined, and the value will be adjusted accordingly.
3457 The special symbol name @samp{.} indicates the location counter. You
3458 may only use this within a @code{SECTIONS} command. @xref{Location Counter}.
3460 The semicolon after @var{expression} is required.
3462 Expressions are defined below; see @ref{Expressions}.
3464 You may write symbol assignments as commands in their own right, or as
3465 statements within a @code{SECTIONS} command, or as part of an output
3466 section description in a @code{SECTIONS} command.
3468 The section of the symbol will be set from the section of the
3469 expression; for more information, see @ref{Expression Section}.
3471 Here is an example showing the three different places that symbol
3472 assignments may be used:
3483 _bdata = (. + 3) & ~ 3;
3484 .data : @{ *(.data) @}
3488 In this example, the symbol @samp{floating_point} will be defined as
3489 zero. The symbol @samp{_etext} will be defined as the address following
3490 the last @samp{.text} input section. The symbol @samp{_bdata} will be
3491 defined as the address following the @samp{.text} output section aligned
3492 upward to a 4 byte boundary.
3497 For ELF targeted ports, define a symbol that will be hidden and won't be
3498 exported. The syntax is @code{HIDDEN(@var{symbol} = @var{expression})}.
3500 Here is the example from @ref{Simple Assignments}, rewritten to use
3504 HIDDEN(floating_point = 0);
3512 HIDDEN(_bdata = (. + 3) & ~ 3);
3513 .data : @{ *(.data) @}
3517 In this case none of the three symbols will be visible outside this module.
3522 In some cases, it is desirable for a linker script to define a symbol
3523 only if it is referenced and is not defined by any object included in
3524 the link. For example, traditional linkers defined the symbol
3525 @samp{etext}. However, ANSI C requires that the user be able to use
3526 @samp{etext} as a function name without encountering an error. The
3527 @code{PROVIDE} keyword may be used to define a symbol, such as
3528 @samp{etext}, only if it is referenced but not defined. The syntax is
3529 @code{PROVIDE(@var{symbol} = @var{expression})}.
3531 Here is an example of using @code{PROVIDE} to define @samp{etext}:
3544 In this example, if the program defines @samp{_etext} (with a leading
3545 underscore), the linker will give a multiple definition error. If, on
3546 the other hand, the program defines @samp{etext} (with no leading
3547 underscore), the linker will silently use the definition in the program.
3548 If the program references @samp{etext} but does not define it, the
3549 linker will use the definition in the linker script.
3551 @node PROVIDE_HIDDEN
3552 @subsection PROVIDE_HIDDEN
3553 @cindex PROVIDE_HIDDEN
3554 Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be
3555 hidden and won't be exported.
3557 @node Source Code Reference
3558 @subsection Source Code Reference
3560 Accessing a linker script defined variable from source code is not
3561 intuitive. In particular a linker script symbol is not equivalent to
3562 a variable declaration in a high level language, it is instead a
3563 symbol that does not have a value.
3565 Before going further, it is important to note that compilers often
3566 transform names in the source code into different names when they are
3567 stored in the symbol table. For example, Fortran compilers commonly
3568 prepend or append an underscore, and C++ performs extensive @samp{name
3569 mangling}. Therefore there might be a discrepancy between the name
3570 of a variable as it is used in source code and the name of the same
3571 variable as it is defined in a linker script. For example in C a
3572 linker script variable might be referred to as:
3578 But in the linker script it might be defined as:
3584 In the remaining examples however it is assumed that no name
3585 transformation has taken place.
3587 When a symbol is declared in a high level language such as C, two
3588 things happen. The first is that the compiler reserves enough space
3589 in the program's memory to hold the @emph{value} of the symbol. The
3590 second is that the compiler creates an entry in the program's symbol
3591 table which holds the symbol's @emph{address}. ie the symbol table
3592 contains the address of the block of memory holding the symbol's
3593 value. So for example the following C declaration, at file scope:
3599 creates a entry called @samp{foo} in the symbol table. This entry
3600 holds the address of an @samp{int} sized block of memory where the
3601 number 1000 is initially stored.
3603 When a program references a symbol the compiler generates code that
3604 first accesses the symbol table to find the address of the symbol's
3605 memory block and then code to read the value from that memory block.
3612 looks up the symbol @samp{foo} in the symbol table, gets the address
3613 associated with this symbol and then writes the value 1 into that
3620 looks up the symbol @samp{foo} in the symbol table, gets it address
3621 and then copies this address into the block of memory associated with
3622 the variable @samp{a}.
3624 Linker scripts symbol declarations, by contrast, create an entry in
3625 the symbol table but do not assign any memory to them. Thus they are
3626 an address without a value. So for example the linker script definition:
3632 creates an entry in the symbol table called @samp{foo} which holds
3633 the address of memory location 1000, but nothing special is stored at
3634 address 1000. This means that you cannot access the @emph{value} of a
3635 linker script defined symbol - it has no value - all you can do is
3636 access the @emph{address} of a linker script defined symbol.
3638 Hence when you are using a linker script defined symbol in source code
3639 you should always take the address of the symbol, and never attempt to
3640 use its value. For example suppose you want to copy the contents of a
3641 section of memory called .ROM into a section called .FLASH and the
3642 linker script contains these declarations:
3646 start_of_ROM = .ROM;
3647 end_of_ROM = .ROM + sizeof (.ROM) - 1;
3648 start_of_FLASH = .FLASH;
3652 Then the C source code to perform the copy would be:
3656 extern char start_of_ROM, end_of_ROM, start_of_FLASH;
3658 memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
3662 Note the use of the @samp{&} operators. These are correct.
3665 @section SECTIONS Command
3667 The @code{SECTIONS} command tells the linker how to map input sections
3668 into output sections, and how to place the output sections in memory.
3670 The format of the @code{SECTIONS} command is:
3674 @var{sections-command}
3675 @var{sections-command}
3680 Each @var{sections-command} may of be one of the following:
3684 an @code{ENTRY} command (@pxref{Entry Point,,Entry command})
3686 a symbol assignment (@pxref{Assignments})
3688 an output section description
3690 an overlay description
3693 The @code{ENTRY} command and symbol assignments are permitted inside the
3694 @code{SECTIONS} command for convenience in using the location counter in
3695 those commands. This can also make the linker script easier to
3696 understand because you can use those commands at meaningful points in
3697 the layout of the output file.
3699 Output section descriptions and overlay descriptions are described
3702 If you do not use a @code{SECTIONS} command in your linker script, the
3703 linker will place each input section into an identically named output
3704 section in the order that the sections are first encountered in the
3705 input files. If all input sections are present in the first file, for
3706 example, the order of sections in the output file will match the order
3707 in the first input file. The first section will be at address zero.
3710 * Output Section Description:: Output section description
3711 * Output Section Name:: Output section name
3712 * Output Section Address:: Output section address
3713 * Input Section:: Input section description
3714 * Output Section Data:: Output section data
3715 * Output Section Keywords:: Output section keywords
3716 * Output Section Discarding:: Output section discarding
3717 * Output Section Attributes:: Output section attributes
3718 * Overlay Description:: Overlay description
3721 @node Output Section Description
3722 @subsection Output Section Description
3723 The full description of an output section looks like this:
3726 @var{section} [@var{address}] [(@var{type})] :
3728 [ALIGN(@var{section_align})]
3729 [SUBALIGN(@var{subsection_align})]
3732 @var{output-section-command}
3733 @var{output-section-command}
3735 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}]
3739 Most output sections do not use most of the optional section attributes.
3741 The whitespace around @var{section} is required, so that the section
3742 name is unambiguous. The colon and the curly braces are also required.
3743 The line breaks and other white space are optional.
3745 Each @var{output-section-command} may be one of the following:
3749 a symbol assignment (@pxref{Assignments})
3751 an input section description (@pxref{Input Section})
3753 data values to include directly (@pxref{Output Section Data})
3755 a special output section keyword (@pxref{Output Section Keywords})
3758 @node Output Section Name
3759 @subsection Output Section Name
3760 @cindex name, section
3761 @cindex section name
3762 The name of the output section is @var{section}. @var{section} must
3763 meet the constraints of your output format. In formats which only
3764 support a limited number of sections, such as @code{a.out}, the name
3765 must be one of the names supported by the format (@code{a.out}, for
3766 example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the
3767 output format supports any number of sections, but with numbers and not
3768 names (as is the case for Oasys), the name should be supplied as a
3769 quoted numeric string. A section name may consist of any sequence of
3770 characters, but a name which contains any unusual characters such as
3771 commas must be quoted.
3773 The output section name @samp{/DISCARD/} is special; @ref{Output Section
3776 @node Output Section Address
3777 @subsection Output Section Address
3778 @cindex address, section
3779 @cindex section address
3780 The @var{address} is an expression for the VMA (the virtual memory
3781 address) of the output section. This address is optional, but if it
3782 is provided then the output address will be set exactly as specified.
3784 If the output address is not specified then one will be chosen for the
3785 section, based on the heuristic below. This address will be adjusted
3786 to fit the alignment requirement of the output section. The
3787 alignment requirement is the strictest alignment of any input section
3788 contained within the output section.
3790 The output section address heuristic is as follows:
3794 If an output memory @var{region} is set for the section then it
3795 is added to this region and its address will be the next free address
3799 If the MEMORY command has been used to create a list of memory
3800 regions then the first region which has attributes compatible with the
3801 section is selected to contain it. The section's output address will
3802 be the next free address in that region; @ref{MEMORY}.
3805 If no memory regions were specified, or none match the section then
3806 the output address will be based on the current value of the location
3814 .text . : @{ *(.text) @}
3821 .text : @{ *(.text) @}
3825 are subtly different. The first will set the address of the
3826 @samp{.text} output section to the current value of the location
3827 counter. The second will set it to the current value of the location
3828 counter aligned to the strictest alignment of any of the @samp{.text}
3831 The @var{address} may be an arbitrary expression; @ref{Expressions}.
3832 For example, if you want to align the section on a 0x10 byte boundary,
3833 so that the lowest four bits of the section address are zero, you could
3834 do something like this:
3836 .text ALIGN(0x10) : @{ *(.text) @}
3839 This works because @code{ALIGN} returns the current location counter
3840 aligned upward to the specified value.
3842 Specifying @var{address} for a section will change the value of the
3843 location counter, provided that the section is non-empty. (Empty
3844 sections are ignored).
3847 @subsection Input Section Description
3848 @cindex input sections
3849 @cindex mapping input sections to output sections
3850 The most common output section command is an input section description.
3852 The input section description is the most basic linker script operation.
3853 You use output sections to tell the linker how to lay out your program
3854 in memory. You use input section descriptions to tell the linker how to
3855 map the input files into your memory layout.
3858 * Input Section Basics:: Input section basics
3859 * Input Section Wildcards:: Input section wildcard patterns
3860 * Input Section Common:: Input section for common symbols
3861 * Input Section Keep:: Input section and garbage collection
3862 * Input Section Example:: Input section example
3865 @node Input Section Basics
3866 @subsubsection Input Section Basics
3867 @cindex input section basics
3868 An input section description consists of a file name optionally followed
3869 by a list of section names in parentheses.
3871 The file name and the section name may be wildcard patterns, which we
3872 describe further below (@pxref{Input Section Wildcards}).
3874 The most common input section description is to include all input
3875 sections with a particular name in the output section. For example, to
3876 include all input @samp{.text} sections, you would write:
3881 Here the @samp{*} is a wildcard which matches any file name. To exclude a list
3882 of files from matching the file name wildcard, EXCLUDE_FILE may be used to
3883 match all files except the ones specified in the EXCLUDE_FILE list. For
3886 *(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
3888 will cause all .ctors sections from all files except @file{crtend.o} and
3889 @file{otherfile.o} to be included.
3891 There are two ways to include more than one section:
3897 The difference between these is the order in which the @samp{.text} and
3898 @samp{.rdata} input sections will appear in the output section. In the
3899 first example, they will be intermingled, appearing in the same order as
3900 they are found in the linker input. In the second example, all
3901 @samp{.text} input sections will appear first, followed by all
3902 @samp{.rdata} input sections.
3904 You can specify a file name to include sections from a particular file.
3905 You would do this if one or more of your files contain special data that
3906 needs to be at a particular location in memory. For example:
3911 To refine the sections that are included based on the section flags
3912 of an input section, INPUT_SECTION_FLAGS may be used.
3914 Here is a simple example for using Section header flags for ELF sections:
3919 .text : @{ INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) @}
3920 .text2 : @{ INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) @}
3925 In this example, the output section @samp{.text} will be comprised of any
3926 input section matching the name *(.text) whose section header flags
3927 @code{SHF_MERGE} and @code{SHF_STRINGS} are set. The output section
3928 @samp{.text2} will be comprised of any input section matching the name *(.text)
3929 whose section header flag @code{SHF_WRITE} is clear.
3931 You can also specify files within archives by writing a pattern
3932 matching the archive, a colon, then the pattern matching the file,
3933 with no whitespace around the colon.
3937 matches file within archive
3939 matches the whole archive
3941 matches file but not one in an archive
3944 Either one or both of @samp{archive} and @samp{file} can contain shell
3945 wildcards. On DOS based file systems, the linker will assume that a
3946 single letter followed by a colon is a drive specifier, so
3947 @samp{c:myfile.o} is a simple file specification, not @samp{myfile.o}
3948 within an archive called @samp{c}. @samp{archive:file} filespecs may
3949 also be used within an @code{EXCLUDE_FILE} list, but may not appear in
3950 other linker script contexts. For instance, you cannot extract a file
3951 from an archive by using @samp{archive:file} in an @code{INPUT}
3954 If you use a file name without a list of sections, then all sections in
3955 the input file will be included in the output section. This is not
3956 commonly done, but it may by useful on occasion. For example:
3961 When you use a file name which is not an @samp{archive:file} specifier
3962 and does not contain any wild card
3963 characters, the linker will first see if you also specified the file
3964 name on the linker command line or in an @code{INPUT} command. If you
3965 did not, the linker will attempt to open the file as an input file, as
3966 though it appeared on the command line. Note that this differs from an
3967 @code{INPUT} command, because the linker will not search for the file in
3968 the archive search path.
3970 @node Input Section Wildcards
3971 @subsubsection Input Section Wildcard Patterns
3972 @cindex input section wildcards
3973 @cindex wildcard file name patterns
3974 @cindex file name wildcard patterns
3975 @cindex section name wildcard patterns
3976 In an input section description, either the file name or the section
3977 name or both may be wildcard patterns.
3979 The file name of @samp{*} seen in many examples is a simple wildcard
3980 pattern for the file name.
3982 The wildcard patterns are like those used by the Unix shell.
3986 matches any number of characters
3988 matches any single character
3990 matches a single instance of any of the @var{chars}; the @samp{-}
3991 character may be used to specify a range of characters, as in
3992 @samp{[a-z]} to match any lower case letter
3994 quotes the following character
3997 When a file name is matched with a wildcard, the wildcard characters
3998 will not match a @samp{/} character (used to separate directory names on
3999 Unix). A pattern consisting of a single @samp{*} character is an
4000 exception; it will always match any file name, whether it contains a
4001 @samp{/} or not. In a section name, the wildcard characters will match
4002 a @samp{/} character.
4004 File name wildcard patterns only match files which are explicitly
4005 specified on the command line or in an @code{INPUT} command. The linker
4006 does not search directories to expand wildcards.
4008 If a file name matches more than one wildcard pattern, or if a file name
4009 appears explicitly and is also matched by a wildcard pattern, the linker
4010 will use the first match in the linker script. For example, this
4011 sequence of input section descriptions is probably in error, because the
4012 @file{data.o} rule will not be used:
4014 .data : @{ *(.data) @}
4015 .data1 : @{ data.o(.data) @}
4018 @cindex SORT_BY_NAME
4019 Normally, the linker will place files and sections matched by wildcards
4020 in the order in which they are seen during the link. You can change
4021 this by using the @code{SORT_BY_NAME} keyword, which appears before a wildcard
4022 pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the
4023 @code{SORT_BY_NAME} keyword is used, the linker will sort the files or sections
4024 into ascending order by name before placing them in the output file.
4026 @cindex SORT_BY_ALIGNMENT
4027 @code{SORT_BY_ALIGNMENT} is very similar to @code{SORT_BY_NAME}. The
4028 difference is @code{SORT_BY_ALIGNMENT} will sort sections into
4029 ascending order by alignment before placing them in the output file.
4031 @cindex SORT_BY_INIT_PRIORITY
4032 @code{SORT_BY_INIT_PRIORITY} is very similar to @code{SORT_BY_NAME}. The
4033 difference is @code{SORT_BY_INIT_PRIORITY} will sort sections into
4034 ascending order by numerical value of the GCC init_priority attribute
4035 encoded in the section name before placing them in the output file.
4038 @code{SORT} is an alias for @code{SORT_BY_NAME}.
4040 When there are nested section sorting commands in linker script, there
4041 can be at most 1 level of nesting for section sorting commands.
4045 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
4046 It will sort the input sections by name first, then by alignment if 2
4047 sections have the same name.
4049 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
4050 It will sort the input sections by alignment first, then by name if 2
4051 sections have the same alignment.
4053 @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is
4054 treated the same as @code{SORT_BY_NAME} (wildcard section pattern).
4056 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern))
4057 is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern).
4059 All other nested section sorting commands are invalid.
4062 When both command line section sorting option and linker script
4063 section sorting command are used, section sorting command always
4064 takes precedence over the command line option.
4066 If the section sorting command in linker script isn't nested, the
4067 command line option will make the section sorting command to be
4068 treated as nested sorting command.
4072 @code{SORT_BY_NAME} (wildcard section pattern ) with
4073 @option{--sort-sections alignment} is equivalent to
4074 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
4076 @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with
4077 @option{--sort-section name} is equivalent to
4078 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
4081 If the section sorting command in linker script is nested, the
4082 command line option will be ignored.
4085 @code{SORT_NONE} disables section sorting by ignoring the command line
4086 section sorting option.
4088 If you ever get confused about where input sections are going, use the
4089 @samp{-M} linker option to generate a map file. The map file shows
4090 precisely how input sections are mapped to output sections.
4092 This example shows how wildcard patterns might be used to partition
4093 files. This linker script directs the linker to place all @samp{.text}
4094 sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}.
4095 The linker will place the @samp{.data} section from all files beginning
4096 with an upper case character in @samp{.DATA}; for all other files, the
4097 linker will place the @samp{.data} section in @samp{.data}.
4101 .text : @{ *(.text) @}
4102 .DATA : @{ [A-Z]*(.data) @}
4103 .data : @{ *(.data) @}
4104 .bss : @{ *(.bss) @}
4109 @node Input Section Common
4110 @subsubsection Input Section for Common Symbols
4111 @cindex common symbol placement
4112 @cindex uninitialized data placement
4113 A special notation is needed for common symbols, because in many object
4114 file formats common symbols do not have a particular input section. The
4115 linker treats common symbols as though they are in an input section
4116 named @samp{COMMON}.
4118 You may use file names with the @samp{COMMON} section just as with any
4119 other input sections. You can use this to place common symbols from a
4120 particular input file in one section while common symbols from other
4121 input files are placed in another section.
4123 In most cases, common symbols in input files will be placed in the
4124 @samp{.bss} section in the output file. For example:
4126 .bss @{ *(.bss) *(COMMON) @}
4129 @cindex scommon section
4130 @cindex small common symbols
4131 Some object file formats have more than one type of common symbol. For
4132 example, the MIPS ELF object file format distinguishes standard common
4133 symbols and small common symbols. In this case, the linker will use a
4134 different special section name for other types of common symbols. In
4135 the case of MIPS ELF, the linker uses @samp{COMMON} for standard common
4136 symbols and @samp{.scommon} for small common symbols. This permits you
4137 to map the different types of common symbols into memory at different
4141 You will sometimes see @samp{[COMMON]} in old linker scripts. This
4142 notation is now considered obsolete. It is equivalent to
4145 @node Input Section Keep
4146 @subsubsection Input Section and Garbage Collection
4148 @cindex garbage collection
4149 When link-time garbage collection is in use (@samp{--gc-sections}),
4150 it is often useful to mark sections that should not be eliminated.
4151 This is accomplished by surrounding an input section's wildcard entry
4152 with @code{KEEP()}, as in @code{KEEP(*(.init))} or
4153 @code{KEEP(SORT_BY_NAME(*)(.ctors))}.
4155 @node Input Section Example
4156 @subsubsection Input Section Example
4157 The following example is a complete linker script. It tells the linker
4158 to read all of the sections from file @file{all.o} and place them at the
4159 start of output section @samp{outputa} which starts at location
4160 @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o}
4161 follows immediately, in the same output section. All of section
4162 @samp{.input2} from @file{foo.o} goes into output section
4163 @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}.
4164 All of the remaining @samp{.input1} and @samp{.input2} sections from any
4165 files are written to output section @samp{outputc}.
4193 @node Output Section Data
4194 @subsection Output Section Data
4196 @cindex section data
4197 @cindex output section data
4198 @kindex BYTE(@var{expression})
4199 @kindex SHORT(@var{expression})
4200 @kindex LONG(@var{expression})
4201 @kindex QUAD(@var{expression})
4202 @kindex SQUAD(@var{expression})
4203 You can include explicit bytes of data in an output section by using
4204 @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as
4205 an output section command. Each keyword is followed by an expression in
4206 parentheses providing the value to store (@pxref{Expressions}). The
4207 value of the expression is stored at the current value of the location
4210 The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands
4211 store one, two, four, and eight bytes (respectively). After storing the
4212 bytes, the location counter is incremented by the number of bytes
4215 For example, this will store the byte 1 followed by the four byte value
4216 of the symbol @samp{addr}:
4222 When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the
4223 same; they both store an 8 byte, or 64 bit, value. When both host and
4224 target are 32 bits, an expression is computed as 32 bits. In this case
4225 @code{QUAD} stores a 32 bit value zero extended to 64 bits, and
4226 @code{SQUAD} stores a 32 bit value sign extended to 64 bits.
4228 If the object file format of the output file has an explicit endianness,
4229 which is the normal case, the value will be stored in that endianness.
4230 When the object file format does not have an explicit endianness, as is
4231 true of, for example, S-records, the value will be stored in the
4232 endianness of the first input object file.
4234 Note---these commands only work inside a section description and not
4235 between them, so the following will produce an error from the linker:
4237 SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@
4239 whereas this will work:
4241 SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@
4244 @kindex FILL(@var{expression})
4245 @cindex holes, filling
4246 @cindex unspecified memory
4247 You may use the @code{FILL} command to set the fill pattern for the
4248 current section. It is followed by an expression in parentheses. Any
4249 otherwise unspecified regions of memory within the section (for example,
4250 gaps left due to the required alignment of input sections) are filled
4251 with the value of the expression, repeated as
4252 necessary. A @code{FILL} statement covers memory locations after the
4253 point at which it occurs in the section definition; by including more
4254 than one @code{FILL} statement, you can have different fill patterns in
4255 different parts of an output section.
4257 This example shows how to fill unspecified regions of memory with the
4263 The @code{FILL} command is similar to the @samp{=@var{fillexp}} output
4264 section attribute, but it only affects the
4265 part of the section following the @code{FILL} command, rather than the
4266 entire section. If both are used, the @code{FILL} command takes
4267 precedence. @xref{Output Section Fill}, for details on the fill
4270 @node Output Section Keywords
4271 @subsection Output Section Keywords
4272 There are a couple of keywords which can appear as output section
4276 @kindex CREATE_OBJECT_SYMBOLS
4277 @cindex input filename symbols
4278 @cindex filename symbols
4279 @item CREATE_OBJECT_SYMBOLS
4280 The command tells the linker to create a symbol for each input file.
4281 The name of each symbol will be the name of the corresponding input
4282 file. The section of each symbol will be the output section in which
4283 the @code{CREATE_OBJECT_SYMBOLS} command appears.
4285 This is conventional for the a.out object file format. It is not
4286 normally used for any other object file format.
4288 @kindex CONSTRUCTORS
4289 @cindex C++ constructors, arranging in link
4290 @cindex constructors, arranging in link
4292 When linking using the a.out object file format, the linker uses an
4293 unusual set construct to support C++ global constructors and
4294 destructors. When linking object file formats which do not support
4295 arbitrary sections, such as ECOFF and XCOFF, the linker will
4296 automatically recognize C++ global constructors and destructors by name.
4297 For these object file formats, the @code{CONSTRUCTORS} command tells the
4298 linker to place constructor information in the output section where the
4299 @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is
4300 ignored for other object file formats.
4302 The symbol @w{@code{__CTOR_LIST__}} marks the start of the global
4303 constructors, and the symbol @w{@code{__CTOR_END__}} marks the end.
4304 Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_END__}} mark
4305 the start and end of the global destructors. The
4306 first word in the list is the number of entries, followed by the address
4307 of each constructor or destructor, followed by a zero word. The
4308 compiler must arrange to actually run the code. For these object file
4309 formats @sc{gnu} C++ normally calls constructors from a subroutine
4310 @code{__main}; a call to @code{__main} is automatically inserted into
4311 the startup code for @code{main}. @sc{gnu} C++ normally runs
4312 destructors either by using @code{atexit}, or directly from the function
4315 For object file formats such as @code{COFF} or @code{ELF} which support
4316 arbitrary section names, @sc{gnu} C++ will normally arrange to put the
4317 addresses of global constructors and destructors into the @code{.ctors}
4318 and @code{.dtors} sections. Placing the following sequence into your
4319 linker script will build the sort of table which the @sc{gnu} C++
4320 runtime code expects to see.
4324 LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
4329 LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
4335 If you are using the @sc{gnu} C++ support for initialization priority,
4336 which provides some control over the order in which global constructors
4337 are run, you must sort the constructors at link time to ensure that they
4338 are executed in the correct order. When using the @code{CONSTRUCTORS}
4339 command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the
4340 @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and
4341 @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and
4344 Normally the compiler and linker will handle these issues automatically,
4345 and you will not need to concern yourself with them. However, you may
4346 need to consider this if you are using C++ and writing your own linker
4351 @node Output Section Discarding
4352 @subsection Output Section Discarding
4353 @cindex discarding sections
4354 @cindex sections, discarding
4355 @cindex removing sections
4356 The linker will not create output sections with no contents. This is
4357 for convenience when referring to input sections that may or may not
4358 be present in any of the input files. For example:
4360 .foo : @{ *(.foo) @}
4363 will only create a @samp{.foo} section in the output file if there is a
4364 @samp{.foo} section in at least one input file, and if the input
4365 sections are not all empty. Other link script directives that allocate
4366 space in an output section will also create the output section.
4368 The linker will ignore address assignments (@pxref{Output Section Address})
4369 on discarded output sections, except when the linker script defines
4370 symbols in the output section. In that case the linker will obey
4371 the address assignments, possibly advancing dot even though the
4372 section is discarded.
4375 The special output section name @samp{/DISCARD/} may be used to discard
4376 input sections. Any input sections which are assigned to an output
4377 section named @samp{/DISCARD/} are not included in the output file.
4379 @node Output Section Attributes
4380 @subsection Output Section Attributes
4381 @cindex output section attributes
4382 We showed above that the full description of an output section looked
4387 @var{section} [@var{address}] [(@var{type})] :
4389 [ALIGN(@var{section_align})]
4390 [SUBALIGN(@var{subsection_align})]
4393 @var{output-section-command}
4394 @var{output-section-command}
4396 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}]
4400 We've already described @var{section}, @var{address}, and
4401 @var{output-section-command}. In this section we will describe the
4402 remaining section attributes.
4405 * Output Section Type:: Output section type
4406 * Output Section LMA:: Output section LMA
4407 * Forced Output Alignment:: Forced Output Alignment
4408 * Forced Input Alignment:: Forced Input Alignment
4409 * Output Section Constraint:: Output section constraint
4410 * Output Section Region:: Output section region
4411 * Output Section Phdr:: Output section phdr
4412 * Output Section Fill:: Output section fill
4415 @node Output Section Type
4416 @subsubsection Output Section Type
4417 Each output section may have a type. The type is a keyword in
4418 parentheses. The following types are defined:
4422 The section should be marked as not loadable, so that it will not be
4423 loaded into memory when the program is run.
4428 These type names are supported for backward compatibility, and are
4429 rarely used. They all have the same effect: the section should be
4430 marked as not allocatable, so that no memory is allocated for the
4431 section when the program is run.
4435 @cindex prevent unnecessary loading
4436 @cindex loading, preventing
4437 The linker normally sets the attributes of an output section based on
4438 the input sections which map into it. You can override this by using
4439 the section type. For example, in the script sample below, the
4440 @samp{ROM} section is addressed at memory location @samp{0} and does not
4441 need to be loaded when the program is run.
4445 ROM 0 (NOLOAD) : @{ @dots{} @}
4451 @node Output Section LMA
4452 @subsubsection Output Section LMA
4453 @kindex AT>@var{lma_region}
4454 @kindex AT(@var{lma})
4455 @cindex load address
4456 @cindex section load address
4457 Every section has a virtual address (VMA) and a load address (LMA); see
4458 @ref{Basic Script Concepts}. The virtual address is specified by the
4459 @pxref{Output Section Address} described earlier. The load address is
4460 specified by the @code{AT} or @code{AT>} keywords. Specifying a load
4461 address is optional.
4463 The @code{AT} keyword takes an expression as an argument. This
4464 specifies the exact load address of the section. The @code{AT>} keyword
4465 takes the name of a memory region as an argument. @xref{MEMORY}. The
4466 load address of the section is set to the next free address in the
4467 region, aligned to the section's alignment requirements.
4469 If neither @code{AT} nor @code{AT>} is specified for an allocatable
4470 section, the linker will use the following heuristic to determine the
4475 If the section has a specific VMA address, then this is used as
4476 the LMA address as well.
4479 If the section is not allocatable then its LMA is set to its VMA.
4482 Otherwise if a memory region can be found that is compatible
4483 with the current section, and this region contains at least one
4484 section, then the LMA is set so the difference between the
4485 VMA and LMA is the same as the difference between the VMA and LMA of
4486 the last section in the located region.
4489 If no memory regions have been declared then a default region
4490 that covers the entire address space is used in the previous step.
4493 If no suitable region could be found, or there was no previous
4494 section then the LMA is set equal to the VMA.
4497 @cindex ROM initialized data
4498 @cindex initialized data in ROM
4499 This feature is designed to make it easy to build a ROM image. For
4500 example, the following linker script creates three output sections: one
4501 called @samp{.text}, which starts at @code{0x1000}, one called
4502 @samp{.mdata}, which is loaded at the end of the @samp{.text} section
4503 even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold
4504 uninitialized data at address @code{0x3000}. The symbol @code{_data} is
4505 defined with the value @code{0x2000}, which shows that the location
4506 counter holds the VMA value, not the LMA value.
4512 .text 0x1000 : @{ *(.text) _etext = . ; @}
4514 AT ( ADDR (.text) + SIZEOF (.text) )
4515 @{ _data = . ; *(.data); _edata = . ; @}
4517 @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@}
4522 The run-time initialization code for use with a program generated with
4523 this linker script would include something like the following, to copy
4524 the initialized data from the ROM image to its runtime address. Notice
4525 how this code takes advantage of the symbols defined by the linker
4530 extern char _etext, _data, _edata, _bstart, _bend;
4531 char *src = &_etext;
4534 /* ROM has data at end of text; copy it. */
4535 while (dst < &_edata)
4539 for (dst = &_bstart; dst< &_bend; dst++)
4544 @node Forced Output Alignment
4545 @subsubsection Forced Output Alignment
4546 @kindex ALIGN(@var{section_align})
4547 @cindex forcing output section alignment
4548 @cindex output section alignment
4549 You can increase an output section's alignment by using ALIGN.
4551 @node Forced Input Alignment
4552 @subsubsection Forced Input Alignment
4553 @kindex SUBALIGN(@var{subsection_align})
4554 @cindex forcing input section alignment
4555 @cindex input section alignment
4556 You can force input section alignment within an output section by using
4557 SUBALIGN. The value specified overrides any alignment given by input
4558 sections, whether larger or smaller.
4560 @node Output Section Constraint
4561 @subsubsection Output Section Constraint
4564 @cindex constraints on output sections
4565 You can specify that an output section should only be created if all
4566 of its input sections are read-only or all of its input sections are
4567 read-write by using the keyword @code{ONLY_IF_RO} and
4568 @code{ONLY_IF_RW} respectively.
4570 @node Output Section Region
4571 @subsubsection Output Section Region
4572 @kindex >@var{region}
4573 @cindex section, assigning to memory region
4574 @cindex memory regions and sections
4575 You can assign a section to a previously defined region of memory by
4576 using @samp{>@var{region}}. @xref{MEMORY}.
4578 Here is a simple example:
4581 MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @}
4582 SECTIONS @{ ROM : @{ *(.text) @} >rom @}
4586 @node Output Section Phdr
4587 @subsubsection Output Section Phdr
4589 @cindex section, assigning to program header
4590 @cindex program headers and sections
4591 You can assign a section to a previously defined program segment by
4592 using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to
4593 one or more segments, then all subsequent allocated sections will be
4594 assigned to those segments as well, unless they use an explicitly
4595 @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the
4596 linker to not put the section in any segment at all.
4598 Here is a simple example:
4601 PHDRS @{ text PT_LOAD ; @}
4602 SECTIONS @{ .text : @{ *(.text) @} :text @}
4606 @node Output Section Fill
4607 @subsubsection Output Section Fill
4608 @kindex =@var{fillexp}
4609 @cindex section fill pattern
4610 @cindex fill pattern, entire section
4611 You can set the fill pattern for an entire section by using
4612 @samp{=@var{fillexp}}. @var{fillexp} is an expression
4613 (@pxref{Expressions}). Any otherwise unspecified regions of memory
4614 within the output section (for example, gaps left due to the required
4615 alignment of input sections) will be filled with the value, repeated as
4616 necessary. If the fill expression is a simple hex number, ie. a string
4617 of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then
4618 an arbitrarily long sequence of hex digits can be used to specify the
4619 fill pattern; Leading zeros become part of the pattern too. For all
4620 other cases, including extra parentheses or a unary @code{+}, the fill
4621 pattern is the four least significant bytes of the value of the
4622 expression. In all cases, the number is big-endian.
4624 You can also change the fill value with a @code{FILL} command in the
4625 output section commands; (@pxref{Output Section Data}).
4627 Here is a simple example:
4630 SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @}
4634 @node Overlay Description
4635 @subsection Overlay Description
4638 An overlay description provides an easy way to describe sections which
4639 are to be loaded as part of a single memory image but are to be run at
4640 the same memory address. At run time, some sort of overlay manager will
4641 copy the overlaid sections in and out of the runtime memory address as
4642 required, perhaps by simply manipulating addressing bits. This approach
4643 can be useful, for example, when a certain region of memory is faster
4646 Overlays are described using the @code{OVERLAY} command. The
4647 @code{OVERLAY} command is used within a @code{SECTIONS} command, like an
4648 output section description. The full syntax of the @code{OVERLAY}
4649 command is as follows:
4652 OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )]
4656 @var{output-section-command}
4657 @var{output-section-command}
4659 @} [:@var{phdr}@dots{}] [=@var{fill}]
4662 @var{output-section-command}
4663 @var{output-section-command}
4665 @} [:@var{phdr}@dots{}] [=@var{fill}]
4667 @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}]
4671 Everything is optional except @code{OVERLAY} (a keyword), and each
4672 section must have a name (@var{secname1} and @var{secname2} above). The
4673 section definitions within the @code{OVERLAY} construct are identical to
4674 those within the general @code{SECTIONS} construct (@pxref{SECTIONS}),
4675 except that no addresses and no memory regions may be defined for
4676 sections within an @code{OVERLAY}.
4678 The sections are all defined with the same starting address. The load
4679 addresses of the sections are arranged such that they are consecutive in
4680 memory starting at the load address used for the @code{OVERLAY} as a
4681 whole (as with normal section definitions, the load address is optional,
4682 and defaults to the start address; the start address is also optional,
4683 and defaults to the current value of the location counter).
4685 If the @code{NOCROSSREFS} keyword is used, and there any references
4686 among the sections, the linker will report an error. Since the sections
4687 all run at the same address, it normally does not make sense for one
4688 section to refer directly to another. @xref{Miscellaneous Commands,
4691 For each section within the @code{OVERLAY}, the linker automatically
4692 provides two symbols. The symbol @code{__load_start_@var{secname}} is
4693 defined as the starting load address of the section. The symbol
4694 @code{__load_stop_@var{secname}} is defined as the final load address of
4695 the section. Any characters within @var{secname} which are not legal
4696 within C identifiers are removed. C (or assembler) code may use these
4697 symbols to move the overlaid sections around as necessary.
4699 At the end of the overlay, the value of the location counter is set to
4700 the start address of the overlay plus the size of the largest section.
4702 Here is an example. Remember that this would appear inside a
4703 @code{SECTIONS} construct.
4706 OVERLAY 0x1000 : AT (0x4000)
4708 .text0 @{ o1/*.o(.text) @}
4709 .text1 @{ o2/*.o(.text) @}
4714 This will define both @samp{.text0} and @samp{.text1} to start at
4715 address 0x1000. @samp{.text0} will be loaded at address 0x4000, and
4716 @samp{.text1} will be loaded immediately after @samp{.text0}. The
4717 following symbols will be defined if referenced: @code{__load_start_text0},
4718 @code{__load_stop_text0}, @code{__load_start_text1},
4719 @code{__load_stop_text1}.
4721 C code to copy overlay @code{.text1} into the overlay area might look
4726 extern char __load_start_text1, __load_stop_text1;
4727 memcpy ((char *) 0x1000, &__load_start_text1,
4728 &__load_stop_text1 - &__load_start_text1);
4732 Note that the @code{OVERLAY} command is just syntactic sugar, since
4733 everything it does can be done using the more basic commands. The above
4734 example could have been written identically as follows.
4738 .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @}
4739 PROVIDE (__load_start_text0 = LOADADDR (.text0));
4740 PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
4741 .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @}
4742 PROVIDE (__load_start_text1 = LOADADDR (.text1));
4743 PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
4744 . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
4749 @section MEMORY Command
4751 @cindex memory regions
4752 @cindex regions of memory
4753 @cindex allocating memory
4754 @cindex discontinuous memory
4755 The linker's default configuration permits allocation of all available
4756 memory. You can override this by using the @code{MEMORY} command.
4758 The @code{MEMORY} command describes the location and size of blocks of
4759 memory in the target. You can use it to describe which memory regions
4760 may be used by the linker, and which memory regions it must avoid. You
4761 can then assign sections to particular memory regions. The linker will
4762 set section addresses based on the memory regions, and will warn about
4763 regions that become too full. The linker will not shuffle sections
4764 around to fit into the available regions.
4766 A linker script may contain at most one use of the @code{MEMORY}
4767 command. However, you can define as many blocks of memory within it as
4768 you wish. The syntax is:
4773 @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len}
4779 The @var{name} is a name used in the linker script to refer to the
4780 region. The region name has no meaning outside of the linker script.
4781 Region names are stored in a separate name space, and will not conflict
4782 with symbol names, file names, or section names. Each memory region
4783 must have a distinct name within the @code{MEMORY} command. However you can
4784 add later alias names to existing memory regions with the @ref{REGION_ALIAS}
4787 @cindex memory region attributes
4788 The @var{attr} string is an optional list of attributes that specify
4789 whether to use a particular memory region for an input section which is
4790 not explicitly mapped in the linker script. As described in
4791 @ref{SECTIONS}, if you do not specify an output section for some input
4792 section, the linker will create an output section with the same name as
4793 the input section. If you define region attributes, the linker will use
4794 them to select the memory region for the output section that it creates.
4796 The @var{attr} string must consist only of the following characters:
4811 Invert the sense of any of the attributes that follow
4814 If a unmapped section matches any of the listed attributes other than
4815 @samp{!}, it will be placed in the memory region. The @samp{!}
4816 attribute reverses this test, so that an unmapped section will be placed
4817 in the memory region only if it does not match any of the listed
4823 The @var{origin} is an numerical expression for the start address of
4824 the memory region. The expression must evaluate to a constant and it
4825 cannot involve any symbols. The keyword @code{ORIGIN} may be
4826 abbreviated to @code{org} or @code{o} (but not, for example,
4832 The @var{len} is an expression for the size in bytes of the memory
4833 region. As with the @var{origin} expression, the expression must
4834 be numerical only and must evaluate to a constant. The keyword
4835 @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
4837 In the following example, we specify that there are two memory regions
4838 available for allocation: one starting at @samp{0} for 256 kilobytes,
4839 and the other starting at @samp{0x40000000} for four megabytes. The
4840 linker will place into the @samp{rom} memory region every section which
4841 is not explicitly mapped into a memory region, and is either read-only
4842 or executable. The linker will place other sections which are not
4843 explicitly mapped into a memory region into the @samp{ram} memory
4850 rom (rx) : ORIGIN = 0, LENGTH = 256K
4851 ram (!rx) : org = 0x40000000, l = 4M
4856 Once you define a memory region, you can direct the linker to place
4857 specific output sections into that memory region by using the
4858 @samp{>@var{region}} output section attribute. For example, if you have
4859 a memory region named @samp{mem}, you would use @samp{>mem} in the
4860 output section definition. @xref{Output Section Region}. If no address
4861 was specified for the output section, the linker will set the address to
4862 the next available address within the memory region. If the combined
4863 output sections directed to a memory region are too large for the
4864 region, the linker will issue an error message.
4866 It is possible to access the origin and length of a memory in an
4867 expression via the @code{ORIGIN(@var{memory})} and
4868 @code{LENGTH(@var{memory})} functions:
4872 _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
4877 @section PHDRS Command
4879 @cindex program headers
4880 @cindex ELF program headers
4881 @cindex program segments
4882 @cindex segments, ELF
4883 The ELF object file format uses @dfn{program headers}, also knows as
4884 @dfn{segments}. The program headers describe how the program should be
4885 loaded into memory. You can print them out by using the @code{objdump}
4886 program with the @samp{-p} option.
4888 When you run an ELF program on a native ELF system, the system loader
4889 reads the program headers in order to figure out how to load the
4890 program. This will only work if the program headers are set correctly.
4891 This manual does not describe the details of how the system loader
4892 interprets program headers; for more information, see the ELF ABI.
4894 The linker will create reasonable program headers by default. However,
4895 in some cases, you may need to specify the program headers more
4896 precisely. You may use the @code{PHDRS} command for this purpose. When
4897 the linker sees the @code{PHDRS} command in the linker script, it will
4898 not create any program headers other than the ones specified.
4900 The linker only pays attention to the @code{PHDRS} command when
4901 generating an ELF output file. In other cases, the linker will simply
4902 ignore @code{PHDRS}.
4904 This is the syntax of the @code{PHDRS} command. The words @code{PHDRS},
4905 @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords.
4911 @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ]
4912 [ FLAGS ( @var{flags} ) ] ;
4917 The @var{name} is used only for reference in the @code{SECTIONS} command
4918 of the linker script. It is not put into the output file. Program
4919 header names are stored in a separate name space, and will not conflict
4920 with symbol names, file names, or section names. Each program header
4921 must have a distinct name. The headers are processed in order and it
4922 is usual for them to map to sections in ascending load address order.
4924 Certain program header types describe segments of memory which the
4925 system loader will load from the file. In the linker script, you
4926 specify the contents of these segments by placing allocatable output
4927 sections in the segments. You use the @samp{:@var{phdr}} output section
4928 attribute to place a section in a particular segment. @xref{Output
4931 It is normal to put certain sections in more than one segment. This
4932 merely implies that one segment of memory contains another. You may
4933 repeat @samp{:@var{phdr}}, using it once for each segment which should
4934 contain the section.
4936 If you place a section in one or more segments using @samp{:@var{phdr}},
4937 then the linker will place all subsequent allocatable sections which do
4938 not specify @samp{:@var{phdr}} in the same segments. This is for
4939 convenience, since generally a whole set of contiguous sections will be
4940 placed in a single segment. You can use @code{:NONE} to override the
4941 default segment and tell the linker to not put the section in any
4946 You may use the @code{FILEHDR} and @code{PHDRS} keywords after
4947 the program header type to further describe the contents of the segment.
4948 The @code{FILEHDR} keyword means that the segment should include the ELF
4949 file header. The @code{PHDRS} keyword means that the segment should
4950 include the ELF program headers themselves. If applied to a loadable
4951 segment (@code{PT_LOAD}), all prior loadable segments must have one of
4954 The @var{type} may be one of the following. The numbers indicate the
4955 value of the keyword.
4958 @item @code{PT_NULL} (0)
4959 Indicates an unused program header.
4961 @item @code{PT_LOAD} (1)
4962 Indicates that this program header describes a segment to be loaded from
4965 @item @code{PT_DYNAMIC} (2)
4966 Indicates a segment where dynamic linking information can be found.
4968 @item @code{PT_INTERP} (3)
4969 Indicates a segment where the name of the program interpreter may be
4972 @item @code{PT_NOTE} (4)
4973 Indicates a segment holding note information.
4975 @item @code{PT_SHLIB} (5)
4976 A reserved program header type, defined but not specified by the ELF
4979 @item @code{PT_PHDR} (6)
4980 Indicates a segment where the program headers may be found.
4982 @item @var{expression}
4983 An expression giving the numeric type of the program header. This may
4984 be used for types not defined above.
4987 You can specify that a segment should be loaded at a particular address
4988 in memory by using an @code{AT} expression. This is identical to the
4989 @code{AT} command used as an output section attribute (@pxref{Output
4990 Section LMA}). The @code{AT} command for a program header overrides the
4991 output section attribute.
4993 The linker will normally set the segment flags based on the sections
4994 which comprise the segment. You may use the @code{FLAGS} keyword to
4995 explicitly specify the segment flags. The value of @var{flags} must be
4996 an integer. It is used to set the @code{p_flags} field of the program
4999 Here is an example of @code{PHDRS}. This shows a typical set of program
5000 headers used on a native ELF system.
5006 headers PT_PHDR PHDRS ;
5008 text PT_LOAD FILEHDR PHDRS ;
5010 dynamic PT_DYNAMIC ;
5016 .interp : @{ *(.interp) @} :text :interp
5017 .text : @{ *(.text) @} :text
5018 .rodata : @{ *(.rodata) @} /* defaults to :text */
5020 . = . + 0x1000; /* move to a new page in memory */
5021 .data : @{ *(.data) @} :data
5022 .dynamic : @{ *(.dynamic) @} :data :dynamic
5029 @section VERSION Command
5030 @kindex VERSION @{script text@}
5031 @cindex symbol versions
5032 @cindex version script
5033 @cindex versions of symbols
5034 The linker supports symbol versions when using ELF. Symbol versions are
5035 only useful when using shared libraries. The dynamic linker can use
5036 symbol versions to select a specific version of a function when it runs
5037 a program that may have been linked against an earlier version of the
5040 You can include a version script directly in the main linker script, or
5041 you can supply the version script as an implicit linker script. You can
5042 also use the @samp{--version-script} linker option.
5044 The syntax of the @code{VERSION} command is simply
5046 VERSION @{ version-script-commands @}
5049 The format of the version script commands is identical to that used by
5050 Sun's linker in Solaris 2.5. The version script defines a tree of
5051 version nodes. You specify the node names and interdependencies in the
5052 version script. You can specify which symbols are bound to which
5053 version nodes, and you can reduce a specified set of symbols to local
5054 scope so that they are not globally visible outside of the shared
5057 The easiest way to demonstrate the version script language is with a few
5083 This example version script defines three version nodes. The first
5084 version node defined is @samp{VERS_1.1}; it has no other dependencies.
5085 The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces
5086 a number of symbols to local scope so that they are not visible outside
5087 of the shared library; this is done using wildcard patterns, so that any
5088 symbol whose name begins with @samp{old}, @samp{original}, or @samp{new}
5089 is matched. The wildcard patterns available are the same as those used
5090 in the shell when matching filenames (also known as ``globbing'').
5091 However, if you specify the symbol name inside double quotes, then the
5092 name is treated as literal, rather than as a glob pattern.
5094 Next, the version script defines node @samp{VERS_1.2}. This node
5095 depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2}
5096 to the version node @samp{VERS_1.2}.
5098 Finally, the version script defines node @samp{VERS_2.0}. This node
5099 depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1}
5100 and @samp{bar2} are bound to the version node @samp{VERS_2.0}.
5102 When the linker finds a symbol defined in a library which is not
5103 specifically bound to a version node, it will effectively bind it to an
5104 unspecified base version of the library. You can bind all otherwise
5105 unspecified symbols to a given version node by using @samp{global: *;}
5106 somewhere in the version script. Note that it's slightly crazy to use
5107 wildcards in a global spec except on the last version node. Global
5108 wildcards elsewhere run the risk of accidentally adding symbols to the
5109 set exported for an old version. That's wrong since older versions
5110 ought to have a fixed set of symbols.
5112 The names of the version nodes have no specific meaning other than what
5113 they might suggest to the person reading them. The @samp{2.0} version
5114 could just as well have appeared in between @samp{1.1} and @samp{1.2}.
5115 However, this would be a confusing way to write a version script.
5117 Node name can be omitted, provided it is the only version node
5118 in the version script. Such version script doesn't assign any versions to
5119 symbols, only selects which symbols will be globally visible out and which
5123 @{ global: foo; bar; local: *; @};
5126 When you link an application against a shared library that has versioned
5127 symbols, the application itself knows which version of each symbol it
5128 requires, and it also knows which version nodes it needs from each
5129 shared library it is linked against. Thus at runtime, the dynamic
5130 loader can make a quick check to make sure that the libraries you have
5131 linked against do in fact supply all of the version nodes that the
5132 application will need to resolve all of the dynamic symbols. In this
5133 way it is possible for the dynamic linker to know with certainty that
5134 all external symbols that it needs will be resolvable without having to
5135 search for each symbol reference.
5137 The symbol versioning is in effect a much more sophisticated way of
5138 doing minor version checking that SunOS does. The fundamental problem
5139 that is being addressed here is that typically references to external
5140 functions are bound on an as-needed basis, and are not all bound when
5141 the application starts up. If a shared library is out of date, a
5142 required interface may be missing; when the application tries to use
5143 that interface, it may suddenly and unexpectedly fail. With symbol
5144 versioning, the user will get a warning when they start their program if
5145 the libraries being used with the application are too old.
5147 There are several GNU extensions to Sun's versioning approach. The
5148 first of these is the ability to bind a symbol to a version node in the
5149 source file where the symbol is defined instead of in the versioning
5150 script. This was done mainly to reduce the burden on the library
5151 maintainer. You can do this by putting something like:
5153 __asm__(".symver original_foo,foo@@VERS_1.1");
5156 in the C source file. This renames the function @samp{original_foo} to
5157 be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}.
5158 The @samp{local:} directive can be used to prevent the symbol
5159 @samp{original_foo} from being exported. A @samp{.symver} directive
5160 takes precedence over a version script.
5162 The second GNU extension is to allow multiple versions of the same
5163 function to appear in a given shared library. In this way you can make
5164 an incompatible change to an interface without increasing the major
5165 version number of the shared library, while still allowing applications
5166 linked against the old interface to continue to function.
5168 To do this, you must use multiple @samp{.symver} directives in the
5169 source file. Here is an example:
5172 __asm__(".symver original_foo,foo@@");
5173 __asm__(".symver old_foo,foo@@VERS_1.1");
5174 __asm__(".symver old_foo1,foo@@VERS_1.2");
5175 __asm__(".symver new_foo,foo@@@@VERS_2.0");
5178 In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the
5179 unspecified base version of the symbol. The source file that contains this
5180 example would define 4 C functions: @samp{original_foo}, @samp{old_foo},
5181 @samp{old_foo1}, and @samp{new_foo}.
5183 When you have multiple definitions of a given symbol, there needs to be
5184 some way to specify a default version to which external references to
5185 this symbol will be bound. You can do this with the
5186 @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only
5187 declare one version of a symbol as the default in this manner; otherwise
5188 you would effectively have multiple definitions of the same symbol.
5190 If you wish to bind a reference to a specific version of the symbol
5191 within the shared library, you can use the aliases of convenience
5192 (i.e., @samp{old_foo}), or you can use the @samp{.symver} directive to
5193 specifically bind to an external version of the function in question.
5195 You can also specify the language in the version script:
5198 VERSION extern "lang" @{ version-script-commands @}
5201 The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}.
5202 The linker will iterate over the list of symbols at the link time and
5203 demangle them according to @samp{lang} before matching them to the
5204 patterns specified in @samp{version-script-commands}. The default
5205 @samp{lang} is @samp{C}.
5207 Demangled names may contains spaces and other special characters. As
5208 described above, you can use a glob pattern to match demangled names,
5209 or you can use a double-quoted string to match the string exactly. In
5210 the latter case, be aware that minor differences (such as differing
5211 whitespace) between the version script and the demangler output will
5212 cause a mismatch. As the exact string generated by the demangler
5213 might change in the future, even if the mangled name does not, you
5214 should check that all of your version directives are behaving as you
5215 expect when you upgrade.
5218 @section Expressions in Linker Scripts
5221 The syntax for expressions in the linker script language is identical to
5222 that of C expressions. All expressions are evaluated as integers. All
5223 expressions are evaluated in the same size, which is 32 bits if both the
5224 host and target are 32 bits, and is otherwise 64 bits.
5226 You can use and set symbol values in expressions.
5228 The linker defines several special purpose builtin functions for use in
5232 * Constants:: Constants
5233 * Symbolic Constants:: Symbolic constants
5234 * Symbols:: Symbol Names
5235 * Orphan Sections:: Orphan Sections
5236 * Location Counter:: The Location Counter
5237 * Operators:: Operators
5238 * Evaluation:: Evaluation
5239 * Expression Section:: The Section of an Expression
5240 * Builtin Functions:: Builtin Functions
5244 @subsection Constants
5245 @cindex integer notation
5246 @cindex constants in linker scripts
5247 All constants are integers.
5249 As in C, the linker considers an integer beginning with @samp{0} to be
5250 octal, and an integer beginning with @samp{0x} or @samp{0X} to be
5251 hexadecimal. Alternatively the linker accepts suffixes of @samp{h} or
5252 @samp{H} for hexadecimal, @samp{o} or @samp{O} for octal, @samp{b} or
5253 @samp{B} for binary and @samp{d} or @samp{D} for decimal. Any integer
5254 value without a prefix or a suffix is considered to be decimal.
5256 @cindex scaled integers
5257 @cindex K and M integer suffixes
5258 @cindex M and K integer suffixes
5259 @cindex suffixes for integers
5260 @cindex integer suffixes
5261 In addition, you can use the suffixes @code{K} and @code{M} to scale a
5265 @c END TEXI2ROFF-KILL
5266 @code{1024} or @code{1024*1024}
5270 ${\rm 1024}$ or ${\rm 1024}^2$
5272 @c END TEXI2ROFF-KILL
5273 respectively. For example, the following
5274 all refer to the same quantity:
5283 Note - the @code{K} and @code{M} suffixes cannot be used in
5284 conjunction with the base suffixes mentioned above.
5286 @node Symbolic Constants
5287 @subsection Symbolic Constants
5288 @cindex symbolic constants
5290 It is possible to refer to target specific constants via the use of
5291 the @code{CONSTANT(@var{name})} operator, where @var{name} is one of:
5296 The target's maximum page size.
5298 @item COMMONPAGESIZE
5299 @kindex COMMONPAGESIZE
5300 The target's default page size.
5306 .text ALIGN (CONSTANT (MAXPAGESIZE)) : @{ *(.text) @}
5309 will create a text section aligned to the largest page boundary
5310 supported by the target.
5313 @subsection Symbol Names
5314 @cindex symbol names
5316 @cindex quoted symbol names
5318 Unless quoted, symbol names start with a letter, underscore, or period
5319 and may include letters, digits, underscores, periods, and hyphens.
5320 Unquoted symbol names must not conflict with any keywords. You can
5321 specify a symbol which contains odd characters or has the same name as a
5322 keyword by surrounding the symbol name in double quotes:
5325 "with a space" = "also with a space" + 10;
5328 Since symbols can contain many non-alphabetic characters, it is safest
5329 to delimit symbols with spaces. For example, @samp{A-B} is one symbol,
5330 whereas @samp{A - B} is an expression involving subtraction.
5332 @node Orphan Sections
5333 @subsection Orphan Sections
5335 Orphan sections are sections present in the input files which
5336 are not explicitly placed into the output file by the linker
5337 script. The linker will still copy these sections into the
5338 output file, but it has to guess as to where they should be
5339 placed. The linker uses a simple heuristic to do this. It
5340 attempts to place orphan sections after non-orphan sections of the
5341 same attribute, such as code vs data, loadable vs non-loadable, etc.
5342 If there is not enough room to do this then it places
5343 at the end of the file.
5345 For ELF targets, the attribute of the section includes section type as
5346 well as section flag.
5348 If an orphaned section's name is representable as a C identifier then
5349 the linker will automatically @pxref{PROVIDE} two symbols:
5350 __start_SECNAME and __stop_SECNAME, where SECNAME is the name of the
5351 section. These indicate the start address and end address of the
5352 orphaned section respectively. Note: most section names are not
5353 representable as C identifiers because they contain a @samp{.}
5356 @node Location Counter
5357 @subsection The Location Counter
5360 @cindex location counter
5361 @cindex current output location
5362 The special linker variable @dfn{dot} @samp{.} always contains the
5363 current output location counter. Since the @code{.} always refers to a
5364 location in an output section, it may only appear in an expression
5365 within a @code{SECTIONS} command. The @code{.} symbol may appear
5366 anywhere that an ordinary symbol is allowed in an expression.
5369 Assigning a value to @code{.} will cause the location counter to be
5370 moved. This may be used to create holes in the output section. The
5371 location counter may not be moved backwards inside an output section,
5372 and may not be moved backwards outside of an output section if so
5373 doing creates areas with overlapping LMAs.
5389 In the previous example, the @samp{.text} section from @file{file1} is
5390 located at the beginning of the output section @samp{output}. It is
5391 followed by a 1000 byte gap. Then the @samp{.text} section from
5392 @file{file2} appears, also with a 1000 byte gap following before the
5393 @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678}
5394 specifies what data to write in the gaps (@pxref{Output Section Fill}).
5396 @cindex dot inside sections
5397 Note: @code{.} actually refers to the byte offset from the start of the
5398 current containing object. Normally this is the @code{SECTIONS}
5399 statement, whose start address is 0, hence @code{.} can be used as an
5400 absolute address. If @code{.} is used inside a section description
5401 however, it refers to the byte offset from the start of that section,
5402 not an absolute address. Thus in a script like this:
5420 The @samp{.text} section will be assigned a starting address of 0x100
5421 and a size of exactly 0x200 bytes, even if there is not enough data in
5422 the @samp{.text} input sections to fill this area. (If there is too
5423 much data, an error will be produced because this would be an attempt to
5424 move @code{.} backwards). The @samp{.data} section will start at 0x500
5425 and it will have an extra 0x600 bytes worth of space after the end of
5426 the values from the @samp{.data} input sections and before the end of
5427 the @samp{.data} output section itself.
5429 @cindex dot outside sections
5430 Setting symbols to the value of the location counter outside of an
5431 output section statement can result in unexpected values if the linker
5432 needs to place orphan sections. For example, given the following:
5438 .text: @{ *(.text) @}
5442 .data: @{ *(.data) @}
5447 If the linker needs to place some input section, e.g. @code{.rodata},
5448 not mentioned in the script, it might choose to place that section
5449 between @code{.text} and @code{.data}. You might think the linker
5450 should place @code{.rodata} on the blank line in the above script, but
5451 blank lines are of no particular significance to the linker. As well,
5452 the linker doesn't associate the above symbol names with their
5453 sections. Instead, it assumes that all assignments or other
5454 statements belong to the previous output section, except for the
5455 special case of an assignment to @code{.}. I.e., the linker will
5456 place the orphan @code{.rodata} section as if the script was written
5463 .text: @{ *(.text) @}
5467 .rodata: @{ *(.rodata) @}
5468 .data: @{ *(.data) @}
5473 This may or may not be the script author's intention for the value of
5474 @code{start_of_data}. One way to influence the orphan section
5475 placement is to assign the location counter to itself, as the linker
5476 assumes that an assignment to @code{.} is setting the start address of
5477 a following output section and thus should be grouped with that
5478 section. So you could write:
5484 .text: @{ *(.text) @}
5489 .data: @{ *(.data) @}
5494 Now, the orphan @code{.rodata} section will be placed between
5495 @code{end_of_text} and @code{start_of_data}.
5499 @subsection Operators
5500 @cindex operators for arithmetic
5501 @cindex arithmetic operators
5502 @cindex precedence in expressions
5503 The linker recognizes the standard C set of arithmetic operators, with
5504 the standard bindings and precedence levels:
5507 @c END TEXI2ROFF-KILL
5509 precedence associativity Operators Notes
5515 5 left == != > < <= >=
5521 11 right &= += -= *= /= (2)
5525 (1) Prefix operators
5526 (2) @xref{Assignments}.
5530 \vskip \baselineskip
5531 %"lispnarrowing" is the extra indent used generally for smallexample
5532 \hskip\lispnarrowing\vbox{\offinterlineskip
5535 {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr
5536 height2pt&\omit&&\omit&&\omit&\cr
5537 &Precedence&& Associativity &&{\rm Operators}&\cr
5538 height2pt&\omit&&\omit&&\omit&\cr
5540 height2pt&\omit&&\omit&&\omit&\cr
5542 % '176 is tilde, '~' in tt font
5543 &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr
5544 &2&&left&&* / \%&\cr
5547 &5&&left&&== != > < <= >=&\cr
5550 &8&&left&&{\&\&}&\cr
5553 &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr
5555 height2pt&\omit&&\omit&&\omit&\cr}
5560 @obeylines@parskip=0pt@parindent=0pt
5561 @dag@quad Prefix operators.
5562 @ddag@quad @xref{Assignments}.
5565 @c END TEXI2ROFF-KILL
5568 @subsection Evaluation
5569 @cindex lazy evaluation
5570 @cindex expression evaluation order
5571 The linker evaluates expressions lazily. It only computes the value of
5572 an expression when absolutely necessary.
5574 The linker needs some information, such as the value of the start
5575 address of the first section, and the origins and lengths of memory
5576 regions, in order to do any linking at all. These values are computed
5577 as soon as possible when the linker reads in the linker script.
5579 However, other values (such as symbol values) are not known or needed
5580 until after storage allocation. Such values are evaluated later, when
5581 other information (such as the sizes of output sections) is available
5582 for use in the symbol assignment expression.
5584 The sizes of sections cannot be known until after allocation, so
5585 assignments dependent upon these are not performed until after
5588 Some expressions, such as those depending upon the location counter
5589 @samp{.}, must be evaluated during section allocation.
5591 If the result of an expression is required, but the value is not
5592 available, then an error results. For example, a script like the
5598 .text 9+this_isnt_constant :
5604 will cause the error message @samp{non constant expression for initial
5607 @node Expression Section
5608 @subsection The Section of an Expression
5609 @cindex expression sections
5610 @cindex absolute expressions
5611 @cindex relative expressions
5612 @cindex absolute and relocatable symbols
5613 @cindex relocatable and absolute symbols
5614 @cindex symbols, relocatable and absolute
5615 Addresses and symbols may be section relative, or absolute. A section
5616 relative symbol is relocatable. If you request relocatable output
5617 using the @samp{-r} option, a further link operation may change the
5618 value of a section relative symbol. On the other hand, an absolute
5619 symbol will retain the same value throughout any further link
5622 Some terms in linker expressions are addresses. This is true of
5623 section relative symbols and for builtin functions that return an
5624 address, such as @code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and
5625 @code{SEGMENT_START}. Other terms are simply numbers, or are builtin
5626 functions that return a non-address value, such as @code{LENGTH}.
5627 One complication is that unless you set @code{LD_FEATURE ("SANE_EXPR")}
5628 (@pxref{Miscellaneous Commands}), numbers and absolute symbols are treated
5629 differently depending on their location, for compatibility with older
5630 versions of @code{ld}. Expressions appearing outside an output
5631 section definition treat all numbers as absolute addresses.
5632 Expressions appearing inside an output section definition treat
5633 absolute symbols as numbers. If @code{LD_FEATURE ("SANE_EXPR")} is
5634 given, then absolute symbols and numbers are simply treated as numbers
5637 In the following simple example,
5644 __executable_start = 0x100;
5648 __data_start = 0x10;
5656 both @code{.} and @code{__executable_start} are set to the absolute
5657 address 0x100 in the first two assignments, then both @code{.} and
5658 @code{__data_start} are set to 0x10 relative to the @code{.data}
5659 section in the second two assignments.
5661 For expressions involving numbers, relative addresses and absolute
5662 addresses, ld follows these rules to evaluate terms:
5666 Unary operations on an absolute address or number, and binary
5667 operations on two absolute addresses or two numbers, or between one
5668 absolute address and a number, apply the operator to the value(s).
5670 Unary operations on a relative address, and binary operations on two
5671 relative addresses in the same section or between one relative address
5672 and a number, apply the operator to the offset part of the address(es).
5674 Other binary operations, that is, between two relative addresses not
5675 in the same section, or between a relative address and an absolute
5676 address, first convert any non-absolute term to an absolute address
5677 before applying the operator.
5680 The result section of each sub-expression is as follows:
5684 An operation involving only numbers results in a number.
5686 The result of comparisons, @samp{&&} and @samp{||} is also a number.
5688 The result of other binary arithmetic and logical operations on two
5689 relative addresses in the same section or two absolute addresses
5690 (after above conversions) is also a number.
5692 The result of other operations on relative addresses or one
5693 relative address and a number, is a relative address in the same
5694 section as the relative operand(s).
5696 The result of other operations on absolute addresses (after above
5697 conversions) is an absolute address.
5700 You can use the builtin function @code{ABSOLUTE} to force an expression
5701 to be absolute when it would otherwise be relative. For example, to
5702 create an absolute symbol set to the address of the end of the output
5703 section @samp{.data}:
5707 .data : @{ *(.data) _edata = ABSOLUTE(.); @}
5711 If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the
5712 @samp{.data} section.
5714 Using @code{LOADADDR} also forces an expression absolute, since this
5715 particular builtin function returns an absolute address.
5717 @node Builtin Functions
5718 @subsection Builtin Functions
5719 @cindex functions in expressions
5720 The linker script language includes a number of builtin functions for
5721 use in linker script expressions.
5724 @item ABSOLUTE(@var{exp})
5725 @kindex ABSOLUTE(@var{exp})
5726 @cindex expression, absolute
5727 Return the absolute (non-relocatable, as opposed to non-negative) value
5728 of the expression @var{exp}. Primarily useful to assign an absolute
5729 value to a symbol within a section definition, where symbol values are
5730 normally section relative. @xref{Expression Section}.
5732 @item ADDR(@var{section})
5733 @kindex ADDR(@var{section})
5734 @cindex section address in expression
5735 Return the address (VMA) of the named @var{section}. Your
5736 script must previously have defined the location of that section. In
5737 the following example, @code{start_of_output_1}, @code{symbol_1} and
5738 @code{symbol_2} are assigned equivalent values, except that
5739 @code{symbol_1} will be relative to the @code{.output1} section while
5740 the other two will be absolute:
5746 start_of_output_1 = ABSOLUTE(.);
5751 symbol_1 = ADDR(.output1);
5752 symbol_2 = start_of_output_1;
5758 @item ALIGN(@var{align})
5759 @itemx ALIGN(@var{exp},@var{align})
5760 @kindex ALIGN(@var{align})
5761 @kindex ALIGN(@var{exp},@var{align})
5762 @cindex round up location counter
5763 @cindex align location counter
5764 @cindex round up expression
5765 @cindex align expression
5766 Return the location counter (@code{.}) or arbitrary expression aligned
5767 to the next @var{align} boundary. The single operand @code{ALIGN}
5768 doesn't change the value of the location counter---it just does
5769 arithmetic on it. The two operand @code{ALIGN} allows an arbitrary
5770 expression to be aligned upwards (@code{ALIGN(@var{align})} is
5771 equivalent to @code{ALIGN(., @var{align})}).
5773 Here is an example which aligns the output @code{.data} section to the
5774 next @code{0x2000} byte boundary after the preceding section and sets a
5775 variable within the section to the next @code{0x8000} boundary after the
5780 .data ALIGN(0x2000): @{
5782 variable = ALIGN(0x8000);
5788 The first use of @code{ALIGN} in this example specifies the location of
5789 a section because it is used as the optional @var{address} attribute of
5790 a section definition (@pxref{Output Section Address}). The second use
5791 of @code{ALIGN} is used to defines the value of a symbol.
5793 The builtin function @code{NEXT} is closely related to @code{ALIGN}.
5795 @item ALIGNOF(@var{section})
5796 @kindex ALIGNOF(@var{section})
5797 @cindex section alignment
5798 Return the alignment in bytes of the named @var{section}, if that section has
5799 been allocated. If the section has not been allocated when this is
5800 evaluated, the linker will report an error. In the following example,
5801 the alignment of the @code{.output} section is stored as the first
5802 value in that section.
5807 LONG (ALIGNOF (.output))
5814 @item BLOCK(@var{exp})
5815 @kindex BLOCK(@var{exp})
5816 This is a synonym for @code{ALIGN}, for compatibility with older linker
5817 scripts. It is most often seen when setting the address of an output
5820 @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
5821 @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
5822 This is equivalent to either
5824 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1)))
5828 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - @var{commonpagesize})))
5831 depending on whether the latter uses fewer @var{commonpagesize} sized pages
5832 for the data segment (area between the result of this expression and
5833 @code{DATA_SEGMENT_END}) than the former or not.
5834 If the latter form is used, it means @var{commonpagesize} bytes of runtime
5835 memory will be saved at the expense of up to @var{commonpagesize} wasted
5836 bytes in the on-disk file.
5838 This expression can only be used directly in @code{SECTIONS} commands, not in
5839 any output section descriptions and only once in the linker script.
5840 @var{commonpagesize} should be less or equal to @var{maxpagesize} and should
5841 be the system page size the object wants to be optimized for (while still
5842 working on system page sizes up to @var{maxpagesize}).
5847 . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
5850 @item DATA_SEGMENT_END(@var{exp})
5851 @kindex DATA_SEGMENT_END(@var{exp})
5852 This defines the end of data segment for @code{DATA_SEGMENT_ALIGN}
5853 evaluation purposes.
5856 . = DATA_SEGMENT_END(.);
5859 @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
5860 @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
5861 This defines the end of the @code{PT_GNU_RELRO} segment when
5862 @samp{-z relro} option is used. Second argument is returned.
5863 When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END}
5864 does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that
5865 @var{exp} + @var{offset} is aligned to the most commonly used page
5866 boundary for particular target. If present in the linker script,
5867 it must always come in between @code{DATA_SEGMENT_ALIGN} and
5868 @code{DATA_SEGMENT_END}.
5871 . = DATA_SEGMENT_RELRO_END(24, .);
5874 @item DEFINED(@var{symbol})
5875 @kindex DEFINED(@var{symbol})
5876 @cindex symbol defaults
5877 Return 1 if @var{symbol} is in the linker global symbol table and is
5878 defined before the statement using DEFINED in the script, otherwise
5879 return 0. You can use this function to provide
5880 default values for symbols. For example, the following script fragment
5881 shows how to set a global symbol @samp{begin} to the first location in
5882 the @samp{.text} section---but if a symbol called @samp{begin} already
5883 existed, its value is preserved:
5889 begin = DEFINED(begin) ? begin : . ;
5897 @item LENGTH(@var{memory})
5898 @kindex LENGTH(@var{memory})
5899 Return the length of the memory region named @var{memory}.
5901 @item LOADADDR(@var{section})
5902 @kindex LOADADDR(@var{section})
5903 @cindex section load address in expression
5904 Return the absolute LMA of the named @var{section}. (@pxref{Output
5908 @item MAX(@var{exp1}, @var{exp2})
5909 Returns the maximum of @var{exp1} and @var{exp2}.
5912 @item MIN(@var{exp1}, @var{exp2})
5913 Returns the minimum of @var{exp1} and @var{exp2}.
5915 @item NEXT(@var{exp})
5916 @kindex NEXT(@var{exp})
5917 @cindex unallocated address, next
5918 Return the next unallocated address that is a multiple of @var{exp}.
5919 This function is closely related to @code{ALIGN(@var{exp})}; unless you
5920 use the @code{MEMORY} command to define discontinuous memory for the
5921 output file, the two functions are equivalent.
5923 @item ORIGIN(@var{memory})
5924 @kindex ORIGIN(@var{memory})
5925 Return the origin of the memory region named @var{memory}.
5927 @item SEGMENT_START(@var{segment}, @var{default})
5928 @kindex SEGMENT_START(@var{segment}, @var{default})
5929 Return the base address of the named @var{segment}. If an explicit
5930 value has been given for this segment (with a command-line @samp{-T}
5931 option) that value will be returned; otherwise the value will be
5932 @var{default}. At present, the @samp{-T} command-line option can only
5933 be used to set the base address for the ``text'', ``data'', and
5934 ``bss'' sections, but you can use @code{SEGMENT_START} with any segment
5937 @item SIZEOF(@var{section})
5938 @kindex SIZEOF(@var{section})
5939 @cindex section size
5940 Return the size in bytes of the named @var{section}, if that section has
5941 been allocated. If the section has not been allocated when this is
5942 evaluated, the linker will report an error. In the following example,
5943 @code{symbol_1} and @code{symbol_2} are assigned identical values:
5952 symbol_1 = .end - .start ;
5953 symbol_2 = SIZEOF(.output);
5958 @item SIZEOF_HEADERS
5959 @itemx sizeof_headers
5960 @kindex SIZEOF_HEADERS
5962 Return the size in bytes of the output file's headers. This is
5963 information which appears at the start of the output file. You can use
5964 this number when setting the start address of the first section, if you
5965 choose, to facilitate paging.
5967 @cindex not enough room for program headers
5968 @cindex program headers, not enough room
5969 When producing an ELF output file, if the linker script uses the
5970 @code{SIZEOF_HEADERS} builtin function, the linker must compute the
5971 number of program headers before it has determined all the section
5972 addresses and sizes. If the linker later discovers that it needs
5973 additional program headers, it will report an error @samp{not enough
5974 room for program headers}. To avoid this error, you must avoid using
5975 the @code{SIZEOF_HEADERS} function, or you must rework your linker
5976 script to avoid forcing the linker to use additional program headers, or
5977 you must define the program headers yourself using the @code{PHDRS}
5978 command (@pxref{PHDRS}).
5981 @node Implicit Linker Scripts
5982 @section Implicit Linker Scripts
5983 @cindex implicit linker scripts
5984 If you specify a linker input file which the linker can not recognize as
5985 an object file or an archive file, it will try to read the file as a
5986 linker script. If the file can not be parsed as a linker script, the
5987 linker will report an error.
5989 An implicit linker script will not replace the default linker script.
5991 Typically an implicit linker script would contain only symbol
5992 assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION}
5995 Any input files read because of an implicit linker script will be read
5996 at the position in the command line where the implicit linker script was
5997 read. This can affect archive searching.
6000 @node Machine Dependent
6001 @chapter Machine Dependent Features
6003 @cindex machine dependencies
6004 @command{ld} has additional features on some platforms; the following
6005 sections describe them. Machines where @command{ld} has no additional
6006 functionality are not listed.
6010 * H8/300:: @command{ld} and the H8/300
6013 * i960:: @command{ld} and the Intel 960 family
6016 * ARM:: @command{ld} and the ARM family
6019 * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF
6022 * M68K:: @command{ld} and the Motorola 68K family
6025 * MMIX:: @command{ld} and MMIX
6028 * MSP430:: @command{ld} and MSP430
6031 * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families
6034 * PowerPC ELF32:: @command{ld} and PowerPC 32-bit ELF Support
6037 * PowerPC64 ELF64:: @command{ld} and PowerPC64 64-bit ELF Support
6040 * SPU ELF:: @command{ld} and SPU ELF Support
6043 * TI COFF:: @command{ld} and TI COFF
6046 * WIN32:: @command{ld} and WIN32 (cygwin/mingw)
6049 * Xtensa:: @command{ld} and Xtensa Processors
6060 @section @command{ld} and the H8/300
6062 @cindex H8/300 support
6063 For the H8/300, @command{ld} can perform these global optimizations when
6064 you specify the @samp{--relax} command-line option.
6067 @cindex relaxing on H8/300
6068 @item relaxing address modes
6069 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
6070 targets are within eight bits, and turns them into eight-bit
6071 program-counter relative @code{bsr} and @code{bra} instructions,
6074 @cindex synthesizing on H8/300
6075 @item synthesizing instructions
6076 @c FIXME: specifically mov.b, or any mov instructions really?
6077 @command{ld} finds all @code{mov.b} instructions which use the
6078 sixteen-bit absolute address form, but refer to the top
6079 page of memory, and changes them to use the eight-bit address form.
6080 (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into
6081 @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the
6082 top page of memory).
6084 @item bit manipulation instructions
6085 @command{ld} finds all bit manipulation instructions like @code{band, bclr,
6086 biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor}
6087 which use 32 bit and 16 bit absolute address form, but refer to the top
6088 page of memory, and changes them to use the 8 bit address form.
6089 (That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into
6090 @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in
6091 the top page of memory).
6093 @item system control instructions
6094 @command{ld} finds all @code{ldc.w, stc.w} instructions which use the
6095 32 bit absolute address form, but refer to the top page of memory, and
6096 changes them to use 16 bit address form.
6097 (That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into
6098 @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in
6099 the top page of memory).
6109 @c This stuff is pointless to say unless you're especially concerned
6110 @c with Renesas chips; don't enable it for generic case, please.
6112 @chapter @command{ld} and Other Renesas Chips
6114 @command{ld} also supports the Renesas (formerly Hitachi) H8/300H,
6115 H8/500, and SH chips. No special features, commands, or command-line
6116 options are required for these chips.
6126 @section @command{ld} and the Intel 960 Family
6128 @cindex i960 support
6130 You can use the @samp{-A@var{architecture}} command line option to
6131 specify one of the two-letter names identifying members of the 960
6132 family; the option specifies the desired output target, and warns of any
6133 incompatible instructions in the input files. It also modifies the
6134 linker's search strategy for archive libraries, to support the use of
6135 libraries specific to each particular architecture, by including in the
6136 search loop names suffixed with the string identifying the architecture.
6138 For example, if your @command{ld} command line included @w{@samp{-ACA}} as
6139 well as @w{@samp{-ltry}}, the linker would look (in its built-in search
6140 paths, and in any paths you specify with @samp{-L}) for a library with
6153 The first two possibilities would be considered in any event; the last
6154 two are due to the use of @w{@samp{-ACA}}.
6156 You can meaningfully use @samp{-A} more than once on a command line, since
6157 the 960 architecture family allows combination of target architectures; each
6158 use will add another pair of name variants to search for when @w{@samp{-l}}
6159 specifies a library.
6161 @cindex @option{--relax} on i960
6162 @cindex relaxing on i960
6163 @command{ld} supports the @samp{--relax} option for the i960 family. If
6164 you specify @samp{--relax}, @command{ld} finds all @code{balx} and
6165 @code{calx} instructions whose targets are within 24 bits, and turns
6166 them into 24-bit program-counter relative @code{bal} and @code{cal}
6167 instructions, respectively. @command{ld} also turns @code{cal}
6168 instructions into @code{bal} instructions when it determines that the
6169 target subroutine is a leaf routine (that is, the target subroutine does
6170 not itself call any subroutines).
6172 @cindex Cortex-A8 erratum workaround
6173 @kindex --fix-cortex-a8
6174 @kindex --no-fix-cortex-a8
6175 The @samp{--fix-cortex-a8} switch enables a link-time workaround for an erratum in certain Cortex-A8 processors. The workaround is enabled by default if you are targeting the ARM v7-A architecture profile. It can be enabled otherwise by specifying @samp{--fix-cortex-a8}, or disabled unconditionally by specifying @samp{--no-fix-cortex-a8}.
6177 The erratum only affects Thumb-2 code. Please contact ARM for further details.
6179 @kindex --merge-exidx-entries
6180 @kindex --no-merge-exidx-entries
6181 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent exidx entries in debuginfo.
6198 @node M68HC11/68HC12
6199 @section @command{ld} and the Motorola 68HC11 and 68HC12 families
6201 @cindex M68HC11 and 68HC12 support
6203 @subsection Linker Relaxation
6205 For the Motorola 68HC11, @command{ld} can perform these global
6206 optimizations when you specify the @samp{--relax} command-line option.
6209 @cindex relaxing on M68HC11
6210 @item relaxing address modes
6211 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
6212 targets are within eight bits, and turns them into eight-bit
6213 program-counter relative @code{bsr} and @code{bra} instructions,
6216 @command{ld} also looks at all 16-bit extended addressing modes and
6217 transforms them in a direct addressing mode when the address is in
6218 page 0 (between 0 and 0x0ff).
6220 @item relaxing gcc instruction group
6221 When @command{gcc} is called with @option{-mrelax}, it can emit group
6222 of instructions that the linker can optimize to use a 68HC11 direct
6223 addressing mode. These instructions consists of @code{bclr} or
6224 @code{bset} instructions.
6228 @subsection Trampoline Generation
6230 @cindex trampoline generation on M68HC11
6231 @cindex trampoline generation on M68HC12
6232 For 68HC11 and 68HC12, @command{ld} can generate trampoline code to
6233 call a far function using a normal @code{jsr} instruction. The linker
6234 will also change the relocation to some far function to use the
6235 trampoline address instead of the function address. This is typically the
6236 case when a pointer to a function is taken. The pointer will in fact
6237 point to the function trampoline.
6245 @section @command{ld} and the ARM family
6247 @cindex ARM interworking support
6248 @kindex --support-old-code
6249 For the ARM, @command{ld} will generate code stubs to allow functions calls
6250 between ARM and Thumb code. These stubs only work with code that has
6251 been compiled and assembled with the @samp{-mthumb-interwork} command
6252 line option. If it is necessary to link with old ARM object files or
6253 libraries, which have not been compiled with the -mthumb-interwork
6254 option then the @samp{--support-old-code} command line switch should be
6255 given to the linker. This will make it generate larger stub functions
6256 which will work with non-interworking aware ARM code. Note, however,
6257 the linker does not support generating stubs for function calls to
6258 non-interworking aware Thumb code.
6260 @cindex thumb entry point
6261 @cindex entry point, thumb
6262 @kindex --thumb-entry=@var{entry}
6263 The @samp{--thumb-entry} switch is a duplicate of the generic
6264 @samp{--entry} switch, in that it sets the program's starting address.
6265 But it also sets the bottom bit of the address, so that it can be
6266 branched to using a BX instruction, and the program will start
6267 executing in Thumb mode straight away.
6269 @cindex PE import table prefixing
6270 @kindex --use-nul-prefixed-import-tables
6271 The @samp{--use-nul-prefixed-import-tables} switch is specifying, that
6272 the import tables idata4 and idata5 have to be generated with a zero
6273 element prefix for import libraries. This is the old style to generate
6274 import tables. By default this option is turned off.
6278 The @samp{--be8} switch instructs @command{ld} to generate BE8 format
6279 executables. This option is only valid when linking big-endian objects.
6280 The resulting image will contain big-endian data and little-endian code.
6283 @kindex --target1-rel
6284 @kindex --target1-abs
6285 The @samp{R_ARM_TARGET1} relocation is typically used for entries in the
6286 @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32}
6287 or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel}
6288 and @samp{--target1-abs} switches override the default.
6291 @kindex --target2=@var{type}
6292 The @samp{--target2=type} switch overrides the default definition of the
6293 @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their
6294 meanings, and target defaults are as follows:
6297 @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi)
6299 @samp{R_ARM_ABS32} (arm*-*-symbianelf)
6301 @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd)
6306 The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF
6307 specification) enables objects compiled for the ARMv4 architecture to be
6308 interworking-safe when linked with other objects compiled for ARMv4t, but
6309 also allows pure ARMv4 binaries to be built from the same ARMv4 objects.
6311 In the latter case, the switch @option{--fix-v4bx} must be passed to the
6312 linker, which causes v4t @code{BX rM} instructions to be rewritten as
6313 @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction.
6315 In the former case, the switch should not be used, and @samp{R_ARM_V4BX}
6316 relocations are ignored.
6318 @cindex FIX_V4BX_INTERWORKING
6319 @kindex --fix-v4bx-interworking
6320 Replace @code{BX rM} instructions identified by @samp{R_ARM_V4BX}
6321 relocations with a branch to the following veneer:
6329 This allows generation of libraries/applications that work on ARMv4 cores
6330 and are still interworking safe. Note that the above veneer clobbers the
6331 condition flags, so may cause incorrect program behavior in rare cases.
6335 The @samp{--use-blx} switch enables the linker to use ARM/Thumb
6336 BLX instructions (available on ARMv5t and above) in various
6337 situations. Currently it is used to perform calls via the PLT from Thumb
6338 code using BLX rather than using BX and a mode-switching stub before
6339 each PLT entry. This should lead to such calls executing slightly faster.
6341 This option is enabled implicitly for SymbianOS, so there is no need to
6342 specify it if you are using that target.
6344 @cindex VFP11_DENORM_FIX
6345 @kindex --vfp11-denorm-fix
6346 The @samp{--vfp11-denorm-fix} switch enables a link-time workaround for a
6347 bug in certain VFP11 coprocessor hardware, which sometimes allows
6348 instructions with denorm operands (which must be handled by support code)
6349 to have those operands overwritten by subsequent instructions before
6350 the support code can read the intended values.
6352 The bug may be avoided in scalar mode if you allow at least one
6353 intervening instruction between a VFP11 instruction which uses a register
6354 and another instruction which writes to the same register, or at least two
6355 intervening instructions if vector mode is in use. The bug only affects
6356 full-compliance floating-point mode: you do not need this workaround if
6357 you are using "runfast" mode. Please contact ARM for further details.
6359 If you know you are using buggy VFP11 hardware, you can
6360 enable this workaround by specifying the linker option
6361 @samp{--vfp-denorm-fix=scalar} if you are using the VFP11 scalar
6362 mode only, or @samp{--vfp-denorm-fix=vector} if you are using
6363 vector mode (the latter also works for scalar code). The default is
6364 @samp{--vfp-denorm-fix=none}.
6366 If the workaround is enabled, instructions are scanned for
6367 potentially-troublesome sequences, and a veneer is created for each
6368 such sequence which may trigger the erratum. The veneer consists of the
6369 first instruction of the sequence and a branch back to the subsequent
6370 instruction. The original instruction is then replaced with a branch to
6371 the veneer. The extra cycles required to call and return from the veneer
6372 are sufficient to avoid the erratum in both the scalar and vector cases.
6374 @cindex ARM1176 erratum workaround
6375 @kindex --fix-arm1176
6376 @kindex --no-fix-arm1176
6377 The @samp{--fix-arm1176} switch enables a link-time workaround for an erratum
6378 in certain ARM1176 processors. The workaround is enabled by default if you
6379 are targeting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled
6380 unconditionally by specifying @samp{--no-fix-arm1176}.
6382 Further information is available in the ``ARM1176JZ-S and ARM1176JZF-S
6383 Programmer Advice Notice'' available on the ARM documentation website at:
6384 http://infocenter.arm.com/.
6386 @cindex NO_ENUM_SIZE_WARNING
6387 @kindex --no-enum-size-warning
6388 The @option{--no-enum-size-warning} switch prevents the linker from
6389 warning when linking object files that specify incompatible EABI
6390 enumeration size attributes. For example, with this switch enabled,
6391 linking of an object file using 32-bit enumeration values with another
6392 using enumeration values fitted into the smallest possible space will
6395 @cindex NO_WCHAR_SIZE_WARNING
6396 @kindex --no-wchar-size-warning
6397 The @option{--no-wchar-size-warning} switch prevents the linker from
6398 warning when linking object files that specify incompatible EABI
6399 @code{wchar_t} size attributes. For example, with this switch enabled,
6400 linking of an object file using 32-bit @code{wchar_t} values with another
6401 using 16-bit @code{wchar_t} values will not be diagnosed.
6404 @kindex --pic-veneer
6405 The @samp{--pic-veneer} switch makes the linker use PIC sequences for
6406 ARM/Thumb interworking veneers, even if the rest of the binary
6407 is not PIC. This avoids problems on uClinux targets where
6408 @samp{--emit-relocs} is used to generate relocatable binaries.
6410 @cindex STUB_GROUP_SIZE
6411 @kindex --stub-group-size=@var{N}
6412 The linker will automatically generate and insert small sequences of
6413 code into a linked ARM ELF executable whenever an attempt is made to
6414 perform a function call to a symbol that is too far away. The
6415 placement of these sequences of instructions - called stubs - is
6416 controlled by the command line option @option{--stub-group-size=N}.
6417 The placement is important because a poor choice can create a need for
6418 duplicate stubs, increasing the code size. The linker will try to
6419 group stubs together in order to reduce interruptions to the flow of
6420 code, but it needs guidance as to how big these groups should be and
6421 where they should be placed.
6423 The value of @samp{N}, the parameter to the
6424 @option{--stub-group-size=} option controls where the stub groups are
6425 placed. If it is negative then all stubs are placed after the first
6426 branch that needs them. If it is positive then the stubs can be
6427 placed either before or after the branches that need them. If the
6428 value of @samp{N} is 1 (either +1 or -1) then the linker will choose
6429 exactly where to place groups of stubs, using its built in heuristics.
6430 A value of @samp{N} greater than 1 (or smaller than -1) tells the
6431 linker that a single group of stubs can service at most @samp{N} bytes
6432 from the input sections.
6434 The default, if @option{--stub-group-size=} is not specified, is
6437 Farcalls stubs insertion is fully supported for the ARM-EABI target
6438 only, because it relies on object files properties not present
6452 @section @command{ld} and HPPA 32-bit ELF Support
6453 @cindex HPPA multiple sub-space stubs
6454 @kindex --multi-subspace
6455 When generating a shared library, @command{ld} will by default generate
6456 import stubs suitable for use with a single sub-space application.
6457 The @samp{--multi-subspace} switch causes @command{ld} to generate export
6458 stubs, and different (larger) import stubs suitable for use with
6459 multiple sub-spaces.
6461 @cindex HPPA stub grouping
6462 @kindex --stub-group-size=@var{N}
6463 Long branch stubs and import/export stubs are placed by @command{ld} in
6464 stub sections located between groups of input sections.
6465 @samp{--stub-group-size} specifies the maximum size of a group of input
6466 sections handled by one stub section. Since branch offsets are signed,
6467 a stub section may serve two groups of input sections, one group before
6468 the stub section, and one group after it. However, when using
6469 conditional branches that require stubs, it may be better (for branch
6470 prediction) that stub sections only serve one group of input sections.
6471 A negative value for @samp{N} chooses this scheme, ensuring that
6472 branches to stubs always use a negative offset. Two special values of
6473 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
6474 @command{ld} to automatically size input section groups for the branch types
6475 detected, with the same behaviour regarding stub placement as other
6476 positive or negative values of @samp{N} respectively.
6478 Note that @samp{--stub-group-size} does not split input sections. A
6479 single input section larger than the group size specified will of course
6480 create a larger group (of one section). If input sections are too
6481 large, it may not be possible for a branch to reach its stub.
6494 @section @command{ld} and the Motorola 68K family
6496 @cindex Motorola 68K GOT generation
6497 @kindex --got=@var{type}
6498 The @samp{--got=@var{type}} option lets you choose the GOT generation scheme.
6499 The choices are @samp{single}, @samp{negative}, @samp{multigot} and
6500 @samp{target}. When @samp{target} is selected the linker chooses
6501 the default GOT generation scheme for the current target.
6502 @samp{single} tells the linker to generate a single GOT with
6503 entries only at non-negative offsets.
6504 @samp{negative} instructs the linker to generate a single GOT with
6505 entries at both negative and positive offsets. Not all environments
6507 @samp{multigot} allows the linker to generate several GOTs in the
6508 output file. All GOT references from a single input object
6509 file access the same GOT, but references from different input object
6510 files might access different GOTs. Not all environments support such GOTs.
6523 @section @code{ld} and MMIX
6524 For MMIX, there is a choice of generating @code{ELF} object files or
6525 @code{mmo} object files when linking. The simulator @code{mmix}
6526 understands the @code{mmo} format. The binutils @code{objcopy} utility
6527 can translate between the two formats.
6529 There is one special section, the @samp{.MMIX.reg_contents} section.
6530 Contents in this section is assumed to correspond to that of global
6531 registers, and symbols referring to it are translated to special symbols,
6532 equal to registers. In a final link, the start address of the
6533 @samp{.MMIX.reg_contents} section corresponds to the first allocated
6534 global register multiplied by 8. Register @code{$255} is not included in
6535 this section; it is always set to the program entry, which is at the
6536 symbol @code{Main} for @code{mmo} files.
6538 Global symbols with the prefix @code{__.MMIX.start.}, for example
6539 @code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special.
6540 The default linker script uses these to set the default start address
6543 Initial and trailing multiples of zero-valued 32-bit words in a section,
6544 are left out from an mmo file.
6557 @section @code{ld} and MSP430
6558 For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]}
6559 will select an appropriate linker script for selected MPU type. (To get a list of known MPUs
6560 just pass @samp{-m help} option to the linker).
6562 @cindex MSP430 extra sections
6563 The linker will recognize some extra sections which are MSP430 specific:
6566 @item @samp{.vectors}
6567 Defines a portion of ROM where interrupt vectors located.
6569 @item @samp{.bootloader}
6570 Defines the bootloader portion of the ROM (if applicable). Any code
6571 in this section will be uploaded to the MPU.
6573 @item @samp{.infomem}
6574 Defines an information memory section (if applicable). Any code in
6575 this section will be uploaded to the MPU.
6577 @item @samp{.infomemnobits}
6578 This is the same as the @samp{.infomem} section except that any code
6579 in this section will not be uploaded to the MPU.
6581 @item @samp{.noinit}
6582 Denotes a portion of RAM located above @samp{.bss} section.
6584 The last two sections are used by gcc.
6598 @section @command{ld} and PowerPC 32-bit ELF Support
6599 @cindex PowerPC long branches
6600 @kindex --relax on PowerPC
6601 Branches on PowerPC processors are limited to a signed 26-bit
6602 displacement, which may result in @command{ld} giving
6603 @samp{relocation truncated to fit} errors with very large programs.
6604 @samp{--relax} enables the generation of trampolines that can access
6605 the entire 32-bit address space. These trampolines are inserted at
6606 section boundaries, so may not themselves be reachable if an input
6607 section exceeds 33M in size. You may combine @samp{-r} and
6608 @samp{--relax} to add trampolines in a partial link. In that case
6609 both branches to undefined symbols and inter-section branches are also
6610 considered potentially out of range, and trampolines inserted.
6612 @cindex PowerPC ELF32 options
6617 Current PowerPC GCC accepts a @samp{-msecure-plt} option that
6618 generates code capable of using a newer PLT and GOT layout that has
6619 the security advantage of no executable section ever needing to be
6620 writable and no writable section ever being executable. PowerPC
6621 @command{ld} will generate this layout, including stubs to access the
6622 PLT, if all input files (including startup and static libraries) were
6623 compiled with @samp{-msecure-plt}. @samp{--bss-plt} forces the old
6624 BSS PLT (and GOT layout) which can give slightly better performance.
6626 @kindex --secure-plt
6628 @command{ld} will use the new PLT and GOT layout if it is linking new
6629 @samp{-fpic} or @samp{-fPIC} code, but does not do so automatically
6630 when linking non-PIC code. This option requests the new PLT and GOT
6631 layout. A warning will be given if some object file requires the old
6637 The new secure PLT and GOT are placed differently relative to other
6638 sections compared to older BSS PLT and GOT placement. The location of
6639 @code{.plt} must change because the new secure PLT is an initialized
6640 section while the old PLT is uninitialized. The reason for the
6641 @code{.got} change is more subtle: The new placement allows
6642 @code{.got} to be read-only in applications linked with
6643 @samp{-z relro -z now}. However, this placement means that
6644 @code{.sdata} cannot always be used in shared libraries, because the
6645 PowerPC ABI accesses @code{.sdata} in shared libraries from the GOT
6646 pointer. @samp{--sdata-got} forces the old GOT placement. PowerPC
6647 GCC doesn't use @code{.sdata} in shared libraries, so this option is
6648 really only useful for other compilers that may do so.
6650 @cindex PowerPC stub symbols
6651 @kindex --emit-stub-syms
6652 @item --emit-stub-syms
6653 This option causes @command{ld} to label linker stubs with a local
6654 symbol that encodes the stub type and destination.
6656 @cindex PowerPC TLS optimization
6657 @kindex --no-tls-optimize
6658 @item --no-tls-optimize
6659 PowerPC @command{ld} normally performs some optimization of code
6660 sequences used to access Thread-Local Storage. Use this option to
6661 disable the optimization.
6674 @node PowerPC64 ELF64
6675 @section @command{ld} and PowerPC64 64-bit ELF Support
6677 @cindex PowerPC64 ELF64 options
6679 @cindex PowerPC64 stub grouping
6680 @kindex --stub-group-size
6681 @item --stub-group-size
6682 Long branch stubs, PLT call stubs and TOC adjusting stubs are placed
6683 by @command{ld} in stub sections located between groups of input sections.
6684 @samp{--stub-group-size} specifies the maximum size of a group of input
6685 sections handled by one stub section. Since branch offsets are signed,
6686 a stub section may serve two groups of input sections, one group before
6687 the stub section, and one group after it. However, when using
6688 conditional branches that require stubs, it may be better (for branch
6689 prediction) that stub sections only serve one group of input sections.
6690 A negative value for @samp{N} chooses this scheme, ensuring that
6691 branches to stubs always use a negative offset. Two special values of
6692 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
6693 @command{ld} to automatically size input section groups for the branch types
6694 detected, with the same behaviour regarding stub placement as other
6695 positive or negative values of @samp{N} respectively.
6697 Note that @samp{--stub-group-size} does not split input sections. A
6698 single input section larger than the group size specified will of course
6699 create a larger group (of one section). If input sections are too
6700 large, it may not be possible for a branch to reach its stub.
6702 @cindex PowerPC64 stub symbols
6703 @kindex --emit-stub-syms
6704 @item --emit-stub-syms
6705 This option causes @command{ld} to label linker stubs with a local
6706 symbol that encodes the stub type and destination.
6708 @cindex PowerPC64 dot symbols
6710 @kindex --no-dotsyms
6711 @item --dotsyms, --no-dotsyms
6712 These two options control how @command{ld} interprets version patterns
6713 in a version script. Older PowerPC64 compilers emitted both a
6714 function descriptor symbol with the same name as the function, and a
6715 code entry symbol with the name prefixed by a dot (@samp{.}). To
6716 properly version a function @samp{foo}, the version script thus needs
6717 to control both @samp{foo} and @samp{.foo}. The option
6718 @samp{--dotsyms}, on by default, automatically adds the required
6719 dot-prefixed patterns. Use @samp{--no-dotsyms} to disable this
6722 @cindex PowerPC64 TLS optimization
6723 @kindex --no-tls-optimize
6724 @item --no-tls-optimize
6725 PowerPC64 @command{ld} normally performs some optimization of code
6726 sequences used to access Thread-Local Storage. Use this option to
6727 disable the optimization.
6729 @cindex PowerPC64 OPD optimization
6730 @kindex --no-opd-optimize
6731 @item --no-opd-optimize
6732 PowerPC64 @command{ld} normally removes @code{.opd} section entries
6733 corresponding to deleted link-once functions, or functions removed by
6734 the action of @samp{--gc-sections} or linker script @code{/DISCARD/}.
6735 Use this option to disable @code{.opd} optimization.
6737 @cindex PowerPC64 OPD spacing
6738 @kindex --non-overlapping-opd
6739 @item --non-overlapping-opd
6740 Some PowerPC64 compilers have an option to generate compressed
6741 @code{.opd} entries spaced 16 bytes apart, overlapping the third word,
6742 the static chain pointer (unused in C) with the first word of the next
6743 entry. This option expands such entries to the full 24 bytes.
6745 @cindex PowerPC64 TOC optimization
6746 @kindex --no-toc-optimize
6747 @item --no-toc-optimize
6748 PowerPC64 @command{ld} normally removes unused @code{.toc} section
6749 entries. Such entries are detected by examining relocations that
6750 reference the TOC in code sections. A reloc in a deleted code section
6751 marks a TOC word as unneeded, while a reloc in a kept code section
6752 marks a TOC word as needed. Since the TOC may reference itself, TOC
6753 relocs are also examined. TOC words marked as both needed and
6754 unneeded will of course be kept. TOC words without any referencing
6755 reloc are assumed to be part of a multi-word entry, and are kept or
6756 discarded as per the nearest marked preceding word. This works
6757 reliably for compiler generated code, but may be incorrect if assembly
6758 code is used to insert TOC entries. Use this option to disable the
6761 @cindex PowerPC64 multi-TOC
6762 @kindex --no-multi-toc
6763 @item --no-multi-toc
6764 If given any toc option besides @code{-mcmodel=medium} or
6765 @code{-mcmodel=large}, PowerPC64 GCC generates code for a TOC model
6767 entries are accessed with a 16-bit offset from r2. This limits the
6768 total TOC size to 64K. PowerPC64 @command{ld} extends this limit by
6769 grouping code sections such that each group uses less than 64K for its
6770 TOC entries, then inserts r2 adjusting stubs between inter-group
6771 calls. @command{ld} does not split apart input sections, so cannot
6772 help if a single input file has a @code{.toc} section that exceeds
6773 64K, most likely from linking multiple files with @command{ld -r}.
6774 Use this option to turn off this feature.
6776 @cindex PowerPC64 TOC sorting
6777 @kindex --no-toc-sort
6779 By default, @command{ld} sorts TOC sections so that those whose file
6780 happens to have a section called @code{.init} or @code{.fini} are
6781 placed first, followed by TOC sections referenced by code generated
6782 with PowerPC64 gcc's @code{-mcmodel=small}, and lastly TOC sections
6783 referenced only by code generated with PowerPC64 gcc's
6784 @code{-mcmodel=medium} or @code{-mcmodel=large} options. Doing this
6785 results in better TOC grouping for multi-TOC. Use this option to turn
6788 @cindex PowerPC64 PLT stub alignment
6790 @kindex --no-plt-align
6792 @itemx --no-plt-align
6793 Use these options to control whether individual PLT call stubs are
6794 aligned to a 32-byte boundary, or to the specified power of two
6795 boundary when using @code{--plt-align=}. By default PLT call stubs
6798 @cindex PowerPC64 PLT call stub static chain
6799 @kindex --plt-static-chain
6800 @kindex --no-plt-static-chain
6801 @item --plt-static-chain
6802 @itemx --no-plt-static-chain
6803 Use these options to control whether PLT call stubs load the static
6804 chain pointer (r11). @code{ld} defaults to not loading the static
6805 chain since there is never any need to do so on a PLT call.
6807 @cindex PowerPC64 PLT call stub thread safety
6808 @kindex --plt-thread-safe
6809 @kindex --no-plt-thread-safe
6810 @item --plt-thread-safe
6811 @itemx --no-thread-safe
6812 With power7's weakly ordered memory model, it is possible when using
6813 lazy binding for ld.so to update a plt entry in one thread and have
6814 another thread see the individual plt entry words update in the wrong
6815 order, despite ld.so carefully writing in the correct order and using
6816 memory write barriers. To avoid this we need some sort of read
6817 barrier in the call stub, or use LD_BIND_NOW=1. By default, @code{ld}
6818 looks for calls to commonly used functions that create threads, and if
6819 seen, adds the necessary barriers. Use these options to change the
6834 @section @command{ld} and SPU ELF Support
6836 @cindex SPU ELF options
6842 This option marks an executable as a PIC plugin module.
6844 @cindex SPU overlays
6845 @kindex --no-overlays
6847 Normally, @command{ld} recognizes calls to functions within overlay
6848 regions, and redirects such calls to an overlay manager via a stub.
6849 @command{ld} also provides a built-in overlay manager. This option
6850 turns off all this special overlay handling.
6852 @cindex SPU overlay stub symbols
6853 @kindex --emit-stub-syms
6854 @item --emit-stub-syms
6855 This option causes @command{ld} to label overlay stubs with a local
6856 symbol that encodes the stub type and destination.
6858 @cindex SPU extra overlay stubs
6859 @kindex --extra-overlay-stubs
6860 @item --extra-overlay-stubs
6861 This option causes @command{ld} to add overlay call stubs on all
6862 function calls out of overlay regions. Normally stubs are not added
6863 on calls to non-overlay regions.
6865 @cindex SPU local store size
6866 @kindex --local-store=lo:hi
6867 @item --local-store=lo:hi
6868 @command{ld} usually checks that a final executable for SPU fits in
6869 the address range 0 to 256k. This option may be used to change the
6870 range. Disable the check entirely with @option{--local-store=0:0}.
6873 @kindex --stack-analysis
6874 @item --stack-analysis
6875 SPU local store space is limited. Over-allocation of stack space
6876 unnecessarily limits space available for code and data, while
6877 under-allocation results in runtime failures. If given this option,
6878 @command{ld} will provide an estimate of maximum stack usage.
6879 @command{ld} does this by examining symbols in code sections to
6880 determine the extents of functions, and looking at function prologues
6881 for stack adjusting instructions. A call-graph is created by looking
6882 for relocations on branch instructions. The graph is then searched
6883 for the maximum stack usage path. Note that this analysis does not
6884 find calls made via function pointers, and does not handle recursion
6885 and other cycles in the call graph. Stack usage may be
6886 under-estimated if your code makes such calls. Also, stack usage for
6887 dynamic allocation, e.g. alloca, will not be detected. If a link map
6888 is requested, detailed information about each function's stack usage
6889 and calls will be given.
6892 @kindex --emit-stack-syms
6893 @item --emit-stack-syms
6894 This option, if given along with @option{--stack-analysis} will result
6895 in @command{ld} emitting stack sizing symbols for each function.
6896 These take the form @code{__stack_<function_name>} for global
6897 functions, and @code{__stack_<number>_<function_name>} for static
6898 functions. @code{<number>} is the section id in hex. The value of
6899 such symbols is the stack requirement for the corresponding function.
6900 The symbol size will be zero, type @code{STT_NOTYPE}, binding
6901 @code{STB_LOCAL}, and section @code{SHN_ABS}.
6915 @section @command{ld}'s Support for Various TI COFF Versions
6916 @cindex TI COFF versions
6917 @kindex --format=@var{version}
6918 The @samp{--format} switch allows selection of one of the various
6919 TI COFF versions. The latest of this writing is 2; versions 0 and 1 are
6920 also supported. The TI COFF versions also vary in header byte-order
6921 format; @command{ld} will read any version or byte order, but the output
6922 header format depends on the default specified by the specific target.
6935 @section @command{ld} and WIN32 (cygwin/mingw)
6937 This section describes some of the win32 specific @command{ld} issues.
6938 See @ref{Options,,Command Line Options} for detailed description of the
6939 command line options mentioned here.
6942 @cindex import libraries
6943 @item import libraries
6944 The standard Windows linker creates and uses so-called import
6945 libraries, which contains information for linking to dll's. They are
6946 regular static archives and are handled as any other static
6947 archive. The cygwin and mingw ports of @command{ld} have specific
6948 support for creating such libraries provided with the
6949 @samp{--out-implib} command line option.
6951 @item exporting DLL symbols
6952 @cindex exporting DLL symbols
6953 The cygwin/mingw @command{ld} has several ways to export symbols for dll's.
6956 @item using auto-export functionality
6957 @cindex using auto-export functionality
6958 By default @command{ld} exports symbols with the auto-export functionality,
6959 which is controlled by the following command line options:
6962 @item --export-all-symbols [This is the default]
6963 @item --exclude-symbols
6964 @item --exclude-libs
6965 @item --exclude-modules-for-implib
6966 @item --version-script
6969 When auto-export is in operation, @command{ld} will export all the non-local
6970 (global and common) symbols it finds in a DLL, with the exception of a few
6971 symbols known to belong to the system's runtime and libraries. As it will
6972 often not be desirable to export all of a DLL's symbols, which may include
6973 private functions that are not part of any public interface, the command-line
6974 options listed above may be used to filter symbols out from the list for
6975 exporting. The @samp{--output-def} option can be used in order to see the
6976 final list of exported symbols with all exclusions taken into effect.
6978 If @samp{--export-all-symbols} is not given explicitly on the
6979 command line, then the default auto-export behavior will be @emph{disabled}
6980 if either of the following are true:
6983 @item A DEF file is used.
6984 @item Any symbol in any object file was marked with the __declspec(dllexport) attribute.
6987 @item using a DEF file
6988 @cindex using a DEF file
6989 Another way of exporting symbols is using a DEF file. A DEF file is
6990 an ASCII file containing definitions of symbols which should be
6991 exported when a dll is created. Usually it is named @samp{<dll
6992 name>.def} and is added as any other object file to the linker's
6993 command line. The file's name must end in @samp{.def} or @samp{.DEF}.
6996 gcc -o <output> <objectfiles> <dll name>.def
6999 Using a DEF file turns off the normal auto-export behavior, unless the
7000 @samp{--export-all-symbols} option is also used.
7002 Here is an example of a DEF file for a shared library called @samp{xyz.dll}:
7005 LIBRARY "xyz.dll" BASE=0x20000000
7011 another_foo = abc.dll.afoo
7017 This example defines a DLL with a non-default base address and seven
7018 symbols in the export table. The third exported symbol @code{_bar} is an
7019 alias for the second. The fourth symbol, @code{another_foo} is resolved
7020 by "forwarding" to another module and treating it as an alias for
7021 @code{afoo} exported from the DLL @samp{abc.dll}. The final symbol
7022 @code{var1} is declared to be a data object. The @samp{doo} symbol in
7023 export library is an alias of @samp{foo}, which gets the string name
7024 in export table @samp{foo2}. The @samp{eoo} symbol is an data export
7025 symbol, which gets in export table the name @samp{var1}.
7027 The optional @code{LIBRARY <name>} command indicates the @emph{internal}
7028 name of the output DLL. If @samp{<name>} does not include a suffix,
7029 the default library suffix, @samp{.DLL} is appended.
7031 When the .DEF file is used to build an application, rather than a
7032 library, the @code{NAME <name>} command should be used instead of
7033 @code{LIBRARY}. If @samp{<name>} does not include a suffix, the default
7034 executable suffix, @samp{.EXE} is appended.
7036 With either @code{LIBRARY <name>} or @code{NAME <name>} the optional
7037 specification @code{BASE = <number>} may be used to specify a
7038 non-default base address for the image.
7040 If neither @code{LIBRARY <name>} nor @code{NAME <name>} is specified,
7041 or they specify an empty string, the internal name is the same as the
7042 filename specified on the command line.
7044 The complete specification of an export symbol is:
7048 ( ( ( <name1> [ = <name2> ] )
7049 | ( <name1> = <module-name> . <external-name>))
7050 [ @@ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
7053 Declares @samp{<name1>} as an exported symbol from the DLL, or declares
7054 @samp{<name1>} as an exported alias for @samp{<name2>}; or declares
7055 @samp{<name1>} as a "forward" alias for the symbol
7056 @samp{<external-name>} in the DLL @samp{<module-name>}.
7057 Optionally, the symbol may be exported by the specified ordinal
7058 @samp{<integer>} alias. The optional @samp{<name3>} is the to be used
7059 string in import/export table for the symbol.
7061 The optional keywords that follow the declaration indicate:
7063 @code{NONAME}: Do not put the symbol name in the DLL's export table. It
7064 will still be exported by its ordinal alias (either the value specified
7065 by the .def specification or, otherwise, the value assigned by the
7066 linker). The symbol name, however, does remain visible in the import
7067 library (if any), unless @code{PRIVATE} is also specified.
7069 @code{DATA}: The symbol is a variable or object, rather than a function.
7070 The import lib will export only an indirect reference to @code{foo} as
7071 the symbol @code{_imp__foo} (ie, @code{foo} must be resolved as
7074 @code{CONSTANT}: Like @code{DATA}, but put the undecorated @code{foo} as
7075 well as @code{_imp__foo} into the import library. Both refer to the
7076 read-only import address table's pointer to the variable, not to the
7077 variable itself. This can be dangerous. If the user code fails to add
7078 the @code{dllimport} attribute and also fails to explicitly add the
7079 extra indirection that the use of the attribute enforces, the
7080 application will behave unexpectedly.
7082 @code{PRIVATE}: Put the symbol in the DLL's export table, but do not put
7083 it into the static import library used to resolve imports at link time. The
7084 symbol can still be imported using the @code{LoadLibrary/GetProcAddress}
7085 API at runtime or by by using the GNU ld extension of linking directly to
7086 the DLL without an import library.
7088 See ld/deffilep.y in the binutils sources for the full specification of
7089 other DEF file statements
7091 @cindex creating a DEF file
7092 While linking a shared dll, @command{ld} is able to create a DEF file
7093 with the @samp{--output-def <file>} command line option.
7095 @item Using decorations
7096 @cindex Using decorations
7097 Another way of marking symbols for export is to modify the source code
7098 itself, so that when building the DLL each symbol to be exported is
7102 __declspec(dllexport) int a_variable
7103 __declspec(dllexport) void a_function(int with_args)
7106 All such symbols will be exported from the DLL. If, however,
7107 any of the object files in the DLL contain symbols decorated in
7108 this way, then the normal auto-export behavior is disabled, unless
7109 the @samp{--export-all-symbols} option is also used.
7111 Note that object files that wish to access these symbols must @emph{not}
7112 decorate them with dllexport. Instead, they should use dllimport,
7116 __declspec(dllimport) int a_variable
7117 __declspec(dllimport) void a_function(int with_args)
7120 This complicates the structure of library header files, because
7121 when included by the library itself the header must declare the
7122 variables and functions as dllexport, but when included by client
7123 code the header must declare them as dllimport. There are a number
7124 of idioms that are typically used to do this; often client code can
7125 omit the __declspec() declaration completely. See
7126 @samp{--enable-auto-import} and @samp{automatic data imports} for more
7130 @cindex automatic data imports
7131 @item automatic data imports
7132 The standard Windows dll format supports data imports from dlls only
7133 by adding special decorations (dllimport/dllexport), which let the
7134 compiler produce specific assembler instructions to deal with this
7135 issue. This increases the effort necessary to port existing Un*x
7136 code to these platforms, especially for large
7137 c++ libraries and applications. The auto-import feature, which was
7138 initially provided by Paul Sokolovsky, allows one to omit the
7139 decorations to achieve a behavior that conforms to that on POSIX/Un*x
7140 platforms. This feature is enabled with the @samp{--enable-auto-import}
7141 command-line option, although it is enabled by default on cygwin/mingw.
7142 The @samp{--enable-auto-import} option itself now serves mainly to
7143 suppress any warnings that are ordinarily emitted when linked objects
7144 trigger the feature's use.
7146 auto-import of variables does not always work flawlessly without
7147 additional assistance. Sometimes, you will see this message
7149 "variable '<var>' can't be auto-imported. Please read the
7150 documentation for ld's @code{--enable-auto-import} for details."
7152 The @samp{--enable-auto-import} documentation explains why this error
7153 occurs, and several methods that can be used to overcome this difficulty.
7154 One of these methods is the @emph{runtime pseudo-relocs} feature, described
7157 @cindex runtime pseudo-relocation
7158 For complex variables imported from DLLs (such as structs or classes),
7159 object files typically contain a base address for the variable and an
7160 offset (@emph{addend}) within the variable--to specify a particular
7161 field or public member, for instance. Unfortunately, the runtime loader used
7162 in win32 environments is incapable of fixing these references at runtime
7163 without the additional information supplied by dllimport/dllexport decorations.
7164 The standard auto-import feature described above is unable to resolve these
7167 The @samp{--enable-runtime-pseudo-relocs} switch allows these references to
7168 be resolved without error, while leaving the task of adjusting the references
7169 themselves (with their non-zero addends) to specialized code provided by the
7170 runtime environment. Recent versions of the cygwin and mingw environments and
7171 compilers provide this runtime support; older versions do not. However, the
7172 support is only necessary on the developer's platform; the compiled result will
7173 run without error on an older system.
7175 @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly
7178 @cindex direct linking to a dll
7179 @item direct linking to a dll
7180 The cygwin/mingw ports of @command{ld} support the direct linking,
7181 including data symbols, to a dll without the usage of any import
7182 libraries. This is much faster and uses much less memory than does the
7183 traditional import library method, especially when linking large
7184 libraries or applications. When @command{ld} creates an import lib, each
7185 function or variable exported from the dll is stored in its own bfd, even
7186 though a single bfd could contain many exports. The overhead involved in
7187 storing, loading, and processing so many bfd's is quite large, and explains the
7188 tremendous time, memory, and storage needed to link against particularly
7189 large or complex libraries when using import libs.
7191 Linking directly to a dll uses no extra command-line switches other than
7192 @samp{-L} and @samp{-l}, because @command{ld} already searches for a number
7193 of names to match each library. All that is needed from the developer's
7194 perspective is an understanding of this search, in order to force ld to
7195 select the dll instead of an import library.
7198 For instance, when ld is called with the argument @samp{-lxxx} it will attempt
7199 to find, in the first directory of its search path,
7211 before moving on to the next directory in the search path.
7213 (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll},
7214 where @samp{<prefix>} is set by the @command{ld} option
7215 @samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec
7216 file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for
7219 Other win32-based unix environments, such as mingw or pw32, may use other
7220 @samp{<prefix>}es, although at present only cygwin makes use of this feature. It
7221 was originally intended to help avoid name conflicts among dll's built for the
7222 various win32/un*x environments, so that (for example) two versions of a zlib dll
7223 could coexist on the same machine.
7225 The generic cygwin/mingw path layout uses a @samp{bin} directory for
7226 applications and dll's and a @samp{lib} directory for the import
7227 libraries (using cygwin nomenclature):
7233 libxxx.dll.a (in case of dll's)
7234 libxxx.a (in case of static archive)
7237 Linking directly to a dll without using the import library can be
7240 1. Use the dll directly by adding the @samp{bin} path to the link line
7242 gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
7245 However, as the dll's often have version numbers appended to their names
7246 (@samp{cygncurses-5.dll}) this will often fail, unless one specifies
7247 @samp{-L../bin -lncurses-5} to include the version. Import libs are generally
7248 not versioned, and do not have this difficulty.
7250 2. Create a symbolic link from the dll to a file in the @samp{lib}
7251 directory according to the above mentioned search pattern. This
7252 should be used to avoid unwanted changes in the tools needed for
7256 ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
7259 Then you can link without any make environment changes.
7262 gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
7265 This technique also avoids the version number problems, because the following is
7272 libxxx.dll.a -> ../bin/cygxxx-5.dll
7275 Linking directly to a dll without using an import lib will work
7276 even when auto-import features are exercised, and even when
7277 @samp{--enable-runtime-pseudo-relocs} is used.
7279 Given the improvements in speed and memory usage, one might justifiably
7280 wonder why import libraries are used at all. There are three reasons:
7282 1. Until recently, the link-directly-to-dll functionality did @emph{not}
7283 work with auto-imported data.
7285 2. Sometimes it is necessary to include pure static objects within the
7286 import library (which otherwise contains only bfd's for indirection
7287 symbols that point to the exports of a dll). Again, the import lib
7288 for the cygwin kernel makes use of this ability, and it is not
7289 possible to do this without an import lib.
7291 3. Symbol aliases can only be resolved using an import lib. This is
7292 critical when linking against OS-supplied dll's (eg, the win32 API)
7293 in which symbols are usually exported as undecorated aliases of their
7294 stdcall-decorated assembly names.
7296 So, import libs are not going away. But the ability to replace
7297 true import libs with a simple symbolic link to (or a copy of)
7298 a dll, in many cases, is a useful addition to the suite of tools
7299 binutils makes available to the win32 developer. Given the
7300 massive improvements in memory requirements during linking, storage
7301 requirements, and linking speed, we expect that many developers
7302 will soon begin to use this feature whenever possible.
7304 @item symbol aliasing
7306 @item adding additional names
7307 Sometimes, it is useful to export symbols with additional names.
7308 A symbol @samp{foo} will be exported as @samp{foo}, but it can also be
7309 exported as @samp{_foo} by using special directives in the DEF file
7310 when creating the dll. This will affect also the optional created
7311 import library. Consider the following DEF file:
7314 LIBRARY "xyz.dll" BASE=0x61000000
7321 The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}.
7323 Another method for creating a symbol alias is to create it in the
7324 source code using the "weak" attribute:
7327 void foo () @{ /* Do something. */; @}
7328 void _foo () __attribute__ ((weak, alias ("foo")));
7331 See the gcc manual for more information about attributes and weak
7334 @item renaming symbols
7335 Sometimes it is useful to rename exports. For instance, the cygwin
7336 kernel does this regularly. A symbol @samp{_foo} can be exported as
7337 @samp{foo} but not as @samp{_foo} by using special directives in the
7338 DEF file. (This will also affect the import library, if it is
7339 created). In the following example:
7342 LIBRARY "xyz.dll" BASE=0x61000000
7348 The line @samp{_foo = foo} maps the exported symbol @samp{foo} to
7352 Note: using a DEF file disables the default auto-export behavior,
7353 unless the @samp{--export-all-symbols} command line option is used.
7354 If, however, you are trying to rename symbols, then you should list
7355 @emph{all} desired exports in the DEF file, including the symbols
7356 that are not being renamed, and do @emph{not} use the
7357 @samp{--export-all-symbols} option. If you list only the
7358 renamed symbols in the DEF file, and use @samp{--export-all-symbols}
7359 to handle the other symbols, then the both the new names @emph{and}
7360 the original names for the renamed symbols will be exported.
7361 In effect, you'd be aliasing those symbols, not renaming them,
7362 which is probably not what you wanted.
7364 @cindex weak externals
7365 @item weak externals
7366 The Windows object format, PE, specifies a form of weak symbols called
7367 weak externals. When a weak symbol is linked and the symbol is not
7368 defined, the weak symbol becomes an alias for some other symbol. There
7369 are three variants of weak externals:
7371 @item Definition is searched for in objects and libraries, historically
7372 called lazy externals.
7373 @item Definition is searched for only in other objects, not in libraries.
7374 This form is not presently implemented.
7375 @item No search; the symbol is an alias. This form is not presently
7378 As a GNU extension, weak symbols that do not specify an alternate symbol
7379 are supported. If the symbol is undefined when linking, the symbol
7380 uses a default value.
7382 @cindex aligned common symbols
7383 @item aligned common symbols
7384 As a GNU extension to the PE file format, it is possible to specify the
7385 desired alignment for a common symbol. This information is conveyed from
7386 the assembler or compiler to the linker by means of GNU-specific commands
7387 carried in the object file's @samp{.drectve} section, which are recognized
7388 by @command{ld} and respected when laying out the common symbols. Native
7389 tools will be able to process object files employing this GNU extension,
7390 but will fail to respect the alignment instructions, and may issue noisy
7391 warnings about unknown linker directives.
7405 @section @code{ld} and Xtensa Processors
7407 @cindex Xtensa processors
7408 The default @command{ld} behavior for Xtensa processors is to interpret
7409 @code{SECTIONS} commands so that lists of explicitly named sections in a
7410 specification with a wildcard file will be interleaved when necessary to
7411 keep literal pools within the range of PC-relative load offsets. For
7412 example, with the command:
7424 @command{ld} may interleave some of the @code{.literal}
7425 and @code{.text} sections from different object files to ensure that the
7426 literal pools are within the range of PC-relative load offsets. A valid
7427 interleaving might place the @code{.literal} sections from an initial
7428 group of files followed by the @code{.text} sections of that group of
7429 files. Then, the @code{.literal} sections from the rest of the files
7430 and the @code{.text} sections from the rest of the files would follow.
7432 @cindex @option{--relax} on Xtensa
7433 @cindex relaxing on Xtensa
7434 Relaxation is enabled by default for the Xtensa version of @command{ld} and
7435 provides two important link-time optimizations. The first optimization
7436 is to combine identical literal values to reduce code size. A redundant
7437 literal will be removed and all the @code{L32R} instructions that use it
7438 will be changed to reference an identical literal, as long as the
7439 location of the replacement literal is within the offset range of all
7440 the @code{L32R} instructions. The second optimization is to remove
7441 unnecessary overhead from assembler-generated ``longcall'' sequences of
7442 @code{L32R}/@code{CALLX@var{n}} when the target functions are within
7443 range of direct @code{CALL@var{n}} instructions.
7445 For each of these cases where an indirect call sequence can be optimized
7446 to a direct call, the linker will change the @code{CALLX@var{n}}
7447 instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R}
7448 instruction, and remove the literal referenced by the @code{L32R}
7449 instruction if it is not used for anything else. Removing the
7450 @code{L32R} instruction always reduces code size but can potentially
7451 hurt performance by changing the alignment of subsequent branch targets.
7452 By default, the linker will always preserve alignments, either by
7453 switching some instructions between 24-bit encodings and the equivalent
7454 density instructions or by inserting a no-op in place of the @code{L32R}
7455 instruction that was removed. If code size is more important than
7456 performance, the @option{--size-opt} option can be used to prevent the
7457 linker from widening density instructions or inserting no-ops, except in
7458 a few cases where no-ops are required for correctness.
7460 The following Xtensa-specific command-line options can be used to
7463 @cindex Xtensa options
7466 When optimizing indirect calls to direct calls, optimize for code size
7467 more than performance. With this option, the linker will not insert
7468 no-ops or widen density instructions to preserve branch target
7469 alignment. There may still be some cases where no-ops are required to
7470 preserve the correctness of the code.
7478 @ifclear SingleFormat
7483 @cindex object file management
7484 @cindex object formats available
7486 The linker accesses object and archive files using the BFD libraries.
7487 These libraries allow the linker to use the same routines to operate on
7488 object files whatever the object file format. A different object file
7489 format can be supported simply by creating a new BFD back end and adding
7490 it to the library. To conserve runtime memory, however, the linker and
7491 associated tools are usually configured to support only a subset of the
7492 object file formats available. You can use @code{objdump -i}
7493 (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to
7494 list all the formats available for your configuration.
7496 @cindex BFD requirements
7497 @cindex requirements for BFD
7498 As with most implementations, BFD is a compromise between
7499 several conflicting requirements. The major factor influencing
7500 BFD design was efficiency: any time used converting between
7501 formats is time which would not have been spent had BFD not
7502 been involved. This is partly offset by abstraction payback; since
7503 BFD simplifies applications and back ends, more time and care
7504 may be spent optimizing algorithms for a greater speed.
7506 One minor artifact of the BFD solution which you should bear in
7507 mind is the potential for information loss. There are two places where
7508 useful information can be lost using the BFD mechanism: during
7509 conversion and during output. @xref{BFD information loss}.
7512 * BFD outline:: How it works: an outline of BFD
7516 @section How It Works: An Outline of BFD
7517 @cindex opening object files
7518 @include bfdsumm.texi
7521 @node Reporting Bugs
7522 @chapter Reporting Bugs
7523 @cindex bugs in @command{ld}
7524 @cindex reporting bugs in @command{ld}
7526 Your bug reports play an essential role in making @command{ld} reliable.
7528 Reporting a bug may help you by bringing a solution to your problem, or
7529 it may not. But in any case the principal function of a bug report is
7530 to help the entire community by making the next version of @command{ld}
7531 work better. Bug reports are your contribution to the maintenance of
7534 In order for a bug report to serve its purpose, you must include the
7535 information that enables us to fix the bug.
7538 * Bug Criteria:: Have you found a bug?
7539 * Bug Reporting:: How to report bugs
7543 @section Have You Found a Bug?
7544 @cindex bug criteria
7546 If you are not sure whether you have found a bug, here are some guidelines:
7549 @cindex fatal signal
7550 @cindex linker crash
7551 @cindex crash of linker
7553 If the linker gets a fatal signal, for any input whatever, that is a
7554 @command{ld} bug. Reliable linkers never crash.
7556 @cindex error on valid input
7558 If @command{ld} produces an error message for valid input, that is a bug.
7560 @cindex invalid input
7562 If @command{ld} does not produce an error message for invalid input, that
7563 may be a bug. In the general case, the linker can not verify that
7564 object files are correct.
7567 If you are an experienced user of linkers, your suggestions for
7568 improvement of @command{ld} are welcome in any case.
7572 @section How to Report Bugs
7574 @cindex @command{ld} bugs, reporting
7576 A number of companies and individuals offer support for @sc{gnu}
7577 products. If you obtained @command{ld} from a support organization, we
7578 recommend you contact that organization first.
7580 You can find contact information for many support companies and
7581 individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs
7585 Otherwise, send bug reports for @command{ld} to
7589 The fundamental principle of reporting bugs usefully is this:
7590 @strong{report all the facts}. If you are not sure whether to state a
7591 fact or leave it out, state it!
7593 Often people omit facts because they think they know what causes the
7594 problem and assume that some details do not matter. Thus, you might
7595 assume that the name of a symbol you use in an example does not
7596 matter. Well, probably it does not, but one cannot be sure. Perhaps
7597 the bug is a stray memory reference which happens to fetch from the
7598 location where that name is stored in memory; perhaps, if the name
7599 were different, the contents of that location would fool the linker
7600 into doing the right thing despite the bug. Play it safe and give a
7601 specific, complete example. That is the easiest thing for you to do,
7602 and the most helpful.
7604 Keep in mind that the purpose of a bug report is to enable us to fix
7605 the bug if it is new to us. Therefore, always write your bug reports
7606 on the assumption that the bug has not been reported previously.
7608 Sometimes people give a few sketchy facts and ask, ``Does this ring a
7609 bell?'' This cannot help us fix a bug, so it is basically useless. We
7610 respond by asking for enough details to enable us to investigate.
7611 You might as well expedite matters by sending them to begin with.
7613 To enable us to fix the bug, you should include all these things:
7617 The version of @command{ld}. @command{ld} announces it if you start it with
7618 the @samp{--version} argument.
7620 Without this, we will not know whether there is any point in looking for
7621 the bug in the current version of @command{ld}.
7624 Any patches you may have applied to the @command{ld} source, including any
7625 patches made to the @code{BFD} library.
7628 The type of machine you are using, and the operating system name and
7632 What compiler (and its version) was used to compile @command{ld}---e.g.
7636 The command arguments you gave the linker to link your example and
7637 observe the bug. To guarantee you will not omit something important,
7638 list them all. A copy of the Makefile (or the output from make) is
7641 If we were to try to guess the arguments, we would probably guess wrong
7642 and then we might not encounter the bug.
7645 A complete input file, or set of input files, that will reproduce the
7646 bug. It is generally most helpful to send the actual object files
7647 provided that they are reasonably small. Say no more than 10K. For
7648 bigger files you can either make them available by FTP or HTTP or else
7649 state that you are willing to send the object file(s) to whomever
7650 requests them. (Note - your email will be going to a mailing list, so
7651 we do not want to clog it up with large attachments). But small
7652 attachments are best.
7654 If the source files were assembled using @code{gas} or compiled using
7655 @code{gcc}, then it may be OK to send the source files rather than the
7656 object files. In this case, be sure to say exactly what version of
7657 @code{gas} or @code{gcc} was used to produce the object files. Also say
7658 how @code{gas} or @code{gcc} were configured.
7661 A description of what behavior you observe that you believe is
7662 incorrect. For example, ``It gets a fatal signal.''
7664 Of course, if the bug is that @command{ld} gets a fatal signal, then we
7665 will certainly notice it. But if the bug is incorrect output, we might
7666 not notice unless it is glaringly wrong. You might as well not give us
7667 a chance to make a mistake.
7669 Even if the problem you experience is a fatal signal, you should still
7670 say so explicitly. Suppose something strange is going on, such as, your
7671 copy of @command{ld} is out of sync, or you have encountered a bug in the
7672 C library on your system. (This has happened!) Your copy might crash
7673 and ours would not. If you told us to expect a crash, then when ours
7674 fails to crash, we would know that the bug was not happening for us. If
7675 you had not told us to expect a crash, then we would not be able to draw
7676 any conclusion from our observations.
7679 If you wish to suggest changes to the @command{ld} source, send us context
7680 diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or
7681 @samp{-p} option. Always send diffs from the old file to the new file.
7682 If you even discuss something in the @command{ld} source, refer to it by
7683 context, not by line number.
7685 The line numbers in our development sources will not match those in your
7686 sources. Your line numbers would convey no useful information to us.
7689 Here are some things that are not necessary:
7693 A description of the envelope of the bug.
7695 Often people who encounter a bug spend a lot of time investigating
7696 which changes to the input file will make the bug go away and which
7697 changes will not affect it.
7699 This is often time consuming and not very useful, because the way we
7700 will find the bug is by running a single example under the debugger
7701 with breakpoints, not by pure deduction from a series of examples.
7702 We recommend that you save your time for something else.
7704 Of course, if you can find a simpler example to report @emph{instead}
7705 of the original one, that is a convenience for us. Errors in the
7706 output will be easier to spot, running under the debugger will take
7707 less time, and so on.
7709 However, simplification is not vital; if you do not want to do this,
7710 report the bug anyway and send us the entire test case you used.
7713 A patch for the bug.
7715 A patch for the bug does help us if it is a good one. But do not omit
7716 the necessary information, such as the test case, on the assumption that
7717 a patch is all we need. We might see problems with your patch and decide
7718 to fix the problem another way, or we might not understand it at all.
7720 Sometimes with a program as complicated as @command{ld} it is very hard to
7721 construct an example that will make the program follow a certain path
7722 through the code. If you do not send us the example, we will not be
7723 able to construct one, so we will not be able to verify that the bug is
7726 And if we cannot understand what bug you are trying to fix, or why your
7727 patch should be an improvement, we will not install it. A test case will
7728 help us to understand.
7731 A guess about what the bug is or what it depends on.
7733 Such guesses are usually wrong. Even we cannot guess right about such
7734 things without first using the debugger to find the facts.
7738 @appendix MRI Compatible Script Files
7739 @cindex MRI compatibility
7740 To aid users making the transition to @sc{gnu} @command{ld} from the MRI
7741 linker, @command{ld} can use MRI compatible linker scripts as an
7742 alternative to the more general-purpose linker scripting language
7743 described in @ref{Scripts}. MRI compatible linker scripts have a much
7744 simpler command set than the scripting language otherwise used with
7745 @command{ld}. @sc{gnu} @command{ld} supports the most commonly used MRI
7746 linker commands; these commands are described here.
7748 In general, MRI scripts aren't of much use with the @code{a.out} object
7749 file format, since it only has three sections and MRI scripts lack some
7750 features to make use of them.
7752 You can specify a file containing an MRI-compatible script using the
7753 @samp{-c} command-line option.
7755 Each command in an MRI-compatible script occupies its own line; each
7756 command line starts with the keyword that identifies the command (though
7757 blank lines are also allowed for punctuation). If a line of an
7758 MRI-compatible script begins with an unrecognized keyword, @command{ld}
7759 issues a warning message, but continues processing the script.
7761 Lines beginning with @samp{*} are comments.
7763 You can write these commands using all upper-case letters, or all
7764 lower case; for example, @samp{chip} is the same as @samp{CHIP}.
7765 The following list shows only the upper-case form of each command.
7768 @cindex @code{ABSOLUTE} (MRI)
7769 @item ABSOLUTE @var{secname}
7770 @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname}
7771 Normally, @command{ld} includes in the output file all sections from all
7772 the input files. However, in an MRI-compatible script, you can use the
7773 @code{ABSOLUTE} command to restrict the sections that will be present in
7774 your output program. If the @code{ABSOLUTE} command is used at all in a
7775 script, then only the sections named explicitly in @code{ABSOLUTE}
7776 commands will appear in the linker output. You can still use other
7777 input sections (whatever you select on the command line, or using
7778 @code{LOAD}) to resolve addresses in the output file.
7780 @cindex @code{ALIAS} (MRI)
7781 @item ALIAS @var{out-secname}, @var{in-secname}
7782 Use this command to place the data from input section @var{in-secname}
7783 in a section called @var{out-secname} in the linker output file.
7785 @var{in-secname} may be an integer.
7787 @cindex @code{ALIGN} (MRI)
7788 @item ALIGN @var{secname} = @var{expression}
7789 Align the section called @var{secname} to @var{expression}. The
7790 @var{expression} should be a power of two.
7792 @cindex @code{BASE} (MRI)
7793 @item BASE @var{expression}
7794 Use the value of @var{expression} as the lowest address (other than
7795 absolute addresses) in the output file.
7797 @cindex @code{CHIP} (MRI)
7798 @item CHIP @var{expression}
7799 @itemx CHIP @var{expression}, @var{expression}
7800 This command does nothing; it is accepted only for compatibility.
7802 @cindex @code{END} (MRI)
7804 This command does nothing whatever; it's only accepted for compatibility.
7806 @cindex @code{FORMAT} (MRI)
7807 @item FORMAT @var{output-format}
7808 Similar to the @code{OUTPUT_FORMAT} command in the more general linker
7809 language, but restricted to one of these output formats:
7813 S-records, if @var{output-format} is @samp{S}
7816 IEEE, if @var{output-format} is @samp{IEEE}
7819 COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is
7823 @cindex @code{LIST} (MRI)
7824 @item LIST @var{anything}@dots{}
7825 Print (to the standard output file) a link map, as produced by the
7826 @command{ld} command-line option @samp{-M}.
7828 The keyword @code{LIST} may be followed by anything on the
7829 same line, with no change in its effect.
7831 @cindex @code{LOAD} (MRI)
7832 @item LOAD @var{filename}
7833 @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename}
7834 Include one or more object file @var{filename} in the link; this has the
7835 same effect as specifying @var{filename} directly on the @command{ld}
7838 @cindex @code{NAME} (MRI)
7839 @item NAME @var{output-name}
7840 @var{output-name} is the name for the program produced by @command{ld}; the
7841 MRI-compatible command @code{NAME} is equivalent to the command-line
7842 option @samp{-o} or the general script language command @code{OUTPUT}.
7844 @cindex @code{ORDER} (MRI)
7845 @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname}
7846 @itemx ORDER @var{secname} @var{secname} @var{secname}
7847 Normally, @command{ld} orders the sections in its output file in the
7848 order in which they first appear in the input files. In an MRI-compatible
7849 script, you can override this ordering with the @code{ORDER} command. The
7850 sections you list with @code{ORDER} will appear first in your output
7851 file, in the order specified.
7853 @cindex @code{PUBLIC} (MRI)
7854 @item PUBLIC @var{name}=@var{expression}
7855 @itemx PUBLIC @var{name},@var{expression}
7856 @itemx PUBLIC @var{name} @var{expression}
7857 Supply a value (@var{expression}) for external symbol
7858 @var{name} used in the linker input files.
7860 @cindex @code{SECT} (MRI)
7861 @item SECT @var{secname}, @var{expression}
7862 @itemx SECT @var{secname}=@var{expression}
7863 @itemx SECT @var{secname} @var{expression}
7864 You can use any of these three forms of the @code{SECT} command to
7865 specify the start address (@var{expression}) for section @var{secname}.
7866 If you have more than one @code{SECT} statement for the same
7867 @var{secname}, only the @emph{first} sets the start address.
7870 @node GNU Free Documentation License
7871 @appendix GNU Free Documentation License
7875 @unnumbered LD Index
7880 % I think something like @colophon should be in texinfo. In the
7882 \long\def\colophon{\hbox to0pt{}\vfill
7883 \centerline{The body of this manual is set in}
7884 \centerline{\fontname\tenrm,}
7885 \centerline{with headings in {\bf\fontname\tenbf}}
7886 \centerline{and examples in {\tt\fontname\tentt}.}
7887 \centerline{{\it\fontname\tenit\/} and}
7888 \centerline{{\sl\fontname\tensl\/}}
7889 \centerline{are used for emphasis.}\vfill}
7891 % Blame: doc@cygnus.com, 28mar91.