3 @c Copyright (C) 1991-2021 Free Software Foundation, Inc.
6 @include configdoc.texi
7 @c (configdoc.texi is generated by the Makefile)
13 @macro gcctabopt{body}
19 @c Configure for the generation of man pages
47 @dircategory Software development
49 * Ld: (ld). The GNU linker.
54 This file documents the @sc{gnu} linker LD
55 @ifset VERSION_PACKAGE
56 @value{VERSION_PACKAGE}
58 version @value{VERSION}.
60 Copyright @copyright{} 1991-2021 Free Software Foundation, Inc.
62 Permission is granted to copy, distribute and/or modify this document
63 under the terms of the GNU Free Documentation License, Version 1.3
64 or any later version published by the Free Software Foundation;
65 with no Invariant Sections, with no Front-Cover Texts, and with no
66 Back-Cover Texts. A copy of the license is included in the
67 section entitled ``GNU Free Documentation License''.
71 @setchapternewpage odd
72 @settitle The GNU linker
77 @ifset VERSION_PACKAGE
78 @subtitle @value{VERSION_PACKAGE}
80 @subtitle Version @value{VERSION}
81 @author Steve Chamberlain
82 @author Ian Lance Taylor
87 \hfill Red Hat Inc\par
88 \hfill nickc\@credhat.com, doc\@redhat.com\par
89 \hfill {\it The GNU linker}\par
90 \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par
92 \global\parindent=0pt % Steve likes it this way.
95 @vskip 0pt plus 1filll
96 @c man begin COPYRIGHT
97 Copyright @copyright{} 1991-2021 Free 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
129 * Plugins:: Linker Plugins
131 * Machine Dependent:: Machine Dependent Features
135 * H8/300:: ld and the H8/300
138 * Renesas:: ld and other Renesas micros
141 * ARM:: ld and the ARM family
144 * M68HC11/68HC12:: ld and the Motorola 68HC11 and 68HC12 families
147 * HPPA ELF32:: ld and HPPA 32-bit ELF
150 * M68K:: ld and Motorola 68K family
153 * MIPS:: ld and MIPS family
156 * PowerPC ELF32:: ld and PowerPC 32-bit ELF Support
159 * PowerPC64 ELF64:: ld and PowerPC64 64-bit ELF Support
162 * S/390 ELF:: ld and S/390 ELF Support
165 * SPU ELF:: ld and SPU ELF Support
168 * TI COFF:: ld and the TI COFF
171 * Win32:: ld and WIN32 (cygwin/mingw)
174 * Xtensa:: ld and Xtensa Processors
177 @ifclear SingleFormat
180 @c Following blank line required for remaining bug in makeinfo conds/menus
182 * Reporting Bugs:: Reporting Bugs
183 * MRI:: MRI Compatible Script Files
184 * GNU Free Documentation License:: GNU Free Documentation License
185 * LD Index:: LD Index
192 @cindex @sc{gnu} linker
193 @cindex what is this?
196 @c man begin SYNOPSIS
197 ld [@b{options}] @var{objfile} @dots{}
201 ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and
202 the Info entries for @file{binutils} and
207 @c man begin DESCRIPTION
209 @command{ld} combines a number of object and archive files, relocates
210 their data and ties up symbol references. Usually the last step in
211 compiling a program is to run @command{ld}.
213 @command{ld} accepts Linker Command Language files written in
214 a superset of AT&T's Link Editor Command Language syntax,
215 to provide explicit and total control over the linking process.
219 This man page does not describe the command language; see the
220 @command{ld} entry in @code{info} for full details on the command
221 language and on other aspects of the GNU linker.
224 @ifclear SingleFormat
225 This version of @command{ld} uses the general purpose BFD libraries
226 to operate on object files. This allows @command{ld} to read, combine, and
227 write object files in many different formats---for example, COFF or
228 @code{a.out}. Different formats may be linked together to produce any
229 available kind of object file. @xref{BFD}, for more information.
232 Aside from its flexibility, the @sc{gnu} linker is more helpful than other
233 linkers in providing diagnostic information. Many linkers abandon
234 execution immediately upon encountering an error; whenever possible,
235 @command{ld} continues executing, allowing you to identify other errors
236 (or, in some cases, to get an output file in spite of the error).
243 @c man begin DESCRIPTION
245 The @sc{gnu} linker @command{ld} is meant to cover a broad range of situations,
246 and to be as compatible as possible with other linkers. As a result,
247 you have many choices to control its behavior.
253 * Options:: Command-line Options
254 * Environment:: Environment Variables
258 @section Command-line Options
266 The linker supports a plethora of command-line options, but in actual
267 practice few of them are used in any particular context.
268 @cindex standard Unix system
269 For instance, a frequent use of @command{ld} is to link standard Unix
270 object files on a standard, supported Unix system. On such a system, to
271 link a file @code{hello.o}:
274 ld -o @var{output} /lib/crt0.o hello.o -lc
277 This tells @command{ld} to produce a file called @var{output} as the
278 result of linking the file @code{/lib/crt0.o} with @code{hello.o} and
279 the library @code{libc.a}, which will come from the standard search
280 directories. (See the discussion of the @samp{-l} option below.)
282 Some of the command-line options to @command{ld} may be specified at any
283 point in the command line. However, options which refer to files, such
284 as @samp{-l} or @samp{-T}, cause the file to be read at the point at
285 which the option appears in the command line, relative to the object
286 files and other file options. Repeating non-file options with a
287 different argument will either have no further effect, or override prior
288 occurrences (those further to the left on the command line) of that
289 option. Options which may be meaningfully specified more than once are
290 noted in the descriptions below.
293 Non-option arguments are object files or archives which are to be linked
294 together. They may follow, precede, or be mixed in with command-line
295 options, except that an object file argument may not be placed between
296 an option and its argument.
298 Usually the linker is invoked with at least one object file, but you can
299 specify other forms of binary input files using @samp{-l}, @samp{-R},
300 and the script command language. If @emph{no} binary input files at all
301 are specified, the linker does not produce any output, and issues the
302 message @samp{No input files}.
304 If the linker cannot recognize the format of an object file, it will
305 assume that it is a linker script. A script specified in this way
306 augments the main linker script used for the link (either the default
307 linker script or the one specified by using @samp{-T}). This feature
308 permits the linker to link against a file which appears to be an object
309 or an archive, but actually merely defines some symbol values, or uses
310 @code{INPUT} or @code{GROUP} to load other objects. Specifying a
311 script in this way merely augments the main linker script, with the
312 extra commands placed after the main script; use the @samp{-T} option
313 to replace the default linker script entirely, but note the effect of
314 the @code{INSERT} command. @xref{Scripts}.
316 For options whose names are a single letter,
317 option arguments must either follow the option letter without intervening
318 whitespace, or be given as separate arguments immediately following the
319 option that requires them.
321 For options whose names are multiple letters, either one dash or two can
322 precede the option name; for example, @samp{-trace-symbol} and
323 @samp{--trace-symbol} are equivalent. Note---there is one exception to
324 this rule. Multiple letter options that start with a lower case 'o' can
325 only be preceded by two dashes. This is to reduce confusion with the
326 @samp{-o} option. So for example @samp{-omagic} sets the output file
327 name to @samp{magic} whereas @samp{--omagic} sets the NMAGIC flag on the
330 Arguments to multiple-letter options must either be separated from the
331 option name by an equals sign, or be given as separate arguments
332 immediately following the option that requires them. For example,
333 @samp{--trace-symbol foo} and @samp{--trace-symbol=foo} are equivalent.
334 Unique abbreviations of the names of multiple-letter options are
337 Note---if the linker is being invoked indirectly, via a compiler driver
338 (e.g. @samp{gcc}) then all the linker command-line options should be
339 prefixed by @samp{-Wl,} (or whatever is appropriate for the particular
340 compiler driver) like this:
343 gcc -Wl,--start-group foo.o bar.o -Wl,--end-group
346 This is important, because otherwise the compiler driver program may
347 silently drop the linker options, resulting in a bad link. Confusion
348 may also arise when passing options that require values through a
349 driver, as the use of a space between option and argument acts as
350 a separator, and causes the driver to pass only the option to the linker
351 and the argument to the compiler. In this case, it is simplest to use
352 the joined forms of both single- and multiple-letter options, such as:
355 gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map
358 Here is a table of the generic command-line switches accepted by the GNU
362 @include at-file.texi
364 @kindex -a @var{keyword}
365 @item -a @var{keyword}
366 This option is supported for HP/UX compatibility. The @var{keyword}
367 argument must be one of the strings @samp{archive}, @samp{shared}, or
368 @samp{default}. @samp{-aarchive} is functionally equivalent to
369 @samp{-Bstatic}, and the other two keywords are functionally equivalent
370 to @samp{-Bdynamic}. This option may be used any number of times.
372 @kindex --audit @var{AUDITLIB}
373 @item --audit @var{AUDITLIB}
374 Adds @var{AUDITLIB} to the @code{DT_AUDIT} entry of the dynamic section.
375 @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME
376 specified in the library. If specified multiple times @code{DT_AUDIT}
377 will contain a colon separated list of audit interfaces to use. If the linker
378 finds an object with an audit entry while searching for shared libraries,
379 it will add a corresponding @code{DT_DEPAUDIT} entry in the output file.
380 This option is only meaningful on ELF platforms supporting the rtld-audit
383 @ifclear SingleFormat
384 @cindex binary input format
385 @kindex -b @var{format}
386 @kindex --format=@var{format}
389 @item -b @var{input-format}
390 @itemx --format=@var{input-format}
391 @command{ld} may be configured to support more than one kind of object
392 file. If your @command{ld} is configured this way, you can use the
393 @samp{-b} option to specify the binary format for input object files
394 that follow this option on the command line. Even when @command{ld} is
395 configured to support alternative object formats, you don't usually need
396 to specify this, as @command{ld} should be configured to expect as a
397 default input format the most usual format on each machine.
398 @var{input-format} is a text string, the name of a particular format
399 supported by the BFD libraries. (You can list the available binary
400 formats with @samp{objdump -i}.)
403 You may want to use this option if you are linking files with an unusual
404 binary format. You can also use @samp{-b} to switch formats explicitly (when
405 linking object files of different formats), by including
406 @samp{-b @var{input-format}} before each group of object files in a
409 The default format is taken from the environment variable
414 You can also define the input format from a script, using the command
417 see @ref{Format Commands}.
421 @kindex -c @var{MRI-cmdfile}
422 @kindex --mri-script=@var{MRI-cmdfile}
423 @cindex compatibility, MRI
424 @item -c @var{MRI-commandfile}
425 @itemx --mri-script=@var{MRI-commandfile}
426 For compatibility with linkers produced by MRI, @command{ld} accepts script
427 files written in an alternate, restricted command language, described in
429 @ref{MRI,,MRI Compatible Script Files}.
432 the MRI Compatible Script Files section of GNU ld documentation.
434 Introduce MRI script files with
435 the option @samp{-c}; use the @samp{-T} option to run linker
436 scripts written in the general-purpose @command{ld} scripting language.
437 If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories
438 specified by any @samp{-L} options.
440 @cindex common allocation
447 These three options are equivalent; multiple forms are supported for
448 compatibility with other linkers. They assign space to common symbols
449 even if a relocatable output file is specified (with @samp{-r}). The
450 script command @code{FORCE_COMMON_ALLOCATION} has the same effect.
451 @xref{Miscellaneous Commands}.
453 @kindex --depaudit @var{AUDITLIB}
454 @kindex -P @var{AUDITLIB}
455 @item --depaudit @var{AUDITLIB}
456 @itemx -P @var{AUDITLIB}
457 Adds @var{AUDITLIB} to the @code{DT_DEPAUDIT} entry of the dynamic section.
458 @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME
459 specified in the library. If specified multiple times @code{DT_DEPAUDIT}
460 will contain a colon separated list of audit interfaces to use. This
461 option is only meaningful on ELF platforms supporting the rtld-audit interface.
462 The -P option is provided for Solaris compatibility.
464 @kindex --enable-non-contiguous-regions
465 @item --enable-non-contiguous-regions
466 This option avoids generating an error if an input section does not
467 fit a matching output section. The linker tries to allocate the input
468 section to subseque nt matching output sections, and generates an
469 error only if no output section is large enough. This is useful when
470 several non-contiguous memory regions are available and the input
471 section does not require a particular one. The order in which input
472 sections are evaluated does not change, for instance:
476 MEM1 (rwx) : ORIGIN : 0x1000, LENGTH = 0x14
477 MEM2 (rwx) : ORIGIN : 0x1000, LENGTH = 0x40
478 MEM3 (rwx) : ORIGIN : 0x2000, LENGTH = 0x40
481 mem1 : @{ *(.data.*); @} > MEM1
482 mem2 : @{ *(.data.*); @} > MEM2
483 mem3 : @{ *(.data.*); @} > MEM2
491 results in .data.1 affected to mem1, and .data.2 and .data.3
492 affected to mem2, even though .data.3 would fit in mem3.
495 This option is incompatible with INSERT statements because it changes
496 the way input sections are mapped to output sections.
498 @kindex --enable-non-contiguous-regions-warnings
499 @item --enable-non-contiguous-regions-warnings
500 This option enables warnings when
501 @code{--enable-non-contiguous-regions} allows possibly unexpected
502 matches in sections mapping, potentially leading to silently
503 discarding a section instead of failing because it does not fit any
506 @cindex entry point, from command line
507 @kindex -e @var{entry}
508 @kindex --entry=@var{entry}
510 @itemx --entry=@var{entry}
511 Use @var{entry} as the explicit symbol for beginning execution of your
512 program, rather than the default entry point. If there is no symbol
513 named @var{entry}, the linker will try to parse @var{entry} as a number,
514 and use that as the entry address (the number will be interpreted in
515 base 10; you may use a leading @samp{0x} for base 16, or a leading
516 @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults
517 and other ways of specifying the entry point.
519 @kindex --exclude-libs
520 @item --exclude-libs @var{lib},@var{lib},...
521 Specifies a list of archive libraries from which symbols should not be automatically
522 exported. The library names may be delimited by commas or colons. Specifying
523 @code{--exclude-libs ALL} excludes symbols in all archive libraries from
524 automatic export. This option is available only for the i386 PE targeted
525 port of the linker and for ELF targeted ports. For i386 PE, symbols
526 explicitly listed in a .def file are still exported, regardless of this
527 option. For ELF targeted ports, symbols affected by this option will
528 be treated as hidden.
530 @kindex --exclude-modules-for-implib
531 @item --exclude-modules-for-implib @var{module},@var{module},...
532 Specifies a list of object files or archive members, from which symbols
533 should not be automatically exported, but which should be copied wholesale
534 into the import library being generated during the link. The module names
535 may be delimited by commas or colons, and must match exactly the filenames
536 used by @command{ld} to open the files; for archive members, this is simply
537 the member name, but for object files the name listed must include and
538 match precisely any path used to specify the input file on the linker's
539 command-line. This option is available only for the i386 PE targeted port
540 of the linker. Symbols explicitly listed in a .def file are still exported,
541 regardless of this option.
543 @cindex dynamic symbol table
545 @kindex --export-dynamic
546 @kindex --no-export-dynamic
548 @itemx --export-dynamic
549 @itemx --no-export-dynamic
550 When creating a dynamically linked executable, using the @option{-E}
551 option or the @option{--export-dynamic} option causes the linker to add
552 all symbols to the dynamic symbol table. The dynamic symbol table is the
553 set of symbols which are visible from dynamic objects at run time.
555 If you do not use either of these options (or use the
556 @option{--no-export-dynamic} option to restore the default behavior), the
557 dynamic symbol table will normally contain only those symbols which are
558 referenced by some dynamic object mentioned in the link.
560 If you use @code{dlopen} to load a dynamic object which needs to refer
561 back to the symbols defined by the program, rather than some other
562 dynamic object, then you will probably need to use this option when
563 linking the program itself.
565 You can also use the dynamic list to control what symbols should
566 be added to the dynamic symbol table if the output format supports it.
567 See the description of @samp{--dynamic-list}.
569 Note that this option is specific to ELF targeted ports. PE targets
570 support a similar function to export all symbols from a DLL or EXE; see
571 the description of @samp{--export-all-symbols} below.
573 @kindex --export-dynamic-symbol=@var{glob}
574 @cindex export dynamic symbol
575 @item --export-dynamic-symbol=@var{glob}
576 When creating a dynamically linked executable, symbols matching
577 @var{glob} will be added to the dynamic symbol table. When creating a
578 shared library, references to symbols matching @var{glob} will not be
579 bound to the definitions within the shared library. This option is a
580 no-op when creating a shared library and @samp{-Bsymbolic} or
581 @samp{--dynamic-list} are not specified. This option is only meaningful
582 on ELF platforms which support shared libraries.
584 @kindex --export-dynamic-symbol-list=@var{file}
585 @cindex export dynamic symbol list
586 @item --export-dynamic-symbol-list=@var{file}
587 Specify a @samp{--export-dynamic-symbol} for each pattern in the file.
588 The format of the file is the same as the version node without
589 scope and node name. See @ref{VERSION} for more information.
591 @ifclear SingleFormat
592 @cindex big-endian objects
596 Link big-endian objects. This affects the default output format.
598 @cindex little-endian objects
601 Link little-endian objects. This affects the default output format.
604 @kindex -f @var{name}
605 @kindex --auxiliary=@var{name}
607 @itemx --auxiliary=@var{name}
608 When creating an ELF shared object, set the internal DT_AUXILIARY field
609 to the specified name. This tells the dynamic linker that the symbol
610 table of the shared object should be used as an auxiliary filter on the
611 symbol table of the shared object @var{name}.
613 If you later link a program against this filter object, then, when you
614 run the program, the dynamic linker will see the DT_AUXILIARY field. If
615 the dynamic linker resolves any symbols from the filter object, it will
616 first check whether there is a definition in the shared object
617 @var{name}. If there is one, it will be used instead of the definition
618 in the filter object. The shared object @var{name} need not exist.
619 Thus the shared object @var{name} may be used to provide an alternative
620 implementation of certain functions, perhaps for debugging or for
621 machine-specific performance.
623 This option may be specified more than once. The DT_AUXILIARY entries
624 will be created in the order in which they appear on the command line.
626 @kindex -F @var{name}
627 @kindex --filter=@var{name}
629 @itemx --filter=@var{name}
630 When creating an ELF shared object, set the internal DT_FILTER field to
631 the specified name. This tells the dynamic linker that the symbol table
632 of the shared object which is being created should be used as a filter
633 on the symbol table of the shared object @var{name}.
635 If you later link a program against this filter object, then, when you
636 run the program, the dynamic linker will see the DT_FILTER field. The
637 dynamic linker will resolve symbols according to the symbol table of the
638 filter object as usual, but it will actually link to the definitions
639 found in the shared object @var{name}. Thus the filter object can be
640 used to select a subset of the symbols provided by the object
643 Some older linkers used the @option{-F} option throughout a compilation
644 toolchain for specifying object-file format for both input and output
646 @ifclear SingleFormat
647 The @sc{gnu} linker uses other mechanisms for this purpose: the
648 @option{-b}, @option{--format}, @option{--oformat} options, the
649 @code{TARGET} command in linker scripts, and the @code{GNUTARGET}
650 environment variable.
652 The @sc{gnu} linker will ignore the @option{-F} option when not
653 creating an ELF shared object.
655 @cindex finalization function
656 @kindex -fini=@var{name}
657 @item -fini=@var{name}
658 When creating an ELF executable or shared object, call NAME when the
659 executable or shared object is unloaded, by setting DT_FINI to the
660 address of the function. By default, the linker uses @code{_fini} as
661 the function to call.
665 Ignored. Provided for compatibility with other tools.
667 @kindex -G @var{value}
668 @kindex --gpsize=@var{value}
671 @itemx --gpsize=@var{value}
672 Set the maximum size of objects to be optimized using the GP register to
673 @var{size}. This is only meaningful for object file formats such as
674 MIPS ELF that support putting large and small objects into different
675 sections. This is ignored for other object file formats.
677 @cindex runtime library name
678 @kindex -h @var{name}
679 @kindex -soname=@var{name}
681 @itemx -soname=@var{name}
682 When creating an ELF shared object, set the internal DT_SONAME field to
683 the specified name. When an executable is linked with a shared object
684 which has a DT_SONAME field, then when the executable is run the dynamic
685 linker will attempt to load the shared object specified by the DT_SONAME
686 field rather than the using the file name given to the linker.
689 @cindex incremental link
691 Perform an incremental link (same as option @samp{-r}).
693 @cindex initialization function
694 @kindex -init=@var{name}
695 @item -init=@var{name}
696 When creating an ELF executable or shared object, call NAME when the
697 executable or shared object is loaded, by setting DT_INIT to the address
698 of the function. By default, the linker uses @code{_init} as the
701 @cindex archive files, from cmd line
702 @kindex -l @var{namespec}
703 @kindex --library=@var{namespec}
704 @item -l @var{namespec}
705 @itemx --library=@var{namespec}
706 Add the archive or object file specified by @var{namespec} to the
707 list of files to link. This option may be used any number of times.
708 If @var{namespec} is of the form @file{:@var{filename}}, @command{ld}
709 will search the library path for a file called @var{filename}, otherwise it
710 will search the library path for a file called @file{lib@var{namespec}.a}.
712 On systems which support shared libraries, @command{ld} may also search for
713 files other than @file{lib@var{namespec}.a}. Specifically, on ELF
714 and SunOS systems, @command{ld} will search a directory for a library
715 called @file{lib@var{namespec}.so} before searching for one called
716 @file{lib@var{namespec}.a}. (By convention, a @code{.so} extension
717 indicates a shared library.) Note that this behavior does not apply
718 to @file{:@var{filename}}, which always specifies a file called
721 The linker will search an archive only once, at the location where it is
722 specified on the command line. If the archive defines a symbol which
723 was undefined in some object which appeared before the archive on the
724 command line, the linker will include the appropriate file(s) from the
725 archive. However, an undefined symbol in an object appearing later on
726 the command line will not cause the linker to search the archive again.
728 See the @option{-(} option for a way to force the linker to search
729 archives multiple times.
731 You may list the same archive multiple times on the command line.
734 This type of archive searching is standard for Unix linkers. However,
735 if you are using @command{ld} on AIX, note that it is different from the
736 behaviour of the AIX linker.
739 @cindex search directory, from cmd line
741 @kindex --library-path=@var{dir}
742 @item -L @var{searchdir}
743 @itemx --library-path=@var{searchdir}
744 Add path @var{searchdir} to the list of paths that @command{ld} will search
745 for archive libraries and @command{ld} control scripts. You may use this
746 option any number of times. The directories are searched in the order
747 in which they are specified on the command line. Directories specified
748 on the command line are searched before the default directories. All
749 @option{-L} options apply to all @option{-l} options, regardless of the
750 order in which the options appear. @option{-L} options do not affect
751 how @command{ld} searches for a linker script unless @option{-T}
754 If @var{searchdir} begins with @code{=} or @code{$SYSROOT}, then this
755 prefix will be replaced by the @dfn{sysroot prefix}, controlled by the
756 @samp{--sysroot} option, or specified when the linker is configured.
759 The default set of paths searched (without being specified with
760 @samp{-L}) depends on which emulation mode @command{ld} is using, and in
761 some cases also on how it was configured. @xref{Environment}.
764 The paths can also be specified in a link script with the
765 @code{SEARCH_DIR} command. Directories specified this way are searched
766 at the point in which the linker script appears in the command line.
769 @kindex -m @var{emulation}
770 @item -m @var{emulation}
771 Emulate the @var{emulation} linker. You can list the available
772 emulations with the @samp{--verbose} or @samp{-V} options.
774 If the @samp{-m} option is not used, the emulation is taken from the
775 @code{LDEMULATION} environment variable, if that is defined.
777 Otherwise, the default emulation depends upon how the linker was
785 Print a link map to the standard output. A link map provides
786 information about the link, including the following:
790 Where object files are mapped into memory.
792 How common symbols are allocated.
794 All archive members included in the link, with a mention of the symbol
795 which caused the archive member to be brought in.
797 The values assigned to symbols.
799 Note - symbols whose values are computed by an expression which
800 involves a reference to a previous value of the same symbol may not
801 have correct result displayed in the link map. This is because the
802 linker discards intermediate results and only retains the final value
803 of an expression. Under such circumstances the linker will display
804 the final value enclosed by square brackets. Thus for example a
805 linker script containing:
813 will produce the following output in the link map if the @option{-M}
818 [0x0000000c] foo = (foo * 0x4)
819 [0x0000000c] foo = (foo + 0x8)
822 See @ref{Expressions} for more information about expressions in linker
826 How GNU properties are merged.
828 When the linker merges input .note.gnu.property sections into one output
829 .note.gnu.property section, some properties are removed or updated.
830 These actions are reported in the link map. For example:
833 Removed property 0xc0000002 to merge foo.o (0x1) and bar.o (not found)
836 This indicates that property 0xc0000002 is removed from output when
837 merging properties in @file{foo.o}, whose property 0xc0000002 value
838 is 0x1, and @file{bar.o}, which doesn't have property 0xc0000002.
841 Updated property 0xc0010001 (0x1) to merge foo.o (0x1) and bar.o (0x1)
844 This indicates that property 0xc0010001 value is updated to 0x1 in output
845 when merging properties in @file{foo.o}, whose 0xc0010001 property value
846 is 0x1, and @file{bar.o}, whose 0xc0010001 property value is 0x1.
849 @cindex link map discarded
850 @kindex --print-map-discarded
851 @kindex --no-print-map-discarded
852 @item --print-map-discarded
853 @itemx --no-print-map-discarded
854 Print (or do not print) the list of discarded and garbage collected sections
855 in the link map. Enabled by default.
858 @cindex read-only text
863 Turn off page alignment of sections, and disable linking against shared
864 libraries. If the output format supports Unix style magic numbers,
865 mark the output as @code{NMAGIC}.
869 @cindex read/write from cmd line
873 Set the text and data sections to be readable and writable. Also, do
874 not page-align the data segment, and disable linking against shared
875 libraries. If the output format supports Unix style magic numbers,
876 mark the output as @code{OMAGIC}. Note: Although a writable text section
877 is allowed for PE-COFF targets, it does not conform to the format
878 specification published by Microsoft.
883 This option negates most of the effects of the @option{-N} option. It
884 sets the text section to be read-only, and forces the data segment to
885 be page-aligned. Note - this option does not enable linking against
886 shared libraries. Use @option{-Bdynamic} for this.
888 @kindex -o @var{output}
889 @kindex --output=@var{output}
890 @cindex naming the output file
891 @item -o @var{output}
892 @itemx --output=@var{output}
893 Use @var{output} as the name for the program produced by @command{ld}; if this
894 option is not specified, the name @file{a.out} is used by default. The
895 script command @code{OUTPUT} can also specify the output file name.
897 @kindex --dependency-file=@var{depfile}
898 @cindex dependency file
899 @item --dependency-file=@var{depfile}
900 Write a @dfn{dependency file} to @var{depfile}. This file contains a rule
901 suitable for @code{make} describing the output file and all the input files
902 that were read to produce it. The output is similar to the compiler's
903 output with @samp{-M -MP} (@pxref{Preprocessor Options,, Options
904 Controlling the Preprocessor, gcc.info, Using the GNU Compiler
905 Collection}). Note that there is no option like the compiler's @samp{-MM},
906 to exclude ``system files'' (which is not a well-specified concept in the
907 linker, unlike ``system headers'' in the compiler). So the output from
908 @samp{--dependency-file} is always specific to the exact state of the
909 installation where it was produced, and should not be copied into
910 distributed makefiles without careful editing.
912 @kindex -O @var{level}
913 @cindex generating optimized output
915 If @var{level} is a numeric values greater than zero @command{ld} optimizes
916 the output. This might take significantly longer and therefore probably
917 should only be enabled for the final binary. At the moment this
918 option only affects ELF shared library generation. Future releases of
919 the linker may make more use of this option. Also currently there is
920 no difference in the linker's behaviour for different non-zero values
921 of this option. Again this may change with future releases.
923 @kindex -plugin @var{name}
924 @item -plugin @var{name}
925 Involve a plugin in the linking process. The @var{name} parameter is
926 the absolute filename of the plugin. Usually this parameter is
927 automatically added by the complier, when using link time
928 optimization, but users can also add their own plugins if they so
931 Note that the location of the compiler originated plugins is different
932 from the place where the @command{ar}, @command{nm} and
933 @command{ranlib} programs search for their plugins. In order for
934 those commands to make use of a compiler based plugin it must first be
935 copied into the @file{$@{libdir@}/bfd-plugins} directory. All gcc
936 based linker plugins are backward compatible, so it is sufficient to
937 just copy in the newest one.
940 @cindex push state governing input file handling
942 The @option{--push-state} allows one to preserve the current state of the
943 flags which govern the input file handling so that they can all be
944 restored with one corresponding @option{--pop-state} option.
946 The option which are covered are: @option{-Bdynamic}, @option{-Bstatic},
947 @option{-dn}, @option{-dy}, @option{-call_shared}, @option{-non_shared},
948 @option{-static}, @option{-N}, @option{-n}, @option{--whole-archive},
949 @option{--no-whole-archive}, @option{-r}, @option{-Ur},
950 @option{--copy-dt-needed-entries}, @option{--no-copy-dt-needed-entries},
951 @option{--as-needed}, @option{--no-as-needed}, and @option{-a}.
953 One target for this option are specifications for @file{pkg-config}. When
954 used with the @option{--libs} option all possibly needed libraries are
955 listed and then possibly linked with all the time. It is better to return
956 something as follows:
959 -Wl,--push-state,--as-needed -libone -libtwo -Wl,--pop-state
963 @cindex pop state governing input file handling
965 Undoes the effect of --push-state, restores the previous values of the
966 flags governing input file handling.
969 @kindex --emit-relocs
970 @cindex retain relocations in final executable
973 Leave relocation sections and contents in fully linked executables.
974 Post link analysis and optimization tools may need this information in
975 order to perform correct modifications of executables. This results
976 in larger executables.
978 This option is currently only supported on ELF platforms.
980 @kindex --force-dynamic
981 @cindex forcing the creation of dynamic sections
982 @item --force-dynamic
983 Force the output file to have dynamic sections. This option is specific
987 @cindex relocatable output
989 @kindex --relocatable
992 Generate relocatable output---i.e., generate an output file that can in
993 turn serve as input to @command{ld}. This is often called @dfn{partial
994 linking}. As a side effect, in environments that support standard Unix
995 magic numbers, this option also sets the output file's magic number to
997 @c ; see @option{-N}.
998 If this option is not specified, an absolute file is produced. When
999 linking C++ programs, this option @emph{will not} resolve references to
1000 constructors; to do that, use @samp{-Ur}.
1002 When an input file does not have the same format as the output file,
1003 partial linking is only supported if that input file does not contain any
1004 relocations. Different output formats can have further restrictions; for
1005 example some @code{a.out}-based formats do not support partial linking
1006 with input files in other formats at all.
1008 This option does the same thing as @samp{-i}.
1010 @kindex -R @var{file}
1011 @kindex --just-symbols=@var{file}
1012 @cindex symbol-only input
1013 @item -R @var{filename}
1014 @itemx --just-symbols=@var{filename}
1015 Read symbol names and their addresses from @var{filename}, but do not
1016 relocate it or include it in the output. This allows your output file
1017 to refer symbolically to absolute locations of memory defined in other
1018 programs. You may use this option more than once.
1020 For compatibility with other ELF linkers, if the @option{-R} option is
1021 followed by a directory name, rather than a file name, it is treated as
1022 the @option{-rpath} option.
1026 @cindex strip all symbols
1029 Omit all symbol information from the output file.
1032 @kindex --strip-debug
1033 @cindex strip debugger symbols
1035 @itemx --strip-debug
1036 Omit debugger symbol information (but not all symbols) from the output file.
1038 @kindex --strip-discarded
1039 @kindex --no-strip-discarded
1040 @item --strip-discarded
1041 @itemx --no-strip-discarded
1042 Omit (or do not omit) global symbols defined in discarded sections.
1047 @cindex input files, displaying
1050 Print the names of the input files as @command{ld} processes them. If
1051 @samp{-t} is given twice then members within archives are also printed.
1052 @samp{-t} output is useful to generate a list of all the object files
1053 and scripts involved in linking, for example, when packaging files for
1054 a linker bug report.
1056 @kindex -T @var{script}
1057 @kindex --script=@var{script}
1058 @cindex script files
1059 @item -T @var{scriptfile}
1060 @itemx --script=@var{scriptfile}
1061 Use @var{scriptfile} as the linker script. This script replaces
1062 @command{ld}'s default linker script (rather than adding to it), so
1063 @var{commandfile} must specify everything necessary to describe the
1064 output file. @xref{Scripts}. If @var{scriptfile} does not exist in
1065 the current directory, @code{ld} looks for it in the directories
1066 specified by any preceding @samp{-L} options. Multiple @samp{-T}
1069 @kindex -dT @var{script}
1070 @kindex --default-script=@var{script}
1071 @cindex script files
1072 @item -dT @var{scriptfile}
1073 @itemx --default-script=@var{scriptfile}
1074 Use @var{scriptfile} as the default linker script. @xref{Scripts}.
1076 This option is similar to the @option{--script} option except that
1077 processing of the script is delayed until after the rest of the
1078 command line has been processed. This allows options placed after the
1079 @option{--default-script} option on the command line to affect the
1080 behaviour of the linker script, which can be important when the linker
1081 command line cannot be directly controlled by the user. (eg because
1082 the command line is being constructed by another tool, such as
1085 @kindex -u @var{symbol}
1086 @kindex --undefined=@var{symbol}
1087 @cindex undefined symbol
1088 @item -u @var{symbol}
1089 @itemx --undefined=@var{symbol}
1090 Force @var{symbol} to be entered in the output file as an undefined
1091 symbol. Doing this may, for example, trigger linking of additional
1092 modules from standard libraries. @samp{-u} may be repeated with
1093 different option arguments to enter additional undefined symbols. This
1094 option is equivalent to the @code{EXTERN} linker script command.
1096 If this option is being used to force additional modules to be pulled
1097 into the link, and if it is an error for the symbol to remain
1098 undefined, then the option @option{--require-defined} should be used
1101 @kindex --require-defined=@var{symbol}
1102 @cindex symbols, require defined
1103 @cindex defined symbol
1104 @item --require-defined=@var{symbol}
1105 Require that @var{symbol} is defined in the output file. This option
1106 is the same as option @option{--undefined} except that if @var{symbol}
1107 is not defined in the output file then the linker will issue an error
1108 and exit. The same effect can be achieved in a linker script by using
1109 @code{EXTERN}, @code{ASSERT} and @code{DEFINED} together. This option
1110 can be used multiple times to require additional symbols.
1113 @cindex constructors
1115 For anything other than C++ programs, this option is equivalent to
1116 @samp{-r}: it generates relocatable output---i.e., an output file that can in
1117 turn serve as input to @command{ld}. When linking C++ programs, @samp{-Ur}
1118 @emph{does} resolve references to constructors, unlike @samp{-r}.
1119 It does not work to use @samp{-Ur} on files that were themselves linked
1120 with @samp{-Ur}; once the constructor table has been built, it cannot
1121 be added to. Use @samp{-Ur} only for the last partial link, and
1122 @samp{-r} for the others.
1124 @kindex --orphan-handling=@var{MODE}
1125 @cindex orphan sections
1126 @cindex sections, orphan
1127 @item --orphan-handling=@var{MODE}
1128 Control how orphan sections are handled. An orphan section is one not
1129 specifically mentioned in a linker script. @xref{Orphan Sections}.
1131 @var{MODE} can have any of the following values:
1135 Orphan sections are placed into a suitable output section following
1136 the strategy described in @ref{Orphan Sections}. The option
1137 @samp{--unique} also affects how sections are placed.
1140 All orphan sections are discarded, by placing them in the
1141 @samp{/DISCARD/} section (@pxref{Output Section Discarding}).
1144 The linker will place the orphan section as for @code{place} and also
1148 The linker will exit with an error if any orphan section is found.
1151 The default if @samp{--orphan-handling} is not given is @code{place}.
1153 @kindex --unique[=@var{SECTION}]
1154 @item --unique[=@var{SECTION}]
1155 Creates a separate output section for every input section matching
1156 @var{SECTION}, or if the optional wildcard @var{SECTION} argument is
1157 missing, for every orphan input section. An orphan section is one not
1158 specifically mentioned in a linker script. You may use this option
1159 multiple times on the command line; It prevents the normal merging of
1160 input sections with the same name, overriding output section assignments
1170 Display the version number for @command{ld}. The @option{-V} option also
1171 lists the supported emulations.
1174 @kindex --discard-all
1175 @cindex deleting local symbols
1177 @itemx --discard-all
1178 Delete all local symbols.
1181 @kindex --discard-locals
1182 @cindex local symbols, deleting
1184 @itemx --discard-locals
1185 Delete all temporary local symbols. (These symbols start with
1186 system-specific local label prefixes, typically @samp{.L} for ELF systems
1187 or @samp{L} for traditional a.out systems.)
1189 @kindex -y @var{symbol}
1190 @kindex --trace-symbol=@var{symbol}
1191 @cindex symbol tracing
1192 @item -y @var{symbol}
1193 @itemx --trace-symbol=@var{symbol}
1194 Print the name of each linked file in which @var{symbol} appears. This
1195 option may be given any number of times. On many systems it is necessary
1196 to prepend an underscore.
1198 This option is useful when you have an undefined symbol in your link but
1199 don't know where the reference is coming from.
1201 @kindex -Y @var{path}
1203 Add @var{path} to the default library search path. This option exists
1204 for Solaris compatibility.
1206 @kindex -z @var{keyword}
1207 @item -z @var{keyword}
1208 The recognized keywords are:
1212 Always generate BND prefix in PLT entries. Supported for Linux/x86_64.
1214 @item call-nop=prefix-addr
1215 @itemx call-nop=suffix-nop
1216 @itemx call-nop=prefix-@var{byte}
1217 @itemx call-nop=suffix-@var{byte}
1218 Specify the 1-byte @code{NOP} padding when transforming indirect call
1219 to a locally defined function, foo, via its GOT slot.
1220 @option{call-nop=prefix-addr} generates @code{0x67 call foo}.
1221 @option{call-nop=suffix-nop} generates @code{call foo 0x90}.
1222 @option{call-nop=prefix-@var{byte}} generates @code{@var{byte} call foo}.
1223 @option{call-nop=suffix-@var{byte}} generates @code{call foo @var{byte}}.
1224 Supported for i386 and x86_64.
1226 @item cet-report=none
1227 @itemx cet-report=warning
1228 @itemx cet-report=error
1229 Specify how to report the missing GNU_PROPERTY_X86_FEATURE_1_IBT and
1230 GNU_PROPERTY_X86_FEATURE_1_SHSTK properties in input .note.gnu.property
1231 section. @option{cet-report=none}, which is the default, will make the
1232 linker not report missing properties in input files.
1233 @option{cet-report=warning} will make the linker issue a warning for
1234 missing properties in input files. @option{cet-report=error} will make
1235 the linker issue an error for missing properties in input files.
1236 Note that @option{ibt} will turn off the missing
1237 GNU_PROPERTY_X86_FEATURE_1_IBT property report and @option{shstk} will
1238 turn off the missing GNU_PROPERTY_X86_FEATURE_1_SHSTK property report.
1239 Supported for Linux/i386 and Linux/x86_64.
1243 Combine multiple dynamic relocation sections and sort to improve
1244 dynamic symbol lookup caching. Do not do this if @samp{nocombreloc}.
1248 Generate common symbols with STT_COMMON type during a relocatable
1249 link. Use STT_OBJECT type if @samp{nocommon}.
1251 @item common-page-size=@var{value}
1252 Set the page size most commonly used to @var{value}. Memory image
1253 layout will be optimized to minimize memory pages if the system is
1254 using pages of this size.
1257 Report unresolved symbol references from regular object files. This
1258 is done even if the linker is creating a non-symbolic shared library.
1259 This option is the inverse of @samp{-z undefs}.
1261 @item dynamic-undefined-weak
1262 @itemx nodynamic-undefined-weak
1263 Make undefined weak symbols dynamic when building a dynamic object,
1264 if they are referenced from a regular object file and not forced local
1265 by symbol visibility or versioning. Do not make them dynamic if
1266 @samp{nodynamic-undefined-weak}. If neither option is given, a target
1267 may default to either option being in force, or make some other
1268 selection of undefined weak symbols dynamic. Not all targets support
1272 Marks the object as requiring executable stack.
1275 This option is only meaningful when building a shared object. It makes
1276 the symbols defined by this shared object available for symbol resolution
1277 of subsequently loaded libraries.
1280 This option is only meaningful when building a dynamic executable.
1281 This option marks the executable as requiring global auditing by
1282 setting the @code{DF_1_GLOBAUDIT} bit in the @code{DT_FLAGS_1} dynamic
1283 tag. Global auditing requires that any auditing library defined via
1284 the @option{--depaudit} or @option{-P} command-line options be run for
1285 all dynamic objects loaded by the application.
1288 Generate Intel Indirect Branch Tracking (IBT) enabled PLT entries.
1289 Supported for Linux/i386 and Linux/x86_64.
1292 Generate GNU_PROPERTY_X86_FEATURE_1_IBT in .note.gnu.property section
1293 to indicate compatibility with IBT. This also implies @option{ibtplt}.
1294 Supported for Linux/i386 and Linux/x86_64.
1297 This option is only meaningful when building a shared object.
1298 It marks the object so that its runtime initialization will occur
1299 before the runtime initialization of any other objects brought into
1300 the process at the same time. Similarly the runtime finalization of
1301 the object will occur after the runtime finalization of any other
1305 Specify that the dynamic loader should modify its symbol search order
1306 so that symbols in this shared library interpose all other shared
1307 libraries not so marked.
1311 When generating a shared library or other dynamically loadable ELF
1312 object mark it as one that should (by default) only ever be loaded once,
1313 and only in the main namespace (when using @code{dlmopen}). This is
1314 primarily used to mark fundamental libraries such as libc, libpthread et
1315 al which do not usually function correctly unless they are the sole instances
1316 of themselves. This behaviour can be overridden by the @code{dlmopen} caller
1317 and does not apply to certain loading mechanisms (such as audit libraries).
1320 Generate GNU_PROPERTY_X86_FEATURE_1_LAM_U48 in .note.gnu.property section
1321 to indicate compatibility with Intel LAM_U48. Supported for Linux/x86_64.
1324 Generate GNU_PROPERTY_X86_FEATURE_1_LAM_U57 in .note.gnu.property section
1325 to indicate compatibility with Intel LAM_U57. Supported for Linux/x86_64.
1327 @item lam-u48-report=none
1328 @itemx lam-u48-report=warning
1329 @itemx lam-u48-report=error
1330 Specify how to report the missing GNU_PROPERTY_X86_FEATURE_1_LAM_U48
1331 property in input .note.gnu.property section.
1332 @option{lam-u48-report=none}, which is the default, will make the
1333 linker not report missing properties in input files.
1334 @option{lam-u48-report=warning} will make the linker issue a warning for
1335 missing properties in input files. @option{lam-u48-report=error} will
1336 make the linker issue an error for missing properties in input files.
1337 Supported for Linux/x86_64.
1339 @item lam-u57-report=none
1340 @itemx lam-u57-report=warning
1341 @itemx lam-u57-report=error
1342 Specify how to report the missing GNU_PROPERTY_X86_FEATURE_1_LAM_U57
1343 property in input .note.gnu.property section.
1344 @option{lam-u57-report=none}, which is the default, will make the
1345 linker not report missing properties in input files.
1346 @option{lam-u57-report=warning} will make the linker issue a warning for
1347 missing properties in input files. @option{lam-u57-report=error} will
1348 make the linker issue an error for missing properties in input files.
1349 Supported for Linux/x86_64.
1351 @item lam-report=none
1352 @itemx lam-report=warning
1353 @itemx lam-report=error
1354 Specify how to report the missing GNU_PROPERTY_X86_FEATURE_1_LAM_U48 and
1355 GNU_PROPERTY_X86_FEATURE_1_LAM_U57 properties in input .note.gnu.property
1356 section. @option{lam-report=none}, which is the default, will make the
1357 linker not report missing properties in input files.
1358 @option{lam-report=warning} will make the linker issue a warning for
1359 missing properties in input files. @option{lam-report=error} will make
1360 the linker issue an error for missing properties in input files.
1361 Supported for Linux/x86_64.
1364 When generating an executable or shared library, mark it to tell the
1365 dynamic linker to defer function call resolution to the point when
1366 the function is called (lazy binding), rather than at load time.
1367 Lazy binding is the default.
1370 Specify that the object's filters be processed immediately at runtime.
1372 @item max-page-size=@var{value}
1373 Set the maximum memory page size supported to @var{value}.
1376 Allow multiple definitions.
1379 Disable linker generated .dynbss variables used in place of variables
1380 defined in shared libraries. May result in dynamic text relocations.
1383 Specify that the dynamic loader search for dependencies of this object
1384 should ignore any default library search paths.
1387 Specify that the object shouldn't be unloaded at runtime.
1390 Specify that the object is not available to @code{dlopen}.
1393 Specify that the object can not be dumped by @code{dldump}.
1396 Marks the object as not requiring executable stack.
1398 @item noextern-protected-data
1399 Don't treat protected data symbols as external when building a shared
1400 library. This option overrides the linker backend default. It can be
1401 used to work around incorrect relocations against protected data symbols
1402 generated by compiler. Updates on protected data symbols by another
1403 module aren't visible to the resulting shared library. Supported for
1406 @item noreloc-overflow
1407 Disable relocation overflow check. This can be used to disable
1408 relocation overflow check if there will be no dynamic relocation
1409 overflow at run-time. Supported for x86_64.
1412 When generating an executable or shared library, mark it to tell the
1413 dynamic linker to resolve all symbols when the program is started, or
1414 when the shared library is loaded by dlopen, instead of deferring
1415 function call resolution to the point when the function is first
1419 Specify that the object requires @samp{$ORIGIN} handling in paths.
1423 Create an ELF @code{PT_GNU_RELRO} segment header in the object. This
1424 specifies a memory segment that should be made read-only after
1425 relocation, if supported. Specifying @samp{common-page-size} smaller
1426 than the system page size will render this protection ineffective.
1427 Don't create an ELF @code{PT_GNU_RELRO} segment if @samp{norelro}.
1429 @item report-relative-reloc
1430 Report dynamic relative relocations generated by linker. Supported for
1431 Linux/i386 and Linux/x86_64.
1434 @itemx noseparate-code
1435 Create separate code @code{PT_LOAD} segment header in the object. This
1436 specifies a memory segment that should contain only instructions and must
1437 be in wholly disjoint pages from any other data. Don't create separate
1438 code @code{PT_LOAD} segment if @samp{noseparate-code} is used.
1441 @itemx nounique-symbol
1442 Avoid duplicated local symbol names in the symbol string table. Append
1443 ".@code{number}" to duplicated local symbol names if @samp{unique-symbol}
1444 is used. @option{nounique-symbol} is the default.
1447 Generate GNU_PROPERTY_X86_FEATURE_1_SHSTK in .note.gnu.property section
1448 to indicate compatibility with Intel Shadow Stack. Supported for
1449 Linux/i386 and Linux/x86_64.
1451 @item stack-size=@var{value}
1452 Specify a stack size for an ELF @code{PT_GNU_STACK} segment.
1453 Specifying zero will override any default non-zero sized
1454 @code{PT_GNU_STACK} segment creation.
1457 @itemx nostart-stop-gc
1458 @cindex start-stop-gc
1459 When @samp{--gc-sections} is in effect, a reference from a retained
1460 section to @code{__start_SECNAME} or @code{__stop_SECNAME} causes all
1461 input sections named @code{SECNAME} to also be retained, if
1462 @code{SECNAME} is representable as a C identifier and either
1463 @code{__start_SECNAME} or @code{__stop_SECNAME} is synthesized by the
1464 linker. @samp{-z start-stop-gc} disables this effect, allowing
1465 sections to be garbage collected as if the special synthesized symbols
1466 were not defined. @samp{-z start-stop-gc} has no effect on a
1467 definition of @code{__start_SECNAME} or @code{__stop_SECNAME} in an
1468 object file or linker script. Such a definition will prevent the
1469 linker providing a synthesized @code{__start_SECNAME} or
1470 @code{__stop_SECNAME} respectively, and therefore the special
1471 treatment by garbage collection for those references.
1473 @item start-stop-visibility=@var{value}
1475 @cindex ELF symbol visibility
1476 Specify the ELF symbol visibility for synthesized
1477 @code{__start_SECNAME} and @code{__stop_SECNAME} symbols (@pxref{Input
1478 Section Example}). @var{value} must be exactly @samp{default},
1479 @samp{internal}, @samp{hidden}, or @samp{protected}. If no @samp{-z
1480 start-stop-visibility} option is given, @samp{protected} is used for
1481 compatibility with historical practice. However, it's highly
1482 recommended to use @samp{-z start-stop-visibility=hidden} in new
1483 programs and shared libraries so that these symbols are not exported
1484 between shared objects, which is not usually what's intended.
1489 Report an error if DT_TEXTREL is set, i.e., if the position-independent
1490 or shared object has dynamic relocations in read-only sections. Don't
1491 report an error if @samp{notext} or @samp{textoff}.
1494 Do not report unresolved symbol references from regular object files,
1495 either when creating an executable, or when creating a shared library.
1496 This option is the inverse of @samp{-z defs}.
1498 @item x86-64-baseline
1502 Specify the x86-64 ISA level needed in .note.gnu.property section.
1503 @option{x86-64-baseline} generates @code{GNU_PROPERTY_X86_ISA_1_BASELINE}.
1504 @option{x86-64-v2} generates @code{GNU_PROPERTY_X86_ISA_1_V2}.
1505 @option{x86-64-v3} generates @code{GNU_PROPERTY_X86_ISA_1_V3}.
1506 @option{x86-64-v4} generates @code{GNU_PROPERTY_X86_ISA_1_V4}.
1507 Supported for Linux/i386 and Linux/x86_64.
1511 Other keywords are ignored for Solaris compatibility.
1514 @cindex groups of archives
1515 @item -( @var{archives} -)
1516 @itemx --start-group @var{archives} --end-group
1517 The @var{archives} should be a list of archive files. They may be
1518 either explicit file names, or @samp{-l} options.
1520 The specified archives are searched repeatedly until no new undefined
1521 references are created. Normally, an archive is searched only once in
1522 the order that it is specified on the command line. If a symbol in that
1523 archive is needed to resolve an undefined symbol referred to by an
1524 object in an archive that appears later on the command line, the linker
1525 would not be able to resolve that reference. By grouping the archives,
1526 they will all be searched repeatedly until all possible references are
1529 Using this option has a significant performance cost. It is best to use
1530 it only when there are unavoidable circular references between two or
1533 @kindex --accept-unknown-input-arch
1534 @kindex --no-accept-unknown-input-arch
1535 @item --accept-unknown-input-arch
1536 @itemx --no-accept-unknown-input-arch
1537 Tells the linker to accept input files whose architecture cannot be
1538 recognised. The assumption is that the user knows what they are doing
1539 and deliberately wants to link in these unknown input files. This was
1540 the default behaviour of the linker, before release 2.14. The default
1541 behaviour from release 2.14 onwards is to reject such input files, and
1542 so the @samp{--accept-unknown-input-arch} option has been added to
1543 restore the old behaviour.
1546 @kindex --no-as-needed
1548 @itemx --no-as-needed
1549 This option affects ELF DT_NEEDED tags for dynamic libraries mentioned
1550 on the command line after the @option{--as-needed} option. Normally
1551 the linker will add a DT_NEEDED tag for each dynamic library mentioned
1552 on the command line, regardless of whether the library is actually
1553 needed or not. @option{--as-needed} causes a DT_NEEDED tag to only be
1554 emitted for a library that @emph{at that point in the link} satisfies a
1555 non-weak undefined symbol reference from a regular object file or, if
1556 the library is not found in the DT_NEEDED lists of other needed libraries, a
1557 non-weak undefined symbol reference from another needed dynamic library.
1558 Object files or libraries appearing on the command line @emph{after}
1559 the library in question do not affect whether the library is seen as
1560 needed. This is similar to the rules for extraction of object files
1561 from archives. @option{--no-as-needed} restores the default behaviour.
1563 Note: On Linux based systems the @option{--as-needed} option also has
1564 an affect on the behaviour of the @option{--rpath} and
1565 @option{--rpath-link} options. See the description of
1566 @option{--rpath-link} for more details.
1568 @kindex --add-needed
1569 @kindex --no-add-needed
1571 @itemx --no-add-needed
1572 These two options have been deprecated because of the similarity of
1573 their names to the @option{--as-needed} and @option{--no-as-needed}
1574 options. They have been replaced by @option{--copy-dt-needed-entries}
1575 and @option{--no-copy-dt-needed-entries}.
1577 @kindex -assert @var{keyword}
1578 @item -assert @var{keyword}
1579 This option is ignored for SunOS compatibility.
1583 @kindex -call_shared
1587 Link against dynamic libraries. This is only meaningful on platforms
1588 for which shared libraries are supported. This option is normally the
1589 default on such platforms. The different variants of this option are
1590 for compatibility with various systems. You may use this option
1591 multiple times on the command line: it affects library searching for
1592 @option{-l} options which follow it.
1596 Set the @code{DF_1_GROUP} flag in the @code{DT_FLAGS_1} entry in the dynamic
1597 section. This causes the runtime linker to handle lookups in this
1598 object and its dependencies to be performed only inside the group.
1599 @option{--unresolved-symbols=report-all} is implied. This option is
1600 only meaningful on ELF platforms which support shared libraries.
1610 Do not link against shared libraries. This is only meaningful on
1611 platforms for which shared libraries are supported. The different
1612 variants of this option are for compatibility with various systems. You
1613 may use this option multiple times on the command line: it affects
1614 library searching for @option{-l} options which follow it. This
1615 option also implies @option{--unresolved-symbols=report-all}. This
1616 option can be used with @option{-shared}. Doing so means that a
1617 shared library is being created but that all of the library's external
1618 references must be resolved by pulling in entries from static
1623 When creating a shared library, bind references to global symbols to the
1624 definition within the shared library, if any. Normally, it is possible
1625 for a program linked against a shared library to override the definition
1626 within the shared library. This option is only meaningful on ELF
1627 platforms which support shared libraries.
1629 @kindex -Bsymbolic-functions
1630 @item -Bsymbolic-functions
1631 When creating a shared library, bind references to global function
1632 symbols to the definition within the shared library, if any.
1633 This option is only meaningful on ELF platforms which support shared
1636 @kindex --dynamic-list=@var{dynamic-list-file}
1637 @item --dynamic-list=@var{dynamic-list-file}
1638 Specify the name of a dynamic list file to the linker. This is
1639 typically used when creating shared libraries to specify a list of
1640 global symbols whose references shouldn't be bound to the definition
1641 within the shared library, or creating dynamically linked executables
1642 to specify a list of symbols which should be added to the symbol table
1643 in the executable. This option is only meaningful on ELF platforms
1644 which support shared libraries.
1646 The format of the dynamic list is the same as the version node without
1647 scope and node name. See @ref{VERSION} for more information.
1649 @kindex --dynamic-list-data
1650 @item --dynamic-list-data
1651 Include all global data symbols to the dynamic list.
1653 @kindex --dynamic-list-cpp-new
1654 @item --dynamic-list-cpp-new
1655 Provide the builtin dynamic list for C++ operator new and delete. It
1656 is mainly useful for building shared libstdc++.
1658 @kindex --dynamic-list-cpp-typeinfo
1659 @item --dynamic-list-cpp-typeinfo
1660 Provide the builtin dynamic list for C++ runtime type identification.
1662 @kindex --check-sections
1663 @kindex --no-check-sections
1664 @item --check-sections
1665 @itemx --no-check-sections
1666 Asks the linker @emph{not} to check section addresses after they have
1667 been assigned to see if there are any overlaps. Normally the linker will
1668 perform this check, and if it finds any overlaps it will produce
1669 suitable error messages. The linker does know about, and does make
1670 allowances for sections in overlays. The default behaviour can be
1671 restored by using the command-line switch @option{--check-sections}.
1672 Section overlap is not usually checked for relocatable links. You can
1673 force checking in that case by using the @option{--check-sections}
1676 @kindex --copy-dt-needed-entries
1677 @kindex --no-copy-dt-needed-entries
1678 @item --copy-dt-needed-entries
1679 @itemx --no-copy-dt-needed-entries
1680 This option affects the treatment of dynamic libraries referred to
1681 by DT_NEEDED tags @emph{inside} ELF dynamic libraries mentioned on the
1682 command line. Normally the linker won't add a DT_NEEDED tag to the
1683 output binary for each library mentioned in a DT_NEEDED tag in an
1684 input dynamic library. With @option{--copy-dt-needed-entries}
1685 specified on the command line however any dynamic libraries that
1686 follow it will have their DT_NEEDED entries added. The default
1687 behaviour can be restored with @option{--no-copy-dt-needed-entries}.
1689 This option also has an effect on the resolution of symbols in dynamic
1690 libraries. With @option{--copy-dt-needed-entries} dynamic libraries
1691 mentioned on the command line will be recursively searched, following
1692 their DT_NEEDED tags to other libraries, in order to resolve symbols
1693 required by the output binary. With the default setting however
1694 the searching of dynamic libraries that follow it will stop with the
1695 dynamic library itself. No DT_NEEDED links will be traversed to resolve
1698 @cindex cross reference table
1701 Output a cross reference table. If a linker map file is being
1702 generated, the cross reference table is printed to the map file.
1703 Otherwise, it is printed on the standard output.
1705 The format of the table is intentionally simple, so that it may be
1706 easily processed by a script if necessary. The symbols are printed out,
1707 sorted by name. For each symbol, a list of file names is given. If the
1708 symbol is defined, the first file listed is the location of the
1709 definition. If the symbol is defined as a common value then any files
1710 where this happens appear next. Finally any files that reference the
1713 @cindex ctf variables
1714 @kindex --ctf-variables
1715 @kindex --no-ctf-variables
1716 @item --ctf-variables
1717 @item --no-ctf-variables
1718 The CTF debuginfo format supports a section which encodes the names and
1719 types of variables found in the program which do not appear in any symbol
1720 table. These variables clearly cannot be looked up by address by
1721 conventional debuggers, so the space used for their types and names is
1722 usually wasted: the types are usually small but the names are often not.
1723 @option{--ctf-variables} causes the generation of such a section.
1724 The default behaviour can be restored with @option{--no-ctf-variables}.
1726 @cindex ctf type sharing
1727 @kindex --ctf-share-types
1728 @item --ctf-share-types=@var{method}
1729 Adjust the method used to share types between translation units in CTF.
1732 @item share-unconflicted
1733 Put all types that do not have ambiguous definitions into the shared dictionary,
1734 where debuggers can easily access them, even if they only occur in one
1735 translation unit. This is the default.
1737 @item share-duplicated
1738 Put only types that occur in multiple translation units into the shared
1739 dictionary: types with only one definition go into per-translation-unit
1740 dictionaries. Types with ambiguous definitions in multiple translation units
1741 always go into per-translation-unit dictionaries. This tends to make the CTF
1742 larger, but may reduce the amount of CTF in the shared dictionary. For very
1743 large projects this may speed up opening the CTF and save memory in the CTF
1744 consumer at runtime.
1747 @cindex common allocation
1748 @kindex --no-define-common
1749 @item --no-define-common
1750 This option inhibits the assignment of addresses to common symbols.
1751 The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect.
1752 @xref{Miscellaneous Commands}.
1754 The @samp{--no-define-common} option allows decoupling
1755 the decision to assign addresses to Common symbols from the choice
1756 of the output file type; otherwise a non-Relocatable output type
1757 forces assigning addresses to Common symbols.
1758 Using @samp{--no-define-common} allows Common symbols that are referenced
1759 from a shared library to be assigned addresses only in the main program.
1760 This eliminates the unused duplicate space in the shared library,
1761 and also prevents any possible confusion over resolving to the wrong
1762 duplicate when there are many dynamic modules with specialized search
1763 paths for runtime symbol resolution.
1765 @cindex group allocation in linker script
1766 @cindex section groups
1768 @kindex --force-group-allocation
1769 @item --force-group-allocation
1770 This option causes the linker to place section group members like
1771 normal input sections, and to delete the section groups. This is the
1772 default behaviour for a final link but this option can be used to
1773 change the behaviour of a relocatable link (@samp{-r}). The script
1774 command @code{FORCE_GROUP_ALLOCATION} has the same
1775 effect. @xref{Miscellaneous Commands}.
1777 @cindex symbols, from command line
1778 @kindex --defsym=@var{symbol}=@var{exp}
1779 @item --defsym=@var{symbol}=@var{expression}
1780 Create a global symbol in the output file, containing the absolute
1781 address given by @var{expression}. You may use this option as many
1782 times as necessary to define multiple symbols in the command line. A
1783 limited form of arithmetic is supported for the @var{expression} in this
1784 context: you may give a hexadecimal constant or the name of an existing
1785 symbol, or use @code{+} and @code{-} to add or subtract hexadecimal
1786 constants or symbols. If you need more elaborate expressions, consider
1787 using the linker command language from a script (@pxref{Assignments}).
1788 @emph{Note:} there should be no white space between @var{symbol}, the
1789 equals sign (``@key{=}''), and @var{expression}.
1791 The linker processes @samp{--defsym} arguments and @samp{-T} arguments
1792 in order, placing @samp{--defsym} before @samp{-T} will define the
1793 symbol before the linker script from @samp{-T} is processed, while
1794 placing @samp{--defsym} after @samp{-T} will define the symbol after
1795 the linker script has been processed. This difference has
1796 consequences for expressions within the linker script that use the
1797 @samp{--defsym} symbols, which order is correct will depend on what
1798 you are trying to achieve.
1800 @cindex demangling, from command line
1801 @kindex --demangle[=@var{style}]
1802 @kindex --no-demangle
1803 @item --demangle[=@var{style}]
1804 @itemx --no-demangle
1805 These options control whether to demangle symbol names in error messages
1806 and other output. When the linker is told to demangle, it tries to
1807 present symbol names in a readable fashion: it strips leading
1808 underscores if they are used by the object file format, and converts C++
1809 mangled symbol names into user readable names. Different compilers have
1810 different mangling styles. The optional demangling style argument can be used
1811 to choose an appropriate demangling style for your compiler. The linker will
1812 demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE}
1813 is set. These options may be used to override the default.
1815 @cindex dynamic linker, from command line
1816 @kindex -I@var{file}
1817 @kindex --dynamic-linker=@var{file}
1819 @itemx --dynamic-linker=@var{file}
1820 Set the name of the dynamic linker. This is only meaningful when
1821 generating dynamically linked ELF executables. The default dynamic
1822 linker is normally correct; don't use this unless you know what you are
1825 @kindex --no-dynamic-linker
1826 @item --no-dynamic-linker
1827 When producing an executable file, omit the request for a dynamic
1828 linker to be used at load-time. This is only meaningful for ELF
1829 executables that contain dynamic relocations, and usually requires
1830 entry point code that is capable of processing these relocations.
1832 @kindex --embedded-relocs
1833 @item --embedded-relocs
1834 This option is similar to the @option{--emit-relocs} option except
1835 that the relocs are stored in a target-specific section. This option
1836 is only supported by the @samp{BFIN}, @samp{CR16} and @emph{M68K}
1839 @kindex --disable-multiple-abs-defs
1840 @item --disable-multiple-abs-defs
1841 Do not allow multiple definitions with symbols included
1842 in filename invoked by -R or --just-symbols
1844 @kindex --fatal-warnings
1845 @kindex --no-fatal-warnings
1846 @item --fatal-warnings
1847 @itemx --no-fatal-warnings
1848 Treat all warnings as errors. The default behaviour can be restored
1849 with the option @option{--no-fatal-warnings}.
1851 @kindex --force-exe-suffix
1852 @item --force-exe-suffix
1853 Make sure that an output file has a .exe suffix.
1855 If a successfully built fully linked output file does not have a
1856 @code{.exe} or @code{.dll} suffix, this option forces the linker to copy
1857 the output file to one of the same name with a @code{.exe} suffix. This
1858 option is useful when using unmodified Unix makefiles on a Microsoft
1859 Windows host, since some versions of Windows won't run an image unless
1860 it ends in a @code{.exe} suffix.
1862 @kindex --gc-sections
1863 @kindex --no-gc-sections
1864 @cindex garbage collection
1866 @itemx --no-gc-sections
1867 Enable garbage collection of unused input sections. It is ignored on
1868 targets that do not support this option. The default behaviour (of not
1869 performing this garbage collection) can be restored by specifying
1870 @samp{--no-gc-sections} on the command line. Note that garbage
1871 collection for COFF and PE format targets is supported, but the
1872 implementation is currently considered to be experimental.
1874 @samp{--gc-sections} decides which input sections are used by
1875 examining symbols and relocations. The section containing the entry
1876 symbol and all sections containing symbols undefined on the
1877 command-line will be kept, as will sections containing symbols
1878 referenced by dynamic objects. Note that when building shared
1879 libraries, the linker must assume that any visible symbol is
1880 referenced. Once this initial set of sections has been determined,
1881 the linker recursively marks as used any section referenced by their
1882 relocations. See @samp{--entry}, @samp{--undefined}, and
1883 @samp{--gc-keep-exported}.
1885 This option can be set when doing a partial link (enabled with option
1886 @samp{-r}). In this case the root of symbols kept must be explicitly
1887 specified either by one of the options @samp{--entry},
1888 @samp{--undefined}, or @samp{--gc-keep-exported} or by a @code{ENTRY}
1889 command in the linker script.
1891 As a GNU extension, ELF input sections marked with the
1892 @code{SHF_GNU_RETAIN} flag will not be garbage collected.
1894 @kindex --print-gc-sections
1895 @kindex --no-print-gc-sections
1896 @cindex garbage collection
1897 @item --print-gc-sections
1898 @itemx --no-print-gc-sections
1899 List all sections removed by garbage collection. The listing is
1900 printed on stderr. This option is only effective if garbage
1901 collection has been enabled via the @samp{--gc-sections}) option. The
1902 default behaviour (of not listing the sections that are removed) can
1903 be restored by specifying @samp{--no-print-gc-sections} on the command
1906 @kindex --gc-keep-exported
1907 @cindex garbage collection
1908 @item --gc-keep-exported
1909 When @samp{--gc-sections} is enabled, this option prevents garbage
1910 collection of unused input sections that contain global symbols having
1911 default or protected visibility. This option is intended to be used for
1912 executables where unreferenced sections would otherwise be garbage
1913 collected regardless of the external visibility of contained symbols.
1914 Note that this option has no effect when linking shared objects since
1915 it is already the default behaviour. This option is only supported for
1918 @kindex --print-output-format
1919 @cindex output format
1920 @item --print-output-format
1921 Print the name of the default output format (perhaps influenced by
1922 other command-line options). This is the string that would appear
1923 in an @code{OUTPUT_FORMAT} linker script command (@pxref{File Commands}).
1925 @kindex --print-memory-usage
1926 @cindex memory usage
1927 @item --print-memory-usage
1928 Print used size, total size and used size of memory regions created with
1929 the @ref{MEMORY} command. This is useful on embedded targets to have a
1930 quick view of amount of free memory. The format of the output has one
1931 headline and one line per region. It is both human readable and easily
1932 parsable by tools. Here is an example of an output:
1935 Memory region Used Size Region Size %age Used
1936 ROM: 256 KB 1 MB 25.00%
1937 RAM: 32 B 2 GB 0.00%
1944 Print a summary of the command-line options on the standard output and exit.
1946 @kindex --target-help
1948 Print a summary of all target-specific options on the standard output and exit.
1950 @kindex -Map=@var{mapfile}
1951 @item -Map=@var{mapfile}
1952 Print a link map to the file @var{mapfile}. See the description of the
1953 @option{-M} option, above. If @var{mapfile} is just the character
1954 @code{-} then the map will be written to stdout.
1956 Specifying a directory as @var{mapfile} causes the linker map to be
1957 written as a file inside the directory. Normally name of the file
1958 inside the directory is computed as the basename of the @var{output}
1959 file with @code{.map} appended. If however the special character
1960 @code{%} is used then this will be replaced by the full path of the
1961 output file. Additionally if there are any characters after the
1962 @var{%} symbol then @code{.map} will no longer be appended.
1965 -o foo.exe -Map=bar [Creates ./bar]
1966 -o ../dir/foo.exe -Map=bar [Creates ./bar]
1967 -o foo.exe -Map=../dir [Creates ../dir/foo.exe.map]
1968 -o ../dir2/foo.exe -Map=../dir [Creates ../dir/foo.exe.map]
1969 -o foo.exe -Map=% [Creates ./foo.exe.map]
1970 -o ../dir/foo.exe -Map=% [Creates ../dir/foo.exe.map]
1971 -o foo.exe -Map=%.bar [Creates ./foo.exe.bar]
1972 -o ../dir/foo.exe -Map=%.bar [Creates ../dir/foo.exe.bar]
1973 -o ../dir2/foo.exe -Map=../dir/% [Creates ../dir/../dir2/foo.exe.map]
1974 -o ../dir2/foo.exe -Map=../dir/%.bar [Creates ../dir/../dir2/foo.exe.bar]
1977 It is an error to specify more than one @code{%} character.
1979 If the map file already exists then it will be overwritten by this
1982 @cindex memory usage
1983 @kindex --no-keep-memory
1984 @item --no-keep-memory
1985 @command{ld} normally optimizes for speed over memory usage by caching the
1986 symbol tables of input files in memory. This option tells @command{ld} to
1987 instead optimize for memory usage, by rereading the symbol tables as
1988 necessary. This may be required if @command{ld} runs out of memory space
1989 while linking a large executable.
1991 @kindex --no-undefined
1994 @item --no-undefined
1996 Report unresolved symbol references from regular object files. This
1997 is done even if the linker is creating a non-symbolic shared library.
1998 The switch @option{--[no-]allow-shlib-undefined} controls the
1999 behaviour for reporting unresolved references found in shared
2000 libraries being linked in.
2002 The effects of this option can be reverted by using @code{-z undefs}.
2004 @kindex --allow-multiple-definition
2006 @item --allow-multiple-definition
2008 Normally when a symbol is defined multiple times, the linker will
2009 report a fatal error. These options allow multiple definitions and the
2010 first definition will be used.
2012 @kindex --allow-shlib-undefined
2013 @kindex --no-allow-shlib-undefined
2014 @item --allow-shlib-undefined
2015 @itemx --no-allow-shlib-undefined
2016 Allows or disallows undefined symbols in shared libraries.
2017 This switch is similar to @option{--no-undefined} except that it
2018 determines the behaviour when the undefined symbols are in a
2019 shared library rather than a regular object file. It does not affect
2020 how undefined symbols in regular object files are handled.
2022 The default behaviour is to report errors for any undefined symbols
2023 referenced in shared libraries if the linker is being used to create
2024 an executable, but to allow them if the linker is being used to create
2027 The reasons for allowing undefined symbol references in shared
2028 libraries specified at link time are that:
2032 A shared library specified at link time may not be the same as the one
2033 that is available at load time, so the symbol might actually be
2034 resolvable at load time.
2036 There are some operating systems, eg BeOS and HPPA, where undefined
2037 symbols in shared libraries are normal.
2039 The BeOS kernel for example patches shared libraries at load time to
2040 select whichever function is most appropriate for the current
2041 architecture. This is used, for example, to dynamically select an
2042 appropriate memset function.
2045 @kindex --error-handling-script=@var{scriptname}
2046 @item --error-handling-script=@var{scriptname}
2047 If this option is provided then the linker will invoke
2048 @var{scriptname} whenever an error is encountered. Currently however
2049 only two kinds of error are supported: missing symbols and missing
2050 libraries. Two arguments will be passed to script: the keyword
2051 ``undefined-symbol'' or `missing-lib'' and the @var{name} of the
2052 undefined symbol or missing library. The intention is that the script
2053 will provide suggestions to the user as to where the symbol or library
2054 might be found. After the script has finished then the normal linker
2055 error message will be displayed.
2057 The availability of this option is controlled by a configure time
2058 switch, so it may not be present in specific implementations.
2060 @kindex --no-undefined-version
2061 @item --no-undefined-version
2062 Normally when a symbol has an undefined version, the linker will ignore
2063 it. This option disallows symbols with undefined version and a fatal error
2064 will be issued instead.
2066 @kindex --default-symver
2067 @item --default-symver
2068 Create and use a default symbol version (the soname) for unversioned
2071 @kindex --default-imported-symver
2072 @item --default-imported-symver
2073 Create and use a default symbol version (the soname) for unversioned
2076 @kindex --no-warn-mismatch
2077 @item --no-warn-mismatch
2078 Normally @command{ld} will give an error if you try to link together input
2079 files that are mismatched for some reason, perhaps because they have
2080 been compiled for different processors or for different endiannesses.
2081 This option tells @command{ld} that it should silently permit such possible
2082 errors. This option should only be used with care, in cases when you
2083 have taken some special action that ensures that the linker errors are
2086 @kindex --no-warn-search-mismatch
2087 @item --no-warn-search-mismatch
2088 Normally @command{ld} will give a warning if it finds an incompatible
2089 library during a library search. This option silences the warning.
2091 @kindex --no-whole-archive
2092 @item --no-whole-archive
2093 Turn off the effect of the @option{--whole-archive} option for subsequent
2096 @cindex output file after errors
2097 @kindex --noinhibit-exec
2098 @item --noinhibit-exec
2099 Retain the executable output file whenever it is still usable.
2100 Normally, the linker will not produce an output file if it encounters
2101 errors during the link process; it exits without writing an output file
2102 when it issues any error whatsoever.
2106 Only search library directories explicitly specified on the
2107 command line. Library directories specified in linker scripts
2108 (including linker scripts specified on the command line) are ignored.
2110 @ifclear SingleFormat
2111 @kindex --oformat=@var{output-format}
2112 @item --oformat=@var{output-format}
2113 @command{ld} may be configured to support more than one kind of object
2114 file. If your @command{ld} is configured this way, you can use the
2115 @samp{--oformat} option to specify the binary format for the output
2116 object file. Even when @command{ld} is configured to support alternative
2117 object formats, you don't usually need to specify this, as @command{ld}
2118 should be configured to produce as a default output format the most
2119 usual format on each machine. @var{output-format} is a text string, the
2120 name of a particular format supported by the BFD libraries. (You can
2121 list the available binary formats with @samp{objdump -i}.) The script
2122 command @code{OUTPUT_FORMAT} can also specify the output format, but
2123 this option overrides it. @xref{BFD}.
2126 @kindex --out-implib
2127 @item --out-implib @var{file}
2128 Create an import library in @var{file} corresponding to the executable
2129 the linker is generating (eg. a DLL or ELF program). This import
2130 library (which should be called @code{*.dll.a} or @code{*.a} for DLLs)
2131 may be used to link clients against the generated executable; this
2132 behaviour makes it possible to skip a separate import library creation
2133 step (eg. @code{dlltool} for DLLs). This option is only available for
2134 the i386 PE and ELF targetted ports of the linker.
2137 @kindex --pic-executable
2139 @itemx --pic-executable
2140 @cindex position independent executables
2141 Create a position independent executable. This is currently only supported on
2142 ELF platforms. Position independent executables are similar to shared
2143 libraries in that they are relocated by the dynamic linker to the virtual
2144 address the OS chooses for them (which can vary between invocations). Like
2145 normal dynamically linked executables they can be executed and symbols
2146 defined in the executable cannot be overridden by shared libraries.
2150 This option is ignored for Linux compatibility.
2154 This option is ignored for SVR4 compatibility.
2157 @cindex synthesizing linker
2158 @cindex relaxing addressing modes
2162 An option with machine dependent effects.
2164 This option is only supported on a few targets.
2167 @xref{H8/300,,@command{ld} and the H8/300}.
2170 @xref{Xtensa,, @command{ld} and Xtensa Processors}.
2173 @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}.
2176 @xref{Nios II,,@command{ld} and the Altera Nios II}.
2179 @xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}.
2182 On some platforms the @option{--relax} option performs target specific,
2183 global optimizations that become possible when the linker resolves
2184 addressing in the program, such as relaxing address modes,
2185 synthesizing new instructions, selecting shorter version of current
2186 instructions, and combining constant values.
2188 On some platforms these link time global optimizations may make symbolic
2189 debugging of the resulting executable impossible.
2191 This is known to be the case for the Matsushita MN10200 and MN10300
2192 family of processors.
2195 On platforms where the feature is supported, the option
2196 @option{--no-relax} will disable it.
2198 On platforms where the feature is not supported, both @option{--relax}
2199 and @option{--no-relax} are accepted, but ignored.
2201 @cindex retaining specified symbols
2202 @cindex stripping all but some symbols
2203 @cindex symbols, retaining selectively
2204 @kindex --retain-symbols-file=@var{filename}
2205 @item --retain-symbols-file=@var{filename}
2206 Retain @emph{only} the symbols listed in the file @var{filename},
2207 discarding all others. @var{filename} is simply a flat file, with one
2208 symbol name per line. This option is especially useful in environments
2212 where a large global symbol table is accumulated gradually, to conserve
2215 @samp{--retain-symbols-file} does @emph{not} discard undefined symbols,
2216 or symbols needed for relocations.
2218 You may only specify @samp{--retain-symbols-file} once in the command
2219 line. It overrides @samp{-s} and @samp{-S}.
2222 @item -rpath=@var{dir}
2223 @cindex runtime library search path
2224 @kindex -rpath=@var{dir}
2225 Add a directory to the runtime library search path. This is used when
2226 linking an ELF executable with shared objects. All @option{-rpath}
2227 arguments are concatenated and passed to the runtime linker, which uses
2228 them to locate shared objects at runtime.
2230 The @option{-rpath} option is also used when locating shared objects which
2231 are needed by shared objects explicitly included in the link; see the
2232 description of the @option{-rpath-link} option. Searching @option{-rpath}
2233 in this way is only supported by native linkers and cross linkers which
2234 have been configured with the @option{--with-sysroot} option.
2236 If @option{-rpath} is not used when linking an ELF executable, the
2237 contents of the environment variable @code{LD_RUN_PATH} will be used if it
2240 The @option{-rpath} option may also be used on SunOS. By default, on
2241 SunOS, the linker will form a runtime search path out of all the
2242 @option{-L} options it is given. If a @option{-rpath} option is used, the
2243 runtime search path will be formed exclusively using the @option{-rpath}
2244 options, ignoring the @option{-L} options. This can be useful when using
2245 gcc, which adds many @option{-L} options which may be on NFS mounted
2248 For compatibility with other ELF linkers, if the @option{-R} option is
2249 followed by a directory name, rather than a file name, it is treated as
2250 the @option{-rpath} option.
2254 @cindex link-time runtime library search path
2255 @kindex -rpath-link=@var{dir}
2256 @item -rpath-link=@var{dir}
2257 When using ELF or SunOS, one shared library may require another. This
2258 happens when an @code{ld -shared} link includes a shared library as one
2261 When the linker encounters such a dependency when doing a non-shared,
2262 non-relocatable link, it will automatically try to locate the required
2263 shared library and include it in the link, if it is not included
2264 explicitly. In such a case, the @option{-rpath-link} option
2265 specifies the first set of directories to search. The
2266 @option{-rpath-link} option may specify a sequence of directory names
2267 either by specifying a list of names separated by colons, or by
2268 appearing multiple times.
2270 The tokens @var{$ORIGIN} and @var{$LIB} can appear in these search
2271 directories. They will be replaced by the full path to the directory
2272 containing the program or shared object in the case of @var{$ORIGIN}
2273 and either @samp{lib} - for 32-bit binaries - or @samp{lib64} - for
2274 64-bit binaries - in the case of @var{$LIB}.
2276 The alternative form of these tokens - @var{$@{ORIGIN@}} and
2277 @var{$@{LIB@}} can also be used. The token @var{$PLATFORM} is not
2280 This option should be used with caution as it overrides the search path
2281 that may have been hard compiled into a shared library. In such a case it
2282 is possible to use unintentionally a different search path than the
2283 runtime linker would do.
2285 The linker uses the following search paths to locate required shared
2290 Any directories specified by @option{-rpath-link} options.
2292 Any directories specified by @option{-rpath} options. The difference
2293 between @option{-rpath} and @option{-rpath-link} is that directories
2294 specified by @option{-rpath} options are included in the executable and
2295 used at runtime, whereas the @option{-rpath-link} option is only effective
2296 at link time. Searching @option{-rpath} in this way is only supported
2297 by native linkers and cross linkers which have been configured with
2298 the @option{--with-sysroot} option.
2300 On an ELF system, for native linkers, if the @option{-rpath} and
2301 @option{-rpath-link} options were not used, search the contents of the
2302 environment variable @code{LD_RUN_PATH}.
2304 On SunOS, if the @option{-rpath} option was not used, search any
2305 directories specified using @option{-L} options.
2307 For a native linker, search the contents of the environment
2308 variable @code{LD_LIBRARY_PATH}.
2310 For a native ELF linker, the directories in @code{DT_RUNPATH} or
2311 @code{DT_RPATH} of a shared library are searched for shared
2312 libraries needed by it. The @code{DT_RPATH} entries are ignored if
2313 @code{DT_RUNPATH} entries exist.
2315 The default directories, normally @file{/lib} and @file{/usr/lib}.
2317 For a linker for a Linux system, if the file @file{/etc/ld.so.conf}
2318 exists, the list of directories found in that file. Note: the path
2319 to this file is prefixed with the @code{sysroot} value, if that is
2320 defined, and then any @code{prefix} string if the linker was
2321 configured with the @command{--prefix=<path>} option.
2323 For a native linker on a FreeBSD system, any directories specified by
2324 the @code{_PATH_ELF_HINTS} macro defined in the @file{elf-hints.h}
2327 Any directories specifed by a @code{SEARCH_DIR} command in the
2328 linker script being used.
2331 Note however on Linux based systems there is an additional caveat: If
2332 the @option{--as-needed} option is active @emph{and} a shared library
2333 is located which would normally satisfy the search @emph{and} this
2334 library does not have DT_NEEDED tag for @file{libc.so}
2335 @emph{and} there is a shared library later on in the set of search
2336 directories which also satisfies the search @emph{and}
2337 this second shared library does have a DT_NEEDED tag for
2338 @file{libc.so} @emph{then} the second library will be selected instead
2341 If the required shared library is not found, the linker will issue a
2342 warning and continue with the link.
2350 @cindex shared libraries
2351 Create a shared library. This is currently only supported on ELF, XCOFF
2352 and SunOS platforms. On SunOS, the linker will automatically create a
2353 shared library if the @option{-e} option is not used and there are
2354 undefined symbols in the link.
2356 @kindex --sort-common
2358 @itemx --sort-common=ascending
2359 @itemx --sort-common=descending
2360 This option tells @command{ld} to sort the common symbols by alignment in
2361 ascending or descending order when it places them in the appropriate output
2362 sections. The symbol alignments considered are sixteen-byte or larger,
2363 eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps
2364 between symbols due to alignment constraints. If no sorting order is
2365 specified, then descending order is assumed.
2367 @kindex --sort-section=name
2368 @item --sort-section=name
2369 This option will apply @code{SORT_BY_NAME} to all wildcard section
2370 patterns in the linker script.
2372 @kindex --sort-section=alignment
2373 @item --sort-section=alignment
2374 This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section
2375 patterns in the linker script.
2377 @kindex --spare-dynamic-tags
2378 @item --spare-dynamic-tags=@var{count}
2379 This option specifies the number of empty slots to leave in the
2380 .dynamic section of ELF shared objects. Empty slots may be needed by
2381 post processing tools, such as the prelinker. The default is 5.
2383 @kindex --split-by-file
2384 @item --split-by-file[=@var{size}]
2385 Similar to @option{--split-by-reloc} but creates a new output section for
2386 each input file when @var{size} is reached. @var{size} defaults to a
2387 size of 1 if not given.
2389 @kindex --split-by-reloc
2390 @item --split-by-reloc[=@var{count}]
2391 Tries to creates extra sections in the output file so that no single
2392 output section in the file contains more than @var{count} relocations.
2393 This is useful when generating huge relocatable files for downloading into
2394 certain real time kernels with the COFF object file format; since COFF
2395 cannot represent more than 65535 relocations in a single section. Note
2396 that this will fail to work with object file formats which do not
2397 support arbitrary sections. The linker will not split up individual
2398 input sections for redistribution, so if a single input section contains
2399 more than @var{count} relocations one output section will contain that
2400 many relocations. @var{count} defaults to a value of 32768.
2404 Compute and display statistics about the operation of the linker, such
2405 as execution time and memory usage.
2407 @kindex --sysroot=@var{directory}
2408 @item --sysroot=@var{directory}
2409 Use @var{directory} as the location of the sysroot, overriding the
2410 configure-time default. This option is only supported by linkers
2411 that were configured using @option{--with-sysroot}.
2415 This is used by COFF/PE based targets to create a task-linked object
2416 file where all of the global symbols have been converted to statics.
2418 @kindex --traditional-format
2419 @cindex traditional format
2420 @item --traditional-format
2421 For some targets, the output of @command{ld} is different in some ways from
2422 the output of some existing linker. This switch requests @command{ld} to
2423 use the traditional format instead.
2426 For example, on SunOS, @command{ld} combines duplicate entries in the
2427 symbol string table. This can reduce the size of an output file with
2428 full debugging information by over 30 percent. Unfortunately, the SunOS
2429 @code{dbx} program can not read the resulting program (@code{gdb} has no
2430 trouble). The @samp{--traditional-format} switch tells @command{ld} to not
2431 combine duplicate entries.
2433 @kindex --section-start=@var{sectionname}=@var{org}
2434 @item --section-start=@var{sectionname}=@var{org}
2435 Locate a section in the output file at the absolute
2436 address given by @var{org}. You may use this option as many
2437 times as necessary to locate multiple sections in the command
2439 @var{org} must be a single hexadecimal integer;
2440 for compatibility with other linkers, you may omit the leading
2441 @samp{0x} usually associated with hexadecimal values. @emph{Note:} there
2442 should be no white space between @var{sectionname}, the equals
2443 sign (``@key{=}''), and @var{org}.
2445 @kindex -Tbss=@var{org}
2446 @kindex -Tdata=@var{org}
2447 @kindex -Ttext=@var{org}
2448 @cindex segment origins, cmd line
2449 @item -Tbss=@var{org}
2450 @itemx -Tdata=@var{org}
2451 @itemx -Ttext=@var{org}
2452 Same as @option{--section-start}, with @code{.bss}, @code{.data} or
2453 @code{.text} as the @var{sectionname}.
2455 @kindex -Ttext-segment=@var{org}
2456 @item -Ttext-segment=@var{org}
2457 @cindex text segment origin, cmd line
2458 When creating an ELF executable, it will set the address of the first
2459 byte of the text segment.
2461 @kindex -Trodata-segment=@var{org}
2462 @item -Trodata-segment=@var{org}
2463 @cindex rodata segment origin, cmd line
2464 When creating an ELF executable or shared object for a target where
2465 the read-only data is in its own segment separate from the executable
2466 text, it will set the address of the first byte of the read-only data segment.
2468 @kindex -Tldata-segment=@var{org}
2469 @item -Tldata-segment=@var{org}
2470 @cindex ldata segment origin, cmd line
2471 When creating an ELF executable or shared object for x86-64 medium memory
2472 model, it will set the address of the first byte of the ldata segment.
2474 @kindex --unresolved-symbols
2475 @item --unresolved-symbols=@var{method}
2476 Determine how to handle unresolved symbols. There are four possible
2477 values for @samp{method}:
2481 Do not report any unresolved symbols.
2484 Report all unresolved symbols. This is the default.
2486 @item ignore-in-object-files
2487 Report unresolved symbols that are contained in shared libraries, but
2488 ignore them if they come from regular object files.
2490 @item ignore-in-shared-libs
2491 Report unresolved symbols that come from regular object files, but
2492 ignore them if they come from shared libraries. This can be useful
2493 when creating a dynamic binary and it is known that all the shared
2494 libraries that it should be referencing are included on the linker's
2498 The behaviour for shared libraries on their own can also be controlled
2499 by the @option{--[no-]allow-shlib-undefined} option.
2501 Normally the linker will generate an error message for each reported
2502 unresolved symbol but the option @option{--warn-unresolved-symbols}
2503 can change this to a warning.
2505 @kindex --verbose[=@var{NUMBER}]
2506 @cindex verbose[=@var{NUMBER}]
2508 @itemx --verbose[=@var{NUMBER}]
2509 Display the version number for @command{ld} and list the linker emulations
2510 supported. Display which input files can and cannot be opened. Display
2511 the linker script being used by the linker. If the optional @var{NUMBER}
2512 argument > 1, plugin symbol status will also be displayed.
2514 @kindex --version-script=@var{version-scriptfile}
2515 @cindex version script, symbol versions
2516 @item --version-script=@var{version-scriptfile}
2517 Specify the name of a version script to the linker. This is typically
2518 used when creating shared libraries to specify additional information
2519 about the version hierarchy for the library being created. This option
2520 is only fully supported on ELF platforms which support shared libraries;
2521 see @ref{VERSION}. It is partially supported on PE platforms, which can
2522 use version scripts to filter symbol visibility in auto-export mode: any
2523 symbols marked @samp{local} in the version script will not be exported.
2526 @kindex --warn-common
2527 @cindex warnings, on combining symbols
2528 @cindex combining symbols, warnings on
2530 Warn when a common symbol is combined with another common symbol or with
2531 a symbol definition. Unix linkers allow this somewhat sloppy practice,
2532 but linkers on some other operating systems do not. This option allows
2533 you to find potential problems from combining global symbols.
2534 Unfortunately, some C libraries use this practice, so you may get some
2535 warnings about symbols in the libraries as well as in your programs.
2537 There are three kinds of global symbols, illustrated here by C examples:
2541 A definition, which goes in the initialized data section of the output
2545 An undefined reference, which does not allocate space.
2546 There must be either a definition or a common symbol for the
2550 A common symbol. If there are only (one or more) common symbols for a
2551 variable, it goes in the uninitialized data area of the output file.
2552 The linker merges multiple common symbols for the same variable into a
2553 single symbol. If they are of different sizes, it picks the largest
2554 size. The linker turns a common symbol into a declaration, if there is
2555 a definition of the same variable.
2558 The @samp{--warn-common} option can produce five kinds of warnings.
2559 Each warning consists of a pair of lines: the first describes the symbol
2560 just encountered, and the second describes the previous symbol
2561 encountered with the same name. One or both of the two symbols will be
2566 Turning a common symbol into a reference, because there is already a
2567 definition for the symbol.
2569 @var{file}(@var{section}): warning: common of `@var{symbol}'
2570 overridden by definition
2571 @var{file}(@var{section}): warning: defined here
2575 Turning a common symbol into a reference, because a later definition for
2576 the symbol is encountered. This is the same as the previous case,
2577 except that the symbols are encountered in a different order.
2579 @var{file}(@var{section}): warning: definition of `@var{symbol}'
2581 @var{file}(@var{section}): warning: common is here
2585 Merging a common symbol with a previous same-sized common symbol.
2587 @var{file}(@var{section}): warning: multiple common
2589 @var{file}(@var{section}): warning: previous common is here
2593 Merging a common symbol with a previous larger common symbol.
2595 @var{file}(@var{section}): warning: common of `@var{symbol}'
2596 overridden by larger common
2597 @var{file}(@var{section}): warning: larger common is here
2601 Merging a common symbol with a previous smaller common symbol. This is
2602 the same as the previous case, except that the symbols are
2603 encountered in a different order.
2605 @var{file}(@var{section}): warning: common of `@var{symbol}'
2606 overriding smaller common
2607 @var{file}(@var{section}): warning: smaller common is here
2611 @kindex --warn-constructors
2612 @item --warn-constructors
2613 Warn if any global constructors are used. This is only useful for a few
2614 object file formats. For formats like COFF or ELF, the linker can not
2615 detect the use of global constructors.
2617 @kindex --warn-multiple-gp
2618 @item --warn-multiple-gp
2619 Warn if multiple global pointer values are required in the output file.
2620 This is only meaningful for certain processors, such as the Alpha.
2621 Specifically, some processors put large-valued constants in a special
2622 section. A special register (the global pointer) points into the middle
2623 of this section, so that constants can be loaded efficiently via a
2624 base-register relative addressing mode. Since the offset in
2625 base-register relative mode is fixed and relatively small (e.g., 16
2626 bits), this limits the maximum size of the constant pool. Thus, in
2627 large programs, it is often necessary to use multiple global pointer
2628 values in order to be able to address all possible constants. This
2629 option causes a warning to be issued whenever this case occurs.
2632 @cindex warnings, on undefined symbols
2633 @cindex undefined symbols, warnings on
2635 Only warn once for each undefined symbol, rather than once per module
2638 @kindex --warn-section-align
2639 @cindex warnings, on section alignment
2640 @cindex section alignment, warnings on
2641 @item --warn-section-align
2642 Warn if the address of an output section is changed because of
2643 alignment. Typically, the alignment will be set by an input section.
2644 The address will only be changed if it not explicitly specified; that
2645 is, if the @code{SECTIONS} command does not specify a start address for
2646 the section (@pxref{SECTIONS}).
2648 @kindex --warn-textrel
2649 @item --warn-textrel
2650 Warn if the linker adds DT_TEXTREL to a position-independent executable
2653 @kindex --warn-alternate-em
2654 @item --warn-alternate-em
2655 Warn if an object has alternate ELF machine code.
2657 @kindex --warn-unresolved-symbols
2658 @item --warn-unresolved-symbols
2659 If the linker is going to report an unresolved symbol (see the option
2660 @option{--unresolved-symbols}) it will normally generate an error.
2661 This option makes it generate a warning instead.
2663 @kindex --error-unresolved-symbols
2664 @item --error-unresolved-symbols
2665 This restores the linker's default behaviour of generating errors when
2666 it is reporting unresolved symbols.
2668 @kindex --whole-archive
2669 @cindex including an entire archive
2670 @item --whole-archive
2671 For each archive mentioned on the command line after the
2672 @option{--whole-archive} option, include every object file in the archive
2673 in the link, rather than searching the archive for the required object
2674 files. This is normally used to turn an archive file into a shared
2675 library, forcing every object to be included in the resulting shared
2676 library. This option may be used more than once.
2678 Two notes when using this option from gcc: First, gcc doesn't know
2679 about this option, so you have to use @option{-Wl,-whole-archive}.
2680 Second, don't forget to use @option{-Wl,-no-whole-archive} after your
2681 list of archives, because gcc will add its own list of archives to
2682 your link and you may not want this flag to affect those as well.
2684 @kindex --wrap=@var{symbol}
2685 @item --wrap=@var{symbol}
2686 Use a wrapper function for @var{symbol}. Any undefined reference to
2687 @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any
2688 undefined reference to @code{__real_@var{symbol}} will be resolved to
2691 This can be used to provide a wrapper for a system function. The
2692 wrapper function should be called @code{__wrap_@var{symbol}}. If it
2693 wishes to call the system function, it should call
2694 @code{__real_@var{symbol}}.
2696 Here is a trivial example:
2700 __wrap_malloc (size_t c)
2702 printf ("malloc called with %zu\n", c);
2703 return __real_malloc (c);
2707 If you link other code with this file using @option{--wrap malloc}, then
2708 all calls to @code{malloc} will call the function @code{__wrap_malloc}
2709 instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will
2710 call the real @code{malloc} function.
2712 You may wish to provide a @code{__real_malloc} function as well, so that
2713 links without the @option{--wrap} option will succeed. If you do this,
2714 you should not put the definition of @code{__real_malloc} in the same
2715 file as @code{__wrap_malloc}; if you do, the assembler may resolve the
2716 call before the linker has a chance to wrap it to @code{malloc}.
2718 Only undefined references are replaced by the linker. So, translation unit
2719 internal references to @var{symbol} are not resolved to
2720 @code{__wrap_@var{symbol}}. In the next example, the call to @code{f} in
2721 @code{g} is not resolved to @code{__wrap_f}.
2737 @kindex --eh-frame-hdr
2738 @kindex --no-eh-frame-hdr
2739 @item --eh-frame-hdr
2740 @itemx --no-eh-frame-hdr
2741 Request (@option{--eh-frame-hdr}) or suppress
2742 (@option{--no-eh-frame-hdr}) the creation of @code{.eh_frame_hdr}
2743 section and ELF @code{PT_GNU_EH_FRAME} segment header.
2745 @kindex --ld-generated-unwind-info
2746 @item --no-ld-generated-unwind-info
2747 Request creation of @code{.eh_frame} unwind info for linker
2748 generated code sections like PLT. This option is on by default
2749 if linker generated unwind info is supported.
2751 @kindex --enable-new-dtags
2752 @kindex --disable-new-dtags
2753 @item --enable-new-dtags
2754 @itemx --disable-new-dtags
2755 This linker can create the new dynamic tags in ELF. But the older ELF
2756 systems may not understand them. If you specify
2757 @option{--enable-new-dtags}, the new dynamic tags will be created as needed
2758 and older dynamic tags will be omitted.
2759 If you specify @option{--disable-new-dtags}, no new dynamic tags will be
2760 created. By default, the new dynamic tags are not created. Note that
2761 those options are only available for ELF systems.
2763 @kindex --hash-size=@var{number}
2764 @item --hash-size=@var{number}
2765 Set the default size of the linker's hash tables to a prime number
2766 close to @var{number}. Increasing this value can reduce the length of
2767 time it takes the linker to perform its tasks, at the expense of
2768 increasing the linker's memory requirements. Similarly reducing this
2769 value can reduce the memory requirements at the expense of speed.
2771 @kindex --hash-style=@var{style}
2772 @item --hash-style=@var{style}
2773 Set the type of linker's hash table(s). @var{style} can be either
2774 @code{sysv} for classic ELF @code{.hash} section, @code{gnu} for
2775 new style GNU @code{.gnu.hash} section or @code{both} for both
2776 the classic ELF @code{.hash} and new style GNU @code{.gnu.hash}
2777 hash tables. The default depends upon how the linker was configured,
2778 but for most Linux based systems it will be @code{both}.
2780 @kindex --compress-debug-sections=none
2781 @kindex --compress-debug-sections=zlib
2782 @kindex --compress-debug-sections=zlib-gnu
2783 @kindex --compress-debug-sections=zlib-gabi
2784 @item --compress-debug-sections=none
2785 @itemx --compress-debug-sections=zlib
2786 @itemx --compress-debug-sections=zlib-gnu
2787 @itemx --compress-debug-sections=zlib-gabi
2788 On ELF platforms, these options control how DWARF debug sections are
2789 compressed using zlib.
2791 @option{--compress-debug-sections=none} doesn't compress DWARF debug
2792 sections. @option{--compress-debug-sections=zlib-gnu} compresses
2793 DWARF debug sections and renames them to begin with @samp{.zdebug}
2794 instead of @samp{.debug}. @option{--compress-debug-sections=zlib-gabi}
2795 also compresses DWARF debug sections, but rather than renaming them it
2796 sets the SHF_COMPRESSED flag in the sections' headers.
2798 The @option{--compress-debug-sections=zlib} option is an alias for
2799 @option{--compress-debug-sections=zlib-gabi}.
2801 Note that this option overrides any compression in input debug
2802 sections, so if a binary is linked with @option{--compress-debug-sections=none}
2803 for example, then any compressed debug sections in input files will be
2804 uncompressed before they are copied into the output binary.
2806 The default compression behaviour varies depending upon the target
2807 involved and the configure options used to build the toolchain. The
2808 default can be determined by examining the output from the linker's
2809 @option{--help} option.
2811 @kindex --reduce-memory-overheads
2812 @item --reduce-memory-overheads
2813 This option reduces memory requirements at ld runtime, at the expense of
2814 linking speed. This was introduced to select the old O(n^2) algorithm
2815 for link map file generation, rather than the new O(n) algorithm which uses
2816 about 40% more memory for symbol storage.
2818 Another effect of the switch is to set the default hash table size to
2819 1021, which again saves memory at the cost of lengthening the linker's
2820 run time. This is not done however if the @option{--hash-size} switch
2823 The @option{--reduce-memory-overheads} switch may be also be used to
2824 enable other tradeoffs in future versions of the linker.
2827 @kindex --build-id=@var{style}
2829 @itemx --build-id=@var{style}
2830 Request the creation of a @code{.note.gnu.build-id} ELF note section
2831 or a @code{.buildid} COFF section. The contents of the note are
2832 unique bits identifying this linked file. @var{style} can be
2833 @code{uuid} to use 128 random bits, @code{sha1} to use a 160-bit
2834 @sc{SHA1} hash on the normative parts of the output contents,
2835 @code{md5} to use a 128-bit @sc{MD5} hash on the normative parts of
2836 the output contents, or @code{0x@var{hexstring}} to use a chosen bit
2837 string specified as an even number of hexadecimal digits (@code{-} and
2838 @code{:} characters between digit pairs are ignored). If @var{style}
2839 is omitted, @code{sha1} is used.
2841 The @code{md5} and @code{sha1} styles produces an identifier
2842 that is always the same in an identical output file, but will be
2843 unique among all nonidentical output files. It is not intended
2844 to be compared as a checksum for the file's contents. A linked
2845 file may be changed later by other tools, but the build ID bit
2846 string identifying the original linked file does not change.
2848 Passing @code{none} for @var{style} disables the setting from any
2849 @code{--build-id} options earlier on the command line.
2854 @subsection Options Specific to i386 PE Targets
2856 @c man begin OPTIONS
2858 The i386 PE linker supports the @option{-shared} option, which causes
2859 the output to be a dynamically linked library (DLL) instead of a
2860 normal executable. You should name the output @code{*.dll} when you
2861 use this option. In addition, the linker fully supports the standard
2862 @code{*.def} files, which may be specified on the linker command line
2863 like an object file (in fact, it should precede archives it exports
2864 symbols from, to ensure that they get linked in, just like a normal
2867 In addition to the options common to all targets, the i386 PE linker
2868 support additional command-line options that are specific to the i386
2869 PE target. Options that take values may be separated from their
2870 values by either a space or an equals sign.
2874 @kindex --add-stdcall-alias
2875 @item --add-stdcall-alias
2876 If given, symbols with a stdcall suffix (@@@var{nn}) will be exported
2877 as-is and also with the suffix stripped.
2878 [This option is specific to the i386 PE targeted port of the linker]
2881 @item --base-file @var{file}
2882 Use @var{file} as the name of a file in which to save the base
2883 addresses of all the relocations needed for generating DLLs with
2885 [This is an i386 PE specific option]
2889 Create a DLL instead of a regular executable. You may also use
2890 @option{-shared} or specify a @code{LIBRARY} in a given @code{.def}
2892 [This option is specific to the i386 PE targeted port of the linker]
2894 @kindex --enable-long-section-names
2895 @kindex --disable-long-section-names
2896 @item --enable-long-section-names
2897 @itemx --disable-long-section-names
2898 The PE variants of the COFF object format add an extension that permits
2899 the use of section names longer than eight characters, the normal limit
2900 for COFF. By default, these names are only allowed in object files, as
2901 fully-linked executable images do not carry the COFF string table required
2902 to support the longer names. As a GNU extension, it is possible to
2903 allow their use in executable images as well, or to (probably pointlessly!)
2904 disallow it in object files, by using these two options. Executable images
2905 generated with these long section names are slightly non-standard, carrying
2906 as they do a string table, and may generate confusing output when examined
2907 with non-GNU PE-aware tools, such as file viewers and dumpers. However,
2908 GDB relies on the use of PE long section names to find Dwarf-2 debug
2909 information sections in an executable image at runtime, and so if neither
2910 option is specified on the command-line, @command{ld} will enable long
2911 section names, overriding the default and technically correct behaviour,
2912 when it finds the presence of debug information while linking an executable
2913 image and not stripping symbols.
2914 [This option is valid for all PE targeted ports of the linker]
2916 @kindex --enable-stdcall-fixup
2917 @kindex --disable-stdcall-fixup
2918 @item --enable-stdcall-fixup
2919 @itemx --disable-stdcall-fixup
2920 If the link finds a symbol that it cannot resolve, it will attempt to
2921 do ``fuzzy linking'' by looking for another defined symbol that differs
2922 only in the format of the symbol name (cdecl vs stdcall) and will
2923 resolve that symbol by linking to the match. For example, the
2924 undefined symbol @code{_foo} might be linked to the function
2925 @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked
2926 to the function @code{_bar}. When the linker does this, it prints a
2927 warning, since it normally should have failed to link, but sometimes
2928 import libraries generated from third-party dlls may need this feature
2929 to be usable. If you specify @option{--enable-stdcall-fixup}, this
2930 feature is fully enabled and warnings are not printed. If you specify
2931 @option{--disable-stdcall-fixup}, this feature is disabled and such
2932 mismatches are considered to be errors.
2933 [This option is specific to the i386 PE targeted port of the linker]
2935 @kindex --leading-underscore
2936 @kindex --no-leading-underscore
2937 @item --leading-underscore
2938 @itemx --no-leading-underscore
2939 For most targets default symbol-prefix is an underscore and is defined
2940 in target's description. By this option it is possible to
2941 disable/enable the default underscore symbol-prefix.
2943 @cindex DLLs, creating
2944 @kindex --export-all-symbols
2945 @item --export-all-symbols
2946 If given, all global symbols in the objects used to build a DLL will
2947 be exported by the DLL. Note that this is the default if there
2948 otherwise wouldn't be any exported symbols. When symbols are
2949 explicitly exported via DEF files or implicitly exported via function
2950 attributes, the default is to not export anything else unless this
2951 option is given. Note that the symbols @code{DllMain@@12},
2952 @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and
2953 @code{impure_ptr} will not be automatically
2954 exported. Also, symbols imported from other DLLs will not be
2955 re-exported, nor will symbols specifying the DLL's internal layout
2956 such as those beginning with @code{_head_} or ending with
2957 @code{_iname}. In addition, no symbols from @code{libgcc},
2958 @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported.
2959 Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will
2960 not be exported, to help with C++ DLLs. Finally, there is an
2961 extensive list of cygwin-private symbols that are not exported
2962 (obviously, this applies on when building DLLs for cygwin targets).
2963 These cygwin-excludes are: @code{_cygwin_dll_entry@@12},
2964 @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12},
2965 @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll},
2966 @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2},
2967 @code{cygwin_premain3}, and @code{environ}.
2968 [This option is specific to the i386 PE targeted port of the linker]
2970 @kindex --exclude-symbols
2971 @item --exclude-symbols @var{symbol},@var{symbol},...
2972 Specifies a list of symbols which should not be automatically
2973 exported. The symbol names may be delimited by commas or colons.
2974 [This option is specific to the i386 PE targeted port of the linker]
2976 @kindex --exclude-all-symbols
2977 @item --exclude-all-symbols
2978 Specifies no symbols should be automatically exported.
2979 [This option is specific to the i386 PE targeted port of the linker]
2981 @kindex --file-alignment
2982 @item --file-alignment
2983 Specify the file alignment. Sections in the file will always begin at
2984 file offsets which are multiples of this number. This defaults to
2986 [This option is specific to the i386 PE targeted port of the linker]
2990 @item --heap @var{reserve}
2991 @itemx --heap @var{reserve},@var{commit}
2992 Specify the number of bytes of memory to reserve (and optionally commit)
2993 to be used as heap for this program. The default is 1MB reserved, 4K
2995 [This option is specific to the i386 PE targeted port of the linker]
2998 @kindex --image-base
2999 @item --image-base @var{value}
3000 Use @var{value} as the base address of your program or dll. This is
3001 the lowest memory location that will be used when your program or dll
3002 is loaded. To reduce the need to relocate and improve performance of
3003 your dlls, each should have a unique base address and not overlap any
3004 other dlls. The default is 0x400000 for executables, and 0x10000000
3006 [This option is specific to the i386 PE targeted port of the linker]
3010 If given, the stdcall suffixes (@@@var{nn}) will be stripped from
3011 symbols before they are exported.
3012 [This option is specific to the i386 PE targeted port of the linker]
3014 @kindex --large-address-aware
3015 @item --large-address-aware
3016 If given, the appropriate bit in the ``Characteristics'' field of the COFF
3017 header is set to indicate that this executable supports virtual addresses
3018 greater than 2 gigabytes. This should be used in conjunction with the /3GB
3019 or /USERVA=@var{value} megabytes switch in the ``[operating systems]''
3020 section of the BOOT.INI. Otherwise, this bit has no effect.
3021 [This option is specific to PE targeted ports of the linker]
3023 @kindex --disable-large-address-aware
3024 @item --disable-large-address-aware
3025 Reverts the effect of a previous @samp{--large-address-aware} option.
3026 This is useful if @samp{--large-address-aware} is always set by the compiler
3027 driver (e.g. Cygwin gcc) and the executable does not support virtual
3028 addresses greater than 2 gigabytes.
3029 [This option is specific to PE targeted ports of the linker]
3031 @kindex --major-image-version
3032 @item --major-image-version @var{value}
3033 Sets the major number of the ``image version''. Defaults to 1.
3034 [This option is specific to the i386 PE targeted port of the linker]
3036 @kindex --major-os-version
3037 @item --major-os-version @var{value}
3038 Sets the major number of the ``os version''. Defaults to 4.
3039 [This option is specific to the i386 PE targeted port of the linker]
3041 @kindex --major-subsystem-version
3042 @item --major-subsystem-version @var{value}
3043 Sets the major number of the ``subsystem version''. Defaults to 4.
3044 [This option is specific to the i386 PE targeted port of the linker]
3046 @kindex --minor-image-version
3047 @item --minor-image-version @var{value}
3048 Sets the minor number of the ``image version''. Defaults to 0.
3049 [This option is specific to the i386 PE targeted port of the linker]
3051 @kindex --minor-os-version
3052 @item --minor-os-version @var{value}
3053 Sets the minor number of the ``os version''. Defaults to 0.
3054 [This option is specific to the i386 PE targeted port of the linker]
3056 @kindex --minor-subsystem-version
3057 @item --minor-subsystem-version @var{value}
3058 Sets the minor number of the ``subsystem version''. Defaults to 0.
3059 [This option is specific to the i386 PE targeted port of the linker]
3061 @cindex DEF files, creating
3062 @cindex DLLs, creating
3063 @kindex --output-def
3064 @item --output-def @var{file}
3065 The linker will create the file @var{file} which will contain a DEF
3066 file corresponding to the DLL the linker is generating. This DEF file
3067 (which should be called @code{*.def}) may be used to create an import
3068 library with @code{dlltool} or may be used as a reference to
3069 automatically or implicitly exported symbols.
3070 [This option is specific to the i386 PE targeted port of the linker]
3072 @cindex DLLs, creating
3073 @kindex --enable-auto-image-base
3074 @item --enable-auto-image-base
3075 @itemx --enable-auto-image-base=@var{value}
3076 Automatically choose the image base for DLLs, optionally starting with base
3077 @var{value}, unless one is specified using the @code{--image-base} argument.
3078 By using a hash generated from the dllname to create unique image bases
3079 for each DLL, in-memory collisions and relocations which can delay program
3080 execution are avoided.
3081 [This option is specific to the i386 PE targeted port of the linker]
3083 @kindex --disable-auto-image-base
3084 @item --disable-auto-image-base
3085 Do not automatically generate a unique image base. If there is no
3086 user-specified image base (@code{--image-base}) then use the platform
3088 [This option is specific to the i386 PE targeted port of the linker]
3090 @cindex DLLs, linking to
3091 @kindex --dll-search-prefix
3092 @item --dll-search-prefix @var{string}
3093 When linking dynamically to a dll without an import library,
3094 search for @code{<string><basename>.dll} in preference to
3095 @code{lib<basename>.dll}. This behaviour allows easy distinction
3096 between DLLs built for the various "subplatforms": native, cygwin,
3097 uwin, pw, etc. For instance, cygwin DLLs typically use
3098 @code{--dll-search-prefix=cyg}.
3099 [This option is specific to the i386 PE targeted port of the linker]
3101 @kindex --enable-auto-import
3102 @item --enable-auto-import
3103 Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for
3104 DATA imports from DLLs, thus making it possible to bypass the dllimport
3105 mechanism on the user side and to reference unmangled symbol names.
3106 [This option is specific to the i386 PE targeted port of the linker]
3108 The following remarks pertain to the original implementation of the
3109 feature and are obsolete nowadays for Cygwin and MinGW targets.
3111 Note: Use of the 'auto-import' extension will cause the text section
3112 of the image file to be made writable. This does not conform to the
3113 PE-COFF format specification published by Microsoft.
3115 Note - use of the 'auto-import' extension will also cause read only
3116 data which would normally be placed into the .rdata section to be
3117 placed into the .data section instead. This is in order to work
3118 around a problem with consts that is described here:
3119 http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
3121 Using 'auto-import' generally will 'just work' -- but sometimes you may
3124 "variable '<var>' can't be auto-imported. Please read the
3125 documentation for ld's @code{--enable-auto-import} for details."
3127 This message occurs when some (sub)expression accesses an address
3128 ultimately given by the sum of two constants (Win32 import tables only
3129 allow one). Instances where this may occur include accesses to member
3130 fields of struct variables imported from a DLL, as well as using a
3131 constant index into an array variable imported from a DLL. Any
3132 multiword variable (arrays, structs, long long, etc) may trigger
3133 this error condition. However, regardless of the exact data type
3134 of the offending exported variable, ld will always detect it, issue
3135 the warning, and exit.
3137 There are several ways to address this difficulty, regardless of the
3138 data type of the exported variable:
3140 One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
3141 of adjusting references in your client code for runtime environment, so
3142 this method works only when runtime environment supports this feature.
3144 A second solution is to force one of the 'constants' to be a variable --
3145 that is, unknown and un-optimizable at compile time. For arrays,
3146 there are two possibilities: a) make the indexee (the array's address)
3147 a variable, or b) make the 'constant' index a variable. Thus:
3150 extern type extern_array[];
3152 @{ volatile type *t=extern_array; t[1] @}
3158 extern type extern_array[];
3160 @{ volatile int t=1; extern_array[t] @}
3163 For structs (and most other multiword data types) the only option
3164 is to make the struct itself (or the long long, or the ...) variable:
3167 extern struct s extern_struct;
3168 extern_struct.field -->
3169 @{ volatile struct s *t=&extern_struct; t->field @}
3175 extern long long extern_ll;
3177 @{ volatile long long * local_ll=&extern_ll; *local_ll @}
3180 A third method of dealing with this difficulty is to abandon
3181 'auto-import' for the offending symbol and mark it with
3182 @code{__declspec(dllimport)}. However, in practice that
3183 requires using compile-time #defines to indicate whether you are
3184 building a DLL, building client code that will link to the DLL, or
3185 merely building/linking to a static library. In making the choice
3186 between the various methods of resolving the 'direct address with
3187 constant offset' problem, you should consider typical real-world usage:
3195 void main(int argc, char **argv)@{
3196 printf("%d\n",arr[1]);
3206 void main(int argc, char **argv)@{
3207 /* This workaround is for win32 and cygwin; do not "optimize" */
3208 volatile int *parr = arr;
3209 printf("%d\n",parr[1]);
3216 /* Note: auto-export is assumed (no __declspec(dllexport)) */
3217 #if (defined(_WIN32) || defined(__CYGWIN__)) && \
3218 !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
3219 #define FOO_IMPORT __declspec(dllimport)
3223 extern FOO_IMPORT int arr[];
3226 void main(int argc, char **argv)@{
3227 printf("%d\n",arr[1]);
3231 A fourth way to avoid this problem is to re-code your
3232 library to use a functional interface rather than a data interface
3233 for the offending variables (e.g. set_foo() and get_foo() accessor
3236 @kindex --disable-auto-import
3237 @item --disable-auto-import
3238 Do not attempt to do sophisticated linking of @code{_symbol} to
3239 @code{__imp__symbol} for DATA imports from DLLs.
3240 [This option is specific to the i386 PE targeted port of the linker]
3242 @kindex --enable-runtime-pseudo-reloc
3243 @item --enable-runtime-pseudo-reloc
3244 If your code contains expressions described in --enable-auto-import section,
3245 that is, DATA imports from DLL with non-zero offset, this switch will create
3246 a vector of 'runtime pseudo relocations' which can be used by runtime
3247 environment to adjust references to such data in your client code.
3248 [This option is specific to the i386 PE targeted port of the linker]
3250 @kindex --disable-runtime-pseudo-reloc
3251 @item --disable-runtime-pseudo-reloc
3252 Do not create pseudo relocations for non-zero offset DATA imports from DLLs.
3253 [This option is specific to the i386 PE targeted port of the linker]
3255 @kindex --enable-extra-pe-debug
3256 @item --enable-extra-pe-debug
3257 Show additional debug info related to auto-import symbol thunking.
3258 [This option is specific to the i386 PE targeted port of the linker]
3260 @kindex --section-alignment
3261 @item --section-alignment
3262 Sets the section alignment. Sections in memory will always begin at
3263 addresses which are a multiple of this number. Defaults to 0x1000.
3264 [This option is specific to the i386 PE targeted port of the linker]
3268 @item --stack @var{reserve}
3269 @itemx --stack @var{reserve},@var{commit}
3270 Specify the number of bytes of memory to reserve (and optionally commit)
3271 to be used as stack for this program. The default is 2MB reserved, 4K
3273 [This option is specific to the i386 PE targeted port of the linker]
3276 @item --subsystem @var{which}
3277 @itemx --subsystem @var{which}:@var{major}
3278 @itemx --subsystem @var{which}:@var{major}.@var{minor}
3279 Specifies the subsystem under which your program will execute. The
3280 legal values for @var{which} are @code{native}, @code{windows},
3281 @code{console}, @code{posix}, and @code{xbox}. You may optionally set
3282 the subsystem version also. Numeric values are also accepted for
3284 [This option is specific to the i386 PE targeted port of the linker]
3286 The following options set flags in the @code{DllCharacteristics} field
3287 of the PE file header:
3288 [These options are specific to PE targeted ports of the linker]
3290 @kindex --high-entropy-va
3291 @item --high-entropy-va
3292 @itemx --disable-high-entropy-va
3293 Image is compatible with 64-bit address space layout randomization
3294 (ASLR). This option is enabled by default for 64-bit PE images.
3296 This option also implies @option{--dynamicbase} and
3297 @option{--enable-reloc-section}.
3299 @kindex --dynamicbase
3301 @itemx --disable-dynamicbase
3302 The image base address may be relocated using address space layout
3303 randomization (ASLR). This feature was introduced with MS Windows
3304 Vista for i386 PE targets. This option is enabled by default but
3305 can be disabled via the @option{--disable-dynamicbase} option.
3306 This option also implies @option{--enable-reloc-section}.
3308 @kindex --forceinteg
3310 @itemx --disable-forceinteg
3311 Code integrity checks are enforced. This option is disabled by
3316 @item --disable-nxcompat
3317 The image is compatible with the Data Execution Prevention.
3318 This feature was introduced with MS Windows XP SP2 for i386 PE
3319 targets. The option is enabled by default.
3321 @kindex --no-isolation
3322 @item --no-isolation
3323 @itemx --disable-no-isolation
3324 Although the image understands isolation, do not isolate the image.
3325 This option is disabled by default.
3329 @itemx --disable-no-seh
3330 The image does not use SEH. No SE handler may be called from
3331 this image. This option is disabled by default.
3335 @itemx --disable-no-bind
3336 Do not bind this image. This option is disabled by default.
3340 @itemx --disable-wdmdriver
3341 The driver uses the MS Windows Driver Model. This option is disabled
3346 @itemx --disable-tsaware
3347 The image is Terminal Server aware. This option is disabled by
3350 @kindex --insert-timestamp
3351 @item --insert-timestamp
3352 @itemx --no-insert-timestamp
3353 Insert a real timestamp into the image. This is the default behaviour
3354 as it matches legacy code and it means that the image will work with
3355 other, proprietary tools. The problem with this default is that it
3356 will result in slightly different images being produced each time the
3357 same sources are linked. The option @option{--no-insert-timestamp}
3358 can be used to insert a zero value for the timestamp, this ensuring
3359 that binaries produced from identical sources will compare
3362 @kindex --enable-reloc-section
3363 @item --enable-reloc-section
3364 @itemx --disable-reloc-section
3365 Create the base relocation table, which is necessary if the image
3366 is loaded at a different image base than specified in the PE header.
3367 This option is enabled by default.
3373 @subsection Options specific to C6X uClinux targets
3375 @c man begin OPTIONS
3377 The C6X uClinux target uses a binary format called DSBT to support shared
3378 libraries. Each shared library in the system needs to have a unique index;
3379 all executables use an index of 0.
3384 @item --dsbt-size @var{size}
3385 This option sets the number of entries in the DSBT of the current executable
3386 or shared library to @var{size}. The default is to create a table with 64
3389 @kindex --dsbt-index
3390 @item --dsbt-index @var{index}
3391 This option sets the DSBT index of the current executable or shared library
3392 to @var{index}. The default is 0, which is appropriate for generating
3393 executables. If a shared library is generated with a DSBT index of 0, the
3394 @code{R_C6000_DSBT_INDEX} relocs are copied into the output file.
3396 @kindex --no-merge-exidx-entries
3397 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent
3398 exidx entries in frame unwind info.
3406 @subsection Options specific to C-SKY targets
3408 @c man begin OPTIONS
3412 @kindex --branch-stub on C-SKY
3414 This option enables linker branch relaxation by inserting branch stub
3415 sections when needed to extend the range of branches. This option is
3416 usually not required since C-SKY supports branch and call instructions that
3417 can access the full memory range and branch relaxation is normally handled by
3418 the compiler or assembler.
3420 @kindex --stub-group-size on C-SKY
3421 @item --stub-group-size=@var{N}
3422 This option allows finer control of linker branch stub creation.
3423 It sets the maximum size of a group of input sections that can
3424 be handled by one stub section. A negative value of @var{N} locates
3425 stub sections after their branches, while a positive value allows stub
3426 sections to appear either before or after the branches. Values of
3427 @samp{1} or @samp{-1} indicate that the
3428 linker should choose suitable defaults.
3436 @subsection Options specific to Motorola 68HC11 and 68HC12 targets
3438 @c man begin OPTIONS
3440 The 68HC11 and 68HC12 linkers support specific options to control the
3441 memory bank switching mapping and trampoline code generation.
3445 @kindex --no-trampoline
3446 @item --no-trampoline
3447 This option disables the generation of trampoline. By default a trampoline
3448 is generated for each far function which is called using a @code{jsr}
3449 instruction (this happens when a pointer to a far function is taken).
3451 @kindex --bank-window
3452 @item --bank-window @var{name}
3453 This option indicates to the linker the name of the memory region in
3454 the @samp{MEMORY} specification that describes the memory bank window.
3455 The definition of such region is then used by the linker to compute
3456 paging and addresses within the memory window.
3464 @subsection Options specific to Motorola 68K target
3466 @c man begin OPTIONS
3468 The following options are supported to control handling of GOT generation
3469 when linking for 68K targets.
3474 @item --got=@var{type}
3475 This option tells the linker which GOT generation scheme to use.
3476 @var{type} should be one of @samp{single}, @samp{negative},
3477 @samp{multigot} or @samp{target}. For more information refer to the
3478 Info entry for @file{ld}.
3486 @subsection Options specific to MIPS targets
3488 @c man begin OPTIONS
3490 The following options are supported to control microMIPS instruction
3491 generation and branch relocation checks for ISA mode transitions when
3492 linking for MIPS targets.
3500 These options control the choice of microMIPS instructions used in code
3501 generated by the linker, such as that in the PLT or lazy binding stubs,
3502 or in relaxation. If @samp{--insn32} is used, then the linker only uses
3503 32-bit instruction encodings. By default or if @samp{--no-insn32} is
3504 used, all instruction encodings are used, including 16-bit ones where
3507 @kindex --ignore-branch-isa
3508 @item --ignore-branch-isa
3509 @kindex --no-ignore-branch-isa
3510 @itemx --no-ignore-branch-isa
3511 These options control branch relocation checks for invalid ISA mode
3512 transitions. If @samp{--ignore-branch-isa} is used, then the linker
3513 accepts any branch relocations and any ISA mode transition required
3514 is lost in relocation calculation, except for some cases of @code{BAL}
3515 instructions which meet relaxation conditions and are converted to
3516 equivalent @code{JALX} instructions as the associated relocation is
3517 calculated. By default or if @samp{--no-ignore-branch-isa} is used
3518 a check is made causing the loss of an ISA mode transition to produce
3521 @kindex --compact-branches
3522 @item --compact-branches
3523 @kindex --no-compact-branches
3524 @itemx --no-compact-branches
3525 These options control the generation of compact instructions by the linker
3526 in the PLT entries for MIPS R6.
3535 @subsection Options specific to PDP11 targets
3537 @c man begin OPTIONS
3539 For the pdp11-aout target, three variants of the output format can be
3540 produced as selected by the following options. The default variant
3541 for pdp11-aout is the @samp{--omagic} option, whereas for other
3542 targets @samp{--nmagic} is the default. The @samp{--imagic} option is
3543 defined only for the pdp11-aout target, while the others are described
3544 here as they apply to the pdp11-aout target.
3553 Mark the output as @code{OMAGIC} (0407) in the @file{a.out} header to
3554 indicate that the text segment is not to be write-protected and
3555 shared. Since the text and data sections are both readable and
3556 writable, the data section is allocated immediately contiguous after
3557 the text segment. This is the oldest format for PDP11 executable
3558 programs and is the default for @command{ld} on PDP11 Unix systems
3559 from the beginning through 2.11BSD.
3566 Mark the output as @code{NMAGIC} (0410) in the @file{a.out} header to
3567 indicate that when the output file is executed, the text portion will
3568 be read-only and shareable among all processes executing the same
3569 file. This involves moving the data areas up to the first possible 8K
3570 byte page boundary following the end of the text. This option creates
3571 a @emph{pure executable} format.
3578 Mark the output as @code{IMAGIC} (0411) in the @file{a.out} header to
3579 indicate that when the output file is executed, the program text and
3580 data areas will be loaded into separate address spaces using the split
3581 instruction and data space feature of the memory management unit in
3582 larger models of the PDP11. This doubles the address space available
3583 to the program. The text segment is again pure, write-protected, and
3584 shareable. The only difference in the output format between this
3585 option and the others, besides the magic number, is that both the text
3586 and data sections start at location 0. The @samp{-z} option selected
3587 this format in 2.11BSD. This option creates a @emph{separate
3593 Equivalent to @samp{--nmagic} for pdp11-aout.
3602 @section Environment Variables
3604 @c man begin ENVIRONMENT
3606 You can change the behaviour of @command{ld} with the environment variables
3607 @ifclear SingleFormat
3610 @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}.
3612 @ifclear SingleFormat
3614 @cindex default input format
3615 @code{GNUTARGET} determines the input-file object format if you don't
3616 use @samp{-b} (or its synonym @samp{--format}). Its value should be one
3617 of the BFD names for an input format (@pxref{BFD}). If there is no
3618 @code{GNUTARGET} in the environment, @command{ld} uses the natural format
3619 of the target. If @code{GNUTARGET} is set to @code{default} then BFD
3620 attempts to discover the input format by examining binary input files;
3621 this method often succeeds, but there are potential ambiguities, since
3622 there is no method of ensuring that the magic number used to specify
3623 object-file formats is unique. However, the configuration procedure for
3624 BFD on each system places the conventional format for that system first
3625 in the search-list, so ambiguities are resolved in favor of convention.
3629 @cindex default emulation
3630 @cindex emulation, default
3631 @code{LDEMULATION} determines the default emulation if you don't use the
3632 @samp{-m} option. The emulation can affect various aspects of linker
3633 behaviour, particularly the default linker script. You can list the
3634 available emulations with the @samp{--verbose} or @samp{-V} options. If
3635 the @samp{-m} option is not used, and the @code{LDEMULATION} environment
3636 variable is not defined, the default emulation depends upon how the
3637 linker was configured.
3639 @kindex COLLECT_NO_DEMANGLE
3640 @cindex demangling, default
3641 Normally, the linker will default to demangling symbols. However, if
3642 @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will
3643 default to not demangling symbols. This environment variable is used in
3644 a similar fashion by the @code{gcc} linker wrapper program. The default
3645 may be overridden by the @samp{--demangle} and @samp{--no-demangle}
3652 @chapter Linker Scripts
3655 @cindex linker scripts
3656 @cindex command files
3657 Every link is controlled by a @dfn{linker script}. This script is
3658 written in the linker command language.
3660 The main purpose of the linker script is to describe how the sections in
3661 the input files should be mapped into the output file, and to control
3662 the memory layout of the output file. Most linker scripts do nothing
3663 more than this. However, when necessary, the linker script can also
3664 direct the linker to perform many other operations, using the commands
3667 The linker always uses a linker script. If you do not supply one
3668 yourself, the linker will use a default script that is compiled into the
3669 linker executable. You can use the @samp{--verbose} command-line option
3670 to display the default linker script. Certain command-line options,
3671 such as @samp{-r} or @samp{-N}, will affect the default linker script.
3673 You may supply your own linker script by using the @samp{-T} command
3674 line option. When you do this, your linker script will replace the
3675 default linker script.
3677 You may also use linker scripts implicitly by naming them as input files
3678 to the linker, as though they were files to be linked. @xref{Implicit
3682 * Basic Script Concepts:: Basic Linker Script Concepts
3683 * Script Format:: Linker Script Format
3684 * Simple Example:: Simple Linker Script Example
3685 * Simple Commands:: Simple Linker Script Commands
3686 * Assignments:: Assigning Values to Symbols
3687 * SECTIONS:: SECTIONS Command
3688 * MEMORY:: MEMORY Command
3689 * PHDRS:: PHDRS Command
3690 * VERSION:: VERSION Command
3691 * Expressions:: Expressions in Linker Scripts
3692 * Implicit Linker Scripts:: Implicit Linker Scripts
3695 @node Basic Script Concepts
3696 @section Basic Linker Script Concepts
3697 @cindex linker script concepts
3698 We need to define some basic concepts and vocabulary in order to
3699 describe the linker script language.
3701 The linker combines input files into a single output file. The output
3702 file and each input file are in a special data format known as an
3703 @dfn{object file format}. Each file is called an @dfn{object file}.
3704 The output file is often called an @dfn{executable}, but for our
3705 purposes we will also call it an object file. Each object file has,
3706 among other things, a list of @dfn{sections}. We sometimes refer to a
3707 section in an input file as an @dfn{input section}; similarly, a section
3708 in the output file is an @dfn{output section}.
3710 Each section in an object file has a name and a size. Most sections
3711 also have an associated block of data, known as the @dfn{section
3712 contents}. A section may be marked as @dfn{loadable}, which means that
3713 the contents should be loaded into memory when the output file is run.
3714 A section with no contents may be @dfn{allocatable}, which means that an
3715 area in memory should be set aside, but nothing in particular should be
3716 loaded there (in some cases this memory must be zeroed out). A section
3717 which is neither loadable nor allocatable typically contains some sort
3718 of debugging information.
3720 Every loadable or allocatable output section has two addresses. The
3721 first is the @dfn{VMA}, or virtual memory address. This is the address
3722 the section will have when the output file is run. The second is the
3723 @dfn{LMA}, or load memory address. This is the address at which the
3724 section will be loaded. In most cases the two addresses will be the
3725 same. An example of when they might be different is when a data section
3726 is loaded into ROM, and then copied into RAM when the program starts up
3727 (this technique is often used to initialize global variables in a ROM
3728 based system). In this case the ROM address would be the LMA, and the
3729 RAM address would be the VMA.
3731 You can see the sections in an object file by using the @code{objdump}
3732 program with the @samp{-h} option.
3734 Every object file also has a list of @dfn{symbols}, known as the
3735 @dfn{symbol table}. A symbol may be defined or undefined. Each symbol
3736 has a name, and each defined symbol has an address, among other
3737 information. If you compile a C or C++ program into an object file, you
3738 will get a defined symbol for every defined function and global or
3739 static variable. Every undefined function or global variable which is
3740 referenced in the input file will become an undefined symbol.
3742 You can see the symbols in an object file by using the @code{nm}
3743 program, or by using the @code{objdump} program with the @samp{-t}
3747 @section Linker Script Format
3748 @cindex linker script format
3749 Linker scripts are text files.
3751 You write a linker script as a series of commands. Each command is
3752 either a keyword, possibly followed by arguments, or an assignment to a
3753 symbol. You may separate commands using semicolons. Whitespace is
3756 Strings such as file or format names can normally be entered directly.
3757 If the file name contains a character such as a comma which would
3758 otherwise serve to separate file names, you may put the file name in
3759 double quotes. There is no way to use a double quote character in a
3762 You may include comments in linker scripts just as in C, delimited by
3763 @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent
3766 @node Simple Example
3767 @section Simple Linker Script Example
3768 @cindex linker script example
3769 @cindex example of linker script
3770 Many linker scripts are fairly simple.
3772 The simplest possible linker script has just one command:
3773 @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the
3774 memory layout of the output file.
3776 The @samp{SECTIONS} command is a powerful command. Here we will
3777 describe a simple use of it. Let's assume your program consists only of
3778 code, initialized data, and uninitialized data. These will be in the
3779 @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively.
3780 Let's assume further that these are the only sections which appear in
3783 For this example, let's say that the code should be loaded at address
3784 0x10000, and that the data should start at address 0x8000000. Here is a
3785 linker script which will do that:
3790 .text : @{ *(.text) @}
3792 .data : @{ *(.data) @}
3793 .bss : @{ *(.bss) @}
3797 You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS},
3798 followed by a series of symbol assignments and output section
3799 descriptions enclosed in curly braces.
3801 The first line inside the @samp{SECTIONS} command of the above example
3802 sets the value of the special symbol @samp{.}, which is the location
3803 counter. If you do not specify the address of an output section in some
3804 other way (other ways are described later), the address is set from the
3805 current value of the location counter. The location counter is then
3806 incremented by the size of the output section. At the start of the
3807 @samp{SECTIONS} command, the location counter has the value @samp{0}.
3809 The second line defines an output section, @samp{.text}. The colon is
3810 required syntax which may be ignored for now. Within the curly braces
3811 after the output section name, you list the names of the input sections
3812 which should be placed into this output section. The @samp{*} is a
3813 wildcard which matches any file name. The expression @samp{*(.text)}
3814 means all @samp{.text} input sections in all input files.
3816 Since the location counter is @samp{0x10000} when the output section
3817 @samp{.text} is defined, the linker will set the address of the
3818 @samp{.text} section in the output file to be @samp{0x10000}.
3820 The remaining lines define the @samp{.data} and @samp{.bss} sections in
3821 the output file. The linker will place the @samp{.data} output section
3822 at address @samp{0x8000000}. After the linker places the @samp{.data}
3823 output section, the value of the location counter will be
3824 @samp{0x8000000} plus the size of the @samp{.data} output section. The
3825 effect is that the linker will place the @samp{.bss} output section
3826 immediately after the @samp{.data} output section in memory.
3828 The linker will ensure that each output section has the required
3829 alignment, by increasing the location counter if necessary. In this
3830 example, the specified addresses for the @samp{.text} and @samp{.data}
3831 sections will probably satisfy any alignment constraints, but the linker
3832 may have to create a small gap between the @samp{.data} and @samp{.bss}
3835 That's it! That's a simple and complete linker script.
3837 @node Simple Commands
3838 @section Simple Linker Script Commands
3839 @cindex linker script simple commands
3840 In this section we describe the simple linker script commands.
3843 * Entry Point:: Setting the entry point
3844 * File Commands:: Commands dealing with files
3845 @ifclear SingleFormat
3846 * Format Commands:: Commands dealing with object file formats
3849 * REGION_ALIAS:: Assign alias names to memory regions
3850 * Miscellaneous Commands:: Other linker script commands
3854 @subsection Setting the Entry Point
3855 @kindex ENTRY(@var{symbol})
3856 @cindex start of execution
3857 @cindex first instruction
3859 The first instruction to execute in a program is called the @dfn{entry
3860 point}. You can use the @code{ENTRY} linker script command to set the
3861 entry point. The argument is a symbol name:
3866 There are several ways to set the entry point. The linker will set the
3867 entry point by trying each of the following methods in order, and
3868 stopping when one of them succeeds:
3871 the @samp{-e} @var{entry} command-line option;
3873 the @code{ENTRY(@var{symbol})} command in a linker script;
3875 the value of a target-specific symbol, if it is defined; For many
3876 targets this is @code{start}, but PE- and BeOS-based systems for example
3877 check a list of possible entry symbols, matching the first one found.
3879 the address of the first byte of the @samp{.text} section, if present;
3881 The address @code{0}.
3885 @subsection Commands Dealing with Files
3886 @cindex linker script file commands
3887 Several linker script commands deal with files.
3890 @item INCLUDE @var{filename}
3891 @kindex INCLUDE @var{filename}
3892 @cindex including a linker script
3893 Include the linker script @var{filename} at this point. The file will
3894 be searched for in the current directory, and in any directory specified
3895 with the @option{-L} option. You can nest calls to @code{INCLUDE} up to
3898 You can place @code{INCLUDE} directives at the top level, in @code{MEMORY} or
3899 @code{SECTIONS} commands, or in output section descriptions.
3901 @item INPUT(@var{file}, @var{file}, @dots{})
3902 @itemx INPUT(@var{file} @var{file} @dots{})
3903 @kindex INPUT(@var{files})
3904 @cindex input files in linker scripts
3905 @cindex input object files in linker scripts
3906 @cindex linker script input object files
3907 The @code{INPUT} command directs the linker to include the named files
3908 in the link, as though they were named on the command line.
3910 For example, if you always want to include @file{subr.o} any time you do
3911 a link, but you can't be bothered to put it on every link command line,
3912 then you can put @samp{INPUT (subr.o)} in your linker script.
3914 In fact, if you like, you can list all of your input files in the linker
3915 script, and then invoke the linker with nothing but a @samp{-T} option.
3917 In case a @dfn{sysroot prefix} is configured, and the filename starts
3918 with the @samp{/} character, and the script being processed was
3919 located inside the @dfn{sysroot prefix}, the filename will be looked
3920 for in the @dfn{sysroot prefix}. The @dfn{sysroot prefix} can also be forced by specifying
3921 @code{=} as the first character in the filename path, or prefixing the
3922 filename path with @code{$SYSROOT}. See also the description of
3923 @samp{-L} in @ref{Options,,Command-line Options}.
3925 If a @dfn{sysroot prefix} is not used then the linker will try to open
3926 the file in the directory containing the linker script. If it is not
3927 found the linker will then search the current directory. If it is still
3928 not found the linker will search through the archive library search
3931 If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the
3932 name to @code{lib@var{file}.a}, as with the command-line argument
3935 When you use the @code{INPUT} command in an implicit linker script, the
3936 files will be included in the link at the point at which the linker
3937 script file is included. This can affect archive searching.
3939 @item GROUP(@var{file}, @var{file}, @dots{})
3940 @itemx GROUP(@var{file} @var{file} @dots{})
3941 @kindex GROUP(@var{files})
3942 @cindex grouping input files
3943 The @code{GROUP} command is like @code{INPUT}, except that the named
3944 files should all be archives, and they are searched repeatedly until no
3945 new undefined references are created. See the description of @samp{-(}
3946 in @ref{Options,,Command-line Options}.
3948 @item AS_NEEDED(@var{file}, @var{file}, @dots{})
3949 @itemx AS_NEEDED(@var{file} @var{file} @dots{})
3950 @kindex AS_NEEDED(@var{files})
3951 This construct can appear only inside of the @code{INPUT} or @code{GROUP}
3952 commands, among other filenames. The files listed will be handled
3953 as if they appear directly in the @code{INPUT} or @code{GROUP} commands,
3954 with the exception of ELF shared libraries, that will be added only
3955 when they are actually needed. This construct essentially enables
3956 @option{--as-needed} option for all the files listed inside of it
3957 and restores previous @option{--as-needed} resp. @option{--no-as-needed}
3960 @item OUTPUT(@var{filename})
3961 @kindex OUTPUT(@var{filename})
3962 @cindex output file name in linker script
3963 The @code{OUTPUT} command names the output file. Using
3964 @code{OUTPUT(@var{filename})} in the linker script is exactly like using
3965 @samp{-o @var{filename}} on the command line (@pxref{Options,,Command
3966 Line Options}). If both are used, the command-line option takes
3969 You can use the @code{OUTPUT} command to define a default name for the
3970 output file other than the usual default of @file{a.out}.
3972 @item SEARCH_DIR(@var{path})
3973 @kindex SEARCH_DIR(@var{path})
3974 @cindex library search path in linker script
3975 @cindex archive search path in linker script
3976 @cindex search path in linker script
3977 The @code{SEARCH_DIR} command adds @var{path} to the list of paths where
3978 @command{ld} looks for archive libraries. Using
3979 @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}}
3980 on the command line (@pxref{Options,,Command-line Options}). If both
3981 are used, then the linker will search both paths. Paths specified using
3982 the command-line option are searched first.
3984 @item STARTUP(@var{filename})
3985 @kindex STARTUP(@var{filename})
3986 @cindex first input file
3987 The @code{STARTUP} command is just like the @code{INPUT} command, except
3988 that @var{filename} will become the first input file to be linked, as
3989 though it were specified first on the command line. This may be useful
3990 when using a system in which the entry point is always the start of the
3994 @ifclear SingleFormat
3995 @node Format Commands
3996 @subsection Commands Dealing with Object File Formats
3997 A couple of linker script commands deal with object file formats.
4000 @item OUTPUT_FORMAT(@var{bfdname})
4001 @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little})
4002 @kindex OUTPUT_FORMAT(@var{bfdname})
4003 @cindex output file format in linker script
4004 The @code{OUTPUT_FORMAT} command names the BFD format to use for the
4005 output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is
4006 exactly like using @samp{--oformat @var{bfdname}} on the command line
4007 (@pxref{Options,,Command-line Options}). If both are used, the command
4008 line option takes precedence.
4010 You can use @code{OUTPUT_FORMAT} with three arguments to use different
4011 formats based on the @samp{-EB} and @samp{-EL} command-line options.
4012 This permits the linker script to set the output format based on the
4015 If neither @samp{-EB} nor @samp{-EL} are used, then the output format
4016 will be the first argument, @var{default}. If @samp{-EB} is used, the
4017 output format will be the second argument, @var{big}. If @samp{-EL} is
4018 used, the output format will be the third argument, @var{little}.
4020 For example, the default linker script for the MIPS ELF target uses this
4023 OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
4025 This says that the default format for the output file is
4026 @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command-line
4027 option, the output file will be created in the @samp{elf32-littlemips}
4030 @item TARGET(@var{bfdname})
4031 @kindex TARGET(@var{bfdname})
4032 @cindex input file format in linker script
4033 The @code{TARGET} command names the BFD format to use when reading input
4034 files. It affects subsequent @code{INPUT} and @code{GROUP} commands.
4035 This command is like using @samp{-b @var{bfdname}} on the command line
4036 (@pxref{Options,,Command-line Options}). If the @code{TARGET} command
4037 is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET}
4038 command is also used to set the format for the output file. @xref{BFD}.
4043 @subsection Assign alias names to memory regions
4044 @kindex REGION_ALIAS(@var{alias}, @var{region})
4045 @cindex region alias
4046 @cindex region names
4048 Alias names can be added to existing memory regions created with the
4049 @ref{MEMORY} command. Each name corresponds to at most one memory region.
4052 REGION_ALIAS(@var{alias}, @var{region})
4055 The @code{REGION_ALIAS} function creates an alias name @var{alias} for the
4056 memory region @var{region}. This allows a flexible mapping of output sections
4057 to memory regions. An example follows.
4059 Suppose we have an application for embedded systems which come with various
4060 memory storage devices. All have a general purpose, volatile memory @code{RAM}
4061 that allows code execution or data storage. Some may have a read-only,
4062 non-volatile memory @code{ROM} that allows code execution and read-only data
4063 access. The last variant is a read-only, non-volatile memory @code{ROM2} with
4064 read-only data access and no code execution capability. We have four output
4069 @code{.text} program code;
4071 @code{.rodata} read-only data;
4073 @code{.data} read-write initialized data;
4075 @code{.bss} read-write zero initialized data.
4078 The goal is to provide a linker command file that contains a system independent
4079 part defining the output sections and a system dependent part mapping the
4080 output sections to the memory regions available on the system. Our embedded
4081 systems come with three different memory setups @code{A}, @code{B} and
4083 @multitable @columnfractions .25 .25 .25 .25
4084 @item Section @tab Variant A @tab Variant B @tab Variant C
4085 @item .text @tab RAM @tab ROM @tab ROM
4086 @item .rodata @tab RAM @tab ROM @tab ROM2
4087 @item .data @tab RAM @tab RAM/ROM @tab RAM/ROM2
4088 @item .bss @tab RAM @tab RAM @tab RAM
4090 The notation @code{RAM/ROM} or @code{RAM/ROM2} means that this section is
4091 loaded into region @code{ROM} or @code{ROM2} respectively. Please note that
4092 the load address of the @code{.data} section starts in all three variants at
4093 the end of the @code{.rodata} section.
4095 The base linker script that deals with the output sections follows. It
4096 includes the system dependent @code{linkcmds.memory} file that describes the
4099 INCLUDE linkcmds.memory
4112 .data : AT (rodata_end)
4117 data_size = SIZEOF(.data);
4118 data_load_start = LOADADDR(.data);
4126 Now we need three different @code{linkcmds.memory} files to define memory
4127 regions and alias names. The content of @code{linkcmds.memory} for the three
4128 variants @code{A}, @code{B} and @code{C}:
4131 Here everything goes into the @code{RAM}.
4135 RAM : ORIGIN = 0, LENGTH = 4M
4138 REGION_ALIAS("REGION_TEXT", RAM);
4139 REGION_ALIAS("REGION_RODATA", RAM);
4140 REGION_ALIAS("REGION_DATA", RAM);
4141 REGION_ALIAS("REGION_BSS", RAM);
4144 Program code and read-only data go into the @code{ROM}. Read-write data goes
4145 into the @code{RAM}. An image of the initialized data is loaded into the
4146 @code{ROM} and will be copied during system start into the @code{RAM}.
4150 ROM : ORIGIN = 0, LENGTH = 3M
4151 RAM : ORIGIN = 0x10000000, LENGTH = 1M
4154 REGION_ALIAS("REGION_TEXT", ROM);
4155 REGION_ALIAS("REGION_RODATA", ROM);
4156 REGION_ALIAS("REGION_DATA", RAM);
4157 REGION_ALIAS("REGION_BSS", RAM);
4160 Program code goes into the @code{ROM}. Read-only data goes into the
4161 @code{ROM2}. Read-write data goes into the @code{RAM}. An image of the
4162 initialized data is loaded into the @code{ROM2} and will be copied during
4163 system start into the @code{RAM}.
4167 ROM : ORIGIN = 0, LENGTH = 2M
4168 ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
4169 RAM : ORIGIN = 0x20000000, LENGTH = 1M
4172 REGION_ALIAS("REGION_TEXT", ROM);
4173 REGION_ALIAS("REGION_RODATA", ROM2);
4174 REGION_ALIAS("REGION_DATA", RAM);
4175 REGION_ALIAS("REGION_BSS", RAM);
4179 It is possible to write a common system initialization routine to copy the
4180 @code{.data} section from @code{ROM} or @code{ROM2} into the @code{RAM} if
4185 extern char data_start [];
4186 extern char data_size [];
4187 extern char data_load_start [];
4189 void copy_data(void)
4191 if (data_start != data_load_start)
4193 memcpy(data_start, data_load_start, (size_t) data_size);
4198 @node Miscellaneous Commands
4199 @subsection Other Linker Script Commands
4200 There are a few other linker scripts commands.
4203 @item ASSERT(@var{exp}, @var{message})
4205 @cindex assertion in linker script
4206 Ensure that @var{exp} is non-zero. If it is zero, then exit the linker
4207 with an error code, and print @var{message}.
4209 Note that assertions are checked before the final stages of linking
4210 take place. This means that expressions involving symbols PROVIDEd
4211 inside section definitions will fail if the user has not set values
4212 for those symbols. The only exception to this rule is PROVIDEd
4213 symbols that just reference dot. Thus an assertion like this:
4218 PROVIDE (__stack = .);
4219 PROVIDE (__stack_size = 0x100);
4220 ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
4224 will fail if @code{__stack_size} is not defined elsewhere. Symbols
4225 PROVIDEd outside of section definitions are evaluated earlier, so they
4226 can be used inside ASSERTions. Thus:
4229 PROVIDE (__stack_size = 0x100);
4232 PROVIDE (__stack = .);
4233 ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
4239 @item EXTERN(@var{symbol} @var{symbol} @dots{})
4241 @cindex undefined symbol in linker script
4242 Force @var{symbol} to be entered in the output file as an undefined
4243 symbol. Doing this may, for example, trigger linking of additional
4244 modules from standard libraries. You may list several @var{symbol}s for
4245 each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This
4246 command has the same effect as the @samp{-u} command-line option.
4248 @item FORCE_COMMON_ALLOCATION
4249 @kindex FORCE_COMMON_ALLOCATION
4250 @cindex common allocation in linker script
4251 This command has the same effect as the @samp{-d} command-line option:
4252 to make @command{ld} assign space to common symbols even if a relocatable
4253 output file is specified (@samp{-r}).
4255 @item INHIBIT_COMMON_ALLOCATION
4256 @kindex INHIBIT_COMMON_ALLOCATION
4257 @cindex common allocation in linker script
4258 This command has the same effect as the @samp{--no-define-common}
4259 command-line option: to make @code{ld} omit the assignment of addresses
4260 to common symbols even for a non-relocatable output file.
4262 @item FORCE_GROUP_ALLOCATION
4263 @kindex FORCE_GROUP_ALLOCATION
4264 @cindex group allocation in linker script
4265 @cindex section groups
4267 This command has the same effect as the
4268 @samp{--force-group-allocation} command-line option: to make
4269 @command{ld} place section group members like normal input sections,
4270 and to delete the section groups even if a relocatable output file is
4271 specified (@samp{-r}).
4273 @item INSERT [ AFTER | BEFORE ] @var{output_section}
4275 @cindex insert user script into default script
4276 This command is typically used in a script specified by @samp{-T} to
4277 augment the default @code{SECTIONS} with, for example, overlays. It
4278 inserts all prior linker script statements after (or before)
4279 @var{output_section}, and also causes @samp{-T} to not override the
4280 default linker script. The exact insertion point is as for orphan
4281 sections. @xref{Location Counter}. The insertion happens after the
4282 linker has mapped input sections to output sections. Prior to the
4283 insertion, since @samp{-T} scripts are parsed before the default
4284 linker script, statements in the @samp{-T} script occur before the
4285 default linker script statements in the internal linker representation
4286 of the script. In particular, input section assignments will be made
4287 to @samp{-T} output sections before those in the default script. Here
4288 is an example of how a @samp{-T} script using @code{INSERT} might look:
4295 .ov1 @{ ov1*(.text) @}
4296 .ov2 @{ ov2*(.text) @}
4302 @item NOCROSSREFS(@var{section} @var{section} @dots{})
4303 @kindex NOCROSSREFS(@var{sections})
4304 @cindex cross references
4305 This command may be used to tell @command{ld} to issue an error about any
4306 references among certain output sections.
4308 In certain types of programs, particularly on embedded systems when
4309 using overlays, when one section is loaded into memory, another section
4310 will not be. Any direct references between the two sections would be
4311 errors. For example, it would be an error if code in one section called
4312 a function defined in the other section.
4314 The @code{NOCROSSREFS} command takes a list of output section names. If
4315 @command{ld} detects any cross references between the sections, it reports
4316 an error and returns a non-zero exit status. Note that the
4317 @code{NOCROSSREFS} command uses output section names, not input section
4320 @item NOCROSSREFS_TO(@var{tosection} @var{fromsection} @dots{})
4321 @kindex NOCROSSREFS_TO(@var{tosection} @var{fromsections})
4322 @cindex cross references
4323 This command may be used to tell @command{ld} to issue an error about any
4324 references to one section from a list of other sections.
4326 The @code{NOCROSSREFS} command is useful when ensuring that two or more
4327 output sections are entirely independent but there are situations where
4328 a one-way dependency is needed. For example, in a multi-core application
4329 there may be shared code that can be called from each core but for safety
4330 must never call back.
4332 The @code{NOCROSSREFS_TO} command takes a list of output section names.
4333 The first section can not be referenced from any of the other sections.
4334 If @command{ld} detects any references to the first section from any of
4335 the other sections, it reports an error and returns a non-zero exit
4336 status. Note that the @code{NOCROSSREFS_TO} command uses output section
4337 names, not input section names.
4339 @ifclear SingleFormat
4340 @item OUTPUT_ARCH(@var{bfdarch})
4341 @kindex OUTPUT_ARCH(@var{bfdarch})
4342 @cindex machine architecture
4343 @cindex architecture
4344 Specify a particular output machine architecture. The argument is one
4345 of the names used by the BFD library (@pxref{BFD}). You can see the
4346 architecture of an object file by using the @code{objdump} program with
4347 the @samp{-f} option.
4350 @item LD_FEATURE(@var{string})
4351 @kindex LD_FEATURE(@var{string})
4352 This command may be used to modify @command{ld} behavior. If
4353 @var{string} is @code{"SANE_EXPR"} then absolute symbols and numbers
4354 in a script are simply treated as numbers everywhere.
4355 @xref{Expression Section}.
4359 @section Assigning Values to Symbols
4360 @cindex assignment in scripts
4361 @cindex symbol definition, scripts
4362 @cindex variables, defining
4363 You may assign a value to a symbol in a linker script. This will define
4364 the symbol and place it into the symbol table with a global scope.
4367 * Simple Assignments:: Simple Assignments
4370 * PROVIDE_HIDDEN:: PROVIDE_HIDDEN
4371 * Source Code Reference:: How to use a linker script defined symbol in source code
4374 @node Simple Assignments
4375 @subsection Simple Assignments
4377 You may assign to a symbol using any of the C assignment operators:
4380 @item @var{symbol} = @var{expression} ;
4381 @itemx @var{symbol} += @var{expression} ;
4382 @itemx @var{symbol} -= @var{expression} ;
4383 @itemx @var{symbol} *= @var{expression} ;
4384 @itemx @var{symbol} /= @var{expression} ;
4385 @itemx @var{symbol} <<= @var{expression} ;
4386 @itemx @var{symbol} >>= @var{expression} ;
4387 @itemx @var{symbol} &= @var{expression} ;
4388 @itemx @var{symbol} |= @var{expression} ;
4391 The first case will define @var{symbol} to the value of
4392 @var{expression}. In the other cases, @var{symbol} must already be
4393 defined, and the value will be adjusted accordingly.
4395 The special symbol name @samp{.} indicates the location counter. You
4396 may only use this within a @code{SECTIONS} command. @xref{Location Counter}.
4398 The semicolon after @var{expression} is required.
4400 Expressions are defined below; see @ref{Expressions}.
4402 You may write symbol assignments as commands in their own right, or as
4403 statements within a @code{SECTIONS} command, or as part of an output
4404 section description in a @code{SECTIONS} command.
4406 The section of the symbol will be set from the section of the
4407 expression; for more information, see @ref{Expression Section}.
4409 Here is an example showing the three different places that symbol
4410 assignments may be used:
4421 _bdata = (. + 3) & ~ 3;
4422 .data : @{ *(.data) @}
4426 In this example, the symbol @samp{floating_point} will be defined as
4427 zero. The symbol @samp{_etext} will be defined as the address following
4428 the last @samp{.text} input section. The symbol @samp{_bdata} will be
4429 defined as the address following the @samp{.text} output section aligned
4430 upward to a 4 byte boundary.
4435 For ELF targeted ports, define a symbol that will be hidden and won't be
4436 exported. The syntax is @code{HIDDEN(@var{symbol} = @var{expression})}.
4438 Here is the example from @ref{Simple Assignments}, rewritten to use
4442 HIDDEN(floating_point = 0);
4450 HIDDEN(_bdata = (. + 3) & ~ 3);
4451 .data : @{ *(.data) @}
4455 In this case none of the three symbols will be visible outside this module.
4460 In some cases, it is desirable for a linker script to define a symbol
4461 only if it is referenced and is not defined by any object included in
4462 the link. For example, traditional linkers defined the symbol
4463 @samp{etext}. However, ANSI C requires that the user be able to use
4464 @samp{etext} as a function name without encountering an error. The
4465 @code{PROVIDE} keyword may be used to define a symbol, such as
4466 @samp{etext}, only if it is referenced but not defined. The syntax is
4467 @code{PROVIDE(@var{symbol} = @var{expression})}.
4469 Here is an example of using @code{PROVIDE} to define @samp{etext}:
4482 In this example, if the program defines @samp{_etext} (with a leading
4483 underscore), the linker will give a multiple definition diagnostic. If,
4484 on the other hand, the program defines @samp{etext} (with no leading
4485 underscore), the linker will silently use the definition in the program.
4486 If the program references @samp{etext} but does not define it, the
4487 linker will use the definition in the linker script.
4489 Note - the @code{PROVIDE} directive considers a common symbol to be
4490 defined, even though such a symbol could be combined with the symbol
4491 that the @code{PROVIDE} would create. This is particularly important
4492 when considering constructor and destructor list symbols such as
4493 @samp{__CTOR_LIST__} as these are often defined as common symbols.
4495 @node PROVIDE_HIDDEN
4496 @subsection PROVIDE_HIDDEN
4497 @cindex PROVIDE_HIDDEN
4498 Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be
4499 hidden and won't be exported.
4501 @node Source Code Reference
4502 @subsection Source Code Reference
4504 Accessing a linker script defined variable from source code is not
4505 intuitive. In particular a linker script symbol is not equivalent to
4506 a variable declaration in a high level language, it is instead a
4507 symbol that does not have a value.
4509 Before going further, it is important to note that compilers often
4510 transform names in the source code into different names when they are
4511 stored in the symbol table. For example, Fortran compilers commonly
4512 prepend or append an underscore, and C++ performs extensive @samp{name
4513 mangling}. Therefore there might be a discrepancy between the name
4514 of a variable as it is used in source code and the name of the same
4515 variable as it is defined in a linker script. For example in C a
4516 linker script variable might be referred to as:
4522 But in the linker script it might be defined as:
4528 In the remaining examples however it is assumed that no name
4529 transformation has taken place.
4531 When a symbol is declared in a high level language such as C, two
4532 things happen. The first is that the compiler reserves enough space
4533 in the program's memory to hold the @emph{value} of the symbol. The
4534 second is that the compiler creates an entry in the program's symbol
4535 table which holds the symbol's @emph{address}. ie the symbol table
4536 contains the address of the block of memory holding the symbol's
4537 value. So for example the following C declaration, at file scope:
4543 creates an entry called @samp{foo} in the symbol table. This entry
4544 holds the address of an @samp{int} sized block of memory where the
4545 number 1000 is initially stored.
4547 When a program references a symbol the compiler generates code that
4548 first accesses the symbol table to find the address of the symbol's
4549 memory block and then code to read the value from that memory block.
4556 looks up the symbol @samp{foo} in the symbol table, gets the address
4557 associated with this symbol and then writes the value 1 into that
4564 looks up the symbol @samp{foo} in the symbol table, gets its address
4565 and then copies this address into the block of memory associated with
4566 the variable @samp{a}.
4568 Linker scripts symbol declarations, by contrast, create an entry in
4569 the symbol table but do not assign any memory to them. Thus they are
4570 an address without a value. So for example the linker script definition:
4576 creates an entry in the symbol table called @samp{foo} which holds
4577 the address of memory location 1000, but nothing special is stored at
4578 address 1000. This means that you cannot access the @emph{value} of a
4579 linker script defined symbol - it has no value - all you can do is
4580 access the @emph{address} of a linker script defined symbol.
4582 Hence when you are using a linker script defined symbol in source code
4583 you should always take the address of the symbol, and never attempt to
4584 use its value. For example suppose you want to copy the contents of a
4585 section of memory called .ROM into a section called .FLASH and the
4586 linker script contains these declarations:
4590 start_of_ROM = .ROM;
4591 end_of_ROM = .ROM + sizeof (.ROM);
4592 start_of_FLASH = .FLASH;
4596 Then the C source code to perform the copy would be:
4600 extern char start_of_ROM, end_of_ROM, start_of_FLASH;
4602 memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
4606 Note the use of the @samp{&} operators. These are correct.
4607 Alternatively the symbols can be treated as the names of vectors or
4608 arrays and then the code will again work as expected:
4612 extern char start_of_ROM[], end_of_ROM[], start_of_FLASH[];
4614 memcpy (start_of_FLASH, start_of_ROM, end_of_ROM - start_of_ROM);
4618 Note how using this method does not require the use of @samp{&}
4622 @section SECTIONS Command
4624 The @code{SECTIONS} command tells the linker how to map input sections
4625 into output sections, and how to place the output sections in memory.
4627 The format of the @code{SECTIONS} command is:
4631 @var{sections-command}
4632 @var{sections-command}
4637 Each @var{sections-command} may of be one of the following:
4641 an @code{ENTRY} command (@pxref{Entry Point,,Entry command})
4643 a symbol assignment (@pxref{Assignments})
4645 an output section description
4647 an overlay description
4650 The @code{ENTRY} command and symbol assignments are permitted inside the
4651 @code{SECTIONS} command for convenience in using the location counter in
4652 those commands. This can also make the linker script easier to
4653 understand because you can use those commands at meaningful points in
4654 the layout of the output file.
4656 Output section descriptions and overlay descriptions are described
4659 If you do not use a @code{SECTIONS} command in your linker script, the
4660 linker will place each input section into an identically named output
4661 section in the order that the sections are first encountered in the
4662 input files. If all input sections are present in the first file, for
4663 example, the order of sections in the output file will match the order
4664 in the first input file. The first section will be at address zero.
4667 * Output Section Description:: Output section description
4668 * Output Section Name:: Output section name
4669 * Output Section Address:: Output section address
4670 * Input Section:: Input section description
4671 * Output Section Data:: Output section data
4672 * Output Section Keywords:: Output section keywords
4673 * Output Section Discarding:: Output section discarding
4674 * Output Section Attributes:: Output section attributes
4675 * Overlay Description:: Overlay description
4678 @node Output Section Description
4679 @subsection Output Section Description
4680 The full description of an output section looks like this:
4683 @var{section} [@var{address}] [(@var{type})] :
4685 [ALIGN(@var{section_align}) | ALIGN_WITH_INPUT]
4686 [SUBALIGN(@var{subsection_align})]
4689 @var{output-section-command}
4690 @var{output-section-command}
4692 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] [,]
4696 Most output sections do not use most of the optional section attributes.
4698 The whitespace around @var{section} is required, so that the section
4699 name is unambiguous. The colon and the curly braces are also required.
4700 The comma at the end may be required if a @var{fillexp} is used and
4701 the next @var{sections-command} looks like a continuation of the expression.
4702 The line breaks and other white space are optional.
4704 Each @var{output-section-command} may be one of the following:
4708 a symbol assignment (@pxref{Assignments})
4710 an input section description (@pxref{Input Section})
4712 data values to include directly (@pxref{Output Section Data})
4714 a special output section keyword (@pxref{Output Section Keywords})
4717 @node Output Section Name
4718 @subsection Output Section Name
4719 @cindex name, section
4720 @cindex section name
4721 The name of the output section is @var{section}. @var{section} must
4722 meet the constraints of your output format. In formats which only
4723 support a limited number of sections, such as @code{a.out}, the name
4724 must be one of the names supported by the format (@code{a.out}, for
4725 example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the
4726 output format supports any number of sections, but with numbers and not
4727 names (as is the case for Oasys), the name should be supplied as a
4728 quoted numeric string. A section name may consist of any sequence of
4729 characters, but a name which contains any unusual characters such as
4730 commas must be quoted.
4732 The output section name @samp{/DISCARD/} is special; @ref{Output Section
4735 @node Output Section Address
4736 @subsection Output Section Address
4737 @cindex address, section
4738 @cindex section address
4739 The @var{address} is an expression for the VMA (the virtual memory
4740 address) of the output section. This address is optional, but if it
4741 is provided then the output address will be set exactly as specified.
4743 If the output address is not specified then one will be chosen for the
4744 section, based on the heuristic below. This address will be adjusted
4745 to fit the alignment requirement of the output section. The
4746 alignment requirement is the strictest alignment of any input section
4747 contained within the output section.
4749 The output section address heuristic is as follows:
4753 If an output memory @var{region} is set for the section then it
4754 is added to this region and its address will be the next free address
4758 If the MEMORY command has been used to create a list of memory
4759 regions then the first region which has attributes compatible with the
4760 section is selected to contain it. The section's output address will
4761 be the next free address in that region; @ref{MEMORY}.
4764 If no memory regions were specified, or none match the section then
4765 the output address will be based on the current value of the location
4773 .text . : @{ *(.text) @}
4780 .text : @{ *(.text) @}
4784 are subtly different. The first will set the address of the
4785 @samp{.text} output section to the current value of the location
4786 counter. The second will set it to the current value of the location
4787 counter aligned to the strictest alignment of any of the @samp{.text}
4790 The @var{address} may be an arbitrary expression; @ref{Expressions}.
4791 For example, if you want to align the section on a 0x10 byte boundary,
4792 so that the lowest four bits of the section address are zero, you could
4793 do something like this:
4795 .text ALIGN(0x10) : @{ *(.text) @}
4798 This works because @code{ALIGN} returns the current location counter
4799 aligned upward to the specified value.
4801 Specifying @var{address} for a section will change the value of the
4802 location counter, provided that the section is non-empty. (Empty
4803 sections are ignored).
4806 @subsection Input Section Description
4807 @cindex input sections
4808 @cindex mapping input sections to output sections
4809 The most common output section command is an input section description.
4811 The input section description is the most basic linker script operation.
4812 You use output sections to tell the linker how to lay out your program
4813 in memory. You use input section descriptions to tell the linker how to
4814 map the input files into your memory layout.
4817 * Input Section Basics:: Input section basics
4818 * Input Section Wildcards:: Input section wildcard patterns
4819 * Input Section Common:: Input section for common symbols
4820 * Input Section Keep:: Input section and garbage collection
4821 * Input Section Example:: Input section example
4824 @node Input Section Basics
4825 @subsubsection Input Section Basics
4826 @cindex input section basics
4827 An input section description consists of a file name optionally followed
4828 by a list of section names in parentheses.
4830 The file name and the section name may be wildcard patterns, which we
4831 describe further below (@pxref{Input Section Wildcards}).
4833 The most common input section description is to include all input
4834 sections with a particular name in the output section. For example, to
4835 include all input @samp{.text} sections, you would write:
4840 Here the @samp{*} is a wildcard which matches any file name. To exclude a list
4841 @cindex EXCLUDE_FILE
4842 of files from matching the file name wildcard, EXCLUDE_FILE may be used to
4843 match all files except the ones specified in the EXCLUDE_FILE list. For
4846 EXCLUDE_FILE (*crtend.o *otherfile.o) *(.ctors)
4849 will cause all .ctors sections from all files except @file{crtend.o}
4850 and @file{otherfile.o} to be included. The EXCLUDE_FILE can also be
4851 placed inside the section list, for example:
4853 *(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
4856 The result of this is identically to the previous example. Supporting
4857 two syntaxes for EXCLUDE_FILE is useful if the section list contains
4858 more than one section, as described below.
4860 There are two ways to include more than one section:
4866 The difference between these is the order in which the @samp{.text} and
4867 @samp{.rdata} input sections will appear in the output section. In the
4868 first example, they will be intermingled, appearing in the same order as
4869 they are found in the linker input. In the second example, all
4870 @samp{.text} input sections will appear first, followed by all
4871 @samp{.rdata} input sections.
4873 When using EXCLUDE_FILE with more than one section, if the exclusion
4874 is within the section list then the exclusion only applies to the
4875 immediately following section, for example:
4877 *(EXCLUDE_FILE (*somefile.o) .text .rdata)
4880 will cause all @samp{.text} sections from all files except
4881 @file{somefile.o} to be included, while all @samp{.rdata} sections
4882 from all files, including @file{somefile.o}, will be included. To
4883 exclude the @samp{.rdata} sections from @file{somefile.o} the example
4884 could be modified to:
4886 *(EXCLUDE_FILE (*somefile.o) .text EXCLUDE_FILE (*somefile.o) .rdata)
4889 Alternatively, placing the EXCLUDE_FILE outside of the section list,
4890 before the input file selection, will cause the exclusion to apply for
4891 all sections. Thus the previous example can be rewritten as:
4893 EXCLUDE_FILE (*somefile.o) *(.text .rdata)
4896 You can specify a file name to include sections from a particular file.
4897 You would do this if one or more of your files contain special data that
4898 needs to be at a particular location in memory. For example:
4903 To refine the sections that are included based on the section flags
4904 of an input section, INPUT_SECTION_FLAGS may be used.
4906 Here is a simple example for using Section header flags for ELF sections:
4911 .text : @{ INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) @}
4912 .text2 : @{ INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) @}
4917 In this example, the output section @samp{.text} will be comprised of any
4918 input section matching the name *(.text) whose section header flags
4919 @code{SHF_MERGE} and @code{SHF_STRINGS} are set. The output section
4920 @samp{.text2} will be comprised of any input section matching the name *(.text)
4921 whose section header flag @code{SHF_WRITE} is clear.
4923 You can also specify files within archives by writing a pattern
4924 matching the archive, a colon, then the pattern matching the file,
4925 with no whitespace around the colon.
4929 matches file within archive
4931 matches the whole archive
4933 matches file but not one in an archive
4936 Either one or both of @samp{archive} and @samp{file} can contain shell
4937 wildcards. On DOS based file systems, the linker will assume that a
4938 single letter followed by a colon is a drive specifier, so
4939 @samp{c:myfile.o} is a simple file specification, not @samp{myfile.o}
4940 within an archive called @samp{c}. @samp{archive:file} filespecs may
4941 also be used within an @code{EXCLUDE_FILE} list, but may not appear in
4942 other linker script contexts. For instance, you cannot extract a file
4943 from an archive by using @samp{archive:file} in an @code{INPUT}
4946 If you use a file name without a list of sections, then all sections in
4947 the input file will be included in the output section. This is not
4948 commonly done, but it may by useful on occasion. For example:
4953 When you use a file name which is not an @samp{archive:file} specifier
4954 and does not contain any wild card
4955 characters, the linker will first see if you also specified the file
4956 name on the linker command line or in an @code{INPUT} command. If you
4957 did not, the linker will attempt to open the file as an input file, as
4958 though it appeared on the command line. Note that this differs from an
4959 @code{INPUT} command, because the linker will not search for the file in
4960 the archive search path.
4962 @node Input Section Wildcards
4963 @subsubsection Input Section Wildcard Patterns
4964 @cindex input section wildcards
4965 @cindex wildcard file name patterns
4966 @cindex file name wildcard patterns
4967 @cindex section name wildcard patterns
4968 In an input section description, either the file name or the section
4969 name or both may be wildcard patterns.
4971 The file name of @samp{*} seen in many examples is a simple wildcard
4972 pattern for the file name.
4974 The wildcard patterns are like those used by the Unix shell.
4978 matches any number of characters
4980 matches any single character
4982 matches a single instance of any of the @var{chars}; the @samp{-}
4983 character may be used to specify a range of characters, as in
4984 @samp{[a-z]} to match any lower case letter
4986 quotes the following character
4989 When a file name is matched with a wildcard, the wildcard characters
4990 will not match a @samp{/} character (used to separate directory names on
4991 Unix). A pattern consisting of a single @samp{*} character is an
4992 exception; it will always match any file name, whether it contains a
4993 @samp{/} or not. In a section name, the wildcard characters will match
4994 a @samp{/} character.
4996 File name wildcard patterns only match files which are explicitly
4997 specified on the command line or in an @code{INPUT} command. The linker
4998 does not search directories to expand wildcards.
5000 If a file name matches more than one wildcard pattern, or if a file name
5001 appears explicitly and is also matched by a wildcard pattern, the linker
5002 will use the first match in the linker script. For example, this
5003 sequence of input section descriptions is probably in error, because the
5004 @file{data.o} rule will not be used:
5006 .data : @{ *(.data) @}
5007 .data1 : @{ data.o(.data) @}
5010 @cindex SORT_BY_NAME
5011 Normally, the linker will place files and sections matched by wildcards
5012 in the order in which they are seen during the link. You can change
5013 this by using the @code{SORT_BY_NAME} keyword, which appears before a wildcard
5014 pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the
5015 @code{SORT_BY_NAME} keyword is used, the linker will sort the files or sections
5016 into ascending order by name before placing them in the output file.
5018 @cindex SORT_BY_ALIGNMENT
5019 @code{SORT_BY_ALIGNMENT} is similar to @code{SORT_BY_NAME}.
5020 @code{SORT_BY_ALIGNMENT} will sort sections into descending order of
5021 alignment before placing them in the output file. Placing larger
5022 alignments before smaller alignments can reduce the amount of padding
5025 @cindex SORT_BY_INIT_PRIORITY
5026 @code{SORT_BY_INIT_PRIORITY} is also similar to @code{SORT_BY_NAME}.
5027 @code{SORT_BY_INIT_PRIORITY} will sort sections into ascending
5028 numerical order of the GCC init_priority attribute encoded in the
5029 section name before placing them in the output file. In
5030 @code{.init_array.NNNNN} and @code{.fini_array.NNNNN}, @code{NNNNN} is
5031 the init_priority. In @code{.ctors.NNNNN} and @code{.dtors.NNNNN},
5032 @code{NNNNN} is 65535 minus the init_priority.
5035 @code{SORT} is an alias for @code{SORT_BY_NAME}.
5037 When there are nested section sorting commands in linker script, there
5038 can be at most 1 level of nesting for section sorting commands.
5042 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
5043 It will sort the input sections by name first, then by alignment if two
5044 sections have the same name.
5046 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
5047 It will sort the input sections by alignment first, then by name if two
5048 sections have the same alignment.
5050 @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is
5051 treated the same as @code{SORT_BY_NAME} (wildcard section pattern).
5053 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern))
5054 is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern).
5056 All other nested section sorting commands are invalid.
5059 When both command-line section sorting option and linker script
5060 section sorting command are used, section sorting command always
5061 takes precedence over the command-line option.
5063 If the section sorting command in linker script isn't nested, the
5064 command-line option will make the section sorting command to be
5065 treated as nested sorting command.
5069 @code{SORT_BY_NAME} (wildcard section pattern ) with
5070 @option{--sort-sections alignment} is equivalent to
5071 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
5073 @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with
5074 @option{--sort-section name} is equivalent to
5075 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
5078 If the section sorting command in linker script is nested, the
5079 command-line option will be ignored.
5082 @code{SORT_NONE} disables section sorting by ignoring the command-line
5083 section sorting option.
5085 If you ever get confused about where input sections are going, use the
5086 @samp{-M} linker option to generate a map file. The map file shows
5087 precisely how input sections are mapped to output sections.
5089 This example shows how wildcard patterns might be used to partition
5090 files. This linker script directs the linker to place all @samp{.text}
5091 sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}.
5092 The linker will place the @samp{.data} section from all files beginning
5093 with an upper case character in @samp{.DATA}; for all other files, the
5094 linker will place the @samp{.data} section in @samp{.data}.
5098 .text : @{ *(.text) @}
5099 .DATA : @{ [A-Z]*(.data) @}
5100 .data : @{ *(.data) @}
5101 .bss : @{ *(.bss) @}
5106 @node Input Section Common
5107 @subsubsection Input Section for Common Symbols
5108 @cindex common symbol placement
5109 @cindex uninitialized data placement
5110 A special notation is needed for common symbols, because in many object
5111 file formats common symbols do not have a particular input section. The
5112 linker treats common symbols as though they are in an input section
5113 named @samp{COMMON}.
5115 You may use file names with the @samp{COMMON} section just as with any
5116 other input sections. You can use this to place common symbols from a
5117 particular input file in one section while common symbols from other
5118 input files are placed in another section.
5120 In most cases, common symbols in input files will be placed in the
5121 @samp{.bss} section in the output file. For example:
5123 .bss @{ *(.bss) *(COMMON) @}
5126 @cindex scommon section
5127 @cindex small common symbols
5128 Some object file formats have more than one type of common symbol. For
5129 example, the MIPS ELF object file format distinguishes standard common
5130 symbols and small common symbols. In this case, the linker will use a
5131 different special section name for other types of common symbols. In
5132 the case of MIPS ELF, the linker uses @samp{COMMON} for standard common
5133 symbols and @samp{.scommon} for small common symbols. This permits you
5134 to map the different types of common symbols into memory at different
5138 You will sometimes see @samp{[COMMON]} in old linker scripts. This
5139 notation is now considered obsolete. It is equivalent to
5142 @node Input Section Keep
5143 @subsubsection Input Section and Garbage Collection
5145 @cindex garbage collection
5146 When link-time garbage collection is in use (@samp{--gc-sections}),
5147 it is often useful to mark sections that should not be eliminated.
5148 This is accomplished by surrounding an input section's wildcard entry
5149 with @code{KEEP()}, as in @code{KEEP(*(.init))} or
5150 @code{KEEP(SORT_BY_NAME(*)(.ctors))}.
5152 @node Input Section Example
5153 @subsubsection Input Section Example
5154 The following example is a complete linker script. It tells the linker
5155 to read all of the sections from file @file{all.o} and place them at the
5156 start of output section @samp{outputa} which starts at location
5157 @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o}
5158 follows immediately, in the same output section. All of section
5159 @samp{.input2} from @file{foo.o} goes into output section
5160 @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}.
5161 All of the remaining @samp{.input1} and @samp{.input2} sections from any
5162 files are written to output section @samp{outputc}.
5190 If an output section's name is the same as the input section's name
5191 and is representable as a C identifier, then the linker will
5192 automatically @pxref{PROVIDE} two symbols: __start_SECNAME and
5193 __stop_SECNAME, where SECNAME is the name of the section. These
5194 indicate the start address and end address of the output section
5195 respectively. Note: most section names are not representable as
5196 C identifiers because they contain a @samp{.} character.
5198 @node Output Section Data
5199 @subsection Output Section Data
5201 @cindex section data
5202 @cindex output section data
5203 @kindex BYTE(@var{expression})
5204 @kindex SHORT(@var{expression})
5205 @kindex LONG(@var{expression})
5206 @kindex QUAD(@var{expression})
5207 @kindex SQUAD(@var{expression})
5208 You can include explicit bytes of data in an output section by using
5209 @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as
5210 an output section command. Each keyword is followed by an expression in
5211 parentheses providing the value to store (@pxref{Expressions}). The
5212 value of the expression is stored at the current value of the location
5215 The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands
5216 store one, two, four, and eight bytes (respectively). After storing the
5217 bytes, the location counter is incremented by the number of bytes
5220 For example, this will store the byte 1 followed by the four byte value
5221 of the symbol @samp{addr}:
5227 When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the
5228 same; they both store an 8 byte, or 64 bit, value. When both host and
5229 target are 32 bits, an expression is computed as 32 bits. In this case
5230 @code{QUAD} stores a 32 bit value zero extended to 64 bits, and
5231 @code{SQUAD} stores a 32 bit value sign extended to 64 bits.
5233 If the object file format of the output file has an explicit endianness,
5234 which is the normal case, the value will be stored in that endianness.
5235 When the object file format does not have an explicit endianness, as is
5236 true of, for example, S-records, the value will be stored in the
5237 endianness of the first input object file.
5239 Note---these commands only work inside a section description and not
5240 between them, so the following will produce an error from the linker:
5242 SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@
5244 whereas this will work:
5246 SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@
5249 @kindex FILL(@var{expression})
5250 @cindex holes, filling
5251 @cindex unspecified memory
5252 You may use the @code{FILL} command to set the fill pattern for the
5253 current section. It is followed by an expression in parentheses. Any
5254 otherwise unspecified regions of memory within the section (for example,
5255 gaps left due to the required alignment of input sections) are filled
5256 with the value of the expression, repeated as
5257 necessary. A @code{FILL} statement covers memory locations after the
5258 point at which it occurs in the section definition; by including more
5259 than one @code{FILL} statement, you can have different fill patterns in
5260 different parts of an output section.
5262 This example shows how to fill unspecified regions of memory with the
5268 The @code{FILL} command is similar to the @samp{=@var{fillexp}} output
5269 section attribute, but it only affects the
5270 part of the section following the @code{FILL} command, rather than the
5271 entire section. If both are used, the @code{FILL} command takes
5272 precedence. @xref{Output Section Fill}, for details on the fill
5275 @node Output Section Keywords
5276 @subsection Output Section Keywords
5277 There are a couple of keywords which can appear as output section
5281 @kindex CREATE_OBJECT_SYMBOLS
5282 @cindex input filename symbols
5283 @cindex filename symbols
5284 @item CREATE_OBJECT_SYMBOLS
5285 The command tells the linker to create a symbol for each input file.
5286 The name of each symbol will be the name of the corresponding input
5287 file. The section of each symbol will be the output section in which
5288 the @code{CREATE_OBJECT_SYMBOLS} command appears.
5290 This is conventional for the a.out object file format. It is not
5291 normally used for any other object file format.
5293 @kindex CONSTRUCTORS
5294 @cindex C++ constructors, arranging in link
5295 @cindex constructors, arranging in link
5297 When linking using the a.out object file format, the linker uses an
5298 unusual set construct to support C++ global constructors and
5299 destructors. When linking object file formats which do not support
5300 arbitrary sections, such as ECOFF and XCOFF, the linker will
5301 automatically recognize C++ global constructors and destructors by name.
5302 For these object file formats, the @code{CONSTRUCTORS} command tells the
5303 linker to place constructor information in the output section where the
5304 @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is
5305 ignored for other object file formats.
5307 The symbol @w{@code{__CTOR_LIST__}} marks the start of the global
5308 constructors, and the symbol @w{@code{__CTOR_END__}} marks the end.
5309 Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_END__}} mark
5310 the start and end of the global destructors. The
5311 first word in the list is the number of entries, followed by the address
5312 of each constructor or destructor, followed by a zero word. The
5313 compiler must arrange to actually run the code. For these object file
5314 formats @sc{gnu} C++ normally calls constructors from a subroutine
5315 @code{__main}; a call to @code{__main} is automatically inserted into
5316 the startup code for @code{main}. @sc{gnu} C++ normally runs
5317 destructors either by using @code{atexit}, or directly from the function
5320 For object file formats such as @code{COFF} or @code{ELF} which support
5321 arbitrary section names, @sc{gnu} C++ will normally arrange to put the
5322 addresses of global constructors and destructors into the @code{.ctors}
5323 and @code{.dtors} sections. Placing the following sequence into your
5324 linker script will build the sort of table which the @sc{gnu} C++
5325 runtime code expects to see.
5329 LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
5334 LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
5340 If you are using the @sc{gnu} C++ support for initialization priority,
5341 which provides some control over the order in which global constructors
5342 are run, you must sort the constructors at link time to ensure that they
5343 are executed in the correct order. When using the @code{CONSTRUCTORS}
5344 command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the
5345 @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and
5346 @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and
5349 Normally the compiler and linker will handle these issues automatically,
5350 and you will not need to concern yourself with them. However, you may
5351 need to consider this if you are using C++ and writing your own linker
5356 @node Output Section Discarding
5357 @subsection Output Section Discarding
5358 @cindex discarding sections
5359 @cindex sections, discarding
5360 @cindex removing sections
5361 The linker will not normally create output sections with no contents.
5362 This is for convenience when referring to input sections that may or
5363 may not be present in any of the input files. For example:
5365 .foo : @{ *(.foo) @}
5368 will only create a @samp{.foo} section in the output file if there is a
5369 @samp{.foo} section in at least one input file, and if the input
5370 sections are not all empty. Other link script directives that allocate
5371 space in an output section will also create the output section. So
5372 too will assignments to dot even if the assignment does not create
5373 space, except for @samp{. = 0}, @samp{. = . + 0}, @samp{. = sym},
5374 @samp{. = . + sym} and @samp{. = ALIGN (. != 0, expr, 1)} when
5375 @samp{sym} is an absolute symbol of value 0 defined in the script.
5376 This allows you to force output of an empty section with @samp{. = .}.
5378 The linker will ignore address assignments (@pxref{Output Section Address})
5379 on discarded output sections, except when the linker script defines
5380 symbols in the output section. In that case the linker will obey
5381 the address assignments, possibly advancing dot even though the
5382 section is discarded.
5385 The special output section name @samp{/DISCARD/} may be used to discard
5386 input sections. Any input sections which are assigned to an output
5387 section named @samp{/DISCARD/} are not included in the output file.
5389 This can be used to discard input sections marked with the ELF flag
5390 @code{SHF_GNU_RETAIN}, which would otherwise have been saved from linker
5393 Note, sections that match the @samp{/DISCARD/} output section will be
5394 discarded even if they are in an ELF section group which has other
5395 members which are not being discarded. This is deliberate.
5396 Discarding takes precedence over grouping.
5398 @node Output Section Attributes
5399 @subsection Output Section Attributes
5400 @cindex output section attributes
5401 We showed above that the full description of an output section looked
5406 @var{section} [@var{address}] [(@var{type})] :
5408 [ALIGN(@var{section_align}) | ALIGN_WITH_INPUT]
5409 [SUBALIGN(@var{subsection_align})]
5412 @var{output-section-command}
5413 @var{output-section-command}
5415 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}]
5419 We've already described @var{section}, @var{address}, and
5420 @var{output-section-command}. In this section we will describe the
5421 remaining section attributes.
5424 * Output Section Type:: Output section type
5425 * Output Section LMA:: Output section LMA
5426 * Forced Output Alignment:: Forced Output Alignment
5427 * Forced Input Alignment:: Forced Input Alignment
5428 * Output Section Constraint:: Output section constraint
5429 * Output Section Region:: Output section region
5430 * Output Section Phdr:: Output section phdr
5431 * Output Section Fill:: Output section fill
5434 @node Output Section Type
5435 @subsubsection Output Section Type
5436 Each output section may have a type. The type is a keyword in
5437 parentheses. The following types are defined:
5441 The section should be marked as not loadable, so that it will not be
5442 loaded into memory when the program is run.
5447 These type names are supported for backward compatibility, and are
5448 rarely used. They all have the same effect: the section should be
5449 marked as not allocatable, so that no memory is allocated for the
5450 section when the program is run.
5454 @cindex prevent unnecessary loading
5455 @cindex loading, preventing
5456 The linker normally sets the attributes of an output section based on
5457 the input sections which map into it. You can override this by using
5458 the section type. For example, in the script sample below, the
5459 @samp{ROM} section is addressed at memory location @samp{0} and does not
5460 need to be loaded when the program is run.
5464 ROM 0 (NOLOAD) : @{ @dots{} @}
5470 @node Output Section LMA
5471 @subsubsection Output Section LMA
5472 @kindex AT>@var{lma_region}
5473 @kindex AT(@var{lma})
5474 @cindex load address
5475 @cindex section load address
5476 Every section has a virtual address (VMA) and a load address (LMA); see
5477 @ref{Basic Script Concepts}. The virtual address is specified by the
5478 @pxref{Output Section Address} described earlier. The load address is
5479 specified by the @code{AT} or @code{AT>} keywords. Specifying a load
5480 address is optional.
5482 The @code{AT} keyword takes an expression as an argument. This
5483 specifies the exact load address of the section. The @code{AT>} keyword
5484 takes the name of a memory region as an argument. @xref{MEMORY}. The
5485 load address of the section is set to the next free address in the
5486 region, aligned to the section's alignment requirements.
5488 If neither @code{AT} nor @code{AT>} is specified for an allocatable
5489 section, the linker will use the following heuristic to determine the
5494 If the section has a specific VMA address, then this is used as
5495 the LMA address as well.
5498 If the section is not allocatable then its LMA is set to its VMA.
5501 Otherwise if a memory region can be found that is compatible
5502 with the current section, and this region contains at least one
5503 section, then the LMA is set so the difference between the
5504 VMA and LMA is the same as the difference between the VMA and LMA of
5505 the last section in the located region.
5508 If no memory regions have been declared then a default region
5509 that covers the entire address space is used in the previous step.
5512 If no suitable region could be found, or there was no previous
5513 section then the LMA is set equal to the VMA.
5516 @cindex ROM initialized data
5517 @cindex initialized data in ROM
5518 This feature is designed to make it easy to build a ROM image. For
5519 example, the following linker script creates three output sections: one
5520 called @samp{.text}, which starts at @code{0x1000}, one called
5521 @samp{.mdata}, which is loaded at the end of the @samp{.text} section
5522 even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold
5523 uninitialized data at address @code{0x3000}. The symbol @code{_data} is
5524 defined with the value @code{0x2000}, which shows that the location
5525 counter holds the VMA value, not the LMA value.
5531 .text 0x1000 : @{ *(.text) _etext = . ; @}
5533 AT ( ADDR (.text) + SIZEOF (.text) )
5534 @{ _data = . ; *(.data); _edata = . ; @}
5536 @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@}
5541 The run-time initialization code for use with a program generated with
5542 this linker script would include something like the following, to copy
5543 the initialized data from the ROM image to its runtime address. Notice
5544 how this code takes advantage of the symbols defined by the linker
5549 extern char _etext, _data, _edata, _bstart, _bend;
5550 char *src = &_etext;
5553 /* ROM has data at end of text; copy it. */
5554 while (dst < &_edata)
5558 for (dst = &_bstart; dst< &_bend; dst++)
5563 @node Forced Output Alignment
5564 @subsubsection Forced Output Alignment
5565 @kindex ALIGN(@var{section_align})
5566 @cindex forcing output section alignment
5567 @cindex output section alignment
5568 You can increase an output section's alignment by using ALIGN. As an
5569 alternative you can enforce that the difference between the VMA and LMA remains
5570 intact throughout this output section with the ALIGN_WITH_INPUT attribute.
5572 @node Forced Input Alignment
5573 @subsubsection Forced Input Alignment
5574 @kindex SUBALIGN(@var{subsection_align})
5575 @cindex forcing input section alignment
5576 @cindex input section alignment
5577 You can force input section alignment within an output section by using
5578 SUBALIGN. The value specified overrides any alignment given by input
5579 sections, whether larger or smaller.
5581 @node Output Section Constraint
5582 @subsubsection Output Section Constraint
5585 @cindex constraints on output sections
5586 You can specify that an output section should only be created if all
5587 of its input sections are read-only or all of its input sections are
5588 read-write by using the keyword @code{ONLY_IF_RO} and
5589 @code{ONLY_IF_RW} respectively.
5591 @node Output Section Region
5592 @subsubsection Output Section Region
5593 @kindex >@var{region}
5594 @cindex section, assigning to memory region
5595 @cindex memory regions and sections
5596 You can assign a section to a previously defined region of memory by
5597 using @samp{>@var{region}}. @xref{MEMORY}.
5599 Here is a simple example:
5602 MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @}
5603 SECTIONS @{ ROM : @{ *(.text) @} >rom @}
5607 @node Output Section Phdr
5608 @subsubsection Output Section Phdr
5610 @cindex section, assigning to program header
5611 @cindex program headers and sections
5612 You can assign a section to a previously defined program segment by
5613 using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to
5614 one or more segments, then all subsequent allocated sections will be
5615 assigned to those segments as well, unless they use an explicitly
5616 @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the
5617 linker to not put the section in any segment at all.
5619 Here is a simple example:
5622 PHDRS @{ text PT_LOAD ; @}
5623 SECTIONS @{ .text : @{ *(.text) @} :text @}
5627 @node Output Section Fill
5628 @subsubsection Output Section Fill
5629 @kindex =@var{fillexp}
5630 @cindex section fill pattern
5631 @cindex fill pattern, entire section
5632 You can set the fill pattern for an entire section by using
5633 @samp{=@var{fillexp}}. @var{fillexp} is an expression
5634 (@pxref{Expressions}). Any otherwise unspecified regions of memory
5635 within the output section (for example, gaps left due to the required
5636 alignment of input sections) will be filled with the value, repeated as
5637 necessary. If the fill expression is a simple hex number, ie. a string
5638 of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then
5639 an arbitrarily long sequence of hex digits can be used to specify the
5640 fill pattern; Leading zeros become part of the pattern too. For all
5641 other cases, including extra parentheses or a unary @code{+}, the fill
5642 pattern is the four least significant bytes of the value of the
5643 expression. In all cases, the number is big-endian.
5645 You can also change the fill value with a @code{FILL} command in the
5646 output section commands; (@pxref{Output Section Data}).
5648 Here is a simple example:
5651 SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @}
5655 @node Overlay Description
5656 @subsection Overlay Description
5659 An overlay description provides an easy way to describe sections which
5660 are to be loaded as part of a single memory image but are to be run at
5661 the same memory address. At run time, some sort of overlay manager will
5662 copy the overlaid sections in and out of the runtime memory address as
5663 required, perhaps by simply manipulating addressing bits. This approach
5664 can be useful, for example, when a certain region of memory is faster
5667 Overlays are described using the @code{OVERLAY} command. The
5668 @code{OVERLAY} command is used within a @code{SECTIONS} command, like an
5669 output section description. The full syntax of the @code{OVERLAY}
5670 command is as follows:
5673 OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )]
5677 @var{output-section-command}
5678 @var{output-section-command}
5680 @} [:@var{phdr}@dots{}] [=@var{fill}]
5683 @var{output-section-command}
5684 @var{output-section-command}
5686 @} [:@var{phdr}@dots{}] [=@var{fill}]
5688 @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] [,]
5692 Everything is optional except @code{OVERLAY} (a keyword), and each
5693 section must have a name (@var{secname1} and @var{secname2} above). The
5694 section definitions within the @code{OVERLAY} construct are identical to
5695 those within the general @code{SECTIONS} construct (@pxref{SECTIONS}),
5696 except that no addresses and no memory regions may be defined for
5697 sections within an @code{OVERLAY}.
5699 The comma at the end may be required if a @var{fill} is used and
5700 the next @var{sections-command} looks like a continuation of the expression.
5702 The sections are all defined with the same starting address. The load
5703 addresses of the sections are arranged such that they are consecutive in
5704 memory starting at the load address used for the @code{OVERLAY} as a
5705 whole (as with normal section definitions, the load address is optional,
5706 and defaults to the start address; the start address is also optional,
5707 and defaults to the current value of the location counter).
5709 If the @code{NOCROSSREFS} keyword is used, and there are any
5710 references among the sections, the linker will report an error. Since
5711 the sections all run at the same address, it normally does not make
5712 sense for one section to refer directly to another.
5713 @xref{Miscellaneous Commands, NOCROSSREFS}.
5715 For each section within the @code{OVERLAY}, the linker automatically
5716 provides two symbols. The symbol @code{__load_start_@var{secname}} is
5717 defined as the starting load address of the section. The symbol
5718 @code{__load_stop_@var{secname}} is defined as the final load address of
5719 the section. Any characters within @var{secname} which are not legal
5720 within C identifiers are removed. C (or assembler) code may use these
5721 symbols to move the overlaid sections around as necessary.
5723 At the end of the overlay, the value of the location counter is set to
5724 the start address of the overlay plus the size of the largest section.
5726 Here is an example. Remember that this would appear inside a
5727 @code{SECTIONS} construct.
5730 OVERLAY 0x1000 : AT (0x4000)
5732 .text0 @{ o1/*.o(.text) @}
5733 .text1 @{ o2/*.o(.text) @}
5738 This will define both @samp{.text0} and @samp{.text1} to start at
5739 address 0x1000. @samp{.text0} will be loaded at address 0x4000, and
5740 @samp{.text1} will be loaded immediately after @samp{.text0}. The
5741 following symbols will be defined if referenced: @code{__load_start_text0},
5742 @code{__load_stop_text0}, @code{__load_start_text1},
5743 @code{__load_stop_text1}.
5745 C code to copy overlay @code{.text1} into the overlay area might look
5750 extern char __load_start_text1, __load_stop_text1;
5751 memcpy ((char *) 0x1000, &__load_start_text1,
5752 &__load_stop_text1 - &__load_start_text1);
5756 Note that the @code{OVERLAY} command is just syntactic sugar, since
5757 everything it does can be done using the more basic commands. The above
5758 example could have been written identically as follows.
5762 .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @}
5763 PROVIDE (__load_start_text0 = LOADADDR (.text0));
5764 PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
5765 .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @}
5766 PROVIDE (__load_start_text1 = LOADADDR (.text1));
5767 PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
5768 . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
5773 @section MEMORY Command
5775 @cindex memory regions
5776 @cindex regions of memory
5777 @cindex allocating memory
5778 @cindex discontinuous memory
5779 The linker's default configuration permits allocation of all available
5780 memory. You can override this by using the @code{MEMORY} command.
5782 The @code{MEMORY} command describes the location and size of blocks of
5783 memory in the target. You can use it to describe which memory regions
5784 may be used by the linker, and which memory regions it must avoid. You
5785 can then assign sections to particular memory regions. The linker will
5786 set section addresses based on the memory regions, and will warn about
5787 regions that become too full. The linker will not shuffle sections
5788 around to fit into the available regions.
5790 A linker script may contain many uses of the @code{MEMORY} command,
5791 however, all memory blocks defined are treated as if they were
5792 specified inside a single @code{MEMORY} command. The syntax for
5798 @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len}
5804 The @var{name} is a name used in the linker script to refer to the
5805 region. The region name has no meaning outside of the linker script.
5806 Region names are stored in a separate name space, and will not conflict
5807 with symbol names, file names, or section names. Each memory region
5808 must have a distinct name within the @code{MEMORY} command. However you can
5809 add later alias names to existing memory regions with the @ref{REGION_ALIAS}
5812 @cindex memory region attributes
5813 The @var{attr} string is an optional list of attributes that specify
5814 whether to use a particular memory region for an input section which is
5815 not explicitly mapped in the linker script. As described in
5816 @ref{SECTIONS}, if you do not specify an output section for some input
5817 section, the linker will create an output section with the same name as
5818 the input section. If you define region attributes, the linker will use
5819 them to select the memory region for the output section that it creates.
5821 The @var{attr} string must consist only of the following characters:
5836 Invert the sense of any of the attributes that follow
5839 If an unmapped section matches any of the listed attributes other than
5840 @samp{!}, it will be placed in the memory region. The @samp{!}
5841 attribute reverses the test for the characters that follow, so that an
5842 unmapped section will be placed in the memory region only if it does
5843 not match any of the attributes listed afterwards. Thus an attribute
5844 string of @samp{RW!X} will match any unmapped section that has either
5845 or both of the @samp{R} and @samp{W} attributes, but only as long as
5846 the section does not also have the @samp{X} attribute.
5851 The @var{origin} is an numerical expression for the start address of
5852 the memory region. The expression must evaluate to a constant and it
5853 cannot involve any symbols. The keyword @code{ORIGIN} may be
5854 abbreviated to @code{org} or @code{o} (but not, for example,
5860 The @var{len} is an expression for the size in bytes of the memory
5861 region. As with the @var{origin} expression, the expression must
5862 be numerical only and must evaluate to a constant. The keyword
5863 @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
5865 In the following example, we specify that there are two memory regions
5866 available for allocation: one starting at @samp{0} for 256 kilobytes,
5867 and the other starting at @samp{0x40000000} for four megabytes. The
5868 linker will place into the @samp{rom} memory region every section which
5869 is not explicitly mapped into a memory region, and is either read-only
5870 or executable. The linker will place other sections which are not
5871 explicitly mapped into a memory region into the @samp{ram} memory
5878 rom (rx) : ORIGIN = 0, LENGTH = 256K
5879 ram (!rx) : org = 0x40000000, l = 4M
5884 Once you define a memory region, you can direct the linker to place
5885 specific output sections into that memory region by using the
5886 @samp{>@var{region}} output section attribute. For example, if you have
5887 a memory region named @samp{mem}, you would use @samp{>mem} in the
5888 output section definition. @xref{Output Section Region}. If no address
5889 was specified for the output section, the linker will set the address to
5890 the next available address within the memory region. If the combined
5891 output sections directed to a memory region are too large for the
5892 region, the linker will issue an error message.
5894 It is possible to access the origin and length of a memory in an
5895 expression via the @code{ORIGIN(@var{memory})} and
5896 @code{LENGTH(@var{memory})} functions:
5900 _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
5905 @section PHDRS Command
5907 @cindex program headers
5908 @cindex ELF program headers
5909 @cindex program segments
5910 @cindex segments, ELF
5911 The ELF object file format uses @dfn{program headers}, also knows as
5912 @dfn{segments}. The program headers describe how the program should be
5913 loaded into memory. You can print them out by using the @code{objdump}
5914 program with the @samp{-p} option.
5916 When you run an ELF program on a native ELF system, the system loader
5917 reads the program headers in order to figure out how to load the
5918 program. This will only work if the program headers are set correctly.
5919 This manual does not describe the details of how the system loader
5920 interprets program headers; for more information, see the ELF ABI.
5922 The linker will create reasonable program headers by default. However,
5923 in some cases, you may need to specify the program headers more
5924 precisely. You may use the @code{PHDRS} command for this purpose. When
5925 the linker sees the @code{PHDRS} command in the linker script, it will
5926 not create any program headers other than the ones specified.
5928 The linker only pays attention to the @code{PHDRS} command when
5929 generating an ELF output file. In other cases, the linker will simply
5930 ignore @code{PHDRS}.
5932 This is the syntax of the @code{PHDRS} command. The words @code{PHDRS},
5933 @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords.
5939 @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ]
5940 [ FLAGS ( @var{flags} ) ] ;
5945 The @var{name} is used only for reference in the @code{SECTIONS} command
5946 of the linker script. It is not put into the output file. Program
5947 header names are stored in a separate name space, and will not conflict
5948 with symbol names, file names, or section names. Each program header
5949 must have a distinct name. The headers are processed in order and it
5950 is usual for them to map to sections in ascending load address order.
5952 Certain program header types describe segments of memory which the
5953 system loader will load from the file. In the linker script, you
5954 specify the contents of these segments by placing allocatable output
5955 sections in the segments. You use the @samp{:@var{phdr}} output section
5956 attribute to place a section in a particular segment. @xref{Output
5959 It is normal to put certain sections in more than one segment. This
5960 merely implies that one segment of memory contains another. You may
5961 repeat @samp{:@var{phdr}}, using it once for each segment which should
5962 contain the section.
5964 If you place a section in one or more segments using @samp{:@var{phdr}},
5965 then the linker will place all subsequent allocatable sections which do
5966 not specify @samp{:@var{phdr}} in the same segments. This is for
5967 convenience, since generally a whole set of contiguous sections will be
5968 placed in a single segment. You can use @code{:NONE} to override the
5969 default segment and tell the linker to not put the section in any
5974 You may use the @code{FILEHDR} and @code{PHDRS} keywords after
5975 the program header type to further describe the contents of the segment.
5976 The @code{FILEHDR} keyword means that the segment should include the ELF
5977 file header. The @code{PHDRS} keyword means that the segment should
5978 include the ELF program headers themselves. If applied to a loadable
5979 segment (@code{PT_LOAD}), all prior loadable segments must have one of
5982 The @var{type} may be one of the following. The numbers indicate the
5983 value of the keyword.
5986 @item @code{PT_NULL} (0)
5987 Indicates an unused program header.
5989 @item @code{PT_LOAD} (1)
5990 Indicates that this program header describes a segment to be loaded from
5993 @item @code{PT_DYNAMIC} (2)
5994 Indicates a segment where dynamic linking information can be found.
5996 @item @code{PT_INTERP} (3)
5997 Indicates a segment where the name of the program interpreter may be
6000 @item @code{PT_NOTE} (4)
6001 Indicates a segment holding note information.
6003 @item @code{PT_SHLIB} (5)
6004 A reserved program header type, defined but not specified by the ELF
6007 @item @code{PT_PHDR} (6)
6008 Indicates a segment where the program headers may be found.
6010 @item @code{PT_TLS} (7)
6011 Indicates a segment containing thread local storage.
6013 @item @var{expression}
6014 An expression giving the numeric type of the program header. This may
6015 be used for types not defined above.
6018 You can specify that a segment should be loaded at a particular address
6019 in memory by using an @code{AT} expression. This is identical to the
6020 @code{AT} command used as an output section attribute (@pxref{Output
6021 Section LMA}). The @code{AT} command for a program header overrides the
6022 output section attribute.
6024 The linker will normally set the segment flags based on the sections
6025 which comprise the segment. You may use the @code{FLAGS} keyword to
6026 explicitly specify the segment flags. The value of @var{flags} must be
6027 an integer. It is used to set the @code{p_flags} field of the program
6030 Here is an example of @code{PHDRS}. This shows a typical set of program
6031 headers used on a native ELF system.
6037 headers PT_PHDR PHDRS ;
6039 text PT_LOAD FILEHDR PHDRS ;
6041 dynamic PT_DYNAMIC ;
6047 .interp : @{ *(.interp) @} :text :interp
6048 .text : @{ *(.text) @} :text
6049 .rodata : @{ *(.rodata) @} /* defaults to :text */
6051 . = . + 0x1000; /* move to a new page in memory */
6052 .data : @{ *(.data) @} :data
6053 .dynamic : @{ *(.dynamic) @} :data :dynamic
6060 @section VERSION Command
6061 @kindex VERSION @{script text@}
6062 @cindex symbol versions
6063 @cindex version script
6064 @cindex versions of symbols
6065 The linker supports symbol versions when using ELF. Symbol versions are
6066 only useful when using shared libraries. The dynamic linker can use
6067 symbol versions to select a specific version of a function when it runs
6068 a program that may have been linked against an earlier version of the
6071 You can include a version script directly in the main linker script, or
6072 you can supply the version script as an implicit linker script. You can
6073 also use the @samp{--version-script} linker option.
6075 The syntax of the @code{VERSION} command is simply
6077 VERSION @{ version-script-commands @}
6080 The format of the version script commands is identical to that used by
6081 Sun's linker in Solaris 2.5. The version script defines a tree of
6082 version nodes. You specify the node names and interdependencies in the
6083 version script. You can specify which symbols are bound to which
6084 version nodes, and you can reduce a specified set of symbols to local
6085 scope so that they are not globally visible outside of the shared
6088 The easiest way to demonstrate the version script language is with a few
6114 This example version script defines three version nodes. The first
6115 version node defined is @samp{VERS_1.1}; it has no other dependencies.
6116 The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces
6117 a number of symbols to local scope so that they are not visible outside
6118 of the shared library; this is done using wildcard patterns, so that any
6119 symbol whose name begins with @samp{old}, @samp{original}, or @samp{new}
6120 is matched. The wildcard patterns available are the same as those used
6121 in the shell when matching filenames (also known as ``globbing'').
6122 However, if you specify the symbol name inside double quotes, then the
6123 name is treated as literal, rather than as a glob pattern.
6125 Next, the version script defines node @samp{VERS_1.2}. This node
6126 depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2}
6127 to the version node @samp{VERS_1.2}.
6129 Finally, the version script defines node @samp{VERS_2.0}. This node
6130 depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1}
6131 and @samp{bar2} are bound to the version node @samp{VERS_2.0}.
6133 When the linker finds a symbol defined in a library which is not
6134 specifically bound to a version node, it will effectively bind it to an
6135 unspecified base version of the library. You can bind all otherwise
6136 unspecified symbols to a given version node by using @samp{global: *;}
6137 somewhere in the version script. Note that it's slightly crazy to use
6138 wildcards in a global spec except on the last version node. Global
6139 wildcards elsewhere run the risk of accidentally adding symbols to the
6140 set exported for an old version. That's wrong since older versions
6141 ought to have a fixed set of symbols.
6143 The names of the version nodes have no specific meaning other than what
6144 they might suggest to the person reading them. The @samp{2.0} version
6145 could just as well have appeared in between @samp{1.1} and @samp{1.2}.
6146 However, this would be a confusing way to write a version script.
6148 Node name can be omitted, provided it is the only version node
6149 in the version script. Such version script doesn't assign any versions to
6150 symbols, only selects which symbols will be globally visible out and which
6154 @{ global: foo; bar; local: *; @};
6157 When you link an application against a shared library that has versioned
6158 symbols, the application itself knows which version of each symbol it
6159 requires, and it also knows which version nodes it needs from each
6160 shared library it is linked against. Thus at runtime, the dynamic
6161 loader can make a quick check to make sure that the libraries you have
6162 linked against do in fact supply all of the version nodes that the
6163 application will need to resolve all of the dynamic symbols. In this
6164 way it is possible for the dynamic linker to know with certainty that
6165 all external symbols that it needs will be resolvable without having to
6166 search for each symbol reference.
6168 The symbol versioning is in effect a much more sophisticated way of
6169 doing minor version checking that SunOS does. The fundamental problem
6170 that is being addressed here is that typically references to external
6171 functions are bound on an as-needed basis, and are not all bound when
6172 the application starts up. If a shared library is out of date, a
6173 required interface may be missing; when the application tries to use
6174 that interface, it may suddenly and unexpectedly fail. With symbol
6175 versioning, the user will get a warning when they start their program if
6176 the libraries being used with the application are too old.
6178 There are several GNU extensions to Sun's versioning approach. The
6179 first of these is the ability to bind a symbol to a version node in the
6180 source file where the symbol is defined instead of in the versioning
6181 script. This was done mainly to reduce the burden on the library
6182 maintainer. You can do this by putting something like:
6184 __asm__(".symver original_foo,foo@@VERS_1.1");
6187 in the C source file. This renames the function @samp{original_foo} to
6188 be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}.
6189 The @samp{local:} directive can be used to prevent the symbol
6190 @samp{original_foo} from being exported. A @samp{.symver} directive
6191 takes precedence over a version script.
6193 The second GNU extension is to allow multiple versions of the same
6194 function to appear in a given shared library. In this way you can make
6195 an incompatible change to an interface without increasing the major
6196 version number of the shared library, while still allowing applications
6197 linked against the old interface to continue to function.
6199 To do this, you must use multiple @samp{.symver} directives in the
6200 source file. Here is an example:
6203 __asm__(".symver original_foo,foo@@");
6204 __asm__(".symver old_foo,foo@@VERS_1.1");
6205 __asm__(".symver old_foo1,foo@@VERS_1.2");
6206 __asm__(".symver new_foo,foo@@@@VERS_2.0");
6209 In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the
6210 unspecified base version of the symbol. The source file that contains this
6211 example would define 4 C functions: @samp{original_foo}, @samp{old_foo},
6212 @samp{old_foo1}, and @samp{new_foo}.
6214 When you have multiple definitions of a given symbol, there needs to be
6215 some way to specify a default version to which external references to
6216 this symbol will be bound. You can do this with the
6217 @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only
6218 declare one version of a symbol as the default in this manner; otherwise
6219 you would effectively have multiple definitions of the same symbol.
6221 If you wish to bind a reference to a specific version of the symbol
6222 within the shared library, you can use the aliases of convenience
6223 (i.e., @samp{old_foo}), or you can use the @samp{.symver} directive to
6224 specifically bind to an external version of the function in question.
6226 You can also specify the language in the version script:
6229 VERSION extern "lang" @{ version-script-commands @}
6232 The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}.
6233 The linker will iterate over the list of symbols at the link time and
6234 demangle them according to @samp{lang} before matching them to the
6235 patterns specified in @samp{version-script-commands}. The default
6236 @samp{lang} is @samp{C}.
6238 Demangled names may contains spaces and other special characters. As
6239 described above, you can use a glob pattern to match demangled names,
6240 or you can use a double-quoted string to match the string exactly. In
6241 the latter case, be aware that minor differences (such as differing
6242 whitespace) between the version script and the demangler output will
6243 cause a mismatch. As the exact string generated by the demangler
6244 might change in the future, even if the mangled name does not, you
6245 should check that all of your version directives are behaving as you
6246 expect when you upgrade.
6249 @section Expressions in Linker Scripts
6252 The syntax for expressions in the linker script language is identical to
6253 that of C expressions. All expressions are evaluated as integers. All
6254 expressions are evaluated in the same size, which is 32 bits if both the
6255 host and target are 32 bits, and is otherwise 64 bits.
6257 You can use and set symbol values in expressions.
6259 The linker defines several special purpose builtin functions for use in
6263 * Constants:: Constants
6264 * Symbolic Constants:: Symbolic constants
6265 * Symbols:: Symbol Names
6266 * Orphan Sections:: Orphan Sections
6267 * Location Counter:: The Location Counter
6268 * Operators:: Operators
6269 * Evaluation:: Evaluation
6270 * Expression Section:: The Section of an Expression
6271 * Builtin Functions:: Builtin Functions
6275 @subsection Constants
6276 @cindex integer notation
6277 @cindex constants in linker scripts
6278 All constants are integers.
6280 As in C, the linker considers an integer beginning with @samp{0} to be
6281 octal, and an integer beginning with @samp{0x} or @samp{0X} to be
6282 hexadecimal. Alternatively the linker accepts suffixes of @samp{h} or
6283 @samp{H} for hexadecimal, @samp{o} or @samp{O} for octal, @samp{b} or
6284 @samp{B} for binary and @samp{d} or @samp{D} for decimal. Any integer
6285 value without a prefix or a suffix is considered to be decimal.
6287 @cindex scaled integers
6288 @cindex K and M integer suffixes
6289 @cindex M and K integer suffixes
6290 @cindex suffixes for integers
6291 @cindex integer suffixes
6292 In addition, you can use the suffixes @code{K} and @code{M} to scale a
6296 @c END TEXI2ROFF-KILL
6297 @code{1024} or @code{1024*1024}
6301 ${\rm 1024}$ or ${\rm 1024}^2$
6303 @c END TEXI2ROFF-KILL
6304 respectively. For example, the following
6305 all refer to the same quantity:
6314 Note - the @code{K} and @code{M} suffixes cannot be used in
6315 conjunction with the base suffixes mentioned above.
6317 @node Symbolic Constants
6318 @subsection Symbolic Constants
6319 @cindex symbolic constants
6321 It is possible to refer to target-specific constants via the use of
6322 the @code{CONSTANT(@var{name})} operator, where @var{name} is one of:
6327 The target's maximum page size.
6329 @item COMMONPAGESIZE
6330 @kindex COMMONPAGESIZE
6331 The target's default page size.
6337 .text ALIGN (CONSTANT (MAXPAGESIZE)) : @{ *(.text) @}
6340 will create a text section aligned to the largest page boundary
6341 supported by the target.
6344 @subsection Symbol Names
6345 @cindex symbol names
6347 @cindex quoted symbol names
6349 Unless quoted, symbol names start with a letter, underscore, or period
6350 and may include letters, digits, underscores, periods, and hyphens.
6351 Unquoted symbol names must not conflict with any keywords. You can
6352 specify a symbol which contains odd characters or has the same name as a
6353 keyword by surrounding the symbol name in double quotes:
6356 "with a space" = "also with a space" + 10;
6359 Since symbols can contain many non-alphabetic characters, it is safest
6360 to delimit symbols with spaces. For example, @samp{A-B} is one symbol,
6361 whereas @samp{A - B} is an expression involving subtraction.
6363 @node Orphan Sections
6364 @subsection Orphan Sections
6366 Orphan sections are sections present in the input files which
6367 are not explicitly placed into the output file by the linker
6368 script. The linker will still copy these sections into the
6369 output file by either finding, or creating a suitable output section
6370 in which to place the orphaned input section.
6372 If the name of an orphaned input section exactly matches the name of
6373 an existing output section, then the orphaned input section will be
6374 placed at the end of that output section.
6376 If there is no output section with a matching name then new output
6377 sections will be created. Each new output section will have the same
6378 name as the orphan section placed within it. If there are multiple
6379 orphan sections with the same name, these will all be combined into
6380 one new output section.
6382 If new output sections are created to hold orphaned input sections,
6383 then the linker must decide where to place these new output sections
6384 in relation to existing output sections. On most modern targets, the
6385 linker attempts to place orphan sections after sections of the same
6386 attribute, such as code vs data, loadable vs non-loadable, etc. If no
6387 sections with matching attributes are found, or your target lacks this
6388 support, the orphan section is placed at the end of the file.
6390 The command-line options @samp{--orphan-handling} and @samp{--unique}
6391 (@pxref{Options,,Command-line Options}) can be used to control which
6392 output sections an orphan is placed in.
6394 @node Location Counter
6395 @subsection The Location Counter
6398 @cindex location counter
6399 @cindex current output location
6400 The special linker variable @dfn{dot} @samp{.} always contains the
6401 current output location counter. Since the @code{.} always refers to a
6402 location in an output section, it may only appear in an expression
6403 within a @code{SECTIONS} command. The @code{.} symbol may appear
6404 anywhere that an ordinary symbol is allowed in an expression.
6407 Assigning a value to @code{.} will cause the location counter to be
6408 moved. This may be used to create holes in the output section. The
6409 location counter may not be moved backwards inside an output section,
6410 and may not be moved backwards outside of an output section if so
6411 doing creates areas with overlapping LMAs.
6427 In the previous example, the @samp{.text} section from @file{file1} is
6428 located at the beginning of the output section @samp{output}. It is
6429 followed by a 1000 byte gap. Then the @samp{.text} section from
6430 @file{file2} appears, also with a 1000 byte gap following before the
6431 @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678}
6432 specifies what data to write in the gaps (@pxref{Output Section Fill}).
6434 @cindex dot inside sections
6435 Note: @code{.} actually refers to the byte offset from the start of the
6436 current containing object. Normally this is the @code{SECTIONS}
6437 statement, whose start address is 0, hence @code{.} can be used as an
6438 absolute address. If @code{.} is used inside a section description
6439 however, it refers to the byte offset from the start of that section,
6440 not an absolute address. Thus in a script like this:
6458 The @samp{.text} section will be assigned a starting address of 0x100
6459 and a size of exactly 0x200 bytes, even if there is not enough data in
6460 the @samp{.text} input sections to fill this area. (If there is too
6461 much data, an error will be produced because this would be an attempt to
6462 move @code{.} backwards). The @samp{.data} section will start at 0x500
6463 and it will have an extra 0x600 bytes worth of space after the end of
6464 the values from the @samp{.data} input sections and before the end of
6465 the @samp{.data} output section itself.
6467 @cindex dot outside sections
6468 Setting symbols to the value of the location counter outside of an
6469 output section statement can result in unexpected values if the linker
6470 needs to place orphan sections. For example, given the following:
6476 .text: @{ *(.text) @}
6480 .data: @{ *(.data) @}
6485 If the linker needs to place some input section, e.g. @code{.rodata},
6486 not mentioned in the script, it might choose to place that section
6487 between @code{.text} and @code{.data}. You might think the linker
6488 should place @code{.rodata} on the blank line in the above script, but
6489 blank lines are of no particular significance to the linker. As well,
6490 the linker doesn't associate the above symbol names with their
6491 sections. Instead, it assumes that all assignments or other
6492 statements belong to the previous output section, except for the
6493 special case of an assignment to @code{.}. I.e., the linker will
6494 place the orphan @code{.rodata} section as if the script was written
6501 .text: @{ *(.text) @}
6505 .rodata: @{ *(.rodata) @}
6506 .data: @{ *(.data) @}
6511 This may or may not be the script author's intention for the value of
6512 @code{start_of_data}. One way to influence the orphan section
6513 placement is to assign the location counter to itself, as the linker
6514 assumes that an assignment to @code{.} is setting the start address of
6515 a following output section and thus should be grouped with that
6516 section. So you could write:
6522 .text: @{ *(.text) @}
6527 .data: @{ *(.data) @}
6532 Now, the orphan @code{.rodata} section will be placed between
6533 @code{end_of_text} and @code{start_of_data}.
6537 @subsection Operators
6538 @cindex operators for arithmetic
6539 @cindex arithmetic operators
6540 @cindex precedence in expressions
6541 The linker recognizes the standard C set of arithmetic operators, with
6542 the standard bindings and precedence levels:
6545 @c END TEXI2ROFF-KILL
6547 precedence associativity Operators Notes
6553 5 left == != > < <= >=
6559 11 right &= += -= *= /= (2)
6563 (1) Prefix operators
6564 (2) @xref{Assignments}.
6568 \vskip \baselineskip
6569 %"lispnarrowing" is the extra indent used generally for smallexample
6570 \hskip\lispnarrowing\vbox{\offinterlineskip
6573 {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr
6574 height2pt&\omit&&\omit&&\omit&\cr
6575 &Precedence&& Associativity &&{\rm Operators}&\cr
6576 height2pt&\omit&&\omit&&\omit&\cr
6578 height2pt&\omit&&\omit&&\omit&\cr
6580 % '176 is tilde, '~' in tt font
6581 &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr
6582 &2&&left&&* / \%&\cr
6585 &5&&left&&== != > < <= >=&\cr
6588 &8&&left&&{\&\&}&\cr
6591 &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr
6593 height2pt&\omit&&\omit&&\omit&\cr}
6598 @obeylines@parskip=0pt@parindent=0pt
6599 @dag@quad Prefix operators.
6600 @ddag@quad @xref{Assignments}.
6603 @c END TEXI2ROFF-KILL
6606 @subsection Evaluation
6607 @cindex lazy evaluation
6608 @cindex expression evaluation order
6609 The linker evaluates expressions lazily. It only computes the value of
6610 an expression when absolutely necessary.
6612 The linker needs some information, such as the value of the start
6613 address of the first section, and the origins and lengths of memory
6614 regions, in order to do any linking at all. These values are computed
6615 as soon as possible when the linker reads in the linker script.
6617 However, other values (such as symbol values) are not known or needed
6618 until after storage allocation. Such values are evaluated later, when
6619 other information (such as the sizes of output sections) is available
6620 for use in the symbol assignment expression.
6622 The sizes of sections cannot be known until after allocation, so
6623 assignments dependent upon these are not performed until after
6626 Some expressions, such as those depending upon the location counter
6627 @samp{.}, must be evaluated during section allocation.
6629 If the result of an expression is required, but the value is not
6630 available, then an error results. For example, a script like the
6636 .text 9+this_isnt_constant :
6642 will cause the error message @samp{non constant expression for initial
6645 @node Expression Section
6646 @subsection The Section of an Expression
6647 @cindex expression sections
6648 @cindex absolute expressions
6649 @cindex relative expressions
6650 @cindex absolute and relocatable symbols
6651 @cindex relocatable and absolute symbols
6652 @cindex symbols, relocatable and absolute
6653 Addresses and symbols may be section relative, or absolute. A section
6654 relative symbol is relocatable. If you request relocatable output
6655 using the @samp{-r} option, a further link operation may change the
6656 value of a section relative symbol. On the other hand, an absolute
6657 symbol will retain the same value throughout any further link
6660 Some terms in linker expressions are addresses. This is true of
6661 section relative symbols and for builtin functions that return an
6662 address, such as @code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and
6663 @code{SEGMENT_START}. Other terms are simply numbers, or are builtin
6664 functions that return a non-address value, such as @code{LENGTH}.
6665 One complication is that unless you set @code{LD_FEATURE ("SANE_EXPR")}
6666 (@pxref{Miscellaneous Commands}), numbers and absolute symbols are treated
6667 differently depending on their location, for compatibility with older
6668 versions of @code{ld}. Expressions appearing outside an output
6669 section definition treat all numbers as absolute addresses.
6670 Expressions appearing inside an output section definition treat
6671 absolute symbols as numbers. If @code{LD_FEATURE ("SANE_EXPR")} is
6672 given, then absolute symbols and numbers are simply treated as numbers
6675 In the following simple example,
6682 __executable_start = 0x100;
6686 __data_start = 0x10;
6694 both @code{.} and @code{__executable_start} are set to the absolute
6695 address 0x100 in the first two assignments, then both @code{.} and
6696 @code{__data_start} are set to 0x10 relative to the @code{.data}
6697 section in the second two assignments.
6699 For expressions involving numbers, relative addresses and absolute
6700 addresses, ld follows these rules to evaluate terms:
6704 Unary operations on an absolute address or number, and binary
6705 operations on two absolute addresses or two numbers, or between one
6706 absolute address and a number, apply the operator to the value(s).
6708 Unary operations on a relative address, and binary operations on two
6709 relative addresses in the same section or between one relative address
6710 and a number, apply the operator to the offset part of the address(es).
6712 Other binary operations, that is, between two relative addresses not
6713 in the same section, or between a relative address and an absolute
6714 address, first convert any non-absolute term to an absolute address
6715 before applying the operator.
6718 The result section of each sub-expression is as follows:
6722 An operation involving only numbers results in a number.
6724 The result of comparisons, @samp{&&} and @samp{||} is also a number.
6726 The result of other binary arithmetic and logical operations on two
6727 relative addresses in the same section or two absolute addresses
6728 (after above conversions) is also a number when
6729 @code{LD_FEATURE ("SANE_EXPR")} or inside an output section definition
6730 but an absolute address otherwise.
6732 The result of other operations on relative addresses or one
6733 relative address and a number, is a relative address in the same
6734 section as the relative operand(s).
6736 The result of other operations on absolute addresses (after above
6737 conversions) is an absolute address.
6740 You can use the builtin function @code{ABSOLUTE} to force an expression
6741 to be absolute when it would otherwise be relative. For example, to
6742 create an absolute symbol set to the address of the end of the output
6743 section @samp{.data}:
6747 .data : @{ *(.data) _edata = ABSOLUTE(.); @}
6751 If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the
6752 @samp{.data} section.
6754 Using @code{LOADADDR} also forces an expression absolute, since this
6755 particular builtin function returns an absolute address.
6757 @node Builtin Functions
6758 @subsection Builtin Functions
6759 @cindex functions in expressions
6760 The linker script language includes a number of builtin functions for
6761 use in linker script expressions.
6764 @item ABSOLUTE(@var{exp})
6765 @kindex ABSOLUTE(@var{exp})
6766 @cindex expression, absolute
6767 Return the absolute (non-relocatable, as opposed to non-negative) value
6768 of the expression @var{exp}. Primarily useful to assign an absolute
6769 value to a symbol within a section definition, where symbol values are
6770 normally section relative. @xref{Expression Section}.
6772 @item ADDR(@var{section})
6773 @kindex ADDR(@var{section})
6774 @cindex section address in expression
6775 Return the address (VMA) of the named @var{section}. Your
6776 script must previously have defined the location of that section. In
6777 the following example, @code{start_of_output_1}, @code{symbol_1} and
6778 @code{symbol_2} are assigned equivalent values, except that
6779 @code{symbol_1} will be relative to the @code{.output1} section while
6780 the other two will be absolute:
6786 start_of_output_1 = ABSOLUTE(.);
6791 symbol_1 = ADDR(.output1);
6792 symbol_2 = start_of_output_1;
6798 @item ALIGN(@var{align})
6799 @itemx ALIGN(@var{exp},@var{align})
6800 @kindex ALIGN(@var{align})
6801 @kindex ALIGN(@var{exp},@var{align})
6802 @cindex round up location counter
6803 @cindex align location counter
6804 @cindex round up expression
6805 @cindex align expression
6806 Return the location counter (@code{.}) or arbitrary expression aligned
6807 to the next @var{align} boundary. The single operand @code{ALIGN}
6808 doesn't change the value of the location counter---it just does
6809 arithmetic on it. The two operand @code{ALIGN} allows an arbitrary
6810 expression to be aligned upwards (@code{ALIGN(@var{align})} is
6811 equivalent to @code{ALIGN(ABSOLUTE(.), @var{align})}).
6813 Here is an example which aligns the output @code{.data} section to the
6814 next @code{0x2000} byte boundary after the preceding section and sets a
6815 variable within the section to the next @code{0x8000} boundary after the
6820 .data ALIGN(0x2000): @{
6822 variable = ALIGN(0x8000);
6828 The first use of @code{ALIGN} in this example specifies the location of
6829 a section because it is used as the optional @var{address} attribute of
6830 a section definition (@pxref{Output Section Address}). The second use
6831 of @code{ALIGN} is used to defines the value of a symbol.
6833 The builtin function @code{NEXT} is closely related to @code{ALIGN}.
6835 @item ALIGNOF(@var{section})
6836 @kindex ALIGNOF(@var{section})
6837 @cindex section alignment
6838 Return the alignment in bytes of the named @var{section}, if that section has
6839 been allocated. If the section has not been allocated when this is
6840 evaluated, the linker will report an error. In the following example,
6841 the alignment of the @code{.output} section is stored as the first
6842 value in that section.
6847 LONG (ALIGNOF (.output))
6854 @item BLOCK(@var{exp})
6855 @kindex BLOCK(@var{exp})
6856 This is a synonym for @code{ALIGN}, for compatibility with older linker
6857 scripts. It is most often seen when setting the address of an output
6860 @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
6861 @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
6862 This is equivalent to either
6864 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1)))
6868 (ALIGN(@var{maxpagesize})
6869 + ((. + @var{commonpagesize} - 1) & (@var{maxpagesize} - @var{commonpagesize})))
6872 depending on whether the latter uses fewer @var{commonpagesize} sized pages
6873 for the data segment (area between the result of this expression and
6874 @code{DATA_SEGMENT_END}) than the former or not.
6875 If the latter form is used, it means @var{commonpagesize} bytes of runtime
6876 memory will be saved at the expense of up to @var{commonpagesize} wasted
6877 bytes in the on-disk file.
6879 This expression can only be used directly in @code{SECTIONS} commands, not in
6880 any output section descriptions and only once in the linker script.
6881 @var{commonpagesize} should be less or equal to @var{maxpagesize} and should
6882 be the system page size the object wants to be optimized for while still
6883 running on system page sizes up to @var{maxpagesize}. Note however
6884 that @samp{-z relro} protection will not be effective if the system
6885 page size is larger than @var{commonpagesize}.
6890 . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
6893 @item DATA_SEGMENT_END(@var{exp})
6894 @kindex DATA_SEGMENT_END(@var{exp})
6895 This defines the end of data segment for @code{DATA_SEGMENT_ALIGN}
6896 evaluation purposes.
6899 . = DATA_SEGMENT_END(.);
6902 @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6903 @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6904 This defines the end of the @code{PT_GNU_RELRO} segment when
6905 @samp{-z relro} option is used.
6906 When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END}
6907 does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that
6908 @var{exp} + @var{offset} is aligned to the @var{commonpagesize}
6909 argument given to @code{DATA_SEGMENT_ALIGN}. If present in the linker
6910 script, it must be placed between @code{DATA_SEGMENT_ALIGN} and
6911 @code{DATA_SEGMENT_END}. Evaluates to the second argument plus any
6912 padding needed at the end of the @code{PT_GNU_RELRO} segment due to
6916 . = DATA_SEGMENT_RELRO_END(24, .);
6919 @item DEFINED(@var{symbol})
6920 @kindex DEFINED(@var{symbol})
6921 @cindex symbol defaults
6922 Return 1 if @var{symbol} is in the linker global symbol table and is
6923 defined before the statement using DEFINED in the script, otherwise
6924 return 0. You can use this function to provide
6925 default values for symbols. For example, the following script fragment
6926 shows how to set a global symbol @samp{begin} to the first location in
6927 the @samp{.text} section---but if a symbol called @samp{begin} already
6928 existed, its value is preserved:
6934 begin = DEFINED(begin) ? begin : . ;
6942 @item LENGTH(@var{memory})
6943 @kindex LENGTH(@var{memory})
6944 Return the length of the memory region named @var{memory}.
6946 @item LOADADDR(@var{section})
6947 @kindex LOADADDR(@var{section})
6948 @cindex section load address in expression
6949 Return the absolute LMA of the named @var{section}. (@pxref{Output
6952 @item LOG2CEIL(@var{exp})
6953 @kindex LOG2CEIL(@var{exp})
6954 Return the binary logarithm of @var{exp} rounded towards infinity.
6955 @code{LOG2CEIL(0)} returns 0.
6958 @item MAX(@var{exp1}, @var{exp2})
6959 Returns the maximum of @var{exp1} and @var{exp2}.
6962 @item MIN(@var{exp1}, @var{exp2})
6963 Returns the minimum of @var{exp1} and @var{exp2}.
6965 @item NEXT(@var{exp})
6966 @kindex NEXT(@var{exp})
6967 @cindex unallocated address, next
6968 Return the next unallocated address that is a multiple of @var{exp}.
6969 This function is closely related to @code{ALIGN(@var{exp})}; unless you
6970 use the @code{MEMORY} command to define discontinuous memory for the
6971 output file, the two functions are equivalent.
6973 @item ORIGIN(@var{memory})
6974 @kindex ORIGIN(@var{memory})
6975 Return the origin of the memory region named @var{memory}.
6977 @item SEGMENT_START(@var{segment}, @var{default})
6978 @kindex SEGMENT_START(@var{segment}, @var{default})
6979 Return the base address of the named @var{segment}. If an explicit
6980 value has already been given for this segment (with a command-line
6981 @samp{-T} option) then that value will be returned otherwise the value
6982 will be @var{default}. At present, the @samp{-T} command-line option
6983 can only be used to set the base address for the ``text'', ``data'', and
6984 ``bss'' sections, but you can use @code{SEGMENT_START} with any segment
6987 @item SIZEOF(@var{section})
6988 @kindex SIZEOF(@var{section})
6989 @cindex section size
6990 Return the size in bytes of the named @var{section}, if that section has
6991 been allocated. If the section has not been allocated when this is
6992 evaluated, the linker will report an error. In the following example,
6993 @code{symbol_1} and @code{symbol_2} are assigned identical values:
7002 symbol_1 = .end - .start ;
7003 symbol_2 = SIZEOF(.output);
7008 @item SIZEOF_HEADERS
7009 @itemx sizeof_headers
7010 @kindex SIZEOF_HEADERS
7012 Return the size in bytes of the output file's headers. This is
7013 information which appears at the start of the output file. You can use
7014 this number when setting the start address of the first section, if you
7015 choose, to facilitate paging.
7017 @cindex not enough room for program headers
7018 @cindex program headers, not enough room
7019 When producing an ELF output file, if the linker script uses the
7020 @code{SIZEOF_HEADERS} builtin function, the linker must compute the
7021 number of program headers before it has determined all the section
7022 addresses and sizes. If the linker later discovers that it needs
7023 additional program headers, it will report an error @samp{not enough
7024 room for program headers}. To avoid this error, you must avoid using
7025 the @code{SIZEOF_HEADERS} function, or you must rework your linker
7026 script to avoid forcing the linker to use additional program headers, or
7027 you must define the program headers yourself using the @code{PHDRS}
7028 command (@pxref{PHDRS}).
7031 @node Implicit Linker Scripts
7032 @section Implicit Linker Scripts
7033 @cindex implicit linker scripts
7034 If you specify a linker input file which the linker can not recognize as
7035 an object file or an archive file, it will try to read the file as a
7036 linker script. If the file can not be parsed as a linker script, the
7037 linker will report an error.
7039 An implicit linker script will not replace the default linker script.
7041 Typically an implicit linker script would contain only symbol
7042 assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION}
7045 Any input files read because of an implicit linker script will be read
7046 at the position in the command line where the implicit linker script was
7047 read. This can affect archive searching.
7050 @chapter Linker Plugins
7053 @cindex linker plugins
7054 The linker can use dynamically loaded plugins to modify its behavior.
7055 For example, the link-time optimization feature that some compilers
7056 support is implemented with a linker plugin.
7058 Currently there is only one plugin shipped by default, but more may
7059 be added here later.
7062 * libdep Plugin:: Static Library Dependencies Plugin
7066 @section Static Library Dependencies Plugin
7067 @cindex static library dependencies
7068 Originally, static libraries were contained in an archive file consisting
7069 just of a collection of relocatable object files. Later they evolved to
7070 optionally include a symbol table, to assist in finding the needed objects
7071 within a library. There their evolution ended, and dynamic libraries
7074 One useful feature of dynamic libraries was that, more than just collecting
7075 multiple objects into a single file, they also included a list of their
7076 dependencies, such that one could specify just the name of a single dynamic
7077 library at link time, and all of its dependencies would be implicitly
7078 referenced as well. But static libraries lacked this feature, so if a
7079 link invocation was switched from using dynamic libraries to static
7080 libraries, the link command would usually fail unless it was rewritten to
7081 explicitly list the dependencies of the static library.
7083 The GNU @command{ar} utility now supports a @option{--record-libdeps} option
7084 to embed dependency lists into static libraries as well, and the @file{libdep}
7085 plugin may be used to read this dependency information at link time. The
7086 dependency information is stored as a single string, carrying @option{-l}
7087 and @option{-L} arguments as they would normally appear in a linker
7088 command line. As such, the information can be written with any text
7089 utility and stored into any archive, even if GNU @command{ar} is not
7090 being used to create the archive. The information is stored in an
7091 archive member named @samp{__.LIBDEP}.
7093 For example, given a library @file{libssl.a} that depends on another
7094 library @file{libcrypto.a} which may be found in @file{/usr/local/lib},
7095 the @samp{__.LIBDEP} member of @file{libssl.a} would contain
7098 -L/usr/local/lib -lcrypto
7102 @node Machine Dependent
7103 @chapter Machine Dependent Features
7105 @cindex machine dependencies
7106 @command{ld} has additional features on some platforms; the following
7107 sections describe them. Machines where @command{ld} has no additional
7108 functionality are not listed.
7112 * H8/300:: @command{ld} and the H8/300
7115 * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families
7118 * ARM:: @command{ld} and the ARM family
7121 * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF
7124 * M68K:: @command{ld} and the Motorola 68K family
7127 * MIPS:: @command{ld} and the MIPS family
7130 * MMIX:: @command{ld} and MMIX
7133 * MSP430:: @command{ld} and MSP430
7136 * NDS32:: @command{ld} and NDS32
7139 * Nios II:: @command{ld} and the Altera Nios II
7142 * PowerPC ELF32:: @command{ld} and PowerPC 32-bit ELF Support
7145 * PowerPC64 ELF64:: @command{ld} and PowerPC64 64-bit ELF Support
7148 * S/390 ELF:: @command{ld} and S/390 ELF Support
7151 * SPU ELF:: @command{ld} and SPU ELF Support
7154 * TI COFF:: @command{ld} and TI COFF
7157 * WIN32:: @command{ld} and WIN32 (cygwin/mingw)
7160 * Xtensa:: @command{ld} and Xtensa Processors
7171 @section @command{ld} and the H8/300
7173 @cindex H8/300 support
7174 For the H8/300, @command{ld} can perform these global optimizations when
7175 you specify the @samp{--relax} command-line option.
7178 @cindex relaxing on H8/300
7179 @item relaxing address modes
7180 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
7181 targets are within eight bits, and turns them into eight-bit
7182 program-counter relative @code{bsr} and @code{bra} instructions,
7185 @cindex synthesizing on H8/300
7186 @item synthesizing instructions
7187 @c FIXME: specifically mov.b, or any mov instructions really? -> mov.b only, at least on H8, H8H, H8S
7188 @command{ld} finds all @code{mov.b} instructions which use the
7189 sixteen-bit absolute address form, but refer to the top
7190 page of memory, and changes them to use the eight-bit address form.
7191 (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into
7192 @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the
7193 top page of memory).
7195 @command{ld} finds all @code{mov} instructions which use the register
7196 indirect with 32-bit displacement addressing mode, but use a small
7197 displacement inside 16-bit displacement range, and changes them to use
7198 the 16-bit displacement form. (That is: the linker turns @samp{mov.b
7199 @code{@@}@var{d}:32,ERx} into @samp{mov.b @code{@@}@var{d}:16,ERx}
7200 whenever the displacement @var{d} is in the 16 bit signed integer
7201 range. Only implemented in ELF-format ld).
7203 @item bit manipulation instructions
7204 @command{ld} finds all bit manipulation instructions like @code{band, bclr,
7205 biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor}
7206 which use 32 bit and 16 bit absolute address form, but refer to the top
7207 page of memory, and changes them to use the 8 bit address form.
7208 (That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into
7209 @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in
7210 the top page of memory).
7212 @item system control instructions
7213 @command{ld} finds all @code{ldc.w, stc.w} instructions which use the
7214 32 bit absolute address form, but refer to the top page of memory, and
7215 changes them to use 16 bit address form.
7216 (That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into
7217 @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in
7218 the top page of memory).
7228 @c This stuff is pointless to say unless you're especially concerned
7229 @c with Renesas chips; don't enable it for generic case, please.
7231 @chapter @command{ld} and Other Renesas Chips
7233 @command{ld} also supports the Renesas (formerly Hitachi) H8/300H,
7234 H8/500, and SH chips. No special features, commands, or command-line
7235 options are required for these chips.
7249 @node M68HC11/68HC12
7250 @section @command{ld} and the Motorola 68HC11 and 68HC12 families
7252 @cindex M68HC11 and 68HC12 support
7254 @subsection Linker Relaxation
7256 For the Motorola 68HC11, @command{ld} can perform these global
7257 optimizations when you specify the @samp{--relax} command-line option.
7260 @cindex relaxing on M68HC11
7261 @item relaxing address modes
7262 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
7263 targets are within eight bits, and turns them into eight-bit
7264 program-counter relative @code{bsr} and @code{bra} instructions,
7267 @command{ld} also looks at all 16-bit extended addressing modes and
7268 transforms them in a direct addressing mode when the address is in
7269 page 0 (between 0 and 0x0ff).
7271 @item relaxing gcc instruction group
7272 When @command{gcc} is called with @option{-mrelax}, it can emit group
7273 of instructions that the linker can optimize to use a 68HC11 direct
7274 addressing mode. These instructions consists of @code{bclr} or
7275 @code{bset} instructions.
7279 @subsection Trampoline Generation
7281 @cindex trampoline generation on M68HC11
7282 @cindex trampoline generation on M68HC12
7283 For 68HC11 and 68HC12, @command{ld} can generate trampoline code to
7284 call a far function using a normal @code{jsr} instruction. The linker
7285 will also change the relocation to some far function to use the
7286 trampoline address instead of the function address. This is typically the
7287 case when a pointer to a function is taken. The pointer will in fact
7288 point to the function trampoline.
7296 @section @command{ld} and the ARM family
7298 @cindex ARM interworking support
7299 @kindex --support-old-code
7300 For the ARM, @command{ld} will generate code stubs to allow functions calls
7301 between ARM and Thumb code. These stubs only work with code that has
7302 been compiled and assembled with the @samp{-mthumb-interwork} command
7303 line option. If it is necessary to link with old ARM object files or
7304 libraries, which have not been compiled with the -mthumb-interwork
7305 option then the @samp{--support-old-code} command-line switch should be
7306 given to the linker. This will make it generate larger stub functions
7307 which will work with non-interworking aware ARM code. Note, however,
7308 the linker does not support generating stubs for function calls to
7309 non-interworking aware Thumb code.
7311 @cindex thumb entry point
7312 @cindex entry point, thumb
7313 @kindex --thumb-entry=@var{entry}
7314 The @samp{--thumb-entry} switch is a duplicate of the generic
7315 @samp{--entry} switch, in that it sets the program's starting address.
7316 But it also sets the bottom bit of the address, so that it can be
7317 branched to using a BX instruction, and the program will start
7318 executing in Thumb mode straight away.
7320 @cindex PE import table prefixing
7321 @kindex --use-nul-prefixed-import-tables
7322 The @samp{--use-nul-prefixed-import-tables} switch is specifying, that
7323 the import tables idata4 and idata5 have to be generated with a zero
7324 element prefix for import libraries. This is the old style to generate
7325 import tables. By default this option is turned off.
7329 The @samp{--be8} switch instructs @command{ld} to generate BE8 format
7330 executables. This option is only valid when linking big-endian
7331 objects - ie ones which have been assembled with the @option{-EB}
7332 option. The resulting image will contain big-endian data and
7336 @kindex --target1-rel
7337 @kindex --target1-abs
7338 The @samp{R_ARM_TARGET1} relocation is typically used for entries in the
7339 @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32}
7340 or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel}
7341 and @samp{--target1-abs} switches override the default.
7344 @kindex --target2=@var{type}
7345 The @samp{--target2=type} switch overrides the default definition of the
7346 @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their
7347 meanings, and target defaults are as follows:
7350 @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi)
7354 @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd)
7359 The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF
7360 specification) enables objects compiled for the ARMv4 architecture to be
7361 interworking-safe when linked with other objects compiled for ARMv4t, but
7362 also allows pure ARMv4 binaries to be built from the same ARMv4 objects.
7364 In the latter case, the switch @option{--fix-v4bx} must be passed to the
7365 linker, which causes v4t @code{BX rM} instructions to be rewritten as
7366 @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction.
7368 In the former case, the switch should not be used, and @samp{R_ARM_V4BX}
7369 relocations are ignored.
7371 @cindex FIX_V4BX_INTERWORKING
7372 @kindex --fix-v4bx-interworking
7373 Replace @code{BX rM} instructions identified by @samp{R_ARM_V4BX}
7374 relocations with a branch to the following veneer:
7382 This allows generation of libraries/applications that work on ARMv4 cores
7383 and are still interworking safe. Note that the above veneer clobbers the
7384 condition flags, so may cause incorrect program behavior in rare cases.
7388 The @samp{--use-blx} switch enables the linker to use ARM/Thumb
7389 BLX instructions (available on ARMv5t and above) in various
7390 situations. Currently it is used to perform calls via the PLT from Thumb
7391 code using BLX rather than using BX and a mode-switching stub before
7392 each PLT entry. This should lead to such calls executing slightly faster.
7394 @cindex VFP11_DENORM_FIX
7395 @kindex --vfp11-denorm-fix
7396 The @samp{--vfp11-denorm-fix} switch enables a link-time workaround for a
7397 bug in certain VFP11 coprocessor hardware, which sometimes allows
7398 instructions with denorm operands (which must be handled by support code)
7399 to have those operands overwritten by subsequent instructions before
7400 the support code can read the intended values.
7402 The bug may be avoided in scalar mode if you allow at least one
7403 intervening instruction between a VFP11 instruction which uses a register
7404 and another instruction which writes to the same register, or at least two
7405 intervening instructions if vector mode is in use. The bug only affects
7406 full-compliance floating-point mode: you do not need this workaround if
7407 you are using "runfast" mode. Please contact ARM for further details.
7409 If you know you are using buggy VFP11 hardware, you can
7410 enable this workaround by specifying the linker option
7411 @samp{--vfp-denorm-fix=scalar} if you are using the VFP11 scalar
7412 mode only, or @samp{--vfp-denorm-fix=vector} if you are using
7413 vector mode (the latter also works for scalar code). The default is
7414 @samp{--vfp-denorm-fix=none}.
7416 If the workaround is enabled, instructions are scanned for
7417 potentially-troublesome sequences, and a veneer is created for each
7418 such sequence which may trigger the erratum. The veneer consists of the
7419 first instruction of the sequence and a branch back to the subsequent
7420 instruction. The original instruction is then replaced with a branch to
7421 the veneer. The extra cycles required to call and return from the veneer
7422 are sufficient to avoid the erratum in both the scalar and vector cases.
7424 @cindex ARM1176 erratum workaround
7425 @kindex --fix-arm1176
7426 @kindex --no-fix-arm1176
7427 The @samp{--fix-arm1176} switch enables a link-time workaround for an erratum
7428 in certain ARM1176 processors. The workaround is enabled by default if you
7429 are targeting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled
7430 unconditionally by specifying @samp{--no-fix-arm1176}.
7432 Further information is available in the ``ARM1176JZ-S and ARM1176JZF-S
7433 Programmer Advice Notice'' available on the ARM documentation website at:
7434 http://infocenter.arm.com/.
7436 @cindex STM32L4xx erratum workaround
7437 @kindex --fix-stm32l4xx-629360
7439 The @samp{--fix-stm32l4xx-629360} switch enables a link-time
7440 workaround for a bug in the bus matrix / memory controller for some of
7441 the STM32 Cortex-M4 based products (STM32L4xx). When accessing
7442 off-chip memory via the affected bus for bus reads of 9 words or more,
7443 the bus can generate corrupt data and/or abort. These are only
7444 core-initiated accesses (not DMA), and might affect any access:
7445 integer loads such as LDM, POP and floating-point loads such as VLDM,
7446 VPOP. Stores are not affected.
7448 The bug can be avoided by splitting memory accesses into the
7449 necessary chunks to keep bus reads below 8 words.
7451 The workaround is not enabled by default, this is equivalent to use
7452 @samp{--fix-stm32l4xx-629360=none}. If you know you are using buggy
7453 STM32L4xx hardware, you can enable the workaround by specifying the
7454 linker option @samp{--fix-stm32l4xx-629360}, or the equivalent
7455 @samp{--fix-stm32l4xx-629360=default}.
7457 If the workaround is enabled, instructions are scanned for
7458 potentially-troublesome sequences, and a veneer is created for each
7459 such sequence which may trigger the erratum. The veneer consists in a
7460 replacement sequence emulating the behaviour of the original one and a
7461 branch back to the subsequent instruction. The original instruction is
7462 then replaced with a branch to the veneer.
7464 The workaround does not always preserve the memory access order for
7465 the LDMDB instruction, when the instruction loads the PC.
7467 The workaround is not able to handle problematic instructions when
7468 they are in the middle of an IT block, since a branch is not allowed
7469 there. In that case, the linker reports a warning and no replacement
7472 The workaround is not able to replace problematic instructions with a
7473 PC-relative branch instruction if the @samp{.text} section is too
7474 large. In that case, when the branch that replaces the original code
7475 cannot be encoded, the linker reports a warning and no replacement
7478 @cindex NO_ENUM_SIZE_WARNING
7479 @kindex --no-enum-size-warning
7480 The @option{--no-enum-size-warning} switch prevents the linker from
7481 warning when linking object files that specify incompatible EABI
7482 enumeration size attributes. For example, with this switch enabled,
7483 linking of an object file using 32-bit enumeration values with another
7484 using enumeration values fitted into the smallest possible space will
7487 @cindex NO_WCHAR_SIZE_WARNING
7488 @kindex --no-wchar-size-warning
7489 The @option{--no-wchar-size-warning} switch prevents the linker from
7490 warning when linking object files that specify incompatible EABI
7491 @code{wchar_t} size attributes. For example, with this switch enabled,
7492 linking of an object file using 32-bit @code{wchar_t} values with another
7493 using 16-bit @code{wchar_t} values will not be diagnosed.
7496 @kindex --pic-veneer
7497 The @samp{--pic-veneer} switch makes the linker use PIC sequences for
7498 ARM/Thumb interworking veneers, even if the rest of the binary
7499 is not PIC. This avoids problems on uClinux targets where
7500 @samp{--emit-relocs} is used to generate relocatable binaries.
7502 @cindex STUB_GROUP_SIZE
7503 @kindex --stub-group-size=@var{N}
7504 The linker will automatically generate and insert small sequences of
7505 code into a linked ARM ELF executable whenever an attempt is made to
7506 perform a function call to a symbol that is too far away. The
7507 placement of these sequences of instructions - called stubs - is
7508 controlled by the command-line option @option{--stub-group-size=N}.
7509 The placement is important because a poor choice can create a need for
7510 duplicate stubs, increasing the code size. The linker will try to
7511 group stubs together in order to reduce interruptions to the flow of
7512 code, but it needs guidance as to how big these groups should be and
7513 where they should be placed.
7515 The value of @samp{N}, the parameter to the
7516 @option{--stub-group-size=} option controls where the stub groups are
7517 placed. If it is negative then all stubs are placed after the first
7518 branch that needs them. If it is positive then the stubs can be
7519 placed either before or after the branches that need them. If the
7520 value of @samp{N} is 1 (either +1 or -1) then the linker will choose
7521 exactly where to place groups of stubs, using its built in heuristics.
7522 A value of @samp{N} greater than 1 (or smaller than -1) tells the
7523 linker that a single group of stubs can service at most @samp{N} bytes
7524 from the input sections.
7526 The default, if @option{--stub-group-size=} is not specified, is
7529 Farcalls stubs insertion is fully supported for the ARM-EABI target
7530 only, because it relies on object files properties not present
7533 @cindex Cortex-A8 erratum workaround
7534 @kindex --fix-cortex-a8
7535 @kindex --no-fix-cortex-a8
7536 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}.
7538 The erratum only affects Thumb-2 code. Please contact ARM for further details.
7540 @cindex Cortex-A53 erratum 835769 workaround
7541 @kindex --fix-cortex-a53-835769
7542 @kindex --no-fix-cortex-a53-835769
7543 The @samp{--fix-cortex-a53-835769} switch enables a link-time workaround for erratum 835769 present on certain early revisions of Cortex-A53 processors. The workaround is disabled by default. It can be enabled by specifying @samp{--fix-cortex-a53-835769}, or disabled unconditionally by specifying @samp{--no-fix-cortex-a53-835769}.
7545 Please contact ARM for further details.
7547 @kindex --merge-exidx-entries
7548 @kindex --no-merge-exidx-entries
7549 @cindex Merging exidx entries
7550 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent exidx entries in debuginfo.
7553 @cindex 32-bit PLT entries
7554 The @samp{--long-plt} option enables the use of 16 byte PLT entries
7555 which support up to 4Gb of code. The default is to use 12 byte PLT
7556 entries which only support 512Mb of code.
7558 @kindex --no-apply-dynamic-relocs
7559 @cindex AArch64 rela addend
7560 The @samp{--no-apply-dynamic-relocs} option makes AArch64 linker do not apply
7561 link-time values for dynamic relocations.
7563 @cindex Placement of SG veneers
7564 All SG veneers are placed in the special output section @code{.gnu.sgstubs}.
7565 Its start address must be set, either with the command-line option
7566 @samp{--section-start} or in a linker script, to indicate where to place these
7569 @kindex --cmse-implib
7570 @cindex Secure gateway import library
7571 The @samp{--cmse-implib} option requests that the import libraries
7572 specified by the @samp{--out-implib} and @samp{--in-implib} options are
7573 secure gateway import libraries, suitable for linking a non-secure
7574 executable against secure code as per ARMv8-M Security Extensions.
7576 @kindex --in-implib=@var{file}
7577 @cindex Input import library
7578 The @samp{--in-implib=file} specifies an input import library whose symbols
7579 must keep the same address in the executable being produced. A warning is
7580 given if no @samp{--out-implib} is given but new symbols have been introduced
7581 in the executable that should be listed in its import library. Otherwise, if
7582 @samp{--out-implib} is specified, the symbols are added to the output import
7583 library. A warning is also given if some symbols present in the input import
7584 library have disappeared from the executable. This option is only effective
7585 for Secure Gateway import libraries, ie. when @samp{--cmse-implib} is
7599 @section @command{ld} and HPPA 32-bit ELF Support
7600 @cindex HPPA multiple sub-space stubs
7601 @kindex --multi-subspace
7602 When generating a shared library, @command{ld} will by default generate
7603 import stubs suitable for use with a single sub-space application.
7604 The @samp{--multi-subspace} switch causes @command{ld} to generate export
7605 stubs, and different (larger) import stubs suitable for use with
7606 multiple sub-spaces.
7608 @cindex HPPA stub grouping
7609 @kindex --stub-group-size=@var{N}
7610 Long branch stubs and import/export stubs are placed by @command{ld} in
7611 stub sections located between groups of input sections.
7612 @samp{--stub-group-size} specifies the maximum size of a group of input
7613 sections handled by one stub section. Since branch offsets are signed,
7614 a stub section may serve two groups of input sections, one group before
7615 the stub section, and one group after it. However, when using
7616 conditional branches that require stubs, it may be better (for branch
7617 prediction) that stub sections only serve one group of input sections.
7618 A negative value for @samp{N} chooses this scheme, ensuring that
7619 branches to stubs always use a negative offset. Two special values of
7620 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
7621 @command{ld} to automatically size input section groups for the branch types
7622 detected, with the same behaviour regarding stub placement as other
7623 positive or negative values of @samp{N} respectively.
7625 Note that @samp{--stub-group-size} does not split input sections. A
7626 single input section larger than the group size specified will of course
7627 create a larger group (of one section). If input sections are too
7628 large, it may not be possible for a branch to reach its stub.
7641 @section @command{ld} and the Motorola 68K family
7643 @cindex Motorola 68K GOT generation
7644 @kindex --got=@var{type}
7645 The @samp{--got=@var{type}} option lets you choose the GOT generation scheme.
7646 The choices are @samp{single}, @samp{negative}, @samp{multigot} and
7647 @samp{target}. When @samp{target} is selected the linker chooses
7648 the default GOT generation scheme for the current target.
7649 @samp{single} tells the linker to generate a single GOT with
7650 entries only at non-negative offsets.
7651 @samp{negative} instructs the linker to generate a single GOT with
7652 entries at both negative and positive offsets. Not all environments
7654 @samp{multigot} allows the linker to generate several GOTs in the
7655 output file. All GOT references from a single input object
7656 file access the same GOT, but references from different input object
7657 files might access different GOTs. Not all environments support such GOTs.
7670 @section @command{ld} and the MIPS family
7672 @cindex MIPS microMIPS instruction choice selection
7675 The @samp{--insn32} and @samp{--no-insn32} options control the choice of
7676 microMIPS instructions used in code generated by the linker, such as that
7677 in the PLT or lazy binding stubs, or in relaxation. If @samp{--insn32} is
7678 used, then the linker only uses 32-bit instruction encodings. By default
7679 or if @samp{--no-insn32} is used, all instruction encodings are used,
7680 including 16-bit ones where possible.
7682 @cindex MIPS branch relocation check control
7683 @kindex --ignore-branch-isa
7684 @kindex --no-ignore-branch-isa
7685 The @samp{--ignore-branch-isa} and @samp{--no-ignore-branch-isa} options
7686 control branch relocation checks for invalid ISA mode transitions. If
7687 @samp{--ignore-branch-isa} is used, then the linker accepts any branch
7688 relocations and any ISA mode transition required is lost in relocation
7689 calculation, except for some cases of @code{BAL} instructions which meet
7690 relaxation conditions and are converted to equivalent @code{JALX}
7691 instructions as the associated relocation is calculated. By default
7692 or if @samp{--no-ignore-branch-isa} is used a check is made causing
7693 the loss of an ISA mode transition to produce an error.
7706 @section @code{ld} and MMIX
7707 For MMIX, there is a choice of generating @code{ELF} object files or
7708 @code{mmo} object files when linking. The simulator @code{mmix}
7709 understands the @code{mmo} format. The binutils @code{objcopy} utility
7710 can translate between the two formats.
7712 There is one special section, the @samp{.MMIX.reg_contents} section.
7713 Contents in this section is assumed to correspond to that of global
7714 registers, and symbols referring to it are translated to special symbols,
7715 equal to registers. In a final link, the start address of the
7716 @samp{.MMIX.reg_contents} section corresponds to the first allocated
7717 global register multiplied by 8. Register @code{$255} is not included in
7718 this section; it is always set to the program entry, which is at the
7719 symbol @code{Main} for @code{mmo} files.
7721 Global symbols with the prefix @code{__.MMIX.start.}, for example
7722 @code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special.
7723 The default linker script uses these to set the default start address
7726 Initial and trailing multiples of zero-valued 32-bit words in a section,
7727 are left out from an mmo file.
7740 @section @code{ld} and MSP430
7741 For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]}
7742 will select an appropriate linker script for selected MPU type. (To get a list of known MPUs
7743 just pass @samp{-m help} option to the linker).
7745 @cindex MSP430 extra sections
7746 The linker will recognize some extra sections which are MSP430 specific:
7749 @item @samp{.vectors}
7750 Defines a portion of ROM where interrupt vectors located.
7752 @item @samp{.bootloader}
7753 Defines the bootloader portion of the ROM (if applicable). Any code
7754 in this section will be uploaded to the MPU.
7756 @item @samp{.infomem}
7757 Defines an information memory section (if applicable). Any code in
7758 this section will be uploaded to the MPU.
7760 @item @samp{.infomemnobits}
7761 This is the same as the @samp{.infomem} section except that any code
7762 in this section will not be uploaded to the MPU.
7764 @item @samp{.noinit}
7765 Denotes a portion of RAM located above @samp{.bss} section.
7767 The last two sections are used by gcc.
7771 @cindex MSP430 Options
7772 @kindex --code-region
7773 @item --code-region=[either,lower,upper,none]
7774 This will transform .text* sections to [either,lower,upper].text* sections. The
7775 argument passed to GCC for -mcode-region is propagated to the linker
7778 @kindex --data-region
7779 @item --data-region=[either,lower,upper,none]
7780 This will transform .data*, .bss* and .rodata* sections to
7781 [either,lower,upper].[data,bss,rodata]* sections. The argument passed to GCC
7782 for -mdata-region is propagated to the linker using this option.
7784 @kindex --disable-sec-transformation
7785 @item --disable-sec-transformation
7786 Prevent the transformation of sections as specified by the @code{--code-region}
7787 and @code{--data-region} options.
7788 This is useful if you are compiling and linking using a single call to the GCC
7789 wrapper, and want to compile the source files using -m[code,data]-region but
7790 not transform the sections for prebuilt libraries and objects.
7804 @section @code{ld} and NDS32
7805 @kindex relaxing on NDS32
7806 For NDS32, there are some options to select relaxation behavior. The linker
7807 relaxes objects according to these options.
7810 @item @samp{--m[no-]fp-as-gp}
7811 Disable/enable fp-as-gp relaxation.
7813 @item @samp{--mexport-symbols=FILE}
7814 Exporting symbols and their address into FILE as linker script.
7816 @item @samp{--m[no-]ex9}
7817 Disable/enable link-time EX9 relaxation.
7819 @item @samp{--mexport-ex9=FILE}
7820 Export the EX9 table after linking.
7822 @item @samp{--mimport-ex9=FILE}
7823 Import the Ex9 table for EX9 relaxation.
7825 @item @samp{--mupdate-ex9}
7826 Update the existing EX9 table.
7828 @item @samp{--mex9-limit=NUM}
7829 Maximum number of entries in the ex9 table.
7831 @item @samp{--mex9-loop-aware}
7832 Avoid generating the EX9 instruction inside the loop.
7834 @item @samp{--m[no-]ifc}
7835 Disable/enable the link-time IFC optimization.
7837 @item @samp{--mifc-loop-aware}
7838 Avoid generating the IFC instruction inside the loop.
7852 @section @command{ld} and the Altera Nios II
7853 @cindex Nios II call relaxation
7854 @kindex --relax on Nios II
7856 Call and immediate jump instructions on Nios II processors are limited to
7857 transferring control to addresses in the same 256MB memory segment,
7858 which may result in @command{ld} giving
7859 @samp{relocation truncated to fit} errors with very large programs.
7860 The command-line option @option{--relax} enables the generation of
7861 trampolines that can access the entire 32-bit address space for calls
7862 outside the normal @code{call} and @code{jmpi} address range. These
7863 trampolines are inserted at section boundaries, so may not themselves
7864 be reachable if an input section and its associated call trampolines are
7867 The @option{--relax} option is enabled by default unless @option{-r}
7868 is also specified. You can disable trampoline generation by using the
7869 @option{--no-relax} linker option. You can also disable this optimization
7870 locally by using the @samp{set .noat} directive in assembly-language
7871 source files, as the linker-inserted trampolines use the @code{at}
7872 register as a temporary.
7874 Note that the linker @option{--relax} option is independent of assembler
7875 relaxation options, and that using the GNU assembler's @option{-relax-all}
7876 option interferes with the linker's more selective call instruction relaxation.
7889 @section @command{ld} and PowerPC 32-bit ELF Support
7890 @cindex PowerPC long branches
7891 @kindex --relax on PowerPC
7892 Branches on PowerPC processors are limited to a signed 26-bit
7893 displacement, which may result in @command{ld} giving
7894 @samp{relocation truncated to fit} errors with very large programs.
7895 @samp{--relax} enables the generation of trampolines that can access
7896 the entire 32-bit address space. These trampolines are inserted at
7897 section boundaries, so may not themselves be reachable if an input
7898 section exceeds 33M in size. You may combine @samp{-r} and
7899 @samp{--relax} to add trampolines in a partial link. In that case
7900 both branches to undefined symbols and inter-section branches are also
7901 considered potentially out of range, and trampolines inserted.
7903 @cindex PowerPC ELF32 options
7908 Current PowerPC GCC accepts a @samp{-msecure-plt} option that
7909 generates code capable of using a newer PLT and GOT layout that has
7910 the security advantage of no executable section ever needing to be
7911 writable and no writable section ever being executable. PowerPC
7912 @command{ld} will generate this layout, including stubs to access the
7913 PLT, if all input files (including startup and static libraries) were
7914 compiled with @samp{-msecure-plt}. @samp{--bss-plt} forces the old
7915 BSS PLT (and GOT layout) which can give slightly better performance.
7917 @kindex --secure-plt
7919 @command{ld} will use the new PLT and GOT layout if it is linking new
7920 @samp{-fpic} or @samp{-fPIC} code, but does not do so automatically
7921 when linking non-PIC code. This option requests the new PLT and GOT
7922 layout. A warning will be given if some object file requires the old
7928 The new secure PLT and GOT are placed differently relative to other
7929 sections compared to older BSS PLT and GOT placement. The location of
7930 @code{.plt} must change because the new secure PLT is an initialized
7931 section while the old PLT is uninitialized. The reason for the
7932 @code{.got} change is more subtle: The new placement allows
7933 @code{.got} to be read-only in applications linked with
7934 @samp{-z relro -z now}. However, this placement means that
7935 @code{.sdata} cannot always be used in shared libraries, because the
7936 PowerPC ABI accesses @code{.sdata} in shared libraries from the GOT
7937 pointer. @samp{--sdata-got} forces the old GOT placement. PowerPC
7938 GCC doesn't use @code{.sdata} in shared libraries, so this option is
7939 really only useful for other compilers that may do so.
7941 @cindex PowerPC stub symbols
7942 @kindex --emit-stub-syms
7943 @item --emit-stub-syms
7944 This option causes @command{ld} to label linker stubs with a local
7945 symbol that encodes the stub type and destination.
7947 @cindex PowerPC TLS optimization
7948 @kindex --no-tls-optimize
7949 @item --no-tls-optimize
7950 PowerPC @command{ld} normally performs some optimization of code
7951 sequences used to access Thread-Local Storage. Use this option to
7952 disable the optimization.
7965 @node PowerPC64 ELF64
7966 @section @command{ld} and PowerPC64 64-bit ELF Support
7968 @cindex PowerPC64 ELF64 options
7970 @cindex PowerPC64 stub grouping
7971 @kindex --stub-group-size
7972 @item --stub-group-size
7973 Long branch stubs, PLT call stubs and TOC adjusting stubs are placed
7974 by @command{ld} in stub sections located between groups of input sections.
7975 @samp{--stub-group-size} specifies the maximum size of a group of input
7976 sections handled by one stub section. Since branch offsets are signed,
7977 a stub section may serve two groups of input sections, one group before
7978 the stub section, and one group after it. However, when using
7979 conditional branches that require stubs, it may be better (for branch
7980 prediction) that stub sections only serve one group of input sections.
7981 A negative value for @samp{N} chooses this scheme, ensuring that
7982 branches to stubs always use a negative offset. Two special values of
7983 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
7984 @command{ld} to automatically size input section groups for the branch types
7985 detected, with the same behaviour regarding stub placement as other
7986 positive or negative values of @samp{N} respectively.
7988 Note that @samp{--stub-group-size} does not split input sections. A
7989 single input section larger than the group size specified will of course
7990 create a larger group (of one section). If input sections are too
7991 large, it may not be possible for a branch to reach its stub.
7993 @cindex PowerPC64 stub symbols
7994 @kindex --emit-stub-syms
7995 @item --emit-stub-syms
7996 This option causes @command{ld} to label linker stubs with a local
7997 symbol that encodes the stub type and destination.
7999 @cindex PowerPC64 dot symbols
8001 @kindex --no-dotsyms
8004 These two options control how @command{ld} interprets version patterns
8005 in a version script. Older PowerPC64 compilers emitted both a
8006 function descriptor symbol with the same name as the function, and a
8007 code entry symbol with the name prefixed by a dot (@samp{.}). To
8008 properly version a function @samp{foo}, the version script thus needs
8009 to control both @samp{foo} and @samp{.foo}. The option
8010 @samp{--dotsyms}, on by default, automatically adds the required
8011 dot-prefixed patterns. Use @samp{--no-dotsyms} to disable this
8014 @cindex PowerPC64 register save/restore functions
8015 @kindex --save-restore-funcs
8016 @kindex --no-save-restore-funcs
8017 @item --save-restore-funcs
8018 @itemx --no-save-restore-funcs
8019 These two options control whether PowerPC64 @command{ld} automatically
8020 provides out-of-line register save and restore functions used by
8021 @samp{-Os} code. The default is to provide any such referenced
8022 function for a normal final link, and to not do so for a relocatable
8025 @cindex PowerPC64 TLS optimization
8026 @kindex --no-tls-optimize
8027 @item --no-tls-optimize
8028 PowerPC64 @command{ld} normally performs some optimization of code
8029 sequences used to access Thread-Local Storage. Use this option to
8030 disable the optimization.
8032 @cindex PowerPC64 __tls_get_addr optimization
8033 @kindex --tls-get-addr-optimize
8034 @kindex --no-tls-get-addr-optimize
8035 @kindex --tls-get-addr-regsave
8036 @kindex --no-tls-get-addr-regsave
8037 @item --tls-get-addr-optimize
8038 @itemx --no-tls-get-addr-optimize
8039 These options control how PowerPC64 @command{ld} uses a special
8040 stub to call __tls_get_addr. PowerPC64 glibc 2.22 and later support
8041 an optimization that allows the second and subsequent calls to
8042 @code{__tls_get_addr} for a given symbol to be resolved by the special
8043 stub without calling in to glibc. By default the linker enables
8044 generation of the stub when glibc advertises the availability of
8046 Using @option{--tls-get-addr-optimize} with an older glibc won't do
8047 much besides slow down your applications, but may be useful if linking
8048 an application against an older glibc with the expectation that it
8049 will normally be used on systems having a newer glibc.
8050 @option{--tls-get-addr-regsave} forces generation of a stub that saves
8051 and restores volatile registers around the call into glibc. Normally,
8052 this is done when the linker detects a call to __tls_get_addr_desc.
8053 Such calls then go via the register saving stub to __tls_get_addr_opt.
8054 @option{--no-tls-get-addr-regsave} disables generation of the
8057 @cindex PowerPC64 OPD optimization
8058 @kindex --no-opd-optimize
8059 @item --no-opd-optimize
8060 PowerPC64 @command{ld} normally removes @code{.opd} section entries
8061 corresponding to deleted link-once functions, or functions removed by
8062 the action of @samp{--gc-sections} or linker script @code{/DISCARD/}.
8063 Use this option to disable @code{.opd} optimization.
8065 @cindex PowerPC64 OPD spacing
8066 @kindex --non-overlapping-opd
8067 @item --non-overlapping-opd
8068 Some PowerPC64 compilers have an option to generate compressed
8069 @code{.opd} entries spaced 16 bytes apart, overlapping the third word,
8070 the static chain pointer (unused in C) with the first word of the next
8071 entry. This option expands such entries to the full 24 bytes.
8073 @cindex PowerPC64 TOC optimization
8074 @kindex --no-toc-optimize
8075 @item --no-toc-optimize
8076 PowerPC64 @command{ld} normally removes unused @code{.toc} section
8077 entries. Such entries are detected by examining relocations that
8078 reference the TOC in code sections. A reloc in a deleted code section
8079 marks a TOC word as unneeded, while a reloc in a kept code section
8080 marks a TOC word as needed. Since the TOC may reference itself, TOC
8081 relocs are also examined. TOC words marked as both needed and
8082 unneeded will of course be kept. TOC words without any referencing
8083 reloc are assumed to be part of a multi-word entry, and are kept or
8084 discarded as per the nearest marked preceding word. This works
8085 reliably for compiler generated code, but may be incorrect if assembly
8086 code is used to insert TOC entries. Use this option to disable the
8089 @cindex PowerPC64 inline PLT call optimization
8090 @kindex --no-inline-optimize
8091 @item --no-inline-optimize
8092 PowerPC64 @command{ld} normally replaces inline PLT call sequences
8093 marked with @code{R_PPC64_PLTSEQ}, @code{R_PPC64_PLTCALL},
8094 @code{R_PPC64_PLT16_HA} and @code{R_PPC64_PLT16_LO_DS} relocations by
8095 a number of @code{nop}s and a direct call when the function is defined
8096 locally and can't be overridden by some other definition. This option
8097 disables that optimization.
8099 @cindex PowerPC64 multi-TOC
8100 @kindex --no-multi-toc
8101 @item --no-multi-toc
8102 If given any toc option besides @code{-mcmodel=medium} or
8103 @code{-mcmodel=large}, PowerPC64 GCC generates code for a TOC model
8105 entries are accessed with a 16-bit offset from r2. This limits the
8106 total TOC size to 64K. PowerPC64 @command{ld} extends this limit by
8107 grouping code sections such that each group uses less than 64K for its
8108 TOC entries, then inserts r2 adjusting stubs between inter-group
8109 calls. @command{ld} does not split apart input sections, so cannot
8110 help if a single input file has a @code{.toc} section that exceeds
8111 64K, most likely from linking multiple files with @command{ld -r}.
8112 Use this option to turn off this feature.
8114 @cindex PowerPC64 TOC sorting
8115 @kindex --no-toc-sort
8117 By default, @command{ld} sorts TOC sections so that those whose file
8118 happens to have a section called @code{.init} or @code{.fini} are
8119 placed first, followed by TOC sections referenced by code generated
8120 with PowerPC64 gcc's @code{-mcmodel=small}, and lastly TOC sections
8121 referenced only by code generated with PowerPC64 gcc's
8122 @code{-mcmodel=medium} or @code{-mcmodel=large} options. Doing this
8123 results in better TOC grouping for multi-TOC. Use this option to turn
8126 @cindex PowerPC64 PLT stub alignment
8128 @kindex --no-plt-align
8130 @itemx --no-plt-align
8131 Use these options to control whether individual PLT call stubs are
8132 aligned to a 32-byte boundary, or to the specified power of two
8133 boundary when using @code{--plt-align=}. A negative value may be
8134 specified to pad PLT call stubs so that they do not cross the
8135 specified power of two boundary (or the minimum number of boundaries
8136 if a PLT stub is so large that it must cross a boundary). By default
8137 PLT call stubs are aligned to 32-byte boundaries.
8139 @cindex PowerPC64 PLT call stub static chain
8140 @kindex --plt-static-chain
8141 @kindex --no-plt-static-chain
8142 @item --plt-static-chain
8143 @itemx --no-plt-static-chain
8144 Use these options to control whether PLT call stubs load the static
8145 chain pointer (r11). @code{ld} defaults to not loading the static
8146 chain since there is never any need to do so on a PLT call.
8148 @cindex PowerPC64 PLT call stub thread safety
8149 @kindex --plt-thread-safe
8150 @kindex --no-plt-thread-safe
8151 @item --plt-thread-safe
8152 @itemx --no-plt-thread-safe
8153 With power7's weakly ordered memory model, it is possible when using
8154 lazy binding for ld.so to update a plt entry in one thread and have
8155 another thread see the individual plt entry words update in the wrong
8156 order, despite ld.so carefully writing in the correct order and using
8157 memory write barriers. To avoid this we need some sort of read
8158 barrier in the call stub, or use LD_BIND_NOW=1. By default, @code{ld}
8159 looks for calls to commonly used functions that create threads, and if
8160 seen, adds the necessary barriers. Use these options to change the
8163 @cindex PowerPC64 ELFv2 PLT localentry optimization
8164 @kindex --plt-localentry
8165 @kindex --no-plt-localentry
8166 @item --plt-localentry
8167 @itemx --no-localentry
8168 ELFv2 functions with localentry:0 are those with a single entry point,
8169 ie. global entry == local entry, and that have no requirement on r2
8170 (the TOC/GOT pointer) or r12, and guarantee r2 is unchanged on return.
8171 Such an external function can be called via the PLT without saving r2
8172 or restoring it on return, avoiding a common load-hit-store for small
8173 functions. The optimization is attractive, with up to 40% reduction
8174 in execution time for a small function, but can result in symbol
8175 interposition failures. Also, minor changes in a shared library,
8176 including system libraries, can cause a function that was localentry:0
8177 to become localentry:8. This will result in a dynamic loader
8178 complaint and failure to run. The option is experimental, use with
8179 care. @option{--no-plt-localentry} is the default.
8181 @cindex PowerPC64 Power10 stubs
8182 @kindex --power10-stubs
8183 @kindex --no-power10-stubs
8184 @item --power10-stubs
8185 @itemx --no-power10-stubs
8186 When PowerPC64 @command{ld} links input object files containing
8187 relocations used on power10 prefixed instructions it normally creates
8188 linkage stubs (PLT call and long branch) using power10 instructions
8189 for @code{@@notoc} PLT calls where @code{r2} is not known. The
8190 power10 notoc stubs are smaller and faster, so are preferred for
8191 power10. @option{--power10-stubs} and @option{--no-power10-stubs}
8192 allow you to override the linker's selection of stub instructions.
8193 @option{--power10-stubs=auto} allows the user to select the default
8208 @section @command{ld} and S/390 ELF Support
8210 @cindex S/390 ELF options
8214 @kindex --s390-pgste
8216 This option marks the result file with a @code{PT_S390_PGSTE}
8217 segment. The Linux kernel is supposed to allocate 4k page tables for
8218 binaries marked that way.
8232 @section @command{ld} and SPU ELF Support
8234 @cindex SPU ELF options
8240 This option marks an executable as a PIC plugin module.
8242 @cindex SPU overlays
8243 @kindex --no-overlays
8245 Normally, @command{ld} recognizes calls to functions within overlay
8246 regions, and redirects such calls to an overlay manager via a stub.
8247 @command{ld} also provides a built-in overlay manager. This option
8248 turns off all this special overlay handling.
8250 @cindex SPU overlay stub symbols
8251 @kindex --emit-stub-syms
8252 @item --emit-stub-syms
8253 This option causes @command{ld} to label overlay stubs with a local
8254 symbol that encodes the stub type and destination.
8256 @cindex SPU extra overlay stubs
8257 @kindex --extra-overlay-stubs
8258 @item --extra-overlay-stubs
8259 This option causes @command{ld} to add overlay call stubs on all
8260 function calls out of overlay regions. Normally stubs are not added
8261 on calls to non-overlay regions.
8263 @cindex SPU local store size
8264 @kindex --local-store=lo:hi
8265 @item --local-store=lo:hi
8266 @command{ld} usually checks that a final executable for SPU fits in
8267 the address range 0 to 256k. This option may be used to change the
8268 range. Disable the check entirely with @option{--local-store=0:0}.
8271 @kindex --stack-analysis
8272 @item --stack-analysis
8273 SPU local store space is limited. Over-allocation of stack space
8274 unnecessarily limits space available for code and data, while
8275 under-allocation results in runtime failures. If given this option,
8276 @command{ld} will provide an estimate of maximum stack usage.
8277 @command{ld} does this by examining symbols in code sections to
8278 determine the extents of functions, and looking at function prologues
8279 for stack adjusting instructions. A call-graph is created by looking
8280 for relocations on branch instructions. The graph is then searched
8281 for the maximum stack usage path. Note that this analysis does not
8282 find calls made via function pointers, and does not handle recursion
8283 and other cycles in the call graph. Stack usage may be
8284 under-estimated if your code makes such calls. Also, stack usage for
8285 dynamic allocation, e.g. alloca, will not be detected. If a link map
8286 is requested, detailed information about each function's stack usage
8287 and calls will be given.
8290 @kindex --emit-stack-syms
8291 @item --emit-stack-syms
8292 This option, if given along with @option{--stack-analysis} will result
8293 in @command{ld} emitting stack sizing symbols for each function.
8294 These take the form @code{__stack_<function_name>} for global
8295 functions, and @code{__stack_<number>_<function_name>} for static
8296 functions. @code{<number>} is the section id in hex. The value of
8297 such symbols is the stack requirement for the corresponding function.
8298 The symbol size will be zero, type @code{STT_NOTYPE}, binding
8299 @code{STB_LOCAL}, and section @code{SHN_ABS}.
8313 @section @command{ld}'s Support for Various TI COFF Versions
8314 @cindex TI COFF versions
8315 @kindex --format=@var{version}
8316 The @samp{--format} switch allows selection of one of the various
8317 TI COFF versions. The latest of this writing is 2; versions 0 and 1 are
8318 also supported. The TI COFF versions also vary in header byte-order
8319 format; @command{ld} will read any version or byte order, but the output
8320 header format depends on the default specified by the specific target.
8333 @section @command{ld} and WIN32 (cygwin/mingw)
8335 This section describes some of the win32 specific @command{ld} issues.
8336 See @ref{Options,,Command-line Options} for detailed description of the
8337 command-line options mentioned here.
8340 @cindex import libraries
8341 @item import libraries
8342 The standard Windows linker creates and uses so-called import
8343 libraries, which contains information for linking to dll's. They are
8344 regular static archives and are handled as any other static
8345 archive. The cygwin and mingw ports of @command{ld} have specific
8346 support for creating such libraries provided with the
8347 @samp{--out-implib} command-line option.
8349 @item exporting DLL symbols
8350 @cindex exporting DLL symbols
8351 The cygwin/mingw @command{ld} has several ways to export symbols for dll's.
8354 @item using auto-export functionality
8355 @cindex using auto-export functionality
8356 By default @command{ld} exports symbols with the auto-export functionality,
8357 which is controlled by the following command-line options:
8360 @item --export-all-symbols [This is the default]
8361 @item --exclude-symbols
8362 @item --exclude-libs
8363 @item --exclude-modules-for-implib
8364 @item --version-script
8367 When auto-export is in operation, @command{ld} will export all the non-local
8368 (global and common) symbols it finds in a DLL, with the exception of a few
8369 symbols known to belong to the system's runtime and libraries. As it will
8370 often not be desirable to export all of a DLL's symbols, which may include
8371 private functions that are not part of any public interface, the command-line
8372 options listed above may be used to filter symbols out from the list for
8373 exporting. The @samp{--output-def} option can be used in order to see the
8374 final list of exported symbols with all exclusions taken into effect.
8376 If @samp{--export-all-symbols} is not given explicitly on the
8377 command line, then the default auto-export behavior will be @emph{disabled}
8378 if either of the following are true:
8381 @item A DEF file is used.
8382 @item Any symbol in any object file was marked with the __declspec(dllexport) attribute.
8385 @item using a DEF file
8386 @cindex using a DEF file
8387 Another way of exporting symbols is using a DEF file. A DEF file is
8388 an ASCII file containing definitions of symbols which should be
8389 exported when a dll is created. Usually it is named @samp{<dll
8390 name>.def} and is added as any other object file to the linker's
8391 command line. The file's name must end in @samp{.def} or @samp{.DEF}.
8394 gcc -o <output> <objectfiles> <dll name>.def
8397 Using a DEF file turns off the normal auto-export behavior, unless the
8398 @samp{--export-all-symbols} option is also used.
8400 Here is an example of a DEF file for a shared library called @samp{xyz.dll}:
8403 LIBRARY "xyz.dll" BASE=0x20000000
8409 another_foo = abc.dll.afoo
8415 This example defines a DLL with a non-default base address and seven
8416 symbols in the export table. The third exported symbol @code{_bar} is an
8417 alias for the second. The fourth symbol, @code{another_foo} is resolved
8418 by "forwarding" to another module and treating it as an alias for
8419 @code{afoo} exported from the DLL @samp{abc.dll}. The final symbol
8420 @code{var1} is declared to be a data object. The @samp{doo} symbol in
8421 export library is an alias of @samp{foo}, which gets the string name
8422 in export table @samp{foo2}. The @samp{eoo} symbol is an data export
8423 symbol, which gets in export table the name @samp{var1}.
8425 The optional @code{LIBRARY <name>} command indicates the @emph{internal}
8426 name of the output DLL. If @samp{<name>} does not include a suffix,
8427 the default library suffix, @samp{.DLL} is appended.
8429 When the .DEF file is used to build an application, rather than a
8430 library, the @code{NAME <name>} command should be used instead of
8431 @code{LIBRARY}. If @samp{<name>} does not include a suffix, the default
8432 executable suffix, @samp{.EXE} is appended.
8434 With either @code{LIBRARY <name>} or @code{NAME <name>} the optional
8435 specification @code{BASE = <number>} may be used to specify a
8436 non-default base address for the image.
8438 If neither @code{LIBRARY <name>} nor @code{NAME <name>} is specified,
8439 or they specify an empty string, the internal name is the same as the
8440 filename specified on the command line.
8442 The complete specification of an export symbol is:
8446 ( ( ( <name1> [ = <name2> ] )
8447 | ( <name1> = <module-name> . <external-name>))
8448 [ @@ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
8451 Declares @samp{<name1>} as an exported symbol from the DLL, or declares
8452 @samp{<name1>} as an exported alias for @samp{<name2>}; or declares
8453 @samp{<name1>} as a "forward" alias for the symbol
8454 @samp{<external-name>} in the DLL @samp{<module-name>}.
8455 Optionally, the symbol may be exported by the specified ordinal
8456 @samp{<integer>} alias. The optional @samp{<name3>} is the to be used
8457 string in import/export table for the symbol.
8459 The optional keywords that follow the declaration indicate:
8461 @code{NONAME}: Do not put the symbol name in the DLL's export table. It
8462 will still be exported by its ordinal alias (either the value specified
8463 by the .def specification or, otherwise, the value assigned by the
8464 linker). The symbol name, however, does remain visible in the import
8465 library (if any), unless @code{PRIVATE} is also specified.
8467 @code{DATA}: The symbol is a variable or object, rather than a function.
8468 The import lib will export only an indirect reference to @code{foo} as
8469 the symbol @code{_imp__foo} (ie, @code{foo} must be resolved as
8472 @code{CONSTANT}: Like @code{DATA}, but put the undecorated @code{foo} as
8473 well as @code{_imp__foo} into the import library. Both refer to the
8474 read-only import address table's pointer to the variable, not to the
8475 variable itself. This can be dangerous. If the user code fails to add
8476 the @code{dllimport} attribute and also fails to explicitly add the
8477 extra indirection that the use of the attribute enforces, the
8478 application will behave unexpectedly.
8480 @code{PRIVATE}: Put the symbol in the DLL's export table, but do not put
8481 it into the static import library used to resolve imports at link time. The
8482 symbol can still be imported using the @code{LoadLibrary/GetProcAddress}
8483 API at runtime or by using the GNU ld extension of linking directly to
8484 the DLL without an import library.
8486 See ld/deffilep.y in the binutils sources for the full specification of
8487 other DEF file statements
8489 @cindex creating a DEF file
8490 While linking a shared dll, @command{ld} is able to create a DEF file
8491 with the @samp{--output-def <file>} command-line option.
8493 @item Using decorations
8494 @cindex Using decorations
8495 Another way of marking symbols for export is to modify the source code
8496 itself, so that when building the DLL each symbol to be exported is
8500 __declspec(dllexport) int a_variable
8501 __declspec(dllexport) void a_function(int with_args)
8504 All such symbols will be exported from the DLL. If, however,
8505 any of the object files in the DLL contain symbols decorated in
8506 this way, then the normal auto-export behavior is disabled, unless
8507 the @samp{--export-all-symbols} option is also used.
8509 Note that object files that wish to access these symbols must @emph{not}
8510 decorate them with dllexport. Instead, they should use dllimport,
8514 __declspec(dllimport) int a_variable
8515 __declspec(dllimport) void a_function(int with_args)
8518 This complicates the structure of library header files, because
8519 when included by the library itself the header must declare the
8520 variables and functions as dllexport, but when included by client
8521 code the header must declare them as dllimport. There are a number
8522 of idioms that are typically used to do this; often client code can
8523 omit the __declspec() declaration completely. See
8524 @samp{--enable-auto-import} and @samp{automatic data imports} for more
8528 @cindex automatic data imports
8529 @item automatic data imports
8530 The standard Windows dll format supports data imports from dlls only
8531 by adding special decorations (dllimport/dllexport), which let the
8532 compiler produce specific assembler instructions to deal with this
8533 issue. This increases the effort necessary to port existing Un*x
8534 code to these platforms, especially for large
8535 c++ libraries and applications. The auto-import feature, which was
8536 initially provided by Paul Sokolovsky, allows one to omit the
8537 decorations to achieve a behavior that conforms to that on POSIX/Un*x
8538 platforms. This feature is enabled with the @samp{--enable-auto-import}
8539 command-line option, although it is enabled by default on cygwin/mingw.
8540 The @samp{--enable-auto-import} option itself now serves mainly to
8541 suppress any warnings that are ordinarily emitted when linked objects
8542 trigger the feature's use.
8544 auto-import of variables does not always work flawlessly without
8545 additional assistance. Sometimes, you will see this message
8547 "variable '<var>' can't be auto-imported. Please read the
8548 documentation for ld's @code{--enable-auto-import} for details."
8550 The @samp{--enable-auto-import} documentation explains why this error
8551 occurs, and several methods that can be used to overcome this difficulty.
8552 One of these methods is the @emph{runtime pseudo-relocs} feature, described
8555 @cindex runtime pseudo-relocation
8556 For complex variables imported from DLLs (such as structs or classes),
8557 object files typically contain a base address for the variable and an
8558 offset (@emph{addend}) within the variable--to specify a particular
8559 field or public member, for instance. Unfortunately, the runtime loader used
8560 in win32 environments is incapable of fixing these references at runtime
8561 without the additional information supplied by dllimport/dllexport decorations.
8562 The standard auto-import feature described above is unable to resolve these
8565 The @samp{--enable-runtime-pseudo-relocs} switch allows these references to
8566 be resolved without error, while leaving the task of adjusting the references
8567 themselves (with their non-zero addends) to specialized code provided by the
8568 runtime environment. Recent versions of the cygwin and mingw environments and
8569 compilers provide this runtime support; older versions do not. However, the
8570 support is only necessary on the developer's platform; the compiled result will
8571 run without error on an older system.
8573 @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly
8576 @cindex direct linking to a dll
8577 @item direct linking to a dll
8578 The cygwin/mingw ports of @command{ld} support the direct linking,
8579 including data symbols, to a dll without the usage of any import
8580 libraries. This is much faster and uses much less memory than does the
8581 traditional import library method, especially when linking large
8582 libraries or applications. When @command{ld} creates an import lib, each
8583 function or variable exported from the dll is stored in its own bfd, even
8584 though a single bfd could contain many exports. The overhead involved in
8585 storing, loading, and processing so many bfd's is quite large, and explains the
8586 tremendous time, memory, and storage needed to link against particularly
8587 large or complex libraries when using import libs.
8589 Linking directly to a dll uses no extra command-line switches other than
8590 @samp{-L} and @samp{-l}, because @command{ld} already searches for a number
8591 of names to match each library. All that is needed from the developer's
8592 perspective is an understanding of this search, in order to force ld to
8593 select the dll instead of an import library.
8596 For instance, when ld is called with the argument @samp{-lxxx} it will attempt
8597 to find, in the first directory of its search path,
8610 before moving on to the next directory in the search path.
8612 (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll},
8613 where @samp{<prefix>} is set by the @command{ld} option
8614 @samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec
8615 file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for
8618 Other win32-based unix environments, such as mingw or pw32, may use other
8619 @samp{<prefix>}es, although at present only cygwin makes use of this feature. It
8620 was originally intended to help avoid name conflicts among dll's built for the
8621 various win32/un*x environments, so that (for example) two versions of a zlib dll
8622 could coexist on the same machine.
8624 The generic cygwin/mingw path layout uses a @samp{bin} directory for
8625 applications and dll's and a @samp{lib} directory for the import
8626 libraries (using cygwin nomenclature):
8632 libxxx.dll.a (in case of dll's)
8633 libxxx.a (in case of static archive)
8636 Linking directly to a dll without using the import library can be
8639 1. Use the dll directly by adding the @samp{bin} path to the link line
8641 gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
8644 However, as the dll's often have version numbers appended to their names
8645 (@samp{cygncurses-5.dll}) this will often fail, unless one specifies
8646 @samp{-L../bin -lncurses-5} to include the version. Import libs are generally
8647 not versioned, and do not have this difficulty.
8649 2. Create a symbolic link from the dll to a file in the @samp{lib}
8650 directory according to the above mentioned search pattern. This
8651 should be used to avoid unwanted changes in the tools needed for
8655 ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
8658 Then you can link without any make environment changes.
8661 gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
8664 This technique also avoids the version number problems, because the following is
8671 libxxx.dll.a -> ../bin/cygxxx-5.dll
8674 Linking directly to a dll without using an import lib will work
8675 even when auto-import features are exercised, and even when
8676 @samp{--enable-runtime-pseudo-relocs} is used.
8678 Given the improvements in speed and memory usage, one might justifiably
8679 wonder why import libraries are used at all. There are three reasons:
8681 1. Until recently, the link-directly-to-dll functionality did @emph{not}
8682 work with auto-imported data.
8684 2. Sometimes it is necessary to include pure static objects within the
8685 import library (which otherwise contains only bfd's for indirection
8686 symbols that point to the exports of a dll). Again, the import lib
8687 for the cygwin kernel makes use of this ability, and it is not
8688 possible to do this without an import lib.
8690 3. Symbol aliases can only be resolved using an import lib. This is
8691 critical when linking against OS-supplied dll's (eg, the win32 API)
8692 in which symbols are usually exported as undecorated aliases of their
8693 stdcall-decorated assembly names.
8695 So, import libs are not going away. But the ability to replace
8696 true import libs with a simple symbolic link to (or a copy of)
8697 a dll, in many cases, is a useful addition to the suite of tools
8698 binutils makes available to the win32 developer. Given the
8699 massive improvements in memory requirements during linking, storage
8700 requirements, and linking speed, we expect that many developers
8701 will soon begin to use this feature whenever possible.
8703 @item symbol aliasing
8705 @item adding additional names
8706 Sometimes, it is useful to export symbols with additional names.
8707 A symbol @samp{foo} will be exported as @samp{foo}, but it can also be
8708 exported as @samp{_foo} by using special directives in the DEF file
8709 when creating the dll. This will affect also the optional created
8710 import library. Consider the following DEF file:
8713 LIBRARY "xyz.dll" BASE=0x61000000
8720 The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}.
8722 Another method for creating a symbol alias is to create it in the
8723 source code using the "weak" attribute:
8726 void foo () @{ /* Do something. */; @}
8727 void _foo () __attribute__ ((weak, alias ("foo")));
8730 See the gcc manual for more information about attributes and weak
8733 @item renaming symbols
8734 Sometimes it is useful to rename exports. For instance, the cygwin
8735 kernel does this regularly. A symbol @samp{_foo} can be exported as
8736 @samp{foo} but not as @samp{_foo} by using special directives in the
8737 DEF file. (This will also affect the import library, if it is
8738 created). In the following example:
8741 LIBRARY "xyz.dll" BASE=0x61000000
8747 The line @samp{_foo = foo} maps the exported symbol @samp{foo} to
8751 Note: using a DEF file disables the default auto-export behavior,
8752 unless the @samp{--export-all-symbols} command-line option is used.
8753 If, however, you are trying to rename symbols, then you should list
8754 @emph{all} desired exports in the DEF file, including the symbols
8755 that are not being renamed, and do @emph{not} use the
8756 @samp{--export-all-symbols} option. If you list only the
8757 renamed symbols in the DEF file, and use @samp{--export-all-symbols}
8758 to handle the other symbols, then the both the new names @emph{and}
8759 the original names for the renamed symbols will be exported.
8760 In effect, you'd be aliasing those symbols, not renaming them,
8761 which is probably not what you wanted.
8763 @cindex weak externals
8764 @item weak externals
8765 The Windows object format, PE, specifies a form of weak symbols called
8766 weak externals. When a weak symbol is linked and the symbol is not
8767 defined, the weak symbol becomes an alias for some other symbol. There
8768 are three variants of weak externals:
8770 @item Definition is searched for in objects and libraries, historically
8771 called lazy externals.
8772 @item Definition is searched for only in other objects, not in libraries.
8773 This form is not presently implemented.
8774 @item No search; the symbol is an alias. This form is not presently
8777 As a GNU extension, weak symbols that do not specify an alternate symbol
8778 are supported. If the symbol is undefined when linking, the symbol
8779 uses a default value.
8781 @cindex aligned common symbols
8782 @item aligned common symbols
8783 As a GNU extension to the PE file format, it is possible to specify the
8784 desired alignment for a common symbol. This information is conveyed from
8785 the assembler or compiler to the linker by means of GNU-specific commands
8786 carried in the object file's @samp{.drectve} section, which are recognized
8787 by @command{ld} and respected when laying out the common symbols. Native
8788 tools will be able to process object files employing this GNU extension,
8789 but will fail to respect the alignment instructions, and may issue noisy
8790 warnings about unknown linker directives.
8805 @section @code{ld} and Xtensa Processors
8807 @cindex Xtensa processors
8808 The default @command{ld} behavior for Xtensa processors is to interpret
8809 @code{SECTIONS} commands so that lists of explicitly named sections in a
8810 specification with a wildcard file will be interleaved when necessary to
8811 keep literal pools within the range of PC-relative load offsets. For
8812 example, with the command:
8824 @command{ld} may interleave some of the @code{.literal}
8825 and @code{.text} sections from different object files to ensure that the
8826 literal pools are within the range of PC-relative load offsets. A valid
8827 interleaving might place the @code{.literal} sections from an initial
8828 group of files followed by the @code{.text} sections of that group of
8829 files. Then, the @code{.literal} sections from the rest of the files
8830 and the @code{.text} sections from the rest of the files would follow.
8832 @cindex @option{--relax} on Xtensa
8833 @cindex relaxing on Xtensa
8834 Relaxation is enabled by default for the Xtensa version of @command{ld} and
8835 provides two important link-time optimizations. The first optimization
8836 is to combine identical literal values to reduce code size. A redundant
8837 literal will be removed and all the @code{L32R} instructions that use it
8838 will be changed to reference an identical literal, as long as the
8839 location of the replacement literal is within the offset range of all
8840 the @code{L32R} instructions. The second optimization is to remove
8841 unnecessary overhead from assembler-generated ``longcall'' sequences of
8842 @code{L32R}/@code{CALLX@var{n}} when the target functions are within
8843 range of direct @code{CALL@var{n}} instructions.
8845 For each of these cases where an indirect call sequence can be optimized
8846 to a direct call, the linker will change the @code{CALLX@var{n}}
8847 instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R}
8848 instruction, and remove the literal referenced by the @code{L32R}
8849 instruction if it is not used for anything else. Removing the
8850 @code{L32R} instruction always reduces code size but can potentially
8851 hurt performance by changing the alignment of subsequent branch targets.
8852 By default, the linker will always preserve alignments, either by
8853 switching some instructions between 24-bit encodings and the equivalent
8854 density instructions or by inserting a no-op in place of the @code{L32R}
8855 instruction that was removed. If code size is more important than
8856 performance, the @option{--size-opt} option can be used to prevent the
8857 linker from widening density instructions or inserting no-ops, except in
8858 a few cases where no-ops are required for correctness.
8860 The following Xtensa-specific command-line options can be used to
8863 @cindex Xtensa options
8866 When optimizing indirect calls to direct calls, optimize for code size
8867 more than performance. With this option, the linker will not insert
8868 no-ops or widen density instructions to preserve branch target
8869 alignment. There may still be some cases where no-ops are required to
8870 preserve the correctness of the code.
8872 @item --abi-windowed
8874 Choose ABI for the output object and for the generated PLT code.
8875 PLT code inserted by the linker must match ABI of the output object
8876 because windowed and call0 ABI use incompatible function call
8878 Default ABI is chosen by the ABI tag in the @code{.xtensa.info} section
8879 of the first input object.
8880 A warning is issued if ABI tags of input objects do not match each other
8881 or the chosen output object ABI.
8889 @ifclear SingleFormat
8894 @cindex object file management
8895 @cindex object formats available
8897 The linker accesses object and archive files using the BFD libraries.
8898 These libraries allow the linker to use the same routines to operate on
8899 object files whatever the object file format. A different object file
8900 format can be supported simply by creating a new BFD back end and adding
8901 it to the library. To conserve runtime memory, however, the linker and
8902 associated tools are usually configured to support only a subset of the
8903 object file formats available. You can use @code{objdump -i}
8904 (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to
8905 list all the formats available for your configuration.
8907 @cindex BFD requirements
8908 @cindex requirements for BFD
8909 As with most implementations, BFD is a compromise between
8910 several conflicting requirements. The major factor influencing
8911 BFD design was efficiency: any time used converting between
8912 formats is time which would not have been spent had BFD not
8913 been involved. This is partly offset by abstraction payback; since
8914 BFD simplifies applications and back ends, more time and care
8915 may be spent optimizing algorithms for a greater speed.
8917 One minor artifact of the BFD solution which you should bear in
8918 mind is the potential for information loss. There are two places where
8919 useful information can be lost using the BFD mechanism: during
8920 conversion and during output. @xref{BFD information loss}.
8923 * BFD outline:: How it works: an outline of BFD
8927 @section How It Works: An Outline of BFD
8928 @cindex opening object files
8929 @include bfdsumm.texi
8932 @node Reporting Bugs
8933 @chapter Reporting Bugs
8934 @cindex bugs in @command{ld}
8935 @cindex reporting bugs in @command{ld}
8937 Your bug reports play an essential role in making @command{ld} reliable.
8939 Reporting a bug may help you by bringing a solution to your problem, or
8940 it may not. But in any case the principal function of a bug report is
8941 to help the entire community by making the next version of @command{ld}
8942 work better. Bug reports are your contribution to the maintenance of
8945 In order for a bug report to serve its purpose, you must include the
8946 information that enables us to fix the bug.
8949 * Bug Criteria:: Have you found a bug?
8950 * Bug Reporting:: How to report bugs
8954 @section Have You Found a Bug?
8955 @cindex bug criteria
8957 If you are not sure whether you have found a bug, here are some guidelines:
8960 @cindex fatal signal
8961 @cindex linker crash
8962 @cindex crash of linker
8964 If the linker gets a fatal signal, for any input whatever, that is a
8965 @command{ld} bug. Reliable linkers never crash.
8967 @cindex error on valid input
8969 If @command{ld} produces an error message for valid input, that is a bug.
8971 @cindex invalid input
8973 If @command{ld} does not produce an error message for invalid input, that
8974 may be a bug. In the general case, the linker can not verify that
8975 object files are correct.
8978 If you are an experienced user of linkers, your suggestions for
8979 improvement of @command{ld} are welcome in any case.
8983 @section How to Report Bugs
8985 @cindex @command{ld} bugs, reporting
8987 A number of companies and individuals offer support for @sc{gnu}
8988 products. If you obtained @command{ld} from a support organization, we
8989 recommend you contact that organization first.
8991 You can find contact information for many support companies and
8992 individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs
8996 Otherwise, send bug reports for @command{ld} to
9000 The fundamental principle of reporting bugs usefully is this:
9001 @strong{report all the facts}. If you are not sure whether to state a
9002 fact or leave it out, state it!
9004 Often people omit facts because they think they know what causes the
9005 problem and assume that some details do not matter. Thus, you might
9006 assume that the name of a symbol you use in an example does not
9007 matter. Well, probably it does not, but one cannot be sure. Perhaps
9008 the bug is a stray memory reference which happens to fetch from the
9009 location where that name is stored in memory; perhaps, if the name
9010 were different, the contents of that location would fool the linker
9011 into doing the right thing despite the bug. Play it safe and give a
9012 specific, complete example. That is the easiest thing for you to do,
9013 and the most helpful.
9015 Keep in mind that the purpose of a bug report is to enable us to fix
9016 the bug if it is new to us. Therefore, always write your bug reports
9017 on the assumption that the bug has not been reported previously.
9019 Sometimes people give a few sketchy facts and ask, ``Does this ring a
9020 bell?'' This cannot help us fix a bug, so it is basically useless. We
9021 respond by asking for enough details to enable us to investigate.
9022 You might as well expedite matters by sending them to begin with.
9024 To enable us to fix the bug, you should include all these things:
9028 The version of @command{ld}. @command{ld} announces it if you start it with
9029 the @samp{--version} argument.
9031 Without this, we will not know whether there is any point in looking for
9032 the bug in the current version of @command{ld}.
9035 Any patches you may have applied to the @command{ld} source, including any
9036 patches made to the @code{BFD} library.
9039 The type of machine you are using, and the operating system name and
9043 What compiler (and its version) was used to compile @command{ld}---e.g.
9047 The command arguments you gave the linker to link your example and
9048 observe the bug. To guarantee you will not omit something important,
9049 list them all. A copy of the Makefile (or the output from make) is
9052 If we were to try to guess the arguments, we would probably guess wrong
9053 and then we might not encounter the bug.
9056 A complete input file, or set of input files, that will reproduce the
9057 bug. It is generally most helpful to send the actual object files
9058 provided that they are reasonably small. Say no more than 10K. For
9059 bigger files you can either make them available by FTP or HTTP or else
9060 state that you are willing to send the object file(s) to whomever
9061 requests them. (Note - your email will be going to a mailing list, so
9062 we do not want to clog it up with large attachments). But small
9063 attachments are best.
9065 If the source files were assembled using @code{gas} or compiled using
9066 @code{gcc}, then it may be OK to send the source files rather than the
9067 object files. In this case, be sure to say exactly what version of
9068 @code{gas} or @code{gcc} was used to produce the object files. Also say
9069 how @code{gas} or @code{gcc} were configured.
9072 A description of what behavior you observe that you believe is
9073 incorrect. For example, ``It gets a fatal signal.''
9075 Of course, if the bug is that @command{ld} gets a fatal signal, then we
9076 will certainly notice it. But if the bug is incorrect output, we might
9077 not notice unless it is glaringly wrong. You might as well not give us
9078 a chance to make a mistake.
9080 Even if the problem you experience is a fatal signal, you should still
9081 say so explicitly. Suppose something strange is going on, such as, your
9082 copy of @command{ld} is out of sync, or you have encountered a bug in the
9083 C library on your system. (This has happened!) Your copy might crash
9084 and ours would not. If you told us to expect a crash, then when ours
9085 fails to crash, we would know that the bug was not happening for us. If
9086 you had not told us to expect a crash, then we would not be able to draw
9087 any conclusion from our observations.
9090 If you wish to suggest changes to the @command{ld} source, send us context
9091 diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or
9092 @samp{-p} option. Always send diffs from the old file to the new file.
9093 If you even discuss something in the @command{ld} source, refer to it by
9094 context, not by line number.
9096 The line numbers in our development sources will not match those in your
9097 sources. Your line numbers would convey no useful information to us.
9100 Here are some things that are not necessary:
9104 A description of the envelope of the bug.
9106 Often people who encounter a bug spend a lot of time investigating
9107 which changes to the input file will make the bug go away and which
9108 changes will not affect it.
9110 This is often time consuming and not very useful, because the way we
9111 will find the bug is by running a single example under the debugger
9112 with breakpoints, not by pure deduction from a series of examples.
9113 We recommend that you save your time for something else.
9115 Of course, if you can find a simpler example to report @emph{instead}
9116 of the original one, that is a convenience for us. Errors in the
9117 output will be easier to spot, running under the debugger will take
9118 less time, and so on.
9120 However, simplification is not vital; if you do not want to do this,
9121 report the bug anyway and send us the entire test case you used.
9124 A patch for the bug.
9126 A patch for the bug does help us if it is a good one. But do not omit
9127 the necessary information, such as the test case, on the assumption that
9128 a patch is all we need. We might see problems with your patch and decide
9129 to fix the problem another way, or we might not understand it at all.
9131 Sometimes with a program as complicated as @command{ld} it is very hard to
9132 construct an example that will make the program follow a certain path
9133 through the code. If you do not send us the example, we will not be
9134 able to construct one, so we will not be able to verify that the bug is
9137 And if we cannot understand what bug you are trying to fix, or why your
9138 patch should be an improvement, we will not install it. A test case will
9139 help us to understand.
9142 A guess about what the bug is or what it depends on.
9144 Such guesses are usually wrong. Even we cannot guess right about such
9145 things without first using the debugger to find the facts.
9149 @appendix MRI Compatible Script Files
9150 @cindex MRI compatibility
9151 To aid users making the transition to @sc{gnu} @command{ld} from the MRI
9152 linker, @command{ld} can use MRI compatible linker scripts as an
9153 alternative to the more general-purpose linker scripting language
9154 described in @ref{Scripts}. MRI compatible linker scripts have a much
9155 simpler command set than the scripting language otherwise used with
9156 @command{ld}. @sc{gnu} @command{ld} supports the most commonly used MRI
9157 linker commands; these commands are described here.
9159 In general, MRI scripts aren't of much use with the @code{a.out} object
9160 file format, since it only has three sections and MRI scripts lack some
9161 features to make use of them.
9163 You can specify a file containing an MRI-compatible script using the
9164 @samp{-c} command-line option.
9166 Each command in an MRI-compatible script occupies its own line; each
9167 command line starts with the keyword that identifies the command (though
9168 blank lines are also allowed for punctuation). If a line of an
9169 MRI-compatible script begins with an unrecognized keyword, @command{ld}
9170 issues a warning message, but continues processing the script.
9172 Lines beginning with @samp{*} are comments.
9174 You can write these commands using all upper-case letters, or all
9175 lower case; for example, @samp{chip} is the same as @samp{CHIP}.
9176 The following list shows only the upper-case form of each command.
9179 @cindex @code{ABSOLUTE} (MRI)
9180 @item ABSOLUTE @var{secname}
9181 @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname}
9182 Normally, @command{ld} includes in the output file all sections from all
9183 the input files. However, in an MRI-compatible script, you can use the
9184 @code{ABSOLUTE} command to restrict the sections that will be present in
9185 your output program. If the @code{ABSOLUTE} command is used at all in a
9186 script, then only the sections named explicitly in @code{ABSOLUTE}
9187 commands will appear in the linker output. You can still use other
9188 input sections (whatever you select on the command line, or using
9189 @code{LOAD}) to resolve addresses in the output file.
9191 @cindex @code{ALIAS} (MRI)
9192 @item ALIAS @var{out-secname}, @var{in-secname}
9193 Use this command to place the data from input section @var{in-secname}
9194 in a section called @var{out-secname} in the linker output file.
9196 @var{in-secname} may be an integer.
9198 @cindex @code{ALIGN} (MRI)
9199 @item ALIGN @var{secname} = @var{expression}
9200 Align the section called @var{secname} to @var{expression}. The
9201 @var{expression} should be a power of two.
9203 @cindex @code{BASE} (MRI)
9204 @item BASE @var{expression}
9205 Use the value of @var{expression} as the lowest address (other than
9206 absolute addresses) in the output file.
9208 @cindex @code{CHIP} (MRI)
9209 @item CHIP @var{expression}
9210 @itemx CHIP @var{expression}, @var{expression}
9211 This command does nothing; it is accepted only for compatibility.
9213 @cindex @code{END} (MRI)
9215 This command does nothing whatever; it's only accepted for compatibility.
9217 @cindex @code{FORMAT} (MRI)
9218 @item FORMAT @var{output-format}
9219 Similar to the @code{OUTPUT_FORMAT} command in the more general linker
9220 language, but restricted to S-records, if @var{output-format} is @samp{S}
9222 @cindex @code{LIST} (MRI)
9223 @item LIST @var{anything}@dots{}
9224 Print (to the standard output file) a link map, as produced by the
9225 @command{ld} command-line option @samp{-M}.
9227 The keyword @code{LIST} may be followed by anything on the
9228 same line, with no change in its effect.
9230 @cindex @code{LOAD} (MRI)
9231 @item LOAD @var{filename}
9232 @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename}
9233 Include one or more object file @var{filename} in the link; this has the
9234 same effect as specifying @var{filename} directly on the @command{ld}
9237 @cindex @code{NAME} (MRI)
9238 @item NAME @var{output-name}
9239 @var{output-name} is the name for the program produced by @command{ld}; the
9240 MRI-compatible command @code{NAME} is equivalent to the command-line
9241 option @samp{-o} or the general script language command @code{OUTPUT}.
9243 @cindex @code{ORDER} (MRI)
9244 @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname}
9245 @itemx ORDER @var{secname} @var{secname} @var{secname}
9246 Normally, @command{ld} orders the sections in its output file in the
9247 order in which they first appear in the input files. In an MRI-compatible
9248 script, you can override this ordering with the @code{ORDER} command. The
9249 sections you list with @code{ORDER} will appear first in your output
9250 file, in the order specified.
9252 @cindex @code{PUBLIC} (MRI)
9253 @item PUBLIC @var{name}=@var{expression}
9254 @itemx PUBLIC @var{name},@var{expression}
9255 @itemx PUBLIC @var{name} @var{expression}
9256 Supply a value (@var{expression}) for external symbol
9257 @var{name} used in the linker input files.
9259 @cindex @code{SECT} (MRI)
9260 @item SECT @var{secname}, @var{expression}
9261 @itemx SECT @var{secname}=@var{expression}
9262 @itemx SECT @var{secname} @var{expression}
9263 You can use any of these three forms of the @code{SECT} command to
9264 specify the start address (@var{expression}) for section @var{secname}.
9265 If you have more than one @code{SECT} statement for the same
9266 @var{secname}, only the @emph{first} sets the start address.
9269 @node GNU Free Documentation License
9270 @appendix GNU Free Documentation License
9274 @unnumbered LD Index
9279 % I think something like @@colophon should be in texinfo. In the
9281 \long\def\colophon{\hbox to0pt{}\vfill
9282 \centerline{The body of this manual is set in}
9283 \centerline{\fontname\tenrm,}
9284 \centerline{with headings in {\bf\fontname\tenbf}}
9285 \centerline{and examples in {\tt\fontname\tentt}.}
9286 \centerline{{\it\fontname\tenit\/} and}
9287 \centerline{{\sl\fontname\tensl\/}}
9288 \centerline{are used for emphasis.}\vfill}
9290 % Blame: doc@@cygnus.com, 28mar91.