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 Generate GNU_PROPERTY_X86_FEATURE_1_SHSTK in .note.gnu.property section
1442 to indicate compatibility with Intel Shadow Stack. Supported for
1443 Linux/i386 and Linux/x86_64.
1445 @item stack-size=@var{value}
1446 Specify a stack size for an ELF @code{PT_GNU_STACK} segment.
1447 Specifying zero will override any default non-zero sized
1448 @code{PT_GNU_STACK} segment creation.
1451 @itemx nostart-stop-gc
1452 @cindex start-stop-gc
1453 When @samp{--gc-sections} is in effect, a reference from a retained
1454 section to @code{__start_SECNAME} or @code{__stop_SECNAME} causes all
1455 input sections named @code{SECNAME} to also be retained, if
1456 @code{SECNAME} is representable as a C identifier and either
1457 @code{__start_SECNAME} or @code{__stop_SECNAME} is synthesized by the
1458 linker. @samp{-z start-stop-gc} disables this effect, allowing
1459 sections to be garbage collected as if the special synthesized symbols
1460 were not defined. @samp{-z start-stop-gc} has no effect on a
1461 definition of @code{__start_SECNAME} or @code{__stop_SECNAME} in an
1462 object file or linker script. Such a definition will prevent the
1463 linker providing a synthesized @code{__start_SECNAME} or
1464 @code{__stop_SECNAME} respectively, and therefore the special
1465 treatment by garbage collection for those references.
1467 @item start-stop-visibility=@var{value}
1469 @cindex ELF symbol visibility
1470 Specify the ELF symbol visibility for synthesized
1471 @code{__start_SECNAME} and @code{__stop_SECNAME} symbols (@pxref{Input
1472 Section Example}). @var{value} must be exactly @samp{default},
1473 @samp{internal}, @samp{hidden}, or @samp{protected}. If no @samp{-z
1474 start-stop-visibility} option is given, @samp{protected} is used for
1475 compatibility with historical practice. However, it's highly
1476 recommended to use @samp{-z start-stop-visibility=hidden} in new
1477 programs and shared libraries so that these symbols are not exported
1478 between shared objects, which is not usually what's intended.
1483 Report an error if DT_TEXTREL is set, i.e., if the position-independent
1484 or shared object has dynamic relocations in read-only sections. Don't
1485 report an error if @samp{notext} or @samp{textoff}.
1488 Do not report unresolved symbol references from regular object files,
1489 either when creating an executable, or when creating a shared library.
1490 This option is the inverse of @samp{-z defs}.
1493 @itemx nounique-symbol
1494 Avoid duplicated local symbol names in the symbol string table. Append
1495 ".@code{number}" to duplicated local symbol names if @samp{unique-symbol}
1496 is used. @option{nounique-symbol} is the default.
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 -Bno-symbolic
1638 This option can cancel previously specified @samp{-Bsymbolic} and
1639 @samp{-Bsymbolic-functions}.
1641 @kindex --dynamic-list=@var{dynamic-list-file}
1642 @item --dynamic-list=@var{dynamic-list-file}
1643 Specify the name of a dynamic list file to the linker. This is
1644 typically used when creating shared libraries to specify a list of
1645 global symbols whose references shouldn't be bound to the definition
1646 within the shared library, or creating dynamically linked executables
1647 to specify a list of symbols which should be added to the symbol table
1648 in the executable. This option is only meaningful on ELF platforms
1649 which support shared libraries.
1651 The format of the dynamic list is the same as the version node without
1652 scope and node name. See @ref{VERSION} for more information.
1654 @kindex --dynamic-list-data
1655 @item --dynamic-list-data
1656 Include all global data symbols to the dynamic list.
1658 @kindex --dynamic-list-cpp-new
1659 @item --dynamic-list-cpp-new
1660 Provide the builtin dynamic list for C++ operator new and delete. It
1661 is mainly useful for building shared libstdc++.
1663 @kindex --dynamic-list-cpp-typeinfo
1664 @item --dynamic-list-cpp-typeinfo
1665 Provide the builtin dynamic list for C++ runtime type identification.
1667 @kindex --check-sections
1668 @kindex --no-check-sections
1669 @item --check-sections
1670 @itemx --no-check-sections
1671 Asks the linker @emph{not} to check section addresses after they have
1672 been assigned to see if there are any overlaps. Normally the linker will
1673 perform this check, and if it finds any overlaps it will produce
1674 suitable error messages. The linker does know about, and does make
1675 allowances for sections in overlays. The default behaviour can be
1676 restored by using the command-line switch @option{--check-sections}.
1677 Section overlap is not usually checked for relocatable links. You can
1678 force checking in that case by using the @option{--check-sections}
1681 @kindex --copy-dt-needed-entries
1682 @kindex --no-copy-dt-needed-entries
1683 @item --copy-dt-needed-entries
1684 @itemx --no-copy-dt-needed-entries
1685 This option affects the treatment of dynamic libraries referred to
1686 by DT_NEEDED tags @emph{inside} ELF dynamic libraries mentioned on the
1687 command line. Normally the linker won't add a DT_NEEDED tag to the
1688 output binary for each library mentioned in a DT_NEEDED tag in an
1689 input dynamic library. With @option{--copy-dt-needed-entries}
1690 specified on the command line however any dynamic libraries that
1691 follow it will have their DT_NEEDED entries added. The default
1692 behaviour can be restored with @option{--no-copy-dt-needed-entries}.
1694 This option also has an effect on the resolution of symbols in dynamic
1695 libraries. With @option{--copy-dt-needed-entries} dynamic libraries
1696 mentioned on the command line will be recursively searched, following
1697 their DT_NEEDED tags to other libraries, in order to resolve symbols
1698 required by the output binary. With the default setting however
1699 the searching of dynamic libraries that follow it will stop with the
1700 dynamic library itself. No DT_NEEDED links will be traversed to resolve
1703 @cindex cross reference table
1706 Output a cross reference table. If a linker map file is being
1707 generated, the cross reference table is printed to the map file.
1708 Otherwise, it is printed on the standard output.
1710 The format of the table is intentionally simple, so that it may be
1711 easily processed by a script if necessary. The symbols are printed out,
1712 sorted by name. For each symbol, a list of file names is given. If the
1713 symbol is defined, the first file listed is the location of the
1714 definition. If the symbol is defined as a common value then any files
1715 where this happens appear next. Finally any files that reference the
1718 @cindex ctf variables
1719 @kindex --ctf-variables
1720 @kindex --no-ctf-variables
1721 @item --ctf-variables
1722 @item --no-ctf-variables
1723 The CTF debuginfo format supports a section which encodes the names and
1724 types of variables found in the program which do not appear in any symbol
1725 table. These variables clearly cannot be looked up by address by
1726 conventional debuggers, so the space used for their types and names is
1727 usually wasted: the types are usually small but the names are often not.
1728 @option{--ctf-variables} causes the generation of such a section.
1729 The default behaviour can be restored with @option{--no-ctf-variables}.
1731 @cindex ctf type sharing
1732 @kindex --ctf-share-types
1733 @item --ctf-share-types=@var{method}
1734 Adjust the method used to share types between translation units in CTF.
1737 @item share-unconflicted
1738 Put all types that do not have ambiguous definitions into the shared dictionary,
1739 where debuggers can easily access them, even if they only occur in one
1740 translation unit. This is the default.
1742 @item share-duplicated
1743 Put only types that occur in multiple translation units into the shared
1744 dictionary: types with only one definition go into per-translation-unit
1745 dictionaries. Types with ambiguous definitions in multiple translation units
1746 always go into per-translation-unit dictionaries. This tends to make the CTF
1747 larger, but may reduce the amount of CTF in the shared dictionary. For very
1748 large projects this may speed up opening the CTF and save memory in the CTF
1749 consumer at runtime.
1752 @cindex common allocation
1753 @kindex --no-define-common
1754 @item --no-define-common
1755 This option inhibits the assignment of addresses to common symbols.
1756 The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect.
1757 @xref{Miscellaneous Commands}.
1759 The @samp{--no-define-common} option allows decoupling
1760 the decision to assign addresses to Common symbols from the choice
1761 of the output file type; otherwise a non-Relocatable output type
1762 forces assigning addresses to Common symbols.
1763 Using @samp{--no-define-common} allows Common symbols that are referenced
1764 from a shared library to be assigned addresses only in the main program.
1765 This eliminates the unused duplicate space in the shared library,
1766 and also prevents any possible confusion over resolving to the wrong
1767 duplicate when there are many dynamic modules with specialized search
1768 paths for runtime symbol resolution.
1770 @cindex group allocation in linker script
1771 @cindex section groups
1773 @kindex --force-group-allocation
1774 @item --force-group-allocation
1775 This option causes the linker to place section group members like
1776 normal input sections, and to delete the section groups. This is the
1777 default behaviour for a final link but this option can be used to
1778 change the behaviour of a relocatable link (@samp{-r}). The script
1779 command @code{FORCE_GROUP_ALLOCATION} has the same
1780 effect. @xref{Miscellaneous Commands}.
1782 @cindex symbols, from command line
1783 @kindex --defsym=@var{symbol}=@var{exp}
1784 @item --defsym=@var{symbol}=@var{expression}
1785 Create a global symbol in the output file, containing the absolute
1786 address given by @var{expression}. You may use this option as many
1787 times as necessary to define multiple symbols in the command line. A
1788 limited form of arithmetic is supported for the @var{expression} in this
1789 context: you may give a hexadecimal constant or the name of an existing
1790 symbol, or use @code{+} and @code{-} to add or subtract hexadecimal
1791 constants or symbols. If you need more elaborate expressions, consider
1792 using the linker command language from a script (@pxref{Assignments}).
1793 @emph{Note:} there should be no white space between @var{symbol}, the
1794 equals sign (``@key{=}''), and @var{expression}.
1796 The linker processes @samp{--defsym} arguments and @samp{-T} arguments
1797 in order, placing @samp{--defsym} before @samp{-T} will define the
1798 symbol before the linker script from @samp{-T} is processed, while
1799 placing @samp{--defsym} after @samp{-T} will define the symbol after
1800 the linker script has been processed. This difference has
1801 consequences for expressions within the linker script that use the
1802 @samp{--defsym} symbols, which order is correct will depend on what
1803 you are trying to achieve.
1805 @cindex demangling, from command line
1806 @kindex --demangle[=@var{style}]
1807 @kindex --no-demangle
1808 @item --demangle[=@var{style}]
1809 @itemx --no-demangle
1810 These options control whether to demangle symbol names in error messages
1811 and other output. When the linker is told to demangle, it tries to
1812 present symbol names in a readable fashion: it strips leading
1813 underscores if they are used by the object file format, and converts C++
1814 mangled symbol names into user readable names. Different compilers have
1815 different mangling styles. The optional demangling style argument can be used
1816 to choose an appropriate demangling style for your compiler. The linker will
1817 demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE}
1818 is set. These options may be used to override the default.
1820 @cindex dynamic linker, from command line
1821 @kindex -I@var{file}
1822 @kindex --dynamic-linker=@var{file}
1824 @itemx --dynamic-linker=@var{file}
1825 Set the name of the dynamic linker. This is only meaningful when
1826 generating dynamically linked ELF executables. The default dynamic
1827 linker is normally correct; don't use this unless you know what you are
1830 @kindex --no-dynamic-linker
1831 @item --no-dynamic-linker
1832 When producing an executable file, omit the request for a dynamic
1833 linker to be used at load-time. This is only meaningful for ELF
1834 executables that contain dynamic relocations, and usually requires
1835 entry point code that is capable of processing these relocations.
1837 @kindex --embedded-relocs
1838 @item --embedded-relocs
1839 This option is similar to the @option{--emit-relocs} option except
1840 that the relocs are stored in a target-specific section. This option
1841 is only supported by the @samp{BFIN}, @samp{CR16} and @emph{M68K}
1844 @kindex --disable-multiple-abs-defs
1845 @item --disable-multiple-abs-defs
1846 Do not allow multiple definitions with symbols included
1847 in filename invoked by -R or --just-symbols
1849 @kindex --fatal-warnings
1850 @kindex --no-fatal-warnings
1851 @item --fatal-warnings
1852 @itemx --no-fatal-warnings
1853 Treat all warnings as errors. The default behaviour can be restored
1854 with the option @option{--no-fatal-warnings}.
1856 @kindex --force-exe-suffix
1857 @item --force-exe-suffix
1858 Make sure that an output file has a .exe suffix.
1860 If a successfully built fully linked output file does not have a
1861 @code{.exe} or @code{.dll} suffix, this option forces the linker to copy
1862 the output file to one of the same name with a @code{.exe} suffix. This
1863 option is useful when using unmodified Unix makefiles on a Microsoft
1864 Windows host, since some versions of Windows won't run an image unless
1865 it ends in a @code{.exe} suffix.
1867 @kindex --gc-sections
1868 @kindex --no-gc-sections
1869 @cindex garbage collection
1871 @itemx --no-gc-sections
1872 Enable garbage collection of unused input sections. It is ignored on
1873 targets that do not support this option. The default behaviour (of not
1874 performing this garbage collection) can be restored by specifying
1875 @samp{--no-gc-sections} on the command line. Note that garbage
1876 collection for COFF and PE format targets is supported, but the
1877 implementation is currently considered to be experimental.
1879 @samp{--gc-sections} decides which input sections are used by
1880 examining symbols and relocations. The section containing the entry
1881 symbol and all sections containing symbols undefined on the
1882 command-line will be kept, as will sections containing symbols
1883 referenced by dynamic objects. Note that when building shared
1884 libraries, the linker must assume that any visible symbol is
1885 referenced. Once this initial set of sections has been determined,
1886 the linker recursively marks as used any section referenced by their
1887 relocations. See @samp{--entry}, @samp{--undefined}, and
1888 @samp{--gc-keep-exported}.
1890 This option can be set when doing a partial link (enabled with option
1891 @samp{-r}). In this case the root of symbols kept must be explicitly
1892 specified either by one of the options @samp{--entry},
1893 @samp{--undefined}, or @samp{--gc-keep-exported} or by a @code{ENTRY}
1894 command in the linker script.
1896 As a GNU extension, ELF input sections marked with the
1897 @code{SHF_GNU_RETAIN} flag will not be garbage collected.
1899 @kindex --print-gc-sections
1900 @kindex --no-print-gc-sections
1901 @cindex garbage collection
1902 @item --print-gc-sections
1903 @itemx --no-print-gc-sections
1904 List all sections removed by garbage collection. The listing is
1905 printed on stderr. This option is only effective if garbage
1906 collection has been enabled via the @samp{--gc-sections}) option. The
1907 default behaviour (of not listing the sections that are removed) can
1908 be restored by specifying @samp{--no-print-gc-sections} on the command
1911 @kindex --gc-keep-exported
1912 @cindex garbage collection
1913 @item --gc-keep-exported
1914 When @samp{--gc-sections} is enabled, this option prevents garbage
1915 collection of unused input sections that contain global symbols having
1916 default or protected visibility. This option is intended to be used for
1917 executables where unreferenced sections would otherwise be garbage
1918 collected regardless of the external visibility of contained symbols.
1919 Note that this option has no effect when linking shared objects since
1920 it is already the default behaviour. This option is only supported for
1923 @kindex --print-output-format
1924 @cindex output format
1925 @item --print-output-format
1926 Print the name of the default output format (perhaps influenced by
1927 other command-line options). This is the string that would appear
1928 in an @code{OUTPUT_FORMAT} linker script command (@pxref{File Commands}).
1930 @kindex --print-memory-usage
1931 @cindex memory usage
1932 @item --print-memory-usage
1933 Print used size, total size and used size of memory regions created with
1934 the @ref{MEMORY} command. This is useful on embedded targets to have a
1935 quick view of amount of free memory. The format of the output has one
1936 headline and one line per region. It is both human readable and easily
1937 parsable by tools. Here is an example of an output:
1940 Memory region Used Size Region Size %age Used
1941 ROM: 256 KB 1 MB 25.00%
1942 RAM: 32 B 2 GB 0.00%
1949 Print a summary of the command-line options on the standard output and exit.
1951 @kindex --target-help
1953 Print a summary of all target-specific options on the standard output and exit.
1955 @kindex -Map=@var{mapfile}
1956 @item -Map=@var{mapfile}
1957 Print a link map to the file @var{mapfile}. See the description of the
1958 @option{-M} option, above. If @var{mapfile} is just the character
1959 @code{-} then the map will be written to stdout.
1961 Specifying a directory as @var{mapfile} causes the linker map to be
1962 written as a file inside the directory. Normally name of the file
1963 inside the directory is computed as the basename of the @var{output}
1964 file with @code{.map} appended. If however the special character
1965 @code{%} is used then this will be replaced by the full path of the
1966 output file. Additionally if there are any characters after the
1967 @var{%} symbol then @code{.map} will no longer be appended.
1970 -o foo.exe -Map=bar [Creates ./bar]
1971 -o ../dir/foo.exe -Map=bar [Creates ./bar]
1972 -o foo.exe -Map=../dir [Creates ../dir/foo.exe.map]
1973 -o ../dir2/foo.exe -Map=../dir [Creates ../dir/foo.exe.map]
1974 -o foo.exe -Map=% [Creates ./foo.exe.map]
1975 -o ../dir/foo.exe -Map=% [Creates ../dir/foo.exe.map]
1976 -o foo.exe -Map=%.bar [Creates ./foo.exe.bar]
1977 -o ../dir/foo.exe -Map=%.bar [Creates ../dir/foo.exe.bar]
1978 -o ../dir2/foo.exe -Map=../dir/% [Creates ../dir/../dir2/foo.exe.map]
1979 -o ../dir2/foo.exe -Map=../dir/%.bar [Creates ../dir/../dir2/foo.exe.bar]
1982 It is an error to specify more than one @code{%} character.
1984 If the map file already exists then it will be overwritten by this
1987 @cindex memory usage
1988 @kindex --no-keep-memory
1989 @item --no-keep-memory
1990 @command{ld} normally optimizes for speed over memory usage by caching the
1991 symbol tables of input files in memory. This option tells @command{ld} to
1992 instead optimize for memory usage, by rereading the symbol tables as
1993 necessary. This may be required if @command{ld} runs out of memory space
1994 while linking a large executable.
1996 @kindex --no-undefined
1999 @item --no-undefined
2001 Report unresolved symbol references from regular object files. This
2002 is done even if the linker is creating a non-symbolic shared library.
2003 The switch @option{--[no-]allow-shlib-undefined} controls the
2004 behaviour for reporting unresolved references found in shared
2005 libraries being linked in.
2007 The effects of this option can be reverted by using @code{-z undefs}.
2009 @kindex --allow-multiple-definition
2011 @item --allow-multiple-definition
2013 Normally when a symbol is defined multiple times, the linker will
2014 report a fatal error. These options allow multiple definitions and the
2015 first definition will be used.
2017 @kindex --allow-shlib-undefined
2018 @kindex --no-allow-shlib-undefined
2019 @item --allow-shlib-undefined
2020 @itemx --no-allow-shlib-undefined
2021 Allows or disallows undefined symbols in shared libraries.
2022 This switch is similar to @option{--no-undefined} except that it
2023 determines the behaviour when the undefined symbols are in a
2024 shared library rather than a regular object file. It does not affect
2025 how undefined symbols in regular object files are handled.
2027 The default behaviour is to report errors for any undefined symbols
2028 referenced in shared libraries if the linker is being used to create
2029 an executable, but to allow them if the linker is being used to create
2032 The reasons for allowing undefined symbol references in shared
2033 libraries specified at link time are that:
2037 A shared library specified at link time may not be the same as the one
2038 that is available at load time, so the symbol might actually be
2039 resolvable at load time.
2041 There are some operating systems, eg BeOS and HPPA, where undefined
2042 symbols in shared libraries are normal.
2044 The BeOS kernel for example patches shared libraries at load time to
2045 select whichever function is most appropriate for the current
2046 architecture. This is used, for example, to dynamically select an
2047 appropriate memset function.
2050 @kindex --error-handling-script=@var{scriptname}
2051 @item --error-handling-script=@var{scriptname}
2052 If this option is provided then the linker will invoke
2053 @var{scriptname} whenever an error is encountered. Currently however
2054 only two kinds of error are supported: missing symbols and missing
2055 libraries. Two arguments will be passed to script: the keyword
2056 ``undefined-symbol'' or `missing-lib'' and the @var{name} of the
2057 undefined symbol or missing library. The intention is that the script
2058 will provide suggestions to the user as to where the symbol or library
2059 might be found. After the script has finished then the normal linker
2060 error message will be displayed.
2062 The availability of this option is controlled by a configure time
2063 switch, so it may not be present in specific implementations.
2065 @kindex --no-undefined-version
2066 @item --no-undefined-version
2067 Normally when a symbol has an undefined version, the linker will ignore
2068 it. This option disallows symbols with undefined version and a fatal error
2069 will be issued instead.
2071 @kindex --default-symver
2072 @item --default-symver
2073 Create and use a default symbol version (the soname) for unversioned
2076 @kindex --default-imported-symver
2077 @item --default-imported-symver
2078 Create and use a default symbol version (the soname) for unversioned
2081 @kindex --no-warn-mismatch
2082 @item --no-warn-mismatch
2083 Normally @command{ld} will give an error if you try to link together input
2084 files that are mismatched for some reason, perhaps because they have
2085 been compiled for different processors or for different endiannesses.
2086 This option tells @command{ld} that it should silently permit such possible
2087 errors. This option should only be used with care, in cases when you
2088 have taken some special action that ensures that the linker errors are
2091 @kindex --no-warn-search-mismatch
2092 @item --no-warn-search-mismatch
2093 Normally @command{ld} will give a warning if it finds an incompatible
2094 library during a library search. This option silences the warning.
2096 @kindex --no-whole-archive
2097 @item --no-whole-archive
2098 Turn off the effect of the @option{--whole-archive} option for subsequent
2101 @cindex output file after errors
2102 @kindex --noinhibit-exec
2103 @item --noinhibit-exec
2104 Retain the executable output file whenever it is still usable.
2105 Normally, the linker will not produce an output file if it encounters
2106 errors during the link process; it exits without writing an output file
2107 when it issues any error whatsoever.
2111 Only search library directories explicitly specified on the
2112 command line. Library directories specified in linker scripts
2113 (including linker scripts specified on the command line) are ignored.
2115 @ifclear SingleFormat
2116 @kindex --oformat=@var{output-format}
2117 @item --oformat=@var{output-format}
2118 @command{ld} may be configured to support more than one kind of object
2119 file. If your @command{ld} is configured this way, you can use the
2120 @samp{--oformat} option to specify the binary format for the output
2121 object file. Even when @command{ld} is configured to support alternative
2122 object formats, you don't usually need to specify this, as @command{ld}
2123 should be configured to produce as a default output format the most
2124 usual format on each machine. @var{output-format} is a text string, the
2125 name of a particular format supported by the BFD libraries. (You can
2126 list the available binary formats with @samp{objdump -i}.) The script
2127 command @code{OUTPUT_FORMAT} can also specify the output format, but
2128 this option overrides it. @xref{BFD}.
2131 @kindex --out-implib
2132 @item --out-implib @var{file}
2133 Create an import library in @var{file} corresponding to the executable
2134 the linker is generating (eg. a DLL or ELF program). This import
2135 library (which should be called @code{*.dll.a} or @code{*.a} for DLLs)
2136 may be used to link clients against the generated executable; this
2137 behaviour makes it possible to skip a separate import library creation
2138 step (eg. @code{dlltool} for DLLs). This option is only available for
2139 the i386 PE and ELF targetted ports of the linker.
2142 @kindex --pic-executable
2144 @itemx --pic-executable
2145 @cindex position independent executables
2146 Create a position independent executable. This is currently only supported on
2147 ELF platforms. Position independent executables are similar to shared
2148 libraries in that they are relocated by the dynamic linker to the virtual
2149 address the OS chooses for them (which can vary between invocations). Like
2150 normal dynamically linked executables they can be executed and symbols
2151 defined in the executable cannot be overridden by shared libraries.
2155 @cindex position dependent executables
2156 Create a position dependent executable. This is the default.
2160 This option is ignored for Linux compatibility.
2164 This option is ignored for SVR4 compatibility.
2167 @cindex synthesizing linker
2168 @cindex relaxing addressing modes
2172 An option with machine dependent effects.
2174 This option is only supported on a few targets.
2177 @xref{H8/300,,@command{ld} and the H8/300}.
2180 @xref{Xtensa,, @command{ld} and Xtensa Processors}.
2183 @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}.
2186 @xref{Nios II,,@command{ld} and the Altera Nios II}.
2189 @xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}.
2192 On some platforms the @option{--relax} option performs target specific,
2193 global optimizations that become possible when the linker resolves
2194 addressing in the program, such as relaxing address modes,
2195 synthesizing new instructions, selecting shorter version of current
2196 instructions, and combining constant values.
2198 On some platforms these link time global optimizations may make symbolic
2199 debugging of the resulting executable impossible.
2201 This is known to be the case for the Matsushita MN10200 and MN10300
2202 family of processors.
2205 On platforms where the feature is supported, the option
2206 @option{--no-relax} will disable it.
2208 On platforms where the feature is not supported, both @option{--relax}
2209 and @option{--no-relax} are accepted, but ignored.
2211 @cindex retaining specified symbols
2212 @cindex stripping all but some symbols
2213 @cindex symbols, retaining selectively
2214 @kindex --retain-symbols-file=@var{filename}
2215 @item --retain-symbols-file=@var{filename}
2216 Retain @emph{only} the symbols listed in the file @var{filename},
2217 discarding all others. @var{filename} is simply a flat file, with one
2218 symbol name per line. This option is especially useful in environments
2222 where a large global symbol table is accumulated gradually, to conserve
2225 @samp{--retain-symbols-file} does @emph{not} discard undefined symbols,
2226 or symbols needed for relocations.
2228 You may only specify @samp{--retain-symbols-file} once in the command
2229 line. It overrides @samp{-s} and @samp{-S}.
2232 @item -rpath=@var{dir}
2233 @cindex runtime library search path
2234 @kindex -rpath=@var{dir}
2235 Add a directory to the runtime library search path. This is used when
2236 linking an ELF executable with shared objects. All @option{-rpath}
2237 arguments are concatenated and passed to the runtime linker, which uses
2238 them to locate shared objects at runtime.
2240 The @option{-rpath} option is also used when locating shared objects which
2241 are needed by shared objects explicitly included in the link; see the
2242 description of the @option{-rpath-link} option. Searching @option{-rpath}
2243 in this way is only supported by native linkers and cross linkers which
2244 have been configured with the @option{--with-sysroot} option.
2246 If @option{-rpath} is not used when linking an ELF executable, the
2247 contents of the environment variable @code{LD_RUN_PATH} will be used if it
2250 The @option{-rpath} option may also be used on SunOS. By default, on
2251 SunOS, the linker will form a runtime search path out of all the
2252 @option{-L} options it is given. If a @option{-rpath} option is used, the
2253 runtime search path will be formed exclusively using the @option{-rpath}
2254 options, ignoring the @option{-L} options. This can be useful when using
2255 gcc, which adds many @option{-L} options which may be on NFS mounted
2258 For compatibility with other ELF linkers, if the @option{-R} option is
2259 followed by a directory name, rather than a file name, it is treated as
2260 the @option{-rpath} option.
2264 @cindex link-time runtime library search path
2265 @kindex -rpath-link=@var{dir}
2266 @item -rpath-link=@var{dir}
2267 When using ELF or SunOS, one shared library may require another. This
2268 happens when an @code{ld -shared} link includes a shared library as one
2271 When the linker encounters such a dependency when doing a non-shared,
2272 non-relocatable link, it will automatically try to locate the required
2273 shared library and include it in the link, if it is not included
2274 explicitly. In such a case, the @option{-rpath-link} option
2275 specifies the first set of directories to search. The
2276 @option{-rpath-link} option may specify a sequence of directory names
2277 either by specifying a list of names separated by colons, or by
2278 appearing multiple times.
2280 The tokens @var{$ORIGIN} and @var{$LIB} can appear in these search
2281 directories. They will be replaced by the full path to the directory
2282 containing the program or shared object in the case of @var{$ORIGIN}
2283 and either @samp{lib} - for 32-bit binaries - or @samp{lib64} - for
2284 64-bit binaries - in the case of @var{$LIB}.
2286 The alternative form of these tokens - @var{$@{ORIGIN@}} and
2287 @var{$@{LIB@}} can also be used. The token @var{$PLATFORM} is not
2290 This option should be used with caution as it overrides the search path
2291 that may have been hard compiled into a shared library. In such a case it
2292 is possible to use unintentionally a different search path than the
2293 runtime linker would do.
2295 The linker uses the following search paths to locate required shared
2300 Any directories specified by @option{-rpath-link} options.
2302 Any directories specified by @option{-rpath} options. The difference
2303 between @option{-rpath} and @option{-rpath-link} is that directories
2304 specified by @option{-rpath} options are included in the executable and
2305 used at runtime, whereas the @option{-rpath-link} option is only effective
2306 at link time. Searching @option{-rpath} in this way is only supported
2307 by native linkers and cross linkers which have been configured with
2308 the @option{--with-sysroot} option.
2310 On an ELF system, for native linkers, if the @option{-rpath} and
2311 @option{-rpath-link} options were not used, search the contents of the
2312 environment variable @code{LD_RUN_PATH}.
2314 On SunOS, if the @option{-rpath} option was not used, search any
2315 directories specified using @option{-L} options.
2317 For a native linker, search the contents of the environment
2318 variable @code{LD_LIBRARY_PATH}.
2320 For a native ELF linker, the directories in @code{DT_RUNPATH} or
2321 @code{DT_RPATH} of a shared library are searched for shared
2322 libraries needed by it. The @code{DT_RPATH} entries are ignored if
2323 @code{DT_RUNPATH} entries exist.
2325 For a linker for a Linux system, if the file @file{/etc/ld.so.conf}
2326 exists, the list of directories found in that file. Note: the path
2327 to this file is prefixed with the @code{sysroot} value, if that is
2328 defined, and then any @code{prefix} string if the linker was
2329 configured with the @command{--prefix=<path>} option.
2331 For a native linker on a FreeBSD system, any directories specified by
2332 the @code{_PATH_ELF_HINTS} macro defined in the @file{elf-hints.h}
2335 Any directories specified by a @code{SEARCH_DIR} command in a
2336 linker script given on the command line, including scripts specified
2337 by @option{-T} (but not @option{-dT}).
2339 The default directories, normally @file{/lib} and @file{/usr/lib}.
2341 Any directories specified by a plugin LDPT_SET_EXTRA_LIBRARY_PATH.
2343 Any directories specified by a @code{SEARCH_DIR} command in a default
2347 Note however on Linux based systems there is an additional caveat: If
2348 the @option{--as-needed} option is active @emph{and} a shared library
2349 is located which would normally satisfy the search @emph{and} this
2350 library does not have DT_NEEDED tag for @file{libc.so}
2351 @emph{and} there is a shared library later on in the set of search
2352 directories which also satisfies the search @emph{and}
2353 this second shared library does have a DT_NEEDED tag for
2354 @file{libc.so} @emph{then} the second library will be selected instead
2357 If the required shared library is not found, the linker will issue a
2358 warning and continue with the link.
2366 @cindex shared libraries
2367 Create a shared library. This is currently only supported on ELF, XCOFF
2368 and SunOS platforms. On SunOS, the linker will automatically create a
2369 shared library if the @option{-e} option is not used and there are
2370 undefined symbols in the link.
2372 @kindex --sort-common
2374 @itemx --sort-common=ascending
2375 @itemx --sort-common=descending
2376 This option tells @command{ld} to sort the common symbols by alignment in
2377 ascending or descending order when it places them in the appropriate output
2378 sections. The symbol alignments considered are sixteen-byte or larger,
2379 eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps
2380 between symbols due to alignment constraints. If no sorting order is
2381 specified, then descending order is assumed.
2383 @kindex --sort-section=name
2384 @item --sort-section=name
2385 This option will apply @code{SORT_BY_NAME} to all wildcard section
2386 patterns in the linker script.
2388 @kindex --sort-section=alignment
2389 @item --sort-section=alignment
2390 This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section
2391 patterns in the linker script.
2393 @kindex --spare-dynamic-tags
2394 @item --spare-dynamic-tags=@var{count}
2395 This option specifies the number of empty slots to leave in the
2396 .dynamic section of ELF shared objects. Empty slots may be needed by
2397 post processing tools, such as the prelinker. The default is 5.
2399 @kindex --split-by-file
2400 @item --split-by-file[=@var{size}]
2401 Similar to @option{--split-by-reloc} but creates a new output section for
2402 each input file when @var{size} is reached. @var{size} defaults to a
2403 size of 1 if not given.
2405 @kindex --split-by-reloc
2406 @item --split-by-reloc[=@var{count}]
2407 Tries to creates extra sections in the output file so that no single
2408 output section in the file contains more than @var{count} relocations.
2409 This is useful when generating huge relocatable files for downloading into
2410 certain real time kernels with the COFF object file format; since COFF
2411 cannot represent more than 65535 relocations in a single section. Note
2412 that this will fail to work with object file formats which do not
2413 support arbitrary sections. The linker will not split up individual
2414 input sections for redistribution, so if a single input section contains
2415 more than @var{count} relocations one output section will contain that
2416 many relocations. @var{count} defaults to a value of 32768.
2420 Compute and display statistics about the operation of the linker, such
2421 as execution time and memory usage.
2423 @kindex --sysroot=@var{directory}
2424 @item --sysroot=@var{directory}
2425 Use @var{directory} as the location of the sysroot, overriding the
2426 configure-time default. This option is only supported by linkers
2427 that were configured using @option{--with-sysroot}.
2431 This is used by COFF/PE based targets to create a task-linked object
2432 file where all of the global symbols have been converted to statics.
2434 @kindex --traditional-format
2435 @cindex traditional format
2436 @item --traditional-format
2437 For some targets, the output of @command{ld} is different in some ways from
2438 the output of some existing linker. This switch requests @command{ld} to
2439 use the traditional format instead.
2442 For example, on SunOS, @command{ld} combines duplicate entries in the
2443 symbol string table. This can reduce the size of an output file with
2444 full debugging information by over 30 percent. Unfortunately, the SunOS
2445 @code{dbx} program can not read the resulting program (@code{gdb} has no
2446 trouble). The @samp{--traditional-format} switch tells @command{ld} to not
2447 combine duplicate entries.
2449 @kindex --section-start=@var{sectionname}=@var{org}
2450 @item --section-start=@var{sectionname}=@var{org}
2451 Locate a section in the output file at the absolute
2452 address given by @var{org}. You may use this option as many
2453 times as necessary to locate multiple sections in the command
2455 @var{org} must be a single hexadecimal integer;
2456 for compatibility with other linkers, you may omit the leading
2457 @samp{0x} usually associated with hexadecimal values. @emph{Note:} there
2458 should be no white space between @var{sectionname}, the equals
2459 sign (``@key{=}''), and @var{org}.
2461 @kindex -Tbss=@var{org}
2462 @kindex -Tdata=@var{org}
2463 @kindex -Ttext=@var{org}
2464 @cindex segment origins, cmd line
2465 @item -Tbss=@var{org}
2466 @itemx -Tdata=@var{org}
2467 @itemx -Ttext=@var{org}
2468 Same as @option{--section-start}, with @code{.bss}, @code{.data} or
2469 @code{.text} as the @var{sectionname}.
2471 @kindex -Ttext-segment=@var{org}
2472 @item -Ttext-segment=@var{org}
2473 @cindex text segment origin, cmd line
2474 When creating an ELF executable, it will set the address of the first
2475 byte of the text segment.
2477 @kindex -Trodata-segment=@var{org}
2478 @item -Trodata-segment=@var{org}
2479 @cindex rodata segment origin, cmd line
2480 When creating an ELF executable or shared object for a target where
2481 the read-only data is in its own segment separate from the executable
2482 text, it will set the address of the first byte of the read-only data segment.
2484 @kindex -Tldata-segment=@var{org}
2485 @item -Tldata-segment=@var{org}
2486 @cindex ldata segment origin, cmd line
2487 When creating an ELF executable or shared object for x86-64 medium memory
2488 model, it will set the address of the first byte of the ldata segment.
2490 @kindex --unresolved-symbols
2491 @item --unresolved-symbols=@var{method}
2492 Determine how to handle unresolved symbols. There are four possible
2493 values for @samp{method}:
2497 Do not report any unresolved symbols.
2500 Report all unresolved symbols. This is the default.
2502 @item ignore-in-object-files
2503 Report unresolved symbols that are contained in shared libraries, but
2504 ignore them if they come from regular object files.
2506 @item ignore-in-shared-libs
2507 Report unresolved symbols that come from regular object files, but
2508 ignore them if they come from shared libraries. This can be useful
2509 when creating a dynamic binary and it is known that all the shared
2510 libraries that it should be referencing are included on the linker's
2514 The behaviour for shared libraries on their own can also be controlled
2515 by the @option{--[no-]allow-shlib-undefined} option.
2517 Normally the linker will generate an error message for each reported
2518 unresolved symbol but the option @option{--warn-unresolved-symbols}
2519 can change this to a warning.
2521 @kindex --verbose[=@var{NUMBER}]
2522 @cindex verbose[=@var{NUMBER}]
2524 @itemx --verbose[=@var{NUMBER}]
2525 Display the version number for @command{ld} and list the linker emulations
2526 supported. Display which input files can and cannot be opened. Display
2527 the linker script being used by the linker. If the optional @var{NUMBER}
2528 argument > 1, plugin symbol status will also be displayed.
2530 @kindex --version-script=@var{version-scriptfile}
2531 @cindex version script, symbol versions
2532 @item --version-script=@var{version-scriptfile}
2533 Specify the name of a version script to the linker. This is typically
2534 used when creating shared libraries to specify additional information
2535 about the version hierarchy for the library being created. This option
2536 is only fully supported on ELF platforms which support shared libraries;
2537 see @ref{VERSION}. It is partially supported on PE platforms, which can
2538 use version scripts to filter symbol visibility in auto-export mode: any
2539 symbols marked @samp{local} in the version script will not be exported.
2542 @kindex --warn-common
2543 @cindex warnings, on combining symbols
2544 @cindex combining symbols, warnings on
2546 Warn when a common symbol is combined with another common symbol or with
2547 a symbol definition. Unix linkers allow this somewhat sloppy practice,
2548 but linkers on some other operating systems do not. This option allows
2549 you to find potential problems from combining global symbols.
2550 Unfortunately, some C libraries use this practice, so you may get some
2551 warnings about symbols in the libraries as well as in your programs.
2553 There are three kinds of global symbols, illustrated here by C examples:
2557 A definition, which goes in the initialized data section of the output
2561 An undefined reference, which does not allocate space.
2562 There must be either a definition or a common symbol for the
2566 A common symbol. If there are only (one or more) common symbols for a
2567 variable, it goes in the uninitialized data area of the output file.
2568 The linker merges multiple common symbols for the same variable into a
2569 single symbol. If they are of different sizes, it picks the largest
2570 size. The linker turns a common symbol into a declaration, if there is
2571 a definition of the same variable.
2574 The @samp{--warn-common} option can produce five kinds of warnings.
2575 Each warning consists of a pair of lines: the first describes the symbol
2576 just encountered, and the second describes the previous symbol
2577 encountered with the same name. One or both of the two symbols will be
2582 Turning a common symbol into a reference, because there is already a
2583 definition for the symbol.
2585 @var{file}(@var{section}): warning: common of `@var{symbol}'
2586 overridden by definition
2587 @var{file}(@var{section}): warning: defined here
2591 Turning a common symbol into a reference, because a later definition for
2592 the symbol is encountered. This is the same as the previous case,
2593 except that the symbols are encountered in a different order.
2595 @var{file}(@var{section}): warning: definition of `@var{symbol}'
2597 @var{file}(@var{section}): warning: common is here
2601 Merging a common symbol with a previous same-sized common symbol.
2603 @var{file}(@var{section}): warning: multiple common
2605 @var{file}(@var{section}): warning: previous common is here
2609 Merging a common symbol with a previous larger common symbol.
2611 @var{file}(@var{section}): warning: common of `@var{symbol}'
2612 overridden by larger common
2613 @var{file}(@var{section}): warning: larger common is here
2617 Merging a common symbol with a previous smaller common symbol. This is
2618 the same as the previous case, except that the symbols are
2619 encountered in a different order.
2621 @var{file}(@var{section}): warning: common of `@var{symbol}'
2622 overriding smaller common
2623 @var{file}(@var{section}): warning: smaller common is here
2627 @kindex --warn-constructors
2628 @item --warn-constructors
2629 Warn if any global constructors are used. This is only useful for a few
2630 object file formats. For formats like COFF or ELF, the linker can not
2631 detect the use of global constructors.
2633 @kindex --warn-multiple-gp
2634 @item --warn-multiple-gp
2635 Warn if multiple global pointer values are required in the output file.
2636 This is only meaningful for certain processors, such as the Alpha.
2637 Specifically, some processors put large-valued constants in a special
2638 section. A special register (the global pointer) points into the middle
2639 of this section, so that constants can be loaded efficiently via a
2640 base-register relative addressing mode. Since the offset in
2641 base-register relative mode is fixed and relatively small (e.g., 16
2642 bits), this limits the maximum size of the constant pool. Thus, in
2643 large programs, it is often necessary to use multiple global pointer
2644 values in order to be able to address all possible constants. This
2645 option causes a warning to be issued whenever this case occurs.
2648 @cindex warnings, on undefined symbols
2649 @cindex undefined symbols, warnings on
2651 Only warn once for each undefined symbol, rather than once per module
2654 @kindex --warn-section-align
2655 @cindex warnings, on section alignment
2656 @cindex section alignment, warnings on
2657 @item --warn-section-align
2658 Warn if the address of an output section is changed because of
2659 alignment. Typically, the alignment will be set by an input section.
2660 The address will only be changed if it not explicitly specified; that
2661 is, if the @code{SECTIONS} command does not specify a start address for
2662 the section (@pxref{SECTIONS}).
2664 @kindex --warn-textrel
2665 @item --warn-textrel
2666 Warn if the linker adds DT_TEXTREL to a position-independent executable
2669 @kindex --warn-alternate-em
2670 @item --warn-alternate-em
2671 Warn if an object has alternate ELF machine code.
2673 @kindex --warn-unresolved-symbols
2674 @item --warn-unresolved-symbols
2675 If the linker is going to report an unresolved symbol (see the option
2676 @option{--unresolved-symbols}) it will normally generate an error.
2677 This option makes it generate a warning instead.
2679 @kindex --error-unresolved-symbols
2680 @item --error-unresolved-symbols
2681 This restores the linker's default behaviour of generating errors when
2682 it is reporting unresolved symbols.
2684 @kindex --whole-archive
2685 @cindex including an entire archive
2686 @item --whole-archive
2687 For each archive mentioned on the command line after the
2688 @option{--whole-archive} option, include every object file in the archive
2689 in the link, rather than searching the archive for the required object
2690 files. This is normally used to turn an archive file into a shared
2691 library, forcing every object to be included in the resulting shared
2692 library. This option may be used more than once.
2694 Two notes when using this option from gcc: First, gcc doesn't know
2695 about this option, so you have to use @option{-Wl,-whole-archive}.
2696 Second, don't forget to use @option{-Wl,-no-whole-archive} after your
2697 list of archives, because gcc will add its own list of archives to
2698 your link and you may not want this flag to affect those as well.
2700 @kindex --wrap=@var{symbol}
2701 @item --wrap=@var{symbol}
2702 Use a wrapper function for @var{symbol}. Any undefined reference to
2703 @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any
2704 undefined reference to @code{__real_@var{symbol}} will be resolved to
2707 This can be used to provide a wrapper for a system function. The
2708 wrapper function should be called @code{__wrap_@var{symbol}}. If it
2709 wishes to call the system function, it should call
2710 @code{__real_@var{symbol}}.
2712 Here is a trivial example:
2716 __wrap_malloc (size_t c)
2718 printf ("malloc called with %zu\n", c);
2719 return __real_malloc (c);
2723 If you link other code with this file using @option{--wrap malloc}, then
2724 all calls to @code{malloc} will call the function @code{__wrap_malloc}
2725 instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will
2726 call the real @code{malloc} function.
2728 You may wish to provide a @code{__real_malloc} function as well, so that
2729 links without the @option{--wrap} option will succeed. If you do this,
2730 you should not put the definition of @code{__real_malloc} in the same
2731 file as @code{__wrap_malloc}; if you do, the assembler may resolve the
2732 call before the linker has a chance to wrap it to @code{malloc}.
2734 Only undefined references are replaced by the linker. So, translation unit
2735 internal references to @var{symbol} are not resolved to
2736 @code{__wrap_@var{symbol}}. In the next example, the call to @code{f} in
2737 @code{g} is not resolved to @code{__wrap_f}.
2753 @kindex --eh-frame-hdr
2754 @kindex --no-eh-frame-hdr
2755 @item --eh-frame-hdr
2756 @itemx --no-eh-frame-hdr
2757 Request (@option{--eh-frame-hdr}) or suppress
2758 (@option{--no-eh-frame-hdr}) the creation of @code{.eh_frame_hdr}
2759 section and ELF @code{PT_GNU_EH_FRAME} segment header.
2761 @kindex --ld-generated-unwind-info
2762 @item --no-ld-generated-unwind-info
2763 Request creation of @code{.eh_frame} unwind info for linker
2764 generated code sections like PLT. This option is on by default
2765 if linker generated unwind info is supported.
2767 @kindex --enable-new-dtags
2768 @kindex --disable-new-dtags
2769 @item --enable-new-dtags
2770 @itemx --disable-new-dtags
2771 This linker can create the new dynamic tags in ELF. But the older ELF
2772 systems may not understand them. If you specify
2773 @option{--enable-new-dtags}, the new dynamic tags will be created as needed
2774 and older dynamic tags will be omitted.
2775 If you specify @option{--disable-new-dtags}, no new dynamic tags will be
2776 created. By default, the new dynamic tags are not created. Note that
2777 those options are only available for ELF systems.
2779 @kindex --hash-size=@var{number}
2780 @item --hash-size=@var{number}
2781 Set the default size of the linker's hash tables to a prime number
2782 close to @var{number}. Increasing this value can reduce the length of
2783 time it takes the linker to perform its tasks, at the expense of
2784 increasing the linker's memory requirements. Similarly reducing this
2785 value can reduce the memory requirements at the expense of speed.
2787 @kindex --hash-style=@var{style}
2788 @item --hash-style=@var{style}
2789 Set the type of linker's hash table(s). @var{style} can be either
2790 @code{sysv} for classic ELF @code{.hash} section, @code{gnu} for
2791 new style GNU @code{.gnu.hash} section or @code{both} for both
2792 the classic ELF @code{.hash} and new style GNU @code{.gnu.hash}
2793 hash tables. The default depends upon how the linker was configured,
2794 but for most Linux based systems it will be @code{both}.
2796 @kindex --compress-debug-sections=none
2797 @kindex --compress-debug-sections=zlib
2798 @kindex --compress-debug-sections=zlib-gnu
2799 @kindex --compress-debug-sections=zlib-gabi
2800 @item --compress-debug-sections=none
2801 @itemx --compress-debug-sections=zlib
2802 @itemx --compress-debug-sections=zlib-gnu
2803 @itemx --compress-debug-sections=zlib-gabi
2804 On ELF platforms, these options control how DWARF debug sections are
2805 compressed using zlib.
2807 @option{--compress-debug-sections=none} doesn't compress DWARF debug
2808 sections. @option{--compress-debug-sections=zlib-gnu} compresses
2809 DWARF debug sections and renames them to begin with @samp{.zdebug}
2810 instead of @samp{.debug}. @option{--compress-debug-sections=zlib-gabi}
2811 also compresses DWARF debug sections, but rather than renaming them it
2812 sets the SHF_COMPRESSED flag in the sections' headers.
2814 The @option{--compress-debug-sections=zlib} option is an alias for
2815 @option{--compress-debug-sections=zlib-gabi}.
2817 Note that this option overrides any compression in input debug
2818 sections, so if a binary is linked with @option{--compress-debug-sections=none}
2819 for example, then any compressed debug sections in input files will be
2820 uncompressed before they are copied into the output binary.
2822 The default compression behaviour varies depending upon the target
2823 involved and the configure options used to build the toolchain. The
2824 default can be determined by examining the output from the linker's
2825 @option{--help} option.
2827 @kindex --reduce-memory-overheads
2828 @item --reduce-memory-overheads
2829 This option reduces memory requirements at ld runtime, at the expense of
2830 linking speed. This was introduced to select the old O(n^2) algorithm
2831 for link map file generation, rather than the new O(n) algorithm which uses
2832 about 40% more memory for symbol storage.
2834 Another effect of the switch is to set the default hash table size to
2835 1021, which again saves memory at the cost of lengthening the linker's
2836 run time. This is not done however if the @option{--hash-size} switch
2839 The @option{--reduce-memory-overheads} switch may be also be used to
2840 enable other tradeoffs in future versions of the linker.
2843 @kindex --build-id=@var{style}
2845 @itemx --build-id=@var{style}
2846 Request the creation of a @code{.note.gnu.build-id} ELF note section
2847 or a @code{.buildid} COFF section. The contents of the note are
2848 unique bits identifying this linked file. @var{style} can be
2849 @code{uuid} to use 128 random bits, @code{sha1} to use a 160-bit
2850 @sc{SHA1} hash on the normative parts of the output contents,
2851 @code{md5} to use a 128-bit @sc{MD5} hash on the normative parts of
2852 the output contents, or @code{0x@var{hexstring}} to use a chosen bit
2853 string specified as an even number of hexadecimal digits (@code{-} and
2854 @code{:} characters between digit pairs are ignored). If @var{style}
2855 is omitted, @code{sha1} is used.
2857 The @code{md5} and @code{sha1} styles produces an identifier
2858 that is always the same in an identical output file, but will be
2859 unique among all nonidentical output files. It is not intended
2860 to be compared as a checksum for the file's contents. A linked
2861 file may be changed later by other tools, but the build ID bit
2862 string identifying the original linked file does not change.
2864 Passing @code{none} for @var{style} disables the setting from any
2865 @code{--build-id} options earlier on the command line.
2870 @subsection Options Specific to i386 PE Targets
2872 @c man begin OPTIONS
2874 The i386 PE linker supports the @option{-shared} option, which causes
2875 the output to be a dynamically linked library (DLL) instead of a
2876 normal executable. You should name the output @code{*.dll} when you
2877 use this option. In addition, the linker fully supports the standard
2878 @code{*.def} files, which may be specified on the linker command line
2879 like an object file (in fact, it should precede archives it exports
2880 symbols from, to ensure that they get linked in, just like a normal
2883 In addition to the options common to all targets, the i386 PE linker
2884 support additional command-line options that are specific to the i386
2885 PE target. Options that take values may be separated from their
2886 values by either a space or an equals sign.
2890 @kindex --add-stdcall-alias
2891 @item --add-stdcall-alias
2892 If given, symbols with a stdcall suffix (@@@var{nn}) will be exported
2893 as-is and also with the suffix stripped.
2894 [This option is specific to the i386 PE targeted port of the linker]
2897 @item --base-file @var{file}
2898 Use @var{file} as the name of a file in which to save the base
2899 addresses of all the relocations needed for generating DLLs with
2901 [This is an i386 PE specific option]
2905 Create a DLL instead of a regular executable. You may also use
2906 @option{-shared} or specify a @code{LIBRARY} in a given @code{.def}
2908 [This option is specific to the i386 PE targeted port of the linker]
2910 @kindex --enable-long-section-names
2911 @kindex --disable-long-section-names
2912 @item --enable-long-section-names
2913 @itemx --disable-long-section-names
2914 The PE variants of the COFF object format add an extension that permits
2915 the use of section names longer than eight characters, the normal limit
2916 for COFF. By default, these names are only allowed in object files, as
2917 fully-linked executable images do not carry the COFF string table required
2918 to support the longer names. As a GNU extension, it is possible to
2919 allow their use in executable images as well, or to (probably pointlessly!)
2920 disallow it in object files, by using these two options. Executable images
2921 generated with these long section names are slightly non-standard, carrying
2922 as they do a string table, and may generate confusing output when examined
2923 with non-GNU PE-aware tools, such as file viewers and dumpers. However,
2924 GDB relies on the use of PE long section names to find Dwarf-2 debug
2925 information sections in an executable image at runtime, and so if neither
2926 option is specified on the command-line, @command{ld} will enable long
2927 section names, overriding the default and technically correct behaviour,
2928 when it finds the presence of debug information while linking an executable
2929 image and not stripping symbols.
2930 [This option is valid for all PE targeted ports of the linker]
2932 @kindex --enable-stdcall-fixup
2933 @kindex --disable-stdcall-fixup
2934 @item --enable-stdcall-fixup
2935 @itemx --disable-stdcall-fixup
2936 If the link finds a symbol that it cannot resolve, it will attempt to
2937 do ``fuzzy linking'' by looking for another defined symbol that differs
2938 only in the format of the symbol name (cdecl vs stdcall) and will
2939 resolve that symbol by linking to the match. For example, the
2940 undefined symbol @code{_foo} might be linked to the function
2941 @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked
2942 to the function @code{_bar}. When the linker does this, it prints a
2943 warning, since it normally should have failed to link, but sometimes
2944 import libraries generated from third-party dlls may need this feature
2945 to be usable. If you specify @option{--enable-stdcall-fixup}, this
2946 feature is fully enabled and warnings are not printed. If you specify
2947 @option{--disable-stdcall-fixup}, this feature is disabled and such
2948 mismatches are considered to be errors.
2949 [This option is specific to the i386 PE targeted port of the linker]
2951 @kindex --leading-underscore
2952 @kindex --no-leading-underscore
2953 @item --leading-underscore
2954 @itemx --no-leading-underscore
2955 For most targets default symbol-prefix is an underscore and is defined
2956 in target's description. By this option it is possible to
2957 disable/enable the default underscore symbol-prefix.
2959 @cindex DLLs, creating
2960 @kindex --export-all-symbols
2961 @item --export-all-symbols
2962 If given, all global symbols in the objects used to build a DLL will
2963 be exported by the DLL. Note that this is the default if there
2964 otherwise wouldn't be any exported symbols. When symbols are
2965 explicitly exported via DEF files or implicitly exported via function
2966 attributes, the default is to not export anything else unless this
2967 option is given. Note that the symbols @code{DllMain@@12},
2968 @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and
2969 @code{impure_ptr} will not be automatically
2970 exported. Also, symbols imported from other DLLs will not be
2971 re-exported, nor will symbols specifying the DLL's internal layout
2972 such as those beginning with @code{_head_} or ending with
2973 @code{_iname}. In addition, no symbols from @code{libgcc},
2974 @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported.
2975 Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will
2976 not be exported, to help with C++ DLLs. Finally, there is an
2977 extensive list of cygwin-private symbols that are not exported
2978 (obviously, this applies on when building DLLs for cygwin targets).
2979 These cygwin-excludes are: @code{_cygwin_dll_entry@@12},
2980 @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12},
2981 @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll},
2982 @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2},
2983 @code{cygwin_premain3}, and @code{environ}.
2984 [This option is specific to the i386 PE targeted port of the linker]
2986 @kindex --exclude-symbols
2987 @item --exclude-symbols @var{symbol},@var{symbol},...
2988 Specifies a list of symbols which should not be automatically
2989 exported. The symbol names may be delimited by commas or colons.
2990 [This option is specific to the i386 PE targeted port of the linker]
2992 @kindex --exclude-all-symbols
2993 @item --exclude-all-symbols
2994 Specifies no symbols should be automatically exported.
2995 [This option is specific to the i386 PE targeted port of the linker]
2997 @kindex --file-alignment
2998 @item --file-alignment
2999 Specify the file alignment. Sections in the file will always begin at
3000 file offsets which are multiples of this number. This defaults to
3002 [This option is specific to the i386 PE targeted port of the linker]
3006 @item --heap @var{reserve}
3007 @itemx --heap @var{reserve},@var{commit}
3008 Specify the number of bytes of memory to reserve (and optionally commit)
3009 to be used as heap for this program. The default is 1MB reserved, 4K
3011 [This option is specific to the i386 PE targeted port of the linker]
3014 @kindex --image-base
3015 @item --image-base @var{value}
3016 Use @var{value} as the base address of your program or dll. This is
3017 the lowest memory location that will be used when your program or dll
3018 is loaded. To reduce the need to relocate and improve performance of
3019 your dlls, each should have a unique base address and not overlap any
3020 other dlls. The default is 0x400000 for executables, and 0x10000000
3022 [This option is specific to the i386 PE targeted port of the linker]
3026 If given, the stdcall suffixes (@@@var{nn}) will be stripped from
3027 symbols before they are exported.
3028 [This option is specific to the i386 PE targeted port of the linker]
3030 @kindex --large-address-aware
3031 @item --large-address-aware
3032 If given, the appropriate bit in the ``Characteristics'' field of the COFF
3033 header is set to indicate that this executable supports virtual addresses
3034 greater than 2 gigabytes. This should be used in conjunction with the /3GB
3035 or /USERVA=@var{value} megabytes switch in the ``[operating systems]''
3036 section of the BOOT.INI. Otherwise, this bit has no effect.
3037 [This option is specific to PE targeted ports of the linker]
3039 @kindex --disable-large-address-aware
3040 @item --disable-large-address-aware
3041 Reverts the effect of a previous @samp{--large-address-aware} option.
3042 This is useful if @samp{--large-address-aware} is always set by the compiler
3043 driver (e.g. Cygwin gcc) and the executable does not support virtual
3044 addresses greater than 2 gigabytes.
3045 [This option is specific to PE targeted ports of the linker]
3047 @kindex --major-image-version
3048 @item --major-image-version @var{value}
3049 Sets the major number of the ``image version''. Defaults to 1.
3050 [This option is specific to the i386 PE targeted port of the linker]
3052 @kindex --major-os-version
3053 @item --major-os-version @var{value}
3054 Sets the major number of the ``os version''. Defaults to 4.
3055 [This option is specific to the i386 PE targeted port of the linker]
3057 @kindex --major-subsystem-version
3058 @item --major-subsystem-version @var{value}
3059 Sets the major number of the ``subsystem version''. Defaults to 4.
3060 [This option is specific to the i386 PE targeted port of the linker]
3062 @kindex --minor-image-version
3063 @item --minor-image-version @var{value}
3064 Sets the minor number of the ``image version''. Defaults to 0.
3065 [This option is specific to the i386 PE targeted port of the linker]
3067 @kindex --minor-os-version
3068 @item --minor-os-version @var{value}
3069 Sets the minor number of the ``os version''. Defaults to 0.
3070 [This option is specific to the i386 PE targeted port of the linker]
3072 @kindex --minor-subsystem-version
3073 @item --minor-subsystem-version @var{value}
3074 Sets the minor number of the ``subsystem version''. Defaults to 0.
3075 [This option is specific to the i386 PE targeted port of the linker]
3077 @cindex DEF files, creating
3078 @cindex DLLs, creating
3079 @kindex --output-def
3080 @item --output-def @var{file}
3081 The linker will create the file @var{file} which will contain a DEF
3082 file corresponding to the DLL the linker is generating. This DEF file
3083 (which should be called @code{*.def}) may be used to create an import
3084 library with @code{dlltool} or may be used as a reference to
3085 automatically or implicitly exported symbols.
3086 [This option is specific to the i386 PE targeted port of the linker]
3088 @cindex DLLs, creating
3089 @kindex --enable-auto-image-base
3090 @item --enable-auto-image-base
3091 @itemx --enable-auto-image-base=@var{value}
3092 Automatically choose the image base for DLLs, optionally starting with base
3093 @var{value}, unless one is specified using the @code{--image-base} argument.
3094 By using a hash generated from the dllname to create unique image bases
3095 for each DLL, in-memory collisions and relocations which can delay program
3096 execution are avoided.
3097 [This option is specific to the i386 PE targeted port of the linker]
3099 @kindex --disable-auto-image-base
3100 @item --disable-auto-image-base
3101 Do not automatically generate a unique image base. If there is no
3102 user-specified image base (@code{--image-base}) then use the platform
3104 [This option is specific to the i386 PE targeted port of the linker]
3106 @cindex DLLs, linking to
3107 @kindex --dll-search-prefix
3108 @item --dll-search-prefix @var{string}
3109 When linking dynamically to a dll without an import library,
3110 search for @code{<string><basename>.dll} in preference to
3111 @code{lib<basename>.dll}. This behaviour allows easy distinction
3112 between DLLs built for the various "subplatforms": native, cygwin,
3113 uwin, pw, etc. For instance, cygwin DLLs typically use
3114 @code{--dll-search-prefix=cyg}.
3115 [This option is specific to the i386 PE targeted port of the linker]
3117 @kindex --enable-auto-import
3118 @item --enable-auto-import
3119 Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for
3120 DATA imports from DLLs, thus making it possible to bypass the dllimport
3121 mechanism on the user side and to reference unmangled symbol names.
3122 [This option is specific to the i386 PE targeted port of the linker]
3124 The following remarks pertain to the original implementation of the
3125 feature and are obsolete nowadays for Cygwin and MinGW targets.
3127 Note: Use of the 'auto-import' extension will cause the text section
3128 of the image file to be made writable. This does not conform to the
3129 PE-COFF format specification published by Microsoft.
3131 Note - use of the 'auto-import' extension will also cause read only
3132 data which would normally be placed into the .rdata section to be
3133 placed into the .data section instead. This is in order to work
3134 around a problem with consts that is described here:
3135 http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
3137 Using 'auto-import' generally will 'just work' -- but sometimes you may
3140 "variable '<var>' can't be auto-imported. Please read the
3141 documentation for ld's @code{--enable-auto-import} for details."
3143 This message occurs when some (sub)expression accesses an address
3144 ultimately given by the sum of two constants (Win32 import tables only
3145 allow one). Instances where this may occur include accesses to member
3146 fields of struct variables imported from a DLL, as well as using a
3147 constant index into an array variable imported from a DLL. Any
3148 multiword variable (arrays, structs, long long, etc) may trigger
3149 this error condition. However, regardless of the exact data type
3150 of the offending exported variable, ld will always detect it, issue
3151 the warning, and exit.
3153 There are several ways to address this difficulty, regardless of the
3154 data type of the exported variable:
3156 One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
3157 of adjusting references in your client code for runtime environment, so
3158 this method works only when runtime environment supports this feature.
3160 A second solution is to force one of the 'constants' to be a variable --
3161 that is, unknown and un-optimizable at compile time. For arrays,
3162 there are two possibilities: a) make the indexee (the array's address)
3163 a variable, or b) make the 'constant' index a variable. Thus:
3166 extern type extern_array[];
3168 @{ volatile type *t=extern_array; t[1] @}
3174 extern type extern_array[];
3176 @{ volatile int t=1; extern_array[t] @}
3179 For structs (and most other multiword data types) the only option
3180 is to make the struct itself (or the long long, or the ...) variable:
3183 extern struct s extern_struct;
3184 extern_struct.field -->
3185 @{ volatile struct s *t=&extern_struct; t->field @}
3191 extern long long extern_ll;
3193 @{ volatile long long * local_ll=&extern_ll; *local_ll @}
3196 A third method of dealing with this difficulty is to abandon
3197 'auto-import' for the offending symbol and mark it with
3198 @code{__declspec(dllimport)}. However, in practice that
3199 requires using compile-time #defines to indicate whether you are
3200 building a DLL, building client code that will link to the DLL, or
3201 merely building/linking to a static library. In making the choice
3202 between the various methods of resolving the 'direct address with
3203 constant offset' problem, you should consider typical real-world usage:
3211 void main(int argc, char **argv)@{
3212 printf("%d\n",arr[1]);
3222 void main(int argc, char **argv)@{
3223 /* This workaround is for win32 and cygwin; do not "optimize" */
3224 volatile int *parr = arr;
3225 printf("%d\n",parr[1]);
3232 /* Note: auto-export is assumed (no __declspec(dllexport)) */
3233 #if (defined(_WIN32) || defined(__CYGWIN__)) && \
3234 !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
3235 #define FOO_IMPORT __declspec(dllimport)
3239 extern FOO_IMPORT int arr[];
3242 void main(int argc, char **argv)@{
3243 printf("%d\n",arr[1]);
3247 A fourth way to avoid this problem is to re-code your
3248 library to use a functional interface rather than a data interface
3249 for the offending variables (e.g. set_foo() and get_foo() accessor
3252 @kindex --disable-auto-import
3253 @item --disable-auto-import
3254 Do not attempt to do sophisticated linking of @code{_symbol} to
3255 @code{__imp__symbol} for DATA imports from DLLs.
3256 [This option is specific to the i386 PE targeted port of the linker]
3258 @kindex --enable-runtime-pseudo-reloc
3259 @item --enable-runtime-pseudo-reloc
3260 If your code contains expressions described in --enable-auto-import section,
3261 that is, DATA imports from DLL with non-zero offset, this switch will create
3262 a vector of 'runtime pseudo relocations' which can be used by runtime
3263 environment to adjust references to such data in your client code.
3264 [This option is specific to the i386 PE targeted port of the linker]
3266 @kindex --disable-runtime-pseudo-reloc
3267 @item --disable-runtime-pseudo-reloc
3268 Do not create pseudo relocations for non-zero offset DATA imports from DLLs.
3269 [This option is specific to the i386 PE targeted port of the linker]
3271 @kindex --enable-extra-pe-debug
3272 @item --enable-extra-pe-debug
3273 Show additional debug info related to auto-import symbol thunking.
3274 [This option is specific to the i386 PE targeted port of the linker]
3276 @kindex --section-alignment
3277 @item --section-alignment
3278 Sets the section alignment. Sections in memory will always begin at
3279 addresses which are a multiple of this number. Defaults to 0x1000.
3280 [This option is specific to the i386 PE targeted port of the linker]
3284 @item --stack @var{reserve}
3285 @itemx --stack @var{reserve},@var{commit}
3286 Specify the number of bytes of memory to reserve (and optionally commit)
3287 to be used as stack for this program. The default is 2MB reserved, 4K
3289 [This option is specific to the i386 PE targeted port of the linker]
3292 @item --subsystem @var{which}
3293 @itemx --subsystem @var{which}:@var{major}
3294 @itemx --subsystem @var{which}:@var{major}.@var{minor}
3295 Specifies the subsystem under which your program will execute. The
3296 legal values for @var{which} are @code{native}, @code{windows},
3297 @code{console}, @code{posix}, and @code{xbox}. You may optionally set
3298 the subsystem version also. Numeric values are also accepted for
3300 [This option is specific to the i386 PE targeted port of the linker]
3302 The following options set flags in the @code{DllCharacteristics} field
3303 of the PE file header:
3304 [These options are specific to PE targeted ports of the linker]
3306 @kindex --high-entropy-va
3307 @item --high-entropy-va
3308 @itemx --disable-high-entropy-va
3309 Image is compatible with 64-bit address space layout randomization
3310 (ASLR). This option is enabled by default for 64-bit PE images.
3312 This option also implies @option{--dynamicbase} and
3313 @option{--enable-reloc-section}.
3315 @kindex --dynamicbase
3317 @itemx --disable-dynamicbase
3318 The image base address may be relocated using address space layout
3319 randomization (ASLR). This feature was introduced with MS Windows
3320 Vista for i386 PE targets. This option is enabled by default but
3321 can be disabled via the @option{--disable-dynamicbase} option.
3322 This option also implies @option{--enable-reloc-section}.
3324 @kindex --forceinteg
3326 @itemx --disable-forceinteg
3327 Code integrity checks are enforced. This option is disabled by
3332 @item --disable-nxcompat
3333 The image is compatible with the Data Execution Prevention.
3334 This feature was introduced with MS Windows XP SP2 for i386 PE
3335 targets. The option is enabled by default.
3337 @kindex --no-isolation
3338 @item --no-isolation
3339 @itemx --disable-no-isolation
3340 Although the image understands isolation, do not isolate the image.
3341 This option is disabled by default.
3345 @itemx --disable-no-seh
3346 The image does not use SEH. No SE handler may be called from
3347 this image. This option is disabled by default.
3351 @itemx --disable-no-bind
3352 Do not bind this image. This option is disabled by default.
3356 @itemx --disable-wdmdriver
3357 The driver uses the MS Windows Driver Model. This option is disabled
3362 @itemx --disable-tsaware
3363 The image is Terminal Server aware. This option is disabled by
3366 @kindex --insert-timestamp
3367 @item --insert-timestamp
3368 @itemx --no-insert-timestamp
3369 Insert a real timestamp into the image. This is the default behaviour
3370 as it matches legacy code and it means that the image will work with
3371 other, proprietary tools. The problem with this default is that it
3372 will result in slightly different images being produced each time the
3373 same sources are linked. The option @option{--no-insert-timestamp}
3374 can be used to insert a zero value for the timestamp, this ensuring
3375 that binaries produced from identical sources will compare
3378 @kindex --enable-reloc-section
3379 @item --enable-reloc-section
3380 @itemx --disable-reloc-section
3381 Create the base relocation table, which is necessary if the image
3382 is loaded at a different image base than specified in the PE header.
3383 This option is enabled by default.
3389 @subsection Options specific to C6X uClinux targets
3391 @c man begin OPTIONS
3393 The C6X uClinux target uses a binary format called DSBT to support shared
3394 libraries. Each shared library in the system needs to have a unique index;
3395 all executables use an index of 0.
3400 @item --dsbt-size @var{size}
3401 This option sets the number of entries in the DSBT of the current executable
3402 or shared library to @var{size}. The default is to create a table with 64
3405 @kindex --dsbt-index
3406 @item --dsbt-index @var{index}
3407 This option sets the DSBT index of the current executable or shared library
3408 to @var{index}. The default is 0, which is appropriate for generating
3409 executables. If a shared library is generated with a DSBT index of 0, the
3410 @code{R_C6000_DSBT_INDEX} relocs are copied into the output file.
3412 @kindex --no-merge-exidx-entries
3413 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent
3414 exidx entries in frame unwind info.
3422 @subsection Options specific to C-SKY targets
3424 @c man begin OPTIONS
3428 @kindex --branch-stub on C-SKY
3430 This option enables linker branch relaxation by inserting branch stub
3431 sections when needed to extend the range of branches. This option is
3432 usually not required since C-SKY supports branch and call instructions that
3433 can access the full memory range and branch relaxation is normally handled by
3434 the compiler or assembler.
3436 @kindex --stub-group-size on C-SKY
3437 @item --stub-group-size=@var{N}
3438 This option allows finer control of linker branch stub creation.
3439 It sets the maximum size of a group of input sections that can
3440 be handled by one stub section. A negative value of @var{N} locates
3441 stub sections after their branches, while a positive value allows stub
3442 sections to appear either before or after the branches. Values of
3443 @samp{1} or @samp{-1} indicate that the
3444 linker should choose suitable defaults.
3452 @subsection Options specific to Motorola 68HC11 and 68HC12 targets
3454 @c man begin OPTIONS
3456 The 68HC11 and 68HC12 linkers support specific options to control the
3457 memory bank switching mapping and trampoline code generation.
3461 @kindex --no-trampoline
3462 @item --no-trampoline
3463 This option disables the generation of trampoline. By default a trampoline
3464 is generated for each far function which is called using a @code{jsr}
3465 instruction (this happens when a pointer to a far function is taken).
3467 @kindex --bank-window
3468 @item --bank-window @var{name}
3469 This option indicates to the linker the name of the memory region in
3470 the @samp{MEMORY} specification that describes the memory bank window.
3471 The definition of such region is then used by the linker to compute
3472 paging and addresses within the memory window.
3480 @subsection Options specific to Motorola 68K target
3482 @c man begin OPTIONS
3484 The following options are supported to control handling of GOT generation
3485 when linking for 68K targets.
3490 @item --got=@var{type}
3491 This option tells the linker which GOT generation scheme to use.
3492 @var{type} should be one of @samp{single}, @samp{negative},
3493 @samp{multigot} or @samp{target}. For more information refer to the
3494 Info entry for @file{ld}.
3502 @subsection Options specific to MIPS targets
3504 @c man begin OPTIONS
3506 The following options are supported to control microMIPS instruction
3507 generation and branch relocation checks for ISA mode transitions when
3508 linking for MIPS targets.
3516 These options control the choice of microMIPS instructions used in code
3517 generated by the linker, such as that in the PLT or lazy binding stubs,
3518 or in relaxation. If @samp{--insn32} is used, then the linker only uses
3519 32-bit instruction encodings. By default or if @samp{--no-insn32} is
3520 used, all instruction encodings are used, including 16-bit ones where
3523 @kindex --ignore-branch-isa
3524 @item --ignore-branch-isa
3525 @kindex --no-ignore-branch-isa
3526 @itemx --no-ignore-branch-isa
3527 These options control branch relocation checks for invalid ISA mode
3528 transitions. If @samp{--ignore-branch-isa} is used, then the linker
3529 accepts any branch relocations and any ISA mode transition required
3530 is lost in relocation calculation, except for some cases of @code{BAL}
3531 instructions which meet relaxation conditions and are converted to
3532 equivalent @code{JALX} instructions as the associated relocation is
3533 calculated. By default or if @samp{--no-ignore-branch-isa} is used
3534 a check is made causing the loss of an ISA mode transition to produce
3537 @kindex --compact-branches
3538 @item --compact-branches
3539 @kindex --no-compact-branches
3540 @itemx --no-compact-branches
3541 These options control the generation of compact instructions by the linker
3542 in the PLT entries for MIPS R6.
3551 @subsection Options specific to PDP11 targets
3553 @c man begin OPTIONS
3555 For the pdp11-aout target, three variants of the output format can be
3556 produced as selected by the following options. The default variant
3557 for pdp11-aout is the @samp{--omagic} option, whereas for other
3558 targets @samp{--nmagic} is the default. The @samp{--imagic} option is
3559 defined only for the pdp11-aout target, while the others are described
3560 here as they apply to the pdp11-aout target.
3569 Mark the output as @code{OMAGIC} (0407) in the @file{a.out} header to
3570 indicate that the text segment is not to be write-protected and
3571 shared. Since the text and data sections are both readable and
3572 writable, the data section is allocated immediately contiguous after
3573 the text segment. This is the oldest format for PDP11 executable
3574 programs and is the default for @command{ld} on PDP11 Unix systems
3575 from the beginning through 2.11BSD.
3582 Mark the output as @code{NMAGIC} (0410) in the @file{a.out} header to
3583 indicate that when the output file is executed, the text portion will
3584 be read-only and shareable among all processes executing the same
3585 file. This involves moving the data areas up to the first possible 8K
3586 byte page boundary following the end of the text. This option creates
3587 a @emph{pure executable} format.
3594 Mark the output as @code{IMAGIC} (0411) in the @file{a.out} header to
3595 indicate that when the output file is executed, the program text and
3596 data areas will be loaded into separate address spaces using the split
3597 instruction and data space feature of the memory management unit in
3598 larger models of the PDP11. This doubles the address space available
3599 to the program. The text segment is again pure, write-protected, and
3600 shareable. The only difference in the output format between this
3601 option and the others, besides the magic number, is that both the text
3602 and data sections start at location 0. The @samp{-z} option selected
3603 this format in 2.11BSD. This option creates a @emph{separate
3609 Equivalent to @samp{--nmagic} for pdp11-aout.
3618 @section Environment Variables
3620 @c man begin ENVIRONMENT
3622 You can change the behaviour of @command{ld} with the environment variables
3623 @ifclear SingleFormat
3626 @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}.
3628 @ifclear SingleFormat
3630 @cindex default input format
3631 @code{GNUTARGET} determines the input-file object format if you don't
3632 use @samp{-b} (or its synonym @samp{--format}). Its value should be one
3633 of the BFD names for an input format (@pxref{BFD}). If there is no
3634 @code{GNUTARGET} in the environment, @command{ld} uses the natural format
3635 of the target. If @code{GNUTARGET} is set to @code{default} then BFD
3636 attempts to discover the input format by examining binary input files;
3637 this method often succeeds, but there are potential ambiguities, since
3638 there is no method of ensuring that the magic number used to specify
3639 object-file formats is unique. However, the configuration procedure for
3640 BFD on each system places the conventional format for that system first
3641 in the search-list, so ambiguities are resolved in favor of convention.
3645 @cindex default emulation
3646 @cindex emulation, default
3647 @code{LDEMULATION} determines the default emulation if you don't use the
3648 @samp{-m} option. The emulation can affect various aspects of linker
3649 behaviour, particularly the default linker script. You can list the
3650 available emulations with the @samp{--verbose} or @samp{-V} options. If
3651 the @samp{-m} option is not used, and the @code{LDEMULATION} environment
3652 variable is not defined, the default emulation depends upon how the
3653 linker was configured.
3655 @kindex COLLECT_NO_DEMANGLE
3656 @cindex demangling, default
3657 Normally, the linker will default to demangling symbols. However, if
3658 @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will
3659 default to not demangling symbols. This environment variable is used in
3660 a similar fashion by the @code{gcc} linker wrapper program. The default
3661 may be overridden by the @samp{--demangle} and @samp{--no-demangle}
3668 @chapter Linker Scripts
3671 @cindex linker scripts
3672 @cindex command files
3673 Every link is controlled by a @dfn{linker script}. This script is
3674 written in the linker command language.
3676 The main purpose of the linker script is to describe how the sections in
3677 the input files should be mapped into the output file, and to control
3678 the memory layout of the output file. Most linker scripts do nothing
3679 more than this. However, when necessary, the linker script can also
3680 direct the linker to perform many other operations, using the commands
3683 The linker always uses a linker script. If you do not supply one
3684 yourself, the linker will use a default script that is compiled into the
3685 linker executable. You can use the @samp{--verbose} command-line option
3686 to display the default linker script. Certain command-line options,
3687 such as @samp{-r} or @samp{-N}, will affect the default linker script.
3689 You may supply your own linker script by using the @samp{-T} command
3690 line option. When you do this, your linker script will replace the
3691 default linker script.
3693 You may also use linker scripts implicitly by naming them as input files
3694 to the linker, as though they were files to be linked. @xref{Implicit
3698 * Basic Script Concepts:: Basic Linker Script Concepts
3699 * Script Format:: Linker Script Format
3700 * Simple Example:: Simple Linker Script Example
3701 * Simple Commands:: Simple Linker Script Commands
3702 * Assignments:: Assigning Values to Symbols
3703 * SECTIONS:: SECTIONS Command
3704 * MEMORY:: MEMORY Command
3705 * PHDRS:: PHDRS Command
3706 * VERSION:: VERSION Command
3707 * Expressions:: Expressions in Linker Scripts
3708 * Implicit Linker Scripts:: Implicit Linker Scripts
3711 @node Basic Script Concepts
3712 @section Basic Linker Script Concepts
3713 @cindex linker script concepts
3714 We need to define some basic concepts and vocabulary in order to
3715 describe the linker script language.
3717 The linker combines input files into a single output file. The output
3718 file and each input file are in a special data format known as an
3719 @dfn{object file format}. Each file is called an @dfn{object file}.
3720 The output file is often called an @dfn{executable}, but for our
3721 purposes we will also call it an object file. Each object file has,
3722 among other things, a list of @dfn{sections}. We sometimes refer to a
3723 section in an input file as an @dfn{input section}; similarly, a section
3724 in the output file is an @dfn{output section}.
3726 Each section in an object file has a name and a size. Most sections
3727 also have an associated block of data, known as the @dfn{section
3728 contents}. A section may be marked as @dfn{loadable}, which means that
3729 the contents should be loaded into memory when the output file is run.
3730 A section with no contents may be @dfn{allocatable}, which means that an
3731 area in memory should be set aside, but nothing in particular should be
3732 loaded there (in some cases this memory must be zeroed out). A section
3733 which is neither loadable nor allocatable typically contains some sort
3734 of debugging information.
3736 Every loadable or allocatable output section has two addresses. The
3737 first is the @dfn{VMA}, or virtual memory address. This is the address
3738 the section will have when the output file is run. The second is the
3739 @dfn{LMA}, or load memory address. This is the address at which the
3740 section will be loaded. In most cases the two addresses will be the
3741 same. An example of when they might be different is when a data section
3742 is loaded into ROM, and then copied into RAM when the program starts up
3743 (this technique is often used to initialize global variables in a ROM
3744 based system). In this case the ROM address would be the LMA, and the
3745 RAM address would be the VMA.
3747 You can see the sections in an object file by using the @code{objdump}
3748 program with the @samp{-h} option.
3750 Every object file also has a list of @dfn{symbols}, known as the
3751 @dfn{symbol table}. A symbol may be defined or undefined. Each symbol
3752 has a name, and each defined symbol has an address, among other
3753 information. If you compile a C or C++ program into an object file, you
3754 will get a defined symbol for every defined function and global or
3755 static variable. Every undefined function or global variable which is
3756 referenced in the input file will become an undefined symbol.
3758 You can see the symbols in an object file by using the @code{nm}
3759 program, or by using the @code{objdump} program with the @samp{-t}
3763 @section Linker Script Format
3764 @cindex linker script format
3765 Linker scripts are text files.
3767 You write a linker script as a series of commands. Each command is
3768 either a keyword, possibly followed by arguments, or an assignment to a
3769 symbol. You may separate commands using semicolons. Whitespace is
3772 Strings such as file or format names can normally be entered directly.
3773 If the file name contains a character such as a comma which would
3774 otherwise serve to separate file names, you may put the file name in
3775 double quotes. There is no way to use a double quote character in a
3778 You may include comments in linker scripts just as in C, delimited by
3779 @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent
3782 @node Simple Example
3783 @section Simple Linker Script Example
3784 @cindex linker script example
3785 @cindex example of linker script
3786 Many linker scripts are fairly simple.
3788 The simplest possible linker script has just one command:
3789 @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the
3790 memory layout of the output file.
3792 The @samp{SECTIONS} command is a powerful command. Here we will
3793 describe a simple use of it. Let's assume your program consists only of
3794 code, initialized data, and uninitialized data. These will be in the
3795 @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively.
3796 Let's assume further that these are the only sections which appear in
3799 For this example, let's say that the code should be loaded at address
3800 0x10000, and that the data should start at address 0x8000000. Here is a
3801 linker script which will do that:
3806 .text : @{ *(.text) @}
3808 .data : @{ *(.data) @}
3809 .bss : @{ *(.bss) @}
3813 You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS},
3814 followed by a series of symbol assignments and output section
3815 descriptions enclosed in curly braces.
3817 The first line inside the @samp{SECTIONS} command of the above example
3818 sets the value of the special symbol @samp{.}, which is the location
3819 counter. If you do not specify the address of an output section in some
3820 other way (other ways are described later), the address is set from the
3821 current value of the location counter. The location counter is then
3822 incremented by the size of the output section. At the start of the
3823 @samp{SECTIONS} command, the location counter has the value @samp{0}.
3825 The second line defines an output section, @samp{.text}. The colon is
3826 required syntax which may be ignored for now. Within the curly braces
3827 after the output section name, you list the names of the input sections
3828 which should be placed into this output section. The @samp{*} is a
3829 wildcard which matches any file name. The expression @samp{*(.text)}
3830 means all @samp{.text} input sections in all input files.
3832 Since the location counter is @samp{0x10000} when the output section
3833 @samp{.text} is defined, the linker will set the address of the
3834 @samp{.text} section in the output file to be @samp{0x10000}.
3836 The remaining lines define the @samp{.data} and @samp{.bss} sections in
3837 the output file. The linker will place the @samp{.data} output section
3838 at address @samp{0x8000000}. After the linker places the @samp{.data}
3839 output section, the value of the location counter will be
3840 @samp{0x8000000} plus the size of the @samp{.data} output section. The
3841 effect is that the linker will place the @samp{.bss} output section
3842 immediately after the @samp{.data} output section in memory.
3844 The linker will ensure that each output section has the required
3845 alignment, by increasing the location counter if necessary. In this
3846 example, the specified addresses for the @samp{.text} and @samp{.data}
3847 sections will probably satisfy any alignment constraints, but the linker
3848 may have to create a small gap between the @samp{.data} and @samp{.bss}
3851 That's it! That's a simple and complete linker script.
3853 @node Simple Commands
3854 @section Simple Linker Script Commands
3855 @cindex linker script simple commands
3856 In this section we describe the simple linker script commands.
3859 * Entry Point:: Setting the entry point
3860 * File Commands:: Commands dealing with files
3861 @ifclear SingleFormat
3862 * Format Commands:: Commands dealing with object file formats
3865 * REGION_ALIAS:: Assign alias names to memory regions
3866 * Miscellaneous Commands:: Other linker script commands
3870 @subsection Setting the Entry Point
3871 @kindex ENTRY(@var{symbol})
3872 @cindex start of execution
3873 @cindex first instruction
3875 The first instruction to execute in a program is called the @dfn{entry
3876 point}. You can use the @code{ENTRY} linker script command to set the
3877 entry point. The argument is a symbol name:
3882 There are several ways to set the entry point. The linker will set the
3883 entry point by trying each of the following methods in order, and
3884 stopping when one of them succeeds:
3887 the @samp{-e} @var{entry} command-line option;
3889 the @code{ENTRY(@var{symbol})} command in a linker script;
3891 the value of a target-specific symbol, if it is defined; For many
3892 targets this is @code{start}, but PE- and BeOS-based systems for example
3893 check a list of possible entry symbols, matching the first one found.
3895 the address of the first byte of the @samp{.text} section, if present;
3897 The address @code{0}.
3901 @subsection Commands Dealing with Files
3902 @cindex linker script file commands
3903 Several linker script commands deal with files.
3906 @item INCLUDE @var{filename}
3907 @kindex INCLUDE @var{filename}
3908 @cindex including a linker script
3909 Include the linker script @var{filename} at this point. The file will
3910 be searched for in the current directory, and in any directory specified
3911 with the @option{-L} option. You can nest calls to @code{INCLUDE} up to
3914 You can place @code{INCLUDE} directives at the top level, in @code{MEMORY} or
3915 @code{SECTIONS} commands, or in output section descriptions.
3917 @item INPUT(@var{file}, @var{file}, @dots{})
3918 @itemx INPUT(@var{file} @var{file} @dots{})
3919 @kindex INPUT(@var{files})
3920 @cindex input files in linker scripts
3921 @cindex input object files in linker scripts
3922 @cindex linker script input object files
3923 The @code{INPUT} command directs the linker to include the named files
3924 in the link, as though they were named on the command line.
3926 For example, if you always want to include @file{subr.o} any time you do
3927 a link, but you can't be bothered to put it on every link command line,
3928 then you can put @samp{INPUT (subr.o)} in your linker script.
3930 In fact, if you like, you can list all of your input files in the linker
3931 script, and then invoke the linker with nothing but a @samp{-T} option.
3933 In case a @dfn{sysroot prefix} is configured, and the filename starts
3934 with the @samp{/} character, and the script being processed was
3935 located inside the @dfn{sysroot prefix}, the filename will be looked
3936 for in the @dfn{sysroot prefix}. The @dfn{sysroot prefix} can also be forced by specifying
3937 @code{=} as the first character in the filename path, or prefixing the
3938 filename path with @code{$SYSROOT}. See also the description of
3939 @samp{-L} in @ref{Options,,Command-line Options}.
3941 If a @dfn{sysroot prefix} is not used then the linker will try to open
3942 the file in the directory containing the linker script. If it is not
3943 found the linker will then search the current directory. If it is still
3944 not found the linker will search through the archive library search
3947 If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the
3948 name to @code{lib@var{file}.a}, as with the command-line argument
3951 When you use the @code{INPUT} command in an implicit linker script, the
3952 files will be included in the link at the point at which the linker
3953 script file is included. This can affect archive searching.
3955 @item GROUP(@var{file}, @var{file}, @dots{})
3956 @itemx GROUP(@var{file} @var{file} @dots{})
3957 @kindex GROUP(@var{files})
3958 @cindex grouping input files
3959 The @code{GROUP} command is like @code{INPUT}, except that the named
3960 files should all be archives, and they are searched repeatedly until no
3961 new undefined references are created. See the description of @samp{-(}
3962 in @ref{Options,,Command-line Options}.
3964 @item AS_NEEDED(@var{file}, @var{file}, @dots{})
3965 @itemx AS_NEEDED(@var{file} @var{file} @dots{})
3966 @kindex AS_NEEDED(@var{files})
3967 This construct can appear only inside of the @code{INPUT} or @code{GROUP}
3968 commands, among other filenames. The files listed will be handled
3969 as if they appear directly in the @code{INPUT} or @code{GROUP} commands,
3970 with the exception of ELF shared libraries, that will be added only
3971 when they are actually needed. This construct essentially enables
3972 @option{--as-needed} option for all the files listed inside of it
3973 and restores previous @option{--as-needed} resp. @option{--no-as-needed}
3976 @item OUTPUT(@var{filename})
3977 @kindex OUTPUT(@var{filename})
3978 @cindex output file name in linker script
3979 The @code{OUTPUT} command names the output file. Using
3980 @code{OUTPUT(@var{filename})} in the linker script is exactly like using
3981 @samp{-o @var{filename}} on the command line (@pxref{Options,,Command
3982 Line Options}). If both are used, the command-line option takes
3985 You can use the @code{OUTPUT} command to define a default name for the
3986 output file other than the usual default of @file{a.out}.
3988 @item SEARCH_DIR(@var{path})
3989 @kindex SEARCH_DIR(@var{path})
3990 @cindex library search path in linker script
3991 @cindex archive search path in linker script
3992 @cindex search path in linker script
3993 The @code{SEARCH_DIR} command adds @var{path} to the list of paths where
3994 @command{ld} looks for archive libraries. Using
3995 @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}}
3996 on the command line (@pxref{Options,,Command-line Options}). If both
3997 are used, then the linker will search both paths. Paths specified using
3998 the command-line option are searched first.
4000 @item STARTUP(@var{filename})
4001 @kindex STARTUP(@var{filename})
4002 @cindex first input file
4003 The @code{STARTUP} command is just like the @code{INPUT} command, except
4004 that @var{filename} will become the first input file to be linked, as
4005 though it were specified first on the command line. This may be useful
4006 when using a system in which the entry point is always the start of the
4010 @ifclear SingleFormat
4011 @node Format Commands
4012 @subsection Commands Dealing with Object File Formats
4013 A couple of linker script commands deal with object file formats.
4016 @item OUTPUT_FORMAT(@var{bfdname})
4017 @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little})
4018 @kindex OUTPUT_FORMAT(@var{bfdname})
4019 @cindex output file format in linker script
4020 The @code{OUTPUT_FORMAT} command names the BFD format to use for the
4021 output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is
4022 exactly like using @samp{--oformat @var{bfdname}} on the command line
4023 (@pxref{Options,,Command-line Options}). If both are used, the command
4024 line option takes precedence.
4026 You can use @code{OUTPUT_FORMAT} with three arguments to use different
4027 formats based on the @samp{-EB} and @samp{-EL} command-line options.
4028 This permits the linker script to set the output format based on the
4031 If neither @samp{-EB} nor @samp{-EL} are used, then the output format
4032 will be the first argument, @var{default}. If @samp{-EB} is used, the
4033 output format will be the second argument, @var{big}. If @samp{-EL} is
4034 used, the output format will be the third argument, @var{little}.
4036 For example, the default linker script for the MIPS ELF target uses this
4039 OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
4041 This says that the default format for the output file is
4042 @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command-line
4043 option, the output file will be created in the @samp{elf32-littlemips}
4046 @item TARGET(@var{bfdname})
4047 @kindex TARGET(@var{bfdname})
4048 @cindex input file format in linker script
4049 The @code{TARGET} command names the BFD format to use when reading input
4050 files. It affects subsequent @code{INPUT} and @code{GROUP} commands.
4051 This command is like using @samp{-b @var{bfdname}} on the command line
4052 (@pxref{Options,,Command-line Options}). If the @code{TARGET} command
4053 is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET}
4054 command is also used to set the format for the output file. @xref{BFD}.
4059 @subsection Assign alias names to memory regions
4060 @kindex REGION_ALIAS(@var{alias}, @var{region})
4061 @cindex region alias
4062 @cindex region names
4064 Alias names can be added to existing memory regions created with the
4065 @ref{MEMORY} command. Each name corresponds to at most one memory region.
4068 REGION_ALIAS(@var{alias}, @var{region})
4071 The @code{REGION_ALIAS} function creates an alias name @var{alias} for the
4072 memory region @var{region}. This allows a flexible mapping of output sections
4073 to memory regions. An example follows.
4075 Suppose we have an application for embedded systems which come with various
4076 memory storage devices. All have a general purpose, volatile memory @code{RAM}
4077 that allows code execution or data storage. Some may have a read-only,
4078 non-volatile memory @code{ROM} that allows code execution and read-only data
4079 access. The last variant is a read-only, non-volatile memory @code{ROM2} with
4080 read-only data access and no code execution capability. We have four output
4085 @code{.text} program code;
4087 @code{.rodata} read-only data;
4089 @code{.data} read-write initialized data;
4091 @code{.bss} read-write zero initialized data.
4094 The goal is to provide a linker command file that contains a system independent
4095 part defining the output sections and a system dependent part mapping the
4096 output sections to the memory regions available on the system. Our embedded
4097 systems come with three different memory setups @code{A}, @code{B} and
4099 @multitable @columnfractions .25 .25 .25 .25
4100 @item Section @tab Variant A @tab Variant B @tab Variant C
4101 @item .text @tab RAM @tab ROM @tab ROM
4102 @item .rodata @tab RAM @tab ROM @tab ROM2
4103 @item .data @tab RAM @tab RAM/ROM @tab RAM/ROM2
4104 @item .bss @tab RAM @tab RAM @tab RAM
4106 The notation @code{RAM/ROM} or @code{RAM/ROM2} means that this section is
4107 loaded into region @code{ROM} or @code{ROM2} respectively. Please note that
4108 the load address of the @code{.data} section starts in all three variants at
4109 the end of the @code{.rodata} section.
4111 The base linker script that deals with the output sections follows. It
4112 includes the system dependent @code{linkcmds.memory} file that describes the
4115 INCLUDE linkcmds.memory
4128 .data : AT (rodata_end)
4133 data_size = SIZEOF(.data);
4134 data_load_start = LOADADDR(.data);
4142 Now we need three different @code{linkcmds.memory} files to define memory
4143 regions and alias names. The content of @code{linkcmds.memory} for the three
4144 variants @code{A}, @code{B} and @code{C}:
4147 Here everything goes into the @code{RAM}.
4151 RAM : ORIGIN = 0, LENGTH = 4M
4154 REGION_ALIAS("REGION_TEXT", RAM);
4155 REGION_ALIAS("REGION_RODATA", RAM);
4156 REGION_ALIAS("REGION_DATA", RAM);
4157 REGION_ALIAS("REGION_BSS", RAM);
4160 Program code and read-only data go into the @code{ROM}. Read-write data goes
4161 into the @code{RAM}. An image of the initialized data is loaded into the
4162 @code{ROM} and will be copied during system start into the @code{RAM}.
4166 ROM : ORIGIN = 0, LENGTH = 3M
4167 RAM : ORIGIN = 0x10000000, LENGTH = 1M
4170 REGION_ALIAS("REGION_TEXT", ROM);
4171 REGION_ALIAS("REGION_RODATA", ROM);
4172 REGION_ALIAS("REGION_DATA", RAM);
4173 REGION_ALIAS("REGION_BSS", RAM);
4176 Program code goes into the @code{ROM}. Read-only data goes into the
4177 @code{ROM2}. Read-write data goes into the @code{RAM}. An image of the
4178 initialized data is loaded into the @code{ROM2} and will be copied during
4179 system start into the @code{RAM}.
4183 ROM : ORIGIN = 0, LENGTH = 2M
4184 ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
4185 RAM : ORIGIN = 0x20000000, LENGTH = 1M
4188 REGION_ALIAS("REGION_TEXT", ROM);
4189 REGION_ALIAS("REGION_RODATA", ROM2);
4190 REGION_ALIAS("REGION_DATA", RAM);
4191 REGION_ALIAS("REGION_BSS", RAM);
4195 It is possible to write a common system initialization routine to copy the
4196 @code{.data} section from @code{ROM} or @code{ROM2} into the @code{RAM} if
4201 extern char data_start [];
4202 extern char data_size [];
4203 extern char data_load_start [];
4205 void copy_data(void)
4207 if (data_start != data_load_start)
4209 memcpy(data_start, data_load_start, (size_t) data_size);
4214 @node Miscellaneous Commands
4215 @subsection Other Linker Script Commands
4216 There are a few other linker scripts commands.
4219 @item ASSERT(@var{exp}, @var{message})
4221 @cindex assertion in linker script
4222 Ensure that @var{exp} is non-zero. If it is zero, then exit the linker
4223 with an error code, and print @var{message}.
4225 Note that assertions are checked before the final stages of linking
4226 take place. This means that expressions involving symbols PROVIDEd
4227 inside section definitions will fail if the user has not set values
4228 for those symbols. The only exception to this rule is PROVIDEd
4229 symbols that just reference dot. Thus an assertion like this:
4234 PROVIDE (__stack = .);
4235 PROVIDE (__stack_size = 0x100);
4236 ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
4240 will fail if @code{__stack_size} is not defined elsewhere. Symbols
4241 PROVIDEd outside of section definitions are evaluated earlier, so they
4242 can be used inside ASSERTions. Thus:
4245 PROVIDE (__stack_size = 0x100);
4248 PROVIDE (__stack = .);
4249 ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
4255 @item EXTERN(@var{symbol} @var{symbol} @dots{})
4257 @cindex undefined symbol in linker script
4258 Force @var{symbol} to be entered in the output file as an undefined
4259 symbol. Doing this may, for example, trigger linking of additional
4260 modules from standard libraries. You may list several @var{symbol}s for
4261 each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This
4262 command has the same effect as the @samp{-u} command-line option.
4264 @item FORCE_COMMON_ALLOCATION
4265 @kindex FORCE_COMMON_ALLOCATION
4266 @cindex common allocation in linker script
4267 This command has the same effect as the @samp{-d} command-line option:
4268 to make @command{ld} assign space to common symbols even if a relocatable
4269 output file is specified (@samp{-r}).
4271 @item INHIBIT_COMMON_ALLOCATION
4272 @kindex INHIBIT_COMMON_ALLOCATION
4273 @cindex common allocation in linker script
4274 This command has the same effect as the @samp{--no-define-common}
4275 command-line option: to make @code{ld} omit the assignment of addresses
4276 to common symbols even for a non-relocatable output file.
4278 @item FORCE_GROUP_ALLOCATION
4279 @kindex FORCE_GROUP_ALLOCATION
4280 @cindex group allocation in linker script
4281 @cindex section groups
4283 This command has the same effect as the
4284 @samp{--force-group-allocation} command-line option: to make
4285 @command{ld} place section group members like normal input sections,
4286 and to delete the section groups even if a relocatable output file is
4287 specified (@samp{-r}).
4289 @item INSERT [ AFTER | BEFORE ] @var{output_section}
4291 @cindex insert user script into default script
4292 This command is typically used in a script specified by @samp{-T} to
4293 augment the default @code{SECTIONS} with, for example, overlays. It
4294 inserts all prior linker script statements after (or before)
4295 @var{output_section}, and also causes @samp{-T} to not override the
4296 default linker script. The exact insertion point is as for orphan
4297 sections. @xref{Location Counter}. The insertion happens after the
4298 linker has mapped input sections to output sections. Prior to the
4299 insertion, since @samp{-T} scripts are parsed before the default
4300 linker script, statements in the @samp{-T} script occur before the
4301 default linker script statements in the internal linker representation
4302 of the script. In particular, input section assignments will be made
4303 to @samp{-T} output sections before those in the default script. Here
4304 is an example of how a @samp{-T} script using @code{INSERT} might look:
4311 .ov1 @{ ov1*(.text) @}
4312 .ov2 @{ ov2*(.text) @}
4318 @item NOCROSSREFS(@var{section} @var{section} @dots{})
4319 @kindex NOCROSSREFS(@var{sections})
4320 @cindex cross references
4321 This command may be used to tell @command{ld} to issue an error about any
4322 references among certain output sections.
4324 In certain types of programs, particularly on embedded systems when
4325 using overlays, when one section is loaded into memory, another section
4326 will not be. Any direct references between the two sections would be
4327 errors. For example, it would be an error if code in one section called
4328 a function defined in the other section.
4330 The @code{NOCROSSREFS} command takes a list of output section names. If
4331 @command{ld} detects any cross references between the sections, it reports
4332 an error and returns a non-zero exit status. Note that the
4333 @code{NOCROSSREFS} command uses output section names, not input section
4336 @item NOCROSSREFS_TO(@var{tosection} @var{fromsection} @dots{})
4337 @kindex NOCROSSREFS_TO(@var{tosection} @var{fromsections})
4338 @cindex cross references
4339 This command may be used to tell @command{ld} to issue an error about any
4340 references to one section from a list of other sections.
4342 The @code{NOCROSSREFS} command is useful when ensuring that two or more
4343 output sections are entirely independent but there are situations where
4344 a one-way dependency is needed. For example, in a multi-core application
4345 there may be shared code that can be called from each core but for safety
4346 must never call back.
4348 The @code{NOCROSSREFS_TO} command takes a list of output section names.
4349 The first section can not be referenced from any of the other sections.
4350 If @command{ld} detects any references to the first section from any of
4351 the other sections, it reports an error and returns a non-zero exit
4352 status. Note that the @code{NOCROSSREFS_TO} command uses output section
4353 names, not input section names.
4355 @ifclear SingleFormat
4356 @item OUTPUT_ARCH(@var{bfdarch})
4357 @kindex OUTPUT_ARCH(@var{bfdarch})
4358 @cindex machine architecture
4359 @cindex architecture
4360 Specify a particular output machine architecture. The argument is one
4361 of the names used by the BFD library (@pxref{BFD}). You can see the
4362 architecture of an object file by using the @code{objdump} program with
4363 the @samp{-f} option.
4366 @item LD_FEATURE(@var{string})
4367 @kindex LD_FEATURE(@var{string})
4368 This command may be used to modify @command{ld} behavior. If
4369 @var{string} is @code{"SANE_EXPR"} then absolute symbols and numbers
4370 in a script are simply treated as numbers everywhere.
4371 @xref{Expression Section}.
4375 @section Assigning Values to Symbols
4376 @cindex assignment in scripts
4377 @cindex symbol definition, scripts
4378 @cindex variables, defining
4379 You may assign a value to a symbol in a linker script. This will define
4380 the symbol and place it into the symbol table with a global scope.
4383 * Simple Assignments:: Simple Assignments
4386 * PROVIDE_HIDDEN:: PROVIDE_HIDDEN
4387 * Source Code Reference:: How to use a linker script defined symbol in source code
4390 @node Simple Assignments
4391 @subsection Simple Assignments
4393 You may assign to a symbol using any of the C assignment operators:
4396 @item @var{symbol} = @var{expression} ;
4397 @itemx @var{symbol} += @var{expression} ;
4398 @itemx @var{symbol} -= @var{expression} ;
4399 @itemx @var{symbol} *= @var{expression} ;
4400 @itemx @var{symbol} /= @var{expression} ;
4401 @itemx @var{symbol} <<= @var{expression} ;
4402 @itemx @var{symbol} >>= @var{expression} ;
4403 @itemx @var{symbol} &= @var{expression} ;
4404 @itemx @var{symbol} |= @var{expression} ;
4407 The first case will define @var{symbol} to the value of
4408 @var{expression}. In the other cases, @var{symbol} must already be
4409 defined, and the value will be adjusted accordingly.
4411 The special symbol name @samp{.} indicates the location counter. You
4412 may only use this within a @code{SECTIONS} command. @xref{Location Counter}.
4414 The semicolon after @var{expression} is required.
4416 Expressions are defined below; see @ref{Expressions}.
4418 You may write symbol assignments as commands in their own right, or as
4419 statements within a @code{SECTIONS} command, or as part of an output
4420 section description in a @code{SECTIONS} command.
4422 The section of the symbol will be set from the section of the
4423 expression; for more information, see @ref{Expression Section}.
4425 Here is an example showing the three different places that symbol
4426 assignments may be used:
4437 _bdata = (. + 3) & ~ 3;
4438 .data : @{ *(.data) @}
4442 In this example, the symbol @samp{floating_point} will be defined as
4443 zero. The symbol @samp{_etext} will be defined as the address following
4444 the last @samp{.text} input section. The symbol @samp{_bdata} will be
4445 defined as the address following the @samp{.text} output section aligned
4446 upward to a 4 byte boundary.
4451 For ELF targeted ports, define a symbol that will be hidden and won't be
4452 exported. The syntax is @code{HIDDEN(@var{symbol} = @var{expression})}.
4454 Here is the example from @ref{Simple Assignments}, rewritten to use
4458 HIDDEN(floating_point = 0);
4466 HIDDEN(_bdata = (. + 3) & ~ 3);
4467 .data : @{ *(.data) @}
4471 In this case none of the three symbols will be visible outside this module.
4476 In some cases, it is desirable for a linker script to define a symbol
4477 only if it is referenced and is not defined by any object included in
4478 the link. For example, traditional linkers defined the symbol
4479 @samp{etext}. However, ANSI C requires that the user be able to use
4480 @samp{etext} as a function name without encountering an error. The
4481 @code{PROVIDE} keyword may be used to define a symbol, such as
4482 @samp{etext}, only if it is referenced but not defined. The syntax is
4483 @code{PROVIDE(@var{symbol} = @var{expression})}.
4485 Here is an example of using @code{PROVIDE} to define @samp{etext}:
4498 In this example, if the program defines @samp{_etext} (with a leading
4499 underscore), the linker will give a multiple definition diagnostic. If,
4500 on the other hand, the program defines @samp{etext} (with no leading
4501 underscore), the linker will silently use the definition in the program.
4502 If the program references @samp{etext} but does not define it, the
4503 linker will use the definition in the linker script.
4505 Note - the @code{PROVIDE} directive considers a common symbol to be
4506 defined, even though such a symbol could be combined with the symbol
4507 that the @code{PROVIDE} would create. This is particularly important
4508 when considering constructor and destructor list symbols such as
4509 @samp{__CTOR_LIST__} as these are often defined as common symbols.
4511 @node PROVIDE_HIDDEN
4512 @subsection PROVIDE_HIDDEN
4513 @cindex PROVIDE_HIDDEN
4514 Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be
4515 hidden and won't be exported.
4517 @node Source Code Reference
4518 @subsection Source Code Reference
4520 Accessing a linker script defined variable from source code is not
4521 intuitive. In particular a linker script symbol is not equivalent to
4522 a variable declaration in a high level language, it is instead a
4523 symbol that does not have a value.
4525 Before going further, it is important to note that compilers often
4526 transform names in the source code into different names when they are
4527 stored in the symbol table. For example, Fortran compilers commonly
4528 prepend or append an underscore, and C++ performs extensive @samp{name
4529 mangling}. Therefore there might be a discrepancy between the name
4530 of a variable as it is used in source code and the name of the same
4531 variable as it is defined in a linker script. For example in C a
4532 linker script variable might be referred to as:
4538 But in the linker script it might be defined as:
4544 In the remaining examples however it is assumed that no name
4545 transformation has taken place.
4547 When a symbol is declared in a high level language such as C, two
4548 things happen. The first is that the compiler reserves enough space
4549 in the program's memory to hold the @emph{value} of the symbol. The
4550 second is that the compiler creates an entry in the program's symbol
4551 table which holds the symbol's @emph{address}. ie the symbol table
4552 contains the address of the block of memory holding the symbol's
4553 value. So for example the following C declaration, at file scope:
4559 creates an entry called @samp{foo} in the symbol table. This entry
4560 holds the address of an @samp{int} sized block of memory where the
4561 number 1000 is initially stored.
4563 When a program references a symbol the compiler generates code that
4564 first accesses the symbol table to find the address of the symbol's
4565 memory block and then code to read the value from that memory block.
4572 looks up the symbol @samp{foo} in the symbol table, gets the address
4573 associated with this symbol and then writes the value 1 into that
4580 looks up the symbol @samp{foo} in the symbol table, gets its address
4581 and then copies this address into the block of memory associated with
4582 the variable @samp{a}.
4584 Linker scripts symbol declarations, by contrast, create an entry in
4585 the symbol table but do not assign any memory to them. Thus they are
4586 an address without a value. So for example the linker script definition:
4592 creates an entry in the symbol table called @samp{foo} which holds
4593 the address of memory location 1000, but nothing special is stored at
4594 address 1000. This means that you cannot access the @emph{value} of a
4595 linker script defined symbol - it has no value - all you can do is
4596 access the @emph{address} of a linker script defined symbol.
4598 Hence when you are using a linker script defined symbol in source code
4599 you should always take the address of the symbol, and never attempt to
4600 use its value. For example suppose you want to copy the contents of a
4601 section of memory called .ROM into a section called .FLASH and the
4602 linker script contains these declarations:
4606 start_of_ROM = .ROM;
4607 end_of_ROM = .ROM + sizeof (.ROM);
4608 start_of_FLASH = .FLASH;
4612 Then the C source code to perform the copy would be:
4616 extern char start_of_ROM, end_of_ROM, start_of_FLASH;
4618 memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
4622 Note the use of the @samp{&} operators. These are correct.
4623 Alternatively the symbols can be treated as the names of vectors or
4624 arrays and then the code will again work as expected:
4628 extern char start_of_ROM[], end_of_ROM[], start_of_FLASH[];
4630 memcpy (start_of_FLASH, start_of_ROM, end_of_ROM - start_of_ROM);
4634 Note how using this method does not require the use of @samp{&}
4638 @section SECTIONS Command
4640 The @code{SECTIONS} command tells the linker how to map input sections
4641 into output sections, and how to place the output sections in memory.
4643 The format of the @code{SECTIONS} command is:
4647 @var{sections-command}
4648 @var{sections-command}
4653 Each @var{sections-command} may of be one of the following:
4657 an @code{ENTRY} command (@pxref{Entry Point,,Entry command})
4659 a symbol assignment (@pxref{Assignments})
4661 an output section description
4663 an overlay description
4666 The @code{ENTRY} command and symbol assignments are permitted inside the
4667 @code{SECTIONS} command for convenience in using the location counter in
4668 those commands. This can also make the linker script easier to
4669 understand because you can use those commands at meaningful points in
4670 the layout of the output file.
4672 Output section descriptions and overlay descriptions are described
4675 If you do not use a @code{SECTIONS} command in your linker script, the
4676 linker will place each input section into an identically named output
4677 section in the order that the sections are first encountered in the
4678 input files. If all input sections are present in the first file, for
4679 example, the order of sections in the output file will match the order
4680 in the first input file. The first section will be at address zero.
4683 * Output Section Description:: Output section description
4684 * Output Section Name:: Output section name
4685 * Output Section Address:: Output section address
4686 * Input Section:: Input section description
4687 * Output Section Data:: Output section data
4688 * Output Section Keywords:: Output section keywords
4689 * Output Section Discarding:: Output section discarding
4690 * Output Section Attributes:: Output section attributes
4691 * Overlay Description:: Overlay description
4694 @node Output Section Description
4695 @subsection Output Section Description
4696 The full description of an output section looks like this:
4699 @var{section} [@var{address}] [(@var{type})] :
4701 [ALIGN(@var{section_align}) | ALIGN_WITH_INPUT]
4702 [SUBALIGN(@var{subsection_align})]
4705 @var{output-section-command}
4706 @var{output-section-command}
4708 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] [,]
4712 Most output sections do not use most of the optional section attributes.
4714 The whitespace around @var{section} is required, so that the section
4715 name is unambiguous. The colon and the curly braces are also required.
4716 The comma at the end may be required if a @var{fillexp} is used and
4717 the next @var{sections-command} looks like a continuation of the expression.
4718 The line breaks and other white space are optional.
4720 Each @var{output-section-command} may be one of the following:
4724 a symbol assignment (@pxref{Assignments})
4726 an input section description (@pxref{Input Section})
4728 data values to include directly (@pxref{Output Section Data})
4730 a special output section keyword (@pxref{Output Section Keywords})
4733 @node Output Section Name
4734 @subsection Output Section Name
4735 @cindex name, section
4736 @cindex section name
4737 The name of the output section is @var{section}. @var{section} must
4738 meet the constraints of your output format. In formats which only
4739 support a limited number of sections, such as @code{a.out}, the name
4740 must be one of the names supported by the format (@code{a.out}, for
4741 example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the
4742 output format supports any number of sections, but with numbers and not
4743 names (as is the case for Oasys), the name should be supplied as a
4744 quoted numeric string. A section name may consist of any sequence of
4745 characters, but a name which contains any unusual characters such as
4746 commas must be quoted.
4748 The output section name @samp{/DISCARD/} is special; @ref{Output Section
4751 @node Output Section Address
4752 @subsection Output Section Address
4753 @cindex address, section
4754 @cindex section address
4755 The @var{address} is an expression for the VMA (the virtual memory
4756 address) of the output section. This address is optional, but if it
4757 is provided then the output address will be set exactly as specified.
4759 If the output address is not specified then one will be chosen for the
4760 section, based on the heuristic below. This address will be adjusted
4761 to fit the alignment requirement of the output section. The
4762 alignment requirement is the strictest alignment of any input section
4763 contained within the output section.
4765 The output section address heuristic is as follows:
4769 If an output memory @var{region} is set for the section then it
4770 is added to this region and its address will be the next free address
4774 If the MEMORY command has been used to create a list of memory
4775 regions then the first region which has attributes compatible with the
4776 section is selected to contain it. The section's output address will
4777 be the next free address in that region; @ref{MEMORY}.
4780 If no memory regions were specified, or none match the section then
4781 the output address will be based on the current value of the location
4789 .text . : @{ *(.text) @}
4796 .text : @{ *(.text) @}
4800 are subtly different. The first will set the address of the
4801 @samp{.text} output section to the current value of the location
4802 counter. The second will set it to the current value of the location
4803 counter aligned to the strictest alignment of any of the @samp{.text}
4806 The @var{address} may be an arbitrary expression; @ref{Expressions}.
4807 For example, if you want to align the section on a 0x10 byte boundary,
4808 so that the lowest four bits of the section address are zero, you could
4809 do something like this:
4811 .text ALIGN(0x10) : @{ *(.text) @}
4814 This works because @code{ALIGN} returns the current location counter
4815 aligned upward to the specified value.
4817 Specifying @var{address} for a section will change the value of the
4818 location counter, provided that the section is non-empty. (Empty
4819 sections are ignored).
4822 @subsection Input Section Description
4823 @cindex input sections
4824 @cindex mapping input sections to output sections
4825 The most common output section command is an input section description.
4827 The input section description is the most basic linker script operation.
4828 You use output sections to tell the linker how to lay out your program
4829 in memory. You use input section descriptions to tell the linker how to
4830 map the input files into your memory layout.
4833 * Input Section Basics:: Input section basics
4834 * Input Section Wildcards:: Input section wildcard patterns
4835 * Input Section Common:: Input section for common symbols
4836 * Input Section Keep:: Input section and garbage collection
4837 * Input Section Example:: Input section example
4840 @node Input Section Basics
4841 @subsubsection Input Section Basics
4842 @cindex input section basics
4843 An input section description consists of a file name optionally followed
4844 by a list of section names in parentheses.
4846 The file name and the section name may be wildcard patterns, which we
4847 describe further below (@pxref{Input Section Wildcards}).
4849 The most common input section description is to include all input
4850 sections with a particular name in the output section. For example, to
4851 include all input @samp{.text} sections, you would write:
4856 Here the @samp{*} is a wildcard which matches any file name. To exclude a list
4857 @cindex EXCLUDE_FILE
4858 of files from matching the file name wildcard, EXCLUDE_FILE may be used to
4859 match all files except the ones specified in the EXCLUDE_FILE list. For
4862 EXCLUDE_FILE (*crtend.o *otherfile.o) *(.ctors)
4865 will cause all .ctors sections from all files except @file{crtend.o}
4866 and @file{otherfile.o} to be included. The EXCLUDE_FILE can also be
4867 placed inside the section list, for example:
4869 *(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
4872 The result of this is identically to the previous example. Supporting
4873 two syntaxes for EXCLUDE_FILE is useful if the section list contains
4874 more than one section, as described below.
4876 There are two ways to include more than one section:
4882 The difference between these is the order in which the @samp{.text} and
4883 @samp{.rdata} input sections will appear in the output section. In the
4884 first example, they will be intermingled, appearing in the same order as
4885 they are found in the linker input. In the second example, all
4886 @samp{.text} input sections will appear first, followed by all
4887 @samp{.rdata} input sections.
4889 When using EXCLUDE_FILE with more than one section, if the exclusion
4890 is within the section list then the exclusion only applies to the
4891 immediately following section, for example:
4893 *(EXCLUDE_FILE (*somefile.o) .text .rdata)
4896 will cause all @samp{.text} sections from all files except
4897 @file{somefile.o} to be included, while all @samp{.rdata} sections
4898 from all files, including @file{somefile.o}, will be included. To
4899 exclude the @samp{.rdata} sections from @file{somefile.o} the example
4900 could be modified to:
4902 *(EXCLUDE_FILE (*somefile.o) .text EXCLUDE_FILE (*somefile.o) .rdata)
4905 Alternatively, placing the EXCLUDE_FILE outside of the section list,
4906 before the input file selection, will cause the exclusion to apply for
4907 all sections. Thus the previous example can be rewritten as:
4909 EXCLUDE_FILE (*somefile.o) *(.text .rdata)
4912 You can specify a file name to include sections from a particular file.
4913 You would do this if one or more of your files contain special data that
4914 needs to be at a particular location in memory. For example:
4919 To refine the sections that are included based on the section flags
4920 of an input section, INPUT_SECTION_FLAGS may be used.
4922 Here is a simple example for using Section header flags for ELF sections:
4927 .text : @{ INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) @}
4928 .text2 : @{ INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) @}
4933 In this example, the output section @samp{.text} will be comprised of any
4934 input section matching the name *(.text) whose section header flags
4935 @code{SHF_MERGE} and @code{SHF_STRINGS} are set. The output section
4936 @samp{.text2} will be comprised of any input section matching the name *(.text)
4937 whose section header flag @code{SHF_WRITE} is clear.
4939 You can also specify files within archives by writing a pattern
4940 matching the archive, a colon, then the pattern matching the file,
4941 with no whitespace around the colon.
4945 matches file within archive
4947 matches the whole archive
4949 matches file but not one in an archive
4952 Either one or both of @samp{archive} and @samp{file} can contain shell
4953 wildcards. On DOS based file systems, the linker will assume that a
4954 single letter followed by a colon is a drive specifier, so
4955 @samp{c:myfile.o} is a simple file specification, not @samp{myfile.o}
4956 within an archive called @samp{c}. @samp{archive:file} filespecs may
4957 also be used within an @code{EXCLUDE_FILE} list, but may not appear in
4958 other linker script contexts. For instance, you cannot extract a file
4959 from an archive by using @samp{archive:file} in an @code{INPUT}
4962 If you use a file name without a list of sections, then all sections in
4963 the input file will be included in the output section. This is not
4964 commonly done, but it may by useful on occasion. For example:
4969 When you use a file name which is not an @samp{archive:file} specifier
4970 and does not contain any wild card
4971 characters, the linker will first see if you also specified the file
4972 name on the linker command line or in an @code{INPUT} command. If you
4973 did not, the linker will attempt to open the file as an input file, as
4974 though it appeared on the command line. Note that this differs from an
4975 @code{INPUT} command, because the linker will not search for the file in
4976 the archive search path.
4978 @node Input Section Wildcards
4979 @subsubsection Input Section Wildcard Patterns
4980 @cindex input section wildcards
4981 @cindex wildcard file name patterns
4982 @cindex file name wildcard patterns
4983 @cindex section name wildcard patterns
4984 In an input section description, either the file name or the section
4985 name or both may be wildcard patterns.
4987 The file name of @samp{*} seen in many examples is a simple wildcard
4988 pattern for the file name.
4990 The wildcard patterns are like those used by the Unix shell.
4994 matches any number of characters
4996 matches any single character
4998 matches a single instance of any of the @var{chars}; the @samp{-}
4999 character may be used to specify a range of characters, as in
5000 @samp{[a-z]} to match any lower case letter
5002 quotes the following character
5005 When a file name is matched with a wildcard, the wildcard characters
5006 will not match a @samp{/} character (used to separate directory names on
5007 Unix). A pattern consisting of a single @samp{*} character is an
5008 exception; it will always match any file name, whether it contains a
5009 @samp{/} or not. In a section name, the wildcard characters will match
5010 a @samp{/} character.
5012 File name wildcard patterns only match files which are explicitly
5013 specified on the command line or in an @code{INPUT} command. The linker
5014 does not search directories to expand wildcards.
5016 If a file name matches more than one wildcard pattern, or if a file name
5017 appears explicitly and is also matched by a wildcard pattern, the linker
5018 will use the first match in the linker script. For example, this
5019 sequence of input section descriptions is probably in error, because the
5020 @file{data.o} rule will not be used:
5022 .data : @{ *(.data) @}
5023 .data1 : @{ data.o(.data) @}
5026 @cindex SORT_BY_NAME
5027 Normally, the linker will place files and sections matched by wildcards
5028 in the order in which they are seen during the link. You can change
5029 this by using the @code{SORT_BY_NAME} keyword, which appears before a wildcard
5030 pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the
5031 @code{SORT_BY_NAME} keyword is used, the linker will sort the files or sections
5032 into ascending order by name before placing them in the output file.
5034 @cindex SORT_BY_ALIGNMENT
5035 @code{SORT_BY_ALIGNMENT} is similar to @code{SORT_BY_NAME}.
5036 @code{SORT_BY_ALIGNMENT} will sort sections into descending order of
5037 alignment before placing them in the output file. Placing larger
5038 alignments before smaller alignments can reduce the amount of padding
5041 @cindex SORT_BY_INIT_PRIORITY
5042 @code{SORT_BY_INIT_PRIORITY} is also similar to @code{SORT_BY_NAME}.
5043 @code{SORT_BY_INIT_PRIORITY} will sort sections into ascending
5044 numerical order of the GCC init_priority attribute encoded in the
5045 section name before placing them in the output file. In
5046 @code{.init_array.NNNNN} and @code{.fini_array.NNNNN}, @code{NNNNN} is
5047 the init_priority. In @code{.ctors.NNNNN} and @code{.dtors.NNNNN},
5048 @code{NNNNN} is 65535 minus the init_priority.
5051 @code{SORT} is an alias for @code{SORT_BY_NAME}.
5053 When there are nested section sorting commands in linker script, there
5054 can be at most 1 level of nesting for section sorting commands.
5058 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
5059 It will sort the input sections by name first, then by alignment if two
5060 sections have the same name.
5062 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
5063 It will sort the input sections by alignment first, then by name if two
5064 sections have the same alignment.
5066 @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is
5067 treated the same as @code{SORT_BY_NAME} (wildcard section pattern).
5069 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern))
5070 is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern).
5072 All other nested section sorting commands are invalid.
5075 When both command-line section sorting option and linker script
5076 section sorting command are used, section sorting command always
5077 takes precedence over the command-line option.
5079 If the section sorting command in linker script isn't nested, the
5080 command-line option will make the section sorting command to be
5081 treated as nested sorting command.
5085 @code{SORT_BY_NAME} (wildcard section pattern ) with
5086 @option{--sort-sections alignment} is equivalent to
5087 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
5089 @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with
5090 @option{--sort-section name} is equivalent to
5091 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
5094 If the section sorting command in linker script is nested, the
5095 command-line option will be ignored.
5098 @code{SORT_NONE} disables section sorting by ignoring the command-line
5099 section sorting option.
5101 If you ever get confused about where input sections are going, use the
5102 @samp{-M} linker option to generate a map file. The map file shows
5103 precisely how input sections are mapped to output sections.
5105 This example shows how wildcard patterns might be used to partition
5106 files. This linker script directs the linker to place all @samp{.text}
5107 sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}.
5108 The linker will place the @samp{.data} section from all files beginning
5109 with an upper case character in @samp{.DATA}; for all other files, the
5110 linker will place the @samp{.data} section in @samp{.data}.
5114 .text : @{ *(.text) @}
5115 .DATA : @{ [A-Z]*(.data) @}
5116 .data : @{ *(.data) @}
5117 .bss : @{ *(.bss) @}
5122 @node Input Section Common
5123 @subsubsection Input Section for Common Symbols
5124 @cindex common symbol placement
5125 @cindex uninitialized data placement
5126 A special notation is needed for common symbols, because in many object
5127 file formats common symbols do not have a particular input section. The
5128 linker treats common symbols as though they are in an input section
5129 named @samp{COMMON}.
5131 You may use file names with the @samp{COMMON} section just as with any
5132 other input sections. You can use this to place common symbols from a
5133 particular input file in one section while common symbols from other
5134 input files are placed in another section.
5136 In most cases, common symbols in input files will be placed in the
5137 @samp{.bss} section in the output file. For example:
5139 .bss @{ *(.bss) *(COMMON) @}
5142 @cindex scommon section
5143 @cindex small common symbols
5144 Some object file formats have more than one type of common symbol. For
5145 example, the MIPS ELF object file format distinguishes standard common
5146 symbols and small common symbols. In this case, the linker will use a
5147 different special section name for other types of common symbols. In
5148 the case of MIPS ELF, the linker uses @samp{COMMON} for standard common
5149 symbols and @samp{.scommon} for small common symbols. This permits you
5150 to map the different types of common symbols into memory at different
5154 You will sometimes see @samp{[COMMON]} in old linker scripts. This
5155 notation is now considered obsolete. It is equivalent to
5158 @node Input Section Keep
5159 @subsubsection Input Section and Garbage Collection
5161 @cindex garbage collection
5162 When link-time garbage collection is in use (@samp{--gc-sections}),
5163 it is often useful to mark sections that should not be eliminated.
5164 This is accomplished by surrounding an input section's wildcard entry
5165 with @code{KEEP()}, as in @code{KEEP(*(.init))} or
5166 @code{KEEP(SORT_BY_NAME(*)(.ctors))}.
5168 @node Input Section Example
5169 @subsubsection Input Section Example
5170 The following example is a complete linker script. It tells the linker
5171 to read all of the sections from file @file{all.o} and place them at the
5172 start of output section @samp{outputa} which starts at location
5173 @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o}
5174 follows immediately, in the same output section. All of section
5175 @samp{.input2} from @file{foo.o} goes into output section
5176 @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}.
5177 All of the remaining @samp{.input1} and @samp{.input2} sections from any
5178 files are written to output section @samp{outputc}.
5206 If an output section's name is the same as the input section's name
5207 and is representable as a C identifier, then the linker will
5208 automatically @pxref{PROVIDE} two symbols: __start_SECNAME and
5209 __stop_SECNAME, where SECNAME is the name of the section. These
5210 indicate the start address and end address of the output section
5211 respectively. Note: most section names are not representable as
5212 C identifiers because they contain a @samp{.} character.
5214 @node Output Section Data
5215 @subsection Output Section Data
5217 @cindex section data
5218 @cindex output section data
5219 @kindex BYTE(@var{expression})
5220 @kindex SHORT(@var{expression})
5221 @kindex LONG(@var{expression})
5222 @kindex QUAD(@var{expression})
5223 @kindex SQUAD(@var{expression})
5224 You can include explicit bytes of data in an output section by using
5225 @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as
5226 an output section command. Each keyword is followed by an expression in
5227 parentheses providing the value to store (@pxref{Expressions}). The
5228 value of the expression is stored at the current value of the location
5231 The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands
5232 store one, two, four, and eight bytes (respectively). After storing the
5233 bytes, the location counter is incremented by the number of bytes
5236 For example, this will store the byte 1 followed by the four byte value
5237 of the symbol @samp{addr}:
5243 When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the
5244 same; they both store an 8 byte, or 64 bit, value. When both host and
5245 target are 32 bits, an expression is computed as 32 bits. In this case
5246 @code{QUAD} stores a 32 bit value zero extended to 64 bits, and
5247 @code{SQUAD} stores a 32 bit value sign extended to 64 bits.
5249 If the object file format of the output file has an explicit endianness,
5250 which is the normal case, the value will be stored in that endianness.
5251 When the object file format does not have an explicit endianness, as is
5252 true of, for example, S-records, the value will be stored in the
5253 endianness of the first input object file.
5255 Note---these commands only work inside a section description and not
5256 between them, so the following will produce an error from the linker:
5258 SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@
5260 whereas this will work:
5262 SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@
5265 @kindex FILL(@var{expression})
5266 @cindex holes, filling
5267 @cindex unspecified memory
5268 You may use the @code{FILL} command to set the fill pattern for the
5269 current section. It is followed by an expression in parentheses. Any
5270 otherwise unspecified regions of memory within the section (for example,
5271 gaps left due to the required alignment of input sections) are filled
5272 with the value of the expression, repeated as
5273 necessary. A @code{FILL} statement covers memory locations after the
5274 point at which it occurs in the section definition; by including more
5275 than one @code{FILL} statement, you can have different fill patterns in
5276 different parts of an output section.
5278 This example shows how to fill unspecified regions of memory with the
5284 The @code{FILL} command is similar to the @samp{=@var{fillexp}} output
5285 section attribute, but it only affects the
5286 part of the section following the @code{FILL} command, rather than the
5287 entire section. If both are used, the @code{FILL} command takes
5288 precedence. @xref{Output Section Fill}, for details on the fill
5291 @node Output Section Keywords
5292 @subsection Output Section Keywords
5293 There are a couple of keywords which can appear as output section
5297 @kindex CREATE_OBJECT_SYMBOLS
5298 @cindex input filename symbols
5299 @cindex filename symbols
5300 @item CREATE_OBJECT_SYMBOLS
5301 The command tells the linker to create a symbol for each input file.
5302 The name of each symbol will be the name of the corresponding input
5303 file. The section of each symbol will be the output section in which
5304 the @code{CREATE_OBJECT_SYMBOLS} command appears.
5306 This is conventional for the a.out object file format. It is not
5307 normally used for any other object file format.
5309 @kindex CONSTRUCTORS
5310 @cindex C++ constructors, arranging in link
5311 @cindex constructors, arranging in link
5313 When linking using the a.out object file format, the linker uses an
5314 unusual set construct to support C++ global constructors and
5315 destructors. When linking object file formats which do not support
5316 arbitrary sections, such as ECOFF and XCOFF, the linker will
5317 automatically recognize C++ global constructors and destructors by name.
5318 For these object file formats, the @code{CONSTRUCTORS} command tells the
5319 linker to place constructor information in the output section where the
5320 @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is
5321 ignored for other object file formats.
5323 The symbol @w{@code{__CTOR_LIST__}} marks the start of the global
5324 constructors, and the symbol @w{@code{__CTOR_END__}} marks the end.
5325 Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_END__}} mark
5326 the start and end of the global destructors. The
5327 first word in the list is the number of entries, followed by the address
5328 of each constructor or destructor, followed by a zero word. The
5329 compiler must arrange to actually run the code. For these object file
5330 formats @sc{gnu} C++ normally calls constructors from a subroutine
5331 @code{__main}; a call to @code{__main} is automatically inserted into
5332 the startup code for @code{main}. @sc{gnu} C++ normally runs
5333 destructors either by using @code{atexit}, or directly from the function
5336 For object file formats such as @code{COFF} or @code{ELF} which support
5337 arbitrary section names, @sc{gnu} C++ will normally arrange to put the
5338 addresses of global constructors and destructors into the @code{.ctors}
5339 and @code{.dtors} sections. Placing the following sequence into your
5340 linker script will build the sort of table which the @sc{gnu} C++
5341 runtime code expects to see.
5345 LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
5350 LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
5356 If you are using the @sc{gnu} C++ support for initialization priority,
5357 which provides some control over the order in which global constructors
5358 are run, you must sort the constructors at link time to ensure that they
5359 are executed in the correct order. When using the @code{CONSTRUCTORS}
5360 command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the
5361 @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and
5362 @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and
5365 Normally the compiler and linker will handle these issues automatically,
5366 and you will not need to concern yourself with them. However, you may
5367 need to consider this if you are using C++ and writing your own linker
5372 @node Output Section Discarding
5373 @subsection Output Section Discarding
5374 @cindex discarding sections
5375 @cindex sections, discarding
5376 @cindex removing sections
5377 The linker will not normally create output sections with no contents.
5378 This is for convenience when referring to input sections that may or
5379 may not be present in any of the input files. For example:
5381 .foo : @{ *(.foo) @}
5384 will only create a @samp{.foo} section in the output file if there is a
5385 @samp{.foo} section in at least one input file, and if the input
5386 sections are not all empty. Other link script directives that allocate
5387 space in an output section will also create the output section. So
5388 too will assignments to dot even if the assignment does not create
5389 space, except for @samp{. = 0}, @samp{. = . + 0}, @samp{. = sym},
5390 @samp{. = . + sym} and @samp{. = ALIGN (. != 0, expr, 1)} when
5391 @samp{sym} is an absolute symbol of value 0 defined in the script.
5392 This allows you to force output of an empty section with @samp{. = .}.
5394 The linker will ignore address assignments (@pxref{Output Section Address})
5395 on discarded output sections, except when the linker script defines
5396 symbols in the output section. In that case the linker will obey
5397 the address assignments, possibly advancing dot even though the
5398 section is discarded.
5401 The special output section name @samp{/DISCARD/} may be used to discard
5402 input sections. Any input sections which are assigned to an output
5403 section named @samp{/DISCARD/} are not included in the output file.
5405 This can be used to discard input sections marked with the ELF flag
5406 @code{SHF_GNU_RETAIN}, which would otherwise have been saved from linker
5409 Note, sections that match the @samp{/DISCARD/} output section will be
5410 discarded even if they are in an ELF section group which has other
5411 members which are not being discarded. This is deliberate.
5412 Discarding takes precedence over grouping.
5414 @node Output Section Attributes
5415 @subsection Output Section Attributes
5416 @cindex output section attributes
5417 We showed above that the full description of an output section looked
5422 @var{section} [@var{address}] [(@var{type})] :
5424 [ALIGN(@var{section_align}) | ALIGN_WITH_INPUT]
5425 [SUBALIGN(@var{subsection_align})]
5428 @var{output-section-command}
5429 @var{output-section-command}
5431 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}]
5435 We've already described @var{section}, @var{address}, and
5436 @var{output-section-command}. In this section we will describe the
5437 remaining section attributes.
5440 * Output Section Type:: Output section type
5441 * Output Section LMA:: Output section LMA
5442 * Forced Output Alignment:: Forced Output Alignment
5443 * Forced Input Alignment:: Forced Input Alignment
5444 * Output Section Constraint:: Output section constraint
5445 * Output Section Region:: Output section region
5446 * Output Section Phdr:: Output section phdr
5447 * Output Section Fill:: Output section fill
5450 @node Output Section Type
5451 @subsubsection Output Section Type
5452 Each output section may have a type. The type is a keyword in
5453 parentheses. The following types are defined:
5457 The section should be marked as not loadable, so that it will not be
5458 loaded into memory when the program is run.
5463 These type names are supported for backward compatibility, and are
5464 rarely used. They all have the same effect: the section should be
5465 marked as not allocatable, so that no memory is allocated for the
5466 section when the program is run.
5470 @cindex prevent unnecessary loading
5471 @cindex loading, preventing
5472 The linker normally sets the attributes of an output section based on
5473 the input sections which map into it. You can override this by using
5474 the section type. For example, in the script sample below, the
5475 @samp{ROM} section is addressed at memory location @samp{0} and does not
5476 need to be loaded when the program is run.
5480 ROM 0 (NOLOAD) : @{ @dots{} @}
5486 @node Output Section LMA
5487 @subsubsection Output Section LMA
5488 @kindex AT>@var{lma_region}
5489 @kindex AT(@var{lma})
5490 @cindex load address
5491 @cindex section load address
5492 Every section has a virtual address (VMA) and a load address (LMA); see
5493 @ref{Basic Script Concepts}. The virtual address is specified by the
5494 @pxref{Output Section Address} described earlier. The load address is
5495 specified by the @code{AT} or @code{AT>} keywords. Specifying a load
5496 address is optional.
5498 The @code{AT} keyword takes an expression as an argument. This
5499 specifies the exact load address of the section. The @code{AT>} keyword
5500 takes the name of a memory region as an argument. @xref{MEMORY}. The
5501 load address of the section is set to the next free address in the
5502 region, aligned to the section's alignment requirements.
5504 If neither @code{AT} nor @code{AT>} is specified for an allocatable
5505 section, the linker will use the following heuristic to determine the
5510 If the section has a specific VMA address, then this is used as
5511 the LMA address as well.
5514 If the section is not allocatable then its LMA is set to its VMA.
5517 Otherwise if a memory region can be found that is compatible
5518 with the current section, and this region contains at least one
5519 section, then the LMA is set so the difference between the
5520 VMA and LMA is the same as the difference between the VMA and LMA of
5521 the last section in the located region.
5524 If no memory regions have been declared then a default region
5525 that covers the entire address space is used in the previous step.
5528 If no suitable region could be found, or there was no previous
5529 section then the LMA is set equal to the VMA.
5532 @cindex ROM initialized data
5533 @cindex initialized data in ROM
5534 This feature is designed to make it easy to build a ROM image. For
5535 example, the following linker script creates three output sections: one
5536 called @samp{.text}, which starts at @code{0x1000}, one called
5537 @samp{.mdata}, which is loaded at the end of the @samp{.text} section
5538 even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold
5539 uninitialized data at address @code{0x3000}. The symbol @code{_data} is
5540 defined with the value @code{0x2000}, which shows that the location
5541 counter holds the VMA value, not the LMA value.
5547 .text 0x1000 : @{ *(.text) _etext = . ; @}
5549 AT ( ADDR (.text) + SIZEOF (.text) )
5550 @{ _data = . ; *(.data); _edata = . ; @}
5552 @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@}
5557 The run-time initialization code for use with a program generated with
5558 this linker script would include something like the following, to copy
5559 the initialized data from the ROM image to its runtime address. Notice
5560 how this code takes advantage of the symbols defined by the linker
5565 extern char _etext, _data, _edata, _bstart, _bend;
5566 char *src = &_etext;
5569 /* ROM has data at end of text; copy it. */
5570 while (dst < &_edata)
5574 for (dst = &_bstart; dst< &_bend; dst++)
5579 @node Forced Output Alignment
5580 @subsubsection Forced Output Alignment
5581 @kindex ALIGN(@var{section_align})
5582 @cindex forcing output section alignment
5583 @cindex output section alignment
5584 You can increase an output section's alignment by using ALIGN. As an
5585 alternative you can enforce that the difference between the VMA and LMA remains
5586 intact throughout this output section with the ALIGN_WITH_INPUT attribute.
5588 @node Forced Input Alignment
5589 @subsubsection Forced Input Alignment
5590 @kindex SUBALIGN(@var{subsection_align})
5591 @cindex forcing input section alignment
5592 @cindex input section alignment
5593 You can force input section alignment within an output section by using
5594 SUBALIGN. The value specified overrides any alignment given by input
5595 sections, whether larger or smaller.
5597 @node Output Section Constraint
5598 @subsubsection Output Section Constraint
5601 @cindex constraints on output sections
5602 You can specify that an output section should only be created if all
5603 of its input sections are read-only or all of its input sections are
5604 read-write by using the keyword @code{ONLY_IF_RO} and
5605 @code{ONLY_IF_RW} respectively.
5607 @node Output Section Region
5608 @subsubsection Output Section Region
5609 @kindex >@var{region}
5610 @cindex section, assigning to memory region
5611 @cindex memory regions and sections
5612 You can assign a section to a previously defined region of memory by
5613 using @samp{>@var{region}}. @xref{MEMORY}.
5615 Here is a simple example:
5618 MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @}
5619 SECTIONS @{ ROM : @{ *(.text) @} >rom @}
5623 @node Output Section Phdr
5624 @subsubsection Output Section Phdr
5626 @cindex section, assigning to program header
5627 @cindex program headers and sections
5628 You can assign a section to a previously defined program segment by
5629 using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to
5630 one or more segments, then all subsequent allocated sections will be
5631 assigned to those segments as well, unless they use an explicitly
5632 @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the
5633 linker to not put the section in any segment at all.
5635 Here is a simple example:
5638 PHDRS @{ text PT_LOAD ; @}
5639 SECTIONS @{ .text : @{ *(.text) @} :text @}
5643 @node Output Section Fill
5644 @subsubsection Output Section Fill
5645 @kindex =@var{fillexp}
5646 @cindex section fill pattern
5647 @cindex fill pattern, entire section
5648 You can set the fill pattern for an entire section by using
5649 @samp{=@var{fillexp}}. @var{fillexp} is an expression
5650 (@pxref{Expressions}). Any otherwise unspecified regions of memory
5651 within the output section (for example, gaps left due to the required
5652 alignment of input sections) will be filled with the value, repeated as
5653 necessary. If the fill expression is a simple hex number, ie. a string
5654 of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then
5655 an arbitrarily long sequence of hex digits can be used to specify the
5656 fill pattern; Leading zeros become part of the pattern too. For all
5657 other cases, including extra parentheses or a unary @code{+}, the fill
5658 pattern is the four least significant bytes of the value of the
5659 expression. In all cases, the number is big-endian.
5661 You can also change the fill value with a @code{FILL} command in the
5662 output section commands; (@pxref{Output Section Data}).
5664 Here is a simple example:
5667 SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @}
5671 @node Overlay Description
5672 @subsection Overlay Description
5675 An overlay description provides an easy way to describe sections which
5676 are to be loaded as part of a single memory image but are to be run at
5677 the same memory address. At run time, some sort of overlay manager will
5678 copy the overlaid sections in and out of the runtime memory address as
5679 required, perhaps by simply manipulating addressing bits. This approach
5680 can be useful, for example, when a certain region of memory is faster
5683 Overlays are described using the @code{OVERLAY} command. The
5684 @code{OVERLAY} command is used within a @code{SECTIONS} command, like an
5685 output section description. The full syntax of the @code{OVERLAY}
5686 command is as follows:
5689 OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )]
5693 @var{output-section-command}
5694 @var{output-section-command}
5696 @} [:@var{phdr}@dots{}] [=@var{fill}]
5699 @var{output-section-command}
5700 @var{output-section-command}
5702 @} [:@var{phdr}@dots{}] [=@var{fill}]
5704 @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] [,]
5708 Everything is optional except @code{OVERLAY} (a keyword), and each
5709 section must have a name (@var{secname1} and @var{secname2} above). The
5710 section definitions within the @code{OVERLAY} construct are identical to
5711 those within the general @code{SECTIONS} construct (@pxref{SECTIONS}),
5712 except that no addresses and no memory regions may be defined for
5713 sections within an @code{OVERLAY}.
5715 The comma at the end may be required if a @var{fill} is used and
5716 the next @var{sections-command} looks like a continuation of the expression.
5718 The sections are all defined with the same starting address. The load
5719 addresses of the sections are arranged such that they are consecutive in
5720 memory starting at the load address used for the @code{OVERLAY} as a
5721 whole (as with normal section definitions, the load address is optional,
5722 and defaults to the start address; the start address is also optional,
5723 and defaults to the current value of the location counter).
5725 If the @code{NOCROSSREFS} keyword is used, and there are any
5726 references among the sections, the linker will report an error. Since
5727 the sections all run at the same address, it normally does not make
5728 sense for one section to refer directly to another.
5729 @xref{Miscellaneous Commands, NOCROSSREFS}.
5731 For each section within the @code{OVERLAY}, the linker automatically
5732 provides two symbols. The symbol @code{__load_start_@var{secname}} is
5733 defined as the starting load address of the section. The symbol
5734 @code{__load_stop_@var{secname}} is defined as the final load address of
5735 the section. Any characters within @var{secname} which are not legal
5736 within C identifiers are removed. C (or assembler) code may use these
5737 symbols to move the overlaid sections around as necessary.
5739 At the end of the overlay, the value of the location counter is set to
5740 the start address of the overlay plus the size of the largest section.
5742 Here is an example. Remember that this would appear inside a
5743 @code{SECTIONS} construct.
5746 OVERLAY 0x1000 : AT (0x4000)
5748 .text0 @{ o1/*.o(.text) @}
5749 .text1 @{ o2/*.o(.text) @}
5754 This will define both @samp{.text0} and @samp{.text1} to start at
5755 address 0x1000. @samp{.text0} will be loaded at address 0x4000, and
5756 @samp{.text1} will be loaded immediately after @samp{.text0}. The
5757 following symbols will be defined if referenced: @code{__load_start_text0},
5758 @code{__load_stop_text0}, @code{__load_start_text1},
5759 @code{__load_stop_text1}.
5761 C code to copy overlay @code{.text1} into the overlay area might look
5766 extern char __load_start_text1, __load_stop_text1;
5767 memcpy ((char *) 0x1000, &__load_start_text1,
5768 &__load_stop_text1 - &__load_start_text1);
5772 Note that the @code{OVERLAY} command is just syntactic sugar, since
5773 everything it does can be done using the more basic commands. The above
5774 example could have been written identically as follows.
5778 .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @}
5779 PROVIDE (__load_start_text0 = LOADADDR (.text0));
5780 PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
5781 .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @}
5782 PROVIDE (__load_start_text1 = LOADADDR (.text1));
5783 PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
5784 . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
5789 @section MEMORY Command
5791 @cindex memory regions
5792 @cindex regions of memory
5793 @cindex allocating memory
5794 @cindex discontinuous memory
5795 The linker's default configuration permits allocation of all available
5796 memory. You can override this by using the @code{MEMORY} command.
5798 The @code{MEMORY} command describes the location and size of blocks of
5799 memory in the target. You can use it to describe which memory regions
5800 may be used by the linker, and which memory regions it must avoid. You
5801 can then assign sections to particular memory regions. The linker will
5802 set section addresses based on the memory regions, and will warn about
5803 regions that become too full. The linker will not shuffle sections
5804 around to fit into the available regions.
5806 A linker script may contain many uses of the @code{MEMORY} command,
5807 however, all memory blocks defined are treated as if they were
5808 specified inside a single @code{MEMORY} command. The syntax for
5814 @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len}
5820 The @var{name} is a name used in the linker script to refer to the
5821 region. The region name has no meaning outside of the linker script.
5822 Region names are stored in a separate name space, and will not conflict
5823 with symbol names, file names, or section names. Each memory region
5824 must have a distinct name within the @code{MEMORY} command. However you can
5825 add later alias names to existing memory regions with the @ref{REGION_ALIAS}
5828 @cindex memory region attributes
5829 The @var{attr} string is an optional list of attributes that specify
5830 whether to use a particular memory region for an input section which is
5831 not explicitly mapped in the linker script. As described in
5832 @ref{SECTIONS}, if you do not specify an output section for some input
5833 section, the linker will create an output section with the same name as
5834 the input section. If you define region attributes, the linker will use
5835 them to select the memory region for the output section that it creates.
5837 The @var{attr} string must consist only of the following characters:
5852 Invert the sense of any of the attributes that follow
5855 If an unmapped section matches any of the listed attributes other than
5856 @samp{!}, it will be placed in the memory region. The @samp{!}
5857 attribute reverses the test for the characters that follow, so that an
5858 unmapped section will be placed in the memory region only if it does
5859 not match any of the attributes listed afterwards. Thus an attribute
5860 string of @samp{RW!X} will match any unmapped section that has either
5861 or both of the @samp{R} and @samp{W} attributes, but only as long as
5862 the section does not also have the @samp{X} attribute.
5867 The @var{origin} is an numerical expression for the start address of
5868 the memory region. The expression must evaluate to a constant and it
5869 cannot involve any symbols. The keyword @code{ORIGIN} may be
5870 abbreviated to @code{org} or @code{o} (but not, for example,
5876 The @var{len} is an expression for the size in bytes of the memory
5877 region. As with the @var{origin} expression, the expression must
5878 be numerical only and must evaluate to a constant. The keyword
5879 @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
5881 In the following example, we specify that there are two memory regions
5882 available for allocation: one starting at @samp{0} for 256 kilobytes,
5883 and the other starting at @samp{0x40000000} for four megabytes. The
5884 linker will place into the @samp{rom} memory region every section which
5885 is not explicitly mapped into a memory region, and is either read-only
5886 or executable. The linker will place other sections which are not
5887 explicitly mapped into a memory region into the @samp{ram} memory
5894 rom (rx) : ORIGIN = 0, LENGTH = 256K
5895 ram (!rx) : org = 0x40000000, l = 4M
5900 Once you define a memory region, you can direct the linker to place
5901 specific output sections into that memory region by using the
5902 @samp{>@var{region}} output section attribute. For example, if you have
5903 a memory region named @samp{mem}, you would use @samp{>mem} in the
5904 output section definition. @xref{Output Section Region}. If no address
5905 was specified for the output section, the linker will set the address to
5906 the next available address within the memory region. If the combined
5907 output sections directed to a memory region are too large for the
5908 region, the linker will issue an error message.
5910 It is possible to access the origin and length of a memory in an
5911 expression via the @code{ORIGIN(@var{memory})} and
5912 @code{LENGTH(@var{memory})} functions:
5916 _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
5921 @section PHDRS Command
5923 @cindex program headers
5924 @cindex ELF program headers
5925 @cindex program segments
5926 @cindex segments, ELF
5927 The ELF object file format uses @dfn{program headers}, also knows as
5928 @dfn{segments}. The program headers describe how the program should be
5929 loaded into memory. You can print them out by using the @code{objdump}
5930 program with the @samp{-p} option.
5932 When you run an ELF program on a native ELF system, the system loader
5933 reads the program headers in order to figure out how to load the
5934 program. This will only work if the program headers are set correctly.
5935 This manual does not describe the details of how the system loader
5936 interprets program headers; for more information, see the ELF ABI.
5938 The linker will create reasonable program headers by default. However,
5939 in some cases, you may need to specify the program headers more
5940 precisely. You may use the @code{PHDRS} command for this purpose. When
5941 the linker sees the @code{PHDRS} command in the linker script, it will
5942 not create any program headers other than the ones specified.
5944 The linker only pays attention to the @code{PHDRS} command when
5945 generating an ELF output file. In other cases, the linker will simply
5946 ignore @code{PHDRS}.
5948 This is the syntax of the @code{PHDRS} command. The words @code{PHDRS},
5949 @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords.
5955 @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ]
5956 [ FLAGS ( @var{flags} ) ] ;
5961 The @var{name} is used only for reference in the @code{SECTIONS} command
5962 of the linker script. It is not put into the output file. Program
5963 header names are stored in a separate name space, and will not conflict
5964 with symbol names, file names, or section names. Each program header
5965 must have a distinct name. The headers are processed in order and it
5966 is usual for them to map to sections in ascending load address order.
5968 Certain program header types describe segments of memory which the
5969 system loader will load from the file. In the linker script, you
5970 specify the contents of these segments by placing allocatable output
5971 sections in the segments. You use the @samp{:@var{phdr}} output section
5972 attribute to place a section in a particular segment. @xref{Output
5975 It is normal to put certain sections in more than one segment. This
5976 merely implies that one segment of memory contains another. You may
5977 repeat @samp{:@var{phdr}}, using it once for each segment which should
5978 contain the section.
5980 If you place a section in one or more segments using @samp{:@var{phdr}},
5981 then the linker will place all subsequent allocatable sections which do
5982 not specify @samp{:@var{phdr}} in the same segments. This is for
5983 convenience, since generally a whole set of contiguous sections will be
5984 placed in a single segment. You can use @code{:NONE} to override the
5985 default segment and tell the linker to not put the section in any
5990 You may use the @code{FILEHDR} and @code{PHDRS} keywords after
5991 the program header type to further describe the contents of the segment.
5992 The @code{FILEHDR} keyword means that the segment should include the ELF
5993 file header. The @code{PHDRS} keyword means that the segment should
5994 include the ELF program headers themselves. If applied to a loadable
5995 segment (@code{PT_LOAD}), all prior loadable segments must have one of
5998 The @var{type} may be one of the following. The numbers indicate the
5999 value of the keyword.
6002 @item @code{PT_NULL} (0)
6003 Indicates an unused program header.
6005 @item @code{PT_LOAD} (1)
6006 Indicates that this program header describes a segment to be loaded from
6009 @item @code{PT_DYNAMIC} (2)
6010 Indicates a segment where dynamic linking information can be found.
6012 @item @code{PT_INTERP} (3)
6013 Indicates a segment where the name of the program interpreter may be
6016 @item @code{PT_NOTE} (4)
6017 Indicates a segment holding note information.
6019 @item @code{PT_SHLIB} (5)
6020 A reserved program header type, defined but not specified by the ELF
6023 @item @code{PT_PHDR} (6)
6024 Indicates a segment where the program headers may be found.
6026 @item @code{PT_TLS} (7)
6027 Indicates a segment containing thread local storage.
6029 @item @var{expression}
6030 An expression giving the numeric type of the program header. This may
6031 be used for types not defined above.
6034 You can specify that a segment should be loaded at a particular address
6035 in memory by using an @code{AT} expression. This is identical to the
6036 @code{AT} command used as an output section attribute (@pxref{Output
6037 Section LMA}). The @code{AT} command for a program header overrides the
6038 output section attribute.
6040 The linker will normally set the segment flags based on the sections
6041 which comprise the segment. You may use the @code{FLAGS} keyword to
6042 explicitly specify the segment flags. The value of @var{flags} must be
6043 an integer. It is used to set the @code{p_flags} field of the program
6046 Here is an example of @code{PHDRS}. This shows a typical set of program
6047 headers used on a native ELF system.
6053 headers PT_PHDR PHDRS ;
6055 text PT_LOAD FILEHDR PHDRS ;
6057 dynamic PT_DYNAMIC ;
6063 .interp : @{ *(.interp) @} :text :interp
6064 .text : @{ *(.text) @} :text
6065 .rodata : @{ *(.rodata) @} /* defaults to :text */
6067 . = . + 0x1000; /* move to a new page in memory */
6068 .data : @{ *(.data) @} :data
6069 .dynamic : @{ *(.dynamic) @} :data :dynamic
6076 @section VERSION Command
6077 @kindex VERSION @{script text@}
6078 @cindex symbol versions
6079 @cindex version script
6080 @cindex versions of symbols
6081 The linker supports symbol versions when using ELF. Symbol versions are
6082 only useful when using shared libraries. The dynamic linker can use
6083 symbol versions to select a specific version of a function when it runs
6084 a program that may have been linked against an earlier version of the
6087 You can include a version script directly in the main linker script, or
6088 you can supply the version script as an implicit linker script. You can
6089 also use the @samp{--version-script} linker option.
6091 The syntax of the @code{VERSION} command is simply
6093 VERSION @{ version-script-commands @}
6096 The format of the version script commands is identical to that used by
6097 Sun's linker in Solaris 2.5. The version script defines a tree of
6098 version nodes. You specify the node names and interdependencies in the
6099 version script. You can specify which symbols are bound to which
6100 version nodes, and you can reduce a specified set of symbols to local
6101 scope so that they are not globally visible outside of the shared
6104 The easiest way to demonstrate the version script language is with a few
6130 This example version script defines three version nodes. The first
6131 version node defined is @samp{VERS_1.1}; it has no other dependencies.
6132 The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces
6133 a number of symbols to local scope so that they are not visible outside
6134 of the shared library; this is done using wildcard patterns, so that any
6135 symbol whose name begins with @samp{old}, @samp{original}, or @samp{new}
6136 is matched. The wildcard patterns available are the same as those used
6137 in the shell when matching filenames (also known as ``globbing'').
6138 However, if you specify the symbol name inside double quotes, then the
6139 name is treated as literal, rather than as a glob pattern.
6141 Next, the version script defines node @samp{VERS_1.2}. This node
6142 depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2}
6143 to the version node @samp{VERS_1.2}.
6145 Finally, the version script defines node @samp{VERS_2.0}. This node
6146 depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1}
6147 and @samp{bar2} are bound to the version node @samp{VERS_2.0}.
6149 When the linker finds a symbol defined in a library which is not
6150 specifically bound to a version node, it will effectively bind it to an
6151 unspecified base version of the library. You can bind all otherwise
6152 unspecified symbols to a given version node by using @samp{global: *;}
6153 somewhere in the version script. Note that it's slightly crazy to use
6154 wildcards in a global spec except on the last version node. Global
6155 wildcards elsewhere run the risk of accidentally adding symbols to the
6156 set exported for an old version. That's wrong since older versions
6157 ought to have a fixed set of symbols.
6159 The names of the version nodes have no specific meaning other than what
6160 they might suggest to the person reading them. The @samp{2.0} version
6161 could just as well have appeared in between @samp{1.1} and @samp{1.2}.
6162 However, this would be a confusing way to write a version script.
6164 Node name can be omitted, provided it is the only version node
6165 in the version script. Such version script doesn't assign any versions to
6166 symbols, only selects which symbols will be globally visible out and which
6170 @{ global: foo; bar; local: *; @};
6173 When you link an application against a shared library that has versioned
6174 symbols, the application itself knows which version of each symbol it
6175 requires, and it also knows which version nodes it needs from each
6176 shared library it is linked against. Thus at runtime, the dynamic
6177 loader can make a quick check to make sure that the libraries you have
6178 linked against do in fact supply all of the version nodes that the
6179 application will need to resolve all of the dynamic symbols. In this
6180 way it is possible for the dynamic linker to know with certainty that
6181 all external symbols that it needs will be resolvable without having to
6182 search for each symbol reference.
6184 The symbol versioning is in effect a much more sophisticated way of
6185 doing minor version checking that SunOS does. The fundamental problem
6186 that is being addressed here is that typically references to external
6187 functions are bound on an as-needed basis, and are not all bound when
6188 the application starts up. If a shared library is out of date, a
6189 required interface may be missing; when the application tries to use
6190 that interface, it may suddenly and unexpectedly fail. With symbol
6191 versioning, the user will get a warning when they start their program if
6192 the libraries being used with the application are too old.
6194 There are several GNU extensions to Sun's versioning approach. The
6195 first of these is the ability to bind a symbol to a version node in the
6196 source file where the symbol is defined instead of in the versioning
6197 script. This was done mainly to reduce the burden on the library
6198 maintainer. You can do this by putting something like:
6200 __asm__(".symver original_foo,foo@@VERS_1.1");
6203 in the C source file. This renames the function @samp{original_foo} to
6204 be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}.
6205 The @samp{local:} directive can be used to prevent the symbol
6206 @samp{original_foo} from being exported. A @samp{.symver} directive
6207 takes precedence over a version script.
6209 The second GNU extension is to allow multiple versions of the same
6210 function to appear in a given shared library. In this way you can make
6211 an incompatible change to an interface without increasing the major
6212 version number of the shared library, while still allowing applications
6213 linked against the old interface to continue to function.
6215 To do this, you must use multiple @samp{.symver} directives in the
6216 source file. Here is an example:
6219 __asm__(".symver original_foo,foo@@");
6220 __asm__(".symver old_foo,foo@@VERS_1.1");
6221 __asm__(".symver old_foo1,foo@@VERS_1.2");
6222 __asm__(".symver new_foo,foo@@@@VERS_2.0");
6225 In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the
6226 unspecified base version of the symbol. The source file that contains this
6227 example would define 4 C functions: @samp{original_foo}, @samp{old_foo},
6228 @samp{old_foo1}, and @samp{new_foo}.
6230 When you have multiple definitions of a given symbol, there needs to be
6231 some way to specify a default version to which external references to
6232 this symbol will be bound. You can do this with the
6233 @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only
6234 declare one version of a symbol as the default in this manner; otherwise
6235 you would effectively have multiple definitions of the same symbol.
6237 If you wish to bind a reference to a specific version of the symbol
6238 within the shared library, you can use the aliases of convenience
6239 (i.e., @samp{old_foo}), or you can use the @samp{.symver} directive to
6240 specifically bind to an external version of the function in question.
6242 You can also specify the language in the version script:
6245 VERSION extern "lang" @{ version-script-commands @}
6248 The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}.
6249 The linker will iterate over the list of symbols at the link time and
6250 demangle them according to @samp{lang} before matching them to the
6251 patterns specified in @samp{version-script-commands}. The default
6252 @samp{lang} is @samp{C}.
6254 Demangled names may contains spaces and other special characters. As
6255 described above, you can use a glob pattern to match demangled names,
6256 or you can use a double-quoted string to match the string exactly. In
6257 the latter case, be aware that minor differences (such as differing
6258 whitespace) between the version script and the demangler output will
6259 cause a mismatch. As the exact string generated by the demangler
6260 might change in the future, even if the mangled name does not, you
6261 should check that all of your version directives are behaving as you
6262 expect when you upgrade.
6265 @section Expressions in Linker Scripts
6268 The syntax for expressions in the linker script language is identical to
6269 that of C expressions. All expressions are evaluated as integers. All
6270 expressions are evaluated in the same size, which is 32 bits if both the
6271 host and target are 32 bits, and is otherwise 64 bits.
6273 You can use and set symbol values in expressions.
6275 The linker defines several special purpose builtin functions for use in
6279 * Constants:: Constants
6280 * Symbolic Constants:: Symbolic constants
6281 * Symbols:: Symbol Names
6282 * Orphan Sections:: Orphan Sections
6283 * Location Counter:: The Location Counter
6284 * Operators:: Operators
6285 * Evaluation:: Evaluation
6286 * Expression Section:: The Section of an Expression
6287 * Builtin Functions:: Builtin Functions
6291 @subsection Constants
6292 @cindex integer notation
6293 @cindex constants in linker scripts
6294 All constants are integers.
6296 As in C, the linker considers an integer beginning with @samp{0} to be
6297 octal, and an integer beginning with @samp{0x} or @samp{0X} to be
6298 hexadecimal. Alternatively the linker accepts suffixes of @samp{h} or
6299 @samp{H} for hexadecimal, @samp{o} or @samp{O} for octal, @samp{b} or
6300 @samp{B} for binary and @samp{d} or @samp{D} for decimal. Any integer
6301 value without a prefix or a suffix is considered to be decimal.
6303 @cindex scaled integers
6304 @cindex K and M integer suffixes
6305 @cindex M and K integer suffixes
6306 @cindex suffixes for integers
6307 @cindex integer suffixes
6308 In addition, you can use the suffixes @code{K} and @code{M} to scale a
6312 @c END TEXI2ROFF-KILL
6313 @code{1024} or @code{1024*1024}
6317 ${\rm 1024}$ or ${\rm 1024}^2$
6319 @c END TEXI2ROFF-KILL
6320 respectively. For example, the following
6321 all refer to the same quantity:
6330 Note - the @code{K} and @code{M} suffixes cannot be used in
6331 conjunction with the base suffixes mentioned above.
6333 @node Symbolic Constants
6334 @subsection Symbolic Constants
6335 @cindex symbolic constants
6337 It is possible to refer to target-specific constants via the use of
6338 the @code{CONSTANT(@var{name})} operator, where @var{name} is one of:
6343 The target's maximum page size.
6345 @item COMMONPAGESIZE
6346 @kindex COMMONPAGESIZE
6347 The target's default page size.
6353 .text ALIGN (CONSTANT (MAXPAGESIZE)) : @{ *(.text) @}
6356 will create a text section aligned to the largest page boundary
6357 supported by the target.
6360 @subsection Symbol Names
6361 @cindex symbol names
6363 @cindex quoted symbol names
6365 Unless quoted, symbol names start with a letter, underscore, or period
6366 and may include letters, digits, underscores, periods, and hyphens.
6367 Unquoted symbol names must not conflict with any keywords. You can
6368 specify a symbol which contains odd characters or has the same name as a
6369 keyword by surrounding the symbol name in double quotes:
6372 "with a space" = "also with a space" + 10;
6375 Since symbols can contain many non-alphabetic characters, it is safest
6376 to delimit symbols with spaces. For example, @samp{A-B} is one symbol,
6377 whereas @samp{A - B} is an expression involving subtraction.
6379 @node Orphan Sections
6380 @subsection Orphan Sections
6382 Orphan sections are sections present in the input files which
6383 are not explicitly placed into the output file by the linker
6384 script. The linker will still copy these sections into the
6385 output file by either finding, or creating a suitable output section
6386 in which to place the orphaned input section.
6388 If the name of an orphaned input section exactly matches the name of
6389 an existing output section, then the orphaned input section will be
6390 placed at the end of that output section.
6392 If there is no output section with a matching name then new output
6393 sections will be created. Each new output section will have the same
6394 name as the orphan section placed within it. If there are multiple
6395 orphan sections with the same name, these will all be combined into
6396 one new output section.
6398 If new output sections are created to hold orphaned input sections,
6399 then the linker must decide where to place these new output sections
6400 in relation to existing output sections. On most modern targets, the
6401 linker attempts to place orphan sections after sections of the same
6402 attribute, such as code vs data, loadable vs non-loadable, etc. If no
6403 sections with matching attributes are found, or your target lacks this
6404 support, the orphan section is placed at the end of the file.
6406 The command-line options @samp{--orphan-handling} and @samp{--unique}
6407 (@pxref{Options,,Command-line Options}) can be used to control which
6408 output sections an orphan is placed in.
6410 @node Location Counter
6411 @subsection The Location Counter
6414 @cindex location counter
6415 @cindex current output location
6416 The special linker variable @dfn{dot} @samp{.} always contains the
6417 current output location counter. Since the @code{.} always refers to a
6418 location in an output section, it may only appear in an expression
6419 within a @code{SECTIONS} command. The @code{.} symbol may appear
6420 anywhere that an ordinary symbol is allowed in an expression.
6423 Assigning a value to @code{.} will cause the location counter to be
6424 moved. This may be used to create holes in the output section. The
6425 location counter may not be moved backwards inside an output section,
6426 and may not be moved backwards outside of an output section if so
6427 doing creates areas with overlapping LMAs.
6443 In the previous example, the @samp{.text} section from @file{file1} is
6444 located at the beginning of the output section @samp{output}. It is
6445 followed by a 1000 byte gap. Then the @samp{.text} section from
6446 @file{file2} appears, also with a 1000 byte gap following before the
6447 @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678}
6448 specifies what data to write in the gaps (@pxref{Output Section Fill}).
6450 @cindex dot inside sections
6451 Note: @code{.} actually refers to the byte offset from the start of the
6452 current containing object. Normally this is the @code{SECTIONS}
6453 statement, whose start address is 0, hence @code{.} can be used as an
6454 absolute address. If @code{.} is used inside a section description
6455 however, it refers to the byte offset from the start of that section,
6456 not an absolute address. Thus in a script like this:
6474 The @samp{.text} section will be assigned a starting address of 0x100
6475 and a size of exactly 0x200 bytes, even if there is not enough data in
6476 the @samp{.text} input sections to fill this area. (If there is too
6477 much data, an error will be produced because this would be an attempt to
6478 move @code{.} backwards). The @samp{.data} section will start at 0x500
6479 and it will have an extra 0x600 bytes worth of space after the end of
6480 the values from the @samp{.data} input sections and before the end of
6481 the @samp{.data} output section itself.
6483 @cindex dot outside sections
6484 Setting symbols to the value of the location counter outside of an
6485 output section statement can result in unexpected values if the linker
6486 needs to place orphan sections. For example, given the following:
6492 .text: @{ *(.text) @}
6496 .data: @{ *(.data) @}
6501 If the linker needs to place some input section, e.g. @code{.rodata},
6502 not mentioned in the script, it might choose to place that section
6503 between @code{.text} and @code{.data}. You might think the linker
6504 should place @code{.rodata} on the blank line in the above script, but
6505 blank lines are of no particular significance to the linker. As well,
6506 the linker doesn't associate the above symbol names with their
6507 sections. Instead, it assumes that all assignments or other
6508 statements belong to the previous output section, except for the
6509 special case of an assignment to @code{.}. I.e., the linker will
6510 place the orphan @code{.rodata} section as if the script was written
6517 .text: @{ *(.text) @}
6521 .rodata: @{ *(.rodata) @}
6522 .data: @{ *(.data) @}
6527 This may or may not be the script author's intention for the value of
6528 @code{start_of_data}. One way to influence the orphan section
6529 placement is to assign the location counter to itself, as the linker
6530 assumes that an assignment to @code{.} is setting the start address of
6531 a following output section and thus should be grouped with that
6532 section. So you could write:
6538 .text: @{ *(.text) @}
6543 .data: @{ *(.data) @}
6548 Now, the orphan @code{.rodata} section will be placed between
6549 @code{end_of_text} and @code{start_of_data}.
6553 @subsection Operators
6554 @cindex operators for arithmetic
6555 @cindex arithmetic operators
6556 @cindex precedence in expressions
6557 The linker recognizes the standard C set of arithmetic operators, with
6558 the standard bindings and precedence levels:
6561 @c END TEXI2ROFF-KILL
6563 precedence associativity Operators Notes
6569 5 left == != > < <= >=
6575 11 right &= += -= *= /= (2)
6579 (1) Prefix operators
6580 (2) @xref{Assignments}.
6584 \vskip \baselineskip
6585 %"lispnarrowing" is the extra indent used generally for smallexample
6586 \hskip\lispnarrowing\vbox{\offinterlineskip
6589 {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr
6590 height2pt&\omit&&\omit&&\omit&\cr
6591 &Precedence&& Associativity &&{\rm Operators}&\cr
6592 height2pt&\omit&&\omit&&\omit&\cr
6594 height2pt&\omit&&\omit&&\omit&\cr
6596 % '176 is tilde, '~' in tt font
6597 &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr
6598 &2&&left&&* / \%&\cr
6601 &5&&left&&== != > < <= >=&\cr
6604 &8&&left&&{\&\&}&\cr
6607 &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr
6609 height2pt&\omit&&\omit&&\omit&\cr}
6614 @obeylines@parskip=0pt@parindent=0pt
6615 @dag@quad Prefix operators.
6616 @ddag@quad @xref{Assignments}.
6619 @c END TEXI2ROFF-KILL
6622 @subsection Evaluation
6623 @cindex lazy evaluation
6624 @cindex expression evaluation order
6625 The linker evaluates expressions lazily. It only computes the value of
6626 an expression when absolutely necessary.
6628 The linker needs some information, such as the value of the start
6629 address of the first section, and the origins and lengths of memory
6630 regions, in order to do any linking at all. These values are computed
6631 as soon as possible when the linker reads in the linker script.
6633 However, other values (such as symbol values) are not known or needed
6634 until after storage allocation. Such values are evaluated later, when
6635 other information (such as the sizes of output sections) is available
6636 for use in the symbol assignment expression.
6638 The sizes of sections cannot be known until after allocation, so
6639 assignments dependent upon these are not performed until after
6642 Some expressions, such as those depending upon the location counter
6643 @samp{.}, must be evaluated during section allocation.
6645 If the result of an expression is required, but the value is not
6646 available, then an error results. For example, a script like the
6652 .text 9+this_isnt_constant :
6658 will cause the error message @samp{non constant expression for initial
6661 @node Expression Section
6662 @subsection The Section of an Expression
6663 @cindex expression sections
6664 @cindex absolute expressions
6665 @cindex relative expressions
6666 @cindex absolute and relocatable symbols
6667 @cindex relocatable and absolute symbols
6668 @cindex symbols, relocatable and absolute
6669 Addresses and symbols may be section relative, or absolute. A section
6670 relative symbol is relocatable. If you request relocatable output
6671 using the @samp{-r} option, a further link operation may change the
6672 value of a section relative symbol. On the other hand, an absolute
6673 symbol will retain the same value throughout any further link
6676 Some terms in linker expressions are addresses. This is true of
6677 section relative symbols and for builtin functions that return an
6678 address, such as @code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and
6679 @code{SEGMENT_START}. Other terms are simply numbers, or are builtin
6680 functions that return a non-address value, such as @code{LENGTH}.
6681 One complication is that unless you set @code{LD_FEATURE ("SANE_EXPR")}
6682 (@pxref{Miscellaneous Commands}), numbers and absolute symbols are treated
6683 differently depending on their location, for compatibility with older
6684 versions of @code{ld}. Expressions appearing outside an output
6685 section definition treat all numbers as absolute addresses.
6686 Expressions appearing inside an output section definition treat
6687 absolute symbols as numbers. If @code{LD_FEATURE ("SANE_EXPR")} is
6688 given, then absolute symbols and numbers are simply treated as numbers
6691 In the following simple example,
6698 __executable_start = 0x100;
6702 __data_start = 0x10;
6710 both @code{.} and @code{__executable_start} are set to the absolute
6711 address 0x100 in the first two assignments, then both @code{.} and
6712 @code{__data_start} are set to 0x10 relative to the @code{.data}
6713 section in the second two assignments.
6715 For expressions involving numbers, relative addresses and absolute
6716 addresses, ld follows these rules to evaluate terms:
6720 Unary operations on an absolute address or number, and binary
6721 operations on two absolute addresses or two numbers, or between one
6722 absolute address and a number, apply the operator to the value(s).
6724 Unary operations on a relative address, and binary operations on two
6725 relative addresses in the same section or between one relative address
6726 and a number, apply the operator to the offset part of the address(es).
6728 Other binary operations, that is, between two relative addresses not
6729 in the same section, or between a relative address and an absolute
6730 address, first convert any non-absolute term to an absolute address
6731 before applying the operator.
6734 The result section of each sub-expression is as follows:
6738 An operation involving only numbers results in a number.
6740 The result of comparisons, @samp{&&} and @samp{||} is also a number.
6742 The result of other binary arithmetic and logical operations on two
6743 relative addresses in the same section or two absolute addresses
6744 (after above conversions) is also a number when
6745 @code{LD_FEATURE ("SANE_EXPR")} or inside an output section definition
6746 but an absolute address otherwise.
6748 The result of other operations on relative addresses or one
6749 relative address and a number, is a relative address in the same
6750 section as the relative operand(s).
6752 The result of other operations on absolute addresses (after above
6753 conversions) is an absolute address.
6756 You can use the builtin function @code{ABSOLUTE} to force an expression
6757 to be absolute when it would otherwise be relative. For example, to
6758 create an absolute symbol set to the address of the end of the output
6759 section @samp{.data}:
6763 .data : @{ *(.data) _edata = ABSOLUTE(.); @}
6767 If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the
6768 @samp{.data} section.
6770 Using @code{LOADADDR} also forces an expression absolute, since this
6771 particular builtin function returns an absolute address.
6773 @node Builtin Functions
6774 @subsection Builtin Functions
6775 @cindex functions in expressions
6776 The linker script language includes a number of builtin functions for
6777 use in linker script expressions.
6780 @item ABSOLUTE(@var{exp})
6781 @kindex ABSOLUTE(@var{exp})
6782 @cindex expression, absolute
6783 Return the absolute (non-relocatable, as opposed to non-negative) value
6784 of the expression @var{exp}. Primarily useful to assign an absolute
6785 value to a symbol within a section definition, where symbol values are
6786 normally section relative. @xref{Expression Section}.
6788 @item ADDR(@var{section})
6789 @kindex ADDR(@var{section})
6790 @cindex section address in expression
6791 Return the address (VMA) of the named @var{section}. Your
6792 script must previously have defined the location of that section. In
6793 the following example, @code{start_of_output_1}, @code{symbol_1} and
6794 @code{symbol_2} are assigned equivalent values, except that
6795 @code{symbol_1} will be relative to the @code{.output1} section while
6796 the other two will be absolute:
6802 start_of_output_1 = ABSOLUTE(.);
6807 symbol_1 = ADDR(.output1);
6808 symbol_2 = start_of_output_1;
6814 @item ALIGN(@var{align})
6815 @itemx ALIGN(@var{exp},@var{align})
6816 @kindex ALIGN(@var{align})
6817 @kindex ALIGN(@var{exp},@var{align})
6818 @cindex round up location counter
6819 @cindex align location counter
6820 @cindex round up expression
6821 @cindex align expression
6822 Return the location counter (@code{.}) or arbitrary expression aligned
6823 to the next @var{align} boundary. The single operand @code{ALIGN}
6824 doesn't change the value of the location counter---it just does
6825 arithmetic on it. The two operand @code{ALIGN} allows an arbitrary
6826 expression to be aligned upwards (@code{ALIGN(@var{align})} is
6827 equivalent to @code{ALIGN(ABSOLUTE(.), @var{align})}).
6829 Here is an example which aligns the output @code{.data} section to the
6830 next @code{0x2000} byte boundary after the preceding section and sets a
6831 variable within the section to the next @code{0x8000} boundary after the
6836 .data ALIGN(0x2000): @{
6838 variable = ALIGN(0x8000);
6844 The first use of @code{ALIGN} in this example specifies the location of
6845 a section because it is used as the optional @var{address} attribute of
6846 a section definition (@pxref{Output Section Address}). The second use
6847 of @code{ALIGN} is used to defines the value of a symbol.
6849 The builtin function @code{NEXT} is closely related to @code{ALIGN}.
6851 @item ALIGNOF(@var{section})
6852 @kindex ALIGNOF(@var{section})
6853 @cindex section alignment
6854 Return the alignment in bytes of the named @var{section}, if that section has
6855 been allocated. If the section has not been allocated when this is
6856 evaluated, the linker will report an error. In the following example,
6857 the alignment of the @code{.output} section is stored as the first
6858 value in that section.
6863 LONG (ALIGNOF (.output))
6870 @item BLOCK(@var{exp})
6871 @kindex BLOCK(@var{exp})
6872 This is a synonym for @code{ALIGN}, for compatibility with older linker
6873 scripts. It is most often seen when setting the address of an output
6876 @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
6877 @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
6878 This is equivalent to either
6880 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1)))
6884 (ALIGN(@var{maxpagesize})
6885 + ((. + @var{commonpagesize} - 1) & (@var{maxpagesize} - @var{commonpagesize})))
6888 depending on whether the latter uses fewer @var{commonpagesize} sized pages
6889 for the data segment (area between the result of this expression and
6890 @code{DATA_SEGMENT_END}) than the former or not.
6891 If the latter form is used, it means @var{commonpagesize} bytes of runtime
6892 memory will be saved at the expense of up to @var{commonpagesize} wasted
6893 bytes in the on-disk file.
6895 This expression can only be used directly in @code{SECTIONS} commands, not in
6896 any output section descriptions and only once in the linker script.
6897 @var{commonpagesize} should be less or equal to @var{maxpagesize} and should
6898 be the system page size the object wants to be optimized for while still
6899 running on system page sizes up to @var{maxpagesize}. Note however
6900 that @samp{-z relro} protection will not be effective if the system
6901 page size is larger than @var{commonpagesize}.
6906 . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
6909 @item DATA_SEGMENT_END(@var{exp})
6910 @kindex DATA_SEGMENT_END(@var{exp})
6911 This defines the end of data segment for @code{DATA_SEGMENT_ALIGN}
6912 evaluation purposes.
6915 . = DATA_SEGMENT_END(.);
6918 @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6919 @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6920 This defines the end of the @code{PT_GNU_RELRO} segment when
6921 @samp{-z relro} option is used.
6922 When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END}
6923 does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that
6924 @var{exp} + @var{offset} is aligned to the @var{commonpagesize}
6925 argument given to @code{DATA_SEGMENT_ALIGN}. If present in the linker
6926 script, it must be placed between @code{DATA_SEGMENT_ALIGN} and
6927 @code{DATA_SEGMENT_END}. Evaluates to the second argument plus any
6928 padding needed at the end of the @code{PT_GNU_RELRO} segment due to
6932 . = DATA_SEGMENT_RELRO_END(24, .);
6935 @item DEFINED(@var{symbol})
6936 @kindex DEFINED(@var{symbol})
6937 @cindex symbol defaults
6938 Return 1 if @var{symbol} is in the linker global symbol table and is
6939 defined before the statement using DEFINED in the script, otherwise
6940 return 0. You can use this function to provide
6941 default values for symbols. For example, the following script fragment
6942 shows how to set a global symbol @samp{begin} to the first location in
6943 the @samp{.text} section---but if a symbol called @samp{begin} already
6944 existed, its value is preserved:
6950 begin = DEFINED(begin) ? begin : . ;
6958 @item LENGTH(@var{memory})
6959 @kindex LENGTH(@var{memory})
6960 Return the length of the memory region named @var{memory}.
6962 @item LOADADDR(@var{section})
6963 @kindex LOADADDR(@var{section})
6964 @cindex section load address in expression
6965 Return the absolute LMA of the named @var{section}. (@pxref{Output
6968 @item LOG2CEIL(@var{exp})
6969 @kindex LOG2CEIL(@var{exp})
6970 Return the binary logarithm of @var{exp} rounded towards infinity.
6971 @code{LOG2CEIL(0)} returns 0.
6974 @item MAX(@var{exp1}, @var{exp2})
6975 Returns the maximum of @var{exp1} and @var{exp2}.
6978 @item MIN(@var{exp1}, @var{exp2})
6979 Returns the minimum of @var{exp1} and @var{exp2}.
6981 @item NEXT(@var{exp})
6982 @kindex NEXT(@var{exp})
6983 @cindex unallocated address, next
6984 Return the next unallocated address that is a multiple of @var{exp}.
6985 This function is closely related to @code{ALIGN(@var{exp})}; unless you
6986 use the @code{MEMORY} command to define discontinuous memory for the
6987 output file, the two functions are equivalent.
6989 @item ORIGIN(@var{memory})
6990 @kindex ORIGIN(@var{memory})
6991 Return the origin of the memory region named @var{memory}.
6993 @item SEGMENT_START(@var{segment}, @var{default})
6994 @kindex SEGMENT_START(@var{segment}, @var{default})
6995 Return the base address of the named @var{segment}. If an explicit
6996 value has already been given for this segment (with a command-line
6997 @samp{-T} option) then that value will be returned otherwise the value
6998 will be @var{default}. At present, the @samp{-T} command-line option
6999 can only be used to set the base address for the ``text'', ``data'', and
7000 ``bss'' sections, but you can use @code{SEGMENT_START} with any segment
7003 @item SIZEOF(@var{section})
7004 @kindex SIZEOF(@var{section})
7005 @cindex section size
7006 Return the size in bytes of the named @var{section}, if that section has
7007 been allocated. If the section has not been allocated when this is
7008 evaluated, the linker will report an error. In the following example,
7009 @code{symbol_1} and @code{symbol_2} are assigned identical values:
7018 symbol_1 = .end - .start ;
7019 symbol_2 = SIZEOF(.output);
7024 @item SIZEOF_HEADERS
7025 @itemx sizeof_headers
7026 @kindex SIZEOF_HEADERS
7028 Return the size in bytes of the output file's headers. This is
7029 information which appears at the start of the output file. You can use
7030 this number when setting the start address of the first section, if you
7031 choose, to facilitate paging.
7033 @cindex not enough room for program headers
7034 @cindex program headers, not enough room
7035 When producing an ELF output file, if the linker script uses the
7036 @code{SIZEOF_HEADERS} builtin function, the linker must compute the
7037 number of program headers before it has determined all the section
7038 addresses and sizes. If the linker later discovers that it needs
7039 additional program headers, it will report an error @samp{not enough
7040 room for program headers}. To avoid this error, you must avoid using
7041 the @code{SIZEOF_HEADERS} function, or you must rework your linker
7042 script to avoid forcing the linker to use additional program headers, or
7043 you must define the program headers yourself using the @code{PHDRS}
7044 command (@pxref{PHDRS}).
7047 @node Implicit Linker Scripts
7048 @section Implicit Linker Scripts
7049 @cindex implicit linker scripts
7050 If you specify a linker input file which the linker can not recognize as
7051 an object file or an archive file, it will try to read the file as a
7052 linker script. If the file can not be parsed as a linker script, the
7053 linker will report an error.
7055 An implicit linker script will not replace the default linker script.
7057 Typically an implicit linker script would contain only symbol
7058 assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION}
7061 Any input files read because of an implicit linker script will be read
7062 at the position in the command line where the implicit linker script was
7063 read. This can affect archive searching.
7066 @chapter Linker Plugins
7069 @cindex linker plugins
7070 The linker can use dynamically loaded plugins to modify its behavior.
7071 For example, the link-time optimization feature that some compilers
7072 support is implemented with a linker plugin.
7074 Currently there is only one plugin shipped by default, but more may
7075 be added here later.
7078 * libdep Plugin:: Static Library Dependencies Plugin
7082 @section Static Library Dependencies Plugin
7083 @cindex static library dependencies
7084 Originally, static libraries were contained in an archive file consisting
7085 just of a collection of relocatable object files. Later they evolved to
7086 optionally include a symbol table, to assist in finding the needed objects
7087 within a library. There their evolution ended, and dynamic libraries
7090 One useful feature of dynamic libraries was that, more than just collecting
7091 multiple objects into a single file, they also included a list of their
7092 dependencies, such that one could specify just the name of a single dynamic
7093 library at link time, and all of its dependencies would be implicitly
7094 referenced as well. But static libraries lacked this feature, so if a
7095 link invocation was switched from using dynamic libraries to static
7096 libraries, the link command would usually fail unless it was rewritten to
7097 explicitly list the dependencies of the static library.
7099 The GNU @command{ar} utility now supports a @option{--record-libdeps} option
7100 to embed dependency lists into static libraries as well, and the @file{libdep}
7101 plugin may be used to read this dependency information at link time. The
7102 dependency information is stored as a single string, carrying @option{-l}
7103 and @option{-L} arguments as they would normally appear in a linker
7104 command line. As such, the information can be written with any text
7105 utility and stored into any archive, even if GNU @command{ar} is not
7106 being used to create the archive. The information is stored in an
7107 archive member named @samp{__.LIBDEP}.
7109 For example, given a library @file{libssl.a} that depends on another
7110 library @file{libcrypto.a} which may be found in @file{/usr/local/lib},
7111 the @samp{__.LIBDEP} member of @file{libssl.a} would contain
7114 -L/usr/local/lib -lcrypto
7118 @node Machine Dependent
7119 @chapter Machine Dependent Features
7121 @cindex machine dependencies
7122 @command{ld} has additional features on some platforms; the following
7123 sections describe them. Machines where @command{ld} has no additional
7124 functionality are not listed.
7128 * H8/300:: @command{ld} and the H8/300
7131 * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families
7134 * ARM:: @command{ld} and the ARM family
7137 * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF
7140 * M68K:: @command{ld} and the Motorola 68K family
7143 * MIPS:: @command{ld} and the MIPS family
7146 * MMIX:: @command{ld} and MMIX
7149 * MSP430:: @command{ld} and MSP430
7152 * NDS32:: @command{ld} and NDS32
7155 * Nios II:: @command{ld} and the Altera Nios II
7158 * PowerPC ELF32:: @command{ld} and PowerPC 32-bit ELF Support
7161 * PowerPC64 ELF64:: @command{ld} and PowerPC64 64-bit ELF Support
7164 * S/390 ELF:: @command{ld} and S/390 ELF Support
7167 * SPU ELF:: @command{ld} and SPU ELF Support
7170 * TI COFF:: @command{ld} and TI COFF
7173 * WIN32:: @command{ld} and WIN32 (cygwin/mingw)
7176 * Xtensa:: @command{ld} and Xtensa Processors
7187 @section @command{ld} and the H8/300
7189 @cindex H8/300 support
7190 For the H8/300, @command{ld} can perform these global optimizations when
7191 you specify the @samp{--relax} command-line option.
7194 @cindex relaxing on H8/300
7195 @item relaxing address modes
7196 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
7197 targets are within eight bits, and turns them into eight-bit
7198 program-counter relative @code{bsr} and @code{bra} instructions,
7201 @cindex synthesizing on H8/300
7202 @item synthesizing instructions
7203 @c FIXME: specifically mov.b, or any mov instructions really? -> mov.b only, at least on H8, H8H, H8S
7204 @command{ld} finds all @code{mov.b} instructions which use the
7205 sixteen-bit absolute address form, but refer to the top
7206 page of memory, and changes them to use the eight-bit address form.
7207 (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into
7208 @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the
7209 top page of memory).
7211 @command{ld} finds all @code{mov} instructions which use the register
7212 indirect with 32-bit displacement addressing mode, but use a small
7213 displacement inside 16-bit displacement range, and changes them to use
7214 the 16-bit displacement form. (That is: the linker turns @samp{mov.b
7215 @code{@@}@var{d}:32,ERx} into @samp{mov.b @code{@@}@var{d}:16,ERx}
7216 whenever the displacement @var{d} is in the 16 bit signed integer
7217 range. Only implemented in ELF-format ld).
7219 @item bit manipulation instructions
7220 @command{ld} finds all bit manipulation instructions like @code{band, bclr,
7221 biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor}
7222 which use 32 bit and 16 bit absolute address form, but refer to the top
7223 page of memory, and changes them to use the 8 bit address form.
7224 (That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into
7225 @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in
7226 the top page of memory).
7228 @item system control instructions
7229 @command{ld} finds all @code{ldc.w, stc.w} instructions which use the
7230 32 bit absolute address form, but refer to the top page of memory, and
7231 changes them to use 16 bit address form.
7232 (That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into
7233 @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in
7234 the top page of memory).
7244 @c This stuff is pointless to say unless you're especially concerned
7245 @c with Renesas chips; don't enable it for generic case, please.
7247 @chapter @command{ld} and Other Renesas Chips
7249 @command{ld} also supports the Renesas (formerly Hitachi) H8/300H,
7250 H8/500, and SH chips. No special features, commands, or command-line
7251 options are required for these chips.
7265 @node M68HC11/68HC12
7266 @section @command{ld} and the Motorola 68HC11 and 68HC12 families
7268 @cindex M68HC11 and 68HC12 support
7270 @subsection Linker Relaxation
7272 For the Motorola 68HC11, @command{ld} can perform these global
7273 optimizations when you specify the @samp{--relax} command-line option.
7276 @cindex relaxing on M68HC11
7277 @item relaxing address modes
7278 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
7279 targets are within eight bits, and turns them into eight-bit
7280 program-counter relative @code{bsr} and @code{bra} instructions,
7283 @command{ld} also looks at all 16-bit extended addressing modes and
7284 transforms them in a direct addressing mode when the address is in
7285 page 0 (between 0 and 0x0ff).
7287 @item relaxing gcc instruction group
7288 When @command{gcc} is called with @option{-mrelax}, it can emit group
7289 of instructions that the linker can optimize to use a 68HC11 direct
7290 addressing mode. These instructions consists of @code{bclr} or
7291 @code{bset} instructions.
7295 @subsection Trampoline Generation
7297 @cindex trampoline generation on M68HC11
7298 @cindex trampoline generation on M68HC12
7299 For 68HC11 and 68HC12, @command{ld} can generate trampoline code to
7300 call a far function using a normal @code{jsr} instruction. The linker
7301 will also change the relocation to some far function to use the
7302 trampoline address instead of the function address. This is typically the
7303 case when a pointer to a function is taken. The pointer will in fact
7304 point to the function trampoline.
7312 @section @command{ld} and the ARM family
7314 @cindex ARM interworking support
7315 @kindex --support-old-code
7316 For the ARM, @command{ld} will generate code stubs to allow functions calls
7317 between ARM and Thumb code. These stubs only work with code that has
7318 been compiled and assembled with the @samp{-mthumb-interwork} command
7319 line option. If it is necessary to link with old ARM object files or
7320 libraries, which have not been compiled with the -mthumb-interwork
7321 option then the @samp{--support-old-code} command-line switch should be
7322 given to the linker. This will make it generate larger stub functions
7323 which will work with non-interworking aware ARM code. Note, however,
7324 the linker does not support generating stubs for function calls to
7325 non-interworking aware Thumb code.
7327 @cindex thumb entry point
7328 @cindex entry point, thumb
7329 @kindex --thumb-entry=@var{entry}
7330 The @samp{--thumb-entry} switch is a duplicate of the generic
7331 @samp{--entry} switch, in that it sets the program's starting address.
7332 But it also sets the bottom bit of the address, so that it can be
7333 branched to using a BX instruction, and the program will start
7334 executing in Thumb mode straight away.
7336 @cindex PE import table prefixing
7337 @kindex --use-nul-prefixed-import-tables
7338 The @samp{--use-nul-prefixed-import-tables} switch is specifying, that
7339 the import tables idata4 and idata5 have to be generated with a zero
7340 element prefix for import libraries. This is the old style to generate
7341 import tables. By default this option is turned off.
7345 The @samp{--be8} switch instructs @command{ld} to generate BE8 format
7346 executables. This option is only valid when linking big-endian
7347 objects - ie ones which have been assembled with the @option{-EB}
7348 option. The resulting image will contain big-endian data and
7352 @kindex --target1-rel
7353 @kindex --target1-abs
7354 The @samp{R_ARM_TARGET1} relocation is typically used for entries in the
7355 @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32}
7356 or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel}
7357 and @samp{--target1-abs} switches override the default.
7360 @kindex --target2=@var{type}
7361 The @samp{--target2=type} switch overrides the default definition of the
7362 @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their
7363 meanings, and target defaults are as follows:
7366 @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi)
7370 @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd)
7375 The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF
7376 specification) enables objects compiled for the ARMv4 architecture to be
7377 interworking-safe when linked with other objects compiled for ARMv4t, but
7378 also allows pure ARMv4 binaries to be built from the same ARMv4 objects.
7380 In the latter case, the switch @option{--fix-v4bx} must be passed to the
7381 linker, which causes v4t @code{BX rM} instructions to be rewritten as
7382 @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction.
7384 In the former case, the switch should not be used, and @samp{R_ARM_V4BX}
7385 relocations are ignored.
7387 @cindex FIX_V4BX_INTERWORKING
7388 @kindex --fix-v4bx-interworking
7389 Replace @code{BX rM} instructions identified by @samp{R_ARM_V4BX}
7390 relocations with a branch to the following veneer:
7398 This allows generation of libraries/applications that work on ARMv4 cores
7399 and are still interworking safe. Note that the above veneer clobbers the
7400 condition flags, so may cause incorrect program behavior in rare cases.
7404 The @samp{--use-blx} switch enables the linker to use ARM/Thumb
7405 BLX instructions (available on ARMv5t and above) in various
7406 situations. Currently it is used to perform calls via the PLT from Thumb
7407 code using BLX rather than using BX and a mode-switching stub before
7408 each PLT entry. This should lead to such calls executing slightly faster.
7410 @cindex VFP11_DENORM_FIX
7411 @kindex --vfp11-denorm-fix
7412 The @samp{--vfp11-denorm-fix} switch enables a link-time workaround for a
7413 bug in certain VFP11 coprocessor hardware, which sometimes allows
7414 instructions with denorm operands (which must be handled by support code)
7415 to have those operands overwritten by subsequent instructions before
7416 the support code can read the intended values.
7418 The bug may be avoided in scalar mode if you allow at least one
7419 intervening instruction between a VFP11 instruction which uses a register
7420 and another instruction which writes to the same register, or at least two
7421 intervening instructions if vector mode is in use. The bug only affects
7422 full-compliance floating-point mode: you do not need this workaround if
7423 you are using "runfast" mode. Please contact ARM for further details.
7425 If you know you are using buggy VFP11 hardware, you can
7426 enable this workaround by specifying the linker option
7427 @samp{--vfp-denorm-fix=scalar} if you are using the VFP11 scalar
7428 mode only, or @samp{--vfp-denorm-fix=vector} if you are using
7429 vector mode (the latter also works for scalar code). The default is
7430 @samp{--vfp-denorm-fix=none}.
7432 If the workaround is enabled, instructions are scanned for
7433 potentially-troublesome sequences, and a veneer is created for each
7434 such sequence which may trigger the erratum. The veneer consists of the
7435 first instruction of the sequence and a branch back to the subsequent
7436 instruction. The original instruction is then replaced with a branch to
7437 the veneer. The extra cycles required to call and return from the veneer
7438 are sufficient to avoid the erratum in both the scalar and vector cases.
7440 @cindex ARM1176 erratum workaround
7441 @kindex --fix-arm1176
7442 @kindex --no-fix-arm1176
7443 The @samp{--fix-arm1176} switch enables a link-time workaround for an erratum
7444 in certain ARM1176 processors. The workaround is enabled by default if you
7445 are targeting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled
7446 unconditionally by specifying @samp{--no-fix-arm1176}.
7448 Further information is available in the ``ARM1176JZ-S and ARM1176JZF-S
7449 Programmer Advice Notice'' available on the ARM documentation website at:
7450 http://infocenter.arm.com/.
7452 @cindex STM32L4xx erratum workaround
7453 @kindex --fix-stm32l4xx-629360
7455 The @samp{--fix-stm32l4xx-629360} switch enables a link-time
7456 workaround for a bug in the bus matrix / memory controller for some of
7457 the STM32 Cortex-M4 based products (STM32L4xx). When accessing
7458 off-chip memory via the affected bus for bus reads of 9 words or more,
7459 the bus can generate corrupt data and/or abort. These are only
7460 core-initiated accesses (not DMA), and might affect any access:
7461 integer loads such as LDM, POP and floating-point loads such as VLDM,
7462 VPOP. Stores are not affected.
7464 The bug can be avoided by splitting memory accesses into the
7465 necessary chunks to keep bus reads below 8 words.
7467 The workaround is not enabled by default, this is equivalent to use
7468 @samp{--fix-stm32l4xx-629360=none}. If you know you are using buggy
7469 STM32L4xx hardware, you can enable the workaround by specifying the
7470 linker option @samp{--fix-stm32l4xx-629360}, or the equivalent
7471 @samp{--fix-stm32l4xx-629360=default}.
7473 If the workaround is enabled, instructions are scanned for
7474 potentially-troublesome sequences, and a veneer is created for each
7475 such sequence which may trigger the erratum. The veneer consists in a
7476 replacement sequence emulating the behaviour of the original one and a
7477 branch back to the subsequent instruction. The original instruction is
7478 then replaced with a branch to the veneer.
7480 The workaround does not always preserve the memory access order for
7481 the LDMDB instruction, when the instruction loads the PC.
7483 The workaround is not able to handle problematic instructions when
7484 they are in the middle of an IT block, since a branch is not allowed
7485 there. In that case, the linker reports a warning and no replacement
7488 The workaround is not able to replace problematic instructions with a
7489 PC-relative branch instruction if the @samp{.text} section is too
7490 large. In that case, when the branch that replaces the original code
7491 cannot be encoded, the linker reports a warning and no replacement
7494 @cindex NO_ENUM_SIZE_WARNING
7495 @kindex --no-enum-size-warning
7496 The @option{--no-enum-size-warning} switch prevents the linker from
7497 warning when linking object files that specify incompatible EABI
7498 enumeration size attributes. For example, with this switch enabled,
7499 linking of an object file using 32-bit enumeration values with another
7500 using enumeration values fitted into the smallest possible space will
7503 @cindex NO_WCHAR_SIZE_WARNING
7504 @kindex --no-wchar-size-warning
7505 The @option{--no-wchar-size-warning} switch prevents the linker from
7506 warning when linking object files that specify incompatible EABI
7507 @code{wchar_t} size attributes. For example, with this switch enabled,
7508 linking of an object file using 32-bit @code{wchar_t} values with another
7509 using 16-bit @code{wchar_t} values will not be diagnosed.
7512 @kindex --pic-veneer
7513 The @samp{--pic-veneer} switch makes the linker use PIC sequences for
7514 ARM/Thumb interworking veneers, even if the rest of the binary
7515 is not PIC. This avoids problems on uClinux targets where
7516 @samp{--emit-relocs} is used to generate relocatable binaries.
7518 @cindex STUB_GROUP_SIZE
7519 @kindex --stub-group-size=@var{N}
7520 The linker will automatically generate and insert small sequences of
7521 code into a linked ARM ELF executable whenever an attempt is made to
7522 perform a function call to a symbol that is too far away. The
7523 placement of these sequences of instructions - called stubs - is
7524 controlled by the command-line option @option{--stub-group-size=N}.
7525 The placement is important because a poor choice can create a need for
7526 duplicate stubs, increasing the code size. The linker will try to
7527 group stubs together in order to reduce interruptions to the flow of
7528 code, but it needs guidance as to how big these groups should be and
7529 where they should be placed.
7531 The value of @samp{N}, the parameter to the
7532 @option{--stub-group-size=} option controls where the stub groups are
7533 placed. If it is negative then all stubs are placed after the first
7534 branch that needs them. If it is positive then the stubs can be
7535 placed either before or after the branches that need them. If the
7536 value of @samp{N} is 1 (either +1 or -1) then the linker will choose
7537 exactly where to place groups of stubs, using its built in heuristics.
7538 A value of @samp{N} greater than 1 (or smaller than -1) tells the
7539 linker that a single group of stubs can service at most @samp{N} bytes
7540 from the input sections.
7542 The default, if @option{--stub-group-size=} is not specified, is
7545 Farcalls stubs insertion is fully supported for the ARM-EABI target
7546 only, because it relies on object files properties not present
7549 @cindex Cortex-A8 erratum workaround
7550 @kindex --fix-cortex-a8
7551 @kindex --no-fix-cortex-a8
7552 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}.
7554 The erratum only affects Thumb-2 code. Please contact ARM for further details.
7556 @cindex Cortex-A53 erratum 835769 workaround
7557 @kindex --fix-cortex-a53-835769
7558 @kindex --no-fix-cortex-a53-835769
7559 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}.
7561 Please contact ARM for further details.
7563 @kindex --merge-exidx-entries
7564 @kindex --no-merge-exidx-entries
7565 @cindex Merging exidx entries
7566 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent exidx entries in debuginfo.
7569 @cindex 32-bit PLT entries
7570 The @samp{--long-plt} option enables the use of 16 byte PLT entries
7571 which support up to 4Gb of code. The default is to use 12 byte PLT
7572 entries which only support 512Mb of code.
7574 @kindex --no-apply-dynamic-relocs
7575 @cindex AArch64 rela addend
7576 The @samp{--no-apply-dynamic-relocs} option makes AArch64 linker do not apply
7577 link-time values for dynamic relocations.
7579 @cindex Placement of SG veneers
7580 All SG veneers are placed in the special output section @code{.gnu.sgstubs}.
7581 Its start address must be set, either with the command-line option
7582 @samp{--section-start} or in a linker script, to indicate where to place these
7585 @kindex --cmse-implib
7586 @cindex Secure gateway import library
7587 The @samp{--cmse-implib} option requests that the import libraries
7588 specified by the @samp{--out-implib} and @samp{--in-implib} options are
7589 secure gateway import libraries, suitable for linking a non-secure
7590 executable against secure code as per ARMv8-M Security Extensions.
7592 @kindex --in-implib=@var{file}
7593 @cindex Input import library
7594 The @samp{--in-implib=file} specifies an input import library whose symbols
7595 must keep the same address in the executable being produced. A warning is
7596 given if no @samp{--out-implib} is given but new symbols have been introduced
7597 in the executable that should be listed in its import library. Otherwise, if
7598 @samp{--out-implib} is specified, the symbols are added to the output import
7599 library. A warning is also given if some symbols present in the input import
7600 library have disappeared from the executable. This option is only effective
7601 for Secure Gateway import libraries, ie. when @samp{--cmse-implib} is
7615 @section @command{ld} and HPPA 32-bit ELF Support
7616 @cindex HPPA multiple sub-space stubs
7617 @kindex --multi-subspace
7618 When generating a shared library, @command{ld} will by default generate
7619 import stubs suitable for use with a single sub-space application.
7620 The @samp{--multi-subspace} switch causes @command{ld} to generate export
7621 stubs, and different (larger) import stubs suitable for use with
7622 multiple sub-spaces.
7624 @cindex HPPA stub grouping
7625 @kindex --stub-group-size=@var{N}
7626 Long branch stubs and import/export stubs are placed by @command{ld} in
7627 stub sections located between groups of input sections.
7628 @samp{--stub-group-size} specifies the maximum size of a group of input
7629 sections handled by one stub section. Since branch offsets are signed,
7630 a stub section may serve two groups of input sections, one group before
7631 the stub section, and one group after it. However, when using
7632 conditional branches that require stubs, it may be better (for branch
7633 prediction) that stub sections only serve one group of input sections.
7634 A negative value for @samp{N} chooses this scheme, ensuring that
7635 branches to stubs always use a negative offset. Two special values of
7636 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
7637 @command{ld} to automatically size input section groups for the branch types
7638 detected, with the same behaviour regarding stub placement as other
7639 positive or negative values of @samp{N} respectively.
7641 Note that @samp{--stub-group-size} does not split input sections. A
7642 single input section larger than the group size specified will of course
7643 create a larger group (of one section). If input sections are too
7644 large, it may not be possible for a branch to reach its stub.
7657 @section @command{ld} and the Motorola 68K family
7659 @cindex Motorola 68K GOT generation
7660 @kindex --got=@var{type}
7661 The @samp{--got=@var{type}} option lets you choose the GOT generation scheme.
7662 The choices are @samp{single}, @samp{negative}, @samp{multigot} and
7663 @samp{target}. When @samp{target} is selected the linker chooses
7664 the default GOT generation scheme for the current target.
7665 @samp{single} tells the linker to generate a single GOT with
7666 entries only at non-negative offsets.
7667 @samp{negative} instructs the linker to generate a single GOT with
7668 entries at both negative and positive offsets. Not all environments
7670 @samp{multigot} allows the linker to generate several GOTs in the
7671 output file. All GOT references from a single input object
7672 file access the same GOT, but references from different input object
7673 files might access different GOTs. Not all environments support such GOTs.
7686 @section @command{ld} and the MIPS family
7688 @cindex MIPS microMIPS instruction choice selection
7691 The @samp{--insn32} and @samp{--no-insn32} options control the choice of
7692 microMIPS instructions used in code generated by the linker, such as that
7693 in the PLT or lazy binding stubs, or in relaxation. If @samp{--insn32} is
7694 used, then the linker only uses 32-bit instruction encodings. By default
7695 or if @samp{--no-insn32} is used, all instruction encodings are used,
7696 including 16-bit ones where possible.
7698 @cindex MIPS branch relocation check control
7699 @kindex --ignore-branch-isa
7700 @kindex --no-ignore-branch-isa
7701 The @samp{--ignore-branch-isa} and @samp{--no-ignore-branch-isa} options
7702 control branch relocation checks for invalid ISA mode transitions. If
7703 @samp{--ignore-branch-isa} is used, then the linker accepts any branch
7704 relocations and any ISA mode transition required is lost in relocation
7705 calculation, except for some cases of @code{BAL} instructions which meet
7706 relaxation conditions and are converted to equivalent @code{JALX}
7707 instructions as the associated relocation is calculated. By default
7708 or if @samp{--no-ignore-branch-isa} is used a check is made causing
7709 the loss of an ISA mode transition to produce an error.
7722 @section @code{ld} and MMIX
7723 For MMIX, there is a choice of generating @code{ELF} object files or
7724 @code{mmo} object files when linking. The simulator @code{mmix}
7725 understands the @code{mmo} format. The binutils @code{objcopy} utility
7726 can translate between the two formats.
7728 There is one special section, the @samp{.MMIX.reg_contents} section.
7729 Contents in this section is assumed to correspond to that of global
7730 registers, and symbols referring to it are translated to special symbols,
7731 equal to registers. In a final link, the start address of the
7732 @samp{.MMIX.reg_contents} section corresponds to the first allocated
7733 global register multiplied by 8. Register @code{$255} is not included in
7734 this section; it is always set to the program entry, which is at the
7735 symbol @code{Main} for @code{mmo} files.
7737 Global symbols with the prefix @code{__.MMIX.start.}, for example
7738 @code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special.
7739 The default linker script uses these to set the default start address
7742 Initial and trailing multiples of zero-valued 32-bit words in a section,
7743 are left out from an mmo file.
7756 @section @code{ld} and MSP430
7757 For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]}
7758 will select an appropriate linker script for selected MPU type. (To get a list of known MPUs
7759 just pass @samp{-m help} option to the linker).
7761 @cindex MSP430 extra sections
7762 The linker will recognize some extra sections which are MSP430 specific:
7765 @item @samp{.vectors}
7766 Defines a portion of ROM where interrupt vectors located.
7768 @item @samp{.bootloader}
7769 Defines the bootloader portion of the ROM (if applicable). Any code
7770 in this section will be uploaded to the MPU.
7772 @item @samp{.infomem}
7773 Defines an information memory section (if applicable). Any code in
7774 this section will be uploaded to the MPU.
7776 @item @samp{.infomemnobits}
7777 This is the same as the @samp{.infomem} section except that any code
7778 in this section will not be uploaded to the MPU.
7780 @item @samp{.noinit}
7781 Denotes a portion of RAM located above @samp{.bss} section.
7783 The last two sections are used by gcc.
7787 @cindex MSP430 Options
7788 @kindex --code-region
7789 @item --code-region=[either,lower,upper,none]
7790 This will transform .text* sections to [either,lower,upper].text* sections. The
7791 argument passed to GCC for -mcode-region is propagated to the linker
7794 @kindex --data-region
7795 @item --data-region=[either,lower,upper,none]
7796 This will transform .data*, .bss* and .rodata* sections to
7797 [either,lower,upper].[data,bss,rodata]* sections. The argument passed to GCC
7798 for -mdata-region is propagated to the linker using this option.
7800 @kindex --disable-sec-transformation
7801 @item --disable-sec-transformation
7802 Prevent the transformation of sections as specified by the @code{--code-region}
7803 and @code{--data-region} options.
7804 This is useful if you are compiling and linking using a single call to the GCC
7805 wrapper, and want to compile the source files using -m[code,data]-region but
7806 not transform the sections for prebuilt libraries and objects.
7820 @section @code{ld} and NDS32
7821 @kindex relaxing on NDS32
7822 For NDS32, there are some options to select relaxation behavior. The linker
7823 relaxes objects according to these options.
7826 @item @samp{--m[no-]fp-as-gp}
7827 Disable/enable fp-as-gp relaxation.
7829 @item @samp{--mexport-symbols=FILE}
7830 Exporting symbols and their address into FILE as linker script.
7832 @item @samp{--m[no-]ex9}
7833 Disable/enable link-time EX9 relaxation.
7835 @item @samp{--mexport-ex9=FILE}
7836 Export the EX9 table after linking.
7838 @item @samp{--mimport-ex9=FILE}
7839 Import the Ex9 table for EX9 relaxation.
7841 @item @samp{--mupdate-ex9}
7842 Update the existing EX9 table.
7844 @item @samp{--mex9-limit=NUM}
7845 Maximum number of entries in the ex9 table.
7847 @item @samp{--mex9-loop-aware}
7848 Avoid generating the EX9 instruction inside the loop.
7850 @item @samp{--m[no-]ifc}
7851 Disable/enable the link-time IFC optimization.
7853 @item @samp{--mifc-loop-aware}
7854 Avoid generating the IFC instruction inside the loop.
7868 @section @command{ld} and the Altera Nios II
7869 @cindex Nios II call relaxation
7870 @kindex --relax on Nios II
7872 Call and immediate jump instructions on Nios II processors are limited to
7873 transferring control to addresses in the same 256MB memory segment,
7874 which may result in @command{ld} giving
7875 @samp{relocation truncated to fit} errors with very large programs.
7876 The command-line option @option{--relax} enables the generation of
7877 trampolines that can access the entire 32-bit address space for calls
7878 outside the normal @code{call} and @code{jmpi} address range. These
7879 trampolines are inserted at section boundaries, so may not themselves
7880 be reachable if an input section and its associated call trampolines are
7883 The @option{--relax} option is enabled by default unless @option{-r}
7884 is also specified. You can disable trampoline generation by using the
7885 @option{--no-relax} linker option. You can also disable this optimization
7886 locally by using the @samp{set .noat} directive in assembly-language
7887 source files, as the linker-inserted trampolines use the @code{at}
7888 register as a temporary.
7890 Note that the linker @option{--relax} option is independent of assembler
7891 relaxation options, and that using the GNU assembler's @option{-relax-all}
7892 option interferes with the linker's more selective call instruction relaxation.
7905 @section @command{ld} and PowerPC 32-bit ELF Support
7906 @cindex PowerPC long branches
7907 @kindex --relax on PowerPC
7908 Branches on PowerPC processors are limited to a signed 26-bit
7909 displacement, which may result in @command{ld} giving
7910 @samp{relocation truncated to fit} errors with very large programs.
7911 @samp{--relax} enables the generation of trampolines that can access
7912 the entire 32-bit address space. These trampolines are inserted at
7913 section boundaries, so may not themselves be reachable if an input
7914 section exceeds 33M in size. You may combine @samp{-r} and
7915 @samp{--relax} to add trampolines in a partial link. In that case
7916 both branches to undefined symbols and inter-section branches are also
7917 considered potentially out of range, and trampolines inserted.
7919 @cindex PowerPC ELF32 options
7924 Current PowerPC GCC accepts a @samp{-msecure-plt} option that
7925 generates code capable of using a newer PLT and GOT layout that has
7926 the security advantage of no executable section ever needing to be
7927 writable and no writable section ever being executable. PowerPC
7928 @command{ld} will generate this layout, including stubs to access the
7929 PLT, if all input files (including startup and static libraries) were
7930 compiled with @samp{-msecure-plt}. @samp{--bss-plt} forces the old
7931 BSS PLT (and GOT layout) which can give slightly better performance.
7933 @kindex --secure-plt
7935 @command{ld} will use the new PLT and GOT layout if it is linking new
7936 @samp{-fpic} or @samp{-fPIC} code, but does not do so automatically
7937 when linking non-PIC code. This option requests the new PLT and GOT
7938 layout. A warning will be given if some object file requires the old
7944 The new secure PLT and GOT are placed differently relative to other
7945 sections compared to older BSS PLT and GOT placement. The location of
7946 @code{.plt} must change because the new secure PLT is an initialized
7947 section while the old PLT is uninitialized. The reason for the
7948 @code{.got} change is more subtle: The new placement allows
7949 @code{.got} to be read-only in applications linked with
7950 @samp{-z relro -z now}. However, this placement means that
7951 @code{.sdata} cannot always be used in shared libraries, because the
7952 PowerPC ABI accesses @code{.sdata} in shared libraries from the GOT
7953 pointer. @samp{--sdata-got} forces the old GOT placement. PowerPC
7954 GCC doesn't use @code{.sdata} in shared libraries, so this option is
7955 really only useful for other compilers that may do so.
7957 @cindex PowerPC stub symbols
7958 @kindex --emit-stub-syms
7959 @item --emit-stub-syms
7960 This option causes @command{ld} to label linker stubs with a local
7961 symbol that encodes the stub type and destination.
7963 @cindex PowerPC TLS optimization
7964 @kindex --no-tls-optimize
7965 @item --no-tls-optimize
7966 PowerPC @command{ld} normally performs some optimization of code
7967 sequences used to access Thread-Local Storage. Use this option to
7968 disable the optimization.
7981 @node PowerPC64 ELF64
7982 @section @command{ld} and PowerPC64 64-bit ELF Support
7984 @cindex PowerPC64 ELF64 options
7986 @cindex PowerPC64 stub grouping
7987 @kindex --stub-group-size
7988 @item --stub-group-size
7989 Long branch stubs, PLT call stubs and TOC adjusting stubs are placed
7990 by @command{ld} in stub sections located between groups of input sections.
7991 @samp{--stub-group-size} specifies the maximum size of a group of input
7992 sections handled by one stub section. Since branch offsets are signed,
7993 a stub section may serve two groups of input sections, one group before
7994 the stub section, and one group after it. However, when using
7995 conditional branches that require stubs, it may be better (for branch
7996 prediction) that stub sections only serve one group of input sections.
7997 A negative value for @samp{N} chooses this scheme, ensuring that
7998 branches to stubs always use a negative offset. Two special values of
7999 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
8000 @command{ld} to automatically size input section groups for the branch types
8001 detected, with the same behaviour regarding stub placement as other
8002 positive or negative values of @samp{N} respectively.
8004 Note that @samp{--stub-group-size} does not split input sections. A
8005 single input section larger than the group size specified will of course
8006 create a larger group (of one section). If input sections are too
8007 large, it may not be possible for a branch to reach its stub.
8009 @cindex PowerPC64 stub symbols
8010 @kindex --emit-stub-syms
8011 @item --emit-stub-syms
8012 This option causes @command{ld} to label linker stubs with a local
8013 symbol that encodes the stub type and destination.
8015 @cindex PowerPC64 dot symbols
8017 @kindex --no-dotsyms
8020 These two options control how @command{ld} interprets version patterns
8021 in a version script. Older PowerPC64 compilers emitted both a
8022 function descriptor symbol with the same name as the function, and a
8023 code entry symbol with the name prefixed by a dot (@samp{.}). To
8024 properly version a function @samp{foo}, the version script thus needs
8025 to control both @samp{foo} and @samp{.foo}. The option
8026 @samp{--dotsyms}, on by default, automatically adds the required
8027 dot-prefixed patterns. Use @samp{--no-dotsyms} to disable this
8030 @cindex PowerPC64 register save/restore functions
8031 @kindex --save-restore-funcs
8032 @kindex --no-save-restore-funcs
8033 @item --save-restore-funcs
8034 @itemx --no-save-restore-funcs
8035 These two options control whether PowerPC64 @command{ld} automatically
8036 provides out-of-line register save and restore functions used by
8037 @samp{-Os} code. The default is to provide any such referenced
8038 function for a normal final link, and to not do so for a relocatable
8041 @cindex PowerPC64 TLS optimization
8042 @kindex --no-tls-optimize
8043 @item --no-tls-optimize
8044 PowerPC64 @command{ld} normally performs some optimization of code
8045 sequences used to access Thread-Local Storage. Use this option to
8046 disable the optimization.
8048 @cindex PowerPC64 __tls_get_addr optimization
8049 @kindex --tls-get-addr-optimize
8050 @kindex --no-tls-get-addr-optimize
8051 @kindex --tls-get-addr-regsave
8052 @kindex --no-tls-get-addr-regsave
8053 @item --tls-get-addr-optimize
8054 @itemx --no-tls-get-addr-optimize
8055 These options control how PowerPC64 @command{ld} uses a special
8056 stub to call __tls_get_addr. PowerPC64 glibc 2.22 and later support
8057 an optimization that allows the second and subsequent calls to
8058 @code{__tls_get_addr} for a given symbol to be resolved by the special
8059 stub without calling in to glibc. By default the linker enables
8060 generation of the stub when glibc advertises the availability of
8062 Using @option{--tls-get-addr-optimize} with an older glibc won't do
8063 much besides slow down your applications, but may be useful if linking
8064 an application against an older glibc with the expectation that it
8065 will normally be used on systems having a newer glibc.
8066 @option{--tls-get-addr-regsave} forces generation of a stub that saves
8067 and restores volatile registers around the call into glibc. Normally,
8068 this is done when the linker detects a call to __tls_get_addr_desc.
8069 Such calls then go via the register saving stub to __tls_get_addr_opt.
8070 @option{--no-tls-get-addr-regsave} disables generation of the
8073 @cindex PowerPC64 OPD optimization
8074 @kindex --no-opd-optimize
8075 @item --no-opd-optimize
8076 PowerPC64 @command{ld} normally removes @code{.opd} section entries
8077 corresponding to deleted link-once functions, or functions removed by
8078 the action of @samp{--gc-sections} or linker script @code{/DISCARD/}.
8079 Use this option to disable @code{.opd} optimization.
8081 @cindex PowerPC64 OPD spacing
8082 @kindex --non-overlapping-opd
8083 @item --non-overlapping-opd
8084 Some PowerPC64 compilers have an option to generate compressed
8085 @code{.opd} entries spaced 16 bytes apart, overlapping the third word,
8086 the static chain pointer (unused in C) with the first word of the next
8087 entry. This option expands such entries to the full 24 bytes.
8089 @cindex PowerPC64 TOC optimization
8090 @kindex --no-toc-optimize
8091 @item --no-toc-optimize
8092 PowerPC64 @command{ld} normally removes unused @code{.toc} section
8093 entries. Such entries are detected by examining relocations that
8094 reference the TOC in code sections. A reloc in a deleted code section
8095 marks a TOC word as unneeded, while a reloc in a kept code section
8096 marks a TOC word as needed. Since the TOC may reference itself, TOC
8097 relocs are also examined. TOC words marked as both needed and
8098 unneeded will of course be kept. TOC words without any referencing
8099 reloc are assumed to be part of a multi-word entry, and are kept or
8100 discarded as per the nearest marked preceding word. This works
8101 reliably for compiler generated code, but may be incorrect if assembly
8102 code is used to insert TOC entries. Use this option to disable the
8105 @cindex PowerPC64 inline PLT call optimization
8106 @kindex --no-inline-optimize
8107 @item --no-inline-optimize
8108 PowerPC64 @command{ld} normally replaces inline PLT call sequences
8109 marked with @code{R_PPC64_PLTSEQ}, @code{R_PPC64_PLTCALL},
8110 @code{R_PPC64_PLT16_HA} and @code{R_PPC64_PLT16_LO_DS} relocations by
8111 a number of @code{nop}s and a direct call when the function is defined
8112 locally and can't be overridden by some other definition. This option
8113 disables that optimization.
8115 @cindex PowerPC64 multi-TOC
8116 @kindex --no-multi-toc
8117 @item --no-multi-toc
8118 If given any toc option besides @code{-mcmodel=medium} or
8119 @code{-mcmodel=large}, PowerPC64 GCC generates code for a TOC model
8121 entries are accessed with a 16-bit offset from r2. This limits the
8122 total TOC size to 64K. PowerPC64 @command{ld} extends this limit by
8123 grouping code sections such that each group uses less than 64K for its
8124 TOC entries, then inserts r2 adjusting stubs between inter-group
8125 calls. @command{ld} does not split apart input sections, so cannot
8126 help if a single input file has a @code{.toc} section that exceeds
8127 64K, most likely from linking multiple files with @command{ld -r}.
8128 Use this option to turn off this feature.
8130 @cindex PowerPC64 TOC sorting
8131 @kindex --no-toc-sort
8133 By default, @command{ld} sorts TOC sections so that those whose file
8134 happens to have a section called @code{.init} or @code{.fini} are
8135 placed first, followed by TOC sections referenced by code generated
8136 with PowerPC64 gcc's @code{-mcmodel=small}, and lastly TOC sections
8137 referenced only by code generated with PowerPC64 gcc's
8138 @code{-mcmodel=medium} or @code{-mcmodel=large} options. Doing this
8139 results in better TOC grouping for multi-TOC. Use this option to turn
8142 @cindex PowerPC64 PLT stub alignment
8144 @kindex --no-plt-align
8146 @itemx --no-plt-align
8147 Use these options to control whether individual PLT call stubs are
8148 aligned to a 32-byte boundary, or to the specified power of two
8149 boundary when using @code{--plt-align=}. A negative value may be
8150 specified to pad PLT call stubs so that they do not cross the
8151 specified power of two boundary (or the minimum number of boundaries
8152 if a PLT stub is so large that it must cross a boundary). By default
8153 PLT call stubs are aligned to 32-byte boundaries.
8155 @cindex PowerPC64 PLT call stub static chain
8156 @kindex --plt-static-chain
8157 @kindex --no-plt-static-chain
8158 @item --plt-static-chain
8159 @itemx --no-plt-static-chain
8160 Use these options to control whether PLT call stubs load the static
8161 chain pointer (r11). @code{ld} defaults to not loading the static
8162 chain since there is never any need to do so on a PLT call.
8164 @cindex PowerPC64 PLT call stub thread safety
8165 @kindex --plt-thread-safe
8166 @kindex --no-plt-thread-safe
8167 @item --plt-thread-safe
8168 @itemx --no-plt-thread-safe
8169 With power7's weakly ordered memory model, it is possible when using
8170 lazy binding for ld.so to update a plt entry in one thread and have
8171 another thread see the individual plt entry words update in the wrong
8172 order, despite ld.so carefully writing in the correct order and using
8173 memory write barriers. To avoid this we need some sort of read
8174 barrier in the call stub, or use LD_BIND_NOW=1. By default, @code{ld}
8175 looks for calls to commonly used functions that create threads, and if
8176 seen, adds the necessary barriers. Use these options to change the
8179 @cindex PowerPC64 ELFv2 PLT localentry optimization
8180 @kindex --plt-localentry
8181 @kindex --no-plt-localentry
8182 @item --plt-localentry
8183 @itemx --no-localentry
8184 ELFv2 functions with localentry:0 are those with a single entry point,
8185 ie. global entry == local entry, and that have no requirement on r2
8186 (the TOC/GOT pointer) or r12, and guarantee r2 is unchanged on return.
8187 Such an external function can be called via the PLT without saving r2
8188 or restoring it on return, avoiding a common load-hit-store for small
8189 functions. The optimization is attractive, with up to 40% reduction
8190 in execution time for a small function, but can result in symbol
8191 interposition failures. Also, minor changes in a shared library,
8192 including system libraries, can cause a function that was localentry:0
8193 to become localentry:8. This will result in a dynamic loader
8194 complaint and failure to run. The option is experimental, use with
8195 care. @option{--no-plt-localentry} is the default.
8197 @cindex PowerPC64 Power10 stubs
8198 @kindex --power10-stubs
8199 @kindex --no-power10-stubs
8200 @item --power10-stubs
8201 @itemx --no-power10-stubs
8202 When PowerPC64 @command{ld} links input object files containing
8203 relocations used on power10 prefixed instructions it normally creates
8204 linkage stubs (PLT call and long branch) using power10 instructions
8205 for @code{@@notoc} PLT calls where @code{r2} is not known. The
8206 power10 notoc stubs are smaller and faster, so are preferred for
8207 power10. @option{--power10-stubs} and @option{--no-power10-stubs}
8208 allow you to override the linker's selection of stub instructions.
8209 @option{--power10-stubs=auto} allows the user to select the default
8224 @section @command{ld} and S/390 ELF Support
8226 @cindex S/390 ELF options
8230 @kindex --s390-pgste
8232 This option marks the result file with a @code{PT_S390_PGSTE}
8233 segment. The Linux kernel is supposed to allocate 4k page tables for
8234 binaries marked that way.
8248 @section @command{ld} and SPU ELF Support
8250 @cindex SPU ELF options
8256 This option marks an executable as a PIC plugin module.
8258 @cindex SPU overlays
8259 @kindex --no-overlays
8261 Normally, @command{ld} recognizes calls to functions within overlay
8262 regions, and redirects such calls to an overlay manager via a stub.
8263 @command{ld} also provides a built-in overlay manager. This option
8264 turns off all this special overlay handling.
8266 @cindex SPU overlay stub symbols
8267 @kindex --emit-stub-syms
8268 @item --emit-stub-syms
8269 This option causes @command{ld} to label overlay stubs with a local
8270 symbol that encodes the stub type and destination.
8272 @cindex SPU extra overlay stubs
8273 @kindex --extra-overlay-stubs
8274 @item --extra-overlay-stubs
8275 This option causes @command{ld} to add overlay call stubs on all
8276 function calls out of overlay regions. Normally stubs are not added
8277 on calls to non-overlay regions.
8279 @cindex SPU local store size
8280 @kindex --local-store=lo:hi
8281 @item --local-store=lo:hi
8282 @command{ld} usually checks that a final executable for SPU fits in
8283 the address range 0 to 256k. This option may be used to change the
8284 range. Disable the check entirely with @option{--local-store=0:0}.
8287 @kindex --stack-analysis
8288 @item --stack-analysis
8289 SPU local store space is limited. Over-allocation of stack space
8290 unnecessarily limits space available for code and data, while
8291 under-allocation results in runtime failures. If given this option,
8292 @command{ld} will provide an estimate of maximum stack usage.
8293 @command{ld} does this by examining symbols in code sections to
8294 determine the extents of functions, and looking at function prologues
8295 for stack adjusting instructions. A call-graph is created by looking
8296 for relocations on branch instructions. The graph is then searched
8297 for the maximum stack usage path. Note that this analysis does not
8298 find calls made via function pointers, and does not handle recursion
8299 and other cycles in the call graph. Stack usage may be
8300 under-estimated if your code makes such calls. Also, stack usage for
8301 dynamic allocation, e.g. alloca, will not be detected. If a link map
8302 is requested, detailed information about each function's stack usage
8303 and calls will be given.
8306 @kindex --emit-stack-syms
8307 @item --emit-stack-syms
8308 This option, if given along with @option{--stack-analysis} will result
8309 in @command{ld} emitting stack sizing symbols for each function.
8310 These take the form @code{__stack_<function_name>} for global
8311 functions, and @code{__stack_<number>_<function_name>} for static
8312 functions. @code{<number>} is the section id in hex. The value of
8313 such symbols is the stack requirement for the corresponding function.
8314 The symbol size will be zero, type @code{STT_NOTYPE}, binding
8315 @code{STB_LOCAL}, and section @code{SHN_ABS}.
8329 @section @command{ld}'s Support for Various TI COFF Versions
8330 @cindex TI COFF versions
8331 @kindex --format=@var{version}
8332 The @samp{--format} switch allows selection of one of the various
8333 TI COFF versions. The latest of this writing is 2; versions 0 and 1 are
8334 also supported. The TI COFF versions also vary in header byte-order
8335 format; @command{ld} will read any version or byte order, but the output
8336 header format depends on the default specified by the specific target.
8349 @section @command{ld} and WIN32 (cygwin/mingw)
8351 This section describes some of the win32 specific @command{ld} issues.
8352 See @ref{Options,,Command-line Options} for detailed description of the
8353 command-line options mentioned here.
8356 @cindex import libraries
8357 @item import libraries
8358 The standard Windows linker creates and uses so-called import
8359 libraries, which contains information for linking to dll's. They are
8360 regular static archives and are handled as any other static
8361 archive. The cygwin and mingw ports of @command{ld} have specific
8362 support for creating such libraries provided with the
8363 @samp{--out-implib} command-line option.
8365 @item exporting DLL symbols
8366 @cindex exporting DLL symbols
8367 The cygwin/mingw @command{ld} has several ways to export symbols for dll's.
8370 @item using auto-export functionality
8371 @cindex using auto-export functionality
8372 By default @command{ld} exports symbols with the auto-export functionality,
8373 which is controlled by the following command-line options:
8376 @item --export-all-symbols [This is the default]
8377 @item --exclude-symbols
8378 @item --exclude-libs
8379 @item --exclude-modules-for-implib
8380 @item --version-script
8383 When auto-export is in operation, @command{ld} will export all the non-local
8384 (global and common) symbols it finds in a DLL, with the exception of a few
8385 symbols known to belong to the system's runtime and libraries. As it will
8386 often not be desirable to export all of a DLL's symbols, which may include
8387 private functions that are not part of any public interface, the command-line
8388 options listed above may be used to filter symbols out from the list for
8389 exporting. The @samp{--output-def} option can be used in order to see the
8390 final list of exported symbols with all exclusions taken into effect.
8392 If @samp{--export-all-symbols} is not given explicitly on the
8393 command line, then the default auto-export behavior will be @emph{disabled}
8394 if either of the following are true:
8397 @item A DEF file is used.
8398 @item Any symbol in any object file was marked with the __declspec(dllexport) attribute.
8401 @item using a DEF file
8402 @cindex using a DEF file
8403 Another way of exporting symbols is using a DEF file. A DEF file is
8404 an ASCII file containing definitions of symbols which should be
8405 exported when a dll is created. Usually it is named @samp{<dll
8406 name>.def} and is added as any other object file to the linker's
8407 command line. The file's name must end in @samp{.def} or @samp{.DEF}.
8410 gcc -o <output> <objectfiles> <dll name>.def
8413 Using a DEF file turns off the normal auto-export behavior, unless the
8414 @samp{--export-all-symbols} option is also used.
8416 Here is an example of a DEF file for a shared library called @samp{xyz.dll}:
8419 LIBRARY "xyz.dll" BASE=0x20000000
8425 another_foo = abc.dll.afoo
8431 This example defines a DLL with a non-default base address and seven
8432 symbols in the export table. The third exported symbol @code{_bar} is an
8433 alias for the second. The fourth symbol, @code{another_foo} is resolved
8434 by "forwarding" to another module and treating it as an alias for
8435 @code{afoo} exported from the DLL @samp{abc.dll}. The final symbol
8436 @code{var1} is declared to be a data object. The @samp{doo} symbol in
8437 export library is an alias of @samp{foo}, which gets the string name
8438 in export table @samp{foo2}. The @samp{eoo} symbol is an data export
8439 symbol, which gets in export table the name @samp{var1}.
8441 The optional @code{LIBRARY <name>} command indicates the @emph{internal}
8442 name of the output DLL. If @samp{<name>} does not include a suffix,
8443 the default library suffix, @samp{.DLL} is appended.
8445 When the .DEF file is used to build an application, rather than a
8446 library, the @code{NAME <name>} command should be used instead of
8447 @code{LIBRARY}. If @samp{<name>} does not include a suffix, the default
8448 executable suffix, @samp{.EXE} is appended.
8450 With either @code{LIBRARY <name>} or @code{NAME <name>} the optional
8451 specification @code{BASE = <number>} may be used to specify a
8452 non-default base address for the image.
8454 If neither @code{LIBRARY <name>} nor @code{NAME <name>} is specified,
8455 or they specify an empty string, the internal name is the same as the
8456 filename specified on the command line.
8458 The complete specification of an export symbol is:
8462 ( ( ( <name1> [ = <name2> ] )
8463 | ( <name1> = <module-name> . <external-name>))
8464 [ @@ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
8467 Declares @samp{<name1>} as an exported symbol from the DLL, or declares
8468 @samp{<name1>} as an exported alias for @samp{<name2>}; or declares
8469 @samp{<name1>} as a "forward" alias for the symbol
8470 @samp{<external-name>} in the DLL @samp{<module-name>}.
8471 Optionally, the symbol may be exported by the specified ordinal
8472 @samp{<integer>} alias. The optional @samp{<name3>} is the to be used
8473 string in import/export table for the symbol.
8475 The optional keywords that follow the declaration indicate:
8477 @code{NONAME}: Do not put the symbol name in the DLL's export table. It
8478 will still be exported by its ordinal alias (either the value specified
8479 by the .def specification or, otherwise, the value assigned by the
8480 linker). The symbol name, however, does remain visible in the import
8481 library (if any), unless @code{PRIVATE} is also specified.
8483 @code{DATA}: The symbol is a variable or object, rather than a function.
8484 The import lib will export only an indirect reference to @code{foo} as
8485 the symbol @code{_imp__foo} (ie, @code{foo} must be resolved as
8488 @code{CONSTANT}: Like @code{DATA}, but put the undecorated @code{foo} as
8489 well as @code{_imp__foo} into the import library. Both refer to the
8490 read-only import address table's pointer to the variable, not to the
8491 variable itself. This can be dangerous. If the user code fails to add
8492 the @code{dllimport} attribute and also fails to explicitly add the
8493 extra indirection that the use of the attribute enforces, the
8494 application will behave unexpectedly.
8496 @code{PRIVATE}: Put the symbol in the DLL's export table, but do not put
8497 it into the static import library used to resolve imports at link time. The
8498 symbol can still be imported using the @code{LoadLibrary/GetProcAddress}
8499 API at runtime or by using the GNU ld extension of linking directly to
8500 the DLL without an import library.
8502 See ld/deffilep.y in the binutils sources for the full specification of
8503 other DEF file statements
8505 @cindex creating a DEF file
8506 While linking a shared dll, @command{ld} is able to create a DEF file
8507 with the @samp{--output-def <file>} command-line option.
8509 @item Using decorations
8510 @cindex Using decorations
8511 Another way of marking symbols for export is to modify the source code
8512 itself, so that when building the DLL each symbol to be exported is
8516 __declspec(dllexport) int a_variable
8517 __declspec(dllexport) void a_function(int with_args)
8520 All such symbols will be exported from the DLL. If, however,
8521 any of the object files in the DLL contain symbols decorated in
8522 this way, then the normal auto-export behavior is disabled, unless
8523 the @samp{--export-all-symbols} option is also used.
8525 Note that object files that wish to access these symbols must @emph{not}
8526 decorate them with dllexport. Instead, they should use dllimport,
8530 __declspec(dllimport) int a_variable
8531 __declspec(dllimport) void a_function(int with_args)
8534 This complicates the structure of library header files, because
8535 when included by the library itself the header must declare the
8536 variables and functions as dllexport, but when included by client
8537 code the header must declare them as dllimport. There are a number
8538 of idioms that are typically used to do this; often client code can
8539 omit the __declspec() declaration completely. See
8540 @samp{--enable-auto-import} and @samp{automatic data imports} for more
8544 @cindex automatic data imports
8545 @item automatic data imports
8546 The standard Windows dll format supports data imports from dlls only
8547 by adding special decorations (dllimport/dllexport), which let the
8548 compiler produce specific assembler instructions to deal with this
8549 issue. This increases the effort necessary to port existing Un*x
8550 code to these platforms, especially for large
8551 c++ libraries and applications. The auto-import feature, which was
8552 initially provided by Paul Sokolovsky, allows one to omit the
8553 decorations to achieve a behavior that conforms to that on POSIX/Un*x
8554 platforms. This feature is enabled with the @samp{--enable-auto-import}
8555 command-line option, although it is enabled by default on cygwin/mingw.
8556 The @samp{--enable-auto-import} option itself now serves mainly to
8557 suppress any warnings that are ordinarily emitted when linked objects
8558 trigger the feature's use.
8560 auto-import of variables does not always work flawlessly without
8561 additional assistance. Sometimes, you will see this message
8563 "variable '<var>' can't be auto-imported. Please read the
8564 documentation for ld's @code{--enable-auto-import} for details."
8566 The @samp{--enable-auto-import} documentation explains why this error
8567 occurs, and several methods that can be used to overcome this difficulty.
8568 One of these methods is the @emph{runtime pseudo-relocs} feature, described
8571 @cindex runtime pseudo-relocation
8572 For complex variables imported from DLLs (such as structs or classes),
8573 object files typically contain a base address for the variable and an
8574 offset (@emph{addend}) within the variable--to specify a particular
8575 field or public member, for instance. Unfortunately, the runtime loader used
8576 in win32 environments is incapable of fixing these references at runtime
8577 without the additional information supplied by dllimport/dllexport decorations.
8578 The standard auto-import feature described above is unable to resolve these
8581 The @samp{--enable-runtime-pseudo-relocs} switch allows these references to
8582 be resolved without error, while leaving the task of adjusting the references
8583 themselves (with their non-zero addends) to specialized code provided by the
8584 runtime environment. Recent versions of the cygwin and mingw environments and
8585 compilers provide this runtime support; older versions do not. However, the
8586 support is only necessary on the developer's platform; the compiled result will
8587 run without error on an older system.
8589 @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly
8592 @cindex direct linking to a dll
8593 @item direct linking to a dll
8594 The cygwin/mingw ports of @command{ld} support the direct linking,
8595 including data symbols, to a dll without the usage of any import
8596 libraries. This is much faster and uses much less memory than does the
8597 traditional import library method, especially when linking large
8598 libraries or applications. When @command{ld} creates an import lib, each
8599 function or variable exported from the dll is stored in its own bfd, even
8600 though a single bfd could contain many exports. The overhead involved in
8601 storing, loading, and processing so many bfd's is quite large, and explains the
8602 tremendous time, memory, and storage needed to link against particularly
8603 large or complex libraries when using import libs.
8605 Linking directly to a dll uses no extra command-line switches other than
8606 @samp{-L} and @samp{-l}, because @command{ld} already searches for a number
8607 of names to match each library. All that is needed from the developer's
8608 perspective is an understanding of this search, in order to force ld to
8609 select the dll instead of an import library.
8612 For instance, when ld is called with the argument @samp{-lxxx} it will attempt
8613 to find, in the first directory of its search path,
8626 before moving on to the next directory in the search path.
8628 (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll},
8629 where @samp{<prefix>} is set by the @command{ld} option
8630 @samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec
8631 file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for
8634 Other win32-based unix environments, such as mingw or pw32, may use other
8635 @samp{<prefix>}es, although at present only cygwin makes use of this feature. It
8636 was originally intended to help avoid name conflicts among dll's built for the
8637 various win32/un*x environments, so that (for example) two versions of a zlib dll
8638 could coexist on the same machine.
8640 The generic cygwin/mingw path layout uses a @samp{bin} directory for
8641 applications and dll's and a @samp{lib} directory for the import
8642 libraries (using cygwin nomenclature):
8648 libxxx.dll.a (in case of dll's)
8649 libxxx.a (in case of static archive)
8652 Linking directly to a dll without using the import library can be
8655 1. Use the dll directly by adding the @samp{bin} path to the link line
8657 gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
8660 However, as the dll's often have version numbers appended to their names
8661 (@samp{cygncurses-5.dll}) this will often fail, unless one specifies
8662 @samp{-L../bin -lncurses-5} to include the version. Import libs are generally
8663 not versioned, and do not have this difficulty.
8665 2. Create a symbolic link from the dll to a file in the @samp{lib}
8666 directory according to the above mentioned search pattern. This
8667 should be used to avoid unwanted changes in the tools needed for
8671 ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
8674 Then you can link without any make environment changes.
8677 gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
8680 This technique also avoids the version number problems, because the following is
8687 libxxx.dll.a -> ../bin/cygxxx-5.dll
8690 Linking directly to a dll without using an import lib will work
8691 even when auto-import features are exercised, and even when
8692 @samp{--enable-runtime-pseudo-relocs} is used.
8694 Given the improvements in speed and memory usage, one might justifiably
8695 wonder why import libraries are used at all. There are three reasons:
8697 1. Until recently, the link-directly-to-dll functionality did @emph{not}
8698 work with auto-imported data.
8700 2. Sometimes it is necessary to include pure static objects within the
8701 import library (which otherwise contains only bfd's for indirection
8702 symbols that point to the exports of a dll). Again, the import lib
8703 for the cygwin kernel makes use of this ability, and it is not
8704 possible to do this without an import lib.
8706 3. Symbol aliases can only be resolved using an import lib. This is
8707 critical when linking against OS-supplied dll's (eg, the win32 API)
8708 in which symbols are usually exported as undecorated aliases of their
8709 stdcall-decorated assembly names.
8711 So, import libs are not going away. But the ability to replace
8712 true import libs with a simple symbolic link to (or a copy of)
8713 a dll, in many cases, is a useful addition to the suite of tools
8714 binutils makes available to the win32 developer. Given the
8715 massive improvements in memory requirements during linking, storage
8716 requirements, and linking speed, we expect that many developers
8717 will soon begin to use this feature whenever possible.
8719 @item symbol aliasing
8721 @item adding additional names
8722 Sometimes, it is useful to export symbols with additional names.
8723 A symbol @samp{foo} will be exported as @samp{foo}, but it can also be
8724 exported as @samp{_foo} by using special directives in the DEF file
8725 when creating the dll. This will affect also the optional created
8726 import library. Consider the following DEF file:
8729 LIBRARY "xyz.dll" BASE=0x61000000
8736 The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}.
8738 Another method for creating a symbol alias is to create it in the
8739 source code using the "weak" attribute:
8742 void foo () @{ /* Do something. */; @}
8743 void _foo () __attribute__ ((weak, alias ("foo")));
8746 See the gcc manual for more information about attributes and weak
8749 @item renaming symbols
8750 Sometimes it is useful to rename exports. For instance, the cygwin
8751 kernel does this regularly. A symbol @samp{_foo} can be exported as
8752 @samp{foo} but not as @samp{_foo} by using special directives in the
8753 DEF file. (This will also affect the import library, if it is
8754 created). In the following example:
8757 LIBRARY "xyz.dll" BASE=0x61000000
8763 The line @samp{_foo = foo} maps the exported symbol @samp{foo} to
8767 Note: using a DEF file disables the default auto-export behavior,
8768 unless the @samp{--export-all-symbols} command-line option is used.
8769 If, however, you are trying to rename symbols, then you should list
8770 @emph{all} desired exports in the DEF file, including the symbols
8771 that are not being renamed, and do @emph{not} use the
8772 @samp{--export-all-symbols} option. If you list only the
8773 renamed symbols in the DEF file, and use @samp{--export-all-symbols}
8774 to handle the other symbols, then the both the new names @emph{and}
8775 the original names for the renamed symbols will be exported.
8776 In effect, you'd be aliasing those symbols, not renaming them,
8777 which is probably not what you wanted.
8779 @cindex weak externals
8780 @item weak externals
8781 The Windows object format, PE, specifies a form of weak symbols called
8782 weak externals. When a weak symbol is linked and the symbol is not
8783 defined, the weak symbol becomes an alias for some other symbol. There
8784 are three variants of weak externals:
8786 @item Definition is searched for in objects and libraries, historically
8787 called lazy externals.
8788 @item Definition is searched for only in other objects, not in libraries.
8789 This form is not presently implemented.
8790 @item No search; the symbol is an alias. This form is not presently
8793 As a GNU extension, weak symbols that do not specify an alternate symbol
8794 are supported. If the symbol is undefined when linking, the symbol
8795 uses a default value.
8797 @cindex aligned common symbols
8798 @item aligned common symbols
8799 As a GNU extension to the PE file format, it is possible to specify the
8800 desired alignment for a common symbol. This information is conveyed from
8801 the assembler or compiler to the linker by means of GNU-specific commands
8802 carried in the object file's @samp{.drectve} section, which are recognized
8803 by @command{ld} and respected when laying out the common symbols. Native
8804 tools will be able to process object files employing this GNU extension,
8805 but will fail to respect the alignment instructions, and may issue noisy
8806 warnings about unknown linker directives.
8821 @section @code{ld} and Xtensa Processors
8823 @cindex Xtensa processors
8824 The default @command{ld} behavior for Xtensa processors is to interpret
8825 @code{SECTIONS} commands so that lists of explicitly named sections in a
8826 specification with a wildcard file will be interleaved when necessary to
8827 keep literal pools within the range of PC-relative load offsets. For
8828 example, with the command:
8840 @command{ld} may interleave some of the @code{.literal}
8841 and @code{.text} sections from different object files to ensure that the
8842 literal pools are within the range of PC-relative load offsets. A valid
8843 interleaving might place the @code{.literal} sections from an initial
8844 group of files followed by the @code{.text} sections of that group of
8845 files. Then, the @code{.literal} sections from the rest of the files
8846 and the @code{.text} sections from the rest of the files would follow.
8848 @cindex @option{--relax} on Xtensa
8849 @cindex relaxing on Xtensa
8850 Relaxation is enabled by default for the Xtensa version of @command{ld} and
8851 provides two important link-time optimizations. The first optimization
8852 is to combine identical literal values to reduce code size. A redundant
8853 literal will be removed and all the @code{L32R} instructions that use it
8854 will be changed to reference an identical literal, as long as the
8855 location of the replacement literal is within the offset range of all
8856 the @code{L32R} instructions. The second optimization is to remove
8857 unnecessary overhead from assembler-generated ``longcall'' sequences of
8858 @code{L32R}/@code{CALLX@var{n}} when the target functions are within
8859 range of direct @code{CALL@var{n}} instructions.
8861 For each of these cases where an indirect call sequence can be optimized
8862 to a direct call, the linker will change the @code{CALLX@var{n}}
8863 instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R}
8864 instruction, and remove the literal referenced by the @code{L32R}
8865 instruction if it is not used for anything else. Removing the
8866 @code{L32R} instruction always reduces code size but can potentially
8867 hurt performance by changing the alignment of subsequent branch targets.
8868 By default, the linker will always preserve alignments, either by
8869 switching some instructions between 24-bit encodings and the equivalent
8870 density instructions or by inserting a no-op in place of the @code{L32R}
8871 instruction that was removed. If code size is more important than
8872 performance, the @option{--size-opt} option can be used to prevent the
8873 linker from widening density instructions or inserting no-ops, except in
8874 a few cases where no-ops are required for correctness.
8876 The following Xtensa-specific command-line options can be used to
8879 @cindex Xtensa options
8882 When optimizing indirect calls to direct calls, optimize for code size
8883 more than performance. With this option, the linker will not insert
8884 no-ops or widen density instructions to preserve branch target
8885 alignment. There may still be some cases where no-ops are required to
8886 preserve the correctness of the code.
8888 @item --abi-windowed
8890 Choose ABI for the output object and for the generated PLT code.
8891 PLT code inserted by the linker must match ABI of the output object
8892 because windowed and call0 ABI use incompatible function call
8894 Default ABI is chosen by the ABI tag in the @code{.xtensa.info} section
8895 of the first input object.
8896 A warning is issued if ABI tags of input objects do not match each other
8897 or the chosen output object ABI.
8905 @ifclear SingleFormat
8910 @cindex object file management
8911 @cindex object formats available
8913 The linker accesses object and archive files using the BFD libraries.
8914 These libraries allow the linker to use the same routines to operate on
8915 object files whatever the object file format. A different object file
8916 format can be supported simply by creating a new BFD back end and adding
8917 it to the library. To conserve runtime memory, however, the linker and
8918 associated tools are usually configured to support only a subset of the
8919 object file formats available. You can use @code{objdump -i}
8920 (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to
8921 list all the formats available for your configuration.
8923 @cindex BFD requirements
8924 @cindex requirements for BFD
8925 As with most implementations, BFD is a compromise between
8926 several conflicting requirements. The major factor influencing
8927 BFD design was efficiency: any time used converting between
8928 formats is time which would not have been spent had BFD not
8929 been involved. This is partly offset by abstraction payback; since
8930 BFD simplifies applications and back ends, more time and care
8931 may be spent optimizing algorithms for a greater speed.
8933 One minor artifact of the BFD solution which you should bear in
8934 mind is the potential for information loss. There are two places where
8935 useful information can be lost using the BFD mechanism: during
8936 conversion and during output. @xref{BFD information loss}.
8939 * BFD outline:: How it works: an outline of BFD
8943 @section How It Works: An Outline of BFD
8944 @cindex opening object files
8945 @include bfdsumm.texi
8948 @node Reporting Bugs
8949 @chapter Reporting Bugs
8950 @cindex bugs in @command{ld}
8951 @cindex reporting bugs in @command{ld}
8953 Your bug reports play an essential role in making @command{ld} reliable.
8955 Reporting a bug may help you by bringing a solution to your problem, or
8956 it may not. But in any case the principal function of a bug report is
8957 to help the entire community by making the next version of @command{ld}
8958 work better. Bug reports are your contribution to the maintenance of
8961 In order for a bug report to serve its purpose, you must include the
8962 information that enables us to fix the bug.
8965 * Bug Criteria:: Have you found a bug?
8966 * Bug Reporting:: How to report bugs
8970 @section Have You Found a Bug?
8971 @cindex bug criteria
8973 If you are not sure whether you have found a bug, here are some guidelines:
8976 @cindex fatal signal
8977 @cindex linker crash
8978 @cindex crash of linker
8980 If the linker gets a fatal signal, for any input whatever, that is a
8981 @command{ld} bug. Reliable linkers never crash.
8983 @cindex error on valid input
8985 If @command{ld} produces an error message for valid input, that is a bug.
8987 @cindex invalid input
8989 If @command{ld} does not produce an error message for invalid input, that
8990 may be a bug. In the general case, the linker can not verify that
8991 object files are correct.
8994 If you are an experienced user of linkers, your suggestions for
8995 improvement of @command{ld} are welcome in any case.
8999 @section How to Report Bugs
9001 @cindex @command{ld} bugs, reporting
9003 A number of companies and individuals offer support for @sc{gnu}
9004 products. If you obtained @command{ld} from a support organization, we
9005 recommend you contact that organization first.
9007 You can find contact information for many support companies and
9008 individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs
9012 Otherwise, send bug reports for @command{ld} to
9016 The fundamental principle of reporting bugs usefully is this:
9017 @strong{report all the facts}. If you are not sure whether to state a
9018 fact or leave it out, state it!
9020 Often people omit facts because they think they know what causes the
9021 problem and assume that some details do not matter. Thus, you might
9022 assume that the name of a symbol you use in an example does not
9023 matter. Well, probably it does not, but one cannot be sure. Perhaps
9024 the bug is a stray memory reference which happens to fetch from the
9025 location where that name is stored in memory; perhaps, if the name
9026 were different, the contents of that location would fool the linker
9027 into doing the right thing despite the bug. Play it safe and give a
9028 specific, complete example. That is the easiest thing for you to do,
9029 and the most helpful.
9031 Keep in mind that the purpose of a bug report is to enable us to fix
9032 the bug if it is new to us. Therefore, always write your bug reports
9033 on the assumption that the bug has not been reported previously.
9035 Sometimes people give a few sketchy facts and ask, ``Does this ring a
9036 bell?'' This cannot help us fix a bug, so it is basically useless. We
9037 respond by asking for enough details to enable us to investigate.
9038 You might as well expedite matters by sending them to begin with.
9040 To enable us to fix the bug, you should include all these things:
9044 The version of @command{ld}. @command{ld} announces it if you start it with
9045 the @samp{--version} argument.
9047 Without this, we will not know whether there is any point in looking for
9048 the bug in the current version of @command{ld}.
9051 Any patches you may have applied to the @command{ld} source, including any
9052 patches made to the @code{BFD} library.
9055 The type of machine you are using, and the operating system name and
9059 What compiler (and its version) was used to compile @command{ld}---e.g.
9063 The command arguments you gave the linker to link your example and
9064 observe the bug. To guarantee you will not omit something important,
9065 list them all. A copy of the Makefile (or the output from make) is
9068 If we were to try to guess the arguments, we would probably guess wrong
9069 and then we might not encounter the bug.
9072 A complete input file, or set of input files, that will reproduce the
9073 bug. It is generally most helpful to send the actual object files
9074 provided that they are reasonably small. Say no more than 10K. For
9075 bigger files you can either make them available by FTP or HTTP or else
9076 state that you are willing to send the object file(s) to whomever
9077 requests them. (Note - your email will be going to a mailing list, so
9078 we do not want to clog it up with large attachments). But small
9079 attachments are best.
9081 If the source files were assembled using @code{gas} or compiled using
9082 @code{gcc}, then it may be OK to send the source files rather than the
9083 object files. In this case, be sure to say exactly what version of
9084 @code{gas} or @code{gcc} was used to produce the object files. Also say
9085 how @code{gas} or @code{gcc} were configured.
9088 A description of what behavior you observe that you believe is
9089 incorrect. For example, ``It gets a fatal signal.''
9091 Of course, if the bug is that @command{ld} gets a fatal signal, then we
9092 will certainly notice it. But if the bug is incorrect output, we might
9093 not notice unless it is glaringly wrong. You might as well not give us
9094 a chance to make a mistake.
9096 Even if the problem you experience is a fatal signal, you should still
9097 say so explicitly. Suppose something strange is going on, such as, your
9098 copy of @command{ld} is out of sync, or you have encountered a bug in the
9099 C library on your system. (This has happened!) Your copy might crash
9100 and ours would not. If you told us to expect a crash, then when ours
9101 fails to crash, we would know that the bug was not happening for us. If
9102 you had not told us to expect a crash, then we would not be able to draw
9103 any conclusion from our observations.
9106 If you wish to suggest changes to the @command{ld} source, send us context
9107 diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or
9108 @samp{-p} option. Always send diffs from the old file to the new file.
9109 If you even discuss something in the @command{ld} source, refer to it by
9110 context, not by line number.
9112 The line numbers in our development sources will not match those in your
9113 sources. Your line numbers would convey no useful information to us.
9116 Here are some things that are not necessary:
9120 A description of the envelope of the bug.
9122 Often people who encounter a bug spend a lot of time investigating
9123 which changes to the input file will make the bug go away and which
9124 changes will not affect it.
9126 This is often time consuming and not very useful, because the way we
9127 will find the bug is by running a single example under the debugger
9128 with breakpoints, not by pure deduction from a series of examples.
9129 We recommend that you save your time for something else.
9131 Of course, if you can find a simpler example to report @emph{instead}
9132 of the original one, that is a convenience for us. Errors in the
9133 output will be easier to spot, running under the debugger will take
9134 less time, and so on.
9136 However, simplification is not vital; if you do not want to do this,
9137 report the bug anyway and send us the entire test case you used.
9140 A patch for the bug.
9142 A patch for the bug does help us if it is a good one. But do not omit
9143 the necessary information, such as the test case, on the assumption that
9144 a patch is all we need. We might see problems with your patch and decide
9145 to fix the problem another way, or we might not understand it at all.
9147 Sometimes with a program as complicated as @command{ld} it is very hard to
9148 construct an example that will make the program follow a certain path
9149 through the code. If you do not send us the example, we will not be
9150 able to construct one, so we will not be able to verify that the bug is
9153 And if we cannot understand what bug you are trying to fix, or why your
9154 patch should be an improvement, we will not install it. A test case will
9155 help us to understand.
9158 A guess about what the bug is or what it depends on.
9160 Such guesses are usually wrong. Even we cannot guess right about such
9161 things without first using the debugger to find the facts.
9165 @appendix MRI Compatible Script Files
9166 @cindex MRI compatibility
9167 To aid users making the transition to @sc{gnu} @command{ld} from the MRI
9168 linker, @command{ld} can use MRI compatible linker scripts as an
9169 alternative to the more general-purpose linker scripting language
9170 described in @ref{Scripts}. MRI compatible linker scripts have a much
9171 simpler command set than the scripting language otherwise used with
9172 @command{ld}. @sc{gnu} @command{ld} supports the most commonly used MRI
9173 linker commands; these commands are described here.
9175 In general, MRI scripts aren't of much use with the @code{a.out} object
9176 file format, since it only has three sections and MRI scripts lack some
9177 features to make use of them.
9179 You can specify a file containing an MRI-compatible script using the
9180 @samp{-c} command-line option.
9182 Each command in an MRI-compatible script occupies its own line; each
9183 command line starts with the keyword that identifies the command (though
9184 blank lines are also allowed for punctuation). If a line of an
9185 MRI-compatible script begins with an unrecognized keyword, @command{ld}
9186 issues a warning message, but continues processing the script.
9188 Lines beginning with @samp{*} are comments.
9190 You can write these commands using all upper-case letters, or all
9191 lower case; for example, @samp{chip} is the same as @samp{CHIP}.
9192 The following list shows only the upper-case form of each command.
9195 @cindex @code{ABSOLUTE} (MRI)
9196 @item ABSOLUTE @var{secname}
9197 @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname}
9198 Normally, @command{ld} includes in the output file all sections from all
9199 the input files. However, in an MRI-compatible script, you can use the
9200 @code{ABSOLUTE} command to restrict the sections that will be present in
9201 your output program. If the @code{ABSOLUTE} command is used at all in a
9202 script, then only the sections named explicitly in @code{ABSOLUTE}
9203 commands will appear in the linker output. You can still use other
9204 input sections (whatever you select on the command line, or using
9205 @code{LOAD}) to resolve addresses in the output file.
9207 @cindex @code{ALIAS} (MRI)
9208 @item ALIAS @var{out-secname}, @var{in-secname}
9209 Use this command to place the data from input section @var{in-secname}
9210 in a section called @var{out-secname} in the linker output file.
9212 @var{in-secname} may be an integer.
9214 @cindex @code{ALIGN} (MRI)
9215 @item ALIGN @var{secname} = @var{expression}
9216 Align the section called @var{secname} to @var{expression}. The
9217 @var{expression} should be a power of two.
9219 @cindex @code{BASE} (MRI)
9220 @item BASE @var{expression}
9221 Use the value of @var{expression} as the lowest address (other than
9222 absolute addresses) in the output file.
9224 @cindex @code{CHIP} (MRI)
9225 @item CHIP @var{expression}
9226 @itemx CHIP @var{expression}, @var{expression}
9227 This command does nothing; it is accepted only for compatibility.
9229 @cindex @code{END} (MRI)
9231 This command does nothing whatever; it's only accepted for compatibility.
9233 @cindex @code{FORMAT} (MRI)
9234 @item FORMAT @var{output-format}
9235 Similar to the @code{OUTPUT_FORMAT} command in the more general linker
9236 language, but restricted to S-records, if @var{output-format} is @samp{S}
9238 @cindex @code{LIST} (MRI)
9239 @item LIST @var{anything}@dots{}
9240 Print (to the standard output file) a link map, as produced by the
9241 @command{ld} command-line option @samp{-M}.
9243 The keyword @code{LIST} may be followed by anything on the
9244 same line, with no change in its effect.
9246 @cindex @code{LOAD} (MRI)
9247 @item LOAD @var{filename}
9248 @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename}
9249 Include one or more object file @var{filename} in the link; this has the
9250 same effect as specifying @var{filename} directly on the @command{ld}
9253 @cindex @code{NAME} (MRI)
9254 @item NAME @var{output-name}
9255 @var{output-name} is the name for the program produced by @command{ld}; the
9256 MRI-compatible command @code{NAME} is equivalent to the command-line
9257 option @samp{-o} or the general script language command @code{OUTPUT}.
9259 @cindex @code{ORDER} (MRI)
9260 @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname}
9261 @itemx ORDER @var{secname} @var{secname} @var{secname}
9262 Normally, @command{ld} orders the sections in its output file in the
9263 order in which they first appear in the input files. In an MRI-compatible
9264 script, you can override this ordering with the @code{ORDER} command. The
9265 sections you list with @code{ORDER} will appear first in your output
9266 file, in the order specified.
9268 @cindex @code{PUBLIC} (MRI)
9269 @item PUBLIC @var{name}=@var{expression}
9270 @itemx PUBLIC @var{name},@var{expression}
9271 @itemx PUBLIC @var{name} @var{expression}
9272 Supply a value (@var{expression}) for external symbol
9273 @var{name} used in the linker input files.
9275 @cindex @code{SECT} (MRI)
9276 @item SECT @var{secname}, @var{expression}
9277 @itemx SECT @var{secname}=@var{expression}
9278 @itemx SECT @var{secname} @var{expression}
9279 You can use any of these three forms of the @code{SECT} command to
9280 specify the start address (@var{expression}) for section @var{secname}.
9281 If you have more than one @code{SECT} statement for the same
9282 @var{secname}, only the @emph{first} sets the start address.
9285 @node GNU Free Documentation License
9286 @appendix GNU Free Documentation License
9290 @unnumbered LD Index
9295 % I think something like @@colophon should be in texinfo. In the
9297 \long\def\colophon{\hbox to0pt{}\vfill
9298 \centerline{The body of this manual is set in}
9299 \centerline{\fontname\tenrm,}
9300 \centerline{with headings in {\bf\fontname\tenbf}}
9301 \centerline{and examples in {\tt\fontname\tentt}.}
9302 \centerline{{\it\fontname\tenit\/} and}
9303 \centerline{{\sl\fontname\tensl\/}}
9304 \centerline{are used for emphasis.}\vfill}
9306 % Blame: doc@@cygnus.com, 28mar91.