3 @c Copyright (C) 1991-2020 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{} 1991-2020 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{} 1991-2020 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
130 * Machine Dependent:: Machine Dependent Features
134 * H8/300:: ld and the H8/300
137 * Renesas:: ld and other Renesas micros
140 * ARM:: ld and the ARM family
143 * M68HC11/68HC12:: ld and the Motorola 68HC11 and 68HC12 families
146 * HPPA ELF32:: ld and HPPA 32-bit ELF
149 * M68K:: ld and Motorola 68K family
152 * MIPS:: ld and MIPS family
155 * PowerPC ELF32:: ld and PowerPC 32-bit ELF Support
158 * PowerPC64 ELF64:: ld and PowerPC64 64-bit ELF Support
161 * S/390 ELF:: ld and S/390 ELF Support
164 * SPU ELF:: ld and SPU ELF Support
167 * TI COFF:: ld and the TI COFF
170 * Win32:: ld and WIN32 (cygwin/mingw)
173 * Xtensa:: ld and Xtensa Processors
176 @ifclear SingleFormat
179 @c Following blank line required for remaining bug in makeinfo conds/menus
181 * Reporting Bugs:: Reporting Bugs
182 * MRI:: MRI Compatible Script Files
183 * GNU Free Documentation License:: GNU Free Documentation License
184 * LD Index:: LD Index
191 @cindex @sc{gnu} linker
192 @cindex what is this?
195 @c man begin SYNOPSIS
196 ld [@b{options}] @var{objfile} @dots{}
200 ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and
201 the Info entries for @file{binutils} and
206 @c man begin DESCRIPTION
208 @command{ld} combines a number of object and archive files, relocates
209 their data and ties up symbol references. Usually the last step in
210 compiling a program is to run @command{ld}.
212 @command{ld} accepts Linker Command Language files written in
213 a superset of AT&T's Link Editor Command Language syntax,
214 to provide explicit and total control over the linking process.
218 This man page does not describe the command language; see the
219 @command{ld} entry in @code{info} for full details on the command
220 language and on other aspects of the GNU linker.
223 @ifclear SingleFormat
224 This version of @command{ld} uses the general purpose BFD libraries
225 to operate on object files. This allows @command{ld} to read, combine, and
226 write object files in many different formats---for example, COFF or
227 @code{a.out}. Different formats may be linked together to produce any
228 available kind of object file. @xref{BFD}, for more information.
231 Aside from its flexibility, the @sc{gnu} linker is more helpful than other
232 linkers in providing diagnostic information. Many linkers abandon
233 execution immediately upon encountering an error; whenever possible,
234 @command{ld} continues executing, allowing you to identify other errors
235 (or, in some cases, to get an output file in spite of the error).
242 @c man begin DESCRIPTION
244 The @sc{gnu} linker @command{ld} is meant to cover a broad range of situations,
245 and to be as compatible as possible with other linkers. As a result,
246 you have many choices to control its behavior.
252 * Options:: Command-line Options
253 * Environment:: Environment Variables
257 @section Command-line Options
265 The linker supports a plethora of command-line options, but in actual
266 practice few of them are used in any particular context.
267 @cindex standard Unix system
268 For instance, a frequent use of @command{ld} is to link standard Unix
269 object files on a standard, supported Unix system. On such a system, to
270 link a file @code{hello.o}:
273 ld -o @var{output} /lib/crt0.o hello.o -lc
276 This tells @command{ld} to produce a file called @var{output} as the
277 result of linking the file @code{/lib/crt0.o} with @code{hello.o} and
278 the library @code{libc.a}, which will come from the standard search
279 directories. (See the discussion of the @samp{-l} option below.)
281 Some of the command-line options to @command{ld} may be specified at any
282 point in the command line. However, options which refer to files, such
283 as @samp{-l} or @samp{-T}, cause the file to be read at the point at
284 which the option appears in the command line, relative to the object
285 files and other file options. Repeating non-file options with a
286 different argument will either have no further effect, or override prior
287 occurrences (those further to the left on the command line) of that
288 option. Options which may be meaningfully specified more than once are
289 noted in the descriptions below.
292 Non-option arguments are object files or archives which are to be linked
293 together. They may follow, precede, or be mixed in with command-line
294 options, except that an object file argument may not be placed between
295 an option and its argument.
297 Usually the linker is invoked with at least one object file, but you can
298 specify other forms of binary input files using @samp{-l}, @samp{-R},
299 and the script command language. If @emph{no} binary input files at all
300 are specified, the linker does not produce any output, and issues the
301 message @samp{No input files}.
303 If the linker cannot recognize the format of an object file, it will
304 assume that it is a linker script. A script specified in this way
305 augments the main linker script used for the link (either the default
306 linker script or the one specified by using @samp{-T}). This feature
307 permits the linker to link against a file which appears to be an object
308 or an archive, but actually merely defines some symbol values, or uses
309 @code{INPUT} or @code{GROUP} to load other objects. Specifying a
310 script in this way merely augments the main linker script, with the
311 extra commands placed after the main script; use the @samp{-T} option
312 to replace the default linker script entirely, but note the effect of
313 the @code{INSERT} command. @xref{Scripts}.
315 For options whose names are a single letter,
316 option arguments must either follow the option letter without intervening
317 whitespace, or be given as separate arguments immediately following the
318 option that requires them.
320 For options whose names are multiple letters, either one dash or two can
321 precede the option name; for example, @samp{-trace-symbol} and
322 @samp{--trace-symbol} are equivalent. Note---there is one exception to
323 this rule. Multiple letter options that start with a lower case 'o' can
324 only be preceded by two dashes. This is to reduce confusion with the
325 @samp{-o} option. So for example @samp{-omagic} sets the output file
326 name to @samp{magic} whereas @samp{--omagic} sets the NMAGIC flag on the
329 Arguments to multiple-letter options must either be separated from the
330 option name by an equals sign, or be given as separate arguments
331 immediately following the option that requires them. For example,
332 @samp{--trace-symbol foo} and @samp{--trace-symbol=foo} are equivalent.
333 Unique abbreviations of the names of multiple-letter options are
336 Note---if the linker is being invoked indirectly, via a compiler driver
337 (e.g. @samp{gcc}) then all the linker command-line options should be
338 prefixed by @samp{-Wl,} (or whatever is appropriate for the particular
339 compiler driver) like this:
342 gcc -Wl,--start-group foo.o bar.o -Wl,--end-group
345 This is important, because otherwise the compiler driver program may
346 silently drop the linker options, resulting in a bad link. Confusion
347 may also arise when passing options that require values through a
348 driver, as the use of a space between option and argument acts as
349 a separator, and causes the driver to pass only the option to the linker
350 and the argument to the compiler. In this case, it is simplest to use
351 the joined forms of both single- and multiple-letter options, such as:
354 gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map
357 Here is a table of the generic command-line switches accepted by the GNU
361 @include at-file.texi
363 @kindex -a @var{keyword}
364 @item -a @var{keyword}
365 This option is supported for HP/UX compatibility. The @var{keyword}
366 argument must be one of the strings @samp{archive}, @samp{shared}, or
367 @samp{default}. @samp{-aarchive} is functionally equivalent to
368 @samp{-Bstatic}, and the other two keywords are functionally equivalent
369 to @samp{-Bdynamic}. This option may be used any number of times.
371 @kindex --audit @var{AUDITLIB}
372 @item --audit @var{AUDITLIB}
373 Adds @var{AUDITLIB} to the @code{DT_AUDIT} entry of the dynamic section.
374 @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME
375 specified in the library. If specified multiple times @code{DT_AUDIT}
376 will contain a colon separated list of audit interfaces to use. If the linker
377 finds an object with an audit entry while searching for shared libraries,
378 it will add a corresponding @code{DT_DEPAUDIT} entry in the output file.
379 This option is only meaningful on ELF platforms supporting the rtld-audit
382 @ifclear SingleFormat
383 @cindex binary input format
384 @kindex -b @var{format}
385 @kindex --format=@var{format}
388 @item -b @var{input-format}
389 @itemx --format=@var{input-format}
390 @command{ld} may be configured to support more than one kind of object
391 file. If your @command{ld} is configured this way, you can use the
392 @samp{-b} option to specify the binary format for input object files
393 that follow this option on the command line. Even when @command{ld} is
394 configured to support alternative object formats, you don't usually need
395 to specify this, as @command{ld} should be configured to expect as a
396 default input format the most usual format on each machine.
397 @var{input-format} is a text string, the name of a particular format
398 supported by the BFD libraries. (You can list the available binary
399 formats with @samp{objdump -i}.)
402 You may want to use this option if you are linking files with an unusual
403 binary format. You can also use @samp{-b} to switch formats explicitly (when
404 linking object files of different formats), by including
405 @samp{-b @var{input-format}} before each group of object files in a
408 The default format is taken from the environment variable
413 You can also define the input format from a script, using the command
416 see @ref{Format Commands}.
420 @kindex -c @var{MRI-cmdfile}
421 @kindex --mri-script=@var{MRI-cmdfile}
422 @cindex compatibility, MRI
423 @item -c @var{MRI-commandfile}
424 @itemx --mri-script=@var{MRI-commandfile}
425 For compatibility with linkers produced by MRI, @command{ld} accepts script
426 files written in an alternate, restricted command language, described in
428 @ref{MRI,,MRI Compatible Script Files}.
431 the MRI Compatible Script Files section of GNU ld documentation.
433 Introduce MRI script files with
434 the option @samp{-c}; use the @samp{-T} option to run linker
435 scripts written in the general-purpose @command{ld} scripting language.
436 If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories
437 specified by any @samp{-L} options.
439 @cindex common allocation
446 These three options are equivalent; multiple forms are supported for
447 compatibility with other linkers. They assign space to common symbols
448 even if a relocatable output file is specified (with @samp{-r}). The
449 script command @code{FORCE_COMMON_ALLOCATION} has the same effect.
450 @xref{Miscellaneous Commands}.
452 @kindex --depaudit @var{AUDITLIB}
453 @kindex -P @var{AUDITLIB}
454 @item --depaudit @var{AUDITLIB}
455 @itemx -P @var{AUDITLIB}
456 Adds @var{AUDITLIB} to the @code{DT_DEPAUDIT} entry of the dynamic section.
457 @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME
458 specified in the library. If specified multiple times @code{DT_DEPAUDIT}
459 will contain a colon separated list of audit interfaces to use. This
460 option is only meaningful on ELF platforms supporting the rtld-audit interface.
461 The -P option is provided for Solaris compatibility.
463 @kindex --enable-non-contiguous-regions
464 @item --enable-non-contiguous-regions
465 This option avoids generating an error if an input section does not
466 fit a matching output section. The linker tries to allocate the input
467 section to subseque nt matching output sections, and generates an
468 error only if no output section is large enough. This is useful when
469 several non-contiguous memory regions are available and the input
470 section does not require a particular one. The order in which input
471 sections are evaluated does not change, for instance:
475 MEM1 (rwx) : ORIGIN : 0x1000, LENGTH = 0x14
476 MEM2 (rwx) : ORIGIN : 0x1000, LENGTH = 0x40
477 MEM3 (rwx) : ORIGIN : 0x2000, LENGTH = 0x40
480 mem1 : @{ *(.data.*); @} > MEM1
481 mem2 : @{ *(.data.*); @} > MEM2
482 mem3 : @{ *(.data.*); @} > MEM2
490 results in .data.1 affected to mem1, and .data.2 and .data.3
491 affected to mem2, even though .data.3 would fit in mem3.
494 This option is incompatible with INSERT statements because it changes
495 the way input sections are mapped to output sections.
497 @kindex --enable-non-contiguous-regions-warnings
498 @item --enable-non-contiguous-regions-warnings
499 This option enables warnings when
500 @code{--enable-non-contiguous-regions} allows possibly unexpected
501 matches in sections mapping, potentially leading to silently
502 discarding a section instead of failing because it does not fit any
505 @cindex entry point, from command line
506 @kindex -e @var{entry}
507 @kindex --entry=@var{entry}
509 @itemx --entry=@var{entry}
510 Use @var{entry} as the explicit symbol for beginning execution of your
511 program, rather than the default entry point. If there is no symbol
512 named @var{entry}, the linker will try to parse @var{entry} as a number,
513 and use that as the entry address (the number will be interpreted in
514 base 10; you may use a leading @samp{0x} for base 16, or a leading
515 @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults
516 and other ways of specifying the entry point.
518 @kindex --exclude-libs
519 @item --exclude-libs @var{lib},@var{lib},...
520 Specifies a list of archive libraries from which symbols should not be automatically
521 exported. The library names may be delimited by commas or colons. Specifying
522 @code{--exclude-libs ALL} excludes symbols in all archive libraries from
523 automatic export. This option is available only for the i386 PE targeted
524 port of the linker and for ELF targeted ports. For i386 PE, symbols
525 explicitly listed in a .def file are still exported, regardless of this
526 option. For ELF targeted ports, symbols affected by this option will
527 be treated as hidden.
529 @kindex --exclude-modules-for-implib
530 @item --exclude-modules-for-implib @var{module},@var{module},...
531 Specifies a list of object files or archive members, from which symbols
532 should not be automatically exported, but which should be copied wholesale
533 into the import library being generated during the link. The module names
534 may be delimited by commas or colons, and must match exactly the filenames
535 used by @command{ld} to open the files; for archive members, this is simply
536 the member name, but for object files the name listed must include and
537 match precisely any path used to specify the input file on the linker's
538 command-line. This option is available only for the i386 PE targeted port
539 of the linker. Symbols explicitly listed in a .def file are still exported,
540 regardless of this option.
542 @cindex dynamic symbol table
544 @kindex --export-dynamic
545 @kindex --no-export-dynamic
547 @itemx --export-dynamic
548 @itemx --no-export-dynamic
549 When creating a dynamically linked executable, using the @option{-E}
550 option or the @option{--export-dynamic} option causes the linker to add
551 all symbols to the dynamic symbol table. The dynamic symbol table is the
552 set of symbols which are visible from dynamic objects at run time.
554 If you do not use either of these options (or use the
555 @option{--no-export-dynamic} option to restore the default behavior), the
556 dynamic symbol table will normally contain only those symbols which are
557 referenced by some dynamic object mentioned in the link.
559 If you use @code{dlopen} to load a dynamic object which needs to refer
560 back to the symbols defined by the program, rather than some other
561 dynamic object, then you will probably need to use this option when
562 linking the program itself.
564 You can also use the dynamic list to control what symbols should
565 be added to the dynamic symbol table if the output format supports it.
566 See the description of @samp{--dynamic-list}.
568 Note that this option is specific to ELF targeted ports. PE targets
569 support a similar function to export all symbols from a DLL or EXE; see
570 the description of @samp{--export-all-symbols} below.
572 @ifclear SingleFormat
573 @cindex big-endian objects
577 Link big-endian objects. This affects the default output format.
579 @cindex little-endian objects
582 Link little-endian objects. This affects the default output format.
585 @kindex -f @var{name}
586 @kindex --auxiliary=@var{name}
588 @itemx --auxiliary=@var{name}
589 When creating an ELF shared object, set the internal DT_AUXILIARY field
590 to the specified name. This tells the dynamic linker that the symbol
591 table of the shared object should be used as an auxiliary filter on the
592 symbol table of the shared object @var{name}.
594 If you later link a program against this filter object, then, when you
595 run the program, the dynamic linker will see the DT_AUXILIARY field. If
596 the dynamic linker resolves any symbols from the filter object, it will
597 first check whether there is a definition in the shared object
598 @var{name}. If there is one, it will be used instead of the definition
599 in the filter object. The shared object @var{name} need not exist.
600 Thus the shared object @var{name} may be used to provide an alternative
601 implementation of certain functions, perhaps for debugging or for
602 machine-specific performance.
604 This option may be specified more than once. The DT_AUXILIARY entries
605 will be created in the order in which they appear on the command line.
607 @kindex -F @var{name}
608 @kindex --filter=@var{name}
610 @itemx --filter=@var{name}
611 When creating an ELF shared object, set the internal DT_FILTER field to
612 the specified name. This tells the dynamic linker that the symbol table
613 of the shared object which is being created should be used as a filter
614 on the symbol table of the shared object @var{name}.
616 If you later link a program against this filter object, then, when you
617 run the program, the dynamic linker will see the DT_FILTER field. The
618 dynamic linker will resolve symbols according to the symbol table of the
619 filter object as usual, but it will actually link to the definitions
620 found in the shared object @var{name}. Thus the filter object can be
621 used to select a subset of the symbols provided by the object
624 Some older linkers used the @option{-F} option throughout a compilation
625 toolchain for specifying object-file format for both input and output
627 @ifclear SingleFormat
628 The @sc{gnu} linker uses other mechanisms for this purpose: the
629 @option{-b}, @option{--format}, @option{--oformat} options, the
630 @code{TARGET} command in linker scripts, and the @code{GNUTARGET}
631 environment variable.
633 The @sc{gnu} linker will ignore the @option{-F} option when not
634 creating an ELF shared object.
636 @cindex finalization function
637 @kindex -fini=@var{name}
638 @item -fini=@var{name}
639 When creating an ELF executable or shared object, call NAME when the
640 executable or shared object is unloaded, by setting DT_FINI to the
641 address of the function. By default, the linker uses @code{_fini} as
642 the function to call.
646 Ignored. Provided for compatibility with other tools.
648 @kindex -G @var{value}
649 @kindex --gpsize=@var{value}
652 @itemx --gpsize=@var{value}
653 Set the maximum size of objects to be optimized using the GP register to
654 @var{size}. This is only meaningful for object file formats such as
655 MIPS ELF that support putting large and small objects into different
656 sections. This is ignored for other object file formats.
658 @cindex runtime library name
659 @kindex -h @var{name}
660 @kindex -soname=@var{name}
662 @itemx -soname=@var{name}
663 When creating an ELF shared object, set the internal DT_SONAME field to
664 the specified name. When an executable is linked with a shared object
665 which has a DT_SONAME field, then when the executable is run the dynamic
666 linker will attempt to load the shared object specified by the DT_SONAME
667 field rather than the using the file name given to the linker.
670 @cindex incremental link
672 Perform an incremental link (same as option @samp{-r}).
674 @cindex initialization function
675 @kindex -init=@var{name}
676 @item -init=@var{name}
677 When creating an ELF executable or shared object, call NAME when the
678 executable or shared object is loaded, by setting DT_INIT to the address
679 of the function. By default, the linker uses @code{_init} as the
682 @cindex archive files, from cmd line
683 @kindex -l @var{namespec}
684 @kindex --library=@var{namespec}
685 @item -l @var{namespec}
686 @itemx --library=@var{namespec}
687 Add the archive or object file specified by @var{namespec} to the
688 list of files to link. This option may be used any number of times.
689 If @var{namespec} is of the form @file{:@var{filename}}, @command{ld}
690 will search the library path for a file called @var{filename}, otherwise it
691 will search the library path for a file called @file{lib@var{namespec}.a}.
693 On systems which support shared libraries, @command{ld} may also search for
694 files other than @file{lib@var{namespec}.a}. Specifically, on ELF
695 and SunOS systems, @command{ld} will search a directory for a library
696 called @file{lib@var{namespec}.so} before searching for one called
697 @file{lib@var{namespec}.a}. (By convention, a @code{.so} extension
698 indicates a shared library.) Note that this behavior does not apply
699 to @file{:@var{filename}}, which always specifies a file called
702 The linker will search an archive only once, at the location where it is
703 specified on the command line. If the archive defines a symbol which
704 was undefined in some object which appeared before the archive on the
705 command line, the linker will include the appropriate file(s) from the
706 archive. However, an undefined symbol in an object appearing later on
707 the command line will not cause the linker to search the archive again.
709 See the @option{-(} option for a way to force the linker to search
710 archives multiple times.
712 You may list the same archive multiple times on the command line.
715 This type of archive searching is standard for Unix linkers. However,
716 if you are using @command{ld} on AIX, note that it is different from the
717 behaviour of the AIX linker.
720 @cindex search directory, from cmd line
722 @kindex --library-path=@var{dir}
723 @item -L @var{searchdir}
724 @itemx --library-path=@var{searchdir}
725 Add path @var{searchdir} to the list of paths that @command{ld} will search
726 for archive libraries and @command{ld} control scripts. You may use this
727 option any number of times. The directories are searched in the order
728 in which they are specified on the command line. Directories specified
729 on the command line are searched before the default directories. All
730 @option{-L} options apply to all @option{-l} options, regardless of the
731 order in which the options appear. @option{-L} options do not affect
732 how @command{ld} searches for a linker script unless @option{-T}
735 If @var{searchdir} begins with @code{=} or @code{$SYSROOT}, then this
736 prefix will be replaced by the @dfn{sysroot prefix}, controlled by the
737 @samp{--sysroot} option, or specified when the linker is configured.
740 The default set of paths searched (without being specified with
741 @samp{-L}) depends on which emulation mode @command{ld} is using, and in
742 some cases also on how it was configured. @xref{Environment}.
745 The paths can also be specified in a link script with the
746 @code{SEARCH_DIR} command. Directories specified this way are searched
747 at the point in which the linker script appears in the command line.
750 @kindex -m @var{emulation}
751 @item -m @var{emulation}
752 Emulate the @var{emulation} linker. You can list the available
753 emulations with the @samp{--verbose} or @samp{-V} options.
755 If the @samp{-m} option is not used, the emulation is taken from the
756 @code{LDEMULATION} environment variable, if that is defined.
758 Otherwise, the default emulation depends upon how the linker was
766 Print a link map to the standard output. A link map provides
767 information about the link, including the following:
771 Where object files are mapped into memory.
773 How common symbols are allocated.
775 All archive members included in the link, with a mention of the symbol
776 which caused the archive member to be brought in.
778 The values assigned to symbols.
780 Note - symbols whose values are computed by an expression which
781 involves a reference to a previous value of the same symbol may not
782 have correct result displayed in the link map. This is because the
783 linker discards intermediate results and only retains the final value
784 of an expression. Under such circumstances the linker will display
785 the final value enclosed by square brackets. Thus for example a
786 linker script containing:
794 will produce the following output in the link map if the @option{-M}
799 [0x0000000c] foo = (foo * 0x4)
800 [0x0000000c] foo = (foo + 0x8)
803 See @ref{Expressions} for more information about expressions in linker
807 How GNU properties are merged.
809 When the linker merges input .note.gnu.property sections into one output
810 .note.gnu.property section, some properties are removed or updated.
811 These actions are reported in the link map. For example:
814 Removed property 0xc0000002 to merge foo.o (0x1) and bar.o (not found)
817 This indicates that property 0xc0000002 is removed from output when
818 merging properties in @file{foo.o}, whose property 0xc0000002 value
819 is 0x1, and @file{bar.o}, which doesn't have property 0xc0000002.
822 Updated property 0xc0010001 (0x1) to merge foo.o (0x1) and bar.o (0x1)
825 This indicates that property 0xc0010001 value is updated to 0x1 in output
826 when merging properties in @file{foo.o}, whose 0xc0010001 property value
827 is 0x1, and @file{bar.o}, whose 0xc0010001 property value is 0x1.
830 @cindex link map discarded
831 @kindex --print-map-discarded
832 @kindex --no-print-map-discarded
833 @item --print-map-discarded
834 @itemx --no-print-map-discarded
835 Print (or do not print) the list of discarded and garbage collected sections
836 in the link map. Enabled by default.
839 @cindex read-only text
844 Turn off page alignment of sections, and disable linking against shared
845 libraries. If the output format supports Unix style magic numbers,
846 mark the output as @code{NMAGIC}.
850 @cindex read/write from cmd line
854 Set the text and data sections to be readable and writable. Also, do
855 not page-align the data segment, and disable linking against shared
856 libraries. If the output format supports Unix style magic numbers,
857 mark the output as @code{OMAGIC}. Note: Although a writable text section
858 is allowed for PE-COFF targets, it does not conform to the format
859 specification published by Microsoft.
864 This option negates most of the effects of the @option{-N} option. It
865 sets the text section to be read-only, and forces the data segment to
866 be page-aligned. Note - this option does not enable linking against
867 shared libraries. Use @option{-Bdynamic} for this.
869 @kindex -o @var{output}
870 @kindex --output=@var{output}
871 @cindex naming the output file
872 @item -o @var{output}
873 @itemx --output=@var{output}
874 Use @var{output} as the name for the program produced by @command{ld}; if this
875 option is not specified, the name @file{a.out} is used by default. The
876 script command @code{OUTPUT} can also specify the output file name.
878 @kindex -O @var{level}
879 @cindex generating optimized output
881 If @var{level} is a numeric values greater than zero @command{ld} optimizes
882 the output. This might take significantly longer and therefore probably
883 should only be enabled for the final binary. At the moment this
884 option only affects ELF shared library generation. Future releases of
885 the linker may make more use of this option. Also currently there is
886 no difference in the linker's behaviour for different non-zero values
887 of this option. Again this may change with future releases.
889 @kindex -plugin @var{name}
890 @item -plugin @var{name}
891 Involve a plugin in the linking process. The @var{name} parameter is
892 the absolute filename of the plugin. Usually this parameter is
893 automatically added by the complier, when using link time
894 optimization, but users can also add their own plugins if they so
897 Note that the location of the compiler originated plugins is different
898 from the place where the @command{ar}, @command{nm} and
899 @command{ranlib} programs search for their plugins. In order for
900 those commands to make use of a compiler based plugin it must first be
901 copied into the @file{$@{libdir@}/bfd-plugins} directory. All gcc
902 based linker plugins are backward compatible, so it is sufficient to
903 just copy in the newest one.
906 @cindex push state governing input file handling
908 The @option{--push-state} allows to preserve the current state of the
909 flags which govern the input file handling so that they can all be
910 restored with one corresponding @option{--pop-state} option.
912 The option which are covered are: @option{-Bdynamic}, @option{-Bstatic},
913 @option{-dn}, @option{-dy}, @option{-call_shared}, @option{-non_shared},
914 @option{-static}, @option{-N}, @option{-n}, @option{--whole-archive},
915 @option{--no-whole-archive}, @option{-r}, @option{-Ur},
916 @option{--copy-dt-needed-entries}, @option{--no-copy-dt-needed-entries},
917 @option{--as-needed}, @option{--no-as-needed}, and @option{-a}.
919 One target for this option are specifications for @file{pkg-config}. When
920 used with the @option{--libs} option all possibly needed libraries are
921 listed and then possibly linked with all the time. It is better to return
922 something as follows:
925 -Wl,--push-state,--as-needed -libone -libtwo -Wl,--pop-state
929 @cindex pop state governing input file handling
931 Undoes the effect of --push-state, restores the previous values of the
932 flags governing input file handling.
935 @kindex --emit-relocs
936 @cindex retain relocations in final executable
939 Leave relocation sections and contents in fully linked executables.
940 Post link analysis and optimization tools may need this information in
941 order to perform correct modifications of executables. This results
942 in larger executables.
944 This option is currently only supported on ELF platforms.
946 @kindex --force-dynamic
947 @cindex forcing the creation of dynamic sections
948 @item --force-dynamic
949 Force the output file to have dynamic sections. This option is specific
953 @cindex relocatable output
955 @kindex --relocatable
958 Generate relocatable output---i.e., generate an output file that can in
959 turn serve as input to @command{ld}. This is often called @dfn{partial
960 linking}. As a side effect, in environments that support standard Unix
961 magic numbers, this option also sets the output file's magic number to
963 @c ; see @option{-N}.
964 If this option is not specified, an absolute file is produced. When
965 linking C++ programs, this option @emph{will not} resolve references to
966 constructors; to do that, use @samp{-Ur}.
968 When an input file does not have the same format as the output file,
969 partial linking is only supported if that input file does not contain any
970 relocations. Different output formats can have further restrictions; for
971 example some @code{a.out}-based formats do not support partial linking
972 with input files in other formats at all.
974 This option does the same thing as @samp{-i}.
976 @kindex -R @var{file}
977 @kindex --just-symbols=@var{file}
978 @cindex symbol-only input
979 @item -R @var{filename}
980 @itemx --just-symbols=@var{filename}
981 Read symbol names and their addresses from @var{filename}, but do not
982 relocate it or include it in the output. This allows your output file
983 to refer symbolically to absolute locations of memory defined in other
984 programs. You may use this option more than once.
986 For compatibility with other ELF linkers, if the @option{-R} option is
987 followed by a directory name, rather than a file name, it is treated as
988 the @option{-rpath} option.
992 @cindex strip all symbols
995 Omit all symbol information from the output file.
998 @kindex --strip-debug
999 @cindex strip debugger symbols
1001 @itemx --strip-debug
1002 Omit debugger symbol information (but not all symbols) from the output file.
1004 @kindex --strip-discarded
1005 @kindex --no-strip-discarded
1006 @item --strip-discarded
1007 @itemx --no-strip-discarded
1008 Omit (or do not omit) global symbols defined in discarded sections.
1013 @cindex input files, displaying
1016 Print the names of the input files as @command{ld} processes them. If
1017 @samp{-t} is given twice then members within archives are also printed.
1018 @samp{-t} output is useful to generate a list of all the object files
1019 and scripts involved in linking, for example, when packaging files for
1020 a linker bug report.
1022 @kindex -T @var{script}
1023 @kindex --script=@var{script}
1024 @cindex script files
1025 @item -T @var{scriptfile}
1026 @itemx --script=@var{scriptfile}
1027 Use @var{scriptfile} as the linker script. This script replaces
1028 @command{ld}'s default linker script (rather than adding to it), so
1029 @var{commandfile} must specify everything necessary to describe the
1030 output file. @xref{Scripts}. If @var{scriptfile} does not exist in
1031 the current directory, @code{ld} looks for it in the directories
1032 specified by any preceding @samp{-L} options. Multiple @samp{-T}
1035 @kindex -dT @var{script}
1036 @kindex --default-script=@var{script}
1037 @cindex script files
1038 @item -dT @var{scriptfile}
1039 @itemx --default-script=@var{scriptfile}
1040 Use @var{scriptfile} as the default linker script. @xref{Scripts}.
1042 This option is similar to the @option{--script} option except that
1043 processing of the script is delayed until after the rest of the
1044 command line has been processed. This allows options placed after the
1045 @option{--default-script} option on the command line to affect the
1046 behaviour of the linker script, which can be important when the linker
1047 command line cannot be directly controlled by the user. (eg because
1048 the command line is being constructed by another tool, such as
1051 @kindex -u @var{symbol}
1052 @kindex --undefined=@var{symbol}
1053 @cindex undefined symbol
1054 @item -u @var{symbol}
1055 @itemx --undefined=@var{symbol}
1056 Force @var{symbol} to be entered in the output file as an undefined
1057 symbol. Doing this may, for example, trigger linking of additional
1058 modules from standard libraries. @samp{-u} may be repeated with
1059 different option arguments to enter additional undefined symbols. This
1060 option is equivalent to the @code{EXTERN} linker script command.
1062 If this option is being used to force additional modules to be pulled
1063 into the link, and if it is an error for the symbol to remain
1064 undefined, then the option @option{--require-defined} should be used
1067 @kindex --require-defined=@var{symbol}
1068 @cindex symbols, require defined
1069 @cindex defined symbol
1070 @item --require-defined=@var{symbol}
1071 Require that @var{symbol} is defined in the output file. This option
1072 is the same as option @option{--undefined} except that if @var{symbol}
1073 is not defined in the output file then the linker will issue an error
1074 and exit. The same effect can be achieved in a linker script by using
1075 @code{EXTERN}, @code{ASSERT} and @code{DEFINED} together. This option
1076 can be used multiple times to require additional symbols.
1079 @cindex constructors
1081 For anything other than C++ programs, this option is equivalent to
1082 @samp{-r}: it generates relocatable output---i.e., an output file that can in
1083 turn serve as input to @command{ld}. When linking C++ programs, @samp{-Ur}
1084 @emph{does} resolve references to constructors, unlike @samp{-r}.
1085 It does not work to use @samp{-Ur} on files that were themselves linked
1086 with @samp{-Ur}; once the constructor table has been built, it cannot
1087 be added to. Use @samp{-Ur} only for the last partial link, and
1088 @samp{-r} for the others.
1090 @kindex --orphan-handling=@var{MODE}
1091 @cindex orphan sections
1092 @cindex sections, orphan
1093 @item --orphan-handling=@var{MODE}
1094 Control how orphan sections are handled. An orphan section is one not
1095 specifically mentioned in a linker script. @xref{Orphan Sections}.
1097 @var{MODE} can have any of the following values:
1101 Orphan sections are placed into a suitable output section following
1102 the strategy described in @ref{Orphan Sections}. The option
1103 @samp{--unique} also affects how sections are placed.
1106 All orphan sections are discarded, by placing them in the
1107 @samp{/DISCARD/} section (@pxref{Output Section Discarding}).
1110 The linker will place the orphan section as for @code{place} and also
1114 The linker will exit with an error if any orphan section is found.
1117 The default if @samp{--orphan-handling} is not given is @code{place}.
1119 @kindex --unique[=@var{SECTION}]
1120 @item --unique[=@var{SECTION}]
1121 Creates a separate output section for every input section matching
1122 @var{SECTION}, or if the optional wildcard @var{SECTION} argument is
1123 missing, for every orphan input section. An orphan section is one not
1124 specifically mentioned in a linker script. You may use this option
1125 multiple times on the command line; It prevents the normal merging of
1126 input sections with the same name, overriding output section assignments
1136 Display the version number for @command{ld}. The @option{-V} option also
1137 lists the supported emulations.
1140 @kindex --discard-all
1141 @cindex deleting local symbols
1143 @itemx --discard-all
1144 Delete all local symbols.
1147 @kindex --discard-locals
1148 @cindex local symbols, deleting
1150 @itemx --discard-locals
1151 Delete all temporary local symbols. (These symbols start with
1152 system-specific local label prefixes, typically @samp{.L} for ELF systems
1153 or @samp{L} for traditional a.out systems.)
1155 @kindex -y @var{symbol}
1156 @kindex --trace-symbol=@var{symbol}
1157 @cindex symbol tracing
1158 @item -y @var{symbol}
1159 @itemx --trace-symbol=@var{symbol}
1160 Print the name of each linked file in which @var{symbol} appears. This
1161 option may be given any number of times. On many systems it is necessary
1162 to prepend an underscore.
1164 This option is useful when you have an undefined symbol in your link but
1165 don't know where the reference is coming from.
1167 @kindex -Y @var{path}
1169 Add @var{path} to the default library search path. This option exists
1170 for Solaris compatibility.
1172 @kindex -z @var{keyword}
1173 @item -z @var{keyword}
1174 The recognized keywords are:
1178 Always generate BND prefix in PLT entries. Supported for Linux/x86_64.
1180 @item call-nop=prefix-addr
1181 @itemx call-nop=suffix-nop
1182 @itemx call-nop=prefix-@var{byte}
1183 @itemx call-nop=suffix-@var{byte}
1184 Specify the 1-byte @code{NOP} padding when transforming indirect call
1185 to a locally defined function, foo, via its GOT slot.
1186 @option{call-nop=prefix-addr} generates @code{0x67 call foo}.
1187 @option{call-nop=suffix-nop} generates @code{call foo 0x90}.
1188 @option{call-nop=prefix-@var{byte}} generates @code{@var{byte} call foo}.
1189 @option{call-nop=suffix-@var{byte}} generates @code{call foo @var{byte}}.
1190 Supported for i386 and x86_64.
1192 @item cet-report=none
1193 @itemx cet-report=warning
1194 @itemx cet-report=error
1195 Specify how to report the missing GNU_PROPERTY_X86_FEATURE_1_IBT and
1196 GNU_PROPERTY_X86_FEATURE_1_SHSTK properties in input .note.gnu.property
1197 section. @option{cet-report=none}, which is the default, will make the
1198 linker not report missing properties in input files.
1199 @option{cet-report=warning} will make the linker issue a warning for
1200 missing properties in input files. @option{cet-report=error} will make
1201 the linker issue an error for missing properties in input files.
1202 Note that @option{ibt} will turn off the missing
1203 GNU_PROPERTY_X86_FEATURE_1_IBT property report and @option{shstk} will
1204 turn off the missing GNU_PROPERTY_X86_FEATURE_1_SHSTK property report.
1205 Supported for Linux/i386 and Linux/x86_64.
1209 Combine multiple dynamic relocation sections and sort to improve
1210 dynamic symbol lookup caching. Do not do this if @samp{nocombreloc}.
1214 Generate common symbols with STT_COMMON type during a relocatable
1215 link. Use STT_OBJECT type if @samp{nocommon}.
1217 @item common-page-size=@var{value}
1218 Set the page size most commonly used to @var{value}. Memory image
1219 layout will be optimized to minimize memory pages if the system is
1220 using pages of this size.
1223 Report unresolved symbol references from regular object files. This
1224 is done even if the linker is creating a non-symbolic shared library.
1225 This option is the inverse of @samp{-z undefs}.
1227 @item dynamic-undefined-weak
1228 @itemx nodynamic-undefined-weak
1229 Make undefined weak symbols dynamic when building a dynamic object,
1230 if they are referenced from a regular object file and not forced local
1231 by symbol visibility or versioning. Do not make them dynamic if
1232 @samp{nodynamic-undefined-weak}. If neither option is given, a target
1233 may default to either option being in force, or make some other
1234 selection of undefined weak symbols dynamic. Not all targets support
1238 Marks the object as requiring executable stack.
1241 This option is only meaningful when building a shared object. It makes
1242 the symbols defined by this shared object available for symbol resolution
1243 of subsequently loaded libraries.
1246 This option is only meaningful when building a dynamic executable.
1247 This option marks the executable as requiring global auditing by
1248 setting the @code{DF_1_GLOBAUDIT} bit in the @code{DT_FLAGS_1} dynamic
1249 tag. Global auditing requires that any auditing library defined via
1250 the @option{--depaudit} or @option{-P} command-line options be run for
1251 all dynamic objects loaded by the application.
1254 Generate Intel Indirect Branch Tracking (IBT) enabled PLT entries.
1255 Supported for Linux/i386 and Linux/x86_64.
1258 Generate GNU_PROPERTY_X86_FEATURE_1_IBT in .note.gnu.property section
1259 to indicate compatibility with IBT. This also implies @option{ibtplt}.
1260 Supported for Linux/i386 and Linux/x86_64.
1263 This option is only meaningful when building a shared object.
1264 It marks the object so that its runtime initialization will occur
1265 before the runtime initialization of any other objects brought into
1266 the process at the same time. Similarly the runtime finalization of
1267 the object will occur after the runtime finalization of any other
1271 Specify that the dynamic loader should modify its symbol search order
1272 so that symbols in this shared library interpose all other shared
1273 libraries not so marked.
1276 When generating an executable or shared library, mark it to tell the
1277 dynamic linker to defer function call resolution to the point when
1278 the function is called (lazy binding), rather than at load time.
1279 Lazy binding is the default.
1282 Specify that the object's filters be processed immediately at runtime.
1284 @item max-page-size=@var{value}
1285 Set the maximum memory page size supported to @var{value}.
1288 Allow multiple definitions.
1291 Disable linker generated .dynbss variables used in place of variables
1292 defined in shared libraries. May result in dynamic text relocations.
1295 Specify that the dynamic loader search for dependencies of this object
1296 should ignore any default library search paths.
1299 Specify that the object shouldn't be unloaded at runtime.
1302 Specify that the object is not available to @code{dlopen}.
1305 Specify that the object can not be dumped by @code{dldump}.
1308 Marks the object as not requiring executable stack.
1310 @item noextern-protected-data
1311 Don't treat protected data symbols as external when building a shared
1312 library. This option overrides the linker backend default. It can be
1313 used to work around incorrect relocations against protected data symbols
1314 generated by compiler. Updates on protected data symbols by another
1315 module aren't visible to the resulting shared library. Supported for
1318 @item noreloc-overflow
1319 Disable relocation overflow check. This can be used to disable
1320 relocation overflow check if there will be no dynamic relocation
1321 overflow at run-time. Supported for x86_64.
1324 When generating an executable or shared library, mark it to tell the
1325 dynamic linker to resolve all symbols when the program is started, or
1326 when the shared library is loaded by dlopen, instead of deferring
1327 function call resolution to the point when the function is first
1331 Specify that the object requires @samp{$ORIGIN} handling in paths.
1335 Create an ELF @code{PT_GNU_RELRO} segment header in the object. This
1336 specifies a memory segment that should be made read-only after
1337 relocation, if supported. Specifying @samp{common-page-size} smaller
1338 than the system page size will render this protection ineffective.
1339 Don't create an ELF @code{PT_GNU_RELRO} segment if @samp{norelro}.
1342 @itemx noseparate-code
1343 Create separate code @code{PT_LOAD} segment header in the object. This
1344 specifies a memory segment that should contain only instructions and must
1345 be in wholly disjoint pages from any other data. Don't create separate
1346 code @code{PT_LOAD} segment if @samp{noseparate-code} is used.
1349 Generate GNU_PROPERTY_X86_FEATURE_1_SHSTK in .note.gnu.property section
1350 to indicate compatibility with Intel Shadow Stack. Supported for
1351 Linux/i386 and Linux/x86_64.
1353 @item stack-size=@var{value}
1354 Specify a stack size for an ELF @code{PT_GNU_STACK} segment.
1355 Specifying zero will override any default non-zero sized
1356 @code{PT_GNU_STACK} segment creation.
1361 Report an error if DT_TEXTREL is set, i.e., if the position-independent
1362 or shared object has dynamic relocations in read-only sections. Don't
1363 report an error if @samp{notext} or @samp{textoff}.
1366 Do not report unresolved symbol references from regular object files,
1367 either when creating an executable, or when creating a shared library.
1368 This option is the inverse of @samp{-z defs}.
1372 Other keywords are ignored for Solaris compatibility.
1375 @cindex groups of archives
1376 @item -( @var{archives} -)
1377 @itemx --start-group @var{archives} --end-group
1378 The @var{archives} should be a list of archive files. They may be
1379 either explicit file names, or @samp{-l} options.
1381 The specified archives are searched repeatedly until no new undefined
1382 references are created. Normally, an archive is searched only once in
1383 the order that it is specified on the command line. If a symbol in that
1384 archive is needed to resolve an undefined symbol referred to by an
1385 object in an archive that appears later on the command line, the linker
1386 would not be able to resolve that reference. By grouping the archives,
1387 they will all be searched repeatedly until all possible references are
1390 Using this option has a significant performance cost. It is best to use
1391 it only when there are unavoidable circular references between two or
1394 @kindex --accept-unknown-input-arch
1395 @kindex --no-accept-unknown-input-arch
1396 @item --accept-unknown-input-arch
1397 @itemx --no-accept-unknown-input-arch
1398 Tells the linker to accept input files whose architecture cannot be
1399 recognised. The assumption is that the user knows what they are doing
1400 and deliberately wants to link in these unknown input files. This was
1401 the default behaviour of the linker, before release 2.14. The default
1402 behaviour from release 2.14 onwards is to reject such input files, and
1403 so the @samp{--accept-unknown-input-arch} option has been added to
1404 restore the old behaviour.
1407 @kindex --no-as-needed
1409 @itemx --no-as-needed
1410 This option affects ELF DT_NEEDED tags for dynamic libraries mentioned
1411 on the command line after the @option{--as-needed} option. Normally
1412 the linker will add a DT_NEEDED tag for each dynamic library mentioned
1413 on the command line, regardless of whether the library is actually
1414 needed or not. @option{--as-needed} causes a DT_NEEDED tag to only be
1415 emitted for a library that @emph{at that point in the link} satisfies a
1416 non-weak undefined symbol reference from a regular object file or, if
1417 the library is not found in the DT_NEEDED lists of other needed libraries, a
1418 non-weak undefined symbol reference from another needed dynamic library.
1419 Object files or libraries appearing on the command line @emph{after}
1420 the library in question do not affect whether the library is seen as
1421 needed. This is similar to the rules for extraction of object files
1422 from archives. @option{--no-as-needed} restores the default behaviour.
1424 @kindex --add-needed
1425 @kindex --no-add-needed
1427 @itemx --no-add-needed
1428 These two options have been deprecated because of the similarity of
1429 their names to the @option{--as-needed} and @option{--no-as-needed}
1430 options. They have been replaced by @option{--copy-dt-needed-entries}
1431 and @option{--no-copy-dt-needed-entries}.
1433 @kindex -assert @var{keyword}
1434 @item -assert @var{keyword}
1435 This option is ignored for SunOS compatibility.
1439 @kindex -call_shared
1443 Link against dynamic libraries. This is only meaningful on platforms
1444 for which shared libraries are supported. This option is normally the
1445 default on such platforms. The different variants of this option are
1446 for compatibility with various systems. You may use this option
1447 multiple times on the command line: it affects library searching for
1448 @option{-l} options which follow it.
1452 Set the @code{DF_1_GROUP} flag in the @code{DT_FLAGS_1} entry in the dynamic
1453 section. This causes the runtime linker to handle lookups in this
1454 object and its dependencies to be performed only inside the group.
1455 @option{--unresolved-symbols=report-all} is implied. This option is
1456 only meaningful on ELF platforms which support shared libraries.
1466 Do not link against shared libraries. This is only meaningful on
1467 platforms for which shared libraries are supported. The different
1468 variants of this option are for compatibility with various systems. You
1469 may use this option multiple times on the command line: it affects
1470 library searching for @option{-l} options which follow it. This
1471 option also implies @option{--unresolved-symbols=report-all}. This
1472 option can be used with @option{-shared}. Doing so means that a
1473 shared library is being created but that all of the library's external
1474 references must be resolved by pulling in entries from static
1479 When creating a shared library, bind references to global symbols to the
1480 definition within the shared library, if any. Normally, it is possible
1481 for a program linked against a shared library to override the definition
1482 within the shared library. This option is only meaningful on ELF
1483 platforms which support shared libraries.
1485 @kindex -Bsymbolic-functions
1486 @item -Bsymbolic-functions
1487 When creating a shared library, bind references to global function
1488 symbols to the definition within the shared library, if any.
1489 This option is only meaningful on ELF platforms which support shared
1492 @kindex --dynamic-list=@var{dynamic-list-file}
1493 @item --dynamic-list=@var{dynamic-list-file}
1494 Specify the name of a dynamic list file to the linker. This is
1495 typically used when creating shared libraries to specify a list of
1496 global symbols whose references shouldn't be bound to the definition
1497 within the shared library, or creating dynamically linked executables
1498 to specify a list of symbols which should be added to the symbol table
1499 in the executable. This option is only meaningful on ELF platforms
1500 which support shared libraries.
1502 The format of the dynamic list is the same as the version node without
1503 scope and node name. See @ref{VERSION} for more information.
1505 @kindex --dynamic-list-data
1506 @item --dynamic-list-data
1507 Include all global data symbols to the dynamic list.
1509 @kindex --dynamic-list-cpp-new
1510 @item --dynamic-list-cpp-new
1511 Provide the builtin dynamic list for C++ operator new and delete. It
1512 is mainly useful for building shared libstdc++.
1514 @kindex --dynamic-list-cpp-typeinfo
1515 @item --dynamic-list-cpp-typeinfo
1516 Provide the builtin dynamic list for C++ runtime type identification.
1518 @kindex --check-sections
1519 @kindex --no-check-sections
1520 @item --check-sections
1521 @itemx --no-check-sections
1522 Asks the linker @emph{not} to check section addresses after they have
1523 been assigned to see if there are any overlaps. Normally the linker will
1524 perform this check, and if it finds any overlaps it will produce
1525 suitable error messages. The linker does know about, and does make
1526 allowances for sections in overlays. The default behaviour can be
1527 restored by using the command-line switch @option{--check-sections}.
1528 Section overlap is not usually checked for relocatable links. You can
1529 force checking in that case by using the @option{--check-sections}
1532 @kindex --copy-dt-needed-entries
1533 @kindex --no-copy-dt-needed-entries
1534 @item --copy-dt-needed-entries
1535 @itemx --no-copy-dt-needed-entries
1536 This option affects the treatment of dynamic libraries referred to
1537 by DT_NEEDED tags @emph{inside} ELF dynamic libraries mentioned on the
1538 command line. Normally the linker won't add a DT_NEEDED tag to the
1539 output binary for each library mentioned in a DT_NEEDED tag in an
1540 input dynamic library. With @option{--copy-dt-needed-entries}
1541 specified on the command line however any dynamic libraries that
1542 follow it will have their DT_NEEDED entries added. The default
1543 behaviour can be restored with @option{--no-copy-dt-needed-entries}.
1545 This option also has an effect on the resolution of symbols in dynamic
1546 libraries. With @option{--copy-dt-needed-entries} dynamic libraries
1547 mentioned on the command line will be recursively searched, following
1548 their DT_NEEDED tags to other libraries, in order to resolve symbols
1549 required by the output binary. With the default setting however
1550 the searching of dynamic libraries that follow it will stop with the
1551 dynamic library itself. No DT_NEEDED links will be traversed to resolve
1554 @cindex cross reference table
1557 Output a cross reference table. If a linker map file is being
1558 generated, the cross reference table is printed to the map file.
1559 Otherwise, it is printed on the standard output.
1561 The format of the table is intentionally simple, so that it may be
1562 easily processed by a script if necessary. The symbols are printed out,
1563 sorted by name. For each symbol, a list of file names is given. If the
1564 symbol is defined, the first file listed is the location of the
1565 definition. If the symbol is defined as a common value then any files
1566 where this happens appear next. Finally any files that reference the
1569 @cindex common allocation
1570 @kindex --no-define-common
1571 @item --no-define-common
1572 This option inhibits the assignment of addresses to common symbols.
1573 The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect.
1574 @xref{Miscellaneous Commands}.
1576 The @samp{--no-define-common} option allows decoupling
1577 the decision to assign addresses to Common symbols from the choice
1578 of the output file type; otherwise a non-Relocatable output type
1579 forces assigning addresses to Common symbols.
1580 Using @samp{--no-define-common} allows Common symbols that are referenced
1581 from a shared library to be assigned addresses only in the main program.
1582 This eliminates the unused duplicate space in the shared library,
1583 and also prevents any possible confusion over resolving to the wrong
1584 duplicate when there are many dynamic modules with specialized search
1585 paths for runtime symbol resolution.
1587 @cindex group allocation in linker script
1588 @cindex section groups
1590 @kindex --force-group-allocation
1591 @item --force-group-allocation
1592 This option causes the linker to place section group members like
1593 normal input sections, and to delete the section groups. This is the
1594 default behaviour for a final link but this option can be used to
1595 change the behaviour of a relocatable link (@samp{-r}). The script
1596 command @code{FORCE_GROUP_ALLOCATION} has the same
1597 effect. @xref{Miscellaneous Commands}.
1599 @cindex symbols, from command line
1600 @kindex --defsym=@var{symbol}=@var{exp}
1601 @item --defsym=@var{symbol}=@var{expression}
1602 Create a global symbol in the output file, containing the absolute
1603 address given by @var{expression}. You may use this option as many
1604 times as necessary to define multiple symbols in the command line. A
1605 limited form of arithmetic is supported for the @var{expression} in this
1606 context: you may give a hexadecimal constant or the name of an existing
1607 symbol, or use @code{+} and @code{-} to add or subtract hexadecimal
1608 constants or symbols. If you need more elaborate expressions, consider
1609 using the linker command language from a script (@pxref{Assignments}).
1610 @emph{Note:} there should be no white space between @var{symbol}, the
1611 equals sign (``@key{=}''), and @var{expression}.
1613 @cindex demangling, from command line
1614 @kindex --demangle[=@var{style}]
1615 @kindex --no-demangle
1616 @item --demangle[=@var{style}]
1617 @itemx --no-demangle
1618 These options control whether to demangle symbol names in error messages
1619 and other output. When the linker is told to demangle, it tries to
1620 present symbol names in a readable fashion: it strips leading
1621 underscores if they are used by the object file format, and converts C++
1622 mangled symbol names into user readable names. Different compilers have
1623 different mangling styles. The optional demangling style argument can be used
1624 to choose an appropriate demangling style for your compiler. The linker will
1625 demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE}
1626 is set. These options may be used to override the default.
1628 @cindex dynamic linker, from command line
1629 @kindex -I@var{file}
1630 @kindex --dynamic-linker=@var{file}
1632 @itemx --dynamic-linker=@var{file}
1633 Set the name of the dynamic linker. This is only meaningful when
1634 generating dynamically linked ELF executables. The default dynamic
1635 linker is normally correct; don't use this unless you know what you are
1638 @kindex --no-dynamic-linker
1639 @item --no-dynamic-linker
1640 When producing an executable file, omit the request for a dynamic
1641 linker to be used at load-time. This is only meaningful for ELF
1642 executables that contain dynamic relocations, and usually requires
1643 entry point code that is capable of processing these relocations.
1645 @kindex --embedded-relocs
1646 @item --embedded-relocs
1647 This option is similar to the @option{--emit-relocs} option except
1648 that the relocs are stored in a target-specific section. This option
1649 is only supported by the @samp{BFIN}, @samp{CR16} and @emph{M68K}
1652 @kindex --disable-multiple-abs-defs
1653 @item --disable-multiple-abs-defs
1654 Do not allow multiple definitions with symbols included
1655 in filename invoked by -R or --just-symbols
1657 @kindex --fatal-warnings
1658 @kindex --no-fatal-warnings
1659 @item --fatal-warnings
1660 @itemx --no-fatal-warnings
1661 Treat all warnings as errors. The default behaviour can be restored
1662 with the option @option{--no-fatal-warnings}.
1664 @kindex --force-exe-suffix
1665 @item --force-exe-suffix
1666 Make sure that an output file has a .exe suffix.
1668 If a successfully built fully linked output file does not have a
1669 @code{.exe} or @code{.dll} suffix, this option forces the linker to copy
1670 the output file to one of the same name with a @code{.exe} suffix. This
1671 option is useful when using unmodified Unix makefiles on a Microsoft
1672 Windows host, since some versions of Windows won't run an image unless
1673 it ends in a @code{.exe} suffix.
1675 @kindex --gc-sections
1676 @kindex --no-gc-sections
1677 @cindex garbage collection
1679 @itemx --no-gc-sections
1680 Enable garbage collection of unused input sections. It is ignored on
1681 targets that do not support this option. The default behaviour (of not
1682 performing this garbage collection) can be restored by specifying
1683 @samp{--no-gc-sections} on the command line. Note that garbage
1684 collection for COFF and PE format targets is supported, but the
1685 implementation is currently considered to be experimental.
1687 @samp{--gc-sections} decides which input sections are used by
1688 examining symbols and relocations. The section containing the entry
1689 symbol and all sections containing symbols undefined on the
1690 command-line will be kept, as will sections containing symbols
1691 referenced by dynamic objects. Note that when building shared
1692 libraries, the linker must assume that any visible symbol is
1693 referenced. Once this initial set of sections has been determined,
1694 the linker recursively marks as used any section referenced by their
1695 relocations. See @samp{--entry}, @samp{--undefined}, and
1696 @samp{--gc-keep-exported}.
1698 This option can be set when doing a partial link (enabled with option
1699 @samp{-r}). In this case the root of symbols kept must be explicitly
1700 specified either by one of the options @samp{--entry},
1701 @samp{--undefined}, or @samp{--gc-keep-exported} or by a @code{ENTRY}
1702 command in the linker script.
1704 @kindex --print-gc-sections
1705 @kindex --no-print-gc-sections
1706 @cindex garbage collection
1707 @item --print-gc-sections
1708 @itemx --no-print-gc-sections
1709 List all sections removed by garbage collection. The listing is
1710 printed on stderr. This option is only effective if garbage
1711 collection has been enabled via the @samp{--gc-sections}) option. The
1712 default behaviour (of not listing the sections that are removed) can
1713 be restored by specifying @samp{--no-print-gc-sections} on the command
1716 @kindex --gc-keep-exported
1717 @cindex garbage collection
1718 @item --gc-keep-exported
1719 When @samp{--gc-sections} is enabled, this option prevents garbage
1720 collection of unused input sections that contain global symbols having
1721 default or protected visibility. This option is intended to be used for
1722 executables where unreferenced sections would otherwise be garbage
1723 collected regardless of the external visibility of contained symbols.
1724 Note that this option has no effect when linking shared objects since
1725 it is already the default behaviour. This option is only supported for
1728 @kindex --print-output-format
1729 @cindex output format
1730 @item --print-output-format
1731 Print the name of the default output format (perhaps influenced by
1732 other command-line options). This is the string that would appear
1733 in an @code{OUTPUT_FORMAT} linker script command (@pxref{File Commands}).
1735 @kindex --print-memory-usage
1736 @cindex memory usage
1737 @item --print-memory-usage
1738 Print used size, total size and used size of memory regions created with
1739 the @ref{MEMORY} command. This is useful on embedded targets to have a
1740 quick view of amount of free memory. The format of the output has one
1741 headline and one line per region. It is both human readable and easily
1742 parsable by tools. Here is an example of an output:
1745 Memory region Used Size Region Size %age Used
1746 ROM: 256 KB 1 MB 25.00%
1747 RAM: 32 B 2 GB 0.00%
1754 Print a summary of the command-line options on the standard output and exit.
1756 @kindex --target-help
1758 Print a summary of all target-specific options on the standard output and exit.
1760 @kindex -Map=@var{mapfile}
1761 @item -Map=@var{mapfile}
1762 Print a link map to the file @var{mapfile}. See the description of the
1763 @option{-M} option, above. Specifying a directory as @var{mapfile}
1764 causes the linker map to be written into a file inside the directory.
1765 The name of the file is based upon the @var{output} filename with
1766 @code{.map} appended.
1768 @cindex memory usage
1769 @kindex --no-keep-memory
1770 @item --no-keep-memory
1771 @command{ld} normally optimizes for speed over memory usage by caching the
1772 symbol tables of input files in memory. This option tells @command{ld} to
1773 instead optimize for memory usage, by rereading the symbol tables as
1774 necessary. This may be required if @command{ld} runs out of memory space
1775 while linking a large executable.
1777 @kindex --no-undefined
1780 @item --no-undefined
1782 Report unresolved symbol references from regular object files. This
1783 is done even if the linker is creating a non-symbolic shared library.
1784 The switch @option{--[no-]allow-shlib-undefined} controls the
1785 behaviour for reporting unresolved references found in shared
1786 libraries being linked in.
1788 The effects of this option can be reverted by using @code{-z undefs}.
1790 @kindex --allow-multiple-definition
1792 @item --allow-multiple-definition
1794 Normally when a symbol is defined multiple times, the linker will
1795 report a fatal error. These options allow multiple definitions and the
1796 first definition will be used.
1798 @kindex --allow-shlib-undefined
1799 @kindex --no-allow-shlib-undefined
1800 @item --allow-shlib-undefined
1801 @itemx --no-allow-shlib-undefined
1802 Allows or disallows undefined symbols in shared libraries.
1803 This switch is similar to @option{--no-undefined} except that it
1804 determines the behaviour when the undefined symbols are in a
1805 shared library rather than a regular object file. It does not affect
1806 how undefined symbols in regular object files are handled.
1808 The default behaviour is to report errors for any undefined symbols
1809 referenced in shared libraries if the linker is being used to create
1810 an executable, but to allow them if the linker is being used to create
1813 The reasons for allowing undefined symbol references in shared
1814 libraries specified at link time are that:
1818 A shared library specified at link time may not be the same as the one
1819 that is available at load time, so the symbol might actually be
1820 resolvable at load time.
1822 There are some operating systems, eg BeOS and HPPA, where undefined
1823 symbols in shared libraries are normal.
1825 The BeOS kernel for example patches shared libraries at load time to
1826 select whichever function is most appropriate for the current
1827 architecture. This is used, for example, to dynamically select an
1828 appropriate memset function.
1831 @kindex --no-undefined-version
1832 @item --no-undefined-version
1833 Normally when a symbol has an undefined version, the linker will ignore
1834 it. This option disallows symbols with undefined version and a fatal error
1835 will be issued instead.
1837 @kindex --default-symver
1838 @item --default-symver
1839 Create and use a default symbol version (the soname) for unversioned
1842 @kindex --default-imported-symver
1843 @item --default-imported-symver
1844 Create and use a default symbol version (the soname) for unversioned
1847 @kindex --no-warn-mismatch
1848 @item --no-warn-mismatch
1849 Normally @command{ld} will give an error if you try to link together input
1850 files that are mismatched for some reason, perhaps because they have
1851 been compiled for different processors or for different endiannesses.
1852 This option tells @command{ld} that it should silently permit such possible
1853 errors. This option should only be used with care, in cases when you
1854 have taken some special action that ensures that the linker errors are
1857 @kindex --no-warn-search-mismatch
1858 @item --no-warn-search-mismatch
1859 Normally @command{ld} will give a warning if it finds an incompatible
1860 library during a library search. This option silences the warning.
1862 @kindex --no-whole-archive
1863 @item --no-whole-archive
1864 Turn off the effect of the @option{--whole-archive} option for subsequent
1867 @cindex output file after errors
1868 @kindex --noinhibit-exec
1869 @item --noinhibit-exec
1870 Retain the executable output file whenever it is still usable.
1871 Normally, the linker will not produce an output file if it encounters
1872 errors during the link process; it exits without writing an output file
1873 when it issues any error whatsoever.
1877 Only search library directories explicitly specified on the
1878 command line. Library directories specified in linker scripts
1879 (including linker scripts specified on the command line) are ignored.
1881 @ifclear SingleFormat
1882 @kindex --oformat=@var{output-format}
1883 @item --oformat=@var{output-format}
1884 @command{ld} may be configured to support more than one kind of object
1885 file. If your @command{ld} is configured this way, you can use the
1886 @samp{--oformat} option to specify the binary format for the output
1887 object file. Even when @command{ld} is configured to support alternative
1888 object formats, you don't usually need to specify this, as @command{ld}
1889 should be configured to produce as a default output format the most
1890 usual format on each machine. @var{output-format} is a text string, the
1891 name of a particular format supported by the BFD libraries. (You can
1892 list the available binary formats with @samp{objdump -i}.) The script
1893 command @code{OUTPUT_FORMAT} can also specify the output format, but
1894 this option overrides it. @xref{BFD}.
1897 @kindex --out-implib
1898 @item --out-implib @var{file}
1899 Create an import library in @var{file} corresponding to the executable
1900 the linker is generating (eg. a DLL or ELF program). This import
1901 library (which should be called @code{*.dll.a} or @code{*.a} for DLLs)
1902 may be used to link clients against the generated executable; this
1903 behaviour makes it possible to skip a separate import library creation
1904 step (eg. @code{dlltool} for DLLs). This option is only available for
1905 the i386 PE and ELF targetted ports of the linker.
1908 @kindex --pic-executable
1910 @itemx --pic-executable
1911 @cindex position independent executables
1912 Create a position independent executable. This is currently only supported on
1913 ELF platforms. Position independent executables are similar to shared
1914 libraries in that they are relocated by the dynamic linker to the virtual
1915 address the OS chooses for them (which can vary between invocations). Like
1916 normal dynamically linked executables they can be executed and symbols
1917 defined in the executable cannot be overridden by shared libraries.
1921 This option is ignored for Linux compatibility.
1925 This option is ignored for SVR4 compatibility.
1928 @cindex synthesizing linker
1929 @cindex relaxing addressing modes
1933 An option with machine dependent effects.
1935 This option is only supported on a few targets.
1938 @xref{H8/300,,@command{ld} and the H8/300}.
1941 @xref{Xtensa,, @command{ld} and Xtensa Processors}.
1944 @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}.
1947 @xref{Nios II,,@command{ld} and the Altera Nios II}.
1950 @xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}.
1953 On some platforms the @samp{--relax} option performs target-specific,
1954 global optimizations that become possible when the linker resolves
1955 addressing in the program, such as relaxing address modes,
1956 synthesizing new instructions, selecting shorter version of current
1957 instructions, and combining constant values.
1959 On some platforms these link time global optimizations may make symbolic
1960 debugging of the resulting executable impossible.
1962 This is known to be the case for the Matsushita MN10200 and MN10300
1963 family of processors.
1967 On platforms where this is not supported, @samp{--relax} is accepted,
1971 On platforms where @samp{--relax} is accepted the option
1972 @samp{--no-relax} can be used to disable the feature.
1974 @cindex retaining specified symbols
1975 @cindex stripping all but some symbols
1976 @cindex symbols, retaining selectively
1977 @kindex --retain-symbols-file=@var{filename}
1978 @item --retain-symbols-file=@var{filename}
1979 Retain @emph{only} the symbols listed in the file @var{filename},
1980 discarding all others. @var{filename} is simply a flat file, with one
1981 symbol name per line. This option is especially useful in environments
1985 where a large global symbol table is accumulated gradually, to conserve
1988 @samp{--retain-symbols-file} does @emph{not} discard undefined symbols,
1989 or symbols needed for relocations.
1991 You may only specify @samp{--retain-symbols-file} once in the command
1992 line. It overrides @samp{-s} and @samp{-S}.
1995 @item -rpath=@var{dir}
1996 @cindex runtime library search path
1997 @kindex -rpath=@var{dir}
1998 Add a directory to the runtime library search path. This is used when
1999 linking an ELF executable with shared objects. All @option{-rpath}
2000 arguments are concatenated and passed to the runtime linker, which uses
2001 them to locate shared objects at runtime.
2003 The @option{-rpath} option is also used when locating shared objects which
2004 are needed by shared objects explicitly included in the link; see the
2005 description of the @option{-rpath-link} option. Searching @option{-rpath}
2006 in this way is only supported by native linkers and cross linkers which
2007 have been configured with the @option{--with-sysroot} option.
2009 If @option{-rpath} is not used when linking an ELF executable, the
2010 contents of the environment variable @code{LD_RUN_PATH} will be used if it
2013 The @option{-rpath} option may also be used on SunOS. By default, on
2014 SunOS, the linker will form a runtime search path out of all the
2015 @option{-L} options it is given. If a @option{-rpath} option is used, the
2016 runtime search path will be formed exclusively using the @option{-rpath}
2017 options, ignoring the @option{-L} options. This can be useful when using
2018 gcc, which adds many @option{-L} options which may be on NFS mounted
2021 For compatibility with other ELF linkers, if the @option{-R} option is
2022 followed by a directory name, rather than a file name, it is treated as
2023 the @option{-rpath} option.
2027 @cindex link-time runtime library search path
2028 @kindex -rpath-link=@var{dir}
2029 @item -rpath-link=@var{dir}
2030 When using ELF or SunOS, one shared library may require another. This
2031 happens when an @code{ld -shared} link includes a shared library as one
2034 When the linker encounters such a dependency when doing a non-shared,
2035 non-relocatable link, it will automatically try to locate the required
2036 shared library and include it in the link, if it is not included
2037 explicitly. In such a case, the @option{-rpath-link} option
2038 specifies the first set of directories to search. The
2039 @option{-rpath-link} option may specify a sequence of directory names
2040 either by specifying a list of names separated by colons, or by
2041 appearing multiple times.
2043 The tokens @var{$ORIGIN} and @var{$LIB} can appear in these search
2044 directories. They will be replaced by the full path to the directory
2045 containing the program or shared object in the case of @var{$ORIGIN}
2046 and either @samp{lib} - for 32-bit binaries - or @samp{lib64} - for
2047 64-bit binaries - in the case of @var{$LIB}.
2049 The alternative form of these tokens - @var{$@{ORIGIN@}} and
2050 @var{$@{LIB@}} can also be used. The token @var{$PLATFORM} is not
2053 This option should be used with caution as it overrides the search path
2054 that may have been hard compiled into a shared library. In such a case it
2055 is possible to use unintentionally a different search path than the
2056 runtime linker would do.
2058 The linker uses the following search paths to locate required shared
2063 Any directories specified by @option{-rpath-link} options.
2065 Any directories specified by @option{-rpath} options. The difference
2066 between @option{-rpath} and @option{-rpath-link} is that directories
2067 specified by @option{-rpath} options are included in the executable and
2068 used at runtime, whereas the @option{-rpath-link} option is only effective
2069 at link time. Searching @option{-rpath} in this way is only supported
2070 by native linkers and cross linkers which have been configured with
2071 the @option{--with-sysroot} option.
2073 On an ELF system, for native linkers, if the @option{-rpath} and
2074 @option{-rpath-link} options were not used, search the contents of the
2075 environment variable @code{LD_RUN_PATH}.
2077 On SunOS, if the @option{-rpath} option was not used, search any
2078 directories specified using @option{-L} options.
2080 For a native linker, search the contents of the environment
2081 variable @code{LD_LIBRARY_PATH}.
2083 For a native ELF linker, the directories in @code{DT_RUNPATH} or
2084 @code{DT_RPATH} of a shared library are searched for shared
2085 libraries needed by it. The @code{DT_RPATH} entries are ignored if
2086 @code{DT_RUNPATH} entries exist.
2088 The default directories, normally @file{/lib} and @file{/usr/lib}.
2090 For a linker for a Linux system, if the file @file{/etc/ld.so.conf}
2091 exists, the list of directories found in that file. Note: the path
2092 to this file is prefixed with the @code{sysroot} value, if that is
2093 defined, and then any @code{prefix} string if the linker was
2094 configured with the @command{--prefix=<path>} option.
2096 For a native linker on a FreeBSD system, any directories specified by
2097 the @code{_PATH_ELF_HINTS} macro defined in the @file{elf-hints.h}
2100 Any directories specifed by a @code{SEARCH_DIR} command in the
2101 linker script being used.
2104 If the required shared library is not found, the linker will issue a
2105 warning and continue with the link.
2112 @cindex shared libraries
2113 Create a shared library. This is currently only supported on ELF, XCOFF
2114 and SunOS platforms. On SunOS, the linker will automatically create a
2115 shared library if the @option{-e} option is not used and there are
2116 undefined symbols in the link.
2118 @kindex --sort-common
2120 @itemx --sort-common=ascending
2121 @itemx --sort-common=descending
2122 This option tells @command{ld} to sort the common symbols by alignment in
2123 ascending or descending order when it places them in the appropriate output
2124 sections. The symbol alignments considered are sixteen-byte or larger,
2125 eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps
2126 between symbols due to alignment constraints. If no sorting order is
2127 specified, then descending order is assumed.
2129 @kindex --sort-section=name
2130 @item --sort-section=name
2131 This option will apply @code{SORT_BY_NAME} to all wildcard section
2132 patterns in the linker script.
2134 @kindex --sort-section=alignment
2135 @item --sort-section=alignment
2136 This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section
2137 patterns in the linker script.
2139 @kindex --spare-dynamic-tags
2140 @item --spare-dynamic-tags=@var{count}
2141 This option specifies the number of empty slots to leave in the
2142 .dynamic section of ELF shared objects. Empty slots may be needed by
2143 post processing tools, such as the prelinker. The default is 5.
2145 @kindex --split-by-file
2146 @item --split-by-file[=@var{size}]
2147 Similar to @option{--split-by-reloc} but creates a new output section for
2148 each input file when @var{size} is reached. @var{size} defaults to a
2149 size of 1 if not given.
2151 @kindex --split-by-reloc
2152 @item --split-by-reloc[=@var{count}]
2153 Tries to creates extra sections in the output file so that no single
2154 output section in the file contains more than @var{count} relocations.
2155 This is useful when generating huge relocatable files for downloading into
2156 certain real time kernels with the COFF object file format; since COFF
2157 cannot represent more than 65535 relocations in a single section. Note
2158 that this will fail to work with object file formats which do not
2159 support arbitrary sections. The linker will not split up individual
2160 input sections for redistribution, so if a single input section contains
2161 more than @var{count} relocations one output section will contain that
2162 many relocations. @var{count} defaults to a value of 32768.
2166 Compute and display statistics about the operation of the linker, such
2167 as execution time and memory usage.
2169 @kindex --sysroot=@var{directory}
2170 @item --sysroot=@var{directory}
2171 Use @var{directory} as the location of the sysroot, overriding the
2172 configure-time default. This option is only supported by linkers
2173 that were configured using @option{--with-sysroot}.
2177 This is used by COFF/PE based targets to create a task-linked object
2178 file where all of the global symbols have been converted to statics.
2180 @kindex --traditional-format
2181 @cindex traditional format
2182 @item --traditional-format
2183 For some targets, the output of @command{ld} is different in some ways from
2184 the output of some existing linker. This switch requests @command{ld} to
2185 use the traditional format instead.
2188 For example, on SunOS, @command{ld} combines duplicate entries in the
2189 symbol string table. This can reduce the size of an output file with
2190 full debugging information by over 30 percent. Unfortunately, the SunOS
2191 @code{dbx} program can not read the resulting program (@code{gdb} has no
2192 trouble). The @samp{--traditional-format} switch tells @command{ld} to not
2193 combine duplicate entries.
2195 @kindex --section-start=@var{sectionname}=@var{org}
2196 @item --section-start=@var{sectionname}=@var{org}
2197 Locate a section in the output file at the absolute
2198 address given by @var{org}. You may use this option as many
2199 times as necessary to locate multiple sections in the command
2201 @var{org} must be a single hexadecimal integer;
2202 for compatibility with other linkers, you may omit the leading
2203 @samp{0x} usually associated with hexadecimal values. @emph{Note:} there
2204 should be no white space between @var{sectionname}, the equals
2205 sign (``@key{=}''), and @var{org}.
2207 @kindex -Tbss=@var{org}
2208 @kindex -Tdata=@var{org}
2209 @kindex -Ttext=@var{org}
2210 @cindex segment origins, cmd line
2211 @item -Tbss=@var{org}
2212 @itemx -Tdata=@var{org}
2213 @itemx -Ttext=@var{org}
2214 Same as @option{--section-start}, with @code{.bss}, @code{.data} or
2215 @code{.text} as the @var{sectionname}.
2217 @kindex -Ttext-segment=@var{org}
2218 @item -Ttext-segment=@var{org}
2219 @cindex text segment origin, cmd line
2220 When creating an ELF executable, it will set the address of the first
2221 byte of the text segment.
2223 @kindex -Trodata-segment=@var{org}
2224 @item -Trodata-segment=@var{org}
2225 @cindex rodata segment origin, cmd line
2226 When creating an ELF executable or shared object for a target where
2227 the read-only data is in its own segment separate from the executable
2228 text, it will set the address of the first byte of the read-only data segment.
2230 @kindex -Tldata-segment=@var{org}
2231 @item -Tldata-segment=@var{org}
2232 @cindex ldata segment origin, cmd line
2233 When creating an ELF executable or shared object for x86-64 medium memory
2234 model, it will set the address of the first byte of the ldata segment.
2236 @kindex --unresolved-symbols
2237 @item --unresolved-symbols=@var{method}
2238 Determine how to handle unresolved symbols. There are four possible
2239 values for @samp{method}:
2243 Do not report any unresolved symbols.
2246 Report all unresolved symbols. This is the default.
2248 @item ignore-in-object-files
2249 Report unresolved symbols that are contained in shared libraries, but
2250 ignore them if they come from regular object files.
2252 @item ignore-in-shared-libs
2253 Report unresolved symbols that come from regular object files, but
2254 ignore them if they come from shared libraries. This can be useful
2255 when creating a dynamic binary and it is known that all the shared
2256 libraries that it should be referencing are included on the linker's
2260 The behaviour for shared libraries on their own can also be controlled
2261 by the @option{--[no-]allow-shlib-undefined} option.
2263 Normally the linker will generate an error message for each reported
2264 unresolved symbol but the option @option{--warn-unresolved-symbols}
2265 can change this to a warning.
2267 @kindex --verbose[=@var{NUMBER}]
2268 @cindex verbose[=@var{NUMBER}]
2270 @itemx --verbose[=@var{NUMBER}]
2271 Display the version number for @command{ld} and list the linker emulations
2272 supported. Display which input files can and cannot be opened. Display
2273 the linker script being used by the linker. If the optional @var{NUMBER}
2274 argument > 1, plugin symbol status will also be displayed.
2276 @kindex --version-script=@var{version-scriptfile}
2277 @cindex version script, symbol versions
2278 @item --version-script=@var{version-scriptfile}
2279 Specify the name of a version script to the linker. This is typically
2280 used when creating shared libraries to specify additional information
2281 about the version hierarchy for the library being created. This option
2282 is only fully supported on ELF platforms which support shared libraries;
2283 see @ref{VERSION}. It is partially supported on PE platforms, which can
2284 use version scripts to filter symbol visibility in auto-export mode: any
2285 symbols marked @samp{local} in the version script will not be exported.
2288 @kindex --warn-common
2289 @cindex warnings, on combining symbols
2290 @cindex combining symbols, warnings on
2292 Warn when a common symbol is combined with another common symbol or with
2293 a symbol definition. Unix linkers allow this somewhat sloppy practice,
2294 but linkers on some other operating systems do not. This option allows
2295 you to find potential problems from combining global symbols.
2296 Unfortunately, some C libraries use this practice, so you may get some
2297 warnings about symbols in the libraries as well as in your programs.
2299 There are three kinds of global symbols, illustrated here by C examples:
2303 A definition, which goes in the initialized data section of the output
2307 An undefined reference, which does not allocate space.
2308 There must be either a definition or a common symbol for the
2312 A common symbol. If there are only (one or more) common symbols for a
2313 variable, it goes in the uninitialized data area of the output file.
2314 The linker merges multiple common symbols for the same variable into a
2315 single symbol. If they are of different sizes, it picks the largest
2316 size. The linker turns a common symbol into a declaration, if there is
2317 a definition of the same variable.
2320 The @samp{--warn-common} option can produce five kinds of warnings.
2321 Each warning consists of a pair of lines: the first describes the symbol
2322 just encountered, and the second describes the previous symbol
2323 encountered with the same name. One or both of the two symbols will be
2328 Turning a common symbol into a reference, because there is already a
2329 definition for the symbol.
2331 @var{file}(@var{section}): warning: common of `@var{symbol}'
2332 overridden by definition
2333 @var{file}(@var{section}): warning: defined here
2337 Turning a common symbol into a reference, because a later definition for
2338 the symbol is encountered. This is the same as the previous case,
2339 except that the symbols are encountered in a different order.
2341 @var{file}(@var{section}): warning: definition of `@var{symbol}'
2343 @var{file}(@var{section}): warning: common is here
2347 Merging a common symbol with a previous same-sized common symbol.
2349 @var{file}(@var{section}): warning: multiple common
2351 @var{file}(@var{section}): warning: previous common is here
2355 Merging a common symbol with a previous larger common symbol.
2357 @var{file}(@var{section}): warning: common of `@var{symbol}'
2358 overridden by larger common
2359 @var{file}(@var{section}): warning: larger common is here
2363 Merging a common symbol with a previous smaller common symbol. This is
2364 the same as the previous case, except that the symbols are
2365 encountered in a different order.
2367 @var{file}(@var{section}): warning: common of `@var{symbol}'
2368 overriding smaller common
2369 @var{file}(@var{section}): warning: smaller common is here
2373 @kindex --warn-constructors
2374 @item --warn-constructors
2375 Warn if any global constructors are used. This is only useful for a few
2376 object file formats. For formats like COFF or ELF, the linker can not
2377 detect the use of global constructors.
2379 @kindex --warn-multiple-gp
2380 @item --warn-multiple-gp
2381 Warn if multiple global pointer values are required in the output file.
2382 This is only meaningful for certain processors, such as the Alpha.
2383 Specifically, some processors put large-valued constants in a special
2384 section. A special register (the global pointer) points into the middle
2385 of this section, so that constants can be loaded efficiently via a
2386 base-register relative addressing mode. Since the offset in
2387 base-register relative mode is fixed and relatively small (e.g., 16
2388 bits), this limits the maximum size of the constant pool. Thus, in
2389 large programs, it is often necessary to use multiple global pointer
2390 values in order to be able to address all possible constants. This
2391 option causes a warning to be issued whenever this case occurs.
2394 @cindex warnings, on undefined symbols
2395 @cindex undefined symbols, warnings on
2397 Only warn once for each undefined symbol, rather than once per module
2400 @kindex --warn-section-align
2401 @cindex warnings, on section alignment
2402 @cindex section alignment, warnings on
2403 @item --warn-section-align
2404 Warn if the address of an output section is changed because of
2405 alignment. Typically, the alignment will be set by an input section.
2406 The address will only be changed if it not explicitly specified; that
2407 is, if the @code{SECTIONS} command does not specify a start address for
2408 the section (@pxref{SECTIONS}).
2410 @kindex --warn-textrel
2411 @item --warn-textrel
2412 Warn if the linker adds DT_TEXTREL to a position-independent executable
2415 @kindex --warn-alternate-em
2416 @item --warn-alternate-em
2417 Warn if an object has alternate ELF machine code.
2419 @kindex --warn-unresolved-symbols
2420 @item --warn-unresolved-symbols
2421 If the linker is going to report an unresolved symbol (see the option
2422 @option{--unresolved-symbols}) it will normally generate an error.
2423 This option makes it generate a warning instead.
2425 @kindex --error-unresolved-symbols
2426 @item --error-unresolved-symbols
2427 This restores the linker's default behaviour of generating errors when
2428 it is reporting unresolved symbols.
2430 @kindex --whole-archive
2431 @cindex including an entire archive
2432 @item --whole-archive
2433 For each archive mentioned on the command line after the
2434 @option{--whole-archive} option, include every object file in the archive
2435 in the link, rather than searching the archive for the required object
2436 files. This is normally used to turn an archive file into a shared
2437 library, forcing every object to be included in the resulting shared
2438 library. This option may be used more than once.
2440 Two notes when using this option from gcc: First, gcc doesn't know
2441 about this option, so you have to use @option{-Wl,-whole-archive}.
2442 Second, don't forget to use @option{-Wl,-no-whole-archive} after your
2443 list of archives, because gcc will add its own list of archives to
2444 your link and you may not want this flag to affect those as well.
2446 @kindex --wrap=@var{symbol}
2447 @item --wrap=@var{symbol}
2448 Use a wrapper function for @var{symbol}. Any undefined reference to
2449 @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any
2450 undefined reference to @code{__real_@var{symbol}} will be resolved to
2453 This can be used to provide a wrapper for a system function. The
2454 wrapper function should be called @code{__wrap_@var{symbol}}. If it
2455 wishes to call the system function, it should call
2456 @code{__real_@var{symbol}}.
2458 Here is a trivial example:
2462 __wrap_malloc (size_t c)
2464 printf ("malloc called with %zu\n", c);
2465 return __real_malloc (c);
2469 If you link other code with this file using @option{--wrap malloc}, then
2470 all calls to @code{malloc} will call the function @code{__wrap_malloc}
2471 instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will
2472 call the real @code{malloc} function.
2474 You may wish to provide a @code{__real_malloc} function as well, so that
2475 links without the @option{--wrap} option will succeed. If you do this,
2476 you should not put the definition of @code{__real_malloc} in the same
2477 file as @code{__wrap_malloc}; if you do, the assembler may resolve the
2478 call before the linker has a chance to wrap it to @code{malloc}.
2480 Only undefined references are replaced by the linker. So, translation unit
2481 internal references to @var{symbol} are not resolved to
2482 @code{__wrap_@var{symbol}}. In the next example, the call to @code{f} in
2483 @code{g} is not resolved to @code{__wrap_f}.
2499 @kindex --eh-frame-hdr
2500 @kindex --no-eh-frame-hdr
2501 @item --eh-frame-hdr
2502 @itemx --no-eh-frame-hdr
2503 Request (@option{--eh-frame-hdr}) or suppress
2504 (@option{--no-eh-frame-hdr}) the creation of @code{.eh_frame_hdr}
2505 section and ELF @code{PT_GNU_EH_FRAME} segment header.
2507 @kindex --ld-generated-unwind-info
2508 @item --no-ld-generated-unwind-info
2509 Request creation of @code{.eh_frame} unwind info for linker
2510 generated code sections like PLT. This option is on by default
2511 if linker generated unwind info is supported.
2513 @kindex --enable-new-dtags
2514 @kindex --disable-new-dtags
2515 @item --enable-new-dtags
2516 @itemx --disable-new-dtags
2517 This linker can create the new dynamic tags in ELF. But the older ELF
2518 systems may not understand them. If you specify
2519 @option{--enable-new-dtags}, the new dynamic tags will be created as needed
2520 and older dynamic tags will be omitted.
2521 If you specify @option{--disable-new-dtags}, no new dynamic tags will be
2522 created. By default, the new dynamic tags are not created. Note that
2523 those options are only available for ELF systems.
2525 @kindex --hash-size=@var{number}
2526 @item --hash-size=@var{number}
2527 Set the default size of the linker's hash tables to a prime number
2528 close to @var{number}. Increasing this value can reduce the length of
2529 time it takes the linker to perform its tasks, at the expense of
2530 increasing the linker's memory requirements. Similarly reducing this
2531 value can reduce the memory requirements at the expense of speed.
2533 @kindex --hash-style=@var{style}
2534 @item --hash-style=@var{style}
2535 Set the type of linker's hash table(s). @var{style} can be either
2536 @code{sysv} for classic ELF @code{.hash} section, @code{gnu} for
2537 new style GNU @code{.gnu.hash} section or @code{both} for both
2538 the classic ELF @code{.hash} and new style GNU @code{.gnu.hash}
2539 hash tables. The default depends upon how the linker was configured,
2540 but for most Linux based systems it will be @code{both}.
2542 @kindex --compress-debug-sections=none
2543 @kindex --compress-debug-sections=zlib
2544 @kindex --compress-debug-sections=zlib-gnu
2545 @kindex --compress-debug-sections=zlib-gabi
2546 @item --compress-debug-sections=none
2547 @itemx --compress-debug-sections=zlib
2548 @itemx --compress-debug-sections=zlib-gnu
2549 @itemx --compress-debug-sections=zlib-gabi
2550 On ELF platforms, these options control how DWARF debug sections are
2551 compressed using zlib.
2553 @option{--compress-debug-sections=none} doesn't compress DWARF debug
2554 sections. @option{--compress-debug-sections=zlib-gnu} compresses
2555 DWARF debug sections and renames them to begin with @samp{.zdebug}
2556 instead of @samp{.debug}. @option{--compress-debug-sections=zlib-gabi}
2557 also compresses DWARF debug sections, but rather than renaming them it
2558 sets the SHF_COMPRESSED flag in the sections' headers.
2560 The @option{--compress-debug-sections=zlib} option is an alias for
2561 @option{--compress-debug-sections=zlib-gabi}.
2563 Note that this option overrides any compression in input debug
2564 sections, so if a binary is linked with @option{--compress-debug-sections=none}
2565 for example, then any compressed debug sections in input files will be
2566 uncompressed before they are copied into the output binary.
2568 The default compression behaviour varies depending upon the target
2569 involved and the configure options used to build the toolchain. The
2570 default can be determined by examining the output from the linker's
2571 @option{--help} option.
2573 @kindex --reduce-memory-overheads
2574 @item --reduce-memory-overheads
2575 This option reduces memory requirements at ld runtime, at the expense of
2576 linking speed. This was introduced to select the old O(n^2) algorithm
2577 for link map file generation, rather than the new O(n) algorithm which uses
2578 about 40% more memory for symbol storage.
2580 Another effect of the switch is to set the default hash table size to
2581 1021, which again saves memory at the cost of lengthening the linker's
2582 run time. This is not done however if the @option{--hash-size} switch
2585 The @option{--reduce-memory-overheads} switch may be also be used to
2586 enable other tradeoffs in future versions of the linker.
2589 @kindex --build-id=@var{style}
2591 @itemx --build-id=@var{style}
2592 Request the creation of a @code{.note.gnu.build-id} ELF note section
2593 or a @code{.buildid} COFF section. The contents of the note are
2594 unique bits identifying this linked file. @var{style} can be
2595 @code{uuid} to use 128 random bits, @code{sha1} to use a 160-bit
2596 @sc{SHA1} hash on the normative parts of the output contents,
2597 @code{md5} to use a 128-bit @sc{MD5} hash on the normative parts of
2598 the output contents, or @code{0x@var{hexstring}} to use a chosen bit
2599 string specified as an even number of hexadecimal digits (@code{-} and
2600 @code{:} characters between digit pairs are ignored). If @var{style}
2601 is omitted, @code{sha1} is used.
2603 The @code{md5} and @code{sha1} styles produces an identifier
2604 that is always the same in an identical output file, but will be
2605 unique among all nonidentical output files. It is not intended
2606 to be compared as a checksum for the file's contents. A linked
2607 file may be changed later by other tools, but the build ID bit
2608 string identifying the original linked file does not change.
2610 Passing @code{none} for @var{style} disables the setting from any
2611 @code{--build-id} options earlier on the command line.
2616 @subsection Options Specific to i386 PE Targets
2618 @c man begin OPTIONS
2620 The i386 PE linker supports the @option{-shared} option, which causes
2621 the output to be a dynamically linked library (DLL) instead of a
2622 normal executable. You should name the output @code{*.dll} when you
2623 use this option. In addition, the linker fully supports the standard
2624 @code{*.def} files, which may be specified on the linker command line
2625 like an object file (in fact, it should precede archives it exports
2626 symbols from, to ensure that they get linked in, just like a normal
2629 In addition to the options common to all targets, the i386 PE linker
2630 support additional command-line options that are specific to the i386
2631 PE target. Options that take values may be separated from their
2632 values by either a space or an equals sign.
2636 @kindex --add-stdcall-alias
2637 @item --add-stdcall-alias
2638 If given, symbols with a stdcall suffix (@@@var{nn}) will be exported
2639 as-is and also with the suffix stripped.
2640 [This option is specific to the i386 PE targeted port of the linker]
2643 @item --base-file @var{file}
2644 Use @var{file} as the name of a file in which to save the base
2645 addresses of all the relocations needed for generating DLLs with
2647 [This is an i386 PE specific option]
2651 Create a DLL instead of a regular executable. You may also use
2652 @option{-shared} or specify a @code{LIBRARY} in a given @code{.def}
2654 [This option is specific to the i386 PE targeted port of the linker]
2656 @kindex --enable-long-section-names
2657 @kindex --disable-long-section-names
2658 @item --enable-long-section-names
2659 @itemx --disable-long-section-names
2660 The PE variants of the COFF object format add an extension that permits
2661 the use of section names longer than eight characters, the normal limit
2662 for COFF. By default, these names are only allowed in object files, as
2663 fully-linked executable images do not carry the COFF string table required
2664 to support the longer names. As a GNU extension, it is possible to
2665 allow their use in executable images as well, or to (probably pointlessly!)
2666 disallow it in object files, by using these two options. Executable images
2667 generated with these long section names are slightly non-standard, carrying
2668 as they do a string table, and may generate confusing output when examined
2669 with non-GNU PE-aware tools, such as file viewers and dumpers. However,
2670 GDB relies on the use of PE long section names to find Dwarf-2 debug
2671 information sections in an executable image at runtime, and so if neither
2672 option is specified on the command-line, @command{ld} will enable long
2673 section names, overriding the default and technically correct behaviour,
2674 when it finds the presence of debug information while linking an executable
2675 image and not stripping symbols.
2676 [This option is valid for all PE targeted ports of the linker]
2678 @kindex --enable-stdcall-fixup
2679 @kindex --disable-stdcall-fixup
2680 @item --enable-stdcall-fixup
2681 @itemx --disable-stdcall-fixup
2682 If the link finds a symbol that it cannot resolve, it will attempt to
2683 do ``fuzzy linking'' by looking for another defined symbol that differs
2684 only in the format of the symbol name (cdecl vs stdcall) and will
2685 resolve that symbol by linking to the match. For example, the
2686 undefined symbol @code{_foo} might be linked to the function
2687 @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked
2688 to the function @code{_bar}. When the linker does this, it prints a
2689 warning, since it normally should have failed to link, but sometimes
2690 import libraries generated from third-party dlls may need this feature
2691 to be usable. If you specify @option{--enable-stdcall-fixup}, this
2692 feature is fully enabled and warnings are not printed. If you specify
2693 @option{--disable-stdcall-fixup}, this feature is disabled and such
2694 mismatches are considered to be errors.
2695 [This option is specific to the i386 PE targeted port of the linker]
2697 @kindex --leading-underscore
2698 @kindex --no-leading-underscore
2699 @item --leading-underscore
2700 @itemx --no-leading-underscore
2701 For most targets default symbol-prefix is an underscore and is defined
2702 in target's description. By this option it is possible to
2703 disable/enable the default underscore symbol-prefix.
2705 @cindex DLLs, creating
2706 @kindex --export-all-symbols
2707 @item --export-all-symbols
2708 If given, all global symbols in the objects used to build a DLL will
2709 be exported by the DLL. Note that this is the default if there
2710 otherwise wouldn't be any exported symbols. When symbols are
2711 explicitly exported via DEF files or implicitly exported via function
2712 attributes, the default is to not export anything else unless this
2713 option is given. Note that the symbols @code{DllMain@@12},
2714 @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and
2715 @code{impure_ptr} will not be automatically
2716 exported. Also, symbols imported from other DLLs will not be
2717 re-exported, nor will symbols specifying the DLL's internal layout
2718 such as those beginning with @code{_head_} or ending with
2719 @code{_iname}. In addition, no symbols from @code{libgcc},
2720 @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported.
2721 Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will
2722 not be exported, to help with C++ DLLs. Finally, there is an
2723 extensive list of cygwin-private symbols that are not exported
2724 (obviously, this applies on when building DLLs for cygwin targets).
2725 These cygwin-excludes are: @code{_cygwin_dll_entry@@12},
2726 @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12},
2727 @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll},
2728 @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2},
2729 @code{cygwin_premain3}, and @code{environ}.
2730 [This option is specific to the i386 PE targeted port of the linker]
2732 @kindex --exclude-symbols
2733 @item --exclude-symbols @var{symbol},@var{symbol},...
2734 Specifies a list of symbols which should not be automatically
2735 exported. The symbol names may be delimited by commas or colons.
2736 [This option is specific to the i386 PE targeted port of the linker]
2738 @kindex --exclude-all-symbols
2739 @item --exclude-all-symbols
2740 Specifies no symbols should be automatically exported.
2741 [This option is specific to the i386 PE targeted port of the linker]
2743 @kindex --file-alignment
2744 @item --file-alignment
2745 Specify the file alignment. Sections in the file will always begin at
2746 file offsets which are multiples of this number. This defaults to
2748 [This option is specific to the i386 PE targeted port of the linker]
2752 @item --heap @var{reserve}
2753 @itemx --heap @var{reserve},@var{commit}
2754 Specify the number of bytes of memory to reserve (and optionally commit)
2755 to be used as heap for this program. The default is 1MB reserved, 4K
2757 [This option is specific to the i386 PE targeted port of the linker]
2760 @kindex --image-base
2761 @item --image-base @var{value}
2762 Use @var{value} as the base address of your program or dll. This is
2763 the lowest memory location that will be used when your program or dll
2764 is loaded. To reduce the need to relocate and improve performance of
2765 your dlls, each should have a unique base address and not overlap any
2766 other dlls. The default is 0x400000 for executables, and 0x10000000
2768 [This option is specific to the i386 PE targeted port of the linker]
2772 If given, the stdcall suffixes (@@@var{nn}) will be stripped from
2773 symbols before they are exported.
2774 [This option is specific to the i386 PE targeted port of the linker]
2776 @kindex --large-address-aware
2777 @item --large-address-aware
2778 If given, the appropriate bit in the ``Characteristics'' field of the COFF
2779 header is set to indicate that this executable supports virtual addresses
2780 greater than 2 gigabytes. This should be used in conjunction with the /3GB
2781 or /USERVA=@var{value} megabytes switch in the ``[operating systems]''
2782 section of the BOOT.INI. Otherwise, this bit has no effect.
2783 [This option is specific to PE targeted ports of the linker]
2785 @kindex --disable-large-address-aware
2786 @item --disable-large-address-aware
2787 Reverts the effect of a previous @samp{--large-address-aware} option.
2788 This is useful if @samp{--large-address-aware} is always set by the compiler
2789 driver (e.g. Cygwin gcc) and the executable does not support virtual
2790 addresses greater than 2 gigabytes.
2791 [This option is specific to PE targeted ports of the linker]
2793 @kindex --major-image-version
2794 @item --major-image-version @var{value}
2795 Sets the major number of the ``image version''. Defaults to 1.
2796 [This option is specific to the i386 PE targeted port of the linker]
2798 @kindex --major-os-version
2799 @item --major-os-version @var{value}
2800 Sets the major number of the ``os version''. Defaults to 4.
2801 [This option is specific to the i386 PE targeted port of the linker]
2803 @kindex --major-subsystem-version
2804 @item --major-subsystem-version @var{value}
2805 Sets the major number of the ``subsystem version''. Defaults to 4.
2806 [This option is specific to the i386 PE targeted port of the linker]
2808 @kindex --minor-image-version
2809 @item --minor-image-version @var{value}
2810 Sets the minor number of the ``image version''. Defaults to 0.
2811 [This option is specific to the i386 PE targeted port of the linker]
2813 @kindex --minor-os-version
2814 @item --minor-os-version @var{value}
2815 Sets the minor number of the ``os version''. Defaults to 0.
2816 [This option is specific to the i386 PE targeted port of the linker]
2818 @kindex --minor-subsystem-version
2819 @item --minor-subsystem-version @var{value}
2820 Sets the minor number of the ``subsystem version''. Defaults to 0.
2821 [This option is specific to the i386 PE targeted port of the linker]
2823 @cindex DEF files, creating
2824 @cindex DLLs, creating
2825 @kindex --output-def
2826 @item --output-def @var{file}
2827 The linker will create the file @var{file} which will contain a DEF
2828 file corresponding to the DLL the linker is generating. This DEF file
2829 (which should be called @code{*.def}) may be used to create an import
2830 library with @code{dlltool} or may be used as a reference to
2831 automatically or implicitly exported symbols.
2832 [This option is specific to the i386 PE targeted port of the linker]
2834 @cindex DLLs, creating
2835 @kindex --enable-auto-image-base
2836 @item --enable-auto-image-base
2837 @itemx --enable-auto-image-base=@var{value}
2838 Automatically choose the image base for DLLs, optionally starting with base
2839 @var{value}, unless one is specified using the @code{--image-base} argument.
2840 By using a hash generated from the dllname to create unique image bases
2841 for each DLL, in-memory collisions and relocations which can delay program
2842 execution are avoided.
2843 [This option is specific to the i386 PE targeted port of the linker]
2845 @kindex --disable-auto-image-base
2846 @item --disable-auto-image-base
2847 Do not automatically generate a unique image base. If there is no
2848 user-specified image base (@code{--image-base}) then use the platform
2850 [This option is specific to the i386 PE targeted port of the linker]
2852 @cindex DLLs, linking to
2853 @kindex --dll-search-prefix
2854 @item --dll-search-prefix @var{string}
2855 When linking dynamically to a dll without an import library,
2856 search for @code{<string><basename>.dll} in preference to
2857 @code{lib<basename>.dll}. This behaviour allows easy distinction
2858 between DLLs built for the various "subplatforms": native, cygwin,
2859 uwin, pw, etc. For instance, cygwin DLLs typically use
2860 @code{--dll-search-prefix=cyg}.
2861 [This option is specific to the i386 PE targeted port of the linker]
2863 @kindex --enable-auto-import
2864 @item --enable-auto-import
2865 Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for
2866 DATA imports from DLLs, thus making it possible to bypass the dllimport
2867 mechanism on the user side and to reference unmangled symbol names.
2868 [This option is specific to the i386 PE targeted port of the linker]
2870 The following remarks pertain to the original implementation of the
2871 feature and are obsolete nowadays for Cygwin and MinGW targets.
2873 Note: Use of the 'auto-import' extension will cause the text section
2874 of the image file to be made writable. This does not conform to the
2875 PE-COFF format specification published by Microsoft.
2877 Note - use of the 'auto-import' extension will also cause read only
2878 data which would normally be placed into the .rdata section to be
2879 placed into the .data section instead. This is in order to work
2880 around a problem with consts that is described here:
2881 http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
2883 Using 'auto-import' generally will 'just work' -- but sometimes you may
2886 "variable '<var>' can't be auto-imported. Please read the
2887 documentation for ld's @code{--enable-auto-import} for details."
2889 This message occurs when some (sub)expression accesses an address
2890 ultimately given by the sum of two constants (Win32 import tables only
2891 allow one). Instances where this may occur include accesses to member
2892 fields of struct variables imported from a DLL, as well as using a
2893 constant index into an array variable imported from a DLL. Any
2894 multiword variable (arrays, structs, long long, etc) may trigger
2895 this error condition. However, regardless of the exact data type
2896 of the offending exported variable, ld will always detect it, issue
2897 the warning, and exit.
2899 There are several ways to address this difficulty, regardless of the
2900 data type of the exported variable:
2902 One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
2903 of adjusting references in your client code for runtime environment, so
2904 this method works only when runtime environment supports this feature.
2906 A second solution is to force one of the 'constants' to be a variable --
2907 that is, unknown and un-optimizable at compile time. For arrays,
2908 there are two possibilities: a) make the indexee (the array's address)
2909 a variable, or b) make the 'constant' index a variable. Thus:
2912 extern type extern_array[];
2914 @{ volatile type *t=extern_array; t[1] @}
2920 extern type extern_array[];
2922 @{ volatile int t=1; extern_array[t] @}
2925 For structs (and most other multiword data types) the only option
2926 is to make the struct itself (or the long long, or the ...) variable:
2929 extern struct s extern_struct;
2930 extern_struct.field -->
2931 @{ volatile struct s *t=&extern_struct; t->field @}
2937 extern long long extern_ll;
2939 @{ volatile long long * local_ll=&extern_ll; *local_ll @}
2942 A third method of dealing with this difficulty is to abandon
2943 'auto-import' for the offending symbol and mark it with
2944 @code{__declspec(dllimport)}. However, in practice that
2945 requires using compile-time #defines to indicate whether you are
2946 building a DLL, building client code that will link to the DLL, or
2947 merely building/linking to a static library. In making the choice
2948 between the various methods of resolving the 'direct address with
2949 constant offset' problem, you should consider typical real-world usage:
2957 void main(int argc, char **argv)@{
2958 printf("%d\n",arr[1]);
2968 void main(int argc, char **argv)@{
2969 /* This workaround is for win32 and cygwin; do not "optimize" */
2970 volatile int *parr = arr;
2971 printf("%d\n",parr[1]);
2978 /* Note: auto-export is assumed (no __declspec(dllexport)) */
2979 #if (defined(_WIN32) || defined(__CYGWIN__)) && \
2980 !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
2981 #define FOO_IMPORT __declspec(dllimport)
2985 extern FOO_IMPORT int arr[];
2988 void main(int argc, char **argv)@{
2989 printf("%d\n",arr[1]);
2993 A fourth way to avoid this problem is to re-code your
2994 library to use a functional interface rather than a data interface
2995 for the offending variables (e.g. set_foo() and get_foo() accessor
2998 @kindex --disable-auto-import
2999 @item --disable-auto-import
3000 Do not attempt to do sophisticated linking of @code{_symbol} to
3001 @code{__imp__symbol} for DATA imports from DLLs.
3002 [This option is specific to the i386 PE targeted port of the linker]
3004 @kindex --enable-runtime-pseudo-reloc
3005 @item --enable-runtime-pseudo-reloc
3006 If your code contains expressions described in --enable-auto-import section,
3007 that is, DATA imports from DLL with non-zero offset, this switch will create
3008 a vector of 'runtime pseudo relocations' which can be used by runtime
3009 environment to adjust references to such data in your client code.
3010 [This option is specific to the i386 PE targeted port of the linker]
3012 @kindex --disable-runtime-pseudo-reloc
3013 @item --disable-runtime-pseudo-reloc
3014 Do not create pseudo relocations for non-zero offset DATA imports from DLLs.
3015 [This option is specific to the i386 PE targeted port of the linker]
3017 @kindex --enable-extra-pe-debug
3018 @item --enable-extra-pe-debug
3019 Show additional debug info related to auto-import symbol thunking.
3020 [This option is specific to the i386 PE targeted port of the linker]
3022 @kindex --section-alignment
3023 @item --section-alignment
3024 Sets the section alignment. Sections in memory will always begin at
3025 addresses which are a multiple of this number. Defaults to 0x1000.
3026 [This option is specific to the i386 PE targeted port of the linker]
3030 @item --stack @var{reserve}
3031 @itemx --stack @var{reserve},@var{commit}
3032 Specify the number of bytes of memory to reserve (and optionally commit)
3033 to be used as stack for this program. The default is 2MB reserved, 4K
3035 [This option is specific to the i386 PE targeted port of the linker]
3038 @item --subsystem @var{which}
3039 @itemx --subsystem @var{which}:@var{major}
3040 @itemx --subsystem @var{which}:@var{major}.@var{minor}
3041 Specifies the subsystem under which your program will execute. The
3042 legal values for @var{which} are @code{native}, @code{windows},
3043 @code{console}, @code{posix}, and @code{xbox}. You may optionally set
3044 the subsystem version also. Numeric values are also accepted for
3046 [This option is specific to the i386 PE targeted port of the linker]
3048 The following options set flags in the @code{DllCharacteristics} field
3049 of the PE file header:
3050 [These options are specific to PE targeted ports of the linker]
3052 @kindex --high-entropy-va
3053 @item --high-entropy-va
3054 Image is compatible with 64-bit address space layout randomization
3056 This option also implies @option{--dynamicbase} and
3057 @option{--enable-reloc-section}.
3059 @kindex --dynamicbase
3061 The image base address may be relocated using address space layout
3062 randomization (ASLR). This feature was introduced with MS Windows
3063 Vista for i386 PE targets.
3064 This option also implies @option{--enable-reloc-section}.
3066 @kindex --forceinteg
3068 Code integrity checks are enforced.
3072 The image is compatible with the Data Execution Prevention.
3073 This feature was introduced with MS Windows XP SP2 for i386 PE targets.
3075 @kindex --no-isolation
3076 @item --no-isolation
3077 Although the image understands isolation, do not isolate the image.
3081 The image does not use SEH. No SE handler may be called from
3086 Do not bind this image.
3090 The driver uses the MS Windows Driver Model.
3094 The image is Terminal Server aware.
3096 @kindex --insert-timestamp
3097 @item --insert-timestamp
3098 @itemx --no-insert-timestamp
3099 Insert a real timestamp into the image. This is the default behaviour
3100 as it matches legacy code and it means that the image will work with
3101 other, proprietary tools. The problem with this default is that it
3102 will result in slightly different images being produced each time the
3103 same sources are linked. The option @option{--no-insert-timestamp}
3104 can be used to insert a zero value for the timestamp, this ensuring
3105 that binaries produced from identical sources will compare
3108 @kindex --enable-reloc-section
3109 @item --enable-reloc-section
3110 Create the base relocation table, which is necessary if the image
3111 is loaded at a different image base than specified in the PE header.
3117 @subsection Options specific to C6X uClinux targets
3119 @c man begin OPTIONS
3121 The C6X uClinux target uses a binary format called DSBT to support shared
3122 libraries. Each shared library in the system needs to have a unique index;
3123 all executables use an index of 0.
3128 @item --dsbt-size @var{size}
3129 This option sets the number of entries in the DSBT of the current executable
3130 or shared library to @var{size}. The default is to create a table with 64
3133 @kindex --dsbt-index
3134 @item --dsbt-index @var{index}
3135 This option sets the DSBT index of the current executable or shared library
3136 to @var{index}. The default is 0, which is appropriate for generating
3137 executables. If a shared library is generated with a DSBT index of 0, the
3138 @code{R_C6000_DSBT_INDEX} relocs are copied into the output file.
3140 @kindex --no-merge-exidx-entries
3141 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent
3142 exidx entries in frame unwind info.
3150 @subsection Options specific to C-SKY targets
3152 @c man begin OPTIONS
3156 @kindex --branch-stub on C-SKY
3158 This option enables linker branch relaxation by inserting branch stub
3159 sections when needed to extend the range of branches. This option is
3160 usually not required since C-SKY supports branch and call instructions that
3161 can access the full memory range and branch relaxation is normally handled by
3162 the compiler or assembler.
3164 @kindex --stub-group-size on C-SKY
3165 @item --stub-group-size=@var{N}
3166 This option allows finer control of linker branch stub creation.
3167 It sets the maximum size of a group of input sections that can
3168 be handled by one stub section. A negative value of @var{N} locates
3169 stub sections after their branches, while a positive value allows stub
3170 sections to appear either before or after the branches. Values of
3171 @samp{1} or @samp{-1} indicate that the
3172 linker should choose suitable defaults.
3180 @subsection Options specific to Motorola 68HC11 and 68HC12 targets
3182 @c man begin OPTIONS
3184 The 68HC11 and 68HC12 linkers support specific options to control the
3185 memory bank switching mapping and trampoline code generation.
3189 @kindex --no-trampoline
3190 @item --no-trampoline
3191 This option disables the generation of trampoline. By default a trampoline
3192 is generated for each far function which is called using a @code{jsr}
3193 instruction (this happens when a pointer to a far function is taken).
3195 @kindex --bank-window
3196 @item --bank-window @var{name}
3197 This option indicates to the linker the name of the memory region in
3198 the @samp{MEMORY} specification that describes the memory bank window.
3199 The definition of such region is then used by the linker to compute
3200 paging and addresses within the memory window.
3208 @subsection Options specific to Motorola 68K target
3210 @c man begin OPTIONS
3212 The following options are supported to control handling of GOT generation
3213 when linking for 68K targets.
3218 @item --got=@var{type}
3219 This option tells the linker which GOT generation scheme to use.
3220 @var{type} should be one of @samp{single}, @samp{negative},
3221 @samp{multigot} or @samp{target}. For more information refer to the
3222 Info entry for @file{ld}.
3230 @subsection Options specific to MIPS targets
3232 @c man begin OPTIONS
3234 The following options are supported to control microMIPS instruction
3235 generation and branch relocation checks for ISA mode transitions when
3236 linking for MIPS targets.
3244 These options control the choice of microMIPS instructions used in code
3245 generated by the linker, such as that in the PLT or lazy binding stubs,
3246 or in relaxation. If @samp{--insn32} is used, then the linker only uses
3247 32-bit instruction encodings. By default or if @samp{--no-insn32} is
3248 used, all instruction encodings are used, including 16-bit ones where
3251 @kindex --ignore-branch-isa
3252 @item --ignore-branch-isa
3253 @kindex --no-ignore-branch-isa
3254 @itemx --no-ignore-branch-isa
3255 These options control branch relocation checks for invalid ISA mode
3256 transitions. If @samp{--ignore-branch-isa} is used, then the linker
3257 accepts any branch relocations and any ISA mode transition required
3258 is lost in relocation calculation, except for some cases of @code{BAL}
3259 instructions which meet relaxation conditions and are converted to
3260 equivalent @code{JALX} instructions as the associated relocation is
3261 calculated. By default or if @samp{--no-ignore-branch-isa} is used
3262 a check is made causing the loss of an ISA mode transition to produce
3265 @kindex --compact-branches
3266 @item --compact-branches
3267 @kindex --no-compact-branches
3268 @itemx --no-compact-branches
3269 These options control the generation of compact instructions by the linker
3270 in the PLT entries for MIPS R6.
3279 @subsection Options specific to PDP11 targets
3281 @c man begin OPTIONS
3283 For the pdp11-aout target, three variants of the output format can be
3284 produced as selected by the following options. The default variant
3285 for pdp11-aout is the @samp{--omagic} option, whereas for other
3286 targets @samp{--nmagic} is the default. The @samp{--imagic} option is
3287 defined only for the pdp11-aout target, while the others are described
3288 here as they apply to the pdp11-aout target.
3297 Mark the output as @code{OMAGIC} (0407) in the @file{a.out} header to
3298 indicate that the text segment is not to be write-protected and
3299 shared. Since the text and data sections are both readable and
3300 writable, the data section is allocated immediately contiguous after
3301 the text segment. This is the oldest format for PDP11 executable
3302 programs and is the default for @command{ld} on PDP11 Unix systems
3303 from the beginning through 2.11BSD.
3310 Mark the output as @code{NMAGIC} (0410) in the @file{a.out} header to
3311 indicate that when the output file is executed, the text portion will
3312 be read-only and shareable among all processes executing the same
3313 file. This involves moving the data areas up to the first possible 8K
3314 byte page boundary following the end of the text. This option creates
3315 a @emph{pure executable} format.
3322 Mark the output as @code{IMAGIC} (0411) in the @file{a.out} header to
3323 indicate that when the output file is executed, the program text and
3324 data areas will be loaded into separate address spaces using the split
3325 instruction and data space feature of the memory management unit in
3326 larger models of the PDP11. This doubles the address space available
3327 to the program. The text segment is again pure, write-protected, and
3328 shareable. The only difference in the output format between this
3329 option and the others, besides the magic number, is that both the text
3330 and data sections start at location 0. The @samp{-z} option selected
3331 this format in 2.11BSD. This option creates a @emph{separate
3337 Equivalent to @samp{--nmagic} for pdp11-aout.
3346 @section Environment Variables
3348 @c man begin ENVIRONMENT
3350 You can change the behaviour of @command{ld} with the environment variables
3351 @ifclear SingleFormat
3354 @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}.
3356 @ifclear SingleFormat
3358 @cindex default input format
3359 @code{GNUTARGET} determines the input-file object format if you don't
3360 use @samp{-b} (or its synonym @samp{--format}). Its value should be one
3361 of the BFD names for an input format (@pxref{BFD}). If there is no
3362 @code{GNUTARGET} in the environment, @command{ld} uses the natural format
3363 of the target. If @code{GNUTARGET} is set to @code{default} then BFD
3364 attempts to discover the input format by examining binary input files;
3365 this method often succeeds, but there are potential ambiguities, since
3366 there is no method of ensuring that the magic number used to specify
3367 object-file formats is unique. However, the configuration procedure for
3368 BFD on each system places the conventional format for that system first
3369 in the search-list, so ambiguities are resolved in favor of convention.
3373 @cindex default emulation
3374 @cindex emulation, default
3375 @code{LDEMULATION} determines the default emulation if you don't use the
3376 @samp{-m} option. The emulation can affect various aspects of linker
3377 behaviour, particularly the default linker script. You can list the
3378 available emulations with the @samp{--verbose} or @samp{-V} options. If
3379 the @samp{-m} option is not used, and the @code{LDEMULATION} environment
3380 variable is not defined, the default emulation depends upon how the
3381 linker was configured.
3383 @kindex COLLECT_NO_DEMANGLE
3384 @cindex demangling, default
3385 Normally, the linker will default to demangling symbols. However, if
3386 @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will
3387 default to not demangling symbols. This environment variable is used in
3388 a similar fashion by the @code{gcc} linker wrapper program. The default
3389 may be overridden by the @samp{--demangle} and @samp{--no-demangle}
3396 @chapter Linker Scripts
3399 @cindex linker scripts
3400 @cindex command files
3401 Every link is controlled by a @dfn{linker script}. This script is
3402 written in the linker command language.
3404 The main purpose of the linker script is to describe how the sections in
3405 the input files should be mapped into the output file, and to control
3406 the memory layout of the output file. Most linker scripts do nothing
3407 more than this. However, when necessary, the linker script can also
3408 direct the linker to perform many other operations, using the commands
3411 The linker always uses a linker script. If you do not supply one
3412 yourself, the linker will use a default script that is compiled into the
3413 linker executable. You can use the @samp{--verbose} command-line option
3414 to display the default linker script. Certain command-line options,
3415 such as @samp{-r} or @samp{-N}, will affect the default linker script.
3417 You may supply your own linker script by using the @samp{-T} command
3418 line option. When you do this, your linker script will replace the
3419 default linker script.
3421 You may also use linker scripts implicitly by naming them as input files
3422 to the linker, as though they were files to be linked. @xref{Implicit
3426 * Basic Script Concepts:: Basic Linker Script Concepts
3427 * Script Format:: Linker Script Format
3428 * Simple Example:: Simple Linker Script Example
3429 * Simple Commands:: Simple Linker Script Commands
3430 * Assignments:: Assigning Values to Symbols
3431 * SECTIONS:: SECTIONS Command
3432 * MEMORY:: MEMORY Command
3433 * PHDRS:: PHDRS Command
3434 * VERSION:: VERSION Command
3435 * Expressions:: Expressions in Linker Scripts
3436 * Implicit Linker Scripts:: Implicit Linker Scripts
3439 @node Basic Script Concepts
3440 @section Basic Linker Script Concepts
3441 @cindex linker script concepts
3442 We need to define some basic concepts and vocabulary in order to
3443 describe the linker script language.
3445 The linker combines input files into a single output file. The output
3446 file and each input file are in a special data format known as an
3447 @dfn{object file format}. Each file is called an @dfn{object file}.
3448 The output file is often called an @dfn{executable}, but for our
3449 purposes we will also call it an object file. Each object file has,
3450 among other things, a list of @dfn{sections}. We sometimes refer to a
3451 section in an input file as an @dfn{input section}; similarly, a section
3452 in the output file is an @dfn{output section}.
3454 Each section in an object file has a name and a size. Most sections
3455 also have an associated block of data, known as the @dfn{section
3456 contents}. A section may be marked as @dfn{loadable}, which means that
3457 the contents should be loaded into memory when the output file is run.
3458 A section with no contents may be @dfn{allocatable}, which means that an
3459 area in memory should be set aside, but nothing in particular should be
3460 loaded there (in some cases this memory must be zeroed out). A section
3461 which is neither loadable nor allocatable typically contains some sort
3462 of debugging information.
3464 Every loadable or allocatable output section has two addresses. The
3465 first is the @dfn{VMA}, or virtual memory address. This is the address
3466 the section will have when the output file is run. The second is the
3467 @dfn{LMA}, or load memory address. This is the address at which the
3468 section will be loaded. In most cases the two addresses will be the
3469 same. An example of when they might be different is when a data section
3470 is loaded into ROM, and then copied into RAM when the program starts up
3471 (this technique is often used to initialize global variables in a ROM
3472 based system). In this case the ROM address would be the LMA, and the
3473 RAM address would be the VMA.
3475 You can see the sections in an object file by using the @code{objdump}
3476 program with the @samp{-h} option.
3478 Every object file also has a list of @dfn{symbols}, known as the
3479 @dfn{symbol table}. A symbol may be defined or undefined. Each symbol
3480 has a name, and each defined symbol has an address, among other
3481 information. If you compile a C or C++ program into an object file, you
3482 will get a defined symbol for every defined function and global or
3483 static variable. Every undefined function or global variable which is
3484 referenced in the input file will become an undefined symbol.
3486 You can see the symbols in an object file by using the @code{nm}
3487 program, or by using the @code{objdump} program with the @samp{-t}
3491 @section Linker Script Format
3492 @cindex linker script format
3493 Linker scripts are text files.
3495 You write a linker script as a series of commands. Each command is
3496 either a keyword, possibly followed by arguments, or an assignment to a
3497 symbol. You may separate commands using semicolons. Whitespace is
3500 Strings such as file or format names can normally be entered directly.
3501 If the file name contains a character such as a comma which would
3502 otherwise serve to separate file names, you may put the file name in
3503 double quotes. There is no way to use a double quote character in a
3506 You may include comments in linker scripts just as in C, delimited by
3507 @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent
3510 @node Simple Example
3511 @section Simple Linker Script Example
3512 @cindex linker script example
3513 @cindex example of linker script
3514 Many linker scripts are fairly simple.
3516 The simplest possible linker script has just one command:
3517 @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the
3518 memory layout of the output file.
3520 The @samp{SECTIONS} command is a powerful command. Here we will
3521 describe a simple use of it. Let's assume your program consists only of
3522 code, initialized data, and uninitialized data. These will be in the
3523 @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively.
3524 Let's assume further that these are the only sections which appear in
3527 For this example, let's say that the code should be loaded at address
3528 0x10000, and that the data should start at address 0x8000000. Here is a
3529 linker script which will do that:
3534 .text : @{ *(.text) @}
3536 .data : @{ *(.data) @}
3537 .bss : @{ *(.bss) @}
3541 You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS},
3542 followed by a series of symbol assignments and output section
3543 descriptions enclosed in curly braces.
3545 The first line inside the @samp{SECTIONS} command of the above example
3546 sets the value of the special symbol @samp{.}, which is the location
3547 counter. If you do not specify the address of an output section in some
3548 other way (other ways are described later), the address is set from the
3549 current value of the location counter. The location counter is then
3550 incremented by the size of the output section. At the start of the
3551 @samp{SECTIONS} command, the location counter has the value @samp{0}.
3553 The second line defines an output section, @samp{.text}. The colon is
3554 required syntax which may be ignored for now. Within the curly braces
3555 after the output section name, you list the names of the input sections
3556 which should be placed into this output section. The @samp{*} is a
3557 wildcard which matches any file name. The expression @samp{*(.text)}
3558 means all @samp{.text} input sections in all input files.
3560 Since the location counter is @samp{0x10000} when the output section
3561 @samp{.text} is defined, the linker will set the address of the
3562 @samp{.text} section in the output file to be @samp{0x10000}.
3564 The remaining lines define the @samp{.data} and @samp{.bss} sections in
3565 the output file. The linker will place the @samp{.data} output section
3566 at address @samp{0x8000000}. After the linker places the @samp{.data}
3567 output section, the value of the location counter will be
3568 @samp{0x8000000} plus the size of the @samp{.data} output section. The
3569 effect is that the linker will place the @samp{.bss} output section
3570 immediately after the @samp{.data} output section in memory.
3572 The linker will ensure that each output section has the required
3573 alignment, by increasing the location counter if necessary. In this
3574 example, the specified addresses for the @samp{.text} and @samp{.data}
3575 sections will probably satisfy any alignment constraints, but the linker
3576 may have to create a small gap between the @samp{.data} and @samp{.bss}
3579 That's it! That's a simple and complete linker script.
3581 @node Simple Commands
3582 @section Simple Linker Script Commands
3583 @cindex linker script simple commands
3584 In this section we describe the simple linker script commands.
3587 * Entry Point:: Setting the entry point
3588 * File Commands:: Commands dealing with files
3589 @ifclear SingleFormat
3590 * Format Commands:: Commands dealing with object file formats
3593 * REGION_ALIAS:: Assign alias names to memory regions
3594 * Miscellaneous Commands:: Other linker script commands
3598 @subsection Setting the Entry Point
3599 @kindex ENTRY(@var{symbol})
3600 @cindex start of execution
3601 @cindex first instruction
3603 The first instruction to execute in a program is called the @dfn{entry
3604 point}. You can use the @code{ENTRY} linker script command to set the
3605 entry point. The argument is a symbol name:
3610 There are several ways to set the entry point. The linker will set the
3611 entry point by trying each of the following methods in order, and
3612 stopping when one of them succeeds:
3615 the @samp{-e} @var{entry} command-line option;
3617 the @code{ENTRY(@var{symbol})} command in a linker script;
3619 the value of a target-specific symbol, if it is defined; For many
3620 targets this is @code{start}, but PE- and BeOS-based systems for example
3621 check a list of possible entry symbols, matching the first one found.
3623 the address of the first byte of the @samp{.text} section, if present;
3625 The address @code{0}.
3629 @subsection Commands Dealing with Files
3630 @cindex linker script file commands
3631 Several linker script commands deal with files.
3634 @item INCLUDE @var{filename}
3635 @kindex INCLUDE @var{filename}
3636 @cindex including a linker script
3637 Include the linker script @var{filename} at this point. The file will
3638 be searched for in the current directory, and in any directory specified
3639 with the @option{-L} option. You can nest calls to @code{INCLUDE} up to
3642 You can place @code{INCLUDE} directives at the top level, in @code{MEMORY} or
3643 @code{SECTIONS} commands, or in output section descriptions.
3645 @item INPUT(@var{file}, @var{file}, @dots{})
3646 @itemx INPUT(@var{file} @var{file} @dots{})
3647 @kindex INPUT(@var{files})
3648 @cindex input files in linker scripts
3649 @cindex input object files in linker scripts
3650 @cindex linker script input object files
3651 The @code{INPUT} command directs the linker to include the named files
3652 in the link, as though they were named on the command line.
3654 For example, if you always want to include @file{subr.o} any time you do
3655 a link, but you can't be bothered to put it on every link command line,
3656 then you can put @samp{INPUT (subr.o)} in your linker script.
3658 In fact, if you like, you can list all of your input files in the linker
3659 script, and then invoke the linker with nothing but a @samp{-T} option.
3661 In case a @dfn{sysroot prefix} is configured, and the filename starts
3662 with the @samp{/} character, and the script being processed was
3663 located inside the @dfn{sysroot prefix}, the filename will be looked
3664 for in the @dfn{sysroot prefix}. The @dfn{sysroot prefix} can also be forced by specifying
3665 @code{=} as the first character in the filename path, or prefixing the
3666 filename path with @code{$SYSROOT}. See also the description of
3667 @samp{-L} in @ref{Options,,Command-line Options}.
3669 If a @dfn{sysroot prefix} is not used then the linker will try to open
3670 the file in the directory containing the linker script. If it is not
3671 found the linker will then search the current directory. If it is still
3672 not found the linker will search through the archive library search
3675 If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the
3676 name to @code{lib@var{file}.a}, as with the command-line argument
3679 When you use the @code{INPUT} command in an implicit linker script, the
3680 files will be included in the link at the point at which the linker
3681 script file is included. This can affect archive searching.
3683 @item GROUP(@var{file}, @var{file}, @dots{})
3684 @itemx GROUP(@var{file} @var{file} @dots{})
3685 @kindex GROUP(@var{files})
3686 @cindex grouping input files
3687 The @code{GROUP} command is like @code{INPUT}, except that the named
3688 files should all be archives, and they are searched repeatedly until no
3689 new undefined references are created. See the description of @samp{-(}
3690 in @ref{Options,,Command-line Options}.
3692 @item AS_NEEDED(@var{file}, @var{file}, @dots{})
3693 @itemx AS_NEEDED(@var{file} @var{file} @dots{})
3694 @kindex AS_NEEDED(@var{files})
3695 This construct can appear only inside of the @code{INPUT} or @code{GROUP}
3696 commands, among other filenames. The files listed will be handled
3697 as if they appear directly in the @code{INPUT} or @code{GROUP} commands,
3698 with the exception of ELF shared libraries, that will be added only
3699 when they are actually needed. This construct essentially enables
3700 @option{--as-needed} option for all the files listed inside of it
3701 and restores previous @option{--as-needed} resp. @option{--no-as-needed}
3704 @item OUTPUT(@var{filename})
3705 @kindex OUTPUT(@var{filename})
3706 @cindex output file name in linker script
3707 The @code{OUTPUT} command names the output file. Using
3708 @code{OUTPUT(@var{filename})} in the linker script is exactly like using
3709 @samp{-o @var{filename}} on the command line (@pxref{Options,,Command
3710 Line Options}). If both are used, the command-line option takes
3713 You can use the @code{OUTPUT} command to define a default name for the
3714 output file other than the usual default of @file{a.out}.
3716 @item SEARCH_DIR(@var{path})
3717 @kindex SEARCH_DIR(@var{path})
3718 @cindex library search path in linker script
3719 @cindex archive search path in linker script
3720 @cindex search path in linker script
3721 The @code{SEARCH_DIR} command adds @var{path} to the list of paths where
3722 @command{ld} looks for archive libraries. Using
3723 @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}}
3724 on the command line (@pxref{Options,,Command-line Options}). If both
3725 are used, then the linker will search both paths. Paths specified using
3726 the command-line option are searched first.
3728 @item STARTUP(@var{filename})
3729 @kindex STARTUP(@var{filename})
3730 @cindex first input file
3731 The @code{STARTUP} command is just like the @code{INPUT} command, except
3732 that @var{filename} will become the first input file to be linked, as
3733 though it were specified first on the command line. This may be useful
3734 when using a system in which the entry point is always the start of the
3738 @ifclear SingleFormat
3739 @node Format Commands
3740 @subsection Commands Dealing with Object File Formats
3741 A couple of linker script commands deal with object file formats.
3744 @item OUTPUT_FORMAT(@var{bfdname})
3745 @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little})
3746 @kindex OUTPUT_FORMAT(@var{bfdname})
3747 @cindex output file format in linker script
3748 The @code{OUTPUT_FORMAT} command names the BFD format to use for the
3749 output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is
3750 exactly like using @samp{--oformat @var{bfdname}} on the command line
3751 (@pxref{Options,,Command-line Options}). If both are used, the command
3752 line option takes precedence.
3754 You can use @code{OUTPUT_FORMAT} with three arguments to use different
3755 formats based on the @samp{-EB} and @samp{-EL} command-line options.
3756 This permits the linker script to set the output format based on the
3759 If neither @samp{-EB} nor @samp{-EL} are used, then the output format
3760 will be the first argument, @var{default}. If @samp{-EB} is used, the
3761 output format will be the second argument, @var{big}. If @samp{-EL} is
3762 used, the output format will be the third argument, @var{little}.
3764 For example, the default linker script for the MIPS ELF target uses this
3767 OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
3769 This says that the default format for the output file is
3770 @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command-line
3771 option, the output file will be created in the @samp{elf32-littlemips}
3774 @item TARGET(@var{bfdname})
3775 @kindex TARGET(@var{bfdname})
3776 @cindex input file format in linker script
3777 The @code{TARGET} command names the BFD format to use when reading input
3778 files. It affects subsequent @code{INPUT} and @code{GROUP} commands.
3779 This command is like using @samp{-b @var{bfdname}} on the command line
3780 (@pxref{Options,,Command-line Options}). If the @code{TARGET} command
3781 is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET}
3782 command is also used to set the format for the output file. @xref{BFD}.
3787 @subsection Assign alias names to memory regions
3788 @kindex REGION_ALIAS(@var{alias}, @var{region})
3789 @cindex region alias
3790 @cindex region names
3792 Alias names can be added to existing memory regions created with the
3793 @ref{MEMORY} command. Each name corresponds to at most one memory region.
3796 REGION_ALIAS(@var{alias}, @var{region})
3799 The @code{REGION_ALIAS} function creates an alias name @var{alias} for the
3800 memory region @var{region}. This allows a flexible mapping of output sections
3801 to memory regions. An example follows.
3803 Suppose we have an application for embedded systems which come with various
3804 memory storage devices. All have a general purpose, volatile memory @code{RAM}
3805 that allows code execution or data storage. Some may have a read-only,
3806 non-volatile memory @code{ROM} that allows code execution and read-only data
3807 access. The last variant is a read-only, non-volatile memory @code{ROM2} with
3808 read-only data access and no code execution capability. We have four output
3813 @code{.text} program code;
3815 @code{.rodata} read-only data;
3817 @code{.data} read-write initialized data;
3819 @code{.bss} read-write zero initialized data.
3822 The goal is to provide a linker command file that contains a system independent
3823 part defining the output sections and a system dependent part mapping the
3824 output sections to the memory regions available on the system. Our embedded
3825 systems come with three different memory setups @code{A}, @code{B} and
3827 @multitable @columnfractions .25 .25 .25 .25
3828 @item Section @tab Variant A @tab Variant B @tab Variant C
3829 @item .text @tab RAM @tab ROM @tab ROM
3830 @item .rodata @tab RAM @tab ROM @tab ROM2
3831 @item .data @tab RAM @tab RAM/ROM @tab RAM/ROM2
3832 @item .bss @tab RAM @tab RAM @tab RAM
3834 The notation @code{RAM/ROM} or @code{RAM/ROM2} means that this section is
3835 loaded into region @code{ROM} or @code{ROM2} respectively. Please note that
3836 the load address of the @code{.data} section starts in all three variants at
3837 the end of the @code{.rodata} section.
3839 The base linker script that deals with the output sections follows. It
3840 includes the system dependent @code{linkcmds.memory} file that describes the
3843 INCLUDE linkcmds.memory
3856 .data : AT (rodata_end)
3861 data_size = SIZEOF(.data);
3862 data_load_start = LOADADDR(.data);
3870 Now we need three different @code{linkcmds.memory} files to define memory
3871 regions and alias names. The content of @code{linkcmds.memory} for the three
3872 variants @code{A}, @code{B} and @code{C}:
3875 Here everything goes into the @code{RAM}.
3879 RAM : ORIGIN = 0, LENGTH = 4M
3882 REGION_ALIAS("REGION_TEXT", RAM);
3883 REGION_ALIAS("REGION_RODATA", RAM);
3884 REGION_ALIAS("REGION_DATA", RAM);
3885 REGION_ALIAS("REGION_BSS", RAM);
3888 Program code and read-only data go into the @code{ROM}. Read-write data goes
3889 into the @code{RAM}. An image of the initialized data is loaded into the
3890 @code{ROM} and will be copied during system start into the @code{RAM}.
3894 ROM : ORIGIN = 0, LENGTH = 3M
3895 RAM : ORIGIN = 0x10000000, LENGTH = 1M
3898 REGION_ALIAS("REGION_TEXT", ROM);
3899 REGION_ALIAS("REGION_RODATA", ROM);
3900 REGION_ALIAS("REGION_DATA", RAM);
3901 REGION_ALIAS("REGION_BSS", RAM);
3904 Program code goes into the @code{ROM}. Read-only data goes into the
3905 @code{ROM2}. Read-write data goes into the @code{RAM}. An image of the
3906 initialized data is loaded into the @code{ROM2} and will be copied during
3907 system start into the @code{RAM}.
3911 ROM : ORIGIN = 0, LENGTH = 2M
3912 ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
3913 RAM : ORIGIN = 0x20000000, LENGTH = 1M
3916 REGION_ALIAS("REGION_TEXT", ROM);
3917 REGION_ALIAS("REGION_RODATA", ROM2);
3918 REGION_ALIAS("REGION_DATA", RAM);
3919 REGION_ALIAS("REGION_BSS", RAM);
3923 It is possible to write a common system initialization routine to copy the
3924 @code{.data} section from @code{ROM} or @code{ROM2} into the @code{RAM} if
3929 extern char data_start [];
3930 extern char data_size [];
3931 extern char data_load_start [];
3933 void copy_data(void)
3935 if (data_start != data_load_start)
3937 memcpy(data_start, data_load_start, (size_t) data_size);
3942 @node Miscellaneous Commands
3943 @subsection Other Linker Script Commands
3944 There are a few other linker scripts commands.
3947 @item ASSERT(@var{exp}, @var{message})
3949 @cindex assertion in linker script
3950 Ensure that @var{exp} is non-zero. If it is zero, then exit the linker
3951 with an error code, and print @var{message}.
3953 Note that assertions are checked before the final stages of linking
3954 take place. This means that expressions involving symbols PROVIDEd
3955 inside section definitions will fail if the user has not set values
3956 for those symbols. The only exception to this rule is PROVIDEd
3957 symbols that just reference dot. Thus an assertion like this:
3962 PROVIDE (__stack = .);
3963 PROVIDE (__stack_size = 0x100);
3964 ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
3968 will fail if @code{__stack_size} is not defined elsewhere. Symbols
3969 PROVIDEd outside of section definitions are evaluated earlier, so they
3970 can be used inside ASSERTions. Thus:
3973 PROVIDE (__stack_size = 0x100);
3976 PROVIDE (__stack = .);
3977 ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
3983 @item EXTERN(@var{symbol} @var{symbol} @dots{})
3985 @cindex undefined symbol in linker script
3986 Force @var{symbol} to be entered in the output file as an undefined
3987 symbol. Doing this may, for example, trigger linking of additional
3988 modules from standard libraries. You may list several @var{symbol}s for
3989 each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This
3990 command has the same effect as the @samp{-u} command-line option.
3992 @item FORCE_COMMON_ALLOCATION
3993 @kindex FORCE_COMMON_ALLOCATION
3994 @cindex common allocation in linker script
3995 This command has the same effect as the @samp{-d} command-line option:
3996 to make @command{ld} assign space to common symbols even if a relocatable
3997 output file is specified (@samp{-r}).
3999 @item INHIBIT_COMMON_ALLOCATION
4000 @kindex INHIBIT_COMMON_ALLOCATION
4001 @cindex common allocation in linker script
4002 This command has the same effect as the @samp{--no-define-common}
4003 command-line option: to make @code{ld} omit the assignment of addresses
4004 to common symbols even for a non-relocatable output file.
4006 @item FORCE_GROUP_ALLOCATION
4007 @kindex FORCE_GROUP_ALLOCATION
4008 @cindex group allocation in linker script
4009 @cindex section groups
4011 This command has the same effect as the
4012 @samp{--force-group-allocation} command-line option: to make
4013 @command{ld} place section group members like normal input sections,
4014 and to delete the section groups even if a relocatable output file is
4015 specified (@samp{-r}).
4017 @item INSERT [ AFTER | BEFORE ] @var{output_section}
4019 @cindex insert user script into default script
4020 This command is typically used in a script specified by @samp{-T} to
4021 augment the default @code{SECTIONS} with, for example, overlays. It
4022 inserts all prior linker script statements after (or before)
4023 @var{output_section}, and also causes @samp{-T} to not override the
4024 default linker script. The exact insertion point is as for orphan
4025 sections. @xref{Location Counter}. The insertion happens after the
4026 linker has mapped input sections to output sections. Prior to the
4027 insertion, since @samp{-T} scripts are parsed before the default
4028 linker script, statements in the @samp{-T} script occur before the
4029 default linker script statements in the internal linker representation
4030 of the script. In particular, input section assignments will be made
4031 to @samp{-T} output sections before those in the default script. Here
4032 is an example of how a @samp{-T} script using @code{INSERT} might look:
4039 .ov1 @{ ov1*(.text) @}
4040 .ov2 @{ ov2*(.text) @}
4046 @item NOCROSSREFS(@var{section} @var{section} @dots{})
4047 @kindex NOCROSSREFS(@var{sections})
4048 @cindex cross references
4049 This command may be used to tell @command{ld} to issue an error about any
4050 references among certain output sections.
4052 In certain types of programs, particularly on embedded systems when
4053 using overlays, when one section is loaded into memory, another section
4054 will not be. Any direct references between the two sections would be
4055 errors. For example, it would be an error if code in one section called
4056 a function defined in the other section.
4058 The @code{NOCROSSREFS} command takes a list of output section names. If
4059 @command{ld} detects any cross references between the sections, it reports
4060 an error and returns a non-zero exit status. Note that the
4061 @code{NOCROSSREFS} command uses output section names, not input section
4064 @item NOCROSSREFS_TO(@var{tosection} @var{fromsection} @dots{})
4065 @kindex NOCROSSREFS_TO(@var{tosection} @var{fromsections})
4066 @cindex cross references
4067 This command may be used to tell @command{ld} to issue an error about any
4068 references to one section from a list of other sections.
4070 The @code{NOCROSSREFS} command is useful when ensuring that two or more
4071 output sections are entirely independent but there are situations where
4072 a one-way dependency is needed. For example, in a multi-core application
4073 there may be shared code that can be called from each core but for safety
4074 must never call back.
4076 The @code{NOCROSSREFS_TO} command takes a list of output section names.
4077 The first section can not be referenced from any of the other sections.
4078 If @command{ld} detects any references to the first section from any of
4079 the other sections, it reports an error and returns a non-zero exit
4080 status. Note that the @code{NOCROSSREFS_TO} command uses output section
4081 names, not input section names.
4083 @ifclear SingleFormat
4084 @item OUTPUT_ARCH(@var{bfdarch})
4085 @kindex OUTPUT_ARCH(@var{bfdarch})
4086 @cindex machine architecture
4087 @cindex architecture
4088 Specify a particular output machine architecture. The argument is one
4089 of the names used by the BFD library (@pxref{BFD}). You can see the
4090 architecture of an object file by using the @code{objdump} program with
4091 the @samp{-f} option.
4094 @item LD_FEATURE(@var{string})
4095 @kindex LD_FEATURE(@var{string})
4096 This command may be used to modify @command{ld} behavior. If
4097 @var{string} is @code{"SANE_EXPR"} then absolute symbols and numbers
4098 in a script are simply treated as numbers everywhere.
4099 @xref{Expression Section}.
4103 @section Assigning Values to Symbols
4104 @cindex assignment in scripts
4105 @cindex symbol definition, scripts
4106 @cindex variables, defining
4107 You may assign a value to a symbol in a linker script. This will define
4108 the symbol and place it into the symbol table with a global scope.
4111 * Simple Assignments:: Simple Assignments
4114 * PROVIDE_HIDDEN:: PROVIDE_HIDDEN
4115 * Source Code Reference:: How to use a linker script defined symbol in source code
4118 @node Simple Assignments
4119 @subsection Simple Assignments
4121 You may assign to a symbol using any of the C assignment operators:
4124 @item @var{symbol} = @var{expression} ;
4125 @itemx @var{symbol} += @var{expression} ;
4126 @itemx @var{symbol} -= @var{expression} ;
4127 @itemx @var{symbol} *= @var{expression} ;
4128 @itemx @var{symbol} /= @var{expression} ;
4129 @itemx @var{symbol} <<= @var{expression} ;
4130 @itemx @var{symbol} >>= @var{expression} ;
4131 @itemx @var{symbol} &= @var{expression} ;
4132 @itemx @var{symbol} |= @var{expression} ;
4135 The first case will define @var{symbol} to the value of
4136 @var{expression}. In the other cases, @var{symbol} must already be
4137 defined, and the value will be adjusted accordingly.
4139 The special symbol name @samp{.} indicates the location counter. You
4140 may only use this within a @code{SECTIONS} command. @xref{Location Counter}.
4142 The semicolon after @var{expression} is required.
4144 Expressions are defined below; see @ref{Expressions}.
4146 You may write symbol assignments as commands in their own right, or as
4147 statements within a @code{SECTIONS} command, or as part of an output
4148 section description in a @code{SECTIONS} command.
4150 The section of the symbol will be set from the section of the
4151 expression; for more information, see @ref{Expression Section}.
4153 Here is an example showing the three different places that symbol
4154 assignments may be used:
4165 _bdata = (. + 3) & ~ 3;
4166 .data : @{ *(.data) @}
4170 In this example, the symbol @samp{floating_point} will be defined as
4171 zero. The symbol @samp{_etext} will be defined as the address following
4172 the last @samp{.text} input section. The symbol @samp{_bdata} will be
4173 defined as the address following the @samp{.text} output section aligned
4174 upward to a 4 byte boundary.
4179 For ELF targeted ports, define a symbol that will be hidden and won't be
4180 exported. The syntax is @code{HIDDEN(@var{symbol} = @var{expression})}.
4182 Here is the example from @ref{Simple Assignments}, rewritten to use
4186 HIDDEN(floating_point = 0);
4194 HIDDEN(_bdata = (. + 3) & ~ 3);
4195 .data : @{ *(.data) @}
4199 In this case none of the three symbols will be visible outside this module.
4204 In some cases, it is desirable for a linker script to define a symbol
4205 only if it is referenced and is not defined by any object included in
4206 the link. For example, traditional linkers defined the symbol
4207 @samp{etext}. However, ANSI C requires that the user be able to use
4208 @samp{etext} as a function name without encountering an error. The
4209 @code{PROVIDE} keyword may be used to define a symbol, such as
4210 @samp{etext}, only if it is referenced but not defined. The syntax is
4211 @code{PROVIDE(@var{symbol} = @var{expression})}.
4213 Here is an example of using @code{PROVIDE} to define @samp{etext}:
4226 In this example, if the program defines @samp{_etext} (with a leading
4227 underscore), the linker will give a multiple definition error. If, on
4228 the other hand, the program defines @samp{etext} (with no leading
4229 underscore), the linker will silently use the definition in the program.
4230 If the program references @samp{etext} but does not define it, the
4231 linker will use the definition in the linker script.
4233 Note - the @code{PROVIDE} directive considers a common symbol to be
4234 defined, even though such a symbol could be combined with the symbol
4235 that the @code{PROVIDE} would create. This is particularly important
4236 when considering constructor and destructor list symbols such as
4237 @samp{__CTOR_LIST__} as these are often defined as common symbols.
4239 @node PROVIDE_HIDDEN
4240 @subsection PROVIDE_HIDDEN
4241 @cindex PROVIDE_HIDDEN
4242 Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be
4243 hidden and won't be exported.
4245 @node Source Code Reference
4246 @subsection Source Code Reference
4248 Accessing a linker script defined variable from source code is not
4249 intuitive. In particular a linker script symbol is not equivalent to
4250 a variable declaration in a high level language, it is instead a
4251 symbol that does not have a value.
4253 Before going further, it is important to note that compilers often
4254 transform names in the source code into different names when they are
4255 stored in the symbol table. For example, Fortran compilers commonly
4256 prepend or append an underscore, and C++ performs extensive @samp{name
4257 mangling}. Therefore there might be a discrepancy between the name
4258 of a variable as it is used in source code and the name of the same
4259 variable as it is defined in a linker script. For example in C a
4260 linker script variable might be referred to as:
4266 But in the linker script it might be defined as:
4272 In the remaining examples however it is assumed that no name
4273 transformation has taken place.
4275 When a symbol is declared in a high level language such as C, two
4276 things happen. The first is that the compiler reserves enough space
4277 in the program's memory to hold the @emph{value} of the symbol. The
4278 second is that the compiler creates an entry in the program's symbol
4279 table which holds the symbol's @emph{address}. ie the symbol table
4280 contains the address of the block of memory holding the symbol's
4281 value. So for example the following C declaration, at file scope:
4287 creates an entry called @samp{foo} in the symbol table. This entry
4288 holds the address of an @samp{int} sized block of memory where the
4289 number 1000 is initially stored.
4291 When a program references a symbol the compiler generates code that
4292 first accesses the symbol table to find the address of the symbol's
4293 memory block and then code to read the value from that memory block.
4300 looks up the symbol @samp{foo} in the symbol table, gets the address
4301 associated with this symbol and then writes the value 1 into that
4308 looks up the symbol @samp{foo} in the symbol table, gets its address
4309 and then copies this address into the block of memory associated with
4310 the variable @samp{a}.
4312 Linker scripts symbol declarations, by contrast, create an entry in
4313 the symbol table but do not assign any memory to them. Thus they are
4314 an address without a value. So for example the linker script definition:
4320 creates an entry in the symbol table called @samp{foo} which holds
4321 the address of memory location 1000, but nothing special is stored at
4322 address 1000. This means that you cannot access the @emph{value} of a
4323 linker script defined symbol - it has no value - all you can do is
4324 access the @emph{address} of a linker script defined symbol.
4326 Hence when you are using a linker script defined symbol in source code
4327 you should always take the address of the symbol, and never attempt to
4328 use its value. For example suppose you want to copy the contents of a
4329 section of memory called .ROM into a section called .FLASH and the
4330 linker script contains these declarations:
4334 start_of_ROM = .ROM;
4335 end_of_ROM = .ROM + sizeof (.ROM);
4336 start_of_FLASH = .FLASH;
4340 Then the C source code to perform the copy would be:
4344 extern char start_of_ROM, end_of_ROM, start_of_FLASH;
4346 memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
4350 Note the use of the @samp{&} operators. These are correct.
4351 Alternatively the symbols can be treated as the names of vectors or
4352 arrays and then the code will again work as expected:
4356 extern char start_of_ROM[], end_of_ROM[], start_of_FLASH[];
4358 memcpy (start_of_FLASH, start_of_ROM, end_of_ROM - start_of_ROM);
4362 Note how using this method does not require the use of @samp{&}
4366 @section SECTIONS Command
4368 The @code{SECTIONS} command tells the linker how to map input sections
4369 into output sections, and how to place the output sections in memory.
4371 The format of the @code{SECTIONS} command is:
4375 @var{sections-command}
4376 @var{sections-command}
4381 Each @var{sections-command} may of be one of the following:
4385 an @code{ENTRY} command (@pxref{Entry Point,,Entry command})
4387 a symbol assignment (@pxref{Assignments})
4389 an output section description
4391 an overlay description
4394 The @code{ENTRY} command and symbol assignments are permitted inside the
4395 @code{SECTIONS} command for convenience in using the location counter in
4396 those commands. This can also make the linker script easier to
4397 understand because you can use those commands at meaningful points in
4398 the layout of the output file.
4400 Output section descriptions and overlay descriptions are described
4403 If you do not use a @code{SECTIONS} command in your linker script, the
4404 linker will place each input section into an identically named output
4405 section in the order that the sections are first encountered in the
4406 input files. If all input sections are present in the first file, for
4407 example, the order of sections in the output file will match the order
4408 in the first input file. The first section will be at address zero.
4411 * Output Section Description:: Output section description
4412 * Output Section Name:: Output section name
4413 * Output Section Address:: Output section address
4414 * Input Section:: Input section description
4415 * Output Section Data:: Output section data
4416 * Output Section Keywords:: Output section keywords
4417 * Output Section Discarding:: Output section discarding
4418 * Output Section Attributes:: Output section attributes
4419 * Overlay Description:: Overlay description
4422 @node Output Section Description
4423 @subsection Output Section Description
4424 The full description of an output section looks like this:
4427 @var{section} [@var{address}] [(@var{type})] :
4429 [ALIGN(@var{section_align}) | ALIGN_WITH_INPUT]
4430 [SUBALIGN(@var{subsection_align})]
4433 @var{output-section-command}
4434 @var{output-section-command}
4436 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] [,]
4440 Most output sections do not use most of the optional section attributes.
4442 The whitespace around @var{section} is required, so that the section
4443 name is unambiguous. The colon and the curly braces are also required.
4444 The comma at the end may be required if a @var{fillexp} is used and
4445 the next @var{sections-command} looks like a continuation of the expression.
4446 The line breaks and other white space are optional.
4448 Each @var{output-section-command} may be one of the following:
4452 a symbol assignment (@pxref{Assignments})
4454 an input section description (@pxref{Input Section})
4456 data values to include directly (@pxref{Output Section Data})
4458 a special output section keyword (@pxref{Output Section Keywords})
4461 @node Output Section Name
4462 @subsection Output Section Name
4463 @cindex name, section
4464 @cindex section name
4465 The name of the output section is @var{section}. @var{section} must
4466 meet the constraints of your output format. In formats which only
4467 support a limited number of sections, such as @code{a.out}, the name
4468 must be one of the names supported by the format (@code{a.out}, for
4469 example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the
4470 output format supports any number of sections, but with numbers and not
4471 names (as is the case for Oasys), the name should be supplied as a
4472 quoted numeric string. A section name may consist of any sequence of
4473 characters, but a name which contains any unusual characters such as
4474 commas must be quoted.
4476 The output section name @samp{/DISCARD/} is special; @ref{Output Section
4479 @node Output Section Address
4480 @subsection Output Section Address
4481 @cindex address, section
4482 @cindex section address
4483 The @var{address} is an expression for the VMA (the virtual memory
4484 address) of the output section. This address is optional, but if it
4485 is provided then the output address will be set exactly as specified.
4487 If the output address is not specified then one will be chosen for the
4488 section, based on the heuristic below. This address will be adjusted
4489 to fit the alignment requirement of the output section. The
4490 alignment requirement is the strictest alignment of any input section
4491 contained within the output section.
4493 The output section address heuristic is as follows:
4497 If an output memory @var{region} is set for the section then it
4498 is added to this region and its address will be the next free address
4502 If the MEMORY command has been used to create a list of memory
4503 regions then the first region which has attributes compatible with the
4504 section is selected to contain it. The section's output address will
4505 be the next free address in that region; @ref{MEMORY}.
4508 If no memory regions were specified, or none match the section then
4509 the output address will be based on the current value of the location
4517 .text . : @{ *(.text) @}
4524 .text : @{ *(.text) @}
4528 are subtly different. The first will set the address of the
4529 @samp{.text} output section to the current value of the location
4530 counter. The second will set it to the current value of the location
4531 counter aligned to the strictest alignment of any of the @samp{.text}
4534 The @var{address} may be an arbitrary expression; @ref{Expressions}.
4535 For example, if you want to align the section on a 0x10 byte boundary,
4536 so that the lowest four bits of the section address are zero, you could
4537 do something like this:
4539 .text ALIGN(0x10) : @{ *(.text) @}
4542 This works because @code{ALIGN} returns the current location counter
4543 aligned upward to the specified value.
4545 Specifying @var{address} for a section will change the value of the
4546 location counter, provided that the section is non-empty. (Empty
4547 sections are ignored).
4550 @subsection Input Section Description
4551 @cindex input sections
4552 @cindex mapping input sections to output sections
4553 The most common output section command is an input section description.
4555 The input section description is the most basic linker script operation.
4556 You use output sections to tell the linker how to lay out your program
4557 in memory. You use input section descriptions to tell the linker how to
4558 map the input files into your memory layout.
4561 * Input Section Basics:: Input section basics
4562 * Input Section Wildcards:: Input section wildcard patterns
4563 * Input Section Common:: Input section for common symbols
4564 * Input Section Keep:: Input section and garbage collection
4565 * Input Section Example:: Input section example
4568 @node Input Section Basics
4569 @subsubsection Input Section Basics
4570 @cindex input section basics
4571 An input section description consists of a file name optionally followed
4572 by a list of section names in parentheses.
4574 The file name and the section name may be wildcard patterns, which we
4575 describe further below (@pxref{Input Section Wildcards}).
4577 The most common input section description is to include all input
4578 sections with a particular name in the output section. For example, to
4579 include all input @samp{.text} sections, you would write:
4584 Here the @samp{*} is a wildcard which matches any file name. To exclude a list
4585 @cindex EXCLUDE_FILE
4586 of files from matching the file name wildcard, EXCLUDE_FILE may be used to
4587 match all files except the ones specified in the EXCLUDE_FILE list. For
4590 EXCLUDE_FILE (*crtend.o *otherfile.o) *(.ctors)
4593 will cause all .ctors sections from all files except @file{crtend.o}
4594 and @file{otherfile.o} to be included. The EXCLUDE_FILE can also be
4595 placed inside the section list, for example:
4597 *(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
4600 The result of this is identically to the previous example. Supporting
4601 two syntaxes for EXCLUDE_FILE is useful if the section list contains
4602 more than one section, as described below.
4604 There are two ways to include more than one section:
4610 The difference between these is the order in which the @samp{.text} and
4611 @samp{.rdata} input sections will appear in the output section. In the
4612 first example, they will be intermingled, appearing in the same order as
4613 they are found in the linker input. In the second example, all
4614 @samp{.text} input sections will appear first, followed by all
4615 @samp{.rdata} input sections.
4617 When using EXCLUDE_FILE with more than one section, if the exclusion
4618 is within the section list then the exclusion only applies to the
4619 immediately following section, for example:
4621 *(EXCLUDE_FILE (*somefile.o) .text .rdata)
4624 will cause all @samp{.text} sections from all files except
4625 @file{somefile.o} to be included, while all @samp{.rdata} sections
4626 from all files, including @file{somefile.o}, will be included. To
4627 exclude the @samp{.rdata} sections from @file{somefile.o} the example
4628 could be modified to:
4630 *(EXCLUDE_FILE (*somefile.o) .text EXCLUDE_FILE (*somefile.o) .rdata)
4633 Alternatively, placing the EXCLUDE_FILE outside of the section list,
4634 before the input file selection, will cause the exclusion to apply for
4635 all sections. Thus the previous example can be rewritten as:
4637 EXCLUDE_FILE (*somefile.o) *(.text .rdata)
4640 You can specify a file name to include sections from a particular file.
4641 You would do this if one or more of your files contain special data that
4642 needs to be at a particular location in memory. For example:
4647 To refine the sections that are included based on the section flags
4648 of an input section, INPUT_SECTION_FLAGS may be used.
4650 Here is a simple example for using Section header flags for ELF sections:
4655 .text : @{ INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) @}
4656 .text2 : @{ INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) @}
4661 In this example, the output section @samp{.text} will be comprised of any
4662 input section matching the name *(.text) whose section header flags
4663 @code{SHF_MERGE} and @code{SHF_STRINGS} are set. The output section
4664 @samp{.text2} will be comprised of any input section matching the name *(.text)
4665 whose section header flag @code{SHF_WRITE} is clear.
4667 You can also specify files within archives by writing a pattern
4668 matching the archive, a colon, then the pattern matching the file,
4669 with no whitespace around the colon.
4673 matches file within archive
4675 matches the whole archive
4677 matches file but not one in an archive
4680 Either one or both of @samp{archive} and @samp{file} can contain shell
4681 wildcards. On DOS based file systems, the linker will assume that a
4682 single letter followed by a colon is a drive specifier, so
4683 @samp{c:myfile.o} is a simple file specification, not @samp{myfile.o}
4684 within an archive called @samp{c}. @samp{archive:file} filespecs may
4685 also be used within an @code{EXCLUDE_FILE} list, but may not appear in
4686 other linker script contexts. For instance, you cannot extract a file
4687 from an archive by using @samp{archive:file} in an @code{INPUT}
4690 If you use a file name without a list of sections, then all sections in
4691 the input file will be included in the output section. This is not
4692 commonly done, but it may by useful on occasion. For example:
4697 When you use a file name which is not an @samp{archive:file} specifier
4698 and does not contain any wild card
4699 characters, the linker will first see if you also specified the file
4700 name on the linker command line or in an @code{INPUT} command. If you
4701 did not, the linker will attempt to open the file as an input file, as
4702 though it appeared on the command line. Note that this differs from an
4703 @code{INPUT} command, because the linker will not search for the file in
4704 the archive search path.
4706 @node Input Section Wildcards
4707 @subsubsection Input Section Wildcard Patterns
4708 @cindex input section wildcards
4709 @cindex wildcard file name patterns
4710 @cindex file name wildcard patterns
4711 @cindex section name wildcard patterns
4712 In an input section description, either the file name or the section
4713 name or both may be wildcard patterns.
4715 The file name of @samp{*} seen in many examples is a simple wildcard
4716 pattern for the file name.
4718 The wildcard patterns are like those used by the Unix shell.
4722 matches any number of characters
4724 matches any single character
4726 matches a single instance of any of the @var{chars}; the @samp{-}
4727 character may be used to specify a range of characters, as in
4728 @samp{[a-z]} to match any lower case letter
4730 quotes the following character
4733 When a file name is matched with a wildcard, the wildcard characters
4734 will not match a @samp{/} character (used to separate directory names on
4735 Unix). A pattern consisting of a single @samp{*} character is an
4736 exception; it will always match any file name, whether it contains a
4737 @samp{/} or not. In a section name, the wildcard characters will match
4738 a @samp{/} character.
4740 File name wildcard patterns only match files which are explicitly
4741 specified on the command line or in an @code{INPUT} command. The linker
4742 does not search directories to expand wildcards.
4744 If a file name matches more than one wildcard pattern, or if a file name
4745 appears explicitly and is also matched by a wildcard pattern, the linker
4746 will use the first match in the linker script. For example, this
4747 sequence of input section descriptions is probably in error, because the
4748 @file{data.o} rule will not be used:
4750 .data : @{ *(.data) @}
4751 .data1 : @{ data.o(.data) @}
4754 @cindex SORT_BY_NAME
4755 Normally, the linker will place files and sections matched by wildcards
4756 in the order in which they are seen during the link. You can change
4757 this by using the @code{SORT_BY_NAME} keyword, which appears before a wildcard
4758 pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the
4759 @code{SORT_BY_NAME} keyword is used, the linker will sort the files or sections
4760 into ascending order by name before placing them in the output file.
4762 @cindex SORT_BY_ALIGNMENT
4763 @code{SORT_BY_ALIGNMENT} is similar to @code{SORT_BY_NAME}.
4764 @code{SORT_BY_ALIGNMENT} will sort sections into descending order of
4765 alignment before placing them in the output file. Placing larger
4766 alignments before smaller alignments can reduce the amount of padding
4769 @cindex SORT_BY_INIT_PRIORITY
4770 @code{SORT_BY_INIT_PRIORITY} is also similar to @code{SORT_BY_NAME}.
4771 @code{SORT_BY_INIT_PRIORITY} will sort sections into ascending
4772 numerical order of the GCC init_priority attribute encoded in the
4773 section name before placing them in the output file. In
4774 @code{.init_array.NNNNN} and @code{.fini_array.NNNNN}, @code{NNNNN} is
4775 the init_priority. In @code{.ctors.NNNNN} and @code{.dtors.NNNNN},
4776 @code{NNNNN} is 65535 minus the init_priority.
4779 @code{SORT} is an alias for @code{SORT_BY_NAME}.
4781 When there are nested section sorting commands in linker script, there
4782 can be at most 1 level of nesting for section sorting commands.
4786 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
4787 It will sort the input sections by name first, then by alignment if two
4788 sections have the same name.
4790 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
4791 It will sort the input sections by alignment first, then by name if two
4792 sections have the same alignment.
4794 @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is
4795 treated the same as @code{SORT_BY_NAME} (wildcard section pattern).
4797 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern))
4798 is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern).
4800 All other nested section sorting commands are invalid.
4803 When both command-line section sorting option and linker script
4804 section sorting command are used, section sorting command always
4805 takes precedence over the command-line option.
4807 If the section sorting command in linker script isn't nested, the
4808 command-line option will make the section sorting command to be
4809 treated as nested sorting command.
4813 @code{SORT_BY_NAME} (wildcard section pattern ) with
4814 @option{--sort-sections alignment} is equivalent to
4815 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
4817 @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with
4818 @option{--sort-section name} is equivalent to
4819 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
4822 If the section sorting command in linker script is nested, the
4823 command-line option will be ignored.
4826 @code{SORT_NONE} disables section sorting by ignoring the command-line
4827 section sorting option.
4829 If you ever get confused about where input sections are going, use the
4830 @samp{-M} linker option to generate a map file. The map file shows
4831 precisely how input sections are mapped to output sections.
4833 This example shows how wildcard patterns might be used to partition
4834 files. This linker script directs the linker to place all @samp{.text}
4835 sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}.
4836 The linker will place the @samp{.data} section from all files beginning
4837 with an upper case character in @samp{.DATA}; for all other files, the
4838 linker will place the @samp{.data} section in @samp{.data}.
4842 .text : @{ *(.text) @}
4843 .DATA : @{ [A-Z]*(.data) @}
4844 .data : @{ *(.data) @}
4845 .bss : @{ *(.bss) @}
4850 @node Input Section Common
4851 @subsubsection Input Section for Common Symbols
4852 @cindex common symbol placement
4853 @cindex uninitialized data placement
4854 A special notation is needed for common symbols, because in many object
4855 file formats common symbols do not have a particular input section. The
4856 linker treats common symbols as though they are in an input section
4857 named @samp{COMMON}.
4859 You may use file names with the @samp{COMMON} section just as with any
4860 other input sections. You can use this to place common symbols from a
4861 particular input file in one section while common symbols from other
4862 input files are placed in another section.
4864 In most cases, common symbols in input files will be placed in the
4865 @samp{.bss} section in the output file. For example:
4867 .bss @{ *(.bss) *(COMMON) @}
4870 @cindex scommon section
4871 @cindex small common symbols
4872 Some object file formats have more than one type of common symbol. For
4873 example, the MIPS ELF object file format distinguishes standard common
4874 symbols and small common symbols. In this case, the linker will use a
4875 different special section name for other types of common symbols. In
4876 the case of MIPS ELF, the linker uses @samp{COMMON} for standard common
4877 symbols and @samp{.scommon} for small common symbols. This permits you
4878 to map the different types of common symbols into memory at different
4882 You will sometimes see @samp{[COMMON]} in old linker scripts. This
4883 notation is now considered obsolete. It is equivalent to
4886 @node Input Section Keep
4887 @subsubsection Input Section and Garbage Collection
4889 @cindex garbage collection
4890 When link-time garbage collection is in use (@samp{--gc-sections}),
4891 it is often useful to mark sections that should not be eliminated.
4892 This is accomplished by surrounding an input section's wildcard entry
4893 with @code{KEEP()}, as in @code{KEEP(*(.init))} or
4894 @code{KEEP(SORT_BY_NAME(*)(.ctors))}.
4896 @node Input Section Example
4897 @subsubsection Input Section Example
4898 The following example is a complete linker script. It tells the linker
4899 to read all of the sections from file @file{all.o} and place them at the
4900 start of output section @samp{outputa} which starts at location
4901 @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o}
4902 follows immediately, in the same output section. All of section
4903 @samp{.input2} from @file{foo.o} goes into output section
4904 @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}.
4905 All of the remaining @samp{.input1} and @samp{.input2} sections from any
4906 files are written to output section @samp{outputc}.
4934 If an output section's name is the same as the input section's name
4935 and is representable as a C identifier, then the linker will
4936 automatically @pxref{PROVIDE} two symbols: __start_SECNAME and
4937 __stop_SECNAME, where SECNAME is the name of the section. These
4938 indicate the start address and end address of the output section
4939 respectively. Note: most section names are not representable as
4940 C identifiers because they contain a @samp{.} character.
4942 @node Output Section Data
4943 @subsection Output Section Data
4945 @cindex section data
4946 @cindex output section data
4947 @kindex BYTE(@var{expression})
4948 @kindex SHORT(@var{expression})
4949 @kindex LONG(@var{expression})
4950 @kindex QUAD(@var{expression})
4951 @kindex SQUAD(@var{expression})
4952 You can include explicit bytes of data in an output section by using
4953 @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as
4954 an output section command. Each keyword is followed by an expression in
4955 parentheses providing the value to store (@pxref{Expressions}). The
4956 value of the expression is stored at the current value of the location
4959 The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands
4960 store one, two, four, and eight bytes (respectively). After storing the
4961 bytes, the location counter is incremented by the number of bytes
4964 For example, this will store the byte 1 followed by the four byte value
4965 of the symbol @samp{addr}:
4971 When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the
4972 same; they both store an 8 byte, or 64 bit, value. When both host and
4973 target are 32 bits, an expression is computed as 32 bits. In this case
4974 @code{QUAD} stores a 32 bit value zero extended to 64 bits, and
4975 @code{SQUAD} stores a 32 bit value sign extended to 64 bits.
4977 If the object file format of the output file has an explicit endianness,
4978 which is the normal case, the value will be stored in that endianness.
4979 When the object file format does not have an explicit endianness, as is
4980 true of, for example, S-records, the value will be stored in the
4981 endianness of the first input object file.
4983 Note---these commands only work inside a section description and not
4984 between them, so the following will produce an error from the linker:
4986 SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@
4988 whereas this will work:
4990 SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@
4993 @kindex FILL(@var{expression})
4994 @cindex holes, filling
4995 @cindex unspecified memory
4996 You may use the @code{FILL} command to set the fill pattern for the
4997 current section. It is followed by an expression in parentheses. Any
4998 otherwise unspecified regions of memory within the section (for example,
4999 gaps left due to the required alignment of input sections) are filled
5000 with the value of the expression, repeated as
5001 necessary. A @code{FILL} statement covers memory locations after the
5002 point at which it occurs in the section definition; by including more
5003 than one @code{FILL} statement, you can have different fill patterns in
5004 different parts of an output section.
5006 This example shows how to fill unspecified regions of memory with the
5012 The @code{FILL} command is similar to the @samp{=@var{fillexp}} output
5013 section attribute, but it only affects the
5014 part of the section following the @code{FILL} command, rather than the
5015 entire section. If both are used, the @code{FILL} command takes
5016 precedence. @xref{Output Section Fill}, for details on the fill
5019 @node Output Section Keywords
5020 @subsection Output Section Keywords
5021 There are a couple of keywords which can appear as output section
5025 @kindex CREATE_OBJECT_SYMBOLS
5026 @cindex input filename symbols
5027 @cindex filename symbols
5028 @item CREATE_OBJECT_SYMBOLS
5029 The command tells the linker to create a symbol for each input file.
5030 The name of each symbol will be the name of the corresponding input
5031 file. The section of each symbol will be the output section in which
5032 the @code{CREATE_OBJECT_SYMBOLS} command appears.
5034 This is conventional for the a.out object file format. It is not
5035 normally used for any other object file format.
5037 @kindex CONSTRUCTORS
5038 @cindex C++ constructors, arranging in link
5039 @cindex constructors, arranging in link
5041 When linking using the a.out object file format, the linker uses an
5042 unusual set construct to support C++ global constructors and
5043 destructors. When linking object file formats which do not support
5044 arbitrary sections, such as ECOFF and XCOFF, the linker will
5045 automatically recognize C++ global constructors and destructors by name.
5046 For these object file formats, the @code{CONSTRUCTORS} command tells the
5047 linker to place constructor information in the output section where the
5048 @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is
5049 ignored for other object file formats.
5051 The symbol @w{@code{__CTOR_LIST__}} marks the start of the global
5052 constructors, and the symbol @w{@code{__CTOR_END__}} marks the end.
5053 Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_END__}} mark
5054 the start and end of the global destructors. The
5055 first word in the list is the number of entries, followed by the address
5056 of each constructor or destructor, followed by a zero word. The
5057 compiler must arrange to actually run the code. For these object file
5058 formats @sc{gnu} C++ normally calls constructors from a subroutine
5059 @code{__main}; a call to @code{__main} is automatically inserted into
5060 the startup code for @code{main}. @sc{gnu} C++ normally runs
5061 destructors either by using @code{atexit}, or directly from the function
5064 For object file formats such as @code{COFF} or @code{ELF} which support
5065 arbitrary section names, @sc{gnu} C++ will normally arrange to put the
5066 addresses of global constructors and destructors into the @code{.ctors}
5067 and @code{.dtors} sections. Placing the following sequence into your
5068 linker script will build the sort of table which the @sc{gnu} C++
5069 runtime code expects to see.
5073 LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
5078 LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
5084 If you are using the @sc{gnu} C++ support for initialization priority,
5085 which provides some control over the order in which global constructors
5086 are run, you must sort the constructors at link time to ensure that they
5087 are executed in the correct order. When using the @code{CONSTRUCTORS}
5088 command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the
5089 @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and
5090 @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and
5093 Normally the compiler and linker will handle these issues automatically,
5094 and you will not need to concern yourself with them. However, you may
5095 need to consider this if you are using C++ and writing your own linker
5100 @node Output Section Discarding
5101 @subsection Output Section Discarding
5102 @cindex discarding sections
5103 @cindex sections, discarding
5104 @cindex removing sections
5105 The linker will not normally create output sections with no contents.
5106 This is for convenience when referring to input sections that may or
5107 may not be present in any of the input files. For example:
5109 .foo : @{ *(.foo) @}
5112 will only create a @samp{.foo} section in the output file if there is a
5113 @samp{.foo} section in at least one input file, and if the input
5114 sections are not all empty. Other link script directives that allocate
5115 space in an output section will also create the output section. So
5116 too will assignments to dot even if the assignment does not create
5117 space, except for @samp{. = 0}, @samp{. = . + 0}, @samp{. = sym},
5118 @samp{. = . + sym} and @samp{. = ALIGN (. != 0, expr, 1)} when
5119 @samp{sym} is an absolute symbol of value 0 defined in the script.
5120 This allows you to force output of an empty section with @samp{. = .}.
5122 The linker will ignore address assignments (@pxref{Output Section Address})
5123 on discarded output sections, except when the linker script defines
5124 symbols in the output section. In that case the linker will obey
5125 the address assignments, possibly advancing dot even though the
5126 section is discarded.
5129 The special output section name @samp{/DISCARD/} may be used to discard
5130 input sections. Any input sections which are assigned to an output
5131 section named @samp{/DISCARD/} are not included in the output file.
5133 Note, sections that match the @samp{/DISCARD/} output section will be
5134 discarded even if they are in an ELF section group which has other
5135 members which are not being discarded. This is deliberate.
5136 Discarding takes precedence over grouping.
5138 @node Output Section Attributes
5139 @subsection Output Section Attributes
5140 @cindex output section attributes
5141 We showed above that the full description of an output section looked
5146 @var{section} [@var{address}] [(@var{type})] :
5148 [ALIGN(@var{section_align}) | ALIGN_WITH_INPUT]
5149 [SUBALIGN(@var{subsection_align})]
5152 @var{output-section-command}
5153 @var{output-section-command}
5155 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}]
5159 We've already described @var{section}, @var{address}, and
5160 @var{output-section-command}. In this section we will describe the
5161 remaining section attributes.
5164 * Output Section Type:: Output section type
5165 * Output Section LMA:: Output section LMA
5166 * Forced Output Alignment:: Forced Output Alignment
5167 * Forced Input Alignment:: Forced Input Alignment
5168 * Output Section Constraint:: Output section constraint
5169 * Output Section Region:: Output section region
5170 * Output Section Phdr:: Output section phdr
5171 * Output Section Fill:: Output section fill
5174 @node Output Section Type
5175 @subsubsection Output Section Type
5176 Each output section may have a type. The type is a keyword in
5177 parentheses. The following types are defined:
5181 The section should be marked as not loadable, so that it will not be
5182 loaded into memory when the program is run.
5187 These type names are supported for backward compatibility, and are
5188 rarely used. They all have the same effect: the section should be
5189 marked as not allocatable, so that no memory is allocated for the
5190 section when the program is run.
5194 @cindex prevent unnecessary loading
5195 @cindex loading, preventing
5196 The linker normally sets the attributes of an output section based on
5197 the input sections which map into it. You can override this by using
5198 the section type. For example, in the script sample below, the
5199 @samp{ROM} section is addressed at memory location @samp{0} and does not
5200 need to be loaded when the program is run.
5204 ROM 0 (NOLOAD) : @{ @dots{} @}
5210 @node Output Section LMA
5211 @subsubsection Output Section LMA
5212 @kindex AT>@var{lma_region}
5213 @kindex AT(@var{lma})
5214 @cindex load address
5215 @cindex section load address
5216 Every section has a virtual address (VMA) and a load address (LMA); see
5217 @ref{Basic Script Concepts}. The virtual address is specified by the
5218 @pxref{Output Section Address} described earlier. The load address is
5219 specified by the @code{AT} or @code{AT>} keywords. Specifying a load
5220 address is optional.
5222 The @code{AT} keyword takes an expression as an argument. This
5223 specifies the exact load address of the section. The @code{AT>} keyword
5224 takes the name of a memory region as an argument. @xref{MEMORY}. The
5225 load address of the section is set to the next free address in the
5226 region, aligned to the section's alignment requirements.
5228 If neither @code{AT} nor @code{AT>} is specified for an allocatable
5229 section, the linker will use the following heuristic to determine the
5234 If the section has a specific VMA address, then this is used as
5235 the LMA address as well.
5238 If the section is not allocatable then its LMA is set to its VMA.
5241 Otherwise if a memory region can be found that is compatible
5242 with the current section, and this region contains at least one
5243 section, then the LMA is set so the difference between the
5244 VMA and LMA is the same as the difference between the VMA and LMA of
5245 the last section in the located region.
5248 If no memory regions have been declared then a default region
5249 that covers the entire address space is used in the previous step.
5252 If no suitable region could be found, or there was no previous
5253 section then the LMA is set equal to the VMA.
5256 @cindex ROM initialized data
5257 @cindex initialized data in ROM
5258 This feature is designed to make it easy to build a ROM image. For
5259 example, the following linker script creates three output sections: one
5260 called @samp{.text}, which starts at @code{0x1000}, one called
5261 @samp{.mdata}, which is loaded at the end of the @samp{.text} section
5262 even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold
5263 uninitialized data at address @code{0x3000}. The symbol @code{_data} is
5264 defined with the value @code{0x2000}, which shows that the location
5265 counter holds the VMA value, not the LMA value.
5271 .text 0x1000 : @{ *(.text) _etext = . ; @}
5273 AT ( ADDR (.text) + SIZEOF (.text) )
5274 @{ _data = . ; *(.data); _edata = . ; @}
5276 @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@}
5281 The run-time initialization code for use with a program generated with
5282 this linker script would include something like the following, to copy
5283 the initialized data from the ROM image to its runtime address. Notice
5284 how this code takes advantage of the symbols defined by the linker
5289 extern char _etext, _data, _edata, _bstart, _bend;
5290 char *src = &_etext;
5293 /* ROM has data at end of text; copy it. */
5294 while (dst < &_edata)
5298 for (dst = &_bstart; dst< &_bend; dst++)
5303 @node Forced Output Alignment
5304 @subsubsection Forced Output Alignment
5305 @kindex ALIGN(@var{section_align})
5306 @cindex forcing output section alignment
5307 @cindex output section alignment
5308 You can increase an output section's alignment by using ALIGN. As an
5309 alternative you can enforce that the difference between the VMA and LMA remains
5310 intact throughout this output section with the ALIGN_WITH_INPUT attribute.
5312 @node Forced Input Alignment
5313 @subsubsection Forced Input Alignment
5314 @kindex SUBALIGN(@var{subsection_align})
5315 @cindex forcing input section alignment
5316 @cindex input section alignment
5317 You can force input section alignment within an output section by using
5318 SUBALIGN. The value specified overrides any alignment given by input
5319 sections, whether larger or smaller.
5321 @node Output Section Constraint
5322 @subsubsection Output Section Constraint
5325 @cindex constraints on output sections
5326 You can specify that an output section should only be created if all
5327 of its input sections are read-only or all of its input sections are
5328 read-write by using the keyword @code{ONLY_IF_RO} and
5329 @code{ONLY_IF_RW} respectively.
5331 @node Output Section Region
5332 @subsubsection Output Section Region
5333 @kindex >@var{region}
5334 @cindex section, assigning to memory region
5335 @cindex memory regions and sections
5336 You can assign a section to a previously defined region of memory by
5337 using @samp{>@var{region}}. @xref{MEMORY}.
5339 Here is a simple example:
5342 MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @}
5343 SECTIONS @{ ROM : @{ *(.text) @} >rom @}
5347 @node Output Section Phdr
5348 @subsubsection Output Section Phdr
5350 @cindex section, assigning to program header
5351 @cindex program headers and sections
5352 You can assign a section to a previously defined program segment by
5353 using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to
5354 one or more segments, then all subsequent allocated sections will be
5355 assigned to those segments as well, unless they use an explicitly
5356 @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the
5357 linker to not put the section in any segment at all.
5359 Here is a simple example:
5362 PHDRS @{ text PT_LOAD ; @}
5363 SECTIONS @{ .text : @{ *(.text) @} :text @}
5367 @node Output Section Fill
5368 @subsubsection Output Section Fill
5369 @kindex =@var{fillexp}
5370 @cindex section fill pattern
5371 @cindex fill pattern, entire section
5372 You can set the fill pattern for an entire section by using
5373 @samp{=@var{fillexp}}. @var{fillexp} is an expression
5374 (@pxref{Expressions}). Any otherwise unspecified regions of memory
5375 within the output section (for example, gaps left due to the required
5376 alignment of input sections) will be filled with the value, repeated as
5377 necessary. If the fill expression is a simple hex number, ie. a string
5378 of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then
5379 an arbitrarily long sequence of hex digits can be used to specify the
5380 fill pattern; Leading zeros become part of the pattern too. For all
5381 other cases, including extra parentheses or a unary @code{+}, the fill
5382 pattern is the four least significant bytes of the value of the
5383 expression. In all cases, the number is big-endian.
5385 You can also change the fill value with a @code{FILL} command in the
5386 output section commands; (@pxref{Output Section Data}).
5388 Here is a simple example:
5391 SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @}
5395 @node Overlay Description
5396 @subsection Overlay Description
5399 An overlay description provides an easy way to describe sections which
5400 are to be loaded as part of a single memory image but are to be run at
5401 the same memory address. At run time, some sort of overlay manager will
5402 copy the overlaid sections in and out of the runtime memory address as
5403 required, perhaps by simply manipulating addressing bits. This approach
5404 can be useful, for example, when a certain region of memory is faster
5407 Overlays are described using the @code{OVERLAY} command. The
5408 @code{OVERLAY} command is used within a @code{SECTIONS} command, like an
5409 output section description. The full syntax of the @code{OVERLAY}
5410 command is as follows:
5413 OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )]
5417 @var{output-section-command}
5418 @var{output-section-command}
5420 @} [:@var{phdr}@dots{}] [=@var{fill}]
5423 @var{output-section-command}
5424 @var{output-section-command}
5426 @} [:@var{phdr}@dots{}] [=@var{fill}]
5428 @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] [,]
5432 Everything is optional except @code{OVERLAY} (a keyword), and each
5433 section must have a name (@var{secname1} and @var{secname2} above). The
5434 section definitions within the @code{OVERLAY} construct are identical to
5435 those within the general @code{SECTIONS} construct (@pxref{SECTIONS}),
5436 except that no addresses and no memory regions may be defined for
5437 sections within an @code{OVERLAY}.
5439 The comma at the end may be required if a @var{fill} is used and
5440 the next @var{sections-command} looks like a continuation of the expression.
5442 The sections are all defined with the same starting address. The load
5443 addresses of the sections are arranged such that they are consecutive in
5444 memory starting at the load address used for the @code{OVERLAY} as a
5445 whole (as with normal section definitions, the load address is optional,
5446 and defaults to the start address; the start address is also optional,
5447 and defaults to the current value of the location counter).
5449 If the @code{NOCROSSREFS} keyword is used, and there are any
5450 references among the sections, the linker will report an error. Since
5451 the sections all run at the same address, it normally does not make
5452 sense for one section to refer directly to another.
5453 @xref{Miscellaneous Commands, NOCROSSREFS}.
5455 For each section within the @code{OVERLAY}, the linker automatically
5456 provides two symbols. The symbol @code{__load_start_@var{secname}} is
5457 defined as the starting load address of the section. The symbol
5458 @code{__load_stop_@var{secname}} is defined as the final load address of
5459 the section. Any characters within @var{secname} which are not legal
5460 within C identifiers are removed. C (or assembler) code may use these
5461 symbols to move the overlaid sections around as necessary.
5463 At the end of the overlay, the value of the location counter is set to
5464 the start address of the overlay plus the size of the largest section.
5466 Here is an example. Remember that this would appear inside a
5467 @code{SECTIONS} construct.
5470 OVERLAY 0x1000 : AT (0x4000)
5472 .text0 @{ o1/*.o(.text) @}
5473 .text1 @{ o2/*.o(.text) @}
5478 This will define both @samp{.text0} and @samp{.text1} to start at
5479 address 0x1000. @samp{.text0} will be loaded at address 0x4000, and
5480 @samp{.text1} will be loaded immediately after @samp{.text0}. The
5481 following symbols will be defined if referenced: @code{__load_start_text0},
5482 @code{__load_stop_text0}, @code{__load_start_text1},
5483 @code{__load_stop_text1}.
5485 C code to copy overlay @code{.text1} into the overlay area might look
5490 extern char __load_start_text1, __load_stop_text1;
5491 memcpy ((char *) 0x1000, &__load_start_text1,
5492 &__load_stop_text1 - &__load_start_text1);
5496 Note that the @code{OVERLAY} command is just syntactic sugar, since
5497 everything it does can be done using the more basic commands. The above
5498 example could have been written identically as follows.
5502 .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @}
5503 PROVIDE (__load_start_text0 = LOADADDR (.text0));
5504 PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
5505 .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @}
5506 PROVIDE (__load_start_text1 = LOADADDR (.text1));
5507 PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
5508 . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
5513 @section MEMORY Command
5515 @cindex memory regions
5516 @cindex regions of memory
5517 @cindex allocating memory
5518 @cindex discontinuous memory
5519 The linker's default configuration permits allocation of all available
5520 memory. You can override this by using the @code{MEMORY} command.
5522 The @code{MEMORY} command describes the location and size of blocks of
5523 memory in the target. You can use it to describe which memory regions
5524 may be used by the linker, and which memory regions it must avoid. You
5525 can then assign sections to particular memory regions. The linker will
5526 set section addresses based on the memory regions, and will warn about
5527 regions that become too full. The linker will not shuffle sections
5528 around to fit into the available regions.
5530 A linker script may contain many uses of the @code{MEMORY} command,
5531 however, all memory blocks defined are treated as if they were
5532 specified inside a single @code{MEMORY} command. The syntax for
5538 @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len}
5544 The @var{name} is a name used in the linker script to refer to the
5545 region. The region name has no meaning outside of the linker script.
5546 Region names are stored in a separate name space, and will not conflict
5547 with symbol names, file names, or section names. Each memory region
5548 must have a distinct name within the @code{MEMORY} command. However you can
5549 add later alias names to existing memory regions with the @ref{REGION_ALIAS}
5552 @cindex memory region attributes
5553 The @var{attr} string is an optional list of attributes that specify
5554 whether to use a particular memory region for an input section which is
5555 not explicitly mapped in the linker script. As described in
5556 @ref{SECTIONS}, if you do not specify an output section for some input
5557 section, the linker will create an output section with the same name as
5558 the input section. If you define region attributes, the linker will use
5559 them to select the memory region for the output section that it creates.
5561 The @var{attr} string must consist only of the following characters:
5576 Invert the sense of any of the attributes that follow
5579 If an unmapped section matches any of the listed attributes other than
5580 @samp{!}, it will be placed in the memory region. The @samp{!}
5581 attribute reverses the test for the characters that follow, so that an
5582 unmapped section will be placed in the memory region only if it does
5583 not match any of the attributes listed afterwards. Thus an attribute
5584 string of @samp{RW!X} will match any unmapped section that has either
5585 or both of the @samp{R} and @samp{W} attributes, but only as long as
5586 the section does not also have the @samp{X} attribute.
5591 The @var{origin} is an numerical expression for the start address of
5592 the memory region. The expression must evaluate to a constant and it
5593 cannot involve any symbols. The keyword @code{ORIGIN} may be
5594 abbreviated to @code{org} or @code{o} (but not, for example,
5600 The @var{len} is an expression for the size in bytes of the memory
5601 region. As with the @var{origin} expression, the expression must
5602 be numerical only and must evaluate to a constant. The keyword
5603 @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
5605 In the following example, we specify that there are two memory regions
5606 available for allocation: one starting at @samp{0} for 256 kilobytes,
5607 and the other starting at @samp{0x40000000} for four megabytes. The
5608 linker will place into the @samp{rom} memory region every section which
5609 is not explicitly mapped into a memory region, and is either read-only
5610 or executable. The linker will place other sections which are not
5611 explicitly mapped into a memory region into the @samp{ram} memory
5618 rom (rx) : ORIGIN = 0, LENGTH = 256K
5619 ram (!rx) : org = 0x40000000, l = 4M
5624 Once you define a memory region, you can direct the linker to place
5625 specific output sections into that memory region by using the
5626 @samp{>@var{region}} output section attribute. For example, if you have
5627 a memory region named @samp{mem}, you would use @samp{>mem} in the
5628 output section definition. @xref{Output Section Region}. If no address
5629 was specified for the output section, the linker will set the address to
5630 the next available address within the memory region. If the combined
5631 output sections directed to a memory region are too large for the
5632 region, the linker will issue an error message.
5634 It is possible to access the origin and length of a memory in an
5635 expression via the @code{ORIGIN(@var{memory})} and
5636 @code{LENGTH(@var{memory})} functions:
5640 _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
5645 @section PHDRS Command
5647 @cindex program headers
5648 @cindex ELF program headers
5649 @cindex program segments
5650 @cindex segments, ELF
5651 The ELF object file format uses @dfn{program headers}, also knows as
5652 @dfn{segments}. The program headers describe how the program should be
5653 loaded into memory. You can print them out by using the @code{objdump}
5654 program with the @samp{-p} option.
5656 When you run an ELF program on a native ELF system, the system loader
5657 reads the program headers in order to figure out how to load the
5658 program. This will only work if the program headers are set correctly.
5659 This manual does not describe the details of how the system loader
5660 interprets program headers; for more information, see the ELF ABI.
5662 The linker will create reasonable program headers by default. However,
5663 in some cases, you may need to specify the program headers more
5664 precisely. You may use the @code{PHDRS} command for this purpose. When
5665 the linker sees the @code{PHDRS} command in the linker script, it will
5666 not create any program headers other than the ones specified.
5668 The linker only pays attention to the @code{PHDRS} command when
5669 generating an ELF output file. In other cases, the linker will simply
5670 ignore @code{PHDRS}.
5672 This is the syntax of the @code{PHDRS} command. The words @code{PHDRS},
5673 @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords.
5679 @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ]
5680 [ FLAGS ( @var{flags} ) ] ;
5685 The @var{name} is used only for reference in the @code{SECTIONS} command
5686 of the linker script. It is not put into the output file. Program
5687 header names are stored in a separate name space, and will not conflict
5688 with symbol names, file names, or section names. Each program header
5689 must have a distinct name. The headers are processed in order and it
5690 is usual for them to map to sections in ascending load address order.
5692 Certain program header types describe segments of memory which the
5693 system loader will load from the file. In the linker script, you
5694 specify the contents of these segments by placing allocatable output
5695 sections in the segments. You use the @samp{:@var{phdr}} output section
5696 attribute to place a section in a particular segment. @xref{Output
5699 It is normal to put certain sections in more than one segment. This
5700 merely implies that one segment of memory contains another. You may
5701 repeat @samp{:@var{phdr}}, using it once for each segment which should
5702 contain the section.
5704 If you place a section in one or more segments using @samp{:@var{phdr}},
5705 then the linker will place all subsequent allocatable sections which do
5706 not specify @samp{:@var{phdr}} in the same segments. This is for
5707 convenience, since generally a whole set of contiguous sections will be
5708 placed in a single segment. You can use @code{:NONE} to override the
5709 default segment and tell the linker to not put the section in any
5714 You may use the @code{FILEHDR} and @code{PHDRS} keywords after
5715 the program header type to further describe the contents of the segment.
5716 The @code{FILEHDR} keyword means that the segment should include the ELF
5717 file header. The @code{PHDRS} keyword means that the segment should
5718 include the ELF program headers themselves. If applied to a loadable
5719 segment (@code{PT_LOAD}), all prior loadable segments must have one of
5722 The @var{type} may be one of the following. The numbers indicate the
5723 value of the keyword.
5726 @item @code{PT_NULL} (0)
5727 Indicates an unused program header.
5729 @item @code{PT_LOAD} (1)
5730 Indicates that this program header describes a segment to be loaded from
5733 @item @code{PT_DYNAMIC} (2)
5734 Indicates a segment where dynamic linking information can be found.
5736 @item @code{PT_INTERP} (3)
5737 Indicates a segment where the name of the program interpreter may be
5740 @item @code{PT_NOTE} (4)
5741 Indicates a segment holding note information.
5743 @item @code{PT_SHLIB} (5)
5744 A reserved program header type, defined but not specified by the ELF
5747 @item @code{PT_PHDR} (6)
5748 Indicates a segment where the program headers may be found.
5750 @item @code{PT_TLS} (7)
5751 Indicates a segment containing thread local storage.
5753 @item @var{expression}
5754 An expression giving the numeric type of the program header. This may
5755 be used for types not defined above.
5758 You can specify that a segment should be loaded at a particular address
5759 in memory by using an @code{AT} expression. This is identical to the
5760 @code{AT} command used as an output section attribute (@pxref{Output
5761 Section LMA}). The @code{AT} command for a program header overrides the
5762 output section attribute.
5764 The linker will normally set the segment flags based on the sections
5765 which comprise the segment. You may use the @code{FLAGS} keyword to
5766 explicitly specify the segment flags. The value of @var{flags} must be
5767 an integer. It is used to set the @code{p_flags} field of the program
5770 Here is an example of @code{PHDRS}. This shows a typical set of program
5771 headers used on a native ELF system.
5777 headers PT_PHDR PHDRS ;
5779 text PT_LOAD FILEHDR PHDRS ;
5781 dynamic PT_DYNAMIC ;
5787 .interp : @{ *(.interp) @} :text :interp
5788 .text : @{ *(.text) @} :text
5789 .rodata : @{ *(.rodata) @} /* defaults to :text */
5791 . = . + 0x1000; /* move to a new page in memory */
5792 .data : @{ *(.data) @} :data
5793 .dynamic : @{ *(.dynamic) @} :data :dynamic
5800 @section VERSION Command
5801 @kindex VERSION @{script text@}
5802 @cindex symbol versions
5803 @cindex version script
5804 @cindex versions of symbols
5805 The linker supports symbol versions when using ELF. Symbol versions are
5806 only useful when using shared libraries. The dynamic linker can use
5807 symbol versions to select a specific version of a function when it runs
5808 a program that may have been linked against an earlier version of the
5811 You can include a version script directly in the main linker script, or
5812 you can supply the version script as an implicit linker script. You can
5813 also use the @samp{--version-script} linker option.
5815 The syntax of the @code{VERSION} command is simply
5817 VERSION @{ version-script-commands @}
5820 The format of the version script commands is identical to that used by
5821 Sun's linker in Solaris 2.5. The version script defines a tree of
5822 version nodes. You specify the node names and interdependencies in the
5823 version script. You can specify which symbols are bound to which
5824 version nodes, and you can reduce a specified set of symbols to local
5825 scope so that they are not globally visible outside of the shared
5828 The easiest way to demonstrate the version script language is with a few
5854 This example version script defines three version nodes. The first
5855 version node defined is @samp{VERS_1.1}; it has no other dependencies.
5856 The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces
5857 a number of symbols to local scope so that they are not visible outside
5858 of the shared library; this is done using wildcard patterns, so that any
5859 symbol whose name begins with @samp{old}, @samp{original}, or @samp{new}
5860 is matched. The wildcard patterns available are the same as those used
5861 in the shell when matching filenames (also known as ``globbing'').
5862 However, if you specify the symbol name inside double quotes, then the
5863 name is treated as literal, rather than as a glob pattern.
5865 Next, the version script defines node @samp{VERS_1.2}. This node
5866 depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2}
5867 to the version node @samp{VERS_1.2}.
5869 Finally, the version script defines node @samp{VERS_2.0}. This node
5870 depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1}
5871 and @samp{bar2} are bound to the version node @samp{VERS_2.0}.
5873 When the linker finds a symbol defined in a library which is not
5874 specifically bound to a version node, it will effectively bind it to an
5875 unspecified base version of the library. You can bind all otherwise
5876 unspecified symbols to a given version node by using @samp{global: *;}
5877 somewhere in the version script. Note that it's slightly crazy to use
5878 wildcards in a global spec except on the last version node. Global
5879 wildcards elsewhere run the risk of accidentally adding symbols to the
5880 set exported for an old version. That's wrong since older versions
5881 ought to have a fixed set of symbols.
5883 The names of the version nodes have no specific meaning other than what
5884 they might suggest to the person reading them. The @samp{2.0} version
5885 could just as well have appeared in between @samp{1.1} and @samp{1.2}.
5886 However, this would be a confusing way to write a version script.
5888 Node name can be omitted, provided it is the only version node
5889 in the version script. Such version script doesn't assign any versions to
5890 symbols, only selects which symbols will be globally visible out and which
5894 @{ global: foo; bar; local: *; @};
5897 When you link an application against a shared library that has versioned
5898 symbols, the application itself knows which version of each symbol it
5899 requires, and it also knows which version nodes it needs from each
5900 shared library it is linked against. Thus at runtime, the dynamic
5901 loader can make a quick check to make sure that the libraries you have
5902 linked against do in fact supply all of the version nodes that the
5903 application will need to resolve all of the dynamic symbols. In this
5904 way it is possible for the dynamic linker to know with certainty that
5905 all external symbols that it needs will be resolvable without having to
5906 search for each symbol reference.
5908 The symbol versioning is in effect a much more sophisticated way of
5909 doing minor version checking that SunOS does. The fundamental problem
5910 that is being addressed here is that typically references to external
5911 functions are bound on an as-needed basis, and are not all bound when
5912 the application starts up. If a shared library is out of date, a
5913 required interface may be missing; when the application tries to use
5914 that interface, it may suddenly and unexpectedly fail. With symbol
5915 versioning, the user will get a warning when they start their program if
5916 the libraries being used with the application are too old.
5918 There are several GNU extensions to Sun's versioning approach. The
5919 first of these is the ability to bind a symbol to a version node in the
5920 source file where the symbol is defined instead of in the versioning
5921 script. This was done mainly to reduce the burden on the library
5922 maintainer. You can do this by putting something like:
5924 __asm__(".symver original_foo,foo@@VERS_1.1");
5927 in the C source file. This renames the function @samp{original_foo} to
5928 be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}.
5929 The @samp{local:} directive can be used to prevent the symbol
5930 @samp{original_foo} from being exported. A @samp{.symver} directive
5931 takes precedence over a version script.
5933 The second GNU extension is to allow multiple versions of the same
5934 function to appear in a given shared library. In this way you can make
5935 an incompatible change to an interface without increasing the major
5936 version number of the shared library, while still allowing applications
5937 linked against the old interface to continue to function.
5939 To do this, you must use multiple @samp{.symver} directives in the
5940 source file. Here is an example:
5943 __asm__(".symver original_foo,foo@@");
5944 __asm__(".symver old_foo,foo@@VERS_1.1");
5945 __asm__(".symver old_foo1,foo@@VERS_1.2");
5946 __asm__(".symver new_foo,foo@@@@VERS_2.0");
5949 In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the
5950 unspecified base version of the symbol. The source file that contains this
5951 example would define 4 C functions: @samp{original_foo}, @samp{old_foo},
5952 @samp{old_foo1}, and @samp{new_foo}.
5954 When you have multiple definitions of a given symbol, there needs to be
5955 some way to specify a default version to which external references to
5956 this symbol will be bound. You can do this with the
5957 @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only
5958 declare one version of a symbol as the default in this manner; otherwise
5959 you would effectively have multiple definitions of the same symbol.
5961 If you wish to bind a reference to a specific version of the symbol
5962 within the shared library, you can use the aliases of convenience
5963 (i.e., @samp{old_foo}), or you can use the @samp{.symver} directive to
5964 specifically bind to an external version of the function in question.
5966 You can also specify the language in the version script:
5969 VERSION extern "lang" @{ version-script-commands @}
5972 The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}.
5973 The linker will iterate over the list of symbols at the link time and
5974 demangle them according to @samp{lang} before matching them to the
5975 patterns specified in @samp{version-script-commands}. The default
5976 @samp{lang} is @samp{C}.
5978 Demangled names may contains spaces and other special characters. As
5979 described above, you can use a glob pattern to match demangled names,
5980 or you can use a double-quoted string to match the string exactly. In
5981 the latter case, be aware that minor differences (such as differing
5982 whitespace) between the version script and the demangler output will
5983 cause a mismatch. As the exact string generated by the demangler
5984 might change in the future, even if the mangled name does not, you
5985 should check that all of your version directives are behaving as you
5986 expect when you upgrade.
5989 @section Expressions in Linker Scripts
5992 The syntax for expressions in the linker script language is identical to
5993 that of C expressions. All expressions are evaluated as integers. All
5994 expressions are evaluated in the same size, which is 32 bits if both the
5995 host and target are 32 bits, and is otherwise 64 bits.
5997 You can use and set symbol values in expressions.
5999 The linker defines several special purpose builtin functions for use in
6003 * Constants:: Constants
6004 * Symbolic Constants:: Symbolic constants
6005 * Symbols:: Symbol Names
6006 * Orphan Sections:: Orphan Sections
6007 * Location Counter:: The Location Counter
6008 * Operators:: Operators
6009 * Evaluation:: Evaluation
6010 * Expression Section:: The Section of an Expression
6011 * Builtin Functions:: Builtin Functions
6015 @subsection Constants
6016 @cindex integer notation
6017 @cindex constants in linker scripts
6018 All constants are integers.
6020 As in C, the linker considers an integer beginning with @samp{0} to be
6021 octal, and an integer beginning with @samp{0x} or @samp{0X} to be
6022 hexadecimal. Alternatively the linker accepts suffixes of @samp{h} or
6023 @samp{H} for hexadecimal, @samp{o} or @samp{O} for octal, @samp{b} or
6024 @samp{B} for binary and @samp{d} or @samp{D} for decimal. Any integer
6025 value without a prefix or a suffix is considered to be decimal.
6027 @cindex scaled integers
6028 @cindex K and M integer suffixes
6029 @cindex M and K integer suffixes
6030 @cindex suffixes for integers
6031 @cindex integer suffixes
6032 In addition, you can use the suffixes @code{K} and @code{M} to scale a
6036 @c END TEXI2ROFF-KILL
6037 @code{1024} or @code{1024*1024}
6041 ${\rm 1024}$ or ${\rm 1024}^2$
6043 @c END TEXI2ROFF-KILL
6044 respectively. For example, the following
6045 all refer to the same quantity:
6054 Note - the @code{K} and @code{M} suffixes cannot be used in
6055 conjunction with the base suffixes mentioned above.
6057 @node Symbolic Constants
6058 @subsection Symbolic Constants
6059 @cindex symbolic constants
6061 It is possible to refer to target-specific constants via the use of
6062 the @code{CONSTANT(@var{name})} operator, where @var{name} is one of:
6067 The target's maximum page size.
6069 @item COMMONPAGESIZE
6070 @kindex COMMONPAGESIZE
6071 The target's default page size.
6077 .text ALIGN (CONSTANT (MAXPAGESIZE)) : @{ *(.text) @}
6080 will create a text section aligned to the largest page boundary
6081 supported by the target.
6084 @subsection Symbol Names
6085 @cindex symbol names
6087 @cindex quoted symbol names
6089 Unless quoted, symbol names start with a letter, underscore, or period
6090 and may include letters, digits, underscores, periods, and hyphens.
6091 Unquoted symbol names must not conflict with any keywords. You can
6092 specify a symbol which contains odd characters or has the same name as a
6093 keyword by surrounding the symbol name in double quotes:
6096 "with a space" = "also with a space" + 10;
6099 Since symbols can contain many non-alphabetic characters, it is safest
6100 to delimit symbols with spaces. For example, @samp{A-B} is one symbol,
6101 whereas @samp{A - B} is an expression involving subtraction.
6103 @node Orphan Sections
6104 @subsection Orphan Sections
6106 Orphan sections are sections present in the input files which
6107 are not explicitly placed into the output file by the linker
6108 script. The linker will still copy these sections into the
6109 output file by either finding, or creating a suitable output section
6110 in which to place the orphaned input section.
6112 If the name of an orphaned input section exactly matches the name of
6113 an existing output section, then the orphaned input section will be
6114 placed at the end of that output section.
6116 If there is no output section with a matching name then new output
6117 sections will be created. Each new output section will have the same
6118 name as the orphan section placed within it. If there are multiple
6119 orphan sections with the same name, these will all be combined into
6120 one new output section.
6122 If new output sections are created to hold orphaned input sections,
6123 then the linker must decide where to place these new output sections
6124 in relation to existing output sections. On most modern targets, the
6125 linker attempts to place orphan sections after sections of the same
6126 attribute, such as code vs data, loadable vs non-loadable, etc. If no
6127 sections with matching attributes are found, or your target lacks this
6128 support, the orphan section is placed at the end of the file.
6130 The command-line options @samp{--orphan-handling} and @samp{--unique}
6131 (@pxref{Options,,Command-line Options}) can be used to control which
6132 output sections an orphan is placed in.
6134 @node Location Counter
6135 @subsection The Location Counter
6138 @cindex location counter
6139 @cindex current output location
6140 The special linker variable @dfn{dot} @samp{.} always contains the
6141 current output location counter. Since the @code{.} always refers to a
6142 location in an output section, it may only appear in an expression
6143 within a @code{SECTIONS} command. The @code{.} symbol may appear
6144 anywhere that an ordinary symbol is allowed in an expression.
6147 Assigning a value to @code{.} will cause the location counter to be
6148 moved. This may be used to create holes in the output section. The
6149 location counter may not be moved backwards inside an output section,
6150 and may not be moved backwards outside of an output section if so
6151 doing creates areas with overlapping LMAs.
6167 In the previous example, the @samp{.text} section from @file{file1} is
6168 located at the beginning of the output section @samp{output}. It is
6169 followed by a 1000 byte gap. Then the @samp{.text} section from
6170 @file{file2} appears, also with a 1000 byte gap following before the
6171 @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678}
6172 specifies what data to write in the gaps (@pxref{Output Section Fill}).
6174 @cindex dot inside sections
6175 Note: @code{.} actually refers to the byte offset from the start of the
6176 current containing object. Normally this is the @code{SECTIONS}
6177 statement, whose start address is 0, hence @code{.} can be used as an
6178 absolute address. If @code{.} is used inside a section description
6179 however, it refers to the byte offset from the start of that section,
6180 not an absolute address. Thus in a script like this:
6198 The @samp{.text} section will be assigned a starting address of 0x100
6199 and a size of exactly 0x200 bytes, even if there is not enough data in
6200 the @samp{.text} input sections to fill this area. (If there is too
6201 much data, an error will be produced because this would be an attempt to
6202 move @code{.} backwards). The @samp{.data} section will start at 0x500
6203 and it will have an extra 0x600 bytes worth of space after the end of
6204 the values from the @samp{.data} input sections and before the end of
6205 the @samp{.data} output section itself.
6207 @cindex dot outside sections
6208 Setting symbols to the value of the location counter outside of an
6209 output section statement can result in unexpected values if the linker
6210 needs to place orphan sections. For example, given the following:
6216 .text: @{ *(.text) @}
6220 .data: @{ *(.data) @}
6225 If the linker needs to place some input section, e.g. @code{.rodata},
6226 not mentioned in the script, it might choose to place that section
6227 between @code{.text} and @code{.data}. You might think the linker
6228 should place @code{.rodata} on the blank line in the above script, but
6229 blank lines are of no particular significance to the linker. As well,
6230 the linker doesn't associate the above symbol names with their
6231 sections. Instead, it assumes that all assignments or other
6232 statements belong to the previous output section, except for the
6233 special case of an assignment to @code{.}. I.e., the linker will
6234 place the orphan @code{.rodata} section as if the script was written
6241 .text: @{ *(.text) @}
6245 .rodata: @{ *(.rodata) @}
6246 .data: @{ *(.data) @}
6251 This may or may not be the script author's intention for the value of
6252 @code{start_of_data}. One way to influence the orphan section
6253 placement is to assign the location counter to itself, as the linker
6254 assumes that an assignment to @code{.} is setting the start address of
6255 a following output section and thus should be grouped with that
6256 section. So you could write:
6262 .text: @{ *(.text) @}
6267 .data: @{ *(.data) @}
6272 Now, the orphan @code{.rodata} section will be placed between
6273 @code{end_of_text} and @code{start_of_data}.
6277 @subsection Operators
6278 @cindex operators for arithmetic
6279 @cindex arithmetic operators
6280 @cindex precedence in expressions
6281 The linker recognizes the standard C set of arithmetic operators, with
6282 the standard bindings and precedence levels:
6285 @c END TEXI2ROFF-KILL
6287 precedence associativity Operators Notes
6293 5 left == != > < <= >=
6299 11 right &= += -= *= /= (2)
6303 (1) Prefix operators
6304 (2) @xref{Assignments}.
6308 \vskip \baselineskip
6309 %"lispnarrowing" is the extra indent used generally for smallexample
6310 \hskip\lispnarrowing\vbox{\offinterlineskip
6313 {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr
6314 height2pt&\omit&&\omit&&\omit&\cr
6315 &Precedence&& Associativity &&{\rm Operators}&\cr
6316 height2pt&\omit&&\omit&&\omit&\cr
6318 height2pt&\omit&&\omit&&\omit&\cr
6320 % '176 is tilde, '~' in tt font
6321 &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr
6322 &2&&left&&* / \%&\cr
6325 &5&&left&&== != > < <= >=&\cr
6328 &8&&left&&{\&\&}&\cr
6331 &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr
6333 height2pt&\omit&&\omit&&\omit&\cr}
6338 @obeylines@parskip=0pt@parindent=0pt
6339 @dag@quad Prefix operators.
6340 @ddag@quad @xref{Assignments}.
6343 @c END TEXI2ROFF-KILL
6346 @subsection Evaluation
6347 @cindex lazy evaluation
6348 @cindex expression evaluation order
6349 The linker evaluates expressions lazily. It only computes the value of
6350 an expression when absolutely necessary.
6352 The linker needs some information, such as the value of the start
6353 address of the first section, and the origins and lengths of memory
6354 regions, in order to do any linking at all. These values are computed
6355 as soon as possible when the linker reads in the linker script.
6357 However, other values (such as symbol values) are not known or needed
6358 until after storage allocation. Such values are evaluated later, when
6359 other information (such as the sizes of output sections) is available
6360 for use in the symbol assignment expression.
6362 The sizes of sections cannot be known until after allocation, so
6363 assignments dependent upon these are not performed until after
6366 Some expressions, such as those depending upon the location counter
6367 @samp{.}, must be evaluated during section allocation.
6369 If the result of an expression is required, but the value is not
6370 available, then an error results. For example, a script like the
6376 .text 9+this_isnt_constant :
6382 will cause the error message @samp{non constant expression for initial
6385 @node Expression Section
6386 @subsection The Section of an Expression
6387 @cindex expression sections
6388 @cindex absolute expressions
6389 @cindex relative expressions
6390 @cindex absolute and relocatable symbols
6391 @cindex relocatable and absolute symbols
6392 @cindex symbols, relocatable and absolute
6393 Addresses and symbols may be section relative, or absolute. A section
6394 relative symbol is relocatable. If you request relocatable output
6395 using the @samp{-r} option, a further link operation may change the
6396 value of a section relative symbol. On the other hand, an absolute
6397 symbol will retain the same value throughout any further link
6400 Some terms in linker expressions are addresses. This is true of
6401 section relative symbols and for builtin functions that return an
6402 address, such as @code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and
6403 @code{SEGMENT_START}. Other terms are simply numbers, or are builtin
6404 functions that return a non-address value, such as @code{LENGTH}.
6405 One complication is that unless you set @code{LD_FEATURE ("SANE_EXPR")}
6406 (@pxref{Miscellaneous Commands}), numbers and absolute symbols are treated
6407 differently depending on their location, for compatibility with older
6408 versions of @code{ld}. Expressions appearing outside an output
6409 section definition treat all numbers as absolute addresses.
6410 Expressions appearing inside an output section definition treat
6411 absolute symbols as numbers. If @code{LD_FEATURE ("SANE_EXPR")} is
6412 given, then absolute symbols and numbers are simply treated as numbers
6415 In the following simple example,
6422 __executable_start = 0x100;
6426 __data_start = 0x10;
6434 both @code{.} and @code{__executable_start} are set to the absolute
6435 address 0x100 in the first two assignments, then both @code{.} and
6436 @code{__data_start} are set to 0x10 relative to the @code{.data}
6437 section in the second two assignments.
6439 For expressions involving numbers, relative addresses and absolute
6440 addresses, ld follows these rules to evaluate terms:
6444 Unary operations on an absolute address or number, and binary
6445 operations on two absolute addresses or two numbers, or between one
6446 absolute address and a number, apply the operator to the value(s).
6448 Unary operations on a relative address, and binary operations on two
6449 relative addresses in the same section or between one relative address
6450 and a number, apply the operator to the offset part of the address(es).
6452 Other binary operations, that is, between two relative addresses not
6453 in the same section, or between a relative address and an absolute
6454 address, first convert any non-absolute term to an absolute address
6455 before applying the operator.
6458 The result section of each sub-expression is as follows:
6462 An operation involving only numbers results in a number.
6464 The result of comparisons, @samp{&&} and @samp{||} is also a number.
6466 The result of other binary arithmetic and logical operations on two
6467 relative addresses in the same section or two absolute addresses
6468 (after above conversions) is also a number when
6469 @code{LD_FEATURE ("SANE_EXPR")} or inside an output section definition
6470 but an absolute address otherwise.
6472 The result of other operations on relative addresses or one
6473 relative address and a number, is a relative address in the same
6474 section as the relative operand(s).
6476 The result of other operations on absolute addresses (after above
6477 conversions) is an absolute address.
6480 You can use the builtin function @code{ABSOLUTE} to force an expression
6481 to be absolute when it would otherwise be relative. For example, to
6482 create an absolute symbol set to the address of the end of the output
6483 section @samp{.data}:
6487 .data : @{ *(.data) _edata = ABSOLUTE(.); @}
6491 If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the
6492 @samp{.data} section.
6494 Using @code{LOADADDR} also forces an expression absolute, since this
6495 particular builtin function returns an absolute address.
6497 @node Builtin Functions
6498 @subsection Builtin Functions
6499 @cindex functions in expressions
6500 The linker script language includes a number of builtin functions for
6501 use in linker script expressions.
6504 @item ABSOLUTE(@var{exp})
6505 @kindex ABSOLUTE(@var{exp})
6506 @cindex expression, absolute
6507 Return the absolute (non-relocatable, as opposed to non-negative) value
6508 of the expression @var{exp}. Primarily useful to assign an absolute
6509 value to a symbol within a section definition, where symbol values are
6510 normally section relative. @xref{Expression Section}.
6512 @item ADDR(@var{section})
6513 @kindex ADDR(@var{section})
6514 @cindex section address in expression
6515 Return the address (VMA) of the named @var{section}. Your
6516 script must previously have defined the location of that section. In
6517 the following example, @code{start_of_output_1}, @code{symbol_1} and
6518 @code{symbol_2} are assigned equivalent values, except that
6519 @code{symbol_1} will be relative to the @code{.output1} section while
6520 the other two will be absolute:
6526 start_of_output_1 = ABSOLUTE(.);
6531 symbol_1 = ADDR(.output1);
6532 symbol_2 = start_of_output_1;
6538 @item ALIGN(@var{align})
6539 @itemx ALIGN(@var{exp},@var{align})
6540 @kindex ALIGN(@var{align})
6541 @kindex ALIGN(@var{exp},@var{align})
6542 @cindex round up location counter
6543 @cindex align location counter
6544 @cindex round up expression
6545 @cindex align expression
6546 Return the location counter (@code{.}) or arbitrary expression aligned
6547 to the next @var{align} boundary. The single operand @code{ALIGN}
6548 doesn't change the value of the location counter---it just does
6549 arithmetic on it. The two operand @code{ALIGN} allows an arbitrary
6550 expression to be aligned upwards (@code{ALIGN(@var{align})} is
6551 equivalent to @code{ALIGN(ABSOLUTE(.), @var{align})}).
6553 Here is an example which aligns the output @code{.data} section to the
6554 next @code{0x2000} byte boundary after the preceding section and sets a
6555 variable within the section to the next @code{0x8000} boundary after the
6560 .data ALIGN(0x2000): @{
6562 variable = ALIGN(0x8000);
6568 The first use of @code{ALIGN} in this example specifies the location of
6569 a section because it is used as the optional @var{address} attribute of
6570 a section definition (@pxref{Output Section Address}). The second use
6571 of @code{ALIGN} is used to defines the value of a symbol.
6573 The builtin function @code{NEXT} is closely related to @code{ALIGN}.
6575 @item ALIGNOF(@var{section})
6576 @kindex ALIGNOF(@var{section})
6577 @cindex section alignment
6578 Return the alignment in bytes of the named @var{section}, if that section has
6579 been allocated. If the section has not been allocated when this is
6580 evaluated, the linker will report an error. In the following example,
6581 the alignment of the @code{.output} section is stored as the first
6582 value in that section.
6587 LONG (ALIGNOF (.output))
6594 @item BLOCK(@var{exp})
6595 @kindex BLOCK(@var{exp})
6596 This is a synonym for @code{ALIGN}, for compatibility with older linker
6597 scripts. It is most often seen when setting the address of an output
6600 @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
6601 @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
6602 This is equivalent to either
6604 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1)))
6608 (ALIGN(@var{maxpagesize})
6609 + ((. + @var{commonpagesize} - 1) & (@var{maxpagesize} - @var{commonpagesize})))
6612 depending on whether the latter uses fewer @var{commonpagesize} sized pages
6613 for the data segment (area between the result of this expression and
6614 @code{DATA_SEGMENT_END}) than the former or not.
6615 If the latter form is used, it means @var{commonpagesize} bytes of runtime
6616 memory will be saved at the expense of up to @var{commonpagesize} wasted
6617 bytes in the on-disk file.
6619 This expression can only be used directly in @code{SECTIONS} commands, not in
6620 any output section descriptions and only once in the linker script.
6621 @var{commonpagesize} should be less or equal to @var{maxpagesize} and should
6622 be the system page size the object wants to be optimized for while still
6623 running on system page sizes up to @var{maxpagesize}. Note however
6624 that @samp{-z relro} protection will not be effective if the system
6625 page size is larger than @var{commonpagesize}.
6630 . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
6633 @item DATA_SEGMENT_END(@var{exp})
6634 @kindex DATA_SEGMENT_END(@var{exp})
6635 This defines the end of data segment for @code{DATA_SEGMENT_ALIGN}
6636 evaluation purposes.
6639 . = DATA_SEGMENT_END(.);
6642 @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6643 @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6644 This defines the end of the @code{PT_GNU_RELRO} segment when
6645 @samp{-z relro} option is used.
6646 When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END}
6647 does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that
6648 @var{exp} + @var{offset} is aligned to the @var{commonpagesize}
6649 argument given to @code{DATA_SEGMENT_ALIGN}. If present in the linker
6650 script, it must be placed between @code{DATA_SEGMENT_ALIGN} and
6651 @code{DATA_SEGMENT_END}. Evaluates to the second argument plus any
6652 padding needed at the end of the @code{PT_GNU_RELRO} segment due to
6656 . = DATA_SEGMENT_RELRO_END(24, .);
6659 @item DEFINED(@var{symbol})
6660 @kindex DEFINED(@var{symbol})
6661 @cindex symbol defaults
6662 Return 1 if @var{symbol} is in the linker global symbol table and is
6663 defined before the statement using DEFINED in the script, otherwise
6664 return 0. You can use this function to provide
6665 default values for symbols. For example, the following script fragment
6666 shows how to set a global symbol @samp{begin} to the first location in
6667 the @samp{.text} section---but if a symbol called @samp{begin} already
6668 existed, its value is preserved:
6674 begin = DEFINED(begin) ? begin : . ;
6682 @item LENGTH(@var{memory})
6683 @kindex LENGTH(@var{memory})
6684 Return the length of the memory region named @var{memory}.
6686 @item LOADADDR(@var{section})
6687 @kindex LOADADDR(@var{section})
6688 @cindex section load address in expression
6689 Return the absolute LMA of the named @var{section}. (@pxref{Output
6692 @item LOG2CEIL(@var{exp})
6693 @kindex LOG2CEIL(@var{exp})
6694 Return the binary logarithm of @var{exp} rounded towards infinity.
6695 @code{LOG2CEIL(0)} returns 0.
6698 @item MAX(@var{exp1}, @var{exp2})
6699 Returns the maximum of @var{exp1} and @var{exp2}.
6702 @item MIN(@var{exp1}, @var{exp2})
6703 Returns the minimum of @var{exp1} and @var{exp2}.
6705 @item NEXT(@var{exp})
6706 @kindex NEXT(@var{exp})
6707 @cindex unallocated address, next
6708 Return the next unallocated address that is a multiple of @var{exp}.
6709 This function is closely related to @code{ALIGN(@var{exp})}; unless you
6710 use the @code{MEMORY} command to define discontinuous memory for the
6711 output file, the two functions are equivalent.
6713 @item ORIGIN(@var{memory})
6714 @kindex ORIGIN(@var{memory})
6715 Return the origin of the memory region named @var{memory}.
6717 @item SEGMENT_START(@var{segment}, @var{default})
6718 @kindex SEGMENT_START(@var{segment}, @var{default})
6719 Return the base address of the named @var{segment}. If an explicit
6720 value has already been given for this segment (with a command-line
6721 @samp{-T} option) then that value will be returned otherwise the value
6722 will be @var{default}. At present, the @samp{-T} command-line option
6723 can only be used to set the base address for the ``text'', ``data'', and
6724 ``bss'' sections, but you can use @code{SEGMENT_START} with any segment
6727 @item SIZEOF(@var{section})
6728 @kindex SIZEOF(@var{section})
6729 @cindex section size
6730 Return the size in bytes of the named @var{section}, if that section has
6731 been allocated. If the section has not been allocated when this is
6732 evaluated, the linker will report an error. In the following example,
6733 @code{symbol_1} and @code{symbol_2} are assigned identical values:
6742 symbol_1 = .end - .start ;
6743 symbol_2 = SIZEOF(.output);
6748 @item SIZEOF_HEADERS
6749 @itemx sizeof_headers
6750 @kindex SIZEOF_HEADERS
6752 Return the size in bytes of the output file's headers. This is
6753 information which appears at the start of the output file. You can use
6754 this number when setting the start address of the first section, if you
6755 choose, to facilitate paging.
6757 @cindex not enough room for program headers
6758 @cindex program headers, not enough room
6759 When producing an ELF output file, if the linker script uses the
6760 @code{SIZEOF_HEADERS} builtin function, the linker must compute the
6761 number of program headers before it has determined all the section
6762 addresses and sizes. If the linker later discovers that it needs
6763 additional program headers, it will report an error @samp{not enough
6764 room for program headers}. To avoid this error, you must avoid using
6765 the @code{SIZEOF_HEADERS} function, or you must rework your linker
6766 script to avoid forcing the linker to use additional program headers, or
6767 you must define the program headers yourself using the @code{PHDRS}
6768 command (@pxref{PHDRS}).
6771 @node Implicit Linker Scripts
6772 @section Implicit Linker Scripts
6773 @cindex implicit linker scripts
6774 If you specify a linker input file which the linker can not recognize as
6775 an object file or an archive file, it will try to read the file as a
6776 linker script. If the file can not be parsed as a linker script, the
6777 linker will report an error.
6779 An implicit linker script will not replace the default linker script.
6781 Typically an implicit linker script would contain only symbol
6782 assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION}
6785 Any input files read because of an implicit linker script will be read
6786 at the position in the command line where the implicit linker script was
6787 read. This can affect archive searching.
6790 @node Machine Dependent
6791 @chapter Machine Dependent Features
6793 @cindex machine dependencies
6794 @command{ld} has additional features on some platforms; the following
6795 sections describe them. Machines where @command{ld} has no additional
6796 functionality are not listed.
6800 * H8/300:: @command{ld} and the H8/300
6803 * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families
6806 * ARM:: @command{ld} and the ARM family
6809 * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF
6812 * M68K:: @command{ld} and the Motorola 68K family
6815 * MIPS:: @command{ld} and the MIPS family
6818 * MMIX:: @command{ld} and MMIX
6821 * MSP430:: @command{ld} and MSP430
6824 * NDS32:: @command{ld} and NDS32
6827 * Nios II:: @command{ld} and the Altera Nios II
6830 * PowerPC ELF32:: @command{ld} and PowerPC 32-bit ELF Support
6833 * PowerPC64 ELF64:: @command{ld} and PowerPC64 64-bit ELF Support
6836 * S/390 ELF:: @command{ld} and S/390 ELF Support
6839 * SPU ELF:: @command{ld} and SPU ELF Support
6842 * TI COFF:: @command{ld} and TI COFF
6845 * WIN32:: @command{ld} and WIN32 (cygwin/mingw)
6848 * Xtensa:: @command{ld} and Xtensa Processors
6859 @section @command{ld} and the H8/300
6861 @cindex H8/300 support
6862 For the H8/300, @command{ld} can perform these global optimizations when
6863 you specify the @samp{--relax} command-line option.
6866 @cindex relaxing on H8/300
6867 @item relaxing address modes
6868 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
6869 targets are within eight bits, and turns them into eight-bit
6870 program-counter relative @code{bsr} and @code{bra} instructions,
6873 @cindex synthesizing on H8/300
6874 @item synthesizing instructions
6875 @c FIXME: specifically mov.b, or any mov instructions really? -> mov.b only, at least on H8, H8H, H8S
6876 @command{ld} finds all @code{mov.b} instructions which use the
6877 sixteen-bit absolute address form, but refer to the top
6878 page of memory, and changes them to use the eight-bit address form.
6879 (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into
6880 @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the
6881 top page of memory).
6883 @command{ld} finds all @code{mov} instructions which use the register
6884 indirect with 32-bit displacement addressing mode, but use a small
6885 displacement inside 16-bit displacement range, and changes them to use
6886 the 16-bit displacement form. (That is: the linker turns @samp{mov.b
6887 @code{@@}@var{d}:32,ERx} into @samp{mov.b @code{@@}@var{d}:16,ERx}
6888 whenever the displacement @var{d} is in the 16 bit signed integer
6889 range. Only implemented in ELF-format ld).
6891 @item bit manipulation instructions
6892 @command{ld} finds all bit manipulation instructions like @code{band, bclr,
6893 biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor}
6894 which use 32 bit and 16 bit absolute address form, but refer to the top
6895 page of memory, and changes them to use the 8 bit address form.
6896 (That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into
6897 @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in
6898 the top page of memory).
6900 @item system control instructions
6901 @command{ld} finds all @code{ldc.w, stc.w} instructions which use the
6902 32 bit absolute address form, but refer to the top page of memory, and
6903 changes them to use 16 bit address form.
6904 (That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into
6905 @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in
6906 the top page of memory).
6916 @c This stuff is pointless to say unless you're especially concerned
6917 @c with Renesas chips; don't enable it for generic case, please.
6919 @chapter @command{ld} and Other Renesas Chips
6921 @command{ld} also supports the Renesas (formerly Hitachi) H8/300H,
6922 H8/500, and SH chips. No special features, commands, or command-line
6923 options are required for these chips.
6937 @node M68HC11/68HC12
6938 @section @command{ld} and the Motorola 68HC11 and 68HC12 families
6940 @cindex M68HC11 and 68HC12 support
6942 @subsection Linker Relaxation
6944 For the Motorola 68HC11, @command{ld} can perform these global
6945 optimizations when you specify the @samp{--relax} command-line option.
6948 @cindex relaxing on M68HC11
6949 @item relaxing address modes
6950 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
6951 targets are within eight bits, and turns them into eight-bit
6952 program-counter relative @code{bsr} and @code{bra} instructions,
6955 @command{ld} also looks at all 16-bit extended addressing modes and
6956 transforms them in a direct addressing mode when the address is in
6957 page 0 (between 0 and 0x0ff).
6959 @item relaxing gcc instruction group
6960 When @command{gcc} is called with @option{-mrelax}, it can emit group
6961 of instructions that the linker can optimize to use a 68HC11 direct
6962 addressing mode. These instructions consists of @code{bclr} or
6963 @code{bset} instructions.
6967 @subsection Trampoline Generation
6969 @cindex trampoline generation on M68HC11
6970 @cindex trampoline generation on M68HC12
6971 For 68HC11 and 68HC12, @command{ld} can generate trampoline code to
6972 call a far function using a normal @code{jsr} instruction. The linker
6973 will also change the relocation to some far function to use the
6974 trampoline address instead of the function address. This is typically the
6975 case when a pointer to a function is taken. The pointer will in fact
6976 point to the function trampoline.
6984 @section @command{ld} and the ARM family
6986 @cindex ARM interworking support
6987 @kindex --support-old-code
6988 For the ARM, @command{ld} will generate code stubs to allow functions calls
6989 between ARM and Thumb code. These stubs only work with code that has
6990 been compiled and assembled with the @samp{-mthumb-interwork} command
6991 line option. If it is necessary to link with old ARM object files or
6992 libraries, which have not been compiled with the -mthumb-interwork
6993 option then the @samp{--support-old-code} command-line switch should be
6994 given to the linker. This will make it generate larger stub functions
6995 which will work with non-interworking aware ARM code. Note, however,
6996 the linker does not support generating stubs for function calls to
6997 non-interworking aware Thumb code.
6999 @cindex thumb entry point
7000 @cindex entry point, thumb
7001 @kindex --thumb-entry=@var{entry}
7002 The @samp{--thumb-entry} switch is a duplicate of the generic
7003 @samp{--entry} switch, in that it sets the program's starting address.
7004 But it also sets the bottom bit of the address, so that it can be
7005 branched to using a BX instruction, and the program will start
7006 executing in Thumb mode straight away.
7008 @cindex PE import table prefixing
7009 @kindex --use-nul-prefixed-import-tables
7010 The @samp{--use-nul-prefixed-import-tables} switch is specifying, that
7011 the import tables idata4 and idata5 have to be generated with a zero
7012 element prefix for import libraries. This is the old style to generate
7013 import tables. By default this option is turned off.
7017 The @samp{--be8} switch instructs @command{ld} to generate BE8 format
7018 executables. This option is only valid when linking big-endian
7019 objects - ie ones which have been assembled with the @option{-EB}
7020 option. The resulting image will contain big-endian data and
7024 @kindex --target1-rel
7025 @kindex --target1-abs
7026 The @samp{R_ARM_TARGET1} relocation is typically used for entries in the
7027 @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32}
7028 or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel}
7029 and @samp{--target1-abs} switches override the default.
7032 @kindex --target2=@var{type}
7033 The @samp{--target2=type} switch overrides the default definition of the
7034 @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their
7035 meanings, and target defaults are as follows:
7038 @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi)
7040 @samp{R_ARM_ABS32} (arm*-*-symbianelf)
7042 @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd)
7047 The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF
7048 specification) enables objects compiled for the ARMv4 architecture to be
7049 interworking-safe when linked with other objects compiled for ARMv4t, but
7050 also allows pure ARMv4 binaries to be built from the same ARMv4 objects.
7052 In the latter case, the switch @option{--fix-v4bx} must be passed to the
7053 linker, which causes v4t @code{BX rM} instructions to be rewritten as
7054 @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction.
7056 In the former case, the switch should not be used, and @samp{R_ARM_V4BX}
7057 relocations are ignored.
7059 @cindex FIX_V4BX_INTERWORKING
7060 @kindex --fix-v4bx-interworking
7061 Replace @code{BX rM} instructions identified by @samp{R_ARM_V4BX}
7062 relocations with a branch to the following veneer:
7070 This allows generation of libraries/applications that work on ARMv4 cores
7071 and are still interworking safe. Note that the above veneer clobbers the
7072 condition flags, so may cause incorrect program behavior in rare cases.
7076 The @samp{--use-blx} switch enables the linker to use ARM/Thumb
7077 BLX instructions (available on ARMv5t and above) in various
7078 situations. Currently it is used to perform calls via the PLT from Thumb
7079 code using BLX rather than using BX and a mode-switching stub before
7080 each PLT entry. This should lead to such calls executing slightly faster.
7082 This option is enabled implicitly for SymbianOS, so there is no need to
7083 specify it if you are using that target.
7085 @cindex VFP11_DENORM_FIX
7086 @kindex --vfp11-denorm-fix
7087 The @samp{--vfp11-denorm-fix} switch enables a link-time workaround for a
7088 bug in certain VFP11 coprocessor hardware, which sometimes allows
7089 instructions with denorm operands (which must be handled by support code)
7090 to have those operands overwritten by subsequent instructions before
7091 the support code can read the intended values.
7093 The bug may be avoided in scalar mode if you allow at least one
7094 intervening instruction between a VFP11 instruction which uses a register
7095 and another instruction which writes to the same register, or at least two
7096 intervening instructions if vector mode is in use. The bug only affects
7097 full-compliance floating-point mode: you do not need this workaround if
7098 you are using "runfast" mode. Please contact ARM for further details.
7100 If you know you are using buggy VFP11 hardware, you can
7101 enable this workaround by specifying the linker option
7102 @samp{--vfp-denorm-fix=scalar} if you are using the VFP11 scalar
7103 mode only, or @samp{--vfp-denorm-fix=vector} if you are using
7104 vector mode (the latter also works for scalar code). The default is
7105 @samp{--vfp-denorm-fix=none}.
7107 If the workaround is enabled, instructions are scanned for
7108 potentially-troublesome sequences, and a veneer is created for each
7109 such sequence which may trigger the erratum. The veneer consists of the
7110 first instruction of the sequence and a branch back to the subsequent
7111 instruction. The original instruction is then replaced with a branch to
7112 the veneer. The extra cycles required to call and return from the veneer
7113 are sufficient to avoid the erratum in both the scalar and vector cases.
7115 @cindex ARM1176 erratum workaround
7116 @kindex --fix-arm1176
7117 @kindex --no-fix-arm1176
7118 The @samp{--fix-arm1176} switch enables a link-time workaround for an erratum
7119 in certain ARM1176 processors. The workaround is enabled by default if you
7120 are targeting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled
7121 unconditionally by specifying @samp{--no-fix-arm1176}.
7123 Further information is available in the ``ARM1176JZ-S and ARM1176JZF-S
7124 Programmer Advice Notice'' available on the ARM documentation website at:
7125 http://infocenter.arm.com/.
7127 @cindex STM32L4xx erratum workaround
7128 @kindex --fix-stm32l4xx-629360
7130 The @samp{--fix-stm32l4xx-629360} switch enables a link-time
7131 workaround for a bug in the bus matrix / memory controller for some of
7132 the STM32 Cortex-M4 based products (STM32L4xx). When accessing
7133 off-chip memory via the affected bus for bus reads of 9 words or more,
7134 the bus can generate corrupt data and/or abort. These are only
7135 core-initiated accesses (not DMA), and might affect any access:
7136 integer loads such as LDM, POP and floating-point loads such as VLDM,
7137 VPOP. Stores are not affected.
7139 The bug can be avoided by splitting memory accesses into the
7140 necessary chunks to keep bus reads below 8 words.
7142 The workaround is not enabled by default, this is equivalent to use
7143 @samp{--fix-stm32l4xx-629360=none}. If you know you are using buggy
7144 STM32L4xx hardware, you can enable the workaround by specifying the
7145 linker option @samp{--fix-stm32l4xx-629360}, or the equivalent
7146 @samp{--fix-stm32l4xx-629360=default}.
7148 If the workaround is enabled, instructions are scanned for
7149 potentially-troublesome sequences, and a veneer is created for each
7150 such sequence which may trigger the erratum. The veneer consists in a
7151 replacement sequence emulating the behaviour of the original one and a
7152 branch back to the subsequent instruction. The original instruction is
7153 then replaced with a branch to the veneer.
7155 The workaround does not always preserve the memory access order for
7156 the LDMDB instruction, when the instruction loads the PC.
7158 The workaround is not able to handle problematic instructions when
7159 they are in the middle of an IT block, since a branch is not allowed
7160 there. In that case, the linker reports a warning and no replacement
7163 The workaround is not able to replace problematic instructions with a
7164 PC-relative branch instruction if the @samp{.text} section is too
7165 large. In that case, when the branch that replaces the original code
7166 cannot be encoded, the linker reports a warning and no replacement
7169 @cindex NO_ENUM_SIZE_WARNING
7170 @kindex --no-enum-size-warning
7171 The @option{--no-enum-size-warning} switch prevents the linker from
7172 warning when linking object files that specify incompatible EABI
7173 enumeration size attributes. For example, with this switch enabled,
7174 linking of an object file using 32-bit enumeration values with another
7175 using enumeration values fitted into the smallest possible space will
7178 @cindex NO_WCHAR_SIZE_WARNING
7179 @kindex --no-wchar-size-warning
7180 The @option{--no-wchar-size-warning} switch prevents the linker from
7181 warning when linking object files that specify incompatible EABI
7182 @code{wchar_t} size attributes. For example, with this switch enabled,
7183 linking of an object file using 32-bit @code{wchar_t} values with another
7184 using 16-bit @code{wchar_t} values will not be diagnosed.
7187 @kindex --pic-veneer
7188 The @samp{--pic-veneer} switch makes the linker use PIC sequences for
7189 ARM/Thumb interworking veneers, even if the rest of the binary
7190 is not PIC. This avoids problems on uClinux targets where
7191 @samp{--emit-relocs} is used to generate relocatable binaries.
7193 @cindex STUB_GROUP_SIZE
7194 @kindex --stub-group-size=@var{N}
7195 The linker will automatically generate and insert small sequences of
7196 code into a linked ARM ELF executable whenever an attempt is made to
7197 perform a function call to a symbol that is too far away. The
7198 placement of these sequences of instructions - called stubs - is
7199 controlled by the command-line option @option{--stub-group-size=N}.
7200 The placement is important because a poor choice can create a need for
7201 duplicate stubs, increasing the code size. The linker will try to
7202 group stubs together in order to reduce interruptions to the flow of
7203 code, but it needs guidance as to how big these groups should be and
7204 where they should be placed.
7206 The value of @samp{N}, the parameter to the
7207 @option{--stub-group-size=} option controls where the stub groups are
7208 placed. If it is negative then all stubs are placed after the first
7209 branch that needs them. If it is positive then the stubs can be
7210 placed either before or after the branches that need them. If the
7211 value of @samp{N} is 1 (either +1 or -1) then the linker will choose
7212 exactly where to place groups of stubs, using its built in heuristics.
7213 A value of @samp{N} greater than 1 (or smaller than -1) tells the
7214 linker that a single group of stubs can service at most @samp{N} bytes
7215 from the input sections.
7217 The default, if @option{--stub-group-size=} is not specified, is
7220 Farcalls stubs insertion is fully supported for the ARM-EABI target
7221 only, because it relies on object files properties not present
7224 @cindex Cortex-A8 erratum workaround
7225 @kindex --fix-cortex-a8
7226 @kindex --no-fix-cortex-a8
7227 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}.
7229 The erratum only affects Thumb-2 code. Please contact ARM for further details.
7231 @cindex Cortex-A53 erratum 835769 workaround
7232 @kindex --fix-cortex-a53-835769
7233 @kindex --no-fix-cortex-a53-835769
7234 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}.
7236 Please contact ARM for further details.
7238 @kindex --merge-exidx-entries
7239 @kindex --no-merge-exidx-entries
7240 @cindex Merging exidx entries
7241 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent exidx entries in debuginfo.
7244 @cindex 32-bit PLT entries
7245 The @samp{--long-plt} option enables the use of 16 byte PLT entries
7246 which support up to 4Gb of code. The default is to use 12 byte PLT
7247 entries which only support 512Mb of code.
7249 @kindex --no-apply-dynamic-relocs
7250 @cindex AArch64 rela addend
7251 The @samp{--no-apply-dynamic-relocs} option makes AArch64 linker do not apply
7252 link-time values for dynamic relocations.
7254 @cindex Placement of SG veneers
7255 All SG veneers are placed in the special output section @code{.gnu.sgstubs}.
7256 Its start address must be set, either with the command-line option
7257 @samp{--section-start} or in a linker script, to indicate where to place these
7260 @kindex --cmse-implib
7261 @cindex Secure gateway import library
7262 The @samp{--cmse-implib} option requests that the import libraries
7263 specified by the @samp{--out-implib} and @samp{--in-implib} options are
7264 secure gateway import libraries, suitable for linking a non-secure
7265 executable against secure code as per ARMv8-M Security Extensions.
7267 @kindex --in-implib=@var{file}
7268 @cindex Input import library
7269 The @samp{--in-implib=file} specifies an input import library whose symbols
7270 must keep the same address in the executable being produced. A warning is
7271 given if no @samp{--out-implib} is given but new symbols have been introduced
7272 in the executable that should be listed in its import library. Otherwise, if
7273 @samp{--out-implib} is specified, the symbols are added to the output import
7274 library. A warning is also given if some symbols present in the input import
7275 library have disappeared from the executable. This option is only effective
7276 for Secure Gateway import libraries, ie. when @samp{--cmse-implib} is
7290 @section @command{ld} and HPPA 32-bit ELF Support
7291 @cindex HPPA multiple sub-space stubs
7292 @kindex --multi-subspace
7293 When generating a shared library, @command{ld} will by default generate
7294 import stubs suitable for use with a single sub-space application.
7295 The @samp{--multi-subspace} switch causes @command{ld} to generate export
7296 stubs, and different (larger) import stubs suitable for use with
7297 multiple sub-spaces.
7299 @cindex HPPA stub grouping
7300 @kindex --stub-group-size=@var{N}
7301 Long branch stubs and import/export stubs are placed by @command{ld} in
7302 stub sections located between groups of input sections.
7303 @samp{--stub-group-size} specifies the maximum size of a group of input
7304 sections handled by one stub section. Since branch offsets are signed,
7305 a stub section may serve two groups of input sections, one group before
7306 the stub section, and one group after it. However, when using
7307 conditional branches that require stubs, it may be better (for branch
7308 prediction) that stub sections only serve one group of input sections.
7309 A negative value for @samp{N} chooses this scheme, ensuring that
7310 branches to stubs always use a negative offset. Two special values of
7311 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
7312 @command{ld} to automatically size input section groups for the branch types
7313 detected, with the same behaviour regarding stub placement as other
7314 positive or negative values of @samp{N} respectively.
7316 Note that @samp{--stub-group-size} does not split input sections. A
7317 single input section larger than the group size specified will of course
7318 create a larger group (of one section). If input sections are too
7319 large, it may not be possible for a branch to reach its stub.
7332 @section @command{ld} and the Motorola 68K family
7334 @cindex Motorola 68K GOT generation
7335 @kindex --got=@var{type}
7336 The @samp{--got=@var{type}} option lets you choose the GOT generation scheme.
7337 The choices are @samp{single}, @samp{negative}, @samp{multigot} and
7338 @samp{target}. When @samp{target} is selected the linker chooses
7339 the default GOT generation scheme for the current target.
7340 @samp{single} tells the linker to generate a single GOT with
7341 entries only at non-negative offsets.
7342 @samp{negative} instructs the linker to generate a single GOT with
7343 entries at both negative and positive offsets. Not all environments
7345 @samp{multigot} allows the linker to generate several GOTs in the
7346 output file. All GOT references from a single input object
7347 file access the same GOT, but references from different input object
7348 files might access different GOTs. Not all environments support such GOTs.
7361 @section @command{ld} and the MIPS family
7363 @cindex MIPS microMIPS instruction choice selection
7366 The @samp{--insn32} and @samp{--no-insn32} options control the choice of
7367 microMIPS instructions used in code generated by the linker, such as that
7368 in the PLT or lazy binding stubs, or in relaxation. If @samp{--insn32} is
7369 used, then the linker only uses 32-bit instruction encodings. By default
7370 or if @samp{--no-insn32} is used, all instruction encodings are used,
7371 including 16-bit ones where possible.
7373 @cindex MIPS branch relocation check control
7374 @kindex --ignore-branch-isa
7375 @kindex --no-ignore-branch-isa
7376 The @samp{--ignore-branch-isa} and @samp{--no-ignore-branch-isa} options
7377 control branch relocation checks for invalid ISA mode transitions. If
7378 @samp{--ignore-branch-isa} is used, then the linker accepts any branch
7379 relocations and any ISA mode transition required is lost in relocation
7380 calculation, except for some cases of @code{BAL} instructions which meet
7381 relaxation conditions and are converted to equivalent @code{JALX}
7382 instructions as the associated relocation is calculated. By default
7383 or if @samp{--no-ignore-branch-isa} is used a check is made causing
7384 the loss of an ISA mode transition to produce an error.
7397 @section @code{ld} and MMIX
7398 For MMIX, there is a choice of generating @code{ELF} object files or
7399 @code{mmo} object files when linking. The simulator @code{mmix}
7400 understands the @code{mmo} format. The binutils @code{objcopy} utility
7401 can translate between the two formats.
7403 There is one special section, the @samp{.MMIX.reg_contents} section.
7404 Contents in this section is assumed to correspond to that of global
7405 registers, and symbols referring to it are translated to special symbols,
7406 equal to registers. In a final link, the start address of the
7407 @samp{.MMIX.reg_contents} section corresponds to the first allocated
7408 global register multiplied by 8. Register @code{$255} is not included in
7409 this section; it is always set to the program entry, which is at the
7410 symbol @code{Main} for @code{mmo} files.
7412 Global symbols with the prefix @code{__.MMIX.start.}, for example
7413 @code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special.
7414 The default linker script uses these to set the default start address
7417 Initial and trailing multiples of zero-valued 32-bit words in a section,
7418 are left out from an mmo file.
7431 @section @code{ld} and MSP430
7432 For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]}
7433 will select an appropriate linker script for selected MPU type. (To get a list of known MPUs
7434 just pass @samp{-m help} option to the linker).
7436 @cindex MSP430 extra sections
7437 The linker will recognize some extra sections which are MSP430 specific:
7440 @item @samp{.vectors}
7441 Defines a portion of ROM where interrupt vectors located.
7443 @item @samp{.bootloader}
7444 Defines the bootloader portion of the ROM (if applicable). Any code
7445 in this section will be uploaded to the MPU.
7447 @item @samp{.infomem}
7448 Defines an information memory section (if applicable). Any code in
7449 this section will be uploaded to the MPU.
7451 @item @samp{.infomemnobits}
7452 This is the same as the @samp{.infomem} section except that any code
7453 in this section will not be uploaded to the MPU.
7455 @item @samp{.noinit}
7456 Denotes a portion of RAM located above @samp{.bss} section.
7458 The last two sections are used by gcc.
7462 @cindex MSP430 Options
7463 @kindex --code-region
7464 @item --code-region=[either,lower,upper,none]
7465 This will transform .text* sections to [either,lower,upper].text* sections. The
7466 argument passed to GCC for -mcode-region is propagated to the linker
7469 @kindex --data-region
7470 @item --data-region=[either,lower,upper,none]
7471 This will transform .data*, .bss* and .rodata* sections to
7472 [either,lower,upper].[data,bss,rodata]* sections. The argument passed to GCC
7473 for -mdata-region is propagated to the linker using this option.
7475 @kindex --disable-sec-transformation
7476 @item --disable-sec-transformation
7477 Prevent the transformation of sections as specified by the @code{--code-region}
7478 and @code{--data-region} options.
7479 This is useful if you are compiling and linking using a single call to the GCC
7480 wrapper, and want to compile the source files using -m[code,data]-region but
7481 not transform the sections for prebuilt libraries and objects.
7495 @section @code{ld} and NDS32
7496 @kindex relaxing on NDS32
7497 For NDS32, there are some options to select relaxation behavior. The linker
7498 relaxes objects according to these options.
7501 @item @samp{--m[no-]fp-as-gp}
7502 Disable/enable fp-as-gp relaxation.
7504 @item @samp{--mexport-symbols=FILE}
7505 Exporting symbols and their address into FILE as linker script.
7507 @item @samp{--m[no-]ex9}
7508 Disable/enable link-time EX9 relaxation.
7510 @item @samp{--mexport-ex9=FILE}
7511 Export the EX9 table after linking.
7513 @item @samp{--mimport-ex9=FILE}
7514 Import the Ex9 table for EX9 relaxation.
7516 @item @samp{--mupdate-ex9}
7517 Update the existing EX9 table.
7519 @item @samp{--mex9-limit=NUM}
7520 Maximum number of entries in the ex9 table.
7522 @item @samp{--mex9-loop-aware}
7523 Avoid generating the EX9 instruction inside the loop.
7525 @item @samp{--m[no-]ifc}
7526 Disable/enable the link-time IFC optimization.
7528 @item @samp{--mifc-loop-aware}
7529 Avoid generating the IFC instruction inside the loop.
7543 @section @command{ld} and the Altera Nios II
7544 @cindex Nios II call relaxation
7545 @kindex --relax on Nios II
7547 Call and immediate jump instructions on Nios II processors are limited to
7548 transferring control to addresses in the same 256MB memory segment,
7549 which may result in @command{ld} giving
7550 @samp{relocation truncated to fit} errors with very large programs.
7551 The command-line option @option{--relax} enables the generation of
7552 trampolines that can access the entire 32-bit address space for calls
7553 outside the normal @code{call} and @code{jmpi} address range. These
7554 trampolines are inserted at section boundaries, so may not themselves
7555 be reachable if an input section and its associated call trampolines are
7558 The @option{--relax} option is enabled by default unless @option{-r}
7559 is also specified. You can disable trampoline generation by using the
7560 @option{--no-relax} linker option. You can also disable this optimization
7561 locally by using the @samp{set .noat} directive in assembly-language
7562 source files, as the linker-inserted trampolines use the @code{at}
7563 register as a temporary.
7565 Note that the linker @option{--relax} option is independent of assembler
7566 relaxation options, and that using the GNU assembler's @option{-relax-all}
7567 option interferes with the linker's more selective call instruction relaxation.
7580 @section @command{ld} and PowerPC 32-bit ELF Support
7581 @cindex PowerPC long branches
7582 @kindex --relax on PowerPC
7583 Branches on PowerPC processors are limited to a signed 26-bit
7584 displacement, which may result in @command{ld} giving
7585 @samp{relocation truncated to fit} errors with very large programs.
7586 @samp{--relax} enables the generation of trampolines that can access
7587 the entire 32-bit address space. These trampolines are inserted at
7588 section boundaries, so may not themselves be reachable if an input
7589 section exceeds 33M in size. You may combine @samp{-r} and
7590 @samp{--relax} to add trampolines in a partial link. In that case
7591 both branches to undefined symbols and inter-section branches are also
7592 considered potentially out of range, and trampolines inserted.
7594 @cindex PowerPC ELF32 options
7599 Current PowerPC GCC accepts a @samp{-msecure-plt} option that
7600 generates code capable of using a newer PLT and GOT layout that has
7601 the security advantage of no executable section ever needing to be
7602 writable and no writable section ever being executable. PowerPC
7603 @command{ld} will generate this layout, including stubs to access the
7604 PLT, if all input files (including startup and static libraries) were
7605 compiled with @samp{-msecure-plt}. @samp{--bss-plt} forces the old
7606 BSS PLT (and GOT layout) which can give slightly better performance.
7608 @kindex --secure-plt
7610 @command{ld} will use the new PLT and GOT layout if it is linking new
7611 @samp{-fpic} or @samp{-fPIC} code, but does not do so automatically
7612 when linking non-PIC code. This option requests the new PLT and GOT
7613 layout. A warning will be given if some object file requires the old
7619 The new secure PLT and GOT are placed differently relative to other
7620 sections compared to older BSS PLT and GOT placement. The location of
7621 @code{.plt} must change because the new secure PLT is an initialized
7622 section while the old PLT is uninitialized. The reason for the
7623 @code{.got} change is more subtle: The new placement allows
7624 @code{.got} to be read-only in applications linked with
7625 @samp{-z relro -z now}. However, this placement means that
7626 @code{.sdata} cannot always be used in shared libraries, because the
7627 PowerPC ABI accesses @code{.sdata} in shared libraries from the GOT
7628 pointer. @samp{--sdata-got} forces the old GOT placement. PowerPC
7629 GCC doesn't use @code{.sdata} in shared libraries, so this option is
7630 really only useful for other compilers that may do so.
7632 @cindex PowerPC stub symbols
7633 @kindex --emit-stub-syms
7634 @item --emit-stub-syms
7635 This option causes @command{ld} to label linker stubs with a local
7636 symbol that encodes the stub type and destination.
7638 @cindex PowerPC TLS optimization
7639 @kindex --no-tls-optimize
7640 @item --no-tls-optimize
7641 PowerPC @command{ld} normally performs some optimization of code
7642 sequences used to access Thread-Local Storage. Use this option to
7643 disable the optimization.
7656 @node PowerPC64 ELF64
7657 @section @command{ld} and PowerPC64 64-bit ELF Support
7659 @cindex PowerPC64 ELF64 options
7661 @cindex PowerPC64 stub grouping
7662 @kindex --stub-group-size
7663 @item --stub-group-size
7664 Long branch stubs, PLT call stubs and TOC adjusting stubs are placed
7665 by @command{ld} in stub sections located between groups of input sections.
7666 @samp{--stub-group-size} specifies the maximum size of a group of input
7667 sections handled by one stub section. Since branch offsets are signed,
7668 a stub section may serve two groups of input sections, one group before
7669 the stub section, and one group after it. However, when using
7670 conditional branches that require stubs, it may be better (for branch
7671 prediction) that stub sections only serve one group of input sections.
7672 A negative value for @samp{N} chooses this scheme, ensuring that
7673 branches to stubs always use a negative offset. Two special values of
7674 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
7675 @command{ld} to automatically size input section groups for the branch types
7676 detected, with the same behaviour regarding stub placement as other
7677 positive or negative values of @samp{N} respectively.
7679 Note that @samp{--stub-group-size} does not split input sections. A
7680 single input section larger than the group size specified will of course
7681 create a larger group (of one section). If input sections are too
7682 large, it may not be possible for a branch to reach its stub.
7684 @cindex PowerPC64 stub symbols
7685 @kindex --emit-stub-syms
7686 @item --emit-stub-syms
7687 This option causes @command{ld} to label linker stubs with a local
7688 symbol that encodes the stub type and destination.
7690 @cindex PowerPC64 dot symbols
7692 @kindex --no-dotsyms
7695 These two options control how @command{ld} interprets version patterns
7696 in a version script. Older PowerPC64 compilers emitted both a
7697 function descriptor symbol with the same name as the function, and a
7698 code entry symbol with the name prefixed by a dot (@samp{.}). To
7699 properly version a function @samp{foo}, the version script thus needs
7700 to control both @samp{foo} and @samp{.foo}. The option
7701 @samp{--dotsyms}, on by default, automatically adds the required
7702 dot-prefixed patterns. Use @samp{--no-dotsyms} to disable this
7705 @cindex PowerPC64 register save/restore functions
7706 @kindex --save-restore-funcs
7707 @kindex --no-save-restore-funcs
7708 @item --save-restore-funcs
7709 @itemx --no-save-restore-funcs
7710 These two options control whether PowerPC64 @command{ld} automatically
7711 provides out-of-line register save and restore functions used by
7712 @samp{-Os} code. The default is to provide any such referenced
7713 function for a normal final link, and to not do so for a relocatable
7716 @cindex PowerPC64 TLS optimization
7717 @kindex --no-tls-optimize
7718 @item --no-tls-optimize
7719 PowerPC64 @command{ld} normally performs some optimization of code
7720 sequences used to access Thread-Local Storage. Use this option to
7721 disable the optimization.
7723 @cindex PowerPC64 __tls_get_addr optimization
7724 @kindex --tls-get-addr-optimize
7725 @kindex --no-tls-get-addr-optimize
7726 @kindex --tls-get-addr-regsave
7727 @kindex --no-tls-get-addr-regsave
7728 @item --tls-get-addr-optimize
7729 @itemx --no-tls-get-addr-optimize
7730 These options control how PowerPC64 @command{ld} uses a special
7731 stub to call __tls_get_addr. PowerPC64 glibc 2.22 and later support
7732 an optimization that allows the second and subsequent calls to
7733 @code{__tls_get_addr} for a given symbol to be resolved by the special
7734 stub without calling in to glibc. By default the linker enables
7735 generation of the stub when glibc advertises the availability of
7737 Using @option{--tls-get-addr-optimize} with an older glibc won't do
7738 much besides slow down your applications, but may be useful if linking
7739 an application against an older glibc with the expectation that it
7740 will normally be used on systems having a newer glibc.
7741 @option{--tls-get-addr-regsave} forces generation of a stub that saves
7742 and restores volatile registers around the call into glibc. Normally,
7743 this is done when the linker detects a call to __tls_get_addr_desc.
7744 Such calls then go via the register saving stub to __tls_get_addr_opt.
7745 @option{--no-tls-get-addr-regsave} disables generation of the
7748 @cindex PowerPC64 OPD optimization
7749 @kindex --no-opd-optimize
7750 @item --no-opd-optimize
7751 PowerPC64 @command{ld} normally removes @code{.opd} section entries
7752 corresponding to deleted link-once functions, or functions removed by
7753 the action of @samp{--gc-sections} or linker script @code{/DISCARD/}.
7754 Use this option to disable @code{.opd} optimization.
7756 @cindex PowerPC64 OPD spacing
7757 @kindex --non-overlapping-opd
7758 @item --non-overlapping-opd
7759 Some PowerPC64 compilers have an option to generate compressed
7760 @code{.opd} entries spaced 16 bytes apart, overlapping the third word,
7761 the static chain pointer (unused in C) with the first word of the next
7762 entry. This option expands such entries to the full 24 bytes.
7764 @cindex PowerPC64 TOC optimization
7765 @kindex --no-toc-optimize
7766 @item --no-toc-optimize
7767 PowerPC64 @command{ld} normally removes unused @code{.toc} section
7768 entries. Such entries are detected by examining relocations that
7769 reference the TOC in code sections. A reloc in a deleted code section
7770 marks a TOC word as unneeded, while a reloc in a kept code section
7771 marks a TOC word as needed. Since the TOC may reference itself, TOC
7772 relocs are also examined. TOC words marked as both needed and
7773 unneeded will of course be kept. TOC words without any referencing
7774 reloc are assumed to be part of a multi-word entry, and are kept or
7775 discarded as per the nearest marked preceding word. This works
7776 reliably for compiler generated code, but may be incorrect if assembly
7777 code is used to insert TOC entries. Use this option to disable the
7780 @cindex PowerPC64 multi-TOC
7781 @kindex --no-multi-toc
7782 @item --no-multi-toc
7783 If given any toc option besides @code{-mcmodel=medium} or
7784 @code{-mcmodel=large}, PowerPC64 GCC generates code for a TOC model
7786 entries are accessed with a 16-bit offset from r2. This limits the
7787 total TOC size to 64K. PowerPC64 @command{ld} extends this limit by
7788 grouping code sections such that each group uses less than 64K for its
7789 TOC entries, then inserts r2 adjusting stubs between inter-group
7790 calls. @command{ld} does not split apart input sections, so cannot
7791 help if a single input file has a @code{.toc} section that exceeds
7792 64K, most likely from linking multiple files with @command{ld -r}.
7793 Use this option to turn off this feature.
7795 @cindex PowerPC64 TOC sorting
7796 @kindex --no-toc-sort
7798 By default, @command{ld} sorts TOC sections so that those whose file
7799 happens to have a section called @code{.init} or @code{.fini} are
7800 placed first, followed by TOC sections referenced by code generated
7801 with PowerPC64 gcc's @code{-mcmodel=small}, and lastly TOC sections
7802 referenced only by code generated with PowerPC64 gcc's
7803 @code{-mcmodel=medium} or @code{-mcmodel=large} options. Doing this
7804 results in better TOC grouping for multi-TOC. Use this option to turn
7807 @cindex PowerPC64 PLT stub alignment
7809 @kindex --no-plt-align
7811 @itemx --no-plt-align
7812 Use these options to control whether individual PLT call stubs are
7813 aligned to a 32-byte boundary, or to the specified power of two
7814 boundary when using @code{--plt-align=}. A negative value may be
7815 specified to pad PLT call stubs so that they do not cross the
7816 specified power of two boundary (or the minimum number of boundaries
7817 if a PLT stub is so large that it must cross a boundary). By default
7818 PLT call stubs are aligned to 32-byte boundaries.
7820 @cindex PowerPC64 PLT call stub static chain
7821 @kindex --plt-static-chain
7822 @kindex --no-plt-static-chain
7823 @item --plt-static-chain
7824 @itemx --no-plt-static-chain
7825 Use these options to control whether PLT call stubs load the static
7826 chain pointer (r11). @code{ld} defaults to not loading the static
7827 chain since there is never any need to do so on a PLT call.
7829 @cindex PowerPC64 PLT call stub thread safety
7830 @kindex --plt-thread-safe
7831 @kindex --no-plt-thread-safe
7832 @item --plt-thread-safe
7833 @itemx --no-plt-thread-safe
7834 With power7's weakly ordered memory model, it is possible when using
7835 lazy binding for ld.so to update a plt entry in one thread and have
7836 another thread see the individual plt entry words update in the wrong
7837 order, despite ld.so carefully writing in the correct order and using
7838 memory write barriers. To avoid this we need some sort of read
7839 barrier in the call stub, or use LD_BIND_NOW=1. By default, @code{ld}
7840 looks for calls to commonly used functions that create threads, and if
7841 seen, adds the necessary barriers. Use these options to change the
7844 @cindex PowerPC64 ELFv2 PLT localentry optimization
7845 @kindex --plt-localentry
7846 @kindex --no-plt-localentry
7847 @item --plt-localentry
7848 @itemx --no-localentry
7849 ELFv2 functions with localentry:0 are those with a single entry point,
7850 ie. global entry == local entry, and that have no requirement on r2
7851 (the TOC/GOT pointer) or r12, and guarantee r2 is unchanged on return.
7852 Such an external function can be called via the PLT without saving r2
7853 or restoring it on return, avoiding a common load-hit-store for small
7854 functions. The optimization is attractive, with up to 40% reduction
7855 in execution time for a small function, but can result in symbol
7856 interposition failures. Also, minor changes in a shared library,
7857 including system libraries, can cause a function that was localentry:0
7858 to become localentry:8. This will result in a dynamic loader
7859 complaint and failure to run. The option is experimental, use with
7860 care. @option{--no-plt-localentry} is the default.
7874 @section @command{ld} and S/390 ELF Support
7876 @cindex S/390 ELF options
7880 @kindex --s390-pgste
7882 This option marks the result file with a @code{PT_S390_PGSTE}
7883 segment. The Linux kernel is supposed to allocate 4k page tables for
7884 binaries marked that way.
7898 @section @command{ld} and SPU ELF Support
7900 @cindex SPU ELF options
7906 This option marks an executable as a PIC plugin module.
7908 @cindex SPU overlays
7909 @kindex --no-overlays
7911 Normally, @command{ld} recognizes calls to functions within overlay
7912 regions, and redirects such calls to an overlay manager via a stub.
7913 @command{ld} also provides a built-in overlay manager. This option
7914 turns off all this special overlay handling.
7916 @cindex SPU overlay stub symbols
7917 @kindex --emit-stub-syms
7918 @item --emit-stub-syms
7919 This option causes @command{ld} to label overlay stubs with a local
7920 symbol that encodes the stub type and destination.
7922 @cindex SPU extra overlay stubs
7923 @kindex --extra-overlay-stubs
7924 @item --extra-overlay-stubs
7925 This option causes @command{ld} to add overlay call stubs on all
7926 function calls out of overlay regions. Normally stubs are not added
7927 on calls to non-overlay regions.
7929 @cindex SPU local store size
7930 @kindex --local-store=lo:hi
7931 @item --local-store=lo:hi
7932 @command{ld} usually checks that a final executable for SPU fits in
7933 the address range 0 to 256k. This option may be used to change the
7934 range. Disable the check entirely with @option{--local-store=0:0}.
7937 @kindex --stack-analysis
7938 @item --stack-analysis
7939 SPU local store space is limited. Over-allocation of stack space
7940 unnecessarily limits space available for code and data, while
7941 under-allocation results in runtime failures. If given this option,
7942 @command{ld} will provide an estimate of maximum stack usage.
7943 @command{ld} does this by examining symbols in code sections to
7944 determine the extents of functions, and looking at function prologues
7945 for stack adjusting instructions. A call-graph is created by looking
7946 for relocations on branch instructions. The graph is then searched
7947 for the maximum stack usage path. Note that this analysis does not
7948 find calls made via function pointers, and does not handle recursion
7949 and other cycles in the call graph. Stack usage may be
7950 under-estimated if your code makes such calls. Also, stack usage for
7951 dynamic allocation, e.g. alloca, will not be detected. If a link map
7952 is requested, detailed information about each function's stack usage
7953 and calls will be given.
7956 @kindex --emit-stack-syms
7957 @item --emit-stack-syms
7958 This option, if given along with @option{--stack-analysis} will result
7959 in @command{ld} emitting stack sizing symbols for each function.
7960 These take the form @code{__stack_<function_name>} for global
7961 functions, and @code{__stack_<number>_<function_name>} for static
7962 functions. @code{<number>} is the section id in hex. The value of
7963 such symbols is the stack requirement for the corresponding function.
7964 The symbol size will be zero, type @code{STT_NOTYPE}, binding
7965 @code{STB_LOCAL}, and section @code{SHN_ABS}.
7979 @section @command{ld}'s Support for Various TI COFF Versions
7980 @cindex TI COFF versions
7981 @kindex --format=@var{version}
7982 The @samp{--format} switch allows selection of one of the various
7983 TI COFF versions. The latest of this writing is 2; versions 0 and 1 are
7984 also supported. The TI COFF versions also vary in header byte-order
7985 format; @command{ld} will read any version or byte order, but the output
7986 header format depends on the default specified by the specific target.
7999 @section @command{ld} and WIN32 (cygwin/mingw)
8001 This section describes some of the win32 specific @command{ld} issues.
8002 See @ref{Options,,Command-line Options} for detailed description of the
8003 command-line options mentioned here.
8006 @cindex import libraries
8007 @item import libraries
8008 The standard Windows linker creates and uses so-called import
8009 libraries, which contains information for linking to dll's. They are
8010 regular static archives and are handled as any other static
8011 archive. The cygwin and mingw ports of @command{ld} have specific
8012 support for creating such libraries provided with the
8013 @samp{--out-implib} command-line option.
8015 @item exporting DLL symbols
8016 @cindex exporting DLL symbols
8017 The cygwin/mingw @command{ld} has several ways to export symbols for dll's.
8020 @item using auto-export functionality
8021 @cindex using auto-export functionality
8022 By default @command{ld} exports symbols with the auto-export functionality,
8023 which is controlled by the following command-line options:
8026 @item --export-all-symbols [This is the default]
8027 @item --exclude-symbols
8028 @item --exclude-libs
8029 @item --exclude-modules-for-implib
8030 @item --version-script
8033 When auto-export is in operation, @command{ld} will export all the non-local
8034 (global and common) symbols it finds in a DLL, with the exception of a few
8035 symbols known to belong to the system's runtime and libraries. As it will
8036 often not be desirable to export all of a DLL's symbols, which may include
8037 private functions that are not part of any public interface, the command-line
8038 options listed above may be used to filter symbols out from the list for
8039 exporting. The @samp{--output-def} option can be used in order to see the
8040 final list of exported symbols with all exclusions taken into effect.
8042 If @samp{--export-all-symbols} is not given explicitly on the
8043 command line, then the default auto-export behavior will be @emph{disabled}
8044 if either of the following are true:
8047 @item A DEF file is used.
8048 @item Any symbol in any object file was marked with the __declspec(dllexport) attribute.
8051 @item using a DEF file
8052 @cindex using a DEF file
8053 Another way of exporting symbols is using a DEF file. A DEF file is
8054 an ASCII file containing definitions of symbols which should be
8055 exported when a dll is created. Usually it is named @samp{<dll
8056 name>.def} and is added as any other object file to the linker's
8057 command line. The file's name must end in @samp{.def} or @samp{.DEF}.
8060 gcc -o <output> <objectfiles> <dll name>.def
8063 Using a DEF file turns off the normal auto-export behavior, unless the
8064 @samp{--export-all-symbols} option is also used.
8066 Here is an example of a DEF file for a shared library called @samp{xyz.dll}:
8069 LIBRARY "xyz.dll" BASE=0x20000000
8075 another_foo = abc.dll.afoo
8081 This example defines a DLL with a non-default base address and seven
8082 symbols in the export table. The third exported symbol @code{_bar} is an
8083 alias for the second. The fourth symbol, @code{another_foo} is resolved
8084 by "forwarding" to another module and treating it as an alias for
8085 @code{afoo} exported from the DLL @samp{abc.dll}. The final symbol
8086 @code{var1} is declared to be a data object. The @samp{doo} symbol in
8087 export library is an alias of @samp{foo}, which gets the string name
8088 in export table @samp{foo2}. The @samp{eoo} symbol is an data export
8089 symbol, which gets in export table the name @samp{var1}.
8091 The optional @code{LIBRARY <name>} command indicates the @emph{internal}
8092 name of the output DLL. If @samp{<name>} does not include a suffix,
8093 the default library suffix, @samp{.DLL} is appended.
8095 When the .DEF file is used to build an application, rather than a
8096 library, the @code{NAME <name>} command should be used instead of
8097 @code{LIBRARY}. If @samp{<name>} does not include a suffix, the default
8098 executable suffix, @samp{.EXE} is appended.
8100 With either @code{LIBRARY <name>} or @code{NAME <name>} the optional
8101 specification @code{BASE = <number>} may be used to specify a
8102 non-default base address for the image.
8104 If neither @code{LIBRARY <name>} nor @code{NAME <name>} is specified,
8105 or they specify an empty string, the internal name is the same as the
8106 filename specified on the command line.
8108 The complete specification of an export symbol is:
8112 ( ( ( <name1> [ = <name2> ] )
8113 | ( <name1> = <module-name> . <external-name>))
8114 [ @@ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
8117 Declares @samp{<name1>} as an exported symbol from the DLL, or declares
8118 @samp{<name1>} as an exported alias for @samp{<name2>}; or declares
8119 @samp{<name1>} as a "forward" alias for the symbol
8120 @samp{<external-name>} in the DLL @samp{<module-name>}.
8121 Optionally, the symbol may be exported by the specified ordinal
8122 @samp{<integer>} alias. The optional @samp{<name3>} is the to be used
8123 string in import/export table for the symbol.
8125 The optional keywords that follow the declaration indicate:
8127 @code{NONAME}: Do not put the symbol name in the DLL's export table. It
8128 will still be exported by its ordinal alias (either the value specified
8129 by the .def specification or, otherwise, the value assigned by the
8130 linker). The symbol name, however, does remain visible in the import
8131 library (if any), unless @code{PRIVATE} is also specified.
8133 @code{DATA}: The symbol is a variable or object, rather than a function.
8134 The import lib will export only an indirect reference to @code{foo} as
8135 the symbol @code{_imp__foo} (ie, @code{foo} must be resolved as
8138 @code{CONSTANT}: Like @code{DATA}, but put the undecorated @code{foo} as
8139 well as @code{_imp__foo} into the import library. Both refer to the
8140 read-only import address table's pointer to the variable, not to the
8141 variable itself. This can be dangerous. If the user code fails to add
8142 the @code{dllimport} attribute and also fails to explicitly add the
8143 extra indirection that the use of the attribute enforces, the
8144 application will behave unexpectedly.
8146 @code{PRIVATE}: Put the symbol in the DLL's export table, but do not put
8147 it into the static import library used to resolve imports at link time. The
8148 symbol can still be imported using the @code{LoadLibrary/GetProcAddress}
8149 API at runtime or by using the GNU ld extension of linking directly to
8150 the DLL without an import library.
8152 See ld/deffilep.y in the binutils sources for the full specification of
8153 other DEF file statements
8155 @cindex creating a DEF file
8156 While linking a shared dll, @command{ld} is able to create a DEF file
8157 with the @samp{--output-def <file>} command-line option.
8159 @item Using decorations
8160 @cindex Using decorations
8161 Another way of marking symbols for export is to modify the source code
8162 itself, so that when building the DLL each symbol to be exported is
8166 __declspec(dllexport) int a_variable
8167 __declspec(dllexport) void a_function(int with_args)
8170 All such symbols will be exported from the DLL. If, however,
8171 any of the object files in the DLL contain symbols decorated in
8172 this way, then the normal auto-export behavior is disabled, unless
8173 the @samp{--export-all-symbols} option is also used.
8175 Note that object files that wish to access these symbols must @emph{not}
8176 decorate them with dllexport. Instead, they should use dllimport,
8180 __declspec(dllimport) int a_variable
8181 __declspec(dllimport) void a_function(int with_args)
8184 This complicates the structure of library header files, because
8185 when included by the library itself the header must declare the
8186 variables and functions as dllexport, but when included by client
8187 code the header must declare them as dllimport. There are a number
8188 of idioms that are typically used to do this; often client code can
8189 omit the __declspec() declaration completely. See
8190 @samp{--enable-auto-import} and @samp{automatic data imports} for more
8194 @cindex automatic data imports
8195 @item automatic data imports
8196 The standard Windows dll format supports data imports from dlls only
8197 by adding special decorations (dllimport/dllexport), which let the
8198 compiler produce specific assembler instructions to deal with this
8199 issue. This increases the effort necessary to port existing Un*x
8200 code to these platforms, especially for large
8201 c++ libraries and applications. The auto-import feature, which was
8202 initially provided by Paul Sokolovsky, allows one to omit the
8203 decorations to achieve a behavior that conforms to that on POSIX/Un*x
8204 platforms. This feature is enabled with the @samp{--enable-auto-import}
8205 command-line option, although it is enabled by default on cygwin/mingw.
8206 The @samp{--enable-auto-import} option itself now serves mainly to
8207 suppress any warnings that are ordinarily emitted when linked objects
8208 trigger the feature's use.
8210 auto-import of variables does not always work flawlessly without
8211 additional assistance. Sometimes, you will see this message
8213 "variable '<var>' can't be auto-imported. Please read the
8214 documentation for ld's @code{--enable-auto-import} for details."
8216 The @samp{--enable-auto-import} documentation explains why this error
8217 occurs, and several methods that can be used to overcome this difficulty.
8218 One of these methods is the @emph{runtime pseudo-relocs} feature, described
8221 @cindex runtime pseudo-relocation
8222 For complex variables imported from DLLs (such as structs or classes),
8223 object files typically contain a base address for the variable and an
8224 offset (@emph{addend}) within the variable--to specify a particular
8225 field or public member, for instance. Unfortunately, the runtime loader used
8226 in win32 environments is incapable of fixing these references at runtime
8227 without the additional information supplied by dllimport/dllexport decorations.
8228 The standard auto-import feature described above is unable to resolve these
8231 The @samp{--enable-runtime-pseudo-relocs} switch allows these references to
8232 be resolved without error, while leaving the task of adjusting the references
8233 themselves (with their non-zero addends) to specialized code provided by the
8234 runtime environment. Recent versions of the cygwin and mingw environments and
8235 compilers provide this runtime support; older versions do not. However, the
8236 support is only necessary on the developer's platform; the compiled result will
8237 run without error on an older system.
8239 @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly
8242 @cindex direct linking to a dll
8243 @item direct linking to a dll
8244 The cygwin/mingw ports of @command{ld} support the direct linking,
8245 including data symbols, to a dll without the usage of any import
8246 libraries. This is much faster and uses much less memory than does the
8247 traditional import library method, especially when linking large
8248 libraries or applications. When @command{ld} creates an import lib, each
8249 function or variable exported from the dll is stored in its own bfd, even
8250 though a single bfd could contain many exports. The overhead involved in
8251 storing, loading, and processing so many bfd's is quite large, and explains the
8252 tremendous time, memory, and storage needed to link against particularly
8253 large or complex libraries when using import libs.
8255 Linking directly to a dll uses no extra command-line switches other than
8256 @samp{-L} and @samp{-l}, because @command{ld} already searches for a number
8257 of names to match each library. All that is needed from the developer's
8258 perspective is an understanding of this search, in order to force ld to
8259 select the dll instead of an import library.
8262 For instance, when ld is called with the argument @samp{-lxxx} it will attempt
8263 to find, in the first directory of its search path,
8276 before moving on to the next directory in the search path.
8278 (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll},
8279 where @samp{<prefix>} is set by the @command{ld} option
8280 @samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec
8281 file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for
8284 Other win32-based unix environments, such as mingw or pw32, may use other
8285 @samp{<prefix>}es, although at present only cygwin makes use of this feature. It
8286 was originally intended to help avoid name conflicts among dll's built for the
8287 various win32/un*x environments, so that (for example) two versions of a zlib dll
8288 could coexist on the same machine.
8290 The generic cygwin/mingw path layout uses a @samp{bin} directory for
8291 applications and dll's and a @samp{lib} directory for the import
8292 libraries (using cygwin nomenclature):
8298 libxxx.dll.a (in case of dll's)
8299 libxxx.a (in case of static archive)
8302 Linking directly to a dll without using the import library can be
8305 1. Use the dll directly by adding the @samp{bin} path to the link line
8307 gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
8310 However, as the dll's often have version numbers appended to their names
8311 (@samp{cygncurses-5.dll}) this will often fail, unless one specifies
8312 @samp{-L../bin -lncurses-5} to include the version. Import libs are generally
8313 not versioned, and do not have this difficulty.
8315 2. Create a symbolic link from the dll to a file in the @samp{lib}
8316 directory according to the above mentioned search pattern. This
8317 should be used to avoid unwanted changes in the tools needed for
8321 ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
8324 Then you can link without any make environment changes.
8327 gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
8330 This technique also avoids the version number problems, because the following is
8337 libxxx.dll.a -> ../bin/cygxxx-5.dll
8340 Linking directly to a dll without using an import lib will work
8341 even when auto-import features are exercised, and even when
8342 @samp{--enable-runtime-pseudo-relocs} is used.
8344 Given the improvements in speed and memory usage, one might justifiably
8345 wonder why import libraries are used at all. There are three reasons:
8347 1. Until recently, the link-directly-to-dll functionality did @emph{not}
8348 work with auto-imported data.
8350 2. Sometimes it is necessary to include pure static objects within the
8351 import library (which otherwise contains only bfd's for indirection
8352 symbols that point to the exports of a dll). Again, the import lib
8353 for the cygwin kernel makes use of this ability, and it is not
8354 possible to do this without an import lib.
8356 3. Symbol aliases can only be resolved using an import lib. This is
8357 critical when linking against OS-supplied dll's (eg, the win32 API)
8358 in which symbols are usually exported as undecorated aliases of their
8359 stdcall-decorated assembly names.
8361 So, import libs are not going away. But the ability to replace
8362 true import libs with a simple symbolic link to (or a copy of)
8363 a dll, in many cases, is a useful addition to the suite of tools
8364 binutils makes available to the win32 developer. Given the
8365 massive improvements in memory requirements during linking, storage
8366 requirements, and linking speed, we expect that many developers
8367 will soon begin to use this feature whenever possible.
8369 @item symbol aliasing
8371 @item adding additional names
8372 Sometimes, it is useful to export symbols with additional names.
8373 A symbol @samp{foo} will be exported as @samp{foo}, but it can also be
8374 exported as @samp{_foo} by using special directives in the DEF file
8375 when creating the dll. This will affect also the optional created
8376 import library. Consider the following DEF file:
8379 LIBRARY "xyz.dll" BASE=0x61000000
8386 The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}.
8388 Another method for creating a symbol alias is to create it in the
8389 source code using the "weak" attribute:
8392 void foo () @{ /* Do something. */; @}
8393 void _foo () __attribute__ ((weak, alias ("foo")));
8396 See the gcc manual for more information about attributes and weak
8399 @item renaming symbols
8400 Sometimes it is useful to rename exports. For instance, the cygwin
8401 kernel does this regularly. A symbol @samp{_foo} can be exported as
8402 @samp{foo} but not as @samp{_foo} by using special directives in the
8403 DEF file. (This will also affect the import library, if it is
8404 created). In the following example:
8407 LIBRARY "xyz.dll" BASE=0x61000000
8413 The line @samp{_foo = foo} maps the exported symbol @samp{foo} to
8417 Note: using a DEF file disables the default auto-export behavior,
8418 unless the @samp{--export-all-symbols} command-line option is used.
8419 If, however, you are trying to rename symbols, then you should list
8420 @emph{all} desired exports in the DEF file, including the symbols
8421 that are not being renamed, and do @emph{not} use the
8422 @samp{--export-all-symbols} option. If you list only the
8423 renamed symbols in the DEF file, and use @samp{--export-all-symbols}
8424 to handle the other symbols, then the both the new names @emph{and}
8425 the original names for the renamed symbols will be exported.
8426 In effect, you'd be aliasing those symbols, not renaming them,
8427 which is probably not what you wanted.
8429 @cindex weak externals
8430 @item weak externals
8431 The Windows object format, PE, specifies a form of weak symbols called
8432 weak externals. When a weak symbol is linked and the symbol is not
8433 defined, the weak symbol becomes an alias for some other symbol. There
8434 are three variants of weak externals:
8436 @item Definition is searched for in objects and libraries, historically
8437 called lazy externals.
8438 @item Definition is searched for only in other objects, not in libraries.
8439 This form is not presently implemented.
8440 @item No search; the symbol is an alias. This form is not presently
8443 As a GNU extension, weak symbols that do not specify an alternate symbol
8444 are supported. If the symbol is undefined when linking, the symbol
8445 uses a default value.
8447 @cindex aligned common symbols
8448 @item aligned common symbols
8449 As a GNU extension to the PE file format, it is possible to specify the
8450 desired alignment for a common symbol. This information is conveyed from
8451 the assembler or compiler to the linker by means of GNU-specific commands
8452 carried in the object file's @samp{.drectve} section, which are recognized
8453 by @command{ld} and respected when laying out the common symbols. Native
8454 tools will be able to process object files employing this GNU extension,
8455 but will fail to respect the alignment instructions, and may issue noisy
8456 warnings about unknown linker directives.
8471 @section @code{ld} and Xtensa Processors
8473 @cindex Xtensa processors
8474 The default @command{ld} behavior for Xtensa processors is to interpret
8475 @code{SECTIONS} commands so that lists of explicitly named sections in a
8476 specification with a wildcard file will be interleaved when necessary to
8477 keep literal pools within the range of PC-relative load offsets. For
8478 example, with the command:
8490 @command{ld} may interleave some of the @code{.literal}
8491 and @code{.text} sections from different object files to ensure that the
8492 literal pools are within the range of PC-relative load offsets. A valid
8493 interleaving might place the @code{.literal} sections from an initial
8494 group of files followed by the @code{.text} sections of that group of
8495 files. Then, the @code{.literal} sections from the rest of the files
8496 and the @code{.text} sections from the rest of the files would follow.
8498 @cindex @option{--relax} on Xtensa
8499 @cindex relaxing on Xtensa
8500 Relaxation is enabled by default for the Xtensa version of @command{ld} and
8501 provides two important link-time optimizations. The first optimization
8502 is to combine identical literal values to reduce code size. A redundant
8503 literal will be removed and all the @code{L32R} instructions that use it
8504 will be changed to reference an identical literal, as long as the
8505 location of the replacement literal is within the offset range of all
8506 the @code{L32R} instructions. The second optimization is to remove
8507 unnecessary overhead from assembler-generated ``longcall'' sequences of
8508 @code{L32R}/@code{CALLX@var{n}} when the target functions are within
8509 range of direct @code{CALL@var{n}} instructions.
8511 For each of these cases where an indirect call sequence can be optimized
8512 to a direct call, the linker will change the @code{CALLX@var{n}}
8513 instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R}
8514 instruction, and remove the literal referenced by the @code{L32R}
8515 instruction if it is not used for anything else. Removing the
8516 @code{L32R} instruction always reduces code size but can potentially
8517 hurt performance by changing the alignment of subsequent branch targets.
8518 By default, the linker will always preserve alignments, either by
8519 switching some instructions between 24-bit encodings and the equivalent
8520 density instructions or by inserting a no-op in place of the @code{L32R}
8521 instruction that was removed. If code size is more important than
8522 performance, the @option{--size-opt} option can be used to prevent the
8523 linker from widening density instructions or inserting no-ops, except in
8524 a few cases where no-ops are required for correctness.
8526 The following Xtensa-specific command-line options can be used to
8529 @cindex Xtensa options
8532 When optimizing indirect calls to direct calls, optimize for code size
8533 more than performance. With this option, the linker will not insert
8534 no-ops or widen density instructions to preserve branch target
8535 alignment. There may still be some cases where no-ops are required to
8536 preserve the correctness of the code.
8544 @ifclear SingleFormat
8549 @cindex object file management
8550 @cindex object formats available
8552 The linker accesses object and archive files using the BFD libraries.
8553 These libraries allow the linker to use the same routines to operate on
8554 object files whatever the object file format. A different object file
8555 format can be supported simply by creating a new BFD back end and adding
8556 it to the library. To conserve runtime memory, however, the linker and
8557 associated tools are usually configured to support only a subset of the
8558 object file formats available. You can use @code{objdump -i}
8559 (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to
8560 list all the formats available for your configuration.
8562 @cindex BFD requirements
8563 @cindex requirements for BFD
8564 As with most implementations, BFD is a compromise between
8565 several conflicting requirements. The major factor influencing
8566 BFD design was efficiency: any time used converting between
8567 formats is time which would not have been spent had BFD not
8568 been involved. This is partly offset by abstraction payback; since
8569 BFD simplifies applications and back ends, more time and care
8570 may be spent optimizing algorithms for a greater speed.
8572 One minor artifact of the BFD solution which you should bear in
8573 mind is the potential for information loss. There are two places where
8574 useful information can be lost using the BFD mechanism: during
8575 conversion and during output. @xref{BFD information loss}.
8578 * BFD outline:: How it works: an outline of BFD
8582 @section How It Works: An Outline of BFD
8583 @cindex opening object files
8584 @include bfdsumm.texi
8587 @node Reporting Bugs
8588 @chapter Reporting Bugs
8589 @cindex bugs in @command{ld}
8590 @cindex reporting bugs in @command{ld}
8592 Your bug reports play an essential role in making @command{ld} reliable.
8594 Reporting a bug may help you by bringing a solution to your problem, or
8595 it may not. But in any case the principal function of a bug report is
8596 to help the entire community by making the next version of @command{ld}
8597 work better. Bug reports are your contribution to the maintenance of
8600 In order for a bug report to serve its purpose, you must include the
8601 information that enables us to fix the bug.
8604 * Bug Criteria:: Have you found a bug?
8605 * Bug Reporting:: How to report bugs
8609 @section Have You Found a Bug?
8610 @cindex bug criteria
8612 If you are not sure whether you have found a bug, here are some guidelines:
8615 @cindex fatal signal
8616 @cindex linker crash
8617 @cindex crash of linker
8619 If the linker gets a fatal signal, for any input whatever, that is a
8620 @command{ld} bug. Reliable linkers never crash.
8622 @cindex error on valid input
8624 If @command{ld} produces an error message for valid input, that is a bug.
8626 @cindex invalid input
8628 If @command{ld} does not produce an error message for invalid input, that
8629 may be a bug. In the general case, the linker can not verify that
8630 object files are correct.
8633 If you are an experienced user of linkers, your suggestions for
8634 improvement of @command{ld} are welcome in any case.
8638 @section How to Report Bugs
8640 @cindex @command{ld} bugs, reporting
8642 A number of companies and individuals offer support for @sc{gnu}
8643 products. If you obtained @command{ld} from a support organization, we
8644 recommend you contact that organization first.
8646 You can find contact information for many support companies and
8647 individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs
8651 Otherwise, send bug reports for @command{ld} to
8655 The fundamental principle of reporting bugs usefully is this:
8656 @strong{report all the facts}. If you are not sure whether to state a
8657 fact or leave it out, state it!
8659 Often people omit facts because they think they know what causes the
8660 problem and assume that some details do not matter. Thus, you might
8661 assume that the name of a symbol you use in an example does not
8662 matter. Well, probably it does not, but one cannot be sure. Perhaps
8663 the bug is a stray memory reference which happens to fetch from the
8664 location where that name is stored in memory; perhaps, if the name
8665 were different, the contents of that location would fool the linker
8666 into doing the right thing despite the bug. Play it safe and give a
8667 specific, complete example. That is the easiest thing for you to do,
8668 and the most helpful.
8670 Keep in mind that the purpose of a bug report is to enable us to fix
8671 the bug if it is new to us. Therefore, always write your bug reports
8672 on the assumption that the bug has not been reported previously.
8674 Sometimes people give a few sketchy facts and ask, ``Does this ring a
8675 bell?'' This cannot help us fix a bug, so it is basically useless. We
8676 respond by asking for enough details to enable us to investigate.
8677 You might as well expedite matters by sending them to begin with.
8679 To enable us to fix the bug, you should include all these things:
8683 The version of @command{ld}. @command{ld} announces it if you start it with
8684 the @samp{--version} argument.
8686 Without this, we will not know whether there is any point in looking for
8687 the bug in the current version of @command{ld}.
8690 Any patches you may have applied to the @command{ld} source, including any
8691 patches made to the @code{BFD} library.
8694 The type of machine you are using, and the operating system name and
8698 What compiler (and its version) was used to compile @command{ld}---e.g.
8702 The command arguments you gave the linker to link your example and
8703 observe the bug. To guarantee you will not omit something important,
8704 list them all. A copy of the Makefile (or the output from make) is
8707 If we were to try to guess the arguments, we would probably guess wrong
8708 and then we might not encounter the bug.
8711 A complete input file, or set of input files, that will reproduce the
8712 bug. It is generally most helpful to send the actual object files
8713 provided that they are reasonably small. Say no more than 10K. For
8714 bigger files you can either make them available by FTP or HTTP or else
8715 state that you are willing to send the object file(s) to whomever
8716 requests them. (Note - your email will be going to a mailing list, so
8717 we do not want to clog it up with large attachments). But small
8718 attachments are best.
8720 If the source files were assembled using @code{gas} or compiled using
8721 @code{gcc}, then it may be OK to send the source files rather than the
8722 object files. In this case, be sure to say exactly what version of
8723 @code{gas} or @code{gcc} was used to produce the object files. Also say
8724 how @code{gas} or @code{gcc} were configured.
8727 A description of what behavior you observe that you believe is
8728 incorrect. For example, ``It gets a fatal signal.''
8730 Of course, if the bug is that @command{ld} gets a fatal signal, then we
8731 will certainly notice it. But if the bug is incorrect output, we might
8732 not notice unless it is glaringly wrong. You might as well not give us
8733 a chance to make a mistake.
8735 Even if the problem you experience is a fatal signal, you should still
8736 say so explicitly. Suppose something strange is going on, such as, your
8737 copy of @command{ld} is out of sync, or you have encountered a bug in the
8738 C library on your system. (This has happened!) Your copy might crash
8739 and ours would not. If you told us to expect a crash, then when ours
8740 fails to crash, we would know that the bug was not happening for us. If
8741 you had not told us to expect a crash, then we would not be able to draw
8742 any conclusion from our observations.
8745 If you wish to suggest changes to the @command{ld} source, send us context
8746 diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or
8747 @samp{-p} option. Always send diffs from the old file to the new file.
8748 If you even discuss something in the @command{ld} source, refer to it by
8749 context, not by line number.
8751 The line numbers in our development sources will not match those in your
8752 sources. Your line numbers would convey no useful information to us.
8755 Here are some things that are not necessary:
8759 A description of the envelope of the bug.
8761 Often people who encounter a bug spend a lot of time investigating
8762 which changes to the input file will make the bug go away and which
8763 changes will not affect it.
8765 This is often time consuming and not very useful, because the way we
8766 will find the bug is by running a single example under the debugger
8767 with breakpoints, not by pure deduction from a series of examples.
8768 We recommend that you save your time for something else.
8770 Of course, if you can find a simpler example to report @emph{instead}
8771 of the original one, that is a convenience for us. Errors in the
8772 output will be easier to spot, running under the debugger will take
8773 less time, and so on.
8775 However, simplification is not vital; if you do not want to do this,
8776 report the bug anyway and send us the entire test case you used.
8779 A patch for the bug.
8781 A patch for the bug does help us if it is a good one. But do not omit
8782 the necessary information, such as the test case, on the assumption that
8783 a patch is all we need. We might see problems with your patch and decide
8784 to fix the problem another way, or we might not understand it at all.
8786 Sometimes with a program as complicated as @command{ld} it is very hard to
8787 construct an example that will make the program follow a certain path
8788 through the code. If you do not send us the example, we will not be
8789 able to construct one, so we will not be able to verify that the bug is
8792 And if we cannot understand what bug you are trying to fix, or why your
8793 patch should be an improvement, we will not install it. A test case will
8794 help us to understand.
8797 A guess about what the bug is or what it depends on.
8799 Such guesses are usually wrong. Even we cannot guess right about such
8800 things without first using the debugger to find the facts.
8804 @appendix MRI Compatible Script Files
8805 @cindex MRI compatibility
8806 To aid users making the transition to @sc{gnu} @command{ld} from the MRI
8807 linker, @command{ld} can use MRI compatible linker scripts as an
8808 alternative to the more general-purpose linker scripting language
8809 described in @ref{Scripts}. MRI compatible linker scripts have a much
8810 simpler command set than the scripting language otherwise used with
8811 @command{ld}. @sc{gnu} @command{ld} supports the most commonly used MRI
8812 linker commands; these commands are described here.
8814 In general, MRI scripts aren't of much use with the @code{a.out} object
8815 file format, since it only has three sections and MRI scripts lack some
8816 features to make use of them.
8818 You can specify a file containing an MRI-compatible script using the
8819 @samp{-c} command-line option.
8821 Each command in an MRI-compatible script occupies its own line; each
8822 command line starts with the keyword that identifies the command (though
8823 blank lines are also allowed for punctuation). If a line of an
8824 MRI-compatible script begins with an unrecognized keyword, @command{ld}
8825 issues a warning message, but continues processing the script.
8827 Lines beginning with @samp{*} are comments.
8829 You can write these commands using all upper-case letters, or all
8830 lower case; for example, @samp{chip} is the same as @samp{CHIP}.
8831 The following list shows only the upper-case form of each command.
8834 @cindex @code{ABSOLUTE} (MRI)
8835 @item ABSOLUTE @var{secname}
8836 @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname}
8837 Normally, @command{ld} includes in the output file all sections from all
8838 the input files. However, in an MRI-compatible script, you can use the
8839 @code{ABSOLUTE} command to restrict the sections that will be present in
8840 your output program. If the @code{ABSOLUTE} command is used at all in a
8841 script, then only the sections named explicitly in @code{ABSOLUTE}
8842 commands will appear in the linker output. You can still use other
8843 input sections (whatever you select on the command line, or using
8844 @code{LOAD}) to resolve addresses in the output file.
8846 @cindex @code{ALIAS} (MRI)
8847 @item ALIAS @var{out-secname}, @var{in-secname}
8848 Use this command to place the data from input section @var{in-secname}
8849 in a section called @var{out-secname} in the linker output file.
8851 @var{in-secname} may be an integer.
8853 @cindex @code{ALIGN} (MRI)
8854 @item ALIGN @var{secname} = @var{expression}
8855 Align the section called @var{secname} to @var{expression}. The
8856 @var{expression} should be a power of two.
8858 @cindex @code{BASE} (MRI)
8859 @item BASE @var{expression}
8860 Use the value of @var{expression} as the lowest address (other than
8861 absolute addresses) in the output file.
8863 @cindex @code{CHIP} (MRI)
8864 @item CHIP @var{expression}
8865 @itemx CHIP @var{expression}, @var{expression}
8866 This command does nothing; it is accepted only for compatibility.
8868 @cindex @code{END} (MRI)
8870 This command does nothing whatever; it's only accepted for compatibility.
8872 @cindex @code{FORMAT} (MRI)
8873 @item FORMAT @var{output-format}
8874 Similar to the @code{OUTPUT_FORMAT} command in the more general linker
8875 language, but restricted to S-records, if @var{output-format} is @samp{S}
8877 @cindex @code{LIST} (MRI)
8878 @item LIST @var{anything}@dots{}
8879 Print (to the standard output file) a link map, as produced by the
8880 @command{ld} command-line option @samp{-M}.
8882 The keyword @code{LIST} may be followed by anything on the
8883 same line, with no change in its effect.
8885 @cindex @code{LOAD} (MRI)
8886 @item LOAD @var{filename}
8887 @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename}
8888 Include one or more object file @var{filename} in the link; this has the
8889 same effect as specifying @var{filename} directly on the @command{ld}
8892 @cindex @code{NAME} (MRI)
8893 @item NAME @var{output-name}
8894 @var{output-name} is the name for the program produced by @command{ld}; the
8895 MRI-compatible command @code{NAME} is equivalent to the command-line
8896 option @samp{-o} or the general script language command @code{OUTPUT}.
8898 @cindex @code{ORDER} (MRI)
8899 @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname}
8900 @itemx ORDER @var{secname} @var{secname} @var{secname}
8901 Normally, @command{ld} orders the sections in its output file in the
8902 order in which they first appear in the input files. In an MRI-compatible
8903 script, you can override this ordering with the @code{ORDER} command. The
8904 sections you list with @code{ORDER} will appear first in your output
8905 file, in the order specified.
8907 @cindex @code{PUBLIC} (MRI)
8908 @item PUBLIC @var{name}=@var{expression}
8909 @itemx PUBLIC @var{name},@var{expression}
8910 @itemx PUBLIC @var{name} @var{expression}
8911 Supply a value (@var{expression}) for external symbol
8912 @var{name} used in the linker input files.
8914 @cindex @code{SECT} (MRI)
8915 @item SECT @var{secname}, @var{expression}
8916 @itemx SECT @var{secname}=@var{expression}
8917 @itemx SECT @var{secname} @var{expression}
8918 You can use any of these three forms of the @code{SECT} command to
8919 specify the start address (@var{expression}) for section @var{secname}.
8920 If you have more than one @code{SECT} statement for the same
8921 @var{secname}, only the @emph{first} sets the start address.
8924 @node GNU Free Documentation License
8925 @appendix GNU Free Documentation License
8929 @unnumbered LD Index
8934 % I think something like @@colophon should be in texinfo. In the
8936 \long\def\colophon{\hbox to0pt{}\vfill
8937 \centerline{The body of this manual is set in}
8938 \centerline{\fontname\tenrm,}
8939 \centerline{with headings in {\bf\fontname\tenbf}}
8940 \centerline{and examples in {\tt\fontname\tentt}.}
8941 \centerline{{\it\fontname\tenit\/} and}
8942 \centerline{{\sl\fontname\tensl\/}}
8943 \centerline{are used for emphasis.}\vfill}
8945 % Blame: doc@@cygnus.com, 28mar91.