3 @c Copyright (C) 1991-2015 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
45 @dircategory Software development
47 * Ld: (ld). The GNU linker.
52 This file documents the @sc{gnu} linker LD
53 @ifset VERSION_PACKAGE
54 @value{VERSION_PACKAGE}
56 version @value{VERSION}.
58 Copyright @copyright{} 1991-2015 Free Software Foundation, Inc.
60 Permission is granted to copy, distribute and/or modify this document
61 under the terms of the GNU Free Documentation License, Version 1.3
62 or any later version published by the Free Software Foundation;
63 with no Invariant Sections, with no Front-Cover Texts, and with no
64 Back-Cover Texts. A copy of the license is included in the
65 section entitled ``GNU Free Documentation License''.
69 @setchapternewpage odd
70 @settitle The GNU linker
75 @ifset VERSION_PACKAGE
76 @subtitle @value{VERSION_PACKAGE}
78 @subtitle Version @value{VERSION}
79 @author Steve Chamberlain
80 @author Ian Lance Taylor
85 \hfill Red Hat Inc\par
86 \hfill nickc\@credhat.com, doc\@redhat.com\par
87 \hfill {\it The GNU linker}\par
88 \hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par
90 \global\parindent=0pt % Steve likes it this way.
93 @vskip 0pt plus 1filll
94 @c man begin COPYRIGHT
95 Copyright @copyright{} 1991-2015 Free Software Foundation, Inc.
97 Permission is granted to copy, distribute and/or modify this document
98 under the terms of the GNU Free Documentation License, Version 1.3
99 or any later version published by the Free Software Foundation;
100 with no Invariant Sections, with no Front-Cover Texts, and with no
101 Back-Cover Texts. A copy of the license is included in the
102 section entitled ``GNU Free Documentation License''.
108 @c FIXME: Talk about importance of *order* of args, cmds to linker!
113 This file documents the @sc{gnu} linker ld
114 @ifset VERSION_PACKAGE
115 @value{VERSION_PACKAGE}
117 version @value{VERSION}.
119 This document is distributed under the terms of the GNU Free
120 Documentation License version 1.3. A copy of the license is included
121 in the section entitled ``GNU Free Documentation License''.
124 * Overview:: Overview
125 * Invocation:: Invocation
126 * Scripts:: Linker Scripts
128 * Machine Dependent:: Machine Dependent Features
132 * H8/300:: ld and the H8/300
135 * Renesas:: ld and other Renesas micros
138 * i960:: ld and the Intel 960 family
141 * ARM:: ld and the ARM family
144 * M68HC11/68HC12:: ld and the Motorola 68HC11 and 68HC12 families
147 * HPPA ELF32:: ld and HPPA 32-bit ELF
150 * M68K:: ld and Motorola 68K family
153 * MIPS:: ld and MIPS family
156 * PowerPC ELF32:: ld and PowerPC 32-bit ELF Support
159 * PowerPC64 ELF64:: ld and PowerPC64 64-bit ELF Support
162 * SPU ELF:: ld and SPU ELF Support
165 * TI COFF:: ld and the TI COFF
168 * Win32:: ld and WIN32 (cygwin/mingw)
171 * Xtensa:: ld and Xtensa Processors
174 @ifclear SingleFormat
177 @c Following blank line required for remaining bug in makeinfo conds/menus
179 * Reporting Bugs:: Reporting Bugs
180 * MRI:: MRI Compatible Script Files
181 * GNU Free Documentation License:: GNU Free Documentation License
182 * LD Index:: LD Index
189 @cindex @sc{gnu} linker
190 @cindex what is this?
193 @c man begin SYNOPSIS
194 ld [@b{options}] @var{objfile} @dots{}
198 ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and
199 the Info entries for @file{binutils} and
204 @c man begin DESCRIPTION
206 @command{ld} combines a number of object and archive files, relocates
207 their data and ties up symbol references. Usually the last step in
208 compiling a program is to run @command{ld}.
210 @command{ld} accepts Linker Command Language files written in
211 a superset of AT&T's Link Editor Command Language syntax,
212 to provide explicit and total control over the linking process.
216 This man page does not describe the command language; see the
217 @command{ld} entry in @code{info} for full details on the command
218 language and on other aspects of the GNU linker.
221 @ifclear SingleFormat
222 This version of @command{ld} uses the general purpose BFD libraries
223 to operate on object files. This allows @command{ld} to read, combine, and
224 write object files in many different formats---for example, COFF or
225 @code{a.out}. Different formats may be linked together to produce any
226 available kind of object file. @xref{BFD}, for more information.
229 Aside from its flexibility, the @sc{gnu} linker is more helpful than other
230 linkers in providing diagnostic information. Many linkers abandon
231 execution immediately upon encountering an error; whenever possible,
232 @command{ld} continues executing, allowing you to identify other errors
233 (or, in some cases, to get an output file in spite of the error).
240 @c man begin DESCRIPTION
242 The @sc{gnu} linker @command{ld} is meant to cover a broad range of situations,
243 and to be as compatible as possible with other linkers. As a result,
244 you have many choices to control its behavior.
250 * Options:: Command Line Options
251 * Environment:: Environment Variables
255 @section Command Line Options
263 The linker supports a plethora of command-line options, but in actual
264 practice few of them are used in any particular context.
265 @cindex standard Unix system
266 For instance, a frequent use of @command{ld} is to link standard Unix
267 object files on a standard, supported Unix system. On such a system, to
268 link a file @code{hello.o}:
271 ld -o @var{output} /lib/crt0.o hello.o -lc
274 This tells @command{ld} to produce a file called @var{output} as the
275 result of linking the file @code{/lib/crt0.o} with @code{hello.o} and
276 the library @code{libc.a}, which will come from the standard search
277 directories. (See the discussion of the @samp{-l} option below.)
279 Some of the command-line options to @command{ld} may be specified at any
280 point in the command line. However, options which refer to files, such
281 as @samp{-l} or @samp{-T}, cause the file to be read at the point at
282 which the option appears in the command line, relative to the object
283 files and other file options. Repeating non-file options with a
284 different argument will either have no further effect, or override prior
285 occurrences (those further to the left on the command line) of that
286 option. Options which may be meaningfully specified more than once are
287 noted in the descriptions below.
290 Non-option arguments are object files or archives which are to be linked
291 together. They may follow, precede, or be mixed in with command-line
292 options, except that an object file argument may not be placed between
293 an option and its argument.
295 Usually the linker is invoked with at least one object file, but you can
296 specify other forms of binary input files using @samp{-l}, @samp{-R},
297 and the script command language. If @emph{no} binary input files at all
298 are specified, the linker does not produce any output, and issues the
299 message @samp{No input files}.
301 If the linker cannot recognize the format of an object file, it will
302 assume that it is a linker script. A script specified in this way
303 augments the main linker script used for the link (either the default
304 linker script or the one specified by using @samp{-T}). This feature
305 permits the linker to link against a file which appears to be an object
306 or an archive, but actually merely defines some symbol values, or uses
307 @code{INPUT} or @code{GROUP} to load other objects. Specifying a
308 script in this way merely augments the main linker script, with the
309 extra commands placed after the main script; use the @samp{-T} option
310 to replace the default linker script entirely, but note the effect of
311 the @code{INSERT} command. @xref{Scripts}.
313 For options whose names are a single letter,
314 option arguments must either follow the option letter without intervening
315 whitespace, or be given as separate arguments immediately following the
316 option that requires them.
318 For options whose names are multiple letters, either one dash or two can
319 precede the option name; for example, @samp{-trace-symbol} and
320 @samp{--trace-symbol} are equivalent. Note---there is one exception to
321 this rule. Multiple letter options that start with a lower case 'o' can
322 only be preceded by two dashes. This is to reduce confusion with the
323 @samp{-o} option. So for example @samp{-omagic} sets the output file
324 name to @samp{magic} whereas @samp{--omagic} sets the NMAGIC flag on the
327 Arguments to multiple-letter options must either be separated from the
328 option name by an equals sign, or be given as separate arguments
329 immediately following the option that requires them. For example,
330 @samp{--trace-symbol foo} and @samp{--trace-symbol=foo} are equivalent.
331 Unique abbreviations of the names of multiple-letter options are
334 Note---if the linker is being invoked indirectly, via a compiler driver
335 (e.g. @samp{gcc}) then all the linker command line options should be
336 prefixed by @samp{-Wl,} (or whatever is appropriate for the particular
337 compiler driver) like this:
340 gcc -Wl,--start-group foo.o bar.o -Wl,--end-group
343 This is important, because otherwise the compiler driver program may
344 silently drop the linker options, resulting in a bad link. Confusion
345 may also arise when passing options that require values through a
346 driver, as the use of a space between option and argument acts as
347 a separator, and causes the driver to pass only the option to the linker
348 and the argument to the compiler. In this case, it is simplest to use
349 the joined forms of both single- and multiple-letter options, such as:
352 gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map
355 Here is a table of the generic command line switches accepted by the GNU
359 @include at-file.texi
361 @kindex -a @var{keyword}
362 @item -a @var{keyword}
363 This option is supported for HP/UX compatibility. The @var{keyword}
364 argument must be one of the strings @samp{archive}, @samp{shared}, or
365 @samp{default}. @samp{-aarchive} is functionally equivalent to
366 @samp{-Bstatic}, and the other two keywords are functionally equivalent
367 to @samp{-Bdynamic}. This option may be used any number of times.
369 @kindex --audit @var{AUDITLIB}
370 @item --audit @var{AUDITLIB}
371 Adds @var{AUDITLIB} to the @code{DT_AUDIT} entry of the dynamic section.
372 @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME
373 specified in the library. If specified multiple times @code{DT_AUDIT}
374 will contain a colon separated list of audit interfaces to use. If the linker
375 finds an object with an audit entry while searching for shared libraries,
376 it will add a corresponding @code{DT_DEPAUDIT} entry in the output file.
377 This option is only meaningful on ELF platforms supporting the rtld-audit
381 @cindex architectures
382 @kindex -A @var{arch}
383 @item -A @var{architecture}
384 @kindex --architecture=@var{arch}
385 @itemx --architecture=@var{architecture}
386 In the current release of @command{ld}, this option is useful only for the
387 Intel 960 family of architectures. In that @command{ld} configuration, the
388 @var{architecture} argument identifies the particular architecture in
389 the 960 family, enabling some safeguards and modifying the
390 archive-library search path. @xref{i960,,@command{ld} and the Intel 960
391 family}, for details.
393 Future releases of @command{ld} may support similar functionality for
394 other architecture families.
397 @ifclear SingleFormat
398 @cindex binary input format
399 @kindex -b @var{format}
400 @kindex --format=@var{format}
403 @item -b @var{input-format}
404 @itemx --format=@var{input-format}
405 @command{ld} may be configured to support more than one kind of object
406 file. If your @command{ld} is configured this way, you can use the
407 @samp{-b} option to specify the binary format for input object files
408 that follow this option on the command line. Even when @command{ld} is
409 configured to support alternative object formats, you don't usually need
410 to specify this, as @command{ld} should be configured to expect as a
411 default input format the most usual format on each machine.
412 @var{input-format} is a text string, the name of a particular format
413 supported by the BFD libraries. (You can list the available binary
414 formats with @samp{objdump -i}.)
417 You may want to use this option if you are linking files with an unusual
418 binary format. You can also use @samp{-b} to switch formats explicitly (when
419 linking object files of different formats), by including
420 @samp{-b @var{input-format}} before each group of object files in a
423 The default format is taken from the environment variable
428 You can also define the input format from a script, using the command
431 see @ref{Format Commands}.
435 @kindex -c @var{MRI-cmdfile}
436 @kindex --mri-script=@var{MRI-cmdfile}
437 @cindex compatibility, MRI
438 @item -c @var{MRI-commandfile}
439 @itemx --mri-script=@var{MRI-commandfile}
440 For compatibility with linkers produced by MRI, @command{ld} accepts script
441 files written in an alternate, restricted command language, described in
443 @ref{MRI,,MRI Compatible Script Files}.
446 the MRI Compatible Script Files section of GNU ld documentation.
448 Introduce MRI script files with
449 the option @samp{-c}; use the @samp{-T} option to run linker
450 scripts written in the general-purpose @command{ld} scripting language.
451 If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories
452 specified by any @samp{-L} options.
454 @cindex common allocation
461 These three options are equivalent; multiple forms are supported for
462 compatibility with other linkers. They assign space to common symbols
463 even if a relocatable output file is specified (with @samp{-r}). The
464 script command @code{FORCE_COMMON_ALLOCATION} has the same effect.
465 @xref{Miscellaneous Commands}.
467 @kindex --depaudit @var{AUDITLIB}
468 @kindex -P @var{AUDITLIB}
469 @item --depaudit @var{AUDITLIB}
470 @itemx -P @var{AUDITLIB}
471 Adds @var{AUDITLIB} to the @code{DT_DEPAUDIT} entry of the dynamic section.
472 @var{AUDITLIB} is not checked for existence, nor will it use the DT_SONAME
473 specified in the library. If specified multiple times @code{DT_DEPAUDIT}
474 will contain a colon separated list of audit interfaces to use. This
475 option is only meaningful on ELF platforms supporting the rtld-audit interface.
476 The -P option is provided for Solaris compatibility.
478 @cindex entry point, from command line
479 @kindex -e @var{entry}
480 @kindex --entry=@var{entry}
482 @itemx --entry=@var{entry}
483 Use @var{entry} as the explicit symbol for beginning execution of your
484 program, rather than the default entry point. If there is no symbol
485 named @var{entry}, the linker will try to parse @var{entry} as a number,
486 and use that as the entry address (the number will be interpreted in
487 base 10; you may use a leading @samp{0x} for base 16, or a leading
488 @samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults
489 and other ways of specifying the entry point.
491 @kindex --exclude-libs
492 @item --exclude-libs @var{lib},@var{lib},...
493 Specifies a list of archive libraries from which symbols should not be automatically
494 exported. The library names may be delimited by commas or colons. Specifying
495 @code{--exclude-libs ALL} excludes symbols in all archive libraries from
496 automatic export. This option is available only for the i386 PE targeted
497 port of the linker and for ELF targeted ports. For i386 PE, symbols
498 explicitly listed in a .def file are still exported, regardless of this
499 option. For ELF targeted ports, symbols affected by this option will
500 be treated as hidden.
502 @kindex --exclude-modules-for-implib
503 @item --exclude-modules-for-implib @var{module},@var{module},...
504 Specifies a list of object files or archive members, from which symbols
505 should not be automatically exported, but which should be copied wholesale
506 into the import library being generated during the link. The module names
507 may be delimited by commas or colons, and must match exactly the filenames
508 used by @command{ld} to open the files; for archive members, this is simply
509 the member name, but for object files the name listed must include and
510 match precisely any path used to specify the input file on the linker's
511 command-line. This option is available only for the i386 PE targeted port
512 of the linker. Symbols explicitly listed in a .def file are still exported,
513 regardless of this option.
515 @cindex dynamic symbol table
517 @kindex --export-dynamic
518 @kindex --no-export-dynamic
520 @itemx --export-dynamic
521 @itemx --no-export-dynamic
522 When creating a dynamically linked executable, using the @option{-E}
523 option or the @option{--export-dynamic} option causes the linker to add
524 all symbols to the dynamic symbol table. The dynamic symbol table is the
525 set of symbols which are visible from dynamic objects at run time.
527 If you do not use either of these options (or use the
528 @option{--no-export-dynamic} option to restore the default behavior), the
529 dynamic symbol table will normally contain only those symbols which are
530 referenced by some dynamic object mentioned in the link.
532 If you use @code{dlopen} to load a dynamic object which needs to refer
533 back to the symbols defined by the program, rather than some other
534 dynamic object, then you will probably need to use this option when
535 linking the program itself.
537 You can also use the dynamic list to control what symbols should
538 be added to the dynamic symbol table if the output format supports it.
539 See the description of @samp{--dynamic-list}.
541 Note that this option is specific to ELF targeted ports. PE targets
542 support a similar function to export all symbols from a DLL or EXE; see
543 the description of @samp{--export-all-symbols} below.
545 @ifclear SingleFormat
546 @cindex big-endian objects
550 Link big-endian objects. This affects the default output format.
552 @cindex little-endian objects
555 Link little-endian objects. This affects the default output format.
558 @kindex -f @var{name}
559 @kindex --auxiliary=@var{name}
561 @itemx --auxiliary=@var{name}
562 When creating an ELF shared object, set the internal DT_AUXILIARY field
563 to the specified name. This tells the dynamic linker that the symbol
564 table of the shared object should be used as an auxiliary filter on the
565 symbol table of the shared object @var{name}.
567 If you later link a program against this filter object, then, when you
568 run the program, the dynamic linker will see the DT_AUXILIARY field. If
569 the dynamic linker resolves any symbols from the filter object, it will
570 first check whether there is a definition in the shared object
571 @var{name}. If there is one, it will be used instead of the definition
572 in the filter object. The shared object @var{name} need not exist.
573 Thus the shared object @var{name} may be used to provide an alternative
574 implementation of certain functions, perhaps for debugging or for
575 machine specific performance.
577 This option may be specified more than once. The DT_AUXILIARY entries
578 will be created in the order in which they appear on the command line.
580 @kindex -F @var{name}
581 @kindex --filter=@var{name}
583 @itemx --filter=@var{name}
584 When creating an ELF shared object, set the internal DT_FILTER field to
585 the specified name. This tells the dynamic linker that the symbol table
586 of the shared object which is being created should be used as a filter
587 on the symbol table of the shared object @var{name}.
589 If you later link a program against this filter object, then, when you
590 run the program, the dynamic linker will see the DT_FILTER field. The
591 dynamic linker will resolve symbols according to the symbol table of the
592 filter object as usual, but it will actually link to the definitions
593 found in the shared object @var{name}. Thus the filter object can be
594 used to select a subset of the symbols provided by the object
597 Some older linkers used the @option{-F} option throughout a compilation
598 toolchain for specifying object-file format for both input and output
600 @ifclear SingleFormat
601 The @sc{gnu} linker uses other mechanisms for this purpose: the
602 @option{-b}, @option{--format}, @option{--oformat} options, the
603 @code{TARGET} command in linker scripts, and the @code{GNUTARGET}
604 environment variable.
606 The @sc{gnu} linker will ignore the @option{-F} option when not
607 creating an ELF shared object.
609 @cindex finalization function
610 @kindex -fini=@var{name}
611 @item -fini=@var{name}
612 When creating an ELF executable or shared object, call NAME when the
613 executable or shared object is unloaded, by setting DT_FINI to the
614 address of the function. By default, the linker uses @code{_fini} as
615 the function to call.
619 Ignored. Provided for compatibility with other tools.
621 @kindex -G @var{value}
622 @kindex --gpsize=@var{value}
625 @itemx --gpsize=@var{value}
626 Set the maximum size of objects to be optimized using the GP register to
627 @var{size}. This is only meaningful for object file formats such as
628 MIPS ELF that support putting large and small objects into different
629 sections. This is ignored for other object file formats.
631 @cindex runtime library name
632 @kindex -h @var{name}
633 @kindex -soname=@var{name}
635 @itemx -soname=@var{name}
636 When creating an ELF shared object, set the internal DT_SONAME field to
637 the specified name. When an executable is linked with a shared object
638 which has a DT_SONAME field, then when the executable is run the dynamic
639 linker will attempt to load the shared object specified by the DT_SONAME
640 field rather than the using the file name given to the linker.
643 @cindex incremental link
645 Perform an incremental link (same as option @samp{-r}).
647 @cindex initialization function
648 @kindex -init=@var{name}
649 @item -init=@var{name}
650 When creating an ELF executable or shared object, call NAME when the
651 executable or shared object is loaded, by setting DT_INIT to the address
652 of the function. By default, the linker uses @code{_init} as the
655 @cindex archive files, from cmd line
656 @kindex -l @var{namespec}
657 @kindex --library=@var{namespec}
658 @item -l @var{namespec}
659 @itemx --library=@var{namespec}
660 Add the archive or object file specified by @var{namespec} to the
661 list of files to link. This option may be used any number of times.
662 If @var{namespec} is of the form @file{:@var{filename}}, @command{ld}
663 will search the library path for a file called @var{filename}, otherwise it
664 will search the library path for a file called @file{lib@var{namespec}.a}.
666 On systems which support shared libraries, @command{ld} may also search for
667 files other than @file{lib@var{namespec}.a}. Specifically, on ELF
668 and SunOS systems, @command{ld} will search a directory for a library
669 called @file{lib@var{namespec}.so} before searching for one called
670 @file{lib@var{namespec}.a}. (By convention, a @code{.so} extension
671 indicates a shared library.) Note that this behavior does not apply
672 to @file{:@var{filename}}, which always specifies a file called
675 The linker will search an archive only once, at the location where it is
676 specified on the command line. If the archive defines a symbol which
677 was undefined in some object which appeared before the archive on the
678 command line, the linker will include the appropriate file(s) from the
679 archive. However, an undefined symbol in an object appearing later on
680 the command line will not cause the linker to search the archive again.
682 See the @option{-(} option for a way to force the linker to search
683 archives multiple times.
685 You may list the same archive multiple times on the command line.
688 This type of archive searching is standard for Unix linkers. However,
689 if you are using @command{ld} on AIX, note that it is different from the
690 behaviour of the AIX linker.
693 @cindex search directory, from cmd line
695 @kindex --library-path=@var{dir}
696 @item -L @var{searchdir}
697 @itemx --library-path=@var{searchdir}
698 Add path @var{searchdir} to the list of paths that @command{ld} will search
699 for archive libraries and @command{ld} control scripts. You may use this
700 option any number of times. The directories are searched in the order
701 in which they are specified on the command line. Directories specified
702 on the command line are searched before the default directories. All
703 @option{-L} options apply to all @option{-l} options, regardless of the
704 order in which the options appear. @option{-L} options do not affect
705 how @command{ld} searches for a linker script unless @option{-T}
708 If @var{searchdir} begins with @code{=}, then the @code{=} will be replaced
709 by the @dfn{sysroot prefix}, controlled by the @samp{--sysroot} option, or
710 specified when the linker is configured.
713 The default set of paths searched (without being specified with
714 @samp{-L}) depends on which emulation mode @command{ld} is using, and in
715 some cases also on how it was configured. @xref{Environment}.
718 The paths can also be specified in a link script with the
719 @code{SEARCH_DIR} command. Directories specified this way are searched
720 at the point in which the linker script appears in the command line.
723 @kindex -m @var{emulation}
724 @item -m @var{emulation}
725 Emulate the @var{emulation} linker. You can list the available
726 emulations with the @samp{--verbose} or @samp{-V} options.
728 If the @samp{-m} option is not used, the emulation is taken from the
729 @code{LDEMULATION} environment variable, if that is defined.
731 Otherwise, the default emulation depends upon how the linker was
739 Print a link map to the standard output. A link map provides
740 information about the link, including the following:
744 Where object files are mapped into memory.
746 How common symbols are allocated.
748 All archive members included in the link, with a mention of the symbol
749 which caused the archive member to be brought in.
751 The values assigned to symbols.
753 Note - symbols whose values are computed by an expression which
754 involves a reference to a previous value of the same symbol may not
755 have correct result displayed in the link map. This is because the
756 linker discards intermediate results and only retains the final value
757 of an expression. Under such circumstances the linker will display
758 the final value enclosed by square brackets. Thus for example a
759 linker script containing:
767 will produce the following output in the link map if the @option{-M}
772 [0x0000000c] foo = (foo * 0x4)
773 [0x0000000c] foo = (foo + 0x8)
776 See @ref{Expressions} for more information about expressions in linker
781 @cindex read-only text
786 Turn off page alignment of sections, and disable linking against shared
787 libraries. If the output format supports Unix style magic numbers,
788 mark the output as @code{NMAGIC}.
792 @cindex read/write from cmd line
796 Set the text and data sections to be readable and writable. Also, do
797 not page-align the data segment, and disable linking against shared
798 libraries. If the output format supports Unix style magic numbers,
799 mark the output as @code{OMAGIC}. Note: Although a writable text section
800 is allowed for PE-COFF targets, it does not conform to the format
801 specification published by Microsoft.
806 This option negates most of the effects of the @option{-N} option. It
807 sets the text section to be read-only, and forces the data segment to
808 be page-aligned. Note - this option does not enable linking against
809 shared libraries. Use @option{-Bdynamic} for this.
811 @kindex -o @var{output}
812 @kindex --output=@var{output}
813 @cindex naming the output file
814 @item -o @var{output}
815 @itemx --output=@var{output}
816 Use @var{output} as the name for the program produced by @command{ld}; if this
817 option is not specified, the name @file{a.out} is used by default. The
818 script command @code{OUTPUT} can also specify the output file name.
820 @kindex -O @var{level}
821 @cindex generating optimized output
823 If @var{level} is a numeric values greater than zero @command{ld} optimizes
824 the output. This might take significantly longer and therefore probably
825 should only be enabled for the final binary. At the moment this
826 option only affects ELF shared library generation. Future releases of
827 the linker may make more use of this option. Also currently there is
828 no difference in the linker's behaviour for different non-zero values
829 of this option. Again this may change with future releases.
832 @cindex push state governing input file handling
834 The @option{--push-state} allows to preserve the current state of the
835 flags which govern the input file handling so that they can all be
836 restored with one corresponding @option{--pop-state} option.
838 The option which are covered are: @option{-Bdynamic}, @option{-Bstatic},
839 @option{-dn}, @option{-dy}, @option{-call_shared}, @option{-non_shared},
840 @option{-static}, @option{-N}, @option{-n}, @option{--whole-archive},
841 @option{--no-whole-archive}, @option{-r}, @option{-Ur},
842 @option{--copy-dt-needed-entries}, @option{--no-copy-dt-needed-entries},
843 @option{--as-needed}, @option{--no-as-needed}, and @option{-a}.
845 One target for this option are specifications for @file{pkg-config}. When
846 used with the @option{--libs} option all possibly needed libraries are
847 listed and then possibly linked with all the time. It is better to return
848 something as follows:
851 -Wl,--push-state,--as-needed -libone -libtwo -Wl,--pop-state
855 @cindex pop state governing input file handling
856 Undoes the effect of --push-state, restores the previous values of the
857 flags governing input file handling.
860 @kindex --emit-relocs
861 @cindex retain relocations in final executable
864 Leave relocation sections and contents in fully linked executables.
865 Post link analysis and optimization tools may need this information in
866 order to perform correct modifications of executables. This results
867 in larger executables.
869 This option is currently only supported on ELF platforms.
871 @kindex --force-dynamic
872 @cindex forcing the creation of dynamic sections
873 @item --force-dynamic
874 Force the output file to have dynamic sections. This option is specific
878 @cindex relocatable output
880 @kindex --relocatable
883 Generate relocatable output---i.e., generate an output file that can in
884 turn serve as input to @command{ld}. This is often called @dfn{partial
885 linking}. As a side effect, in environments that support standard Unix
886 magic numbers, this option also sets the output file's magic number to
888 @c ; see @option{-N}.
889 If this option is not specified, an absolute file is produced. When
890 linking C++ programs, this option @emph{will not} resolve references to
891 constructors; to do that, use @samp{-Ur}.
893 When an input file does not have the same format as the output file,
894 partial linking is only supported if that input file does not contain any
895 relocations. Different output formats can have further restrictions; for
896 example some @code{a.out}-based formats do not support partial linking
897 with input files in other formats at all.
899 This option does the same thing as @samp{-i}.
901 @kindex -R @var{file}
902 @kindex --just-symbols=@var{file}
903 @cindex symbol-only input
904 @item -R @var{filename}
905 @itemx --just-symbols=@var{filename}
906 Read symbol names and their addresses from @var{filename}, but do not
907 relocate it or include it in the output. This allows your output file
908 to refer symbolically to absolute locations of memory defined in other
909 programs. You may use this option more than once.
911 For compatibility with other ELF linkers, if the @option{-R} option is
912 followed by a directory name, rather than a file name, it is treated as
913 the @option{-rpath} option.
917 @cindex strip all symbols
920 Omit all symbol information from the output file.
923 @kindex --strip-debug
924 @cindex strip debugger symbols
927 Omit debugger symbol information (but not all symbols) from the output file.
931 @cindex input files, displaying
934 Print the names of the input files as @command{ld} processes them.
936 @kindex -T @var{script}
937 @kindex --script=@var{script}
939 @item -T @var{scriptfile}
940 @itemx --script=@var{scriptfile}
941 Use @var{scriptfile} as the linker script. This script replaces
942 @command{ld}'s default linker script (rather than adding to it), so
943 @var{commandfile} must specify everything necessary to describe the
944 output file. @xref{Scripts}. If @var{scriptfile} does not exist in
945 the current directory, @code{ld} looks for it in the directories
946 specified by any preceding @samp{-L} options. Multiple @samp{-T}
949 @kindex -dT @var{script}
950 @kindex --default-script=@var{script}
952 @item -dT @var{scriptfile}
953 @itemx --default-script=@var{scriptfile}
954 Use @var{scriptfile} as the default linker script. @xref{Scripts}.
956 This option is similar to the @option{--script} option except that
957 processing of the script is delayed until after the rest of the
958 command line has been processed. This allows options placed after the
959 @option{--default-script} option on the command line to affect the
960 behaviour of the linker script, which can be important when the linker
961 command line cannot be directly controlled by the user. (eg because
962 the command line is being constructed by another tool, such as
965 @kindex -u @var{symbol}
966 @kindex --undefined=@var{symbol}
967 @cindex undefined symbol
968 @item -u @var{symbol}
969 @itemx --undefined=@var{symbol}
970 Force @var{symbol} to be entered in the output file as an undefined
971 symbol. Doing this may, for example, trigger linking of additional
972 modules from standard libraries. @samp{-u} may be repeated with
973 different option arguments to enter additional undefined symbols. This
974 option is equivalent to the @code{EXTERN} linker script command.
979 For anything other than C++ programs, this option is equivalent to
980 @samp{-r}: it generates relocatable output---i.e., an output file that can in
981 turn serve as input to @command{ld}. When linking C++ programs, @samp{-Ur}
982 @emph{does} resolve references to constructors, unlike @samp{-r}.
983 It does not work to use @samp{-Ur} on files that were themselves linked
984 with @samp{-Ur}; once the constructor table has been built, it cannot
985 be added to. Use @samp{-Ur} only for the last partial link, and
986 @samp{-r} for the others.
988 @kindex --unique[=@var{SECTION}]
989 @item --unique[=@var{SECTION}]
990 Creates a separate output section for every input section matching
991 @var{SECTION}, or if the optional wildcard @var{SECTION} argument is
992 missing, for every orphan input section. An orphan section is one not
993 specifically mentioned in a linker script. You may use this option
994 multiple times on the command line; It prevents the normal merging of
995 input sections with the same name, overriding output section assignments
1005 Display the version number for @command{ld}. The @option{-V} option also
1006 lists the supported emulations.
1009 @kindex --discard-all
1010 @cindex deleting local symbols
1012 @itemx --discard-all
1013 Delete all local symbols.
1016 @kindex --discard-locals
1017 @cindex local symbols, deleting
1019 @itemx --discard-locals
1020 Delete all temporary local symbols. (These symbols start with
1021 system-specific local label prefixes, typically @samp{.L} for ELF systems
1022 or @samp{L} for traditional a.out systems.)
1024 @kindex -y @var{symbol}
1025 @kindex --trace-symbol=@var{symbol}
1026 @cindex symbol tracing
1027 @item -y @var{symbol}
1028 @itemx --trace-symbol=@var{symbol}
1029 Print the name of each linked file in which @var{symbol} appears. This
1030 option may be given any number of times. On many systems it is necessary
1031 to prepend an underscore.
1033 This option is useful when you have an undefined symbol in your link but
1034 don't know where the reference is coming from.
1036 @kindex -Y @var{path}
1038 Add @var{path} to the default library search path. This option exists
1039 for Solaris compatibility.
1041 @kindex -z @var{keyword}
1042 @item -z @var{keyword}
1043 The recognized keywords are:
1047 Combines multiple reloc sections and sorts them to make dynamic symbol
1048 lookup caching possible.
1051 Disallows undefined symbols in object files. Undefined symbols in
1052 shared libraries are still allowed.
1055 Marks the object as requiring executable stack.
1058 This option is only meaningful when building a shared object. It makes
1059 the symbols defined by this shared object available for symbol resolution
1060 of subsequently loaded libraries.
1063 This option is only meaningful when building a shared object.
1064 It marks the object so that its runtime initialization will occur
1065 before the runtime initialization of any other objects brought into
1066 the process at the same time. Similarly the runtime finalization of
1067 the object will occur after the runtime finalization of any other
1071 Marks the object that its symbol table interposes before all symbols
1072 but the primary executable.
1075 When generating an executable or shared library, mark it to tell the
1076 dynamic linker to defer function call resolution to the point when
1077 the function is called (lazy binding), rather than at load time.
1078 Lazy binding is the default.
1081 Marks the object that its filters be processed immediately at
1085 Allows multiple definitions.
1088 Disables multiple reloc sections combining.
1091 Disable linker generated .dynbss variables used in place of variables
1092 defined in shared libraries. May result in dynamic text relocations.
1095 Marks the object that the search for dependencies of this object will
1096 ignore any default library search paths.
1099 Marks the object shouldn't be unloaded at runtime.
1102 Marks the object not available to @code{dlopen}.
1105 Marks the object can not be dumped by @code{dldump}.
1108 Marks the object as not requiring executable stack.
1111 Treat DT_TEXTREL in shared object as error.
1114 Don't treat DT_TEXTREL in shared object as error.
1117 Don't treat DT_TEXTREL in shared object as error.
1120 Don't create an ELF @code{PT_GNU_RELRO} segment header in the object.
1123 When generating an executable or shared library, mark it to tell the
1124 dynamic linker to resolve all symbols when the program is started, or
1125 when the shared library is linked to using dlopen, instead of
1126 deferring function call resolution to the point when the function is
1130 Marks the object may contain $ORIGIN.
1133 Create an ELF @code{PT_GNU_RELRO} segment header in the object.
1135 @item max-page-size=@var{value}
1136 Set the emulation maximum page size to @var{value}.
1138 @item common-page-size=@var{value}
1139 Set the emulation common page size to @var{value}.
1141 @item stack-size=@var{value}
1142 Specify a stack size for in an ELF @code{PT_GNU_STACK} segment.
1143 Specifying zero will override any default non-zero sized
1144 @code{PT_GNU_STACK} segment creation.
1147 Always generate BND prefix in PLT entries. Supported for Linux/x86_64.
1149 @item noextern-protected-data
1150 Don't treat protected data symbol as external when building shared
1151 library. This option overrides linker backend default. It can be used
1152 to workaround incorrect relocations against protected data symbols
1153 generated by compiler. Updates on protected data symbols by another
1154 module aren't visibile to the resulting shared library. Supported for
1159 Other keywords are ignored for Solaris compatibility.
1162 @cindex groups of archives
1163 @item -( @var{archives} -)
1164 @itemx --start-group @var{archives} --end-group
1165 The @var{archives} should be a list of archive files. They may be
1166 either explicit file names, or @samp{-l} options.
1168 The specified archives are searched repeatedly until no new undefined
1169 references are created. Normally, an archive is searched only once in
1170 the order that it is specified on the command line. If a symbol in that
1171 archive is needed to resolve an undefined symbol referred to by an
1172 object in an archive that appears later on the command line, the linker
1173 would not be able to resolve that reference. By grouping the archives,
1174 they all be searched repeatedly until all possible references are
1177 Using this option has a significant performance cost. It is best to use
1178 it only when there are unavoidable circular references between two or
1181 @kindex --accept-unknown-input-arch
1182 @kindex --no-accept-unknown-input-arch
1183 @item --accept-unknown-input-arch
1184 @itemx --no-accept-unknown-input-arch
1185 Tells the linker to accept input files whose architecture cannot be
1186 recognised. The assumption is that the user knows what they are doing
1187 and deliberately wants to link in these unknown input files. This was
1188 the default behaviour of the linker, before release 2.14. The default
1189 behaviour from release 2.14 onwards is to reject such input files, and
1190 so the @samp{--accept-unknown-input-arch} option has been added to
1191 restore the old behaviour.
1194 @kindex --no-as-needed
1196 @itemx --no-as-needed
1197 This option affects ELF DT_NEEDED tags for dynamic libraries mentioned
1198 on the command line after the @option{--as-needed} option. Normally
1199 the linker will add a DT_NEEDED tag for each dynamic library mentioned
1200 on the command line, regardless of whether the library is actually
1201 needed or not. @option{--as-needed} causes a DT_NEEDED tag to only be
1202 emitted for a library that @emph{at that point in the link} satisfies a
1203 non-weak undefined symbol reference from a regular object file or, if
1204 the library is not found in the DT_NEEDED lists of other needed libraries, a
1205 non-weak undefined symbol reference from another needed dynamic library.
1206 Object files or libraries appearing on the command line @emph{after}
1207 the library in question do not affect whether the library is seen as
1208 needed. This is similar to the rules for extraction of object files
1209 from archives. @option{--no-as-needed} restores the default behaviour.
1211 @kindex --add-needed
1212 @kindex --no-add-needed
1214 @itemx --no-add-needed
1215 These two options have been deprecated because of the similarity of
1216 their names to the @option{--as-needed} and @option{--no-as-needed}
1217 options. They have been replaced by @option{--copy-dt-needed-entries}
1218 and @option{--no-copy-dt-needed-entries}.
1220 @kindex -assert @var{keyword}
1221 @item -assert @var{keyword}
1222 This option is ignored for SunOS compatibility.
1226 @kindex -call_shared
1230 Link against dynamic libraries. This is only meaningful on platforms
1231 for which shared libraries are supported. This option is normally the
1232 default on such platforms. The different variants of this option are
1233 for compatibility with various systems. You may use this option
1234 multiple times on the command line: it affects library searching for
1235 @option{-l} options which follow it.
1239 Set the @code{DF_1_GROUP} flag in the @code{DT_FLAGS_1} entry in the dynamic
1240 section. This causes the runtime linker to handle lookups in this
1241 object and its dependencies to be performed only inside the group.
1242 @option{--unresolved-symbols=report-all} is implied. This option is
1243 only meaningful on ELF platforms which support shared libraries.
1253 Do not link against shared libraries. This is only meaningful on
1254 platforms for which shared libraries are supported. The different
1255 variants of this option are for compatibility with various systems. You
1256 may use this option multiple times on the command line: it affects
1257 library searching for @option{-l} options which follow it. This
1258 option also implies @option{--unresolved-symbols=report-all}. This
1259 option can be used with @option{-shared}. Doing so means that a
1260 shared library is being created but that all of the library's external
1261 references must be resolved by pulling in entries from static
1266 When creating a shared library, bind references to global symbols to the
1267 definition within the shared library, if any. Normally, it is possible
1268 for a program linked against a shared library to override the definition
1269 within the shared library. This option is only meaningful on ELF
1270 platforms which support shared libraries.
1272 @kindex -Bsymbolic-functions
1273 @item -Bsymbolic-functions
1274 When creating a shared library, bind references to global function
1275 symbols to the definition within the shared library, if any.
1276 This option is only meaningful on ELF platforms which support shared
1279 @kindex --dynamic-list=@var{dynamic-list-file}
1280 @item --dynamic-list=@var{dynamic-list-file}
1281 Specify the name of a dynamic list file to the linker. This is
1282 typically used when creating shared libraries to specify a list of
1283 global symbols whose references shouldn't be bound to the definition
1284 within the shared library, or creating dynamically linked executables
1285 to specify a list of symbols which should be added to the symbol table
1286 in the executable. This option is only meaningful on ELF platforms
1287 which support shared libraries.
1289 The format of the dynamic list is the same as the version node without
1290 scope and node name. See @ref{VERSION} for more information.
1292 @kindex --dynamic-list-data
1293 @item --dynamic-list-data
1294 Include all global data symbols to the dynamic list.
1296 @kindex --dynamic-list-cpp-new
1297 @item --dynamic-list-cpp-new
1298 Provide the builtin dynamic list for C++ operator new and delete. It
1299 is mainly useful for building shared libstdc++.
1301 @kindex --dynamic-list-cpp-typeinfo
1302 @item --dynamic-list-cpp-typeinfo
1303 Provide the builtin dynamic list for C++ runtime type identification.
1305 @kindex --check-sections
1306 @kindex --no-check-sections
1307 @item --check-sections
1308 @itemx --no-check-sections
1309 Asks the linker @emph{not} to check section addresses after they have
1310 been assigned to see if there are any overlaps. Normally the linker will
1311 perform this check, and if it finds any overlaps it will produce
1312 suitable error messages. The linker does know about, and does make
1313 allowances for sections in overlays. The default behaviour can be
1314 restored by using the command line switch @option{--check-sections}.
1315 Section overlap is not usually checked for relocatable links. You can
1316 force checking in that case by using the @option{--check-sections}
1319 @kindex --copy-dt-needed-entries
1320 @kindex --no-copy-dt-needed-entries
1321 @item --copy-dt-needed-entries
1322 @itemx --no-copy-dt-needed-entries
1323 This option affects the treatment of dynamic libraries referred to
1324 by DT_NEEDED tags @emph{inside} ELF dynamic libraries mentioned on the
1325 command line. Normally the linker won't add a DT_NEEDED tag to the
1326 output binary for each library mentioned in a DT_NEEDED tag in an
1327 input dynamic library. With @option{--copy-dt-needed-entries}
1328 specified on the command line however any dynamic libraries that
1329 follow it will have their DT_NEEDED entries added. The default
1330 behaviour can be restored with @option{--no-copy-dt-needed-entries}.
1332 This option also has an effect on the resolution of symbols in dynamic
1333 libraries. With @option{--copy-dt-needed-entries} dynamic libraries
1334 mentioned on the command line will be recursively searched, following
1335 their DT_NEEDED tags to other libraries, in order to resolve symbols
1336 required by the output binary. With the default setting however
1337 the searching of dynamic libraries that follow it will stop with the
1338 dynamic library itself. No DT_NEEDED links will be traversed to resolve
1341 @cindex cross reference table
1344 Output a cross reference table. If a linker map file is being
1345 generated, the cross reference table is printed to the map file.
1346 Otherwise, it is printed on the standard output.
1348 The format of the table is intentionally simple, so that it may be
1349 easily processed by a script if necessary. The symbols are printed out,
1350 sorted by name. For each symbol, a list of file names is given. If the
1351 symbol is defined, the first file listed is the location of the
1352 definition. If the symbol is defined as a common value then any files
1353 where this happens appear next. Finally any files that reference the
1356 @cindex common allocation
1357 @kindex --no-define-common
1358 @item --no-define-common
1359 This option inhibits the assignment of addresses to common symbols.
1360 The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect.
1361 @xref{Miscellaneous Commands}.
1363 The @samp{--no-define-common} option allows decoupling
1364 the decision to assign addresses to Common symbols from the choice
1365 of the output file type; otherwise a non-Relocatable output type
1366 forces assigning addresses to Common symbols.
1367 Using @samp{--no-define-common} allows Common symbols that are referenced
1368 from a shared library to be assigned addresses only in the main program.
1369 This eliminates the unused duplicate space in the shared library,
1370 and also prevents any possible confusion over resolving to the wrong
1371 duplicate when there are many dynamic modules with specialized search
1372 paths for runtime symbol resolution.
1374 @cindex symbols, from command line
1375 @kindex --defsym=@var{symbol}=@var{exp}
1376 @item --defsym=@var{symbol}=@var{expression}
1377 Create a global symbol in the output file, containing the absolute
1378 address given by @var{expression}. You may use this option as many
1379 times as necessary to define multiple symbols in the command line. A
1380 limited form of arithmetic is supported for the @var{expression} in this
1381 context: you may give a hexadecimal constant or the name of an existing
1382 symbol, or use @code{+} and @code{-} to add or subtract hexadecimal
1383 constants or symbols. If you need more elaborate expressions, consider
1384 using the linker command language from a script (@pxref{Assignments}).
1385 @emph{Note:} there should be no white space between @var{symbol}, the
1386 equals sign (``@key{=}''), and @var{expression}.
1388 @cindex demangling, from command line
1389 @kindex --demangle[=@var{style}]
1390 @kindex --no-demangle
1391 @item --demangle[=@var{style}]
1392 @itemx --no-demangle
1393 These options control whether to demangle symbol names in error messages
1394 and other output. When the linker is told to demangle, it tries to
1395 present symbol names in a readable fashion: it strips leading
1396 underscores if they are used by the object file format, and converts C++
1397 mangled symbol names into user readable names. Different compilers have
1398 different mangling styles. The optional demangling style argument can be used
1399 to choose an appropriate demangling style for your compiler. The linker will
1400 demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE}
1401 is set. These options may be used to override the default.
1403 @cindex dynamic linker, from command line
1404 @kindex -I@var{file}
1405 @kindex --dynamic-linker=@var{file}
1407 @itemx --dynamic-linker=@var{file}
1408 Set the name of the dynamic linker. This is only meaningful when
1409 generating dynamically linked ELF executables. The default dynamic
1410 linker is normally correct; don't use this unless you know what you are
1413 @kindex --fatal-warnings
1414 @kindex --no-fatal-warnings
1415 @item --fatal-warnings
1416 @itemx --no-fatal-warnings
1417 Treat all warnings as errors. The default behaviour can be restored
1418 with the option @option{--no-fatal-warnings}.
1420 @kindex --force-exe-suffix
1421 @item --force-exe-suffix
1422 Make sure that an output file has a .exe suffix.
1424 If a successfully built fully linked output file does not have a
1425 @code{.exe} or @code{.dll} suffix, this option forces the linker to copy
1426 the output file to one of the same name with a @code{.exe} suffix. This
1427 option is useful when using unmodified Unix makefiles on a Microsoft
1428 Windows host, since some versions of Windows won't run an image unless
1429 it ends in a @code{.exe} suffix.
1431 @kindex --gc-sections
1432 @kindex --no-gc-sections
1433 @cindex garbage collection
1435 @itemx --no-gc-sections
1436 Enable garbage collection of unused input sections. It is ignored on
1437 targets that do not support this option. The default behaviour (of not
1438 performing this garbage collection) can be restored by specifying
1439 @samp{--no-gc-sections} on the command line.
1441 @samp{--gc-sections} decides which input sections are used by
1442 examining symbols and relocations. The section containing the entry
1443 symbol and all sections containing symbols undefined on the
1444 command-line will be kept, as will sections containing symbols
1445 referenced by dynamic objects. Note that when building shared
1446 libraries, the linker must assume that any visible symbol is
1447 referenced. Once this initial set of sections has been determined,
1448 the linker recursively marks as used any section referenced by their
1449 relocations. See @samp{--entry} and @samp{--undefined}.
1451 This option can be set when doing a partial link (enabled with option
1452 @samp{-r}). In this case the root of symbols kept must be explicitly
1453 specified either by an @samp{--entry} or @samp{--undefined} option or by
1454 a @code{ENTRY} command in the linker script.
1456 @kindex --print-gc-sections
1457 @kindex --no-print-gc-sections
1458 @cindex garbage collection
1459 @item --print-gc-sections
1460 @itemx --no-print-gc-sections
1461 List all sections removed by garbage collection. The listing is
1462 printed on stderr. This option is only effective if garbage
1463 collection has been enabled via the @samp{--gc-sections}) option. The
1464 default behaviour (of not listing the sections that are removed) can
1465 be restored by specifying @samp{--no-print-gc-sections} on the command
1468 @kindex --print-output-format
1469 @cindex output format
1470 @item --print-output-format
1471 Print the name of the default output format (perhaps influenced by
1472 other command-line options). This is the string that would appear
1473 in an @code{OUTPUT_FORMAT} linker script command (@pxref{File Commands}).
1475 @kindex --print-memory-usage
1476 @cindex memory usage
1477 @item --print-memory-usage
1478 Print used size, total size and used size of memory regions created with
1479 the @ref{MEMORY} command. This is useful on embedded targets to have a
1480 quick view of amount of free memory. The format of the output has one
1481 headline and one line per region. It is both human readable and easily
1482 parsable by tools. Here is an example of an output:
1485 Memory region Used Size Region Size %age Used
1486 ROM: 256 KB 1 MB 25.00%
1487 RAM: 32 B 2 GB 0.00%
1494 Print a summary of the command-line options on the standard output and exit.
1496 @kindex --target-help
1498 Print a summary of all target specific options on the standard output and exit.
1500 @kindex -Map=@var{mapfile}
1501 @item -Map=@var{mapfile}
1502 Print a link map to the file @var{mapfile}. See the description of the
1503 @option{-M} option, above.
1505 @cindex memory usage
1506 @kindex --no-keep-memory
1507 @item --no-keep-memory
1508 @command{ld} normally optimizes for speed over memory usage by caching the
1509 symbol tables of input files in memory. This option tells @command{ld} to
1510 instead optimize for memory usage, by rereading the symbol tables as
1511 necessary. This may be required if @command{ld} runs out of memory space
1512 while linking a large executable.
1514 @kindex --no-undefined
1516 @item --no-undefined
1518 Report unresolved symbol references from regular object files. This
1519 is done even if the linker is creating a non-symbolic shared library.
1520 The switch @option{--[no-]allow-shlib-undefined} controls the
1521 behaviour for reporting unresolved references found in shared
1522 libraries being linked in.
1524 @kindex --allow-multiple-definition
1526 @item --allow-multiple-definition
1528 Normally when a symbol is defined multiple times, the linker will
1529 report a fatal error. These options allow multiple definitions and the
1530 first definition will be used.
1532 @kindex --allow-shlib-undefined
1533 @kindex --no-allow-shlib-undefined
1534 @item --allow-shlib-undefined
1535 @itemx --no-allow-shlib-undefined
1536 Allows or disallows undefined symbols in shared libraries.
1537 This switch is similar to @option{--no-undefined} except that it
1538 determines the behaviour when the undefined symbols are in a
1539 shared library rather than a regular object file. It does not affect
1540 how undefined symbols in regular object files are handled.
1542 The default behaviour is to report errors for any undefined symbols
1543 referenced in shared libraries if the linker is being used to create
1544 an executable, but to allow them if the linker is being used to create
1547 The reasons for allowing undefined symbol references in shared
1548 libraries specified at link time are that:
1552 A shared library specified at link time may not be the same as the one
1553 that is available at load time, so the symbol might actually be
1554 resolvable at load time.
1556 There are some operating systems, eg BeOS and HPPA, where undefined
1557 symbols in shared libraries are normal.
1559 The BeOS kernel for example patches shared libraries at load time to
1560 select whichever function is most appropriate for the current
1561 architecture. This is used, for example, to dynamically select an
1562 appropriate memset function.
1565 @kindex --no-undefined-version
1566 @item --no-undefined-version
1567 Normally when a symbol has an undefined version, the linker will ignore
1568 it. This option disallows symbols with undefined version and a fatal error
1569 will be issued instead.
1571 @kindex --default-symver
1572 @item --default-symver
1573 Create and use a default symbol version (the soname) for unversioned
1576 @kindex --default-imported-symver
1577 @item --default-imported-symver
1578 Create and use a default symbol version (the soname) for unversioned
1581 @kindex --no-warn-mismatch
1582 @item --no-warn-mismatch
1583 Normally @command{ld} will give an error if you try to link together input
1584 files that are mismatched for some reason, perhaps because they have
1585 been compiled for different processors or for different endiannesses.
1586 This option tells @command{ld} that it should silently permit such possible
1587 errors. This option should only be used with care, in cases when you
1588 have taken some special action that ensures that the linker errors are
1591 @kindex --no-warn-search-mismatch
1592 @item --no-warn-search-mismatch
1593 Normally @command{ld} will give a warning if it finds an incompatible
1594 library during a library search. This option silences the warning.
1596 @kindex --no-whole-archive
1597 @item --no-whole-archive
1598 Turn off the effect of the @option{--whole-archive} option for subsequent
1601 @cindex output file after errors
1602 @kindex --noinhibit-exec
1603 @item --noinhibit-exec
1604 Retain the executable output file whenever it is still usable.
1605 Normally, the linker will not produce an output file if it encounters
1606 errors during the link process; it exits without writing an output file
1607 when it issues any error whatsoever.
1611 Only search library directories explicitly specified on the
1612 command line. Library directories specified in linker scripts
1613 (including linker scripts specified on the command line) are ignored.
1615 @ifclear SingleFormat
1616 @kindex --oformat=@var{output-format}
1617 @item --oformat=@var{output-format}
1618 @command{ld} may be configured to support more than one kind of object
1619 file. If your @command{ld} is configured this way, you can use the
1620 @samp{--oformat} option to specify the binary format for the output
1621 object file. Even when @command{ld} is configured to support alternative
1622 object formats, you don't usually need to specify this, as @command{ld}
1623 should be configured to produce as a default output format the most
1624 usual format on each machine. @var{output-format} is a text string, the
1625 name of a particular format supported by the BFD libraries. (You can
1626 list the available binary formats with @samp{objdump -i}.) The script
1627 command @code{OUTPUT_FORMAT} can also specify the output format, but
1628 this option overrides it. @xref{BFD}.
1632 @kindex --pic-executable
1634 @itemx --pic-executable
1635 @cindex position independent executables
1636 Create a position independent executable. This is currently only supported on
1637 ELF platforms. Position independent executables are similar to shared
1638 libraries in that they are relocated by the dynamic linker to the virtual
1639 address the OS chooses for them (which can vary between invocations). Like
1640 normal dynamically linked executables they can be executed and symbols
1641 defined in the executable cannot be overridden by shared libraries.
1645 This option is ignored for Linux compatibility.
1649 This option is ignored for SVR4 compatibility.
1652 @cindex synthesizing linker
1653 @cindex relaxing addressing modes
1657 An option with machine dependent effects.
1659 This option is only supported on a few targets.
1662 @xref{H8/300,,@command{ld} and the H8/300}.
1665 @xref{i960,, @command{ld} and the Intel 960 family}.
1668 @xref{Xtensa,, @command{ld} and Xtensa Processors}.
1671 @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}.
1674 @xref{Nios II,,@command{ld} and the Altera Nios II}.
1677 @xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}.
1680 On some platforms the @samp{--relax} option performs target specific,
1681 global optimizations that become possible when the linker resolves
1682 addressing in the program, such as relaxing address modes,
1683 synthesizing new instructions, selecting shorter version of current
1684 instructions, and combining constant values.
1686 On some platforms these link time global optimizations may make symbolic
1687 debugging of the resulting executable impossible.
1689 This is known to be the case for the Matsushita MN10200 and MN10300
1690 family of processors.
1694 On platforms where this is not supported, @samp{--relax} is accepted,
1698 On platforms where @samp{--relax} is accepted the option
1699 @samp{--no-relax} can be used to disable the feature.
1701 @cindex retaining specified symbols
1702 @cindex stripping all but some symbols
1703 @cindex symbols, retaining selectively
1704 @kindex --retain-symbols-file=@var{filename}
1705 @item --retain-symbols-file=@var{filename}
1706 Retain @emph{only} the symbols listed in the file @var{filename},
1707 discarding all others. @var{filename} is simply a flat file, with one
1708 symbol name per line. This option is especially useful in environments
1712 where a large global symbol table is accumulated gradually, to conserve
1715 @samp{--retain-symbols-file} does @emph{not} discard undefined symbols,
1716 or symbols needed for relocations.
1718 You may only specify @samp{--retain-symbols-file} once in the command
1719 line. It overrides @samp{-s} and @samp{-S}.
1722 @item -rpath=@var{dir}
1723 @cindex runtime library search path
1724 @kindex -rpath=@var{dir}
1725 Add a directory to the runtime library search path. This is used when
1726 linking an ELF executable with shared objects. All @option{-rpath}
1727 arguments are concatenated and passed to the runtime linker, which uses
1728 them to locate shared objects at runtime. The @option{-rpath} option is
1729 also used when locating shared objects which are needed by shared
1730 objects explicitly included in the link; see the description of the
1731 @option{-rpath-link} option. If @option{-rpath} is not used when linking an
1732 ELF executable, the contents of the environment variable
1733 @code{LD_RUN_PATH} will be used if it is defined.
1735 The @option{-rpath} option may also be used on SunOS. By default, on
1736 SunOS, the linker will form a runtime search patch out of all the
1737 @option{-L} options it is given. If a @option{-rpath} option is used, the
1738 runtime search path will be formed exclusively using the @option{-rpath}
1739 options, ignoring the @option{-L} options. This can be useful when using
1740 gcc, which adds many @option{-L} options which may be on NFS mounted
1743 For compatibility with other ELF linkers, if the @option{-R} option is
1744 followed by a directory name, rather than a file name, it is treated as
1745 the @option{-rpath} option.
1749 @cindex link-time runtime library search path
1750 @kindex -rpath-link=@var{dir}
1751 @item -rpath-link=@var{dir}
1752 When using ELF or SunOS, one shared library may require another. This
1753 happens when an @code{ld -shared} link includes a shared library as one
1756 When the linker encounters such a dependency when doing a non-shared,
1757 non-relocatable link, it will automatically try to locate the required
1758 shared library and include it in the link, if it is not included
1759 explicitly. In such a case, the @option{-rpath-link} option
1760 specifies the first set of directories to search. The
1761 @option{-rpath-link} option may specify a sequence of directory names
1762 either by specifying a list of names separated by colons, or by
1763 appearing multiple times.
1765 This option should be used with caution as it overrides the search path
1766 that may have been hard compiled into a shared library. In such a case it
1767 is possible to use unintentionally a different search path than the
1768 runtime linker would do.
1770 The linker uses the following search paths to locate required shared
1774 Any directories specified by @option{-rpath-link} options.
1776 Any directories specified by @option{-rpath} options. The difference
1777 between @option{-rpath} and @option{-rpath-link} is that directories
1778 specified by @option{-rpath} options are included in the executable and
1779 used at runtime, whereas the @option{-rpath-link} option is only effective
1780 at link time. Searching @option{-rpath} in this way is only supported
1781 by native linkers and cross linkers which have been configured with
1782 the @option{--with-sysroot} option.
1784 On an ELF system, for native linkers, if the @option{-rpath} and
1785 @option{-rpath-link} options were not used, search the contents of the
1786 environment variable @code{LD_RUN_PATH}.
1788 On SunOS, if the @option{-rpath} option was not used, search any
1789 directories specified using @option{-L} options.
1791 For a native linker, search the contents of the environment
1792 variable @code{LD_LIBRARY_PATH}.
1794 For a native ELF linker, the directories in @code{DT_RUNPATH} or
1795 @code{DT_RPATH} of a shared library are searched for shared
1796 libraries needed by it. The @code{DT_RPATH} entries are ignored if
1797 @code{DT_RUNPATH} entries exist.
1799 The default directories, normally @file{/lib} and @file{/usr/lib}.
1801 For a native linker on an ELF system, if the file @file{/etc/ld.so.conf}
1802 exists, the list of directories found in that file.
1805 If the required shared library is not found, the linker will issue a
1806 warning and continue with the link.
1813 @cindex shared libraries
1814 Create a shared library. This is currently only supported on ELF, XCOFF
1815 and SunOS platforms. On SunOS, the linker will automatically create a
1816 shared library if the @option{-e} option is not used and there are
1817 undefined symbols in the link.
1819 @kindex --sort-common
1821 @itemx --sort-common=ascending
1822 @itemx --sort-common=descending
1823 This option tells @command{ld} to sort the common symbols by alignment in
1824 ascending or descending order when it places them in the appropriate output
1825 sections. The symbol alignments considered are sixteen-byte or larger,
1826 eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps
1827 between symbols due to alignment constraints. If no sorting order is
1828 specified, then descending order is assumed.
1830 @kindex --sort-section=name
1831 @item --sort-section=name
1832 This option will apply @code{SORT_BY_NAME} to all wildcard section
1833 patterns in the linker script.
1835 @kindex --sort-section=alignment
1836 @item --sort-section=alignment
1837 This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section
1838 patterns in the linker script.
1840 @kindex --split-by-file
1841 @item --split-by-file[=@var{size}]
1842 Similar to @option{--split-by-reloc} but creates a new output section for
1843 each input file when @var{size} is reached. @var{size} defaults to a
1844 size of 1 if not given.
1846 @kindex --split-by-reloc
1847 @item --split-by-reloc[=@var{count}]
1848 Tries to creates extra sections in the output file so that no single
1849 output section in the file contains more than @var{count} relocations.
1850 This is useful when generating huge relocatable files for downloading into
1851 certain real time kernels with the COFF object file format; since COFF
1852 cannot represent more than 65535 relocations in a single section. Note
1853 that this will fail to work with object file formats which do not
1854 support arbitrary sections. The linker will not split up individual
1855 input sections for redistribution, so if a single input section contains
1856 more than @var{count} relocations one output section will contain that
1857 many relocations. @var{count} defaults to a value of 32768.
1861 Compute and display statistics about the operation of the linker, such
1862 as execution time and memory usage.
1864 @kindex --sysroot=@var{directory}
1865 @item --sysroot=@var{directory}
1866 Use @var{directory} as the location of the sysroot, overriding the
1867 configure-time default. This option is only supported by linkers
1868 that were configured using @option{--with-sysroot}.
1870 @kindex --traditional-format
1871 @cindex traditional format
1872 @item --traditional-format
1873 For some targets, the output of @command{ld} is different in some ways from
1874 the output of some existing linker. This switch requests @command{ld} to
1875 use the traditional format instead.
1878 For example, on SunOS, @command{ld} combines duplicate entries in the
1879 symbol string table. This can reduce the size of an output file with
1880 full debugging information by over 30 percent. Unfortunately, the SunOS
1881 @code{dbx} program can not read the resulting program (@code{gdb} has no
1882 trouble). The @samp{--traditional-format} switch tells @command{ld} to not
1883 combine duplicate entries.
1885 @kindex --section-start=@var{sectionname}=@var{org}
1886 @item --section-start=@var{sectionname}=@var{org}
1887 Locate a section in the output file at the absolute
1888 address given by @var{org}. You may use this option as many
1889 times as necessary to locate multiple sections in the command
1891 @var{org} must be a single hexadecimal integer;
1892 for compatibility with other linkers, you may omit the leading
1893 @samp{0x} usually associated with hexadecimal values. @emph{Note:} there
1894 should be no white space between @var{sectionname}, the equals
1895 sign (``@key{=}''), and @var{org}.
1897 @kindex -Tbss=@var{org}
1898 @kindex -Tdata=@var{org}
1899 @kindex -Ttext=@var{org}
1900 @cindex segment origins, cmd line
1901 @item -Tbss=@var{org}
1902 @itemx -Tdata=@var{org}
1903 @itemx -Ttext=@var{org}
1904 Same as @option{--section-start}, with @code{.bss}, @code{.data} or
1905 @code{.text} as the @var{sectionname}.
1907 @kindex -Ttext-segment=@var{org}
1908 @item -Ttext-segment=@var{org}
1909 @cindex text segment origin, cmd line
1910 When creating an ELF executable, it will set the address of the first
1911 byte of the text segment.
1913 @kindex -Trodata-segment=@var{org}
1914 @item -Trodata-segment=@var{org}
1915 @cindex rodata segment origin, cmd line
1916 When creating an ELF executable or shared object for a target where
1917 the read-only data is in its own segment separate from the executable
1918 text, it will set the address of the first byte of the read-only data segment.
1920 @kindex -Tldata-segment=@var{org}
1921 @item -Tldata-segment=@var{org}
1922 @cindex ldata segment origin, cmd line
1923 When creating an ELF executable or shared object for x86-64 medium memory
1924 model, it will set the address of the first byte of the ldata segment.
1926 @kindex --unresolved-symbols
1927 @item --unresolved-symbols=@var{method}
1928 Determine how to handle unresolved symbols. There are four possible
1929 values for @samp{method}:
1933 Do not report any unresolved symbols.
1936 Report all unresolved symbols. This is the default.
1938 @item ignore-in-object-files
1939 Report unresolved symbols that are contained in shared libraries, but
1940 ignore them if they come from regular object files.
1942 @item ignore-in-shared-libs
1943 Report unresolved symbols that come from regular object files, but
1944 ignore them if they come from shared libraries. This can be useful
1945 when creating a dynamic binary and it is known that all the shared
1946 libraries that it should be referencing are included on the linker's
1950 The behaviour for shared libraries on their own can also be controlled
1951 by the @option{--[no-]allow-shlib-undefined} option.
1953 Normally the linker will generate an error message for each reported
1954 unresolved symbol but the option @option{--warn-unresolved-symbols}
1955 can change this to a warning.
1957 @kindex --verbose[=@var{NUMBER}]
1958 @cindex verbose[=@var{NUMBER}]
1960 @itemx --verbose[=@var{NUMBER}]
1961 Display the version number for @command{ld} and list the linker emulations
1962 supported. Display which input files can and cannot be opened. Display
1963 the linker script being used by the linker. If the optional @var{NUMBER}
1964 argument > 1, plugin symbol status will also be displayed.
1966 @kindex --version-script=@var{version-scriptfile}
1967 @cindex version script, symbol versions
1968 @item --version-script=@var{version-scriptfile}
1969 Specify the name of a version script to the linker. This is typically
1970 used when creating shared libraries to specify additional information
1971 about the version hierarchy for the library being created. This option
1972 is only fully supported on ELF platforms which support shared libraries;
1973 see @ref{VERSION}. It is partially supported on PE platforms, which can
1974 use version scripts to filter symbol visibility in auto-export mode: any
1975 symbols marked @samp{local} in the version script will not be exported.
1978 @kindex --warn-common
1979 @cindex warnings, on combining symbols
1980 @cindex combining symbols, warnings on
1982 Warn when a common symbol is combined with another common symbol or with
1983 a symbol definition. Unix linkers allow this somewhat sloppy practice,
1984 but linkers on some other operating systems do not. This option allows
1985 you to find potential problems from combining global symbols.
1986 Unfortunately, some C libraries use this practice, so you may get some
1987 warnings about symbols in the libraries as well as in your programs.
1989 There are three kinds of global symbols, illustrated here by C examples:
1993 A definition, which goes in the initialized data section of the output
1997 An undefined reference, which does not allocate space.
1998 There must be either a definition or a common symbol for the
2002 A common symbol. If there are only (one or more) common symbols for a
2003 variable, it goes in the uninitialized data area of the output file.
2004 The linker merges multiple common symbols for the same variable into a
2005 single symbol. If they are of different sizes, it picks the largest
2006 size. The linker turns a common symbol into a declaration, if there is
2007 a definition of the same variable.
2010 The @samp{--warn-common} option can produce five kinds of warnings.
2011 Each warning consists of a pair of lines: the first describes the symbol
2012 just encountered, and the second describes the previous symbol
2013 encountered with the same name. One or both of the two symbols will be
2018 Turning a common symbol into a reference, because there is already a
2019 definition for the symbol.
2021 @var{file}(@var{section}): warning: common of `@var{symbol}'
2022 overridden by definition
2023 @var{file}(@var{section}): warning: defined here
2027 Turning a common symbol into a reference, because a later definition for
2028 the symbol is encountered. This is the same as the previous case,
2029 except that the symbols are encountered in a different order.
2031 @var{file}(@var{section}): warning: definition of `@var{symbol}'
2033 @var{file}(@var{section}): warning: common is here
2037 Merging a common symbol with a previous same-sized common symbol.
2039 @var{file}(@var{section}): warning: multiple common
2041 @var{file}(@var{section}): warning: previous common is here
2045 Merging a common symbol with a previous larger common symbol.
2047 @var{file}(@var{section}): warning: common of `@var{symbol}'
2048 overridden by larger common
2049 @var{file}(@var{section}): warning: larger common is here
2053 Merging a common symbol with a previous smaller common symbol. This is
2054 the same as the previous case, except that the symbols are
2055 encountered in a different order.
2057 @var{file}(@var{section}): warning: common of `@var{symbol}'
2058 overriding smaller common
2059 @var{file}(@var{section}): warning: smaller common is here
2063 @kindex --warn-constructors
2064 @item --warn-constructors
2065 Warn if any global constructors are used. This is only useful for a few
2066 object file formats. For formats like COFF or ELF, the linker can not
2067 detect the use of global constructors.
2069 @kindex --warn-multiple-gp
2070 @item --warn-multiple-gp
2071 Warn if multiple global pointer values are required in the output file.
2072 This is only meaningful for certain processors, such as the Alpha.
2073 Specifically, some processors put large-valued constants in a special
2074 section. A special register (the global pointer) points into the middle
2075 of this section, so that constants can be loaded efficiently via a
2076 base-register relative addressing mode. Since the offset in
2077 base-register relative mode is fixed and relatively small (e.g., 16
2078 bits), this limits the maximum size of the constant pool. Thus, in
2079 large programs, it is often necessary to use multiple global pointer
2080 values in order to be able to address all possible constants. This
2081 option causes a warning to be issued whenever this case occurs.
2084 @cindex warnings, on undefined symbols
2085 @cindex undefined symbols, warnings on
2087 Only warn once for each undefined symbol, rather than once per module
2090 @kindex --warn-orphan
2091 @kindex --no-warn-orphan
2092 @cindex warnings, on orphan sections
2093 @cindex orphan sections, warnings on
2095 The @option{--warn-orphan} option tells the linker to generate a
2096 warning message whenever it has to place an orphan section into the
2097 output file. @xref{Orphan Sections} The @option{--no-warn-orphan}
2098 option restores the default behaviour of just silently placing these
2101 @kindex --warn-section-align
2102 @cindex warnings, on section alignment
2103 @cindex section alignment, warnings on
2104 @item --warn-section-align
2105 Warn if the address of an output section is changed because of
2106 alignment. Typically, the alignment will be set by an input section.
2107 The address will only be changed if it not explicitly specified; that
2108 is, if the @code{SECTIONS} command does not specify a start address for
2109 the section (@pxref{SECTIONS}).
2111 @kindex --warn-shared-textrel
2112 @item --warn-shared-textrel
2113 Warn if the linker adds a DT_TEXTREL to a shared object.
2115 @kindex --warn-alternate-em
2116 @item --warn-alternate-em
2117 Warn if an object has alternate ELF machine code.
2119 @kindex --warn-unresolved-symbols
2120 @item --warn-unresolved-symbols
2121 If the linker is going to report an unresolved symbol (see the option
2122 @option{--unresolved-symbols}) it will normally generate an error.
2123 This option makes it generate a warning instead.
2125 @kindex --error-unresolved-symbols
2126 @item --error-unresolved-symbols
2127 This restores the linker's default behaviour of generating errors when
2128 it is reporting unresolved symbols.
2130 @kindex --whole-archive
2131 @cindex including an entire archive
2132 @item --whole-archive
2133 For each archive mentioned on the command line after the
2134 @option{--whole-archive} option, include every object file in the archive
2135 in the link, rather than searching the archive for the required object
2136 files. This is normally used to turn an archive file into a shared
2137 library, forcing every object to be included in the resulting shared
2138 library. This option may be used more than once.
2140 Two notes when using this option from gcc: First, gcc doesn't know
2141 about this option, so you have to use @option{-Wl,-whole-archive}.
2142 Second, don't forget to use @option{-Wl,-no-whole-archive} after your
2143 list of archives, because gcc will add its own list of archives to
2144 your link and you may not want this flag to affect those as well.
2146 @kindex --wrap=@var{symbol}
2147 @item --wrap=@var{symbol}
2148 Use a wrapper function for @var{symbol}. Any undefined reference to
2149 @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any
2150 undefined reference to @code{__real_@var{symbol}} will be resolved to
2153 This can be used to provide a wrapper for a system function. The
2154 wrapper function should be called @code{__wrap_@var{symbol}}. If it
2155 wishes to call the system function, it should call
2156 @code{__real_@var{symbol}}.
2158 Here is a trivial example:
2162 __wrap_malloc (size_t c)
2164 printf ("malloc called with %zu\n", c);
2165 return __real_malloc (c);
2169 If you link other code with this file using @option{--wrap malloc}, then
2170 all calls to @code{malloc} will call the function @code{__wrap_malloc}
2171 instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will
2172 call the real @code{malloc} function.
2174 You may wish to provide a @code{__real_malloc} function as well, so that
2175 links without the @option{--wrap} option will succeed. If you do this,
2176 you should not put the definition of @code{__real_malloc} in the same
2177 file as @code{__wrap_malloc}; if you do, the assembler may resolve the
2178 call before the linker has a chance to wrap it to @code{malloc}.
2180 @kindex --eh-frame-hdr
2181 @item --eh-frame-hdr
2182 Request creation of @code{.eh_frame_hdr} section and ELF
2183 @code{PT_GNU_EH_FRAME} segment header.
2185 @kindex --ld-generated-unwind-info
2186 @item --no-ld-generated-unwind-info
2187 Request creation of @code{.eh_frame} unwind info for linker
2188 generated code sections like PLT. This option is on by default
2189 if linker generated unwind info is supported.
2191 @kindex --enable-new-dtags
2192 @kindex --disable-new-dtags
2193 @item --enable-new-dtags
2194 @itemx --disable-new-dtags
2195 This linker can create the new dynamic tags in ELF. But the older ELF
2196 systems may not understand them. If you specify
2197 @option{--enable-new-dtags}, the new dynamic tags will be created as needed
2198 and older dynamic tags will be omitted.
2199 If you specify @option{--disable-new-dtags}, no new dynamic tags will be
2200 created. By default, the new dynamic tags are not created. Note that
2201 those options are only available for ELF systems.
2203 @kindex --hash-size=@var{number}
2204 @item --hash-size=@var{number}
2205 Set the default size of the linker's hash tables to a prime number
2206 close to @var{number}. Increasing this value can reduce the length of
2207 time it takes the linker to perform its tasks, at the expense of
2208 increasing the linker's memory requirements. Similarly reducing this
2209 value can reduce the memory requirements at the expense of speed.
2211 @kindex --hash-style=@var{style}
2212 @item --hash-style=@var{style}
2213 Set the type of linker's hash table(s). @var{style} can be either
2214 @code{sysv} for classic ELF @code{.hash} section, @code{gnu} for
2215 new style GNU @code{.gnu.hash} section or @code{both} for both
2216 the classic ELF @code{.hash} and new style GNU @code{.gnu.hash}
2217 hash tables. The default is @code{sysv}.
2219 @kindex --compress-debug-sections=none
2220 @kindex --compress-debug-sections=zlib
2221 @kindex --compress-debug-sections=zlib-gnu
2222 @kindex --compress-debug-sections=zlib-gabi
2223 @item --compress-debug-sections=none
2224 @itemx --compress-debug-sections=zlib
2225 @itemx --compress-debug-sections=zlib-gnu
2226 @itemx --compress-debug-sections=zlib-gabi
2227 On ELF platforms , these options control how DWARF debug sections are
2228 compressed using zlib. @option{--compress-debug-sections=none} doesn't
2229 compress DWARF debug sections. @option{--compress-debug-sections=zlib}
2230 and @option{--compress-debug-sections=zlib-gnu} compress DWARF debug
2231 sections and rename debug section names to begin with @samp{.zdebug}
2232 instead of @samp{.debug}. @option{--compress-debug-sections=zlib-gabi}
2233 compresses DWARF debug sections with SHF_COMPRESSED from the ELF ABI.
2235 @kindex --reduce-memory-overheads
2236 @item --reduce-memory-overheads
2237 This option reduces memory requirements at ld runtime, at the expense of
2238 linking speed. This was introduced to select the old O(n^2) algorithm
2239 for link map file generation, rather than the new O(n) algorithm which uses
2240 about 40% more memory for symbol storage.
2242 Another effect of the switch is to set the default hash table size to
2243 1021, which again saves memory at the cost of lengthening the linker's
2244 run time. This is not done however if the @option{--hash-size} switch
2247 The @option{--reduce-memory-overheads} switch may be also be used to
2248 enable other tradeoffs in future versions of the linker.
2251 @kindex --build-id=@var{style}
2253 @itemx --build-id=@var{style}
2254 Request the creation of a @code{.note.gnu.build-id} ELF note section
2255 or a @code{.build-id} COFF section. The contents of the note are
2256 unique bits identifying this linked file. @var{style} can be
2257 @code{uuid} to use 128 random bits, @code{sha1} to use a 160-bit
2258 @sc{SHA1} hash on the normative parts of the output contents,
2259 @code{md5} to use a 128-bit @sc{MD5} hash on the normative parts of
2260 the output contents, or @code{0x@var{hexstring}} to use a chosen bit
2261 string specified as an even number of hexadecimal digits (@code{-} and
2262 @code{:} characters between digit pairs are ignored). If @var{style}
2263 is omitted, @code{sha1} is used.
2265 The @code{md5} and @code{sha1} styles produces an identifier
2266 that is always the same in an identical output file, but will be
2267 unique among all nonidentical output files. It is not intended
2268 to be compared as a checksum for the file's contents. A linked
2269 file may be changed later by other tools, but the build ID bit
2270 string identifying the original linked file does not change.
2272 Passing @code{none} for @var{style} disables the setting from any
2273 @code{--build-id} options earlier on the command line.
2278 @subsection Options Specific to i386 PE Targets
2280 @c man begin OPTIONS
2282 The i386 PE linker supports the @option{-shared} option, which causes
2283 the output to be a dynamically linked library (DLL) instead of a
2284 normal executable. You should name the output @code{*.dll} when you
2285 use this option. In addition, the linker fully supports the standard
2286 @code{*.def} files, which may be specified on the linker command line
2287 like an object file (in fact, it should precede archives it exports
2288 symbols from, to ensure that they get linked in, just like a normal
2291 In addition to the options common to all targets, the i386 PE linker
2292 support additional command line options that are specific to the i386
2293 PE target. Options that take values may be separated from their
2294 values by either a space or an equals sign.
2298 @kindex --add-stdcall-alias
2299 @item --add-stdcall-alias
2300 If given, symbols with a stdcall suffix (@@@var{nn}) will be exported
2301 as-is and also with the suffix stripped.
2302 [This option is specific to the i386 PE targeted port of the linker]
2305 @item --base-file @var{file}
2306 Use @var{file} as the name of a file in which to save the base
2307 addresses of all the relocations needed for generating DLLs with
2309 [This is an i386 PE specific option]
2313 Create a DLL instead of a regular executable. You may also use
2314 @option{-shared} or specify a @code{LIBRARY} in a given @code{.def}
2316 [This option is specific to the i386 PE targeted port of the linker]
2318 @kindex --enable-long-section-names
2319 @kindex --disable-long-section-names
2320 @item --enable-long-section-names
2321 @itemx --disable-long-section-names
2322 The PE variants of the Coff object format add an extension that permits
2323 the use of section names longer than eight characters, the normal limit
2324 for Coff. By default, these names are only allowed in object files, as
2325 fully-linked executable images do not carry the Coff string table required
2326 to support the longer names. As a GNU extension, it is possible to
2327 allow their use in executable images as well, or to (probably pointlessly!)
2328 disallow it in object files, by using these two options. Executable images
2329 generated with these long section names are slightly non-standard, carrying
2330 as they do a string table, and may generate confusing output when examined
2331 with non-GNU PE-aware tools, such as file viewers and dumpers. However,
2332 GDB relies on the use of PE long section names to find Dwarf-2 debug
2333 information sections in an executable image at runtime, and so if neither
2334 option is specified on the command-line, @command{ld} will enable long
2335 section names, overriding the default and technically correct behaviour,
2336 when it finds the presence of debug information while linking an executable
2337 image and not stripping symbols.
2338 [This option is valid for all PE targeted ports of the linker]
2340 @kindex --enable-stdcall-fixup
2341 @kindex --disable-stdcall-fixup
2342 @item --enable-stdcall-fixup
2343 @itemx --disable-stdcall-fixup
2344 If the link finds a symbol that it cannot resolve, it will attempt to
2345 do ``fuzzy linking'' by looking for another defined symbol that differs
2346 only in the format of the symbol name (cdecl vs stdcall) and will
2347 resolve that symbol by linking to the match. For example, the
2348 undefined symbol @code{_foo} might be linked to the function
2349 @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked
2350 to the function @code{_bar}. When the linker does this, it prints a
2351 warning, since it normally should have failed to link, but sometimes
2352 import libraries generated from third-party dlls may need this feature
2353 to be usable. If you specify @option{--enable-stdcall-fixup}, this
2354 feature is fully enabled and warnings are not printed. If you specify
2355 @option{--disable-stdcall-fixup}, this feature is disabled and such
2356 mismatches are considered to be errors.
2357 [This option is specific to the i386 PE targeted port of the linker]
2359 @kindex --leading-underscore
2360 @kindex --no-leading-underscore
2361 @item --leading-underscore
2362 @itemx --no-leading-underscore
2363 For most targets default symbol-prefix is an underscore and is defined
2364 in target's description. By this option it is possible to
2365 disable/enable the default underscore symbol-prefix.
2367 @cindex DLLs, creating
2368 @kindex --export-all-symbols
2369 @item --export-all-symbols
2370 If given, all global symbols in the objects used to build a DLL will
2371 be exported by the DLL. Note that this is the default if there
2372 otherwise wouldn't be any exported symbols. When symbols are
2373 explicitly exported via DEF files or implicitly exported via function
2374 attributes, the default is to not export anything else unless this
2375 option is given. Note that the symbols @code{DllMain@@12},
2376 @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and
2377 @code{impure_ptr} will not be automatically
2378 exported. Also, symbols imported from other DLLs will not be
2379 re-exported, nor will symbols specifying the DLL's internal layout
2380 such as those beginning with @code{_head_} or ending with
2381 @code{_iname}. In addition, no symbols from @code{libgcc},
2382 @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported.
2383 Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will
2384 not be exported, to help with C++ DLLs. Finally, there is an
2385 extensive list of cygwin-private symbols that are not exported
2386 (obviously, this applies on when building DLLs for cygwin targets).
2387 These cygwin-excludes are: @code{_cygwin_dll_entry@@12},
2388 @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12},
2389 @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll},
2390 @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2},
2391 @code{cygwin_premain3}, and @code{environ}.
2392 [This option is specific to the i386 PE targeted port of the linker]
2394 @kindex --exclude-symbols
2395 @item --exclude-symbols @var{symbol},@var{symbol},...
2396 Specifies a list of symbols which should not be automatically
2397 exported. The symbol names may be delimited by commas or colons.
2398 [This option is specific to the i386 PE targeted port of the linker]
2400 @kindex --exclude-all-symbols
2401 @item --exclude-all-symbols
2402 Specifies no symbols should be automatically exported.
2403 [This option is specific to the i386 PE targeted port of the linker]
2405 @kindex --file-alignment
2406 @item --file-alignment
2407 Specify the file alignment. Sections in the file will always begin at
2408 file offsets which are multiples of this number. This defaults to
2410 [This option is specific to the i386 PE targeted port of the linker]
2414 @item --heap @var{reserve}
2415 @itemx --heap @var{reserve},@var{commit}
2416 Specify the number of bytes of memory to reserve (and optionally commit)
2417 to be used as heap for this program. The default is 1MB reserved, 4K
2419 [This option is specific to the i386 PE targeted port of the linker]
2422 @kindex --image-base
2423 @item --image-base @var{value}
2424 Use @var{value} as the base address of your program or dll. This is
2425 the lowest memory location that will be used when your program or dll
2426 is loaded. To reduce the need to relocate and improve performance of
2427 your dlls, each should have a unique base address and not overlap any
2428 other dlls. The default is 0x400000 for executables, and 0x10000000
2430 [This option is specific to the i386 PE targeted port of the linker]
2434 If given, the stdcall suffixes (@@@var{nn}) will be stripped from
2435 symbols before they are exported.
2436 [This option is specific to the i386 PE targeted port of the linker]
2438 @kindex --large-address-aware
2439 @item --large-address-aware
2440 If given, the appropriate bit in the ``Characteristics'' field of the COFF
2441 header is set to indicate that this executable supports virtual addresses
2442 greater than 2 gigabytes. This should be used in conjunction with the /3GB
2443 or /USERVA=@var{value} megabytes switch in the ``[operating systems]''
2444 section of the BOOT.INI. Otherwise, this bit has no effect.
2445 [This option is specific to PE targeted ports of the linker]
2447 @kindex --disable-large-address-aware
2448 @item --disable-large-address-aware
2449 Reverts the effect of a previous @samp{--large-address-aware} option.
2450 This is useful if @samp{--large-address-aware} is always set by the compiler
2451 driver (e.g. Cygwin gcc) and the executable does not support virtual
2452 addresses greater than 2 gigabytes.
2453 [This option is specific to PE targeted ports of the linker]
2455 @kindex --major-image-version
2456 @item --major-image-version @var{value}
2457 Sets the major number of the ``image version''. Defaults to 1.
2458 [This option is specific to the i386 PE targeted port of the linker]
2460 @kindex --major-os-version
2461 @item --major-os-version @var{value}
2462 Sets the major number of the ``os version''. Defaults to 4.
2463 [This option is specific to the i386 PE targeted port of the linker]
2465 @kindex --major-subsystem-version
2466 @item --major-subsystem-version @var{value}
2467 Sets the major number of the ``subsystem version''. Defaults to 4.
2468 [This option is specific to the i386 PE targeted port of the linker]
2470 @kindex --minor-image-version
2471 @item --minor-image-version @var{value}
2472 Sets the minor number of the ``image version''. Defaults to 0.
2473 [This option is specific to the i386 PE targeted port of the linker]
2475 @kindex --minor-os-version
2476 @item --minor-os-version @var{value}
2477 Sets the minor number of the ``os version''. Defaults to 0.
2478 [This option is specific to the i386 PE targeted port of the linker]
2480 @kindex --minor-subsystem-version
2481 @item --minor-subsystem-version @var{value}
2482 Sets the minor number of the ``subsystem version''. Defaults to 0.
2483 [This option is specific to the i386 PE targeted port of the linker]
2485 @cindex DEF files, creating
2486 @cindex DLLs, creating
2487 @kindex --output-def
2488 @item --output-def @var{file}
2489 The linker will create the file @var{file} which will contain a DEF
2490 file corresponding to the DLL the linker is generating. This DEF file
2491 (which should be called @code{*.def}) may be used to create an import
2492 library with @code{dlltool} or may be used as a reference to
2493 automatically or implicitly exported symbols.
2494 [This option is specific to the i386 PE targeted port of the linker]
2496 @cindex DLLs, creating
2497 @kindex --out-implib
2498 @item --out-implib @var{file}
2499 The linker will create the file @var{file} which will contain an
2500 import lib corresponding to the DLL the linker is generating. This
2501 import lib (which should be called @code{*.dll.a} or @code{*.a}
2502 may be used to link clients against the generated DLL; this behaviour
2503 makes it possible to skip a separate @code{dlltool} import library
2505 [This option is specific to the i386 PE targeted port of the linker]
2507 @kindex --enable-auto-image-base
2508 @item --enable-auto-image-base
2509 @itemx --enable-auto-image-base=@var{value}
2510 Automatically choose the image base for DLLs, optionally starting with base
2511 @var{value}, unless one is specified using the @code{--image-base} argument.
2512 By using a hash generated from the dllname to create unique image bases
2513 for each DLL, in-memory collisions and relocations which can delay program
2514 execution are avoided.
2515 [This option is specific to the i386 PE targeted port of the linker]
2517 @kindex --disable-auto-image-base
2518 @item --disable-auto-image-base
2519 Do not automatically generate a unique image base. If there is no
2520 user-specified image base (@code{--image-base}) then use the platform
2522 [This option is specific to the i386 PE targeted port of the linker]
2524 @cindex DLLs, linking to
2525 @kindex --dll-search-prefix
2526 @item --dll-search-prefix @var{string}
2527 When linking dynamically to a dll without an import library,
2528 search for @code{<string><basename>.dll} in preference to
2529 @code{lib<basename>.dll}. This behaviour allows easy distinction
2530 between DLLs built for the various "subplatforms": native, cygwin,
2531 uwin, pw, etc. For instance, cygwin DLLs typically use
2532 @code{--dll-search-prefix=cyg}.
2533 [This option is specific to the i386 PE targeted port of the linker]
2535 @kindex --enable-auto-import
2536 @item --enable-auto-import
2537 Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for
2538 DATA imports from DLLs, and create the necessary thunking symbols when
2539 building the import libraries with those DATA exports. Note: Use of the
2540 'auto-import' extension will cause the text section of the image file
2541 to be made writable. This does not conform to the PE-COFF format
2542 specification published by Microsoft.
2544 Note - use of the 'auto-import' extension will also cause read only
2545 data which would normally be placed into the .rdata section to be
2546 placed into the .data section instead. This is in order to work
2547 around a problem with consts that is described here:
2548 http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
2550 Using 'auto-import' generally will 'just work' -- but sometimes you may
2553 "variable '<var>' can't be auto-imported. Please read the
2554 documentation for ld's @code{--enable-auto-import} for details."
2556 This message occurs when some (sub)expression accesses an address
2557 ultimately given by the sum of two constants (Win32 import tables only
2558 allow one). Instances where this may occur include accesses to member
2559 fields of struct variables imported from a DLL, as well as using a
2560 constant index into an array variable imported from a DLL. Any
2561 multiword variable (arrays, structs, long long, etc) may trigger
2562 this error condition. However, regardless of the exact data type
2563 of the offending exported variable, ld will always detect it, issue
2564 the warning, and exit.
2566 There are several ways to address this difficulty, regardless of the
2567 data type of the exported variable:
2569 One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
2570 of adjusting references in your client code for runtime environment, so
2571 this method works only when runtime environment supports this feature.
2573 A second solution is to force one of the 'constants' to be a variable --
2574 that is, unknown and un-optimizable at compile time. For arrays,
2575 there are two possibilities: a) make the indexee (the array's address)
2576 a variable, or b) make the 'constant' index a variable. Thus:
2579 extern type extern_array[];
2581 @{ volatile type *t=extern_array; t[1] @}
2587 extern type extern_array[];
2589 @{ volatile int t=1; extern_array[t] @}
2592 For structs (and most other multiword data types) the only option
2593 is to make the struct itself (or the long long, or the ...) variable:
2596 extern struct s extern_struct;
2597 extern_struct.field -->
2598 @{ volatile struct s *t=&extern_struct; t->field @}
2604 extern long long extern_ll;
2606 @{ volatile long long * local_ll=&extern_ll; *local_ll @}
2609 A third method of dealing with this difficulty is to abandon
2610 'auto-import' for the offending symbol and mark it with
2611 @code{__declspec(dllimport)}. However, in practice that
2612 requires using compile-time #defines to indicate whether you are
2613 building a DLL, building client code that will link to the DLL, or
2614 merely building/linking to a static library. In making the choice
2615 between the various methods of resolving the 'direct address with
2616 constant offset' problem, you should consider typical real-world usage:
2624 void main(int argc, char **argv)@{
2625 printf("%d\n",arr[1]);
2635 void main(int argc, char **argv)@{
2636 /* This workaround is for win32 and cygwin; do not "optimize" */
2637 volatile int *parr = arr;
2638 printf("%d\n",parr[1]);
2645 /* Note: auto-export is assumed (no __declspec(dllexport)) */
2646 #if (defined(_WIN32) || defined(__CYGWIN__)) && \
2647 !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
2648 #define FOO_IMPORT __declspec(dllimport)
2652 extern FOO_IMPORT int arr[];
2655 void main(int argc, char **argv)@{
2656 printf("%d\n",arr[1]);
2660 A fourth way to avoid this problem is to re-code your
2661 library to use a functional interface rather than a data interface
2662 for the offending variables (e.g. set_foo() and get_foo() accessor
2664 [This option is specific to the i386 PE targeted port of the linker]
2666 @kindex --disable-auto-import
2667 @item --disable-auto-import
2668 Do not attempt to do sophisticated linking of @code{_symbol} to
2669 @code{__imp__symbol} for DATA imports from DLLs.
2670 [This option is specific to the i386 PE targeted port of the linker]
2672 @kindex --enable-runtime-pseudo-reloc
2673 @item --enable-runtime-pseudo-reloc
2674 If your code contains expressions described in --enable-auto-import section,
2675 that is, DATA imports from DLL with non-zero offset, this switch will create
2676 a vector of 'runtime pseudo relocations' which can be used by runtime
2677 environment to adjust references to such data in your client code.
2678 [This option is specific to the i386 PE targeted port of the linker]
2680 @kindex --disable-runtime-pseudo-reloc
2681 @item --disable-runtime-pseudo-reloc
2682 Do not create pseudo relocations for non-zero offset DATA imports from
2684 [This option is specific to the i386 PE targeted port of the linker]
2686 @kindex --enable-extra-pe-debug
2687 @item --enable-extra-pe-debug
2688 Show additional debug info related to auto-import symbol thunking.
2689 [This option is specific to the i386 PE targeted port of the linker]
2691 @kindex --section-alignment
2692 @item --section-alignment
2693 Sets the section alignment. Sections in memory will always begin at
2694 addresses which are a multiple of this number. Defaults to 0x1000.
2695 [This option is specific to the i386 PE targeted port of the linker]
2699 @item --stack @var{reserve}
2700 @itemx --stack @var{reserve},@var{commit}
2701 Specify the number of bytes of memory to reserve (and optionally commit)
2702 to be used as stack for this program. The default is 2MB reserved, 4K
2704 [This option is specific to the i386 PE targeted port of the linker]
2707 @item --subsystem @var{which}
2708 @itemx --subsystem @var{which}:@var{major}
2709 @itemx --subsystem @var{which}:@var{major}.@var{minor}
2710 Specifies the subsystem under which your program will execute. The
2711 legal values for @var{which} are @code{native}, @code{windows},
2712 @code{console}, @code{posix}, and @code{xbox}. You may optionally set
2713 the subsystem version also. Numeric values are also accepted for
2715 [This option is specific to the i386 PE targeted port of the linker]
2717 The following options set flags in the @code{DllCharacteristics} field
2718 of the PE file header:
2719 [These options are specific to PE targeted ports of the linker]
2721 @kindex --high-entropy-va
2722 @item --high-entropy-va
2723 Image is compatible with 64-bit address space layout randomization
2726 @kindex --dynamicbase
2728 The image base address may be relocated using address space layout
2729 randomization (ASLR). This feature was introduced with MS Windows
2730 Vista for i386 PE targets.
2732 @kindex --forceinteg
2734 Code integrity checks are enforced.
2738 The image is compatible with the Data Execution Prevention.
2739 This feature was introduced with MS Windows XP SP2 for i386 PE targets.
2741 @kindex --no-isolation
2742 @item --no-isolation
2743 Although the image understands isolation, do not isolate the image.
2747 The image does not use SEH. No SE handler may be called from
2752 Do not bind this image.
2756 The driver uses the MS Windows Driver Model.
2760 The image is Terminal Server aware.
2762 @kindex --insert-timestamp
2763 @item --insert-timestamp
2764 @itemx --no-insert-timestamp
2765 Insert a real timestamp into the image. This is the default behaviour
2766 as it matches legacy code and it means that the image will work with
2767 other, proprietary tools. The problem with this default is that it
2768 will result in slightly different images being produced each tiem the
2769 same sources are linked. The option @option{--no-insert-timestamp}
2770 can be used to insert a zero value for the timestamp, this ensuring
2771 that binaries produced from indentical sources will compare
2778 @subsection Options specific to C6X uClinux targets
2780 @c man begin OPTIONS
2782 The C6X uClinux target uses a binary format called DSBT to support shared
2783 libraries. Each shared library in the system needs to have a unique index;
2784 all executables use an index of 0.
2789 @item --dsbt-size @var{size}
2790 This option sets the number of entires in the DSBT of the current executable
2791 or shared library to @var{size}. The default is to create a table with 64
2794 @kindex --dsbt-index
2795 @item --dsbt-index @var{index}
2796 This option sets the DSBT index of the current executable or shared library
2797 to @var{index}. The default is 0, which is appropriate for generating
2798 executables. If a shared library is generated with a DSBT index of 0, the
2799 @code{R_C6000_DSBT_INDEX} relocs are copied into the output file.
2801 @kindex --no-merge-exidx-entries
2802 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent
2803 exidx entries in frame unwind info.
2811 @subsection Options specific to Motorola 68HC11 and 68HC12 targets
2813 @c man begin OPTIONS
2815 The 68HC11 and 68HC12 linkers support specific options to control the
2816 memory bank switching mapping and trampoline code generation.
2820 @kindex --no-trampoline
2821 @item --no-trampoline
2822 This option disables the generation of trampoline. By default a trampoline
2823 is generated for each far function which is called using a @code{jsr}
2824 instruction (this happens when a pointer to a far function is taken).
2826 @kindex --bank-window
2827 @item --bank-window @var{name}
2828 This option indicates to the linker the name of the memory region in
2829 the @samp{MEMORY} specification that describes the memory bank window.
2830 The definition of such region is then used by the linker to compute
2831 paging and addresses within the memory window.
2839 @subsection Options specific to Motorola 68K target
2841 @c man begin OPTIONS
2843 The following options are supported to control handling of GOT generation
2844 when linking for 68K targets.
2849 @item --got=@var{type}
2850 This option tells the linker which GOT generation scheme to use.
2851 @var{type} should be one of @samp{single}, @samp{negative},
2852 @samp{multigot} or @samp{target}. For more information refer to the
2853 Info entry for @file{ld}.
2861 @subsection Options specific to MIPS targets
2863 @c man begin OPTIONS
2865 The following options are supported to control microMIPS instruction
2866 generation when linking for MIPS targets.
2874 These options control the choice of microMIPS instructions used in code
2875 generated by the linker, such as that in the PLT or lazy binding stubs,
2876 or in relaxation. If @samp{--insn32} is used, then the linker only uses
2877 32-bit instruction encodings. By default or if @samp{--no-insn32} is
2878 used, all instruction encodings are used, including 16-bit ones where
2888 @section Environment Variables
2890 @c man begin ENVIRONMENT
2892 You can change the behaviour of @command{ld} with the environment variables
2893 @ifclear SingleFormat
2896 @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}.
2898 @ifclear SingleFormat
2900 @cindex default input format
2901 @code{GNUTARGET} determines the input-file object format if you don't
2902 use @samp{-b} (or its synonym @samp{--format}). Its value should be one
2903 of the BFD names for an input format (@pxref{BFD}). If there is no
2904 @code{GNUTARGET} in the environment, @command{ld} uses the natural format
2905 of the target. If @code{GNUTARGET} is set to @code{default} then BFD
2906 attempts to discover the input format by examining binary input files;
2907 this method often succeeds, but there are potential ambiguities, since
2908 there is no method of ensuring that the magic number used to specify
2909 object-file formats is unique. However, the configuration procedure for
2910 BFD on each system places the conventional format for that system first
2911 in the search-list, so ambiguities are resolved in favor of convention.
2915 @cindex default emulation
2916 @cindex emulation, default
2917 @code{LDEMULATION} determines the default emulation if you don't use the
2918 @samp{-m} option. The emulation can affect various aspects of linker
2919 behaviour, particularly the default linker script. You can list the
2920 available emulations with the @samp{--verbose} or @samp{-V} options. If
2921 the @samp{-m} option is not used, and the @code{LDEMULATION} environment
2922 variable is not defined, the default emulation depends upon how the
2923 linker was configured.
2925 @kindex COLLECT_NO_DEMANGLE
2926 @cindex demangling, default
2927 Normally, the linker will default to demangling symbols. However, if
2928 @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will
2929 default to not demangling symbols. This environment variable is used in
2930 a similar fashion by the @code{gcc} linker wrapper program. The default
2931 may be overridden by the @samp{--demangle} and @samp{--no-demangle}
2938 @chapter Linker Scripts
2941 @cindex linker scripts
2942 @cindex command files
2943 Every link is controlled by a @dfn{linker script}. This script is
2944 written in the linker command language.
2946 The main purpose of the linker script is to describe how the sections in
2947 the input files should be mapped into the output file, and to control
2948 the memory layout of the output file. Most linker scripts do nothing
2949 more than this. However, when necessary, the linker script can also
2950 direct the linker to perform many other operations, using the commands
2953 The linker always uses a linker script. If you do not supply one
2954 yourself, the linker will use a default script that is compiled into the
2955 linker executable. You can use the @samp{--verbose} command line option
2956 to display the default linker script. Certain command line options,
2957 such as @samp{-r} or @samp{-N}, will affect the default linker script.
2959 You may supply your own linker script by using the @samp{-T} command
2960 line option. When you do this, your linker script will replace the
2961 default linker script.
2963 You may also use linker scripts implicitly by naming them as input files
2964 to the linker, as though they were files to be linked. @xref{Implicit
2968 * Basic Script Concepts:: Basic Linker Script Concepts
2969 * Script Format:: Linker Script Format
2970 * Simple Example:: Simple Linker Script Example
2971 * Simple Commands:: Simple Linker Script Commands
2972 * Assignments:: Assigning Values to Symbols
2973 * SECTIONS:: SECTIONS Command
2974 * MEMORY:: MEMORY Command
2975 * PHDRS:: PHDRS Command
2976 * VERSION:: VERSION Command
2977 * Expressions:: Expressions in Linker Scripts
2978 * Implicit Linker Scripts:: Implicit Linker Scripts
2981 @node Basic Script Concepts
2982 @section Basic Linker Script Concepts
2983 @cindex linker script concepts
2984 We need to define some basic concepts and vocabulary in order to
2985 describe the linker script language.
2987 The linker combines input files into a single output file. The output
2988 file and each input file are in a special data format known as an
2989 @dfn{object file format}. Each file is called an @dfn{object file}.
2990 The output file is often called an @dfn{executable}, but for our
2991 purposes we will also call it an object file. Each object file has,
2992 among other things, a list of @dfn{sections}. We sometimes refer to a
2993 section in an input file as an @dfn{input section}; similarly, a section
2994 in the output file is an @dfn{output section}.
2996 Each section in an object file has a name and a size. Most sections
2997 also have an associated block of data, known as the @dfn{section
2998 contents}. A section may be marked as @dfn{loadable}, which means that
2999 the contents should be loaded into memory when the output file is run.
3000 A section with no contents may be @dfn{allocatable}, which means that an
3001 area in memory should be set aside, but nothing in particular should be
3002 loaded there (in some cases this memory must be zeroed out). A section
3003 which is neither loadable nor allocatable typically contains some sort
3004 of debugging information.
3006 Every loadable or allocatable output section has two addresses. The
3007 first is the @dfn{VMA}, or virtual memory address. This is the address
3008 the section will have when the output file is run. The second is the
3009 @dfn{LMA}, or load memory address. This is the address at which the
3010 section will be loaded. In most cases the two addresses will be the
3011 same. An example of when they might be different is when a data section
3012 is loaded into ROM, and then copied into RAM when the program starts up
3013 (this technique is often used to initialize global variables in a ROM
3014 based system). In this case the ROM address would be the LMA, and the
3015 RAM address would be the VMA.
3017 You can see the sections in an object file by using the @code{objdump}
3018 program with the @samp{-h} option.
3020 Every object file also has a list of @dfn{symbols}, known as the
3021 @dfn{symbol table}. A symbol may be defined or undefined. Each symbol
3022 has a name, and each defined symbol has an address, among other
3023 information. If you compile a C or C++ program into an object file, you
3024 will get a defined symbol for every defined function and global or
3025 static variable. Every undefined function or global variable which is
3026 referenced in the input file will become an undefined symbol.
3028 You can see the symbols in an object file by using the @code{nm}
3029 program, or by using the @code{objdump} program with the @samp{-t}
3033 @section Linker Script Format
3034 @cindex linker script format
3035 Linker scripts are text files.
3037 You write a linker script as a series of commands. Each command is
3038 either a keyword, possibly followed by arguments, or an assignment to a
3039 symbol. You may separate commands using semicolons. Whitespace is
3042 Strings such as file or format names can normally be entered directly.
3043 If the file name contains a character such as a comma which would
3044 otherwise serve to separate file names, you may put the file name in
3045 double quotes. There is no way to use a double quote character in a
3048 You may include comments in linker scripts just as in C, delimited by
3049 @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent
3052 @node Simple Example
3053 @section Simple Linker Script Example
3054 @cindex linker script example
3055 @cindex example of linker script
3056 Many linker scripts are fairly simple.
3058 The simplest possible linker script has just one command:
3059 @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the
3060 memory layout of the output file.
3062 The @samp{SECTIONS} command is a powerful command. Here we will
3063 describe a simple use of it. Let's assume your program consists only of
3064 code, initialized data, and uninitialized data. These will be in the
3065 @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively.
3066 Let's assume further that these are the only sections which appear in
3069 For this example, let's say that the code should be loaded at address
3070 0x10000, and that the data should start at address 0x8000000. Here is a
3071 linker script which will do that:
3076 .text : @{ *(.text) @}
3078 .data : @{ *(.data) @}
3079 .bss : @{ *(.bss) @}
3083 You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS},
3084 followed by a series of symbol assignments and output section
3085 descriptions enclosed in curly braces.
3087 The first line inside the @samp{SECTIONS} command of the above example
3088 sets the value of the special symbol @samp{.}, which is the location
3089 counter. If you do not specify the address of an output section in some
3090 other way (other ways are described later), the address is set from the
3091 current value of the location counter. The location counter is then
3092 incremented by the size of the output section. At the start of the
3093 @samp{SECTIONS} command, the location counter has the value @samp{0}.
3095 The second line defines an output section, @samp{.text}. The colon is
3096 required syntax which may be ignored for now. Within the curly braces
3097 after the output section name, you list the names of the input sections
3098 which should be placed into this output section. The @samp{*} is a
3099 wildcard which matches any file name. The expression @samp{*(.text)}
3100 means all @samp{.text} input sections in all input files.
3102 Since the location counter is @samp{0x10000} when the output section
3103 @samp{.text} is defined, the linker will set the address of the
3104 @samp{.text} section in the output file to be @samp{0x10000}.
3106 The remaining lines define the @samp{.data} and @samp{.bss} sections in
3107 the output file. The linker will place the @samp{.data} output section
3108 at address @samp{0x8000000}. After the linker places the @samp{.data}
3109 output section, the value of the location counter will be
3110 @samp{0x8000000} plus the size of the @samp{.data} output section. The
3111 effect is that the linker will place the @samp{.bss} output section
3112 immediately after the @samp{.data} output section in memory.
3114 The linker will ensure that each output section has the required
3115 alignment, by increasing the location counter if necessary. In this
3116 example, the specified addresses for the @samp{.text} and @samp{.data}
3117 sections will probably satisfy any alignment constraints, but the linker
3118 may have to create a small gap between the @samp{.data} and @samp{.bss}
3121 That's it! That's a simple and complete linker script.
3123 @node Simple Commands
3124 @section Simple Linker Script Commands
3125 @cindex linker script simple commands
3126 In this section we describe the simple linker script commands.
3129 * Entry Point:: Setting the entry point
3130 * File Commands:: Commands dealing with files
3131 @ifclear SingleFormat
3132 * Format Commands:: Commands dealing with object file formats
3135 * REGION_ALIAS:: Assign alias names to memory regions
3136 * Miscellaneous Commands:: Other linker script commands
3140 @subsection Setting the Entry Point
3141 @kindex ENTRY(@var{symbol})
3142 @cindex start of execution
3143 @cindex first instruction
3145 The first instruction to execute in a program is called the @dfn{entry
3146 point}. You can use the @code{ENTRY} linker script command to set the
3147 entry point. The argument is a symbol name:
3152 There are several ways to set the entry point. The linker will set the
3153 entry point by trying each of the following methods in order, and
3154 stopping when one of them succeeds:
3157 the @samp{-e} @var{entry} command-line option;
3159 the @code{ENTRY(@var{symbol})} command in a linker script;
3161 the value of a target specific symbol, if it is defined; For many
3162 targets this is @code{start}, but PE and BeOS based systems for example
3163 check a list of possible entry symbols, matching the first one found.
3165 the address of the first byte of the @samp{.text} section, if present;
3167 The address @code{0}.
3171 @subsection Commands Dealing with Files
3172 @cindex linker script file commands
3173 Several linker script commands deal with files.
3176 @item INCLUDE @var{filename}
3177 @kindex INCLUDE @var{filename}
3178 @cindex including a linker script
3179 Include the linker script @var{filename} at this point. The file will
3180 be searched for in the current directory, and in any directory specified
3181 with the @option{-L} option. You can nest calls to @code{INCLUDE} up to
3184 You can place @code{INCLUDE} directives at the top level, in @code{MEMORY} or
3185 @code{SECTIONS} commands, or in output section descriptions.
3187 @item INPUT(@var{file}, @var{file}, @dots{})
3188 @itemx INPUT(@var{file} @var{file} @dots{})
3189 @kindex INPUT(@var{files})
3190 @cindex input files in linker scripts
3191 @cindex input object files in linker scripts
3192 @cindex linker script input object files
3193 The @code{INPUT} command directs the linker to include the named files
3194 in the link, as though they were named on the command line.
3196 For example, if you always want to include @file{subr.o} any time you do
3197 a link, but you can't be bothered to put it on every link command line,
3198 then you can put @samp{INPUT (subr.o)} in your linker script.
3200 In fact, if you like, you can list all of your input files in the linker
3201 script, and then invoke the linker with nothing but a @samp{-T} option.
3203 In case a @dfn{sysroot prefix} is configured, and the filename starts
3204 with the @samp{/} character, and the script being processed was
3205 located inside the @dfn{sysroot prefix}, the filename will be looked
3206 for in the @dfn{sysroot prefix}. Otherwise, the linker will try to
3207 open the file in the current directory. If it is not found, the
3208 linker will search through the archive library search path.
3209 The @dfn{sysroot prefix} can also be forced by specifying @code{=}
3210 as the first character in the filename path. See also the
3211 description of @samp{-L} in @ref{Options,,Command Line Options}.
3213 If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the
3214 name to @code{lib@var{file}.a}, as with the command line argument
3217 When you use the @code{INPUT} command in an implicit linker script, the
3218 files will be included in the link at the point at which the linker
3219 script file is included. This can affect archive searching.
3221 @item GROUP(@var{file}, @var{file}, @dots{})
3222 @itemx GROUP(@var{file} @var{file} @dots{})
3223 @kindex GROUP(@var{files})
3224 @cindex grouping input files
3225 The @code{GROUP} command is like @code{INPUT}, except that the named
3226 files should all be archives, and they are searched repeatedly until no
3227 new undefined references are created. See the description of @samp{-(}
3228 in @ref{Options,,Command Line Options}.
3230 @item AS_NEEDED(@var{file}, @var{file}, @dots{})
3231 @itemx AS_NEEDED(@var{file} @var{file} @dots{})
3232 @kindex AS_NEEDED(@var{files})
3233 This construct can appear only inside of the @code{INPUT} or @code{GROUP}
3234 commands, among other filenames. The files listed will be handled
3235 as if they appear directly in the @code{INPUT} or @code{GROUP} commands,
3236 with the exception of ELF shared libraries, that will be added only
3237 when they are actually needed. This construct essentially enables
3238 @option{--as-needed} option for all the files listed inside of it
3239 and restores previous @option{--as-needed} resp. @option{--no-as-needed}
3242 @item OUTPUT(@var{filename})
3243 @kindex OUTPUT(@var{filename})
3244 @cindex output file name in linker script
3245 The @code{OUTPUT} command names the output file. Using
3246 @code{OUTPUT(@var{filename})} in the linker script is exactly like using
3247 @samp{-o @var{filename}} on the command line (@pxref{Options,,Command
3248 Line Options}). If both are used, the command line option takes
3251 You can use the @code{OUTPUT} command to define a default name for the
3252 output file other than the usual default of @file{a.out}.
3254 @item SEARCH_DIR(@var{path})
3255 @kindex SEARCH_DIR(@var{path})
3256 @cindex library search path in linker script
3257 @cindex archive search path in linker script
3258 @cindex search path in linker script
3259 The @code{SEARCH_DIR} command adds @var{path} to the list of paths where
3260 @command{ld} looks for archive libraries. Using
3261 @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}}
3262 on the command line (@pxref{Options,,Command Line Options}). If both
3263 are used, then the linker will search both paths. Paths specified using
3264 the command line option are searched first.
3266 @item STARTUP(@var{filename})
3267 @kindex STARTUP(@var{filename})
3268 @cindex first input file
3269 The @code{STARTUP} command is just like the @code{INPUT} command, except
3270 that @var{filename} will become the first input file to be linked, as
3271 though it were specified first on the command line. This may be useful
3272 when using a system in which the entry point is always the start of the
3276 @ifclear SingleFormat
3277 @node Format Commands
3278 @subsection Commands Dealing with Object File Formats
3279 A couple of linker script commands deal with object file formats.
3282 @item OUTPUT_FORMAT(@var{bfdname})
3283 @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little})
3284 @kindex OUTPUT_FORMAT(@var{bfdname})
3285 @cindex output file format in linker script
3286 The @code{OUTPUT_FORMAT} command names the BFD format to use for the
3287 output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is
3288 exactly like using @samp{--oformat @var{bfdname}} on the command line
3289 (@pxref{Options,,Command Line Options}). If both are used, the command
3290 line option takes precedence.
3292 You can use @code{OUTPUT_FORMAT} with three arguments to use different
3293 formats based on the @samp{-EB} and @samp{-EL} command line options.
3294 This permits the linker script to set the output format based on the
3297 If neither @samp{-EB} nor @samp{-EL} are used, then the output format
3298 will be the first argument, @var{default}. If @samp{-EB} is used, the
3299 output format will be the second argument, @var{big}. If @samp{-EL} is
3300 used, the output format will be the third argument, @var{little}.
3302 For example, the default linker script for the MIPS ELF target uses this
3305 OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
3307 This says that the default format for the output file is
3308 @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line
3309 option, the output file will be created in the @samp{elf32-littlemips}
3312 @item TARGET(@var{bfdname})
3313 @kindex TARGET(@var{bfdname})
3314 @cindex input file format in linker script
3315 The @code{TARGET} command names the BFD format to use when reading input
3316 files. It affects subsequent @code{INPUT} and @code{GROUP} commands.
3317 This command is like using @samp{-b @var{bfdname}} on the command line
3318 (@pxref{Options,,Command Line Options}). If the @code{TARGET} command
3319 is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET}
3320 command is also used to set the format for the output file. @xref{BFD}.
3325 @subsection Assign alias names to memory regions
3326 @kindex REGION_ALIAS(@var{alias}, @var{region})
3327 @cindex region alias
3328 @cindex region names
3330 Alias names can be added to existing memory regions created with the
3331 @ref{MEMORY} command. Each name corresponds to at most one memory region.
3334 REGION_ALIAS(@var{alias}, @var{region})
3337 The @code{REGION_ALIAS} function creates an alias name @var{alias} for the
3338 memory region @var{region}. This allows a flexible mapping of output sections
3339 to memory regions. An example follows.
3341 Suppose we have an application for embedded systems which come with various
3342 memory storage devices. All have a general purpose, volatile memory @code{RAM}
3343 that allows code execution or data storage. Some may have a read-only,
3344 non-volatile memory @code{ROM} that allows code execution and read-only data
3345 access. The last variant is a read-only, non-volatile memory @code{ROM2} with
3346 read-only data access and no code execution capability. We have four output
3351 @code{.text} program code;
3353 @code{.rodata} read-only data;
3355 @code{.data} read-write initialized data;
3357 @code{.bss} read-write zero initialized data.
3360 The goal is to provide a linker command file that contains a system independent
3361 part defining the output sections and a system dependent part mapping the
3362 output sections to the memory regions available on the system. Our embedded
3363 systems come with three different memory setups @code{A}, @code{B} and
3365 @multitable @columnfractions .25 .25 .25 .25
3366 @item Section @tab Variant A @tab Variant B @tab Variant C
3367 @item .text @tab RAM @tab ROM @tab ROM
3368 @item .rodata @tab RAM @tab ROM @tab ROM2
3369 @item .data @tab RAM @tab RAM/ROM @tab RAM/ROM2
3370 @item .bss @tab RAM @tab RAM @tab RAM
3372 The notation @code{RAM/ROM} or @code{RAM/ROM2} means that this section is
3373 loaded into region @code{ROM} or @code{ROM2} respectively. Please note that
3374 the load address of the @code{.data} section starts in all three variants at
3375 the end of the @code{.rodata} section.
3377 The base linker script that deals with the output sections follows. It
3378 includes the system dependent @code{linkcmds.memory} file that describes the
3381 INCLUDE linkcmds.memory
3394 .data : AT (rodata_end)
3399 data_size = SIZEOF(.data);
3400 data_load_start = LOADADDR(.data);
3408 Now we need three different @code{linkcmds.memory} files to define memory
3409 regions and alias names. The content of @code{linkcmds.memory} for the three
3410 variants @code{A}, @code{B} and @code{C}:
3413 Here everything goes into the @code{RAM}.
3417 RAM : ORIGIN = 0, LENGTH = 4M
3420 REGION_ALIAS("REGION_TEXT", RAM);
3421 REGION_ALIAS("REGION_RODATA", RAM);
3422 REGION_ALIAS("REGION_DATA", RAM);
3423 REGION_ALIAS("REGION_BSS", RAM);
3426 Program code and read-only data go into the @code{ROM}. Read-write data goes
3427 into the @code{RAM}. An image of the initialized data is loaded into the
3428 @code{ROM} and will be copied during system start into the @code{RAM}.
3432 ROM : ORIGIN = 0, LENGTH = 3M
3433 RAM : ORIGIN = 0x10000000, LENGTH = 1M
3436 REGION_ALIAS("REGION_TEXT", ROM);
3437 REGION_ALIAS("REGION_RODATA", ROM);
3438 REGION_ALIAS("REGION_DATA", RAM);
3439 REGION_ALIAS("REGION_BSS", RAM);
3442 Program code goes into the @code{ROM}. Read-only data goes into the
3443 @code{ROM2}. Read-write data goes into the @code{RAM}. An image of the
3444 initialized data is loaded into the @code{ROM2} and will be copied during
3445 system start into the @code{RAM}.
3449 ROM : ORIGIN = 0, LENGTH = 2M
3450 ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
3451 RAM : ORIGIN = 0x20000000, LENGTH = 1M
3454 REGION_ALIAS("REGION_TEXT", ROM);
3455 REGION_ALIAS("REGION_RODATA", ROM2);
3456 REGION_ALIAS("REGION_DATA", RAM);
3457 REGION_ALIAS("REGION_BSS", RAM);
3461 It is possible to write a common system initialization routine to copy the
3462 @code{.data} section from @code{ROM} or @code{ROM2} into the @code{RAM} if
3467 extern char data_start [];
3468 extern char data_size [];
3469 extern char data_load_start [];
3471 void copy_data(void)
3473 if (data_start != data_load_start)
3475 memcpy(data_start, data_load_start, (size_t) data_size);
3480 @node Miscellaneous Commands
3481 @subsection Other Linker Script Commands
3482 There are a few other linker scripts commands.
3485 @item ASSERT(@var{exp}, @var{message})
3487 @cindex assertion in linker script
3488 Ensure that @var{exp} is non-zero. If it is zero, then exit the linker
3489 with an error code, and print @var{message}.
3491 Note that assertions are checked before the final stages of linking
3492 take place. This means that expressions involving symbols PROVIDEd
3493 inside section definitions will fail if the user has not set values
3494 for those symbols. The only exception to this rule is PROVIDEd
3495 symbols that just reference dot. Thus an assertion like this:
3500 PROVIDE (__stack = .);
3501 PROVIDE (__stack_size = 0x100);
3502 ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
3506 will fail if @code{__stack_size} is not defined elsewhere. Symbols
3507 PROVIDEd outside of section definitions are evaluated earlier, so they
3508 can be used inside ASSERTions. Thus:
3511 PROVIDE (__stack_size = 0x100);
3514 PROVIDE (__stack = .);
3515 ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack");
3521 @item EXTERN(@var{symbol} @var{symbol} @dots{})
3523 @cindex undefined symbol in linker script
3524 Force @var{symbol} to be entered in the output file as an undefined
3525 symbol. Doing this may, for example, trigger linking of additional
3526 modules from standard libraries. You may list several @var{symbol}s for
3527 each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This
3528 command has the same effect as the @samp{-u} command-line option.
3530 @item FORCE_COMMON_ALLOCATION
3531 @kindex FORCE_COMMON_ALLOCATION
3532 @cindex common allocation in linker script
3533 This command has the same effect as the @samp{-d} command-line option:
3534 to make @command{ld} assign space to common symbols even if a relocatable
3535 output file is specified (@samp{-r}).
3537 @item INHIBIT_COMMON_ALLOCATION
3538 @kindex INHIBIT_COMMON_ALLOCATION
3539 @cindex common allocation in linker script
3540 This command has the same effect as the @samp{--no-define-common}
3541 command-line option: to make @code{ld} omit the assignment of addresses
3542 to common symbols even for a non-relocatable output file.
3544 @item INSERT [ AFTER | BEFORE ] @var{output_section}
3546 @cindex insert user script into default script
3547 This command is typically used in a script specified by @samp{-T} to
3548 augment the default @code{SECTIONS} with, for example, overlays. It
3549 inserts all prior linker script statements after (or before)
3550 @var{output_section}, and also causes @samp{-T} to not override the
3551 default linker script. The exact insertion point is as for orphan
3552 sections. @xref{Location Counter}. The insertion happens after the
3553 linker has mapped input sections to output sections. Prior to the
3554 insertion, since @samp{-T} scripts are parsed before the default
3555 linker script, statements in the @samp{-T} script occur before the
3556 default linker script statements in the internal linker representation
3557 of the script. In particular, input section assignments will be made
3558 to @samp{-T} output sections before those in the default script. Here
3559 is an example of how a @samp{-T} script using @code{INSERT} might look:
3566 .ov1 @{ ov1*(.text) @}
3567 .ov2 @{ ov2*(.text) @}
3573 @item NOCROSSREFS(@var{section} @var{section} @dots{})
3574 @kindex NOCROSSREFS(@var{sections})
3575 @cindex cross references
3576 This command may be used to tell @command{ld} to issue an error about any
3577 references among certain output sections.
3579 In certain types of programs, particularly on embedded systems when
3580 using overlays, when one section is loaded into memory, another section
3581 will not be. Any direct references between the two sections would be
3582 errors. For example, it would be an error if code in one section called
3583 a function defined in the other section.
3585 The @code{NOCROSSREFS} command takes a list of output section names. If
3586 @command{ld} detects any cross references between the sections, it reports
3587 an error and returns a non-zero exit status. Note that the
3588 @code{NOCROSSREFS} command uses output section names, not input section
3591 @ifclear SingleFormat
3592 @item OUTPUT_ARCH(@var{bfdarch})
3593 @kindex OUTPUT_ARCH(@var{bfdarch})
3594 @cindex machine architecture
3595 @cindex architecture
3596 Specify a particular output machine architecture. The argument is one
3597 of the names used by the BFD library (@pxref{BFD}). You can see the
3598 architecture of an object file by using the @code{objdump} program with
3599 the @samp{-f} option.
3602 @item LD_FEATURE(@var{string})
3603 @kindex LD_FEATURE(@var{string})
3604 This command may be used to modify @command{ld} behavior. If
3605 @var{string} is @code{"SANE_EXPR"} then absolute symbols and numbers
3606 in a script are simply treated as numbers everywhere.
3607 @xref{Expression Section}.
3611 @section Assigning Values to Symbols
3612 @cindex assignment in scripts
3613 @cindex symbol definition, scripts
3614 @cindex variables, defining
3615 You may assign a value to a symbol in a linker script. This will define
3616 the symbol and place it into the symbol table with a global scope.
3619 * Simple Assignments:: Simple Assignments
3622 * PROVIDE_HIDDEN:: PROVIDE_HIDDEN
3623 * Source Code Reference:: How to use a linker script defined symbol in source code
3626 @node Simple Assignments
3627 @subsection Simple Assignments
3629 You may assign to a symbol using any of the C assignment operators:
3632 @item @var{symbol} = @var{expression} ;
3633 @itemx @var{symbol} += @var{expression} ;
3634 @itemx @var{symbol} -= @var{expression} ;
3635 @itemx @var{symbol} *= @var{expression} ;
3636 @itemx @var{symbol} /= @var{expression} ;
3637 @itemx @var{symbol} <<= @var{expression} ;
3638 @itemx @var{symbol} >>= @var{expression} ;
3639 @itemx @var{symbol} &= @var{expression} ;
3640 @itemx @var{symbol} |= @var{expression} ;
3643 The first case will define @var{symbol} to the value of
3644 @var{expression}. In the other cases, @var{symbol} must already be
3645 defined, and the value will be adjusted accordingly.
3647 The special symbol name @samp{.} indicates the location counter. You
3648 may only use this within a @code{SECTIONS} command. @xref{Location Counter}.
3650 The semicolon after @var{expression} is required.
3652 Expressions are defined below; see @ref{Expressions}.
3654 You may write symbol assignments as commands in their own right, or as
3655 statements within a @code{SECTIONS} command, or as part of an output
3656 section description in a @code{SECTIONS} command.
3658 The section of the symbol will be set from the section of the
3659 expression; for more information, see @ref{Expression Section}.
3661 Here is an example showing the three different places that symbol
3662 assignments may be used:
3673 _bdata = (. + 3) & ~ 3;
3674 .data : @{ *(.data) @}
3678 In this example, the symbol @samp{floating_point} will be defined as
3679 zero. The symbol @samp{_etext} will be defined as the address following
3680 the last @samp{.text} input section. The symbol @samp{_bdata} will be
3681 defined as the address following the @samp{.text} output section aligned
3682 upward to a 4 byte boundary.
3687 For ELF targeted ports, define a symbol that will be hidden and won't be
3688 exported. The syntax is @code{HIDDEN(@var{symbol} = @var{expression})}.
3690 Here is the example from @ref{Simple Assignments}, rewritten to use
3694 HIDDEN(floating_point = 0);
3702 HIDDEN(_bdata = (. + 3) & ~ 3);
3703 .data : @{ *(.data) @}
3707 In this case none of the three symbols will be visible outside this module.
3712 In some cases, it is desirable for a linker script to define a symbol
3713 only if it is referenced and is not defined by any object included in
3714 the link. For example, traditional linkers defined the symbol
3715 @samp{etext}. However, ANSI C requires that the user be able to use
3716 @samp{etext} as a function name without encountering an error. The
3717 @code{PROVIDE} keyword may be used to define a symbol, such as
3718 @samp{etext}, only if it is referenced but not defined. The syntax is
3719 @code{PROVIDE(@var{symbol} = @var{expression})}.
3721 Here is an example of using @code{PROVIDE} to define @samp{etext}:
3734 In this example, if the program defines @samp{_etext} (with a leading
3735 underscore), the linker will give a multiple definition error. If, on
3736 the other hand, the program defines @samp{etext} (with no leading
3737 underscore), the linker will silently use the definition in the program.
3738 If the program references @samp{etext} but does not define it, the
3739 linker will use the definition in the linker script.
3741 @node PROVIDE_HIDDEN
3742 @subsection PROVIDE_HIDDEN
3743 @cindex PROVIDE_HIDDEN
3744 Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be
3745 hidden and won't be exported.
3747 @node Source Code Reference
3748 @subsection Source Code Reference
3750 Accessing a linker script defined variable from source code is not
3751 intuitive. In particular a linker script symbol is not equivalent to
3752 a variable declaration in a high level language, it is instead a
3753 symbol that does not have a value.
3755 Before going further, it is important to note that compilers often
3756 transform names in the source code into different names when they are
3757 stored in the symbol table. For example, Fortran compilers commonly
3758 prepend or append an underscore, and C++ performs extensive @samp{name
3759 mangling}. Therefore there might be a discrepancy between the name
3760 of a variable as it is used in source code and the name of the same
3761 variable as it is defined in a linker script. For example in C a
3762 linker script variable might be referred to as:
3768 But in the linker script it might be defined as:
3774 In the remaining examples however it is assumed that no name
3775 transformation has taken place.
3777 When a symbol is declared in a high level language such as C, two
3778 things happen. The first is that the compiler reserves enough space
3779 in the program's memory to hold the @emph{value} of the symbol. The
3780 second is that the compiler creates an entry in the program's symbol
3781 table which holds the symbol's @emph{address}. ie the symbol table
3782 contains the address of the block of memory holding the symbol's
3783 value. So for example the following C declaration, at file scope:
3789 creates an entry called @samp{foo} in the symbol table. This entry
3790 holds the address of an @samp{int} sized block of memory where the
3791 number 1000 is initially stored.
3793 When a program references a symbol the compiler generates code that
3794 first accesses the symbol table to find the address of the symbol's
3795 memory block and then code to read the value from that memory block.
3802 looks up the symbol @samp{foo} in the symbol table, gets the address
3803 associated with this symbol and then writes the value 1 into that
3810 looks up the symbol @samp{foo} in the symbol table, gets its address
3811 and then copies this address into the block of memory associated with
3812 the variable @samp{a}.
3814 Linker scripts symbol declarations, by contrast, create an entry in
3815 the symbol table but do not assign any memory to them. Thus they are
3816 an address without a value. So for example the linker script definition:
3822 creates an entry in the symbol table called @samp{foo} which holds
3823 the address of memory location 1000, but nothing special is stored at
3824 address 1000. This means that you cannot access the @emph{value} of a
3825 linker script defined symbol - it has no value - all you can do is
3826 access the @emph{address} of a linker script defined symbol.
3828 Hence when you are using a linker script defined symbol in source code
3829 you should always take the address of the symbol, and never attempt to
3830 use its value. For example suppose you want to copy the contents of a
3831 section of memory called .ROM into a section called .FLASH and the
3832 linker script contains these declarations:
3836 start_of_ROM = .ROM;
3837 end_of_ROM = .ROM + sizeof (.ROM) - 1;
3838 start_of_FLASH = .FLASH;
3842 Then the C source code to perform the copy would be:
3846 extern char start_of_ROM, end_of_ROM, start_of_FLASH;
3848 memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
3852 Note the use of the @samp{&} operators. These are correct.
3855 @section SECTIONS Command
3857 The @code{SECTIONS} command tells the linker how to map input sections
3858 into output sections, and how to place the output sections in memory.
3860 The format of the @code{SECTIONS} command is:
3864 @var{sections-command}
3865 @var{sections-command}
3870 Each @var{sections-command} may of be one of the following:
3874 an @code{ENTRY} command (@pxref{Entry Point,,Entry command})
3876 a symbol assignment (@pxref{Assignments})
3878 an output section description
3880 an overlay description
3883 The @code{ENTRY} command and symbol assignments are permitted inside the
3884 @code{SECTIONS} command for convenience in using the location counter in
3885 those commands. This can also make the linker script easier to
3886 understand because you can use those commands at meaningful points in
3887 the layout of the output file.
3889 Output section descriptions and overlay descriptions are described
3892 If you do not use a @code{SECTIONS} command in your linker script, the
3893 linker will place each input section into an identically named output
3894 section in the order that the sections are first encountered in the
3895 input files. If all input sections are present in the first file, for
3896 example, the order of sections in the output file will match the order
3897 in the first input file. The first section will be at address zero.
3900 * Output Section Description:: Output section description
3901 * Output Section Name:: Output section name
3902 * Output Section Address:: Output section address
3903 * Input Section:: Input section description
3904 * Output Section Data:: Output section data
3905 * Output Section Keywords:: Output section keywords
3906 * Output Section Discarding:: Output section discarding
3907 * Output Section Attributes:: Output section attributes
3908 * Overlay Description:: Overlay description
3911 @node Output Section Description
3912 @subsection Output Section Description
3913 The full description of an output section looks like this:
3916 @var{section} [@var{address}] [(@var{type})] :
3918 [ALIGN(@var{section_align}) | ALIGN_WITH_INPUT]
3919 [SUBALIGN(@var{subsection_align})]
3922 @var{output-section-command}
3923 @var{output-section-command}
3925 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] [,]
3929 Most output sections do not use most of the optional section attributes.
3931 The whitespace around @var{section} is required, so that the section
3932 name is unambiguous. The colon and the curly braces are also required.
3933 The comma at the end may be required if a @var{fillexp} is used and
3934 the next @var{sections-command} looks like a continuation of the expression.
3935 The line breaks and other white space are optional.
3937 Each @var{output-section-command} may be one of the following:
3941 a symbol assignment (@pxref{Assignments})
3943 an input section description (@pxref{Input Section})
3945 data values to include directly (@pxref{Output Section Data})
3947 a special output section keyword (@pxref{Output Section Keywords})
3950 @node Output Section Name
3951 @subsection Output Section Name
3952 @cindex name, section
3953 @cindex section name
3954 The name of the output section is @var{section}. @var{section} must
3955 meet the constraints of your output format. In formats which only
3956 support a limited number of sections, such as @code{a.out}, the name
3957 must be one of the names supported by the format (@code{a.out}, for
3958 example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the
3959 output format supports any number of sections, but with numbers and not
3960 names (as is the case for Oasys), the name should be supplied as a
3961 quoted numeric string. A section name may consist of any sequence of
3962 characters, but a name which contains any unusual characters such as
3963 commas must be quoted.
3965 The output section name @samp{/DISCARD/} is special; @ref{Output Section
3968 @node Output Section Address
3969 @subsection Output Section Address
3970 @cindex address, section
3971 @cindex section address
3972 The @var{address} is an expression for the VMA (the virtual memory
3973 address) of the output section. This address is optional, but if it
3974 is provided then the output address will be set exactly as specified.
3976 If the output address is not specified then one will be chosen for the
3977 section, based on the heuristic below. This address will be adjusted
3978 to fit the alignment requirement of the output section. The
3979 alignment requirement is the strictest alignment of any input section
3980 contained within the output section.
3982 The output section address heuristic is as follows:
3986 If an output memory @var{region} is set for the section then it
3987 is added to this region and its address will be the next free address
3991 If the MEMORY command has been used to create a list of memory
3992 regions then the first region which has attributes compatible with the
3993 section is selected to contain it. The section's output address will
3994 be the next free address in that region; @ref{MEMORY}.
3997 If no memory regions were specified, or none match the section then
3998 the output address will be based on the current value of the location
4006 .text . : @{ *(.text) @}
4013 .text : @{ *(.text) @}
4017 are subtly different. The first will set the address of the
4018 @samp{.text} output section to the current value of the location
4019 counter. The second will set it to the current value of the location
4020 counter aligned to the strictest alignment of any of the @samp{.text}
4023 The @var{address} may be an arbitrary expression; @ref{Expressions}.
4024 For example, if you want to align the section on a 0x10 byte boundary,
4025 so that the lowest four bits of the section address are zero, you could
4026 do something like this:
4028 .text ALIGN(0x10) : @{ *(.text) @}
4031 This works because @code{ALIGN} returns the current location counter
4032 aligned upward to the specified value.
4034 Specifying @var{address} for a section will change the value of the
4035 location counter, provided that the section is non-empty. (Empty
4036 sections are ignored).
4039 @subsection Input Section Description
4040 @cindex input sections
4041 @cindex mapping input sections to output sections
4042 The most common output section command is an input section description.
4044 The input section description is the most basic linker script operation.
4045 You use output sections to tell the linker how to lay out your program
4046 in memory. You use input section descriptions to tell the linker how to
4047 map the input files into your memory layout.
4050 * Input Section Basics:: Input section basics
4051 * Input Section Wildcards:: Input section wildcard patterns
4052 * Input Section Common:: Input section for common symbols
4053 * Input Section Keep:: Input section and garbage collection
4054 * Input Section Example:: Input section example
4057 @node Input Section Basics
4058 @subsubsection Input Section Basics
4059 @cindex input section basics
4060 An input section description consists of a file name optionally followed
4061 by a list of section names in parentheses.
4063 The file name and the section name may be wildcard patterns, which we
4064 describe further below (@pxref{Input Section Wildcards}).
4066 The most common input section description is to include all input
4067 sections with a particular name in the output section. For example, to
4068 include all input @samp{.text} sections, you would write:
4073 Here the @samp{*} is a wildcard which matches any file name. To exclude a list
4074 of files from matching the file name wildcard, EXCLUDE_FILE may be used to
4075 match all files except the ones specified in the EXCLUDE_FILE list. For
4078 *(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
4080 will cause all .ctors sections from all files except @file{crtend.o} and
4081 @file{otherfile.o} to be included.
4083 There are two ways to include more than one section:
4089 The difference between these is the order in which the @samp{.text} and
4090 @samp{.rdata} input sections will appear in the output section. In the
4091 first example, they will be intermingled, appearing in the same order as
4092 they are found in the linker input. In the second example, all
4093 @samp{.text} input sections will appear first, followed by all
4094 @samp{.rdata} input sections.
4096 You can specify a file name to include sections from a particular file.
4097 You would do this if one or more of your files contain special data that
4098 needs to be at a particular location in memory. For example:
4103 To refine the sections that are included based on the section flags
4104 of an input section, INPUT_SECTION_FLAGS may be used.
4106 Here is a simple example for using Section header flags for ELF sections:
4111 .text : @{ INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) @}
4112 .text2 : @{ INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) @}
4117 In this example, the output section @samp{.text} will be comprised of any
4118 input section matching the name *(.text) whose section header flags
4119 @code{SHF_MERGE} and @code{SHF_STRINGS} are set. The output section
4120 @samp{.text2} will be comprised of any input section matching the name *(.text)
4121 whose section header flag @code{SHF_WRITE} is clear.
4123 You can also specify files within archives by writing a pattern
4124 matching the archive, a colon, then the pattern matching the file,
4125 with no whitespace around the colon.
4129 matches file within archive
4131 matches the whole archive
4133 matches file but not one in an archive
4136 Either one or both of @samp{archive} and @samp{file} can contain shell
4137 wildcards. On DOS based file systems, the linker will assume that a
4138 single letter followed by a colon is a drive specifier, so
4139 @samp{c:myfile.o} is a simple file specification, not @samp{myfile.o}
4140 within an archive called @samp{c}. @samp{archive:file} filespecs may
4141 also be used within an @code{EXCLUDE_FILE} list, but may not appear in
4142 other linker script contexts. For instance, you cannot extract a file
4143 from an archive by using @samp{archive:file} in an @code{INPUT}
4146 If you use a file name without a list of sections, then all sections in
4147 the input file will be included in the output section. This is not
4148 commonly done, but it may by useful on occasion. For example:
4153 When you use a file name which is not an @samp{archive:file} specifier
4154 and does not contain any wild card
4155 characters, the linker will first see if you also specified the file
4156 name on the linker command line or in an @code{INPUT} command. If you
4157 did not, the linker will attempt to open the file as an input file, as
4158 though it appeared on the command line. Note that this differs from an
4159 @code{INPUT} command, because the linker will not search for the file in
4160 the archive search path.
4162 @node Input Section Wildcards
4163 @subsubsection Input Section Wildcard Patterns
4164 @cindex input section wildcards
4165 @cindex wildcard file name patterns
4166 @cindex file name wildcard patterns
4167 @cindex section name wildcard patterns
4168 In an input section description, either the file name or the section
4169 name or both may be wildcard patterns.
4171 The file name of @samp{*} seen in many examples is a simple wildcard
4172 pattern for the file name.
4174 The wildcard patterns are like those used by the Unix shell.
4178 matches any number of characters
4180 matches any single character
4182 matches a single instance of any of the @var{chars}; the @samp{-}
4183 character may be used to specify a range of characters, as in
4184 @samp{[a-z]} to match any lower case letter
4186 quotes the following character
4189 When a file name is matched with a wildcard, the wildcard characters
4190 will not match a @samp{/} character (used to separate directory names on
4191 Unix). A pattern consisting of a single @samp{*} character is an
4192 exception; it will always match any file name, whether it contains a
4193 @samp{/} or not. In a section name, the wildcard characters will match
4194 a @samp{/} character.
4196 File name wildcard patterns only match files which are explicitly
4197 specified on the command line or in an @code{INPUT} command. The linker
4198 does not search directories to expand wildcards.
4200 If a file name matches more than one wildcard pattern, or if a file name
4201 appears explicitly and is also matched by a wildcard pattern, the linker
4202 will use the first match in the linker script. For example, this
4203 sequence of input section descriptions is probably in error, because the
4204 @file{data.o} rule will not be used:
4206 .data : @{ *(.data) @}
4207 .data1 : @{ data.o(.data) @}
4210 @cindex SORT_BY_NAME
4211 Normally, the linker will place files and sections matched by wildcards
4212 in the order in which they are seen during the link. You can change
4213 this by using the @code{SORT_BY_NAME} keyword, which appears before a wildcard
4214 pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the
4215 @code{SORT_BY_NAME} keyword is used, the linker will sort the files or sections
4216 into ascending order by name before placing them in the output file.
4218 @cindex SORT_BY_ALIGNMENT
4219 @code{SORT_BY_ALIGNMENT} is very similar to @code{SORT_BY_NAME}. The
4220 difference is @code{SORT_BY_ALIGNMENT} will sort sections into
4221 descending order by alignment before placing them in the output file.
4222 Larger alignments are placed before smaller alignments in order to
4223 reduce the amount of padding necessary.
4225 @cindex SORT_BY_INIT_PRIORITY
4226 @code{SORT_BY_INIT_PRIORITY} is very similar to @code{SORT_BY_NAME}. The
4227 difference is @code{SORT_BY_INIT_PRIORITY} will sort sections into
4228 ascending order by numerical value of the GCC init_priority attribute
4229 encoded in the section name before placing them in the output file.
4232 @code{SORT} is an alias for @code{SORT_BY_NAME}.
4234 When there are nested section sorting commands in linker script, there
4235 can be at most 1 level of nesting for section sorting commands.
4239 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
4240 It will sort the input sections by name first, then by alignment if two
4241 sections have the same name.
4243 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
4244 It will sort the input sections by alignment first, then by name if two
4245 sections have the same alignment.
4247 @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is
4248 treated the same as @code{SORT_BY_NAME} (wildcard section pattern).
4250 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern))
4251 is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern).
4253 All other nested section sorting commands are invalid.
4256 When both command line section sorting option and linker script
4257 section sorting command are used, section sorting command always
4258 takes precedence over the command line option.
4260 If the section sorting command in linker script isn't nested, the
4261 command line option will make the section sorting command to be
4262 treated as nested sorting command.
4266 @code{SORT_BY_NAME} (wildcard section pattern ) with
4267 @option{--sort-sections alignment} is equivalent to
4268 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
4270 @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with
4271 @option{--sort-section name} is equivalent to
4272 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
4275 If the section sorting command in linker script is nested, the
4276 command line option will be ignored.
4279 @code{SORT_NONE} disables section sorting by ignoring the command line
4280 section sorting option.
4282 If you ever get confused about where input sections are going, use the
4283 @samp{-M} linker option to generate a map file. The map file shows
4284 precisely how input sections are mapped to output sections.
4286 This example shows how wildcard patterns might be used to partition
4287 files. This linker script directs the linker to place all @samp{.text}
4288 sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}.
4289 The linker will place the @samp{.data} section from all files beginning
4290 with an upper case character in @samp{.DATA}; for all other files, the
4291 linker will place the @samp{.data} section in @samp{.data}.
4295 .text : @{ *(.text) @}
4296 .DATA : @{ [A-Z]*(.data) @}
4297 .data : @{ *(.data) @}
4298 .bss : @{ *(.bss) @}
4303 @node Input Section Common
4304 @subsubsection Input Section for Common Symbols
4305 @cindex common symbol placement
4306 @cindex uninitialized data placement
4307 A special notation is needed for common symbols, because in many object
4308 file formats common symbols do not have a particular input section. The
4309 linker treats common symbols as though they are in an input section
4310 named @samp{COMMON}.
4312 You may use file names with the @samp{COMMON} section just as with any
4313 other input sections. You can use this to place common symbols from a
4314 particular input file in one section while common symbols from other
4315 input files are placed in another section.
4317 In most cases, common symbols in input files will be placed in the
4318 @samp{.bss} section in the output file. For example:
4320 .bss @{ *(.bss) *(COMMON) @}
4323 @cindex scommon section
4324 @cindex small common symbols
4325 Some object file formats have more than one type of common symbol. For
4326 example, the MIPS ELF object file format distinguishes standard common
4327 symbols and small common symbols. In this case, the linker will use a
4328 different special section name for other types of common symbols. In
4329 the case of MIPS ELF, the linker uses @samp{COMMON} for standard common
4330 symbols and @samp{.scommon} for small common symbols. This permits you
4331 to map the different types of common symbols into memory at different
4335 You will sometimes see @samp{[COMMON]} in old linker scripts. This
4336 notation is now considered obsolete. It is equivalent to
4339 @node Input Section Keep
4340 @subsubsection Input Section and Garbage Collection
4342 @cindex garbage collection
4343 When link-time garbage collection is in use (@samp{--gc-sections}),
4344 it is often useful to mark sections that should not be eliminated.
4345 This is accomplished by surrounding an input section's wildcard entry
4346 with @code{KEEP()}, as in @code{KEEP(*(.init))} or
4347 @code{KEEP(SORT_BY_NAME(*)(.ctors))}.
4349 @node Input Section Example
4350 @subsubsection Input Section Example
4351 The following example is a complete linker script. It tells the linker
4352 to read all of the sections from file @file{all.o} and place them at the
4353 start of output section @samp{outputa} which starts at location
4354 @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o}
4355 follows immediately, in the same output section. All of section
4356 @samp{.input2} from @file{foo.o} goes into output section
4357 @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}.
4358 All of the remaining @samp{.input1} and @samp{.input2} sections from any
4359 files are written to output section @samp{outputc}.
4387 @node Output Section Data
4388 @subsection Output Section Data
4390 @cindex section data
4391 @cindex output section data
4392 @kindex BYTE(@var{expression})
4393 @kindex SHORT(@var{expression})
4394 @kindex LONG(@var{expression})
4395 @kindex QUAD(@var{expression})
4396 @kindex SQUAD(@var{expression})
4397 You can include explicit bytes of data in an output section by using
4398 @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as
4399 an output section command. Each keyword is followed by an expression in
4400 parentheses providing the value to store (@pxref{Expressions}). The
4401 value of the expression is stored at the current value of the location
4404 The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands
4405 store one, two, four, and eight bytes (respectively). After storing the
4406 bytes, the location counter is incremented by the number of bytes
4409 For example, this will store the byte 1 followed by the four byte value
4410 of the symbol @samp{addr}:
4416 When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the
4417 same; they both store an 8 byte, or 64 bit, value. When both host and
4418 target are 32 bits, an expression is computed as 32 bits. In this case
4419 @code{QUAD} stores a 32 bit value zero extended to 64 bits, and
4420 @code{SQUAD} stores a 32 bit value sign extended to 64 bits.
4422 If the object file format of the output file has an explicit endianness,
4423 which is the normal case, the value will be stored in that endianness.
4424 When the object file format does not have an explicit endianness, as is
4425 true of, for example, S-records, the value will be stored in the
4426 endianness of the first input object file.
4428 Note---these commands only work inside a section description and not
4429 between them, so the following will produce an error from the linker:
4431 SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@
4433 whereas this will work:
4435 SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@
4438 @kindex FILL(@var{expression})
4439 @cindex holes, filling
4440 @cindex unspecified memory
4441 You may use the @code{FILL} command to set the fill pattern for the
4442 current section. It is followed by an expression in parentheses. Any
4443 otherwise unspecified regions of memory within the section (for example,
4444 gaps left due to the required alignment of input sections) are filled
4445 with the value of the expression, repeated as
4446 necessary. A @code{FILL} statement covers memory locations after the
4447 point at which it occurs in the section definition; by including more
4448 than one @code{FILL} statement, you can have different fill patterns in
4449 different parts of an output section.
4451 This example shows how to fill unspecified regions of memory with the
4457 The @code{FILL} command is similar to the @samp{=@var{fillexp}} output
4458 section attribute, but it only affects the
4459 part of the section following the @code{FILL} command, rather than the
4460 entire section. If both are used, the @code{FILL} command takes
4461 precedence. @xref{Output Section Fill}, for details on the fill
4464 @node Output Section Keywords
4465 @subsection Output Section Keywords
4466 There are a couple of keywords which can appear as output section
4470 @kindex CREATE_OBJECT_SYMBOLS
4471 @cindex input filename symbols
4472 @cindex filename symbols
4473 @item CREATE_OBJECT_SYMBOLS
4474 The command tells the linker to create a symbol for each input file.
4475 The name of each symbol will be the name of the corresponding input
4476 file. The section of each symbol will be the output section in which
4477 the @code{CREATE_OBJECT_SYMBOLS} command appears.
4479 This is conventional for the a.out object file format. It is not
4480 normally used for any other object file format.
4482 @kindex CONSTRUCTORS
4483 @cindex C++ constructors, arranging in link
4484 @cindex constructors, arranging in link
4486 When linking using the a.out object file format, the linker uses an
4487 unusual set construct to support C++ global constructors and
4488 destructors. When linking object file formats which do not support
4489 arbitrary sections, such as ECOFF and XCOFF, the linker will
4490 automatically recognize C++ global constructors and destructors by name.
4491 For these object file formats, the @code{CONSTRUCTORS} command tells the
4492 linker to place constructor information in the output section where the
4493 @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is
4494 ignored for other object file formats.
4496 The symbol @w{@code{__CTOR_LIST__}} marks the start of the global
4497 constructors, and the symbol @w{@code{__CTOR_END__}} marks the end.
4498 Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_END__}} mark
4499 the start and end of the global destructors. The
4500 first word in the list is the number of entries, followed by the address
4501 of each constructor or destructor, followed by a zero word. The
4502 compiler must arrange to actually run the code. For these object file
4503 formats @sc{gnu} C++ normally calls constructors from a subroutine
4504 @code{__main}; a call to @code{__main} is automatically inserted into
4505 the startup code for @code{main}. @sc{gnu} C++ normally runs
4506 destructors either by using @code{atexit}, or directly from the function
4509 For object file formats such as @code{COFF} or @code{ELF} which support
4510 arbitrary section names, @sc{gnu} C++ will normally arrange to put the
4511 addresses of global constructors and destructors into the @code{.ctors}
4512 and @code{.dtors} sections. Placing the following sequence into your
4513 linker script will build the sort of table which the @sc{gnu} C++
4514 runtime code expects to see.
4518 LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
4523 LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
4529 If you are using the @sc{gnu} C++ support for initialization priority,
4530 which provides some control over the order in which global constructors
4531 are run, you must sort the constructors at link time to ensure that they
4532 are executed in the correct order. When using the @code{CONSTRUCTORS}
4533 command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the
4534 @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and
4535 @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and
4538 Normally the compiler and linker will handle these issues automatically,
4539 and you will not need to concern yourself with them. However, you may
4540 need to consider this if you are using C++ and writing your own linker
4545 @node Output Section Discarding
4546 @subsection Output Section Discarding
4547 @cindex discarding sections
4548 @cindex sections, discarding
4549 @cindex removing sections
4550 The linker will not normally create output sections with no contents.
4551 This is for convenience when referring to input sections that may or
4552 may not be present in any of the input files. For example:
4554 .foo : @{ *(.foo) @}
4557 will only create a @samp{.foo} section in the output file if there is a
4558 @samp{.foo} section in at least one input file, and if the input
4559 sections are not all empty. Other link script directives that allocate
4560 space in an output section will also create the output section. So
4561 too will assignments to dot even if the assignment does not create
4562 space, except for @samp{. = 0}, @samp{. = . + 0}, @samp{. = sym},
4563 @samp{. = . + sym} and @samp{. = ALIGN (. != 0, expr, 1)} when
4564 @samp{sym} is an absolute symbol of value 0 defined in the script.
4565 This allows you to force output of an empty section with @samp{. = .}.
4567 The linker will ignore address assignments (@pxref{Output Section Address})
4568 on discarded output sections, except when the linker script defines
4569 symbols in the output section. In that case the linker will obey
4570 the address assignments, possibly advancing dot even though the
4571 section is discarded.
4574 The special output section name @samp{/DISCARD/} may be used to discard
4575 input sections. Any input sections which are assigned to an output
4576 section named @samp{/DISCARD/} are not included in the output file.
4578 @node Output Section Attributes
4579 @subsection Output Section Attributes
4580 @cindex output section attributes
4581 We showed above that the full description of an output section looked
4586 @var{section} [@var{address}] [(@var{type})] :
4588 [ALIGN(@var{section_align})]
4589 [SUBALIGN(@var{subsection_align})]
4592 @var{output-section-command}
4593 @var{output-section-command}
4595 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}]
4599 We've already described @var{section}, @var{address}, and
4600 @var{output-section-command}. In this section we will describe the
4601 remaining section attributes.
4604 * Output Section Type:: Output section type
4605 * Output Section LMA:: Output section LMA
4606 * Forced Output Alignment:: Forced Output Alignment
4607 * Forced Input Alignment:: Forced Input Alignment
4608 * Output Section Constraint:: Output section constraint
4609 * Output Section Region:: Output section region
4610 * Output Section Phdr:: Output section phdr
4611 * Output Section Fill:: Output section fill
4614 @node Output Section Type
4615 @subsubsection Output Section Type
4616 Each output section may have a type. The type is a keyword in
4617 parentheses. The following types are defined:
4621 The section should be marked as not loadable, so that it will not be
4622 loaded into memory when the program is run.
4627 These type names are supported for backward compatibility, and are
4628 rarely used. They all have the same effect: the section should be
4629 marked as not allocatable, so that no memory is allocated for the
4630 section when the program is run.
4634 @cindex prevent unnecessary loading
4635 @cindex loading, preventing
4636 The linker normally sets the attributes of an output section based on
4637 the input sections which map into it. You can override this by using
4638 the section type. For example, in the script sample below, the
4639 @samp{ROM} section is addressed at memory location @samp{0} and does not
4640 need to be loaded when the program is run.
4644 ROM 0 (NOLOAD) : @{ @dots{} @}
4650 @node Output Section LMA
4651 @subsubsection Output Section LMA
4652 @kindex AT>@var{lma_region}
4653 @kindex AT(@var{lma})
4654 @cindex load address
4655 @cindex section load address
4656 Every section has a virtual address (VMA) and a load address (LMA); see
4657 @ref{Basic Script Concepts}. The virtual address is specified by the
4658 @pxref{Output Section Address} described earlier. The load address is
4659 specified by the @code{AT} or @code{AT>} keywords. Specifying a load
4660 address is optional.
4662 The @code{AT} keyword takes an expression as an argument. This
4663 specifies the exact load address of the section. The @code{AT>} keyword
4664 takes the name of a memory region as an argument. @xref{MEMORY}. The
4665 load address of the section is set to the next free address in the
4666 region, aligned to the section's alignment requirements.
4668 If neither @code{AT} nor @code{AT>} is specified for an allocatable
4669 section, the linker will use the following heuristic to determine the
4674 If the section has a specific VMA address, then this is used as
4675 the LMA address as well.
4678 If the section is not allocatable then its LMA is set to its VMA.
4681 Otherwise if a memory region can be found that is compatible
4682 with the current section, and this region contains at least one
4683 section, then the LMA is set so the difference between the
4684 VMA and LMA is the same as the difference between the VMA and LMA of
4685 the last section in the located region.
4688 If no memory regions have been declared then a default region
4689 that covers the entire address space is used in the previous step.
4692 If no suitable region could be found, or there was no previous
4693 section then the LMA is set equal to the VMA.
4696 @cindex ROM initialized data
4697 @cindex initialized data in ROM
4698 This feature is designed to make it easy to build a ROM image. For
4699 example, the following linker script creates three output sections: one
4700 called @samp{.text}, which starts at @code{0x1000}, one called
4701 @samp{.mdata}, which is loaded at the end of the @samp{.text} section
4702 even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold
4703 uninitialized data at address @code{0x3000}. The symbol @code{_data} is
4704 defined with the value @code{0x2000}, which shows that the location
4705 counter holds the VMA value, not the LMA value.
4711 .text 0x1000 : @{ *(.text) _etext = . ; @}
4713 AT ( ADDR (.text) + SIZEOF (.text) )
4714 @{ _data = . ; *(.data); _edata = . ; @}
4716 @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@}
4721 The run-time initialization code for use with a program generated with
4722 this linker script would include something like the following, to copy
4723 the initialized data from the ROM image to its runtime address. Notice
4724 how this code takes advantage of the symbols defined by the linker
4729 extern char _etext, _data, _edata, _bstart, _bend;
4730 char *src = &_etext;
4733 /* ROM has data at end of text; copy it. */
4734 while (dst < &_edata)
4738 for (dst = &_bstart; dst< &_bend; dst++)
4743 @node Forced Output Alignment
4744 @subsubsection Forced Output Alignment
4745 @kindex ALIGN(@var{section_align})
4746 @cindex forcing output section alignment
4747 @cindex output section alignment
4748 You can increase an output section's alignment by using ALIGN. As an
4749 alternative you can enforce that the difference between the VMA and LMA remains
4750 intact throughout this output section with the ALIGN_WITH_INPUT attribute.
4752 @node Forced Input Alignment
4753 @subsubsection Forced Input Alignment
4754 @kindex SUBALIGN(@var{subsection_align})
4755 @cindex forcing input section alignment
4756 @cindex input section alignment
4757 You can force input section alignment within an output section by using
4758 SUBALIGN. The value specified overrides any alignment given by input
4759 sections, whether larger or smaller.
4761 @node Output Section Constraint
4762 @subsubsection Output Section Constraint
4765 @cindex constraints on output sections
4766 You can specify that an output section should only be created if all
4767 of its input sections are read-only or all of its input sections are
4768 read-write by using the keyword @code{ONLY_IF_RO} and
4769 @code{ONLY_IF_RW} respectively.
4771 @node Output Section Region
4772 @subsubsection Output Section Region
4773 @kindex >@var{region}
4774 @cindex section, assigning to memory region
4775 @cindex memory regions and sections
4776 You can assign a section to a previously defined region of memory by
4777 using @samp{>@var{region}}. @xref{MEMORY}.
4779 Here is a simple example:
4782 MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @}
4783 SECTIONS @{ ROM : @{ *(.text) @} >rom @}
4787 @node Output Section Phdr
4788 @subsubsection Output Section Phdr
4790 @cindex section, assigning to program header
4791 @cindex program headers and sections
4792 You can assign a section to a previously defined program segment by
4793 using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to
4794 one or more segments, then all subsequent allocated sections will be
4795 assigned to those segments as well, unless they use an explicitly
4796 @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the
4797 linker to not put the section in any segment at all.
4799 Here is a simple example:
4802 PHDRS @{ text PT_LOAD ; @}
4803 SECTIONS @{ .text : @{ *(.text) @} :text @}
4807 @node Output Section Fill
4808 @subsubsection Output Section Fill
4809 @kindex =@var{fillexp}
4810 @cindex section fill pattern
4811 @cindex fill pattern, entire section
4812 You can set the fill pattern for an entire section by using
4813 @samp{=@var{fillexp}}. @var{fillexp} is an expression
4814 (@pxref{Expressions}). Any otherwise unspecified regions of memory
4815 within the output section (for example, gaps left due to the required
4816 alignment of input sections) will be filled with the value, repeated as
4817 necessary. If the fill expression is a simple hex number, ie. a string
4818 of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then
4819 an arbitrarily long sequence of hex digits can be used to specify the
4820 fill pattern; Leading zeros become part of the pattern too. For all
4821 other cases, including extra parentheses or a unary @code{+}, the fill
4822 pattern is the four least significant bytes of the value of the
4823 expression. In all cases, the number is big-endian.
4825 You can also change the fill value with a @code{FILL} command in the
4826 output section commands; (@pxref{Output Section Data}).
4828 Here is a simple example:
4831 SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @}
4835 @node Overlay Description
4836 @subsection Overlay Description
4839 An overlay description provides an easy way to describe sections which
4840 are to be loaded as part of a single memory image but are to be run at
4841 the same memory address. At run time, some sort of overlay manager will
4842 copy the overlaid sections in and out of the runtime memory address as
4843 required, perhaps by simply manipulating addressing bits. This approach
4844 can be useful, for example, when a certain region of memory is faster
4847 Overlays are described using the @code{OVERLAY} command. The
4848 @code{OVERLAY} command is used within a @code{SECTIONS} command, like an
4849 output section description. The full syntax of the @code{OVERLAY}
4850 command is as follows:
4853 OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )]
4857 @var{output-section-command}
4858 @var{output-section-command}
4860 @} [:@var{phdr}@dots{}] [=@var{fill}]
4863 @var{output-section-command}
4864 @var{output-section-command}
4866 @} [:@var{phdr}@dots{}] [=@var{fill}]
4868 @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] [,]
4872 Everything is optional except @code{OVERLAY} (a keyword), and each
4873 section must have a name (@var{secname1} and @var{secname2} above). The
4874 section definitions within the @code{OVERLAY} construct are identical to
4875 those within the general @code{SECTIONS} construct (@pxref{SECTIONS}),
4876 except that no addresses and no memory regions may be defined for
4877 sections within an @code{OVERLAY}.
4879 The comma at the end may be required if a @var{fill} is used and
4880 the next @var{sections-command} looks like a continuation of the expression.
4882 The sections are all defined with the same starting address. The load
4883 addresses of the sections are arranged such that they are consecutive in
4884 memory starting at the load address used for the @code{OVERLAY} as a
4885 whole (as with normal section definitions, the load address is optional,
4886 and defaults to the start address; the start address is also optional,
4887 and defaults to the current value of the location counter).
4889 If the @code{NOCROSSREFS} keyword is used, and there are any
4890 references among the sections, the linker will report an error. Since
4891 the sections all run at the same address, it normally does not make
4892 sense for one section to refer directly to another.
4893 @xref{Miscellaneous Commands, NOCROSSREFS}.
4895 For each section within the @code{OVERLAY}, the linker automatically
4896 provides two symbols. The symbol @code{__load_start_@var{secname}} is
4897 defined as the starting load address of the section. The symbol
4898 @code{__load_stop_@var{secname}} is defined as the final load address of
4899 the section. Any characters within @var{secname} which are not legal
4900 within C identifiers are removed. C (or assembler) code may use these
4901 symbols to move the overlaid sections around as necessary.
4903 At the end of the overlay, the value of the location counter is set to
4904 the start address of the overlay plus the size of the largest section.
4906 Here is an example. Remember that this would appear inside a
4907 @code{SECTIONS} construct.
4910 OVERLAY 0x1000 : AT (0x4000)
4912 .text0 @{ o1/*.o(.text) @}
4913 .text1 @{ o2/*.o(.text) @}
4918 This will define both @samp{.text0} and @samp{.text1} to start at
4919 address 0x1000. @samp{.text0} will be loaded at address 0x4000, and
4920 @samp{.text1} will be loaded immediately after @samp{.text0}. The
4921 following symbols will be defined if referenced: @code{__load_start_text0},
4922 @code{__load_stop_text0}, @code{__load_start_text1},
4923 @code{__load_stop_text1}.
4925 C code to copy overlay @code{.text1} into the overlay area might look
4930 extern char __load_start_text1, __load_stop_text1;
4931 memcpy ((char *) 0x1000, &__load_start_text1,
4932 &__load_stop_text1 - &__load_start_text1);
4936 Note that the @code{OVERLAY} command is just syntactic sugar, since
4937 everything it does can be done using the more basic commands. The above
4938 example could have been written identically as follows.
4942 .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @}
4943 PROVIDE (__load_start_text0 = LOADADDR (.text0));
4944 PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
4945 .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @}
4946 PROVIDE (__load_start_text1 = LOADADDR (.text1));
4947 PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
4948 . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
4953 @section MEMORY Command
4955 @cindex memory regions
4956 @cindex regions of memory
4957 @cindex allocating memory
4958 @cindex discontinuous memory
4959 The linker's default configuration permits allocation of all available
4960 memory. You can override this by using the @code{MEMORY} command.
4962 The @code{MEMORY} command describes the location and size of blocks of
4963 memory in the target. You can use it to describe which memory regions
4964 may be used by the linker, and which memory regions it must avoid. You
4965 can then assign sections to particular memory regions. The linker will
4966 set section addresses based on the memory regions, and will warn about
4967 regions that become too full. The linker will not shuffle sections
4968 around to fit into the available regions.
4970 A linker script may contain at most one use of the @code{MEMORY}
4971 command. However, you can define as many blocks of memory within it as
4972 you wish. The syntax is:
4977 @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len}
4983 The @var{name} is a name used in the linker script to refer to the
4984 region. The region name has no meaning outside of the linker script.
4985 Region names are stored in a separate name space, and will not conflict
4986 with symbol names, file names, or section names. Each memory region
4987 must have a distinct name within the @code{MEMORY} command. However you can
4988 add later alias names to existing memory regions with the @ref{REGION_ALIAS}
4991 @cindex memory region attributes
4992 The @var{attr} string is an optional list of attributes that specify
4993 whether to use a particular memory region for an input section which is
4994 not explicitly mapped in the linker script. As described in
4995 @ref{SECTIONS}, if you do not specify an output section for some input
4996 section, the linker will create an output section with the same name as
4997 the input section. If you define region attributes, the linker will use
4998 them to select the memory region for the output section that it creates.
5000 The @var{attr} string must consist only of the following characters:
5015 Invert the sense of any of the attributes that follow
5018 If a unmapped section matches any of the listed attributes other than
5019 @samp{!}, it will be placed in the memory region. The @samp{!}
5020 attribute reverses this test, so that an unmapped section will be placed
5021 in the memory region only if it does not match any of the listed
5027 The @var{origin} is an numerical expression for the start address of
5028 the memory region. The expression must evaluate to a constant and it
5029 cannot involve any symbols. The keyword @code{ORIGIN} may be
5030 abbreviated to @code{org} or @code{o} (but not, for example,
5036 The @var{len} is an expression for the size in bytes of the memory
5037 region. As with the @var{origin} expression, the expression must
5038 be numerical only and must evaluate to a constant. The keyword
5039 @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
5041 In the following example, we specify that there are two memory regions
5042 available for allocation: one starting at @samp{0} for 256 kilobytes,
5043 and the other starting at @samp{0x40000000} for four megabytes. The
5044 linker will place into the @samp{rom} memory region every section which
5045 is not explicitly mapped into a memory region, and is either read-only
5046 or executable. The linker will place other sections which are not
5047 explicitly mapped into a memory region into the @samp{ram} memory
5054 rom (rx) : ORIGIN = 0, LENGTH = 256K
5055 ram (!rx) : org = 0x40000000, l = 4M
5060 Once you define a memory region, you can direct the linker to place
5061 specific output sections into that memory region by using the
5062 @samp{>@var{region}} output section attribute. For example, if you have
5063 a memory region named @samp{mem}, you would use @samp{>mem} in the
5064 output section definition. @xref{Output Section Region}. If no address
5065 was specified for the output section, the linker will set the address to
5066 the next available address within the memory region. If the combined
5067 output sections directed to a memory region are too large for the
5068 region, the linker will issue an error message.
5070 It is possible to access the origin and length of a memory in an
5071 expression via the @code{ORIGIN(@var{memory})} and
5072 @code{LENGTH(@var{memory})} functions:
5076 _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
5081 @section PHDRS Command
5083 @cindex program headers
5084 @cindex ELF program headers
5085 @cindex program segments
5086 @cindex segments, ELF
5087 The ELF object file format uses @dfn{program headers}, also knows as
5088 @dfn{segments}. The program headers describe how the program should be
5089 loaded into memory. You can print them out by using the @code{objdump}
5090 program with the @samp{-p} option.
5092 When you run an ELF program on a native ELF system, the system loader
5093 reads the program headers in order to figure out how to load the
5094 program. This will only work if the program headers are set correctly.
5095 This manual does not describe the details of how the system loader
5096 interprets program headers; for more information, see the ELF ABI.
5098 The linker will create reasonable program headers by default. However,
5099 in some cases, you may need to specify the program headers more
5100 precisely. You may use the @code{PHDRS} command for this purpose. When
5101 the linker sees the @code{PHDRS} command in the linker script, it will
5102 not create any program headers other than the ones specified.
5104 The linker only pays attention to the @code{PHDRS} command when
5105 generating an ELF output file. In other cases, the linker will simply
5106 ignore @code{PHDRS}.
5108 This is the syntax of the @code{PHDRS} command. The words @code{PHDRS},
5109 @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords.
5115 @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ]
5116 [ FLAGS ( @var{flags} ) ] ;
5121 The @var{name} is used only for reference in the @code{SECTIONS} command
5122 of the linker script. It is not put into the output file. Program
5123 header names are stored in a separate name space, and will not conflict
5124 with symbol names, file names, or section names. Each program header
5125 must have a distinct name. The headers are processed in order and it
5126 is usual for them to map to sections in ascending load address order.
5128 Certain program header types describe segments of memory which the
5129 system loader will load from the file. In the linker script, you
5130 specify the contents of these segments by placing allocatable output
5131 sections in the segments. You use the @samp{:@var{phdr}} output section
5132 attribute to place a section in a particular segment. @xref{Output
5135 It is normal to put certain sections in more than one segment. This
5136 merely implies that one segment of memory contains another. You may
5137 repeat @samp{:@var{phdr}}, using it once for each segment which should
5138 contain the section.
5140 If you place a section in one or more segments using @samp{:@var{phdr}},
5141 then the linker will place all subsequent allocatable sections which do
5142 not specify @samp{:@var{phdr}} in the same segments. This is for
5143 convenience, since generally a whole set of contiguous sections will be
5144 placed in a single segment. You can use @code{:NONE} to override the
5145 default segment and tell the linker to not put the section in any
5150 You may use the @code{FILEHDR} and @code{PHDRS} keywords after
5151 the program header type to further describe the contents of the segment.
5152 The @code{FILEHDR} keyword means that the segment should include the ELF
5153 file header. The @code{PHDRS} keyword means that the segment should
5154 include the ELF program headers themselves. If applied to a loadable
5155 segment (@code{PT_LOAD}), all prior loadable segments must have one of
5158 The @var{type} may be one of the following. The numbers indicate the
5159 value of the keyword.
5162 @item @code{PT_NULL} (0)
5163 Indicates an unused program header.
5165 @item @code{PT_LOAD} (1)
5166 Indicates that this program header describes a segment to be loaded from
5169 @item @code{PT_DYNAMIC} (2)
5170 Indicates a segment where dynamic linking information can be found.
5172 @item @code{PT_INTERP} (3)
5173 Indicates a segment where the name of the program interpreter may be
5176 @item @code{PT_NOTE} (4)
5177 Indicates a segment holding note information.
5179 @item @code{PT_SHLIB} (5)
5180 A reserved program header type, defined but not specified by the ELF
5183 @item @code{PT_PHDR} (6)
5184 Indicates a segment where the program headers may be found.
5186 @item @var{expression}
5187 An expression giving the numeric type of the program header. This may
5188 be used for types not defined above.
5191 You can specify that a segment should be loaded at a particular address
5192 in memory by using an @code{AT} expression. This is identical to the
5193 @code{AT} command used as an output section attribute (@pxref{Output
5194 Section LMA}). The @code{AT} command for a program header overrides the
5195 output section attribute.
5197 The linker will normally set the segment flags based on the sections
5198 which comprise the segment. You may use the @code{FLAGS} keyword to
5199 explicitly specify the segment flags. The value of @var{flags} must be
5200 an integer. It is used to set the @code{p_flags} field of the program
5203 Here is an example of @code{PHDRS}. This shows a typical set of program
5204 headers used on a native ELF system.
5210 headers PT_PHDR PHDRS ;
5212 text PT_LOAD FILEHDR PHDRS ;
5214 dynamic PT_DYNAMIC ;
5220 .interp : @{ *(.interp) @} :text :interp
5221 .text : @{ *(.text) @} :text
5222 .rodata : @{ *(.rodata) @} /* defaults to :text */
5224 . = . + 0x1000; /* move to a new page in memory */
5225 .data : @{ *(.data) @} :data
5226 .dynamic : @{ *(.dynamic) @} :data :dynamic
5233 @section VERSION Command
5234 @kindex VERSION @{script text@}
5235 @cindex symbol versions
5236 @cindex version script
5237 @cindex versions of symbols
5238 The linker supports symbol versions when using ELF. Symbol versions are
5239 only useful when using shared libraries. The dynamic linker can use
5240 symbol versions to select a specific version of a function when it runs
5241 a program that may have been linked against an earlier version of the
5244 You can include a version script directly in the main linker script, or
5245 you can supply the version script as an implicit linker script. You can
5246 also use the @samp{--version-script} linker option.
5248 The syntax of the @code{VERSION} command is simply
5250 VERSION @{ version-script-commands @}
5253 The format of the version script commands is identical to that used by
5254 Sun's linker in Solaris 2.5. The version script defines a tree of
5255 version nodes. You specify the node names and interdependencies in the
5256 version script. You can specify which symbols are bound to which
5257 version nodes, and you can reduce a specified set of symbols to local
5258 scope so that they are not globally visible outside of the shared
5261 The easiest way to demonstrate the version script language is with a few
5287 This example version script defines three version nodes. The first
5288 version node defined is @samp{VERS_1.1}; it has no other dependencies.
5289 The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces
5290 a number of symbols to local scope so that they are not visible outside
5291 of the shared library; this is done using wildcard patterns, so that any
5292 symbol whose name begins with @samp{old}, @samp{original}, or @samp{new}
5293 is matched. The wildcard patterns available are the same as those used
5294 in the shell when matching filenames (also known as ``globbing'').
5295 However, if you specify the symbol name inside double quotes, then the
5296 name is treated as literal, rather than as a glob pattern.
5298 Next, the version script defines node @samp{VERS_1.2}. This node
5299 depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2}
5300 to the version node @samp{VERS_1.2}.
5302 Finally, the version script defines node @samp{VERS_2.0}. This node
5303 depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1}
5304 and @samp{bar2} are bound to the version node @samp{VERS_2.0}.
5306 When the linker finds a symbol defined in a library which is not
5307 specifically bound to a version node, it will effectively bind it to an
5308 unspecified base version of the library. You can bind all otherwise
5309 unspecified symbols to a given version node by using @samp{global: *;}
5310 somewhere in the version script. Note that it's slightly crazy to use
5311 wildcards in a global spec except on the last version node. Global
5312 wildcards elsewhere run the risk of accidentally adding symbols to the
5313 set exported for an old version. That's wrong since older versions
5314 ought to have a fixed set of symbols.
5316 The names of the version nodes have no specific meaning other than what
5317 they might suggest to the person reading them. The @samp{2.0} version
5318 could just as well have appeared in between @samp{1.1} and @samp{1.2}.
5319 However, this would be a confusing way to write a version script.
5321 Node name can be omitted, provided it is the only version node
5322 in the version script. Such version script doesn't assign any versions to
5323 symbols, only selects which symbols will be globally visible out and which
5327 @{ global: foo; bar; local: *; @};
5330 When you link an application against a shared library that has versioned
5331 symbols, the application itself knows which version of each symbol it
5332 requires, and it also knows which version nodes it needs from each
5333 shared library it is linked against. Thus at runtime, the dynamic
5334 loader can make a quick check to make sure that the libraries you have
5335 linked against do in fact supply all of the version nodes that the
5336 application will need to resolve all of the dynamic symbols. In this
5337 way it is possible for the dynamic linker to know with certainty that
5338 all external symbols that it needs will be resolvable without having to
5339 search for each symbol reference.
5341 The symbol versioning is in effect a much more sophisticated way of
5342 doing minor version checking that SunOS does. The fundamental problem
5343 that is being addressed here is that typically references to external
5344 functions are bound on an as-needed basis, and are not all bound when
5345 the application starts up. If a shared library is out of date, a
5346 required interface may be missing; when the application tries to use
5347 that interface, it may suddenly and unexpectedly fail. With symbol
5348 versioning, the user will get a warning when they start their program if
5349 the libraries being used with the application are too old.
5351 There are several GNU extensions to Sun's versioning approach. The
5352 first of these is the ability to bind a symbol to a version node in the
5353 source file where the symbol is defined instead of in the versioning
5354 script. This was done mainly to reduce the burden on the library
5355 maintainer. You can do this by putting something like:
5357 __asm__(".symver original_foo,foo@@VERS_1.1");
5360 in the C source file. This renames the function @samp{original_foo} to
5361 be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}.
5362 The @samp{local:} directive can be used to prevent the symbol
5363 @samp{original_foo} from being exported. A @samp{.symver} directive
5364 takes precedence over a version script.
5366 The second GNU extension is to allow multiple versions of the same
5367 function to appear in a given shared library. In this way you can make
5368 an incompatible change to an interface without increasing the major
5369 version number of the shared library, while still allowing applications
5370 linked against the old interface to continue to function.
5372 To do this, you must use multiple @samp{.symver} directives in the
5373 source file. Here is an example:
5376 __asm__(".symver original_foo,foo@@");
5377 __asm__(".symver old_foo,foo@@VERS_1.1");
5378 __asm__(".symver old_foo1,foo@@VERS_1.2");
5379 __asm__(".symver new_foo,foo@@@@VERS_2.0");
5382 In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the
5383 unspecified base version of the symbol. The source file that contains this
5384 example would define 4 C functions: @samp{original_foo}, @samp{old_foo},
5385 @samp{old_foo1}, and @samp{new_foo}.
5387 When you have multiple definitions of a given symbol, there needs to be
5388 some way to specify a default version to which external references to
5389 this symbol will be bound. You can do this with the
5390 @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only
5391 declare one version of a symbol as the default in this manner; otherwise
5392 you would effectively have multiple definitions of the same symbol.
5394 If you wish to bind a reference to a specific version of the symbol
5395 within the shared library, you can use the aliases of convenience
5396 (i.e., @samp{old_foo}), or you can use the @samp{.symver} directive to
5397 specifically bind to an external version of the function in question.
5399 You can also specify the language in the version script:
5402 VERSION extern "lang" @{ version-script-commands @}
5405 The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}.
5406 The linker will iterate over the list of symbols at the link time and
5407 demangle them according to @samp{lang} before matching them to the
5408 patterns specified in @samp{version-script-commands}. The default
5409 @samp{lang} is @samp{C}.
5411 Demangled names may contains spaces and other special characters. As
5412 described above, you can use a glob pattern to match demangled names,
5413 or you can use a double-quoted string to match the string exactly. In
5414 the latter case, be aware that minor differences (such as differing
5415 whitespace) between the version script and the demangler output will
5416 cause a mismatch. As the exact string generated by the demangler
5417 might change in the future, even if the mangled name does not, you
5418 should check that all of your version directives are behaving as you
5419 expect when you upgrade.
5422 @section Expressions in Linker Scripts
5425 The syntax for expressions in the linker script language is identical to
5426 that of C expressions. All expressions are evaluated as integers. All
5427 expressions are evaluated in the same size, which is 32 bits if both the
5428 host and target are 32 bits, and is otherwise 64 bits.
5430 You can use and set symbol values in expressions.
5432 The linker defines several special purpose builtin functions for use in
5436 * Constants:: Constants
5437 * Symbolic Constants:: Symbolic constants
5438 * Symbols:: Symbol Names
5439 * Orphan Sections:: Orphan Sections
5440 * Location Counter:: The Location Counter
5441 * Operators:: Operators
5442 * Evaluation:: Evaluation
5443 * Expression Section:: The Section of an Expression
5444 * Builtin Functions:: Builtin Functions
5448 @subsection Constants
5449 @cindex integer notation
5450 @cindex constants in linker scripts
5451 All constants are integers.
5453 As in C, the linker considers an integer beginning with @samp{0} to be
5454 octal, and an integer beginning with @samp{0x} or @samp{0X} to be
5455 hexadecimal. Alternatively the linker accepts suffixes of @samp{h} or
5456 @samp{H} for hexadecimal, @samp{o} or @samp{O} for octal, @samp{b} or
5457 @samp{B} for binary and @samp{d} or @samp{D} for decimal. Any integer
5458 value without a prefix or a suffix is considered to be decimal.
5460 @cindex scaled integers
5461 @cindex K and M integer suffixes
5462 @cindex M and K integer suffixes
5463 @cindex suffixes for integers
5464 @cindex integer suffixes
5465 In addition, you can use the suffixes @code{K} and @code{M} to scale a
5469 @c END TEXI2ROFF-KILL
5470 @code{1024} or @code{1024*1024}
5474 ${\rm 1024}$ or ${\rm 1024}^2$
5476 @c END TEXI2ROFF-KILL
5477 respectively. For example, the following
5478 all refer to the same quantity:
5487 Note - the @code{K} and @code{M} suffixes cannot be used in
5488 conjunction with the base suffixes mentioned above.
5490 @node Symbolic Constants
5491 @subsection Symbolic Constants
5492 @cindex symbolic constants
5494 It is possible to refer to target specific constants via the use of
5495 the @code{CONSTANT(@var{name})} operator, where @var{name} is one of:
5500 The target's maximum page size.
5502 @item COMMONPAGESIZE
5503 @kindex COMMONPAGESIZE
5504 The target's default page size.
5510 .text ALIGN (CONSTANT (MAXPAGESIZE)) : @{ *(.text) @}
5513 will create a text section aligned to the largest page boundary
5514 supported by the target.
5517 @subsection Symbol Names
5518 @cindex symbol names
5520 @cindex quoted symbol names
5522 Unless quoted, symbol names start with a letter, underscore, or period
5523 and may include letters, digits, underscores, periods, and hyphens.
5524 Unquoted symbol names must not conflict with any keywords. You can
5525 specify a symbol which contains odd characters or has the same name as a
5526 keyword by surrounding the symbol name in double quotes:
5529 "with a space" = "also with a space" + 10;
5532 Since symbols can contain many non-alphabetic characters, it is safest
5533 to delimit symbols with spaces. For example, @samp{A-B} is one symbol,
5534 whereas @samp{A - B} is an expression involving subtraction.
5536 @node Orphan Sections
5537 @subsection Orphan Sections
5539 Orphan sections are sections present in the input files which
5540 are not explicitly placed into the output file by the linker
5541 script. The linker will still copy these sections into the
5542 output file, but it has to guess as to where they should be
5543 placed. The linker uses a simple heuristic to do this. It
5544 attempts to place orphan sections after non-orphan sections of the
5545 same attribute, such as code vs data, loadable vs non-loadable, etc.
5546 If there is not enough room to do this then it places
5547 at the end of the file.
5549 For ELF targets, the attribute of the section includes section type as
5550 well as section flag.
5552 If an orphaned section's name is representable as a C identifier then
5553 the linker will automatically @pxref{PROVIDE} two symbols:
5554 __start_SECNAME and __stop_SECNAME, where SECNAME is the name of the
5555 section. These indicate the start address and end address of the
5556 orphaned section respectively. Note: most section names are not
5557 representable as C identifiers because they contain a @samp{.}
5560 @node Location Counter
5561 @subsection The Location Counter
5564 @cindex location counter
5565 @cindex current output location
5566 The special linker variable @dfn{dot} @samp{.} always contains the
5567 current output location counter. Since the @code{.} always refers to a
5568 location in an output section, it may only appear in an expression
5569 within a @code{SECTIONS} command. The @code{.} symbol may appear
5570 anywhere that an ordinary symbol is allowed in an expression.
5573 Assigning a value to @code{.} will cause the location counter to be
5574 moved. This may be used to create holes in the output section. The
5575 location counter may not be moved backwards inside an output section,
5576 and may not be moved backwards outside of an output section if so
5577 doing creates areas with overlapping LMAs.
5593 In the previous example, the @samp{.text} section from @file{file1} is
5594 located at the beginning of the output section @samp{output}. It is
5595 followed by a 1000 byte gap. Then the @samp{.text} section from
5596 @file{file2} appears, also with a 1000 byte gap following before the
5597 @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678}
5598 specifies what data to write in the gaps (@pxref{Output Section Fill}).
5600 @cindex dot inside sections
5601 Note: @code{.} actually refers to the byte offset from the start of the
5602 current containing object. Normally this is the @code{SECTIONS}
5603 statement, whose start address is 0, hence @code{.} can be used as an
5604 absolute address. If @code{.} is used inside a section description
5605 however, it refers to the byte offset from the start of that section,
5606 not an absolute address. Thus in a script like this:
5624 The @samp{.text} section will be assigned a starting address of 0x100
5625 and a size of exactly 0x200 bytes, even if there is not enough data in
5626 the @samp{.text} input sections to fill this area. (If there is too
5627 much data, an error will be produced because this would be an attempt to
5628 move @code{.} backwards). The @samp{.data} section will start at 0x500
5629 and it will have an extra 0x600 bytes worth of space after the end of
5630 the values from the @samp{.data} input sections and before the end of
5631 the @samp{.data} output section itself.
5633 @cindex dot outside sections
5634 Setting symbols to the value of the location counter outside of an
5635 output section statement can result in unexpected values if the linker
5636 needs to place orphan sections. For example, given the following:
5642 .text: @{ *(.text) @}
5646 .data: @{ *(.data) @}
5651 If the linker needs to place some input section, e.g. @code{.rodata},
5652 not mentioned in the script, it might choose to place that section
5653 between @code{.text} and @code{.data}. You might think the linker
5654 should place @code{.rodata} on the blank line in the above script, but
5655 blank lines are of no particular significance to the linker. As well,
5656 the linker doesn't associate the above symbol names with their
5657 sections. Instead, it assumes that all assignments or other
5658 statements belong to the previous output section, except for the
5659 special case of an assignment to @code{.}. I.e., the linker will
5660 place the orphan @code{.rodata} section as if the script was written
5667 .text: @{ *(.text) @}
5671 .rodata: @{ *(.rodata) @}
5672 .data: @{ *(.data) @}
5677 This may or may not be the script author's intention for the value of
5678 @code{start_of_data}. One way to influence the orphan section
5679 placement is to assign the location counter to itself, as the linker
5680 assumes that an assignment to @code{.} is setting the start address of
5681 a following output section and thus should be grouped with that
5682 section. So you could write:
5688 .text: @{ *(.text) @}
5693 .data: @{ *(.data) @}
5698 Now, the orphan @code{.rodata} section will be placed between
5699 @code{end_of_text} and @code{start_of_data}.
5703 @subsection Operators
5704 @cindex operators for arithmetic
5705 @cindex arithmetic operators
5706 @cindex precedence in expressions
5707 The linker recognizes the standard C set of arithmetic operators, with
5708 the standard bindings and precedence levels:
5711 @c END TEXI2ROFF-KILL
5713 precedence associativity Operators Notes
5719 5 left == != > < <= >=
5725 11 right &= += -= *= /= (2)
5729 (1) Prefix operators
5730 (2) @xref{Assignments}.
5734 \vskip \baselineskip
5735 %"lispnarrowing" is the extra indent used generally for smallexample
5736 \hskip\lispnarrowing\vbox{\offinterlineskip
5739 {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr
5740 height2pt&\omit&&\omit&&\omit&\cr
5741 &Precedence&& Associativity &&{\rm Operators}&\cr
5742 height2pt&\omit&&\omit&&\omit&\cr
5744 height2pt&\omit&&\omit&&\omit&\cr
5746 % '176 is tilde, '~' in tt font
5747 &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr
5748 &2&&left&&* / \%&\cr
5751 &5&&left&&== != > < <= >=&\cr
5754 &8&&left&&{\&\&}&\cr
5757 &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr
5759 height2pt&\omit&&\omit&&\omit&\cr}
5764 @obeylines@parskip=0pt@parindent=0pt
5765 @dag@quad Prefix operators.
5766 @ddag@quad @xref{Assignments}.
5769 @c END TEXI2ROFF-KILL
5772 @subsection Evaluation
5773 @cindex lazy evaluation
5774 @cindex expression evaluation order
5775 The linker evaluates expressions lazily. It only computes the value of
5776 an expression when absolutely necessary.
5778 The linker needs some information, such as the value of the start
5779 address of the first section, and the origins and lengths of memory
5780 regions, in order to do any linking at all. These values are computed
5781 as soon as possible when the linker reads in the linker script.
5783 However, other values (such as symbol values) are not known or needed
5784 until after storage allocation. Such values are evaluated later, when
5785 other information (such as the sizes of output sections) is available
5786 for use in the symbol assignment expression.
5788 The sizes of sections cannot be known until after allocation, so
5789 assignments dependent upon these are not performed until after
5792 Some expressions, such as those depending upon the location counter
5793 @samp{.}, must be evaluated during section allocation.
5795 If the result of an expression is required, but the value is not
5796 available, then an error results. For example, a script like the
5802 .text 9+this_isnt_constant :
5808 will cause the error message @samp{non constant expression for initial
5811 @node Expression Section
5812 @subsection The Section of an Expression
5813 @cindex expression sections
5814 @cindex absolute expressions
5815 @cindex relative expressions
5816 @cindex absolute and relocatable symbols
5817 @cindex relocatable and absolute symbols
5818 @cindex symbols, relocatable and absolute
5819 Addresses and symbols may be section relative, or absolute. A section
5820 relative symbol is relocatable. If you request relocatable output
5821 using the @samp{-r} option, a further link operation may change the
5822 value of a section relative symbol. On the other hand, an absolute
5823 symbol will retain the same value throughout any further link
5826 Some terms in linker expressions are addresses. This is true of
5827 section relative symbols and for builtin functions that return an
5828 address, such as @code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and
5829 @code{SEGMENT_START}. Other terms are simply numbers, or are builtin
5830 functions that return a non-address value, such as @code{LENGTH}.
5831 One complication is that unless you set @code{LD_FEATURE ("SANE_EXPR")}
5832 (@pxref{Miscellaneous Commands}), numbers and absolute symbols are treated
5833 differently depending on their location, for compatibility with older
5834 versions of @code{ld}. Expressions appearing outside an output
5835 section definition treat all numbers as absolute addresses.
5836 Expressions appearing inside an output section definition treat
5837 absolute symbols as numbers. If @code{LD_FEATURE ("SANE_EXPR")} is
5838 given, then absolute symbols and numbers are simply treated as numbers
5841 In the following simple example,
5848 __executable_start = 0x100;
5852 __data_start = 0x10;
5860 both @code{.} and @code{__executable_start} are set to the absolute
5861 address 0x100 in the first two assignments, then both @code{.} and
5862 @code{__data_start} are set to 0x10 relative to the @code{.data}
5863 section in the second two assignments.
5865 For expressions involving numbers, relative addresses and absolute
5866 addresses, ld follows these rules to evaluate terms:
5870 Unary operations on an absolute address or number, and binary
5871 operations on two absolute addresses or two numbers, or between one
5872 absolute address and a number, apply the operator to the value(s).
5874 Unary operations on a relative address, and binary operations on two
5875 relative addresses in the same section or between one relative address
5876 and a number, apply the operator to the offset part of the address(es).
5878 Other binary operations, that is, between two relative addresses not
5879 in the same section, or between a relative address and an absolute
5880 address, first convert any non-absolute term to an absolute address
5881 before applying the operator.
5884 The result section of each sub-expression is as follows:
5888 An operation involving only numbers results in a number.
5890 The result of comparisons, @samp{&&} and @samp{||} is also a number.
5892 The result of other binary arithmetic and logical operations on two
5893 relative addresses in the same section or two absolute addresses
5894 (after above conversions) is also a number.
5896 The result of other operations on relative addresses or one
5897 relative address and a number, is a relative address in the same
5898 section as the relative operand(s).
5900 The result of other operations on absolute addresses (after above
5901 conversions) is an absolute address.
5904 You can use the builtin function @code{ABSOLUTE} to force an expression
5905 to be absolute when it would otherwise be relative. For example, to
5906 create an absolute symbol set to the address of the end of the output
5907 section @samp{.data}:
5911 .data : @{ *(.data) _edata = ABSOLUTE(.); @}
5915 If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the
5916 @samp{.data} section.
5918 Using @code{LOADADDR} also forces an expression absolute, since this
5919 particular builtin function returns an absolute address.
5921 @node Builtin Functions
5922 @subsection Builtin Functions
5923 @cindex functions in expressions
5924 The linker script language includes a number of builtin functions for
5925 use in linker script expressions.
5928 @item ABSOLUTE(@var{exp})
5929 @kindex ABSOLUTE(@var{exp})
5930 @cindex expression, absolute
5931 Return the absolute (non-relocatable, as opposed to non-negative) value
5932 of the expression @var{exp}. Primarily useful to assign an absolute
5933 value to a symbol within a section definition, where symbol values are
5934 normally section relative. @xref{Expression Section}.
5936 @item ADDR(@var{section})
5937 @kindex ADDR(@var{section})
5938 @cindex section address in expression
5939 Return the address (VMA) of the named @var{section}. Your
5940 script must previously have defined the location of that section. In
5941 the following example, @code{start_of_output_1}, @code{symbol_1} and
5942 @code{symbol_2} are assigned equivalent values, except that
5943 @code{symbol_1} will be relative to the @code{.output1} section while
5944 the other two will be absolute:
5950 start_of_output_1 = ABSOLUTE(.);
5955 symbol_1 = ADDR(.output1);
5956 symbol_2 = start_of_output_1;
5962 @item ALIGN(@var{align})
5963 @itemx ALIGN(@var{exp},@var{align})
5964 @kindex ALIGN(@var{align})
5965 @kindex ALIGN(@var{exp},@var{align})
5966 @cindex round up location counter
5967 @cindex align location counter
5968 @cindex round up expression
5969 @cindex align expression
5970 Return the location counter (@code{.}) or arbitrary expression aligned
5971 to the next @var{align} boundary. The single operand @code{ALIGN}
5972 doesn't change the value of the location counter---it just does
5973 arithmetic on it. The two operand @code{ALIGN} allows an arbitrary
5974 expression to be aligned upwards (@code{ALIGN(@var{align})} is
5975 equivalent to @code{ALIGN(., @var{align})}).
5977 Here is an example which aligns the output @code{.data} section to the
5978 next @code{0x2000} byte boundary after the preceding section and sets a
5979 variable within the section to the next @code{0x8000} boundary after the
5984 .data ALIGN(0x2000): @{
5986 variable = ALIGN(0x8000);
5992 The first use of @code{ALIGN} in this example specifies the location of
5993 a section because it is used as the optional @var{address} attribute of
5994 a section definition (@pxref{Output Section Address}). The second use
5995 of @code{ALIGN} is used to defines the value of a symbol.
5997 The builtin function @code{NEXT} is closely related to @code{ALIGN}.
5999 @item ALIGNOF(@var{section})
6000 @kindex ALIGNOF(@var{section})
6001 @cindex section alignment
6002 Return the alignment in bytes of the named @var{section}, if that section has
6003 been allocated. If the section has not been allocated when this is
6004 evaluated, the linker will report an error. In the following example,
6005 the alignment of the @code{.output} section is stored as the first
6006 value in that section.
6011 LONG (ALIGNOF (.output))
6018 @item BLOCK(@var{exp})
6019 @kindex BLOCK(@var{exp})
6020 This is a synonym for @code{ALIGN}, for compatibility with older linker
6021 scripts. It is most often seen when setting the address of an output
6024 @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
6025 @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
6026 This is equivalent to either
6028 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1)))
6032 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - @var{commonpagesize})))
6035 depending on whether the latter uses fewer @var{commonpagesize} sized pages
6036 for the data segment (area between the result of this expression and
6037 @code{DATA_SEGMENT_END}) than the former or not.
6038 If the latter form is used, it means @var{commonpagesize} bytes of runtime
6039 memory will be saved at the expense of up to @var{commonpagesize} wasted
6040 bytes in the on-disk file.
6042 This expression can only be used directly in @code{SECTIONS} commands, not in
6043 any output section descriptions and only once in the linker script.
6044 @var{commonpagesize} should be less or equal to @var{maxpagesize} and should
6045 be the system page size the object wants to be optimized for (while still
6046 working on system page sizes up to @var{maxpagesize}).
6051 . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
6054 @item DATA_SEGMENT_END(@var{exp})
6055 @kindex DATA_SEGMENT_END(@var{exp})
6056 This defines the end of data segment for @code{DATA_SEGMENT_ALIGN}
6057 evaluation purposes.
6060 . = DATA_SEGMENT_END(.);
6063 @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6064 @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6065 This defines the end of the @code{PT_GNU_RELRO} segment when
6066 @samp{-z relro} option is used.
6067 When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END}
6068 does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that
6069 @var{exp} + @var{offset} is aligned to the most commonly used page
6070 boundary for particular target. If present in the linker script,
6071 it must always come in between @code{DATA_SEGMENT_ALIGN} and
6072 @code{DATA_SEGMENT_END}. Evaluates to the second argument plus any
6073 padding needed at the end of the @code{PT_GNU_RELRO} segment due to
6077 . = DATA_SEGMENT_RELRO_END(24, .);
6080 @item DEFINED(@var{symbol})
6081 @kindex DEFINED(@var{symbol})
6082 @cindex symbol defaults
6083 Return 1 if @var{symbol} is in the linker global symbol table and is
6084 defined before the statement using DEFINED in the script, otherwise
6085 return 0. You can use this function to provide
6086 default values for symbols. For example, the following script fragment
6087 shows how to set a global symbol @samp{begin} to the first location in
6088 the @samp{.text} section---but if a symbol called @samp{begin} already
6089 existed, its value is preserved:
6095 begin = DEFINED(begin) ? begin : . ;
6103 @item LENGTH(@var{memory})
6104 @kindex LENGTH(@var{memory})
6105 Return the length of the memory region named @var{memory}.
6107 @item LOADADDR(@var{section})
6108 @kindex LOADADDR(@var{section})
6109 @cindex section load address in expression
6110 Return the absolute LMA of the named @var{section}. (@pxref{Output
6113 @item LOG2CEIL(@var{exp})
6114 @kindex LOG2CEIL(@var{exp})
6115 Return the binary logarithm of @var{exp} rounded towards infinity.
6116 @code{LOG2CEIL(0)} returns 0.
6119 @item MAX(@var{exp1}, @var{exp2})
6120 Returns the maximum of @var{exp1} and @var{exp2}.
6123 @item MIN(@var{exp1}, @var{exp2})
6124 Returns the minimum of @var{exp1} and @var{exp2}.
6126 @item NEXT(@var{exp})
6127 @kindex NEXT(@var{exp})
6128 @cindex unallocated address, next
6129 Return the next unallocated address that is a multiple of @var{exp}.
6130 This function is closely related to @code{ALIGN(@var{exp})}; unless you
6131 use the @code{MEMORY} command to define discontinuous memory for the
6132 output file, the two functions are equivalent.
6134 @item ORIGIN(@var{memory})
6135 @kindex ORIGIN(@var{memory})
6136 Return the origin of the memory region named @var{memory}.
6138 @item SEGMENT_START(@var{segment}, @var{default})
6139 @kindex SEGMENT_START(@var{segment}, @var{default})
6140 Return the base address of the named @var{segment}. If an explicit
6141 value has already been given for this segment (with a command-line
6142 @samp{-T} option) then that value will be returned otherwise the value
6143 will be @var{default}. At present, the @samp{-T} command-line option
6144 can only be used to set the base address for the ``text'', ``data'', and
6145 ``bss'' sections, but you can use @code{SEGMENT_START} with any segment
6148 @item SIZEOF(@var{section})
6149 @kindex SIZEOF(@var{section})
6150 @cindex section size
6151 Return the size in bytes of the named @var{section}, if that section has
6152 been allocated. If the section has not been allocated when this is
6153 evaluated, the linker will report an error. In the following example,
6154 @code{symbol_1} and @code{symbol_2} are assigned identical values:
6163 symbol_1 = .end - .start ;
6164 symbol_2 = SIZEOF(.output);
6169 @item SIZEOF_HEADERS
6170 @itemx sizeof_headers
6171 @kindex SIZEOF_HEADERS
6173 Return the size in bytes of the output file's headers. This is
6174 information which appears at the start of the output file. You can use
6175 this number when setting the start address of the first section, if you
6176 choose, to facilitate paging.
6178 @cindex not enough room for program headers
6179 @cindex program headers, not enough room
6180 When producing an ELF output file, if the linker script uses the
6181 @code{SIZEOF_HEADERS} builtin function, the linker must compute the
6182 number of program headers before it has determined all the section
6183 addresses and sizes. If the linker later discovers that it needs
6184 additional program headers, it will report an error @samp{not enough
6185 room for program headers}. To avoid this error, you must avoid using
6186 the @code{SIZEOF_HEADERS} function, or you must rework your linker
6187 script to avoid forcing the linker to use additional program headers, or
6188 you must define the program headers yourself using the @code{PHDRS}
6189 command (@pxref{PHDRS}).
6192 @node Implicit Linker Scripts
6193 @section Implicit Linker Scripts
6194 @cindex implicit linker scripts
6195 If you specify a linker input file which the linker can not recognize as
6196 an object file or an archive file, it will try to read the file as a
6197 linker script. If the file can not be parsed as a linker script, the
6198 linker will report an error.
6200 An implicit linker script will not replace the default linker script.
6202 Typically an implicit linker script would contain only symbol
6203 assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION}
6206 Any input files read because of an implicit linker script will be read
6207 at the position in the command line where the implicit linker script was
6208 read. This can affect archive searching.
6211 @node Machine Dependent
6212 @chapter Machine Dependent Features
6214 @cindex machine dependencies
6215 @command{ld} has additional features on some platforms; the following
6216 sections describe them. Machines where @command{ld} has no additional
6217 functionality are not listed.
6221 * H8/300:: @command{ld} and the H8/300
6224 * i960:: @command{ld} and the Intel 960 family
6227 * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families
6230 * ARM:: @command{ld} and the ARM family
6233 * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF
6236 * M68K:: @command{ld} and the Motorola 68K family
6239 * MIPS:: @command{ld} and the MIPS family
6242 * MMIX:: @command{ld} and MMIX
6245 * MSP430:: @command{ld} and MSP430
6248 * NDS32:: @command{ld} and NDS32
6251 * Nios II:: @command{ld} and the Altera Nios II
6254 * PowerPC ELF32:: @command{ld} and PowerPC 32-bit ELF Support
6257 * PowerPC64 ELF64:: @command{ld} and PowerPC64 64-bit ELF Support
6260 * SPU ELF:: @command{ld} and SPU ELF Support
6263 * TI COFF:: @command{ld} and TI COFF
6266 * WIN32:: @command{ld} and WIN32 (cygwin/mingw)
6269 * Xtensa:: @command{ld} and Xtensa Processors
6280 @section @command{ld} and the H8/300
6282 @cindex H8/300 support
6283 For the H8/300, @command{ld} can perform these global optimizations when
6284 you specify the @samp{--relax} command-line option.
6287 @cindex relaxing on H8/300
6288 @item relaxing address modes
6289 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
6290 targets are within eight bits, and turns them into eight-bit
6291 program-counter relative @code{bsr} and @code{bra} instructions,
6294 @cindex synthesizing on H8/300
6295 @item synthesizing instructions
6296 @c FIXME: specifically mov.b, or any mov instructions really? -> mov.b only, at least on H8, H8H, H8S
6297 @command{ld} finds all @code{mov.b} instructions which use the
6298 sixteen-bit absolute address form, but refer to the top
6299 page of memory, and changes them to use the eight-bit address form.
6300 (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into
6301 @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the
6302 top page of memory).
6304 @command{ld} finds all @code{mov} instructions which use the register
6305 indirect with 32-bit displacement addressing mode, but use a small
6306 displacement inside 16-bit displacement range, and changes them to use
6307 the 16-bit displacement form. (That is: the linker turns @samp{mov.b
6308 @code{@@}@var{d}:32,ERx} into @samp{mov.b @code{@@}@var{d}:16,ERx}
6309 whenever the displacement @var{d} is in the 16 bit signed integer
6310 range. Only implemented in ELF-format ld).
6312 @item bit manipulation instructions
6313 @command{ld} finds all bit manipulation instructions like @code{band, bclr,
6314 biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor}
6315 which use 32 bit and 16 bit absolute address form, but refer to the top
6316 page of memory, and changes them to use the 8 bit address form.
6317 (That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into
6318 @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in
6319 the top page of memory).
6321 @item system control instructions
6322 @command{ld} finds all @code{ldc.w, stc.w} instructions which use the
6323 32 bit absolute address form, but refer to the top page of memory, and
6324 changes them to use 16 bit address form.
6325 (That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into
6326 @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in
6327 the top page of memory).
6337 @c This stuff is pointless to say unless you're especially concerned
6338 @c with Renesas chips; don't enable it for generic case, please.
6340 @chapter @command{ld} and Other Renesas Chips
6342 @command{ld} also supports the Renesas (formerly Hitachi) H8/300H,
6343 H8/500, and SH chips. No special features, commands, or command-line
6344 options are required for these chips.
6354 @section @command{ld} and the Intel 960 Family
6356 @cindex i960 support
6358 You can use the @samp{-A@var{architecture}} command line option to
6359 specify one of the two-letter names identifying members of the 960
6360 family; the option specifies the desired output target, and warns of any
6361 incompatible instructions in the input files. It also modifies the
6362 linker's search strategy for archive libraries, to support the use of
6363 libraries specific to each particular architecture, by including in the
6364 search loop names suffixed with the string identifying the architecture.
6366 For example, if your @command{ld} command line included @w{@samp{-ACA}} as
6367 well as @w{@samp{-ltry}}, the linker would look (in its built-in search
6368 paths, and in any paths you specify with @samp{-L}) for a library with
6381 The first two possibilities would be considered in any event; the last
6382 two are due to the use of @w{@samp{-ACA}}.
6384 You can meaningfully use @samp{-A} more than once on a command line, since
6385 the 960 architecture family allows combination of target architectures; each
6386 use will add another pair of name variants to search for when @w{@samp{-l}}
6387 specifies a library.
6389 @cindex @option{--relax} on i960
6390 @cindex relaxing on i960
6391 @command{ld} supports the @samp{--relax} option for the i960 family. If
6392 you specify @samp{--relax}, @command{ld} finds all @code{balx} and
6393 @code{calx} instructions whose targets are within 24 bits, and turns
6394 them into 24-bit program-counter relative @code{bal} and @code{cal}
6395 instructions, respectively. @command{ld} also turns @code{cal}
6396 instructions into @code{bal} instructions when it determines that the
6397 target subroutine is a leaf routine (that is, the target subroutine does
6398 not itself call any subroutines).
6415 @node M68HC11/68HC12
6416 @section @command{ld} and the Motorola 68HC11 and 68HC12 families
6418 @cindex M68HC11 and 68HC12 support
6420 @subsection Linker Relaxation
6422 For the Motorola 68HC11, @command{ld} can perform these global
6423 optimizations when you specify the @samp{--relax} command-line option.
6426 @cindex relaxing on M68HC11
6427 @item relaxing address modes
6428 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
6429 targets are within eight bits, and turns them into eight-bit
6430 program-counter relative @code{bsr} and @code{bra} instructions,
6433 @command{ld} also looks at all 16-bit extended addressing modes and
6434 transforms them in a direct addressing mode when the address is in
6435 page 0 (between 0 and 0x0ff).
6437 @item relaxing gcc instruction group
6438 When @command{gcc} is called with @option{-mrelax}, it can emit group
6439 of instructions that the linker can optimize to use a 68HC11 direct
6440 addressing mode. These instructions consists of @code{bclr} or
6441 @code{bset} instructions.
6445 @subsection Trampoline Generation
6447 @cindex trampoline generation on M68HC11
6448 @cindex trampoline generation on M68HC12
6449 For 68HC11 and 68HC12, @command{ld} can generate trampoline code to
6450 call a far function using a normal @code{jsr} instruction. The linker
6451 will also change the relocation to some far function to use the
6452 trampoline address instead of the function address. This is typically the
6453 case when a pointer to a function is taken. The pointer will in fact
6454 point to the function trampoline.
6462 @section @command{ld} and the ARM family
6464 @cindex ARM interworking support
6465 @kindex --support-old-code
6466 For the ARM, @command{ld} will generate code stubs to allow functions calls
6467 between ARM and Thumb code. These stubs only work with code that has
6468 been compiled and assembled with the @samp{-mthumb-interwork} command
6469 line option. If it is necessary to link with old ARM object files or
6470 libraries, which have not been compiled with the -mthumb-interwork
6471 option then the @samp{--support-old-code} command line switch should be
6472 given to the linker. This will make it generate larger stub functions
6473 which will work with non-interworking aware ARM code. Note, however,
6474 the linker does not support generating stubs for function calls to
6475 non-interworking aware Thumb code.
6477 @cindex thumb entry point
6478 @cindex entry point, thumb
6479 @kindex --thumb-entry=@var{entry}
6480 The @samp{--thumb-entry} switch is a duplicate of the generic
6481 @samp{--entry} switch, in that it sets the program's starting address.
6482 But it also sets the bottom bit of the address, so that it can be
6483 branched to using a BX instruction, and the program will start
6484 executing in Thumb mode straight away.
6486 @cindex PE import table prefixing
6487 @kindex --use-nul-prefixed-import-tables
6488 The @samp{--use-nul-prefixed-import-tables} switch is specifying, that
6489 the import tables idata4 and idata5 have to be generated with a zero
6490 element prefix for import libraries. This is the old style to generate
6491 import tables. By default this option is turned off.
6495 The @samp{--be8} switch instructs @command{ld} to generate BE8 format
6496 executables. This option is only valid when linking big-endian
6497 objects - ie ones which have been assembled with the @option{-EB}
6498 option. The resulting image will contain big-endian data and
6502 @kindex --target1-rel
6503 @kindex --target1-abs
6504 The @samp{R_ARM_TARGET1} relocation is typically used for entries in the
6505 @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32}
6506 or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel}
6507 and @samp{--target1-abs} switches override the default.
6510 @kindex --target2=@var{type}
6511 The @samp{--target2=type} switch overrides the default definition of the
6512 @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their
6513 meanings, and target defaults are as follows:
6516 @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi)
6518 @samp{R_ARM_ABS32} (arm*-*-symbianelf)
6520 @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd)
6525 The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF
6526 specification) enables objects compiled for the ARMv4 architecture to be
6527 interworking-safe when linked with other objects compiled for ARMv4t, but
6528 also allows pure ARMv4 binaries to be built from the same ARMv4 objects.
6530 In the latter case, the switch @option{--fix-v4bx} must be passed to the
6531 linker, which causes v4t @code{BX rM} instructions to be rewritten as
6532 @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction.
6534 In the former case, the switch should not be used, and @samp{R_ARM_V4BX}
6535 relocations are ignored.
6537 @cindex FIX_V4BX_INTERWORKING
6538 @kindex --fix-v4bx-interworking
6539 Replace @code{BX rM} instructions identified by @samp{R_ARM_V4BX}
6540 relocations with a branch to the following veneer:
6548 This allows generation of libraries/applications that work on ARMv4 cores
6549 and are still interworking safe. Note that the above veneer clobbers the
6550 condition flags, so may cause incorrect program behavior in rare cases.
6554 The @samp{--use-blx} switch enables the linker to use ARM/Thumb
6555 BLX instructions (available on ARMv5t and above) in various
6556 situations. Currently it is used to perform calls via the PLT from Thumb
6557 code using BLX rather than using BX and a mode-switching stub before
6558 each PLT entry. This should lead to such calls executing slightly faster.
6560 This option is enabled implicitly for SymbianOS, so there is no need to
6561 specify it if you are using that target.
6563 @cindex VFP11_DENORM_FIX
6564 @kindex --vfp11-denorm-fix
6565 The @samp{--vfp11-denorm-fix} switch enables a link-time workaround for a
6566 bug in certain VFP11 coprocessor hardware, which sometimes allows
6567 instructions with denorm operands (which must be handled by support code)
6568 to have those operands overwritten by subsequent instructions before
6569 the support code can read the intended values.
6571 The bug may be avoided in scalar mode if you allow at least one
6572 intervening instruction between a VFP11 instruction which uses a register
6573 and another instruction which writes to the same register, or at least two
6574 intervening instructions if vector mode is in use. The bug only affects
6575 full-compliance floating-point mode: you do not need this workaround if
6576 you are using "runfast" mode. Please contact ARM for further details.
6578 If you know you are using buggy VFP11 hardware, you can
6579 enable this workaround by specifying the linker option
6580 @samp{--vfp-denorm-fix=scalar} if you are using the VFP11 scalar
6581 mode only, or @samp{--vfp-denorm-fix=vector} if you are using
6582 vector mode (the latter also works for scalar code). The default is
6583 @samp{--vfp-denorm-fix=none}.
6585 If the workaround is enabled, instructions are scanned for
6586 potentially-troublesome sequences, and a veneer is created for each
6587 such sequence which may trigger the erratum. The veneer consists of the
6588 first instruction of the sequence and a branch back to the subsequent
6589 instruction. The original instruction is then replaced with a branch to
6590 the veneer. The extra cycles required to call and return from the veneer
6591 are sufficient to avoid the erratum in both the scalar and vector cases.
6593 @cindex ARM1176 erratum workaround
6594 @kindex --fix-arm1176
6595 @kindex --no-fix-arm1176
6596 The @samp{--fix-arm1176} switch enables a link-time workaround for an erratum
6597 in certain ARM1176 processors. The workaround is enabled by default if you
6598 are targeting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled
6599 unconditionally by specifying @samp{--no-fix-arm1176}.
6601 Further information is available in the ``ARM1176JZ-S and ARM1176JZF-S
6602 Programmer Advice Notice'' available on the ARM documentation website at:
6603 http://infocenter.arm.com/.
6605 @cindex NO_ENUM_SIZE_WARNING
6606 @kindex --no-enum-size-warning
6607 The @option{--no-enum-size-warning} switch prevents the linker from
6608 warning when linking object files that specify incompatible EABI
6609 enumeration size attributes. For example, with this switch enabled,
6610 linking of an object file using 32-bit enumeration values with another
6611 using enumeration values fitted into the smallest possible space will
6614 @cindex NO_WCHAR_SIZE_WARNING
6615 @kindex --no-wchar-size-warning
6616 The @option{--no-wchar-size-warning} switch prevents the linker from
6617 warning when linking object files that specify incompatible EABI
6618 @code{wchar_t} size attributes. For example, with this switch enabled,
6619 linking of an object file using 32-bit @code{wchar_t} values with another
6620 using 16-bit @code{wchar_t} values will not be diagnosed.
6623 @kindex --pic-veneer
6624 The @samp{--pic-veneer} switch makes the linker use PIC sequences for
6625 ARM/Thumb interworking veneers, even if the rest of the binary
6626 is not PIC. This avoids problems on uClinux targets where
6627 @samp{--emit-relocs} is used to generate relocatable binaries.
6629 @cindex STUB_GROUP_SIZE
6630 @kindex --stub-group-size=@var{N}
6631 The linker will automatically generate and insert small sequences of
6632 code into a linked ARM ELF executable whenever an attempt is made to
6633 perform a function call to a symbol that is too far away. The
6634 placement of these sequences of instructions - called stubs - is
6635 controlled by the command line option @option{--stub-group-size=N}.
6636 The placement is important because a poor choice can create a need for
6637 duplicate stubs, increasing the code size. The linker will try to
6638 group stubs together in order to reduce interruptions to the flow of
6639 code, but it needs guidance as to how big these groups should be and
6640 where they should be placed.
6642 The value of @samp{N}, the parameter to the
6643 @option{--stub-group-size=} option controls where the stub groups are
6644 placed. If it is negative then all stubs are placed after the first
6645 branch that needs them. If it is positive then the stubs can be
6646 placed either before or after the branches that need them. If the
6647 value of @samp{N} is 1 (either +1 or -1) then the linker will choose
6648 exactly where to place groups of stubs, using its built in heuristics.
6649 A value of @samp{N} greater than 1 (or smaller than -1) tells the
6650 linker that a single group of stubs can service at most @samp{N} bytes
6651 from the input sections.
6653 The default, if @option{--stub-group-size=} is not specified, is
6656 Farcalls stubs insertion is fully supported for the ARM-EABI target
6657 only, because it relies on object files properties not present
6660 @cindex Cortex-A8 erratum workaround
6661 @kindex --fix-cortex-a8
6662 @kindex --no-fix-cortex-a8
6663 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}.
6665 The erratum only affects Thumb-2 code. Please contact ARM for further details.
6667 @cindex Cortex-A53 erratum 835769 workaround
6668 @kindex --fix-cortex-a53-835769
6669 @kindex --no-fix-cortex-a53-835769
6670 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}.
6672 Please contact ARM for further details.
6674 @kindex --merge-exidx-entries
6675 @kindex --no-merge-exidx-entries
6676 @cindex Merging exidx entries
6677 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent exidx entries in debuginfo.
6680 @cindex 32-bit PLT entries
6681 The @samp{--long-plt} option enables the use of 16 byte PLT entries
6682 which support up to 4Gb of code. The default is to use 12 byte PLT
6683 entries which only support 512Mb of code.
6696 @section @command{ld} and HPPA 32-bit ELF Support
6697 @cindex HPPA multiple sub-space stubs
6698 @kindex --multi-subspace
6699 When generating a shared library, @command{ld} will by default generate
6700 import stubs suitable for use with a single sub-space application.
6701 The @samp{--multi-subspace} switch causes @command{ld} to generate export
6702 stubs, and different (larger) import stubs suitable for use with
6703 multiple sub-spaces.
6705 @cindex HPPA stub grouping
6706 @kindex --stub-group-size=@var{N}
6707 Long branch stubs and import/export stubs are placed by @command{ld} in
6708 stub sections located between groups of input sections.
6709 @samp{--stub-group-size} specifies the maximum size of a group of input
6710 sections handled by one stub section. Since branch offsets are signed,
6711 a stub section may serve two groups of input sections, one group before
6712 the stub section, and one group after it. However, when using
6713 conditional branches that require stubs, it may be better (for branch
6714 prediction) that stub sections only serve one group of input sections.
6715 A negative value for @samp{N} chooses this scheme, ensuring that
6716 branches to stubs always use a negative offset. Two special values of
6717 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
6718 @command{ld} to automatically size input section groups for the branch types
6719 detected, with the same behaviour regarding stub placement as other
6720 positive or negative values of @samp{N} respectively.
6722 Note that @samp{--stub-group-size} does not split input sections. A
6723 single input section larger than the group size specified will of course
6724 create a larger group (of one section). If input sections are too
6725 large, it may not be possible for a branch to reach its stub.
6738 @section @command{ld} and the Motorola 68K family
6740 @cindex Motorola 68K GOT generation
6741 @kindex --got=@var{type}
6742 The @samp{--got=@var{type}} option lets you choose the GOT generation scheme.
6743 The choices are @samp{single}, @samp{negative}, @samp{multigot} and
6744 @samp{target}. When @samp{target} is selected the linker chooses
6745 the default GOT generation scheme for the current target.
6746 @samp{single} tells the linker to generate a single GOT with
6747 entries only at non-negative offsets.
6748 @samp{negative} instructs the linker to generate a single GOT with
6749 entries at both negative and positive offsets. Not all environments
6751 @samp{multigot} allows the linker to generate several GOTs in the
6752 output file. All GOT references from a single input object
6753 file access the same GOT, but references from different input object
6754 files might access different GOTs. Not all environments support such GOTs.
6767 @section @command{ld} and the MIPS family
6769 @cindex MIPS microMIPS instruction choice selection
6772 The @samp{--insn32} and @samp{--no-insn32} options control the choice of
6773 microMIPS instructions used in code generated by the linker, such as that
6774 in the PLT or lazy binding stubs, or in relaxation. If @samp{--insn32} is
6775 used, then the linker only uses 32-bit instruction encodings. By default
6776 or if @samp{--no-insn32} is used, all instruction encodings are used,
6777 including 16-bit ones where possible.
6790 @section @code{ld} and MMIX
6791 For MMIX, there is a choice of generating @code{ELF} object files or
6792 @code{mmo} object files when linking. The simulator @code{mmix}
6793 understands the @code{mmo} format. The binutils @code{objcopy} utility
6794 can translate between the two formats.
6796 There is one special section, the @samp{.MMIX.reg_contents} section.
6797 Contents in this section is assumed to correspond to that of global
6798 registers, and symbols referring to it are translated to special symbols,
6799 equal to registers. In a final link, the start address of the
6800 @samp{.MMIX.reg_contents} section corresponds to the first allocated
6801 global register multiplied by 8. Register @code{$255} is not included in
6802 this section; it is always set to the program entry, which is at the
6803 symbol @code{Main} for @code{mmo} files.
6805 Global symbols with the prefix @code{__.MMIX.start.}, for example
6806 @code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special.
6807 The default linker script uses these to set the default start address
6810 Initial and trailing multiples of zero-valued 32-bit words in a section,
6811 are left out from an mmo file.
6824 @section @code{ld} and MSP430
6825 For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]}
6826 will select an appropriate linker script for selected MPU type. (To get a list of known MPUs
6827 just pass @samp{-m help} option to the linker).
6829 @cindex MSP430 extra sections
6830 The linker will recognize some extra sections which are MSP430 specific:
6833 @item @samp{.vectors}
6834 Defines a portion of ROM where interrupt vectors located.
6836 @item @samp{.bootloader}
6837 Defines the bootloader portion of the ROM (if applicable). Any code
6838 in this section will be uploaded to the MPU.
6840 @item @samp{.infomem}
6841 Defines an information memory section (if applicable). Any code in
6842 this section will be uploaded to the MPU.
6844 @item @samp{.infomemnobits}
6845 This is the same as the @samp{.infomem} section except that any code
6846 in this section will not be uploaded to the MPU.
6848 @item @samp{.noinit}
6849 Denotes a portion of RAM located above @samp{.bss} section.
6851 The last two sections are used by gcc.
6865 @section @code{ld} and NDS32
6866 @kindex relaxing on NDS32
6867 For NDS32, there are some options to select relaxation behavior. The linker
6868 relaxes objects according to these options.
6871 @item @samp{--m[no-]fp-as-gp}
6872 Disable/enable fp-as-gp relaxation.
6874 @item @samp{--mexport-symbols=FILE}
6875 Exporting symbols and their address into FILE as linker script.
6877 @item @samp{--m[no-]ex9}
6878 Disable/enable link-time EX9 relaxation.
6880 @item @samp{--mexport-ex9=FILE}
6881 Export the EX9 table after linking.
6883 @item @samp{--mimport-ex9=FILE}
6884 Import the Ex9 table for EX9 relaxation.
6886 @item @samp{--mupdate-ex9}
6887 Update the existing EX9 table.
6889 @item @samp{--mex9-limit=NUM}
6890 Maximum number of entries in the ex9 table.
6892 @item @samp{--mex9-loop-aware}
6893 Avoid generating the EX9 instruction inside the loop.
6895 @item @samp{--m[no-]ifc}
6896 Disable/enable the link-time IFC optimization.
6898 @item @samp{--mifc-loop-aware}
6899 Avoid generating the IFC instruction inside the loop.
6913 @section @command{ld} and the Altera Nios II
6914 @cindex Nios II call relaxation
6915 @kindex --relax on Nios II
6917 Call and immediate jump instructions on Nios II processors are limited to
6918 transferring control to addresses in the same 256MB memory segment,
6919 which may result in @command{ld} giving
6920 @samp{relocation truncated to fit} errors with very large programs.
6921 The command-line option @option{--relax} enables the generation of
6922 trampolines that can access the entire 32-bit address space for calls
6923 outside the normal @code{call} and @code{jmpi} address range. These
6924 trampolines are inserted at section boundaries, so may not themselves
6925 be reachable if an input section and its associated call trampolines are
6928 The @option{--relax} option is enabled by default unless @option{-r}
6929 is also specified. You can disable trampoline generation by using the
6930 @option{--no-relax} linker option. You can also disable this optimization
6931 locally by using the @samp{set .noat} directive in assembly-language
6932 source files, as the linker-inserted trampolines use the @code{at}
6933 register as a temporary.
6935 Note that the linker @option{--relax} option is independent of assembler
6936 relaxation options, and that using the GNU assembler's @option{-relax-all}
6937 option interferes with the linker's more selective call instruction relaxation.
6950 @section @command{ld} and PowerPC 32-bit ELF Support
6951 @cindex PowerPC long branches
6952 @kindex --relax on PowerPC
6953 Branches on PowerPC processors are limited to a signed 26-bit
6954 displacement, which may result in @command{ld} giving
6955 @samp{relocation truncated to fit} errors with very large programs.
6956 @samp{--relax} enables the generation of trampolines that can access
6957 the entire 32-bit address space. These trampolines are inserted at
6958 section boundaries, so may not themselves be reachable if an input
6959 section exceeds 33M in size. You may combine @samp{-r} and
6960 @samp{--relax} to add trampolines in a partial link. In that case
6961 both branches to undefined symbols and inter-section branches are also
6962 considered potentially out of range, and trampolines inserted.
6964 @cindex PowerPC ELF32 options
6969 Current PowerPC GCC accepts a @samp{-msecure-plt} option that
6970 generates code capable of using a newer PLT and GOT layout that has
6971 the security advantage of no executable section ever needing to be
6972 writable and no writable section ever being executable. PowerPC
6973 @command{ld} will generate this layout, including stubs to access the
6974 PLT, if all input files (including startup and static libraries) were
6975 compiled with @samp{-msecure-plt}. @samp{--bss-plt} forces the old
6976 BSS PLT (and GOT layout) which can give slightly better performance.
6978 @kindex --secure-plt
6980 @command{ld} will use the new PLT and GOT layout if it is linking new
6981 @samp{-fpic} or @samp{-fPIC} code, but does not do so automatically
6982 when linking non-PIC code. This option requests the new PLT and GOT
6983 layout. A warning will be given if some object file requires the old
6989 The new secure PLT and GOT are placed differently relative to other
6990 sections compared to older BSS PLT and GOT placement. The location of
6991 @code{.plt} must change because the new secure PLT is an initialized
6992 section while the old PLT is uninitialized. The reason for the
6993 @code{.got} change is more subtle: The new placement allows
6994 @code{.got} to be read-only in applications linked with
6995 @samp{-z relro -z now}. However, this placement means that
6996 @code{.sdata} cannot always be used in shared libraries, because the
6997 PowerPC ABI accesses @code{.sdata} in shared libraries from the GOT
6998 pointer. @samp{--sdata-got} forces the old GOT placement. PowerPC
6999 GCC doesn't use @code{.sdata} in shared libraries, so this option is
7000 really only useful for other compilers that may do so.
7002 @cindex PowerPC stub symbols
7003 @kindex --emit-stub-syms
7004 @item --emit-stub-syms
7005 This option causes @command{ld} to label linker stubs with a local
7006 symbol that encodes the stub type and destination.
7008 @cindex PowerPC TLS optimization
7009 @kindex --no-tls-optimize
7010 @item --no-tls-optimize
7011 PowerPC @command{ld} normally performs some optimization of code
7012 sequences used to access Thread-Local Storage. Use this option to
7013 disable the optimization.
7026 @node PowerPC64 ELF64
7027 @section @command{ld} and PowerPC64 64-bit ELF Support
7029 @cindex PowerPC64 ELF64 options
7031 @cindex PowerPC64 stub grouping
7032 @kindex --stub-group-size
7033 @item --stub-group-size
7034 Long branch stubs, PLT call stubs and TOC adjusting stubs are placed
7035 by @command{ld} in stub sections located between groups of input sections.
7036 @samp{--stub-group-size} specifies the maximum size of a group of input
7037 sections handled by one stub section. Since branch offsets are signed,
7038 a stub section may serve two groups of input sections, one group before
7039 the stub section, and one group after it. However, when using
7040 conditional branches that require stubs, it may be better (for branch
7041 prediction) that stub sections only serve one group of input sections.
7042 A negative value for @samp{N} chooses this scheme, ensuring that
7043 branches to stubs always use a negative offset. Two special values of
7044 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
7045 @command{ld} to automatically size input section groups for the branch types
7046 detected, with the same behaviour regarding stub placement as other
7047 positive or negative values of @samp{N} respectively.
7049 Note that @samp{--stub-group-size} does not split input sections. A
7050 single input section larger than the group size specified will of course
7051 create a larger group (of one section). If input sections are too
7052 large, it may not be possible for a branch to reach its stub.
7054 @cindex PowerPC64 stub symbols
7055 @kindex --emit-stub-syms
7056 @item --emit-stub-syms
7057 This option causes @command{ld} to label linker stubs with a local
7058 symbol that encodes the stub type and destination.
7060 @cindex PowerPC64 dot symbols
7062 @kindex --no-dotsyms
7063 @item --dotsyms, --no-dotsyms
7064 These two options control how @command{ld} interprets version patterns
7065 in a version script. Older PowerPC64 compilers emitted both a
7066 function descriptor symbol with the same name as the function, and a
7067 code entry symbol with the name prefixed by a dot (@samp{.}). To
7068 properly version a function @samp{foo}, the version script thus needs
7069 to control both @samp{foo} and @samp{.foo}. The option
7070 @samp{--dotsyms}, on by default, automatically adds the required
7071 dot-prefixed patterns. Use @samp{--no-dotsyms} to disable this
7074 @cindex PowerPC64 TLS optimization
7075 @kindex --no-tls-optimize
7076 @item --no-tls-optimize
7077 PowerPC64 @command{ld} normally performs some optimization of code
7078 sequences used to access Thread-Local Storage. Use this option to
7079 disable the optimization.
7081 @cindex PowerPC64 OPD optimization
7082 @kindex --no-opd-optimize
7083 @item --no-opd-optimize
7084 PowerPC64 @command{ld} normally removes @code{.opd} section entries
7085 corresponding to deleted link-once functions, or functions removed by
7086 the action of @samp{--gc-sections} or linker script @code{/DISCARD/}.
7087 Use this option to disable @code{.opd} optimization.
7089 @cindex PowerPC64 OPD spacing
7090 @kindex --non-overlapping-opd
7091 @item --non-overlapping-opd
7092 Some PowerPC64 compilers have an option to generate compressed
7093 @code{.opd} entries spaced 16 bytes apart, overlapping the third word,
7094 the static chain pointer (unused in C) with the first word of the next
7095 entry. This option expands such entries to the full 24 bytes.
7097 @cindex PowerPC64 TOC optimization
7098 @kindex --no-toc-optimize
7099 @item --no-toc-optimize
7100 PowerPC64 @command{ld} normally removes unused @code{.toc} section
7101 entries. Such entries are detected by examining relocations that
7102 reference the TOC in code sections. A reloc in a deleted code section
7103 marks a TOC word as unneeded, while a reloc in a kept code section
7104 marks a TOC word as needed. Since the TOC may reference itself, TOC
7105 relocs are also examined. TOC words marked as both needed and
7106 unneeded will of course be kept. TOC words without any referencing
7107 reloc are assumed to be part of a multi-word entry, and are kept or
7108 discarded as per the nearest marked preceding word. This works
7109 reliably for compiler generated code, but may be incorrect if assembly
7110 code is used to insert TOC entries. Use this option to disable the
7113 @cindex PowerPC64 multi-TOC
7114 @kindex --no-multi-toc
7115 @item --no-multi-toc
7116 If given any toc option besides @code{-mcmodel=medium} or
7117 @code{-mcmodel=large}, PowerPC64 GCC generates code for a TOC model
7119 entries are accessed with a 16-bit offset from r2. This limits the
7120 total TOC size to 64K. PowerPC64 @command{ld} extends this limit by
7121 grouping code sections such that each group uses less than 64K for its
7122 TOC entries, then inserts r2 adjusting stubs between inter-group
7123 calls. @command{ld} does not split apart input sections, so cannot
7124 help if a single input file has a @code{.toc} section that exceeds
7125 64K, most likely from linking multiple files with @command{ld -r}.
7126 Use this option to turn off this feature.
7128 @cindex PowerPC64 TOC sorting
7129 @kindex --no-toc-sort
7131 By default, @command{ld} sorts TOC sections so that those whose file
7132 happens to have a section called @code{.init} or @code{.fini} are
7133 placed first, followed by TOC sections referenced by code generated
7134 with PowerPC64 gcc's @code{-mcmodel=small}, and lastly TOC sections
7135 referenced only by code generated with PowerPC64 gcc's
7136 @code{-mcmodel=medium} or @code{-mcmodel=large} options. Doing this
7137 results in better TOC grouping for multi-TOC. Use this option to turn
7140 @cindex PowerPC64 PLT stub alignment
7142 @kindex --no-plt-align
7144 @itemx --no-plt-align
7145 Use these options to control whether individual PLT call stubs are
7146 padded so that they don't cross a 32-byte boundary, or to the
7147 specified power of two boundary when using @code{--plt-align=}. Note
7148 that this isn't alignment in the usual sense. By default PLT call
7149 stubs are packed tightly.
7151 @cindex PowerPC64 PLT call stub static chain
7152 @kindex --plt-static-chain
7153 @kindex --no-plt-static-chain
7154 @item --plt-static-chain
7155 @itemx --no-plt-static-chain
7156 Use these options to control whether PLT call stubs load the static
7157 chain pointer (r11). @code{ld} defaults to not loading the static
7158 chain since there is never any need to do so on a PLT call.
7160 @cindex PowerPC64 PLT call stub thread safety
7161 @kindex --plt-thread-safe
7162 @kindex --no-plt-thread-safe
7163 @item --plt-thread-safe
7164 @itemx --no-thread-safe
7165 With power7's weakly ordered memory model, it is possible when using
7166 lazy binding for ld.so to update a plt entry in one thread and have
7167 another thread see the individual plt entry words update in the wrong
7168 order, despite ld.so carefully writing in the correct order and using
7169 memory write barriers. To avoid this we need some sort of read
7170 barrier in the call stub, or use LD_BIND_NOW=1. By default, @code{ld}
7171 looks for calls to commonly used functions that create threads, and if
7172 seen, adds the necessary barriers. Use these options to change the
7187 @section @command{ld} and SPU ELF Support
7189 @cindex SPU ELF options
7195 This option marks an executable as a PIC plugin module.
7197 @cindex SPU overlays
7198 @kindex --no-overlays
7200 Normally, @command{ld} recognizes calls to functions within overlay
7201 regions, and redirects such calls to an overlay manager via a stub.
7202 @command{ld} also provides a built-in overlay manager. This option
7203 turns off all this special overlay handling.
7205 @cindex SPU overlay stub symbols
7206 @kindex --emit-stub-syms
7207 @item --emit-stub-syms
7208 This option causes @command{ld} to label overlay stubs with a local
7209 symbol that encodes the stub type and destination.
7211 @cindex SPU extra overlay stubs
7212 @kindex --extra-overlay-stubs
7213 @item --extra-overlay-stubs
7214 This option causes @command{ld} to add overlay call stubs on all
7215 function calls out of overlay regions. Normally stubs are not added
7216 on calls to non-overlay regions.
7218 @cindex SPU local store size
7219 @kindex --local-store=lo:hi
7220 @item --local-store=lo:hi
7221 @command{ld} usually checks that a final executable for SPU fits in
7222 the address range 0 to 256k. This option may be used to change the
7223 range. Disable the check entirely with @option{--local-store=0:0}.
7226 @kindex --stack-analysis
7227 @item --stack-analysis
7228 SPU local store space is limited. Over-allocation of stack space
7229 unnecessarily limits space available for code and data, while
7230 under-allocation results in runtime failures. If given this option,
7231 @command{ld} will provide an estimate of maximum stack usage.
7232 @command{ld} does this by examining symbols in code sections to
7233 determine the extents of functions, and looking at function prologues
7234 for stack adjusting instructions. A call-graph is created by looking
7235 for relocations on branch instructions. The graph is then searched
7236 for the maximum stack usage path. Note that this analysis does not
7237 find calls made via function pointers, and does not handle recursion
7238 and other cycles in the call graph. Stack usage may be
7239 under-estimated if your code makes such calls. Also, stack usage for
7240 dynamic allocation, e.g. alloca, will not be detected. If a link map
7241 is requested, detailed information about each function's stack usage
7242 and calls will be given.
7245 @kindex --emit-stack-syms
7246 @item --emit-stack-syms
7247 This option, if given along with @option{--stack-analysis} will result
7248 in @command{ld} emitting stack sizing symbols for each function.
7249 These take the form @code{__stack_<function_name>} for global
7250 functions, and @code{__stack_<number>_<function_name>} for static
7251 functions. @code{<number>} is the section id in hex. The value of
7252 such symbols is the stack requirement for the corresponding function.
7253 The symbol size will be zero, type @code{STT_NOTYPE}, binding
7254 @code{STB_LOCAL}, and section @code{SHN_ABS}.
7268 @section @command{ld}'s Support for Various TI COFF Versions
7269 @cindex TI COFF versions
7270 @kindex --format=@var{version}
7271 The @samp{--format} switch allows selection of one of the various
7272 TI COFF versions. The latest of this writing is 2; versions 0 and 1 are
7273 also supported. The TI COFF versions also vary in header byte-order
7274 format; @command{ld} will read any version or byte order, but the output
7275 header format depends on the default specified by the specific target.
7288 @section @command{ld} and WIN32 (cygwin/mingw)
7290 This section describes some of the win32 specific @command{ld} issues.
7291 See @ref{Options,,Command Line Options} for detailed description of the
7292 command line options mentioned here.
7295 @cindex import libraries
7296 @item import libraries
7297 The standard Windows linker creates and uses so-called import
7298 libraries, which contains information for linking to dll's. They are
7299 regular static archives and are handled as any other static
7300 archive. The cygwin and mingw ports of @command{ld} have specific
7301 support for creating such libraries provided with the
7302 @samp{--out-implib} command line option.
7304 @item exporting DLL symbols
7305 @cindex exporting DLL symbols
7306 The cygwin/mingw @command{ld} has several ways to export symbols for dll's.
7309 @item using auto-export functionality
7310 @cindex using auto-export functionality
7311 By default @command{ld} exports symbols with the auto-export functionality,
7312 which is controlled by the following command line options:
7315 @item --export-all-symbols [This is the default]
7316 @item --exclude-symbols
7317 @item --exclude-libs
7318 @item --exclude-modules-for-implib
7319 @item --version-script
7322 When auto-export is in operation, @command{ld} will export all the non-local
7323 (global and common) symbols it finds in a DLL, with the exception of a few
7324 symbols known to belong to the system's runtime and libraries. As it will
7325 often not be desirable to export all of a DLL's symbols, which may include
7326 private functions that are not part of any public interface, the command-line
7327 options listed above may be used to filter symbols out from the list for
7328 exporting. The @samp{--output-def} option can be used in order to see the
7329 final list of exported symbols with all exclusions taken into effect.
7331 If @samp{--export-all-symbols} is not given explicitly on the
7332 command line, then the default auto-export behavior will be @emph{disabled}
7333 if either of the following are true:
7336 @item A DEF file is used.
7337 @item Any symbol in any object file was marked with the __declspec(dllexport) attribute.
7340 @item using a DEF file
7341 @cindex using a DEF file
7342 Another way of exporting symbols is using a DEF file. A DEF file is
7343 an ASCII file containing definitions of symbols which should be
7344 exported when a dll is created. Usually it is named @samp{<dll
7345 name>.def} and is added as any other object file to the linker's
7346 command line. The file's name must end in @samp{.def} or @samp{.DEF}.
7349 gcc -o <output> <objectfiles> <dll name>.def
7352 Using a DEF file turns off the normal auto-export behavior, unless the
7353 @samp{--export-all-symbols} option is also used.
7355 Here is an example of a DEF file for a shared library called @samp{xyz.dll}:
7358 LIBRARY "xyz.dll" BASE=0x20000000
7364 another_foo = abc.dll.afoo
7370 This example defines a DLL with a non-default base address and seven
7371 symbols in the export table. The third exported symbol @code{_bar} is an
7372 alias for the second. The fourth symbol, @code{another_foo} is resolved
7373 by "forwarding" to another module and treating it as an alias for
7374 @code{afoo} exported from the DLL @samp{abc.dll}. The final symbol
7375 @code{var1} is declared to be a data object. The @samp{doo} symbol in
7376 export library is an alias of @samp{foo}, which gets the string name
7377 in export table @samp{foo2}. The @samp{eoo} symbol is an data export
7378 symbol, which gets in export table the name @samp{var1}.
7380 The optional @code{LIBRARY <name>} command indicates the @emph{internal}
7381 name of the output DLL. If @samp{<name>} does not include a suffix,
7382 the default library suffix, @samp{.DLL} is appended.
7384 When the .DEF file is used to build an application, rather than a
7385 library, the @code{NAME <name>} command should be used instead of
7386 @code{LIBRARY}. If @samp{<name>} does not include a suffix, the default
7387 executable suffix, @samp{.EXE} is appended.
7389 With either @code{LIBRARY <name>} or @code{NAME <name>} the optional
7390 specification @code{BASE = <number>} may be used to specify a
7391 non-default base address for the image.
7393 If neither @code{LIBRARY <name>} nor @code{NAME <name>} is specified,
7394 or they specify an empty string, the internal name is the same as the
7395 filename specified on the command line.
7397 The complete specification of an export symbol is:
7401 ( ( ( <name1> [ = <name2> ] )
7402 | ( <name1> = <module-name> . <external-name>))
7403 [ @@ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
7406 Declares @samp{<name1>} as an exported symbol from the DLL, or declares
7407 @samp{<name1>} as an exported alias for @samp{<name2>}; or declares
7408 @samp{<name1>} as a "forward" alias for the symbol
7409 @samp{<external-name>} in the DLL @samp{<module-name>}.
7410 Optionally, the symbol may be exported by the specified ordinal
7411 @samp{<integer>} alias. The optional @samp{<name3>} is the to be used
7412 string in import/export table for the symbol.
7414 The optional keywords that follow the declaration indicate:
7416 @code{NONAME}: Do not put the symbol name in the DLL's export table. It
7417 will still be exported by its ordinal alias (either the value specified
7418 by the .def specification or, otherwise, the value assigned by the
7419 linker). The symbol name, however, does remain visible in the import
7420 library (if any), unless @code{PRIVATE} is also specified.
7422 @code{DATA}: The symbol is a variable or object, rather than a function.
7423 The import lib will export only an indirect reference to @code{foo} as
7424 the symbol @code{_imp__foo} (ie, @code{foo} must be resolved as
7427 @code{CONSTANT}: Like @code{DATA}, but put the undecorated @code{foo} as
7428 well as @code{_imp__foo} into the import library. Both refer to the
7429 read-only import address table's pointer to the variable, not to the
7430 variable itself. This can be dangerous. If the user code fails to add
7431 the @code{dllimport} attribute and also fails to explicitly add the
7432 extra indirection that the use of the attribute enforces, the
7433 application will behave unexpectedly.
7435 @code{PRIVATE}: Put the symbol in the DLL's export table, but do not put
7436 it into the static import library used to resolve imports at link time. The
7437 symbol can still be imported using the @code{LoadLibrary/GetProcAddress}
7438 API at runtime or by by using the GNU ld extension of linking directly to
7439 the DLL without an import library.
7441 See ld/deffilep.y in the binutils sources for the full specification of
7442 other DEF file statements
7444 @cindex creating a DEF file
7445 While linking a shared dll, @command{ld} is able to create a DEF file
7446 with the @samp{--output-def <file>} command line option.
7448 @item Using decorations
7449 @cindex Using decorations
7450 Another way of marking symbols for export is to modify the source code
7451 itself, so that when building the DLL each symbol to be exported is
7455 __declspec(dllexport) int a_variable
7456 __declspec(dllexport) void a_function(int with_args)
7459 All such symbols will be exported from the DLL. If, however,
7460 any of the object files in the DLL contain symbols decorated in
7461 this way, then the normal auto-export behavior is disabled, unless
7462 the @samp{--export-all-symbols} option is also used.
7464 Note that object files that wish to access these symbols must @emph{not}
7465 decorate them with dllexport. Instead, they should use dllimport,
7469 __declspec(dllimport) int a_variable
7470 __declspec(dllimport) void a_function(int with_args)
7473 This complicates the structure of library header files, because
7474 when included by the library itself the header must declare the
7475 variables and functions as dllexport, but when included by client
7476 code the header must declare them as dllimport. There are a number
7477 of idioms that are typically used to do this; often client code can
7478 omit the __declspec() declaration completely. See
7479 @samp{--enable-auto-import} and @samp{automatic data imports} for more
7483 @cindex automatic data imports
7484 @item automatic data imports
7485 The standard Windows dll format supports data imports from dlls only
7486 by adding special decorations (dllimport/dllexport), which let the
7487 compiler produce specific assembler instructions to deal with this
7488 issue. This increases the effort necessary to port existing Un*x
7489 code to these platforms, especially for large
7490 c++ libraries and applications. The auto-import feature, which was
7491 initially provided by Paul Sokolovsky, allows one to omit the
7492 decorations to achieve a behavior that conforms to that on POSIX/Un*x
7493 platforms. This feature is enabled with the @samp{--enable-auto-import}
7494 command-line option, although it is enabled by default on cygwin/mingw.
7495 The @samp{--enable-auto-import} option itself now serves mainly to
7496 suppress any warnings that are ordinarily emitted when linked objects
7497 trigger the feature's use.
7499 auto-import of variables does not always work flawlessly without
7500 additional assistance. Sometimes, you will see this message
7502 "variable '<var>' can't be auto-imported. Please read the
7503 documentation for ld's @code{--enable-auto-import} for details."
7505 The @samp{--enable-auto-import} documentation explains why this error
7506 occurs, and several methods that can be used to overcome this difficulty.
7507 One of these methods is the @emph{runtime pseudo-relocs} feature, described
7510 @cindex runtime pseudo-relocation
7511 For complex variables imported from DLLs (such as structs or classes),
7512 object files typically contain a base address for the variable and an
7513 offset (@emph{addend}) within the variable--to specify a particular
7514 field or public member, for instance. Unfortunately, the runtime loader used
7515 in win32 environments is incapable of fixing these references at runtime
7516 without the additional information supplied by dllimport/dllexport decorations.
7517 The standard auto-import feature described above is unable to resolve these
7520 The @samp{--enable-runtime-pseudo-relocs} switch allows these references to
7521 be resolved without error, while leaving the task of adjusting the references
7522 themselves (with their non-zero addends) to specialized code provided by the
7523 runtime environment. Recent versions of the cygwin and mingw environments and
7524 compilers provide this runtime support; older versions do not. However, the
7525 support is only necessary on the developer's platform; the compiled result will
7526 run without error on an older system.
7528 @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly
7531 @cindex direct linking to a dll
7532 @item direct linking to a dll
7533 The cygwin/mingw ports of @command{ld} support the direct linking,
7534 including data symbols, to a dll without the usage of any import
7535 libraries. This is much faster and uses much less memory than does the
7536 traditional import library method, especially when linking large
7537 libraries or applications. When @command{ld} creates an import lib, each
7538 function or variable exported from the dll is stored in its own bfd, even
7539 though a single bfd could contain many exports. The overhead involved in
7540 storing, loading, and processing so many bfd's is quite large, and explains the
7541 tremendous time, memory, and storage needed to link against particularly
7542 large or complex libraries when using import libs.
7544 Linking directly to a dll uses no extra command-line switches other than
7545 @samp{-L} and @samp{-l}, because @command{ld} already searches for a number
7546 of names to match each library. All that is needed from the developer's
7547 perspective is an understanding of this search, in order to force ld to
7548 select the dll instead of an import library.
7551 For instance, when ld is called with the argument @samp{-lxxx} it will attempt
7552 to find, in the first directory of its search path,
7564 before moving on to the next directory in the search path.
7566 (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll},
7567 where @samp{<prefix>} is set by the @command{ld} option
7568 @samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec
7569 file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for
7572 Other win32-based unix environments, such as mingw or pw32, may use other
7573 @samp{<prefix>}es, although at present only cygwin makes use of this feature. It
7574 was originally intended to help avoid name conflicts among dll's built for the
7575 various win32/un*x environments, so that (for example) two versions of a zlib dll
7576 could coexist on the same machine.
7578 The generic cygwin/mingw path layout uses a @samp{bin} directory for
7579 applications and dll's and a @samp{lib} directory for the import
7580 libraries (using cygwin nomenclature):
7586 libxxx.dll.a (in case of dll's)
7587 libxxx.a (in case of static archive)
7590 Linking directly to a dll without using the import library can be
7593 1. Use the dll directly by adding the @samp{bin} path to the link line
7595 gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
7598 However, as the dll's often have version numbers appended to their names
7599 (@samp{cygncurses-5.dll}) this will often fail, unless one specifies
7600 @samp{-L../bin -lncurses-5} to include the version. Import libs are generally
7601 not versioned, and do not have this difficulty.
7603 2. Create a symbolic link from the dll to a file in the @samp{lib}
7604 directory according to the above mentioned search pattern. This
7605 should be used to avoid unwanted changes in the tools needed for
7609 ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
7612 Then you can link without any make environment changes.
7615 gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
7618 This technique also avoids the version number problems, because the following is
7625 libxxx.dll.a -> ../bin/cygxxx-5.dll
7628 Linking directly to a dll without using an import lib will work
7629 even when auto-import features are exercised, and even when
7630 @samp{--enable-runtime-pseudo-relocs} is used.
7632 Given the improvements in speed and memory usage, one might justifiably
7633 wonder why import libraries are used at all. There are three reasons:
7635 1. Until recently, the link-directly-to-dll functionality did @emph{not}
7636 work with auto-imported data.
7638 2. Sometimes it is necessary to include pure static objects within the
7639 import library (which otherwise contains only bfd's for indirection
7640 symbols that point to the exports of a dll). Again, the import lib
7641 for the cygwin kernel makes use of this ability, and it is not
7642 possible to do this without an import lib.
7644 3. Symbol aliases can only be resolved using an import lib. This is
7645 critical when linking against OS-supplied dll's (eg, the win32 API)
7646 in which symbols are usually exported as undecorated aliases of their
7647 stdcall-decorated assembly names.
7649 So, import libs are not going away. But the ability to replace
7650 true import libs with a simple symbolic link to (or a copy of)
7651 a dll, in many cases, is a useful addition to the suite of tools
7652 binutils makes available to the win32 developer. Given the
7653 massive improvements in memory requirements during linking, storage
7654 requirements, and linking speed, we expect that many developers
7655 will soon begin to use this feature whenever possible.
7657 @item symbol aliasing
7659 @item adding additional names
7660 Sometimes, it is useful to export symbols with additional names.
7661 A symbol @samp{foo} will be exported as @samp{foo}, but it can also be
7662 exported as @samp{_foo} by using special directives in the DEF file
7663 when creating the dll. This will affect also the optional created
7664 import library. Consider the following DEF file:
7667 LIBRARY "xyz.dll" BASE=0x61000000
7674 The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}.
7676 Another method for creating a symbol alias is to create it in the
7677 source code using the "weak" attribute:
7680 void foo () @{ /* Do something. */; @}
7681 void _foo () __attribute__ ((weak, alias ("foo")));
7684 See the gcc manual for more information about attributes and weak
7687 @item renaming symbols
7688 Sometimes it is useful to rename exports. For instance, the cygwin
7689 kernel does this regularly. A symbol @samp{_foo} can be exported as
7690 @samp{foo} but not as @samp{_foo} by using special directives in the
7691 DEF file. (This will also affect the import library, if it is
7692 created). In the following example:
7695 LIBRARY "xyz.dll" BASE=0x61000000
7701 The line @samp{_foo = foo} maps the exported symbol @samp{foo} to
7705 Note: using a DEF file disables the default auto-export behavior,
7706 unless the @samp{--export-all-symbols} command line option is used.
7707 If, however, you are trying to rename symbols, then you should list
7708 @emph{all} desired exports in the DEF file, including the symbols
7709 that are not being renamed, and do @emph{not} use the
7710 @samp{--export-all-symbols} option. If you list only the
7711 renamed symbols in the DEF file, and use @samp{--export-all-symbols}
7712 to handle the other symbols, then the both the new names @emph{and}
7713 the original names for the renamed symbols will be exported.
7714 In effect, you'd be aliasing those symbols, not renaming them,
7715 which is probably not what you wanted.
7717 @cindex weak externals
7718 @item weak externals
7719 The Windows object format, PE, specifies a form of weak symbols called
7720 weak externals. When a weak symbol is linked and the symbol is not
7721 defined, the weak symbol becomes an alias for some other symbol. There
7722 are three variants of weak externals:
7724 @item Definition is searched for in objects and libraries, historically
7725 called lazy externals.
7726 @item Definition is searched for only in other objects, not in libraries.
7727 This form is not presently implemented.
7728 @item No search; the symbol is an alias. This form is not presently
7731 As a GNU extension, weak symbols that do not specify an alternate symbol
7732 are supported. If the symbol is undefined when linking, the symbol
7733 uses a default value.
7735 @cindex aligned common symbols
7736 @item aligned common symbols
7737 As a GNU extension to the PE file format, it is possible to specify the
7738 desired alignment for a common symbol. This information is conveyed from
7739 the assembler or compiler to the linker by means of GNU-specific commands
7740 carried in the object file's @samp{.drectve} section, which are recognized
7741 by @command{ld} and respected when laying out the common symbols. Native
7742 tools will be able to process object files employing this GNU extension,
7743 but will fail to respect the alignment instructions, and may issue noisy
7744 warnings about unknown linker directives.
7759 @section @code{ld} and Xtensa Processors
7761 @cindex Xtensa processors
7762 The default @command{ld} behavior for Xtensa processors is to interpret
7763 @code{SECTIONS} commands so that lists of explicitly named sections in a
7764 specification with a wildcard file will be interleaved when necessary to
7765 keep literal pools within the range of PC-relative load offsets. For
7766 example, with the command:
7778 @command{ld} may interleave some of the @code{.literal}
7779 and @code{.text} sections from different object files to ensure that the
7780 literal pools are within the range of PC-relative load offsets. A valid
7781 interleaving might place the @code{.literal} sections from an initial
7782 group of files followed by the @code{.text} sections of that group of
7783 files. Then, the @code{.literal} sections from the rest of the files
7784 and the @code{.text} sections from the rest of the files would follow.
7786 @cindex @option{--relax} on Xtensa
7787 @cindex relaxing on Xtensa
7788 Relaxation is enabled by default for the Xtensa version of @command{ld} and
7789 provides two important link-time optimizations. The first optimization
7790 is to combine identical literal values to reduce code size. A redundant
7791 literal will be removed and all the @code{L32R} instructions that use it
7792 will be changed to reference an identical literal, as long as the
7793 location of the replacement literal is within the offset range of all
7794 the @code{L32R} instructions. The second optimization is to remove
7795 unnecessary overhead from assembler-generated ``longcall'' sequences of
7796 @code{L32R}/@code{CALLX@var{n}} when the target functions are within
7797 range of direct @code{CALL@var{n}} instructions.
7799 For each of these cases where an indirect call sequence can be optimized
7800 to a direct call, the linker will change the @code{CALLX@var{n}}
7801 instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R}
7802 instruction, and remove the literal referenced by the @code{L32R}
7803 instruction if it is not used for anything else. Removing the
7804 @code{L32R} instruction always reduces code size but can potentially
7805 hurt performance by changing the alignment of subsequent branch targets.
7806 By default, the linker will always preserve alignments, either by
7807 switching some instructions between 24-bit encodings and the equivalent
7808 density instructions or by inserting a no-op in place of the @code{L32R}
7809 instruction that was removed. If code size is more important than
7810 performance, the @option{--size-opt} option can be used to prevent the
7811 linker from widening density instructions or inserting no-ops, except in
7812 a few cases where no-ops are required for correctness.
7814 The following Xtensa-specific command-line options can be used to
7817 @cindex Xtensa options
7820 When optimizing indirect calls to direct calls, optimize for code size
7821 more than performance. With this option, the linker will not insert
7822 no-ops or widen density instructions to preserve branch target
7823 alignment. There may still be some cases where no-ops are required to
7824 preserve the correctness of the code.
7832 @ifclear SingleFormat
7837 @cindex object file management
7838 @cindex object formats available
7840 The linker accesses object and archive files using the BFD libraries.
7841 These libraries allow the linker to use the same routines to operate on
7842 object files whatever the object file format. A different object file
7843 format can be supported simply by creating a new BFD back end and adding
7844 it to the library. To conserve runtime memory, however, the linker and
7845 associated tools are usually configured to support only a subset of the
7846 object file formats available. You can use @code{objdump -i}
7847 (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to
7848 list all the formats available for your configuration.
7850 @cindex BFD requirements
7851 @cindex requirements for BFD
7852 As with most implementations, BFD is a compromise between
7853 several conflicting requirements. The major factor influencing
7854 BFD design was efficiency: any time used converting between
7855 formats is time which would not have been spent had BFD not
7856 been involved. This is partly offset by abstraction payback; since
7857 BFD simplifies applications and back ends, more time and care
7858 may be spent optimizing algorithms for a greater speed.
7860 One minor artifact of the BFD solution which you should bear in
7861 mind is the potential for information loss. There are two places where
7862 useful information can be lost using the BFD mechanism: during
7863 conversion and during output. @xref{BFD information loss}.
7866 * BFD outline:: How it works: an outline of BFD
7870 @section How It Works: An Outline of BFD
7871 @cindex opening object files
7872 @include bfdsumm.texi
7875 @node Reporting Bugs
7876 @chapter Reporting Bugs
7877 @cindex bugs in @command{ld}
7878 @cindex reporting bugs in @command{ld}
7880 Your bug reports play an essential role in making @command{ld} reliable.
7882 Reporting a bug may help you by bringing a solution to your problem, or
7883 it may not. But in any case the principal function of a bug report is
7884 to help the entire community by making the next version of @command{ld}
7885 work better. Bug reports are your contribution to the maintenance of
7888 In order for a bug report to serve its purpose, you must include the
7889 information that enables us to fix the bug.
7892 * Bug Criteria:: Have you found a bug?
7893 * Bug Reporting:: How to report bugs
7897 @section Have You Found a Bug?
7898 @cindex bug criteria
7900 If you are not sure whether you have found a bug, here are some guidelines:
7903 @cindex fatal signal
7904 @cindex linker crash
7905 @cindex crash of linker
7907 If the linker gets a fatal signal, for any input whatever, that is a
7908 @command{ld} bug. Reliable linkers never crash.
7910 @cindex error on valid input
7912 If @command{ld} produces an error message for valid input, that is a bug.
7914 @cindex invalid input
7916 If @command{ld} does not produce an error message for invalid input, that
7917 may be a bug. In the general case, the linker can not verify that
7918 object files are correct.
7921 If you are an experienced user of linkers, your suggestions for
7922 improvement of @command{ld} are welcome in any case.
7926 @section How to Report Bugs
7928 @cindex @command{ld} bugs, reporting
7930 A number of companies and individuals offer support for @sc{gnu}
7931 products. If you obtained @command{ld} from a support organization, we
7932 recommend you contact that organization first.
7934 You can find contact information for many support companies and
7935 individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs
7939 Otherwise, send bug reports for @command{ld} to
7943 The fundamental principle of reporting bugs usefully is this:
7944 @strong{report all the facts}. If you are not sure whether to state a
7945 fact or leave it out, state it!
7947 Often people omit facts because they think they know what causes the
7948 problem and assume that some details do not matter. Thus, you might
7949 assume that the name of a symbol you use in an example does not
7950 matter. Well, probably it does not, but one cannot be sure. Perhaps
7951 the bug is a stray memory reference which happens to fetch from the
7952 location where that name is stored in memory; perhaps, if the name
7953 were different, the contents of that location would fool the linker
7954 into doing the right thing despite the bug. Play it safe and give a
7955 specific, complete example. That is the easiest thing for you to do,
7956 and the most helpful.
7958 Keep in mind that the purpose of a bug report is to enable us to fix
7959 the bug if it is new to us. Therefore, always write your bug reports
7960 on the assumption that the bug has not been reported previously.
7962 Sometimes people give a few sketchy facts and ask, ``Does this ring a
7963 bell?'' This cannot help us fix a bug, so it is basically useless. We
7964 respond by asking for enough details to enable us to investigate.
7965 You might as well expedite matters by sending them to begin with.
7967 To enable us to fix the bug, you should include all these things:
7971 The version of @command{ld}. @command{ld} announces it if you start it with
7972 the @samp{--version} argument.
7974 Without this, we will not know whether there is any point in looking for
7975 the bug in the current version of @command{ld}.
7978 Any patches you may have applied to the @command{ld} source, including any
7979 patches made to the @code{BFD} library.
7982 The type of machine you are using, and the operating system name and
7986 What compiler (and its version) was used to compile @command{ld}---e.g.
7990 The command arguments you gave the linker to link your example and
7991 observe the bug. To guarantee you will not omit something important,
7992 list them all. A copy of the Makefile (or the output from make) is
7995 If we were to try to guess the arguments, we would probably guess wrong
7996 and then we might not encounter the bug.
7999 A complete input file, or set of input files, that will reproduce the
8000 bug. It is generally most helpful to send the actual object files
8001 provided that they are reasonably small. Say no more than 10K. For
8002 bigger files you can either make them available by FTP or HTTP or else
8003 state that you are willing to send the object file(s) to whomever
8004 requests them. (Note - your email will be going to a mailing list, so
8005 we do not want to clog it up with large attachments). But small
8006 attachments are best.
8008 If the source files were assembled using @code{gas} or compiled using
8009 @code{gcc}, then it may be OK to send the source files rather than the
8010 object files. In this case, be sure to say exactly what version of
8011 @code{gas} or @code{gcc} was used to produce the object files. Also say
8012 how @code{gas} or @code{gcc} were configured.
8015 A description of what behavior you observe that you believe is
8016 incorrect. For example, ``It gets a fatal signal.''
8018 Of course, if the bug is that @command{ld} gets a fatal signal, then we
8019 will certainly notice it. But if the bug is incorrect output, we might
8020 not notice unless it is glaringly wrong. You might as well not give us
8021 a chance to make a mistake.
8023 Even if the problem you experience is a fatal signal, you should still
8024 say so explicitly. Suppose something strange is going on, such as, your
8025 copy of @command{ld} is out of sync, or you have encountered a bug in the
8026 C library on your system. (This has happened!) Your copy might crash
8027 and ours would not. If you told us to expect a crash, then when ours
8028 fails to crash, we would know that the bug was not happening for us. If
8029 you had not told us to expect a crash, then we would not be able to draw
8030 any conclusion from our observations.
8033 If you wish to suggest changes to the @command{ld} source, send us context
8034 diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or
8035 @samp{-p} option. Always send diffs from the old file to the new file.
8036 If you even discuss something in the @command{ld} source, refer to it by
8037 context, not by line number.
8039 The line numbers in our development sources will not match those in your
8040 sources. Your line numbers would convey no useful information to us.
8043 Here are some things that are not necessary:
8047 A description of the envelope of the bug.
8049 Often people who encounter a bug spend a lot of time investigating
8050 which changes to the input file will make the bug go away and which
8051 changes will not affect it.
8053 This is often time consuming and not very useful, because the way we
8054 will find the bug is by running a single example under the debugger
8055 with breakpoints, not by pure deduction from a series of examples.
8056 We recommend that you save your time for something else.
8058 Of course, if you can find a simpler example to report @emph{instead}
8059 of the original one, that is a convenience for us. Errors in the
8060 output will be easier to spot, running under the debugger will take
8061 less time, and so on.
8063 However, simplification is not vital; if you do not want to do this,
8064 report the bug anyway and send us the entire test case you used.
8067 A patch for the bug.
8069 A patch for the bug does help us if it is a good one. But do not omit
8070 the necessary information, such as the test case, on the assumption that
8071 a patch is all we need. We might see problems with your patch and decide
8072 to fix the problem another way, or we might not understand it at all.
8074 Sometimes with a program as complicated as @command{ld} it is very hard to
8075 construct an example that will make the program follow a certain path
8076 through the code. If you do not send us the example, we will not be
8077 able to construct one, so we will not be able to verify that the bug is
8080 And if we cannot understand what bug you are trying to fix, or why your
8081 patch should be an improvement, we will not install it. A test case will
8082 help us to understand.
8085 A guess about what the bug is or what it depends on.
8087 Such guesses are usually wrong. Even we cannot guess right about such
8088 things without first using the debugger to find the facts.
8092 @appendix MRI Compatible Script Files
8093 @cindex MRI compatibility
8094 To aid users making the transition to @sc{gnu} @command{ld} from the MRI
8095 linker, @command{ld} can use MRI compatible linker scripts as an
8096 alternative to the more general-purpose linker scripting language
8097 described in @ref{Scripts}. MRI compatible linker scripts have a much
8098 simpler command set than the scripting language otherwise used with
8099 @command{ld}. @sc{gnu} @command{ld} supports the most commonly used MRI
8100 linker commands; these commands are described here.
8102 In general, MRI scripts aren't of much use with the @code{a.out} object
8103 file format, since it only has three sections and MRI scripts lack some
8104 features to make use of them.
8106 You can specify a file containing an MRI-compatible script using the
8107 @samp{-c} command-line option.
8109 Each command in an MRI-compatible script occupies its own line; each
8110 command line starts with the keyword that identifies the command (though
8111 blank lines are also allowed for punctuation). If a line of an
8112 MRI-compatible script begins with an unrecognized keyword, @command{ld}
8113 issues a warning message, but continues processing the script.
8115 Lines beginning with @samp{*} are comments.
8117 You can write these commands using all upper-case letters, or all
8118 lower case; for example, @samp{chip} is the same as @samp{CHIP}.
8119 The following list shows only the upper-case form of each command.
8122 @cindex @code{ABSOLUTE} (MRI)
8123 @item ABSOLUTE @var{secname}
8124 @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname}
8125 Normally, @command{ld} includes in the output file all sections from all
8126 the input files. However, in an MRI-compatible script, you can use the
8127 @code{ABSOLUTE} command to restrict the sections that will be present in
8128 your output program. If the @code{ABSOLUTE} command is used at all in a
8129 script, then only the sections named explicitly in @code{ABSOLUTE}
8130 commands will appear in the linker output. You can still use other
8131 input sections (whatever you select on the command line, or using
8132 @code{LOAD}) to resolve addresses in the output file.
8134 @cindex @code{ALIAS} (MRI)
8135 @item ALIAS @var{out-secname}, @var{in-secname}
8136 Use this command to place the data from input section @var{in-secname}
8137 in a section called @var{out-secname} in the linker output file.
8139 @var{in-secname} may be an integer.
8141 @cindex @code{ALIGN} (MRI)
8142 @item ALIGN @var{secname} = @var{expression}
8143 Align the section called @var{secname} to @var{expression}. The
8144 @var{expression} should be a power of two.
8146 @cindex @code{BASE} (MRI)
8147 @item BASE @var{expression}
8148 Use the value of @var{expression} as the lowest address (other than
8149 absolute addresses) in the output file.
8151 @cindex @code{CHIP} (MRI)
8152 @item CHIP @var{expression}
8153 @itemx CHIP @var{expression}, @var{expression}
8154 This command does nothing; it is accepted only for compatibility.
8156 @cindex @code{END} (MRI)
8158 This command does nothing whatever; it's only accepted for compatibility.
8160 @cindex @code{FORMAT} (MRI)
8161 @item FORMAT @var{output-format}
8162 Similar to the @code{OUTPUT_FORMAT} command in the more general linker
8163 language, but restricted to one of these output formats:
8167 S-records, if @var{output-format} is @samp{S}
8170 IEEE, if @var{output-format} is @samp{IEEE}
8173 COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is
8177 @cindex @code{LIST} (MRI)
8178 @item LIST @var{anything}@dots{}
8179 Print (to the standard output file) a link map, as produced by the
8180 @command{ld} command-line option @samp{-M}.
8182 The keyword @code{LIST} may be followed by anything on the
8183 same line, with no change in its effect.
8185 @cindex @code{LOAD} (MRI)
8186 @item LOAD @var{filename}
8187 @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename}
8188 Include one or more object file @var{filename} in the link; this has the
8189 same effect as specifying @var{filename} directly on the @command{ld}
8192 @cindex @code{NAME} (MRI)
8193 @item NAME @var{output-name}
8194 @var{output-name} is the name for the program produced by @command{ld}; the
8195 MRI-compatible command @code{NAME} is equivalent to the command-line
8196 option @samp{-o} or the general script language command @code{OUTPUT}.
8198 @cindex @code{ORDER} (MRI)
8199 @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname}
8200 @itemx ORDER @var{secname} @var{secname} @var{secname}
8201 Normally, @command{ld} orders the sections in its output file in the
8202 order in which they first appear in the input files. In an MRI-compatible
8203 script, you can override this ordering with the @code{ORDER} command. The
8204 sections you list with @code{ORDER} will appear first in your output
8205 file, in the order specified.
8207 @cindex @code{PUBLIC} (MRI)
8208 @item PUBLIC @var{name}=@var{expression}
8209 @itemx PUBLIC @var{name},@var{expression}
8210 @itemx PUBLIC @var{name} @var{expression}
8211 Supply a value (@var{expression}) for external symbol
8212 @var{name} used in the linker input files.
8214 @cindex @code{SECT} (MRI)
8215 @item SECT @var{secname}, @var{expression}
8216 @itemx SECT @var{secname}=@var{expression}
8217 @itemx SECT @var{secname} @var{expression}
8218 You can use any of these three forms of the @code{SECT} command to
8219 specify the start address (@var{expression}) for section @var{secname}.
8220 If you have more than one @code{SECT} statement for the same
8221 @var{secname}, only the @emph{first} sets the start address.
8224 @node GNU Free Documentation License
8225 @appendix GNU Free Documentation License
8229 @unnumbered LD Index
8234 % I think something like @@colophon should be in texinfo. In the
8236 \long\def\colophon{\hbox to0pt{}\vfill
8237 \centerline{The body of this manual is set in}
8238 \centerline{\fontname\tenrm,}
8239 \centerline{with headings in {\bf\fontname\tenbf}}
8240 \centerline{and examples in {\tt\fontname\tentt}.}
8241 \centerline{{\it\fontname\tenit\/} and}
8242 \centerline{{\sl\fontname\tensl\/}}
8243 \centerline{are used for emphasis.}\vfill}
8245 % Blame: doc@@cygnus.com, 28mar91.