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}).
1479 Print a summary of the command-line options on the standard output and exit.
1481 @kindex --target-help
1483 Print a summary of all target specific options on the standard output and exit.
1485 @kindex -Map=@var{mapfile}
1486 @item -Map=@var{mapfile}
1487 Print a link map to the file @var{mapfile}. See the description of the
1488 @option{-M} option, above.
1490 @cindex memory usage
1491 @kindex --no-keep-memory
1492 @item --no-keep-memory
1493 @command{ld} normally optimizes for speed over memory usage by caching the
1494 symbol tables of input files in memory. This option tells @command{ld} to
1495 instead optimize for memory usage, by rereading the symbol tables as
1496 necessary. This may be required if @command{ld} runs out of memory space
1497 while linking a large executable.
1499 @kindex --no-undefined
1501 @item --no-undefined
1503 Report unresolved symbol references from regular object files. This
1504 is done even if the linker is creating a non-symbolic shared library.
1505 The switch @option{--[no-]allow-shlib-undefined} controls the
1506 behaviour for reporting unresolved references found in shared
1507 libraries being linked in.
1509 @kindex --allow-multiple-definition
1511 @item --allow-multiple-definition
1513 Normally when a symbol is defined multiple times, the linker will
1514 report a fatal error. These options allow multiple definitions and the
1515 first definition will be used.
1517 @kindex --allow-shlib-undefined
1518 @kindex --no-allow-shlib-undefined
1519 @item --allow-shlib-undefined
1520 @itemx --no-allow-shlib-undefined
1521 Allows or disallows undefined symbols in shared libraries.
1522 This switch is similar to @option{--no-undefined} except that it
1523 determines the behaviour when the undefined symbols are in a
1524 shared library rather than a regular object file. It does not affect
1525 how undefined symbols in regular object files are handled.
1527 The default behaviour is to report errors for any undefined symbols
1528 referenced in shared libraries if the linker is being used to create
1529 an executable, but to allow them if the linker is being used to create
1532 The reasons for allowing undefined symbol references in shared
1533 libraries specified at link time are that:
1537 A shared library specified at link time may not be the same as the one
1538 that is available at load time, so the symbol might actually be
1539 resolvable at load time.
1541 There are some operating systems, eg BeOS and HPPA, where undefined
1542 symbols in shared libraries are normal.
1544 The BeOS kernel for example patches shared libraries at load time to
1545 select whichever function is most appropriate for the current
1546 architecture. This is used, for example, to dynamically select an
1547 appropriate memset function.
1550 @kindex --no-undefined-version
1551 @item --no-undefined-version
1552 Normally when a symbol has an undefined version, the linker will ignore
1553 it. This option disallows symbols with undefined version and a fatal error
1554 will be issued instead.
1556 @kindex --default-symver
1557 @item --default-symver
1558 Create and use a default symbol version (the soname) for unversioned
1561 @kindex --default-imported-symver
1562 @item --default-imported-symver
1563 Create and use a default symbol version (the soname) for unversioned
1566 @kindex --no-warn-mismatch
1567 @item --no-warn-mismatch
1568 Normally @command{ld} will give an error if you try to link together input
1569 files that are mismatched for some reason, perhaps because they have
1570 been compiled for different processors or for different endiannesses.
1571 This option tells @command{ld} that it should silently permit such possible
1572 errors. This option should only be used with care, in cases when you
1573 have taken some special action that ensures that the linker errors are
1576 @kindex --no-warn-search-mismatch
1577 @item --no-warn-search-mismatch
1578 Normally @command{ld} will give a warning if it finds an incompatible
1579 library during a library search. This option silences the warning.
1581 @kindex --no-whole-archive
1582 @item --no-whole-archive
1583 Turn off the effect of the @option{--whole-archive} option for subsequent
1586 @cindex output file after errors
1587 @kindex --noinhibit-exec
1588 @item --noinhibit-exec
1589 Retain the executable output file whenever it is still usable.
1590 Normally, the linker will not produce an output file if it encounters
1591 errors during the link process; it exits without writing an output file
1592 when it issues any error whatsoever.
1596 Only search library directories explicitly specified on the
1597 command line. Library directories specified in linker scripts
1598 (including linker scripts specified on the command line) are ignored.
1600 @ifclear SingleFormat
1601 @kindex --oformat=@var{output-format}
1602 @item --oformat=@var{output-format}
1603 @command{ld} may be configured to support more than one kind of object
1604 file. If your @command{ld} is configured this way, you can use the
1605 @samp{--oformat} option to specify the binary format for the output
1606 object file. Even when @command{ld} is configured to support alternative
1607 object formats, you don't usually need to specify this, as @command{ld}
1608 should be configured to produce as a default output format the most
1609 usual format on each machine. @var{output-format} is a text string, the
1610 name of a particular format supported by the BFD libraries. (You can
1611 list the available binary formats with @samp{objdump -i}.) The script
1612 command @code{OUTPUT_FORMAT} can also specify the output format, but
1613 this option overrides it. @xref{BFD}.
1617 @kindex --pic-executable
1619 @itemx --pic-executable
1620 @cindex position independent executables
1621 Create a position independent executable. This is currently only supported on
1622 ELF platforms. Position independent executables are similar to shared
1623 libraries in that they are relocated by the dynamic linker to the virtual
1624 address the OS chooses for them (which can vary between invocations). Like
1625 normal dynamically linked executables they can be executed and symbols
1626 defined in the executable cannot be overridden by shared libraries.
1630 This option is ignored for Linux compatibility.
1634 This option is ignored for SVR4 compatibility.
1637 @cindex synthesizing linker
1638 @cindex relaxing addressing modes
1642 An option with machine dependent effects.
1644 This option is only supported on a few targets.
1647 @xref{H8/300,,@command{ld} and the H8/300}.
1650 @xref{i960,, @command{ld} and the Intel 960 family}.
1653 @xref{Xtensa,, @command{ld} and Xtensa Processors}.
1656 @xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}.
1659 @xref{Nios II,,@command{ld} and the Altera Nios II}.
1662 @xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}.
1665 On some platforms the @samp{--relax} option performs target specific,
1666 global optimizations that become possible when the linker resolves
1667 addressing in the program, such as relaxing address modes,
1668 synthesizing new instructions, selecting shorter version of current
1669 instructions, and combining constant values.
1671 On some platforms these link time global optimizations may make symbolic
1672 debugging of the resulting executable impossible.
1674 This is known to be the case for the Matsushita MN10200 and MN10300
1675 family of processors.
1679 On platforms where this is not supported, @samp{--relax} is accepted,
1683 On platforms where @samp{--relax} is accepted the option
1684 @samp{--no-relax} can be used to disable the feature.
1686 @cindex retaining specified symbols
1687 @cindex stripping all but some symbols
1688 @cindex symbols, retaining selectively
1689 @kindex --retain-symbols-file=@var{filename}
1690 @item --retain-symbols-file=@var{filename}
1691 Retain @emph{only} the symbols listed in the file @var{filename},
1692 discarding all others. @var{filename} is simply a flat file, with one
1693 symbol name per line. This option is especially useful in environments
1697 where a large global symbol table is accumulated gradually, to conserve
1700 @samp{--retain-symbols-file} does @emph{not} discard undefined symbols,
1701 or symbols needed for relocations.
1703 You may only specify @samp{--retain-symbols-file} once in the command
1704 line. It overrides @samp{-s} and @samp{-S}.
1707 @item -rpath=@var{dir}
1708 @cindex runtime library search path
1709 @kindex -rpath=@var{dir}
1710 Add a directory to the runtime library search path. This is used when
1711 linking an ELF executable with shared objects. All @option{-rpath}
1712 arguments are concatenated and passed to the runtime linker, which uses
1713 them to locate shared objects at runtime. The @option{-rpath} option is
1714 also used when locating shared objects which are needed by shared
1715 objects explicitly included in the link; see the description of the
1716 @option{-rpath-link} option. If @option{-rpath} is not used when linking an
1717 ELF executable, the contents of the environment variable
1718 @code{LD_RUN_PATH} will be used if it is defined.
1720 The @option{-rpath} option may also be used on SunOS. By default, on
1721 SunOS, the linker will form a runtime search patch out of all the
1722 @option{-L} options it is given. If a @option{-rpath} option is used, the
1723 runtime search path will be formed exclusively using the @option{-rpath}
1724 options, ignoring the @option{-L} options. This can be useful when using
1725 gcc, which adds many @option{-L} options which may be on NFS mounted
1728 For compatibility with other ELF linkers, if the @option{-R} option is
1729 followed by a directory name, rather than a file name, it is treated as
1730 the @option{-rpath} option.
1734 @cindex link-time runtime library search path
1735 @kindex -rpath-link=@var{dir}
1736 @item -rpath-link=@var{dir}
1737 When using ELF or SunOS, one shared library may require another. This
1738 happens when an @code{ld -shared} link includes a shared library as one
1741 When the linker encounters such a dependency when doing a non-shared,
1742 non-relocatable link, it will automatically try to locate the required
1743 shared library and include it in the link, if it is not included
1744 explicitly. In such a case, the @option{-rpath-link} option
1745 specifies the first set of directories to search. The
1746 @option{-rpath-link} option may specify a sequence of directory names
1747 either by specifying a list of names separated by colons, or by
1748 appearing multiple times.
1750 This option should be used with caution as it overrides the search path
1751 that may have been hard compiled into a shared library. In such a case it
1752 is possible to use unintentionally a different search path than the
1753 runtime linker would do.
1755 The linker uses the following search paths to locate required shared
1759 Any directories specified by @option{-rpath-link} options.
1761 Any directories specified by @option{-rpath} options. The difference
1762 between @option{-rpath} and @option{-rpath-link} is that directories
1763 specified by @option{-rpath} options are included in the executable and
1764 used at runtime, whereas the @option{-rpath-link} option is only effective
1765 at link time. Searching @option{-rpath} in this way is only supported
1766 by native linkers and cross linkers which have been configured with
1767 the @option{--with-sysroot} option.
1769 On an ELF system, for native linkers, if the @option{-rpath} and
1770 @option{-rpath-link} options were not used, search the contents of the
1771 environment variable @code{LD_RUN_PATH}.
1773 On SunOS, if the @option{-rpath} option was not used, search any
1774 directories specified using @option{-L} options.
1776 For a native linker, search the contents of the environment
1777 variable @code{LD_LIBRARY_PATH}.
1779 For a native ELF linker, the directories in @code{DT_RUNPATH} or
1780 @code{DT_RPATH} of a shared library are searched for shared
1781 libraries needed by it. The @code{DT_RPATH} entries are ignored if
1782 @code{DT_RUNPATH} entries exist.
1784 The default directories, normally @file{/lib} and @file{/usr/lib}.
1786 For a native linker on an ELF system, if the file @file{/etc/ld.so.conf}
1787 exists, the list of directories found in that file.
1790 If the required shared library is not found, the linker will issue a
1791 warning and continue with the link.
1798 @cindex shared libraries
1799 Create a shared library. This is currently only supported on ELF, XCOFF
1800 and SunOS platforms. On SunOS, the linker will automatically create a
1801 shared library if the @option{-e} option is not used and there are
1802 undefined symbols in the link.
1804 @kindex --sort-common
1806 @itemx --sort-common=ascending
1807 @itemx --sort-common=descending
1808 This option tells @command{ld} to sort the common symbols by alignment in
1809 ascending or descending order when it places them in the appropriate output
1810 sections. The symbol alignments considered are sixteen-byte or larger,
1811 eight-byte, four-byte, two-byte, and one-byte. This is to prevent gaps
1812 between symbols due to alignment constraints. If no sorting order is
1813 specified, then descending order is assumed.
1815 @kindex --sort-section=name
1816 @item --sort-section=name
1817 This option will apply @code{SORT_BY_NAME} to all wildcard section
1818 patterns in the linker script.
1820 @kindex --sort-section=alignment
1821 @item --sort-section=alignment
1822 This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section
1823 patterns in the linker script.
1825 @kindex --split-by-file
1826 @item --split-by-file[=@var{size}]
1827 Similar to @option{--split-by-reloc} but creates a new output section for
1828 each input file when @var{size} is reached. @var{size} defaults to a
1829 size of 1 if not given.
1831 @kindex --split-by-reloc
1832 @item --split-by-reloc[=@var{count}]
1833 Tries to creates extra sections in the output file so that no single
1834 output section in the file contains more than @var{count} relocations.
1835 This is useful when generating huge relocatable files for downloading into
1836 certain real time kernels with the COFF object file format; since COFF
1837 cannot represent more than 65535 relocations in a single section. Note
1838 that this will fail to work with object file formats which do not
1839 support arbitrary sections. The linker will not split up individual
1840 input sections for redistribution, so if a single input section contains
1841 more than @var{count} relocations one output section will contain that
1842 many relocations. @var{count} defaults to a value of 32768.
1846 Compute and display statistics about the operation of the linker, such
1847 as execution time and memory usage.
1849 @kindex --sysroot=@var{directory}
1850 @item --sysroot=@var{directory}
1851 Use @var{directory} as the location of the sysroot, overriding the
1852 configure-time default. This option is only supported by linkers
1853 that were configured using @option{--with-sysroot}.
1855 @kindex --traditional-format
1856 @cindex traditional format
1857 @item --traditional-format
1858 For some targets, the output of @command{ld} is different in some ways from
1859 the output of some existing linker. This switch requests @command{ld} to
1860 use the traditional format instead.
1863 For example, on SunOS, @command{ld} combines duplicate entries in the
1864 symbol string table. This can reduce the size of an output file with
1865 full debugging information by over 30 percent. Unfortunately, the SunOS
1866 @code{dbx} program can not read the resulting program (@code{gdb} has no
1867 trouble). The @samp{--traditional-format} switch tells @command{ld} to not
1868 combine duplicate entries.
1870 @kindex --section-start=@var{sectionname}=@var{org}
1871 @item --section-start=@var{sectionname}=@var{org}
1872 Locate a section in the output file at the absolute
1873 address given by @var{org}. You may use this option as many
1874 times as necessary to locate multiple sections in the command
1876 @var{org} must be a single hexadecimal integer;
1877 for compatibility with other linkers, you may omit the leading
1878 @samp{0x} usually associated with hexadecimal values. @emph{Note:} there
1879 should be no white space between @var{sectionname}, the equals
1880 sign (``@key{=}''), and @var{org}.
1882 @kindex -Tbss=@var{org}
1883 @kindex -Tdata=@var{org}
1884 @kindex -Ttext=@var{org}
1885 @cindex segment origins, cmd line
1886 @item -Tbss=@var{org}
1887 @itemx -Tdata=@var{org}
1888 @itemx -Ttext=@var{org}
1889 Same as @option{--section-start}, with @code{.bss}, @code{.data} or
1890 @code{.text} as the @var{sectionname}.
1892 @kindex -Ttext-segment=@var{org}
1893 @item -Ttext-segment=@var{org}
1894 @cindex text segment origin, cmd line
1895 When creating an ELF executable, it will set the address of the first
1896 byte of the text segment.
1898 @kindex -Trodata-segment=@var{org}
1899 @item -Trodata-segment=@var{org}
1900 @cindex rodata segment origin, cmd line
1901 When creating an ELF executable or shared object for a target where
1902 the read-only data is in its own segment separate from the executable
1903 text, it will set the address of the first byte of the read-only data segment.
1905 @kindex -Tldata-segment=@var{org}
1906 @item -Tldata-segment=@var{org}
1907 @cindex ldata segment origin, cmd line
1908 When creating an ELF executable or shared object for x86-64 medium memory
1909 model, it will set the address of the first byte of the ldata segment.
1911 @kindex --unresolved-symbols
1912 @item --unresolved-symbols=@var{method}
1913 Determine how to handle unresolved symbols. There are four possible
1914 values for @samp{method}:
1918 Do not report any unresolved symbols.
1921 Report all unresolved symbols. This is the default.
1923 @item ignore-in-object-files
1924 Report unresolved symbols that are contained in shared libraries, but
1925 ignore them if they come from regular object files.
1927 @item ignore-in-shared-libs
1928 Report unresolved symbols that come from regular object files, but
1929 ignore them if they come from shared libraries. This can be useful
1930 when creating a dynamic binary and it is known that all the shared
1931 libraries that it should be referencing are included on the linker's
1935 The behaviour for shared libraries on their own can also be controlled
1936 by the @option{--[no-]allow-shlib-undefined} option.
1938 Normally the linker will generate an error message for each reported
1939 unresolved symbol but the option @option{--warn-unresolved-symbols}
1940 can change this to a warning.
1942 @kindex --verbose[=@var{NUMBER}]
1943 @cindex verbose[=@var{NUMBER}]
1945 @itemx --verbose[=@var{NUMBER}]
1946 Display the version number for @command{ld} and list the linker emulations
1947 supported. Display which input files can and cannot be opened. Display
1948 the linker script being used by the linker. If the optional @var{NUMBER}
1949 argument > 1, plugin symbol status will also be displayed.
1951 @kindex --version-script=@var{version-scriptfile}
1952 @cindex version script, symbol versions
1953 @item --version-script=@var{version-scriptfile}
1954 Specify the name of a version script to the linker. This is typically
1955 used when creating shared libraries to specify additional information
1956 about the version hierarchy for the library being created. This option
1957 is only fully supported on ELF platforms which support shared libraries;
1958 see @ref{VERSION}. It is partially supported on PE platforms, which can
1959 use version scripts to filter symbol visibility in auto-export mode: any
1960 symbols marked @samp{local} in the version script will not be exported.
1963 @kindex --warn-common
1964 @cindex warnings, on combining symbols
1965 @cindex combining symbols, warnings on
1967 Warn when a common symbol is combined with another common symbol or with
1968 a symbol definition. Unix linkers allow this somewhat sloppy practice,
1969 but linkers on some other operating systems do not. This option allows
1970 you to find potential problems from combining global symbols.
1971 Unfortunately, some C libraries use this practice, so you may get some
1972 warnings about symbols in the libraries as well as in your programs.
1974 There are three kinds of global symbols, illustrated here by C examples:
1978 A definition, which goes in the initialized data section of the output
1982 An undefined reference, which does not allocate space.
1983 There must be either a definition or a common symbol for the
1987 A common symbol. If there are only (one or more) common symbols for a
1988 variable, it goes in the uninitialized data area of the output file.
1989 The linker merges multiple common symbols for the same variable into a
1990 single symbol. If they are of different sizes, it picks the largest
1991 size. The linker turns a common symbol into a declaration, if there is
1992 a definition of the same variable.
1995 The @samp{--warn-common} option can produce five kinds of warnings.
1996 Each warning consists of a pair of lines: the first describes the symbol
1997 just encountered, and the second describes the previous symbol
1998 encountered with the same name. One or both of the two symbols will be
2003 Turning a common symbol into a reference, because there is already a
2004 definition for the symbol.
2006 @var{file}(@var{section}): warning: common of `@var{symbol}'
2007 overridden by definition
2008 @var{file}(@var{section}): warning: defined here
2012 Turning a common symbol into a reference, because a later definition for
2013 the symbol is encountered. This is the same as the previous case,
2014 except that the symbols are encountered in a different order.
2016 @var{file}(@var{section}): warning: definition of `@var{symbol}'
2018 @var{file}(@var{section}): warning: common is here
2022 Merging a common symbol with a previous same-sized common symbol.
2024 @var{file}(@var{section}): warning: multiple common
2026 @var{file}(@var{section}): warning: previous common is here
2030 Merging a common symbol with a previous larger common symbol.
2032 @var{file}(@var{section}): warning: common of `@var{symbol}'
2033 overridden by larger common
2034 @var{file}(@var{section}): warning: larger common is here
2038 Merging a common symbol with a previous smaller common symbol. This is
2039 the same as the previous case, except that the symbols are
2040 encountered in a different order.
2042 @var{file}(@var{section}): warning: common of `@var{symbol}'
2043 overriding smaller common
2044 @var{file}(@var{section}): warning: smaller common is here
2048 @kindex --warn-constructors
2049 @item --warn-constructors
2050 Warn if any global constructors are used. This is only useful for a few
2051 object file formats. For formats like COFF or ELF, the linker can not
2052 detect the use of global constructors.
2054 @kindex --warn-multiple-gp
2055 @item --warn-multiple-gp
2056 Warn if multiple global pointer values are required in the output file.
2057 This is only meaningful for certain processors, such as the Alpha.
2058 Specifically, some processors put large-valued constants in a special
2059 section. A special register (the global pointer) points into the middle
2060 of this section, so that constants can be loaded efficiently via a
2061 base-register relative addressing mode. Since the offset in
2062 base-register relative mode is fixed and relatively small (e.g., 16
2063 bits), this limits the maximum size of the constant pool. Thus, in
2064 large programs, it is often necessary to use multiple global pointer
2065 values in order to be able to address all possible constants. This
2066 option causes a warning to be issued whenever this case occurs.
2069 @cindex warnings, on undefined symbols
2070 @cindex undefined symbols, warnings on
2072 Only warn once for each undefined symbol, rather than once per module
2075 @kindex --warn-orphan
2076 @kindex --no-warn-orphan
2077 @cindex warnings, on orphan sections
2078 @cindex orphan sections, warnings on
2080 The @option{--warn-orphan} option tells the linker to generate a
2081 warning message whenever it has to place an orphan section into the
2082 output file. @xref{Orphan Sections} The @option{--no-warn-orphan}
2083 option restores the default behaviour of just silently placing these
2086 @kindex --warn-section-align
2087 @cindex warnings, on section alignment
2088 @cindex section alignment, warnings on
2089 @item --warn-section-align
2090 Warn if the address of an output section is changed because of
2091 alignment. Typically, the alignment will be set by an input section.
2092 The address will only be changed if it not explicitly specified; that
2093 is, if the @code{SECTIONS} command does not specify a start address for
2094 the section (@pxref{SECTIONS}).
2096 @kindex --warn-shared-textrel
2097 @item --warn-shared-textrel
2098 Warn if the linker adds a DT_TEXTREL to a shared object.
2100 @kindex --warn-alternate-em
2101 @item --warn-alternate-em
2102 Warn if an object has alternate ELF machine code.
2104 @kindex --warn-unresolved-symbols
2105 @item --warn-unresolved-symbols
2106 If the linker is going to report an unresolved symbol (see the option
2107 @option{--unresolved-symbols}) it will normally generate an error.
2108 This option makes it generate a warning instead.
2110 @kindex --error-unresolved-symbols
2111 @item --error-unresolved-symbols
2112 This restores the linker's default behaviour of generating errors when
2113 it is reporting unresolved symbols.
2115 @kindex --whole-archive
2116 @cindex including an entire archive
2117 @item --whole-archive
2118 For each archive mentioned on the command line after the
2119 @option{--whole-archive} option, include every object file in the archive
2120 in the link, rather than searching the archive for the required object
2121 files. This is normally used to turn an archive file into a shared
2122 library, forcing every object to be included in the resulting shared
2123 library. This option may be used more than once.
2125 Two notes when using this option from gcc: First, gcc doesn't know
2126 about this option, so you have to use @option{-Wl,-whole-archive}.
2127 Second, don't forget to use @option{-Wl,-no-whole-archive} after your
2128 list of archives, because gcc will add its own list of archives to
2129 your link and you may not want this flag to affect those as well.
2131 @kindex --wrap=@var{symbol}
2132 @item --wrap=@var{symbol}
2133 Use a wrapper function for @var{symbol}. Any undefined reference to
2134 @var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any
2135 undefined reference to @code{__real_@var{symbol}} will be resolved to
2138 This can be used to provide a wrapper for a system function. The
2139 wrapper function should be called @code{__wrap_@var{symbol}}. If it
2140 wishes to call the system function, it should call
2141 @code{__real_@var{symbol}}.
2143 Here is a trivial example:
2147 __wrap_malloc (size_t c)
2149 printf ("malloc called with %zu\n", c);
2150 return __real_malloc (c);
2154 If you link other code with this file using @option{--wrap malloc}, then
2155 all calls to @code{malloc} will call the function @code{__wrap_malloc}
2156 instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will
2157 call the real @code{malloc} function.
2159 You may wish to provide a @code{__real_malloc} function as well, so that
2160 links without the @option{--wrap} option will succeed. If you do this,
2161 you should not put the definition of @code{__real_malloc} in the same
2162 file as @code{__wrap_malloc}; if you do, the assembler may resolve the
2163 call before the linker has a chance to wrap it to @code{malloc}.
2165 @kindex --eh-frame-hdr
2166 @item --eh-frame-hdr
2167 Request creation of @code{.eh_frame_hdr} section and ELF
2168 @code{PT_GNU_EH_FRAME} segment header.
2170 @kindex --ld-generated-unwind-info
2171 @item --no-ld-generated-unwind-info
2172 Request creation of @code{.eh_frame} unwind info for linker
2173 generated code sections like PLT. This option is on by default
2174 if linker generated unwind info is supported.
2176 @kindex --enable-new-dtags
2177 @kindex --disable-new-dtags
2178 @item --enable-new-dtags
2179 @itemx --disable-new-dtags
2180 This linker can create the new dynamic tags in ELF. But the older ELF
2181 systems may not understand them. If you specify
2182 @option{--enable-new-dtags}, the new dynamic tags will be created as needed
2183 and older dynamic tags will be omitted.
2184 If you specify @option{--disable-new-dtags}, no new dynamic tags will be
2185 created. By default, the new dynamic tags are not created. Note that
2186 those options are only available for ELF systems.
2188 @kindex --hash-size=@var{number}
2189 @item --hash-size=@var{number}
2190 Set the default size of the linker's hash tables to a prime number
2191 close to @var{number}. Increasing this value can reduce the length of
2192 time it takes the linker to perform its tasks, at the expense of
2193 increasing the linker's memory requirements. Similarly reducing this
2194 value can reduce the memory requirements at the expense of speed.
2196 @kindex --hash-style=@var{style}
2197 @item --hash-style=@var{style}
2198 Set the type of linker's hash table(s). @var{style} can be either
2199 @code{sysv} for classic ELF @code{.hash} section, @code{gnu} for
2200 new style GNU @code{.gnu.hash} section or @code{both} for both
2201 the classic ELF @code{.hash} and new style GNU @code{.gnu.hash}
2202 hash tables. The default is @code{sysv}.
2204 @kindex --reduce-memory-overheads
2205 @item --reduce-memory-overheads
2206 This option reduces memory requirements at ld runtime, at the expense of
2207 linking speed. This was introduced to select the old O(n^2) algorithm
2208 for link map file generation, rather than the new O(n) algorithm which uses
2209 about 40% more memory for symbol storage.
2211 Another effect of the switch is to set the default hash table size to
2212 1021, which again saves memory at the cost of lengthening the linker's
2213 run time. This is not done however if the @option{--hash-size} switch
2216 The @option{--reduce-memory-overheads} switch may be also be used to
2217 enable other tradeoffs in future versions of the linker.
2220 @kindex --build-id=@var{style}
2222 @itemx --build-id=@var{style}
2223 Request the creation of a @code{.note.gnu.build-id} ELF note section
2224 or a @code{.build-id} COFF section. The contents of the note are
2225 unique bits identifying this linked file. @var{style} can be
2226 @code{uuid} to use 128 random bits, @code{sha1} to use a 160-bit
2227 @sc{SHA1} hash on the normative parts of the output contents,
2228 @code{md5} to use a 128-bit @sc{MD5} hash on the normative parts of
2229 the output contents, or @code{0x@var{hexstring}} to use a chosen bit
2230 string specified as an even number of hexadecimal digits (@code{-} and
2231 @code{:} characters between digit pairs are ignored). If @var{style}
2232 is omitted, @code{sha1} is used.
2234 The @code{md5} and @code{sha1} styles produces an identifier
2235 that is always the same in an identical output file, but will be
2236 unique among all nonidentical output files. It is not intended
2237 to be compared as a checksum for the file's contents. A linked
2238 file may be changed later by other tools, but the build ID bit
2239 string identifying the original linked file does not change.
2241 Passing @code{none} for @var{style} disables the setting from any
2242 @code{--build-id} options earlier on the command line.
2247 @subsection Options Specific to i386 PE Targets
2249 @c man begin OPTIONS
2251 The i386 PE linker supports the @option{-shared} option, which causes
2252 the output to be a dynamically linked library (DLL) instead of a
2253 normal executable. You should name the output @code{*.dll} when you
2254 use this option. In addition, the linker fully supports the standard
2255 @code{*.def} files, which may be specified on the linker command line
2256 like an object file (in fact, it should precede archives it exports
2257 symbols from, to ensure that they get linked in, just like a normal
2260 In addition to the options common to all targets, the i386 PE linker
2261 support additional command line options that are specific to the i386
2262 PE target. Options that take values may be separated from their
2263 values by either a space or an equals sign.
2267 @kindex --add-stdcall-alias
2268 @item --add-stdcall-alias
2269 If given, symbols with a stdcall suffix (@@@var{nn}) will be exported
2270 as-is and also with the suffix stripped.
2271 [This option is specific to the i386 PE targeted port of the linker]
2274 @item --base-file @var{file}
2275 Use @var{file} as the name of a file in which to save the base
2276 addresses of all the relocations needed for generating DLLs with
2278 [This is an i386 PE specific option]
2282 Create a DLL instead of a regular executable. You may also use
2283 @option{-shared} or specify a @code{LIBRARY} in a given @code{.def}
2285 [This option is specific to the i386 PE targeted port of the linker]
2287 @kindex --enable-long-section-names
2288 @kindex --disable-long-section-names
2289 @item --enable-long-section-names
2290 @itemx --disable-long-section-names
2291 The PE variants of the Coff object format add an extension that permits
2292 the use of section names longer than eight characters, the normal limit
2293 for Coff. By default, these names are only allowed in object files, as
2294 fully-linked executable images do not carry the Coff string table required
2295 to support the longer names. As a GNU extension, it is possible to
2296 allow their use in executable images as well, or to (probably pointlessly!)
2297 disallow it in object files, by using these two options. Executable images
2298 generated with these long section names are slightly non-standard, carrying
2299 as they do a string table, and may generate confusing output when examined
2300 with non-GNU PE-aware tools, such as file viewers and dumpers. However,
2301 GDB relies on the use of PE long section names to find Dwarf-2 debug
2302 information sections in an executable image at runtime, and so if neither
2303 option is specified on the command-line, @command{ld} will enable long
2304 section names, overriding the default and technically correct behaviour,
2305 when it finds the presence of debug information while linking an executable
2306 image and not stripping symbols.
2307 [This option is valid for all PE targeted ports of the linker]
2309 @kindex --enable-stdcall-fixup
2310 @kindex --disable-stdcall-fixup
2311 @item --enable-stdcall-fixup
2312 @itemx --disable-stdcall-fixup
2313 If the link finds a symbol that it cannot resolve, it will attempt to
2314 do ``fuzzy linking'' by looking for another defined symbol that differs
2315 only in the format of the symbol name (cdecl vs stdcall) and will
2316 resolve that symbol by linking to the match. For example, the
2317 undefined symbol @code{_foo} might be linked to the function
2318 @code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked
2319 to the function @code{_bar}. When the linker does this, it prints a
2320 warning, since it normally should have failed to link, but sometimes
2321 import libraries generated from third-party dlls may need this feature
2322 to be usable. If you specify @option{--enable-stdcall-fixup}, this
2323 feature is fully enabled and warnings are not printed. If you specify
2324 @option{--disable-stdcall-fixup}, this feature is disabled and such
2325 mismatches are considered to be errors.
2326 [This option is specific to the i386 PE targeted port of the linker]
2328 @kindex --leading-underscore
2329 @kindex --no-leading-underscore
2330 @item --leading-underscore
2331 @itemx --no-leading-underscore
2332 For most targets default symbol-prefix is an underscore and is defined
2333 in target's description. By this option it is possible to
2334 disable/enable the default underscore symbol-prefix.
2336 @cindex DLLs, creating
2337 @kindex --export-all-symbols
2338 @item --export-all-symbols
2339 If given, all global symbols in the objects used to build a DLL will
2340 be exported by the DLL. Note that this is the default if there
2341 otherwise wouldn't be any exported symbols. When symbols are
2342 explicitly exported via DEF files or implicitly exported via function
2343 attributes, the default is to not export anything else unless this
2344 option is given. Note that the symbols @code{DllMain@@12},
2345 @code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and
2346 @code{impure_ptr} will not be automatically
2347 exported. Also, symbols imported from other DLLs will not be
2348 re-exported, nor will symbols specifying the DLL's internal layout
2349 such as those beginning with @code{_head_} or ending with
2350 @code{_iname}. In addition, no symbols from @code{libgcc},
2351 @code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported.
2352 Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will
2353 not be exported, to help with C++ DLLs. Finally, there is an
2354 extensive list of cygwin-private symbols that are not exported
2355 (obviously, this applies on when building DLLs for cygwin targets).
2356 These cygwin-excludes are: @code{_cygwin_dll_entry@@12},
2357 @code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12},
2358 @code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll},
2359 @code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2},
2360 @code{cygwin_premain3}, and @code{environ}.
2361 [This option is specific to the i386 PE targeted port of the linker]
2363 @kindex --exclude-symbols
2364 @item --exclude-symbols @var{symbol},@var{symbol},...
2365 Specifies a list of symbols which should not be automatically
2366 exported. The symbol names may be delimited by commas or colons.
2367 [This option is specific to the i386 PE targeted port of the linker]
2369 @kindex --exclude-all-symbols
2370 @item --exclude-all-symbols
2371 Specifies no symbols should be automatically exported.
2372 [This option is specific to the i386 PE targeted port of the linker]
2374 @kindex --file-alignment
2375 @item --file-alignment
2376 Specify the file alignment. Sections in the file will always begin at
2377 file offsets which are multiples of this number. This defaults to
2379 [This option is specific to the i386 PE targeted port of the linker]
2383 @item --heap @var{reserve}
2384 @itemx --heap @var{reserve},@var{commit}
2385 Specify the number of bytes of memory to reserve (and optionally commit)
2386 to be used as heap for this program. The default is 1MB reserved, 4K
2388 [This option is specific to the i386 PE targeted port of the linker]
2391 @kindex --image-base
2392 @item --image-base @var{value}
2393 Use @var{value} as the base address of your program or dll. This is
2394 the lowest memory location that will be used when your program or dll
2395 is loaded. To reduce the need to relocate and improve performance of
2396 your dlls, each should have a unique base address and not overlap any
2397 other dlls. The default is 0x400000 for executables, and 0x10000000
2399 [This option is specific to the i386 PE targeted port of the linker]
2403 If given, the stdcall suffixes (@@@var{nn}) will be stripped from
2404 symbols before they are exported.
2405 [This option is specific to the i386 PE targeted port of the linker]
2407 @kindex --large-address-aware
2408 @item --large-address-aware
2409 If given, the appropriate bit in the ``Characteristics'' field of the COFF
2410 header is set to indicate that this executable supports virtual addresses
2411 greater than 2 gigabytes. This should be used in conjunction with the /3GB
2412 or /USERVA=@var{value} megabytes switch in the ``[operating systems]''
2413 section of the BOOT.INI. Otherwise, this bit has no effect.
2414 [This option is specific to PE targeted ports of the linker]
2416 @kindex --disable-large-address-aware
2417 @item --disable-large-address-aware
2418 Reverts the effect of a previous @samp{--large-address-aware} option.
2419 This is useful if @samp{--large-address-aware} is always set by the compiler
2420 driver (e.g. Cygwin gcc) and the executable does not support virtual
2421 addresses greater than 2 gigabytes.
2422 [This option is specific to PE targeted ports of the linker]
2424 @kindex --major-image-version
2425 @item --major-image-version @var{value}
2426 Sets the major number of the ``image version''. Defaults to 1.
2427 [This option is specific to the i386 PE targeted port of the linker]
2429 @kindex --major-os-version
2430 @item --major-os-version @var{value}
2431 Sets the major number of the ``os version''. Defaults to 4.
2432 [This option is specific to the i386 PE targeted port of the linker]
2434 @kindex --major-subsystem-version
2435 @item --major-subsystem-version @var{value}
2436 Sets the major number of the ``subsystem version''. Defaults to 4.
2437 [This option is specific to the i386 PE targeted port of the linker]
2439 @kindex --minor-image-version
2440 @item --minor-image-version @var{value}
2441 Sets the minor number of the ``image version''. Defaults to 0.
2442 [This option is specific to the i386 PE targeted port of the linker]
2444 @kindex --minor-os-version
2445 @item --minor-os-version @var{value}
2446 Sets the minor number of the ``os version''. Defaults to 0.
2447 [This option is specific to the i386 PE targeted port of the linker]
2449 @kindex --minor-subsystem-version
2450 @item --minor-subsystem-version @var{value}
2451 Sets the minor number of the ``subsystem version''. Defaults to 0.
2452 [This option is specific to the i386 PE targeted port of the linker]
2454 @cindex DEF files, creating
2455 @cindex DLLs, creating
2456 @kindex --output-def
2457 @item --output-def @var{file}
2458 The linker will create the file @var{file} which will contain a DEF
2459 file corresponding to the DLL the linker is generating. This DEF file
2460 (which should be called @code{*.def}) may be used to create an import
2461 library with @code{dlltool} or may be used as a reference to
2462 automatically or implicitly exported symbols.
2463 [This option is specific to the i386 PE targeted port of the linker]
2465 @cindex DLLs, creating
2466 @kindex --out-implib
2467 @item --out-implib @var{file}
2468 The linker will create the file @var{file} which will contain an
2469 import lib corresponding to the DLL the linker is generating. This
2470 import lib (which should be called @code{*.dll.a} or @code{*.a}
2471 may be used to link clients against the generated DLL; this behaviour
2472 makes it possible to skip a separate @code{dlltool} import library
2474 [This option is specific to the i386 PE targeted port of the linker]
2476 @kindex --enable-auto-image-base
2477 @item --enable-auto-image-base
2478 @itemx --enable-auto-image-base=@var{value}
2479 Automatically choose the image base for DLLs, optionally starting with base
2480 @var{value}, unless one is specified using the @code{--image-base} argument.
2481 By using a hash generated from the dllname to create unique image bases
2482 for each DLL, in-memory collisions and relocations which can delay program
2483 execution are avoided.
2484 [This option is specific to the i386 PE targeted port of the linker]
2486 @kindex --disable-auto-image-base
2487 @item --disable-auto-image-base
2488 Do not automatically generate a unique image base. If there is no
2489 user-specified image base (@code{--image-base}) then use the platform
2491 [This option is specific to the i386 PE targeted port of the linker]
2493 @cindex DLLs, linking to
2494 @kindex --dll-search-prefix
2495 @item --dll-search-prefix @var{string}
2496 When linking dynamically to a dll without an import library,
2497 search for @code{<string><basename>.dll} in preference to
2498 @code{lib<basename>.dll}. This behaviour allows easy distinction
2499 between DLLs built for the various "subplatforms": native, cygwin,
2500 uwin, pw, etc. For instance, cygwin DLLs typically use
2501 @code{--dll-search-prefix=cyg}.
2502 [This option is specific to the i386 PE targeted port of the linker]
2504 @kindex --enable-auto-import
2505 @item --enable-auto-import
2506 Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for
2507 DATA imports from DLLs, and create the necessary thunking symbols when
2508 building the import libraries with those DATA exports. Note: Use of the
2509 'auto-import' extension will cause the text section of the image file
2510 to be made writable. This does not conform to the PE-COFF format
2511 specification published by Microsoft.
2513 Note - use of the 'auto-import' extension will also cause read only
2514 data which would normally be placed into the .rdata section to be
2515 placed into the .data section instead. This is in order to work
2516 around a problem with consts that is described here:
2517 http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
2519 Using 'auto-import' generally will 'just work' -- but sometimes you may
2522 "variable '<var>' can't be auto-imported. Please read the
2523 documentation for ld's @code{--enable-auto-import} for details."
2525 This message occurs when some (sub)expression accesses an address
2526 ultimately given by the sum of two constants (Win32 import tables only
2527 allow one). Instances where this may occur include accesses to member
2528 fields of struct variables imported from a DLL, as well as using a
2529 constant index into an array variable imported from a DLL. Any
2530 multiword variable (arrays, structs, long long, etc) may trigger
2531 this error condition. However, regardless of the exact data type
2532 of the offending exported variable, ld will always detect it, issue
2533 the warning, and exit.
2535 There are several ways to address this difficulty, regardless of the
2536 data type of the exported variable:
2538 One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
2539 of adjusting references in your client code for runtime environment, so
2540 this method works only when runtime environment supports this feature.
2542 A second solution is to force one of the 'constants' to be a variable --
2543 that is, unknown and un-optimizable at compile time. For arrays,
2544 there are two possibilities: a) make the indexee (the array's address)
2545 a variable, or b) make the 'constant' index a variable. Thus:
2548 extern type extern_array[];
2550 @{ volatile type *t=extern_array; t[1] @}
2556 extern type extern_array[];
2558 @{ volatile int t=1; extern_array[t] @}
2561 For structs (and most other multiword data types) the only option
2562 is to make the struct itself (or the long long, or the ...) variable:
2565 extern struct s extern_struct;
2566 extern_struct.field -->
2567 @{ volatile struct s *t=&extern_struct; t->field @}
2573 extern long long extern_ll;
2575 @{ volatile long long * local_ll=&extern_ll; *local_ll @}
2578 A third method of dealing with this difficulty is to abandon
2579 'auto-import' for the offending symbol and mark it with
2580 @code{__declspec(dllimport)}. However, in practice that
2581 requires using compile-time #defines to indicate whether you are
2582 building a DLL, building client code that will link to the DLL, or
2583 merely building/linking to a static library. In making the choice
2584 between the various methods of resolving the 'direct address with
2585 constant offset' problem, you should consider typical real-world usage:
2593 void main(int argc, char **argv)@{
2594 printf("%d\n",arr[1]);
2604 void main(int argc, char **argv)@{
2605 /* This workaround is for win32 and cygwin; do not "optimize" */
2606 volatile int *parr = arr;
2607 printf("%d\n",parr[1]);
2614 /* Note: auto-export is assumed (no __declspec(dllexport)) */
2615 #if (defined(_WIN32) || defined(__CYGWIN__)) && \
2616 !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
2617 #define FOO_IMPORT __declspec(dllimport)
2621 extern FOO_IMPORT int arr[];
2624 void main(int argc, char **argv)@{
2625 printf("%d\n",arr[1]);
2629 A fourth way to avoid this problem is to re-code your
2630 library to use a functional interface rather than a data interface
2631 for the offending variables (e.g. set_foo() and get_foo() accessor
2633 [This option is specific to the i386 PE targeted port of the linker]
2635 @kindex --disable-auto-import
2636 @item --disable-auto-import
2637 Do not attempt to do sophisticated linking of @code{_symbol} to
2638 @code{__imp__symbol} for DATA imports from DLLs.
2639 [This option is specific to the i386 PE targeted port of the linker]
2641 @kindex --enable-runtime-pseudo-reloc
2642 @item --enable-runtime-pseudo-reloc
2643 If your code contains expressions described in --enable-auto-import section,
2644 that is, DATA imports from DLL with non-zero offset, this switch will create
2645 a vector of 'runtime pseudo relocations' which can be used by runtime
2646 environment to adjust references to such data in your client code.
2647 [This option is specific to the i386 PE targeted port of the linker]
2649 @kindex --disable-runtime-pseudo-reloc
2650 @item --disable-runtime-pseudo-reloc
2651 Do not create pseudo relocations for non-zero offset DATA imports from
2653 [This option is specific to the i386 PE targeted port of the linker]
2655 @kindex --enable-extra-pe-debug
2656 @item --enable-extra-pe-debug
2657 Show additional debug info related to auto-import symbol thunking.
2658 [This option is specific to the i386 PE targeted port of the linker]
2660 @kindex --section-alignment
2661 @item --section-alignment
2662 Sets the section alignment. Sections in memory will always begin at
2663 addresses which are a multiple of this number. Defaults to 0x1000.
2664 [This option is specific to the i386 PE targeted port of the linker]
2668 @item --stack @var{reserve}
2669 @itemx --stack @var{reserve},@var{commit}
2670 Specify the number of bytes of memory to reserve (and optionally commit)
2671 to be used as stack for this program. The default is 2MB reserved, 4K
2673 [This option is specific to the i386 PE targeted port of the linker]
2676 @item --subsystem @var{which}
2677 @itemx --subsystem @var{which}:@var{major}
2678 @itemx --subsystem @var{which}:@var{major}.@var{minor}
2679 Specifies the subsystem under which your program will execute. The
2680 legal values for @var{which} are @code{native}, @code{windows},
2681 @code{console}, @code{posix}, and @code{xbox}. You may optionally set
2682 the subsystem version also. Numeric values are also accepted for
2684 [This option is specific to the i386 PE targeted port of the linker]
2686 The following options set flags in the @code{DllCharacteristics} field
2687 of the PE file header:
2688 [These options are specific to PE targeted ports of the linker]
2690 @kindex --high-entropy-va
2691 @item --high-entropy-va
2692 Image is compatible with 64-bit address space layout randomization
2695 @kindex --dynamicbase
2697 The image base address may be relocated using address space layout
2698 randomization (ASLR). This feature was introduced with MS Windows
2699 Vista for i386 PE targets.
2701 @kindex --forceinteg
2703 Code integrity checks are enforced.
2707 The image is compatible with the Data Execution Prevention.
2708 This feature was introduced with MS Windows XP SP2 for i386 PE targets.
2710 @kindex --no-isolation
2711 @item --no-isolation
2712 Although the image understands isolation, do not isolate the image.
2716 The image does not use SEH. No SE handler may be called from
2721 Do not bind this image.
2725 The driver uses the MS Windows Driver Model.
2729 The image is Terminal Server aware.
2731 @kindex --insert-timestamp
2732 @item --insert-timestamp
2733 @itemx --no-insert-timestamp
2734 Insert a real timestamp into the image. This is the default behaviour
2735 as it matches legacy code and it means that the image will work with
2736 other, proprietary tools. The problem with this default is that it
2737 will result in slightly different images being produced each tiem the
2738 same sources are linked. The option @option{--no-insert-timestamp}
2739 can be used to insert a zero value for the timestamp, this ensuring
2740 that binaries produced from indentical sources will compare
2747 @subsection Options specific to C6X uClinux targets
2749 @c man begin OPTIONS
2751 The C6X uClinux target uses a binary format called DSBT to support shared
2752 libraries. Each shared library in the system needs to have a unique index;
2753 all executables use an index of 0.
2758 @item --dsbt-size @var{size}
2759 This option sets the number of entires in the DSBT of the current executable
2760 or shared library to @var{size}. The default is to create a table with 64
2763 @kindex --dsbt-index
2764 @item --dsbt-index @var{index}
2765 This option sets the DSBT index of the current executable or shared library
2766 to @var{index}. The default is 0, which is appropriate for generating
2767 executables. If a shared library is generated with a DSBT index of 0, the
2768 @code{R_C6000_DSBT_INDEX} relocs are copied into the output file.
2770 @kindex --no-merge-exidx-entries
2771 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent
2772 exidx entries in frame unwind info.
2780 @subsection Options specific to Motorola 68HC11 and 68HC12 targets
2782 @c man begin OPTIONS
2784 The 68HC11 and 68HC12 linkers support specific options to control the
2785 memory bank switching mapping and trampoline code generation.
2789 @kindex --no-trampoline
2790 @item --no-trampoline
2791 This option disables the generation of trampoline. By default a trampoline
2792 is generated for each far function which is called using a @code{jsr}
2793 instruction (this happens when a pointer to a far function is taken).
2795 @kindex --bank-window
2796 @item --bank-window @var{name}
2797 This option indicates to the linker the name of the memory region in
2798 the @samp{MEMORY} specification that describes the memory bank window.
2799 The definition of such region is then used by the linker to compute
2800 paging and addresses within the memory window.
2808 @subsection Options specific to Motorola 68K target
2810 @c man begin OPTIONS
2812 The following options are supported to control handling of GOT generation
2813 when linking for 68K targets.
2818 @item --got=@var{type}
2819 This option tells the linker which GOT generation scheme to use.
2820 @var{type} should be one of @samp{single}, @samp{negative},
2821 @samp{multigot} or @samp{target}. For more information refer to the
2822 Info entry for @file{ld}.
2830 @subsection Options specific to MIPS targets
2832 @c man begin OPTIONS
2834 The following options are supported to control microMIPS instruction
2835 generation when linking for MIPS targets.
2843 These options control the choice of microMIPS instructions used in code
2844 generated by the linker, such as that in the PLT or lazy binding stubs,
2845 or in relaxation. If @samp{--insn32} is used, then the linker only uses
2846 32-bit instruction encodings. By default or if @samp{--no-insn32} is
2847 used, all instruction encodings are used, including 16-bit ones where
2857 @section Environment Variables
2859 @c man begin ENVIRONMENT
2861 You can change the behaviour of @command{ld} with the environment variables
2862 @ifclear SingleFormat
2865 @code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}.
2867 @ifclear SingleFormat
2869 @cindex default input format
2870 @code{GNUTARGET} determines the input-file object format if you don't
2871 use @samp{-b} (or its synonym @samp{--format}). Its value should be one
2872 of the BFD names for an input format (@pxref{BFD}). If there is no
2873 @code{GNUTARGET} in the environment, @command{ld} uses the natural format
2874 of the target. If @code{GNUTARGET} is set to @code{default} then BFD
2875 attempts to discover the input format by examining binary input files;
2876 this method often succeeds, but there are potential ambiguities, since
2877 there is no method of ensuring that the magic number used to specify
2878 object-file formats is unique. However, the configuration procedure for
2879 BFD on each system places the conventional format for that system first
2880 in the search-list, so ambiguities are resolved in favor of convention.
2884 @cindex default emulation
2885 @cindex emulation, default
2886 @code{LDEMULATION} determines the default emulation if you don't use the
2887 @samp{-m} option. The emulation can affect various aspects of linker
2888 behaviour, particularly the default linker script. You can list the
2889 available emulations with the @samp{--verbose} or @samp{-V} options. If
2890 the @samp{-m} option is not used, and the @code{LDEMULATION} environment
2891 variable is not defined, the default emulation depends upon how the
2892 linker was configured.
2894 @kindex COLLECT_NO_DEMANGLE
2895 @cindex demangling, default
2896 Normally, the linker will default to demangling symbols. However, if
2897 @code{COLLECT_NO_DEMANGLE} is set in the environment, then it will
2898 default to not demangling symbols. This environment variable is used in
2899 a similar fashion by the @code{gcc} linker wrapper program. The default
2900 may be overridden by the @samp{--demangle} and @samp{--no-demangle}
2907 @chapter Linker Scripts
2910 @cindex linker scripts
2911 @cindex command files
2912 Every link is controlled by a @dfn{linker script}. This script is
2913 written in the linker command language.
2915 The main purpose of the linker script is to describe how the sections in
2916 the input files should be mapped into the output file, and to control
2917 the memory layout of the output file. Most linker scripts do nothing
2918 more than this. However, when necessary, the linker script can also
2919 direct the linker to perform many other operations, using the commands
2922 The linker always uses a linker script. If you do not supply one
2923 yourself, the linker will use a default script that is compiled into the
2924 linker executable. You can use the @samp{--verbose} command line option
2925 to display the default linker script. Certain command line options,
2926 such as @samp{-r} or @samp{-N}, will affect the default linker script.
2928 You may supply your own linker script by using the @samp{-T} command
2929 line option. When you do this, your linker script will replace the
2930 default linker script.
2932 You may also use linker scripts implicitly by naming them as input files
2933 to the linker, as though they were files to be linked. @xref{Implicit
2937 * Basic Script Concepts:: Basic Linker Script Concepts
2938 * Script Format:: Linker Script Format
2939 * Simple Example:: Simple Linker Script Example
2940 * Simple Commands:: Simple Linker Script Commands
2941 * Assignments:: Assigning Values to Symbols
2942 * SECTIONS:: SECTIONS Command
2943 * MEMORY:: MEMORY Command
2944 * PHDRS:: PHDRS Command
2945 * VERSION:: VERSION Command
2946 * Expressions:: Expressions in Linker Scripts
2947 * Implicit Linker Scripts:: Implicit Linker Scripts
2950 @node Basic Script Concepts
2951 @section Basic Linker Script Concepts
2952 @cindex linker script concepts
2953 We need to define some basic concepts and vocabulary in order to
2954 describe the linker script language.
2956 The linker combines input files into a single output file. The output
2957 file and each input file are in a special data format known as an
2958 @dfn{object file format}. Each file is called an @dfn{object file}.
2959 The output file is often called an @dfn{executable}, but for our
2960 purposes we will also call it an object file. Each object file has,
2961 among other things, a list of @dfn{sections}. We sometimes refer to a
2962 section in an input file as an @dfn{input section}; similarly, a section
2963 in the output file is an @dfn{output section}.
2965 Each section in an object file has a name and a size. Most sections
2966 also have an associated block of data, known as the @dfn{section
2967 contents}. A section may be marked as @dfn{loadable}, which means that
2968 the contents should be loaded into memory when the output file is run.
2969 A section with no contents may be @dfn{allocatable}, which means that an
2970 area in memory should be set aside, but nothing in particular should be
2971 loaded there (in some cases this memory must be zeroed out). A section
2972 which is neither loadable nor allocatable typically contains some sort
2973 of debugging information.
2975 Every loadable or allocatable output section has two addresses. The
2976 first is the @dfn{VMA}, or virtual memory address. This is the address
2977 the section will have when the output file is run. The second is the
2978 @dfn{LMA}, or load memory address. This is the address at which the
2979 section will be loaded. In most cases the two addresses will be the
2980 same. An example of when they might be different is when a data section
2981 is loaded into ROM, and then copied into RAM when the program starts up
2982 (this technique is often used to initialize global variables in a ROM
2983 based system). In this case the ROM address would be the LMA, and the
2984 RAM address would be the VMA.
2986 You can see the sections in an object file by using the @code{objdump}
2987 program with the @samp{-h} option.
2989 Every object file also has a list of @dfn{symbols}, known as the
2990 @dfn{symbol table}. A symbol may be defined or undefined. Each symbol
2991 has a name, and each defined symbol has an address, among other
2992 information. If you compile a C or C++ program into an object file, you
2993 will get a defined symbol for every defined function and global or
2994 static variable. Every undefined function or global variable which is
2995 referenced in the input file will become an undefined symbol.
2997 You can see the symbols in an object file by using the @code{nm}
2998 program, or by using the @code{objdump} program with the @samp{-t}
3002 @section Linker Script Format
3003 @cindex linker script format
3004 Linker scripts are text files.
3006 You write a linker script as a series of commands. Each command is
3007 either a keyword, possibly followed by arguments, or an assignment to a
3008 symbol. You may separate commands using semicolons. Whitespace is
3011 Strings such as file or format names can normally be entered directly.
3012 If the file name contains a character such as a comma which would
3013 otherwise serve to separate file names, you may put the file name in
3014 double quotes. There is no way to use a double quote character in a
3017 You may include comments in linker scripts just as in C, delimited by
3018 @samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent
3021 @node Simple Example
3022 @section Simple Linker Script Example
3023 @cindex linker script example
3024 @cindex example of linker script
3025 Many linker scripts are fairly simple.
3027 The simplest possible linker script has just one command:
3028 @samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the
3029 memory layout of the output file.
3031 The @samp{SECTIONS} command is a powerful command. Here we will
3032 describe a simple use of it. Let's assume your program consists only of
3033 code, initialized data, and uninitialized data. These will be in the
3034 @samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively.
3035 Let's assume further that these are the only sections which appear in
3038 For this example, let's say that the code should be loaded at address
3039 0x10000, and that the data should start at address 0x8000000. Here is a
3040 linker script which will do that:
3045 .text : @{ *(.text) @}
3047 .data : @{ *(.data) @}
3048 .bss : @{ *(.bss) @}
3052 You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS},
3053 followed by a series of symbol assignments and output section
3054 descriptions enclosed in curly braces.
3056 The first line inside the @samp{SECTIONS} command of the above example
3057 sets the value of the special symbol @samp{.}, which is the location
3058 counter. If you do not specify the address of an output section in some
3059 other way (other ways are described later), the address is set from the
3060 current value of the location counter. The location counter is then
3061 incremented by the size of the output section. At the start of the
3062 @samp{SECTIONS} command, the location counter has the value @samp{0}.
3064 The second line defines an output section, @samp{.text}. The colon is
3065 required syntax which may be ignored for now. Within the curly braces
3066 after the output section name, you list the names of the input sections
3067 which should be placed into this output section. The @samp{*} is a
3068 wildcard which matches any file name. The expression @samp{*(.text)}
3069 means all @samp{.text} input sections in all input files.
3071 Since the location counter is @samp{0x10000} when the output section
3072 @samp{.text} is defined, the linker will set the address of the
3073 @samp{.text} section in the output file to be @samp{0x10000}.
3075 The remaining lines define the @samp{.data} and @samp{.bss} sections in
3076 the output file. The linker will place the @samp{.data} output section
3077 at address @samp{0x8000000}. After the linker places the @samp{.data}
3078 output section, the value of the location counter will be
3079 @samp{0x8000000} plus the size of the @samp{.data} output section. The
3080 effect is that the linker will place the @samp{.bss} output section
3081 immediately after the @samp{.data} output section in memory.
3083 The linker will ensure that each output section has the required
3084 alignment, by increasing the location counter if necessary. In this
3085 example, the specified addresses for the @samp{.text} and @samp{.data}
3086 sections will probably satisfy any alignment constraints, but the linker
3087 may have to create a small gap between the @samp{.data} and @samp{.bss}
3090 That's it! That's a simple and complete linker script.
3092 @node Simple Commands
3093 @section Simple Linker Script Commands
3094 @cindex linker script simple commands
3095 In this section we describe the simple linker script commands.
3098 * Entry Point:: Setting the entry point
3099 * File Commands:: Commands dealing with files
3100 @ifclear SingleFormat
3101 * Format Commands:: Commands dealing with object file formats
3104 * REGION_ALIAS:: Assign alias names to memory regions
3105 * Miscellaneous Commands:: Other linker script commands
3109 @subsection Setting the Entry Point
3110 @kindex ENTRY(@var{symbol})
3111 @cindex start of execution
3112 @cindex first instruction
3114 The first instruction to execute in a program is called the @dfn{entry
3115 point}. You can use the @code{ENTRY} linker script command to set the
3116 entry point. The argument is a symbol name:
3121 There are several ways to set the entry point. The linker will set the
3122 entry point by trying each of the following methods in order, and
3123 stopping when one of them succeeds:
3126 the @samp{-e} @var{entry} command-line option;
3128 the @code{ENTRY(@var{symbol})} command in a linker script;
3130 the value of a target specific symbol, if it is defined; For many
3131 targets this is @code{start}, but PE and BeOS based systems for example
3132 check a list of possible entry symbols, matching the first one found.
3134 the address of the first byte of the @samp{.text} section, if present;
3136 The address @code{0}.
3140 @subsection Commands Dealing with Files
3141 @cindex linker script file commands
3142 Several linker script commands deal with files.
3145 @item INCLUDE @var{filename}
3146 @kindex INCLUDE @var{filename}
3147 @cindex including a linker script
3148 Include the linker script @var{filename} at this point. The file will
3149 be searched for in the current directory, and in any directory specified
3150 with the @option{-L} option. You can nest calls to @code{INCLUDE} up to
3153 You can place @code{INCLUDE} directives at the top level, in @code{MEMORY} or
3154 @code{SECTIONS} commands, or in output section descriptions.
3156 @item INPUT(@var{file}, @var{file}, @dots{})
3157 @itemx INPUT(@var{file} @var{file} @dots{})
3158 @kindex INPUT(@var{files})
3159 @cindex input files in linker scripts
3160 @cindex input object files in linker scripts
3161 @cindex linker script input object files
3162 The @code{INPUT} command directs the linker to include the named files
3163 in the link, as though they were named on the command line.
3165 For example, if you always want to include @file{subr.o} any time you do
3166 a link, but you can't be bothered to put it on every link command line,
3167 then you can put @samp{INPUT (subr.o)} in your linker script.
3169 In fact, if you like, you can list all of your input files in the linker
3170 script, and then invoke the linker with nothing but a @samp{-T} option.
3172 In case a @dfn{sysroot prefix} is configured, and the filename starts
3173 with the @samp{/} character, and the script being processed was
3174 located inside the @dfn{sysroot prefix}, the filename will be looked
3175 for in the @dfn{sysroot prefix}. Otherwise, the linker will try to
3176 open the file in the current directory. If it is not found, the
3177 linker will search through the archive library search path.
3178 The @dfn{sysroot prefix} can also be forced by specifying @code{=}
3179 as the first character in the filename path. See also the
3180 description of @samp{-L} in @ref{Options,,Command Line Options}.
3182 If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the
3183 name to @code{lib@var{file}.a}, as with the command line argument
3186 When you use the @code{INPUT} command in an implicit linker script, the
3187 files will be included in the link at the point at which the linker
3188 script file is included. This can affect archive searching.
3190 @item GROUP(@var{file}, @var{file}, @dots{})
3191 @itemx GROUP(@var{file} @var{file} @dots{})
3192 @kindex GROUP(@var{files})
3193 @cindex grouping input files
3194 The @code{GROUP} command is like @code{INPUT}, except that the named
3195 files should all be archives, and they are searched repeatedly until no
3196 new undefined references are created. See the description of @samp{-(}
3197 in @ref{Options,,Command Line Options}.
3199 @item AS_NEEDED(@var{file}, @var{file}, @dots{})
3200 @itemx AS_NEEDED(@var{file} @var{file} @dots{})
3201 @kindex AS_NEEDED(@var{files})
3202 This construct can appear only inside of the @code{INPUT} or @code{GROUP}
3203 commands, among other filenames. The files listed will be handled
3204 as if they appear directly in the @code{INPUT} or @code{GROUP} commands,
3205 with the exception of ELF shared libraries, that will be added only
3206 when they are actually needed. This construct essentially enables
3207 @option{--as-needed} option for all the files listed inside of it
3208 and restores previous @option{--as-needed} resp. @option{--no-as-needed}
3211 @item OUTPUT(@var{filename})
3212 @kindex OUTPUT(@var{filename})
3213 @cindex output file name in linker script
3214 The @code{OUTPUT} command names the output file. Using
3215 @code{OUTPUT(@var{filename})} in the linker script is exactly like using
3216 @samp{-o @var{filename}} on the command line (@pxref{Options,,Command
3217 Line Options}). If both are used, the command line option takes
3220 You can use the @code{OUTPUT} command to define a default name for the
3221 output file other than the usual default of @file{a.out}.
3223 @item SEARCH_DIR(@var{path})
3224 @kindex SEARCH_DIR(@var{path})
3225 @cindex library search path in linker script
3226 @cindex archive search path in linker script
3227 @cindex search path in linker script
3228 The @code{SEARCH_DIR} command adds @var{path} to the list of paths where
3229 @command{ld} looks for archive libraries. Using
3230 @code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}}
3231 on the command line (@pxref{Options,,Command Line Options}). If both
3232 are used, then the linker will search both paths. Paths specified using
3233 the command line option are searched first.
3235 @item STARTUP(@var{filename})
3236 @kindex STARTUP(@var{filename})
3237 @cindex first input file
3238 The @code{STARTUP} command is just like the @code{INPUT} command, except
3239 that @var{filename} will become the first input file to be linked, as
3240 though it were specified first on the command line. This may be useful
3241 when using a system in which the entry point is always the start of the
3245 @ifclear SingleFormat
3246 @node Format Commands
3247 @subsection Commands Dealing with Object File Formats
3248 A couple of linker script commands deal with object file formats.
3251 @item OUTPUT_FORMAT(@var{bfdname})
3252 @itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little})
3253 @kindex OUTPUT_FORMAT(@var{bfdname})
3254 @cindex output file format in linker script
3255 The @code{OUTPUT_FORMAT} command names the BFD format to use for the
3256 output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is
3257 exactly like using @samp{--oformat @var{bfdname}} on the command line
3258 (@pxref{Options,,Command Line Options}). If both are used, the command
3259 line option takes precedence.
3261 You can use @code{OUTPUT_FORMAT} with three arguments to use different
3262 formats based on the @samp{-EB} and @samp{-EL} command line options.
3263 This permits the linker script to set the output format based on the
3266 If neither @samp{-EB} nor @samp{-EL} are used, then the output format
3267 will be the first argument, @var{default}. If @samp{-EB} is used, the
3268 output format will be the second argument, @var{big}. If @samp{-EL} is
3269 used, the output format will be the third argument, @var{little}.
3271 For example, the default linker script for the MIPS ELF target uses this
3274 OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
3276 This says that the default format for the output file is
3277 @samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line
3278 option, the output file will be created in the @samp{elf32-littlemips}
3281 @item TARGET(@var{bfdname})
3282 @kindex TARGET(@var{bfdname})
3283 @cindex input file format in linker script
3284 The @code{TARGET} command names the BFD format to use when reading input
3285 files. It affects subsequent @code{INPUT} and @code{GROUP} commands.
3286 This command is like using @samp{-b @var{bfdname}} on the command line
3287 (@pxref{Options,,Command Line Options}). If the @code{TARGET} command
3288 is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET}
3289 command is also used to set the format for the output file. @xref{BFD}.
3294 @subsection Assign alias names to memory regions
3295 @kindex REGION_ALIAS(@var{alias}, @var{region})
3296 @cindex region alias
3297 @cindex region names
3299 Alias names can be added to existing memory regions created with the
3300 @ref{MEMORY} command. Each name corresponds to at most one memory region.
3303 REGION_ALIAS(@var{alias}, @var{region})
3306 The @code{REGION_ALIAS} function creates an alias name @var{alias} for the
3307 memory region @var{region}. This allows a flexible mapping of output sections
3308 to memory regions. An example follows.
3310 Suppose we have an application for embedded systems which come with various
3311 memory storage devices. All have a general purpose, volatile memory @code{RAM}
3312 that allows code execution or data storage. Some may have a read-only,
3313 non-volatile memory @code{ROM} that allows code execution and read-only data
3314 access. The last variant is a read-only, non-volatile memory @code{ROM2} with
3315 read-only data access and no code execution capability. We have four output
3320 @code{.text} program code;
3322 @code{.rodata} read-only data;
3324 @code{.data} read-write initialized data;
3326 @code{.bss} read-write zero initialized data.
3329 The goal is to provide a linker command file that contains a system independent
3330 part defining the output sections and a system dependent part mapping the
3331 output sections to the memory regions available on the system. Our embedded
3332 systems come with three different memory setups @code{A}, @code{B} and
3334 @multitable @columnfractions .25 .25 .25 .25
3335 @item Section @tab Variant A @tab Variant B @tab Variant C
3336 @item .text @tab RAM @tab ROM @tab ROM
3337 @item .rodata @tab RAM @tab ROM @tab ROM2
3338 @item .data @tab RAM @tab RAM/ROM @tab RAM/ROM2
3339 @item .bss @tab RAM @tab RAM @tab RAM
3341 The notation @code{RAM/ROM} or @code{RAM/ROM2} means that this section is
3342 loaded into region @code{ROM} or @code{ROM2} respectively. Please note that
3343 the load address of the @code{.data} section starts in all three variants at
3344 the end of the @code{.rodata} section.
3346 The base linker script that deals with the output sections follows. It
3347 includes the system dependent @code{linkcmds.memory} file that describes the
3350 INCLUDE linkcmds.memory
3363 .data : AT (rodata_end)
3368 data_size = SIZEOF(.data);
3369 data_load_start = LOADADDR(.data);
3377 Now we need three different @code{linkcmds.memory} files to define memory
3378 regions and alias names. The content of @code{linkcmds.memory} for the three
3379 variants @code{A}, @code{B} and @code{C}:
3382 Here everything goes into the @code{RAM}.
3386 RAM : ORIGIN = 0, LENGTH = 4M
3389 REGION_ALIAS("REGION_TEXT", RAM);
3390 REGION_ALIAS("REGION_RODATA", RAM);
3391 REGION_ALIAS("REGION_DATA", RAM);
3392 REGION_ALIAS("REGION_BSS", RAM);
3395 Program code and read-only data go into the @code{ROM}. Read-write data goes
3396 into the @code{RAM}. An image of the initialized data is loaded into the
3397 @code{ROM} and will be copied during system start into the @code{RAM}.
3401 ROM : ORIGIN = 0, LENGTH = 3M
3402 RAM : ORIGIN = 0x10000000, LENGTH = 1M
3405 REGION_ALIAS("REGION_TEXT", ROM);
3406 REGION_ALIAS("REGION_RODATA", ROM);
3407 REGION_ALIAS("REGION_DATA", RAM);
3408 REGION_ALIAS("REGION_BSS", RAM);
3411 Program code goes into the @code{ROM}. Read-only data goes into the
3412 @code{ROM2}. Read-write data goes into the @code{RAM}. An image of the
3413 initialized data is loaded into the @code{ROM2} and will be copied during
3414 system start into the @code{RAM}.
3418 ROM : ORIGIN = 0, LENGTH = 2M
3419 ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
3420 RAM : ORIGIN = 0x20000000, LENGTH = 1M
3423 REGION_ALIAS("REGION_TEXT", ROM);
3424 REGION_ALIAS("REGION_RODATA", ROM2);
3425 REGION_ALIAS("REGION_DATA", RAM);
3426 REGION_ALIAS("REGION_BSS", RAM);
3430 It is possible to write a common system initialization routine to copy the
3431 @code{.data} section from @code{ROM} or @code{ROM2} into the @code{RAM} if
3436 extern char data_start [];
3437 extern char data_size [];
3438 extern char data_load_start [];
3440 void copy_data(void)
3442 if (data_start != data_load_start)
3444 memcpy(data_start, data_load_start, (size_t) data_size);
3449 @node Miscellaneous Commands
3450 @subsection Other Linker Script Commands
3451 There are a few other linker scripts commands.
3454 @item ASSERT(@var{exp}, @var{message})
3456 @cindex assertion in linker script
3457 Ensure that @var{exp} is non-zero. If it is zero, then exit the linker
3458 with an error code, and print @var{message}.
3460 @item EXTERN(@var{symbol} @var{symbol} @dots{})
3462 @cindex undefined symbol in linker script
3463 Force @var{symbol} to be entered in the output file as an undefined
3464 symbol. Doing this may, for example, trigger linking of additional
3465 modules from standard libraries. You may list several @var{symbol}s for
3466 each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This
3467 command has the same effect as the @samp{-u} command-line option.
3469 @item FORCE_COMMON_ALLOCATION
3470 @kindex FORCE_COMMON_ALLOCATION
3471 @cindex common allocation in linker script
3472 This command has the same effect as the @samp{-d} command-line option:
3473 to make @command{ld} assign space to common symbols even if a relocatable
3474 output file is specified (@samp{-r}).
3476 @item INHIBIT_COMMON_ALLOCATION
3477 @kindex INHIBIT_COMMON_ALLOCATION
3478 @cindex common allocation in linker script
3479 This command has the same effect as the @samp{--no-define-common}
3480 command-line option: to make @code{ld} omit the assignment of addresses
3481 to common symbols even for a non-relocatable output file.
3483 @item INSERT [ AFTER | BEFORE ] @var{output_section}
3485 @cindex insert user script into default script
3486 This command is typically used in a script specified by @samp{-T} to
3487 augment the default @code{SECTIONS} with, for example, overlays. It
3488 inserts all prior linker script statements after (or before)
3489 @var{output_section}, and also causes @samp{-T} to not override the
3490 default linker script. The exact insertion point is as for orphan
3491 sections. @xref{Location Counter}. The insertion happens after the
3492 linker has mapped input sections to output sections. Prior to the
3493 insertion, since @samp{-T} scripts are parsed before the default
3494 linker script, statements in the @samp{-T} script occur before the
3495 default linker script statements in the internal linker representation
3496 of the script. In particular, input section assignments will be made
3497 to @samp{-T} output sections before those in the default script. Here
3498 is an example of how a @samp{-T} script using @code{INSERT} might look:
3505 .ov1 @{ ov1*(.text) @}
3506 .ov2 @{ ov2*(.text) @}
3512 @item NOCROSSREFS(@var{section} @var{section} @dots{})
3513 @kindex NOCROSSREFS(@var{sections})
3514 @cindex cross references
3515 This command may be used to tell @command{ld} to issue an error about any
3516 references among certain output sections.
3518 In certain types of programs, particularly on embedded systems when
3519 using overlays, when one section is loaded into memory, another section
3520 will not be. Any direct references between the two sections would be
3521 errors. For example, it would be an error if code in one section called
3522 a function defined in the other section.
3524 The @code{NOCROSSREFS} command takes a list of output section names. If
3525 @command{ld} detects any cross references between the sections, it reports
3526 an error and returns a non-zero exit status. Note that the
3527 @code{NOCROSSREFS} command uses output section names, not input section
3530 @ifclear SingleFormat
3531 @item OUTPUT_ARCH(@var{bfdarch})
3532 @kindex OUTPUT_ARCH(@var{bfdarch})
3533 @cindex machine architecture
3534 @cindex architecture
3535 Specify a particular output machine architecture. The argument is one
3536 of the names used by the BFD library (@pxref{BFD}). You can see the
3537 architecture of an object file by using the @code{objdump} program with
3538 the @samp{-f} option.
3541 @item LD_FEATURE(@var{string})
3542 @kindex LD_FEATURE(@var{string})
3543 This command may be used to modify @command{ld} behavior. If
3544 @var{string} is @code{"SANE_EXPR"} then absolute symbols and numbers
3545 in a script are simply treated as numbers everywhere.
3546 @xref{Expression Section}.
3550 @section Assigning Values to Symbols
3551 @cindex assignment in scripts
3552 @cindex symbol definition, scripts
3553 @cindex variables, defining
3554 You may assign a value to a symbol in a linker script. This will define
3555 the symbol and place it into the symbol table with a global scope.
3558 * Simple Assignments:: Simple Assignments
3561 * PROVIDE_HIDDEN:: PROVIDE_HIDDEN
3562 * Source Code Reference:: How to use a linker script defined symbol in source code
3565 @node Simple Assignments
3566 @subsection Simple Assignments
3568 You may assign to a symbol using any of the C assignment operators:
3571 @item @var{symbol} = @var{expression} ;
3572 @itemx @var{symbol} += @var{expression} ;
3573 @itemx @var{symbol} -= @var{expression} ;
3574 @itemx @var{symbol} *= @var{expression} ;
3575 @itemx @var{symbol} /= @var{expression} ;
3576 @itemx @var{symbol} <<= @var{expression} ;
3577 @itemx @var{symbol} >>= @var{expression} ;
3578 @itemx @var{symbol} &= @var{expression} ;
3579 @itemx @var{symbol} |= @var{expression} ;
3582 The first case will define @var{symbol} to the value of
3583 @var{expression}. In the other cases, @var{symbol} must already be
3584 defined, and the value will be adjusted accordingly.
3586 The special symbol name @samp{.} indicates the location counter. You
3587 may only use this within a @code{SECTIONS} command. @xref{Location Counter}.
3589 The semicolon after @var{expression} is required.
3591 Expressions are defined below; see @ref{Expressions}.
3593 You may write symbol assignments as commands in their own right, or as
3594 statements within a @code{SECTIONS} command, or as part of an output
3595 section description in a @code{SECTIONS} command.
3597 The section of the symbol will be set from the section of the
3598 expression; for more information, see @ref{Expression Section}.
3600 Here is an example showing the three different places that symbol
3601 assignments may be used:
3612 _bdata = (. + 3) & ~ 3;
3613 .data : @{ *(.data) @}
3617 In this example, the symbol @samp{floating_point} will be defined as
3618 zero. The symbol @samp{_etext} will be defined as the address following
3619 the last @samp{.text} input section. The symbol @samp{_bdata} will be
3620 defined as the address following the @samp{.text} output section aligned
3621 upward to a 4 byte boundary.
3626 For ELF targeted ports, define a symbol that will be hidden and won't be
3627 exported. The syntax is @code{HIDDEN(@var{symbol} = @var{expression})}.
3629 Here is the example from @ref{Simple Assignments}, rewritten to use
3633 HIDDEN(floating_point = 0);
3641 HIDDEN(_bdata = (. + 3) & ~ 3);
3642 .data : @{ *(.data) @}
3646 In this case none of the three symbols will be visible outside this module.
3651 In some cases, it is desirable for a linker script to define a symbol
3652 only if it is referenced and is not defined by any object included in
3653 the link. For example, traditional linkers defined the symbol
3654 @samp{etext}. However, ANSI C requires that the user be able to use
3655 @samp{etext} as a function name without encountering an error. The
3656 @code{PROVIDE} keyword may be used to define a symbol, such as
3657 @samp{etext}, only if it is referenced but not defined. The syntax is
3658 @code{PROVIDE(@var{symbol} = @var{expression})}.
3660 Here is an example of using @code{PROVIDE} to define @samp{etext}:
3673 In this example, if the program defines @samp{_etext} (with a leading
3674 underscore), the linker will give a multiple definition error. If, on
3675 the other hand, the program defines @samp{etext} (with no leading
3676 underscore), the linker will silently use the definition in the program.
3677 If the program references @samp{etext} but does not define it, the
3678 linker will use the definition in the linker script.
3680 @node PROVIDE_HIDDEN
3681 @subsection PROVIDE_HIDDEN
3682 @cindex PROVIDE_HIDDEN
3683 Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be
3684 hidden and won't be exported.
3686 @node Source Code Reference
3687 @subsection Source Code Reference
3689 Accessing a linker script defined variable from source code is not
3690 intuitive. In particular a linker script symbol is not equivalent to
3691 a variable declaration in a high level language, it is instead a
3692 symbol that does not have a value.
3694 Before going further, it is important to note that compilers often
3695 transform names in the source code into different names when they are
3696 stored in the symbol table. For example, Fortran compilers commonly
3697 prepend or append an underscore, and C++ performs extensive @samp{name
3698 mangling}. Therefore there might be a discrepancy between the name
3699 of a variable as it is used in source code and the name of the same
3700 variable as it is defined in a linker script. For example in C a
3701 linker script variable might be referred to as:
3707 But in the linker script it might be defined as:
3713 In the remaining examples however it is assumed that no name
3714 transformation has taken place.
3716 When a symbol is declared in a high level language such as C, two
3717 things happen. The first is that the compiler reserves enough space
3718 in the program's memory to hold the @emph{value} of the symbol. The
3719 second is that the compiler creates an entry in the program's symbol
3720 table which holds the symbol's @emph{address}. ie the symbol table
3721 contains the address of the block of memory holding the symbol's
3722 value. So for example the following C declaration, at file scope:
3728 creates an entry called @samp{foo} in the symbol table. This entry
3729 holds the address of an @samp{int} sized block of memory where the
3730 number 1000 is initially stored.
3732 When a program references a symbol the compiler generates code that
3733 first accesses the symbol table to find the address of the symbol's
3734 memory block and then code to read the value from that memory block.
3741 looks up the symbol @samp{foo} in the symbol table, gets the address
3742 associated with this symbol and then writes the value 1 into that
3749 looks up the symbol @samp{foo} in the symbol table, gets its address
3750 and then copies this address into the block of memory associated with
3751 the variable @samp{a}.
3753 Linker scripts symbol declarations, by contrast, create an entry in
3754 the symbol table but do not assign any memory to them. Thus they are
3755 an address without a value. So for example the linker script definition:
3761 creates an entry in the symbol table called @samp{foo} which holds
3762 the address of memory location 1000, but nothing special is stored at
3763 address 1000. This means that you cannot access the @emph{value} of a
3764 linker script defined symbol - it has no value - all you can do is
3765 access the @emph{address} of a linker script defined symbol.
3767 Hence when you are using a linker script defined symbol in source code
3768 you should always take the address of the symbol, and never attempt to
3769 use its value. For example suppose you want to copy the contents of a
3770 section of memory called .ROM into a section called .FLASH and the
3771 linker script contains these declarations:
3775 start_of_ROM = .ROM;
3776 end_of_ROM = .ROM + sizeof (.ROM) - 1;
3777 start_of_FLASH = .FLASH;
3781 Then the C source code to perform the copy would be:
3785 extern char start_of_ROM, end_of_ROM, start_of_FLASH;
3787 memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
3791 Note the use of the @samp{&} operators. These are correct.
3794 @section SECTIONS Command
3796 The @code{SECTIONS} command tells the linker how to map input sections
3797 into output sections, and how to place the output sections in memory.
3799 The format of the @code{SECTIONS} command is:
3803 @var{sections-command}
3804 @var{sections-command}
3809 Each @var{sections-command} may of be one of the following:
3813 an @code{ENTRY} command (@pxref{Entry Point,,Entry command})
3815 a symbol assignment (@pxref{Assignments})
3817 an output section description
3819 an overlay description
3822 The @code{ENTRY} command and symbol assignments are permitted inside the
3823 @code{SECTIONS} command for convenience in using the location counter in
3824 those commands. This can also make the linker script easier to
3825 understand because you can use those commands at meaningful points in
3826 the layout of the output file.
3828 Output section descriptions and overlay descriptions are described
3831 If you do not use a @code{SECTIONS} command in your linker script, the
3832 linker will place each input section into an identically named output
3833 section in the order that the sections are first encountered in the
3834 input files. If all input sections are present in the first file, for
3835 example, the order of sections in the output file will match the order
3836 in the first input file. The first section will be at address zero.
3839 * Output Section Description:: Output section description
3840 * Output Section Name:: Output section name
3841 * Output Section Address:: Output section address
3842 * Input Section:: Input section description
3843 * Output Section Data:: Output section data
3844 * Output Section Keywords:: Output section keywords
3845 * Output Section Discarding:: Output section discarding
3846 * Output Section Attributes:: Output section attributes
3847 * Overlay Description:: Overlay description
3850 @node Output Section Description
3851 @subsection Output Section Description
3852 The full description of an output section looks like this:
3855 @var{section} [@var{address}] [(@var{type})] :
3857 [ALIGN(@var{section_align}) | ALIGN_WITH_INPUT]
3858 [SUBALIGN(@var{subsection_align})]
3861 @var{output-section-command}
3862 @var{output-section-command}
3864 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}] [,]
3868 Most output sections do not use most of the optional section attributes.
3870 The whitespace around @var{section} is required, so that the section
3871 name is unambiguous. The colon and the curly braces are also required.
3872 The comma at the end may be required if a @var{fillexp} is used and
3873 the next @var{sections-command} looks like a continuation of the expression.
3874 The line breaks and other white space are optional.
3876 Each @var{output-section-command} may be one of the following:
3880 a symbol assignment (@pxref{Assignments})
3882 an input section description (@pxref{Input Section})
3884 data values to include directly (@pxref{Output Section Data})
3886 a special output section keyword (@pxref{Output Section Keywords})
3889 @node Output Section Name
3890 @subsection Output Section Name
3891 @cindex name, section
3892 @cindex section name
3893 The name of the output section is @var{section}. @var{section} must
3894 meet the constraints of your output format. In formats which only
3895 support a limited number of sections, such as @code{a.out}, the name
3896 must be one of the names supported by the format (@code{a.out}, for
3897 example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the
3898 output format supports any number of sections, but with numbers and not
3899 names (as is the case for Oasys), the name should be supplied as a
3900 quoted numeric string. A section name may consist of any sequence of
3901 characters, but a name which contains any unusual characters such as
3902 commas must be quoted.
3904 The output section name @samp{/DISCARD/} is special; @ref{Output Section
3907 @node Output Section Address
3908 @subsection Output Section Address
3909 @cindex address, section
3910 @cindex section address
3911 The @var{address} is an expression for the VMA (the virtual memory
3912 address) of the output section. This address is optional, but if it
3913 is provided then the output address will be set exactly as specified.
3915 If the output address is not specified then one will be chosen for the
3916 section, based on the heuristic below. This address will be adjusted
3917 to fit the alignment requirement of the output section. The
3918 alignment requirement is the strictest alignment of any input section
3919 contained within the output section.
3921 The output section address heuristic is as follows:
3925 If an output memory @var{region} is set for the section then it
3926 is added to this region and its address will be the next free address
3930 If the MEMORY command has been used to create a list of memory
3931 regions then the first region which has attributes compatible with the
3932 section is selected to contain it. The section's output address will
3933 be the next free address in that region; @ref{MEMORY}.
3936 If no memory regions were specified, or none match the section then
3937 the output address will be based on the current value of the location
3945 .text . : @{ *(.text) @}
3952 .text : @{ *(.text) @}
3956 are subtly different. The first will set the address of the
3957 @samp{.text} output section to the current value of the location
3958 counter. The second will set it to the current value of the location
3959 counter aligned to the strictest alignment of any of the @samp{.text}
3962 The @var{address} may be an arbitrary expression; @ref{Expressions}.
3963 For example, if you want to align the section on a 0x10 byte boundary,
3964 so that the lowest four bits of the section address are zero, you could
3965 do something like this:
3967 .text ALIGN(0x10) : @{ *(.text) @}
3970 This works because @code{ALIGN} returns the current location counter
3971 aligned upward to the specified value.
3973 Specifying @var{address} for a section will change the value of the
3974 location counter, provided that the section is non-empty. (Empty
3975 sections are ignored).
3978 @subsection Input Section Description
3979 @cindex input sections
3980 @cindex mapping input sections to output sections
3981 The most common output section command is an input section description.
3983 The input section description is the most basic linker script operation.
3984 You use output sections to tell the linker how to lay out your program
3985 in memory. You use input section descriptions to tell the linker how to
3986 map the input files into your memory layout.
3989 * Input Section Basics:: Input section basics
3990 * Input Section Wildcards:: Input section wildcard patterns
3991 * Input Section Common:: Input section for common symbols
3992 * Input Section Keep:: Input section and garbage collection
3993 * Input Section Example:: Input section example
3996 @node Input Section Basics
3997 @subsubsection Input Section Basics
3998 @cindex input section basics
3999 An input section description consists of a file name optionally followed
4000 by a list of section names in parentheses.
4002 The file name and the section name may be wildcard patterns, which we
4003 describe further below (@pxref{Input Section Wildcards}).
4005 The most common input section description is to include all input
4006 sections with a particular name in the output section. For example, to
4007 include all input @samp{.text} sections, you would write:
4012 Here the @samp{*} is a wildcard which matches any file name. To exclude a list
4013 of files from matching the file name wildcard, EXCLUDE_FILE may be used to
4014 match all files except the ones specified in the EXCLUDE_FILE list. For
4017 *(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
4019 will cause all .ctors sections from all files except @file{crtend.o} and
4020 @file{otherfile.o} to be included.
4022 There are two ways to include more than one section:
4028 The difference between these is the order in which the @samp{.text} and
4029 @samp{.rdata} input sections will appear in the output section. In the
4030 first example, they will be intermingled, appearing in the same order as
4031 they are found in the linker input. In the second example, all
4032 @samp{.text} input sections will appear first, followed by all
4033 @samp{.rdata} input sections.
4035 You can specify a file name to include sections from a particular file.
4036 You would do this if one or more of your files contain special data that
4037 needs to be at a particular location in memory. For example:
4042 To refine the sections that are included based on the section flags
4043 of an input section, INPUT_SECTION_FLAGS may be used.
4045 Here is a simple example for using Section header flags for ELF sections:
4050 .text : @{ INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) @}
4051 .text2 : @{ INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) @}
4056 In this example, the output section @samp{.text} will be comprised of any
4057 input section matching the name *(.text) whose section header flags
4058 @code{SHF_MERGE} and @code{SHF_STRINGS} are set. The output section
4059 @samp{.text2} will be comprised of any input section matching the name *(.text)
4060 whose section header flag @code{SHF_WRITE} is clear.
4062 You can also specify files within archives by writing a pattern
4063 matching the archive, a colon, then the pattern matching the file,
4064 with no whitespace around the colon.
4068 matches file within archive
4070 matches the whole archive
4072 matches file but not one in an archive
4075 Either one or both of @samp{archive} and @samp{file} can contain shell
4076 wildcards. On DOS based file systems, the linker will assume that a
4077 single letter followed by a colon is a drive specifier, so
4078 @samp{c:myfile.o} is a simple file specification, not @samp{myfile.o}
4079 within an archive called @samp{c}. @samp{archive:file} filespecs may
4080 also be used within an @code{EXCLUDE_FILE} list, but may not appear in
4081 other linker script contexts. For instance, you cannot extract a file
4082 from an archive by using @samp{archive:file} in an @code{INPUT}
4085 If you use a file name without a list of sections, then all sections in
4086 the input file will be included in the output section. This is not
4087 commonly done, but it may by useful on occasion. For example:
4092 When you use a file name which is not an @samp{archive:file} specifier
4093 and does not contain any wild card
4094 characters, the linker will first see if you also specified the file
4095 name on the linker command line or in an @code{INPUT} command. If you
4096 did not, the linker will attempt to open the file as an input file, as
4097 though it appeared on the command line. Note that this differs from an
4098 @code{INPUT} command, because the linker will not search for the file in
4099 the archive search path.
4101 @node Input Section Wildcards
4102 @subsubsection Input Section Wildcard Patterns
4103 @cindex input section wildcards
4104 @cindex wildcard file name patterns
4105 @cindex file name wildcard patterns
4106 @cindex section name wildcard patterns
4107 In an input section description, either the file name or the section
4108 name or both may be wildcard patterns.
4110 The file name of @samp{*} seen in many examples is a simple wildcard
4111 pattern for the file name.
4113 The wildcard patterns are like those used by the Unix shell.
4117 matches any number of characters
4119 matches any single character
4121 matches a single instance of any of the @var{chars}; the @samp{-}
4122 character may be used to specify a range of characters, as in
4123 @samp{[a-z]} to match any lower case letter
4125 quotes the following character
4128 When a file name is matched with a wildcard, the wildcard characters
4129 will not match a @samp{/} character (used to separate directory names on
4130 Unix). A pattern consisting of a single @samp{*} character is an
4131 exception; it will always match any file name, whether it contains a
4132 @samp{/} or not. In a section name, the wildcard characters will match
4133 a @samp{/} character.
4135 File name wildcard patterns only match files which are explicitly
4136 specified on the command line or in an @code{INPUT} command. The linker
4137 does not search directories to expand wildcards.
4139 If a file name matches more than one wildcard pattern, or if a file name
4140 appears explicitly and is also matched by a wildcard pattern, the linker
4141 will use the first match in the linker script. For example, this
4142 sequence of input section descriptions is probably in error, because the
4143 @file{data.o} rule will not be used:
4145 .data : @{ *(.data) @}
4146 .data1 : @{ data.o(.data) @}
4149 @cindex SORT_BY_NAME
4150 Normally, the linker will place files and sections matched by wildcards
4151 in the order in which they are seen during the link. You can change
4152 this by using the @code{SORT_BY_NAME} keyword, which appears before a wildcard
4153 pattern in parentheses (e.g., @code{SORT_BY_NAME(.text*)}). When the
4154 @code{SORT_BY_NAME} keyword is used, the linker will sort the files or sections
4155 into ascending order by name before placing them in the output file.
4157 @cindex SORT_BY_ALIGNMENT
4158 @code{SORT_BY_ALIGNMENT} is very similar to @code{SORT_BY_NAME}. The
4159 difference is @code{SORT_BY_ALIGNMENT} will sort sections into
4160 descending order by alignment before placing them in the output file.
4161 Larger alignments are placed before smaller alignments in order to
4162 reduce the amount of padding necessary.
4164 @cindex SORT_BY_INIT_PRIORITY
4165 @code{SORT_BY_INIT_PRIORITY} is very similar to @code{SORT_BY_NAME}. The
4166 difference is @code{SORT_BY_INIT_PRIORITY} will sort sections into
4167 ascending order by numerical value of the GCC init_priority attribute
4168 encoded in the section name before placing them in the output file.
4171 @code{SORT} is an alias for @code{SORT_BY_NAME}.
4173 When there are nested section sorting commands in linker script, there
4174 can be at most 1 level of nesting for section sorting commands.
4178 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
4179 It will sort the input sections by name first, then by alignment if two
4180 sections have the same name.
4182 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
4183 It will sort the input sections by alignment first, then by name if two
4184 sections have the same alignment.
4186 @code{SORT_BY_NAME} (@code{SORT_BY_NAME} (wildcard section pattern)) is
4187 treated the same as @code{SORT_BY_NAME} (wildcard section pattern).
4189 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern))
4190 is treated the same as @code{SORT_BY_ALIGNMENT} (wildcard section pattern).
4192 All other nested section sorting commands are invalid.
4195 When both command line section sorting option and linker script
4196 section sorting command are used, section sorting command always
4197 takes precedence over the command line option.
4199 If the section sorting command in linker script isn't nested, the
4200 command line option will make the section sorting command to be
4201 treated as nested sorting command.
4205 @code{SORT_BY_NAME} (wildcard section pattern ) with
4206 @option{--sort-sections alignment} is equivalent to
4207 @code{SORT_BY_NAME} (@code{SORT_BY_ALIGNMENT} (wildcard section pattern)).
4209 @code{SORT_BY_ALIGNMENT} (wildcard section pattern) with
4210 @option{--sort-section name} is equivalent to
4211 @code{SORT_BY_ALIGNMENT} (@code{SORT_BY_NAME} (wildcard section pattern)).
4214 If the section sorting command in linker script is nested, the
4215 command line option will be ignored.
4218 @code{SORT_NONE} disables section sorting by ignoring the command line
4219 section sorting option.
4221 If you ever get confused about where input sections are going, use the
4222 @samp{-M} linker option to generate a map file. The map file shows
4223 precisely how input sections are mapped to output sections.
4225 This example shows how wildcard patterns might be used to partition
4226 files. This linker script directs the linker to place all @samp{.text}
4227 sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}.
4228 The linker will place the @samp{.data} section from all files beginning
4229 with an upper case character in @samp{.DATA}; for all other files, the
4230 linker will place the @samp{.data} section in @samp{.data}.
4234 .text : @{ *(.text) @}
4235 .DATA : @{ [A-Z]*(.data) @}
4236 .data : @{ *(.data) @}
4237 .bss : @{ *(.bss) @}
4242 @node Input Section Common
4243 @subsubsection Input Section for Common Symbols
4244 @cindex common symbol placement
4245 @cindex uninitialized data placement
4246 A special notation is needed for common symbols, because in many object
4247 file formats common symbols do not have a particular input section. The
4248 linker treats common symbols as though they are in an input section
4249 named @samp{COMMON}.
4251 You may use file names with the @samp{COMMON} section just as with any
4252 other input sections. You can use this to place common symbols from a
4253 particular input file in one section while common symbols from other
4254 input files are placed in another section.
4256 In most cases, common symbols in input files will be placed in the
4257 @samp{.bss} section in the output file. For example:
4259 .bss @{ *(.bss) *(COMMON) @}
4262 @cindex scommon section
4263 @cindex small common symbols
4264 Some object file formats have more than one type of common symbol. For
4265 example, the MIPS ELF object file format distinguishes standard common
4266 symbols and small common symbols. In this case, the linker will use a
4267 different special section name for other types of common symbols. In
4268 the case of MIPS ELF, the linker uses @samp{COMMON} for standard common
4269 symbols and @samp{.scommon} for small common symbols. This permits you
4270 to map the different types of common symbols into memory at different
4274 You will sometimes see @samp{[COMMON]} in old linker scripts. This
4275 notation is now considered obsolete. It is equivalent to
4278 @node Input Section Keep
4279 @subsubsection Input Section and Garbage Collection
4281 @cindex garbage collection
4282 When link-time garbage collection is in use (@samp{--gc-sections}),
4283 it is often useful to mark sections that should not be eliminated.
4284 This is accomplished by surrounding an input section's wildcard entry
4285 with @code{KEEP()}, as in @code{KEEP(*(.init))} or
4286 @code{KEEP(SORT_BY_NAME(*)(.ctors))}.
4288 @node Input Section Example
4289 @subsubsection Input Section Example
4290 The following example is a complete linker script. It tells the linker
4291 to read all of the sections from file @file{all.o} and place them at the
4292 start of output section @samp{outputa} which starts at location
4293 @samp{0x10000}. All of section @samp{.input1} from file @file{foo.o}
4294 follows immediately, in the same output section. All of section
4295 @samp{.input2} from @file{foo.o} goes into output section
4296 @samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}.
4297 All of the remaining @samp{.input1} and @samp{.input2} sections from any
4298 files are written to output section @samp{outputc}.
4326 @node Output Section Data
4327 @subsection Output Section Data
4329 @cindex section data
4330 @cindex output section data
4331 @kindex BYTE(@var{expression})
4332 @kindex SHORT(@var{expression})
4333 @kindex LONG(@var{expression})
4334 @kindex QUAD(@var{expression})
4335 @kindex SQUAD(@var{expression})
4336 You can include explicit bytes of data in an output section by using
4337 @code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as
4338 an output section command. Each keyword is followed by an expression in
4339 parentheses providing the value to store (@pxref{Expressions}). The
4340 value of the expression is stored at the current value of the location
4343 The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands
4344 store one, two, four, and eight bytes (respectively). After storing the
4345 bytes, the location counter is incremented by the number of bytes
4348 For example, this will store the byte 1 followed by the four byte value
4349 of the symbol @samp{addr}:
4355 When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the
4356 same; they both store an 8 byte, or 64 bit, value. When both host and
4357 target are 32 bits, an expression is computed as 32 bits. In this case
4358 @code{QUAD} stores a 32 bit value zero extended to 64 bits, and
4359 @code{SQUAD} stores a 32 bit value sign extended to 64 bits.
4361 If the object file format of the output file has an explicit endianness,
4362 which is the normal case, the value will be stored in that endianness.
4363 When the object file format does not have an explicit endianness, as is
4364 true of, for example, S-records, the value will be stored in the
4365 endianness of the first input object file.
4367 Note---these commands only work inside a section description and not
4368 between them, so the following will produce an error from the linker:
4370 SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@
4372 whereas this will work:
4374 SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@
4377 @kindex FILL(@var{expression})
4378 @cindex holes, filling
4379 @cindex unspecified memory
4380 You may use the @code{FILL} command to set the fill pattern for the
4381 current section. It is followed by an expression in parentheses. Any
4382 otherwise unspecified regions of memory within the section (for example,
4383 gaps left due to the required alignment of input sections) are filled
4384 with the value of the expression, repeated as
4385 necessary. A @code{FILL} statement covers memory locations after the
4386 point at which it occurs in the section definition; by including more
4387 than one @code{FILL} statement, you can have different fill patterns in
4388 different parts of an output section.
4390 This example shows how to fill unspecified regions of memory with the
4396 The @code{FILL} command is similar to the @samp{=@var{fillexp}} output
4397 section attribute, but it only affects the
4398 part of the section following the @code{FILL} command, rather than the
4399 entire section. If both are used, the @code{FILL} command takes
4400 precedence. @xref{Output Section Fill}, for details on the fill
4403 @node Output Section Keywords
4404 @subsection Output Section Keywords
4405 There are a couple of keywords which can appear as output section
4409 @kindex CREATE_OBJECT_SYMBOLS
4410 @cindex input filename symbols
4411 @cindex filename symbols
4412 @item CREATE_OBJECT_SYMBOLS
4413 The command tells the linker to create a symbol for each input file.
4414 The name of each symbol will be the name of the corresponding input
4415 file. The section of each symbol will be the output section in which
4416 the @code{CREATE_OBJECT_SYMBOLS} command appears.
4418 This is conventional for the a.out object file format. It is not
4419 normally used for any other object file format.
4421 @kindex CONSTRUCTORS
4422 @cindex C++ constructors, arranging in link
4423 @cindex constructors, arranging in link
4425 When linking using the a.out object file format, the linker uses an
4426 unusual set construct to support C++ global constructors and
4427 destructors. When linking object file formats which do not support
4428 arbitrary sections, such as ECOFF and XCOFF, the linker will
4429 automatically recognize C++ global constructors and destructors by name.
4430 For these object file formats, the @code{CONSTRUCTORS} command tells the
4431 linker to place constructor information in the output section where the
4432 @code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is
4433 ignored for other object file formats.
4435 The symbol @w{@code{__CTOR_LIST__}} marks the start of the global
4436 constructors, and the symbol @w{@code{__CTOR_END__}} marks the end.
4437 Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_END__}} mark
4438 the start and end of the global destructors. The
4439 first word in the list is the number of entries, followed by the address
4440 of each constructor or destructor, followed by a zero word. The
4441 compiler must arrange to actually run the code. For these object file
4442 formats @sc{gnu} C++ normally calls constructors from a subroutine
4443 @code{__main}; a call to @code{__main} is automatically inserted into
4444 the startup code for @code{main}. @sc{gnu} C++ normally runs
4445 destructors either by using @code{atexit}, or directly from the function
4448 For object file formats such as @code{COFF} or @code{ELF} which support
4449 arbitrary section names, @sc{gnu} C++ will normally arrange to put the
4450 addresses of global constructors and destructors into the @code{.ctors}
4451 and @code{.dtors} sections. Placing the following sequence into your
4452 linker script will build the sort of table which the @sc{gnu} C++
4453 runtime code expects to see.
4457 LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
4462 LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
4468 If you are using the @sc{gnu} C++ support for initialization priority,
4469 which provides some control over the order in which global constructors
4470 are run, you must sort the constructors at link time to ensure that they
4471 are executed in the correct order. When using the @code{CONSTRUCTORS}
4472 command, use @samp{SORT_BY_NAME(CONSTRUCTORS)} instead. When using the
4473 @code{.ctors} and @code{.dtors} sections, use @samp{*(SORT_BY_NAME(.ctors))} and
4474 @samp{*(SORT_BY_NAME(.dtors))} instead of just @samp{*(.ctors)} and
4477 Normally the compiler and linker will handle these issues automatically,
4478 and you will not need to concern yourself with them. However, you may
4479 need to consider this if you are using C++ and writing your own linker
4484 @node Output Section Discarding
4485 @subsection Output Section Discarding
4486 @cindex discarding sections
4487 @cindex sections, discarding
4488 @cindex removing sections
4489 The linker will not normally create output sections with no contents.
4490 This is for convenience when referring to input sections that may or
4491 may not be present in any of the input files. For example:
4493 .foo : @{ *(.foo) @}
4496 will only create a @samp{.foo} section in the output file if there is a
4497 @samp{.foo} section in at least one input file, and if the input
4498 sections are not all empty. Other link script directives that allocate
4499 space in an output section will also create the output section. So
4500 too will assignments to dot even if the assignment does not create
4501 space, except for @samp{. = 0}, @samp{. = . + 0}, @samp{. = sym},
4502 @samp{. = . + sym} and @samp{. = ALIGN (. != 0, expr, 1)} when
4503 @samp{sym} is an absolute symbol of value 0 defined in the script.
4504 This allows you to force output of an empty section with @samp{. = .}.
4506 The linker will ignore address assignments (@pxref{Output Section Address})
4507 on discarded output sections, except when the linker script defines
4508 symbols in the output section. In that case the linker will obey
4509 the address assignments, possibly advancing dot even though the
4510 section is discarded.
4513 The special output section name @samp{/DISCARD/} may be used to discard
4514 input sections. Any input sections which are assigned to an output
4515 section named @samp{/DISCARD/} are not included in the output file.
4517 @node Output Section Attributes
4518 @subsection Output Section Attributes
4519 @cindex output section attributes
4520 We showed above that the full description of an output section looked
4525 @var{section} [@var{address}] [(@var{type})] :
4527 [ALIGN(@var{section_align})]
4528 [SUBALIGN(@var{subsection_align})]
4531 @var{output-section-command}
4532 @var{output-section-command}
4534 @} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}]
4538 We've already described @var{section}, @var{address}, and
4539 @var{output-section-command}. In this section we will describe the
4540 remaining section attributes.
4543 * Output Section Type:: Output section type
4544 * Output Section LMA:: Output section LMA
4545 * Forced Output Alignment:: Forced Output Alignment
4546 * Forced Input Alignment:: Forced Input Alignment
4547 * Output Section Constraint:: Output section constraint
4548 * Output Section Region:: Output section region
4549 * Output Section Phdr:: Output section phdr
4550 * Output Section Fill:: Output section fill
4553 @node Output Section Type
4554 @subsubsection Output Section Type
4555 Each output section may have a type. The type is a keyword in
4556 parentheses. The following types are defined:
4560 The section should be marked as not loadable, so that it will not be
4561 loaded into memory when the program is run.
4566 These type names are supported for backward compatibility, and are
4567 rarely used. They all have the same effect: the section should be
4568 marked as not allocatable, so that no memory is allocated for the
4569 section when the program is run.
4573 @cindex prevent unnecessary loading
4574 @cindex loading, preventing
4575 The linker normally sets the attributes of an output section based on
4576 the input sections which map into it. You can override this by using
4577 the section type. For example, in the script sample below, the
4578 @samp{ROM} section is addressed at memory location @samp{0} and does not
4579 need to be loaded when the program is run.
4583 ROM 0 (NOLOAD) : @{ @dots{} @}
4589 @node Output Section LMA
4590 @subsubsection Output Section LMA
4591 @kindex AT>@var{lma_region}
4592 @kindex AT(@var{lma})
4593 @cindex load address
4594 @cindex section load address
4595 Every section has a virtual address (VMA) and a load address (LMA); see
4596 @ref{Basic Script Concepts}. The virtual address is specified by the
4597 @pxref{Output Section Address} described earlier. The load address is
4598 specified by the @code{AT} or @code{AT>} keywords. Specifying a load
4599 address is optional.
4601 The @code{AT} keyword takes an expression as an argument. This
4602 specifies the exact load address of the section. The @code{AT>} keyword
4603 takes the name of a memory region as an argument. @xref{MEMORY}. The
4604 load address of the section is set to the next free address in the
4605 region, aligned to the section's alignment requirements.
4607 If neither @code{AT} nor @code{AT>} is specified for an allocatable
4608 section, the linker will use the following heuristic to determine the
4613 If the section has a specific VMA address, then this is used as
4614 the LMA address as well.
4617 If the section is not allocatable then its LMA is set to its VMA.
4620 Otherwise if a memory region can be found that is compatible
4621 with the current section, and this region contains at least one
4622 section, then the LMA is set so the difference between the
4623 VMA and LMA is the same as the difference between the VMA and LMA of
4624 the last section in the located region.
4627 If no memory regions have been declared then a default region
4628 that covers the entire address space is used in the previous step.
4631 If no suitable region could be found, or there was no previous
4632 section then the LMA is set equal to the VMA.
4635 @cindex ROM initialized data
4636 @cindex initialized data in ROM
4637 This feature is designed to make it easy to build a ROM image. For
4638 example, the following linker script creates three output sections: one
4639 called @samp{.text}, which starts at @code{0x1000}, one called
4640 @samp{.mdata}, which is loaded at the end of the @samp{.text} section
4641 even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold
4642 uninitialized data at address @code{0x3000}. The symbol @code{_data} is
4643 defined with the value @code{0x2000}, which shows that the location
4644 counter holds the VMA value, not the LMA value.
4650 .text 0x1000 : @{ *(.text) _etext = . ; @}
4652 AT ( ADDR (.text) + SIZEOF (.text) )
4653 @{ _data = . ; *(.data); _edata = . ; @}
4655 @{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@}
4660 The run-time initialization code for use with a program generated with
4661 this linker script would include something like the following, to copy
4662 the initialized data from the ROM image to its runtime address. Notice
4663 how this code takes advantage of the symbols defined by the linker
4668 extern char _etext, _data, _edata, _bstart, _bend;
4669 char *src = &_etext;
4672 /* ROM has data at end of text; copy it. */
4673 while (dst < &_edata)
4677 for (dst = &_bstart; dst< &_bend; dst++)
4682 @node Forced Output Alignment
4683 @subsubsection Forced Output Alignment
4684 @kindex ALIGN(@var{section_align})
4685 @cindex forcing output section alignment
4686 @cindex output section alignment
4687 You can increase an output section's alignment by using ALIGN. As an
4688 alternative you can enforce that the difference between the VMA and LMA remains
4689 intact throughout this output section with the ALIGN_WITH_INPUT attribute.
4691 @node Forced Input Alignment
4692 @subsubsection Forced Input Alignment
4693 @kindex SUBALIGN(@var{subsection_align})
4694 @cindex forcing input section alignment
4695 @cindex input section alignment
4696 You can force input section alignment within an output section by using
4697 SUBALIGN. The value specified overrides any alignment given by input
4698 sections, whether larger or smaller.
4700 @node Output Section Constraint
4701 @subsubsection Output Section Constraint
4704 @cindex constraints on output sections
4705 You can specify that an output section should only be created if all
4706 of its input sections are read-only or all of its input sections are
4707 read-write by using the keyword @code{ONLY_IF_RO} and
4708 @code{ONLY_IF_RW} respectively.
4710 @node Output Section Region
4711 @subsubsection Output Section Region
4712 @kindex >@var{region}
4713 @cindex section, assigning to memory region
4714 @cindex memory regions and sections
4715 You can assign a section to a previously defined region of memory by
4716 using @samp{>@var{region}}. @xref{MEMORY}.
4718 Here is a simple example:
4721 MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @}
4722 SECTIONS @{ ROM : @{ *(.text) @} >rom @}
4726 @node Output Section Phdr
4727 @subsubsection Output Section Phdr
4729 @cindex section, assigning to program header
4730 @cindex program headers and sections
4731 You can assign a section to a previously defined program segment by
4732 using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to
4733 one or more segments, then all subsequent allocated sections will be
4734 assigned to those segments as well, unless they use an explicitly
4735 @code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the
4736 linker to not put the section in any segment at all.
4738 Here is a simple example:
4741 PHDRS @{ text PT_LOAD ; @}
4742 SECTIONS @{ .text : @{ *(.text) @} :text @}
4746 @node Output Section Fill
4747 @subsubsection Output Section Fill
4748 @kindex =@var{fillexp}
4749 @cindex section fill pattern
4750 @cindex fill pattern, entire section
4751 You can set the fill pattern for an entire section by using
4752 @samp{=@var{fillexp}}. @var{fillexp} is an expression
4753 (@pxref{Expressions}). Any otherwise unspecified regions of memory
4754 within the output section (for example, gaps left due to the required
4755 alignment of input sections) will be filled with the value, repeated as
4756 necessary. If the fill expression is a simple hex number, ie. a string
4757 of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then
4758 an arbitrarily long sequence of hex digits can be used to specify the
4759 fill pattern; Leading zeros become part of the pattern too. For all
4760 other cases, including extra parentheses or a unary @code{+}, the fill
4761 pattern is the four least significant bytes of the value of the
4762 expression. In all cases, the number is big-endian.
4764 You can also change the fill value with a @code{FILL} command in the
4765 output section commands; (@pxref{Output Section Data}).
4767 Here is a simple example:
4770 SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @}
4774 @node Overlay Description
4775 @subsection Overlay Description
4778 An overlay description provides an easy way to describe sections which
4779 are to be loaded as part of a single memory image but are to be run at
4780 the same memory address. At run time, some sort of overlay manager will
4781 copy the overlaid sections in and out of the runtime memory address as
4782 required, perhaps by simply manipulating addressing bits. This approach
4783 can be useful, for example, when a certain region of memory is faster
4786 Overlays are described using the @code{OVERLAY} command. The
4787 @code{OVERLAY} command is used within a @code{SECTIONS} command, like an
4788 output section description. The full syntax of the @code{OVERLAY}
4789 command is as follows:
4792 OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )]
4796 @var{output-section-command}
4797 @var{output-section-command}
4799 @} [:@var{phdr}@dots{}] [=@var{fill}]
4802 @var{output-section-command}
4803 @var{output-section-command}
4805 @} [:@var{phdr}@dots{}] [=@var{fill}]
4807 @} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}] [,]
4811 Everything is optional except @code{OVERLAY} (a keyword), and each
4812 section must have a name (@var{secname1} and @var{secname2} above). The
4813 section definitions within the @code{OVERLAY} construct are identical to
4814 those within the general @code{SECTIONS} construct (@pxref{SECTIONS}),
4815 except that no addresses and no memory regions may be defined for
4816 sections within an @code{OVERLAY}.
4818 The comma at the end may be required if a @var{fill} is used and
4819 the next @var{sections-command} looks like a continuation of the expression.
4821 The sections are all defined with the same starting address. The load
4822 addresses of the sections are arranged such that they are consecutive in
4823 memory starting at the load address used for the @code{OVERLAY} as a
4824 whole (as with normal section definitions, the load address is optional,
4825 and defaults to the start address; the start address is also optional,
4826 and defaults to the current value of the location counter).
4828 If the @code{NOCROSSREFS} keyword is used, and there are any
4829 references among the sections, the linker will report an error. Since
4830 the sections all run at the same address, it normally does not make
4831 sense for one section to refer directly to another.
4832 @xref{Miscellaneous Commands, NOCROSSREFS}.
4834 For each section within the @code{OVERLAY}, the linker automatically
4835 provides two symbols. The symbol @code{__load_start_@var{secname}} is
4836 defined as the starting load address of the section. The symbol
4837 @code{__load_stop_@var{secname}} is defined as the final load address of
4838 the section. Any characters within @var{secname} which are not legal
4839 within C identifiers are removed. C (or assembler) code may use these
4840 symbols to move the overlaid sections around as necessary.
4842 At the end of the overlay, the value of the location counter is set to
4843 the start address of the overlay plus the size of the largest section.
4845 Here is an example. Remember that this would appear inside a
4846 @code{SECTIONS} construct.
4849 OVERLAY 0x1000 : AT (0x4000)
4851 .text0 @{ o1/*.o(.text) @}
4852 .text1 @{ o2/*.o(.text) @}
4857 This will define both @samp{.text0} and @samp{.text1} to start at
4858 address 0x1000. @samp{.text0} will be loaded at address 0x4000, and
4859 @samp{.text1} will be loaded immediately after @samp{.text0}. The
4860 following symbols will be defined if referenced: @code{__load_start_text0},
4861 @code{__load_stop_text0}, @code{__load_start_text1},
4862 @code{__load_stop_text1}.
4864 C code to copy overlay @code{.text1} into the overlay area might look
4869 extern char __load_start_text1, __load_stop_text1;
4870 memcpy ((char *) 0x1000, &__load_start_text1,
4871 &__load_stop_text1 - &__load_start_text1);
4875 Note that the @code{OVERLAY} command is just syntactic sugar, since
4876 everything it does can be done using the more basic commands. The above
4877 example could have been written identically as follows.
4881 .text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @}
4882 PROVIDE (__load_start_text0 = LOADADDR (.text0));
4883 PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
4884 .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @}
4885 PROVIDE (__load_start_text1 = LOADADDR (.text1));
4886 PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
4887 . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
4892 @section MEMORY Command
4894 @cindex memory regions
4895 @cindex regions of memory
4896 @cindex allocating memory
4897 @cindex discontinuous memory
4898 The linker's default configuration permits allocation of all available
4899 memory. You can override this by using the @code{MEMORY} command.
4901 The @code{MEMORY} command describes the location and size of blocks of
4902 memory in the target. You can use it to describe which memory regions
4903 may be used by the linker, and which memory regions it must avoid. You
4904 can then assign sections to particular memory regions. The linker will
4905 set section addresses based on the memory regions, and will warn about
4906 regions that become too full. The linker will not shuffle sections
4907 around to fit into the available regions.
4909 A linker script may contain at most one use of the @code{MEMORY}
4910 command. However, you can define as many blocks of memory within it as
4911 you wish. The syntax is:
4916 @var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len}
4922 The @var{name} is a name used in the linker script to refer to the
4923 region. The region name has no meaning outside of the linker script.
4924 Region names are stored in a separate name space, and will not conflict
4925 with symbol names, file names, or section names. Each memory region
4926 must have a distinct name within the @code{MEMORY} command. However you can
4927 add later alias names to existing memory regions with the @ref{REGION_ALIAS}
4930 @cindex memory region attributes
4931 The @var{attr} string is an optional list of attributes that specify
4932 whether to use a particular memory region for an input section which is
4933 not explicitly mapped in the linker script. As described in
4934 @ref{SECTIONS}, if you do not specify an output section for some input
4935 section, the linker will create an output section with the same name as
4936 the input section. If you define region attributes, the linker will use
4937 them to select the memory region for the output section that it creates.
4939 The @var{attr} string must consist only of the following characters:
4954 Invert the sense of any of the attributes that follow
4957 If a unmapped section matches any of the listed attributes other than
4958 @samp{!}, it will be placed in the memory region. The @samp{!}
4959 attribute reverses this test, so that an unmapped section will be placed
4960 in the memory region only if it does not match any of the listed
4966 The @var{origin} is an numerical expression for the start address of
4967 the memory region. The expression must evaluate to a constant and it
4968 cannot involve any symbols. The keyword @code{ORIGIN} may be
4969 abbreviated to @code{org} or @code{o} (but not, for example,
4975 The @var{len} is an expression for the size in bytes of the memory
4976 region. As with the @var{origin} expression, the expression must
4977 be numerical only and must evaluate to a constant. The keyword
4978 @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
4980 In the following example, we specify that there are two memory regions
4981 available for allocation: one starting at @samp{0} for 256 kilobytes,
4982 and the other starting at @samp{0x40000000} for four megabytes. The
4983 linker will place into the @samp{rom} memory region every section which
4984 is not explicitly mapped into a memory region, and is either read-only
4985 or executable. The linker will place other sections which are not
4986 explicitly mapped into a memory region into the @samp{ram} memory
4993 rom (rx) : ORIGIN = 0, LENGTH = 256K
4994 ram (!rx) : org = 0x40000000, l = 4M
4999 Once you define a memory region, you can direct the linker to place
5000 specific output sections into that memory region by using the
5001 @samp{>@var{region}} output section attribute. For example, if you have
5002 a memory region named @samp{mem}, you would use @samp{>mem} in the
5003 output section definition. @xref{Output Section Region}. If no address
5004 was specified for the output section, the linker will set the address to
5005 the next available address within the memory region. If the combined
5006 output sections directed to a memory region are too large for the
5007 region, the linker will issue an error message.
5009 It is possible to access the origin and length of a memory in an
5010 expression via the @code{ORIGIN(@var{memory})} and
5011 @code{LENGTH(@var{memory})} functions:
5015 _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
5020 @section PHDRS Command
5022 @cindex program headers
5023 @cindex ELF program headers
5024 @cindex program segments
5025 @cindex segments, ELF
5026 The ELF object file format uses @dfn{program headers}, also knows as
5027 @dfn{segments}. The program headers describe how the program should be
5028 loaded into memory. You can print them out by using the @code{objdump}
5029 program with the @samp{-p} option.
5031 When you run an ELF program on a native ELF system, the system loader
5032 reads the program headers in order to figure out how to load the
5033 program. This will only work if the program headers are set correctly.
5034 This manual does not describe the details of how the system loader
5035 interprets program headers; for more information, see the ELF ABI.
5037 The linker will create reasonable program headers by default. However,
5038 in some cases, you may need to specify the program headers more
5039 precisely. You may use the @code{PHDRS} command for this purpose. When
5040 the linker sees the @code{PHDRS} command in the linker script, it will
5041 not create any program headers other than the ones specified.
5043 The linker only pays attention to the @code{PHDRS} command when
5044 generating an ELF output file. In other cases, the linker will simply
5045 ignore @code{PHDRS}.
5047 This is the syntax of the @code{PHDRS} command. The words @code{PHDRS},
5048 @code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords.
5054 @var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ]
5055 [ FLAGS ( @var{flags} ) ] ;
5060 The @var{name} is used only for reference in the @code{SECTIONS} command
5061 of the linker script. It is not put into the output file. Program
5062 header names are stored in a separate name space, and will not conflict
5063 with symbol names, file names, or section names. Each program header
5064 must have a distinct name. The headers are processed in order and it
5065 is usual for them to map to sections in ascending load address order.
5067 Certain program header types describe segments of memory which the
5068 system loader will load from the file. In the linker script, you
5069 specify the contents of these segments by placing allocatable output
5070 sections in the segments. You use the @samp{:@var{phdr}} output section
5071 attribute to place a section in a particular segment. @xref{Output
5074 It is normal to put certain sections in more than one segment. This
5075 merely implies that one segment of memory contains another. You may
5076 repeat @samp{:@var{phdr}}, using it once for each segment which should
5077 contain the section.
5079 If you place a section in one or more segments using @samp{:@var{phdr}},
5080 then the linker will place all subsequent allocatable sections which do
5081 not specify @samp{:@var{phdr}} in the same segments. This is for
5082 convenience, since generally a whole set of contiguous sections will be
5083 placed in a single segment. You can use @code{:NONE} to override the
5084 default segment and tell the linker to not put the section in any
5089 You may use the @code{FILEHDR} and @code{PHDRS} keywords after
5090 the program header type to further describe the contents of the segment.
5091 The @code{FILEHDR} keyword means that the segment should include the ELF
5092 file header. The @code{PHDRS} keyword means that the segment should
5093 include the ELF program headers themselves. If applied to a loadable
5094 segment (@code{PT_LOAD}), all prior loadable segments must have one of
5097 The @var{type} may be one of the following. The numbers indicate the
5098 value of the keyword.
5101 @item @code{PT_NULL} (0)
5102 Indicates an unused program header.
5104 @item @code{PT_LOAD} (1)
5105 Indicates that this program header describes a segment to be loaded from
5108 @item @code{PT_DYNAMIC} (2)
5109 Indicates a segment where dynamic linking information can be found.
5111 @item @code{PT_INTERP} (3)
5112 Indicates a segment where the name of the program interpreter may be
5115 @item @code{PT_NOTE} (4)
5116 Indicates a segment holding note information.
5118 @item @code{PT_SHLIB} (5)
5119 A reserved program header type, defined but not specified by the ELF
5122 @item @code{PT_PHDR} (6)
5123 Indicates a segment where the program headers may be found.
5125 @item @var{expression}
5126 An expression giving the numeric type of the program header. This may
5127 be used for types not defined above.
5130 You can specify that a segment should be loaded at a particular address
5131 in memory by using an @code{AT} expression. This is identical to the
5132 @code{AT} command used as an output section attribute (@pxref{Output
5133 Section LMA}). The @code{AT} command for a program header overrides the
5134 output section attribute.
5136 The linker will normally set the segment flags based on the sections
5137 which comprise the segment. You may use the @code{FLAGS} keyword to
5138 explicitly specify the segment flags. The value of @var{flags} must be
5139 an integer. It is used to set the @code{p_flags} field of the program
5142 Here is an example of @code{PHDRS}. This shows a typical set of program
5143 headers used on a native ELF system.
5149 headers PT_PHDR PHDRS ;
5151 text PT_LOAD FILEHDR PHDRS ;
5153 dynamic PT_DYNAMIC ;
5159 .interp : @{ *(.interp) @} :text :interp
5160 .text : @{ *(.text) @} :text
5161 .rodata : @{ *(.rodata) @} /* defaults to :text */
5163 . = . + 0x1000; /* move to a new page in memory */
5164 .data : @{ *(.data) @} :data
5165 .dynamic : @{ *(.dynamic) @} :data :dynamic
5172 @section VERSION Command
5173 @kindex VERSION @{script text@}
5174 @cindex symbol versions
5175 @cindex version script
5176 @cindex versions of symbols
5177 The linker supports symbol versions when using ELF. Symbol versions are
5178 only useful when using shared libraries. The dynamic linker can use
5179 symbol versions to select a specific version of a function when it runs
5180 a program that may have been linked against an earlier version of the
5183 You can include a version script directly in the main linker script, or
5184 you can supply the version script as an implicit linker script. You can
5185 also use the @samp{--version-script} linker option.
5187 The syntax of the @code{VERSION} command is simply
5189 VERSION @{ version-script-commands @}
5192 The format of the version script commands is identical to that used by
5193 Sun's linker in Solaris 2.5. The version script defines a tree of
5194 version nodes. You specify the node names and interdependencies in the
5195 version script. You can specify which symbols are bound to which
5196 version nodes, and you can reduce a specified set of symbols to local
5197 scope so that they are not globally visible outside of the shared
5200 The easiest way to demonstrate the version script language is with a few
5226 This example version script defines three version nodes. The first
5227 version node defined is @samp{VERS_1.1}; it has no other dependencies.
5228 The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces
5229 a number of symbols to local scope so that they are not visible outside
5230 of the shared library; this is done using wildcard patterns, so that any
5231 symbol whose name begins with @samp{old}, @samp{original}, or @samp{new}
5232 is matched. The wildcard patterns available are the same as those used
5233 in the shell when matching filenames (also known as ``globbing'').
5234 However, if you specify the symbol name inside double quotes, then the
5235 name is treated as literal, rather than as a glob pattern.
5237 Next, the version script defines node @samp{VERS_1.2}. This node
5238 depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2}
5239 to the version node @samp{VERS_1.2}.
5241 Finally, the version script defines node @samp{VERS_2.0}. This node
5242 depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1}
5243 and @samp{bar2} are bound to the version node @samp{VERS_2.0}.
5245 When the linker finds a symbol defined in a library which is not
5246 specifically bound to a version node, it will effectively bind it to an
5247 unspecified base version of the library. You can bind all otherwise
5248 unspecified symbols to a given version node by using @samp{global: *;}
5249 somewhere in the version script. Note that it's slightly crazy to use
5250 wildcards in a global spec except on the last version node. Global
5251 wildcards elsewhere run the risk of accidentally adding symbols to the
5252 set exported for an old version. That's wrong since older versions
5253 ought to have a fixed set of symbols.
5255 The names of the version nodes have no specific meaning other than what
5256 they might suggest to the person reading them. The @samp{2.0} version
5257 could just as well have appeared in between @samp{1.1} and @samp{1.2}.
5258 However, this would be a confusing way to write a version script.
5260 Node name can be omitted, provided it is the only version node
5261 in the version script. Such version script doesn't assign any versions to
5262 symbols, only selects which symbols will be globally visible out and which
5266 @{ global: foo; bar; local: *; @};
5269 When you link an application against a shared library that has versioned
5270 symbols, the application itself knows which version of each symbol it
5271 requires, and it also knows which version nodes it needs from each
5272 shared library it is linked against. Thus at runtime, the dynamic
5273 loader can make a quick check to make sure that the libraries you have
5274 linked against do in fact supply all of the version nodes that the
5275 application will need to resolve all of the dynamic symbols. In this
5276 way it is possible for the dynamic linker to know with certainty that
5277 all external symbols that it needs will be resolvable without having to
5278 search for each symbol reference.
5280 The symbol versioning is in effect a much more sophisticated way of
5281 doing minor version checking that SunOS does. The fundamental problem
5282 that is being addressed here is that typically references to external
5283 functions are bound on an as-needed basis, and are not all bound when
5284 the application starts up. If a shared library is out of date, a
5285 required interface may be missing; when the application tries to use
5286 that interface, it may suddenly and unexpectedly fail. With symbol
5287 versioning, the user will get a warning when they start their program if
5288 the libraries being used with the application are too old.
5290 There are several GNU extensions to Sun's versioning approach. The
5291 first of these is the ability to bind a symbol to a version node in the
5292 source file where the symbol is defined instead of in the versioning
5293 script. This was done mainly to reduce the burden on the library
5294 maintainer. You can do this by putting something like:
5296 __asm__(".symver original_foo,foo@@VERS_1.1");
5299 in the C source file. This renames the function @samp{original_foo} to
5300 be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}.
5301 The @samp{local:} directive can be used to prevent the symbol
5302 @samp{original_foo} from being exported. A @samp{.symver} directive
5303 takes precedence over a version script.
5305 The second GNU extension is to allow multiple versions of the same
5306 function to appear in a given shared library. In this way you can make
5307 an incompatible change to an interface without increasing the major
5308 version number of the shared library, while still allowing applications
5309 linked against the old interface to continue to function.
5311 To do this, you must use multiple @samp{.symver} directives in the
5312 source file. Here is an example:
5315 __asm__(".symver original_foo,foo@@");
5316 __asm__(".symver old_foo,foo@@VERS_1.1");
5317 __asm__(".symver old_foo1,foo@@VERS_1.2");
5318 __asm__(".symver new_foo,foo@@@@VERS_2.0");
5321 In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the
5322 unspecified base version of the symbol. The source file that contains this
5323 example would define 4 C functions: @samp{original_foo}, @samp{old_foo},
5324 @samp{old_foo1}, and @samp{new_foo}.
5326 When you have multiple definitions of a given symbol, there needs to be
5327 some way to specify a default version to which external references to
5328 this symbol will be bound. You can do this with the
5329 @samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only
5330 declare one version of a symbol as the default in this manner; otherwise
5331 you would effectively have multiple definitions of the same symbol.
5333 If you wish to bind a reference to a specific version of the symbol
5334 within the shared library, you can use the aliases of convenience
5335 (i.e., @samp{old_foo}), or you can use the @samp{.symver} directive to
5336 specifically bind to an external version of the function in question.
5338 You can also specify the language in the version script:
5341 VERSION extern "lang" @{ version-script-commands @}
5344 The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}.
5345 The linker will iterate over the list of symbols at the link time and
5346 demangle them according to @samp{lang} before matching them to the
5347 patterns specified in @samp{version-script-commands}. The default
5348 @samp{lang} is @samp{C}.
5350 Demangled names may contains spaces and other special characters. As
5351 described above, you can use a glob pattern to match demangled names,
5352 or you can use a double-quoted string to match the string exactly. In
5353 the latter case, be aware that minor differences (such as differing
5354 whitespace) between the version script and the demangler output will
5355 cause a mismatch. As the exact string generated by the demangler
5356 might change in the future, even if the mangled name does not, you
5357 should check that all of your version directives are behaving as you
5358 expect when you upgrade.
5361 @section Expressions in Linker Scripts
5364 The syntax for expressions in the linker script language is identical to
5365 that of C expressions. All expressions are evaluated as integers. All
5366 expressions are evaluated in the same size, which is 32 bits if both the
5367 host and target are 32 bits, and is otherwise 64 bits.
5369 You can use and set symbol values in expressions.
5371 The linker defines several special purpose builtin functions for use in
5375 * Constants:: Constants
5376 * Symbolic Constants:: Symbolic constants
5377 * Symbols:: Symbol Names
5378 * Orphan Sections:: Orphan Sections
5379 * Location Counter:: The Location Counter
5380 * Operators:: Operators
5381 * Evaluation:: Evaluation
5382 * Expression Section:: The Section of an Expression
5383 * Builtin Functions:: Builtin Functions
5387 @subsection Constants
5388 @cindex integer notation
5389 @cindex constants in linker scripts
5390 All constants are integers.
5392 As in C, the linker considers an integer beginning with @samp{0} to be
5393 octal, and an integer beginning with @samp{0x} or @samp{0X} to be
5394 hexadecimal. Alternatively the linker accepts suffixes of @samp{h} or
5395 @samp{H} for hexadecimal, @samp{o} or @samp{O} for octal, @samp{b} or
5396 @samp{B} for binary and @samp{d} or @samp{D} for decimal. Any integer
5397 value without a prefix or a suffix is considered to be decimal.
5399 @cindex scaled integers
5400 @cindex K and M integer suffixes
5401 @cindex M and K integer suffixes
5402 @cindex suffixes for integers
5403 @cindex integer suffixes
5404 In addition, you can use the suffixes @code{K} and @code{M} to scale a
5408 @c END TEXI2ROFF-KILL
5409 @code{1024} or @code{1024*1024}
5413 ${\rm 1024}$ or ${\rm 1024}^2$
5415 @c END TEXI2ROFF-KILL
5416 respectively. For example, the following
5417 all refer to the same quantity:
5426 Note - the @code{K} and @code{M} suffixes cannot be used in
5427 conjunction with the base suffixes mentioned above.
5429 @node Symbolic Constants
5430 @subsection Symbolic Constants
5431 @cindex symbolic constants
5433 It is possible to refer to target specific constants via the use of
5434 the @code{CONSTANT(@var{name})} operator, where @var{name} is one of:
5439 The target's maximum page size.
5441 @item COMMONPAGESIZE
5442 @kindex COMMONPAGESIZE
5443 The target's default page size.
5449 .text ALIGN (CONSTANT (MAXPAGESIZE)) : @{ *(.text) @}
5452 will create a text section aligned to the largest page boundary
5453 supported by the target.
5456 @subsection Symbol Names
5457 @cindex symbol names
5459 @cindex quoted symbol names
5461 Unless quoted, symbol names start with a letter, underscore, or period
5462 and may include letters, digits, underscores, periods, and hyphens.
5463 Unquoted symbol names must not conflict with any keywords. You can
5464 specify a symbol which contains odd characters or has the same name as a
5465 keyword by surrounding the symbol name in double quotes:
5468 "with a space" = "also with a space" + 10;
5471 Since symbols can contain many non-alphabetic characters, it is safest
5472 to delimit symbols with spaces. For example, @samp{A-B} is one symbol,
5473 whereas @samp{A - B} is an expression involving subtraction.
5475 @node Orphan Sections
5476 @subsection Orphan Sections
5478 Orphan sections are sections present in the input files which
5479 are not explicitly placed into the output file by the linker
5480 script. The linker will still copy these sections into the
5481 output file, but it has to guess as to where they should be
5482 placed. The linker uses a simple heuristic to do this. It
5483 attempts to place orphan sections after non-orphan sections of the
5484 same attribute, such as code vs data, loadable vs non-loadable, etc.
5485 If there is not enough room to do this then it places
5486 at the end of the file.
5488 For ELF targets, the attribute of the section includes section type as
5489 well as section flag.
5491 If an orphaned section's name is representable as a C identifier then
5492 the linker will automatically @pxref{PROVIDE} two symbols:
5493 __start_SECNAME and __stop_SECNAME, where SECNAME is the name of the
5494 section. These indicate the start address and end address of the
5495 orphaned section respectively. Note: most section names are not
5496 representable as C identifiers because they contain a @samp{.}
5499 @node Location Counter
5500 @subsection The Location Counter
5503 @cindex location counter
5504 @cindex current output location
5505 The special linker variable @dfn{dot} @samp{.} always contains the
5506 current output location counter. Since the @code{.} always refers to a
5507 location in an output section, it may only appear in an expression
5508 within a @code{SECTIONS} command. The @code{.} symbol may appear
5509 anywhere that an ordinary symbol is allowed in an expression.
5512 Assigning a value to @code{.} will cause the location counter to be
5513 moved. This may be used to create holes in the output section. The
5514 location counter may not be moved backwards inside an output section,
5515 and may not be moved backwards outside of an output section if so
5516 doing creates areas with overlapping LMAs.
5532 In the previous example, the @samp{.text} section from @file{file1} is
5533 located at the beginning of the output section @samp{output}. It is
5534 followed by a 1000 byte gap. Then the @samp{.text} section from
5535 @file{file2} appears, also with a 1000 byte gap following before the
5536 @samp{.text} section from @file{file3}. The notation @samp{= 0x12345678}
5537 specifies what data to write in the gaps (@pxref{Output Section Fill}).
5539 @cindex dot inside sections
5540 Note: @code{.} actually refers to the byte offset from the start of the
5541 current containing object. Normally this is the @code{SECTIONS}
5542 statement, whose start address is 0, hence @code{.} can be used as an
5543 absolute address. If @code{.} is used inside a section description
5544 however, it refers to the byte offset from the start of that section,
5545 not an absolute address. Thus in a script like this:
5563 The @samp{.text} section will be assigned a starting address of 0x100
5564 and a size of exactly 0x200 bytes, even if there is not enough data in
5565 the @samp{.text} input sections to fill this area. (If there is too
5566 much data, an error will be produced because this would be an attempt to
5567 move @code{.} backwards). The @samp{.data} section will start at 0x500
5568 and it will have an extra 0x600 bytes worth of space after the end of
5569 the values from the @samp{.data} input sections and before the end of
5570 the @samp{.data} output section itself.
5572 @cindex dot outside sections
5573 Setting symbols to the value of the location counter outside of an
5574 output section statement can result in unexpected values if the linker
5575 needs to place orphan sections. For example, given the following:
5581 .text: @{ *(.text) @}
5585 .data: @{ *(.data) @}
5590 If the linker needs to place some input section, e.g. @code{.rodata},
5591 not mentioned in the script, it might choose to place that section
5592 between @code{.text} and @code{.data}. You might think the linker
5593 should place @code{.rodata} on the blank line in the above script, but
5594 blank lines are of no particular significance to the linker. As well,
5595 the linker doesn't associate the above symbol names with their
5596 sections. Instead, it assumes that all assignments or other
5597 statements belong to the previous output section, except for the
5598 special case of an assignment to @code{.}. I.e., the linker will
5599 place the orphan @code{.rodata} section as if the script was written
5606 .text: @{ *(.text) @}
5610 .rodata: @{ *(.rodata) @}
5611 .data: @{ *(.data) @}
5616 This may or may not be the script author's intention for the value of
5617 @code{start_of_data}. One way to influence the orphan section
5618 placement is to assign the location counter to itself, as the linker
5619 assumes that an assignment to @code{.} is setting the start address of
5620 a following output section and thus should be grouped with that
5621 section. So you could write:
5627 .text: @{ *(.text) @}
5632 .data: @{ *(.data) @}
5637 Now, the orphan @code{.rodata} section will be placed between
5638 @code{end_of_text} and @code{start_of_data}.
5642 @subsection Operators
5643 @cindex operators for arithmetic
5644 @cindex arithmetic operators
5645 @cindex precedence in expressions
5646 The linker recognizes the standard C set of arithmetic operators, with
5647 the standard bindings and precedence levels:
5650 @c END TEXI2ROFF-KILL
5652 precedence associativity Operators Notes
5658 5 left == != > < <= >=
5664 11 right &= += -= *= /= (2)
5668 (1) Prefix operators
5669 (2) @xref{Assignments}.
5673 \vskip \baselineskip
5674 %"lispnarrowing" is the extra indent used generally for smallexample
5675 \hskip\lispnarrowing\vbox{\offinterlineskip
5678 {\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ #\ \hfil&\vrule#&\strut\hfil\ {\tt #}\ \hfil&\vrule#\cr
5679 height2pt&\omit&&\omit&&\omit&\cr
5680 &Precedence&& Associativity &&{\rm Operators}&\cr
5681 height2pt&\omit&&\omit&&\omit&\cr
5683 height2pt&\omit&&\omit&&\omit&\cr
5685 % '176 is tilde, '~' in tt font
5686 &1&&left&&\qquad- \char'176\ !\qquad\dag&\cr
5687 &2&&left&&* / \%&\cr
5690 &5&&left&&== != > < <= >=&\cr
5693 &8&&left&&{\&\&}&\cr
5696 &11&&right&&\qquad\&= += -= *= /=\qquad\ddag&\cr
5698 height2pt&\omit&&\omit&&\omit&\cr}
5703 @obeylines@parskip=0pt@parindent=0pt
5704 @dag@quad Prefix operators.
5705 @ddag@quad @xref{Assignments}.
5708 @c END TEXI2ROFF-KILL
5711 @subsection Evaluation
5712 @cindex lazy evaluation
5713 @cindex expression evaluation order
5714 The linker evaluates expressions lazily. It only computes the value of
5715 an expression when absolutely necessary.
5717 The linker needs some information, such as the value of the start
5718 address of the first section, and the origins and lengths of memory
5719 regions, in order to do any linking at all. These values are computed
5720 as soon as possible when the linker reads in the linker script.
5722 However, other values (such as symbol values) are not known or needed
5723 until after storage allocation. Such values are evaluated later, when
5724 other information (such as the sizes of output sections) is available
5725 for use in the symbol assignment expression.
5727 The sizes of sections cannot be known until after allocation, so
5728 assignments dependent upon these are not performed until after
5731 Some expressions, such as those depending upon the location counter
5732 @samp{.}, must be evaluated during section allocation.
5734 If the result of an expression is required, but the value is not
5735 available, then an error results. For example, a script like the
5741 .text 9+this_isnt_constant :
5747 will cause the error message @samp{non constant expression for initial
5750 @node Expression Section
5751 @subsection The Section of an Expression
5752 @cindex expression sections
5753 @cindex absolute expressions
5754 @cindex relative expressions
5755 @cindex absolute and relocatable symbols
5756 @cindex relocatable and absolute symbols
5757 @cindex symbols, relocatable and absolute
5758 Addresses and symbols may be section relative, or absolute. A section
5759 relative symbol is relocatable. If you request relocatable output
5760 using the @samp{-r} option, a further link operation may change the
5761 value of a section relative symbol. On the other hand, an absolute
5762 symbol will retain the same value throughout any further link
5765 Some terms in linker expressions are addresses. This is true of
5766 section relative symbols and for builtin functions that return an
5767 address, such as @code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and
5768 @code{SEGMENT_START}. Other terms are simply numbers, or are builtin
5769 functions that return a non-address value, such as @code{LENGTH}.
5770 One complication is that unless you set @code{LD_FEATURE ("SANE_EXPR")}
5771 (@pxref{Miscellaneous Commands}), numbers and absolute symbols are treated
5772 differently depending on their location, for compatibility with older
5773 versions of @code{ld}. Expressions appearing outside an output
5774 section definition treat all numbers as absolute addresses.
5775 Expressions appearing inside an output section definition treat
5776 absolute symbols as numbers. If @code{LD_FEATURE ("SANE_EXPR")} is
5777 given, then absolute symbols and numbers are simply treated as numbers
5780 In the following simple example,
5787 __executable_start = 0x100;
5791 __data_start = 0x10;
5799 both @code{.} and @code{__executable_start} are set to the absolute
5800 address 0x100 in the first two assignments, then both @code{.} and
5801 @code{__data_start} are set to 0x10 relative to the @code{.data}
5802 section in the second two assignments.
5804 For expressions involving numbers, relative addresses and absolute
5805 addresses, ld follows these rules to evaluate terms:
5809 Unary operations on an absolute address or number, and binary
5810 operations on two absolute addresses or two numbers, or between one
5811 absolute address and a number, apply the operator to the value(s).
5813 Unary operations on a relative address, and binary operations on two
5814 relative addresses in the same section or between one relative address
5815 and a number, apply the operator to the offset part of the address(es).
5817 Other binary operations, that is, between two relative addresses not
5818 in the same section, or between a relative address and an absolute
5819 address, first convert any non-absolute term to an absolute address
5820 before applying the operator.
5823 The result section of each sub-expression is as follows:
5827 An operation involving only numbers results in a number.
5829 The result of comparisons, @samp{&&} and @samp{||} is also a number.
5831 The result of other binary arithmetic and logical operations on two
5832 relative addresses in the same section or two absolute addresses
5833 (after above conversions) is also a number.
5835 The result of other operations on relative addresses or one
5836 relative address and a number, is a relative address in the same
5837 section as the relative operand(s).
5839 The result of other operations on absolute addresses (after above
5840 conversions) is an absolute address.
5843 You can use the builtin function @code{ABSOLUTE} to force an expression
5844 to be absolute when it would otherwise be relative. For example, to
5845 create an absolute symbol set to the address of the end of the output
5846 section @samp{.data}:
5850 .data : @{ *(.data) _edata = ABSOLUTE(.); @}
5854 If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the
5855 @samp{.data} section.
5857 Using @code{LOADADDR} also forces an expression absolute, since this
5858 particular builtin function returns an absolute address.
5860 @node Builtin Functions
5861 @subsection Builtin Functions
5862 @cindex functions in expressions
5863 The linker script language includes a number of builtin functions for
5864 use in linker script expressions.
5867 @item ABSOLUTE(@var{exp})
5868 @kindex ABSOLUTE(@var{exp})
5869 @cindex expression, absolute
5870 Return the absolute (non-relocatable, as opposed to non-negative) value
5871 of the expression @var{exp}. Primarily useful to assign an absolute
5872 value to a symbol within a section definition, where symbol values are
5873 normally section relative. @xref{Expression Section}.
5875 @item ADDR(@var{section})
5876 @kindex ADDR(@var{section})
5877 @cindex section address in expression
5878 Return the address (VMA) of the named @var{section}. Your
5879 script must previously have defined the location of that section. In
5880 the following example, @code{start_of_output_1}, @code{symbol_1} and
5881 @code{symbol_2} are assigned equivalent values, except that
5882 @code{symbol_1} will be relative to the @code{.output1} section while
5883 the other two will be absolute:
5889 start_of_output_1 = ABSOLUTE(.);
5894 symbol_1 = ADDR(.output1);
5895 symbol_2 = start_of_output_1;
5901 @item ALIGN(@var{align})
5902 @itemx ALIGN(@var{exp},@var{align})
5903 @kindex ALIGN(@var{align})
5904 @kindex ALIGN(@var{exp},@var{align})
5905 @cindex round up location counter
5906 @cindex align location counter
5907 @cindex round up expression
5908 @cindex align expression
5909 Return the location counter (@code{.}) or arbitrary expression aligned
5910 to the next @var{align} boundary. The single operand @code{ALIGN}
5911 doesn't change the value of the location counter---it just does
5912 arithmetic on it. The two operand @code{ALIGN} allows an arbitrary
5913 expression to be aligned upwards (@code{ALIGN(@var{align})} is
5914 equivalent to @code{ALIGN(., @var{align})}).
5916 Here is an example which aligns the output @code{.data} section to the
5917 next @code{0x2000} byte boundary after the preceding section and sets a
5918 variable within the section to the next @code{0x8000} boundary after the
5923 .data ALIGN(0x2000): @{
5925 variable = ALIGN(0x8000);
5931 The first use of @code{ALIGN} in this example specifies the location of
5932 a section because it is used as the optional @var{address} attribute of
5933 a section definition (@pxref{Output Section Address}). The second use
5934 of @code{ALIGN} is used to defines the value of a symbol.
5936 The builtin function @code{NEXT} is closely related to @code{ALIGN}.
5938 @item ALIGNOF(@var{section})
5939 @kindex ALIGNOF(@var{section})
5940 @cindex section alignment
5941 Return the alignment in bytes of the named @var{section}, if that section has
5942 been allocated. If the section has not been allocated when this is
5943 evaluated, the linker will report an error. In the following example,
5944 the alignment of the @code{.output} section is stored as the first
5945 value in that section.
5950 LONG (ALIGNOF (.output))
5957 @item BLOCK(@var{exp})
5958 @kindex BLOCK(@var{exp})
5959 This is a synonym for @code{ALIGN}, for compatibility with older linker
5960 scripts. It is most often seen when setting the address of an output
5963 @item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
5964 @kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
5965 This is equivalent to either
5967 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1)))
5971 (ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - @var{commonpagesize})))
5974 depending on whether the latter uses fewer @var{commonpagesize} sized pages
5975 for the data segment (area between the result of this expression and
5976 @code{DATA_SEGMENT_END}) than the former or not.
5977 If the latter form is used, it means @var{commonpagesize} bytes of runtime
5978 memory will be saved at the expense of up to @var{commonpagesize} wasted
5979 bytes in the on-disk file.
5981 This expression can only be used directly in @code{SECTIONS} commands, not in
5982 any output section descriptions and only once in the linker script.
5983 @var{commonpagesize} should be less or equal to @var{maxpagesize} and should
5984 be the system page size the object wants to be optimized for (while still
5985 working on system page sizes up to @var{maxpagesize}).
5990 . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
5993 @item DATA_SEGMENT_END(@var{exp})
5994 @kindex DATA_SEGMENT_END(@var{exp})
5995 This defines the end of data segment for @code{DATA_SEGMENT_ALIGN}
5996 evaluation purposes.
5999 . = DATA_SEGMENT_END(.);
6002 @item DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6003 @kindex DATA_SEGMENT_RELRO_END(@var{offset}, @var{exp})
6004 This defines the end of the @code{PT_GNU_RELRO} segment when
6005 @samp{-z relro} option is used.
6006 When @samp{-z relro} option is not present, @code{DATA_SEGMENT_RELRO_END}
6007 does nothing, otherwise @code{DATA_SEGMENT_ALIGN} is padded so that
6008 @var{exp} + @var{offset} is aligned to the most commonly used page
6009 boundary for particular target. If present in the linker script,
6010 it must always come in between @code{DATA_SEGMENT_ALIGN} and
6011 @code{DATA_SEGMENT_END}. Evaluates to the second argument plus any
6012 padding needed at the end of the @code{PT_GNU_RELRO} segment due to
6016 . = DATA_SEGMENT_RELRO_END(24, .);
6019 @item DEFINED(@var{symbol})
6020 @kindex DEFINED(@var{symbol})
6021 @cindex symbol defaults
6022 Return 1 if @var{symbol} is in the linker global symbol table and is
6023 defined before the statement using DEFINED in the script, otherwise
6024 return 0. You can use this function to provide
6025 default values for symbols. For example, the following script fragment
6026 shows how to set a global symbol @samp{begin} to the first location in
6027 the @samp{.text} section---but if a symbol called @samp{begin} already
6028 existed, its value is preserved:
6034 begin = DEFINED(begin) ? begin : . ;
6042 @item LENGTH(@var{memory})
6043 @kindex LENGTH(@var{memory})
6044 Return the length of the memory region named @var{memory}.
6046 @item LOADADDR(@var{section})
6047 @kindex LOADADDR(@var{section})
6048 @cindex section load address in expression
6049 Return the absolute LMA of the named @var{section}. (@pxref{Output
6052 @item LOG2CEIL(@var{exp})
6053 @kindex LOG2CEIL(@var{exp})
6054 Return the binary logarithm of @var{exp} rounded towards infinity.
6055 @code{LOG2CEIL(0)} returns 0.
6058 @item MAX(@var{exp1}, @var{exp2})
6059 Returns the maximum of @var{exp1} and @var{exp2}.
6062 @item MIN(@var{exp1}, @var{exp2})
6063 Returns the minimum of @var{exp1} and @var{exp2}.
6065 @item NEXT(@var{exp})
6066 @kindex NEXT(@var{exp})
6067 @cindex unallocated address, next
6068 Return the next unallocated address that is a multiple of @var{exp}.
6069 This function is closely related to @code{ALIGN(@var{exp})}; unless you
6070 use the @code{MEMORY} command to define discontinuous memory for the
6071 output file, the two functions are equivalent.
6073 @item ORIGIN(@var{memory})
6074 @kindex ORIGIN(@var{memory})
6075 Return the origin of the memory region named @var{memory}.
6077 @item SEGMENT_START(@var{segment}, @var{default})
6078 @kindex SEGMENT_START(@var{segment}, @var{default})
6079 Return the base address of the named @var{segment}. If an explicit
6080 value has already been given for this segment (with a command-line
6081 @samp{-T} option) then that value will be returned otherwise the value
6082 will be @var{default}. At present, the @samp{-T} command-line option
6083 can only be used to set the base address for the ``text'', ``data'', and
6084 ``bss'' sections, but you can use @code{SEGMENT_START} with any segment
6087 @item SIZEOF(@var{section})
6088 @kindex SIZEOF(@var{section})
6089 @cindex section size
6090 Return the size in bytes of the named @var{section}, if that section has
6091 been allocated. If the section has not been allocated when this is
6092 evaluated, the linker will report an error. In the following example,
6093 @code{symbol_1} and @code{symbol_2} are assigned identical values:
6102 symbol_1 = .end - .start ;
6103 symbol_2 = SIZEOF(.output);
6108 @item SIZEOF_HEADERS
6109 @itemx sizeof_headers
6110 @kindex SIZEOF_HEADERS
6112 Return the size in bytes of the output file's headers. This is
6113 information which appears at the start of the output file. You can use
6114 this number when setting the start address of the first section, if you
6115 choose, to facilitate paging.
6117 @cindex not enough room for program headers
6118 @cindex program headers, not enough room
6119 When producing an ELF output file, if the linker script uses the
6120 @code{SIZEOF_HEADERS} builtin function, the linker must compute the
6121 number of program headers before it has determined all the section
6122 addresses and sizes. If the linker later discovers that it needs
6123 additional program headers, it will report an error @samp{not enough
6124 room for program headers}. To avoid this error, you must avoid using
6125 the @code{SIZEOF_HEADERS} function, or you must rework your linker
6126 script to avoid forcing the linker to use additional program headers, or
6127 you must define the program headers yourself using the @code{PHDRS}
6128 command (@pxref{PHDRS}).
6131 @node Implicit Linker Scripts
6132 @section Implicit Linker Scripts
6133 @cindex implicit linker scripts
6134 If you specify a linker input file which the linker can not recognize as
6135 an object file or an archive file, it will try to read the file as a
6136 linker script. If the file can not be parsed as a linker script, the
6137 linker will report an error.
6139 An implicit linker script will not replace the default linker script.
6141 Typically an implicit linker script would contain only symbol
6142 assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION}
6145 Any input files read because of an implicit linker script will be read
6146 at the position in the command line where the implicit linker script was
6147 read. This can affect archive searching.
6150 @node Machine Dependent
6151 @chapter Machine Dependent Features
6153 @cindex machine dependencies
6154 @command{ld} has additional features on some platforms; the following
6155 sections describe them. Machines where @command{ld} has no additional
6156 functionality are not listed.
6160 * H8/300:: @command{ld} and the H8/300
6163 * i960:: @command{ld} and the Intel 960 family
6166 * M68HC11/68HC12:: @code{ld} and the Motorola 68HC11 and 68HC12 families
6169 * ARM:: @command{ld} and the ARM family
6172 * HPPA ELF32:: @command{ld} and HPPA 32-bit ELF
6175 * M68K:: @command{ld} and the Motorola 68K family
6178 * MIPS:: @command{ld} and the MIPS family
6181 * MMIX:: @command{ld} and MMIX
6184 * MSP430:: @command{ld} and MSP430
6187 * NDS32:: @command{ld} and NDS32
6190 * Nios II:: @command{ld} and the Altera Nios II
6193 * PowerPC ELF32:: @command{ld} and PowerPC 32-bit ELF Support
6196 * PowerPC64 ELF64:: @command{ld} and PowerPC64 64-bit ELF Support
6199 * SPU ELF:: @command{ld} and SPU ELF Support
6202 * TI COFF:: @command{ld} and TI COFF
6205 * WIN32:: @command{ld} and WIN32 (cygwin/mingw)
6208 * Xtensa:: @command{ld} and Xtensa Processors
6219 @section @command{ld} and the H8/300
6221 @cindex H8/300 support
6222 For the H8/300, @command{ld} can perform these global optimizations when
6223 you specify the @samp{--relax} command-line option.
6226 @cindex relaxing on H8/300
6227 @item relaxing address modes
6228 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
6229 targets are within eight bits, and turns them into eight-bit
6230 program-counter relative @code{bsr} and @code{bra} instructions,
6233 @cindex synthesizing on H8/300
6234 @item synthesizing instructions
6235 @c FIXME: specifically mov.b, or any mov instructions really? -> mov.b only, at least on H8, H8H, H8S
6236 @command{ld} finds all @code{mov.b} instructions which use the
6237 sixteen-bit absolute address form, but refer to the top
6238 page of memory, and changes them to use the eight-bit address form.
6239 (That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into
6240 @samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the
6241 top page of memory).
6243 @command{ld} finds all @code{mov} instructions which use the register
6244 indirect with 32-bit displacement addressing mode, but use a small
6245 displacement inside 16-bit displacement range, and changes them to use
6246 the 16-bit displacement form. (That is: the linker turns @samp{mov.b
6247 @code{@@}@var{d}:32,ERx} into @samp{mov.b @code{@@}@var{d}:16,ERx}
6248 whenever the displacement @var{d} is in the 16 bit signed integer
6249 range. Only implemented in ELF-format ld).
6251 @item bit manipulation instructions
6252 @command{ld} finds all bit manipulation instructions like @code{band, bclr,
6253 biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor}
6254 which use 32 bit and 16 bit absolute address form, but refer to the top
6255 page of memory, and changes them to use the 8 bit address form.
6256 (That is: the linker turns @samp{bset #xx:3,@code{@@}@var{aa}:32} into
6257 @samp{bset #xx:3,@code{@@}@var{aa}:8} whenever the address @var{aa} is in
6258 the top page of memory).
6260 @item system control instructions
6261 @command{ld} finds all @code{ldc.w, stc.w} instructions which use the
6262 32 bit absolute address form, but refer to the top page of memory, and
6263 changes them to use 16 bit address form.
6264 (That is: the linker turns @samp{ldc.w @code{@@}@var{aa}:32,ccr} into
6265 @samp{ldc.w @code{@@}@var{aa}:16,ccr} whenever the address @var{aa} is in
6266 the top page of memory).
6276 @c This stuff is pointless to say unless you're especially concerned
6277 @c with Renesas chips; don't enable it for generic case, please.
6279 @chapter @command{ld} and Other Renesas Chips
6281 @command{ld} also supports the Renesas (formerly Hitachi) H8/300H,
6282 H8/500, and SH chips. No special features, commands, or command-line
6283 options are required for these chips.
6293 @section @command{ld} and the Intel 960 Family
6295 @cindex i960 support
6297 You can use the @samp{-A@var{architecture}} command line option to
6298 specify one of the two-letter names identifying members of the 960
6299 family; the option specifies the desired output target, and warns of any
6300 incompatible instructions in the input files. It also modifies the
6301 linker's search strategy for archive libraries, to support the use of
6302 libraries specific to each particular architecture, by including in the
6303 search loop names suffixed with the string identifying the architecture.
6305 For example, if your @command{ld} command line included @w{@samp{-ACA}} as
6306 well as @w{@samp{-ltry}}, the linker would look (in its built-in search
6307 paths, and in any paths you specify with @samp{-L}) for a library with
6320 The first two possibilities would be considered in any event; the last
6321 two are due to the use of @w{@samp{-ACA}}.
6323 You can meaningfully use @samp{-A} more than once on a command line, since
6324 the 960 architecture family allows combination of target architectures; each
6325 use will add another pair of name variants to search for when @w{@samp{-l}}
6326 specifies a library.
6328 @cindex @option{--relax} on i960
6329 @cindex relaxing on i960
6330 @command{ld} supports the @samp{--relax} option for the i960 family. If
6331 you specify @samp{--relax}, @command{ld} finds all @code{balx} and
6332 @code{calx} instructions whose targets are within 24 bits, and turns
6333 them into 24-bit program-counter relative @code{bal} and @code{cal}
6334 instructions, respectively. @command{ld} also turns @code{cal}
6335 instructions into @code{bal} instructions when it determines that the
6336 target subroutine is a leaf routine (that is, the target subroutine does
6337 not itself call any subroutines).
6354 @node M68HC11/68HC12
6355 @section @command{ld} and the Motorola 68HC11 and 68HC12 families
6357 @cindex M68HC11 and 68HC12 support
6359 @subsection Linker Relaxation
6361 For the Motorola 68HC11, @command{ld} can perform these global
6362 optimizations when you specify the @samp{--relax} command-line option.
6365 @cindex relaxing on M68HC11
6366 @item relaxing address modes
6367 @command{ld} finds all @code{jsr} and @code{jmp} instructions whose
6368 targets are within eight bits, and turns them into eight-bit
6369 program-counter relative @code{bsr} and @code{bra} instructions,
6372 @command{ld} also looks at all 16-bit extended addressing modes and
6373 transforms them in a direct addressing mode when the address is in
6374 page 0 (between 0 and 0x0ff).
6376 @item relaxing gcc instruction group
6377 When @command{gcc} is called with @option{-mrelax}, it can emit group
6378 of instructions that the linker can optimize to use a 68HC11 direct
6379 addressing mode. These instructions consists of @code{bclr} or
6380 @code{bset} instructions.
6384 @subsection Trampoline Generation
6386 @cindex trampoline generation on M68HC11
6387 @cindex trampoline generation on M68HC12
6388 For 68HC11 and 68HC12, @command{ld} can generate trampoline code to
6389 call a far function using a normal @code{jsr} instruction. The linker
6390 will also change the relocation to some far function to use the
6391 trampoline address instead of the function address. This is typically the
6392 case when a pointer to a function is taken. The pointer will in fact
6393 point to the function trampoline.
6401 @section @command{ld} and the ARM family
6403 @cindex ARM interworking support
6404 @kindex --support-old-code
6405 For the ARM, @command{ld} will generate code stubs to allow functions calls
6406 between ARM and Thumb code. These stubs only work with code that has
6407 been compiled and assembled with the @samp{-mthumb-interwork} command
6408 line option. If it is necessary to link with old ARM object files or
6409 libraries, which have not been compiled with the -mthumb-interwork
6410 option then the @samp{--support-old-code} command line switch should be
6411 given to the linker. This will make it generate larger stub functions
6412 which will work with non-interworking aware ARM code. Note, however,
6413 the linker does not support generating stubs for function calls to
6414 non-interworking aware Thumb code.
6416 @cindex thumb entry point
6417 @cindex entry point, thumb
6418 @kindex --thumb-entry=@var{entry}
6419 The @samp{--thumb-entry} switch is a duplicate of the generic
6420 @samp{--entry} switch, in that it sets the program's starting address.
6421 But it also sets the bottom bit of the address, so that it can be
6422 branched to using a BX instruction, and the program will start
6423 executing in Thumb mode straight away.
6425 @cindex PE import table prefixing
6426 @kindex --use-nul-prefixed-import-tables
6427 The @samp{--use-nul-prefixed-import-tables} switch is specifying, that
6428 the import tables idata4 and idata5 have to be generated with a zero
6429 element prefix for import libraries. This is the old style to generate
6430 import tables. By default this option is turned off.
6434 The @samp{--be8} switch instructs @command{ld} to generate BE8 format
6435 executables. This option is only valid when linking big-endian
6436 objects - ie ones which have been assembled with the @option{-EB}
6437 option. The resulting image will contain big-endian data and
6441 @kindex --target1-rel
6442 @kindex --target1-abs
6443 The @samp{R_ARM_TARGET1} relocation is typically used for entries in the
6444 @samp{.init_array} section. It is interpreted as either @samp{R_ARM_REL32}
6445 or @samp{R_ARM_ABS32}, depending on the target. The @samp{--target1-rel}
6446 and @samp{--target1-abs} switches override the default.
6449 @kindex --target2=@var{type}
6450 The @samp{--target2=type} switch overrides the default definition of the
6451 @samp{R_ARM_TARGET2} relocation. Valid values for @samp{type}, their
6452 meanings, and target defaults are as follows:
6455 @samp{R_ARM_REL32} (arm*-*-elf, arm*-*-eabi)
6457 @samp{R_ARM_ABS32} (arm*-*-symbianelf)
6459 @samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd)
6464 The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF
6465 specification) enables objects compiled for the ARMv4 architecture to be
6466 interworking-safe when linked with other objects compiled for ARMv4t, but
6467 also allows pure ARMv4 binaries to be built from the same ARMv4 objects.
6469 In the latter case, the switch @option{--fix-v4bx} must be passed to the
6470 linker, which causes v4t @code{BX rM} instructions to be rewritten as
6471 @code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction.
6473 In the former case, the switch should not be used, and @samp{R_ARM_V4BX}
6474 relocations are ignored.
6476 @cindex FIX_V4BX_INTERWORKING
6477 @kindex --fix-v4bx-interworking
6478 Replace @code{BX rM} instructions identified by @samp{R_ARM_V4BX}
6479 relocations with a branch to the following veneer:
6487 This allows generation of libraries/applications that work on ARMv4 cores
6488 and are still interworking safe. Note that the above veneer clobbers the
6489 condition flags, so may cause incorrect program behavior in rare cases.
6493 The @samp{--use-blx} switch enables the linker to use ARM/Thumb
6494 BLX instructions (available on ARMv5t and above) in various
6495 situations. Currently it is used to perform calls via the PLT from Thumb
6496 code using BLX rather than using BX and a mode-switching stub before
6497 each PLT entry. This should lead to such calls executing slightly faster.
6499 This option is enabled implicitly for SymbianOS, so there is no need to
6500 specify it if you are using that target.
6502 @cindex VFP11_DENORM_FIX
6503 @kindex --vfp11-denorm-fix
6504 The @samp{--vfp11-denorm-fix} switch enables a link-time workaround for a
6505 bug in certain VFP11 coprocessor hardware, which sometimes allows
6506 instructions with denorm operands (which must be handled by support code)
6507 to have those operands overwritten by subsequent instructions before
6508 the support code can read the intended values.
6510 The bug may be avoided in scalar mode if you allow at least one
6511 intervening instruction between a VFP11 instruction which uses a register
6512 and another instruction which writes to the same register, or at least two
6513 intervening instructions if vector mode is in use. The bug only affects
6514 full-compliance floating-point mode: you do not need this workaround if
6515 you are using "runfast" mode. Please contact ARM for further details.
6517 If you know you are using buggy VFP11 hardware, you can
6518 enable this workaround by specifying the linker option
6519 @samp{--vfp-denorm-fix=scalar} if you are using the VFP11 scalar
6520 mode only, or @samp{--vfp-denorm-fix=vector} if you are using
6521 vector mode (the latter also works for scalar code). The default is
6522 @samp{--vfp-denorm-fix=none}.
6524 If the workaround is enabled, instructions are scanned for
6525 potentially-troublesome sequences, and a veneer is created for each
6526 such sequence which may trigger the erratum. The veneer consists of the
6527 first instruction of the sequence and a branch back to the subsequent
6528 instruction. The original instruction is then replaced with a branch to
6529 the veneer. The extra cycles required to call and return from the veneer
6530 are sufficient to avoid the erratum in both the scalar and vector cases.
6532 @cindex ARM1176 erratum workaround
6533 @kindex --fix-arm1176
6534 @kindex --no-fix-arm1176
6535 The @samp{--fix-arm1176} switch enables a link-time workaround for an erratum
6536 in certain ARM1176 processors. The workaround is enabled by default if you
6537 are targeting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled
6538 unconditionally by specifying @samp{--no-fix-arm1176}.
6540 Further information is available in the ``ARM1176JZ-S and ARM1176JZF-S
6541 Programmer Advice Notice'' available on the ARM documentation website at:
6542 http://infocenter.arm.com/.
6544 @cindex NO_ENUM_SIZE_WARNING
6545 @kindex --no-enum-size-warning
6546 The @option{--no-enum-size-warning} switch prevents the linker from
6547 warning when linking object files that specify incompatible EABI
6548 enumeration size attributes. For example, with this switch enabled,
6549 linking of an object file using 32-bit enumeration values with another
6550 using enumeration values fitted into the smallest possible space will
6553 @cindex NO_WCHAR_SIZE_WARNING
6554 @kindex --no-wchar-size-warning
6555 The @option{--no-wchar-size-warning} switch prevents the linker from
6556 warning when linking object files that specify incompatible EABI
6557 @code{wchar_t} size attributes. For example, with this switch enabled,
6558 linking of an object file using 32-bit @code{wchar_t} values with another
6559 using 16-bit @code{wchar_t} values will not be diagnosed.
6562 @kindex --pic-veneer
6563 The @samp{--pic-veneer} switch makes the linker use PIC sequences for
6564 ARM/Thumb interworking veneers, even if the rest of the binary
6565 is not PIC. This avoids problems on uClinux targets where
6566 @samp{--emit-relocs} is used to generate relocatable binaries.
6568 @cindex STUB_GROUP_SIZE
6569 @kindex --stub-group-size=@var{N}
6570 The linker will automatically generate and insert small sequences of
6571 code into a linked ARM ELF executable whenever an attempt is made to
6572 perform a function call to a symbol that is too far away. The
6573 placement of these sequences of instructions - called stubs - is
6574 controlled by the command line option @option{--stub-group-size=N}.
6575 The placement is important because a poor choice can create a need for
6576 duplicate stubs, increasing the code size. The linker will try to
6577 group stubs together in order to reduce interruptions to the flow of
6578 code, but it needs guidance as to how big these groups should be and
6579 where they should be placed.
6581 The value of @samp{N}, the parameter to the
6582 @option{--stub-group-size=} option controls where the stub groups are
6583 placed. If it is negative then all stubs are placed after the first
6584 branch that needs them. If it is positive then the stubs can be
6585 placed either before or after the branches that need them. If the
6586 value of @samp{N} is 1 (either +1 or -1) then the linker will choose
6587 exactly where to place groups of stubs, using its built in heuristics.
6588 A value of @samp{N} greater than 1 (or smaller than -1) tells the
6589 linker that a single group of stubs can service at most @samp{N} bytes
6590 from the input sections.
6592 The default, if @option{--stub-group-size=} is not specified, is
6595 Farcalls stubs insertion is fully supported for the ARM-EABI target
6596 only, because it relies on object files properties not present
6599 @cindex Cortex-A8 erratum workaround
6600 @kindex --fix-cortex-a8
6601 @kindex --no-fix-cortex-a8
6602 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}.
6604 The erratum only affects Thumb-2 code. Please contact ARM for further details.
6606 @cindex Cortex-A53 erratum 835769 workaround
6607 @kindex --fix-cortex-a53-835769
6608 @kindex --no-fix-cortex-a53-835769
6609 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}.
6611 Please contact ARM for further details.
6613 @kindex --merge-exidx-entries
6614 @kindex --no-merge-exidx-entries
6615 @cindex Merging exidx entries
6616 The @samp{--no-merge-exidx-entries} switch disables the merging of adjacent exidx entries in debuginfo.
6619 @cindex 32-bit PLT entries
6620 The @samp{--long-plt} option enables the use of 16 byte PLT entries
6621 which support up to 4Gb of code. The default is to use 12 byte PLT
6622 entries which only support 512Mb of code.
6635 @section @command{ld} and HPPA 32-bit ELF Support
6636 @cindex HPPA multiple sub-space stubs
6637 @kindex --multi-subspace
6638 When generating a shared library, @command{ld} will by default generate
6639 import stubs suitable for use with a single sub-space application.
6640 The @samp{--multi-subspace} switch causes @command{ld} to generate export
6641 stubs, and different (larger) import stubs suitable for use with
6642 multiple sub-spaces.
6644 @cindex HPPA stub grouping
6645 @kindex --stub-group-size=@var{N}
6646 Long branch stubs and import/export stubs are placed by @command{ld} in
6647 stub sections located between groups of input sections.
6648 @samp{--stub-group-size} specifies the maximum size of a group of input
6649 sections handled by one stub section. Since branch offsets are signed,
6650 a stub section may serve two groups of input sections, one group before
6651 the stub section, and one group after it. However, when using
6652 conditional branches that require stubs, it may be better (for branch
6653 prediction) that stub sections only serve one group of input sections.
6654 A negative value for @samp{N} chooses this scheme, ensuring that
6655 branches to stubs always use a negative offset. Two special values of
6656 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
6657 @command{ld} to automatically size input section groups for the branch types
6658 detected, with the same behaviour regarding stub placement as other
6659 positive or negative values of @samp{N} respectively.
6661 Note that @samp{--stub-group-size} does not split input sections. A
6662 single input section larger than the group size specified will of course
6663 create a larger group (of one section). If input sections are too
6664 large, it may not be possible for a branch to reach its stub.
6677 @section @command{ld} and the Motorola 68K family
6679 @cindex Motorola 68K GOT generation
6680 @kindex --got=@var{type}
6681 The @samp{--got=@var{type}} option lets you choose the GOT generation scheme.
6682 The choices are @samp{single}, @samp{negative}, @samp{multigot} and
6683 @samp{target}. When @samp{target} is selected the linker chooses
6684 the default GOT generation scheme for the current target.
6685 @samp{single} tells the linker to generate a single GOT with
6686 entries only at non-negative offsets.
6687 @samp{negative} instructs the linker to generate a single GOT with
6688 entries at both negative and positive offsets. Not all environments
6690 @samp{multigot} allows the linker to generate several GOTs in the
6691 output file. All GOT references from a single input object
6692 file access the same GOT, but references from different input object
6693 files might access different GOTs. Not all environments support such GOTs.
6706 @section @command{ld} and the MIPS family
6708 @cindex MIPS microMIPS instruction choice selection
6711 The @samp{--insn32} and @samp{--no-insn32} options control the choice of
6712 microMIPS instructions used in code generated by the linker, such as that
6713 in the PLT or lazy binding stubs, or in relaxation. If @samp{--insn32} is
6714 used, then the linker only uses 32-bit instruction encodings. By default
6715 or if @samp{--no-insn32} is used, all instruction encodings are used,
6716 including 16-bit ones where possible.
6729 @section @code{ld} and MMIX
6730 For MMIX, there is a choice of generating @code{ELF} object files or
6731 @code{mmo} object files when linking. The simulator @code{mmix}
6732 understands the @code{mmo} format. The binutils @code{objcopy} utility
6733 can translate between the two formats.
6735 There is one special section, the @samp{.MMIX.reg_contents} section.
6736 Contents in this section is assumed to correspond to that of global
6737 registers, and symbols referring to it are translated to special symbols,
6738 equal to registers. In a final link, the start address of the
6739 @samp{.MMIX.reg_contents} section corresponds to the first allocated
6740 global register multiplied by 8. Register @code{$255} is not included in
6741 this section; it is always set to the program entry, which is at the
6742 symbol @code{Main} for @code{mmo} files.
6744 Global symbols with the prefix @code{__.MMIX.start.}, for example
6745 @code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special.
6746 The default linker script uses these to set the default start address
6749 Initial and trailing multiples of zero-valued 32-bit words in a section,
6750 are left out from an mmo file.
6763 @section @code{ld} and MSP430
6764 For the MSP430 it is possible to select the MPU architecture. The flag @samp{-m [mpu type]}
6765 will select an appropriate linker script for selected MPU type. (To get a list of known MPUs
6766 just pass @samp{-m help} option to the linker).
6768 @cindex MSP430 extra sections
6769 The linker will recognize some extra sections which are MSP430 specific:
6772 @item @samp{.vectors}
6773 Defines a portion of ROM where interrupt vectors located.
6775 @item @samp{.bootloader}
6776 Defines the bootloader portion of the ROM (if applicable). Any code
6777 in this section will be uploaded to the MPU.
6779 @item @samp{.infomem}
6780 Defines an information memory section (if applicable). Any code in
6781 this section will be uploaded to the MPU.
6783 @item @samp{.infomemnobits}
6784 This is the same as the @samp{.infomem} section except that any code
6785 in this section will not be uploaded to the MPU.
6787 @item @samp{.noinit}
6788 Denotes a portion of RAM located above @samp{.bss} section.
6790 The last two sections are used by gcc.
6804 @section @code{ld} and NDS32
6805 @kindex relaxing on NDS32
6806 For NDS32, there are some options to select relaxation behavior. The linker
6807 relaxes objects according to these options.
6810 @item @samp{--m[no-]fp-as-gp}
6811 Disable/enable fp-as-gp relaxation.
6813 @item @samp{--mexport-symbols=FILE}
6814 Exporting symbols and their address into FILE as linker script.
6816 @item @samp{--m[no-]ex9}
6817 Disable/enable link-time EX9 relaxation.
6819 @item @samp{--mexport-ex9=FILE}
6820 Export the EX9 table after linking.
6822 @item @samp{--mimport-ex9=FILE}
6823 Import the Ex9 table for EX9 relaxation.
6825 @item @samp{--mupdate-ex9}
6826 Update the existing EX9 table.
6828 @item @samp{--mex9-limit=NUM}
6829 Maximum number of entries in the ex9 table.
6831 @item @samp{--mex9-loop-aware}
6832 Avoid generating the EX9 instruction inside the loop.
6834 @item @samp{--m[no-]ifc}
6835 Disable/enable the link-time IFC optimization.
6837 @item @samp{--mifc-loop-aware}
6838 Avoid generating the IFC instruction inside the loop.
6852 @section @command{ld} and the Altera Nios II
6853 @cindex Nios II call relaxation
6854 @kindex --relax on Nios II
6856 Call and immediate jump instructions on Nios II processors are limited to
6857 transferring control to addresses in the same 256MB memory segment,
6858 which may result in @command{ld} giving
6859 @samp{relocation truncated to fit} errors with very large programs.
6860 The command-line option @option{--relax} enables the generation of
6861 trampolines that can access the entire 32-bit address space for calls
6862 outside the normal @code{call} and @code{jmpi} address range. These
6863 trampolines are inserted at section boundaries, so may not themselves
6864 be reachable if an input section and its associated call trampolines are
6867 The @option{--relax} option is enabled by default unless @option{-r}
6868 is also specified. You can disable trampoline generation by using the
6869 @option{--no-relax} linker option. You can also disable this optimization
6870 locally by using the @samp{set .noat} directive in assembly-language
6871 source files, as the linker-inserted trampolines use the @code{at}
6872 register as a temporary.
6874 Note that the linker @option{--relax} option is independent of assembler
6875 relaxation options, and that using the GNU assembler's @option{-relax-all}
6876 option interferes with the linker's more selective call instruction relaxation.
6889 @section @command{ld} and PowerPC 32-bit ELF Support
6890 @cindex PowerPC long branches
6891 @kindex --relax on PowerPC
6892 Branches on PowerPC processors are limited to a signed 26-bit
6893 displacement, which may result in @command{ld} giving
6894 @samp{relocation truncated to fit} errors with very large programs.
6895 @samp{--relax} enables the generation of trampolines that can access
6896 the entire 32-bit address space. These trampolines are inserted at
6897 section boundaries, so may not themselves be reachable if an input
6898 section exceeds 33M in size. You may combine @samp{-r} and
6899 @samp{--relax} to add trampolines in a partial link. In that case
6900 both branches to undefined symbols and inter-section branches are also
6901 considered potentially out of range, and trampolines inserted.
6903 @cindex PowerPC ELF32 options
6908 Current PowerPC GCC accepts a @samp{-msecure-plt} option that
6909 generates code capable of using a newer PLT and GOT layout that has
6910 the security advantage of no executable section ever needing to be
6911 writable and no writable section ever being executable. PowerPC
6912 @command{ld} will generate this layout, including stubs to access the
6913 PLT, if all input files (including startup and static libraries) were
6914 compiled with @samp{-msecure-plt}. @samp{--bss-plt} forces the old
6915 BSS PLT (and GOT layout) which can give slightly better performance.
6917 @kindex --secure-plt
6919 @command{ld} will use the new PLT and GOT layout if it is linking new
6920 @samp{-fpic} or @samp{-fPIC} code, but does not do so automatically
6921 when linking non-PIC code. This option requests the new PLT and GOT
6922 layout. A warning will be given if some object file requires the old
6928 The new secure PLT and GOT are placed differently relative to other
6929 sections compared to older BSS PLT and GOT placement. The location of
6930 @code{.plt} must change because the new secure PLT is an initialized
6931 section while the old PLT is uninitialized. The reason for the
6932 @code{.got} change is more subtle: The new placement allows
6933 @code{.got} to be read-only in applications linked with
6934 @samp{-z relro -z now}. However, this placement means that
6935 @code{.sdata} cannot always be used in shared libraries, because the
6936 PowerPC ABI accesses @code{.sdata} in shared libraries from the GOT
6937 pointer. @samp{--sdata-got} forces the old GOT placement. PowerPC
6938 GCC doesn't use @code{.sdata} in shared libraries, so this option is
6939 really only useful for other compilers that may do so.
6941 @cindex PowerPC stub symbols
6942 @kindex --emit-stub-syms
6943 @item --emit-stub-syms
6944 This option causes @command{ld} to label linker stubs with a local
6945 symbol that encodes the stub type and destination.
6947 @cindex PowerPC TLS optimization
6948 @kindex --no-tls-optimize
6949 @item --no-tls-optimize
6950 PowerPC @command{ld} normally performs some optimization of code
6951 sequences used to access Thread-Local Storage. Use this option to
6952 disable the optimization.
6965 @node PowerPC64 ELF64
6966 @section @command{ld} and PowerPC64 64-bit ELF Support
6968 @cindex PowerPC64 ELF64 options
6970 @cindex PowerPC64 stub grouping
6971 @kindex --stub-group-size
6972 @item --stub-group-size
6973 Long branch stubs, PLT call stubs and TOC adjusting stubs are placed
6974 by @command{ld} in stub sections located between groups of input sections.
6975 @samp{--stub-group-size} specifies the maximum size of a group of input
6976 sections handled by one stub section. Since branch offsets are signed,
6977 a stub section may serve two groups of input sections, one group before
6978 the stub section, and one group after it. However, when using
6979 conditional branches that require stubs, it may be better (for branch
6980 prediction) that stub sections only serve one group of input sections.
6981 A negative value for @samp{N} chooses this scheme, ensuring that
6982 branches to stubs always use a negative offset. Two special values of
6983 @samp{N} are recognized, @samp{1} and @samp{-1}. These both instruct
6984 @command{ld} to automatically size input section groups for the branch types
6985 detected, with the same behaviour regarding stub placement as other
6986 positive or negative values of @samp{N} respectively.
6988 Note that @samp{--stub-group-size} does not split input sections. A
6989 single input section larger than the group size specified will of course
6990 create a larger group (of one section). If input sections are too
6991 large, it may not be possible for a branch to reach its stub.
6993 @cindex PowerPC64 stub symbols
6994 @kindex --emit-stub-syms
6995 @item --emit-stub-syms
6996 This option causes @command{ld} to label linker stubs with a local
6997 symbol that encodes the stub type and destination.
6999 @cindex PowerPC64 dot symbols
7001 @kindex --no-dotsyms
7002 @item --dotsyms, --no-dotsyms
7003 These two options control how @command{ld} interprets version patterns
7004 in a version script. Older PowerPC64 compilers emitted both a
7005 function descriptor symbol with the same name as the function, and a
7006 code entry symbol with the name prefixed by a dot (@samp{.}). To
7007 properly version a function @samp{foo}, the version script thus needs
7008 to control both @samp{foo} and @samp{.foo}. The option
7009 @samp{--dotsyms}, on by default, automatically adds the required
7010 dot-prefixed patterns. Use @samp{--no-dotsyms} to disable this
7013 @cindex PowerPC64 TLS optimization
7014 @kindex --no-tls-optimize
7015 @item --no-tls-optimize
7016 PowerPC64 @command{ld} normally performs some optimization of code
7017 sequences used to access Thread-Local Storage. Use this option to
7018 disable the optimization.
7020 @cindex PowerPC64 OPD optimization
7021 @kindex --no-opd-optimize
7022 @item --no-opd-optimize
7023 PowerPC64 @command{ld} normally removes @code{.opd} section entries
7024 corresponding to deleted link-once functions, or functions removed by
7025 the action of @samp{--gc-sections} or linker script @code{/DISCARD/}.
7026 Use this option to disable @code{.opd} optimization.
7028 @cindex PowerPC64 OPD spacing
7029 @kindex --non-overlapping-opd
7030 @item --non-overlapping-opd
7031 Some PowerPC64 compilers have an option to generate compressed
7032 @code{.opd} entries spaced 16 bytes apart, overlapping the third word,
7033 the static chain pointer (unused in C) with the first word of the next
7034 entry. This option expands such entries to the full 24 bytes.
7036 @cindex PowerPC64 TOC optimization
7037 @kindex --no-toc-optimize
7038 @item --no-toc-optimize
7039 PowerPC64 @command{ld} normally removes unused @code{.toc} section
7040 entries. Such entries are detected by examining relocations that
7041 reference the TOC in code sections. A reloc in a deleted code section
7042 marks a TOC word as unneeded, while a reloc in a kept code section
7043 marks a TOC word as needed. Since the TOC may reference itself, TOC
7044 relocs are also examined. TOC words marked as both needed and
7045 unneeded will of course be kept. TOC words without any referencing
7046 reloc are assumed to be part of a multi-word entry, and are kept or
7047 discarded as per the nearest marked preceding word. This works
7048 reliably for compiler generated code, but may be incorrect if assembly
7049 code is used to insert TOC entries. Use this option to disable the
7052 @cindex PowerPC64 multi-TOC
7053 @kindex --no-multi-toc
7054 @item --no-multi-toc
7055 If given any toc option besides @code{-mcmodel=medium} or
7056 @code{-mcmodel=large}, PowerPC64 GCC generates code for a TOC model
7058 entries are accessed with a 16-bit offset from r2. This limits the
7059 total TOC size to 64K. PowerPC64 @command{ld} extends this limit by
7060 grouping code sections such that each group uses less than 64K for its
7061 TOC entries, then inserts r2 adjusting stubs between inter-group
7062 calls. @command{ld} does not split apart input sections, so cannot
7063 help if a single input file has a @code{.toc} section that exceeds
7064 64K, most likely from linking multiple files with @command{ld -r}.
7065 Use this option to turn off this feature.
7067 @cindex PowerPC64 TOC sorting
7068 @kindex --no-toc-sort
7070 By default, @command{ld} sorts TOC sections so that those whose file
7071 happens to have a section called @code{.init} or @code{.fini} are
7072 placed first, followed by TOC sections referenced by code generated
7073 with PowerPC64 gcc's @code{-mcmodel=small}, and lastly TOC sections
7074 referenced only by code generated with PowerPC64 gcc's
7075 @code{-mcmodel=medium} or @code{-mcmodel=large} options. Doing this
7076 results in better TOC grouping for multi-TOC. Use this option to turn
7079 @cindex PowerPC64 PLT stub alignment
7081 @kindex --no-plt-align
7083 @itemx --no-plt-align
7084 Use these options to control whether individual PLT call stubs are
7085 padded so that they don't cross a 32-byte boundary, or to the
7086 specified power of two boundary when using @code{--plt-align=}. Note
7087 that this isn't alignment in the usual sense. By default PLT call
7088 stubs are packed tightly.
7090 @cindex PowerPC64 PLT call stub static chain
7091 @kindex --plt-static-chain
7092 @kindex --no-plt-static-chain
7093 @item --plt-static-chain
7094 @itemx --no-plt-static-chain
7095 Use these options to control whether PLT call stubs load the static
7096 chain pointer (r11). @code{ld} defaults to not loading the static
7097 chain since there is never any need to do so on a PLT call.
7099 @cindex PowerPC64 PLT call stub thread safety
7100 @kindex --plt-thread-safe
7101 @kindex --no-plt-thread-safe
7102 @item --plt-thread-safe
7103 @itemx --no-thread-safe
7104 With power7's weakly ordered memory model, it is possible when using
7105 lazy binding for ld.so to update a plt entry in one thread and have
7106 another thread see the individual plt entry words update in the wrong
7107 order, despite ld.so carefully writing in the correct order and using
7108 memory write barriers. To avoid this we need some sort of read
7109 barrier in the call stub, or use LD_BIND_NOW=1. By default, @code{ld}
7110 looks for calls to commonly used functions that create threads, and if
7111 seen, adds the necessary barriers. Use these options to change the
7126 @section @command{ld} and SPU ELF Support
7128 @cindex SPU ELF options
7134 This option marks an executable as a PIC plugin module.
7136 @cindex SPU overlays
7137 @kindex --no-overlays
7139 Normally, @command{ld} recognizes calls to functions within overlay
7140 regions, and redirects such calls to an overlay manager via a stub.
7141 @command{ld} also provides a built-in overlay manager. This option
7142 turns off all this special overlay handling.
7144 @cindex SPU overlay stub symbols
7145 @kindex --emit-stub-syms
7146 @item --emit-stub-syms
7147 This option causes @command{ld} to label overlay stubs with a local
7148 symbol that encodes the stub type and destination.
7150 @cindex SPU extra overlay stubs
7151 @kindex --extra-overlay-stubs
7152 @item --extra-overlay-stubs
7153 This option causes @command{ld} to add overlay call stubs on all
7154 function calls out of overlay regions. Normally stubs are not added
7155 on calls to non-overlay regions.
7157 @cindex SPU local store size
7158 @kindex --local-store=lo:hi
7159 @item --local-store=lo:hi
7160 @command{ld} usually checks that a final executable for SPU fits in
7161 the address range 0 to 256k. This option may be used to change the
7162 range. Disable the check entirely with @option{--local-store=0:0}.
7165 @kindex --stack-analysis
7166 @item --stack-analysis
7167 SPU local store space is limited. Over-allocation of stack space
7168 unnecessarily limits space available for code and data, while
7169 under-allocation results in runtime failures. If given this option,
7170 @command{ld} will provide an estimate of maximum stack usage.
7171 @command{ld} does this by examining symbols in code sections to
7172 determine the extents of functions, and looking at function prologues
7173 for stack adjusting instructions. A call-graph is created by looking
7174 for relocations on branch instructions. The graph is then searched
7175 for the maximum stack usage path. Note that this analysis does not
7176 find calls made via function pointers, and does not handle recursion
7177 and other cycles in the call graph. Stack usage may be
7178 under-estimated if your code makes such calls. Also, stack usage for
7179 dynamic allocation, e.g. alloca, will not be detected. If a link map
7180 is requested, detailed information about each function's stack usage
7181 and calls will be given.
7184 @kindex --emit-stack-syms
7185 @item --emit-stack-syms
7186 This option, if given along with @option{--stack-analysis} will result
7187 in @command{ld} emitting stack sizing symbols for each function.
7188 These take the form @code{__stack_<function_name>} for global
7189 functions, and @code{__stack_<number>_<function_name>} for static
7190 functions. @code{<number>} is the section id in hex. The value of
7191 such symbols is the stack requirement for the corresponding function.
7192 The symbol size will be zero, type @code{STT_NOTYPE}, binding
7193 @code{STB_LOCAL}, and section @code{SHN_ABS}.
7207 @section @command{ld}'s Support for Various TI COFF Versions
7208 @cindex TI COFF versions
7209 @kindex --format=@var{version}
7210 The @samp{--format} switch allows selection of one of the various
7211 TI COFF versions. The latest of this writing is 2; versions 0 and 1 are
7212 also supported. The TI COFF versions also vary in header byte-order
7213 format; @command{ld} will read any version or byte order, but the output
7214 header format depends on the default specified by the specific target.
7227 @section @command{ld} and WIN32 (cygwin/mingw)
7229 This section describes some of the win32 specific @command{ld} issues.
7230 See @ref{Options,,Command Line Options} for detailed description of the
7231 command line options mentioned here.
7234 @cindex import libraries
7235 @item import libraries
7236 The standard Windows linker creates and uses so-called import
7237 libraries, which contains information for linking to dll's. They are
7238 regular static archives and are handled as any other static
7239 archive. The cygwin and mingw ports of @command{ld} have specific
7240 support for creating such libraries provided with the
7241 @samp{--out-implib} command line option.
7243 @item exporting DLL symbols
7244 @cindex exporting DLL symbols
7245 The cygwin/mingw @command{ld} has several ways to export symbols for dll's.
7248 @item using auto-export functionality
7249 @cindex using auto-export functionality
7250 By default @command{ld} exports symbols with the auto-export functionality,
7251 which is controlled by the following command line options:
7254 @item --export-all-symbols [This is the default]
7255 @item --exclude-symbols
7256 @item --exclude-libs
7257 @item --exclude-modules-for-implib
7258 @item --version-script
7261 When auto-export is in operation, @command{ld} will export all the non-local
7262 (global and common) symbols it finds in a DLL, with the exception of a few
7263 symbols known to belong to the system's runtime and libraries. As it will
7264 often not be desirable to export all of a DLL's symbols, which may include
7265 private functions that are not part of any public interface, the command-line
7266 options listed above may be used to filter symbols out from the list for
7267 exporting. The @samp{--output-def} option can be used in order to see the
7268 final list of exported symbols with all exclusions taken into effect.
7270 If @samp{--export-all-symbols} is not given explicitly on the
7271 command line, then the default auto-export behavior will be @emph{disabled}
7272 if either of the following are true:
7275 @item A DEF file is used.
7276 @item Any symbol in any object file was marked with the __declspec(dllexport) attribute.
7279 @item using a DEF file
7280 @cindex using a DEF file
7281 Another way of exporting symbols is using a DEF file. A DEF file is
7282 an ASCII file containing definitions of symbols which should be
7283 exported when a dll is created. Usually it is named @samp{<dll
7284 name>.def} and is added as any other object file to the linker's
7285 command line. The file's name must end in @samp{.def} or @samp{.DEF}.
7288 gcc -o <output> <objectfiles> <dll name>.def
7291 Using a DEF file turns off the normal auto-export behavior, unless the
7292 @samp{--export-all-symbols} option is also used.
7294 Here is an example of a DEF file for a shared library called @samp{xyz.dll}:
7297 LIBRARY "xyz.dll" BASE=0x20000000
7303 another_foo = abc.dll.afoo
7309 This example defines a DLL with a non-default base address and seven
7310 symbols in the export table. The third exported symbol @code{_bar} is an
7311 alias for the second. The fourth symbol, @code{another_foo} is resolved
7312 by "forwarding" to another module and treating it as an alias for
7313 @code{afoo} exported from the DLL @samp{abc.dll}. The final symbol
7314 @code{var1} is declared to be a data object. The @samp{doo} symbol in
7315 export library is an alias of @samp{foo}, which gets the string name
7316 in export table @samp{foo2}. The @samp{eoo} symbol is an data export
7317 symbol, which gets in export table the name @samp{var1}.
7319 The optional @code{LIBRARY <name>} command indicates the @emph{internal}
7320 name of the output DLL. If @samp{<name>} does not include a suffix,
7321 the default library suffix, @samp{.DLL} is appended.
7323 When the .DEF file is used to build an application, rather than a
7324 library, the @code{NAME <name>} command should be used instead of
7325 @code{LIBRARY}. If @samp{<name>} does not include a suffix, the default
7326 executable suffix, @samp{.EXE} is appended.
7328 With either @code{LIBRARY <name>} or @code{NAME <name>} the optional
7329 specification @code{BASE = <number>} may be used to specify a
7330 non-default base address for the image.
7332 If neither @code{LIBRARY <name>} nor @code{NAME <name>} is specified,
7333 or they specify an empty string, the internal name is the same as the
7334 filename specified on the command line.
7336 The complete specification of an export symbol is:
7340 ( ( ( <name1> [ = <name2> ] )
7341 | ( <name1> = <module-name> . <external-name>))
7342 [ @@ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
7345 Declares @samp{<name1>} as an exported symbol from the DLL, or declares
7346 @samp{<name1>} as an exported alias for @samp{<name2>}; or declares
7347 @samp{<name1>} as a "forward" alias for the symbol
7348 @samp{<external-name>} in the DLL @samp{<module-name>}.
7349 Optionally, the symbol may be exported by the specified ordinal
7350 @samp{<integer>} alias. The optional @samp{<name3>} is the to be used
7351 string in import/export table for the symbol.
7353 The optional keywords that follow the declaration indicate:
7355 @code{NONAME}: Do not put the symbol name in the DLL's export table. It
7356 will still be exported by its ordinal alias (either the value specified
7357 by the .def specification or, otherwise, the value assigned by the
7358 linker). The symbol name, however, does remain visible in the import
7359 library (if any), unless @code{PRIVATE} is also specified.
7361 @code{DATA}: The symbol is a variable or object, rather than a function.
7362 The import lib will export only an indirect reference to @code{foo} as
7363 the symbol @code{_imp__foo} (ie, @code{foo} must be resolved as
7366 @code{CONSTANT}: Like @code{DATA}, but put the undecorated @code{foo} as
7367 well as @code{_imp__foo} into the import library. Both refer to the
7368 read-only import address table's pointer to the variable, not to the
7369 variable itself. This can be dangerous. If the user code fails to add
7370 the @code{dllimport} attribute and also fails to explicitly add the
7371 extra indirection that the use of the attribute enforces, the
7372 application will behave unexpectedly.
7374 @code{PRIVATE}: Put the symbol in the DLL's export table, but do not put
7375 it into the static import library used to resolve imports at link time. The
7376 symbol can still be imported using the @code{LoadLibrary/GetProcAddress}
7377 API at runtime or by by using the GNU ld extension of linking directly to
7378 the DLL without an import library.
7380 See ld/deffilep.y in the binutils sources for the full specification of
7381 other DEF file statements
7383 @cindex creating a DEF file
7384 While linking a shared dll, @command{ld} is able to create a DEF file
7385 with the @samp{--output-def <file>} command line option.
7387 @item Using decorations
7388 @cindex Using decorations
7389 Another way of marking symbols for export is to modify the source code
7390 itself, so that when building the DLL each symbol to be exported is
7394 __declspec(dllexport) int a_variable
7395 __declspec(dllexport) void a_function(int with_args)
7398 All such symbols will be exported from the DLL. If, however,
7399 any of the object files in the DLL contain symbols decorated in
7400 this way, then the normal auto-export behavior is disabled, unless
7401 the @samp{--export-all-symbols} option is also used.
7403 Note that object files that wish to access these symbols must @emph{not}
7404 decorate them with dllexport. Instead, they should use dllimport,
7408 __declspec(dllimport) int a_variable
7409 __declspec(dllimport) void a_function(int with_args)
7412 This complicates the structure of library header files, because
7413 when included by the library itself the header must declare the
7414 variables and functions as dllexport, but when included by client
7415 code the header must declare them as dllimport. There are a number
7416 of idioms that are typically used to do this; often client code can
7417 omit the __declspec() declaration completely. See
7418 @samp{--enable-auto-import} and @samp{automatic data imports} for more
7422 @cindex automatic data imports
7423 @item automatic data imports
7424 The standard Windows dll format supports data imports from dlls only
7425 by adding special decorations (dllimport/dllexport), which let the
7426 compiler produce specific assembler instructions to deal with this
7427 issue. This increases the effort necessary to port existing Un*x
7428 code to these platforms, especially for large
7429 c++ libraries and applications. The auto-import feature, which was
7430 initially provided by Paul Sokolovsky, allows one to omit the
7431 decorations to achieve a behavior that conforms to that on POSIX/Un*x
7432 platforms. This feature is enabled with the @samp{--enable-auto-import}
7433 command-line option, although it is enabled by default on cygwin/mingw.
7434 The @samp{--enable-auto-import} option itself now serves mainly to
7435 suppress any warnings that are ordinarily emitted when linked objects
7436 trigger the feature's use.
7438 auto-import of variables does not always work flawlessly without
7439 additional assistance. Sometimes, you will see this message
7441 "variable '<var>' can't be auto-imported. Please read the
7442 documentation for ld's @code{--enable-auto-import} for details."
7444 The @samp{--enable-auto-import} documentation explains why this error
7445 occurs, and several methods that can be used to overcome this difficulty.
7446 One of these methods is the @emph{runtime pseudo-relocs} feature, described
7449 @cindex runtime pseudo-relocation
7450 For complex variables imported from DLLs (such as structs or classes),
7451 object files typically contain a base address for the variable and an
7452 offset (@emph{addend}) within the variable--to specify a particular
7453 field or public member, for instance. Unfortunately, the runtime loader used
7454 in win32 environments is incapable of fixing these references at runtime
7455 without the additional information supplied by dllimport/dllexport decorations.
7456 The standard auto-import feature described above is unable to resolve these
7459 The @samp{--enable-runtime-pseudo-relocs} switch allows these references to
7460 be resolved without error, while leaving the task of adjusting the references
7461 themselves (with their non-zero addends) to specialized code provided by the
7462 runtime environment. Recent versions of the cygwin and mingw environments and
7463 compilers provide this runtime support; older versions do not. However, the
7464 support is only necessary on the developer's platform; the compiled result will
7465 run without error on an older system.
7467 @samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly
7470 @cindex direct linking to a dll
7471 @item direct linking to a dll
7472 The cygwin/mingw ports of @command{ld} support the direct linking,
7473 including data symbols, to a dll without the usage of any import
7474 libraries. This is much faster and uses much less memory than does the
7475 traditional import library method, especially when linking large
7476 libraries or applications. When @command{ld} creates an import lib, each
7477 function or variable exported from the dll is stored in its own bfd, even
7478 though a single bfd could contain many exports. The overhead involved in
7479 storing, loading, and processing so many bfd's is quite large, and explains the
7480 tremendous time, memory, and storage needed to link against particularly
7481 large or complex libraries when using import libs.
7483 Linking directly to a dll uses no extra command-line switches other than
7484 @samp{-L} and @samp{-l}, because @command{ld} already searches for a number
7485 of names to match each library. All that is needed from the developer's
7486 perspective is an understanding of this search, in order to force ld to
7487 select the dll instead of an import library.
7490 For instance, when ld is called with the argument @samp{-lxxx} it will attempt
7491 to find, in the first directory of its search path,
7503 before moving on to the next directory in the search path.
7505 (*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll},
7506 where @samp{<prefix>} is set by the @command{ld} option
7507 @samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec
7508 file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for
7511 Other win32-based unix environments, such as mingw or pw32, may use other
7512 @samp{<prefix>}es, although at present only cygwin makes use of this feature. It
7513 was originally intended to help avoid name conflicts among dll's built for the
7514 various win32/un*x environments, so that (for example) two versions of a zlib dll
7515 could coexist on the same machine.
7517 The generic cygwin/mingw path layout uses a @samp{bin} directory for
7518 applications and dll's and a @samp{lib} directory for the import
7519 libraries (using cygwin nomenclature):
7525 libxxx.dll.a (in case of dll's)
7526 libxxx.a (in case of static archive)
7529 Linking directly to a dll without using the import library can be
7532 1. Use the dll directly by adding the @samp{bin} path to the link line
7534 gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
7537 However, as the dll's often have version numbers appended to their names
7538 (@samp{cygncurses-5.dll}) this will often fail, unless one specifies
7539 @samp{-L../bin -lncurses-5} to include the version. Import libs are generally
7540 not versioned, and do not have this difficulty.
7542 2. Create a symbolic link from the dll to a file in the @samp{lib}
7543 directory according to the above mentioned search pattern. This
7544 should be used to avoid unwanted changes in the tools needed for
7548 ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
7551 Then you can link without any make environment changes.
7554 gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
7557 This technique also avoids the version number problems, because the following is
7564 libxxx.dll.a -> ../bin/cygxxx-5.dll
7567 Linking directly to a dll without using an import lib will work
7568 even when auto-import features are exercised, and even when
7569 @samp{--enable-runtime-pseudo-relocs} is used.
7571 Given the improvements in speed and memory usage, one might justifiably
7572 wonder why import libraries are used at all. There are three reasons:
7574 1. Until recently, the link-directly-to-dll functionality did @emph{not}
7575 work with auto-imported data.
7577 2. Sometimes it is necessary to include pure static objects within the
7578 import library (which otherwise contains only bfd's for indirection
7579 symbols that point to the exports of a dll). Again, the import lib
7580 for the cygwin kernel makes use of this ability, and it is not
7581 possible to do this without an import lib.
7583 3. Symbol aliases can only be resolved using an import lib. This is
7584 critical when linking against OS-supplied dll's (eg, the win32 API)
7585 in which symbols are usually exported as undecorated aliases of their
7586 stdcall-decorated assembly names.
7588 So, import libs are not going away. But the ability to replace
7589 true import libs with a simple symbolic link to (or a copy of)
7590 a dll, in many cases, is a useful addition to the suite of tools
7591 binutils makes available to the win32 developer. Given the
7592 massive improvements in memory requirements during linking, storage
7593 requirements, and linking speed, we expect that many developers
7594 will soon begin to use this feature whenever possible.
7596 @item symbol aliasing
7598 @item adding additional names
7599 Sometimes, it is useful to export symbols with additional names.
7600 A symbol @samp{foo} will be exported as @samp{foo}, but it can also be
7601 exported as @samp{_foo} by using special directives in the DEF file
7602 when creating the dll. This will affect also the optional created
7603 import library. Consider the following DEF file:
7606 LIBRARY "xyz.dll" BASE=0x61000000
7613 The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}.
7615 Another method for creating a symbol alias is to create it in the
7616 source code using the "weak" attribute:
7619 void foo () @{ /* Do something. */; @}
7620 void _foo () __attribute__ ((weak, alias ("foo")));
7623 See the gcc manual for more information about attributes and weak
7626 @item renaming symbols
7627 Sometimes it is useful to rename exports. For instance, the cygwin
7628 kernel does this regularly. A symbol @samp{_foo} can be exported as
7629 @samp{foo} but not as @samp{_foo} by using special directives in the
7630 DEF file. (This will also affect the import library, if it is
7631 created). In the following example:
7634 LIBRARY "xyz.dll" BASE=0x61000000
7640 The line @samp{_foo = foo} maps the exported symbol @samp{foo} to
7644 Note: using a DEF file disables the default auto-export behavior,
7645 unless the @samp{--export-all-symbols} command line option is used.
7646 If, however, you are trying to rename symbols, then you should list
7647 @emph{all} desired exports in the DEF file, including the symbols
7648 that are not being renamed, and do @emph{not} use the
7649 @samp{--export-all-symbols} option. If you list only the
7650 renamed symbols in the DEF file, and use @samp{--export-all-symbols}
7651 to handle the other symbols, then the both the new names @emph{and}
7652 the original names for the renamed symbols will be exported.
7653 In effect, you'd be aliasing those symbols, not renaming them,
7654 which is probably not what you wanted.
7656 @cindex weak externals
7657 @item weak externals
7658 The Windows object format, PE, specifies a form of weak symbols called
7659 weak externals. When a weak symbol is linked and the symbol is not
7660 defined, the weak symbol becomes an alias for some other symbol. There
7661 are three variants of weak externals:
7663 @item Definition is searched for in objects and libraries, historically
7664 called lazy externals.
7665 @item Definition is searched for only in other objects, not in libraries.
7666 This form is not presently implemented.
7667 @item No search; the symbol is an alias. This form is not presently
7670 As a GNU extension, weak symbols that do not specify an alternate symbol
7671 are supported. If the symbol is undefined when linking, the symbol
7672 uses a default value.
7674 @cindex aligned common symbols
7675 @item aligned common symbols
7676 As a GNU extension to the PE file format, it is possible to specify the
7677 desired alignment for a common symbol. This information is conveyed from
7678 the assembler or compiler to the linker by means of GNU-specific commands
7679 carried in the object file's @samp{.drectve} section, which are recognized
7680 by @command{ld} and respected when laying out the common symbols. Native
7681 tools will be able to process object files employing this GNU extension,
7682 but will fail to respect the alignment instructions, and may issue noisy
7683 warnings about unknown linker directives.
7698 @section @code{ld} and Xtensa Processors
7700 @cindex Xtensa processors
7701 The default @command{ld} behavior for Xtensa processors is to interpret
7702 @code{SECTIONS} commands so that lists of explicitly named sections in a
7703 specification with a wildcard file will be interleaved when necessary to
7704 keep literal pools within the range of PC-relative load offsets. For
7705 example, with the command:
7717 @command{ld} may interleave some of the @code{.literal}
7718 and @code{.text} sections from different object files to ensure that the
7719 literal pools are within the range of PC-relative load offsets. A valid
7720 interleaving might place the @code{.literal} sections from an initial
7721 group of files followed by the @code{.text} sections of that group of
7722 files. Then, the @code{.literal} sections from the rest of the files
7723 and the @code{.text} sections from the rest of the files would follow.
7725 @cindex @option{--relax} on Xtensa
7726 @cindex relaxing on Xtensa
7727 Relaxation is enabled by default for the Xtensa version of @command{ld} and
7728 provides two important link-time optimizations. The first optimization
7729 is to combine identical literal values to reduce code size. A redundant
7730 literal will be removed and all the @code{L32R} instructions that use it
7731 will be changed to reference an identical literal, as long as the
7732 location of the replacement literal is within the offset range of all
7733 the @code{L32R} instructions. The second optimization is to remove
7734 unnecessary overhead from assembler-generated ``longcall'' sequences of
7735 @code{L32R}/@code{CALLX@var{n}} when the target functions are within
7736 range of direct @code{CALL@var{n}} instructions.
7738 For each of these cases where an indirect call sequence can be optimized
7739 to a direct call, the linker will change the @code{CALLX@var{n}}
7740 instruction to a @code{CALL@var{n}} instruction, remove the @code{L32R}
7741 instruction, and remove the literal referenced by the @code{L32R}
7742 instruction if it is not used for anything else. Removing the
7743 @code{L32R} instruction always reduces code size but can potentially
7744 hurt performance by changing the alignment of subsequent branch targets.
7745 By default, the linker will always preserve alignments, either by
7746 switching some instructions between 24-bit encodings and the equivalent
7747 density instructions or by inserting a no-op in place of the @code{L32R}
7748 instruction that was removed. If code size is more important than
7749 performance, the @option{--size-opt} option can be used to prevent the
7750 linker from widening density instructions or inserting no-ops, except in
7751 a few cases where no-ops are required for correctness.
7753 The following Xtensa-specific command-line options can be used to
7756 @cindex Xtensa options
7759 When optimizing indirect calls to direct calls, optimize for code size
7760 more than performance. With this option, the linker will not insert
7761 no-ops or widen density instructions to preserve branch target
7762 alignment. There may still be some cases where no-ops are required to
7763 preserve the correctness of the code.
7771 @ifclear SingleFormat
7776 @cindex object file management
7777 @cindex object formats available
7779 The linker accesses object and archive files using the BFD libraries.
7780 These libraries allow the linker to use the same routines to operate on
7781 object files whatever the object file format. A different object file
7782 format can be supported simply by creating a new BFD back end and adding
7783 it to the library. To conserve runtime memory, however, the linker and
7784 associated tools are usually configured to support only a subset of the
7785 object file formats available. You can use @code{objdump -i}
7786 (@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to
7787 list all the formats available for your configuration.
7789 @cindex BFD requirements
7790 @cindex requirements for BFD
7791 As with most implementations, BFD is a compromise between
7792 several conflicting requirements. The major factor influencing
7793 BFD design was efficiency: any time used converting between
7794 formats is time which would not have been spent had BFD not
7795 been involved. This is partly offset by abstraction payback; since
7796 BFD simplifies applications and back ends, more time and care
7797 may be spent optimizing algorithms for a greater speed.
7799 One minor artifact of the BFD solution which you should bear in
7800 mind is the potential for information loss. There are two places where
7801 useful information can be lost using the BFD mechanism: during
7802 conversion and during output. @xref{BFD information loss}.
7805 * BFD outline:: How it works: an outline of BFD
7809 @section How It Works: An Outline of BFD
7810 @cindex opening object files
7811 @include bfdsumm.texi
7814 @node Reporting Bugs
7815 @chapter Reporting Bugs
7816 @cindex bugs in @command{ld}
7817 @cindex reporting bugs in @command{ld}
7819 Your bug reports play an essential role in making @command{ld} reliable.
7821 Reporting a bug may help you by bringing a solution to your problem, or
7822 it may not. But in any case the principal function of a bug report is
7823 to help the entire community by making the next version of @command{ld}
7824 work better. Bug reports are your contribution to the maintenance of
7827 In order for a bug report to serve its purpose, you must include the
7828 information that enables us to fix the bug.
7831 * Bug Criteria:: Have you found a bug?
7832 * Bug Reporting:: How to report bugs
7836 @section Have You Found a Bug?
7837 @cindex bug criteria
7839 If you are not sure whether you have found a bug, here are some guidelines:
7842 @cindex fatal signal
7843 @cindex linker crash
7844 @cindex crash of linker
7846 If the linker gets a fatal signal, for any input whatever, that is a
7847 @command{ld} bug. Reliable linkers never crash.
7849 @cindex error on valid input
7851 If @command{ld} produces an error message for valid input, that is a bug.
7853 @cindex invalid input
7855 If @command{ld} does not produce an error message for invalid input, that
7856 may be a bug. In the general case, the linker can not verify that
7857 object files are correct.
7860 If you are an experienced user of linkers, your suggestions for
7861 improvement of @command{ld} are welcome in any case.
7865 @section How to Report Bugs
7867 @cindex @command{ld} bugs, reporting
7869 A number of companies and individuals offer support for @sc{gnu}
7870 products. If you obtained @command{ld} from a support organization, we
7871 recommend you contact that organization first.
7873 You can find contact information for many support companies and
7874 individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs
7878 Otherwise, send bug reports for @command{ld} to
7882 The fundamental principle of reporting bugs usefully is this:
7883 @strong{report all the facts}. If you are not sure whether to state a
7884 fact or leave it out, state it!
7886 Often people omit facts because they think they know what causes the
7887 problem and assume that some details do not matter. Thus, you might
7888 assume that the name of a symbol you use in an example does not
7889 matter. Well, probably it does not, but one cannot be sure. Perhaps
7890 the bug is a stray memory reference which happens to fetch from the
7891 location where that name is stored in memory; perhaps, if the name
7892 were different, the contents of that location would fool the linker
7893 into doing the right thing despite the bug. Play it safe and give a
7894 specific, complete example. That is the easiest thing for you to do,
7895 and the most helpful.
7897 Keep in mind that the purpose of a bug report is to enable us to fix
7898 the bug if it is new to us. Therefore, always write your bug reports
7899 on the assumption that the bug has not been reported previously.
7901 Sometimes people give a few sketchy facts and ask, ``Does this ring a
7902 bell?'' This cannot help us fix a bug, so it is basically useless. We
7903 respond by asking for enough details to enable us to investigate.
7904 You might as well expedite matters by sending them to begin with.
7906 To enable us to fix the bug, you should include all these things:
7910 The version of @command{ld}. @command{ld} announces it if you start it with
7911 the @samp{--version} argument.
7913 Without this, we will not know whether there is any point in looking for
7914 the bug in the current version of @command{ld}.
7917 Any patches you may have applied to the @command{ld} source, including any
7918 patches made to the @code{BFD} library.
7921 The type of machine you are using, and the operating system name and
7925 What compiler (and its version) was used to compile @command{ld}---e.g.
7929 The command arguments you gave the linker to link your example and
7930 observe the bug. To guarantee you will not omit something important,
7931 list them all. A copy of the Makefile (or the output from make) is
7934 If we were to try to guess the arguments, we would probably guess wrong
7935 and then we might not encounter the bug.
7938 A complete input file, or set of input files, that will reproduce the
7939 bug. It is generally most helpful to send the actual object files
7940 provided that they are reasonably small. Say no more than 10K. For
7941 bigger files you can either make them available by FTP or HTTP or else
7942 state that you are willing to send the object file(s) to whomever
7943 requests them. (Note - your email will be going to a mailing list, so
7944 we do not want to clog it up with large attachments). But small
7945 attachments are best.
7947 If the source files were assembled using @code{gas} or compiled using
7948 @code{gcc}, then it may be OK to send the source files rather than the
7949 object files. In this case, be sure to say exactly what version of
7950 @code{gas} or @code{gcc} was used to produce the object files. Also say
7951 how @code{gas} or @code{gcc} were configured.
7954 A description of what behavior you observe that you believe is
7955 incorrect. For example, ``It gets a fatal signal.''
7957 Of course, if the bug is that @command{ld} gets a fatal signal, then we
7958 will certainly notice it. But if the bug is incorrect output, we might
7959 not notice unless it is glaringly wrong. You might as well not give us
7960 a chance to make a mistake.
7962 Even if the problem you experience is a fatal signal, you should still
7963 say so explicitly. Suppose something strange is going on, such as, your
7964 copy of @command{ld} is out of sync, or you have encountered a bug in the
7965 C library on your system. (This has happened!) Your copy might crash
7966 and ours would not. If you told us to expect a crash, then when ours
7967 fails to crash, we would know that the bug was not happening for us. If
7968 you had not told us to expect a crash, then we would not be able to draw
7969 any conclusion from our observations.
7972 If you wish to suggest changes to the @command{ld} source, send us context
7973 diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or
7974 @samp{-p} option. Always send diffs from the old file to the new file.
7975 If you even discuss something in the @command{ld} source, refer to it by
7976 context, not by line number.
7978 The line numbers in our development sources will not match those in your
7979 sources. Your line numbers would convey no useful information to us.
7982 Here are some things that are not necessary:
7986 A description of the envelope of the bug.
7988 Often people who encounter a bug spend a lot of time investigating
7989 which changes to the input file will make the bug go away and which
7990 changes will not affect it.
7992 This is often time consuming and not very useful, because the way we
7993 will find the bug is by running a single example under the debugger
7994 with breakpoints, not by pure deduction from a series of examples.
7995 We recommend that you save your time for something else.
7997 Of course, if you can find a simpler example to report @emph{instead}
7998 of the original one, that is a convenience for us. Errors in the
7999 output will be easier to spot, running under the debugger will take
8000 less time, and so on.
8002 However, simplification is not vital; if you do not want to do this,
8003 report the bug anyway and send us the entire test case you used.
8006 A patch for the bug.
8008 A patch for the bug does help us if it is a good one. But do not omit
8009 the necessary information, such as the test case, on the assumption that
8010 a patch is all we need. We might see problems with your patch and decide
8011 to fix the problem another way, or we might not understand it at all.
8013 Sometimes with a program as complicated as @command{ld} it is very hard to
8014 construct an example that will make the program follow a certain path
8015 through the code. If you do not send us the example, we will not be
8016 able to construct one, so we will not be able to verify that the bug is
8019 And if we cannot understand what bug you are trying to fix, or why your
8020 patch should be an improvement, we will not install it. A test case will
8021 help us to understand.
8024 A guess about what the bug is or what it depends on.
8026 Such guesses are usually wrong. Even we cannot guess right about such
8027 things without first using the debugger to find the facts.
8031 @appendix MRI Compatible Script Files
8032 @cindex MRI compatibility
8033 To aid users making the transition to @sc{gnu} @command{ld} from the MRI
8034 linker, @command{ld} can use MRI compatible linker scripts as an
8035 alternative to the more general-purpose linker scripting language
8036 described in @ref{Scripts}. MRI compatible linker scripts have a much
8037 simpler command set than the scripting language otherwise used with
8038 @command{ld}. @sc{gnu} @command{ld} supports the most commonly used MRI
8039 linker commands; these commands are described here.
8041 In general, MRI scripts aren't of much use with the @code{a.out} object
8042 file format, since it only has three sections and MRI scripts lack some
8043 features to make use of them.
8045 You can specify a file containing an MRI-compatible script using the
8046 @samp{-c} command-line option.
8048 Each command in an MRI-compatible script occupies its own line; each
8049 command line starts with the keyword that identifies the command (though
8050 blank lines are also allowed for punctuation). If a line of an
8051 MRI-compatible script begins with an unrecognized keyword, @command{ld}
8052 issues a warning message, but continues processing the script.
8054 Lines beginning with @samp{*} are comments.
8056 You can write these commands using all upper-case letters, or all
8057 lower case; for example, @samp{chip} is the same as @samp{CHIP}.
8058 The following list shows only the upper-case form of each command.
8061 @cindex @code{ABSOLUTE} (MRI)
8062 @item ABSOLUTE @var{secname}
8063 @itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname}
8064 Normally, @command{ld} includes in the output file all sections from all
8065 the input files. However, in an MRI-compatible script, you can use the
8066 @code{ABSOLUTE} command to restrict the sections that will be present in
8067 your output program. If the @code{ABSOLUTE} command is used at all in a
8068 script, then only the sections named explicitly in @code{ABSOLUTE}
8069 commands will appear in the linker output. You can still use other
8070 input sections (whatever you select on the command line, or using
8071 @code{LOAD}) to resolve addresses in the output file.
8073 @cindex @code{ALIAS} (MRI)
8074 @item ALIAS @var{out-secname}, @var{in-secname}
8075 Use this command to place the data from input section @var{in-secname}
8076 in a section called @var{out-secname} in the linker output file.
8078 @var{in-secname} may be an integer.
8080 @cindex @code{ALIGN} (MRI)
8081 @item ALIGN @var{secname} = @var{expression}
8082 Align the section called @var{secname} to @var{expression}. The
8083 @var{expression} should be a power of two.
8085 @cindex @code{BASE} (MRI)
8086 @item BASE @var{expression}
8087 Use the value of @var{expression} as the lowest address (other than
8088 absolute addresses) in the output file.
8090 @cindex @code{CHIP} (MRI)
8091 @item CHIP @var{expression}
8092 @itemx CHIP @var{expression}, @var{expression}
8093 This command does nothing; it is accepted only for compatibility.
8095 @cindex @code{END} (MRI)
8097 This command does nothing whatever; it's only accepted for compatibility.
8099 @cindex @code{FORMAT} (MRI)
8100 @item FORMAT @var{output-format}
8101 Similar to the @code{OUTPUT_FORMAT} command in the more general linker
8102 language, but restricted to one of these output formats:
8106 S-records, if @var{output-format} is @samp{S}
8109 IEEE, if @var{output-format} is @samp{IEEE}
8112 COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is
8116 @cindex @code{LIST} (MRI)
8117 @item LIST @var{anything}@dots{}
8118 Print (to the standard output file) a link map, as produced by the
8119 @command{ld} command-line option @samp{-M}.
8121 The keyword @code{LIST} may be followed by anything on the
8122 same line, with no change in its effect.
8124 @cindex @code{LOAD} (MRI)
8125 @item LOAD @var{filename}
8126 @itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename}
8127 Include one or more object file @var{filename} in the link; this has the
8128 same effect as specifying @var{filename} directly on the @command{ld}
8131 @cindex @code{NAME} (MRI)
8132 @item NAME @var{output-name}
8133 @var{output-name} is the name for the program produced by @command{ld}; the
8134 MRI-compatible command @code{NAME} is equivalent to the command-line
8135 option @samp{-o} or the general script language command @code{OUTPUT}.
8137 @cindex @code{ORDER} (MRI)
8138 @item ORDER @var{secname}, @var{secname}, @dots{} @var{secname}
8139 @itemx ORDER @var{secname} @var{secname} @var{secname}
8140 Normally, @command{ld} orders the sections in its output file in the
8141 order in which they first appear in the input files. In an MRI-compatible
8142 script, you can override this ordering with the @code{ORDER} command. The
8143 sections you list with @code{ORDER} will appear first in your output
8144 file, in the order specified.
8146 @cindex @code{PUBLIC} (MRI)
8147 @item PUBLIC @var{name}=@var{expression}
8148 @itemx PUBLIC @var{name},@var{expression}
8149 @itemx PUBLIC @var{name} @var{expression}
8150 Supply a value (@var{expression}) for external symbol
8151 @var{name} used in the linker input files.
8153 @cindex @code{SECT} (MRI)
8154 @item SECT @var{secname}, @var{expression}
8155 @itemx SECT @var{secname}=@var{expression}
8156 @itemx SECT @var{secname} @var{expression}
8157 You can use any of these three forms of the @code{SECT} command to
8158 specify the start address (@var{expression}) for section @var{secname}.
8159 If you have more than one @code{SECT} statement for the same
8160 @var{secname}, only the @emph{first} sets the start address.
8163 @node GNU Free Documentation License
8164 @appendix GNU Free Documentation License
8168 @unnumbered LD Index
8173 % I think something like @@colophon should be in texinfo. In the
8175 \long\def\colophon{\hbox to0pt{}\vfill
8176 \centerline{The body of this manual is set in}
8177 \centerline{\fontname\tenrm,}
8178 \centerline{with headings in {\bf\fontname\tenbf}}
8179 \centerline{and examples in {\tt\fontname\tentt}.}
8180 \centerline{{\it\fontname\tenit\/} and}
8181 \centerline{{\sl\fontname\tensl\/}}
8182 \centerline{are used for emphasis.}\vfill}
8184 % Blame: doc@@cygnus.com, 28mar91.