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@setfilename ld.info
@c Copyright 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
@c 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
@syncodeindex ky cp
@c man begin INCLUDE
@include configdoc.texi
@c (configdoc.texi is generated by the Makefile)
@include ldver.texi
@c man end
@c @smallbook
@macro gcctabopt{body}
@code{\body\}
@end macro
@c man begin NAME
@ifset man
@c Configure for the generation of man pages
@set UsesEnvVars
@set GENERIC
@set ARC
@set ARM
@set D10V
@set D30V
@set H8/300
@set H8/500
@set HPPA
@set I370
@set I80386
@set I860
@set I960
@set M32R
@set M68HC11
@set M680X0
@set MCORE
@set MIPS
@set MMIX
@set MSP430
@set PDP11
@set PJ
@set POWERPC
@set POWERPC64
@set SH
@set SPARC
@set TIC54X
@set V850
@set VAX
@set WIN32
@set XTENSA
@end ifset
@c man end
@ifinfo
@format
START-INFO-DIR-ENTRY
* Ld: (ld). The GNU linker.
END-INFO-DIR-ENTRY
@end format
@end ifinfo
@ifinfo
This file documents the @sc{gnu} linker LD version @value{VERSION}.
Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000,
2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
@ignore
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts. A copy of the license is included in the
section entitled ``GNU Free Documentation License''.
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
@end ignore
@end ifinfo
@iftex
@finalout
@setchapternewpage odd
@settitle The GNU linker
@titlepage
@title The GNU linker
@sp 1
@subtitle @code{ld} version 2
@subtitle Version @value{VERSION}
@author Steve Chamberlain
@author Ian Lance Taylor
@page
@tex
{\parskip=0pt
\hfill Red Hat Inc\par
\hfill nickc\@credhat.com, doc\@redhat.com\par
\hfill {\it The GNU linker}\par
\hfill Edited by Jeffrey Osier (jeffrey\@cygnus.com)\par
}
\global\parindent=0pt % Steve likes it this way.
@end tex
@vskip 0pt plus 1filll
@c man begin COPYRIGHT
Copyright @copyright{} 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001,
2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts. A copy of the license is included in the
section entitled ``GNU Free Documentation License''.
@c man end
@end titlepage
@end iftex
@c FIXME: Talk about importance of *order* of args, cmds to linker!
@ifnottex
@node Top
@top LD
This file documents the @sc{gnu} linker ld version @value{VERSION}.
This document is distributed under the terms of the GNU Free
Documentation License. A copy of the license is included in the
section entitled ``GNU Free Documentation License''.
@menu
* Overview:: Overview
* Invocation:: Invocation
* Scripts:: Linker Scripts
@ifset GENERIC
* Machine Dependent:: Machine Dependent Features
@end ifset
@ifclear GENERIC
@ifset H8300
* H8/300:: ld and the H8/300
@end ifset
@ifset Renesas
* Renesas:: ld and other Renesas micros
@end ifset
@ifset I960
* i960:: ld and the Intel 960 family
@end ifset
@ifset ARM
* ARM:: ld and the ARM family
@end ifset
@ifset HPPA
* HPPA ELF32:: ld and HPPA 32-bit ELF
@end ifset
@ifset M68HC11
* M68HC11/68HC12:: ld and the Motorola 68HC11 and 68HC12 families
@end ifset
@ifset POWERPC
* PowerPC ELF32:: ld and PowerPC 32-bit ELF Support
@end ifset
@ifset POWERPC64
* PowerPC64 ELF64:: ld and PowerPC64 64-bit ELF Support
@end ifset
@ifset TICOFF
* TI COFF:: ld and the TI COFF
@end ifset
@ifset WIN32
* Win32:: ld and WIN32 (cygwin/mingw)
@end ifset
@ifset XTENSA
* Xtensa:: ld and Xtensa Processors
@end ifset
@end ifclear
@ifclear SingleFormat
* BFD:: BFD
@end ifclear
@c Following blank line required for remaining bug in makeinfo conds/menus
* Reporting Bugs:: Reporting Bugs
* MRI:: MRI Compatible Script Files
* GNU Free Documentation License:: GNU Free Documentation License
* LD Index:: LD Index
@end menu
@end ifnottex
@node Overview
@chapter Overview
@cindex @sc{gnu} linker
@cindex what is this?
@ifset man
@c man begin SYNOPSIS
ld [@b{options}] @var{objfile} @dots{}
@c man end
@c man begin SEEALSO
ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and
the Info entries for @file{binutils} and
@file{ld}.
@c man end
@end ifset
@c man begin DESCRIPTION
@command{ld} combines a number of object and archive files, relocates
their data and ties up symbol references. Usually the last step in
compiling a program is to run @command{ld}.
@command{ld} accepts Linker Command Language files written in
a superset of AT&T's Link Editor Command Language syntax,
to provide explicit and total control over the linking process.
@ifset man
@c For the man only
This man page does not describe the command language; see the
@command{ld} entry in @code{info} for full details on the command
language and on other aspects of the GNU linker.
@end ifset
@ifclear SingleFormat
This version of @command{ld} uses the general purpose BFD libraries
to operate on object files. This allows @command{ld} to read, combine, and
write object files in many different formats---for example, COFF or
@code{a.out}. Different formats may be linked together to produce any
available kind of object file. @xref{BFD}, for more information.
@end ifclear
Aside from its flexibility, the @sc{gnu} linker is more helpful than other
linkers in providing diagnostic information. Many linkers abandon
execution immediately upon encountering an error; whenever possible,
@command{ld} continues executing, allowing you to identify other errors
(or, in some cases, to get an output file in spite of the error).
@c man end
@node Invocation
@chapter Invocation
@c man begin DESCRIPTION
The @sc{gnu} linker @command{ld} is meant to cover a broad range of situations,
and to be as compatible as possible with other linkers. As a result,
you have many choices to control its behavior.
@c man end
@ifset UsesEnvVars
@menu
* Options:: Command Line Options
* Environment:: Environment Variables
@end menu
@node Options
@section Command Line Options
@end ifset
@cindex command line
@cindex options
@c man begin OPTIONS
The linker supports a plethora of command-line options, but in actual
practice few of them are used in any particular context.
@cindex standard Unix system
For instance, a frequent use of @command{ld} is to link standard Unix
object files on a standard, supported Unix system. On such a system, to
link a file @code{hello.o}:
@smallexample
ld -o @var{output} /lib/crt0.o hello.o -lc
@end smallexample
This tells @command{ld} to produce a file called @var{output} as the
result of linking the file @code{/lib/crt0.o} with @code{hello.o} and
the library @code{libc.a}, which will come from the standard search
directories. (See the discussion of the @samp{-l} option below.)
Some of the command-line options to @command{ld} may be specified at any
point in the command line. However, options which refer to files, such
as @samp{-l} or @samp{-T}, cause the file to be read at the point at
which the option appears in the command line, relative to the object
files and other file options. Repeating non-file options with a
different argument will either have no further effect, or override prior
occurrences (those further to the left on the command line) of that
option. Options which may be meaningfully specified more than once are
noted in the descriptions below.
@cindex object files
Non-option arguments are object files or archives which are to be linked
together. They may follow, precede, or be mixed in with command-line
options, except that an object file argument may not be placed between
an option and its argument.
Usually the linker is invoked with at least one object file, but you can
specify other forms of binary input files using @samp{-l}, @samp{-R},
and the script command language. If @emph{no} binary input files at all
are specified, the linker does not produce any output, and issues the
message @samp{No input files}.
If the linker cannot recognize the format of an object file, it will
assume that it is a linker script. A script specified in this way
augments the main linker script used for the link (either the default
linker script or the one specified by using @samp{-T}). This feature
permits the linker to link against a file which appears to be an object
or an archive, but actually merely defines some symbol values, or uses
@code{INPUT} or @code{GROUP} to load other objects. Note that
specifying a script in this way merely augments the main linker script;
use the @samp{-T} option to replace the default linker script entirely.
@xref{Scripts}.
For options whose names are a single letter,
option arguments must either follow the option letter without intervening
whitespace, or be given as separate arguments immediately following the
option that requires them.
For options whose names are multiple letters, either one dash or two can
precede the option name; for example, @samp{-trace-symbol} and
@samp{--trace-symbol} are equivalent. Note---there is one exception to
this rule. Multiple letter options that start with a lower case 'o' can
only be preceded by two dashes. This is to reduce confusion with the
@samp{-o} option. So for example @samp{-omagic} sets the output file
name to @samp{magic} whereas @samp{--omagic} sets the NMAGIC flag on the
output.
Arguments to multiple-letter options must either be separated from the
option name by an equals sign, or be given as separate arguments
immediately following the option that requires them. For example,
@samp{--trace-symbol foo} and @samp{--trace-symbol=foo} are equivalent.
Unique abbreviations of the names of multiple-letter options are
accepted.
Note---if the linker is being invoked indirectly, via a compiler driver
(e.g. @samp{gcc}) then all the linker command line options should be
prefixed by @samp{-Wl,} (or whatever is appropriate for the particular
compiler driver) like this:
@smallexample
gcc -Wl,--startgroup foo.o bar.o -Wl,--endgroup
@end smallexample
This is important, because otherwise the compiler driver program may
silently drop the linker options, resulting in a bad link.
Here is a table of the generic command line switches accepted by the GNU
linker:
@table @gcctabopt
@include at-file.texi
@kindex -a@var{keyword}
@item -a@var{keyword}
This option is supported for HP/UX compatibility. The @var{keyword}
argument must be one of the strings @samp{archive}, @samp{shared}, or
@samp{default}. @samp{-aarchive} is functionally equivalent to
@samp{-Bstatic}, and the other two keywords are functionally equivalent
to @samp{-Bdynamic}. This option may be used any number of times.
@ifset I960
@cindex architectures
@kindex -A@var{arch}
@item -A@var{architecture}
@kindex --architecture=@var{arch}
@itemx --architecture=@var{architecture}
In the current release of @command{ld}, this option is useful only for the
Intel 960 family of architectures. In that @command{ld} configuration, the
@var{architecture} argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path. @xref{i960,,@command{ld} and the Intel 960
family}, for details.
Future releases of @command{ld} may support similar functionality for
other architecture families.
@end ifset
@ifclear SingleFormat
@cindex binary input format
@kindex -b @var{format}
@kindex --format=@var{format}
@cindex input format
@cindex input format
@item -b @var{input-format}
@itemx --format=@var{input-format}
@command{ld} may be configured to support more than one kind of object
file. If your @command{ld} is configured this way, you can use the
@samp{-b} option to specify the binary format for input object files
that follow this option on the command line. Even when @command{ld} is
configured to support alternative object formats, you don't usually need
to specify this, as @command{ld} should be configured to expect as a
default input format the most usual format on each machine.
@var{input-format} is a text string, the name of a particular format
supported by the BFD libraries. (You can list the available binary
formats with @samp{objdump -i}.)
@xref{BFD}.
You may want to use this option if you are linking files with an unusual
binary format. You can also use @samp{-b} to switch formats explicitly (when
linking object files of different formats), by including
@samp{-b @var{input-format}} before each group of object files in a
particular format.
The default format is taken from the environment variable
@code{GNUTARGET}.
@ifset UsesEnvVars
@xref{Environment}.
@end ifset
You can also define the input format from a script, using the command
@code{TARGET};
@ifclear man
see @ref{Format Commands}.
@end ifclear
@end ifclear
@kindex -c @var{MRI-cmdfile}
@kindex --mri-script=@var{MRI-cmdfile}
@cindex compatibility, MRI
@item -c @var{MRI-commandfile}
@itemx --mri-script=@var{MRI-commandfile}
For compatibility with linkers produced by MRI, @command{ld} accepts script
files written in an alternate, restricted command language, described in
@ifclear man
@ref{MRI,,MRI Compatible Script Files}.
@end ifclear
@ifset man
the MRI Compatible Script Files section of GNU ld documentation.
@end ifset
Introduce MRI script files with
the option @samp{-c}; use the @samp{-T} option to run linker
scripts written in the general-purpose @command{ld} scripting language.
If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories
specified by any @samp{-L} options.
@cindex common allocation
@kindex -d
@kindex -dc
@kindex -dp
@item -d
@itemx -dc
@itemx -dp
These three options are equivalent; multiple forms are supported for
compatibility with other linkers. They assign space to common symbols
even if a relocatable output file is specified (with @samp{-r}). The
script command @code{FORCE_COMMON_ALLOCATION} has the same effect.
@xref{Miscellaneous Commands}.
@cindex entry point, from command line
@kindex -e @var{entry}
@kindex --entry=@var{entry}
@item -e @var{entry}
@itemx --entry=@var{entry}
Use @var{entry} as the explicit symbol for beginning execution of your
program, rather than the default entry point. If there is no symbol
named @var{entry}, the linker will try to parse @var{entry} as a number,
and use that as the entry address (the number will be interpreted in
base 10; you may use a leading @samp{0x} for base 16, or a leading
@samp{0} for base 8). @xref{Entry Point}, for a discussion of defaults
and other ways of specifying the entry point.
@kindex --exclude-libs
@item --exclude-libs @var{lib},@var{lib},...
Specifies a list of archive libraries from which symbols should not be automatically
exported. The library names may be delimited by commas or colons. Specifying
@code{--exclude-libs ALL} excludes symbols in all archive libraries from
automatic export. This option is available only for the i386 PE targeted
port of the linker and for ELF targeted ports. For i386 PE, symbols
explicitly listed in a .def file are still exported, regardless of this
option. For ELF targeted ports, symbols affected by this option will
be treated as hidden.
@cindex dynamic symbol table
@kindex -E
@kindex --export-dynamic
@item -E
@itemx --export-dynamic
When creating a dynamically linked executable, add all symbols to the
dynamic symbol table. The dynamic symbol table is the set of symbols
which are visible from dynamic objects at run time.
If you do not use this option, the dynamic symbol table will normally
contain only those symbols which are referenced by some dynamic object
mentioned in the link.
If you use @code{dlopen} to load a dynamic object which needs to refer
back to the symbols defined by the program, rather than some other
dynamic object, then you will probably need to use this option when
linking the program itself.
You can also use the dynamic list to control what symbols should
be added to the dynamic symbol table if the output format supports it.
See the description of @samp{--dynamic-list}.
@ifclear SingleFormat
@cindex big-endian objects
@cindex endianness
@kindex -EB
@item -EB
Link big-endian objects. This affects the default output format.
@cindex little-endian objects
@kindex -EL
@item -EL
Link little-endian objects. This affects the default output format.
@end ifclear
@kindex -f
@kindex --auxiliary
@item -f
@itemx --auxiliary @var{name}
When creating an ELF shared object, set the internal DT_AUXILIARY field
to the specified name. This tells the dynamic linker that the symbol
table of the shared object should be used as an auxiliary filter on the
symbol table of the shared object @var{name}.
If you later link a program against this filter object, then, when you
run the program, the dynamic linker will see the DT_AUXILIARY field. If
the dynamic linker resolves any symbols from the filter object, it will
first check whether there is a definition in the shared object
@var{name}. If there is one, it will be used instead of the definition
in the filter object. The shared object @var{name} need not exist.
Thus the shared object @var{name} may be used to provide an alternative
implementation of certain functions, perhaps for debugging or for
machine specific performance.
This option may be specified more than once. The DT_AUXILIARY entries
will be created in the order in which they appear on the command line.
@kindex -F
@kindex --filter
@item -F @var{name}
@itemx --filter @var{name}
When creating an ELF shared object, set the internal DT_FILTER field to
the specified name. This tells the dynamic linker that the symbol table
of the shared object which is being created should be used as a filter
on the symbol table of the shared object @var{name}.
If you later link a program against this filter object, then, when you
run the program, the dynamic linker will see the DT_FILTER field. The
dynamic linker will resolve symbols according to the symbol table of the
filter object as usual, but it will actually link to the definitions
found in the shared object @var{name}. Thus the filter object can be
used to select a subset of the symbols provided by the object
@var{name}.
Some older linkers used the @option{-F} option throughout a compilation
toolchain for specifying object-file format for both input and output
object files.
@ifclear SingleFormat
The @sc{gnu} linker uses other mechanisms for this purpose: the
@option{-b}, @option{--format}, @option{--oformat} options, the
@code{TARGET} command in linker scripts, and the @code{GNUTARGET}
environment variable.
@end ifclear
The @sc{gnu} linker will ignore the @option{-F} option when not
creating an ELF shared object.
@cindex finalization function
@kindex -fini
@item -fini @var{name}
When creating an ELF executable or shared object, call NAME when the
executable or shared object is unloaded, by setting DT_FINI to the
address of the function. By default, the linker uses @code{_fini} as
the function to call.
@kindex -g
@item -g
Ignored. Provided for compatibility with other tools.
@kindex -G
@kindex --gpsize
@cindex object size
@item -G@var{value}
@itemx --gpsize=@var{value}
Set the maximum size of objects to be optimized using the GP register to
@var{size}. This is only meaningful for object file formats such as
MIPS ECOFF which supports putting large and small objects into different
sections. This is ignored for other object file formats.
@cindex runtime library name
@kindex -h@var{name}
@kindex -soname=@var{name}
@item -h@var{name}
@itemx -soname=@var{name}
When creating an ELF shared object, set the internal DT_SONAME field to
the specified name. When an executable is linked with a shared object
which has a DT_SONAME field, then when the executable is run the dynamic
linker will attempt to load the shared object specified by the DT_SONAME
field rather than the using the file name given to the linker.
@kindex -i
@cindex incremental link
@item -i
Perform an incremental link (same as option @samp{-r}).
@cindex initialization function
@kindex -init
@item -init @var{name}
When creating an ELF executable or shared object, call NAME when the
executable or shared object is loaded, by setting DT_INIT to the address
of the function. By default, the linker uses @code{_init} as the
function to call.
@cindex archive files, from cmd line
@kindex -l@var{archive}
@kindex --library=@var{archive}
@item -l@var{archive}
@itemx --library=@var{archive}
Add archive file @var{archive} to the list of files to link. This
option may be used any number of times. @command{ld} will search its
path-list for occurrences of @code{lib@var{archive}.a} for every
@var{archive} specified.
On systems which support shared libraries, @command{ld} may also search for
libraries with extensions other than @code{.a}. Specifically, on ELF
and SunOS systems, @command{ld} will search a directory for a library with
an extension of @code{.so} before searching for one with an extension of
@code{.a}. By convention, a @code{.so} extension indicates a shared
library.
The linker will search an archive only once, at the location where it is
specified on the command line. If the archive defines a symbol which
was undefined in some object which appeared before the archive on the
command line, the linker will include the appropriate file(s) from the
archive. However, an undefined symbol in an object appearing later on
the command line will not cause the linker to search the archive again.
See the @option{-(} option for a way to force the linker to search
archives multiple times.
You may list the same archive multiple times on the command line.
@ifset GENERIC
This type of archive searching is standard for Unix linkers. However,
if you are using @command{ld} on AIX, note that it is different from the
behaviour of the AIX linker.
@end ifset
@cindex search directory, from cmd line
@kindex -L@var{dir}
@kindex --library-path=@var{dir}
@item -L@var{searchdir}
@itemx --library-path=@var{searchdir}
Add path @var{searchdir} to the list of paths that @command{ld} will search
for archive libraries and @command{ld} control scripts. You may use this
option any number of times. The directories are searched in the order
in which they are specified on the command line. Directories specified
on the command line are searched before the default directories. All
@option{-L} options apply to all @option{-l} options, regardless of the
order in which the options appear.
If @var{searchdir} begins with @code{=}, then the @code{=} will be replaced
by the @dfn{sysroot prefix}, a path specified when the linker is configured.
@ifset UsesEnvVars
The default set of paths searched (without being specified with
@samp{-L}) depends on which emulation mode @command{ld} is using, and in
some cases also on how it was configured. @xref{Environment}.
@end ifset
The paths can also be specified in a link script with the
@code{SEARCH_DIR} command. Directories specified this way are searched
at the point in which the linker script appears in the command line.
@cindex emulation
@kindex -m @var{emulation}
@item -m@var{emulation}
Emulate the @var{emulation} linker. You can list the available
emulations with the @samp{--verbose} or @samp{-V} options.
If the @samp{-m} option is not used, the emulation is taken from the
@code{LDEMULATION} environment variable, if that is defined.
Otherwise, the default emulation depends upon how the linker was
configured.
@cindex link map
@kindex -M
@kindex --print-map
@item -M
@itemx --print-map
Print a link map to the standard output. A link map provides
information about the link, including the following:
@itemize @bullet
@item
Where object files are mapped into memory.
@item
How common symbols are allocated.
@item
All archive members included in the link, with a mention of the symbol
which caused the archive member to be brought in.
@item
The values assigned to symbols.
Note - symbols whose values are computed by an expression which
involves a reference to a previous value of the same symbol may not
have correct result displayed in the link map. This is because the
linker discards intermediate results and only retains the final value
of an expression. Under such circumstances the linker will display
the final value enclosed by square brackets. Thus for example a
linker script containing:
@smallexample
foo = 1
foo = foo * 4
foo = foo + 8
@end smallexample
will produce the following output in the link map if the @option{-M}
option is used:
@smallexample
0x00000001 foo = 0x1
[0x0000000c] foo = (foo * 0x4)
[0x0000000c] foo = (foo + 0x8)
@end smallexample
See @ref{Expressions} for more information about expressions in linker
scripts.
@end itemize
@kindex -n
@cindex read-only text
@cindex NMAGIC
@kindex --nmagic
@item -n
@itemx --nmagic
Turn off page alignment of sections, and mark the output as
@code{NMAGIC} if possible.
@kindex -N
@kindex --omagic
@cindex read/write from cmd line
@cindex OMAGIC
@item -N
@itemx --omagic
Set the text and data sections to be readable and writable. Also, do
not page-align the data segment, and disable linking against shared
libraries. If the output format supports Unix style magic numbers,
mark the output as @code{OMAGIC}. Note: Although a writable text section
is allowed for PE-COFF targets, it does not conform to the format
specification published by Microsoft.
@kindex --no-omagic
@cindex OMAGIC
@item --no-omagic
This option negates most of the effects of the @option{-N} option. It
sets the text section to be read-only, and forces the data segment to
be page-aligned. Note - this option does not enable linking against
shared libraries. Use @option{-Bdynamic} for this.
@kindex -o @var{output}
@kindex --output=@var{output}
@cindex naming the output file
@item -o @var{output}
@itemx --output=@var{output}
Use @var{output} as the name for the program produced by @command{ld}; if this
option is not specified, the name @file{a.out} is used by default. The
script command @code{OUTPUT} can also specify the output file name.
@kindex -O @var{level}
@cindex generating optimized output
@item -O @var{level}
If @var{level} is a numeric values greater than zero @command{ld} optimizes
the output. This might take significantly longer and therefore probably
should only be enabled for the final binary.
@kindex -q
@kindex --emit-relocs
@cindex retain relocations in final executable
@item -q
@itemx --emit-relocs
Leave relocation sections and contents in fully linked executables.
Post link analysis and optimization tools may need this information in
order to perform correct modifications of executables. This results
in larger executables.
This option is currently only supported on ELF platforms.
@kindex --force-dynamic
@cindex forcing the creation of dynamic sections
@item --force-dynamic
Force the output file to have dynamic sections. This option is specific
to VxWorks targets.
@cindex partial link
@cindex relocatable output
@kindex -r
@kindex --relocatable
@item -r
@itemx --relocatable
Generate relocatable output---i.e., generate an output file that can in
turn serve as input to @command{ld}. This is often called @dfn{partial
linking}. As a side effect, in environments that support standard Unix
magic numbers, this option also sets the output file's magic number to
@code{OMAGIC}.
@c ; see @option{-N}.
If this option is not specified, an absolute file is produced. When
linking C++ programs, this option @emph{will not} resolve references to
constructors; to do that, use @samp{-Ur}.
When an input file does not have the same format as the output file,
partial linking is only supported if that input file does not contain any
relocations. Different output formats can have further restrictions; for
example some @code{a.out}-based formats do not support partial linking
with input files in other formats at all.
This option does the same thing as @samp{-i}.
@kindex -R @var{file}
@kindex --just-symbols=@var{file}
@cindex symbol-only input
@item -R @var{filename}
@itemx --just-symbols=@var{filename}
Read symbol names and their addresses from @var{filename}, but do not
relocate it or include it in the output. This allows your output file
to refer symbolically to absolute locations of memory defined in other
programs. You may use this option more than once.
For compatibility with other ELF linkers, if the @option{-R} option is
followed by a directory name, rather than a file name, it is treated as
the @option{-rpath} option.
@kindex -s
@kindex --strip-all
@cindex strip all symbols
@item -s
@itemx --strip-all
Omit all symbol information from the output file.
@kindex -S
@kindex --strip-debug
@cindex strip debugger symbols
@item -S
@itemx --strip-debug
Omit debugger symbol information (but not all symbols) from the output file.
@kindex -t
@kindex --trace
@cindex input files, displaying
@item -t
@itemx --trace
Print the names of the input files as @command{ld} processes them.
@kindex -T @var{script}
@kindex --script=@var{script}
@cindex script files
@item -T @var{scriptfile}
@itemx --script=@var{scriptfile}
Use @var{scriptfile} as the linker script. This script replaces
@command{ld}'s default linker script (rather than adding to it), so
@var{commandfile} must specify everything necessary to describe the
output file. @xref{Scripts}. If @var{scriptfile} does not exist in
the current directory, @code{ld} looks for it in the directories
specified by any preceding @samp{-L} options. Multiple @samp{-T}
options accumulate.
@kindex -dT @var{script}
@kindex --default-script=@var{script}
@cindex script files
@item -dT @var{scriptfile}
@itemx --default-script=@var{scriptfile}
Use @var{scriptfile} as the default linker script. @xref{Scripts}.
This option is similar to the @option{--script} option except that
processing of the script is delayed until after the rest of the
command line has been processed. This allows options placed after the
@option{--default-script} option on the command line to affect the
behaviour of the linker script, which can be important when the linker
command line cannot be directly controlled by the user. (eg because
the command line is being constructed by another tool, such as
@samp{gcc}).
@kindex -u @var{symbol}
@kindex --undefined=@var{symbol}
@cindex undefined symbol
@item -u @var{symbol}
@itemx --undefined=@var{symbol}
Force @var{symbol} to be entered in the output file as an undefined
symbol. Doing this may, for example, trigger linking of additional
modules from standard libraries. @samp{-u} may be repeated with
different option arguments to enter additional undefined symbols. This
option is equivalent to the @code{EXTERN} linker script command.
@kindex -Ur
@cindex constructors
@item -Ur
For anything other than C++ programs, this option is equivalent to
@samp{-r}: it generates relocatable output---i.e., an output file that can in
turn serve as input to @command{ld}. When linking C++ programs, @samp{-Ur}
@emph{does} resolve references to constructors, unlike @samp{-r}.
It does not work to use @samp{-Ur} on files that were themselves linked
with @samp{-Ur}; once the constructor table has been built, it cannot
be added to. Use @samp{-Ur} only for the last partial link, and
@samp{-r} for the others.
@kindex --unique[=@var{SECTION}]
@item --unique[=@var{SECTION}]
Creates a separate output section for every input section matching
@var{SECTION}, or if the optional wildcard @var{SECTION} argument is
missing, for every orphan input section. An orphan section is one not
specifically mentioned in a linker script. You may use this option
multiple times on the command line; It prevents the normal merging of
input sections with the same name, overriding output section assignments
in a linker script.
@kindex -v
@kindex -V
@kindex --version
@cindex version
@item -v
@itemx --version
@itemx -V
Display the version number for @command{ld}. The @option{-V} option also
lists the supported emulations.
@kindex -x
@kindex --discard-all
@cindex deleting local symbols
@item -x
@itemx --discard-all
Delete all local symbols.
@kindex -X
@kindex --discard-locals
@cindex local symbols, deleting
@item -X
@itemx --discard-locals
Delete all temporary local symbols. (These symbols start with
system-specific local label prefixes, typically @samp{.L} for ELF systems
or @samp{L} for traditional a.out systems.)
@kindex -y @var{symbol}
@kindex --trace-symbol=@var{symbol}
@cindex symbol tracing
@item -y @var{symbol}
@itemx --trace-symbol=@var{symbol}
Print the name of each linked file in which @var{symbol} appears. This
option may be given any number of times. On many systems it is necessary
to prepend an underscore.
This option is useful when you have an undefined symbol in your link but
don't know where the reference is coming from.
@kindex -Y @var{path}
@item -Y @var{path}
Add @var{path} to the default library search path. This option exists
for Solaris compatibility.
@kindex -z @var{keyword}
@item -z @var{keyword}
The recognized keywords are:
@table @samp
@item combreloc
Combines multiple reloc sections and sorts them to make dynamic symbol
lookup caching possible.
@item defs
Disallows undefined symbols in object files. Undefined symbols in
shared libraries are still allowed.
@item execstack
Marks the object as requiring executable stack.
@item initfirst
This option is only meaningful when building a shared object.
It marks the object so that its runtime initialization will occur
before the runtime initialization of any other objects brought into
the process at the same time. Similarly the runtime finalization of
the object will occur after the runtime finalization of any other
objects.
@item interpose
Marks the object that its symbol table interposes before all symbols
but the primary executable.
@item lazy
When generating an executable or shared library, mark it to tell the
dynamic linker to defer function call resolution to the point when
the function is called (lazy binding), rather than at load time.
Lazy binding is the default.
@item loadfltr
Marks the object that its filters be processed immediately at
runtime.
@item muldefs
Allows multiple definitions.
@item nocombreloc
Disables multiple reloc sections combining.
@item nocopyreloc
Disables production of copy relocs.
@item nodefaultlib
Marks the object that the search for dependencies of this object will
ignore any default library search paths.
@item nodelete
Marks the object shouldn't be unloaded at runtime.
@item nodlopen
Marks the object not available to @code{dlopen}.
@item nodump
Marks the object can not be dumped by @code{dldump}.
@item noexecstack
Marks the object as not requiring executable stack.
@item norelro
Don't create an ELF @code{PT_GNU_RELRO} segment header in the object.
@item now
When generating an executable or shared library, mark it to tell the
dynamic linker to resolve all symbols when the program is started, or
when the shared library is linked to using dlopen, instead of
deferring function call resolution to the point when the function is
first called.
@item origin
Marks the object may contain $ORIGIN.
@item relro
Create an ELF @code{PT_GNU_RELRO} segment header in the object.
@item max-page-size=@var{value}
Set the emulation maximum page size to @var{value}.
@item common-page-size=@var{value}
Set the emulation common page size to @var{value}.
@end table
Other keywords are ignored for Solaris compatibility.
@kindex -(
@cindex groups of archives
@item -( @var{archives} -)
@itemx --start-group @var{archives} --end-group
The @var{archives} should be a list of archive files. They may be
either explicit file names, or @samp{-l} options.
The specified archives are searched repeatedly until no new undefined
references are created. Normally, an archive is searched only once in
the order that it is specified on the command line. If a symbol in that
archive is needed to resolve an undefined symbol referred to by an
object in an archive that appears later on the command line, the linker
would not be able to resolve that reference. By grouping the archives,
they all be searched repeatedly until all possible references are
resolved.
Using this option has a significant performance cost. It is best to use
it only when there are unavoidable circular references between two or
more archives.
@kindex --accept-unknown-input-arch
@kindex --no-accept-unknown-input-arch
@item --accept-unknown-input-arch
@itemx --no-accept-unknown-input-arch
Tells the linker to accept input files whose architecture cannot be
recognised. The assumption is that the user knows what they are doing
and deliberately wants to link in these unknown input files. This was
the default behaviour of the linker, before release 2.14. The default
behaviour from release 2.14 onwards is to reject such input files, and
so the @samp{--accept-unknown-input-arch} option has been added to
restore the old behaviour.
@kindex --as-needed
@kindex --no-as-needed
@item --as-needed
@itemx --no-as-needed
This option affects ELF DT_NEEDED tags for dynamic libraries mentioned
on the command line after the @option{--as-needed} option. Normally,
the linker will add a DT_NEEDED tag for each dynamic library mentioned
on the command line, regardless of whether the library is actually
needed. @option{--as-needed} causes DT_NEEDED tags to only be emitted
for libraries that satisfy some symbol reference from regular objects
which is undefined at the point that the library was linked.
@option{--no-as-needed} restores the default behaviour.
@kindex --add-needed
@kindex --no-add-needed
@item --add-needed
@itemx --no-add-needed
This option affects the treatment of dynamic libraries from ELF
DT_NEEDED tags in dynamic libraries mentioned on the command line after
the @option{--no-add-needed} option. Normally, the linker will add
a DT_NEEDED tag for each dynamic library from DT_NEEDED tags.
@option{--no-add-needed} causes DT_NEEDED tags will never be emitted
for those libraries from DT_NEEDED tags. @option{--add-needed} restores
the default behaviour.
@kindex -assert @var{keyword}
@item -assert @var{keyword}
This option is ignored for SunOS compatibility.
@kindex -Bdynamic
@kindex -dy
@kindex -call_shared
@item -Bdynamic
@itemx -dy
@itemx -call_shared
Link against dynamic libraries. This is only meaningful on platforms
for which shared libraries are supported. This option is normally the
default on such platforms. The different variants of this option are
for compatibility with various systems. You may use this option
multiple times on the command line: it affects library searching for
@option{-l} options which follow it.
@kindex -Bgroup
@item -Bgroup
Set the @code{DF_1_GROUP} flag in the @code{DT_FLAGS_1} entry in the dynamic
section. This causes the runtime linker to handle lookups in this
object and its dependencies to be performed only inside the group.
@option{--unresolved-symbols=report-all} is implied. This option is
only meaningful on ELF platforms which support shared libraries.
@kindex -Bstatic
@kindex -dn
@kindex -non_shared
@kindex -static
@item -Bstatic
@itemx -dn
@itemx -non_shared
@itemx -static
Do not link against shared libraries. This is only meaningful on
platforms for which shared libraries are supported. The different
variants of this option are for compatibility with various systems. You
may use this option multiple times on the command line: it affects
library searching for @option{-l} options which follow it. This
option also implies @option{--unresolved-symbols=report-all}. This
option can be used with @option{-shared}. Doing so means that a
shared library is being created but that all of the library's external
references must be resolved by pulling in entries from static
libraries.
@kindex -Bsymbolic
@item -Bsymbolic
When creating a shared library, bind references to global symbols to the
definition within the shared library, if any. Normally, it is possible
for a program linked against a shared library to override the definition
within the shared library. This option is only meaningful on ELF
platforms which support shared libraries.
@kindex -Bsymbolic-functions
@item -Bsymbolic-functions
When creating a shared library, bind references to global function
symbols to the definition within the shared library, if any.
This option is only meaningful on ELF platforms which support shared
libraries.
@kindex --dynamic-list=@var{dynamic-list-file}
@item --dynamic-list=@var{dynamic-list-file}
Specify the name of a dynamic list file to the linker. This is
typically used when creating shared libraries to specify a list of
global symbols whose references shouldn't be bound to the definition
within the shared library, or creating dynamically linked executables
to specify a list of symbols which should be added to the symbol table
in the executable. This option is only meaningful on ELF platforms
which support shared libraries.
The format of the dynamic list is the same as the version node without
scope and node name. See @ref{VERSION} for more information.
@kindex --dynamic-list-data
@item --dynamic-list-data
Include all global data symbols to the dynamic list.
@kindex --dynamic-list-cpp-new
@item --dynamic-list-cpp-new
Provide the builtin dynamic list for C++ operator new and delete. It
is mainly useful for building shared libstdc++.
@kindex --dynamic-list-cpp-typeinfo
@item --dynamic-list-cpp-typeinfo
Provide the builtin dynamic list for C++ runtime type identification.
@kindex --check-sections
@kindex --no-check-sections
@item --check-sections
@itemx --no-check-sections
Asks the linker @emph{not} to check section addresses after they have
been assigned to see if there are any overlaps. Normally the linker will
perform this check, and if it finds any overlaps it will produce
suitable error messages. The linker does know about, and does make
allowances for sections in overlays. The default behaviour can be
restored by using the command line switch @option{--check-sections}.
@cindex cross reference table
@kindex --cref
@item --cref
Output a cross reference table. If a linker map file is being
generated, the cross reference table is printed to the map file.
Otherwise, it is printed on the standard output.
The format of the table is intentionally simple, so that it may be
easily processed by a script if necessary. The symbols are printed out,
sorted by name. For each symbol, a list of file names is given. If the
symbol is defined, the first file listed is the location of the
definition. The remaining files contain references to the symbol.
@cindex common allocation
@kindex --no-define-common
@item --no-define-common
This option inhibits the assignment of addresses to common symbols.
The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect.
@xref{Miscellaneous Commands}.
The @samp{--no-define-common} option allows decoupling
the decision to assign addresses to Common symbols from the choice
of the output file type; otherwise a non-Relocatable output type
forces assigning addresses to Common symbols.
Using @samp{--no-define-common} allows Common symbols that are referenced
from a shared library to be assigned addresses only in the main program.
This eliminates the unused duplicate space in the shared library,
and also prevents any possible confusion over resolving to the wrong
duplicate when there are many dynamic modules with specialized search
paths for runtime symbol resolution.
@cindex symbols, from command line
@kindex --defsym @var{symbol}=@var{exp}
@item --defsym @var{symbol}=@var{expression}
Create a global symbol in the output file, containing the absolute
address given by @var{expression}. You may use this option as many
times as necessary to define multiple symbols in the command line. A
limited form of arithmetic is supported for the @var{expression} in this
context: you may give a hexadecimal constant or the name of an existing
symbol, or use @code{+} and @code{-} to add or subtract hexadecimal
constants or symbols. If you need more elaborate expressions, consider
using the linker command language from a script (@pxref{Assignments,,
Assignment: Symbol Definitions}). @emph{Note:} there should be no white
space between @var{symbol}, the equals sign (``@key{=}''), and
@var{expression}.
@cindex demangling, from command line
@kindex --demangle[=@var{style}]
@kindex --no-demangle
@item --demangle[=@var{style}]
@itemx --no-demangle
These options control whether to demangle symbol names in error messages
and other output. When the linker is told to demangle, it tries to
present symbol names in a readable fashion: it strips leading
underscores if they are used by the object file format, and converts C++
mangled symbol names into user readable names. Different compilers have
different mangling styles. The optional demangling style argument can be used
to choose an appropriate demangling style for your compiler. The linker will
demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE}
is set. These options may be used to override the default.
@cindex dynamic linker, from command line
@kindex -I@var{file}
@kindex --dynamic-linker @var{file}
@item --dynamic-linker @var{file}
Set the name of the dynamic linker. This is only meaningful when
generating dynamically linked ELF executables. The default dynamic
linker is normally correct; don't use this unless you know what you are
doing.
@kindex --fatal-warnings
@item --fatal-warnings
Treat all warnings as errors.
@kindex --force-exe-suffix
@item --force-exe-suffix
Make sure that an output file has a .exe suffix.
If a successfully built fully linked output file does not have a
@code{.exe} or @code{.dll} suffix, this option forces the linker to copy
the output file to one of the same name with a @code{.exe} suffix. This
option is useful when using unmodified Unix makefiles on a Microsoft
Windows host, since some versions of Windows won't run an image unless
it ends in a @code{.exe} suffix.
@kindex --gc-sections
@kindex --no-gc-sections
@cindex garbage collection
@item --gc-sections
@itemx --no-gc-sections
Enable garbage collection of unused input sections. It is ignored on
targets that do not support this option. This option is not compatible
with @samp{-r} or @samp{--emit-relocs}. The default behaviour (of not
performing this garbage collection) can be restored by specifying
@samp{--no-gc-sections} on the command line.
@kindex --print-gc-sections
@kindex --no-print-gc-sections
@cindex garbage collection
@item --print-gc-sections
@itemx --no-print-gc-sections
List all sections removed by garbage collection. The listing is
printed on stderr. This option is only effective if garbage
collection has been enabled via the @samp{--gc-sections}) option. The
default behaviour (of not listing the sections that are removed) can
be restored by specifying @samp{--no-print-gc-sections} on the command
line.
@cindex help
@cindex usage
@kindex --help
@item --help
Print a summary of the command-line options on the standard output and exit.
@kindex --target-help
@item --target-help
Print a summary of all target specific options on the standard output and exit.
@kindex -Map
@item -Map @var{mapfile}
Print a link map to the file @var{mapfile}. See the description of the
@option{-M} option, above.
@cindex memory usage
@kindex --no-keep-memory
@item --no-keep-memory
@command{ld} normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells @command{ld} to
instead optimize for memory usage, by rereading the symbol tables as
necessary. This may be required if @command{ld} runs out of memory space
while linking a large executable.
@kindex --no-undefined
@kindex -z defs
@item --no-undefined
@itemx -z defs
Report unresolved symbol references from regular object files. This
is done even if the linker is creating a non-symbolic shared library.
The switch @option{--[no-]allow-shlib-undefined} controls the
behaviour for reporting unresolved references found in shared
libraries being linked in.
@kindex --allow-multiple-definition
@kindex -z muldefs
@item --allow-multiple-definition
@itemx -z muldefs
Normally when a symbol is defined multiple times, the linker will
report a fatal error. These options allow multiple definitions and the
first definition will be used.
@kindex --allow-shlib-undefined
@kindex --no-allow-shlib-undefined
@item --allow-shlib-undefined
@itemx --no-allow-shlib-undefined
Allows (the default) or disallows undefined symbols in shared libraries.
This switch is similar to @option{--no-undefined} except that it
determines the behaviour when the undefined symbols are in a
shared library rather than a regular object file. It does not affect
how undefined symbols in regular object files are handled.
The reason that @option{--allow-shlib-undefined} is the default is that
the shared library being specified at link time may not be the same as
the one that is available at load time, so the symbols might actually be
resolvable at load time. Plus there are some systems, (eg BeOS) where
undefined symbols in shared libraries is normal. (The kernel patches
them at load time to select which function is most appropriate
for the current architecture. This is used for example to dynamically
select an appropriate memset function). Apparently it is also normal
for HPPA shared libraries to have undefined symbols.
@kindex --no-undefined-version
@item --no-undefined-version
Normally when a symbol has an undefined version, the linker will ignore
it. This option disallows symbols with undefined version and a fatal error
will be issued instead.
@kindex --default-symver
@item --default-symver
Create and use a default symbol version (the soname) for unversioned
exported symbols.
@kindex --default-imported-symver
@item --default-imported-symver
Create and use a default symbol version (the soname) for unversioned
imported symbols.
@kindex --no-warn-mismatch
@item --no-warn-mismatch
Normally @command{ld} will give an error if you try to link together input
files that are mismatched for some reason, perhaps because they have
been compiled for different processors or for different endiannesses.
This option tells @command{ld} that it should silently permit such possible
errors. This option should only be used with care, in cases when you
have taken some special action that ensures that the linker errors are
inappropriate.
@kindex --no-whole-archive
@item --no-whole-archive
Turn off the effect of the @option{--whole-archive} option for subsequent
archive files.
@cindex output file after errors
@kindex --noinhibit-exec
@item --noinhibit-exec
Retain the executable output file whenever it is still usable.
Normally, the linker will not produce an output file if it encounters
errors during the link process; it exits without writing an output file
when it issues any error whatsoever.
@kindex -nostdlib
@item -nostdlib
Only search library directories explicitly specified on the
command line. Library directories specified in linker scripts
(including linker scripts specified on the command line) are ignored.
@ifclear SingleFormat
@kindex --oformat
@item --oformat @var{output-format}
@command{ld} may be configured to support more than one kind of object
file. If your @command{ld} is configured this way, you can use the
@samp{--oformat} option to specify the binary format for the output
object file. Even when @command{ld} is configured to support alternative
object formats, you don't usually need to specify this, as @command{ld}
should be configured to produce as a default output format the most
usual format on each machine. @var{output-format} is a text string, the
name of a particular format supported by the BFD libraries. (You can
list the available binary formats with @samp{objdump -i}.) The script
command @code{OUTPUT_FORMAT} can also specify the output format, but
this option overrides it. @xref{BFD}.
@end ifclear
@kindex -pie
@kindex --pic-executable
@item -pie
@itemx --pic-executable
@cindex position independent executables
Create a position independent executable. This is currently only supported on
ELF platforms. Position independent executables are similar to shared
libraries in that they are relocated by the dynamic linker to the virtual
address the OS chooses for them (which can vary between invocations). Like
normal dynamically linked executables they can be executed and symbols
defined in the executable cannot be overridden by shared libraries.
@kindex -qmagic
@item -qmagic
This option is ignored for Linux compatibility.
@kindex -Qy
@item -Qy
This option is ignored for SVR4 compatibility.
@kindex --relax
@cindex synthesizing linker
@cindex relaxing addressing modes
@item --relax
An option with machine dependent effects.
@ifset GENERIC
This option is only supported on a few targets.
@end ifset
@ifset H8300
@xref{H8/300,,@command{ld} and the H8/300}.
@end ifset
@ifset I960
@xref{i960,, @command{ld} and the Intel 960 family}.
@end ifset
@ifset XTENSA
@xref{Xtensa,, @command{ld} and Xtensa Processors}.
@end ifset
@ifset M68HC11
@xref{M68HC11/68HC12,,@command{ld} and the 68HC11 and 68HC12}.
@end ifset
@ifset POWERPC
@xref{PowerPC ELF32,,@command{ld} and PowerPC 32-bit ELF Support}.
@end ifset
On some platforms, the @samp{--relax} option performs global
optimizations that become possible when the linker resolves addressing
in the program, such as relaxing address modes and synthesizing new
instructions in the output object file.
On some platforms these link time global optimizations may make symbolic
debugging of the resulting executable impossible.
@ifset GENERIC
This is known to be
the case for the Matsushita MN10200 and MN10300 family of processors.
@end ifset
@ifset GENERIC
On platforms where this is not supported, @samp{--relax} is accepted,
but ignored.
@end ifset
@cindex retaining specified symbols
@cindex stripping all but some symbols
@cindex symbols, retaining selectively
@item --retain-symbols-file @var{filename}
Retain @emph{only} the symbols listed in the file @var{filename},
discarding all others. @var{filename} is simply a flat file, with one
symbol name per line. This option is especially useful in environments
@ifset GENERIC
(such as VxWorks)
@end ifset
where a large global symbol table is accumulated gradually, to conserve
run-time memory.
@samp{--retain-symbols-file} does @emph{not} discard undefined symbols,
or symbols needed for relocations.
You may only specify @samp{--retain-symbols-file} once in the command
line. It overrides @samp{-s} and @samp{-S}.
@ifset GENERIC
@item -rpath @var{dir}
@cindex runtime library search path
@kindex -rpath
Add a directory to the runtime library search path. This is used when
linking an ELF executable with shared objects. All @option{-rpath}
arguments are concatenated and passed to the runtime linker, which uses
them to locate shared objects at runtime. The @option{-rpath} option is
also used when locating shared objects which are needed by shared
objects explicitly included in the link; see the description of the
@option{-rpath-link} option. If @option{-rpath} is not used when linking an
ELF executable, the contents of the environment variable
@code{LD_RUN_PATH} will be used if it is defined.
The @option{-rpath} option may also be used on SunOS. By default, on
SunOS, the linker will form a runtime search patch out of all the
@option{-L} options it is given. If a @option{-rpath} option is used, the
runtime search path will be formed exclusively using the @option{-rpath}
options, ignoring the @option{-L} options. This can be useful when using
gcc, which adds many @option{-L} options which may be on NFS mounted
file systems.
For compatibility with other ELF linkers, if the @option{-R} option is
followed by a directory name, rather than a file name, it is treated as
the @option{-rpath} option.
@end ifset
@ifset GENERIC
@cindex link-time runtime library search path
@kindex -rpath-link
@item -rpath-link @var{DIR}
When using ELF or SunOS, one shared library may require another. This
happens when an @code{ld -shared} link includes a shared library as one
of the input files.
When the linker encounters such a dependency when doing a non-shared,
non-relocatable link, it will automatically try to locate the required
shared library and include it in the link, if it is not included
explicitly. In such a case, the @option{-rpath-link} option
specifies the first set of directories to search. The
@option{-rpath-link} option may specify a sequence of directory names
either by specifying a list of names separated by colons, or by
appearing multiple times.
This option should be used with caution as it overrides the search path
that may have been hard compiled into a shared library. In such a case it
is possible to use unintentionally a different search path than the
runtime linker would do.
The linker uses the following search paths to locate required shared
libraries:
@enumerate
@item
Any directories specified by @option{-rpath-link} options.
@item
Any directories specified by @option{-rpath} options. The difference
between @option{-rpath} and @option{-rpath-link} is that directories
specified by @option{-rpath} options are included in the executable and
used at runtime, whereas the @option{-rpath-link} option is only effective
at link time. Searching @option{-rpath} in this way is only supported
by native linkers and cross linkers which have been configured with
the @option{--with-sysroot} option.
@item
On an ELF system, if the @option{-rpath} and @code{rpath-link} options
were not used, search the contents of the environment variable
@code{LD_RUN_PATH}. It is for the native linker only.
@item
On SunOS, if the @option{-rpath} option was not used, search any
directories specified using @option{-L} options.
@item
For a native linker, the contents of the environment variable
@code{LD_LIBRARY_PATH}.
@item
For a native ELF linker, the directories in @code{DT_RUNPATH} or
@code{DT_RPATH} of a shared library are searched for shared
libraries needed by it. The @code{DT_RPATH} entries are ignored if
@code{DT_RUNPATH} entries exist.
@item
The default directories, normally @file{/lib} and @file{/usr/lib}.
@item
For a native linker on an ELF system, if the file @file{/etc/ld.so.conf}
exists, the list of directories found in that file.
@end enumerate
If the required shared library is not found, the linker will issue a
warning and continue with the link.
@end ifset
@kindex -shared
@kindex -Bshareable
@item -shared
@itemx -Bshareable
@cindex shared libraries
Create a shared library. This is currently only supported on ELF, XCOFF
and SunOS platforms. On SunOS, the linker will automatically create a
shared library if the @option{-e} option is not used and there are
undefined symbols in the link.
@item --sort-common
@kindex --sort-common
This option tells @command{ld} to sort the common symbols by size when it
places them in the appropriate output sections. First come all the one
byte symbols, then all the two byte, then all the four byte, and then
everything else. This is to prevent gaps between symbols due to
alignment constraints.
@kindex --sort-section name
@item --sort-section name
This option will apply @code{SORT_BY_NAME} to all wildcard section
patterns in the linker script.
@kindex --sort-section alignment
@item --sort-section alignment
This option will apply @code{SORT_BY_ALIGNMENT} to all wildcard section
patterns in the linker script.
@kindex --split-by-file
@item --split-by-file [@var{size}]
Similar to @option{--split-by-reloc} but creates a new output section for
each input file when @var{size} is reached. @var{size} defaults to a
size of 1 if not given.
@kindex --split-by-reloc
@item --split-by-reloc [@var{count}]
Tries to creates extra sections in the output file so that no single
output section in the file contains more than @var{count} relocations.
This is useful when generating huge relocatable files for downloading into
certain real time kernels with the COFF object file format; since COFF
cannot represent more than 65535 relocations in a single section. Note
that this will fail to work with object file formats which do not
support arbitrary sections. The linker will not split up individual
input sections for redistribution, so if a single input section contains
more than @var{count} relocations one output section will contain that
many relocations. @var{count} defaults to a value of 32768.
@kindex --stats
@item --stats
Compute and display statistics about the operation of the linker, such
as execution time and memory usage.
@kindex --sysroot
@item --sysroot=@var{directory}
Use @var{directory} as the location of the sysroot, overriding the
configure-time default. This option is only supported by linkers
that were configured using @option{--with-sysroot}.
@kindex --traditional-format
@cindex traditional format
@item --traditional-format
For some targets, the output of @command{ld} is different in some ways from
the output of some existing linker. This switch requests @command{ld} to
use the traditional format instead.
@cindex dbx
For example, on SunOS, @command{ld} combines duplicate entries in the
symbol string table. This can reduce the size of an output file with
full debugging information by over 30 percent. Unfortunately, the SunOS
@code{dbx} program can not read the resulting program (@code{gdb} has no
trouble). The @samp{--traditional-format} switch tells @command{ld} to not
combine duplicate entries.
@kindex --section-start @var{sectionname}=@var{org}
@item --section-start @var{sectionname}=@var{org}
Locate a section in the output file at the absolute
address given by @var{org}. You may use this option as many
times as necessary to locate multiple sections in the command
line.
@var{org} must be a single hexadecimal integer;
for compatibility with other linkers, you may omit the leading
@samp{0x} usually associated with hexadecimal values. @emph{Note:} there
should be no white space between @var{sectionname}, the equals
sign (``@key{=}''), and @var{org}.
@kindex -Tbss @var{org}
@kindex -Tdata @var{org}
@kindex -Ttext @var{org}
@cindex segment origins, cmd line
@item -Tbss @var{org}
@itemx -Tdata @var{org}
@itemx -Ttext @var{org}
Same as --section-start, with @code{.bss}, @code{.data} or
@code{.text} as the @var{sectionname}.
@kindex --unresolved-symbols
@item --unresolved-symbols=@var{method}
Determine how to handle unresolved symbols. There are four possible
values for @samp{method}:
@table @samp
@item ignore-all
Do not report any unresolved symbols.
@item report-all
Report all unresolved symbols. This is the default.
@item ignore-in-object-files
Report unresolved symbols that are contained in shared libraries, but
ignore them if they come from regular object files.
@item ignore-in-shared-libs
Report unresolved symbols that come from regular object files, but
ignore them if they come from shared libraries. This can be useful
when creating a dynamic binary and it is known that all the shared
libraries that it should be referencing are included on the linker's
command line.
@end table
The behaviour for shared libraries on their own can also be controlled
by the @option{--[no-]allow-shlib-undefined} option.
Normally the linker will generate an error message for each reported
unresolved symbol but the option @option{--warn-unresolved-symbols}
can change this to a warning.
@kindex --verbose
@cindex verbose
@item --dll-verbose
@itemx --verbose
Display the version number for @command{ld} and list the linker emulations
supported. Display which input files can and cannot be opened. Display
the linker script being used by the linker.
@kindex --version-script=@var{version-scriptfile}
@cindex version script, symbol versions
@itemx --version-script=@var{version-scriptfile}
Specify the name of a version script to the linker. This is typically
used when creating shared libraries to specify additional information
about the version hierarchy for the library being created. This option
is only meaningful on ELF platforms which support shared libraries.
@xref{VERSION}.
@kindex --warn-common
@cindex warnings, on combining symbols
@cindex combining symbols, warnings on
@item --warn-common
Warn when a common symbol is combined with another common symbol or with
a symbol definition. Unix linkers allow this somewhat sloppy practise,
but linkers on some other operating systems do not. This option allows
you to find potential problems from combining global symbols.
Unfortunately, some C libraries use this practise, so you may get some
warnings about symbols in the libraries as well as in your programs.
There are three kinds of global symbols, illustrated here by C examples:
@table @samp
@item int i = 1;
A definition, which goes in the initialized data section of the output
file.
@item extern int i;
An undefined reference, which does not allocate space.
There must be either a definition or a common symbol for the
variable somewhere.
@item int i;
A common symbol. If there are only (one or more) common symbols for a
variable, it goes in the uninitialized data area of the output file.
The linker merges multiple common symbols for the same variable into a
single symbol. If they are of different sizes, it picks the largest
size. The linker turns a common symbol into a declaration, if there is
a definition of the same variable.
@end table
The @samp{--warn-common} option can produce five kinds of warnings.
Each warning consists of a pair of lines: the first describes the symbol
just encountered, and the second describes the previous symbol
encountered with the same name. One or both of the two symbols will be
a common symbol.
@enumerate
@item
Turning a common symbol into a reference, because there is already a
definition for the symbol.
@smallexample
@var{file}(@var{section}): warning: common of `@var{symbol}'
overridden by definition
@var{file}(@var{section}): warning: defined here
@end smallexample
@item
Turning a common symbol into a reference, because a later definition for
the symbol is encountered. This is the same as the previous case,
except that the symbols are encountered in a different order.
@smallexample
@var{file}(@var{section}): warning: definition of `@var{symbol}'
overriding common
@var{file}(@var{section}): warning: common is here
@end smallexample
@item
Merging a common symbol with a previous same-sized common symbol.
@smallexample
@var{file}(@var{section}): warning: multiple common
of `@var{symbol}'
@var{file}(@var{section}): warning: previous common is here
@end smallexample
@item
Merging a common symbol with a previous larger common symbol.
@smallexample
@var{file}(@var{section}): warning: common of `@var{symbol}'
overridden by larger common
@var{file}(@var{section}): warning: larger common is here
@end smallexample
@item
Merging a common symbol with a previous smaller common symbol. This is
the same as the previous case, except that the symbols are
encountered in a different order.
@smallexample
@var{file}(@var{section}): warning: common of `@var{symbol}'
overriding smaller common
@var{file}(@var{section}): warning: smaller common is here
@end smallexample
@end enumerate
@kindex --warn-constructors
@item --warn-constructors
Warn if any global constructors are used. This is only useful for a few
object file formats. For formats like COFF or ELF, the linker can not
detect the use of global constructors.
@kindex --warn-multiple-gp
@item --warn-multiple-gp
Warn if multiple global pointer values are required in the output file.
This is only meaningful for certain processors, such as the Alpha.
Specifically, some processors put large-valued constants in a special
section. A special register (the global pointer) points into the middle
of this section, so that constants can be loaded efficiently via a
base-register relative addressing mode. Since the offset in
base-register relative mode is fixed and relatively small (e.g., 16
bits), this limits the maximum size of the constant pool. Thus, in
large programs, it is often necessary to use multiple global pointer
values in order to be able to address all possible constants. This
option causes a warning to be issued whenever this case occurs.
@kindex --warn-once
@cindex warnings, on undefined symbols
@cindex undefined symbols, warnings on
@item --warn-once
Only warn once for each undefined symbol, rather than once per module
which refers to it.
@kindex --warn-section-align
@cindex warnings, on section alignment
@cindex section alignment, warnings on
@item --warn-section-align
Warn if the address of an output section is changed because of
alignment. Typically, the alignment will be set by an input section.
The address will only be changed if it not explicitly specified; that
is, if the @code{SECTIONS} command does not specify a start address for
the section (@pxref{SECTIONS}).
@kindex --warn-shared-textrel
@item --warn-shared-textrel
Warn if the linker adds a DT_TEXTREL to a shared object.
@kindex --warn-unresolved-symbols
@item --warn-unresolved-symbols
If the linker is going to report an unresolved symbol (see the option
@option{--unresolved-symbols}) it will normally generate an error.
This option makes it generate a warning instead.
@kindex --error-unresolved-symbols
@item --error-unresolved-symbols
This restores the linker's default behaviour of generating errors when
it is reporting unresolved symbols.
@kindex --whole-archive
@cindex including an entire archive
@item --whole-archive
For each archive mentioned on the command line after the
@option{--whole-archive} option, include every object file in the archive
in the link, rather than searching the archive for the required object
files. This is normally used to turn an archive file into a shared
library, forcing every object to be included in the resulting shared
library. This option may be used more than once.
Two notes when using this option from gcc: First, gcc doesn't know
about this option, so you have to use @option{-Wl,-whole-archive}.
Second, don't forget to use @option{-Wl,-no-whole-archive} after your
list of archives, because gcc will add its own list of archives to
your link and you may not want this flag to affect those as well.
@kindex --wrap
@item --wrap @var{symbol}
Use a wrapper function for @var{symbol}. Any undefined reference to
@var{symbol} will be resolved to @code{__wrap_@var{symbol}}. Any
undefined reference to @code{__real_@var{symbol}} will be resolved to
@var{symbol}.
This can be used to provide a wrapper for a system function. The
wrapper function should be called @code{__wrap_@var{symbol}}. If it
wishes to call the system function, it should call
@code{__real_@var{symbol}}.
Here is a trivial example:
@smallexample
void *
__wrap_malloc (size_t c)
@{
printf ("malloc called with %zu\n", c);
return __real_malloc (c);
@}
@end smallexample
If you link other code with this file using @option{--wrap malloc}, then
all calls to @code{malloc} will call the function @code{__wrap_malloc}
instead. The call to @code{__real_malloc} in @code{__wrap_malloc} will
call the real @code{malloc} function.
You may wish to provide a @code{__real_malloc} function as well, so that
links without the @option{--wrap} option will succeed. If you do this,
you should not put the definition of @code{__real_malloc} in the same
file as @code{__wrap_malloc}; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to @code{malloc}.
@kindex --eh-frame-hdr
@item --eh-frame-hdr
Request creation of @code{.eh_frame_hdr} section and ELF
@code{PT_GNU_EH_FRAME} segment header.
@kindex --enable-new-dtags
@kindex --disable-new-dtags
@item --enable-new-dtags
@itemx --disable-new-dtags
This linker can create the new dynamic tags in ELF. But the older ELF
systems may not understand them. If you specify
@option{--enable-new-dtags}, the dynamic tags will be created as needed.
If you specify @option{--disable-new-dtags}, no new dynamic tags will be
created. By default, the new dynamic tags are not created. Note that
those options are only available for ELF systems.
@kindex --hash-size=@var{number}
@item --hash-size=@var{number}
Set the default size of the linker's hash tables to a prime number
close to @var{number}. Increasing this value can reduce the length of
time it takes the linker to perform its tasks, at the expense of
increasing the linker's memory requirements. Similarly reducing this
value can reduce the memory requirements at the expense of speed.
@kindex --hash-style=@var{style}
@item --hash-style=@var{style}
Set the type of linker's hash table(s). @var{style} can be either
@code{sysv} for classic ELF @code{.hash} section, @code{gnu} for
new style GNU @code{.gnu.hash} section or @code{both} for both
the classic ELF @code{.hash} and new style GNU @code{.gnu.hash}
hash tables. The default is @code{sysv}.
@kindex --reduce-memory-overheads
@item --reduce-memory-overheads
This option reduces memory requirements at ld runtime, at the expense of
linking speed. This was introduced to select the old O(n^2) algorithm
for link map file generation, rather than the new O(n) algorithm which uses
about 40% more memory for symbol storage.
Another effect of the switch is to set the default hash table size to
1021, which again saves memory at the cost of lengthening the linker's
run time. This is not done however if the @option{--hash-size} switch
has been used.
The @option{--reduce-memory-overheads} switch may be also be used to
enable other tradeoffs in future versions of the linker.
@end table
@c man end
@subsection Options Specific to i386 PE Targets
@c man begin OPTIONS
The i386 PE linker supports the @option{-shared} option, which causes
the output to be a dynamically linked library (DLL) instead of a
normal executable. You should name the output @code{*.dll} when you
use this option. In addition, the linker fully supports the standard
@code{*.def} files, which may be specified on the linker command line
like an object file (in fact, it should precede archives it exports
symbols from, to ensure that they get linked in, just like a normal
object file).
In addition to the options common to all targets, the i386 PE linker
support additional command line options that are specific to the i386
PE target. Options that take values may be separated from their
values by either a space or an equals sign.
@table @gcctabopt
@kindex --add-stdcall-alias
@item --add-stdcall-alias
If given, symbols with a stdcall suffix (@@@var{nn}) will be exported
as-is and also with the suffix stripped.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --base-file
@item --base-file @var{file}
Use @var{file} as the name of a file in which to save the base
addresses of all the relocations needed for generating DLLs with
@file{dlltool}.
[This is an i386 PE specific option]
@kindex --dll
@item --dll
Create a DLL instead of a regular executable. You may also use
@option{-shared} or specify a @code{LIBRARY} in a given @code{.def}
file.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --enable-stdcall-fixup
@kindex --disable-stdcall-fixup
@item --enable-stdcall-fixup
@itemx --disable-stdcall-fixup
If the link finds a symbol that it cannot resolve, it will attempt to
do ``fuzzy linking'' by looking for another defined symbol that differs
only in the format of the symbol name (cdecl vs stdcall) and will
resolve that symbol by linking to the match. For example, the
undefined symbol @code{_foo} might be linked to the function
@code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked
to the function @code{_bar}. When the linker does this, it prints a
warning, since it normally should have failed to link, but sometimes
import libraries generated from third-party dlls may need this feature
to be usable. If you specify @option{--enable-stdcall-fixup}, this
feature is fully enabled and warnings are not printed. If you specify
@option{--disable-stdcall-fixup}, this feature is disabled and such
mismatches are considered to be errors.
[This option is specific to the i386 PE targeted port of the linker]
@cindex DLLs, creating
@kindex --export-all-symbols
@item --export-all-symbols
If given, all global symbols in the objects used to build a DLL will
be exported by the DLL. Note that this is the default if there
otherwise wouldn't be any exported symbols. When symbols are
explicitly exported via DEF files or implicitly exported via function
attributes, the default is to not export anything else unless this
option is given. Note that the symbols @code{DllMain@@12},
@code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and
@code{impure_ptr} will not be automatically
exported. Also, symbols imported from other DLLs will not be
re-exported, nor will symbols specifying the DLL's internal layout
such as those beginning with @code{_head_} or ending with
@code{_iname}. In addition, no symbols from @code{libgcc},
@code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported.
Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will
not be exported, to help with C++ DLLs. Finally, there is an
extensive list of cygwin-private symbols that are not exported
(obviously, this applies on when building DLLs for cygwin targets).
These cygwin-excludes are: @code{_cygwin_dll_entry@@12},
@code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12},
@code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll},
@code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2},
@code{cygwin_premain3}, and @code{environ}.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --exclude-symbols
@item --exclude-symbols @var{symbol},@var{symbol},...
Specifies a list of symbols which should not be automatically
exported. The symbol names may be delimited by commas or colons.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --file-alignment
@item --file-alignment
Specify the file alignment. Sections in the file will always begin at
file offsets which are multiples of this number. This defaults to
512.
[This option is specific to the i386 PE targeted port of the linker]
@cindex heap size
@kindex --heap
@item --heap @var{reserve}
@itemx --heap @var{reserve},@var{commit}
Specify the amount of memory to reserve (and optionally commit) to be
used as heap for this program. The default is 1Mb reserved, 4K
committed.
[This option is specific to the i386 PE targeted port of the linker]
@cindex image base
@kindex --image-base
@item --image-base @var{value}
Use @var{value} as the base address of your program or dll. This is
the lowest memory location that will be used when your program or dll
is loaded. To reduce the need to relocate and improve performance of
your dlls, each should have a unique base address and not overlap any
other dlls. The default is 0x400000 for executables, and 0x10000000
for dlls.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --kill-at
@item --kill-at
If given, the stdcall suffixes (@@@var{nn}) will be stripped from
symbols before they are exported.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --large-address-aware
@item --large-address-aware
If given, the appropriate bit in the ``Characteristics'' field of the COFF
header is set to indicate that this executable supports virtual addresses
greater than 2 gigabytes. This should be used in conjunction with the /3GB
or /USERVA=@var{value} megabytes switch in the ``[operating systems]''
section of the BOOT.INI. Otherwise, this bit has no effect.
[This option is specific to PE targeted ports of the linker]
@kindex --major-image-version
@item --major-image-version @var{value}
Sets the major number of the ``image version''. Defaults to 1.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --major-os-version
@item --major-os-version @var{value}
Sets the major number of the ``os version''. Defaults to 4.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --major-subsystem-version
@item --major-subsystem-version @var{value}
Sets the major number of the ``subsystem version''. Defaults to 4.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --minor-image-version
@item --minor-image-version @var{value}
Sets the minor number of the ``image version''. Defaults to 0.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --minor-os-version
@item --minor-os-version @var{value}
Sets the minor number of the ``os version''. Defaults to 0.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --minor-subsystem-version
@item --minor-subsystem-version @var{value}
Sets the minor number of the ``subsystem version''. Defaults to 0.
[This option is specific to the i386 PE targeted port of the linker]
@cindex DEF files, creating
@cindex DLLs, creating
@kindex --output-def
@item --output-def @var{file}
The linker will create the file @var{file} which will contain a DEF
file corresponding to the DLL the linker is generating. This DEF file
(which should be called @code{*.def}) may be used to create an import
library with @code{dlltool} or may be used as a reference to
automatically or implicitly exported symbols.
[This option is specific to the i386 PE targeted port of the linker]
@cindex DLLs, creating
@kindex --out-implib
@item --out-implib @var{file}
The linker will create the file @var{file} which will contain an
import lib corresponding to the DLL the linker is generating. This
import lib (which should be called @code{*.dll.a} or @code{*.a}
may be used to link clients against the generated DLL; this behaviour
makes it possible to skip a separate @code{dlltool} import library
creation step.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --enable-auto-image-base
@item --enable-auto-image-base
Automatically choose the image base for DLLs, unless one is specified
using the @code{--image-base} argument. By using a hash generated
from the dllname to create unique image bases for each DLL, in-memory
collisions and relocations which can delay program execution are
avoided.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --disable-auto-image-base
@item --disable-auto-image-base
Do not automatically generate a unique image base. If there is no
user-specified image base (@code{--image-base}) then use the platform
default.
[This option is specific to the i386 PE targeted port of the linker]
@cindex DLLs, linking to
@kindex --dll-search-prefix
@item --dll-search-prefix @var{string}
When linking dynamically to a dll without an import library,
search for @code{<string><basename>.dll} in preference to
@code{lib<basename>.dll}. This behaviour allows easy distinction
between DLLs built for the various "subplatforms": native, cygwin,
uwin, pw, etc. For instance, cygwin DLLs typically use
@code{--dll-search-prefix=cyg}.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --enable-auto-import
@item --enable-auto-import
Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for
DATA imports from DLLs, and create the necessary thunking symbols when
building the import libraries with those DATA exports. Note: Use of the
'auto-import' extension will cause the text section of the image file
to be made writable. This does not conform to the PE-COFF format
specification published by Microsoft.
Using 'auto-import' generally will 'just work' -- but sometimes you may
see this message:
"variable '<var>' can't be auto-imported. Please read the
documentation for ld's @code{--enable-auto-import} for details."
This message occurs when some (sub)expression accesses an address
ultimately given by the sum of two constants (Win32 import tables only
allow one). Instances where this may occur include accesses to member
fields of struct variables imported from a DLL, as well as using a
constant index into an array variable imported from a DLL. Any
multiword variable (arrays, structs, long long, etc) may trigger
this error condition. However, regardless of the exact data type
of the offending exported variable, ld will always detect it, issue
the warning, and exit.
There are several ways to address this difficulty, regardless of the
data type of the exported variable:
One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
of adjusting references in your client code for runtime environment, so
this method works only when runtime environment supports this feature.
A second solution is to force one of the 'constants' to be a variable --
that is, unknown and un-optimizable at compile time. For arrays,
there are two possibilities: a) make the indexee (the array's address)
a variable, or b) make the 'constant' index a variable. Thus:
@example
extern type extern_array[];
extern_array[1] -->
@{ volatile type *t=extern_array; t[1] @}
@end example
or
@example
extern type extern_array[];
extern_array[1] -->
@{ volatile int t=1; extern_array[t] @}
@end example
For structs (and most other multiword data types) the only option
is to make the struct itself (or the long long, or the ...) variable:
@example
extern struct s extern_struct;
extern_struct.field -->
@{ volatile struct s *t=&extern_struct; t->field @}
@end example
or
@example
extern long long extern_ll;
extern_ll -->
@{ volatile long long * local_ll=&extern_ll; *local_ll @}
@end example
A third method of dealing with this difficulty is to abandon
'auto-import' for the offending symbol and mark it with
@code{__declspec(dllimport)}. However, in practise that
requires using compile-time #defines to indicate whether you are
building a DLL, building client code that will link to the DLL, or
merely building/linking to a static library. In making the choice
between the various methods of resolving the 'direct address with
constant offset' problem, you should consider typical real-world usage:
Original:
@example
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv)@{
printf("%d\n",arr[1]);
@}
@end example
Solution 1:
@example
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv)@{
/* This workaround is for win32 and cygwin; do not "optimize" */
volatile int *parr = arr;
printf("%d\n",parr[1]);
@}
@end example
Solution 2:
@example
--foo.h
/* Note: auto-export is assumed (no __declspec(dllexport)) */
#if (defined(_WIN32) || defined(__CYGWIN__)) && \
!(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
#define FOO_IMPORT __declspec(dllimport)
#else
#define FOO_IMPORT
#endif
extern FOO_IMPORT int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv)@{
printf("%d\n",arr[1]);
@}
@end example
A fourth way to avoid this problem is to re-code your
library to use a functional interface rather than a data interface
for the offending variables (e.g. set_foo() and get_foo() accessor
functions).
[This option is specific to the i386 PE targeted port of the linker]
@kindex --disable-auto-import
@item --disable-auto-import
Do not attempt to do sophisticated linking of @code{_symbol} to
@code{__imp__symbol} for DATA imports from DLLs.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --enable-runtime-pseudo-reloc
@item --enable-runtime-pseudo-reloc
If your code contains expressions described in --enable-auto-import section,
that is, DATA imports from DLL with non-zero offset, this switch will create
a vector of 'runtime pseudo relocations' which can be used by runtime
environment to adjust references to such data in your client code.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --disable-runtime-pseudo-reloc
@item --disable-runtime-pseudo-reloc
Do not create pseudo relocations for non-zero offset DATA imports from
DLLs. This is the default.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --enable-extra-pe-debug
@item --enable-extra-pe-debug
Show additional debug info related to auto-import symbol thunking.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --section-alignment
@item --section-alignment
Sets the section alignment. Sections in memory will always begin at
addresses which are a multiple of this number. Defaults to 0x1000.
[This option is specific to the i386 PE targeted port of the linker]
@cindex stack size
@kindex --stack
@item --stack @var{reserve}
@itemx --stack @var{reserve},@var{commit}
Specify the amount of memory to reserve (and optionally commit) to be
used as stack for this program. The default is 2Mb reserved, 4K
committed.
[This option is specific to the i386 PE targeted port of the linker]
@kindex --subsystem
@item --subsystem @var{which}
@itemx --subsystem @var{which}:@var{major}
@itemx --subsystem @var{which}:@var{major}.@var{minor}
Specifies the subsystem under which your program will execute. The
legal values for @var{which} are @code{native}, @code{windows},
@code{console}, @code{posix}, and @code{xbox}. You may optionally set
the subsystem version also. Numeric values are also accepted for
@var{which}.
[This option is specific to the i386 PE targeted port of the linker]
@end table
@c man end
@ifset M68HC11
@subsection Options specific to Motorola 68HC11 and 68HC12 targets
@c man begin OPTIONS
The 68HC11 and 68HC12 linkers support specific options to control the
memory bank switching mapping and trampoline code generation.
@table @gcctabopt
@kindex --no-trampoline
@item --no-trampoline
This option disables the generation of trampoline. By default a trampoline
is generated for each far function which is called using a @code{jsr}
instruction (this happens when a pointer to a far function is taken).
@kindex --bank-window
@item --bank-window @var{name}
This option indicates to the linker the name of the memory region in
the @samp{MEMORY} specification that describes the memory bank window.
The definition of such region is then used by the linker to compute
paging and addresses within the memory window.
@end table
@c man end
@end ifset
@ifset UsesEnvVars
@node Environment
@section Environment Variables
@c man begin ENVIRONMENT
You can change the behaviour of @command{ld} with the environment variables
@ifclear SingleFormat
@code{GNUTARGET},
@end ifclear
@code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}.
@ifclear SingleFormat
@kindex GNUTARGET
@cindex default input format
@code{GNUTARGET} determines the input-file object format if you don't
use @samp{-b} (or its synonym @samp{--format}). Its value should be one
of the BFD names for an input format (@pxref{BFD}). If there is no
@code{GNUTARGET} in the environment, @command{ld} uses the natural format
of the target. If @code{GNUTARGET} is set to @code{default} then BFD
attempts to discover the input format by examining binary input files;
this method often succeeds, but there are potential ambiguities, since
there is no method of ensuring that the magic number used to specify
object-file formats is unique. However, the configuration procedure for
BFD on each system places the conventional format for that system first
in the search-list, so ambiguities are resolved in favor of convention.
@end ifclear
@kindex LDEMULATION
@cindex default emulation
@cindex emulation, default
@code{LDEMULATION} determines the default emulation if you don't use the
@samp{-m} option. The emulation can affect various aspects of linker
behaviour, particularly the default linker script. You can list the
available emulations with the @samp{--verbose} or @samp{-V} options. If
the @samp{-m} option is not used, and the @code{LDEMULATION} environment
variable is not defined, the default emulation depends upon how the
linker was configured.
@kindex COLLECT_NO_DEMANGLE
@cindex demangling, default
Normally, the linker will default to demangling symbols. However, if
@code{COLLECT_NO_DEMANGLE} is set in the environment, then it will
default to not demangling symbols. This environment variable is used in
a similar fashion by the @code{gcc} linker wrapper program. The default
may be overridden by the @samp{--demangle} and @samp{--no-demangle}
options.
@c man end
@end ifset
@node Scripts
@chapter Linker Scripts
@cindex scripts
@cindex linker scripts
@cindex command files
Every link is controlled by a @dfn{linker script}. This script is
written in the linker command language.
The main purpose of the linker script is to describe how the sections in
the input files should be mapped into the output file, and to control
the memory layout of the output file. Most linker scripts do nothing
more than this. However, when necessary, the linker script can also
direct the linker to perform many other operations, using the commands
described below.
The linker always uses a linker script. If you do not supply one
yourself, the linker will use a default script that is compiled into the
linker executable. You can use the @samp{--verbose} command line option
to display the default linker script. Certain command line options,
such as @samp{-r} or @samp{-N}, will affect the default linker script.
You may supply your own linker script by using the @samp{-T} command
line option. When you do this, your linker script will replace the
default linker script.
You may also use linker scripts implicitly by naming them as input files
to the linker, as though they were files to be linked. @xref{Implicit
Linker Scripts}.
@menu
* Basic Script Concepts:: Basic Linker Script Concepts
* Script Format:: Linker Script Format
* Simple Example:: Simple Linker Script Example
* Simple Commands:: Simple Linker Script Commands
* Assignments:: Assigning Values to Symbols
* SECTIONS:: SECTIONS Command
* MEMORY:: MEMORY Command
* PHDRS:: PHDRS Command
* VERSION:: VERSION Command
* Expressions:: Expressions in Linker Scripts
* Implicit Linker Scripts:: Implicit Linker Scripts
@end menu
@node Basic Script Concepts
@section Basic Linker Script Concepts
@cindex linker script concepts
We need to define some basic concepts and vocabulary in order to
describe the linker script language.
The linker combines input files into a single output file. The output
file and each input file are in a special data format known as an
@dfn{object file format}. Each file is called an @dfn{object file}.
The output file is often called an @dfn{executable}, but for our
purposes we will also call it an object file. Each object file has,
among other things, a list of @dfn{sections}. We sometimes refer to a
section in an input file as an @dfn{input section}; similarly, a section
in the output file is an @dfn{output section}.
Each section in an object file has a name and a size. Most sections
also have an associated block of data, known as the @dfn{section
contents}. A section may be marked as @dfn{loadable}, which mean that
the contents should be loaded into memory when the output file is run.
A section with no contents may be @dfn{allocatable}, which means that an
area in memory should be set aside, but nothing in particular should be
loaded there (in some cases this memory must be zeroed out). A section
which is neither loadable nor allocatable typically contains some sort
of debugging information.
Every loadable or allocatable output section has two addresses. The
first is the @dfn{VMA}, or virtual memory address. This is the address
the section will have when the output file is run. The second is the
@dfn{LMA}, or load memory address. This is the address at which the
section will be loaded. In most cases the two addresses will be the
same. An example of when they might be different is when a data section
is loaded into ROM, and then copied into RAM when the program starts up
(this technique is often used to initialize global variables in a ROM
based system). In this case the ROM address would be the LMA, and the
RAM address would be the VMA.
You can see the sections in an object file by using the @code{objdump}
program with the @samp{-h} option.
Every object file also has a list of @dfn{symbols}, known as the
@dfn{symbol table}. A symbol may be defined or undefined. Each symbol
has a name, and each defined symbol has an address, among other
information. If you compile a C or C++ program into an object file, you
will get a defined symbol for every defined function and global or
static variable. Every undefined function or global variable which is
referenced in the input file will become an undefined symbol.
You can see the symbols in an object file by using the @code{nm}
program, or by using the @code{objdump} program with the @samp{-t}
option.
@node Script Format
@section Linker Script Format
@cindex linker script format
Linker scripts are text files.
You write a linker script as a series of commands. Each command is
either a keyword, possibly followed by arguments, or an assignment to a
symbol. You may separate commands using semicolons. Whitespace is
generally ignored.
Strings such as file or format names can normally be entered directly.
If the file name contains a character such as a comma which would
otherwise serve to separate file names, you may put the file name in
double quotes. There is no way to use a double quote character in a
file name.
You may include comments in linker scripts just as in C, delimited by
@samp{/*} and @samp{*/}. As in C, comments are syntactically equivalent
to whitespace.
@node Simple Example
@section Simple Linker Script Example
@cindex linker script example
@cindex example of linker script
Many linker scripts are fairly simple.
The simplest possible linker script has just one command:
@samp{SECTIONS}. You use the @samp{SECTIONS} command to describe the
memory layout of the output file.
The @samp{SECTIONS} command is a powerful command. Here we will
describe a simple use of it. Let's assume your program consists only of
code, initialized data, and uninitialized data. These will be in the
@samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively.
Let's assume further that these are the only sections which appear in
your input files.
For this example, let's say that the code should be loaded at address
0x10000, and that the data should start at address 0x8000000. Here is a
linker script which will do that:
@smallexample
SECTIONS
@{
. = 0x10000;
.text : @{ *(.text) @}
. = 0x8000000;
.data : @{ *(.data) @}
.bss : @{ *(.bss) @}
@}
@end smallexample
You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS},
followed by a series of symbol assignments and output section
descriptions enclosed in curly braces.
The first line inside the @samp{SECTIONS} command of the above example
sets the value of the special symbol @samp{.}, which is the location
counter. If you do not specify the address of an output section in some
other way (other ways are described later), the address is set from the
current value of the location counter. The location counter is then
incremented by the size of the output section. At the start of the
@samp{SECTIONS} command, the location counter has the value @samp{0}.
The second line defines an output section, @samp{.text}. The colon is
required syntax which may be ignored for now. Within the curly braces
after the output section name, you list the names of the input sections
which should be placed into this output section. The @samp{*} is a
wildcard which matches any file name. The expression @samp{*(.text)}
means all @samp{.text} input sections in all input files.
Since the location counter is @samp{0x10000} when the output section
@samp{.text} is defined, the linker will set the address of the
@samp{.text} section in the output file to be @samp{0x10000}.
The remaining lines define the @samp{.data} and @samp{.bss} sections in
the output file. The linker will place the @samp{.data} output section
at address @samp{0x8000000}. After the linker places the @samp{.data}
output section, the value of the location counter will be
@samp{0x8000000} plus the size of the @samp{.data} output section. The
effect is that the linker will place the @samp{.bss} output section
immediately after the @samp{.data} output section in memory.
The linker will ensure that each output section has the required
alignment, by increasing the location counter if necessary. In this
example, the specified addresses for the @samp{.text} and @samp{.data}
sections will probably satisfy any alignment constraints, but the linker
may have to create a small gap between the @samp{.data} and @samp{.bss}
sections.
That's it! That's a simple and complete linker script.
@node Simple Commands
@section Simple Linker Script Commands
@cindex linker script simple commands
In this section we describe the simple linker script commands.
@menu
* Entry Point:: Setting the entry point
* File Commands:: Commands dealing with files
@ifclear SingleFormat
* Format Commands:: Commands dealing with object file formats
@end ifclear
* Miscellaneous Commands:: Other linker script commands
@end menu
@node Entry Point
@subsection Setting the Entry Point
@kindex ENTRY(@var{symbol})
@cindex start of execution
@cindex first instruction
@cindex entry point
The first instruction to execute in a program is called the @dfn{entry
point}. You can use the @code{ENTRY} linker script command to set the
entry point. The argument is a symbol name:
@smallexample
ENTRY(@var{symbol})
@end smallexample
There are several ways to set the entry point. The linker will set the
entry point by trying each of the following methods in order, and
stopping when one of them succeeds:
@itemize @bullet
@item
the @samp{-e} @var{entry} command-line option;
@item
the @code{ENTRY(@var{symbol})} command in a linker script;
@item
the value of the symbol @code{start}, if defined;
@item
the address of the first byte of the @samp{.text} section, if present;
@item
The address @code{0}.
@end itemize
@node File Commands
@subsection Commands Dealing with Files
@cindex linker script file commands
Several linker script commands deal with files.
@table @code
@item INCLUDE @var{filename}
@kindex INCLUDE @var{filename}
@cindex including a linker script
Include the linker script @var{filename} at this point. The file will
be searched for in the current directory, and in any directory specified
with the @option{-L} option. You can nest calls to @code{INCLUDE} up to
10 levels deep.
@item INPUT(@var{file}, @var{file}, @dots{})
@itemx INPUT(@var{file} @var{file} @dots{})
@kindex INPUT(@var{files})
@cindex input files in linker scripts
@cindex input object files in linker scripts
@cindex linker script input object files
The @code{INPUT} command directs the linker to include the named files
in the link, as though they were named on the command line.
For example, if you always want to include @file{subr.o} any time you do
a link, but you can't be bothered to put it on every link command line,
then you can put @samp{INPUT (subr.o)} in your linker script.
In fact, if you like, you can list all of your input files in the linker
script, and then invoke the linker with nothing but a @samp{-T} option.
In case a @dfn{sysroot prefix} is configured, and the filename starts
with the @samp{/} character, and the script being processed was
located inside the @dfn{sysroot prefix}, the filename will be looked
for in the @dfn{sysroot prefix}. Otherwise, the linker will try to
open the file in the current directory. If it is not found, the
linker will search through the archive library search path. See the
description of @samp{-L} in @ref{Options,,Command Line Options}.
If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the
name to @code{lib@var{file}.a}, as with the command line argument
@samp{-l}.
When you use the @code{INPUT} command in an implicit linker script, the
files will be included in the link at the point at which the linker
script file is included. This can affect archive searching.
@item GROUP(@var{file}, @var{file}, @dots{})
@itemx GROUP(@var{file} @var{file} @dots{})
@kindex GROUP(@var{files})
@cindex grouping input files
The @code{GROUP} command is like @code{INPUT}, except that the named
files should all be archives, and they are searched repeatedly until no
new undefined references are created. See the description of @samp{-(}
in @ref{Options,,Command Line Options}.
@item AS_NEEDED(@var{file}, @var{file}, @dots{})
@itemx AS_NEEDED(@var{file} @var{file} @dots{})
@kindex AS_NEEDED(@var{files})
This construct can appear only inside of the @code{INPUT} or @code{GROUP}
commands, among other filenames. The files listed will be handled
as if they appear directly in the @code{INPUT} or @code{GROUP} commands,
with the exception of ELF shared libraries, that will be added only
when they are actually needed. This construct essentially enables
@option{--as-needed} option for all the files listed inside of it
and restores previous @option{--as-needed} resp. @option{--no-as-needed}
setting afterwards.
@item OUTPUT(@var{filename})
@kindex OUTPUT(@var{filename})
@cindex output file name in linker script
The @code{OUTPUT} command names the output file. Using
@code{OUTPUT(@var{filename})} in the linker script is exactly like using
@samp{-o @var{filename}} on the command line (@pxref{Options,,Command
Line Options}). If both are used, the command line option takes
precedence.
You can use the @code{OUTPUT} command to define a default name for the
output file other than the usual default of @file{a.out}.
@item SEARCH_DIR(@var{path})
@kindex SEARCH_DIR(@var{path})
@cindex library search path in linker script
@cindex archive search path in linker script
@cindex search path in linker script
The @code{SEARCH_DIR} command adds @var{path} to the list of paths where
@command{ld} looks for archive libraries. Using
@code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}}
on the command line (@pxref{Options,,Command Line Options}). If both
are used, then the linker will search both paths. Paths specified using
the command line option are searched first.
@item STARTUP(@var{filename})
@kindex STARTUP(@var{filename})
@cindex first input file
The @code{STARTUP} command is just like the @code{INPUT} command, except
that @var{filename} will become the first input file to be linked, as
though it were specified first on the command line. This may be useful
when using a system in which the entry point is always the start of the
first file.
@end table
@ifclear SingleFormat
@node Format Commands
@subsection Commands Dealing with Object File Formats
A couple of linker script commands deal with object file formats.
@table @code
@item OUTPUT_FORMAT(@var{bfdname})
@itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little})
@kindex OUTPUT_FORMAT(@var{bfdname})
@cindex output file format in linker script
The @code{OUTPUT_FORMAT} command names the BFD format to use for the
output file (@pxref{BFD}). Using @code{OUTPUT_FORMAT(@var{bfdname})} is
exactly like using @samp{--oformat @var{bfdname}} on the command line
(@pxref{Options,,Command Line Options}). If both are used, the command
line option takes precedence.
You can use @code{OUTPUT_FORMAT} with three arguments to use different
formats based on the @samp{-EB} and @samp{-EL} command line options.
This permits the linker script to set the output format based on the
desired endianness.
If neither @samp{-EB} nor @samp{-EL} are used, then the output format
will be the first argument, @var{default}. If @samp{-EB} is used, the
output format will be the second argument, @var{big}. If @samp{-EL} is
used, the output format will be the third argument, @var{little}.
For example, the default linker script for the MIPS ELF target uses this
command:
@smallexample
OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
@end smallexample
This says that the default format for the output file is
@samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line
option, the output file will be created in the @samp{elf32-littlemips}
format.
@item TARGET(@var{bfdname})
@kindex TARGET(@var{bfdname})
@cindex input file format in linker script
The @code{TARGET} command names the BFD format to use when reading input
files. It affects subsequent @code{INPUT} and @code{GROUP} commands.
This command is like using @samp{-b @var{bfdname}} on the command line
(@pxref{Options,,Command Line Options}). If the @code{TARGET} command
is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET}
command is also used to set the format for the output file. @xref{BFD}.
@end table
@end ifclear
@node Miscellaneous Commands
@subsection Other Linker Script Commands
There are a few other linker scripts commands.
@table @code
@item ASSERT(@var{exp}, @var{message})
@kindex ASSERT
@cindex assertion in linker script
Ensure that @var{exp} is non-zero. If it is zero, then exit the linker
with an error code, and print @var{message}.
@item EXTERN(@var{symbol} @var{symbol} @dots{})
@kindex EXTERN
@cindex undefined symbol in linker script
Force @var{symbol} to be entered in the output file as an undefined
symbol. Doing this may, for example, trigger linking of additional
modules from standard libraries. You may list several @var{symbol}s for
each @code{EXTERN}, and you may use @code{EXTERN} multiple times. This
command has the same effect as the @samp{-u} command-line option.
@item FORCE_COMMON_ALLOCATION
@kindex FORCE_COMMON_ALLOCATION
@cindex common allocation in linker script
This command has the same effect as the @samp{-d} command-line option:
to make @command{ld} assign space to common symbols even if a relocatable
output file is specified (@samp{-r}).
@item INHIBIT_COMMON_ALLOCATION
@kindex INHIBIT_COMMON_ALLOCATION
@cindex common allocation in linker script
This command has the same effect as the @samp{--no-define-common}
command-line option: to make @code{ld} omit the assignment of addresses
to common symbols even for a non-relocatable output file.
@item NOCROSSREFS(@var{section} @var{section} @dots{})
@kindex NOCROSSREFS(@var{sections})
@cindex cross references
This command may be used to tell @command{ld} to issue an error about any
references among certain output sections.
In certain types of programs, particularly on embedded systems when
using overlays, when one section is loaded into memory, another section
will not be. Any direct references between the two sections would be
errors. For example, it would be an error if code in one section called
a function defined in the other section.
The @code{NOCROSSREFS} command takes a list of output section names. If
@command{ld} detects any cross references between the sections, it reports
an error and returns a non-zero exit status. Note that the
@code{NOCROSSREFS} command uses output section names, not input section
names.
@ifclear SingleFormat
@item OUTPUT_ARCH(@var{bfdarch})
@kindex OUTPUT_ARCH(@var{bfdarch})
@cindex machine architecture
@cindex architecture
Specify a particular output machine architecture. The argument is one
of the names used by the BFD library (@pxref{BFD}). You can see the
architecture of an object file by using the @code{objdump} program with
the @samp{-f} option.
@end ifclear
@end table
@node Assignments
@section Assigning Values to Symbols
@cindex assignment in scripts
@cindex symbol definition, scripts
@cindex variables, defining
You may assign a value to a symbol in a linker script. This will define
the symbol and place it into the symbol table with a global scope.
@menu
* Simple Assignments:: Simple Assignments
* PROVIDE:: PROVIDE
* PROVIDE_HIDDEN:: PROVIDE_HIDDEN
* Source Code Reference:: How to use a linker script defined symbol in source code
@end menu
@node Simple Assignments
@subsection Simple Assignments
You may assign to a symbol using any of the C assignment operators:
@table @code
@item @var{symbol} = @var{expression} ;
@itemx @var{symbol} += @var{expression} ;
@itemx @var{symbol} -= @var{expression} ;
@itemx @var{symbol} *= @var{expression} ;
@itemx @var{symbol} /= @var{expression} ;
@itemx @var{symbol} <<= @var{expression} ;
@itemx @var{symbol} >>= @var{expression} ;
@itemx @var{symbol} &= @var{expression} ;
@itemx @var{symbol} |= @var{expression} ;
@end table
The first case will define @var{symbol} to the value of
@var{expression}. In the other cases, @var{symbol} must already be
defined, and the value will be adjusted accordingly.
The special symbol name @samp{.} indicates the location counter. You
may only use this within a @code{SECTIONS} command. @xref{Location Counter}.
The semicolon after @var{expression} is required.
Expressions are defined below; see @ref{Expressions}.
You may write symbol assignments as commands in their own right, or as
statements within a @code{SECTIONS} command, or as part of an output
section description in a @code{SECTIONS} command.
The section of the symbol will be set from the section of the
expression; for more information, see @ref{Expression Section}.
Here is an example showing the three different places that symbol
assignments may be used:
@smallexample
floating_point = 0;
SECTIONS
@{
.text :
@{
*(.text)
_etext = .;
@}
_bdata = (. + 3) & ~ 3;
.data : @{ *(.data) @}
@}
@end smallexample
@noindent
In this example, the symbol @samp{floating_point} will be defined as
zero. The symbol @samp{_etext} will be defined as the address following
the last @samp{.text} input section. The symbol @samp{_bdata} will be
defined as the address following the @samp{.text} output section aligned
upward to a 4 byte boundary.
@node PROVIDE
@subsection PROVIDE
@cindex PROVIDE
In some cases, it is desirable for a linker script to define a symbol
only if it is referenced and is not defined by any object included in
the link. For example, traditional linkers defined the symbol
@samp{etext}. However, ANSI C requires that the user be able to use
@samp{etext} as a function name without encountering an error. The
@code{PROVIDE} keyword may be used to define a symbol, such as
@samp{etext}, only if it is referenced but not defined. The syntax is
@code{PROVIDE(@var{symbol} = @var{expression})}.
Here is an example of using @code{PROVIDE} to define @samp{etext}:
@smallexample
SECTIONS
@{
.text :
@{
*(.text)
_etext = .;
PROVIDE(etext = .);
@}
@}
@end smallexample
In this example, if the program defines @samp{_etext} (with a leading
underscore), the linker will give a multiple definition error. If, on
the other hand, the program defines @samp{etext} (with no leading
underscore), the linker will silently use the definition in the program.
If the program references @samp{etext} but does not define it, the
linker will use the definition in the linker script.
@node PROVIDE_HIDDEN
@subsection PROVIDE_HIDDEN
@cindex PROVIDE_HIDDEN
Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be
hidden and won't be exported.
@node Source Code Reference
@subsection Source Code Reference
Accessing a linker script defined variable from source code is not
intuitive. In particular a linker script symbol is not equivalent to
a variable declaration in a high level language, it is instead a
symbol that does not have a value.
Before going further, it is important to note that compilers often
transform names in the source code into different names when they are
stored in the symbol table. For example, Fortran compilers commonly
prepend or append an underscore, and C++ performs extensive @samp{name
mangling}. Therefore there might be a discrepancy between the name
of a variable as it is used in source code and the name of the same
variable as it is defined in a linker script. For example in C a
linker script variable might be referred to as:
@smallexample
extern int foo;
@end smallexample
But in the linker script it might be defined as:
@smallexample
_foo = 1000;
@end smallexample
In the remaining examples however it is assumed that no name
transformation has taken place.
When a symbol is declared in a high level language such as C, two
things happen. The first is that the compiler reserves enough space
in the program's memory to hold the @emph{value} of the symbol. The
second is that the compiler creates an entry in the program's symbol
table which holds the symbol's @emph{address}. ie the symbol table
contains the address of the block of memory holding the symbol's
value. So for example the following C declaration, at file scope:
@smallexample
int foo = 1000;
@end smallexample
creates a entry called @samp{foo} in the symbol table. This entry
holds the address of an @samp{int} sized block of memory where the
number 1000 is initially stored.
When a program references a symbol the compiler generates code that
first accesses the symbol table to find the address of the symbol's
memory block and then code to read the value from that memory block.
So:
@smallexample
foo = 1;
@end smallexample
looks up the symbol @samp{foo} in the symbol table, gets the address
associated with this symbol and then writes the value 1 into that
address. Whereas:
@smallexample
int * a = & foo;
@end smallexample
looks up the symbol @samp{foo} in the symbol table, gets it address
and then copies this address into the block of memory associated with
the variable @samp{a}.
Linker scripts symbol declarations, by contrast, create an entry in
the symbol table but do not assign any memory to them. Thus they are
an address without a value. So for example the linker script definition:
@smallexample
foo = 1000;
@end smallexample
creates an entry in the symbol table called @samp{foo} which holds
the address of memory location 1000, but nothing special is stored at
address 1000. This means that you cannot access the @emph{value} of a
linker script defined symbol - it has no value - all you can do is
access the @emph{address} of a linker script defined symbol.
Hence when you are using a linker script defined symbol in source code
you should always take the address of the symbol, and never attempt to
use its value. For example suppose you want to copy the contents of a
section of memory called .ROM into a section called .FLASH and the
linker script contains these declarations:
@smallexample
@group
start_of_ROM = .ROM;
end_of_ROM = .ROM + sizeof (.ROM) - 1;
start_of_FLASH = .FLASH;
@end group
@end smallexample
Then the C source code to perform the copy would be:
@smallexample
@group
extern char start_of_ROM, end_of_ROM, start_of_FLASH;
memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
@end group
@end smallexample
Note the use of the @samp{&} operators. These are correct.
@node SECTIONS
@section SECTIONS Command
@kindex SECTIONS
The @code{SECTIONS} command tells the linker how to map input sections
into output sections, and how to place the output sections in memory.
The format of the @code{SECTIONS} command is:
@smallexample
SECTIONS
@{
@var{sections-command}
@var{sections-command}
@dots{}
@}
@end smallexample
Each @var{sections-command} may of be one of the following:
@itemize @bullet
@item
an @code{ENTRY} command (@pxref{Entry Point,,Entry command})
@item
a symbol assignment (@pxref{Assignments})
@item
an output section description
@item
an overlay description
@end itemize
The @code{ENTRY} command and symbol assignments are permitted inside the
@code{SECTIONS} command for convenience in using the location counter in
those commands. This can also make the linker script easier to
understand because you can use those commands at meaningful points in
the layout of the output file.
Output section descriptions and overlay descriptions are described
below.
If you do not use a @code{SECTIONS} command in your linker script, the
linker will place each input section into an identically named output
section in the order that the sections are first encountered in the
input files. If all input sections are present in the first file, for
example, the order of sections in the output file will match the order
in the first input file. The first section will be at address zero.
@menu
* Output Section Description:: Output section description
* Output Section Name:: Output section name
* Output Section Address:: Output section address
* Input Section:: Input section description
* Output Section Data:: Output section data
* Output Section Keywords:: Output section keywords
* Output Section Discarding:: Output section discarding
* Output Section Attributes:: Output section attributes
* Overlay Description:: Overlay description
@end menu
@node Output Section Description
@subsection Output Section Description
The full description of an output section looks like this:
@smallexample
@group
@var{section} [@var{address}] [(@var{type})] :
[AT(@var{lma})] [ALIGN(@var{section_align})] [SUBALIGN(@var{subsection_align})]
@{
@var{output-section-command}
@var{output-section-command}
@dots{}
@} [>@var{region}] [AT>@var{lma_region}] [:@var{phdr} :@var{phdr} @dots{}] [=@var{fillexp}]
@end group
@end smallexample
Most output sections do not use most of the optional section attributes.
The whitespace around @var{section} is required, so that the section
name is unambiguous. The colon and the curly braces are also required.
The line breaks and other white space are optional.
Each @var{output-section-command} may be one of the following:
@itemize @bullet
@item
a symbol assignment (@pxref{Assignments})
@item
an input section description (@pxref{Input Section})
@item
data values to include directly (@pxref{Output Section Data})
@item
a special output section keyword (@pxref{Output Section Keywords})
@end itemize
@node Output Section Name
@subsection Output Section Name
@cindex name, section
@cindex section name
The name of the output section is @var{section}. @var{section} must
meet the constraints of your output format. In formats which only
support a limited number of sections, such as @code{a.out}, the name
must be one of the names supported by the format (@code{a.out}, for
example, allows only @samp{.text}, @samp{.data} or @samp{.bss}). If the
output format supports any number of sections, but with numbers and not
names (as is the case for Oasys), the name should be supplied as a
quoted numeric string. A section name may consist of any sequence of
characters, but a name which contains any unusual characters such as
commas must be quoted.
The output section name @samp{/DISCARD/} is special; @ref{Output Section
Discarding}.
@node Output Section Address
@subsection Output Section Address
@cindex address, section
@cindex section address
The @var{address} is an expression for the VMA (the virtual memory
address) of the output section. If you do not provide @var{address},
the linker will set it based on @var{region} if present, or otherwise
based on the current value of the location counter.
If you provide @var{address}, the address of the output section will be
set to precisely that. If you provide neither @var{address} nor
@var{region}, then the address of the output section will be set to the
current value of the location counter aligned to the alignment
requirements of the output section. The alignment requirement of the
output section is the strictest alignment of any input section contained
within the output section.
For example,
@smallexample
.text . : @{ *(.text) @}
@end smallexample
@noindent
and
@smallexample
.text : @{ *(.text) @}
@end smallexample
@noindent
are subtly different. The first will set the address of the
@samp{.text} output section to the current value of the location
counter. The second will set it to the current value of the location
counter aligned to the strictest alignment of a @samp{.text} input
section.
The @var{address} may be an arbitrary expression; @ref{Expressions}.
For example, if you want to align the section on a 0x10 byte boundary,
so that the lowest four bits of the section address are zero, you could
do something like this:
@smallexample
.text ALIGN(0x10) : @{ *(.text) @}
@end smallexample
@noindent
This works because @code{ALIGN} returns the current location counter
aligned upward to the specified value.
Specifying @var{address} for a section will change the value of the
location counter.
@node Input Section
@subsection Input Section Description
@cindex input sections
@cindex mapping input sections to output sections
The most common output section command is an input section description.
The input section description is the most basic linker script operation.
You use output sections to tell the linker h