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\input texinfo
@setfilename ld.info
@c Copyright 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
@c 2001 Free Software Foundation, Inc.
@syncodeindex ky cp
@include configdoc.texi
@c (configdoc.texi is generated by the Makefile)
@include ldver.texi
@c @smallbook
@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 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 Using LD, the GNU linker
@titlepage
@title Using ld
@subtitle 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 Using LD, 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
Copyright @copyright{} 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000 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".
@end titlepage
@end iftex
@c FIXME: Talk about importance of *order* of args, cmds to linker!
@ifinfo
@node Top
@top Using 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 Hitachi
* Hitachi:: ld and other Hitachi micros
@end ifset
@ifset I960
* i960:: ld and the Intel 960 family
@end ifset
@ifset TICOFF
* TI COFF:: ld and the TI COFF
@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
* Index:: Index
@end menu
@end ifinfo
@node Overview
@chapter Overview
@cindex @sc{gnu} linker
@cindex what is this?
@code{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 @code{ld}.
@code{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.
@ifclear SingleFormat
This version of @code{ld} uses the general purpose BFD libraries
to operate on object files. This allows @code{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,
@code{ld} continues executing, allowing you to identify other errors
(or, in some cases, to get an output file in spite of the error).
@node Invocation
@chapter Invocation
The @sc{gnu} linker @code{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.
@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
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 @code{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 @code{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 @code{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 can not 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 should only be used to augment the main
linker script; if you want to use some command that logically can only
appear once, such as the @code{SECTIONS} or @code{MEMORY} command, you
must replace the default linker script using the @samp{-T} option.
@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 preceeded 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
(eg @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 @code
@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 @code{ld}, this option is useful only for the
Intel 960 family of architectures. In that @code{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,,@code{ld} and the Intel 960
family}, for details.
Future releases of @code{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}
@code{ld} may be configured to support more than one kind of object
file. If your @code{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 @code{ld} is
configured to support alternative object formats, you don't usually need
to specify this, as @code{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}; see @ref{Format Commands}.
@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, @code{ld} accepts script
files written in an alternate, restricted command language, described in
@ref{MRI,,MRI Compatible Script Files}. Introduce MRI script files with
the option @samp{-c}; use the @samp{-T} option to run linker
scripts written in the general-purpose @code{ld} scripting language.
If @var{MRI-cmdfile} does not exist, @code{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.
@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.
@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.
@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 @code{-F} option throughout a compilation
toolchain for specifying object-file format for both input and output
object files. The @sc{gnu} linker uses other mechanisms for this
purpose: the @code{-b}, @code{--format}, @code{--oformat} options, the
@code{TARGET} command in linker scripts, and the @code{GNUTARGET}
environment variable. The @sc{gnu} linker will ignore the @code{-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. @code{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, @code{ld} may also search for
libraries with extensions other than @code{.a}. Specifically, on ELF
and SunOS systems, @code{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 @code{-(} 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 @code{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 @code{ld} will search
for archive libraries and @code{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
@code{-L} options apply to all @code{-l} options, regardless of the
order in which the options appear.
@ifset UsesEnvVars
The default set of paths searched (without being specified with
@samp{-L}) depends on which emulation mode @code{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 and symbols 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.
@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. If the output format supports Unix
style magic numbers, mark the output as @code{OMAGIC}.
@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 @code{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 @code{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 exececutables.
Post link analysis and optimization tools may need this information in
order to perform correct modifications of executables. This results
in larger executables.
@cindex partial link
@cindex relocatable output
@kindex -r
@kindex --relocateable
@item -r
@itemx --relocateable
Generate relocatable output---i.e., generate an output file that can in
turn serve as input to @code{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 @code{-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}.
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 @code{-R} option is
followed by a directory name, rather than a file name, it is treated as
the @code{-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 @code{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
@code{ld}'s default linker script (rather than adding to it), so
@var{commandfile} must specify everything necessary to describe the
output file. You must use this option if you want to use a command
which can only appear once in a linker script, such as the
@code{SECTIONS} or @code{MEMORY} command. @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 -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 @code{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 @code{ld}. The @code{-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
@cindex L, deleting symbols beginning
@item -X
@itemx --discard-locals
Delete all temporary local symbols. For most targets, this is all local
symbols whose names begin with @samp{L}.
@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 @code{initfirst}, @code{interpose},
@code{loadfltr}, @code{nodefaultlib}, @code{nodelete}, @code{nodlopen},
@code{nodump}, @code{now} and @code{origin}. The other keywords are
ignored for Solaris compatibility. @code{initfirst} marks the object
to be initialized first at runtime before any other objects.
@code{interpose} marks the object that its symbol table interposes
before all symbols but the primary executable. @code{loadfltr} marks
the object that its filtees be processed immediately at runtime.
@code{nodefaultlib} marks the object that the search for dependencies
of this object will ignore any default library search paths.
@code{nodelete} marks the object shouldn't be unloaded at runtime.
@code{nodlopen} marks the object not available to @code{dlopen}.
@code{nodump} marks the object can not be dumped by @code{dldump}.
@code{now} marks the object with the non-lazy runtime binding.
@code{origin} marks the object may contain $ORIGIN.
@code{defs} disallows undefined symbols.
@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 -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
@code{-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.
@code{--no-undefined} 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 @code{-l} options which follow it.
@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 --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 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 @samp{--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 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 --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.
@cindex MIPS embedded PIC code
@kindex --embedded-relocs
@item --embedded-relocs
This option is only meaningful when linking MIPS embedded PIC code,
generated by the -membedded-pic option to the @sc{gnu} compiler and
assembler. It causes the linker to create a table which may be used at
runtime to relocate any data which was statically initialized to pointer
values. See the code in testsuite/ld-empic for details.
@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 --no-gc-sections
@itemx --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}, nor should it be used with dynamic linking. The default
behaviour (of not performing this garbage collection) can be restored by
specifying @samp{--no-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
@samp{-M} option, above.
@cindex memory usage
@kindex --no-keep-memory
@item --no-keep-memory
@code{ld} normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells @code{ld} to
instead optimize for memory usage, by rereading the symbol tables as
necessary. This may be required if @code{ld} runs out of memory space
while linking a large executable.
@kindex --no-undefined
@kindex -z defs
@item --no-undefined
@itemx -z defs
Normally when creating a non-symbolic shared library, undefined symbols
are allowed and left to be resolved by the runtime loader. These options
disallow such undefined symbols.
@kindex --allow-shlib-undefined
@item --allow-shlib-undefined
Allow undefined symbols in shared objects even when --no-undefined is
set. The net result will be that undefined symbols in regular objects
will still trigger an error, but undefined symbols in shared objects
will be ignored. The implementation of no_undefined makes the
assumption that the runtime linker will choke on undefined symbols.
However there is at least one system (BeOS) where undefined symbols in
shared libraries is normal since the kernel patches them at load time to
select which function is most appropriate for the current architecture.
I.E. dynamically select an appropriate memset function. Apparently it
is also normal for HPPA shared libraries to have undefined symbols.
@kindex --no-warn-mismatch
@item --no-warn-mismatch
Normally @code{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 @code{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 @code{--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.
@ifclear SingleFormat
@kindex --oformat
@item --oformat @var{output-format}
@code{ld} may be configured to support more than one kind of object
file. If your @code{ld} is configured this way, you can use the
@samp{--oformat} option to specify the binary format for the output
object file. Even when @code{ld} is configured to support alternative
object formats, you don't usually need to specify this, as @code{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 -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,,@code{ld} and the H8/300}.
@end ifset
@ifset I960
@xref{i960,, @code{ld} and the Intel 960 family}.
@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 @code{-rpath}
arguments are concatenated and passed to the runtime linker, which uses
them to locate shared objects at runtime. The @code{-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
@code{-rpath-link} option. If @code{-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 @code{-rpath} option may also be used on SunOS. By default, on
SunOS, the linker will form a runtime search patch out of all the
@code{-L} options it is given. If a @code{-rpath} option is used, the
runtime search path will be formed exclusively using the @code{-rpath}
options, ignoring the @code{-L} options. This can be useful when using
gcc, which adds many @code{-L} options which may be on NFS mounted
filesystems.
For compatibility with other ELF linkers, if the @code{-R} option is
followed by a directory name, rather than a file name, it is treated as
the @code{-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 @code{-rpath-link} option
specifies the first set of directories to search. The
@code{-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 @code{-rpath-link} options.
@item
Any directories specified by @code{-rpath} options. The difference
between @code{-rpath} and @code{-rpath-link} is that directories
specified by @code{-rpath} options are included in the executable and
used at runtime, whereas the @code{-rpath-link} option is only effective
at link time. It is for the native linker only.
@item
On an ELF system, if the @code{-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 @code{-rpath} option was not used, search any
directories specified using @code{-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 @code{-e} option is not used and there are
undefined symbols in the link.
@item --sort-common
@kindex --sort-common
This option tells @code{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 bytes, then all the four bytes, and then
everything else. This is to prevent gaps between symbols due to
alignment constraints.
@kindex --split-by-file
@item --split-by-file [@var{size}]
Similar to @code{--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 --traditional-format
@cindex traditional format
@item --traditional-format
For some targets, the output of @code{ld} is different in some ways from
the output of some existing linker. This switch requests @code{ld} to
use the traditional format instead.
@cindex dbx
For example, on SunOS, @code{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 @code{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}
Use @var{org} as the starting address for---respectively---the
@code{bss}, @code{data}, or the @code{text} segment of the output file.
@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.
@kindex --verbose
@cindex verbose
@item --dll-verbose
@itemx --verbose
Display the version number for @code{ld} and list the linker emulations
supported. Display which input files can and cannot be opened. Display
the linker script if using a default builtin script.
@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 heirarchy for the library being created. This option
is only meaningful on ELF platforms which support shared libraries.
@xref{VERSION}.
@kindex --warn-comon
@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 practice,
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 practice, 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 --whole-archive
@cindex including an entire archive
@item --whole-archive
For each archive mentioned on the command line after the
@code{--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 @code{-Wl,-whole-archive}.
Second, don't forget to use @code{-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 (int c)
@{
printf ("malloc called with %ld\n", c);
return __real_malloc (c);
@}
@end smallexample
If you link other code with this file using @code{--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 @code{--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 --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
@code{--enable-new-dtags}, the dynamic tags will be created as needed.
If you specify @code{--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.
@end table
@subsection Options specific to i386 PE targets
The i386 PE linker supports the @code{-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 @code
@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.
@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}.
@kindex --dll
@item --dll
Create a DLL instead of a regular executable. You may also use
@code{-shared} or specify a @code{LIBRARY} in a given @code{.def}
file.
@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 @code{--enable-stdcall-fixup}, this
feature is fully enabled and warnings are not printed. If you specify
@code{--disable-stdcall-fixup}, this feature is disabled and such
mismatches are considered to be errors.
@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}, and @code{impure_ptr} will not be automatically
exported.
@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.
@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.
@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.
@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.
@kindex --kill-at
@item --kill-at
If given, the stdcall suffixes (@@@var{nn}) will be stripped from
symbols before they are exported.
@kindex --major-image-version
@item --major-image-version @var{value}
Sets the major number of the "image version". Defaults to 1.
@kindex --major-os-version
@item --major-os-version @var{value}
Sets the major number of the "os version". Defaults to 4.
@kindex --major-subsystem-version
@item --major-subsystem-version @var{value}
Sets the major number of the "subsystem version". Defaults to 4.
@kindex --minor-image-version
@item --minor-image-version @var{value}
Sets the minor number of the "image version". Defaults to 0.
@kindex --minor-os-version
@item --minor-os-version @var{value}
Sets the minor number of the "os version". Defaults to 0.
@kindex --minor-subsystem-version
@item --minor-subsystem-version @var{value}
Sets the minor number of the "subsystem version". Defaults to 0.
@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.
@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.
@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 32Mb reserved, 4K
committed.
@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}, and @code{posix}. You may optionally set the
subsystem version also.
@end table
@ifset UsesEnvVars
@node Environment
@section Environment Variables
You can change the behavior of @code{ld} with the environment variables
@code{GNUTARGET}, @code{LDEMULATION}, and @code{COLLECT_NO_DEMANGLE}.
@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, @code{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.
@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.
@end ifset
@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.
@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 @code{-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.
The linker will first 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})}, @code{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 OUTPUT(@var{filename})
@kindex OUTPUT(@var{filename})
@cindex output file name in linker scripot
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
@code{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 @code{ld} assign space to common symbols even if a relocatable
output file is specified (@samp{-r}).
@item NOCROSSREFS(@var{section} @var{section} @dots{})
@kindex NOCROSSREFS(@var{sections})
@cindex cross references
This command may be used to tell @code{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
@code{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 as a global symbol.
@menu
* Simple Assignments:: Simple Assignments
* PROVIDE:: PROVIDE
@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.
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 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})]
@{
@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 how to lay out your program
in memory. You use input section descriptions to tell the linker how to
map the input files into your memory layout.
@menu
* Input Section Basics:: Input section basics
* Input Section Wildcards:: Input section wildcard patterns
* Input Section Common:: Input section for common symbols
* Input Section Keep:: Input section and garbage collection
* Input Section Example:: Input section example
@end menu
@node Input Section Basics
@subsubsection Input section basics
@cindex input section basics
An input section description consists of a file name optionally followed
by a list of section names in parentheses.
The file name and the section name may be wildcard patterns, which we
describe further below (@pxref{Input Section Wildcards}).
The most common input section description is to include all input
sections with a particular name in the output section. For example, to
include all input @samp{.text} sections, you would write:
@smallexample
*(.text)
@end smallexample
@noindent
Here the @samp{*} is a wildcard which matches any file name. To exclude a list
of files from matching the file name wildcard, EXCLUDE_FILE may be used to
match all files except the ones specified in the EXCLUDE_FILE list. For
example:
@smallexample
(*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors))
@end smallexample
will cause all .ctors sections from all files except @file{crtend.o} and
@file{otherfile.o} to be included.
There are two ways to include more than one section:
@smallexample
*(.text .rdata)
*(.text) *(.rdata)
@end smallexample
@noindent
The difference between these is the order in which the @samp{.text} and
@samp{.rdata} input sections will appear in the output section. In the
first example, they will be intermingled. In the second example, all
@samp{.text} input sections will appear first, followed by all
@samp{.rdata} input sections.
You can specify a file name to include sections from a particular file.
You would do this if one or more of your files contain special data that
needs to be at a particular location in memory. For example:
@smallexample
data.o(.data)
@end smallexample
If you use a file name without a list of sections, then all sections in
the input file will be included in the output section. This is not
commonly done, but it may by useful on occasion. For example:
@smallexample
data.o
@end smallexample
When you use a file name which does not contain any wild card
characters, the linker will first see if you also specified the file
name on the linker command line or in an @code{INPUT} command. If you
did not, the linker will attempt to open the file as an input file, as
though it appeared on the command line. Note that this differs from an
@code{INPUT} command, because the linker will not search for the file in
the archive search path.
@node Input Section Wildcards
@subsubsection Input section wildcard patterns
@cindex input section wildcards
@cindex wildcard file name patterns
@cindex file name wildcard patterns
@cindex section name wildcard patterns
In an input section description, either the file name or the section
name or both may be wildcard patterns.
The file name of @samp{*} seen in many examples is a simple wildcard
pattern for the file name.
The wildcard patterns are like those used by the Unix shell.
@table @samp
@item *
matches any number of characters
@item ?
matches any single character
@item [@var{chars}]
matches a single instance of any of the @var{chars}; the @samp{-}
character may be used to specify a range of characters, as in
@samp{[a-z]} to match any lower case letter
@item \
quotes the following character
@end table
When a file name is matched with a wildcard, the wildcard characters
will not match a @samp{/} character (used to separate directory names on
Unix). A pattern consisting of a single @samp{*} character is an
exception; it will always match any file name, whether it contains a
@samp{/} or not. In a section name, the wildcard characters will match
a @samp{/} character.
File name wildcard patterns only match files which are explicitly
specified on the command line or in an @code{INPUT} command. The linker
does not search directories to expand wildcards.
If a file name matches more than one wildcard pattern, or if a file name
appears explicitly and is also matched by a wildcard pattern, the linker
will use the first match in the linker script. For example, this
sequence of input section descriptions is probably in error, because the
@file{data.o} rule will not be used:
@smallexample
.data : @{ *(.data) @}
.data1 : @{ data.o(.data) @}
@end smallexample
@cindex SORT
Normally, the linker will place files and sections matched by wildcards
in the order in which they are seen during the link. You can change
this by using the @code{SORT} keyword, which appears before a wildcard
pattern in parentheses (e.g., @code{SORT(.text*)}). When the
@code{SORT} keyword is used, the linker will sort the files or sections
into ascending order by name before placing them in the output file.
If you ever get confused about where input sections are going, use the
@samp{-M} linker option to generate a map file. The map file shows
precisely how input sections are mapped to output sections.
This example shows how wildcard patterns might be used to partition
files. This linker script directs the linker to place all @samp{.text}
sections in @samp{.text} and all @samp{.bss} sections in @samp{.bss}.
The linker will place the @samp{.data} section from all files beginning
with an upper case character in @samp{.DATA}; for all other files, the
linker will place the @samp{.data} section in @samp{.data}.
@smallexample
@group
SECTIONS @{
.text : @{ *(.text) @}
.DATA : @{ [A-Z]*(.data) @}
.data : @{ *(.data) @}
.bss : @{ *(.bss) @}
@}
@end group
@end smallexample
@node Input Section Common
@subsubsection Input section for common symbols
@cindex common symbol placement
@cindex uninitialized data placement
A special notation is needed for common symbols, because in many object
file formats common symbols do not have a particular input section. The
linker treats common symbols as though they are in an input section
named @samp{COMMON}.
You may use file names with the @samp{COMMON} section just as with any
other input sections. You can use this to place common symbols from a
particular input file in one section while common symbols from other
input files are placed in another section.
In most cases, common symbols in input files will be placed in the
@samp{.bss} section in the output file. For example:
@smallexample
.bss @{ *(.bss) *(COMMON) @}
@end smallexample
@cindex scommon section
@cindex small common symbols
Some object file formats have more than one type of common symbol. For
example, the MIPS ELF object file format distinguishes standard common
symbols and small common symbols. In this case, the linker will use a
different special section name for other types of common symbols. In
the case of MIPS ELF, the linker uses @samp{COMMON} for standard common
symbols and @samp{.scommon} for small common symbols. This permits you
to map the different types of common symbols into memory at different
locations.
@cindex [COMMON]
You will sometimes see @samp{[COMMON]} in old linker scripts. This
notation is now considered obsolete. It is equivalent to
@samp{*(COMMON)}.
@node Input Section Keep
@subsubsection Input section and garbage collection
@cindex KEEP
@cindex garbage collection
When link-time garbage collection is in use (@samp{--gc-sections}),
it is often useful to mark sections that should not be eliminated.
This is accomplished by surrounding an input section's wildcard entry
with @code{KEEP()}, as in @code{KEEP(*(.init))} or
@code{KEEP(SORT(*)(.ctors))}.
@node Input Section Example
@subsubsection Input section example
The following example is a complete linker script. It tells the linker
to read all of the sections from file @file{all.o} and place them at the
start of output section @samp{outputa} which starts at location
@samp{0x10000}. All of section @samp{.input1} from file @file{foo.o}
follows immediately, in the same output section. All of section
@samp{.input2} from @file{foo.o} goes into output section
@samp{outputb}, followed by section @samp{.input1} from @file{foo1.o}.
All of the remaining @samp{.input1} and @samp{.input2} sections from any
files are written to output section @samp{outputc}.
@smallexample
@group
SECTIONS @{
outputa 0x10000 :
@{
all.o
foo.o (.input1)
@}
outputb :
@{
foo.o (.input2)
foo1.o (.input1)
@}
outputc :
@{
*(.input1)
*(.input2)
@}
@}
@end group
@end smallexample
@node Output Section Data
@subsection Output section data
@cindex data
@cindex section data
@cindex output section data
@kindex BYTE(@var{expression})
@kindex SHORT(@var{expression})
@kindex LONG(@var{expression})
@kindex QUAD(@var{expression})
@kindex SQUAD(@var{expression})
You can include explicit bytes of data in an output section by using
@code{BYTE}, @code{SHORT}, @code{LONG}, @code{QUAD}, or @code{SQUAD} as
an output section command. Each keyword is followed by an expression in
parentheses providing the value to store (@pxref{Expressions}). The
value of the expression is stored at the current value of the location
counter.
The @code{BYTE}, @code{SHORT}, @code{LONG}, and @code{QUAD} commands
store one, two, four, and eight bytes (respectively). After storing the
bytes, the location counter is incremented by the number of bytes
stored.
For example, this will store the byte 1 followed by the four byte value
of the symbol @samp{addr}:
@smallexample
BYTE(1)
LONG(addr)
@end smallexample
When using a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the
same; they both store an 8 byte, or 64 bit, value. When both host and
target are 32 bits, an expression is computed as 32 bits. In this case
@code{QUAD} stores a 32 bit value zero extended to 64 bits, and
@code{SQUAD} stores a 32 bit value sign extended to 64 bits.
If the object file format of the output file has an explicit endianness,
which is the normal case, the value will be stored in that endianness.
When the object file format does not have an explicit endianness, as is
true of, for example, S-records, the value will be stored in the
endianness of the first input object file.
Note - these commands only work inside a section description and not
between them, so the following will produce an error from the linker:
@smallexample
SECTIONS @{@ .text : @{@ *(.text) @}@ LONG(1) .data : @{@ *(.data) @}@ @}@
@end smallexample
whereas this will work:
@smallexample
SECTIONS @{@ .text : @{@ *(.text) ; LONG(1) @}@ .data : @{@ *(.data) @}@ @}@
@end smallexample
@kindex FILL(@var{expression})
@cindex holes, filling
@cindex unspecified memory
You may use the @code{FILL} command to set the fill pattern for the
current section. It is followed by an expression in parentheses. Any
otherwise unspecified regions of memory within the section (for example,
gaps left due to the required alignment of input sections) are filled
with the two least significant bytes of the expression, repeated as
necessary. A @code{FILL} statement covers memory locations after the
point at which it occurs in the section definition; by including more
than one @code{FILL} statement, you can have different fill patterns in
different parts of an output section.
This example shows how to fill unspecified regions of memory with the
value @samp{0x9090}:
@smallexample
FILL(0x9090)
@end smallexample
The @code{FILL} command is similar to the @samp{=@var{fillexp}} output
section attribute (@pxref{Output Section Fill}), but it only affects the
part of the section following the @code{FILL} command, rather than the
entire section. If both are used, the @code{FILL} command takes
precedence.
@node Output Section Keywords
@subsection Output section keywords
There are a couple of keywords which can appear as output section
commands.
@table @code
@kindex CREATE_OBJECT_SYMBOLS
@cindex input filename symbols
@cindex filename symbols
@item CREATE_OBJECT_SYMBOLS
The command tells the linker to create a symbol for each input file.
The name of each symbol will be the name of the corresponding input
file. The section of each symbol will be the output section in which
the @code{CREATE_OBJECT_SYMBOLS} command appears.
This is conventional for the a.out object file format. It is not
normally used for any other object file format.
@kindex CONSTRUCTORS
@cindex C++ constructors, arranging in link
@cindex constructors, arranging in link
@item CONSTRUCTORS
When linking using the a.out object file format, the linker uses an
unusual set construct to support C++ global constructors and
destructors. When linking object file formats which do not support
arbitrary sections, such as ECOFF and XCOFF, the linker will
automatically recognize C++ global constructors and destructors by name.
For these object file formats, the @code{CONSTRUCTORS} command tells the
linker to place constructor information in the output section where the
@code{CONSTRUCTORS} command appears. The @code{CONSTRUCTORS} command is
ignored for other object file formats.
The symbol @w{@code{__CTOR_LIST__}} marks the start of the global
constructors, and the symbol @w{@code{__DTOR_LIST}} marks the end. The
first word in the list is the number of entries, followed by the address
of each constructor or destructor, followed by a zero word. The
compiler must arrange to actually run the code. For these object file
formats @sc{gnu} C++ normally calls constructors from a subroutine
@code{__main}; a call to @code{__main} is automatically inserted into
the startup code for @code{main}. @sc{gnu} C++ normally runs
destructors either by using @code{atexit}, or directly from the function
@code{exit}.
For object file formats such as @code{COFF} or @code{ELF} which support
arbitrary section names, @sc{gnu} C++ will normally arrange to put the
addresses of global constructors and destructors into the @code{.ctors}
and @code{.dtors} sections. Placing the following sequence into your
linker script will build the sort of table which the @sc{gnu} C++
runtime code expects to see.
@smallexample
__CTOR_LIST__ = .;
LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
*(.ctors)
LONG(0)
__CTOR_END__ = .;
__DTOR_LIST__ = .;
LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
*(.dtors)
LONG(0)
__DTOR_END__ = .;
@end smallexample
If you are using the @sc{gnu} C++ support for initialization priority,
which provides some control over the order in which global constructors
are run, you must sort the constructors at link time to ensure that they
are executed in the correct order. When using the @code{CONSTRUCTORS}
command, use @samp{SORT(CONSTRUCTORS)} instead. When using the
@code{.ctors} and @code{.dtors} sections, use @samp{*(SORT(.ctors))} and
@samp{*(SORT(.dtors))} instead of just @samp{*(.ctors)} and
@samp{*(.dtors)}.
Normally the compiler and linker will handle these issues automatically,
and you will not need to concern yourself with them. However, you may
need to consider this if you are using C++ and writing your own linker
scripts.
@end table
@node Output Section Discarding
@subsection Output section discarding
@cindex discarding sections
@cindex sections, discarding
@cindex removing sections
The linker will not create output section which do not have any
contents. This is for convenience when referring to input sections that
may or may not be present in any of the input files. For example:
@smallexample
.foo @{ *(.foo) @}
@end smallexample
@noindent
will only create a @samp{.foo} section in the output file if there is a
@samp{.foo} section in at least one input file.
If you use anything other than an input section description as an output
section command, such as a symbol assignment, then the output section
will always be created, even if there are no matching input sections.
@cindex /DISCARD/
The special output section name @samp{/DISCARD/} may be used to discard
input sections. Any input sections which are assigned to an output
section named @samp{/DISCARD/} are not included in the output file.
@node Output Section Attributes
@subsection Output section attributes
@cindex output section attributes
We showed above that the full description of an output section looked
like this:
@smallexample
@group
@var{section} [@var{address}] [(@var{type})] : [AT(@var{lma})]
@{
@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
We've already described @var{section}, @var{address}, and
@var{output-section-command}. In this section we will describe the
remaining section attributes.
@menu
* Output Section Type:: Output section type
* Output Section LMA:: Output section LMA
* Output Section Region:: Output section region
* Output Section Phdr:: Output section phdr
* Output Section Fill:: Output section fill
@end menu
@node Output Section Type
@subsubsection Output section type
Each output section may have a type. The type is a keyword in
parentheses. The following types are defined:
@table @code
@item NOLOAD
The section should be marked as not loadable, so that it will not be
loaded into memory when the program is run.
@item DSECT
@itemx COPY
@itemx INFO
@itemx OVERLAY
These type names are supported for backward compatibility, and are
rarely used. They all have the same effect: the section should be
marked as not allocatable, so that no memory is allocated for the
section when the program is run.
@end table
@kindex NOLOAD
@cindex prevent unnecessary loading
@cindex loading, preventing
The linker normally sets the attributes of an output section based on
the input sections which map into it. You can override this by using
the section type. For example, in the script sample below, the
@samp{ROM} section is addressed at memory location @samp{0} and does not
need to be loaded when the program is run. The contents of the
@samp{ROM} section will appear in the linker output file as usual.
@smallexample
@group
SECTIONS @{
ROM 0 (NOLOAD) : @{ @dots{} @}
@dots{}
@}
@end group
@end smallexample
@node Output Section LMA
@subsubsection Output section LMA
@kindex AT>@var{lma_region}
@kindex AT(@var{lma})
@cindex load address
@cindex section load address
Every section has a virtual address (VMA) and a load address (LMA); see
@ref{Basic Script Concepts}. The address expression which may appear in
an output section description sets the VMA (@pxref{Output Section
Address}).
The linker will normally set the LMA equal to the VMA. You can change
that by using the @code{AT} keyword. The expression @var{lma} that
follows the @code{AT} keyword specifies the load address of the
section. Alternatively, with @samp{AT>@var{lma_region}} expression,
you may specify a memory region for the section's load address. @xref{MEMORY}.
@cindex ROM initialized data
@cindex initialized data in ROM
This feature is designed to make it easy to build a ROM image. For
example, the following linker script creates three output sections: one
called @samp{.text}, which starts at @code{0x1000}, one called
@samp{.mdata}, which is loaded at the end of the @samp{.text} section
even though its VMA is @code{0x2000}, and one called @samp{.bss} to hold
uninitialized data at address @code{0x3000}. The symbol @code{_data} is
defined with the value @code{0x2000}, which shows that the location
counter holds the VMA value, not the LMA value.
@smallexample
@group
SECTIONS
@{
.text 0x1000 : @{ *(.text) _etext = . ; @}
.mdata 0x2000 :
AT ( ADDR (.text) + SIZEOF (.text) )
@{ _data = . ; *(.data); _edata = . ; @}
.bss 0x3000 :
@{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;@}
@}
@end group
@end smallexample
The run-time initialization code for use with a program generated with
this linker script would include something like the following, to copy
the initialized data from the ROM image to its runtime address. Notice
how this code takes advantage of the symbols defined by the linker
script.
@smallexample
@group
extern char _etext, _data, _edata, _bstart, _bend;
char *src = &_etext;
char *dst = &_data;
/* ROM has data at end of text; copy it. */
while (dst < &_edata) @{
*dst++ = *src++;
@}
/* Zero bss */
for (dst = &_bstart; dst< &_bend; dst++)
*dst = 0;
@end group
@end smallexample
@node Output Section Region
@subsubsection Output section region
@kindex >@var{region}
@cindex section, assigning to memory region
@cindex memory regions and sections
You can assign a section to a previously defined region of memory by
using @samp{>@var{region}}. @xref{MEMORY}.
Here is a simple example:
@smallexample
@group
MEMORY @{ rom : ORIGIN = 0x1000, LENGTH = 0x1000 @}
SECTIONS @{ ROM : @{ *(.text) @} >rom @}
@end group
@end smallexample
@node Output Section Phdr
@subsubsection Output section phdr
@kindex :@var{phdr}
@cindex section, assigning to program header
@cindex program headers and sections
You can assign a section to a previously defined program segment by
using @samp{:@var{phdr}}. @xref{PHDRS}. If a section is assigned to
one or more segments, then all subsequent allocated sections will be
assigned to those segments as well, unless they use an explicitly
@code{:@var{phdr}} modifier. You can use @code{:NONE} to tell the
linker to not put the section in any segment at all.
Here is a simple example:
@smallexample
@group
PHDRS @{ text PT_LOAD ; @}
SECTIONS @{ .text : @{ *(.text) @} :text @}
@end group
@end smallexample
@node Output Section Fill
@subsubsection Output section fill
@kindex =@var{fillexp}
@cindex section fill pattern
@cindex fill pattern, entire section
You can set the fill pattern for an entire section by using
@samp{=@var{fillexp}}. @var{fillexp} is an expression
(@pxref{Expressions}). Any otherwise unspecified regions of memory
within the output section (for example, gaps left due to the required
alignment of input sections) will be filled with the two least
significant bytes of the value, repeated as necessary.
You can also change the fill value with a @code{FILL} command in the
output section commands; see @ref{Output Section Data}.
Here is a simple example:
@smallexample
@group
SECTIONS @{ .text : @{ *(.text) @} =0x9090 @}
@end group
@end smallexample
@node Overlay Description
@subsection Overlay description
@kindex OVERLAY
@cindex overlays
An overlay description provides an easy way to describe sections which
are to be loaded as part of a single memory image but are to be run at
the same memory address. At run time, some sort of overlay manager will
copy the overlaid sections in and out of the runtime memory address as
required, perhaps by simply manipulating addressing bits. This approach
can be useful, for example, when a certain region of memory is faster
than another.
Overlays are described using the @code{OVERLAY} command. The
@code{OVERLAY} command is used within a @code{SECTIONS} command, like an
output section description. The full syntax of the @code{OVERLAY}
command is as follows:
@smallexample
@group
OVERLAY [@var{start}] : [NOCROSSREFS] [AT ( @var{ldaddr} )]
@{
@var{secname1}
@{
@var{output-section-command}
@var{output-section-command}
@dots{}
@} [:@var{phdr}@dots{}] [=@var{fill}]
@var{secname2}
@{
@var{output-section-command}
@var{output-section-command}
@dots{}
@} [:@var{phdr}@dots{}] [=@var{fill}]
@dots{}
@} [>@var{region}] [:@var{phdr}@dots{}] [=@var{fill}]
@end group
@end smallexample
Everything is optional except @code{OVERLAY} (a keyword), and each
section must have a name (@var{secname1} and @var{secname2} above). The
section definitions within the @code{OVERLAY} construct are identical to
those within the general @code{SECTIONS} contruct (@pxref{SECTIONS}),
except that no addresses and no memory regions may be defined for
sections within an @code{OVERLAY}.
The sections are all defined with the same starting address. The load
addresses of the sections are arranged such that they are consecutive in
memory starting at the load address used for the @code{OVERLAY} as a
whole (as with normal section definitions, the load address is optional,
and defaults to the start address; the start address is also optional,
and defaults to the current value of the location counter).
If the @code{NOCROSSREFS} keyword is used, and there any references
among the sections, the linker will report an error. Since the sections
all run at the same address, it normally does not make sense for one
section to refer directly to another. @xref{Miscellaneous Commands,
NOCROSSREFS}.
For each section within the @code{OVERLAY}, the linker automatically
defines two symbols. The symbol @code{__load_start_@var{secname}} is
defined as the starting load address of the section. The symbol
@code{__load_stop_@var{secname}} is defined as the final load address of
the section. Any characters within @var{secname} which are not legal
within C identifiers are removed. C (or assembler) code may use these
symbols to move the overlaid sections around as necessary.
At the end of the overlay, the value of the location counter is set to
the start address of the overlay plus the size of the largest section.
Here is an example. Remember that this would appear inside a
@code{SECTIONS} construct.
@smallexample
@group
OVERLAY 0x1000 : AT (0x4000)
@{
.text0 @{ o1/*.o(.text) @}
.text1 @{ o2/*.o(.text) @}
@}
@end group
@end smallexample
@noindent
This will define both @samp{.text0} and @samp{.text1} to start at
address 0x1000. @samp{.text0} will be loaded at address 0x4000, and
@samp{.text1} will be loaded immediately after @samp{.text0}. The
following symbols will be defined: @code{__load_start_text0},
@code{__load_stop_text0}, @code{__load_start_text1},
@code{__load_stop_text1}.
C code to copy overlay @code{.text1} into the overlay area might look
like the following.
@smallexample
@group
extern char __load_start_text1, __load_stop_text1;
memcpy ((char *) 0x1000, &__load_start_text1,
&__load_stop_text1 - &__load_start_text1);
@end group
@end smallexample
Note that the @code{OVERLAY} command is just syntactic sugar, since
everything it does can be done using the more basic commands. The above
example could have been written identically as follows.
@smallexample
@group
.text0 0x1000 : AT (0x4000) @{ o1/*.o(.text) @}
__load_start_text0 = LOADADDR (.text0);
__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0);
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) @{ o2/*.o(.text) @}
__load_start_text1 = LOADADDR (.text1);
__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1);
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
@end group
@end smallexample
@node MEMORY
@section MEMORY command
@kindex MEMORY
@cindex memory regions
@cindex regions of memory
@cindex allocating memory
@cindex discontinuous memory
The linker's default configuration permits allocation of all available
memory. You can override this by using the @code{MEMORY} command.
The @code{MEMORY} command describes the location and size of blocks of
memory in the target. You can use it to describe which memory regions
may be used by the linker, and which memory regions it must avoid. You
can then assign sections to particular memory regions. The linker will
set section addresses based on the memory regions, and will warn about
regions that become too full. The linker will not shuffle sections
around to fit into the available regions.
A linker script may contain at most one use of the @code{MEMORY}
command. However, you can define as many blocks of memory within it as
you wish. The syntax is:
@smallexample
@group
MEMORY
@{
@var{name} [(@var{attr})] : ORIGIN = @var{origin}, LENGTH = @var{len}
@dots{}
@}
@end group
@end smallexample
The @var{name} is a name used in the linker script to refer to the
region. The region name has no meaning outside of the linker script.
Region names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each memory region
must have a distinct name.
@cindex memory region attributes
The @var{attr} string is an optional list of attributes that specify
whether to use a particular memory region for an input section which is
not explicitly mapped in the linker script. As described in
@ref{SECTIONS}, if you do not specify an output section for some input
section, the linker will create an output section with the same name as
the input section. If you define region attributes, the linker will use
them to select the memory region for the output section that it creates.
The @var{attr} string must consist only of the following characters:
@table @samp
@item R
Read-only section
@item W
Read/write section
@item X
Executable section
@item A
Allocatable section
@item I
Initialized section
@item L
Same as @samp{I}
@item !
Invert the sense of any of the preceding attributes
@end table
If a unmapped section matches any of the listed attributes other than
@samp{!}, it will be placed in the memory region. The @samp{!}
attribute reverses this test, so that an unmapped section will be placed
in the memory region only if it does not match any of the listed
attributes.
@kindex ORIGIN =
@kindex o =
@kindex org =
The @var{origin} is an expression for the start address of the memory
region. The expression must evaluate to a constant before memory
allocation is performed, which means that you may not use any section
relative symbols. The keyword @code{ORIGIN} may be abbreviated to
@code{org} or @code{o} (but not, for example, @code{ORG}).
@kindex LENGTH =
@kindex len =
@kindex l =
The @var{len} is an expression for the size in bytes of the memory
region. As with the @var{origin} expression, the expression must
evaluate to a constant before memory allocation is performed. The
keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
In the following example, we specify that there are two memory regions
available for allocation: one starting at @samp{0} for 256 kilobytes,
and the other starting at @samp{0x40000000} for four megabytes. The
linker will place into the @samp{rom} memory region every section which
is not explicitly mapped into a memory region, and is either read-only
or executable. The linker will place other sections which are not
explicitly mapped into a memory region into the @samp{ram} memory
region.
@smallexample
@group
MEMORY
@{
rom (rx) : ORIGIN = 0, LENGTH = 256K
ram (!rx) : org = 0x40000000, l = 4M
@}
@end group
@end smallexample
Once you define a memory region, you can direct the linker to place
specific output sections into that memory region by using the
@samp{>@var{region}} output section attribute. For example, if you have
a memory region named @samp{mem}, you would use @samp{>mem} in the
output section definition. @xref{Output Section Region}. If no address
was specified for the output section, the linker will set the address to
the next available address within the memory region. If the combined
output sections directed to a memory region are too large for the
region, the linker will issue an error message.
@node PHDRS
@section PHDRS Command
@kindex PHDRS
@cindex program headers
@cindex ELF program headers
@cindex program segments
@cindex segments, ELF
The ELF object file format uses @dfn{program headers}, also knows as
@dfn{segments}. The program headers describe how the program should be
loaded into memory. You can print them out by using the @code{objdump}
program with the @samp{-p} option.
When you run an ELF program on a native ELF system, the system loader
reads the program headers in order to figure out how to load the
program. This will only work if the program headers are set correctly.
This manual does not describe the details of how the system loader
interprets program headers; for more information, see the ELF ABI.
The linker will create reasonable program headers by default. However,
in some cases, you may need to specify the program headers more
precisely. You may use the @code{PHDRS} command for this purpose. When
the linker sees the @code{PHDRS} command in the linker script, it will
not create any program headers other than the ones specified.
The linker only pays attention to the @code{PHDRS} command when
generating an ELF output file. In other cases, the linker will simply
ignore @code{PHDRS}.
This is the syntax of the @code{PHDRS} command. The words @code{PHDRS},
@code{FILEHDR}, @code{AT}, and @code{FLAGS} are keywords.
@smallexample
@group
PHDRS
@{
@var{name} @var{type} [ FILEHDR ] [ PHDRS ] [ AT ( @var{address} ) ]
[ FLAGS ( @var{flags} ) ] ;
@}
@end group
@end smallexample
The @var{name} is used only for reference in the @code{SECTIONS} command
of the linker script. It is not put into the output file. Program
header names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each program header
must have a distinct name.
Certain program header types describe segments of memory which the
system loader will load from the file. In the linker script, you
specify the contents of these segments by placing allocatable output
sections in the segments. You use the @samp{:@var{phdr}} output section
attribute to place a section in a particular segment. @xref{Output
Section Phdr}.
It is normal to put certain sections in more than one segment. This
merely implies that one segment of memory contains another. You may
repeat @samp{:@var{phdr}}, using it once for each segment which should
contain the section.
If you place a section in one or more segments using @samp{:@var{phdr}},
then the linker will place all subsequent allocatable sections which do
not specify @samp{:@var{phdr}} in the same segments. This is for
convenience, since generally a whole set of contiguous sections will be
placed in a single segment. You can use @code{:NONE} to override the
default segment and tell the linker to not put the section in any
segment at all.
@kindex FILEHDR
@kindex PHDRS
You may use the @code{FILEHDR} and @code{PHDRS} keywords appear after
the program header type to further describe the contents of the segment.
The @code{FILEHDR} keyword means that the segment should include the ELF
file header. The @code{PHDRS} keyword means that the segment should
include the ELF program headers themselves.
The @var{type} may be one of the following. The numbers indicate the
value of the keyword.
@table @asis
@item @code{PT_NULL} (0)
Indicates an unused program header.
@item @code{PT_LOAD} (1)
Indicates that this program header describes a segment to be loaded from
the file.
@item @code{PT_DYNAMIC} (2)
Indicates a segment where dynamic linking information can be found.
@item @code{PT_INTERP} (3)
Indicates a segment where the name of the program interpreter may be
found.
@item @code{PT_NOTE} (4)
Indicates a segment holding note information.
@item @code{PT_SHLIB} (5)
A reserved program header type, defined but not specified by the ELF
ABI.
@item @code{PT_PHDR} (6)
Indicates a segment where the program headers may be found.
@item @var{expression}
An expression giving the numeric type of the program header. This may
be used for types not defined above.
@end table
You can specify that a segment should be loaded at a particular address
in memory by using an @code{AT} expression. This is identical to the
@code{AT} command used as an output section attribute (@pxref{Output
Section LMA}). The @code{AT} command for a program header overrides the
output section attribute.
The linker will normally set the segment flags based on the sections
which comprise the segment. You may use the @code{FLAGS} keyword to
explicitly specify the segment flags. The value of @var{flags} must be
an integer. It is used to set the @code{p_flags} field of the program
header.
Here is an example of @code{PHDRS}. This shows a typical set of program
headers used on a native ELF system.
@example
@group
PHDRS
@{
headers PT_PHDR PHDRS ;
interp PT_INTERP ;
text PT_LOAD FILEHDR PHDRS ;
data PT_LOAD ;
dynamic PT_DYNAMIC ;
@}
SECTIONS
@{
. = SIZEOF_HEADERS;
.interp : @{ *(.interp) @} :text :interp
.text : @{ *(.text) @} :text
.rodata : @{ *(.rodata) @} /* defaults to :text */
@dots{}
. = . + 0x1000; /* move to a new page in memory */
.data : @{ *(.data) @} :data
.dynamic : @{ *(.dynamic) @} :data :dynamic
@dots{}
@}
@end group
@end example
@node VERSION
@section VERSION Command
@kindex VERSION @{script text@}
@cindex symbol versions
@cindex version script
@cindex versions of symbols
The linker supports symbol versions when using ELF. Symbol versions are
only useful when using shared libraries. The dynamic linker can use
symbol versions to select a specific version of a function when it runs
a program that may have been linked against an earlier version of the
shared library.
You can include a version script directly in the main linker script, or
you can supply the version script as an implicit linker script. You can
also use the @samp{--version-script} linker option.
The syntax of the @code{VERSION} command is simply
@smallexample
VERSION @{ version-script-commands @}
@end smallexample
The format of the version script commands is identical to that used by
Sun's linker in Solaris 2.5. The version script defines a tree of
version nodes. You specify the node names and interdependencies in the
version script. You can specify which symbols are bound to which
version nodes, and you can reduce a specified set of symbols to local
scope so that they are not globally visible outside of the shared
library.
The easiest way to demonstrate the version script language is with a few
examples.
@smallexample
VERS_1.1 @{
global:
foo1;
local:
old*;
original*;
new*;
@};
VERS_1.2 @{
foo2;
@} VERS_1.1;
VERS_2.0 @{
bar1; bar2;
@} VERS_1.2;
@end smallexample
This example version script defines three version nodes. The first
version node defined is @samp{VERS_1.1}; it has no other dependencies.
The script binds the symbol @samp{foo1} to @samp{VERS_1.1}. It reduces
a number of symbols to local scope so that they are not visible outside
of the shared library.
Next, the version script defines node @samp{VERS_1.2}. This node
depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2}
to the version node @samp{VERS_1.2}.
Finally, the version script defines node @samp{VERS_2.0}. This node
depends upon @samp{VERS_1.2}. The scripts binds the symbols @samp{bar1}
and @samp{bar2} are bound to the version node @samp{VERS_2.0}.
When the linker finds a symbol defined in a library which is not
specifically bound to a version node, it will effectively bind it to an
unspecified base version of the library. You can bind all otherwise
unspecified symbols to a given version node by using @samp{global: *}
somewhere in the version script.
The names of the version nodes have no specific meaning other than what
they might suggest to the person reading them. The @samp{2.0} version
could just as well have appeared in between @samp{1.1} and @samp{1.2}.
However, this would be a confusing way to write a version script.
When you link an application against a shared library that has versioned
symbols, the application itself knows which version of each symbol it
requires, and it also knows which version nodes it needs from each
shared library it is linked against. Thus at runtime, the dynamic
loader can make a quick check to make sure that the libraries you have
linked against do in fact supply all of the version nodes that the
application will need to resolve all of the dynamic symbols. In this
way it is possible for the dynamic linker to know with certainty that
all external symbols that it needs will be resolvable without having to
search for each symbol reference.
The symbol versioning is in effect a much more sophisticated way of
doing minor version checking that SunOS does. The fundamental problem
that is being addressed here is that typically references to external
functions are bound on an as-needed basis, and are not all bound when
the application starts up. If a shared library is out of date, a
required interface may be missing; when the application tries to use
that interface, it may suddenly and unexpectedly fail. With symbol
versioning, the user will get a warning when they start their program if
the libraries being used with the application are too old.
There are several GNU extensions to Sun's versioning approach. The
first of these is the ability to bind a symbol to a version node in the
source file where the symbol is defined instead of in the versioning
script. This was done mainly to reduce the burden on the library
maintainer. You can do this by putting something like:
@smallexample
__asm__(".symver original_foo,foo@@VERS_1.1");
@end smallexample
@noindent
in the C source file. This renames the function @samp{original_foo} to
be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}.
The @samp{local:} directive can be used to prevent the symbol
@samp{original_foo} from being exported.
The second GNU extension is to allow multiple versions of the same
function to appear in a given shared library. In this way you can make
an incompatible change to an interface without increasing the major
version number of the shared library, while still allowing applications
linked against the old interface to continue to function.
To do this, you must use multiple @samp{.symver} directives in the
source file. Here is an example:
@smallexample
__asm__(".symver original_foo,foo@@");
__asm__(".symver old_foo,foo@@VERS_1.1");
__asm__(".symver old_foo1,foo@@VERS_1.2");
__asm__(".symver new_foo,foo@@@@VERS_2.0");
@end smallexample
In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the
unspecified base version of the symbol. The source file that contains this
example would define 4 C functions: @samp{original_foo}, @samp{old_foo},
@samp{old_foo1}, and @samp{new_foo}.
When you have multiple definitions of a given symbol, there needs to be
some way to specify a default version to which external references to
this symbol will be bound. You can do this with the
@samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. You can only
declare one version of a symbol as the default in this manner; otherwise
you would effectively have multiple definitions of the same symbol.
If you wish to bind a reference to a specific version of the symbol
within the shared library, you can use the aliases of convenience
(i.e. @samp{old_foo}), or you can use the @samp{.symver} directive to
specifically bind to an external version of the function in question.
@node Expressions
@section Expressions in Linker Scripts
@cindex expressions
@cindex arithmetic
The syntax for expressions in the linker script language is identical to
that of C expressions. All expressions are evaluated as integers. All
expressions are evaluated in the same size, which is 32 bits if both the
host and target are 32 bits, and is otherwise 64 bits.
You can use and set symbol values in expressions.
The linker defines several special purpose builtin functions for use in
expressions.
@menu
* Constants:: Constants
* Symbols:: Symbol Names
* Location Counter:: The Location Counter
* Operators:: Operators
* Evaluation:: Evaluation
* Expression Section:: The Section of an Expression
* Builtin Functions:: Builtin Functions
@end menu
@node Constants
@subsection Constants
@cindex integer notation
@cindex constants in linker scripts
All constants are integers.
As in C, the linker considers an integer beginning with @samp{0} to be
octal, and an integer beginning with @samp{0x} or @samp{0X} to be
hexadecimal. The linker considers other integers to be decimal.
@cindex scaled integers
@cindex K and M integer suffixes
@cindex M and K integer suffixes
@cindex suffixes for integers
@cindex integer suffixes
In addition, you can use the suffixes @code{K} and @code{M} to scale a
constant by
@c TEXI2ROFF-KILL
@ifinfo
@c END TEXI2ROFF-KILL
@code{1024} or @code{1024*1024}
@c TEXI2ROFF-KILL
@end ifinfo
@tex
${\rm 1024}$ or ${\rm 1024}^2$
@end tex
@c END TEXI2ROFF-KILL
respectively. For example, the following all refer to the same quantity:
@smallexample
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;
@end smallexample
@node Symbols
@subsection Symbol Names
@cindex symbol names
@cindex names
@cindex quoted symbol names
@kindex "
Unless quoted, symbol names start with a letter, underscore, or period
and may include letters, digits, underscores, periods, and hyphens.
Unquoted symbol names must not conflict with any keywords. You can
specify a symbol which contains odd characters or has the same name as a
keyword by surrounding the symbol name in double quotes:
@smallexample
"SECTION" = 9;
"with a space" = "also with a space" + 10;
@end smallexample
Since symbols can contain many non-alphabetic characters, it is safest
to delimit symbols with spaces. For example, @samp{A-B} is one symbol,
whereas @samp{A - B} is an expression involving subtraction.
@node Location Counter
@subsection The Location Counter
@kindex .
@cindex dot
@cindex location counter
@cindex current output location
The special linker variable @dfn{dot} @samp{.} always contains the
current output location counter. Since the @code{.} always refers to a
location in an output section, it may only appear in an expression
within a @code{SECTIONS} command. The @code{.} symbol may appear
anywhere that an ordinary symbol is allowed in an expression.
@cindex holes
Assigning a value to @code{.} will cause the location counter to be
moved. This may be used to create holes in the output section. The
location counter may never be moved backwards.
@smallexample
SECTIONS
@{
output :
@{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
@} = 0x1234;
@}
@end smallexample
@noindent
In the previous example, the @samp{.text} section from @file{file1} is
located at the beginning of the output section @samp{output}. It is
followed by a 1000 byte gap. Then the @samp{.text} section from
@file{file2} appears, also with a 1000 byte gap following before the
@samp{.text} section from @file{file3}. The notation @samp{= 0x1234}
specifies what data to write in the gaps (@pxref{Output Section Fill}).
@cindex dot inside sections
Note: @code{.} actually refers to the byte offset from the start of the
current containing object. Normally this is the @code{SECTIONS}
statement, whoes start address is 0, hence @code{.} can be used as an
absolute address. If @code{.} is used inside a section description
however, it refers to the byte offset from the start of that section,
not an absolute address. Thus in a script like this:
@smallexample
SECTIONS
@{
. = 0x100
.text: @{
*(.text)
. = 0x200
@}
. = 0x500
.data: @{
*(.data)
. += 0x600
@}
@}
@end smallexample
The @samp{.text} section will be assigned a starting address of 0x100
and a size of exactly 0x200 bytes, even if there is not enough data in
the @samp{.text} input sections to fill this area. (If there is too
much data, an error will be produced because this would be an attempt to
move @code{.} backwards). The @samp{.data} section will start at 0x500
and it will have an extra 0x600 bytes worth of space after the end of
the values from the @samp{.data} input sections and before the end of
the @samp{.data} output section itself.
@need 2000
@node Operators
@subsection Operators
@cindex operators for arithmetic
@cindex arithmetic operators
@cindex precedence in expressions
The linker recognizes the standard C set of arithmetic operators, with
the standard bindings and precedence levels:
@c TEXI2ROFF-KILL
@ifinfo
@c END TEXI2ROFF-KILL
@smallexample
precedence associativity Operators Notes
(highest)
1 left ! - ~ (1)
2 left * / %
3 left + -
4 left >> <<
5 left == != > < <= >=
6 left &
7 left |
8 left &&
9 left ||
10 right ? :
11 right &= += -= *= /= (2)
(lowest)
@end smallexample
Notes:
(1) Prefix operators
(2) @xref{Assignments}.
@c TEXI2ROFF-KILL
@end ifinfo
@tex
\vskip \baselineskip
%"lispnarrowing" is the extra indent used generally for smallexample
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\halign
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height2pt&\omit&&\omit&&\omit&\cr
&Precedence&& Associativity &&{\rm Operators}&\cr
height2pt&\omit&&\omit&&\omit&\cr
\noalign{\hrule}
height2pt&\omit&&\omit&&\omit&\cr
&highest&&&&&\cr
% '176 is tilde, '~' in tt font
&1&&left&&\qquad- \char'176\ !\qquad\dag&\cr
&2&&left&&* / \%&\cr
&3&&left&&+ -&\cr
&4&&left&&>> <<&\cr