path: root/ld/ld.texinfo

\input texinfo
@setfilename ld.info
@c Copyright 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
@c 2001, 2002, 2003 Free Software Foundation, Inc.
@syncodeindex ky cp
@include configdoc.texi
@c (configdoc.texi is generated by the Makefile)
@include ldver.texi

@c @smallbook

@macro gcctabopt{body}
@code{\body\}
@end macro

@c man begin NAME
@ifset man
@c Configure for the generation of man pages
@set UsesEnvVars
@set GENERIC
@set A29K
@set ARC
@set ARM
@set D10V
@set D30V
@set H8/300
@set H8/500
@set HPPA
@set I370
@set I80386
@set I860
@set I960
@set M32R
@set M68HC11
@set M680X0
@set MCORE
@set MIPS
@set MMIX
@set MSP430
@set PDP11
@set PJ
@set SH
@set SPARC
@set TIC54X
@set V850
@set VAX
@set WIN32
@set XTENSA
@end ifset
@c man end

@ifinfo
@format
START-INFO-DIR-ENTRY
* Ld: (ld).                       The GNU linker.
END-INFO-DIR-ENTRY
@end format
@end ifinfo

@ifinfo
This file documents the @sc{gnu} linker LD version @value{VERSION}.

Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000,
2001, 2002, 2003 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
@c man begin COPYRIGHT
Copyright @copyright{} 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001,
2002, 2003 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts.  A copy of the license is included in the
section entitled ``GNU Free Documentation License''.
@c man end

@end titlepage
@end iftex
@c FIXME: Talk about importance of *order* of args, cmds to linker!

@ifnottex
@node Top
@top 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 Renesas
* Renesas::                     ld and other Renesas micros
@end ifset
@ifset I960
* i960::                        ld and the Intel 960 family
@end ifset
@ifset ARM
* ARM::				ld and the ARM family
@end ifset
@ifset HPPA
* HPPA ELF32::                  ld and HPPA 32-bit ELF
@end ifset
@ifset TICOFF
* TI COFF::                     ld and the TI COFF
@end ifset
@ifset WIN32
* Win32::                       ld and WIN32 (cygwin/mingw)
@end ifset
@ifset XTENSA
* Xtensa::                      ld and Xtensa Processors
@end ifset
@end ifclear
@ifclear SingleFormat
* BFD::                         BFD
@end ifclear
@c Following blank line required for remaining bug in makeinfo conds/menus

* Reporting Bugs::              Reporting Bugs
* MRI::                         MRI Compatible Script Files
* GNU Free Documentation License::  GNU Free Documentation License
* Index::                       Index
@end menu
@end ifnottex

@node Overview
@chapter Overview

@cindex @sc{gnu} linker
@cindex what is this?

@ifset man
@c man begin SYNOPSIS
ld [@b{options}] @var{objfile} @dots{}
@c man end

@c man begin SEEALSO
ar(1), nm(1), objcopy(1), objdump(1), readelf(1) and
the Info entries for @file{binutils} and
@file{ld}.
@c man end
@end ifset

@c man begin DESCRIPTION

@command{ld} combines a number of object and archive files, relocates
their data and ties up symbol references. Usually the last step in
compiling a program is to run @command{ld}.

@command{ld} accepts Linker Command Language files written in
a superset of AT&T's Link Editor Command Language syntax,
to provide explicit and total control over the linking process.

@ifset man
@c For the man only
This man page does not describe the command language; see the 
@command{ld} entry in @code{info}, or the manual
ld: the GNU linker, for full details on the command language and 
on other aspects of the GNU linker. 
@end ifset

@ifclear SingleFormat
This version of @command{ld} uses the general purpose BFD libraries
to operate on object files. This allows @command{ld} to read, combine, and
write object files in many different formats---for example, COFF or
@code{a.out}.  Different formats may be linked together to produce any
available kind of object file.  @xref{BFD}, for more information.
@end ifclear

Aside from its flexibility, the @sc{gnu} linker is more helpful than other
linkers in providing diagnostic information.  Many linkers abandon
execution immediately upon encountering an error; whenever possible,
@command{ld} continues executing, allowing you to identify other errors
(or, in some cases, to get an output file in spite of the error).

@c man end

@node Invocation
@chapter Invocation

@c man begin DESCRIPTION

The @sc{gnu} linker @command{ld} is meant to cover a broad range of situations,
and to be as compatible as possible with other linkers.  As a result,
you have many choices to control its behavior.

@c man end

@ifset UsesEnvVars
@menu
* Options::                     Command Line Options
* Environment::                 Environment Variables
@end menu

@node Options
@section Command Line Options
@end ifset

@cindex command line
@cindex options

@c man begin OPTIONS

The linker supports a plethora of command-line options, but in actual
practice few of them are used in any particular context.
@cindex standard Unix system
For instance, a frequent use of @command{ld} is to link standard Unix
object files on a standard, supported Unix system.  On such a system, to
link a file @code{hello.o}:

@smallexample
ld -o @var{output} /lib/crt0.o hello.o -lc
@end smallexample

This tells @command{ld} to produce a file called @var{output} as the
result of linking the file @code{/lib/crt0.o} with @code{hello.o} and
the library @code{libc.a}, which will come from the standard search
directories.  (See the discussion of the @samp{-l} option below.)

Some of the command-line options to @command{ld} may be specified at any
point in the command line.  However, options which refer to files, such
as @samp{-l} or @samp{-T}, cause the file to be read at the point at
which the option appears in the command line, relative to the object
files and other file options.  Repeating non-file options with a
different argument will either have no further effect, or override prior
occurrences (those further to the left on the command line) of that
option.  Options which may be meaningfully specified more than once are
noted in the descriptions below.

@cindex object files
Non-option arguments are object files or archives which are to be linked
together.  They may follow, precede, or be mixed in with command-line
options, except that an object file argument may not be placed between
an option and its argument.

Usually the linker is invoked with at least one object file, but you can
specify other forms of binary input files using @samp{-l}, @samp{-R},
and the script command language.  If @emph{no} binary input files at all
are specified, the linker does not produce any output, and issues the
message @samp{No input files}.

If the linker cannot recognize the format of an object file, it will
assume that it is a linker script.  A script specified in this way
augments the main linker script used for the link (either the default
linker script or the one specified by using @samp{-T}).  This feature
permits the linker to link against a file which appears to be an object
or an archive, but actually merely defines some symbol values, or uses
@code{INPUT} or @code{GROUP} to load other objects.  Note that
specifying a script in this way merely augments the main linker script;
use the @samp{-T} option to replace the default linker script entirely.
@xref{Scripts}.

For options whose names are a single letter,
option arguments must either follow the option letter without intervening
whitespace, or be given as separate arguments immediately following the
option that requires them.

For options whose names are multiple letters, either one dash or two can
precede the option name; for example, @samp{-trace-symbol} and
@samp{--trace-symbol} are equivalent.  Note---there is one exception to
this rule.  Multiple letter options that start with a lower case 'o' can
only be 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
(e.g. @samp{gcc}) then all the linker command line options should be
prefixed by @samp{-Wl,} (or whatever is appropriate for the particular
compiler driver) like this:

@smallexample
  gcc -Wl,--startgroup foo.o bar.o -Wl,--endgroup
@end smallexample

This is important, because otherwise the compiler driver program may
silently drop the linker options, resulting in a bad link.

Here is a table of the generic command line switches accepted by the GNU
linker:

@table @gcctabopt
@kindex -a@var{keyword}
@item -a@var{keyword}
This option is supported for HP/UX compatibility.  The @var{keyword}
argument must be one of the strings @samp{archive}, @samp{shared}, or
@samp{default}.  @samp{-aarchive} is functionally equivalent to
@samp{-Bstatic}, and the other two keywords are functionally equivalent
to @samp{-Bdynamic}.  This option may be used any number of times.

@ifset I960
@cindex architectures
@kindex -A@var{arch}
@item -A@var{architecture}
@kindex --architecture=@var{arch}
@itemx --architecture=@var{architecture}
In the current release of @command{ld}, this option is useful only for the
Intel 960 family of architectures.  In that @command{ld} configuration, the
@var{architecture} argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path.  @xref{i960,,@command{ld} and the Intel 960
family}, for details.

Future releases of @command{ld} may support similar functionality for
other architecture families.
@end ifset

@ifclear SingleFormat
@cindex binary input format
@kindex -b @var{format}
@kindex --format=@var{format}
@cindex input format
@cindex input format
@item -b @var{input-format}
@itemx --format=@var{input-format}
@command{ld} may be configured to support more than one kind of object
file.  If your @command{ld} is configured this way, you can use the
@samp{-b} option to specify the binary format for input object files
that follow this option on the command line.  Even when @command{ld} is
configured to support alternative object formats, you don't usually need
to specify this, as @command{ld} should be configured to expect as a
default input format the most usual format on each machine.
@var{input-format} is a text string, the name of a particular format
supported by the BFD libraries.  (You can list the available binary
formats with @samp{objdump -i}.)
@xref{BFD}.

You may want to use this option if you are linking files with an unusual
binary format.  You can also use @samp{-b} to switch formats explicitly (when
linking object files of different formats), by including
@samp{-b @var{input-format}} before each group of object files in a
particular format.

The default format is taken from the environment variable
@code{GNUTARGET}.
@ifset UsesEnvVars
@xref{Environment}.
@end ifset
You can also define the input format from a script, using the command
@code{TARGET};
@ifclear man
see @ref{Format Commands}.
@end ifclear
@end ifclear

@kindex -c @var{MRI-cmdfile}
@kindex --mri-script=@var{MRI-cmdfile}
@cindex compatibility, MRI
@item -c @var{MRI-commandfile}
@itemx --mri-script=@var{MRI-commandfile}
For compatibility with linkers produced by MRI, @command{ld} accepts script
files written in an alternate, restricted command language, described in
@ifclear man
@ref{MRI,,MRI Compatible Script Files}.
@end ifclear
@ifset man
the MRI Compatible Script Files section of GNU ld documentation.
@end ifset
Introduce MRI script files with
the option @samp{-c}; use the @samp{-T} option to run linker
scripts written in the general-purpose @command{ld} scripting language.
If @var{MRI-cmdfile} does not exist, @command{ld} looks for it in the directories
specified by any @samp{-L} options.

@cindex common allocation
@kindex -d
@kindex -dc
@kindex -dp
@item -d
@itemx -dc
@itemx -dp
These three options are equivalent; multiple forms are supported for
compatibility with other linkers.  They assign space to common symbols
even if a relocatable output file is specified (with @samp{-r}).  The
script command @code{FORCE_COMMON_ALLOCATION} has the same effect.
@xref{Miscellaneous Commands}.

@cindex entry point, from command line
@kindex -e @var{entry}
@kindex --entry=@var{entry}
@item -e @var{entry}
@itemx --entry=@var{entry}
Use @var{entry} as the explicit symbol for beginning execution of your
program, rather than the default entry point.  If there is no symbol
named @var{entry}, the linker will try to parse @var{entry} as a number,
and use that as the entry address (the number will be interpreted in
base 10; you may use a leading @samp{0x} for base 16, or a leading
@samp{0} for base 8).  @xref{Entry Point}, for a discussion of defaults
and other ways of specifying the entry point.

@cindex dynamic symbol table
@kindex -E
@kindex --export-dynamic
@item -E
@itemx --export-dynamic
When creating a dynamically linked executable, add all symbols to the
dynamic symbol table.  The dynamic symbol table is the set of symbols
which are visible from dynamic objects at run time.

If you do not use this option, the dynamic symbol table will normally
contain only those symbols which are referenced by some dynamic object
mentioned in the link.

If you use @code{dlopen} to load a dynamic object which needs to refer
back to the symbols defined by the program, rather than some other
dynamic object, then you will probably need to use this option when
linking the program itself.

You can also use the version script to control what symbols should
be added to the dynamic symbol table if the output format supports it.
See the description of @samp{--version-script} in @ref{VERSION}.

@ifclear SingleFormat
@cindex big-endian objects
@cindex endianness
@kindex -EB
@item -EB
Link big-endian objects.  This affects the default output format.

@cindex little-endian objects
@kindex -EL
@item -EL
Link little-endian objects.  This affects the default output format.
@end ifclear

@kindex -f
@kindex --auxiliary
@item -f
@itemx --auxiliary @var{name}
When creating an ELF shared object, set the internal DT_AUXILIARY field
to the specified name.  This tells the dynamic linker that the symbol
table of the shared object should be used as an auxiliary filter on the
symbol table of the shared object @var{name}.

If you later link a program against this filter object, then, when you
run the program, the dynamic linker will see the DT_AUXILIARY field.  If
the dynamic linker resolves any symbols from the filter object, it will
first check whether there is a definition in the shared object
@var{name}.  If there is one, it will be used instead of the definition
in the filter object.  The shared object @var{name} need not exist.
Thus the shared object @var{name} may be used to provide an alternative
implementation of certain functions, perhaps for debugging or for
machine specific performance.

This option may be specified more than once.  The DT_AUXILIARY entries
will be created in the order in which they appear on the command line.

@kindex -F
@kindex --filter
@item -F @var{name}
@itemx --filter @var{name}
When creating an ELF shared object, set the internal DT_FILTER field to
the specified name.  This tells the dynamic linker that the symbol table
of the shared object which is being created should be used as a filter
on the symbol table of the shared object @var{name}.

If you later link a program against this filter object, then, when you
run the program, the dynamic linker will see the DT_FILTER field.  The
dynamic linker will resolve symbols according to the symbol table of the
filter object as usual, but it will actually link to the definitions
found in the shared object @var{name}.  Thus the filter object can be
used to select a subset of the symbols provided by the object
@var{name}.

Some older linkers used the @option{-F} option throughout a compilation
toolchain for specifying object-file format for both input and output
object files.
@ifclear SingleFormat
The @sc{gnu} linker uses other mechanisms for this purpose: the
@option{-b}, @option{--format}, @option{--oformat} options, the 
@code{TARGET} command in linker scripts, and the @code{GNUTARGET}
environment variable.
@end ifclear
The @sc{gnu} linker will ignore the @option{-F} option when not
creating an ELF shared object.

@cindex finalization function
@kindex -fini
@item -fini @var{name}
When creating an ELF executable or shared object, call NAME when the
executable or shared object is unloaded, by setting DT_FINI to the
address of the function.  By default, the linker uses @code{_fini} as
the function to call.

@kindex -g
@item -g
Ignored.  Provided for compatibility with other tools.

@kindex -G
@kindex --gpsize
@cindex object size
@item -G@var{value}
@itemx --gpsize=@var{value}
Set the maximum size of objects to be optimized using the GP register to
@var{size}.  This is only meaningful for object file formats such as
MIPS ECOFF which supports putting large and small objects into different
sections.  This is ignored for other object file formats.

@cindex runtime library name
@kindex -h@var{name}
@kindex -soname=@var{name}
@item -h@var{name}
@itemx -soname=@var{name}
When creating an ELF shared object, set the internal DT_SONAME field to
the specified name.  When an executable is linked with a shared object
which has a DT_SONAME field, then when the executable is run the dynamic
linker will attempt to load the shared object specified by the DT_SONAME
field rather than the using the file name given to the linker.

@kindex -i
@cindex incremental link
@item -i
Perform an incremental link (same as option @samp{-r}).

@cindex initialization function
@kindex -init
@item -init @var{name}
When creating an ELF executable or shared object, call NAME when the
executable or shared object is loaded, by setting DT_INIT to the address
of the function.  By default, the linker uses @code{_init} as the
function to call.

@cindex archive files, from cmd line
@kindex -l@var{archive}
@kindex --library=@var{archive}
@item -l@var{archive}
@itemx --library=@var{archive}
Add archive file @var{archive} to the list of files to link.  This
option may be used any number of times.  @command{ld} will search its
path-list for occurrences of @code{lib@var{archive}.a} for every
@var{archive} specified.

On systems which support shared libraries, @command{ld} may also search for
libraries with extensions other than @code{.a}.  Specifically, on ELF
and SunOS systems, @command{ld} will search a directory for a library with
an extension of @code{.so} before searching for one with an extension of
@code{.a}.  By convention, a @code{.so} extension indicates a shared
library.

The linker will search an archive only once, at the location where it is
specified on the command line.  If the archive defines a symbol which
was undefined in some object which appeared before the archive on the
command line, the linker will include the appropriate file(s) from the
archive.  However, an undefined symbol in an object appearing later on
the command line will not cause the linker to search the archive again.

See the @option{-(} option for a way to force the linker to search
archives multiple times.

You may list the same archive multiple times on the command line.

@ifset GENERIC
This type of archive searching is standard for Unix linkers.  However,
if you are using @command{ld} on AIX, note that it is different from the
behaviour of the AIX linker.
@end ifset

@cindex search directory, from cmd line
@kindex -L@var{dir}
@kindex --library-path=@var{dir}
@item -L@var{searchdir}
@itemx --library-path=@var{searchdir}
Add path @var{searchdir} to the list of paths that @command{ld} will search
for archive libraries and @command{ld} control scripts.  You may use this
option any number of times.  The directories are searched in the order
in which they are specified on the command line.  Directories specified
on the command line are searched before the default directories.  All
@option{-L} options apply to all @option{-l} options, regardless of the
order in which the options appear.

If @var{searchdir} begins with @code{=}, then the @code{=} will be replaced
by the @dfn{sysroot prefix}, a path specified when the linker is configured.

@ifset UsesEnvVars
The default set of paths searched (without being specified with
@samp{-L}) depends on which emulation mode @command{ld} is using, and in
some cases also on how it was configured.  @xref{Environment}.
@end ifset

The paths can also be specified in a link script with the
@code{SEARCH_DIR} command.  Directories specified this way are searched
at the point in which the linker script appears in the command line.

@cindex emulation
@kindex -m @var{emulation}
@item -m@var{emulation}
Emulate the @var{emulation} linker.  You can list the available
emulations with the @samp{--verbose} or @samp{-V} options.

If the @samp{-m} option is not used, the emulation is taken from the
@code{LDEMULATION} environment variable, if that is defined.

Otherwise, the default emulation depends upon how the linker was
configured.

@cindex link map
@kindex -M
@kindex --print-map
@item -M
@itemx --print-map
Print a link map to the standard output.  A link map provides
information about the link, including the following:

@itemize @bullet
@item
Where object files 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, and disable linking against shared
libraries.  If the output format supports Unix style magic numbers,
mark the output as @code{OMAGIC}.

@kindex --no-omagic
@cindex OMAGIC
@item --no-omagic
This option negates most of the effects of the @option{-N} option.  It
sets the text section to be read-only, and forces the data segment to
be page-aligned.  Note - this option does not enable linking against
shared libraries.  Use @option{-Bdynamic} for this.

@kindex -o @var{output}
@kindex --output=@var{output}
@cindex naming the output file
@item -o @var{output}
@itemx --output=@var{output}
Use @var{output} as the name for the program produced by @command{ld}; if this
option is not specified, the name @file{a.out} is used by default.  The
script command @code{OUTPUT} can also specify the output file name.

@kindex -O @var{level}
@cindex generating optimized output
@item -O @var{level}
If @var{level} is a numeric values greater than zero @command{ld} optimizes
the output.  This might take significantly longer and therefore probably
should only be enabled for the final binary.

@kindex -q
@kindex --emit-relocs
@cindex retain relocations in final executable
@item -q
@itemx --emit-relocs
Leave relocation sections and contents in fully linked exececutables.
Post link analysis and optimization tools may need this information in
order to perform correct modifications of executables.  This results
in larger executables.

This option is currently only supported on ELF platforms.

@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 @command{ld}.  This is often called @dfn{partial
linking}.  As a side effect, in environments that support standard Unix
magic numbers, this option also sets the output file's magic number to
@code{OMAGIC}.
@c ; see @option{-N}.
If this option is not specified, an absolute file is produced.  When
linking C++ programs, this option @emph{will not} resolve references to
constructors; to do that, use @samp{-Ur}.

When an input file does not have the same format as the output file,
partial linking is only supported if that input file does not contain any
relocations.  Different output formats can have further restrictions; for
example some @code{a.out}-based formats do not support partial linking
with input files in other formats at all.

This option does the same thing as @samp{-i}.

@kindex -R @var{file}
@kindex --just-symbols=@var{file}
@cindex symbol-only input
@item -R @var{filename}
@itemx --just-symbols=@var{filename}
Read symbol names and their addresses from @var{filename}, but do not
relocate it or include it in the output.  This allows your output file
to refer symbolically to absolute locations of memory defined in other
programs.  You may use this option more than once.

For compatibility with other ELF linkers, if the @option{-R} option is
followed by a directory name, rather than a file name, it is treated as
the @option{-rpath} option.

@kindex -s
@kindex --strip-all
@cindex strip all symbols
@item -s
@itemx --strip-all
Omit all symbol information from the output file.

@kindex -S
@kindex --strip-debug
@cindex strip debugger symbols
@item -S
@itemx --strip-debug
Omit debugger symbol information (but not all symbols) from the output file.

@kindex -t
@kindex --trace
@cindex input files, displaying
@item -t
@itemx --trace
Print the names of the input files as @command{ld} processes them.

@kindex -T @var{script}
@kindex --script=@var{script}
@cindex script files
@item -T @var{scriptfile}
@itemx --script=@var{scriptfile}
Use @var{scriptfile} as the linker script.  This script replaces
@command{ld}'s default linker script (rather than adding to it), so
@var{commandfile} must specify everything necessary to describe the
output file.  @xref{Scripts}.  If @var{scriptfile} does not exist in
the current directory, @code{ld} looks for it in the directories
specified by any preceding @samp{-L} options.  Multiple @samp{-T}
options accumulate.

@kindex -u @var{symbol}
@kindex --undefined=@var{symbol}
@cindex undefined symbol
@item -u @var{symbol}
@itemx --undefined=@var{symbol}
Force @var{symbol} to be entered in the output file as an undefined
symbol.  Doing this may, for example, trigger linking of additional
modules from standard libraries.  @samp{-u} may be repeated with
different option arguments to enter additional undefined symbols.  This
option is equivalent to the @code{EXTERN} linker script command.

@kindex -Ur
@cindex constructors
@item -Ur
For anything other than C++ programs, this option is equivalent to
@samp{-r}: it generates relocatable output---i.e., an output file that can in
turn serve as input to @command{ld}.  When linking C++ programs, @samp{-Ur}
@emph{does} resolve references to constructors, unlike @samp{-r}.
It does not work to use @samp{-Ur} on files that were themselves linked
with @samp{-Ur}; once the constructor table has been built, it cannot
be added to.  Use @samp{-Ur} only for the last partial link, and
@samp{-r} for the others.

@kindex --unique[=@var{SECTION}]
@item --unique[=@var{SECTION}]
Creates a separate output section for every input section matching
@var{SECTION}, or if the optional wildcard @var{SECTION} argument is
missing, for every orphan input section.  An orphan section is one not
specifically mentioned in a linker script.  You may use this option
multiple times on the command line;  It prevents the normal merging of
input sections with the same name, overriding output section assignments
in a linker script.

@kindex -v
@kindex -V
@kindex --version
@cindex version
@item -v
@itemx --version
@itemx -V
Display the version number for @command{ld}.  The @option{-V} option also
lists the supported emulations.

@kindex -x
@kindex --discard-all
@cindex deleting local symbols
@item -x
@itemx --discard-all
Delete all local symbols.

@kindex -X
@kindex --discard-locals
@cindex local symbols, deleting
@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}, @code{origin}, @code{combreloc}, @code{nocombreloc} 
and @code{nocopyreloc}.
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.
@code{muldefs} allows multiple definitions.
@code{combreloc} combines multiple reloc sections and sorts them
to make dynamic symbol lookup caching possible.
@code{nocombreloc} disables multiple reloc sections combining.
@code{nocopyreloc} disables production of copy relocs.

@kindex -(
@cindex groups of archives
@item -( @var{archives} -)
@itemx --start-group @var{archives} --end-group
The @var{archives} should be a list of archive files.  They may be
either explicit file names, or @samp{-l} options.

The specified archives are searched repeatedly until no new undefined
references are created.  Normally, an archive is searched only once in
the order that it is specified on the command line.  If a symbol in that
archive is needed to resolve an undefined symbol referred to by an
object in an archive that appears later on the command line, the linker
would not be able to resolve that reference.  By grouping the archives,
they all be searched repeatedly until all possible references are
resolved.

Using this option has a significant performance cost.  It is best to use
it only when there are unavoidable circular references between two or
more archives.

@kindex --accept-unknown-input-arch
@kindex --no-accept-unknown-input-arch
@item --accept-unknown-input-arch
@itemx --no-accept-unknown-input-arch
Tells the linker to accept input files whose architecture cannot be
recognised.  The assumption is that the user knows what they are doing
and deliberately wants to link in these unknown input files.  This was
the default behaviour of the linker, before release 2.14.  The default
behaviour from release 2.14 onwards is to reject such input files, and
so the @samp{--accept-unknown-input-arch} option has been added to
restore the old behaviour.

@kindex -assert @var{keyword}
@item -assert @var{keyword}
This option is ignored for SunOS compatibility.

@kindex -Bdynamic
@kindex -dy
@kindex -call_shared
@item -Bdynamic
@itemx -dy
@itemx -call_shared
Link against dynamic libraries.  This is only meaningful on platforms
for which shared libraries are supported.  This option is normally the
default on such platforms.  The different variants of this option are
for compatibility with various systems.  You may use this option
multiple times on the command line: it affects library searching for
@option{-l} options which follow it.

@kindex -Bgroup
@item -Bgroup
Set the @code{DF_1_GROUP} flag in the @code{DT_FLAGS_1} entry in the dynamic
section.  This causes the runtime linker to handle lookups in this
object and its dependencies to be performed only inside the group.
@option{--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 @option{-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 common allocation
@kindex --no-define-common
@item --no-define-common
This option inhibits the assignment of addresses to common symbols.
The script command @code{INHIBIT_COMMON_ALLOCATION} has the same effect.
@xref{Miscellaneous Commands}.

The @samp{--no-define-common} option allows decoupling
the decision to assign addresses to Common symbols from the choice
of the output file type; otherwise a non-Relocatable output type
forces assigning addresses to Common symbols.
Using @samp{--no-define-common} allows Common symbols that are referenced
from a shared library to be assigned addresses only in the main program.
This eliminates the unused duplicate space in the shared library,
and also prevents any possible confusion over resolving to the wrong
duplicate when there are many dynamic modules with specialized search
paths for runtime symbol resolution.

@cindex symbols, from command line
@kindex --defsym @var{symbol}=@var{exp}
@item --defsym @var{symbol}=@var{expression}
Create a global symbol in the output file, containing the absolute
address given by @var{expression}.  You may use this option as many
times as necessary to define multiple symbols in the command line.  A
limited form of arithmetic is supported for the @var{expression} in this
context: you may give a hexadecimal constant or the name of an existing
symbol, or use @code{+} and @code{-} to add or subtract hexadecimal
constants or symbols.  If you need more elaborate expressions, consider
using the linker command language from a script (@pxref{Assignments,,
Assignment: Symbol Definitions}).  @emph{Note:} there should be no white
space between @var{symbol}, the equals sign (``@key{=}''), and
@var{expression}.

@cindex demangling, from command line
@kindex --demangle[=@var{style}]
@kindex --no-demangle
@item --demangle[=@var{style}]
@itemx --no-demangle
These options control whether to demangle symbol names in error messages
and other output.  When the linker is told to demangle, it tries to
present symbol names in a readable fashion: it strips leading
underscores if they are used by the object file format, and converts C++
mangled symbol names into user readable names.  Different compilers have
different mangling styles.  The optional demangling style argument can be used
to choose an appropriate demangling style for your compiler.  The linker will
demangle by default unless the environment variable @samp{COLLECT_NO_DEMANGLE}
is set.  These options may be used to override the default.

@cindex dynamic linker, from command line
@kindex -I@var{file}
@kindex --dynamic-linker @var{file}
@item --dynamic-linker @var{file}
Set the name of the dynamic linker.  This is only meaningful when
generating dynamically linked ELF executables.  The default dynamic
linker is normally correct; don't use this unless you know what you are
doing.

@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 --fatal-warnings
@item --fatal-warnings
Treat all warnings as errors.

@kindex --force-exe-suffix
@item  --force-exe-suffix
Make sure that an output file has a .exe suffix.

If a successfully built fully linked output file does not have a
@code{.exe} or @code{.dll} suffix, this option forces the linker to copy
the output file to one of the same name with a @code{.exe} suffix. This
option is useful when using unmodified Unix makefiles on a Microsoft
Windows host, since some versions of Windows won't run an image unless
it ends in a @code{.exe} suffix.

@kindex --gc-sections
@kindex --no-gc-sections
@cindex garbage collection
@item --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
@command{ld} normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory.  This option tells @command{ld} to
instead optimize for memory usage, by rereading the symbol tables as
necessary.  This may be required if @command{ld} runs out of memory space
while linking a large executable.

@kindex --no-undefined
@kindex -z defs
@item --no-undefined
@itemx -z defs
Normally when creating a non-symbolic shared library, undefined symbols
are allowed and left to be resolved by the runtime loader.  This option
disallows such undefined symbols if they come from regular object
files.  The switch @samp{--no-allow-shlib-undefined} controls the
behaviour for shared objects being linked into the shared library.

@kindex --allow-multiple-definition
@kindex -z muldefs
@item --allow-multiple-definition
@itemx -z muldefs
Normally when a symbol is defined multiple times, the linker will
report a fatal error. These options allow multiple definitions and the
first definition will be used.

@kindex --allow-shlib-undefined
@kindex --no-allow-shlib-undefined
@item --allow-shlib-undefined
@itemx --no-allow-shlib-undefined
Allow (the default) or disallow undefined symbols in shared objects.
The setting of this switch overrides @samp{--no-undefined} where
shared objects are concerned.  Thus if @samp{--no-undefined} is set
but @samp{--no-allow-shlib-undefined} is not, the net result will be
that undefined symbols in regular object files will trigger an error,
but undefined symbols in shared objects will be ignored.

The reason that @samp{--allow-shlib-undefined} is the default is that
the shared object being specified at link time may not be the same one
that is available at load time, so the symbols might actually be
resolvable at load time.  Plus there are some systems, (eg BeOS) where
undefined symbols in shared libraries is normal since the kernel
patches them at load time to select which function is most appropriate
for the current architecture. eg. to dynamically select an appropriate
memset function.  Apparently it is also normal for HPPA shared
libraries to have undefined symbols.

@kindex --no-undefined-version
@item --no-undefined-version
Normally when a symbol has an undefined version, the linker will ignore
it. This option disallows symbols with undefined version and a fatal error
will be issued instead.

@kindex --no-warn-mismatch
@item --no-warn-mismatch
Normally @command{ld} will give an error if you try to link together input
files that are mismatched for some reason, perhaps because they have
been compiled for different processors or for different endiannesses.
This option tells @command{ld} that it should silently permit such possible
errors.  This option should only be used with care, in cases when you
have taken some special action that ensures that the linker errors are
inappropriate.

@kindex --no-whole-archive
@item --no-whole-archive
Turn off the effect of the @option{--whole-archive} option for subsequent
archive files.

@cindex output file after errors
@kindex --noinhibit-exec
@item --noinhibit-exec
Retain the executable output file whenever it is still usable.
Normally, the linker will not produce an output file if it encounters
errors during the link process; it exits without writing an output file
when it issues any error whatsoever.

@kindex -nostdlib
@item -nostdlib
Only search library directories explicitly specified on the
command line.  Library directories specified in linker scripts
(including linker scripts specified on the command line) are ignored.

@ifclear SingleFormat
@kindex --oformat
@item --oformat @var{output-format}
@command{ld} may be configured to support more than one kind of object
file.  If your @command{ld} is configured this way, you can use the
@samp{--oformat} option to specify the binary format for the output
object file.  Even when @command{ld} is configured to support alternative
object formats, you don't usually need to specify this, as @command{ld}
should be configured to produce as a default output format the most
usual format on each machine.  @var{output-format} is a text string, the
name of a particular format supported by the BFD libraries.  (You can
list the available binary formats with @samp{objdump -i}.)  The script
command @code{OUTPUT_FORMAT} can also specify the output format, but
this option overrides it.  @xref{BFD}.
@end ifclear

@kindex -qmagic
@item -qmagic
This option is ignored for Linux compatibility.

@kindex -Qy
@item -Qy
This option is ignored for SVR4 compatibility.

@kindex --relax
@cindex synthesizing linker
@cindex relaxing addressing modes
@item --relax
An option with machine dependent effects.
@ifset GENERIC
This option is only supported on a few targets.
@end ifset
@ifset H8300
@xref{H8/300,,@command{ld} and the H8/300}.
@end ifset
@ifset I960
@xref{i960,, @command{ld} and the Intel 960 family}.
@end ifset
@ifset XTENSA
@xref{Xtensa,, @command{ld} and Xtensa Processors}.
@end ifset

On some platforms, the @samp{--relax} option performs global
optimizations that become possible when the linker resolves addressing
in the program, such as relaxing address modes and synthesizing new
instructions in the output object file.

On some platforms these link time global optimizations may make symbolic
debugging of the resulting executable impossible.
@ifset GENERIC
This is known to be
the case for the Matsushita MN10200 and MN10300 family of processors.
@end ifset

@ifset GENERIC
On platforms where this is not supported, @samp{--relax} is accepted,
but ignored.
@end ifset

@cindex retaining specified symbols
@cindex stripping all but some symbols
@cindex symbols, retaining selectively
@item --retain-symbols-file @var{filename}
Retain @emph{only} the symbols listed in the file @var{filename},
discarding all others.  @var{filename} is simply a flat file, with one
symbol name per line.  This option is especially useful in environments
@ifset GENERIC
(such as VxWorks)
@end ifset
where a large global symbol table is accumulated gradually, to conserve
run-time memory.

@samp{--retain-symbols-file} does @emph{not} discard undefined symbols,
or symbols needed for relocations.

You may only specify @samp{--retain-symbols-file} once in the command
line.  It overrides @samp{-s} and @samp{-S}.

@ifset GENERIC
@item -rpath @var{dir}
@cindex runtime library search path
@kindex -rpath
Add a directory to the runtime library search path.  This is used when
linking an ELF executable with shared objects.  All @option{-rpath}
arguments are concatenated and passed to the runtime linker, which uses
them to locate shared objects at runtime.  The @option{-rpath} option is
also used when locating shared objects which are needed by shared
objects explicitly included in the link; see the description of the
@option{-rpath-link} option.  If @option{-rpath} is not used when linking an
ELF executable, the contents of the environment variable
@code{LD_RUN_PATH} will be used if it is defined.

The @option{-rpath} option may also be used on SunOS.  By default, on
SunOS, the linker will form a runtime search patch out of all the
@option{-L} options it is given.  If a @option{-rpath} option is used, the
runtime search path will be formed exclusively using the @option{-rpath}
options, ignoring the @option{-L} options.  This can be useful when using
gcc, which adds many @option{-L} options which may be on NFS mounted
filesystems.

For compatibility with other ELF linkers, if the @option{-R} option is
followed by a directory name, rather than a file name, it is treated as
the @option{-rpath} option.
@end ifset

@ifset GENERIC
@cindex link-time runtime library search path
@kindex -rpath-link
@item -rpath-link @var{DIR}
When using ELF or SunOS, one shared library may require another.  This
happens when an @code{ld -shared} link includes a shared library as one
of the input files.

When the linker encounters such a dependency when doing a non-shared,
non-relocatable link, it will automatically try to locate the required
shared library and include it in the link, if it is not included
explicitly.  In such a case, the @option{-rpath-link} option
specifies the first set of directories to search.  The
@option{-rpath-link} option may specify a sequence of directory names
either by specifying a list of names separated by colons, or by
appearing multiple times.

This option should be used with caution as it overrides the search path
that may have been hard compiled into a shared library. In such a case it
is possible to use unintentionally a different search path than the
runtime linker would do.

The linker uses the following search paths to locate required shared
libraries.
@enumerate
@item
Any directories specified by @option{-rpath-link} options.
@item
Any directories specified by @option{-rpath} options.  The difference
between @option{-rpath} and @option{-rpath-link} is that directories
specified by @option{-rpath} options are included in the executable and
used at runtime, whereas the @option{-rpath-link} option is only effective
at link time. It is for the native linker only.
@item
On an ELF system, if the @option{-rpath} and @code{rpath-link} options
were not used, search the contents of the environment variable
@code{LD_RUN_PATH}. It is for the native linker only.
@item
On SunOS, if the @option{-rpath} option was not used, search any
directories specified using @option{-L} options.
@item
For a native linker, the contents of the environment variable
@code{LD_LIBRARY_PATH}.
@item
For a native ELF linker, the directories in @code{DT_RUNPATH} or
@code{DT_RPATH} of a shared library are searched for shared
libraries needed by it. The @code{DT_RPATH} entries are ignored if
@code{DT_RUNPATH} entries exist.
@item
The default directories, normally @file{/lib} and @file{/usr/lib}.
@item
For a native linker on an ELF system, if the file @file{/etc/ld.so.conf}
exists, the list of directories found in that file.
@end enumerate

If the required shared library is not found, the linker will issue a
warning and continue with the link.
@end ifset

@kindex -shared
@kindex -Bshareable
@item -shared
@itemx -Bshareable
@cindex shared libraries
Create a shared library.  This is currently only supported on ELF, XCOFF
and SunOS platforms.  On SunOS, the linker will automatically create a
shared library if the @option{-e} option is not used and there are
undefined symbols in the link.

@item --sort-common
@kindex --sort-common
This option tells @command{ld} to sort the common symbols by size when it
places them in the appropriate output sections.  First come all the one
byte symbols, then all the two byte, then all the four byte, and then
everything else.  This is to prevent gaps between symbols due to
alignment constraints.

@kindex --split-by-file
@item --split-by-file [@var{size}]
Similar to @option{--split-by-reloc} but creates a new output section for
each input file when @var{size} is reached.  @var{size} defaults to a
size of 1 if not given.

@kindex --split-by-reloc
@item --split-by-reloc [@var{count}]
Tries to creates extra sections in the output file so that no single
output section in the file contains more than @var{count} relocations.
This is useful when generating huge relocatable files for downloading into
certain real time kernels with the COFF object file format; since COFF
cannot represent more than 65535 relocations in a single section.  Note
that this will fail to work with object file formats which do not
support arbitrary sections.  The linker will not split up individual
input sections for redistribution, so if a single input section contains
more than @var{count} relocations one output section will contain that
many relocations.  @var{count} defaults to a value of 32768.

@kindex --stats
@item --stats
Compute and display statistics about the operation of the linker, such
as execution time and memory usage.

@kindex --traditional-format
@cindex traditional format
@item --traditional-format
For some targets, the output of @command{ld} is different in some ways from
the output of some existing linker.  This switch requests @command{ld} to
use the traditional format instead.

@cindex dbx
For example, on SunOS, @command{ld} combines duplicate entries in the
symbol string table.  This can reduce the size of an output file with
full debugging information by over 30 percent.  Unfortunately, the SunOS
@code{dbx} program can not read the resulting program (@code{gdb} has no
trouble).  The @samp{--traditional-format} switch tells @command{ld} to not
combine duplicate entries.

@kindex --section-start @var{sectionname}=@var{org}
@item --section-start @var{sectionname}=@var{org}
Locate a section in the output file at the absolute
address given by @var{org}.  You may use this option as many
times as necessary to locate multiple sections in the command
line.
@var{org} must be a single hexadecimal integer;
for compatibility with other linkers, you may omit the leading
@samp{0x} usually associated with hexadecimal values.  @emph{Note:} there
should be no white space between @var{sectionname}, the equals
sign (``@key{=}''), and @var{org}.

@kindex -Tbss @var{org}
@kindex -Tdata @var{org}
@kindex -Ttext @var{org}
@cindex segment origins, cmd line
@item -Tbss @var{org}
@itemx -Tdata @var{org}
@itemx -Ttext @var{org}
Same as --section-start, with @code{.bss}, @code{.data} or
@code{.text} as the @var{sectionname}.

@kindex --verbose
@cindex verbose
@item --dll-verbose
@itemx --verbose
Display the version number for @command{ld} and list the linker emulations
supported.  Display which input files can and cannot be opened.  Display
the linker script being used by the linker.

@kindex --version-script=@var{version-scriptfile}
@cindex version script, symbol versions
@itemx --version-script=@var{version-scriptfile}
Specify the name of a version script to the linker.  This is typically
used when creating shared libraries to specify additional information
about the version hierarchy for the library being created.  This option
is only meaningful on ELF platforms which support shared libraries.
@xref{VERSION}.

@kindex --warn-common
@cindex warnings, on combining symbols
@cindex combining symbols, warnings on
@item --warn-common
Warn when a common symbol is combined with another common symbol or with
a symbol definition.  Unix linkers allow this somewhat sloppy 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
@option{--whole-archive} option, include every object file in the archive
in the link, rather than searching the archive for the required object
files.  This is normally used to turn an archive file into a shared
library, forcing every object to be included in the resulting shared
library.  This option may be used more than once.

Two notes when using this option from gcc: First, gcc doesn't know
about this option, so you have to use @option{-Wl,-whole-archive}.
Second, don't forget to use @option{-Wl,-no-whole-archive} after your
list of archives, because gcc will add its own list of archives to
your link and you may not want this flag to affect those as well.

@kindex --wrap
@item --wrap @var{symbol}
Use a wrapper function for @var{symbol}.  Any undefined reference to
@var{symbol} will be resolved to @code{__wrap_@var{symbol}}.  Any
undefined reference to @code{__real_@var{symbol}} will be resolved to
@var{symbol}.

This can be used to provide a wrapper for a system function.  The
wrapper function should be called @code{__wrap_@var{symbol}}.  If it
wishes to call the system function, it should call
@code{__real_@var{symbol}}.

Here is a trivial example:

@smallexample
void *
__wrap_malloc (int c)
@{
  printf ("malloc called with %ld\n", c);
  return __real_malloc (c);
@}
@end smallexample

If you link other code with this file using @option{--wrap malloc}, then
all calls to @code{malloc} will call the function @code{__wrap_malloc}
instead.  The call to @code{__real_malloc} in @code{__wrap_malloc} will
call the real @code{malloc} function.

You may wish to provide a @code{__real_malloc} function as well, so that
links without the @option{--wrap} option will succeed.  If you do this,
you should not put the definition of @code{__real_malloc} in the same
file as @code{__wrap_malloc}; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to @code{malloc}.

@kindex --enable-new-dtags
@kindex --disable-new-dtags
@item --enable-new-dtags
@itemx --disable-new-dtags
This linker can create the new dynamic tags in ELF. But the older ELF
systems may not understand them. If you specify
@option{--enable-new-dtags}, the dynamic tags will be created as needed.
If you specify @option{--disable-new-dtags}, no new dynamic tags will be
created. By default, the new dynamic tags are not created. Note that
those options are only available for ELF systems.

@end table

@c man end

@subsection Options Specific to i386 PE Targets

@c man begin OPTIONS

The i386 PE linker supports the @option{-shared} option, which causes
the output to be a dynamically linked library (DLL) instead of a
normal executable.  You should name the output @code{*.dll} when you
use this option.  In addition, the linker fully supports the standard
@code{*.def} files, which may be specified on the linker command line
like an object file (in fact, it should precede archives it exports
symbols from, to ensure that they get linked in, just like a normal
object file).

In addition to the options common to all targets, the i386 PE linker
support additional command line options that are specific to the i386
PE target.  Options that take values may be separated from their
values by either a space or an equals sign.

@table @gcctabopt

@kindex --add-stdcall-alias
@item --add-stdcall-alias
If given, symbols with a stdcall suffix (@@@var{nn}) will be exported
as-is and also with the suffix stripped.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --base-file
@item --base-file @var{file}
Use @var{file} as the name of a file in which to save the base
addresses of all the relocations needed for generating DLLs with
@file{dlltool}.
[This is an i386 PE specific option]

@kindex --dll
@item --dll
Create a DLL instead of a regular executable.  You may also use
@option{-shared} or specify a @code{LIBRARY} in a given @code{.def}
file.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --enable-stdcall-fixup
@kindex --disable-stdcall-fixup
@item --enable-stdcall-fixup
@itemx --disable-stdcall-fixup
If the link finds a symbol that it cannot resolve, it will attempt to
do ``fuzzy linking'' by looking for another defined symbol that differs
only in the format of the symbol name (cdecl vs stdcall) and will
resolve that symbol by linking to the match.  For example, the
undefined symbol @code{_foo} might be linked to the function
@code{_foo@@12}, or the undefined symbol @code{_bar@@16} might be linked
to the function @code{_bar}.  When the linker does this, it prints a
warning, since it normally should have failed to link, but sometimes
import libraries generated from third-party dlls may need this feature
to be usable.  If you specify @option{--enable-stdcall-fixup}, this
feature is fully enabled and warnings are not printed.  If you specify
@option{--disable-stdcall-fixup}, this feature is disabled and such
mismatches are considered to be errors.
[This option is specific to the i386 PE targeted port of the linker]

@cindex DLLs, creating
@kindex --export-all-symbols
@item --export-all-symbols
If given, all global symbols in the objects used to build a DLL will
be exported by the DLL.  Note that this is the default if there
otherwise wouldn't be any exported symbols.  When symbols are
explicitly exported via DEF files or implicitly exported via function
attributes, the default is to not export anything else unless this
option is given.  Note that the symbols @code{DllMain@@12},
@code{DllEntryPoint@@0}, @code{DllMainCRTStartup@@12}, and 
@code{impure_ptr} will not be automatically
exported.  Also, symbols imported from other DLLs will not be 
re-exported, nor will symbols specifying the DLL's internal layout 
such as those beginning with @code{_head_} or ending with 
@code{_iname}.  In addition, no symbols from @code{libgcc}, 
@code{libstd++}, @code{libmingw32}, or @code{crtX.o} will be exported.
Symbols whose names begin with @code{__rtti_} or @code{__builtin_} will
not be exported, to help with C++ DLLs.  Finally, there is an
extensive list of cygwin-private symbols that are not exported 
(obviously, this applies on when building DLLs for cygwin targets).
These cygwin-excludes are: @code{_cygwin_dll_entry@@12}, 
@code{_cygwin_crt0_common@@8}, @code{_cygwin_noncygwin_dll_entry@@12},
@code{_fmode}, @code{_impure_ptr}, @code{cygwin_attach_dll}, 
@code{cygwin_premain0}, @code{cygwin_premain1}, @code{cygwin_premain2},
@code{cygwin_premain3}, and @code{environ}. 
[This option is specific to the i386 PE targeted port of the linker]

@kindex --exclude-symbols
@item --exclude-symbols @var{symbol},@var{symbol},...
Specifies a list of symbols which should not be automatically
exported.  The symbol names may be delimited by commas or colons.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --exclude-libs
@item --exclude-libs @var{lib},@var{lib},...
Specifies a list of archive libraries from which symbols should not be automatically
exported. The library names may be delimited by commas or colons.  Specifying
@code{--exclude-libs ALL} excludes symbols in all archive libraries from
automatic export. Symbols explicitly listed in a .def file are still exported,
regardless of this option. 
[This option is specific to the i386 PE targeted port of the linker]

@kindex --file-alignment
@item --file-alignment
Specify the file alignment.  Sections in the file will always begin at
file offsets which are multiples of this number.  This defaults to
512.
[This option is specific to the i386 PE targeted port of the linker]

@cindex heap size
@kindex --heap
@item --heap @var{reserve}
@itemx --heap @var{reserve},@var{commit}
Specify the amount of memory to reserve (and optionally commit) to be
used as heap for this program.  The default is 1Mb reserved, 4K
committed.
[This option is specific to the i386 PE targeted port of the linker]

@cindex image base
@kindex --image-base
@item --image-base @var{value}
Use @var{value} as the base address of your program or dll.  This is
the lowest memory location that will be used when your program or dll
is loaded.  To reduce the need to relocate and improve performance of
your dlls, each should have a unique base address and not overlap any
other dlls.  The default is 0x400000 for executables, and 0x10000000
for dlls.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --kill-at
@item --kill-at
If given, the stdcall suffixes (@@@var{nn}) will be stripped from
symbols before they are exported.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --major-image-version
@item --major-image-version @var{value}
Sets the major number of the ``image version''.  Defaults to 1.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --major-os-version
@item --major-os-version @var{value}
Sets the major number of the ``os version''.  Defaults to 4.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --major-subsystem-version
@item --major-subsystem-version @var{value}
Sets the major number of the ``subsystem version''.  Defaults to 4.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --minor-image-version
@item --minor-image-version @var{value}
Sets the minor number of the ``image version''.  Defaults to 0.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --minor-os-version
@item --minor-os-version @var{value}
Sets the minor number of the ``os version''.  Defaults to 0.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --minor-subsystem-version
@item --minor-subsystem-version @var{value}
Sets the minor number of the ``subsystem version''.  Defaults to 0.
[This option is specific to the i386 PE targeted port of the linker]

@cindex DEF files, creating
@cindex DLLs, creating
@kindex --output-def
@item --output-def @var{file}
The linker will create the file @var{file} which will contain a DEF
file corresponding to the DLL the linker is generating.  This DEF file
(which should be called @code{*.def}) may be used to create an import
library with @code{dlltool} or may be used as a reference to
automatically or implicitly exported symbols.
[This option is specific to the i386 PE targeted port of the linker]

@cindex DLLs, creating
@kindex --out-implib
@item --out-implib @var{file}
The linker will create the file @var{file} which will contain an
import lib corresponding to the DLL the linker is generating. This
import lib (which should be called @code{*.dll.a} or @code{*.a}
may be used to link clients against the generated DLL; this behavior
makes it possible to skip a separate @code{dlltool} import library
creation step.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --enable-auto-image-base
@item --enable-auto-image-base
Automatically choose the image base for DLLs, unless one is specified
using the @code{--image-base} argument.  By using a hash generated
from the dllname to create unique image bases for each DLL, in-memory
collisions and relocations which can delay program execution are
avoided.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --disable-auto-image-base
@item --disable-auto-image-base
Do not automatically generate a unique image base.  If there is no
user-specified image base (@code{--image-base}) then use the platform
default.
[This option is specific to the i386 PE targeted port of the linker]

@cindex DLLs, linking to
@kindex --dll-search-prefix
@item --dll-search-prefix @var{string}
When linking dynamically to a dll without an import library,
search for @code{<string><basename>.dll} in preference to 
@code{lib<basename>.dll}. This behavior allows easy distinction
between DLLs built for the various "subplatforms": native, cygwin,
uwin, pw, etc.  For instance, cygwin DLLs typically use
@code{--dll-search-prefix=cyg}. 
[This option is specific to the i386 PE targeted port of the linker]

@kindex --enable-auto-import
@item --enable-auto-import
Do sophisticated linking of @code{_symbol} to @code{__imp__symbol} for 
DATA imports from DLLs, and create the necessary thunking symbols when 
building the import libraries with those DATA exports.  This generally 
will 'just work' -- but sometimes you may see this message:

"variable '<var>' can't be auto-imported. Please read the 
documentation for ld's @code{--enable-auto-import} for details."

This message occurs when some (sub)expression accesses an address 
ultimately given by the sum of two constants (Win32 import tables only 
allow one).  Instances where this may occur include accesses to member 
fields of struct variables imported from a DLL, as well as using a 
constant index into an array variable imported from a DLL.  Any 
multiword variable (arrays, structs, long long, etc) may trigger
this error condition.  However, regardless of the exact data type
of the offending exported variable, ld will always detect it, issue
the warning, and exit.

There are several ways to address this difficulty, regardless of the
data type of the exported variable:

One way is to use --enable-runtime-pseudo-reloc switch. This leaves the task
of adjusting references in your client code for runtime environment, so
this method works only when runtime environtment supports this feature.

A second solution is to force one of the 'constants' to be a variable -- 
that is, unknown and un-optimizable at compile time.  For arrays, 
there are two possibilities: a) make the indexee (the array's address) 
a variable, or b) make the 'constant' index a variable.  Thus:

@example
extern type extern_array[];
extern_array[1] --> 
   @{ volatile type *t=extern_array; t[1] @}
@end example

or

@example
extern type extern_array[];
extern_array[1] --> 
   @{ volatile int t=1; extern_array[t] @}
@end example

For structs (and most other multiword data types) the only option 
is to make the struct itself (or the long long, or the ...) variable:

@example
extern struct s extern_struct;
extern_struct.field --> 
   @{ volatile struct s *t=&extern_struct; t->field @}
@end example

or

@example
extern long long extern_ll;
extern_ll -->
  @{ volatile long long * local_ll=&extern_ll; *local_ll @}
@end example

A third method of dealing with this difficulty is to abandon
'auto-import' for the offending symbol and mark it with 
@code{__declspec(dllimport)}.  However, in practice that
requires using compile-time #defines to indicate whether you are
building a DLL, building client code that will link to the DLL, or 
merely building/linking to a static library.   In making the choice 
between the various methods of resolving the 'direct address with 
constant offset' problem, you should consider typical real-world usage:

Original:
@example
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv)@{
  printf("%d\n",arr[1]);
@}
@end example

Solution 1:
@example
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv)@{
  /* This workaround is for win32 and cygwin; do not "optimize" */
  volatile int *parr = arr;
  printf("%d\n",parr[1]);
@}
@end example

Solution 2:
@example
--foo.h
/* Note: auto-export is assumed (no __declspec(dllexport)) */
#if (defined(_WIN32) || defined(__CYGWIN__)) && \
  !(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
#define FOO_IMPORT __declspec(dllimport)
#else
#define FOO_IMPORT
#endif
extern FOO_IMPORT int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv)@{
  printf("%d\n",arr[1]);
@}
@end example

A fourth way to avoid this problem is to re-code your 
library to use a functional interface rather than a data interface
for the offending variables (e.g. set_foo() and get_foo() accessor
functions).
[This option is specific to the i386 PE targeted port of the linker]

@kindex --disable-auto-import
@item --disable-auto-import
Do not attempt to do sophisticalted linking of @code{_symbol} to 
@code{__imp__symbol} for DATA imports from DLLs.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --enable-runtime-pseudo-reloc
@item --enable-runtime-pseudo-reloc
If your code contains expressions described in --enable-auto-import section,
that is, DATA imports from DLL with non-zero offset, this switch will create
a vector of 'runtime pseudo relocations' which can be used by runtime
environment to adjust references to such data in your client code. 
[This option is specific to the i386 PE targeted port of the linker]

@kindex --disable-runtime-pseudo-reloc
@item --disable-runtime-pseudo-reloc
Do not create pseudo relocations for non-zero offset DATA imports from
DLLs.  This is the default.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --enable-extra-pe-debug
@item --enable-extra-pe-debug
Show additional debug info related to auto-import symbol thunking.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --section-alignment
@item --section-alignment
Sets the section alignment.  Sections in memory will always begin at
addresses which are a multiple of this number.  Defaults to 0x1000.
[This option is specific to the i386 PE targeted port of the linker]

@cindex stack size
@kindex --stack
@item --stack @var{reserve}
@itemx --stack @var{reserve},@var{commit}
Specify the amount of memory to reserve (and optionally commit) to be
used as stack for this program.  The default is 2Mb reserved, 4K
committed.
[This option is specific to the i386 PE targeted port of the linker]

@kindex --subsystem
@item --subsystem @var{which}
@itemx --subsystem @var{which}:@var{major}
@itemx --subsystem @var{which}:@var{major}.@var{minor}
Specifies the subsystem under which your program will execute.  The
legal values for @var{which} are @code{native}, @code{windows},
@code{console}, and @code{posix}.  You may optionally set the
subsystem version also.
[This option is specific to the i386 PE targeted port of the linker]

@end table

@c man end

@ifset UsesEnvVars
@node Environment
@section Environment Variables

@c man begin ENVIRONMENT

You can change the behavior of @command{ld} with the environment variables
@ifclear SingleFormat
@code{GNUTARGET},
@end ifclear
@code{LDEMULATION} and @code{COLLECT_NO_DEMANGLE}.

@ifclear SingleFormat
@kindex GNUTARGET
@cindex default input format
@code{GNUTARGET} determines the input-file object format if you don't
use @samp{-b} (or its synonym @samp{--format}).  Its value should be one
of the BFD names for an input format (@pxref{BFD}).  If there is no
@code{GNUTARGET} in the environment, @command{ld} uses the natural format
of the target. If @code{GNUTARGET} is set to @code{default} then BFD
attempts to discover the input format by examining binary input files;
this method often succeeds, but there are potential ambiguities, since
there is no method of ensuring that the magic number used to specify
object-file formats is unique.  However, the configuration procedure for
BFD on each system places the conventional format for that system first
in the search-list, so ambiguities are resolved in favor of convention.
@end ifclear

@kindex LDEMULATION
@cindex default emulation
@cindex emulation, default
@code{LDEMULATION} determines the default emulation if you don't use the
@samp{-m} option.  The emulation can affect various aspects of linker
behaviour, particularly the default linker script.  You can list the
available emulations with the @samp{--verbose} or @samp{-V} options.  If
the @samp{-m} option is not used, and the @code{LDEMULATION} environment
variable is not defined, the default emulation depends upon how the
linker was configured.

@kindex COLLECT_NO_DEMANGLE
@cindex demangling, default
Normally, the linker will default to demangling symbols.  However, if
@code{COLLECT_NO_DEMANGLE} is set in the environment, then it will
default to not demangling symbols.  This environment variable is used in
a similar fashion by the @code{gcc} linker wrapper program.  The default
may be overridden by the @samp{--demangle} and @samp{--no-demangle}
options.

@c man end
@end ifset

@node Scripts
@chapter Linker Scripts

@cindex scripts
@cindex linker scripts
@cindex command files
Every link is controlled by a @dfn{linker script}.  This script is
written in the linker command language.

The main purpose of the linker script is to describe how the sections in
the input files should be mapped into the output file, and to control
the memory layout of the output file.  Most linker scripts do nothing
more than this.  However, when necessary, the linker script can also
direct the linker to perform many other operations, using the commands
described below.

The linker always uses a linker script.  If you do not supply one
yourself, the linker will use a default script that is compiled into the
linker executable.  You can use the @samp{--verbose} command line option
to display the default linker script.  Certain command line options,
such as @samp{-r} or @samp{-N}, will affect the default linker script.

You may supply your own linker script by using the @samp{-T} command
line option.  When you do this, your linker script will replace the
default linker script.

You may also use linker scripts implicitly by naming them as input files
to the linker, as though they were files to be linked.  @xref{Implicit
Linker Scripts}.

@menu
* Basic Script Concepts::	Basic Linker Script Concepts
* Script Format::		Linker Script Format
* Simple Example::		Simple Linker Script Example
* Simple Commands::		Simple Linker Script Commands
* Assignments::			Assigning Values to Symbols
* SECTIONS::			SECTIONS Command
* MEMORY::			MEMORY Command
* PHDRS::			PHDRS Command
* VERSION::			VERSION Command
* Expressions::			Expressions in Linker Scripts
* Implicit Linker Scripts::	Implicit Linker Scripts
@end menu

@node Basic Script Concepts
@section Basic Linker Script Concepts
@cindex linker script concepts
We need to define some basic concepts and vocabulary in order to
describe the linker script language.

The linker combines input files into a single output file.  The output
file and each input file are in a special data format known as an
@dfn{object file format}.  Each file is called an @dfn{object file}.
The output file is often called an @dfn{executable}, but for our
purposes we will also call it an object file.  Each object file has,
among other things, a list of @dfn{sections}.  We sometimes refer to a
section in an input file as an @dfn{input section}; similarly, a section
in the output file is an @dfn{output section}.

Each section in an object file has a name and a size.  Most sections
also have an associated block of data, known as the @dfn{section
contents}.  A section may be marked as @dfn{loadable}, which mean that
the contents should be loaded into memory when the output file is run.
A section with no contents may be @dfn{allocatable}, which means that an
area in memory should be set aside, but nothing in particular should be
loaded there (in some cases this memory must be zeroed out).  A section
which is neither loadable nor allocatable typically contains some sort
of debugging information.

Every loadable or allocatable output section has two addresses.  The
first is the @dfn{VMA}, or virtual memory address.  This is the address
the section will have when the output file is run.  The second is the
@dfn{LMA}, or load memory address.  This is the address at which the
section will be loaded.  In most cases the two addresses will be the
same.  An example of when they might be different is when a data section
is loaded into ROM, and then copied into RAM when the program starts up
(this technique is often used to initialize global variables in a ROM
based system).  In this case the ROM address would be the LMA, and the
RAM address would be the VMA.

You can see the sections in an object file by using the @code{objdump}
program with the @samp{-h} option.

Every object file also has a list of @dfn{symbols}, known as the
@dfn{symbol table}.  A symbol may be defined or undefined.  Each symbol
has a name, and each defined symbol has an address, among other
information.  If you compile a C or C++ program into an object file, you
will get a defined symbol for every defined function and global or
static variable.  Every undefined function or global variable which is
referenced in the input file will become an undefined symbol.

You can see the symbols in an object file by using the @code{nm}
program, or by using the @code{objdump} program with the @samp{-t}
option.

@node Script Format
@section Linker Script Format
@cindex linker script format
Linker scripts are text files.

You write a linker script as a series of commands.  Each command is
either a keyword, possibly followed by arguments, or an assignment to a
symbol.  You may separate commands using semicolons.  Whitespace is
generally ignored.

Strings such as file or format names can normally be entered directly.
If the file name contains a character such as a comma which would
otherwise serve to separate file names, you may put the file name in
double quotes.  There is no way to use a double quote character in a
file name.

You may include comments in linker scripts just as in C, delimited by
@samp{/*} and @samp{*/}.  As in C, comments are syntactically equivalent
to whitespace.

@node Simple Example
@section Simple Linker Script Example
@cindex linker script example
@cindex example of linker script
Many linker scripts are fairly simple.

The simplest possible linker script has just one command:
@samp{SECTIONS}.  You use the @samp{SECTIONS} command to describe the
memory layout of the output file.

The @samp{SECTIONS} command is a powerful command.  Here we will
describe a simple use of it.  Let's assume your program consists only of
code, initialized data, and uninitialized data.  These will be in the
@samp{.text}, @samp{.data}, and @samp{.bss} sections, respectively.
Let's assume further that these are the only sections which appear in
your input files.

For this example, let's say that the code should be loaded at address
0x10000, and that the data should start at address 0x8000000.  Here is a
linker script which will do that:
@smallexample
SECTIONS
@{
  . = 0x10000;
  .text : @{ *(.text) @}
  . = 0x8000000;
  .data : @{ *(.data) @}
  .bss : @{ *(.bss) @}
@}
@end smallexample

You write the @samp{SECTIONS} command as the keyword @samp{SECTIONS},
followed by a series of symbol assignments and output section
descriptions enclosed in curly braces.

The first line inside the @samp{SECTIONS} command of the above example
sets the value of the special symbol @samp{.}, which is the location
counter.  If you do not specify the address of an output section in some
other way (other ways are described later), the address is set from the
current value of the location counter.  The location counter is then
incremented by the size of the output section.  At the start of the
@samp{SECTIONS} command, the location counter has the value @samp{0}.

The second line defines an output section, @samp{.text}.  The colon is
required syntax which may be ignored for now.  Within the curly braces
after the output section name, you list the names of the input sections
which should be placed into this output section.  The @samp{*} is a
wildcard which matches any file name.  The expression @samp{*(.text)}
means all @samp{.text} input sections in all input files.

Since the location counter is @samp{0x10000} when the output section
@samp{.text} is defined, the linker will set the address of the
@samp{.text} section in the output file to be @samp{0x10000}.

The remaining lines define the @samp{.data} and @samp{.bss} sections in
the output file.  The linker will place the @samp{.data} output section
at address @samp{0x8000000}.  After the linker places the @samp{.data}
output section, the value of the location counter will be
@samp{0x8000000} plus the size of the @samp{.data} output section.  The
effect is that the linker will place the @samp{.bss} output section
immediately after the @samp{.data} output section in memory

The linker will ensure that each output section has the required
alignment, by increasing the location counter if necessary.  In this
example, the specified addresses for the @samp{.text} and @samp{.data}
sections will probably satisfy any alignment constraints, but the linker
may have to create a small gap between the @samp{.data} and @samp{.bss}
sections.

That's it!  That's a simple and complete linker script.

@node Simple Commands
@section Simple Linker Script Commands
@cindex linker script simple commands
In this section we describe the simple linker script commands.

@menu
* Entry Point::			Setting the entry point
* File Commands::		Commands dealing with files
@ifclear SingleFormat
* Format Commands::		Commands dealing with object file formats
@end ifclear

* Miscellaneous Commands::	Other linker script commands
@end menu

@node Entry Point
@subsection Setting the Entry Point
@kindex ENTRY(@var{symbol})
@cindex start of execution
@cindex first instruction
@cindex entry point
The first instruction to execute in a program is called the @dfn{entry
point}.  You can use the @code{ENTRY} linker script command to set the
entry point.  The argument is a symbol name:
@smallexample
ENTRY(@var{symbol})
@end smallexample

There are several ways to set the entry point.  The linker will set the
entry point by trying each of the following methods in order, and
stopping when one of them succeeds:
@itemize @bullet
@item
the @samp{-e} @var{entry} command-line option;
@item
the @code{ENTRY(@var{symbol})} command in a linker script;
@item
the value of the symbol @code{start}, if defined;
@item
the address of the first byte of the @samp{.text} section, if present;
@item
The address @code{0}.
@end itemize

@node File Commands
@subsection Commands Dealing with Files
@cindex linker script file commands
Several linker script commands deal with files.

@table @code
@item INCLUDE @var{filename}
@kindex INCLUDE @var{filename}
@cindex including a linker script
Include the linker script @var{filename} at this point.  The file will
be searched for in the current directory, and in any directory specified
with the @option{-L} option.  You can nest calls to @code{INCLUDE} up to
10 levels deep.

@item INPUT(@var{file}, @var{file}, @dots{})
@itemx INPUT(@var{file} @var{file} @dots{})
@kindex INPUT(@var{files})
@cindex input files in linker scripts
@cindex input object files in linker scripts
@cindex linker script input object files
The @code{INPUT} command directs the linker to include the named files
in the link, as though they were named on the command line.

For example, if you always want to include @file{subr.o} any time you do
a link, but you can't be bothered to put it on every link command line,
then you can put @samp{INPUT (subr.o)} in your linker script.

In fact, if you like, you can list all of your input files in the linker
script, and then invoke the linker with nothing but a @samp{-T} option.

In case a @dfn{sysroot prefix} is configured, and the filename starts
with the @samp{/} character, and the script being processed was
located inside the @dfn{sysroot prefix}, the filename will be looked
for in the @dfn{sysroot prefix}.  Otherwise, the linker will try to
open the file in the current directory.  If it is not found, the
linker will search through the archive library search path.  See the
description of @samp{-L} in @ref{Options,,Command Line Options}.

If you use @samp{INPUT (-l@var{file})}, @command{ld} will transform the
name to @code{lib@var{file}.a}, as with the command line argument
@samp{-l}.

When you use the @code{INPUT} command in an implicit linker script, the
files will be included in the link at the point at which the linker
script file is included.  This can affect archive searching.

@item GROUP(@var{file}, @var{file}, @dots{})
@itemx GROUP(@var{file} @var{file} @dots{})
@kindex GROUP(@var{files})
@cindex grouping input files
The @code{GROUP} command is like @code{INPUT}, except that the named
files should all be archives, and they are searched repeatedly until no
new undefined references are created.  See the description of @samp{-(}
in @ref{Options,,Command Line Options}.

@item 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
@command{ld} looks for archive libraries.  Using
@code{SEARCH_DIR(@var{path})} is exactly like using @samp{-L @var{path}}
on the command line (@pxref{Options,,Command Line Options}).  If both
are used, then the linker will search both paths.  Paths specified using
the command line option are searched first.

@item STARTUP(@var{filename})
@kindex STARTUP(@var{filename})
@cindex first input file
The @code{STARTUP} command is just like the @code{INPUT} command, except
that @var{filename} will become the first input file to be linked, as
though it were specified first on the command line.  This may be useful
when using a system in which the entry point is always the start of the
first file.
@end table

@ifclear SingleFormat
@node Format Commands
@subsection Commands Dealing with Object File Formats
A couple of linker script commands deal with object file formats.

@table @code
@item OUTPUT_FORMAT(@var{bfdname})
@itemx OUTPUT_FORMAT(@var{default}, @var{big}, @var{little})
@kindex OUTPUT_FORMAT(@var{bfdname})
@cindex output file format in linker script
The @code{OUTPUT_FORMAT} command names the BFD format to use for the
output file (@pxref{BFD}).  Using @code{OUTPUT_FORMAT(@var{bfdname})} is
exactly like using @samp{--oformat @var{bfdname}} on the command line
(@pxref{Options,,Command Line Options}).  If both are used, the command
line option takes precedence.

You can use @code{OUTPUT_FORMAT} with three arguments to use different
formats based on the @samp{-EB} and @samp{-EL} command line options.
This permits the linker script to set the output format based on the
desired endianness.

If neither @samp{-EB} nor @samp{-EL} are used, then the output format
will be the first argument, @var{default}.  If @samp{-EB} is used, the
output format will be the second argument, @var{big}.  If @samp{-EL} is
used, the output format will be the third argument, @var{little}.

For example, the default linker script for the MIPS ELF target uses this
command:
@smallexample
OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
@end smallexample
This says that the default format for the output file is
@samp{elf32-bigmips}, but if the user uses the @samp{-EL} command line
option, the output file will be created in the @samp{elf32-littlemips}
format.

@item TARGET(@var{bfdname})
@kindex TARGET(@var{bfdname})
@cindex input file format in linker script
The @code{TARGET} command names the BFD format to use when reading input
files.  It affects subsequent @code{INPUT} and @code{GROUP} commands.
This command is like using @samp{-b @var{bfdname}} on the command line
(@pxref{Options,,Command Line Options}).  If the @code{TARGET} command
is used but @code{OUTPUT_FORMAT} is not, then the last @code{TARGET}
command is also used to set the format for the output file.  @xref{BFD}.
@end table
@end ifclear

@node Miscellaneous Commands
@subsection Other Linker Script Commands
There are a few other linker scripts commands.

@table @code
@item ASSERT(@var{exp}, @var{message})
@kindex ASSERT
@cindex assertion in linker script
Ensure that @var{exp} is non-zero.  If it is zero, then exit the linker
with an error code, and print @var{message}.

@item EXTERN(@var{symbol} @var{symbol} @dots{})
@kindex EXTERN
@cindex undefined symbol in linker script
Force @var{symbol} to be entered in the output file as an undefined
symbol.  Doing this may, for example, trigger linking of additional
modules from standard libraries.  You may list several @var{symbol}s for
each @code{EXTERN}, and you may use @code{EXTERN} multiple times.  This
command has the same effect as the @samp{-u} command-line option.

@item FORCE_COMMON_ALLOCATION
@kindex FORCE_COMMON_ALLOCATION
@cindex common allocation in linker script
This command has the same effect as the @samp{-d} command-line option:
to make @command{ld} assign space to common symbols even if a relocatable
output file is specified (@samp{-r}).

@item INHIBIT_COMMON_ALLOCATION
@kindex INHIBIT_COMMON_ALLOCATION
@cindex common allocation in linker script
This command has the same effect as the @samp{--no-define-common}
command-line option: to make @code{ld} omit the assignment of addresses
to common symbols even for a non-relocatable output file.

@item NOCROSSREFS(@var{section} @var{section} @dots{})
@kindex NOCROSSREFS(@var{sections})
@cindex cross references
This command may be used to tell @command{ld} to issue an error about any
references among certain output sections.

In certain types of programs, particularly on embedded systems when
using overlays, when one section is loaded into memory, another section
will not be.  Any direct references between the two sections would be
errors.  For example, it would be an error if code in one section called
a function defined in the other section.

The @code{NOCROSSREFS} command takes a list of output section names.  If
@command{ld} detects any cross references between the sections, it reports
an error and returns a non-zero exit status.  Note that the
@code{NOCROSSREFS} command uses output section names, not input section
names.

@ifclear SingleFormat
@item OUTPUT_ARCH(@var{bfdarch})
@kindex OUTPUT_ARCH(@var{bfdarch})
@cindex machine architecture
@cindex architecture
Specify a particular output machine architecture.  The argument is one
of the names used by the BFD library (@pxref{BFD}).  You can see the
architecture of an object file by using the @code{objdump} program with
the @samp{-f} option.
@end ifclear
@end table

@node Assignments
@section Assigning Values to Symbols
@cindex assignment in scripts
@cindex symbol definition, scripts
@cindex variables, defining
You may assign a value to a symbol in a linker script.  This will define
the symbol 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 Description
@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, appearing in the same order as
they are found in the linker input.  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)
    @}
@end group
@group
  outputb :
    @{
    foo.o (.input2)
    foo1.o (.input1)
    @}
@end group
@group
  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 value 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{0x90}:
@smallexample
FILL(0x90909090)
@end smallexample

The @code{FILL} command is similar to the @samp{=@var{fillexp}} output
section attribute, 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.  @xref{Output Section Fill}, for details on the fill
expression.

@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 value, repeated as
necessary.  If the fill expression is a simple hex number, ie. a string
of hex digit starting with @samp{0x} and without a trailing @samp{k} or @samp{M}, then
an arbitrarily long sequence of hex digits can be used to specify the
fill pattern;  Leading zeros become part of the pattern too.  For all
other cases, including extra parentheses or a unary @code{+}, the fill
pattern is the four least significant bytes of the value of the
expression.  In all cases, the number is big-endian.

You can also change the fill value with a @code{FILL} command in the
output section commands; (@pxref{Output Section Data}).

Here is a simple example:
@smallexample
@group
SECTIONS @{ .text : @{ *(.text) @} =0x90909090 @}
@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; this is done using wildcard patterns, so that any
symbol whose name begins with @samp{old}, @samp{original}, or @samp{new}
is matched.  The wildcard patterns available are the same as those used
in the shell when matching filenames (also known as ``globbing'').

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.

Node name can be omited, provided it is the only version node
in the version script.  Such version script doesn't assign any versions to
symbols, only selects which symbols will be globally visible out and which
won't.

@smallexample
@{ global: foo; bar; local: *; @};
@end smallexample

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. A @samp{.symver} directive
takes precedence over a version script.

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.

You can also specify the language in the version script:

@smallexample
VERSION extern "lang" @{ version-script-commands @}
@end smallexample

The supported @samp{lang}s are @samp{C}, @samp{C++}, and @samp{Java}. 
The linker will iterate over the list of symbols at the link time and
demangle them according to @samp{lang} before matching them to the
patterns specified in @samp{version-script-commands}.

@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
@ifnottex
@c END TEXI2ROFF-KILL
@code{1024} or @code{1024*1024}
@c TEXI2ROFF-KILL
@end ifnottex
@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)
    @} = 0x12345678;
@}
@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{= 0x12345678}
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, whose 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
@ifnottex
@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 ifnottex
@tex
\vskip \baselineskip
%"lispnarrowing" is the extra indent used generally for smallexample
\hskip\lispnarrowing\vbox{\offinterlineskip
\hrule
\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
&5&&left&&==         !=       >      <      <=      >=&\cr
&6&&left&&\&&\cr
&7&&left&&|&\cr
&8&&left&&{\&\&}&\cr
&9&&left&&||&\cr
&10&&right&&?        :&\cr
&11&&right&&\qquad\&=      +=       -=     *=     /=\qquad\ddag&\cr
&lowest&&&&&\cr
height2pt&\omit&&\omit&&\omit&\cr}
\hrule}
@end tex
@iftex
{
@obeylines@parskip=0pt@parindent=0pt
@dag@quad Prefix operators.
@ddag@quad @xref{Assignments}.
}
@end iftex
@c END TEXI2ROFF-KILL

@node Evaluation
@subsection Evaluation
@cindex lazy evaluation
@cindex expression evaluation order
The linker evaluates expressions lazily.  It only computes the value of
an expression when absolutely necessary.

The linker needs some information, such as the value of the start
address of the first section, and the origins and lengths of memory
regions, in order to do any linking at all.  These values are computed
as soon as possible when the linker reads in the linker script.

However, other values (such as symbol values) are not known or needed
until after storage allocation.  Such values are evaluated later, when
other information (such as the sizes of output sections) is available
for use in the symbol assignment expression.

The sizes of sections cannot be known until after allocation, so
assignments dependent upon these are not performed until after
allocation.

Some expressions, such as those depending upon the location counter
@samp{.}, must be evaluated during section allocation.

If the result of an expression is required, but the value is not
available, then an error results.  For example, a script like the
following
@smallexample
@group
SECTIONS
  @{
    .text 9+this_isnt_constant :
      @{ *(.text) @}
  @}
@end group
@end smallexample
@noindent
will cause the error message @samp{non constant expression for initial
address}.

@node Expression Section
@subsection The Section of an Expression
@cindex expression sections
@cindex absolute expressions
@cindex relative expressions
@cindex absolute and relocatable symbols
@cindex relocatable and absolute symbols
@cindex symbols, relocatable and absolute
When the linker evaluates an expression, the result is either absolute
or relative to some section.  A relative expression is expressed as a
fixed offset from the base of a section.

The position of the expression within the linker script determines
whether it is absolute or relative.  An expression which appears within
an output section definition is relative to the base of the output
section.  An expression which appears elsewhere will be absolute.

A symbol set to a relative expression will be relocatable if you request
relocatable output using the @samp{-r} option.  That means that a
further link operation may change the value of the symbol.  The symbol's
section will be the section of the relative expression.

A symbol set to an absolute expression will retain the same value
through any further link operation.  The symbol will be absolute, and
will not have any particular associated section.

You can use the builtin function @code{ABSOLUTE} to force an expression
to be absolute when it would otherwise be relative.  For example, to
create an absolute symbol set to the address of the end of the output
section @samp{.data}:
@smallexample
SECTIONS
  @{
    .data : @{ *(.data) _edata = ABSOLUTE(.); @}
  @}
@end smallexample
@noindent
If @samp{ABSOLUTE} were not used, @samp{_edata} would be relative to the
@samp{.data} section.

@node Builtin Functions
@subsection Builtin Functions
@cindex functions in expressions
The linker script language includes a number of builtin functions for
use in linker script expressions.

@table @code
@item ABSOLUTE(@var{exp})
@kindex ABSOLUTE(@var{exp})
@cindex expression, absolute
Return the absolute (non-relocatable, as opposed to non-negative) value
of the expression @var{exp}.  Primarily useful to assign an absolute
value to a symbol within a section definition, where symbol values are
normally section relative.  @xref{Expression Section}.

@item ADDR(@var{section})
@kindex ADDR(@var{section})
@cindex section address in expression
Return the absolute address (the VMA) of the named @var{section}.  Your
script must previously have defined the location of that section.  In
the following example, @code{symbol_1} and @code{symbol_2} are assigned
identical values:
@smallexample
@group
SECTIONS @{ @dots{}
  .output1 :
    @{
    start_of_output_1 = ABSOLUTE(.);
    @dots{}
    @}
  .output :
    @{
    symbol_1 = ADDR(.output1);
    symbol_2 = start_of_output_1;
    @}
@dots{} @}
@end group
@end smallexample

@item ALIGN(@var{exp})
@kindex ALIGN(@var{exp})
@cindex round up location counter
@cindex align location counter
Return the location counter (@code{.}) aligned to the next @var{exp}
boundary.
@code{ALIGN} doesn't change the value of the location counter---it just
does arithmetic on it.  Here is an example which aligns the output
@code{.data} section to the next @code{0x2000} byte boundary after the
preceding section and sets a variable within the section to the next
@code{0x8000} boundary after the input sections:
@smallexample
@group
SECTIONS @{ @dots{}
  .data ALIGN(0x2000): @{
    *(.data)
    variable = ALIGN(0x8000);
  @}
@dots{} @}
@end group
@end smallexample
@noindent
The first use of @code{ALIGN} in this example specifies the location of
a section because it is used as the optional @var{address} attribute of
a section definition (@pxref{Output Section Address}).  The second use
of @code{ALIGN} is used to defines the value of a symbol.

The builtin function @code{NEXT} is closely related to @code{ALIGN}.

@item BLOCK(@var{exp})
@kindex BLOCK(@var{exp})
This is a synonym for @code{ALIGN}, for compatibility with older linker
scripts.  It is most often seen when setting the address of an output
section.

@item DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
@kindex DATA_SEGMENT_ALIGN(@var{maxpagesize}, @var{commonpagesize})
This is equivalent to either
@smallexample
(ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - 1)))
@end smallexample
or
@smallexample
(ALIGN(@var{maxpagesize}) + (. & (@var{maxpagesize} - @var{commonpagesize})))
@end smallexample
@noindent
depending on whether the latter uses fewer @var{commonpagesize} sized pages
for the data segment (area between the result of this expression and
@code{DATA_SEGMENT_END}) than the former or not.
If the latter form is used, it means @var{commonpagesize} bytes of runtime
memory will be saved at the expense of up to @var{commonpagesize} wasted
bytes in the on-disk file.

This expression can only be used directly in @code{SECTIONS} commands, not in
any output section descriptions and only once in the linker script.
@var{commonpagesize} should be less or equal to @var{maxpagesize} and should
be the system page size the object wants to be optimized for (while still
working on system page sizes up to @var{maxpagesize}).

@noindent
Example:
@smallexample
  . = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
@end smallexample

@item DATA_SEGMENT_END(@var{exp})
@kindex DATA_SEGMENT_END(@var{exp})
This defines the end of data segment for @code{DATA_SEGMENT_ALIGN}
evaluation purposes.

@smallexample
  . = DATA_SEGMENT_END(.);
@end smallexample

@item DEFINED(@var{symbol})
@kindex DEFINED(@var{symbol})
@cindex symbol defaults
Return 1 if @var{symbol} is in the linker global symbol table and is
defined, otherwise return 0.  You can use this function to provide
default values for symbols.  For example, the following script fragment
shows how to set a global symbol @samp{begin} to the first location in
the @samp{.text} section---but if a symbol called @samp{begin} already
existed, its value is preserved:

@smallexample
@group
SECTIONS @{ @dots{}
  .text : @{
    begin = DEFINED(begin) ? begin : . ;
    @dots{}
  @}
  @dots{}
@}
@end group
@end smallexample

@item LOADADDR(@var{section})
@kindex LOADADDR(@var{section})
@cindex section load address in expression
Return the absolute LMA of the named @var{section}.  This is normally
the same as @code{ADDR}, but it may be different if the @code{AT}
attribute is used in the output section definition (@pxref{Output
Section LMA}).

@kindex MAX
@item MAX(@var{exp1}, @var{exp2})
Returns the maximum of @var{exp1} and @var{exp2}.

@kindex MIN
@item MIN(@var{exp1}, @var{exp2})
Returns the minimum of @var{exp1} and @var{exp2}.

@item NEXT(@var{exp})
@kindex NEXT(@var{exp})
@cindex unallocated address, next
Return the next unallocated address that is a multiple of @var{exp}.
This function is closely related to @code{ALIGN(@var{exp})}; unless you
use the @code{MEMORY} command to define discontinuous memory for the
output file, the two functions are equivalent.

@item SIZEOF(@var{section})
@kindex SIZEOF(@var{section})
@cindex section size
Return the size in bytes of the named @var{section}, if that section has
been allocated.  If the section has not been allocated when this is
evaluated, the linker will report an error.  In the following example,
@code{symbol_1} and @code{symbol_2} are assigned identical values:
@smallexample
@group
SECTIONS@{ @dots{}
  .output @{
    .start = . ;
    @dots{}
    .end = . ;
    @}
  symbol_1 = .end - .start ;
  symbol_2 = SIZEOF(.output);
@dots{} @}
@end group
@end smallexample

@item SIZEOF_HEADERS
@itemx sizeof_headers
@kindex SIZEOF_HEADERS
@cindex header size
Return the size in bytes of the output file's headers.  This is
information which appears at the start of the output file.  You can use
this number when setting the start address of the first section, if you
choose, to facilitate paging.

@cindex not enough room for program headers
@cindex program headers, not enough room
When producing an ELF output file, if the linker script uses the
@code{SIZEOF_HEADERS} builtin function, the linker must compute the
number of program headers before it has determined all the section
addresses and sizes.  If the linker later discovers that it needs
additional program headers, it will report an error @samp{not enough
room for program headers}.  To avoid this error, you must avoid using
the @code{SIZEOF_HEADERS} function, or you must rework your linker
script to avoid forcing the linker to use additional program headers, or
you must define the program headers yourself using the @code{PHDRS}
command (@pxref{PHDRS}).
@end table

@node Implicit Linker Scripts
@section Implicit Linker Scripts
@cindex implicit linker scripts
If you specify a linker input file which the linker can not recognize as
an object file or an archive file, it will try to read the file as a
linker script.  If the file can not be parsed as a linker script, the
linker will report an error.

An implicit linker script will not replace the default linker script.

Typically an implicit linker script would contain only symbol
assignments, or the @code{INPUT}, @code{GROUP}, or @code{VERSION}
commands.

Any input files read because of an implicit linker script will be read
at the position in the command line where the implicit linker script was
read.  This can affect archive searching.

@ifset GENERIC
@node Machine Dependent
@chapter Machine Dependent Features

@cindex machine dependencies
@command{ld} has additional features on some platforms; the following
sections describe them.  Machines where @command{ld} has no additional
functionality are not listed.

@menu
@ifset H8300
* H8/300::                      @command{ld} and the H8/300
@end ifset
@ifset I960
* i960::                        @command{ld} and the Intel 960 family
@end ifset
@ifset ARM
* ARM::				@command{ld} and the ARM family
@end ifset
@ifset HPPA
* HPPA ELF32::                  @command{ld} and HPPA 32-bit ELF
@end ifset
@ifset MMIX
* MMIX::			@command{ld} and MMIX
@end ifset
@ifset MSP430
* MSP430::			@command{ld} and MSP430
@end ifset
@ifset TICOFF
* TI COFF::                     @command{ld} and TI COFF
@end ifset
@ifset WIN32
* WIN32::                       @command{ld} and WIN32 (cygwin/mingw)
@end ifset
@ifset XTENSA
* Xtensa::                      @command{ld} and Xtensa Processors
@end ifset
@end menu
@end ifset

@ifset H8300
@ifclear GENERIC
@raisesections
@end ifclear

@node H8/300
@section @command{ld} and the H8/300

@cindex H8/300 support
For the H8/300, @command{ld} can perform these global optimizations when
you specify the @samp{--relax} command-line option.

@table @emph
@cindex relaxing on H8/300
@item relaxing address modes
@command{ld} finds all @code{jsr} and @code{jmp} instructions whose
targets are within eight bits, and turns them into eight-bit
program-counter relative @code{bsr} and @code{bra} instructions,
respectively.

@cindex synthesizing on H8/300
@item synthesizing instructions
@c FIXME: specifically mov.b, or any mov instructions really?
@command{ld} finds all @code{mov.b} instructions which use the
sixteen-bit absolute address form, but refer to the top
page of memory, and changes them to use the eight-bit address form.
(That is: the linker turns @samp{mov.b @code{@@}@var{aa}:16} into
@samp{mov.b @code{@@}@var{aa}:8} whenever the address @var{aa} is in the
top page of memory).
@end table

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifclear GENERIC
@ifset Renesas
@c This stuff is pointless to say unless you're especially concerned
@c with Renesas chips; don't enable it for generic case, please.
@node Renesas
@chapter @command{ld} and Other Renesas Chips

@command{ld} also supports the Renesas (formerly Hitachi) H8/300H,
H8/500, and SH chips.  No special features, commands, or command-line
options are required for these chips.
@end ifset
@end ifclear

@ifset I960
@ifclear GENERIC
@raisesections
@end ifclear

@node i960
@section @command{ld} and the Intel 960 Family

@cindex i960 support

You can use the @samp{-A@var{architecture}} command line option to
specify one of the two-letter names identifying members of the 960
family; the option specifies the desired output target, and warns of any
incompatible instructions in the input files.  It also modifies the
linker's search strategy for archive libraries, to support the use of
libraries specific to each particular architecture, by including in the
search loop names suffixed with the string identifying the architecture.

For example, if your @command{ld} command line included @w{@samp{-ACA}} as
well as @w{@samp{-ltry}}, the linker would look (in its built-in search
paths, and in any paths you specify with @samp{-L}) for a library with
the names

@smallexample
@group
try
libtry.a
tryca
libtryca.a
@end group
@end smallexample

@noindent
The first two possibilities would be considered in any event; the last
two are due to the use of @w{@samp{-ACA}}.

You can meaningfully use @samp{-A} more than once on a command line, since
the 960 architecture family allows combination of target architectures; each
use will add another pair of name variants to search for when @w{@samp{-l}}
specifies a library.

@cindex @option{--relax} on i960
@cindex relaxing on i960
@command{ld} supports the @samp{--relax} option for the i960 family.  If
you specify @samp{--relax}, @command{ld} finds all @code{balx} and
@code{calx} instructions whose targets are within 24 bits, and turns
them into 24-bit program-counter relative @code{bal} and @code{cal}
instructions, respectively.  @command{ld} also turns @code{cal}
instructions into @code{bal} instructions when it determines that the
target subroutine is a leaf routine (that is, the target subroutine does
not itself call any subroutines).

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifset ARM
@ifclear GENERIC
@raisesections
@end ifclear

@node ARM
@section @command{ld}'s Support for Interworking Between ARM and Thumb Code

@cindex ARM interworking support
@kindex --support-old-code
For the ARM, @command{ld} will generate code stubs to allow functions calls
betweem ARM and Thumb code.  These stubs only work with code that has
been compiled and assembled with the @samp{-mthumb-interwork} command
line option.  If it is necessary to link with old ARM object files or
libraries, which have not been compiled with the -mthumb-interwork
option then the @samp{--support-old-code} command line switch should be
given to the linker.  This will make it generate larger stub functions
which will work with non-interworking aware ARM code.  Note, however,
the linker does not support generating stubs for function calls to
non-interworking aware Thumb code.

@cindex thumb entry point
@cindex entry point, thumb
@kindex --thumb-entry=@var{entry}
The @samp{--thumb-entry} switch is a duplicate of the generic
@samp{--entry} switch, in that it sets the program's starting address.
But it also sets the bottom bit of the address, so that it can be
branched to using a BX instruction, and the program will start
executing in Thumb mode straight away.

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifset HPPA
@ifclear GENERIC
@raisesections
@end ifclear

@node HPPA ELF32
@section @command{ld} and HPPA 32-bit ELF Support
@cindex HPPA multiple sub-space stubs
@kindex --multi-subspace
When generating a shared library, @command{ld} will by default generate
import stubs suitable for use with a single sub-space application.
The @samp{--multi-subspace} switch causes @command{ld} to generate export
stubs, and different (larger) import stubs suitable for use with
multiple sub-spaces.

@cindex HPPA stub grouping
@kindex --stub-group-size=@var{N}
Long branch stubs and import/export stubs are placed by @command{ld} in
stub sections located between groups of input sections.
@samp{--stub-group-size} specifies the maximum size of a group of input
sections handled by one stub section.  Since branch offsets are signed,
a stub section may serve two groups of input sections, one group before
the stub section, and one group after it.  However, when using
conditional branches that require stubs, it may be better (for branch
prediction) that stub sections only serve one group of input sections.
A negative value for @samp{N} chooses this scheme, ensuring that
branches to stubs always use a negative offset.  Two special values of
@samp{N} are recognized, @samp{1} and @samp{-1}.  These both instruct
@command{ld} to automatically size input section groups for the branch types
detected, with the same behaviour regarding stub placement as other
positive or negative values of @samp{N} respectively.

Note that @samp{--stub-group-size} does not split input sections.  A
single input section larger than the group size specified will of course
create a larger group (of one section).  If input sections are too
large, it may not be possible for a branch to reach its stub.

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifset MMIX
@ifclear GENERIC
@raisesections
@end ifclear

@node MMIX
@section @code{ld} and MMIX
For MMIX, there is a choice of generating @code{ELF} object files or
@code{mmo} object files when linking.  The simulator @code{mmix}
understands the @code{mmo} format.  The binutils @code{objcopy} utility
can translate between the two formats.

There is one special section, the @samp{.MMIX.reg_contents} section.
Contents in this section is assumed to correspond to that of global
registers, and symbols referring to it are translated to special symbols,
equal to registers.  In a final link, the start address of the
@samp{.MMIX.reg_contents} section corresponds to the first allocated
global register multiplied by 8.  Register @code{$255} is not included in
this section; it is always set to the program entry, which is at the
symbol @code{Main} for @code{mmo} files.

Symbols with the prefix @code{__.MMIX.start.}, for example
@code{__.MMIX.start..text} and @code{__.MMIX.start..data} are special;
there must be only one each, even if they are local.  The default linker
script uses these to set the default start address of a section.

Initial and trailing multiples of zero-valued 32-bit words in a section,
are left out from an mmo file.

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifset MSP430
@ifclear GENERIC
@raisesections
@end ifclear

@node  MSP430
@section @code{ld} and MSP430
For the MSP430 it is possible to select the MPU architecture.  The flag @samp{-m [mpu type]}
will select an appropriate linker script for selected MPU type.  (To get a list of known MPUs
just pass @samp{-m help} option to the linker).

@cindex MSP430 extra sections
The linker will recognize some extra sections which are MSP430 specific:

@table @code
@item @samp{.vectors}
Defines a portion of ROM where interrupt vectors located.

@item @samp{.bootloader}
Defines the bootloader portion of the ROM (if applicable).  Any code
in this section will be uploaded to the MPU.

@item @samp{.infomem}
Defines an information memory section (if applicable).  Any code in
this section will be uploaded to the MPU.

@item @samp{.infomemnobits} 
This is the same as the @samp{.infomem} section except that any code
in this section will not be uploaded to the MPU.

@item @samp{.noinit}
Denotes a portion of RAM located above @samp{.bss} section.

The last two sections are used by gcc. 
@end table

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifset TICOFF
@ifclear GENERIC
@raisesections
@end ifclear

@node TI COFF
@section @command{ld}'s Support for Various TI COFF Versions
@cindex TI COFF versions
@kindex --format=@var{version}
The @samp{--format} switch allows selection of one of the various
TI COFF versions.  The latest of this writing is 2; versions 0 and 1 are
also supported.  The TI COFF versions also vary in header byte-order
format; @command{ld} will read any version or byte order, but the output
header format depends on the default specified by the specific target.

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifset WIN32
@ifclear GENERIC
@raisesections
@end ifclear

@node WIN32
@section @command{ld} and WIN32 (cygwin/mingw)

This section describes some of the win32 specific @command{ld} issues. 
See @ref{Options,,Command Line Options} for detailed decription of the
command line options mentioned here.

@table @emph
@cindex import libraries 
@item import libraries 
The standard Windows linker creates and uses so-called import
libraries, which contains information for linking to dll's.  They are
regular static archives and are handled as any other static
archive.  The cygwin and mingw ports of @command{ld} have specific
support for creating such libraries provided with the
@samp{--out-implib} command line option.

@item   exporting DLL symbols 
@cindex exporting DLL symbols 
The cygwin/mingw @command{ld} has several ways to export symbols for dll's.

@table @emph
@item   using auto-export functionality
@cindex using auto-export functionality
By default @command{ld} exports symbols with the auto-export functionality,
which is controlled by the following command line options:

@itemize
@item --export-all-symbols   [This is the default]
@item --exclude-symbols
@item --exclude-libs
@end itemize

If, however, @samp{--export-all-symbols} is not given explicitly on the 
command line, then the default auto-export behavior will be @emph{disabled}
if either of the following are true:

@itemize
@item A DEF file is used.
@item Any symbol in any object file was marked with the __declspec(dllexport) attribute.
@end itemize

@item   using a DEF file 
@cindex using a DEF file 
Another way of exporting symbols is using a DEF file.  A DEF file is
an ASCII file containing definitions of symbols which should be
exported when a dll is created.  Usually it is named @samp{<dll
name>.def} and is added as any other object file to the linker's
command line.  The file's name must end in @samp{.def} or @samp{.DEF}.

@example
gcc -o <output> <objectfiles> <dll name>.def
@end example

Using a DEF file turns off the normal auto-export behavior, unless the
@samp{--export-all-symbols} option is also used.

Here is an example of a DEF file for a shared library called @samp{xyz.dll}:

@example
LIBRARY "xyz.dll" BASE=0x10000000

EXPORTS
foo
bar
_bar = bar
@end example 

This example defines a base address and three symbols.  The third
symbol is an alias for the second.  For the complete format
specification see ld/deffilep.y in the binutils sources.

@cindex creating a DEF file
While linking a shared dll, @command{ld} is able to create a DEF file
with the @samp{--output-def <file>} command line option.

@item   Using decorations
@cindex Using decorations
Another way of marking symbols for export is to modify the source code
itself, so that when building the DLL each symbol to be exported is
declared as:

@example
__declspec(dllexport) int a_variable
__declspec(dllexport) void a_function(int with_args)
@end example

All such symbols will be exported from the DLL.  If, however,
any of the object files in the DLL contain symbols decorated in
this way, then the normal auto-export behavior is disabled, unless
the @samp{--export-all-symbols} option is also used.

Note that object files that wish to access these symbols must @emph{not}
decorate them with dllexport.  Instead, they should use dllimport, 
instead:

@example
__declspec(dllimport) int a_variable
__declspec(dllimport) void a_function(int with_args)
@end example

This complicates the structure of library header files, because 
when included by the library itself the header must declare the 
variables and functions as dllexport, but when included by client
code the header must declare them as dllimport.  There are a number
of idioms that are typically used to do this; often client code can 
omit the __declspec() declaration completely.  See
@samp{--enable-auto-import} and @samp{automatic data imports} for more
imformation.
@end table 

@cindex automatic data imports
@item automatic data imports
The standard Windows dll format supports data imports from dlls only
by adding special decorations (dllimport/dllexport), which let the
compiler produce specific assembler instructions to deal with this
issue.  This increases the effort necessary to port existing Un*x 
code to these platforms, especially for large
c++ libraries and applications.  The auto-import feature, which was
initially provided by Paul Sokolovsky, allows one to omit the 
decorations to archieve a behavior that conforms to that on POSIX/Un*x
platforms. This feature is enabled with the @samp{--enable-auto-import} 
command-line option, although it is enabled by default on cygwin/mingw.
The @samp{--enable-auto-import} option itself now serves mainly to
suppress any warnings that are ordinarily emitted when linked objects
trigger the feature's use.

auto-import of variables does not always work flawlessly without 
additional assistance.  Sometimes, you will see this message

"variable '<var>' can't be auto-imported. Please read the 
documentation for ld's @code{--enable-auto-import} for details."

The @samp{--enable-auto-import} documentation explains why this error 
occurs, and several methods that can be used to overcome this difficulty.  
One of these methods is the @emph{runtime pseudo-relocs} feature, described 
below.

@cindex runtime pseudo-relocation
For complex variables imported from DLLs (such as structs or classes), 
object files typically contain a base address for the variable and an 
offset (@emph{addend}) within the variable--to specify a particular 
field or public member, for instance.  Unfortunately, the runtime loader used 
in win32 environments is incapable of fixing these references at runtime 
without the additional information supplied by dllimport/dllexport decorations.
The standard auto-import feature described above is unable to resolve these 
references.

The @samp{--enable-runtime-pseudo-relocs} switch allows these references to 
be resolved without error, while leaving the task of adjusting the references 
themselves (with their non-zero addends) to specialized code provided by the 
runtime environment.  Recent versions of the cygwin and mingw environments and 
compilers provide this runtime support; older versions do not.  However, the 
support is only necessary on the developer's platform; the compiled result will 
run without error on an older system.

@samp{--enable-runtime-pseudo-relocs} is not the default; it must be explicitly 
enabled as needed. 

@cindex direct linking to a dll
@item direct linking to a dll
The cygwin/mingw ports of @command{ld} support the direct linking,
including data symbols, to a dll without the usage of any import
libraries.  This is much faster and uses much less memory than does the
traditional import library method, expecially when linking large
libraries or applications.  When @command{ld} creates an import lib, each 
function or variable exported from the dll is stored in its own bfd, even 
though a single bfd could contain many exports.  The overhead involved in 
storing, loading, and processing so many bfd's is quite large, and explains the
tremendous time, memory, and storage needed to link against particularly 
large or complex libraries when using import libs.

Linking directly to a dll uses no extra command-line switches other than 
@samp{-L} and @samp{-l}, because @command{ld} already searches for a number
of names to match each library.  All that is needed from the developer's 
perspective is an understanding of this search, in order to force ld to
select the dll instead of an import library.


For instance, when ld is called with the argument @samp{-lxxx} it will attempt
to find, in the first directory of its search path,

@example
libxxx.dll.a 
xxx.dll.a 
libxxx.a 
cygxxx.dll (*)
libxxx.dll 
xxx.dll 
@end example

before moving on to the next directory in the search path.

(*) Actually, this is not @samp{cygxxx.dll} but in fact is @samp{<prefix>xxx.dll}, 
where @samp{<prefix>} is set by the @command{ld} option 
@samp{--dll-search-prefix=<prefix>}. In the case of cygwin, the standard gcc spec 
file includes @samp{--dll-search-prefix=cyg}, so in effect we actually search for 
@samp{cygxxx.dll}.

Other win32-based unix environments, such as mingw or pw32, may use other 
@samp{<prefix>}es, although at present only cygwin makes use of this feature.  It 
was originally intended to help avoid name conflicts among dll's built for the
various win32/un*x environments, so that (for example) two versions of a zlib dll
could coexist on the same machine.

The generic cygwin/mingw path layout uses a @samp{bin} directory for
applications and dll's and a @samp{lib} directory for the import
libraries (using cygwin nomenclature):

@example
bin/
	cygxxx.dll
lib/
	libxxx.dll.a   (in case of dll's)
	libxxx.a       (in case of static archive) 
@end example

Linking directly to a dll without using the import library can be 
done two ways: 

1. Use the dll directly by adding the @samp{bin} path to the link line
@example
gcc -Wl,-verbose  -o a.exe -L../bin/ -lxxx
@end example 

However, as the dll's often have version numbers appended to their names
(@samp{cygncurses-5.dll}) this will often fail, unless one specifies
@samp{-L../bin -lncurses-5} to include the version.  Import libs are generally
not versioned, and do not have this difficulty.

2. Create a symbolic link from the dll to a file in the @samp{lib}
directory according to the above mentioned search pattern.  This
should be used to avoid unwanted changes in the tools needed for
making the app/dll.

@example
ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
@end example 

Then you can link without any make environment changes.

@example
gcc -Wl,-verbose  -o a.exe -L../lib/ -lxxx
@end example 

This technique also avoids the version number problems, because the following is
perfectly legal

@example
bin/
	cygxxx-5.dll
lib/
	libxxx.dll.a -> ../bin/cygxxx-5.dll 
@end example

Linking directly to a dll without using an import lib will work
even when auto-import features are exercised, and even when
@samp{--enable-runtime-pseudo-relocs} is used.

Given the improvements in speed and memory usage, one might justifiably
wonder why import libraries are used at all.  There are two reasons:

1. Until recently, the link-directly-to-dll functionality did @emph{not}
work with auto-imported data.

2. Sometimes it is necessary to include pure static objects within the
import library (which otherwise contains only bfd's for indirection
symbols that point to the exports of a dll).  Again, the import lib
for the cygwin kernel makes use of this ability, and it is not
possible to do this without an import lib.

So, import libs are not going away.  But the ability to replace
true import libs with a simple symbolic link to (or a copy of) 
a dll, in most cases, is a useful addition to the suite of tools 
binutils makes available to the win32 developer.  Given the 
massive improvements in memory requirements during linking, storage
requirements, and linking speed, we expect that many developers
will soon begin to use this feature whenever possible.

@item symbol aliasing  
@table @emph
@item adding additional names 
Sometimes, it is useful to export symbols with additional names.  
A symbol @samp{foo} will be exported as @samp{foo}, but it can also be
exported as @samp{_foo} by using special directives in the DEF file
when creating the dll.  This will affect also the optional created
import library.  Consider the following DEF file: 

@example 
LIBRARY "xyz.dll" BASE=0x61000000

EXPORTS
foo 
_foo = foo
@end example 

The line @samp{_foo = foo} maps the symbol @samp{foo} to @samp{_foo}.

Another method for creating a symbol alias is to create it in the
source code using the "weak" attribute:

@example 
void foo () @{ /* Do something.  */; @} 
void _foo () __attribute__ ((weak, alias ("foo")));
@end example 

See the gcc manual for more information about attributes and weak
symbols.

@item renaming symbols
Sometimes it is useful to rename exports.  For instance, the cygwin
kernel does this regularly.  A symbol @samp{_foo} can be exported as 
@samp{foo} but not as @samp{_foo} by using special directives in the
DEF file. (This will also affect the import library, if it is
created).  In the following example: 

@example 
LIBRARY "xyz.dll" BASE=0x61000000

EXPORTS
_foo = foo
@end example 

The line @samp{_foo = foo} maps the exported symbol @samp{foo} to
@samp{_foo}.
@end table 

Note: using a DEF file disables the default auto-export behavior,
unless the @samp{--export-all-symbols} command line option is used. 
If, however, you are trying to rename symbols, then you should list
@emph{all} desired exports in the DEF file, including the symbols 
that are not being renamed, and do @emph{not} use the 
@samp{--export-all-symbols} option.  If you list only the 
renamed symbols in the DEF file, and use @samp{--export-all-symbols} 
to handle the other symbols, then the both the new names @emph{and} 
the original names for the the renamed symbols will be exported.  
In effect, you'd be aliasing those symbols, not renaming them, 
which is probably not what you wanted.
@end table

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifset XTENSA
@ifclear GENERIC
@raisesections
@end ifclear

@node Xtensa
@section @code{ld} and Xtensa Processors

@cindex Xtensa processors
The default @command{ld} behavior for Xtensa processors is to interpret
@code{SECTIONS} commands so that lists of explicitly named sections in a
specification with a wildcard file will be interleaved when necessary to
keep literal pools within the range of PC-relative load offsets.  For
example, with the command:

@smallexample
SECTIONS
@{
  .text : @{
    *(.literal .text)
  @}
@}
@end smallexample

@noindent
@command{ld} may interleave some of the @code{.literal}
and @code{.text} sections from different object files to ensure that the
literal pools are within the range of PC-relative load offsets.  A valid
interleaving might place the @code{.literal} sections from an initial
group of files followed by the @code{.text} sections of that group of
files.  Then, the @code{.literal} sections from the rest of the files
and the @code{.text} sections from the rest of the files would follow.
The non-interleaved order can still be specified as:

@smallexample
SECTIONS
@{
  .text : @{
    *(.literal) *(.text)
  @}
@}
@end smallexample

@cindex @code{--relax} on Xtensa
@cindex relaxing on Xtensa
@kindex --no-relax
The Xtensa version of @command{ld} enables the @option{--relax} option by
default to attempt to reduce space in the output image by combining
literals with identical values.  It also provides the
@option{--no-relax} option to disable this optimization.  When enabled,
the relaxation algorithm ensures that a literal will only be merged with
another literal when the new merged literal location is within the
offset range of all of its uses.

The relaxation mechanism will also attempt to optimize
assembler-generated ``longcall'' sequences of
@code{L32R}/@code{CALLX@var{n}} when the target is known to fit into a
@code{CALL@var{n}} instruction encoding.  The current optimization
converts the sequence into @code{NOP}/@code{CALL@var{n}} and removes the
literal referenced by the @code{L32R} instruction.

@ifclear GENERIC
@lowersections
@end ifclear
@end ifset

@ifclear SingleFormat
@node BFD
@chapter BFD

@cindex back end
@cindex object file management
@cindex object formats available
@kindex objdump -i
The linker accesses object and archive files using the BFD libraries.
These libraries allow the linker to use the same routines to operate on
object files whatever the object file format.  A different object file
format can be supported simply by creating a new BFD back end and adding
it to the library.  To conserve runtime memory, however, the linker and
associated tools are usually configured to support only a subset of the
object file formats available.  You can use @code{objdump -i}
(@pxref{objdump,,objdump,binutils.info,The GNU Binary Utilities}) to
list all the formats available for your configuration.

@cindex BFD requirements
@cindex requirements for BFD
As with most implementations, BFD is a compromise between
several conflicting requirements. The major factor influencing
BFD design was efficiency: any time used converting between
formats is time which would not have been spent had BFD not
been involved. This is partly offset by abstraction payback; since
BFD simplifies applications and back ends, more time and care
may be spent optimizing algorithms for a greater speed.

One minor artifact of the BFD solution which you should bear in
mind is the potential for information loss.  There are two places where
useful information can be lost using the BFD mechanism: during
conversion and during output. @xref{BFD information loss}.

@menu
* BFD outline::                 How it works: an outline of BFD
@end menu

@node BFD outline
@section How It Works: An Outline of BFD
@cindex opening object files
@include bfdsumm.texi
@end ifclear

@node Reporting Bugs
@chapter Reporting Bugs
@cindex bugs in @command{ld}
@cindex reporting bugs in @command{ld}

Your bug reports play an essential role in making @command{ld} reliable.

Reporting a bug may help you by bringing a solution to your problem, or
it may not.  But in any case the principal function of a bug report is
to help the entire community by making the next version of @command{ld}
work better.  Bug reports are your contribution to the maintenance of
@command{ld}.

In order for a bug report to serve its purpose, you must include the
information that enables us to fix the bug.

@menu
* Bug Criteria::                Have you found a bug?
* Bug Reporting::               How to report bugs
@end menu

@node Bug Criteria
@section Have You Found a Bug?
@cindex bug criteria

If you are not sure whether you have found a bug, here are some guidelines:

@itemize @bullet
@cindex fatal signal
@cindex linker crash
@cindex crash of linker
@item
If the linker gets a fatal signal, for any input whatever, that is a
@command{ld} bug.  Reliable linkers never crash.

@cindex error on valid input
@item
If @command{ld} produces an error message for valid input, that is a bug.

@cindex invalid input
@item
If @command{ld} does not produce an error message for invalid input, that
may be a bug.  In the general case, the linker can not verify that
object files are correct.

@item
If you are an experienced user of linkers, your suggestions for
improvement of @command{ld} are welcome in any case.
@end itemize

@node Bug Reporting
@section How to Report Bugs
@cindex bug reports
@cindex @command{ld} bugs, reporting

A number of companies and individuals offer support for @sc{gnu}
products.  If you obtained @command{ld} from a support organization, we
recommend you contact that organization first.

You can find contact information for many support companies and
individuals in the file @file{etc/SERVICE} in the @sc{gnu} Emacs
distribution.

Otherwise, send bug reports for @command{ld} to
@samp{bug-binutils@@gnu.org}.

The fundamental principle of reporting bugs usefully is this:
@strong{report all the facts}.  If you are not sure whether to state a
fact or leave it out, state it!

Often people omit facts because they think they know what causes the
problem and assume that some details do not matter.  Thus, you might
assume that the name of a symbol you use in an example does not
matter.  Well, probably it does not, but one cannot be sure.  Perhaps
the bug is a stray memory reference which happens to fetch from the
location where that name is stored in memory; perhaps, if the name
were different, the contents of that location would fool the linker
into doing the right thing despite the bug.  Play it safe and give a
specific, complete example.  That is the easiest thing for you to do,
and the most helpful. 

Keep in mind that the purpose of a bug report is to enable us to fix
the bug if it is new to us.  Therefore, always write your bug reports
on the assumption that the bug has not been reported previously.

Sometimes people give a few sketchy facts and ask, ``Does this ring a
bell?''  This cannot help us fix a bug, so it is basically useless.  We
respond by asking for enough details to enable us to investigate.
You might as well expedite matters by sending them to begin with.

To enable us to fix the bug, you should include all these things:

@itemize @bullet
@item
The version of @command{ld}.  @command{ld} announces it if you start it with
the @samp{--version} argument.

Without this, we will not know whether there is any point in looking for
the bug in the current version of @command{ld}.

@item
Any patches you may have applied to the @command{ld} source, including any
patches made to the @code{BFD} library.

@item
The type of machine you are using, and the operating system name and
version number.

@item
What compiler (and its version) was used to compile @command{ld}---e.g.
``@code{gcc-2.7}''.

@item
The command arguments you gave the linker to link your example and
observe the bug.  To guarantee you will not omit something important,
list them all.  A copy of the Makefile (or the output from make) is
sufficient.

If we were to try to guess the arguments, we would probably guess wrong
and then we might not encounter the bug.

@item
A complete input file, or set of input files, that will reproduce the
bug.  It is generally most helpful to send the actual object files
provided that they are reasonably small.  Say no more than 10K.  For
bigger files you can either make them available by FTP or HTTP or else
state that you are willing to send the object file(s) to whomever
requests them.  (Note - your email will be going to a mailing list, so
we do not want to clog it up with large attachments).  But small
attachments are best.

If the source files were assembled using @code{gas} or compiled using
@code{gcc}, then it may be OK to send the source files rather than the
object files.  In this case, be sure to say exactly what version of
@code{gas} or @code{gcc} was used to produce the object files.  Also say
how @code{gas} or @code{gcc} were configured.

@item
A description of what behavior you observe that you believe is
incorrect.  For example, ``It gets a fatal signal.''

Of course, if the bug is that @command{ld} gets a fatal signal, then we
will certainly notice it.  But if the bug is incorrect output, we might
not notice unless it is glaringly wrong.  You might as well not give us
a chance to make a mistake.

Even if the problem you experience is a fatal signal, you should still
say so explicitly.  Suppose something strange is going on, such as, your
copy of @command{ld} is out of synch, or you have encountered a bug in the
C library on your system.  (This has happened!)  Your copy might crash
and ours would not.  If you told us to expect a crash, then when ours
fails to crash, we would know that the bug was not happening for us.  If
you had not told us to expect a crash, then we would not be able to draw
any conclusion from our observations.

@item
If you wish to suggest changes to the @command{ld} source, send us context
diffs, as generated by @code{diff} with the @samp{-u}, @samp{-c}, or
@samp{-p} option.  Always send diffs from the old file to the new file.
If you even discuss something in the @command{ld} source, refer to it by
context, not by line number.

The line numbers in our development sources will not match those in your
sources.  Your line numbers would convey no useful information to us.
@end itemize

Here are some things that are not necessary:

@itemize @bullet
@item
A description of the envelope of the bug.

Often people who encounter a bug spend a lot of time investigating
which changes to the input file will make the bug go away and which
changes will not affect it.

This is often time consuming and not very useful, because the way we
will find the bug is by running a single example under the debugger
with breakpoints, not by pure deduction from a series of examples.
We recommend that you save your time for something else.

Of course, if you can find a simpler example to report @emph{instead}
of the original one, that is a convenience for us.  Errors in the
output will be easier to spot, running under the debugger will take
less time, and so on.

However, simplification is not vital; if you do not want to do this,
report the bug anyway and send us the entire test case you used.

@item
A patch for the bug.

A patch for the bug does help us if it is a good one.  But do not omit
the necessary information, such as the test case, on the assumption that
a patch is all we need.  We might see problems with your patch and decide
to fix the problem another way, or we might not understand it at all.

Sometimes with a program as complicated as @command{ld} it is very hard to
construct an example that will make the program follow a certain path
through the code.  If you do not send us the example, we will not be
able to construct one, so we will not be able to verify that the bug is
fixed.

And if we cannot understand what bug you are trying to fix, or why your
patch should be an improvement, we will not install it.  A test case will
help us to understand.

@item
A guess about what the bug is or what it depends on.

Such guesses are usually wrong.  Even we cannot guess right about such
things without first using the debugger to find the facts.
@end itemize

@node MRI
@appendix MRI Compatible Script Files
@cindex MRI compatibility
To aid users making the transition to @sc{gnu} @command{ld} from the MRI
linker, @command{ld} can use MRI compatible linker scripts as an
alternative to the more general-purpose linker scripting language
described in @ref{Scripts}.  MRI compatible linker scripts have a much
simpler command set than the scripting language otherwise used with
@command{ld}.  @sc{gnu} @command{ld} supports the most commonly used MRI
linker commands; these commands are described here.

In general, MRI scripts aren't of much use with the @code{a.out} object
file format, since it only has three sections and MRI scripts lack some
features to make use of them.

You can specify a file containing an MRI-compatible script using the
@samp{-c} command-line option.

Each command in an MRI-compatible script occupies its own line; each
command line starts with the keyword that identifies the command (though
blank lines are also allowed for punctuation).  If a line of an
MRI-compatible script begins with an unrecognized keyword, @command{ld}
issues a warning message, but continues processing the script.

Lines beginning with @samp{*} are comments.

You can write these commands using all upper-case letters, or all
lower case; for example, @samp{chip} is the same as @samp{CHIP}.
The following list shows only the upper-case form of each command.

@table @code
@cindex @code{ABSOLUTE} (MRI)
@item ABSOLUTE @var{secname}
@itemx ABSOLUTE @var{secname}, @var{secname}, @dots{} @var{secname}
Normally, @command{ld} includes in the output file all sections from all
the input files.  However, in an MRI-compatible script, you can use the
@code{ABSOLUTE} command to restrict the sections that will be present in
your output program.  If the @code{ABSOLUTE} command is used at all in a
script, then only the sections named explicitly in @code{ABSOLUTE}
commands will appear in the linker output.  You can still use other
input sections (whatever you select on the command line, or using
@code{LOAD}) to resolve addresses in the output file.

@cindex @code{ALIAS} (MRI)
@item ALIAS @var{out-secname}, @var{in-secname}
Use this command to place the data from input section @var{in-secname}
in a section called @var{out-secname} in the linker output file.

@var{in-secname} may be an integer.

@cindex @code{ALIGN} (MRI)
@item ALIGN @var{secname} = @var{expression}
Align the section called @var{secname} to @var{expression}.  The
@var{expression} should be a power of two.

@cindex @code{BASE} (MRI)
@item BASE @var{expression}
Use the value of @var{expression} as the lowest address (other than
absolute addresses) in the output file.

@cindex @code{CHIP} (MRI)
@item CHIP @var{expression}
@itemx CHIP @var{expression}, @var{expression}
This command does nothing; it is accepted only for compatibility.

@cindex @code{END} (MRI)
@item END
This command does nothing whatever; it's only accepted for compatibility.

@cindex @code{FORMAT} (MRI)
@item FORMAT @var{output-format}
Similar to the @code{OUTPUT_FORMAT} command in the more general linker
language, but restricted to one of these output formats:

@enumerate
@item
S-records, if @var{output-format} is @samp{S}

@item
IEEE, if @var{output-format} is @samp{IEEE}

@item
COFF (the @samp{coff-m68k} variant in BFD), if @var{output-format} is
@samp{COFF}
@end enumerate

@cindex @code{LIST} (MRI)
@item LIST @var{anything}@dots{}
Print (to the standard output file) a link map, as produced by the
@command{ld} command-line option @samp{-M}.

The keyword @code{LIST} may be followed by anything on the
same line, with no change in its effect.

@cindex @code{LOAD} (MRI)
@item LOAD @var{filename}
@itemx LOAD @var{filename}, @var{filename}, @dots{} @var{filename}
Include one or more object file @var{filename} in the link; this has the
same effect as specifying @var{filename} directly on the @command{ld}
command line.

@cindex @code{NAME} (MRI)
@item NAME @var{output-name}
@var{output-name} is the name for the program produced by @command{ld}; the
MRI-compatible command @code{NAME} is equivalent to the command-line
option @samp{-o} or the general script language command @code{OUTPUT}.

@cindex @code{ORDER} (MRI)
@item ORDER @var{secname}, @var{secname}, @dots{} @var{secname}
@itemx ORDER @var{secname} @var{secname} @var{secname}
Normally, @command{ld} orders the sections in its output file in the
order in which they first appear in the input files.  In an MRI-compatible
script, you can override this ordering with the @code{ORDER} command.  The
sections you list with @code{ORDER} will appear first in your output
file, in the order specified.

@cindex @code{PUBLIC} (MRI)
@item PUBLIC @var{name}=@var{expression}
@itemx PUBLIC @var{name},@var{expression}
@itemx PUBLIC @var{name} @var{expression}
Supply a value (@var{expression}) for external symbol
@var{name} used in the linker input files.

@cindex @code{SECT} (MRI)
@item SECT @var{secname}, @var{expression}
@itemx SECT @var{secname}=@var{expression}
@itemx SECT @var{secname} @var{expression}
You can use any of these three forms of the @code{SECT} command to
specify the start address (@var{expression}) for section @var{secname}.
If you have more than one @code{SECT} statement for the same
@var{secname}, only the @emph{first} sets the start address.
@end table

@include fdl.texi

@node Index
@unnumbered Index

@printindex cp

@tex
% I think something like @colophon should be in texinfo.  In the
% meantime:
\long\def\colophon{\hbox to0pt{}\vfill
\centerline{The body of this manual is set in}
\centerline{\fontname\tenrm,}
\centerline{with headings in {\bf\fontname\tenbf}}
\centerline{and examples in {\tt\fontname\tentt}.}
\centerline{{\it\fontname\tenit\/} and}
\centerline{{\sl\fontname\tensl\/}}
\centerline{are used for emphasis.}\vfill}
\page\colophon
% Blame: doc@cygnus.com, 28mar91.
@end tex


@contents
@bye