This is ld.info, produced by makeinfo version 4.13 from
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INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Ld: (ld). The GNU linker.
END-INFO-DIR-ENTRY
This file documents the GNU linker LD (GNU Binutils) version 2.21.
Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 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 1.3 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".
File: ld.info, Node: Top, Next: Overview, Up: (dir)
LD
**
This file documents the GNU linker ld (GNU Binutils) version 2.21.
This document is distributed under the terms of the GNU Free
Documentation License version 1.3. A copy of the license is included
in the section entitled "GNU Free Documentation License".
* Menu:
* Overview:: Overview
* Invocation:: Invocation
* Scripts:: Linker Scripts
* Machine Dependent:: Machine Dependent Features
* BFD:: BFD
* Reporting Bugs:: Reporting Bugs
* MRI:: MRI Compatible Script Files
* GNU Free Documentation License:: GNU Free Documentation License
* LD Index:: LD Index
File: ld.info, Node: Overview, Next: Invocation, Prev: Top, Up: Top
1 Overview
**********
`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 `ld'.
`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.
This version of `ld' uses the general purpose BFD libraries to
operate on object files. This allows `ld' to read, combine, and write
object files in many different formats--for example, COFF or `a.out'.
Different formats may be linked together to produce any available kind
of object file. *Note BFD::, for more information.
Aside from its flexibility, the GNU linker is more helpful than other
linkers in providing diagnostic information. Many linkers abandon
execution immediately upon encountering an error; whenever possible,
`ld' continues executing, allowing you to identify other errors (or, in
some cases, to get an output file in spite of the error).
File: ld.info, Node: Invocation, Next: Scripts, Prev: Overview, Up: Top
2 Invocation
************
The GNU linker `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.
* Menu:
* Options:: Command Line Options
* Environment:: Environment Variables
File: ld.info, Node: Options, Next: Environment, Up: Invocation
2.1 Command Line Options
========================
The linker supports a plethora of command-line options, but in actual
practice few of them are used in any particular context. For instance,
a frequent use of `ld' is to link standard Unix object files on a
standard, supported Unix system. On such a system, to link a file
`hello.o':
ld -o OUTPUT /lib/crt0.o hello.o -lc
This tells `ld' to produce a file called OUTPUT as the result of
linking the file `/lib/crt0.o' with `hello.o' and the library `libc.a',
which will come from the standard search directories. (See the
discussion of the `-l' option below.)
Some of the command-line options to `ld' may be specified at any
point in the command line. However, options which refer to files, such
as `-l' or `-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.
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 `-l', `-R', and the
script command language. If _no_ binary input files at all are
specified, the linker does not produce any output, and issues the
message `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 `-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
`INPUT' or `GROUP' to load other objects. Specifying a script in this
way merely augments the main linker script, with the extra commands
placed after the main script; use the `-T' option to replace the
default linker script entirely, but note the effect of the `INSERT'
command. *Note 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, `-trace-symbol' and
`--trace-symbol' are equivalent. Note--there is one exception to this
rule. Multiple letter options that start with a lower case 'o' can
only be preceded by two dashes. This is to reduce confusion with the
`-o' option. So for example `-omagic' sets the output file name to
`magic' whereas `--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,
`--trace-symbol foo' and `--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. `gcc') then all the linker command line options should be
prefixed by `-Wl,' (or whatever is appropriate for the particular
compiler driver) like this:
gcc -Wl,--start-group foo.o bar.o -Wl,--end-group
This is important, because otherwise the compiler driver program may
silently drop the linker options, resulting in a bad link. Confusion
may also arise when passing options that require values through a
driver, as the use of a space between option and argument acts as a
separator, and causes the driver to pass only the option to the linker
and the argument to the compiler. In this case, it is simplest to use
the joined forms of both single- and multiple-letter options, such as:
gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map
Here is a table of the generic command line switches accepted by the
GNU linker:
`@FILE'
Read command-line options from FILE. The options read are
inserted in place of the original @FILE option. If FILE does not
exist, or cannot be read, then the option will be treated
literally, and not removed.
Options in FILE are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character
(including a backslash) may be included by prefixing the character
to be included with a backslash. The FILE may itself contain
additional @FILE options; any such options will be processed
recursively.
`-a KEYWORD'
This option is supported for HP/UX compatibility. The KEYWORD
argument must be one of the strings `archive', `shared', or
`default'. `-aarchive' is functionally equivalent to `-Bstatic',
and the other two keywords are functionally equivalent to
`-Bdynamic'. This option may be used any number of times.
`--audit AUDITLIB'
Adds AUDITLIB to the `DT_AUDIT' entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the
DT_SONAME specified in the library. If specified multiple times
`DT_AUDIT' will contain a colon separated list of audit interfaces
to use. If the linker finds an object with an audit entry while
searching for shared libraries, it will add a corresponding
`DT_DEPAUDIT' entry in the output file. This option is only
meaningful on ELF platforms supporting the rtld-audit interface.
`-A ARCHITECTURE'
`--architecture=ARCHITECTURE'
In the current release of `ld', this option is useful only for the
Intel 960 family of architectures. In that `ld' configuration, the
ARCHITECTURE argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path. *Note `ld' and the Intel 960 family:
i960, for details.
Future releases of `ld' may support similar functionality for
other architecture families.
`-b INPUT-FORMAT'
`--format=INPUT-FORMAT'
`ld' may be configured to support more than one kind of object
file. If your `ld' is configured this way, you can use the `-b'
option to specify the binary format for input object files that
follow this option on the command line. Even when `ld' is
configured to support alternative object formats, you don't
usually need to specify this, as `ld' should be configured to
expect as a default input format the most usual format on each
machine. 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 `objdump -i'.) *Note BFD::.
You may want to use this option if you are linking files with an
unusual binary format. You can also use `-b' to switch formats
explicitly (when linking object files of different formats), by
including `-b INPUT-FORMAT' before each group of object files in a
particular format.
The default format is taken from the environment variable
`GNUTARGET'. *Note Environment::. You can also define the input
format from a script, using the command `TARGET'; see *note Format
Commands::.
`-c MRI-COMMANDFILE'
`--mri-script=MRI-COMMANDFILE'
For compatibility with linkers produced by MRI, `ld' accepts script
files written in an alternate, restricted command language,
described in *note MRI Compatible Script Files: MRI. Introduce
MRI script files with the option `-c'; use the `-T' option to run
linker scripts written in the general-purpose `ld' scripting
language. If MRI-CMDFILE does not exist, `ld' looks for it in the
directories specified by any `-L' options.
`-d'
`-dc'
`-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
`-r'). The script command `FORCE_COMMON_ALLOCATION' has the same
effect. *Note Miscellaneous Commands::.
`--depaudit AUDITLIB'
`-P AUDITLIB'
Adds AUDITLIB to the `DT_DEPAUDIT' entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the
DT_SONAME specified in the library. If specified multiple times
`DT_DEPAUDIT' will contain a colon separated list of audit
interfaces to use. This option is only meaningful on ELF
platforms supporting the rtld-audit interface. The -P option is
provided for Solaris compatibility.
`-e ENTRY'
`--entry=ENTRY'
Use ENTRY as the explicit symbol for beginning execution of your
program, rather than the default entry point. If there is no
symbol named ENTRY, the linker will try to parse ENTRY as a number,
and use that as the entry address (the number will be interpreted
in base 10; you may use a leading `0x' for base 16, or a leading
`0' for base 8). *Note Entry Point::, for a discussion of defaults
and other ways of specifying the entry point.
`--exclude-libs LIB,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 `--exclude-libs ALL' excludes
symbols in all archive libraries from automatic export. This
option is available only for the i386 PE targeted port of the
linker and for ELF targeted ports. For i386 PE, symbols
explicitly listed in a .def file are still exported, regardless of
this option. For ELF targeted ports, symbols affected by this
option will be treated as hidden.
`--exclude-modules-for-implib MODULE,MODULE,...'
Specifies a list of object files or archive members, from which
symbols should not be automatically exported, but which should be
copied wholesale into the import library being generated during
the link. The module names may be delimited by commas or colons,
and must match exactly the filenames used by `ld' to open the
files; for archive members, this is simply the member name, but
for object files the name listed must include and match precisely
any path used to specify the input file on the linker's
command-line. This option is available only for the i386 PE
targeted port of the linker. Symbols explicitly listed in a .def
file are still exported, regardless of this option.
`-E'
`--export-dynamic'
`--no-export-dynamic'
When creating a dynamically linked executable, using the `-E'
option or the `--export-dynamic' option causes the linker to 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 either of these options (or use the
`--no-export-dynamic' option to restore the default behavior), the
dynamic symbol table will normally contain only those symbols
which are referenced by some dynamic object mentioned in the link.
If you use `dlopen' to load a dynamic object which needs to refer
back to the symbols defined by the program, rather than some other
dynamic object, then you will probably need to use this option when
linking the program itself.
You can also use the dynamic list to control what symbols should
be added to the dynamic symbol table if the output format supports
it. See the description of `--dynamic-list'.
Note that this option is specific to ELF targeted ports. PE
targets support a similar function to export all symbols from a
DLL or EXE; see the description of `--export-all-symbols' below.
`-EB'
Link big-endian objects. This affects the default output format.
`-EL'
Link little-endian objects. This affects the default output
format.
`-f NAME'
`--auxiliary=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 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 NAME. If there is one, it will be used instead of
the definition in the filter object. The shared object NAME need
not exist. Thus the shared object 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.
`-F NAME'
`--filter=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 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 NAME. Thus the
filter object can be used to select a subset of the symbols
provided by the object NAME.
Some older linkers used the `-F' option throughout a compilation
toolchain for specifying object-file format for both input and
output object files. The GNU linker uses other mechanisms for
this purpose: the `-b', `--format', `--oformat' options, the
`TARGET' command in linker scripts, and the `GNUTARGET'
environment variable. The GNU linker will ignore the `-F' option
when not creating an ELF shared object.
`-fini=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 `_fini'
as the function to call.
`-g'
Ignored. Provided for compatibility with other tools.
`-G VALUE'
`--gpsize=VALUE'
Set the maximum size of objects to be optimized using the GP
register to 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.
`-h NAME'
`-soname=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.
`-i'
Perform an incremental link (same as option `-r').
`-init=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 `_init'
as the function to call.
`-l NAMESPEC'
`--library=NAMESPEC'
Add the archive or object file specified by NAMESPEC to the list
of files to link. This option may be used any number of times.
If NAMESPEC is of the form `:FILENAME', `ld' will search the
library path for a file called FILENAME, otherwise it will search
the library path for a file called `libNAMESPEC.a'.
On systems which support shared libraries, `ld' may also search for
files other than `libNAMESPEC.a'. Specifically, on ELF and SunOS
systems, `ld' will search a directory for a library called
`libNAMESPEC.so' before searching for one called `libNAMESPEC.a'.
(By convention, a `.so' extension indicates a shared library.)
Note that this behavior does not apply to `:FILENAME', which
always specifies a file called FILENAME.
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 for a way to force the linker to search
archives multiple times.
You may list the same archive multiple times on the command line.
This type of archive searching is standard for Unix linkers.
However, if you are using `ld' on AIX, note that it is different
from the behaviour of the AIX linker.
`-L SEARCHDIR'
`--library-path=SEARCHDIR'
Add path SEARCHDIR to the list of paths that `ld' will search for
archive libraries and `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 `-L' options apply to all `-l' options,
regardless of the order in which the options appear. `-L' options
do not affect how `ld' searches for a linker script unless `-T'
option is specified.
If SEARCHDIR begins with `=', then the `=' will be replaced by the
"sysroot prefix", a path specified when the linker is configured.
The default set of paths searched (without being specified with
`-L') depends on which emulation mode `ld' is using, and in some
cases also on how it was configured. *Note Environment::.
The paths can also be specified in a link script with the
`SEARCH_DIR' command. Directories specified this way are searched
at the point in which the linker script appears in the command
line.
`-m EMULATION'
Emulate the EMULATION linker. You can list the available
emulations with the `--verbose' or `-V' options.
If the `-m' option is not used, the emulation is taken from the
`LDEMULATION' environment variable, if that is defined.
Otherwise, the default emulation depends upon how the linker was
configured.
`-M'
`--print-map'
Print a link map to the standard output. A link map provides
information about the link, including the following:
* Where object files are mapped into memory.
* How common symbols are allocated.
* All archive members included in the link, with a mention of
the symbol which caused the archive member to be brought in.
* The values assigned to symbols.
Note - symbols whose values are computed by an expression
which involves a reference to a previous value of the same
symbol may not have correct result displayed in the link map.
This is because the linker discards intermediate results and
only retains the final value of an expression. Under such
circumstances the linker will display the final value
enclosed by square brackets. Thus for example a linker
script containing:
foo = 1
foo = foo * 4
foo = foo + 8
will produce the following output in the link map if the `-M'
option is used:
0x00000001 foo = 0x1
[0x0000000c] foo = (foo * 0x4)
[0x0000000c] foo = (foo + 0x8)
See *note Expressions:: for more information about
expressions in linker scripts.
`-n'
`--nmagic'
Turn off page alignment of sections, and disable linking against
shared libraries. If the output format supports Unix style magic
numbers, mark the output as `NMAGIC'.
`-N'
`--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 `OMAGIC'. Note: Although a writable
text section is allowed for PE-COFF targets, it does not conform
to the format specification published by Microsoft.
`--no-omagic'
This option negates most of the effects of the `-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 `-Bdynamic' for this.
`-o OUTPUT'
`--output=OUTPUT'
Use OUTPUT as the name for the program produced by `ld'; if this
option is not specified, the name `a.out' is used by default. The
script command `OUTPUT' can also specify the output file name.
`-O LEVEL'
If LEVEL is a numeric values greater than zero `ld' optimizes the
output. This might take significantly longer and therefore
probably should only be enabled for the final binary. At the
moment this option only affects ELF shared library generation.
Future releases of the linker may make more use of this option.
Also currently there is no difference in the linker's behaviour
for different non-zero values of this option. Again this may
change with future releases.
`-q'
`--emit-relocs'
Leave relocation sections and contents in fully linked executables.
Post link analysis and optimization tools may need this
information in order to perform correct modifications of
executables. This results in larger executables.
This option is currently only supported on ELF platforms.
`--force-dynamic'
Force the output file to have dynamic sections. This option is
specific to VxWorks targets.
`-r'
`--relocatable'
Generate relocatable output--i.e., generate an output file that
can in turn serve as input to `ld'. This is often called "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 `OMAGIC'. If this option is not specified, an absolute
file is produced. When linking C++ programs, this option _will
not_ resolve references to constructors; to do that, use `-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 `a.out'-based formats do
not support partial linking with input files in other formats at
all.
This option does the same thing as `-i'.
`-R FILENAME'
`--just-symbols=FILENAME'
Read symbol names and their addresses from 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 `-R' option is
followed by a directory name, rather than a file name, it is
treated as the `-rpath' option.
`-s'
`--strip-all'
Omit all symbol information from the output file.
`-S'
`--strip-debug'
Omit debugger symbol information (but not all symbols) from the
output file.
`-t'
`--trace'
Print the names of the input files as `ld' processes them.
`-T SCRIPTFILE'
`--script=SCRIPTFILE'
Use SCRIPTFILE as the linker script. This script replaces `ld''s
default linker script (rather than adding to it), so COMMANDFILE
must specify everything necessary to describe the output file.
*Note Scripts::. If SCRIPTFILE does not exist in the current
directory, `ld' looks for it in the directories specified by any
preceding `-L' options. Multiple `-T' options accumulate.
`-dT SCRIPTFILE'
`--default-script=SCRIPTFILE'
Use SCRIPTFILE as the default linker script. *Note Scripts::.
This option is similar to the `--script' option except that
processing of the script is delayed until after the rest of the
command line has been processed. This allows options placed after
the `--default-script' option on the command line to affect the
behaviour of the linker script, which can be important when the
linker command line cannot be directly controlled by the user.
(eg because the command line is being constructed by another tool,
such as `gcc').
`-u SYMBOL'
`--undefined=SYMBOL'
Force 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. `-u' may be repeated with
different option arguments to enter additional undefined symbols.
This option is equivalent to the `EXTERN' linker script command.
`-Ur'
For anything other than C++ programs, this option is equivalent to
`-r': it generates relocatable output--i.e., an output file that
can in turn serve as input to `ld'. When linking C++ programs,
`-Ur' _does_ resolve references to constructors, unlike `-r'. It
does not work to use `-Ur' on files that were themselves linked
with `-Ur'; once the constructor table has been built, it cannot
be added to. Use `-Ur' only for the last partial link, and `-r'
for the others.
`--unique[=SECTION]'
Creates a separate output section for every input section matching
SECTION, or if the optional wildcard 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.
`-v'
`--version'
`-V'
Display the version number for `ld'. The `-V' option also lists
the supported emulations.
`-x'
`--discard-all'
Delete all local symbols.
`-X'
`--discard-locals'
Delete all temporary local symbols. (These symbols start with
system-specific local label prefixes, typically `.L' for ELF
systems or `L' for traditional a.out systems.)
`-y SYMBOL'
`--trace-symbol=SYMBOL'
Print the name of each linked file in which 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.
`-Y PATH'
Add PATH to the default library search path. This option exists
for Solaris compatibility.
`-z KEYWORD'
The recognized keywords are:
`combreloc'
Combines multiple reloc sections and sorts them to make
dynamic symbol lookup caching possible.
`defs'
Disallows undefined symbols in object files. Undefined
symbols in shared libraries are still allowed.
`execstack'
Marks the object as requiring executable stack.
`initfirst'
This option is only meaningful when building a shared object.
It marks the object so that its runtime initialization will
occur before the runtime initialization of any other objects
brought into the process at the same time. Similarly the
runtime finalization of the object will occur after the
runtime finalization of any other objects.
`interpose'
Marks the object that its symbol table interposes before all
symbols but the primary executable.
`lazy'
When generating an executable or shared library, mark it to
tell the dynamic linker to defer function call resolution to
the point when the function is called (lazy binding), rather
than at load time. Lazy binding is the default.
`loadfltr'
Marks the object that its filters be processed immediately at
runtime.
`muldefs'
Allows multiple definitions.
`nocombreloc'
Disables multiple reloc sections combining.
`nocopyreloc'
Disables production of copy relocs.
`nodefaultlib'
Marks the object that the search for dependencies of this
object will ignore any default library search paths.
`nodelete'
Marks the object shouldn't be unloaded at runtime.
`nodlopen'
Marks the object not available to `dlopen'.
`nodump'
Marks the object can not be dumped by `dldump'.
`noexecstack'
Marks the object as not requiring executable stack.
`norelro'
Don't create an ELF `PT_GNU_RELRO' segment header in the
object.
`now'
When generating an executable or shared library, mark it to
tell the dynamic linker to resolve all symbols when the
program is started, or when the shared library is linked to
using dlopen, instead of deferring function call resolution
to the point when the function is first called.
`origin'
Marks the object may contain $ORIGIN.
`relro'
Create an ELF `PT_GNU_RELRO' segment header in the object.
`max-page-size=VALUE'
Set the emulation maximum page size to VALUE.
`common-page-size=VALUE'
Set the emulation common page size to VALUE.
Other keywords are ignored for Solaris compatibility.
`-( ARCHIVES -)'
`--start-group ARCHIVES --end-group'
The ARCHIVES should be a list of archive files. They may be
either explicit file names, or `-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.
`--accept-unknown-input-arch'
`--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 `--accept-unknown-input-arch' option has
been added to restore the old behaviour.
`--as-needed'
`--no-as-needed'
This option affects ELF DT_NEEDED tags for dynamic libraries
mentioned on the command line after the `--as-needed' option.
Normally the linker will add a DT_NEEDED tag for each dynamic
library mentioned on the command line, regardless of whether the
library is actually needed or not. `--as-needed' causes a
DT_NEEDED tag to only be emitted for a library that satisfies an
undefined symbol reference from a regular object file or, if the
library is not found in the DT_NEEDED lists of other libraries
linked up to that point, an undefined symbol reference from
another dynamic library. `--no-as-needed' restores the default
behaviour.
`--add-needed'
`--no-add-needed'
These two options have been deprecated because of the similarity of
their names to the `--as-needed' and `--no-as-needed' options.
They have been replaced by `--copy-dt-needed-entries' and
`--no-copy-dt-needed-entries'.
`-assert KEYWORD'
This option is ignored for SunOS compatibility.
`-Bdynamic'
`-dy'
`-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 `-l' options which follow it.
`-Bgroup'
Set the `DF_1_GROUP' flag in the `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.
`--unresolved-symbols=report-all' is implied. This option is only
meaningful on ELF platforms which support shared libraries.
`-Bstatic'
`-dn'
`-non_shared'
`-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 `-l' options which follow
it. This option also implies `--unresolved-symbols=report-all'.
This option can be used with `-shared'. Doing so means that a
shared library is being created but that all of the library's
external references must be resolved by pulling in entries from
static libraries.
`-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.
`-Bsymbolic-functions'
When creating a shared library, bind references to global function
symbols to the definition within the shared library, if any. This
option is only meaningful on ELF platforms which support shared
libraries.
`--dynamic-list=DYNAMIC-LIST-FILE'
Specify the name of a dynamic list file to the linker. This is
typically used when creating shared libraries to specify a list of
global symbols whose references shouldn't be bound to the
definition within the shared library, or creating dynamically
linked executables to specify a list of symbols which should be
added to the symbol table in the executable. This option is only
meaningful on ELF platforms which support shared libraries.
The format of the dynamic list is the same as the version node
without scope and node name. See *note VERSION:: for more
information.
`--dynamic-list-data'
Include all global data symbols to the dynamic list.
`--dynamic-list-cpp-new'
Provide the builtin dynamic list for C++ operator new and delete.
It is mainly useful for building shared libstdc++.
`--dynamic-list-cpp-typeinfo'
Provide the builtin dynamic list for C++ runtime type
identification.
`--check-sections'
`--no-check-sections'
Asks the linker _not_ to check section addresses after they have
been assigned to see if there are any overlaps. Normally the
linker will perform this check, and if it finds any overlaps it
will produce suitable error messages. The linker does know about,
and does make allowances for sections in overlays. The default
behaviour can be restored by using the command line switch
`--check-sections'. Section overlap is not usually checked for
relocatable links. You can force checking in that case by using
the `--check-sections' option.
`--copy-dt-needed-entries'
`--no-copy-dt-needed-entries'
This option affects the treatment of dynamic libraries referred to
by DT_NEEDED tags _inside_ ELF dynamic libraries mentioned on the
command line. Normally the linker will add a DT_NEEDED tag to the
output binary for each library mentioned in a DT_NEEDED tag in an
input dynamic library. With `--no-copy-dt-needed-entries'
specified on the command line however any dynamic libraries that
follow it will have their DT_NEEDED entries ignored. The default
behaviour can be restored with `--copy-dt-needed-entries'.
This option also has an effect on the resolution of symbols in
dynamic libraries. With the default setting dynamic libraries
mentioned on the command line will be recursively searched,
following their DT_NEEDED tags to other libraries, in order to
resolve symbols required by the output binary. With
`--no-copy-dt-needed-entries' specified however the searching of
dynamic libraries that follow it will stop with the dynamic
library itself. No DT_NEEDED links will be traversed to resolve
symbols.
`--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.
`--no-define-common'
This option inhibits the assignment of addresses to common symbols.
The script command `INHIBIT_COMMON_ALLOCATION' has the same effect.
*Note Miscellaneous Commands::.
The `--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 `--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.
`--defsym=SYMBOL=EXPRESSION'
Create a global symbol in the output file, containing the absolute
address given by 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 EXPRESSION in
this context: you may give a hexadecimal constant or the name of
an existing symbol, or use `+' and `-' to add or subtract
hexadecimal constants or symbols. If you need more elaborate
expressions, consider using the linker command language from a
script (*note Assignment: Symbol Definitions: Assignments.).
_Note:_ there should be no white space between SYMBOL, the equals
sign ("<=>"), and EXPRESSION.
`--demangle[=STYLE]'
`--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 `COLLECT_NO_DEMANGLE' is
set. These options may be used to override the default.
`-IFILE'
`--dynamic-linker=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.
`--fatal-warnings'
`--no-fatal-warnings'
Treat all warnings as errors. The default behaviour can be
restored with the option `--no-fatal-warnings'.
`--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
`.exe' or `.dll' suffix, this option forces the linker to copy the
output file to one of the same name with a `.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 `.exe' suffix.
`--gc-sections'
`--no-gc-sections'
Enable garbage collection of unused input sections. It is ignored
on targets that do not support this option. The default behaviour
(of not performing this garbage collection) can be restored by
specifying `--no-gc-sections' on the command line.
`--gc-sections' decides which input sections are used by examining
symbols and relocations. The section containing the entry symbol
and all sections containing symbols undefined on the command-line
will be kept, as will sections containing symbols referenced by
dynamic objects. Note that when building shared libraries, the
linker must assume that any visible symbol is referenced. Once
this initial set of sections has been determined, the linker
recursively marks as used any section referenced by their
relocations. See `--entry' and `--undefined'.
This option can be set when doing a partial link (enabled with
option `-r'). In this case the root of symbols kept must be
explicitly specified either by an `--entry' or `--undefined'
option or by a `ENTRY' command in the linker script.
`--print-gc-sections'
`--no-print-gc-sections'
List all sections removed by garbage collection. The listing is
printed on stderr. This option is only effective if garbage
collection has been enabled via the `--gc-sections') option. The
default behaviour (of not listing the sections that are removed)
can be restored by specifying `--no-print-gc-sections' on the
command line.
`--help'
Print a summary of the command-line options on the standard output
and exit.
`--target-help'
Print a summary of all target specific options on the standard
output and exit.
`-Map=MAPFILE'
Print a link map to the file MAPFILE. See the description of the
`-M' option, above.
`--no-keep-memory'
`ld' normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells `ld' to
instead optimize for memory usage, by rereading the symbol tables
as necessary. This may be required if `ld' runs out of memory
space while linking a large executable.
`--no-undefined'
`-z defs'
Report unresolved symbol references from regular object files.
This is done even if the linker is creating a non-symbolic shared
library. The switch `--[no-]allow-shlib-undefined' controls the
behaviour for reporting unresolved references found in shared
libraries being linked in.
`--allow-multiple-definition'
`-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.
`--allow-shlib-undefined'
`--no-allow-shlib-undefined'
Allows or disallows undefined symbols in shared libraries. This
switch is similar to `--no-undefined' except that it determines
the behaviour when the undefined symbols are in a shared library
rather than a regular object file. It does not affect how
undefined symbols in regular object files are handled.
The default behaviour is to report errors for any undefined symbols
referenced in shared libraries if the linker is being used to
create an executable, but to allow them if the linker is being
used to create a shared library.
The reasons for allowing undefined symbol references in shared
libraries specified at link time are that:
* A shared library specified at link time may not be the same
as the one that is available at load time, so the symbol
might actually be resolvable at load time.
* There are some operating systems, eg BeOS and HPPA, where
undefined symbols in shared libraries are normal.
The BeOS kernel for example patches shared libraries at load
time to select whichever function is most appropriate for the
current architecture. This is used, for example, to
dynamically select an appropriate memset function.
`--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.
`--default-symver'
Create and use a default symbol version (the soname) for
unversioned exported symbols.
`--default-imported-symver'
Create and use a default symbol version (the soname) for
unversioned imported symbols.
`--no-warn-mismatch'
Normally `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 `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.
`--no-warn-search-mismatch'
Normally `ld' will give a warning if it finds an incompatible
library during a library search. This option silences the warning.
`--no-whole-archive'
Turn off the effect of the `--whole-archive' option for subsequent
archive files.
`--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.
`-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.
`--oformat=OUTPUT-FORMAT'
`ld' may be configured to support more than one kind of object
file. If your `ld' is configured this way, you can use the
`--oformat' option to specify the binary format for the output
object file. Even when `ld' is configured to support alternative
object formats, you don't usually need to specify this, as `ld'
should be configured to produce as a default output format the most
usual format on each machine. 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 `objdump -i'.) The
script command `OUTPUT_FORMAT' can also specify the output format,
but this option overrides it. *Note BFD::.
`-pie'
`--pic-executable'
Create a position independent executable. This is currently only
supported on ELF platforms. Position independent executables are
similar to shared libraries in that they are relocated by the
dynamic linker to the virtual address the OS chooses for them
(which can vary between invocations). Like normal dynamically
linked executables they can be executed and symbols defined in the
executable cannot be overridden by shared libraries.
`-qmagic'
This option is ignored for Linux compatibility.
`-Qy'
This option is ignored for SVR4 compatibility.
`--relax'
`--no-relax'
An option with machine dependent effects. This option is only
supported on a few targets. *Note `ld' and the H8/300: H8/300.
*Note `ld' and the Intel 960 family: i960. *Note `ld' and Xtensa
Processors: Xtensa. *Note `ld' and the 68HC11 and 68HC12:
M68HC11/68HC12. *Note `ld' and PowerPC 32-bit ELF Support:
PowerPC ELF32.
On some platforms the `--relax' option performs target specific,
global optimizations that become possible when the linker resolves
addressing in the program, such as relaxing address modes,
synthesizing new instructions, selecting shorter version of current
instructions, and combinig constant values.
On some platforms these link time global optimizations may make
symbolic debugging of the resulting executable impossible. This
is known to be the case for the Matsushita MN10200 and MN10300
family of processors.
On platforms where this is not supported, `--relax' is accepted,
but ignored.
On platforms where `--relax' is accepted the option `--no-relax'
can be used to disable the feature.
`--retain-symbols-file=FILENAME'
Retain _only_ the symbols listed in the file FILENAME, discarding
all others. FILENAME is simply a flat file, with one symbol name
per line. This option is especially useful in environments (such
as VxWorks) where a large global symbol table is accumulated
gradually, to conserve run-time memory.
`--retain-symbols-file' does _not_ discard undefined symbols, or
symbols needed for relocations.
You may only specify `--retain-symbols-file' once in the command
line. It overrides `-s' and `-S'.
`-rpath=DIR'
Add a directory to the runtime library search path. This is used
when linking an ELF executable with shared objects. All `-rpath'
arguments are concatenated and passed to the runtime linker, which
uses them to locate shared objects at runtime. The `-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 `-rpath-link' option. If `-rpath' is not used
when linking an ELF executable, the contents of the environment
variable `LD_RUN_PATH' will be used if it is defined.
The `-rpath' option may also be used on SunOS. By default, on
SunOS, the linker will form a runtime search patch out of all the
`-L' options it is given. If a `-rpath' option is used, the
runtime search path will be formed exclusively using the `-rpath'
options, ignoring the `-L' options. This can be useful when using
gcc, which adds many `-L' options which may be on NFS mounted file
systems.
For compatibility with other ELF linkers, if the `-R' option is
followed by a directory name, rather than a file name, it is
treated as the `-rpath' option.
`-rpath-link=DIR'
When using ELF or SunOS, one shared library may require another.
This happens when an `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 `-rpath-link'
option specifies the first set of directories to search. The
`-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:
1. Any directories specified by `-rpath-link' options.
2. Any directories specified by `-rpath' options. The difference
between `-rpath' and `-rpath-link' is that directories
specified by `-rpath' options are included in the executable
and used at runtime, whereas the `-rpath-link' option is only
effective at link time. Searching `-rpath' in this way is
only supported by native linkers and cross linkers which have
been configured with the `--with-sysroot' option.
3. On an ELF system, for native linkers, if the `-rpath' and
`-rpath-link' options were not used, search the contents of
the environment variable `LD_RUN_PATH'.
4. On SunOS, if the `-rpath' option was not used, search any
directories specified using `-L' options.
5. For a native linker, the search the contents of the
environment variable `LD_LIBRARY_PATH'.
6. For a native ELF linker, the directories in `DT_RUNPATH' or
`DT_RPATH' of a shared library are searched for shared
libraries needed by it. The `DT_RPATH' entries are ignored if
`DT_RUNPATH' entries exist.
7. The default directories, normally `/lib' and `/usr/lib'.
8. For a native linker on an ELF system, if the file
`/etc/ld.so.conf' exists, the list of directories found in
that file.
If the required shared library is not found, the linker will issue
a warning and continue with the link.
`-shared'
`-Bshareable'
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 `-e' option is not
used and there are undefined symbols in the link.
`--sort-common'
`--sort-common=ascending'
`--sort-common=descending'
This option tells `ld' to sort the common symbols by alignment in
ascending or descending order when it places them in the
appropriate output sections. The symbol alignments considered are
sixteen-byte or larger, eight-byte, four-byte, two-byte, and
one-byte. This is to prevent gaps between symbols due to alignment
constraints. If no sorting order is specified, then descending
order is assumed.
`--sort-section=name'
This option will apply `SORT_BY_NAME' to all wildcard section
patterns in the linker script.
`--sort-section=alignment'
This option will apply `SORT_BY_ALIGNMENT' to all wildcard section
patterns in the linker script.
`--split-by-file[=SIZE]'
Similar to `--split-by-reloc' but creates a new output section for
each input file when SIZE is reached. SIZE defaults to a size of
1 if not given.
`--split-by-reloc[=COUNT]'
Tries to creates extra sections in the output file so that no
single output section in the file contains more than 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
COUNT relocations one output section will contain that many
relocations. COUNT defaults to a value of 32768.
`--stats'
Compute and display statistics about the operation of the linker,
such as execution time and memory usage.
`--sysroot=DIRECTORY'
Use DIRECTORY as the location of the sysroot, overriding the
configure-time default. This option is only supported by linkers
that were configured using `--with-sysroot'.
`--traditional-format'
For some targets, the output of `ld' is different in some ways from
the output of some existing linker. This switch requests `ld' to
use the traditional format instead.
For example, on SunOS, `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 `dbx' program can not read the resulting
program (`gdb' has no trouble). The `--traditional-format' switch
tells `ld' to not combine duplicate entries.
`--section-start=SECTIONNAME=ORG'
Locate a section in the output file at the absolute address given
by ORG. You may use this option as many times as necessary to
locate multiple sections in the command line. ORG must be a
single hexadecimal integer; for compatibility with other linkers,
you may omit the leading `0x' usually associated with hexadecimal
values. _Note:_ there should be no white space between
SECTIONNAME, the equals sign ("<=>"), and ORG.
`-Tbss=ORG'
`-Tdata=ORG'
`-Ttext=ORG'
Same as `--section-start', with `.bss', `.data' or `.text' as the
SECTIONNAME.
`-Ttext-segment=ORG'
When creating an ELF executable or shared object, it will set the
address of the first byte of the text segment.
`--unresolved-symbols=METHOD'
Determine how to handle unresolved symbols. There are four
possible values for `method':
`ignore-all'
Do not report any unresolved symbols.
`report-all'
Report all unresolved symbols. This is the default.
`ignore-in-object-files'
Report unresolved symbols that are contained in shared
libraries, but ignore them if they come from regular object
files.
`ignore-in-shared-libs'
Report unresolved symbols that come from regular object
files, but ignore them if they come from shared libraries.
This can be useful when creating a dynamic binary and it is
known that all the shared libraries that it should be
referencing are included on the linker's command line.
The behaviour for shared libraries on their own can also be
controlled by the `--[no-]allow-shlib-undefined' option.
Normally the linker will generate an error message for each
reported unresolved symbol but the option
`--warn-unresolved-symbols' can change this to a warning.
`--dll-verbose'
`--verbose'
Display the version number for `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.
`--version-script=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 fully supported on ELF
platforms which support shared libraries; see *note VERSION::. It
is partially supported on PE platforms, which can use version
scripts to filter symbol visibility in auto-export mode: any
symbols marked `local' in the version script will not be exported.
*Note WIN32::.
`--warn-common'
Warn when a common symbol is combined with another common symbol
or with a symbol definition. Unix linkers allow this somewhat
sloppy practise, but linkers on some other operating systems do
not. This option allows you to find potential problems from
combining global symbols. Unfortunately, some C libraries use
this practise, so you may get some warnings about symbols in the
libraries as well as in your programs.
There are three kinds of global symbols, illustrated here by C
examples:
`int i = 1;'
A definition, which goes in the initialized data section of
the output file.
`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.
`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.
The `--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.
1. Turning a common symbol into a reference, because there is
already a definition for the symbol.
FILE(SECTION): warning: common of `SYMBOL'
overridden by definition
FILE(SECTION): warning: defined here
2. 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.
FILE(SECTION): warning: definition of `SYMBOL'
overriding common
FILE(SECTION): warning: common is here
3. Merging a common symbol with a previous same-sized common
symbol.
FILE(SECTION): warning: multiple common
of `SYMBOL'
FILE(SECTION): warning: previous common is here
4. Merging a common symbol with a previous larger common symbol.
FILE(SECTION): warning: common of `SYMBOL'
overridden by larger common
FILE(SECTION): warning: larger common is here
5. 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.
FILE(SECTION): warning: common of `SYMBOL'
overriding smaller common
FILE(SECTION): warning: smaller common is here
`--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.
`--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.
`--warn-once'
Only warn once for each undefined symbol, rather than once per
module which refers to it.
`--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 `SECTIONS' command does not specify a
start address for the section (*note SECTIONS::).
`--warn-shared-textrel'
Warn if the linker adds a DT_TEXTREL to a shared object.
`--warn-alternate-em'
Warn if an object has alternate ELF machine code.
`--warn-unresolved-symbols'
If the linker is going to report an unresolved symbol (see the
option `--unresolved-symbols') it will normally generate an error.
This option makes it generate a warning instead.
`--error-unresolved-symbols'
This restores the linker's default behaviour of generating errors
when it is reporting unresolved symbols.
`--whole-archive'
For each archive mentioned on the command line after the
`--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 `-Wl,-whole-archive'.
Second, don't forget to use `-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.
`--wrap=SYMBOL'
Use a wrapper function for SYMBOL. Any undefined reference to
SYMBOL will be resolved to `__wrap_SYMBOL'. Any undefined
reference to `__real_SYMBOL' will be resolved to SYMBOL.
This can be used to provide a wrapper for a system function. The
wrapper function should be called `__wrap_SYMBOL'. If it wishes
to call the system function, it should call `__real_SYMBOL'.
Here is a trivial example:
void *
__wrap_malloc (size_t c)
{
printf ("malloc called with %zu\n", c);
return __real_malloc (c);
}
If you link other code with this file using `--wrap malloc', then
all calls to `malloc' will call the function `__wrap_malloc'
instead. The call to `__real_malloc' in `__wrap_malloc' will call
the real `malloc' function.
You may wish to provide a `__real_malloc' function as well, so that
links without the `--wrap' option will succeed. If you do this,
you should not put the definition of `__real_malloc' in the same
file as `__wrap_malloc'; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to `malloc'.
`--eh-frame-hdr'
Request creation of `.eh_frame_hdr' section and ELF
`PT_GNU_EH_FRAME' segment header.
`--enable-new-dtags'
`--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
`--enable-new-dtags', the dynamic tags will be created as needed.
If you specify `--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.
`--hash-size=NUMBER'
Set the default size of the linker's hash tables to a prime number
close to NUMBER. Increasing this value can reduce the length of
time it takes the linker to perform its tasks, at the expense of
increasing the linker's memory requirements. Similarly reducing
this value can reduce the memory requirements at the expense of
speed.
`--hash-style=STYLE'
Set the type of linker's hash table(s). STYLE can be either
`sysv' for classic ELF `.hash' section, `gnu' for new style GNU
`.gnu.hash' section or `both' for both the classic ELF `.hash' and
new style GNU `.gnu.hash' hash tables. The default is `sysv'.
`--reduce-memory-overheads'
This option reduces memory requirements at ld runtime, at the
expense of linking speed. This was introduced to select the old
O(n^2) algorithm for link map file generation, rather than the new
O(n) algorithm which uses about 40% more memory for symbol storage.
Another effect of the switch is to set the default hash table size
to 1021, which again saves memory at the cost of lengthening the
linker's run time. This is not done however if the `--hash-size'
switch has been used.
The `--reduce-memory-overheads' switch may be also be used to
enable other tradeoffs in future versions of the linker.
`--build-id'
`--build-id=STYLE'
Request creation of `.note.gnu.build-id' ELF note section. The
contents of the note are unique bits identifying this linked file.
STYLE can be `uuid' to use 128 random bits, `sha1' to use a
160-bit SHA1 hash on the normative parts of the output contents,
`md5' to use a 128-bit MD5 hash on the normative parts of the
output contents, or `0xHEXSTRING' to use a chosen bit string
specified as an even number of hexadecimal digits (`-' and `:'
characters between digit pairs are ignored). If STYLE is omitted,
`sha1' is used.
The `md5' and `sha1' styles produces an identifier that is always
the same in an identical output file, but will be unique among all
nonidentical output files. It is not intended to be compared as a
checksum for the file's contents. A linked file may be changed
later by other tools, but the build ID bit string identifying the
original linked file does not change.
Passing `none' for STYLE disables the setting from any
`--build-id' options earlier on the command line.
2.1.1 Options Specific to i386 PE Targets
-----------------------------------------
The i386 PE linker supports the `-shared' option, which causes the
output to be a dynamically linked library (DLL) instead of a normal
executable. You should name the output `*.dll' when you use this
option. In addition, the linker fully supports the standard `*.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.
`--add-stdcall-alias'
If given, symbols with a stdcall suffix (@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]
`--base-file FILE'
Use FILE as the name of a file in which to save the base addresses
of all the relocations needed for generating DLLs with `dlltool'.
[This is an i386 PE specific option]
`--dll'
Create a DLL instead of a regular executable. You may also use
`-shared' or specify a `LIBRARY' in a given `.def' file. [This
option is specific to the i386 PE targeted port of the linker]
`--enable-long-section-names'
`--disable-long-section-names'
The PE variants of the Coff object format add an extension that
permits the use of section names longer than eight characters, the
normal limit for Coff. By default, these names are only allowed
in object files, as fully-linked executable images do not carry
the Coff string table required to support the longer names. As a
GNU extension, it is possible to allow their use in executable
images as well, or to (probably pointlessly!) disallow it in
object files, by using these two options. Executable images
generated with these long section names are slightly non-standard,
carrying as they do a string table, and may generate confusing
output when examined with non-GNU PE-aware tools, such as file
viewers and dumpers. However, GDB relies on the use of PE long
section names to find Dwarf-2 debug information sections in an
executable image at runtime, and so if neither option is specified
on the command-line, `ld' will enable long section names,
overriding the default and technically correct behaviour, when it
finds the presence of debug information while linking an executable
image and not stripping symbols. [This option is valid for all PE
targeted ports of the linker]
`--enable-stdcall-fixup'
`--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 `_foo' might be linked to the
function `_foo@12', or the undefined symbol `_bar@16' might be
linked to the function `_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
`--enable-stdcall-fixup', this feature is fully enabled and
warnings are not printed. If you specify
`--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]
`--leading-underscore'
`--no-leading-underscore'
For most targets default symbol-prefix is an underscore and is
defined in target's description. By this option it is possible to
disable/enable the default underscore symbol-prefix.
`--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 `DllMain@12',
`DllEntryPoint@0', `DllMainCRTStartup@12', and `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 `_head_' or
ending with `_iname'. In addition, no symbols from `libgcc',
`libstd++', `libmingw32', or `crtX.o' will be exported. Symbols
whose names begin with `__rtti_' or `__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: `_cygwin_dll_entry@12',
`_cygwin_crt0_common@8', `_cygwin_noncygwin_dll_entry@12',
`_fmode', `_impure_ptr', `cygwin_attach_dll', `cygwin_premain0',
`cygwin_premain1', `cygwin_premain2', `cygwin_premain3', and
`environ'. [This option is specific to the i386 PE targeted port
of the linker]
`--exclude-symbols SYMBOL,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]
`--exclude-all-symbols'
Specifies no symbols should be automatically exported. [This
option is specific to the i386 PE targeted port of the linker]
`--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]
`--heap RESERVE'
`--heap RESERVE,COMMIT'
Specify the number of bytes 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]
`--image-base VALUE'
Use 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]
`--kill-at'
If given, the stdcall suffixes (@NN) will be stripped from symbols
before they are exported. [This option is specific to the i386 PE
targeted port of the linker]
`--large-address-aware'
If given, the appropriate bit in the "Characteristics" field of
the COFF header is set to indicate that this executable supports
virtual addresses greater than 2 gigabytes. This should be used
in conjunction with the /3GB or /USERVA=VALUE megabytes switch in
the "[operating systems]" section of the BOOT.INI. Otherwise,
this bit has no effect. [This option is specific to PE targeted
ports of the linker]
`--major-image-version 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]
`--major-os-version 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]
`--major-subsystem-version 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]
`--minor-image-version 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]
`--minor-os-version 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]
`--minor-subsystem-version 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]
`--output-def FILE'
The linker will create the file FILE which will contain a DEF file
corresponding to the DLL the linker is generating. This DEF file
(which should be called `*.def') may be used to create an import
library with `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]
`--out-implib FILE'
The linker will create the file FILE which will contain an import
lib corresponding to the DLL the linker is generating. This import
lib (which should be called `*.dll.a' or `*.a' may be used to link
clients against the generated DLL; this behaviour makes it
possible to skip a separate `dlltool' import library creation step.
[This option is specific to the i386 PE targeted port of the
linker]
`--enable-auto-image-base'
Automatically choose the image base for DLLs, unless one is
specified using the `--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]
`--disable-auto-image-base'
Do not automatically generate a unique image base. If there is no
user-specified image base (`--image-base') then use the platform
default. [This option is specific to the i386 PE targeted port of
the linker]
`--dll-search-prefix STRING'
When linking dynamically to a dll without an import library,
search for `<string><basename>.dll' in preference to
`lib<basename>.dll'. This behaviour allows easy distinction
between DLLs built for the various "subplatforms": native, cygwin,
uwin, pw, etc. For instance, cygwin DLLs typically use
`--dll-search-prefix=cyg'. [This option is specific to the i386
PE targeted port of the linker]
`--enable-auto-import'
Do sophisticated linking of `_symbol' to `__imp__symbol' for DATA
imports from DLLs, and create the necessary thunking symbols when
building the import libraries with those DATA exports. Note: Use
of the 'auto-import' extension will cause the text section of the
image file to be made writable. This does not conform to the
PE-COFF format specification published by Microsoft.
Note - use of the 'auto-import' extension will also cause read only
data which would normally be placed into the .rdata section to be
placed into the .data section instead. This is in order to work
around a problem with consts that is described here:
http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
Using 'auto-import' generally will 'just work' - but sometimes you
may see this message:
"variable '<var>' can't be auto-imported. Please read the
documentation for ld's `--enable-auto-import' for details."
This message occurs when some (sub)expression accesses an address
ultimately given by the sum of two constants (Win32 import tables
only allow one). Instances where this may occur include accesses
to member fields of struct variables imported from a DLL, as well
as using a constant index into an array variable imported from a
DLL. Any multiword variable (arrays, structs, long long, etc) may
trigger this error condition. However, regardless of the exact
data type of the offending exported variable, ld will always
detect it, issue the warning, and exit.
There are several ways to address this difficulty, regardless of
the data type of the exported variable:
One way is to use -enable-runtime-pseudo-reloc switch. This leaves
the task of adjusting references in your client code for runtime
environment, so this method works only when runtime environment
supports this feature.
A second solution is to force one of the 'constants' to be a
variable - that is, unknown and un-optimizable at compile time.
For arrays, there are two possibilities: a) make the indexee (the
array's address) a variable, or b) make the 'constant' index a
variable. Thus:
extern type extern_array[];
extern_array[1] -->
{ volatile type *t=extern_array; t[1] }
or
extern type extern_array[];
extern_array[1] -->
{ volatile int t=1; extern_array[t] }
For structs (and most other multiword data types) the only option
is to make the struct itself (or the long long, or the ...)
variable:
extern struct s extern_struct;
extern_struct.field -->
{ volatile struct s *t=&extern_struct; t->field }
or
extern long long extern_ll;
extern_ll -->
{ volatile long long * local_ll=&extern_ll; *local_ll }
A third method of dealing with this difficulty is to abandon
'auto-import' for the offending symbol and mark it with
`__declspec(dllimport)'. However, in practise that requires using
compile-time #defines to indicate whether you are building a DLL,
building client code that will link to the DLL, or merely
building/linking to a static library. In making the choice
between the various methods of resolving the 'direct address with
constant offset' problem, you should consider typical real-world
usage:
Original:
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}
Solution 1:
--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]);
}
Solution 2:
--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]);
}
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]
`--disable-auto-import'
Do not attempt to do sophisticated linking of `_symbol' to
`__imp__symbol' for DATA imports from DLLs. [This option is
specific to the i386 PE targeted port of the linker]
`--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]
`--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]
`--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]
`--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]
`--stack RESERVE'
`--stack RESERVE,COMMIT'
Specify the number of bytes 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]
`--subsystem WHICH'
`--subsystem WHICH:MAJOR'
`--subsystem WHICH:MAJOR.MINOR'
Specifies the subsystem under which your program will execute. The
legal values for WHICH are `native', `windows', `console',
`posix', and `xbox'. You may optionally set the subsystem version
also. Numeric values are also accepted for WHICH. [This option
is specific to the i386 PE targeted port of the linker]
The following options set flags in the `DllCharacteristics' field
of the PE file header: [These options are specific to PE targeted
ports of the linker]
`--dynamicbase'
The image base address may be relocated using address space layout
randomization (ASLR). This feature was introduced with MS Windows
Vista for i386 PE targets.
`--forceinteg'
Code integrity checks are enforced.
`--nxcompat'
The image is compatible with the Data Execution Prevention. This
feature was introduced with MS Windows XP SP2 for i386 PE targets.
`--no-isolation'
Although the image understands isolation, do not isolate the image.
`--no-seh'
The image does not use SEH. No SE handler may be called from this
image.
`--no-bind'
Do not bind this image.
`--wdmdriver'
The driver uses the MS Windows Driver Model.
`--tsaware'
The image is Terminal Server aware.
2.1.2 Options specific to Motorola 68HC11 and 68HC12 targets
------------------------------------------------------------
The 68HC11 and 68HC12 linkers support specific options to control the
memory bank switching mapping and trampoline code generation.
`--no-trampoline'
This option disables the generation of trampoline. By default a
trampoline is generated for each far function which is called
using a `jsr' instruction (this happens when a pointer to a far
function is taken).
`--bank-window NAME'
This option indicates to the linker the name of the memory region
in the `MEMORY' specification that describes the memory bank
window. The definition of such region is then used by the linker
to compute paging and addresses within the memory window.
2.1.3 Options specific to Motorola 68K target
---------------------------------------------
The following options are supported to control handling of GOT
generation when linking for 68K targets.
`--got=TYPE'
This option tells the linker which GOT generation scheme to use.
TYPE should be one of `single', `negative', `multigot' or
`target'. For more information refer to the Info entry for `ld'.
File: ld.info, Node: Environment, Prev: Options, Up: Invocation
2.2 Environment Variables
=========================
You can change the behaviour of `ld' with the environment variables
`GNUTARGET', `LDEMULATION' and `COLLECT_NO_DEMANGLE'.
`GNUTARGET' determines the input-file object format if you don't use
`-b' (or its synonym `--format'). Its value should be one of the BFD
names for an input format (*note BFD::). If there is no `GNUTARGET' in
the environment, `ld' uses the natural format of the target. If
`GNUTARGET' is set to `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.
`LDEMULATION' determines the default emulation if you don't use the
`-m' option. The emulation can affect various aspects of linker
behaviour, particularly the default linker script. You can list the
available emulations with the `--verbose' or `-V' options. If the `-m'
option is not used, and the `LDEMULATION' environment variable is not
defined, the default emulation depends upon how the linker was
configured.
Normally, the linker will default to demangling symbols. However, if
`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 `gcc' linker wrapper program. The default may
be overridden by the `--demangle' and `--no-demangle' options.
File: ld.info, Node: Scripts, Next: Machine Dependent, Prev: Invocation, Up: Top
3 Linker Scripts
****************
Every link is controlled by a "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 `--verbose' command line option to
display the default linker script. Certain command line options, such
as `-r' or `-N', will affect the default linker script.
You may supply your own linker script by using the `-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. *Note
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
File: ld.info, Node: Basic Script Concepts, Next: Script Format, Up: Scripts
3.1 Basic 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
"object file format". Each file is called an "object file". The
output file is often called an "executable", but for our purposes we
will also call it an object file. Each object file has, among other
things, a list of "sections". We sometimes refer to a section in an
input file as an "input section"; similarly, a section in the output
file is an "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 "section contents".
A section may be marked as "loadable", which mean that the contents
should be loaded into memory when the output file is run. A section
with no contents may be "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 "VMA", or virtual memory address. This is the address the
section will have when the output file is run. The second is the
"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 `objdump'
program with the `-h' option.
Every object file also has a list of "symbols", known as the "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 `nm' program,
or by using the `objdump' program with the `-t' option.
File: ld.info, Node: Script Format, Next: Simple Example, Prev: Basic Script Concepts, Up: Scripts
3.2 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
`/*' and `*/'. As in C, comments are syntactically equivalent to
whitespace.
File: ld.info, Node: Simple Example, Next: Simple Commands, Prev: Script Format, Up: Scripts
3.3 Simple Linker Script Example
================================
Many linker scripts are fairly simple.
The simplest possible linker script has just one command:
`SECTIONS'. You use the `SECTIONS' command to describe the memory
layout of the output file.
The `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
`.text', `.data', and `.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:
SECTIONS
{
. = 0x10000;
.text : { *(.text) }
. = 0x8000000;
.data : { *(.data) }
.bss : { *(.bss) }
}
You write the `SECTIONS' command as the keyword `SECTIONS', followed
by a series of symbol assignments and output section descriptions
enclosed in curly braces.
The first line inside the `SECTIONS' command of the above example
sets the value of the special symbol `.', 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
`SECTIONS' command, the location counter has the value `0'.
The second line defines an output section, `.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 `*' is a wildcard
which matches any file name. The expression `*(.text)' means all
`.text' input sections in all input files.
Since the location counter is `0x10000' when the output section
`.text' is defined, the linker will set the address of the `.text'
section in the output file to be `0x10000'.
The remaining lines define the `.data' and `.bss' sections in the
output file. The linker will place the `.data' output section at
address `0x8000000'. After the linker places the `.data' output
section, the value of the location counter will be `0x8000000' plus the
size of the `.data' output section. The effect is that the linker will
place the `.bss' output section immediately after the `.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 `.text' and `.data' sections
will probably satisfy any alignment constraints, but the linker may
have to create a small gap between the `.data' and `.bss' sections.
That's it! That's a simple and complete linker script.
File: ld.info, Node: Simple Commands, Next: Assignments, Prev: Simple Example, Up: Scripts
3.4 Simple Linker Script Commands
=================================
In this section we describe the simple linker script commands.
* Menu:
* Entry Point:: Setting the entry point
* File Commands:: Commands dealing with files
* Format Commands:: Commands dealing with object file formats
* REGION_ALIAS:: Assign alias names to memory regions
* Miscellaneous Commands:: Other linker script commands
File: ld.info, Node: Entry Point, Next: File Commands, Up: Simple Commands
3.4.1 Setting the Entry Point
-----------------------------
The first instruction to execute in a program is called the "entry
point". You can use the `ENTRY' linker script command to set the entry
point. The argument is a symbol name:
ENTRY(SYMBOL)
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:
* the `-e' ENTRY command-line option;
* the `ENTRY(SYMBOL)' command in a linker script;
* the value of a target specific symbol, if it is defined; For many
targets this is `start', but PE and BeOS based systems for example
check a list of possible entry symbols, matching the first one
found.
* the address of the first byte of the `.text' section, if present;
* The address `0'.
File: ld.info, Node: File Commands, Next: Format Commands, Prev: Entry Point, Up: Simple Commands
3.4.2 Commands Dealing with Files
---------------------------------
Several linker script commands deal with files.
`INCLUDE FILENAME'
Include the linker script FILENAME at this point. The file will
be searched for in the current directory, and in any directory
specified with the `-L' option. You can nest calls to `INCLUDE'
up to 10 levels deep.
You can place `INCLUDE' directives at the top level, in `MEMORY' or
`SECTIONS' commands, or in output section descriptions.
`INPUT(FILE, FILE, ...)'
`INPUT(FILE FILE ...)'
The `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 `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 `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 `-T'
option.
In case a "sysroot prefix" is configured, and the filename starts
with the `/' character, and the script being processed was located
inside the "sysroot prefix", the filename will be looked for in
the "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 `-L' in *note Command Line Options: Options.
If you use `INPUT (-lFILE)', `ld' will transform the name to
`libFILE.a', as with the command line argument `-l'.
When you use the `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.
`GROUP(FILE, FILE, ...)'
`GROUP(FILE FILE ...)'
The `GROUP' command is like `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 `-('
in *note Command Line Options: Options.
`AS_NEEDED(FILE, FILE, ...)'
`AS_NEEDED(FILE FILE ...)'
This construct can appear only inside of the `INPUT' or `GROUP'
commands, among other filenames. The files listed will be handled
as if they appear directly in the `INPUT' or `GROUP' commands,
with the exception of ELF shared libraries, that will be added only
when they are actually needed. This construct essentially enables
`--as-needed' option for all the files listed inside of it and
restores previous `--as-needed' resp. `--no-as-needed' setting
afterwards.
`OUTPUT(FILENAME)'
The `OUTPUT' command names the output file. Using
`OUTPUT(FILENAME)' in the linker script is exactly like using `-o
FILENAME' on the command line (*note Command Line Options:
Options.). If both are used, the command line option takes
precedence.
You can use the `OUTPUT' command to define a default name for the
output file other than the usual default of `a.out'.
`SEARCH_DIR(PATH)'
The `SEARCH_DIR' command adds PATH to the list of paths where `ld'
looks for archive libraries. Using `SEARCH_DIR(PATH)' is exactly
like using `-L PATH' on the command line (*note Command Line
Options: Options.). If both are used, then the linker will search
both paths. Paths specified using the command line option are
searched first.
`STARTUP(FILENAME)'
The `STARTUP' command is just like the `INPUT' command, except
that 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.
File: ld.info, Node: Format Commands, Next: REGION_ALIAS, Prev: File Commands, Up: Simple Commands
3.4.3 Commands Dealing with Object File Formats
-----------------------------------------------
A couple of linker script commands deal with object file formats.
`OUTPUT_FORMAT(BFDNAME)'
`OUTPUT_FORMAT(DEFAULT, BIG, LITTLE)'
The `OUTPUT_FORMAT' command names the BFD format to use for the
output file (*note BFD::). Using `OUTPUT_FORMAT(BFDNAME)' is
exactly like using `--oformat BFDNAME' on the command line (*note
Command Line Options: Options.). If both are used, the command
line option takes precedence.
You can use `OUTPUT_FORMAT' with three arguments to use different
formats based on the `-EB' and `-EL' command line options. This
permits the linker script to set the output format based on the
desired endianness.
If neither `-EB' nor `-EL' are used, then the output format will
be the first argument, DEFAULT. If `-EB' is used, the output
format will be the second argument, BIG. If `-EL' is used, the
output format will be the third argument, LITTLE.
For example, the default linker script for the MIPS ELF target
uses this command:
OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
This says that the default format for the output file is
`elf32-bigmips', but if the user uses the `-EL' command line
option, the output file will be created in the `elf32-littlemips'
format.
`TARGET(BFDNAME)'
The `TARGET' command names the BFD format to use when reading input
files. It affects subsequent `INPUT' and `GROUP' commands. This
command is like using `-b BFDNAME' on the command line (*note
Command Line Options: Options.). If the `TARGET' command is used
but `OUTPUT_FORMAT' is not, then the last `TARGET' command is also
used to set the format for the output file. *Note BFD::.
File: ld.info, Node: REGION_ALIAS, Next: Miscellaneous Commands, Prev: Format Commands, Up: Simple Commands
3.4.4 Assign alias names to memory regions
------------------------------------------
Alias names can be added to existing memory regions created with the
*note MEMORY:: command. Each name corresponds to at most one memory
region.
REGION_ALIAS(ALIAS, REGION)
The `REGION_ALIAS' function creates an alias name ALIAS for the
memory region REGION. This allows a flexible mapping of output sections
to memory regions. An example follows.
Suppose we have an application for embedded systems which come with
various memory storage devices. All have a general purpose, volatile
memory `RAM' that allows code execution or data storage. Some may have
a read-only, non-volatile memory `ROM' that allows code execution and
read-only data access. The last variant is a read-only, non-volatile
memory `ROM2' with read-only data access and no code execution
capability. We have four output sections:
* `.text' program code;
* `.rodata' read-only data;
* `.data' read-write initialized data;
* `.bss' read-write zero initialized data.
The goal is to provide a linker command file that contains a system
independent part defining the output sections and a system dependent
part mapping the output sections to the memory regions available on the
system. Our embedded systems come with three different memory setups
`A', `B' and `C':
Section Variant A Variant B Variant C
.text RAM ROM ROM
.rodata RAM ROM ROM2
.data RAM RAM/ROM RAM/ROM2
.bss RAM RAM RAM
The notation `RAM/ROM' or `RAM/ROM2' means that this section is
loaded into region `ROM' or `ROM2' respectively. Please note that the
load address of the `.data' section starts in all three variants at the
end of the `.rodata' section.
The base linker script that deals with the output sections follows.
It includes the system dependent `linkcmds.memory' file that describes
the memory layout:
INCLUDE linkcmds.memory
SECTIONS
{
.text :
{
*(.text)
} > REGION_TEXT
.rodata :
{
*(.rodata)
rodata_end = .;
} > REGION_RODATA
.data : AT (rodata_end)
{
data_start = .;
*(.data)
} > REGION_DATA
data_size = SIZEOF(.data);
data_load_start = LOADADDR(.data);
.bss :
{
*(.bss)
} > REGION_BSS
}
Now we need three different `linkcmds.memory' files to define memory
regions and alias names. The content of `linkcmds.memory' for the three
variants `A', `B' and `C':
`A'
Here everything goes into the `RAM'.
MEMORY
{
RAM : ORIGIN = 0, LENGTH = 4M
}
REGION_ALIAS("REGION_TEXT", RAM);
REGION_ALIAS("REGION_RODATA", RAM);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
`B'
Program code and read-only data go into the `ROM'. Read-write
data goes into the `RAM'. An image of the initialized data is
loaded into the `ROM' and will be copied during system start into
the `RAM'.
MEMORY
{
ROM : ORIGIN = 0, LENGTH = 3M
RAM : ORIGIN = 0x10000000, LENGTH = 1M
}
REGION_ALIAS("REGION_TEXT", ROM);
REGION_ALIAS("REGION_RODATA", ROM);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
`C'
Program code goes into the `ROM'. Read-only data goes into the
`ROM2'. Read-write data goes into the `RAM'. An image of the
initialized data is loaded into the `ROM2' and will be copied
during system start into the `RAM'.
MEMORY
{
ROM : ORIGIN = 0, LENGTH = 2M
ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
RAM : ORIGIN = 0x20000000, LENGTH = 1M
}
REGION_ALIAS("REGION_TEXT", ROM);
REGION_ALIAS("REGION_RODATA", ROM2);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
It is possible to write a common system initialization routine to
copy the `.data' section from `ROM' or `ROM2' into the `RAM' if
necessary:
#include <string.h>
extern char data_start [];
extern char data_size [];
extern char data_load_start [];
void copy_data(void)
{
if (data_start != data_load_start)
{
memcpy(data_start, data_load_start, (size_t) data_size);
}
}
File: ld.info, Node: Miscellaneous Commands, Prev: REGION_ALIAS, Up: Simple Commands
3.4.5 Other Linker Script Commands
----------------------------------
There are a few other linker scripts commands.
`ASSERT(EXP, MESSAGE)'
Ensure that EXP is non-zero. If it is zero, then exit the linker
with an error code, and print MESSAGE.
`EXTERN(SYMBOL SYMBOL ...)'
Force 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 SYMBOLs for
each `EXTERN', and you may use `EXTERN' multiple times. This
command has the same effect as the `-u' command-line option.
`FORCE_COMMON_ALLOCATION'
This command has the same effect as the `-d' command-line option:
to make `ld' assign space to common symbols even if a relocatable
output file is specified (`-r').
`INHIBIT_COMMON_ALLOCATION'
This command has the same effect as the `--no-define-common'
command-line option: to make `ld' omit the assignment of addresses
to common symbols even for a non-relocatable output file.
`INSERT [ AFTER | BEFORE ] OUTPUT_SECTION'
This command is typically used in a script specified by `-T' to
augment the default `SECTIONS' with, for example, overlays. It
inserts all prior linker script statements after (or before)
OUTPUT_SECTION, and also causes `-T' to not override the default
linker script. The exact insertion point is as for orphan
sections. *Note Location Counter::. The insertion happens after
the linker has mapped input sections to output sections. Prior to
the insertion, since `-T' scripts are parsed before the default
linker script, statements in the `-T' script occur before the
default linker script statements in the internal linker
representation of the script. In particular, input section
assignments will be made to `-T' output sections before those in
the default script. Here is an example of how a `-T' script using
`INSERT' might look:
SECTIONS
{
OVERLAY :
{
.ov1 { ov1*(.text) }
.ov2 { ov2*(.text) }
}
}
INSERT AFTER .text;
`NOCROSSREFS(SECTION SECTION ...)'
This command may be used to tell `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 `NOCROSSREFS' command takes a list of output section names. If
`ld' detects any cross references between the sections, it reports
an error and returns a non-zero exit status. Note that the
`NOCROSSREFS' command uses output section names, not input section
names.
`OUTPUT_ARCH(BFDARCH)'
Specify a particular output machine architecture. The argument is
one of the names used by the BFD library (*note BFD::). You can
see the architecture of an object file by using the `objdump'
program with the `-f' option.
File: ld.info, Node: Assignments, Next: SECTIONS, Prev: Simple Commands, Up: Scripts
3.5 Assigning Values to Symbols
===============================
You may assign a value to a symbol in a linker script. This will define
the symbol and place it into the symbol table with a global scope.
* Menu:
* Simple Assignments:: Simple Assignments
* PROVIDE:: PROVIDE
* PROVIDE_HIDDEN:: PROVIDE_HIDDEN
* Source Code Reference:: How to use a linker script defined symbol in source code
File: ld.info, Node: Simple Assignments, Next: PROVIDE, Up: Assignments
3.5.1 Simple Assignments
------------------------
You may assign to a symbol using any of the C assignment operators:
`SYMBOL = EXPRESSION ;'
`SYMBOL += EXPRESSION ;'
`SYMBOL -= EXPRESSION ;'
`SYMBOL *= EXPRESSION ;'
`SYMBOL /= EXPRESSION ;'
`SYMBOL <<= EXPRESSION ;'
`SYMBOL >>= EXPRESSION ;'
`SYMBOL &= EXPRESSION ;'
`SYMBOL |= EXPRESSION ;'
The first case will define SYMBOL to the value of EXPRESSION. In
the other cases, SYMBOL must already be defined, and the value will be
adjusted accordingly.
The special symbol name `.' indicates the location counter. You may
only use this within a `SECTIONS' command. *Note Location Counter::.
The semicolon after EXPRESSION is required.
Expressions are defined below; see *note Expressions::.
You may write symbol assignments as commands in their own right, or
as statements within a `SECTIONS' command, or as part of an output
section description in a `SECTIONS' command.
The section of the symbol will be set from the section of the
expression; for more information, see *note Expression Section::.
Here is an example showing the three different places that symbol
assignments may be used:
floating_point = 0;
SECTIONS
{
.text :
{
*(.text)
_etext = .;
}
_bdata = (. + 3) & ~ 3;
.data : { *(.data) }
}
In this example, the symbol `floating_point' will be defined as
zero. The symbol `_etext' will be defined as the address following the
last `.text' input section. The symbol `_bdata' will be defined as the
address following the `.text' output section aligned upward to a 4 byte
boundary.
File: ld.info, Node: PROVIDE, Next: PROVIDE_HIDDEN, Prev: Simple Assignments, Up: Assignments
3.5.2 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 `etext'.
However, ANSI C requires that the user be able to use `etext' as a
function name without encountering an error. The `PROVIDE' keyword may
be used to define a symbol, such as `etext', only if it is referenced
but not defined. The syntax is `PROVIDE(SYMBOL = EXPRESSION)'.
Here is an example of using `PROVIDE' to define `etext':
SECTIONS
{
.text :
{
*(.text)
_etext = .;
PROVIDE(etext = .);
}
}
In this example, if the program defines `_etext' (with a leading
underscore), the linker will give a multiple definition error. If, on
the other hand, the program defines `etext' (with no leading
underscore), the linker will silently use the definition in the program.
If the program references `etext' but does not define it, the linker
will use the definition in the linker script.
File: ld.info, Node: PROVIDE_HIDDEN, Next: Source Code Reference, Prev: PROVIDE, Up: Assignments
3.5.3 PROVIDE_HIDDEN
--------------------
Similar to `PROVIDE'. For ELF targeted ports, the symbol will be
hidden and won't be exported.
File: ld.info, Node: Source Code Reference, Prev: PROVIDE_HIDDEN, Up: Assignments
3.5.4 Source Code Reference
---------------------------
Accessing a linker script defined variable from source code is not
intuitive. In particular a linker script symbol is not equivalent to a
variable declaration in a high level language, it is instead a symbol
that does not have a value.
Before going further, it is important to note that compilers often
transform names in the source code into different names when they are
stored in the symbol table. For example, Fortran compilers commonly
prepend or append an underscore, and C++ performs extensive `name
mangling'. Therefore there might be a discrepancy between the name of
a variable as it is used in source code and the name of the same
variable as it is defined in a linker script. For example in C a
linker script variable might be referred to as:
extern int foo;
But in the linker script it might be defined as:
_foo = 1000;
In the remaining examples however it is assumed that no name
transformation has taken place.
When a symbol is declared in a high level language such as C, two
things happen. The first is that the compiler reserves enough space in
the program's memory to hold the _value_ of the symbol. The second is
that the compiler creates an entry in the program's symbol table which
holds the symbol's _address_. ie the symbol table contains the address
of the block of memory holding the symbol's value. So for example the
following C declaration, at file scope:
int foo = 1000;
creates a entry called `foo' in the symbol table. This entry holds
the address of an `int' sized block of memory where the number 1000 is
initially stored.
When a program references a symbol the compiler generates code that
first accesses the symbol table to find the address of the symbol's
memory block and then code to read the value from that memory block.
So:
foo = 1;
looks up the symbol `foo' in the symbol table, gets the address
associated with this symbol and then writes the value 1 into that
address. Whereas:
int * a = & foo;
looks up the symbol `foo' in the symbol table, gets it address and
then copies this address into the block of memory associated with the
variable `a'.
Linker scripts symbol declarations, by contrast, create an entry in
the symbol table but do not assign any memory to them. Thus they are
an address without a value. So for example the linker script
definition:
foo = 1000;
creates an entry in the symbol table called `foo' which holds the
address of memory location 1000, but nothing special is stored at
address 1000. This means that you cannot access the _value_ of a
linker script defined symbol - it has no value - all you can do is
access the _address_ of a linker script defined symbol.
Hence when you are using a linker script defined symbol in source
code you should always take the address of the symbol, and never
attempt to use its value. For example suppose you want to copy the
contents of a section of memory called .ROM into a section called
.FLASH and the linker script contains these declarations:
start_of_ROM = .ROM;
end_of_ROM = .ROM + sizeof (.ROM) - 1;
start_of_FLASH = .FLASH;
Then the C source code to perform the copy would be:
extern char start_of_ROM, end_of_ROM, start_of_FLASH;
memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
Note the use of the `&' operators. These are correct.
File: ld.info, Node: SECTIONS, Next: MEMORY, Prev: Assignments, Up: Scripts
3.6 SECTIONS Command
====================
The `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 `SECTIONS' command is:
SECTIONS
{
SECTIONS-COMMAND
SECTIONS-COMMAND
...
}
Each SECTIONS-COMMAND may of be one of the following:
* an `ENTRY' command (*note Entry command: Entry Point.)
* a symbol assignment (*note Assignments::)
* an output section description
* an overlay description
The `ENTRY' command and symbol assignments are permitted inside the
`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 `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
File: ld.info, Node: Output Section Description, Next: Output Section Name, Up: SECTIONS
3.6.1 Output Section Description
--------------------------------
The full description of an output section looks like this:
SECTION [ADDRESS] [(TYPE)] :
[AT(LMA)]
[ALIGN(SECTION_ALIGN)]
[SUBALIGN(SUBSECTION_ALIGN)]
[CONSTRAINT]
{
OUTPUT-SECTION-COMMAND
OUTPUT-SECTION-COMMAND
...
} [>REGION] [AT>LMA_REGION] [:PHDR :PHDR ...] [=FILLEXP]
Most output sections do not use most of the optional section
attributes.
The whitespace around 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 OUTPUT-SECTION-COMMAND may be one of the following:
* a symbol assignment (*note Assignments::)
* an input section description (*note Input Section::)
* data values to include directly (*note Output Section Data::)
* a special output section keyword (*note Output Section Keywords::)
File: ld.info, Node: Output Section Name, Next: Output Section Address, Prev: Output Section Description, Up: SECTIONS
3.6.2 Output Section Name
-------------------------
The name of the output section is SECTION. SECTION must meet the
constraints of your output format. In formats which only support a
limited number of sections, such as `a.out', the name must be one of
the names supported by the format (`a.out', for example, allows only
`.text', `.data' or `.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 `/DISCARD/' is special; *note Output Section
Discarding::.
File: ld.info, Node: Output Section Address, Next: Input Section, Prev: Output Section Name, Up: SECTIONS
3.6.3 Output Section Address
----------------------------
The ADDRESS is an expression for the VMA (the virtual memory address)
of the output section. This address is optional, but if it is provided
then the output address will be set exactly as specified.
If the output address is not specified then one will be chosen for
the section, based on the heuristic below. This address will be
adjusted to fit the alignment requirement of the output section. The
alignment requirement is the strictest alignment of any input section
contained within the output section.
The output section address heuristic is as follows:
* If an output memory REGION is set for the section then it is added
to this region and its address will be the next free address in
that region.
* If the MEMORY command has been used to create a list of memory
regions then the first region which has attributes compatible with
the section is selected to contain it. The section's output
address will be the next free address in that region; *note
MEMORY::.
* If no memory regions were specified, or none match the section then
the output address will be based on the current value of the
location counter.
For example:
.text . : { *(.text) }
and
.text : { *(.text) }
are subtly different. The first will set the address of the `.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 any of the `.text' input sections.
The ADDRESS may be an arbitrary expression; *note 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:
.text ALIGN(0x10) : { *(.text) }
This works because `ALIGN' returns the current location counter
aligned upward to the specified value.
Specifying ADDRESS for a section will change the value of the
location counter, provided that the section is non-empty. (Empty
sections are ignored).
File: ld.info, Node: Input Section, Next: Output Section Data, Prev: Output Section Address, Up: SECTIONS
3.6.4 Input Section Description
-------------------------------
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
File: ld.info, Node: Input Section Basics, Next: Input Section Wildcards, Up: Input Section
3.6.4.1 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 (*note 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 `.text' sections, you would write:
*(.text)
Here the `*' 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:
*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
will cause all .ctors sections from all files except `crtend.o' and
`otherfile.o' to be included.
There are two ways to include more than one section:
*(.text .rdata)
*(.text) *(.rdata)
The difference between these is the order in which the `.text' and
`.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 `.text'
input sections will appear first, followed by all `.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:
data.o(.data)
You can also specify files within archives by writing a pattern
matching the archive, a colon, then the pattern matching the file, with
no whitespace around the colon.
`archive:file'
matches file within archive
`archive:'
matches the whole archive
`:file'
matches file but not one in an archive
Either one or both of `archive' and `file' can contain shell
wildcards. On DOS based file systems, the linker will assume that a
single letter followed by a colon is a drive specifier, so `c:myfile.o'
is a simple file specification, not `myfile.o' within an archive called
`c'. `archive:file' filespecs may also be used within an
`EXCLUDE_FILE' list, but may not appear in other linker script
contexts. For instance, you cannot extract a file from an archive by
using `archive:file' in an `INPUT' command.
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:
data.o
When you use a file name which is not an `archive:file' specifier
and 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 `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 `INPUT' command, because the linker will
not search for the file in the archive search path.
File: ld.info, Node: Input Section Wildcards, Next: Input Section Common, Prev: Input Section Basics, Up: Input Section
3.6.4.2 Input Section 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 `*' seen in many examples is a simple wildcard
pattern for the file name.
The wildcard patterns are like those used by the Unix shell.
`*'
matches any number of characters
`?'
matches any single character
`[CHARS]'
matches a single instance of any of the CHARS; the `-' character
may be used to specify a range of characters, as in `[a-z]' to
match any lower case letter
`\'
quotes the following character
When a file name is matched with a wildcard, the wildcard characters
will not match a `/' character (used to separate directory names on
Unix). A pattern consisting of a single `*' character is an exception;
it will always match any file name, whether it contains a `/' or not.
In a section name, the wildcard characters will match a `/' character.
File name wildcard patterns only match files which are explicitly
specified on the command line or in an `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
`data.o' rule will not be used:
.data : { *(.data) }
.data1 : { data.o(.data) }
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 `SORT_BY_NAME' keyword, which appears before a
wildcard pattern in parentheses (e.g., `SORT_BY_NAME(.text*)'). When
the `SORT_BY_NAME' keyword is used, the linker will sort the files or
sections into ascending order by name before placing them in the output
file.
`SORT_BY_ALIGNMENT' is very similar to `SORT_BY_NAME'. The
difference is `SORT_BY_ALIGNMENT' will sort sections into ascending
order by alignment before placing them in the output file.
`SORT' is an alias for `SORT_BY_NAME'.
When there are nested section sorting commands in linker script,
there can be at most 1 level of nesting for section sorting commands.
1. `SORT_BY_NAME' (`SORT_BY_ALIGNMENT' (wildcard section pattern)).
It will sort the input sections by name first, then by alignment
if 2 sections have the same name.
2. `SORT_BY_ALIGNMENT' (`SORT_BY_NAME' (wildcard section pattern)).
It will sort the input sections by alignment first, then by name
if 2 sections have the same alignment.
3. `SORT_BY_NAME' (`SORT_BY_NAME' (wildcard section pattern)) is
treated the same as `SORT_BY_NAME' (wildcard section pattern).
4. `SORT_BY_ALIGNMENT' (`SORT_BY_ALIGNMENT' (wildcard section
pattern)) is treated the same as `SORT_BY_ALIGNMENT' (wildcard
section pattern).
5. All other nested section sorting commands are invalid.
When both command line section sorting option and linker script
section sorting command are used, section sorting command always takes
precedence over the command line option.
If the section sorting command in linker script isn't nested, the
command line option will make the section sorting command to be treated
as nested sorting command.
1. `SORT_BY_NAME' (wildcard section pattern ) with `--sort-sections
alignment' is equivalent to `SORT_BY_NAME' (`SORT_BY_ALIGNMENT'
(wildcard section pattern)).
2. `SORT_BY_ALIGNMENT' (wildcard section pattern) with
`--sort-section name' is equivalent to `SORT_BY_ALIGNMENT'
(`SORT_BY_NAME' (wildcard section pattern)).
If the section sorting command in linker script is nested, the
command line option will be ignored.
If you ever get confused about where input sections are going, use
the `-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 `.text'
sections in `.text' and all `.bss' sections in `.bss'. The linker will
place the `.data' section from all files beginning with an upper case
character in `.DATA'; for all other files, the linker will place the
`.data' section in `.data'.
SECTIONS {
.text : { *(.text) }
.DATA : { [A-Z]*(.data) }
.data : { *(.data) }
.bss : { *(.bss) }
}
File: ld.info, Node: Input Section Common, Next: Input Section Keep, Prev: Input Section Wildcards, Up: Input Section
3.6.4.3 Input Section for Common Symbols
........................................
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 `COMMON'.
You may use file names with the `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
`.bss' section in the output file. For example:
.bss { *(.bss) *(COMMON) }
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 `COMMON' for standard common
symbols and `.scommon' for small common symbols. This permits you to
map the different types of common symbols into memory at different
locations.
You will sometimes see `[COMMON]' in old linker scripts. This
notation is now considered obsolete. It is equivalent to `*(COMMON)'.
File: ld.info, Node: Input Section Keep, Next: Input Section Example, Prev: Input Section Common, Up: Input Section
3.6.4.4 Input Section and Garbage Collection
............................................
When link-time garbage collection is in use (`--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
`KEEP()', as in `KEEP(*(.init))' or `KEEP(SORT_BY_NAME(*)(.ctors))'.
File: ld.info, Node: Input Section Example, Prev: Input Section Keep, Up: Input Section
3.6.4.5 Input Section Example
.............................
The following example is a complete linker script. It tells the linker
to read all of the sections from file `all.o' and place them at the
start of output section `outputa' which starts at location `0x10000'.
All of section `.input1' from file `foo.o' follows immediately, in the
same output section. All of section `.input2' from `foo.o' goes into
output section `outputb', followed by section `.input1' from `foo1.o'.
All of the remaining `.input1' and `.input2' sections from any files
are written to output section `outputc'.
SECTIONS {
outputa 0x10000 :
{
all.o
foo.o (.input1)
}
outputb :
{
foo.o (.input2)
foo1.o (.input1)
}
outputc :
{
*(.input1)
*(.input2)
}
}
File: ld.info, Node: Output Section Data, Next: Output Section Keywords, Prev: Input Section, Up: SECTIONS
3.6.5 Output Section Data
-------------------------
You can include explicit bytes of data in an output section by using
`BYTE', `SHORT', `LONG', `QUAD', or `SQUAD' as an output section
command. Each keyword is followed by an expression in parentheses
providing the value to store (*note Expressions::). The value of the
expression is stored at the current value of the location counter.
The `BYTE', `SHORT', `LONG', and `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 `addr':
BYTE(1)
LONG(addr)
When using a 64 bit host or target, `QUAD' and `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 `QUAD'
stores a 32 bit value zero extended to 64 bits, and `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:
SECTIONS { .text : { *(.text) } LONG(1) .data : { *(.data) } }
whereas this will work:
SECTIONS { .text : { *(.text) ; LONG(1) } .data : { *(.data) } }
You may use the `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 `FILL'
statement covers memory locations after the point at which it occurs in
the section definition; by including more than one `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 `0x90':
FILL(0x90909090)
The `FILL' command is similar to the `=FILLEXP' output section
attribute, but it only affects the part of the section following the
`FILL' command, rather than the entire section. If both are used, the
`FILL' command takes precedence. *Note Output Section Fill::, for
details on the fill expression.
File: ld.info, Node: Output Section Keywords, Next: Output Section Discarding, Prev: Output Section Data, Up: SECTIONS
3.6.6 Output Section Keywords
-----------------------------
There are a couple of keywords which can appear as output section
commands.
`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 `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.
`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 `CONSTRUCTORS' command
tells the linker to place constructor information in the output
section where the `CONSTRUCTORS' command appears. The
`CONSTRUCTORS' command is ignored for other object file formats.
The symbol `__CTOR_LIST__' marks the start of the global
constructors, and the symbol `__CTOR_END__' marks the end.
Similarly, `__DTOR_LIST__' and `__DTOR_END__' mark the start and
end of the global destructors. 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 GNU C++
normally calls constructors from a subroutine `__main'; a call to
`__main' is automatically inserted into the startup code for
`main'. GNU C++ normally runs destructors either by using
`atexit', or directly from the function `exit'.
For object file formats such as `COFF' or `ELF' which support
arbitrary section names, GNU C++ will normally arrange to put the
addresses of global constructors and destructors into the `.ctors'
and `.dtors' sections. Placing the following sequence into your
linker script will build the sort of table which the GNU C++
runtime code expects to see.
__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__ = .;
If you are using the 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 `CONSTRUCTORS' command, use `SORT_BY_NAME(CONSTRUCTORS)'
instead. When using the `.ctors' and `.dtors' sections, use
`*(SORT_BY_NAME(.ctors))' and `*(SORT_BY_NAME(.dtors))' instead of
just `*(.ctors)' and `*(.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.
File: ld.info, Node: Output Section Discarding, Next: Output Section Attributes, Prev: Output Section Keywords, Up: SECTIONS
3.6.7 Output Section Discarding
-------------------------------
The linker will not create output sections with no 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:
.foo : { *(.foo) }
will only create a `.foo' section in the output file if there is a
`.foo' section in at least one input file, and if the input sections
are not all empty. Other link script directives that allocate space in
an output section will also create the output section.
The linker will ignore address assignments (*note Output Section
Address::) on discarded output sections, except when the linker script
defines symbols in the output section. In that case the linker will
obey the address assignments, possibly advancing dot even though the
section is discarded.
The special output section name `/DISCARD/' may be used to discard
input sections. Any input sections which are assigned to an output
section named `/DISCARD/' are not included in the output file.
File: ld.info, Node: Output Section Attributes, Next: Overlay Description, Prev: Output Section Discarding, Up: SECTIONS
3.6.8 Output Section Attributes
-------------------------------
We showed above that the full description of an output section looked
like this:
SECTION [ADDRESS] [(TYPE)] :
[AT(LMA)]
[ALIGN(SECTION_ALIGN)]
[SUBALIGN(SUBSECTION_ALIGN)]
[CONSTRAINT]
{
OUTPUT-SECTION-COMMAND
OUTPUT-SECTION-COMMAND
...
} [>REGION] [AT>LMA_REGION] [:PHDR :PHDR ...] [=FILLEXP]
We've already described SECTION, ADDRESS, and
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
* Forced Output Alignment:: Forced Output Alignment
* Forced Input Alignment:: Forced Input Alignment
* Output Section Constraint:: Output section constraint
* Output Section Region:: Output section region
* Output Section Phdr:: Output section phdr
* Output Section Fill:: Output section fill
File: ld.info, Node: Output Section Type, Next: Output Section LMA, Up: Output Section Attributes
3.6.8.1 Output Section Type
...........................
Each output section may have a type. The type is a keyword in
parentheses. The following types are defined:
`NOLOAD'
The section should be marked as not loadable, so that it will not
be loaded into memory when the program is run.
`DSECT'
`COPY'
`INFO'
`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.
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 `ROM'
section is addressed at memory location `0' and does not need to be
loaded when the program is run.
SECTIONS {
ROM 0 (NOLOAD) : { ... }
...
}
File: ld.info, Node: Output Section LMA, Next: Forced Output Alignment, Prev: Output Section Type, Up: Output Section Attributes
3.6.8.2 Output Section LMA
..........................
Every section has a virtual address (VMA) and a load address (LMA); see
*note Basic Script Concepts::. The virtual address is specified by the
*note Output Section Address:: described earlier. The load address is
specified by the `AT' or `AT>' keywords. Specifying a load address is
optional.
The `AT' keyword takes an expression as an argument. This specifies
the exact load address of the section. The `AT>' keyword takes the
name of a memory region as an argument. *Note MEMORY::. The load
address of the section is set to the next free address in the region,
aligned to the section's alignment requirements.
If neither `AT' nor `AT>' is specified for an allocatable section,
the linker will use the following heuristic to determine the load
address:
* If the section has a specific VMA address, then this is used as
the LMA address as well.
* If the section is not allocatable then its LMA is set to its VMA.
* Otherwise if a memory region can be found that is compatible with
the current section, and this region contains at least one
section, then the LMA is set so the difference between the VMA and
LMA is the same as the difference between the VMA and LMA of the
last section in the located region.
* If no memory regions have been declared then a default region that
covers the entire address space is used in the previous step.
* If no suitable region could be found, or there was no previous
section then the LMA is set equal to the VMA.
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 `.text', which starts at `0x1000', one called `.mdata', which is
loaded at the end of the `.text' section even though its VMA is
`0x2000', and one called `.bss' to hold uninitialized data at address
`0x3000'. The symbol `_data' is defined with the value `0x2000', which
shows that the location counter holds the VMA value, not the LMA value.
SECTIONS
{
.text 0x1000 : { *(.text) _etext = . ; }
.mdata 0x2000 :
AT ( ADDR (.text) + SIZEOF (.text) )
{ _data = . ; *(.data); _edata = . ; }
.bss 0x3000 :
{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;}
}
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.
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;
File: ld.info, Node: Forced Output Alignment, Next: Forced Input Alignment, Prev: Output Section LMA, Up: Output Section Attributes
3.6.8.3 Forced Output Alignment
...............................
You can increase an output section's alignment by using ALIGN.
File: ld.info, Node: Forced Input Alignment, Next: Output Section Constraint, Prev: Forced Output Alignment, Up: Output Section Attributes
3.6.8.4 Forced Input Alignment
..............................
You can force input section alignment within an output section by using
SUBALIGN. The value specified overrides any alignment given by input
sections, whether larger or smaller.
File: ld.info, Node: Output Section Constraint, Next: Output Section Region, Prev: Forced Input Alignment, Up: Output Section Attributes
3.6.8.5 Output Section Constraint
.................................
You can specify that an output section should only be created if all of
its input sections are read-only or all of its input sections are
read-write by using the keyword `ONLY_IF_RO' and `ONLY_IF_RW'
respectively.
File: ld.info, Node: Output Section Region, Next: Output Section Phdr, Prev: Output Section Constraint, Up: Output Section Attributes
3.6.8.6 Output Section Region
.............................
You can assign a section to a previously defined region of memory by
using `>REGION'. *Note MEMORY::.
Here is a simple example:
MEMORY { rom : ORIGIN = 0x1000, LENGTH = 0x1000 }
SECTIONS { ROM : { *(.text) } >rom }
File: ld.info, Node: Output Section Phdr, Next: Output Section Fill, Prev: Output Section Region, Up: Output Section Attributes
3.6.8.7 Output Section Phdr
...........................
You can assign a section to a previously defined program segment by
using `:PHDR'. *Note 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 `:PHDR' modifier.
You can use `:NONE' to tell the linker to not put the section in any
segment at all.
Here is a simple example:
PHDRS { text PT_LOAD ; }
SECTIONS { .text : { *(.text) } :text }
File: ld.info, Node: Output Section Fill, Prev: Output Section Phdr, Up: Output Section Attributes
3.6.8.8 Output Section Fill
...........................
You can set the fill pattern for an entire section by using `=FILLEXP'.
FILLEXP is an expression (*note 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 `0x'
and without a trailing `k' or `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 `+', 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 `FILL' command in the
output section commands; (*note Output Section Data::).
Here is a simple example:
SECTIONS { .text : { *(.text) } =0x90909090 }
File: ld.info, Node: Overlay Description, Prev: Output Section Attributes, Up: SECTIONS
3.6.9 Overlay Description
-------------------------
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 `OVERLAY' command. The `OVERLAY'
command is used within a `SECTIONS' command, like an output section
description. The full syntax of the `OVERLAY' command is as follows:
OVERLAY [START] : [NOCROSSREFS] [AT ( LDADDR )]
{
SECNAME1
{
OUTPUT-SECTION-COMMAND
OUTPUT-SECTION-COMMAND
...
} [:PHDR...] [=FILL]
SECNAME2
{
OUTPUT-SECTION-COMMAND
OUTPUT-SECTION-COMMAND
...
} [:PHDR...] [=FILL]
...
} [>REGION] [:PHDR...] [=FILL]
Everything is optional except `OVERLAY' (a keyword), and each
section must have a name (SECNAME1 and SECNAME2 above). The section
definitions within the `OVERLAY' construct are identical to those
within the general `SECTIONS' contruct (*note SECTIONS::), except that
no addresses and no memory regions may be defined for sections within
an `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
`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 `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. *Note NOCROSSREFS: Miscellaneous
Commands.
For each section within the `OVERLAY', the linker automatically
provides two symbols. The symbol `__load_start_SECNAME' is defined as
the starting load address of the section. The symbol
`__load_stop_SECNAME' is defined as the final load address of the
section. Any characters within 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
`SECTIONS' construct.
OVERLAY 0x1000 : AT (0x4000)
{
.text0 { o1/*.o(.text) }
.text1 { o2/*.o(.text) }
}
This will define both `.text0' and `.text1' to start at address 0x1000.
`.text0' will be loaded at address 0x4000, and `.text1' will be loaded
immediately after `.text0'. The following symbols will be defined if
referenced: `__load_start_text0', `__load_stop_text0',
`__load_start_text1', `__load_stop_text1'.
C code to copy overlay `.text1' into the overlay area might look
like the following.
extern char __load_start_text1, __load_stop_text1;
memcpy ((char *) 0x1000, &__load_start_text1,
&__load_stop_text1 - &__load_start_text1);
Note that the `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.
.text0 0x1000 : AT (0x4000) { o1/*.o(.text) }
PROVIDE (__load_start_text0 = LOADADDR (.text0));
PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
PROVIDE (__load_start_text1 = LOADADDR (.text1));
PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
File: ld.info, Node: MEMORY, Next: PHDRS, Prev: SECTIONS, Up: Scripts
3.7 MEMORY Command
==================
The linker's default configuration permits allocation of all available
memory. You can override this by using the `MEMORY' command.
The `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 `MEMORY' command.
However, you can define as many blocks of memory within it as you wish.
The syntax is:
MEMORY
{
NAME [(ATTR)] : ORIGIN = ORIGIN, LENGTH = LEN
...
}
The 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 within the `MEMORY' command. However you can add
later alias names to existing memory regions with the *note
REGION_ALIAS:: command.
The 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 *note
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 ATTR string must consist only of the following characters:
`R'
Read-only section
`W'
Read/write section
`X'
Executable section
`A'
Allocatable section
`I'
Initialized section
`L'
Same as `I'
`!'
Invert the sense of any of the attributes that follow
If a unmapped section matches any of the listed attributes other than
`!', it will be placed in the memory region. The `!' 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.
The ORIGIN is an numerical expression for the start address of the
memory region. The expression must evaluate to a constant and it
cannot involve any symbols. The keyword `ORIGIN' may be abbreviated to
`org' or `o' (but not, for example, `ORG').
The LEN is an expression for the size in bytes of the memory region.
As with the ORIGIN expression, the expression must be numerical only
and must evaluate to a constant. The keyword `LENGTH' may be
abbreviated to `len' or `l'.
In the following example, we specify that there are two memory
regions available for allocation: one starting at `0' for 256 kilobytes,
and the other starting at `0x40000000' for four megabytes. The linker
will place into the `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 `ram' memory region.
MEMORY
{
rom (rx) : ORIGIN = 0, LENGTH = 256K
ram (!rx) : org = 0x40000000, l = 4M
}
Once you define a memory region, you can direct the linker to place
specific output sections into that memory region by using the `>REGION'
output section attribute. For example, if you have a memory region
named `mem', you would use `>mem' in the output section definition.
*Note 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.
It is possible to access the origin and length of a memory in an
expression via the `ORIGIN(MEMORY)' and `LENGTH(MEMORY)' functions:
_fstack = ORIGIN(ram) + LENGTH(ram) - 4;
File: ld.info, Node: PHDRS, Next: VERSION, Prev: MEMORY, Up: Scripts
3.8 PHDRS Command
=================
The ELF object file format uses "program headers", also knows as
"segments". The program headers describe how the program should be
loaded into memory. You can print them out by using the `objdump'
program with the `-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 `PHDRS' command for this purpose. When the
linker sees the `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 `PHDRS' command when
generating an ELF output file. In other cases, the linker will simply
ignore `PHDRS'.
This is the syntax of the `PHDRS' command. The words `PHDRS',
`FILEHDR', `AT', and `FLAGS' are keywords.
PHDRS
{
NAME TYPE [ FILEHDR ] [ PHDRS ] [ AT ( ADDRESS ) ]
[ FLAGS ( FLAGS ) ] ;
}
The NAME is used only for reference in the `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. The headers are processed in order and it is
usual for them to map to sections in ascending load address order.
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 `:PHDR' output section attribute
to place a section in a particular segment. *Note 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 `:PHDR', using it once for each segment which should contain the
section.
If you place a section in one or more segments using `:PHDR', then
the linker will place all subsequent allocatable sections which do not
specify `: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 `:NONE' to override the default segment and tell
the linker to not put the section in any segment at all.
You may use the `FILEHDR' and `PHDRS' keywords after the program
header type to further describe the contents of the segment. The
`FILEHDR' keyword means that the segment should include the ELF file
header. The `PHDRS' keyword means that the segment should include the
ELF program headers themselves. If applied to a loadable segment
(`PT_LOAD'), all prior loadable segments must have one of these
keywords.
The TYPE may be one of the following. The numbers indicate the
value of the keyword.
`PT_NULL' (0)
Indicates an unused program header.
`PT_LOAD' (1)
Indicates that this program header describes a segment to be
loaded from the file.
`PT_DYNAMIC' (2)
Indicates a segment where dynamic linking information can be found.
`PT_INTERP' (3)
Indicates a segment where the name of the program interpreter may
be found.
`PT_NOTE' (4)
Indicates a segment holding note information.
`PT_SHLIB' (5)
A reserved program header type, defined but not specified by the
ELF ABI.
`PT_PHDR' (6)
Indicates a segment where the program headers may be found.
EXPRESSION
An expression giving the numeric type of the program header. This
may be used for types not defined above.
You can specify that a segment should be loaded at a particular
address in memory by using an `AT' expression. This is identical to the
`AT' command used as an output section attribute (*note Output Section
LMA::). The `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 `FLAGS' keyword to
explicitly specify the segment flags. The value of FLAGS must be an
integer. It is used to set the `p_flags' field of the program header.
Here is an example of `PHDRS'. This shows a typical set of program
headers used on a native ELF system.
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 */
...
. = . + 0x1000; /* move to a new page in memory */
.data : { *(.data) } :data
.dynamic : { *(.dynamic) } :data :dynamic
...
}
File: ld.info, Node: VERSION, Next: Expressions, Prev: PHDRS, Up: Scripts
3.9 VERSION Command
===================
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 `--version-script' linker option.
The syntax of the `VERSION' command is simply
VERSION { version-script-commands }
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.
VERS_1.1 {
global:
foo1;
local:
old*;
original*;
new*;
};
VERS_1.2 {
foo2;
} VERS_1.1;
VERS_2.0 {
bar1; bar2;
extern "C++" {
ns::*;
"f(int, double)";
};
} VERS_1.2;
This example version script defines three version nodes. The first
version node defined is `VERS_1.1'; it has no other dependencies. The
script binds the symbol `foo1' to `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 `old', `original', or `new' is matched.
The wildcard patterns available are the same as those used in the shell
when matching filenames (also known as "globbing"). However, if you
specify the symbol name inside double quotes, then the name is treated
as literal, rather than as a glob pattern.
Next, the version script defines node `VERS_1.2'. This node depends
upon `VERS_1.1'. The script binds the symbol `foo2' to the version
node `VERS_1.2'.
Finally, the version script defines node `VERS_2.0'. This node
depends upon `VERS_1.2'. The scripts binds the symbols `bar1' and
`bar2' are bound to the version node `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 `global: *;'
somewhere in the version script. Note that it's slightly crazy to use
wildcards in a global spec except on the last version node. Global
wildcards elsewhere run the risk of accidentally adding symbols to the
set exported for an old version. That's wrong since older versions
ought to have a fixed set of symbols.
The names of the version nodes have no specific meaning other than
what they might suggest to the person reading them. The `2.0' version
could just as well have appeared in between `1.1' and `1.2'. However,
this would be a confusing way to write a version script.
Node name can be omitted, 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.
{ global: foo; bar; local: *; };
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:
__asm__(".symver original_foo,foo@VERS_1.1");
in the C source file. This renames the function `original_foo' to
be an alias for `foo' bound to the version node `VERS_1.1'. The
`local:' directive can be used to prevent the symbol `original_foo'
from being exported. A `.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 `.symver' directives in the source
file. Here is an example:
__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");
In this example, `foo@' represents the symbol `foo' bound to the
unspecified base version of the symbol. The source file that contains
this example would define 4 C functions: `original_foo', `old_foo',
`old_foo1', and `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 `foo@@VERS_2.0'
type of `.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., `old_foo'), or you can use the `.symver' directive to
specifically bind to an external version of the function in question.
You can also specify the language in the version script:
VERSION extern "lang" { version-script-commands }
The supported `lang's are `C', `C++', and `Java'. The linker will
iterate over the list of symbols at the link time and demangle them
according to `lang' before matching them to the patterns specified in
`version-script-commands'. The default `lang' is `C'.
Demangled names may contains spaces and other special characters. As
described above, you can use a glob pattern to match demangled names,
or you can use a double-quoted string to match the string exactly. In
the latter case, be aware that minor differences (such as differing
whitespace) between the version script and the demangler output will
cause a mismatch. As the exact string generated by the demangler might
change in the future, even if the mangled name does not, you should
check that all of your version directives are behaving as you expect
when you upgrade.
File: ld.info, Node: Expressions, Next: Implicit Linker Scripts, Prev: VERSION, Up: Scripts
3.10 Expressions in Linker Scripts
==================================
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
* Symbolic Constants:: Symbolic constants
* Symbols:: Symbol Names
* Orphan Sections:: Orphan Sections
* Location Counter:: The Location Counter
* Operators:: Operators
* Evaluation:: Evaluation
* Expression Section:: The Section of an Expression
* Builtin Functions:: Builtin Functions
File: ld.info, Node: Constants, Next: Symbolic Constants, Up: Expressions
3.10.1 Constants
----------------
All constants are integers.
As in C, the linker considers an integer beginning with `0' to be
octal, and an integer beginning with `0x' or `0X' to be hexadecimal.
Alternatively the linker accepts suffixes of `h' or `H' for
hexadeciaml, `o' or `O' for octal, `b' or `B' for binary and `d' or `D'
for decimal. Any integer value without a prefix or a suffix is
considered to be decimal.
In addition, you can use the suffixes `K' and `M' to scale a
constant by `1024' or `1024*1024' respectively. For example, the
following all refer to the same quantity:
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;
_fourk_4 = 10000o;
Note - the `K' and `M' suffixes cannot be used in conjunction with
the base suffixes mentioned above.
File: ld.info, Node: Symbolic Constants, Next: Symbols, Prev: Constants, Up: Expressions
3.10.2 Symbolic Constants
-------------------------
It is possible to refer to target specific constants via the use of the
`CONSTANT(NAME)' operator, where NAME is one of:
`MAXPAGESIZE'
The target's maximum page size.
`COMMONPAGESIZE'
The target's default page size.
So for example:
.text ALIGN (CONSTANT (MAXPAGESIZE)) : { *(.text) }
will create a text section aligned to the largest page boundary
supported by the target.
File: ld.info, Node: Symbols, Next: Orphan Sections, Prev: Symbolic Constants, Up: Expressions
3.10.3 Symbol Names
-------------------
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:
"SECTION" = 9;
"with a space" = "also with a space" + 10;
Since symbols can contain many non-alphabetic characters, it is
safest to delimit symbols with spaces. For example, `A-B' is one
symbol, whereas `A - B' is an expression involving subtraction.
File: ld.info, Node: Orphan Sections, Next: Location Counter, Prev: Symbols, Up: Expressions
3.10.4 Orphan Sections
----------------------
Orphan sections are sections present in the input files which are not
explicitly placed into the output file by the linker script. The
linker will still copy these sections into the output file, but it has
to guess as to where they should be placed. The linker uses a simple
heuristic to do this. It attempts to place orphan sections after
non-orphan sections of the same attribute, such as code vs data,
loadable vs non-loadable, etc. If there is not enough room to do this
then it places at the end of the file.
For ELF targets, the attribute of the section includes section type
as well as section flag.
If an orphaned section's name is representable as a C identifier then
the linker will automatically *note PROVIDE:: two symbols:
__start_SECNAME and __end_SECNAME, where SECNAME is the name of the
section. These indicate the start address and end address of the
orphaned section respectively. Note: most section names are not
representable as C identifiers because they contain a `.' character.
File: ld.info, Node: Location Counter, Next: Operators, Prev: Orphan Sections, Up: Expressions
3.10.5 The Location Counter
---------------------------
The special linker variable "dot" `.' always contains the current
output location counter. Since the `.' always refers to a location in
an output section, it may only appear in an expression within a
`SECTIONS' command. The `.' symbol may appear anywhere that an
ordinary symbol is allowed in an expression.
Assigning a value to `.' will cause the location counter to be
moved. This may be used to create holes in the output section. The
location counter may not be moved backwards inside an output section,
and may not be moved backwards outside of an output section if so doing
creates areas with overlapping LMAs.
SECTIONS
{
output :
{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
} = 0x12345678;
}
In the previous example, the `.text' section from `file1' is located
at the beginning of the output section `output'. It is followed by a
1000 byte gap. Then the `.text' section from `file2' appears, also
with a 1000 byte gap following before the `.text' section from `file3'.
The notation `= 0x12345678' specifies what data to write in the gaps
(*note Output Section Fill::).
Note: `.' actually refers to the byte offset from the start of the
current containing object. Normally this is the `SECTIONS' statement,
whose start address is 0, hence `.' can be used as an absolute address.
If `.' 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:
SECTIONS
{
. = 0x100
.text: {
*(.text)
. = 0x200
}
. = 0x500
.data: {
*(.data)
. += 0x600
}
}
The `.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
`.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 `.'
backwards). The `.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 `.data' input sections and before the end of the `.data' output
section itself.
Setting symbols to the value of the location counter outside of an
output section statement can result in unexpected values if the linker
needs to place orphan sections. For example, given the following:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
start_of_data = . ;
.data: { *(.data) }
end_of_data = . ;
}
If the linker needs to place some input section, e.g. `.rodata', not
mentioned in the script, it might choose to place that section between
`.text' and `.data'. You might think the linker should place `.rodata'
on the blank line in the above script, but blank lines are of no
particular significance to the linker. As well, the linker doesn't
associate the above symbol names with their sections. Instead, it
assumes that all assignments or other statements belong to the previous
output section, except for the special case of an assignment to `.'.
I.e., the linker will place the orphan `.rodata' section as if the
script was written as follows:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
start_of_data = . ;
.rodata: { *(.rodata) }
.data: { *(.data) }
end_of_data = . ;
}
This may or may not be the script author's intention for the value of
`start_of_data'. One way to influence the orphan section placement is
to assign the location counter to itself, as the linker assumes that an
assignment to `.' is setting the start address of a following output
section and thus should be grouped with that section. So you could
write:
SECTIONS
{
start_of_text = . ;
.text: { *(.text) }
end_of_text = . ;
. = . ;
start_of_data = . ;
.data: { *(.data) }
end_of_data = . ;
}
Now, the orphan `.rodata' section will be placed between
`end_of_text' and `start_of_data'.
File: ld.info, Node: Operators, Next: Evaluation, Prev: Location Counter, Up: Expressions
3.10.6 Operators
----------------
The linker recognizes the standard C set of arithmetic operators, with
the standard bindings and precedence levels:
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)
Notes: (1) Prefix operators (2) *Note Assignments::.
File: ld.info, Node: Evaluation, Next: Expression Section, Prev: Operators, Up: Expressions
3.10.7 Evaluation
-----------------
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
`.', 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
SECTIONS
{
.text 9+this_isnt_constant :
{ *(.text) }
}
will cause the error message `non constant expression for initial
address'.
File: ld.info, Node: Expression Section, Next: Builtin Functions, Prev: Evaluation, Up: Expressions
3.10.8 The Section of an Expression
-----------------------------------
Addresses and symbols may be section relative, or absolute. A section
relative symbol is relocatable. If you request relocatable output
using the `-r' option, a further link operation may change the value of
a section relative symbol. On the other hand, an absolute symbol will
retain the same value throughout any further link operations.
Some terms in linker expressions are addresses. This is true of
section relative symbols and for builtin functions that return an
address, such as `ADDR', `LOADADDR', `ORIGIN' and `SEGMENT_START'.
Other terms are simply numbers, or are builtin functions that return a
non-address value, such as `LENGTH'.
When the linker evaluates an expression, the result depends on where
the expression is located in a linker script. Expressions appearing
outside an output section definitions are evaluated with all terms
first being converted to absolute addresses before applying operators,
and evaluate to an absolute address result. Expressions appearing
inside an output section definition are evaluated with more complex
rules, but the aim is to treat terms as relative addresses and produce
a relative address result. In particular, an assignment of a number to
a symbol results in a symbol relative to the output section with an
offset given by the number. So, in the following simple example,
SECTIONS
{
. = 0x100;
__executable_start = 0x100;
.data :
{
. = 0x10;
__data_start = 0x10;
*(.data)
}
...
}
both `.' and `__executable_start' are set to the absolute address
0x100 in the first two assignments, then both `.' and `__data_start'
are set to 0x10 relative to the `.data' section in the second two
assignments.
For expressions appearing inside an output section definition
involving numbers, relative addresses and absolute addresses, ld
follows these rules to evaluate terms:
* Unary operations on a relative address, and binary operations on
two relative addresses in the same section or between one relative
address and a number, apply the operator to the offset part of the
address(es).
* Unary operations on an absolute address, and binary operations on
one or more absolute addresses or on two relative addresses not in
the same section, first convert any non-absolute term to an
absolute address before applying the operator.
The result section of each sub-expression is as follows:
* An operation involving only numbers results in a number.
* The result of comparisons, `&&' and `||' is also a number.
* The result of other operations on relative addresses (after above
conversions) is a relative address in the same section as the
operand(s).
* The result of other operations on absolute addresses (after above
conversions) is an absolute address.
You can use the builtin function `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 `.data':
SECTIONS
{
.data : { *(.data) _edata = ABSOLUTE(.); }
}
If `ABSOLUTE' were not used, `_edata' would be relative to the
`.data' section.
Using `LOADADDR' also forces an expression absolute, since this
particular builtin function returns an absolute address.
File: ld.info, Node: Builtin Functions, Prev: Expression Section, Up: Expressions
3.10.9 Builtin Functions
------------------------
The linker script language includes a number of builtin functions for
use in linker script expressions.
`ABSOLUTE(EXP)'
Return the absolute (non-relocatable, as opposed to non-negative)
value of the expression EXP. Primarily useful to assign an
absolute value to a symbol within a section definition, where
symbol values are normally section relative. *Note Expression
Section::.
`ADDR(SECTION)'
Return the address (VMA) of the named SECTION. Your script must
previously have defined the location of that section. In the
following example, `start_of_output_1', `symbol_1' and `symbol_2'
are assigned equivalent values, except that `symbol_1' will be
relative to the `.output1' section while the other two will be
absolute:
SECTIONS { ...
.output1 :
{
start_of_output_1 = ABSOLUTE(.);
...
}
.output :
{
symbol_1 = ADDR(.output1);
symbol_2 = start_of_output_1;
}
... }
`ALIGN(ALIGN)'
`ALIGN(EXP,ALIGN)'
Return the location counter (`.') or arbitrary expression aligned
to the next ALIGN boundary. The single operand `ALIGN' doesn't
change the value of the location counter--it just does arithmetic
on it. The two operand `ALIGN' allows an arbitrary expression to
be aligned upwards (`ALIGN(ALIGN)' is equivalent to `ALIGN(.,
ALIGN)').
Here is an example which aligns the output `.data' section to the
next `0x2000' byte boundary after the preceding section and sets a
variable within the section to the next `0x8000' boundary after the
input sections:
SECTIONS { ...
.data ALIGN(0x2000): {
*(.data)
variable = ALIGN(0x8000);
}
... }
The first use of `ALIGN' in this example specifies the location of
a section because it is used as the optional ADDRESS attribute of
a section definition (*note Output Section Address::). The second
use of `ALIGN' is used to defines the value of a symbol.
The builtin function `NEXT' is closely related to `ALIGN'.
`ALIGNOF(SECTION)'
Return the alignment in bytes of the named 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, the alignment of the `.output' section is
stored as the first value in that section.
SECTIONS{ ...
.output {
LONG (ALIGNOF (.output))
...
}
... }
`BLOCK(EXP)'
This is a synonym for `ALIGN', for compatibility with older linker
scripts. It is most often seen when setting the address of an
output section.
`DATA_SEGMENT_ALIGN(MAXPAGESIZE, COMMONPAGESIZE)'
This is equivalent to either
(ALIGN(MAXPAGESIZE) + (. & (MAXPAGESIZE - 1)))
or
(ALIGN(MAXPAGESIZE) + (. & (MAXPAGESIZE - COMMONPAGESIZE)))
depending on whether the latter uses fewer COMMONPAGESIZE sized
pages for the data segment (area between the result of this
expression and `DATA_SEGMENT_END') than the former or not. If the
latter form is used, it means COMMONPAGESIZE bytes of runtime
memory will be saved at the expense of up to COMMONPAGESIZE wasted
bytes in the on-disk file.
This expression can only be used directly in `SECTIONS' commands,
not in any output section descriptions and only once in the linker
script. COMMONPAGESIZE should be less or equal to MAXPAGESIZE and
should be the system page size the object wants to be optimized
for (while still working on system page sizes up to MAXPAGESIZE).
Example:
. = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
`DATA_SEGMENT_END(EXP)'
This defines the end of data segment for `DATA_SEGMENT_ALIGN'
evaluation purposes.
. = DATA_SEGMENT_END(.);
`DATA_SEGMENT_RELRO_END(OFFSET, EXP)'
This defines the end of the `PT_GNU_RELRO' segment when `-z relro'
option is used. Second argument is returned. When `-z relro'
option is not present, `DATA_SEGMENT_RELRO_END' does nothing,
otherwise `DATA_SEGMENT_ALIGN' is padded so that EXP + OFFSET is
aligned to the most commonly used page boundary for particular
target. If present in the linker script, it must always come in
between `DATA_SEGMENT_ALIGN' and `DATA_SEGMENT_END'.
. = DATA_SEGMENT_RELRO_END(24, .);
`DEFINED(SYMBOL)'
Return 1 if SYMBOL is in the linker global symbol table and is
defined before the statement using DEFINED in the script, 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 `begin' to the first location in the `.text'
section--but if a symbol called `begin' already existed, its value
is preserved:
SECTIONS { ...
.text : {
begin = DEFINED(begin) ? begin : . ;
...
}
...
}
`LENGTH(MEMORY)'
Return the length of the memory region named MEMORY.
`LOADADDR(SECTION)'
Return the absolute LMA of the named SECTION. (*note Output
Section LMA::).
`MAX(EXP1, EXP2)'
Returns the maximum of EXP1 and EXP2.
`MIN(EXP1, EXP2)'
Returns the minimum of EXP1 and EXP2.
`NEXT(EXP)'
Return the next unallocated address that is a multiple of EXP.
This function is closely related to `ALIGN(EXP)'; unless you use
the `MEMORY' command to define discontinuous memory for the output
file, the two functions are equivalent.
`ORIGIN(MEMORY)'
Return the origin of the memory region named MEMORY.
`SEGMENT_START(SEGMENT, DEFAULT)'
Return the base address of the named SEGMENT. If an explicit
value has been given for this segment (with a command-line `-T'
option) that value will be returned; otherwise the value will be
DEFAULT. At present, the `-T' command-line option can only be
used to set the base address for the "text", "data", and "bss"
sections, but you can use `SEGMENT_START' with any segment name.
`SIZEOF(SECTION)'
Return the size in bytes of the named 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, `symbol_1' and `symbol_2' are assigned identical values:
SECTIONS{ ...
.output {
.start = . ;
...
.end = . ;
}
symbol_1 = .end - .start ;
symbol_2 = SIZEOF(.output);
... }
`SIZEOF_HEADERS'
`sizeof_headers'
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.
When producing an ELF output file, if the linker script uses the
`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 `not enough
room for program headers'. To avoid this error, you must avoid
using the `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
`PHDRS' command (*note PHDRS::).
File: ld.info, Node: Implicit Linker Scripts, Prev: Expressions, Up: Scripts
3.11 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 `INPUT', `GROUP', or `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.
File: ld.info, Node: Machine Dependent, Next: BFD, Prev: Scripts, Up: Top
4 Machine Dependent Features
****************************
`ld' has additional features on some platforms; the following sections
describe them. Machines where `ld' has no additional functionality are
not listed.
* Menu:
* H8/300:: `ld' and the H8/300
* i960:: `ld' and the Intel 960 family
* ARM:: `ld' and the ARM family
* HPPA ELF32:: `ld' and HPPA 32-bit ELF
* M68K:: `ld' and the Motorola 68K family
* MMIX:: `ld' and MMIX
* MSP430:: `ld' and MSP430
* M68HC11/68HC12:: `ld' and the Motorola 68HC11 and 68HC12 families
* PowerPC ELF32:: `ld' and PowerPC 32-bit ELF Support
* PowerPC64 ELF64:: `ld' and PowerPC64 64-bit ELF Support
* SPU ELF:: `ld' and SPU ELF Support
* TI COFF:: `ld' and TI COFF
* WIN32:: `ld' and WIN32 (cygwin/mingw)
* Xtensa:: `ld' and Xtensa Processors
File: ld.info, Node: H8/300, Next: i960, Up: Machine Dependent
4.1 `ld' and the H8/300
=======================
For the H8/300, `ld' can perform these global optimizations when you
specify the `--relax' command-line option.
_relaxing address modes_
`ld' finds all `jsr' and `jmp' instructions whose targets are
within eight bits, and turns them into eight-bit program-counter
relative `bsr' and `bra' instructions, respectively.
_synthesizing instructions_
`ld' finds all `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 `mov.b `@'AA:16' into `mov.b `@'AA:8' whenever the
address AA is in the top page of memory).
_bit manipulation instructions_
`ld' finds all bit manipulation instructions like `band, bclr,
biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst,
bxor' which use 32 bit and 16 bit absolute address form, but refer
to the top page of memory, and changes them to use the 8 bit
address form. (That is: the linker turns `bset #xx:3,`@'AA:32'
into `bset #xx:3,`@'AA:8' whenever the address AA is in the top
page of memory).
_system control instructions_
`ld' finds all `ldc.w, stc.w' instructions which use the 32 bit
absolute address form, but refer to the top page of memory, and
changes them to use 16 bit address form. (That is: the linker
turns `ldc.w `@'AA:32,ccr' into `ldc.w `@'AA:16,ccr' whenever the
address AA is in the top page of memory).
File: ld.info, Node: i960, Next: ARM, Prev: H8/300, Up: Machine Dependent
4.2 `ld' and the Intel 960 Family
=================================
You can use the `-AARCHITECTURE' 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 `ld' command line included `-ACA' as well as
`-ltry', the linker would look (in its built-in search paths, and in
any paths you specify with `-L') for a library with the names
try
libtry.a
tryca
libtryca.a
The first two possibilities would be considered in any event; the last
two are due to the use of `-ACA'.
You can meaningfully use `-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 `-l'
specifies a library.
`ld' supports the `--relax' option for the i960 family. If you
specify `--relax', `ld' finds all `balx' and `calx' instructions whose
targets are within 24 bits, and turns them into 24-bit program-counter
relative `bal' and `cal' instructions, respectively. `ld' also turns
`cal' instructions into `bal' instructions when it determines that the
target subroutine is a leaf routine (that is, the target subroutine does
not itself call any subroutines).
The `--fix-cortex-a8' switch enables a link-time workaround for an
erratum in certain Cortex-A8 processors. The workaround is enabled by
default if you are targeting the ARM v7-A architecture profile. It can
be enabled otherwise by specifying `--fix-cortex-a8', or disabled
unconditionally by specifying `--no-fix-cortex-a8'.
The erratum only affects Thumb-2 code. Please contact ARM for
further details.
The `--no-merge-exidx-entries' switch disables the merging of
adjacent exidx entries in debuginfo.
File: ld.info, Node: M68HC11/68HC12, Next: PowerPC ELF32, Prev: MSP430, Up: Machine Dependent
4.3 `ld' and the Motorola 68HC11 and 68HC12 families
====================================================
4.3.1 Linker Relaxation
-----------------------
For the Motorola 68HC11, `ld' can perform these global optimizations
when you specify the `--relax' command-line option.
_relaxing address modes_
`ld' finds all `jsr' and `jmp' instructions whose targets are
within eight bits, and turns them into eight-bit program-counter
relative `bsr' and `bra' instructions, respectively.
`ld' also looks at all 16-bit extended addressing modes and
transforms them in a direct addressing mode when the address is in
page 0 (between 0 and 0x0ff).
_relaxing gcc instruction group_
When `gcc' is called with `-mrelax', it can emit group of
instructions that the linker can optimize to use a 68HC11 direct
addressing mode. These instructions consists of `bclr' or `bset'
instructions.
4.3.2 Trampoline Generation
---------------------------
For 68HC11 and 68HC12, `ld' can generate trampoline code to call a far
function using a normal `jsr' instruction. The linker will also change
the relocation to some far function to use the trampoline address
instead of the function address. This is typically the case when a
pointer to a function is taken. The pointer will in fact point to the
function trampoline.
File: ld.info, Node: ARM, Next: HPPA ELF32, Prev: i960, Up: Machine Dependent
4.4 `ld' and the ARM family
===========================
For the ARM, `ld' will generate code stubs to allow functions calls
between ARM and Thumb code. These stubs only work with code that has
been compiled and assembled with the `-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 `--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.
The `--thumb-entry' switch is a duplicate of the generic `--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.
The `--use-nul-prefixed-import-tables' switch is specifying, that
the import tables idata4 and idata5 have to be generated with a zero
elememt prefix for import libraries. This is the old style to generate
import tables. By default this option is turned off.
The `--be8' switch instructs `ld' to generate BE8 format
executables. This option is only valid when linking big-endian objects.
The resulting image will contain big-endian data and little-endian code.
The `R_ARM_TARGET1' relocation is typically used for entries in the
`.init_array' section. It is interpreted as either `R_ARM_REL32' or
`R_ARM_ABS32', depending on the target. The `--target1-rel' and
`--target1-abs' switches override the default.
The `--target2=type' switch overrides the default definition of the
`R_ARM_TARGET2' relocation. Valid values for `type', their meanings,
and target defaults are as follows:
`rel'
`R_ARM_REL32' (arm*-*-elf, arm*-*-eabi)
`abs'
`R_ARM_ABS32' (arm*-*-symbianelf)
`got-rel'
`R_ARM_GOT_PREL' (arm*-*-linux, arm*-*-*bsd)
The `R_ARM_V4BX' relocation (defined by the ARM AAELF specification)
enables objects compiled for the ARMv4 architecture to be
interworking-safe when linked with other objects compiled for ARMv4t,
but also allows pure ARMv4 binaries to be built from the same ARMv4
objects.
In the latter case, the switch `--fix-v4bx' must be passed to the
linker, which causes v4t `BX rM' instructions to be rewritten as `MOV
PC,rM', since v4 processors do not have a `BX' instruction.
In the former case, the switch should not be used, and `R_ARM_V4BX'
relocations are ignored.
Replace `BX rM' instructions identified by `R_ARM_V4BX' relocations
with a branch to the following veneer:
TST rM, #1
MOVEQ PC, rM
BX Rn
This allows generation of libraries/applications that work on ARMv4
cores and are still interworking safe. Note that the above veneer
clobbers the condition flags, so may cause incorrect progrm behavior in
rare cases.
The `--use-blx' switch enables the linker to use ARM/Thumb BLX
instructions (available on ARMv5t and above) in various situations.
Currently it is used to perform calls via the PLT from Thumb code using
BLX rather than using BX and a mode-switching stub before each PLT
entry. This should lead to such calls executing slightly faster.
This option is enabled implicitly for SymbianOS, so there is no need
to specify it if you are using that target.
The `--vfp11-denorm-fix' switch enables a link-time workaround for a
bug in certain VFP11 coprocessor hardware, which sometimes allows
instructions with denorm operands (which must be handled by support
code) to have those operands overwritten by subsequent instructions
before the support code can read the intended values.
The bug may be avoided in scalar mode if you allow at least one
intervening instruction between a VFP11 instruction which uses a
register and another instruction which writes to the same register, or
at least two intervening instructions if vector mode is in use. The bug
only affects full-compliance floating-point mode: you do not need this
workaround if you are using "runfast" mode. Please contact ARM for
further details.
If you know you are using buggy VFP11 hardware, you can enable this
workaround by specifying the linker option `--vfp-denorm-fix=scalar' if
you are using the VFP11 scalar mode only, or `--vfp-denorm-fix=vector'
if you are using vector mode (the latter also works for scalar code).
The default is `--vfp-denorm-fix=none'.
If the workaround is enabled, instructions are scanned for
potentially-troublesome sequences, and a veneer is created for each
such sequence which may trigger the erratum. The veneer consists of the
first instruction of the sequence and a branch back to the subsequent
instruction. The original instruction is then replaced with a branch to
the veneer. The extra cycles required to call and return from the veneer
are sufficient to avoid the erratum in both the scalar and vector cases.
The `--no-enum-size-warning' switch prevents the linker from warning
when linking object files that specify incompatible EABI enumeration
size attributes. For example, with this switch enabled, linking of an
object file using 32-bit enumeration values with another using
enumeration values fitted into the smallest possible space will not be
diagnosed.
The `--no-wchar-size-warning' switch prevents the linker from
warning when linking object files that specify incompatible EABI
`wchar_t' size attributes. For example, with this switch enabled,
linking of an object file using 32-bit `wchar_t' values with another
using 16-bit `wchar_t' values will not be diagnosed.
The `--pic-veneer' switch makes the linker use PIC sequences for
ARM/Thumb interworking veneers, even if the rest of the binary is not
PIC. This avoids problems on uClinux targets where `--emit-relocs' is
used to generate relocatable binaries.
The linker will automatically generate and insert small sequences of
code into a linked ARM ELF executable whenever an attempt is made to
perform a function call to a symbol that is too far away. The
placement of these sequences of instructions - called stubs - is
controlled by the command line option `--stub-group-size=N'. The
placement is important because a poor choice can create a need for
duplicate stubs, increasing the code sizw. The linker will try to
group stubs together in order to reduce interruptions to the flow of
code, but it needs guidance as to how big these groups should be and
where they should be placed.
The value of `N', the parameter to the `--stub-group-size=' option
controls where the stub groups are placed. If it is negative then all
stubs are placed after the first branch that needs them. If it is
positive then the stubs can be placed either before or after the
branches that need them. If the value of `N' is 1 (either +1 or -1)
then the linker will choose exactly where to place groups of stubs,
using its built in heuristics. A value of `N' greater than 1 (or
smaller than -1) tells the linker that a single group of stubs can
service at most `N' bytes from the input sections.
The default, if `--stub-group-size=' is not specified, is `N = +1'.
Farcalls stubs insertion is fully supported for the ARM-EABI target
only, because it relies on object files properties not present
otherwise.
File: ld.info, Node: HPPA ELF32, Next: M68K, Prev: ARM, Up: Machine Dependent
4.5 `ld' and HPPA 32-bit ELF Support
====================================
When generating a shared library, `ld' will by default generate import
stubs suitable for use with a single sub-space application. The
`--multi-subspace' switch causes `ld' to generate export stubs, and
different (larger) import stubs suitable for use with multiple
sub-spaces.
Long branch stubs and import/export stubs are placed by `ld' in stub
sections located between groups of input sections. `--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
`N' chooses this scheme, ensuring that branches to stubs always use a
negative offset. Two special values of `N' are recognized, `1' and
`-1'. These both instruct `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 `N' respectively.
Note that `--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.
File: ld.info, Node: M68K, Next: MMIX, Prev: HPPA ELF32, Up: Machine Dependent
4.6 `ld' and the Motorola 68K family
====================================
The `--got=TYPE' option lets you choose the GOT generation scheme. The
choices are `single', `negative', `multigot' and `target'. When
`target' is selected the linker chooses the default GOT generation
scheme for the current target. `single' tells the linker to generate a
single GOT with entries only at non-negative offsets. `negative'
instructs the linker to generate a single GOT with entries at both
negative and positive offsets. Not all environments support such GOTs.
`multigot' allows the linker to generate several GOTs in the output
file. All GOT references from a single input object file access the
same GOT, but references from different input object files might access
different GOTs. Not all environments support such GOTs.
File: ld.info, Node: MMIX, Next: MSP430, Prev: M68K, Up: Machine Dependent
4.7 `ld' and MMIX
=================
For MMIX, there is a choice of generating `ELF' object files or `mmo'
object files when linking. The simulator `mmix' understands the `mmo'
format. The binutils `objcopy' utility can translate between the two
formats.
There is one special section, the `.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
`.MMIX.reg_contents' section corresponds to the first allocated global
register multiplied by 8. Register `$255' is not included in this
section; it is always set to the program entry, which is at the symbol
`Main' for `mmo' files.
Global symbols with the prefix `__.MMIX.start.', for example
`__.MMIX.start..text' and `__.MMIX.start..data' are special. 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.
File: ld.info, Node: MSP430, Next: M68HC11/68HC12, Prev: MMIX, Up: Machine Dependent
4.8 `ld' and MSP430
===================
For the MSP430 it is possible to select the MPU architecture. The flag
`-m [mpu type]' will select an appropriate linker script for selected
MPU type. (To get a list of known MPUs just pass `-m help' option to
the linker).
The linker will recognize some extra sections which are MSP430
specific:
``.vectors''
Defines a portion of ROM where interrupt vectors located.
``.bootloader''
Defines the bootloader portion of the ROM (if applicable). Any
code in this section will be uploaded to the MPU.
``.infomem''
Defines an information memory section (if applicable). Any code in
this section will be uploaded to the MPU.
``.infomemnobits''
This is the same as the `.infomem' section except that any code in
this section will not be uploaded to the MPU.
``.noinit''
Denotes a portion of RAM located above `.bss' section.
The last two sections are used by gcc.
File: ld.info, Node: PowerPC ELF32, Next: PowerPC64 ELF64, Prev: M68HC11/68HC12, Up: Machine Dependent
4.9 `ld' and PowerPC 32-bit ELF Support
=======================================
Branches on PowerPC processors are limited to a signed 26-bit
displacement, which may result in `ld' giving `relocation truncated to
fit' errors with very large programs. `--relax' enables the generation
of trampolines that can access the entire 32-bit address space. These
trampolines are inserted at section boundaries, so may not themselves
be reachable if an input section exceeds 33M in size. You may combine
`-r' and `--relax' to add trampolines in a partial link. In that case
both branches to undefined symbols and inter-section branches are also
considered potentially out of range, and trampolines inserted.
`--bss-plt'
Current PowerPC GCC accepts a `-msecure-plt' option that generates
code capable of using a newer PLT and GOT layout that has the
security advantage of no executable section ever needing to be
writable and no writable section ever being executable. PowerPC
`ld' will generate this layout, including stubs to access the PLT,
if all input files (including startup and static libraries) were
compiled with `-msecure-plt'. `--bss-plt' forces the old BSS PLT
(and GOT layout) which can give slightly better performance.
`--secure-plt'
`ld' will use the new PLT and GOT layout if it is linking new
`-fpic' or `-fPIC' code, but does not do so automatically when
linking non-PIC code. This option requests the new PLT and GOT
layout. A warning will be given if some object file requires the
old style BSS PLT.
`--sdata-got'
The new secure PLT and GOT are placed differently relative to other
sections compared to older BSS PLT and GOT placement. The
location of `.plt' must change because the new secure PLT is an
initialized section while the old PLT is uninitialized. The
reason for the `.got' change is more subtle: The new placement
allows `.got' to be read-only in applications linked with `-z
relro -z now'. However, this placement means that `.sdata' cannot
always be used in shared libraries, because the PowerPC ABI
accesses `.sdata' in shared libraries from the GOT pointer.
`--sdata-got' forces the old GOT placement. PowerPC GCC doesn't
use `.sdata' in shared libraries, so this option is really only
useful for other compilers that may do so.
`--emit-stub-syms'
This option causes `ld' to label linker stubs with a local symbol
that encodes the stub type and destination.
`--no-tls-optimize'
PowerPC `ld' normally performs some optimization of code sequences
used to access Thread-Local Storage. Use this option to disable
the optimization.
File: ld.info, Node: PowerPC64 ELF64, Next: SPU ELF, Prev: PowerPC ELF32, Up: Machine Dependent
4.10 `ld' and PowerPC64 64-bit ELF Support
==========================================
`--stub-group-size'
Long branch stubs, PLT call stubs and TOC adjusting stubs are
placed by `ld' in stub sections located between groups of input
sections. `--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 `N'
chooses this scheme, ensuring that branches to stubs always use a
negative offset. Two special values of `N' are recognized, `1'
and `-1'. These both instruct `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 `N' respectively.
Note that `--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.
`--emit-stub-syms'
This option causes `ld' to label linker stubs with a local symbol
that encodes the stub type and destination.
`--dotsyms, --no-dotsyms'
These two options control how `ld' interprets version patterns in
a version script. Older PowerPC64 compilers emitted both a
function descriptor symbol with the same name as the function, and
a code entry symbol with the name prefixed by a dot (`.'). To
properly version a function `foo', the version script thus needs
to control both `foo' and `.foo'. The option `--dotsyms', on by
default, automatically adds the required dot-prefixed patterns.
Use `--no-dotsyms' to disable this feature.
`--no-tls-optimize'
PowerPC64 `ld' normally performs some optimization of code
sequences used to access Thread-Local Storage. Use this option to
disable the optimization.
`--no-opd-optimize'
PowerPC64 `ld' normally removes `.opd' section entries
corresponding to deleted link-once functions, or functions removed
by the action of `--gc-sections' or linker script `/DISCARD/'.
Use this option to disable `.opd' optimization.
`--non-overlapping-opd'
Some PowerPC64 compilers have an option to generate compressed
`.opd' entries spaced 16 bytes apart, overlapping the third word,
the static chain pointer (unused in C) with the first word of the
next entry. This option expands such entries to the full 24 bytes.
`--no-toc-optimize'
PowerPC64 `ld' normally removes unused `.toc' section entries.
Such entries are detected by examining relocations that reference
the TOC in code sections. A reloc in a deleted code section marks
a TOC word as unneeded, while a reloc in a kept code section marks
a TOC word as needed. Since the TOC may reference itself, TOC
relocs are also examined. TOC words marked as both needed and
unneeded will of course be kept. TOC words without any referencing
reloc are assumed to be part of a multi-word entry, and are kept or
discarded as per the nearest marked preceding word. This works
reliably for compiler generated code, but may be incorrect if
assembly code is used to insert TOC entries. Use this option to
disable the optimization.
`--no-multi-toc'
By default, PowerPC64 GCC generates code for a TOC model where TOC
entries are accessed with a 16-bit offset from r2. This limits the
total TOC size to 64K. PowerPC64 `ld' extends this limit by
grouping code sections such that each group uses less than 64K for
its TOC entries, then inserts r2 adjusting stubs between
inter-group calls. `ld' does not split apart input sections, so
cannot help if a single input file has a `.toc' section that
exceeds 64K, most likely from linking multiple files with `ld -r'.
Use this option to turn off this feature.
File: ld.info, Node: SPU ELF, Next: TI COFF, Prev: PowerPC64 ELF64, Up: Machine Dependent
4.11 `ld' and SPU ELF Support
=============================
`--plugin'
This option marks an executable as a PIC plugin module.
`--no-overlays'
Normally, `ld' recognizes calls to functions within overlay
regions, and redirects such calls to an overlay manager via a stub.
`ld' also provides a built-in overlay manager. This option turns
off all this special overlay handling.
`--emit-stub-syms'
This option causes `ld' to label overlay stubs with a local symbol
that encodes the stub type and destination.
`--extra-overlay-stubs'
This option causes `ld' to add overlay call stubs on all function
calls out of overlay regions. Normally stubs are not added on
calls to non-overlay regions.
`--local-store=lo:hi'
`ld' usually checks that a final executable for SPU fits in the
address range 0 to 256k. This option may be used to change the
range. Disable the check entirely with `--local-store=0:0'.
`--stack-analysis'
SPU local store space is limited. Over-allocation of stack space
unnecessarily limits space available for code and data, while
under-allocation results in runtime failures. If given this
option, `ld' will provide an estimate of maximum stack usage.
`ld' does this by examining symbols in code sections to determine
the extents of functions, and looking at function prologues for
stack adjusting instructions. A call-graph is created by looking
for relocations on branch instructions. The graph is then searched
for the maximum stack usage path. Note that this analysis does not
find calls made via function pointers, and does not handle
recursion and other cycles in the call graph. Stack usage may be
under-estimated if your code makes such calls. Also, stack usage
for dynamic allocation, e.g. alloca, will not be detected. If a
link map is requested, detailed information about each function's
stack usage and calls will be given.
`--emit-stack-syms'
This option, if given along with `--stack-analysis' will result in
`ld' emitting stack sizing symbols for each function. These take
the form `__stack_<function_name>' for global functions, and
`__stack_<number>_<function_name>' for static functions.
`<number>' is the section id in hex. The value of such symbols is
the stack requirement for the corresponding function. The symbol
size will be zero, type `STT_NOTYPE', binding `STB_LOCAL', and
section `SHN_ABS'.
File: ld.info, Node: TI COFF, Next: WIN32, Prev: SPU ELF, Up: Machine Dependent
4.12 `ld''s Support for Various TI COFF Versions
================================================
The `--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;
`ld' will read any version or byte order, but the output header format
depends on the default specified by the specific target.
File: ld.info, Node: WIN32, Next: Xtensa, Prev: TI COFF, Up: Machine Dependent
4.13 `ld' and WIN32 (cygwin/mingw)
==================================
This section describes some of the win32 specific `ld' issues. See
*note Command Line Options: Options. for detailed description of the
command line options mentioned here.
_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 `ld' have specific support
for creating such libraries provided with the `--out-implib'
command line option.
_exporting DLL symbols_
The cygwin/mingw `ld' has several ways to export symbols for dll's.
_using auto-export functionality_
By default `ld' exports symbols with the auto-export
functionality, which is controlled by the following command
line options:
* -export-all-symbols [This is the default]
* -exclude-symbols
* -exclude-libs
* -exclude-modules-for-implib
* -version-script
When auto-export is in operation, `ld' will export all the
non-local (global and common) symbols it finds in a DLL, with
the exception of a few symbols known to belong to the
system's runtime and libraries. As it will often not be
desirable to export all of a DLL's symbols, which may include
private functions that are not part of any public interface,
the command-line options listed above may be used to filter
symbols out from the list for exporting. The `--output-def'
option can be used in order to see the final list of exported
symbols with all exclusions taken into effect.
If `--export-all-symbols' is not given explicitly on the
command line, then the default auto-export behavior will be
_disabled_ if either of the following are true:
* A DEF file is used.
* Any symbol in any object file was marked with the
__declspec(dllexport) attribute.
_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 `<dll name>.def' and is added as any other object file
to the linker's command line. The file's name must end in
`.def' or `.DEF'.
gcc -o <output> <objectfiles> <dll name>.def
Using a DEF file turns off the normal auto-export behavior,
unless the `--export-all-symbols' option is also used.
Here is an example of a DEF file for a shared library called
`xyz.dll':
LIBRARY "xyz.dll" BASE=0x20000000
EXPORTS
foo
bar
_bar = bar
another_foo = abc.dll.afoo
var1 DATA
doo = foo == foo2
eoo DATA == var1
This example defines a DLL with a non-default base address
and seven symbols in the export table. The third exported
symbol `_bar' is an alias for the second. The fourth symbol,
`another_foo' is resolved by "forwarding" to another module
and treating it as an alias for `afoo' exported from the DLL
`abc.dll'. The final symbol `var1' is declared to be a data
object. The `doo' symbol in export library is an alias of
`foo', which gets the string name in export table `foo2'. The
`eoo' symbol is an data export symbol, which gets in export
table the name `var1'.
The optional `LIBRARY <name>' command indicates the _internal_
name of the output DLL. If `<name>' does not include a suffix,
the default library suffix, `.DLL' is appended.
When the .DEF file is used to build an application, rather
than a library, the `NAME <name>' command should be used
instead of `LIBRARY'. If `<name>' does not include a suffix,
the default executable suffix, `.EXE' is appended.
With either `LIBRARY <name>' or `NAME <name>' the optional
specification `BASE = <number>' may be used to specify a
non-default base address for the image.
If neither `LIBRARY <name>' nor `NAME <name>' is specified,
or they specify an empty string, the internal name is the
same as the filename specified on the command line.
The complete specification of an export symbol is:
EXPORTS
( ( ( <name1> [ = <name2> ] )
| ( <name1> = <module-name> . <external-name>))
[ @ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
Declares `<name1>' as an exported symbol from the DLL, or
declares `<name1>' as an exported alias for `<name2>'; or
declares `<name1>' as a "forward" alias for the symbol
`<external-name>' in the DLL `<module-name>'. Optionally,
the symbol may be exported by the specified ordinal
`<integer>' alias. The optional `<name3>' is the to be used
string in import/export table for the symbol.
The optional keywords that follow the declaration indicate:
`NONAME': Do not put the symbol name in the DLL's export
table. It will still be exported by its ordinal alias
(either the value specified by the .def specification or,
otherwise, the value assigned by the linker). The symbol
name, however, does remain visible in the import library (if
any), unless `PRIVATE' is also specified.
`DATA': The symbol is a variable or object, rather than a
function. The import lib will export only an indirect
reference to `foo' as the symbol `_imp__foo' (ie, `foo' must
be resolved as `*_imp__foo').
`CONSTANT': Like `DATA', but put the undecorated `foo' as
well as `_imp__foo' into the import library. Both refer to the
read-only import address table's pointer to the variable, not
to the variable itself. This can be dangerous. If the user
code fails to add the `dllimport' attribute and also fails to
explicitly add the extra indirection that the use of the
attribute enforces, the application will behave unexpectedly.
`PRIVATE': Put the symbol in the DLL's export table, but do
not put it into the static import library used to resolve
imports at link time. The symbol can still be imported using
the `LoadLibrary/GetProcAddress' API at runtime or by by
using the GNU ld extension of linking directly to the DLL
without an import library.
See ld/deffilep.y in the binutils sources for the full
specification of other DEF file statements
While linking a shared dll, `ld' is able to create a DEF file
with the `--output-def <file>' command line option.
_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:
__declspec(dllexport) int a_variable
__declspec(dllexport) void a_function(int with_args)
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 `--export-all-symbols' option is also
used.
Note that object files that wish to access these symbols must
_not_ decorate them with dllexport. Instead, they should use
dllimport, instead:
__declspec(dllimport) int a_variable
__declspec(dllimport) void a_function(int with_args)
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 `--enable-auto-import' and
`automatic data imports' for more information.
_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 achieve a behavior that conforms to that on
POSIX/Un*x platforms. This feature is enabled with the
`--enable-auto-import' command-line option, although it is enabled
by default on cygwin/mingw. The `--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 `--enable-auto-import' for details."
The `--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 _runtime pseudo-relocs_
feature, described below.
For complex variables imported from DLLs (such as structs or
classes), object files typically contain a base address for the
variable and an offset (_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 `--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.
`--enable-runtime-pseudo-relocs' is not the default; it must be
explicitly enabled as needed.
_direct linking to a dll_
The cygwin/mingw ports of `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, especially when
linking large libraries or applications. When `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 `-L' and `-l', because `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 `-lxxx' it will
attempt to find, in the first directory of its search path,
libxxx.dll.a
xxx.dll.a
libxxx.a
xxx.lib
cygxxx.dll (*)
libxxx.dll
xxx.dll
before moving on to the next directory in the search path.
(*) Actually, this is not `cygxxx.dll' but in fact is
`<prefix>xxx.dll', where `<prefix>' is set by the `ld' option
`--dll-search-prefix=<prefix>'. In the case of cygwin, the
standard gcc spec file includes `--dll-search-prefix=cyg', so in
effect we actually search for `cygxxx.dll'.
Other win32-based unix environments, such as mingw or pw32, may
use other `<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 `bin' directory for
applications and dll's and a `lib' directory for the import
libraries (using cygwin nomenclature):
bin/
cygxxx.dll
lib/
libxxx.dll.a (in case of dll's)
libxxx.a (in case of static archive)
Linking directly to a dll without using the import library can be
done two ways:
1. Use the dll directly by adding the `bin' path to the link line
gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
However, as the dll's often have version numbers appended to their
names (`cygncurses-5.dll') this will often fail, unless one
specifies `-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 `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.
ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
Then you can link without any make environment changes.
gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
This technique also avoids the version number problems, because
the following is perfectly legal
bin/
cygxxx-5.dll
lib/
libxxx.dll.a -> ../bin/cygxxx-5.dll
Linking directly to a dll without using an import lib will work
even when auto-import features are exercised, and even when
`--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 three reasons:
1. Until recently, the link-directly-to-dll functionality did _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.
3. Symbol aliases can only be resolved using an import lib. This
is critical when linking against OS-supplied dll's (eg, the win32
API) in which symbols are usually exported as undecorated aliases
of their stdcall-decorated assembly names.
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 many 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.
_symbol aliasing_
_adding additional names_
Sometimes, it is useful to export symbols with additional
names. A symbol `foo' will be exported as `foo', but it can
also be exported as `_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:
LIBRARY "xyz.dll" BASE=0x61000000
EXPORTS
foo
_foo = foo
The line `_foo = foo' maps the symbol `foo' to `_foo'.
Another method for creating a symbol alias is to create it in
the source code using the "weak" attribute:
void foo () { /* Do something. */; }
void _foo () __attribute__ ((weak, alias ("foo")));
See the gcc manual for more information about attributes and
weak symbols.
_renaming symbols_
Sometimes it is useful to rename exports. For instance, the
cygwin kernel does this regularly. A symbol `_foo' can be
exported as `foo' but not as `_foo' by using special
directives in the DEF file. (This will also affect the import
library, if it is created). In the following example:
LIBRARY "xyz.dll" BASE=0x61000000
EXPORTS
_foo = foo
The line `_foo = foo' maps the exported symbol `foo' to
`_foo'.
Note: using a DEF file disables the default auto-export behavior,
unless the `--export-all-symbols' command line option is used.
If, however, you are trying to rename symbols, then you should list
_all_ desired exports in the DEF file, including the symbols that
are not being renamed, and do _not_ use the `--export-all-symbols'
option. If you list only the renamed symbols in the DEF file, and
use `--export-all-symbols' to handle the other symbols, then the
both the new names _and_ the original names for the renamed
symbols will be exported. In effect, you'd be aliasing those
symbols, not renaming them, which is probably not what you wanted.
_weak externals_
The Windows object format, PE, specifies a form of weak symbols
called weak externals. When a weak symbol is linked and the
symbol is not defined, the weak symbol becomes an alias for some
other symbol. There are three variants of weak externals:
* Definition is searched for in objects and libraries,
historically called lazy externals.
* Definition is searched for only in other objects, not in
libraries. This form is not presently implemented.
* No search; the symbol is an alias. This form is not presently
implemented.
As a GNU extension, weak symbols that do not specify an alternate
symbol are supported. If the symbol is undefined when linking,
the symbol uses a default value.
_aligned common symbols_
As a GNU extension to the PE file format, it is possible to
specify the desired alignment for a common symbol. This
information is conveyed from the assembler or compiler to the
linker by means of GNU-specific commands carried in the object
file's `.drectve' section, which are recognized by `ld' and
respected when laying out the common symbols. Native tools will
be able to process object files employing this GNU extension, but
will fail to respect the alignment instructions, and may issue
noisy warnings about unknown linker directives.
File: ld.info, Node: Xtensa, Prev: WIN32, Up: Machine Dependent
4.14 `ld' and Xtensa Processors
===============================
The default `ld' behavior for Xtensa processors is to interpret
`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:
SECTIONS
{
.text : {
*(.literal .text)
}
}
`ld' may interleave some of the `.literal' and `.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 `.literal' sections from an initial group of files followed by the
`.text' sections of that group of files. Then, the `.literal' sections
from the rest of the files and the `.text' sections from the rest of
the files would follow.
Relaxation is enabled by default for the Xtensa version of `ld' and
provides two important link-time optimizations. The first optimization
is to combine identical literal values to reduce code size. A redundant
literal will be removed and all the `L32R' instructions that use it
will be changed to reference an identical literal, as long as the
location of the replacement literal is within the offset range of all
the `L32R' instructions. The second optimization is to remove
unnecessary overhead from assembler-generated "longcall" sequences of
`L32R'/`CALLXN' when the target functions are within range of direct
`CALLN' instructions.
For each of these cases where an indirect call sequence can be
optimized to a direct call, the linker will change the `CALLXN'
instruction to a `CALLN' instruction, remove the `L32R' instruction,
and remove the literal referenced by the `L32R' instruction if it is
not used for anything else. Removing the `L32R' instruction always
reduces code size but can potentially hurt performance by changing the
alignment of subsequent branch targets. By default, the linker will
always preserve alignments, either by switching some instructions
between 24-bit encodings and the equivalent density instructions or by
inserting a no-op in place of the `L32R' instruction that was removed.
If code size is more important than performance, the `--size-opt'
option can be used to prevent the linker from widening density
instructions or inserting no-ops, except in a few cases where no-ops
are required for correctness.
The following Xtensa-specific command-line options can be used to
control the linker:
`--size-opt'
When optimizing indirect calls to direct calls, optimize for code
size more than performance. With this option, the linker will not
insert no-ops or widen density instructions to preserve branch
target alignment. There may still be some cases where no-ops are
required to preserve the correctness of the code.
File: ld.info, Node: BFD, Next: Reporting Bugs, Prev: Machine Dependent, Up: Top
5 BFD
*****
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 `objdump -i' (*note
objdump: (binutils.info)objdump.) to list all the formats available for
your configuration.
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. *Note BFD information loss::.
* Menu:
* BFD outline:: How it works: an outline of BFD
File: ld.info, Node: BFD outline, Up: BFD
5.1 How It Works: An Outline of BFD
===================================
When an object file is opened, BFD subroutines automatically determine
the format of the input object file. They then build a descriptor in
memory with pointers to routines that will be used to access elements of
the object file's data structures.
As different information from the object files is required, BFD
reads from different sections of the file and processes them. For
example, a very common operation for the linker is processing symbol
tables. Each BFD back end provides a routine for converting between
the object file's representation of symbols and an internal canonical
format. When the linker asks for the symbol table of an object file, it
calls through a memory pointer to the routine from the relevant BFD
back end which reads and converts the table into a canonical form. The
linker then operates upon the canonical form. When the link is finished
and the linker writes the output file's symbol table, another BFD back
end routine is called to take the newly created symbol table and
convert it into the chosen output format.
* Menu:
* BFD information loss:: Information Loss
* Canonical format:: The BFD canonical object-file format
File: ld.info, Node: BFD information loss, Next: Canonical format, Up: BFD outline
5.1.1 Information Loss
----------------------
_Information can be lost during output._ The output formats supported
by BFD do not provide identical facilities, and information which can
be described in one form has nowhere to go in another format. One
example of this is alignment information in `b.out'. There is nowhere
in an `a.out' format file to store alignment information on the
contained data, so when a file is linked from `b.out' and an `a.out'
image is produced, alignment information will not propagate to the
output file. (The linker will still use the alignment information
internally, so the link is performed correctly).
Another example is COFF section names. COFF files may contain an
unlimited number of sections, each one with a textual section name. If
the target of the link is a format which does not have many sections
(e.g., `a.out') or has sections without names (e.g., the Oasys format),
the link cannot be done simply. You can circumvent this problem by
describing the desired input-to-output section mapping with the linker
command language.
_Information can be lost during canonicalization._ The BFD internal
canonical form of the external formats is not exhaustive; there are
structures in input formats for which there is no direct representation
internally. This means that the BFD back ends cannot maintain all
possible data richness through the transformation between external to
internal and back to external formats.
This limitation is only a problem when an application reads one
format and writes another. Each BFD back end is responsible for
maintaining as much data as possible, and the internal BFD canonical
form has structures which are opaque to the BFD core, and exported only
to the back ends. When a file is read in one format, the canonical form
is generated for BFD and the application. At the same time, the back
end saves away any information which may otherwise be lost. If the data
is then written back in the same format, the back end routine will be
able to use the canonical form provided by the BFD core as well as the
information it prepared earlier. Since there is a great deal of
commonality between back ends, there is no information lost when
linking or copying big endian COFF to little endian COFF, or `a.out' to
`b.out'. When a mixture of formats is linked, the information is only
lost from the files whose format differs from the destination.
File: ld.info, Node: Canonical format, Prev: BFD information loss, Up: BFD outline
5.1.2 The BFD canonical object-file format
------------------------------------------
The greatest potential for loss of information occurs when there is the
least overlap between the information provided by the source format,
that stored by the canonical format, and that needed by the destination
format. A brief description of the canonical form may help you
understand which kinds of data you can count on preserving across
conversions.
_files_
Information stored on a per-file basis includes target machine
architecture, particular implementation format type, a demand
pageable bit, and a write protected bit. Information like Unix
magic numbers is not stored here--only the magic numbers' meaning,
so a `ZMAGIC' file would have both the demand pageable bit and the
write protected text bit set. The byte order of the target is
stored on a per-file basis, so that big- and little-endian object
files may be used with one another.
_sections_
Each section in the input file contains the name of the section,
the section's original address in the object file, size and
alignment information, various flags, and pointers into other BFD
data structures.
_symbols_
Each symbol contains a pointer to the information for the object
file which originally defined it, its name, its value, and various
flag bits. When a BFD back end reads in a symbol table, it
relocates all symbols to make them relative to the base of the
section where they were defined. Doing this ensures that each
symbol points to its containing section. Each symbol also has a
varying amount of hidden private data for the BFD back end. Since
the symbol points to the original file, the private data format
for that symbol is accessible. `ld' can operate on a collection
of symbols of wildly different formats without problems.
Normal global and simple local symbols are maintained on output,
so an output file (no matter its format) will retain symbols
pointing to functions and to global, static, and common variables.
Some symbol information is not worth retaining; in `a.out', type
information is stored in the symbol table as long symbol names.
This information would be useless to most COFF debuggers; the
linker has command line switches to allow users to throw it away.
There is one word of type information within the symbol, so if the
format supports symbol type information within symbols (for
example, COFF, IEEE, Oasys) and the type is simple enough to fit
within one word (nearly everything but aggregates), the
information will be preserved.
_relocation level_
Each canonical BFD relocation record contains a pointer to the
symbol to relocate to, the offset of the data to relocate, the
section the data is in, and a pointer to a relocation type
descriptor. Relocation is performed by passing messages through
the relocation type descriptor and the symbol pointer. Therefore,
relocations can be performed on output data using a relocation
method that is only available in one of the input formats. For
instance, Oasys provides a byte relocation format. A relocation
record requesting this relocation type would point indirectly to a
routine to perform this, so the relocation may be performed on a
byte being written to a 68k COFF file, even though 68k COFF has no
such relocation type.
_line numbers_
Object formats can contain, for debugging purposes, some form of
mapping between symbols, source line numbers, and addresses in the
output file. These addresses have to be relocated along with the
symbol information. Each symbol with an associated list of line
number records points to the first record of the list. The head
of a line number list consists of a pointer to the symbol, which
allows finding out the address of the function whose line number
is being described. The rest of the list is made up of pairs:
offsets into the section and line numbers. Any format which can
simply derive this information can pass it successfully between
formats (COFF, IEEE and Oasys).
File: ld.info, Node: Reporting Bugs, Next: MRI, Prev: BFD, Up: Top
6 Reporting Bugs
****************
Your bug reports play an essential role in making `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 `ld' work
better. Bug reports are your contribution to the maintenance of `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
File: ld.info, Node: Bug Criteria, Next: Bug Reporting, Up: Reporting Bugs
6.1 Have You Found a Bug?
=========================
If you are not sure whether you have found a bug, here are some
guidelines:
* If the linker gets a fatal signal, for any input whatever, that is
a `ld' bug. Reliable linkers never crash.
* If `ld' produces an error message for valid input, that is a bug.
* If `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.
* If you are an experienced user of linkers, your suggestions for
improvement of `ld' are welcome in any case.
File: ld.info, Node: Bug Reporting, Prev: Bug Criteria, Up: Reporting Bugs
6.2 How to Report Bugs
======================
A number of companies and individuals offer support for GNU products.
If you obtained `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 `etc/SERVICE' in the GNU Emacs distribution.
Otherwise, send bug reports for `ld' to
`http://www.sourceware.org/bugzilla/'.
The fundamental principle of reporting bugs usefully is this:
*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:
* The version of `ld'. `ld' announces it if you start it with the
`--version' argument.
Without this, we will not know whether there is any point in
looking for the bug in the current version of `ld'.
* Any patches you may have applied to the `ld' source, including any
patches made to the `BFD' library.
* The type of machine you are using, and the operating system name
and version number.
* What compiler (and its version) was used to compile `ld'--e.g.
"`gcc-2.7'".
* 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.
* 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 `gas' or compiled using
`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
`gas' or `gcc' was used to produce the object files. Also say how
`gas' or `gcc' were configured.
* 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 `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 `ld' is out of sync, 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.
* If you wish to suggest changes to the `ld' source, send us context
diffs, as generated by `diff' with the `-u', `-c', or `-p' option.
Always send diffs from the old file to the new file. If you even
discuss something in the `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.
Here are some things that are not necessary:
* 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 _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.
* 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 `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.
* 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.
File: ld.info, Node: MRI, Next: GNU Free Documentation License, Prev: Reporting Bugs, Up: Top
Appendix A MRI Compatible Script Files
**************************************
To aid users making the transition to GNU `ld' from the MRI linker,
`ld' can use MRI compatible linker scripts as an alternative to the
more general-purpose linker scripting language described in *note
Scripts::. MRI compatible linker scripts have a much simpler command
set than the scripting language otherwise used with `ld'. GNU `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 `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
`-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, `ld' issues
a warning message, but continues processing the script.
Lines beginning with `*' are comments.
You can write these commands using all upper-case letters, or all
lower case; for example, `chip' is the same as `CHIP'. The following
list shows only the upper-case form of each command.
`ABSOLUTE SECNAME'
`ABSOLUTE SECNAME, SECNAME, ... SECNAME'
Normally, `ld' includes in the output file all sections from all
the input files. However, in an MRI-compatible script, you can
use the `ABSOLUTE' command to restrict the sections that will be
present in your output program. If the `ABSOLUTE' command is used
at all in a script, then only the sections named explicitly in
`ABSOLUTE' commands will appear in the linker output. You can
still use other input sections (whatever you select on the command
line, or using `LOAD') to resolve addresses in the output file.
`ALIAS OUT-SECNAME, IN-SECNAME'
Use this command to place the data from input section IN-SECNAME
in a section called OUT-SECNAME in the linker output file.
IN-SECNAME may be an integer.
`ALIGN SECNAME = EXPRESSION'
Align the section called SECNAME to EXPRESSION. The EXPRESSION
should be a power of two.
`BASE EXPRESSION'
Use the value of EXPRESSION as the lowest address (other than
absolute addresses) in the output file.
`CHIP EXPRESSION'
`CHIP EXPRESSION, EXPRESSION'
This command does nothing; it is accepted only for compatibility.
`END'
This command does nothing whatever; it's only accepted for
compatibility.
`FORMAT OUTPUT-FORMAT'
Similar to the `OUTPUT_FORMAT' command in the more general linker
language, but restricted to one of these output formats:
1. S-records, if OUTPUT-FORMAT is `S'
2. IEEE, if OUTPUT-FORMAT is `IEEE'
3. COFF (the `coff-m68k' variant in BFD), if OUTPUT-FORMAT is
`COFF'
`LIST ANYTHING...'
Print (to the standard output file) a link map, as produced by the
`ld' command-line option `-M'.
The keyword `LIST' may be followed by anything on the same line,
with no change in its effect.
`LOAD FILENAME'
`LOAD FILENAME, FILENAME, ... FILENAME'
Include one or more object file FILENAME in the link; this has the
same effect as specifying FILENAME directly on the `ld' command
line.
`NAME OUTPUT-NAME'
OUTPUT-NAME is the name for the program produced by `ld'; the
MRI-compatible command `NAME' is equivalent to the command-line
option `-o' or the general script language command `OUTPUT'.
`ORDER SECNAME, SECNAME, ... SECNAME'
`ORDER SECNAME SECNAME SECNAME'
Normally, `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
`ORDER' command. The sections you list with `ORDER' will appear
first in your output file, in the order specified.
`PUBLIC NAME=EXPRESSION'
`PUBLIC NAME,EXPRESSION'
`PUBLIC NAME EXPRESSION'
Supply a value (EXPRESSION) for external symbol NAME used in the
linker input files.
`SECT SECNAME, EXPRESSION'
`SECT SECNAME=EXPRESSION'
`SECT SECNAME EXPRESSION'
You can use any of these three forms of the `SECT' command to
specify the start address (EXPRESSION) for section SECNAME. If
you have more than one `SECT' statement for the same SECNAME, only
the _first_ sets the start address.
File: ld.info, Node: GNU Free Documentation License, Next: LD Index, Prev: MRI, Up: Top
Appendix B GNU Free Documentation License
*****************************************
Version 1.3, 3 November 2008
Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
`http://fsf.org/'
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other
functional and useful document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not
being considered responsible for modifications made by others.
This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense.
It complements the GNU General Public License, which is a copyleft
license designed for free software.
We have designed this License in order to use it for manuals for
free software, because free software needs free documentation: a
free program should come with manuals providing the same freedoms
that the software does. But this License is not limited to
software manuals; it can be used for any textual work, regardless
of subject matter or whether it is published as a printed book.
We recommend this License principally for works whose purpose is
instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium,
that contains a notice placed by the copyright holder saying it
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"Document", below, refers to any such manual or work. Any member
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A "Modified Version" of the Document means any work containing the
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A "Secondary Section" is a named appendix or a front-matter section
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2. VERBATIM COPYING
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may not use technical measures to obstruct or control the reading
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You may also lend copies, under the same conditions stated above,
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3. COPYING IN QUANTITY
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enclose the copies in covers that carry, clearly and legibly, all
these Cover Texts: Front-Cover Texts on the front cover, and
Back-Cover Texts on the back cover. Both covers must also clearly
and legibly identify you as the publisher of these copies. The
front cover must present the full title with all words of the
title equally prominent and visible. You may add other material
on the covers in addition. Copying with changes limited to the
covers, as long as they preserve the title of the Document and
satisfy these conditions, can be treated as verbatim copying in
other respects.
If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto
adjacent pages.
If you publish or distribute Opaque copies of the Document
numbering more than 100, you must either include a
machine-readable Transparent copy along with each Opaque copy, or
state in or with each Opaque copy a computer-network location from
which the general network-using public has access to download
using public-standard network protocols a complete Transparent
copy of the Document, free of added material. If you use the
latter option, you must take reasonably prudent steps, when you
begin distribution of Opaque copies in quantity, to ensure that
this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you
distribute an Opaque copy (directly or through your agents or
retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of
the Document well before redistributing any large number of
copies, to give them a chance to provide you with an updated
version of the Document.
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document
under the conditions of sections 2 and 3 above, provided that you
release the Modified Version under precisely this License, with
the Modified Version filling the role of the Document, thus
licensing distribution and modification of the Modified Version to
whoever possesses a copy of it. In addition, you must do these
things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title
distinct from that of the Document, and from those of
previous versions (which should, if there were any, be listed
in the History section of the Document). You may use the
same title as a previous version if the original publisher of
that version gives permission.
B. List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in
the Modified Version, together with at least five of the
principal authors of the Document (all of its principal
authors, if it has fewer than five), unless they release you
from this requirement.
C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.
F. Include, immediately after the copyright notices, a license
notice giving the public permission to use the Modified
Version under the terms of this License, in the form shown in
the Addendum below.
G. Preserve in that license notice the full lists of Invariant
Sections and required Cover Texts given in the Document's
license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled "History", Preserve its Title,
and add to it an item stating at least the title, year, new
authors, and publisher of the Modified Version as given on
the Title Page. If there is no section Entitled "History" in
the Document, create one stating the title, year, authors,
and publisher of the Document as given on its Title Page,
then add an item describing the Modified Version as stated in
the previous sentence.
J. Preserve the network location, if any, given in the Document
for public access to a Transparent copy of the Document, and
likewise the network locations given in the Document for
previous versions it was based on. These may be placed in
the "History" section. You may omit a network location for a
work that was published at least four years before the
Document itself, or if the original publisher of the version
it refers to gives permission.
K. For any section Entitled "Acknowledgements" or "Dedications",
Preserve the Title of the section, and preserve in the
section all the substance and tone of each of the contributor
acknowledgements and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers
or the equivalent are not considered part of the section
titles.
M. Delete any section Entitled "Endorsements". Such a section
may not be included in the Modified Version.
N. Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant
Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no
material copied from the Document, you may at your option
designate some or all of these sections as invariant. To do this,
add their titles to the list of Invariant Sections in the Modified
Version's license notice. These titles must be distinct from any
other section titles.
You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text
has been approved by an organization as the authoritative
definition of a standard.
You may add a passage of up to five words as a Front-Cover Text,
and a passage of up to 25 words as a Back-Cover Text, to the end
of the list of Cover Texts in the Modified Version. Only one
passage of Front-Cover Text and one of Back-Cover Text may be
added by (or through arrangements made by) any one entity. If the
Document already includes a cover text for the same cover,
previously added by you or by arrangement made by the same entity
you are acting on behalf of, you may not add another; but you may
replace the old one, on explicit permission from the previous
publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this
License give permission to use their names for publicity for or to
assert or imply endorsement of any Modified Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under
this License, under the terms defined in section 4 above for
modified versions, provided that you include in the combination
all of the Invariant Sections of all of the original documents,
unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all
their Warranty Disclaimers.
The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name
but different contents, make the title of each such section unique
by adding at the end of it, in parentheses, the name of the
original author or publisher of that section if known, or else a
unique number. Make the same adjustment to the section titles in
the list of Invariant Sections in the license notice of the
combined work.
In the combination, you must combine any sections Entitled
"History" in the various original documents, forming one section
Entitled "History"; likewise combine any sections Entitled
"Acknowledgements", and any sections Entitled "Dedications". You
must delete all sections Entitled "Endorsements."
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other
documents released under this License, and replace the individual
copies of this License in the various documents with a single copy
that is included in the collection, provided that you follow the
rules of this License for verbatim copying of each of the
documents in all other respects.
You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert
a copy of this License into the extracted document, and follow
this License in all other respects regarding verbatim copying of
that document.
7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other
separate and independent documents or works, in or on a volume of
a storage or distribution medium, is called an "aggregate" if the
copyright resulting from the compilation is not used to limit the
legal rights of the compilation's users beyond what the individual
works permit. When the Document is included in an aggregate, this
License does not apply to the other works in the aggregate which
are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half
of the entire aggregate, the Document's Cover Texts may be placed
on covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic
form. Otherwise they must appear on printed covers that bracket
the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section
4. Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also
include the original English version of this License and the
original versions of those notices and disclaimers. In case of a
disagreement between the translation and the original version of
this License or a notice or disclaimer, the original version will
prevail.
If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to
Preserve its Title (section 1) will typically require changing the
actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense, or distribute it is void,
and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your
license from a particular copyright holder is reinstated (a)
provisionally, unless and until the copyright holder explicitly
and finally terminates your license, and (b) permanently, if the
copyright holder fails to notify you of the violation by some
reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from
that copyright holder, and you cure the violation prior to 30 days
after your receipt of the notice.
Termination of your rights under this section does not terminate
the licenses of parties who have received copies or rights from
you under this License. If your rights have been terminated and
not permanently reinstated, receipt of a copy of some or all of
the same material does not give you any rights to use it.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of
the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
`http://www.gnu.org/copyleft/'.
Each version of the License is given a distinguishing version
number. If the Document specifies that a particular numbered
version of this License "or any later version" applies to it, you
have the option of following the terms and conditions either of
that specified version or of any later version that has been
published (not as a draft) by the Free Software Foundation. If
the Document does not specify a version number of this License,
you may choose any version ever published (not as a draft) by the
Free Software Foundation. If the Document specifies that a proxy
can decide which future versions of this License can be used, that
proxy's public statement of acceptance of a version permanently
authorizes you to choose that version for the Document.
11. RELICENSING
"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server.
A "Massive Multiauthor Collaboration" (or "MMC") contained in the
site means any set of copyrightable works thus published on the MMC
site.
"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.
"Incorporate" means to publish or republish a Document, in whole or
in part, as part of another Document.
An MMC is "eligible for relicensing" if it is licensed under this
License, and if all works that were first published under this
License somewhere other than this MMC, and subsequently
incorporated in whole or in part into the MMC, (1) had no cover
texts or invariant sections, and (2) were thus incorporated prior
to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the
site under CC-BY-SA on the same site at any time before August 1,
2009, provided the MMC is eligible for relicensing.
ADDENDUM: How to use this License for your documents
====================================================
To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:
Copyright (C) YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:
with the Invariant Sections being LIST THEIR TITLES, with
the Front-Cover Texts being LIST, and with the Back-Cover Texts
being LIST.
If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.
If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License, to
permit their use in free software.
File: ld.info, Node: LD Index, Prev: GNU Free Documentation License, Up: Top
LD Index
********
[index ]
* Menu:
* ": Symbols. (line 6)
* -(: Options. (line 696)
* --accept-unknown-input-arch: Options. (line 714)
* --add-needed: Options. (line 738)
* --add-stdcall-alias: Options. (line 1571)
* --allow-multiple-definition: Options. (line 984)
* --allow-shlib-undefined: Options. (line 990)
* --architecture=ARCH: Options. (line 123)
* --as-needed: Options. (line 724)
* --audit AUDITLIB: Options. (line 112)
* --auxiliary=NAME: Options. (line 255)
* --bank-window: Options. (line 1980)
* --base-file: Options. (line 1576)
* --be8: ARM. (line 28)
* --bss-plt: PowerPC ELF32. (line 16)
* --build-id: Options. (line 1533)
* --build-id=STYLE: Options. (line 1533)
* --check-sections: Options. (line 817)
* --copy-dt-needed-entries: Options. (line 829)
* --cref: Options. (line 850)
* --default-imported-symver: Options. (line 1027)
* --default-script=SCRIPT: Options. (line 541)
* --default-symver: Options. (line 1023)
* --defsym=SYMBOL=EXP: Options. (line 878)
* --demangle[=STYLE]: Options. (line 891)
* --depaudit AUDITLIB: Options. (line 177)
* --disable-auto-image-base: Options. (line 1755)
* --disable-auto-import: Options. (line 1890)
* --disable-long-section-names: Options. (line 1586)
* --disable-new-dtags: Options. (line 1496)
* --disable-runtime-pseudo-reloc: Options. (line 1903)
* --disable-stdcall-fixup: Options. (line 1608)
* --discard-all: Options. (line 587)
* --discard-locals: Options. (line 591)
* --dll: Options. (line 1581)
* --dll-search-prefix: Options. (line 1761)
* --dotsyms: PowerPC64 ELF64. (line 33)
* --dynamic-linker=FILE: Options. (line 904)
* --dynamic-list-cpp-new: Options. (line 809)
* --dynamic-list-cpp-typeinfo: Options. (line 813)
* --dynamic-list-data: Options. (line 806)
* --dynamic-list=DYNAMIC-LIST-FILE: Options. (line 793)
* --dynamicbase: Options. (line 1939)
* --eh-frame-hdr: Options. (line 1492)
* --emit-relocs: Options. (line 476)
* --emit-stack-syms: SPU ELF. (line 46)
* --emit-stub-syms <1>: SPU ELF. (line 15)
* --emit-stub-syms <2>: PowerPC ELF32. (line 47)
* --emit-stub-syms: PowerPC64 ELF64. (line 29)
* --enable-auto-image-base: Options. (line 1747)
* --enable-auto-import: Options. (line 1770)
* --enable-extra-pe-debug: Options. (line 1908)
* --enable-long-section-names: Options. (line 1586)
* --enable-new-dtags: Options. (line 1496)
* --enable-runtime-pseudo-reloc: Options. (line 1895)
* --enable-stdcall-fixup: Options. (line 1608)
* --entry=ENTRY: Options. (line 187)
* --error-unresolved-symbols: Options. (line 1445)
* --exclude-all-symbols: Options. (line 1662)
* --exclude-libs: Options. (line 197)
* --exclude-modules-for-implib: Options. (line 208)
* --exclude-symbols: Options. (line 1656)
* --export-all-symbols: Options. (line 1632)
* --export-dynamic: Options. (line 221)
* --extra-overlay-stubs: SPU ELF. (line 19)
* --fatal-warnings: Options. (line 911)
* --file-alignment: Options. (line 1666)
* --filter=NAME: Options. (line 276)
* --fix-cortex-a8: i960. (line 39)
* --fix-v4bx: ARM. (line 49)
* --fix-v4bx-interworking: ARM. (line 62)
* --force-dynamic: Options. (line 485)
* --force-exe-suffix: Options. (line 916)
* --forceinteg: Options. (line 1944)
* --format=FORMAT: Options. (line 134)
* --format=VERSION: TI COFF. (line 6)
* --gc-sections: Options. (line 926)
* --got: Options. (line 1993)
* --got=TYPE: M68K. (line 6)
* --gpsize=VALUE: Options. (line 309)
* --hash-size=NUMBER: Options. (line 1505)
* --hash-style=STYLE: Options. (line 1513)
* --heap: Options. (line 1672)
* --help: Options. (line 957)
* --image-base: Options. (line 1679)
* --just-symbols=FILE: Options. (line 508)
* --kill-at: Options. (line 1688)
* --large-address-aware: Options. (line 1693)
* --leading-underscore: Options. (line 1626)
* --library-path=DIR: Options. (line 368)
* --library=NAMESPEC: Options. (line 335)
* --local-store=lo:hi: SPU ELF. (line 24)
* --major-image-version: Options. (line 1702)
* --major-os-version: Options. (line 1707)
* --major-subsystem-version: Options. (line 1711)
* --merge-exidx-entries: i960. (line 48)
* --minor-image-version: Options. (line 1716)
* --minor-os-version: Options. (line 1721)
* --minor-subsystem-version: Options. (line 1725)
* --mri-script=MRI-CMDFILE: Options. (line 158)
* --multi-subspace: HPPA ELF32. (line 6)
* --nmagic: Options. (line 439)
* --no-accept-unknown-input-arch: Options. (line 714)
* --no-add-needed: Options. (line 738)
* --no-allow-shlib-undefined: Options. (line 990)
* --no-as-needed: Options. (line 724)
* --no-bind: Options. (line 1958)
* --no-check-sections: Options. (line 817)
* --no-copy-dt-needed-entries: Options. (line 829)
* --no-define-common: Options. (line 862)
* --no-demangle: Options. (line 891)
* --no-dotsyms: PowerPC64 ELF64. (line 33)
* --no-enum-size-warning: ARM. (line 111)
* --no-export-dynamic: Options. (line 221)
* --no-fatal-warnings: Options. (line 911)
* --no-fix-cortex-a8: i960. (line 39)
* --no-gc-sections: Options. (line 926)
* --no-isolation: Options. (line 1951)
* --no-keep-memory: Options. (line 969)
* --no-leading-underscore: Options. (line 1626)
* --no-merge-exidx-entries: i960. (line 48)
* --no-multi-toc: PowerPC64 ELF64. (line 74)
* --no-omagic: Options. (line 454)
* --no-opd-optimize: PowerPC64 ELF64. (line 48)
* --no-overlays: SPU ELF. (line 9)
* --no-print-gc-sections: Options. (line 948)
* --no-seh: Options. (line 1954)
* --no-tls-optimize <1>: PowerPC ELF32. (line 51)
* --no-tls-optimize: PowerPC64 ELF64. (line 43)
* --no-toc-optimize: PowerPC64 ELF64. (line 60)
* --no-trampoline: Options. (line 1974)
* --no-undefined: Options. (line 976)
* --no-undefined-version: Options. (line 1018)
* --no-warn-mismatch: Options. (line 1031)
* --no-warn-search-mismatch: Options. (line 1040)
* --no-wchar-size-warning: ARM. (line 118)
* --no-whole-archive: Options. (line 1044)
* --noinhibit-exec: Options. (line 1048)
* --non-overlapping-opd: PowerPC64 ELF64. (line 54)
* --nxcompat: Options. (line 1947)
* --oformat=OUTPUT-FORMAT: Options. (line 1060)
* --omagic: Options. (line 445)
* --out-implib: Options. (line 1738)
* --output-def: Options. (line 1730)
* --output=OUTPUT: Options. (line 460)
* --pic-executable: Options. (line 1073)
* --pic-veneer: ARM. (line 124)
* --plugin: SPU ELF. (line 6)
* --print-gc-sections: Options. (line 948)
* --print-map: Options. (line 402)
* --reduce-memory-overheads: Options. (line 1519)
* --relax: Options. (line 1089)
* --relax on i960: i960. (line 31)
* --relax on PowerPC: PowerPC ELF32. (line 6)
* --relax on Xtensa: Xtensa. (line 27)
* --relocatable: Options. (line 489)
* --retain-symbols-file=FILENAME: Options. (line 1115)
* --script=SCRIPT: Options. (line 532)
* --sdata-got: PowerPC ELF32. (line 33)
* --section-alignment: Options. (line 1913)
* --section-start=SECTIONNAME=ORG: Options. (line 1271)
* --secure-plt: PowerPC ELF32. (line 26)
* --sort-common: Options. (line 1213)
* --sort-section=alignment: Options. (line 1228)
* --sort-section=name: Options. (line 1224)
* --split-by-file: Options. (line 1232)
* --split-by-reloc: Options. (line 1237)
* --stack: Options. (line 1919)
* --stack-analysis: SPU ELF. (line 29)
* --stats: Options. (line 1250)
* --strip-all: Options. (line 519)
* --strip-debug: Options. (line 523)
* --stub-group-size: PowerPC64 ELF64. (line 6)
* --stub-group-size=N <1>: ARM. (line 129)
* --stub-group-size=N: HPPA ELF32. (line 12)
* --subsystem: Options. (line 1926)
* --support-old-code: ARM. (line 6)
* --sysroot=DIRECTORY: Options. (line 1254)
* --target-help: Options. (line 961)
* --target1-abs: ARM. (line 32)
* --target1-rel: ARM. (line 32)
* --target2=TYPE: ARM. (line 37)
* --thumb-entry=ENTRY: ARM. (line 17)
* --trace: Options. (line 528)
* --trace-symbol=SYMBOL: Options. (line 597)
* --traditional-format: Options. (line 1259)
* --tsaware: Options. (line 1964)
* --undefined=SYMBOL: Options. (line 554)
* --unique[=SECTION]: Options. (line 572)
* --unresolved-symbols: Options. (line 1290)
* --use-blx: ARM. (line 74)
* --use-nul-prefixed-import-tables: ARM. (line 23)
* --verbose: Options. (line 1319)
* --version: Options. (line 581)
* --version-script=VERSION-SCRIPTFILE: Options. (line 1325)
* --vfp11-denorm-fix: ARM. (line 83)
* --warn-alternate-em: Options. (line 1437)
* --warn-common: Options. (line 1336)
* --warn-constructors: Options. (line 1404)
* --warn-multiple-gp: Options. (line 1409)
* --warn-once: Options. (line 1423)
* --warn-section-align: Options. (line 1427)
* --warn-shared-textrel: Options. (line 1434)
* --warn-unresolved-symbols: Options. (line 1440)
* --wdmdriver: Options. (line 1961)
* --whole-archive: Options. (line 1449)
* --wrap=SYMBOL: Options. (line 1463)
* -A ARCH: Options. (line 122)
* -a KEYWORD: Options. (line 105)
* -assert KEYWORD: Options. (line 745)
* -b FORMAT: Options. (line 134)
* -Bdynamic: Options. (line 748)
* -Bgroup: Options. (line 758)
* -Bshareable: Options. (line 1206)
* -Bstatic: Options. (line 765)
* -Bsymbolic: Options. (line 780)
* -Bsymbolic-functions: Options. (line 787)
* -c MRI-CMDFILE: Options. (line 158)
* -call_shared: Options. (line 748)
* -d: Options. (line 168)
* -dc: Options. (line 168)
* -dn: Options. (line 765)
* -dp: Options. (line 168)
* -dT SCRIPT: Options. (line 541)
* -dy: Options. (line 748)
* -E: Options. (line 221)
* -e ENTRY: Options. (line 187)
* -EB: Options. (line 248)
* -EL: Options. (line 251)
* -F NAME: Options. (line 276)
* -f NAME: Options. (line 255)
* -fini=NAME: Options. (line 300)
* -g: Options. (line 306)
* -G VALUE: Options. (line 309)
* -h NAME: Options. (line 317)
* -i: Options. (line 326)
* -IFILE: Options. (line 904)
* -init=NAME: Options. (line 329)
* -L DIR: Options. (line 368)
* -l NAMESPEC: Options. (line 335)
* -M: Options. (line 402)
* -m EMULATION: Options. (line 392)
* -Map=MAPFILE: Options. (line 965)
* -N: Options. (line 445)
* -n: Options. (line 439)
* -no-relax: Options. (line 1089)
* -non_shared: Options. (line 765)
* -nostdlib: Options. (line 1054)
* -O LEVEL: Options. (line 466)
* -o OUTPUT: Options. (line 460)
* -P AUDITLIB: Options. (line 177)
* -pie: Options. (line 1073)
* -q: Options. (line 476)
* -qmagic: Options. (line 1083)
* -Qy: Options. (line 1086)
* -r: Options. (line 489)
* -R FILE: Options. (line 508)
* -rpath-link=DIR: Options. (line 1151)
* -rpath=DIR: Options. (line 1129)
* -s: Options. (line 519)
* -S: Options. (line 523)
* -shared: Options. (line 1206)
* -soname=NAME: Options. (line 317)
* -static: Options. (line 765)
* -t: Options. (line 528)
* -T SCRIPT: Options. (line 532)
* -Tbss=ORG: Options. (line 1280)
* -Tdata=ORG: Options. (line 1280)
* -Ttext-segment=ORG: Options. (line 1286)
* -Ttext=ORG: Options. (line 1280)
* -u SYMBOL: Options. (line 554)
* -Ur: Options. (line 562)
* -V: Options. (line 581)
* -v: Options. (line 581)
* -X: Options. (line 591)
* -x: Options. (line 587)
* -Y PATH: Options. (line 606)
* -y SYMBOL: Options. (line 597)
* -z defs: Options. (line 976)
* -z KEYWORD: Options. (line 610)
* -z muldefs: Options. (line 984)
* .: Location Counter. (line 6)
* /DISCARD/: Output Section Discarding.
(line 21)
* :PHDR: Output Section Phdr.
(line 6)
* =FILLEXP: Output Section Fill.
(line 6)
* >REGION: Output Section Region.
(line 6)
* [COMMON]: Input Section Common.
(line 29)
* ABSOLUTE (MRI): MRI. (line 33)
* absolute and relocatable symbols: Expression Section. (line 6)
* absolute expressions: Expression Section. (line 6)
* ABSOLUTE(EXP): Builtin Functions. (line 10)
* ADDR(SECTION): Builtin Functions. (line 17)
* address, section: Output Section Address.
(line 6)
* ALIAS (MRI): MRI. (line 44)
* ALIGN (MRI): MRI. (line 50)
* align expression: Builtin Functions. (line 38)
* align location counter: Builtin Functions. (line 38)
* ALIGN(ALIGN): Builtin Functions. (line 38)
* ALIGN(EXP,ALIGN): Builtin Functions. (line 38)
* ALIGN(SECTION_ALIGN): Forced Output Alignment.
(line 6)
* aligned common symbols: WIN32. (line 424)
* ALIGNOF(SECTION): Builtin Functions. (line 63)
* allocating memory: MEMORY. (line 6)
* architecture: Miscellaneous Commands.
(line 72)
* architectures: Options. (line 122)
* archive files, from cmd line: Options. (line 335)
* archive search path in linker script: File Commands. (line 74)
* arithmetic: Expressions. (line 6)
* arithmetic operators: Operators. (line 6)
* ARM interworking support: ARM. (line 6)
* AS_NEEDED(FILES): File Commands. (line 54)
* ASSERT: Miscellaneous Commands.
(line 9)
* assertion in linker script: Miscellaneous Commands.
(line 9)
* assignment in scripts: Assignments. (line 6)
* AT(LMA): Output Section LMA. (line 6)
* AT>LMA_REGION: Output Section LMA. (line 6)
* automatic data imports: WIN32. (line 191)
* back end: BFD. (line 6)
* BASE (MRI): MRI. (line 54)
* BE8: ARM. (line 28)
* BFD canonical format: Canonical format. (line 11)
* BFD requirements: BFD. (line 16)
* big-endian objects: Options. (line 248)
* binary input format: Options. (line 134)
* BLOCK(EXP): Builtin Functions. (line 76)
* bug criteria: Bug Criteria. (line 6)
* bug reports: Bug Reporting. (line 6)
* bugs in ld: Reporting Bugs. (line 6)
* BYTE(EXPRESSION): Output Section Data.
(line 6)
* C++ constructors, arranging in link: Output Section Keywords.
(line 19)
* CHIP (MRI): MRI. (line 58)
* COLLECT_NO_DEMANGLE: Environment. (line 29)
* combining symbols, warnings on: Options. (line 1336)
* command files: Scripts. (line 6)
* command line: Options. (line 6)
* common allocation: Options. (line 862)
* common allocation in linker script: Miscellaneous Commands.
(line 25)
* common symbol placement: Input Section Common.
(line 6)
* COMMONPAGESIZE: Symbolic Constants. (line 13)
* compatibility, MRI: Options. (line 158)
* CONSTANT: Symbolic Constants. (line 6)
* constants in linker scripts: Constants. (line 6)
* constraints on output sections: Output Section Constraint.
(line 6)
* CONSTRUCTORS: Output Section Keywords.
(line 19)
* constructors: Options. (line 562)
* constructors, arranging in link: Output Section Keywords.
(line 19)
* Cortex-A8 erratum workaround: i960. (line 39)
* crash of linker: Bug Criteria. (line 9)
* CREATE_OBJECT_SYMBOLS: Output Section Keywords.
(line 9)
* creating a DEF file: WIN32. (line 158)
* cross reference table: Options. (line 850)
* cross references: Miscellaneous Commands.
(line 56)
* current output location: Location Counter. (line 6)
* data: Output Section Data.
(line 6)
* DATA_SEGMENT_ALIGN(MAXPAGESIZE, COMMONPAGESIZE): Builtin Functions.
(line 81)
* DATA_SEGMENT_END(EXP): Builtin Functions. (line 102)
* DATA_SEGMENT_RELRO_END(OFFSET, EXP): Builtin Functions. (line 108)
* dbx: Options. (line 1264)
* DEF files, creating: Options. (line 1730)
* default emulation: Environment. (line 21)
* default input format: Environment. (line 9)
* DEFINED(SYMBOL): Builtin Functions. (line 119)
* deleting local symbols: Options. (line 587)
* demangling, default: Environment. (line 29)
* demangling, from command line: Options. (line 891)
* direct linking to a dll: WIN32. (line 239)
* discarding sections: Output Section Discarding.
(line 6)
* discontinuous memory: MEMORY. (line 6)
* DLLs, creating: Options. (line 1738)
* DLLs, linking to: Options. (line 1761)
* dot: Location Counter. (line 6)
* dot inside sections: Location Counter. (line 36)
* dot outside sections: Location Counter. (line 66)
* dynamic linker, from command line: Options. (line 904)
* dynamic symbol table: Options. (line 221)
* ELF program headers: PHDRS. (line 6)
* emulation: Options. (line 392)
* emulation, default: Environment. (line 21)
* END (MRI): MRI. (line 62)
* endianness: Options. (line 248)
* entry point: Entry Point. (line 6)
* entry point, from command line: Options. (line 187)
* entry point, thumb: ARM. (line 17)
* ENTRY(SYMBOL): Entry Point. (line 6)
* error on valid input: Bug Criteria. (line 12)
* example of linker script: Simple Example. (line 6)
* exporting DLL symbols: WIN32. (line 19)
* expression evaluation order: Evaluation. (line 6)
* expression sections: Expression Section. (line 6)
* expression, absolute: Builtin Functions. (line 10)
* expressions: Expressions. (line 6)
* EXTERN: Miscellaneous Commands.
(line 13)
* fatal signal: Bug Criteria. (line 9)
* file name wildcard patterns: Input Section Wildcards.
(line 6)
* FILEHDR: PHDRS. (line 62)
* filename symbols: Output Section Keywords.
(line 9)
* fill pattern, entire section: Output Section Fill.
(line 6)
* FILL(EXPRESSION): Output Section Data.
(line 39)
* finalization function: Options. (line 300)
* first input file: File Commands. (line 82)
* first instruction: Entry Point. (line 6)
* FIX_V4BX: ARM. (line 49)
* FIX_V4BX_INTERWORKING: ARM. (line 62)
* FORCE_COMMON_ALLOCATION: Miscellaneous Commands.
(line 20)
* forcing input section alignment: Forced Input Alignment.
(line 6)
* forcing output section alignment: Forced Output Alignment.
(line 6)
* forcing the creation of dynamic sections: Options. (line 485)
* FORMAT (MRI): MRI. (line 66)
* functions in expressions: Builtin Functions. (line 6)
* garbage collection <1>: Input Section Keep. (line 6)
* garbage collection: Options. (line 948)
* generating optimized output: Options. (line 466)
* GNU linker: Overview. (line 6)
* GNUTARGET: Environment. (line 9)
* GROUP(FILES): File Commands. (line 47)
* grouping input files: File Commands. (line 47)
* groups of archives: Options. (line 696)
* H8/300 support: H8/300. (line 6)
* header size: Builtin Functions. (line 182)
* heap size: Options. (line 1672)
* help: Options. (line 957)
* holes: Location Counter. (line 12)
* holes, filling: Output Section Data.
(line 39)
* HPPA multiple sub-space stubs: HPPA ELF32. (line 6)
* HPPA stub grouping: HPPA ELF32. (line 12)
* i960 support: i960. (line 6)
* image base: Options. (line 1679)
* implicit linker scripts: Implicit Linker Scripts.
(line 6)
* import libraries: WIN32. (line 10)
* INCLUDE FILENAME: File Commands. (line 9)
* including a linker script: File Commands. (line 9)
* including an entire archive: Options. (line 1449)
* incremental link: Options. (line 326)
* INHIBIT_COMMON_ALLOCATION: Miscellaneous Commands.
(line 25)
* initialization function: Options. (line 329)
* initialized data in ROM: Output Section LMA. (line 39)
* input file format in linker script: Format Commands. (line 35)
* input filename symbols: Output Section Keywords.
(line 9)
* input files in linker scripts: File Commands. (line 19)
* input files, displaying: Options. (line 528)
* input format: Options. (line 134)
* input object files in linker scripts: File Commands. (line 19)
* input section alignment: Forced Input Alignment.
(line 6)
* input section basics: Input Section Basics.
(line 6)
* input section wildcards: Input Section Wildcards.
(line 6)
* input sections: Input Section. (line 6)
* INPUT(FILES): File Commands. (line 19)
* INSERT: Miscellaneous Commands.
(line 30)
* insert user script into default script: Miscellaneous Commands.
(line 30)
* integer notation: Constants. (line 6)
* integer suffixes: Constants. (line 15)
* internal object-file format: Canonical format. (line 11)
* invalid input: Bug Criteria. (line 14)
* K and M integer suffixes: Constants. (line 15)
* KEEP: Input Section Keep. (line 6)
* l =: MEMORY. (line 74)
* lazy evaluation: Evaluation. (line 6)
* ld bugs, reporting: Bug Reporting. (line 6)
* LDEMULATION: Environment. (line 21)
* len =: MEMORY. (line 74)
* LENGTH =: MEMORY. (line 74)
* LENGTH(MEMORY): Builtin Functions. (line 136)
* library search path in linker script: File Commands. (line 74)
* link map: Options. (line 402)
* link-time runtime library search path: Options. (line 1151)
* linker crash: Bug Criteria. (line 9)
* linker script concepts: Basic Script Concepts.
(line 6)
* linker script example: Simple Example. (line 6)
* linker script file commands: File Commands. (line 6)
* linker script format: Script Format. (line 6)
* linker script input object files: File Commands. (line 19)
* linker script simple commands: Simple Commands. (line 6)
* linker scripts: Scripts. (line 6)
* LIST (MRI): MRI. (line 77)
* little-endian objects: Options. (line 251)
* LOAD (MRI): MRI. (line 84)
* load address: Output Section LMA. (line 6)
* LOADADDR(SECTION): Builtin Functions. (line 139)
* loading, preventing: Output Section Type.
(line 22)
* local symbols, deleting: Options. (line 591)
* location counter: Location Counter. (line 6)
* LONG(EXPRESSION): Output Section Data.
(line 6)
* M and K integer suffixes: Constants. (line 15)
* M68HC11 and 68HC12 support: M68HC11/68HC12. (line 6)
* machine architecture: Miscellaneous Commands.
(line 72)
* machine dependencies: Machine Dependent. (line 6)
* mapping input sections to output sections: Input Section. (line 6)
* MAX: Builtin Functions. (line 142)
* MAXPAGESIZE: Symbolic Constants. (line 10)
* MEMORY: MEMORY. (line 6)
* memory region attributes: MEMORY. (line 34)
* memory regions: MEMORY. (line 6)
* memory regions and sections: Output Section Region.
(line 6)
* memory usage: Options. (line 969)
* MIN: Builtin Functions. (line 145)
* Motorola 68K GOT generation: M68K. (line 6)
* MRI compatibility: MRI. (line 6)
* MSP430 extra sections: MSP430. (line 11)
* NAME (MRI): MRI. (line 90)
* name, section: Output Section Name.
(line 6)
* names: Symbols. (line 6)
* naming the output file: Options. (line 460)
* NEXT(EXP): Builtin Functions. (line 149)
* NMAGIC: Options. (line 439)
* NO_ENUM_SIZE_WARNING: ARM. (line 111)
* NO_WCHAR_SIZE_WARNING: ARM. (line 118)
* NOCROSSREFS(SECTIONS): Miscellaneous Commands.
(line 56)
* NOLOAD: Output Section Type.
(line 22)
* not enough room for program headers: Builtin Functions. (line 187)
* o =: MEMORY. (line 69)
* objdump -i: BFD. (line 6)
* object file management: BFD. (line 6)
* object files: Options. (line 29)
* object formats available: BFD. (line 6)
* object size: Options. (line 309)
* OMAGIC: Options. (line 454)
* ONLY_IF_RO: Output Section Constraint.
(line 6)
* ONLY_IF_RW: Output Section Constraint.
(line 6)
* opening object files: BFD outline. (line 6)
* operators for arithmetic: Operators. (line 6)
* options: Options. (line 6)
* ORDER (MRI): MRI. (line 95)
* org =: MEMORY. (line 69)
* ORIGIN =: MEMORY. (line 69)
* ORIGIN(MEMORY): Builtin Functions. (line 155)
* orphan: Orphan Sections. (line 6)
* output file after errors: Options. (line 1048)
* output file format in linker script: Format Commands. (line 10)
* output file name in linker script: File Commands. (line 64)
* output section alignment: Forced Output Alignment.
(line 6)
* output section attributes: Output Section Attributes.
(line 6)
* output section data: Output Section Data.
(line 6)
* OUTPUT(FILENAME): File Commands. (line 64)
* OUTPUT_ARCH(BFDARCH): Miscellaneous Commands.
(line 72)
* OUTPUT_FORMAT(BFDNAME): Format Commands. (line 10)
* OVERLAY: Overlay Description.
(line 6)
* overlays: Overlay Description.
(line 6)
* partial link: Options. (line 489)
* PE import table prefixing: ARM. (line 23)
* PHDRS: PHDRS. (line 62)
* PIC_VENEER: ARM. (line 124)
* position independent executables: Options. (line 1075)
* PowerPC ELF32 options: PowerPC ELF32. (line 16)
* PowerPC GOT: PowerPC ELF32. (line 33)
* PowerPC long branches: PowerPC ELF32. (line 6)
* PowerPC PLT: PowerPC ELF32. (line 16)
* PowerPC stub symbols: PowerPC ELF32. (line 47)
* PowerPC TLS optimization: PowerPC ELF32. (line 51)
* PowerPC64 dot symbols: PowerPC64 ELF64. (line 33)
* PowerPC64 ELF64 options: PowerPC64 ELF64. (line 6)
* PowerPC64 multi-TOC: PowerPC64 ELF64. (line 74)
* PowerPC64 OPD optimization: PowerPC64 ELF64. (line 48)
* PowerPC64 OPD spacing: PowerPC64 ELF64. (line 54)
* PowerPC64 stub grouping: PowerPC64 ELF64. (line 6)
* PowerPC64 stub symbols: PowerPC64 ELF64. (line 29)
* PowerPC64 TLS optimization: PowerPC64 ELF64. (line 43)
* PowerPC64 TOC optimization: PowerPC64 ELF64. (line 60)
* precedence in expressions: Operators. (line 6)
* prevent unnecessary loading: Output Section Type.
(line 22)
* program headers: PHDRS. (line 6)
* program headers and sections: Output Section Phdr.
(line 6)
* program headers, not enough room: Builtin Functions. (line 187)
* program segments: PHDRS. (line 6)
* PROVIDE: PROVIDE. (line 6)
* PROVIDE_HIDDEN: PROVIDE_HIDDEN. (line 6)
* PUBLIC (MRI): MRI. (line 103)
* QUAD(EXPRESSION): Output Section Data.
(line 6)
* quoted symbol names: Symbols. (line 6)
* read-only text: Options. (line 439)
* read/write from cmd line: Options. (line 445)
* region alias: REGION_ALIAS. (line 6)
* region names: REGION_ALIAS. (line 6)
* REGION_ALIAS(ALIAS, REGION): REGION_ALIAS. (line 6)
* regions of memory: MEMORY. (line 6)
* relative expressions: Expression Section. (line 6)
* relaxing addressing modes: Options. (line 1089)
* relaxing on H8/300: H8/300. (line 9)
* relaxing on i960: i960. (line 31)
* relaxing on M68HC11: M68HC11/68HC12. (line 12)
* relaxing on Xtensa: Xtensa. (line 27)
* relocatable and absolute symbols: Expression Section. (line 6)
* relocatable output: Options. (line 489)
* removing sections: Output Section Discarding.
(line 6)
* reporting bugs in ld: Reporting Bugs. (line 6)
* requirements for BFD: BFD. (line 16)
* retain relocations in final executable: Options. (line 476)
* retaining specified symbols: Options. (line 1115)
* ROM initialized data: Output Section LMA. (line 39)
* round up expression: Builtin Functions. (line 38)
* round up location counter: Builtin Functions. (line 38)
* runtime library name: Options. (line 317)
* runtime library search path: Options. (line 1129)
* runtime pseudo-relocation: WIN32. (line 217)
* scaled integers: Constants. (line 15)
* scommon section: Input Section Common.
(line 20)
* script files: Options. (line 532)
* scripts: Scripts. (line 6)
* search directory, from cmd line: Options. (line 368)
* search path in linker script: File Commands. (line 74)
* SEARCH_DIR(PATH): File Commands. (line 74)
* SECT (MRI): MRI. (line 109)
* section address: Output Section Address.
(line 6)
* section address in expression: Builtin Functions. (line 17)
* section alignment: Builtin Functions. (line 63)
* section alignment, warnings on: Options. (line 1427)
* section data: Output Section Data.
(line 6)
* section fill pattern: Output Section Fill.
(line 6)
* section load address: Output Section LMA. (line 6)
* section load address in expression: Builtin Functions. (line 139)
* section name: Output Section Name.
(line 6)
* section name wildcard patterns: Input Section Wildcards.
(line 6)
* section size: Builtin Functions. (line 166)
* section, assigning to memory region: Output Section Region.
(line 6)
* section, assigning to program header: Output Section Phdr.
(line 6)
* SECTIONS: SECTIONS. (line 6)
* sections, discarding: Output Section Discarding.
(line 6)
* segment origins, cmd line: Options. (line 1280)
* SEGMENT_START(SEGMENT, DEFAULT): Builtin Functions. (line 158)
* segments, ELF: PHDRS. (line 6)
* shared libraries: Options. (line 1208)
* SHORT(EXPRESSION): Output Section Data.
(line 6)
* SIZEOF(SECTION): Builtin Functions. (line 166)
* SIZEOF_HEADERS: Builtin Functions. (line 182)
* small common symbols: Input Section Common.
(line 20)
* SORT: Input Section Wildcards.
(line 58)
* SORT_BY_ALIGNMENT: Input Section Wildcards.
(line 54)
* SORT_BY_NAME: Input Section Wildcards.
(line 46)
* SPU: SPU ELF. (line 46)
* SPU ELF options: SPU ELF. (line 6)
* SPU extra overlay stubs: SPU ELF. (line 19)
* SPU local store size: SPU ELF. (line 24)
* SPU overlay stub symbols: SPU ELF. (line 15)
* SPU overlays: SPU ELF. (line 9)
* SPU plugins: SPU ELF. (line 6)
* SQUAD(EXPRESSION): Output Section Data.
(line 6)
* stack size: Options. (line 1919)
* standard Unix system: Options. (line 7)
* start of execution: Entry Point. (line 6)
* STARTUP(FILENAME): File Commands. (line 82)
* strip all symbols: Options. (line 519)
* strip debugger symbols: Options. (line 523)
* stripping all but some symbols: Options. (line 1115)
* STUB_GROUP_SIZE: ARM. (line 129)
* SUBALIGN(SUBSECTION_ALIGN): Forced Input Alignment.
(line 6)
* suffixes for integers: Constants. (line 15)
* symbol defaults: Builtin Functions. (line 119)
* symbol definition, scripts: Assignments. (line 6)
* symbol names: Symbols. (line 6)
* symbol tracing: Options. (line 597)
* symbol versions: VERSION. (line 6)
* symbol-only input: Options. (line 508)
* symbolic constants: Symbolic Constants. (line 6)
* symbols, from command line: Options. (line 878)
* symbols, relocatable and absolute: Expression Section. (line 6)
* symbols, retaining selectively: Options. (line 1115)
* synthesizing linker: Options. (line 1089)
* synthesizing on H8/300: H8/300. (line 14)
* TARGET(BFDNAME): Format Commands. (line 35)
* TARGET1: ARM. (line 32)
* TARGET2: ARM. (line 37)
* text segment origin, cmd line: Options. (line 1287)
* thumb entry point: ARM. (line 17)
* TI COFF versions: TI COFF. (line 6)
* traditional format: Options. (line 1259)
* trampoline generation on M68HC11: M68HC11/68HC12. (line 31)
* trampoline generation on M68HC12: M68HC11/68HC12. (line 31)
* unallocated address, next: Builtin Functions. (line 149)
* undefined symbol: Options. (line 554)
* undefined symbol in linker script: Miscellaneous Commands.
(line 13)
* undefined symbols, warnings on: Options. (line 1423)
* uninitialized data placement: Input Section Common.
(line 6)
* unspecified memory: Output Section Data.
(line 39)
* usage: Options. (line 957)
* USE_BLX: ARM. (line 74)
* using a DEF file: WIN32. (line 57)
* using auto-export functionality: WIN32. (line 22)
* Using decorations: WIN32. (line 162)
* variables, defining: Assignments. (line 6)
* verbose: Options. (line 1319)
* version: Options. (line 581)
* version script: VERSION. (line 6)
* version script, symbol versions: Options. (line 1325)
* VERSION {script text}: VERSION. (line 6)
* versions of symbols: VERSION. (line 6)
* VFP11_DENORM_FIX: ARM. (line 83)
* warnings, on combining symbols: Options. (line 1336)
* warnings, on section alignment: Options. (line 1427)
* warnings, on undefined symbols: Options. (line 1423)
* weak externals: WIN32. (line 407)
* what is this?: Overview. (line 6)
* wildcard file name patterns: Input Section Wildcards.
(line 6)
* Xtensa options: Xtensa. (line 56)
* Xtensa processors: Xtensa. (line 6)
Tag Table:
Node: Top891
Node: Overview1674
Node: Invocation2788
Node: Options3196
Node: Environment93412
Node: Scripts95172
Node: Basic Script Concepts96906
Node: Script Format99613
Node: Simple Example100476
Node: Simple Commands103572
Node: Entry Point104078
Node: File Commands105011
Node: Format Commands109012
Node: REGION_ALIAS110968
Node: Miscellaneous Commands115800
Node: Assignments119176
Node: Simple Assignments119667
Node: PROVIDE121403
Node: PROVIDE_HIDDEN122608
Node: Source Code Reference122852
Node: SECTIONS126432
Node: Output Section Description128323
Node: Output Section Name129410
Node: Output Section Address130286
Node: Input Section132521
Node: Input Section Basics133322
Node: Input Section Wildcards136540
Node: Input Section Common141273
Node: Input Section Keep142755
Node: Input Section Example143245
Node: Output Section Data144213
Node: Output Section Keywords146990
Node: Output Section Discarding150559
Node: Output Section Attributes151740
Node: Output Section Type152841
Node: Output Section LMA153912
Node: Forced Output Alignment156983
Node: Forced Input Alignment157251
Node: Output Section Constraint157640
Node: Output Section Region158068
Node: Output Section Phdr158501
Node: Output Section Fill159165
Node: Overlay Description160307
Node: MEMORY164609
Node: PHDRS168943
Node: VERSION174197
Node: Expressions182290
Node: Constants183219
Node: Symbolic Constants184094
Node: Symbols184645
Node: Orphan Sections185392
Node: Location Counter186556
Node: Operators190992
Node: Evaluation191914
Node: Expression Section193278
Node: Builtin Functions196879
Node: Implicit Linker Scripts204835
Node: Machine Dependent205610
Node: H8/300206626
Node: i960208251
Node: M68HC11/68HC12210455
Node: ARM211909
Node: HPPA ELF32219421
Node: M68K221044
Node: MMIX221953
Node: MSP430223118
Node: PowerPC ELF32224167
Node: PowerPC64 ELF64227003
Node: SPU ELF231419
Node: TI COFF234051
Node: WIN32234577
Node: Xtensa254702
Node: BFD257667
Node: BFD outline259122
Node: BFD information loss260408
Node: Canonical format262925
Node: Reporting Bugs267282
Node: Bug Criteria267976
Node: Bug Reporting268675
Node: MRI275714
Node: GNU Free Documentation License280357
Node: LD Index305513
End Tag Table