ld.bfd
The GNU linker
see also :
ar - nm - objcopy - objdump - readelf
Synopsis
ld
[options] objfile ...
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examples
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description
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 man page
does not describe the command language; see the ld
entry in "info" for full details on the
command language and on other aspects of the
GNU linker.
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.
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).
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.
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.
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.
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.)
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".
You can also
define the input format from a script, using the command
"TARGET";
-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 the MRI Compatible Script Files
section of GNU ld documentation. 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.
--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).
--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.
--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.
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.
--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
Expressions for more information about expressions in
linker scripts.
--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".
--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.
--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.
--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.
--strip-all
Omit all symbol information
from the output file.
--strip-debug
Omit debugger symbol
information (but not all symbols) from the output file.
--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. 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.
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.
--version
-V
Display the version number for ld. The
-V option also lists the supported
emulations.
-x
--discard-all
Delete all local symbols.
--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
-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
-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 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 won’t add a
DT_NEEDED tag to the output binary for each
library mentioned in a DT_NEEDED tag in an
input dynamic library. With
--copy-dt-needed-entries
specified on the command line however any dynamic libraries
that follow it will have their DT_NEEDED
entries added. The default behaviour can be restored with
--no-copy-dt-needed-entries.
This option
also has an effect on the resolution of symbols in dynamic
libraries. With
--copy-dt-needed-entries
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 the
default setting 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.
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: 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.
--print-output-format
Print the name of the default
output format (perhaps influenced by other command-line
options). This is the string that would appear in an
"OUTPUT_FORMAT" linker script
command.
--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.
-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.
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.
-Trodata-segment=org
When creating an
ELF executable or shared object for a target
where the read-only data is in its own segment separate from
the executable text, it will set the address of the first
byte of the read-only data 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[= NUMBER
]
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. If the optional
NUMBER argument > 1, plugin symbol
status will also be displayed.
--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
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.
--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.
--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.
--no-ld-generated-unwind-info
Request creation of
".eh_frame" unwind info for linker
generated code sections like PLT . This
option is on by default if linker generated unwind info is
supported.
--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.
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.
The C6X uClinux
target uses a binary format called DSBT to
support shared libraries. Each shared library in the system
needs to have a unique index; all executables use an index
of 0.
--dsbt-size size
This option sets the number of
entires in the DSBT of the current executable
or shared library to size. The default is to create a
table with 64 entries.
--dsbt-index
index
This option sets the
DSBT index of the current executable or
shared library to index. The default is 0, which is
appropriate for generating executables. If a shared library
is generated with a DSBT index of 0, the
"R_C6000_DSBT_INDEX" relocs are copied
into the output file.
The
--no-merge-exidx-entries
switch disables the merging of adjacent exidx entries in
frame unwind info.
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.
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.
copyright
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".
environment
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. 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.
see also
ar ,
nm , objcopy , objdump ,
readelf and the Info entries for binutils
and ld.