LD(1) GNU Development Tools LD(1)NAMEld - Using LD, the GNU linker
SYNOPSISld [options] objfile ...
DESCRIPTIONld 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 super-
set of AT&T's Link Editor Command Language syntax, to pro-
vide explicit and total control over the linking process.
This man page does not describe the command language; see
the ld entry in "info", or the manual ld: the GNU linker,
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 exam-
ple, 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, allow-
ing 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 situa-
tions, and to be as compatible as possible with other link-
ers. 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 stan-
dard 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.)
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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, rela-
tive 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 speci-
fied in this way augments the main linker script used for
the link (either the default linker script or the one speci-
fied 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. Note that speci-
fying a script in this way merely augments the main linker
script; use the -T option to replace the default linker
script entirely.
For options whose names are a single letter, option argu-
ments must either follow the option letter without interven-
ing whitespace, or be given as separate arguments immedi-
ately 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 pre-
ceeded 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 out-
put.
Arguments to multiple-letter options must either be
separated from the option name by an equals sign, or be
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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 com-
piler driver (e.g. gcc) then all the linker command line
options should be prefixed by -Wl, (or whatever is appropri-
ate for the particular compiler driver) like this:
gcc -Wl,--startgroup foo.o bar.o -Wl,--endgroup
This is important, because otherwise the compiler driver
program may silently drop the linker options, resulting in a
bad link.
Here is a table of the generic command line switches
accepted by the GNU linker:
-akeyword
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.
-Aarchitecture
--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.)
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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 vari-
able "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.
-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 avail-
able only for the i386 PE targeted port of the linker
and for ELF targeted ports. For i386 PE, symbols expli-
citly listed in a .def file are still exported, regard-
less of this option. For ELF targeted ports, symbols
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affected by this option will be treated as hidden.
-E
--export-dynamic
When creating a dynamically linked executable, add all
symbols to the dynamic symbol table. The dynamic symbol
table is the set of symbols which are visible from
dynamic objects at run time.
If you do not use this option, the dynamic symbol table
will normally contain only those symbols which are
referenced by some dynamic object mentioned in the link.
If you use "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 pro-
gram itself.
You can also use the version script to control what sym-
bols should be added to the dynamic symbol table if the
output format supports it. See the description of
--version-script in @ref{VERSION}.
-EB Link big-endian objects. This affects the default out-
put format.
-EL Link little-endian objects. This affects the default
output format.
-f
--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 sym-
bol 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
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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 sym-
bols provided by the object name.
Some older linkers used the -F option throughout a com-
pilation 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.
-Gvalue
--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.
-hname
-soname=name
When creating an ELF shared object, set the internal
DT_SONAME field to the specified name. When an execut-
able 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
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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.
-larchive
--library=archive
Add archive file archive to the list of files to link.
This option may be used any number of times. ld will
search its path-list for occurrences of "libarchive.a"
for every archive specified.
On systems which support shared libraries, ld may also
search for libraries with extensions other than ".a".
Specifically, on ELF and SunOS systems, ld will search a
directory for a library with an extension of ".so"
before searching for one with an extension of ".a". By
convention, a ".so" extension indicates a shared
library.
The linker will search an archive only once, at the
location where it is specified on the command line. If
the archive defines a symbol which was undefined in some
object which appeared before the archive on the command
line, the linker will include the appropriate file(s)
from the archive. However, an undefined symbol in an
object appearing later on the command line will not
cause the linker to search the archive again.
See the -( option for a way to force the linker to
search archives multiple times.
You may list the same archive multiple times on the com-
mand 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.
-Lsearchdir
--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 direc-
tories are searched in the order in which they are
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specified on the command line. Directories specified on
the command line are searched before the default direc-
tories. All -L options apply to all -l options, regard-
less of the order in which the options appear.
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 speci-
fied 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.
-memulation
Emulate the emulation linker. You can list the avail-
able 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 fol-
lowing:
* 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
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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 @ref{Expressions} for more information about
expressions in linker scripts.
-n
--nmagic
Turn off page alignment of sections, and mark the output
as "NMAGIC" if possible.
-N
--omagic
Set the text and data sections to be readable and writ-
able. 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 out-
put 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 optim-
izes the output. This might take significantly longer
and therefore probably should only be enabled for the
final binary.
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--emit-relocs
Leave relocation sections and contents in fully linked
exececutables. Post link analysis and optimization tools
may need this information in order to perform correct
modifications of executables. This results in larger
executables.
This option is currently only supported on ELF plat-
forms.
-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 exam-
ple 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 abso-
lute 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.
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--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. Mul-
tiple -T options accumulate.
-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 sec-
tion matching SECTION, or if the optional wildcard SEC-
TION argument is missing, for every orphan input sec-
tion. An orphan section is one not specifically men-
tioned in a linker script. You may use this option mul-
tiple 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.
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--discard-locals
Delete all temporary local symbols. For most targets,
this is all local symbols whose names begin with L.
-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 under-
score.
This option is useful when you have an undefined symbol
in your link but don't know where the reference is com-
ing 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. Unde-
fined symbols in shared libraries are still allowed.
initfirst
This option is only meaningful when building a
shared object. It marks the object so that its run-
time 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 run-
time finalization of any other objects.
interpose
Marks the object that its symbol table interposes
before all symbols but the primary executable.
loadfltr
Marks the object that its filters be processed
immediately at runtime.
muldefs
Allows multiple definitions.
nocombreloc
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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".
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.
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
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Tells the linker to accept input files whose architec-
ture 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 actu-
ally needed. --as-needed causes DT_NEEDED tags to only
be emitted for libraries that satisfy some symbol refer-
ence from regular objects which is undefined at the
point that the library was linked. --no-as-needed
restores the default behaviour.
--add-needed
--no-add-needed
This option affects the treatment of dynamic libraries
from ELF DT_NEEDED tags in dynamic libraries mentioned
on the command line after the --no-add-needed option.
Normally, the linker will add a DT_NEEDED tag for each
dynamic library from DT_NEEDED tags. --no-add-needed
causes DT_NEEDED tags will never be emitted for those
libraries from DT_NEEDED tags. --add-needed restores the
default behaviour.
-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 compati-
bility 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
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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.
-Bsymbolic
When creating a shared library, bind references to glo-
bal 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 meaning-
ful on ELF platforms which support shared libraries.
--check-sections
--no-check-sections
Asks the linker not to check section addresses after
they have been assigned to see if there any overlaps.
Normally the linker will perform this check, and if it
finds any overlaps it will produce suitable error mes-
sages. 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.
--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
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This option inhibits the assignment of addresses to com-
mon 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 elim-
inates 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 multi-
ple symbols in the command line. A limited form of
arithmetic is supported for the expression in this con-
text: you may give a hexadecimal constant or the name of
an existing symbol, or use "+" and "-" to add or sub-
tract 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. Dif-
ferent 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.
--dynamic-linker file
Set the name of the dynamic linker. This is only mean-
ingful when generating dynamically linked ELF execut-
ables. The default dynamic linker is normally correct;
don't use this unless you know what you are doing.
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LD(1) GNU Development Tools LD(1)--fatal-warnings
Treat all warnings as errors.
--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.
--no-gc-sections
--gc-sections
Enable garbage collection of unused input sections. It
is ignored on targets that do not support this option.
This option is not compatible with -r. The default
behaviour (of not performing this garbage collection)
can be restored by specifying --no-gc-sections on the
command line.
--help
Print a summary of the command-line options on the stan-
dard 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 descrip-
tion 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.
binutils-050707 2011-08-18 17
LD(1) GNU Development Tools LD(1)--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 (the default) 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 han-
dled.
The reason that --allow-shlib-undefined is the default
is that the shared library being specified at link time
may not be the same as the one that is available at load
time, so the symbols might actually be resolvable at
load time. Plus there are some systems, (eg BeOS) where
undefined symbols in shared libraries is normal. (The
kernel patches them at load time to select which func-
tion is most appropriate for the current architecture.
This is used for example to dynamically select an
appropriate memset function). Apparently it is also
normal for HPPA shared libraries to have undefined sym-
bols.
--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 rea-
son, perhaps because they have been compiled for dif-
ferent processors or for different endiannesses. This
option tells ld that it should silently permit such pos-
sible errors. This option should only be used with
care, in cases when you have taken some special action
binutils-050707 2011-08-18 18
LD(1) GNU Development Tools LD(1)
that ensures that the linker errors are inappropriate.
--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 pro-
duce 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
An option with machine dependent effects. This option is
only supported on a few targets.
binutils-050707 2011-08-18 19
LD(1) GNU Development Tools LD(1)
On some platforms, the --relax option performs global
optimizations that become possible when the linker
resolves addressing in the program, such as relaxing
address modes and synthesizing new instructions in the
output object file.
On some platforms these link time global optimizations
may make symbolic debugging of the resulting executable
impossible. 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.
--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 espe-
cially 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 sym-
bols, 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
filesystems.
For compatibility with other ELF linkers, if the -R
option is followed by a directory name, rather than a
binutils-050707 2011-08-18 20
LD(1) GNU Development Tools LD(1)
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 automati-
cally try to locate the required shared library and
include it in the link, if it is not included expli-
citly. 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.
It is for the native linker only.
3. On an ELF system, if the -rpath and "rpath-link"
options were not used, search the contents of the
environment variable "LD_RUN_PATH". It is for the
native linker only.
4. On SunOS, if the -rpath option was not used, search
any directories specified using -L options.
5. For a native linker, 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.
binutils-050707 2011-08-18 21
LD(1) GNU Development Tools LD(1)
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 sup-
ported 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
This option tells ld to sort the common symbols by size
when it places them in the appropriate output sections.
First come all the one byte symbols, then all the two
byte, then all the four byte, and then everything else.
This is to prevent gaps between symbols due to alignment
constraints.
--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 wild-
card 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 sec-
tions. The linker will not split up individual input
sections for redistribution, so if a single input sec-
tion contains more than count relocations one output
section will contain that many relocations. count
defaults to a value of 32768.
binutils-050707 2011-08-18 22
LD(1) GNU Development Tools LD(1)--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 sup-
ported 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 section-
name, the equals sign (``=''), and org.
-Tbss org
-Tdata org
-Ttext org
Same as --section-start, with ".bss", ".data" or ".text"
as the sectionname.
--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
binutils-050707 2011-08-18 23
LD(1) GNU Development Tools LD(1)
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 hierar-
chy for the library being created. This option is only
meaningful on ELF platforms which support shared
libraries.
--warn-common
Warn when a common symbol is combined with another com-
mon 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 sym-
bols. 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;
binutils-050707 2011-08-18 24
LD(1) GNU Development Tools LD(1)
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 unini-
tialized 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 warn-
ings. 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 com-
mon 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
binutils-050707 2011-08-18 25
LD(1) GNU Development Tools LD(1)
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 pro-
cessors 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 lim-
its 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-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 warn-
ing instead.
--error-unresolved-symbols
binutils-050707 2011-08-18 26
LD(1) GNU Development Tools LD(1)
This restores the linker's default behaviour of generat-
ing 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 refer-
ence 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 func-
tion. The wrapper function should be called
"__wrap_symbol". If it wishes to call the system func-
tion, 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 mal-
loc, 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 defini-
tion 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".
binutils-050707 2011-08-18 27
LD(1) GNU Development Tools LD(1)--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.
--reduce-memory-overheads
This option reduces memory requirements at ld runtime,
at the expense of linking speed. This was introduced to
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 affect 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.
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
binutils-050707 2011-08-18 28
LD(1) GNU Development Tools LD(1)
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-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]
--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 attri-
butes, 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-
binutils-050707 2011-08-18 29
LD(1) GNU Development Tools LD(1)
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 automat-
ically exported. The symbol names may be delimited by
commas or colons. [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 amount of memory to reserve (and optionally
commit) to be used as heap for this program. The
default is 1Mb reserved, 4K committed. [This option is
specific to the i386 PE targeted port of the linker]
--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 ``Charateristics''
field of the COFF header is set to indicate that this
executable supports virtual addresses greater than 2
gigabytes. This should be used in conjuction with the
/3GB or /USERVA=value megabytes switch in the ``[operat-
ing systems]'' section of the BOOT.INI. Otherwise, this
bit has no effect. [This option is specific to PE
binutils-050707 2011-08-18 30
LD(1) GNU Development Tools LD(1)
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 gen-
erating. 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 impli-
citly 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
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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 relo-
cations 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 prefer-
ence to "lib<basename>.dll". This behaviour allows easy
distinction between DLLs built for the various "subplat-
forms": 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.
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.
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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:
binutils-050707 2011-08-18 33
LD(1) GNU Development Tools LD(1)--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
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LD(1) GNU Development Tools LD(1)
DLL with non-zero offset, this switch will create a vec-
tor 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 tar-
geted 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 amount of memory to reserve (and optionally
commit) to be used as stack for this program. The
default is 2Mb reserved, 4K committed. [This option is
specific to the i386 PE targeted port of the linker]
--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", "win-
dows", "console", "posix", and "xbox". You may option-
ally 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 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
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LD(1) GNU Development Tools LD(1)
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.
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 "LDEMULA-
TION" environment variable is not defined, the default emu-
lation 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 ALSOar(1), nm(1), objcopy(1), objdump(1), readelf(1) and the
Info entries for binutils and ld.
COPYRIGHT
Copyright (c) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000,
2001, 2002, 2003, 2004 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation
License, Version 1.1 or any later version published by the
Free Software Foundation; with no Invariant Sections, with
no Front-Cover Texts, and with no Back-Cover Texts. A copy
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LD(1) GNU Development Tools LD(1)
of the license is included in the section entitled ``GNU
Free Documentation License''.
binutils-050707 2011-08-18 37
