1:ld
From Linux Man Pages
ld - The GNU linker
ld [options] objfile ...
Contents |
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 informa-
tion. 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 par-
ticular context. For instance, a frequent use of ld is to link standard Unix object files on a standard, sup-
ported 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 com-
mand 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 com-
mand line) of that option. Options which may be meaningfully specified more than once are noted in the descrip-
tions 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.
Note that specifying 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 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,--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:
@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 sur-
rounding 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.
-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 func-
tionally 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 com-
mand line. Even when ld is configured to support alternative object formats, you don't usually need to spec-
ify 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.
-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 hid-
den.
-E
--export-dynamic
When creating a dynamically linked executable, add all symbols to the dynamic symbol table. The dynamic sym-
bol 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 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.
-EB Link big-endian objects. This affects the default output 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
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 vari-
able. 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 sec-
tions. 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 exe-
cutable 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.
-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 com-
mand 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.
-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 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.
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.
-memulation
Emulate the emulation linker. You can list the available emulations with the --verbose or -V options.
If the -m option is not used, the emulation is taken from the "LDEMULATION" environment variable, if that is
defined.
Otherwise, the default emulation depends upon how the linker was configured.
-M
--print-map
Print a link map to the standard output. A link map provides information about the link, including the 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 circum-
stances 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.
-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 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 con-
form 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.
-q
--emit-relocs
Leave relocation sections and contents in fully linked executables. Post link analysis and optimization
tools may need this information in order to perform correct modifications of executables. This results in
larger executables.
This option is currently only supported on ELF platforms.
--force-dynamic
Force the output file to have dynamic sections. This option is specific to VxWorks targets.
-r
--relocatable
Generate relocatable output---i.e., generate an output file that can in turn serve as input to ld. This is
often called partial linking. As a side effect, in environments that support standard Unix magic numbers,
this option also sets the output file's magic number to "OMAGIC". If this option is not specified, an abso-
lute file is produced. When linking C++ programs, this option will not resolve references to constructors;
to do that, use -Ur.
When an input file does not have the same format as the output file, partial linking is only supported if
that input file does not contain any relocations. Different output formats can have further restrictions;
for example some "a.out"-based formats do not support partial linking with input files in other formats at
all.
This option does the same thing as -i.
-R filename
--just-symbols=filename
Read symbol names and their addresses from filename, but do not relocate it or include it in the output.
This allows your output file to refer symbolically to absolute locations of memory defined in other programs.
You may use this option more than once.
For compatibility with other ELF linkers, if the -R option is followed by a directory name, rather than a
file name, it is treated as the -rpath option.
-s
--strip-all
Omit all symbol information from the output file.
-S
--strip-debug
Omit debugger symbol information (but not all symbols) from the output file.
-t
--trace
Print the names of the input files as ld processes them.
-T scriptfile
--script=scriptfile
Use scriptfile as the linker script. This script replaces ld's default linker script (rather than adding to
it), so commandfile must specify everything necessary to describe the output file. 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.
-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 out-
put---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 SEC-
TION argument is missing, for every orphan input section. An orphan section is one not specifically men-
tioned in a linker script. You may use this option multiple times on the command line; It prevents the nor-
mal merging of input sections with the same name, overriding output section assignments in a linker script.
-v
--version
-V Display the version number for ld. The -V option also lists the supported emulations.
-x
--discard-all
Delete all local symbols.
-X
--discard-locals
Delete all temporary local symbols. (These symbols start with system-specific local label prefixes, typi-
cally .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.
dynsort
Sorts dynamic link sections, to reduce cache misses during linking.
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 final-
ization 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".
nodynsort
Disables dynamic link section sorting.
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 sym-
bols when the program is started, or when the shared library is linked to using dlopen, instead of defer-
ring 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. --as-needed causes DT_NEEDED tags to
only be emitted for libraries that satisfy some symbol reference 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 sup-
ported. 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. --unre-
solved-symbols=report-all is implied. This option is only meaningful on ELF platforms which support shared
libraries.
-Bstatic
-dn
-non_shared
-static
Do not link against shared libraries. This is only meaningful on platforms for which shared libraries are
supported. The different variants of this option are for compatibility with various systems. You may use
this option multiple times on the command line: it affects library searching for -l options which follow it.
This option also implies --unresolved-symbols=report-all. This option can be used with -shared. Doing so
means that a shared library is being created but that all of the library's external references must be
resolved by pulling in entries from static libraries.
-Bsymbolic
When creating a shared library, bind references to global symbols to the definition within the shared
library, if any. Normally, it is possible for a program linked against a shared library to override the def-
inition within the shared library. 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-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 over-
laps. 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.
--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_ALLO-
CATION" 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.
--dynamic-linker file
Set the name of the dynamic linker. This is only meaningful when generating dynamically linked ELF executa-
bles. The default dynamic linker is normally correct; don't use this unless you know what you are doing.
--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.
--gc-sections
--no-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 collec-
tion) can be restored by specifying --no-gc-sections on the command line.
--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 com-
mand line.
--help
Print a summary of the command-line options on the standard output and exit.
--target-help
Print a summary of all target specific options on the standard output and exit.
-Map mapfile
Print a link map to the file mapfile. See the description of the -M option, above.
--no-keep-memory
ld normally optimizes for speed over memory usage by caching the symbol tables of input files in memory.
This option tells ld to instead optimize for memory usage, by rereading the symbol tables as necessary. This
may be required if ld runs out of memory space while linking a large executable.
--no-undefined
-z defs
Report unresolved symbol references from regular object files. This is done even if the linker is creating a
non-symbolic shared library. The switch --[no-]allow-shlib-undefined controls the behaviour for reporting
unresolved references found in shared libraries being linked in.
--allow-multiple-definition
-z muldefs
Normally when a symbol is defined multiple times, the linker will report a fatal error. These options allow
multiple definitions and the first definition will be used.
--allow-shlib-undefined
--no-allow-shlib-undefined
Allows (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 resolv-
able 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 function 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 symbols.
--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-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 out-
put 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 con-
figured to support alternative object formats, you don't usually need to specify this, as ld should be con-
figured 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 inde-
pendent 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.
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 exe-
cutable impossible. This is known to be the case for the Matsushita MN10200 and MN10300 family of proces-
sors.
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 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 automati-
cally 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 run-
time 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, if the -rpath and "rpath-link" options were not used, search the contents of the envi-
ronment 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.
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 sym-
bols in the link.
--sort-common
This option tells ld to sort the common symbols by size when it places them in the appropriate output sec-
tions. First come all the one byte symbols, then all the two byte, then all the four byte, and then every-
thing 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 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 arbi-
trary 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 inte-
ger; 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.
--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 regu-
lar object files.
ignore-in-shared-libs
Report unresolved symbols that come from regular object files, but ignore them if they come from shared
libraries. This can be useful when creating a dynamic binary and it is known that all the shared
libraries that it should be referencing are included on the linker's command line.
The behaviour for shared libraries on their own can also be controlled by the --[no-]allow-shlib-undefined
option.
Normally the linker will generate an error message for each reported unresolved symbol but the option
--warn-unresolved-symbols can change this to a warning.
--dll-verbose
--verbose
Display the version number for ld and list the linker emulations supported. Display which input files can
and cannot be opened. Display the linker script being used by the linker.
--version-script=version-scriptfile
Specify the name of a version script to the linker. This is typically used when creating shared libraries to
specify additional information about the version hierarchy for the library being created. This option is
only meaningful on ELF platforms which support shared libraries.
--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 prac-
tise, 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 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 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 sec-
tion. 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 rela-
tive 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-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" func-
tion.
You may wish to provide a "__real_malloc" function as well, so that links without the --wrap option will suc-
ceed. If you do this, you should not put the definition of "__real_malloc" in the same file as "__wrap_mal-
loc"; if you do, the assembler may resolve the call before the linker has a chance to wrap it to "malloc".
--eh-frame-hdr
Request creation of ".eh_frame_hdr" section and ELF "PT_GNU_EH_FRAME" segment header.
--enable-new-dtags
--disable-new-dtags
This linker can create the new dynamic tags in ELF. But the older ELF systems may not understand them. If you
specify --enable-new-dtags, the dynamic tags will be created as needed. If you specify --disable-new-dtags,
no new dynamic tags will be created. By default, the new dynamic tags are 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.
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 generat-
ing 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 func-
tion "_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 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", "_cyg-
win_crt0_common@8", "_cygwin_noncygwin_dll_entry@12", "_fmode", "_impure_ptr", "cygwin_attach_dll", "cyg-
win_premain0", "cygwin_premain1", "cygwin_premain2", "cygwin_premain3", and "environ". [This option is spe-
cific 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]
--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 executa-
bles, 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 tar-
geted 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 tar-
geted 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 prefer-
ence to "lib<basename>.dll". This behaviour allows easy distinction between DLLs built for the various "sub-
platforms": native, cygwin, uwin, pw, etc. For instance, cygwin DLLs typically use "--dll-search-pre-
fix=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 con-
stants (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 condi-
tion. 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 fea-
ture.
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 vari-
able, 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 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",
"windows", "console", "posix", and "xbox". You may optionally set the subsystem version also. Numeric val-
ues 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 trampo-
line 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.
ENVIRONMENT
You can change the behaviour of ld with the environment variables "GNUTARGET", "LDEMULATION" and "COL-
LECT_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 configu-
ration 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 envi-
ronment, 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.
RELATED
ar(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, 2005, 2006 Free Software Foun-
dation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documenta-
tion License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sec-
tions, 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".
CATEGORY
Categories: Executable programs or shell commands | POSIX | Linux | UNIX | Solaris | AIX | IBM | I386