ld
and MMIX
ld
and MSP430
ld
and Xtensa Processors
This file documents the gnu linker ld (GNU Binutils) version 2.38.50.
This document is distributed under the terms of the GNU Free Documentation License version 1.3. A copy of the license is included in the section entitled “GNU Free Documentation License”.
ld combines a number of object and archive files, relocates their data and ties up symbol references. Usually the last step in compiling a program is to run ld.
ld accepts Linker Command Language files written in a superset of AT&T's Link Editor Command Language syntax, to provide explicit and total control over the linking process.
This version of ld uses the general purpose BFD libraries
to operate on object files. This allows ld to read, combine, and
write object files in many different formats—for example, COFF or
a.out
. Different formats may be linked together to produce any
available kind of object file. See BFD, for more information.
Aside from its flexibility, the gnu linker is more helpful than other linkers in providing diagnostic information. Many linkers abandon execution immediately upon encountering an error; whenever possible, ld continues executing, allowing you to identify other errors (or, in some cases, to get an output file in spite of the error).
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.
The linker supports a plethora of command-line options, but in actual
practice few of them are used in any particular context.
For instance, a frequent use of ld is to link standard Unix
object files on a standard, supported Unix system. On such a system, to
link a file hello.o
:
ld -o output /lib/crt0.o hello.o -lc
This tells ld to produce a file called output as the
result of linking the file /lib/crt0.o
with hello.o
and
the library libc.a
, which will come from the standard search
directories. (See the discussion of the ‘-l’ option below.)
Some of the command-line options to ld may be specified at any point in the command line. However, options which refer to files, such as ‘-l’ or ‘-T’, cause the file to be read at the point at which the option appears in the command line, relative to the object files and other file options. Repeating non-file options with a different argument will either have no further effect, or override prior occurrences (those further to the left on the command line) of that option. Options which may be meaningfully specified more than once are noted in the descriptions below.
Non-option arguments are object files or archives which are to be linked together. They may follow, precede, or be mixed in with command-line options, except that an object file argument may not be placed between an option and its argument.
Usually the linker is invoked with at least one object file, but you can specify other forms of binary input files using ‘-l’, ‘-R’, and the script command language. If no binary input files at all are specified, the linker does not produce any output, and issues the message ‘No input files’.
If the linker cannot recognize the format of an object file, it will
assume that it is a linker script. A script specified in this way
augments the main linker script used for the link (either the default
linker script or the one specified by using ‘-T’). This feature
permits the linker to link against a file which appears to be an object
or an archive, but actually merely defines some symbol values, or uses
INPUT
or GROUP
to load other objects. Specifying a
script in this way merely augments the main linker script, with the
extra commands placed after the main script; use the ‘-T’ option
to replace the default linker script entirely, but note the effect of
the INSERT
command. See Scripts.
For options whose names are a single letter, option arguments must either follow the option letter without intervening whitespace, or be given as separate arguments immediately following the option that requires them.
For options whose names are multiple letters, either one dash or two can precede the option name; for example, ‘-trace-symbol’ and ‘--trace-symbol’ are equivalent. Note—there is one exception to this rule. Multiple letter options that start with a lower case 'o' can only be preceded by two dashes. This is to reduce confusion with the ‘-o’ option. So for example ‘-omagic’ sets the output file name to ‘magic’ whereas ‘--omagic’ sets the NMAGIC flag on the output.
Arguments to multiple-letter options must either be separated from the option name by an equals sign, or be given as separate arguments immediately following the option that requires them. For example, ‘--trace-symbol foo’ and ‘--trace-symbol=foo’ are equivalent. Unique abbreviations of the names of multiple-letter options are accepted.
Note—if the linker is being invoked indirectly, via a compiler driver (e.g. ‘gcc’) then all the linker command-line options should be prefixed by ‘-Wl,’ (or whatever is appropriate for the particular compiler driver) like this:
gcc -Wl,--start-group foo.o bar.o -Wl,--end-group
This is important, because otherwise the compiler driver program may silently drop the linker options, resulting in a bad link. Confusion may also arise when passing options that require values through a driver, as the use of a space between option and argument acts as a separator, and causes the driver to pass only the option to the linker and the argument to the compiler. In this case, it is simplest to use the joined forms of both single- and multiple-letter options, such as:
gcc foo.o bar.o -Wl,-eENTRY -Wl,-Map=a.map
Here is a table of the generic command-line switches accepted by the GNU linker:
@
fileOptions in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.
-a
keyword--audit
AUDITLIBDT_AUDIT
entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the DT_SONAME
specified in the library. If specified multiple times DT_AUDIT
will contain a colon separated list of audit interfaces to use. If the linker
finds an object with an audit entry while searching for shared libraries,
it will add a corresponding DT_DEPAUDIT
entry in the output file.
This option is only meaningful on ELF platforms supporting the rtld-audit
interface.
-b
input-format--format=
input-formatYou 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
.
See Environment.
You can also define the input format from a script, using the command
TARGET
;
see Format Commands.
-c
MRI-commandfile--mri-script=
MRI-commandfile-d
-dc
-dp
FORCE_COMMON_ALLOCATION
has the same effect.
See Miscellaneous Commands.
--depaudit
AUDITLIB-P
AUDITLIBDT_DEPAUDIT
entry of the dynamic section.
AUDITLIB is not checked for existence, nor will it use the DT_SONAME
specified in the library. If specified multiple times DT_DEPAUDIT
will contain a colon separated list of audit interfaces to use. This
option is only meaningful on ELF platforms supporting the rtld-audit interface.
The -P option is provided for Solaris compatibility.
--enable-non-contiguous-regions
MEMORY { MEM1 (rwx) : ORIGIN : 0x1000, LENGTH = 0x14 MEM2 (rwx) : ORIGIN : 0x1000, LENGTH = 0x40 MEM3 (rwx) : ORIGIN : 0x2000, LENGTH = 0x40 } SECTIONS { mem1 : { *(.data.*); } > MEM1 mem2 : { *(.data.*); } > MEM2 mem3 : { *(.data.*); } > MEM2 } with input sections: .data.1: size 8 .data.2: size 0x10 .data.3: size 4 results in .data.1 affected to mem1, and .data.2 and .data.3 affected to mem2, even though .data.3 would fit in mem3.
This option is incompatible with INSERT statements because it changes the way input sections are mapped to output sections.
--enable-non-contiguous-regions-warnings
--enable-non-contiguous-regions
allows possibly unexpected
matches in sections mapping, potentially leading to silently
discarding a section instead of failing because it does not fit any
output region.
-e
entry--entry=
entry--exclude-libs
lib,
lib,...
--exclude-libs ALL
excludes symbols in all archive libraries from
automatic export. This option is available only for the i386 PE targeted
port of the linker and for ELF targeted ports. For i386 PE, symbols
explicitly listed in a .def file are still exported, regardless of this
option. For ELF targeted ports, symbols affected by this option will
be treated as hidden.
--exclude-modules-for-implib
module,
module,...
-E
--export-dynamic
--no-export-dynamic
If you do not use either of these options (or use the --no-export-dynamic option to restore the default behavior), the dynamic symbol table will normally contain only those symbols which are referenced by some dynamic object mentioned in the link.
If you use dlopen
to load a dynamic object which needs to refer
back to the symbols defined by the program, rather than some other
dynamic object, then you will probably need to use this option when
linking the program itself.
You can also use the dynamic list to control what symbols should be added to the dynamic symbol table if the output format supports it. See the description of ‘--dynamic-list’.
Note that this option is specific to ELF targeted ports. PE targets support a similar function to export all symbols from a DLL or EXE; see the description of ‘--export-all-symbols’ below.
--export-dynamic-symbol=
glob--export-dynamic-symbol-list=
file-EB
-EL
-f
name--auxiliary=
nameIf 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=
nameIf you later link a program against this filter object, then, when you run the program, the dynamic linker will see the DT_FILTER field. The dynamic linker will resolve symbols according to the symbol table of the filter object as usual, but it will actually link to the definitions found in the shared object name. Thus the filter object can be used to select a subset of the symbols provided by the object name.
Some older linkers used the -F option throughout a compilation
toolchain for specifying object-file format for both input and output
object files.
The gnu linker uses other mechanisms for this purpose: the
-b, --format, --oformat options, the
TARGET
command in linker scripts, and the GNUTARGET
environment variable.
The gnu linker will ignore the -F option when not
creating an ELF shared object.
-fini=
name_fini
as
the function to call.
-g
-G
value--gpsize=
value-h
name-soname=
name-i
-init=
name_init
as the
function to call.
-l
namespec--library=
namespecOn systems which support shared libraries, ld may also search for
files other than libnamespec.a. Specifically, on ELF
and SunOS systems, ld will search a directory for a library
called libnamespec.so before searching for one called
libnamespec.a. (By convention, a .so
extension
indicates a shared library.) Note that this behavior does not apply
to :filename, which always specifies a file called
filename.
The linker will search an archive only once, at the location where it is specified on the command line. If the archive defines a symbol which was undefined in some object which appeared before the archive on the command line, the linker will include the appropriate file(s) from the archive. However, an undefined symbol in an object appearing later on the command line will not cause the linker to search the archive again.
See the -( option for a way to force the linker to search archives multiple times.
You may list the same archive multiple times on the command line.
This type of archive searching is standard for Unix linkers. However, if you are using ld on AIX, note that it is different from the behaviour of the AIX linker.
-L
searchdir--library-path=
searchdirIf searchdir begins with =
or $SYSROOT
, then this
prefix will be replaced by the sysroot prefix, controlled by the
‘--sysroot’ option, or 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. See Environment.
The paths can also be specified in a link script with the
SEARCH_DIR
command. Directories specified this way are searched
at the point in which the linker script appears in the command line.
-m
emulationIf 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
Note - symbols whose values are computed by an expression which involves a reference to a previous value of the same symbol may not have correct result displayed in the link map. This is because the linker discards intermediate results and only retains the final value of an expression. Under such circumstances the linker will display the final value enclosed by square brackets. Thus for example a linker script containing:
foo = 1 foo = foo * 4 foo = foo + 8
will produce the following output in the link map if the -M option is used:
0x00000001 foo = 0x1 [0x0000000c] foo = (foo * 0x4) [0x0000000c] foo = (foo + 0x8)
See Expressions for more information about expressions in linker scripts.
When the linker merges input .note.gnu.property sections into one output .note.gnu.property section, some properties are removed or updated. These actions are reported in the link map. For example:
Removed property 0xc0000002 to merge foo.o (0x1) and bar.o (not found)
This indicates that property 0xc0000002 is removed from output when merging properties in foo.o, whose property 0xc0000002 value is 0x1, and bar.o, which doesn't have property 0xc0000002.
Updated property 0xc0010001 (0x1) to merge foo.o (0x1) and bar.o (0x1)
This indicates that property 0xc0010001 value is updated to 0x1 in output when merging properties in foo.o, whose 0xc0010001 property value is 0x1, and bar.o, whose 0xc0010001 property value is 0x1.
--print-map-discarded
--no-print-map-discarded
-n
--nmagic
NMAGIC
.
-N
--omagic
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
-o
output--output=
outputOUTPUT
can also specify the output file name.
--dependency-file=
depfilemake
describing the output file and all the input files
that were read to produce it. The output is similar to the compiler's
output with ‘-M -MP’ (see Options Controlling the Preprocessor). Note that there is no option like the compiler's ‘-MM’,
to exclude “system files” (which is not a well-specified concept in the
linker, unlike “system headers” in the compiler). So the output from
‘--dependency-file’ is always specific to the exact state of the
installation where it was produced, and should not be copied into
distributed makefiles without careful editing.
-O
level-plugin
nameNote that the location of the compiler originated plugins is different from the place where the ar, nm and ranlib programs search for their plugins. In order for those commands to make use of a compiler based plugin it must first be copied into the ${libdir}/bfd-plugins directory. All gcc based linker plugins are backward compatible, so it is sufficient to just copy in the newest one.
--push-state
The option which are covered are: -Bdynamic, -Bstatic, -dn, -dy, -call_shared, -non_shared, -static, -N, -n, --whole-archive, --no-whole-archive, -r, -Ur, --copy-dt-needed-entries, --no-copy-dt-needed-entries, --as-needed, --no-as-needed, and -a.
One target for this option are specifications for pkg-config. When used with the --libs option all possibly needed libraries are listed and then possibly linked with all the time. It is better to return something as follows:
-Wl,--push-state,--as-needed -libone -libtwo -Wl,--pop-state
--pop-state
-q
--emit-relocs
This option is currently only supported on ELF platforms.
--force-dynamic
-r
--relocatable
OMAGIC
.
If this option is not specified, an absolute file is produced. When
linking C++ programs, this option will not resolve references to
constructors; to do that, use ‘-Ur’.
When an input file does not have the same format as the output file,
partial linking is only supported if that input file does not contain any
relocations. Different output formats can have further restrictions; for
example some a.out
-based formats do not support partial linking
with input files in other formats at all.
This option does the same thing as ‘-i’.
-R
filename--just-symbols=
filenameFor 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
-S
--strip-debug
--strip-discarded
--no-strip-discarded
-t
--trace
-T
scriptfile--script=
scriptfileld
looks for it in the directories
specified by any preceding ‘-L’ options. Multiple ‘-T’
options accumulate.
-dT
scriptfile--default-script=
scriptfileThis option is similar to the --script option except that processing of the script is delayed until after the rest of the command line has been processed. This allows options placed after the --default-script option on the command line to affect the behaviour of the linker script, which can be important when the linker command line cannot be directly controlled by the user. (eg because the command line is being constructed by another tool, such as ‘gcc’).
-u
symbol--undefined=
symbolEXTERN
linker script command.
If this option is being used to force additional modules to be pulled into the link, and if it is an error for the symbol to remain undefined, then the option --require-defined should be used instead.
--require-defined=
symbolEXTERN
, ASSERT
and DEFINED
together. This option
can be used multiple times to require additional symbols.
-Ur
--orphan-handling=
MODEMODE can have any of the following values:
place
discard
warn
place
and also
issue a warning.
error
The default if ‘--orphan-handling’ is not given is place
.
--unique[=
SECTION]
-v
--version
-V
-x
--discard-all
-X
--discard-locals
-y
symbol--trace-symbol=
symbolThis option is useful when you have an undefined symbol in your link but don't know where the reference is coming from.
-Y
path-z
keywordNOP
padding when transforming indirect call
to a locally defined function, foo, via its GOT slot.
call-nop=prefix-addr generates 0x67 call foo
.
call-nop=suffix-nop generates call foo 0x90
.
call-nop=prefix-byte generates byte call foo
.
call-nop=suffix-byte generates call foo
byte.
Supported for i386 and x86_64.
DF_1_GLOBAUDIT
bit in the DT_FLAGS_1
dynamic
tag. Global auditing requires that any auditing library defined via
the --depaudit or -P command-line options be run for
all dynamic objects loaded by the application.
noindirect-extern-access removes
GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS from .note.gnu.property
section.
dlmopen
). This is
primarily used to mark fundamental libraries such as libc, libpthread et
al which do not usually function correctly unless they are the sole instances
of themselves. This behaviour can be overridden by the dlmopen
caller
and does not apply to certain loading mechanisms (such as audit libraries).
dlopen
.
dldump
.
DT_RELR
, DT_RELRSZ
and
DT_RELRENT
entries to the dynamic section. It is ignored when
building position-dependent executable and relocatable output.
nopack-relative-relocs is the default, which disables compact
relative relocation. When linked against the GNU C Library, a
GLIBC_ABI_DT_RELR symbol version dependency on the shared C Library is
added to the output. Supported for i386 and x86-64.
PT_GNU_RELRO
segment header in the object. This
specifies a memory segment that should be made read-only after
relocation, if supported. Specifying ‘common-page-size’ smaller
than the system page size will render this protection ineffective.
Don't create an ELF PT_GNU_RELRO
segment if ‘norelro’.
PT_LOAD
segment header in the object. This
specifies a memory segment that should contain only instructions and must
be in wholly disjoint pages from any other data. Don't create separate
code PT_LOAD
segment if ‘noseparate-code’ is used.
PT_GNU_STACK
segment.
Specifying zero will override any default non-zero sized
PT_GNU_STACK
segment creation.
__start_SECNAME
or __stop_SECNAME
causes all
input sections named SECNAME
to also be retained, if
SECNAME
is representable as a C identifier and either
__start_SECNAME
or __stop_SECNAME
is synthesized by the
linker. ‘-z start-stop-gc’ disables this effect, allowing
sections to be garbage collected as if the special synthesized symbols
were not defined. ‘-z start-stop-gc’ has no effect on a
definition of __start_SECNAME
or __stop_SECNAME
in an
object file or linker script. Such a definition will prevent the
linker providing a synthesized __start_SECNAME
or
__stop_SECNAME
respectively, and therefore the special
treatment by garbage collection for those references.
__start_SECNAME
and __stop_SECNAME
symbols (see Input Section Example). value must be exactly ‘default’,
‘internal’, ‘hidden’, or ‘protected’. If no ‘-z
start-stop-visibility’ option is given, ‘protected’ is used for
compatibility with historical practice. However, it's highly
recommended to use ‘-z start-stop-visibility=hidden’ in new
programs and shared libraries so that these symbols are not exported
between shared objects, which is not usually what's intended.
number
" to duplicated local symbol names if ‘unique-symbol’
is used. nounique-symbol is the default.
GNU_PROPERTY_X86_ISA_1_BASELINE
.
x86-64-v2 generates GNU_PROPERTY_X86_ISA_1_V2
.
x86-64-v3 generates GNU_PROPERTY_X86_ISA_1_V3
.
x86-64-v4 generates GNU_PROPERTY_X86_ISA_1_V4
.
Supported for Linux/i386 and Linux/x86_64.
Other keywords are ignored for Solaris compatibility.
-(
archives -)
--start-group
archives --end-group
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 will 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
--as-needed
--no-as-needed
Note: On Linux based systems the --as-needed option also has an affect on the behaviour of the --rpath and --rpath-link options. See the description of --rpath-link for more details.
--add-needed
--no-add-needed
-assert
keyword-Bdynamic
-dy
-call_shared
-Bgroup
DF_1_GROUP
flag in the DT_FLAGS_1
entry in the dynamic
section. This causes the runtime linker to handle lookups in this
object and its dependencies to be performed only inside the group.
--unresolved-symbols=report-all is implied. This option is
only meaningful on ELF platforms which support shared libraries.
-Bstatic
-dn
-non_shared
-static
-Bsymbolic
-Bsymbolic-functions
-Bno-symbolic
--dynamic-list=
dynamic-list-fileThe format of the dynamic list is the same as the version node without scope and node name. See VERSION for more information.
--dynamic-list-data
--dynamic-list-cpp-new
--dynamic-list-cpp-typeinfo
--check-sections
--no-check-sections
--copy-dt-needed-entries
--no-copy-dt-needed-entries
This option also has an effect on the resolution of symbols in dynamic libraries. With --copy-dt-needed-entries dynamic libraries mentioned on the command line will be recursively searched, following their DT_NEEDED tags to other libraries, in order to resolve symbols required by the output binary. With the default setting however the searching of dynamic libraries that follow it will stop with the dynamic library itself. No DT_NEEDED links will be traversed to resolve symbols.
--cref
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. If the symbol is defined as a common value then any files where this happens appear next. Finally any files that reference the symbol are listed.
--ctf-variables
--no-ctf-variables
--ctf-share-types=
method--no-define-common
INHIBIT_COMMON_ALLOCATION
has the same effect.
See Miscellaneous Commands.
The ‘--no-define-common’ option allows decoupling the decision to assign addresses to Common symbols from the choice of the output file type; otherwise a non-Relocatable output type forces assigning addresses to Common symbols. Using ‘--no-define-common’ allows Common symbols that are referenced from a shared library to be assigned addresses only in the main program. This eliminates the unused duplicate space in the shared library, and also prevents any possible confusion over resolving to the wrong duplicate when there are many dynamic modules with specialized search paths for runtime symbol resolution.
--force-group-allocation
FORCE_GROUP_ALLOCATION
has the same
effect. See Miscellaneous Commands.
--defsym=
symbol=
expression+
and -
to add or subtract hexadecimal
constants or symbols. If you need more elaborate expressions, consider
using the linker command language from a script (see Assignments).
Note: there should be no white space between symbol, the
equals sign (“<=>”), and expression.
The linker processes ‘--defsym’ arguments and ‘-T’ arguments in order, placing ‘--defsym’ before ‘-T’ will define the symbol before the linker script from ‘-T’ is processed, while placing ‘--defsym’ after ‘-T’ will define the symbol after the linker script has been processed. This difference has consequences for expressions within the linker script that use the ‘--defsym’ symbols, which order is correct will depend on what you are trying to achieve.
--demangle[=
style]
--no-demangle
-I
file--dynamic-linker=
file--no-dynamic-linker
--embedded-relocs
--disable-multiple-abs-defs
--fatal-warnings
--no-fatal-warnings
--force-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
‘--gc-sections’ decides which input sections are used by examining symbols and relocations. The section containing the entry symbol and all sections containing symbols undefined on the command-line will be kept, as will sections containing symbols referenced by dynamic objects. Note that when building shared libraries, the linker must assume that any visible symbol is referenced. Once this initial set of sections has been determined, the linker recursively marks as used any section referenced by their relocations. See ‘--entry’, ‘--undefined’, and ‘--gc-keep-exported’.
This option can be set when doing a partial link (enabled with option ‘-r’). In this case the root of symbols kept must be explicitly specified either by one of the options ‘--entry’, ‘--undefined’ or ‘--gc-keep-exported’.
As a GNU extension, ELF input sections marked with the
SHF_GNU_RETAIN
flag will not be garbage collected.
--print-gc-sections
--no-print-gc-sections
--gc-keep-exported
--print-output-format
OUTPUT_FORMAT
linker script command (see File Commands).
--print-memory-usage
Memory region Used Size Region Size %age Used ROM: 256 KB 1 MB 25.00% RAM: 32 B 2 GB 0.00%
--help
--target-help
-Map=
mapfile-
then the map will be written to stdout.
Specifying a directory as mapfile causes the linker map to be
written as a file inside the directory. Normally name of the file
inside the directory is computed as the basename of the output
file with .map
appended. If however the special character
%
is used then this will be replaced by the full path of the
output file. Additionally if there are any characters after the
% symbol then .map
will no longer be appended.
-o foo.exe -Map=bar [Creates ./bar] -o ../dir/foo.exe -Map=bar [Creates ./bar] -o foo.exe -Map=../dir [Creates ../dir/foo.exe.map] -o ../dir2/foo.exe -Map=../dir [Creates ../dir/foo.exe.map] -o foo.exe -Map=% [Creates ./foo.exe.map] -o ../dir/foo.exe -Map=% [Creates ../dir/foo.exe.map] -o foo.exe -Map=%.bar [Creates ./foo.exe.bar] -o ../dir/foo.exe -Map=%.bar [Creates ../dir/foo.exe.bar] -o ../dir2/foo.exe -Map=../dir/% [Creates ../dir/../dir2/foo.exe.map] -o ../dir2/foo.exe -Map=../dir/%.bar [Creates ../dir/../dir2/foo.exe.bar]
It is an error to specify more than one %
character.
If the map file already exists then it will be overwritten by this operation.
--no-keep-memory
--no-undefined
-z defs
The effects of this option can be reverted by using -z undefs
.
--allow-multiple-definition
-z muldefs
--allow-shlib-undefined
--no-allow-shlib-undefined
The default behaviour is to report errors for any undefined symbols referenced in shared libraries if the linker is being used to create an executable, but to allow them if the linker is being used to create a shared library.
The reasons for allowing undefined symbol references in shared libraries specified at link time are that:
The BeOS kernel for example patches shared libraries at load time to select whichever function is most appropriate for the current architecture. This is used, for example, to dynamically select an appropriate memset function.
--error-handling-script=
scriptnameThe availability of this option is controlled by a configure time switch, so it may not be present in specific implementations.
--no-undefined-version
--default-symver
--default-imported-symver
--no-warn-mismatch
--no-warn-search-mismatch
--no-whole-archive
--noinhibit-exec
-nostdlib
--oformat=
output-formatOUTPUT_FORMAT
can also specify the output format, but
this option overrides it. See BFD.
--out-implib
file*.dll.a
or *.a
for DLLs)
may be used to link clients against the generated executable; this
behaviour makes it possible to skip a separate import library creation
step (eg. dlltool
for DLLs). This option is only available for
the i386 PE and ELF targetted ports of the linker.
-pie
--pic-executable
-no-pie
-qmagic
-Qy
--relax
--no-relax
On some platforms the --relax option performs target specific, global optimizations that become possible when the linker resolves addressing in the program, such as relaxing address modes, synthesizing new instructions, selecting shorter version of current instructions, and combining constant values.
On some platforms these link time global optimizations may make symbolic debugging of the resulting executable impossible. This is known to be the case for the Matsushita MN10200 and MN10300 family of processors.
On platforms where the feature is supported, the option --no-relax will disable it.
On platforms where the feature is not supported, both --relax and --no-relax are accepted, but ignored.
--retain-symbols-file=
filename‘--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=
dirThe -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. Searching -rpath in this way is only supported by native linkers and cross linkers which have been configured with the --with-sysroot 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 path 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=
dirld -shared
link includes a shared library as one
of the input files.
When the linker encounters such a dependency when doing a non-shared, non-relocatable link, it will automatically try to locate the required shared library and include it in the link, if it is not included explicitly. In such a case, the -rpath-link option specifies the first set of directories to search. The -rpath-link option may specify a sequence of directory names either by specifying a list of names separated by colons, or by appearing multiple times.
The tokens $ORIGIN and $LIB can appear in these search directories. They will be replaced by the full path to the directory containing the program or shared object in the case of $ORIGIN and either ‘lib’ - for 32-bit binaries - or ‘lib64’ - for 64-bit binaries - in the case of $LIB.
The alternative form of these tokens - ${ORIGIN} and ${LIB} can also be used. The token $PLATFORM is not supported.
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:
LD_RUN_PATH
.
LD_LIBRARY_PATH
.
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.
sysroot
value, if that is
defined, and then any prefix
string if the linker was
configured with the --prefix=<path> option.
_PATH_ELF_HINTS
macro defined in the elf-hints.h
header file.
SEARCH_DIR
command in a
linker script given on the command line, including scripts specified
by -T (but not -dT).
SEARCH_DIR
command in a default
linker script.
Note however on Linux based systems there is an additional caveat: If the --as-needed option is active and a shared library is located which would normally satisfy the search and this library does not have DT_NEEDED tag for libc.so and there is a shared library later on in the set of search directories which also satisfies the search and this second shared library does have a DT_NEEDED tag for libc.so then the second library will be selected instead of the first.
If the required shared library is not found, the linker will issue a warning and continue with the link.
-shared
-Bshareable
--sort-common
--sort-common=ascending
--sort-common=descending
--sort-section=name
SORT_BY_NAME
to all wildcard section
patterns in the linker script.
--sort-section=alignment
SORT_BY_ALIGNMENT
to all wildcard section
patterns in the linker script.
--spare-dynamic-tags=
count--split-by-file[=
size]
--split-by-reloc[=
count]
--stats
--sysroot=
directory--task-link
--traditional-format
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-Tbss=
org-Tdata=
org-Ttext=
org.bss
, .data
or
.text
as the sectionname.
-Ttext-segment=
org-Trodata-segment=
org-Tldata-segment=
org--unresolved-symbols=
methodThe behaviour for shared libraries on their own can also be controlled by the --[no-]allow-shlib-undefined option.
Normally the linker will generate an error message for each reported unresolved symbol but the option --warn-unresolved-symbols can change this to a warning.
--dll-verbose
--verbose[=
NUMBER]
--version-script=
version-scriptfile--warn-common
There are three kinds of global symbols, illustrated here by C examples:
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.
file(section): warning: common of `symbol' overridden by definition file(section): warning: defined here
file(section): warning: definition of `symbol' overriding common file(section): warning: common is here
file(section): warning: multiple common of `symbol' file(section): warning: previous common is here
file(section): warning: common of `symbol' overridden by larger common file(section): warning: larger common is here
file(section): warning: common of `symbol' overriding smaller common file(section): warning: smaller common is here
--warn-constructors
--warn-execstack
--no-warn-execstack
On the other hand the linker will normally warn if the stack is made executable because one or more of the input files need an execuable stack and neither of the -z execstack or -z noexecstack comman line options have been specified. This warning can be disabled via the --no-warn-execstack option.
Note: ELF format input files specify that they need an executable stack by having a .note.GNU-stack section with the executable bit set in its section flags. They can specify that they do not need an executable stack by having that section, but without the executable flag bit set. If an input file does not have a .note.GNU-stack section present then the default behaviour is target specific. For some targets, then absence of such a section implies that an executable stack is required. This is often a problem for hand crafted assembler files.
--warn-multiple-gp
--warn-once
--warn-rwx-segments
--no-warn-rwx-segments
These warnings are enabled by default. They can be disabled via the --no-warn-rwx-segments option and re-enabled via the --warn-rwx-segments option.
--warn-section-align
SECTIONS
command does not specify a start address for
the section (see SECTIONS).
--warn-textrel
--warn-alternate-em
--warn-unresolved-symbols
--error-unresolved-symbols
--whole-archive
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__wrap_
symbol. Any
undefined reference to __real_
symbol will be resolved to
symbol.
This can be used to provide a wrapper for a system function. The
wrapper function should be called __wrap_
symbol. If it
wishes to call the system function, it should call
__real_
symbol.
Here is a trivial example:
void * __wrap_malloc (size_t c) { printf ("malloc called with %zu\n", c); return __real_malloc (c); }
If you link other code with this file using --wrap malloc, then
all calls to malloc
will call the function __wrap_malloc
instead. The call to __real_malloc
in __wrap_malloc
will
call the real malloc
function.
You may wish to provide a __real_malloc
function as well, so that
links without the --wrap option will succeed. If you do this,
you should not put the definition of __real_malloc
in the same
file as __wrap_malloc
; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to malloc
.
Only undefined references are replaced by the linker. So, translation unit
internal references to symbol are not resolved to
__wrap_
symbol. In the next example, the call to f
in
g
is not resolved to __wrap_f
.
int f (void) { return 123; } int g (void) { return f(); }
--eh-frame-hdr
--no-eh-frame-hdr
.eh_frame_hdr
section and ELF PT_GNU_EH_FRAME
segment header.
--no-ld-generated-unwind-info
.eh_frame
unwind info for linker
generated code sections like PLT. This option is on by default
if linker generated unwind info is supported.
--enable-new-dtags
--disable-new-dtags
--hash-size=
number--hash-style=
stylesysv
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 depends upon how the linker was configured,
but for most Linux based systems it will be both
.
--compress-debug-sections=none
--compress-debug-sections=zlib
--compress-debug-sections=zlib-gnu
--compress-debug-sections=zlib-gabi
--compress-debug-sections=none doesn't compress DWARF debug sections. --compress-debug-sections=zlib-gnu compresses DWARF debug sections and renames them to begin with ‘.zdebug’ instead of ‘.debug’. --compress-debug-sections=zlib-gabi also compresses DWARF debug sections, but rather than renaming them it sets the SHF_COMPRESSED flag in the sections' headers.
The --compress-debug-sections=zlib option is an alias for --compress-debug-sections=zlib-gabi.
Note that this option overrides any compression in input debug sections, so if a binary is linked with --compress-debug-sections=none for example, then any compressed debug sections in input files will be uncompressed before they are copied into the output binary.
The default compression behaviour varies depending upon the target involved and the configure options used to build the toolchain. The default can be determined by examining the output from the linker's --help option.
--reduce-memory-overheads
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.
--max-cache-size=
size--build-id
--build-id=
style.note.gnu.build-id
ELF note section
or a .buildid
COFF section. The contents of the note are
unique bits identifying this linked file. style can be
uuid
to use 128 random bits, sha1
to use a 160-bit
SHA1 hash on the normative parts of the output contents,
md5
to use a 128-bit MD5 hash on the normative parts of
the output contents, or 0x
hexstring to use a chosen bit
string specified as an even number of hexadecimal digits (-
and
:
characters between digit pairs are ignored). If style
is omitted, sha1
is used.
The md5
and sha1
styles produces an identifier
that is always the same in an identical output file, but will be
unique among all nonidentical output files. It is not intended
to be compared as a checksum for the file's contents. A linked
file may be changed later by other tools, but the build ID bit
string identifying the original linked file does not change.
Passing none
for style disables the setting from any
--build-id
options earlier on the command line.
The i386 PE linker supports the -shared option, which causes
the output to be a dynamically linked library (DLL) instead of a
normal executable. You should name the output *.dll
when you
use this option. In addition, the linker fully supports the standard
*.def
files, which may be specified on the linker command line
like an object file (in fact, it should precede archives it exports
symbols from, to ensure that they get linked in, just like a normal
object file).
In addition to the options common to all targets, the i386 PE linker support additional command-line options that are specific to the i386 PE target. Options that take values may be separated from their values by either a space or an equals sign.
--add-stdcall-alias
--base-file
file--dll
LIBRARY
in a given .def
file.
[This option is specific to the i386 PE targeted port of the linker]
--enable-long-section-names
--disable-long-section-names
--enable-stdcall-fixup
--disable-stdcall-fixup
_foo
might be linked to the function
_foo@12
, or the undefined symbol _bar@16
might be linked
to the function _bar
. When the linker does this, it prints a
warning, since it normally should have failed to link, but sometimes
import libraries generated from third-party dlls may need this feature
to be usable. If you specify --enable-stdcall-fixup, this
feature is fully enabled and warnings are not printed. If you specify
--disable-stdcall-fixup, this feature is disabled and such
mismatches are considered to be errors.
[This option is specific to the i386 PE targeted port of the linker]
--leading-underscore
--no-leading-underscore
--export-all-symbols
DllMain@12
,
DllEntryPoint@0
, DllMainCRTStartup@12
, and
impure_ptr
will not be automatically
exported. Also, symbols imported from other DLLs will not be
re-exported, nor will symbols specifying the DLL's internal layout
such as those beginning with _head_
or ending with
_iname
. In addition, no symbols from libgcc
,
libstd++
, libmingw32
, or crtX.o
will be exported.
Symbols whose names begin with __rtti_
or __builtin_
will
not be exported, to help with C++ DLLs. Finally, there is an
extensive list of cygwin-private symbols that are not exported
(obviously, this applies on when building DLLs for cygwin targets).
These cygwin-excludes are: _cygwin_dll_entry@12
,
_cygwin_crt0_common@8
, _cygwin_noncygwin_dll_entry@12
,
_fmode
, _impure_ptr
, cygwin_attach_dll
,
cygwin_premain0
, cygwin_premain1
, cygwin_premain2
,
cygwin_premain3
, and environ
.
[This option is specific to the i386 PE targeted port of the linker]
--exclude-symbols
symbol,
symbol,...
--exclude-all-symbols
--file-alignment
--heap
reserve--heap
reserve,
commit--image-base
value--kill-at
--large-address-aware
--disable-large-address-aware
--major-image-version
value--major-os-version
value--major-subsystem-version
value--minor-image-version
value--minor-os-version
value--minor-subsystem-version
value--output-def
file*.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]
--enable-auto-image-base
--enable-auto-image-base=
value--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
--image-base
) then use the platform
default.
[This option is specific to the i386 PE targeted port of the linker]
--dll-search-prefix
string<string><basename>.dll
in preference to
lib<basename>.dll
. This behaviour allows easy distinction
between DLLs built for the various "subplatforms": native, cygwin,
uwin, pw, etc. For instance, cygwin DLLs typically use
--dll-search-prefix=cyg
.
[This option is specific to the i386 PE targeted port of the linker]
--enable-auto-import
_symbol
to __imp__symbol
for
DATA imports from DLLs, thus making it possible to bypass the dllimport
mechanism on the user side and to reference unmangled symbol names.
[This option is specific to the i386 PE targeted port of the linker]
The following remarks pertain to the original implementation of the feature and are obsolete nowadays for Cygwin and MinGW targets.
Note: Use of the 'auto-import' extension will cause the text section of the image file to be made writable. This does not conform to the PE-COFF format specification published by Microsoft.
Note - use of the 'auto-import' extension will also cause read only data which would normally be placed into the .rdata section to be placed into the .data section instead. This is in order to work around a problem with consts that is described here: http://www.cygwin.com/ml/cygwin/2004-09/msg01101.html
Using 'auto-import' generally will 'just work' – but sometimes you may see this message:
"variable '<var>' can't be auto-imported. Please read the
documentation for ld's --enable-auto-import
for details."
This message occurs when some (sub)expression accesses an address ultimately given by the sum of two constants (Win32 import tables only allow one). Instances where this may occur include accesses to member fields of struct variables imported from a DLL, as well as using a constant index into an array variable imported from a DLL. Any multiword variable (arrays, structs, long long, etc) may trigger this error condition. However, regardless of the exact data type of the offending exported variable, ld will always detect it, issue the warning, and exit.
There are several ways to address this difficulty, regardless of the data type of the exported variable:
One way is to use –enable-runtime-pseudo-reloc switch. This leaves the task of adjusting references in your client code for runtime environment, so this method works only when runtime environment supports this feature.
A second solution is to force one of the 'constants' to be a variable – that is, unknown and un-optimizable at compile time. For arrays, there are two possibilities: a) make the indexee (the array's address) a variable, or b) make the 'constant' index a variable. Thus:
extern type extern_array[]; extern_array[1] --> { volatile type *t=extern_array; t[1] }
or
extern type extern_array[]; extern_array[1] --> { volatile int t=1; extern_array[t] }
For structs (and most other multiword data types) the only option is to make the struct itself (or the long long, or the ...) variable:
extern struct s extern_struct; extern_struct.field --> { volatile struct s *t=&extern_struct; t->field }
or
extern long long extern_ll; extern_ll --> { volatile long long * local_ll=&extern_ll; *local_ll }
A third method of dealing with this difficulty is to abandon
'auto-import' for the offending symbol and mark it with
__declspec(dllimport)
. However, in practice that
requires using compile-time #defines to indicate whether you are
building a DLL, building client code that will link to the DLL, or
merely building/linking to a static library. In making the choice
between the various methods of resolving the 'direct address with
constant offset' problem, you should consider typical real-world usage:
Original:
--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).
--disable-auto-import
_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
--disable-runtime-pseudo-reloc
--enable-extra-pe-debug
--section-alignment
--stack
reserve--stack
reserve,
commit--subsystem
which--subsystem
which:
major--subsystem
which:
major.
minornative
, windows
,
console
, posix
, and xbox
. You may optionally set
the subsystem version also. Numeric values are also accepted for
which.
[This option is specific to the i386 PE targeted port of the linker]
The following options set flags in the DllCharacteristics
field
of the PE file header:
[These options are specific to PE targeted ports of the linker]
--high-entropy-va
--disable-high-entropy-va
This option also implies --dynamicbase and --enable-reloc-section.
--dynamicbase
--disable-dynamicbase
--forceinteg
--disable-forceinteg
--nxcompat
--disable-nxcompat
--no-isolation
--disable-no-isolation
--no-seh
--disable-no-seh
--no-bind
--disable-no-bind
--wdmdriver
--disable-wdmdriver
--tsaware
--disable-tsaware
--insert-timestamp
--no-insert-timestamp
--enable-reloc-section
--disable-reloc-section
The C6X uClinux target uses a binary format called DSBT to support shared libraries. Each shared library in the system needs to have a unique index; all executables use an index of 0.
--dsbt-size
size--dsbt-index
indexR_C6000_DSBT_INDEX
relocs are copied into the output file.
The ‘--no-merge-exidx-entries’ switch disables the merging of adjacent exidx entries in frame unwind info.
The 68HC11 and 68HC12 linkers support specific options to control the memory bank switching mapping and trampoline code generation.
--no-trampoline
jsr
instruction (this happens when a pointer to a far function is taken).
--bank-window
nameThe following options are supported to control handling of GOT generation when linking for 68K targets.
--got=
typeYou 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 (see BFD). If there is no
GNUTARGET
in the environment, ld uses the natural format
of the target. If GNUTARGET
is set to default
then BFD
attempts to discover the input format by examining binary input files;
this method often succeeds, but there are potential ambiguities, since
there is no method of ensuring that the magic number used to specify
object-file formats is unique. However, the configuration procedure for
BFD on each system places the conventional format for that system first
in the search-list, so ambiguities are resolved in favor of convention.
LDEMULATION
determines the default emulation if you don't use the
‘-m’ option. The emulation can affect various aspects of linker
behaviour, particularly the default linker script. You can list the
available emulations with the ‘--verbose’ or ‘-V’ options. If
the ‘-m’ option is not used, and the LDEMULATION
environment
variable is not defined, the default emulation depends upon how the
linker was configured.
Normally, the linker will default to demangling symbols. However, if
COLLECT_NO_DEMANGLE
is set in the environment, then it will
default to not demangling symbols. This environment variable is used in
a similar fashion by the gcc
linker wrapper program. The default
may be overridden by the ‘--demangle’ and ‘--no-demangle’
options.
Every link is controlled by a linker script. This script is written in the linker command language.
The main purpose of the linker script is to describe how the sections in the input files should be mapped into the output file, and to control the memory layout of the output file. Most linker scripts do nothing more than this. However, when necessary, the linker script can also direct the linker to perform many other operations, using the commands described below.
The linker always uses a linker script. If you do not supply one yourself, the linker will use a default script that is compiled into the linker executable. You can use the ‘--verbose’ command-line option to display the default linker script. Certain command-line options, such as ‘-r’ or ‘-N’, will affect the default linker script.
You may supply your own linker script by using the ‘-T’ command line option. When you do this, your linker script will replace the default linker script.
You may also use linker scripts implicitly by naming them as input files to the linker, as though they were files to be linked. See Implicit Linker Scripts.
We need to define some basic concepts and vocabulary in order to describe the linker script language.
The linker combines input files into a single output file. The output file and each input file are in a special data format known as an object file format. Each file is called an object file. The output file is often called an executable, but for our purposes we will also call it an object file. Each object file has, among other things, a list of sections. We sometimes refer to a section in an input file as an input section; similarly, a section in the output file is an output section.
Each section in an object file has a name and a size. Most sections also have an associated block of data, known as the section contents. A section may be marked as loadable, which means that the contents should be loaded into memory when the output file is run. A section with no contents may be allocatable, which means that an area in memory should be set aside, but nothing in particular should be loaded there (in some cases this memory must be zeroed out). A section which is neither loadable nor allocatable typically contains some sort of debugging information.
Every loadable or allocatable output section has two addresses. The first is the VMA, or virtual memory address. This is the address the section will have when the output file is run. The second is the LMA, or load memory address. This is the address at which the section will be loaded. In most cases the two addresses will be the same. An example of when they might be different is when a data section is loaded into ROM, and then copied into RAM when the program starts up (this technique is often used to initialize global variables in a ROM based system). In this case the ROM address would be the LMA, and the RAM address would be the VMA.
You can see the sections in an object file by using the objdump
program with the ‘-h’ option.
Every object file also has a list of symbols, known as the symbol table. A symbol may be defined or undefined. Each symbol has a name, and each defined symbol has an address, among other information. If you compile a C or C++ program into an object file, you will get a defined symbol for every defined function and global or static variable. Every undefined function or global variable which is referenced in the input file will become an undefined symbol.
You can see the symbols in an object file by using the nm
program, or by using the objdump
program with the ‘-t’
option.
Linker scripts are text files.
You write a linker script as a series of commands. Each command is either a keyword, possibly followed by arguments, or an assignment to a symbol. You may separate commands using semicolons. Whitespace is generally ignored.
Strings such as file or format names can normally be entered directly. If the file name contains a character such as a comma which would otherwise serve to separate file names, you may put the file name in double quotes. There is no way to use a double quote character in a file name.
You may include comments in linker scripts just as in C, delimited by ‘/*’ and ‘*/’. As in C, comments are syntactically equivalent to whitespace.
Many linker scripts are fairly simple.
The simplest possible linker script has just one command: ‘SECTIONS’. You use the ‘SECTIONS’ command to describe the memory layout of the output file.
The ‘SECTIONS’ command is a powerful command. Here we will describe a simple use of it. Let's assume your program consists only of code, initialized data, and uninitialized data. These will be in the ‘.text’, ‘.data’, and ‘.bss’ sections, respectively. Let's assume further that these are the only sections which appear in your input files.
For this example, let's say that the code should be loaded at address 0x10000, and that the data should start at address 0x8000000. Here is a linker script which will do that:
SECTIONS { . = 0x10000; .text : { *(.text) } . = 0x8000000; .data : { *(.data) } .bss : { *(.bss) } }
You write the ‘SECTIONS’ command as the keyword ‘SECTIONS’, followed by a series of symbol assignments and output section descriptions enclosed in curly braces.
The first line inside the ‘SECTIONS’ command of the above example sets the value of the special symbol ‘.’, which is the location counter. If you do not specify the address of an output section in some other way (other ways are described later), the address is set from the current value of the location counter. The location counter is then incremented by the size of the output section. At the start of the ‘SECTIONS’ command, the location counter has the value ‘0’.
The second line defines an output section, ‘.text’. The colon is required syntax which may be ignored for now. Within the curly braces after the output section name, you list the names of the input sections which should be placed into this output section. The ‘*’ is a wildcard which matches any file name. The expression ‘*(.text)’ means all ‘.text’ input sections in all input files.
Since the location counter is ‘0x10000’ when the output section ‘.text’ is defined, the linker will set the address of the ‘.text’ section in the output file to be ‘0x10000’.
The remaining lines define the ‘.data’ and ‘.bss’ sections in the output file. The linker will place the ‘.data’ output section at address ‘0x8000000’. After the linker places the ‘.data’ output section, the value of the location counter will be ‘0x8000000’ plus the size of the ‘.data’ output section. The effect is that the linker will place the ‘.bss’ output section immediately after the ‘.data’ output section in memory.
The linker will ensure that each output section has the required alignment, by increasing the location counter if necessary. In this example, the specified addresses for the ‘.text’ and ‘.data’ sections will probably satisfy any alignment constraints, but the linker may have to create a small gap between the ‘.data’ and ‘.bss’ sections.
That's it! That's a simple and complete linker script.
In this section we describe the simple linker script commands.
The first instruction to execute in a program is called the entry
point. You can use the ENTRY
linker script command to set the
entry point. The argument is a symbol name:
ENTRY(symbol)
There are several ways to set the entry point. The linker will set the entry point by trying each of the following methods in order, and stopping when one of them succeeds:
ENTRY(
symbol)
command in a linker script;
start
, but PE- and BeOS-based systems for example
check a list of possible entry symbols, matching the first one found.
0
.
Several linker script commands deal with files.
INCLUDE
filenameINCLUDE
up to
10 levels deep.
You can place INCLUDE
directives at the top level, in MEMORY
or
SECTIONS
commands, or in output section descriptions.
INPUT(
file,
file, ...)
INPUT(
file file ...)
INPUT
command directs the linker to include the named files
in the link, as though they were named on the command line.
For example, if you always want to include subr.o any time you do a link, but you can't be bothered to put it on every link command line, then you can put ‘INPUT (subr.o)’ in your linker script.
In fact, if you like, you can list all of your input files in the linker script, and then invoke the linker with nothing but a ‘-T’ option.
In case a sysroot prefix is configured, and the filename starts
with the ‘/’ character, and the script being processed was
located inside the sysroot prefix, the filename will be looked
for in the sysroot prefix. The sysroot prefix can also be forced by specifying
=
as the first character in the filename path, or prefixing the
filename path with $SYSROOT
. See also the description of
‘-L’ in Command-line Options.
If a sysroot prefix is not used then the linker will try to open the file in the directory containing the linker script. If it is not found the linker will then search the current directory. If it is still not found the linker will search through the archive library search path.
If you use ‘INPUT (-lfile)’, ld will transform the
name to lib
file.a
, as with the command-line argument
‘-l’.
When you use the INPUT
command in an implicit linker script, the
files will be included in the link at the point at which the linker
script file is included. This can affect archive searching.
GROUP(
file,
file, ...)
GROUP(
file file ...)
GROUP
command is like INPUT
, except that the named
files should all be archives, and they are searched repeatedly until no
new undefined references are created. See the description of ‘-(’
in Command-line Options.
AS_NEEDED(
file,
file, ...)
AS_NEEDED(
file file ...)
INPUT
or GROUP
commands, among other filenames. The files listed will be handled
as if they appear directly in the INPUT
or GROUP
commands,
with the exception of ELF shared libraries, that will be added only
when they are actually needed. This construct essentially enables
--as-needed option for all the files listed inside of it
and restores previous --as-needed resp. --no-as-needed
setting afterwards.
OUTPUT(
filename)
OUTPUT
command names the output file. Using
OUTPUT(
filename)
in the linker script is exactly like using
‘-o filename’ on the command line (see Command Line Options). If both are used, the command-line option takes
precedence.
You can use the OUTPUT
command to define a default name for the
output file other than the usual default of a.out.
SEARCH_DIR(
path)
SEARCH_DIR
command adds path to the list of paths where
ld looks for archive libraries. Using
SEARCH_DIR(
path)
is exactly like using ‘-L path’
on the command line (see Command-line Options). If both
are used, then the linker will search both paths. Paths specified using
the command-line option are searched first.
STARTUP(
filename)
STARTUP
command is just like the INPUT
command, except
that filename will become the first input file to be linked, as
though it were specified first on the command line. This may be useful
when using a system in which the entry point is always the start of the
first file.
A couple of linker script commands deal with object file formats.
OUTPUT_FORMAT(
bfdname)
OUTPUT_FORMAT(
default,
big,
little)
OUTPUT_FORMAT
command names the BFD format to use for the
output file (see BFD). Using OUTPUT_FORMAT(
bfdname)
is
exactly like using ‘--oformat bfdname’ on the command line
(see Command-line Options). If both are used, the command
line option takes precedence.
You can use OUTPUT_FORMAT
with three arguments to use different
formats based on the ‘-EB’ and ‘-EL’ command-line options.
This permits the linker script to set the output format based on the
desired endianness.
If neither ‘-EB’ nor ‘-EL’ are used, then the output format will be the first argument, default. If ‘-EB’ is used, the output format will be the second argument, big. If ‘-EL’ is used, the output format will be the third argument, little.
For example, the default linker script for the MIPS ELF target uses this command:
OUTPUT_FORMAT(elf32-bigmips, elf32-bigmips, elf32-littlemips)
This says that the default format for the output file is
‘elf32-bigmips’, but if the user uses the ‘-EL’ command-line
option, the output file will be created in the ‘elf32-littlemips’
format.
TARGET(
bfdname)
TARGET
command names the BFD format to use when reading input
files. It affects subsequent INPUT
and GROUP
commands.
This command is like using ‘-b bfdname’ on the command line
(see Command-line Options). If the TARGET
command
is used but OUTPUT_FORMAT
is not, then the last TARGET
command is also used to set the format for the output file. See BFD.
Alias names can be added to existing memory regions created with the MEMORY command. Each name corresponds to at most one memory region.
REGION_ALIAS(alias, region)
The REGION_ALIAS
function creates an alias name alias for the
memory region region. This allows a flexible mapping of output sections
to memory regions. An example follows.
Suppose we have an application for embedded systems which come with various
memory storage devices. All have a general purpose, volatile memory RAM
that allows code execution or data storage. Some may have a read-only,
non-volatile memory ROM
that allows code execution and read-only data
access. The last variant is a read-only, non-volatile memory ROM2
with
read-only data access and no code execution capability. We have four output
sections:
.text
program code;
.rodata
read-only data;
.data
read-write initialized data;
.bss
read-write zero initialized data.
The goal is to provide a linker command file that contains a system independent
part defining the output sections and a system dependent part mapping the
output sections to the memory regions available on the system. Our embedded
systems come with three different memory setups A
, B
and
C
:
Section | Variant A | Variant B | Variant C
|
.text | RAM | ROM | ROM
|
.rodata | RAM | ROM | ROM2
|
.data | RAM | RAM/ROM | RAM/ROM2
|
.bss | RAM | RAM | RAM
|
RAM/ROM
or RAM/ROM2
means that this section is
loaded into region ROM
or ROM2
respectively. Please note that
the load address of the .data
section starts in all three variants at
the end of the .rodata
section.
The base linker script that deals with the output sections follows. It
includes the system dependent linkcmds.memory
file that describes the
memory layout:
INCLUDE linkcmds.memory SECTIONS { .text : { *(.text) } > REGION_TEXT .rodata : { *(.rodata) rodata_end = .; } > REGION_RODATA .data : AT (rodata_end) { data_start = .; *(.data) } > REGION_DATA data_size = SIZEOF(.data); data_load_start = LOADADDR(.data); .bss : { *(.bss) } > REGION_BSS }
Now we need three different linkcmds.memory
files to define memory
regions and alias names. The content of linkcmds.memory
for the three
variants A
, B
and C
:
A
RAM
.
MEMORY { RAM : ORIGIN = 0, LENGTH = 4M } REGION_ALIAS("REGION_TEXT", RAM); REGION_ALIAS("REGION_RODATA", RAM); REGION_ALIAS("REGION_DATA", RAM); REGION_ALIAS("REGION_BSS", RAM);
B
ROM
. Read-write data goes
into the RAM
. An image of the initialized data is loaded into the
ROM
and will be copied during system start into the RAM
.
MEMORY { ROM : ORIGIN = 0, LENGTH = 3M RAM : ORIGIN = 0x10000000, LENGTH = 1M } REGION_ALIAS("REGION_TEXT", ROM); REGION_ALIAS("REGION_RODATA", ROM); REGION_ALIAS("REGION_DATA", RAM); REGION_ALIAS("REGION_BSS", RAM);
C
ROM
. Read-only data goes into the
ROM2
. Read-write data goes into the RAM
. An image of the
initialized data is loaded into the ROM2
and will be copied during
system start into the RAM
.
MEMORY { ROM : ORIGIN = 0, LENGTH = 2M ROM2 : ORIGIN = 0x10000000, LENGTH = 1M RAM : ORIGIN = 0x20000000, LENGTH = 1M } REGION_ALIAS("REGION_TEXT", ROM); REGION_ALIAS("REGION_RODATA", ROM2); REGION_ALIAS("REGION_DATA", RAM); REGION_ALIAS("REGION_BSS", RAM);
It is possible to write a common system initialization routine to copy the
.data
section from ROM
or ROM2
into the RAM
if
necessary:
#include <string.h> extern char data_start []; extern char data_size []; extern char data_load_start []; void copy_data(void) { if (data_start != data_load_start) { memcpy(data_start, data_load_start, (size_t) data_size); } }
There are a few other linker scripts commands.
ASSERT(
exp,
message)
Note that assertions are checked before the final stages of linking take place. This means that expressions involving symbols PROVIDEd inside section definitions will fail if the user has not set values for those symbols. The only exception to this rule is PROVIDEd symbols that just reference dot. Thus an assertion like this:
.stack : { PROVIDE (__stack = .); PROVIDE (__stack_size = 0x100); ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack"); }
will fail if __stack_size
is not defined elsewhere. Symbols
PROVIDEd outside of section definitions are evaluated earlier, so they
can be used inside ASSERTions. Thus:
PROVIDE (__stack_size = 0x100); .stack : { PROVIDE (__stack = .); ASSERT ((__stack > (_end + __stack_size)), "Error: No room left for the stack"); }
will work.
EXTERN(
symbol symbol ...)
EXTERN
, and you may use EXTERN
multiple times. This
command has the same effect as the ‘-u’ command-line option.
FORCE_COMMON_ALLOCATION
INHIBIT_COMMON_ALLOCATION
ld
omit the assignment of addresses
to common symbols even for a non-relocatable output file.
FORCE_GROUP_ALLOCATION
INSERT [ AFTER | BEFORE ]
output_sectionSECTIONS
with, for example, overlays. It
inserts all prior linker script statements after (or before)
output_section, and also causes ‘-T’ to not override the
default linker script. The exact insertion point is as for orphan
sections. See Location Counter. The insertion happens after the
linker has mapped input sections to output sections. Prior to the
insertion, since ‘-T’ scripts are parsed before the default
linker script, statements in the ‘-T’ script occur before the
default linker script statements in the internal linker representation
of the script. In particular, input section assignments will be made
to ‘-T’ output sections before those in the default script. Here
is an example of how a ‘-T’ script using INSERT
might look:
SECTIONS { OVERLAY : { .ov1 { ov1*(.text) } .ov2 { ov2*(.text) } } } INSERT AFTER .text;
NOCROSSREFS(
section section ...)
In certain types of programs, particularly on embedded systems when using overlays, when one section is loaded into memory, another section will not be. Any direct references between the two sections would be errors. For example, it would be an error if code in one section called a function defined in the other section.
The NOCROSSREFS
command takes a list of output section names. If
ld detects any cross references between the sections, it reports
an error and returns a non-zero exit status. Note that the
NOCROSSREFS
command uses output section names, not input section
names.
NOCROSSREFS_TO(
tosection fromsection ...)
The NOCROSSREFS
command is useful when ensuring that two or more
output sections are entirely independent but there are situations where
a one-way dependency is needed. For example, in a multi-core application
there may be shared code that can be called from each core but for safety
must never call back.
The NOCROSSREFS_TO
command takes a list of output section names.
The first section can not be referenced from any of the other sections.
If ld detects any references to the first section from any of
the other sections, it reports an error and returns a non-zero exit
status. Note that the NOCROSSREFS_TO
command uses output section
names, not input section names.
OUTPUT_ARCH(
bfdarch)
objdump
program with
the ‘-f’ option.
LD_FEATURE(
string)
"SANE_EXPR"
then absolute symbols and numbers
in a script are simply treated as numbers everywhere.
See Expression Section.
You may assign a value to a symbol in a linker script. This will define the symbol and place it into the symbol table with a global scope.
You may assign to a symbol using any of the C assignment operators:
=
expression ;
+=
expression ;
-=
expression ;
*=
expression ;
/=
expression ;
<<=
expression ;
>>=
expression ;
&=
expression ;
|=
expression ;
The first case will define symbol to the value of expression. In the other cases, symbol must already be defined, and the value will be adjusted accordingly.
The special symbol name ‘.’ indicates the location counter. You
may only use this within a SECTIONS
command. See Location Counter.
The semicolon after expression is required.
Expressions are defined below; see Expressions.
You may write symbol assignments as commands in their own right, or as
statements within a SECTIONS
command, or as part of an output
section description in a SECTIONS
command.
The section of the symbol will be set from the section of the expression; for more information, see Expression Section.
Here is an example showing the three different places that symbol assignments may be used:
floating_point = 0; SECTIONS { .text : { *(.text) _etext = .; } _bdata = (. + 3) & ~ 3; .data : { *(.data) } }
In this example, the symbol ‘floating_point’ will be defined as zero. The symbol ‘_etext’ will be defined as the address following the last ‘.text’ input section. The symbol ‘_bdata’ will be defined as the address following the ‘.text’ output section aligned upward to a 4 byte boundary.
For ELF targeted ports, define a symbol that will be hidden and won't be
exported. The syntax is HIDDEN(
symbol =
expression)
.
Here is the example from Simple Assignments, rewritten to use
HIDDEN
:
HIDDEN(floating_point = 0); SECTIONS { .text : { *(.text) HIDDEN(_etext = .); } HIDDEN(_bdata = (. + 3) & ~ 3); .data : { *(.data) } }
In this case none of the three symbols will be visible outside this module.
In some cases, it is desirable for a linker script to define a symbol
only if it is referenced and is not defined by any object included in
the link. For example, traditional linkers defined the symbol
‘etext’. However, ANSI C requires that the user be able to use
‘etext’ as a function name without encountering an error. The
PROVIDE
keyword may be used to define a symbol, such as
‘etext’, only if it is referenced but not defined. The syntax is
PROVIDE(
symbol =
expression)
.
Here is an example of using PROVIDE
to define ‘etext’:
SECTIONS { .text : { *(.text) _etext = .; PROVIDE(etext = .); } }
In this example, if the program defines ‘_etext’ (with a leading underscore), the linker will give a multiple definition diagnostic. If, on the other hand, the program defines ‘etext’ (with no leading underscore), the linker will silently use the definition in the program. If the program references ‘etext’ but does not define it, the linker will use the definition in the linker script.
Note - the PROVIDE
directive considers a common symbol to be
defined, even though such a symbol could be combined with the symbol
that the PROVIDE
would create. This is particularly important
when considering constructor and destructor list symbols such as
‘__CTOR_LIST__’ as these are often defined as common symbols.
Similar to PROVIDE
. For ELF targeted ports, the symbol will be
hidden and won't be exported.
Accessing a linker script defined variable from source code is not intuitive. In particular a linker script symbol is not equivalent to a variable declaration in a high level language, it is instead a symbol that does not have a value.
Before going further, it is important to note that compilers often transform names in the source code into different names when they are stored in the symbol table. For example, Fortran compilers commonly prepend or append an underscore, and C++ performs extensive ‘name mangling’. Therefore there might be a discrepancy between the name of a variable as it is used in source code and the name of the same variable as it is defined in a linker script. For example in C a linker script variable might be referred to as:
extern int foo;
But in the linker script it might be defined as:
_foo = 1000;
In the remaining examples however it is assumed that no name transformation has taken place.
When a symbol is declared in a high level language such as C, two things happen. The first is that the compiler reserves enough space in the program's memory to hold the value of the symbol. The second is that the compiler creates an entry in the program's symbol table which holds the symbol's address. ie the symbol table contains the address of the block of memory holding the symbol's value. So for example the following C declaration, at file scope:
int foo = 1000;
creates an entry called ‘foo’ in the symbol table. This entry holds the address of an ‘int’ sized block of memory where the number 1000 is initially stored.
When a program references a symbol the compiler generates code that first accesses the symbol table to find the address of the symbol's memory block and then code to read the value from that memory block. So:
foo = 1;
looks up the symbol ‘foo’ in the symbol table, gets the address associated with this symbol and then writes the value 1 into that address. Whereas:
int * a = & foo;
looks up the symbol ‘foo’ in the symbol table, gets its address and then copies this address into the block of memory associated with the variable ‘a’.
Linker scripts symbol declarations, by contrast, create an entry in the symbol table but do not assign any memory to them. Thus they are an address without a value. So for example the linker script definition:
foo = 1000;
creates an entry in the symbol table called ‘foo’ which holds the address of memory location 1000, but nothing special is stored at address 1000. This means that you cannot access the value of a linker script defined symbol - it has no value - all you can do is access the address of a linker script defined symbol.
Hence when you are using a linker script defined symbol in source code you should always take the address of the symbol, and never attempt to use its value. For example suppose you want to copy the contents of a section of memory called .ROM into a section called .FLASH and the linker script contains these declarations:
start_of_ROM = .ROM; end_of_ROM = .ROM + sizeof (.ROM); start_of_FLASH = .FLASH;
Then the C source code to perform the copy would be:
extern char start_of_ROM, end_of_ROM, start_of_FLASH; memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
Note the use of the ‘&’ operators. These are correct. Alternatively the symbols can be treated as the names of vectors or arrays and then the code will again work as expected:
extern char start_of_ROM[], end_of_ROM[], start_of_FLASH[]; memcpy (start_of_FLASH, start_of_ROM, end_of_ROM - start_of_ROM);
Note how using this method does not require the use of ‘&’ operators.
The SECTIONS
command tells the linker how to map input sections
into output sections, and how to place the output sections in memory.
The format of the SECTIONS
command is:
SECTIONS { sections-command sections-command ... }
Each sections-command may of be one of the following:
ENTRY
command (see Entry command)
The ENTRY
command and symbol assignments are permitted inside the
SECTIONS
command for convenience in using the location counter in
those commands. This can also make the linker script easier to
understand because you can use those commands at meaningful points in
the layout of the output file.
Output section descriptions and overlay descriptions are described below.
If you do not use a SECTIONS
command in your linker script, the
linker will place each input section into an identically named output
section in the order that the sections are first encountered in the
input files. If all input sections are present in the first file, for
example, the order of sections in the output file will match the order
in the first input file. The first section will be at address zero.
The full description of an output section looks like this:
section [address] [(type)] : [AT(lma)] [ALIGN(section_align) | ALIGN_WITH_INPUT] [SUBALIGN(subsection_align)] [constraint] { output-section-command output-section-command ... } [>region] [AT>lma_region] [:phdr :phdr ...] [=fillexp] [,]
Most output sections do not use most of the optional section attributes.
The whitespace around section is required, so that the section name is unambiguous. The colon and the curly braces are also required. The comma at the end may be required if a fillexp is used and the next sections-command looks like a continuation of the expression. The line breaks and other white space are optional.
Each output-section-command may be one of the following:
The name of the output section is section. section must
meet the constraints of your output format. In formats which only
support a limited number of sections, such as a.out
, the name
must be one of the names supported by the format (a.out
, for
example, allows only ‘.text’, ‘.data’ or ‘.bss’). If the
output format supports any number of sections, but with numbers and not
names (as is the case for Oasys), the name should be supplied as a
quoted numeric string. A section name may consist of any sequence of
characters, but a name which contains any unusual characters such as
commas must be quoted.
The output section name ‘/DISCARD/’ is special; Output Section Discarding.
The address is an expression for the VMA (the virtual memory address) of the output section. This address is optional, but if it is provided then the output address will be set exactly as specified.
If the output address is not specified then one will be chosen for the section, based on the heuristic below. This address will be adjusted to fit the alignment requirement of the output section. The alignment requirement is the strictest alignment of any input section contained within the output section.
The output section address heuristic is as follows:
For example:
.text . : { *(.text) }
and
.text : { *(.text) }
are subtly different. The first will set the address of the ‘.text’ output section to the current value of the location counter. The second will set it to the current value of the location counter aligned to the strictest alignment of any of the ‘.text’ input sections.
The address may be an arbitrary expression; Expressions. For example, if you want to align the section on a 0x10 byte boundary, so that the lowest four bits of the section address are zero, you could do something like this:
.text ALIGN(0x10) : { *(.text) }
This works because ALIGN
returns the current location counter
aligned upward to the specified value.
Specifying address for a section will change the value of the location counter, provided that the section is non-empty. (Empty sections are ignored).
The most common output section command is an input section description.
The input section description is the most basic linker script operation. You use output sections to tell the linker how to lay out your program in memory. You use input section descriptions to tell the linker how to map the input files into your memory layout.
An input section description consists of a file name optionally followed by a list of section names in parentheses.
The file name and the section name may be wildcard patterns, which we describe further below (see Input Section Wildcards).
The most common input section description is to include all input sections with a particular name in the output section. For example, to include all input ‘.text’ sections, you would write:
*(.text)
Here the ‘*’ is a wildcard which matches any file name. To exclude a list of files from matching the file name wildcard, EXCLUDE_FILE may be used to match all files except the ones specified in the EXCLUDE_FILE list. For example:
EXCLUDE_FILE (*crtend.o *otherfile.o) *(.ctors)
will cause all .ctors sections from all files except crtend.o and otherfile.o to be included. The EXCLUDE_FILE can also be placed inside the section list, for example:
*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors)
The result of this is identically to the previous example. Supporting two syntaxes for EXCLUDE_FILE is useful if the section list contains more than one section, as described below.
There are two ways to include more than one section:
*(.text .rdata) *(.text) *(.rdata)
The difference between these is the order in which the ‘.text’ and ‘.rdata’ input sections will appear in the output section. In the first example, they will be intermingled, appearing in the same order as they are found in the linker input. In the second example, all ‘.text’ input sections will appear first, followed by all ‘.rdata’ input sections.
When using EXCLUDE_FILE with more than one section, if the exclusion is within the section list then the exclusion only applies to the immediately following section, for example:
*(EXCLUDE_FILE (*somefile.o) .text .rdata)
will cause all ‘.text’ sections from all files except somefile.o to be included, while all ‘.rdata’ sections from all files, including somefile.o, will be included. To exclude the ‘.rdata’ sections from somefile.o the example could be modified to:
*(EXCLUDE_FILE (*somefile.o) .text EXCLUDE_FILE (*somefile.o) .rdata)
Alternatively, placing the EXCLUDE_FILE outside of the section list, before the input file selection, will cause the exclusion to apply for all sections. Thus the previous example can be rewritten as:
EXCLUDE_FILE (*somefile.o) *(.text .rdata)
You can specify a file name to include sections from a particular file. You would do this if one or more of your files contain special data that needs to be at a particular location in memory. For example:
data.o(.data)
To refine the sections that are included based on the section flags of an input section, INPUT_SECTION_FLAGS may be used.
Here is a simple example for using Section header flags for ELF sections:
SECTIONS { .text : { INPUT_SECTION_FLAGS (SHF_MERGE & SHF_STRINGS) *(.text) } .text2 : { INPUT_SECTION_FLAGS (!SHF_WRITE) *(.text) } }
In this example, the output section ‘.text’ will be comprised of any
input section matching the name *(.text) whose section header flags
SHF_MERGE
and SHF_STRINGS
are set. The output section
‘.text2’ will be comprised of any input section matching the name *(.text)
whose section header flag SHF_WRITE
is clear.
You can also specify files within archives by writing a pattern matching the archive, a colon, then the pattern matching the file, with no whitespace around the colon.
Either one or both of ‘archive’ and ‘file’ can contain shell
wildcards. On DOS based file systems, the linker will assume that a
single letter followed by a colon is a drive specifier, so
‘c:myfile.o’ is a simple file specification, not ‘myfile.o’
within an archive called ‘c’. ‘archive:file’ filespecs may
also be used within an EXCLUDE_FILE
list, but may not appear in
other linker script contexts. For instance, you cannot extract a file
from an archive by using ‘archive:file’ in an INPUT
command.
If you use a file name without a list of sections, then all sections in the input file will be included in the output section. This is not commonly done, but it may by useful on occasion. For example:
data.o
When you use a file name which is not an ‘archive:file’ specifier
and does not contain any wild card
characters, the linker will first see if you also specified the file
name on the linker command line or in an INPUT
command. If you
did not, the linker will attempt to open the file as an input file, as
though it appeared on the command line. Note that this differs from an
INPUT
command, because the linker will not search for the file in
the archive search path.
In an input section description, either the file name or the section name or both may be wildcard patterns.
The file name of ‘*’ seen in many examples is a simple wildcard pattern for the file name.
The wildcard patterns are like those used by the Unix shell.
File name wildcard patterns only match files which are explicitly
specified on the command line or in an INPUT
command. The linker
does not search directories to expand wildcards.
If a file name matches more than one wildcard pattern, or if a file name appears explicitly and is also matched by a wildcard pattern, the linker will use the first match in the linker script. For example, this sequence of input section descriptions is probably in error, because the data.o rule will not be used:
.data : { *(.data) } .data1 : { data.o(.data) }
Normally, the linker will place files and sections matched by wildcards
in the order in which they are seen during the link. You can change
this by using the SORT_BY_NAME
keyword, which appears before a wildcard
pattern in parentheses (e.g., SORT_BY_NAME(.text*)
). When the
SORT_BY_NAME
keyword is used, the linker will sort the files or sections
into ascending order by name before placing them in the output file.
SORT_BY_ALIGNMENT
is similar to SORT_BY_NAME
.
SORT_BY_ALIGNMENT
will sort sections into descending order of
alignment before placing them in the output file. Placing larger
alignments before smaller alignments can reduce the amount of padding
needed.
SORT_BY_INIT_PRIORITY
is also similar to SORT_BY_NAME
.
SORT_BY_INIT_PRIORITY
will sort sections into ascending
numerical order of the GCC init_priority attribute encoded in the
section name before placing them in the output file. In
.init_array.NNNNN
and .fini_array.NNNNN
, NNNNN
is
the init_priority. In .ctors.NNNNN
and .dtors.NNNNN
,
NNNNN
is 65535 minus the init_priority.
SORT
is an alias for SORT_BY_NAME
.
When there are nested section sorting commands in linker script, there can be at most 1 level of nesting for section sorting commands.
SORT_BY_NAME
(SORT_BY_ALIGNMENT
(wildcard section pattern)).
It will sort the input sections by name first, then by alignment if two
sections have the same name.
SORT_BY_ALIGNMENT
(SORT_BY_NAME
(wildcard section pattern)).
It will sort the input sections by alignment first, then by name if two
sections have the same alignment.
SORT_BY_NAME
(SORT_BY_NAME
(wildcard section pattern)) is
treated the same as SORT_BY_NAME
(wildcard section pattern).
SORT_BY_ALIGNMENT
(SORT_BY_ALIGNMENT
(wildcard section pattern))
is treated the same as SORT_BY_ALIGNMENT
(wildcard section pattern).
When both command-line section sorting option and linker script section sorting command are used, section sorting command always takes precedence over the command-line option.
If the section sorting command in linker script isn't nested, the command-line option will make the section sorting command to be treated as nested sorting command.
SORT_BY_NAME
(wildcard section pattern ) with
--sort-sections alignment is equivalent to
SORT_BY_NAME
(SORT_BY_ALIGNMENT
(wildcard section pattern)).
SORT_BY_ALIGNMENT
(wildcard section pattern) with
--sort-section name is equivalent to
SORT_BY_ALIGNMENT
(SORT_BY_NAME
(wildcard section pattern)).
If the section sorting command in linker script is nested, the command-line option will be ignored.
SORT_NONE
disables section sorting by ignoring the command-line
section sorting option.
If you ever get confused about where input sections are going, use the ‘-M’ linker option to generate a map file. The map file shows precisely how input sections are mapped to output sections.
This example shows how wildcard patterns might be used to partition files. This linker script directs the linker to place all ‘.text’ sections in ‘.text’ and all ‘.bss’ sections in ‘.bss’. The linker will place the ‘.data’ section from all files beginning with an upper case character in ‘.DATA’; for all other files, the linker will place the ‘.data’ section in ‘.data’.
SECTIONS { .text : { *(.text) } .DATA : { [A-Z]*(.data) } .data : { *(.data) } .bss : { *(.bss) } }
A special notation is needed for common symbols, because in many object file formats common symbols do not have a particular input section. The linker treats common symbols as though they are in an input section named ‘COMMON’.
You may use file names with the ‘COMMON’ section just as with any other input sections. You can use this to place common symbols from a particular input file in one section while common symbols from other input files are placed in another section.
In most cases, common symbols in input files will be placed in the ‘.bss’ section in the output file. For example:
.bss { *(.bss) *(COMMON) }
Some object file formats have more than one type of common symbol. For example, the MIPS ELF object file format distinguishes standard common symbols and small common symbols. In this case, the linker will use a different special section name for other types of common symbols. In the case of MIPS ELF, the linker uses ‘COMMON’ for standard common symbols and ‘.scommon’ for small common symbols. This permits you to map the different types of common symbols into memory at different locations.
You will sometimes see ‘[COMMON]’ in old linker scripts. This notation is now considered obsolete. It is equivalent to ‘*(COMMON)’.
When link-time garbage collection is in use (‘--gc-sections’),
it is often useful to mark sections that should not be eliminated.
This is accomplished by surrounding an input section's wildcard entry
with KEEP()
, as in KEEP(*(.init))
or
KEEP(SORT_BY_NAME(*)(.ctors))
.
The following example is a complete linker script. It tells the linker to read all of the sections from file all.o and place them at the start of output section ‘outputa’ which starts at location ‘0x10000’. All of section ‘.input1’ from file foo.o follows immediately, in the same output section. All of section ‘.input2’ from foo.o goes into output section ‘outputb’, followed by section ‘.input1’ from foo1.o. All of the remaining ‘.input1’ and ‘.input2’ sections from any files are written to output section ‘outputc’.
SECTIONS { outputa 0x10000 : { all.o foo.o (.input1) } outputb : { foo.o (.input2) foo1.o (.input1) } outputc : { *(.input1) *(.input2) } }
If an output section's name is the same as the input section's name and is representable as a C identifier, then the linker will automatically see PROVIDE two symbols: __start_SECNAME and __stop_SECNAME, where SECNAME is the name of the section. These indicate the start address and end address of the output section respectively. Note: most section names are not representable as C identifiers because they contain a ‘.’ character.
You can include explicit bytes of data in an output section by using
BYTE
, SHORT
, LONG
, QUAD
, or SQUAD
as
an output section command. Each keyword is followed by an expression in
parentheses providing the value to store (see Expressions). The
value of the expression is stored at the current value of the location
counter.
The BYTE
, SHORT
, LONG
, and QUAD
commands
store one, two, four, and eight bytes (respectively). After storing the
bytes, the location counter is incremented by the number of bytes
stored.
For example, this will store the byte 1 followed by the four byte value of the symbol ‘addr’:
BYTE(1) LONG(addr)
When using a 64 bit host or target, QUAD
and SQUAD
are the
same; they both store an 8 byte, or 64 bit, value. When both host and
target are 32 bits, an expression is computed as 32 bits. In this case
QUAD
stores a 32 bit value zero extended to 64 bits, and
SQUAD
stores a 32 bit value sign extended to 64 bits.
If the object file format of the output file has an explicit endianness, which is the normal case, the value will be stored in that endianness. When the object file format does not have an explicit endianness, as is true of, for example, S-records, the value will be stored in the endianness of the first input object file.
Note—these commands only work inside a section description and not between them, so the following will produce an error from the linker:
SECTIONS { .text : { *(.text) } LONG(1) .data : { *(.data) } }
whereas this will work:
SECTIONS { .text : { *(.text) ; LONG(1) } .data : { *(.data) } }
You may use the FILL
command to set the fill pattern for the
current section. It is followed by an expression in parentheses. Any
otherwise unspecified regions of memory within the section (for example,
gaps left due to the required alignment of input sections) are filled
with the value of the expression, repeated as
necessary. A FILL
statement covers memory locations after the
point at which it occurs in the section definition; by including more
than one FILL
statement, you can have different fill patterns in
different parts of an output section.
This example shows how to fill unspecified regions of memory with the value ‘0x90’:
FILL(0x90909090)
The FILL
command is similar to the ‘=fillexp’ output
section attribute, but it only affects the
part of the section following the FILL
command, rather than the
entire section. If both are used, the FILL
command takes
precedence. See Output Section Fill, for details on the fill
expression.
There are a couple of keywords which can appear as output section commands.
CREATE_OBJECT_SYMBOLS
CREATE_OBJECT_SYMBOLS
command appears.
This is conventional for the a.out object file format. It is not normally used for any other object file format.
CONSTRUCTORS
CONSTRUCTORS
command tells the
linker to place constructor information in the output section where the
CONSTRUCTORS
command appears. The CONSTRUCTORS
command is
ignored for other object file formats.
The symbol __CTOR_LIST__
marks the start of the global
constructors, and the symbol __CTOR_END__
marks the end.
Similarly, __DTOR_LIST__
and __DTOR_END__
mark
the start and end of the global destructors. The
first word in the list is the number of entries, followed by the address
of each constructor or destructor, followed by a zero word. The
compiler must arrange to actually run the code. For these object file
formats gnu C++ normally calls constructors from a subroutine
__main
; a call to __main
is automatically inserted into
the startup code for main
. gnu C++ normally runs
destructors either by using atexit
, or directly from the function
exit
.
For object file formats such as COFF
or ELF
which support
arbitrary section names, gnu C++ will normally arrange to put the
addresses of global constructors and destructors into the .ctors
and .dtors
sections. Placing the following sequence into your
linker script will build the sort of table which the gnu C++
runtime code expects to see.
__CTOR_LIST__ = .; LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2) *(.ctors) LONG(0) __CTOR_END__ = .; __DTOR_LIST__ = .; LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2) *(.dtors) LONG(0) __DTOR_END__ = .;
If you are using the gnu C++ support for initialization priority,
which provides some control over the order in which global constructors
are run, you must sort the constructors at link time to ensure that they
are executed in the correct order. When using the CONSTRUCTORS
command, use ‘SORT_BY_NAME(CONSTRUCTORS)’ instead. When using the
.ctors
and .dtors
sections, use ‘*(SORT_BY_NAME(.ctors))’ and
‘*(SORT_BY_NAME(.dtors))’ instead of just ‘*(.ctors)’ and
‘*(.dtors)’.
Normally the compiler and linker will handle these issues automatically, and you will not need to concern yourself with them. However, you may need to consider this if you are using C++ and writing your own linker scripts.
The linker will not normally create output sections with no contents. This is for convenience when referring to input sections that may or may not be present in any of the input files. For example:
.foo : { *(.foo) }
will only create a ‘.foo’ section in the output file if there is a ‘.foo’ section in at least one input file, and if the input sections are not all empty. Other link script directives that allocate space in an output section will also create the output section. So too will assignments to dot even if the assignment does not create space, except for ‘. = 0’, ‘. = . + 0’, ‘. = sym’, ‘. = . + sym’ and ‘. = ALIGN (. != 0, expr, 1)’ when ‘sym’ is an absolute symbol of value 0 defined in the script. This allows you to force output of an empty section with ‘. = .’.
The linker will ignore address assignments (see Output Section Address) on discarded output sections, except when the linker script defines symbols in the output section. In that case the linker will obey the address assignments, possibly advancing dot even though the section is discarded.
The special output section name ‘/DISCARD/’ may be used to discard input sections. Any input sections which are assigned to an output section named ‘/DISCARD/’ are not included in the output file.
This can be used to discard input sections marked with the ELF flag
SHF_GNU_RETAIN
, which would otherwise have been saved from linker
garbage collection.
Note, sections that match the ‘/DISCARD/’ output section will be discarded even if they are in an ELF section group which has other members which are not being discarded. This is deliberate. Discarding takes precedence over grouping.
We showed above that the full description of an output section looked like this:
section [address] [(type)] : [AT(lma)] [ALIGN(section_align) | ALIGN_WITH_INPUT] [SUBALIGN(subsection_align)] [constraint] { output-section-command output-section-command ... } [>region] [AT>lma_region] [:phdr :phdr ...] [=fillexp]
We've already described section, address, and output-section-command. In this section we will describe the remaining section attributes.
Each output section may have a type. The type is a keyword in parentheses. The following types are defined:
NOLOAD
READONLY
DSECT
COPY
INFO
OVERLAY
TYPE =
typeSHT_PROGBITS
, SHT_STRTAB
,
SHT_NOTE
, SHT_NOBITS
, SHT_INIT_ARRAY
,
SHT_FINI_ARRAY
, and SHT_PREINIT_ARRAY
are also allowed
for type. It is the user's responsibility to ensure that any
special requirements of the section type are met.
READONLY ( TYPE =
type )
The linker normally sets the attributes of an output section based on the input sections which map into it. You can override this by using the section type. For example, in the script sample below, the ‘ROM’ section is addressed at memory location ‘0’ and does not need to be loaded when the program is run.
SECTIONS { ROM 0 (NOLOAD) : { ... } ... }
Every section has a virtual address (VMA) and a load address (LMA); see
Basic Script Concepts. The virtual address is specified by the
see Output Section Address described earlier. The load address is
specified by the AT
or AT>
keywords. Specifying a load
address is optional.
The AT
keyword takes an expression as an argument. This
specifies the exact load address of the section. The AT>
keyword
takes the name of a memory region as an argument. See MEMORY. The
load address of the section is set to the next free address in the
region, aligned to the section's alignment requirements.
If neither AT
nor AT>
is specified for an allocatable
section, the linker will use the following heuristic to determine the
load address:
This feature is designed to make it easy to build a ROM image. For
example, the following linker script creates three output sections: one
called ‘.text’, which starts at 0x1000
, one called
‘.mdata’, which is loaded at the end of the ‘.text’ section
even though its VMA is 0x2000
, and one called ‘.bss’ to hold
uninitialized data at address 0x3000
. The symbol _data
is
defined with the value 0x2000
, which shows that the location
counter holds the VMA value, not the LMA value.
SECTIONS { .text 0x1000 : { *(.text) _etext = . ; } .mdata 0x2000 : AT ( ADDR (.text) + SIZEOF (.text) ) { _data = . ; *(.data); _edata = . ; } .bss 0x3000 : { _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;} }
The run-time initialization code for use with a program generated with this linker script would include something like the following, to copy the initialized data from the ROM image to its runtime address. Notice how this code takes advantage of the symbols defined by the linker script.
extern char _etext, _data, _edata, _bstart, _bend; char *src = &_etext; char *dst = &_data; /* ROM has data at end of text; copy it. */ while (dst < &_edata) *dst++ = *src++; /* Zero bss. */ for (dst = &_bstart; dst< &_bend; dst++) *dst = 0;
You can increase an output section's alignment by using ALIGN. As an alternative you can enforce that the difference between the VMA and LMA remains intact throughout this output section with the ALIGN_WITH_INPUT attribute.
You can force input section alignment within an output section by using SUBALIGN. The value specified overrides any alignment given by input sections, whether larger or smaller.
You can specify that an output section should only be created if all
of its input sections are read-only or all of its input sections are
read-write by using the keyword ONLY_IF_RO
and
ONLY_IF_RW
respectively.
You can assign a section to a previously defined region of memory by using ‘>region’. See MEMORY.
Here is a simple example:
MEMORY { rom : ORIGIN = 0x1000, LENGTH = 0x1000 } SECTIONS { ROM : { *(.text) } >rom }
You can assign a section to a previously defined program segment by
using ‘:phdr’. See PHDRS. If a section is assigned to
one or more segments, then all subsequent allocated sections will be
assigned to those segments as well, unless they use an explicitly
:
phdr modifier. You can use :NONE
to tell the
linker to not put the section in any segment at all.
Here is a simple example:
PHDRS { text PT_LOAD ; } SECTIONS { .text : { *(.text) } :text }
You can set the fill pattern for an entire section by using
‘=fillexp’. fillexp is an expression
(see Expressions). Any otherwise unspecified regions of memory
within the output section (for example, gaps left due to the required
alignment of input sections) will be filled with the value, repeated as
necessary. If the fill expression is a simple hex number, ie. a string
of hex digit starting with ‘0x’ and without a trailing ‘k’ or ‘M’, then
an arbitrarily long sequence of hex digits can be used to specify the
fill pattern; Leading zeros become part of the pattern too. For all
other cases, including extra parentheses or a unary +
, the fill
pattern is the four least significant bytes of the value of the
expression. In all cases, the number is big-endian.
You can also change the fill value with a FILL
command in the
output section commands; (see Output Section Data).
Here is a simple example:
SECTIONS { .text : { *(.text) } =0x90909090 }
An overlay description provides an easy way to describe sections which are to be loaded as part of a single memory image but are to be run at the same memory address. At run time, some sort of overlay manager will copy the overlaid sections in and out of the runtime memory address as required, perhaps by simply manipulating addressing bits. This approach can be useful, for example, when a certain region of memory is faster than another.
Overlays are described using the OVERLAY
command. The
OVERLAY
command is used within a SECTIONS
command, like an
output section description. The full syntax of the OVERLAY
command is as follows:
OVERLAY [start] : [NOCROSSREFS] [AT ( ldaddr )] { secname1 { output-section-command output-section-command ... } [:phdr...] [=fill] secname2 { output-section-command output-section-command ... } [:phdr...] [=fill] ... } [>region] [:phdr...] [=fill] [,]
Everything is optional except OVERLAY
(a keyword), and each
section must have a name (secname1 and secname2 above). The
section definitions within the OVERLAY
construct are identical to
those within the general SECTIONS
construct (see SECTIONS),
except that no addresses and no memory regions may be defined for
sections within an OVERLAY
.
The comma at the end may be required if a fill is used and the next sections-command looks like a continuation of the expression.
The sections are all defined with the same starting address. The load
addresses of the sections are arranged such that they are consecutive in
memory starting at the load address used for the OVERLAY
as a
whole (as with normal section definitions, the load address is optional,
and defaults to the start address; the start address is also optional,
and defaults to the current value of the location counter).
If the NOCROSSREFS
keyword is used, and there are any
references among the sections, the linker will report an error. Since
the sections all run at the same address, it normally does not make
sense for one section to refer directly to another.
See NOCROSSREFS.
For each section within the OVERLAY
, the linker automatically
provides two symbols. The symbol __load_start_
secname is
defined as the starting load address of the section. The symbol
__load_stop_
secname is defined as the final load address of
the section. Any characters within secname which are not legal
within C identifiers are removed. C (or assembler) code may use these
symbols to move the overlaid sections around as necessary.
At the end of the overlay, the value of the location counter is set to the start address of the overlay plus the size of the largest section.
Here is an example. Remember that this would appear inside a
SECTIONS
construct.
OVERLAY 0x1000 : AT (0x4000) { .text0 { o1/*.o(.text) } .text1 { o2/*.o(.text) } }
This will define both ‘.text0’ and ‘.text1’ to start at
address 0x1000. ‘.text0’ will be loaded at address 0x4000, and
‘.text1’ will be loaded immediately after ‘.text0’. The
following symbols will be defined if referenced: __load_start_text0
,
__load_stop_text0
, __load_start_text1
,
__load_stop_text1
.
C code to copy overlay .text1
into the overlay area might look
like the following.
extern char __load_start_text1, __load_stop_text1; memcpy ((char *) 0x1000, &__load_start_text1, &__load_stop_text1 - &__load_start_text1);
Note that the OVERLAY
command is just syntactic sugar, since
everything it does can be done using the more basic commands. The above
example could have been written identically as follows.
.text0 0x1000 : AT (0x4000) { o1/*.o(.text) } PROVIDE (__load_start_text0 = LOADADDR (.text0)); PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0)); .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) } PROVIDE (__load_start_text1 = LOADADDR (.text1)); PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1)); . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
The linker's default configuration permits allocation of all available
memory. You can override this by using the MEMORY
command.
The MEMORY
command describes the location and size of blocks of
memory in the target. You can use it to describe which memory regions
may be used by the linker, and which memory regions it must avoid. You
can then assign sections to particular memory regions. The linker will
set section addresses based on the memory regions, and will warn about
regions that become too full. The linker will not shuffle sections
around to fit into the available regions.
A linker script may contain many uses of the MEMORY
command,
however, all memory blocks defined are treated as if they were
specified inside a single MEMORY
command. The syntax for
MEMORY
is:
MEMORY { name [(attr)] : ORIGIN = origin, LENGTH = len ... }
The name is a name used in the linker script to refer to the
region. The region name has no meaning outside of the linker script.
Region names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each memory region
must have a distinct name within the MEMORY
command. However you can
add later alias names to existing memory regions with the REGION_ALIAS
command.
The attr string is an optional list of attributes that specify whether to use a particular memory region for an input section which is not explicitly mapped in the linker script. As described in SECTIONS, if you do not specify an output section for some input section, the linker will create an output section with the same name as the input section. If you define region attributes, the linker will use them to select the memory region for the output section that it creates.
The attr string must consist only of the following characters:
If an unmapped section matches any of the listed attributes other than ‘!’, it will be placed in the memory region. The ‘!’ attribute reverses the test for the characters that follow, so that an unmapped section will be placed in the memory region only if it does not match any of the attributes listed afterwards. Thus an attribute string of ‘RW!X’ will match any unmapped section that has either or both of the ‘R’ and ‘W’ attributes, but only as long as the section does not also have the ‘X’ attribute.
The origin is an numerical expression for the start address of
the memory region. The expression must evaluate to a constant and it
cannot involve any symbols. The keyword ORIGIN
may be
abbreviated to org
or o
(but not, for example,
ORG
).
The len is an expression for the size in bytes of the memory
region. As with the origin expression, the expression must
be numerical only and must evaluate to a constant. The keyword
LENGTH
may be abbreviated to len
or l
.
In the following example, we specify that there are two memory regions available for allocation: one starting at ‘0’ for 256 kilobytes, and the other starting at ‘0x40000000’ for four megabytes. The linker will place into the ‘rom’ memory region every section which is not explicitly mapped into a memory region, and is either read-only or executable. The linker will place other sections which are not explicitly mapped into a memory region into the ‘ram’ memory region.
MEMORY { rom (rx) : ORIGIN = 0, LENGTH = 256K ram (!rx) : org = 0x40000000, l = 4M }
Once you define a memory region, you can direct the linker to place specific output sections into that memory region by using the ‘>region’ output section attribute. For example, if you have a memory region named ‘mem’, you would use ‘>mem’ in the output section definition. See Output Section Region. If no address was specified for the output section, the linker will set the address to the next available address within the memory region. If the combined output sections directed to a memory region are too large for the region, the linker will issue an error message.
It is possible to access the origin and length of a memory in an
expression via the ORIGIN(
memory)
and
LENGTH(
memory)
functions:
_fstack = ORIGIN(ram) + LENGTH(ram) - 4;
The ELF object file format uses program headers, also knows as
segments. The program headers describe how the program should be
loaded into memory. You can print them out by using the objdump
program with the ‘-p’ option.
When you run an ELF program on a native ELF system, the system loader reads the program headers in order to figure out how to load the program. This will only work if the program headers are set correctly. This manual does not describe the details of how the system loader interprets program headers; for more information, see the ELF ABI.
The linker will create reasonable program headers by default. However,
in some cases, you may need to specify the program headers more
precisely. You may use the PHDRS
command for this purpose. When
the linker sees the PHDRS
command in the linker script, it will
not create any program headers other than the ones specified.
The linker only pays attention to the PHDRS
command when
generating an ELF output file. In other cases, the linker will simply
ignore PHDRS
.
This is the syntax of the PHDRS
command. The words PHDRS
,
FILEHDR
, AT
, and FLAGS
are keywords.
PHDRS { name type [ FILEHDR ] [ PHDRS ] [ AT ( address ) ] [ FLAGS ( flags ) ] ; }
The name is used only for reference in the SECTIONS
command
of the linker script. It is not put into the output file. Program
header names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each program header
must have a distinct name. The headers are processed in order and it
is usual for them to map to sections in ascending load address order.
Certain program header types describe segments of memory which the system loader will load from the file. In the linker script, you specify the contents of these segments by placing allocatable output sections in the segments. You use the ‘:phdr’ output section attribute to place a section in a particular segment. See Output Section Phdr.
It is normal to put certain sections in more than one segment. This merely implies that one segment of memory contains another. You may repeat ‘:phdr’, using it once for each segment which should contain the section.
If you place a section in one or more segments using ‘:phdr’,
then the linker will place all subsequent allocatable sections which do
not specify ‘:phdr’ in the same segments. This is for
convenience, since generally a whole set of contiguous sections will be
placed in a single segment. You can use :NONE
to override the
default segment and tell the linker to not put the section in any
segment at all.
You may use the FILEHDR
and PHDRS
keywords after
the program header type to further describe the contents of the segment.
The FILEHDR
keyword means that the segment should include the ELF
file header. The PHDRS
keyword means that the segment should
include the ELF program headers themselves. If applied to a loadable
segment (PT_LOAD
), all prior loadable segments must have one of
these keywords.
The type may be one of the following. The numbers indicate the value of the keyword.
PT_NULL
(0)PT_LOAD
(1)PT_DYNAMIC
(2)PT_INTERP
(3)PT_NOTE
(4)PT_SHLIB
(5)PT_PHDR
(6)PT_TLS
(7)You can specify that a segment should be loaded at a particular address
in memory by using an AT
expression. This is identical to the
AT
command used as an output section attribute (see Output Section LMA). The AT
command for a program header overrides the
output section attribute.
The linker will normally set the segment flags based on the sections
which comprise the segment. You may use the FLAGS
keyword to
explicitly specify the segment flags. The value of flags must be
an integer. It is used to set the p_flags
field of the program
header.
Here is an example of PHDRS
. This shows a typical set of program
headers used on a native ELF system.
PHDRS { headers PT_PHDR PHDRS ; interp PT_INTERP ; text PT_LOAD FILEHDR PHDRS ; data PT_LOAD ; dynamic PT_DYNAMIC ; } SECTIONS { . = SIZEOF_HEADERS; .interp : { *(.interp) } :text :interp .text : { *(.text) } :text .rodata : { *(.rodata) } /* defaults to :text */ ... . = . + 0x1000; /* move to a new page in memory */ .data : { *(.data) } :data .dynamic : { *(.dynamic) } :data :dynamic ... }
The linker supports symbol versions when using ELF. Symbol versions are only useful when using shared libraries. The dynamic linker can use symbol versions to select a specific version of a function when it runs a program that may have been linked against an earlier version of the shared library.
You can include a version script directly in the main linker script, or you can supply the version script as an implicit linker script. You can also use the ‘--version-script’ linker option.
The syntax of the VERSION
command is simply
VERSION { version-script-commands }
The format of the version script commands is identical to that used by Sun's linker in Solaris 2.5. The version script defines a tree of version nodes. You specify the node names and interdependencies in the version script. You can specify which symbols are bound to which version nodes, and you can reduce a specified set of symbols to local scope so that they are not globally visible outside of the shared library.
The easiest way to demonstrate the version script language is with a few examples.
VERS_1.1 { global: foo1; local: old*; original*; new*; }; VERS_1.2 { foo2; } VERS_1.1; VERS_2.0 { bar1; bar2; extern "C++" { ns::*; "f(int, double)"; }; } VERS_1.2;
This example version script defines three version nodes. The first version node defined is ‘VERS_1.1’; it has no other dependencies. The script binds the symbol ‘foo1’ to ‘VERS_1.1’. It reduces a number of symbols to local scope so that they are not visible outside of the shared library; this is done using wildcard patterns, so that any symbol whose name begins with ‘old’, ‘original’, or ‘new’ is matched. The wildcard patterns available are the same as those used in the shell when matching filenames (also known as “globbing”). However, if you specify the symbol name inside double quotes, then the name is treated as literal, rather than as a glob pattern.
Next, the version script defines node ‘VERS_1.2’. This node depends upon ‘VERS_1.1’. The script binds the symbol ‘foo2’ to the version node ‘VERS_1.2’.
Finally, the version script defines node ‘VERS_2.0’. This node depends upon ‘VERS_1.2’. The scripts binds the symbols ‘bar1’ and ‘bar2’ are bound to the version node ‘VERS_2.0’.
When the linker finds a symbol defined in a library which is not specifically bound to a version node, it will effectively bind it to an unspecified base version of the library. You can bind all otherwise unspecified symbols to a given version node by using ‘global: *;’ somewhere in the version script. Note that it's slightly crazy to use wildcards in a global spec except on the last version node. Global wildcards elsewhere run the risk of accidentally adding symbols to the set exported for an old version. That's wrong since older versions ought to have a fixed set of symbols.
The names of the version nodes have no specific meaning other than what they might suggest to the person reading them. The ‘2.0’ version could just as well have appeared in between ‘1.1’ and ‘1.2’. However, this would be a confusing way to write a version script.
Node name can be omitted, provided it is the only version node in the version script. Such version script doesn't assign any versions to symbols, only selects which symbols will be globally visible out and which won't.
{ global: foo; bar; local: *; };
When you link an application against a shared library that has versioned symbols, the application itself knows which version of each symbol it requires, and it also knows which version nodes it needs from each shared library it is linked against. Thus at runtime, the dynamic loader can make a quick check to make sure that the libraries you have linked against do in fact supply all of the version nodes that the application will need to resolve all of the dynamic symbols. In this way it is possible for the dynamic linker to know with certainty that all external symbols that it needs will be resolvable without having to search for each symbol reference.
The symbol versioning is in effect a much more sophisticated way of doing minor version checking that SunOS does. The fundamental problem that is being addressed here is that typically references to external functions are bound on an as-needed basis, and are not all bound when the application starts up. If a shared library is out of date, a required interface may be missing; when the application tries to use that interface, it may suddenly and unexpectedly fail. With symbol versioning, the user will get a warning when they start their program if the libraries being used with the application are too old.
There are several GNU extensions to Sun's versioning approach. The first of these is the ability to bind a symbol to a version node in the source file where the symbol is defined instead of in the versioning script. This was done mainly to reduce the burden on the library maintainer. You can do this by putting something like:
__asm__(".symver original_foo,foo@VERS_1.1");
in the C source file. This renames the function ‘original_foo’ to be an alias for ‘foo’ bound to the version node ‘VERS_1.1’. The ‘local:’ directive can be used to prevent the symbol ‘original_foo’ from being exported. A ‘.symver’ directive takes precedence over a version script.
The second GNU extension is to allow multiple versions of the same function to appear in a given shared library. In this way you can make an incompatible change to an interface without increasing the major version number of the shared library, while still allowing applications linked against the old interface to continue to function.
To do this, you must use multiple ‘.symver’ directives in the source file. Here is an example:
__asm__(".symver original_foo,foo@"); __asm__(".symver old_foo,foo@VERS_1.1"); __asm__(".symver old_foo1,foo@VERS_1.2"); __asm__(".symver new_foo,foo@@VERS_2.0");
In this example, ‘foo@’ represents the symbol ‘foo’ bound to the unspecified base version of the symbol. The source file that contains this example would define 4 C functions: ‘original_foo’, ‘old_foo’, ‘old_foo1’, and ‘new_foo’.
When you have multiple definitions of a given symbol, there needs to be some way to specify a default version to which external references to this symbol will be bound. You can do this with the ‘foo@@VERS_2.0’ type of ‘.symver’ directive. You can only declare one version of a symbol as the default in this manner; otherwise you would effectively have multiple definitions of the same symbol.
If you wish to bind a reference to a specific version of the symbol within the shared library, you can use the aliases of convenience (i.e., ‘old_foo’), or you can use the ‘.symver’ directive to specifically bind to an external version of the function in question.
You can also specify the language in the version script:
VERSION extern "lang" { version-script-commands }
The supported ‘lang’s are ‘C’, ‘C++’, and ‘Java’. The linker will iterate over the list of symbols at the link time and demangle them according to ‘lang’ before matching them to the patterns specified in ‘version-script-commands’. The default ‘lang’ is ‘C’.
Demangled names may contains spaces and other special characters. As described above, you can use a glob pattern to match demangled names, or you can use a double-quoted string to match the string exactly. In the latter case, be aware that minor differences (such as differing whitespace) between the version script and the demangler output will cause a mismatch. As the exact string generated by the demangler might change in the future, even if the mangled name does not, you should check that all of your version directives are behaving as you expect when you upgrade.
The syntax for expressions in the linker script language is identical to that of C expressions, except that whitespace is required in some places to resolve syntactic ambiguities. All expressions are evaluated as integers. All expressions are evaluated in the same size, which is 32 bits if both the host and target are 32 bits, and is otherwise 64 bits.
You can use and set symbol values in expressions.
The linker defines several special purpose builtin functions for use in expressions.
As in C, the linker considers an integer beginning with ‘0’ to be octal, and an integer beginning with ‘0x’ or ‘0X’ to be hexadecimal. Alternatively the linker accepts suffixes of ‘h’ or ‘H’ for hexadecimal, ‘o’ or ‘O’ for octal, ‘b’ or ‘B’ for binary and ‘d’ or ‘D’ for decimal. Any integer value without a prefix or a suffix is considered to be decimal.
In addition, you can use the suffixes K
and M
to scale a
constant by
1024
or 1024*1024
respectively. For example, the following
all refer to the same quantity:
_fourk_1 = 4K; _fourk_2 = 4096; _fourk_3 = 0x1000; _fourk_4 = 10000o;
Note - the K
and M
suffixes cannot be used in
conjunction with the base suffixes mentioned above.
It is possible to refer to target-specific constants via the use of
the CONSTANT(
name)
operator, where name is one of:
MAXPAGESIZE
COMMONPAGESIZE
So for example:
.text ALIGN (CONSTANT (MAXPAGESIZE)) : { *(.text) }
will create a text section aligned to the largest page boundary supported by the target.
Unless quoted, symbol names start with a letter, underscore, or period and may include letters, digits, underscores, periods, and hyphens. Unquoted symbol names must not conflict with any keywords. You can specify a symbol which contains odd characters or has the same name as a keyword by surrounding the symbol name in double quotes:
"SECTION" = 9; "with a space" = "also with a space" + 10;
Since symbols can contain many non-alphabetic characters, it is safest to delimit symbols with spaces. For example, ‘A-B’ is one symbol, whereas ‘A - B’ is an expression involving subtraction.
Orphan sections are sections present in the input files which are not explicitly placed into the output file by the linker script. The linker will still copy these sections into the output file by either finding, or creating a suitable output section in which to place the orphaned input section.
If the name of an orphaned input section exactly matches the name of an existing output section, then the orphaned input section will be placed at the end of that output section.
If there is no output section with a matching name then new output sections will be created. Each new output section will have the same name as the orphan section placed within it. If there are multiple orphan sections with the same name, these will all be combined into one new output section.
If new output sections are created to hold orphaned input sections, then the linker must decide where to place these new output sections in relation to existing output sections. On most modern targets, the linker attempts to place orphan sections after sections of the same attribute, such as code vs data, loadable vs non-loadable, etc. If no sections with matching attributes are found, or your target lacks this support, the orphan section is placed at the end of the file.
The command-line options ‘--orphan-handling’ and ‘--unique’ (see Command-line Options) can be used to control which output sections an orphan is placed in.
The special linker variable dot ‘.’ always contains the
current output location counter. Since the .
always refers to a
location in an output section, it may only appear in an expression
within a SECTIONS
command. The .
symbol may appear
anywhere that an ordinary symbol is allowed in an expression.
Assigning a value to .
will cause the location counter to be
moved. This may be used to create holes in the output section. The
location counter may not be moved backwards inside an output section,
and may not be moved backwards outside of an output section if so
doing creates areas with overlapping LMAs.
SECTIONS { output : { file1(.text) . = . + 1000; file2(.text) . += 1000; file3(.text) } = 0x12345678; }
In the previous example, the ‘.text’ section from file1 is located at the beginning of the output section ‘output’. It is followed by a 1000 byte gap. Then the ‘.text’ section from file2 appears, also with a 1000 byte gap following before the ‘.text’ section from file3. The notation ‘= 0x12345678’ specifies what data to write in the gaps (see Output Section Fill).
Note: .
actually refers to the byte offset from the start of the
current containing object. Normally this is the SECTIONS
statement, whose start address is 0, hence .
can be used as an
absolute address. If .
is used inside a section description
however, it refers to the byte offset from the start of that section,
not an absolute address. Thus in a script like this:
SECTIONS { . = 0x100 .text: { *(.text) . = 0x200 } . = 0x500 .data: { *(.data) . += 0x600 } }
The ‘.text’ section will be assigned a starting address of 0x100
and a size of exactly 0x200 bytes, even if there is not enough data in
the ‘.text’ input sections to fill this area. (If there is too
much data, an error will be produced because this would be an attempt to
move .
backwards). The ‘.data’ section will start at 0x500
and it will have an extra 0x600 bytes worth of space after the end of
the values from the ‘.data’ input sections and before the end of
the ‘.data’ output section itself.
Setting symbols to the value of the location counter outside of an output section statement can result in unexpected values if the linker needs to place orphan sections. For example, given the following:
SECTIONS { start_of_text = . ; .text: { *(.text) } end_of_text = . ; start_of_data = . ; .data: { *(.data) } end_of_data = . ; }
If the linker needs to place some input section, e.g. .rodata
,
not mentioned in the script, it might choose to place that section
between .text
and .data
. You might think the linker
should place .rodata
on the blank line in the above script, but
blank lines are of no particular significance to the linker. As well,
the linker doesn't associate the above symbol names with their
sections. Instead, it assumes that all assignments or other
statements belong to the previous output section, except for the
special case of an assignment to .
. I.e., the linker will
place the orphan .rodata
section as if the script was written
as follows:
SECTIONS { start_of_text = . ; .text: { *(.text) } end_of_text = . ; start_of_data = . ; .rodata: { *(.rodata) } .data: { *(.data) } end_of_data = . ; }
This may or may not be the script author's intention for the value of
start_of_data
. One way to influence the orphan section
placement is to assign the location counter to itself, as the linker
assumes that an assignment to .
is setting the start address of
a following output section and thus should be grouped with that
section. So you could write:
SECTIONS { start_of_text = . ; .text: { *(.text) } end_of_text = . ; . = . ; start_of_data = . ; .data: { *(.data) } end_of_data = . ; }
Now, the orphan .rodata
section will be placed between
end_of_text
and start_of_data
.
The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels:
precedence associativity Operators Notes (highest) 1 left ! - ~ (1) 2 left * / % 3 left + - 4 left >> << 5 left == != > < <= >= 6 left & 7 left | 8 left && 9 left || 10 right ? : 11 right &= += -= *= /= (2) (lowest)
Notes: (1) Prefix operators (2) See Assignments.
The linker evaluates expressions lazily. It only computes the value of an expression when absolutely necessary.
The linker needs some information, such as the value of the start address of the first section, and the origins and lengths of memory regions, in order to do any linking at all. These values are computed as soon as possible when the linker reads in the linker script.
However, other values (such as symbol values) are not known or needed until after storage allocation. Such values are evaluated later, when other information (such as the sizes of output sections) is available for use in the symbol assignment expression.
The sizes of sections cannot be known until after allocation, so assignments dependent upon these are not performed until after allocation.
Some expressions, such as those depending upon the location counter ‘.’, must be evaluated during section allocation.
If the result of an expression is required, but the value is not available, then an error results. For example, a script like the following
SECTIONS { .text 9+this_isnt_constant : { *(.text) } }
will cause the error message ‘non constant expression for initial address’.
Addresses and symbols may be section relative, or absolute. A section relative symbol is relocatable. If you request relocatable output using the ‘-r’ option, a further link operation may change the value of a section relative symbol. On the other hand, an absolute symbol will retain the same value throughout any further link operations.
Some terms in linker expressions are addresses. This is true of
section relative symbols and for builtin functions that return an
address, such as ADDR
, LOADADDR
, ORIGIN
and
SEGMENT_START
. Other terms are simply numbers, or are builtin
functions that return a non-address value, such as LENGTH
.
One complication is that unless you set LD_FEATURE ("SANE_EXPR")
(see Miscellaneous Commands), numbers and absolute symbols are treated
differently depending on their location, for compatibility with older
versions of ld
. Expressions appearing outside an output
section definition treat all numbers as absolute addresses.
Expressions appearing inside an output section definition treat
absolute symbols as numbers. If LD_FEATURE ("SANE_EXPR")
is
given, then absolute symbols and numbers are simply treated as numbers
everywhere.
In the following simple example,
SECTIONS { . = 0x100; __executable_start = 0x100; .data : { . = 0x10; __data_start = 0x10; *(.data) } ... }
both .
and __executable_start
are set to the absolute
address 0x100 in the first two assignments, then both .
and
__data_start
are set to 0x10 relative to the .data
section in the second two assignments.
For expressions involving numbers, relative addresses and absolute addresses, ld follows these rules to evaluate terms:
The result section of each sub-expression is as follows:
LD_FEATURE ("SANE_EXPR")
or inside an output section definition
but an absolute address otherwise.
You can use the builtin function ABSOLUTE
to force an expression
to be absolute when it would otherwise be relative. For example, to
create an absolute symbol set to the address of the end of the output
section ‘.data’:
SECTIONS { .data : { *(.data) _edata = ABSOLUTE(.); } }
If ‘ABSOLUTE’ were not used, ‘_edata’ would be relative to the ‘.data’ section.
Using LOADADDR
also forces an expression absolute, since this
particular builtin function returns an absolute address.
The linker script language includes a number of builtin functions for use in linker script expressions.
ABSOLUTE(
exp)
ADDR(
section)
start_of_output_1
, symbol_1
and
symbol_2
are assigned equivalent values, except that
symbol_1
will be relative to the .output1
section while
the other two will be absolute:
SECTIONS { ... .output1 : { start_of_output_1 = ABSOLUTE(.); ... } .output : { symbol_1 = ADDR(.output1); symbol_2 = start_of_output_1; } ... }
ALIGN(
align)
ALIGN(
exp,
align)
.
) or arbitrary expression aligned
to the next align boundary. The single operand ALIGN
doesn't change the value of the location counter—it just does
arithmetic on it. The two operand ALIGN
allows an arbitrary
expression to be aligned upwards (ALIGN(
align)
is
equivalent to ALIGN(ABSOLUTE(.),
align)
).
Here is an example which aligns the output .data
section to the
next 0x2000
byte boundary after the preceding section and sets a
variable within the section to the next 0x8000
boundary after the
input sections:
SECTIONS { ... .data ALIGN(0x2000): { *(.data) variable = ALIGN(0x8000); } ... }
The first use of ALIGN
in this example specifies the location of
a section because it is used as the optional address attribute of
a section definition (see Output Section Address). The second use
of ALIGN
is used to defines the value of a symbol.
The builtin function NEXT
is closely related to ALIGN
.
ALIGNOF(
section)
.output
section is stored as the first
value in that section.
SECTIONS{ ... .output { LONG (ALIGNOF (.output)) ... } ... }
BLOCK(
exp)
ALIGN
, for compatibility with older linker
scripts. It is most often seen when setting the address of an output
section.
DATA_SEGMENT_ALIGN(
maxpagesize,
commonpagesize)
(ALIGN(maxpagesize) + (. & (maxpagesize - 1)))
or
(ALIGN(maxpagesize) + ((. + commonpagesize - 1) & (maxpagesize - commonpagesize)))
depending on whether the latter uses fewer commonpagesize sized pages
for the data segment (area between the result of this expression and
DATA_SEGMENT_END
) than the former or not.
If the latter form is used, it means commonpagesize bytes of runtime
memory will be saved at the expense of up to commonpagesize wasted
bytes in the on-disk file.
This expression can only be used directly in SECTIONS
commands, not in
any output section descriptions and only once in the linker script.
commonpagesize should be less or equal to maxpagesize and should
be the system page size the object wants to be optimized for while still
running on system page sizes up to maxpagesize. Note however
that ‘-z relro’ protection will not be effective if the system
page size is larger than commonpagesize.
Example:
. = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
DATA_SEGMENT_END(
exp)
DATA_SEGMENT_ALIGN
evaluation purposes.
. = DATA_SEGMENT_END(.);
DATA_SEGMENT_RELRO_END(
offset,
exp)
PT_GNU_RELRO
segment when
‘-z relro’ option is used.
When ‘-z relro’ option is not present, DATA_SEGMENT_RELRO_END
does nothing, otherwise DATA_SEGMENT_ALIGN
is padded so that
exp + offset is aligned to the commonpagesize
argument given to DATA_SEGMENT_ALIGN
. If present in the linker
script, it must be placed between DATA_SEGMENT_ALIGN
and
DATA_SEGMENT_END
. Evaluates to the second argument plus any
padding needed at the end of the PT_GNU_RELRO
segment due to
section alignment.
. = DATA_SEGMENT_RELRO_END(24, .);
DEFINED(
symbol)
SECTIONS { ... .text : { begin = DEFINED(begin) ? begin : . ; ... } ... }
LENGTH(
memory)
LOADADDR(
section)
LOG2CEIL(
exp)
LOG2CEIL(0)
returns 0.
MAX(
exp1,
exp2)
MIN(
exp1,
exp2)
NEXT(
exp)
ALIGN(
exp)
; unless you
use the MEMORY
command to define discontinuous memory for the
output file, the two functions are equivalent.
ORIGIN(
memory)
SEGMENT_START(
segment,
default)
SEGMENT_START
with any segment
name.
SIZEOF(
section)
symbol_1
and symbol_2
are assigned identical values:
SECTIONS{ ... .output { .start = . ; ... .end = . ; } symbol_1 = .end - .start ; symbol_2 = SIZEOF(.output); ... }
SIZEOF_HEADERS
When producing an ELF output file, if the linker script uses the
SIZEOF_HEADERS
builtin function, the linker must compute the
number of program headers before it has determined all the section
addresses and sizes. If the linker later discovers that it needs
additional program headers, it will report an error ‘not enough
room for program headers’. To avoid this error, you must avoid using
the SIZEOF_HEADERS
function, or you must rework your linker
script to avoid forcing the linker to use additional program headers, or
you must define the program headers yourself using the PHDRS
command (see PHDRS).
If you specify a linker input file which the linker can not recognize as an object file or an archive file, it will try to read the file as a linker script. If the file can not be parsed as a linker script, the linker will report an error.
An implicit linker script will not replace the default linker script.
Typically an implicit linker script would contain only symbol
assignments, or the INPUT
, GROUP
, or VERSION
commands.
Any input files read because of an implicit linker script will be read at the position in the command line where the implicit linker script was read. This can affect archive searching.
The linker can use dynamically loaded plugins to modify its behavior. For example, the link-time optimization feature that some compilers support is implemented with a linker plugin.
Currently there is only one plugin shipped by default, but more may be added here later.
Originally, static libraries were contained in an archive file consisting just of a collection of relocatable object files. Later they evolved to optionally include a symbol table, to assist in finding the needed objects within a library. There their evolution ended, and dynamic libraries rose to ascendance.
One useful feature of dynamic libraries was that, more than just collecting multiple objects into a single file, they also included a list of their dependencies, such that one could specify just the name of a single dynamic library at link time, and all of its dependencies would be implicitly referenced as well. But static libraries lacked this feature, so if a link invocation was switched from using dynamic libraries to static libraries, the link command would usually fail unless it was rewritten to explicitly list the dependencies of the static library.
The GNU ar utility now supports a --record-libdeps option to embed dependency lists into static libraries as well, and the libdep plugin may be used to read this dependency information at link time. The dependency information is stored as a single string, carrying -l and -L arguments as they would normally appear in a linker command line. As such, the information can be written with any text utility and stored into any archive, even if GNU ar is not being used to create the archive. The information is stored in an archive member named ‘__.LIBDEP’.
For example, given a library libssl.a that depends on another library libcrypto.a which may be found in /usr/local/lib, the ‘__.LIBDEP’ member of libssl.a would contain
-L/usr/local/lib -lcrypto
ld has additional features on some platforms; the following sections describe them. Machines where ld has no additional functionality are not listed.
For the H8/300, ld can perform these global optimizations when you specify the ‘--relax’ command-line option.
jsr
and jmp
instructions whose
targets are within eight bits, and turns them into eight-bit
program-counter relative bsr
and bra
instructions,
respectively.
mov.b
instructions which use the
sixteen-bit absolute address form, but refer to the top
page of memory, and changes them to use the eight-bit address form.
(That is: the linker turns ‘mov.b @
aa:16’ into
‘mov.b @
aa:8’ whenever the address aa is in the
top page of memory).
ld finds all mov
instructions which use the register
indirect with 32-bit displacement addressing mode, but use a small
displacement inside 16-bit displacement range, and changes them to use
the 16-bit displacement form. (That is: the linker turns ‘mov.b
@
d:32,ERx’ into ‘mov.b @
d:16,ERx’
whenever the displacement d is in the 16 bit signed integer
range. Only implemented in ELF-format ld).
band, bclr,
biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor
which use 32 bit and 16 bit absolute address form, but refer to the top
page of memory, and changes them to use the 8 bit address form.
(That is: the linker turns ‘bset #xx:3,@
aa:32’ into
‘bset #xx:3,@
aa:8’ whenever the address aa is in
the top page of memory).
ldc.w, stc.w
instructions which use the
32 bit absolute address form, but refer to the top page of memory, and
changes them to use 16 bit address form.
(That is: the linker turns ‘ldc.w @
aa:32,ccr’ into
‘ldc.w @
aa:16,ccr’ whenever the address aa is in
the top page of memory).
For the Motorola 68HC11, ld can perform these global optimizations when you specify the ‘--relax’ command-line option.
jsr
and jmp
instructions whose
targets are within eight bits, and turns them into eight-bit
program-counter relative bsr
and bra
instructions,
respectively.
ld also looks at all 16-bit extended addressing modes and
transforms them in a direct addressing mode when the address is in
page 0 (between 0 and 0x0ff).
bclr
or
bset
instructions.
For 68HC11 and 68HC12, ld can generate trampoline code to
call a far function using a normal jsr
instruction. The linker
will also change the relocation to some far function to use the
trampoline address instead of the function address. This is typically the
case when a pointer to a function is taken. The pointer will in fact
point to the function trampoline.
For the ARM, ld will generate code stubs to allow functions calls between ARM and Thumb code. These stubs only work with code that has been compiled and assembled with the ‘-mthumb-interwork’ command line option. If it is necessary to link with old ARM object files or libraries, which have not been compiled with the -mthumb-interwork option then the ‘--support-old-code’ command-line switch should be given to the linker. This will make it generate larger stub functions which will work with non-interworking aware ARM code. Note, however, the linker does not support generating stubs for function calls to non-interworking aware Thumb code.
The ‘--thumb-entry’ switch is a duplicate of the generic ‘--entry’ switch, in that it sets the program's starting address. But it also sets the bottom bit of the address, so that it can be branched to using a BX instruction, and the program will start executing in Thumb mode straight away.
The ‘--use-nul-prefixed-import-tables’ switch is specifying, that the import tables idata4 and idata5 have to be generated with a zero element prefix for import libraries. This is the old style to generate import tables. By default this option is turned off.
The ‘--be8’ switch instructs ld to generate BE8 format executables. This option is only valid when linking big-endian objects - ie ones which have been assembled with the -EB option. The resulting image will contain big-endian data and little-endian code.
The ‘R_ARM_TARGET1’ relocation is typically used for entries in the ‘.init_array’ section. It is interpreted as either ‘R_ARM_REL32’ or ‘R_ARM_ABS32’, depending on the target. The ‘--target1-rel’ and ‘--target1-abs’ switches override the default.
The ‘--target2=type’ switch overrides the default definition of the ‘R_ARM_TARGET2’ relocation. Valid values for ‘type’, their meanings, and target defaults are as follows:
The ‘R_ARM_V4BX’ relocation (defined by the ARM AAELF specification) enables objects compiled for the ARMv4 architecture to be interworking-safe when linked with other objects compiled for ARMv4t, but also allows pure ARMv4 binaries to be built from the same ARMv4 objects.
In the latter case, the switch --fix-v4bx must be passed to the
linker, which causes v4t BX rM
instructions to be rewritten as
MOV PC,rM
, since v4 processors do not have a BX
instruction.
In the former case, the switch should not be used, and ‘R_ARM_V4BX’ relocations are ignored.
Replace BX rM
instructions identified by ‘R_ARM_V4BX’
relocations with a branch to the following veneer:
TST rM, #1 MOVEQ PC, rM BX Rn
This allows generation of libraries/applications that work on ARMv4 cores and are still interworking safe. Note that the above veneer clobbers the condition flags, so may cause incorrect program behavior in rare cases.
The ‘--use-blx’ switch enables the linker to use ARM/Thumb BLX instructions (available on ARMv5t and above) in various situations. Currently it is used to perform calls via the PLT from Thumb code using BLX rather than using BX and a mode-switching stub before each PLT entry. This should lead to such calls executing slightly faster.
The ‘--vfp11-denorm-fix’ switch enables a link-time workaround for a bug in certain VFP11 coprocessor hardware, which sometimes allows instructions with denorm operands (which must be handled by support code) to have those operands overwritten by subsequent instructions before the support code can read the intended values.
The bug may be avoided in scalar mode if you allow at least one intervening instruction between a VFP11 instruction which uses a register and another instruction which writes to the same register, or at least two intervening instructions if vector mode is in use. The bug only affects full-compliance floating-point mode: you do not need this workaround if you are using "runfast" mode. Please contact ARM for further details.
If you know you are using buggy VFP11 hardware, you can enable this workaround by specifying the linker option ‘--vfp-denorm-fix=scalar’ if you are using the VFP11 scalar mode only, or ‘--vfp-denorm-fix=vector’ if you are using vector mode (the latter also works for scalar code). The default is ‘--vfp-denorm-fix=none’.
If the workaround is enabled, instructions are scanned for potentially-troublesome sequences, and a veneer is created for each such sequence which may trigger the erratum. The veneer consists of the first instruction of the sequence and a branch back to the subsequent instruction. The original instruction is then replaced with a branch to the veneer. The extra cycles required to call and return from the veneer are sufficient to avoid the erratum in both the scalar and vector cases.
The ‘--fix-arm1176’ switch enables a link-time workaround for an erratum in certain ARM1176 processors. The workaround is enabled by default if you are targeting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled unconditionally by specifying ‘--no-fix-arm1176’.
Further information is available in the “ARM1176JZ-S and ARM1176JZF-S Programmer Advice Notice” available on the ARM documentation website at: http://infocenter.arm.com/.
The ‘--fix-stm32l4xx-629360’ switch enables a link-time workaround for a bug in the bus matrix / memory controller for some of the STM32 Cortex-M4 based products (STM32L4xx). When accessing off-chip memory via the affected bus for bus reads of 9 words or more, the bus can generate corrupt data and/or abort. These are only core-initiated accesses (not DMA), and might affect any access: integer loads such as LDM, POP and floating-point loads such as VLDM, VPOP. Stores are not affected.
The bug can be avoided by splitting memory accesses into the necessary chunks to keep bus reads below 8 words.
The workaround is not enabled by default, this is equivalent to use ‘--fix-stm32l4xx-629360=none’. If you know you are using buggy STM32L4xx hardware, you can enable the workaround by specifying the linker option ‘--fix-stm32l4xx-629360’, or the equivalent ‘--fix-stm32l4xx-629360=default’.
If the workaround is enabled, instructions are scanned for potentially-troublesome sequences, and a veneer is created for each such sequence which may trigger the erratum. The veneer consists in a replacement sequence emulating the behaviour of the original one and a branch back to the subsequent instruction. The original instruction is then replaced with a branch to the veneer.
The workaround does not always preserve the memory access order for the LDMDB instruction, when the instruction loads the PC.
The workaround is not able to handle problematic instructions when they are in the middle of an IT block, since a branch is not allowed there. In that case, the linker reports a warning and no replacement occurs.
The workaround is not able to replace problematic instructions with a PC-relative branch instruction if the ‘.text’ section is too large. In that case, when the branch that replaces the original code cannot be encoded, the linker reports a warning and no replacement occurs.
The --no-enum-size-warning switch prevents the linker from warning when linking object files that specify incompatible EABI enumeration size attributes. For example, with this switch enabled, linking of an object file using 32-bit enumeration values with another using enumeration values fitted into the smallest possible space will not be diagnosed.
The --no-wchar-size-warning switch prevents the linker from
warning when linking object files that specify incompatible EABI
wchar_t
size attributes. For example, with this switch enabled,
linking of an object file using 32-bit wchar_t
values with another
using 16-bit wchar_t
values will not be diagnosed.
The ‘--pic-veneer’ switch makes the linker use PIC sequences for ARM/Thumb interworking veneers, even if the rest of the binary is not PIC. This avoids problems on uClinux targets where ‘--emit-relocs’ is used to generate relocatable binaries.
The linker will automatically generate and insert small sequences of code into a linked ARM ELF executable whenever an attempt is made to perform a function call to a symbol that is too far away. The placement of these sequences of instructions - called stubs - is controlled by the command-line option --stub-group-size=N. The placement is important because a poor choice can create a need for duplicate stubs, increasing the code size. The linker will try to group stubs together in order to reduce interruptions to the flow of code, but it needs guidance as to how big these groups should be and where they should be placed.
The value of ‘N’, the parameter to the --stub-group-size= option controls where the stub groups are placed. If it is negative then all stubs are placed after the first branch that needs them. If it is positive then the stubs can be placed either before or after the branches that need them. If the value of ‘N’ is 1 (either +1 or -1) then the linker will choose exactly where to place groups of stubs, using its built in heuristics. A value of ‘N’ greater than 1 (or smaller than -1) tells the linker that a single group of stubs can service at most ‘N’ bytes from the input sections.
The default, if --stub-group-size= is not specified, is ‘N = +1’.
Farcalls stubs insertion is fully supported for the ARM-EABI target only, because it relies on object files properties not present otherwise.
The ‘--fix-cortex-a8’ switch enables a link-time workaround for an erratum in certain Cortex-A8 processors. The workaround is enabled by default if you are targeting the ARM v7-A architecture profile. It can be enabled otherwise by specifying ‘--fix-cortex-a8’, or disabled unconditionally by specifying ‘--no-fix-cortex-a8’.
The erratum only affects Thumb-2 code. Please contact ARM for further details.
The ‘--fix-cortex-a53-835769’ switch enables a link-time workaround for erratum 835769 present on certain early revisions of Cortex-A53 processors. The workaround is disabled by default. It can be enabled by specifying ‘--fix-cortex-a53-835769’, or disabled unconditionally by specifying ‘--no-fix-cortex-a53-835769’.
Please contact ARM for further details.
The ‘--no-merge-exidx-entries’ switch disables the merging of adjacent exidx entries in debuginfo.
The ‘--long-plt’ option enables the use of 16 byte PLT entries which support up to 4Gb of code. The default is to use 12 byte PLT entries which only support 512Mb of code.
The ‘--no-apply-dynamic-relocs’ option makes AArch64 linker do not apply link-time values for dynamic relocations.
All SG veneers are placed in the special output section .gnu.sgstubs
.
Its start address must be set, either with the command-line option
‘--section-start’ or in a linker script, to indicate where to place these
veneers in memory.
The ‘--cmse-implib’ option requests that the import libraries specified by the ‘--out-implib’ and ‘--in-implib’ options are secure gateway import libraries, suitable for linking a non-secure executable against secure code as per ARMv8-M Security Extensions.
The ‘--in-implib=file’ specifies an input import library whose symbols must keep the same address in the executable being produced. A warning is given if no ‘--out-implib’ is given but new symbols have been introduced in the executable that should be listed in its import library. Otherwise, if ‘--out-implib’ is specified, the symbols are added to the output import library. A warning is also given if some symbols present in the input import library have disappeared from the executable. This option is only effective for Secure Gateway import libraries, ie. when ‘--cmse-implib’ is specified.
When generating a shared library, ld will by default generate import stubs suitable for use with a single sub-space application. The ‘--multi-subspace’ switch causes ld to generate export stubs, and different (larger) import stubs suitable for use with multiple sub-spaces.
Long branch stubs and import/export stubs are placed by ld in stub sections located between groups of input sections. ‘--stub-group-size’ specifies the maximum size of a group of input sections handled by one stub section. Since branch offsets are signed, a stub section may serve two groups of input sections, one group before the stub section, and one group after it. However, when using conditional branches that require stubs, it may be better (for branch prediction) that stub sections only serve one group of input sections. A negative value for ‘N’ chooses this scheme, ensuring that branches to stubs always use a negative offset. Two special values of ‘N’ are recognized, ‘1’ and ‘-1’. These both instruct ld to automatically size input section groups for the branch types detected, with the same behaviour regarding stub placement as other positive or negative values of ‘N’ respectively.
Note that ‘--stub-group-size’ does not split input sections. A single input section larger than the group size specified will of course create a larger group (of one section). If input sections are too large, it may not be possible for a branch to reach its stub.
The ‘--got=type’ option lets you choose the GOT generation scheme. The choices are ‘single’, ‘negative’, ‘multigot’ and ‘target’. When ‘target’ is selected the linker chooses the default GOT generation scheme for the current target. ‘single’ tells the linker to generate a single GOT with entries only at non-negative offsets. ‘negative’ instructs the linker to generate a single GOT with entries at both negative and positive offsets. Not all environments support such GOTs. ‘multigot’ allows the linker to generate several GOTs in the output file. All GOT references from a single input object file access the same GOT, but references from different input object files might access different GOTs. Not all environments support such GOTs.
ld
and MMIXFor MMIX, there is a choice of generating ELF
object files or
mmo
object files when linking. The simulator mmix
understands the mmo
format. The binutils objcopy
utility
can translate between the two formats.
There is one special section, the ‘.MMIX.reg_contents’ section.
Contents in this section is assumed to correspond to that of global
registers, and symbols referring to it are translated to special symbols,
equal to registers. In a final link, the start address of the
‘.MMIX.reg_contents’ section corresponds to the first allocated
global register multiplied by 8. Register $255
is not included in
this section; it is always set to the program entry, which is at the
symbol Main
for mmo
files.
Global symbols with the prefix __.MMIX.start.
, for example
__.MMIX.start..text
and __.MMIX.start..data
are special.
The default linker script uses these to set the default start address
of a section.
Initial and trailing multiples of zero-valued 32-bit words in a section, are left out from an mmo file.
ld
and MSP430For the MSP430 it is possible to select the MPU architecture. The flag ‘-m [mpu type]’ will select an appropriate linker script for selected MPU type. (To get a list of known MPUs just pass ‘-m help’ option to the linker).
The linker will recognize some extra sections which are MSP430 specific:
‘
.vectors’
‘
.bootloader’
‘
.infomem’
‘
.infomemnobits’
‘
.noinit’
The last two sections are used by gcc.
--code-region
and --data-region
options.
This is useful if you are compiling and linking using a single call to the GCC
wrapper, and want to compile the source files using -m[code,data]-region but
not transform the sections for prebuilt libraries and objects.
Branches on PowerPC processors are limited to a signed 26-bit displacement, which may result in ld giving ‘relocation truncated to fit’ errors with very large programs. ‘--relax’ enables the generation of trampolines that can access the entire 32-bit address space. These trampolines are inserted at section boundaries, so may not themselves be reachable if an input section exceeds 33M in size. You may combine ‘-r’ and ‘--relax’ to add trampolines in a partial link. In that case both branches to undefined symbols and inter-section branches are also considered potentially out of range, and trampolines inserted.
.plt
must change because the new secure PLT is an initialized
section while the old PLT is uninitialized. The reason for the
.got
change is more subtle: The new placement allows
.got
to be read-only in applications linked with
‘-z relro -z now’. However, this placement means that
.sdata
cannot always be used in shared libraries, because the
PowerPC ABI accesses .sdata
in shared libraries from the GOT
pointer. ‘--sdata-got’ forces the old GOT placement. PowerPC
GCC doesn't use .sdata
in shared libraries, so this option is
really only useful for other compilers that may do so.
Note that ‘--stub-group-size’ does not split input sections. A single input section larger than the group size specified will of course create a larger group (of one section). If input sections are too large, it may not be possible for a branch to reach its stub.
__tls_get_addr
for a given symbol to be resolved by the special
stub without calling in to glibc. By default the linker enables
generation of the stub when glibc advertises the availability of
__tls_get_addr_opt.
Using --tls-get-addr-optimize with an older glibc won't do
much besides slow down your applications, but may be useful if linking
an application against an older glibc with the expectation that it
will normally be used on systems having a newer glibc.
--tls-get-addr-regsave forces generation of a stub that saves
and restores volatile registers around the call into glibc. Normally,
this is done when the linker detects a call to __tls_get_addr_desc.
Such calls then go via the register saving stub to __tls_get_addr_opt.
--no-tls-get-addr-regsave disables generation of the
register saves.
.opd
section entries
corresponding to deleted link-once functions, or functions removed by
the action of ‘--gc-sections’ or linker script /DISCARD/
.
Use this option to disable .opd
optimization.
.opd
entries spaced 16 bytes apart, overlapping the third word,
the static chain pointer (unused in C) with the first word of the next
entry. This option expands such entries to the full 24 bytes.
.toc
section
entries. Such entries are detected by examining relocations that
reference the TOC in code sections. A reloc in a deleted code section
marks a TOC word as unneeded, while a reloc in a kept code section
marks a TOC word as needed. Since the TOC may reference itself, TOC
relocs are also examined. TOC words marked as both needed and
unneeded will of course be kept. TOC words without any referencing
reloc are assumed to be part of a multi-word entry, and are kept or
discarded as per the nearest marked preceding word. This works
reliably for compiler generated code, but may be incorrect if assembly
code is used to insert TOC entries. Use this option to disable the
optimization.
R_PPC64_PLTSEQ
, R_PPC64_PLTCALL
,
R_PPC64_PLT16_HA
and R_PPC64_PLT16_LO_DS
relocations by
a number of nop
s and a direct call when the function is defined
locally and can't be overridden by some other definition. This option
disables that optimization.
-mcmodel=medium
or
-mcmodel=large
, PowerPC64 GCC generates code for a TOC model
where TOC
entries are accessed with a 16-bit offset from r2. This limits the
total TOC size to 64K. PowerPC64 ld extends this limit by
grouping code sections such that each group uses less than 64K for its
TOC entries, then inserts r2 adjusting stubs between inter-group
calls. ld does not split apart input sections, so cannot
help if a single input file has a .toc
section that exceeds
64K, most likely from linking multiple files with ld -r.
Use this option to turn off this feature.
.init
or .fini
are
placed first, followed by TOC sections referenced by code generated
with PowerPC64 gcc's -mcmodel=small
, and lastly TOC sections
referenced only by code generated with PowerPC64 gcc's
-mcmodel=medium
or -mcmodel=large
options. Doing this
results in better TOC grouping for multi-TOC. Use this option to turn
off this feature.
--plt-align=
. A negative value may be
specified to pad PLT call stubs so that they do not cross the
specified power of two boundary (or the minimum number of boundaries
if a PLT stub is so large that it must cross a boundary). By default
PLT call stubs are aligned to 32-byte boundaries.
ld
defaults to not loading the static
chain since there is never any need to do so on a PLT call.
ld
looks for calls to commonly used functions that create threads, and if
seen, adds the necessary barriers. Use these options to change the
default behaviour.
@notoc
PLT calls where r2
is not known. The
power10 notoc stubs are smaller and faster, so are preferred for
power10. --power10-stubs and --no-power10-stubs
allow you to override the linker's selection of stub instructions.
--power10-stubs=auto allows the user to select the default
auto mode.
__stack_<function_name>
for global
functions, and __stack_<number>_<function_name>
for static
functions. <number>
is the section id in hex. The value of
such symbols is the stack requirement for the corresponding function.
The symbol size will be zero, type STT_NOTYPE
, binding
STB_LOCAL
, and section SHN_ABS
.
The ‘--format’ switch allows selection of one of the various TI COFF versions. The latest of this writing is 2; versions 0 and 1 are also supported. The TI COFF versions also vary in header byte-order format; ld will read any version or byte order, but the output header format depends on the default specified by the specific target.
This section describes some of the win32 specific ld issues. See Command-line Options for detailed description of the command-line options mentioned here.
When auto-export is in operation, ld will export all the non-local (global and common) symbols it finds in a DLL, with the exception of a few symbols known to belong to the system's runtime and libraries. As it will often not be desirable to export all of a DLL's symbols, which may include private functions that are not part of any public interface, the command-line options listed above may be used to filter symbols out from the list for exporting. The ‘--output-def’ option can be used in order to see the final list of exported symbols with all exclusions taken into effect.
If ‘--export-all-symbols’ is not given explicitly on the command line, then the default auto-export behavior will be disabled if either of the following are true:
gcc -o <output> <objectfiles> <dll name>.def
Using a DEF file turns off the normal auto-export behavior, unless the ‘--export-all-symbols’ option is also used.
Here is an example of a DEF file for a shared library called ‘xyz.dll’:
LIBRARY "xyz.dll" BASE=0x20000000 EXPORTS foo bar _bar = bar another_foo = abc.dll.afoo var1 DATA doo = foo == foo2 eoo DATA == var1
This example defines a DLL with a non-default base address and seven
symbols in the export table. The third exported symbol _bar
is an
alias for the second. The fourth symbol, another_foo
is resolved
by "forwarding" to another module and treating it as an alias for
afoo
exported from the DLL ‘abc.dll’. The final symbol
var1
is declared to be a data object. The ‘doo’ symbol in
export library is an alias of ‘foo’, which gets the string name
in export table ‘foo2’. The ‘eoo’ symbol is an data export
symbol, which gets in export table the name ‘var1’.
The optional LIBRARY <name>
command indicates the internal
name of the output DLL. If ‘<name>’ does not include a suffix,
the default library suffix, ‘.DLL’ is appended.
When the .DEF file is used to build an application, rather than a
library, the NAME <name>
command should be used instead of
LIBRARY
. If ‘<name>’ does not include a suffix, the default
executable suffix, ‘.EXE’ is appended.
With either LIBRARY <name>
or NAME <name>
the optional
specification BASE = <number>
may be used to specify a
non-default base address for the image.
If neither LIBRARY <name>
nor NAME <name>
is specified,
or they specify an empty string, the internal name is the same as the
filename specified on the command line.
The complete specification of an export symbol is:
EXPORTS ( ( ( <name1> [ = <name2> ] ) | ( <name1> = <module-name> . <external-name>)) [ @ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *
Declares ‘<name1>’ as an exported symbol from the DLL, or declares ‘<name1>’ as an exported alias for ‘<name2>’; or declares ‘<name1>’ as a "forward" alias for the symbol ‘<external-name>’ in the DLL ‘<module-name>’. Optionally, the symbol may be exported by the specified ordinal ‘<integer>’ alias. The optional ‘<name3>’ is the to be used string in import/export table for the symbol.
The optional keywords that follow the declaration indicate:
NONAME
: Do not put the symbol name in the DLL's export table. It
will still be exported by its ordinal alias (either the value specified
by the .def specification or, otherwise, the value assigned by the
linker). The symbol name, however, does remain visible in the import
library (if any), unless PRIVATE
is also specified.
DATA
: The symbol is a variable or object, rather than a function.
The import lib will export only an indirect reference to foo
as
the symbol _imp__foo
(ie, foo
must be resolved as
*_imp__foo
).
CONSTANT
: Like DATA
, but put the undecorated foo
as
well as _imp__foo
into the import library. Both refer to the
read-only import address table's pointer to the variable, not to the
variable itself. This can be dangerous. If the user code fails to add
the dllimport
attribute and also fails to explicitly add the
extra indirection that the use of the attribute enforces, the
application will behave unexpectedly.
PRIVATE
: Put the symbol in the DLL's export table, but do not put
it into the static import library used to resolve imports at link time. The
symbol can still be imported using the LoadLibrary/GetProcAddress
API at runtime or by using the GNU ld extension of linking directly to
the DLL without an import library.
See ld/deffilep.y in the binutils sources for the full specification of other DEF file statements
While linking a shared dll, ld is able to create a DEF file
with the ‘--output-def <file>’ command-line option.
__declspec(dllexport) int a_variable __declspec(dllexport) void a_function(int with_args)
All such symbols will be exported from the DLL. If, however, any of the object files in the DLL contain symbols decorated in this way, then the normal auto-export behavior is disabled, unless the ‘--export-all-symbols’ option is also used.
Note that object files that wish to access these symbols must not decorate them with dllexport. Instead, they should use dllimport, instead:
__declspec(dllimport) int a_variable __declspec(dllimport) void a_function(int with_args)
This complicates the structure of library header files, because when included by the library itself the header must declare the variables and functions as dllexport, but when included by client code the header must declare them as dllimport. There are a number of idioms that are typically used to do this; often client code can omit the __declspec() declaration completely. See ‘--enable-auto-import’ and ‘automatic data imports’ for more information.
auto-import of variables does not always work flawlessly without additional assistance. Sometimes, you will see this message
"variable '<var>' can't be auto-imported. Please read the
documentation for ld's --enable-auto-import
for details."
The ‘--enable-auto-import’ documentation explains why this error occurs, and several methods that can be used to overcome this difficulty. One of these methods is the runtime pseudo-relocs feature, described below.
For complex variables imported from DLLs (such as structs or classes), object files typically contain a base address for the variable and an offset (addend) within the variable–to specify a particular field or public member, for instance. Unfortunately, the runtime loader used in win32 environments is incapable of fixing these references at runtime without the additional information supplied by dllimport/dllexport decorations. The standard auto-import feature described above is unable to resolve these references.
The ‘--enable-runtime-pseudo-relocs’ switch allows these references to be resolved without error, while leaving the task of adjusting the references themselves (with their non-zero addends) to specialized code provided by the runtime environment. Recent versions of the cygwin and mingw environments and compilers provide this runtime support; older versions do not. However, the support is only necessary on the developer's platform; the compiled result will run without error on an older system.
‘--enable-runtime-pseudo-relocs’ is not the default; it must be explicitly enabled as needed.
Linking directly to a dll uses no extra command-line switches other than ‘-L’ and ‘-l’, because ld already searches for a number of names to match each library. All that is needed from the developer's perspective is an understanding of this search, in order to force ld to select the dll instead of an import library.
For instance, when ld is called with the argument ‘-lxxx’ it will attempt to find, in the first directory of its search path,
libxxx.dll.a xxx.dll.a libxxx.a xxx.lib libxxx.lib cygxxx.dll (*) libxxx.dll xxx.dll
before moving on to the next directory in the search path.
(*) Actually, this is not ‘cygxxx.dll’ but in fact is ‘<prefix>xxx.dll’, where ‘<prefix>’ is set by the ld option ‘--dll-search-prefix=<prefix>’. In the case of cygwin, the standard gcc spec file includes ‘--dll-search-prefix=cyg’, so in effect we actually search for ‘cygxxx.dll’.
Other win32-based unix environments, such as mingw or pw32, may use other ‘<prefix>’es, although at present only cygwin makes use of this feature. It was originally intended to help avoid name conflicts among dll's built for the various win32/un*x environments, so that (for example) two versions of a zlib dll could coexist on the same machine.
The generic cygwin/mingw path layout uses a ‘bin’ directory for applications and dll's and a ‘lib’ directory for the import libraries (using cygwin nomenclature):
bin/ cygxxx.dll lib/ libxxx.dll.a (in case of dll's) libxxx.a (in case of static archive)
Linking directly to a dll without using the import library can be done two ways:
1. Use the dll directly by adding the ‘bin’ path to the link line
gcc -Wl,-verbose -o a.exe -L../bin/ -lxxx
However, as the dll's often have version numbers appended to their names (‘cygncurses-5.dll’) this will often fail, unless one specifies ‘-L../bin -lncurses-5’ to include the version. Import libs are generally not versioned, and do not have this difficulty.
2. Create a symbolic link from the dll to a file in the ‘lib’ directory according to the above mentioned search pattern. This should be used to avoid unwanted changes in the tools needed for making the app/dll.
ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
Then you can link without any make environment changes.
gcc -Wl,-verbose -o a.exe -L../lib/ -lxxx
This technique also avoids the version number problems, because the following is perfectly legal
bin/ cygxxx-5.dll lib/ libxxx.dll.a -> ../bin/cygxxx-5.dll
Linking directly to a dll without using an import lib will work even when auto-import features are exercised, and even when ‘--enable-runtime-pseudo-relocs’ is used.
Given the improvements in speed and memory usage, one might justifiably wonder why import libraries are used at all. There are three reasons:
1. Until recently, the link-directly-to-dll functionality did not work with auto-imported data.
2. Sometimes it is necessary to include pure static objects within the import library (which otherwise contains only bfd's for indirection symbols that point to the exports of a dll). Again, the import lib for the cygwin kernel makes use of this ability, and it is not possible to do this without an import lib.
3. Symbol aliases can only be resolved using an import lib. This is critical when linking against OS-supplied dll's (eg, the win32 API) in which symbols are usually exported as undecorated aliases of their stdcall-decorated assembly names.
So, import libs are not going away. But the ability to replace
true import libs with a simple symbolic link to (or a copy of)
a dll, in many cases, is a useful addition to the suite of tools
binutils makes available to the win32 developer. Given the
massive improvements in memory requirements during linking, storage
requirements, and linking speed, we expect that many developers
will soon begin to use this feature whenever possible.
LIBRARY "xyz.dll" BASE=0x61000000 EXPORTS foo _foo = foo
The line ‘_foo = foo’ maps the symbol ‘foo’ to ‘_foo’.
Another method for creating a symbol alias is to create it in the source code using the "weak" attribute:
void foo () { /* Do something. */; } void _foo () __attribute__ ((weak, alias ("foo")));
See the gcc manual for more information about attributes and weak
symbols.
LIBRARY "xyz.dll" BASE=0x61000000 EXPORTS _foo = foo
The line ‘_foo = foo’ maps the exported symbol ‘foo’ to ‘_foo’.
Note: using a DEF file disables the default auto-export behavior, unless the ‘--export-all-symbols’ command-line option is used. If, however, you are trying to rename symbols, then you should list all desired exports in the DEF file, including the symbols that are not being renamed, and do not use the ‘--export-all-symbols’ option. If you list only the renamed symbols in the DEF file, and use ‘--export-all-symbols’ to handle the other symbols, then the both the new names and the original names for the renamed symbols will be exported. In effect, you'd be aliasing those symbols, not renaming them, which is probably not what you wanted.
ld
and Xtensa ProcessorsThe default ld behavior for Xtensa processors is to interpret
SECTIONS
commands so that lists of explicitly named sections in a
specification with a wildcard file will be interleaved when necessary to
keep literal pools within the range of PC-relative load offsets. For
example, with the command:
SECTIONS { .text : { *(.literal .text) } }
ld may interleave some of the .literal
and .text
sections from different object files to ensure that the
literal pools are within the range of PC-relative load offsets. A valid
interleaving might place the .literal
sections from an initial
group of files followed by the .text
sections of that group of
files. Then, the .literal
sections from the rest of the files
and the .text
sections from the rest of the files would follow.
Relaxation is enabled by default for the Xtensa version of ld and
provides two important link-time optimizations. The first optimization
is to combine identical literal values to reduce code size. A redundant
literal will be removed and all the L32R
instructions that use it
will be changed to reference an identical literal, as long as the
location of the replacement literal is within the offset range of all
the L32R
instructions. The second optimization is to remove
unnecessary overhead from assembler-generated “longcall” sequences of
L32R
/CALLX
n when the target functions are within
range of direct CALL
n instructions.
For each of these cases where an indirect call sequence can be optimized
to a direct call, the linker will change the CALLX
n
instruction to a CALL
n instruction, remove the L32R
instruction, and remove the literal referenced by the L32R
instruction if it is not used for anything else. Removing the
L32R
instruction always reduces code size but can potentially
hurt performance by changing the alignment of subsequent branch targets.
By default, the linker will always preserve alignments, either by
switching some instructions between 24-bit encodings and the equivalent
density instructions or by inserting a no-op in place of the L32R
instruction that was removed. If code size is more important than
performance, the --size-opt option can be used to prevent the
linker from widening density instructions or inserting no-ops, except in
a few cases where no-ops are required for correctness.
The following Xtensa-specific command-line options can be used to control the linker:
.xtensa.info
section
of the first input object.
A warning is issued if ABI tags of input objects do not match each other
or the chosen output object ABI.
The linker accesses object and archive files using the BFD libraries.
These libraries allow the linker to use the same routines to operate on
object files whatever the object file format. A different object file
format can be supported simply by creating a new BFD back end and adding
it to the library. To conserve runtime memory, however, the linker and
associated tools are usually configured to support only a subset of the
object file formats available. You can use objdump -i
(see objdump) to
list all the formats available for your configuration.
As with most implementations, BFD is a compromise between several conflicting requirements. The major factor influencing BFD design was efficiency: any time used converting between formats is time which would not have been spent had BFD not been involved. This is partly offset by abstraction payback; since BFD simplifies applications and back ends, more time and care may be spent optimizing algorithms for a greater speed.
One minor artifact of the BFD solution which you should bear in mind is the potential for information loss. There are two places where useful information can be lost using the BFD mechanism: during conversion and during output. See BFD information loss.
When an object file is opened, BFD subroutines automatically determine the format of the input object file. They then build a descriptor in memory with pointers to routines that will be used to access elements of the object file's data structures.
As different information from the object files is required, BFD reads from different sections of the file and processes them. For example, a very common operation for the linker is processing symbol tables. Each BFD back end provides a routine for converting between the object file's representation of symbols and an internal canonical format. When the linker asks for the symbol table of an object file, it calls through a memory pointer to the routine from the relevant BFD back end which reads and converts the table into a canonical form. The linker then operates upon the canonical form. When the link is finished and the linker writes the output file's symbol table, another BFD back end routine is called to take the newly created symbol table and convert it into the chosen output format.
Information can be lost during output. The output formats
supported by BFD do not provide identical facilities, and
information which can be described in one form has nowhere to go in
another format. One example of this is alignment information in
b.out
. There is nowhere in an a.out
format file to store
alignment information on the contained data, so when a file is linked
from b.out
and an a.out
image is produced, alignment
information will not propagate to the output file. (The linker will
still use the alignment information internally, so the link is performed
correctly).
Another example is COFF section names. COFF files may contain an
unlimited number of sections, each one with a textual section name. If
the target of the link is a format which does not have many sections (e.g.,
a.out
) or has sections without names (e.g., the Oasys format), the
link cannot be done simply. You can circumvent this problem by
describing the desired input-to-output section mapping with the linker command
language.
Information can be lost during canonicalization. The BFD internal canonical form of the external formats is not exhaustive; there are structures in input formats for which there is no direct representation internally. This means that the BFD back ends cannot maintain all possible data richness through the transformation between external to internal and back to external formats.
This limitation is only a problem when an application reads one
format and writes another. Each BFD back end is responsible for
maintaining as much data as possible, and the internal BFD
canonical form has structures which are opaque to the BFD core,
and exported only to the back ends. When a file is read in one format,
the canonical form is generated for BFD and the application. At the
same time, the back end saves away any information which may otherwise
be lost. If the data is then written back in the same format, the back
end routine will be able to use the canonical form provided by the
BFD core as well as the information it prepared earlier. Since
there is a great deal of commonality between back ends,
there is no information lost when
linking or copying big endian COFF to little endian COFF, or a.out
to
b.out
. When a mixture of formats is linked, the information is
only lost from the files whose format differs from the destination.
The greatest potential for loss of information occurs when there is the least overlap between the information provided by the source format, that stored by the canonical format, and that needed by the destination format. A brief description of the canonical form may help you understand which kinds of data you can count on preserving across conversions.
ZMAGIC
file would have both the demand pageable bit and the write protected
text bit set. The byte order of the target is stored on a per-file
basis, so that big- and little-endian object files may be used with one
another.
ld
can
operate on a collection of symbols of wildly different formats without
problems.
Normal global and simple local symbols are maintained on output, so an
output file (no matter its format) will retain symbols pointing to
functions and to global, static, and common variables. Some symbol
information is not worth retaining; in a.out
, type information is
stored in the symbol table as long symbol names. This information would
be useless to most COFF debuggers; the linker has command-line switches
to allow users to throw it away.
There is one word of type information within the symbol, so if the
format supports symbol type information within symbols (for example, COFF,
Oasys) and the type is simple enough to fit within one word
(nearly everything but aggregates), the information will be preserved.
Your bug reports play an essential role in making ld reliable.
Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case the principal function of a bug report is to help the entire community by making the next version of ld work better. Bug reports are your contribution to the maintenance of ld.
In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.
If you are not sure whether you have found a bug, here are some guidelines:
A number of companies and individuals offer support for gnu products. If you obtained ld from a support organization, we recommend you contact that organization first.
You can find contact information for many support companies and individuals in the file etc/SERVICE in the gnu Emacs distribution.
Otherwise, send bug reports for ld to https://sourceware.org/bugzilla/.
The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and assume that some details do not matter. Thus, you might assume that the name of a symbol you use in an example does not matter. Well, probably it does not, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the linker into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us. Therefore, always write your bug reports on the assumption that the bug has not been reported previously.
Sometimes people give a few sketchy facts and ask, “Does this ring a bell?” This cannot help us fix a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with.
To enable us to fix the bug, you should include all these things:
Without this, we will not know whether there is any point in looking for the bug in the current version of ld.
BFD
library.
gcc-2.7
”.
If we were to try to guess the arguments, we would probably guess wrong and then we might not encounter the bug.
If the source files were assembled using gas
or compiled using
gcc
, then it may be OK to send the source files rather than the
object files. In this case, be sure to say exactly what version of
gas
or gcc
was used to produce the object files. Also say
how gas
or gcc
were configured.
Of course, if the bug is that ld gets a fatal signal, then we will certainly notice it. But if the bug is incorrect output, we might not notice unless it is glaringly wrong. You might as well not give us a chance to make a mistake.
Even if the problem you experience is a fatal signal, you should still say so explicitly. Suppose something strange is going on, such as, your copy of ld is out of sync, or you have encountered a bug in the C library on your system. (This has happened!) Your copy might crash and ours would not. If you told us to expect a crash, then when ours fails to crash, we would know that the bug was not happening for us. If you had not told us to expect a crash, then we would not be able to draw any conclusion from our observations.
diff
with the ‘-u’, ‘-c’, or
‘-p’ option. Always send diffs from the old file to the new file.
If you even discuss something in the ld source, refer to it by
context, not by line number.
The line numbers in our development sources will not match those in your sources. Your line numbers would convey no useful information to us.
Here are some things that are not necessary:
Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.
This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. We recommend that you save your time for something else.
Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us. Errors in the output will be easier to spot, running under the debugger will take less time, and so on.
However, simplification is not vital; if you do not want to do this, report the bug anyway and send us the entire test case you used.
A patch for the bug does help us if it is a good one. But do not omit the necessary information, such as the test case, on the assumption that a patch is all we need. We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as ld it is very hard to construct an example that will make the program follow a certain path through the code. If you do not send us the example, we will not be able to construct one, so we will not be able to verify that the bug is fixed.
And if we cannot understand what bug you are trying to fix, or why your patch should be an improvement, we will not install it. A test case will help us to understand.
Such guesses are usually wrong. Even we cannot guess right about such things without first using the debugger to find the facts.
To aid users making the transition to gnu ld from the MRI linker, ld can use MRI compatible linker scripts as an alternative to the more general-purpose linker scripting language described in Scripts. MRI compatible linker scripts have a much simpler command set than the scripting language otherwise used with ld. gnu ld supports the most commonly used MRI linker commands; these commands are described here.
In general, MRI scripts aren't of much use with the a.out
object
file format, since it only has three sections and MRI scripts lack some
features to make use of them.
You can specify a file containing an MRI-compatible script using the ‘-c’ command-line option.
Each command in an MRI-compatible script occupies its own line; each command line starts with the keyword that identifies the command (though blank lines are also allowed for punctuation). If a line of an MRI-compatible script begins with an unrecognized keyword, ld issues a warning message, but continues processing the script.
Lines beginning with ‘*’ are comments.
You can write these commands using all upper-case letters, or all lower case; for example, ‘chip’ is the same as ‘CHIP’. The following list shows only the upper-case form of each command.
ABSOLUTE
secnameABSOLUTE
secname,
secname, ...
secnameABSOLUTE
command to restrict the sections that will be present in
your output program. If the ABSOLUTE
command is used at all in a
script, then only the sections named explicitly in ABSOLUTE
commands will appear in the linker output. You can still use other
input sections (whatever you select on the command line, or using
LOAD
) to resolve addresses in the output file.
ALIAS
out-secname,
in-secnamein-secname may be an integer.
ALIGN
secname =
expressionBASE
expressionCHIP
expressionCHIP
expression,
expressionEND
FORMAT
output-formatOUTPUT_FORMAT
command in the more general linker
language, but restricted to S-records, if output-format is ‘S’
LIST
anything...
The keyword LIST
may be followed by anything on the
same line, with no change in its effect.
LOAD
filenameLOAD
filename,
filename, ...
filenameNAME
output-nameNAME
is equivalent to the command-line
option ‘-o’ or the general script language command OUTPUT
.
ORDER
secname,
secname, ...
secnameORDER
secname secname secnameORDER
command. The
sections you list with ORDER
will appear first in your output
file, in the order specified.
PUBLIC
name=
expressionPUBLIC
name,
expressionPUBLIC
name expressionSECT
secname,
expressionSECT
secname=
expressionSECT
secname expressionSECT
command to
specify the start address (expression) for section secname.
If you have more than one SECT
statement for the same
secname, only the first sets the start address.
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
Examples of suitable formats for Transparent copies include plain ascii without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being list their titles, with the Front-Cover Texts being list, and with the Back-Cover Texts being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
"
: Symbols-(
: Options--accept-unknown-input-arch
: Options--add-needed
: Options--add-stdcall-alias
: Options--allow-multiple-definition
: Options--allow-shlib-undefined
: Options--as-needed
: Options--audit
AUDITLIB: Options--auxiliary=
name: Options--bank-window
: Options--base-file
: Options--be8
: ARM--bss-plt
: PowerPC ELF32--build-id
: Options--build-id=
style: Options--check-sections
: Options--cmse-implib
: ARM--code-region
: MSP430--compress-debug-sections=none
: Options--compress-debug-sections=zlib
: Options--compress-debug-sections=zlib-gabi
: Options--compress-debug-sections=zlib-gnu
: Options--copy-dt-needed-entries
: Options--cref
: Options--ctf-share-types
: Options--ctf-variables
: Options--data-region
: MSP430--default-imported-symver
: Options--default-script=
script: Options--default-symver
: Options--defsym=
symbol=
exp: Options--demangle[=
style]
: Options--depaudit
AUDITLIB: Options--dependency-file=
depfile: Options--disable-auto-image-base
: Options--disable-auto-import
: Options--disable-large-address-aware
: Options--disable-long-section-names
: Options--disable-multiple-abs-defs
: Options--disable-new-dtags
: Options--disable-runtime-pseudo-reloc
: Options--disable-sec-transformation
: MSP430--disable-stdcall-fixup
: Options--discard-all
: Options--discard-locals
: Options--dll
: Options--dll-search-prefix
: Options--dotsyms
: PowerPC64 ELF64--dsbt-index
: Options--dsbt-size
: Options--dynamic-linker=
file: Options--dynamic-list-cpp-new
: Options--dynamic-list-cpp-typeinfo
: Options--dynamic-list-data
: Options--dynamic-list=
dynamic-list-file: Options--dynamicbase
: Options--eh-frame-hdr
: Options--embedded-relocs
: Options--emit-relocs
: Options--emit-stack-syms
: SPU ELF--emit-stub-syms
: SPU ELF--emit-stub-syms
: PowerPC64 ELF64--emit-stub-syms
: PowerPC ELF32--enable-auto-image-base
: Options--enable-auto-import
: Options--enable-extra-pe-debug
: Options--enable-long-section-names
: Options--enable-new-dtags
: Options--enable-non-contiguous-regions
: Options--enable-non-contiguous-regions-warnings
: Options--enable-reloc-section
: Options--enable-runtime-pseudo-reloc
: Options--enable-stdcall-fixup
: Options--entry=
entry: Options--error-handling-script=
scriptname: Options--error-unresolved-symbols
: Options--exclude-all-symbols
: Options--exclude-libs
: Options--exclude-modules-for-implib
: Options--exclude-symbols
: Options--export-all-symbols
: Options--export-dynamic
: Options--export-dynamic-symbol-list=
file: Options--export-dynamic-symbol=
glob: Options--extra-overlay-stubs
: SPU ELF--fatal-warnings
: Options--file-alignment
: Options--filter=
name: Options--fix-arm1176
: ARM--fix-cortex-a53-835769
: ARM--fix-cortex-a8
: ARM--fix-stm32l4xx-629360
: ARM--fix-v4bx
: ARM--fix-v4bx-interworking
: ARM--force-dynamic
: Options--force-exe-suffix
: Options--force-group-allocation
: Options--forceinteg
: Options--format=
format: Options--format=
version: TI COFF--gc-keep-exported
: Options--gc-sections
: Options--got
: Options--got=
type: M68K--gpsize=
value: Options--hash-size=
number: Options--hash-style=
style: Options--heap
: Options--help
: Options--high-entropy-va
: Options--image-base
: Options--in-implib=
file: ARM--insert-timestamp
: Options--just-symbols=
file: Options--kill-at
: Options--large-address-aware
: Options--ld-generated-unwind-info
: Options--leading-underscore
: Options--library-path=
dir: Options--library=
namespec: Options--local-store=lo:hi
: SPU ELF--long-plt
: ARM--major-image-version
: Options--major-os-version
: Options--major-subsystem-version
: Options--max-cache-size=
size: Options--merge-exidx-entries
: ARM--minor-image-version
: Options--minor-os-version
: Options--minor-subsystem-version
: Options--mri-script=
MRI-cmdfile: Options--multi-subspace
: HPPA ELF32--nmagic
: Options--no-accept-unknown-input-arch
: Options--no-add-needed
: Options--no-allow-shlib-undefined
: Options--no-apply-dynamic-relocs
: ARM--no-as-needed
: Options--no-bind
: Options--no-check-sections
: Options--no-copy-dt-needed-entries
: Options--no-ctf-variables
: Options--no-define-common
: Options--no-demangle
: Options--no-dotsyms
: PowerPC64 ELF64--no-dynamic-linker
: Options--no-eh-frame-hdr
: Options--no-enum-size-warning
: ARM--no-export-dynamic
: Options--no-fatal-warnings
: Options--no-fix-arm1176
: ARM--no-fix-cortex-a53-835769
: ARM--no-fix-cortex-a8
: ARM--no-gc-sections
: Options--no-inline-optimize
: PowerPC64 ELF64--no-isolation
: Options--no-keep-memory
: Options--no-leading-underscore
: Options--no-merge-exidx-entries
: ARM--no-merge-exidx-entries
: Options--no-multi-toc
: PowerPC64 ELF64--no-omagic
: Options--no-opd-optimize
: PowerPC64 ELF64--no-overlays
: SPU ELF--no-plt-align
: PowerPC64 ELF64--no-plt-localentry
: PowerPC64 ELF64--no-plt-static-chain
: PowerPC64 ELF64--no-plt-thread-safe
: PowerPC64 ELF64--no-power10-stubs
: PowerPC64 ELF64--no-print-gc-sections
: Options--no-print-map-discarded
: Options--no-save-restore-funcs
: PowerPC64 ELF64--no-seh
: Options--no-strip-discarded
: Options--no-tls-get-addr-optimize
: PowerPC64 ELF64--no-tls-get-addr-regsave
: PowerPC64 ELF64--no-tls-optimize
: PowerPC64 ELF64--no-tls-optimize
: PowerPC ELF32--no-toc-optimize
: PowerPC64 ELF64--no-toc-sort
: PowerPC64 ELF64--no-trampoline
: Options--no-undefined
: Options--no-undefined-version
: Options--no-warn-mismatch
: Options--no-warn-search-mismatch
: Options--no-wchar-size-warning
: ARM--no-whole-archive
: Options--noinhibit-exec
: Options--non-overlapping-opd
: PowerPC64 ELF64--nxcompat
: Options--oformat=
output-format: Options--omagic
: Options--orphan-handling=
MODE: Options--out-implib
: Options--output-def
: Options--output=
output: Options--pic-executable
: Options--pic-veneer
: ARM--plt-align
: PowerPC64 ELF64--plt-localentry
: PowerPC64 ELF64--plt-static-chain
: PowerPC64 ELF64--plt-thread-safe
: PowerPC64 ELF64--plugin
: SPU ELF--pop-state
: Options--power10-stubs
: PowerPC64 ELF64--print-gc-sections
: Options--print-map
: Options--print-map-discarded
: Options--print-memory-usage
: Options--print-output-format
: Options--push-state
: Options--reduce-memory-overheads
: Options--relax
: Options--relax on PowerPC
: PowerPC ELF32--relocatable
: Options--require-defined=
symbol: Options--retain-symbols-file=
filename: Options--save-restore-funcs
: PowerPC64 ELF64--script=
script: Options--sdata-got
: PowerPC ELF32--section-alignment
: Options--section-start=
sectionname=
org: Options--secure-plt
: PowerPC ELF32--sort-common
: Options--sort-section=alignment
: Options--sort-section=name
: Options--spare-dynamic-tags
: Options--split-by-file
: Options--split-by-reloc
: Options--stack
: Options--stack-analysis
: SPU ELF--stats
: Options--strip-all
: Options--strip-debug
: Options--strip-discarded
: Options--stub-group-size
: PowerPC64 ELF64--stub-group-size=
N: HPPA ELF32--stub-group-size=
N: ARM--subsystem
: Options--support-old-code
: ARM--sysroot=
directory: Options--target-help
: Options--target1-abs
: ARM--target1-rel
: ARM--target2=
type: ARM--task-link
: Options--thumb-entry=
entry: ARM--tls-get-addr-optimize
: PowerPC64 ELF64--tls-get-addr-regsave
: PowerPC64 ELF64--trace
: Options--trace-symbol=
symbol: Options--traditional-format
: Options--tsaware
: Options--undefined=
symbol: Options--unique[=
SECTION]
: Options--unresolved-symbols
: Options--use-blx
: ARM--use-nul-prefixed-import-tables
: ARM--verbose[=
NUMBER]
: Options--version
: Options--version-script=
version-scriptfile: Options--vfp11-denorm-fix
: ARM--warn-alternate-em
: Options--warn-common
: Options--warn-constructors
: Options--warn-execstack
: Options--warn-multiple-gp
: Options--warn-once
: Options--warn-rwx-segments
: Options--warn-section-align
: Options--warn-textrel
: Options--warn-unresolved-symbols
: Options--wdmdriver
: Options--whole-archive
: Options--wrap=
symbol: Options-a
keyword: Options-assert
keyword: Options-b
format: Options-Bdynamic
: Options-Bgroup
: Options-Bno-symbolic
: Options-Bshareable
: Options-Bstatic
: Options-Bsymbolic
: Options-Bsymbolic-functions
: Options-c
MRI-cmdfile: Options-call_shared
: Options-d
: Options-dc
: Options-dn
: Options-dp
: Options-dT
script: Options-dy
: Options-E
: Options-e
entry: Options-EB
: Options-EL
: Options-F
name: Options-f
name: Options-fini=
name: Options-g
: Options-G
value: Options-h
name: Options-i
: Options-I
file: Options-init=
name: Options-L
dir: Options-l
namespec: Options-M
: Options-m
emulation: Options-Map=
mapfile: Options-N
: Options-n
: Options-no-pie
: Options-non_shared
: Options-nostdlib
: Options-O
level: Options-o
output: Options-P
AUDITLIB: Options-pie
: Options-plugin
name: Options-q
: Options-qmagic
: Options-Qy
: Options-r
: Options-R
file: Options-rpath-link=
dir: Options-rpath=
dir: Options-S
: Options-s
: Options-shared
: Options-soname=
name: Options-static
: Options-t
: Options-T
script: Options-Tbss=
org: Options-Tdata=
org: Options-Tldata-segment=
org: Options-Trodata-segment=
org: Options-Ttext-segment=
org: Options-Ttext=
org: Options-u
symbol: Options-Ur
: Options-V
: Options-v
: Options-X
: Options-x
: Options-Y
path: Options-y
symbol: Options-z defs
: Options-z
keyword: Options-z muldefs
: Options-z undefs
: Options.
: Location Counter:
phdr: Output Section Phdr=
fillexp: Output Section Fill>
region: Output Section RegionABSOLUTE
(MRI): MRIABSOLUTE(
exp)
: Builtin FunctionsADDR(
section)
: Builtin FunctionsALIAS
(MRI): MRIALIGN
(MRI): MRIALIGN(
align)
: Builtin FunctionsALIGN(
exp,
align)
: Builtin FunctionsALIGN(
section_align)
: Forced Output AlignmentALIGNOF(
section)
: Builtin FunctionsAS_NEEDED(
files)
: File CommandsASSERT
: Miscellaneous CommandsAT(
lma)
: Output Section LMAAT>
lma_region: Output Section LMABASE
(MRI): MRIBLOCK(
exp)
: Builtin FunctionsBYTE(
expression)
: Output Section DataCHIP
(MRI): MRICOLLECT_NO_DEMANGLE
: EnvironmentCOMMONPAGESIZE
: Symbolic ConstantsCONSTANT
: Symbolic ConstantsCONSTRUCTORS
: Output Section KeywordsCREATE_OBJECT_SYMBOLS
: Output Section KeywordsDATA_SEGMENT_ALIGN(
maxpagesize,
commonpagesize)
: Builtin FunctionsDATA_SEGMENT_END(
exp)
: Builtin FunctionsDATA_SEGMENT_RELRO_END(
offset,
exp)
: Builtin FunctionsDEFINED(
symbol)
: Builtin FunctionsEND
(MRI): MRIENTRY(
symbol)
: Entry PointEXTERN
: Miscellaneous CommandsFILEHDR
: PHDRSFILL(
expression)
: Output Section DataFORCE_COMMON_ALLOCATION
: Miscellaneous CommandsFORCE_GROUP_ALLOCATION
: Miscellaneous CommandsFORMAT
(MRI): MRIGNUTARGET
: EnvironmentGROUP(
files)
: File CommandsINCLUDE
filename: File CommandsINHIBIT_COMMON_ALLOCATION
: Miscellaneous CommandsINPUT(
files)
: File CommandsINSERT
: Miscellaneous Commandsl =
: MEMORYLD_FEATURE(
string)
: Miscellaneous CommandsLDEMULATION
: Environmentlen =
: MEMORYLENGTH =
: MEMORYLENGTH(
memory)
: Builtin FunctionsLIST
(MRI): MRILOAD
(MRI): MRILOADADDR(
section)
: Builtin FunctionsLOG2CEIL(
exp)
: Builtin FunctionsLONG(
expression)
: Output Section DataMAX
: Builtin FunctionsMAXPAGESIZE
: Symbolic ConstantsMEMORY
: MEMORYMIN
: Builtin FunctionsNAME
(MRI): MRINEXT(
exp)
: Builtin FunctionsNOCROSSREFS(
sections)
: Miscellaneous CommandsNOCROSSREFS_TO(
tosection fromsections)
: Miscellaneous CommandsNOLOAD
: Output Section Typeo =
: MEMORYobjdump -i
: BFDONLY_IF_RO
: Output Section ConstraintONLY_IF_RW
: Output Section ConstraintORDER
(MRI): MRIorg =
: MEMORYORIGIN =
: MEMORYORIGIN(
memory)
: Builtin FunctionsOUTPUT(
filename)
: File CommandsOUTPUT_ARCH(
bfdarch)
: Miscellaneous CommandsOUTPUT_FORMAT(
bfdname)
: Format CommandsOVERLAY
: Overlay DescriptionPHDRS
: PHDRSPUBLIC
(MRI): MRIQUAD(
expression)
: Output Section DataREGION_ALIAS(
alias,
region)
: REGION_ALIASSEARCH_DIR(
path)
: File CommandsSECT
(MRI): MRISECTIONS
: SECTIONSSEGMENT_START(
segment,
default)
: Builtin FunctionsSHORT(
expression)
: Output Section DataSIZEOF(
section)
: Builtin FunctionsSIZEOF_HEADERS
: Builtin FunctionsSQUAD(
expression)
: Output Section DataSTARTUP(
filename)
: File CommandsSUBALIGN(
subsection_align)
: Forced Input AlignmentTARGET(
bfdname)
: Format CommandsVERSION {script text}
: VERSION