ld version 2
Copyright (C) 1991, 92, 93, 94, 1995 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
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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 section 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.
Here is a summary of the options you can use on the ld command
line:
ld [ -o output ] objfile... [ -Aarchitecture ] [ -b input-format ] [ -Bstatic ] [ -Bdynamic ] [ -Bsymbolic ] [ -c MRI-commandfile ] [ -d | -dc | -dp ] [ -defsym symbol=expression ] [ -dynamic-linker file ] [ -embedded-relocs ] [ -export-dynamic ] [ -e entry ] [ -F ] [ -F format ] [ -format input-format ] [ -g ] [ -G size ] [ -help ] [ -i ] [ -larchive ] [ -Lsearchdir ] [ -M ] [ -Map mapfile ] [ -m emulation ] [ -N | -n ] [ -noinhibit-exec ] [ -no-keep-memory ] [ -oformat output-format ] [ -R filename ] [ -relax ] [ -retain-symbols-file filename ] [ -r | -Ur ] [ -rpath dir ] [-rpath-link dir ] [ -S ] [ -s ] [ -soname name ] [ -shared ] [ -sort-common ] [ -stats ] [ -T commandfile ] [ -Ttext org ] [ -Tdata org ] [ -Tbss org ] [ -t ] [ -traditional-format ] [ -u symbol] [-V] [-v] [ -verbose] [ -version ] [ -warn-common ] [ -warn-constructors] [ -warn-once ] [ -y symbol ] [ -X ] [-x ] [ -( [ archives ] -) ] [ --start-group [ archives ] --end-group ] [ -split-by-reloc count ] [ -split-by-file ] [ --whole-archive ]
This plethora of command-line options may seem intimidating, 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.)
The command-line options to ld may be specified in any order, and
may be repeated at will. Repeating most 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.
The exceptions--which may meaningfully be used more than once--are `-A', `-b' (or its synonym `-format'), `-defsym', `-L', `-l', `-R', `-u', and `-(' (or its synonym `--start-group').
The list of object files to be linked together, shown as objfile...,
may follow, precede, or be mixed in with command-line options, except that
an objfile argument may not be placed between an option and
its argument. Alternatively a list of files may be specified using the
syntax @listfile. Here listfile is the name of a file
containing a list of file names, one per line.
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 can not 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. See section Command Language.
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, `--oformat' and `-oformat' are equivalent. 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, `--oformat srec' and `--oformat=srec' are equivalent. Unique abbreviations of the names of multiple-letter options are accepted.
-Aarchitecture
ld, this option is useful only for the
Intel 960 family of architectures. In that ld configuration, the
architecture argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path. See section ld and the Intel 960 family, for details.
Future releases of ld may support similar functionality for
other architecture families.
-b input-format
ld may be configured to support more than one kind of object
file. If your ld is configured this way, you can use the
`-b' option to specify the binary format for input object files
that follow this option on the command line. Even when ld is
configured to support alternative object formats, you don't usually need
to specify this, as ld should be configured to expect as a
default input format the most usual format on each machine.
input-format is a text string, the name of a particular format
supported by the BFD libraries. (You can list the available binary
formats with `objdump -i'.) `-format input-format'
has the same effect, as does the script command TARGET.
See section BFD.
You may want to use this option if you are linking files with an unusual
binary format. You can also use `-b' to switch formats explicitly (when
linking object files of different formats), by including
`-b input-format' before each group of object files in a
particular format.
The default format is taken from the environment variable
GNUTARGET.
See section Environment Variables.
You can also define the input
format from a script, using the command TARGET; see section Option Commands.
-Bstatic
-Bdynamic
-Bsymbolic
-c MRI-commandfile
ld accepts script
files written in an alternate, restricted command language, described in
section MRI Compatible Script Files. Introduce MRI script files with
the option `-c'; use the `-T' option to run linker
scripts written in the general-purpose ld scripting language.
If MRI-cmdfile does not exist, ld looks for it in the directories
specified by any `-L' options.
-d
-dc
-dp
FORCE_COMMON_ALLOCATION has the same effect. See section Option 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 section Assignment: Defining Symbols). Note: there should be no
white space between symbol, the equals sign ("="), and
expression.
-dynamic-linker file
-embedded-relocs
-e entry
-export-dynamic
dlopen.
-F
-Fformat
ld uses for this purpose (the
`-b' or `-format' options for input files, `-oformat'
option or the TARGET command in linker scripts for output files,
the GNUTARGET environment variable) are more flexible, but
ld accepts the `-F' option for compatibility with scripts
written to call the old linker.
-format input-format
-g
-Gvalue
-G value
-help
-i
-lar
ld will search its
path-list for occurrences of libar.a for every archive
specified.
-Lsearchdir
-L searchdir
ld will search
for archive libraries and ld control scripts. You may use this
option any number of times. The directories are searched in the order
in which they are specified on the command line. Directories specified
on the command line are searched before the default directories. All
-L options apply to all -l options, regardless of the
order in which the options appear.
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 section Environment Variables.
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
ld, and information on global common
storage allocation.
-Map mapfile
ld, and information on global
common storage allocation.
-memulation
-m emulation
ld was configured.
-N
OMAGIC.
-n
NMAGIC if possible.
-noinhibit-exec
-no-keep-memory
ld normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells ld to
instead optimize for memory usage, by rereading the symbol tables as
necessary. This may be required if ld runs out of memory space
while linking a large executable.
-o output
ld; if this
option is not specified, the name `a.out' is used by default. The
script command OUTPUT can also specify the output file name.
-oformat output-format
ld may be configured to support more than one kind of object
file. If your ld is configured this way, you can use the
`-oformat' option to specify the binary format for the output
object file. Even when ld is configured to support alternative
object formats, you don't usually need to specify this, as ld
should be configured to produce as a default output format the most
usual format on each machine. output-format is a text string, the
name of a particular format supported by the BFD libraries. (You can
list the available binary formats with `objdump -i'.) The script
command OUTPUT_FORMAT can also specify the output format, but
this option overrides it. See section BFD.
-R filename
-relax
ld and the Intel 960 family.
On some platforms, the `-relax' option performs global optimizations that
become possible when the linker resolves addressing in the program, such
as relaxing address modes and synthesizing new instructions in the
output object file.
On platforms where this is not supported, `-relax' is accepted, but
ignored.
-retain-symbols-file filename
-rpath dir
-rpath
arguments are concatenated and passed to the runtime linker, which uses
them to locate shared objects at runtime. The -rpath option is
also used when locating shared objects which are needed by shared
objects explicitly included in the link; see the description of the
-rpath-link option. If -rpath is not used when linking an
ELF executable, the contents of the environment variable
LD_RUN_PATH will be used if it is defined.
The -rpath option may also be used on SunOS. By default, on
SunOS, the linker will form a runtime search patch out of all the
-L options it is given. If a -rpath option is used, the
runtime search path will be formed exclusively using the -rpath
options, ignoring the -L options. This can be useful when using
gcc, which adds many -L options which may be on NFS mounted
filesystems.
-rpath-link DIR
ld -shared link includes a shared library as one
of the input files.
When the linker encounters such a dependency when doing a non-shared,
non-relocateable 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 linker uses the following search paths to locate required shared
libraries.
-rpath-link options.
-rpath options. The difference
between -rpath and -rpath-link is that directories
specified by -rpath options are included in the executable and
used at runtime, whereas the -rpath-link option is only effective
at link time.
-rpath and rpath-link options
were not used, search the contents of the environment variable
LD_RUN_PATH.
-rpath option was not used, search any
directories specified using -L options.
LD_LIBRARY_PATH.
-r
ld. This is often called partial
linking. As a side effect, in environments that support standard Unix
magic numbers, this option also sets the output file's magic number to
OMAGIC.
If this option is not specified, an absolute file is produced. When
linking C++ programs, this option will not resolve references to
constructors; to do that, use `-Ur'.
This option does the same thing as `-i'.
-S
-s
-soname name
-shared
-e option is not used and there are
undefined symbols in the link.
-sort-common
ld places the global common symbols in the
appropriate output sections, it sorts them by size. First come all the
one byte symbols, then all the two bytes, then all the four bytes, and
then everything else. This is to prevent gaps between symbols due to
alignment constraints. This option disables that sorting.
-split-by-reloc count
-split-by-file
-stats
-Tbss org
-Tdata org
-Ttext org
bss, data, or the text segment of the output file.
org must be a single hexadecimal integer;
for compatibility with other linkers, you may omit the leading
`0x' usually associated with hexadecimal values.
-T commandfile
-Tcommandfile
ld's default link script (rather than adding
to it), so commandfile must specify everything necessary to describe
the target format. See section Command Language. If commandfile does not
exist, ld looks for it in the directories specified by any
preceding `-L' options. Multiple `-T' options accumulate.
-t
ld processes them.
-traditional-format
ld is different in some ways from
the output of some existing linker. This switch requests ld to
use the traditional format instead.
For example, on SunOS, ld combines duplicate entries in the
symbol string table. This can reduce the size of an output file with
full debugging information by over 30 percent. Unfortunately, the SunOS
dbx program can not read the resulting program (gdb has no
trouble). The `-traditional-format' switch tells ld to not
combine duplicate entries.
-u symbol
-Ur
ld. When linking C++ programs, `-Ur'
does resolve references to constructors, unlike `-r'.
It does not work to use `-Ur' on files that were themselves linked
with `-Ur'; once the constructor table has been built, it cannot
be added to. Use `-Ur' only for the last partial link, and
`-r' for the others.
--verbose
ld and list the linker emulations
supported. Display which input files can and cannot be opened.
-v
-V
ld. The -V option also
lists the supported emulations.
-version
ld and exit.
-warn-common
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-once
-X
-x
-y symbol
-( archives -)
--start-group archives --end-group
You can change the behavior of ld with the environment
variable GNUTARGET.
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 section 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.
The command language provides explicit control over the link process, allowing complete specification of the mapping between the linker's input files and its output. It controls:
You may supply a command file (also known as a link script) to the linker either explicitly through the `-T' option, or implicitly as an ordinary file. If the linker opens a file which it cannot recognize as a supported object or archive format, it reports an error.
The ld command language is a collection of statements; some are
simple keywords setting a particular option, some are used to select and
group input files or name output files; and two statement
types have a fundamental and pervasive impact on the linking process.
The most fundamental command of the ld command language is the
SECTIONS command (see section Specifying Output Sections). Every meaningful command
script must have a SECTIONS command: it specifies a
"picture" of the output file's layout, in varying degrees of detail.
No other command is required in all cases.
The MEMORY command complements SECTIONS by describing the
available memory in the target architecture. This command is optional;
if you don't use a MEMORY command, ld assumes sufficient
memory is available in a contiguous block for all output.
See section Memory Layout.
You may include comments in linker scripts just as in C: delimited by `/*' and `*/'. As in C, comments are syntactically equivalent to whitespace.
Many useful commands involve arithmetic expressions. The syntax for expressions in the command language is identical to that of C expressions, with the following features:
An octal integer is `0' followed by zero or more of the octal digits (`01234567').
_as_octal = 0157255;
A decimal integer starts with a non-zero digit followed by zero or more digits (`0123456789').
_as_decimal = 57005;
A hexadecimal integer is `0x' or `0X' followed by one or more hexadecimal digits chosen from `0123456789abcdefABCDEF'.
_as_hex = 0xdead;
To write a negative integer, use the prefix operator `-'; see section Operators.
_as_neg = -57005;
Additionally the suffixes K and M may be used to scale a
constant by
respectively. For example, the following all refer to the same quantity:
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;
Unless quoted, symbol names start with a letter, underscore, or point and may include any letters, underscores, digits, points, 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.
The special linker variable dot `.' always contains the
current output location counter. Since the . always refers to
a location in an output section, it must always appear in an
expression within a SECTIONS command. The . symbol
may appear anywhere that an ordinary symbol is allowed in an
expression, but its assignments have a side effect. Assigning a value
to the . symbol will cause the location counter to be moved.
This may be used to create holes in the output section. The location
counter may never be moved backwards.
SECTIONS
{
output :
{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
} = 0x1234;
}
In the previous example, file1 is located at the beginning of the
output section, then there is a 1000 byte gap. Then file2
appears, also with a 1000 byte gap following before file3 is
loaded. The notation `= 0x1234' specifies what data to write in
the gaps (see section Optional Section Attributes).
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 | >> &l;< | |
| 5 | left | == != > < <= >= | |
| 6 | left | & | |
| 7 | left | | | |
| 8 | left | && | |
| 9 | left | || | |
| 10 | right | ? : | |
| 11 | right | &= += -= *= /= | (2) |
| (lowest) | |||
| (1) | Prefix operators. | ||
| (2) | See section Assignment: Defining Symbols. | ||
The linker uses "lazy evaluation" for expressions; it only calculates an expression when absolutely necessary. The linker needs the value of the start address, and the 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 command file. 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.
You may create global symbols, and assign values (addresses) to global symbols, using any of the C assignment operators:
symbol = expression ;
symbol &= expression ;
symbol += expression ;
symbol -= expression ;
symbol *= expression ;
symbol /= expression ;
Two things distinguish assignment from other operators in ld
expressions.
Assignment statements may appear:
ld script; or
SECTIONS command; or
SECTIONS command.
The first two cases are equivalent in effect--both define a symbol with an absolute address. The last case defines a symbol whose address is relative to a particular section (see section Specifying Output Sections).
When a linker expression is evaluated and assigned to a variable, it is given either an absolute or a relocatable type. An absolute expression type is one in which the symbol contains the value that it will have in the output file; a relocatable expression type is one in which the value is expressed as a fixed offset from the base of a section.
The type of the expression is controlled by its position in the script
file. A symbol assigned within a section definition is created relative
to the base of the section; a symbol assigned in any other place is
created as an absolute symbol. Since a symbol created within a
section definition is relative to the base of the section, it
will remain relocatable if relocatable output is requested. A symbol
may be created with an absolute value even when assigned to within a
section definition by using the absolute assignment function
ABSOLUTE. For example, to create an absolute symbol whose address
is the last byte of an output section named .data:
SECTIONS{ ...
.data :
{
*(.data)
_edata = ABSOLUTE(.) ;
}
... }
The linker tries to put off the evaluation of an assignment until all the terms in the source expression are known (see section Evaluation). For instance, 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 dot, `.' must be evaluated during 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 :
{ ...
}
... }
will cause the error message "Non constant expression for initial
address".
In some cases, it is desirable for a linker script to define a symbol
only if it is referenced, and only if it 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).
The command language includes a number of built-in functions for use in link script expressions.
ABSOLUTE(exp)
ADDR(section)
symbol_1 and symbol_2 are assigned identical
values:
SECTIONS{ ...
.output1 :
{
start_of_output_1 = ABSOLUTE(.);
...
}
.output :
{
symbol_1 = ADDR(.output1);
symbol_2 = start_of_output_1;
}
... }
ALIGN(exp)
.) aligned to
the next exp boundary. exp must be an expression whose
value is a power of two. This is equivalent to
(. + exp - 1) & ~(exp - 1)
ALIGN doesn't change the value of the location counter--it just
does arithmetic on it. As an example, to align the output .data
section to the next 0x2000 byte boundary after the preceding
section and to set 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 start attribute of a
section definition (see section Optional Section Attributes). The second use simply
defines the value of a variable.
The built-in NEXT is closely related to ALIGN.
DEFINED(symbol)
begin to the first location in the
.text section--but if a symbol called begin already
existed, its value is preserved:
SECTIONS{ ...
.text : {
begin = DEFINED(begin) ? begin : . ;
...
}
... }
NEXT(exp)
ALIGN(exp); unless you
use the MEMORY command to define discontinuous memory for the
output file, the two functions are equivalent.
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
sizeof_headers
The linker's default configuration permits allocation of all available memory.
You can override this configuration by using the MEMORY command. The
MEMORY command describes the location and size of blocks of
memory in the target. By using it carefully, you can describe which
memory regions may be used by the linker, and which memory regions it
must avoid. The linker does not shuffle sections to fit into the
available regions, but does move the requested sections into the correct
regions and issue errors when the regions become too full.
A command file may contain at most one use of the MEMORY
command; however, you can define as many blocks of memory within it as
you wish. The syntax is:
MEMORY
{
name (attr) : ORIGIN = origin, LENGTH = len
...
}
name
(attr)
ld beyond checking that the
attribute list is valid. Valid attribute lists must be made up of the
characters "LIRWX". If you omit the attribute list, you may
omit the parentheses around it as well.
origin
ORIGIN may be
abbreviated to org or o (but not, for example, `ORG').
len
LENGTH may be abbreviated to len or l.
For example, to specify that memory has two regions available for
allocation--one starting at 0 for 256 kilobytes, and the other
starting at 0x40000000 for four megabytes:
MEMORY
{
rom : ORIGIN = 0, LENGTH = 256K
ram : org = 0x40000000, l = 4M
}
Once you have defined a region of memory named mem, you can direct
specific output sections there by using a command ending in
`>mem' within the SECTIONS command (see section Optional Section Attributes). If the combined output sections directed to a region are too
big for the region, the linker will issue an error message.
The SECTIONS command controls exactly where input sections are
placed into output sections, their order in the output file, and to
which output sections they are allocated.
You may use at most one SECTIONS command in a script file,
but you can have as many statements within it as you wish. Statements
within the SECTIONS command can do one of three things:
You can also use the first two operations--defining the entry point and
defining symbols--outside the SECTIONS command: see section The Entry Point, and see section Assignment: Defining Symbols. They are permitted here as well for
your convenience in reading the script, so that symbols and the entry
point can be defined at meaningful points in your output-file layout.
If you do not use a SECTIONS command, the linker places 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 most frequently used statement in the SECTIONS command is
the section definition, which specifies the
properties of an output section: its location, alignment, contents,
fill pattern, and target memory region. Most of
these specifications are optional; the simplest form of a section
definition is
SECTIONS { ...
secname : {
contents
}
... }
secname is the name of the output section, and contents a specification of what goes there--for example, a list of input files or sections of input files (see section Section Placement). As you might assume, the whitespace shown is optional. You do need the colon `:' and the braces `{}', however.
secname 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 any name which does not conform to the standard
ld symbol name syntax must be quoted.
See section Symbol Names.
The linker will not create output sections which do not have any contents. This is for convenience when referring to input sections that may or may not exist. 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.
In a section definition, you can specify the contents of an output section by listing particular input files, by listing particular input-file sections, or by a combination of the two. You can also place arbitrary data in the section, and define symbols relative to the beginning of the section.
The contents of a section definition may include any of the following kinds of statement. You can include as many of these as you like in a single section definition, separated from one another by whitespace.
filename
.data : { afile.o bfile.o cfile.o }
The example also illustrates that multiple statements can be included in
the contents of a section definition, since each file name is a separate
statement.
filename( section )
filename( section, section, ... )
filename( section section ... )
* (section)
* (section, section, ...)
* (section section ...)
ld command
line: use `*' instead of a particular file name before the
parenthesized input-file section list.
If you have already explicitly included some files by name, `*'
refers to all remaining files--those whose places in the output
file have not yet been defined.
For example, to copy sections 1 through 4 from an Oasys file
into the .text section of an a.out file, and sections 13
and 14 into the .data section:
SECTIONS {
.text :{
*("1" "2" "3" "4")
}
.data :{
*("13" "14")
}
}
`[ section ... ]' used to be accepted as an alternate way
to specify named sections from all unallocated input files. Because
some operating systems (VMS) allow brackets in file names, that notation
is no longer supported.
filename( COMMON )
*( COMMON )
*(COMMON) by itself refers to all
uninitialized data from all input files (so far as it is not yet
allocated); filename(COMMON) refers to uninitialized data
from a particular file. Both are special cases of the general
mechanisms for specifying where to place input-file sections:
ld permits you to refer to uninitialized data as if it
were in an input-file section named COMMON, regardless of the
input file's format.
For example, the following command script arranges the output file into
three consecutive sections, named .text, .data, and
.bss, taking the input for each from the correspondingly named
sections of all the input files:
SECTIONS {
.text : { *(.text) }
.data : { *(.data) }
.bss : { *(.bss) *(COMMON) }
}
The following example reads all of the sections from file all.o
and places 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)
}
}
The foregoing statements arrange, in your output file, data originating
from your input files. You can also place data directly in an output
section from the link command script. Most of these additional
statements involve expressions; see section Expressions. Although these
statements are shown separately here for ease of presentation, no such
segregation is needed within a section definition in the SECTIONS
command; you can intermix them freely with any of the statements we've
just described.
CREATE_OBJECT_SYMBOLS
a.out
files it is conventional to have a symbol for each input file. You can
accomplish this by defining the output .text section as follows:
SECTIONS {
.text 0x2020 :
{
CREATE_OBJECT_SYMBOLS
*(.text)
_etext = ALIGN(0x2000);
}
...
}
If sample.ld is a file containing this script, and a.o,
b.o, c.o, and d.o are four input files with
contents like the following---
/* a.c */
afunction() { }
int adata=1;
int abss;
`ld -M -T sample.ld a.o b.o c.o d.o' would create a map like this,
containing symbols matching the object file names:
00000000 A __DYNAMIC 00004020 B _abss 00004000 D _adata 00002020 T _afunction 00004024 B _bbss 00004008 D _bdata 00002038 T _bfunction 00004028 B _cbss 00004010 D _cdata 00002050 T _cfunction 0000402c B _dbss 00004018 D _ddata 00002068 T _dfunction 00004020 D _edata 00004030 B _end 00004000 T _etext 00002020 t a.o 00002038 t b.o 00002050 t c.o 00002068 t d.o
symbol = expression ;
symbol f= expression ;
&= += -= *= /= which combine
arithmetic and assignment.
When you assign a value to a symbol within a particular section
definition, the value is relative to the beginning of the section
(see section Assignment: Defining Symbols). If you write
SECTIONS {
abs = 14 ;
...
.data : { ... rel = 14 ; ... }
abs2 = 14 + ADDR(.data);
...
}
abs and rel do not have the same value; rel has the
same value as abs2.
BYTE(expression)
SHORT(expression)
LONG(expression)
QUAD(expression)
QUAD is only supported when
using a 64 bit host or target.
Multiple-byte quantities are represented in whatever byte order is
appropriate for the output file format (see section BFD).
FILL(expression)
FILL statement covers memory
locations after the point 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.
Here is the full syntax of a section definition, including all the optional portions:
SECTIONS {
...
secname start BLOCK(align) (NOLOAD) : AT ( ldadr )
{ contents } >region :phdr =fill
...
}
secname and contents are required. See section Section Definitions, and see section Section Placement for details on
contents. The remaining elements---start,
BLOCK(align), (NOLOAD), AT ( ldadr ),
>region, :phdr, and =fill---are
all optional.
start
0x40000000:
SECTIONS {
...
output 0x40000000: {
...
}
...
}
BLOCK(align)
BLOCK() specification to advance
the location counter . prior to the beginning of the section, so
that the section will begin at the specified alignment. align is
an expression.
(NOLOAD)
ROM segment is addressed at memory location `0' and does not
need to be loaded into each object file:
SECTIONS {
ROM 0 (NOLOAD) : { ... }
...
}
AT ( ldadr )
AT keyword specifies
the load address of the section. The default (if you do not use the
AT keyword) is to make the load address the same as the
relocation address. This feature is designed to make it easy to build a
ROM image. For example, this SECTIONS definition creates two
output sections: one called `.text', which starts at 0x1000,
and one called `.mdata', which is loaded at the end of the
`.text' section even though its relocation address is
0x2000. The symbol _data is defined with the value
0x2000:
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 C programs, usually crt0)
for use with a ROM generated this way has to include something like
the following, to copy the initialized data from the ROM image to its runtime
address:
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;
>region
:phdr
:phdr modifier. To
prevent a section from being assigned to a segment when it would
normally default to one, use :NONE.
=fill
=fill in a section definition specifies the
initial fill value for that section. You may use any expression to
specify fill. Any unallocated holes in the current output section
when written to the output file will be filled with the two least
significant bytes of the value, repeated as necessary. You can also
change the fill value with a FILL statement in the contents
of a section definition.
The ELF object file format uses program headers, which are read by
the system loader and describe how the program should be loaded into
memory. These program headers must be set correctly in order to run the
program on a native ELF system. The linker will create reasonable
program headers by default. However, in some cases, it is desirable to
specify the program headers more precisely; the PHDRS command may
be used for this purpose. When the PHDRS command is used, the
linker will not generate any program headers itself.
The PHDRS command is only meaningful when generating an ELF
output file. It is ignored in other cases. This manual does not
describe the details of how the system loader interprets program
headers; for more information, see the ELF ABI. The program headers of
an ELF file may be displayed using the `-p' option of the
objdump command.
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 does not get put into the output file.
Certain program header types describe segments of memory which are
loaded from the file by the system loader. In the linker script, the
contents of these segments are specified by directing allocated output
sections to be placed in the segment. To do this, the command
describing the output section in the SECTIONS command should use
`:name', where name is the name of the program header
as it appears in the PHDRS command. See section Optional Section Attributes.
It is normal for certain sections to appear in more than one segment. This merely implies that one segment of memory contains another. This is specified by repeating `:name', using it once for each program header in which the section is to appear.
If a section is placed in one or more segments using `:name',
then all subsequent allocated sections which do not specify
`:name' are placed in the same segments. This is for
convenience, since generally a whole set of contiguous sections will be
placed in a single segment. To prevent a section from being assigned to
a segment when it would normally default to one, use :NONE.
The FILEHDR and PHDRS keywords which may appear after the
program header type also indicate contents of the segment of memory.
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.
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)
It is possible to specify that a segment should be loaded at a
particular address in memory. This is done using an AT
expression. This is identical to the AT command used in the
SECTIONS command (see section Optional Section Attributes). Using the AT
command for a program header overrides any information in the
SECTIONS command.
Normally the segment flags are set based on the sections. The
FLAGS keyword may be used 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 the use 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 command language includes a command specifically for defining the first executable instruction in an output file (its entry point). Its argument is a symbol name:
ENTRY(symbol)
Like symbol assignments, the ENTRY command may be placed either
as an independent command in the command file, or among the section
definitions within the SECTIONS command--whatever makes the most
sense for your layout.
ENTRY is only one of several ways of choosing the entry point.
You may indicate it in any of the following ways (shown in descending
order of priority: methods higher in the list override methods lower down).
ENTRY(symbol) command in a linker control script;
start, if present;
.text section, if present;
0.
For example, you can use these rules to generate an entry point with an
assignment statement: if no symbol start is defined within your
input files, you can simply define it, assigning it an appropriate
value---
start = 0x2020;
The example shows an absolute address, but you can use any expression.
For example, if your input object files use some other symbol-name
convention for the entry point, you can just assign the value of
whatever symbol contains the start address to start:
start = other_symbol ;
The command language includes a number of other commands that you can use for specialized purposes. They are similar in purpose to command-line options.
CONSTRUCTORS
CONSTRUCTORS command specifies where the
linker is to place the data from these sections, relative to the rest of
the linked output. Constructor data is marked by the symbol
__CTOR_LIST__ at the start, and __CTOR_LIST_END at
the end; destructor data is bracketed similarly, between
__DTOR_LIST__ and __DTOR_LIST_END. (The compiler
must arrange to actually run this code; GNU C++ calls constructors from
a subroutine __main, which it inserts automatically into the
startup code for main, and destructors from _exit.)
FLOAT
NOFLOAT
ld doesn't use the keywords, assuming
instead that any necessary subroutines are in libraries specified using
the general mechanisms for linking to archives; but to permit the use of
scripts that were written for the older linkers, the keywords
FLOAT and NOFLOAT are accepted and ignored.
FORCE_COMMON_ALLOCATION
ld assign space to common symbols even if a relocatable
output file is specified (`-r').
INPUT ( file, file, ... )
INPUT ( file file ... )
ld searches for each file through the archive-library
search path, just as for files you specify on the command line.
See the description of `-L' in section Command Line Options.
If you use `-lfile', ld will transform the name to
libfile.a as with the command line argument `-l'.
GROUP ( file, file, ... )
GROUP ( file file ... )
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
section Command Line Options.
OUTPUT ( filename )
OUTPUT(filename) is identical to the effect of
`-o filename', which overrides it. You can use this
command to supply a default output-file name other than a.out.
OUTPUT_ARCH ( bfdname )
OUTPUT_FORMAT
command.
OUTPUT_FORMAT ( bfdname )
ld is configured to support multiple object code formats,
you can use this command to specify a particular output format.
bfdname is one of the names used by the BFD back-end routines
(see section BFD). The effect is identical to the effect of the
`-oformat' command-line option. This selection affects only
the output file; the related command TARGET affects primarily
input files.
SEARCH_DIR ( path )
ld looks for
archive libraries. SEARCH_DIR(path) has the same
effect as `-Lpath' on the command line.
STARTUP ( filename )
TARGET ( format )
ld is configured to support multiple object code formats,
you can use this command to change the input-file object code format
(like the command-line option `-b' or its synonym `-format').
The argument format is one of the strings used by BFD to name
binary formats. If TARGET is specified but OUTPUT_FORMAT
is not, the last TARGET argument is also used as the default
format for the ld output file. See section BFD.
If you don't use the TARGET command, ld uses the value of
the environment variable GNUTARGET, if available, to select the
output file format. If that variable is also absent, ld uses
the default format configured for your machine in the BFD libraries.
ld has additional features on some platforms; the following
sections describe them. Machines where ld has no additional
functionality are not listed.
ld and the Intel 960 familyYou can use the `-Aarchitecture' command line option to specify one of the two-letter names identifying members of the 960 family; the option specifies the desired output target, and warns of any incompatible instructions in the input files. It also modifies the linker's search strategy for archive libraries, to support the use of libraries specific to each particular architecture, by including in the search loop names suffixed with the string identifying the architecture.
For example, if your ld command line included `-ACA' as
well as `-ltry', the linker would look (in its built-in search
paths, and in any paths you specify with `-L') for a library with
the names
try libtry.a tryca libtryca.a
The first two possibilities would be considered in any event; the last two are due to the use of `-ACA'.
You can meaningfully use `-A' more than once on a command line, since the 960 architecture family allows combination of target architectures; each use will add another pair of name variants to search for when `-l' specifies a library.
ld supports the `-relax' option for the i960 family. If you
specify `-relax', ld finds all balx and calx
instructions whose targets are within 24 bits, and turns them into
24-bit program-counter relative bal and cal
instructions, respectively. ld also turns cal
instructions into bal instructions when it determines that the
target subroutine is a leaf routine (that is, the target subroutine does
not itself call any subroutines).
The 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 section `objdump' in The GNU Binary Utilities) 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 section 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 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,
IEEE, Oasys) and the type is simple enough to fit within one word
(nearly everything but aggregates), the information will be preserved.
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 section Command Language. MRI compatible linker
scripts have a much simpler command set than the scripting language
otherwise used with ld. GNU ld supports the most
commonly used MRI linker commands; these commands are described here.
In general, MRI scripts aren't of much use with the a.out object
file format, since it only has three sections and MRI scripts lack some
features to make use of them.
You can specify a file containing an MRI-compatible script using the `-c' command-line option.
Each command in an MRI-compatible script occupies its own line; each
command line starts with the keyword that identifies the command (though
blank lines are also allowed for punctuation). If a line of an
MRI-compatible script begins with an unrecognized keyword, ld
issues a warning message, but continues processing the script.
Lines beginning with `*' are comments.
You can write these commands using all upper-case letters, or all lower case; for example, `chip' is the same as `CHIP'. The following list shows only the upper-case form of each command.
ABSOLUTE secname
ABSOLUTE secname, secname, ... secname
ld includes in the output file all sections from all
the input files. However, in an MRI-compatible script, you can use the
ABSOLUTE command to restrict the sections that will be present in
your output program. If the ABSOLUTE command is used at all in a
script, then only the sections named explicitly in ABSOLUTE
commands will appear in the linker output. You can still use other
input sections (whatever you select on the command line, or using
LOAD) to resolve addresses in the output file.
ALIAS out-secname, in-secname
ALIGN secname = expression
BASE expression
CHIP expression
CHIP expression, expression
END
FORMAT output-format
OUTPUT_FORMAT command in the more general linker
language, but restricted to one of these output formats:
LIST anything...
ld command-line option `-M'.
The keyword LIST may be followed by anything on the
same line, with no change in its effect.
LOAD filename
LOAD filename, filename, ... filename
ld
command line.
NAME output-name
ld; the
MRI-compatible command NAME is equivalent to the command-line
option `-o' or the general script language command OUTPUT.
ORDER secname, secname, ... secname
ORDER secname secname secname
ld orders the sections in its output file in the
order in which they first appear in the input files. In an MRI-compatible
script, you can override this ordering with the ORDER command. The
sections you list with ORDER will appear first in your output
file, in the order specified.
PUBLIC name=expression
PUBLIC name,expression
PUBLIC name expression
SECT secname, expression
SECT secname=expression
SECT secname expression
SECT command to
specify the start address (expression) for section secname.
If you have more than one SECT statement for the same
secname, only the first sets the start address.
Jump to: " - * - - - . - 0 - : - ; - = - > - [ - a - b - c - d - e - f - g - h - i - k - l - m - n - o - p - q - r - s - t - u - v - w
-relax on i960
[section...], not supported
ABSOLUTE (MRI)
ALIAS (MRI)
ALIGN (MRI)
BASE (MRI)
CHIP (MRI)
END (MRI)
FORMAT (MRI)
LIST (MRI)
LOAD (MRI)
NAME (MRI)
ORDER (MRI)
PUBLIC (MRI)
SECT (MRI)
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