Application
Development
Michael K. Johnson
Erik W. Troan
|
|
Linux
Application Development |
Michael K. Johnson Erik W. Troan |
ld version 2
Copyright (C) 1991, 92, 93, 94, 95, 96, 1997 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.
Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.
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.
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.)
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. Options which may be meaningfully specified more than once are
noted in the descriptions below.
Non-option arguments are objects files 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 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. Note that
specifying a script in this way should only be used to augment the main
linker script; if you want to use some command that logically can only
appear once, such as the SECTIONS or MEMORY command, you
must replace the default linker script using the `-T' option.
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.
-akeyword
-Aarchitecture
--architecture=architecture
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
--format=input-format
ld may be configured to support more than one kind of object
file. If your ld is configured this way, you can use the
`-b' option to specify the binary format for input object files
that follow this option on the command line. Even when ld is
configured to support alternative object formats, you don't usually need
to specify this, as ld should be configured to expect as a
default input format the most usual format on each machine.
input-format is a text string, the name of a particular format
supported by the BFD libraries. (You can list the available binary
formats with `objdump -i'.)
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.
-c MRI-commandfile
--mri-script=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.
-e entry
--entry=entry
-E
--export-dynamic
dlopen.
-f
--auxiliary name
-F name
--filter name
-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.
--force-exe-suffix
.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.
-g
-Gvalue
--gpsize=value
-hname
-soname=name
-i
-larchive
--library=archive
ld will search its
path-list for occurrences of libarchive.a for every
archive specified.
On systems which support shared libraries, ld may also search for
libraries with extensions other than .a. Specifically, on ELF
and SunOS systems, ld will search a directory for a library with
an extension of .so before searching for one with an extension of
.a. By convention, a .so extension indicates a shared
library.
The linker will search an archive only once, at the location where it is
specified on the command line. If the archive defines a symbol which
was undefined in some object which appeared before the archive on the
command line, the linker will include the appropriate file(s) from the
archive. However, an undefined symbol in an object appearing later on
the command line will not cause the linker to search the archive again.
See the -( option for a way to force the linker to search
archives multiple times.
You may list the same archive multiple times on the command line.
This type of archive searching is standard for Unix linkers. However,
if you are using ld on AIX, note that it is different from the
behaviour of the AIX linker.
-Lsearchdir
--library-path=searchdir
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.
-memulation
ld was configured.
-M
--print-map
ld, and information on global common
storage allocation.
-n
--nmagic
NMAGIC if possible.
-N
--omagic
OMAGIC.
-o output
--output=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.
-r
--relocateable
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'.
-R filename
--just-symbols=filename
-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
-t
--trace
ld processes them.
-T commandfile
--script=commandfile
ld's default link script (rather than adding to it), so
commandfile must specify everything necessary to describe the
target format. You must use this option if you want to use a command
which can only appear once in a linker script, such as the
SECTIONS or MEMORY command. 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.
-u symbol
--undefined=symbol
-v
--version
-V
ld. The -V option also
lists the supported emulations.
-x
--discard-all
-X
--discard-locals
-y symbol
--trace-symbol=symbol
-Y path
-z keyword
-( archives -)
--start-group archives --end-group
-assert keyword
-Bdynamic
-dy
-call_shared
-l options which follow it.
-Bstatic
-dn
-non_shared
-static
-l options which follow it.
-Bsymbolic
--cref
--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
-EB
-EL
--embedded-relocs
--help
-Map mapfile
ld, and information on global
common storage allocation.
--no-keep-memory
ld normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells ld to
instead optimize for memory usage, by rereading the symbol tables as
necessary. This may be required if ld runs out of memory space
while linking a large executable.
--no-whole-archive
--whole-archive option for subsequent
archive files.
--noinhibit-exec
--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.
-qmagic
-Qy
--relax
ld and the H8/300.
See section 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.
For compatibility with other ELF linkers, if the -R option is
followed by a directory name, rather than a file name, it is treated as
the -rpath option.
-rpath-link DIR
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.
-shared
-Bshareable
-e option is not used and there are
undefined symbols in the link.
--sort-common
ld to sort the common symbols by size when it
places them in the appropriate output sections. First come all the one
byte symbols, then all the two bytes, then all the four bytes, and then
everything else. This is to prevent gaps between symbols due to
alignment constraints.
--split-by-file
--split-by-reloc but creates a new output section for
each input file.
--split-by-reloc count
--stats
--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.
-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.
-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. Display
the linker script if using a default builtin script.
--version-script=version-scriptfile
--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-multiple-gp
--warn-once
--warn-section-align
SECTIONS command does not specify a start address for
the section (see section Specifying Output Sections).
--whole-archive
--whole-archive option, include every object file in the archive
in the link, rather than searching the archive for the required object
files. This is normally used to turn an archive file into a shared
library, forcing every object to be included in the resulting shared
library. This option may be used more than once.
--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 (int c)
{
printf ("malloc called with %ld\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.
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 linker script) to the
linker either explicitly through the `-T' option, or implicitly as
an ordinary file. Normally you should use the `-T' option. An
implicit linker script should only be used when you want to augment,
rather than replace, the default linker script; typically an implicit
linker script would consist only of INPUT or GROUP
commands.
If the linker opens a file which it cannot recognize as a supported object or archive format, nor as a linker script, 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).
@vfill
The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels: { @obeylines@parskip=0pt@parindent=0pt @dag@quad Prefix operators. @ddag@quad 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;
}
... }
LOADADDR(section)
ADDR, but it may be different if the
AT keyword is used in the section definition (see section Optional Section Attributes).
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
MAX(exp1, exp2)
MIN(exp1, exp2)
Semicolons (";") are required in the following places. In all other places they can appear for aesthetic reasons but are otherwise ignored.
Assignment
PHDRS
PHDRS statement.
See section ELF Program 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 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 special secname `/DISCARD/' may be used to discard input sections. Any sections which are assigned to an output section named `/DISCARD/' are not included in the final link output.
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.
In any place where you may use a specific file or section name, you may also use a wildcard pattern. The linker handles wildcards much as the Unix shell does. A `*' character matches any number of characters. A `?' character matches any single character. The sequence `[chars]' will match a single instance of any of the chars; the `-' character may be used to specify a range of characters, as in `[a-z]' to match any lower case letter. A `\' character may be used to quote the following character.
When a file name is matched with a wildcard, the wildcard characters will not match a `/' character (used to separate directory names on Unix). A pattern consisting of a single `*' character is an exception; it will always match any file name. In a section name, the wildcard characters will match a `/' character.
Wildcards only match files which are explicitly specified on the command line. The linker does not search directories to expand wildcards. However, if you specify a simple file name--a name with no wildcard characters--in a linker script, and the file name is not also specified on the command line, the linker will attempt to open the file as though it appeared on the command line.
In the following example, the 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)
}
}
This example shows how wildcard patterns might be used to partition
files. All .text sections are placed in .text, and all
.bss sections are placed in .bss. For all files beginning
with an upper case character, the .data section is placed into
.DATA; for all other files, the .data section is placed
into .data.
SECTIONS {
.text : { *(.text) }
.DATA : { [A-Z]*(.data) }
.data : { *(.data) }
.bss : { *(.bss) }
}
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 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 OVERLAY command 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.
The OVERLAY command is used within a SECTIONS command. It
appears as follows:
OVERLAY start : [ NOCROSSREFS ] AT ( ldaddr )
{
secname1 { contents } :phdr =fill
secname2 { contents } :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 contruct (see section Specifying Output Sections),
except that no addresses and no memory regions may be defined for
sections within an OVERLAY.
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 .).
If the NOCROSSREFS keyword is used, and there 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 section Option Commands.
For each section within the OVERLAY, the linker automatically
defines 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 . 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: __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) }
__load_start_text0 = LOADADDR (.text0);
__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0);
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
__load_start_text1 = LOADADDR (.text1);
__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1);
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
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 linker command script includes a command specifically for specifying a version script, and is only meaningful for ELF platforms that support shared libraries. A version script can be build directly into the linker script that you are using, or you can supply the version script as just another input file to the linker at the time that you link. The command script syntax is:
VERSION { version script contents }
The version script can also be specified to the linker by means of the `--version-script' linker command line option. Version scripts are only meaningful when creating shared libraries.
The format of the version script itself is identical to that used by Sun's linker in Solaris 2.5. Versioning is done by defining a tree of version nodes with the names and interdependencies specified in the version script. The version script can specify which symbols are bound to which version nodes, and it 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;
} VERS_1.2;
In this example, three version nodes are defined. `VERS_1.1' is the first version node defined, and has no other dependencies. The symbol `foo1' is bound to this version node, and a number of symbols that have appeared within various object files are reduced in scope to local so that they are not visible outside of the shared library.
Next, the node `VERS_1.2' is defined. It depends upon `VERS_1.1'. The symbol `foo2' is bound to this version node.
Finally, the node `VERS_2.0' is defined. It depends upon `VERS_1.2'. The symbols `bar1' and `bar2' are bound to this version node.
Symbols defined in the library which aren't specifically bound to a version node are effectively bound to an unspecified base version of the library. It is possible to bind all otherwise unspecified symbols to a given version node using `global: *' somewhere in the version script.
Lexically 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.
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. This can be done by putting something like:
__asm__(".symver original_foo,foo@VERS_1.1");
in the C source file. This renamed 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.
The second GNU extension is to allow multiple versions of the same function to appear in a given shared library. In this way an incompatible change to an interface can take place without increasing the major version number of the shared library, while still allowing applications linked against the old interface to continue to function.
This can only be accomplished by using multiple `.symver' directives in the assembler. An example of this would be:
__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. This can be accomplished with the `foo@@VERS_2.0' type of `.symver' directive. Only one version of a symbol can be declared '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.
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
a.out object file format, the linker uses
an unusual set construct to support C++ global constructors and
destructors. When linking object file formats which do not support
arbitrary sections, such as ECOFF and XCOFF, the linker
will automatically recognize C++ global constructors and destructors by
name. For these object file formats, the CONSTRUCTORS command
tells the linker where this information should be placed. The
CONSTRUCTORS command is ignored for other object file formats.
The symbol __CTOR_LIST__ marks the start of the global
constructors, and the symbol __DTOR_LIST marks the end. 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++ calls constructors from a subroutine __main;
a call to __main is automatically inserted into the startup code
for main. GNU C++ runs destructors either by using
atexit, or directly from the function exit.
For object file formats such as COFF or ELF which support
multiple sections, 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__ = .;
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.
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').
INCLUDE filename
-L option. You can nest calls to INCLUDE up to
10 levels deep.
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.
NOCROSSREFS ( section section ... )
ld to issue an error about any
references among certain sections.
In certain types of programs, particularly on embedded systems, 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 section names. If
ld detects any cross references between the sections, it reports
an error and returns a non-zero exit status. The NOCROSSREFS
command uses output section names, defined in the SECTIONS
command. It does not use the names of input sections.
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 H8/300
For the H8/300, ld can perform these global optimizations when
you specify the `--relax' command-line option.
ld finds all 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 finds all 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 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. @xref{BFD information loss}.
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:
ld bug. Reliable linkers never crash.
ld produces an error message for valid input, that is a bug.
ld does not produce an error message for invalid input, that
may be a bug. In the general case, the linker can not verify that
object files are correct.
ld are welcome in any case.
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.
In any event, we also recommend that you send bug reports for ld
to `bug-gnu-utils@prep.ai.mit.edu'.
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?" Those bug reports are useless, and we urge everyone to refuse to respond to them except to chide the sender to report bugs properly.
To enable us to fix the bug, you should include all these things:
ld. ld announces it if you start it with
the `--version' argument.
Without this, we will not know whether there is any point in looking for
the bug in the current version of ld.
ld source, including any
patches made to the BFD library.
ld---e.g.
"gcc-2.7".
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.
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 synch, 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.
ld source, send us context
diffs, as generated by 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:
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.
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.
--relax on i960
[section...], not supported
ABSOLUTE (MRI)
ALIAS (MRI)
ALIGN (MRI)
BASE (MRI)
ld
CHIP (MRI)
END (MRI)
FORMAT (MRI)
ld bugs, reporting
LIST (MRI)
LOAD (MRI)
NAME (MRI)
ORDER (MRI)
PUBLIC (MRI)
ld
SECT (MRI)
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