FreeChainXenon/binutils-gdb
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions
binutils-gdb/ld/ld.info-1

1255 lines
48 KiB
Text
Raw Blame History

This file contains invisible Unicode characters

This file contains invisible Unicode characters that are indistinguishable to humans but may be processed differently by a computer. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

This is Info file ld.info, produced by Makeinfo-1.55 from the input
file ./ld.texinfo.
START-INFO-DIR-ENTRY
* Ld:: The GNU linker.
END-INFO-DIR-ENTRY
This file documents the GNU linker LD.
Copyright (C) 1991, 1992, 1993 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.

File: ld.info, Node: Top, Next: Overview, Prev: (DIR), Up: (DIR)
Using ld
********
This file documents the GNU linker ld.
* Menu:
* Overview:: Overview
* Invocation:: Invocation
* Commands:: Command Language
* Machine Dependent:: Machine Dependent Features
* BFD:: BFD
* MRI:: MRI Compatible Script Files
* Index:: Index

File: ld.info, Node: Overview, Next: Invocation, Prev: Top, Up: Top
Overview
********
`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. *Note BFD:: for a list of formats supported on various
architectures.
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).

File: ld.info, Node: Invocation, Next: Commands, Prev: Overview, Up: Top
Invocation
**********
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.
* Menu:
* Options:: Command Line Options
* Environment:: Environment Variables

File: ld.info, Node: Options, Next: Environment, Up: Invocation
Command Line Options
====================
Here is a summary of the options you can use on the `ld' command
line:
ld [ -o OUTPUT ] OBJFILE...
[ -AARCHITECTURE ] [ -b INPUT-FORMAT ] [ -Bstatic ]
[ -c MRI-COMMANDFILE ] [ -d | -dc | -dp ]
[ -defsym SYMBOL=EXPRESSION ]
[ -e ENTRY ] [ -F ] [ -F FORMAT ]
[ -format INPUT-FORMAT ] [ -g ] [ -G SIZE ] [ --help ] [ -i ]
[ -lARCHIVE ] [ -LSEARCHDIR ] [ -M ] [ -Map MAPFILE ]
[ -m EMULATION ] [ -N | -n ] [ -noinhibit-exec ]
[ -oformat OUTPUT-FORMAT ] [ -R FILENAME ] [ -relax ]
[ -r | -Ur ] [ -S ] [ -s ] [ -sort-common ] [ -T COMMANDFILE ]
[ -Ttext ORG ] [ -Tdata ORG ]
[ -Tbss ORG ] [ -t ] [ -u SYMBOL] [-V] [-v] [ --version ]
[ -warn-common ] [ -ySYMBOL ] [ -X ] [-x ]
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', and
`-u'.
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.
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'.
Option arguments must either follow the option letter without
intervening whitespace, or be given as separate arguments immediately
following the option that requires them.
`-AARCHITECTURE'
In the current release of `ld', this option is useful only for the
Intel 960 family of architectures. In that `ld' configuration, the
ARCHITECTURE argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path. *Note `ld' and the Intel 960 family:
i960, for details.
Future releases of `ld' may support similar functionality for
other architecture families.
`-b INPUT-FORMAT'
Specify the binary format for input object files that follow this
option on the command line. You don't usually need to specify
this, as `ld' is 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'. *Note 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'. *Note Environment::. You can also define the input
format from a script, using the command `TARGET'; see *Note Option
Commands::.
`-Bstatic'
Ignored. This option is accepted for command-line compatibility
with the SunOS linker.
`-c MRI-COMMANDFILE'
For compatibility with linkers produced by MRI, `ld' accepts script
files written in an alternate, restricted command language,
described in *Note MRI Compatible Script Files: MRI. Introduce
MRI script files with the option `-c'; use the `-T' option to run
linker scripts written in the general-purpose `ld' scripting
language. If MRI-CMDFILE does not exist, `ld' looks for it in the
directories specified by any `-L' options.
`-d'
`-dc'
`-dp'
These three options are equivalent; multiple forms are supported
for compatibility with other linkers. They assign space to common
symbols even if a relocatable output file is specified (with
`-r'). The script command `FORCE_COMMON_ALLOCATION' has the same
effect. *Note Option Commands::.
`-defsym SYMBOL=EXPRESSION'
Create a global symbol in the output file, containing the absolute
address given by EXPRESSION. You may use this option as many
times as necessary to define multiple symbols in the command line.
A limited form of arithmetic is supported for the EXPRESSION in
this context: you may give a hexadecimal constant or the name of
an existing symbol, or use `+' and `-' to add or subtract
hexadecimal constants or symbols. If you need more elaborate
expressions, consider using the linker command language from a
script (*note Assignment: Symbol Definitions: Assignment.).
*Note:* there should be no white space between SYMBOL, the equals
sign ("="), and EXPRESSION.
`-e ENTRY'
Use ENTRY as the explicit symbol for beginning execution of your
program, rather than the default entry point. *Note Entry Point::,
for a discussion of defaults and other ways of specifying the
entry point.
`-F'
`-FFORMAT'
Ignored. Some older linkers used this option throughout a
compilation toolchain for specifying object-file format for both
input and output object files. The mechanisms `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'
Synonym for `-b INPUT-FORMAT'.
`-g'
Ignored. Provided for compatibility with other tools.
`-GVALUE'
`-G VALUE'
Set the maximum size of objects to be optimized using the GP
register to SIZE under MIPS ECOFF. Ignored for other object file
formats.
`--help'
Print a summary of the command-line options on the standard output
and exit. This option and `--version' begin with two dashes
instead of one for compatibility with other GNU programs. The
other options start with only one dash for compatibility with
other linkers.
`-i'
Perform an incremental link (same as option `-r').
`-lAR'
Add archive file ARCHIVE to the list of files to link. This
option may be used any number of times. `ld' will search its
path-list for occurrences of `libAR.a' for every ARCHIVE specified.
`-LSEARCHDIR'
`-L SEARCHDIR'
Add path SEARCHDIR to the list of paths that `ld' will search for
archive libraries and `ld' control scripts. You may use this
option any number of times.
The 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. *Note Environment::.
The paths can also be specified in a link script with the
`SEARCH_DIR' command.
`-M'
Print (to the standard output) a link map--diagnostic information
about where symbols are mapped by `ld', and information on global
common storage allocation.
`-Map MAPFILE'
Print to the file MAPFILE a link map--diagnostic information about
where symbols are mapped by `ld', and information on global common
storage allocation.
`-mEMULATION'
`-m EMULATION'
Emulate the EMULATION linker. You can list the available
emulations with the `-V' option. The default is the system for
which you configured `ld'.
`-N'
Set the text and data sections to be readable and writable. Also,
do not page-align the data segment. If the output format supports
Unix style magic numbers, mark the output as `OMAGIC'.
`-n'
Set the text segment to be read only, and mark the output as
`NMAGIC' if possible.
`-noinhibit-exec'
Retain the executable output file whenever it is still usable.
Normally, the linker will not produce an output file if it
encounters errors during the link process; it exits without
writing an output file when it issues any error whatsoever.
`-o OUTPUT'
Use OUTPUT as the name for the program produced by `ld'; if this
option is not specified, the name `a.out' is used by default. The
script command `OUTPUT' can also specify the output file name.
`-oformat OUTPUT-FORMAT'
Specify the binary format for the output object file. You don't
usually need to specify this, as `ld' is 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.
*Note BFD::.
`-R FILENAME'
Read symbol names and their addresses from FILENAME, but do not
relocate it or include it in the output. This allows your output
file to refer symbolically to absolute locations of memory defined
in other programs.
`-relax'
An option with machine dependent effects. Currently this option
is only supported on the H8/300. *Note `ld' and the H8/300:
H8/300.
On some platforms, use 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.
`-r'
Generate relocatable output--i.e., generate an output file that
can in turn serve as input to `ld'. This is often called "partial
linking". As a side effect, in environments that support standard
Unix magic numbers, this option also sets the output file's magic
number to `OMAGIC'. If this option is not specified, an absolute
file is produced. When linking C++ programs, this option *will
not* resolve references to constructors; to do that, use `-Ur'.
This option does the same thing as `-i'.
`-S'
Omit debugger symbol information (but not all symbols) from the
output file.
`-s'
Omit all symbol information from the output file.
`-sort-common'
Normally, when `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.
`-Tbss ORG'
`-Tdata ORG'
`-Ttext ORG'
Use ORG as the starting address for--respectively--the `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'
Read link commands from the file COMMANDFILE. These commands
completely override `ld''s default link format (rather than adding
to it); COMMANDFILE must specify everything necessary to describe
the target format. *Note Commands::. If COMMANDFILE does not
exist, `ld' looks for it in the directories specified by any
preceding `-L' options. Multiple `-T' options accumulate.
`-t'
Print the names of the input files as `ld' processes them.
`-u SYMBOL'
Force SYMBOL to be entered in the output file as an undefined
symbol. Doing this may, for example, trigger linking of
additional modules from standard libraries. `-u' may be repeated
with different option arguments to enter additional undefined
symbols.
`-Ur'
For anything other than C++ programs, this option is equivalent to
`-r': it generates relocatable output--i.e., an output file that
can in turn serve as input to `ld'. When linking C++ programs,
`-Ur' *will* 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 can not
be added to. Use `-Ur' only for the last partial link, and `-r'
for the others.
`-V'
Display the version number for `ld' and list the supported
emulations. Display which input files can and can not be opened.
`-v'
Display the version number for `ld'.
`--version'
Display the version number for `ld' and exit.
`-warn-common'
Warn when a common symbol is combined with another common symbol
or with a symbol definition. Unix linkers allow this somewhat
sloppy practice, but linkers on some other operating systems do
not. This option allows you to find potential problems from
combining global symbols. Unfortunately, some C libraries use
this practice, so you may get some warnings about symbols in the
libraries as well as in your programs.
There are three kinds of global symbols, illustrated here by C
examples:
`int i = 1;'
A definition, which goes in the initialized data section of
the output file.
`extern int i;'
An undefined reference, which does not allocate space. There
must be either a definition or a common symbol for the
variable somewhere.
`int i;'
A common symbol. If there are only (one or more) common
symbols for a variable, it goes in the uninitialized data
area of the output file. The linker merges multiple common
symbols for the same variable into a single symbol. If they
are of different sizes, it picks the largest size. The
linker turns a common symbol into a declaration, if there is
a definition of the same variable.
The `-warn-common' option can produce five kinds of warnings. Each
warning consists of a pair of lines: the first describes the
symbol just encountered, and the second describes the previous
symbol encountered with the same name. One or both of the two
symbols will be a common symbol.
1. Turning a common symbol into a reference, because there is
already a definition for the symbol.
FILE(SECTION): warning: common of `SYMBOL'
overridden by definition
FILE(SECTION): warning: defined here
2. Turning a common symbol into a reference, because a later
definition for the symbol is encountered. This is the same
as the previous case, except that the symbols are encountered
in a different order.
FILE(SECTION): warning: definition of `SYMBOL'
overriding common
FILE(SECTION): warning: common is here
3. Merging a common symbol with a previous same-sized common
symbol.
FILE(SECTION): warning: multiple common
of `SYMBOL'
FILE(SECTION): warning: previous common is here
4. Merging a common symbol with a previous larger common symbol.
FILE(SECTION): warning: common of `SYMBOL'
overridden by larger common
FILE(SECTION): warning: larger common is here
5. Merging a common symbol with a previous smaller common
symbol. This is the same as the previous case, except that
the symbols are encountered in a different order.
FILE(SECTION): warning: common of `SYMBOL'
overriding smaller common
FILE(SECTION): warning: smaller common is here
`-X'
If `-s' or `-S' is also specified, delete only local symbols
beginning with `L'.
`-x'
If `-s' or `-S' is also specified, delete all local symbols, not
just those beginning with `L'.
`-ySYMBOL'
Print the name of each linked file in which SYMBOL appears. This
option may be given any number of times. On many systems it is
necessary to prepend an underscore.
This option is useful when you have an undefined symbol in your
link but don't know where the reference is coming from.

File: ld.info, Node: Environment, Prev: Options, Up: Invocation
Environment Variables
=====================
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 (*note 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.

File: ld.info, Node: Commands, Next: Machine Dependent, Prev: Invocation, Up: Top
Command Language
****************
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:
* input files
* file formats
* output file layout
* addresses of sections
* placement of common blocks
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.
* Menu:
* Scripts:: Linker Scripts
* Expressions:: Expressions
* MEMORY:: MEMORY Command
* SECTIONS:: SECTIONS Command
* Entry Point:: The Entry Point
* Option Commands:: Option Commands

File: ld.info, Node: Scripts, Next: Expressions, Up: Commands
Linker Scripts
==============
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 (*note 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. *Note MEMORY::.
You may include comments in linker scripts just as in C: delimited
by `/*' and `*/'. As in C, comments are syntactically equivalent to
whitespace.

File: ld.info, Node: Expressions, Next: MEMORY, Prev: Scripts, Up: Commands
Expressions
===========
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:
* All expressions evaluated as integers and are of "long" or
"unsigned long" type.
* All constants are integers.
* All of the C arithmetic operators are provided.
* You may reference, define, and create global variables.
* You may call special purpose built-in functions.
* Menu:
* Integers:: Integers
* Symbols:: Symbol Names
* Location Counter:: The Location Counter
* Operators:: Operators
* Evaluation:: Evaluation
* Assignment:: Assignment: Defining Symbols
* Arithmetic Functions:: Built-In Functions

File: ld.info, Node: Integers, Next: Symbols, Up: Expressions
Integers
--------
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 `-'; *note
Operators::..
_as_neg = -57005;
Additionally the suffixes `K' and `M' may be used to scale a
constant by `1024' or `1024*1024' respectively. For example, the
following all refer to the same quantity:
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;

File: ld.info, Node: Symbols, Next: Location Counter, Prev: Integers, Up: Expressions
Symbol Names
------------
Unless quoted, symbol names start with a letter, underscore, point or
hyphen and may include any letters, underscores, digits, points, and
minus signs. 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;

File: ld.info, Node: Location Counter, Next: Operators, Prev: Symbols, Up: Expressions
The Location Counter
--------------------
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 (*note
Section Options::.).

File: ld.info, Node: Operators, Next: Evaluation, Prev: Location Counter, Up: Expressions
Operators
---------
The linker recognizes the standard C set of arithmetic operators,
with the standard bindings and precedence levels:
precedence associativity Operators Notes
(highest)
1 left ! - ~ (1)
2 left * / %
3 left + -
4 left >> <<
5 left == != > < <= >=
6 left &
7 left |
8 left &&
9 left ||
10 right ? :
11 right &= += -= *= /= (2)
(lowest)
Notes: (1) Prefix operators (2) *Note Assignment::

File: ld.info, Node: Evaluation, Next: Assignment, Prev: Operators, Up: Expressions
Evaluation
----------
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.

File: ld.info, Node: Assignment, Next: Arithmetic Functions, Prev: Evaluation, Up: Expressions
Assignment: Defining Symbols
----------------------------
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 may only be used at the root of an expression; `a=b+3;'
is allowed, but `a+b=3;' is an error.
* You must place a trailing semicolon (";") at the end of an
assignment statement.
Assignment statements may appear:
* as commands in their own right in an `ld' script; or
* as independent statements within a `SECTIONS' command; or
* as part of the contents of a section definition in a `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 (*note 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 (*note 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'".

File: ld.info, Node: Arithmetic Functions, Prev: Assignment, Up: Expressions
Arithmetic Functions
--------------------
The command language includes a number of built-in functions for use
in link script expressions.
`ABSOLUTE(EXP)'
Return the absolute (non-relocatable, as opposed to non-negative)
value of the expression EXP. Primarily useful to assign an
absolute value to a symbol within a section definition, where
symbol values are normally section-relative.
`ADDR(SECTION)'
Return the absolute address of the named SECTION. Your script must
previously have defined the location of that section. In the
following example, `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)'
Return the result of the current location counter (`.') 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 (*note Section Options::.). The second use
simply defines the value of a variable.
The built-in `NEXT' is closely related to `ALIGN'.
`DEFINED(SYMBOL)'
Return 1 if SYMBOL is in the linker global symbol table and is
defined, otherwise return 0. You can use this function to provide
default values for symbols. For example, the following
command-file fragment shows how to set a global 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)'
Return the next unallocated address that is a multiple of EXP.
This function is closely related to `ALIGN(EXP)'; unless you use
the `MEMORY' command to define discontinuous memory for the output
file, the two functions are equivalent.
`SIZEOF(SECTION)'
Return the size in bytes of the named SECTION, if that section has
been allocated. In the following example, `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'
Return the size in bytes of the output file's headers. You can
use this number as the start address of the first section, if you
choose, to facilitate paging.

File: ld.info, Node: MEMORY, Next: SECTIONS, Prev: Expressions, Up: Commands
Memory Layout
=============
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'
is a name used internally by the linker to refer to the region. Any
symbol name may be used. The region names are stored in a separate
name space, and will not conflict with symbols, file names or
section names. Use distinct names to specify multiple regions.
`(ATTR)'
is an optional list of attributes, permitted for compatibility
with the AT&T linker but not used by `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'
is the start address of the region in physical memory. It is an
expression that must evaluate to a constant before memory
allocation is performed. The keyword `ORIGIN' may be abbreviated
to `org' or `o' (but not, for example, `ORG').
`LEN'
is the size in bytes of the region (an expression). The keyword
`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 (*note Section Options::.). If the
combined output sections directed to a region are too big for the
region, the linker will issue an error message.

File: ld.info, Node: SECTIONS, Next: Entry Point, Prev: MEMORY, Up: Commands
Specifying Output Sections
==========================
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:
* define the entry point;
* assign a value to a symbol;
* describe the placement of a named output section, and which input
sections go into it.
You can also use the first two operations--defining the entry point
and defining symbols--outside the `SECTIONS' command: *note Entry
Point::., and *note Assignment::.. 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.
When no `SECTIONS' command is given, 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.
* Menu:
* Section Definition:: Section Definitions
* Section Placement:: Section Placement
* Section Data Expressions:: Section Data Expressions
* Section Options:: Optional Section Attributes

File: ld.info, Node: Section Definition, Next: Section Placement, Up: SECTIONS
Section Definitions
-------------------
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 (*note 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. *Note Symbol Names: Symbols.

File: ld.info, Node: Section Placement, Next: Section Data Expressions, Prev: Section Definition, Up: SECTIONS
Section Placement
-----------------
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'
You may simply name a particular input file to be placed in the
current output section; *all* sections from that file are placed
in the current section definition. If the file name has already
been mentioned in another section definition, with an explicit
section name list, then only those sections which have not yet
been allocated are used.
To specify a list of particular files by name:
.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 ... )'
You can name one or more sections from your input files, for
insertion in the current output section. If you wish to specify a
list of input-file sections inside the parentheses, you may
separate the section names by either commas or whitespace.
`* (SECTION)'
`* (SECTION, SECTION, ...)'
`* (SECTION SECTION ...)'
Instead of explicitly naming particular input files in a link
control script, you can refer to *all* files from the `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 )'
Specify where in your output file to place uninitialized data with
this notation. `*(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)
}
}

File: ld.info, Node: Section Data Expressions, Next: Section Options, Prev: Section Placement, Up: SECTIONS
Section Data Expressions
------------------------
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; *note 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'
Create a symbol for each input file in the current section, set to
the address of the first byte of data written from that input
file. For instance, with `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 ;'
SYMBOL is any symbol name (*note Symbols::.). "F=" refers to any
of the operators `&= += -= *= /=' 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
(*note Assignment::.). 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)'
By including one of these three statements in a section
definition, you can explicitly place one, two, or four bytes
(respectively) at the current address of that section.
Multiple-byte quantities are represented in whatever byte order is
appropriate for the output file format (*note BFD::.).
`FILL(EXPRESSION)'
Specify the "fill pattern" for the current section. Any otherwise
unspecified regions of memory within the section (for example,
regions you skip over by assigning a new value to the location
counter `.') are filled with the two least significant bytes from
the EXPRESSION argument. A `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.
Reference in a new issue View git blame Copy permalink