The command language provides explicit control over the link process, allowing complete specification of the mapping between the linker's input files and its output. It controls:
You may supply a command file (also known as a link script) to the linker either explicitly through the `-T' option, or implicitly as an ordinary file. If the linker opens a file which it cannot recognize as a supported object or archive format, it reports an error.
The ld command language is a collection of statements; some are
simple keywords setting a particular option, some are used to select and
group input files or name output files; and two statement
types have a fundamental and pervasive impact on the linking process.
The most fundamental command of the ld command language is the
SECTIONS command (see section Specifying Output Sections). Every meaningful command
script must have a SECTIONS command: it specifies a
"picture" of the output file's layout, in varying degrees of detail.
No other command is required in all cases.
The MEMORY command complements SECTIONS by describing the
available memory in the target architecture. This command is optional;
if you don't use a MEMORY command, ld assumes sufficient
memory is available in a contiguous block for all output.
See section Memory Layout.
You may include comments in linker scripts just as in C: delimited by `/*' and `*/'. As in C, comments are syntactically equivalent to whitespace.
Many useful commands involve arithmetic expressions. The syntax for expressions in the command language is identical to that of C expressions, with the following features:
An octal integer is `0' followed by zero or more of the octal digits (`01234567').
_as_octal = 0157255;
A decimal integer starts with a non-zero digit followed by zero or more digits (`0123456789').
_as_decimal = 57005;
A hexadecimal integer is `0x' or `0X' followed by one or more hexadecimal digits chosen from `0123456789abcdefABCDEF'.
_as_hex = 0xdead;
To write a negative integer, use the prefix operator `-'; see section Operators.
_as_neg = -57005;
Additionally the suffixes K and M may be used to scale a
constant by
respectively. For example, the following all refer to the same quantity:
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;
Unless quoted, symbol names start with a letter, underscore, or point and may include any letters, underscores, digits, points, and hyphens. Unquoted symbol names must not conflict with any keywords. You can specify a symbol which contains odd characters or has the same name as a keyword, by surrounding the symbol name in double quotes:
"SECTION" = 9;
"with a space" = "also with a space" + 10;
Since symbols can contain many non-alphabetic characters, it is safest to delimit symbols with spaces. For example, `A-B' is one symbol, whereas `A - B' is an expression involving subtraction.
The special linker variable dot `.' always contains the
current output location counter. Since the . always refers to
a location in an output section, it must always appear in an
expression within a SECTIONS command. The . symbol
may appear anywhere that an ordinary symbol is allowed in an
expression, but its assignments have a side effect. Assigning a value
to the . symbol will cause the location counter to be moved.
This may be used to create holes in the output section. The location
counter may never be moved backwards.
SECTIONS
{
output :
{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
} = 0x1234;
}
In the previous example, file1 is located at the beginning of the
output section, then there is a 1000 byte gap. Then file2
appears, also with a 1000 byte gap following before file3 is
loaded. The notation `= 0x1234' specifies what data to write in
the gaps (see section Optional Section Attributes).
@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;
}
... }
ALIGN(exp)
.) aligned to
the next exp boundary. exp must be an expression whose
value is a power of two. This is equivalent to
(. + exp - 1) & ~(exp - 1)
ALIGN doesn't change the value of the location counter--it just
does arithmetic on it. As an example, to align the output .data
section to the next 0x2000 byte boundary after the preceding
section and to set a variable within the section to the next
0x8000 boundary after the input sections:
SECTIONS{ ...
.data ALIGN(0x2000): {
*(.data)
variable = ALIGN(0x8000);
}
... }
The first use of ALIGN in this example specifies the location of
a section because it is used as the optional start attribute of a
section definition (see section Optional Section Attributes). The second use simply
defines the value of a variable.
The built-in NEXT is closely related to ALIGN.
DEFINED(symbol)
begin to the first location in the
.text section--but if a symbol called begin already
existed, its value is preserved:
SECTIONS{ ...
.text : {
begin = DEFINED(begin) ? begin : . ;
...
}
... }
NEXT(exp)
ALIGN(exp); unless you
use the MEMORY command to define discontinuous memory for the
output file, the two functions are equivalent.
SIZEOF(section)
symbol_1 and
symbol_2 are assigned identical values:
SECTIONS{ ...
.output {
.start = . ;
...
.end = . ;
}
symbol_1 = .end - .start ;
symbol_2 = SIZEOF(.output);
... }
SIZEOF_HEADERS
sizeof_headers
The linker's default configuration permits allocation of all available memory.
You can override this configuration by using the MEMORY command. The
MEMORY command describes the location and size of blocks of
memory in the target. By using it carefully, you can describe which
memory regions may be used by the linker, and which memory regions it
must avoid. The linker does not shuffle sections to fit into the
available regions, but does move the requested sections into the correct
regions and issue errors when the regions become too full.
A command file may contain at most one use of the MEMORY
command; however, you can define as many blocks of memory within it as
you wish. The syntax is:
MEMORY
{
name (attr) : ORIGIN = origin, LENGTH = len
...
}
name
(attr)
ld beyond checking that the
attribute list is valid. Valid attribute lists must be made up of the
characters "LIRWX". If you omit the attribute list, you may
omit the parentheses around it as well.
origin
ORIGIN may be
abbreviated to org or o (but not, for example, `ORG').
len
LENGTH may be abbreviated to len or l.
For example, to specify that memory has two regions available for
allocation--one starting at 0 for 256 kilobytes, and the other
starting at 0x40000000 for four megabytes:
MEMORY
{
rom : ORIGIN = 0, LENGTH = 256K
ram : org = 0x40000000, l = 4M
}
Once you have defined a region of memory named mem, you can direct
specific output sections there by using a command ending in
`>mem' within the SECTIONS command (see section Optional Section Attributes). If the combined output sections directed to a region are too
big for the region, the linker will issue an error message.
The SECTIONS command controls exactly where input sections are
placed into output sections, their order in the output file, and to
which output sections they are allocated.
You may use at most one SECTIONS command in a script file,
but you can have as many statements within it as you wish. Statements
within the SECTIONS command can do one of three things:
You can also use the first two operations--defining the entry point and
defining symbols--outside the SECTIONS command: see section The Entry Point, and see section Assignment: Defining Symbols. They are permitted here as well for
your convenience in reading the script, so that symbols and the entry
point can be defined at meaningful points in your output-file layout.
If you do not use a SECTIONS command, the linker places each input
section into an identically named output section in the order that the
sections are first encountered in the input files. If all input sections
are present in the first file, for example, the order of sections in the
output file will match the order in the first input file.
The most frequently used statement in the SECTIONS command is
the section definition, which specifies the
properties of an output section: its location, alignment, contents,
fill pattern, and target memory region. Most of
these specifications are optional; the simplest form of a section
definition is
SECTIONS { ...
secname : {
contents
}
... }
secname is the name of the output section, and contents a specification of what goes there--for example, a list of input files or sections of input files (see section Section Placement). As you might assume, the whitespace shown is optional. You do need the colon `:' and the braces `{}', however.
secname must meet the constraints of your output format. In
formats which only support a limited number of sections, such as
a.out, the name must be one of the names supported by the format
(a.out, for example, allows only .text, .data or
.bss). If the output format supports any number of sections, but
with numbers and not names (as is the case for Oasys), the name should be
supplied as a quoted numeric string. A section name may consist of any
sequence of characters, but any name which does not conform to the standard
ld symbol name syntax must be quoted.
See section Symbol Names.
The linker will not create output sections which do not have any contents. This is for convenience when referring to input sections that may or may not exist. For example,
.foo { *(.foo }
will only create a `.foo' section in the output file if there is a `.foo' section in at least one input file.
In a section definition, you can specify the contents of an output section by listing particular input files, by listing particular input-file sections, or by a combination of the two. You can also place arbitrary data in the section, and define symbols relative to the beginning of the section.
The contents of a section definition may include any of the following kinds of statement. You can include as many of these as you like in a single section definition, separated from one another by whitespace.
filename
.data : { afile.o bfile.o cfile.o }
The example also illustrates that multiple statements can be included in
the contents of a section definition, since each file name is a separate
statement.
filename( section )
filename( section, section, ... )
filename( section section ... )
* (section)
* (section, section, ...)
* (section section ...)
ld command
line: use `*' instead of a particular file name before the
parenthesized input-file section list.
If you have already explicitly included some files by name, `*'
refers to all remaining files--those whose places in the output
file have not yet been defined.
For example, to copy sections 1 through 4 from an Oasys file
into the .text section of an a.out file, and sections 13
and 14 into the .data section:
SECTIONS {
.text :{
*("1" "2" "3" "4")
}
.data :{
*("13" "14")
}
}
`[ section ... ]' used to be accepted as an alternate way
to specify named sections from all unallocated input files. Because
some operating systems (VMS) allow brackets in file names, that notation
is no longer supported.
filename( COMMON )
*( COMMON )
*(COMMON) by itself refers to all
uninitialized data from all input files (so far as it is not yet
allocated); filename(COMMON) refers to uninitialized data
from a particular file. Both are special cases of the general
mechanisms for specifying where to place input-file sections:
ld permits you to refer to uninitialized data as if it
were in an input-file section named COMMON, regardless of the
input file's format.
For example, the following command script arranges the output file into
three consecutive sections, named .text, .data, and
.bss, taking the input for each from the correspondingly named
sections of all the input files:
SECTIONS {
.text : { *(.text) }
.data : { *(.data) }
.bss : { *(.bss) *(COMMON) }
}
The following example reads all of the sections from file all.o
and places them at the start of output section outputa which
starts at location 0x10000. All of section .input1 from
file foo.o follows immediately, in the same output section. All
of section .input2 from foo.o goes into output section
outputb, followed by section .input1 from foo1.o.
All of the remaining .input1 and .input2 sections from any
files are written to output section outputc.
SECTIONS {
outputa 0x10000 :
{
all.o
foo.o (.input1)
}
outputb :
{
foo.o (.input2)
foo1.o (.input1)
}
outputc :
{
*(.input1)
*(.input2)
}
}
The foregoing statements arrange, in your output file, data originating
from your input files. You can also place data directly in an output
section from the link command script. Most of these additional
statements involve expressions; see section Expressions. Although these
statements are shown separately here for ease of presentation, no such
segregation is needed within a section definition in the SECTIONS
command; you can intermix them freely with any of the statements we've
just described.
CREATE_OBJECT_SYMBOLS
a.out
files it is conventional to have a symbol for each input file. You can
accomplish this by defining the output .text section as follows:
SECTIONS {
.text 0x2020 :
{
CREATE_OBJECT_SYMBOLS
*(.text)
_etext = ALIGN(0x2000);
}
...
}
If sample.ld is a file containing this script, and a.o,
b.o, c.o, and d.o are four input files with
contents like the following---
/* a.c */
afunction() { }
int adata=1;
int abss;
`ld -M -T sample.ld a.o b.o c.o d.o' would create a map like this,
containing symbols matching the object file names:
00000000 A __DYNAMIC 00004020 B _abss 00004000 D _adata 00002020 T _afunction 00004024 B _bbss 00004008 D _bdata 00002038 T _bfunction 00004028 B _cbss 00004010 D _cdata 00002050 T _cfunction 0000402c B _dbss 00004018 D _ddata 00002068 T _dfunction 00004020 D _edata 00004030 B _end 00004000 T _etext 00002020 t a.o 00002038 t b.o 00002050 t c.o 00002068 t d.o
symbol = expression ;
symbol f= expression ;
&= += -= *= /= which combine
arithmetic and assignment.
When you assign a value to a symbol within a particular section
definition, the value is relative to the beginning of the section
(see section Assignment: Defining Symbols). If you write
SECTIONS {
abs = 14 ;
...
.data : { ... rel = 14 ; ... }
abs2 = 14 + ADDR(.data);
...
}
abs and rel do not have the same value; rel has the
same value as abs2.
BYTE(expression)
SHORT(expression)
LONG(expression)
QUAD(expression)
QUAD is only supported when
using a 64 bit host or target.
Multiple-byte quantities are represented in whatever byte order is
appropriate for the output file format (see section BFD).
FILL(expression)
FILL statement covers memory
locations after the point it occurs in the section definition; by
including more than one FILL statement, you can have different
fill patterns in different parts of an output section.
Here is the full syntax of a section definition, including all the optional portions:
SECTIONS {
...
secname start BLOCK(align) (NOLOAD) : AT ( ldadr )
{ contents } >region =fill
...
}
secname and contents are required. See section Section Definitions, and see section Section Placement for details on
contents. The remaining elements---start,
BLOCK(align), (NOLOAD), AT ( ldadr ),
>region, 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
=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 linker command language includes a command specifically for defining the first executable instruction in an output file (its entry point). Its argument is a symbol name:
ENTRY(symbol)
Like symbol assignments, the ENTRY command may be placed either
as an independent command in the command file, or among the section
definitions within the SECTIONS command--whatever makes the most
sense for your layout.
ENTRY is only one of several ways of choosing the entry point.
You may indicate it in any of the following ways (shown in descending
order of priority: methods higher in the list override methods lower down).
ENTRY(symbol) command in a linker control script;
start, if present;
.text section, if present;
0.
For example, you can use these rules to generate an entry point with an
assignment statement: if no symbol start is defined within your
input files, you can simply define it, assigning it an appropriate
value---
start = 0x2020;
The example shows an absolute address, but you can use any expression.
For example, if your input object files use some other symbol-name
convention for the entry point, you can just assign the value of
whatever symbol contains the start address to start:
start = other_symbol ;
The command language includes a number of other commands that you can use for specialized purposes. They are similar in purpose to command-line options.
CONSTRUCTORS
CONSTRUCTORS command specifies where the
linker is to place the data from these sections, relative to the rest of
the linked output. Constructor data is marked by the symbol
__CTOR_LIST__ at the start, and __CTOR_LIST_END at
the end; destructor data is bracketed similarly, between
__DTOR_LIST__ and __DTOR_LIST_END. (The compiler
must arrange to actually run this code; GNU C++ calls constructors from
a subroutine __main, which it inserts automatically into the
startup code for main, and destructors from _exit.)
FLOAT
NOFLOAT
ld doesn't use the keywords, assuming
instead that any necessary subroutines are in libraries specified using
the general mechanisms for linking to archives; but to permit the use of
scripts that were written for the older linkers, the keywords
FLOAT and NOFLOAT are accepted and ignored.
FORCE_COMMON_ALLOCATION
ld assign space to common symbols even if a relocatable
output file is specified (`-r').
INPUT ( file, file, ... )
INPUT ( file file ... )
ld searches for each file through the archive-library
search path, just as for files you specify on the command line.
See the description of `-L' in @xref{Options,,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
@xref{Options,,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.