A linker script controls
every link. Such a script is written in the linker command language. The
main purpose of the linker script is to describe how the sections in the
input files should be mapped into the output file, and to control the memory
layout of the output file. However, when necessary, the linker script can
also direct the linker to perform many other operations, using the linker
commands. The following documentation discusses using the linker script
and its commands.
Basic
linker script concepts
The linker always uses a linker
script. If you do not supply one yourself, the linker will use a default
script that is compiled into the linker executable. You can use the
--verbose command line option to display the default linker script.
Certain command line options, such as -r or -N,
will affect the default linker script. You may supply your own linker script
by using the -T command line option. When you do this, your linker
script will replace the default linker script.
You may also
use linker scripts implicitly by naming them as input files to the linker,
as though they were files to be linked. If the linker opens a file, which
it can not recognize as an object file or as an archive file, it will try
to read it as a linker script. If the file can not be parsed as a linker
script, the linker will report an error. An implicit linker script will
not replace the default linker script. Typically an implicit linker script
would contain only the INPUT, GROUP, or VERSION
commands.
We need to define some basic
concepts and vocabulary in order to describe the linker script language.
The linker
combines input files into a single output file. The output file and each
input file are in a special data format known as an object file format .
Each file is called an object file .
The output file is often called an executable ,
but for our purposes we will also call it an object file. Each object file
has, among other things, a list of sections. We sometimes refer to a section
in an input file as an input section ;
similarly, a section in the output file is an output section .
Each section
in an object file has a name and a size. Most sections also have an associated
block of data, known as the section contents .
A section may be marked as loadable ,
meaning that the contents should be loaded into memory when the output
file is run. A section with no contents may be allocatable ,
which means that an area in memory should be set aside, but nothing in
particular should be loaded there (in some cases this memory must be zeroed
out). A section, which is neither loadable nor allocatable, typically contains
some sort of debugging information.
Every loadable
or allocatable output section has two addresses. The first is the VMA ,
or virtual memory address . This is the
address the section will have when the output file is run. The second is
the LMA , or load memory address .
This is the address at which the section will be loaded. In most cases
the two addresses will be the same. An example of when they might be different
is when a data section is loaded into ROM, and then copied into RAM when
the program starts up (this technique is often used to initialize global
variables in a ROM based system). In this case the ROM address would be
the LMA, and the RAM address would be
the VMA.
You can see
the sections in an object file by using the objdump program with
the -h option.
Every object
file also has a list of symbols , known
as the symbol table . A symbol may be
defined or undefined. Each symbol has a name, and each defined symbol has
an address, among other information. If you compile a C or C++ program
into an object file, you will get a defined symbol for every defined function
and global or static variable. Every undefined function or global variable,
which is referenced in the input file, will become an undefined symbol.
You can see the symbols in an object file by using the nm
program, or by using the objdump
program with the -t option.
Linker scripts are text
files. You write a linker script as a series of commands. Each command
is either a keyword, possibly followed by arguments or an assignment to
a symbol. You may separate commands using semicolons. Whitespace is generally
ignored.
Strings such
as file or format names can normally be entered directly. If the file name
contains a character such as a comma, which would otherwise serve to separate
file names, you may put the file name in double quotes. There is no way
to use a double quote character in a file name.
You may include
comments in linker scripts just as in C, delimited by /*
and */. As in C, comments are syntactically equivalent to whitespace.
Simple
linker script example
Many linker scripts
are fairly simple. The simplest possible linker script has just one command: SECTIONS.
You use the SECTIONS command to describe the memory layout of
the output file. The SECTIONS command is a powerful command. Here
we will describe a simple use of it. Lets assume your program consists
only of code, initialized data, and uninitialized data. These will be in
the .text
, .data, and .bss sections, respectively. Lets assume
further that these are the only sections, which appear in your input files.
For this example,
lets say that the code should be loaded at address 0x10000 , and
that the data should start at address 0x8000000. The following
linker script will do this function.
SECTIONS
{
. = 0x10000;
.text : { *(.text) }
. = 0x8000000;
.data : { *(.data) }
.bss : { *(.bss) }
}
You write the
SECTIONS command as the keyword SECTIONS, followed by
a series of symbol assignments and output section descriptions enclosed
in curly braces. The first line in the above example sets the special symbol
., which is the location counter. If you do not specify the address
of an output section in some other way (other ways are described later),
the address is set from the current value of the location counter. The
location counter is then incremented by the size of the output section.
The second line defines an output section, .text. The colon is
required syntax, which may be ignored for now. Within the curly braces
after the output section name, you list the names of the input sections,
which should be placed into this output section. The * is a wildcard
which matches any file name. The expression *(.text)
means all .text input sections
in all input files.
Since the location
counter is 0x10000 when the output section .text is defined,
the linker will set the address of the .text
section in the output file to be 0x10000. The remaining lines
define the .data and .bss sections in the output file.
The .data output section will be at address 0x8000000
. When the .bss output section is defined, the value of the location
counter will be 0x8000000 plus the size of the ` .data
output section. The effect is that the .bss
output section will follow immediately after the .data
output section in memory.
Thats it! Thats
a simple and complete linker script.
Simple
linker script commands
In the following documentation,
the discussion describes the simple linker script commands. See also Command
line options for ld and BFD.
Setting
the entry point
The first instruction
to execute in a program is called the entry point.
You can use the ENTRY linker script command to set the entry point.
The argument is a symbol name:
ENTRY (symbol)
There are several
ways to set the entry point. The linker will set the entry point by trying
each of the following methods in order, and stopping when one of them succeeds:
-
The -e entry command-line
option;
-
The ENTRY (symbol)
command in a linker script;
-
The value of the symbol, start,
if defined;
-
The address of the first byte
of the .text section, if present;
-
The address, 0.
Commands
dealing with files
Several linker script
commands deal with files. See also Command
line options for ld and BFD.
INCLUDE filename
Include the linker script, filename,
at this point. The file will be searched for in the current directory,
and in any directory specified with the -L option. You can nest
calls to INCLUDE up to 10 levels deep.
INPUT (file, file,
...)
INPUT (file file
...)
The INPUT command directs the linker to include
the named files in the link, as though they were named on the command line.
For example, if you always
want to include subr.o any time you do a link, but you cant be
bothered to put it on every link command line, then you can put INPUT
(subr.o) in your linker script. In fact, if you like, you can list
all of your input files in the linker script, and then invoke the linker
with nothing but a -T option. The linker will first try to open
the file in the current directory. If it is not found, the linker will
search through the archive library search path. See the description of
-L . If you use INPUT (-l file), ld
will transform the name to lib file.a , as with
the command line argument, -l. When you use the INPUT
command in an implicit linker script, the files will be included in the
link at the point at which the linker script file is included. This can
affect archive searching.
GROUP(FILE, FILE, ...)
GROUP (file file
...)
The GROUP command is like INPUT , except
that the named files should all be archives, and they are searched repeatedly
until no new undefined references are created.
OUTPUT (filename)
The OUTPUT command names the output file. Using
OUTPUT(FILENAME) in the linker script is exactly like using -o
filename on the command line. If both are used, the command line option
takes precedence.
You can use the OUTPUT
command to define a default name for the output file other than the usual
default of a.out.
SEARCH_DIR (path)
The SEARCH_DIR command adds path to the list
of paths where ld looks for archive libraries. Using SEARCH_DIR
(path) is exactly like using -L path on the command
line; see Command line
options for ld. If both are used, then the linker will search both
paths. Paths specified using the command line option are searched first.
STARTUP ( filename
)
The STARTUP command is just like the INPUT
command, except that filename will become the first input file to be linked,
as though it were specified first on the command line. This may be useful
when using a system in which the entry point is always the start of the
first file.
Commands
dealing with object file formats
A couple of linker script
commands deal with object file formats. See also Command
line options for ld and BFD.
OUTPUT_FORMAT (bfdname)
OUTPUT_FORMAT(default,
big, little)
The OUTPUT_FORMAT command names which BFD format
to use for the output file. Using OUTPUT_FORMAT (bfdname)
is exactly like using -oformat bfdname on
the command line . If both are used, the command line option takes precedence.
You
can use OUTPUT_FORMAT with three arguments to use different formats
based on the -EB and -EL command line options. This permits
the linker script to set the output format based on the desired endianness.
If neither -EB nor -EL is used, then the output format
will be the first argument, DEFAULT . If -EB is used,
the output format will be the second argument, BIG. If -EL
is used, the output format will be the third argument, LITTLE.
For example, the default linker script for the MIPS ELF target uses the
following command:
OUTPUT_FORMAT(elf32-bigmips,
elf32-bigmips, elf32-littlemips)
This says that the default
format for the output file is elf32-bigmips, but if the user uses
the -EL command line option, the output file will be created in
the elf32-littlemips format.
TARGET(bfdname)
The TARGET command names which BFD format to
use when reading input files. It affects subsequent INPUT and
GROUP commands. This command is like using -b bfdname
on the command line. If the TARGET command is used but OUTPUT_FORMAT
is not, then the last TARGET command is also used to set the format
for the output file.
Other
linker script commands
The following options
are a few other linker scripts commands. See also Command
line options for ld and BFD.
-
FORCE_COMMON_ALLOCATION
This command has the same effect as the -d command-line
option: to make ld assign space to common symbols even if a relocatable
output file is specified (using the option, -r).
-
NOCROSSREFS( section
section ...)
This command may be used to tell ld to issue
an error about any references among certain output sections.
In certain types of
programs, particularly on embedded systems when using overlays, when one
section is loaded into memory, another section will not be. Any direct
references between the two sections would be errors. For example, it would
be an error if code in one section called a function defined in the other
section. The NOCROSSREFS command takes a list of output section
names. If ld detects any cross-references between the sections,
it reports an error and returns a non-zero exit status. Remember that the
NOCROSSREFS command uses output section names, not input section
names.
-
OUTPUT_ARCH( bfdarch
)
Specify a particular output machine architecture, bfdarch.
The argument is one of the names used by the BFD library. You can see the
architecture of an object file by using the objdump program with
the -f option.
Assigning
values to symbols
You may assign a value
to a symbol in a linker script. This will define the symbol as a global
symbol.
Simple
assignments
You may assign to a symbol
using any of the C assignment operators:
symbol = expression;
symbol += expression;
symbol -= expression;
symbol *= expression;
symbol /= expression;
symbol <<= expression;
symbol >>= expression;
symbol &= expression;
symbol |= expression;
-
The first case will define symbol
to the value of expression. In the other cases, symbol
must already be defined, and the value will be accordingly adjusted.
-
The special symbol name .
indicates the location counter. You may only use this within a SECTIONS
command.
-
The semicolon after expression
is required.
-
See Expressions
in linker scripts.
-
You may write symbol assignments
as commands in their own right, or as statements within a SECTIONS
command, or as part of an output section description in a SECTIONS
command.
-
The section of the symbol
will be set from the section of the expression; for more
information, see Expressions
in linker scripts.
-
The following is an example showing
the three different places that symbol assignments may be used:
floating_point = 0;
SECTIONS
{
.text :
{
*(.text)
_etext = .;
}
_bdata = (. + 3) & ~ 4;
.data : { *(.data) }
}
In the previous
example, the floating_point symbol will be defined as zero. The
_etext symbol will be defined as the address following the last
.text input section. The _bdata symbol will be defined
as the address following the .text output section aligned upward
to a 4 byte boundary.
In some cases, it is desirable
for a linker script to define a symbol only if it is referenced and is
not defined by any object included in the link. For example, traditional
linkers defined the symbol, etext. However, ANSI C requires that
the user be able to use etext as a function name without encountering
an error. The PROVIDE keyword
may be used to define a symbol, such as etext, only if it is referenced
but not defined.
The syntax is PROVIDE( symbol
= expression).
The following
example shows how to use PROVIDE to define etext.
SECTIONS
{
.text :
{
*(.text)
_etext = .;
PROVIDE(etext = .);
}
}
In the previous
example, if the program defines _etext, the linker will give a
multiple definition error. If, on the other hand, the program defines etext,
the linker will silently use the definition in the program. If the program
references etext but does not define it, the linker will use the
definition in the linker script.
SECTIONS
command
The SECTIONS
command tells the linker how to map input sections into output sections,
and how to place the output sections in memory. The format of the SECTIONS
command is:
SECTIONS
{
sections - command
sections - command
...
}
Each sections
- command may of be one of the following:
The ENTRY
command and symbol assignments are permitted inside the SECTIONS
command for convenience in using the location counter in those commands.
This can also make the linker script easier to understand because you can
use those commands at meaningful points in the layout of the output file.
See Output section description
and Overlay description.
If you do not
use a SECTIONS command in your linker script, the linker will
place each input section into an identically named output section in the
order that the sections are first encountered in the input files. If all
input sections are present in the first file, for example, the order of
sections in the output file will match the order in the first input file.
The first section will be at address-zero.
Output
section description
The full description
of an output section looks like the following statement.
SECTION [address] [(type)] : [AT(LMA)]
{
output-sections-command
output-sections-command
...
} [>region] [: phdr : phdr ...] [=fillexp]
Most output
sections do not use most of the optional section attributes. The whitespace
around SECTION is required, so that the section name is unambiguous.
The colon and the curly braces are also required. The line breaks and other
white space are optional. The expression, LMA, that follows
the AT keyword specifies the load address of the section. This
feature is designed to make it easy to build a ROM image; see also Output
section LMA.
Each output-sections-command
may be one of the following:
The name of
the output section is section.
section must meet the constraints of your output format.
In formats which only support a limited number of sections, such as a.out,
the name must be one of the names supported by the format (a.out,
for example, allows only .text, .data or .bss).
If the output format supports any number of sections, but with numbers
and not names (as is the case for Oasys), the name should be supplied as
a quoted numeric string . A section name
may consist of any sequence of characters, but a name, which contains any
unusual characters such as commas, must be quoted. The output section name
/DISCARD/ is special. See Output
section discarding.
The address
is an expression for the VMA
(the virtual memory address) of the output section. If you do not provide address,
the linker will set it based on REGION if present, or otherwise
based on the current value of the location counter.
If you provide
address, the address of the output section will be set
to precisely that specification. If you provide neither address
nor region, then the address of the output section will
be set to the current value of the location counter aligned to the alignment
requirements of the output section. The alignment requirement of the output
section is the strictest alignment of any input section contained within
the output section.
For example,
the following two declarations are subtly different.
The first will
set the address of the .text output section to the current value
of the location counter. The second will set it to the current value of
the location counter aligned to the strictest alignment of a .text
input section.
The address
may be an arbitrary expression. See Expressions
in linker scripts. For example, if you want to align the section on
a 0x10 byte boundary, so that the lowest four bits of the section address
are zero, you could use something like the following declaration.
.text ALIGN(0x10) : { *(.text) }
This works
because ALIGN returns the current location counter aligned upward
to the specified value.
Specifying
address for a section will change the value of the location counter.
Input
section description
The most common
output section command is an input section description .
The input section description is the most basic linker script operation.
You use output sections to tell the linker how to lay out your program
in memory. You use input section descriptions to tell the linker how to
map the input files into your memory layout.
An input section
description consists of a file name optionally
followed by a list of section names in parentheses. The file name and the
section name may be wildcard patterns, which we describe; see Input
section wildcard patterns. The most common input section description
is to include all input sections with a particular name in the output section.
For example, to include all input .text sections, you would write:
*(.text)
Here the *
is a wildcard which matches any file name.
There are two
ways to include more than one section:
-
*(.text .rdata)
-
*(.text) *(.rdata)
The difference
between these is the order in which the .text and .rdata
input sections will appear in the output section. In the first example,
they will be intermingled. In the second example, all .text input
sections will appear first, followed by all .rdata input sections.
You can specify
a file name to include sections from a particular file. You would do this
if one or more of your files contain special data that needs to be at a
particular location in memory. For example, use the following input.
data.o(.data)
If you use
a file name without a list of sections, then all sections in the input
file will be included in the output section. This is not commonly done,
but it may by useful on occasion. For example, use the following input.
data.o
When you use
a file name, which does not contain any wild card characters, the linker
will first see if you also specified the file name on the linker command
line or in an INPUT command. If you did not, the linker will attempt
to open the file as an input file, as though it appeared on the command
line. This differs from an INPUT command, because the linker will
not search for the file in the archive search path.
Input
section wildcard patterns
In an input
section description, either the file name or the section name or both may
be wildcard patterns. The file name of *
seen in many examples is a simple wildcard pattern for the file name. The
wildcard patterns are like those used by the Unix shell.
*
Matches any number of characters.
?
Matches any single character.
[chars]
Matches 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.
\
Quotes 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, whether it contains a / or not. In a section
name, the wildcard characters will match a / character.
File name wildcard patterns
only match files which are explicitly specified on the command line or
in an INPUT command. The linker
does not search directories to expand wildcards.
If a file name matches more
than one wildcard pattern, or if a file name appears explicitly and is
also matched by a wildcard pattern, the linker will use the first match
in the linker script. For example, the following input uses a sequence
of input section descriptions that is probably in error, because the data.o
rule will not be used.
If you ever get confused about
where input sections are going, use the -M linker option to generate
a map file. The map file shows precisely how input sections are mapped
to output sections.
This example
shows how wildcard patterns might be used to partition files. This linker
script directs the linker to place all .text sections in .text
and all .bss sections in .bss. The linker will place
the .data section from all files beginning with an upper case
character in .DATA; for all other files, the linker will place
the .data section in .data.
SECTIONS {
.text : { *(.text) }
.DATA : { [A-Z]*(.data) }
.data : { *(.data) }
.bss : { *(.bss) }
}
Input
section for common symbols
A special notation is
needed for common symbols because, in many object file formats, common
symbols do not have a particular input section. The linker treats common
symbols as though they are in an input section named COMMON.
You may use
file names with the COMMON section just as with any other input
sections. You can use this to place common symbols from a particular input
file in one section while common symbols from other input files are placed
in another section.
In most cases,
common symbols in input files will be placed in the .bss section
in the output file. For example, use the following input.
.bss { *(.bss) *(COMMON) }
Some object
file formats have more than one type of common symbol. For example, the
MIPS ELF object file format distinguishes standard common symbols and small
common symbols. In this case, the linker will use a different special section
name for other types of common symbols. In the case of MIPS ELF, the linker
uses COMMON for standard common symbols and .scommon
for small common symbols. This permits you to map the different types of
common symbols into memory at different locations.
You will sometimes
see [COMMON] in old linker scripts. This notation is now considered
obsolete. It is equivalent to *(COMMON).
The following example is a
complete linker script that tells the linker to read all of the sections
from file, all.o, and place them at the start of output section,
outputa, which starts at a 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)
}
}
You can include
explicit bytes of data in an output section by using BYTE, SHORT,
LONG, QUAD, or SQUAD as an output section command.
Each keyword is followed by an expression in parentheses providing the
value to store; see Expressions
in linker scripts. The value of the expression is stored at the current
value of the location counter.
The BYTE
, SHORT, LONG, and QUAD commands store one,
two, four, and eight bytes (respectively). After storing the bytes, the
location counter is incremented by the number of bytes stored. For example,
this will store the byte 1 followed by the four byte value of the symbol,
addr:
BYTE(1)
LONG(addr)
When using
a 64-bit host or target, QUAD and SQUAD are the same;
they both store an 8-byte, or 64-bit, value. When both host and target
are 32 bits, an expression is computed as 32 bits. In this case QUAD
stores a 32-bit value zero extended to 64 bits, and SQUAD stores
a 32-bit value sign extended to 64 bits.
If the object
file format of the output file has an explicit endianness, which is the
normal case, the value will be stored in that endianness. When the object
file format does not have an explicit endianness, as is true of, for example,
S-records, the value will be stored in the endianness of the first input
object file.
You may use
the FILL command to set the fill pattern for the current section.
It is followed by an expression in parentheses. Any otherwise unspecified
regions of memory within the section (for example, gaps left due to the
required alignment of input sections) are filled with the two least significant
bytes of the expression, repeated as necessary. A FILL statement
covers memory locations after the point at which it occurs in the section
definition; by including more than one FILL statement, you can
have different fill patterns in different parts of an output section.
This example
shows how to fill unspecified regions of memory with the value, 0x9090
:
FILL(0x9090)
The FILL
command is similar to the =fillexp output section attribute
(see Output section fill); but it only
affects the part of the section following the FILL command, rather
than the entire section. If both are used, the FILL command takes
precedence.
There are a
couple of keywords, which can appear as output section commands.
-
CREATE_OBJECT_SYMBOLS
The command tells the linker
to create a symbol for each input file. The name of each symbol will be
the name of the corresponding input file. The section of each symbol will
be the output section in which the CREATE_OBJECT_SYMBOLS command
appears.
This
is conventional for the a.out object file format. It is not normally
used for any other object file format.
-
CONSTRUCTORS
When linking, using the
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 to place constructor information in the output
section where the CONSTRUCTORS command appears. The CONSTRUCTORS
command is ignored for other object file formats.
The
__CTOR_LIST__ symbol marks the start of the global constructors,
and the __DTOR_LIST symbol 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++ normally calls constructors
from a subroutine, __main;
a call to __main is automatically inserted into the startup code
for main. GNU C++ normally runs destructors either by using atexit,
or directly from the function, exit.
For object file formats such
as COFF or ELF, which support arbitrary section names,
GNU C++ will normally arrange to put the addresses of global constructors
and destructors into the .ctors and .dtors sections.
Placing the following sequence into your linker script will build the sort
of table that 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 occurrence,
if you are using C++ and writing your own linker scripts.
Output
section discarding
The linker will not create
output section which do not have any contents. This is for convenience
when referring to input sections that may or may not be present in any
of the input files. For example, the following input will
only create a .foo section in the output file if there is a .foo
section in at least one input file.
.foo { *(.foo) }
If you use
anything other than an input section description as an output section command,
such as a symbol assignment, then the output section will always be created,
even if there are no matching input sections.
The special
output section name, /DISCARD/ may be used to discard input sections.
Any input sections which are assigned to an output section named /DISCARD/
are not included in the output file.
Output
section attributes
We showed with Output
section description that the full description of an output section
looked like the following input.
SECTION [address] [(type)] : [AT(LMA)]
{
output-sections-command
output-sections-command
...
} [> region ] [: phdr : phdr ...] [=fillexp]
We’ve already
described section, address, and output-sections-command.
In the following discussion, we will describe the remaining section attributes.
Each output
section may have a type. The type is a keyword in parentheses. The following
types are defined:
NOLOAD
The section should be marked
as not loadable, so that it will not be loaded into memory when the program
is run.
DSECT
COPY
INFO
OVERLAY
These type names are supported
for backward compatibility, and are rarely used. They all have the same
effect: the section should be marked as not allocatable, so that no memory
is allocated for the section when the program is run.
The
linker normally sets the attributes of an output section, based on the
input sections, which map into it. You can override this by using the section
type. For example, in the script sample below, the ROM section
is addressed at memory location 0 and does not need to be loaded
when the program is run. The contents of the ROM section will
appear in the linker output file as usual.
SECTIONS {
ROM 0 (NOLOAD) : { ... }
...
}
Every section has a virtual
address (VMA) and a load address (LMA); see Basic linker
script concepts. The address expression that, may appear in an output
section description sets the VMA. The linker will normally set the LMA
equal to the VMA. You can change that by using the AT keyword.
The expression, LMA, that follows the AT keyword
specifies the load address of the section. This feature is designed to
make it easy to build a ROM image. For example, the following linker script
creates three output sections: one called .text, which starts
at 0x1000, one called .mdata, which is loaded at the
end of the .textsection even though its VMA is 0x2000,
and one called .bssto hold uninitialized data at address, 0x3000.
The symbol _data is defined with the value, 0x2000, which
shows that the location counter holds the VMA value, not the LMA value.
SECTIONS
{
.text 0x1000 : { *(.text) _etext = . ; }
.mdata 0x2000 :
AT ( ADDR (.text) + SIZEOF (.text) )
{ _data = . ; *(.data); _edata = . ; }
.bss 0x3000 :
{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;}
}
The run-time
initialization code for use with a program generated with this linker script
would include something like the following, to copy the initialized data
from the ROM image to its runtime address. Notice how this code takes advantage
of the symbols defined by the linker script.
extern char _etext, _data, _edata, _bstart, _bend;
char *src = &_etext;
char *dst = &_data;
/* ROM has data at end of text; copy it. */
while (dst < &_edata) {
*dst++ = *src++;
}
/* Zero bss */
for (dst = &_bstart; dst< &_bend; dst++)
*dst = 0;
You can assign
a section to a previously defined region of memory by using >region,
where region is the defined region for the section. The
following example shows usage.
MEMORY { rom : ORIGIN = 0x1000, LENGTH = 0x1000 }
SECTIONS { ROM : { *(.text) } >rom }
You can assign
a section to a previously defined program segment by using :phdr.
If a section is assigned to one or more segments, then all subsequent allocated
sections will be assigned to those segments as well, unless they use an
explicitly :phdr modifier. To prevent a section from being
assigned to a segment when it would normally default to one, use :NONE.
See PHDRS command.
The following
example shows usage.
PHDRS { text PT_LOAD ; }
SECTIONS { .text : { *(.text) } :text }
You can set
the fill pattern for an entire section by using =fillexp.
fillexp is an expression; see Expressions
in linker scripts. Any otherwise unspecified regions of memory within
the output section (for example, gaps left due to the required alignment
of input sections) will be filled with the two least significant bytes
of the value, repeated as necessary.
You can also
change the fill value with a FILL command in the output section
commands. See Output section data.
Here is a simple
example:
SECTIONS { .text : { *(.text) } =0x9090 }
An overlay description provides
an easy way to describe sections, which are to be loaded as part of a single
memory image but are to be run at the same memory address. At run time,
some sort of overlay manager will copy the overlaid sections in and out
of the runtime memory address as required, perhaps by simply manipulating
addressing bits. This approach can be useful, for example, when a certain
region of memory is faster than another region of memory.
Overlays are
described using the OVERLAY command. The OVERLAY command
is used within a SECTIONS command, like an output section description.
The full syntax of the OVERLAY command is as follows:
OVERLAY [ start ] : [NOCROSSREFS] [AT (ldaddr)]
{
secname1
{
output-section-command
output-section-command
...
} [:PHDR...] [=FILL]
secname2
{
output-section-command
output-section-command
...
} [:phdr ...] [=fill]
...
} [>region ] [:phdr ...] [=fill]
Everything
is optional except OVERLAY
(a keyword), and each section must have a name (secname1 and secname2
in the previous examples input). The section definitions within the OVERLAY
construct are identical to those within the general SECTIONS
construct, except that no addresses and no memory regions may be defined
for sections within an OVERLAY. See SECTIONS
command.
The sections
are all defined with the same starting address. The load addresses of the
sections are arranged, so that they are consecutive in memory, starting
at the load address used for the OVERLAY
as a whole (as with normal section definitions. The load address is optional,
and defaults to the start address. The start address is also optional,
and defaults to the current value of the location counter).
If the NOCROSSREFS
keyword is used, and there 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.
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 that are not legal within C identifiers
are removed. C (or assembler) code may use these symbols to move the overlaid
sections around as necessary.
At the end
of the overlay, the value of the location counter is set to the start address
of the overlay plus the size of the largest section.
Here is an
example. Remember that this would appear inside a SECTIONS construct.
OVERLAY 0x1000 : AT (0x4000)
{
.text0 { o1/*.o(.text) }
.text1 { o2/*.o(.text) }
}
This will define
both .text0 and .text1 to start at address 0x1000.
.text0 will be loaded at address 0x4000, and .text1will
be loaded immediately after .text0. The following symbols will
be defined: __load_start_text0, __load_stop_text0 , __load_start_text1,
and __load_stop_text1.
C code to copy
overlay .text1 into the overlay area might look like the following
example input.
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 previous example could have
been input 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 linkers default configuration
permits allocation of all available memory. You can override this by using
the MEMORY
command.
The MEMORY
command describes the location and size of blocks of memory in the target.
You can use it to describe which memory regions may be used by the linker,
and which memory regions it must avoid. You can then assign sections to
particular memory regions. The linker will set section addresses based
on the memory regions, and will warn about regions that become too full.
The linker will not shuffle sections around to fit into the available regions.
A linker script
may contain 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
...
}
The name
is a name used in the linker script to refer to the region. The region
name has no meaning outside of the linker script. Region names are stored
in a separate name space, and will not conflict with symbol names, file
names, or section names. Each memory region must have a distinct name.
The attr
string is an optional list of attributes that specify whether to use
a particular memory region for an input section, which is not explicitly
mapped in the linker script. If you do not specify an output section for
some input section, the linker will create an output section with the same
name as the input section. If you define region attributes, the linker
will use them to select the memory region for the output section that it
creates. See SECTIONS command.
The attr
string must consist only of the following characters:
-
A
-
I
-
L
-
!
Invert the sense of any of the preceding attributes.
If an unmapped
section matches any of the listed attributes other than !, it
will be placed in the memory region. The ! attribute reverses
this test, so that an unmapped section will be placed in the memory region
only if it does not match any of the listed attributes.
The ORIGIN
is an expression for the start address of the memory region. The expression
must evaluate to a constant before memory allocation is performed, which
means that you may not use any section relative symbols. The ORIGIN
keyword may be abbreviated to org or o (but not, for
example, to ORG).
The len
is an expression for the size in bytes of the memory region. As with the
origin expression, the expression must evaluate to a constant before memory
allocation is performed. The LENGTH keyword may be abbreviated
to len or l.
In the following
example, we specify that there are two memory regions available for allocation:
one starting at 0 for 256 kilobytes, and the other starting at
0x40000000 for four megabytes. The linker will place into the
rom memory region every section, which is not explicitly mapped
into a memory region, and is either read-only or executable. The linker
will place other sections, which are not explicitly mapped into a memory
region into the ram memory region.
MEMORY
{
rom (rx) : ORIGIN = 0, LENGTH = 256K
ram (!rx) : org = 0x40000000, l = 4M
}
If you have
defined a memory region named mem, you can direct the linker to
place specific output sections into that memory region by using the >region
output section attribute. If no address was specified for the output section,
the linker will set the address to the next available address within the
memory region. If the combined output sections directed to a memory region
are too large for the region, the linker will issue an error message. See
Output section region.
The ELF object file format
uses program headers, also knows as segments .
The program headers describe how the program should be loaded into memory.
You can print them out by using the objdump program with the -p
option.
When you run
an ELF program on a native ELF system, the system loader reads the program
headers in order to figure out how to load the program. This will only
work if the program headers are set correctly. This documentation does
not describe the details of how the system loader interprets program headers;
for more information, see the ELF ABI.
The linker
will create reasonable program headers by default. However, in some cases,
you may need to specify the program headers more precisely. You may use
the PHDRS command for this purpose. When the linker sees the PHDRS
command in the linker script, it will not create any program headers other
than the ones specified.
The linker
only pays attention to the PHDRS command when generating an ELF
output file. In other cases, the linker will simply ignore PHDRS.
This is the
syntax of the PHDRS command. The words,
PHDRS, FILEHDR, AT, and FLAGS, are
keywords.
{
name type [ FILEHDR ] [ PHDRS ] [ AT (address) ]
[ FLAGS (flags) ] ;
}
The name is
used only for reference in the SECTIONS command of the linker
script. It is not put into the output file. Program header names are stored
in a separate name space, and will not conflict with symbol names, file
names, or section names. Each program header must have a distinct name.
Certain program
header types describe segments of memory, which the system loader will
load from the file. In the linker script, you specify the contents of these
segments by placing allocatable output sections in the segments. You use
the : phdr output section attribute to place a section
in a particular segment. See Output section
phdr.
It is normal
to put certain sections in more than one segment. This merely implies that
one segment of memory contains another. You may repeat : phdr,
using it once for each segment which should contain the section.
If you place
a section in one or more segments using : phdr, then the linker
will place all subsequent allocatable sections which do not specify :
phdr in the same segments. This is for convenience, since generally
a whole set of contiguous sections will be placed in a single segment.
To prevent a section from being assigned to a segment when it would normally
default to one, use :NONE.
You may use
the FILEHDR and PHDRS keywords appear after the program
header type to further describe the contents of the segment. The FILEHDR
keyword means that the segment should include the ELF file header. The
PHDRS keyword means that the segment should include the ELF program
headers themselves.
The type may
be one of the following. The numbers indicate the value of the keyword.
Indicates an unused program header.
Indicates that this program header
describes a segment to be loaded from the file.
Indicates a segment where dynamic
linking information can be found.
Indicates a segment where the
name of the program interpreter may be found.
Indicates a segment holding note
information.
A reserved program header type,
defined but not specified by the ELF ABI.
Indicates a segment where the program
headers may be found.
An expression giving the numeric type
of the program header. This may be used for types not defined above.
You can specify
that a segment should be loaded at a particular address in memory by using
an AT expression. This is identical to the AT command
used as an output section attribute. The AT command for a program
header, overrides the output section attribute. See Output
section, LMA.
The linker
will normally set the segment flags based on the sections, which comprise
the segment. You may use the FLAGS keyword to specify the segment
flags, explicitly. The value of flags must be an integer. It is used to
set the p_flags field of the program header.
The following
example shows the script for PHDRS. This shows a typical set of
program headers used on a native ELF system.
PHDRS
{
headers PT_PHDR PHDRS ;
interp PT_INTERP ;
text PT_LOAD FILEHDR PHDRS ;
data PT_LOAD ;
dynamic PT_DYNAMIC ;
}
SECTIONS
{
. = SIZEOF_HEADERS;
.interp : { *(.interp) } :text :interp
.text : { *(.text) } :text
.rodata : { *(.rodata) } /* defaults to :text */
...
. = . + 0x1000; /* move to a new page in memory */
.data : { *(.data) } :data
.dynamic : { *(.dynamic) } :data :dynamic
...
}
The linker supports symbol
versions when using ELF. Symbol versions are only useful when using shared
libraries. The dynamic linker can use symbol versions to select a specific
version of a function when it runs a program that may have been linked
against an earlier version of the shared library.
You can include
a version script directly in the main linker script, or you can supply
the version script as an implicit linker script. You can also use the --version-script
linker option.
The syntax
of the VERSION command is a simple declaration like the following
example.
VERSION {version-script-commands}
The format
of the version script commands is identical to that used by Suns linker
in Solaris 2.5. The version script defines a tree of version nodes. You
specify the node names and interdependencies in the version script. You
can specify which symbols are bound to which version nodes, and you can
reduce a specified set of symbols to local scope so that they are not globally
visible outside of the shared library.
The easiest
way to demonstrate the version script language is with a few examples.
VERS_1.1 {
global:
foo1;
local:
old*;
original*;
new*;
};
VERS_1.2 {
foo2;
} VERS_1.1;
VERS_2.0 {
bar1; bar2;
} VERS_1.2;
This example
version script defines three version nodes. The first version node defined
is VERS_1.1; it has no other dependencies. The script binds the
symbol, foo1, to VERS_1.1. It reduces a number
of symbols to local scope so that they are not visible outside of the shared
library.
Next, the version
script defines node, VERS_1.2. This node depends upon VERS_1.1
. The script binds the symbol, foo2, to the version node,
VERS_1.2.
Finally, the
version script defines node, VERS_2.0. This node depends upon
VERS_1.2. The script binds the symbols, bar1 and bar2,
to the version node, VERS_2.0.
When the linker
finds a symbol defined in a library, which is not specifically bound to
a version node, it will effectively bind it to an unspecified base version
of the library. You can bind all otherwise unspecified symbols to a given
version node by using global: * somewhere in the version script.
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 Suns versioning approach. The first of these is the ability
to bind a symbol to a version node in the source file where the symbol
is defined instead of in the versioning script. This was done mainly to
reduce the burden on the library maintainer. You can do this by putting
something like this in the C source file:
__asm__(".symver original_foo,foo@VERS_1.1");
This renames
the function, original_foo, to be an alias for foo bound
to the version node VERS_1.1. The local: directive can
be used to prevent the symbol, original_foo, from being exported.
The second
GNU extension is to allow multiple versions of the same function to appear
in a given, shared library. In this way you can make an incompatible change
to an interface without increasing the major version number of the shared
library, while still allowing applications linked against the old interface
to continue to function.
To do this,
you must use multiple .symver directives in the source file. The
following example shows the usage.
__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 four C functions: original_foo, old_foo
, old_foo1, and new_foo.
When you have
multiple definitions of a given symbol, there needs to be some way to specify
a default version to which external references to this symbol will be bound.
You can do this with the foo@@VERS_2.0 type of .symver
directive. You can only declare one version of a symbol as the default
in this manner; otherwise you would effectively have multiple definitions
of the same symbol.
If you wish
to bind a reference to a specific version of the symbol within the shared
library, you can use the aliases of convenience (i.e., old_foo
), or you can use the .symver directive to bind specifically to
an external version of the function in question.
Expressions
in linker scripts
The syntax for expressions
in the linker script language is identical to that of C expressions. All
expressions are evaluated as integers. All expressions are evaluated in
the same size, which is 32 bits if both the host and target are 32 bits,
and is otherwise 64 bits. You can use and set symbol values in expressions.
The linker defines several special purpose builtin functions for use in
expressions.
All constants are integers.
As in C, the linker considers an integer beginning with 0
to be octal, and an integer beginning with 0x or 0X to
be hexadecimal.
The linker
considers other integers to be decimal.
In addition,
you can use the suffixes K and M to scale a constant
by 1024, or 1024*1024, respectively. For example, the
following example declarations 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 period and may include letters, digits,
underscores, periods, and hyphens. Unquoted symbol names must not conflict
with any keywords. You can specify a symbol, which contains odd characters
or has the same name as a keyword by surrounding the symbol name in double
quotes.
"SECTION" = 9;
"with a space" = "also with a space" + 10;
Since symbols
can contain many non-alphabetic characters, it is safest to delimit symbols
with spaces. For example, A-B is one symbol, whereas A - B
is an expression involving subtraction.
The special linker dot (.)
variable always contains the current output location counter. Since the .
symbol always refers to a location in an output section, it may only appear
in an expression within a SECTIONS command. The . symbol
may appear anywhere that an ordinary symbol is allowed in an expression.
Assigning a
value to . 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, the .text section from file1 is located at the
beginning of the output section, output. It is followed by a 1000
byte gap. Then the .text section from file2 appears,
also with a 1000 byte gap following before the .text section from
file3. The notation, = 0x1234, specifies data to write
in the gaps.
The linker recognizes the
standard C set of arithmetic operators, with the standard bindings and
precedence levels; see Table
1: Arithmetic operators with precedence levels and bindings associations.
Table
1: Arithmetic operators with precedence levels and bindings associations
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The linker evaluates expressions
lazily. It only computes the value of an expression when absolutely necessary.
The linker
needs some information, such as the value of the start address of the first
section, and the origins and lengths of memory regions, in order to do
any linking at all. These values are computed as soon as possible when
the linker reads in the linker script.
However, other
values (such as symbol values) are not known or needed until after storage
allocation. Such values are evaluated later, when other information (such
as the sizes of output sections) is available for use in the symbol assignment
expression.
The sizes of
sections cannot be known until after allocation, so assignments dependent
upon these are not performed until after allocation.
Some expressions,
such as those depending upon the location counter ., must be evaluated
during section allocation.
If the result
of an expression is required, but the value is not available, then an error
results. For example, a script, like the following, will cause the error
message, non constant expression for initial address:
SECTIONS
{
.text 9+this_isnt_constant :
{ *(.text) }
}
The
section of an expression
When the linker evaluates
an expression, the result is either absolute or relative to some section.
A relative expression is expressed as a fixed offset from the base of a
section.
The position
of the expression within the linker script determines whether it is absolute
or relative. An expression, which appears within an output section definition,
is relative to the base of the output section. An expression, which appears
elsewhere, will be absolute.
A symbol set
to a relative expression will be relocatable if you request relocatable
output using the -r option. That means that a further link operation
may change the value of the symbol. The symbols section will be the section
of the relative expression.
A symbol set
to an absolute expression will retain the same value through any further
link operation. The symbol will be absolute, and will not have any particular
associated section.
You can use
the builtin function ABSOLUTE to force an expression to be absolute
when it would otherwise be relative. For example, to create an absolute
symbol set to the address of the end of the output section .data
:
SECTIONS
{
.data : { *(.data) _edata = ABSOLUTE(.); }
}
If ABSOLUTE
were not used, _edata would be relative to the .data
section.
The linker script language
includes a number of builtin functions for use in linker script expressions.
Return the absolute (non-relocatable, as opposed to non-negative)
value of the exp expression. Primarily useful to assign
an absolute value to a symbol within a section definition, where symbol
values are normally section relative. See Expressions
in linker scripts.
ADDR( section
)
Return the absolute address (the VMA) 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:
.output1 :
{
start_of_output_1 = ABSOLUTE(.);
...
}
.output :
{
symbol_1 = ADDR(.output1);
symbol_2 = start_of_output_1;
}
...
}
ALIGN(exp)
Return the 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 doesnt change
the value of the location counter, it just does arithmetic on it. The following
example aligns the output .data section to the next 0x2000
byte boundary after the preceding section and sets a variable within the
section to the next 0x8000 boundary after the input sections:
SECTIONS { ...
.data ALIGN(0x2000): {
*(.data)
variable = ALIGN(0x8000);
}
...
}
The first use of ALIGN
in this example specifies the location of a section because it is used
as the optional ADDRESS attribute of a section definition. The
second use of ALIGN is to define the value of a symbol. The builtin
function, NEXT, is closely related to ALIGN. See Output
section address.
BLOCK(exp)
This is a synonym for ALIGN, for compatibility
with older linker scripts. It is most often seen when setting the address
of an output section.
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 script 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 : . ;
...
}
...
}
LOADADDR(section)
Return the absolute LMA of the named
SECTION. This is normally the same as ADDR, but it may
be different if the AT attribute is used in the output section
definition.
MAX(exp1,
exp2)
Returns the maximum of exp1 and exp2.
MIN(exp1,
exp2)
Returns the minimum of exp1 and exp2.
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. If the section has not been allocated
when this is evaluated, the linker will report an error. See PHDRS
command. In the following example, symbol_1 and symbol_2
are assigned identical values:
SECTIONS{ ...
.output {
.start = . ;
...
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