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This is Info file ld.info, produced by Makeinfo-1.64 from the input
file ./ld.texinfo.
START-INFO-DIR-ENTRY
* Ld: (ld). The GNU linker.
END-INFO-DIR-ENTRY
This file documents the GNU linker LD.
Copyright (C) 1991, 92, 93, 94, 95, 96, 1997 Free Software
Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions.

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'".
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)'.

File: ld.info, Node: Arithmetic Functions, Next: Semicolons, 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;
}
... }
`LOADADDR(SECTION)'
Return the absolute load address of the named SECTION. This is
normally the same as `ADDR', but it may be different if the `AT'
keyword is used in the section definition (*note Section
Options::.).
`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.
`MAX(EXP1, EXP2)'
Returns the maximum of EXP1 and EXP2.
`MIN(EXP1, EXP2)'
Returns the minimum of EXP1 and EXP2.

File: ld.info, Node: Semicolons, Prev: Arithmetic Functions, Up: Expressions
Semicolons
----------
Semicolons (";") are required in the following places. In all other
places they can appear for aesthetic reasons but are otherwise ignored.
`Assignment'
Semicolons must appear at the end of assignment expressions.
*Note Assignment::
`PHDRS'
Semicolons must appear at the end of a `PHDRS' statement. *Note
PHDRS::

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: PHDRS, 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.
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.
* Menu:
* Section Definition:: Section Definitions
* Section Placement:: Section Placement
* Section Data Expressions:: Section Data Expressions
* Section Options:: Optional Section Attributes
* Overlays:: Overlays

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.
The special SECNAME `/DISCARD/' may be used to discard input
sections. Any sections which are assigned to an output section named
`/DISCARD/' are not included in the final link output.
The linker will not create output sections which do not have any
contents. This is for convenience when referring to input sections that
may or may not exist. For example,
.foo { *(.foo) }
will only create a `.foo' section in the output file if there is a
`.foo' section in at least one input file.

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.
In any place where you may use a specific file or section name, you
may also use a wildcard pattern. The linker handles wildcards much as
the Unix shell does. A `*' character matches any number of characters.
A `?' character matches any single character. The sequence `[CHARS]'
will match a single instance of any of the CHARS; the `-' character may
be used to specify a range of characters, as in `[a-z]' to match any
lower case letter. A `\' character may be used to quote the following
character.
When a file name is matched with a wildcard, the wildcard characters
will not match a `/' character (used to separate directory names on
Unix). A pattern consisting of a single `*' character is an exception;
it will always match any file name. In a section name, the wildcard
characters will match a `/' character.
Wildcards only match files which are explicitly specified on the
command line. The linker does not search directories to expand
wildcards. However, if you specify a simple file name--a name with no
wildcard characters--in a linker script, and the file name is not also
specified on the command line, the linker will attempt to open the file
as though it appeared on the command line.
In the following example, the command script arranges the output file
into three consecutive sections, named `.text', `.data', and `.bss',
taking the input for each from the correspondingly named sections of
all the input files:
SECTIONS {
.text : { *(.text) }
.data : { *(.data) }
.bss : { *(.bss) *(COMMON) }
}
The following example reads all of the sections from file `all.o'
and places them at the start of output section `outputa' which starts
at location `0x10000'. All of section `.input1' from file `foo.o'
follows immediately, in the same output section. All of section
`.input2' from `foo.o' goes into output section `outputb', followed by
section `.input1' from `foo1.o'. All of the remaining `.input1' and
`.input2' sections from any files are written to output section
`outputc'.
SECTIONS {
outputa 0x10000 :
{
all.o
foo.o (.input1)
}
outputb :
{
foo.o (.input2)
foo1.o (.input1)
}
outputc :
{
*(.input1)
*(.input2)
}
}
This example shows how wildcard patterns might be used to partition
files. All `.text' sections are placed in `.text', and all `.bss'
sections are placed in `.bss'. For all files beginning with an upper
case character, the `.data' section is placed into `.DATA'; for all
other files, the `.data' section is placed into `.data'.
SECTIONS {
.text : { *(.text) }
.DATA : { [A-Z]*(.data) }
.data : { *(.data) }
.bss : { *(.bss) }
}

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)'
`QUAD(EXPRESSION)'
By including one of these four statements in a section definition,
you can explicitly place one, two, four, or eight bytes
(respectively) at the current address of that section. `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 (*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.

File: ld.info, Node: Section Options, Next: Overlays, Prev: Section Data Expressions, Up: SECTIONS
Optional Section Attributes
---------------------------
Here is the full syntax of a section definition, including all the
optional portions:
SECTIONS {
...
SECNAME START BLOCK(ALIGN) (NOLOAD) : AT ( LDADR )
{ CONTENTS } >REGION :PHDR =FILL
...
}
SECNAME and CONTENTS are required. *Note Section Definition::, and
*Note Section Placement::, for details on CONTENTS. The remaining
elements--START, `BLOCK(ALIGN)', `(NOLOAD)', `AT ( LDADR )', `>REGION',
`:PHDR', and `=FILL'--are all optional.
`START'
You can force the output section to be loaded at a specified
address by specifying START immediately following the section name.
START can be represented as any expression. The following example
generates section OUTPUT at location `0x40000000':
SECTIONS {
...
output 0x40000000: {
...
}
...
}
`BLOCK(ALIGN)'
You can include `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)'
Use `(NOLOAD)' to prevent a section from being loaded into memory
each time it is accessed. For example, in the script sample
below, the `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 )'
The expression LDADR that follows the `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'
Assign this section to a previously defined region of memory.
*Note MEMORY::.
`:PHDR'
Assign this section to a segment described by a program header.
*Note PHDRS::. 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'.
`=FILL'
Including `=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.

File: ld.info, Node: Overlays, Prev: Section Options, Up: SECTIONS
Overlays
--------
The `OVERLAY' command provides an easy way to describe sections
which are to be loaded as part of a single memory image but are to be
run at the same memory address. At run time, some sort of overlay
manager will copy the overlaid sections in and out of the runtime memory
address as required, perhaps by simply manipulating addressing bits.
This approach can be useful, for example, when a certain region of
memory is faster than another.
The `OVERLAY' command is used within a `SECTIONS' command. It
appears as follows:
OVERLAY START : [ NOCROSSREFS ] AT ( LDADDR )
{
SECNAME1 { CONTENTS } :PHDR =FILL
SECNAME2 { CONTENTS } :PHDR =FILL
...
} >REGION :PHDR =FILL
Everything is optional except `OVERLAY' (a keyword), and each
section must have a name (SECNAME1 and SECNAME2 above). The section
definitions within the `OVERLAY' construct are identical to those
within the general `SECTIONS' contruct (*note SECTIONS::.), except that
no addresses and no memory regions may be defined for sections within
an `OVERLAY'.
The sections are all defined with the same starting address. The
load addresses of the sections are arranged such that they are
consecutive in memory starting at the load address used for the
`OVERLAY' as a whole (as with normal section definitions, the load
address is optional, and defaults to the start address; the start
address is also optional, and defaults to `.').
If the `NOCROSSREFS' keyword is used, and there any references among
the sections, the linker will report an error. Since the sections all
run at the same address, it normally does not make sense for one
section to refer directly to another. *Note NOCROSSREFS: Option
Commands.
For each section within the `OVERLAY', the linker automatically
defines two symbols. The symbol `__load_start_SECNAME' is defined as
the starting load address of the section. The symbol
`__load_stop_SECNAME' is defined as the final load address of the
section. Any characters within SECNAME which are not legal within C
identifiers are removed. C (or assembler) code may use these symbols
to move the overlaid sections around as necessary.
At the end of the overlay, the value of `.' is set to the start
address of the overlay plus the size of the largest section.
Here is an example. Remember that this would appear inside a
`SECTIONS' construct.
OVERLAY 0x1000 : AT (0x4000)
{
.text0 { o1/*.o(.text) }
.text1 { o2/*.o(.text) }
}
This will define both `.text0' and `.text1' to start at address
0x1000. `.text0' will be loaded at address 0x4000, and `.text1' will
be loaded immediately after `.text0'. The following symbols will be
defined: `__load_start_text0', `__load_stop_text0',
`__load_start_text1', `__load_stop_text1'.
C code to copy overlay `.text1' into the overlay area might look
like the following.
extern char __load_start_text1, __load_stop_text1;
memcpy ((char *) 0x1000, &__load_start_text1,
&__load_stop_text1 - &__load_start_text1);
Note that the `OVERLAY' command is just syntactic sugar, since
everything it does can be done using the more basic commands. The above
example could have been written identically as follows.
.text0 0x1000 : AT (0x4000) { o1/*.o(.text) }
__load_start_text0 = LOADADDR (.text0);
__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0);
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
__load_start_text1 = LOADADDR (.text1);
__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1);
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));

File: ld.info, Node: PHDRS, Next: Entry Point, Prev: SECTIONS, Up: Commands
ELF Program Headers
===================
The ELF object file format uses "program headers", which are read by
the system loader and describe how the program should be loaded into
memory. These program headers must be set correctly in order to run the
program on a native ELF system. The linker will create reasonable
program headers by default. However, in some cases, it is desirable to
specify the program headers more precisely; the `PHDRS' command may be
used for this purpose. When the `PHDRS' command is used, the linker
will not generate any program headers itself.
The `PHDRS' command is only meaningful when generating an ELF output
file. It is ignored in other cases. This manual does not describe the
details of how the system loader interprets program headers; for more
information, see the ELF ABI. The program headers of an ELF file may
be displayed using the `-p' option of the `objdump' command.
This is the syntax of the `PHDRS' command. The words `PHDRS',
`FILEHDR', `AT', and `FLAGS' are keywords.
PHDRS
{
NAME TYPE [ FILEHDR ] [ PHDRS ] [ AT ( ADDRESS ) ]
[ FLAGS ( FLAGS ) ] ;
}
The NAME is used only for reference in the `SECTIONS' command of the
linker script. It does not get put into the output file.
Certain program header types describe segments of memory which are
loaded from the file by the system loader. In the linker script, the
contents of these segments are specified by directing allocated output
sections to be placed in the segment. To do this, the command
describing the output section in the `SECTIONS' command should use
`:NAME', where NAME is the name of the program header as it appears in
the `PHDRS' command. *Note Section Options::.
It is normal for certain sections to appear in more than one segment.
This merely implies that one segment of memory contains another. This
is specified by repeating `:NAME', using it once for each program
header in which the section is to appear.
If a section is placed in one or more segments using `:NAME', then
all subsequent allocated sections which do not specify `:NAME' are
placed in the same segments. This is for convenience, since generally
a whole set of contiguous sections will be placed in a single segment.
To prevent a section from being assigned to a segment when it would
normally default to one, use `:NONE'.
The `FILEHDR' and `PHDRS' keywords which may appear after the
program header type also indicate contents of the segment of memory.
The `FILEHDR' keyword means that the segment should include the ELF
file header. The `PHDRS' keyword means that the segment should include
the ELF program headers themselves.
The TYPE may be one of the following. The numbers indicate the
value of the keyword.
`PT_NULL' (0)
Indicates an unused program header.
`PT_LOAD' (1)
Indicates that this program header describes a segment to be
loaded from the file.
`PT_DYNAMIC' (2)
Indicates a segment where dynamic linking information can be found.
`PT_INTERP' (3)
Indicates a segment where the name of the program interpreter may
be found.
`PT_NOTE' (4)
Indicates a segment holding note information.
`PT_SHLIB' (5)
A reserved program header type, defined but not specified by the
ELF ABI.
`PT_PHDR' (6)
Indicates a segment where the program headers may be found.
EXPRESSION
An expression giving the numeric type of the program header. This
may be used for types not defined above.
It is possible to specify that a segment should be loaded at a
particular address in memory. This is done using an `AT' expression.
This is identical to the `AT' command used in the `SECTIONS' command
(*note Section Options::.). Using the `AT' command for a program
header overrides any information in the `SECTIONS' command.
Normally the segment flags are set based on the sections. The
`FLAGS' keyword may be used to explicitly specify the segment flags.
The value of FLAGS must be an integer. It is used to set the `p_flags'
field of the program header.
Here is an example of the use of `PHDRS'. This shows a typical set
of program headers used on a native ELF system.
PHDRS
{
headers PT_PHDR PHDRS ;
interp PT_INTERP ;
text PT_LOAD FILEHDR PHDRS ;
data PT_LOAD ;
dynamic PT_DYNAMIC ;
}
SECTIONS
{
. = SIZEOF_HEADERS;
.interp : { *(.interp) } :text :interp
.text : { *(.text) } :text
.rodata : { *(.rodata) } /* defaults to :text */
...
. = . + 0x1000; /* move to a new page in memory */
.data : { *(.data) } :data
.dynamic : { *(.dynamic) } :data :dynamic
...
}

File: ld.info, Node: Entry Point, Next: Version Script, Prev: PHDRS, Up: Commands
The Entry Point
===============
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).
* the `-e' ENTRY command-line option;
* the `ENTRY(SYMBOL)' command in a linker control script;
* the value of the symbol `start', if present;
* the address of the first byte of the `.text' section, if present;
* The address `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 ;

File: ld.info, Node: Version Script, Next: Option Commands, Prev: Entry Point, Up: Commands
Version Script
==============
The linker command script includes a command specifically for
specifying a version script, and is only meaningful for ELF platforms
that support shared libraries. A version script can be build directly
into the linker script that you are using, or you can supply the
version script as just another input file to the linker at the time
that you link. The command script syntax is:
VERSION { version script contents }
The version script can also be specified to the linker by means of
the `--version-script' linker command line option. Version scripts are
only meaningful when creating shared libraries.
The format of the version script itself is identical to that used by
Sun's linker in Solaris 2.5. Versioning is done by defining a tree of
version nodes with the names and interdependencies specified in the
version script. The version script can specify which symbols are bound
to which version nodes, and it can reduce a specified set of symbols to
local scope so that they are not globally visible outside of the shared
library.
The easiest way to demonstrate the version script language is with a
few examples.
VERS_1.1 {
global:
foo1;
local:
old*;
original*;
new*;
};
VERS_1.2 {
foo2;
} VERS_1.1;
VERS_2.0 {
bar1; bar2;
} VERS_1.2;
In this example, three version nodes are defined. `VERS_1.1' is the
first version node defined, and has no other dependencies. The symbol
`foo1' is bound to this version node, and a number of symbols that have
appeared within various object files are reduced in scope to local so
that they are not visible outside of the shared library.
Next, the node `VERS_1.2' is defined. It depends upon `VERS_1.1'.
The symbol `foo2' is bound to this version node.
Finally, the node `VERS_2.0' is defined. It depends upon
`VERS_1.2'. The symbols `bar1' and `bar2' are bound to this version
node.
Symbols defined in the library which aren't specifically bound to a
version node are effectively bound to an unspecified base version of the
library. It is possible to bind all otherwise unspecified symbols to a
given version node using `global: *' somewhere in the version script.
Lexically the names of the version nodes have no specific meaning
other than what they might suggest to the person reading them. The
`2.0' version could just as well have appeared in between `1.1' and
`1.2'. However, this would be a confusing way to write a version
script.
When you link an application against a shared library that has
versioned symbols, the application itself knows which version of each
symbol it requires, and it also knows which version nodes it needs from
each shared library it is linked against. Thus at runtime, the dynamic
loader can make a quick check to make sure that the libraries you have
linked against do in fact supply all of the version nodes that the
application will need to resolve all of the dynamic symbols. In this
way it is possible for the dynamic linker to know with certainty that
all external symbols that it needs will be resolvable without having to
search for each symbol reference.
The symbol versioning is in effect a much more sophisticated way of
doing minor version checking that SunOS does. The fundamental problem
that is being addressed here is that typically references to external
functions are bound on an as-needed basis, and are not all bound when
the application starts up. If a shared library is out of date, a
required interface may be missing; when the application tries to use
that interface, it may suddenly and unexpectedly fail. With symbol
versioning, the user will get a warning when they start their program if
the libraries being used with the application are too old.
There are several GNU extensions to Sun's versioning approach. The
first of these is the ability to bind a symbol to a version node in the
source file where the symbol is defined instead of in the versioning
script. This was done mainly to reduce the burden on the library
maintainer. This can be done by putting something like:
__asm__(".symver original_foo,foo@VERS_1.1");
in the C source file. This renamed the function `original_foo' to
be an alias for `foo' bound to the version node `VERS_1.1'. The
`local:' directive can be used to prevent the symbol `original_foo'
from being exported.
The second GNU extension is to allow multiple versions of the same
function to appear in a given shared library. In this way an
incompatible change to an interface can take place without increasing
the major version number of the shared library, while still allowing
applications linked against the old interface to continue to function.
This can only be accomplished by using multiple `.symver' directives
in the assembler. An example of this would be:
__asm__(".symver original_foo,foo@");
__asm__(".symver old_foo,foo@VERS_1.1");
__asm__(".symver old_foo1,foo@VERS_1.2");
__asm__(".symver new_foo,foo@@VERS_2.0");
In this example, `foo@' represents the symbol `foo' bound to the
unspecified base version of the symbol. The source file that contains
this example would define 4 C functions: `original_foo', `old_foo',
`old_foo1', and `new_foo'.
When you have multiple definitions of a given symbol, there needs to
be some way to specify a default version to which external references to
this symbol will be bound. This can be accomplished with the
`foo@@VERS_2.0' type of `.symver' directive. Only one version of a
symbol can be declared 'default' in this manner - otherwise you would
effectively have multiple definitions of the same symbol.
If you wish to bind a reference to a specific version of the symbol
within the shared library, you can use the aliases of convenience (i.e.
`old_foo'), or you can use the `.symver' directive to specifically bind
to an external version of the function in question.
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