Red Hat Enterprise Linux 4 Using ld, the Gnu Linker

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Red Hat Enterprise Linux 4
Using ld, the Gnu Linker
Red Hat Enterprise Linux 4: Using ld, the Gnu Linker
Copyright © 1987, 1989, 1991-2004 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation
License, Version 1.1 or any later version published by the Free Software Foundation. A copy of the license is included in the
section entitled "GNU Free Documentation License".
This manual contains no Invariant Sections. The Front-Cover Texts are (a) (see below), and the Back-Cover Texts are (b) (see
below).
(a) The FSF’s Front-Cover Text is: A GNU Manual
(b) The FSF’s Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies
published by the Free Software Foundation raise funds for GNU development.
This version is by Red Hat, Inc.
Red Hat, Inc.
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Table of Contents
1. Using ld ............................................................................................................................................ 1
2. Overview .......................................................................................................................................... 3
3. Invocation ........................................................................................................................................ 5
3.1. Command Line Options..................................................................................................... 5
3.1.1. Options Specific to i386 PE Targets ................................................................. 21
3.2. Environment Variables..................................................................................................... 27
4. Linker Scripts................................................................................................................................ 29
4.1. Basic Linker Script Concepts........................................................................................... 29
4.2. Linker Script Format........................................................................................................ 29
4.3. Simple Linker Script Example......................................................................................... 30
4.4. Simple Linker Script Commands..................................................................................... 31
4.4.1. Setting the Entry Point ...................................................................................... 31
4.4.2. Commands Dealing with Files.......................................................................... 31
4.4.3. Commands Dealing with Object File Formats.................................................. 32
4.4.4. Other Linker Script Commands ........................................................................ 33
4.5. Assigning Values to Symbols .......................................................................................... 33
4.5.1. Simple Assignments ......................................................................................... 33
4.5.2. PROVIDE.......................................................................................................... 34
4.6. SECTIONS Command..................................................................................................... 35
4.6.1. Output Section Description............................................................................... 35
4.6.2. Output Section Name........................................................................................ 36
4.6.3. Output Section Description............................................................................... 36
4.6.4. Input Section Description ................................................................................. 37
4.6.5. Output Section Data.......................................................................................... 39
4.6.6. Output Section Keywords ................................................................................. 40
4.6.7. Output Section Discarding................................................................................ 41
4.6.8. Output Section Attributes ................................................................................. 42
4.6.9. Overlay Description .......................................................................................... 44
4.7. MEMORY Command ...................................................................................................... 45
4.8. PHDRS Command ........................................................................................................... 47
4.9. VERSION Command....................................................................................................... 49
4.10. Expressions in Linker Scripts ........................................................................................ 51
4.10.1. Constants......................................................................................................... 51
4.10.2. Symbol Names ................................................................................................ 51
4.10.3. The Location Counter ..................................................................................... 51
4.10.4. Operators......................................................................................................... 52
4.10.5. Evaluation ....................................................................................................... 53
4.10.6. The Section of an Expression ......................................................................... 53
4.10.7. Builtin Functions............................................................................................. 54
4.11. Implicit Linker Scripts ................................................................................................... 56
5. Machine Dependent Features ...................................................................................................... 59
5.1. ld and the H8/300............................................................................................................ 59
5.2. ld and the Intel 960 Family ............................................................................................. 59
5.3. ld’s Support for Interworking Between ARM and Thumb Code ................................... 59
5.4. ld and HPPA 32-bit ELF Support ................................................................................... 60
5.5. ld and MMIX .................................................................................................................. 60
5.6. ld and MSP430 ............................................................................................................... 60
5.7. ld’s Support for Various TI COFF Versions ................................................................... 61
5.8. ld and WIN32 (cygwin/mingw)...................................................................................... 61
5.9. ld and Xtensa Processors ................................................................................................ 65
6. BFD................................................................................................................................................. 67
6.1. How It Works: An Outline of BFD .................................................................................. 67
6.1.1. Information Loss............................................................................................... 67
6.1.2. The BFD canonical object-file format .............................................................. 68
7. Reporting Bugs.............................................................................................................................. 71
7.1. Have You Found a Bug? .................................................................................................. 71
7.2. How to Report Bugs......................................................................................................... 71
A. MRI Compatible Script Files ...................................................................................................... 75
B. GNU Free Documentation License ............................................................................................. 77
B.1. ADDENDUM: How to use this License for your documents......................................... 81
Index................................................................................................................................................... 83
Chapter 1.
Using ld
This file documents the gnu linker ld version 2.14.90.0.4.
This document is distributed under the terms of the GNU Free Documentation License. A copy of the
license is included in the section entitled "GNU Free Documentation License".
2
Chapter 1. Using ld
Chapter 2.
Overview
ld combines a number of object and archive files, relocates their data and ties up symbol references.
Usually the last step in compiling a program is to run ld.
ld accepts Linker Command Language files written in a superset of AT&T’s Link Editor Command
Language syntax, to provide explicit and total control over the linking process.
This version of ld uses the general purpose BFD libraries to operate on object files. This allows ld
to read, combine, and write object files in many different formats--for example, COFF or a.out.
Different formats may be linked together to produce any available kind of object file. Refer to Chapter
6 BFD, for more information.
Aside from its flexibility, the gnu linker is more helpful than other linkers in providing diagnostic
information. Many linkers abandon execution immediately upon encountering an error; whenever
possible, ld continues executing, allowing you to identify other errors (or, in some cases, to get an
output file in spite of the error).
4
Chapter 2. Overview
Chapter 3.
Invocation
The gnu linker ld is meant to cover a broad range of situations, and to be as compatible as possible
with other linkers. As a result, you have many choices to control its behavior.
3.1. Command Line Options
The linker supports a plethora of command-line options, but in actual practice few of them are used
in any particular context. For instance, a frequent use of ld is to link standard Unix object files on a
standard, supported Unix system. On such a system, to link a file hello.o:
ld -o output /lib/crt0.o hello.o -lc
This tells ld to produce a file called output as the result of linking the file /lib/crt0.o with
hello.o and the library libc.a, which will come from the standard search directories. (See the
discussion of the -l option below.)
Some of the command-line options to ld may be specified at any point in the command line. However,
options which refer to files, such as -l or -T, cause the file to be read at the point at which the option
appears in the command line, relative to the object files and other file options. Repeating non-file
options with a different argument will either have no further effect, or override prior occurrences
(those further to the left on the command line) of that option. Options which may be meaningfully
specified more than once are noted in the descriptions below.
Non-option arguments are object files or archives which are to be linked together. They may follow,
precede, or be mixed in with command-line options, except that an object file argument may not be
placed between an option and its argument.
Usually the linker is invoked with at least one object file, but you can specify other forms of binary
input files using -l, -R, and the script command language. If no binary input files at all are specified,
the linker does not produce any output, and issues the message No input files.
If the linker cannot recognize the format of an object file, it will assume that it is a linker script. A
script specified in this way augments the main linker script used for the link (either the default linker
script or the one specified by using -T). This feature permits the linker to link against a file which
appears to be an object or an archive, but actually merely defines some symbol values, or uses INPUT
or GROUP to load other objects. Note that specifying a script in this way merely augments the main
linker script; use the -T option to replace the default linker script entirely. Chapter 4 Linker Scripts.
For options whose names are a single letter, option arguments must either follow the option letter
without intervening whitespace, or be given as separate arguments immediately following the option
that requires them.
For options whose names are multiple letters, either one dash or two can precede the option name;
for example, -trace-symbol and -trace-symbol are equivalent. Note--there is one exception to
this rule. Multiple letter options that start with a lower case ’o’ can only be preceeded by two dashes.
This is to reduce confusion with the -o option. So for example -omagic sets the output file name to
magic whereas -omagic sets the NMAGIC flag on the output.
Arguments to multiple-letter options must either be separated from the option name by an equals sign,
or be given as separate arguments immediately following the option that requires them. For example,
-trace-symbol foo and -trace-symbol=foo are equivalent. Unique abbreviations of the names
of multiple-letter options are accepted.
6
Chapter 3. Invocation
Note--if the linker is being invoked indirectly, via a compiler driver (e.g. gcc) then all the linker command line options should be prefixed by -Wl, (or whatever is appropriate for the particular compiler
driver) like this:
gcc -Wl,--startgroup foo.o bar.o -Wl,--endgroup
This is important, because otherwise the compiler driver program may silently drop the linker options,
resulting in a bad link.
Here is a table of the generic command line switches accepted by the GNU linker:
-akeyword
This option is supported for HP/UX compatibility. The keyword argument must be one of the
strings archive, shared, or default. -aarchive is functionally equivalent to -Bstatic,
and the other two keywords are functionally equivalent to -Bdynamic. This option may be used
any number of times.
-Aarchitecture
architecture
-architecture=architecture
In the current release of ld, this option is useful only for the Intel 960 family of architectures. In
that ld configuration, the architecture argument identifies the particular architecture in the
960 family, enabling some safeguards and modifying the archive-library search path. Refer to
Section 5.2 ld and the Intel 960 Family, for details.
Future releases of ld may support similar functionality for other architecture families.
-b input-format
-format=input-format
ld may be configured to support more than one kind of object file. If your ld is configured this
way, you can use the -b option to specify the binary format for input object files that follow this
option on the command line. Even when ld is configured to support alternative object formats,
you don’t usually need to specify this, as ld should be configured to expect as a default input
format the most usual format on each machine. input-format is a text string, the name of a
particular format supported by the BFD libraries. (You can list the available binary formats with
objdump -i.) Chapter 6 BFD.
You may want to use this option if you are linking files with an unusual binary format. You
can also use -b to switch formats explicitly (when linking object files of different formats), by
including -b input-format before each group of object files in a particular format.
The default format is taken from the environment variable GNUTARGET. Section 3.2 Environment
Variables. You can also define the input format from a script, using the command TARGET; see
Section 4.4.3 Commands Dealing with Object File Formats.
-c MRI-commandfile
-mri-script=MRI-commandfile
For compatibility with linkers produced by MRI, ld accepts script files written in an alternate,
restricted command language, described in Appendix A MRI Compatible Script Files. Introduce
MRI script files with the option -c; use the -T option to run linker scripts written in the generalpurpose ld scripting language. If MRI-cmdfile does not exist, ld looks for it in the directories
specified by any -L options.
Chapter 3. Invocation
7
-d
-dc
-dp
These three options are equivalent; multiple forms are supported for compatibility with other
linkers. They assign space to common symbols even if a relocatable output file is specified (with
-r). The script command FORCE_COMMON_ALLOCATION has the same effect. Refer to Section
4.4.4 Other Linker Script Commands.
-e entry
-entry=entry
Use entry as the explicit symbol for beginning execution of your program, rather than the
default entry point. If there is no symbol named entry, the linker will try to parse entry as a
number, and use that as the entry address (the number will be interpreted in base 10; you may
use a leading 0x for base 16, or a leading 0 for base 8). Refer to Section 4.4.1 Setting the Entry
Point for a discussion of defaults and other ways of specifying the entry point.
-E
-export-dynamic
When creating a dynamically linked executable, add all symbols to the dynamic symbol table.
The dynamic symbol table is the set of symbols which are visible from dynamic objects at run
time.
If you do not use this option, the dynamic symbol table will normally contain only those symbols
which are referenced by some dynamic object mentioned in the link.
If you use dlopen to load a dynamic object which needs to refer back to the symbols defined
by the program, rather than some other dynamic object, then you will probably need to use this
option when linking the program itself.
You can also use the version script to control what symbols should be added to the dynamic symbol table if the output format supports it. See the description of -version-script in Section
4.9 VERSION Command.
-EB
Link big-endian objects. This affects the default output format.
-EL
Link little-endian objects. This affects the default output format.
-f
-auxiliary name
When creating an ELF shared object, set the internal DT_AUXILIARY field to the specified
name. This tells the dynamic linker that the symbol table of the shared object should be used as
an auxiliary filter on the symbol table of the shared object name.
If you later link a program against this filter object, then, when you run the program, the dynamic
linker will see the DT_AUXILIARY field. If the dynamic linker resolves any symbols from the
filter object, it will first check whether there is a definition in the shared object name. If there
is one, it will be used instead of the definition in the filter object. The shared object name need
not exist. Thus the shared object name may be used to provide an alternative implementation of
certain functions, perhaps for debugging or for machine specific performance.
This option may be specified more than once. The DT_AUXILIARY entries will be created in
the order in which they appear on the command line.
8
Chapter 3. Invocation
-F name
-filter name
When creating an ELF shared object, set the internal DT_FILTER field to the specified name.
This tells the dynamic linker that the symbol table of the shared object which is being created
should be used as a filter on the symbol table of the shared object name.
If you later link a program against this filter object, then, when you run the program, the dynamic
linker will see the DT_FILTER field. The dynamic linker will resolve symbols according to the
symbol table of the filter object as usual, but it will actually link to the definitions found in the
shared object name. Thus the filter object can be used to select a subset of the symbols provided
by the object name.
Some older linkers used the -F option throughout a compilation toolchain for specifying objectfile format for both input and output object files. The gnu linker uses other mechanisms for this
purpose: the -b, -format, -oformat options, the TARGET command in linker scripts, and the
GNUTARGET environment variable. The gnu linker will ignore the -F option when not creating an
ELF shared object.
-fini name
When creating an ELF executable or shared object, call NAME when the executable or shared
object is unloaded, by setting DT_FINI to the address of the function. By default, the linker uses
_fini as the function to call.
-g
Ignored. Provided for compatibility with other tools.
-Gvalue
-gpsize=value
Set the maximum size of objects to be optimized using the GP register to size. This is only
meaningful for object file formats such as MIPS ECOFF which supports putting large and small
objects into different sections. This is ignored for other object file formats.
-hname
-soname=name
When creating an ELF shared object, set the internal DT_SONAME field to the specified name.
When an executable is linked with a shared object which has a DT_SONAME field, then when
the executable is run the dynamic linker will attempt to load the shared object specified by the
DT_SONAME field rather than the using the file name given to the linker.
-i
Perform an incremental link (same as option -r).
-init name
When creating an ELF executable or shared object, call NAME when the executable or shared
object is loaded, by setting DT_INIT to the address of the function. By default, the linker uses
_init as the function to call.
-larchive
-library=archive
Add archive file archive to the list of files to link. This option may be used any number of times.
ld will search its path-list for occurrences of libarchive.a for every archive specified.
Chapter 3. Invocation
9
On systems which support shared libraries, ld may also search for libraries with extensions other
than .a. Specifically, on ELF and SunOS systems, ld will search a directory for a library with
an extension of .so before searching for one with an extension of .a. By convention, a .so
extension indicates a shared library.
The linker will search an archive only once, at the location where it is specified on the command
line. If the archive defines a symbol which was undefined in some object which appeared before
the archive on the command line, the linker will include the appropriate file(s) from the archive.
However, an undefined symbol in an object appearing later on the command line will not cause
the linker to search the archive again.
See the -( option for a way to force the linker to search archives multiple times.
You may list the same archive multiple times on the command line.
This type of archive searching is standard for Unix linkers. However, if you are using ld on AIX,
note that it is different from the behaviour of the AIX linker.
-Lsearchdir
-library-path=searchdir
Add path searchdir to the list of paths that ld will search for archive libraries and ld control
scripts. You may use this option any number of times. The directories are searched in the order
in which they are specified on the command line. Directories specified on the command line are
searched before the default directories. All -L options apply to all -l options, regardless of the
order in which the options appear.
If searchdir begins with =, then the = will be replaced by the sysroot prefix, a path specified
when the linker is configured.
The default set of paths searched (without being specified with -L) depends on which emulation mode ld is using, and in some cases also on how it was configured. Refer to Section 3.2
Environment Variables.
The paths can also be specified in a link script with the SEARCH_DIR command. Directories
specified this way are searched at the point in which the linker script appears in the command
line.
-memulation
Emulate the emulation linker. You can list the available emulations with the -verbose or -V
options.
If the -m option is not used, the emulation is taken from the LDEMULATION environment variable,
if that is defined.
Otherwise, the default emulation depends upon how the linker was configured.
-M
-print-map
Print a link map to the standard output. A link map provides information about the link, including
the following:
•
Where object files and symbols are mapped into memory.
•
How common symbols are allocated.
•
All archive members included in the link, with a mention of the symbol which caused the
archive member to be brought in.
10
Chapter 3. Invocation
-n
-nmagic
Turn off page alignment of sections, and mark the output as NMAGIC if possible.
-N
-omagic
Set the text and data sections to be readable and writable. Also, do not page-align the data segment, and disable linking against shared libraries. If the output format supports Unix style magic
numbers, mark the output as OMAGIC.
-no-omagic
This option negates most of the effects of the -N option. It sets the text section to be read-only,
and forces the data segment to be page-aligned. Note - this option does not enable linking against
shared libraries. Use -Bdynamic for this.
-o output
-output=output
Use output as the name for the program produced by ld; if this option is not specified, the name
a.out is used by default. The script command OUTPUT can also specify the output file name.
-O level
If level is a numeric values greater than zero ld optimizes the output. This might take significantly longer and therefore probably should only be enabled for the final binary.
-q
-emit-relocs
Leave relocation sections and contents in fully linked exececutables. Post link analysis and optimization tools may need this information in order to perform correct modifications of executables. This results in larger executables.
This option is currently only supported on ELF platforms.
-r
-relocateable
Generate relocatable output--i.e., generate an output file that can in turn serve as input to ld.
This is often called partial linking. As a side effect, in environments that support standard Unix
magic numbers, this option also sets the output file’s magic number to OMAGIC. If this option
is not specified, an absolute file is produced. When linking C++ programs, this option will not
resolve references to constructors; to do that, use -Ur.
When an input file does not have the same format as the output file, partial linking is only supported if that input file does not contain any relocations. Different output formats can have further
restrictions; for example some a.out-based formats do not support partial linking with input files
in other formats at all.
This option does the same thing as -i.
Chapter 3. Invocation
11
-R filename
-just-symbols=filename
Read symbol names and their addresses from filename, but do not relocate it or include it in
the output. This allows your output file to refer symbolically to absolute locations of memory
defined in other programs. You may use this option more than once.
For compatibility with other ELF linkers, if the -R option is followed by a directory name, rather
than a file name, it is treated as the -rpath option.
-s
-strip-all
Omit all symbol information from the output file.
-S
-strip-debug
Omit debugger symbol information (but not all symbols) from the output file.
-t
-trace
Print the names of the input files as ld processes them.
-T scriptfile
-script=scriptfile
Use scriptfile as the linker script. This script replaces ld’s default linker script (rather than
adding to it), so commandfile must specify everything necessary to describe the output file.
Refer to Chapter 4 Linker Scripts. If scriptfile does not exist in the current directory, ld looks
for it in the directories specified by any preceding -L options. Multiple -T options accumulate.
-u symbol
-undefined=symbol
Force symbol to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. -u may be repeated with
different option arguments to enter additional undefined symbols. This option is equivalent to the
EXTERN linker script command.
-Ur
For anything other than C++ programs, this option is equivalent to -r: it generates relocatable
output--i.e., an output file that can in turn serve as input to ld. When linking C++ programs, -Ur
does resolve references to constructors, unlike -r. It does not work to use -Ur on files that were
themselves linked with -Ur; once the constructor table has been built, it cannot be added to. Use
-Ur only for the last partial link, and -r for the others.
-unique[=SECTION]
Creates a separate output section for every input section matching SECTION, or if the optional
wildcard SECTION argument is missing, for every orphan input section. An orphan section is
one not specifically mentioned in a linker script. You may use this option multiple times on the
command line; It prevents the normal merging of input sections with the same name, overriding
output section assignments in a linker script.
12
Chapter 3. Invocation
-v
-version
-V
Display the version number for ld. The -V option also lists the supported emulations.
-x
-discard-all
Delete all local symbols.
-X
-discard-locals
Delete all temporary local symbols. For most targets, this is all local symbols whose names begin
with L.
-y symbol
-trace-symbol=symbol
Print the name of each linked file in which symbol appears. This option may be given any
number of times. On many systems it is necessary to prepend an underscore.
This option is useful when you have an undefined symbol in your link but don’t know where the
reference is coming from.
-Y path
Add path to the default library search path. This option exists for Solaris compatibility.
-z keyword
The recognized keywords are initfirst, interpose, loadfltr, nodefaultlib,
nodelete, nodlopen, nodump, now, origin, combreloc, nocombreloc and
nocopyreloc. The other keywords are ignored for Solaris compatibility. initfirst marks
the object to be initialized first at runtime before any other objects. interpose marks the
object that its symbol table interposes before all symbols but the primary executable. loadfltr
marks the object that its filtees be processed immediately at runtime. nodefaultlib marks the
object that the search for dependencies of this object will ignore any default library search
paths. nodelete marks the object shouldn’t be unloaded at runtime. nodlopen marks the
object not available to dlopen. nodump marks the object can not be dumped by dldump. now
marks the object with the non-lazy runtime binding. origin marks the object may contain
$ORIGIN. defs disallows undefined symbols. muldefs allows multiple definitions.
combreloc combines multiple reloc sections and sorts them to make dynamic symbol lookup
caching possible. nocombreloc disables multiple reloc sections combining. nocopyreloc
disables production of copy relocs.
-( archives -)
-start-group archives -end-group
The archives should be a list of archive files. They may be either explicit file names, or -l
options.
The specified archives are searched repeatedly until no new undefined references are created.
Normally, an archive is searched only once in the order that it is specified on the command line.
If a symbol in that archive is needed to resolve an undefined symbol referred to by an object in
an archive that appears later on the command line, the linker would not be able to resolve that
reference. By grouping the archives, they all be searched repeatedly until all possible references
are resolved.
Chapter 3. Invocation
13
Using this option has a significant performance cost. It is best to use it only when there are
unavoidable circular references between two or more archives.
-accept-unknown-input-arch
-no-accept-unknown-input-arch
Tells the linker to accept input files whose architecture cannot be recognised. The assumption is
that the user knows what they are doing and deliberately wants to link in these unknown input
files. This was the default behaviour of the linker, before release 2.14. The default behaviour from
release 2.14 onwards is to reject such input files, and so the -accept-unknown-input-arch
option has been added to restore the old behaviour.
-assert keyword
This option is ignored for SunOS compatibility.
-Bdynamic
-dy
-call_shared
Link against dynamic libraries. This is only meaningful on platforms for which shared libraries
are supported. This option is normally the default on such platforms. The different variants of
this option are for compatibility with various systems. You may use this option multiple times on
the command line: it affects library searching for -l options which follow it.
-Bgroup
Set the DF_1_GROUP flag in the DT_FLAGS_1 entry in the dynamic section. This causes the
runtime linker to handle lookups in this object and its dependencies to be performed only inside
the group. -no-undefined is implied. This option is only meaningful on ELF platforms which
support shared libraries.
-Bstatic
-dn
-non_shared
-static
Do not link against shared libraries. This is only meaningful on platforms for which shared
libraries are supported. The different variants of this option are for compatibility with various
systems. You may use this option multiple times on the command line: it affects library searching
for -l options which follow it.
-Bsymbolic
When creating a shared library, bind references to global symbols to the definition within the
shared library, if any. Normally, it is possible for a program linked against a shared library to
override the definition within the shared library. This option is only meaningful on ELF platforms
which support shared libraries.
-check-sections
-no-check-sections
Asks the linker not to check section addresses after they have been assigned to see if there
any overlaps. Normally the linker will perform this check, and if it finds any overlaps it will
produce suitable error messages. The linker does know about, and does make allowances for
sections in overlays. The default behaviour can be restored by using the command line switch
-check-sections.
14
Chapter 3. Invocation
-cref
Output a cross reference table. If a linker map file is being generated, the cross reference table is
printed to the map file. Otherwise, it is printed on the standard output.
The format of the table is intentionally simple, so that it may be easily processed by a script if
necessary. The symbols are printed out, sorted by name. For each symbol, a list of file names is
given. If the symbol is defined, the first file listed is the location of the definition. The remaining
files contain references to the symbol.
-no-define-common
This option inhibits the assignment of addresses to common symbols. The script command
INHIBIT_COMMON_ALLOCATION has the same effect. Refer to Section 4.4.4 Other Linker Script
Commands.
The -no-define-common option allows decoupling the decision to assign addresses to Common symbols from the choice of the output file type; otherwise a non-Relocatable output type
forces assigning addresses to Common symbols. Using -no-define-common allows Common
symbols that are referenced from a shared library to be assigned addresses only in the main program. This eliminates the unused duplicate space in the shared library, and also prevents any
possible confusion over resolving to the wrong duplicate when there are many dynamic modules
with specialized search paths for runtime symbol resolution.
-defsym symbol=expression
Create a global symbol in the output file, containing the absolute address given by expression.
You may use this option as many times as necessary to define multiple symbols in the command
line. A limited form of arithmetic is supported for the expression in this context: you may
give a hexadecimal constant or the name of an existing symbol, or use + and - to add or subtract
hexadecimal constants or symbols. If you need more elaborate expressions, consider using the
linker command language from a script (refer to Section 4.5 Assigning Values to Symbols). Note:
there should be no white space between symbol, the equals sign ("[=]"), and expression.
-demangle[=style]
-no-demangle
These options control whether to demangle symbol names in error messages and other output. When the linker is told to demangle, it tries to present symbol names in a readable fashion: it strips leading underscores if they are used by the object file format, and converts C++
mangled symbol names into user readable names. Different compilers have different mangling
styles. The optional demangling style argument can be used to choose an appropriate demangling
style for your compiler. The linker will demangle by default unless the environment variable
COLLECT_NO_DEMANGLE is set. These options may be used to override the default.
-dynamic-linker file
Set the name of the dynamic linker. This is only meaningful when generating dynamically linked
ELF executables. The default dynamic linker is normally correct; don’t use this unless you know
what you are doing.
-embedded-relocs
This option is only meaningful when linking MIPS embedded PIC code, generated by the membedded-pic option to the gnu compiler and assembler. It causes the linker to create a table
which may be used at runtime to relocate any data which was statically initialized to pointer
values. See the code in testsuite/ld-empic for details.
Chapter 3. Invocation
15
-fatal-warnings
Treat all warnings as errors.
-force-exe-suffix
Make sure that an output file has a .exe suffix.
If a successfully built fully linked output file does not have a .exe or .dll suffix, this option
forces the linker to copy the output file to one of the same name with a .exe suffix. This option is
useful when using unmodified Unix makefiles on a Microsoft Windows host, since some versions
of Windows won’t run an image unless it ends in a .exe suffix.
-no-gc-sections
-gc-sections
Enable garbage collection of unused input sections. It is ignored on targets that do not support
this option. This option is not compatible with -r, nor should it be used with dynamic linking.
The default behaviour (of not performing this garbage collection) can be restored by specifying
-no-gc-sections on the command line.
-help
Print a summary of the command-line options on the standard output and exit.
-target-help
Print a summary of all target specific options on the standard output and exit.
-Map mapfile
Print a link map to the file mapfile. See the description of the -M option, above.
-no-keep-memory
ld normally optimizes for speed over memory usage by caching the symbol tables of input files
in memory. This option tells ld to instead optimize for memory usage, by rereading the symbol
tables as necessary. This may be required if ld runs out of memory space while linking a large
executable.
-no-undefined
-z defs
Normally when creating a non-symbolic shared library, undefined symbols are allowed and left
to be resolved by the runtime loader. This option disallows such undefined symbols if they come
from regular object files. The switch -no-allow-shlib-undefined controls the behaviour
for shared objects being linked into the shared library.
-allow-multiple-definition
-z muldefs
Normally when a symbol is defined multiple times, the linker will report a fatal error. These
options allow multiple definitions and the first definition will be used.
-allow-shlib-undefined
-no-allow-shlib-undefined
Allow (the default) or disallow undefined symbols in shared objects. The setting of this switch
overrides -no-undefined where shared objects are concerned. Thus if -no-undefined is
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Chapter 3. Invocation
set but -no-allow-shlib-undefined is not, the net result will be that undefined symbols in
regular object files will trigger an error, but undefined symbols in shared objects will be ignored.
The reason that -allow-shlib-undefined is the default is that the shared object being specified at link time may not be the same one that is available at load time, so the symbols might
actually be resolvable at load time. Plus there are some systems, (eg BeOS) where undefined
symbols in shared libraries is normal since the kernel patches them at load time to select which
function is most appropriate for the current architecture. eg. to dynamically select an appropriate memset function. Apparently it is also normal for HPPA shared libraries to have undefined
symbols.
-no-undefined-version
Normally when a symbol has an undefined version, the linker will ignore it. This option disallows
symbols with undefined version and a fatal error will be issued instead.
-no-warn-mismatch
Normally ld will give an error if you try to link together input files that are mismatched for
some reason, perhaps because they have been compiled for different processors or for different
endiannesses. This option tells ld that it should silently permit such possible errors. This option
should only be used with care, in cases when you have taken some special action that ensures
that the linker errors are inappropriate.
-no-whole-archive
Turn off the effect of the -whole-archive option for subsequent archive files.
-noinhibit-exec
Retain the executable output file whenever it is still usable. Normally, the linker will not produce
an output file if it encounters errors during the link process; it exits without writing an output file
when it issues any error whatsoever.
-nostdlib
Only search library directories explicitly specified on the command line. Library directories specified in linker scripts (including linker scripts specified on the command line) are ignored.
-oformat output-format
ld may be configured to support more than one kind of object file. If your ld is configured
this way, you can use the -oformat option to specify the binary format for the output object
file. Even when ld is configured to support alternative object formats, you don’t usually need
to specify this, as ld should be configured to produce as a default output format the most usual
format on each machine. output-format is a text string, the name of a particular format supported by the BFD libraries. (You can list the available binary formats with objdump -i.) The
script command OUTPUT_FORMAT can also specify the output format, but this option overrides
it. Refer to Chapter 6 BFD.
-pie
-pic-executable
Create a position independent executable. This is currently only supported on ELF platforms.
Position independent executables are similar to shared libraries in that they are relocated by the
dynamic linker to the virtual address OS chooses for them (which can varry between invocations),
like normal dynamically linked executables they can be executed and symbols defined in the
executable cannot be overridden by shared libraries.
Chapter 3. Invocation
17
-qmagic
This option is ignored for Linux compatibility.
-Qy
This option is ignored for SVR4 compatibility.
-relax
An option with machine dependent effects. This option is only supported on a few targets. Refer
to Section 5.1 ld and the H8/300, Section 5.2 ld and the Intel 960 Family and Section 5.9 ld
and Xtensa Processors.
On some platforms, the -relax option performs global optimizations that become possible when
the linker resolves addressing in the program, such as relaxing address modes and synthesizing
new instructions in the output object file.
On some platforms these link time global optimizations may make symbolic debugging of the
resulting executable impossible. This is known to be the case for the Matsushita MN10200 and
MN10300 family of processors.
On platforms where this is not supported, -relax is accepted, but ignored.
-retain-symbols-file filename
Retain only the symbols listed in the file filename, discarding all others. filename is simply
a flat file, with one symbol name per line. This option is especially useful in environments (such
as VxWorks) where a large global symbol table is accumulated gradually, to conserve run-time
memory.
-retain-symbols-file does not discard undefined symbols, or symbols needed for reloca-
tions.
You may only specify -retain-symbols-file once in the command line. It overrides -s and
-S.
-rpath dir
Add a directory to the runtime library search path. This is used when linking an ELF executable
with shared objects. All -rpath arguments are concatenated and passed to the runtime linker,
which uses them to locate shared objects at runtime. The -rpath option is also used when
locating shared objects which are needed by shared objects explicitly included in the link; see the
description of the -rpath-link option. If -rpath is not used when linking an ELF executable,
the contents of the environment variable LD_RUN_PATH will be used if it is defined.
The -rpath option may also be used on SunOS. By default, on SunOS, the linker will form a
runtime search patch out of all the -L options it is given. If a -rpath option is used, the runtime
search path will be formed exclusively using the -rpath options, ignoring the -L options. This
can be useful when using gcc, which adds many -L options which may be on NFS mounted
filesystems.
For compatibility with other ELF linkers, if the -R option is followed by a directory name, rather
than a file name, it is treated as the -rpath option.
-rpath-link DIR
When using ELF or SunOS, one shared library may require another. This happens when an ld
-shared link includes a shared library as one of the input files.
When the linker encounters such a dependency when doing a non-shared, non-relocatable link,
it will automatically try to locate the required shared library and include it in the link, if it is
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Chapter 3. Invocation
not included explicitly. In such a case, the -rpath-link option specifies the first set of directories to search. The -rpath-link option may specify a sequence of directory names either by
specifying a list of names separated by colons, or by appearing multiple times.
This option should be used with caution as it overrides the search path that may have been hard
compiled into a shared library. In such a case it is possible to use unintentionally a different
search path than the runtime linker would do.
The linker uses the following search paths to locate required shared libraries.
1. Any directories specified by -rpath-link options.
2. Any directories specified by -rpath options. The difference between -rpath and
-rpath-link is that directories specified by -rpath options are included in the
executable and used at runtime, whereas the -rpath-link option is only effective at link
time. It is for the native linker only.
3. On an ELF system, if the -rpath and rpath-link options were not used, search the
contents of the environment variable LD_RUN_PATH. It is for the native linker only.
4. On SunOS, if the -rpath option was not used, search any directories specified using -L
options.
5. For a native linker, the contents of the environment variable LD_LIBRARY_PATH.
6. For a native ELF linker, the directories in DT_RUNPATH or DT_RPATH of a shared library are searched for shared libraries needed by it. The DT_RPATH entries are ignored
if DT_RUNPATH entries exist.
7. The default directories, normally /lib and /usr/lib.
8. For a native linker on an ELF system, if the file /etc/ld.so.conf exists, the list of
directories found in that file.
If the required shared library is not found, the linker will issue a warning and continue with the
link.
-shared
-Bshareable
Create a shared library. This is currently only supported on ELF, XCOFF and SunOS platforms.
On SunOS, the linker will automatically create a shared library if the -e option is not used and
there are undefined symbols in the link.
-sort-common
This option tells ld to sort the common symbols by size when it places them in the appropriate
output sections. First come all the one byte symbols, then all the two byte, then all the four byte,
and then everything else. This is to prevent gaps between symbols due to alignment constraints.
-split-by-file [size]
Similar to -split-by-reloc but creates a new output section for each input file when size is
reached. size defaults to a size of 1 if not given.
-split-by-reloc [count]
Tries to creates extra sections in the output file so that no single output section in the file contains more than count relocations. This is useful when generating huge relocatable files for
downloading into certain real time kernels with the COFF object file format; since COFF cannot
represent more than 65535 relocations in a single section. Note that this will fail to work with
object file formats which do not support arbitrary sections. The linker will not split up individual
Chapter 3. Invocation
19
input sections for redistribution, so if a single input section contains more than count relocations
one output section will contain that many relocations. count defaults to a value of 32768.
-stats
Compute and display statistics about the operation of the linker, such as execution time and
memory usage.
-traditional-format
For some targets, the output of ld is different in some ways from the output of some existing
linker. This switch requests ld to use the traditional format instead.
For example, on SunOS, ld combines duplicate entries in the symbol string table. This can
reduce the size of an output file with full debugging information by over 30 percent. Unfortunately, the SunOS dbx program can not read the resulting program (gdb has no trouble). The
-traditional-format switch tells ld to not combine duplicate entries.
-section-start sectionname=org
Locate a section in the output file at the absolute address given by org. You may use this option as many times as necessary to locate multiple sections in the command line. org must be
a single hexadecimal integer; for compatibility with other linkers, you may omit the leading
0x usually associated with hexadecimal values. Note: there should be no white space between
sectionname, the equals sign ("[=]"), and org.
-Tbss org
-Tdata org
-Ttext org
Same as -section-start, with .bss, .data or .text as the sectionname.
-dll-verbose
-verbose
Display the version number for ld and list the linker emulations supported. Display which input
files can and cannot be opened. Display the linker script being used by the linker.
-version-script=version-scriptfile
Specify the name of a version script to the linker. This is typically used when creating shared
libraries to specify additional information about the version hierarchy for the library being created. This option is only meaningful on ELF platforms which support shared libraries. Refer to
Section 4.9 VERSION Command.
-warn-common
Warn when a common symbol is combined with another common symbol or with a symbol definition. Unix linkers allow this somewhat sloppy practice, but linkers on some other operating
systems do not. This option allows you to find potential problems from combining global symbols. Unfortunately, some C libraries use this practice, so you may get some warnings about
symbols in the libraries as well as in your programs.
There are three kinds of global symbols, illustrated here by C examples:
int i = 1;
A definition, which goes in the initialized data section of the output file.
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Chapter 3. Invocation
extern int i;
An undefined reference, which does not allocate space. There must be either a definition or
a common symbol for the variable somewhere.
int i;
A common symbol. If there are only (one or more) common symbols for a variable, it goes
in the uninitialized data area of the output file. The linker merges multiple common symbols
for the same variable into a single symbol. If they are of different sizes, it picks the largest
size. The linker turns a common symbol into a declaration, if there is a definition of the
same variable.
The -warn-common option can produce five kinds of warnings. Each warning consists of a pair
of lines: the first describes the symbol just encountered, and the second describes the previous
symbol encountered with the same name. One or both of the two symbols will be a common
symbol.
1. Turning a common symbol into a reference, because there is already a definition for the
symbol.
file(section): warning: common of ‘symbol’
overridden by definition
file(section): warning: defined here
2. Turning a common symbol into a reference, because a later definition for the symbol is
encountered. This is the same as the previous case, except that the symbols are encountered
in a different order.
file(section): warning: definition of ‘symbol’
overriding common
file(section): warning: common is here
3. Merging a common symbol with a previous same-sized common symbol.
file(section): warning: multiple common
of ‘symbol’
file(section): warning: previous common is here
4. Merging a common symbol with a previous larger common symbol.
file(section): warning: common of ‘symbol’
overridden by larger common
file(section): warning: larger common is here
5. Merging a common symbol with a previous smaller common symbol. This is the same as
the previous case, except that the symbols are encountered in a different order.
file(section): warning: common of ‘symbol’
overriding smaller common
file(section): warning: smaller common is here
-warn-constructors
Warn if any global constructors are used. This is only useful for a few object file formats. For
formats like COFF or ELF, the linker can not detect the use of global constructors.
-warn-multiple-gp
Warn if multiple global pointer values are required in the output file. This is only meaningful for
certain processors, such as the Alpha. Specifically, some processors put large-valued constants
in a special section. A special register (the global pointer) points into the middle of this section,
so that constants can be loaded efficiently via a base-register relative addressing mode. Since the
offset in base-register relative mode is fixed and relatively small (e.g., 16 bits), this limits the
maximum size of the constant pool. Thus, in large programs, it is often necessary to use multiple
global pointer values in order to be able to address all possible constants. This option causes a
warning to be issued whenever this case occurs.
Chapter 3. Invocation
21
-warn-once
Only warn once for each undefined symbol, rather than once per module which refers to it.
-warn-section-align
Warn if the address of an output section is changed because of alignment. Typically, the alignment
will be set by an input section. The address will only be changed if it not explicitly specified; that
is, if the SECTIONS command does not specify a start address for the section (refer to Section
4.6 SECTIONS Command).
-whole-archive
For each archive mentioned on the command line after the -whole-archive option, include
every object file in the archive in the link, rather than searching the archive for the required object
files. This is normally used to turn an archive file into a shared library, forcing every object to be
included in the resulting shared library. This option may be used more than once.
Two notes when using this option from gcc: First, gcc doesn’t know about this option, so you
have to use -Wl,-whole-archive. Second, don’t forget to use -Wl,-no-whole-archive
after your list of archives, because gcc will add its own list of archives to your link and you may
not want this flag to affect those as well.
-wrap symbol
Use a wrapper function for symbol. Any undefined reference to symbol will be resolved to
__wrap_symbol. Any undefined reference to __real_symbol will be resolved to symbol.
This can be used to provide a wrapper for a system function. The wrapper function should be
called __wrap_symbol. If it wishes to call the system function, it should call __real_symbol.
Here is a trivial example:
void *
__wrap_malloc (int c)
{
printf ("malloc called with %ld\n", c);
return __real_malloc (c);
}
If you link other code with this file using -wrap malloc, then all calls to malloc will call the
function __wrap_malloc instead. The call to __real_malloc in __wrap_malloc will call
the real malloc function.
You may wish to provide a __real_malloc function as well, so that links without the -wrap
option will succeed. If you do this, you should not put the definition of __real_malloc in the
same file as __wrap_malloc; if you do, the assembler may resolve the call before the linker has
a chance to wrap it to malloc.
-enable-new-dtags
-disable-new-dtags
This linker can create the new dynamic tags in ELF. But the older ELF systems may not understand them. If you specify -enable-new-dtags, the dynamic tags will be created as needed. If
you specify -disable-new-dtags, no new dynamic tags will be created. By default, the new
dynamic tags are not created. Note that those options are only available for ELF systems.
3.1.1. Options Specific to i386 PE Targets
The i386 PE linker supports the -shared option, which causes the output to be a dynamically linked
library (DLL) instead of a normal executable. You should name the output *.dll when you use this
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Chapter 3. Invocation
option. In addition, the linker fully supports the standard *.def files, which may be specified on the
linker command line like an object file (in fact, it should precede archives it exports symbols from, to
ensure that they get linked in, just like a normal object file).
In addition to the options common to all targets, the i386 PE linker support additional command line
options that are specific to the i386 PE target. Options that take values may be separated from their
values by either a space or an equals sign.
-add-stdcall-alias
If given, symbols with a stdcall suffix (@nn) will be exported as-is and also with the suffix
stripped. [This option is specific to the i386 PE targeted port of the linker]
-base-file file
Use file as the name of a file in which to save the base addresses of all the relocations needed
for generating DLLs with dlltool. [This is an i386 PE specific option]
-dll
Create a DLL instead of a regular executable. You may also use -shared or specify a LIBRARY
in a given .def file. [This option is specific to the i386 PE targeted port of the linker]
-enable-stdcall-fixup
-disable-stdcall-fixup
If the link finds a symbol that it cannot resolve, it will attempt to do "fuzzy linking" by looking
for another defined symbol that differs only in the format of the symbol name (cdecl vs stdcall)
and will resolve that symbol by linking to the match. For example, the undefined symbol _foo
might be linked to the function [email protected], or the undefined symbol [email protected] might be linked
to the function _bar. When the linker does this, it prints a warning, since it normally should
have failed to link, but sometimes import libraries generated from third-party dlls may need this
feature to be usable. If you specify -enable-stdcall-fixup, this feature is fully enabled and
warnings are not printed. If you specify -disable-stdcall-fixup, this feature is disabled
and such mismatches are considered to be errors. [This option is specific to the i386 PE targeted
port of the linker]
-export-all-symbols
If given, all global symbols in the objects used to build a DLL will be exported by the DLL.
Note that this is the default if there otherwise wouldn’t be any exported symbols. When
symbols are explicitly exported via DEF files or implicitly exported via function attributes,
the default is to not export anything else unless this option is given. Note that the symbols
[email protected], [email protected], [email protected], and impure_ptr will not be
automatically exported. Also, symbols imported from other DLLs will not be re-exported, nor
will symbols specifying the DLL’s internal layout such as those beginning with _head_ or
ending with _iname. In addition, no symbols from libgcc, libstd++, libmingw32, or
crtX.o will be exported. Symbols whose names begin with __rtti_ or __builtin_ will not
be exported, to help with C++ DLLs. Finally, there is an extensive list of cygwin-private
symbols that are not exported (obviously, this applies on when building DLLs for cygwin
targets). These cygwin-excludes are: [email protected], [email protected],
[email protected], _fmode, _impure_ptr, cygwin_attach_dll,
cygwin_premain0, cygwin_premain1, cygwin_premain2, cygwin_premain3, and
environ. [This option is specific to the i386 PE targeted port of the linker]
Chapter 3. Invocation
23
-exclude-symbols symbol,symbol,...
Specifies a list of symbols which should not be automatically exported. The symbol names may
be delimited by commas or colons. [This option is specific to the i386 PE targeted port of the
linker]
-exclude-libs lib,lib,...
Specifies a list of archive libraries from which symbols should not be automatically exported.
The library names may be delimited by commas or colons. Specifying -exclude-libs ALL
excludes symbols in all archive libraries from automatic export. Symbols explicitly listed in a
.def file are still exported, regardless of this option. [This option is specific to the i386 PE targeted
port of the linker]
-file-alignment
Specify the file alignment. Sections in the file will always begin at file offsets which are multiples
of this number. This defaults to 512. [This option is specific to the i386 PE targeted port of the
linker]
-heap reserve
-heap reserve,commit
Specify the amount of memory to reserve (and optionally commit) to be used as heap for this
program. The default is 1Mb reserved, 4K committed. [This option is specific to the i386 PE
targeted port of the linker]
-image-base value
Use value as the base address of your program or dll. This is the lowest memory location that
will be used when your program or dll is loaded. To reduce the need to relocate and improve
performance of your dlls, each should have a unique base address and not overlap any other dlls.
The default is 0x400000 for executables, and 0x10000000 for dlls. [This option is specific to the
i386 PE targeted port of the linker]
-kill-at
If given, the stdcall suffixes (@nn) will be stripped from symbols before they are exported. [This
option is specific to the i386 PE targeted port of the linker]
-major-image-version value
Sets the major number of the "image version". Defaults to 1. [This option is specific to the i386
PE targeted port of the linker]
-major-os-version value
Sets the major number of the "os version". Defaults to 4. [This option is specific to the i386 PE
targeted port of the linker]
-major-subsystem-version value
Sets the major number of the "subsystem version". Defaults to 4. [This option is specific to the
i386 PE targeted port of the linker]
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Chapter 3. Invocation
-minor-image-version value
Sets the minor number of the "image version". Defaults to 0. [This option is specific to the i386
PE targeted port of the linker]
-minor-os-version value
Sets the minor number of the "os version". Defaults to 0. [This option is specific to the i386 PE
targeted port of the linker]
-minor-subsystem-version value
Sets the minor number of the "subsystem version". Defaults to 0. [This option is specific to the
i386 PE targeted port of the linker]
-output-def file
The linker will create the file file which will contain a DEF file corresponding to the DLL
the linker is generating. This DEF file (which should be called *.def) may be used to create
an import library with dlltool or may be used as a reference to automatically or implicitly
exported symbols. [This option is specific to the i386 PE targeted port of the linker]
-out-implib file
The linker will create the file file which will contain an import lib corresponding to the DLL the
linker is generating. This import lib (which should be called *.dll.a or *.a may be used to link
clients against the generated DLL; this behavior makes it possible to skip a separate dlltool
import library creation step. [This option is specific to the i386 PE targeted port of the linker]
-enable-auto-image-base
Automatically choose the image base for DLLs, unless one is specified using the -image-base
argument. By using a hash generated from the dllname to create unique image bases for each
DLL, in-memory collisions and relocations which can delay program execution are avoided.
[This option is specific to the i386 PE targeted port of the linker]
-disable-auto-image-base
Do not automatically generate a unique image base. If there is no user-specified image base
(-image-base) then use the platform default. [This option is specific to the i386 PE targeted
port of the linker]
-dll-search-prefix string
When
linking
string
dynamically
to
a
dll
without
an
import
library,
search
for
basename .dll in preference to lib basename .dll. This behavior allows
easy distinction between DLLs built for the various "subplatforms": native, cygwin, uwin, pw,
etc. For instance, cygwin DLLs typically use -dll-search-prefix=cyg. [This option is
specific to the i386 PE targeted port of the linker]
-enable-auto-import
Do sophisticated linking of _symbol to __imp__symbol for DATA imports from DLLs, and
create the necessary thunking symbols when building the import libraries with those DATA exports. This generally will ’just work’ - but sometimes you may see this message:
"variable ’ var ’ can’t be auto-imported. Please read the documentation for ld’s
-enable-auto-import for details."
Chapter 3. Invocation
25
This message occurs when some (sub)expression accesses an address ultimately given by the
sum of two constants (Win32 import tables only allow one). Instances where this may occur
include accesses to member fields of struct variables imported from a DLL, as well as using
a constant index into an array variable imported from a DLL. Any multiword variable (arrays,
structs, long long, etc) may trigger this error condition. However, regardless of the exact data
type of the offending exported variable, ld will always detect it, issue the warning, and exit.
There are several ways to address this difficulty, regardless of the data type of the exported
variable:
One way is to use -enable-runtime-pseudo-reloc switch. This leaves the task of adjusting references in your client code for runtime environment, so this method works only when runtime
environtment supports this feature.
A second solution is to force one of the ’constants’ to be a variable - that is, unknown and unoptimizable at compile time. For arrays, there are two possibilities: a) make the indexee (the
array’s address) a variable, or b) make the ’constant’ index a variable. Thus:
extern type extern_array[];
extern_array[1] -{ volatile type *t=extern_array; t[1] }
or
extern type extern_array[];
extern_array[1] -{ volatile int t=1; extern_array[t] }
For structs (and most other multiword data types) the only option is to make the struct itself (or
the long long, or the ...) variable:
extern struct s extern_struct;
extern_struct.field -{ volatile struct s *t=&extern_struct; t- field }
or
extern long long extern_ll;
extern_ll -{ volatile long long * local_ll=&extern_ll; *local_ll }
A third method of dealing with this difficulty is to abandon ’auto-import’ for the offending
symbol and mark it with __declspec(dllimport). However, in practice that requires using compile-time #defines to indicate whether you are building a DLL, building client code that
will link to the DLL, or merely building/linking to a static library. In making the choice between
the various methods of resolving the ’direct address with constant offset’ problem, you should
consider typical real-world usage:
Original:
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}
Solution 1:
--foo.h
extern int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
/* This workaround is for win32 and cygwin; do not "optimize" */
volatile int *parr = arr;
printf("%d\n",parr[1]);
}
26
Chapter 3. Invocation
Solution 2:
--foo.h
/* Note: auto-export is assumed (no __declspec(dllexport)) */
#if (defined(_WIN32) || defined(__CYGWIN__)) && \
!(defined(FOO_BUILD_DLL) || defined(FOO_STATIC))
#define FOO_IMPORT __declspec(dllimport)
#else
#define FOO_IMPORT
#endif
extern FOO_IMPORT int arr[];
--foo.c
#include "foo.h"
void main(int argc, char **argv){
printf("%d\n",arr[1]);
}
A fourth way to avoid this problem is to re-code your library to use a functional interface rather
than a data interface for the offending variables (e.g. set_foo() and get_foo() accessor functions).
[This option is specific to the i386 PE targeted port of the linker]
-disable-auto-import
Do not attempt to do sophisticalted linking of _symbol to __imp__symbol for DATA imports
from DLLs. [This option is specific to the i386 PE targeted port of the linker]
-enable-runtime-pseudo-reloc
If your code contains expressions described in -enable-auto-import section, that is, DATA imports
from DLL with non-zero offset, this switch will create a vector of ’runtime pseudo relocations’
which can be used by runtime environment to adjust references to such data in your client code.
[This option is specific to the i386 PE targeted port of the linker]
-disable-runtime-pseudo-reloc
Do not create pseudo relocations for non-zero offset DATA imports from DLLs. This is the
default. [This option is specific to the i386 PE targeted port of the linker]
-enable-extra-pe-debug
Show additional debug info related to auto-import symbol thunking. [This option is specific to
the i386 PE targeted port of the linker]
-section-alignment
Sets the section alignment. Sections in memory will always begin at addresses which are a multiple of this number. Defaults to 0x1000. [This option is specific to the i386 PE targeted port of
the linker]
-stack reserve
-stack reserve,commit
Specify the amount of memory to reserve (and optionally commit) to be used as stack for this
program. The default is 2Mb reserved, 4K committed. [This option is specific to the i386 PE
targeted port of the linker]
Chapter 3. Invocation
27
-subsystem which
-subsystem which:major
-subsystem which:major.minor
Specifies the subsystem under which your program will execute. The legal values for which are
native, windows, console, and posix. You may optionally set the subsystem version also.
[This option is specific to the i386 PE targeted port of the linker]
3.2. Environment Variables
You can change the behavior of ld with the environment variables GNUTARGET, LDEMULATION and
COLLECT_NO_DEMANGLE.
GNUTARGET determines the input-file object format if you don’t use -b (or its synonym -format).
Its value should be one of the BFD names for an input format (refer to Chapter 6 BFD). If there is
no GNUTARGET in the environment, ld uses the natural format of the target. If GNUTARGET is set to
default then BFD attempts to discover the input format by examining binary input files; this method
often succeeds, but there are potential ambiguities, since there is no method of ensuring that the magic
number used to specify object-file formats is unique. However, the configuration procedure for BFD
on each system places the conventional format for that system first in the search-list, so ambiguities
are resolved in favor of convention.
LDEMULATION determines the default emulation if you don’t use the -m option. The emulation can af-
fect various aspects of linker behaviour, particularly the default linker script. You can list the available
emulations with the -verbose or -V options. If the -m option is not used, and the LDEMULATION environment variable is not defined, the default emulation depends upon how the linker was configured.
Normally, the linker will default to demangling symbols. However, if COLLECT_NO_DEMANGLE is
set in the environment, then it will default to not demangling symbols. This environment variable is
used in a similar fashion by the gcc linker wrapper program. The default may be overridden by the
-demangle and -no-demangle options.
28
Chapter 3. Invocation
Chapter 4.
Linker Scripts
Every link is controlled by a linker script. This 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. Most linker scripts
do nothing more than this. However, when necessary, the linker script can also direct the linker to
perform many other operations, using the commands described below.
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. Refer to Section 4.11 Implicit Linker Scripts.
4.1. Basic Linker Script Concepts
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, which mean 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.
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Chapter 4. Linker Scripts
4.2. Linker Script Format
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.
4.3. 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. Let’s
assume your program consists only of code, initialized data, and uninitialized data. These will be
in the .text, .data, and .bss sections, respectively. Let’s assume further that these are the only
sections which appear in your input files.
For this example, let’s say that the code should be loaded at address 0x10000, and that the data should
start at address 0x8000000. Here is a linker script which will do that:
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 inside the SECTIONS command of the above example sets the value of 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. At the start of the SECTIONS
command, the location counter has the value 0.
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 linker will place the
.data output section at address 0x8000000. After the linker places the .data output section, the
value of the location counter will be 0x8000000 plus the size of the .data output section. The effect
is that the linker will place the .bss output section immediately after the .data output section in
memory
Chapter 4. Linker Scripts
31
The linker will ensure that each output section has the required alignment, by increasing the location
counter if necessary. In this example, the specified addresses for the .text and .data sections will
probably satisfy any alignment constraints, but the linker may have to create a small gap between the
.data and .bss sections.
That’s it! That’s a simple and complete linker script.
4.4. Simple Linker Script Commands
In this section we describe the simple linker script commands.
4.4.1. 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.
4.4.2. Commands Dealing with Files
Several linker script commands deal with files.
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 can’t 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.
In case a sysroot prefix is configured, and the filename starts with the / character, and the script
being processed was located inside the sysroot prefix, the filename will be looked for in the
sysroot prefix. Otherwise, the linker will 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 in
Section 3.1 Command Line Options.
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Chapter 4. Linker Scripts
If you use INPUT (-lfile), ld will transform the name to libfile.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. See the description of -(
in Section 3.1 Command Line Options.
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 (refer to Section 3.1 Command Line
Options). 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 (refer to Section 3.1 Command Line Options). 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.
4.4.3. Commands Dealing with Object File Formats
A couple of linker script commands deal with object file formats.
OUTPUT_FORMAT(bfdname)
OUTPUT_FORMAT(default, big, little)
The OUTPUT_FORMAT command names the BFD format to use for the output file (refer to Chapter
6 BFD). Using OUTPUT_FORMAT(bfdname) is exactly like using -oformat bfdname on the
command line (refer to Section 3.1 Command Line Options). 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 are 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 this command:
UTPUT_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.
Chapter 4. Linker Scripts
33
TARGET(bfdname)
The TARGET command names the 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 (refer to Section 3.1 Command Line Options). 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. Refer to Chapter 6 BFD.
4.4.4. Other Linker Script Commands
There are a few other linker scripts commands.
ASSERT(exp, message)
Ensure that exp is non-zero. If it is zero, then exit the linker with an error code, and print
message.
EXTERN(symbol symbol ...)
Force symbol to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. You may list several symbols
for each EXTERN, and you may use EXTERN multiple times. This command has the same effect
as the -u command-line option.
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 (-r).
INHIBIT_COMMON_ALLOCATION
This command has the same effect as the -no-define-common command-line option: to make
ld omit the assignment of addresses to common symbols even for a non-relocatable output file.
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. Note that the
NOCROSSREFS command uses output section names, not input section names.
OUTPUT_ARCH(bfdarch)
Specify a particular output machine architecture. The argument is one of the names used by the
BFD library (refer to Chapter 6 BFD). You can see the architecture of an object file by using the
objdump program with the -f option.
4.5. 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.
34
Chapter 4. Linker Scripts
4.5.1. 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
• symbol
= expression ;
= 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 adjusted accordingly.
The special symbol name . indicates the location counter. You may only use this within a SECTIONS
command.
The semicolon after expression is required.
Expressions are defined below; refer to Section 4.10 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, refer
to Section 4.10.6 The Section of an Expression.
Here is an example showing the three different places that symbol assignments may be used:
floating_point = 0;
SECTIONS
{
.text :
{
*(.text)
_etext = .;
}
_bdata = (. + 3) & ~ 3;
.data : { *(.data) }
}
In this example, the symbol floating_point will be defined as zero. The symbol _etext will be
defined as the address following the last .text input section. The symbol _bdata will be defined as
the address following the .text output section aligned upward to a 4 byte boundary.
4.5.2. PROVIDE
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).
Here is an example of using PROVIDE to define etext:
Chapter 4. Linker Scripts
35
SECTIONS
{
.text :
{
*(.text)
_etext = .;
PROVIDE(etext = .);
}
}
In this example, if the program defines _etext (with a leading underscore), the linker will give a multiple definition error. If, on the other hand, the program defines etext (with no leading underscore),
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.
4.6. 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:
•
an ENTRY command (refer to Section 4.4.1 Setting the Entry Point)
•
a symbol assignment (refer to Section 4.5 Assigning Values to Symbols)
•
an output section description
•
an overlay description
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.
Output section descriptions and overlay descriptions are described below.
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.
4.6.1. Output Section Description
The full description of an output section looks like this:
section [address] [(type)] : [AT(lma)]
{
output-section-command
output-section-command
36
Chapter 4. Linker Scripts
...
} [ region] [AT lma_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.
Each output-section-command may be one of the following:
•
a symbol assignment (refer to Section 4.5 Assigning Values to Symbols)
•
an input section description (refer to Section 4.6.4 Input Section Description)
•
data values to include directly (refer to Section 4.6.5 Output Section Data)
•
a special output section keyword (refer to Section 4.6.6 Output Section Keywords)
4.6.2. Output Section Name
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; refer to Section 4.6.7 Output Section Discarding.
4.6.3. Output Section Description
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. 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,
.text . : { *(.text) }
and
.text : { *(.text) }
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; refer to Section 4.10 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 do something like this:
.text ALIGN(0x10) : { *(.text) }
Chapter 4. Linker Scripts
37
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.
4.6.4. 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.
4.6.4.1. Input Section Basics
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 further below (refer
to Section 4.6.4.2 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. To exclude a list of files from matching the
file name wildcard, EXCLUDE_FILE may be used to match all files except the ones specified in the
EXCLUDE_FILE list. For example:
(*(EXCLUDE_FILE (*crtend.o *otherfile.o) .ctors))
will cause all .ctors sections from all files except crtend.o and otherfile.o to be included.
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, appearing in the same order as
they are found in the linker input. 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:
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:
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.
Note that this differs from an INPUT command, because the linker will not search for the file in the
archive search path.
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Chapter 4. Linker Scripts
4.6.4.2. 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, this
sequence of input section descriptions is probably in error, because the data.o rule will not be used:
.data : { *(.data) }
.data1 : { data.o(.data) }
Normally, the linker will place files and sections matched by wildcards in the order in which they
are seen during the link. You can change this by using the SORT keyword, which appears before a
wildcard pattern in parentheses (e.g., SORT(.text*)). When the SORT keyword is used, the linker
will sort the files or sections into ascending order by name before placing them in the output file.
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) }
}
Chapter 4. Linker Scripts
39
4.6.4.3. 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:
.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).
4.6.4.4. Input Section and Garbage Collection
When link-time garbage collection is in use (-gc-sections), it is often useful to mark sections that
should not be eliminated. This is accomplished by surrounding an input section’s wildcard entry with
KEEP(), as in KEEP(*(.init)) or KEEP(SORT(*)(.ctors)).
4.6.4.5. Input Section Example
The following example is a complete linker script. It 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 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)
}
}
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Chapter 4. Linker Scripts
4.6.5. Output Section Data
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 (refer to Section 4.10 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.
Note--these commands only work inside a section description and not between them, so the following
will produce an error from the linker:
SECTIONS { .text : { *(.text) } LONG(1) .data : { *(.data) } }
whereas this will work:
SECTIONS { .text : { *(.text) ; LONG(1) } .data : { *(.data) } }
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 value 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 0x90:
FILL(0x90909090)
The FILL command is similar to the =fillexp output section attribute, 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. Refer to Section 4.6.8.5 Output Section Fill for details on the fill expression.
4.6.6. Output Section Keywords
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.
Chapter 4. Linker Scripts
41
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 symbol __CTOR_LIST__ marks the start of the global constructors, and the symbol
__DTOR_LIST marks the end. The first word in the list is the number of entries, followed by the
address of each constructor or destructor, followed by a zero word. The compiler must arrange
to actually run the code. For these object file formats gnu C++ 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 which the gnu C++ runtime code expects to see.
__CTOR_LIST__ = .;
LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
*(.ctors)
LONG(0)
__CTOR_END__ = .;
__DTOR_LIST__ = .;
LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
*(.dtors)
LONG(0)
__DTOR_END__ = .;
If you are using the gnu C++ support for initialization priority, which provides some control
over the order in which global constructors are run, you must sort the constructors at link time
to ensure that they are executed in the correct order. When using the CONSTRUCTORS command, use SORT(CONSTRUCTORS) instead. When using the .ctors and .dtors sections, use
*(SORT(.ctors)) and *(SORT(.dtors)) instead of just *(.ctors) and *(.dtors).
Normally the compiler and linker will handle these issues automatically, and you will not need
to concern yourself with them. However, you may need to consider this if you are using C++ and
writing your own linker scripts.
4.6.7. 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:
.foo { *(.foo) }
will only create a .foo section in the output file if there is a .foo section in at least one input file.
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.
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Chapter 4. Linker Scripts
4.6.8. Output Section Attributes
We showed above that the full description of an output section looked like this:
section [address] [(type)] : [AT(lma)]
{
output-section-command
output-section-command
...
} [ region] [AT lma_region] [:phdr :phdr ...] [=fillexp]
We’ve already described section, address, and output-section-command. In this section we
will describe the remaining section attributes.
4.6.8.1. Output Section Type
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) : { ... }
...
}
4.6.8.2. Output Section LMA
Every section has a virtual address (VMA) and a load address (LMA); see Section 4.1 Basic Linker
Script Concepts. The address expression which may appear in an output section description sets the
VMA (refer to Section 4.6.3 Output Section Description).
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.
Alternatively, with AT lma_region expression, you may specify a memory region for the section’s
load address. Refer to Section 4.7 MEMORY Command.
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 .text section even though its VMA is 0x2000, and one called .bss
to 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.
Chapter 4. Linker Scripts
43
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
*dst = 0;
&_bend; dst++)
4.6.8.3. Output Section Region
You can assign a section to a previously defined region of memory by using
4.7 MEMORY Command.
region. Refer to Section
Here is a simple example:
MEMORY { rom : ORIGIN = 0x1000, LENGTH = 0x1000 }
SECTIONS { ROM : { *(.text) } rom }
4.6.8.4. Output Section Phdr
You can assign a section to a previously defined program segment by using :phdr. Refer to Section
4.8 PHDRS Command. 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. You
can use :NONE to tell the linker to not put the section in any segment at all.
Here is a simple example:
PHDRS { text PT_LOAD ; }
SECTIONS { .text : { *(.text) } :text }
4.6.8.5. Output Section Fill
You can set the fill pattern for an entire section by using =fillexp. fillexp is an expression (refer
to Section 4.10 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 value, repeated as necessary. If the fill expression is a simple hex number, ie. a string of hex
digit starting with 0x and without a trailing k or M, then an arbitrarily long sequence of hex digits can
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Chapter 4. Linker Scripts
be used to specify the fill pattern; Leading zeros become part of the pattern too. For all other cases,
including extra parentheses or a unary +, the fill pattern is the four least significant bytes of the value
of the expression. In all cases, the number is big-endian.
You can also change the fill value with a FILL command in the output section commands; (refer to
Section 4.6.5 Output Section Data).
Here is a simple example:
SECTIONS { .text : { *(.text) } =0x90909090 }
4.6.9. Overlay Description
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.
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 above). The section definitions within the OVERLAY construct are identical to those
within the general SECTIONS contruct (refer to Section 4.6 SECTIONS Command), 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 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. NOCROSSREFS.
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.
Chapter 4. Linker Scripts
45
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 .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));
4.7. MEMORY Command
The linker’s 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. As described in Section 4.6
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Chapter 4. Linker Scripts
SECTIONS Command, 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.
The attr string must consist only of the following characters:
R
Read-only section
W
Read/write section
X
Executable section
A
Allocatable section
I
Initialized section
L
Same as I
!
Invert the sense of any of the preceding attributes
If a 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 keyword ORIGIN may be abbreviated to org or o (but not, for example, 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 keyword
LENGTH 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
}
Once you define a memory region, you can direct the linker to place specific output sections into that
memory region by using the region output section attribute. For example, if you have a memory
region named mem, you would use mem in the output section definition. Refer to Section 4.6.8.3
Output Section Region. If no address was specified for the output section, the linker will set the address
Chapter 4. Linker Scripts
47
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.
4.8. PHDRS Command
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 manual 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.
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 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. Refer to Section 4.6.8.4 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. You can use :NONE
to override the default segment and tell the linker to not put the section in any segment at all.
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.
PT_NULL (0)
Indicates an unused program header.
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Chapter 4. Linker Scripts
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.
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 (refer to Section
4.6.8.2 Output Section LMA). The AT command for a program header overrides the output section
attribute.
The linker will normally set the segment flags based on the sections which comprise the segment. You
may use the FLAGS keyword 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 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
...
}
Chapter 4. Linker Scripts
49
4.9. VERSION Command
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 simply
VERSION { version-script-commands }
The format of the version script commands is identical to that used by Sun’s 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; this is done using
wildcard patterns, so that any symbol whose name begins with old, original, or new is matched.
The wildcard patterns available are the same as those used in the shell when matching filenames (also
known as "globbing").
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 scripts
binds the symbols bar1 and bar2 are bound 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.
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Chapter 4. Linker Scripts
Node name can be omited, provided it is the only version node in the version script. Such version
script doesn’t assign any versions to symbols, only selects which symbols will be globally visible out
and which won’t.
{ global: foo; bar; local: *; };
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. You can do this by putting
something like:
__asm__(".symver original_foo,[email protected]_1.1");
in the C source file. 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. A .symver directive takes precedence over a version script.
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. Here is an example:
__asm__(".symver
__asm__(".symver
__asm__(".symver
__asm__(".symver
original_foo,[email protected]");
old_foo,[email protected]_1.1");
old_foo1,[email protected]_1.2");
new_foo,[email protected]@VERS_2.0");
In this example, [email protected] 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. You can do this with the
[email protected]@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.
Chapter 4. Linker Scripts
51
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.
You can also specify the language in the version script:
VERSION extern "lang" { version-script-commands }
The supported langs are C, C++, and Java. The linker will iterate over the list of symbols at the
link time and demangle them according to lang before matching them to the patterns specified in
version-script-commands.
4.10. 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.
4.10.1. Constants
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 all refer to the same quantity:
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;
4.10.2. Symbol Names
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.
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Chapter 4. Linker Scripts
4.10.3. The Location Counter
The special linker variable dot . always contains the current output location counter. Since the .
always refers to a location in an output section, it 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 . 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)
} = 0x12345678;
}
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 = 0x12345678
specifies what data to write in the gaps (refer to Section 4.6.8.5 Output Section Fill).
Note: . actually refers to the byte offset from the start of the current containing object. Normally this
is the SECTIONS statement, whose start address is 0, hence . can be used as an absolute address. If .
is used inside a section description however, it refers to the byte offset from the start of that section,
not an absolute address. Thus in a script like this:
SECTIONS
{
. = 0x100
.text: {
*(.text)
. = 0x200
}
. = 0x500
.data: {
*(.data)
. += 0x600
}
}
The .text section will be assigned a starting address of 0x100 and a size of exactly 0x200 bytes,
even if there is not enough data in the .text input sections to fill this area. (If there is too much data,
an error will be produced because this would be an attempt to move . backwards). The .data section
will start at 0x500 and it will have an extra 0x600 bytes worth of space after the end of the values
from the .data input sections and before the end of the .data output section itself.
Chapter 4. Linker Scripts
53
4.10.4. Operators
The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels:
precedence
(highest)
1
2
3
4
5
6
7
8
9
10
11
(lowest)
associativity
Operators
Notes
left
left
left
left
left
left
left
left
left
right
right
!
*
+
(1)
/
-
~
%
== !=
&
|
&&
||
? :
&= +=
=
-=
/=
*=
=
(2)
Notes: (1) Prefix operators (2) Refer to Section 4.5 Assigning Values to Symbols.
4.10.5. Evaluation
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
SECTIONS
{
.text 9+this_isnt_constant :
{ *(.text) }
}
will cause the error message non constant expression for initial address.
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Chapter 4. Linker Scripts
4.10.6. 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 symbol’s
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.
4.10.7. Builtin Functions
The linker script language includes a number of builtin functions for use in linker 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. Refer to Section 4.10.6 The Section of an Expression.
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:
SECTIONS { ...
.output1 :
{
start_of_output_1 = ABSOLUTE(.);
...
}
.output :
{
symbol_1 = ADDR(.output1);
symbol_2 = start_of_output_1;
}
... }
Chapter 4. Linker Scripts
55
ALIGN(exp)
Return the location counter (.) aligned to the next exp boundary. ALIGN doesn’t change the
value of the location counter--it just does arithmetic on it. Here is an example which 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 (refer to Section 4.6.3 Output Section
Description). The second use of ALIGN is used to defines the value of a symbol.
The builtin function NEXT is closely related to ALIGN.
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.
DATA_SEGMENT_ALIGN(maxpagesize, commonpagesize)
This is equivalent to either
(ALIGN(maxpagesize) + (. & (maxpagesize - 1)))
or
(ALIGN(maxpagesize) + (. & (maxpagesize - commonpagesize)))
depending on whether the latter uses fewer commonpagesize sized pages for the data segment
(area between the result of this expression and DATA_SEGMENT_END) than the former or not. If
the latter form is used, it means commonpagesize bytes of runtime memory will be saved at the
expense of up to commonpagesize wasted bytes in the on-disk file.
This expression can only be used directly in SECTIONS commands, not in any output section
descriptions and only once in the linker script. commonpagesize should be less or equal to
maxpagesize and should be the system page size the object wants to be optimized for (while
still working on system page sizes up to maxpagesize).
Example:
. = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
DATA_SEGMENT_END(exp)
This defines the end of data segment for DATA_SEGMENT_ALIGN evaluation purposes.
. = DATA_SEGMENT_END(.);
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 : . ;
...
}
...
}
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Chapter 4. Linker Scripts
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 (refer to Section 4.6.8.2
Output Section LMA).
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. 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. This is information which appears at the start
of the output file. You can use this number when setting the start address of the first section, if
you choose, to facilitate paging.
When producing an ELF output file, if the linker script uses the SIZEOF_HEADERS builtin function, the linker must compute the number of program headers before it has determined all the
section addresses and sizes. If the linker later discovers that it needs additional program headers,
it will report an error not enough room for program headers. To avoid this error, you
must avoid using the SIZEOF_HEADERS function, or you must rework your linker script to avoid
forcing the linker to use additional program headers, or you must define the program headers
yourself using the PHDRS command (refer to Section 4.8 PHDRS Command).
4.11. Implicit Linker Scripts
If you specify a linker input file which the linker can not recognize as an object file or an archive file,
it will try to read the file 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.
Chapter 4. Linker Scripts
57
Typically an implicit linker script would contain only symbol assignments, or the INPUT, GROUP, or
VERSION commands.
Any input files read because of an implicit linker script will be read at the position in the command
line where the implicit linker script was read. This can affect archive searching.
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Chapter 4. Linker Scripts
Chapter 5.
Machine Dependent Features
ld has additional features on some platforms; the following sections describe them. Machines where
ld has no additional functionality are not listed.
5.1. ld and the H8/300
For the H8/300, ld can perform these global optimizations when you specify the -relax commandline option.
relaxing address modes
ld finds all jsr and jmp instructions whose targets are within eight bits, and turns them into
eight-bit program-counter relative bsr and bra instructions, respectively.
synthesizing instructions
ld finds all mov.b instructions which use the sixteen-bit absolute address form, but refer to the
top page of memory, and changes them to use the eight-bit address form. (That is: the linker turns
mov.b @aa:16 into mov.b @aa:8 whenever the address aa is in the top page of memory).
5.2. ld and the Intel 960 Family
You can use the -Aarchitecture command line option to specify one of the two-letter names identifying members of the 960 family; the option specifies the desired output target, and warns of any
incompatible instructions in the input files. It also modifies the linker’s search strategy for archive
libraries, to support the use of libraries specific to each particular architecture, by including in the
search loop names suffixed with the string identifying the architecture.
For example, if your ld command line included -ACA as well as -ltry, the linker would look (in its
built-in search paths, and in any paths you specify with -L) for a library with the names
try
libtry.a
tryca
libtryca.a
The first two possibilities would be considered in any event; the last two are due to the use of -ACA.
You can meaningfully use -A more than once on a command line, since the 960 architecture family
allows combination of target architectures; each use will add another pair of name variants to search
for when -l specifies a library.
ld supports the -relax option for the i960 family. If you specify -relax, ld finds all balx and
calx instructions whose targets are within 24 bits, and turns them into 24-bit program-counter relative
bal and cal instructions, respectively. ld also turns cal instructions into bal instructions when it
determines that the target subroutine is a leaf routine (that is, the target subroutine does not itself call
any subroutines).
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Chapter 5. Machine Dependent Features
5.3. ld’s Support for Interworking Between ARM and Thumb Code
For the ARM, ld will generate code stubs to allow functions calls betweem ARM and Thumb
code. These stubs only work with code that has been compiled and assembled with the
-mthumb-interwork command line option. If it is necessary to link with old ARM object
files or libraries, which have not been compiled with the -mthumb-interwork option then the
-support-old-code command line switch should be given to the linker. This will make it generate
larger stub functions which will work with non-interworking aware ARM code. Note, however, the
linker does not support generating stubs for function calls to non-interworking aware Thumb code.
The -thumb-entry switch is a duplicate of the generic -entry switch, in that it sets the program’s
starting address. But it also sets the bottom bit of the address, so that it can be branched to using a BX
instruction, and the program will start executing in Thumb mode straight away.
5.4. ld and HPPA 32-bit ELF Support
When generating a shared library, ld will by default generate import stubs suitable for use with a
single sub-space application. The -multi-subspace switch causes ld to generate export stubs, and
different (larger) import stubs suitable for use with multiple sub-spaces.
Long branch stubs and import/export stubs are placed by ld in stub sections located between groups of
input sections. -stub-group-size specifies the maximum size of a group of input sections handled
by one stub section. Since branch offsets are signed, a stub section may serve two groups of input
sections, one group before the stub section, and one group after it. However, when using conditional
branches that require stubs, it may be better (for branch prediction) that stub sections only serve one
group of input sections. A negative value for N chooses this scheme, ensuring that branches to stubs
always use a negative offset. Two special values of N are recognized, 1 and -1. These both instruct
ld to automatically size input section groups for the branch types detected, with the same behaviour
regarding stub placement as other positive or negative values of N respectively.
Note that -stub-group-size does not split input sections. A single input section larger than the
group size specified will of course create a larger group (of one section). If input sections are too
large, it may not be possible for a branch to reach its stub.
5.5. ld and MMIX
For MMIX, there is a choice of generating ELF object files or mmo object files when linking. The
simulator mmix understands the mmo format. The binutils objcopy utility can translate between the
two formats.
There is one special section, the .MMIX.reg_contents section. Contents in this section is assumed
to correspond to that of global registers, and symbols referring to it are translated to special symbols,
equal to registers. In a final link, the start address of the .MMIX.reg_contents section corresponds
to the first allocated global register multiplied by 8. Register $255 is not included in this section; it is
always set to the program entry, which is at the symbol Main for mmo files.
Symbols with the prefix __.MMIX.start., for example __.MMIX.start..text and
__.MMIX.start..data are special; there must be only one each, even if they are local. The default
linker script uses these to set the default start address of a section.
Initial and trailing multiples of zero-valued 32-bit words in a section, are left out from an mmo file.
5.6. ld and MSP430
For the MSP430 it is possible to select the MPU architecture. The flag -m [mpu type] will select
an appropriate linker script for selected MPU type. (To get a list of known MPUs just pass -m help
option to the linker).
Chapter 5. Machine Dependent Features
61
The linker will recognize some extra sections which are MSP430 specific:
.vectors
Defines a portion of ROM where interrupt vectors located.
.bootloader
Defines the bootloader portion of the ROM (if applicable). Any code in this section will be
uploaded to the MPU.
.infomem
Defines an information memory section (if applicable). Any code in this section will be uploaded
to the MPU.
.infomemnobits
This is the same as the .infomem section except that any code in this section will not be uploaded
to the MPU.
.noinit
Denotes a portion of RAM located above .bss section.
The last two sections are used by gcc.
5.7. ld’s Support for Various TI COFF Versions
The -format switch allows selection of one of the various TI COFF versions. The latest of this
writing is 2; versions 0 and 1 are also supported. The TI COFF versions also vary in header byte-order
format; ld will read any version or byte order, but the output header format depends on the default
specified by the specific target.
5.8. ld and WIN32 (cygwin/mingw)
This section describes some of the win32 specific ld issues. See Section 3.1 Command Line Options
for detailed decription of the command line options mentioned here.
import libraries
The standard Windows linker creates and uses so-called import libraries, which contains information for linking to dll’s. They are regular static archives and are handled as any other static
archive. The cygwin and mingw ports of ld have specific support for creating such libraries
provided with the -out-implib command line option.
exporting DLL symbols
The cygwin/mingw ld has several ways to export symbols for dll’s.
using auto-export functionality
By default ld exports symbols with the auto-export functionality, which is controlled by the
following command line options:
•
-export-all-symbols [This is the default]
•
-exclude-symbols
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Chapter 5. Machine Dependent Features
•
-exclude-libs
If, however, -export-all-symbols is not given explicitly on the command line, then the
default auto-export behavior will be disabled if either of the following are true:
•
A DEF file is used.
•
Any symbol in any object file was marked with the __declspec(dllexport) attribute.
using a DEF file
Another way of exporting symbols is using a DEF file. A DEF file is an ASCII file containing definitions of symbols which should be exported when a dll is created. Usually it
is named dll name .def and is added as any other object file to the linker’s command
line. The file’s name must end in .def or .DEF.
gcc -o
Using
a
output
DEF
objectfiles
file
dll name .def
turns off the normal
-export-all-symbols option is also used.
auto-export
behavior,
unless
the
Here is an example of a DEF file for a shared library called xyz.dll:
LIBRARY "xyz.dll" BASE=0x10000000
EXPORTS
foo
bar
_bar = bar
This example defines a base address and three symbols. The third symbol is an alias for the
second. For the complete format specification see ld/deffilep.y in the binutils sources.
While linking a shared dll, ld is able to create a DEF file with the -output-def
command line option.
file
Using decorations
Another way of marking symbols for export is to modify the source code itself, so that when
building the DLL each symbol to be exported is declared as:
__declspec(dllexport) int a_variable
__declspec(dllexport) void a_function(int with_args)
All such symbols will be exported from the DLL. If, however, any of the object files in
the DLL contain symbols decorated in this way, then the normal auto-export behavior is
disabled, unless the -export-all-symbols option is also used.
Note that object files that wish to access these symbols must not decorate them with dllexport. Instead, they should use dllimport, instead:
__declspec(dllimport) int a_variable
__declspec(dllimport) void a_function(int with_args)
This complicates the structure of library header files, because when included by the library
itself the header must declare the variables and functions as dllexport, but when included by
client code the header must declare them as dllimport. There are a number of idioms that are
typically used to do this; often client code can omit the __declspec() declaration completely.
See -enable-auto-import and automatic data imports for more imformation.
automatic data imports
The standard Windows dll format supports data imports from dlls only by adding special
decorations (dllimport/dllexport), which let the compiler produce specific assembler instructions
to deal with this issue. This increases the effort necessary to port existing Un*x code to these
platforms, especially for large c++ libraries and applications. The auto-import feature, which
Chapter 5. Machine Dependent Features
63
was initially provided by Paul Sokolovsky, allows one to omit the decorations to archieve
a behavior that conforms to that on POSIX/Un*x platforms. This feature is enabled with
the -enable-auto-import command-line option, although it is enabled by default on
cygwin/mingw. The -enable-auto-import option itself now serves mainly to suppress any
warnings that are ordinarily emitted when linked objects trigger the feature’s use.
auto-import of variables does not always work flawlessly without additional assistance. Sometimes, you will see this message
"variable ’ var ’ can’t be auto-imported. Please read the documentation for ld’s
-enable-auto-import for details."
The -enable-auto-import documentation explains why this error occurs, and several methods that can be used to overcome this difficulty. One of these methods is the runtime pseudorelocs feature, described below.
For complex variables imported from DLLs (such as structs or classes), object files typically
contain a base address for the variable and an offset (addend) within the variable-to specify a
particular field or public member, for instance. Unfortunately, the runtime loader used in win32
environments is incapable of fixing these references at runtime without the additional information
supplied by dllimport/dllexport decorations. The standard auto-import feature described above is
unable to resolve these references.
The -enable-runtime-pseudo-relocs switch allows these references to be resolved without error, while leaving the task of adjusting the references themselves (with their non-zero addends) to specialized code provided by the runtime environment. Recent versions of the cygwin
and mingw environments and compilers provide this runtime support; older versions do not.
However, the support is only necessary on the developer’s platform; the compiled result will run
without error on an older system.
-enable-runtime-pseudo-relocs is not the default; it must be explicitly enabled as needed.
direct linking to a dll
The cygwin/mingw ports of ld support the direct linking, including data symbols, to a dll without
the usage of any import libraries. This is much faster and uses much less memory than does the
traditional import library method, expecially when linking large libraries or applications. When
ld creates an import lib, each function or variable exported from the dll is stored in its own bfd,
even though a single bfd could contain many exports. The overhead involved in storing, loading,
and processing so many bfd’s is quite large, and explains the tremendous time, memory, and
storage needed to link against particularly large or complex libraries when using import libs.
Linking directly to a dll uses no extra command-line switches other than -L and -l, because
ld already searches for a number of names to match each library. All that is needed from the
developer’s perspective is an understanding of this search, in order to force ld to select the dll
instead of an import library.
For instance, when ld is called with the argument -lxxx it will attempt to find, in the first
directory of its search path,
libxxx.dll.a
xxx.dll.a
libxxx.a
cygxxx.dll (*)
libxxx.dll
xxx.dll
before moving on to the next directory in the search path.
(*) Actually, this is not cygxxx.dll but in fact is prefix xxx.dll, where prefix is set
by the ld option -dll-search-prefix= prefix . In the case of cygwin, the standard gcc
spec file includes -dll-search-prefix=cyg, so in effect we actually search for cygxxx.dll.
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Chapter 5. Machine Dependent Features
Other win32-based unix environments, such as mingw or pw32, may use other prefix es,
although at present only cygwin makes use of this feature. It was originally intended to help avoid
name conflicts among dll’s built for the various win32/un*x environments, so that (for example)
two versions of a zlib dll could coexist on the same machine.
The generic cygwin/mingw path layout uses a bin directory for applications and dll’s and a lib
directory for the import libraries (using cygwin nomenclature):
bin/
cygxxx.dll
lib/
libxxx.dll.a
libxxx.a
(in case of dll’s)
(in case of static archive)
Linking directly to a dll without using the import library can be done two ways:
1. Use the dll directly by adding the bin path to the link line
gcc -Wl,-verbose
-o a.exe -L../bin/ -lxxx
However, as the dll’s often have version numbers appended to their names
(cygncurses-5.dll) this will often fail, unless one specifies -L../bin -lncurses-5 to
include the version. Import libs are generally not versioned, and do not have this difficulty.
2. Create a symbolic link from the dll to a file in the lib directory according to the above
mentioned search pattern. This should be used to avoid unwanted changes in the tools needed for
making the app/dll.
ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]
Then you can link without any make environment changes.
gcc -Wl,-verbose
-o a.exe -L../lib/ -lxxx
This technique also avoids the version number problems, because the following is perfectly legal
bin/
cygxxx-5.dll
lib/
libxxx.dll.a -
../bin/cygxxx-5.dll
Linking directly to a dll without using an import lib will work even when auto-import features
are exercised, and even when -enable-runtime-pseudo-relocs is used.
Given the improvements in speed and memory usage, one might justifiably wonder why import
libraries are used at all. There are two reasons:
1. Until recently, the link-directly-to-dll functionality did not work with auto-imported data.
2. Sometimes it is necessary to include pure static objects within the import library (which otherwise contains only bfd’s for indirection symbols that point to the exports of a dll). Again, the
import lib for the cygwin kernel makes use of this ability, and it is not possible to do this without
an import lib.
So, import libs are not going away. But the ability to replace true import libs with a simple symbolic link to (or a copy of) a dll, in most cases, is a useful addition to the suite of tools binutils
makes available to the win32 developer. Given the massive improvements in memory requirements during linking, storage requirements, and linking speed, we expect that many developers
will soon begin to use this feature whenever possible.
symbol aliasing
adding additional names
Sometimes, it is useful to export symbols with additional names. A symbol foo will be
exported as foo, but it can also be exported as _foo by using special directives in the DEF
Chapter 5. Machine Dependent Features
65
file when creating the dll. This will affect also the optional created import library. Consider
the following DEF file:
LIBRARY "xyz.dll" BASE=0x61000000
EXPORTS
foo
_foo = foo
The line _foo = foo maps the symbol foo to _foo.
Another method for creating a symbol alias is to create it in the source code using the "weak"
attribute:
void foo () { /* Do something. */; }
void _foo () __attribute__ ((weak, alias ("foo")));
See the gcc manual for more information about attributes and weak symbols.
renaming symbols
Sometimes it is useful to rename exports. For instance, the cygwin kernel does this regularly.
A symbol _foo can be exported as foo but not as _foo by using special directives in the
DEF file. (This will also affect the import library, if it is created). In the following example:
LIBRARY "xyz.dll" BASE=0x61000000
EXPORTS
_foo = foo
The line _foo = foo maps the exported symbol foo to _foo.
Note:
using
a
DEF
file
disables
the
default
auto-export
behavior,
unless
the
-export-all-symbols command line option is used. If, however, you are trying to rename
symbols, then you should list all desired exports in the DEF file, including the symbols that
are not being renamed, and do not use the -export-all-symbols option. If you list only
the renamed symbols in the DEF file, and use -export-all-symbols to handle the other
symbols, then the both the new names and the original names for the the renamed symbols will
be exported. In effect, you’d be aliasing those symbols, not renaming them, which is probably
not what you wanted.
5.9. ld and Xtensa Processors
The default ld behavior for Xtensa processors is to interpret SECTIONS commands so that lists of
explicitly named sections in a specification with a wildcard file will be interleaved when necessary to
keep literal pools within the range of PC-relative load offsets. For example, with the command:
SECTIONS
{
.text : {
*(.literal .text)
}
}
ld may interleave some of the .literal and .text sections from different object files to ensure
that the literal pools are within the range of PC-relative load offsets. A valid interleaving might place
the .literal sections from an initial group of files followed by the .text sections of that group of
files. Then, the .literal sections from the rest of the files and the .text sections from the rest of
the files would follow. The non-interleaved order can still be specified as:
SECTIONS
{
66
Chapter 5. Machine Dependent Features
.text : {
*(.literal) *(.text)
}
}
The Xtensa version of ld enables the -relax option by default to attempt to reduce space in the
output image by combining literals with identical values. It also provides the -no-relax option to
disable this optimization. When enabled, the relaxation algorithm ensures that a literal will only be
merged with another literal when the new merged literal location is within the offset range of all of its
uses.
The relaxation mechanism will also attempt to optimize assembler-generated "longcall" sequences of
L32R/CALLXn when the target is known to fit into a CALLn instruction encoding. The current optimization converts the sequence into NOP/CALLn and removes the literal referenced by the L32R instruction.
Chapter 6.
BFD
The linker accesses object and archive files using the BFD libraries. These libraries allow the linker
to use the same routines to operate on object files whatever the object file format. A different object
file format can be supported simply by creating a new BFD back end and adding it to the library. To
conserve runtime memory, however, the linker and associated tools are usually configured to support
only a subset of the object file formats available. You can use objdump -i () to list all the formats
available for your configuration.
As with most implementations, BFD is a compromise between several conflicting requirements. The
major factor influencing BFD design was efficiency: any time used converting between formats is time
which would not have been spent had BFD not been involved. This is partly offset by abstraction payback; since BFD simplifies applications and back ends, more time and care may be spent optimizing
algorithms for a greater speed.
One minor artifact of the BFD solution which you should bear in mind is the potential for information
loss. There are two places where useful information can be lost using the BFD mechanism: during
conversion and during output. Refer to Section 6.1.1 Information Loss.
6.1. How It Works: An Outline of BFD
When an object file is opened, BFD subroutines automatically determine the format of the input
object file. They then build a descriptor in memory with pointers to routines that will be used to
access elements of the object file’s data structures.
As different information from the object files is required, BFD reads from different sections of the
file and processes them. For example, a very common operation for the linker is processing symbol
tables. Each BFD back end provides a routine for converting between the object file’s representation
of symbols and an internal canonical format. When the linker asks for the symbol table of an object
file, it calls through a memory pointer to the routine from the relevant BFD back end which reads and
converts the table into a canonical form. The linker then operates upon the canonical form. When the
link is finished and the linker writes the output file’s symbol table, another BFD back end routine is
called to take the newly created symbol table and convert it into the chosen output format.
6.1.1. Information Loss
Information can be lost during output. The output formats supported by BFD do not provide identical
facilities, and information which can be described in one form has nowhere to go in another format.
One example of this is alignment information in b.out. There is nowhere in an a.out format file to
store alignment information on the contained data, so when a file is linked from b.out and an a.out
image is produced, alignment information will not propagate to the output file. (The linker will still
use the alignment information internally, so the link is performed correctly).
Another example is COFF section names. COFF files may contain an unlimited number of sections,
each one with a textual section name. If the target of the link is a format which does not have many
sections (e.g., a.out) or has sections without names (e.g., the Oasys format), the link cannot be done
simply. You can circumvent this problem by describing the desired input-to-output section mapping
with the linker command language.
Information can be lost during canonicalization. The BFD internal canonical form of the external formats is not exhaustive; there are structures in input formats for which there is no direct representation
internally. This means that the BFD back ends cannot maintain all possible data richness through the
transformation between external to internal and back to external formats.
68
Chapter 6. BFD
This limitation is only a problem when an application reads one format and writes another. Each BFD
back end is responsible for maintaining as much data as possible, and the internal BFD canonical form
has structures which are opaque to the BFD core, and exported only to the back ends. When a file is
read in one format, the canonical form is generated for BFD and the application. At the same time, the
back end saves away any information which may otherwise be lost. If the data is then written back in
the same format, the back end routine will be able to use the canonical form provided by the BFD core
as well as the information it prepared earlier. Since there is a great deal of commonality between back
ends, there is no information lost when linking or copying big endian COFF to little endian COFF,
or a.out to b.out. When a mixture of formats is linked, the information is only lost from the files
whose format differs from the destination.
6.1.2. The BFD canonical object-file format
The greatest potential for loss of information occurs when there is the least overlap between the
information provided by the source format, that stored by the canonical format, and that needed by
the destination format. A brief description of the canonical form may help you understand which kinds
of data you can count on preserving across conversions.
files
Information stored on a per-file basis includes target machine architecture, particular implementation format type, a demand pageable bit, and a write protected bit. Information like Unix magic
numbers is not stored here--only the magic numbers’ meaning, so a ZMAGIC file would have both
the demand pageable bit and the write protected text bit set. The byte order of the target is stored
on a per-file basis, so that big- and little-endian object files may be used with one another.
sections
Each section in the input file contains the name of the section, the section’s original address in
the object file, size and alignment information, various flags, and pointers into other BFD data
structures.
symbols
Each symbol contains a pointer to the information for the object file which originally defined
it, its name, its value, and various flag bits. When a BFD back end reads in a symbol table, it
relocates all symbols to make them relative to the base of the section where they were defined.
Doing this ensures that each symbol points to its containing section. Each symbol also has a
varying amount of hidden private data for the BFD back end. Since the symbol points to the
original file, the private data format for that symbol is accessible. ld can operate on a collection
of symbols of wildly different formats without problems.
Normal global and simple local symbols are maintained on output, so an output file (no matter
its format) will retain symbols pointing to functions and to global, static, and common variables.
Some symbol information is not worth retaining; in a.out, type information is stored in the
symbol table as long symbol names. This information would be useless to most COFF debuggers;
the linker has command line switches to allow users to throw it away.
There is one word of type information within the symbol, so if the format supports symbol type
information within symbols (for example, COFF, IEEE, Oasys) and the type is simple enough to
fit within one word (nearly everything but aggregates), the information will be preserved.
relocation level
Each canonical BFD relocation record contains a pointer to the symbol to relocate to, the offset
of the data to relocate, the section the data is in, and a pointer to a relocation type descriptor.
Relocation is performed by passing messages through the relocation type descriptor and the
symbol pointer. Therefore, relocations can be performed on output data using a relocation method
Chapter 6. BFD
69
that is only available in one of the input formats. For instance, Oasys provides a byte relocation
format. A relocation record requesting this relocation type would point indirectly to a routine to
perform this, so the relocation may be performed on a byte being written to a 68k COFF file,
even though 68k COFF has no such relocation type.
line numbers
Object formats can contain, for debugging purposes, some form of mapping between symbols,
source line numbers, and addresses in the output file. These addresses have to be relocated along
with the symbol information. Each symbol with an associated list of line number records points
to the first record of the list. The head of a line number list consists of a pointer to the symbol,
which allows finding out the address of the function whose line number is being described. The
rest of the list is made up of pairs: offsets into the section and line numbers. Any format which
can simply derive this information can pass it successfully between formats (COFF, IEEE and
Oasys).
70
Chapter 6. BFD
Chapter 7.
Reporting Bugs
Your bug reports play an essential role in making ld reliable.
Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case
the principal function of a bug report is to help the entire community by making the next version of
ld work better. Bug reports are your contribution to the maintenance of ld.
In order for a bug report to serve its purpose, you must include the information that enables us to fix
the bug.
7.1. Have You Found a Bug?
If you are not sure whether you have found a bug, here are some guidelines:
•
If the linker gets a fatal signal, for any input whatever, that is a ld bug. Reliable linkers never crash.
•
If ld produces an error message for valid input, that is a bug.
•
If ld does not produce an error message for invalid input, that may be a bug. In the general case,
the linker can not verify that object files are correct.
•
If you are an experienced user of linkers, your suggestions for improvement of ld are welcome in
any case.
7.2. How to Report Bugs
A number of companies and individuals offer support for gnu products. If you obtained ld from a
support organization, we recommend you contact that organization first.
You can find contact information for many support companies and individuals in the file
etc/SERVICE in the gnu Emacs distribution.
Otherwise, send bug reports for ld to [email protected]
The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure
whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and assume that some
details do not matter. Thus, you might assume that the name of a symbol you use in an example does
not matter. Well, probably it does not, but one cannot be sure. Perhaps the bug is a stray memory
reference which happens to fetch from the location where that name is stored in memory; perhaps, if
the name were different, the contents of that location would fool the linker into doing the right thing
despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to
do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us. Therefore,
always write your bug reports on the assumption that the bug has not been reported previously.
Sometimes people give a few sketchy facts and ask, "Does this ring a bell?" This cannot help us fix
a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate.
You might as well expedite matters by sending them to begin with.
To enable us to fix the bug, you should include all these things:
•
The version of ld. ld announces it if you start it with the -version argument.
72
Chapter 7. Reporting Bugs
Without this, we will not know whether there is any point in looking for the bug in the current
version of ld.
•
Any patches you may have applied to the ld source, including any patches made to the BFD library.
•
The type of machine you are using, and the operating system name and version number.
•
What compiler (and its version) was used to compile ld--e.g. "gcc-2.7".
•
The command arguments you gave the linker to link your example and observe the bug. To guarantee you will not omit something important, list them all. A copy of the Makefile (or the output from
make) is sufficient.
If we were to try to guess the arguments, we would probably guess wrong and then we might not
encounter the bug.
•
A complete input file, or set of input files, that will reproduce the bug. It is generally most helpful
to send the actual object files provided that they are reasonably small. Say no more than 10K. For
bigger files you can either make them available by FTP or HTTP or else state that you are willing
to send the object file(s) to whomever requests them. (Note - your email will be going to a mailing
list, so we do not want to clog it up with large attachments). But small attachments are best.
If the source files were assembled using gas or compiled using gcc, then it may be OK to send the
source files rather than the object files. In this case, be sure to say exactly what version of gas or
gcc was used to produce the object files. Also say how gas or gcc were configured.
•
A description of what behavior you observe that you believe is incorrect. For example, "It gets a
fatal signal."
Of course, if the bug is that ld gets a fatal signal, then we will certainly notice it. But if the bug is
incorrect output, we might not notice unless it is glaringly wrong. You might as well not give us a
chance to make a mistake.
Even if the problem you experience is a fatal signal, you should still say so explicitly. Suppose
something strange is going on, such as, your copy of ld is out of synch, or you have encountered a
bug in the C library on your system. (This has happened!) Your copy might crash and ours would
not. If you told us to expect a crash, then when ours fails to crash, we would know that the bug was
not happening for us. If you had not told us to expect a crash, then we would not be able to draw
any conclusion from our observations.
•
If you wish to suggest changes to the ld source, send us context diffs, as generated by diff with
the -u, -c, or -p option. Always send diffs from the old file to the new file. If you even discuss
something in the ld source, refer to it by context, not by line number.
The line numbers in our development sources will not match those in your sources. Your line
numbers would convey no useful information to us.
Here are some things that are not necessary:
•
A description of the envelope of the bug.
Often people who encounter a bug spend a lot of time investigating which changes to the input file
will make the bug go away and which changes will not affect it.
This is often time consuming and not very useful, because the way we will find the bug is by
running a single example under the debugger with breakpoints, not by pure deduction from a series
of examples. We recommend that you save your time for something else.
Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us. Errors in the output will be easier to spot, running under the debugger will take less
time, and so on.
However, simplification is not vital; if you do not want to do this, report the bug anyway and send
us the entire test case you used.
Chapter 7. Reporting Bugs
•
73
A patch for the bug.
A patch for the bug does help us if it is a good one. But do not omit the necessary information, such
as the test case, on the assumption that a patch is all we need. We might see problems with your
patch and decide to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as ld it is very hard to construct an example that will
make the program follow a certain path through the code. If you do not send us the example, we
will not be able to construct one, so we will not be able to verify that the bug is fixed.
And if we cannot understand what bug you are trying to fix, or why your patch should be an
improvement, we will not install it. A test case will help us to understand.
•
A guess about what the bug is or what it depends on.
Such guesses are usually wrong. Even we cannot guess right about such things without first using
the debugger to find the facts.
74
Chapter 7. Reporting Bugs
Appendix A.
MRI Compatible Script Files
To aid users making the transition to gnu ld from the MRI linker, ld can use MRI compatible linker
scripts as an alternative to the more general-purpose linker scripting language described in Chapter
4 Linker Scripts. MRI compatible linker scripts have a much simpler command set than the scripting
language otherwise used with ld. gnu ld supports the most commonly used MRI linker commands;
these commands are described here.
In general, MRI scripts aren’t of much use with the a.out object file format, since it only has three
sections and MRI scripts lack some features to make use of them.
You can specify a file containing an MRI-compatible script using the -c command-line option.
Each command in an MRI-compatible script occupies its own line; each command line starts with the
keyword that identifies the command (though blank lines are also allowed for punctuation). If a line
of an MRI-compatible script begins with an unrecognized keyword, ld issues a warning message, but
continues processing the script.
Lines beginning with * are comments.
You can write these commands using all upper-case letters, or all lower case; for example, chip is the
same as CHIP. The following list shows only the upper-case form of each command.
ABSOLUTE secname
ABSOLUTE secname, secname, ... secname
Normally, ld includes in the output file all sections from all the input files. However, in an
MRI-compatible script, you can use the ABSOLUTE command to restrict the sections that will be
present in your output program. If the ABSOLUTE command is used at all in a script, then only
the sections named explicitly in ABSOLUTE commands will appear in the linker output. You can
still use other input sections (whatever you select on the command line, or using LOAD) to resolve
addresses in the output file.
ALIAS out-secname, in-secname
Use this command to place the data from input section in-secname in a section called
out-secname in the linker output file.
in-secname may be an integer.
ALIGN secname = expression
Align the section called secname to expression. The expression should be a power of two.
BASE expression
Use the value of expression as the lowest address (other than absolute addresses) in the output
file.
CHIP expression
CHIP expression, expression
This command does nothing; it is accepted only for compatibility.
76
Appendix A. MRI Compatible Script Files
END
This command does nothing whatever; it’s only accepted for compatibility.
FORMAT output-format
Similar to the OUTPUT_FORMAT command in the more general linker language, but restricted to
one of these output formats:
1. S-records, if output-format is S
2. IEEE, if output-format is IEEE
3. COFF (the coff-m68k variant in BFD), if output-format is COFF
LIST anything...
Print (to the standard output file) a link map, as produced by the ld command-line option -M.
The keyword LIST may be followed by anything on the same line, with no change in its effect.
LOAD filename
LOAD filename, filename, ... filename
Include one or more object file filename in the link; this has the same effect as specifying
filename directly on the ld command line.
NAME output-name
output-name is the name for the program produced by ld; the MRI-compatible command NAME
is equivalent to the command-line option -o or the general script language command OUTPUT.
ORDER secname, secname, ... secname
ORDER secname secname secname
Normally, ld orders the sections in its output file in the order in which they first appear in
the input files. In an MRI-compatible script, you can override this ordering with the ORDER
command. The sections you list with ORDER will appear first in your output file, in the order
specified.
PUBLIC name=expression
PUBLIC name,expression
PUBLIC name expression
Supply a value (expression) for external symbol name used in the linker input files.
SECT secname, expression
SECT secname=expression
SECT secname expression
You can use any of these three forms of the SECT command to specify the start address
(expression) for section secname. If you have more than one SECT statement for the same
secname, only the first sets the start address.
Appendix B.
GNU Free Documentation License
Version 1.1, March 2000
Copyright (C) 2000, Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111-1307
USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
1. PREAMBLE
The purpose of this License is to make a manual, textbook, or other written document "free"
in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being
considered responsible for modifications made by others.
This License is a kind of "copyleft", which means that derivative works of the document must
themselves be free in the same sense. It complements the GNU General Public License, which
is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free
software needs free documentation: a free program should come with manuals providing the
same freedoms that the software does. But this License is not limited to software manuals; it
can be used for any textual work, regardless of subject matter or whether it is published as a
printed book. We recommend this License principally for works whose purpose is instruction
or reference.
2. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work that contains a notice placed by the copyright
holder saying it can be distributed under the terms of this License. The "Document", below,
refers to any such manual or work. Any member of the public is a licensee, and is addressed as
"you."
A "Modified Version" of the Document means any work containing the Document or a portion
of it, either copied verbatim, or with modifications and/or translated into another language.
A "Secondary Section" is a named appendix or a front-matter section of the Document that deals
exclusively with the relationship of the publishers or authors of the Document to the Document’s
overall subject (or to related matters) and contains nothing that could fall directly within that
overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary
Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical
or political position regarding them.
The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being
those of Invariant Sections, in the notice that says that the Document is released under this
License.
The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or
Back-Cover Texts, in the notice that says that the Document is released under this License.
A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and
78
Appendix B. GNU Free Documentation License
edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that
is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose
markup has been designed to thwart or discourage subsequent modification by readers is not
Transparent. A copy that is not "Transparent" is called "Opaque."
Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and
standard-conforming simple HTML designed for human modification. Opaque formats include
PostScript, PDF, proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available,
and the machine-generated HTML produced by some word processors for output purposes only.
The "Title Page" means, for a printed book, the title page itself, plus such following pages as are
needed to hold, legibly, the material this License requires to appear in the title page. For works
in formats which do not have any title page as such, "Title Page" means the text near the most
prominent appearance of the work’s title, preceding the beginning of the body of the text.
3. VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this
License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or
control the reading or further copying of the copies you make or distribute. However, you may
accept compensation in exchange for copies. If you distribute a large enough number of copies
you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display
copies.
4. COPYING IN QUANTITY
If you publish printed copies of the Document numbering more than 100, and the Document’s
license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and
legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify you as the publisher of these
copies. The front cover must present the full title with all words of the title equally prominent
and visible. You may add other material on the covers in addition. Copying with changes limited
to the covers, as long as they preserve the title of the Document and satisfy these conditions,
can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first
ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent
pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you
must either include a machine-readable Transparent copy along with each Opaque copy, or
state in or with each Opaque copy a publicly-accessible computer-network location containing a
complete Transparent copy of the Document, free of added material, which the general networkusing public has access to download anonymously at no charge using public-standard network
protocols. If you use the latter option, you must take reasonably prudent steps, when you begin
distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus
accessible at the stated location until at least one year after the last time you distribute an Opaque
copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before
redistributing any large number of copies, to give them a chance to provide you with an updated
version of the Document.
5. MODIFICATIONS
Appendix B. GNU Free Documentation License
79
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License,
with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do
these things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and
from those of previous versions (which should, if there were any, be listed in the History section
of the Document). You may use the same title as a previous version if the original publisher
of that version gives permission. B. List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in the Modified Version, together with at
least five of the principal authors of the Document (all of its principal authors, if it has less than
five). C. State on the Title page the name of the publisher of the Modified Version, as the publisher. D. Preserve all the copyright notices of the Document. E. Add an appropriate copyright
notice for your modifications adjacent to the other copyright notices. F. Include, immediately
after the copyright notices, a license notice giving the public permission to use the Modified
Version under the terms of this License, in the form shown in the Addendum below. G. Preserve
in that license notice the full lists of Invariant Sections and required Cover Texts given in the
Document’s license notice. H. Include an unaltered copy of this License. I. Preserve the section
entitled "History", and its title, and add to it an item stating at least the title, year, new authors,
and publisher of the Modified Version as given on the Title Page. If there is no section entitled
"History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in
the previous sentence. J. Preserve the network location, if any, given in the Document for public
access to a Transparent copy of the Document, and likewise the network locations given in the
Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before
the Document itself, or if the original publisher of the version it refers to gives permission. K.
In any section entitled "Acknowledgements" or "Dedications", preserve the section’s title, and
preserve in the section all the substance and tone of each of the contributor acknowledgements
and/or dedications given therein. L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part
of the section titles. M. Delete any section entitled "Endorsements." Such a section may not be
included in the Modified Version. N. Do not retitle any existing section as "Endorsements" or
to conflict in title with any Invariant Section. If the Modified Version includes new front-matter
sections or appendices that qualify as Secondary Sections and contain no material copied from
the Document, you may at your option designate some or all of these sections as invariant. To
do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice.
These titles must be distinct from any other section titles.
You may add a section entitled "Endorsements", provided it contains nothing but endorsements
of your Modified Version by various parties-for example, statements of peer review or that the
text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25
words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only
one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through
arrangements made by) any one entity. If the Document already includes a cover text for the
same cover, previously added by you or by arrangement made by the same entity you are acting
on behalf of, you may not add another; but you may replace the old one, on explicit permission
from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use
their names for publicity for or to assert or imply endorsement of any Modified Version.
6. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under the
terms defined in section 4 above for modified versions, provided that you include in the combi-
80
Appendix B. GNU Free Documentation License
nation all of the Invariant Sections of all of the original documents, unmodified, and list them
all as Invariant Sections of your combined work in its license notice.
The combined work need only contain one copy of this License, and multiple identical Invariant
Sections may be replaced with a single copy. If there are multiple Invariant Sections with the
same name but different contents, make the title of each such section unique by adding at the
end of it, in parentheses, the name of the original author or publisher of that section if known,
or else a unique number. Make the same adjustment to the section titles in the list of Invariant
Sections in the license notice of the combined work.
In the combination, you must combine any sections entitled "History" in the various original
documents, forming one section entitled "History"; likewise combine any sections entitled "Acknowledgements", and any sections entitled "Dedications." You must delete all sections entitled
"Endorsements."
7. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under
this License, and replace the individual copies of this License in the various documents with a
single copy that is included in the collection, provided that you follow the rules of this License
for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under
this License, provided you insert a copy of this License into the extracted document, and follow
this License in all other respects regarding verbatim copying of that document.
8. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole
count as a Modified Version of the Document, provided no compilation copyright is claimed for
the compilation. Such a compilation is called an "aggregate", and this License does not apply
to the other self-contained works thus compiled with the Document, on account of their being
thus compiled, if they are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then
if the Document is less than one quarter of the entire aggregate, the Document’s Cover Texts
may be placed on covers that surround only the Document within the aggregate. Otherwise they
must appear on covers around the whole aggregate.
9. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include
a translation of this License provided that you also include the original English version of this
License. In case of a disagreement between the translation and the original English version of
this License, the original English version will prevail.
10. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided
for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties
who have received copies, or rights, from you under this License will not have their licenses
terminated so long as such parties remain in full compliance.
11. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version,
but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Appendix B. GNU Free Documentation License
81
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you
have the option of following the terms and conditions either of that specified version or of any
later version that has been published (not as a draft) by the Free Software Foundation. If the
Document does not specify a version number of this License, you may choose any version ever
published (not as a draft) by the Free Software Foundation.
B.1. ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document
and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with the Invariant Sections being list their titles, with the
Front-Cover Texts being list, and with the Back-Cover Texts being list.
A copy of the license is included in the section entitled "GNU
Free Documentation License."
If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones
are invariant. If you have no Front-Cover Texts, write "no Front-Cover Texts" instead of "Front-Cover
Texts being list"; likewise for Back-Cover Texts.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License,
to permit their use in free software.
82
Appendix B. GNU Free Documentation License
Index
", see Section 4.10.2 Symbol Names
-(, see Section 3.1 Command Line Options
-Aarch, see Section 3.1 Command Line Options
-accept-unknown-input-arch, see Section 3.1 Command Line Options
-add-stdcall-alias, see Section 3.1 Command Line Options
-akeyword, see Section 3.1 Command Line Options
-allow-multiple-definition, see Section 3.1 Command
Line Options
-allow-shlib-undefined, see Section 3.1 Command
Line Options
-architecture=arch, see Section 3.1 Command Line
Options
-assert keyword, see Section 3.1 Command Line Options
-auxiliary, see Section 3.1 Command Line Options
-b format, see Section 3.1 Command Line Options
-base-file, see Section 3.1 Command Line Options
-Bdynamic, see Section 3.1 Command Line Options
-Bgroup, see Section 3.1 Command Line Options
-Bshareable, see Section 3.1 Command Line Options
-Bstatic, see Section 3.1 Command Line Options
-Bsymbolic, see Section 3.1 Command Line Options
-c MRI-cmdfile, see Section 3.1 Command Line Options
-call_shared, see Section 3.1 Command Line Options
-check-sections, see Section 3.1 Command Line Options
-cref, see Section 3.1 Command Line Options
-d, see Section 3.1 Command Line Options
-dc, see Section 3.1 Command Line Options
-defsym symbol=exp, see Section 3.1 Command
Line Options
-demangle[=style], see Section 3.1 Command Line
Options
-disable-auto-image-base, see Section 3.1 Command
Line Options
-disable-auto-import, see Section 3.1 Command Line
Options
-disable-new-dtags, see Section 3.1 Command Line
Options
-disable-runtime-pseudo-reloc, see Section 3.1 Command Line Options
-disable-stdcall-fixup, see Section 3.1 Command Line
Options
-discard-all, see Section 3.1 Command Line Options
-discard-locals, see Section 3.1 Command Line Options
-dll, see Section 3.1 Command Line Options
-dll-search-prefix, see Section 3.1 Command Line Options
-dn, see Section 3.1 Command Line Options
-dp, see Section 3.1 Command Line Options
-dy, see Section 3.1 Command Line Options
-dynamic-linker file, see Section 3.1 Command Line
Options
-E, see Section 3.1 Command Line Options
-e entry, see Section 3.1 Command Line Options
-EB, see Section 3.1 Command Line Options
-EL, see Section 3.1 Command Line Options
-embedded-relocs, see Section 3.1 Command Line
Options
-emit-relocs, see Section 3.1 Command Line Options
-enable-auto-image-base, see Section 3.1 Command
Line Options
-enable-auto-import, see Section 3.1 Command Line
Options
-enable-extra-pe-debug, see Section 3.1 Command
Line Options
-enable-new-dtags, see Section 3.1 Command Line
Options
-enable-runtime-pseudo-reloc, see Section 3.1 Command Line Options
-enable-stdcall-fixup, see Section 3.1 Command Line
Options
-entry=entry, see Section 3.1 Command Line Options
-exclude-libs, see Section 3.1 Command Line Options
-exclude-symbols, see Section 3.1 Command Line
Options
-export-all-symbols, see Section 3.1 Command Line
Options
-export-dynamic, see Section 3.1 Command Line Options
-F, see Section 3.1 Command Line Options
-f, see Section 3.1 Command Line Options
-fatal-warnings, see Section 3.1 Command Line Options
-file-alignment, see Section 3.1 Command Line Options
-filter, see Section 3.1 Command Line Options
-fini, see Section 3.1 Command Line Options
-force-exe-suffix, see Section 3.1 Command Line Options
-format=format, see Section 3.1 Command Line Options
-format=version, see Section 5.7 ld ’s Support for
Various TI COFF Versions
-G, see Section 3.1 Command Line Options
-g, see Section 3.1 Command Line Options
-gc-sections, see Section 3.1 Command Line Options
-gpsize, see Section 3.1 Command Line Options
-heap, see Section 3.1 Command Line Options
-help, see Section 3.1 Command Line Options
-hname, see Section 3.1 Command Line Options
-i, see Section 3.1 Command Line Options
-Ifile, see Section 3.1 Command Line Options
84
-image-base, see Section 3.1 Command Line Options
-init, see Section 3.1 Command Line Options
-just-symbols=file, see Section 3.1 Command Line
Options
-kill-at, see Section 3.1 Command Line Options
-larchive, see Section 3.1 Command Line Options
-Ldir, see Section 3.1 Command Line Options
-library-path=dir, see Section 3.1 Command Line
Options
-library=archive, see Section 3.1 Command Line
Options
-M, see Section 3.1 Command Line Options
-m emulation, see Section 3.1 Command Line Options
-major-image-version, see Section 3.1 Command Line
Options
-major-os-version, see Section 3.1 Command Line
Options
-major-subsystem-version, see Section 3.1 Command
Line Options
-Map, see Section 3.1 Command Line Options
-minor-image-version, see Section 3.1 Command Line
Options
-minor-os-version, see Section 3.1 Command Line
Options
-minor-subsystem-version, see Section 3.1 Command
Line Options
-mri-script=MRI-cmdfile, see Section 3.1 Command
Line Options
-multi-subspace, see Section 5.4 ld and HPPA 32-bit
ELF Support
-N, see Section 3.1 Command Line Options
-n, see Section 3.1 Command Line Options
-nmagic, see Section 3.1 Command Line Options
-no-accept-unknown-input-arch, see Section 3.1
Command Line Options
-no-allow-shlib-undefined, see Section 3.1 Command
Line Options
-no-check-sections, see Section 3.1 Command Line
Options
-no-define-common, see Section 3.1 Command Line
Options
-no-demangle, see Section 3.1 Command Line Options
-no-gc-sections, see Section 3.1 Command Line Options
-no-keep-memory, see Section 3.1 Command Line
Options
-no-omagic, see Section 3.1 Command Line Options
-no-relax, see Section 5.9 ld and Xtensa Processors
-no-undefined, see Section 3.1 Command Line Options
-no-undefined-version, see Section 3.1 Command
Line Options
-no-warn-mismatch, see Section 3.1 Command Line
Options
Index
-no-whole-archive, see Section 3.1 Command Line
Options
-noinhibit-exec, see Section 3.1 Command Line Options
-non_shared, see Section 3.1 Command Line Options
-nostdlib, see Section 3.1 Command Line Options
-O level, see Section 3.1 Command Line Options
-o output, see Section 3.1 Command Line Options
-oformat, see Section 3.1 Command Line Options
-omagic, see Section 3.1 Command Line Options
-out-implib, see Section 3.1 Command Line Options
-output-def, see Section 3.1 Command Line Options
-output=output, see Section 3.1 Command Line Options
-pic-executable, see Section 3.1 Command Line Options
-pie, see Section 3.1 Command Line Options
-print-map, see Section 3.1 Command Line Options
-q, see Section 3.1 Command Line Options
-qmagic, see Section 3.1 Command Line Options
-Qy, see Section 3.1 Command Line Options
-r, see Section 3.1 Command Line Options
-R file, see Section 3.1 Command Line Options
-relax, see Section 3.1 Command Line Options
-relax on i960, see Section 5.2 ld and the Intel 960
Family
-relax on Xtensa, see Section 5.9 ld and Xtensa
Processors
-relocateable, see Section 3.1 Command Line Options
-rpath, see Section 3.1 Command Line Options
-rpath-link, see Section 3.1 Command Line Options
-S, see Section 3.1 Command Line Options
-s, see Section 3.1 Command Line Options
-script=script, see Section 3.1 Command Line Options
-section-alignment, see Section 3.1 Command Line
Options
-section-start sectionname=org, see Section 3.1
Command Line Options
-shared, see Section 3.1 Command Line Options
-soname=name, see Section 3.1 Command Line Options
-sort-common, see Section 3.1 Command Line Options
-split-by-file, see Section 3.1 Command Line Options
-split-by-reloc, see Section 3.1 Command Line Options
-stack, see Section 3.1 Command Line Options
-static, see Section 3.1 Command Line Options
-stats, see Section 3.1 Command Line Options
-strip-all, see Section 3.1 Command Line Options
-strip-debug, see Section 3.1 Command Line Options
-stub-group-size=N, see Section 5.4 ld and HPPA 32bit ELF Support
-subsystem, see Section 3.1 Command Line Options
Index
-support-old-code, see Section 5.3 ld ’s Support for
Interworking Between ARM and Thumb Code
-t, see Section 3.1 Command Line Options
-T script, see Section 3.1 Command Line Options
-target-help, see Section 3.1 Command Line Options
-Tbss org, see Section 3.1 Command Line Options
-Tdata org, see Section 3.1 Command Line Options
-thumb-entry=entry, see Section 5.3 ld ’s Support
for Interworking Between ARM and Thumb Code
-trace, see Section 3.1 Command Line Options
-trace-symbol=symbol, see Section 3.1 Command
Line Options
-traditional-format, see Section 3.1 Command Line
Options
-Ttext org, see Section 3.1 Command Line Options
-u symbol, see Section 3.1 Command Line Options
-undefined=symbol, see Section 3.1 Command Line
Options
-unique[=SECTION], see Section 3.1 Command Line
Options
-Ur, see Section 3.1 Command Line Options
-V, see Section 3.1 Command Line Options
-v, see Section 3.1 Command Line Options
-verbose, see Section 3.1 Command Line Options
-version, see Section 3.1 Command Line Options
-version-script=version-scriptfile, see Section
3.1 Command Line Options
-warn-common, see Section 3.1 Command Line Options
-warn-constructors, see Section 3.1 Command Line
Options
-warn-multiple-gp, see Section 3.1 Command Line
Options
-warn-once, see Section 3.1 Command Line Options
-warn-section-align, see Section 3.1 Command Line
Options
-whole-archive, see Section 3.1 Command Line Options
-wrap, see Section 3.1 Command Line Options
-X, see Section 3.1 Command Line Options
-x, see Section 3.1 Command Line Options
-Y path, see Section 3.1 Command Line Options
-y symbol, see Section 3.1 Command Line Options
-z defs, see Section 3.1 Command Line Options
-z keyword, see Section 3.1 Command Line Options
-z muldefs, see Section 3.1 Command Line Options
., see Section 4.10.3 The Location Counter
/DISCARD/, see Section 4.6.7 Output Section Discarding
:phdr, see Section 4.6.8.4 Output Section Phdr
=fillexp, see Section 4.6.8.5 Output Section Fill
region, see Section 4.6.8.3 Output Section Region
[COMMON], see Section 4.6.4.3 Input Section for
Common Symbols
ABSOLUTE (MRI), see Appendix A MRI Compatible
Script Files
85
A
absolute and relocatable symbols, see Section 4.10.6
The Section of an Expression
absolute expressions, see Section 4.10.6 The Section
of an Expression
ABSOLUTE(exp), see Section 4.10.7 Builtin Functions
ADDR(section), see Section 4.10.7 Builtin Functions
address, section, see Section 4.6.3 Output Section Description
ALIAS (MRI), see Appendix A MRI Compatible
Script Files
ALIGN (MRI), see Appendix A MRI Compatible
Script Files
align location counter, see Section 4.10.7 Builtin
Functions
ALIGN(exp), see Section 4.10.7 Builtin Functions
allocating memory, see Section 4.7 MEMORY Command
architecture, see Section 4.4.4 Other Linker Script
Commands
architectures, see Section 3.1 Command Line Options
archive files, from cmd line, see Section 3.1 Command
Line Options
archive search path in linker script, see Section 4.4.2
Commands Dealing with Files
arithmetic, see Section 4.10 Expressions in Linker
Scripts
arithmetic operators, see Section 4.10.4 Operators
ARM interworking support, see Section 5.3 ld ’s Support for Interworking Between ARM and Thumb Code
ASSERT, see Section 4.4.4 Other Linker Script Commands
assertion in linker script, see Section 4.4.4 Other
Linker Script Commands
assignment in scripts, see Section 4.5 Assigning Values to Symbols
AT(lma), see Section 4.6.8.2 Output Section LMA
AT lma_region, see Section 4.6.8.2 Output Section
LMA
automatic data imports, see Section 5.8 ld and WIN32
(cygwin/mingw)
86
Index
B
cross references, see Section 4.4.4 Other Linker Script
Commands
back end, see Chapter 6 BFD
BASE (MRI), see Appendix A MRI Compatible Script
Files
BFD canonical format, see Section 6.1.2 The BFD
canonical object-file format
BFD requirements, see Chapter 6 BFD
big-endian objects, see Section 3.1 Command Line
Options
binary input format, see Section 3.1 Command Line
Options
BLOCK(exp), see Section 4.10.7 Builtin Functions
bug criteria, see Section 7.1 Have You Found a Bug?
bug reports, see Section 7.2 How to Report Bugs
bugs in ld, see Chapter 7 Reporting Bugs
BYTE(expression), see Section 4.6.5 Output Section Data
C
C++ constructors, arranging in link, see Section 4.6.6
Output Section Keywords
CHIP (MRI), see Appendix A MRI Compatible Script
Files
COLLECT_NO_DEMANGLE, see Section 3.2 Environment Variables
combining symbols, warnings on, see Section 3.1
Command Line Options
command files, see Chapter 4 Linker Scripts
command line, see Section 3.1 Command Line Options
common allocation, see Section 3.1 Command Line
Options
common allocation in linker script, see Section 4.4.4
Other Linker Script Commands
common symbol placement, see Section 4.6.4.3 Input
Section for Common Symbols
compatibility, MRI, see Section 3.1 Command Line
Options
constants in linker scripts, see Section 4.10.1 Constants
CONSTRUCTORS, see Section 4.6.6 Output Section
Keywords
constructors, see Section 3.1 Command Line Options
constructors, arranging in link, see Section 4.6.6 Output Section Keywords
crash of linker, see Section 7.1 Have You Found a
Bug?
CREATE_OBJECT_SYMBOLS, see Section 4.6.6
Output Section Keywords
creating a DEF file, see Section 5.8 ld and WIN32
(cygwin/mingw)
cross reference table, see Section 3.1 Command Line
Options
current output location, see Section 4.10.3 The Location Counter
D
data, see Section 4.6.5 Output Section Data
DATA_SEGMENT_ALIGN(maxpagesize,
commonpagesize), see Section 4.10.7 Builtin Functions
DATA_SEGMENT_END(exp), see Section 4.10.7
Builtin Functions
dbx, see Section 3.1 Command Line Options
DEF files, creating, see Section 3.1 Command Line
Options
default emulation, see Section 3.2 Environment Variables
default input format, see Section 3.2 Environment
Variables
DEFINED(symbol), see Section 4.10.7 Builtin Functions
deleting local symbols, see Section 3.1 Command
Line Options
demangling, default, see Section 3.2 Environment
Variables
demangling, from command line, see Section 3.1
Command Line Options
direct linking to a dll, see Section 5.8 ld and WIN32
(cygwin/mingw)
discarding sections, see Section 4.6.7 Output Section
Discarding
discontinuous memory, see Section 4.7 MEMORY
Command
DLLs, creating, see Section 3.1 Command Line Options
DLLs, linking to, see Section 3.1 Command Line Options
dot, see Section 4.10.3 The Location Counter
dot inside sections, see Section 4.10.3 The Location
Counter
dynamic linker, from command line, see Section 3.1
Command Line Options
dynamic symbol table, see Section 3.1 Command Line
Options
Index
E
ELF program headers, see Section 4.8 PHDRS Command
emulation, see Section 3.1 Command Line Options
emulation, default, see Section 3.2 Environment Variables
END (MRI), see Appendix A MRI Compatible Script
Files
endianness, see Section 3.1 Command Line Options
entry point, see Section 4.4.1 Setting the Entry Point
entry point, from command line, see Section 3.1 Command Line Options
entry point, thumb, see Section 5.3 ld ’s Support for
Interworking Between ARM and Thumb Code
ENTRY(symbol), see Section 4.4.1 Setting the Entry
Point
error on valid input, see Section 7.1 Have You Found
a Bug?
example of linker script, see Section 4.3 Simple Linker
Script Example
exporting DLL symbols, see Section 5.8 ld and
WIN32 (cygwin/mingw)
expression evaluation order, see Section 4.10.5 Evaluation
expression sections, see Section 4.10.6 The Section of
an Expression
expression, absolute, see Section 4.10.7 Builtin Functions
expressions, see Section 4.10 Expressions in Linker
Scripts
EXTERN, see Section 4.4.4 Other Linker Script Commands
F
fatal signal, see Section 7.1 Have You Found a Bug?
file name wildcard patterns, see Section 4.6.4.2 Input
Section Wildcard Patterns
FILEHDR, see Section 4.8 PHDRS Command
filename symbols, see Section 4.6.6 Output Section
Keywords
fill pattern, entire section, see Section 4.6.8.5 Output
Section Fill
FILL(expression), see Section 4.6.5 Output Section
Data
finalization function, see Section 3.1 Command Line
Options
first input file, see Section 4.4.2 Commands Dealing
with Files
first instruction, see Section 4.4.1 Setting the Entry
Point
FORCE_COMMON_ALLOCATION, see Section
4.4.4 Other Linker Script Commands
FORMAT (MRI), see Appendix A MRI Compatible
Script Files
87
functions in expressions, see Section 4.10.7 Builtin
Functions
G
garbage collection, see Section 4.6.4.4 Input Section
and Garbage Collection
garbage collection, see Section 3.1 Command Line
Options
generating optimized output, see Section 3.1 Command Line Options
gnu linker, see Chapter 2 Overview
GNUTARGET, see Section 3.2 Environment Variables
GROUP(files), see Section 4.4.2 Commands Dealing with Files
grouping input files, see Section 4.4.2 Commands
Dealing with Files
groups of archives, see Section 3.1 Command Line
Options
H
H8/300 support, see Section 5.1 ld and the H8/300
header size, see Section 4.10.7 Builtin Functions
heap size, see Section 3.1 Command Line Options
help, see Section 3.1 Command Line Options
holes, see Section 4.10.3 The Location Counter
holes, filling, see Section 4.6.5 Output Section Data
HPPA multiple sub-space stubs, see Section 5.4 ld
and HPPA 32-bit ELF Support
HPPA stub grouping, see Section 5.4 ld and HPPA
32-bit ELF Support
I
i960 support, see Section 5.2 ld and the Intel 960
Family
image base, see Section 3.1 Command Line Options
implicit linker scripts, see Section 4.11 Implicit Linker
Scripts
import libraries, see Section 5.8 ld and WIN32 (cygwin/mingw)
INCLUDE filename, see Section 4.4.2 Commands
Dealing with Files
including a linker script, see Section 4.4.2 Commands
Dealing with Files
including an entire archive, see Section 3.1 Command
Line Options
incremental link, see Section 3.1 Command Line Options
INHIBIT_COMMON_ALLOCATION, see Section
4.4.4 Other Linker Script Commands
88
Index
initialization function, see Section 3.1 Command Line
Options
initialized data in ROM, see Section 4.6.8.2 Output
Section LMA
input file format in linker script, see Section 4.4.3
Commands Dealing with Object File Formats
input filename symbols, see Section 4.6.6 Output Section Keywords
input files in linker scripts, see Section 4.4.2 Commands Dealing with Files
input files, displaying, see Section 3.1 Command Line
Options
input format, see Section 3.1 Command Line Options
input object files in linker scripts, see Section 4.4.2
Commands Dealing with Files
input section basics, see Section 4.6.4.1 Input Section
Basics
input section wildcards, see Section 4.6.4.2 Input Section Wildcard Patterns
input sections, see Section 4.6.4 Input Section Description
INPUT(files), see Section 4.4.2 Commands Dealing with Files
integer notation, see Section 4.10.1 Constants
integer suffixes, see Section 4.10.1 Constants
internal object-file format, see Section 6.1.2 The BFD
canonical object-file format
invalid input, see Section 7.1 Have You Found a Bug?
linker script example, see Section 4.3 Simple Linker
Script Example
linker script file commands, see Section 4.4.2 Commands Dealing with Files
linker script format, see Section 4.2 Linker Script Format
linker script input object files, see Section 4.4.2 Commands Dealing with Files
linker script simple commands, see Section 4.4 Simple
Linker Script Commands
linker scripts, see Chapter 4 Linker Scripts
LIST (MRI), see Appendix A MRI Compatible Script
Files
little-endian objects, see Section 3.1 Command Line
Options
LOAD (MRI), see Appendix A MRI Compatible Script
Files
load address, see Section 4.6.8.2 Output Section LMA
LOADADDR(section), see Section 4.10.7 Builtin
Functions
loading, preventing, see Section 4.6.8.1 Output Section Type
local symbols, deleting, see Section 3.1 Command
Line Options
location counter, see Section 4.10.3 The Location
Counter
LONG(expression), see Section 4.6.5 Output Section Data
K
M
K and M integer suffixes, see Section 4.10.1 Constants
KEEP, see Section 4.6.4.4 Input Section and Garbage
Collection
M and K integer suffixes, see Section 4.10.1 Constants
machine architecture, see Section 4.4.4 Other Linker
Script Commands
machine dependencies, see Chapter 5 Machine Dependent Features
mapping input sections to output sections, see Section
4.6.4 Input Section Description
MAX, see Section 4.10.7 Builtin Functions
MEMORY, see Section 4.7 MEMORY Command
memory region attributes, see Section 4.7 MEMORY
Command
memory regions, see Section 4.7 MEMORY Command
memory regions and sections, see Section 4.6.8.3 Output Section Region
memory usage, see Section 3.1 Command Line Options
MIN, see Section 4.10.7 Builtin Functions
MIPS embedded PIC code, see Section 3.1 Command
Line Options
MRI compatibility, see Appendix A MRI Compatible
Script Files
MSP430 extra sections, see Section 5.6 ld and
MSP430
L
l =, see Section 4.7 MEMORY Command
L, deleting symbols beginning, see Section 3.1 Command Line Options
lazy evaluation, see Section 4.10.5 Evaluation
ld bugs, reporting, see Section 7.2 How to Report
Bugs
LDEMULATION, see Section 3.2 Environment Variables
len =, see Section 4.7 MEMORY Command
LENGTH =, see Section 4.7 MEMORY Command
library search path in linker script, see Section 4.4.2
Commands Dealing with Files
link map, see Section 3.1 Command Line Options
link-time runtime library search path, see Section 3.1
Command Line Options
linker crash, see Section 7.1 Have You Found a Bug?
linker script concepts, see Section 4.1 Basic Linker
Script Concepts
Index
NAME (MRI), see Appendix A MRI Compatible Script
Files
N
name, section, see Section 4.6.2 Output Section Name
names, see Section 4.10.2 Symbol Names
naming the output file, see Section 3.1 Command Line
Options
NEXT(exp), see Section 4.10.7 Builtin Functions
NMAGIC, see Section 3.1 Command Line Options
NOCROSSREFS(sections), see Section 4.4.4
Other Linker Script Commands
NOLOAD, see Section 4.6.8.1 Output Section Type
not enough room for program headers, see Section
4.10.7 Builtin Functions
89
P
partial link, see Section 3.1 Command Line Options
PHDRS, see Section 4.8 PHDRS Command
position independent executables, see Section 3.1
Command Line Options
precedence in expressions, see Section 4.10.4 Operators
prevent unnecessary loading, see Section 4.6.8.1 Output Section Type
program headers, see Section 4.8 PHDRS Command
program headers and sections, see Section 4.6.8.4
Output Section Phdr
program headers, not enough room, see Section 4.10.7
Builtin Functions
program segments, see Section 4.8 PHDRS Command
PROVIDE, see Section 4.5.2 PROVIDE
PUBLIC (MRI), see Appendix A MRI Compatible
Script Files
O
o =, see Section 4.7 MEMORY Command
objdump -i, see Chapter 6 BFD
object file management, see Chapter 6 BFD
object files, see Section 3.1 Command Line Options
object formats available, see Chapter 6 BFD
object size, see Section 3.1 Command Line Options
OMAGIC, see Section 3.1 Command Line Options
opening object files, see Section 6.1 How It Works: An
Outline of BFD
operators for arithmetic, see Section 4.10.4 Operators
options, see Section 3.1 Command Line Options
ORDER (MRI), see Appendix A MRI Compatible
Script Files
org =, see Section 4.7 MEMORY Command
ORIGIN =, see Section 4.7 MEMORY Command
output file after errors, see Section 3.1 Command Line
Options
output file format in linker script, see Section 4.4.3
Commands Dealing with Object File Formats
output file name in linker scripot, see Section 4.4.2
Commands Dealing with Files
output section attributes, see Section 4.6.8 Output Section Attributes
output section data, see Section 4.6.5 Output Section
Data
OUTPUT(filename), see Section 4.4.2 Commands
Dealing with Files
OUTPUT_ARCH(bfdarch), see Section 4.4.4 Other
Linker Script Commands
OUTPUT_FORMAT(bfdname), see Section 4.4.3
Commands Dealing with Object File Formats
OVERLAY, see Section 4.6.9 Overlay Description
overlays, see Section 4.6.9 Overlay Description
Q
QUAD(expression), see Section 4.6.5 Output Section Data
quoted symbol names, see Section 4.10.2 Symbol
Names
R
read-only text, see Section 3.1 Command Line Options
read/write from cmd line, see Section 3.1 Command
Line Options
regions of memory, see Section 4.7 MEMORY Command
relative expressions, see Section 4.10.6 The Section of
an Expression
relaxing addressing modes, see Section 3.1 Command
Line Options
relaxing on H8/300, see Section 5.1 ld and the
H8/300
relaxing on i960, see Section 5.2 ld and the Intel 960
Family
relaxing on Xtensa, see Section 5.9 ld and Xtensa
Processors
relocatable and absolute symbols, see Section 4.10.6
The Section of an Expression
relocatable output, see Section 3.1 Command Line
Options
removing sections, see Section 4.6.7 Output Section
Discarding
reporting bugs in ld, see Chapter 7 Reporting Bugs
requirements for BFD, see Chapter 6 BFD
retain relocations in final executable, see Section 3.1
Command Line Options
90
retaining specified symbols, see Section 3.1 Command Line Options
ROM initialized data, see Section 4.6.8.2 Output Section LMA
round up location counter, see Section 4.10.7 Builtin
Functions
runtime library name, see Section 3.1 Command Line
Options
runtime library search path, see Section 3.1 Command
Line Options
runtime pseudo-relocation, see Section 5.8 ld and
WIN32 (cygwin/mingw)
S
scaled integers, see Section 4.10.1 Constants
scommon section, see Section 4.6.4.3 Input Section
for Common Symbols
script files, see Section 3.1 Command Line Options
scripts, see Chapter 4 Linker Scripts
search directory, from cmd line, see Section 3.1 Command Line Options
search path in linker script, see Section 4.4.2 Commands Dealing with Files
SEARCH_DIR(path), see Section 4.4.2 Commands
Dealing with Files
SECT (MRI), see Appendix A MRI Compatible Script
Files
section address, see Section 4.6.3 Output Section Description
section address in expression, see Section 4.10.7
Builtin Functions
section alignment, warnings on, see Section 3.1 Command Line Options
section data, see Section 4.6.5 Output Section Data
section fill pattern, see Section 4.6.8.5 Output Section
Fill
section load address, see Section 4.6.8.2 Output Section LMA
section load address in expression, see Section 4.10.7
Builtin Functions
section name, see Section 4.6.2 Output Section Name
section name wildcard patterns, see Section 4.6.4.2 Input Section Wildcard Patterns
section size, see Section 4.10.7 Builtin Functions
section, assigning to memory region, see Section
4.6.8.3 Output Section Region
section, assigning to program header, see Section
4.6.8.4 Output Section Phdr
SECTIONS, see Section 4.6 SECTIONS Command
sections, discarding, see Section 4.6.7 Output Section
Discarding
segment origins, cmd line, see Section 3.1 Command
Line Options
segments, ELF, see Section 4.8 PHDRS Command
Index
shared libraries, see Section 3.1 Command Line Options
SHORT(expression), see Section 4.6.5 Output Section Data
SIZEOF(section), see Section 4.10.7 Builtin Functions
SIZEOF_HEADERS, see Section 4.10.7 Builtin
Functions
small common symbols, see Section 4.6.4.3 Input Section for Common Symbols
SORT, see Section 4.6.4.2 Input Section Wildcard Patterns
SQUAD(expression), see Section 4.6.5 Output Section Data
stack size, see Section 3.1 Command Line Options
standard Unix system, see Section 3.1 Command Line
Options
start of execution, see Section 4.4.1 Setting the Entry
Point
STARTUP(filename), see Section 4.4.2 Commands
Dealing with Files
strip all symbols, see Section 3.1 Command Line Options
strip debugger symbols, see Section 3.1 Command
Line Options
stripping all but some symbols, see Section 3.1 Command Line Options
suffixes for integers, see Section 4.10.1 Constants
symbol defaults, see Section 4.10.7 Builtin Functions
symbol definition, scripts, see Section 4.5 Assigning
Values to Symbols
symbol names, see Section 4.10.2 Symbol Names
symbol tracing, see Section 3.1 Command Line Options
symbol versions, see Section 4.9 VERSION Command
symbol-only input, see Section 3.1 Command Line
Options
symbols, from command line, see Section 3.1 Command Line Options
symbols, relocatable and absolute, see Section 4.10.6
The Section of an Expression
symbols, retaining selectively, see Section 3.1 Command Line Options
synthesizing linker, see Section 3.1 Command Line
Options
synthesizing on H8/300, see Section 5.1 ld and the
H8/300
Index
T
TARGET(bfdname), see Section 4.4.3 Commands
Dealing with Object File Formats
thumb entry point, see Section 5.3 ld ’s Support for
Interworking Between ARM and Thumb Code
TI COFF versions, see Section 5.7 ld ’s Support for
Various TI COFF Versions
traditional format, see Section 3.1 Command Line Options
U
unallocated address, next, see Section 4.10.7 Builtin
Functions
undefined symbol, see Section 3.1 Command Line Options
undefined symbol in linker script, see Section 4.4.4
Other Linker Script Commands
undefined symbols, warnings on, see Section 3.1
Command Line Options
uninitialized data placement, see Section 4.6.4.3 Input
Section for Common Symbols
unspecified memory, see Section 4.6.5 Output Section
Data
usage, see Section 3.1 Command Line Options
using a DEF file, see Section 5.8 ld and WIN32 (cygwin/mingw)
using auto-export functionality, see Section 5.8 ld
and WIN32 (cygwin/mingw)
Using decorations, see Section 5.8 ld and WIN32
(cygwin/mingw)
V
variables, defining, see Section 4.5 Assigning Values
to Symbols
verbose, see Section 3.1 Command Line Options
version, see Section 3.1 Command Line Options
version script, see Section 4.9 VERSION Command
version script, symbol versions, see Section 3.1 Command Line Options
VERSION {script text}, see Section 4.9 VERSION
Command
versions of symbols, see Section 4.9 VERSION Command
W
warnings, on combining symbols, see Section 3.1
Command Line Options
warnings, on section alignment, see Section 3.1 Command Line Options
warnings, on undefined symbols, see Section 3.1
Command Line Options
91
what is this?, see Chapter 2 Overview
wildcard file name patterns, see Section 4.6.4.2 Input
Section Wildcard Patterns
X
Xtensa processors, see Section 5.9 ld and Xtensa Processors

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