"GNU链接器教程与规范，详解使用和链接脚本，命令行说明"

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本文是《GNU链接器教程和规范》的总结，全文共2000字。《GNU链接器教程和规范》详细说明了GNU链接器（ld）的使用方法以及链接脚本的规范，并提供了命令行的命令说明。首先，该文档由Steve Chamberlain、Ian Lance Taylor和Red Hat Inc的nickc@credhat.com、doc@redhat.com等人撰写，由Jeffrey Osier (jeffrey@cygnus.com)进行编辑。版权归自由软件基金会所有，根据GNU自由文档许可证的条款，允许复制、分发和修改本文档。《GNU链接器教程和规范》主要分为以下几个部分： 1. 引言：介绍了链接器的作用和基本概念，包括链接过程、符号解析、重定位、共享库等内容。此部分对链接器的工作原理进行了简要说明，为后续内容的理解打下基础。 2. 链接器命令行选项：详细介绍了GNU链接器ld的各种命令行选项及其用法。其中包括输入文件选项、输出文件选项、链接类型选项、弱符号选项等等。这些选项可以通过命令行来指定，用于控制链接器的行为。 3. 链接脚本规范：介绍了GNU链接器的链接脚本规范，即ld脚本的语法和用法。链接脚本是一个文本文件，用于控制链接器的各种行为，如内存布局、段的排列顺序、符号的定位等等。本节详细说明了链接脚本的各种命令和指令，并提供了一些示例脚本。 4. 高级链接器特性：介绍了一些高级的链接器特性，包括版本符号、符号版本控制、插件、节选项等。这些特性可以帮助开发者更灵活地控制链接过程，实现一些特定的需求。 5. 示例和常见问题解答：提供了一些实际的示例和常见问题的解答，帮助读者更好地理解和应用链接器。通过对《GNU链接器教程和规范》的总结，我们可以了解到GNU链接器ld是一个功能强大且灵活的工具，可用于将目标文件、库文件等进行链接，生成可执行文件或共享库。本文详细介绍了链接器的使用方法、链接脚本的规范，以及各种命令行选项的说明。无论是初学者还是有经验的开发者，都可以通过本文掌握GNU链接器ld的基本原理和高级特性，从而更好地应用于实际开发中。总之，《GNU链接器教程和规范》是一本权威的链接器教程，内容全面详细，可作为学习和使用GNU链接器的参考资料。无论是对链接器感兴趣的开发者，还是需要使用链接器进行项目开发的人员，都能够从中获得实际帮助。希望本文的总结能够为读者提供一个简洁而全面的概述，并激发大家对链接器的进一步探索和应用。

10 The GNU linker

-m emulation

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 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.

• The values assigned to symbols.

Note - symbols whose values are computed by an expression which involves

a reference to a previous value of the same symbol may not have correct

result displayed in the link map. This is because the linker discards inter-

mediate results and only retains the ﬁnal value of an expression. Under

such circumstances the linker will display the ﬁnal value enclosed by square

brackets. Thus for example a linker script containing:

foo = 1

foo = foo * 4

foo = foo + 8

will produce the following output in the link map if the ‘-M’ option is used:

0x00000001 foo = 0x1

[0x0000000c] foo = (foo * 0x4)

[0x0000000c] foo = (foo + 0x8)

See Section 3.10 [Expressions], page 86 for more information about expres-

sions in linker scripts.

• How GNU properties are merged.

When the linker merges input .note.gnu.property sections into one output

.note.gnu.property section, some properties are removed or updated. These

actions are reported in the link map. For example:

Removed property 0xc0000002 to merge foo.o (0x1) and bar.o (not found)

This indicates that property 0xc0000002 is removed from output when

merging properties in ‘foo.o’, whose property 0xc0000002 value is 0x1,

and ‘bar.o’, which doesn’t have property 0xc0000002.

Updated property 0xc0010001 (0x1) to merge foo.o (0x1) and bar.o (0x1)

This indicates that property 0xc0010001 value is updated to 0x1 in output

when merging properties in ‘foo.o’, whose 0xc0010001 property value is

0x1, and ‘bar.o’, whose 0xc0010001 property value is 0x1.

Chapter 2: Invocation 11

--print-map-discarded

--no-print-map-discarded

Print (or do not print) the list of discarded and garbage collected sections in

the link map. Enabled by default.

-n

--nmagic Turn oﬀ page alignment of sections, and disable linking against shared libraries.

If the output format supports Unix style magic numbers, mark the output as

NMAGIC.

-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.

Note: Although a writable text section is allowed for PE-COFF targets, it does

not conform to the format speciﬁcation published by Microsoft.

--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.

--dependency-file=depfile

Write a dependency ﬁle to depﬁle. This ﬁle contains a rule suitable for make

describing the output ﬁle and all the input ﬁles that were read to produce it. The

output is similar to the compiler’s output with ‘-M -MP’ (see Section “Options

Controlling the Preprocessor” in Using the GNU Compiler Collection). Note

that there is no option like the compiler’s ‘-MM’, to exclude “system ﬁles” (which

is not a well-speciﬁed concept in the linker, unlike “system headers” in the

compiler). So the output from ‘--dependency-file’ is always speciﬁc to the

exact state of the installation where it was produced, and should not be copied

into distributed makeﬁles without careful editing.

-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. At the moment this option only aﬀects ELF shared library

generation. Future releases of the linker may make more use of this option. Also

currently there is no diﬀerence in the linker’s behaviour for diﬀerent non-zero

values of this option. Again this may change with future releases.

-plugin name

Involve a plugin in the linking process. The name parameter is the absolute

ﬁlename of the plugin. Usually this parameter is automatically added by the

12 The GNU linker

complier, when using link time optimization, but users can also add their own

plugins if they so wish.

Note that the location of the compiler originated plugins is diﬀerent from the

place where the ar, nm and ranlib programs search for their plugins. In or-

der for those commands to make use of a compiler based plugin it must ﬁrst

be copied into the ‘${libdir}/bfd-plugins’ directory. All gcc based linker

plugins are backward compatible, so it is suﬃcient to just copy in the newest

one.

--push-state

The ‘--push-state’ allows one to preserve the current state of the ﬂags which

govern the input ﬁle handling so that they can all be restored with one corre-

sponding ‘--pop-state’ option.

The option which are covered are: ‘-Bdynamic’, ‘-Bstatic’, ‘-dn’, ‘-dy’,

‘-call_shared’, ‘-non_shared’, ‘-static’, ‘-N’, ‘-n’, ‘--whole-archive’,

‘--no-whole-archive’, ‘-r’, ‘-Ur’, ‘--copy-dt-needed-entries’,

‘--no-copy-dt-needed-entries’, ‘--as-needed’, ‘--no-as-needed’, and

‘-a’.

One target for this option are speciﬁcations for ‘pkg-config’. When used with

the ‘--libs’ option all possibly needed libraries are listed and then possibly

linked with all the time. It is better to return something as follows:

-Wl,--push-state,--as-needed -libone -libtwo -Wl,--pop-state

--pop-state

Undoes the eﬀect of –push-state, restores the previous values of the ﬂags gov-

erning input ﬁle handling.

-q

--emit-relocs

Leave relocation sections and contents in fully linked executables. 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.

--force-dynamic

Force the output ﬁle to have dynamic sections. This option is speciﬁc to Vx-

Works targets.

-r

--relocatable

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 envi-

ronments 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 abso-

lute 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.

Chapter 2: Invocation 13

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’.

-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.

--strip-discarded

--no-strip-discarded

Omit (or do not omit) global symbols deﬁned in discarded sections. Enabled

by default.

-t

--trace Print the names of the input ﬁles as ld processes them. If ‘-t’ is given twice

then members within archives are also printed. ‘-t’ output is useful to generate

a list of all the object ﬁles and scripts involved in linking, for example, when

packaging ﬁles for a linker bug report.

-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. See Chapter 3 [Scripts], page 53. 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.

-dT scriptfile

--default-script=scriptfile

Use scriptﬁle as the default linker script. See Chapter 3 [Scripts], page 53.

This option is similar to the ‘--script’ option except that processing of the

script is delayed until after the rest of the command line has been processed.

This allows options placed after the ‘--default-script’ option on the com-

mand line to aﬀect the behaviour of the linker script, which can be important

when the linker command line cannot be directly controlled by the user. (eg

because the command line is being constructed by another tool, such as ‘gcc’).

14 The GNU linker

-u symbol

--undefined=symbol

Force symbol to be entered in the output ﬁle as an undeﬁned symbol. Do-

ing this may, for example, trigger linking of additional modules from standard

libraries. ‘-u’ may be repeated with diﬀerent option arguments to enter addi-

tional undeﬁned symbols. This option is equivalent to the EXTERN linker script

command.

If this option is being used to force additional modules to be pulled into the

link, and if it is an error for the symbol to remain undeﬁned, then the option

‘--require-defined’ should be used instead.

--require-defined=symbol

Require that symbol is deﬁned in the output ﬁle. This option is the same as

option ‘--undefined’ except that if symbol is not deﬁned in the output ﬁle

then the linker will issue an error and exit. The same eﬀect can be achieved

in a linker script by using EXTERN, ASSERT and DEFINED together. This option

can be used multiple times to require additional symbols.

-Ur For anything other than C++ programs, this option is equivalent to ‘-r’: it gen-

erates 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.

--orphan-handling=MODE

Control how orphan sections are handled. An orphan section is one not speciﬁ-

cally mentioned in a linker script. See Section 3.10.4 [Orphan Sections], page 87.

MODE can have any of the following values:

place Orphan sections are placed into a suitable output section following

the strategy described in Section 3.10.4 [Orphan Sections], page 87.

The option ‘--unique’ also aﬀects how sections are placed.

discard All orphan sections are discarded, by placing them in the

‘/DISCARD/’ section (see Section 3.6.7 [Output Section Discarding],

page 74).

warn The linker will place the orphan section as for place and also issue

a warning.

error The linker will exit with an error if any orphan section is found.

The default if ‘--orphan-handling’ is not given is place.

--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.

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