GNU链接器详解：Sourcery G++ Lite 2.19.51 版本文档

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GNU linker, 或者通常称为ld，是GNU工具链中的关键组成部分，它是用于链接可执行文件和共享库的程序链接器。该文档（版本2.19.51）由Steve Chamberlain和Ian Lance Taylor编写，最初由Sourcery G++ Lite 2010q1-188发行，且在2015年7月28日进行了更新。Red Hat Inc.的nickc和doc负责维护，同时Jeffrey Osier（jeffrey@cygnus.com）也对文档进行了编辑。文档的主体部分分为几个主要章节： 1. **概述**：这部分介绍了GNU linker的基本功能和作用，它将编译过的目标代码模块、对象文件、静态库等链接在一起，形成最终的可执行文件或动态链接库。它支持多种架构，如i386、Motorola 68HC11/68HC12和68K。 2. **调用与选项**： - **命令行选项**：文档详细列出了针对不同架构（如i386、摩托罗拉68HC11/68HC12和68K）的特定链接器选项。这些选项控制了链接过程中的行为，如符号解析、输入文件的选择、输出文件的生成以及链接时的优化级别。 - **环境变量**：还提到了环境变量在链接过程中的作用，用户可以通过设置这些变量来调整链接器的行为，如LD_LIBRARY_PATH，用于指定搜索共享库的路径。 3. **链接器脚本**： - **基本概念**：这部分阐述了链接器脚本的作用，它们是文本文件，允许用户自定义链接过程中的细节，包括内存分配、初始化和处理特殊链接要求。 - **脚本格式**：讲解了链接器脚本的语法和结构，包括如何定义段、数据区域、入口点、重定位等。 - **示例**：提供了简单的链接器脚本示例，帮助读者理解如何创建和使用脚本来定制链接过程。 4. **更深入的内容**：除了以上概述，文档还可能包含了其他高级主题，如链接器的工作原理、链接器的行为模式、链接器警告和错误处理、以及如何处理链接依赖和链接时间优化。 GNU linker文档是程序员在开发过程中必不可少的参考资料，它详细解释了如何使用这个强大的工具来链接源代码，并提供了丰富的选项和脚本控制，以满足各种复杂的应用场景需求。对于那些使用GCC和Linux环境的开发者来说，理解和掌握GNU linker的使用方法是至关重要的。

10 The GNU linker

-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.

-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.

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.

Chapter 2: Invocation 11

-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. See Chapter 3 [Scripts], page 37. 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 37.

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’).

-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.

-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.

--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.

-v

--version

-V Display the version number for ld. The ‘-V’ option also lists the supported

emulations.

Chapter 2: Invocation 13

‘muldefs’ Allows multiple deﬁnitions.

‘nocombreloc’

Disables multiple reloc sections combining.

‘nocopyreloc’

Disables production of copy relocs.

‘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.

‘noexecstack’

Marks the object as not requiring executable stack.

‘norelro’ Don’t create an ELF PT_GNU_RELRO segment header in the object.

‘now’ When generating an executable or shared library, mark it to tell the

dynamic linker to resolve all symbols when the program is started,

or when the shared library is linked to using dlopen, instead of

deferring function call resolution to the point when the function is

ﬁrst called.

‘origin’ Marks the object may contain $ORIGIN.

‘relro’ Create an ELF PT_GNU_RELRO segment header in the object.

‘max-page-size=value ’

Set the emulation maximum page size to value.

‘common-page-size=value ’

Set the emulation common page size to value.

Other keywords are ignored for Solaris compatibility.

-( 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.

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.

14 The GNU linker

--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.

--as-needed

--no-as-needed

This option aﬀects ELF DT NEEDED tags for dynamic libraries mentioned

on the command line after the ‘--as-needed’ option. Normally, the linker will

add a DT NEEDED tag for each dynamic library mentioned on the command

line, regardless of whether the library is actually needed. ‘--as-needed’ causes

a DT NEEDED tag to only be emitted for a library that satisﬁes a symbol

reference from regular objects which is undeﬁned at the point that the library

was linked, or, if the library is not found in the DT NEEDED lists of other

libraries linked up to that point, a reference from another dynamic library.

‘--no-as-needed’ restores the default behaviour.

--add-needed

--no-add-needed

This option aﬀects the treatment of dynamic libraries from ELF DT NEEDED

tags in dynamic libraries mentioned on the command line after the

‘--no-add-needed’ option. Normally, the linker will add a DT NEEDED

tag for each dynamic library from DT NEEDED tags. ‘--no-add-needed’

causes DT NEEDED tags will never be emitted for those libraries from

DT NEEDED tags. ‘--add-needed’ restores the default 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 vari-

ous 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. ‘--unresolved-symbols=report-all’

is implied. This option is only meaningful on ELF platforms which support

shared libraries.

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