GNU Linker (ld) V2.19.51 更新与官方文档

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"The GNU Linker V2.19.51是GNU工具链中的一部分，主要用于连接编译器产生的对象文件，形成可执行程序或库。这个版本修复了《Using ld》文档中的错误，并增加了新的内容，使得用户能够更好地理解和使用ld链接器。文档由Steve Chamberlain和Ian Lance Taylor等人编写，由Red Hat Inc维护，适用于Sourcery G++ Lite 2010q1-188版本。GNU链接器遵循GNU Free Documentation License，允许自由复制、分发和修改。" 在GNU链接器（ld）V2.19.51中，主要包含以下几个方面的知识点： 1. **概述**：链接器的作用是在编译过程中将多个目标文件（.o文件）合并成一个单一的可执行文件或库。ld处理这些目标文件，解决符号引用，合并代码和数据，并生成最终的二进制文件。 2. **命令行选项**：链接器支持多种命令行选项来定制链接过程。例如，可以设置输出文件名、指定输入文件、选择特定的目标架构等。其中，2.1.1至2.1.3分别列举了针对i386 PET目标、Motorola 68HC11和68HC12目标以及Motorola 68K目标的特定选项。 3. **环境变量**：在链接过程中，可以使用环境变量来影响链接器的行为，如`LD_LIBRARY_PATH`用于查找动态库，`LDFLAGS`用于传递额外的链接选项等。 4. **链接脚本**： 3.1和3.2章节详细介绍了基本的链接脚本概念和格式。链接脚本是一种控制链接器如何组合输入文件和如何定位段（如代码、数据）的自定义方式。3.3和3.4章节提供了简单链接脚本的示例，帮助用户理解如何编写和使用它们。 5. **链接过程**：链接器的工作包括：解析输入文件、解决符号引用（外部函数和全局变量）、重定位（根据地址信息更新代码和数据）、处理节区（如.text、.data等）的合并，以及生成可执行文件的头部信息。 6. **兼容性和移植性**： GNU链接器支持多种处理器架构和操作系统，因此它具有广泛的兼容性和可移植性，可以在不同的平台和系统环境下工作。 7. **错误和警告处理**：当链接过程中遇到问题，如未定义的符号、重定义的符号或不匹配的架构，ld会发出错误或警告信息，帮助开发者调试和解决问题。 8. **版本更新**：版本2.19.51相对于之前的2.14版进行了错误修正，提高了用户体验，同时可能添加了对新功能的支持。通过学习这个文档，用户不仅可以了解ld链接器的基本操作，还可以深入理解链接过程的细节，从而更好地进行软件开发和调试工作。

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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