GNU链接器ld：ARM Linux开发入门与实战

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"Using LD: The GNU Linker指南" 本资源是一份针对GNU链接器ld的详细介绍文档，主要针对ARM Linux开发环境，版本为2.11.90。作者包括Steve Chamberlain、Ian Lance Taylor以及来自Red Hat Inc的成员。文档由Jeffrey Osier编辑，版权由Free Software Foundation所有，遵循GNU Free Documentation License v1.1或更高版本。第一章“概述”解释了ld的主要功能。作为编译过程中的关键步骤，ld的作用是将多个目标对象文件和归档文件合并，进行数据重定位，并解决符号引用。它接受Linker Command Language (LCB) 文件，这是一种扩展自AT&T Link Editor命令语言的语法，提供了对链接过程的精细控制。值得注意的是，此版本的ld利用了通用的Binary File Descriptor (BFD) 库，使其能够处理各种类型的可执行文件和目标文件，增强了其兼容性和灵活性。通过学习这份文档，开发者可以掌握如何编写和理解LCB文件，以便在构建Linux应用程序时定制链接策略。这不仅涉及基本的链接选项设置，还包括如何处理共享库（如动态链接）、如何定义和管理符号表，以及如何优化代码的大小和性能。对于那些从事嵌入式系统开发、系统级编程或者维护Linux内核的工程师来说，深入理解ld的工作原理是至关重要的。学习内容可能包括但不限于： 1. **链接器工作原理**：理解ld如何解析和连接各个模块，以及如何处理依赖关系。 2. **LCB文件编写**：掌握链接器命令语法，如`-T`选项指定链接脚本，`-l`选项引用库等。 3. **动态链接与静态链接**：了解如何决定何时选择哪种链接方式，以及链接器在处理这两种情况下的差异。 4. **重定位和符号寻址**：学习如何正确地处理地址空间布局，避免符号冲突和虚实地址转换。 5. **链接器优化**：学习如何通过链接器调整程序的大小、速度和内存使用。这份文档为深入理解和使用ld提供了一个全面的基础，无论你是Linux初学者还是经验丰富的开发人员，都将从中获益匪浅。通过阅读和实践，你将能提升自己在ARM Linux开发中的链接器管理能力，确保软件的稳定性和性能。

12 Using LD, the GNU linker

--check-sections

--no-check-sections

Asks the linker not to check section addresse s 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’.

--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 e asily 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.

--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 neces sary to deﬁne

multiple symbols in the command line. A limited form of arithmetic is sup-

ported 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 hexadec-

imal constants or symbols. If you need m ore elaborate expressions, consider

using the linker com mand language from a script (see Section 3.5 [Assignment:

Symbol Deﬁnitions], page 30). 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 s ymbol

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 read-

able 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 envi-

ronment 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, gen-

erated 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

Chapter 2: Invocation 13

any data which was statically initialized to pointer values. See the code in

testsuite/ld-empic for details.

--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 make ﬁles

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. These options disallows

such undeﬁned symbols.

--allow-shlib-undefined

Allow undeﬁned s ymbols in shared objects even when –no-undeﬁned is set. The

net result will be that undeﬁned symbols in regular objects will still trigger an

error, but undeﬁned symbols in shared objects will be ignored. The implemen-

tation of no undeﬁned makes the assumption that the runtime linker will choke

on undeﬁned symbols. However there is at least one system (BeOS) where un-

deﬁned symbols in shared libraries is normal since the kernel patches them at

load time to select which function is most appropriate for the current architec-

ture. I.E. dynamically select an appropriate memset function. Apparently it is

also normal for HPPA shared libraries to have undeﬁned symb ols.

14 Using LD, the GNU linker

--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 s till 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.

--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. See Chapter 5 [BFD], page 57.

-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. See Section 4.1 [ld and the H8/300], page 55. See Section 4.2 [ld

and the Intel 960 family], page 55.

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. ﬁle-

name is simply a ﬂat ﬁle, with one symbol name per line. This option is esp e -

cially 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 relocations.

Chapter 2: Invocation 15

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 exe-

cutable, 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 b e 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 not included explicitly. In s uch 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 e nvironment variable LD_RUN_PATH. It is for the

native linker only.

4. On SunOS, if the -rpath option was not used, search any directories spec-

iﬁed using -L options.

5. For a native linker, the contents of the environment variable LD_LIBRARY_

PATH.

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GNU链接器ld：ARM Linux开发入门与实战

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