"深入理解ARM GCC链接器的链接脚本语法"

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ARM GCC linker 链接脚本语法是对GNU linkerld(GNU Arm Embedded Toolchain 10-2020-q4-major)Version 2.35.1的一种描述。该链接器由Steve Chamberlain、Ian Lance Taylor和Red Hat Inc共同编写，版权归Free Software Foundation, Inc所有。根据GNU Free Documentation License，任何人都可以复制、分发和/或修改这份文档，但必须遵守相关条款和条件。该链接脚本语法的设计目的是为了帮助开发者更好地理解和使用GNU Arm Embedded Toolchain在ARM架构下的链接过程。在ARM架构下，链接脚本语法起着至关重要的作用。它定义了链接器如何将目标文件和库文件组合成最终的可执行文件或可链接文件。链接脚本语法包括了一系列的命令和规则，用于指导链接器在链接过程中的具体行为。开发者可以通过编写链接脚本来定制链接过程，例如指定程序的起始地址、定义数据在内存中的布局、配置代码段和数据段的属性等。在ARM GCC linker 链接脚本语法中，有一些重要的关键字和命令需要开发者了解。比如，MEMORY命令用于定义目标设备的内存布局，包括内存区域的起始地址和大小；SECTIONS命令用于定义不同类型的段，并指示链接器如何处理这些段；以及OUTPUT命令用于指定构建输出文件的格式和属性等。除了这些基本的命令外，链接脚本语法还支持一些高级特性，例如通配符、条件判断、循环等，这些特性可以帮助开发者更灵活地完成链接脚本的编写。另外，链接脚本语法还支持使用变量和函数，以及内联汇编等功能，这些功能为开发者提供了更多的自定义和扩展空间。总的来说，ARM GCC linker 链接脚本语法是GNU Arm Embedded Toolchain在ARM架构下链接过程中的必备工具。开发者可以通过编写链接脚本来定制和控制链接器的行为，从而更好地满足项目的需求。然而，链接脚本语法相对复杂，需要开发者有一定的经验和技能才能熟练掌握。因此，建议开发者在使用链接脚本时，结合官方文档和示例，多进行实践和尝试，逐渐掌握其精髓，提高链接过程的效率和质量。

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 82 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 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 49. 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 49.

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 83.

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 83.

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 70).

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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