GCC编译器使用指南

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"GCC编译器文档" GCC（GNU Compiler Collection）是GNU项目的一部分，由Richard M. Stallman和GCC开发者社区编写，是一个广泛使用的开源编译器套件，支持多种编程语言，包括C、C++、Objective-C、Fortran、Ada和Go等。此文档详细介绍了GCC 4.9.1版本的使用方法和特性。 GCC的主要功能是将源代码编译成可执行程序。它首先将高级语言转换为中间表示（Intermediate Representation），然后再将其优化和转换为目标机器码。GCC提供了多级优化选项，允许用户在速度和代码大小之间进行权衡。例如，"-O0"选项用于禁用大部分优化，而"-O3"则开启最高级别的优化。文档中会涵盖如何配置、安装和使用GCC，包括设置头文件路径、库路径以及链接选项。对于开发者来说，理解编译过程中的错误消息和警告是非常重要的，这些信息通常可以帮助定位和修复代码问题。 GCC还支持跨平台编译，这意味着可以在一个系统上构建为另一个系统生成代码。这对于软件开发和部署在不同架构上的应用非常有用。文档中会解释如何配置构建目标，并提供交叉编译的指导。在C++方面，GCC支持标准模板库（STL）和其他现代C++特性，如模板元编程、命名空间、异常处理和C++11及更高版本的新特性。此外，文档还会介绍如何使用G++，这是GCC为C++编程提供的接口。 GCC的文档还会包含关于调试信息生成的章节，这对于使用GDB等调试工具是必不可少的。通过适当的编译选项，可以生成用于调试的详细信息，帮助开发者更好地理解程序运行时的状态。除了编译器本身，GCC还包括了诸如gcov这样的代码覆盖率工具，帮助开发者评估测试的完整性。还有ld链接器，它是构建最终可执行文件的关键部分，允许组合多个编译单元并解决符号引用。 GCC编译器文档是一份详尽的指南，涵盖了从基本使用到高级特性的方方面面，对于任何使用GCC的人来说都是宝贵的参考资料。无论你是初学者还是经验丰富的开发者，都能从中找到解决问题的答案和提升效率的技巧。虽然这份文档是针对4.9.1版本的，但很多核心概念和流程对于后来的GCC版本也是适用的。

Chapter 2: Language Standards Supported by GCC 5

2 Language Standards Supported by GCC

For each language compiled by GCC for which there is a standard, GCC attempts to follow

one or more versions of that standard, possibly with some exceptions, and possibly with

some extensions.

2.1 C language

GCC supports three versions of the C standard, although support for the most recent version

is not yet complete.

The original ANSI C standard (X3.159-1989) was ratiﬁed in 1989 and published in 1990.

This standard was ratiﬁed as an ISO standard (ISO/IEC 9899:1990) later in 1990. There

were no technical diﬀerences between these publications, although the sections of the ANSI

standard were renumbered and became clauses in the ISO standard. This standard, in

both its forms, is commonly known as C89, or occasionally as C90, from the dates of

ratiﬁcation. The ANSI standard, but not the ISO standard, also came with a Rationale

document. To select this standard in GCC, use one of the options ‘-ansi’, ‘-std=c90’ or

‘-std=iso9899:1990’; to obtain all the diagnostics required by the standard, you should

also specify ‘-pedantic’ (or ‘-pedantic-errors’ if you want them to be errors rather than

warnings). See Section 3.4 [Options Controlling C Dialect], page 31.

Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published

in 1994 and 1996. GCC does not support the uncorrected version.

An amendment to the 1990 standard was published in 1995. This amendment added

digraphs and __STDC_VERSION__ to the language, but otherwise concerned the library. This

amendment is commonly known as AMD1; the amended standard is sometimes known as

C94 or C95. To select this standard in GCC, use the option ‘-std=iso9899:199409’ (with,

as for other standard versions, ‘-pedantic’ to receive all required diagnostics).

A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and

is commonly known as C99. GCC has substantially complete support for this standard

version; see http://gcc.gnu.org/c99status.html for details. To select this standard,

use ‘-std=c99’ or ‘-std=iso9899:1999’. (While in development, drafts of this standard

version were referred to as C9X.)

Errors in the 1999 ISO C standard were corrected in three Technical Corrigenda published

in 2001, 2004 and 2007. GCC does not support the uncorrected version.

A fourth version of the C standard, known as C11, was published in 2011 as ISO/IEC

9899:2011. GCC has substantially complete support for this standard, enabled with

‘-std=c11’ or ‘-std=iso9899:2011’. (While in development, drafts of this standard

version were referred to as C1X.)

By default, GCC provides some extensions to the C language that on rare occasions con-

ﬂict with the C standard. See Chapter 6 [Extensions to the C Language Family], page 349.

Use of the ‘-std’ options listed above will disable these extensions where they conﬂict with

the C standard version selected. You may also select an extended version of the C language

explicitly with ‘-std=gnu90’ (for C90 with GNU extensions), ‘-std=gnu99’ (for C99 with

GNU extensions) or ‘-std=gnu11’ (for C11 with GNU extensions). The default, if no C lan-

guage dialect options are given, is ‘-std=gnu90’; this is intended to change to ‘-std=gnu11’

in some future release. Some features that are part of the C99 standard are accepted as

6 Using the GNU Compiler Collection (GCC)

extensions in C90 mode, and some features that are part of the C11 standard are accepted

as extensions in C90 and C99 modes.

The ISO C standard deﬁnes (in clause 4) two classes of conforming implementation. A

conforming hosted implementation supports the whole standard including all the library fa-

cilities; a conforming freestanding implementation is only required to provide certain library

facilities: those in <float.h>, <limits.h>, <stdarg.h>, and <stddef.h>; since AMD1,

also those in <iso646.h>; since C99, also those in <stdbool.h> and <stdint.h>; and since

C11, also those in <stdalign.h> and <stdnoreturn.h>. In addition, complex types, added

in C99, are not required for freestanding implementations. The standard also deﬁnes two

environments for programs, a freestanding environment, required of all implementations and

which may not have library facilities beyond those required of freestanding implementations,

where the handling of program startup and termination are implementation-deﬁned, and a

hosted environment, which is not required, in which all the library facilities are provided

and startup is through a function int main (void) or int main (int, char *[]). An OS

kernel would be a freestanding environment; a program using the facilities of an operating

system would normally be in a hosted implementation.

GCC aims towards being usable as a conforming freestanding implementation, or as the

compiler for a conforming hosted implementation. By default, it will act as the compiler for a

hosted implementation, deﬁning __STDC_HOSTED__ as 1 and presuming that when the names

of ISO C functions are used, they have the semantics deﬁned in the standard. To make it act

as a conforming freestanding implementation for a freestanding environment, use the option

‘-ffreestanding’; it will then deﬁne __STDC_HOSTED__ to 0 and not make assumptions

about the meanings of function names from the standard library, with exceptions noted

below. To build an OS kernel, you may well still need to make your own arrangements for

linking and startup. See Section 3.4 [Options Controlling C Dialect], page 31.

GCC does not provide the library facilities required only of hosted implementations, nor

yet all the facilities required by C99 of freestanding implementations on all platforms; to use

the facilities of a hosted environment, you will need to ﬁnd them elsewhere (for example,

in the GNU C library). See Section 11.5 [Standard Libraries], page 752.

Most of the compiler support routines used by GCC are present in ‘libgcc’, but there

are a few exceptions. GCC requires the freestanding environment provide memcpy, memmove,

memset and memcmp. Finally, if __builtin_trap is used, and the target does not implement

the trap pattern, then GCC will emit a call to abort.

For references to Technical Corrigenda, Rationale documents and information concerning

the history of C that is available online, see http://gcc.gnu.org/readings.html

2.2 C++ language

GCC supports the original ISO C++ standard (1998) and contains experimental support for

the second ISO C++ standard (2011).

The original ISO C++ standard was published as the ISO standard (ISO/IEC 14882:1998)

and amended by a Technical Corrigenda published in 2003 (ISO/IEC 14882:2003). These

standards are referred to as C++98 and C++03, respectively. GCC implements the majority

of C++98 (export is a notable exception) and most of the changes in C++03. To select

this standard in GCC, use one of the options ‘-ansi’, ‘-std=c++98’, or ‘-std=c++03’; to

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