详解GCC 4.9.4编译器工具链

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本文档是关于GCC（GNU Compiler Collection）版本4.9.4的官方介绍，由Richard Stallman和GCC开发者社区共同编写，由GNUPress出版。GCC是一款广泛使用的开源编译器，由自由软件基金会（Free Software Foundation, FSF）维护，旨在支持各种编程语言，如C、C++、Objective-C等。这份文档不仅提供了详细的安装和配置指南，还涵盖了GCC的核心功能、优化选项以及与Linux开发环境的集成。在文档的开头部分，强调了版权信息，指出版权由Free Software Foundation自1988年至2015年持有，并且遵循GNU Free Documentation License Version 1.3或更高版本。此许可证允许用户自由复制、分发和修改文档内容，只要满足特定条件，如保留版权声明、使用指定的前言和后记文本，并包含GNU Free Documentation License全文。文档的重要知识点包括： 1. **概述**：介绍了GCC作为自由软件的特性，其设计理念是提供一个强大、可定制的编译器，支持软件自由开发。 2. **版本介绍**：特别关注的是4.9.4版本，这可能是当时的一个稳定版本，文档可能包含了该版本的新特性、改进和性能提升。 3. **作者和组织**：Richard Stallman和GCC开发者社区在文档创作中的角色，体现了开源社区的协作精神。 4. **获取途径**：文档通过GNUPress网站发行，用户可以通过购买印刷版或在线获取，价格为45美元，同时也提供了联系销售部门的渠道。 5. **版权和许可**：详细阐述了文档的版权归属和使用权限，强调了读者可以按照GNU Free Documentation License进行复制和传播，但必须遵守相关规定。 6. **版权信息的法律依据**：引用了具体的版权许可条款，明确了前言和后记的文本内容，这些都是理解文档使用权限的关键。 7. **文档用途**：文档的目标用户是程序员和系统管理员，它旨在帮助他们理解和利用GCC进行高效、自由的软件开发。在阅读这份文档时，用户可以了解到如何在Linux平台上正确安装和配置GCC，如何处理源代码编译过程，以及如何通过不同选项优化编译结果。对于深入研究或从事开源项目开发的人来说，这是不可或缺的一份参考资料。

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 353.

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 758.

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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