C语言标准ISO-IEC 9899-202x：编程规范与移植性指南

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ISO-IEC 9899-202x (E)是一份由国际标准化组织（ISO）和国际电工委员会（IEC）联合发布的国际标准文档，全称为《编程语言 - C》，最新版本发布于2020年2月5日，工作草案状态。该标准的主要目标是规范C语言的表达形式，确立其在各种计算机系统上的程序执行方式，以促进程序的可移植性、可靠性、可维护性和高效执行。 ISO/IEC 9899-202x定义了C语言本身的核心特性，包括语法、数据类型、控制结构、函数等基本元素。标准还详细规定了C语言执行库的内容，这些库包含了一系列用于处理内存管理、输入输出、数学运算等操作的函数。附录部分汇总了这些内容的关键要素，并探讨了影响C程序可移植性的因素，例如编译器特性和平台依赖性。这份文档的目标读者包括有经验的C语言程序员和C语言翻译系统的实现者。它并非一个入门教程，而是为专业人员提供指导，帮助他们编写出可在不同环境下的稳定且高效的代码。接收草案的人被邀请提交他们的意见和建议，同时告知他们所知的相关专利信息，并提供支持文档，以便于标准的修订和完善。与前一版本C17相比，C2x标准引入了多项更新和改进，这些变化通过文档中的注释以删除线形式表示，以反映技术演进和最佳实践的变迁。对于C语言开发者和标准维护者来说，理解和遵循ISO-IEC 9899-202x至关重要，因为它不仅规定了语言规范，还为确保代码的互操作性和长期兼容性提供了准则。遵守这一标准有助于提高软件开发的质量和效率，尤其是在跨平台环境下。

N2479 working draft — February 5, 2020 ISO/IEC 9899:202x (E)

INTERNATIONAL STANDARD ©ISO/IEC ISO/IEC 9899:yyyy

Programming languages — C

1. Scope

This document speciﬁes the form and establishes the interpretation of programs written in the C

programming language.

It speciﬁes

— the representation of C programs;

— the syntax and constraints of the C language;

— the semantic rules for interpreting C programs;

— the representation of input data to be processed by C programs;

— the representation of output data produced by C programs;

— the restrictions and limits imposed by a conforming implementation of C.

2 This document does not specify

— the mechanism by which C programs are transformed for use by a data-processing system;

— the mechanism by which C programs are invoked for use by a data-processing system;

— the mechanism by which input data are transformed for use by a C program;

— the mechanism by which output data are transformed after being produced by a C program;

—

the size or complexity of a program and its data that will exceed the capacity of any speciﬁc

data-processing system or the capacity of a particular processor;

—

all minimal requirements of a data-processing system that is capable of supporting a conform-

ing implementation.

This document is designed to promote the portability of C programs among a variety of data-processing systems. It is

intended for use by implementors and programmers. Annex J gives an overview of portability issues that a C program might

encounter.

§ 1 General 1

N2479 working draft — February 5, 2020 ISO/IEC 9899:202x (E)

3. Terms, deﬁnitions, and symbols

For the purposes of this document, the terms and deﬁnitions given in ISO/IEC 2382, ISO 80000–2,

and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at http://www.electropedia.org/

Additional terms are deﬁned where they appear in italic type or on the left side of a syntax rule.

Terms explicitly deﬁned in this document are not to be presumed to refer implicitly to similar terms

deﬁned elsewhere.

3.1

1 access (verb)

⟨execution-time action⟩ to read or modify the value of an object

2 Note 1 to entry: Where only one of these two actions is meant, “read” or “modify” is used.

3 Note 2 to entry: “Modify” includes the case where the new value being stored is the same as the previous value.

4 Note 3 to entry: Expressions that are not evaluated do not access objects.

3.2

1 alignment

requirement that objects of a particular type be located on storage boundaries with addresses that

are particular multiples of a byte address

3.3

1 argument

actual argument

DEPRECATED: actual parameter

expression in the comma-separated list bounded by the parentheses in a function call expression, or

a sequence of preprocessing tokens in the comma-separated list bounded by the parentheses in a

function-like macro invocation

3.4

1 behavior

external appearance or action

3.4.1

1 implementation-deﬁned behavior

unspeciﬁed behavior where each implementation documents how the choice is made

2 Note 1 to entry: J.3 gives an overview over properties of C programs that lead to implementation-deﬁned behavior.

3 EXAMPLE

An example of implementation-deﬁned behavior is the propagation of the high-order bit when a signed integer

is shifted right.

3.4.2

1 locale-speciﬁc behavior

behavior that depends on local conventions of nationality, culture, and language that each implemen-

tation documents

§ 3.4.2 General 3

ISO/IEC 9899:202x (E) working draft — February 5, 2020 N2479

2 Note 1 to entry: J.4 gives an overview over properties of C programs that lead to locale-speciﬁc behavior.

3 EXAMPLE

An example of locale-speciﬁc behavior is whether the

islower

function returns true for characters other than

the 26 lowercase Latin letters.

3.4.3

1 undeﬁned behavior

behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which

this document imposes no requirements

2 Note 1 to entry:

Possible undeﬁned behavior ranges from ignoring the situation completely with unpredictable results,

to behaving during translation or program execution in a documented manner characteristic of the environment (with or

without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic

message).

3 Note 2 to entry: J.2 gives an overview over properties of C programs that lead to undeﬁned behavior.

4 EXAMPLE An example of undeﬁned behavior is the behavior on integer overﬂow.

3.4.4

1 unspeciﬁed behavior

behavior, that results from the use of an unspeciﬁed value, or other behavior upon which this

document provides two or more possibilities and imposes no further requirements on which is

chosen in any instance

2 Note 1 to entry: J.1 gives an overview over properties of C programs that lead to unspeciﬁed behavior.

3 EXAMPLE An example of unspeciﬁed behavior is the order in which the arguments to a function are evaluated.

3.5

1 bit

unit of data storage in the execution environment large enough to hold an object that can have one

of two values

2 Note 1 to entry: It need not be possible to express the address of each individual bit of an object.

3.6

1 byte

addressable unit of data storage large enough to hold any member of the basic character set of the

execution environment

2 Note 1 to entry: It is possible to express the address of each individual byte of an object uniquely.

3 Note 2 to entry:

A byte is composed of a contiguous sequence of bits, the number of which is implementation-deﬁned. The

least signiﬁcant bit is called the low-order bit; the most signiﬁcant bit is called the high-order bit.

3.7

1 character

⟨abstract⟩ member of a set of elements used for the organization, control, or representation of data

3.7.1

1 character

single-byte character

⟨C⟩ bit representation that ﬁts in a byte

3.7.2

1 multibyte character

sequence of one or more bytes representing a member of the extended character set of either the

source or the execution environment

2 Note 1 to entry: The extended character set is a superset of the basic character set.

4 General § 3.7.2

N2479 working draft — February 5, 2020 ISO/IEC 9899:202x (E)

3.7.3

1 wide character

value representable by an object of type

wchar

, capable of representing any character in the

current locale

3.8

1 constraint

restriction, either syntactic or semantic, by which the exposition of language elements is to be

interpreted

3.9

1 correctly rounded result

representation in the result format that is nearest in value, subject to the current rounding mode, to

what the result would be given unlimited range and precision

2 Note 1 to entry:

In this document, when the words “correctly rounded” are not immediately followed by “result”, this is the

intended usage.

3.10

1 diagnostic message

message belonging to an implementation-deﬁned subset of the implementation’s message output

3.11

1 forward reference

reference to a later subclause of this document that contains additional information relevant to this

subclause

3.12

1 implementation

particular set of software, running in a particular translation environment under particular con-

trol options, that performs translation of programs for, and supports execution of functions in, a

particular execution environment

3.13

1 implementation limit

restriction imposed upon programs by the implementation

3.14

1 memory location

either an object of scalar type, or a maximal sequence of adjacent bit-ﬁelds all having nonzero width

2 Note 1 to entry:

Two threads of execution can update and access separate memory locations without interfering with each

other.

3 Note 2 to entry:

A bit-ﬁeld and an adjacent non-bit-ﬁeld member are in separate memory locations. The same applies to

two bit-ﬁelds, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a

zero-length bit-ﬁeld declaration, or if they are separated by a non-bit-ﬁeld member declaration. It is not safe to concurrently

update two non-atomic bit-ﬁelds in the same structure if all members declared between them are also (nonzero-length)

bit-ﬁelds, no matter what the sizes of those intervening bit-ﬁelds happen to be.

4 EXAMPLE A structure declared as

struct {

char a;

int b:5, c:11,:0, d:8;

struct { int ee:8; } e;

}

§ 3.14 General 5

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