C语言新标准草案N2596：ISO/IEC 9899:202x(E)解读

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"ISO∕IEC 9899-working draft：n2596 - C18后的C语言最新标准草案" ISO/IEC 9899是国际标准化组织和国际电工委员会联合制定的C语言标准，其工作草案n2596是自C18标准（2018年版）以来的最新版本。这份文档对于C语言的规范、解释及其执行库的内容进行了详细的规定，旨在提升C语言程序在各种计算系统上的可移植性、可靠性和维护性，同时保证高效的执行效率。 C语言标准分为多个条款，详细阐述了语言本身的各种特性，包括但不限于数据类型、控制结构、函数、预处理器等。此外，它还涵盖了C语言执行库的内容，如数学函数、输入输出操作等。附录部分则对这些内容进行了总结，并列举了影响C程序可移植性的因素，这对于开发者理解和优化代码至关重要。虽然这个标准草案是针对有经验的C语言程序员和C语言翻译系统的实现者，但文档本身并不作为初学者教程设计。因此，它包含了大量的技术细节和规定，需要读者具有一定的编程基础来理解和应用。为了确保标准的公正性和广泛性，接收此草案的个人或组织被邀请提交与之相关的专利权信息，以及支持性文档。这意味着标准的制定过程考虑到了知识产权的问题，以避免潜在的法律纠纷。 ISO/IEC 9899的每个新版本都反映了C语言的发展和改进，n2596草案可能包含了新的特性、修正了已有的问题，或者对原有规则进行了更明确的定义。对于C语言的开发者和研究者来说，跟踪学习这样的标准草案是非常有价值的，因为它能够帮助他们保持与时俱进，理解最新的编程实践和技术趋势。

N2596 working draft — December 11, 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

N2596 working draft — December 11, 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

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

§ 3.4.2 General 3

ISO/IEC 9899:202x (E) working draft — December 11, 2020 N2596

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 dereferencing a null pointer.

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

N2596 working draft — December 11, 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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