C99标准解析：编程语言C的理性说明

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“C99RationaleV5.10.pdf”是关于C语言标准C99的一份文档，提供了该标准的理据和解释，主要针对C99的规范、环境、语言特性和词法元素等方面进行详细阐述。 C99是C语言的一个重要版本，它在C90的基础上进行了多方面的改进和扩展。这份文档的目的是帮助开发者理解C99标准的设计决策和目的，以便更好地遵循标准编写代码。文档的组织结构清晰，分为多个章节，涵盖以下几个关键领域： 1. **范围**：这部分说明了C99标准适用的范围，包括语言、库以及它们的实现和使用场景。 2. **规范参考**：列出对理解和实现C99标准至关重要的其他标准或规范，如ISO/IEC 9899（C语言国际标准）。 3. **术语和定义**：定义了C99中使用的专业术语，这对于理解标准中的规定至关重要。 4. **符合性**：讨论了程序如何符合C99标准，包括不同级别的符合性类别以及对实现的要求。 5. **环境**：分为翻译环境和执行环境两部分，详细描述了编译和运行时环境的特点，如字符集、显示语义、信号处理和系统限制。 6. **语言**：深入解析C99的语言特性，包括标识符的作用域、链接性、命名空间、对象的存储持续时间、类型系统、类型表示、兼容类型以及转换规则等。 - **概念**：如标识符的范围和链接性，不同类型对象的存储持续时间，以及类型系统的基本概念。 - **转换**：详述了算术运算符的转换规则，包括复数类型和实数类型的处理，以及其他类型的转换。 7. **词法元素**：涵盖关键字、标识符、通用字符名称和常量等构成C语言源代码的基本单元。这份文档对于理解C99标准的细节，特别是新引入的特性，如复数类型、变量长度数组、内联函数、弱符号等，非常有帮助。通过阅读和学习，开发者可以编写出更高效、更健壮、更符合标准的C程序。同时，它也提供了对标准制定背后思考的洞察，有助于在实际编程中做出明智的选择。

Normative References

2. Normative References

Terms and Definitions

3. Terms and definitions

The definitions of object, bit, byte, and alignment reflect a strong consensus, reached after

considerable discussion, about the fundamental nature of the memory organization of a C

environment:

• All objects in C must be representable as a contiguous sequence of bytes, each of which 5

is at least 8 bits wide.

• A char whether signed or unsigned, occupies exactly one byte.

(Thus, for instance, on a machine with 36-bit words, a byte can be defined to consist of 9, 12, 18,

or 36 bits, these numbers being all the exact divisors of 36 which are not less than 8.) These

strictures codify the widespread presumption that any object can be treated as an array of 10

characters, the size of which is given by the sizeof operator with that object’s type as its

operand.

These definitions do not preclude “holes” in struct objects. Such holes are in fact often

mandated by alignment and packing requirements. The holes simply do not participate in

representing the composite value of an object. 15

The definition of object does not employ the notion of type. Thus an object has no type in and of

itself. However, since an object may only be designated by an lvalue (see §6.3.2.1), the phrase

“the type of an object” is taken to mean, here and in the Standard, “the type of the lvalue

designating this object,” and “the value of an object” means “the contents of the object

interpreted as a value of the type of the lvalue designating the object.” 20

The concepts of multibyte character, wide character, universal character, and extended

character have been added to C to support very large character sets (see §5.2.1 and §MSE.1).

The terms unspecified behavior, undefined behavior, and implementation-defined behavior are

used to categorize the result of writing programs whose properties the Standard does not, or

cannot, completely describe. The goal of adopting this categorization is to allow a certain 25

variety among implementations which permits quality of implementation to be an active force in

the marketplace as well as to allow certain popular extensions, without removing the cachet of

conformance to the Standard. Informative Annex J of the Standard catalogs those behaviors

which fall into one of these three categories.

Unspecified behavior gives the implementor some latitude in translating programs. This latitude 30

does not extend as far as failing to translate the program, however, because all possible behaviors

are “correct” in the sense that they don’t cause undefined behavior in any implementation.

Undefined behavior gives the implementor license not to catch certain program errors that are

difficult to diagnose. It also identifies areas of possible conforming language extension: the

implementor may augment the language by providing a definition of the officially undefined 35

behavior.

Conformance

4. Conformance

The three-fold definition of conformance is used to broaden the population of conforming

programs and distinguish between conforming programs using a single implementation and

portable conforming programs.

A strictly conforming program is another term for a maximally portable program. The goal is to 5

give the programmer a fighting chance to make powerful C programs that are also highly

portable, without seeming to demean perfectly useful C programs that happen not to be portable,

thus the adverb strictly.

By defining conforming implementations in terms of the programs they accept, the Standard

leaves open the door for a broad class of extensions as part of a conforming implementation. By 10

defining both conforming hosted and conforming freestanding implementations, the Standard

recognizes the use of C to write such programs as operating systems and ROM-based

applications, as well as more conventional hosted applications. Beyond this two-level scheme,

no additional subsetting is defined for C, since the C89 Committee felt strongly that too many

levels dilutes the effectiveness of a standard. 15

Conforming program is thus the most tolerant of all categories, since only one conforming

implementation need accept it. The primary limitation on this license is §5.1.1.3.

Diverse parts of the Standard comprise the “treaty” between programmers and implementors

regarding various name spaces: if the programmer follows the rules of the Standard the

implementation will not impose any further restrictions or surprises: 20

• A strictly conforming program can use only a restricted subset of the identifiers that

begin with underscore (§7.1.3). Identifiers and keywords are distinct (§6.4.1).

Otherwise, programmers can use whatever internal names they wish; a conforming

implementation is guaranteed not to use conflicting names of the form reserved for the

programmer. (Note, however, the class of identifiers which are identified in §7.26 as 25

possible future library names.)

• The external functions defined in, or called within, a portable program can be named

whatever the programmer wishes, as long as these names are distinct from the external

names defined by the Standard library (§7). External names in a maximally portable

program must be distinct within the first 31 characters (in C89, the first 6 characters 30

mapped into one case) (see §5.2.4.1 and §6.4.2).

• A maximally portable program cannot, of course, assume any language keywords other

than those defined in the Standard.

• Each function called within a maximally portable program must either be defined within

some source file of the program or else be a function in the Standard library. 35

One proposal long entertained by the C89 Committee was to mandate that each implementation

have a translation-time switch for turning off extensions and making a pure Standard-conforming

implementation. It was pointed out, however, that virtually every translation-time switch setting

effectively creates a different “implementation,” however close may be the effect of translating

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