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C编程常见问题指南：Steve Summit著作精华

FAQs

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"C Programming FAQs: Frequently Asked Questions" 是一本由 Steve Summit 在 1995 年出版的专业书籍，由 Addison-Wesley 出版社发行，ISBN 为 0-201-84519-9。这本著作汇集了 C 语言编程中的常见问题和解答，旨在为程序员社区提供一个方便的服务。作者和出版社允许这些内容在此处免费访问，以便于个人学习，但未经书面许可不得进行出版或重新传播，以保护其版权。这本书的核心是 comp.lang.c 新闻组的 FAQ（Frequently Asked Questions）列表，这是一个长期存在的在线资源，它汇总了在 C 语言讨论区中最常被问到的问题。这些问题涵盖了从基础语法、数据类型、控制结构、函数、指针、内存管理到高级主题，如错误处理、文件操作和网络编程等各个方面。FAQ 的目的是减少重复提问，使讨论更专注于技术深度和创新性话题，使得它们成为程序员日常工作中不可或缺的参考文档。在 HTML 版本的 comp.lang.c FAQ 中，每一项问题都经过精心挑选和整理，确保答案准确且易于理解。这些内容不仅帮助初学者快速入门，也能让经验丰富的开发者不断更新他们的知识库，解决实际编程过程中遇到的难题。由于 FAQ 是基于广泛讨论和实践经验总结，因此它们不仅限于书本知识，而是反映了实际编程场景中的最佳实践和解决方案。 C Programming FAQs 是一个宝贵的资源库，对于学习者和开发者来说，无论是为了提高技能，还是查找快速答案，都是一个高效的学习和参考工具。理解和掌握这个 FAQ 列表中的知识，将有助于提升你在 C 语言编程领域的专业素养和解决问题的能力。

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Question 1.25

My compiler is complaining about an invalid redeclaration of a function, but I only define it once and

call it once.

Functions which are called without a declaration in scope (perhaps because the first call precedes the

function's definition) are assumed to be declared as returning int (and without any argument type

information), leading to discrepancies if the function is later declared or defined otherwise. Non-int

functions must be declared before they are called.

Another possible source of this problem is that the function has the same name as another one declared in

some header file.

See also questions

11.3 and 15.1.

References: K&R1 Sec. 4.2 p. 70

K&R2 Sec. 4.2 p. 72

ANSI Sec. 3.3.2.2

ISO Sec. 6.3.2.2

H&S Sec. 4.7 p. 101

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Question 1.30

What can I safely assume about the initial values of variables which are not explicitly initialized? If

global variables start out as ``zero,'' is that good enough for null pointers and floating-point zeroes?

Variables with static duration (that is, those declared outside of functions, and those declared with the

storage class static), are guaranteed initialized (just once, at program startup) to zero, as if the

programmer had typed ``= 0''. Therefore, such variables are initialized to the null pointer (of the correct

type; see also section

5) if they are pointers, and to 0.0 if they are floating-point.

Variables with automatic duration (i.e. local variables without the static storage class) start out

containing garbage, unless they are explicitly initialized. (Nothing useful can be predicted about the

garbage.)

Dynamically-allocated memory obtained with malloc and realloc is also likely to contain garbage,

and must be initialized by the calling program, as appropriate. Memory obtained with calloc is all-bits-

0, but this is not necessarily useful for pointer or floating-point values (see question

7.31, and section 5).

References: K&R1 Sec. 4.9 pp. 82-4

K&R2 Sec. 4.9 pp. 85-86

ANSI Sec. 3.5.7, Sec. 4.10.3.1, Sec. 4.10.5.3

ISO Sec. 6.5.7, Sec. 7.10.3.1, Sec. 7.10.5.3

H&S Sec. 4.2.8 pp. 72-3, Sec. 4.6 pp. 92-3, Sec. 4.6.2 pp. 94-5, Sec. 4.6.3 p. 96, Sec. 16.1 p. 386

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Question 1.31

This code, straight out of a book, isn't compiling:

f()

{

char a[] = "Hello, world!";

}

Perhaps you have a pre-ANSI compiler, which doesn't allow initialization of ``automatic aggregates'' (i.e.

non-static local arrays, structures, and unions). As a workaround, you can make the array global or

static (if you won't need a fresh copy during any subsequent calls), or replace it with a pointer (if the

array won't be written to). (You can always initialize local char * variables to point to string literals,

but see question

1.32.) If neither of these conditions hold, you'll have to initialize the array by hand with

strcpy when f is called. See also question

11.29.

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Question 1.32

What is the difference between these initializations?

char a[] = "string literal";

char *p = "string literal";

My program crashes if I try to assign a new value to p[i].

A string literal can be used in two slightly different ways. As an array initializer (as in the declaration of

char a[]), it specifies the initial values of the characters in that array. Anywhere else, it turns into an

unnamed, static array of characters, which may be stored in read-only memory, which is why you can't

safely modify it. In an expression context, the array is converted at once to a pointer, as usual (see section

6), so the second declaration initializes p to point to the unnamed array's first element.

(For compiling old code, some compilers have a switch controlling whether strings are writable or not.)

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