C语言标准库详解

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"C标准库.pdf" C标准库是C编程语言不可或缺的一部分，它提供了一系列预定义的函数和数据类型，方便程序员进行系统级和应用程序级别的开发。这份文档详细介绍了C语言的标准库，包括了所有由ANSI（美国国家标准学会）和ISO（国际标准化组织）在C语言标准中规定的函数和头文件。头文件是C标准库的核心组成部分，它们包含了各种函数声明和宏定义。例如： - `<stdio.h>`：这是输入/输出操作的基础，提供了如`printf`、`scanf`等基本的I/O功能。 - `<stdlib.h>`：包含了通用的实用函数，如内存分配`malloc`、释放`free`，以及随机数生成`rand`等。 - `<string.h>`：用于字符串处理，如复制`strcpy`、比较`strcmp`和填充`memset`等。 - `<math.h>`：提供了数学运算，如平方根`sqrt`、指数`pow`和三角函数`sin`, `cos`等。 - `<time.h>`：处理时间和日期，包含`time`函数获取当前时间，以及`strftime`格式化时间等。 - `<float.h>`：定义了浮点数相关的常量，如最大值、最小值和精度等。 - `<stdarg.h>`：支持可变参数列表的函数，如`printf`家族的变体`vprintf`。 - `<setjmp.h>`：提供了非局部跳转功能，用于实现异常处理和递归调用的退出。 - `<signal.h>`：处理程序信号，如设置信号处理器`signal`和暂停接收信号`pause`。 - `<errno.h>`：包含了错误码，允许程序获取和设置错误信息。 - `<locale.h>`：支持多语言环境，如货币、日期格式的本地化。书中详细展示了如何使用这些库函数，并且通过约9,000行经过测试的代码，演示了它们在不同计算机架构上的高度可移植性。特别强调了C标准中新增的功能，如对多语种环境（locale）的支持和大字符集（如汉字 Kanji）的处理，这些都是现代C编程中重要的国际化和本地化特性。这本书不仅适合初学者了解C标准库的使用方法，也对有经验的开发者具有很高的参考价值，因为它揭示了库函数的内部工作原理，有助于深入理解C语言的本质和机制。通过阅读和实践书中的示例，程序员可以更有效地利用C标准库，提高代码的可靠性和效率。

Preface

Finally,

happily acknowledge the contributions made by my family. My

son, Geoffrey, helped with the layout and typographic design of this book.

My wife, Tana, provided much

needed moral and logistical support over

many long months. They, more than anybody, kept this project fun for me.

P.J.

Plauger

Bondi, New South Wales

Chapter

Introduction

Background

libra

is a collection of program components that can be reused in

many programs. Most programming languages include some form of

library. The programming language

is no exception. It began accreting

useful

functions

right from the start. These functions help you classify

characters, manipulate character strings, read input, and write output

to name just a few categories of services.

few

You must

declare

a typical function before you use it in a program. The

definitions

easiest way to do so is to incorporate into the program a

header

that declares

all the library functions in a given category.

header can also define any

associated

type definitions

and

macros.

A header is as much a part of the

library

the functions themselves. Most often, a header is a

text file

just

like the you write to make a program.

You use the

#include

directive in a

source file to make a header part

of the

translation unit.

For example, the header

<stdio.

declares functions

that perform input and output. A program that prints a simple message

with the function

printf

consists of the single

source file:

a simple test program

#include <stdio

int main (void)

{

say hello

printf

(

"Hello\n"

)

;

return

(0);

translator

converts each translation unit to an

object module,

a form

suitable for use with a given

computer architecture

(or

machine).

linker

combines all the object modules that make up a program. It incorporates

any object modules you use from the

library

well. The most popular

form of translator is a

compilw.

It produces an

executable file.

Ideally at least,

an executable file contains only those object modules from the library that

contain functions actually used by the program. That way, the program

suffers no size penalty as the

library grows more extensive. (Another

form of translator is an

interpreter.

It may include the entire

library as part

of the program that interprets your program.)

Chapter

making a

You can construct your own libraries. Atypical

compiler has a

librarian,

library

a program that assembles a library from the object modules you specify.

The linker knows

select from

any

library only the object modules used

by the program. The

library is not a special case.

You can write part or all of a library in

The translation unit you write

to make a library object module is not that unusual:

Alibrary object module should contain no definition of the

functionmain

with external linkage. Aprogrammer is unlikely to reuse code that insists

on taking control at program startup.

The object module should contain only functions that are easy to declare

and use. Provide a header that declares the functions and defines any

associated types and

maaos.

Most important, a library object module should be usable in a variety of

contexts. Writing code that is highly reusable is a skill you develop only

with practice and by studying successful libraries.

After you have read this book, you should be comfortable designing,

writing, and constructing specialized libraries in

the

The

library itself is typically written in

That is often

not

the case

library

with other programming languages. Earlier languages had libraries writ

ten in

assembly language.

Different computer architectures have different

assembly languages. To move the library to another computer architecture,

you had to rewrite it completely.

lets you write powerful and efficient

code that is also highly

portable.

You can move portable code simply by

translating it with a different

translator.

Here, for example, is the library function

strlen,

declared in

<string.

h>.

The function returns the length of a null

terminated string. Its pointer

argument points to the first element of the string:

strlen

function

#include

<string.h>

size-t

(strlen)

(const

char

*s)

{

find

length

s[]

const

char

*sc;

for

(sc

*sc

'\O'

;

++sc)

return

(sc

;

strlen

is a small function, one fairly easy to write. It is also fairly easy to

write incorrectly in many small ways.

strlen

is widely used. You might

want to provide a special version tuned to a given computer architecture.

But you don't have to. This version is correct, portable, and reasonably

efficient.

Other

contemporary languages cannot be used to write significant

portions of their own libraries. You cannot, for example, write the Pascal

library function

writeln

in portable Pascal. By contrast, you

can

write the

Introduction

equivalent C library function

printf

in portable C. The comparison is a bit

unfair because C type

checking is weaker. Nevertheless, the underlying

point is significant

the C library has been expressible from its earliest

beginnings almost completely in C.

nonpatable

Sometimes the code for a library function cannot be written in

portable

code

C. The code you write in

may work for a large class of computer

architectures, but not all. In such a case, the important thing is to document

clearly the nonportable portions that may have to change. You should also

isolate

nonportable code

much as possible. Even nonportable C code is

easier to write, debug, and maintain than assembly language. You write

assembly language only where it is unavoidable. Those places are few and

far between in the C library.

This book shows you how to use the C library in its current, standardized

form. Along the way, it also shows you how to write the

library in C. That

can help you understand how the library works. And it illustrates many

aspects of designing and writing a nontrivial library in

What the

Standard Says

Dennis Ritchie developed the original version of the programming

IanguageC at AT&T Bell Laboratories in the early 1970s. At first it appeared

to be little more than a

UNIX-specific system

implementation language for

the DEC PDP

11 computer architecture. Others soon discovered, however,

that it modeled a broad class of modern computers rather well. By the late

1970s, several other compiler writers had implemented C for a variety of

popular targets, from microcomputers to mainframes. By the early

1980s,

hundreds of implementations of C were being used by a rapidly growing

community of programmers. It was time to standardize the language.

standards

The American National Standards Institute, or ANSI, standardizes com-

for

puter programming languages in the United States. X3Jll is the name of

the ANSI

authorized committee that developed the standard for C, starting

in 1983. The language is now defined by ANSI Standard X3.159

1989.

The International Standards Organization, or

KO, C has a similar re

sponsibility in the international arena. IS0 formed the technical committee

JTCl/SC22/WG14 to review and augment the work of X3Jll. Currently,

IS0 has adopted a standard for C that is essentially identical to X3.159. It

is called

IS0 9899:1990. The C Standards differ only in format and in the

numbering of sections. The wording differs in a few small places but makes

no substantive change to the language definition.

I quote extensively from the

IS0 C Standard throughout this book. That

way you can see exactly what the

Standard says about every aspect of

the Standard C library. It is the final authority on what constitutes the

programming language. If you think my interpretation disagrees with the

C Standard, trust the C Standard.

may very well be wrong.

Chapter

string

letter

decimal

point

standard

headers

resewed

Identifiers

You will find the

Standard hard to read from time to time. Remember

that it is cast intentionally in a kind of legalese.

standard must be precise

and accurate first. Readability comes a distant second. The document is not

intended to be tutorial.

X3Jll

also produced a Rationale to accompany the

Standard. If you are curious about why

X3Jll

made certain decisions, go

read that document. It might help. I emphasize, however, that the Rationale

is also not a tutorial on the

language.

Here are two quotes from the

IS0

Standard. The first quote introduces

the Library section of the

Standard. It provides a few definitions and lays

down several important ground rules that affect the library

a whole.

Library

7.1 Introduction

7.1.1 Definitions of terms

string

is a contiguous sequence of characters terminated by and including the fust null

character.

AUpointer to

a string is a pointer to its initial (lowest addressed) character. The

length

of a string is the number of characters preceding the null character and its

value

is the sequence

of the values of the contained characters, in order.

letter

is a printing character in the execution character set corresponding to any of the

required lowercase and uppercase letters in the source character set, listed in 5.2.1.

The

decimal

point character

is the character used by functions that convert floating

point

numbers to or from character sequences to denote the beginning of the fractional part of such

character It is represented in the text and examples by a period, but may be changed

by the

setlocale

function.

Forward references: character handling (7.3). the

setlocale

function (7.4.1.1).

7.1.2 Standard headers

Each library function is declared in a

header.89

whose contents are made available by the

#include

preprocessing directive. The header declares a set of related functions, plus any

necessary types and additional macros needed

to facilitate their use.

The standard headers are

If a file with the same name as one of the above

and >delimited sequences, not provided as

part

of the implementation. is placed in any of the standard places for a source file to be included.

the behavior is undefined.

Headers may be included in any order, each may be included more than once in a given

scope,

with no effect different from being included only once, except that the effect of including

<assert.

depends on the definition of

NDEBUG.

If used, a header shall be included outside

of any external declaration or definition, and it shall first be included before the first reference to

any of the functions or objects it declares, or to any of the types or macros it defines. However,

if the identifier is declared or defined in more than one header, the second and subsequent

associated headers may be included after the initial reference to the identifier. The program shall

not have any macros with names lexically identical to keywords currently

defmed prior to the

inclusion.

Forward references: diagnostics (7.2).

7.1.3 Reserved identifiers

Each header declares or defines all identifiers listed in its associated subclause, and optionally

declares or defines identifiers listed in its associated future library directions subclause and

identifiers which are always resewed either for any use or for use as file scope identifiers.

All identifiers that begin with an underscore and either an uppercase letter or another underscore

are always resewed for any use.

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