C language knowledge (4) -- Library (C语言 基本知识4)

C language knowledge (4) -- Library (C语言 基本知识4)

C (programming language)

From Wikipedia, the free encyclopedia


The name and characteristic of each function are included into a computer file called a header file but the actual implementation of functions are separated into a library file. The naming and scope of headers have become common but the organization of libraries still remains diverse. The standard library is usually shipped along with a compiler. Since C compilers often provide extra functionalities that are not specified in ANSI C, a standard library with a particular compiler is mostly incompatible with standard libraries of other compilers.

Much of the C standard library has been shown to have been well-designed. A few parts, with the benefit of hindsight, are regarded as mistakes. The string input functions gets() (and the use of scanf() to read string input) are the source of many buffer overflows, and most programming guides recommend avoiding this usage. Another oddity is strtok(), a function that is designed as a primitive lexical analyser but is highly "fragile" and difficult to use.


The C programming language, before it was standardized, did not provide built-in functionalities such as I/O operations (unlike traditional languages such as Cobol and Fortran). Over time, user communities of C shared ideas and implementations of what is now called C standard libraries to provide that functionality. Many of these ideas were incorporated eventually into the definition of the standardized C programming language.

Both Unix and C were created at AT&T's Bell Laboratories in the late 1960s and early 1970s. During the 1970s the C programming language became increasingly popular. Many universities and organizations began creating their own variations of the language for their own projects. By the beginning of the 1980s compatibility problems between the various C implementations became apparent. In 1983 the American National Standards Institute (ANSI) formed a committee to establish a standard specification of C known as "ANSI C". This work culminated in the creation of the so-called C89 standard in 1989. Part of the resulting standard was a set of software libraries called the ANSI C standard library.

Later revisions of the C standard have added several new required header files to the library. Support for these new extensions varies between implementations.

The headers <iso646.h>, <wchar.h>, and <wctype.h> were added with Normative Addendum 1 (hereafter abbreviated as NA1), an addition to the C Standard ratified in 1995.

The headers <complex.h>, <fenv.h>, <inttypes.h>, <stdbool.h>, <stdint.h>, and <tgmath.h> were added with C99, a revision to the C Standard published in 1999.

ANSI Standard

The ANSI C standard library consists of 24 C header files which can be included into a programmer's project with a single directive. Each header file contains one or more function declarations, data type definitions and macros. The contents of these header files follows.

In comparison to some other languages (for example Java) the standard library is minuscule. The library provides a basic set of mathematical functions, string manipulation, type conversions, and file and console-based I/O. It does not include a standard set of "container types" like the C++ Standard Template Library, let alone the complete graphical user interface (GUI) toolkits, networking tools, and profusion of other functionality that Java provides as standard. The main advantage of the small standard library is that providing a working ANSI C environment is much easier than it is with other languages, and consequently porting C to a new platform is relatively easy.

Many other libraries have been developed to supply equivalent functionality to that provided by other languages in their standard library. For instance, the GNOME desktop environment project has developed the GTK+ graphics toolkit and GLib, a library of container data structures, and there are many other well-known examples. The variety of libraries available has meant that some superior toolkits have proven themselves through history. The considerable downside is that they often do not work particularly well together, programmers are often familiar with different sets of libraries, and a different set of them may be available on any particular platform.


ANSI C library header files

<assert.h> Contains the assert macro, used to assist with detecting logical errors and other types of bug in debugging versions of a program.
<complex.h> A set of functions for manipulating complex numbers. (New with C99)
<ctype.h> This header file contains functions used to classify characters by their types or to convert between upper and lower case in a way that is independent of the used character set (typically ASCII or one of its extensions, although implementations utilizing EBCDIC are also known).
<errno.h> For testing error codes reported by library functions.
<fenv.h> For controlling floating-point environment. (New with C99)
<float.h> Contains defined constants specifying the implementation-specific properties of the floating-point library, such as the minimum difference between two different floating-point numbers (_EPSILON), the maximum number of digits of accuracy (_DIG) and the range of numbers which can be represented (_MIN, _MAX).
<inttypes.h> For precise conversion between integer types. (New with C99)
<iso646.h> For programming in ISO 646 variant character sets. (New with NA1)
<limits.h> Contains defined constants specifying the implementation-specific properties of the integer types, such as the range of numbers which can be represented (_MIN, _MAX).
<locale.h> For setlocale() and related constants. This is used to choose an appropriate locale.
<math.h> For computing common mathematical functions
<setjmp.h> Declares the macros setjmp and longjmp, which are used for non-local exits
<signal.h> For controlling various exceptional conditions
<stdarg.h> For accessing a varying number of arguments passed to functions.
<stdbool.h> For a boolean data type. (New with C99)
<stdint.h> For defining various integer types. (New with C99)
<stddef.h> For defining several useful types and macros.
<stdio.h> Provides the core input and output capabilities of the C language. This file includes the venerable printf function.
<stdlib.h> For performing a variety of operations, including conversion, pseudo-random numbers, memory allocation, process control, environment, signalling, searching, and sorting.
<string.h> For manipulating several kinds of strings.
<tgmath.h> For type-generic mathematical functions. (New with C99)
<time.h> For converting between various time and date formats.
<wchar.h> For manipulating wide streams and several kinds of strings using wide characters - key to supporting a range of languages. (New with NA1)
<wctype.h> For classifying wide characters. (New with NA1)

The C standard library in C++

The C++ programming language includes the functionality of the ANSI C standard library, but makes several modifications, such as changing the names of the header files from <xxx.h> to <cxxx> (however, the C-style names are still available, although deprecated), and placing all identifiers into the std namespace.

Common support libraries

While not standardized, C programs may depend on a runtime library of routines which contain code the compiler uses at runtime. The code that initializes the process for the operating system, for example, before calling main(), is implemented in the C Run-Time Library for a given vendor's compiler. The Run-Time Library code might help with other language feature implementations, like handling uncaught exceptions or implementing floating point code.

The C standard library only documents that the specific routines mentioned in this article are available, and how they behave. Because the compiler implementation might depend on these additional implementation-level functions to be available, it is likely the vendor-specific routines are packaged with the C Standard Library in the same module, because they're both likely to be needed by any program built with their toolset.

Though often confused with the C Standard Library because of this packaging, the C Runtime Library is not a standardized part of the language and is vendor-specific.

Compiler built-in functions

Some compilers (for example, GCC[1]) provide built-in versions of many of the functions in the C standard library; that is, the implementations of the functions are written into the compiled object file, and the program calls the built-in versions instead of the functions in the C library shared object file. This reduces function call overhead, especially if function calls are replaced with inline variants, and allows other forms of optimisation (as the compiler knows the control-flow characteristics of the built-in variants), but may cause confusion when debugging (for example, the built-in versions cannot be replaced with instrumented variants).


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