ANSI C standard library

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 implementation of C known as "ANSI C". Part of the resulting standard was a set of software libraries called the ANSI C standard library.

The ANSI C standard library consists of 18 C header files which can be included into a programmer's project with a single statement. Each header file contains one or more functions, function prototypes, data type definitions and macros. The contents of these header files follows.

Later revisions of the C standard have added several new required definitions to the library, notably the , adding a standard boolean type, and functions for dealing with Unicode in and . Support for these new extensions varies between implementations.

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 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 for other languages, and consquently 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.

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 very difficult to use.