The GNU C Library Reference Manual Version 2.26

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The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are defined in a standard library, which you compile and link with your programs.
The GNU C Library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to GNU systems.
The purpose of this manual is to explain how to use the facilities of the GNU C Library. We have mentioned which features belong to which standards to help you identify things that are potentially non-portable to other systems. But the emphasis in this manual is not on strict portability.