“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 the GNU system.”
“The GNU C library includes several header files, each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header filestdio.hdeclares facilities for performing input and output, and the header filestring.hdeclares string processing utilities. The organization of this manual generally follows the same division as the header files.”
The manual for glibc on gnu.org:
HTML version with entirely on one web page.
HTML version with one web page per node.