The most common C libraries are C standard library and the ISO and ANSI C standard provides the standard specification, which is largely shared among the libraries. The ANSI C standard library includes routines for file input and output, allocating memory, and working with common data such as mathematical functions, character stings, and time values.
Another common set of C libraries are those used for developing Unix and Unix-like systems, which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.
Since many programs have been written in C, there are a wide variety of other libraries available. Libraries are often written in C because C generates fast object code; programmers then create interfaces to the library so that the routines can be used from higher-level languages like Java, Perl, and Python.