In programming, a library is a collection of precompiled routines that are available for use by programs. The use of libraries makes programs more modular. Many programming languages come with their own standard libraries. The standard library contains standard, tested functions available within the programming language. The programmer must know the name of the function, the arguments or parameters required by the function, and the data type of the result returned (if any).

In addition to using the standard library, programmers can create their own libraries of common functions and use libraries developed by third-party vendors. Before writing a new function, programmers should determine whether a library function is already available. This saves programmers time since library functions have already been tested. When a library function is used within a program, the programmer must provide the name of the library to the linker to resolve the function name. Some linkers automatically search the standard library for the names of functions not specified elsewhere.