Library (computing)

Library (computing)

In computer science, a library is a collection of non-volatile resources used by programs on a computer, often to develop software. These may include configuration data, documentation, help data, message templates, pre-written code and subroutines, classes, values or type specifications. In IBM's OS/360 and its successors they are referred to as partitioned data sets.

Libraries contain code and data that provide services to independent programs. This encourages the sharing and changing of code and data in a modular fashion, and eases the distribution of the code and data. Library files are not executable programs. They are either static libraries that are merged with an executable when the executable is being compiled and linked, making them "statically linked", or they are dynamic libraries that are loaded by a dynamic linker while the executable is running, making them "dynamically linked". The dynamic linker may also allow an application to explicitly request that a module be loaded and to obtain references to routines in the module; this can be used to implement plug-ins.

Most compiled languages have a standard library although programmers can also create their own custom libraries. Most modern software systems provide libraries that implement the majority of system services. Such libraries have commoditized the services which a modern application requires. As such, most code used by modern applications is provided in these system libraries.

Libraries often contain a jump table of all the methods within it, known as entry points. Calls into the library use this table, looking up the location of the code in memory, then calling it. This introduces overhead in calling into the library, but the delay is so small as to be negligible.