A library function is a function that can be called by a program to perform some task, but it is not part of the program itself. Typically library functions are collected together into libraries, which comprise suites of functions that are loosely related in some way. An example might be a collection of functions that deal with dates and times and how they can be formatted or represented. Libraries save programmers the bother of writing code to do the same tasks time and time again; in short, libraries encourage code reuse.

The code the library comprises is usually in the form of "machine code" that the computer can understand rather than human-readable source code, although this is not always the case, especially with "open source" libraries like those from the GNU project.

There are two main types of libraries: static libraries that are read by the...