In programming and software design, a binding is an application programming interface (API) that provides glue code specifically made to allow a programming language to use a foreign library or operating system service (one that is not native to that language).
Binding generally refers to a mapping of one thing to another. In the context of software libraries, bindings are wrapper libraries that bridge two programming languages, so that a library written for one language can be used in another language. Many software libraries are written in system programming languages such as C or C++. To use such libraries from another language, usually of higher-level, such as Java, Common Lisp, Scheme, Python, or Lua, a binding to the library must be created in that language, possibly requiring recompiling the language's code, depending on the amount of modification needed. However, most languages offer a foreign function interface, such as Python's and OCaml's <code>ctypes</code>, and Embeddable Common Lisp's <code>cffi</code> and <code>uffi</code>.
For example, Python bindings are used when an extant C library, written for some purpose, is to be used from Python. Another example is <code>libsvn</code> which is written in C to provide an API to access the Subversion software repository. To access Subversion from within Java code, <code>libsvnjavahl</code> can be used, which depends on <code>libsvn</code> being installed and acts as a bridge between the language Java and <code>libsvn</code>, thus providing an API that invokes functions from <code>libsvn</code> to do the work.
Major motives to create library bindings include software reuse, to reduce reimplementing a library in several languages, and the difficulty of implementing some algorithms efficiently in some high-level languages.