API shim

In computer programming, an API shim is a small library (computer science) that converts one API into another. They typically come about when the functions of one API become a part of another, larger, library. In these cases the older API can still be supported as a thin layer on top of the newer, and more general, code.

One example was the support of AppleTalk on Apple Macintosh computers during the brief period in which Apple Computer supported the OpenTransport networking system. Thousands of Mac programs were based on the universal AppleTalk protocol, in order to support these programs AppleTalk was re-implemented as an OpenTransport stack , and then re-implemented as an API shim on top of this new library.

A less successful example occurred when Silicon Graphics repeatedly attempted to merge Open Inventor with OpenGL Performer. The later was an offshoot project from people involved in the former, and after a few years of parallel development it became clear there was no reason they could not be re-merged. This led to Cosmo 3D, a universal higher-level library over OpenGL, with Inventor and Performer re-implemented as API shims on top. However internal politics and a variety of failed industry partnerships doomed the project as it was handed first to the OpenGL plus plus group, and then the Fahrenheit graphics API project, before being cancelled outright in 2000. Today Inventor and Performer remain separate projects, neither one well supported.