It is often desirable to modify the behavior of existing code bases by wrapping or replacing functions. When editing the source code of those functions is a viable option, this can be a straight-forward process. When the source of the functions cannot be edited (e.g., if the functions are provided by the system C library), then alternative techniques are required. Here, we present such techniques for UNIX, Windows, and Macintosh OS X platforms. We have used these techniques to update bioinformatics applications to call the application program interface (API) provided by the Berkeley Open Infrastructure for Network Computing (BOINC), a distributed computing toolkit.