The tools library fptools defines many functions that aren't standard on all systems. Most of them do not differ in behavior from their originals, but might be slightly slower. But since they are usually only needed in non-speed-critical sections, the replacements are used throughout the library. For a full listing of the available replacement functions, see section 11.
However, there are two functions, strerror and tempnam, that aren't fully implemented. The replacement strerror does not have a table of error messages and only produces the error number as string, and the ``fake'' tempnam does not necessarily use a proper temp directory.
Because some functionality is missing, the replacement functions should only be used if the original is not available.