The author of the profiling library needs a way to provide some control over the behavior of the profiling library to the application program. Since profiling libraries will have quite different types of controls, there is no way to completely standardize the control calls. MPI does require the existence of a single library function MPI_Pcontrol, which has an undefined, variable-length argument list. This function doesn't do anything, but, just like the other MPI functions, can be redefined in order to provide whatever controls the profiling library needs.