The Unix linker traditionally operates in one pass. The effect of this is that functions from libraries are only included in the image if they are needed at the time the library is scanned. When combined with weak symbols, or multiple definitions of the same function, this can cause odd (and unexpected) effects.
Consider, for instance, an implementation of MPI in which the Fortran binding is achieved by using wrapper functions on top of the C implementation. The author of the profile library then assumes that it is reasonable to provide profile functions only for the C binding, since Fortran will eventually call these, and the cost of the wrappers is assumed to be small. However, if the wrapper functions are not in the profiling library, then none of the profiled entry points will be undefined when the profiling library is called. Therefore none of the profiling code will be included in the image. When the standard MPI library is scanned, the Fortran wrappers will be resolved, and will also pull in the base versions of the MPI functions. The overall effect is that the code will link successfully, but will not be profiled.
To overcome this we must ensure that the Fortran wrapper functions are included in the profiling version of the library. We ensure that this is possible by requiring that these be separable from the rest of the base MPI library. This allows them to be extracted out of the base library and placed into the profiling library using the Unix ar command.