Since the C language imposes little restriction on the use of function pointers, the task of call graph construction for C programs is far more dificult than that found in Fortran programs. From the experience of implementing a call graph extractor in the IMPACT compiler, we found the call graph construction problem has evolved into an interprocedural pointer analysis problem. In this paper, we report several interesting function pointer usage patterns found in the SPECint92 and SPECint95 benchmarks. They can be considered as critical issues in the design of a complete call graph extractor.