Even simpler than this, we could just forward all calls to the MPI_Count interface (see below). The int count argument should type convert to MPI_Count without issue. Note that it still needs to be a function-like macro so that function pointers work. Don't give up yet! :D ~Jim. #include <stdio.h> typedef int MPI_Datatype; typedef int MPI_Comm; int MPI_Send(const void* buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm) { printf("MPI_Send(count = %d)\n", count); return 0; } int MPI_Send_x(const void* buf, long long count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm) { printf("MPI_Send_x(count = %lld)\n", count); return 0; } #define MPI_Send(buf, count, datatype, dest, tag, comm) MPI_Send_x(buf, count, datatype, dest, tag, comm) int main(int argc, char *argv[]) { /* 8589934592LL == 2^33 */ long long i = 8589934592LL + 11; int ret; int (*snd_ptr)(const void*, int, MPI_Datatype, int, int, MPI_Comm) = &MPI_Send; ret = MPI_Send(NULL, i, 0, 0, 0, 0); ret = MPI_Send(NULL, 5, 0, 0, 0, 0); ret = (*snd_ptr)(NULL, i, 0, 0, 0, 0); ret = (*snd_ptr)(NULL, 5, 0, 0, 0, 0); return 0; } MPI_Send_x(count = 8589934603) MPI_Send_x(count = 5) MPI_Send(count = 11) MPI_Send(count = 5)