The definition for MPI_Pack looks like this: int MPI_Pack(const void* inbuf, int incount, MPI_Datatype datatype, void *outbuf, int outsize, int *position, MPI_Comm comm) What happens if incount and datatype describe something larger than an int? Looks to me like 'outsize' and 'posistion' will overflow, but I'm hoping that I'm just missing something. Likewise, MPI_Unpack: int MPI_Unpack(const void* inbuf, int insize, int *position, void *outbuf, int outcount, MPI_Datatype datatype, MPI_Comm comm) ==rob -- Rob Latham Mathematics and Computer Science Division Argonne National Lab, IL USA