This entire problem comes from the natural but mistaken idea that the MPI library should assign some default padding when creating a datatype. This is a bad idea because the alignment rules can depend on compiler switches or other compiler-specific choices; for example, some compilers allow the user to select between always 8 byte alignment, leading to structures that might have gaps but permit the uniform use of faster load operations or 4 byte alignment, but at a potential performance cost. In other words, even for a single compiler, there need not be a unique choice of extent, and hence this problem is insoluble. Further, it is not necessary - the user can either create the datatype with the appropriate extent (by using MPI_UB) or resize it appropriately (with MPI_TYPE_CREATE_RESIZED since MPI 2.0). Rather than continue to get this wrong, I recommend simply telling the user not to assume that the MPI library can always divine how the compiler is going to chose to pad a derived type or struct. We can add an advice to users here that explains the problem and recommends using the existing MPI features to handle this problem. This is better coding and will affect few users. Bill William Gropp Director, Parallel Computing Institute Deputy Director for Research Institute for Advanced Computing Applications and Technologies Paul and Cynthia Saylor Professor of Computer Science University of Illinois Urbana-Champaign