To be more specific as to the reason, this is the C idiom - it you pass it as a pointer to a pointer, you have to cast it as a void pointer to a pointer. If it is just a void *, C views this as an anonymous pointer, and you can use the address - as in
int *intp; MPI_Alloc_mem(1000, MPI_INFO_NULL, &intp); If baseptr is a void**, you need to do this: MPI_Alloc_mem(1000, MPI_INFO_NULL, (void **)&intp); I don’t like it either, but this is the choice in C. Bill William Gropp Director and Chief Scientist, NCSA Thomas M. Siebel Chair in Computer Science University of Illinois Urbana-Champaign > On Oct 14, 2020, at 12:01 PM, Anthony Skjellum via mpi-forum > <mpi-forum@lists.mpi-forum.org> wrote: > > Folks, I know we have had this function for a long time, and I've implemented > ports of MPI that actually use it (e.g., with pre-pinned memory). But, I am > trying to understand the logic for why baseptr is passed by value, instead of > by reference. In C, everything is by value, so the last argument in normal C > programs would be void **baseptr. > > The standard has: > int MPI_Alloc_mem(MPI_Aint size, MPI_Info info, void *baseptr); > Now, MPICC/GCC takes this with > void *memory = (void *)0; > int error = MPI_Alloc_mem(1024, MPI_INFO_NULL, &memory); > and you get the memory allocated properly. > What is more, this is incorrect: > int error = MPI_Alloc_mem(1024, MPI_INFO_NULL, memory); > although it compiles fine because that is, indeed, the API. > Why would we have the pointer going in by value, when it is coming out > as an OUT argument? > Isn't this plain wrong? void **baseptr means ---> I am passing you the > address of a void pointer. > Every viable implementation must do *(void **)baseptr = ... > when providing the "malloc'd/special" memory. So... why did we fudge > the C API. Is there some tie-in with the Fortran API? > Thanks in advance, > Tony Skjellum > > > -- > Anthony Skjellum, PhD > skjel...@gmail.com <mailto:skjel...@gmail.com> > Cell: +1-205-807-4968 > > > _______________________________________________ > mpi-forum mailing list > mpi-forum@lists.mpi-forum.org > https://lists.mpi-forum.org/mailman/listinfo/mpi-forum
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