[Mpi-forum] Question about MPI_Alloc_mem

Anthony Skjellum skjellum at gmail.com
Wed Oct 14 12:01:54 CDT 2020

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
skjellum at gmail.com
Cell: +1-205-807-4968
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