Question

I have to transfer a number of elements of type:
typedef struct
{
float w;
int a, b;
} edge;
to different processes, hence I am creating an MPI derived type like this:

unsigned int typecount;
MPI_Datatype PEDGE, types[2] = { MPI_FLOAT, MPI_INT };
MPI_Aint offsets[2], extent;
int blocklen[2] = { 1, 2 };

typecount     = 2;
offsets[0]    = 0;
MPI_Type_extent(MPI_FLOAT, &extent);
offsets[1]    = (1*extent);
MPI_Type_struct (typecount, blocklen, offsets, types, &PEDGE);
MPI_Type_commit(&PEDGE);

When I do a sizeof(edge) I get 12 bytes, but I am getting only 8 bytes when I do sizeof(PEDGE)…why is that? Apart from this, my code for sending some elements of PEDGE type to arrays of edge type are failing, probably because of this mismatch.

Answer 1

The problem here is that an MPI_Datatype object such as PEDGE is not itself the new datatype, merely an opaque handle to some implementation-specific entity that MPI can interpret as a datatype. As such, sizeof() will not be able to return its accurate size. Use MPI_Type_size() instead.

As for the sends failing, I can’t say much without seeing your code, but your datatype definition does look correct.