Structures in MPI

In MPI we can exchange struct type data. We can do this by first creating a new MPI data type (MPI_Datatype), using MPI_Type_create_struct to which we specify the number of fields of each type and their size in the structure, the placement of the fields in memory (using *offsets * and MPI_Aint).

The signature of the MPI_Type_create_struct function is as follows:

int MPI_Type_create_struct(int count, const int array_of_blocklengths[], const MPI_Aint array_of_displacements[], const MPI_Datatype array_of_types[], MPI_Datatype *newtype)

And it has the following parameters:

• count (↓) - number of blocks (also number of entries in array_of_types, array_of_displacements and array_of_blocklengths vectors)
• array_of_blocklengths[] (↓) - number of elements in each block
• MPI_Aint array_of_displacements[] (↓) - byte displacement for each block
• array_of_types[] (↓) - the type of elements in each block
• *newtype (↑) - the new data type

tip

The byte offset for each block is calculated using the offsetof function. More details on its use can be found here: tutorialspoint.

Example of use:

#include <mpi.h>
#include <stdio.h>
#include <stddef.h>
 
// the structure at the base of the custom type
typedef struct {
    float f1, f2;
    char c;
    int i[2];
} custom_type;
 
int main(int argc, char *argv[])  {
    int numtasks, rank, source = 0, dest = 1, tag = 1;
 
    custom_type t;
    MPI_Datatype customtype, oldtypes[4];
    int blockcounts[4];
    MPI_Aint offsets[4];
    MPI_Status status;
 
    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &numtasks);
 
    // field f1
    offsets[0] = offsetof(custom_type, f1);
    oldtypes[0] = MPI_FLOAT;
    blockcounts[0] = 1;
 
    // field f2
    offsets[1] = offsetof(custom_type, f2);
    oldtypes[1] = MPI_FLOAT;
    blockcounts[1] = 1;
 
    // field c
    offsets[2] = offsetof(custom_type, c);
    oldtypes[2] = MPI_CHAR;
    blockcounts[2] = 1;
 
    // field i
    offsets[3] = offsetof(custom_type, i);
    oldtypes[3] = MPI_INT;
    blockcounts[3] = 2;
 
    // define the new type and commit
    MPI_Type_create_struct(4, blockcounts, offsets, oldtypes, &customtype);
    MPI_Type_commit(&customtype);
 
    if (rank == 0) {
        t.f1 = 0.5;
        t.f2 = -1.2;
        t.c = 'a';
        t.i[0] = 0;
        t.i[1] = 1;
        MPI_Send(&t, 1, customtype, dest, tag, MPI_COMM_WORLD);
    } else if (rank == 1) {
        MPI_Recv(&t, 1, customtype, source, tag, MPI_COMM_WORLD, &status);
        printf("Received custom type with f1=%.1f, f2=%.1f, c=%c, i[0]=%d, i[1]=%d\n", t.f1, t.f2, t.c, t.i[0], t.i[1]);
    }
 
    // the new type is released when it is no longer used
    MPI_Type_free(&customtype);
 
    MPI_Finalize();
}