MPI_Send works only with statically allocated buffer - c

Question

If I want to define my own type, and use it as a datatype with MPI_Send to take only even rows from a matrix, does that matrix (send buffer) have to be allocated statically?
I seem to have problems when I allocate it dynamically. Is this because addresses need to be successive for data to be sent?

Answer 1

No, memory to be sent with MPI_Send does not have to be statically allocated.
To send array subsets, you likely want to use MPI_Type_indexed. Here is a slightly modified version of the example from the mpi.deino.net article on MPI_Type_indexed, where I have replaced the statically allocated buffer
int buffer[27];
to a dynamically allocated buffer
int* buffer = (int*)malloc(27 * sizeof(int));
I hope it helps:
#include <mpi.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
int rank, size, i;
MPI_Datatype type, type2;
int blocklen[3] = { 2, 3, 1 };
int displacement[3] = { 0, 3, 8 };
int* buffer = (int*)malloc(27 * sizeof(int)); //int buffer[27];
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
if (size < 2)
{
printf("Please run with 2 processes.\n");
MPI_Finalize();
return 1;
}
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Type_contiguous(3, MPI_INT, &type2);
MPI_Type_commit(&type2);
MPI_Type_indexed(3, blocklen, displacement, type2, &type);
MPI_Type_commit(&type);
if (rank == 0)
{
for (i=0; i<27; i++)
buffer[i] = i;
MPI_Send(buffer, 1, type, 1, 123, MPI_COMM_WORLD);
}
if (rank == 1)
{
for (i=0; i<27; i++)
buffer[i] = -1;
MPI_Recv(buffer, 1, type, 0, 123, MPI_COMM_WORLD, &status);
for (i=0; i<27; i++)
printf("buffer[%d] = %d\n", i, buffer[i]);
fflush(stdout);
}
MPI_Finalize();
free(buffer);
return 0;
}