Extract 2D subarray using MPI_Type_vector()

Hi all. The following code (which I am typing in, excuse my typos please) is not working as expected.

Code:

#include <stdlib.h>
#include <mpi.h>
#include <stdio.h>

int main(int argc, char **argv){
double u[4][4] = {{1,2,3,4},{5,6,7,8},{9,10,11,12},{13,14,15,16}};
double *recv_buff;
int size, rank;
MPI_Datatype matrix;
MPI_Status status;

MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);

MPI_Type_vector(4,2,4, MPI_DOUBLE, &matrix);

if(rank ==0){
   for (i = 0; i < 4; i++){
      for (j = 0; j<4; j++){
          printf("u[%i][%j]=%4.3f\n", u[i][j]);
      }
   }
MPI_Send(u[0][0],1,matrix, 1,0, MPI_COMM_WORD);
}
if (rank == 1){
recv_buff = malloc(sizeof(double *) * 2);
for (i = 0; i<2; i++)
   recv_buff[i] = malloc(sizeof(double) * 2);

   MPI_Recv(recv_buff[0][0], 1, matrix, 0,0,MPI_COMM_WORLD, &status);
   for (i = 0; i < 2; i++){
      for (j = 0; j<2; j++){
          printf("recv_buff[%i][%j]=%4.3f\n", recv_buff[i][j]);
      }
   }
}

}

I was expecting the recv_buffer to contain the 4 top-left entries in the original grid, that is the values 1, 2, 5, 6

This code doesnt seem to be working for me. I can't seem to figure out the values for the stride, or the allocations, or something, that will give me those values. Am I forgetting something? It had occured to me maybe i am declaring the arrays wrong, or something, but I am out of ideas. Thanks

MPI_Send
> MPI_Send(u[0][0],1,matrix, 1,0, MPI_COMM_WORD);
You're passing the top-left element BY VALUE.

Do you even get a warning?
I get - error: incompatible type for argument 1 of ‘MPI_Send’

> MPI_Recv(recv_buff[0][0], 1, matrix, 0,0,MPI_COMM_WORLD, &status);
Again, passing by value rather than by reference.
Even worse, MPI_Recv will assume that the single pointer is to CONTIGUOUS memory. Your fancy 2D pointer allocation isn't.

Fix the send to be

Code:

MPI_Send(u,1,matrix, 1,0, MPI_COMM_WORD);

Fix the recv to be (for now)

Code:

double temp[16] = { 0.0 };
MPI_Recv(temp, 1, matrix, 0,0,MPI_COMM_WORLD, &status);
// now print out all 16 entries

When you're reliably receiving the values of interest in the first 4 entries, then you can think about how to allocate the memory for it.

Thanks for the suggestions, I will check into it monday when i get access back to the compiler/system.