How to send an array of pointers to arrays in C MPI - c

Question

So let's say I'm using MPI and I want to be able to send a number of rows of a matrix of integers from the main process to other processes. It's relatively easy to do so, like this:
MPI_Send(&matrix[start_row][0], amount_of_cells, MPI_INT, target_process, 1, MPI_COMM_WORLD);
Now let's say that in our matrix, instead of each cell holding an integer, each cell holds a reference to an integer array of size 2. How could we send a number of rows of the new matrix to subprocesses?
I was thinking of doing the same thing as the code above but doubling the amount_of_cells variable because each cell holds a reference to an integer array of size 2. However, it doesn't seem to work, I'm at a bit of a loss here.
Any tips or advice on how to approach this would be helpful?
Old matrix:
_________
| 1 | 2 |
--------
| 3 | 4 |
_________
New matrix:
___________________
| [1, 0] | [2, 0] |
--------------------
| [3, 0] | [4, 0] |
___________________
So instead of holding integers, each cell holds a reference to an array of size 2 of integers created using malloc(). How could I send rows of this new matrix to subprocesses?
I was considering defining my own MPI datatype which could be a place to start.

Answer 1

Please try this code,To send an array of pointers to arrays in C MPI
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define n 10
int a[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int a2[1000];
int main(int argc, char* argv[])
{
int pid, np,
elements_per_process,
n_elements_recieved;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &pid);
MPI_Comm_size(MPI_COMM_WORLD, &np);
if (pid == 0) {
int index, i;
elements_per_process = n / np;
if (np > 1) {
for (i = 1; i < np - 1; i++) {
index = i * elements_per_process;
MPI_Send(&elements_per_process,
1, MPI_INT, i, 0,
MPI_COMM_WORLD);
MPI_Send(&a[index],
elements_per_process,
MPI_INT, i, 0,
MPI_COMM_WORLD);
}
index = i * elements_per_process;
int elements_left = n - index;
MPI_Send(&elements_left,
1, MPI_INT,
i, 0,
MPI_COMM_WORLD);
MPI_Send(&a[index],
elements_left,
MPI_INT, i, 0,
MPI_COMM_WORLD);
}
int sum = 0;
for (i = 0; i < elements_per_process; i++)
sum += a[i];
int tmp;
for (i = 1; i < np; i++) {
MPI_Recv(&tmp, 1, MPI_INT,
MPI_ANY_SOURCE, 0,
MPI_COMM_WORLD,
&status);
int sender = status.MPI_SOURCE;
sum += tmp;
}
printf("Sum of array is : %d\n", sum);
}
else {
MPI_Recv(&n_elements_recieved,
1, MPI_INT, 0, 0,
MPI_COMM_WORLD,
&status);
MPI_Recv(&a2, n_elements_recieved,
MPI_INT, 0, 0,
MPI_COMM_WORLD,
&status);
int partial_sum = 0;
for (int i = 0; i < n_elements_recieved; i++)
partial_sum += a2[i];
MPI_Send(&partial_sum, 1, MPI_INT,
0, 0, MPI_COMM_WORLD);
}
MPI_Finalize();
return 0;
}
I hope this code will be usefull.
Thank you.