(MPI) Segmentation fault with dynamic allocated 2D array

[lancer6238]

Hi all,
I'm getting segmentation fault errors when I try to send/receive 2 rows of a matrix as a block, and I can't figure out why.

Basically, I have a 4x5 matrix with an extra top row and an extra leftmost column, making it a (4+1)x(5+1) matrix stored in P0. I'm trying to send the 2nd and 3rd row as a block to P1, print out the contents, send the 4th and 5th row as a block to P1, then print out the contents again. I have dynamically allocated a (4+1)x(5+1) memory block to cellblock, then used the pointers cell[i] to point to each "row" of cellblock, so I can access the 2D array by cell[i][j].

I get the correct output, but with segmentation fault at the end.

Here is my code:

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

int main(int argc, char** argv)
{
   int rank, i, j, k = 1;
   int **cell, *row1, *cellblock;
   MPI_Status status;
   MPI_Init(&argc, &argv);
   MPI_Comm_rank(MPI_COMM_WORLD, &rank);
   row1 = (int*)calloc((5+1) * 2, sizeof(int));
   cellblock = (int*)calloc((4+1)*(5+1), sizeof(int));
   cell = &cellblock;
   for (i = 0 ; i < (4+1) ; i++)
   	cell[i] = &cellblock[i*(5+1)];
   if (rank == 0)
   {
   	for (i = 0 ; i < (4+1) ; i++)
      	      for (j = 0 ; j < (5+1) ; j++)
                   cell[i][j] = k++;
        for (i = 1 ; i < 5 ; i+=2)
	      MPI_Send(&(cell[i][0]), (5+1)*2, MPI_INT, 1, 1, MPI_COMM_WORLD);
   }
   if (rank == 1)
   {
   	for (i = 0 ; i < 2 ; i++)
        {
      	    MPI_Recv(row1, (5+1)*2, MPI_INT, 0, 1, MPI_COMM_WORLD, &status);
            for (j = 0 ; j < (5+1)*2 ; j++)
         	printf("%d ", row1[j]);
            printf("\n");
       }
   }
   free(row1);
   free(cellblock);
   MPI_Finalize();
   return 0;
}

Please advise. Thank you.

Regards,
Rayne

[dargueta]

You have several problems with your code that you need to fix.

(1) You're allocating your matrix wrong. Do this:

int **matrix = (int **)calloc(numberOfRows,sizeof(int *));
for(int i = 0; i < numberOfColumns; ++i)
    matrix[i] = (int *)calloc(numberOfColumns,sizeof(int));

(2) The reason why you're getting a segmentation fault is because you're freein part of an array and then trying to free the entire array. That's a no-no. Contrary to what you might believe, cell and cellblock are actually pointing to the same memory.

(3) I have no idea what you're doing with the for loop in lines 15-16, but it appears that you're copying values in an array on top of themselves. Again, I think it's because you think that cell and cellblock are different. They're not.

(4) Try putting comments in your code, because as of right now, I have no idea what's going on in your if(rank==...) section. That might also be a part of the problem.