(MPI) Segmentation fault with dynamic allocated 2D array
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:
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;
}
Then remove all the code that refers to arrays of ints and just prove that you can pass a buffer of characters around, effectively an MPI Hello World. Confirm that this cleanly exits.
Then be careful with your type casting whereever there is a void *, for instance the first argument to MPI_Send.
If you want to send 'ints' then change that to a
Code:
int *mySendData=&(cell[i][0]);
MPI_Send(mySendData,.....
and similarly on the read, this lets the compiler do type checking for you.
Also, run the thing under a debugger or use the core dump to find out where the thing is failing. Have you not closed the library down correctly? Is this a build issue? Is it compiler options or selection of libraries?
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