MPI_Scatter a 2D array in other 2D arrays - c++

Question

I want to scatter a 2D array in other 2D arrays (one for each process) using this specific way of allocating memory.
int (*matrix)[cols] = malloc(sizeof *matrix* rows);
I keep getting this error:
One of the processes started by mpirun has exited with a nonzero exit
code. This typically indicates that the process finished in error.
If your process did not finish in error, be sure to include a "return
0" or "exit(0)" in your C code before exiting the application.
PID 7035 failed on node n0 (127.0.0.1) due to signal 11.
I think the problem is on scatter but I am new to parallel programming so if anyone knows what the issue is please help me.
Thanks in advance.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "mpi.h"
int main(int argc, char** argv) {
int my_rank;
int p;
int root;
int rows = 0;
int cols = 0;
int **matrix;
int i, j;
int local_rows;
int answer = 0;
int broke = 0;
MPI_Init(& argc, & argv);
MPI_Comm_rank(MPI_COMM_WORLD, & my_rank);
MPI_Comm_size(MPI_COMM_WORLD, & p);
if (my_rank == 0) {
do {
printf("Enter Dimensions NxN\n");
scanf("%d", & rows);
scanf("%d", & cols);
if (cols != rows) {
printf("Columns must be the same as rows,enter dimensions again.\n");
}
} while (rows != cols);
int (*matrix)[cols] = malloc(sizeof *matrix* rows);
printf("Fill array %dx%d\n", rows, cols);
for (i = 0; i < rows; i++) {
for (j = 0; j < cols; j++) {
scanf("%d",&matrix[i][j]);
}
}
printf("\n");
for (i = 0; i < rows; i++) {
for (j = 0; j < cols; j++) {
printf("%d ",matrix[i][j]);
}
printf("\n");
}
}
root = 0;
MPI_Bcast(&rows, 1, MPI_INT, root, MPI_COMM_WORLD);
MPI_Bcast(&cols, 1, MPI_INT, root, MPI_COMM_WORLD);
local_rows = rows / p;
int (*local_matrix)[rows] = malloc(sizeof *local_matrix* local_rows);
MPI_Scatter(matrix, local_rows*rows, MPI_INT,local_matrix, local_rows*rows, MPI_INT, 0, MPI_COMM_WORLD);
printf("\nLocal matrix fo the process %d is :\n", my_rank);
for (i = 0; i < local_rows; i++) {
for (j = 0; j < cols; j++) {
printf("%d ", local_matrix[i][j]);
}
printf("\n");
}
if (my_rank==0){
free(matrix);
free(local_matrix);
}
MPI_Finalize();
}

Answer 1

The problem with your code is that you declared two variables with the name matrix:
int **matrix;
and
int (*matrix)[cols] = malloc(sizeof *matrix* rows);
and since the latter was declared inside the if (my_rank == 0) {..} the variable begin used in the scatter MPI_Scatter(matrix, local_rows*rows, MPI_INT,local_matrix, local_rows*rows, MPI_INT, 0, MPI_COMM_WORLD);
is the first one, the not allocated one, and not the one you allocated space for. That is why you are getting the error.
Try this:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "mpi.h"
int main(int argc, char** argv) {
int my_rank;
int p;
int root;
int rows = 0;
int cols = 0;
int i, j;
int local_rows;
int answer = 0;
int broke = 0;
MPI_Init(& argc, & argv);
MPI_Comm_rank(MPI_COMM_WORLD, & my_rank);
MPI_Comm_size(MPI_COMM_WORLD, & p);
int (*matrix)[cols];
if (my_rank == 0) {
do {
printf("Enter Dimensions NxN\n");
scanf("%d", & rows);
scanf("%d", & cols);
if (cols != rows) {
printf("Columns must be the same as rows,enter dimensions again.\n");
}
} while (rows != cols);
matrix = malloc(sizeof *matrix * rows);
printf("Fill array %dx%d\n", rows, cols);
for (i = 0; i < rows; i++) {
for (j = 0; j < cols; j++) {
scanf("%d",&matrix[i][j]);
}
}
printf("\n");
for (i = 0; i < rows; i++) {
for (j = 0; j < cols; j++) {
printf("%d ",matrix[i][j]);
}
printf("\n");
}
}
root = 0;
MPI_Bcast(&rows, 1, MPI_INT, root, MPI_COMM_WORLD);
MPI_Bcast(&cols, 1, MPI_INT, root, MPI_COMM_WORLD);
local_rows = rows / p;
// Changed from the original
int (*local_matrix)[cols] = malloc(sizeof *local_matrix* local_rows);
printf("R = (%d, %d, %d) \n",my_rank, local_rows, cols);
if(my_rank == 0)
{
printf("\n");
for (i = 0; i < rows; i++) {
for (j = 0; j < cols; j++) {
printf("%d ",matrix[i][j]);
}
printf("\n");
}
}
MPI_Scatter(matrix, local_rows*cols, MPI_INT,local_matrix,
local_rows*cols, MPI_INT, 0, MPI_COMM_WORLD);
...
Btw I think you meant:
int (*local_matrix)[cols] = malloc(sizeof *local_matrix* local_rows);
and not
int (*local_matrix)[rows] = malloc(sizeof *local_matrix* local_rows);
Also do not forget to free the "local_matrix" for the slaves also.