Segmentation fault by using MPI_Scatter and MPI_Gather - c++

Question

I was trying to calculate elementwise multiplication of matrix elements.
But I've got this error and don't know what to do.
===================================================================================
= BAD TERMINATION OF ONE OF YOUR APPLICATION PROCESSES
= PID 16855 RUNNING AT kevlinsky-PC
= EXIT CODE: 139
= CLEANING UP REMAINING PROCESSES
= YOU CAN IGNORE THE BELOW CLEANUP MESSAGES
===================================================================================
YOUR APPLICATION TERMINATED WITH THE EXIT STRING: Segmentation fault (signal 11)
This typically refers to a problem with your application.
Please see the FAQ page for debugging suggestions
The task was to split it between processes, calculate the result and return it to the zero process.
Code example:
#include <iostream>
#include <math.h>
#include "mpi.h"
int main(int argc, char *argv[]){
MPI_Init(&argc, &argv);
int rank, size;
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
const int n = 4;
int arrayA[n][n];
int arrayB[n][n];
for (int i = 0; i < n; i++){
for (int j = 0; j < n; i++) {
arrayA[i][j] = (rand() % 1000) - 500;
}
for (int j = 0; j < n; i++) {
arrayB[i][j] = (rand() % 1000) - 500;
}
}
int getbufA[n];
int getbufB[n];
int arrayC[n][n];
int bufC[n];
MPI_Scatter(&arrayA, n, MPI_INT, &getbufA, n, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Scatter(&arrayB, n, MPI_INT, &getbufB, n, MPI_INT, 0, MPI_COMM_WORLD);
for (int i = 0; i < n; i++) {
bufC[i] = getbufA[i] * getbufB[i];
}
MPI_Gather(&bufC, n, MPI_INT, &arrayC, n, MPI_INT, 0, MPI_COMM_WORLD);
if (rank == 0) {
printf("MATRIX C \n");
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
printf("%d ", arrayC[i][j]);
}
printf("\n");
}
}
MPI_Finalize();
}
Can someone help with this?

Answer 1

I think this is your error:
for (int j = 0; j < n; i++) {
arrayA[i][j] = (rand() % 1000) - 500;
}
You need j++ in this loop. And you have this error in two places. j is never incremented and stays 0, and i is incremented indefinitely (because the condition for the loop is based on j), so very soon you go out of bounds for the array, hence the segmentation fault.