Question

#include <stdio.h>
#define N 1024
int main(){
  int i, j;
  int a[N][N];
  int b[N][N];
  for (i=0;i<N;i++){
    a[i][i]=i;
    b[i][i]=i;
  }
  for (i=0;i<N;i++)
    for(j=0;j<N;j++)
    {
         printf("%d", a[i][j]);
         printf("%d", b[i][j]);
    }
  return 0;
}

This program is a reason of segmentation fault, but if I define N as 1023, program will work correctly. Why it happens?

Answer 1

You are overflowing the stack. 2 * 1024 * 1024 * sizeof(int) is a lot for most systems.

The simplest solution would be to make the arrays static.

static int a[N][N];
static int b[N][N];

Other methods:

• Make the arrays global (this is essentially the same as the above)

• Use malloc in a loop and of course remember to free

  int **a = malloc(N * sizeof *a);
  for (i = 0; i < N; i++)
      a[i] = malloc(N * sizeof *a[i]);