Segmentation Fault, large arrays

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]);