Trying to access array element in loop causes segmentation fault, why?

Question

I am trying to create a two dimensional array, which has either 1 or 0 randomly assigned to each coordinate. It works just fine until it gets to the coordinates [20][3]. After that it just throws out "segmentation fault 11".

I am absolutely clueless how or why. Especially since I can create a matrix with 200 * 200 for instance but it still gets the same Problem only at the coordinates [200][3]. So it is somehow always the third y coordinate in the last x coordinate where the error occurs.

#include <stdio.h>
#include <stdlib.h>

int main() {

  int x, y, i, j ;
  x = 20;
  y = 20;

  int grid [x][y];

  for ( i = 0; i <= x; i++) {
    for ( j = 0; j <= y; j++) {

      grid[i][j] = rand() % 2 ;

      printf("grid [%d][%d]: %d\n", i, j, grid[i][j]);

    }
  }
  return 0;
}

There is an error. Your index can only reach 19, you have to change the loops: for ( i = 0; i < x; i++) { // Note the "<" instead "<=" for ( j = 0; j < y; j++) { — Alexi
– Alexi, Commented Sep 5, 2016 at 14:35

Sourav Ghosh · Accepted Answer · 2016-09-05 14:41:18Z

2

C uses 0-based indexing for arrays. So, for an array defined as

int grid [x][y]

looping for

 for ( i = 0; i <= x; i++) 
   for ( j = 0; j <= y; j++)

if off-by-one. (Note the <= part).

to elaborate, for an array of dimension p, the valid indexes are 0 to p-1, inclusive.

You should change your loop conditions as i < x and j < y to stay withing the bounds. Accessing out of bound memory causes undefined behavior.

That said,

int main() should be int main(void), at least, to conform to C standards for hosted environments.
There is no need to make grid as VLA here. If the dimensions are already known, better approach is to use a compile-time constant (#define) to generate the array dimensions.

edited Sep 5, 2016 at 14:41

answered Sep 5, 2016 at 14:34

Sourav Ghosh

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Comments

Bathsheba · Accepted Answer · 2016-09-05 14:34:39Z

2

You're running past the bounds of the array. That's undefined behaviour in C, and is manifesting itself as a crash.

Change i <= x to i < x etc, or increase the grid size.

answered Sep 5, 2016 at 14:34

Bathsheba

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