Segmentation fault on relatively small array

Question

I have an array populated with integers:

#include <stdio.h>

#define MAX 4000000

int main() {
  int primes[MAX + 1];

  // init array with all numbers
  for(int i = 1; i <= MAX; i++)
    primes[i] = i;

  return 0;
}

With 4 byte integers, it seems this should only use up 4M * 4 or 16 MB. But I'm getting a segmentation fault when I run this program. I don't understand why.

Tip: Don't allocate large structures on the stack, use dynamic allocation (e.g. `malloc` or `calloc`). — tadman, Mar 28 '20 at 01:19
I can't reproduce that here (macOS + clang), but it could be compiler dependent, or an artifact of your OS. — tadman, Mar 28 '20 at 01:20
The cause of your problem is stated in the address bar of your browser, right in between the `https://` and the `.com`. Also in the upper left corner of this web page. :-) — Nate Eldredge, Mar 28 '20 at 01:20
You should make minimal use of the stack in your program as the default is usually 8MB. Anything more and you get a literal *stack overflow*. `ulimit` limits apply. — tadman, Mar 28 '20 at 01:25
OK, making this an extern variable by moving it outside main did the trick. Thanks @tadman — StevieD, Mar 28 '20 at 01:26
That's not exactly a solution, either. Why not dynamically allocate it? — tadman, Mar 28 '20 at 01:27
Weak compiler puts `int primes[MAX + 1];` on stack with limited space. — chux - Reinstate Monica, Mar 28 '20 at 01:31
I haven't learned about malloc yet. These are just toy programs I'm writing to learn C so just interested in getting it working for now. — StevieD, Mar 28 '20 at 01:32
@M.M If by that you mean "just use global variables" then that's not really a solution so much as trading one anti-pattern for another. — tadman, Mar 28 '20 at 19:39

score 1 · Accepted Answer · answered Mar 28 '20 at 01:29

As with any structure too large to accommodate on the stack, the solution is to dynamically allocate:

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

#define MAX 4000000

int main() {
  int* primes = calloc(MAX + 1, sizeof(int));

  // init array with all numbers
  for(int i = 1; i <= MAX; i++)
    primes[i] = i;

  free(primes);

  return 0;
}

Segmentation fault on relatively small array

1 Answers1