Question

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C programming, why does this large array declaration produce a segmentation fault?

Why does this program crash? It works fine for a max of 1e6, and works fine if I don’t set every value to 0.

Doesn’t the program get all of the memory allocated?

int main() {
    const int max = 10000000; // 1e7
    int end[max];
    unsigned int i;
    for ( i = 0; i < max; i++ )
        end[i] = 0;
}


$ gcc test.c && ./a.out 
Segmentation fault (core dumped)

Answer 1

Variable-length arrays are usually allocated on the stack. Stack space is limited; you are probably getting a stack overflow here. You can’t allocate a very large size like that on such memory location. Your array is too big to fit in your program’s stack. A better idea to allocate such size is to allocate on the heap.

#include <stdlib.h>

int *end = malloc(sizeof *end * max);

/* some stuff */

free(end);

The heap is many times larger than the stack.