Using realloc() crahes program

Question

I have some problems using realloc(), so I made a sample program to illustrate the problem using as less code as possible.

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

int main(void)
{
    unsigned int i;
    unsigned long long *a;
    srand(time(NULL));
    a = malloc(sizeof(unsigned long long));
    for (i = 0; i < 20; ++i)
    {
        a[i] = rand() % 32;
        printf("%llu\n", a[i]);
        a = realloc(a, (i + 1) * sizeof(unsigned long long));
    }
    return 0;
}

This outputs:

* glibc detected demo: realloc(): invalid next size: 0x0000000000dc3010 **

Why does this crash?

Edit: I tried chaning (i + 1) to (i + 2) and then the program worked, but I do not understand why. I only request to extend the memory space by one unsigned long long.

Answer 1

The first time your loop runs, i is equal to 0. You realloc a to hold i + 1 elements, which is... 1 ! The second time your loop runs, you try to write to a[i] with i == 1, which is the second element of your array. But since your array can only hold 1 element, that can cause a crash.