An array on the heap

malloc asks for a block of memory on the heap and returns its address. The block lives on after the line that made it, until you call free.

Watch the heap block appear on the malloc line, fill up in the loop, then vanish at free. After free, data still holds the old address but points at nothing valid (a dangling pointer), so the program sets it to NULL.

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

int main(void) {
    int n = 4;
    int *data = malloc(n * sizeof *data);
    if (data == NULL) {
        return 1;
    }
    for (int i = 0; i < n; i++) {
        data[i] = (i + 1) * 10;
    }
    printf("last = %d\n", data[n - 1]);
    free(data);
    data = NULL;
    return 0;
}

Output:

last = 40

From the lesson: Dynamic memory