Using p[-1], and getting defined behaviour

NegativeIndex.c

#include <stdio.h>
#include <stdlib.h>  
int main(void) {
    int *p = (int*)malloc(sizeof(int)*2);
    *p = 4;
    ++p;
    *p = 2;
    printf("%d", p[-1]); /* Prints 4 */
    return 0;
}