void free(void *);

    long * array_of_100 = (long *)malloc(sizeof(long) * 100);

    /* Do something that only needs 20 of them */
    array_of_100[0] = 0;
    array_of_100[1] = 1;
        .....
    array_of_100[18] = 19;
    array_of_100[19] = 20;

    free(&array_of_100[20]); /* This is NOT!!! valid */