#include <stdio.h>

    void foo(int *pi)
    {
        printf("[foo] `i` was %d\n", *pi);
        (*pi)++; /* please be careful ^o^ */
        printf("[foo] Now `i` is %d\n", *pi);
    }

    int main()
    {
        int i = 0;
        printf("[main] `i` was %d\n", i);
        foo(&i);
        printf("[main] Now `i` is %d\n", i);
    }