int foo( volatile int *x, int v ) { return *x - v; }
int bar( volatile int *x, int v ) { return *x + v; }

int main() {
    volatile int x = 4;
    int (*fun)( volatile int *, int );
    fun = bar;
    fun( &x, 4 );
    fun = foo;
    return fun( &x, 4 );
}