void callback(){
    int a = 10;
    int b = 20;
    dSP;
    ENTER;
    SAVETMPS;
    PUSHMARK(SP);
    XPUSHs(sv_2mortal(newSViv(a)));
    XPUSHs(sv_2mortal(newSViv(b)));
    PUTBACK;

    call_pv("p_callback", G_DISCARD|G_NOARGS);

    SPAGAIN;

    printf("%d to the %dth power is %d.\n", a, b, POPi);

    PUTBACK;
    FREETMPS;
    LEAVE;
}