process.c:1132
static VALUE
proc_getpgid(obj, pid)
    VALUE obj, pid;
{
#if defined(HAVE_GETPGID) && !defined(__CHECKER__)
    int i = getpgid(NUM2INT(pid));

    if (i < 0) rb_sys_fail(0);
    return INT2NUM(i);
#else
    rb_notimplement();
#endif
}
