rt，形如下面的调试方法原理是什么？

#ifdef xxx
    freopen("ex_a.in", "r", stdin);
    // freopen("a.out", "w", stdout);
    // freopen("a.err", "w", stderr);
#else
    freopen("b.in", "r", stdin);
    freopen("b.out", "w", stdout);
#endif