c++ - OS X 下使用 cin / cout 为什么会过慢？

Question

1. 描述你的问题
   向下面这个程序输入 10000000 个随机数后再输出需要用掉约 18 秒时间，其中输入用掉约 15 秒，输出用掉约 3 秒。这个速度比一台配置较低的 Linux 系统（g++ 和 clang++ 都试过）和一台 Windows XP 虚拟机（g++）慢得多（不到 4 秒）。printf() 和 scanf() 的速度是正常的。

2. 贴上相关代码
   test.cpp:（random.txt 是随机数文件。编译选项为 clang++ test.cpp -o test。）

   #include <iostream>
   #include <ctime>
   #include <cstdio>
   using namespace std;
   
   const int length = 10000000;
   int array[length] = {};
   
   int main()
   {
       freopen("random.txt", "r", stdin);
       freopen("out.txt", "w", stdout);
       clock_t start1, end1, end2;
       start1 = clock();
       for (int i = 0; i < length; i++) cin >> array[i];
       end1 = clock();
   
       for (int i = 0; i < length; i++) cout << array[i];
       end2 = clock();
       freopen("result.txt", "w", stdout);
       cout << (double)(end1 - start1) / CLOCKS_PER_SEC << endl;
       cout << (double)(end2 - end1) / CLOCKS_PER_SEC << endl;
       return 0;
   }

3. 贴上报错信息
   无

4. 贴上相关截图
   无

5. 已经尝试过哪些方法仍然没解决（附上相关链接）
   用 Instruments.app 的 Time Profiler 看过，但是看不懂……网上搜索过也没有结果。

更新：刚才又看了一遍 Instruments.app，发现 _pthread_mutex_unlock_slow 和 _pthread_mutex_lock_slow 在输入中花掉了很多时间。但我不知道它们是做什么用的。

附：OS X 版本 10.11.3，Xcode 版本为 7.1.1。
clang 版本：

clang++ --version
Apple LLVM version 7.0.0 (clang-700.1.76)
Target: x86_64-apple-darwin15.3.0
Thread model: posix

Answer 1

The reasons may be:

1. For unknown reasons, the synchronization time with stdio is too long, please use std::ios::sync_with_stdio(false);

2. The previous point is really a cliché. According to my own observation, libc++'s implementation of iostream is very slow. When both stdio synchronization is turned off at the same time, libstdc++ is 1100% faster. The default -stdlib specified by clang is libc++.

3. (Explanation for the previous point) According to the results of Instruments.app, it is possible that libc++ implements thread safety for iostream. I have not looked at the code, but the provisions of thread safety in the standard are implementation-dependent.

Since the two versions tested above are shipped with OS X, and the libstdc++ that comes with OS X is very old, maybe the new version of libstdc++ also implements thread safety. Therefore, please re-run the test specifying -stdlib=libstdc++ and improve the test results of Instruments.app.