用三个线程按顺序循环打印ABC三个字母

• 有两种方法：semaphore信号量和mutex互斥锁。需要注意的是C++11已经没有semaphore。
  • C++ 并发编程（六）：信号量（Semaphore） - 止于至善 - SegmentFault 思否
    • https://segmentfault.com/a/1190000006818772
• 线程按顺序执行（迅雷笔试题） - CSDN博客
  • http://blog.csdn.net/mafuli007/article/details/8488534
• mutex - C++ Reference
  • http://www.cplusplus.com/reference/mutex/mutex/

 //
 //  main.cpp
 //  LeetCode
 //
 //  Created by Hao on 2017/3/16.
 //  Copyright © 2017年 Hao. All rights reserved.
 //
 #include <iostream>       // std::cout
 #include <thread>         // std::thread, std::this_thread::sleep_for
 #include <chrono>         // std::chrono::seconds
 #include <mutex>          // std::mutex

 std::mutex mtx;           // mutex for critical section
 int count = ;
 int num_count = ;

 void print_block_0 (int n) {
     // critical section (exclusive access to std::cout signaled by locking mtx):
     while (true) {
         if (count %  == ) {
             mtx.lock();

             ++ count;

             std::cout << "A" << std::endl;

             mtx.unlock();
         }

         if (num_count == ) break;
     }
 }

 void print_block_1 (int n) {
     // critical section (exclusive access to std::cout signaled by locking mtx):
     while (true) {
         if (count %  == ) {
             mtx.lock();

             ++ count;

             std::cout << "B" << std::endl;

             mtx.unlock();
         }

         if (num_count == ) break;
     }
 }

 void print_block_2 (int n) {
     // critical section (exclusive access to std::cout signaled by locking mtx):
     while (true) {
         if (count %  == ) {
             mtx.lock();

             ++ num_count;   // must be executed prior to the following statement, or else an extra "A" would be printed

             ++ count;

             std::cout << "C" << std::endl;

 //            std::this_thread::sleep_for (std::chrono::seconds(1)); // sleep in case that an extra "A" is printed

             mtx.unlock();
         }

         if (num_count == ) break;
     }
 }

 int main()
 {
     std::thread threads[];                         // default-constructed threads

     std::cout << "Spawning 3 threads...\n";

     threads[] = std::thread(print_block_0, );   // move-assign threads
     threads[] = std::thread(print_block_1, );   // move-assign threads
     threads[] = std::thread(print_block_2, );   // move-assign threads

     std::cout << "Done spawning threads. Now waiting for them to join:\n";
     for (int i=; i<; ++i)
         threads[i].join();

     std::cout << "All threads joined!\n";

     return ;
 }