用三个线程按顺序循环打印ABC三个字母

 1 //
 2 //  main.cpp
 3 //  LeetCode
 4 //
 5 //  Created by Hao on 2017/3/16.
 6 //  Copyright © 2017年 Hao. All rights reserved.
 7 //
 8 #include <iostream>       // std::cout
 9 #include <thread>         // std::thread, std::this_thread::sleep_for
10 #include <chrono>         // std::chrono::seconds
11 #include <mutex>          // std::mutex
12 
13 std::mutex mtx;           // mutex for critical section
14 int count = 0;
15 int num_count = 0;
16 
17 void print_block_0 (int n) {
18     // critical section (exclusive access to std::cout signaled by locking mtx):
19     while (true) {
20         if (count % 3 == 0) {
21             mtx.lock();
22             
23             ++ count;
24             
25             std::cout << "A" << std::endl;
26             
27             mtx.unlock();
28         }
29 
30         if (num_count == 3) break;
31     }
32 }
33 
34 void print_block_1 (int n) {
35     // critical section (exclusive access to std::cout signaled by locking mtx):
36     while (true) {
37         if (count % 3 == 1) {
38             mtx.lock();
39             
40             ++ count;
41             
42             std::cout << "B" << std::endl;
43 
44             mtx.unlock();
45         }
46 
47         if (num_count == 3) break;
48     }
49 }
50 
51 void print_block_2 (int n) {
52     // critical section (exclusive access to std::cout signaled by locking mtx):
53     while (true) {
54         if (count % 3 == 2) {
55             mtx.lock();
56             
57             ++ num_count;   // must be executed prior to the following statement, or else an extra "A" would be printed
58 
59             ++ count;
60 
61             std::cout << "C" << std::endl;
62             
63 //            std::this_thread::sleep_for (std::chrono::seconds(1)); // sleep in case that an extra "A" is printed
64             
65             mtx.unlock();
66         }
67         
68         if (num_count == 3) break;
69     }
70 }
71 
72 int main()
73 {
74     std::thread threads[3];                         // default-constructed threads
75     
76     std::cout << "Spawning 3 threads...
";
77     
78     threads[0] = std::thread(print_block_0, 10);   // move-assign threads
79     threads[1] = std::thread(print_block_1, 10);   // move-assign threads
80     threads[2] = std::thread(print_block_2, 10);   // move-assign threads
81 
82     std::cout << "Done spawning threads. Now waiting for them to join:
";
83     for (int i=0; i<3; ++i)
84         threads[i].join();
85     
86     std::cout << "All threads joined!
";
87     
88     return 0;
89 }