update 5.4 Thread-synchronization-with-condition-variable

Author: forhappy

zh/chapter5-Condition-Variable/5.4 Thread-synchronization-with-condition-variable.md

@@ -1 +1,67 @@
-
+## 利用条件变量进行线程同步 ##
+
+在多线程同步时，条件变量通常和锁(Lock)一起使用。
+
+请看下例([参考](http://en.cppreference.com/w/cpp/thread/condition_variable))：
+
+    #include <iostream>
+    #include <string>
+    #include <thread>
+    #include <mutex>
+    #include <condition_variable>
+     
+    std::mutex m;
+    std::condition_variable cv;
+    std::string data;
+    bool ready = false;
+    bool processed = false;
+     
+    void worker_thread()
+    {
+        // Wait until main() sends data
+        {
+            std::unique_lock<std::mutex> lk(m);
+            cv.wait(lk, []{return ready;});
+        }
+     
+        std::cout << "Worker thread is processing data\n";
+        data += " after processing";
+     
+        // Send data back to main()
+        {
+            std::lock_guard<std::mutex> lk(m);
+            processed = true;
+            std::cout << "Worker thread signals data processing completed\n";
+        }
+        cv.notify_one();
+    }
+     
+    int main()
+    {
+        std::thread worker(worker_thread);
+     
+        data = "Example data";
+        // send data to the worker thread
+        {
+            std::lock_guard<std::mutex> lk(m);
+            ready = true;
+            std::cout << "main() signals data ready for processing\n";
+        }
+        cv.notify_one();
+     
+        // wait for the worker
+        {
+            std::unique_lock<std::mutex> lk(m);
+            cv.wait(lk, []{return processed;});
+        }
+        std::cout << "Back in main(), data = " << data << '\n';
+     
+        worker.join();
+    }
+
+执行结果如下：
+
+    main() signals data ready for processing
+    Worker thread is processing data
+    Worker thread signals data processing completed
+    Back in main(), data = Example data after processing