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--- c:c_threads:event_handling [2021/03/05 19:48] – created peter
+++ c:c_threads:event_handling [2021/06/09 11:04] (current) – peter
@@ Line 3: / Line 3: @@
 To have a thread wait for an event to happen like a condition to become true or a task to be completed by another thread.
-Say 2 threads:
+----
-  * thread1 performs some actions, then sets a flag.
+===== Option 1 - Use a boolean global variable =====
-  * thread2 is waiting for this flag to be set.
+Two threads:
+  - thread1 performs some actions, then sets a flag.
+  - thread2 is waiting for this flag to be set.
 <code cpp>
@@ Line 23: / Line 27: @@
     m_bDataLoaded = false;
   }
   void loadData()
   {
@@ Line 75: / Line 80: @@
 </code>
+<WRAP info>
+**NOTE:**  It has following disadvantages:
+  * Thread will keep on acquiring the lock and releasing it just to check the value, therefore it will consume CPU cycles and will also make Thread 1 slow, because it needs to acquire same lock to update the bool flag.
+A better choice is to use Condition Variables; which is a kind of Event used for signaling between two or more threads.  One or more thread can wait on it to get signaled, while an another thread can signal it.
+</WRAP>
+----
+===== Option 2 - Using a Condition Variable =====
+<code cpp>
+#include <iostream>
+#include <thread>
+#include <functional>
+#include <mutex>
+#include <condition_variable>
+using namespace std::placeholders;
+class Application
+{
+  std::mutex m_mutex;
+  std::condition_variable m_condVar;
+  bool m_bDataLoaded;
+public:
+  Application()
+  {
+    m_bDataLoaded = false;
+  }
+  void loadData()
+  {
+    // Make This Thread sleep for 1 Second.
+    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
+    std::cout<<"Loading Data from XML"<<std::endl;
+    // Lock The Data structure.
+    std::lock_guard<std::mutex> guard(m_mutex);
+    // Set the flag to true, means data is loaded.
+    m_bDataLoaded = true;
+    // Notify the condition variable.
+    m_condVar.notify_one();
+  }
+  bool isDataLoaded()
+  {
+    return m_bDataLoaded;
+  }
+  void mainTask()
+  {
+    std::cout<<"Do Some Handshaking"<<std::endl;
+    // Acquire the lock
+    std::unique_lock<std::mutex> mlock(m_mutex);
+    // Start waiting for the Condition Variable to get signaled
+    // Wait() will internally release the lock and make the thread to block
+    // As soon as condition variable get signaled, resume the thread and
+    // again acquire the lock. Then check if condition is met or not
+    // If condition is met then continue else again go in wait.
+    m_condVar.wait(mlock, std::bind(&Application::isDataLoaded, this));
+    std::cout<<"Do Processing On loaded Data"<<std::endl;
+  }
+};
+int main()
+{
+  Application app;
+  std::thread thread_1(&Application::mainTask, &app);
+  std::thread thread_2(&Application::loadData, &app);
+  thread_2.join();
+  thread_1.join();
+  return 0;
+}
+</code>