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--- for_developers:thread_safety [2019/09/10 11:47] – created kunz
+++ for_developers:thread_safety [2019/11/01 10:11] (current) – Add link to source code kunz
@@ Line 5: / Line 5: @@
 Concurrency can not be tested by Google Test framework. Therefore, test executables are needed which can be executed manually or via bash scripts.
-To provide an example, a test program testing the Gain block is described in the following. The full source code is available at GitHub at:
+To provide an example, a test program testing the Gain block is described below. The full source code is available on GitHub at: https://github.com/eeros-project/eeros-framework/blob/master/examples/development/multithreading.cpp
+===== The problem =====
+The following code snipped and image show the issue simulated in the test program. The main thread calls **g1.run()** on the gain object. This method is doing a long calculation. Thread t2 is unfortunately changing the gain by calling **g1.setGain(gM2)** during the main threads calculation. This will finally lead to an unexpected result.
+<code>
+Matrix<M,N> gM1;
+Matrix<M,N> gM2;
+Gain<Matrix<M,N>, Matrix<M,N>,true> g1{gM1};
+Constant<Matrix<M,N>> c1{m2};
+std::thread t2{[&] {
+  g1.setGain(gM2);
+}};
+g1.run();
+Matrix<M,N> res = g1.getOut().getSignal().getValue();
+// Note: The above code is simplified for a better understanding.
+</code>
+{{:for_developers:concurrencyissue.jpg?800|}}
+===== The solution =====
+The Gain block is now implemented thread safe by using the class [[https://en.cppreference.com/w/cpp/thread/lock_guard|lock_guard]] and a [[https://en.cppreference.com/w/cpp/thread/mutex|mutex]]. The code snipped below shows the **setGain()** method as an example.
+<code>
+virtual void setGain(Tgain c) {
+  std::lock_guard<std::mutex> lock(mtx);
+  gain = c;
+}
+</code>
+The test program will not show an unexpected result anymore. To see the unexpected result, the locking mechanism must be removed (//comment std::lock_guard<std::mutex> lock(mtx);//).