.tutorial

git-svn-id: svn://ultimatepp.org/upp/trunk@10557 f0d560ea-af0d-0410-9eb7-867de7ffcac7

tutorial/CoreTutorial/Mutex.cpp

@@ -0,0 +1,51 @@
+#include "Tutorial.h"
+
+void MutexTutorial()
+{
+	/// .`Mutex`
+	
+	/// Mutex ("mutual exclusion") is a well known concept in multithreaded programming: When
+	/// multiple threads write and read the same data, the access has to be serialized using
+	/// Mutex. Following invalid code demonstrates why:
+	
+	Thread t;
+	
+	int sum = 0;
+	t.Run([&sum] {
+		for(int i = 0; i < 1000000; i++)
+			sum++;
+	});
+	
+	for(int i = 0; i < 1000000; i++)
+		sum++;
+	
+	t.Wait();
+	DUMP(sum);
+	
+	/// While the expected value is 2000000, produced value is different. The problem is that
+	/// both thread read / modify / write `sum` value without any locking. Using `Mutex` locks
+	/// the `sum` and thus serializes access to it - read / modify / write sequence  is now
+	/// exclusive for the thread that has `Mutex` locked, this fixing the issue. `Mutex` can be
+	/// locked / unlocked with `Enter` / `Leave` methods. Alternatively, `Mutex::Lock` helper
+	/// class locks `Mutex` in constructor and unlocks it in destructor:
+	
+	Mutex m;
+	sum = 0;
+	t.Run([&sum, &m] {
+		for(int i = 0; i < 1000000; i++) {
+			m.Enter();
+			sum++;
+			m.Leave();
+		}
+	});
+	
+	for(int i = 0; i < 1000000; i++) {
+		Mutex::Lock __(m); // Lock m till the end of scope
+		sum++;
+	}
+	
+	t.Wait();
+	DUMP(sum);
+	
+	///
+}