see #2: revision of parallelism and concurrency

code/7/7.3.critical.section.b.cpp

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+//
+// 7.3.critical.section.b.cpp
+// chapter 7 parallelism and concurrency
+// modern c++ tutorial
+//
+// created by changkun at changkun.de
+// https://github.com/changkun/modern-cpp-tutorial
+//
+
+#include <iostream>
+#include <thread>
+
+int v = 1;
+
+void critical_section(int change_v) {
+    static std::mutex mtx;
+    std::unique_lock<std::mutex> lock(mtx);
+    // do contention operations
+    v = change_v;
+    std::cout << v << std::endl;
+    // release the lock
+    lock.unlock();
+
+    // during this period,
+    // others are allowed to acquire v
+
+    // start another group of contention operations
+    // lock again
+    lock.lock();
+    v += 1;
+    std::cout << v << std::endl;
+}
+
+int main() {
+    std::thread t1(critical_section, 2), t2(critical_section, 3);
+    t1.join();
+    t2.join();
+    return 0;
+}