C++编程思想 第2卷 第11章 并发 共享有限资源 访问控制

有时候 在试图使用某一资源时
并不关心它在同一时刻是否正在被访问
对于多线程处理的工作
需要一些方法来防止两个线程同时访问一个资源
至少要防止两个线程在临界期 critical period内访问同一资源

识别临界区 critical section
在临界区必须应用相互排斥机制
在进入临界区之前获得互斥锁
并在临界区的终点释放

//: C11:MutexEvenGenerator.cpp {RunByHand}
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Preventing thread collisions with mutexes.
//{L} ZThread
#include <iostream>
#include "EvenChecker.h"
#include "zthread/ThreadedExecutor.h"
#include "zthread/Mutex.h"
using namespace ZThread;
using namespace std;

class MutexEvenGenerator : public Generator {
  unsigned int currentEvenValue;
  Mutex lock;
public:
  MutexEvenGenerator() { currentEvenValue = 0; }
  ~MutexEvenGenerator() {
    cout << "~MutexEvenGenerator" << endl;
  }
  int nextValue() {
    lock.acquire();
    ++currentEvenValue;
    Thread::yield(); // Cause failure faster
    ++currentEvenValue;
    int rval = currentEvenValue;
    lock.release();
    return rval;
  }
};

int main() {
  EvenChecker::test<MutexEvenGenerator>();
  getchar();
} ///:~

输出
ThreadQueue created
Press Control-C to exit
User thread created.
Reference thread created.
1 reference-thread added.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting...
User thread created.
pollPendingThreads()
1 user-thread added.
Thread starting... 按回车键

ThreadQueue waiting on remaining threads...
Reference thread created.
Reference thread created.
1 reference-thread added.
1 reference-thread added.
pollUserThreads()
Thread joining...
1 user-thread reclaimed.
Thread exiting...
1 user-thread reclaimed.
insertPendingThread()
1 user-thread reclaimed.
1 user-thread reclaimed.
1 user-thread reclaimed.
1 user-thread reclaimed.
1 user-thread reclaimed.
1 user-thread reclaimed.
1 user-thread reclaimed.
1 user-thread reclaimed.
1 pending-thread added.
~EvenChecker 4

在MutexEvenGenerator中增加了一个叫做lock的Mutex型变量
并且在nextValue()函数中使用acquire()和release()创建了临界区
为了在currentEvenValue处于奇数状态时提高语句切换的可能性
一个Thread::yield()调用被插入到两个增1语句之间