Poco线程同步问题

Question

以下是Sample Poco线程程序，用于理解互斥和线程同步。仍然看到同一程序的不同输出。

#include "Poco/ThreadPool.h"
#include "Poco/Thread.h"
#include "Poco/Runnable.h"
#include "Poco/Mutex.h"
#include <iostream>
#include <unistd.h>
using namespace std;

class HelloRunnable: public Poco::Runnable
{
    public:
        static int a;
        HelloRunnable(){
        }
        HelloRunnable(unsigned long n):_tID(n){
        }
        void run()
        {
            Poco::Mutex::ScopedLock lock(_mutex);
            std::cout << "==>> In Mutex thread " << _tID << endl;
            int i;
            for (i=0;i<50000;i++)
            {
                a = a+1;
            }
            Poco::Mutex::ScopedLock unlock(_mutex);
        }
    private:
        unsigned long _tID;
        Poco::Mutex _mutex;
};
int HelloRunnable::a = 0;
int main(int argc, char** argv)
{
    Poco::Thread thread1("one"), thread2("two"), thread3("three");

    HelloRunnable runnable1(thread1.id());
    HelloRunnable runnable2(thread2.id());
    HelloRunnable runnable3(thread3.id());

    thread1.start(runnable1);
    thread2.start(runnable2);
    thread3.start(runnable3);

    thread1.join();
    thread2.join();
    thread3.join();
    cout << "****>> Done and a: " << HelloRunnable::a  << endl;
    return 0;
}

获得如下输出：

OUTPUT1：

==＆GT;＆GT;在Mutex线程中1 ==＆GT;＆GT;在Mutex线程中2 ==＆GT;＆GT;在互斥线程3中 ****＆GT;＆GT;完成和a：142436

OUTPUT2：

==＆GT;＆GT;在Mutex线程2 ==＆gt;＆gt;在Mutex线程3中

==＆GT;＆GT;在Mutex线程中1 ****＆GT;＆GT;完成并且：a：143671

OUTPUT3：

==＆GT;＆GT;在Mutex线程中2 ==＆GT;＆GT;在互斥线程3中 ==＆GT;＆GT;在Mutex线程中1 ****＆GT;＆GT;完成和a：150000

我总是期待OutPut3成为结果。上述课程有什么问题？

Answer 1

互斥锁是类的非静态成员变量，这意味着类的每个实例都有自己的互斥锁。如果要进行同步，则需要在线程之间共享互斥锁。你需要把它static。

同样在run函数中，变量unlock没有做任何事情。对象lock将在超出函数返回范围时解锁互斥锁。