互斥信号量如何工作？

Question

我正在尝试多核编程，以实现 Mutex 信号量： 初始化；请求所有权；释放所有权；尝试所有权；毁灭.

我创建了 3 个线程。每个线程都对一个全局变量 Answer 进行操作。 10 次线程中有 9 次顺序执行。但是有时候线程3没有执行。第二个线程互斥锁没有释放吗？

这是我的代码：thread_tasks.cc

void *task1(void *X)
{

    int MValue = pthread_mutex_init(&Mutex,NULL); //Initialization
    pthread_mutex_lock(&Mutex); //Request ownership
    Answer = Answer * 32; //critical section
    pthread_mutex_unlock(&Mutex); //Release ownership
    flag1=1;
    cout << "\nthread A Answer = " << Answer << endl;
    cout<<"\nthread 1 executed successfully";
}

 void *task2(void *X)
 {
    if(flag1==1)
 {
    int RC = pthread_mutex_trylock(&Mutex); //Try ownership
    cout<<"\nRC: "<<RC;
    Answer = Answer/2;
    flag2=1;
    cout<<"\nthread 2 executed successfully";
    pthread_mutex_unlock(&Mutex); //Release ownership
    pthread_mutex_destroy(&Mutex); //Destruction

}
}



void *task3(void *X)
{
    Answer = Answer+5;
    cout<<"\nthread 3 executed successfully";
}

头文件：thread_tasks.h

using namespace std;
#include <iostream>
#include <pthread.h>

void *task1(void *X);
void *task2(void *X);
void *task3(void *X);

mutex_example.cpp

int Answer = 10;
int flag1 = 0;
int flag2 = 0;
int main(int argc, char *argv[])
{

 pthread_t ThreadA, ThreadB, ThreadC;

 cout << "Answer = " << Answer << endl;
 pthread_create( &ThreadA,NULL,task1,NULL);
 pthread_create( &ThreadB,NULL,task2,NULL);
 pthread_create( &ThreadC,NULL,task3,NULL);
 pthread_join(ThreadA,NULL);
 pthread_join(ThreadB,NULL);
 pthread_join(ThreadC,NULL);

 cout << "Answer = " << Answer << endl;

 return(0);
}

Answer 1

线程3函数的真正执行——task3需要flag2==1，此时可能线程2还没有完成flag2重置任务。所以任务 3 可能不会进入 if 块。

Answer 2

解决了问题：使用 thread_1done 和 thread_2done 来维护序列。

void *task1(void *X)
     {
        int MValue = pthread_mutex_init(&Mutex,NULL); //Initialization
        pthread_mutex_lock(&Mutex); //Request ownership
        Answer = Answer * 32; //critical section
        pthread_mutex_unlock(&Mutex); //Release ownership
        thread_1done=1;
        cout << "\nthread A Answer = " << Answer << endl;
        cout<<"thread 1 executed successfully"<<endl;
     }


    void *task2(void *X)
    {
        while(1)
            if(thread_1done==1)
            {
                int RC = pthread_mutex_trylock(&Mutex); //Try ownership
                Answer = Answer/2;
                pthread_mutex_unlock(&Mutex);
                thread_2done=1;
                cout << "\nthread B Answer = " << Answer << endl;
                cout<<"thread 2 executed successfully"<<endl;
                break;
            }
    }



    void *task3(void *X)
    {
        while(1)
            if (thread_2done==1 && thread_1done == 1)
            {
                int RC = pthread_mutex_trylock(&Mutex); //Try ownership
                Answer = Answer+5;
                pthread_mutex_unlock(&Mutex);
                pthread_mutex_destroy(&Mutex); //Destruction
                cout << "\nthread C Answer = " << Answer << endl;
                cout<<"thread 3 executed successfully"<<endl;
                break;
            }
    }

互斥信号量如何工作？

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