C++ 11线程:等待条件变量的多个线程

Question

我目前正在研究一个模拟扩展的Producer-Worker模型的问题.在这个问题中,有3个工人和3个工具可供使用,而对于工人来说,他们需要2个工具(和材料,但那些是无关紧要的).如果保险库中有> = 2个工具,则工作人员将采用2.否则,他们将等待条件变量,当> = 2时将发出信号.

对于2名工人来说这很好:一个工作然后将工具返回到金库,另一个等待工人将被唤醒并使用2个工具.问题是,有3名工人,总会有一个人渴望得到这些工具.

经过一些测试后,我注意到等待条件变量的线程以堆栈形式构建.有没有可能使它排队？(1等待,2等待,3等待.当1被唤醒并想要制造另一个时,他必须在2和3后面等待.)

这是一个示例输出.代码太长,所以如果真的有必要,我会发布它.有3个工作线程和1个工具互斥锁.任何挨饿的人都不同于其他人.

1 Tools taken. Remaining: 1
2 Waiting on tools...
3 Waiting on tools...
1 Operator Product made. Tools returned. Tools now:3
3 Tools taken. Remaining: 1
1 Waiting on tools...
3 Materials returned for switch.
3 Operator Product made. Tools returned. Tools now:3
1 Tools taken. Remaining: 1
3 Waiting on tools...
1 Materials returned for switch.
1 Operator Product made. Tools returned. Tools now:3
3 Tools taken. Remaining: 1
1 Waiting on tools...
3 Materials returned for switch.
3 Operator Product made. Tools returned. Tools now:3
1 Tools taken. Remaining: 1
3 Waiting on tools...
1 Materials returned for switch.
1 Operator Product made. Tools returned. Tools now:3
3 Tools taken. Remaining: 1
1 Waiting on tools...
3 Materials returned for switch.
3 Operator Product made. Tools returned. Tools now:3
1 Tools taken. Remaining: 1
3 Waiting on tools...
1 Materials returned for switch.
...

(你可以看到2永远不会得到工具......)

更新:2013/07/05 我添加了一些代码.

int tools = 3; //global
string last; //current last product on output buffer
mutex toolsMutex;
mutex matSearchMutex;

int main(){
//Initializing Producers
    Producer prod1(1);
    Producer prod2(2);
        Producer prod3(3);



    thread p1(processor,1);
    thread p2(processor,2);
    thread p3(processor,3);

    p1.detach();
    p2.detach();
    p3.detach();

    while(true){//forever running

    }

    return 0;
}

处理器:

  //Processor method
void processor(int i){
    srand(time(NULL)); 

    while (true){ //forever running


    bool hasTools = false;
    bool productMade = false;
    while (productMade == false){ //while product has yet to be made.
        //choose what to make...



        if (hasTools == false){
            thread matT(getMaterials,whatToMake);
            thread toolT(getTools,i);
            toolT.join();           
            matT.join();
            hasTools = true;
        }
        else{ //tools acquired but no materials
            thread matT(getMaterials,whatToMake);
            matT.join();
        }

        if (recordedLast.compare(last) != 0){

            //return materials and acquire new ones the next run

            continue;
        }
        else {
            makeProduct(whatToMake);
            unique_lock<mutex> locker(toolMutex); 
            tools = tools + 2;
            cout << i << " Operator Product made. Tools returned. Tools now:" << tools << endl;
            productMade = true;
            if (tools >=2)  toolsCV.notify_one();
        }

    //done processing

    }


}   

}

makeProducts:

void makeProduct(int i){
    unique_lock<mutex> mainMatLock(matSearchMutex); 
    // make product according to i
    this_thread::sleep_for(chrono::milliseconds(rand() % 1000 + 10));   
}

GETTOOLS:

void getTools(int i){
    unique_lock<mutex> locker(toolMutex); 
    if (tools <2){
        cout << i << " Waiting on tools..." << endl;
        toolsCV.wait(locker);}
    tools = tools - 2;//tools acquired
    cout << i <<" Tools taken. Remaining: " << tools << endl;

}

感谢那些回复的人.我将尝试使用多个条件变量在今晚实现等待队列.

(PS有没有更好的方法在Stack Overflow上进行代码格式化？除了四个空格...

Answer 1

std::condition_variable没有指定你打电话时唤醒哪个等待线程notify_one.因此,您应该编写不关心哪个线程被唤醒的代码.标准模式是无论哪个线程被唤醒,该线程都应该完成需要完成的工作.

如果您要求以特定顺序唤醒线程,则使用其他机制.例如,您可以std::condition_variable为每个线程分别使用,然后在需要工具时将线程放入队列中.当一个线程交给工具时,它可以发信号通知对应于队列前面的线程的条件变量.然后该线程将被唤醒,其他线程将保持睡眠状态(模拟虚假唤醒).