How can I synchronize three threads?

Question

My app consist of the main-process and two threads, all running concurrently and making use of three fifo-queues:

The fifo-q\'s are Qmain, Q1 and Q2. Internally the

Answer 1

An example of how I would adapt the design and lock the queue access the posix way. Remark that I would wrap the mutex to use RAII or use boost-threading and that I would use stl::deque or stl::queue as queue, but staying as close as possible to your code:

main-process:
...
start thread Monitor
...
while (!quit)
{
    ...
    if (Qmain.count() > 0)
    {
        X = Qmain.get();
        process(X) 
            delete X;
    }
    ...
    //at some random time:
    QMain.put(Y);
    ...
}

Monitor:
{
    while (1)
    {
        //obtain & package data
        QMain.put(data)
    }
}

fifo_q:
template < class X* > class fifo_q
{
    struct item
    {
        X* data;
        item *next;
        item() { data=NULL; next=NULL; }
    }
    item *head, *tail;
    int count;
    pthread_mutex_t m;
public:
    fifo_q() { head=tail=NULL; count=0; }
    ~fifo_q() { clear(); /*deletes all items*/ }
    void put(X x) 
    { 
      pthread_mutex_lock(&m);
      item i=new item(); 
      (... adds to tail...); 
      count++; 
      pthread_mutex_unlock(&m);
    }
    X* get() 
    { 
      pthread_mutex_lock(&m);
      X *d = h.data; 
      (...deletes head ...); 
      count--; 
      pthread_mutex_unlock(&m);
      return d; 
    }
    clear() {...}
};

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Remark too that the mutex still needs to be initialized as in the example here and that count() should also use the mutex