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Question

I am working on Single Producer Single Consumer Problem.Here producer thread will write to the list and consumer thread remove it from list. I have dialog box where i maintain two list 1.consumer list box 2.producer list box, which will list the message posted by both thread.I am getting strange problem here.Here message are get mixed up with each other.Consumer thread is also getting message of the Producer apart from consumer thread message thread and vice versa.

I am passing THREADINFO from main thread

can any body suggest me what went wrong here.I am passing correct thread name from main thread but when it comes to Producer or Consumer Thread sometime value getting changed. I have two thread given below

typedef struct THREADINFO{  
    CEventQueue* pEventQueue;
    HWND hWndHandle;
    char* pThreadName;
}THREADINFO
DWORD WINAPI    ProducerThrdFunc ( LPVOID n )
{
    THREADINFO* stThreadInfoProd = (THREADINFO*)n;
    char* pMsg1 = new char[100];
    while(1)
    {

        strcpy(pMsg1,stThreadInfoProd->pThreadName);
        strcat(pMsg1," Thread No:");        
        strcat(pMsg1,"Adding Msg");

        PostMessage(stThreadInfoProd->hWndHandle,UWM_ONUPDATEPRODUCERLIST,(WPARAM)pMsg1,0);

        stThreadInfoProd->pEventQueue->AddTail(pMsg1);
        memset(pMsg1,0,100);

        strcpy(pMsg1,stThreadInfoProd->pThreadName);
        strcat(pMsg1,"Thread No:");     
        strcat(pMsg1,"Added Msg");
        char*p = "Producer";
        PostMessage(stThreadInfoProd->hWndHandle,UWM_ONUPDATEPRODUCERLIST,(WPARAM)pMsg1,0);

        Sleep(3000);
    }   
    return 0;
}

DWORD WINAPI    ConsumerThrdFunc ( LPVOID n )
{
    THREADINFO* stThreadInfoCons = (THREADINFO*)n;

    char* pMsg = new char[100];
    memset(pMsg,0,100);

    while(1)
    {
        strcpy(pMsg,stThreadInfoCons->pThreadName);
        strcat(pMsg," Thread No:");
        strcat(pMsg,"Removing Msg");        
        PostMessage(stThreadInfoCons->hWndHandle,UWM_ONUPADTECONSUMERLIST,(WPARAM)pMsg,0);

        memset(pMsg,0,100);

        char *pMsg = (char*)stThreadInfoCons->pEventQueue->RemoveHead();        
        strcpy(pMsg,stThreadInfoCons->pThreadName);
        strcat(pMsg,"Thread No:");
        strcat(pMsg,"Removed Msg");

        PostMessage(stThreadInfoCons->hWndHandle,UWM_ONUPADTECONSUMERLIST,(WPARAM)pMsg,0);
        Sleep(3000);
    }
    return 0;
}

Answer 1

How is your THREADINFO struct constructed? On the local stack in the function where you create the struct, on a global level of the stack, or on the heap? It's possible that the THREADINFO instance go out of scope before it's accesed on the worker threads.

Answer 2

There is a lot wrong with this code.

In your producer thread, you are doing the following:

stThreadInfoProd->pEventQueue->AddTail(pMsg1);

and unless the AddTail is being clever, this is adding the pointer to the queue, it is not duplicating the string. Then, in your consumer you're doing this:

char *pMsg = (char*)stThreadInfoCons->pEventQueue->RemoveHead();

which is getting the pointer you added to the queue. This points to the buffer you're using for the producer messages, so when you do this in the consumer:

strcpy(pMsg,stThreadInfoCons->pThreadName);
strcat(pMsg,"Thread No:");
strcat(pMsg,"Removed Msg");

you are overwriting the producer's buffer. I think what you wanted is more like this:

char *pProducerMsg = (char*)stThreadInfoCons->pEventQueue->RemoveHead();

and then the following lines won't overwrite the producer's data.

But this introduces another problem. In your producer you do this:

stThreadInfoProd->pEventQueue->AddTail(pMsg1);

which adds the pointer to the queue and then immediately do this:

memset(pMsg1,0,100);
strcpy(pMsg1,stThreadInfoProd->pThreadName);
strcat(pMsg1,"Thread No:");     
strcat(pMsg1,"Added Msg");

which overwrites the buffer. This overwrite will almost certainly occur before the consumer has had a chance to consume the message, so the consumer will read the modified buffer, not the message you originally sent. To solve this, change the producer thus:

stThreadInfoProd->pEventQueue->AddTail(strdup(pMsg1));

The strdup creates a copy of the contents of the buffer pMsg1 points to.

The consumer then needs to be:

char *pProducerMsg = (char*)stThreadInfoCons->pEventQueue->RemoveHead();
// do something with pProducerMsg
free (pProducerMsg); // strdup calls malloc, so a matching free is required