Question

I am creating 5 thread here using ThrdFunc. Here each thread update the listBox.

I was expecting message in this way. Initially come in this way but after some time

Thread1:Adding msg
Thread2:Adding msg
Thread3:Adding msg

But after some time I get message like

Thread0:Adding msg
Thread18967654:Adding msg
Thread18967654:Adding msg
Thread18967654:Adding msg

This is the code:

for (int i = 0;i<6;i++)
    {   
        nThreadNo = i+1;
        hWndProducer[i] = CreateThread(NULL,0,(LPTHREAD_START_ROUTINE)ProducerThrdFunc,(void*)&nThreadNo,0,&dwProducerThreadID[i]);     
        if (hWndProducer[i] == NULL) 
        {
           //ErrorHandler(TEXT("CreateThread"));
           ExitProcess(3);
        }
    }   

DWORD WINAPI    ThrdFunc ( LPVOID n )
{
    int *nThreadNo = (int*)n;       
    char chThreadNo[3];
    memset(chThreadNo,0,3);

    while(1)
    {
        itoa(*nThreadNo,chThreadNo,10);
        char* pMsg1 = new char[100];
        char* pMsg2 = new char[100];
        memset(pMsg1,0,100);
        memset(pMsg2,0,100);

        strcpy(pMsg1," Thread No:");        
        strcat(pMsg1,chThreadNo);
        strcat(pMsg1," Adding Msg:");

        PostMessage(stThreadInfoProd.hWndHandle,UWM_ONUPDATEPRODUCERLIST,(WPARAM)pMsg1,0);


    }   
    return 0;
}

Answer 1

Most likely nThreadNo is allocated on the stack. You’re giving each thread a pointer to one of it’s elements.

Once the function creating the threads returns, the array is no longer valid, but the thread functions are still pointing to it. The memory the threads are holding pointers to will most likely be overwritten, causing what was originally the thread ID to be overwritten with garbage.

Anything you pass another thread should generally be allocated on heap, either via malloc type functions or new, preferably new since this is C++.

For example, instead of int nThreadNo[6], use int* nThreadNo = new int[6]. However, keep in mind that you will have to delete[] the memory nThreadNo points to when you’re done with it.