Question

I am trying to use a class based implementation of Win32 threads to create a Producer thread and a Consumer thread. Information of type int x in the consumer is updated by the producer.

Producer and Consumer both inherit from IRunnable
struct IRunnable {
    virtual unsigned long run() = 0;
    virtual void stop() = 0;
};

Which creates an interface for class Thread,

class Thread {
public:
    Thread(IRunnable *ptr=0) {
        _runnable = ptr;
        _started = false;
        _threadHandle = 0;
    }

a thread is created in class thread by

DWORD threadID=0;
_threadHandle = ::CreateThread(0, 0, ThreadProc, this, 0, &threadID);

And

static unsigned long __stdcall ThreadProc(void* ptr) 
{
return ((Thread *)ptr)->run();
}

How I have used it is

    int _tmain(int argc, _TCHAR* argv[]) {
     //example of usage



    Consumer *obj1=0;

    Thread *consumerThread=0;

    try {
        // create the threadable object first
    Consumer *obj1 = new Consumer();

        // create and start the thread the thread
        Thread *consumerThread = new Thread(obj1);
        consumerThread->start();


        printf("OkCon.\n");

    } 
    catch (ThreadException &e)
    {
        printf(e.Message.c_str());  
    }


    Producer *obj=0;
    Thread *ProducerThread=0;

    try {
        // create the threadable object first
        Producer *obj = new Producer();
        obj->Init(obj1);

        // create and start the thread the thread
        Thread *ProducerThread = new Thread(obj);
        ProducerThread->start();

        printf("OkProdu.\n");

    } 
    catch (ThreadException &e)
    {
        printf(e.Message.c_str());  
    }



    for(int i = 0; i<1000000; i++)
    {int a = i;}// just lets the program run on a bit so the threads can run and do a bit more work

    delete obj;
    delete ProducerThread;
    delete obj1;
    delete consumerThread;

    return 0;
}

The run function for consumer is

unsigned long Consumer::run()
{
    while(_continue)
    {
        printf("readX, %d \n",x);

    }

    return 0;
}

The init function and run function for producer are

void Producer::Init(Consumer* aConsumer)
{
    consData = aConsumer;

}

unsigned long Producer::run()
{ 
    while(_continue)
    {       
        this->consData->x = 1;
    }
    return 0;
}

Thread::Run is

unsigned long run() {
        _started = true;
        unsigned long threadExitCode = _runnable->run();
        _started = false;
        return threadExitCode;
    }

When I run the code I get an Unhandled exception. Access violation writing location 0X… at line this->consData->x = 1;

Any help would be greatly appreciated.

Answer 1

In first try block you are assigning Consumer instance to newly created local variable Consumer *obj1 instead of using existing variable that was created just before try block. Try something like this instead:

Consumer *obj1=0;
Thread *consumerThread=0;

try {
    // create the threadable object first
    obj1 = new Consumer();

This modify existing variable instead of creating new one. Same story with Producer *obj, Thread *consumerThread and Thread *ProducerThread. Please read something about scope and lifetime of variables in C++.