cannot convert '*void(MyClass::*)(void*) to void*(*)(void*) in pthread_create function

Question

i\'m trying to create a new thread with a class \"CameraManager\" but i have the following error:

cannot convert \'*void(CameraManager:: * )(void*) to

Answer 1

If you want the function to be a member of the class, it must be static. It's because the thread function will be called directly and will not have a valid this pointer. This can be solved by having a wrapper function, that gets passed the actual object and then calls the proper member function:

void *dequeueLoopWrapper(void *p)
{
    CameraManager *cameraManager = static_cast<CameraManager*>(p);
    camereraManager->dequeueLoop();
    return nullptr;
}

// ...

void CameraManager::startDequeuing()
{
    dequeuing = true;
    dequeueThreadId = pthread_create(&dequeueThread, NULL, dequeueLoopWrapper, this);
}

However, I would recommend you start using the threading support in the new standard library:

void CameraManager::startDequeuing()
{
    dequeuing = true;
    myThread = std::thread(&CameraManager::dequeueLoop, this);
}

Answer 2

I don't want to declare dequeueLoop as a static function

If you want to use pthreads, then you'll need a static or non-member function for the entry point. You can pass a pointer to your object to this function, using it as a trampoline into the non-static member function:

static void * dequeueEntry(void * self) {
    return static_cast<CameraManager*>(self)->dequeueLoop();
}

dequeueThreadId = pthread_create(
    &dequeueThread, NULL, 
    &CameraManager::dequeueEntry, // <-- pointer to trampoline function
    this);                        // <-- pointer to object for member function

Alternatively, if you have a modern compiler, you could use the standard thread library instead:

std::thread thread(&CameraManager::dequeLoop, this);

Answer 3

You can't use a pointer to member function as a function pointer unless it's static. You'll have to make dequeueLoop a free function, or write a free function as a wrapper to it.

To access the class members in a free function, you should have the function pass it's this pointer as the final argument of pthread_create. Then have the free function cast it's argument to a pointer to the class.