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 void*( * )(void*) in pthread_create function

i defined in the cameramanager.h file:

public:
void *dequeueLoop(void *ptr);

and in the cameramanager.cpp

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

void *CameraManager::dequeueLoop(void *ptr){
while(dequeuing){
    highSpeedCamera->dequeue();
    highSpeedCamera->enqueue();
}

I don’t want to declare dequeueLoop as a static function i also tried to declare dequeueLoop as a class friend function in the following way but then it doesn’t have scope on class variables ‘highSpeedCamera’ and ‘dequeuing’ and the compiler also tell me that ‘dequeueLoop’ was not declared in this scope

to make dequeueLoop a friend function i did:

cameramanager.h

public:
friend void *dequeueLoop(void *ptr);

cameramanager.cpp

void CameraManager::startDequeuing(){
    dequeuing = true;
    dequeueThreadId = pthread_create(&dequeueThread, NULL, &CameraManager::dequeueLoop, NULL);
}
void *dequeueLoop(void *ptr){
    while(dequeuing){
        highSpeedCamera->dequeue();
        highSpeedCamera->enqueue();
    }
}

Where i’m doing wrong?

Answer 1

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);