Question

I am trying to create a thread in the following code but the pointer to a function parameter of the pthread_create method call is just not letting me compile my code.

Please let me know what I am doing wrong and how can I fix it in the following code:

#include "RobotNodes.cpp"

int main(int argc, char** argv){

int i, numRobotsToInit = 7;

//declare run function pointer
void (*run)();

//create array of robot nodes 
RobotNodes* robots[numRobotsToInit]; 

//init robot nodes
for(i = 0; i<numRobotsToInit; i++){
    robots[i] = new RobotNodes(i, 0.2, 0.2);
}


for(i = 0; i<numRobotsToInit; i++){
        run = &robots[i]->run;
        pthread_t thread; 
    pthread_create(&thread, NULL, (void*(*)(void*))run, NULL);       
}
}

The error that I get is the following:
error: lvalue required as unary ‘&’ operand

Edit: run() is a method from class RobotNodes.cpp that is included on the top of this class.

Answer 1

There seems to be a non-static member function in the class RobotNodes and you seem to think that the type of member function is void (*)(). If so, then you are wrong.

The type of non-static member functon and free function are not same, even if they have exactly same signature!

So I would suggest you to define a static function called start, as:

class RobotNodes
{
   public:
        void run(); //define it as you like

        static void* start(void *pdata)
        {
             RobotNodes *robot = static_cast<RobotNodes*>(pdata);
             robot->run(); //forward it
             return 0; //or return as the documentation says
        }
};

Then use start as :

std::vector<pthread_t> threads(numRobotsToInit);
for(i = 0; i<numRobotsToInit; i++)
{
    pthread_create(&threads[i], NULL, RobotNodes::start, &robots[i]);       
}

Also, notice that I have created a vector of pthread_t outside the loop; it is because each thread instance has to be different if they are different thread, and furthermore, each thread instance must continue to exist even after the loop stops.