pthread_create parameter function pointer

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.

Are you sure you're not confusing free functions with member functions? — Kerrek SB
– Kerrek SB, Commented Oct 19, 2011 at 11:41

Sarfaraz Nawaz · Accepted Answer · 2011-10-19 11:56:05Z

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.

justin · Accepted Answer · 2011-10-19 11:45:39Z

0

the problem is that you are attempting to specify a pointer to member function, rather than a standard function.

Instead, pass the robot as the parameter to a function which tells the robot to run:

extern "C" void* run_robot(void* robot) __attribute__((noreturn));
void* run_robot(void* robot) {
    RobotNodes* node(static_cast<RobotNodes*>(robot));
    node->run();
    pthread_exit(0);
}
...
pthread_create(&thread, NULL, run_robot, &robots[i]);

answered Oct 19, 2011 at 11:45

justin

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Comments

RedX · Accepted Answer · 2011-10-19 11:45:44Z

0

Where is run defined?

Using pthread_create(&thread, ...) is bad since thread a variable local to the for loop and will go out of scope at the end of the for.

try instead:

for(i = 0; i<numRobotsToInit; i++){
  run = &robots[i]->run;
  pthread_t* thread = new pthread_t;  //you will get a memory leak unless you store these to delete later
  pthread_create(thread, NULL, run, NULL);       
}

If i understood correctly the run function is a member of RobotNodes, in this case you need to make a dispatcher to call this function since you cannot pass member functions the pthread_create.

Create a static function called dispatch or something, pass it a pointer to the class instance you want to invoke the function on.

edited Oct 19, 2011 at 11:45

answered Oct 19, 2011 at 11:37

RedX

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1 Comment

freakii Over a year ago

Sorry forgot to include that run() is from another class(RobotNodes) that is included in this class. Get the following errors when I run the code with your changes: error: cannot convert ‘void (RobotNodes::*)()’ to ‘void ()()’ in assignment error: invalid conversion from ‘void ()()’ to ‘void* ()(void)’ error: initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* ()(void), void*)’

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