Pass an array list as an argument to pthread_create

Question

I am attempting to pass an Array List as below as an argument to a class member thread function which in turn calls another member function but i am failing to retrieve the values inside the thread function.

Code :-

 pthread_t t2;
 int Coordinates[] = {(0,0), (1,1), (2,2)};
 pthread_create(&t2, NULL, &Class::thread_func,(void*) Coordinates);


void* Class::thread_func(void *arg)
{
  int *coord = (int *) arg;
  for(int i=0; i< sizeof(coord); i++)
  {
  classObj->doSomething(coord[i][0], coord[i][1]);
  }
  pthread_exit(0);
}

Please let me know what i'm doing wrong, been struggling with this for a while and I am not too experienced with Multithreading. Thanks in advance.

Answer 1

The problem is that Coordinates is passed void * so there is ONLY one address stored. No size of initial array. You MUST pass size extra, by passing custom struct, std::pair or std::vector or append something that is invalid coordinate and stop on that.

If you can use std::vector and so on you can do it like this (just pass vector by pointer):

void* thread_func(std::vector<std::array<int, 2>> *coord) {
  for(int i=0; i < coord->size(); i++)
  {
    cout << (*coord)[i][0] << " " << (*coord)[i][1]<<endl;;
  }
  pthread_exit(0);
}

int main(){
  pthread_t t2;
  std::vector<std::array<int, 2>> Coordinates= {{0,10}, {1,11}, {2,22}};
  pthread_create(&t2, NULL, (void * (*)(void *))thread_func, &Coordinates);
  pthread_join(t2, NULL);
}