pthread function from a class

Question

Let's say I have a class such as

class c {      // ...     void *print(void *){ cout << "Hello"; } } 

And then I have a vector of c

vector<c> classes; pthread_t t1; classes.push_back(c()); classes.push_back(c()); 

Now, I want to create a thread on c.print();

And the following is giving me the problem below: pthread_create(&t1, NULL, &c[0].print, NULL);

Error Ouput: cannot convert ‘void* (tree_item::)(void)’ to ‘void* ()(void)’ for argument ‘3’ to ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* ()(void), void*)’

Answer 1

You can't do it the way you've written it because C++ class member functions have a hidden this parameter passed in. pthread_create() has no idea what value of this to use, so if you try to get around the compiler by casting the method to a function pointer of the appropriate type, you'll get a segmetnation fault. You have to use a static class method (which has no this parameter), or a plain ordinary function to bootstrap the class:

class C { public:     void *hello(void)     {         std::cout << "Hello, world!" << std::endl;         return 0;     }      static void *hello_helper(void *context)     {         return ((C *)context)->hello();     } }; ... C c; pthread_t t; pthread_create(&t, NULL, &C::hello_helper, &c);