Cast member function for create_pthread() call

Question

I want to stop the warning

server.cpp:823: warning: converting from 'void* (ClientHandler:: )()' to 'void ( )(void )'

in the call:

pthread_create(th, NULL,
    (void* (*)(void*)) &ClientHandler::handle,
    (void *) clientHandler);

where handle() is a member function of ClientHandler :

void* ClientHandler::handle();

I have difficulties deciphering the function-type message from the compiler.

The question is:

• Should I change the handle() interface? Can I get rid of casting overall?
• Should I change the cast? To what exactly?
• Something completely different?

Answer 1

You can't do that directly, pointers to member functions are not plain pointers to functions and can't be handed over to C callbacks directly.

You'll need one level of indirection:

void callHandle(void *data) {
  ClientHandle *h = static_cast<ClientHandle*>(data);
  h->handle();
}

pthread_create(th, 0, &callHandle, static_cast<void*>(handle));

See the Pointers to members section of the C++FAQ for more information / alternatives.

For the validity of the cast in callHandle , see this question . You are sole responsible for making sure that handle is still alive and well when callHandle is called of course (and for the fact that it actually points to a ClientHandle ).

Answer 2

You need to pass a static cdecl function to pthread_create as given in this signature:

void* handler(void* data);

The optional argument can be used to pass your ClientHandler object into the thread.

class ClientHandler()
{
public:
  static void* handle(void* data);
}

extern "C" {

void* ClientHandler::handle(void* data)
{
  ClientHandler* handler = reinterpret_cast<ClientHandler*>(data)
  // fancy stuff with handler object here
}

} /* extern "C" */

Answer 3

If you're looking to invoke the method ClientHandler::handle from a specific instance of the ClientHandler class, it's unfortunately a bit more complicated than your example as pointers to member functions are different than pointers to functions in general. See here for a complete description of what needs to be done to create pthreads in such a manner.