casting to void* to pass objects to pthread in c++

Question

I\'m a little confused about how to pass an object to the pthread_create function. I\'ve found a lot of piecemeal information concerning casting to void*, passing arguments

Answer 1

If you want to go this route, I believe you want something like this:

Edit: Based on James Kanze's answer, add a separate activate method to launch the thread after construction is finished.

class GenericThread {
protected:
    GenericThread () {
      //...
    }
    virtual ~GenericThread () {}

    int activate () {
        return pthread_create(..., GenericThreadEntry, this);
    }

    virtual void * thread_routine () = 0;

    #if 0
    // This code is wrong, because the C routine callback will do so using the
    // C ABI, but there is no guarantee that the C++ ABI for static class methods
    // is the same as the C ABI.
    static void * thread_entry (void *arg) {
        GenericThread *t = static_cast(arg);
        return t->thread_routine();
    }
    #endif
};

extern "C" void * GenericThreadEntry (void *) {
    GenericThread *t = static_cast(arg);
    return t->thread_routine();
}

Then, ProducerThread would derive from GenericThread.

Edit: Searching for extern "C" in the C++ Standard. revealed no requirement that a function pointer must point to a function with C linkage to be callable by a C library routine. Since pointers are being passed, linkage requirements do not apply, as linkage is used to resolve names. A pointer to a static method is a function pointer, according to C++ 2011 draft (n3242), Sec. 3.9.2p3:

Except for pointers to static members, text referring to pointers does not apply to pointers to members.

Edit: Mea culpa. The C library will invoke the callback function assuming the C application binary interface. A function with C++ linkage may use a different ABI than the C ABI. This is why it is required to use a function with extern "C" linkage when passing to a callback function to a C library. My sincere apologies to James Kanze for doubting him, and my sincere thanks to Loki Astari for setting me straignt.