Question

I’m trying to set a callback function in Jack. This is usually quite simple, using the function “jack_set_process_callback” (in the case of setting the “process” callback).

The definition in the API is this:

int jack_set_process_callback(jack_client_t* client, JackProcessCallback process_callback, void* arg).

In the example code (which has C components) the global function below is declared.

int process(jack_nframes_t nframes, void *arg){

  // do something in the callback
  return 0;
}

It is then set as the Jack process callback by the line:

jack_set_process_callback(client, process, 0)

.. and this compiles and works perfectly.

What I’m doing now is trying to build a class-based audio program. I’m putting all the Jack stuff inside a class imaginatively called “Sound_Module”.

The definition of the function that I want to set as my callback is now a member function of this class:

int Sound_Module::process(jack_nframes_t nframes, void *arg){

  // do something in the callback
  return 0;
}

I’m trying to set this method as my callback function within the class constructor:

(83) jack_set_process_callback(client, process, 0)

but on compilation this leads to the following error:

sound_module.cpp:83: error: argument of type ‘int (Sound_Module::)(jack_nframes_t, void*)’ does not match ‘int (*)(jack_nframes_t, void*)’

I get the impression from this error message that I need to cast the callback function to something else. I’ve tried a few things, such as casting to the Jack type designed for callbacks with no luck – below is the line dug out of the API for “JackProcessCallback”.

typedef int(* JackProcessCallback)(jack_nframes_t nframes, void *arg)

Can anyone shed any light on what this compiler error might be hinting at, or what I could be doing wrong?

Thanks!

Answer 1

Member functions are different from non-member functions because you can only call them on an object of the class type of which the function is a member.

You need to write a non-member function that delegates to the member function. When you call jack_set_process_callback, you tell the library what to pass to the void* parameter of your callback, so you can tell it to pass back the address of the Sound_Module object for which you want to call the member function callback.

The non-member callback is quite simple:

int Sound_Module_Process_Callback(jack_nframes_t x, void* p)
{
    return static_cast<Sound_Module*>(p)->process(x);
}

Registered as:

jack_set_process_callback(client, Sound_Module_Process_Callback, this);