Declaring extern C functions with void return type

Hi,

I'm working on a simple SDL2 test, creating bindings with the help of
Cigloo (and posterior manual adjustments). One use case I'm interested in
is generating audio procedurally, which involves providing a callback
function that will be invoked periodically by SDL2, where a buffer of a
given size is meant to be filled with the desired audio samples. Currently
I'm having trouble finding the right directives for the callback function
and a structure that is passed as a parameter to the function that is used
to register it.

The callback function is of the form:

*void audio_callback(void *userdata, Uint8 *stream, int len);*


The function used to register this callback is SDL_OpenAudioDevice, which
receives a SDL_AudioSpec* (among other parameters). This structure's
"callback" field is used set the callback up.

The definitions generated by Cigloo are:

*(type Uint8 uint8_t "Uint8")*
*(type Uint8* (pointer Uint8) "Uint8 *")*
*(type SDL_AudioCallback *void*,Uint8*,int->void "SDL_AudioCallback")*
*(type SDL_AudioSpec (struct ... (callback::SDL_AudioCallback "callback")
...) **"struct SDL_AudioSpec"**)*


On my program I tried the following definitions:

*(module audio*
*    (include "sdl.scm")*
*        (export (audio-callback::void ::*void* ::Uint8* ::int))*
*        (extern (export audio-callback "scheme_audio_callback")))*


The problem I have in that case is that I have not found a way of
"returning" void from the function (since all Scheme functions implicitly
return some value). Returning any value gives a compilation error of the
form "Type error: 'void' expected, 'x' provided". I tried using pragma with
a return type of void, but the following example gives this compilation
error: "Type error --  "obj" expected, "void" provided".

This is the function and the way I set the structure's callback field (req
is an instance of SDL_AudioSpec*).

*(define (audio-callback userdata stream len)*
*  (pragma::void "return;"))*


*(SDL_AudioSpec*-callback-set! req audio-callback) *


The other option I tried is using the type defined by Cigloo:


*(module audio*
*    (include "sdl.scm")**        (export
(audio-callback::SDL_AudioCallback))*
*        (extern (export audio-callback "scheme_audio_callback")))*


But this one fails with the following error (which is logical, after all):

*ERROR:audio-callback:Prototype and definition don't match (arity differs)*
*(audio-callback::SDL_AudioCallback) -- (define (audio-callback userdata
stream len)*


Is it possible to "return" void from a function? What is the proper way for
defining these types/functions?

Thank you!

Regards,
Francisco