How to initialize audio with Vala/SDL

Question

I've been trying to figure this out for a few hours now.

In order to start up the audio, I need to create an SDL.AudioSpec object and pass it to SDL.Audio.Open. The problem is, AudioSpec is a class with a private constructor, so when I try to create one I get:

sdl.vala:18.25-18.43: error: `SDL.AudioSpec' does not have a default constructor
        AudioSpec audiospec = new SDL.AudioSpec();
                              ^^^^^^^^^^^^^^^^^^^

And if I try to just assign values to it's member vars like a struct (it's a struct in normal sdl) I get:

sdl.vala:20.3-20.25: error: use of possibly unassigned local variable `audiospec'
        audiospec.freq = 22050;
        ^^^^^^^^^^^^^^^^^^^^^^^

I found the valac doc here: http://valadoc.org/sdl/SDL.AudioSpec.html But it isn't much help at all.

The offending code block looks like this:

// setup the audio configuration
AudioSpec audiospec;
AudioSpec specback;
audiospec.freq = 22050;
audiospec.format = SDL.AudioFormat.S16LSB;
audiospec.channels = 2;
audiospec.samples = 512;

// try to initialize sound with these values
if (SDL.Audio.open(audiospec, specback) < 0)
{
    stdout.printf("ERROR! Check audio settings!\n");
    return 1;
}

Any help would be greatly appreciated!

Another update, as I'm still having some trouble. I've changed the vapi file, and this is what I have now:

public delegate void AudioCallback (out void* userdata, out uchar stream, int len);

[CCode (cname="SDL_AudioSpec")]
[Compact]
public struct AudioSpec {
    public int freq;
    public AudioFormat format;
    public uchar channels;
    public uchar silence;
    public uint16 samples;
    public uint16 padding;
    public uint32 size;
    public AudioCallback callback;
    public void* userdata;
}// AudioSpec

I have a method that (tries?) to meet this function signature:

public void callback(out void* userdata, out uchar stream, int len)
{
    stream = 0;
    userdata = null;
}

And assigning it as:

    audiospec.callback = gen.callback;

Needless to say, this still isn't working, get lots of errors:

/home/gukid/vala/soundgen.vala.c: In function ‘sound_gen_main’:
/home/gukid/vala/soundgen.vala.c:766:12: error: ‘SDL_AudioSpec’ has no member named ‘callback_target_destroy_notify’
/home/gukid/vala/soundgen.vala.c:766:72: error: ‘SDL_AudioSpec’ has no member named ‘callback_target_destroy_notify’
/home/gukid/vala/soundgen.vala.c:766:114: error: ‘SDL_AudioSpec’ has no member named ‘callback_target’
/home/gukid/vala/soundgen.vala.c:768:11: error: ‘SDL_AudioSpec’ has no member named ‘callback_target’
/home/gukid/vala/soundgen.vala.c:769:11: error: ‘SDL_AudioSpec’ has no member named ‘callback_target_destroy_notify’
/home/gukid/vala/soundgen.vala.c:770:21: warning: assignment from incompatible pointer type [enabled by default]
/home/gukid/vala/soundgen.vala.c:771:11: error: ‘SDL_AudioSpec’ has no member named ‘callback_target’
/home/gukid/vala/soundgen.vala.c:772:11: error: ‘SDL_AudioSpec’ has no member named ‘callback_target_destroy_notify’
error: cc exited with status 256

So I'm at a bit of another sticky point.

3rd post: EUREKA! I have a solution! (debatable :P)

First off, the sdl.vapi looks like:

[CCode (cheader_filename = "SDL.h")]
public delegate void AudioCallback (void* userdata, uchar* stream, int len);

[CCode (cname="SDL_AudioSpec", has_type_id=false)]
public struct AudioSpec {
    public int freq;
    public AudioFormat format;
    public uchar channels;
    public uchar silence;
    public uint16 samples;
    public uint16 padding;
    public uint32 size;
    [CCode (delegate_target = false, type = "void*")]
    public weak AudioCallback callback;
    public void* userdata;
}// AudioSpec

And then I can just create a function:

public static void callback(void* userdata, uchar* stream, int len)

And:

audiospec.callback = callback;

Ahhh, finally my beautiful white noise generator is complete!

Answer 1

I think it's a bug in the VAPI. In sdl.vapi, try changing

[Compact]
public class AudioSpec {

to

public struct AudioSpec {

and

[CCode (cname="SDL_OpenAudio")]
public static int open(AudioSpec desired, AudioSpec obtained);

to

[CCode (cname="SDL_OpenAudio")]
public static int open(AudioSpec desired, out AudioSpec obtained);

and make your code look like:

AudioSpec audiospec = AudioSpec();
AudioSpec specback;
audiospec.freq = 22050;
audiospec.format = SDL.AudioFormat.S16LSB;
audiospec.channels = 2;
audiospec.samples = 512;

// try to initialize sound with these values
if (SDL.Audio.open(audiospec, out specback) < 0)
{
    stdout.printf("ERROR! Check audio settings!\n");
    return 1;
}

and give it a test. It seems to generate the correct code according to the SDL docs. If it works, consider submitting the VAPI changes to the Vala bugzilla.