Technical Q&A QA1646

Always check the result code from AudioUnitGetProperty when used with kAudioUnitProperty_FastDispatch

Q: I'm using `kAudioUnitProperty_FastDispatch` with `AudioUnitGetProperty` to get function pointers back for some Audio Unit selectors such as `kAudioUnitRenderSelect`, `kAudioUnitSetParameterSelect` and so on. How important is it to check the return code from `AudioUnitGetProperty`?

A: I'm using kAudioUnitProperty_FastDispatch with AudioUnitGetProperty to get function pointers back for some Audio Unit selectors such as kAudioUnitRenderSelect, kAudioUnitSetParameterSelect and so on. How important is it to check the return code from AudioUnitGetProperty?

While it goes without saying that checking for errors is good programming practice, it is especially important that you always check the return code from AudioUnitGetProperty when asking for function pointers from an Audio Unit using the kAudioUnitProperty_FastDispatch property.

The normal usage pattern is to check the return code and only assume that a valid function pointer was returned if AudioUnitGetProperty returns noErr. When noErr is returned, there is a guarantee that you have a valid function pointer for the specified selector.

Not checking the return code and assuming a valid function pointer in all instances will lead to crashing since some Audio Units may not implement the kAudioUnitProperty_FastDispatch property.

Listing 1 Correctly retrieving and using Audio Unit function pointers.

AudioUnit myAUnit;
ProcPtr myRenderProc, myGetParamProc, mySetParamProc;
void *myConnInstanceStorage;

void GetFastDispatchPointers(void)
{ 
    UInt32 size = sizeof(AudioUnitRenderProc);
    if (AudioUnitGetProperty(myAUnit, kAudioUnitProperty_FastDispatch,
                             kAudioUnitScope_Global, kAudioUnitRenderSelect,
                             &myRenderProc, &size) != noErr)
        myRenderProc = NULL;

    if (AudioUnitGetProperty(myAUnit, kAudioUnitProperty_FastDispatch,
                             kAudioUnitScope_Global, kAudioUnitGetParameterSelect,
                             &myGetParamProc, &size) != noErr)
        myGetParamProc = NULL;

    if (AudioUnitGetProperty(myAUnit, kAudioUnitProperty_FastDispatch,
                             kAudioUnitScope_Global, kAudioUnitSetParameterSelect,
                             &mySetParamProc, &size) != noErr)
        mySetParamProc = NULL;

    if (myRenderProc || myGetParamProc || mySetParamProc) {
        myConnInstanceStorage = GetComponentInstanceStorage(myAUnit);
    } else {
        myConnInstanceStorage = NULL;
    }
}

...

OSStatus Render (AudioUnitRenderActionFlags *  ioActionFlags,
                 const AudioTimeStamp *        inTimeStamp,
                 UInt32                        inOutputBusNumber,
                 UInt32                        inNumberFrames,
                 AudioBufferList *             ioData)

{

    if (myRenderProc != NULL) {
        return reinterpret_cast<AudioUnitRenderProc>(myRenderProc) (myConnInstanceStorage,
                                                                    ioActionFlags, inTimeStamp,
                                                                    inOutputBusNumber, inNumberFrames,
                                                                    ioData);
    }

    return AudioUnitRender(myAUnit, ioActionFlags, inTimeStamp, inOutputBusNumber, inNumberFrames, ioData);
}

References:

See CAAudioUnit.cpp and CAAudioUnit.h included with the 10.6 Xcode Tools located in Developer/Extras/CoreAudio/PublicUtility.

Document Revision History

Date	Notes
2010-02-24	New document that describes why you should always check the return code from AudioUnitGetProperty when using the kAudioUnitProperty_FastDispatch property.

Always check the result code from AudioUnitGetProperty when used with kAudioUnitProperty_FastDispatch

Q: I'm using kAudioUnitProperty_FastDispatch with AudioUnitGetProperty to get function pointers back for some Audio Unit selectors such as kAudioUnitRenderSelect, kAudioUnitSetParameterSelect and so on. How important is it to check the return code from AudioUnitGetProperty?

References:

Document Revision History

Q: I'm using `kAudioUnitProperty_FastDispatch` with `AudioUnitGetProperty` to get function pointers back for some Audio Unit selectors such as `kAudioUnitRenderSelect`, `kAudioUnitSetParameterSelect` and so on. How important is it to check the return code from `AudioUnitGetProperty`?