EXC_BAS_ACCESS in Core Audio - writing mic data to file w/ Extended AudioFile Services - core-audio

Question

I am attempting to write incoming mic audio to a file. Because the audio samples are delivered 4096 frames (the set frame rate for my project) at a time in a time-critical callback I cannot simply write the bytes to a file with AudioFileWriteBytes. I also did not wish to go through the effort and complexity of setting up my own ring buffer to store samples to write elsewhere. So I am using Extended Audio File API for its ExtAudioFileWriteAsync function.
As per instructed by the documentation I create the ExtAudioFileRef with a CFURL and than run it once with a null buffer and 0 frames in main. Then I initiate my AUHAL unit and the input callback begins to be called.
ExtAudioFileWriteAsync(player.recordFile, 0, NULL);
There I have my code to write to this file asynchronously. I have the call nested in a dispatch queue so that it runs after the callback function exits scope (tho not sure if that is necessary but I get this error with all without the enclosing dispatch block). This is the callback as it is right now.
OSStatus InputRenderProc(void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList * ioData)
{
MyAUGraphPlayer *player = (MyAUGraphPlayer*) inRefCon;
// rendering incoming mic samples to player->inputBuffer
OSStatus inputProcErr = noErr;
inputProcErr = AudioUnitRender(player->inputUnit,
ioActionFlags,
inTimeStamp,
inBusNumber,
inNumberFrames,
player->inputBuffer);
printf("%i", inNumberFrames);
dispatch_async(player->fileWritingQueue, ^{
ExtAudioFileWriteAsync(player->recordFile, 4096, player->inputBuffer);
});
return inputProcErr;
}
It immediately bails out with the bad access exception on the first callback invocation. For clarity these are the settings I have for creating the file to begin with.
// describe a PCM format for audio file
AudioStreamBasicDescription format = { 0 };
format.mBytesPerFrame = 4;
format.mBytesPerPacket = 4;
format.mChannelsPerFrame = 2;
format.mBitsPerChannel = 16;
format.mFramesPerPacket = 1;
format.mFormatFlags = kAudioFormatFlagIsPacked | kAudioFormatFlagIsFloat;
format.mFormatID = kAudioFormatLinearPCM;
CFURLRef myFileURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, CFSTR("./test2.wav"), kCFURLPOSIXPathStyle, false);
ExtAudioFileCreateWithURL(myFileURL,
kAudioFileWAVEType,
&format,
NULL,
kAudioFileFlags_EraseFile,
&player.recordFile);
player.fileWritingQueue = dispatch_queue_create("myQueue", NULL);
ExtAudioFileWriteAsync(player.recordFile, 0, NULL);