[SDL] Transferring Decoded Audio

[Scott Harper]

Hello, (last one sent from the wrong address...)

I crossposted this on the ffmpeg mailing lists, but upon thinking  
today I realized that since my problem might involve threads, or  
likely has something to do with getting audio to SDL (and since you  
guys respond infinitely better to requests than anyone on the ffmpeg  
lists), I'd give it a shot here, too.  Sorry if anyone received this  
already.

I'm using the ffplay source to try to help me with decoding and  
playing back audio using SDL and the libavcodec and its associated  
libraries.

I'm using C++, and I have a class which maintains the ffmpeg  
interface-type code (AVCodecContext, etc...) and runs a thread to  
grab the next frame whilst the current one draws.  The video fetching  
code works fine, but I'll post it anyway for completeness' sake.   
Here's the threaded function:

int VE_Media::thread_func() {
	mMutex = SDL_CreateMutex();
	
	while( mThreadLives ) {
		if( needNextFrame ) {
			int frameFinished;
			SDL_mutexP(mMutex);	//lock
			av_read_frame(pFormatCtx, &packet);// >= 0) {
				if(packet.stream_index == videoStream ) {
					// Decode video frame
					avcodec_decode_video(pCodecCtx, pFrameYUV, &frameFinished,
											packet.data, packet.size);
					
					if(frameFinished) {
						sws_scale( pSwsCtx, pFrameYUV->data, pFrameYUV->linesize,
									0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize );
					}
				} else if( packet.stream_index == audioStream ) {
					mAudioBuffer = (int16_t *)av_fast_realloc(mAudioBuffer,  
&mAudioBufferSize, VEMAX(packet.size, AVCODEC_MAX_AUDIO_FRAME_SIZE));
					mAudioDataSize = mAudioBufferSize;
					int result = avcodec_decode_audio2( mAudioCodecCtx,  
mAudioBuffer, &mAudioDataSize,
																							packet.data, packet.size );
					if( result == -1 )
						printf( "error decoding audio\n" );
					//else
					//	printf( "decoded audio packet just fine!\n" );
				}
				av_free_packet( &packet );
				SDL_mutexV(mMutex);	//unlock
			}
		SDL_Delay(1);
	}
	
	printf( "VE_Media:  thread dying\n" );
	SDL_DestroyMutex( mMutex );
	
	return 0;
}

...the code I call to grab the audio buffer is as follows:

int VE_Media::getCurrentAudioFrame(int16_t *audioBuffer) {
	if( audStreamExists ) {
		audioBuffer = mAudioBuffer;
		return mAudioDataSize;
	} else
		return NULL;
}

...In my sound class, I have created the callback function for SDL's  
audio, basing it largely on the code within ffplay.c:

void VE_Sound::audio_callback( void *userdata, Uint8 *stream, int  
len ) {
	int dLen;
	
	//grab audio from all media objects and mix together
	if( mMediaList.size() > 0 ) {
		while( len > 0 ) {
			//check if we need to grab more audio...
			if( mAudioBufferIndex >= mAudioBufferSize ) {
				mAudioBufferSize = mMediaList[0]->getCurrentAudioFrame 
( mAudioBuffer );
				if( mAudioBufferSize < 0 ) {
					mAudioBufferSize = 1024;
					memset(mAudioBuffer, 0, 1024);
				} else {
					//synchronize for audio to transfer...
					//...look at ffplay.c for help
				}
				mAudioBufferIndex = 0;
			}
			dLen = mAudioBufferSize - mAudioBufferIndex;
			if( dLen > len )
				dLen = len;		//is this even necessary?
			
			memcpy(stream, (uint8_t *)mAudioBuffer + mAudioBufferIndex, dLen);
			len -= dLen;
			mAudioBufferIndex += dLen;
			printf( "length of audio buffer: %i\nnew index: %i\n",  
mAudioBufferSize, mAudioBufferIndex );
		}
	}
}

...What's happening is my program invariably crashed on the "memcpy 
()" function call, stating EXC_BAD_ACCESS, which I think means that  
I'm trying to write/read memory from an array which is not assigned  
to that array.

If someone could offer me some help on this, I would greatly  
appreciate it.  I'm sure it's something I've missed with either  
threading or memory/data shuffling that I've screwed up.

-- Scott Harper