Programmatic read Audio packets and decode from MP4

audio-read-ffmpeg.cpp

// Modified from - https://gist.github.com/cedricve/a7f6817c18eeb19a0716ed0ca0e037c4

// Open the initial context variables that are needed
	SwsContext *img_convert_ctx;
	AVFormatContext* format_ctx = avformat_alloc_context();
	AVCodecContext* codec_ctx = NULL;
	int audio_stream_index;
	std::ofstream output_file;
	int64_t last_time = 0, first_time = 0;

	std::cout << std::endl << "Starting audio thread" << std::endl;

	//open TCP
	if (avformat_open_input(&format_ctx, "some-url or mp4 full path",
		NULL, NULL) != 0) {
		return EXIT_FAILURE;
	}

	if (avformat_find_stream_info(format_ctx, NULL) < 0) {
		return EXIT_FAILURE;
	}

	//search audio stream
	for (int i = 0; i < format_ctx->nb_streams; i++) {
		if (format_ctx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO)
			audio_stream_index = i;
	}

	AVPacket packet;
	av_init_packet(&packet);

	//open output file
	AVFormatContext* output_ctx = avformat_alloc_context();

	AVStream* stream = NULL;
	int cnt = 0;

	//start reading packets from stream and write them to file
	av_read_play(format_ctx);    //play

	// Get the codec
	AVCodec *codec = NULL;

	/* Find the audio Stream, if no audio stream are found, clean and exit */
	AVCodec* cdc = NULL;
	int streamIndex = av_find_best_stream(format_ctx, AVMEDIA_TYPE_AUDIO, audio_stream_index, -1, &cdc, 0);
	if (streamIndex < 0)
	{
		char errBuf[256];
		std::cout << av_strerror(streamIndex, errBuf, 256);
		std::cout << errBuf << std::endl;
		avformat_close_input(&format_ctx);
		std::cout << "Could not find any supported audio stream in the file" << std::endl;
		return 1;
	}

	/* Open the audio stream to read data  in audioStream */
	AVStream* audioStream = format_ctx->streams[streamIndex];

	/* Initialize the codec context */
	codec_ctx = audioStream->codec;
	codec_ctx->codec = cdc;
	/* Open the codec, and verify if it has opened */
	if (avcodec_open2(codec_ctx, codec_ctx->codec, NULL) != 0)
	{
		avformat_close_input(&format_ctx);
		std::cout << "Couldn't open the context with the audio decoder" << std::endl;
		return 1;
	}

	if (avcodec_open2(codec_ctx, codec, NULL) < 0)
	{
		std::cout << "audio: avcodec_open2 failed";
		exit(1);
	}
	AVFrame* audio = av_frame_alloc();
	// size = samples/ch * 2 bytes/sample * 2 ch
	int audio_buffer_size = (codec_ctx->frame_size * 2) * 2; // TODO
	uint8_t* audio_buffer = (uint8_t*)(av_malloc(audio_buffer_size));
	avcodec_fill_audio_frame(audio, 2, codec_ctx->sample_fmt, audio_buffer, audio_buffer_size, 0);


	double timeBase = av_q2d(format_ctx->streams[audio_stream_index]->time_base);
	std::cout << "this is the timebase: " << timeBase << std::endl;

	while (av_read_frame(format_ctx, &packet) >= 0 && !bQuit) //&& cnt < 1000) { //read ~ 1000 frames
	{
		// std::cout << "1 Frame: " << cnt << std::endl;
		if (packet.stream_index == audio_stream_index)
		{
			//packet is audio
			int check = 0;
			packet.stream_index = audio_stream_index;
			int result = avcodec_decode_audio4(codec_ctx, audio, &check, &packet);
			if (result <= 0)
			{
				std::cout << "No audio decode result" << std::endl;
				continue;
			}
			last_time = packet.dts * timeBase;

// Register everything
av_register_all();
avformat_network_init();