All RtAudio clients must create a function of type RtAudioCallback to read and/or write data from/to the audio stream. When the underlying audio system is ready for new input or output data, this function will be invoked.
To continue normal stream operation, the RtAudioCallback function should return a value of zero. To stop the stream and drain the output buffer, the function should return a value of one. To abort the stream immediately, the client should return a value of two.