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# Realtime Audio (blocking)

In this section, we modify the `sineosc.cpp` program in order to send the output to the default audio playback device on your computer system. We also make use of the stk::SineWave class as a sine-wave oscillator. stk::SineWave computes an internal, static sine-wave table when its first instance is created. Subsequent instances make use of the same table. The default table length, specified in SineWave.h, is 2048 samples.

    // rtsine.cpp STK tutorial program

    #include "SineWave.h"
    #include "RtWvOut.h"
    #include <cstdlib>

    using namespace stk;

    int main()
    {
      // Set the global sample rate before creating class instances.
      Stk::setSampleRate( 44100.0 );
      Stk::showWarnings( true );

      int nFrames = 100000;
      SineWave sine;
      RtWvOut *dac = 0;

      try {
        // Define and open the default realtime output device for one-channel playback
        dac = new RtWvOut( 1 );
      }
      catch ( StkError & ) {
        exit( 1 );
      }

      sine.setFrequency( 441.0 );

      // Option 1: Use StkFrames
      /*
      StkFrames frames( nFrames, 1 );
      try {
        dac->tick( sine.tick( frames ) );
      }
      catch ( StkError & ) {
        goto cleanup;
      }
      */

      // Option 2: Single-sample computations
      for ( int i=0; i<nFrames; i++ ) {
        try {
          dac->tick( sine.tick() );
         }
        catch ( StkError & ) {
          goto cleanup;
        }
      }

     cleanup:
      delete dac;

      return 0;
    }

The class stk::RtWvOut is a protected subclass of stk::WvOut. A number of optional constructor arguments can be used to fine tune its performance for a given system. stk::RtWvOut provides a "single-sample", blocking interface to the RtAudio class. Note that stk::RtWvOut (as well as the stk::RtWvIn class described below) makes use of RtAudio's callback input/output functionality by creating a large ring-buffer into which data is written. These classes should not be used when low-latency and robust performance is necessary

Though not used here, an stk::RtWvIn class exists as well that can be used to read realtime audio data from an input device. See the `record.cpp` example program in the `examples` project for more information.

It may be possible to use an instance of stk::RtWvOut and an instance of stk::RtWvIn to simultaneously read and write realtime audio to and from a hardware device or devices. However, it is recommended to instead use a single instance of RtAudio to achieve this behavior, as described in the next section. See the `effects` project or the `duplex.cpp` example program in the `examples` project for more information.

When using any realtime STK class (RtAudio, stk::RtWvOut, stk::RtWvIn, RtMidi, stk::InetWvIn, stk::InetWvOut, stk::Socket, stk::UdpSocket, stk::TcpServer, stk::TcpClient, and stk::Thread), it is necessary to specify an audio/MIDI API preprocessor definition and link with the appropriate libraries or frameworks. For example, the above program could be compiled on a Linux system using the GNU g++ compiler and the ALSA audio API as follows (assuming all necessary files exist in the project directory):

    g++ -Wall -D__LINUX_ALSA__ -D__LITTLE_ENDIAN__ -o rtsine Stk.cpp Generator.cpp SineWave.cpp WvOut.cpp \
        RtWvOut.cpp RtAudio.cpp rtsine.cpp -lpthread -lasound

On a Macintosh OS X system, the syntax would be:

    g++ -Wall -D__MACOSX_CORE__ -o rtsine Stk.cpp Generator.cpp SineWave.cpp WvOut.cpp RtWvOut.cpp RtAudio.cpp \
       rtsine.cpp -lpthread -framework CoreAudio -framework CoreMIDI -framework CoreFoundation

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