Blocking Read/Write Functions

PortAudio V19 adds a huge advance over previous versions with a feature called Blocking I/O. Although it may have lower performance that the callback method described earlier in this tutorial, blocking I/O is easier to understand and is, in some cases, more compatible with third party systems than the callback method. Most people starting audio programming also find Blocking I/O easier to learn.

Blocking I/O works in much the same way as the callback method except that instead of providing a function to provide (or consume) audio data, you must feed data to (or consume data from) PortAudio at regular intervals, usually inside a loop. The example below, excepted from patest_read_write_wire.c, shows how to open the default device, and pass data from its input to its output for a set period of time. Note that we use the default high latency values to help avoid underruns since we are usually reading and writing audio data from a relatively low priority thread, and there is usually extra buffering required to make blocking I/O work.

Note that not all API's implement Blocking I/O at this point, so for maximum portability or performance, you'll still want to use callbacks.

/* -- initialize PortAudio -- */
err = Pa_Initialize();
if( err != paNoError ) goto error;
 
/* -- setup input and output -- */
inputParameters.device = Pa_GetDefaultInputDevice(); /* default input device */
inputParameters.channelCount = NUM_CHANNELS;
inputParameters.sampleFormat = PA_SAMPLE_TYPE;
inputParameters.suggestedLatency = Pa_GetDeviceInfo( inputParameters.device )->defaultHighInputLatency ;
inputParameters.hostApiSpecificStreamInfo = NULL;
 
outputParameters.device = Pa_GetDefaultOutputDevice(); /* default output device */
outputParameters.channelCount = NUM_CHANNELS;
outputParameters.sampleFormat = PA_SAMPLE_TYPE;
outputParameters.suggestedLatency = Pa_GetDeviceInfo( outputParameters.device )->defaultHighOutputLatency;
outputParameters.hostApiSpecificStreamInfo = NULL;
 
/* -- setup stream -- */
err = Pa_OpenStream(
          &stream,
          &inputParameters,
          &outputParameters,
          SAMPLE_RATE,
          FRAMES_PER_BUFFER,
          paClipOff,      /* we won't output out of range samples so don't bother clipping them */
          NULL, /* no callback, use blocking API */
          NULL ); /* no callback, so no callback userData */
if( err != paNoError ) goto error;
 
/* -- start stream -- */
err = Pa_StartStream( stream );
if( err != paNoError ) goto error;
printf("Wire on. Will run one minute.\n"); fflush(stdout);
 
/* -- Here's the loop where we pass data from input to output -- */
for( i=0; i<(60*SAMPLE_RATE)/FRAMES_PER_BUFFER; ++i )
{
   err = Pa_WriteStream( stream, sampleBlock, FRAMES_PER_BUFFER );
   if( err ) goto xrun;
   err = Pa_ReadStream( stream, sampleBlock, FRAMES_PER_BUFFER );
   if( err ) goto xrun;
}
/* -- Now we stop the stream -- */
err = Pa_StopStream( stream );
if( err != paNoError ) goto error;
 
/* -- don't forget to cleanup! -- */
err = Pa_CloseStream( stream );
if( err != paNoError ) goto error;
 
Pa_Terminate();
return 0;

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