[alsa-devel] Alsa "short read"

20 Nov 2008


      Hi all,
Is there anybody out there who can help me to answer the following
questions:
I have converted an application called Linrad
( http://www.nitehawk.com/sm5bsz/linuxdsp/linrad.htm ) from OSS to ALSA.
Linrad is a remarkable DSP application used worldwide by many
radio-hams.
I am facing hereby a problem with a "short read" :
An ALSA device is opened in blocking mode:
*********************************************
err=snd_pcm_open(&rx_ad_handle, 
                 rx_sndin_plughw_pcm_name,
                 SND_PCM_STREAM_CAPTURE,
                 0);
*********************************************
When I subsequently read the device with following statement I sometimes
get a "short read":
********************************************
      err=snd_pcm_readi(rx_ad_handle,rxin_isho, rxad.block_frames);
      if((unsigned int)err != rxad.block_frames)
        {
        if(err == -EPIPE)           // EPIPE, try recovery  
          { 
          err= snd_pcm_recover(rx_ad_handle, err, 0);
          if(err < 0)lirerr(1108);  // recovery failed
          }
        else 
          {
          if(err < 0)             
            {
            lirerr(1113);           // catch all
            }
          else
            {
            lirerr(1116);            // short read
            }
          }
        }
      }
**************************************************
Here are my questions:
1. How is it possible to get a "short read" in blocking mode ?
I would think that in blocking mode the control is only returned after
the read is completed.
2. What are the possible causes for a "short read" ?
3. What can be done to  avoid a "short read" ?
4. How am I supposed to recover from a "short read"?
Best regards,
Pierre Vanhoucke/ ON5GN