Re: faster return from read()

Hi!

No need to wait for the data. You can fetch it whenever you want.

This is a code fragment from qtscope:

http://www.cn.stir.ac.uk/~mhh1/qtscope/

--------------------------

   while (comedi_get_buffer_contents(comediDevice,comediSubdevice))
     {
       // read data from device
       if (read(comedi_fileno(comediDevice), dataBuffer, 
comediChannels*2) == 0)
         {
           cerr << "Error: end of Aquisition\n";
           return;
         }
       if( num < MAX_DATA_BUFFER )
         {
           // fill the data buffer
           for( i = 0; i < comediChannels; i++)
             convDataBuffer[i][num] = ((sampl_t *)dataBuffer)[i]* \
 
(myRanges[i]->max-myRanges[i]->min)/4095.0+
myRanges[i]->min;
           num++;
         }
       else
         cout << "data buffer overflow\n";cout.flush();
     }

   // announce data to plugins

---------------------------

Basically this routine is called from a QT-timer which is terribly 
unreliable. Thus, the buffer contains more or less data or is even 
empty. This is checked by comedi_get_buffer_contents. The while loop 
loops as long as data is there ("read"). The data is then plotted on the 
screen as soon as the buffer has been emptied.

Hope you got the idea. There's also a simpler version of the program:

http://www.linux-usb-daq.co.uk/applications/comedi-scope/

/Bernd



www:    http://www.berndporr.me.uk/
         http://www.linux-usb-daq.co.uk/
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Christian Fleischer wrote:
> Hello!
> 
> I've comedi v0.7.68, comedilib 0.7.21 and wonder about something when I use 
> the "cmd.c" test-program:
> 
> When I set the number of samples to deliver to 4000, the frequency to 1000 and 
> the number of channels to 1, the read command always waits for 1024 bytes.
> Is it possible to make this queue smaller, so that the read command returns 
> faster with less bytes?
> The command line for the test-program is:
> cmd -N 4000 -F 1000 -n 1 -v
> 
> Is there any other option for a comedi command to specify how long this 
> internal buffer is?
> 
> Thanks a lot,
> Christian
> 
> 
> 
>

Received on 2004-07-16Z14:09:28