[cups.development] Re: Backchannel usage

[Michael Sweet]

Josh Mathis wrote:
> After doing some more testing, it appears that the USB backend is not 
> fetching the data from the printer until my rastertoxxx filter has 
> exited (in most cases).
> 
> Just as a recap - I modified my filter to just send a status request to 
> the printer. The printer should return 1 byte in reponse, which I 
> attempt to read, then exit the filter.
> 
> In the error log, I see:
> 
> [Job xx] **MY Debug** Wrote status request
> [Job xx] **MY Debug** Read -1 backchannel bytes
> PID # (my filter) has exited with no errors.
> [Job xx] *****DEBUG: Received 1 bytes of back-channel data!
> 
> In my filter, I write the status request and try reading from the 
> backchannel. Once my filter exits, the USB backend gets the response. Is 
> this the intended behavior, or is there a way I can trigger the USB 
> backend to read any backchannel data from the printer without exiting my 
> filter?

The USB backend checks for backchannel data every time it writes.
It could be as simple as the USB backend blocking on a read() on
the pipe from your driver filter...  Yup, that's the problem, and
it affects all of the backends by the looks of things... :(

Can you file a bug report for this at:

     http://www.cups.org/str.php

Thanks!

-- 
______________________________________________________________________
Michael Sweet, Easy Software Products           mike at easysw dot com
Internet Printing and Document Software          http://www.easysw.com