Trying to get status via cupsBackChannelRead does not work with thiscode.

[Johan Henselmans]

Op 2/6/12 9:53 PM, Johan Henselmans schreef:
> I am having problems trying to get the status from the printer I am writing a filter for.
> 

I have found out the answer myself: as it appears, the printer does not send replies directly, but withing .5 to 2 seconds.

That explains why I am not getting the reply directly. This also implies that I will not be able to use the filter to
pick up status codes. Sometimes the filter executable is already shut down when the status codes will arrive.

Now I have to find another way to pick up the status messages from the printer. First I was thinking about setting up a
port monitor to pick up the status messages, as per http://www.cups.org/documentation.php/spec-design.html.

However, I suppose that the port monitor as described in this spec will also shut down after all the info has been sent
to the printer.

Should I run a separate daemon to pick up the status codes that the printer sends via the backend?




> The code I am using is:
> 
> 				
> 	status = cupsSideChannelDoRequest(CUPS_SC_CMD_GET_BIDI, &data, &datalen, 5.0);
> 
> 	// Read the response if the output was sent
> 	if (status == CUPS_SC_STATUS_OK & datalen==1){
> 	  bidi=(cups_sc_bidi_t)data;
> 	  // drain the pending output
> 	  status = cupsSideChannelDoRequest(CUPS_SC_CMD_DRAIN_OUTPUT, &data, &datalen, 5.0);
> 
> 	  fprintf(stdout, "   <S2>"); //send status request, returns seven digit ticket count and prom version
> 	  fflush(stdout);
> 	  count = cupsBackChannelRead(backbuffer, sizeof(backbuffer), 0.0);
> 	  fprintf(stderr, "DEBUG: ***** backbuf  Read %i backchannel bytes in Setup*****\n", count);
> 	  if (count > 0){
> 	    fprintf(stderr, "DEBUG: ***** backbuffer: [");
>             int i = 0;
>             while (i < count){
> 	      fprintf(stderr, "%c", backbuffer[i]);
> 	      i++;
> 	    }
> 	    fprintf(stderr, "] *****\n");
> 	  }
> 	fflush(stderr);
> 	}
> 		
> 
> This should return 23 bytes of data. The result is that I do not get 23 bytes of data, but -1, and after that I get a
> bunch of messages that some data is received. It seems that the cupsBackChannelRead is not waiting long enough for the
> answer. I varied with different settings for the time-out, from 20.0 to 0.0 to -1.0 as per
> 
> http://www.cups.org/documentation.php/api-filter.html#cupsBackChannelRead
> 
> But the results are not what I would expect, sometimes I get 1 bytes, sometimes -1 bytes, sometime I get the full 23
> bytes. Is there something that I am missing here?
> 
> 
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] ***** backbuf  Read -1 backchannel bytes in Setup*****
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 2 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> D [06/Feb/2012:20:59:09 +0100] [Job 1786] Read 1 bytes of back-channel data...
> 


Kind Regards,

Johan Henselmans