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CUPS and error dialogs

Okay, I answered my own question. I may not have all the details correct, so if you see an error, please let me know.

Printing from a cups filter or backend to stderr has special effects, as it is processed by CUPS and by the PrinterProxy application.

(A PrinterProxy is created by the system when a printer is installed. It is named the same as the printer, and placed in ~/Library/Printers/ This is the app that runs the print job, and it monitors stderr messages by polling.

If you open this application, either from the Printer Setup Utility or by task switching, you will see that any message printed to stderr will, after a few seconds, appear in the dialog box above the progress bar.

If the message printed is of the form "ERROR: ...", the message is written to /var/cups/error_log. If several seconds pass before the job finishes or another message is printed, the same message but with "ERROR: " stripped off, will appear above the progress bar in the dialog box.

If the message printed is of the form "ERROR: recoverable: ...", the message is written to /var/cups/error_log, "ERROR: " If several seconds pass before the job finishes or another message is printed, the same message but with "ERROR: " stripped off, be picked up by the PrinterProxy. PrinterProxy will in turn strip off "recoverable: ", and the remainder of the message will be displayed in a sheet that drops down from the top of the dialog box. (If the PrinterProxy was not already opened, this will cause it to be opened.) The sheet gives options to Delete Job, Stop All Jobs, or Stop Job.

It is supposed to be possible to write messages of the form "ERROR: fatal: ..." also. But when I tried that the dropdown sheet kept dropping down, sliding up, dropping down, sliding up, endlessly. I had to terminate the task to get it to stop.


Earlier this month there was a thread about CUPS and error dialogs, but it was dropped with no satisfactory answer. The problem is, a printer may have a paper jam or run out of paper during printing. somehow, the user must be notified. Some printers are successful in doing so, but some of us writng new drivers have not been able to figure out how to make this happen.
Printing to stderr does not do it, all that does is add a line to the cups error_log.
So what is the secret?

--
= Bob Sander-Cederlof <email@hidden>, <email@hidden>
= On the web at <http://bobsc5.home.comcast.net>
= Macromedia, 1500 N. Greenville, Suite 500, Richardson, TX 75081
= Phone: (972) 664-7260 FAX: (972) 680-0537
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