Technical Q&A QA1182

Why does logging keep my Printer Module from working?

Q: Why does logging keep my Printer Module from working? This used to work fine in Mac OS X 10.0 and 10.1.

A: Unlike in previous versions of Mac OS X, in Mac OS X 10.2 (Jaguar) the printing system (CUPS) makes use of stdout to route information between modules. Any code in a Printer Module (PM) or I/O Module (IOM) that tries to log to stdout via printf, etc., will interfere with the printing system. If you must write to a log file, use fprintf to write to a specific file or send your log information to stderr. See Listing 1 for examples.

//    Write to /private/var/log/cups/error_log
fprintf( stderr, "This message will appear in the CUPS error log." );

//    Write to your own log file
fprintf( file, "This message will appear in your own log file" );

Be aware that on Jaguar your PMs and IOMs run as daemon and not the currently logged in user, so if you are logging to a file via fprintf you'll need to set the appropriate permissions on the destination directory.

As an additional debugging aid, you can adjust the logging level for the CUPS "error_log" file by editing the CUPS configuration file "/etc/cups/cupsd.conf" and changing LogLevel. Listing 2 shows the relevant section from "cupsd.conf" that you will need to edit.

#
# LogLevel: controls the number of messages logged to the ErrorLog
# file and can be one of the following:
#
#     debug2    Log everything.
#     debug    Log almost everything.
#     info      Log all requests and state changes.
#     warn      Log errors and warnings.
#     error     Log only errors.
#     none      Log nothing.
#

LogLevel info

[Nov 06 2002]