Fault Messages

You can use the lp.tell program to send fault messages to the LP print service. This program is referenced by the LPTELL shell variable in the standard printer interface code. The program takes standard input and sends it to the LP print service, where it is put into the message that alerts the administrator to the printer fault. If its standard input is empty, lp.tell does not initiate an alert. For an example of how the lp.tell program is used, examine the standard printer interface code immediately after the following comment:

# Set up the $LPTELL program to capture fault messages here

If you use the special exit code 129 or the lp.tell program, the printer interface program does not need to disable the printer itself. The interface program can disable the printer directly, but doing so will override the fault-alerting mechanism. Alerts are sent only if the LP print service detects that the printer has a fault, and the special exit code and the lp.tell program are its main detection tools.

If the LP print service has to interrupt printing of a file at any time, it kills the interface program with a signal TERM (trap number 15). (For more information, see kill(1) and signal(3C).) If the printer interface program dies from receipt of any other signal, the LP print service assumes that future print requests will not be affected, and continues to use the printer. The LP print service notifies the user who submitted the request that the request has not been finished successfully.

When the interface is first invoked, the signals HUP, INT, QUIT, and PIPE (trap numbers 1, 2, 3, and 13) are ignored. The standard interface changes this so the signals are trapped at appropriate times. The standard interface interprets receipt of these signals as warnings that the printer has a problem; when it receives a signal, it issues a fault alert.