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. The LP print service puts standard input into the message that alerts the administrator to the printer fault. If its standard input is empty, the lp.tell program 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. However, doing so overrides 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 the kill(1) and signal(3C) man pages. If the printer interface program dies from receipt of any other signal, the LP print service assumes that future print requests are not be affected. As a result, the LP print service 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 default behavior, so that the signals are trapped at appropriate times. The standard interface interprets receipt of these signals as warnings that the printer has a problem. When the standard interface receives a signal, it issues a fault alert.