Writing Signal Handlers

A signal handler should do very little. A signal can be generated at any time so the routine that was being executed could be in an "unsafe" state (eg. in the middle of modifying global variables) and so trying to execute it in your signal handler could cause problems. The routine being executed could be a C library routine such as malloc().

There is a small subset of C library routines that are safe to call from signal handlers. Do not try to call any other C library routines, or any COBOL routines, from your signal handler. See your system documentation for more information on the safe C library routines.

Some signals are generated by the operating system when a serious error has occurred. Catching these signals can cause unexpected and potentially dangerous results. You should not catch any of the following signals:

Preventing the run-time system from processing certain signals can break some run-time system functionality. If you catch any of these signals (with a priority of 140 or more), your signal handler must return a non-zero value so that the run-time system's handler is also executed. The signals concerned include:

Note: We reserve the right to change our default signal handlers or use extra signals in the future.

For an example, see Example of Posting a Signal Handler.