More than One Handler for the Same Signal

The cobpostsighandler() routine enables two or more handlers to be posted for the same signal. When that signal is generated by the operating system (such as a SIGINT generated when the interrupt key is pressed), each handler can take appropriate action and then let other handlers be executed (by returning a non-zero value). Alternatively, they can decide to make the signal "ignored" by returning a value of zero to prevent other handlers from being executed. However, so that your handler can work with other handlers for the same signal, a handler of priority 140 (or more) should return non-zero to let other handlers have the opportunity to also be notified. A handler of priority 128 can return zero if you do not want the run-time system default action to be executed (which may cause termination).

When you raise a signal (such as by using CTRL+C to raise a SIGINT signal, or when raising a signal to a different run-unit or application), then it is probable that only your signal handler wants to be called and other posted signal handlers are not expecting the signal. Hence, in this case, you should set a global resource before raising the signal and then check that resource in your signal handler. If the resource is set (as expected), then the signal was raised by you, so you can take the appropriate action and return zero to prevent other handlers from being unexpectedly called. If the resource is unset, then someone else (or something else, such as the operating system) raised the signal and so you can take no action and should let other handlers execute by returning a non-zero value.

Examples of global resources are global variables (if in the same run unit), shared memory, pipes, files, and so on. Hence, in the above example, we could have set a global variable before raising the alarm() signal. We would then need to check the global variable in the signal handler, mysigcatch(). If it was set, we would carry out our action, reset the variable and return zero. However, if the global variable had not been set, then some other code is likely to have raised the signal. Hence, we would not want to carry out our action and would instead need to return a non-zero value immediately. This allows the appropriate action to be taken in some other handler.