The system defines a set of signals that may be delivered to a process. A process may specify a handler to which a signal is delivered, or specify that a signal is to be blocked or ignored . A process may also specify that a default action is to be taken by the system when a signal occurs. Normally, signal handlers execute on the current stack of the process. This may be changed, on a per-handler basis, so that signals are taken on a special signal stack .
All signals have the same priority .
A global signal mask defines the set of signals currently blocked from delivery to a process. When a signal condition arises for a process, the signal is added to a set of signals pending for the process. If the signal is not currently blocked by the process then it is delivered to the process. The call to the handler is arranged so that if the signal han- dling routine returns normally the process will resume execution in the context from before the signal's delivery. If the process wishes to re- sume in a different context, then it must arrange to restore the previous context itself.