22.8. Realtime Signals

Realtime signals were defined in POSIX.1b to remedy a number of limitations of standard signals. They have the following advantages over standard signals:

• Realtime signals provide an increased range of signals that can be used for application-defined purposes. Only two standard signals are freely available for application-defined purposes: SIGUSR1 and SIGUSR2.

• Realtime signals are queued. If multiple instances of a realtime signal are sent to a process, then the signal is delivered multiple times. By contrast, if we send further instances of a standard signal that is already pending for a process, that signal is delivered only once.

• When sending a realtime signal, it is possible to specify data (an integer or pointer value) that accompanies the signal. The signal handler in the receiving process can retrieve this data.

• The order of delivery of different realtime signals is guaranteed. If multiple different realtime signals are pending, then the lowest-numbered signal is delivered first. In other words, signals are prioritized, with lower-numbered signals having higher priority. When multiple signals of the same type are queued, they are delivered—along with their accompanying data—in the order in which they were sent.