Real-time signals

The POSIX specification defines so called real-time signals and Linux supports it. They are to be used by the programmer and have no predefined meaning. Two macros are available: SIGRTMIN and SIGRTMAX that tells the range of these signals. You can use one using SIGRTMIN+n where n is some number. Never hard code their numbers, real time signals are used by threading library (both LinuxThreads and NTPL), so they adjust SIGRTMIN at run time.

Whats the difference between RT signals and standard signals? There are couple:

• More than one RT signal can be queued for the process if it has the signal blocked while someone sends it. In standard signals only one of a given type is queued, the rest is ignored.
• Order of delivery of RT signal is guaranteed to be the same as the sending order.
• PID and UID of sending process is written to si_pid and si_uid fields of siginfo_t. For more information see section about Real time signals in signal(7).