RTEMS POSIX API User's Guide
POSIX signals are an asynchronous event mechanism. Each process and thread has a set of signals associated with it. Individual signals may be enabled (e.g. unmasked) or blocked (e.g. ignored) on both a per-thread and process level. Signals which are enabled have a signal handler associated with them. When the signal is generated and conditions are met, then the signal handler is invoked in the proper process or thread context asynchronous relative to the logical thread of execution.
If a signal has been blocked when it is generated, then it is queued and kept pending until the thread or process unblocks the signal or explicitly checks for it. Traditional, non-real-time POSIX signals do not queue. Thus if a process or thread has blocked a particular signal, then multiple occurrences of that signal are recorded as a single occurrence of that signal.
One can check for the set of outstanding signals that have been blocked. Services are provided to check for outstanding process or thread directed signals.
RTEMS POSIX API User's Guide
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