Real-time Signals
       Linux supports real-time signals as originally defined in the POSIX.1b real-time extensions (and now included in POSIX.1-2001).  The range of  supported
       real-time  signals  is  defined by the macros SIGRTMIN and SIGRTMAX.  POSIX.1-2001 requires that an implementation support at least _POSIX_RTSIG_MAX (8)
       real-time signals.

       The Linux kernel supports a range of 32 different real-time signals, numbered 33 to 64.  However, the glibc POSIX threads implementation internally uses
       two  (for  NPTL) or three (for LinuxThreads) real-time signals (see pthreads(7)), and adjusts the value of SIGRTMIN suitably (to 34 or 35).  Because the
       range of available real-time signals varies according to the glibc threading implementation (and this variation can occur at run time according  to  the
       available  kernel  and  glibc),  and  indeed the range of real-time signals varies across UNIX systems, programs should never refer to real-time signals
       using hard-coded numbers, but instead should always refer to real-time signals using the notation SIGRTMIN+n, and  include  suitable  (run-time)  checks
       that SIGRTMIN+n does not exceed SIGRTMAX.

       Unlike  standard  signals,  real-time  signals have no predefined meanings: the entire set of real-time signals can be used for application-defined pur-
       poses.  (Note, however, that the LinuxThreads implementation uses the first three real-time signals.)

       The default action for an unhandled real-time signal is to terminate the receiving process.

       Real-time signals are distinguished by the following:

       1.  Multiple instances of real-time signals can be queued.  By contrast, if multiple instances of a standard signal are delivered while that  signal  is
           currently blocked, then only one instance is queued.

       2.  If  the signal is sent using sigqueue(2), an accompanying value (either an integer or a pointer) can be sent with the signal.  If the receiving pro-
           cess establishes a handler for this signal using the SA_SIGINFO flag to sigaction(2) then it can obtain this data via the si_value field of the sig-
           info_t  structure  passed  as the second argument to the handler.  Furthermore, the si_pid and si_uid fields of this structure can be used to obtain
           the PID and real user ID of the process sending the signal.

       3.  Real-time signals are delivered in a guaranteed order.  Multiple real-time signals of the same type are delivered in the order they were  sent.   If
           different  real-time  signals  are sent to a process, they are delivered starting with the lowest-numbered signal.  (I.e., low-numbered signals have
           highest priority.)  By contrast, if multiple standard signals are pending for a process, the order in which they are delivered is unspecified.