SIGRTMIN
SIGRTMIN is the symbolic name for the lowest real-time signal number in POSIX-compliant systems. It marks the lower bound of the range of real-time signals, which on a given system run from SIGRTMIN through SIGRTMAX inclusive. The exact numeric values are implementation-defined and may vary by architecture and C library; the macros SIGRTMIN and SIGRTMAX are provided so programs can adapt to the system’s range.
Real-time signals differ from standard signals in that they are queued: multiple instances and multiple distinct
Usage and semantics: Real-time signals are commonly employed for asynchronous notifications between processes or between threads
Notes: The range of real-time signals and the behavior of their queuing are defined by the system