SIGRTMAX

SIGRTMAX is a macro used in POSIX-compliant systems to denote the maximum number of real-time signals available to user-space processes. It is defined alongside SIGRTMIN, which marks the lower bound of the real-time signal range. Real-time signals are a dedicated class of signals designed for asynchronous notifications that can carry data and be queued.

The exact numeric values of SIGRTMIN and SIGRTMAX are implementation-defined and can vary between architectures and kernel versions. In Linux, for example, the common range on many architectures is from SIGRTMIN to SIGRTMAX inclusive, with SIGRTMIN often at 34 and SIGRTMAX at 64, yielding 31 real-time signals. Because the numbers are not fixed across systems, software should rely on the macros rather than hard-coded constants.

Delivery semantics for real-time signals differ from standard signals. Real-time signals are queued, allowing multiple instances to be pending, and are delivered in a first-in, first-out order for a given signal type. They can carry accompanying data via the siginfo_t structure when the signal handler is installed with SA_SIGINFO, and signals can be sent with additional data using sigqueue. Receiving processes typically set up handlers using sigaction.

Sending and handling real-time signals usually involve functions such as kill, tkill, or pthread_kill to target a process with a number in the SIGRTMIN..SIGRTMAX range, and sigqueue to attach data. Real-time signals are intended for high-priority, low-traffic IPC needs where queuing and data delivery are advantageous, but they are limited by the finite range of signals.

See also: SIGRTMIN.