sigactionsignum

Sigactionsignum refers to the signal number parameter used with the sigaction system call in POSIX-compliant systems. It designates which signal’s handling behavior the programmer wishes to configure or inspect. The term combines “sigaction,” the routine that installs or queries signal handlers, with “signum,” the numeric identifier of a particular signal.

In POSIX, the sigaction interface takes the form int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact). The signum argument specifies the target signal. It is typically one of the standard signal macros such as SIGINT, SIGTERM, or SIGUSR1. Most signals can be caught, ignored, or handled with a custom routine, but a few signals (notably SIGKILL and SIGSTOP) cannot be caught or ignored; attempting to install a handler for them yields an error, commonly EINVAL.

The sigaction structure defines how a signal is handled. It includes sa_handler or sa_sigaction (the function to be invoked, with or without extended information), sa_mask (a set of signals to block during execution of the handler), and sa_flags (options like SA_RESTART or SA_SIGINFO). When SA_SIGINFO is set, the handler is sa_sigaction with a signature that receives additional context via a siginfo_t parameter.

Usage typically involves filling a struct sigaction, setting the desired handler, clearing or configuring sa_mask, and invoking sigaction(signum, &act, &oldact). The previous action can be stored in oldact if needed.

Platform behavior can vary for real-time signals and certain flags, but the signum parameter remains the primary identifier for selecting the signal whose behavior is modified.