SIGIGN

SIGIGN is commonly used to refer to the constant SIG_IGN in POSIX-compliant systems. It designates a signal handler that causes the receiving process to ignore the specified signal. The SIG_IGN macro is defined in signal.h as a special value of the function pointer type used for signal handling. When SIG_IGN is installed for a given signal, the signal is not delivered to the process; it is effectively ignored and not added to the pending signals.

Common usage involves installing SIG_IGN as the signal handler. For example, using the C standard library: signal(SIGINT, SIG_IGN) or, with sigaction, sa.sa_handler = SIG_IGN; sigaction(SIGINT, &sa, NULL). This tells the system to discard the specified signal rather than invoking a handler.

Effects and caveats: For most signals, ignoring them means the process continues as if they did not occur. For SIGCHLD, ignoring may cause child processes to be automatically reaped on some systems, preventing zombie processes; however, this behavior is not guaranteed by all standards, so portable code should manage child processes explicitly. Some signals cannot be ignored on certain systems or may have different effects depending on the process’s signal mask and system specifics.

Portability: SIG_IGN is part of the POSIX standard, but behavior can vary across non-POSIX systems. Windows signal support differs from POSIX, so the concept of ignoring signals may not translate directly.

See also: SIG_DFL, signal, sigaction, SIGCHLD.