sigaction

Sigaction is a POSIX API used to define and control how a process handles signals. It provides a more consistent and portable mechanism than the older signal() interface, allowing precise specification of the handler and the context in which it runs.

The core data type is struct sigaction, which includes:

- sa_handler, a pointer to a function taking an int (the signal number); or sa_sigaction, a pointer to a function taking (int, siginfo_t *, void *) when extended information is needed.

- sa_mask, a signal set that specifies which signals should be blocked during execution of the signal handler.

- sa_flags, a set of options that modify handler behavior (see below).

- sa_restorer, a historical field present on some systems (often unused and considered obsolete).

To install a handler, a program fills a struct sigaction and calls sigaction(signum, &act, oldact). If oldact is non-NULL, the previous action for signum is returned. If SA_SIGINFO is set in sa_flags, the handler pointed to by sa_sigaction is used; otherwise sa_handler is used.

Common flags include:

- SA_RESTART, to automatically restart certain interrupted system calls.

- SA_NODEFER, to avoid blocking the signal while its handler runs.

- SA_RESETHAND, to reset the handler to default behavior after the signal is handled.

- SA_SIGINFO, to supply additional information to the handler via the siginfo_t structure.

- SA_ONSTACK, to use an alternate signal stack.

Notes: sigaction is the standard, portable interface for signal handling across POSIX systems (Linux, BSD, macOS, etc.). Differences may exist in edge cases and real-time signals. It supersedes signal() by offering precise control over handler type, masking, and behavior.