Sigevent

Sigevent refers to a POSIX structure that describes how a process or thread should be notified when a specified asynchronous event occurs. It is most commonly used with POSIX timers (via timer_create) and message queue notifications (via mq_notify), but can be involved with other asynchronous mechanisms as well. The notification method is selected by the sigev_notify field and can be delivered as a signal, by invoking a function in a new thread, or not delivered at all.

The typical members of struct sigevent include:

- sigev_notify: the notification type, such as SIGEV_SIGNAL, SIGEV_THREAD, or SIGEV_NONE. Some systems also support extended options like SIGEV_THREAD_ID.

- sigev_signo: the signal number to deliver when using SIGEV_SIGNAL.

- sigev_value: a union (sigval) that carries data to the notification. For signals, this data is accessible via si_value in the signal handler; for threads, it is passed to the thread function.

- sigev_notify_function: a pointer to the function to invoke when using SIGEV_THREAD.

- sigev_notify_attributes: a pointer to thread attributes to use when creating the notification thread (platform-dependent).

The related type sigval, defined as union sigval, can hold either an int or a void* (commonly sival_int and sival_ptr) and is used to pass user data to the notification handler.

Implementation details vary by system. In timer_create, the kernel copies the sigevent content when establishing the timer; in mq_notify, the notification is similarly determined by the structure. While the core concept is portable, the exact fields and behavior may differ across Unix-like platforms, so programmers should consult their system’s documentation for precise semantics and supported notification types.