SIGEVSIGNAL

SIGEV_SIGNAL, sometimes written as SIGEVSIGNAL, is a notification method defined in the POSIX sigevent interface. It specifies that a signal should be generated to notify a process (and, depending on the system, a particular thread) when an associated asynchronous event occurs, such as a timer expiration, I/O completion, or a message queue state change.

In the sigevent structure, SIGEV_SIGNAL is selected by setting the sigev_notify field to SIGEV_SIGNAL. The signal number to deliver is provided in sigev_signo, and an accompanying payload can be supplied in sigev_value (a union sigval) for retrieval by the signal handler. The actual delivery follows the platform’s standard signal delivery rules; on multi-threaded systems, the signal may be delivered to a particular thread or to the process as a whole, subject to the process’s signal disposition and masking.

SIGEV_SIGNAL is one of several notification methods for asynchronous events. Others include SIGEV_NONE (no notification) and SIGEV_THREAD (invoke a user-space function in a new thread). Some systems also define extensions such as SIGEV_THREAD_ID to target a specific thread. The mechanism is commonly used with APIs like timer_create and mq_notify, enabling asynchronous event handling via signals.

Usage considerations include ensuring a signal handler is installed for the chosen signal (often with SA_SIGINFO to access the payload), and understanding how signal delivery interacts with the program’s existing threading and signal-masking strategy. SIGEV_SIGNAL provides a lightweight way to receive notifications without spawning new threads.