POSIXTimers

POSIX timers are a portable API defined by the POSIX real-time extensions that enable a process to create and manage timers that deliver asynchronous notifications when they expire. A timer is created with timer_create, which associates the timer with a clock (for example CLOCK_REALTIME or CLOCK_MONOTONIC) and a notification method described by a struct sigevent. The timer is referenced by a timer_t handle and controlled with timer_settime, timer_gettime, and timer_delete.

The timing and repetition are described by an itimerspec structure, containing it_value (initial expiration) and it_interval (period). If it_value is nonzero, the timer first expires after that interval; if it_interval is nonzero, the timer repeats with that interval, yielding periodic expirations. If it_interval is zero, the timer expires only once unless reset.

Notification methods are specified in the sigevent object. The common choice is SIGEV_SIGNAL, which sends a specified signal (sigev_signo) to the process or a thread upon expiration. Some implementations support SIGEV_THREAD, which invokes a user function in a newly created thread when the timer expires; others may support targeting a specific thread.

Clocks determine how time is measured. CLOCK_REALTIME follows wall clock time; CLOCK_MONOTONIC measures time since an unspecified starting point and is not affected by system clock changes. Timers are widely supported on UNIX-like systems, though available clocks and notification options can vary by implementation.

POSIX timers are used for timeouts, periodic tasks, and precise scheduling in real-time and application software. Developers must consider signal-safety in handlers and resource limits when creating multiple timers.