POSIX Timers: Introduction#

Overview#

  • One-shot and periodic variants

  • Nanosecond granularity

  • Parameterizable clock (see Clocks, And Points In Time)

  • Absolute time-points and relative time-intervals

  • Dynamically created (timer_t, see here)

  • Notification using freely choosable signals (which can be waited for synchronously for that matter; see Synchronous Signal Handling)

  • Notification using threads (see POSIX Threads)

Creation And Deletion#

#include <time.h>

int timer_create(clockid_t clockid,
                 struct sigevent *_Nullable restrict sevp,
                 timer_t *restrict timerid);
int timer_delete(timer_t timerid);

  • “Callback” configuration via struct sigevent

  • Signal or thread

  • Optional callback parameter

struct sigevent#

  • struct sigevent - timer “callback” specification

  • A bit convoluted

  • Not all fields are valid in all circumstances

Notification type

  • sigev_notify = SIGEV_SIGNAL

    • sigev_signo: signal number - user selectable, e.g. SIGRTMIN+7, or SIGUSR1

    • Important: even when a realtime signal is chosen (see here), multiple expirations are not queued

  • sigev_notify = SIGEV_THREAD

    • sigev_notify_function: callback function called in a separate thread

    • sigev_notify_attributes: attributes for thread creation (see here)

  • sigev_notify = SIGEV_NONE: must poll using timer_gettime()

More callback information

  • sigev_value, of type

    union sigval {
    int sival_int;
    void *sival_ptr;
};

  • SIGEV_SIGVAL: passed to signal handler when installed as SA_SIGINFO (see later)

  • SIGEV_THREAD: passed as argument to thread callback function, sigev_notify_function (see later)