POSIX timers

How

System Calls

Compile with cc -lrt to link against real-time library

System calls

timer_create()
timer_settime()
timer_gettime()
timer_getoverrun()
timer_delete()

int timer_create(clockid_t clockid, struct sigevent *evp,
timer_t *timerid);

Creates a timer.
clockid specifies clock used for measuring time; typical is CLOCK_REALTIME (wallclock time).

timerid returns unique timer ID (integer).
evp specifies how process should be notified when timer expires.
struct sigevent {   /* Abridged structure defn */
    int          sigev_notify;  /* Notification method */
    int          sigev_signo;   /* Timer expiration signal */
    union sigval sigev_value;   /* Value passed to signal 
                                   handler or thread function */
    void (*sigev_notify_function) (union sigval);	 
                                /* Thread notification function */
    ...
};
Various notification methods possible, e.g., SIGEV_SIGNAL (send signal) or SIGEV_THREAD (call a function in a new thread).
sigev_value specifies data to be sent to signal handler, or passed to thread function.
union sigval {
    int   sival_int;   /* Integer value for accompanying data */
    void *sival_ptr;   /* Pointer value for accompanying data */
};

int timer_settime(timer_t timerid, int flags,
const struct itimerspec *value, struct itimerspec *oldvalue);

Starts timer timerid.
value specifies an initial timer expiration, and a repeat interval:
struct itimerspec {
    struct timespec it_interval;   /* Interval for periodic timer */
    struct timespec it_value;      /* First expiration */
};

struct timespec {
    time_t tv_sec;                 /* Seconds */
    long   tv_nsec;                /* Nanoseconds */
};
oldvalue returns previous timer settings.

By default (flags == 0), timer is relative.

Specify flags as TIMER_ABSTIME for absolute timer (measured since Epoch).

int timer_getoverrun(timer_t timerid)

Returns timer overrun value.

Useful because timer might expire multiple times before signal is delivered or thread function is started.

Reset to 0 each time we receive the timer signal, or thread function is started.

int timer_gettime(timer_t timerid, struct itimerspec *value)

Retrieve current settings of a timer.

int timer_delete(timer_t timerid)

Delete a timer, allowing the associated resources to be re-used.