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NAME

     timer_getoverrun, timer_gettime, timer_settime — per-process timers (REALTIME)

LIBRARY

     POSIX Real-time Library (librt, -lrt)

SYNOPSIS

     #include <time.h>

     int
     timer_getoverrun(timer_t timerid);

     int
     timer_gettime(timer_t timerid, struct itimerspec *value);

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

DESCRIPTION

     The timer_gettime() system call stores the amount of time until the specified timer,
     timerid, expires and the reload value of the timer into the space pointed to by the value
     argument.  The it_value member of this structure contains the amount of time before the
     timer expires, or zero if the timer is disarmed.  This value is returned as the interval
     until timer expiration, even if the timer was armed with absolute time.  The it_interval
     member of value contains the reload value last set by timer_settime().

     The timer_settime() system call sets the time until the next expiration of the timer
     specified by timerid from the it_value member of the value argument and arms the timer if
     the it_value member of value is non-zero.  If the specified timer was already armed when
     timer_settime() is called, this call resets the time until next expiration to the value
     specified.  If the it_value member of value is zero, the timer is disarmed.  If the timer is
     disarmed, then pending signal is removed.

     If the flag TIMER_ABSTIME is not set in the argument flags, timer_settime() behaves as if
     the time until next expiration is set to be equal to the interval specified by the it_value
     member of value.  That is, the timer expires in it_value nanoseconds from when the call is
     made.  If the flag TIMER_ABSTIME is set in the argument flags, timer_settime() behaves as if
     the time until next expiration is set to be equal to the difference between the absolute
     time specified by the it_value member of value and the current value of the clock associated
     with timerid.  That is, the timer expires when the clock reaches the value specified by the
     it_value member of value.  If the specified time has already passed, the system call
     succeeds and the expiration notification is made.

     The reload value of the timer is set to the value specified by the it_interval member of
     value.  When a timer is armed with a non-zero it_interval, a periodic (or repetitive) timer
     is specified.

     Time values that are between two consecutive non-negative integer multiples of the
     resolution of the specified timer are rounded up to the larger multiple of the resolution.
     Quantization error will not cause the timer to expire earlier than the rounded time value.

     If the argument ovalue is not NULL, the timer_settime() system call stores, in the location
     referenced by ovalue, a value representing the previous amount of time before the timer
     would have expired, or zero if the timer was disarmed, together with the previous timer
     reload value.  Timers do not expire before their scheduled time.

     Only a single signal is queued to the process for a given timer at any point in time.  When
     a timer for which a signal is still pending expires, no signal is queued, and a timer
     overrun will occur.  When a timer expiration signal is accepted by a process, the
     timer_getoverrun() system call returns the timer expiration overrun count for the specified
     timer.  The overrun count returned contains the number of extra timer expirations that
     occurred between the time the signal was generated (queued) and when it was accepted, up to
     but not including an maximum of {DELAYTIMER_MAX}.  If the number of such extra expirations
     is greater than or equal to {DELAYTIMER_MAX}, then the overrun count is set to
     {DELAYTIMER_MAX}.  The value returned by timer_getoverrun() applies to the most recent
     expiration signal acceptance for the timer.  If no expiration signal has been delivered for
     the timer, the return value of timer_getoverrun() is unspecified.

RETURN VALUES

     If the timer_getoverrun() system call succeeds, it returns the timer expiration overrun
     count as explained above.  Otherwise the value -1 is returned, and the global variable errno
     is set to indicate the error.

     The timer_gettime() and timer_settime() functions return the value 0 if successful;
     otherwise the value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS

     The timer_settime() system call will fail if:

     [EINVAL]           A value structure specified a nanosecond value less than zero or greater
                        than or equal to 1000 million, and the it_value member of that structure
                        did not specify zero seconds and nanoseconds.

     These system calls may fail if:

     [EINVAL]           The timerid argument does not correspond to an ID returned by
                        timer_create() but not yet deleted by timer_delete().

     The timer_settime() system call may fail if:

     [EINVAL]           The it_interval member of value is not zero and the timer was created
                        with notification by creation of a new thread (sigev_sigev_notify was
                        SIGEV_THREAD) and a fixed stack address has been set in the thread
                        attribute pointed to by sigev_notify_attributes.

     The timer_gettime() and timer_settime() system calls may fail if:

     [EFAULT]           Any arguments point outside the allocated address space or there is a
                        memory protection fault.

SEE ALSO

     clock_getres(2), timer_create(2), siginfo(3)

STANDARDS

     The timer_getoverrun(), timer_gettime(), and timer_settime() system calls conform to IEEE
     Std 1003.1-2004 (“POSIX.1”)

HISTORY

     Support for POSIX per-process timer first appeared in FreeBSD 7.0.