NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

GETITIMER(2)              Linux Programmer's Manual             GETITIMER(2)

NAME         top

       getitimer, setitimer - get or set value of an interval timer

SYNOPSIS         top

       #include <sys/time.h>

       int getitimer(int which, struct itimerval *curr_value);
       int setitimer(int which, const struct itimerval *new_value,
                     struct itimerval *old_value);

DESCRIPTION         top

       The system provides each process with three interval timers, each
       decrementing in a distinct time domain.  When a timer expires, a
       signal is sent to the process, and the timer is reset to the
       specified interval (if nonzero).

       ITIMER_REAL    decrements in real time, and delivers SIGALRM upon
                      expiration.

       ITIMER_VIRTUAL decrements only when the process is executing, and
                      delivers SIGVTALRM upon expiration.

       ITIMER_PROF    decrements both when the process executes and when the
                      system is executing on behalf of the process.  Coupled
                      with ITIMER_VIRTUAL, this timer is usually used to
                      profile the time spent by the application in user and
                      kernel space.  SIGPROF is delivered upon expiration.

       Timer values are defined by the following structures:

           struct itimerval {
               struct timeval it_interval; /* Interval for periodic timer */
               struct timeval it_value;    /* Time until next expiration */
           };

           struct timeval {
               time_t      tv_sec;         /* seconds */
               suseconds_t tv_usec;        /* microseconds */
           };

       The function getitimer() fills the structure pointed to by curr_value
       with the current value (i.e., the amount of time remaining until the
       next expiration) of the timer specified by which (one of ITIMER_REAL,
       ITIMER_VIRTUAL, or ITIMER_PROF).  The subfields of the field it_value
       are set to the amount of time remaining on the timer, or zero if the
       timer is disabled.  The it_interval field is set to the timer
       interval (period); a value of zero returned in (both subfields of)
       this field indicates that this is a single-shot timer.

       The function setitimer() sets the specified timer to the value in
       new_value.  If old_value is non-NULL, the old value of the timer
       (i.e., the same information as returned by getitimer()) is stored
       there.

       Timers decrement from it_value to zero, generate a signal, and reset
       to it_interval.  A timer which is set to zero (it_value is zero or
       the timer expires and it_interval is zero) stops.

       Both tv_sec and tv_usec are significant in determining the duration
       of a timer.

       Timers will never expire before the requested time, but may expire
       some (short) time afterward, which depends on the system timer
       resolution and on the system load; see time(7).  (But see BUGS
       below.)  Upon expiration, a signal will be generated and the timer
       reset.  If the timer expires while the process is active (always true
       for ITIMER_VIRTUAL), the signal will be delivered immediately when
       generated.  Otherwise, the delivery will be offset by a small time
       dependent on the system loading.

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

ERRORS         top

       EFAULT new_value, old_value, or curr_value is not valid a pointer.

       EINVAL which is not one of ITIMER_REAL, ITIMER_VIRTUAL, or
              ITIMER_PROF; or (since Linux 2.6.22) one of the tv_usec fields
              in the structure pointed to by new_value contains a value
              outside the range 0 to 999999.

CONFORMING TO         top

       POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
       POSIX.1-2008 marks getitimer() and setitimer() obsolete, recommending
       the use of the POSIX timers API (timer_gettime(2), timer_settime(2),
       etc.) instead.

NOTES         top

       A child created via fork(2) does not inherit its parent's interval
       timers.  Interval timers are preserved across an execve(2).

       POSIX.1 leaves the interaction between setitimer() and the three
       interfaces alarm(2), sleep(3), and usleep(3) unspecified.

       The standards are silent on the meaning of the call:

           setitimer(which, NULL, &old_value);

       Many systems (Solaris, the BSDs, and perhaps others) treat this as
       equivalent to:

           getitimer(which, &old_value);

       In Linux, this is treated as being equivalent to a call in which the
       new_value fields are zero; that is, the timer is disabled.  Don't use
       this Linux misfeature: it is nonportable and unnecessary.

BUGS         top

       The generation and delivery of a signal are distinct, and only one
       instance of each of the signals listed above may be pending for a
       process.  Under very heavy loading, an ITIMER_REAL timer may expire
       before the signal from a previous expiration has been delivered.  The
       second signal in such an event will be lost.

       On Linux kernels before 2.6.16, timer values are represented in
       jiffies.  If a request is made set a timer with a value whose jiffies
       representation exceeds MAX_SEC_IN_JIFFIES (defined in
       include/linux/jiffies.h), then the timer is silently truncated to
       this ceiling value.  On Linux/i386 (where, since Linux 2.6.13, the
       default jiffy is 0.004 seconds), this means that the ceiling value
       for a timer is approximately 99.42 days.  Since Linux 2.6.16, the
       kernel uses a different internal representation for times, and this
       ceiling is removed.

       On certain systems (including i386), Linux kernels before version
       2.6.12 have a bug which will produce premature timer expirations of
       up to one jiffy under some circumstances.  This bug is fixed in
       kernel 2.6.12.

       POSIX.1-2001 says that setitimer() should fail if a tv_usec value is
       specified that is outside of the range 0 to 999999.  However, in
       kernels up to and including 2.6.21, Linux does not give an error, but
       instead silently adjusts the corresponding seconds value for the
       timer.  From kernel 2.6.22 onward, this nonconformance has been
       repaired: an improper tv_usec value results in an EINVAL error.

SEE ALSO         top

       gettimeofday(2), sigaction(2), signal(2), timer_create(2),
       timerfd_create(2), time(7)

COLOPHON         top

       This page is part of release 3.75 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-07-08                     GETITIMER(2)