xenial (2) setitimer.2.gz

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NAME

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

SYNOPSIS

       #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

       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

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

ERRORS

       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

       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

       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

       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

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

COLOPHON

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