setitimer

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

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
       These  system  calls provide access to interval timers, that is, timers
       that initially expire at some point in the future, and (optionally)  at
       regular intervals after that.  When a timer expires, a signal is gener-
       ated for the calling process, and the timer is reset to  the  specified
       interval (if the interval is nonzero).

       Three  types of timers--specified via the which argument--are provided,
       each of which counts against a different clock and generates a  differ-
       ent signal on timer expiration:

       ITIMER_REAL    This  timer counts down in real (i.e., wall clock) time.
                      At each expiration, a SIGALRM signal is generated.

       ITIMER_VIRTUAL This timer counts down against the  user-mode  CPU  time
                      consumed  by the process.  (The measurement includes CPU
                      time consumed by all threads in the process.)   At  each
                      expiration, a SIGVTALRM signal is generated.

       ITIMER_PROF    This  timer  counts  down  against the total (i.e., both
                      user and system) CPU time consumed by the process.  (The
                      measurement includes CPU time consumed by all threads in
                      the process.)  At each expiration, a SIGPROF  signal  is
                      generated.

                      In  conjunction  with  ITIMER_VIRTUAL, this timer can be
                      used to profile user and system CPU time consumed by the
                      process.

       A process has only one of each of the three types of timers.

       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 */
           };

   getitimer()
       The  function  getitimer() places the current value of the timer speci-
       fied by which in the buffer pointed to by curr_value.

       The it_value substructure is populated with the amount of time  remain-
       ing  until  the  next  expiration  of  the specified timer.  This value
       changes as the timer counts down, and will be reset to it_interval when
       the  timer  expires.   If  both  fields of it_value are zero, then this
       timer is currently disarmed (inactive).

       The it_interval substructure is populated with the timer interval.   If
       both  fields  of it_interval are zero, then this is a single-shot timer
       (i.e., it expires just once).

   setitimer()
       The function setitimer() arms or disarms the timer specified by  which,
       by setting the timer to the value specified by new_value.  If old_value
       is non-NULL, the buffer it points to is used  to  return  the  previous
       value  of  the  timer  (i.e.,  the same information that is returned by
       getitimer()).

       If either field in new_value.it_value is nonzero,  then  the  timer  is
       armed  to  initially  expire  at the specified time.  If both fields in
       new_value.it_value are zero, then the timer is disarmed.

       The new_value.it_interval field specifies  the  new  interval  for  the
       timer; if both of its subfields are zero, the timer is single-shot.

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 struc-
              ture 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
       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.)   If  the
       timer  expires while the process is active (always true for ITIMER_VIR-
       TUAL), the signal will be delivered immediately when generated.

       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 inter-
       faces 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 ker-
       nels 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_cre-
       ate(2), time(7)

COLOPHON
       This page is part of release 4.15 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                      GETITIMER(2)
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