setitimer(2) — Linux manual page


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

       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
       generated 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
       different signal on timer expiration:

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

              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.

              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 */

       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 re‐
       maining 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).

       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 re‐
       turned 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         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

       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_VIRTUAL), 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
       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 5.09 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

Linux                            2020-04-11                     GETITIMER(2)

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