getitimer(2) — Linux manual page

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 *restrict new_value,
                     struct itimerval *restrict 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:

       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
       specified by which in the buffer pointed to by curr_value.

       The it_value substructure is populated with the amount of time
       remaining 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         top

       On success, zero is returned.  On error, -1 is returned, and
       errno is set to indicate the error.

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 5.13 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                          2021-03-22                   GETITIMER(2)

Pages that refer to this page: alarm(2)fork(2)syscalls(2)timer_create(2)timerfd_create(2)__pmaf(3)profil(3)ualarm(3)usleep(3)pthreads(7)signal(7)socket(7)time(7)