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

       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 pro-
                      cess.   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; /* next value */
               struct timeval it_value;    /* current value */
           };

           struct timeval {
               long tv_sec;                /* seconds */
               long tv_usec;               /* microseconds */
           };

       The function getitimer() fills the structure pointed to by curr_value with the current setting  for  the  timer
       specified  by  which  (one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF).  The element it_value is set to the
       amount of time remaining on the timer, or zero if the timer is disabled.  Similarly, it_interval is set to  the
       reset value.

       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 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 afterwards, 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.

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 prema-
       ture 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 onwards,  this  non-conformance  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
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-03-15                      GETITIMER(2)