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
       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
       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
       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
       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
       This page is part of release 5.10 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                              2020-04-11                       GETITIMER(2)