Ubuntu Manpage: getitimer, setitimer - get or set value of an interval timer

NAME

       getitimer, setitimer - get or set value of an interval timer

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/time.h>

       int getitimer(int which, struct itimerval *curr_value);
       int setitimer(int which, const struct itimerval *restrict new_value,
                     struct itimerval *_Nullable restrict 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 to indicate  the
       error.

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, 999999].

STANDARDS

       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.

Before Linux 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 Linux 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 Linux 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, 999999]. However, up to and including Linux 2.6.21, Linux does
not give an error, but instead silently adjusts the corresponding seconds value for the
timer. From Linux 2.6.22 onward, this nonconformance has been repaired: an improper
tv_usec value results in an EINVAL error.