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].

VERSIONS

       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.

STANDARDS

       POSIX.1-2008.

HISTORY

       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.

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.