# getitimer

## 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.
    


## 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.
    


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