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       timer_settime, timer_gettime - arm/disarm and fetch state of POSIX per-process timer


       Real-time library (librt, -lrt)


       #include <time.h>

       int timer_settime(timer_t timerid, int flags,
                         const struct itimerspec *restrict new_value,
                         struct itimerspec *_Nullable restrict old_value);
       int timer_gettime(timer_t timerid, struct itimerspec *curr_value);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       timer_settime(), timer_gettime():
           _POSIX_C_SOURCE >= 199309L


       timer_settime()  arms  or disarms the timer identified by timerid.  The new_value argument
       is pointer to an itimerspec structure that specifies the new initial  value  and  the  new
       interval for the timer.  The itimerspec structure is described in itimerspec(3type).

       Each  of  the  substructures  of  the itimerspec structure is a timespec(3) structure that
       allows a time value to be specified in seconds and nanoseconds.   These  time  values  are
       measured  according  to  the  clock  that  was  specified  when  the  timer was created by

       If new_value->it_value specifies a nonzero value (i.e., either subfield is nonzero),  then
       timer_settime() arms (starts) the timer, setting it to initially expire at the given time.
       (If the timer was  already  armed,  then  the  previous  settings  are  overwritten.)   If
       new_value->it_value specifies a zero value (i.e., both subfields are zero), then the timer
       is disarmed.

       The new_value->it_interval field specifies  the  period  of  the  timer,  in  seconds  and
       nanoseconds.   If  this  field is nonzero, then each time that an armed timer expires, the
       timer  is   reloaded   from   the   value   specified   in   new_value->it_interval.    If
       new_value->it_interval  specifies  a  zero value, then the timer expires just once, at the
       time specified by it_value.

       By default, the initial expiration time specified in  new_value->it_value  is  interpreted
       relative  to  the  current time on the timer's clock at the time of the call.  This can be
       modified by specifying TIMER_ABSTIME  in  flags,  in  which  case  new_value->it_value  is
       interpreted as an absolute value as measured on the timer's clock; that is, the timer will
       expire when the clock value reaches the value specified by  new_value->it_value.   If  the
       specified  absolute  time  has already passed, then the timer expires immediately, and the
       overrun count (see timer_getoverrun(2)) will be set correctly.

       If the value of the CLOCK_REALTIME clock is adjusted while an absolute timer based on that
       clock  is  armed,  then  the  expiration  of  the  timer  will  be appropriately adjusted.
       Adjustments to the CLOCK_REALTIME clock have no effect on relative timers  based  on  that

       If  old_value  is not NULL, then it points to a buffer that is used to return the previous
       interval of the timer (in old_value->it_interval) and the amount of time until  the  timer
       would previously have next expired (in old_value->it_value).

       timer_gettime()  returns  the  time until next expiration, and the interval, for the timer
       specified by timerid, in the buffer pointed to by curr_value.  The  time  remaining  until
       the  next  timer expiration is returned in curr_value->it_value; this is always a relative
       value, regardless of whether the TIMER_ABSTIME flag was used when arming  the  timer.   If
       the  value returned in curr_value->it_value is zero, then the timer is currently disarmed.
       The timer interval is returned in  curr_value->it_interval.   If  the  value  returned  in
       curr_value->it_interval is zero, then this is a "one-shot" timer.


       On  success,  timer_settime() and timer_gettime() return 0.  On error, -1 is returned, and
       errno is set to indicate the error.


       These functions may fail with the following errors:

       EFAULT new_value, old_value, or curr_value is not a valid pointer.

       EINVAL timerid is invalid.

       timer_settime() may fail with the following errors:

       EINVAL new_value.it_value  is  negative;  or  new_value.it_value.tv_nsec  is  negative  or
              greater than 999,999,999.


       These system calls are available since Linux 2.6.


       POSIX.1-2001, POSIX.1-2008.


       See timer_create(2).


       timer_create(2), timer_getoverrun(2), timespec(3), time(7)