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


       #include <time.h>

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

       Link with -lrt.

   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 defined as follows:

           struct timespec {
               time_t tv_sec;                /* Seconds */
               long   tv_nsec;               /* Nanoseconds */

           struct itimerspec {
               struct timespec it_interval;  /* Timer interval */
               struct timespec it_value;     /* Initial expiration */

       Each  of the substructures of the itimerspec structure is a timespec 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 timer_create(2).

       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), time(7)


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