TIMER_SETTIME(2)          (2020-06-09)           TIMER_SETTIME(2)

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

     SYNOPSIS
          #include <time.h>

          int timer_settime(timer_t timerid, int flags,
                            const struct itimerspec *new_value,
                            struct itimerspec *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

     DESCRIPTION
          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 pre-
          vious settings are overwritten.)  If new_value->it_value
          specifies a zero value (i.e., both subfields are zero), then
          the timer is disarmed.

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          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 speci-
          fied 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 cor-
          rectly.

          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 clock.

          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.

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

     ERRORS
          These functions may fail with the following errors:

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     TIMER_SETTIME(2)          (2020-06-09)           TIMER_SETTIME(2)

          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.

     VERSIONS
          These system calls are available since Linux 2.6.

     CONFORMING TO
          POSIX.1-2001, POSIX.1-2008.

     EXAMPLES
          See timer_create(2).

     SEE ALSO
          timer_create(2), timer_getoverrun(2), time(7)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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