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       clock_getres, clock_gettime, clock_settime - clock and time functions


       #include <time.h>

       int clock_getres(clockid_t clk_id, struct timespec *res);

       int clock_gettime(clockid_t clk_id, struct timespec *tp);

       int clock_settime(clockid_t clk_id, const struct timespec *tp);

       Link with -lrt.

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

       clock_getres(), clock_gettime(), clock_settime():
       _POSIX_C_SOURCE >= 199309L


       The function clock_getres() finds the  resolution  (precision)  of  the
       specified  clock  clk_id,  and,  if  res  is non-NULL, stores it in the
       struct timespec pointed to by res.  The resolution of clocks depends on
       the  implementation  and  cannot be configured by a particular process.
       If the time value pointed to by the argument tp of  clock_settime()  is
       not a multiple of res, then it is truncated to a multiple of res.

       The  functions clock_gettime() and clock_settime() retrieve and set the
       time of the specified clock clk_id.

       The res and tp arguments are  timespec  structures,  as  specified   in

           struct timespec {
               time_t   tv_sec;        /* seconds */
               long     tv_nsec;       /* nanoseconds */

       The  clk_id argument is the identifier of the particular clock on which
       to act.   A  clock  may  be  system-wide  and  hence  visible  for  all
       processes,  or  per-process  if  it  measures time only within a single

       All implementations support the system-wide real-time clock,  which  is
       identified   by   CLOCK_REALTIME.   Its  time  represents  seconds  and
       nanoseconds since the Epoch.  When its time is changed,  timers  for  a
       relative  interval  are unaffected, but timers for an absolute point in
       time are affected.

       More  clocks  may  be   implemented.    The   interpretation   of   the
       corresponding time values and the effect on timers is unspecified.

       Sufficiently  recent versions of glibc and the Linux kernel support the
       following clocks:

              System-wide  real-time  clock.   Setting  this  clock   requires
              appropriate privileges.

              Clock  that  cannot  be  set and represents monotonic time since
              some unspecified starting point.

              High-resolution per-process timer from the CPU.

              Thread-specific CPU-time clock.


       clock_gettime(),  clock_settime()  and  clock_getres()  return  0   for
       success,  or -1 for failure (in which case errno is set appropriately).


       EFAULT tp points outside the accessible address space.

       EINVAL The clk_id specified is not supported on this system.

       EPERM  clock_settime() does  not  have  permission  to  set  the  clock


       SUSv2, POSIX.1-2001.


       On  POSIX  systems  on  which these functions are available, the symbol
       _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0.   The
       indicate      that      CLOCK_MONOTONIC,      CLOCK_PROCESS_CPUTIME_ID,
       CLOCK_THREAD_CPUTIME_ID are available.  (See also sysconf(3).)


   Note for SMP systems
       realized on many platforms using timers from the  CPUs  (TSC  on  i386,
       AR.ITC  on  Itanium).  These registers may differ between CPUs and as a
       consequence these clocks may return  bogus  results  if  a  process  is
       migrated to another CPU.

       If the CPUs in an SMP system have different clock sources then there is
       no way to maintain a correlation between the timer registers since each
       CPU  will  run  at a slightly different frequency.  If that is the case
       then  clock_getcpuclockid(0)  will  return  ENOENT  to   signify   this
       condition.   The  two  clocks  will  then  only  be useful if it can be
       ensured that a process stays on a certain CPU.

       The processors in an SMP system do not start all at  exactly  the  same
       time  and  therefore  the  timer  registers are typically running at an
       offset.  Some architectures include code that attempts to  limit  these
       offsets  on  bootup.   However, the code cannot guarantee to accurately
       tune the offsets.  Glibc contains no  provisions  to  deal  with  these
       offsets  (unlike  the Linux Kernel).  Typically these offsets are small
       and therefore the effects may be negligible in most cases.


       According   to   POSIX.1-2001,   the    CLOCK_PROCESS_CPUTIME_ID    and
       CLOCK_THREAD_CPUTIME_ID     clocks    should    be    settable    using
       clock_settime().  However, the clocks currently are not settable.


       date(1),  adjtimex(2),   gettimeofday(2),   settimeofday(2),   time(2),
       clock_getcpuclockid(3), ctime(3), ftime(3), sysconf(3), time(7)


       This  page  is  part of release 3.15 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

                                  2008-11-07                   CLOCK_GETRES(3)