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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 (only for glibc versions before 2.17).

   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  clock  that  measures real (i.e., wall-clock) time.
              Setting this clock requires appropriate privileges.  This  clock
              is  affected by discontinuous jumps in the system time (e.g., if
              the system administrator manually changes the clock), and by the
              incremental adjustments performed by adjtime(3) and NTP.

       CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)
              A  faster  but less precise version of CLOCK_REALTIME.  Use when
              you need very fast, but not fine-grained timestamps.

              Clock that cannot be set and  represents  monotonic  time
              since some unspecified starting point.  This clock is not
              affected by discontinuous jumps in the system time (e.g.,
              if  the system administrator manually changes the clock),
              but is affected by the incremental adjustments  performed
              by adjtime(3) and NTP.

       CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific)
              A  faster  but  less  precise version of CLOCK_MONOTONIC.
              Use  when  you  need  very  fast,  but  not  fine-grained

       CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
              Similar  to CLOCK_MONOTONIC, but provides access to a raw
              hardware-based  time  that  is   not   subject   to   NTP
              adjustments  or  the incremental adjustments performed by

       CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
              Identical to CLOCK_MONOTONIC, except it also includes any
              time   that   the   system  is  suspended.   This  allows
              applications  to  get  a  suspend-aware  monotonic  clock
              without   having   to  deal  with  the  complications  of
              CLOCK_REALTIME, which may  have  discontinuities  if  the
              time is changed using settimeofday(2).

       CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
              Per-process CPU-time clock (measures CPU time consumed by
              all threads in the process).

       CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
              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


       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


       These system calls first appeared in Linux 2.6.


       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  symbols  _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME,
       _POSIX_THREAD_CPUTIME     indicate     that     CLOCK_MONOTONIC,
       (See also sysconf(3).)


   Historical note for SMP systems
       Before Linux added kernel support  for  CLOCK_PROCESS_CPUTIME_ID
       and  CLOCK_THREAD_CPUTIME_ID,  glibc implemented these clocks on
       many platforms using timer registers 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 be useful only 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.

       Since glibc 2.4, the wrapper  functions  for  the  system  calls
       described  in  this  page  avoid  the abovementioned problems by
       employing the kernel implementation of  CLOCK_PROCESS_CPUTIME_ID
       and  CLOCK_THREAD_CPUTIME_ID,  on  systems  that provide such an
       implementation (i.e., Linux 2.6.12 and later).


       According  to  POSIX.1-2001,   a   process   with   "appropriate
       privileges"    may    set   the   CLOCK_PROCESS_CPUTIME_ID   and
       CLOCK_THREAD_CPUTIME_ID clocks using clock_settime().  On Linux,
       these clocks are not settable (i.e., no process has "appropriate


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


       This page is  part  of  release  3.65  of  the  Linux  man-pages
       project.   A  description  of the project, and information about
       reporting       bugs,       can        be        found        at

                                  2013-12-28                   CLOCK_GETRES(2)

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