CLOCK_GETRES(2)           (2020-12-21)            CLOCK_GETRES(2)

          clock_getres, clock_gettime, clock_settime - clock and time

          #include <time.h>

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

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

          int clock_settime(clockid_t clockid, const struct timespec

          Link with -lrt (only for glibc versions before 2.17).

     Feature Test Macro Requirements for glibc (see

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

          The function clock_getres() finds the resolution (precision)
          of  the  specified  clock  clockid, and, if res is non-NULL,
          stores it in the struct timespec pointed to by res. The res-
          olution  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 clockid.

          The res and tp arguments are timespec structures, as  speci-
          fied in <time.h>:

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

          The clockid 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 process.

          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

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          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

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

               A settable system-wide clock that measures real  (i.e.,
               wall-clock)  time.   Setting this clock requires appro-
               priate privileges.  This clock is affected  by  discon-
               tinuous  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

          CLOCK_REALTIME_ALARM (since Linux 3.0; Linux-specific)
               Like   CLOCK_REALTIME,   but   not    settable.     See
               timer_create(2) for further details.

          CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)
               A faster but less precise  version  of  CLOCK_REALTIME.
               This  clock  is  not  settable.  Use when you need very
               fast, but not fine-grained timestamps.   Requires  per-
               architecture  support,  and  probably also architecture
               support for this flag in the vdso(7).

          CLOCK_TAI (since Linux 3.10; Linux-specific)
               A nonsettable system-wide clock derived from wall-clock
               time  but  ignoring  leap seconds.  This clock does not
               experience discontinuities and backwards  jumps  caused
               by NTP inserting leap seconds as CLOCK_REALTIME does.

               The acronym TAI refers to International Atomic Time.

               A nonsettable system-wide clock that  represents  mono-
               tonic  time  since-as described by POSIX-"some unspeci-
               fied point in the past".  On Linux, that  point  corre-
               sponds  to  the  number  of seconds that the system has
               been running since it was booted.

               The CLOCK_MONOTONIC clock is not affected by discontin-
               uous  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.  This clock  does  not  count  time
               that  the  system  is  suspended.   All CLOCK_MONOTONIC
               variants  guarantee   that   the   time   returned   by

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               consecutive calls will not go backwards, but successive
               calls may-depending on the architecture-return  identi-
               cal (not-increased) time values.

          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
               timestamps.   Requires  per-architecture  support,  and
               probably also architecture support for this flag in the

          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
               adjtime(3).   This  clock  does not count time that the
               system is suspended.

          CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
               A nonsettable system-wide clock that  is  identical  to
               CLOCK_MONOTONIC,  except that 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) or similar.

          CLOCK_BOOTTIME_ALARM (since Linux 3.0; Linux-specific)
               Like CLOCK_BOOTTIME.  See timer_create(2)  for  further

          CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
               This is a clock that measures CPU time consumed by this
               process  (i.e., CPU time consumed by all threads in the
               process).  On Linux, this clock is not settable.

          CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
               This is a clock that measures CPU time consumed by this
               thread.  On Linux, this clock is not settable.

          Linux also implements dynamic clock instances  as  described

        Dynamic clocks
          In addition to  the  hard-coded  System-V  style  clock  IDs
          described  above, Linux also supports POSIX clock operations
          on certain  character  devices.   Such  devices  are  called
          "dynamic" clocks, and are supported since Linux 2.6.39.

          Using the appropriate macros, open file descriptors  may  be
          converted  into  clock  IDs  and  passed to clock_gettime(),
          clock_settime(),  and   clock_adjtime(2).    The   following

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          example  shows  how  to  convert  a  file  descriptor into a
          dynamic clock ID.

              #define CLOCKFD 3
              #define FD_TO_CLOCKID(fd)   ((ti(clockid_t) (fd) << 3) | CLOCKFD)
              #define CLOCKID_TO_FD(clk)  ((unsigned int) ti((clk) >> 3))

              struct timespec ts;
              clockid_t clkid;
              int fd;

              fd = open("/dev/ptp0", O_RDWR);
              clkid = FD_TO_CLOCKID(fd);
              clock_gettime(clkid, &ts);

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

               clock_settime() does not have write permission for  the
               dynamic POSIX clock device indicated.

               tp points outside the accessible address space.

               The clockid specified is invalid for one  of  two  rea-
               sons.   Either  the  System-V style hard coded positive
               value is out of range, or the dynamic clock ID does not
               refer to a valid instance of a clock object.

               (clock_settime()): tp.tv_sec is negative or  tp.tv_nsec
               is outside the range [0..999,999,999].

               The clockid specified in a call to  clock_settime()  is
               not a settable clock.

          EINVAL (since Linux 4.3)
               A  call  to   clock_settime()   with   a   clockid   of
               CLOCK_REALTIME  attempted  to  set  the time to a value
               less than the  current  value  of  the  CLOCK_MONOTONIC

               The hot-pluggable device (like USB for example)  repre-
               sented  by  a  dynamic clk_id has disappeared after its
               character device was opened.

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               The operation is not supported  by  the  dynamic  POSIX
               clock device specified.

               clock_settime() does not have  permission  to  set  the
               clock indicated.

          These system calls first appeared in Linux 2.6.

          For an explanation of the terms used in  this  section,  see
          attributes(7).     allbox;    lbw32    lb    lb   l   l   l.
          Interface Attribute Value         T{         clock_getres(),
          clock_gettime(), clock_settime() T}   Thread safety  MT-Safe

          POSIX.1-2001, POSIX.1-2008, SUSv2.

          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,                   CLOCK_PROCESS_CPUTIME_ID,
          CLOCK_THREAD_CPUTIME_ID    are    available.     (See   also

          POSIX.1 specifies the following:

               Setting the  value  of  the  CLOCK_REALTIME  clock  via
               clock_settime()  shall  have  no effect on threads that
               are blocked waiting for a relative time  service  based
               upon  this  clock,  including the nanosleep() function;
               nor on the expiration of  relative  timers  based  upon
               this  clock.   Consequently,  these time services shall
               expire when the requested  relative  interval  elapses,
               independently of the new or old value of the clock.

          According to POSIX.1-2001, a process with "appropriate priv-
          ileges"    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 privileges").

        C library/kernel differences
          On some architectures, an implementation of  clock_gettime()
          is provided in the vdso(7).

        Historical note for SMP systems

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          Before     Linux     added      kernel      support      for
          implemented these clocks on many platforms using timer  reg-
          isters  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  dif-
          ferent    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 typi-
          cally 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

          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
          systems that provide such  an  implementation  (i.e.,  Linux
          2.6.12 and later).

          The program below demonstrates the  use  of  clock_gettime()
          and  clock_getres() with various clocks.  This is an example
          of what we might see when running the program:

              $ ./clock_times x
              CLOCK_REALTIME : 1585985459.446 (18356 days +  7h 30m 59s)
                   resolution:          0.000000001
              CLOCK_TAI      : 1585985496.447 (18356 days +  7h 31m 36s)
                   resolution:          0.000000001
              CLOCK_MONOTONIC:      52395.722 (14h 33m 15s)
                   resolution:          0.000000001
              CLOCK_BOOTTIME :      72691.019 (20h 11m 31s)
                   resolution:          0.000000001

        Program source

          /* clock_times.c

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             Licensed under GNU General Public License v2 or later.
          #define _XOPEN_SOURCE 600
          #include <time.h>
          #include <stdint.h>
          #include <stdio.h>
          #include <stdlib.h>
          #include <stdbool.h>
          #include <unistd.h>

          #define SECS_IN_DAY (24 * 60 * 60)

          static void
          displayClock(clockid_t clock, const char *name, bool showRes)
              struct timespec ts;

              if (clock_gettime(clock, &ts) == -1) {

              printf("%-15s: %10jd.%03ld (", name,
                      (intmax_t) ts.tv_sec, ts.tv_nsec / 1000000);

              long days = ts.tv_sec / SECS_IN_DAY;
              if (days > 0)
                  printf("%ld days + ", days);

              printf("%2dh %2dm %2ds",
                      (int) (ts.tv_sec % SECS_IN_DAY) / 3600,
                      (int) (ts.tv_sec % 3600) / 60,
                      (int) ts.tv_sec % 60);

              if (clock_getres(clock, &ts) == -1) {

              if (showRes)
                  printf("     resolution: %10jd.%09ld\n",
                          (intmax_t) ts.tv_sec, ts.tv_nsec);

          main(int argc, char *argv[])
              bool showRes = argc > 1;

              displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes);
          #ifdef CLOCK_TAI

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              displayClock(CLOCK_TAI, "CLOCK_TAI", showRes);
              displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes);
          #ifdef CLOCK_BOOTTIME
              displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes);

          date(1),    gettimeofday(2),    settimeofday(2),    time(2),
          adjtime(3),   clock_getcpuclockid(3),   ctime(3),  ftime(3),
          pthread_getcpuclockid(3),        sysconf(3),        time(7),
          time_namespaces(7), vdso(7), hwclock(8)

          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

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