CLOCK_GETRES(2)            Linux Programmer's Manual           CLOCK_GETRES(2)

       clock_getres, clock_gettime, clock_settime - clock and time functions

       #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 *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 clockid, 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 clockid.

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

           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 pro-
       cesses, 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 nanosec-
       onds  since the Epoch.  When its time is changed, timers for a relative
       interval are unaffected, but timers for an absolute point in  time  are

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

       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  appropriate  privi-
              leges.   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 adj-
              time(3) and NTP.

       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 disconti-
              nuities 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 monotonic time
              since--as described by POSIX--"some  unspecified  point  in  the
              past".   On  Linux, that point corresponds to the number of sec-
              onds that the system has been running since it was booted.

              The CLOCK_MONOTONIC 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  adjust-
              ments  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 consecutive calls
              will not go backwards, but successive  calls  may--depending  on
              the architecture--return identical (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 vdso(7).

       CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
              Similar to CLOCK_MONOTONIC, but provides access to a  raw  hard-
              ware-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_MONO-
              TONIC, 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 details.

       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 below.

   Dynamic clocks
       In addition to the hard-coded System-V style clock IDs described above,
       Linux also supports POSIX clock operations  on  certain  character  de-
       vices.   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 example shows how to  convert  a  file
       descriptor into a dynamic clock ID.

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

           struct timeval tv;
           clockid_t clkid;
           int fd;

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

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

       EFAULT tp points outside the accessible address space.

       EINVAL The clockid specified is invalid for one of two reasons.  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.

       EINVAL (clock_settime()):  tp.tv_sec  is negative or tp.tv_nsec is out-
              side the range [0..999,999,999].

       EINVAL The clockid specified in a call to clock_settime() is not a set-
              table clock.

              The operation is not supported by the dynamic POSIX clock device

       EINVAL (since Linux 4.3)
              A call to clock_settime() with a clockid of  CLOCK_REALTIME  at-
              tempted  to  set the time to a value less than the current value
              of the CLOCK_MONOTONIC clock.

       ENODEV The hot-pluggable device (like USB for example) represented by a
              dynamic  clk_id  has  disappeared after its character device was

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

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

       These system calls first appeared in Linux 2.6.

       For an  explanation  of  the  terms  used  in  this  section,  see  at-

       |Interface                        | Attribute     | Value   |
       |clock_getres(), clock_gettime(), | Thread safety | MT-Safe |
       |clock_settime()                  |               |         |

       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
       indicate      that      CLOCK_MONOTONIC,      CLOCK_PROCESS_CPUTIME_ID,
       CLOCK_THREAD_CPUTIME_ID are available.  (See also sysconf(3).)

       POSIX.1 specifies the following:

              Setting the value of the  CLOCK_REALTIME  clock  via  clock_set-
              time()  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, indepen-
              dently of the new or old value of the clock.

       According to POSIX.1-2001, a process with "appropriate privileges"  may
       ing 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
       Before Linux added  kernel  support  for  CLOCK_PROCESS_CPUTIME_ID  and
       CLOCK_THREAD_CPUTIME_ID,  glibc  implemented these clocks on many plat-
       forms using timer registers from the CPUs (TSC on i386, AR.ITC on  Ita-
       nium).   These  registers  may differ between CPUs and as a consequence
       these clocks may return bogus results if a process is migrated  to  an-
       other 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 en-
       sured 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 off-
       set.  Some architectures include code that attempts to limit these off-
       sets  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
       on  systems that provide such an implementation (i.e., Linux 2.6.12 and

       The program below demonstrates the use of clock_gettime() and clock_ge-
       tres()  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

          Licensed under GNU General Public License v2 or later.
       #define _XOPEN_SOURCE 600
       #include <time.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, char *name, bool showRes)
           struct timespec ts;

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

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

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

           printf("%2ldh %2ldm %2lds", (ts.tv_sec % SECS_IN_DAY) / 3600,
                   (ts.tv_sec % 3600) / 60, ts.tv_sec % 60);

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

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

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

           displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes);
       #ifdef CLOCK_TAI
           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 5.07 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

                                  2020-04-11                   CLOCK_GETRES(2)

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