clock_getres(3) — Linux manual page


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

NAME         top

       clock_getres, clock_gettime, clock_settime - clock and time functions

SYNOPSIS         top

       #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

DESCRIPTION         top

       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
       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 corre‐
       sponding 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 per‐
              formed by adjtime(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

       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 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 incremen‐
              tal 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
              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 architec‐
              ture 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_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 discontinu‐
              ities 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 charac‐
       ter devices.  Such devices are called "dynamic" clocks, and are sup‐
       ported 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);

RETURN VALUE         top

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

ERRORS         top

       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
              outside the range [0..999,999,999].

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

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

       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 clock.

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

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

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

VERSIONS         top

       These system calls first appeared in Linux 2.6.

ATTRIBUTES         top

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

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

CONFORMING TO         top

       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.
       (See also sysconf(3).)

NOTES         top

       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 privileges"
       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
       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).

EXAMPLES         top

       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

          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);

SEE ALSO         top

       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)

COLOPHON         top

       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)