clock_getres(2) — Linux manual page


clock_getres(2)            System Calls Manual           clock_getres(2)

NAME         top

       clock_getres, clock_gettime, clock_settime - clock and time

LIBRARY         top

       Standard C library (libc, -lc), since glibc 2.17

       Before glibc 2.17, Real-time library (librt, -lrt)

SYNOPSIS         top

       #include <time.h>

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

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

   Feature Test Macro Requirements for glibc (see

       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(3) structures.

       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
       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 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_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 monotonic
              time since—as described by POSIX—"some unspecified point
              in the past".  On Linux, that point corresponds to the
              number of seconds 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 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
              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
              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

       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 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 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 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 timespec ts;
           clockid_t clkid;
           int fd;

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

RETURN VALUE         top

       clock_gettime(), clock_settime(), and clock_getres() return 0 for
       success.  On error, -1 is returned and errno is set to indicate
       the error.

ERRORS         top

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

       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.

       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.

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

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

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()                     │               │         │

VERSIONS         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" 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

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

STANDARDS         top


HISTORY         top

       POSIX.1-2001, SUSv2.  Linux 2.6.

       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,
       (See also sysconf(3).)  POSIX.1-2008 makes these APIs mandatory.

   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 <stdbool.h>
       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <time.h>

       #define SECS_IN_DAY (24 * 60 * 60)

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

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

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

           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
           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), timespec(3), time(7),
       time_namespaces(7), vdso(7), hwclock(8)

Linux man-pages (unreleased)     (date)                  clock_getres(2)

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