clock_settime

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



NAME
       clock_getres, clock_gettime, clock_settime - clock and time functions

SYNOPSIS
       #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
       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
       <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 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:

       CLOCK_REALTIME
              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.

       CLOCK_MONOTONIC
              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 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 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.

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

ERRORS
       EFAULT tp points outside the accessible address space.

       EINVAL The clockid specified is not supported on this system.

       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.

       ENOTSUP
              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.

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

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

VERSIONS
       These system calls first appeared in Linux 2.6.

ATTRIBUTES
       For an explanation of the terms used in this section, see
       attributes(7).

       ┌─────────────────────────────────┬───────────────┬─────────┐
       │Interface                        Attribute     Value   │
       ├─────────────────────────────────┼───────────────┼─────────┤
       │clock_getres(), clock_gettime(), │ Thread safety │ MT-Safe │
       │clock_settime()                  │               │         │
       └─────────────────────────────────┴───────────────┴─────────┘

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

AVAILABILITY
       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 sysconf(3).)

NOTES
       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 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).

EXAMPLE
       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) {
               perror("clock_gettime");
               exit(EXIT_FAILURE);
           }

           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);
           printf(")\n");

           if (clock_getres(clock, &ts) == -1) {
               perror("clock_getres");
               exit(EXIT_FAILURE);
           }

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

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

           displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes);
       #ifdef CLOCK_TAI
           displayClock(CLOCK_TAI, "CLOCK_TAI", showRes);
       #endif
           displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes);
       #ifdef CLOCK_BOOTTIME
           displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes);
       #endif
           exit(EXIT_SUCCESS);
       }

SEE ALSO
       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
       This page is part of release 5.06 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
       https://www.kernel.org/doc/man-pages/.




                                  2020-04-11                   CLOCK_GETRES(2)