clock_getres

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.

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

ERRORS
       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.

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

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

       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).

EXAMPLES
       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 <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) {
               perror("clock_gettime");
               exit(EXIT_FAILURE);
           }

           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);
           printf(")\n");

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

           if (showRes)
               printf("     resolution: %10jd.%09ld\n",
                       (intmax_t) 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 5.10 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
       https://www.kernel.org/doc/man-pages/.



                                   2020-12-21                    CLOCK_GETRES(2)