Ubuntu Manpage: clock_getres, clock_gettime, clock_settime

NAME

       clock_getres, clock_gettime, clock_settime - clock and time functions

LIBRARY

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

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

SYNOPSIS

       #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 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(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 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  frequency  adjustments performed by NTP and similar
              applications via adjtime(3),  adjtimex(2),  clock_adjtime(2),  and  ntp_adjtime(3).
              This  clock  normally  counts  the  number  of  seconds  since  1970-01-01 00:00:00
              Coordinated Universal Time (UTC) except that it ignores leap seconds; near  a  leap
              second it is typically adjusted by NTP to stay roughly in sync with UTC.

       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  counting  leap
              seconds.   This  clock does not experience discontinuities or frequency adjustments
              caused by 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
              frequency adjustments.   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 frequency adjustments.  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.  On error, -1
       is returned and errno is set to indicate the error.

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.

EOVERFLOW
The timestamp would not fit in time_t range. This can happen if an executable with
32-bit time_t is run on a 64-bit kernel when the time is 2038-01-19 03:14:08 UTC or
later. However, when the system time is out of time_t range in other situations,
the behavior is undefined.

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

ATTRIBUTES

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

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

VERSIONS

       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).

STANDARDS

       POSIX.1-2008.

HISTORY

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. POSIX.1-2008 makes these functions
mandatory.

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).)

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

           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);
           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);
       }