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);
       }