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 clk_id, struct timespec *res);

       int clock_gettime(clockid_t clk_id, struct timespec *tp);

       int clock_settime(clockid_t clk_id, 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 clk_id, 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 clk_id.

       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 clk_id 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
              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_COARSE (since Linux 2.6.32; Linux-specific)
              A faster but less precise version of CLOCK_REALTIME.  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
              Clock that cannot be set and 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.

       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)
              Identical to CLOCK_MONOTONIC, except 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_PROCESS_CPUTIME_ID (since Linux 2.6.12)
              Per-process CPU-time clock (measures CPU time consumed by all
              threads in the process).

       CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
              Thread-specific CPU-time clock.

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 clk_id 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 (since Linux 4.3)
              A call to clock_settime() with a clk_id 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.

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.

   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).

BUGS
       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").

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), vdso(7), hwclock(8)

COLOPHON
       This page is part of release 5.04 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/.



                                  2019-03-06                   CLOCK_GETRES(2)