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


     clock_getres, clock_gettime, clock_settime - clock and time functions


     #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


     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
         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 pro-
     cesses, or per-process  if  it  measures  time  only  within  a  single
     All  implementations  support the system-wide real-time clock, which is
     identified by CLOCK_REALTIME.  Its time represents seconds and nanosec-
     onds  since the Epoch.  When its time is changed, timers for a relative
     interval are unaffected, but timers for an absolute point in  time  are
     More  clocks may be implemented.  The interpretation of the correspond-
     ing time values and the effect on timers is unspecified.
     Sufficiently recent versions of glibc and the Linux kernel support  the
     following clocks:
            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 that cannot be set and  represents  monotonic  time  since
            some  unspecified starting point.  This 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.
     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 hard-
            ware-based time that is not subject to NTP  adjustments  or  the
            incremental adjustments performed by adjtime(3).
     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.


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


     EFAULT tp points outside the accessible address space.
     EINVAL The clk_id specified is not supported on this system.
     EPERM  clock_settime()  does not have permission to set the clock indi-


     These system calls first appeared in Linux 2.6.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     allbox;   lbw32   lb   lb   l   l   l.    Interface Attribute Value  T{
     clock_getres(),    clock_gettime(),     clock_settime()     T}   Thread
     safety  MT-Safe


     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
     indicate      that      CLOCK_MONOTONIC,      CLOCK_PROCESS_CPUTIME_ID,
     CLOCK_THREAD_CPUTIME_ID are available.  (See also sysconf(3).)


     POSIX.1 specifies the following:
            Setting  the  value  of  the CLOCK_REALTIME clock via clock_set-
            time() 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,  indepen-
            dently 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 plat-
     forms using timer registers from the CPUs (TSC on i386, AR.ITC on  Ita-
     nium).   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 off-
     set.  Some architectures include code that attempts to limit these off-
     sets  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
     on  systems that provide such an implementation (i.e., Linux 2.6.12 and


     According to POSIX.1-2001, a process with "appropriate privileges"  may
     using clock_settime().  On Linux, these clocks are not settable  (i.e.,
     no process has "appropriate privileges").


     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)


     This  page  is  part of release 4.16 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
                                2017-09-15                   CLOCK_GETRES(2)
/data/webs/external/dokuwiki/data/pages/man/clock_getres.txt · Last modified: 2019/05/17 09:47 by

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