clock_gettime(3)



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 pro-
       cesses, 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 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
       affected.

       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:

       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.

       CLOCK_MONOTONIC
              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  time-
              stamps.

       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  adjust-
              ments  or  the  incremental adjustments performed by adj-
              time(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 applica-
              tions to get a suspend-aware monotonic clock without hav-
              ing  to  deal  with  the complications of CLOCK_REALTIME,
              which may have discontinuities if  the  time  is  changed
              using settimeofday(2).

       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 appropri-
       ately).

ERRORS
       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
              indicated.

VERSIONS
       These system calls first appeared in Linux 2.6.

CONFORMING TO
       SUSv2, POSIX.1-2001.

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
   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 reg-
       isters  since  each  CPU  will  run at a slightly different fre-
       quency.  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  run-
       ning  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  Ker-
       nel).   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  privi-
       leges"     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_getcpu-
       clockid(3), sysconf(3), time(7)

COLOPHON
       This page is  part  of  release  3.74  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 http://www.kernel.org/doc/man-pages/.

                                  2013-12-28                   CLOCK_GETRES(2)

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