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CLOCK_GETRES(3)            Linux Programmer's Manual           CLOCK_GETRES(3)



NAME
       clock_getres, clock_gettime, clock_settime - clock and time functions

SYNOPSIS
       #include <&lt;time.h>&gt;

       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);

   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  structs,  as  specified   in
       &lt;time.h&gt;:

           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 real-time clock.  Setting this clock requires appro-
              priate privileges.

       CLOCK_MONOTONIC
              Clock  that  cannot  be  set and represents monotonic time since
              some unspecified starting point.

       CLOCK_PROCESS_CPUTIME_ID
              High-resolution per-process timer from the CPU.

       CLOCK_THREAD_CPUTIME_ID
              Thread-specific CPU-time clock.

RETURN VALUE
       clock_gettime(), clock_settime() and clock_getres() return 0  for  suc-
       cess, 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.

       EPERM  clock_settime()  does not have permission to set the clock indi-
              cated.

CONFORMING TO
       SUSv2, POSIX.1-2001.

AVAILABILITY
       On POSIX systems on which these functions  are  available,  the  symbol
       _POSIX_TIMERS  is defined in &lt;unistd.h&gt; 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
       Most systems require the program be linked with the  librt  library  to
       use these functions.

   Note for SMP systems
       The  CLOCK_PROCESS_CPUTIME_ID  and  CLOCK_THREAD_CPUTIME_ID  clocks are
       realized on many platforms using timers 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  condi-
       tion.   The  two  clocks  will then only be useful 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.

SEE ALSO
       date(1),  adjtimex(2),   gettimeofday(2),   settimeofday(2),   time(2),
       ctime(3), ftime(3), sysconf(3), time(7)

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



                                  2007-07-26                   CLOCK_GETRES(3)