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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.

   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   real-time  clock.   Setting  this  clock  requires
              appropriate privileges.

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

       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.

       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
       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.

       EPERM  clock_settime()  does  not  have  permission  to  set  the clock
              indicated.

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

   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
       condition.  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
       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.

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),  adjtimex(2),   gettimeofday(2),   settimeofday(2),   time(2),
       clock_getcpuclockid(3),  ctime(3),  ftime(3), pthread_getcpuclockid(3),
       sysconf(3), time(7)

COLOPHON

       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.

                                  2010-02-03                   CLOCK_GETRES(2)