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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.

   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  implementa-
       tion  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, the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks should be settable
       using clock_settime().  However, the clocks currently are not settable.

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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



                                  2009-02-05                   CLOCK_GETRES(2)