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 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 vis-
       ible 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 unspeci-
       fied.

       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  adjust-
              ments.

       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_getcpu-
       clockid(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 3.25 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/.



                                                           2010-02-03                                            CLOCK_GETRES(2)