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 particu‐
       lar 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 speci‐
       fied 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 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  unspeci‐
              fied  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 adj‐
              time(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 timestamps.

       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 or the incremental  adjustments  per‐
              formed by adjtime(3).

       CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
              Identical to CLOCK_MONOTONIC, except it also includes any time that the sys‐
              tem is suspended.  This allows applications to get a suspend-aware monotonic
              clock without having 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 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.

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_MONO‐
       TONIC_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 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 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 running at an offset.  Some architec‐
       tures 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.

       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  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  privi‐
       leges").

SEE ALSO
       date(1),   gettimeofday(2),  settimeofday(2),  time(2),  adjtime(3),  clock_getcpu‐
       clockid(3), ctime(3), ftime(3), pthread_getcpuclockid(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)