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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 (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  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 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  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 unspecified 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 adjtime(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  time‐
	      stamps.

       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  or	 the  incremental adjustments performed by adj‐
	      time(3).

       CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
	      Identical to CLOCK_MONOTONIC, except it also includes any
	      time  that the system is suspended.  This allows applica‐
	      tions to get a suspend-aware monotonic clock without hav‐
	      ing  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)
	      High-resolution per-process timer from the CPU.

       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 appropri‐
       ately).

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_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
   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 reg‐
       isters since each CPU will run  at  a  slightly	different  fre‐
       quency.	 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 run‐
       ning  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 Ker‐
       nel).  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 privi‐
       leges"	 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),	 gettimeofday(2), settimeofday(2), time(2), adjtime(3),
       clock_getcpuclockid(3),	ctime(3),   ftime(3),	pthread_getcpu‐
       clockid(3), sysconf(3), time(7)

COLOPHON
       This  page  is  part  of	 release  3.54	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/.

				  2013-09-04		       CLOCK_GETRES(2)
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