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CentOS 7.0 - man page for clock_getres (centos section 2)

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  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 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 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 applications to get a suspend-aware monotonic clock without
	      having to deal with the complications of CLOCK_REALTIME, which may have discontinu-
	      ities if the time is changed using settimeofday(2).

       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 fail-
       ure (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 plat-
       forms 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 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 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),  gettimeofday(2),  settimeofday(2),  time(2), adjtime(3), clock_getcpuclockid(3),
       ctime(3), ftime(3), pthread_getcpuclockid(3), sysconf(3), time(7)

COLOPHON
       This page is part of release 3.53 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/.

					    2013-02-25				  CLOCK_GETRES(2)


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