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

CLOCK_GETRES(2) 		    Linux Programmer's Manual			  CLOCK_GETRES(2)

       clock_getres, clock_gettime, clock_settime - clock and time functions

       #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

       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:

	      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 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).

	      High-resolution per-process timer from the CPU.

	      Thread-specific CPU-time clock.

       clock_gettime(), clock_settime() and clock_getres() return 0 for success, or -1 for
       failure (in which case errno is set appropriately).

       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.

       SUSv2, POSIX.1-2001.

       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-
       CLOCK_PROCESS_CPUTIME_ID,  CLOCK_THREAD_CPUTIME_ID  are	 available.    (See   also

   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 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.

       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-

       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)

       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

					    2013-02-25				  CLOCK_GETRES(2)

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