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Linux 2.6 - man page for clock_gettime (linux 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  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)
	      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 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 3.55 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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