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.

   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
       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 real-time clock.  Setting this clock requires appropriate privileges.

       CLOCK_MONOTONIC
	      Clock that cannot be set and represents monotonic time since some unspecified starting point.

       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.

       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 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.

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 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 only be useful 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),  adjtimex(2),  gettimeofday(2),  settimeofday(2),  time(2),  clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3),
       sysconf(3), time(7)

COLOPHON

       This page is part of release 3.27 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/.

								    2010-02-03							   CLOCK_GETRES(2)