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Linux 2.6 - man page for clock_settime (linux section 3posix)

CLOCK_GETRES(P) 		    POSIX Programmer's Manual			  CLOCK_GETRES(P)

NAME
       clock_getres, clock_gettime, clock_settime - clock and timer functions (REALTIME)

SYNOPSIS
       #include <time.h>

       int clock_getres(clockid_t clock_id, struct timespec *res);
       int clock_gettime(clockid_t clock_id, struct timespec *tp);
       int clock_settime(clockid_t clock_id, const struct timespec *tp);

DESCRIPTION
       The  clock_getres()  function  shall return the resolution of any clock. Clock resolutions
       are implementation-defined and cannot be set by a process. If  the  argument  res  is  not
       NULL,  the resolution of the specified clock shall be stored in the location pointed to by
       res. If res is NULL, the clock resolution  is  not  returned.  If  the  time  argument  of
       clock_settime()	is  not  a  multiple of res, then the value is truncated to a multiple of
       res.

       The clock_gettime() function shall return the current value tp for  the	specified  clock,
       clock_id.

       The  clock_settime() function shall set the specified clock, clock_id, to the value speci-
       fied by tp. Time values that are between two consecutive non-negative integer multiples of
       the  resolution	of the specified clock shall be truncated down to the smaller multiple of
       the resolution.

       A clock may be system-wide (that is, visible to all processes) or  per-process  (measuring
       time  that  is  meaningful  only  within  a  process). All implementations shall support a
       clock_id of CLOCK_REALTIME as defined in <time.h>.  This  clock	represents  the  realtime
       clock for the system. For this clock, the values returned by clock_gettime() and specified
       by clock_settime() represent the amount of time (in seconds  and  nanoseconds)  since  the
       Epoch.  An  implementation  may also support additional clocks. The interpretation of time
       values for these clocks is unspecified.

       If the value of the CLOCK_REALTIME clock is set via clock_settime(), the new value of  the
       clock  shall  be used to determine the time of expiration for absolute time services based
       upon the CLOCK_REALTIME clock. This applies to the time at  which  armed  absolute  timers
       expire.	If the absolute time requested at the invocation of such a time service is before
       the new value of the clock, the time service shall expire immediately as if the clock  had
       reached the requested time normally.

       Setting	the value of the CLOCK_REALTIME clock via clock_settime() shall have no effect on
       threads that are blocked waiting for a  relative  time  service	based  upon  this  clock,
       including  the  nanosleep()  function; nor on the expiration of relative timers based upon
       this clock.  Consequently, these time services shall expire when  the  requested  relative
       interval elapses, independently of the new or old value of the clock.

       If  the	Monotonic Clock option is supported, all implementations shall support a clock_id
       of CLOCK_MONOTONIC defined in <time.h>.	This clock represents the monotonic clock for the
       system.	For  this  clock,  the value returned by clock_gettime() represents the amount of
       time (in seconds and nanoseconds) since an unspecified point in	the  past  (for  example,
       system  start-up  time,	or  the  Epoch). This point does not change after system start-up
       time. The value of the CLOCK_MONOTONIC clock cannot  be	set  via  clock_settime().   This
       function shall fail if it is invoked with a clock_id argument of CLOCK_MONOTONIC.

       The  effect  of setting a clock via clock_settime() on armed per-process timers associated
       with a clock other than CLOCK_REALTIME is implementation-defined.

       If the value of the CLOCK_REALTIME clock is set via clock_settime(), the new value of  the
       clock  shall  be  used  to  determine  the  time at which the system shall awaken a thread
       blocked on an absolute clock_nanosleep() call based upon the CLOCK_REALTIME clock.  If the
       absolute  time  requested at the invocation of such a time service is before the new value
       of the clock, the call shall return immediately as if the clock had reached the	requested
       time normally.

       Setting	the value of the CLOCK_REALTIME clock via clock_settime() shall have no effect on
       any thread that is blocked on a relative clock_nanosleep() call.  Consequently,	the  call
       shall return when the requested relative interval elapses, independently of the new or old
       value of the clock.

       The appropriate privilege to set a particular clock is implementation-defined.

       If _POSIX_CPUTIME is defined, implementations shall support clock ID  values  obtained  by
       invoking  clock_getcpuclockid(),  which	represent  the CPU-time clock of a given process.
       Implementations shall also support the special clockid_t  value	CLOCK_PROCESS_CPUTIME_ID,
       which  represents  the  CPU-time  clock	of  the  calling process when invoking one of the
       clock_*() or timer_*() functions. For these clock IDs, the values returned  by  clock_get-
       time()  and  specified  by  clock_settime()  represent the amount of execution time of the
       process associated with the clock. Changing the value of a CPU-time clock  via  clock_set-
       time()  shall have no effect on the behavior of the sporadic server scheduling policy (see
       Scheduling Policies ).

       If _POSIX_THREAD_CPUTIME  is  defined,  implementations	shall  support	clock  ID  values
       obtained  by  invoking  pthread_getcpuclockid(),  which	represent the CPU-time clock of a
       given  thread.	Implementations  shall	also  support	the   special	clockid_t   value
       CLOCK_THREAD_CPUTIME_ID,  which	represents  the CPU-time clock of the calling thread when
       invoking one of the clock_*() or timer_*() functions. For  these  clock	IDs,  the  values
       returned by clock_gettime() and specified by clock_settime() shall represent the amount of
       execution time of the thread associated with the clock. Changing the value of  a  CPU-time
       clock  via  clock_settime()  shall  have  no effect on the behavior of the sporadic server
       scheduling policy (see Scheduling Policies ).

RETURN VALUE
       A return value of 0 shall indicate that the call succeeded. A return  value  of	-1  shall
       indicate that an error occurred, and errno shall be set to indicate the error.

ERRORS
       The clock_getres(), clock_gettime(), and clock_settime() functions shall fail if:

       EINVAL The clock_id argument does not specify a known clock.

       The clock_settime() function shall fail if:

       EINVAL The tp argument to clock_settime() is outside the range for the given clock ID.

       EINVAL The  tp  argument  specified  a  nanosecond value less than zero or greater than or
	      equal to 1000 million.

       EINVAL The value of the clock_id argument is CLOCK_MONOTONIC.

       The clock_settime() function may fail if:

       EPERM  The requesting process does not have the appropriate privilege to set the specified
	      clock.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       These functions are part of the Timers option and need not be available on all implementa-
       tions.

       Note that the absolute value of the monotonic clock is meaningless (because its origin  is
       arbitrary),  and  thus  there is no need to set it. Furthermore, realtime applications can
       rely on the fact that the value of this clock is  never	set  and,  therefore,  that  time
       intervals measured with this clock will not be affected by calls to clock_settime().

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       clock_getcpuclockid() , clock_nanosleep() , ctime() , mq_timedreceive() , mq_timedsend() ,
       nanosleep() , pthread_mutex_timedlock() , sem_timedwait()  ,  time()  ,	timer_create()	,
       timer_getoverrun() , the Base Definitions volume of IEEE Std 1003.1-2001, <time.h>

COPYRIGHT
       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				  CLOCK_GETRES(P)


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