bionic (3) clock_getres.3posix.gz

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PROLOG
       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.


NAME
       clock_getres, clock_gettime, clock_settime — clock and timer functions


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 specified 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 clock measuring real time 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.

       Appropriate privileges to set a particular clock are 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_gettime() 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_settime()
       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_gettime() function shall fail if:

       EOVERFLOW
              The number of seconds will not fit in an object of type time_t.

       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 appropriate privileges to set the specified clock.

       The following sections are informative.


EXAMPLES
       None.


APPLICATION USAGE
       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
       Scheduling Policies, clock_getcpuclockid(), clock_nanosleep(), ctime(), mq_receive(), mq_send(),
       nanosleep(), pthread_mutex_timedlock(), sem_timedwait(), time(), timer_create(), timer_getoverrun()

       The Base Definitions volume of POSIX.1‐2008, <time.h>


       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers,  Inc
       and  The  Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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 original Standard can be obtained online at
       http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .