PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

CLOCK_GETRES(3P)          POSIX Programmer's Manual         CLOCK_GETRES(3P)

PROLOG         top

       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         top

       clock_getres, clock_gettime, clock_settime — clock and timer
       functions

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

       None.

APPLICATION USAGE         top

       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         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       Scheduling Policies, clock_getcpuclockid(3p), clock_nanosleep(3p),
       ctime(3p), mq_receive(3p), mq_send(3p), nanosleep(3p),
       pthread_mutex_timedlock(3p), sem_timedwait(3p), time(3p),
       timer_create(3p), timer_getoverrun(3p)

       The Base Definitions volume of POSIX.1‐2008, time.h(0p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                    CLOCK_GETRES(3P)