clock_getres(3p) — Linux manual page

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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.1‐2017, time.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 original Standard can be
       obtained online at http:https://www.opengroup.org/unix/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               2017                  CLOCK_GETRES(3P)

Pages that refer to this page: time.h(0p)clock(3p)clock_getcpuclockid(3p)clock_nanosleep(3p)clock_settime(3p)gettimeofday(3p)posix_trace_create(3p)pthread_getcpuclockid(3p)time(3p)timer_create(3p)timer_getoverrun(3p)