TIMER_GETOVERRUN(3)        POSIX Programmer's Manual       TIMER_GETOVERRUN(3)



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

NAME
       timer_getoverrun, timer_gettime, timer_settime -- per-process timers

SYNOPSIS
       #include <time.h>

       int timer_getoverrun(timer_t timerid);
       int timer_gettime(timer_t timerid, struct itimerspec *value);
       int timer_settime(timer_t timerid, int flags,
           const struct itimerspec *restrict value,
           struct itimerspec *restrict ovalue);

DESCRIPTION
       The timer__gettime() function shall store the amount of time until the
       specified timer, timerid, expires and the reload value of the timer
       into the space pointed to by the value argument. The it__value member of
       this structure shall contain the amount of time before the timer
       expires, or zero if the timer is disarmed. This value is returned as
       the interval until timer expiration, even if the timer was armed with
       absolute time. The it__interval member of value shall contain the reload
       value last set by timer__settime().

       The timer__settime() function shall set the time until the next
       expiration of the timer specified by timerid from the it__value member
       of the value argument and arm the timer if the it__value member of value
       is non-zero. If the specified timer was already armed when
       timer__settime() is called, this call shall reset the time until next
       expiration to the value specified. If the it__value member of value is
       zero, the timer shall be disarmed. The effect of disarming or resetting
       a timer with pending expiration notifications is unspecified.

       If the flag TIMER_ABSTIME is not set in the argument flags,
       timer__settime() shall behave as if the time until next expiration is
       set to be equal to the interval specified by the it__value member of
       value.  That is, the timer shall expire in it__value nanoseconds from
       when the call is made. If the flag TIMER_ABSTIME is set in the argument
       flags, timer__settime() shall behave as if the time until next
       expiration is set to be equal to the difference between the absolute
       time specified by the it__value member of value and the current value of
       the clock associated with timerid.  That is, the timer shall expire
       when the clock reaches the value specified by the it__value member of
       value.  If the specified time has already passed, the function shall
       succeed and the expiration notification shall be made.

       The reload value of the timer shall be set to the value specified by
       the it__interval member of value.  When a timer is armed with a non-zero
       it__interval, a periodic (or repetitive) timer is specified.

       Time values that are between two consecutive non-negative integer
       multiples of the resolution of the specified timer shall be rounded up
       to the larger multiple of the resolution. Quantization error shall not
       cause the timer to expire earlier than the rounded time value.

       If the argument ovalue is not NULL, the timer__settime() function shall
       store, in the location referenced by ovalue, a value representing the
       previous amount of time before the timer would have expired, or zero if
       the timer was disarmed, together with the previous timer reload value.
       Timers shall not expire before their scheduled time.

       Only a single signal shall be queued to the process for a given timer
       at any point in time. When a timer for which a signal is still pending
       expires, no signal shall be queued, and a timer overrun shall occur.
       When a timer expiration signal is delivered to or accepted by a
       process, the timer__getoverrun() function shall return the timer
       expiration overrun count for the specified timer. The overrun count
       returned contains the number of extra timer expirations that occurred
       between the time the signal was generated (queued) and when it was
       delivered or accepted, up to but not including an implementation-
       defined maximum of {DELAYTIMER_MAX}.  If the number of such extra
       expirations is greater than or equal to {DELAYTIMER_MAX}, then the
       overrun count shall be set to {DELAYTIMER_MAX}.  The value returned by
       timer__getoverrun() shall apply to the most recent expiration signal
       delivery or acceptance for the timer. If no expiration signal has been
       delivered for the timer, the return value of timer__getoverrun() is
       unspecified.

       The behavior is undefined if the value specified by the timerid
       argument to timer__getoverrun(), timer__gettime(), or timer__settime()
       does not correspond to a timer ID returned by timer__create() but not
       yet deleted by timer__delete().

RETURN VALUE
       If the timer__getoverrun() function succeeds, it shall return the timer
       expiration overrun count as explained above.

       If the timer__gettime() or timer__settime() functions succeed, a value of
       0 shall be returned.

       If an error occurs for any of these functions, the value -1 shall be
       returned, and errno set to indicate the error.

ERRORS
       The timer__settime() function shall fail if:

       EINVAL A value structure specified a nanosecond value less than zero or
              greater than or equal to 1000 million, and the it__value member
              of that structure did not specify zero seconds and nanoseconds.

       The timer__settime() function may fail if:

       EINVAL The it__interval member of value is not zero and the timer was
              created with notification by creation of a new thread
              (sigev__sigev__notify was SIGEV_THREAD) and a fixed stack address
              has been set in the thread attribute pointed to by
              sigev__notify__attributes.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       Using fixed stack addresses is problematic when timer expiration is
       signaled by the creation of a new thread. Since it cannot be assumed
       that the thread created for one expiration is finished before the next
       expiration of the timer, it could happen that two threads use the same
       memory as a stack at the same time. This is invalid and produces
       undefined results.

RATIONALE
       Practical clocks tick at a finite rate, with rates of 100 hertz and
       1000 hertz being common. The inverse of this tick rate is the clock
       resolution, also called the clock granularity, which in either case is
       expressed as a time duration, being 10 milliseconds and 1 millisecond
       respectively for these common rates. The granularity of practical
       clocks implies that if one reads a given clock twice in rapid
       succession, one may get the same time value twice; and that timers must
       wait for the next clock tick after the theoretical expiration time, to
       ensure that a timer never returns too soon. Note also that the
       granularity of the clock may be significantly coarser than the
       resolution of the data format used to set and get time and interval
       values. Also note that some implementations may choose to adjust time
       and/or interval values to exactly match the ticks of the underlying
       clock.

       This volume of POSIX.1-2017 defines functions that allow an application
       to determine the implementation-supported resolution for the clocks and
       requires an implementation to document the resolution supported for
       timers and nanosleep() if they differ from the supported clock
       resolution. This is more of a procurement issue than a runtime
       application issue.

       If an implementation detects that the value specified by the timerid
       argument to timer__getoverrun(), timer__gettime(), or timer__settime()
       does not correspond to a timer ID returned by timer__create() but not
       yet deleted by timer__delete(), it is recommended that the function
       should fail and report an [EINVAL] error.

FUTURE DIRECTIONS
       None.

SEE ALSO
       clock__getres(), timer__create()

       The Base Definitions volume of POSIX.1-2017, <time.h>

COPYRIGHT
       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://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                  TIMER_GETOVERRUN(3)
