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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
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 .