таймеры процесса (process timers)
Пролог (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.
Имя (Name)
timer_getoverrun, timer_gettime, timer_settime — per-process
timers
Синопсис (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);
Описание (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().
Возвращаемое значение (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.
Ошибки (Error)
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.
Примеры (Examples)
None.
Использование в приложениях (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.
Обоснование (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.
Будущие направления (Future directions)
None.
Смотри также (See also)
clock_getres(3p), timer_create(3p)
The Base Definitions volume of POSIX.1‐2017, time.h(0p)
