NAME
timer_getoverrun, timer_gettime, timer_settime - per-process timers
(REALTIME)
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, if the implementation supports the Realtime Signals Extension,
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,
or if the Realtime Signals Extension is not supported, the return value
of timer_getoverrun() is unspecified.
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_getoverrun(), timer_gettime(), and timer_settime() functions
shall fail if:
EINVAL The timerid argument does not correspond to an ID returned by
timer_create() but not yet deleted by timer_delete().
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 following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
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 IEEE Std 1003.1-2001 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.
FUTURE DIRECTIONS
None.
SEE ALSO
clock_getres() , timer_create() , the Base Definitions volume of
IEEE Std 1003.1-2001, <time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .