NAME [Toc] [Back]
getitimer, setitimer - get and set value of interval timer
SYNOPSIS [Toc] [Back]
int getitimer(int which, struct itimerval *value);
const struct itimerval *value,
struct itimerval *ovalue
DESCRIPTION [Toc] [Back]
The getitimer() function stores the current value of the timer
specified by which into the structure pointed to by value. The
setitimer() function sets the timer specified by which to the value
specified in the structure pointed to by value, and if ovalue is not a
null pointer, stores the previous value of the timer in the structure
pointed to by ovalue.
The <sys/time.h> header declares the itimerval structure:
struct timeval it_interval; /* timer interval */
struct timeval it_value; /* current value */
If it_value is non-zero, it indicates the time to the next timer
expiration. If it_interval is non-zero, it specifies a value to be
used in reloading it_value when the timer expires. Setting it_value
to 0 disables a timer, regardless of the value of it_interval.
Setting it_interval to 0 disables a timer after its next expiration
(assuming it_value is non-zero).
Implementations may place limitations on the granularity of timer
values. For each interval timer, if the requested timer value
requires a finer granularity than the implementation supports, the
actual timer value will be rounded up to the next supported value.
Time values smaller than the resolution of the system clock are
rounded up to this resolution. The machine-dependent clock resolution
is 1/HZ seconds, where the constant HZ is defined in <sys/param.h>.
To make sure that a process gets at least as much time as requested,
the timer value is rounded up to the next timer tick (a timer tick is
the smallest supported value). The timer value is rounded up to the
next timer tick, because the timer may be initialized somewhere
between timer ticks. If a setitimer() is followed by a getitimer()
without a timer tick in between, it is possible that the value
returned by getitimer() may be more than the initial value requested
by setitimer() due to this rounding.
Hewlett-Packard Company - 1 - HP-UX 11i Version 2: August 2003
Implementations may place limitations on the timer value. Time values
larger than an implementation-specific maximum value are rounded down
to this maximum. The maximum values for the three interval timers are
specified by the constants MAX_ALARM, MAX_VTALARM, and MAX_PROF
defined in <sys/param.h>. On all implementations, these values are
guaranteed to be at least 31 days (in seconds).
An XSI-conforming implementation provides each process with at least
three interval timers, which are indicated by the which argument:
ITIMER_REAL Decrements in real time. A SIGALRM
signal is delivered when this timer
ITIMER_VIRTUAL Decrements in process virtual time. It
runs only when the process is executing.
A SIGVTALRM signal is delivered when it
ITIMER_PROF Decrements both in process virtual time
and when the system is running on behalf
of the process. It is designed to be
used by interpreters in statistically
profiling the execution of interpreted
programs. Each time the ITIMER_PROF
timer expires, the SIGPROF signal is
Since SIGPROF signal can interrupt in-progress system calls, programs
using this timer must be prepared to restart interrupted system calls.
Interval timers are not inherited by a child process across a fork(),
but are inherited across an exec().
Three macros for manipulating time values are defined in <sys/time.h>:
timerclear Set a time value to zero.
timerisset Test if a time value is non-zero.
timercmp Compare two time values. (Beware that >= and
<= do not work with the timercmp macro.)
The timer used with ITIMER_REAL is also used by alarm() (see
alarm(2)). Thus successive calls to alarm(), getitimer(), and
setitimer() set and return the state of a single timer. In addition,
a call to alarm() sets the timer interval to zero.
The interaction between setitimer() and any of alarm(), sleep() or
usleep() is unspecified.
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RETURN VALUE [Toc] [Back]
Upon successful completion, getitimer() or setitimer() returns 0.
Otherwise, -1 is returned and errno is set to indicate the error.
ERRORS [Toc] [Back]
The setitimer() function will fail if:
[EINVAL] The value argument is not in canonical form. (In
canonical form, the number of microseconds is a
non-negative integer less than 1,000,000 and the
number of seconds is a non-negative integer.)
The getitimer() and setitimer() functions may fail if:
[EINVAL] The which argument is not recognized.
[EFAULT] The value structure specified a bad address.
Reliable detection of this error is implementation
EXAMPLES [Toc] [Back]
The following call to setitimer() sets the real-time interval timer to
expire initially after 10 seconds and every 0.5 seconds thereafter:
struct itimerval rttimer;
struct itimerval old_rttimer;
rttimer.it_value.tv_sec = 10;
rttimer.it_value.tv_usec = 0;
rttimer.it_interval.tv_sec = 0;
rttimer.it_interval.tv_usec = 500000;
setitimer (ITIMER_REAL, &rttimer, &old_rttimer);
AUTHOR [Toc] [Back]
getitimer() was developed by the University of California, Berkeley.
SEE ALSO [Toc] [Back]
alarm(2), sleep(3C), ualarm(2), usleep(2), exec(2), signal(5).
CHANGE HISTORY [Toc] [Back]
First released in Issue 4, Version 2.
Hewlett-Packard Company - 3 - HP-UX 11i Version 2: August 2003 [ Back ]