GETITIMER(2)GETITIMER(2)NAME
getitimer, setitimer - get/set value of interval timer
SYNOPSIS
#include <sys/time.h>
#define ITIMER_REAL 0 /* real time intervals */
#define ITIMER_VIRTUAL 1 /* virtual time intervals */
#define ITIMER_PROF 2 /* user and system virtual time */
int getitimer(int which, struct itimerval *value);
int setitimer(int which, struct itimerval *value,
struct itimerval *ovalue);
DESCRIPTION
The system provides each process with three interval timers, defined in
<sys/time.h>. The getitimer call returns the current value for the timer
specified in which, while the setitimer call sets the value of a timer
(optionally returning the previous value of the timer).
A timer value is defined by the itimerval structure:
struct itimerval {
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 and not the time the timer was set originally. 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. Setting
it_interval to 0 causes a timer to be disabled after its next expiration
(assuming it_value is non-zero).
For ITIMER_VIRTUAL and ITIMER_PROF, the timer resolution is 100 HZ (which
equals 10 milliseconds). Time values that are not a multiple of 100 HZ
will be rounded to a multiple.
For ITIMER_REAL, the timer resolution is 100 HZ (which equals 10
milliseconds) for normal processes. For processes with a real time
scheduling policy (see sched setscheduler (2)), actual resolution of the
clock depends on the timer capability of the underlying hardware (see
timers (5)). To avoid a reduction in overall system performance, normal
processes are not permitted to use the high resolution timers, since fast
itimer requests can add considerable system overhead.
The ITIMER_REAL timer decrements in real time. A SIGALRM signal is
delivered when this timer expires.
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GETITIMER(2)GETITIMER(2)
The ITIMER_VIRTUAL timer decrements in process virtual time. It runs
only when the process is executing. A SIGVTALRM signal is delivered when
it expires.
The ITIMER_PROF timer 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 delivered. Because this signal may interrupt in-
progress system calls, programs using this timer must be prepared to
restart interrupted system calls.
NOTES
Three macros for manipulating time values are defined in <sys/time.h>.
timerclear sets a time value to zero, timerisset tests if a time value is
non-zero, and timercmp compares two time values (beware that >= and <= do
not work with this macro).
SEE ALSOftimer(1), lboot(1M), npri(1), systune(1M), schedctl(2), sigset(2),
timers(5).
FILES
/var/sysgen/mtune/kernel fast itimer configuration variables.
DIAGNOSTICS
If the calls succeed, a value of 0 is returned. If an error occurs, the
value -1 is returned, and errno will be set. setitimer and getitimer
will fail if any of the following are true:
[EFAULT] The value or ovalue structure specified a bad address.
[EINVAL] A value structure specified a time was too large to be
handled.
BUGSsetitimer calls reset the alarm clock (see alarm(2)) of the calling
process.
Programs cannot use getitimer(2) to accurately determine how much time
remains until the timer expires. On the Challenge/Onyx hardware family,
it_value is not updated at a constant rate.
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