GETITIMER(2) Linux Programmer's Manual GETITIMER(2)
NAME
getitimer, setitimer - get or set value of an interval timer
SYNOPSIS
#include<sys/time.h>
intgetitimer(int which,structitimerval*value);
intsetitimer(int which,conststructitimerval*value,structitimer-
val*ovalue);
DESCRIPTION
The system provides each process with three interval timers, each
decrementing in a distinct time domain. When any timer expires, a sig-
nal is sent to the process, and the timer (potentially) restarts.
ITIMER_REAL decrements in real time, and delivers SIGALRM upon expi-
ration.
ITIMER_VIRTUAL decrements only when the process is executing, and
delivers SIGVTALRM upon expiration.
ITIMER_PROF decrements both when the process executes and when the
system is executing on behalf of the process. Coupled
with ITIMER_VIRTUAL, this timer is usually used to pro-
file the time spent by the application in user and ker-
nel space. SIGPROF is delivered upon expiration.
Timer values are defined by the following structures:
struct itimerval {
struct timeval it_interval; /* next value */
struct timeval it_value; /* current value */
};
struct timeval {
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
The function getitimer fills the structure indicated by value with the
current setting for the timer indicated by which (one of ITIMER_REAL,
ITIMER_VIRTUAL, or ITIMER_PROF). The element it_value is set to the
amount of time remaining on the timer, or zero if the timer is dis-
abled. Similarly, it_interval is set to the reset value. The function
setitimer sets the indicated timer to the value in value. If ovalue is
nonzero, the old value of the timer is stored there.
Timers decrement from it_value to zero, generate a signal, and reset to
it_interval. A timer which is set to zero (it_value is zero or the
timer expires and it_interval is zero) stops.
Both tv_sec and tv_usec are significant in determining the duration of
a timer.
Timers will never expire before the requested time, instead expiring
some short, constant time afterwards, dependent on the system timer
resolution (currently 10ms). Upon expiration, a signal will be gener-
ated and the timer reset. If the timer expires while the process is
active (always true for ITIMER_VIRT) the signal will be delivered imme-
diately when generated. Otherwise the delivery will be offset by a
small time dependent on the system loading.
RETURNVALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
EFAULT value or ovalue are not valid pointers.
EINVAL which is not one of ITIMER_REAL, ITIMER_VIRT, or ITIMER_PROF.
CONFORMINGTO
SVr4, 4.4BSD (This call first appeared in 4.2BSD).
SEEALSO
gettimeofday(2), sigaction(2), signal(2)
BUGS
Under Linux, the generation and delivery of a signal are distinct, and
there each signal is permitted only one outstanding event. It's there-
fore conceivable that under pathologically heavy loading, ITIMER_REAL
will expire before the signal from a previous expiration has been
delivered. The second signal in such an event will be lost.
Linux 0.99.11 1993-08-05 GETITIMER(2)
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