GETITIMER(2)

NAME
getitimer, setitimer - get or set value of an interval timer

SYNOPSIS
#include <sys/time.h>

int getitimer(int which, struct itimerval *value);
int setitimer(int which, const struct itimerval *value,
              struct itimerval *ovalue);

DESCRIPTION
The system provides each process with three interval timers, each decrementing in a distinct time domain. When any timer expires, a signal is sent to the process, and the timer (potentially) restarts.

ITIMER_REAL
    decrements in real time, and delivers SIGALRM upon expiration.

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 profile the time spent by the application in user and kernel 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 disabled. Similarly, it_interval is set to the reset value. The function setitimer() sets the indicated timer to the value in value. If ovalue is non-zero, 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, but may expire some (short) time afterwards, which depends on the system timer resolution and on the system load. (But see BUGS below.) Upon expiration, a signal will be generated and the timer reset. If the timer expires while the process is active (always true for ITIMER_VIRTUAL) the signal will be delivered immediately when generated. Otherwise the delivery will be offset by a small time dependent on the system loading.

RETURN VALUE
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_VIRTUAL, or ITIMER_PROF.

CONFORMING TO
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).

SEE ALSO
gettimeofday(2), sigaction(2), signal(2), time(7)

BUGS
The generation and delivery of a signal are distinct, and only one instance of each of the signals listed above may be pending for a process. Under very heavy loading, an ITIMER_REAL timer may expire before the signal from a previous expiration has been delivered. The second signal in such an event will be lost.

On Linux, timer values are represented in jiffies. If a request is made set a timer with a value whose jiffies representation exceeds MAX_SEC_IN_JIFFIES (defined in include/linux/jiffies.h), then the timer is silently truncated to this ceiling value. On Linux/x86 (where, since kernel 2.6.13, the default jiffy is 0.004 seconds), this means that the ceiling value for a timer is approximately 99.42 days.

On certain systems (including x86), Linux kernels before version 2.6.12 have a bug which will produce premature timer expirations of up to one jiffy under some circumstances. This bug is fixed in kernel 2.6.12.

POSIX.1 says that setitimer() should fail if a tv_usec value is specified that is outside of the range 0 to 999999. However, Linux does not give an error, but instead silently adjusts the corresponding seconds value for the timer. In the future (scheduled for March 2007), this non-conformance will be repaired: existing applications should be fixed now to ensure that they supply a properly formed tv_usec value.