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 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 generated and the timer reset. If the timer expires while the process is active (always true for ITIMER_VIRT) 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_VIRT, 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)

BUGS
Under Linux, the generation and delivery of a signal are distinct, and there each signal is permitted only one outstanding event. It's therefore 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. 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 2.6 on x86 (where a jiffy is 0.001 seconds), this means that the ceiling value for a timer is approximately 24.86 days. 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.