NAME

getitimer, setitimer — get/set value of interval timer

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/time.h>

int
getitimer(int which, struct itimerval *value);

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

DESCRIPTION

The system provides each process with multiple interval timers, defined in ⟨sys/time.h⟩. The getitimer() call returns the current value for the timer specified in which in the structure at value. The setitimer() call sets a timer to the specified value, returning the previous value of the timer if ovalue is not NULL.

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. 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).

The which parameter specifies the type of the timer:

ITIMER_REAL: timer decrements in real time. This timer is affected by adjtime(2) and settimeofday(2). A SIGALRM signal is delivered when this timer expires.
ITIMER_VIRTUAL: timer decrements in process virtual time. It runs only when the process is executing. A SIGVTALRM signal is delivered when it expires.
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.
ITIMER_MONOTONIC: timer decrements in monotonic time. This timer is not affected by adjtime(2) and settimeofday(2). A SIGALRM signal is delivered when this timer expires.

Note that:

Time values smaller than the resolution of the system clock are rounded up to this resolution (typically 10 milliseconds).
The interaction between setitimer() and alarm(3) or sleep(3) is unspecified by the specification.

RETURN VALUES

If the calls succeed, a value of 0 is returned. If an error occurs, the value -1 is returned, and a more precise error code is placed in the global variable errno.

ERRORS

Both functions may fail if:

[EFAULT]: The value parameter specified a bad address.
[EINVAL]: The which parameter was not a known timer type, or the value parameter specified a time that was too large to be handled.

STANDARDS

The functions conform to IEEE Std 1003.1-2001 (“POSIX.1”). The later IEEE Std 1003.1-2008 (“POSIX.1”) revision however marked both as obsolescent, recommending the use of timer_gettime(2) and timer_settime(2) instead.

HISTORY

The getitimer() function call appeared in 4.2BSD. The ITIMER_MONOTONIC functionality appeared in NetBSD 6.0.