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HP-UX 11i Version 3: February 2007

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getitimer, setitimer — get and set value of interval timer


#include <sys/time.h> int getitimer(int which, struct itimerval *value); int setitimer( int which, const struct itimerval *value, struct itimerval *ovalue );


The getitimer() function stores the current value of the timer specified by which into the structure pointed to by value. The setitimer() function sets the timer specified by which to the value specified in the structure pointed to by value, and if ovalue is not a null pointer, stores the previous value of the timer in the structure pointed to by ovalue.

The <sys/time.h> header declares the itimerval structure:

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, regardless of the value of it_interval. Setting it_interval to 0 disables a timer after its next expiration (assuming it_value is non-zero).

Implementations may place limitations on the granularity of timer values. For each interval timer, if the requested timer value requires a finer granularity than the implementation supports, the actual timer value will be rounded up to the next supported value. Time values smaller than the resolution of the system clock are rounded up to this resolution. The machine-dependent clock resolution is 1/HZ seconds, where the constant HZ is defined in <sys/param.h>. To make sure that a process gets at least as much time as requested, the timer value is rounded up to the next timer tick (a timer tick is the smallest supported value). The timer value is rounded up to the next timer tick, because the timer may be initialized somewhere between timer ticks. If a setitimer() is followed by a getitimer() without a timer tick in between, it is possible that the value returned by getitimer() may be more than the initial value requested by setitimer() due to this rounding.

Implementations may place limitations on the timer value. Time values larger than an implementation-specific maximum value are rounded down to this maximum. The maximum values for the three interval timers are specified by the constants MAX_ALARM, MAX_VTALARM, and MAX_PROF defined in <sys/param.h>. On all implementations, these values are guaranteed to be at least 31 days (in seconds).

An XSI-conforming implementation provides each process with at least three interval timers, which are indicated by the which argument:


Decrements in real time. A SIGALRM signal is delivered when this timer expires.


Decrements in process virtual time. It runs only when the process is executing. A SIGVTALRM signal is delivered when it expires.


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.

Since SIGPROF signal can interrupt in-progress system calls, programs using this timer must be prepared to restart interrupted system calls.

Interval timers are not inherited by a child process across a fork(), but are inherited across an exec().

Three macros for manipulating time values are defined in <sys/time.h>:


Set a time value to zero.


Test if a time value is non-zero.


Compare two time values. (Beware that >= and <= do not work with the timercmp macro.)

The timer used with ITIMER_REAL is also used by alarm() (see alarm(2)). Thus successive calls to alarm(), getitimer(), and setitimer() set and return the state of a single timer. In addition, a call to alarm() sets the timer interval to zero.

The interaction between setitimer() and any of alarm(), sleep() or usleep() is unspecified.


Upon successful completion, getitimer() or setitimer() returns 0. Otherwise, -1 is returned and errno is set to indicate the error.


The setitimer() function will fail if:


The value argument is not in canonical form. (In canonical form, the number of microseconds is a non-negative integer less than 1,000,000 and the number of seconds is a non-negative integer.)

The getitimer() and setitimer() functions may fail if:


The which argument is not recognized.


The value structure specified a bad address. Reliable detection of this error is implementation dependent.


The following call to setitimer() sets the real-time interval timer to expire initially after 10 seconds and every 0.5 seconds thereafter:

struct itimerval rttimer; struct itimerval old_rttimer; rttimer.it_value.tv_sec = 10; rttimer.it_value.tv_usec = 0; rttimer.it_interval.tv_sec = 0; rttimer.it_interval.tv_usec = 500000; setitimer (ITIMER_REAL, &rttimer, &old_rttimer);


getitimer() was developed by the University of California, Berkeley.


First released in Issue 4, Version 2.

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