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BSD 2.11 - man page for setitimer (bsd section 2)

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GETITIMER(2)									     GETITIMER(2)

       getitimer, setitimer - get/set value of interval timer

       #include <sys/time.h>

       #define ITIMER_REAL	0    /* real time intervals */
       #define ITIMER_VIRTUAL	1    /* virtual time intervals */
       #define ITIMER_PROF	2    /* user and system virtual time */

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

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

       The system provides each process with three 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 nonzero).

       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_inter-
       val  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).

       Time values smaller than the resolution of the system clock are rounded up to this resolu-
       tion (on the VAX, 10 milliseconds).

       The ITIMER_REAL timer decrements in real time.  A SIGALRM signal is  delivered  when  this
       timer expires.

       The  ITIMER_VIRTUAL  timer  decrements  in  process  virtual  time.  It runs only when the
       process is executing.  A SIGVTALRM signal is delivered when it expires.

       The ITIMER_PROF timer decrements both in process virtual time and when the system is  run-
       ning 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.

       Three macros for manipulating time values are defined in <sys/time.h>.  Timerclear sets	a
       time  value  to	zero, timerisset tests if a time value is non-zero, and timercmp compares
       two time values (beware that >= and <= do not work with this macro).

       On the PDP-11, setitimer rounds timer values up to seconds resolution.  (This  saves  some
       space and computation in the overburdened PDP-11 kernel.)

       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.

       The possible errors are:

       [EFAULT]       The value parameter specified a bad address.

       [EINVAL]       A value parameter specified a time was too large to be handled.

       sigvec(2), gettimeofday(2)

4.2 Berkeley Distribution		 August 26, 1985			     GETITIMER(2)
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