bsd man page for getitimer

GETITIMER(2)							System Calls Manual						      GETITIMER(2)

NAME
       getitimer, setitimer - get/set value of interval timer

SYNOPSIS
       #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;

DESCRIPTION
       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_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  dis-
       abled 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 resolution (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 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.

NOTES
       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).

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

RETURN VALUE
       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
       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.

SEE ALSO
       sigvec(2), gettimeofday(2)

4.2 Berkeley Distribution					  August 26, 1985						      GETITIMER(2)