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CALLOUT(9)               BSD Kernel Developer's Manual              CALLOUT(9)

NAME
     callout_init, callout_reset, callout_schedule, callout_setfunc,
     callout_stop, callout_expired, callout_invoking, callout_ack,
     CALLOUT_INITIALIZER, CALLOUT_INITIALIZER_SETFUNC -- execute a function
     after a specified length of time

SYNOPSIS
     #include <&lt;sys/callout.h>&gt;

     void
     callout_init(struct callout *c);

     void
     callout_reset(struct callout *c, int ticks, void (*func)(void *),
         void *arg);

     void
     callout_schedule(struct callout *c, int ticks);

     void
     callout_setfunc(struct callout *c, void (*func)(void *), void *arg);

     void
     callout_stop(struct callout *c);

     int
     callout_pending(struct callout *c);

     int
     callout_expired(struct callout *c);

     int
     callout_invoking(struct callout *c);

     void
     callout_ack(struct callout *c);

     CALLOUT_INITIALIZER

     CALLOUT_INITIALIZER_SETFUNC(func, arg);

DESCRIPTION
     The callout facility provides a mechanism to execute a function at a
     given time.  The timer is based on the hardclock timer which ticks hz
     times per second.  The function is called at softclock interrupt level.

     Clients of the callout facility are responsible for providing pre-allo-
     cated callout structures, or ``handles''.  The callout facility replaces
     the historic BSD functions timeout() and untimeout().

     The callout_init() function initializes the callout handle c for use.  If
     it is inconvenient to call callout_init(), statically-allocated callout
     handles may be initialized by assigning the value CALLOUT_INITIALIZER to
     them.

     The callout_reset() function resets and starts the timer associated with
     the callout handle c.  When the timer expires after ticks/hz seconds, the
     function specified by func will be called with the argument arg.  If the
     timer associated with the callout handle is already running, the callout
     will simply be rescheduled to execute at the newly specified time.  Once
     the timer is started, the callout handle is marked as PENDING.  Once the
     timer expires, the handle is marked as EXPIRED and INVOKING, and the
     PENDING status is cleared.

     The callout_setfunc() function sets the function and argument of the
     callout handle c to func and arg respectively.  The callout handle must
     already be initialized.  If a callout will always be used with the same
     function and argument, then callout_setfunc() used in conjunction with
     callout_schedule() is slightly more efficient than using callout_reset().
     If it is inconvenient to call callout_setfunc(), statically-allocated
     callout handles may be initialized by assigning the value
     CALLOUT_INITIALIZER_SETFUNC to them, passing the function and argument to
     the initializer.

     The callout_stop() function stops the timer associated the callout handle
     c.  The PENDING and EXPIRED status for the callout handle is cleared.  It
     is safe to call callout_stop() on a callout handle that is not pending,
     so long as it is initialized.

     The callout_pending() function tests the PENDING status of the callout
     handle c.  A PENDING callout is one that has been started and whose func-
     tion has not yet been called.  Note that it is possible for a callout's
     timer to have expired without its function being called if interrupt
     level has not dropped low enough to let softclock interrupts through.
     Note that it is only safe to test PENDING status when at softclock inter-
     rupt level or higher.

     The callout_expired() function tests to see if the callout's timer has
     expired and its function called.

     The callout_invoking() function tests the INVOKING status of the callout
     handle c.  This flag is set just before a callout's function is being
     called.  Since the priority level is lowered prior to invocation of the
     callout function, other pending higher-priority code may run before the
     callout function is allowed to run.  This may create a race condition if
     this higher-priority code deallocates storage containing one or more
     callout structures whose callout functions are about to be run.  In such
     cases, one technique to prevent references to deallocated storage would
     be to test whether any callout functions are in the INVOKING state using
     callout_invoking(), and if so, to mark the data structure and defer stor-
     age deallocation until the callout function is allowed to run.  For this
     handshake protocol to work, the callout function will have to use the
     callout_ack() function to clear this flag.

     The callout_ack() function clears the INVOKING state in the callout han-
     dle c.  This is used in situations where it is necessary to protect
     against the race condition described under callout_invoking().

SEE ALSO
     hz(9)

HISTORY
     The callout facility was implemented by Artur Grabowski and Thomas
     Nordin, based on the work of G. Varghese and A. Lauck, described in the
     paper Hashed and Hierarchical Timing Wheels: Data Structures for the
     Efficient Implementation of a Timer Facility in the Proceedings of the
     11th ACM Annual Symposium on Operating System Principles, Austin, Texas,
     November 1987.  It was adapted to the NetBSD kernel by Jason R. Thorpe.

BSD                            October 26, 2003                            BSD