CALLOUT(9) BSD Kernel Developer's Manual CALLOUT(9)
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
callout_init(struct callout *c);
callout_reset(struct callout *c, int ticks, void (*func)(void *),
callout_schedule(struct callout *c, int ticks);
callout_setfunc(struct callout *c, void (*func)(void *), void *arg);
callout_stop(struct callout *c);
callout_pending(struct callout *c);
callout_expired(struct callout *c);
callout_invoking(struct callout *c);
callout_ack(struct callout *c);
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
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 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().
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