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 <sys/callout.h>
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_active(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-allocated 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 function 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 interrupt level or higher.

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

The callout_active() function returns true if a timer has been started but not explicitly stopped, even if it has already fired. callout_active(foo) is logically the same as callout_pending(foo) || callout_expired(foo); it is implemented as a separate function for compatibility with FreeBSD and for the special case of TCP_TIMER_ISARMED(). Its use is not recommended.

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 storage 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 handle 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.