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.Dd October 26, 2003 .Dt CALLOUT 9 .Os .Sh NAME .Nm callout_init , .Nm callout_reset , .Nm callout_schedule , .Nm callout_setfunc , .Nm callout_stop , .Nm callout_expired , .Nm callout_invoking , .Nm callout_ack , .Nm CALLOUT_INITIALIZER , .Nm CALLOUT_INITIALIZER_SETFUNC .Nd execute a function after a specified length of time .Sh SYNOPSIS n sys/callout.h .Ft void .Fn "callout_init" "struct callout *c" .Ft void .Fn "callout_reset" "struct callout *c" "int ticks" \ "void (*func)(void *)" "void *arg" .Ft void .Fn "callout_schedule" "struct callout *c" "int ticks" .Ft void .Fn "callout_setfunc" "struct callout *c" "void (*func)(void *)" "void *arg" .Ft void .Fn "callout_stop" "struct callout *c" .Ft int .Fn "callout_pending" "struct callout *c" .Ft int .Fn "callout_expired" "struct callout *c" .Ft int .Fn "callout_invoking" "struct callout *c" .Ft void .Fn "callout_ack" "struct callout *c" .Fd CALLOUT_INITIALIZER
p .Fn CALLOUT_INITIALIZER_SETFUNC "func" "arg" .Sh DESCRIPTION The .Nm callout facility provides a mechanism to execute a function at a given time. The timer is based on the hardclock timer which ticks .Dv hz times per second. The function is called at softclock interrupt level.
p Clients of the .Nm callout facility are responsible for providing pre-allocated callout structures, or .Dq handles . The .Nm callout facility replaces the historic x functions .Fn timeout and .Fn untimeout .
p The .Fn callout_init function initializes the callout handle .Fa c for use. If it is inconvenient to call .Fn callout_init , statically-allocated callout handles may be initialized by assigning the value .Dv CALLOUT_INITIALIZER to them.
p The .Fn callout_reset function resets and starts the timer associated with the callout handle .Fa c . When the timer expires after .Fa ticks Ns No /hz seconds, the function specified by .Fa func will be called with the argument .Fa 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 .Em PENDING . Once the timer expires, the handle is marked as .Em EXPIRED and .Em INVOKING , and the .Em PENDING status is cleared.
p The .Fn callout_setfunc function sets the function and argument of the callout handle .Fa c to .Fa func and .Fa arg respectively. The callout handle must already be initialized. If a callout will always be used with the same function and argument, then .Fn callout_setfunc used in conjunction with .Fn callout_schedule is slightly more efficient than using .Fn callout_reset . If it is inconvenient to call .Fn callout_setfunc , statically-allocated callout handles may be initialized by assigning the value .Dv CALLOUT_INITIALIZER_SETFUNC to them, passing the function and argument to the initializer.
p The .Fn callout_stop function stops the timer associated the callout handle .Fa c . The .Em PENDING and .Em EXPIRED status for the callout handle is cleared. It is safe to call .Fn callout_stop on a callout handle that is not pending, so long as it is initialized.
p The .Fn callout_pending function tests the .Em PENDING status of the callout handle .Fa c . A .Em 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 .Em PENDING status when at softclock interrupt level or higher.
p The .Fn callout_expired function tests to see if the callout's timer has expired and its function called.
p The .Fn callout_invoking function tests the .Em INVOKING status of the callout handle .Fa 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 .Em INVOKING state using .Fn 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 .Fn callout_ack function to clear this flag.
p The .Fn callout_ack function clears the .Em INVOKING state in the callout handle .Fa c . This is used in situations where it is necessary to protect against the race condition described under .Fn callout_invoking . .Sh SEE ALSO .Xr hz 9 .Sh HISTORY The .Nm 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 .Nx kernel by Jason R. Thorpe.