man/man9/callout.9

.\"	$NetBSD: callout.9,v 1.6 2002/02/13 08:18:38 ross Exp $
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.\" by Jason R. Thorpe.
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.Dd March 21, 2000
.Dt CALLOUT 9
.Os
.Sh NAME
.Nm callout_init ,
.Nm callout_reset ,
.Nm callout_stop
.Nd execute a function after a specified length of time
.Sh SYNOPSIS
.Fd #include \*[Lt]sys/callout.h\*[Gt]
.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_stop" "struct callout *c"
.Ft int
.Fn "callout_active" "struct callout *c"
.Ft int
.Fn "callout_pending" "struct callout *c"
.Ft int
.Fn "callout_expired" "struct callout *c"
.Ft void
.Fn "callout_deactivate" "struct callout *c"
.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.
.Pp
Clients of the
.Nm callout
facility are responsible for providing pre-allocated
callout structures, or
.Dq handles .
The
.Nm callout
facility replaces the historic
.Bx
functions
.Fn timeout
and
.Fn untimeout .
.Pp
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.
.Pp
The
.Fn callout_reset
function starts (or resets) 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 .
Note that if the timer associated with the callout handle is already running,
it will be implicitly stopped before being reset.
Once the timer is started, the callout handle is marked as
.Em ACTIVE
and
.Em PENDING .
Once the timer expires,
.Em PENDING
status is cleared.  Expiration of the timer does not affect
.Em ACTIVE
status.
.Pp
The
.Fn callout_stop
function stops the timer associated the callout handle
.Fa c .
The
.Em ACTIVE
and
.Em PENDING
status for the callout handle is cleared.  It is safe to call
.Fn callout_stop
on a callout handle that is not active, so long as it is initialized.
.Pp
The
.Fn callout_active
function tests the
.Em ACTIVE
status of the callout handle
.Fa c .
An
.Em ACTIVE
callout is one that has been started but not explicitly stopped.
Testing
.Em ACTIVE
status is a way to determine if a callout has been started.  Once the
callout fires, the executed function may clear
.Em ACTIVE
status.  See
.Fn callout_deactivate
below.
.Pp
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.
.Pp
The
.Fn callout_expired
function tests the opposite of
.Fn callout_pending .
That is to say that
.Fn callout_expired
returns true when the callout function has been called.
.Pp
The
.Fn callout_deactivate
function clears the
.Em ACTIVE
status of the callout handle
.Fa c .
Note that is only safe to call
.Fn callout_deactivate
if the callout function has already been executed, i.e. the callout
is no longer
.Em PENDING .
.Sh SEE ALSO
.Xr hz 9
.Sh HISTORY
The
.Nm callout
facility is based on the work of Adam M. Costello and George Varghese,
published in a technical report entitled
.Dq Redesigning the BSD Callout and Timer Facilities ,
and Justin Gibbs's subsequent integration into
.Fx .
It was modified for
.Nx
by Jason R. Thorpe, who also added optional statistics gathering and
an alternate sorting mode for the callwheel.
.Pp
The original work on the data structures used in this implementation was
published by G. Varghese and A. Lauck 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.