freebsd/share/man/man9/timeout.9

.\"	$NetBSD: timeout.9,v 1.2 1996/06/23 22:32:34 pk Exp $
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.Dd September 10, 1996
.Dt TIMEOUT 9
.Os FreeBSD
.Sh NAME
.Nm timeout
.Nd execute a function after a specified length of time
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/systm.h>
.Ft typedef void timeout_t \*(lpvoid *\*(rp;

.Ft struct callout_handle
.Fn timeout "timeout_t *func" "void *arg" "int ticks"
.Ft void
.Fn callout_handle_init "struct callout_handle *"

.Li struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle)
.Ft void
.Fn untimeout "timeout_t *func" "void *arg" "struct callout_handle handle"
.Sh DESCRIPTION
The function
.Fn timeout
schedules a call to the function given by the argument
.Fa func
to take place after
.Fa ticks Ns No /hz
seconds. Non-positive values of
.Fa ticks
are silently converted to the value
.Sq 1 .
.Fa func
should be a pointer to a function that takes a
.Fa void *
argument.
Upon invocation,
.Fa func
will receive
.Fa arg
as it's only argument.
The return value from
.Fn timeout
is a
.Ft struct callout_handle
which can be used in conjunction with the 
.Fn untimeout
function to request that a scheduled timeout be canceled.
.Pp
The function
.Fn callout_handle_init
can be used to initialize a handle to a state which will cause
any calls to untimeout with that handle to return with no side
effects.
.Pp
Assigning a callout handle the value of
.Fn CALLOUT_HANDLE_INITIALIZER
performs the same function as
.Fn callout_handle_init
and is provided for use on statically declared or global callout handles.
.Pp
The function
.Fn untimeout
cancels the timeout associated with
.Fa handle
using the
.Fa func
and
.Fa arg
arguments to validate the handle.
If the handle does not correspond to a timeout with
the function
.Fa func
taking the argument
.Fa arg
no action is taken.
.Fa handle
must be initialized by a previous call to
.Fn timeout ,
.Fn callout_handle_init ,
or assigned the value of
.Fn CALLOUT_HANDLE_INITIALIZER "&handle"
before being passed to
.Fn untimeout .
The behavior of calling untimeout without a previously initialized handle
is undefined.
.Pp
As handles are recycled by the system, it is possible (although unlikely)
that a handle from one invocation of
.Fn timeout
may match the handle of another invocation of
.Fn timeout
if both calls used the same function pointer and argument, and the first
timeout is expired or canceled before the second call.
The timeout facility offers O(1) running time for
.Fn timeout
and
.Fn untimeout .
Timeouts are executed from
.Fn softclock
at spl0 and are protected from re-entrancy.
.Sh HISTORY
The current timeout and untimeout routines are based on the work of
Adam M. Costello and George Varghese, published in a technical report entitled
.%T "Redesigning the BSD Callout and Timer Facilities"
and modified slightly for inclusion in FreeBSD by Justin T. Gibbs.
The original work on the data structures used in this implementation
was published by G.Varghese and A. Lauck in the paper
.%T "Hashed and Hierarchical Timing Wheels: Data Structures for the Efficient Implementation of a Timer Facility" 
in the
.%B "Proceedings of the 11th ACM Annual Symposium on Operating Systems Principles" .
The current implementation replaces the long standing BSD linked list
callout mechanism which offered O(n) insertion and removal running time
but did not generate or require handles for untimeout operations.