freebsd-nq/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
.Sh NAME
.Nm timeout ,
.Nm untimeout ,
.Nm callout_handle_init ,
.Nm callout_init ,
.Nm callout_stop ,
.Nm callout_reset
.Nd execute a function after a specified length of time
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.Pp
.Bd -literal
typedef void timeout_t (void *);
.Ed
.Ft struct callout_handle
.Fn timeout "timeout_t *func" "void *arg" "int ticks"
.Ft void
.Fn callout_handle_init "struct callout_handle *handle"
.Pp
.Bd -literal
struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle)
.Ed
.Ft void
.Fn untimeout "timeout_t *func" "void *arg" "struct callout_handle handle"
.Ft void
.Fn callout_init "struct callout *c" "int mpsafe"
.Ft int
.Fn callout_stop "struct callout *c"
.Ft void
.Fn callout_reset "struct callout *c" "int ticks" "timeout_t *func" "void *arg"
.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 its 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
.Fn splsoftclock .
Thus they are protected from re-entrancy.
.Pp
The functions
.Fn callout_init ,
.Fn callout_stop
and
.Fn callout_reset
are low-level routines for clients who wish to allocate their own
callout structures.
.Pp
The function
.Fn callout_init
initializes a callout so it can be passed to
.Fn callout_stop
or
.Fn callout_reset
without any side effects.
If the
.Fa mpsafe
argument is zero,
the callout structure is not considered to be
.Dq multi-processor safe ;
that is,
the Giant lock will be acquired before calling the callout function,
and released when the callout function returns.
.Pp
The function
.Fn callout_stop
cancels a callout if it is currently pending.
If the callout is pending, then
.Fn callout_stop
will return a non-zero value.
If the callout has already been serviced or is currently being serviced,
then zero will be returned.
.Pp
The function
.Fn callout_reset
first calls
.Fn callout_stop
to disestablish the callout, and then establishes a new callout in the
same manner as
.Fn timeout .
.Sh RETURN VALUES
The
.Fn timeout
function returns a
.Ft struct callout_handle
that can be passed to
.Fn untimeout .
The
.Fn callout_stop
function returns non-zero if the callout is still pending or zero otherwise.
.Sh BUGS
This API has no way to cancel a callback and ensure that if it was
cancelled too late that the callback has actually finished.
.Fn callout_stop 
only guarantees that the callback has started when it returns 0.
It does not guarnatee that the callback has finished.
This can create a race when one wishes to ensure that no threads are
executing before returning from a driver detach routine.
The locking scheme currently used ensures that a
.Fn callout_stop 
will not return until after the callout has been dequeued or has
finished running.
This locking is an accident of the implementation and is not
guarnateed by the interface.
.Sh HISTORY
The current timeout and untimeout routines are based on the work of
.An Adam M. Costello
and
.An George Varghese ,
published in a technical report entitled
.%T "Redesigning the BSD Callout and Timer Facilities"
and modified slightly for inclusion in
.Fx
by
.An Justin T. Gibbs .
The original work on the data structures used in this implementation
was published by
.An G. Varghese
and
.An 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
.Bx
linked list
callout mechanism which offered O(n) insertion and removal running time
but did not generate or require handles for untimeout operations.