195 lines
5.0 KiB
Groff
195 lines
5.0 KiB
Groff
.\" $NetBSD: pcq.9,v 1.9 2023/02/23 03:03:23 riastradh Exp $
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.\"
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.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Matt Thomas.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd January 22, 2012
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.Dt PCQ 9
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.Os
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.Sh NAME
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.Nm pcq
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.Nd producer/consumer queue
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.Sh SYNOPSIS
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.In sys/pcq.h
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.Ft pcq_t *
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.Fn pcq_create "size_t maxlen" "km_flags_t kmflags"
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.Ft void
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.Fn pcq_destroy "pcq_t *pcq"
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.Ft void *
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.Fn pcq_get "pcq_t *pcq"
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.Ft size_t
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.Fn pcq_maxitems "pcq_t *pcq"
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.Ft void *
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.Fn pcq_peek "pcq_t *pcq"
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.Ft bool
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.Fn pcq_put "pcq_t *pcq" "void *item"
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.Sh DESCRIPTION
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The machine-independent
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.Nm
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interface provides lockless producer/consumer queues.
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A queue
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.Po
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.Vt pcq_t
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.Pc
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allows multiple writers
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.Pq producers ,
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but only a single reader
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.Pq consumer .
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The consumer is expected to be protected by a lock that covers
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the structure that the
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.Vt pcq_t
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is embedded into
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.Po
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e.g., socket lock, ifnet hwlock
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.Pc .
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These queues operate in a first-in, first-out
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.Pq FIFO
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manner.
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The act of inserting or removing an item from a
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.Vt pcq_t
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does not modify the item in any way.
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.Nm
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does not prevent an item from being inserted multiple times into a single
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.Vt pcq_t .
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.Sh FUNCTIONS
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.Bl -tag -width compact
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.It Fn pcq_create "maxlen" "kmflags"
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Create a queue that can store at most
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.Fa maxlen
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items at one time.
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.Fa kmflags
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should be either
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.Dv KM_SLEEP ,
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if
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.Fn pcq_create
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is allowed to sleep until resources are available, or
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.Dv KM_NOSLEEP
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if it should return
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.Dv NULL
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immediately, if resources are unavailable.
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.It Fn pcq_destroy "pcq"
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Free the resources held by
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.Fa pcq .
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.It Fn pcq_get "pcq"
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Remove the next item to be consumed from the queue and return it.
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If the queue is empty,
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return
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.Dv NULL .
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The caller must prevent concurrent gets from occurring.
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.It Fn pcq_maxitems "pcq"
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Return the maximum number of items that the queue can store at
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any one time.
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.It Fn pcq_peek "pcq"
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Return the next item to be consumed from the queue but do not remove
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it from the queue.
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If the queue is empty,
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return
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.Dv NULL .
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.It Fn pcq_put "pcq" "item"
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Place an item at the end of the queue.
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If there is no room in the queue for the item, return
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.Dv false ;
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otherwise, return
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.Dv true .
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The item must not have the value of
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.Dv NULL .
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.El
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.Ss Memory ordering
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Any memory operations sequenced before
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.Fn pcq_put
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of an item in one thread happen before all memory operations with data
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dependencies on the item returned by
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.Fn pcq_get
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or
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.Fn pcq_peek
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in another thread.
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For example:
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.Bd -literal -offset indent
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int mumble;
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/* producer */
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mumble = 42; // A
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foo->x = 123; // B
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refcnt = foo->refcnt; // C
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pcq_put(pcq, foo);
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KASSERT(refcnt == 0);
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/* consumer */
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foo = pcq_get(pcq);
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if (foo == NULL)
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return;
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atomic_inc_uint(&foo->refcnt); // D
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x = foo->x; // E
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if (x == 123)
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KASSERT(mumble == 42); // F
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.Ed
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.Pp
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In this example, memory operations B and C happen-before D and E.
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However, no ordering is guaranteed for A or F relative to any other
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memory operations, because the memory location of
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.Fa mumble
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is independent of the pointer
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.Fa foo
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returned by
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.Fn pcq_get .
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.Pp
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If you must guarantee A happens before F, then on the consumer side,
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after
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.Fn pcq_get
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or
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.Fn pcq_peek ,
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you can call
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.Fn membar_acquire
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to turn it into an acquire operation instead of a consume operation;
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.Fn pcq_put
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serves as the matching release operation.
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.Po
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This is a little dicey.
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Perhaps there should be separate
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.Fn pcq_peek_acq
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and
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.Fn pcq_get_acq
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operations if this semantics is necessary.
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.Pc
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.Sh CODE REFERENCES
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The
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.Nm
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interface is implemented within the file
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.Pa sys/kern/subr_pcq.c .
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.\" .Sh EXAMPLES
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.Sh SEE ALSO
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.Xr atomic_ops 3 ,
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.Xr queue 3
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.Sh HISTORY
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The
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.Nm
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interface first appeared in
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.Nx 6.0 .
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.\" .Sh CAVEATS
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.\" .Sh BUGS
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.\" .Sh SECURITY CONSIDERATIONS
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