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+.\"	$NetBSD: queue.3,v 1.61 2020/10/20 23:27:57 kamil Exp $
+.\"
+.\" Copyright (c) 2000, 2002 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.\" Copyright (c) 1993 The Regents of the University of California.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"	@(#)queue.3	8.1 (Berkeley) 12/13/93
+.\"
+.Dd October 1, 2017
+.Dt QUEUE 3
+.Os
+.Sh NAME
+.Nm SLIST_HEAD ,
+.Nm SLIST_HEAD_INITIALIZER ,
+.Nm SLIST_ENTRY ,
+.Nm SLIST_FIRST ,
+.Nm SLIST_EMPTY ,
+.Nm SLIST_NEXT ,
+.Nm SLIST_FOREACH ,
+.Nm SLIST_FOREACH_SAFE ,
+.Nm SLIST_INIT ,
+.Nm SLIST_INSERT_AFTER ,
+.Nm SLIST_INSERT_HEAD ,
+.Nm SLIST_REMOVE_AFTER ,
+.Nm SLIST_REMOVE_HEAD ,
+.Nm SLIST_REMOVE ,
+.Nm LIST_HEAD ,
+.Nm LIST_HEAD_INITIALIZER ,
+.Nm LIST_ENTRY ,
+.Nm LIST_FIRST ,
+.Nm LIST_EMPTY ,
+.Nm LIST_NEXT ,
+.Nm LIST_FOREACH ,
+.Nm LIST_FOREACH_SAFE ,
+.Nm LIST_INIT ,
+.Nm LIST_INSERT_AFTER ,
+.Nm LIST_INSERT_BEFORE ,
+.Nm LIST_INSERT_HEAD ,
+.Nm LIST_REMOVE ,
+.Nm LIST_REPLACE ,
+.Nm LIST_MOVE ,
+.Nm SIMPLEQ_HEAD ,
+.Nm SIMPLEQ_HEAD_INITIALIZER ,
+.Nm SIMPLEQ_ENTRY ,
+.Nm SIMPLEQ_FIRST ,
+.Nm SIMPLEQ_EMPTY ,
+.Nm SIMPLEQ_NEXT ,
+.Nm SIMPLEQ_LAST ,
+.Nm SIMPLEQ_FOREACH ,
+.Nm SIMPLEQ_FOREACH_SAFE ,
+.Nm SIMPLEQ_INIT ,
+.Nm SIMPLEQ_INSERT_AFTER ,
+.Nm SIMPLEQ_INSERT_HEAD ,
+.Nm SIMPLEQ_INSERT_TAIL ,
+.Nm SIMPLEQ_REMOVE_AFTER ,
+.Nm SIMPLEQ_REMOVE_HEAD ,
+.Nm SIMPLEQ_REMOVE ,
+.Nm SIMPLEQ_CONCAT ,
+.Nm TAILQ_HEAD ,
+.Nm TAILQ_HEAD_INITIALIZER ,
+.Nm TAILQ_ENTRY ,
+.Nm TAILQ_FIRST ,
+.Nm TAILQ_NEXT ,
+.Nm TAILQ_LAST ,
+.Nm TAILQ_PREV ,
+.Nm TAILQ_EMPTY ,
+.Nm TAILQ_FOREACH ,
+.Nm TAILQ_FOREACH_SAFE ,
+.Nm TAILQ_FOREACH_REVERSE ,
+.Nm TAILQ_FOREACH_REVERSE_SAFE ,
+.Nm TAILQ_INIT ,
+.Nm TAILQ_INSERT_AFTER ,
+.Nm TAILQ_INSERT_BEFORE ,
+.Nm TAILQ_INSERT_HEAD ,
+.Nm TAILQ_INSERT_TAIL ,
+.Nm TAILQ_REMOVE ,
+.Nm TAILQ_REPLACE ,
+.Nm TAILQ_CONCAT ,
+.Nm STAILQ_HEAD ,
+.Nm STAILQ_HEAD_INITIALIZER ,
+.Nm STAILQ_ENTRY ,
+.Nm STAILQ_FIRST ,
+.Nm STAILQ_EMPTY ,
+.Nm STAILQ_NEXT ,
+.Nm STAILQ_LAST ,
+.Nm STAILQ_FOREACH ,
+.Nm STAILQ_FOREACH_SAFE ,
+.Nm STAILQ_INIT ,
+.Nm STAILQ_INSERT_AFTER ,
+.Nm STAILQ_INSERT_HEAD ,
+.Nm STAILQ_INSERT_TAIL ,
+.Nm STAILQ_REMOVE_HEAD ,
+.Nm STAILQ_REMOVE ,
+.Nm STAILQ_CONCAT
+.Nd implementations of singly-linked lists, lists, simple queues, tail queues, and singly-linked tail queues
+.Sh SYNOPSIS
+.In sys/queue.h
+.Pp
+.Fn SLIST_HEAD "HEADNAME" "TYPE"
+.Fn SLIST_HEAD_INITIALIZER "head"
+.Fn SLIST_ENTRY "TYPE"
+.Ft TYPE *
+.Fn SLIST_FIRST "SLIST_HEAD *head"
+.Ft int
+.Fn SLIST_EMPTY "SLIST_HEAD *head"
+.Ft TYPE *
+.Fn SLIST_NEXT "TYPE *elm" "SLIST_ENTRY NAME"
+.Fn SLIST_FOREACH "TYPE *var" "SLIST_HEAD *head" "SLIST_ENTRY NAME"
+.Fn SLIST_FOREACH_SAFE "TYPE *var" "SLIST_HEAD *head" "SLIST_ENTRY NAME" "TYPE *tmp"
+.Fn SLIST_INIT "SLIST_HEAD *head"
+.Fn SLIST_INSERT_HEAD "SLIST_HEAD *head" "TYPE *elm" "SLIST_ENTRY NAME"
+.Fn SLIST_INSERT_AFTER "TYPE *listelm" "TYPE *elm" "SLIST_ENTRY NAME"
+.Fn SLIST_REMOVE "SLIST_HEAD *head" "TYPE *elm" "TYPE" "SLIST_ENTRY NAME"
+.Fn SLIST_REMOVE_HEAD "SLIST_HEAD *head" "SLIST_ENTRY NAME"
+.Pp
+.Fn LIST_HEAD "HEADNAME" "TYPE"
+.Fn LIST_HEAD_INITIALIZER "head"
+.Fn LIST_ENTRY "TYPE"
+.Ft TYPE *
+.Fn LIST_FIRST "LIST_HEAD *head"
+.Ft TYPE *
+.Fn LIST_NEXT "TYPE *elm" "LIST_ENTRY NAME"
+.Ft int
+.Fn LIST_EMPTY "LIST_HEAD *head"
+.Fn LIST_FOREACH "TYPE *var" "LIST_HEAD *head" "LIST_ENTRY NAME"
+.Fn LIST_FOREACH_SAFE "TYPE *var" "LIST_HEAD *head" "LIST_ENTRY NAME" "TYPE *tmp"
+.Fn LIST_INIT "LIST_HEAD *head"
+.Fn LIST_INSERT_AFTER "TYPE *listelm" "TYPE *elm" "LIST_ENTRY NAME"
+.Fn LIST_INSERT_BEFORE "TYPE *listelm" "TYPE *elm" "LIST_ENTRY NAME"
+.Fn LIST_INSERT_HEAD "LIST_HEAD *head" "TYPE *elm" "LIST_ENTRY NAME"
+.Fn LIST_REMOVE "TYPE *elm" "LIST_ENTRY NAME"
+.Fn LIST_REPLACE "TYPE *elm" "TYPE *new" "LIST_ENTRY NAME"
+.Fn LIST_MOVE "LIST_HEAD *head1" "LIST_HEAD *head2" "LIST_ENTRY NAME"
+.Pp
+.Fn SIMPLEQ_HEAD "HEADNAME" "TYPE"
+.Fn SIMPLEQ_HEAD_INITIALIZER "head"
+.Fn SIMPLEQ_ENTRY "TYPE"
+.Ft TYPE *
+.Fn SIMPLEQ_FIRST "SIMPLEQ_HEAD *head"
+.Ft int
+.Fn SIMPLEQ_EMPTY "SIMPLEQ_HEAD *head"
+.Ft TYPE *
+.Fn SIMPLEQ_NEXT "TYPE *elm" "SIMPLEQ_ENTRY NAME"
+.Ft TYPE *
+.Fn SIMPLEQ_LAST "SIMPLEQ_HEAD *head" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
+.Fn SIMPLEQ_FOREACH "TYPE *var" "SIMPLEQ_HEAD *head" "SIMPLEQ_ENTRY NAME"
+.Fn SIMPLEQ_FOREACH_SAFE "TYPE *var" "SIMPLEQ_HEAD *head" "SIMPLEQ_ENTRY NAME" "TYPE *tmp"
+.Fn SIMPLEQ_INIT "SIMPLEQ_HEAD *head"
+.Fn SIMPLEQ_INSERT_HEAD "SIMPLEQ_HEAD *head" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
+.Fn SIMPLEQ_INSERT_TAIL "SIMPLEQ_HEAD *head" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
+.Fn SIMPLEQ_INSERT_AFTER "SIMPLEQ_HEAD *head" "TYPE *listelm" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
+.Fn SIMPLEQ_REMOVE_HEAD "SIMPLEQ_HEAD *head" "SIMPLEQ_ENTRY NAME"
+.Fn SIMPLEQ_REMOVE_AFTER "SIMPLEQ_HEAD *head" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
+.Fn SIMPLEQ_REMOVE "SIMPLEQ_HEAD *head" "TYPE *elm" "TYPE" "SIMPLEQ_ENTRY NAME"
+.Fn SIMPLEQ_CONCAT "SIMPLEQ_HEAD *head1" "SIMPLEQ_HEAD *head2"
+.Pp
+.Fn TAILQ_HEAD "HEADNAME" "TYPE"
+.Fn TAILQ_HEAD_INITIALIZER "head"
+.Fn TAILQ_ENTRY "TYPE"
+.Ft TYPE *
+.Fn TAILQ_FIRST "TAILQ_HEAD *head"
+.Ft TYPE *
+.Fn TAILQ_NEXT "TYPE *elm" "TAILQ_ENTRY NAME"
+.Ft TYPE *
+.Fn TAILQ_LAST "TAILQ_HEAD *head" "HEADNAME"
+.Ft TYPE *
+.Fn TAILQ_PREV "TYPE *elm" "HEADNAME" "TAILQ_ENTRY NAME"
+.Ft int
+.Fn TAILQ_EMPTY "TAILQ_HEAD *head"
+.Fn TAILQ_FOREACH "TYPE *var" "TAILQ_HEAD *head" "TAILQ_ENTRY NAME"
+.Fn TAILQ_FOREACH_SAFE "TYPE *var" "TAILQ_HEAD *head" "TAILQ_ENTRY NAME" "TYPE *tmp"
+.Fn TAILQ_FOREACH_REVERSE "TYPE *var" "TAILQ_HEAD *head" "HEADNAME" "TAILQ_ENTRY NAME"
+.Fn TAILQ_FOREACH_REVERSE_SAFE "TYPE *var" "TAILQ_HEAD *head" "HEADNAME" "TAILQ_ENTRY NAME" "TYPE *tmp"
+.Fn TAILQ_INIT "TAILQ_HEAD *head"
+.Fn TAILQ_INSERT_HEAD "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
+.Fn TAILQ_INSERT_TAIL "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
+.Fn TAILQ_INSERT_AFTER "TAILQ_HEAD *head" "TYPE *listelm" "TYPE *elm" "TAILQ_ENTRY NAME"
+.Fn TAILQ_INSERT_BEFORE "TYPE *listelm" "TYPE *elm" "TAILQ_ENTRY NAME"
+.Fn TAILQ_REMOVE "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
+.Fn TAILQ_REPLACE "TAILQ_HEAD *head" "TYPE *elm" "TYPE *new" "TAILQ_ENTRY NAME"
+.Fn TAILQ_CONCAT "TAILQ_HEAD *head1" "TAILQ_HEAD *head2" "TAILQ_ENTRY NAME"
+.Pp
+.Fn STAILQ_HEAD "HEADNAME" "TYPE"
+.Fn STAILQ_HEAD_INITIALIZER "head"
+.Fn STAILQ_ENTRY "TYPE"
+.Ft TYPE *
+.Fn STAILQ_FIRST "STAILQ_HEAD *head"
+.Ft int
+.Fn STAILQ_EMPTY "STAILQ_HEAD *head"
+.Ft TYPE *
+.Fn STAILQ_NEXT "TYPE *elm" "STAILQ_ENTRY NAME"
+.Ft TYPE *
+.Fn STAILQ_LAST "STAILQ_HEAD *head" "TYPE *elm" "STAILQ_ENTRY NAME"
+.Fn STAILQ_FOREACH "TYPE *var" "STAILQ_HEAD *head" "STAILQ_ENTRY NAME"
+.Fn STAILQ_FOREACH_SAFE "TYPE *var" "STAILQ_HEAD *head" "STAILQ_ENTRY NAME" "TYPE *tmp"
+.Fn STAILQ_INIT "STAILQ_HEAD *head"
+.Fn STAILQ_INSERT_HEAD "STAILQ_HEAD *head" "TYPE *elm" "STAILQ_ENTRY NAME"
+.Fn STAILQ_INSERT_TAIL "STAILQ_HEAD *head" "TYPE *elm" "STAILQ_ENTRY NAME"
+.Fn STAILQ_INSERT_AFTER "STAILQ_HEAD *head" "TYPE *listelm" "TYPE *elm" "STAILQ_ENTRY NAME"
+.Fn STAILQ_REMOVE_HEAD "STAILQ_HEAD *head" "STAILQ_ENTRY NAME"
+.Fn STAILQ_REMOVE "STAILQ_HEAD *head" "TYPE *elm" "TYPE" "STAILQ_ENTRY NAME"
+.Fn STAILQ_CONCAT "STAILQ_HEAD *head1" "STAILQ_HEAD *head2"
+.Sh DESCRIPTION
+These macros define and operate on five types of data structures:
+singly-linked lists, simple queues, lists, tail queues, and singly-linked
+tail queues.
+All five structures support the following functionality:
+.Bl -enum -compact -offset indent
+.It
+Insertion of a new entry at the head of the list.
+.It
+Insertion of a new entry after any element in the list.
+.It
+Removal of any entry in the list.
+.It
+Forward traversal through the list.
+.El
+.Pp
+Singly-linked lists are the simplest of the four data structures and
+support only the above functionality.
+Singly-linked lists are ideal for applications with large datasets and
+few or no removals,
+or for implementing a LIFO queue.
+.Pp
+Simple queues add the following functionality:
+.Bl -enum -compact -offset indent
+.It
+Entries can be added at the end of a list.
+.It
+They may be concatenated.
+.El
+However:
+.Bl -enum -compact -offset indent
+.It
+Entries may not be added before any element in the list.
+.It
+All list insertions and removals must specify the head of the list.
+.It
+Each head entry requires two pointers rather than one.
+.El
+.Pp
+Simple queues are ideal for applications with large datasets and few or
+no removals, or for implementing a FIFO queue.
+.Pp
+All doubly linked types of data structures (lists and tail queues)
+additionally allow:
+.Bl -enum -compact -offset indent
+.It
+Insertion of a new entry before any element in the list.
+.It
+O(1) removal of any entry in the list.
+.El
+However:
+.Bl -enum -compact -offset indent
+.It
+Each element requires two pointers rather than one.
+.It
+Code size and execution time of operations (except for removal) is about
+twice that of the singly-linked data-structures.
+.El
+.Pp
+Linked lists are the simplest of the doubly linked data structures and
+support only the above functionality over singly-linked lists.
+.Pp
+Tail queues add the following functionality:
+.Bl -enum -compact -offset indent
+.It
+Entries can be added at the end of a list.
+.It
+They may be concatenated.
+.El
+However:
+.Bl -enum -compact -offset indent
+.It
+All list insertions and removals, except insertion before another element, must
+specify the head of the list.
+.It
+Each head entry requires two pointers rather than one.
+.It
+Code size is about 15% greater and operations run about 20% slower
+than lists.
+.El
+.Pp
+Circular queues add the following functionality:
+.Bl -enum -compact -offset indent
+.It
+Entries can be added at the end of a list.
+.It
+They may be traversed backwards, from tail to head.
+.El
+However:
+.Bl -enum -compact -offset indent
+.It
+All list insertions and removals must specify the head of the list.
+.It
+Each head entry requires two pointers rather than one.
+.It
+The termination condition for traversal is more complex.
+.It
+Code size is about 40% greater and operations run about 45% slower
+than lists.
+.El
+.Pp
+In the macro definitions,
+.Fa TYPE
+is the name of a user defined structure,
+that must contain a field of type
+.Li SLIST_ENTRY ,
+.Li LIST_ENTRY ,
+.Li SIMPLEQ_ENTRY ,
+.Li TAILQ_ENTRY ,
+or
+.Li STAILQ_ENTRY ,
+named
+.Fa NAME .
+The argument
+.Fa HEADNAME
+is the name of a user defined structure that must be declared
+using the macros
+.Li LIST_HEAD ,
+.Li SIMPLEQ_HEAD ,
+.Li SLIST_HEAD ,
+or
+.Li TAILQ_HEAD .
+See the examples below for further explanation of how these
+macros are used.
+.Ss Summary of Operations
+The following table summarizes the supported macros for each type
+of data structure.
+.Pp
+.TS
+box tab(:);
+l | c | c | c | c | c
+l | c | c | c | c | c
+l | c | c | c | c | c
+l | c | c | c | c | c
+l | c | c | c | c | c
+l | c | c | c | c | c.
+:SLIST:LIST:SIMPLEQ:TAILQ:STAILQ
+_
+_FIRST:+:+:+:+:+
+_EMPTY:+:+:+:+:+
+_NEXT:+:+:+:+:+
+_PREV:-:-:-:+:-
+_LAST:-:-:+:+:+
+_FOREACH:+:+:+:+:+
+_FOREACH_SAFE:+:+:+:+:+
+_FOREACH_REVERSE:-:-:-:+:-
+_FOREACH_REVERSE_SAFE:-:-:-:+:-
+_INSERT_HEAD:+:+:+:+:+
+_INSERT_AFTER:+:+:+:+:+
+_INSERT_BEFORE:-:+:-:+:-
+_INSERT_TAIL:-:-:+:+:+
+_REMOVE:+:+:+:+:+
+_REMOVE_HEAD:+:-:+:-:+
+_REMOVE_AFTER:-:-:+:-:+
+_REPLACE:-:+:-:+:-
+_CONCAT:-:-:+:+:+
+.TE
+.Sh SINGLY-LINKED LISTS
+A singly-linked list is headed by a structure defined by the
+.Fn SLIST_HEAD
+macro.
+This structure contains a single pointer to the first element
+on the list.
+The elements are singly linked for minimum space and pointer manipulation
+overhead at the expense of O(n) removal for arbitrary elements.
+New elements can be added to the list after an existing element or
+at the head of the list.
+An
+.Fa SLIST_HEAD
+structure is declared as follows:
+.Bd -literal -offset indent
+SLIST_HEAD(HEADNAME, TYPE) head;
+.Ed
+.Pp
+where
+.Fa HEADNAME
+is the name of the structure to be defined, and
+.Fa TYPE
+is the type of the elements to be linked into the list.
+A pointer to the head of the list can later be declared as:
+.Bd -literal -offset indent
+struct HEADNAME *headp;
+.Ed
+.Pp
+(The names
+.Li head
+and
+.Li headp
+are user selectable.)
+.Pp
+The macro
+.Fn SLIST_HEAD_INITIALIZER
+evaluates to an initializer for the list
+.Fa head .
+.Pp
+The macro
+.Fn SLIST_ENTRY
+declares a structure that connects the elements in
+the list.
+.Pp
+The macro
+.Fn SLIST_FIRST
+returns the first element in the list or NULL if the list is empty.
+.Pp
+The macro
+.Fn SLIST_EMPTY
+evaluates to true if there are no elements in the list.
+.Pp
+The macro
+.Fn SLIST_NEXT
+returns the next element in the list.
+.Pp
+.Fn SLIST_FOREACH
+traverses the list referenced by
+.Fa head
+in the forward direction, assigning each element in
+turn to
+.Fa var .
+.Pp
+The SAFE version uses
+.Fa tmp
+to hold the next element, so
+.Fa var
+may be freed or removed from the list.
+.Pp
+The macro
+.Fn SLIST_INIT
+initializes the list referenced by
+.Fa head .
+.Pp
+The macro
+.Fn SLIST_INSERT_HEAD
+inserts the new element
+.Fa elm
+at the head of the list.
+.Pp
+The macro
+.Fn SLIST_INSERT_AFTER
+inserts the new element
+.Fa elm
+after the element
+.Fa listelm .
+.Pp
+The macro
+.Fn SLIST_REMOVE
+removes the element
+.Fa elm
+from the list.
+.Pp
+The macro
+.Fn SLIST_REMOVE_HEAD
+removes the first element from the head of the list.
+For optimum efficiency,
+elements being removed from the head of the list should explicitly use
+this macro instead of the generic
+.Fn SLIST_REMOVE
+macro.
+.Pp
+The macro
+.Fn SLIST_REMOVE_AFTER
+removes the element after the one specified.
+For optimum efficiency,
+elements being removed after a specified one should explicitly use
+this macro instead of the generic
+.Fn SLIST_REMOVE
+.Sh SINGLY-LINKED LIST EXAMPLE
+.Bd -literal
+SLIST_HEAD(slisthead, entry) head =
+    SLIST_HEAD_INITIALIZER(head);
+struct slisthead *headp;                /* Singly-linked List head. */
+struct entry {
+        ...
+        SLIST_ENTRY(entry) entries;     /* Singly-linked List. */
+        ...
+} *n1, *n2, *n3, *np;
+
+SLIST_INIT(&head);                      /* Initialize the list. */
+
+n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
+SLIST_INSERT_HEAD(&head, n1, entries);
+
+n2 = malloc(sizeof(struct entry));      /* Insert after. */
+SLIST_INSERT_AFTER(n1, n2, entries);
+
+SLIST_REMOVE(&head, n2, entry, entries);/* Deletion. */
+free(n2);
+
+n3 = SLIST_FIRST(&head);
+SLIST_REMOVE_HEAD(&head, entries);      /* Deletion from the head. */
+free(n3);
+
+SLIST_FOREACH(np, &head, entries)       /* Forward traversal. */
+        np-> ...
+
+while (!SLIST_EMPTY(&head)) {           /* List Deletion. */
+        n1 = SLIST_FIRST(&head);
+        SLIST_REMOVE_HEAD(&head, entries);
+        free(n1);
+}
+.Ed
+.Sh LISTS
+A list is headed by a structure defined by the
+.Fn LIST_HEAD
+macro.
+This structure contains a single pointer to the first element
+on the list.
+The elements are doubly linked so that an arbitrary element can be
+removed without traversing the list.
+New elements can be added to the list after an existing element,
+before an existing element, or at the head of the list.
+A
+.Fa LIST_HEAD
+structure is declared as follows:
+.Bd -literal -offset indent
+LIST_HEAD(HEADNAME, TYPE) head;
+.Ed
+.Pp
+where
+.Fa HEADNAME
+is the name of the structure to be defined, and
+.Fa TYPE
+is the type of the elements to be linked into the list.
+A pointer to the head of the list can later be declared as:
+.Bd -literal -offset indent
+struct HEADNAME *headp;
+.Ed
+.Pp
+(The names
+.Li head
+and
+.Li headp
+are user selectable.)
+.Pp
+The macro
+.Fn LIST_ENTRY
+declares a structure that connects the elements in
+the list.
+.Pp
+The macro
+.Fn LIST_HEAD_INITIALIZER
+provides a value which can be used to initialize a list head at
+compile time, and is used at the point that the list head
+variable is declared, like:
+.Bd -literal -offset indent
+struct HEADNAME head = LIST_HEAD_INITIALIZER(head);
+.Ed
+.Pp
+The macro
+.Fn LIST_FIRST
+returns the first element of the list
+.Fa head .
+.Pp
+The macro
+.Fn LIST_EMPTY
+returns true if the list
+.Fa head
+has no elements.
+.Pp
+The macro
+.Fn LIST_NEXT
+returns the element after the element
+.Fa elm .
+.Pp
+The macro
+.Fn LIST_FOREACH
+traverses the list referenced by
+.Fa head
+in the forward direction, assigning each element in turn to
+.Fa var .
+.Pp
+The SAFE version uses
+.Fa tmp
+to hold the next element, so
+.Fa var
+may be freed or removed from the list.
+.Pp
+The macro
+.Fn LIST_INIT
+initializes the list referenced by
+.Fa head .
+.Pp
+The macro
+.Fn LIST_INSERT_AFTER
+inserts the new element
+.Fa elm
+after the element
+.Fa listelm .
+.Pp
+The macro
+.Fn LIST_INSERT_BEFORE
+inserts the new element
+.Fa elm
+before the element
+.Fa listelm .
+.Pp
+The macro
+.Fn LIST_INSERT_HEAD
+inserts the new element
+.Fa elm
+at the head of the list.
+.Pp
+The macro
+.Fn LIST_REMOVE
+removes the element
+.Fa elm
+from the list.
+.Pp
+The macro
+.Fn LIST_REPLACE
+replaces the element
+.Fa elm
+with
+.Fa new
+in the list.
+.Pp
+The macro
+.Fn LIST_MOVE
+moves the list headed by
+.Fa head1
+onto the list headed by
+.Fa head2 ,
+always making the former empty.
+.Sh LIST EXAMPLE
+.Bd -literal
+LIST_HEAD(listhead, entry) head;
+struct listhead *headp;			/* List head. */
+struct entry {
+	...
+	LIST_ENTRY(entry) entries;	/* List. */
+	...
+} *n1, *n2, *np;
+
+LIST_INIT(&head);			/* Initialize the list. */
+
+n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
+LIST_INSERT_HEAD(&head, n1, entries);
+
+n2 = malloc(sizeof(struct entry));	/* Insert after. */
+LIST_INSERT_AFTER(n1, n2, entries);
+
+n2 = malloc(sizeof(struct entry));	/* Insert before. */
+LIST_INSERT_BEFORE(n1, n2, entries);
+
+LIST_FOREACH(np, &head, entries)	/* Forward traversal. */
+	np-> ...
+
+while (LIST_FIRST(&head) != NULL)	/* Delete. */
+	LIST_REMOVE(LIST_FIRST(&head), entries);
+if (LIST_EMPTY(&head))			/* Test for emptiness. */
+	printf("nothing to do\\n");
+.Ed
+.Sh SIMPLE QUEUES
+A simple queue is headed by a structure defined by the
+.Fn SIMPLEQ_HEAD
+macro.
+This structure contains a pair of pointers,
+one to the first element in the simple queue and the other to
+the last element in the simple queue.
+The elements are singly linked for minimum space and pointer manipulation
+overhead at the expense of O(n) removal for arbitrary elements.
+New elements can be added to the queue after an existing element,
+at the head of the queue, or at the end of the queue.
+A
+.Fa SIMPLEQ_HEAD
+structure is declared as follows:
+.Bd -literal -offset indent
+SIMPLEQ_HEAD(HEADNAME, TYPE) head;
+.Ed
+.Pp
+where
+.Li HEADNAME
+is the name of the structure to be defined, and
+.Li TYPE
+is the type of the elements to be linked into the simple queue.
+A pointer to the head of the simple queue can later be declared as:
+.Bd -literal -offset indent
+struct HEADNAME *headp;
+.Ed
+.Pp
+(The names
+.Li head
+and
+.Li headp
+are user selectable.)
+.Pp
+The macro
+.Fn SIMPLEQ_ENTRY
+declares a structure that connects the elements in
+the simple queue.
+.Pp
+The macro
+.Fn SIMPLEQ_HEAD_INITIALIZER
+provides a value which can be used to initialize a simple queue head at
+compile time, and is used at the point that the simple queue head
+variable is declared, like:
+.Bd -literal -offset indent
+struct HEADNAME head = SIMPLEQ_HEAD_INITIALIZER(head);
+.Ed
+.Pp
+The macro
+.Fn SIMPLEQ_FIRST
+returns the first element of the simple queue
+.Fa head .
+.Pp
+The macro
+.Fn SIMPLEQ_EMPTY
+returns true if the simple queue
+.Fa head
+has no elements.
+.Pp
+The macro
+.Fn SIMPLEQ_NEXT
+returns the element after the element
+.Fa elm .
+.Pp
+The macro
+.Fn SIMPLEQ_LAST
+returns the last item on the simple queue.
+If the simple queue is empty the return value is
+.Dv NULL .
+.Pp
+The macro
+.Fn SIMPLEQ_FOREACH
+traverses the simple queue referenced by
+.Fa head
+in the forward direction, assigning each element
+in turn to
+.Fa var .
+.Pp
+The SAFE version uses
+.Fa tmp
+to hold the next element, so
+.Fa var
+may be freed or removed from the list.
+.Pp
+The macro
+.Fn SIMPLEQ_INIT
+initializes the simple queue referenced by
+.Fa head .
+.Pp
+The macro
+.Fn SIMPLEQ_INSERT_HEAD
+inserts the new element
+.Fa elm
+at the head of the simple queue.
+.Pp
+The macro
+.Fn SIMPLEQ_INSERT_TAIL
+inserts the new element
+.Fa elm
+at the end of the simple queue.
+.Pp
+The macro
+.Fn SIMPLEQ_INSERT_AFTER
+inserts the new element
+.Fa elm
+after the element
+.Fa listelm .
+.Pp
+The macro
+.Fn SIMPLEQ_REMOVE_HEAD
+removes the first element from the head of the simple queue.
+For optimum efficiency,
+elements being removed from the head of the queue should explicitly use
+this macro instead of the generic
+.Fn SIMPLEQ_REMOVE
+macro.
+.Pp
+The macro
+.Fn SIMPLEQ_REMOVE_AFTER
+removes the element after the one specified from the simple queue.
+For optimum efficiency,
+elements being removed after specified elements should explicitly use
+this macro instead of the generic
+.Fn SIMPLEQ_REMOVE
+macro.
+.Pp
+The macro
+.Fn SIMPLEQ_REMOVE
+removes
+.Fa elm
+from the simple queue.
+.Pp
+The macro
+.Fn SIMPLEQ_CONCAT
+concatenates the simple queue headed by
+.Fa head2
+onto the end of the one headed by
+.Fa head1 ,
+removing all entries from the former.
+.Sh SIMPLE QUEUE EXAMPLE
+.Bd -literal
+SIMPLEQ_HEAD(simplehead, entry) head;
+struct simplehead *headp;		/* Simple queue head. */
+struct entry {
+	...
+	SIMPLEQ_ENTRY(entry) entries;	/* Simple queue. */
+	...
+} *n1, *n2, *np;
+
+SIMPLEQ_INIT(&head);			/* Initialize the queue. */
+
+n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
+SIMPLEQ_INSERT_HEAD(&head, n1, entries);
+
+n1 = malloc(sizeof(struct entry));	/* Insert at the tail. */
+SIMPLEQ_INSERT_TAIL(&head, n1, entries);
+
+n2 = malloc(sizeof(struct entry));	/* Insert after. */
+SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);
+
+SIMPLEQ_FOREACH(np, &head, entries)	/* Forward traversal. */
+	np-> ...
+
+while (SIMPLEQ_FIRST(&head) != NULL)	/* Delete. */
+	SIMPLEQ_REMOVE_HEAD(&head, entries);
+if (SIMPLEQ_EMPTY(&head))		/* Test for emptiness. */
+	printf("nothing to do\\n");
+.Ed
+.Sh TAIL QUEUES
+A tail queue is headed by a structure defined by the
+.Fn TAILQ_HEAD
+macro.
+This structure contains a pair of pointers,
+one to the first element in the tail queue and the other to
+the last element in the tail queue.
+The elements are doubly linked so that an arbitrary element can be
+removed without traversing the tail queue.
+New elements can be added to the queue after an existing element,
+before an existing element, at the head of the queue, or at the end
+the queue.
+A
+.Fa TAILQ_HEAD
+structure is declared as follows:
+.Bd -literal -offset indent
+TAILQ_HEAD(HEADNAME, TYPE) head;
+.Ed
+.Pp
+where
+.Li HEADNAME
+is the name of the structure to be defined, and
+.Li TYPE
+is the type of the elements to be linked into the tail queue.
+A pointer to the head of the tail queue can later be declared as:
+.Bd -literal -offset indent
+struct HEADNAME *headp;
+.Ed
+.Pp
+(The names
+.Li head
+and
+.Li headp
+are user selectable.)
+.Pp
+The macro
+.Fn TAILQ_ENTRY
+declares a structure that connects the elements in
+the tail queue.
+.Pp
+The macro
+.Fn TAILQ_HEAD_INITIALIZER
+provides a value which can be used to initialize a tail queue head at
+compile time, and is used at the point that the tail queue head
+variable is declared, like:
+.Bd -literal -offset indent
+struct HEADNAME head = TAILQ_HEAD_INITIALIZER(head);
+.Ed
+.Pp
+The macro
+.Fn TAILQ_FIRST
+returns the first element of the tail queue
+.Fa head .
+.Pp
+The macro
+.Fn TAILQ_NEXT
+returns the element after the element
+.Fa elm .
+.Pp
+The macro
+.Fn TAILQ_LAST
+returns the last item on the tail queue.
+If the tail queue is empty the return value is
+.Dv NULL .
+.Pp
+The macro
+.Fn TAILQ_PREV
+returns the previous item on the tail queue, from the one specified.
+If the tail queue is empty the return value is
+.Dv NULL .
+.Pp
+The macro
+.Fn TAILQ_EMPTY
+returns true if the tail queue
+.Fa head
+has no elements.
+.Pp
+The macros
+.Fn TAILQ_FOREACH ,
+.Fn TAILQ_FOREACH_REVERSE ,
+.Fn TAILQ_FOREACH_SAFE ,
+and
+.Fn TAILQ_FOREACH_REVERSE_SAFE
+traverse the tail queue referenced by
+.Fa head
+in the forward or reverse direction, assigning each element in turn to
+.Fa var .
+.Pp
+The SAFE versions use
+.Fa tmp
+to hold the next element, so
+.Fa var
+may be freed or removed from the list.
+.Pp
+The macro
+.Fn TAILQ_INIT
+initializes the tail queue referenced by
+.Fa head .
+.Pp
+The macro
+.Fn TAILQ_INSERT_HEAD
+inserts the new element
+.Fa elm
+at the head of the tail queue.
+.Pp
+The macro
+.Fn TAILQ_INSERT_TAIL
+inserts the new element
+.Fa elm
+at the end of the tail queue.
+.Pp
+The macro
+.Fn TAILQ_INSERT_AFTER
+inserts the new element
+.Fa elm
+after the element
+.Fa listelm .
+.Pp
+The macro
+.Fn TAILQ_INSERT_BEFORE
+inserts the new element
+.Fa elm
+before the element
+.Fa listelm .
+.Pp
+The macro
+.Fn TAILQ_REMOVE
+removes the element
+.Fa elm
+from the tail queue.
+.Pp
+The macro
+.Fn TAILQ_REPLACE
+replaces the element
+.Fa elm
+with the
+.Fa new
+one specified in the tail queue.
+.Pp
+The macro
+.Fn TAILQ_CONCAT
+concatenates the tail queue headed by
+.Fa head2
+onto the end of the one headed by
+.Fa head1 ,
+removing all entries from the former.
+.Sh TAIL QUEUE EXAMPLE
+.Bd -literal
+TAILQ_HEAD(tailhead, entry) head;
+struct tailhead *headp;			/* Tail queue head. */
+struct entry {
+	...
+	TAILQ_ENTRY(entry) entries;	/* Tail queue. */
+	...
+} *n1, *n2, *np;
+
+TAILQ_INIT(&head);			/* Initialize the queue. */
+
+n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
+TAILQ_INSERT_HEAD(&head, n1, entries);
+
+n1 = malloc(sizeof(struct entry));	/* Insert at the tail. */
+TAILQ_INSERT_TAIL(&head, n1, entries);
+
+n2 = malloc(sizeof(struct entry));	/* Insert after. */
+TAILQ_INSERT_AFTER(&head, n1, n2, entries);
+
+n2 = malloc(sizeof(struct entry));	/* Insert before. */
+TAILQ_INSERT_BEFORE(n1, n2, entries);
+
+TAILQ_FOREACH(np, &head, entries)	/* Forward traversal. */
+	np-> ...
+					/* Reverse traversal. */
+TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries)
+	np-> ...
+
+while (TAILQ_FIRST(&head) != NULL)	/* Delete. */
+	TAILQ_REMOVE(&head, TAILQ_FIRST(&head), entries);
+if (TAILQ_EMPTY(&head))			/* Test for emptiness. */
+	printf("nothing to do\\n");
+.Ed
+.Sh SINGLY LINKED TAIL QUEUES
+The macros prefixed with
+.Do Nm STAILQ_ Dc (
+.Fn STAILQ_HEAD ,
+.Fn STAILQ_HEAD_INITIALIZER ,
+.Fn STAILQ_ENTRY ,
+.Fn STAILQ_FOREACH ,
+.Fn STAILQ_FOREACH_SAFE ,
+.Fn STAILQ_FIRST ,
+.Fn STAILQ_EMPTY ,
+.Fn STAILQ_NEXT ,
+.Fn STAILQ_LAST ,
+.Fn STAILQ_INIT ,
+.Fn STAILQ_INSERT_HEAD ,
+.Fn STAILQ_INSERT_TAIL ,
+.Fn STAILQ_INSERT_AFTER ,
+.Fn STAILQ_REMOVE_HEAD ,
+.Fn STAILQ_REMOVE ,
+and
+.Fn STAILQ_CONCAT )
+are functionally identical to these simple queue functions,
+and are provided for compatibility with
+.Fx .
+.Sh NOTES
+Some of these macros or functions perform no error checking,
+and invalid usage leads to undefined behaviour.
+In the case of macros or functions that expect their arguments
+to be elements that are present in the list or queue, passing an element
+that is not present is invalid.
+.Sh HISTORY
+The
+.Nm queue
+functions first appeared in
+.Bx 4.4 .
+The
+.Nm SIMPLEQ
+functions first appeared in
+.Nx 1.2 .
+The
+.Nm SLIST
+and
+.Nm STAILQ
+functions first appeared in
+.Fx 2.1.5 .
+.Pp
+The
+.Nm CIRCLEQ
+functions first appeared in
+.Bx 4.4
+and were deprecated in
+.Nx 7
+and removed in
+.Nx 10
+due to the pointer aliasing violations.