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QUEUE(3)	       FreeBSD Library Functions Manual		      QUEUE(3)

NAME
     SLIST_EMPTY, SLIST_ENTRY, SLIST_FIRST, SLIST_FOREACH, SLIST_HEAD,
     SLIST_HEAD_INITIALIZER, SLIST_INIT, SLIST_INSERT_AFTER,
     SLIST_INSERT_HEAD,	SLIST_NEXT, SLIST_REMOVE_HEAD, SLIST_REMOVE,
     STAILQ_EMPTY, STAILQ_ENTRY, STAILQ_FIRST, STAILQ_FOREACH, STAILQ_HEAD,
     STAILQ_HEAD_INITIALIZER, STAILQ_INIT, STAILQ_INSERT_AFTER,
     STAILQ_INSERT_HEAD, STAILQ_INSERT_TAIL, STAILQ_LAST, STAILQ_NEXT,
     STAILQ_REMOVE_HEAD, STAILQ_REMOVE,	LIST_EMPTY, LIST_ENTRY,	LIST_FIRST,
     LIST_FOREACH, LIST_HEAD, LIST_HEAD_INITIALIZER, LIST_INIT,
     LIST_INSERT_AFTER,	LIST_INSERT_BEFORE, LIST_INSERT_HEAD, LIST_NEXT,
     LIST_REMOVE, TAILQ_EMPTY, TAILQ_ENTRY, TAILQ_FIRST, TAILQ_FOREACH,
     TAILQ_FOREACH_REVERSE, TAILQ_HEAD,	TAILQ_HEAD_INITIALIZER,	TAILQ_INIT,
     TAILQ_INSERT_AFTER, TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD,
     TAILQ_INSERT_TAIL,	TAILQ_LAST, TAILQ_NEXT,	TAILQ_PREV, TAILQ_REMOVE,
     CIRCLEQ_EMPTY, CIRCLEQ_ENTRY, CIRCLEQ_FIRST, CIRCLEQ_FOREACH,
     CIRCLEQ_FOREACH_REVERSE, CIRCLEQ_HEAD, CIRCLEQ_HEAD_INITIALIZER,
     CIRCLEQ_INIT, CIRCLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE,
     CIRCLEQ_INSERT_HEAD, CIRCLEQ_INSERT_TAIL, CIRCLE_LAST, CIRCLE_NEXT,
     CIRCLE_PREV, CIRCLEQ_REMOVE -- implementations of singly-linked lists,
     singly-linked tail	queues,	lists, tail queues, and	circular queues

SYNOPSIS
     #include <sys/queue.h>

     SLIST_EMPTY(SLIST_HEAD *head);

     SLIST_ENTRY(TYPE);

     SLIST_FIRST(SLIST_HEAD *head);

     SLIST_FOREACH(TYPE	*var, SLIST_HEAD *head,	SLIST_ENTRY NAME);

     SLIST_HEAD(HEADNAME, TYPE);

     SLIST_HEAD_INITIALIZER(SLIST_HEAD head);

     SLIST_INIT(SLIST_HEAD *head);

     SLIST_INSERT_AFTER(TYPE *listelm, TYPE *elm, SLIST_ENTRY NAME);

     SLIST_INSERT_HEAD(SLIST_HEAD *head, TYPE *elm, SLIST_ENTRY	NAME);

     SLIST_NEXT(TYPE *elm, SLIST_ENTRY NAME);

     SLIST_REMOVE_HEAD(SLIST_HEAD *head, SLIST_ENTRY NAME);

     SLIST_REMOVE(SLIST_HEAD *head, TYPE *elm, TYPE, SLIST_ENTRY NAME);

     STAILQ_EMPTY(STAILQ_HEAD *head);

     STAILQ_ENTRY(TYPE);

     STAILQ_FIRST(STAILQ_HEAD *head);

     STAILQ_FOREACH(TYPE *var, STAILQ_HEAD *head, STAILQ_ENTRY NAME);

     STAILQ_HEAD(HEADNAME, TYPE);

     STAILQ_HEAD_INITIALIZER(STAILQ_HEAD head);

     STAILQ_INIT(STAILQ_HEAD *head);

     STAILQ_INSERT_AFTER(STAILQ_HEAD *head, TYPE *listelm, TYPE	*elm,
	 STAILQ_ENTRY NAME);

     STAILQ_INSERT_HEAD(STAILQ_HEAD *head, TYPE	*elm, STAILQ_ENTRY NAME);

     STAILQ_INSERT_TAIL(STAILQ_HEAD *head, TYPE	*elm, STAILQ_ENTRY NAME);

     STAILQ_LAST(STAILQ_HEAD *head, TYPE, STAILQ_ENTRY NAME);

     STAILQ_NEXT(TYPE *elm, STAILQ_ENTRY NAME);

     STAILQ_REMOVE_HEAD(STAILQ_HEAD *head, STAILQ_ENTRY	NAME);

     STAILQ_REMOVE(STAILQ_HEAD *head, TYPE *elm, TYPE, STAILQ_ENTRY NAME);

     LIST_EMPTY(LIST_HEAD *head);

     LIST_ENTRY(TYPE);

     LIST_FIRST(LIST_HEAD *head);

     LIST_FOREACH(TYPE *var, LIST_HEAD *head, LIST_ENTRY NAME);

     LIST_HEAD(HEADNAME, TYPE);

     LIST_HEAD_INITIALIZER(LIST_HEAD head);

     LIST_INIT(LIST_HEAD *head);

     LIST_INSERT_AFTER(TYPE *listelm, TYPE *elm, LIST_ENTRY NAME);

     LIST_INSERT_BEFORE(TYPE *listelm, TYPE *elm, LIST_ENTRY NAME);

     LIST_INSERT_HEAD(LIST_HEAD	*head, TYPE *elm, LIST_ENTRY NAME);

     LIST_NEXT(TYPE *elm, LIST_ENTRY NAME);

     LIST_REMOVE(TYPE *elm, LIST_ENTRY NAME);

     TAILQ_EMPTY(TAILQ_HEAD *head);

     TAILQ_ENTRY(TYPE);

     TAILQ_FIRST(TAILQ_HEAD *head);

     TAILQ_FOREACH(TYPE	*var, TAILQ_HEAD *head,	TAILQ_ENTRY NAME);

     TAILQ_FOREACH_REVERSE(TYPE	*var, TAILQ_HEAD *head,	HEADNAME,
	 TAILQ_ENTRY NAME);

     TAILQ_HEAD(HEADNAME, TYPE);

     TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);

     TAILQ_INIT(TAILQ_HEAD *head);

     TAILQ_INSERT_AFTER(TAILQ_HEAD *head, TYPE *listelm, TYPE *elm,
	 TAILQ_ENTRY NAME);

     TAILQ_INSERT_BEFORE(TYPE *listelm,	TYPE *elm, TAILQ_ENTRY NAME);

     TAILQ_INSERT_HEAD(TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY	NAME);

     TAILQ_INSERT_TAIL(TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY	NAME);

     TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);

     TAILQ_NEXT(TYPE *elm, TAILQ_ENTRY NAME);

     TAILQ_PREV(TYPE *elm, HEADNAME, TAILQ_ENTRY NAME);

     TAILQ_REMOVE(TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);

     CIRCLEQ_EMPTY(CIRCLEQ_HEAD	*head);

     CIRCLEQ_ENTRY(TYPE);

     CIRCLEQ_FIRST(CIRCLEQ_HEAD	*head);

     CIRCLEQ_FOREACH(TYPE *var,	CIRCLEQ_HEAD *head, CIRCLEQ_ENTRY NAME);

     CIRCLEQ_FOREACH_REVERSE(TYPE *var,	CIRCLEQ_HEAD *head,
	 CIRCLEQ_ENTRY NAME);

     CIRCLEQ_HEAD(HEADNAME, TYPE);

     CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD head);

     CIRCLEQ_INIT(CIRCLEQ_HEAD *head);

     CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *head, TYPE *listelm, TYPE *elm,
	 CIRCLEQ_ENTRY NAME);

     CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD	*head, TYPE *listelm, TYPE *elm,
	 CIRCLEQ_ENTRY NAME);

     CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head, TYPE *elm,	CIRCLEQ_ENTRY NAME);

     CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head, TYPE *elm,	CIRCLEQ_ENTRY NAME);

     CIRCLEQ_LAST(CIRCLEQ_HEAD *head);

     CIRCLEQ_NEXT(TYPE *elm, CIRCLEQ_ENTRY NAME);

     CIRCLE_PREV(TYPE *elm, CIRCLEQ_ENTRY NAME);

     CIRCLEQ_REMOVE(CIRCLEQ_HEAD *head,	TYPE *elm, CIRCLEQ_ENTRY NAME);

DESCRIPTION
     These macros define and operate on	five types of data structures: singly-
     linked lists, singly-linked tail queues, lists, tail queues, and circular
     queues.  All five structures support the following	functionality:
	   1.	Insertion of a new entry at the	head of	the list.
	   2.	Insertion of a new entry after any element in the list.
	   3.	O(1) removal of	an entry from the head of the list.
	   4.	O(n) removal of	any entry in the list.
	   5.	Forward	traversal through the list.

     Singly-linked lists are the simplest of the five data structures and sup-
     port only the above functionality.	 Singly-linked lists are ideal for
     applications with large datasets and few or no removals, or for imple-
     menting a LIFO queue.

     Singly-linked tail	queues add the following functionality:
	   1.	Entries	can be added at	the end	of a list.
     However:
	   1.	All list insertions must specify the head of the list.
	   2.	Each head entry	requires two pointers rather than one.
	   3.	Code size is about 15% greater and operations run about	20%
		slower than singly-linked lists.

     Singly-linked tailqs are ideal for	applications with large	datasets and
     few or no removals, or for	implementing a FIFO queue.

     All doubly	linked types of	data structures	(lists,	tail queues, and cir-
     cle queues) additionally allow:
	   1.	Insertion of a new entry before	any element in the list.
	   2.	O(1) removal of	any entry in the list.
     However:
	   1.	Each elements requires two pointers rather than	one.
	   2.	Code size and execution	time of	operations (except for
		removal) is about twice	that of	the singly-linked data-struc-
		tures.

     Linked lists are the simplest of the doubly linked	data structures	and
     support only the above functionality over singly-linked lists.

     Tail queues add the following functionality:
	   1.	Entries	can be added at	the end	of a list.
	   2.	They may be traversed backwards, from tail to head.
     However:
	   1.	All list insertions and	removals must specify the head of the
		list.
	   2.	Each head entry	requires two pointers rather than one.
	   3.	Code size is about 15% greater and operations run about	20%
		slower than singly-linked lists.

     Circular queues add the following functionality:
	   1.	Entries	can be added at	the end	of a list.
	   2.	They may be traversed backwards, from tail to head.
     However:
	   1.	All list insertions and	removals must specify the head of the
		list.
	   2.	Each head entry	requires two pointers rather than one.
	   3.	The termination	condition for traversal	is more	complex.
	   4.	Code size is about 40% greater and operations run about	45%
		slower than lists.

     In	the macro definitions, TYPE is the name	of a user defined structure,
     that must contain a field of type SLIST_ENTRY, STAILQ_ENTRY, LIST_ENTRY,
     TAILQ_ENTRY, or CIRCLEQ_ENTRY, named NAME.	 The argument HEADNAME is the
     name of a user defined structure that must	be declared using the macros
     SLIST_HEAD, STAILQ_HEAD, LIST_HEAD, TAILQ_HEAD, or	CIRCLEQ_HEAD.  See the
     examples below for	further	explanation of how these macros	are used.

SINGLY-LINKED LISTS
     A singly-linked list is headed by a structure defined by the 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	ele-
     ments.  New elements can be added to the list after an existing element
     or	at the head of the list.  An SLIST_HEAD	structure is declared as fol-
     lows:

	   SLIST_HEAD(HEADNAME,	TYPE) head;

     where HEADNAME is the name	of the structure to be defined,	and 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:

	   struct HEADNAME *headp;

     (The names	head and headp are user	selectable.)

     The macro SLIST_HEAD_INITIALIZER evaluates	to an initializer for the list
     head.

     The macro SLIST_EMPTY evaluates to	true if	there are no elements in the
     list.

     The macro SLIST_ENTRY declares a structure	that connects the elements in
     the list.

     The macro SLIST_FIRST returns the first element in	the list or NULL if
     the list is empty.

     The macro SLIST_FOREACH traverses the list	referenced by head in the for-
     ward direction, assigning each element in turn to var.

     The macro SLIST_INIT initializes the list referenced by head.

     The macro SLIST_INSERT_HEAD inserts the new element elm at	the head of
     the list.

     The macro SLIST_INSERT_AFTER inserts the new element elm after the	ele-
     ment listelm.

     The macro SLIST_NEXT returns the next element in the list.

     The macro SLIST_REMOVE_HEAD removes the element elm 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
     SLIST_REMOVE macro.

     The macro SLIST_REMOVE removes the	element	elm from the list.

SINGLY-LINKED LIST EXAMPLE
     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);
					     /*	Forward	traversal. */
     SLIST_FOREACH(np, &head, entries)
	     np-> ...

     while (!SLIST_EMPTY(&head)) {	     /*	List Deletion. */
	     n1	= SLIST_FIRST(&head);
	     SLIST_REMOVE_HEAD(&head, entries);
	     free(n1);
     }

SINGLY-LINKED TAIL QUEUES
     A singly-linked tail queue	is headed by a structure defined by the
     STAILQ_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 singly linked for minimum space and
     pointer manipulation overhead at the expense of O(n) removal for arbi-
     trary elements.  New elements can be added	to the tail queue after	an
     existing element, at the head of the tail queue, or at the	end of the
     tail queue.  A STAILQ_HEAD	structure is declared as follows:

	   STAILQ_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name	of the structure to be defined,	and 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:

	   struct HEADNAME *headp;

     (The names	head and headp are user	selectable.)

     The macro STAILQ_HEAD_INITIALIZER evaluates to an initializer for the
     tail queue	head.

     The macro STAILQ_EMPTY evaluates to true if there are no items on the
     tail queue.

     The macro STAILQ_ENTRY declares a structure that connects the elements in
     the tail queue.

     The macro STAILQ_FIRST returns the	first item on the tail queue or	NULL
     if	the tail queue is empty.

     The macro STAILQ_FOREACH traverses	the tail queue referenced by head in
     the forward direction, assigning each element in turn to var.

     The macro STAILQ_INIT initializes the tail	queue referenced by head.

     The macro STAILQ_INSERT_HEAD inserts the new element elm at the head of
     the tail queue.

     The macro STAILQ_INSERT_TAIL inserts the new element elm at the end of
     the tail queue.

     The macro STAILQ_INSERT_AFTER inserts the new element elm after the ele-
     ment listelm.

     The macro STAILQ_LAST returns the last item on the	tail queue.  If	the
     tail queue	is empty the return value is undefined.

     The macro STAILQ_NEXT returns the next item on the	tail queue, or NULL
     this item is the last.

     The macro STAILQ_REMOVE_HEAD removes the element at the head of the tail
     queue.  For optimum efficiency, elements being removed from the head of
     the tail queue should use this macro explicitly rather than the generic
     STAILQ_REMOVE macro.

     The macro STAILQ_REMOVE removes the element elm from the tail queue.

SINGLY-LINKED TAIL QUEUE EXAMPLE
     STAILQ_HEAD(stailhead, entry) head	=
	 STAILQ_HEAD_INITIALIZER(head);
     struct stailhead *headp;		     /*	Singly-linked tail queue head. */
     struct entry {
	     ...
	     STAILQ_ENTRY(entry) entries;    /*	Tail queue. */
	     ...
     } *n1, *n2, *n3, *np;

     STAILQ_INIT(&head);		     /*	Initialize the queue. */

     n1	= malloc(sizeof(struct entry));	     /*	Insert at the head. */
     STAILQ_INSERT_HEAD(&head, n1, entries);

     n1	= malloc(sizeof(struct entry));	     /*	Insert at the tail. */
     STAILQ_INSERT_TAIL(&head, n1, entries);

     n2	= malloc(sizeof(struct entry));	     /*	Insert after. */
     STAILQ_INSERT_AFTER(&head,	n1, n2,	entries);
					     /*	Deletion. */
     STAILQ_REMOVE(&head, n2, entry, entries);
     free(n2);
					     /*	Deletion from the head.	*/
     n3	= STAILQ_FIRST(&head);
     STAILQ_REMOVE_HEAD(&head, entries);
     free(n3);
					     /*	Forward	traversal. */
     STAILQ_FOREACH(np,	&head, entries)
	     np-> ...
					     /*	TailQ Deletion.	*/
     while (!STAILQ_EMPTY(&head)) {
	     n1	= STAILQ_FIRST(&head);
	     STAILQ_REMOVE_HEAD(&head, entries);
	     free(n1);
     }
					     /*	Faster TailQ Deletion. */
     n1	= STAILQ_FIRST(&head);
     while (n1 != NULL)	{
	     n2	= STAILQ_NEXT(n1, entries);
	     free(n1);
	     n1	= n2;
     }
     STAILQ_INIT(&head);

LISTS
     A list is headed by a structure defined by	the 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 LIST_HEAD structure is declared as follows:

	   LIST_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name	of the structure to be defined,	and 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:

	   struct HEADNAME *headp;

     (The names	head and headp are user	selectable.)

     The macro LIST_HEAD_INITIALIZER evaluates to an initializer for the list
     head.

     The macro LIST_EMPTY evaluates to true if their are no elements in	the
     list.

     The macro LIST_ENTRY declares a structure that connects the elements in
     the list.

     The macro LIST_FIRST returns the first element in the list	or NULL	if the
     list is empty.

     The macro LIST_FOREACH traverses the list referenced by head in the for-
     ward direction, assigning each element in turn to var.

     The macro LIST_INIT initializes the list referenced by head.

     The macro LIST_INSERT_HEAD	inserts	the new	element	elm at the head	of the
     list.

     The macro LIST_INSERT_AFTER inserts the new element elm after the element
     listelm.

     The macro LIST_INSERT_BEFORE inserts the new element elm before the ele-
     ment listelm.

     The macro LIST_NEXT returns the next element in the list, or NULL if this
     is	the last.

     The macro LIST_REMOVE removes the element elm from	the list.

LIST EXAMPLE
     LIST_HEAD(listhead, entry)	head =
	 LIST_HEAD_INITIALIZER(head);
     struct listhead *headp;		     /*	List head. */
     struct entry {
	     ...
	     LIST_ENTRY(entry) entries;	     /*	List. */
	     ...
     } *n1, *n2, *n3, *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);

     n3	= malloc(sizeof(struct entry));	     /*	Insert before. */
     LIST_INSERT_BEFORE(n2, n3,	entries);

     LIST_REMOVE(n2, entries);		     /*	Deletion. */
     free(n2);
					     /*	Forward	traversal. */
     LIST_FOREACH(np, &head, entries)
	     np-> ...

     while (!LIST_EMPTY(&head))	{	     /*	List Deletion. */
	     n1	= LIST_FIRST(&head);
	     LIST_REMOVE(n1, entries);
	     free(n1);
     }

     n1	= LIST_FIRST(&head);		     /*	Faster List Deletion. */
     while (n1 != NULL)	{
	     n2	= LIST_NEXT(n1,	entries);
	     free(n1);
	     n1	= n2;
     }
     LIST_INIT(&head);

TAIL QUEUES
     A tail queue is headed by a structure defined by the 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 tail
     queue after an existing element, before an	existing element, at the head
     of	the tail queue,	or at the end of the tail queue.  A TAILQ_HEAD struc-
     ture is declared as follows:

	   TAILQ_HEAD(HEADNAME,	TYPE) head;

     where HEADNAME is the name	of the structure to be defined,	and 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:

	   struct HEADNAME *headp;

     (The names	head and headp are user	selectable.)

     The macro TAILQ_HEAD_INITIALIZER evaluates	to an initializer for the tail
     queue head.

     The macro TAILQ_EMPTY evaluates to	true if	there are no items on the tail
     queue.

     The macro TAILQ_ENTRY declares a structure	that connects the elements in
     the tail queue.

     The macro TAILQ_FIRST returns the first item on the tail queue or NULL if
     the tail queue is empty.

     The macro TAILQ_FOREACH traverses the tail	queue referenced by head in
     the forward direction, assigning each element in turn to var.

     The macro TAILQ_FOREACH_REVERSE traverses the tail	queue referenced by
     head in the reverse direction, assigning each element in turn to var.

     The macro TAILQ_INIT initializes the tail queue referenced	by head.

     The macro TAILQ_INSERT_HEAD inserts the new element elm at	the head of
     the tail queue.

     The macro TAILQ_INSERT_TAIL inserts the new element elm at	the end	of the
     tail queue.

     The macro TAILQ_INSERT_AFTER inserts the new element elm after the	ele-
     ment listelm.

     The macro TAILQ_INSERT_BEFORE inserts the new element elm before the ele-
     ment listelm.

     The macro TAILQ_LAST returns the last item	on the tail queue.  If the
     tail queue	is empty the return value is undefined.

     The macro TAILQ_NEXT returns the next item	on the tail queue, or NULL if
     this item is the last.

     The macro TAILQ_PREV returns the previous item on the tail	queue, or NULL
     if	this item is the first.

     The macro TAILQ_REMOVE removes the	element	elm from the tail queue.

TAIL QUEUE EXAMPLE
     TAILQ_HEAD(tailhead, entry) head =
	 TAILQ_HEAD_INITIALIZER(head);
     struct tailhead *headp;		     /*	Tail queue head. */
     struct entry {
	     ...
	     TAILQ_ENTRY(entry)	entries;     /*	Tail queue. */
	     ...
     } *n1, *n2, *n3, *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);

     n3	= malloc(sizeof(struct entry));	     /*	Insert before. */
     TAILQ_INSERT_BEFORE(n2, n3, entries);

     TAILQ_REMOVE(&head, n2, entries);	     /*	Deletion. */
     free(n2);
					     /*	Forward	traversal. */
     TAILQ_FOREACH(np, &head, entries)
	     np-> ...
					     /*	Reverse	traversal. */
     TAILQ_FOREACH_REVERSE(np, &head, tailhead,	entries)
	     np-> ...
					     /*	TailQ Deletion.	*/
     while (!TAILQ_EMPTY(&head)) {
	     n1	= TAILQ_FIRST(&head);
	     TAILQ_REMOVE(&head, n1, entries);
	     free(n1);
     }
					     /*	Faster TailQ Deletion. */
     n1	= TAILQ_FIRST(&head);
     while (n1 != NULL)	{
	     n2	= TAILQ_NEXT(n1, entries);
	     free(n1);
	     n1	= n2;
     }
     TAILQ_INIT(&head);

CIRCULAR QUEUES
     A circular	queue is headed	by a structure defined by the CIRCLEQ_HEAD
     macro.  This structure contains a pair of pointers, one to	the first ele-
     ment in the circular queue	and the	other to the last element in the cir-
     cular queue.  The elements	are doubly linked so that an arbitrary element
     can be removed without traversing the 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 of the queue.	 A CIRCLEQ_HEAD	struc-
     ture is declared as follows:

	   CIRCLEQ_HEAD(HEADNAME, TYPE)	head;

     where HEADNAME is the name	of the structure to be defined,	and TYPE is
     the type of the elements to be linked into	the circular queue.  A pointer
     to	the head of the	circular queue can later be declared as:

	   struct HEADNAME *headp;

     (The names	head and headp are user	selectable.)

     The macro CIRCLEQ_HEAD_INITIALIZER	evaluates to an	initializer for	the
     circle queue head.

     The macro CIRCLEQ_EMPTY evaluates to true if there	are no items on	the
     circle queue.

     The macro CIRCLEQ_ENTRY declares a	structure that connects	the elements
     in	the circular queue.

     The macro CIRCLEQ_FIRST returns the first item on the circle queue.

     The macro CICRLEQ_FOREACH traverses the circle queue referenced by	head
     in	the forward direction, assigning each element in turn to var.

     The macro CICRLEQ_FOREACH_REVERSE traverses the circle queue referenced
     by	head in	the reverse direction, assigning each element in turn to var.

     The macro CIRCLEQ_INIT initializes	the circular queue referenced by head.

     The macro CIRCLEQ_INSERT_HEAD inserts the new element elm at the head of
     the circular queue.

     The macro CIRCLEQ_INSERT_TAIL inserts the new element elm at the end of
     the circular queue.

     The macro CIRCLEQ_INSERT_AFTER inserts the	new element elm	after the ele-
     ment listelm.

     The macro CIRCLEQ_INSERT_BEFORE inserts the new element elm before	the
     element listelm.

     The macro CIRCLEQ_LAST returns the	last item on the circle	queue.

     The macro CIRCLEQ_NEXT returns the	next item on the circle	queue.

     The macro CIRCLEQ_PREV returns the	previous item on the circle queue.

     The macro CIRCLEQ_REMOVE removes the element elm from the circular	queue.

CIRCULAR QUEUE EXAMPLE
     CIRCLEQ_HEAD(circlehead, entry) head =
	 CIRCLEQ_HEAD_INITIALIZER(head);
     struct circlehead *headp;		     /*	Circular queue head. */
     struct entry {
	     ...
	     CIRCLEQ_ENTRY(entry) entries;   /*	Circular queue.	*/
	     ...
     } *n1, *n2, *np;

     CIRCLEQ_INIT(&head);		     /*	Initialize the circular	queue. */

     n1	= malloc(sizeof(struct entry));	     /*	Insert at the head. */
     CIRCLEQ_INSERT_HEAD(&head,	n1, entries);

     n1	= malloc(sizeof(struct entry));	     /*	Insert at the tail. */
     CIRCLEQ_INSERT_TAIL(&head,	n1, entries);

     n2	= malloc(sizeof(struct entry));	     /*	Insert after. */
     CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);

     n2	= malloc(sizeof(struct entry));	     /*	Insert before. */
     CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);

     CIRCLEQ_REMOVE(&head, n1, entries);     /*	Deletion. */
     free(n1);
					     /*	Forward	traversal. */
     CIRCLEQ_FOREACH(np, &head,	entries)
	     np-> ...
					     /*	Reverse	traversal. */
     CIRCLEQ_FOREACH_REVERSE(np, &head,	entries)
	     np-> ...
					     /*	CircleQ	Deletion. */
     while (CIRCLEQ_FIRST(&head) != (void *)&head) {
	     n1	= CIRCLEQ_HEAD(&head);
	     CIRCLEQ_REMOVE(&head, n1, entries);
	     free(n1);
     }
					     /*	Faster CircleQ Deletion. */
     n1	= CIRCLEQ_FIRST(&head);
     while (n1 != (void	*)&head) {
	     n2	= CIRCLEQ_NEXT(n1, entries);
	     free(n1);
	     n1	= n2;
     }
     CIRCLEQ_INIT(&head);

HISTORY
     The queue functions first appeared	in 4.4BSD.

FreeBSD	9.2		       January 24, 1994			   FreeBSD 9.2

NAME | SYNOPSIS | DESCRIPTION | SINGLY-LINKED LISTS | SINGLY-LINKED LIST EXAMPLE | SINGLY-LINKED TAIL QUEUES | SINGLY-LINKED TAIL QUEUE EXAMPLE | LISTS | LIST EXAMPLE | TAIL QUEUES | TAIL QUEUE EXAMPLE | CIRCULAR QUEUES | CIRCULAR QUEUE EXAMPLE | HISTORY

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