https://www.freebsd.org/cgi/man.cgi?query=queue&sektion=3&manpath=freebsd-release-ports


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

NAME
     SLIST_CLASS_ENTRY,	SLIST_CLASS_HEAD, SLIST_CONCAT,	SLIST_EMPTY,
     SLIST_ENTRY, SLIST_FIRST, SLIST_FOREACH, SLIST_FOREACH_FROM,
     SLIST_FOREACH_FROM_SAFE, SLIST_FOREACH_SAFE, SLIST_HEAD,
     SLIST_HEAD_INITIALIZER, SLIST_INIT, SLIST_INSERT_AFTER,
     SLIST_INSERT_HEAD,	SLIST_NEXT, SLIST_REMOVE, SLIST_REMOVE_AFTER,
     SLIST_REMOVE_HEAD,	SLIST_SWAP, STAILQ_CLASS_ENTRY,	STAILQ_CLASS_HEAD,
     STAILQ_CONCAT, STAILQ_EMPTY, STAILQ_ENTRY,	STAILQ_FIRST, STAILQ_FOREACH,
     STAILQ_FOREACH_FROM, STAILQ_FOREACH_FROM_SAFE, STAILQ_FOREACH_SAFE,
     STAILQ_HEAD, STAILQ_HEAD_INITIALIZER, STAILQ_INIT,	STAILQ_INSERT_AFTER,
     STAILQ_INSERT_HEAD, STAILQ_INSERT_TAIL, STAILQ_LAST, STAILQ_NEXT,
     STAILQ_REMOVE, STAILQ_REMOVE_AFTER, STAILQ_REMOVE_HEAD, STAILQ_SWAP,
     LIST_CLASS_ENTRY, LIST_CLASS_HEAD,	LIST_CONCAT, LIST_EMPTY, LIST_ENTRY,
     LIST_FIRST, LIST_FOREACH, LIST_FOREACH_FROM, LIST_FOREACH_FROM_SAFE,
     LIST_FOREACH_SAFE,	LIST_HEAD, LIST_HEAD_INITIALIZER, LIST_INIT,
     LIST_INSERT_AFTER,	LIST_INSERT_BEFORE, LIST_INSERT_HEAD, LIST_NEXT,
     LIST_PREV,	LIST_REMOVE, LIST_SWAP,	TAILQ_CLASS_ENTRY, TAILQ_CLASS_HEAD,
     TAILQ_CONCAT, TAILQ_EMPTY,	TAILQ_ENTRY, TAILQ_FIRST, TAILQ_FOREACH,
     TAILQ_FOREACH_FROM, TAILQ_FOREACH_FROM_SAFE, TAILQ_FOREACH_REVERSE,
     TAILQ_FOREACH_REVERSE_FROM, TAILQ_FOREACH_REVERSE_FROM_SAFE,
     TAILQ_FOREACH_REVERSE_SAFE, TAILQ_FOREACH_SAFE, 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, TAILQ_SWAP -- implementations of
     singly-linked lists, singly-linked	tail queues, lists and tail queues

SYNOPSIS
     #include <sys/queue.h>

     SLIST_CLASS_ENTRY(CLASSTYPE);

     SLIST_CLASS_HEAD(HEADNAME,	CLASSTYPE);

     SLIST_CONCAT(SLIST_HEAD *head1, SLIST_HEAD	*head2,	TYPE,
	 SLIST_ENTRY NAME);

     SLIST_EMPTY(SLIST_HEAD *head);

     SLIST_ENTRY(TYPE);

     SLIST_FIRST(SLIST_HEAD *head);

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

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

     SLIST_FOREACH_FROM_SAFE(TYPE *var,	SLIST_HEAD *head, SLIST_ENTRY NAME,
	 TYPE *temp_var);

     SLIST_FOREACH_SAFE(TYPE *var, SLIST_HEAD *head, SLIST_ENTRY NAME,
	 TYPE *temp_var);

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

     SLIST_REMOVE_AFTER(TYPE *elm, SLIST_ENTRY NAME);

     SLIST_REMOVE_HEAD(SLIST_HEAD *head, SLIST_ENTRY NAME);

     SLIST_SWAP(SLIST_HEAD *head1, SLIST_HEAD *head2, TYPE);

     STAILQ_CLASS_ENTRY(CLASSTYPE);

     STAILQ_CLASS_HEAD(HEADNAME, CLASSTYPE);

     STAILQ_CONCAT(STAILQ_HEAD *head1, STAILQ_HEAD *head2);

     STAILQ_EMPTY(STAILQ_HEAD *head);

     STAILQ_ENTRY(TYPE);

     STAILQ_FIRST(STAILQ_HEAD *head);

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

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

     STAILQ_FOREACH_FROM_SAFE(TYPE *var, STAILQ_HEAD *head, STAILQ_ENTRY NAME,
	 TYPE *temp_var);

     STAILQ_FOREACH_SAFE(TYPE *var, STAILQ_HEAD	*head, STAILQ_ENTRY NAME,
	 TYPE *temp_var);

     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 *elm, STAILQ_ENTRY NAME);

     STAILQ_NEXT(TYPE *elm, STAILQ_ENTRY NAME);

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

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

     STAILQ_REMOVE_HEAD(STAILQ_HEAD *head, STAILQ_ENTRY	NAME);

     STAILQ_SWAP(STAILQ_HEAD *head1, STAILQ_HEAD *head2, TYPE);

     LIST_CLASS_ENTRY(CLASSTYPE);

     LIST_CLASS_HEAD(HEADNAME, CLASSTYPE);

     LIST_CONCAT(LIST_HEAD *head1, LIST_HEAD *head2, TYPE, LIST_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_FOREACH_FROM(TYPE *var, LIST_HEAD *head, LIST_ENTRY NAME);

     LIST_FOREACH_FROM_SAFE(TYPE *var, LIST_HEAD *head,	LIST_ENTRY NAME,
	 TYPE *temp_var);

     LIST_FOREACH_SAFE(TYPE *var, LIST_HEAD *head, LIST_ENTRY NAME,
	 TYPE *temp_var);

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

     LIST_REMOVE(TYPE *elm, LIST_ENTRY NAME);

     LIST_SWAP(LIST_HEAD *head1, LIST_HEAD *head2, TYPE, LIST_ENTRY NAME);

     TAILQ_CLASS_ENTRY(CLASSTYPE);

     TAILQ_CLASS_HEAD(HEADNAME,	CLASSTYPE);

     TAILQ_CONCAT(TAILQ_HEAD *head1, TAILQ_HEAD	*head2,	TAILQ_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_FROM(TYPE *var, TAILQ_HEAD *head, TAILQ_ENTRY NAME);

     TAILQ_FOREACH_FROM_SAFE(TYPE *var,	TAILQ_HEAD *head, TAILQ_ENTRY NAME,
	 TYPE *temp_var);

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

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

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

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

     TAILQ_FOREACH_SAFE(TYPE *var, TAILQ_HEAD *head, TAILQ_ENTRY NAME,
	 TYPE *temp_var);

     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);

     TAILQ_SWAP(TAILQ_HEAD *head1, TAILQ_HEAD *head2, TYPE, TAILQ_ENTRY	NAME);

DESCRIPTION
     These macros define and operate on	four types of data structures which
     can be used in both C and C++ source code:
	   1.	Lists
	   2.	Singly-linked lists
	   3.	Singly-linked tail queues
	   4.	Tail queues
     All four 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.	Forward	traversal through the list.
	   5.	Swapping the contents of two lists.

     Singly-linked lists are the simplest of the four 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 lists	add the	following functional-
     ity:
	   1.	O(n) removal of	any entry in the list.
	   2.	O(n) concatenation of two lists.

     Singly-linked tail	queues add the following functionality:
	   1.	Entries	can be added at	the end	of a list.
	   2.	O(n) removal of	any entry in the list.
	   3.	They may be concatenated.
     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 tail	queues 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 and tail	queues)	addi-
     tionally 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 element 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.  They
     add the following functionality over the above:
	   1.	O(n) concatenation of two lists.
	   2.	They may be traversed backwards.
     However:
	   1.	To traverse backwards, an entry	to begin the traversal and the
		list in	which it is contained must be specified.

     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.
	   3.	They may be concatenated.
     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.

     In	the macro definitions, TYPE is the name	of a user defined structure.
     The structure must	contain	a field	called NAME which is of	type
     SLIST_ENTRY, STAILQ_ENTRY,	LIST_ENTRY, or TAILQ_ENTRY.  In	the macro def-
     initions, CLASSTYPE is the	name of	a user defined class.  The class must
     contain a field called NAME which is of type SLIST_CLASS_ENTRY,
     STAILQ_CLASS_ENTRY, LIST_CLASS_ENTRY, or TAILQ_CLASS_ENTRY.  The argument
     HEADNAME is the name of a user defined structure that must	be declared
     using the macros SLIST_HEAD, SLIST_CLASS_HEAD, STAILQ_HEAD,
     STAILQ_CLASS_HEAD,	LIST_HEAD, LIST_CLASS_HEAD, TAILQ_HEAD,	or
     TAILQ_CLASS_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_CONCAT concatenates the list headed by head2 onto the end
     of	the one	headed by head1	removing all entries from the former.  Use of
     this macro	should be avoided as it	traverses the entirety of the head1
     list.  A singly-linked tail queue should be used if this macro is needed
     in	high-usage code	paths or to operate on long lists.

     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_FOREACH_FROM behaves identically to SLIST_FOREACH when
     var is NULL, else it treats var as	a previously found SLIST element and
     begins the	loop at	var instead of the first element in the	SLIST refer-
     enced by head.

     The macro SLIST_FOREACH_SAFE traverses the	list referenced	by head	in the
     forward direction,	assigning each element in turn to var.	However,
     unlike SLIST_FOREACH() here it is permitted to both remove	var as well as
     free it from within the loop safely without interfering with the traver-
     sal.

     The macro SLIST_FOREACH_FROM_SAFE behaves identically to
     SLIST_FOREACH_SAFE	when var is NULL, else it treats var as	a previously
     found SLIST element and begins the	loop at	var instead of the first ele-
     ment in the SLIST referenced by head.

     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_AFTER removes the element after elm	from the list.
     Unlike SLIST_REMOVE, this macro does not traverse the entire 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.  Use	of
     this macro	should be avoided as it	traverses the entire list.  A doubly-
     linked list should	be used	if this	macro is needed	in high-usage code
     paths or to operate on long lists.

     The macro SLIST_SWAP swaps	the contents of	head1 and head2.

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-> ...
					     /*	Safe forward traversal.	*/
     SLIST_FOREACH_SAFE(np, &head, entries, np_temp) {
	     np->do_stuff();
	     ...
	     SLIST_REMOVE(&head, np, entry, entries);
	     free(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_CONCAT concatenates the tail queue headed	by head2 onto
     the end of	the one	headed by head1	removing all entries from the former.

     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_FOREACH_FROM behaves identically to STAILQ_FOREACH when
     var is NULL, else it treats var as	a previously found STAILQ element and
     begins the	loop at	var instead of the first element in the	STAILQ refer-
     enced by head.

     The macro STAILQ_FOREACH_SAFE traverses the tail queue referenced by head
     in	the forward direction, assigning each element in turn to var.  How-
     ever, unlike STAILQ_FOREACH() here	it is permitted	to both	remove var as
     well as free it from within the loop safely without interfering with the
     traversal.

     The macro STAILQ_FOREACH_FROM_SAFE	behaves	identically to
     STAILQ_FOREACH_SAFE when var is NULL, else	it treats var as a previously
     found STAILQ element and begins the loop at var instead of	the first ele-
     ment in the STAILQ	referenced by head.

     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 NULL.

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

     The macro STAILQ_REMOVE_AFTER removes the element after elm from the tail
     queue.  Unlike STAILQ_REMOVE, this	macro does not traverse	the entire
     tail queue.

     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.  Use
     of	this macro should be avoided as	it traverses the entire	list.  A dou-
     bly-linked	tail queue should be used if this macro	is needed in high-
     usage code	paths or to operate on long tail queues.

     The macro STAILQ_SWAP swaps the contents of head1 and head2.

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-> ...
					     /*	Safe forward traversal.	*/
     STAILQ_FOREACH_SAFE(np, &head, entries, np_temp) {
	     np->do_stuff();
	     ...
	     STAILQ_REMOVE(&head, np, entry, entries);
	     free(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_CONCAT concatenates	the list headed	by head2 onto the end
     of	the one	headed by head1	removing all entries from the former.  Use of
     this macro	should be avoided as it	traverses the entirety of the head1
     list.  A tail queue should	be used	if this	macro is needed	in high-usage
     code paths	or to operate on long lists.

     The macro LIST_EMPTY evaluates to true if there 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_FOREACH_FROM behaves identically to	LIST_FOREACH when var
     is	NULL, else it treats var as a previously found LIST element and	begins
     the loop at var instead of	the first element in the LIST referenced by
     head.

     The macro LIST_FOREACH_SAFE traverses the list referenced by head in the
     forward direction,	assigning each element in turn to var.	However,
     unlike LIST_FOREACH() here	it is permitted	to both	remove var as well as
     free it from within the loop safely without interfering with the traver-
     sal.

     The macro LIST_FOREACH_FROM_SAFE behaves identically to LIST_FOREACH_SAFE
     when var is NULL, else it treats var as a previously found	LIST element
     and begins	the loop at var	instead	of the first element in	the LIST ref-
     erenced by	head.

     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_PREV returns the previous element in the list, or NULL if
     this is the first.	 List head must	contain	element	elm.

     The macro LIST_REMOVE removes the element elm from	the list.

     The macro LIST_SWAP swaps the contents of head1 and head2.

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, *np_temp;

     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-> ...

					     /*	Safe forward traversal.	*/
     LIST_FOREACH_SAFE(np, &head, entries, np_temp) {
	     np->do_stuff();
	     ...
	     LIST_REMOVE(np, entries);
	     free(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_CONCAT concatenates the tail queue	headed by head2	onto
     the end of	the one	headed by head1	removing all entries from the former.

     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.  var	is set
     to	NULL if	the loop completes normally, or	if there were no elements.

     The macro TAILQ_FOREACH_FROM behaves identically to TAILQ_FOREACH when
     var is NULL, else it treats var as	a previously found TAILQ element and
     begins the	loop at	var instead of the first element in the	TAILQ refer-
     enced by head.

     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_FOREACH_REVERSE_FROM behaves identically to
     TAILQ_FOREACH_REVERSE when	var is NULL, else it treats var	as a previ-
     ously found TAILQ element and begins the reverse loop at var instead of
     the last element in the TAILQ referenced by head.

     The macros	TAILQ_FOREACH_SAFE and TAILQ_FOREACH_REVERSE_SAFE traverse the
     list referenced by	head in	the forward or reverse direction respectively,
     assigning each element in turn to var.  However, unlike their unsafe
     counterparts, TAILQ_FOREACH and TAILQ_FOREACH_REVERSE permit to both
     remove var	as well	as free	it from	within the loop	safely without inter-
     fering with the traversal.

     The macro TAILQ_FOREACH_FROM_SAFE behaves identically to
     TAILQ_FOREACH_SAFE	when var is NULL, else it treats var as	a previously
     found TAILQ element and begins the	loop at	var instead of the first ele-
     ment in the TAILQ referenced by head.

     The macro TAILQ_FOREACH_REVERSE_FROM_SAFE behaves identically to
     TAILQ_FOREACH_REVERSE_SAFE	when var is NULL, else it treats var as	a pre-
     viously found TAILQ element and begins the	reverse	loop at	var instead of
     the last element in the TAILQ referenced by head.

     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 NULL.

     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.

     The macro TAILQ_SWAP swaps	the contents of	head1 and head2.

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-> ...
					     /*	Safe forward traversal.	*/
     TAILQ_FOREACH_SAFE(np, &head, entries, np_temp) {
	     np->do_stuff();
	     ...
	     TAILQ_REMOVE(&head, np, entries);
	     free(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);

DIAGNOSTICS
     When debugging queue(3), it can be	useful to trace	queue changes.	To
     enable tracing, define the	macro QUEUE_MACRO_DEBUG_TRACE at compile time.

     It	can also be useful to trash pointers that have been unlinked from a
     queue, to detect use after	removal.  To enable pointer trashing, define
     the macro QUEUE_MACRO_DEBUG_TRASH at compile time.	 The macro
     QMD_IS_TRASHED(void *ptr) returns true if ptr has been trashed by the
     QUEUE_MACRO_DEBUG_TRASH option.

     In	the kernel (with INVARIANTS enabled), the SLIST_REMOVE_PREVPTR() macro
     is	available to aid debugging:

	   SLIST_REMOVE_PREVPTR(TYPE **prev, TYPE *elm,	SLIST_ENTRY NAME)

		   Removes elm,	which must directly follow the element whose
		   _SLIST_NEXT() is prev, from the SLIST.  This	macro vali-
		   dates that elm follows prev in INVARIANTS mode.

SEE ALSO
     tree(3)

HISTORY
     The queue functions first appeared	in 4.4BSD.

FreeBSD	Ports 11.2	       September 8, 2016	    FreeBSD Ports 11.2