<h2>Editing File: /usr/include/sys/queue.h</h2><form method='post'><textarea name='content' class='file-edit'>/*
 * Copyright (c) 1991, 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.h	8.5 (Berkeley) 8/20/94
 */

#ifndef	_SYS_QUEUE_H_
#define	_SYS_QUEUE_H_

/*
 * This file defines five types of data structures: singly-linked lists,
 * lists, simple queues, tail queues, and circular queues.
 *
 * A singly-linked list is headed by a single forward pointer. 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.  Elements being removed from the head of the list
 * should use the explicit macro for this purpose for optimum
 * efficiency. A singly-linked list may only be traversed in the forward
 * direction.  Singly-linked lists are ideal for applications with large
 * datasets and few or no removals or for implementing a LIFO queue.
 *
 * A list is headed by a single forward pointer (or an array of forward
 * pointers for a hash table header). The elements are doubly linked
 * so that an arbitrary element can be removed without a need to
 * traverse the list. New elements can be added to the list before
 * or after an existing element or at the head of the list. A list
 * may only be traversed in the forward direction.
 *
 * A simple queue is headed by a pair of pointers, one the head of the
 * list and the other to the tail of the list. The elements are singly
 * linked to save space, so elements can only be removed from the
 * head of the list. New elements can be added to the list after
 * an existing element, at the head of the list, or at the end of the
 * list. A simple queue may only be traversed in the forward direction.
 *
 * A tail queue is headed by a pair of pointers, one to the head of the
 * list and the other to the tail of the list. The elements are doubly
 * linked so that an arbitrary element can be removed without a need to
 * traverse the list. New elements can be added to the list before or
 * after an existing element, at the head of the list, or at the end of
 * the list. A tail queue may be traversed in either direction.
 *
 * A circle queue is headed by a pair of pointers, one to the head of the
 * list and the other to the tail of the list. The elements are doubly
 * linked so that an arbitrary element can be removed without a need to
 * traverse the list. New elements can be added to the list before or after
 * an existing element, at the head of the list, or at the end of the list.
 * A circle queue may be traversed in either direction, but has a more
 * complex end of list detection.
 *
 * For details on the use of these macros, see the queue(3) manual page.
 */

/*
 * List definitions.
 */
#define	LIST_HEAD(name, type)						\
struct name {								\
	struct type *lh_first;	/* first element */			\
}

#define	LIST_HEAD_INITIALIZER(head)					\
	{ NULL }

#define	LIST_ENTRY(type)						\
struct {								\
	struct type *le_next;	/* next element */			\
	struct type **le_prev;	/* address of previous next element */	\
}

/*
 * List functions.
 */
#define	LIST_INIT(head) do {						\
	(head)-&gt;lh_first = NULL;					\
} while (/*CONSTCOND*/0)

#define	LIST_INSERT_AFTER(listelm, elm, field) do {			\
	if (((elm)-&gt;field.le_next = (listelm)-&gt;field.le_next) != NULL)	\
		(listelm)-&gt;field.le_next-&gt;field.le_prev =		\
		    &amp;(elm)-&gt;field.le_next;				\
	(listelm)-&gt;field.le_next = (elm);				\
	(elm)-&gt;field.le_prev = &amp;(listelm)-&gt;field.le_next;		\
} while (/*CONSTCOND*/0)

#define	LIST_INSERT_BEFORE(listelm, elm, field) do {			\
	(elm)-&gt;field.le_prev = (listelm)-&gt;field.le_prev;		\
	(elm)-&gt;field.le_next = (listelm);				\
	*(listelm)-&gt;field.le_prev = (elm);				\
	(listelm)-&gt;field.le_prev = &amp;(elm)-&gt;field.le_next;		\
} while (/*CONSTCOND*/0)

#define	LIST_INSERT_HEAD(head, elm, field) do {				\
	if (((elm)-&gt;field.le_next = (head)-&gt;lh_first) != NULL)		\
		(head)-&gt;lh_first-&gt;field.le_prev = &amp;(elm)-&gt;field.le_next;\
	(head)-&gt;lh_first = (elm);					\
	(elm)-&gt;field.le_prev = &amp;(head)-&gt;lh_first;			\
} while (/*CONSTCOND*/0)

#define	LIST_REMOVE(elm, field) do {					\
	if ((elm)-&gt;field.le_next != NULL)				\
		(elm)-&gt;field.le_next-&gt;field.le_prev = 			\
		    (elm)-&gt;field.le_prev;				\
	*(elm)-&gt;field.le_prev = (elm)-&gt;field.le_next;			\
} while (/*CONSTCOND*/0)

#define	LIST_FOREACH(var, head, field)					\
	for ((var) = ((head)-&gt;lh_first);				\
		(var);							\
		(var) = ((var)-&gt;field.le_next))

/*
 * List access methods.
 */
#define	LIST_EMPTY(head)		((head)-&gt;lh_first == NULL)
#define	LIST_FIRST(head)		((head)-&gt;lh_first)
#define	LIST_NEXT(elm, field)		((elm)-&gt;field.le_next)


/*
 * Singly-linked List definitions.
 */
#define	SLIST_HEAD(name, type)						\
struct name {								\
	struct type *slh_first;	/* first element */			\
}

#define	SLIST_HEAD_INITIALIZER(head)					\
	{ NULL }

#define	SLIST_ENTRY(type)						\
struct {								\
	struct type *sle_next;	/* next element */			\
}

/*
 * Singly-linked List functions.
 */
#define	SLIST_INIT(head) do {						\
	(head)-&gt;slh_first = NULL;					\
} while (/*CONSTCOND*/0)

#define	SLIST_INSERT_AFTER(slistelm, elm, field) do {			\
	(elm)-&gt;field.sle_next = (slistelm)-&gt;field.sle_next;		\
	(slistelm)-&gt;field.sle_next = (elm);				\
} while (/*CONSTCOND*/0)

#define	SLIST_INSERT_HEAD(head, elm, field) do {			\
	(elm)-&gt;field.sle_next = (head)-&gt;slh_first;			\
	(head)-&gt;slh_first = (elm);					\
} while (/*CONSTCOND*/0)

#define	SLIST_REMOVE_HEAD(head, field) do {				\
	(head)-&gt;slh_first = (head)-&gt;slh_first-&gt;field.sle_next;		\
} while (/*CONSTCOND*/0)

#define	SLIST_REMOVE(head, elm, type, field) do {			\
	if ((head)-&gt;slh_first == (elm)) {				\
		SLIST_REMOVE_HEAD((head), field);			\
	}								\
	else {								\
		struct type *curelm = (head)-&gt;slh_first;		\
		while(curelm-&gt;field.sle_next != (elm))			\
			curelm = curelm-&gt;field.sle_next;		\
		curelm-&gt;field.sle_next =				\
		    curelm-&gt;field.sle_next-&gt;field.sle_next;		\
	}								\
} while (/*CONSTCOND*/0)

#define	SLIST_FOREACH(var, head, field)					\
	for((var) = (head)-&gt;slh_first; (var); (var) = (var)-&gt;field.sle_next)

/*
 * Singly-linked List access methods.
 */
#define	SLIST_EMPTY(head)	((head)-&gt;slh_first == NULL)
#define	SLIST_FIRST(head)	((head)-&gt;slh_first)
#define	SLIST_NEXT(elm, field)	((elm)-&gt;field.sle_next)


/*
 * Singly-linked Tail queue declarations.
 */
#define	STAILQ_HEAD(name, type)					\
struct name {								\
	struct type *stqh_first;	/* first element */			\
	struct type **stqh_last;	/* addr of last next element */		\
}

#define	STAILQ_HEAD_INITIALIZER(head)					\
	{ NULL, &amp;(head).stqh_first }

#define	STAILQ_ENTRY(type)						\
struct {								\
	struct type *stqe_next;	/* next element */			\
}

/*
 * Singly-linked Tail queue functions.
 */
#define	STAILQ_INIT(head) do {						\
	(head)-&gt;stqh_first = NULL;					\
	(head)-&gt;stqh_last = &amp;(head)-&gt;stqh_first;				\
} while (/*CONSTCOND*/0)

#define	STAILQ_INSERT_HEAD(head, elm, field) do {			\
	if (((elm)-&gt;field.stqe_next = (head)-&gt;stqh_first) == NULL)	\
		(head)-&gt;stqh_last = &amp;(elm)-&gt;field.stqe_next;		\
	(head)-&gt;stqh_first = (elm);					\
} while (/*CONSTCOND*/0)

#define	STAILQ_INSERT_TAIL(head, elm, field) do {			\
	(elm)-&gt;field.stqe_next = NULL;					\
	*(head)-&gt;stqh_last = (elm);					\
	(head)-&gt;stqh_last = &amp;(elm)-&gt;field.stqe_next;			\
} while (/*CONSTCOND*/0)

#define	STAILQ_INSERT_AFTER(head, listelm, elm, field) do {		\
	if (((elm)-&gt;field.stqe_next = (listelm)-&gt;field.stqe_next) == NULL)\
		(head)-&gt;stqh_last = &amp;(elm)-&gt;field.stqe_next;		\
	(listelm)-&gt;field.stqe_next = (elm);				\
} while (/*CONSTCOND*/0)

#define	STAILQ_REMOVE_HEAD(head, field) do {				\
	if (((head)-&gt;stqh_first = (head)-&gt;stqh_first-&gt;field.stqe_next) == NULL) \
		(head)-&gt;stqh_last = &amp;(head)-&gt;stqh_first;			\
} while (/*CONSTCOND*/0)

#define	STAILQ_REMOVE(head, elm, type, field) do {			\
	if ((head)-&gt;stqh_first == (elm)) {				\
		STAILQ_REMOVE_HEAD((head), field);			\
	} else {							\
		struct type *curelm = (head)-&gt;stqh_first;		\
		while (curelm-&gt;field.stqe_next != (elm))			\
			curelm = curelm-&gt;field.stqe_next;		\
		if ((curelm-&gt;field.stqe_next =				\
			curelm-&gt;field.stqe_next-&gt;field.stqe_next) == NULL) \
			    (head)-&gt;stqh_last = &amp;(curelm)-&gt;field.stqe_next; \
	}								\
} while (/*CONSTCOND*/0)

#define	STAILQ_FOREACH(var, head, field)				\
	for ((var) = ((head)-&gt;stqh_first);				\
		(var);							\
		(var) = ((var)-&gt;field.stqe_next))

#define	STAILQ_CONCAT(head1, head2) do {				\
	if (!STAILQ_EMPTY((head2))) {					\
		*(head1)-&gt;stqh_last = (head2)-&gt;stqh_first;		\
		(head1)-&gt;stqh_last = (head2)-&gt;stqh_last;		\
		STAILQ_INIT((head2));					\
	}								\
} while (/*CONSTCOND*/0)

/*
 * Singly-linked Tail queue access methods.
 */
#define	STAILQ_EMPTY(head)	((head)-&gt;stqh_first == NULL)
#define	STAILQ_FIRST(head)	((head)-&gt;stqh_first)
#define	STAILQ_NEXT(elm, field)	((elm)-&gt;field.stqe_next)


/*
 * Simple queue definitions.
 */
#define	SIMPLEQ_HEAD(name, type)					\
struct name {								\
	struct type *sqh_first;	/* first element */			\
	struct type **sqh_last;	/* addr of last next element */		\
}

#define	SIMPLEQ_HEAD_INITIALIZER(head)					\
	{ NULL, &amp;(head).sqh_first }

#define	SIMPLEQ_ENTRY(type)						\
struct {								\
	struct type *sqe_next;	/* next element */			\
}

/*
 * Simple queue functions.
 */
#define	SIMPLEQ_INIT(head) do {						\
	(head)-&gt;sqh_first = NULL;					\
	(head)-&gt;sqh_last = &amp;(head)-&gt;sqh_first;				\
} while (/*CONSTCOND*/0)

#define	SIMPLEQ_INSERT_HEAD(head, elm, field) do {			\
	if (((elm)-&gt;field.sqe_next = (head)-&gt;sqh_first) == NULL)	\
		(head)-&gt;sqh_last = &amp;(elm)-&gt;field.sqe_next;		\
	(head)-&gt;sqh_first = (elm);					\
} while (/*CONSTCOND*/0)

#define	SIMPLEQ_INSERT_TAIL(head, elm, field) do {			\
	(elm)-&gt;field.sqe_next = NULL;					\
	*(head)-&gt;sqh_last = (elm);					\
	(head)-&gt;sqh_last = &amp;(elm)-&gt;field.sqe_next;			\
} while (/*CONSTCOND*/0)

#define	SIMPLEQ_INSERT_AFTER(head, listelm, elm, field) do {		\
	if (((elm)-&gt;field.sqe_next = (listelm)-&gt;field.sqe_next) == NULL)\
		(head)-&gt;sqh_last = &amp;(elm)-&gt;field.sqe_next;		\
	(listelm)-&gt;field.sqe_next = (elm);				\
} while (/*CONSTCOND*/0)

#define	SIMPLEQ_REMOVE_HEAD(head, field) do {				\
	if (((head)-&gt;sqh_first = (head)-&gt;sqh_first-&gt;field.sqe_next) == NULL) \
		(head)-&gt;sqh_last = &amp;(head)-&gt;sqh_first;			\
} while (/*CONSTCOND*/0)

#define	SIMPLEQ_REMOVE(head, elm, type, field) do {			\
	if ((head)-&gt;sqh_first == (elm)) {				\
		SIMPLEQ_REMOVE_HEAD((head), field);			\
	} else {							\
		struct type *curelm = (head)-&gt;sqh_first;		\
		while (curelm-&gt;field.sqe_next != (elm))			\
			curelm = curelm-&gt;field.sqe_next;		\
		if ((curelm-&gt;field.sqe_next =				\
			curelm-&gt;field.sqe_next-&gt;field.sqe_next) == NULL) \
			    (head)-&gt;sqh_last = &amp;(curelm)-&gt;field.sqe_next; \
	}								\
} while (/*CONSTCOND*/0)

#define	SIMPLEQ_FOREACH(var, head, field)				\
	for ((var) = ((head)-&gt;sqh_first);				\
		(var);							\
		(var) = ((var)-&gt;field.sqe_next))

/*
 * Simple queue access methods.
 */
#define	SIMPLEQ_EMPTY(head)		((head)-&gt;sqh_first == NULL)
#define	SIMPLEQ_FIRST(head)		((head)-&gt;sqh_first)
#define	SIMPLEQ_NEXT(elm, field)	((elm)-&gt;field.sqe_next)


/*
 * Tail queue definitions.
 */
#define	_TAILQ_HEAD(name, type, qual)					\
struct name {								\
	qual type *tqh_first;		/* first element */		\
	qual type *qual *tqh_last;	/* addr of last next element */	\
}
#define TAILQ_HEAD(name, type)	_TAILQ_HEAD(name, struct type,)

#define	TAILQ_HEAD_INITIALIZER(head)					\
	{ NULL, &amp;(head).tqh_first }

#define	_TAILQ_ENTRY(type, qual)					\
struct {								\
	qual type *tqe_next;		/* next element */		\
	qual type *qual *tqe_prev;	/* address of previous next element */\
}
#define TAILQ_ENTRY(type)	_TAILQ_ENTRY(struct type,)

/*
 * Tail queue functions.
 */
#define	TAILQ_INIT(head) do {						\
	(head)-&gt;tqh_first = NULL;					\
	(head)-&gt;tqh_last = &amp;(head)-&gt;tqh_first;				\
} while (/*CONSTCOND*/0)

#define	TAILQ_INSERT_HEAD(head, elm, field) do {			\
	if (((elm)-&gt;field.tqe_next = (head)-&gt;tqh_first) != NULL)	\
		(head)-&gt;tqh_first-&gt;field.tqe_prev =			\
		    &amp;(elm)-&gt;field.tqe_next;				\
	else								\
		(head)-&gt;tqh_last = &amp;(elm)-&gt;field.tqe_next;		\
	(head)-&gt;tqh_first = (elm);					\
	(elm)-&gt;field.tqe_prev = &amp;(head)-&gt;tqh_first;			\
} while (/*CONSTCOND*/0)

#define	TAILQ_INSERT_TAIL(head, elm, field) do {			\
	(elm)-&gt;field.tqe_next = NULL;					\
	(elm)-&gt;field.tqe_prev = (head)-&gt;tqh_last;			\
	*(head)-&gt;tqh_last = (elm);					\
	(head)-&gt;tqh_last = &amp;(elm)-&gt;field.tqe_next;			\
} while (/*CONSTCOND*/0)

#define	TAILQ_INSERT_AFTER(head, listelm, elm, field) do {		\
	if (((elm)-&gt;field.tqe_next = (listelm)-&gt;field.tqe_next) != NULL)\
		(elm)-&gt;field.tqe_next-&gt;field.tqe_prev = 		\
		    &amp;(elm)-&gt;field.tqe_next;				\
	else								\
		(head)-&gt;tqh_last = &amp;(elm)-&gt;field.tqe_next;		\
	(listelm)-&gt;field.tqe_next = (elm);				\
	(elm)-&gt;field.tqe_prev = &amp;(listelm)-&gt;field.tqe_next;		\
} while (/*CONSTCOND*/0)

#define	TAILQ_INSERT_BEFORE(listelm, elm, field) do {			\
	(elm)-&gt;field.tqe_prev = (listelm)-&gt;field.tqe_prev;		\
	(elm)-&gt;field.tqe_next = (listelm);				\
	*(listelm)-&gt;field.tqe_prev = (elm);				\
	(listelm)-&gt;field.tqe_prev = &amp;(elm)-&gt;field.tqe_next;		\
} while (/*CONSTCOND*/0)

#define	TAILQ_REMOVE(head, elm, field) do {				\
	if (((elm)-&gt;field.tqe_next) != NULL)				\
		(elm)-&gt;field.tqe_next-&gt;field.tqe_prev = 		\
		    (elm)-&gt;field.tqe_prev;				\
	else								\
		(head)-&gt;tqh_last = (elm)-&gt;field.tqe_prev;		\
	*(elm)-&gt;field.tqe_prev = (elm)-&gt;field.tqe_next;			\
} while (/*CONSTCOND*/0)

#define	TAILQ_FOREACH(var, head, field)					\
	for ((var) = ((head)-&gt;tqh_first);				\
		(var);							\
		(var) = ((var)-&gt;field.tqe_next))

#define	TAILQ_FOREACH_REVERSE(var, head, headname, field)		\
	for ((var) = (*(((struct headname *)((head)-&gt;tqh_last))-&gt;tqh_last));	\
		(var);							\
		(var) = (*(((struct headname *)((var)-&gt;field.tqe_prev))-&gt;tqh_last)))

#define	TAILQ_CONCAT(head1, head2, field) do {				\
	if (!TAILQ_EMPTY(head2)) {					\
		*(head1)-&gt;tqh_last = (head2)-&gt;tqh_first;		\
		(head2)-&gt;tqh_first-&gt;field.tqe_prev = (head1)-&gt;tqh_last;	\
		(head1)-&gt;tqh_last = (head2)-&gt;tqh_last;			\
		TAILQ_INIT((head2));					\
	}								\
} while (/*CONSTCOND*/0)

/*
 * Tail queue access methods.
 */
#define	TAILQ_EMPTY(head)		((head)-&gt;tqh_first == NULL)
#define	TAILQ_FIRST(head)		((head)-&gt;tqh_first)
#define	TAILQ_NEXT(elm, field)		((elm)-&gt;field.tqe_next)

#define	TAILQ_LAST(head, headname) \
	(*(((struct headname *)((head)-&gt;tqh_last))-&gt;tqh_last))
#define	TAILQ_PREV(elm, headname, field) \
	(*(((struct headname *)((elm)-&gt;field.tqe_prev))-&gt;tqh_last))


/*
 * Circular queue definitions.
 */
#define	CIRCLEQ_HEAD(name, type)					\
struct name {								\
	struct type *cqh_first;		/* first element */		\
	struct type *cqh_last;		/* last element */		\
}

#define	CIRCLEQ_HEAD_INITIALIZER(head)					\
	{ (void *)&amp;head, (void *)&amp;head }

#define	CIRCLEQ_ENTRY(type)						\
struct {								\
	struct type *cqe_next;		/* next element */		\
	struct type *cqe_prev;		/* previous element */		\
}

/*
 * Circular queue functions.
 */
#define	CIRCLEQ_INIT(head) do {						\
	(head)-&gt;cqh_first = (void *)(head);				\
	(head)-&gt;cqh_last = (void *)(head);				\
} while (/*CONSTCOND*/0)

#define	CIRCLEQ_INSERT_AFTER(head, listelm, elm, field) do {		\
	(elm)-&gt;field.cqe_next = (listelm)-&gt;field.cqe_next;		\
	(elm)-&gt;field.cqe_prev = (listelm);				\
	if ((listelm)-&gt;field.cqe_next == (void *)(head))		\
		(head)-&gt;cqh_last = (elm);				\
	else								\
		(listelm)-&gt;field.cqe_next-&gt;field.cqe_prev = (elm);	\
	(listelm)-&gt;field.cqe_next = (elm);				\
} while (/*CONSTCOND*/0)

#define	CIRCLEQ_INSERT_BEFORE(head, listelm, elm, field) do {		\
	(elm)-&gt;field.cqe_next = (listelm);				\
	(elm)-&gt;field.cqe_prev = (listelm)-&gt;field.cqe_prev;		\
	if ((listelm)-&gt;field.cqe_prev == (void *)(head))		\
		(head)-&gt;cqh_first = (elm);				\
	else								\
		(listelm)-&gt;field.cqe_prev-&gt;field.cqe_next = (elm);	\
	(listelm)-&gt;field.cqe_prev = (elm);				\
} while (/*CONSTCOND*/0)

#define	CIRCLEQ_INSERT_HEAD(head, elm, field) do {			\
	(elm)-&gt;field.cqe_next = (head)-&gt;cqh_first;			\
	(elm)-&gt;field.cqe_prev = (void *)(head);				\
	if ((head)-&gt;cqh_last == (void *)(head))				\
		(head)-&gt;cqh_last = (elm);				\
	else								\
		(head)-&gt;cqh_first-&gt;field.cqe_prev = (elm);		\
	(head)-&gt;cqh_first = (elm);					\
} while (/*CONSTCOND*/0)

#define	CIRCLEQ_INSERT_TAIL(head, elm, field) do {			\
	(elm)-&gt;field.cqe_next = (void *)(head);				\
	(elm)-&gt;field.cqe_prev = (head)-&gt;cqh_last;			\
	if ((head)-&gt;cqh_first == (void *)(head))			\
		(head)-&gt;cqh_first = (elm);				\
	else								\
		(head)-&gt;cqh_last-&gt;field.cqe_next = (elm);		\
	(head)-&gt;cqh_last = (elm);					\
} while (/*CONSTCOND*/0)

#define	CIRCLEQ_REMOVE(head, elm, field) do {				\
	if ((elm)-&gt;field.cqe_next == (void *)(head))			\
		(head)-&gt;cqh_last = (elm)-&gt;field.cqe_prev;		\
	else								\
		(elm)-&gt;field.cqe_next-&gt;field.cqe_prev =			\
		    (elm)-&gt;field.cqe_prev;				\
	if ((elm)-&gt;field.cqe_prev == (void *)(head))			\
		(head)-&gt;cqh_first = (elm)-&gt;field.cqe_next;		\
	else								\
		(elm)-&gt;field.cqe_prev-&gt;field.cqe_next =			\
		    (elm)-&gt;field.cqe_next;				\
} while (/*CONSTCOND*/0)

#define	CIRCLEQ_FOREACH(var, head, field)				\
	for ((var) = ((head)-&gt;cqh_first);				\
		(var) != (const void *)(head);				\
		(var) = ((var)-&gt;field.cqe_next))

#define	CIRCLEQ_FOREACH_REVERSE(var, head, field)			\
	for ((var) = ((head)-&gt;cqh_last);				\
		(var) != (const void *)(head);				\
		(var) = ((var)-&gt;field.cqe_prev))

/*
 * Circular queue access methods.
 */
#define	CIRCLEQ_EMPTY(head)		((head)-&gt;cqh_first == (void *)(head))
#define	CIRCLEQ_FIRST(head)		((head)-&gt;cqh_first)
#define	CIRCLEQ_LAST(head)		((head)-&gt;cqh_last)
#define	CIRCLEQ_NEXT(elm, field)	((elm)-&gt;field.cqe_next)
#define	CIRCLEQ_PREV(elm, field)	((elm)-&gt;field.cqe_prev)

#define CIRCLEQ_LOOP_NEXT(head, elm, field)				\
	(((elm)-&gt;field.cqe_next == (void *)(head))			\
	    ? ((head)-&gt;cqh_first)					\
	    : (elm-&gt;field.cqe_next))
#define CIRCLEQ_LOOP_PREV(head, elm, field)				\
	(((elm)-&gt;field.cqe_prev == (void *)(head))			\
	    ? ((head)-&gt;cqh_last)					\
	    : (elm-&gt;field.cqe_prev))

#endif	/* sys/queue.h */
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