/*
 * 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)->lh_first = NULL;                                                 \
   } while (/*CONSTCOND*/ 0)

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

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

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

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

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

#define LIST_FOREACH_SAFE(var, head, field, tvar)                              \
   for ((var) = LIST_FIRST((head));                                            \
        (var) && ((tvar) = LIST_NEXT((var), field), 1); (var) = (tvar))

/*
 * List access methods.
 */
#define LIST_EMPTY(head) ((head)->lh_first == NULL)
#define LIST_FIRST(head) ((head)->lh_first)
#define LIST_NEXT(elm, field) ((elm)->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)->slh_first = NULL;                                                \
   } while (/*CONSTCOND*/ 0)

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

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

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

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

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

#define SLIST_FOREACH_SAFE(var, head, field, tvar)                             \
   for ((var) = SLIST_FIRST((head));                                           \
        (var) && ((tvar) = SLIST_NEXT((var), field), 1); (var) = (tvar))

#define SLIST_FOREACH_PREVPTR(var, varp, head, field)                          \
   for ((varp) = &SLIST_FIRST((head)); ((var) = *(varp)) != NULL;              \
        (varp) = &SLIST_NEXT((var), field))

/*
 * Singly-linked List access methods.
 */
#define SLIST_EMPTY(head) ((head)->slh_first == NULL)
#define SLIST_FIRST(head) ((head)->slh_first)
#define SLIST_NEXT(elm, field) ((elm)->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, &(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)->stqh_first = NULL;                                               \
      (head)->stqh_last = &(head)->stqh_first;                                 \
   } while (/*CONSTCOND*/ 0)

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

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

#define STAILQ_LAST(head, type, field)                                         \
   (STAILQ_EMPTY((head))                                                       \
        ? NULL                                                                 \
        : ((struct type*)(void*)((char*)((head)->stqh_last)                    \
                                 - offsetof(struct type, field))))

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

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

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

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

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

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

#define STAILQ_FOREACH_SAFE(var, head, field, tvar)                            \
   for ((var) = STAILQ_FIRST((head));                                          \
        (var) && ((tvar) = STAILQ_NEXT((var), field), 1); (var) = (tvar))

/*
 * 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, &(head).sqh_first                                                  \
   }

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

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

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

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

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

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

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

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

/*
 * Simple queue access methods.
 */
#define SIMPLEQ_EMPTY(head) ((head)->sqh_first == NULL)
#define SIMPLEQ_FIRST(head) ((head)->sqh_first)
#define SIMPLEQ_NEXT(elm, field) ((elm)->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, &(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)->tqh_first = NULL;                                                \
      (head)->tqh_last = &(head)->tqh_first;                                   \
   } while (/*CONSTCOND*/ 0)

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

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

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

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

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

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

#define TAILQ_FOREACH_SAFE(var, head, field, tvar)                             \
   for ((var) = TAILQ_FIRST((head));                                           \
        (var) && ((tvar) = TAILQ_NEXT((var), field), 1); (var) = (tvar))

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

#define TAILQ_FOREACH_REVERSE_SAFE(var, head, headname, field, tvar)           \
   for ((var) = TAILQ_LAST((head), headname);                                  \
        (var) && ((tvar) = TAILQ_PREV((var), headname, field), 1);             \
        (var) = (tvar))

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

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

#define TAILQ_LAST(head, headname)                                             \
   (*(((struct headname*)((head)->tqh_last))->tqh_last))
#define TAILQ_PREV(elm, headname, field)                                       \
   (*(((struct headname*)((elm)->field.tqe_prev))->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*)&head, (void*)&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)->cqh_first = (void*)(head);                                       \
      (head)->cqh_last = (void*)(head);                                        \
   } while (/*CONSTCOND*/ 0)

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

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

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

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

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

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

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

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

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

#endif /* sys/queue.h */
