/*      $NetBSD: queue.h,v 1.77 2024/05/12 10:34:56 rillig Exp $        */

/*
 * 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.
 *
 * For details on the use of these macros, see the queue(3) manual page.
 */

/*
 * Include the definition of NULL only on NetBSD because sys/null.h
 * is not available elsewhere.  This conditional makes the header
 * portable and it can simply be dropped verbatim into any system.
 * The caveat is that on other systems some other header
 * must provide NULL before the macros can be used.
 */
#ifdef __NetBSD__
#include <sys/null.h>
#endif

#if defined(_KERNEL) && defined(DIAGNOSTIC)
#define QUEUEDEBUG      1
#endif

#if defined(QUEUEDEBUG)
# if defined(_KERNEL)
#  define QUEUEDEBUG_ABORT(...) panic(__VA_ARGS__)
# else
#  include <err.h>
#  define QUEUEDEBUG_ABORT(...) err(1, __VA_ARGS__)
# endif
#endif

/*
 * 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 access methods.
 */
#define SLIST_FIRST(head)       ((head)->slh_first)
#define SLIST_END(head)         NULL
#define SLIST_EMPTY(head)       ((head)->slh_first == NULL)
#define SLIST_NEXT(elm, field)  ((elm)->field.sle_next)

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

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

/*
 * Singly-linked List functions.
 */
#define SLIST_INIT(head) do {                                           \
        (head)->slh_first = SLIST_END(head);                            \
} while (0)

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

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

#define SLIST_REMOVE_AFTER(slistelm, field) do {                        \
        (slistelm)->field.sle_next =                                    \
            SLIST_NEXT(SLIST_NEXT((slistelm), field), field);           \
} while (0)

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


/*
 * 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 access methods.
 */
#define LIST_FIRST(head)                ((head)->lh_first)
#define LIST_END(head)                  NULL
#define LIST_EMPTY(head)                ((head)->lh_first == LIST_END(head))
#define LIST_NEXT(elm, field)           ((elm)->field.le_next)

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

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

#define LIST_MOVE(head1, head2, field) do {                             \
        LIST_INIT((head2));                                             \
        if (!LIST_EMPTY((head1))) {                                     \
                (head2)->lh_first = (head1)->lh_first;                  \
                (head2)->lh_first->field.le_prev = &(head2)->lh_first;  \
                LIST_INIT((head1));                                     \
        }                                                               \
} while (0)

/*
 * List functions.
 */
#if defined(QUEUEDEBUG)
#define QUEUEDEBUG_LIST_INSERT_HEAD(head, elm, field)                   \
        if ((head)->lh_first &&                                         \
            (head)->lh_first->field.le_prev != &(head)->lh_first)       \
                QUEUEDEBUG_ABORT("LIST_INSERT_HEAD %p %s:%d", (head),   \
                    __FILE__, __LINE__);
#define QUEUEDEBUG_LIST_OP(elm, field)                                  \
        if ((elm)->field.le_next &&                                     \
            (elm)->field.le_next->field.le_prev !=                      \
            &(elm)->field.le_next)                                      \
                QUEUEDEBUG_ABORT("LIST_* forw %p %s:%d", (elm),         \
                    __FILE__, __LINE__);                                \
        if (*(elm)->field.le_prev != (elm))                             \
                QUEUEDEBUG_ABORT("LIST_* back %p %s:%d", (elm),         \
                    __FILE__, __LINE__);
#define QUEUEDEBUG_LIST_POSTREMOVE(elm, field)                          \
        (elm)->field.le_next = (void *)1L;                              \
        (elm)->field.le_prev = (void *)1L;
#else
#define QUEUEDEBUG_LIST_INSERT_HEAD(head, elm, field)
#define QUEUEDEBUG_LIST_OP(elm, field)
#define QUEUEDEBUG_LIST_POSTREMOVE(elm, field)
#endif

#define LIST_INIT(head) do {                                            \
        (head)->lh_first = LIST_END(head);                              \
} while (0)

#define LIST_INSERT_AFTER(listelm, elm, field) do {                     \
        QUEUEDEBUG_LIST_OP((listelm), field)                            \
        if (((elm)->field.le_next = (listelm)->field.le_next) !=        \
            LIST_END(head))                                             \
                (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 (0)

#define LIST_INSERT_BEFORE(listelm, elm, field) do {                    \
        QUEUEDEBUG_LIST_OP((listelm), field)                            \
        (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 (0)

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

#define LIST_REMOVE(elm, field) do {                                    \
        QUEUEDEBUG_LIST_OP((elm), field)                                \
        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;                   \
        QUEUEDEBUG_LIST_POSTREMOVE((elm), field)                        \
} while (0)

#define LIST_REPLACE(elm, elm2, field) do {                             \
        if (((elm2)->field.le_next = (elm)->field.le_next) != NULL)     \
                (elm2)->field.le_next->field.le_prev =                  \
                    &(elm2)->field.le_next;                             \
        (elm2)->field.le_prev = (elm)->field.le_prev;                   \
        *(elm2)->field.le_prev = (elm2);                                \
        QUEUEDEBUG_LIST_POSTREMOVE((elm), field)                        \
} while (0)

/*
 * 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 access methods.
 */
#define SIMPLEQ_FIRST(head)             ((head)->sqh_first)
#define SIMPLEQ_END(head)               NULL
#define SIMPLEQ_EMPTY(head)             ((head)->sqh_first == SIMPLEQ_END(head))
#define SIMPLEQ_NEXT(elm, field)        ((elm)->field.sqe_next)

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

#define SIMPLEQ_FOREACH_SAFE(var, head, field, next)                    \
        for ((var) = ((head)->sqh_first);                               \
            (var) != SIMPLEQ_END(head) &&                               \
            ((next = ((var)->field.sqe_next)), 1);                      \
            (var) = (next))

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

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

#define SIMPLEQ_CONCAT(head1, head2) do {                               \
        if (!SIMPLEQ_EMPTY((head2))) {                                  \
                *(head1)->sqh_last = (head2)->sqh_first;                \
                (head1)->sqh_last = (head2)->sqh_last;          \
                SIMPLEQ_INIT((head2));                                  \
        }                                                               \
} while (0)

#define SIMPLEQ_LAST(head, type, field)                                 \
        (SIMPLEQ_EMPTY((head)) ?                                                \
                NULL :                                                  \
                ((struct type *)(void *)                                \
                ((char *)((head)->sqh_last) - offsetof(struct type, field))))

/*
 * 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)                                    \
        { TAILQ_END(head), &(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 access methods.
 */
#define TAILQ_FIRST(head)               ((head)->tqh_first)
#define TAILQ_END(head)                 (NULL)
#define TAILQ_NEXT(elm, field)          ((elm)->field.tqe_next)
#define TAILQ_LAST(head, headname) \
        (*(((struct headname *)(void *)((head)->tqh_last))->tqh_last))
#define TAILQ_PREV(elm, headname, field) \
        (*(((struct headname *)(void *)((elm)->field.tqe_prev))->tqh_last))
#define TAILQ_EMPTY(head)               (TAILQ_FIRST(head) == TAILQ_END(head))


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

#define TAILQ_FOREACH_SAFE(var, head, field, next)                      \
        for ((var) = ((head)->tqh_first);                               \
            (var) != TAILQ_END(head) &&                                 \
            ((next) = TAILQ_NEXT(var, field), 1); (var) = (next))

#define TAILQ_FOREACH_REVERSE(var, head, headname, field)               \
        for ((var) = TAILQ_LAST((head), headname);                      \
            (var) != TAILQ_END(head);                                   \
            (var) = TAILQ_PREV((var), headname, field))

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

/*
 * Tail queue functions.
 */
#if defined(QUEUEDEBUG)
#define QUEUEDEBUG_TAILQ_INSERT_HEAD(head, elm, field)                  \
        if ((head)->tqh_first &&                                        \
            (head)->tqh_first->field.tqe_prev != &(head)->tqh_first)    \
                QUEUEDEBUG_ABORT("TAILQ_INSERT_HEAD %p %s:%d", (head),  \
                    __FILE__, __LINE__);
#define QUEUEDEBUG_TAILQ_INSERT_TAIL(head, elm, field)                  \
        if (*(head)->tqh_last != NULL)                                  \
                QUEUEDEBUG_ABORT("TAILQ_INSERT_TAIL %p %s:%d", (head),  \
                    __FILE__, __LINE__);
#define QUEUEDEBUG_TAILQ_OP(elm, field)                                 \
        if ((elm)->field.tqe_next &&                                    \
            (elm)->field.tqe_next->field.tqe_prev !=                    \
            &(elm)->field.tqe_next)                                     \
                QUEUEDEBUG_ABORT("TAILQ_* forw %p %s:%d", (elm),        \
                    __FILE__, __LINE__);                                \
        if (*(elm)->field.tqe_prev != (elm))                            \
                QUEUEDEBUG_ABORT("TAILQ_* back %p %s:%d", (elm),        \
                    __FILE__, __LINE__);
#define QUEUEDEBUG_TAILQ_PREREMOVE(head, elm, field)                    \
        if ((elm)->field.tqe_next == NULL &&                            \
            (head)->tqh_last != &(elm)->field.tqe_next)                 \
                QUEUEDEBUG_ABORT("TAILQ_PREREMOVE head %p elm %p %s:%d",\
                    (head), (elm), __FILE__, __LINE__);
#define QUEUEDEBUG_TAILQ_POSTREMOVE(elm, field)                         \
        (elm)->field.tqe_next = (void *)1L;                             \
        (elm)->field.tqe_prev = (void *)1L;
#else
#define QUEUEDEBUG_TAILQ_INSERT_HEAD(head, elm, field)
#define QUEUEDEBUG_TAILQ_INSERT_TAIL(head, elm, field)
#define QUEUEDEBUG_TAILQ_OP(elm, field)
#define QUEUEDEBUG_TAILQ_PREREMOVE(head, elm, field)
#define QUEUEDEBUG_TAILQ_POSTREMOVE(elm, field)
#endif

#define TAILQ_INIT(head) do {                                           \
        (head)->tqh_first = TAILQ_END(head);                            \
        (head)->tqh_last = &(head)->tqh_first;                          \
} while (0)

#define TAILQ_INSERT_HEAD(head, elm, field) do {                        \
        QUEUEDEBUG_TAILQ_INSERT_HEAD((head), (elm), field)              \
        if (((elm)->field.tqe_next = (head)->tqh_first) != TAILQ_END(head))\
                (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 (0)

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

#define TAILQ_INSERT_AFTER(head, listelm, elm, field) do {              \
        QUEUEDEBUG_TAILQ_OP((listelm), field)                           \
        if (((elm)->field.tqe_next = (listelm)->field.tqe_next) !=      \
            TAILQ_END(head))                                            \
                (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 (0)

#define TAILQ_INSERT_BEFORE(listelm, elm, field) do {                   \
        QUEUEDEBUG_TAILQ_OP((listelm), field)                           \
        (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 (0)

#define TAILQ_REMOVE(head, elm, field) do {                             \
        QUEUEDEBUG_TAILQ_PREREMOVE((head), (elm), field)                \
        QUEUEDEBUG_TAILQ_OP((elm), field)                               \
        if (((elm)->field.tqe_next) != TAILQ_END(head))                 \
                (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;                 \
        QUEUEDEBUG_TAILQ_POSTREMOVE((elm), field);                      \
} while (0)

#define TAILQ_REPLACE(head, elm, elm2, field) do {                      \
        if (((elm2)->field.tqe_next = (elm)->field.tqe_next) !=         \
            TAILQ_END(head))                                            \
                (elm2)->field.tqe_next->field.tqe_prev =                \
                    &(elm2)->field.tqe_next;                            \
        else                                                            \
                (head)->tqh_last = &(elm2)->field.tqe_next;             \
        (elm2)->field.tqe_prev = (elm)->field.tqe_prev;                 \
        *(elm2)->field.tqe_prev = (elm2);                               \
        QUEUEDEBUG_TAILQ_POSTREMOVE((elm), field);                      \
} while (0)

#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 (0)

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

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

#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 (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 (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 (0)

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

#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 (0)

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

#endif  /* !_SYS_QUEUE_H_ */