bsd_queue.h

/*-
 *  (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.
 * 4. 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
 * $FreeBSD: src/sys/sys/queue.h,v 1.68.2.1.2.1 2010/02/10 00:26:20 kensmith Exp $
 */

#ifndef _SYS_QUEUE_H_
#define _SYS_QUEUE_H_

// #include <sys/cdefs.h>

/*
 * This file defines four types of data structures: singly-linked lists,
 * singly-linked tail queues, lists and tail 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 singly-linked 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
 * 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, at the head of the list, or at the
 * end of the list. Elements being removed from the head of the tail queue
 * should use the explicit macro for this purpose for optimum efficiency.
 * A singly-linked tail queue may only be traversed in the forward direction.
 * Singly-linked tail queues are ideal for applications with large datasets
 * and few or no removals or for implementing a FIFO 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 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.
 *
 *
 *                              SLLIST   LIST    STAILQ  TAILQ
 * _HEAD                        +       +       +       +
 * _HEAD_INITIALIZER            +       +       +       +
 * _ENTRY                       +       +       +       +
 * _INIT                        +       +       +       +
 * _EMPTY                       +       +       +       +
 * _FIRST                       +       +       +       +
 * _NEXT                        +       +       +       +
 * _PREV                        -       -       -       +
 * _LAST                        -       -       +       +
 * _FOREACH                     +       +       +       +
 * _FOREACH_SAFE                +       +       +       +
 * _FOREACH_REVERSE             -       -       -       +
 * _FOREACH_REVERSE_SAFE        -       -       -       +
 * _INSERT_HEAD                 +       +       +       +
 * _INSERT_BEFORE               -       +       -       +
 * _INSERT_AFTER                +       +       +       +
 * _INSERT_TAIL                 -       -       +       +
 * _CONCAT                      -       -       +       +
 * _REMOVE_AFTER                +       -       +       -
 * _REMOVE_HEAD                 +       -       +       -
 * _REMOVE                      +       +       +       +
 *
 */
#ifdef QUEUE_MACRO_DEBUG
/* Store the last 2 places the queue element or head was altered */
struct qm_trace {
        char * lastfile;
        int lastline;
        char * prevfile;
        int prevline;
};

#define TRACEBUF        struct qm_trace trace;
#define TRASHIT(x)      do {(x) = (void *)-1;} while (0)

#define QMD_TRACE_HEAD(head) do {                                       \
        (head)->trace.prevline = (head)->trace.lastline;                \
        (head)->trace.prevfile = (head)->trace.lastfile;                \
        (head)->trace.lastline = __LINE__;                              \
        (head)->trace.lastfile = __FILE__;                              \
} while (0)

#define QMD_TRACE_ELEM(elem) do {                                       \
        (elem)->trace.prevline = (elem)->trace.lastline;                \
        (elem)->trace.prevfile = (elem)->trace.lastfile;                \
        (elem)->trace.lastline = __LINE__;                              \
        (elem)->trace.lastfile = __FILE__;                              \
} while (0)

#else
#define QMD_TRACE_ELEM(elem)
#define QMD_TRACE_HEAD(head)
#define TRACEBUF
#define TRASHIT(x)
#endif  /* QUEUE_MACRO_DEBUG */

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

#define SLLIST_HEAD_INITIALIZER(head)                                    \
        { NULL }

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

/*
 * Singly-linked List functions.
 */
#define SLLIST_EMPTY(head)       ((head)->slh_first == NULL)

#define SLLIST_FIRST(head)       ((head)->slh_first)

#define SLLIST_FOREACH(var, head, field)                                 \
        for ((var) = SLLIST_FIRST((head));                               \
            (var);                                                      \
            (var) = SLLIST_NEXT((var), field))

#define SLLIST_FOREACH_SAFE(var, head, field, tvar)                      \
        for ((var) = SLLIST_FIRST((head));                               \
            (var) && ((tvar) = SLLIST_NEXT((var), field), 1);            \
            (var) = (tvar))

#define SLLIST_FOREACH_PREVPTR(var, varp, head, field)                   \
        for ((varp) = &SLLIST_FIRST((head));                             \
            ((var) = *(varp)) != NULL;                                  \
            (varp) = &SLLIST_NEXT((var), field))

#define SLLIST_INIT(head) do {                                           \
        SLLIST_FIRST((head)) = NULL;                                     \
} while (0)

#define SLLIST_INSERT_AFTER(slistelm, elm, field) do {                   \
        SLLIST_NEXT((elm), field) = SLLIST_NEXT((slistelm), field);       \
        SLLIST_NEXT((slistelm), field) = (elm);                          \
} while (0)

#define SLLIST_INSERT_HEAD(head, elm, field) do {                        \
        SLLIST_NEXT((elm), field) = SLLIST_FIRST((head));                 \
        SLLIST_FIRST((head)) = (elm);                                    \
} while (0)

#define SLLIST_NEXT(elm, field)  ((elm)->field.sle_next)

#define SLLIST_REMOVE(head, elm, type, field) do {                       \
        if (SLLIST_FIRST((head)) == (elm)) {                             \
                SLLIST_REMOVE_HEAD((head), field);                       \
        }                                                               \
        else {                                                          \
                struct type *curelm = SLLIST_FIRST((head));              \
                while (SLLIST_NEXT(curelm, field) != (elm))              \
                        curelm = SLLIST_NEXT(curelm, field);             \
                SLLIST_REMOVE_AFTER(curelm, field);                      \
        }                                                               \
        TRASHIT((elm)->field.sle_next);                                 \
} while (0)

#define SLLIST_REMOVE_AFTER(elm, field) do {                             \
        SLLIST_NEXT(elm, field) =                                        \
            SLLIST_NEXT(SLLIST_NEXT(elm, field), field);                  \
} while (0)

#define SLLIST_REMOVE_HEAD(head, field) do {                             \
        SLLIST_FIRST((head)) = SLLIST_NEXT(SLLIST_FIRST((head)), field);   \
} 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 functions.
 */
#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_EMPTY(head)      ((head)->stqh_first == NULL)

#define STAILQ_FIRST(head)      ((head)->stqh_first)

#define STAILQ_FOREACH(var, head, field)                                \
        for((var) = STAILQ_FIRST((head));                               \
           (var);                                                       \
           (var) = STAILQ_NEXT((var), field))


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

#define STAILQ_INIT(head) do {                                          \
        STAILQ_FIRST((head)) = NULL;                                    \
        (head)->stqh_last = &STAILQ_FIRST((head));                      \
} while (0)

#define STAILQ_INSERT_AFTER(head, tqelm, elm, field) do {               \
        if ((STAILQ_NEXT((elm), field) = STAILQ_NEXT((tqelm), field)) == NULL)\
                (head)->stqh_last = &STAILQ_NEXT((elm), field);         \
        STAILQ_NEXT((tqelm), field) = (elm);                            \
} while (0)

#define STAILQ_INSERT_HEAD(head, elm, field) do {                       \
        if ((STAILQ_NEXT((elm), field) = STAILQ_FIRST((head))) == NULL) \
                (head)->stqh_last = &STAILQ_NEXT((elm), field);         \
        STAILQ_FIRST((head)) = (elm);                                   \
} while (0)

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

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

#define STAILQ_NEXT(elm, field) ((elm)->field.stqe_next)

#define STAILQ_REMOVE(head, elm, type, field) do {                      \
        if (STAILQ_FIRST((head)) == (elm)) {                            \
                STAILQ_REMOVE_HEAD((head), field);                      \
        }                                                               \
        else {                                                          \
                struct type *curelm = STAILQ_FIRST((head));             \
                while (STAILQ_NEXT(curelm, field) != (elm))             \
                        curelm = STAILQ_NEXT(curelm, field);            \
                STAILQ_REMOVE_AFTER(head, curelm, field);               \
        }                                                               \
        TRASHIT((elm)->field.stqe_next);                                \
} while (0)

#define STAILQ_REMOVE_HEAD(head, field) do {                            \
        if ((STAILQ_FIRST((head)) =                                     \
             STAILQ_NEXT(STAILQ_FIRST((head)), field)) == NULL)         \
                (head)->stqh_last = &STAILQ_FIRST((head));              \
} while (0)

#define STAILQ_REMOVE_AFTER(head, elm, field) do {                      \
        if ((STAILQ_NEXT(elm, field) =                                  \
             STAILQ_NEXT(STAILQ_NEXT(elm, field), field)) == NULL)      \
                (head)->stqh_last = &STAILQ_NEXT((elm), field);         \
} while (0)

/*
 * List declarations.
 */
#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.
 */

#if (defined(_KERNEL) && defined(INVARIANTS))
#define QMD_LIST_CHECK_HEAD(head, field) do {                           \
        if (LIST_FIRST((head)) != NULL &&                               \
            LIST_FIRST((head))->field.le_prev !=                        \
             &LIST_FIRST((head)))                                       \
                panic("Bad list head %p first->prev != head", (head));  \
} while (0)

#define QMD_LIST_CHECK_NEXT(elm, field) do {                            \
        if (LIST_NEXT((elm), field) != NULL &&                          \
            LIST_NEXT((elm), field)->field.le_prev !=                   \
             &((elm)->field.le_next))                                   \
                panic("Bad link elm %p next->prev != elm", (elm));      \
} while (0)

#define QMD_LIST_CHECK_PREV(elm, field) do {                            \
        if (*(elm)->field.le_prev != (elm))                             \
                panic("Bad link elm %p prev->next != elm", (elm));      \
} while (0)
#else
#define QMD_LIST_CHECK_HEAD(head, field)
#define QMD_LIST_CHECK_NEXT(elm, field)
#define QMD_LIST_CHECK_PREV(elm, field)
#endif /* (_KERNEL && INVARIANTS) */

#define LIST_EMPTY(head)        ((head)->lh_first == NULL)

#define LIST_FIRST(head)        ((head)->lh_first)

#define LIST_FOREACH(var, head, field)                                  \
        for ((var) = LIST_FIRST((head));                                \
            (var);                                                      \
            (var) = LIST_NEXT((var), field))

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

#define LIST_INIT(head) do {                                            \
        LIST_FIRST((head)) = NULL;                                      \
} while (0)

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

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

#define LIST_INSERT_HEAD(head, elm, field) do {                         \
        QMD_LIST_CHECK_HEAD((head), field);                             \
        if ((LIST_NEXT((elm), field) = LIST_FIRST((head))) != NULL)     \
                LIST_FIRST((head))->field.le_prev = &LIST_NEXT((elm), field);\
        LIST_FIRST((head)) = (elm);                                     \
        (elm)->field.le_prev = &LIST_FIRST((head));                     \
} while (0)

#define LIST_NEXT(elm, field)   ((elm)->field.le_next)

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

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

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

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

/*
 * Tail queue functions.
 */
#if (defined(_KERNEL) && defined(INVARIANTS))
#define QMD_TAILQ_CHECK_HEAD(head, field) do {                          \
        if (!TAILQ_EMPTY(head) &&                                       \
            TAILQ_FIRST((head))->field.tqe_prev !=                      \
             &TAILQ_FIRST((head)))                                      \
                panic("Bad tailq head %p first->prev != head", (head)); \
} while (0)

#define QMD_TAILQ_CHECK_TAIL(head, field) do {                          \
        if (*(head)->tqh_last != NULL)                                  \
                panic("Bad tailq NEXT(%p->tqh_last) != NULL", (head));  \
} while (0)

#define QMD_TAILQ_CHECK_NEXT(elm, field) do {                           \
        if (TAILQ_NEXT((elm), field) != NULL &&                         \
            TAILQ_NEXT((elm), field)->field.tqe_prev !=                 \
             &((elm)->field.tqe_next))                                  \
                panic("Bad link elm %p next->prev != elm", (elm));      \
} while (0)

#define QMD_TAILQ_CHECK_PREV(elm, field) do {                           \
        if (*(elm)->field.tqe_prev != (elm))                            \
                panic("Bad link elm %p prev->next != elm", (elm));      \
} while (0)
#else
#define QMD_TAILQ_CHECK_HEAD(head, field)
#define QMD_TAILQ_CHECK_TAIL(head, headname)
#define QMD_TAILQ_CHECK_NEXT(elm, field)
#define QMD_TAILQ_CHECK_PREV(elm, field)
#endif /* (_KERNEL && INVARIANTS) */

#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));                                    \
                QMD_TRACE_HEAD(head1);                                  \
                QMD_TRACE_HEAD(head2);                                  \
        }                                                               \
} while (0)

#define TAILQ_EMPTY(head)       ((head)->tqh_first == NULL)

#define TAILQ_FIRST(head)       ((head)->tqh_first)

#define TAILQ_FOREACH(var, head, field)                                 \
        for ((var) = TAILQ_FIRST((head));                               \
            (var);                                                      \
            (var) = TAILQ_NEXT((var), field))

#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) = TAILQ_LAST((head), headname);                      \
            (var);                                                      \
            (var) = TAILQ_PREV((var), headname, field))

#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_INIT(head) do {                                           \
        TAILQ_FIRST((head)) = NULL;                                     \
        (head)->tqh_last = &TAILQ_FIRST((head));                        \
        QMD_TRACE_HEAD(head);                                           \
} while (0)

#define TAILQ_INSERT_AFTER(head, listelm, elm, field) do {              \
        QMD_TAILQ_CHECK_NEXT(listelm, field);                           \
        if ((TAILQ_NEXT((elm), field) = TAILQ_NEXT((listelm), field)) != NULL)\
                TAILQ_NEXT((elm), field)->field.tqe_prev =              \
                    &TAILQ_NEXT((elm), field);                          \
        else {                                                          \
                (head)->tqh_last = &TAILQ_NEXT((elm), field);           \
                QMD_TRACE_HEAD(head);                                   \
        }                                                               \
        TAILQ_NEXT((listelm), field) = (elm);                           \
        (elm)->field.tqe_prev = &TAILQ_NEXT((listelm), field);          \
        QMD_TRACE_ELEM(&(elm)->field);                                  \
        QMD_TRACE_ELEM(&listelm->field);                                \
} while (0)

#define TAILQ_INSERT_BEFORE(listelm, elm, field) do {                   \
        QMD_TAILQ_CHECK_PREV(listelm, field);                           \
        (elm)->field.tqe_prev = (listelm)->field.tqe_prev;              \
        TAILQ_NEXT((elm), field) = (listelm);                           \
        *(listelm)->field.tqe_prev = (elm);                             \
        (listelm)->field.tqe_prev = &TAILQ_NEXT((elm), field);          \
        QMD_TRACE_ELEM(&(elm)->field);                                  \
        QMD_TRACE_ELEM(&listelm->field);                                \
} while (0)

#define TAILQ_INSERT_HEAD(head, elm, field) do {                        \
        QMD_TAILQ_CHECK_HEAD(head, field);                              \
        if ((TAILQ_NEXT((elm), field) = TAILQ_FIRST((head))) != NULL)   \
                TAILQ_FIRST((head))->field.tqe_prev =                   \
                    &TAILQ_NEXT((elm), field);                          \
        else                                                            \
                (head)->tqh_last = &TAILQ_NEXT((elm), field);           \
        TAILQ_FIRST((head)) = (elm);                                    \
        (elm)->field.tqe_prev = &TAILQ_FIRST((head));                   \
        QMD_TRACE_HEAD(head);                                           \
        QMD_TRACE_ELEM(&(elm)->field);                                  \
} while (0)

#define TAILQ_INSERT_TAIL(head, elm, field) do {                        \
        QMD_TAILQ_CHECK_TAIL(head, field);                              \
        TAILQ_NEXT((elm), field) = NULL;                                \
        (elm)->field.tqe_prev = (head)->tqh_last;                       \
        *(head)->tqh_last = (elm);                                      \
        (head)->tqh_last = &TAILQ_NEXT((elm), field);                   \
        QMD_TRACE_HEAD(head);                                           \
        QMD_TRACE_ELEM(&(elm)->field);                                  \
} while (0)

#define TAILQ_LAST(head, headname)                                      \
        (*(((struct headname *)((head)->tqh_last))->tqh_last))

#define TAILQ_NEXT(elm, field) ((elm)->field.tqe_next)

#define TAILQ_PREV(elm, headname, field)                                \
        (*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))

#define TAILQ_REMOVE(head, elm, field) do {                             \
        QMD_TAILQ_CHECK_NEXT(elm, field);                               \
        QMD_TAILQ_CHECK_PREV(elm, field);                               \
        if ((TAILQ_NEXT((elm), field)) != NULL)                         \
                TAILQ_NEXT((elm), field)->field.tqe_prev =              \
                    (elm)->field.tqe_prev;                              \
        else {                                                          \
                (head)->tqh_last = (elm)->field.tqe_prev;               \
                QMD_TRACE_HEAD(head);                                   \
        }                                                               \
        *(elm)->field.tqe_prev = TAILQ_NEXT((elm), field);              \
        TRASHIT((elm)->field.tqe_next);                                 \
        TRASHIT((elm)->field.tqe_prev);                                 \
        QMD_TRACE_ELEM(&(elm)->field);                                  \
} while (0)


#ifdef _KERNEL

/*
 * XXX insque() and remque() are an old way of handling certain queues.
 * They bogusly assumes that all queue heads look alike.
 */

struct quehead {
        struct quehead *qh_link;
        struct quehead *qh_rlink;
};

#ifdef __CC_SUPPORTS___INLINE

static __inline void
insque(void *a, void *b)
{
        struct quehead *element = (struct quehead *)a,
                 *head = (struct quehead *)b;

        element->qh_link = head->qh_link;
        element->qh_rlink = head;
        head->qh_link = element;
        element->qh_link->qh_rlink = element;
}

static __inline void
remque(void *a)
{
        struct quehead *element = (struct quehead *)a;

        element->qh_link->qh_rlink = element->qh_rlink;
        element->qh_rlink->qh_link = element->qh_link;
        element->qh_rlink = 0;
}

#else /* !__CC_SUPPORTS___INLINE */

void    insque(void *a, void *b);
void    remque(void *a);

#endif /* __CC_SUPPORTS___INLINE */

#endif /* _KERNEL */

#endif /* !_SYS_QUEUE_H_ */