Libevent0.1学习之queue.h

最近打算研究一下libevent网络库，发现用了FreeBSD的queue.h中的尾队列来存储各种事件队列。下面就是一些学习queue.h的一些笔记。

queue.h中定义了一系列的宏操作去实现单链表、双链表、简单队列、尾队列和循环链表(queue.h的内容见文章结尾)。他们的特点对比如下：
单链表：
头部定义：一个指向第一个元素的指针
entry定义：一个指向下一个元素的指针
删除任意元素时的开销：O(n)
可插入的位置：头部、元素后面

双链表：
头部定义：一个指向第一个元素的指针
entry定义：一个指向下一个元素的指针，和一个指向前一个元素的le_next的地址的指针
删除任意元素时的开销：O(1)
可插入的位置：头部、元素前面、元素后面

简单队列
头部定义：一个指向第一个元素的指针和一个指向最后一个元素的sqe_next地址的指针
entry定义：一个指向下一个元素的指针
删除任意元素时的开销：O(n)
可插入的位置：头部、尾部、元素后面

尾队列
头部定义：一个指向第一个元素的指针和一个指向最后一个元素的tqe_next地址的指针
entry定义：一个指向下一个元素的指针和一个指向前一个元素的tqe_next地址的指针
删除任意元素时的开销：O(1)
可插入的位置：头部、尾部、元素后面、元素前面

循环队列
头部定义：一个指向第一个元素的指针和一个指向最后一个元素的指针
entry定义：一个指向下一个元素的指针和一个指向前一个元素的指针
删除任意元素时的开销：O(1)
可插入的位置：头部、尾部、元素后面、元素前面

其中尾队列和双链表的元素entry定义都包含了一个指针*_prev，它指向前一个元素的*_next变量的地址，也就是二级指针，这么设计我想是因为：
他们都有一个头部，这个头部都包含有一个指向第一个元素的指针。所以当在他们头部插入一个元素时（执行*_INSERT_HEAD宏），使用这种二级指针可以很方便的插入。

libevent中尾队列的使用

libevent中使用了尾队列来存储各种事件。

event结构体定义

struct event {
    TAILQ_ENTRY (event) ev_read_next;
    TAILQ_ENTRY (event) ev_write_next;
    TAILQ_ENTRY (event) ev_timeout_next;
    TAILQ_ENTRY (event) ev_add_next;

    int ev_fd;
    short ev_events;

    struct timeval ev_timeout;

    void (*ev_callback)(int, short, void *arg);
    void *ev_arg;

    int ev_flags;
};

可以看出event中包含4种事件队列，ev_read_next变量中的tqe_next指向下一个event，这样就形成了一个队列。

各种事件队列的头部定义

TAILQ_HEAD (timeout_list, event) timequeue;
TAILQ_HEAD (event_wlist, event) writequeue;
TAILQ_HEAD (event_rlist, event) readqueue;
TAILQ_HEAD (event_ilist, event) addqueue;

上面定义了4种事件队列头，定时器事件队列，写事件队列，读事件队列，待添加事件队列。待添加事件队列中的事件为需要延时加入的事件。

queue.h内容

/*
 * 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))

/*
 * 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)

/*
 * 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_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)


/*
 * 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_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_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 */