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    QUEUE(3)                  OpenBSD Programmer's Manual                 QUEUE(3)

    NAME
         SLIST_ENTRY, SLIST_HEAD, SLIST_HEAD_INITIALIZER, SLIST_FIRST, SLIST_NEXT,
         SLIST_END, SLIST_EMPTY, SLIST_FOREACH, SLIST_FOREACH_PREVPTR, SLIST_INIT,
         SLIST_INSERT_AFTER, SLIST_INSERT_HEAD, SLIST_REMOVE_HEAD,
         SLIST_REMOVE_NEXT, SLIST_REMOVE, LIST_ENTRY, LIST_HEAD,
         LIST_HEAD_INITIALIZER, LIST_FIRST, LIST_NEXT, LIST_END, LIST_EMPTY,
         LIST_FOREACH, LIST_INIT, LIST_INSERT_AFTER, LIST_INSERT_BEFORE,
         LIST_INSERT_HEAD, LIST_REMOVE, LIST_REPLACE, SIMPLEQ_ENTRY, SIMPLEQ_HEAD,
         SIMPLEQ_HEAD_INITIALIZER, SIMPLEQ_FIRST, SIMPLEQ_NEXT, SIMPLEQ_END,
         SIMPLEQ_EMPTY, SIMPLEQ_FOREACH, SIMPLEQ_INIT, SIMPLEQ_INSERT_HEAD,
         SIMPLEQ_INSERT_TAIL, SIMPLEQ_INSERT_AFTER, SIMPLEQ_REMOVE_HEAD,
         TAILQ_ENTRY, TAILQ_HEAD, TAILQ_HEAD_INITIALIZER, TAILQ_FIRST, TAILQ_NEXT,
         TAILQ_END, TAILQ_LAST, TAILQ_PREV, TAILQ_EMPTY, TAILQ_FOREACH,
         TAILQ_FOREACH_REVERSE, TAILQ_INIT, TAILQ_INSERT_AFTER,
         TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD, TAILQ_INSERT_TAIL, TAILQ_REMOVE,
         TAILQ_REPLACE, CIRCLEQ_ENTRY, CIRCLEQ_HEAD, CIRCLEQ_HEAD_INITIALIZER,
         CIRCLEQ_FIRST, CIRCLEQ_LAST, CIRCLEQ_END, CIRCLEQ_NEXT, CIRCLEQ_PREV,
         CIRCLEQ_EMPTY, CIRCLEQ_FOREACH, CIRCLEQ_FOREACH_REVERSE, CIRCLEQ_INIT,
         CIRCLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE, CIRCLEQ_INSERT_HEAD,
         CIRCLEQ_INSERT_TAIL, CIRCLEQ_REMOVE, CIRCLEQ_REPLACE - implementations of
         singly-linked lists, doubly-linked lists, simple queues, tail queues, and
         circular queues

    SYNOPSIS
         #include <sys/queue.h>

         SLIST_ENTRY(TYPE);

         SLIST_HEAD(HEADNAME, TYPE);

         SLIST_HEAD_INITIALIZER(SLIST_HEAD head);

         struct TYPE *
         SLIST_FIRST(SLIST_HEAD *head);

         struct TYPE *
         SLIST_NEXT(struct TYPE *listelm, SLIST_ENTRY NAME);

         struct TYPE *
         SLIST_END(SLIST_HEAD *head);

         int
         SLIST_EMPTY(SLIST_HEAD *head);

         SLIST_FOREACH(VARNAME, SLIST_HEAD *head, SLIST_ENTRY NAME);

         SLIST_FOREACH_PREVPTR(VARNAME, VARNAMEP, SLIST_HEAD *head,
                 SLIST_ENTRY NAME);

         void
         SLIST_INIT(SLIST_HEAD *head);

         void
         SLIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm,
                 SLIST_ENTRY NAME);

         void
         SLIST_INSERT_HEAD(SLIST_HEAD *head, struct TYPE *elm, SLIST_ENTRY NAME);

         void
         SLIST_REMOVE_HEAD(SLIST_HEAD *head, SLIST_ENTRY NAME);

         void
         SLIST_REMOVE_NEXT(SLIST_HEAD *head, struct TYPE *elm, SLIST_ENTRY NAME);

         void
         SLIST_REMOVE(SLIST_HEAD *head, struct TYPE *elm, TYPE, SLIST_ENTRY NAME);

         LIST_ENTRY(TYPE);

         LIST_HEAD(HEADNAME, TYPE);

         LIST_HEAD_INITIALIZER(LIST_HEAD head);

         struct TYPE *
         LIST_FIRST(LIST_HEAD *head);

         struct TYPE *
         LIST_NEXT(struct TYPE *listelm, LIST_ENTRY NAME);

         struct TYPE *
         LIST_END(LIST_HEAD *head);

         int
         LIST_EMPTY(LIST_HEAD *head);

         LIST_FOREACH(VARNAME, LIST_HEAD *head, LIST_ENTRY NAME);

         void
         LIST_INIT(LIST_HEAD *head);

         void
         LIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm,
                 LIST_ENTRY NAME);

         void
         LIST_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm,
                 LIST_ENTRY NAME);

         void
         LIST_INSERT_HEAD(LIST_HEAD *head, struct TYPE *elm, LIST_ENTRY NAME);

         void
         LIST_REMOVE(struct TYPE *elm, LIST_ENTRY NAME);

         void
         LIST_REPLACE(struct TYPE *elm, struct TYPE *elm2, LIST_ENTRY NAME);

         SIMPLEQ_ENTRY(TYPE);

         SIMPLEQ_HEAD(HEADNAME, TYPE);

         SIMPLEQ_HEAD_INITIALIZER(SIMPLEQ_HEAD head);

         struct TYPE *
         SIMPLEQ_FIRST(SIMPLEQ_HEAD *head);

         struct TYPE *
         SIMPLEQ_NEXT(struct TYPE *listelm, SIMPLEQ_ENTRY NAME);

         struct TYPE *
         SIMPLEQ_END(SIMPLEQ_HEAD *head);

         int
         SIMPLEQ_EMPTY(SIMPLEQ_HEAD *head);

         SIMPLEQ_FOREACH(VARNAME, SIMPLEQ_HEAD *head, SIMPLEQ_ENTRY NAME);

         void
         SIMPLEQ_INIT(SIMPLEQ_HEAD *head);

         void
         SIMPLEQ_INSERT_HEAD(SIMPLEQ_HEAD *head, struct TYPE *elm,
                 SIMPLEQ_ENTRY NAME);

         void
         SIMPLEQ_INSERT_TAIL(SIMPLEQ_HEAD *head, struct TYPE *elm,
                 SIMPLEQ_ENTRY NAME);

         void
         SIMPLEQ_INSERT_AFTER(SIMPLEQ_HEAD *head, struct TYPE *listelm,
                 struct TYPE *elm, SIMPLEQ_ENTRY NAME);

         void
         SIMPLEQ_REMOVE_HEAD(SIMPLEQ_HEAD *head, SIMPLEQ_ENTRY NAME);

         TAILQ_ENTRY(TYPE);

         TAILQ_HEAD(HEADNAME, TYPE);

         TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);

         struct TYPE *
         TAILQ_FIRST(TAILQ_HEAD *head);

         struct TYPE *
         TAILQ_NEXT(struct TYPE *listelm, TAILQ_ENTRY NAME);

         struct TYPE *
         TAILQ_END(TAILQ_HEAD *head);

         struct TYPE *
         TAILQ_LAST(TAILQ_HEAD *head, HEADNAME NAME);

         struct TYPE *
         TAILQ_PREV(struct TYPE *listelm, HEADNAME NAME, TAILQ_ENTRY NAME);

         int
         TAILQ_EMPTY(TAILQ_HEAD *head);

         TAILQ_FOREACH(VARNAME, TAILQ_HEAD *head, TAILQ_ENTRY NAME);

         TAILQ_FOREACH_REVERSE(VARNAME, TAILQ_HEAD *head, HEADNAME,
                 TAILQ_ENTRY NAME);

         void
         TAILQ_INIT(TAILQ_HEAD *head);

         void
         TAILQ_INSERT_AFTER(TAILQ_HEAD *head, struct TYPE *listelm,
                 struct TYPE *elm, TAILQ_ENTRY NAME);

         void
         TAILQ_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm,
                 TAILQ_ENTRY NAME);

         void
         TAILQ_INSERT_HEAD(TAILQ_HEAD *head, struct TYPE *elm, TAILQ_ENTRY NAME);

         void
         TAILQ_INSERT_TAIL(TAILQ_HEAD *head, struct TYPE *elm, TAILQ_ENTRY NAME);

         void
         TAILQ_REMOVE(TAILQ_HEAD *head, struct TYPE *elm, TAILQ_ENTRY NAME);

         void
         TAILQ_REPLACE(TAILQ_HEAD *head, struct TYPE *elm, struct TYPE *elm2,
                 TAILQ_ENTRY NAME);

         CIRCLEQ_ENTRY(TYPE);

         CIRCLEQ_HEAD(HEADNAME, TYPE);

         CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD head);

         struct TYPE *
         CIRCLEQ_FIRST(CIRCLEQ_HEAD *head);

         struct TYPE *
         CIRCLEQ_LAST(CIRCLEQ_HEAD *head);

         struct TYPE *
         CIRCLEQ_END(CIRCLEQ_HEAD *head);

         struct TYPE *
         CIRCLEQ_NEXT(struct TYPE *listelm, CIRCLEQ_ENTRY NAME);

         struct TYPE *
         CIRCLEQ_PREV(struct TYPE *listelm, CIRCLEQ_ENTRY NAME);

         int
         CIRCLEQ_EMPTY(CIRCLEQ_HEAD *head);

         CIRCLEQ_FOREACH(VARNAME, CIRCLEQ_HEAD *head, CIRCLEQ_ENTRY NAME);

         CIRCLEQ_FOREACH_REVERSE(VARNAME, CIRCLEQ_HEAD *head, CIRCLEQ_ENTRY NAME);

         void
         CIRCLEQ_INIT(CIRCLEQ_HEAD *head);

         void
         CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *head, struct TYPE *listelm,
                 struct TYPE *elm, CIRCLEQ_ENTRY NAME);

         void
         CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *head, struct TYPE *listelm,
                 struct TYPE *elm, CIRCLEQ_ENTRY NAME);

         void
         CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head, struct TYPE *elm,
                 CIRCLEQ_ENTRY NAME);

         void
         CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head, struct TYPE *elm,
                 CIRCLEQ_ENTRY NAME);

         void
         CIRCLEQ_REMOVE(CIRCLEQ_HEAD *head, struct TYPE *elm, CIRCLEQ_ENTRY NAME);

         void
         CIRCLEQ_REPLACE(CIRCLEQ_HEAD *head, struct TYPE *elm, struct TYPE *elm2,
                 CIRCLEQ_ENTRY NAME);

    DESCRIPTION
         These macros define and operate on five types of data structures: singly-
         linked lists, simple queues, lists, tail queues, and circular queues.
         All five structures support the following functionality:

               1.   Insertion of a new entry at the head of the list.
               2.   Insertion of a new entry after any element in the list.
               3.   Removal of an entry from the head of the list.
               4.   Forward traversal through the list.

         Singly-linked lists are the simplest of the five data structures and sup-
         port only the above functionality.  Singly-linked lists are ideal for ap-
         plications with large datasets and few or no removals, or for implement-
         ing a LIFO queue.

         Simple queues add the following functionality:

               1.   Entries can be added at the end of a list.

         However:

               1.   All list insertions must specify the head of the list.
               2.   Each head entry requires two pointers rather than one.
               3.   Code size is about 15% greater and operations run about 20%
                    slower than singly-linked lists.

         Simple queues are ideal for applications with large datasets and few or
         no removals, or for implementing a FIFO queue.

         All doubly linked types of data structures (lists, tail queues, and cir-
         cle queues) additionally allow:

               1.   Insertion of a new entry before any element in the list.
               2.   Removal of any entry in the list.

         However:

               1.   Each element requires two pointers rather than one.
               2.   Code size and execution time of operations (except for re-
                    moval) is about twice that of the singly-linked data-struc-
                    tures.

         Lists are the simplest of the doubly linked data structures and support
         only the above functionality over singly-linked lists.

         Tail queues add the following functionality:

               1.   Entries can be added at the end of a list.
               2.   They may be traversed backwards, at a cost.

         However:

               1.   All list insertions and removals must specify the head of the
                    list.
               2.   Each head entry requires two pointers rather than one.
               3.   Code size is about 15% greater and operations run about 20%
                    slower than singly-linked lists.

         Circular queues add the following functionality:

               1.   Entries can be added at the end of a list.
               2.   They may be traversed backwards, from tail to head.

         However:

               1.   All list insertions and removals must specify the head of the
                    list.
               2.   Each head entry requires two pointers rather than one.
               3.   The termination condition for traversal is more complex.
               4.   Code size is about 40% greater and operations run about 45%
                    slower than lists.

         In the macro definitions, TYPE is the name tag of a user defined struc-
         ture that must contain a field of type SLIST_ENTRY, LIST_ENTRY,
         SIMPLEQ_ENTRY, TAILQ_ENTRY, or CIRCLEQ_ENTRY, named NAME.  The argument
         HEADNAME is the name tag of a user defined structure that must be de-
         clared using the macros SLIST_HEAD(), LIST_HEAD(), SIMPLEQ_HEAD(),
         TAILQ_HEAD(), or CIRCLEQ_HEAD().  See the examples below for further ex-
         planation of how these macros are used.

    SINGLY-LINKED LISTS
         A singly-linked list is headed by a structure defined by the SLIST_HEAD()
         macro.  This structure contains a single pointer to the first element on
         the list.  The elements are singly linked for minimum space and pointer
         manipulation overhead at the expense of O(n) removal for arbitrary ele-
         ments.  New elements can be added to the list after an existing element
         or at the head of the list.  A SLIST_HEAD structure is declared as fol-
         lows:

               SLIST_HEAD(HEADNAME, TYPE) head;

         where HEADNAME is the name of the structure to be defined, and struct
         TYPE is the type of the elements to be linked into the list.  A pointer
         to the head of the list can later be declared as:

               struct HEADNAME *headp;

         (The names head and headp are user selectable.)

         The HEADNAME facility is often not used, leading to the following bizarre
         code:

               SLIST_HEAD(, TYPE) head, *headp;

         The SLIST_ENTRY() macro declares a structure that connects the elements
         in the list.

         The SLIST_INIT() macro initializes the list referenced by head.

         The list can also be initialized statically by using the
         SLIST_HEAD_INITIALIZER() macro like this:

               SLIST_HEAD(HEADNAME, TYPE) head = SLIST_HEAD_INITIALIZER(head);

         The SLIST_INSERT_HEAD() macro inserts the new element elm at the head of
         the list.

         The SLIST_INSERT_AFTER() macro inserts the new element elm after the ele-
         ment listelm.

         The SLIST_REMOVE_HEAD() macro removes the first element of the list
         pointed by head.

         The SLIST_REMOVE_NEXT() macro removes the list element immediately fol-
         lowing elm.

         The SLIST_REMOVE() macro removes the element elm of the list pointed by
         head.

         The SLIST_FIRST() and SLIST_NEXT() macros can be used to traverse the
         list:

               for (np = SLIST_FIRST(&head); np != NULL; np = SLIST_NEXT(np, NAME))

         Or, for simplicity, one can use the SLIST_FOREACH() macro:

               SLIST_FOREACH(np, head, NAME)

         The SLIST_FOREACH_PREVPTR() macro is similar to SLIST_FOREACH() except
         that it stores a pointer to the previous element in VARNAMEP.  This pro-
         vides access to the previous element while traversing the list, as one
         would have with a doubly-linked list.

         The SLIST_EMPTY() macro should be used to check whether a simple list is
         empty.

    SINGLY-LINKED LIST EXAMPLE
         SLIST_HEAD(listhead, entry) head;
         struct entry {
                 ...
                 SLIST_ENTRY(entry) entries;     /* Simple list. */
                 ...
         } *n1, *n2, *np;

         SLIST_INIT(&head);                      /* Initialize simple list. */

         n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
         SLIST_INSERT_HEAD(&head, n1, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert after. */
         SLIST_INSERT_AFTER(n1, n2, entries);

         SLIST_FOREACH(np, &head, entries)       /* Forward traversal. */
                 np-> ...

         while (!SLIST_EMPTY(&head)) {           /* Delete. */
                 n1 = SLIST_FIRST(&head);
                 SLIST_REMOVE_HEAD(&head, entries);
                 free(n1);
         }

    LISTS
         A list is headed by a structure defined by the LIST_HEAD() macro.  This
         structure contains a single pointer to the first element on the list.
         The elements are doubly linked so that an arbitrary element can be re-
         moved without traversing the list.  New elements can be added to the list
         after an existing element, before an existing element, or at the head of
         the list.  A LIST_HEAD structure is declared as follows:

               LIST_HEAD(HEADNAME, TYPE) head;

         where HEADNAME is the name of the structure to be defined, and struct
         TYPE is the type of the elements to be linked into the list.  A pointer
         to the head of the list can later be declared as:

               struct HEADNAME *headp;

         (The names head and headp are user selectable.)

         The HEADNAME facility is often not used, leading to the following bizarre
         code:

               LIST_HEAD(, TYPE) head, *headp;

         The LIST_ENTRY() macro declares a structure that connects the elements in
         the list.

         The LIST_INIT() macro initializes the list referenced by head.

         The list can also be initialized statically by using the
         LIST_HEAD_INITIALIZER() macro like this:

               LIST_HEAD(HEADNAME, TYPE) head = LIST_HEAD_INITIALIZER(head);

         The LIST_INSERT_HEAD() macro inserts the new element elm at the head of
         the list.

         The LIST_INSERT_AFTER() macro inserts the new element elm after the ele-
         ment listelm.

         The LIST_INSERT_BEFORE() macro inserts the new element elm before the el-
         ement listelm.

         The LIST_REMOVE() macro removes the element elm from the list.

         The LIST_REPLACE() macro replaces the list element elm with the new ele-
         ment elm2.

         The LIST_FIRST() and LIST_NEXT() macros can be used to traverse the list:

               for (np = LIST_FIRST(&head); np != NULL; np = LIST_NEXT(np, NAME))

         Or, for simplicity, one can use the LIST_FOREACH() macro:

               LIST_FOREACH(np, head, NAME)

         The LIST_EMPTY() macro should be used to check whether a list is empty.

    LIST EXAMPLE
         LIST_HEAD(listhead, entry) head;
         struct entry {
                 ...
                 LIST_ENTRY(entry) entries;      /* List. */
                 ...
         } *n1, *n2, *np;

         LIST_INIT(&head);                       /* Initialize list. */

         n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
         LIST_INSERT_HEAD(&head, n1, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert after. */
         LIST_INSERT_AFTER(n1, n2, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert before. */
         LIST_INSERT_BEFORE(n1, n2, entries);
                                                 /* Forward traversal. */
         LIST_FOREACH(np, &head, entries)
                 np-> ...

         while (!LIST_EMPTY(&head))              /* Delete. */
                 n1 = LIST_FIRST(&head);
                 LIST_REMOVE(n1, entries);
                 free(n1);
         }

    SIMPLE QUEUES
         A simple queue is headed by a structure defined by the SIMPLEQ_HEAD()
         macro.  This structure contains a pair of pointers, one to the first ele-
         ment in the simple queue and the other to the last element in the simple
         queue.  The elements are singly linked.  New elements can be added to the
         queue after an existing element, at the head of the queue or at the tail
         of the queue.  A SIMPLEQ_HEAD structure is declared as follows:

               SIMPLEQ_HEAD(HEADNAME, TYPE) head;

         where HEADNAME is the name of the structure to be defined, and struct
         TYPE is the type of the elements to be linked into the queue.  A pointer
         to the head of the queue can later be declared as:

               struct HEADNAME *headp;

         (The names head and headp are user selectable.)

         The SIMPLEQ_ENTRY() macro declares a structure that connects the elements
         in the queue.

         The SIMPLEQ_INIT() macro initializes the queue referenced by head.

         The queue can also be initialized statically by using the
         SIMPLEQ_HEAD_INITIALIZER() macro like this:

               SIMPLEQ_HEAD(HEADNAME, TYPE) head = SIMPLEQ_HEAD_INITIALIZER(head);

         The SIMPLEQ_INSERT_HEAD() macro inserts the new element elm at the head
         of the queue.

         The SIMPLEQ_INSERT_TAIL() macro inserts the new element elm at the end of
         the queue.

         The SIMPLEQ_INSERT_AFTER() macro inserts the new element elm after the
         element listelm.

         The SIMPLEQ_REMOVE_HEAD() macro removes the first element from the queue.

         The SIMPLEQ_FIRST() and SIMPLEQ_NEXT() macros can be used to traverse the
         queue.  The SIMPLEQ_FOREACH() is used for queue traversal:

               SIMPLEQ_FOREACH(np, head, NAME)

         The SIMPLEQ_EMPTY() macro should be used to check whether a list is emp-
         ty.

    SIMPLE QUEUE EXAMPLE
         SIMPLEQ_HEAD(listhead, entry) head = SIMPLEQ_HEAD_INITIALIZER(head);
         struct entry {
                 ...
                 SIMPLEQ_ENTRY(entry) entries;   /* Simple queue. */
                 ...
         } *n1, *n2, *np;

         n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
         SIMPLEQ_INSERT_HEAD(&head, n1, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert after. */
         SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert at the tail. */
         SIMPLEQ_INSERT_TAIL(&head, n2, entries);
                                                 /* Forward traversal. */
         SIMPLEQ_FOREACH(np, &head, entries)
                 np-> ...
                                                 /* Delete. */
         while (!SIMPLEQ_EMPTY(&head)) {
                 n1 = SIMPLEQ_FIRST(&head);
                 SIMPLEQ_REMOVE_HEAD(&head, entries);
                 free(n1);
         }

    TAIL QUEUES
         A tail queue is headed by a structure defined by the TAILQ_HEAD() macro.
         This structure contains a pair of pointers, one to the first element in
         the tail queue and the other to the last element in the tail queue.  The
         elements are doubly linked so that an arbitrary element can be removed
         without traversing the tail queue.  New elements can be added to the
         queue after an existing element, before an existing element, at the head
         of the queue, or at the end of the queue.  A TAILQ_HEAD structure is de-
         clared as follows:

               TAILQ_HEAD(HEADNAME, TYPE) head;

         where HEADNAME is the name of the structure to be defined, and struct
         TYPE is the type of the elements to be linked into the tail queue.  A
         pointer to the head of the tail queue can later be declared as:

               struct HEADNAME *headp;

         (The names head and headp are user selectable.)

         The TAILQ_ENTRY() macro declares a structure that connects the elements
         in the tail queue.

         The TAILQ_INIT() macro initializes the tail queue referenced by head.

         The tail queue can also be initialized statically by using the
         TAILQ_HEAD_INITIALIZER() macro.

         The TAILQ_INSERT_HEAD() macro inserts the new element elm at the head of
         the tail queue.

         The TAILQ_INSERT_TAIL() macro inserts the new element elm at the end of
         the tail queue.

         The TAILQ_INSERT_AFTER() macro inserts the new element elm after the ele-
         ment listelm.

         The TAILQ_INSERT_BEFORE() macro inserts the new element elm before the
         element listelm.

         The TAILQ_REMOVE() macro removes the element elm from the tail queue.

         The TAILQ_REPLACE() macro replaces the list element elm with the new ele-
         ment elm2.

         TAILQ_FOREACH() and TAILQ_FOREACH_REVERSE() are used for traversing a
         tail queue.  TAILQ_FOREACH() starts at the first element and proceeds to-
         wards the last.  TAILQ_FOREACH_REVERSE() starts at the last element and
         proceeds towards the first.

               TAILQ_FOREACH(np, &head, NAME)
               TAILQ_FOREACH_REVERSE(np, &head, HEADNAME, NAME)

         The TAILQ_FIRST(), TAILQ_NEXT(), TAILQ_LAST() and TAILQ_PREV() macros can
         be used to manually traverse a tail queue or an arbitrary part of one.

         The TAILQ_EMPTY() macro should be used to check whether a tail queue is
         empty.

    TAIL QUEUE EXAMPLE
         TAILQ_HEAD(tailhead, entry) head;
         struct entry {
                 ...
                 TAILQ_ENTRY(entry) entries;     /* Tail queue. */
                 ...
         } *n1, *n2, *np;

         TAILQ_INIT(&head);                      /* Initialize queue. */

         n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
         TAILQ_INSERT_HEAD(&head, n1, entries);

         n1 = malloc(sizeof(struct entry));      /* Insert at the tail. */
         TAILQ_INSERT_TAIL(&head, n1, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert after. */
         TAILQ_INSERT_AFTER(&head, n1, n2, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert before. */
         TAILQ_INSERT_BEFORE(n1, n2, entries);
                                                 /* Forward traversal. */
         TAILQ_FOREACH(np, &head, entries)
                 np-> ...
                                                 /* Manual forward traversal. */
         for (np = n2; np != NULL; np = TAILQ_NEXT(np, entries))
                 np-> ...
                                                 /* Delete. */
         while (np = TAILQ_FIRST(&head)) {
                 TAILQ_REMOVE(&head, np, entries);
                 free(np);
         }

    CIRCULAR QUEUES
         A circular queue is headed by a structure defined by the CIRCLEQ_HEAD()
         macro.  This structure contains a pair of pointers, one to the first ele-
         ment in the circular queue and the other to the last element in the cir-
         cular queue.  The elements are doubly linked so that an arbitrary element
         can be removed without traversing the queue.  New elements can be added
         to the queue after an existing element, before an existing element, at
         the head of the queue, or at the end of the queue.  A CIRCLEQ_HEAD struc-
         ture is declared as follows:

               CIRCLEQ_HEAD(HEADNAME, TYPE) head;

         where HEADNAME is the name of the structure to be defined, and struct
         TYPE is the type of the elements to be linked into the circular queue.  A
         pointer to the head of the circular queue can later be declared as:

               struct HEADNAME *headp;

         (The names head and headp are user selectable.)

         The CIRCLEQ_ENTRY() macro declares a structure that connects the elements
         in the circular queue.

         The CIRCLEQ_INIT() macro initializes the circular queue referenced by
         head.

         The circular queue can also be initialized statically by using the
         CIRCLEQ_HEAD_INITIALIZER() macro.

         The CIRCLEQ_INSERT_HEAD() macro inserts the new element elm at the head
         of the circular queue.

         The CIRCLEQ_INSERT_TAIL() macro inserts the new element elm at the end of
         the circular queue.

         The CIRCLEQ_INSERT_AFTER() macro inserts the new element elm after the
         element listelm.

         The CIRCLEQ_INSERT_BEFORE() macro inserts the new element elm before the
         element listelm.

         The CIRCLEQ_REMOVE() macro removes the element elm from the circular
         queue.

         The CIRCLEQ_REPLACE() macro replaces the list element elm with the new
         element elm2.

         The CIRCLEQ_FIRST(), CIRCLEQ_LAST(), CIRCLEQ_END(), CIRCLEQ_NEXT() and
         CIRCLEQ_PREV() macros can be used to traverse a circular queue.  The
         CIRCLEQ_FOREACH() is used for circular queue forward traversal:

               CIRCLEQ_FOREACH(np, head, NAME)

         The CIRCLEQ_FOREACH_REVERSE() macro acts like CIRCLEQ_FOREACH() but tra-
         verses the circular queue backwards.

         The CIRCLEQ_EMPTY() macro should be used to check whether a circular
         queue is empty.

    CIRCULAR QUEUE EXAMPLE
         CIRCLEQ_HEAD(circleq, entry) head;
         struct entry {
                 ...
                 CIRCLEQ_ENTRY(entry) entries;   /* Circular queue. */
                 ...
         } *n1, *n2, *np;

         CIRCLEQ_INIT(&head);                    /* Initialize circular queue. */

         n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
         CIRCLEQ_INSERT_HEAD(&head, n1, entries);

         n1 = malloc(sizeof(struct entry));      /* Insert at the tail. */
         CIRCLEQ_INSERT_TAIL(&head, n1, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert after. */
         CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);

         n2 = malloc(sizeof(struct entry));      /* Insert before. */
         CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
                                                 /* Forward traversal. */
         CIRCLEQ_FOREACH(np, &head, entries)
                 np-> ...
                                                 /* Reverse traversal. */
         CIRCLEQ_FOREACH_REVERSE(np, &head, entries)
                 np-> ...
                                                 /* Delete. */
         while (!CIRCLEQ_EMPTY(&head)) {
                 n1 = CIRCLEQ_FIRST(&head);
                 CIRCLEQ_REMOVE(&head, n1, entries);
                 free(n1);
         }

    NOTES
         It is an error to assume the next and previous fields are preserved after
         an element has been removed from a list or queue.  Using any macro (ex-
         cept the various forms of insertion) on an element removed from a list or
         queue is incorrect.  An example of erroneous usage is removing the same
         element twice.

         The SLIST_END(), LIST_END(), SIMPLEQ_END() and TAILQ_END() macros are
         provided for symmetry with CIRCLEQ_END().  They expand to NULL and don't
         serve any useful purpose.

         Trying to free a list in the following way is a common error:

               LIST_FOREACH(var, head, entry)
                       free(var);
               free(head);

         Since var is free'd, the FOREACH() macro refers to a pointer that may
         have been reallocated already.  Proper code needs a second variable.

               for (var = LIST_FIRST(head); var != LIST_END(head); var = nxt) {
                       nxt = LIST_NEXT(var, entry);
                       free(var);
               }
               LIST_INIT(head);        /* to put the list back in order */

         A similar situation occurs when the current element is deleted from the
         list.  Correct code saves a pointer to the next element in the list be-
         fore removing the element:

               for (var = LIST_FIRST(head); var != LIST_END(head); var = nxt) {
                       nxt = LIST_NEXT(var, entry);
                       if (some_condition) {
                               LIST_REMOVE(var, entry);
                               some_function(var);
                       }
               }

    HISTORY
         The queue functions first appeared in 4.4BSD.

    OpenBSD 4.8                      March 1, 2009                              13

NAME | SYNOPSIS | DESCRIPTION | SINGLY-LINKED LISTS | SINGLY-LINKED LIST | LISTS | LIST EXAMPLE | SIMPLE QUEUES | SIMPLE QUEUE | TAIL QUEUES | TAIL QUEUE | CIRCULAR QUEUES | CIRCULAR QUEUE | NOTES | HISTORY