QUEUE(3)                NetBSD Library Functions Manual               QUEUE(3)

NAME
     SLIST_HEAD, SLIST_HEAD_INITIALIZER, SLIST_ENTRY, SLIST_FIRST,
     SLIST_EMPTY, SLIST_NEXT, SLIST_FOREACH, SLIST_FOREACH_SAFE, SLIST_INIT,
     SLIST_INSERT_AFTER, SLIST_INSERT_HEAD, SLIST_REMOVE_AFTER,
     SLIST_REMOVE_HEAD, SLIST_REMOVE, LIST_HEAD, LIST_HEAD_INITIALIZER,
     LIST_ENTRY, LIST_FIRST, LIST_EMPTY, LIST_NEXT, LIST_FOREACH,
     LIST_FOREACH_SAFE, LIST_INIT, LIST_INSERT_AFTER, LIST_INSERT_BEFORE,
     LIST_INSERT_HEAD, LIST_REMOVE, LIST_REPLACE, LIST_MOVE, SIMPLEQ_HEAD,
     SIMPLEQ_HEAD_INITIALIZER, SIMPLEQ_ENTRY, SIMPLEQ_FIRST, SIMPLEQ_EMPTY,
     SIMPLEQ_NEXT, SIMPLEQ_LAST, SIMPLEQ_FOREACH, SIMPLEQ_FOREACH_SAFE,
     SIMPLEQ_INIT, SIMPLEQ_INSERT_AFTER, SIMPLEQ_INSERT_HEAD,
     SIMPLEQ_INSERT_TAIL, SIMPLEQ_REMOVE_AFTER, SIMPLEQ_REMOVE_HEAD,
     SIMPLEQ_REMOVE, SIMPLEQ_CONCAT, TAILQ_HEAD, TAILQ_HEAD_INITIALIZER,
     TAILQ_ENTRY, TAILQ_FIRST, TAILQ_NEXT, TAILQ_LAST, TAILQ_PREV,
     TAILQ_EMPTY, TAILQ_FOREACH, TAILQ_FOREACH_SAFE, TAILQ_FOREACH_REVERSE,
     TAILQ_FOREACH_REVERSE_SAFE, TAILQ_INIT, TAILQ_INSERT_AFTER,
     TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD, TAILQ_INSERT_TAIL, TAILQ_REMOVE,
     TAILQ_REPLACE, TAILQ_CONCAT, STAILQ_HEAD, STAILQ_HEAD_INITIALIZER,
     STAILQ_ENTRY, STAILQ_FIRST, STAILQ_EMPTY, STAILQ_NEXT, STAILQ_LAST,
     STAILQ_FOREACH, STAILQ_FOREACH_SAFE, STAILQ_INIT, STAILQ_INSERT_AFTER,
     STAILQ_INSERT_HEAD, STAILQ_INSERT_TAIL, STAILQ_REMOVE_HEAD,
     STAILQ_REMOVE, STAILQ_CONCAT, -- implementations of singly-linked lists,
     lists, simple queues, tail queues, and singly-linked tail queues

SYNOPSIS
     #include <sys/queue.h>

     SLIST_HEAD(HEADNAME, TYPE);

     SLIST_HEAD_INITIALIZER(head);

     SLIST_ENTRY(TYPE);

     TYPE *
     SLIST_FIRST(SLIST_HEAD *head);

     int
     SLIST_EMPTY(SLIST_HEAD *head);

     TYPE *
     SLIST_NEXT(TYPE *elm, SLIST_ENTRY NAME);

     SLIST_FOREACH(TYPE *var, SLIST_HEAD *head, SLIST_ENTRY NAME);

     SLIST_FOREACH_SAFE(TYPE *var, SLIST_HEAD *head, SLIST_ENTRY NAME,
         TYPE *tmp);

     SLIST_INIT(SLIST_HEAD *head);

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

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

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

     SLIST_REMOVE_HEAD(SLIST_HEAD *head, SLIST_ENTRY NAME);

     LIST_HEAD(HEADNAME, TYPE);

     LIST_HEAD_INITIALIZER(head);

     LIST_ENTRY(TYPE);

     TYPE *
     LIST_FIRST(LIST_HEAD *head);

     TYPE *
     LIST_NEXT(TYPE *elm, LIST_ENTRY NAME);

     int
     LIST_EMPTY(LIST_HEAD *head);

     LIST_FOREACH(TYPE *var, LIST_HEAD *head, LIST_ENTRY NAME);

     LIST_FOREACH_SAFE(TYPE *var, LIST_HEAD *head, LIST_ENTRY NAME,
         TYPE *tmp);

     LIST_INIT(LIST_HEAD *head);

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

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

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

     LIST_REMOVE(TYPE *elm, LIST_ENTRY NAME);

     LIST_REPLACE(TYPE *elm, TYPE *new, LIST_ENTRY NAME);

     LIST_MOVE(LIST_HEAD *head1, LIST_HEAD *head2);

     SIMPLEQ_HEAD(HEADNAME, TYPE);

     SIMPLEQ_HEAD_INITIALIZER(head);

     SIMPLEQ_ENTRY(TYPE);

     TYPE *
     SIMPLEQ_FIRST(SIMPLEQ_HEAD *head);

     int
     SIMPLEQ_EMPTY(SIMPLEQ_HEAD *head);

     TYPE *
     SIMPLEQ_NEXT(TYPE *elm, SIMPLEQ_ENTRY NAME);

     TYPE *
     SIMPLEQ_LAST(SIMPLEQ_HEAD *head, TYPE *elm, SIMPLEQ_ENTRY NAME);

     SIMPLEQ_FOREACH(TYPE *var, SIMPLEQ_HEAD *head, SIMPLEQ_ENTRY NAME);

     SIMPLEQ_FOREACH_SAFE(TYPE *var, SIMPLEQ_HEAD *head, SIMPLEQ_ENTRY NAME,
         TYPE *tmp);

     SIMPLEQ_INIT(SIMPLEQ_HEAD *head);

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

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

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

     SIMPLEQ_REMOVE_HEAD(SIMPLEQ_HEAD *head, SIMPLEQ_ENTRY NAME);

     SIMPLEQ_REMOVE_AFTER(SIMPLEQ_HEAD *head, TYPE *elm, SIMPLEQ_ENTRY NAME);

     SIMPLEQ_REMOVE(SIMPLEQ_HEAD *head, TYPE *elm, TYPE, SIMPLEQ_ENTRY NAME);

     SIMPLEQ_CONCAT(SIMPLEQ_HEAD *head1, SIMPLEQ_HEAD *head2);

     TAILQ_HEAD(HEADNAME, TYPE);

     TAILQ_HEAD_INITIALIZER(head);

     TAILQ_ENTRY(TYPE);

     TYPE *
     TAILQ_FIRST(TAILQ_HEAD *head);

     TYPE *
     TAILQ_NEXT(TYPE *elm, TAILQ_ENTRY NAME);

     TYPE *
     TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);

     TYPE *
     TAILQ_PREV(TYPE *elm, HEADNAME, TAILQ_ENTRY NAME);

     int
     TAILQ_EMPTY(TAILQ_HEAD *head);

     TAILQ_FOREACH(TYPE *var, TAILQ_HEAD *head, TAILQ_ENTRY NAME);

     TAILQ_FOREACH_SAFE(TYPE *var, TAILQ_HEAD *head, TAILQ_ENTRY NAME,
         TYPE *tmp);

     TAILQ_FOREACH_REVERSE(TYPE *var, TAILQ_HEAD *head, HEADNAME,
         TAILQ_ENTRY NAME);

     TAILQ_FOREACH_REVERSE_SAFE(TYPE *var, TAILQ_HEAD *head, HEADNAME,
         TAILQ_ENTRY NAME, TYPE *tmp);

     TAILQ_INIT(TAILQ_HEAD *head);

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

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

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

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

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

     TAILQ_REPLACE(TAILQ_HEAD *head, TYPE *elm, TYPE *new, TAILQ_ENTRY NAME);

     TAILQ_CONCAT(TAILQ_HEAD *head1, TAILQ_HEAD *head2, TAILQ_ENTRY NAME);

     STAILQ_HEAD(HEADNAME, TYPE);

     STAILQ_HEAD_INITIALIZER(head);

     STAILQ_ENTRY(TYPE);

     TYPE *
     STAILQ_FIRST(STAILQ_HEAD *head);

     int
     STAILQ_EMPTY(STAILQ_HEAD *head);

     TYPE *
     STAILQ_NEXT(TYPE *elm, STAILQ_ENTRY NAME);

     TYPE *
     STAILQ_LAST(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);

     STAILQ_FOREACH(TYPE *var, STAILQ_HEAD *head, STAILQ_ENTRY NAME);

     STAILQ_FOREACH_SAFE(TYPE *var, STAILQ_HEAD *head, STAILQ_ENTRY NAME,
         TYPE *tmp);

     STAILQ_INIT(STAILQ_HEAD *head);

     STAILQ_INSERT_HEAD(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);

     STAILQ_INSERT_TAIL(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);

     STAILQ_INSERT_AFTER(STAILQ_HEAD *head, TYPE *listelm, TYPE *elm,
         STAILQ_ENTRY NAME);

     STAILQ_REMOVE_HEAD(STAILQ_HEAD *head, STAILQ_ENTRY NAME);

     STAILQ_REMOVE(STAILQ_HEAD *head, TYPE *elm, TYPE, STAILQ_ENTRY NAME);

     STAILQ_CONCAT(STAILQ_HEAD *head1, STAILQ_HEAD *head2);

DESCRIPTION
     These macros define and operate on five types of data structures: singly-
     linked lists, simple queues, lists, tail queues, and singly-linked tail
     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 before or after any element in the
                list.
           3.   Removal of any entry in the list.
           4.   Forward traversal through the list.

     Singly-linked lists are the simplest of the four data structures and sup-
     port only the above functionality.  Singly-linked lists are ideal for
     applications with large datasets and few or no removals, or for imple-
     menting a LIFO queue.

     Simple queues add the following functionality:
           1.   Entries can be added at the end of a list.
           2.   They may be concatenated.
     However:
           1.   Entries may not be added before any element in the list.
           2.   All list insertions and removals must specify the head of the
                list.
           3.   Each head entry requires two pointers rather than one.

     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 and tail queues) addi-
     tionally allow:
           1.   Insertion of a new entry before any element in the list.
           2.   O(1) 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
                removal) is about twice that of the singly-linked data-struc-
                tures.

     Linked 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 concatenated.
     However:
           1.   All list insertions and removals, except insertion before
                another element, 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 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 of a user defined structure,
     that must contain a field of type SLIST_ENTRY, LIST_ENTRY, SIMPLEQ_ENTRY,
     TAILQ_ENTRY, or STAILQ_ENTRY, named NAME.  The argument HEADNAME is the
     name of a user defined structure that must be declared using the macros
     LIST_HEAD, SIMPLEQ_HEAD, SLIST_HEAD, or TAILQ_HEAD, See the examples
     below for further explanation of how these macros are used.

   Summary of Operations
     The following table summarizes the supported macros for each type of data
     structure.

     box tab(:); l | c | c | c | c | c l | c | c | c | c | c l | c | c | c | c
     | c l | c | c | c | c | c l | c | c | c | c | c l | c | c | c | c | c.
     :SLIST:LIST:SIMPLEQ:TAILQ:STAILQ _ _FIRST:+:+:+:+:+ _EMPTY:+:+:+:+:+
     _NEXT:+:+:+:+:+ _PREV:-:-:-:+:- _LAST:-:-:+:+:+ _FOREACH:+:+:+:+:+ _FORE-
     ACH_SAFE:+:+:+:+:+ _FOREACH_REVERSE:-:-:-:+:- _FORE-
     ACH_REVERSE_SAFE:-:-:-:+:- _INSERT_HEAD:+:+:+:+:+ _INSERT_AFTER:+:+:+:+:+
     _INSERT_BEFORE:-:+:-:+:- _INSERT_TAIL:-:-:+:+:+ _REMOVE:+:+:+:+:+
     _REMOVE_HEAD:+:-:+:-:+ _REMOVE_AFTER:-:-:+:-:+ _REPLACE:-:+:-:+:- _CON-
     CAT:-:-:+:+:+

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.  An 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 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 macro SLIST_HEAD_INITIALIZER evaluates to an initializer for the list
     head.

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

     The macro SLIST_FIRST returns the first element in the list or NULL if
     the list is empty.

     The macro

     The macro SLIST_EMPTY evaluates to true if there are no elements in the
     list.

     The macro SLIST_NEXT returns the next element in the list.

     SLIST_FOREACH traverses the list referenced by head in the forward direc-
     tion, assigning each element in turn to var.

     The SAFE version uses tmp to hold the next element, so var may be freed
     or removed from the list.

     The macro SLIST_INIT initializes the list referenced by head.

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

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

     The macro SLIST_REMOVE removes the element elm from the list.

     The macro SLIST_REMOVE_HEAD removes the first element from the head of
     the list.  For optimum efficiency, elements being removed from the head
     of the list should explicitly use this macro instead of the generic
     SLIST_REMOVE macro.

     The macros SLIST_REMOVE_AFTER removes the element after the one speci-
     fied.  For optimum efficiency, elements being removed after a specified
     one should explicitly use this macro instead of the generic SLIST_REMOVE

SINGLY-LINKED LIST EXAMPLE
     SLIST_HEAD(slisthead, entry) head =
         SLIST_HEAD_INITIALIZER(head);
     struct slisthead *headp;                /* Singly-linked List head. */
     struct entry {
             ...
             SLIST_ENTRY(entry) entries;     /* Singly-linked List. */
             ...
     } *n1, *n2, *n3, *np;

     SLIST_INIT(&head);                      /* Initialize the 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_REMOVE(&head, n2, entry, entries);/* Deletion. */
     free(n2);

     n3 = SLIST_FIRST(&head);
     SLIST_REMOVE_HEAD(&head, entries);      /* Deletion from the head. */
     free(n3);
                                             /* Forward traversal. */
     SLIST_FOREACH(np, &head, entries)
             np-> ...

     while (!SLIST_EMPTY(&head)) {           /* List Deletion. */
             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
     removed 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 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 macro LIST_ENTRY declares a structure that connects the elements in
     the list.

     The macro LIST_HEAD_INITIALIZER provides a value which can be used to
     initialize a list head at compile time, and is used at the point that the
     list head variable is declared, like:

           struct HEADNAME head = LIST_HEAD_INITIALIZER(head);

     The macro LIST_FIRST returns the first element of the list head.

     The macro LIST_EMPTY return true if the list head has no elements.

     The macro LIST_NEXT returns the element after the element elm.

     The macro LIST_FOREACH traverses the list referenced by head in the for-
     ward direction, assigning each element in turn to var.

     The SAFE version uses tmp to hold the next element, so var may be freed
     or removed from the list.

     The macro LIST_INIT initializes the list referenced by head.

     The macro LIST_INSERT_AFTER inserts the new element elm after the element
     listelm.

     The macro LIST_INSERT_BEFORE inserts the new element elm before the ele-
     ment listelm.

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

     The macro LIST_REMOVE removes the element elm from the list.

     The macro LIST_REPLACE replaces the element elm with new in the list.

     The macro LIST_MOVE moves the list headed by head1 onto the list headed
     by head2, always making the former empty.

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

     LIST_INIT(&head);                       /* Initialize the 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-> ...
                                             /* Delete. */
     while (LIST_FIRST(&head) != NULL)
             LIST_REMOVE(LIST_FIRST(&head), entries);
     if (LIST_EMPTY(&head))                  /* Test for emptiness. */
             printf("nothing to do\n");

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 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 queue after an existing element,
     at the head of the queue, or at the end 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 TYPE is
     the type of the elements to be linked into the simple queue.  A pointer
     to the head of the simple queue can later be declared as:

           struct HEADNAME *headp;

     (The names head and headp are user selectable.)

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

     The macro SIMPLEQ_HEAD_INITIALIZER provides a value which can be used to
     initialize a simple queue head at compile time, and is used at the point
     that the simple queue head variable is declared, like:

           struct HEADNAME head = SIMPLEQ_HEAD_INITIALIZER(head);

     The macro SIMPLEQ_FIRST returns the first element of the simple queue
     head.

     The macro SIMPLEQ_EMPTY return true if the simple queue head has no ele-
     ments.

     The macro SIMPLEQ_NEXT returns the element after the element elm.

     The macro SIMPLEQ_LAST returns the last item on the tail queue.  If the
     tail queue is empty the return value is NULL.

     The macro SIMPLEQ_FOREACH traverses the tail queue referenced by head in
     the forward direction, assigning each element in turn to var.

     The SAFE version uses tmp to hold the next element, so var may be freed
     or removed from the list.

     The macro SIMPLEQ_INIT initializes the simple queue referenced by head.

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

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

     The macro SIMPLEQ_INSERT_AFTER inserts the new element elm after the ele-
     ment listelm.

     The macro SIMPLEQ_REMOVE_HEAD removes the first element from the head of
     the simple queue.  For optimum efficiency, elements being removed from
     the head of the queue should explicitly use this macro instead of the
     generic SIMPLQ_REMOVE macro.

     The macro SIMPLEQ_REMOVE_AFTER removes the element after the one speci-
     fied from the simple queue.  For optimum efficiency, elements being
     removed after specified elements should explicitly use this macro instead
     of the generic SIMPLQ_REMOVE macro.

     The macro SIMPLEQ_REMOVE removes elm from the simple queue.

     The macro SIMPLEQ_CONCAT concatenates the tail queue headed by head2 onto
     the end of the one headed by head1 removing all entries from the former.

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

     SIMPLEQ_INIT(&head);                    /* Initialize the queue. */

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

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

     n2 = malloc(sizeof(struct entry));      /* Insert after. */
     SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);
                                             /* Forward traversal. */
     SIMPLEQ_FOREACH(np, &head, entries)
             np-> ...
                                             /* Delete. */
     while (SIMPLEQ_FIRST(&head) != NULL)
             SIMPLEQ_REMOVE_HEAD(&head, entries);
     if (SIMPLEQ_EMPTY(&head))               /* Test for emptiness. */
             printf("nothing to do\n");

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 the queue.  A TAILQ_HEAD structure is
     declared as follows:

           TAILQ_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name of the structure to be defined, and 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 macro TAILQ_ENTRY declares a structure that connects the elements in
     the tail queue.

     The macro TAILQ_HEAD_INITIALIZER provides a value which can be used to
     initialize a tail queue head at compile time, and is used at the point
     that the tail queue head variable is declared, like:

           struct HEADNAME head = TAILQ_HEAD_INITIALIZER(head);

     The macro TAILQ_FIRST returns the first element of the tail queue head.

     The macro TAILQ_NEXT returns the element after the element elm.

     The macro TAILQ_LAST returns the last item on the tail queue.  If the
     tail queue is empty the return value is NULL.

     The macro TAILQ_PREV returns the previous item on the tail queue, from
     the one specified.  If the tail queue is empty the return value is NULL.

     The macro TAILQ_EMPTY return true if the tail queue head has no elements.

     The macros TAILQ_FOREACH, TAILQ_FOREACH_REVERSE, TAILQ_FOREACH_SAFE, and
     TAILQ_FOREACH_REVERSE_SAFE traverse the tail queue referenced by head in
     the forward or reverse direction direction, assigning each element in
     turn to var.

     The SAFE versions use tmp to hold the next element, so var may be freed
     or removed from the list.

     The macro TAILQ_INIT initializes the tail queue referenced by head.

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

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

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

     The macro TAILQ_INSERT_BEFORE inserts the new element elm before the ele-
     ment listelm.

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

     The macro TAILQ_REPLACE replace the element elm with the new one speci-
     fied in the tail queue.

     The macro TAILQ_CONCAT concatenates the tail queue headed by head2 onto
     the end of the one headed by head1 removing all entries from the former.

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

     TAILQ_INIT(&head);                      /* Initialize the 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-> ...
                                             /* Reverse traversal. */
     TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries)
             np-> ...
                                             /* Delete. */
     while (TAILQ_FIRST(&head) != NULL)
             TAILQ_REMOVE(&head, TAILQ_FIRST(&head), entries);
     if (TAILQ_EMPTY(&head))                 /* Test for emptiness. */
             printf("nothing to do\n");

SINGLY LINKED TAIL QUEUES
     The macros prefixed with ``STAILQ_'' (STAILQ_HEAD,
     STAILQ_HEAD_INITIALIZER, STAILQ_ENTRY, STAILQ_FOREACH,
     STAILQ_FOREACH_SAFE, STAILQ_FIRST, STAILQ_EMPTY, STAILQ_NEXT,
     STAILQ_LAST, STAILQ_INIT, STAILQ_INSERT_HEAD, STAILQ_INSERT_TAIL,
     STAILQ_INSERT_AFTER, STAILQ_REMOVE_HEAD, STAILQ_REMOVE, and
     STAILQ_CONCAT) are functionally identical to these simple queue func-
     tions, and are provided for compatibility with FreeBSD.

NOTES
     Some of these macros or functions perform no error checking, and invalid
     usage leads to undefined behaviour.  In the case of macros or functions
     that expect their arguments to be elements that are present in the list
     or queue, passing an element that is not present is invalid.

HISTORY
     The queue functions first appeared in 4.4BSD.  The SIMPLEQ functions
     first appeared in NetBSD 1.2.  The SLIST and STAILQ functions first
     appeared in FreeBSD 2.1.5.

NetBSD 7.0                       May 17, 2014                       NetBSD 7.0

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