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NAME

       TAILQ_CONCAT, TAILQ_EMPTY, TAILQ_ENTRY, TAILQ_FIRST, TAILQ_FOREACH, TAILQ_FOREACH_REVERSE,
       TAILQ_HEAD, TAILQ_HEAD_INITIALIZER, TAILQ_INIT,  TAILQ_INSERT_AFTER,  TAILQ_INSERT_BEFORE,
       TAILQ_INSERT_HEAD,  TAILQ_INSERT_TAIL,  TAILQ_LAST, TAILQ_NEXT, TAILQ_PREV, TAILQ_REMOVE -
       implementation of a doubly linked tail queue

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/queue.h>

       TAILQ_ENTRY(TYPE);

       TAILQ_HEAD(HEADNAME, TYPE);
       TAILQ_HEAD TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);
       void TAILQ_INIT(TAILQ_HEAD *head);

       int TAILQ_EMPTY(TAILQ_HEAD *head);

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

       struct TYPE *TAILQ_FIRST(TAILQ_HEAD *head);
       struct TYPE *TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);
       struct TYPE *TAILQ_PREV(struct TYPE *elm, HEADNAME, TAILQ_ENTRY NAME);
       struct TYPE *TAILQ_NEXT(struct TYPE *elm, TAILQ_ENTRY NAME);

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

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

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

DESCRIPTION

       These macros define and operate on doubly linked tail queues.

       In the macro definitions, TYPE is the name of a user defined structure, that must  contain
       a  field  of  type  TAILQ_ENTRY,  named NAME.  The argument HEADNAME is the name of a user
       defined structure that must be declared using the macro TAILQ_HEAD().

   Creation
       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 queue and the other to the
       last element in the 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 TAILQ_HEAD structure is declared as follows:

           TAILQ_HEAD(HEADNAME, TYPE) head;

       where  struct  HEADNAME is 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.)

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

       TAILQ_HEAD_INITIALIZER() evaluates to an initializer for the queue head.

       TAILQ_INIT() initializes the queue referenced by

       TAILQ_EMPTY() evaluates to true if there are no items on the queue.  head.

   Insertion
       TAILQ_INSERT_HEAD() inserts the new element elm at the head of the queue.

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

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

       TAILQ_INSERT_AFTER() inserts the new element elm after the element listelm.

   Traversal
       TAILQ_FIRST() returns the first item on the queue, or NULL if the queue is empty.

       TAILQ_LAST()  returns  the last item on the queue.  If the queue is empty the return value
       is NULL.

       TAILQ_PREV() returns the previous item on the queue, or NULL if this item is the first.

       TAILQ_NEXT() returns the next item on the queue, or NULL if this item is the last.

       TAILQ_FOREACH() traverses the queue referenced by head in the forward direction, assigning
       each  element  in  turn  to var.  var is set to NULL if the loop completes normally, or if
       there were no elements.

       TAILQ_FOREACH_REVERSE() traverses the queue referenced by head in the  reverse  direction,
       assigning each element in turn to var.

   Removal
       TAILQ_REMOVE() removes the element elm from the queue.

   Other features
       TAILQ_CONCAT()  concatenates  the  queue headed by head2 onto the end of the one headed by
       head1 removing all entries from the former.

RETURN VALUE

       TAILQ_EMPTY() returns nonzero if the queue is empty, and zero if  the  queue  contains  at
       least one entry.

       TAILQ_FIRST(), TAILQ_LAST(), TAILQ_PREV(), and TAILQ_NEXT() return a pointer to the first,
       last, previous, or next TYPE structure, respectively.

       TAILQ_HEAD_INITIALIZER() returns an initializer that can be assigned to the queue head.

STANDARDS

       Not in POSIX.1, POSIX.1-2001, or POSIX.1-2008.  Present on  the  BSDs.   (TAILQ  functions
       first appeared in 4.4BSD).

BUGS

       TAILQ_FOREACH()  and TAILQ_FOREACH_REVERSE() don't allow var to be removed or freed within
       the  loop,  as  it  would  interfere  with  the   traversal.    TAILQ_FOREACH_SAFE()   and
       TAILQ_FOREACH_REVERSE_SAFE(),  which are present on the BSDs but are not present in glibc,
       fix this limitation by allowing var to safely be removed from  the  list  and  freed  from
       within the loop without interfering with the traversal.

EXAMPLES

       #include <stddef.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/queue.h>

       struct entry {
           int data;
           TAILQ_ENTRY(entry) entries;             /* Tail queue */
       };

       TAILQ_HEAD(tailhead, entry);

       int
       main(void)
       {
           struct entry *n1, *n2, *n3, *np;
           struct tailhead head;                   /* Tail queue head */
           int i;

           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);

           n3 = malloc(sizeof(struct entry));      /* Insert before */
           TAILQ_INSERT_BEFORE(n2, n3, entries);

           TAILQ_REMOVE(&head, n2, entries);       /* Deletion */
           free(n2);
                                                   /* Forward traversal */
           i = 0;
           TAILQ_FOREACH(np, &head, entries)
               np->data = i++;
                                                   /* Reverse traversal */
           TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries)
               printf("%i\n", np->data);
                                                   /* TailQ deletion */
           n1 = TAILQ_FIRST(&head);
           while (n1 != NULL) {
               n2 = TAILQ_NEXT(n1, entries);
               free(n1);
               n1 = n2;
           }
           TAILQ_INIT(&head);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       insque(3), queue(7)