tailq(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | BUGS | EXAMPLES | SEE ALSO | COLOPHON

TAILQ(3)                Linux Programmer's Manual               TAILQ(3)

NAME         top

       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

SYNOPSIS         top

       #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         top

       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         top

       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.

CONFORMING TO         top

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

BUGS         top

       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         top

       #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         top

       insque(3), queue(7)

COLOPHON         top

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GNU                            2021-03-22                       TAILQ(3)

Pages that refer to this page: queue(7)