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_IN‐
       SERT_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>

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

       int TAILQ_EMPTY(TAILQ_HEAD *head);

       TAILQ_ENTRY(TYPE);

       struct TYPE *TAILQ_FIRST(TAILQ_HEAD *head);

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

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

       TAILQ_HEAD(HEADNAME, TYPE);

       TAILQ_HEAD TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);

       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);

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

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

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

       void TAILQ_REMOVE(TAILQ_HEAD *head, struct TYPE *elm, 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().

       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 tail queue after an existing element,
       before an existing element, at the head of the tail queue, or at the
       end of the tail 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 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_HEAD_INITIALIZER() evaluates to an initializer for
       the tail queue head.

       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.

       The macro TAILQ_EMPTY() evaluates to true if there are no items on
       the tail queue.

       The macro TAILQ_ENTRY() declares a structure that connects the ele‐
       ments in the tail queue.

       The macro TAILQ_FIRST() returns the first item on the tail queue or
       NULL if the tail queue is empty.

       The macro TAILQ_FOREACH() traverses the tail 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.

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

       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
       element listelm.

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

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

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

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

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_NEXT(), and TAILQ_PREV() return a
       pointer to the first, last, next or previous 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

       The macros 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.  The macros 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

       This page is part of release 5.09 of the Linux man-pages project.  A
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       latest version of this page, can be found at
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GNU                              2020-10-21                         TAILQ(3)

Pages that refer to this page: queue(3)queue(7)