circleq(3) — Linux manual page

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

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

NAME         top

       CIRCLEQ_EMPTY, CIRCLEQ_ENTRY, CIRCLEQ_FIRST, CIRCLEQ_FOREACH,
       CIRCLEQ_FOREACH_REVERSE, CIRCLEQ_HEAD, CIRCLEQ_HEAD_INITIALIZER,
       CIRCLEQ_INIT, CIRCLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE,
       CIRCLEQ_INSERT_HEAD, CIRCLEQ_INSERT_TAIL, CIRCLEQ_LAST,
       CIRCLEQ_LOOP_NEXT, CIRCLEQ_LOOP_PREV, CIRCLEQ_NEXT, CIRCLEQ_PREV,
       CIRCLEQ_REMOVE - implementation of a doubly linked circular queue

SYNOPSIS         top

       #include <sys/queue.h>

       CIRCLEQ_ENTRY(TYPE);

       CIRCLEQ_HEAD(HEADNAME, TYPE);
       CIRCLEQ_HEAD CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD head);
       void CIRCLEQ_INIT(CIRCLEQ_HEAD *head);

       int CIRCLEQ_EMPTY(CIRCLEQ_HEAD *head);

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

       struct TYPE *CIRCLEQ_FIRST(CIRCLEQ_HEAD *head);
       struct TYPE *CIRCLEQ_LAST(CIRCLEQ_HEAD *head);
       struct TYPE *CIRCLEQ_PREV(struct TYPE *elm, CIRCLEQ_ENTRY NAME);
       struct TYPE *CIRCLEQ_NEXT(struct TYPE *elm, CIRCLEQ_ENTRY NAME);
       struct TYPE *CIRCLEQ_LOOP_PREV(CIRCLEQ_HEAD *head,
                                  struct TYPE *elm, CIRCLEQ_ENTRY NAME);
       struct TYPE *CIRCLEQ_LOOP_NEXT(CIRCLEQ_HEAD *head,
                                  struct TYPE *elm, CIRCLEQ_ENTRY NAME);

       CIRCLEQ_FOREACH(struct TYPE *var, CIRCLEQ_HEAD *head,
                                  CIRCLEQ_ENTRY NAME);
       CIRCLEQ_FOREACH_REVERSE(struct TYPE *var, CIRCLEQ_HEAD *head,
                                  CIRCLEQ_ENTRY NAME);

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

DESCRIPTION         top

       These macros define and operate on doubly linked circular queues.

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

   Creation
       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 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 CIRCLEQ_HEAD structure is declared as
       follows:

           CIRCLEQ_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.)

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

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

       CIRCLEQ_INIT() initializes the queue referenced by head.

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

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

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

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

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

   Traversal
       CIRCLEQ_FIRST() returns the first item on the queue.

       CIRCLEQ_LAST() returns the last item on the queue.

       CIRCLEQ_PREV() returns the previous item on the queue, or &head
       if this item is the first one.

       CIRCLEQ_NEXT() returns the next item on the queue, or &head if
       this item is the last one.

       CIRCLEQ_LOOP_PREV() returns the previous item on the queue.  If
       elm is the first element on the queue, the last element is
       returned.

       CIRCLEQ_LOOP_NEXT() returns the next item on the queue.  If elm
       is the last element on the queue, the first element is returned.

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

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

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

RETURN VALUE         top

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

       CIRCLEQ_FIRST(), CIRCLEQ_LAST(), CIRCLEQ_LOOP_PREV(), and
       CIRCLEQ_LOOP_NEXT() return a pointer to the first, last,
       previous, or next TYPE structure, respectively.

       CIRCLEQ_PREV(), and CIRCLEQ_NEXT() are similar to their
       CIRCLEQ_LOOP_*() counterparts, except that if the argument is the
       first or last element, respectively, they return &head.

       CIRCLEQ_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 (CIRCLEQ macros first appeared in 4.4BSD).

BUGS         top

       CIRCLEQ_FOREACH() and CIRCLEQ_FOREACH_REVERSE() don't allow var
       to be removed or freed within the loop, as it would interfere
       with the traversal.  CIRCLEQ_FOREACH_SAFE() and
       CIRCLEQ_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;
           CIRCLEQ_ENTRY(entry) entries;           /* Queue */
       };

       CIRCLEQ_HEAD(circlehead, entry);

       int
       main(void)
       {
           struct entry *n1, *n2, *n3, *np;
           struct circlehead head;                 /* Queue head */
           int i;

           CIRCLEQ_INIT(&head);                    /* Initialize the 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);

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

           CIRCLEQ_REMOVE(&head, n2, entries);     /* Deletion */
           free(n2);
                                                   /* Forward traversal */
           i = 0;
           CIRCLEQ_FOREACH(np, &head, entries)
               np->data = i++;
                                                   /* Reverse traversal */
           CIRCLEQ_FOREACH_REVERSE(np, &head, entries)
               printf("%i\n", np->data);
                                                   /* Queue deletion */
           n1 = CIRCLEQ_FIRST(&head);
           while (n1 != (void *)&head) {
               n2 = CIRCLEQ_NEXT(n1, entries);
               free(n1);
               n1 = n2;
           }
           CIRCLEQ_INIT(&head);

           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       insque(3), queue(7)

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

GNU                            2021-03-22                     CIRCLEQ(3)

Pages that refer to this page: queue(7)