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, CIR‐
       CLEQ_FOREACH_REVERSE,  CIRCLEQ_HEAD,  CIRCLEQ_HEAD_INITIALIZER,  CIR‐
       CLEQ_INIT,  CIRCLEQ_INSERT_AFTER,  CIRCLEQ_INSERT_BEFORE, CIRCLEQ_IN‐
       SERT_HEAD, CIRCLEQ_INSERT_TAIL, CIRCLEQ_LAST, CIRCLEQ_LOOP_NEXT, CIR‐
       CLEQ_LOOP_PREV,  CIRCLEQ_NEXT,  CIRCLEQ_PREV, CIRCLEQ_REMOVE - imple‐
       mentation of a doubly linked circular queue

SYNOPSIS         top

       #include <sys/queue.h>

       int CIRCLEQ_EMPTY(CIRCLEQ_HEAD *head);

       CIRCLEQ_ENTRY(TYPE);

       struct TYPE *CIRCLEQ_FIRST(CIRCLEQ_HEAD *head);

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

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

       CIRCLEQ_HEAD(HEADNAME, TYPE);

       CIRCLEQ_HEAD CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD head);

       void CIRCLEQ_INIT(CIRCLEQ_HEAD *head);

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

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

       struct TYPE *CIRCLEQ_LAST(CIRCLEQ_HEAD *head);

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

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

       struct TYPE *CIRCLEQ_NEXT(struct TYPE *elm, CIRCLEQ_ENTRY NAME);

       struct TYPE *CIRCLEQ_PREV(struct TYPE *elm, 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().

       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 circular queue and the other to the
       last element in the circular queue.  The elements are doubly linked
       so that an arbitrary element can be removed without traversing the
       circular queue.  New elements can be added to the circular queue
       after an existing element, before an existing element, at the head of
       the circular queue, or at the end of the circular 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 circular queue.  A
       pointer to the head of the circular queue can later be declared as:

           struct HEADNAME *headp;

       (The names head and headp are user selectable.)

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

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

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

       The macro CIRCLEQ_FIRST() returns the first item on the circular
       queue.

       The macro CIRCLEQ_FOREACH() traverses the circular 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.

       The macro CIRCLEQ_FOREACH_REVERSE() traverses the circular queue ref‐
       erenced by head in the reverse direction, assigning each element in
       turn to var.

       The macro CIRCLEQ_INIT() initializes the circular queue referenced by
       head.

       The macro CIRCLEQ_INSERT_HEAD() inserts the new element elm at the
       head of the circular queue.

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

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

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

       The macro CIRCLEQ_LAST() returns the last item on the circular queue.

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

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

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

       The macro CIRCLEQ_LOOP_PREV() returns the previous item on the circu‐
       lar queue.  If elm is the first element on the circular queue, the
       last element is returned.

       The macro CIRCLEQ_REMOVE() removes the element elm from the circular
       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_NEXT(), and CIRCLEQ_PREV()
       return a pointer to the first, last, next or previous TYPE structure,
       respectively.

       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

       The macros 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.  The macros 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

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       latest version of this page, can be found at
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GNU                              2020-10-21                       CIRCLEQ(3)

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