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NAME

       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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

       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

       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.

STANDARDS

       BSD.

BUGS

       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

       #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

       insque(3), queue(7)