# circleq

## 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);
           }
    


* * *