# tailq

## NAME



           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_INSERT_HEAD, TAILQ_INSERT_TAIL,
           TAILQ_LAST, TAILQ_NEXT, TAILQ_PREV, TAILQ_REMOVE - implementation
           of a doubly linked tail queue
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/queue.h>**
    
           **TAILQ_ENTRY(TYPE);**
    
           **TAILQ_HEAD(HEADNAME, TYPE);**
           **TAILQ_HEAD TAILQ_HEAD_INITIALIZER(TAILQ_HEAD** _head_**);**
           **void TAILQ_INIT(TAILQ_HEAD ***_head_**);**
    
           **int TAILQ_EMPTY(TAILQ_HEAD ***_head_**);**
    
           **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_**);**
           **void TAILQ_INSERT_BEFORE(struct TYPE ***_listelm_**,**
                                    **struct TYPE ***_elm_**, TAILQ_ENTRY** _NAME_**);**
           **void TAILQ_INSERT_AFTER(TAILQ_HEAD ***_head_**, struct TYPE ***_listelm_**,**
                                    **struct TYPE ***_elm_**, TAILQ_ENTRY** _NAME_**);**
    
           **struct TYPE *TAILQ_FIRST(TAILQ_HEAD ***_head_**);**
           **struct TYPE *TAILQ_LAST(TAILQ_HEAD ***_head_**, HEADNAME);**
           **struct TYPE *TAILQ_PREV(struct TYPE ***_elm_**, HEADNAME, TAILQ_ENTRY** _NAME_**);**
           **struct TYPE *TAILQ_NEXT(struct TYPE ***_elm_**, TAILQ_ENTRY** _NAME_**);**
    
           **TAILQ_FOREACH(struct TYPE ***_var_**, TAILQ_HEAD ***_head_**,**
                                    **TAILQ_ENTRY** _NAME_**);**
           **TAILQ_FOREACH_REVERSE(struct TYPE ***_var_**, TAILQ_HEAD ***_head_**, HEADNAME,**
                                    **TAILQ_ENTRY** _NAME_**);**
    
           **void TAILQ_REMOVE(TAILQ_HEAD ***_head_**, struct TYPE ***_elm_**,**
                                    **TAILQ_ENTRY** _NAME_**);**
    
           **void TAILQ_CONCAT(TAILQ_HEAD ***_head1_**, TAILQ_HEAD ***_head2_**,**
                                    **TAILQ_ENTRY** _NAME_**);**
    


## DESCRIPTION



           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**().
    
       **Creation**
           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 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 _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 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.)
    
           **TAILQ_ENTRY**() declares a structure that connects the elements in
           the queue.
    
           **TAILQ_HEAD_INITIALIZER**() evaluates to an initializer for the queue
           _head_.
    
           **TAILQ_INIT**() initializes the queue referenced by
    
           **TAILQ_EMPTY**() evaluates to true if there are no items on the
           queue.  _head_.
    
       **Insertion**
           **TAILQ_INSERT_HEAD**() inserts the new element _elm_ at the head of the
           queue.
    
           **TAILQ_INSERT_TAIL**() inserts the new element _elm_ at the end of the
           queue.
    
           **TAILQ_INSERT_BEFORE**() inserts the new element _elm_ before the
           element _listelm_.
    
           **TAILQ_INSERT_AFTER**() inserts the new element _elm_ after the element
           _listelm_.
    
       **Traversal**
           **TAILQ_FIRST**() returns the first item on the queue, or NULL if the
           queue is empty.
    
           **TAILQ_LAST**() returns the last item on the queue.  If the queue is
           empty the return value is NULL.
    
           **TAILQ_PREV**() returns the previous item on the queue, or NULL if
           this item is the first.
    
           **TAILQ_NEXT**() returns the next item on the queue, or NULL if this
           item is the last.
    
           **TAILQ_FOREACH**() traverses the 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.
    
           **TAILQ_FOREACH_REVERSE**() traverses the queue referenced by _head_ in
           the reverse direction, assigning each element in turn to _var_.
    
       **Removal**
           **TAILQ_REMOVE**() removes the element _elm_ from the queue.
    
       **Other features**
           **TAILQ_CONCAT**() concatenates the queue headed by _head2_ onto the end
           of the one headed by _head1_ removing all entries from the former.
    


## RETURN VALUE



           **TAILQ_EMPTY**() returns nonzero if the queue is empty, and zero if
           the queue contains at least one entry.
    
           **TAILQ_FIRST**(), **TAILQ_LAST**(), **TAILQ_PREV**(), and **TAILQ_NEXT**() return
           a pointer to the first, last, previous, or next _TYPE_ structure,
           respectively.
    
           **TAILQ_HEAD_INITIALIZER**() returns an initializer that can be
           assigned to the queue _head_.
    


## STANDARDS



           BSD.
    


## CAVEATS



           **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.  **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



           #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);
           }
    


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