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

NAME
       LIST_ENTRY,   LIST_HEAD,   LIST_INIT,  LIST_INSERT_AFTER,  LIST_INSERT_HEAD,  LIST_REMOVE,
       TAILQ_ENTRY,    TAILQ_HEAD,     TAILQ_INIT,     TAILQ_INSERT_AFTER,     TAILQ_INSERT_HEAD,
       TAILQ_INSERT_TAIL,   TAILQ_REMOVE,   CIRCLEQ_ENTRY,   CIRCLEQ_HEAD,   CIRCLEQ_INIT,   CIR-
       CLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE, CIRCLEQ_INSERT_HEAD,  CIRCLEQ_INSERT_TAIL,  CIR-
       CLEQ_REMOVE - implementations of lists, tail queues, and circular queues

SYNOPSIS
       #include <sys/queue.h>

       LIST_ENTRY(TYPE);
       LIST_HEAD(HEADNAME, TYPE);
       LIST_INIT(LIST_HEAD *head);
       LIST_INSERT_AFTER(LIST_ENTRY *listelm,
		       TYPE *elm, LIST_ENTRY NAME);
       LIST_INSERT_HEAD(LIST_HEAD *head,
		       TYPE *elm, LIST_ENTRY NAME);
       LIST_REMOVE(TYPE *elm, LIST_ENTRY NAME);

       TAILQ_ENTRY(TYPE);
       TAILQ_HEAD(HEADNAME, TYPE);
       TAILQ_INIT(TAILQ_HEAD *head);
       TAILQ_INSERT_AFTER(TAILQ_HEAD *head, TYPE *listelm,
		       TYPE *elm, TAILQ_ENTRY NAME);
       TAILQ_INSERT_HEAD(TAILQ_HEAD *head,
		       TYPE *elm, TAILQ_ENTRY NAME);
       TAILQ_INSERT_TAIL(TAILQ_HEAD *head,
		       TYPE *elm, TAILQ_ENTRY NAME);
       TAILQ_REMOVE(TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);

       CIRCLEQ_ENTRY(TYPE);
       CIRCLEQ_HEAD(HEADNAME, TYPE);
       CIRCLEQ_INIT(CIRCLEQ_HEAD *head);
       CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *head, TYPE *listelm,
		       TYPE *elm, CIRCLEQ_ENTRY NAME);
       CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *head, TYPE *listelm,
		       TYPE *elm, CIRCLEQ_ENTRY NAME);
       CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head,
		       TYPE *elm, CIRCLEQ_ENTRY NAME);
       CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head,
		       TYPE *elm, CIRCLEQ_ENTRY NAME);
       CIRCLEQ_REMOVE(CIRCLEQ_HEAD *head,
		       TYPE *elm, CIRCLEQ_ENTRY NAME);

DESCRIPTION
       These macros define and operate on three types of data structures: lists, tail queues, and
       circular queues.  All three structures support the following functionality:

	   *   Insertion of a new entry at the head of the list.
	   *   Insertion of a new entry after any element in the list.
	   *   Removal of any entry in the list.
	   *   Forward traversal through the list.

       Lists are the simplest of the three data structures and support only the above functional-
       ity.

       Tail queues add the following functionality:

	   *   Entries can be added at the end of a list.

       However:

	   1.  All list insertions and removals must specify the head of the list.
	   2.  Each head entry requires two pointers rather than one.
	   3.  Code size is about 15% greater and operations run about 20% slower than lists.

       Circular queues add the following functionality:

	   *   Entries can be added at the end of a list.
	   *   Entries can be added before another entry.
	   *   They may be traversed backward, from tail to head.

       However:

	   1.  All list insertions and removals must specify the head of the list.
	   2.  Each head entry requires two pointers rather than one.
	   3.  The termination condition for traversal is more complex.
	   4.  Code size is about 40% greater and operations run about 45% slower than lists.

       In  the macro definitions, TYPE is the name of a user-defined structure, that must contain
       a field of type LIST_ENTRY, TAILQ_ENTRY, or CIRCLEQ_ENTRY, named NAME.  The argument HEAD-
       NAME  is  the  name  of	a  user-defined  structure that must be declared using the macros
       LIST_HEAD, TAILQ_HEAD, or CIRCLEQ_HEAD.	See the examples below for further explanation of
       how these macros are used.

   Lists
       A list is headed by a structure defined by the LIST_HEAD macro.	This structure contains a
       single pointer to the first element on the list.  The elements are doubly linked  so  that
       an  arbitrary  element  can  be	removed without traversing the list.  New elements can be
       added to the list after an existing element or at the  head  of	the  list.   A	LIST_HEAD
       structure is declared as follows:

	   LIST_HEAD(HEADNAME, TYPE) head;

       where  HEADNAME	is  the  name of the structure to be defined, and TYPE is the type of the
       elements to be linked into the list.  A pointer to the head  of	the  list  can	later  be
       declared as:

	   struct HEADNAME *headp;

       (The names head and headp are user selectable.)

       The macro LIST_ENTRY declares a structure that connects the elements in the list.

       The macro LIST_INIT initializes the list referenced by head.

       The macro LIST_INSERT_HEAD inserts the new element elm at the head of the list.

       The macro LIST_INSERT_AFTER inserts the new element elm after the element listelm.

       The macro LIST_REMOVE removes the element elm from the list.

   List example
       LIST_HEAD(listhead, entry) head;
       struct listhead *headp;		       /* List head. */
       struct entry {
	   ...
	   LIST_ENTRY(entry) entries;	       /* List. */
	   ...
       } *n1, *n2, *np;

       LIST_INIT(&head);		       /* Initialize the list. */

       n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
       LIST_INSERT_HEAD(&head, n1, entries);

       n2 = malloc(sizeof(struct entry));      /* Insert after. */
       LIST_INSERT_AFTER(n1, n2, entries);
					       /* Forward traversal. */
       for (np = head.lh_first; np != NULL; np = np->entries.le_next)
	   np-> ...

       while (head.lh_first != NULL)	       /* Delete. */
	   LIST_REMOVE(head.lh_first, entries);

   Tail queues
       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 tail queue and the	other  to
       the  last  element in the tail queue.  The elements are doubly linked so that an arbitrary
       element can be removed without traversing the tail queue.  New elements can  be	added  to
       the  tail queue after an existing element, at the head of the tail queue, or at the end of
       the tail queue.	A TAILQ_HEAD structure is declared as follows:

	   TAILQ_HEAD(HEADNAME, TYPE) head;

       where HEADNAME is the name of the structure to be defined, and TYPE is  the  type  of  the
       elements  to  be  linked into the tail queue.  A pointer to the head of the tail queue can
       later be declared as:

	   struct HEADNAME *headp;

       (The names head and headp are user selectable.)

       The macro TAILQ_ENTRY declares a structure that connects the elements in the tail queue.

       The macro TAILQ_INIT initializes the tail queue referenced by head.

       The macro TAILQ_INSERT_HEAD inserts the new element elm at the head of the tail queue.

       The macro TAILQ_INSERT_TAIL inserts the new element elm at the end of the tail queue.

       The macro TAILQ_INSERT_AFTER inserts the new element elm after the element listelm.

       The macro TAILQ_REMOVE removes the element elm from the tail queue.

   Tail queue example
       TAILQ_HEAD(tailhead, entry) head;
       struct tailhead *headp;		       /* Tail queue head. */
       struct entry {
	   ...
	   TAILQ_ENTRY(entry) entries;	       /* Tail queue. */
	   ...
       } *n1, *n2, *np;

       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);
					       /* Forward traversal. */
       for (np = head.tqh_first; np != NULL; np = np->entries.tqe_next)
	   np-> ...
					       /* Delete. */
       while (head.tqh_first != NULL)
	   TAILQ_REMOVE(&head, head.tqh_first, entries);

   Circular queues
       A circular queue is headed by a structure defined by the CIRCLEQ_HEAD macro.  This  struc-
       ture  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 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  HEADNAME	is  the  name of the structure to be defined, and 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_ENTRY  declares a structure that connects the elements in the circular
       queue.

       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_REMOVE removes the element elm from the circular queue.

   Circular queue example
       CIRCLEQ_HEAD(circleq, entry) head;
       struct circleq *headp;		   /* Circular queue head. */
       struct entry {
	   ...
	   CIRCLEQ_ENTRY(entry) entries;   /* Circular queue. */
	   ...
       } *n1, *n2, *np;

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

       n2 = malloc(sizeof(struct entry));  /* Insert before. */
       CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
					   /* Forward traversal. */
       for (np = head.cqh_first; np != (void *)&head;
	       np = np->entries.cqe_next)
	   np-> ...
					   /* Reverse traversal. */
       for (np = head.cqh_last; np != (void *)&head; np = np->entries.cqe_prev)
	   np-> ...
					   /* Delete. */
       while (head.cqh_first != (void *)&head)
	   CIRCLEQ_REMOVE(&head, head.cqh_first, entries);

CONFORMING TO
       Not in POSIX.1-2001.  Present on the BSDs.  The queue functions first appeared in 4.4BSD.

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux					    2007-12-28					 QUEUE(3)
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