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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, CIR‐
       CLEQ_HEAD, CIRCLEQ_INIT,	 CIRCLEQ_INSERT_AFTER,	CIRCLEQ_INSERT_BEFORE,
       CIRCLEQ_INSERT_HEAD,  CIRCLEQ_INSERT_TAIL, CIRCLEQ_REMOVE - implementa‐
       tions 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 functionality.

       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  CIR‐
       CLEQ_ENTRY,  named  NAME.  The argument HEADNAME 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 expla‐
       nation 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 ele‐
       ment 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 ele‐
       ment 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 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 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 CIR‐
       CLEQ_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 3.54 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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