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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,            CIRCLEQ_INSERT_AFTER,
       CIRCLEQ_INSERT_BEFORE,     CIRCLEQ_INSERT_HEAD,    CIRCLEQ_INSERT_TAIL,
       CIRCLEQ_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 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 backwards, 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 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 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  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
       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 3.15 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/.