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, 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 3.25 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)