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QUEUE(3)

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NAME    [Toc]    [Back]

     SLIST_ENTRY,       SLIST_HEAD,       SLIST_HEAD_INITIALIZER,
SLIST_FIRST, SLIST_NEXT,
     SLIST_END,            SLIST_EMPTY,            SLIST_FOREACH,
SLIST_FOREACH_PREVPTR, SLIST_INIT,
     SLIST_INSERT_AFTER, SLIST_INSERT_HEAD, SLIST_REMOVE_HEAD,
     SLIST_REMOVE_NEXT, SLIST_REMOVE, LIST_ENTRY, LIST_HEAD,
     LIST_HEAD_INITIALIZER,  LIST_FIRST,   LIST_NEXT,   LIST_END,
LIST_EMPTY,
     LIST_FOREACH,          LIST_INIT,         LIST_INSERT_AFTER,
LIST_INSERT_BEFORE,
     LIST_INSERT_HEAD, LIST_REMOVE, LIST_REPLACE,  SIMPLEQ_ENTRY,
SIMPLEQ_HEAD,
     SIMPLEQ_HEAD_INITIALIZER,    SIMPLEQ_FIRST,    SIMPLEQ_NEXT,
SIMPLEQ_END,
     SIMPLEQ_EMPTY,        SIMPLEQ_FOREACH,         SIMPLEQ_INIT,
SIMPLEQ_INSERT_HEAD,
     SIMPLEQ_INSERT_TAIL,                   SIMPLEQ_INSERT_AFTER,
SIMPLEQ_REMOVE_HEAD,
     TAILQ_ENTRY,       TAILQ_HEAD,       TAILQ_HEAD_INITIALIZER,
TAILQ_FIRST, TAILQ_NEXT,
     TAILQ_END,      TAILQ_LAST,     TAILQ_PREV,     TAILQ_EMPTY,
TAILQ_FOREACH,
     TAILQ_FOREACH_REVERSE, TAILQ_INIT, TAILQ_INSERT_AFTER,
     TAILQ_INSERT_BEFORE,  TAILQ_INSERT_HEAD,  TAILQ_INSERT_TAIL,
TAILQ_REMOVE,
     CIRCLEQ_ENTRY,    CIRCLEQ_HEAD,    CIRCLEQ_HEAD_INITIALIZER,
CIRCLEQ_FIRST,
     CIRCLEQ_LAST,   CIRCLEQ_END,   CIRCLEQ_NEXT,   CIRCLEQ_PREV,
CIRCLEQ_EMPTY,
     CIRCLEQ_FOREACH, CIRCLEQ_FOREACH_REVERSE, CIRCLEQ_INIT,
     CIRCLEQ_INSERT_AFTER,                 CIRCLEQ_INSERT_BEFORE,
CIRCLEQ_INSERT_HEAD,
     CIRCLEQ_INSERT_TAIL,  CIRCLEQ_REMOVE  -  implementations  of
singly-linked
     lists,  doubly-linked lists, simple queues, tail queues, and
circular
     queues

SYNOPSIS    [Toc]    [Back]

     #include <sys/queue.h>

     SLIST_ENTRY(TYPE);

     SLIST_HEAD(HEADNAME, TYPE);

     SLIST_HEAD_INITIALIZER(SLIST_HEAD head);

     struct TYPE *
     SLIST_FIRST(SLIST_HEAD *head);

     struct TYPE *
     SLIST_NEXT(struct TYPE *listelm, SLIST_ENTRY NAME);

     struct TYPE *
     SLIST_END(SLIST_HEAD *head);

     bool
     SLIST_EMPTY(SLIST_HEAD *head);

     SLIST_FOREACH(VARNAME, SLIST_HEAD *head, SLIST_ENTRY NAME);

     SLIST_FOREACH_PREVPTR(VARNAME, VARNAMEP, SLIST_HEAD *head,
             SLIST_ENTRY NAME);

     void
     SLIST_INIT(SLIST_HEAD *head);

     void
     SLIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm,
             SLIST_ENTRY NAME);

     void
     SLIST_INSERT_HEAD(SLIST_HEAD  *head,   struct   TYPE   *elm,
SLIST_ENTRY NAME);

     void
     SLIST_REMOVE_HEAD(SLIST_HEAD *head, SLIST_ENTRY NAME);

     void
     SLIST_REMOVE_NEXT(SLIST_HEAD   *head,   struct   TYPE  *elm,
SLIST_ENTRY NAME);

     void
     SLIST_REMOVE(SLIST_HEAD  *head,  struct  TYPE  *elm,   TYPE,
SLIST_ENTRY NAME);

     LIST_ENTRY(TYPE);

     LIST_HEAD(HEADNAME, TYPE);

     LIST_HEAD_INITIALIZER(LIST_HEAD head);

     struct TYPE *
     LIST_FIRST(LIST_HEAD *head);

     struct TYPE *
     LIST_NEXT(struct TYPE *listelm, LIST_ENTRY NAME);

     struct TYPE *
     LIST_END(LIST_HEAD *head);

     bool
     LIST_EMPTY(LIST_HEAD *head);

     LIST_FOREACH(VARNAME, LIST_HEAD *head, LIST_ENTRY NAME);

     void
     LIST_INIT(LIST_HEAD *head);

     void
     LIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm,
             LIST_ENTRY NAME);

     void
     LIST_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm,
             LIST_ENTRY NAME);

     void
     LIST_INSERT_HEAD(LIST_HEAD    *head,   struct   TYPE   *elm,
LIST_ENTRY NAME);

     void
     LIST_REMOVE(struct TYPE *elm, LIST_ENTRY NAME);

     void
     LIST_REPLACE(struct TYPE *elm, struct TYPE *elm2, LIST_ENTRY
NAME);

     SIMPLEQ_ENTRY(TYPE);

     SIMPLEQ_HEAD(HEADNAME, TYPE);

     SIMPLEQ_HEAD_INITIALIZER(SIMPLEQ_HEAD head);

     struct TYPE *
     SIMPLEQ_FIRST(SIMPLEQ_HEAD *head);

     struct TYPE *
     SIMPLEQ_NEXT(struct TYPE *listelm, SIMPLEQ_ENTRY NAME);

     struct TYPE *
     SIMPLEQ_END(SIMPLEQ_HEAD *head);

     void
     SIMPLEQ_INIT(SIMPLEQ_HEAD *head);

     void
     SIMPLEQ_INSERT_HEAD(SIMPLEQ_HEAD *head, struct TYPE *elm,
             SIMPLEQ_ENTRY NAME);

     void
     SIMPLEQ_INSERT_TAIL(SIMPLEQ_HEAD *head, struct TYPE *elm,
             SIMPLEQ_ENTRY NAME);

     void
     SIMPLEQ_INSERT_AFTER(SIMPLEQ_HEAD    *head,    struct   TYPE
*listelm,
             struct TYPE *elm, SIMPLEQ_ENTRY NAME);

     void
     SIMPLEQ_REMOVE_HEAD(SIMPLEQ_HEAD *head, SIMPLEQ_ENTRY NAME);

     TAILQ_ENTRY(TYPE);

     TAILQ_HEAD(HEADNAME, TYPE);

     TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);

     struct TYPE *
     TAILQ_FIRST(TAILQ_HEAD *head);

     struct TYPE *
     TAILQ_NEXT(struct TYPE *listelm, TAILQ_ENTRY NAME);

     struct TYPE *
     TAILQ_END(TAILQ_HEAD *head);

     struct TYPE *
     TAILQ_LAST(TAILQ_HEAD *head, HEADNAME NAME);

     TAILQ_PREV(struct  TYPE *listelm, HEADNAME NAME, TAILQ_ENTRY
NAME);

     bool
     TAILQ_EMPTY(TAILQ_HEAD *head);

     TAILQ_FOREACH(VARNAME, TAILQ_HEAD *head, TAILQ_ENTRY NAME);

     TAILQ_FOREACH_REVERSE(VARNAME, TAILQ_HEAD *head, HEADNAME,
             TAILQ_ENTRY NAME);

     void
     TAILQ_INIT(TAILQ_HEAD *head);

     void
     TAILQ_INSERT_AFTER(TAILQ_HEAD *head, struct TYPE *listelm,
             struct TYPE *elm, TAILQ_ENTRY NAME);

     void
     TAILQ_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm,
             TAILQ_ENTRY NAME);

     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_REMOVE(TAILQ_HEAD *head, struct TYPE *elm, TAILQ_ENTRY
NAME);

     CIRCLEQ_ENTRY(TYPE);

     CIRCLEQ_HEAD(HEADNAME, TYPE);

     CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD head);

     struct TYPE *
     CIRCLEQ_FIRST(CIRCLEQ_HEAD *head);

     struct TYPE *
     CIRCLEQ_LAST(CIRCLEQ_HEAD *head);

     struct TYPE *
     CIRCLEQ_END(CIRCLEQ_HEAD *head);

     struct TYPE *
     CIRCLEQ_NEXT(struct TYPE *listelm, CIRCLEQ_ENTRY NAME);

     struct TYPE *
     CIRCLEQ_PREV(struct TYPE *listelm, CIRCLEQ_ENTRY NAME);

     bool
     CIRCLEQ_EMPTY(CIRCLEQ_HEAD *head);

     CIRCLEQ_FOREACH(VARNAME,  CIRCLEQ_HEAD  *head, CIRCLEQ_ENTRY
NAME);

     CIRCLEQ_FOREACH_REVERSE(VARNAME,     CIRCLEQ_HEAD     *head,
CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_INIT(CIRCLEQ_HEAD *head);

     void
     CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD    *head,    struct   TYPE
*listelm,
             struct TYPE *elm, CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD   *head,   struct    TYPE
*listelm,
             struct TYPE *elm, CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head, struct TYPE *elm,
             CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head, struct TYPE *elm,
             CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_REMOVE(CIRCLEQ_HEAD   *head,   struct   TYPE   *elm,
CIRCLEQ_ENTRY NAME);

DESCRIPTION    [Toc]    [Back]

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

           1.   Insertion of a new entry at the head of the list.
           2.   Insertion of a new entry after any element in the
list.
           3.   Removal of an entry from the head of the list.
           4.   Forward traversal through the list.

     Singly-linked lists are the simplest of the five data structures and support
  only the above functionality.  Singly-linked lists are
ideal for applications
 with large datasets and few or  no  removals,  or
for implementing
 a LIFO queue.

     Simple queues add the following functionality:

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

     However:

           1.    All list insertions 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 singly-linked lists.

     Simple queues are ideal for applications with large datasets
and few or
     no removals, or for implementing a FIFO queue.

     All  doubly  linked  types  of  data structures (lists, tail
queues, and circle
 queues) additionally allow:

           1.   Insertion of a new entry before  any  element  in
the list.
           2.   Removal of any entry in the list.

     However:

           1.    Each  element  requires two pointers rather than
one.
           2.   Code size and execution time of  operations  (except for removal)
  is  about twice that of the singly-linked
data-structures.


     Lists are the simplest of the doubly linked data  structures
and support
     only the above functionality over singly-linked lists.

     Tail queues add the following functionality:

           1.   Entries can be added at the end of a list.
           2.   They may be traversed backwards, at a cost.

     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 singly-linked lists.

     Circular queues add the following functionality:

           1.   Entries can be added at the end of a list.
           2.   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 tag of a user defined structure
 that must contain a field of type SLIST_ENTRY, LIST_ENTRY,
     SIMPLEQ_ENTRY,  TAILQ_ENTRY,  or  CIRCLEQ_ENTRY, named NAME.
The argument
     HEADNAME is the name tag of a user  defined  structure  that
must be declared
   using   the   macros   SLIST_HEAD(),   LIST_HEAD(),
SIMPLEQ_HEAD(),
     TAILQ_HEAD(), or CIRCLEQ_HEAD().  See the examples below for
further explanation
 of how these macros are used.

SINGLY_LINKED LISTS    [Toc]    [Back]

     A singly-linked list is headed by a structure defined by the
SLIST_HEAD()
     macro.  This structure contains  a  single  pointer  to  the
first element on
     the  list.  The elements are singly linked for minimum space
and pointer
     manipulation overhead at the expense of O(n) removal for arbitrary elements.
   New  elements can be added to the list after an existing element
     or at the head of the list.  A SLIST_HEAD structure  is  declared as follows:


           SLIST_HEAD(HEADNAME, TYPE) head;

     where  HEADNAME  is the name of the structure to be defined,
and struct
     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 HEADNAME facility is often not used, leading to the following bizarre
     code:

           SLIST_HEAD(, TYPE) head, *headp;

     The SLIST_ENTRY() macro declares a structure  that  connects
the elements
     in the list.

     The  SLIST_INIT()  macro  initializes the list referenced by
head.

     The list can also be initialized statically by using the
     SLIST_HEAD_INITIALIZER() macro like this:

           SLIST_HEAD(HEADNAME, TYPE) head =  SLIST_HEAD_INITIALIZER(head);

     The SLIST_INSERT_HEAD() macro inserts the new element elm at
the head of
     the list.

     The SLIST_INSERT_AFTER() macro inserts the new  element  elm
after the element
 listelm.

     The  SLIST_REMOVE_HEAD()  macro removes the first element of
the list
     pointed by head.

     The SLIST_REMOVE_NEXT() macro removes the list element immediately following
 elm.

     The SLIST_REMOVE() macro removes the element elm of the list
pointed by
     head.

     The SLIST_FIRST() and SLIST_NEXT() macros  can  be  used  to
traverse the
     list:

           for  (np  =  SLIST_FIRST(&head);  np  !=  NULL;  np  =
SLIST_NEXT(np, NAME))

     Or, for simplicity, one can use the SLIST_FOREACH() macro:

           SLIST_FOREACH(np, head, NAME)

     The   SLIST_FOREACH_PREVPTR()   macro    is    similar    to
SLIST_FOREACH() except
     that  it  stores  a  pointer  to  the  previous  element  in
VARNAMEP.  This provides
 access to the previous element  while  traversing  the
list, as one
     would have with a doubly-linked list.

     The  SLIST_EMPTY()  macro  should be used to check whether a
simple list is
     empty.

LISTS    [Toc]    [Back]

     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, before 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 struct
     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 HEADNAME facility is often not used, leading to the following bizarre
     code:

           LIST_HEAD(, TYPE) head, *headp;

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

     The  LIST_INIT()  macro  initializes  the list referenced by
head.

     The list can also be initialized statically by using the
     LIST_HEAD_INITIALIZER() macro like this:

           LIST_HEAD(HEADNAME, TYPE) head =  LIST_HEAD_INITIALIZER(head);

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

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

     The  LIST_INSERT_BEFORE()  macro inserts the new element elm
before the element
 listelm.

     The LIST_REMOVE() macro removes the  element  elm  from  the
list.

     The  LIST_REPLACE() macro replaces the list element elm with
the new element
 elm2.

     The LIST_FIRST() and LIST_NEXT() macros can be used to  traverse the list:

           for   (np  =  LIST_FIRST(&head);  np  !=  NULL;  np  =
LIST_NEXT(np, NAME))

     Or, for simplicity, one can use the LIST_FOREACH() macro:

           LIST_FOREACH(np, head, NAME)

     The LIST_EMPTY() macro should be used  to  check  whether  a
list is empty.

LIST EXAMPLE    [Toc]    [Back]

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

     LIST_INIT(&head);                        /* Initialize 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);

     n2 = malloc(sizeof(struct entry));      /* Insert before. */
     LIST_INSERT_BEFORE(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);

SIMPLE QUEUES    [Toc]    [Back]

     A simple queue is headed  by  a  structure  defined  by  the
SIMPLEQ_HEAD()
     macro.   This  structure contains a pair of pointers, one to
the first element
 in the simple queue and the other to the  last  element
in the simple
     queue.  The elements are singly linked.  New elements can be
added to the
     queue after an existing element, at the head of the queue or
at the tail
     of  the queue.  A SIMPLEQ_HEAD structure is declared as follows:

           SIMPLEQ_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name of 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.)

     The SIMPLEQ_ENTRY() macro declares a structure that connects
the elements
     in the queue.

     The SIMPLEQ_INIT() macro initializes the queue referenced by
head.

     The queue can also be initialized statically by using the
     SIMPLEQ_HEAD_INITIALIZER() macro like this:

           SIMPLEQ_HEAD(HEADNAME, TYPE) head =  SIMPLEQ_HEAD_INITIALIZER(head);

     The  SIMPLEQ_INSERT_HEAD() macro inserts the new element elm
at the head
     of the queue.

     The SIMPLEQ_INSERT_TAIL() macro inserts the new element  elm
at the end of
     the queue.

     The SIMPLEQ_INSERT_AFTER() macro inserts the new element elm
after the
     element listelm.

     The SIMPLEQ_REMOVE_HEAD() macro removes  the  first  element
from the queue.

     The SIMPLEQ_FIRST() and SIMPLEQ_NEXT() macros can be used to
traverse the
     queue.  The SIMPLEQ_FOREACH() is used for queue traversal:

           SIMPLEQ_FOREACH(np, head, NAME)

     The SIMPLEQ_EMPTY() macro should be used to check whether  a
list is empty.

SIMPLE QUEUE EXAMPLE    [Toc]    [Back]

     SIMPLEQ_HEAD(listhead, entry) head = SIMPLEQ_HEAD_INITIALIZER(head);
     struct entry {
             ...
             SIMPLEQ_ENTRY(entry) entries;   /* List. */
             ...
     } *n1, *n2, *np;

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

     n2 = malloc(sizeof(struct entry));      /* Insert after. */
     SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);

     n2  =  malloc(sizeof(struct  entry));       /* Insert at the
tail. */
     SIMPLEQ_INSERT_TAIL(&head, n2, entries);
                                             /*  Forward  traversal. */
     for  (np  =  SIMPLEQ_FIRST(&head);  np  !=  NULL;  np = SIMPLEQ_NEXT(np, entries))
             np-> ...
                                             /* Delete. */
     while ((n1 = SIMPLEQ_FIRST(&head)) != NULL)
             SIMPLEQ_REMOVE_HEAD(&head, entries);

TAIL QUEUES    [Toc]    [Back]

     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
     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  HEADNAME  is the name of the structure to be defined,
and struct
     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 TAILQ_ENTRY() macro declares a structure  that  connects
the elements
     in the tail queue.

     The TAILQ_INIT() macro initializes the tail queue referenced
by head.

     The tail queue can also be initialized statically  by  using
the
     TAILQ_HEAD_INITIALIZER() macro.

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

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

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

     The TAILQ_INSERT_BEFORE() macro inserts the new element  elm
before the
     element listelm.

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

     TAILQ_FOREACH() and  TAILQ_FOREACH_REVERSE()  are  used  for
traversing a
     tail queue.  TAILQ_FOREACH() starts at the first element and
proceeds towards
 the last.  TAILQ_FOREACH_REVERSE() starts at the  last
element and
     proceeds towards the first.

           TAILQ_FOREACH(np, &head, NAME)
           TAILQ_FOREACH_REVERSE(np, &head, HEADNAME, NAME)

     The    TAILQ_FIRST(),    TAILQ_NEXT(),    TAILQ_LAST()   and
TAILQ_PREV() macros can
     be used to manually traverse a tail queue  or  an  arbitrary
part of one.

     The  TAILQ_EMPTY()  macro  should be used to check whether a
tail queue is
     empty.

TAIL QUEUE EXAMPLE    [Toc]    [Back]

     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 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);

     n2 = malloc(sizeof(struct entry));      /* Insert before. */
     TAILQ_INSERT_BEFORE(n1, n2, entries);
                                             /*  Forward  traversal. */
     TAILQ_FOREACH(np, &head, entries)
             np-> ...
                                             /*   Manual  forward
traversal */
     for (np = n2; np != NULL; np = TAILQ_NEXT(np, entries))
             np-> ...
                                             /* Delete. */
     while (np = TAILQ_FIRST(&head))
             TAILQ_REMOVE(&head, np, entries);

CIRCULAR QUEUES    [Toc]    [Back]

     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 struct
     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 CIRCLEQ_ENTRY() macro declares a structure that connects
the elements
     in the circular queue.

     The CIRCLEQ_INIT() macro initializes the circular queue referenced by
     head.

     The circular queue can also be initialized statically by using the
     CIRCLEQ_HEAD_INITIALIZER() macro.

     The  CIRCLEQ_INSERT_HEAD() macro inserts the new element elm
at the head
     of the circular queue.

     The CIRCLEQ_INSERT_TAIL() macro inserts the new element  elm
at the end of
     the circular queue.

     The CIRCLEQ_INSERT_AFTER() macro inserts the new element elm
after the
     element listelm.

     The CIRCLEQ_INSERT_BEFORE() macro inserts  the  new  element
elm before the
     element listelm.

     The  CIRCLEQ_REMOVE() macro removes the element elm from the
circular
     queue.

     The    CIRCLEQ_FIRST(),    CIRCLEQ_LAST(),    CIRCLEQ_END(),
CIRCLEQ_NEXT() and
     CIRCLEQ_PREV()  macros  can  be  used to traverse a circular
queue.  The
     CIRCLEQ_FOREACH() is used for circular queue forward traversal:

           CIRCLEQ_FOREACH(np, head, NAME)

     The     CIRCLEQ_FOREACH_REVERSE()     macro     acts    like
CIRCLEQ_FOREACH() but traverses
 the circular queue backwards.

     The CIRCLEQ_EMPTY() macro should be used to check whether  a
circular
     queue is empty.

CIRCULAR QUEUE EXAMPLE    [Toc]    [Back]

     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 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 = CIRCLEQ_FIRST(&head); np != CIRCLEQ_END(&head);
         np = CIRCLEQ_NEXT(np, entries))
             np-> ...
                                             /*  Reverse  traversal. */
     for (np = CIRCLEQ_LAST(&head); np != CIRCLEQ_END(&head);
         np = CIRCLEQ_PREV(np, entries))
             np-> ...
                                             /* Delete. */
     while (CIRCLEQ_FIRST(&head) != CIRCLEQ_END(&head))
             CIRCLEQ_REMOVE(&head,    CIRCLEQ_FIRST(&head),   entries);

NOTES    [Toc]    [Back]

     The SLIST_END(), LIST_END(), SIMPLEQ_END()  and  TAILQ_END()
macros are
     provided  for  symmetry  with CIRCLEQ_END().  They expand to
NULL and don't
     serve any useful purpose.

     Trying to free a list in the following way is a  common  error:

           LIST_FOREACH(var, head, entry)
                   free(var);
           free(head);

     Since var is free'd, the FOREACH() macro refers to a pointer
that may
     have been reallocated already.  Proper code needs  a  second
variable.

           for  (var  =  LIST_FIRST(head); var != LIST_END(head);
var = nxt) {
                   nxt = LIST_NEXT(var, entry);
                   free(var);
           }
           LIST_INIT(head);        /* to put the list back in order */

HISTORY    [Toc]    [Back]

     The queue functions first appeared in 4.4BSD.

OpenBSD      3.6                        December     13,     1993
[ Back ]
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