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.\"	@(#)queue.3	8.1 (Berkeley) 12/13/93
.\"
.Dd October 1, 2017
.Dt QUEUE 3
.Os
.Sh NAME
.Nm SLIST_HEAD ,
.Nm SLIST_HEAD_INITIALIZER ,
.Nm SLIST_ENTRY ,
.Nm SLIST_FIRST ,
.Nm SLIST_EMPTY ,
.Nm SLIST_NEXT ,
.Nm SLIST_FOREACH ,
.Nm SLIST_FOREACH_SAFE ,
.Nm SLIST_INIT ,
.Nm SLIST_INSERT_AFTER ,
.Nm SLIST_INSERT_HEAD ,
.Nm SLIST_REMOVE_AFTER ,
.Nm SLIST_REMOVE_HEAD ,
.Nm SLIST_REMOVE ,
.Nm LIST_HEAD ,
.Nm LIST_HEAD_INITIALIZER ,
.Nm LIST_ENTRY ,
.Nm LIST_FIRST ,
.Nm LIST_EMPTY ,
.Nm LIST_NEXT ,
.Nm LIST_FOREACH ,
.Nm LIST_FOREACH_SAFE ,
.Nm LIST_INIT ,
.Nm LIST_INSERT_AFTER ,
.Nm LIST_INSERT_BEFORE ,
.Nm LIST_INSERT_HEAD ,
.Nm LIST_REMOVE ,
.Nm LIST_REPLACE ,
.Nm LIST_MOVE ,
.Nm SIMPLEQ_HEAD ,
.Nm SIMPLEQ_HEAD_INITIALIZER ,
.Nm SIMPLEQ_ENTRY ,
.Nm SIMPLEQ_FIRST ,
.Nm SIMPLEQ_EMPTY ,
.Nm SIMPLEQ_NEXT ,
.Nm SIMPLEQ_LAST ,
.Nm SIMPLEQ_FOREACH ,
.Nm SIMPLEQ_FOREACH_SAFE ,
.Nm SIMPLEQ_INIT ,
.Nm SIMPLEQ_INSERT_AFTER ,
.Nm SIMPLEQ_INSERT_HEAD ,
.Nm SIMPLEQ_INSERT_TAIL ,
.Nm SIMPLEQ_REMOVE_AFTER ,
.Nm SIMPLEQ_REMOVE_HEAD ,
.Nm SIMPLEQ_REMOVE ,
.Nm SIMPLEQ_CONCAT ,
.Nm TAILQ_HEAD ,
.Nm TAILQ_HEAD_INITIALIZER ,
.Nm TAILQ_ENTRY ,
.Nm TAILQ_FIRST ,
.Nm TAILQ_NEXT ,
.Nm TAILQ_LAST ,
.Nm TAILQ_PREV ,
.Nm TAILQ_EMPTY ,
.Nm TAILQ_FOREACH ,
.Nm TAILQ_FOREACH_SAFE ,
.Nm TAILQ_FOREACH_REVERSE ,
.Nm TAILQ_FOREACH_REVERSE_SAFE ,
.Nm TAILQ_INIT ,
.Nm TAILQ_INSERT_AFTER ,
.Nm TAILQ_INSERT_BEFORE ,
.Nm TAILQ_INSERT_HEAD ,
.Nm TAILQ_INSERT_TAIL ,
.Nm TAILQ_REMOVE ,
.Nm TAILQ_REPLACE ,
.Nm TAILQ_CONCAT ,
.Nm STAILQ_HEAD ,
.Nm STAILQ_HEAD_INITIALIZER ,
.Nm STAILQ_ENTRY ,
.Nm STAILQ_FIRST ,
.Nm STAILQ_EMPTY ,
.Nm STAILQ_NEXT ,
.Nm STAILQ_LAST ,
.Nm STAILQ_FOREACH ,
.Nm STAILQ_FOREACH_SAFE ,
.Nm STAILQ_INIT ,
.Nm STAILQ_INSERT_AFTER ,
.Nm STAILQ_INSERT_HEAD ,
.Nm STAILQ_INSERT_TAIL ,
.Nm STAILQ_REMOVE_HEAD ,
.Nm STAILQ_REMOVE ,
.Nm STAILQ_CONCAT
.Nd implementations of singly-linked lists, lists, simple queues, tail queues, and singly-linked tail queues
.Sh SYNOPSIS
.In sys/queue.h
.Pp
.Fn SLIST_HEAD "HEADNAME" "TYPE"
.Fn SLIST_HEAD_INITIALIZER "head"
.Fn SLIST_ENTRY "TYPE"
.Ft TYPE *
.Fn SLIST_FIRST "SLIST_HEAD *head"
.Ft int
.Fn SLIST_EMPTY "SLIST_HEAD *head"
.Ft TYPE *
.Fn SLIST_NEXT "TYPE *elm" "SLIST_ENTRY NAME"
.Fn SLIST_FOREACH "TYPE *var" "SLIST_HEAD *head" "SLIST_ENTRY NAME"
.Fn SLIST_FOREACH_SAFE "TYPE *var" "SLIST_HEAD *head" "SLIST_ENTRY NAME" "TYPE *tmp"
.Fn SLIST_INIT "SLIST_HEAD *head"
.Fn SLIST_INSERT_HEAD "SLIST_HEAD *head" "TYPE *elm" "SLIST_ENTRY NAME"
.Fn SLIST_INSERT_AFTER "TYPE *listelm" "TYPE *elm" "SLIST_ENTRY NAME"
.Fn SLIST_REMOVE "SLIST_HEAD *head" "TYPE *elm" "TYPE" "SLIST_ENTRY NAME"
.Fn SLIST_REMOVE_HEAD "SLIST_HEAD *head" "SLIST_ENTRY NAME"
.Pp
.Fn LIST_HEAD "HEADNAME" "TYPE"
.Fn LIST_HEAD_INITIALIZER "head"
.Fn LIST_ENTRY "TYPE"
.Ft TYPE *
.Fn LIST_FIRST "LIST_HEAD *head"
.Ft TYPE *
.Fn LIST_NEXT "TYPE *elm" "LIST_ENTRY NAME"
.Ft int
.Fn LIST_EMPTY "LIST_HEAD *head"
.Fn LIST_FOREACH "TYPE *var" "LIST_HEAD *head" "LIST_ENTRY NAME"
.Fn LIST_FOREACH_SAFE "TYPE *var" "LIST_HEAD *head" "LIST_ENTRY NAME" "TYPE *tmp"
.Fn LIST_INIT "LIST_HEAD *head"
.Fn LIST_INSERT_AFTER "TYPE *listelm" "TYPE *elm" "LIST_ENTRY NAME"
.Fn LIST_INSERT_BEFORE "TYPE *listelm" "TYPE *elm" "LIST_ENTRY NAME"
.Fn LIST_INSERT_HEAD "LIST_HEAD *head" "TYPE *elm" "LIST_ENTRY NAME"
.Fn LIST_REMOVE "TYPE *elm" "LIST_ENTRY NAME"
.Fn LIST_REPLACE "TYPE *elm" "TYPE *new" "LIST_ENTRY NAME"
.Fn LIST_MOVE "LIST_HEAD *head1" "LIST_HEAD *head2" "LIST_ENTRY NAME"
.Pp
.Fn SIMPLEQ_HEAD "HEADNAME" "TYPE"
.Fn SIMPLEQ_HEAD_INITIALIZER "head"
.Fn SIMPLEQ_ENTRY "TYPE"
.Ft TYPE *
.Fn SIMPLEQ_FIRST "SIMPLEQ_HEAD *head"
.Ft int
.Fn SIMPLEQ_EMPTY "SIMPLEQ_HEAD *head"
.Ft TYPE *
.Fn SIMPLEQ_NEXT "TYPE *elm" "SIMPLEQ_ENTRY NAME"
.Ft TYPE *
.Fn SIMPLEQ_LAST "SIMPLEQ_HEAD *head" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
.Fn SIMPLEQ_FOREACH "TYPE *var" "SIMPLEQ_HEAD *head" "SIMPLEQ_ENTRY NAME"
.Fn SIMPLEQ_FOREACH_SAFE "TYPE *var" "SIMPLEQ_HEAD *head" "SIMPLEQ_ENTRY NAME" "TYPE *tmp"
.Fn SIMPLEQ_INIT "SIMPLEQ_HEAD *head"
.Fn SIMPLEQ_INSERT_HEAD "SIMPLEQ_HEAD *head" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
.Fn SIMPLEQ_INSERT_TAIL "SIMPLEQ_HEAD *head" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
.Fn SIMPLEQ_INSERT_AFTER "SIMPLEQ_HEAD *head" "TYPE *listelm" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
.Fn SIMPLEQ_REMOVE_HEAD "SIMPLEQ_HEAD *head" "SIMPLEQ_ENTRY NAME"
.Fn SIMPLEQ_REMOVE_AFTER "SIMPLEQ_HEAD *head" "TYPE *elm" "SIMPLEQ_ENTRY NAME"
.Fn SIMPLEQ_REMOVE "SIMPLEQ_HEAD *head" "TYPE *elm" "TYPE" "SIMPLEQ_ENTRY NAME"
.Fn SIMPLEQ_CONCAT "SIMPLEQ_HEAD *head1" "SIMPLEQ_HEAD *head2"
.Pp
.Fn TAILQ_HEAD "HEADNAME" "TYPE"
.Fn TAILQ_HEAD_INITIALIZER "head"
.Fn TAILQ_ENTRY "TYPE"
.Ft TYPE *
.Fn TAILQ_FIRST "TAILQ_HEAD *head"
.Ft TYPE *
.Fn TAILQ_NEXT "TYPE *elm" "TAILQ_ENTRY NAME"
.Ft TYPE *
.Fn TAILQ_LAST "TAILQ_HEAD *head" "HEADNAME"
.Ft TYPE *
.Fn TAILQ_PREV "TYPE *elm" "HEADNAME" "TAILQ_ENTRY NAME"
.Ft int
.Fn TAILQ_EMPTY "TAILQ_HEAD *head"
.Fn TAILQ_FOREACH "TYPE *var" "TAILQ_HEAD *head" "TAILQ_ENTRY NAME"
.Fn TAILQ_FOREACH_SAFE "TYPE *var" "TAILQ_HEAD *head" "TAILQ_ENTRY NAME" "TYPE *tmp"
.Fn TAILQ_FOREACH_REVERSE "TYPE *var" "TAILQ_HEAD *head" "HEADNAME" "TAILQ_ENTRY NAME"
.Fn TAILQ_FOREACH_REVERSE_SAFE "TYPE *var" "TAILQ_HEAD *head" "HEADNAME" "TAILQ_ENTRY NAME" "TYPE *tmp"
.Fn TAILQ_INIT "TAILQ_HEAD *head"
.Fn TAILQ_INSERT_HEAD "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
.Fn TAILQ_INSERT_TAIL "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
.Fn TAILQ_INSERT_AFTER "TAILQ_HEAD *head" "TYPE *listelm" "TYPE *elm" "TAILQ_ENTRY NAME"
.Fn TAILQ_INSERT_BEFORE "TYPE *listelm" "TYPE *elm" "TAILQ_ENTRY NAME"
.Fn TAILQ_REMOVE "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
.Fn TAILQ_REPLACE "TAILQ_HEAD *head" "TYPE *elm" "TYPE *new" "TAILQ_ENTRY NAME"
.Fn TAILQ_CONCAT "TAILQ_HEAD *head1" "TAILQ_HEAD *head2" "TAILQ_ENTRY NAME"
.Pp
.Fn STAILQ_HEAD "HEADNAME" "TYPE"
.Fn STAILQ_HEAD_INITIALIZER "head"
.Fn STAILQ_ENTRY "TYPE"
.Ft TYPE *
.Fn STAILQ_FIRST "STAILQ_HEAD *head"
.Ft int
.Fn STAILQ_EMPTY "STAILQ_HEAD *head"
.Ft TYPE *
.Fn STAILQ_NEXT "TYPE *elm" "STAILQ_ENTRY NAME"
.Ft TYPE *
.Fn STAILQ_LAST "STAILQ_HEAD *head" "TYPE *elm" "STAILQ_ENTRY NAME"
.Fn STAILQ_FOREACH "TYPE *var" "STAILQ_HEAD *head" "STAILQ_ENTRY NAME"
.Fn STAILQ_FOREACH_SAFE "TYPE *var" "STAILQ_HEAD *head" "STAILQ_ENTRY NAME" "TYPE *tmp"
.Fn STAILQ_INIT "STAILQ_HEAD *head"
.Fn STAILQ_INSERT_HEAD "STAILQ_HEAD *head" "TYPE *elm" "STAILQ_ENTRY NAME"
.Fn STAILQ_INSERT_TAIL "STAILQ_HEAD *head" "TYPE *elm" "STAILQ_ENTRY NAME"
.Fn STAILQ_INSERT_AFTER "STAILQ_HEAD *head" "TYPE *listelm" "TYPE *elm" "STAILQ_ENTRY NAME"
.Fn STAILQ_REMOVE_HEAD "STAILQ_HEAD *head" "STAILQ_ENTRY NAME"
.Fn STAILQ_REMOVE "STAILQ_HEAD *head" "TYPE *elm" "TYPE" "STAILQ_ENTRY NAME"
.Fn STAILQ_CONCAT "STAILQ_HEAD *head1" "STAILQ_HEAD *head2"
.Sh DESCRIPTION
These macros define and operate on five types of data structures:
singly-linked lists, simple queues, lists, tail queues, and singly-linked
tail queues.
All five structures support the following functionality:
.Bl -enum -compact -offset indent
.It
Insertion of a new entry at the head of the list.
.It
Insertion of a new entry after any element in the list.
.It
Removal of any entry in the list.
.It
Forward traversal through the list.
.El
.Pp
Singly-linked lists are the simplest of the four 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.
.Pp
Simple queues add the following functionality:
.Bl -enum -compact -offset indent
.It
Entries can be added at the end of a list.
.It
They may be concatenated.
.El
However:
.Bl -enum -compact -offset indent
.It
Entries may not be added before any element in the list.
.It
All list insertions and removals must specify the head of the list.
.It
Each head entry requires two pointers rather than one.
.El
.Pp
Simple queues are ideal for applications with large datasets and few or
no removals, or for implementing a FIFO queue.
.Pp
All doubly linked types of data structures (lists and tail queues)
additionally allow:
.Bl -enum -compact -offset indent
.It
Insertion of a new entry before any element in the list.
.It
O(1) removal of any entry in the list.
.El
However:
.Bl -enum -compact -offset indent
.It
Each element requires two pointers rather than one.
.It
Code size and execution time of operations (except for removal) is about
twice that of the singly-linked data-structures.
.El
.Pp
Linked lists are the simplest of the doubly linked data structures and
support only the above functionality over singly-linked lists.
.Pp
Tail queues add the following functionality:
.Bl -enum -compact -offset indent
.It
Entries can be added at the end of a list.
.It
They may be concatenated.
.El
However:
.Bl -enum -compact -offset indent
.It
All list insertions and removals, except insertion before another element, must
specify the head of the list.
.It
Each head entry requires two pointers rather than one.
.It
Code size is about 15% greater and operations run about 20% slower
than lists.
.El
.Pp
Circular queues add the following functionality:
.Bl -enum -compact -offset indent
.It
Entries can be added at the end of a list.
.It
They may be traversed backwards, from tail to head.
.El
However:
.Bl -enum -compact -offset indent
.It
All list insertions and removals must specify the head of the list.
.It
Each head entry requires two pointers rather than one.
.It
The termination condition for traversal is more complex.
.It
Code size is about 40% greater and operations run about 45% slower
than lists.
.El
.Pp
In the macro definitions,
.Fa TYPE
is the name of a user defined structure,
that must contain a field of type
.Li SLIST_ENTRY ,
.Li LIST_ENTRY ,
.Li SIMPLEQ_ENTRY ,
.Li TAILQ_ENTRY ,
or
.Li STAILQ_ENTRY ,
named
.Fa NAME .
The argument
.Fa HEADNAME
is the name of a user defined structure that must be declared
using the macros
.Li LIST_HEAD ,
.Li SIMPLEQ_HEAD ,
.Li SLIST_HEAD ,
or
.Li TAILQ_HEAD .
See the examples below for further explanation of how these
macros are used.
.Ss Summary of Operations
The following table summarizes the supported macros for each type
of data structure.
.Pp
.TS
box tab(:);
l | c | c | c | c | c
l | c | c | c | c | c
l | c | c | c | c | c
l | c | c | c | c | c
l | c | c | c | c | c
l | c | c | c | c | c.
:SLIST:LIST:SIMPLEQ:TAILQ:STAILQ
_
_FIRST:+:+:+:+:+
_EMPTY:+:+:+:+:+
_NEXT:+:+:+:+:+
_PREV:-:-:-:+:-
_LAST:-:-:+:+:+
_FOREACH:+:+:+:+:+
_FOREACH_SAFE:+:+:+:+:+
_FOREACH_REVERSE:-:-:-:+:-
_FOREACH_REVERSE_SAFE:-:-:-:+:-
_INSERT_HEAD:+:+:+:+:+
_INSERT_AFTER:+:+:+:+:+
_INSERT_BEFORE:-:+:-:+:-
_INSERT_TAIL:-:-:+:+:+
_REMOVE:+:+:+:+:+
_REMOVE_HEAD:+:-:+:-:+
_REMOVE_AFTER:-:-:+:-:+
_REPLACE:-:+:-:+:-
_CONCAT:-:-:+:+:+
.TE
.Sh SINGLY-LINKED LISTS
A singly-linked list is headed by a structure defined by the
.Fn 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.
An
.Fa SLIST_HEAD
structure is declared as follows:
.Bd -literal -offset indent
SLIST_HEAD(HEADNAME, TYPE) head;
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and
.Fa 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:
.Bd -literal -offset indent
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The macro
.Fn SLIST_HEAD_INITIALIZER
evaluates to an initializer for the list
.Fa head .
.Pp
The macro
.Fn SLIST_ENTRY
declares a structure that connects the elements in
the list.
.Pp
The macro
.Fn SLIST_FIRST
returns the first element in the list or NULL if the list is empty.
.Pp
The macro
.Fn SLIST_EMPTY
evaluates to true if there are no elements in the list.
.Pp
The macro
.Fn SLIST_NEXT
returns the next element in the list.
.Pp
.Fn SLIST_FOREACH
traverses the list referenced by
.Fa head
in the forward direction, assigning each element in
turn to
.Fa var .
.Pp
The SAFE version uses
.Fa tmp
to hold the next element, so
.Fa var
may be freed or removed from the list.
.Pp
The macro
.Fn SLIST_INIT
initializes the list referenced by
.Fa head .
.Pp
The macro
.Fn SLIST_INSERT_HEAD
inserts the new element
.Fa elm
at the head of the list.
.Pp
The macro
.Fn SLIST_INSERT_AFTER
inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The macro
.Fn SLIST_REMOVE
removes the element
.Fa elm
from the list.
.Pp
The macro
.Fn SLIST_REMOVE_HEAD
removes the first element from the head of the list.
For optimum efficiency,
elements being removed from the head of the list should explicitly use
this macro instead of the generic
.Fn SLIST_REMOVE
macro.
.Pp
The macro
.Fn SLIST_REMOVE_AFTER
removes the element after the one specified.
For optimum efficiency,
elements being removed after a specified one should explicitly use
this macro instead of the generic
.Fn SLIST_REMOVE
.Sh SINGLY-LINKED LIST EXAMPLE
.Bd -literal
SLIST_HEAD(slisthead, entry) head =
    SLIST_HEAD_INITIALIZER(head);
struct slisthead *headp;                /* Singly-linked List head. */
struct entry {
        ...
        SLIST_ENTRY(entry) entries;     /* Singly-linked List. */
        ...
} *n1, *n2, *n3, *np;

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

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

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

SLIST_REMOVE(&head, n2, entry, entries);/* Deletion. */
free(n2);

n3 = SLIST_FIRST(&head);
SLIST_REMOVE_HEAD(&head, entries);      /* Deletion from the head. */
free(n3);

SLIST_FOREACH(np, &head, entries)       /* Forward traversal. */
        np-> ...

while (!SLIST_EMPTY(&head)) {           /* List Deletion. */
        n1 = SLIST_FIRST(&head);
        SLIST_REMOVE_HEAD(&head, entries);
        free(n1);
}
.Ed
.Sh LISTS
A list is headed by a structure defined by the
.Fn 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
.Fa LIST_HEAD
structure is declared as follows:
.Bd -literal -offset indent
LIST_HEAD(HEADNAME, TYPE) head;
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and
.Fa 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:
.Bd -literal -offset indent
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The macro
.Fn LIST_ENTRY
declares a structure that connects the elements in
the list.
.Pp
The macro
.Fn LIST_HEAD_INITIALIZER
provides a value which can be used to initialize a list head at
compile time, and is used at the point that the list head
variable is declared, like:
.Bd -literal -offset indent
struct HEADNAME head = LIST_HEAD_INITIALIZER(head);
.Ed
.Pp
The macro
.Fn LIST_FIRST
returns the first element of the list
.Fa head .
.Pp
The macro
.Fn LIST_EMPTY
returns true if the list
.Fa head
has no elements.
.Pp
The macro
.Fn LIST_NEXT
returns the element after the element
.Fa elm .
.Pp
The macro
.Fn LIST_FOREACH
traverses the list referenced by
.Fa head
in the forward direction, assigning each element in turn to
.Fa var .
.Pp
The SAFE version uses
.Fa tmp
to hold the next element, so
.Fa var
may be freed or removed from the list.
.Pp
The macro
.Fn LIST_INIT
initializes the list referenced by
.Fa head .
.Pp
The macro
.Fn LIST_INSERT_AFTER
inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The macro
.Fn LIST_INSERT_BEFORE
inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The macro
.Fn LIST_INSERT_HEAD
inserts the new element
.Fa elm
at the head of the list.
.Pp
The macro
.Fn LIST_REMOVE
removes the element
.Fa elm
from the list.
.Pp
The macro
.Fn LIST_REPLACE
replaces the element
.Fa elm
with
.Fa new
in the list.
.Pp
The macro
.Fn LIST_MOVE
moves the list headed by
.Fa head1
onto the list headed by
.Fa head2 ,
always making the former empty.
.Sh LIST EXAMPLE
.Bd -literal
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);

n2 = malloc(sizeof(struct entry));	/* Insert before. */
LIST_INSERT_BEFORE(n1, n2, entries);

LIST_FOREACH(np, &head, entries)	/* Forward traversal. */
	np-> ...

while (LIST_FIRST(&head) != NULL)	/* Delete. */
	LIST_REMOVE(LIST_FIRST(&head), entries);
if (LIST_EMPTY(&head))			/* Test for emptiness. */
	printf("nothing to do\\n");
.Ed
.Sh SIMPLE QUEUES
A simple queue is headed by a structure defined by the
.Fn 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 for minimum space and pointer manipulation
overhead at the expense of O(n) removal for arbitrary elements.
New elements can be added to the queue after an existing element,
at the head of the queue, or at the end of the queue.
A
.Fa SIMPLEQ_HEAD
structure is declared as follows:
.Bd -literal -offset indent
SIMPLEQ_HEAD(HEADNAME, TYPE) head;
.Ed
.Pp
where
.Li HEADNAME
is the name of the structure to be defined, and
.Li TYPE
is the type of the elements to be linked into the simple queue.
A pointer to the head of the simple queue can later be declared as:
.Bd -literal -offset indent
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The macro
.Fn SIMPLEQ_ENTRY
declares a structure that connects the elements in
the simple queue.
.Pp
The macro
.Fn SIMPLEQ_HEAD_INITIALIZER
provides a value which can be used to initialize a simple queue head at
compile time, and is used at the point that the simple queue head
variable is declared, like:
.Bd -literal -offset indent
struct HEADNAME head = SIMPLEQ_HEAD_INITIALIZER(head);
.Ed
.Pp
The macro
.Fn SIMPLEQ_FIRST
returns the first element of the simple queue
.Fa head .
.Pp
The macro
.Fn SIMPLEQ_EMPTY
returns true if the simple queue
.Fa head
has no elements.
.Pp
The macro
.Fn SIMPLEQ_NEXT
returns the element after the element
.Fa elm .
.Pp
The macro
.Fn SIMPLEQ_LAST
returns the last item on the simple queue.
If the simple queue is empty the return value is
.Dv NULL .
.Pp
The macro
.Fn SIMPLEQ_FOREACH
traverses the simple queue referenced by
.Fa head
in the forward direction, assigning each element
in turn to
.Fa var .
.Pp
The SAFE version uses
.Fa tmp
to hold the next element, so
.Fa var
may be freed or removed from the list.
.Pp
The macro
.Fn SIMPLEQ_INIT
initializes the simple queue referenced by
.Fa head .
.Pp
The macro
.Fn SIMPLEQ_INSERT_HEAD
inserts the new element
.Fa elm
at the head of the simple queue.
.Pp
The macro
.Fn SIMPLEQ_INSERT_TAIL
inserts the new element
.Fa elm
at the end of the simple queue.
.Pp
The macro
.Fn SIMPLEQ_INSERT_AFTER
inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The macro
.Fn SIMPLEQ_REMOVE_HEAD
removes the first element from the head of the simple queue.
For optimum efficiency,
elements being removed from the head of the queue should explicitly use
this macro instead of the generic
.Fn SIMPLEQ_REMOVE
macro.
.Pp
The macro
.Fn SIMPLEQ_REMOVE_AFTER
removes the element after the one specified from the simple queue.
For optimum efficiency,
elements being removed after specified elements should explicitly use
this macro instead of the generic
.Fn SIMPLEQ_REMOVE
macro.
.Pp
The macro
.Fn SIMPLEQ_REMOVE
removes
.Fa elm
from the simple queue.
.Pp
The macro
.Fn SIMPLEQ_CONCAT
concatenates the simple queue headed by
.Fa head2
onto the end of the one headed by
.Fa head1 ,
removing all entries from the former.
.Sh SIMPLE QUEUE EXAMPLE
.Bd -literal
SIMPLEQ_HEAD(simplehead, entry) head;
struct simplehead *headp;		/* Simple queue head. */
struct entry {
	...
	SIMPLEQ_ENTRY(entry) entries;	/* Simple queue. */
	...
} *n1, *n2, *np;

SIMPLEQ_INIT(&head);			/* Initialize the queue. */

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

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

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

SIMPLEQ_FOREACH(np, &head, entries)	/* Forward traversal. */
	np-> ...

while (SIMPLEQ_FIRST(&head) != NULL)	/* Delete. */
	SIMPLEQ_REMOVE_HEAD(&head, entries);
if (SIMPLEQ_EMPTY(&head))		/* Test for emptiness. */
	printf("nothing to do\\n");
.Ed
.Sh TAIL QUEUES
A tail queue is headed by a structure defined by the
.Fn 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
the queue.
A
.Fa TAILQ_HEAD
structure is declared as follows:
.Bd -literal -offset indent
TAILQ_HEAD(HEADNAME, TYPE) head;
.Ed
.Pp
where
.Li HEADNAME
is the name of the structure to be defined, and
.Li 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:
.Bd -literal -offset indent
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The macro
.Fn TAILQ_ENTRY
declares a structure that connects the elements in
the tail queue.
.Pp
The macro
.Fn TAILQ_HEAD_INITIALIZER
provides a value which can be used to initialize a tail queue head at
compile time, and is used at the point that the tail queue head
variable is declared, like:
.Bd -literal -offset indent
struct HEADNAME head = TAILQ_HEAD_INITIALIZER(head);
.Ed
.Pp
The macro
.Fn TAILQ_FIRST
returns the first element of the tail queue
.Fa head .
.Pp
The macro
.Fn TAILQ_NEXT
returns the element after the element
.Fa elm .
.Pp
The macro
.Fn TAILQ_LAST
returns the last item on the tail queue.
If the tail queue is empty the return value is
.Dv NULL .
.Pp
The macro
.Fn TAILQ_PREV
returns the previous item on the tail queue, from the one specified.
If the tail queue is empty the return value is
.Dv NULL .
.Pp
The macro
.Fn TAILQ_EMPTY
returns true if the tail queue
.Fa head
has no elements.
.Pp
The macros
.Fn TAILQ_FOREACH ,
.Fn TAILQ_FOREACH_REVERSE ,
.Fn TAILQ_FOREACH_SAFE ,
and
.Fn TAILQ_FOREACH_REVERSE_SAFE
traverse the tail queue referenced by
.Fa head
in the forward or reverse direction, assigning each element in turn to
.Fa var .
.Pp
The SAFE versions use
.Fa tmp
to hold the next element, so
.Fa var
may be freed or removed from the list.
.Pp
The macro
.Fn TAILQ_INIT
initializes the tail queue referenced by
.Fa head .
.Pp
The macro
.Fn TAILQ_INSERT_HEAD
inserts the new element
.Fa elm
at the head of the tail queue.
.Pp
The macro
.Fn TAILQ_INSERT_TAIL
inserts the new element
.Fa elm
at the end of the tail queue.
.Pp
The macro
.Fn TAILQ_INSERT_AFTER
inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The macro
.Fn TAILQ_INSERT_BEFORE
inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The macro
.Fn TAILQ_REMOVE
removes the element
.Fa elm
from the tail queue.
.Pp
The macro
.Fn TAILQ_REPLACE
replaces the element
.Fa elm
with the
.Fa new
one specified in the tail queue.
.Pp
The macro
.Fn TAILQ_CONCAT
concatenates the tail queue headed by
.Fa head2
onto the end of the one headed by
.Fa head1 ,
removing all entries from the former.
.Sh TAIL QUEUE EXAMPLE
.Bd -literal
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);

n2 = malloc(sizeof(struct entry));	/* Insert before. */
TAILQ_INSERT_BEFORE(n1, n2, entries);

TAILQ_FOREACH(np, &head, entries)	/* Forward traversal. */
	np-> ...
					/* Reverse traversal. */
TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries)
	np-> ...

while (TAILQ_FIRST(&head) != NULL)	/* Delete. */
	TAILQ_REMOVE(&head, TAILQ_FIRST(&head), entries);
if (TAILQ_EMPTY(&head))			/* Test for emptiness. */
	printf("nothing to do\\n");
.Ed
.Sh SINGLY LINKED TAIL QUEUES
The macros prefixed with
.Do Nm STAILQ_ Dc (
.Fn STAILQ_HEAD ,
.Fn STAILQ_HEAD_INITIALIZER ,
.Fn STAILQ_ENTRY ,
.Fn STAILQ_FOREACH ,
.Fn STAILQ_FOREACH_SAFE ,
.Fn STAILQ_FIRST ,
.Fn STAILQ_EMPTY ,
.Fn STAILQ_NEXT ,
.Fn STAILQ_LAST ,
.Fn STAILQ_INIT ,
.Fn STAILQ_INSERT_HEAD ,
.Fn STAILQ_INSERT_TAIL ,
.Fn STAILQ_INSERT_AFTER ,
.Fn STAILQ_REMOVE_HEAD ,
.Fn STAILQ_REMOVE ,
and
.Fn STAILQ_CONCAT )
are functionally identical to these simple queue functions,
and are provided for compatibility with
.Fx .
.Sh NOTES
Some of these macros or functions perform no error checking,
and invalid usage leads to undefined behaviour.
In the case of macros or functions that expect their arguments
to be elements that are present in the list or queue, passing an element
that is not present is invalid.
.Sh HISTORY
The
.Nm queue
functions first appeared in
.Bx 4.4 .
The
.Nm SIMPLEQ
functions first appeared in
.Nx 1.2 .
The
.Nm SLIST
and
.Nm STAILQ
functions first appeared in
.Fx 2.1.5 .
.Pp
The
.Nm CIRCLEQ
functions first appeared in
.Bx 4.4
and were deprecated in
.Nx 7
and removed in
.Nx 10
due to the pointer aliasing violations.