merge fix for boot-time hang on centos' xen

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.\"     @(#)queue.3     8.2 (Berkeley) 1/24/94
.\" $FreeBSD$
.\"
.Dd January 24, 1994
.Dt QUEUE 3
.Os
.Sh NAME
.Nm SLIST_EMPTY ,
.Nm SLIST_ENTRY ,
.Nm SLIST_FIRST ,
.Nm SLIST_FOREACH ,
.Nm SLIST_FOREACH_SAFE ,
.Nm SLIST_HEAD ,
.Nm SLIST_HEAD_INITIALIZER ,
.Nm SLIST_INIT ,
.Nm SLIST_INSERT_AFTER ,
.Nm SLIST_INSERT_HEAD ,
.Nm SLIST_NEXT ,
.Nm SLIST_REMOVE_HEAD ,
.Nm SLIST_REMOVE ,
.Nm STAILQ_CONCAT ,
.Nm STAILQ_EMPTY ,
.Nm STAILQ_ENTRY ,
.Nm STAILQ_FIRST ,
.Nm STAILQ_FOREACH ,
.Nm STAILQ_FOREACH_SAFE ,
.Nm STAILQ_HEAD ,
.Nm STAILQ_HEAD_INITIALIZER ,
.Nm STAILQ_INIT ,
.Nm STAILQ_INSERT_AFTER ,
.Nm STAILQ_INSERT_HEAD ,
.Nm STAILQ_INSERT_TAIL ,
.Nm STAILQ_LAST ,
.Nm STAILQ_NEXT ,
.Nm STAILQ_REMOVE_HEAD ,
.Nm STAILQ_REMOVE ,
.Nm LIST_EMPTY ,
.Nm LIST_ENTRY ,
.Nm LIST_FIRST ,
.Nm LIST_FOREACH ,
.Nm LIST_FOREACH_SAFE ,
.Nm LIST_HEAD ,
.Nm LIST_HEAD_INITIALIZER ,
.Nm LIST_INIT ,
.Nm LIST_INSERT_AFTER ,
.Nm LIST_INSERT_BEFORE ,
.Nm LIST_INSERT_HEAD ,
.Nm LIST_NEXT ,
.Nm LIST_REMOVE ,
.Nm TAILQ_CONCAT ,
.Nm TAILQ_EMPTY ,
.Nm TAILQ_ENTRY ,
.Nm TAILQ_FIRST ,
.Nm TAILQ_FOREACH ,
.Nm TAILQ_FOREACH_SAFE ,
.Nm TAILQ_FOREACH_REVERSE ,
.Nm TAILQ_FOREACH_REVERSE_SAFE ,
.Nm TAILQ_HEAD ,
.Nm TAILQ_HEAD_INITIALIZER ,
.Nm TAILQ_INIT ,
.Nm TAILQ_INSERT_AFTER ,
.Nm TAILQ_INSERT_BEFORE ,
.Nm TAILQ_INSERT_HEAD ,
.Nm TAILQ_INSERT_TAIL ,
.Nm TAILQ_LAST ,
.Nm TAILQ_NEXT ,
.Nm TAILQ_PREV ,
.Nm TAILQ_REMOVE
.Nd implementations of singly-linked lists, singly-linked tail queues,
lists and tail queues
.Sh SYNOPSIS
.In sys/queue.h
.\"
.Fn SLIST_EMPTY "SLIST_HEAD *head"
.Fn SLIST_ENTRY "TYPE"
.Fn SLIST_FIRST "SLIST_HEAD *head"
.Fn SLIST_FOREACH "TYPE *var" "SLIST_HEAD *head" "SLIST_ENTRY NAME"
.Fn SLIST_FOREACH_SAFE "TYPE *var" "SLIST_HEAD *head" "SLIST_ENTRY NAME" "TYPE *temp_var"
.Fn SLIST_HEAD "HEADNAME" "TYPE"
.Fn SLIST_HEAD_INITIALIZER "SLIST_HEAD head"
.Fn SLIST_INIT "SLIST_HEAD *head"
.Fn SLIST_INSERT_AFTER "TYPE *listelm" "TYPE *elm" "SLIST_ENTRY NAME"
.Fn SLIST_INSERT_HEAD "SLIST_HEAD *head" "TYPE *elm" "SLIST_ENTRY NAME"
.Fn SLIST_NEXT "TYPE *elm" "SLIST_ENTRY NAME"
.Fn SLIST_REMOVE_HEAD "SLIST_HEAD *head" "SLIST_ENTRY NAME"
.Fn SLIST_REMOVE "SLIST_HEAD *head" "TYPE *elm" "TYPE" "SLIST_ENTRY NAME"
.\"
.Fn STAILQ_CONCAT "STAILQ_HEAD *head1" "STAILQ_HEAD *head2"
.Fn STAILQ_EMPTY "STAILQ_HEAD *head"
.Fn STAILQ_ENTRY "TYPE"
.Fn STAILQ_FIRST "STAILQ_HEAD *head"
.Fn STAILQ_FOREACH "TYPE *var" "STAILQ_HEAD *head" "STAILQ_ENTRY NAME"
.Fn STAILQ_FOREACH_SAFE "TYPE *var" "STAILQ_HEAD *head" "STAILQ_ENTRY NAME" "TYPE *temp_var"
.Fn STAILQ_HEAD "HEADNAME" "TYPE"
.Fn STAILQ_HEAD_INITIALIZER "STAILQ_HEAD head"
.Fn STAILQ_INIT "STAILQ_HEAD *head"
.Fn STAILQ_INSERT_AFTER "STAILQ_HEAD *head" "TYPE *listelm" "TYPE *elm" "STAILQ_ENTRY NAME"
.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_LAST "STAILQ_HEAD *head" "TYPE" "STAILQ_ENTRY NAME"
.Fn STAILQ_NEXT "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 LIST_EMPTY "LIST_HEAD *head"
.Fn LIST_ENTRY "TYPE"
.Fn LIST_FIRST "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 *temp_var"
.Fn LIST_HEAD "HEADNAME" "TYPE"
.Fn LIST_HEAD_INITIALIZER "LIST_HEAD head"
.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_NEXT "TYPE *elm" "LIST_ENTRY NAME"
.Fn LIST_REMOVE "TYPE *elm" "LIST_ENTRY NAME"
.\"
.Fn TAILQ_CONCAT "TAILQ_HEAD *head1" "TAILQ_HEAD *head2" "TAILQ_ENTRY NAME"
.Fn TAILQ_EMPTY "TAILQ_HEAD *head"
.Fn TAILQ_ENTRY "TYPE"
.Fn TAILQ_FIRST "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 *temp_var"
.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 *temp_var"
.Fn TAILQ_HEAD "HEADNAME" "TYPE"
.Fn TAILQ_HEAD_INITIALIZER "TAILQ_HEAD head"
.Fn TAILQ_INIT "TAILQ_HEAD *head"
.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_INSERT_HEAD "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
.Fn TAILQ_INSERT_TAIL "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
.Fn TAILQ_LAST "TAILQ_HEAD *head" "HEADNAME"
.Fn TAILQ_NEXT "TYPE *elm" "TAILQ_ENTRY NAME"
.Fn TAILQ_PREV "TYPE *elm" "HEADNAME" "TAILQ_ENTRY NAME"
.Fn TAILQ_REMOVE "TAILQ_HEAD *head" "TYPE *elm" "TAILQ_ENTRY NAME"
.\"
.Sh DESCRIPTION
These macros define and operate on four types of data structures:
singly-linked lists, singly-linked tail queues, lists, and tail queues.
All four 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
O(1) removal of an entry from the head of the list.
.It
O(n) 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
Singly-linked 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 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 singly-linked lists.
.El
.Pp
Singly-linked tailqs 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 elements 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 traversed backwards, from tail to head.
.It
They may be concatenated.
.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
Code size is about 15% greater and operations run about 20% slower
than singly-linked 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 STAILQ_ENTRY ,
.Li LIST_ENTRY ,
or
.Li TAILQ_ENTRY ,
named
.Fa NAME .
The argument
.Fa HEADNAME
is the name of a user defined structure that must be declared
using the macros
.Li SLIST_HEAD ,
.Li STAILQ_HEAD ,
.Li LIST_HEAD ,
or
.Li TAILQ_HEAD .
See the examples below for further explanation of how these
macros are used.
.Sh SINGLY-LINKED LISTS
A singly-linked list is headed by a structure defined by the
.Nm 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
.Nm SLIST_HEAD_INITIALIZER
evaluates to an initializer for the list
.Fa head .
.Pp
The macro
.Nm SLIST_EMPTY
evaluates to true if there are no elements in the list.
.Pp
The macro
.Nm SLIST_ENTRY
declares a structure that connects the elements in
the list.
.Pp
The macro
.Nm SLIST_FIRST
returns the first element in the list or NULL if the list is empty.
.Pp
The macro
.Nm SLIST_FOREACH
traverses the list referenced by
.Fa head
in the forward direction, assigning each element in
turn to
.Fa var .
.Pp
The macro
.Nm SLIST_FOREACH_SAFE
traverses the list referenced by
.Fa head
in the forward direction, assigning each element in
turn to
.Fa var .
However, unlike
.Fn SLIST_FOREACH
here it is permitted to both remove
.Fa var
as well as free it from within the loop safely without interfering with the
traversal.
.Pp
The macro
.Nm SLIST_INIT
initializes the list referenced by
.Fa head .
.Pp
The macro
.Nm SLIST_INSERT_HEAD
inserts the new element
.Fa elm
at the head of the list.
.Pp
The macro
.Nm SLIST_INSERT_AFTER
inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The macro
.Nm SLIST_NEXT
returns the next element in the list.
.Pp
The macro
.Nm SLIST_REMOVE_HEAD
removes the element
.Fa elm
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
.Fa SLIST_REMOVE
macro.
.Pp
The macro
.Nm SLIST_REMOVE
removes the element
.Fa elm
from the list.
.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);
                                        /* Forward traversal. */
SLIST_FOREACH(np, &head, entries)
        np-> ...
                                        /* Safe forward traversal. */
SLIST_FOREACH_SAFE(np, &head, entries, np_temp) {
        np->do_stuff();
        ...
        SLIST_REMOVE(&head, np, entry, entries);
        free(np);
}

while (!SLIST_EMPTY(&head)) {           /* List Deletion. */
        n1 = SLIST_FIRST(&head);
        SLIST_REMOVE_HEAD(&head, entries);
        free(n1);
}
.Ed
.Sh SINGLY-LINKED TAIL QUEUES
A singly-linked tail queue is headed by a structure defined by the
.Nm STAILQ_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 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 tail queue after an existing element,
at the head of the tail queue, or at the end of the tail queue.
A
.Fa STAILQ_HEAD
structure is declared as follows:
.Bd -literal -offset indent
STAILQ_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
.Nm STAILQ_HEAD_INITIALIZER
evaluates to an initializer for the tail queue
.Fa head .
.Pp
The macro
.Nm STAILQ_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.
.Pp
The macro
.Nm STAILQ_EMPTY
evaluates to true if there are no items on the tail queue.
.Pp
The macro
.Nm STAILQ_ENTRY
declares a structure that connects the elements in
the tail queue.
.Pp
The macro
.Nm STAILQ_FIRST
returns the first item on the tail queue or NULL if the tail queue
is empty.
.Pp
The macro
.Nm STAILQ_FOREACH
traverses the tail queue referenced by
.Fa head
in the forward direction, assigning each element
in turn to
.Fa var .
.Pp
The macro
.Nm STAILQ_FOREACH_SAFE
traverses the tail queue referenced by
.Fa head
in the forward direction, assigning each element
in turn to
.Fa var .
However, unlike
.Fn STAILQ_FOREACH
here it is permitted to both remove
.Fa var
as well as free it from within the loop safely without interfering with the
traversal.
.Pp
The macro
.Nm STAILQ_INIT
initializes the tail queue referenced by
.Fa head .
.Pp
The macro
.Nm STAILQ_INSERT_HEAD
inserts the new element
.Fa elm
at the head of the tail queue.
.Pp
The macro
.Nm STAILQ_INSERT_TAIL
inserts the new element
.Fa elm
at the end of the tail queue.
.Pp
The macro
.Nm STAILQ_INSERT_AFTER
inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The macro
.Nm STAILQ_LAST
returns the last item on the tail queue.
If the tail queue is empty the return value is undefined.
.Pp
The macro
.Nm STAILQ_NEXT
returns the next item on the tail queue, or NULL this item is the last.
.Pp
The macro
.Nm STAILQ_REMOVE_HEAD
removes the element at the head of the tail queue.
For optimum efficiency,
elements being removed from the head of the tail queue should
use this macro explicitly rather than the generic
.Fa STAILQ_REMOVE
macro.
.Pp
The macro
.Nm STAILQ_REMOVE
removes the element
.Fa elm
from the tail queue.
.Sh SINGLY-LINKED TAIL QUEUE EXAMPLE
.Bd -literal
STAILQ_HEAD(stailhead, entry) head =
    STAILQ_HEAD_INITIALIZER(head);
struct stailhead *headp;                /* Singly-linked tail queue head. */
struct entry {
        ...
        STAILQ_ENTRY(entry) entries;    /* Tail queue. */
        ...
} *n1, *n2, *n3, *np;

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

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

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

n2 = malloc(sizeof(struct entry));      /* Insert after. */
STAILQ_INSERT_AFTER(&head, n1, n2, entries);
                                        /* Deletion. */
STAILQ_REMOVE(&head, n2, entry, entries);
free(n2);
                                        /* Deletion from the head. */
n3 = STAILQ_FIRST(&head);
STAILQ_REMOVE_HEAD(&head, entries);
free(n3);
                                        /* Forward traversal. */
STAILQ_FOREACH(np, &head, entries)
        np-> ...
                                        /* Safe forward traversal. */
STAILQ_FOREACH_SAFE(np, &head, entries, np_temp) {
        np->do_stuff();
        ...
        STAILQ_REMOVE(&head, np, entry, entries);
        free(np);
}
                                        /* TailQ Deletion. */
while (!STAILQ_EMPTY(&head)) {
        n1 = STAILQ_FIRST(&head);
        STAILQ_REMOVE_HEAD(&head, entries);
        free(n1);
}
                                        /* Faster TailQ Deletion. */
n1 = STAILQ_FIRST(&head);
while (n1 != NULL) {
        n2 = STAILQ_NEXT(n1, entries);
        free(n1);
        n1 = n2;
}
STAILQ_INIT(&head);
.Ed
.Sh LISTS
A list is headed by a structure defined by the
.Nm 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
.Nm LIST_HEAD_INITIALIZER
evaluates to an initializer for the list
.Fa head .
.Pp
The macro
.Nm LIST_EMPTY
evaluates to true if there are no elements in the list.
.Pp
The macro
.Nm LIST_ENTRY
declares a structure that connects the elements in
the list.
.Pp
The macro
.Nm LIST_FIRST
returns the first element in the list or NULL if the list
is empty.
.Pp
The macro
.Nm LIST_FOREACH
traverses the list referenced by
.Fa head
in the forward direction, assigning each element in turn to
.Fa var .
.Pp
The macro
.Nm LIST_FOREACH_SAFE
traverses the list referenced by
.Fa head
in the forward direction, assigning each element in turn to
.Fa var .
However, unlike
.Fn LIST_FOREACH
here it is permitted to both remove
.Fa var
as well as free it from within the loop safely without interfering with the
traversal.
.Pp
The macro
.Nm LIST_INIT
initializes the list referenced by
.Fa head .
.Pp
The macro
.Nm LIST_INSERT_HEAD
inserts the new element
.Fa elm
at the head of the list.
.Pp
The macro
.Nm LIST_INSERT_AFTER
inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The macro
.Nm LIST_INSERT_BEFORE
inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The macro
.Nm LIST_NEXT
returns the next element in the list, or NULL if this is the last.
.Pp
The macro
.Nm LIST_REMOVE
removes the element
.Fa elm
from the list.
.Sh LIST EXAMPLE
.Bd -literal
LIST_HEAD(listhead, entry) head =
    LIST_HEAD_INITIALIZER(head);
struct listhead *headp;                 /* List head. */
struct entry {
        ...
        LIST_ENTRY(entry) entries;      /* List. */
        ...
} *n1, *n2, *n3, *np, *np_temp;

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);

n3 = malloc(sizeof(struct entry));      /* Insert before. */
LIST_INSERT_BEFORE(n2, n3, entries);

LIST_REMOVE(n2, entries);               /* Deletion. */
free(n2);
                                        /* Forward traversal. */
LIST_FOREACH(np, &head, entries)
        np-> ...

                                        /* Safe forward traversal. */
LIST_FOREACH_SAFE(np, &head, entries, np_temp) {
        np->do_stuff();
        ...
        LIST_REMOVE(np, entries);
        free(np);
}

while (!LIST_EMPTY(&head)) {            /* List Deletion. */
        n1 = LIST_FIRST(&head);
        LIST_REMOVE(n1, entries);
        free(n1);
}

n1 = LIST_FIRST(&head);                 /* Faster List Deletion. */
while (n1 != NULL) {
        n2 = LIST_NEXT(n1, entries);
        free(n1);
        n1 = n2;
}
LIST_INIT(&head);
.Ed
.Sh TAIL QUEUES
A tail queue is headed by a structure defined by the
.Nm 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,
before an existing element, at the head of the tail queue,
or at the end of the tail 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
.Nm TAILQ_HEAD_INITIALIZER
evaluates to an initializer for the tail queue
.Fa head .
.Pp
The macro
.Nm 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.
.Pp
The macro
.Nm TAILQ_EMPTY
evaluates to true if there are no items on the tail queue.
.Pp
The macro
.Nm TAILQ_ENTRY
declares a structure that connects the elements in
the tail queue.
.Pp
The macro
.Nm TAILQ_FIRST
returns the first item on the tail queue or NULL if the tail queue
is empty.
.Pp
The macro
.Nm TAILQ_FOREACH
traverses the tail queue referenced by
.Fa head
in the forward direction, assigning each element in turn to
.Fa var .
.Fa var
is set to
.Dv NULL
if the loop completes normally, or if there were no elements.
.Pp
The macro
.Nm TAILQ_FOREACH_REVERSE
traverses the tail queue referenced by
.Fa head
in the reverse direction, assigning each element in turn to
.Fa var .
.Pp
The macros
.Nm TAILQ_FOREACH_SAFE
and
.Nm TAILQ_FOREACH_REVERSE_SAFE
traverse the list referenced by
.Fa head
in the forward or reverse direction respectively,
assigning each element in turn to
.Fa var .
However, unlike their unsafe counterparts,
.Nm TAILQ_FOREACH
and
.Nm TAILQ_FOREACH_REVERSE
permit to both remove
.Fa var
as well as free it from within the loop safely without interfering with the
traversal.
.Pp
The macro
.Nm TAILQ_INIT
initializes the tail queue referenced by
.Fa head .
.Pp
The macro
.Nm TAILQ_INSERT_HEAD
inserts the new element
.Fa elm
at the head of the tail queue.
.Pp
The macro
.Nm TAILQ_INSERT_TAIL
inserts the new element
.Fa elm
at the end of the tail queue.
.Pp
The macro
.Nm TAILQ_INSERT_AFTER
inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The macro
.Nm TAILQ_INSERT_BEFORE
inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The macro
.Nm TAILQ_LAST
returns the last item on the tail queue.
If the tail queue is empty the return value is undefined.
.Pp
The macro
.Nm TAILQ_NEXT
returns the next item on the tail queue, or NULL if this item is the last.
.Pp
The macro
.Nm TAILQ_PREV
returns the previous item on the tail queue, or NULL if this item
is the first.
.Pp
The macro
.Nm TAILQ_REMOVE
removes the element
.Fa elm
from the tail queue.
.Sh TAIL QUEUE EXAMPLE
.Bd -literal
TAILQ_HEAD(tailhead, entry) head =
    TAILQ_HEAD_INITIALIZER(head);
struct tailhead *headp;                 /* Tail queue head. */
struct entry {
        ...
        TAILQ_ENTRY(entry) entries;     /* Tail queue. */
        ...
} *n1, *n2, *n3, *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);

n3 = malloc(sizeof(struct entry));      /* Insert before. */
TAILQ_INSERT_BEFORE(n2, n3, entries);

TAILQ_REMOVE(&head, n2, entries);       /* Deletion. */
free(n2);
                                        /* Forward traversal. */
TAILQ_FOREACH(np, &head, entries)
        np-> ...
                                        /* Safe forward traversal. */
TAILQ_FOREACH_SAFE(np, &head, entries, np_temp) {
        np->do_stuff();
        ...
        TAILQ_REMOVE(&head, np, entries);
        free(np);
}
                                        /* Reverse traversal. */
TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries)
        np-> ...
                                        /* TailQ Deletion. */
while (!TAILQ_EMPTY(&head)) {
        n1 = TAILQ_FIRST(&head);
        TAILQ_REMOVE(&head, n1, entries);
        free(n1);
}
                                        /* Faster TailQ Deletion. */
n1 = TAILQ_FIRST(&head);
while (n1 != NULL) {
        n2 = TAILQ_NEXT(n1, entries);
        free(n1);
        n1 = n2;
}
TAILQ_INIT(&head);
.Ed
.Sh HISTORY
The
.Nm queue
functions first appeared in
.Bx 4.4 .