.\" Copyright (c) 1993

.\"	The Regents of the University of California.  All rights reserved.

.\"

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.\" modification, are permitted provided that the following conditions

.\" are met:

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.\"    notice, this list of conditions and the following disclaimer.

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.\"    may be used to endorse or promote products derived from this software

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.\"

.\"	@(#)queue.3	8.2 (Berkeley) 1/24/94

.\" $FreeBSD$

.\"

.Dd February 7, 2015

.Dt QUEUE 3

.Os

.Sh NAME

.Nm SLIST_EMPTY ,

.Nm SLIST_ENTRY ,

.Nm SLIST_FIRST ,

.Nm SLIST_FOREACH ,

.\" .Nm SLIST_FOREACH_FROM ,

.\" .Nm SLIST_FOREACH_SAFE ,

.\" .Nm SLIST_FOREACH_FROM_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_AFTER ,

.Nm SLIST_REMOVE_HEAD ,

.Nm SLIST_REMOVE ,

.\" .Nm SLIST_SWAP ,

.Nm STAILQ_CONCAT ,

.Nm STAILQ_EMPTY ,

.Nm STAILQ_ENTRY ,

.Nm STAILQ_FIRST ,

.Nm STAILQ_FOREACH ,

.\" .Nm STAILQ_FOREACH_FROM ,

.\" .Nm STAILQ_FOREACH_SAFE ,

.\" .Nm STAILQ_FOREACH_FROM_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_AFTER ,

.Nm STAILQ_REMOVE_HEAD ,

.Nm STAILQ_REMOVE ,

.\" .Nm STAILQ_SWAP ,

.Nm LIST_EMPTY ,

.Nm LIST_ENTRY ,

.Nm LIST_FIRST ,

.Nm LIST_FOREACH ,

.\" .Nm LIST_FOREACH_FROM ,

.\" .Nm LIST_FOREACH_SAFE ,

.\" .Nm LIST_FOREACH_FROM_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_PREV ,

.Nm LIST_REMOVE ,

.\" .Nm LIST_SWAP ,

.Nm TAILQ_CONCAT ,

.Nm TAILQ_EMPTY ,

.Nm TAILQ_ENTRY ,

.Nm TAILQ_FIRST ,

.Nm TAILQ_FOREACH ,

.\" .Nm TAILQ_FOREACH_FROM ,

.\" .Nm TAILQ_FOREACH_SAFE ,

.\" .Nm TAILQ_FOREACH_FROM_SAFE ,

.Nm TAILQ_FOREACH_REVERSE ,

.\" .Nm TAILQ_FOREACH_REVERSE_FROM ,

.\" .Nm TAILQ_FOREACH_REVERSE_SAFE ,

.\" .Nm TAILQ_FOREACH_REVERSE_FROM_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 ,

.Nm TAILQ_SWAP

.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_FROM "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_FOREACH_FROM_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_AFTER "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 SLIST_SWAP "SLIST_HEAD *head1" "SLIST_HEAD *head2" "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_FROM "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_FOREACH_FROM_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_AFTER "STAILQ_HEAD *head" "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_SWAP "STAILQ_HEAD *head1" "STAILQ_HEAD *head2" "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_FROM "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_FOREACH_FROM_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_PREV "TYPE *elm" "LIST_HEAD *head" "TYPE" "LIST_ENTRY NAME"

.Fn LIST_REMOVE "TYPE *elm" "LIST_ENTRY NAME"

.Fn LIST_SWAP "LIST_HEAD *head1" "LIST_HEAD *head2" "TYPE" "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_FROM "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_FROM_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_FROM "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_FOREACH_REVERSE_FROM_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"

.Fn TAILQ_SWAP "TAILQ_HEAD *head1" "TAILQ_HEAD *head2" "TYPE" "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:

.Pp

.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

Forward traversal through the list.

.It

Swapping the contents of two lists.

.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.

Singly-linked lists add the following functionality:

.Pp

.Bl -enum -compact -offset indent

.It

O(n) removal of any entry in the list.

.El

.Pp

Singly-linked tail queues add the following functionality:

.Pp

.Bl -enum -compact -offset indent

.It

Entries can be added at the end of a list.

.It

O(n) removal of any entry in the list.

.It

They may be concatenated.

.El

.Pp

However:

.Pp

.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 tail 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:

.Pp

.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

.Pp

However:

.Pp

.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.

They add the following functionality over the above:

.Pp

.Bl -enum -compact -offset indent

.It

They may be traversed backwards.

.El

.Pp

However:

.Pp

.Bl -enum -compact -offset indent

.It

To traverse backwards, an entry to begin the traversal and the list in

which it is contained must be specified.

.El

.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

.Pp

However:

.Pp

.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.

.Ss 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_FROM

.\" behaves identically to

.\" .Nm SLIST_FOREACH

.\" when

.\" .Fa var

.\" is NULL, else it treats

.\" .Fa var

.\" as a previously found SLIST element and begins the loop at

.\" .Fa var

.\" instead of the first element in the SLIST referenced by

.\" .Fa head .

.\" .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_FOREACH_FROM_SAFE

.\" behaves identically to

.\" .Nm SLIST_FOREACH_SAFE

.\" when

.\" .Fa var

.\" is NULL, else it treats

.\" .Fa var

.\" as a previously found SLIST element and begins the loop at

.\" .Fa var

.\" instead of the first element in the SLIST referenced by

.\" .Fa head .

.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_AFTER

.\" removes the element after

.\" .Fa elm

.\" from the list.

.\" Unlike

.\" .Fa SLIST_REMOVE ,

.\" this macro does not traverse the entire 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.

.\" .Pp

.\" The macro

.\" .Nm SLIST_SWAP

.\" swaps the contents of

.\" .Fa head1

.\" and

.\" .Fa head2 .

.Ss 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

.Ss 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_FROM

.\" behaves identically to

.\" .Nm STAILQ_FOREACH

.\" when

.\" .Fa var

.\" is NULL, else it treats

.\" .Fa var

.\" as a previously found STAILQ element and begins the loop at

.\" .Fa var

.\" instead of the first element in the STAILQ referenced by

.\" .Fa head .

.\" .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_FOREACH_FROM_SAFE

.\" behaves identically to

.\" .Nm STAILQ_FOREACH_SAFE

.\" when

.\" .Fa var

.\" is NULL, else it treats

.\" .Fa var

.\" as a previously found STAILQ element and begins the loop at

.\" .Fa var

.\" instead of the first element in the STAILQ referenced by

.\" .Fa head .

.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

.\" .Dv NULL .

.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_AFTER

.\" removes the element after

.\" .Fa elm

.\" from the tail queue.

.\" Unlike

.\" .Fa STAILQ_REMOVE ,

.\" this macro does not traverse the entire tail queue.

.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.

.\" .Pp

.\" The macro

.\" .Nm STAILQ_SWAP

.\" swaps the contents of

.\" .Fa head1

.\" and

.\" .Fa head2 .

.Ss 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

.Ss 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_FROM

.\" behaves identically to

.\" .Nm LIST_FOREACH

.\" when

.\" .Fa var

.\" is NULL, else it treats

.\" .Fa var

.\" as a previously found LIST element and begins the loop at

.\" .Fa var

.\" instead of the first element in the LIST referenced by

.\" .Fa head .

.\" .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_FOREACH_FROM_SAFE

.\" behaves identically to

.\" .Nm LIST_FOREACH_SAFE

.\" when

.\" .Fa var

.\" is NULL, else it treats

.\" .Fa var

.\" as a previously found LIST element and begins the loop at

.\" .Fa var

.\" instead of the first element in the LIST referenced by

.\" .Fa head .

.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