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.\"     @(#)queue.3     8.2 (Berkeley) 1/24/94
.\"
.\" hch, 2002-03-25
.\" 2007-12-08, mtk, Converted from mdoc to man macros
.\"
.TH QUEUE 3 2007-12-28 "Linux" "Linux Programmer's Manual"
.SH NAME
LIST_ENTRY, LIST_HEAD, LIST_INIT, LIST_INSERT_AFTER,
LIST_INSERT_HEAD, LIST_REMOVE, TAILQ_ENTRY, TAILQ_HEAD,
TAILQ_INIT, TAILQ_INSERT_AFTER, TAILQ_INSERT_HEAD, TAILQ_INSERT_TAIL,
TAILQ_REMOVE, CIRCLEQ_ENTRY, CIRCLEQ_HEAD, CIRCLEQ_INIT,
CIRCLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE,
CIRCLEQ_INSERT_HEAD, CIRCLEQ_INSERT_TAIL,
CIRCLEQ_REMOVE \- implementations of lists, tail queues, and circular queues
.SH SYNOPSIS
.nf
.B #include <sys/queue.h>

.BI "LIST_ENTRY(" TYPE );
.BI "LIST_HEAD(" HEADNAME ", " TYPE );
.BI "LIST_INIT(LIST_HEAD *" head );
.BI "LIST_INSERT_AFTER(LIST_ENTRY *" listelm ", "
.BI "                TYPE *" elm ", LIST_ENTRY " NAME );
.BI "LIST_INSERT_HEAD(LIST_HEAD *" head ", "
.BI "                TYPE *" elm ", LIST_ENTRY " NAME );
.BI "LIST_REMOVE(TYPE *" elm ", LIST_ENTRY " NAME );

.BI "TAILQ_ENTRY(" TYPE );
.BI "TAILQ_HEAD("HEADNAME ", " TYPE );
.BI "TAILQ_INIT(TAILQ_HEAD *" head );
.BI "TAILQ_INSERT_AFTER(TAILQ_HEAD *" head ", TYPE *" listelm ", "
.BI "                TYPE *" elm ", TAILQ_ENTRY " NAME );
.BI "TAILQ_INSERT_HEAD(TAILQ_HEAD *" head ", "
.BI "                TYPE *" elm ", TAILQ_ENTRY " NAME );
.BI "TAILQ_INSERT_TAIL(TAILQ_HEAD *" head ", "
.BI "                TYPE *" elm ", TAILQ_ENTRY " NAME );
.BI "TAILQ_REMOVE(TAILQ_HEAD *" head ", TYPE *" elm ", TAILQ_ENTRY " NAME );

.BI CIRCLEQ_ENTRY( TYPE );
.BI "CIRCLEQ_HEAD(" HEADNAME ", " TYPE );
.BI "CIRCLEQ_INIT(CIRCLEQ_HEAD *" head );
.BI "CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *" head ", TYPE *" listelm ", "
.BI "                TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *" head ", TYPE *" listelm ", "
.BI "                TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *" head ", "
.BI "                TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *" head ", "
.BI "                TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.BI "CIRCLEQ_REMOVE(CIRCLEQ_HEAD *" head ", "
.BI "                TYPE *" elm ", CIRCLEQ_ENTRY " NAME );
.fi
.SH DESCRIPTION
These macros define and operate on three types of data structures:
lists, tail queues, and circular queues.
All three structures support the following functionality:
.sp
.RS 4
.PD 0
.IP * 4
Insertion of a new entry at the head of the list.
.IP *
Insertion of a new entry after any element in the list.
.IP *
Removal of any entry in the list.
.IP *
Forward traversal through the list.
.PD
.RE
.PP
Lists are the simplest of the three data structures and support
only the above functionality.

Tail queues add the following functionality:
.RS 4
.IP * 4
Entries can be added at the end of a list.
.RE
.PP
However:
.sp
.RS 4
.PD 0
.IP 1. 4
All list insertions and removals must specify the head of the list.
.IP 2.
Each head entry requires two pointers rather than one.
.IP 3.
Code size is about 15% greater and operations run about 20% slower
than lists.
.PD
.RE
.PP
Circular queues add the following functionality:
.sp
.RS 4
.PD 0
.IP * 4
Entries can be added at the end of a list.
.IP *
Entries can be added before another entry.
.IP *
They may be traversed backward, from tail to head.
.PD
.RE
.PP
However:
.sp
.RS 4
.PD 0
.IP 1. 4
All list insertions and removals must specify the head of the list.
.IP 2.
Each head entry requires two pointers rather than one.
.IP 3.
The termination condition for traversal is more complex.
.IP 4.
Code size is about 40% greater and operations run about 45% slower
than lists.
.PD
.RE
.PP
In the macro definitions,
.I TYPE
is the name of a user-defined structure,
that must contain a field of type
.BR "LIST_ENTRY" ,
.BR "TAILQ_ENTRY" ,
or
.BR "CIRCLEQ_ENTRY" ,
named
.IR NAME .
The argument
.I HEADNAME
is the name of a user-defined structure that must be declared
using the macros
.BR "LIST_HEAD" ,
.BR "TAILQ_HEAD" ,
or
.BR "CIRCLEQ_HEAD" .
See the examples below for further explanation of how these
macros are used.
.SS Lists
A list is headed by a structure defined by the
.B LIST_HEAD
macro.
This structure contains a single pointer to the first element
on the list.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the list.
New elements can be added to the list after an existing element or
at the head of the list.
A
.B LIST_HEAD
structure is declared as follows:
.in +4n
.nf

LIST_HEAD(HEADNAME, TYPE) head;
.fi
.in
.PP
where
.I HEADNAME
is the name of the structure to be defined, and
.I 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:
.in +4n
.nf

struct HEADNAME *headp;
.fi
.in
.PP
(The names
.IR "head"
and
.IR "headp"
are user selectable.)
.PP
The macro
.B LIST_ENTRY
declares a structure that connects the elements in
the list.
.PP
The macro
.B LIST_INIT
initializes the list referenced by
.IR head .
.PP
The macro
.B LIST_INSERT_HEAD
inserts the new element
.I elm
at the head of the list.
.PP
The macro
.B LIST_INSERT_AFTER
inserts the new element
.I elm
after the element
.IR listelm .
.PP
The macro
.B LIST_REMOVE
removes the element
.I elm
from the list.
.SS List example
.nf
LIST_HEAD(listhead, entry) head;
struct listhead *headp;                 /* List head. */
struct entry {
    ...
    LIST_ENTRY(entry) entries;          /* List. */
    ...
} *n1, *n2, *np;

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

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

n2 = malloc(sizeof(struct entry));      /* Insert after. */
LIST_INSERT_AFTER(n1, n2, entries);
                                        /* Forward traversal. */
for (np = head.lh_first; np != NULL; np = np\->entries.le_next)
    np\-> ...

.\" FIXME . http://sourceware.org/bugzilla/show_bug.cgi?id=1506
while (head.lh_first != NULL)           /* Delete. */
    LIST_REMOVE(head.lh_first, entries);
.fi
.SS Tail queues
A tail queue is headed by a structure defined by the
.B TAILQ_HEAD
macro.
This structure contains a pair of pointers,
one to the first element in the tail queue and the other to
the last element in the tail queue.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the tail queue.
New elements can be added to the tail queue after an existing element,
at the head of the tail queue, or at the end of the tail queue.
A
.B TAILQ_HEAD
structure is declared as follows:
.in +4n
.nf

TAILQ_HEAD(HEADNAME, TYPE) head;
.fi
.in
.PP
where
.IR "HEADNAME"
is the name of the structure to be defined, and
.IR "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:
.in +4n
.nf

struct HEADNAME *headp;
.fi
.in
.PP
(The names
.IR "head"
and
.IR "headp"
are user selectable.)
.PP
The macro
.B TAILQ_ENTRY
declares a structure that connects the elements in
the tail queue.
.PP
The macro
.B TAILQ_INIT
initializes the tail queue referenced by
.IR head .
.PP
The macro
.B TAILQ_INSERT_HEAD
inserts the new element
.I elm
at the head of the tail queue.
.PP
The macro
.B TAILQ_INSERT_TAIL
inserts the new element
.I elm
at the end of the tail queue.
.PP
The macro
.B TAILQ_INSERT_AFTER
inserts the new element
.I elm
after the element
.IR listelm .
.PP
The macro
.B TAILQ_REMOVE
removes the element
.I elm
from the tail queue.
.SS Tail queue example
.nf
TAILQ_HEAD(tailhead, entry) head;
struct tailhead *headp;                 /* Tail queue head. */
struct entry {
    ...
    TAILQ_ENTRY(entry) entries;         /* Tail queue. */
    ...
} *n1, *n2, *np;

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

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

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

n2 = malloc(sizeof(struct entry));      /* Insert after. */
TAILQ_INSERT_AFTER(&head, n1, n2, entries);
                                        /* Forward traversal. */
for (np = head.tqh_first; np != NULL; np = np\->entries.tqe_next)
    np\-> ...
                                        /* Delete. */
while (head.tqh_first != NULL)
    TAILQ_REMOVE(&head, head.tqh_first, entries);
.fi
.SS Circular queues
A circular queue is headed by a structure defined by the
.B 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
.B CIRCLEQ_HEAD
structure is declared as follows:
.in +4n
.nf

CIRCLEQ_HEAD(HEADNAME, TYPE) head;
.fi
.in
.PP
where
.IR "HEADNAME"
is the name of the structure to be defined, and
.IR "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:
.in +4n
.nf

struct HEADNAME *headp;
.fi
.in
.PP
(The names
.IR "head"
and
.IR "headp"
are user selectable.)
.PP
The macro
.B CIRCLEQ_ENTRY
declares a structure that connects the elements in
the circular queue.
.PP
The macro
.B CIRCLEQ_INIT
initializes the circular queue referenced by
.IR head .
.PP
The macro
.B CIRCLEQ_INSERT_HEAD
inserts the new element
.I elm
at the head of the circular queue.
.PP
The macro
.B CIRCLEQ_INSERT_TAIL
inserts the new element
.I elm
at the end of the circular queue.
.PP
The macro
.B CIRCLEQ_INSERT_AFTER
inserts the new element
.I elm
after the element
.IR listelm .
.PP
The macro
.B CIRCLEQ_INSERT_BEFORE
inserts the new element
.I elm
before the element
.IR listelm .
.PP
The macro
.B CIRCLEQ_REMOVE
removes the element
.I elm
from the circular queue.
.SS Circular queue example
.nf
CIRCLEQ_HEAD(circleq, entry) head;
struct circleq *headp;              /* Circular queue head. */
struct entry {
    ...
    CIRCLEQ_ENTRY(entry) entries;   /* Circular queue. */
    ...
} *n1, *n2, *np;

CIRCLEQ_INIT(&head);                /* Initialize the circular queue. */

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

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

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

n2 = malloc(sizeof(struct entry));  /* Insert before. */
CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
                                    /* Forward traversal. */
for (np = head.cqh_first; np != (void *)&head;
        np = np\->entries.cqe_next)
    np\-> ...
                                    /* Reverse traversal. */
for (np = head.cqh_last; np != (void *)&head; np = np\->entries.cqe_prev)
    np\-> ...
                                    /* Delete. */
while (head.cqh_first != (void *)&head)
    CIRCLEQ_REMOVE(&head, head.cqh_first, entries);
.fi
.SH CONFORMING TO
Not in POSIX.1-2001.
Present on the BSDs.
The
queue functions first appeared in
4.4BSD.
.SH COLOPHON
This page is part of release 3.74 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%http://www.kernel.org/doc/man\-pages/.