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.\" Copyright (c) 1993
.\" The Regents of the University of California. All rights reserved.
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.\"
.\" @(#)queue.3 8.2 (Berkeley) 1/24/94
.\" $FreeBSD$
.\"
.Dd February 7, 2015
.Dt QUEUE 3
.Os
.Sh NAME
.Nm TAILQ_CONCAT ,
.Nm TAILQ_EMPTY ,
.Nm TAILQ_ENTRY ,
.Nm TAILQ_FIRST ,
.Nm TAILQ_FOREACH ,
.\" .Nm TAILQ_FOREACH_FROM ,
.\" .Nm TAILQ_FOREACH_FROM_SAFE ,
.Nm TAILQ_FOREACH_REVERSE ,
.\" .Nm TAILQ_FOREACH_REVERSE_FROM ,
.\" .Nm TAILQ_FOREACH_REVERSE_FROM_SAFE ,
.\" .Nm TAILQ_FOREACH_REVERSE_SAFE ,
.\" .Nm TAILQ_FOREACH_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, tail queues, and circular queues
.Sh SYNOPSIS
.In sys/queue.h
.\"
.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_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_FROM_SAFE "TYPE *var" "TAILQ_HEAD *head" "HEADNAME" "TAILQ_ENTRY NAME" "TYPE *temp_var"
.\" .Fn TAILQ_FOREACH_REVERSE_SAFE "TYPE *var" "TAILQ_HEAD *head" "HEADNAME" "TAILQ_ENTRY NAME" "TYPE *temp_var"
.\" .Fn TAILQ_FOREACH_SAFE "TYPE *var" "TAILQ_HEAD *head" "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 five types of data structures:
singly-linked lists, singly-linked tail queues, lists, tail queues, and
circular queues.
All five 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:
.Pp
.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
Circular queues add the following functionality over the above:
.Pp
.Bl -enum -compact -offset indent
.It
The first and last entries are connected.
.El
.Pp
However:
.Pp
.Bl -enum -compact -offset indent
.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 STAILQ_ENTRY ,
.Li LIST_ENTRY ,
.Li TAILQ_ENTRY ,
or
.Li CIRCLEQ_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 ,
.Li TAILQ_HEAD ,
or
.Li CIRCLEQ_HEAD .
See the examples below for further explanation of how these
macros are used.
.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 .
.Pp
See the EXAMPLES section below for an example program
using a singly-linked tail queue.
.Ss 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_FROM
.\" behaves identically to
.\" .Nm TAILQ_FOREACH
.\" when
.\" .Fa var
.\" is NULL, else it treats
.\" .Fa var
.\" as a previously found TAILQ element and begins the loop at
.\" .Fa var
.\" instead of the first element in the TAILQ referenced by
.\" .Fa head .
.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 macro
.\" .Nm TAILQ_FOREACH_REVERSE_FROM
.\" behaves identically to
.\" .Nm TAILQ_FOREACH_REVERSE
.\" when
.\" .Fa var
.\" is NULL, else it treats
.\" .Fa var
.\" as a previously found TAILQ element and begins the reverse loop at
.\" .Fa var
.\" instead of the last element in the TAILQ referenced by
.\" .Fa head .
.\" .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_FOREACH_FROM_SAFE
.\" behaves identically to
.\" .Nm TAILQ_FOREACH_SAFE
.\" when
.\" .Fa var
.\" is NULL, else it treats
.\" .Fa var
.\" as a previously found TAILQ element and begins the loop at
.\" .Fa var
.\" instead of the first element in the TAILQ referenced by
.\" .Fa head .
.\" .Pp
.\" The macro
.\" .Nm TAILQ_FOREACH_REVERSE_FROM_SAFE
.\" behaves identically to
.\" .Nm TAILQ_FOREACH_REVERSE_SAFE
.\" when
.\" .Fa var
.\" is NULL, else it treats
.\" .Fa var
.\" as a previously found TAILQ element and begins the reverse loop at
.\" .Fa var
.\" instead of the last element in the TAILQ referenced by
.\" .Fa head .
.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
.Dv NULL .
.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.
.\" .Pp
.\" The macro
.\" .Nm TAILQ_SWAP
.\" swaps the contents of
.\" .Fa head1
.\" and
.\" .Fa head2 .
.Pp
See the EXAMPLES section below for an example program using a tail queue.
.Sh EXAMPLES
.Ss Singly-linked tail queue example
.Bd -literal
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
struct entry {
int data;
STAILQ_ENTRY(entry) entries; /* Singly-linked tail queue. */
};
STAILQ_HEAD(stailhead, entry);
int
main(void)
{
struct entry *n1, *n2, *n3, *np;
struct stailhead head; /* Singly-linked tail queue
head. */
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);
STAILQ_REMOVE(&head, n2, entry, entries);/* Deletion. */
free(n2);
n3 = STAILQ_FIRST(&head);
STAILQ_REMOVE_HEAD(&head, entries); /* Deletion from the head. */
free(n3);
n1 = STAILQ_FIRST(&head);
n1->data = 0;
for (int i = 1; i < 5; i++) {
n1 = malloc(sizeof(struct entry));
STAILQ_INSERT_HEAD(&head, n1, entries);
n1->data = i;
}
/* Forward traversal. */
STAILQ_FOREACH(np, &head, entries)
printf("%i\en", np->data);
/* TailQ Deletion. */
n1 = STAILQ_FIRST(&head);
while (n1 != NULL) {
n2 = STAILQ_NEXT(n1, entries);
free(n1);
n1 = n2;
}
STAILQ_INIT(&head);
exit(EXIT_SUCCESS);
}
.Ed
.Ss Tail queue example
.Bd -literal
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
struct entry {
int data;
TAILQ_ENTRY(entry) entries; /* Tail queue. */
};
TAILQ_HEAD(tailhead, entry);
int
main(void)
{
struct entry *n1, *n2, *n3, *np;
struct tailhead head; /* Tail queue head. */
int i;
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. */
i = 0;
TAILQ_FOREACH(np, &head, entries)
np->data = i++;
/* Reverse traversal. */
TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries)
printf("%i\en", np->data);
/* TailQ Deletion. */
n1 = TAILQ_FIRST(&head);
while (n1 != NULL) {
n2 = TAILQ_NEXT(n1, entries);
free(n1);
n1 = n2;
}
TAILQ_INIT(&head);
exit(EXIT_SUCCESS);
}
.Ed
.Sh CONFORMING TO
Not in POSIX.1, POSIX.1-2001 or POSIX.1-2008.
Present on the BSDs.
.Nm queue
functions first appeared in
.Bx 4.4 .
.Sh SEE ALSO
.Xr circleq 3
.Xr insque 3
.Xr list 3
.Xr slist 3
.\" .Xr tree 3
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