QUEUE(3) BSD Library Functions Manual QUEUE(3)
NAME
SLIST_EMPTY, SLIST_ENTRY, SLIST_FIRST, SLIST_FOREACH, SLIST_HEAD,
SLIST_HEAD_INITIALIZER, SLIST_INIT, SLIST_INSERT_AFTER,
SLIST_INSERT_HEAD, SLIST_NEXT, SLIST_REMOVE_HEAD, SLIST_REMOVE,
STAILQ_CONCAT, STAILQ_EMPTY, STAILQ_ENTRY, STAILQ_FIRST, STAILQ_FOREACH,
STAILQ_HEAD, STAILQ_HEAD_INITIALIZER, STAILQ_INIT, STAILQ_INSERT_AFTER,
STAILQ_INSERT_HEAD, STAILQ_INSERT_TAIL, STAILQ_NEXT, STAILQ_REMOVE_HEAD,
STAILQ_REMOVE, LIST_EMPTY, LIST_ENTRY, LIST_FIRST, LIST_FOREACH,
LIST_HEAD, LIST_HEAD_INITIALIZER, LIST_INIT, LIST_INSERT_AFTER,
LIST_INSERT_BEFORE, LIST_INSERT_HEAD, LIST_NEXT, LIST_REMOVE,
TAILQ_CONCAT, TAILQ_EMPTY, TAILQ_ENTRY, TAILQ_FIRST, TAILQ_FOREACH,
TAILQ_FOREACH_REVERSE, TAILQ_HEAD, TAILQ_HEAD_INITIALIZER, TAILQ_INIT,
TAILQ_INSERT_AFTER, TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD,
TAILQ_INSERT_TAIL, TAILQ_LAST, TAILQ_NEXT, TAILQ_PREV, TAILQ_REMOVE,
TAILQ_SWAP -- implementations of singly-linked lists, singly-linked tail
queues, lists and tail queues
SYNOPSIS
#include <sys/queue.h>
SLIST_EMPTY(SLIST_HEAD *head);
SLIST_ENTRY(TYPE);
SLIST_FIRST(SLIST_HEAD *head);
SLIST_FOREACH(TYPE *var, SLIST_HEAD *head, SLIST_ENTRY NAME);
SLIST_HEAD(HEADNAME, TYPE);
SLIST_HEAD_INITIALIZER(SLIST_HEAD head);
SLIST_INIT(SLIST_HEAD *head);
SLIST_INSERT_AFTER(TYPE *listelm, TYPE *elm, SLIST_ENTRY NAME);
SLIST_INSERT_HEAD(SLIST_HEAD *head, TYPE *elm, SLIST_ENTRY NAME);
SLIST_NEXT(TYPE *elm, SLIST_ENTRY NAME);
SLIST_REMOVE_HEAD(SLIST_HEAD *head, SLIST_ENTRY NAME);
SLIST_REMOVE(SLIST_HEAD *head, TYPE *elm, TYPE, SLIST_ENTRY NAME);
STAILQ_CONCAT(STAILQ_HEAD *head1, STAILQ_HEAD *head2);
STAILQ_EMPTY(STAILQ_HEAD *head);
STAILQ_ENTRY(TYPE);
STAILQ_FIRST(STAILQ_HEAD *head);
STAILQ_FOREACH(TYPE *var, STAILQ_HEAD *head, STAILQ_ENTRY NAME);
STAILQ_HEAD(HEADNAME, TYPE);
STAILQ_HEAD_INITIALIZER(STAILQ_HEAD head);
STAILQ_INIT(STAILQ_HEAD *head);
STAILQ_INSERT_AFTER(STAILQ_HEAD *head, TYPE *listelm, TYPE *elm,
STAILQ_ENTRY NAME);
STAILQ_INSERT_HEAD(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);
STAILQ_INSERT_TAIL(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);
STAILQ_NEXT(TYPE *elm, STAILQ_ENTRY NAME);
STAILQ_REMOVE_HEAD(STAILQ_HEAD *head, STAILQ_ENTRY NAME);
STAILQ_REMOVE(STAILQ_HEAD *head, TYPE *elm, TYPE, STAILQ_ENTRY NAME);
LIST_EMPTY(LIST_HEAD *head);
LIST_ENTRY(TYPE);
LIST_FIRST(LIST_HEAD *head);
LIST_FOREACH(TYPE *var, LIST_HEAD *head, LIST_ENTRY NAME);
LIST_HEAD(HEADNAME, TYPE);
LIST_HEAD_INITIALIZER(LIST_HEAD head);
LIST_INIT(LIST_HEAD *head);
LIST_INSERT_AFTER(TYPE *listelm, TYPE *elm, LIST_ENTRY NAME);
LIST_INSERT_BEFORE(TYPE *listelm, TYPE *elm, LIST_ENTRY NAME);
LIST_INSERT_HEAD(LIST_HEAD *head, TYPE *elm, LIST_ENTRY NAME);
LIST_NEXT(TYPE *elm, LIST_ENTRY NAME);
LIST_REMOVE(TYPE *elm, LIST_ENTRY NAME);
LIST_SWAP(LIST_HEAD *head1, LIST_HEAD *head2, TYPE, LIST_ENTRY NAME);
TAILQ_CONCAT(TAILQ_HEAD *head1, TAILQ_HEAD *head2, TAILQ_ENTRY NAME);
TAILQ_EMPTY(TAILQ_HEAD *head);
TAILQ_ENTRY(TYPE);
TAILQ_FIRST(TAILQ_HEAD *head);
TAILQ_FOREACH(TYPE *var, TAILQ_HEAD *head, TAILQ_ENTRY NAME);
TAILQ_FOREACH_REVERSE(TYPE *var, TAILQ_HEAD *head, HEADNAME,
TAILQ_ENTRY NAME);
TAILQ_HEAD(HEADNAME, TYPE);
TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);
TAILQ_INIT(TAILQ_HEAD *head);
TAILQ_INSERT_AFTER(TAILQ_HEAD *head, TYPE *listelm, TYPE *elm,
TAILQ_ENTRY NAME);
TAILQ_INSERT_BEFORE(TYPE *listelm, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_INSERT_HEAD(TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_INSERT_TAIL(TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);
TAILQ_NEXT(TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_PREV(TYPE *elm, HEADNAME, TAILQ_ENTRY NAME);
TAILQ_REMOVE(TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_SWAP(TAILQ_HEAD *head1, TAILQ_HEAD *head2, TYPE, TAILQ_ENTRY NAME);
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:
1. Insertion of a new entry at the head of the list.
2. Insertion of a new entry after any element in the list.
3. O(1) removal of an entry from the head of the list.
4. Forward traversal through the list.
5. Swapping the contents of two lists.
Singly-linked lists are the simplest of the four data structures and sup-
port only the above functionality. Singly-linked lists are ideal for
applications with large datasets and few or no removals, or for imple-
menting a LIFO queue. Singly-linked lists add the following functional-
ity:
1. O(n) removal of any entry in the list.
Singly-linked tail queues add the following functionality:
1. Entries can be added at the end of a list.
2. O(n) removal of any entry in the list.
3. They may be concatenated.
However:
1. All list insertions must specify the head of the list.
2. Each head entry requires two pointers rather than one.
3. Code size is about 15% greater and operations run about 20%
slower than singly-linked lists.
Singly-linked tail queues are ideal for applications with large datasets
and few or no removals, or for implementing a FIFO queue.
All doubly linked types of data structures (lists and tail queues) addi-
tionally allow:
1. Insertion of a new entry before any element in the list.
2. O(1) removal of any entry in the list.
However:
1. Each element requires two pointers rather than one.
2. Code size and execution time of operations (except for
removal) is about twice that of the singly-linked data-struc-
tures.
Linked lists are the simplest of the doubly linked data structures. They
add the following functionality over the above:
1. They may be traversed backwards.
However:
1. To traverse backwards, an entry to begin the traversal and the
list in which it is contained must be specified.
Tail queues add the following functionality:
1. Entries can be added at the end of a list.
2. They may be traversed backwards, from tail to head.
3. They may be concatenated.
However:
1. All list insertions and removals must specify the head of the
list.
2. Each head entry requires two pointers rather than one.
3. Code size is about 15% greater and operations run about 20%
slower than singly-linked lists.
In the macro definitions, TYPE is the name of a user defined structure,
that must contain a field of type SLIST_ENTRY, STAILQ_ENTRY, LIST_ENTRY,
or TAILQ_ENTRY, named NAME. The argument HEADNAME is the name of a user
defined structure that must be declared using the macros SLIST_HEAD,
STAILQ_HEAD, LIST_HEAD, or TAILQ_HEAD. See the examples below for fur-
ther explanation of how these macros are used.
Singly-linked lists
A singly-linked list is headed by a structure defined by the 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 ele-
ments. New elements can be added to the list after an existing element
or at the head of the list. An SLIST_HEAD structure is declared as fol-
lows:
SLIST_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and 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:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The macro SLIST_HEAD_INITIALIZER evaluates to an initializer for the list
head.
The macro SLIST_EMPTY evaluates to true if there are no elements in the
list.
The macro SLIST_ENTRY declares a structure that connects the elements in
the list.
The macro SLIST_FIRST returns the first element in the list or NULL if
the list is empty.
The macro SLIST_FOREACH traverses the list referenced by head in the for-
ward direction, assigning each element in turn to var.
The macro SLIST_INIT initializes the list referenced by head.
The macro SLIST_INSERT_HEAD inserts the new element elm at the head of
the list.
The macro SLIST_INSERT_AFTER inserts the new element elm after the ele-
ment listelm.
The macro SLIST_NEXT returns the next element in the list.
The macro SLIST_REMOVE_HEAD removes the element 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
SLIST_REMOVE macro.
The macro SLIST_REMOVE removes the element elm from the list.
Singly-linked list example
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-> ...
while (!SLIST_EMPTY(&head)) { /* List Deletion. */
n1 = SLIST_FIRST(&head);
SLIST_REMOVE_HEAD(&head, entries);
free(n1);
}
Singly-linked tail queues
A singly-linked tail queue is headed by a structure defined by the
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 arbi-
trary 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 STAILQ_HEAD structure is declared as follows:
STAILQ_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and 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:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The macro STAILQ_HEAD_INITIALIZER evaluates to an initializer for the
tail queue head.
The macro STAILQ_CONCAT concatenates the tail queue headed by head2 onto
the end of the one headed by head1 removing all entries from the former.
The macro STAILQ_EMPTY evaluates to true if there are no items on the
tail queue.
The macro STAILQ_ENTRY declares a structure that connects the elements in
the tail queue.
The macro STAILQ_FIRST returns the first item on the tail queue or NULL
if the tail queue is empty.
The macro STAILQ_FOREACH traverses the tail queue referenced by head in
the forward direction, assigning each element in turn to var.
The macro STAILQ_INIT initializes the tail queue referenced by head.
The macro STAILQ_INSERT_HEAD inserts the new element elm at the head of
the tail queue.
The macro STAILQ_INSERT_TAIL inserts the new element elm at the end of
the tail queue.
The macro STAILQ_INSERT_AFTER inserts the new element elm after the ele-
ment listelm.
The macro STAILQ_NEXT returns the next item on the tail queue, or NULL
this item is the last.
The macro 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
STAILQ_REMOVE macro.
The macro STAILQ_REMOVE removes the element elm from the tail queue.
Singly-linked tail queue example
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-> ...
/* 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);
Lists
A list is headed by a structure defined by the 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 LIST_HEAD structure is declared as follows:
LIST_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and 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:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The macro LIST_HEAD_INITIALIZER evaluates to an initializer for the list
head.
The macro LIST_EMPTY evaluates to true if there are no elements in the
list.
The macro LIST_ENTRY declares a structure that connects the elements in
the list.
The macro LIST_FIRST returns the first element in the list or NULL if the
list is empty.
The macro LIST_FOREACH traverses the list referenced by head in the for-
ward direction, assigning each element in turn to var.
The macro LIST_INIT initializes the list referenced by head.
The macro LIST_INSERT_HEAD inserts the new element elm at the head of the
list.
The macro LIST_INSERT_AFTER inserts the new element elm after the element
listelm.
The macro LIST_INSERT_BEFORE inserts the new element elm before the ele-
ment listelm.
The macro LIST_NEXT returns the next element in the list, or NULL if this
is the last.
The macro LIST_REMOVE removes the element elm from the list.
List example
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-> ...
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);
Tail queues
A tail queue is headed by a structure defined by the 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 TAILQ_HEAD struc-
ture is declared as follows:
TAILQ_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and 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:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The macro TAILQ_HEAD_INITIALIZER evaluates to an initializer for the tail
queue head.
The macro TAILQ_CONCAT concatenates the tail queue headed by head2 onto
the end of the one headed by head1 removing all entries from the former.
The macro TAILQ_EMPTY evaluates to true if there are no items on the tail
queue.
The macro TAILQ_ENTRY declares a structure that connects the elements in
the tail queue.
The macro TAILQ_FIRST returns the first item on the tail queue or NULL if
the tail queue is empty.
The macro TAILQ_FOREACH traverses the tail queue referenced by head in
the forward direction, assigning each element in turn to var. var is set
to NULL if the loop completes normally, or if there were no elements.
The macro TAILQ_FOREACH_REVERSE traverses the tail queue referenced by
head in the reverse direction, assigning each element in turn to var.
The macro TAILQ_INIT initializes the tail queue referenced by head.
The macro TAILQ_INSERT_HEAD inserts the new element elm at the head of
the tail queue.
The macro TAILQ_INSERT_TAIL inserts the new element elm at the end of the
tail queue.
The macro TAILQ_INSERT_AFTER inserts the new element elm after the ele-
ment listelm.
The macro TAILQ_INSERT_BEFORE inserts the new element elm before the ele-
ment listelm.
The macro TAILQ_LAST returns the last item on the tail queue. If the
tail queue is empty the return value is NULL.
The macro TAILQ_NEXT returns the next item on the tail queue, or NULL if
this item is the last.
The macro TAILQ_PREV returns the previous item on the tail queue, or NULL
if this item is the first.
The macro TAILQ_REMOVE removes the element elm from the tail queue.
The macro TAILQ_SWAP swaps the contents of head1 and head2.
Tail queue example
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-> ...
/* 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);
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);
CONFORMING TO
Not in POSIX.1, POSIX.1-2001 or POSIX.1-2008. Present on the BSDs.
queue functions first appeared in 4.4BSD.
SEE ALSO
insque(3)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
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