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queue(3)
lists and tail queues 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: Insertion of a new entry at the head of
the list. Insertion of a new entry after any element in the
list. O(1) removal of an entry from the head of the list. For‐
ward traversal through the list. Swapping the contents of two
lists. 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: O(n) removal of any
entry in the list. Singly-linked tail queues add the following
functionality: Entries can be added at the end of a list. O(n)
removal of any entry in the list. They may be concatenated.
However: All list insertions must specify the head of the list.
Each head entry requires two pointers rather than one. Code size
is about 15% greater and operations run about 20% slower than
singly-linked lists. Singly-linked tail queues are ideal for ap‐
plications with large datasets and few or no removals, or for im‐
plementing a FIFO queue. All doubly linked types of data struc‐
tures (lists and tail queues) additionally allow: Insertion of a
new entry before any element in the list. O(1) removal of any
entry in the list. However: Each element requires two pointers
rather than one. Code size and execution time of operations (ex‐
cept for removal) is about twice that of the singly-linked data-
structures. Linked lists are the simplest of the doubly linked
data structures. They add the following functionality over the
above: They may be traversed backwards. However: 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: Entries can be added at the end of a list. They
may be traversed backwards, from tail to head. They may be con‐
catenated. However: All list insertions and removals must spec‐
ify the head of the list. Each head entry requires two pointers
rather than one. Code size is about 15% greater and operations
run about 20% slower than singly-linked lists. In the macro def‐
initions, is the name of a user defined structure, that must con‐
tain a field of type or named The argument is the name of a user
defined structure that must be declared using the macros or See
the examples below for further explanation of how these macros
are used. A singly-linked list is headed by a structure defined
by the 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 structure is declared as follows: SLIST_HEAD(HEADNAME, TYPE)
head; where is the name of the structure to be defined, and 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 and are user selectable.) The macro evaluates
to an initializer for the list The macro evaluates to true if
there are no elements in the list. The macro declares a struc‐
ture that connects the elements in the list. The macro returns
the first element in the list or NULL if the list is empty. The
macro traverses the list referenced by in the forward direction,
assigning each element in turn to The macro initializes the list
referenced by The macro inserts the new element at the head of
the list. The macro inserts the new element after the element
The macro returns the next element in the list. The macro re‐
moves the element from the head of the list. For optimum effi‐
ciency, elements being removed from the head of the list should
explicitly use this macro instead of the generic macro. The
macro removes the element from the list. 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, en‐
tries); /* 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); } A singly-linked tail queue is
headed by a structure defined by the macro. This structure con‐
tains 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 manipu‐
lation overhead at the expense of O(n) removal for arbitrary ele‐
ments. New elements can be added to the tail queue after an ex‐
isting element, at the head of the tail queue, or at the end of
the tail queue. A structure is declared as follows:
STAILQ_HEAD(HEADNAME, TYPE) head; where is the name of the struc‐
ture to be defined, and 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 and are
user selectable.) The macro evaluates to an initializer for the
tail queue The macro concatenates the tail queue headed by onto
the end of the one headed by removing all entries from the for‐
mer. The macro evaluates to true if there are no items on the
tail queue. The macro declares a structure that connects the el‐
ements in the tail queue. The macro returns the first item on
the tail queue or NULL if the tail queue is empty. The macro
traverses the tail queue referenced by in the forward direction,
assigning each element in turn to The macro initializes the tail
queue referenced by The macro inserts the new element at the head
of the tail queue. The macro inserts the new element at the end
of the tail queue. The macro inserts the new element after the
element The macro returns the next item on the tail queue, or
NULL this item is the last. The macro 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 ex‐
plicitly rather than the generic macro. The macro removes the
element from the tail queue. STAILQ_HEAD(stailhead, entry) head
=
STAILQ_HEAD_INITIALIZER(head); struct stailhead
*headp; /* Singly-linked tail queue head. */
struct entry { ... STAILQ_ENTRY(entry) en‐
tries; /* 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, en‐
tries); free(n3); /* For‐
ward 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); A list is headed by a
structure defined by the macro. This structure contains a single
pointer to the first element on the list. The elements are dou‐
bly 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 structure is declared as follows: LIST_HEAD(HEAD‐
NAME, TYPE) head; where is the name of the structure to be de‐
fined, and 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 and are user selectable.)
The macro evaluates to an initializer for the list The macro
evaluates to true if there are no elements in the list. The
macro declares a structure that connects the elements in the
list. The macro returns the first element in the list or NULL if
the list is empty. The macro traverses the list referenced by in
the forward direction, assigning each element in turn to The
macro initializes the list referenced by The macro inserts the
new element at the head of the list. The macro inserts the new
element after the element The macro inserts the new element be‐
fore the element The macro returns the next element in the list,
or NULL if this is the last. The macro removes the element from
the list. 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); A tail
queue is headed by a structure defined by the macro. This struc‐
ture 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 structure is declared as follows:
TAILQ_HEAD(HEADNAME, TYPE) head; where is the name of the struc‐
ture to be defined, and 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 and are
user selectable.) The macro evaluates to an initializer for the
tail queue The macro concatenates the tail queue headed by onto
the end of the one headed by removing all entries from the for‐
mer. The macro evaluates to true if there are no items on the
tail queue. The macro declares a structure that connects the el‐
ements in the tail queue. The macro returns the first item on
the tail queue or NULL if the tail queue is empty. The macro
traverses the tail queue referenced by in the forward direction,
assigning each element in turn to is set to if the loop completes
normally, or if there were no elements. The macro traverses the
tail queue referenced by in the reverse direction, assigning each
element in turn to The macro initializes the tail queue refer‐
enced by The macro inserts the new element at the head of the
tail queue. The macro inserts the new element at the end of the
tail queue. The macro inserts the new element after the element
The macro inserts the new element before the element The macro
returns the last item on the tail queue. If the tail queue is
empty the return value is The macro returns the next item on the
tail queue, or NULL if this item is the last. The macro returns
the previous item on the tail queue, or NULL if this item is the
first. The macro removes the element from the tail queue. The
macro swaps the contents of and 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->en‐
tries.cqe_prev)
np-> ...
/* Delete. */ while
(head.cqh_first != (void *)&head)
CIRCLEQ_REMOVE(&head, head.cqh_first, entries); Not in
POSIX.1, POSIX.1-2001 or POSIX.1-2008. Present on the BSDs.
functions first appeared in This page is part of release 5.02 of
the Linux 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/.