Manpages - queue.3bsd
lists and tail queues
(See
for include usage.)
These macros define and operate on four types of data structures which can be used in both C and C++ source code:
Lists
Singly-linked lists
Singly-linked tail queues
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.
Forward 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.
O(n) concatenation of two lists.
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 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) 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 (except 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:
O(n) concatenation of two lists.
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 concatenated.
However:
All list insertions and removals 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.
In the macro definitions,
is the name of a user defined structure. The structure must contain a field called
which is of type
or
In the macro definitions,
is the name of a user defined class. The class must contain a field called
which is of type
or
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
concatenates the list headed by
onto the end of the one headed by
removing all entries from the former. Use of this macro should be avoided as it traverses the entirety of the
list. A singly-linked tail queue should be used if this macro is needed in high-usage code paths or to operate on long lists.
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
behaves identically to
when
is NULL, else it treats
as a previously found SLIST element and begins the loop at
instead of the first element in the SLIST referenced by
The macro
traverses the list referenced by
in the forward direction, assigning each element in turn to
However, unlike
here it is permitted to both remove
as well as free it from within the loop safely without interfering with the traversal.
The macro
behaves identically to
when
is NULL, else it treats
as a previously found SLIST element and begins the loop at
instead of the first element in the SLIST referenced by
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
removes the element after
from the list. Unlike
this macro does not traverse the entire list.
The macro
removes the element
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
macro.
The macro
removes the element
from the list. Use of this macro should be avoided as it traverses the entire list. A doubly-linked list should be used if this macro is needed in high-usage code paths or to operate on long lists.
The macro
swaps the contents of
and
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); }
A singly-linked tail queue is headed by a structure defined by the
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
structure is declared as follows:
STAILQ_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 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 former.
The macro
evaluates to true if there are no items on the tail queue.
The macro
declares a structure that connects the elements 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
behaves identically to
when
is NULL, else it treats
as a previously found STAILQ element and begins the loop at
instead of the first element in the STAILQ referenced by
The macro
traverses the tail queue referenced by
in the forward direction, assigning each element in turn to
However, unlike
here it is permitted to both remove
as well as free it from within the loop safely without interfering with the traversal.
The macro
behaves identically to
when
is NULL, else it treats
as a previously found STAILQ element and begins the loop at
instead of the first element in the STAILQ referenced by
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 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 this item is the last.
The macro
removes the element after
from the tail queue. Unlike
this macro does not traverse the entire tail queue.
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 explicitly rather than the generic
macro.
The macro
removes the element
from the tail queue. Use of this macro should be avoided as it traverses the entire list. A doubly-linked tail queue should be used if this macro is needed in high-usage code paths or to operate on long tail queues.
The macro
swaps the contents of
and
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);
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 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
structure is declared as follows:
LIST_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
concatenates the list headed by
onto the end of the one headed by
removing all entries from the former. Use of this macro should be avoided as it traverses the entirety of the
list. A tail queue should be used if this macro is needed in high-usage code paths or to operate on long lists.
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
behaves identically to
when
is NULL, else it treats
as a previously found LIST element and begins the loop at
instead of the first element in the LIST referenced by
The macro
traverses the list referenced by
in the forward direction, assigning each element in turn to
However, unlike
here it is permitted to both remove
as well as free it from within the loop safely without interfering with the traversal.
The macro
behaves identically to
when
is NULL, else it treats
as a previously found LIST element and begins the loop at
instead of the first element in the LIST referenced by
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
before the element
The macro
returns the next element in the list, or NULL if this is the last.
The macro
returns the previous element in the list, or NULL if this is the first. List
must contain element
The macro
removes the element
from the list.
The macro
swaps the contents of
and
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-> …
* Safe forward traversal. * LIST_FOREACH_SAFE(np, &head, entries, np_temp) { np->do_stuff(); … LIST_REMOVE(np, entries); free(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 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
structure is declared as follows:
TAILQ_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 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 former.
The macro
evaluates to true if there are no items on the tail queue.
The macro
declares a structure that connects the elements 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
behaves identically to
when
is NULL, else it treats
as a previously found TAILQ element and begins the loop at
instead of the first element in the TAILQ referenced by
The macro
traverses the tail queue referenced by
in the reverse direction, assigning each element in turn to
The macro
behaves identically to
when
is NULL, else it treats
as a previously found TAILQ element and begins the reverse loop at
instead of the last element in the TAILQ referenced by
The macros
and
traverse the list referenced by
in the forward or reverse direction respectively, assigning each element in turn to
However, unlike their unsafe counterparts,
and
make it possible to both remove
as well as free it from within the loop safely without interfering with the traversal.
The macro
behaves identically to
when
is NULL, else it treats
as a previously found TAILQ element and begins the loop at
instead of the first element in the TAILQ referenced by
The macro
behaves identically to
when
is NULL, else it treats
as a previously found TAILQ element and begins the reverse loop at
instead of the last element in the TAILQ referenced by
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
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-> … * Safe forward traversal. * TAILQ_FOREACH_SAFE(np, &head, entries, np_temp) { np->do_stuff(); … TAILQ_REMOVE(&head, np, entries); free(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);
When debugging
it can be useful to trace queue changes. To enable tracing, define the macro
at compile time.
It can also be useful to trash pointers that have been unlinked from a queue, to detect use after removal. To enable pointer trashing, define the macro
at compile time. The macro
returns true if
has been trashed by the
option.
The
functions first appeared in