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 - implementation of a doubly linked tail queue
#include <sys/queue.h>
void TAILQ_CONCAT(TAILQ_HEAD *head1, TAILQ_HEAD *head2,
TAILQ_ENTRY NAME);
int TAILQ_EMPTY(TAILQ_HEAD *head);
TAILQ_ENTRY(TYPE);
struct TYPE *TAILQ_FIRST(TAILQ_HEAD *head);
TAILQ_FOREACH(struct TYPE *var, TAILQ_HEAD *head, TAILQ_ENTRY NAME);
TAILQ_FOREACH_REVERSE(struct TYPE *var, TAILQ_HEAD *head, HEADNAME,
TAILQ_ENTRY NAME);
TAILQ_HEAD(HEADNAME, TYPE);
TAILQ_HEAD TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);
void TAILQ_INIT(TAILQ_HEAD *head);
void TAILQ_INSERT_AFTER(TAILQ_HEAD *head, struct TYPE *listelm,
struct TYPE *elm, TAILQ_ENTRY NAME);
void TAILQ_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm,
TAILQ_ENTRY NAME);
void TAILQ_INSERT_HEAD(TAILQ_HEAD *head, struct TYPE *elm,
TAILQ_ENTRY NAME);
void TAILQ_INSERT_TAIL(TAILQ_HEAD *head, struct TYPE *elm,
TAILQ_ENTRY NAME);
struct TYPE *TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);
struct TYPE *TAILQ_NEXT(struct TYPE *elm, TAILQ_ENTRY NAME);
struct TYPE *TAILQ_PREV(struct TYPE *elm, HEADNAME, TAILQ_ENTRY NAME);
void TAILQ_REMOVE(TAILQ_HEAD *head, struct TYPE *elm, TAILQ_ENTRY NAME);
These macros define and operate on doubly linked tail queues.
In the macro definitions, TYPE
is the name of a user defined
structure, that must contain a field of type TAILQ_ENTRY
, named
NAME
. The argument HEADNAME
is the name of a user
defined structure that must be declared using the macro
TAILQ_HEAD().
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
structure is
declared as follows:
TAILQ_HEAD(HEADNAME, TYPE) head;
where struct HEADNAME
is the structure to be defined, and
struct 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 element listelm
.
The macro TAILQ_INSERT_BEFORE() inserts the new
element elm
before the element 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.
TAILQ_EMPTY() returns nonzero if the queue is empty, and zero if the queue contains at least one entry.
TAILQ_FIRST(), TAILQ_LAST(),
TAILQ_NEXT(), and TAILQ_PREV() return
a pointer to the first, last, next or previous TYPE
structure,
respectively.
TAILQ_HEAD_INITIALIZER() returns an initializer that
can be assigned to the queue head
.
#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\n", 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);
}
Not in POSIX.1, POSIX.1-2001 or POSIX.1-2008. Present on the BSDs. (TAILQ functions first appeared in 4.4BSD).
The macros TAILQ_FOREACH() and
TAILQ_FOREACH_REVERSE() don't allow var
to be
removed or freed within the loop, as it would interfere with the
traversal. The macros TAILQ_FOREACH_SAFE() and
TAILQ_FOREACH_REVERSE_SAFE(), which are present on the
BSDs but are not present in glibc, fix this limitation by allowing
var
to safely be removed from the list and freed from within
the loop without interfering with the traversal.
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