CIRCLEQ(3) Library Functions Manual CIRCLEQ(3)
NAME
CIRCLEQ_EMPTY, CIRCLEQ_ENTRY, CIRCLEQ_FIRST, CIRCLEQ_FOREACH, CIR-
CLEQ_FOREACH_REVERSE, CIRCLEQ_HEAD, CIRCLEQ_HEAD_INITIALIZER, CIR-
CLEQ_INIT, CIRCLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE, CIRCLEQ_IN-
SERT_HEAD, CIRCLEQ_INSERT_TAIL, CIRCLEQ_LAST, CIRCLEQ_LOOP_NEXT, CIR-
CLEQ_LOOP_PREV, CIRCLEQ_NEXT, CIRCLEQ_PREV, CIRCLEQ_REMOVE - implemen-
tation of a doubly linked circular queue
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/queue.h>
CIRCLEQ_ENTRY(TYPE);
CIRCLEQ_HEAD(HEADNAME, TYPE);
CIRCLEQ_HEAD CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD head);
void CIRCLEQ_INIT(CIRCLEQ_HEAD *head);
int CIRCLEQ_EMPTY(CIRCLEQ_HEAD *head);
void CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head,
struct TYPE *elm, CIRCLEQ_ENTRY NAME);
void CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head,
struct TYPE *elm, CIRCLEQ_ENTRY NAME);
void CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *head, struct TYPE *listelm,
struct TYPE *elm, CIRCLEQ_ENTRY NAME);
void CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *head, struct TYPE *listelm,
struct TYPE *elm, CIRCLEQ_ENTRY NAME);
struct TYPE *CIRCLEQ_FIRST(CIRCLEQ_HEAD *head);
struct TYPE *CIRCLEQ_LAST(CIRCLEQ_HEAD *head);
struct TYPE *CIRCLEQ_PREV(struct TYPE *elm, CIRCLEQ_ENTRY NAME);
struct TYPE *CIRCLEQ_NEXT(struct TYPE *elm, CIRCLEQ_ENTRY NAME);
struct TYPE *CIRCLEQ_LOOP_PREV(CIRCLEQ_HEAD *head,
struct TYPE *elm, CIRCLEQ_ENTRY NAME);
struct TYPE *CIRCLEQ_LOOP_NEXT(CIRCLEQ_HEAD *head,
struct TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_FOREACH(struct TYPE *var, CIRCLEQ_HEAD *head,
CIRCLEQ_ENTRY NAME);
CIRCLEQ_FOREACH_REVERSE(struct TYPE *var, CIRCLEQ_HEAD *head,
CIRCLEQ_ENTRY NAME);
void CIRCLEQ_REMOVE(CIRCLEQ_HEAD *head, struct TYPE *elm,
CIRCLEQ_ENTRY NAME);
DESCRIPTION
These macros define and operate on doubly linked circular queues.
In the macro definitions, TYPE is the name of a user-defined structure,
that must contain a field of type CIRCLEQ_ENTRY, named NAME. The argu-
ment HEADNAME is the name of a user-defined structure that must be de-
clared using the macro CIRCLEQ_HEAD().
Creation
A circular queue is headed by a structure defined by the CIRCLEQ_HEAD()
macro. This structure contains a pair of pointers, one to the first
element in the queue and the other to the last element in the queue.
The elements are doubly linked so that an arbitrary element can be re-
moved without traversing the queue. New elements can be added to the
queue after an existing element, before an existing element, at the
head of the queue, or at the end of the queue. A CIRCLEQ_HEAD struc-
ture is declared as follows:
CIRCLEQ_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 queue. A pointer to
the head of the queue can later be declared as:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
CIRCLEQ_ENTRY() declares a structure that connects the elements in the
queue.
CIRCLEQ_HEAD_INITIALIZER() evaluates to an initializer for the queue
head.
CIRCLEQ_INIT() initializes the queue referenced by head.
CIRCLEQ_EMPTY() evaluates to true if there are no items on the queue.
Insertion
CIRCLEQ_INSERT_HEAD() inserts the new element elm at the head of the
queue.
CIRCLEQ_INSERT_TAIL() inserts the new element elm at the end of the
queue.
CIRCLEQ_INSERT_BEFORE() inserts the new element elm before the element
listelm.
CIRCLEQ_INSERT_AFTER() inserts the new element elm after the element
listelm.
Traversal
CIRCLEQ_FIRST() returns the first item on the queue.
CIRCLEQ_LAST() returns the last item on the queue.
CIRCLEQ_PREV() returns the previous item on the queue, or &head if this
item is the first one.
CIRCLEQ_NEXT() returns the next item on the queue, or &head if this
item is the last one.
CIRCLEQ_LOOP_PREV() returns the previous item on the queue. If elm is
the first element on the queue, the last element is returned.
CIRCLEQ_LOOP_NEXT() returns the next item on the queue. If elm is the
last element on the queue, the first element is returned.
CIRCLEQ_FOREACH() traverses the queue referenced by head in the forward
direction, assigning each element in turn to var. var is set to &head
if the loop completes normally, or if there were no elements.
CIRCLEQ_FOREACH_REVERSE() traverses the queue referenced by head in the
reverse direction, assigning each element in turn to var.
Removal
CIRCLEQ_REMOVE() removes the element elm from the queue.
RETURN VALUE
CIRCLEQ_EMPTY() returns nonzero if the queue is empty, and zero if the
queue contains at least one entry.
CIRCLEQ_FIRST(), CIRCLEQ_LAST(), CIRCLEQ_LOOP_PREV(), and CIR-
CLEQ_LOOP_NEXT() return a pointer to the first, last, previous, or next
TYPE structure, respectively.
CIRCLEQ_PREV(), and CIRCLEQ_NEXT() are similar to their CIR-
CLEQ_LOOP_*() counterparts, except that if the argument is the first or
last element, respectively, they return &head.
CIRCLEQ_HEAD_INITIALIZER() returns an initializer that can be assigned
to the queue head.
STANDARDS
Not in POSIX.1, POSIX.1-2001, or POSIX.1-2008. Present on the BSDs
(CIRCLEQ macros first appeared in 4.4BSD).
BUGS
CIRCLEQ_FOREACH() and CIRCLEQ_FOREACH_REVERSE() don't allow var to be
removed or freed within the loop, as it would interfere with the tra-
versal. CIRCLEQ_FOREACH_SAFE() and CIRCLEQ_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.
EXAMPLES
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
struct entry {
int data;
CIRCLEQ_ENTRY(entry) entries; /* Queue */
};
CIRCLEQ_HEAD(circlehead, entry);
int
main(void)
{
struct entry *n1, *n2, *n3, *np;
struct circlehead head; /* Queue head */
int i;
CIRCLEQ_INIT(&head); /* Initialize the queue */
n1 = malloc(sizeof(struct entry)); /* Insert at the head */
CIRCLEQ_INSERT_HEAD(&head, n1, entries);
n1 = malloc(sizeof(struct entry)); /* Insert at the tail */
CIRCLEQ_INSERT_TAIL(&head, n1, entries);
n2 = malloc(sizeof(struct entry)); /* Insert after */
CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);
n3 = malloc(sizeof(struct entry)); /* Insert before */
CIRCLEQ_INSERT_BEFORE(&head, n2, n3, entries);
CIRCLEQ_REMOVE(&head, n2, entries); /* Deletion */
free(n2);
/* Forward traversal */
i = 0;
CIRCLEQ_FOREACH(np, &head, entries)
np->data = i++;
/* Reverse traversal */
CIRCLEQ_FOREACH_REVERSE(np, &head, entries)
printf("%i\n", np->data);
/* Queue deletion */
n1 = CIRCLEQ_FIRST(&head);
while (n1 != (void *)&head) {
n2 = CIRCLEQ_NEXT(n1, entries);
free(n1);
n1 = n2;
}
CIRCLEQ_INIT(&head);
exit(EXIT_SUCCESS);
}
SEE ALSO
insque(3), queue(7)
Linux man-pages 6.03 2022-10-30 CIRCLEQ(3)
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