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.TH CIRCLEQ 3 2020-10-21 "GNU" "Linux Programmer's Manual"
.SH NAME
CIRCLEQ_EMPTY,
CIRCLEQ_ENTRY,
CIRCLEQ_FIRST,
CIRCLEQ_FOREACH,
CIRCLEQ_FOREACH_REVERSE,
CIRCLEQ_HEAD,
CIRCLEQ_HEAD_INITIALIZER,
CIRCLEQ_INIT,
CIRCLEQ_INSERT_AFTER,
CIRCLEQ_INSERT_BEFORE,
CIRCLEQ_INSERT_HEAD,
CIRCLEQ_INSERT_TAIL,
CIRCLEQ_LAST,
CIRCLEQ_LOOP_NEXT,
CIRCLEQ_LOOP_PREV,
CIRCLEQ_NEXT,
CIRCLEQ_PREV,
CIRCLEQ_REMOVE
\- implementation of a doubly linked circular queue
.SH SYNOPSIS
.nf
.B #include <sys/queue.h>
.PP
.BI "int CIRCLEQ_EMPTY(CIRCLEQ_HEAD *" head ");"
.PP
.B CIRCLEQ_ENTRY(TYPE);
.PP
.BI "struct TYPE *CIRCLEQ_FIRST(CIRCLEQ_HEAD *" head ");"
.PP
.BI "CIRCLEQ_FOREACH(struct TYPE *" var ", CIRCLEQ_HEAD *" head ,
.BI "                CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "CIRCLEQ_FOREACH_REVERSE(struct TYPE *" var ", CIRCLEQ_HEAD *" head ","
.BI "                CIRCLEQ_ENTRY " NAME ");"
.PP
.B CIRCLEQ_HEAD(HEADNAME, TYPE);
.PP
.BI "CIRCLEQ_HEAD CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD " head ");"
.PP
.BI "void CIRCLEQ_INIT(CIRCLEQ_HEAD *" head ");"
.PP
.BI "void CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *" head ", struct TYPE *" listelm ,
.BI "                struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "void CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *" head ", struct TYPE *" listelm ,
.BI "                struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "void CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *" head ", struct TYPE *" elm ","
.BI "                CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "void CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *" head ", struct TYPE *" elm ","
.BI "                CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "struct TYPE *CIRCLEQ_LAST(CIRCLEQ_HEAD *" head ");"
.PP
.BI "void CIRCLEQ_LOOP_NEXT(CIRCLEQ_HEAD *" head ", struct TYPE *" elm ","
.BI "                CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "void CIRCLEQ_LOOP_PREV(CIRCLEQ_HEAD *" head ", struct TYPE *" elm ","
.BI "                CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "struct TYPE *CIRCLEQ_NEXT(struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "struct TYPE *CIRCLEQ_PREV(struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME ");"
.PP
.BI "void CIRCLEQ_REMOVE(CIRCLEQ_HEAD *" head ", struct TYPE *" elm ,
.BI "                CIRCLEQ_ENTRY " NAME ");"
.fi
.SH DESCRIPTION
These macros define and operate on doubly linked circular queues.
.PP
In the macro definitions,
.I TYPE
is the name of a user-defined structure,
that must contain a field of type
.IR CIRCLEQ_ENTRY ,
named
.IR NAME .
The argument
.I HEADNAME
is the name of a user-defined structure that must be declared
using the macro
.BR CIRCLEQ_HEAD ().
.PP
A circular queue is headed by a structure defined by the
.BR CIRCLEQ_HEAD ()
macro.
This structure contains a pair of pointers,
one to the first element in the circular queue and the other to
the last element in the circular queue.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the circular queue.
New elements can be added to the circular queue after an existing element,
before an existing element, at the head of the circular queue,
or at the end of the circular queue.
A
.I CIRCLEQ_HEAD
structure is declared as follows:
.PP
.in +4
.EX
CIRCLEQ_HEAD(HEADNAME, TYPE) head;
.EE
.in
.PP
where
.I struct HEADNAME
is the structure to be defined, and
.I struct TYPE
is the type of the elements to be linked into the circular queue.
A pointer to the head of the circular queue can later be declared as:
.PP
.in +4
.EX
struct HEADNAME *headp;
.EE
.in
.PP
(The names
.I head
and
.I headp
are user selectable.)
.PP
The macro
.BR CIRCLEQ_HEAD_INITIALIZER ()
evaluates to an initializer for the circular queue
.IR head .
.PP
The macro
.BR CIRCLEQ_EMPTY ()
evaluates to true if there are no items on the circular queue.
.PP
The macro
.BR CIRCLEQ_ENTRY ()
declares a structure that connects the elements in
the circular queue.
.PP
The macro
.BR CIRCLEQ_FIRST ()
returns the first item on the circular queue.
.PP
The macro
.BR CIRCLEQ_FOREACH ()
traverses the circular queue referenced by
.I head
in the forward direction, assigning each element in turn to
.IR var .
.I var
is set to
.I &head
if the loop completes normally, or if there were no elements.
.PP
The macro
.BR CIRCLEQ_FOREACH_REVERSE ()
traverses the circular queue referenced by
.I head
in the reverse direction, assigning each element in turn to
.IR var .
.PP
The macro
.BR CIRCLEQ_INIT ()
initializes the circular queue referenced by
.IR head .
.PP
The macro
.BR CIRCLEQ_INSERT_HEAD ()
inserts the new element
.I elm
at the head of the circular queue.
.PP
The macro
.BR CIRCLEQ_INSERT_TAIL ()
inserts the new element
.I elm
at the end of the circular queue.
.PP
The macro
.BR CIRCLEQ_INSERT_AFTER ()
inserts the new element
.I elm
after the element
.IR listelm .
.PP
The macro
.BR CIRCLEQ_INSERT_BEFORE ()
inserts the new element
.I elm
before the element
.IR listelm .
.PP
The macro
.BR CIRCLEQ_LAST ()
returns the last item on the circular queue.
.PP
The macro
.BR CIRCLEQ_NEXT ()
returns the next item on the circular queue, or
.I &head
if this item is the last one.
.PP
The macro
.BR CIRCLEQ_PREV ()
returns the previous item on the circular queue, or
.I &head
if this item is the first one.
.PP
The macro
.BR CIRCLEQ_LOOP_NEXT ()
returns the next item on the circular queue.
If
.I elm
is the last element on the circular queue, the first element is returned.
.PP
The macro
.BR CIRCLEQ_LOOP_PREV ()
returns the previous item on the circular queue.
If
.I elm
is the first element on the circular queue, the last element is returned.
.PP
The macro
.BR CIRCLEQ_REMOVE ()
removes the element
.I elm
from the circular queue.
.SH RETURN VALUE
.BR CIRCLEQ_EMPTY ()
returns nonzero if the queue is empty,
and zero if the queue contains at least one entry.
.PP
.BR CIRCLEQ_FIRST (),
.BR CIRCLEQ_LAST (),
.BR CIRCLEQ_NEXT (),
and
.BR CIRCLEQ_PREV ()
return a pointer to the first, last, next or previous
.I TYPE
structure, respectively.
.PP
.BR CIRCLEQ_HEAD_INITIALIZER ()
returns an initializer that can be assigned to the queue
.IR head .
.SH CONFORMING TO
Not in POSIX.1, POSIX.1-2001 or POSIX.1-2008.
Present on the BSDs
(CIRCLEQ macros first appeared in 4.4BSD).
.SH BUGS
The macros
.BR CIRCLEQ_FOREACH ()
and
.BR CIRCLEQ_FOREACH_REVERSE ()
don't allow
.I var
to be removed or freed within the loop,
as it would interfere with the traversal.
The macros
.BR CIRCLEQ_FOREACH_SAFE ()
and
.BR CIRCLEQ_FOREACH_REVERSE_SAFE (),
which are present on the BSDs but are not present in glibc,
fix this limitation by allowing
.I var
to safely be removed from the list and freed from within the loop
without interfering with the traversal.
.SH EXAMPLES
.EX
#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\en", 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);
}
.EE
.SH SEE ALSO
.BR insque (3),
.BR queue (7)
.SH COLOPHON
This page is part of release 5.10 of the Linux
.I 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/.