.\" Copyright (c) 1993
.\"    The Regents of the University of California.  All rights reserved.
.\" and Copyright (c) 2020 by Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: BSD-3-Clause
.\"
.\"
.TH TAILQ 3 2022-10-30 "Linux man-pages 6.03"
.SH NAME
TAILQ_CONCAT,
TAILQ_EMPTY,
TAILQ_ENTRY,
TAILQ_FIRST,
TAILQ_FOREACH,
.\"TAILQ_FOREACH_FROM,
.\"TAILQ_FOREACH_FROM_SAFE,
TAILQ_FOREACH_REVERSE,
.\"TAILQ_FOREACH_REVERSE_FROM,
.\"TAILQ_FOREACH_REVERSE_FROM_SAFE,
.\"TAILQ_FOREACH_REVERSE_SAFE,
.\"TAILQ_FOREACH_SAFE,
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
.\"TAILQ_SWAP
\- implementation of a doubly linked tail queue
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/queue.h>
.PP
.B TAILQ_ENTRY(TYPE);
.PP
.B TAILQ_HEAD(HEADNAME, TYPE);
.BI "TAILQ_HEAD TAILQ_HEAD_INITIALIZER(TAILQ_HEAD " head );
.BI "void TAILQ_INIT(TAILQ_HEAD *" head );
.PP
.BI "int TAILQ_EMPTY(TAILQ_HEAD *" head );
.PP
.BI "void TAILQ_INSERT_HEAD(TAILQ_HEAD *" head ,
.BI "                         struct TYPE *" elm ", TAILQ_ENTRY " NAME );
.BI "void TAILQ_INSERT_TAIL(TAILQ_HEAD *" head ,
.BI "                         struct TYPE *" elm ", TAILQ_ENTRY " NAME );
.BI "void TAILQ_INSERT_BEFORE(struct TYPE *" listelm ,
.BI "                         struct TYPE *" elm ", TAILQ_ENTRY " NAME );
.BI "void TAILQ_INSERT_AFTER(TAILQ_HEAD *" head ", struct TYPE *" listelm ,
.BI "                         struct TYPE *" elm ", TAILQ_ENTRY " NAME );
.PP
.BI "struct TYPE *TAILQ_FIRST(TAILQ_HEAD *" head );
.BI "struct TYPE *TAILQ_LAST(TAILQ_HEAD *" head ", HEADNAME);"
.BI "struct TYPE *TAILQ_PREV(struct TYPE *" elm ", HEADNAME, TAILQ_ENTRY " NAME );
.BI "struct TYPE *TAILQ_NEXT(struct TYPE *" elm ", TAILQ_ENTRY " NAME );
.PP
.BI "TAILQ_FOREACH(struct TYPE *" var ", TAILQ_HEAD *" head ,
.BI "                         TAILQ_ENTRY " NAME );
.\" .BI "TAILQ_FOREACH_FROM(struct TYPE *" var ", TAILQ_HEAD *" head ,
.\" .BI "                                TAILQ_ENTRY " NAME );
.BI "TAILQ_FOREACH_REVERSE(struct TYPE *" var ", TAILQ_HEAD *" head ", HEADNAME,"
.BI "                         TAILQ_ENTRY " NAME );
.\" .BI "TAILQ_FOREACH_REVERSE_FROM(struct TYPE *" var ", TAILQ_HEAD *" head ", HEADNAME,"
.\" .BI "                                TAILQ_ENTRY " NAME );
.\" .PP
.\" .BI "TAILQ_FOREACH_SAFE(struct TYPE *" var ", TAILQ_HEAD *" head ,
.\" .BI "                                TAILQ_ENTRY " NAME ,
.\" .BI "                                struct TYPE *" temp_var );
.\" .BI "TAILQ_FOREACH_FROM_SAFE(struct TYPE *" var ", TAILQ_HEAD *" head ,
.\" .BI "                                TAILQ_ENTRY " NAME ,
.\" .BI "                                struct TYPE *" temp_var );
.\" .BI "TAILQ_FOREACH_REVERSE_SAFE(struct TYPE *" var ", TAILQ_HEAD *" head ,
.\" .BI "                                HEADNAME, TAILQ_ENTRY " NAME ,
.\" .BI "                                struct TYPE *" temp_var );
.\" .BI "TAILQ_FOREACH_REVERSE_FROM_SAFE(struct TYPE *" var ", TAILQ_HEAD *" head ,
.\" .BI "                                HEADNAME, TAILQ_ENTRY " NAME ,
.\" .BI "                                struct TYPE *" temp_var );
.PP
.BI "void TAILQ_REMOVE(TAILQ_HEAD *" head ", struct TYPE *" elm ,
.BI "                         TAILQ_ENTRY " NAME );
.PP
.BI "void TAILQ_CONCAT(TAILQ_HEAD *" head1 ", TAILQ_HEAD *" head2 ,
.BI "                         TAILQ_ENTRY " NAME );
.\" .BI "void TAILQ_SWAP(TAILQ_HEAD *" head1 ", TAILQ_HEAD *" head2 ", TYPE,"
.\" .BI "                                TAILQ_ENTRY " NAME );
.fi
.SH DESCRIPTION
These macros define and operate on doubly linked tail queues.
.PP
In the macro definitions,
.I TYPE
is the name of a user defined structure,
that must contain a field of type
.IR TAILQ_ENTRY ,
named
.IR NAME .
The argument
.I HEADNAME
is the name of a user defined structure that must be declared
using the macro
.BR TAILQ_HEAD ().
.SS Creation
A tail queue is headed by a structure defined by the
.BR TAILQ_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 removed 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
.I TAILQ_HEAD
structure is declared as follows:
.PP
.in +4
.EX
TAILQ_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 queue.
A pointer to the head of the 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
.BR TAILQ_ENTRY ()
declares a structure that connects the elements in the queue.
.PP
.BR TAILQ_HEAD_INITIALIZER ()
evaluates to an initializer for the queue
.IR head .
.PP
.BR TAILQ_INIT ()
initializes the queue referenced by
.PP
.BR TAILQ_EMPTY ()
evaluates to true if there are no items on the queue.
.IR head .
.SS Insertion
.BR TAILQ_INSERT_HEAD ()
inserts the new element
.I elm
at the head of the queue.
.PP
.BR TAILQ_INSERT_TAIL ()
inserts the new element
.I elm
at the end of the queue.
.PP
.BR TAILQ_INSERT_BEFORE ()
inserts the new element
.I elm
before the element
.IR listelm .
.PP
.BR TAILQ_INSERT_AFTER ()
inserts the new element
.I elm
after the element
.IR listelm .
.SS Traversal
.BR TAILQ_FIRST ()
returns the first item on the queue, or NULL if the queue is empty.
.PP
.BR TAILQ_LAST ()
returns the last item on the queue.
If the queue is empty the return value is NULL.
.PP
.BR TAILQ_PREV ()
returns the previous item on the queue, or NULL if this item is the first.
.PP
.BR TAILQ_NEXT ()
returns the next item on the queue, or NULL if this item is the last.
.PP
.BR TAILQ_FOREACH ()
traverses the queue referenced by
.I head
in the forward direction,
assigning each element in turn to
.IR var .
.I var
is set to NULL if the loop completes normally,
or if there were no elements.
.\" .PP
.\" .BR TAILQ_FOREACH_FROM ()
.\" behaves identically to
.\" .BR TAILQ_FOREACH ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found TAILQ element and begins the loop at
.\" .I var
.\" instead of the first element in the TAILQ referenced by
.\" .IR head .
.PP
.BR TAILQ_FOREACH_REVERSE ()
traverses the queue referenced by
.I head
in the reverse direction,
assigning each element in turn to
.IR var .
.\" .PP
.\" .BR TAILQ_FOREACH_REVERSE_FROM ()
.\" behaves identically to
.\" .BR TAILQ_FOREACH_REVERSE ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found TAILQ element and begins the reverse loop at
.\" .I var
.\" instead of the last element in the TAILQ referenced by
.\" .IR head .
.\" .PP
.\" .BR TAILQ_FOREACH_SAFE ()
.\" and
.\" .BR TAILQ_FOREACH_REVERSE_SAFE ()
.\" traverse the list referenced by
.\" .I head
.\" in the forward or reverse direction respectively,
.\" assigning each element in turn to
.\" .IR var .
.\" However, unlike their unsafe counterparts,
.\" .BR TAILQ_FOREACH ()
.\" and
.\" .BR TAILQ_FOREACH_REVERSE ()
.\" permit to both remove
.\" .I var
.\" as well as free it from within the loop safely without interfering with the
.\" traversal.
.\" .PP
.\" .BR TAILQ_FOREACH_FROM_SAFE ()
.\" behaves identically to
.\" .BR TAILQ_FOREACH_SAFE ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found TAILQ element and begins the loop at
.\" .I var
.\" instead of the first element in the TAILQ referenced by
.\" .IR head .
.\" .PP
.\" .BR TAILQ_FOREACH_REVERSE_FROM_SAFE ()
.\" behaves identically to
.\" .BR TAILQ_FOREACH_REVERSE_SAFE ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found TAILQ element and begins the reverse loop at
.\" .I var
.\" instead of the last element in the TAILQ referenced by
.\" .IR head .
.SS Removal
.BR TAILQ_REMOVE ()
removes the element
.I elm
from the queue.
.SS Other features
.\" .BR TAILQ_SWAP ()
.\" swaps the contents of
.\" .I head1
.\" and
.\" .IR head2 .
.\" .PP
.BR TAILQ_CONCAT ()
concatenates the queue headed by
.I head2
onto the end of the one headed by
.I head1
removing all entries from the former.
.SH RETURN VALUE
.BR TAILQ_EMPTY ()
returns nonzero if the queue is empty,
and zero if the queue contains at least one entry.
.PP
.BR TAILQ_FIRST (),
.BR TAILQ_LAST (),
.BR TAILQ_PREV (),
and
.BR TAILQ_NEXT ()
return a pointer to the first, last, previous, or next
.I TYPE
structure, respectively.
.PP
.BR TAILQ_HEAD_INITIALIZER ()
returns an initializer that can be assigned to the queue
.IR head .
.SH STANDARDS
Not in POSIX.1, POSIX.1-2001, or POSIX.1-2008.
Present on the BSDs.
(TAILQ functions first appeared in 4.4BSD).
.SH BUGS
.BR TAILQ_FOREACH ()
and
.BR TAILQ_FOREACH_REVERSE ()
don't allow
.I var
to be removed or freed within the loop,
as it would interfere with the traversal.
.BR TAILQ_FOREACH_SAFE ()
and
.BR TAILQ_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
.\" SRC BEGIN (tailq.c)
.EX
#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\en", 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);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR insque (3),
.BR queue (7)