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.TH TAILQ 3 2020-12-21 "GNU" "Linux Programmer's Manual"
.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 SYNOPSIS
.nf
.B #include <sys/queue.h>
.PP
.BI "void TAILQ_CONCAT(TAILQ_HEAD *" head1 ", TAILQ_HEAD *" head2 ","
.BI "                TAILQ_ENTRY " NAME ");"
.PP
.BI "int TAILQ_EMPTY(TAILQ_HEAD *" head ");"
.PP
.B TAILQ_ENTRY(TYPE);
.PP
.BI "struct TYPE *TAILQ_FIRST(TAILQ_HEAD *" head ");"
.PP
.BI "TAILQ_FOREACH(struct TYPE *" var ", TAILQ_HEAD *" head ", TAILQ_ENTRY " NAME ");"
.\" .PP
.\" .BI "TAILQ_FOREACH_FROM(struct TYPE *" var ", TAILQ_HEAD *" head ", TAILQ_ENTRY " NAME ");"
.\" .PP
.\" .BI "TAILQ_FOREACH_FROM_SAFE(struct TYPE *" var ", TAILQ_HEAD *" head ", TAILQ_ENTRY " NAME ", struct TYPE *" temp_var ");"
.PP
.BI "TAILQ_FOREACH_REVERSE(struct TYPE *" var ", TAILQ_HEAD *" head ", HEADNAME,"
.BI "                TAILQ_ENTRY " NAME ");"
.\" .PP
.\" .BI "TAILQ_FOREACH_REVERSE_FROM(struct TYPE *" var ", TAILQ_HEAD *" head ", HEADNAME, TAILQ_ENTRY " NAME ");"
.\" .PP
.\" .BI "TAILQ_FOREACH_REVERSE_FROM_SAFE(struct TYPE *" var ", TAILQ_HEAD *" head ", HEADNAME, TAILQ_ENTRY " NAME ", struct TYPE *" temp_var ");"
.\" .PP
.\" .BI "TAILQ_FOREACH_REVERSE_SAFE(struct TYPE *" var ", TAILQ_HEAD *" head ", HEADNAME, TAILQ_ENTRY " NAME ", TYPE *" temp_var ");"
.\" .PP
.\" .BI "TAILQ_FOREACH_SAFE(struct TYPE *" var ", TAILQ_HEAD *" head ", TAILQ_ENTRY " NAME ", struct TYPE *" temp_var ");"
.PP
.B TAILQ_HEAD(HEADNAME, TYPE);
.PP
.BI "TAILQ_HEAD TAILQ_HEAD_INITIALIZER(TAILQ_HEAD " head ");"
.PP
.BI "void TAILQ_INIT(TAILQ_HEAD *" head ");"
.PP
.BI "void TAILQ_INSERT_AFTER(TAILQ_HEAD *" head ", struct TYPE *" listelm ","
.BI "                struct TYPE *" elm ", TAILQ_ENTRY " NAME ");"
.PP
.BI "void TAILQ_INSERT_BEFORE(struct TYPE *" listelm ", struct TYPE *" elm ","
.BI "                TAILQ_ENTRY " NAME ");"
.PP
.BI "void TAILQ_INSERT_HEAD(TAILQ_HEAD *" head ", struct TYPE *" elm ","
.BI "                TAILQ_ENTRY " NAME ");"
.PP
.BI "void TAILQ_INSERT_TAIL(TAILQ_HEAD *" head ", struct TYPE *" elm ","
.BI "                TAILQ_ENTRY " NAME ");"
.PP
.BI "struct TYPE *TAILQ_LAST(TAILQ_HEAD *" head ", HEADNAME);"
.PP
.BI "struct TYPE *TAILQ_NEXT(struct TYPE *" elm ", TAILQ_ENTRY " NAME ");"
.PP
.BI "struct TYPE *TAILQ_PREV(struct TYPE *" elm ", HEADNAME, TAILQ_ENTRY " NAME ");"
.PP
.BI "void TAILQ_REMOVE(TAILQ_HEAD *" head ", struct TYPE *" elm ", TAILQ_ENTRY " NAME ");"
.\" .PP
.\" .BI "void TAILQ_SWAP(TAILQ_HEAD *" head1 ", TAILQ_HEAD *" head2 ", TYPE, 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 ().
.PP
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 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
.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 tail queue.
A pointer to the head of the tail 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 TAILQ_HEAD_INITIALIZER ()
evaluates to an initializer for the tail queue
.IR head .
.PP
The macro
.BR TAILQ_CONCAT ()
concatenates the tail queue headed by
.I head2
onto the end of the one headed by
.I head1
removing all entries from the former.
.PP
The macro
.BR TAILQ_EMPTY ()
evaluates to true if there are no items on the tail queue.
.PP
The macro
.BR TAILQ_ENTRY ()
declares a structure that connects the elements in
the tail queue.
.PP
The macro
.BR TAILQ_FIRST ()
returns the first item on the tail queue or NULL if the tail queue
is empty.
.PP
The macro
.BR TAILQ_FOREACH ()
traverses the tail 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
.\" The macro
.\" .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
The macro
.BR TAILQ_FOREACH_REVERSE ()
traverses the tail queue referenced by
.I head
in the reverse direction, assigning each element in turn to
.IR var .
.\" .PP
.\" The macro
.\" .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
.\" The macros
.\" .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
.\" The macro
.\" .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
.\" The macro
.\" .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 .
.PP
The macro
.BR TAILQ_INIT ()
initializes the tail queue referenced by
.IR head .
.PP
The macro
.BR TAILQ_INSERT_HEAD ()
inserts the new element
.I elm
at the head of the tail queue.
.PP
The macro
.BR TAILQ_INSERT_TAIL ()
inserts the new element
.I elm
at the end of the tail queue.
.PP
The macro
.BR TAILQ_INSERT_AFTER ()
inserts the new element
.I elm
after the element
.IR listelm .
.PP
The macro
.BR TAILQ_INSERT_BEFORE ()
inserts the new element
.I elm
before the element
.IR listelm .
.PP
The macro
.BR TAILQ_LAST ()
returns the last item on the tail queue.
If the tail queue is empty the return value is NULL.
.PP
The macro
.BR TAILQ_NEXT ()
returns the next item on the tail queue, or NULL if this item is the last.
.PP
The macro
.BR TAILQ_PREV ()
returns the previous item on the tail queue, or NULL if this item
is the first.
.PP
The macro
.BR TAILQ_REMOVE ()
removes the element
.I elm
from the tail queue.
.\" .PP
.\" The macro
.\" .BR TAILQ_SWAP ()
.\" swaps the contents of
.\" .I head1
.\" and
.\" .IR head2 .
.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_NEXT (),
and
.BR TAILQ_PREV ()
return a pointer to the first, last, next or previous
.I TYPE
structure, respectively.
.PP
.BR TAILQ_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.
(TAILQ functions first appeared in 4.4BSD).
.SH BUGS
The macros
.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.
The macros
.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
.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
.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/.