.\" 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 STAILQ 3 2024-06-16 "Linux man-pages (unreleased)"
.SH NAME
.\"SIMPLEQ_CONCAT,
SIMPLEQ_EMPTY,
SIMPLEQ_ENTRY,
SIMPLEQ_FIRST,
SIMPLEQ_FOREACH,
.\"SIMPLEQ_FOREACH_FROM,
.\"SIMPLEQ_FOREACH_FROM_SAFE,
.\"SIMPLEQ_FOREACH_SAFE,
SIMPLEQ_HEAD,
SIMPLEQ_HEAD_INITIALIZER,
SIMPLEQ_INIT,
SIMPLEQ_INSERT_AFTER,
SIMPLEQ_INSERT_HEAD,
SIMPLEQ_INSERT_TAIL,
.\"SIMPLEQ_LAST,
SIMPLEQ_NEXT,
SIMPLEQ_REMOVE,
.\"SIMPLEQ_REMOVE_AFTER,
SIMPLEQ_REMOVE_HEAD,
.\"SIMPLEQ_SWAP,
STAILQ_CONCAT,
STAILQ_EMPTY,
STAILQ_ENTRY,
STAILQ_FIRST,
STAILQ_FOREACH,
.\"STAILQ_FOREACH_FROM,
.\"STAILQ_FOREACH_FROM_SAFE,
.\"STAILQ_FOREACH_SAFE,
STAILQ_HEAD,
STAILQ_HEAD_INITIALIZER,
STAILQ_INIT,
STAILQ_INSERT_AFTER,
STAILQ_INSERT_HEAD,
STAILQ_INSERT_TAIL,
.\"STAILQ_LAST,
STAILQ_NEXT,
STAILQ_REMOVE,
.\"STAILQ_REMOVE_AFTER,
STAILQ_REMOVE_HEAD,
.\"STAILQ_SWAP
\- implementation of a singly linked tail queue
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/queue.h>
.P
.B STAILQ_ENTRY(TYPE);
.P
.B STAILQ_HEAD(HEADNAME, TYPE);
.BI "STAILQ_HEAD STAILQ_HEAD_INITIALIZER(STAILQ_HEAD " head );
.BI "void STAILQ_INIT(STAILQ_HEAD *" head );
.P
.BI "int STAILQ_EMPTY(STAILQ_HEAD *" head );
.P
.BI "void STAILQ_INSERT_HEAD(STAILQ_HEAD *" head ,
.BI "                         struct TYPE *" elm ", STAILQ_ENTRY " NAME );
.BI "void STAILQ_INSERT_TAIL(STAILQ_HEAD *" head ,
.BI "                         struct TYPE *" elm ", STAILQ_ENTRY " NAME );
.BI "void STAILQ_INSERT_AFTER(STAILQ_HEAD *" head ", struct TYPE *" listelm ,
.BI "                         struct TYPE *" elm ", STAILQ_ENTRY " NAME );
.P
.BI "struct TYPE *STAILQ_FIRST(STAILQ_HEAD *" head );
.\" .BI "struct TYPE *STAILQ_LAST(STAILQ_HEAD *" head ", struct TYPE *" elm ,
.\" .BI "                          STAILQ_ENTRY " NAME );
.BI "struct TYPE *STAILQ_NEXT(struct TYPE *" elm ", STAILQ_ENTRY " NAME );
.P
.BI "STAILQ_FOREACH(struct TYPE *" var ", STAILQ_HEAD *" head ", STAILQ_ENTRY " NAME );
.\" .BI "STAILQ_FOREACH_FROM(struct TYPE *" var ", STAILQ_HEAD *" head ,
.\" .BI "                          STAILQ_ENTRY " NAME );
.\" .P
.\" .BI "STAILQ_FOREACH_SAFE(struct TYPE *" var ", STAILQ_HEAD *" head ,
.\" .BI "                          STAILQ_ENTRY " NAME ", struct TYPE *" temp_var );
.\" .BI "STAILQ_FOREACH_FROM_SAFE(struct TYPE *" var ", STAILQ_HEAD *" head ,
.\" .BI "                          STAILQ_ENTRY " NAME ", struct TYPE *" temp_var );
.P
.BI "void STAILQ_REMOVE(STAILQ_HEAD *" head ", struct TYPE *" elm ", TYPE,"
.BI "                         STAILQ_ENTRY " NAME );
.BI "void STAILQ_REMOVE_HEAD(STAILQ_HEAD *" head ,
.BI "                         STAILQ_ENTRY " NAME );
.\" .BI "void STAILQ_REMOVE_AFTER(STAILQ_HEAD *" head ", struct TYPE *" elm ,
.\" .BI "                          STAILQ_ENTRY " NAME );
.P
.BI "void STAILQ_CONCAT(STAILQ_HEAD *" head1 ", STAILQ_HEAD *" head2 );
.\" .BI "void STAILQ_SWAP(STAILQ_HEAD *" head1 ", STAILQ_HEAD *" head2 ,
.\" .BI "                          STAILQ_ENTRY " NAME );
.fi
.IR Note :
Identical macros prefixed with SIMPLEQ instead of STAILQ exist; see VERSIONS.
.SH DESCRIPTION
These macros define and operate on singly linked tail queues.
.P
In the macro definitions,
.I TYPE
is the name of a user-defined structure,
that must contain a field of type
.IR STAILQ_ENTRY ,
named
.IR NAME .
The argument
.I HEADNAME
is the name of a user-defined structure that must be declared
using the macro
.BR STAILQ_HEAD ().
.SS Creation
A singly linked tail queue is headed by a structure defined by the
.BR STAILQ_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 singly linked for minimum space and pointer
manipulation overhead at the expense of O(n) removal for arbitrary elements.
New elements can be added to the tail queue after an existing element,
at the head of the tail queue, or at the end of the tail queue.
A
.I STAILQ_HEAD
structure is declared as follows:
.P
.in +4
.EX
STAILQ_HEAD(HEADNAME, TYPE) head;
.EE
.in
.P
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:
.P
.in +4
.EX
struct HEADNAME *headp;
.EE
.in
.P
(The names
.I head
and
.I headp
are user selectable.)
.P
.BR STAILQ_ENTRY ()
declares a structure that connects the elements in the tail queue.
.P
.BR STAILQ_HEAD_INITIALIZER ()
evaluates to an initializer for the tail queue
.IR head .
.P
.BR STAILQ_INIT ()
initializes the tail queue referenced by
.IR head .
.P
.BR STAILQ_EMPTY ()
evaluates to true if there are no items on the tail queue.
.SS Insertion
.BR STAILQ_INSERT_HEAD ()
inserts the new element
.I elm
at the head of the tail queue.
.P
.BR STAILQ_INSERT_TAIL ()
inserts the new element
.I elm
at the end of the tail queue.
.P
.BR STAILQ_INSERT_AFTER ()
inserts the new element
.I elm
after the element
.IR listelm .
.SS Traversal
.BR STAILQ_FIRST ()
returns the first item on the tail queue or NULL if the tail queue is empty.
.\" .P
.\" .BR STAILQ_LAST ()
.\" returns the last item on the tail queue.
.\" If the tail queue is empty the return value is NULL .
.P
.BR STAILQ_NEXT ()
returns the next item on the tail queue, or NULL this item is the last.
.P
.BR STAILQ_FOREACH ()
traverses the tail queue referenced by
.I head
in the forward direction,
assigning each element in turn to
.IR var .
.\" .P
.\" .BR STAILQ_FOREACH_FROM ()
.\" behaves identically to
.\" .BR STAILQ_FOREACH ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found STAILQ element and begins the loop at
.\" .I var
.\" instead of the first element in the STAILQ referenced by
.\" .IR head .
.\" .P
.\" .BR STAILQ_FOREACH_SAFE ()
.\" traverses the tail queue referenced by
.\" .I head
.\" in the forward direction, assigning each element
.\" in turn to
.\" .IR var .
.\" However, unlike
.\" .BR STAILQ_FOREACH ()
.\" here it is permitted to both remove
.\" .I var
.\" as well as free it from within the loop safely without interfering with the
.\" traversal.
.\" .P
.\" .BR STAILQ_FOREACH_FROM_SAFE ()
.\" behaves identically to
.\" .BR STAILQ_FOREACH_SAFE ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found STAILQ element and begins the loop at
.\" .I var
.\" instead of the first element in the STAILQ referenced by
.\" .IR head .
.SS Removal
.BR STAILQ_REMOVE ()
removes the element
.I elm
from the tail queue.
.P
.BR STAILQ_REMOVE_HEAD ()
removes the element at the head of the tail queue.
For optimum efficiency,
elements being removed from the head of the tail queue should
use this macro explicitly rather than the generic
.BR STAILQ_REMOVE ()
macro.
.\" .P
.\" .BR STAILQ_REMOVE_AFTER ()
.\" removes the element after
.\" .I elm
.\" from the tail queue.
.\" Unlike
.\" .BR STAILQ_REMOVE (),
.\" this macro does not traverse the entire tail queue.
.SS Other features
.BR STAILQ_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.
.\" .P
.\" .BR STAILQ_SWAP ()
.\" swaps the contents of
.\" .I head1
.\" and
.\" .IR head2 .
.SH RETURN VALUE
.BR STAILQ_EMPTY ()
returns nonzero if the queue is empty,
and zero if the queue contains at least one entry.
.P
.BR STAILQ_FIRST (),
and
.BR STAILQ_NEXT ()
return a pointer to the first or next
.I TYPE
structure, respectively.
.P
.BR STAILQ_HEAD_INITIALIZER ()
returns an initializer that can be assigned to the queue
.IR head .
.SH VERSIONS
Some BSDs provide SIMPLEQ instead of STAILQ.
They are identical, but for historical reasons
they were named differently on different BSDs.
STAILQ originated on FreeBSD, and SIMPLEQ originated on NetBSD.
For compatibility reasons, some systems provide both sets of macros.
glibc provides both STAILQ and SIMPLEQ,
which are identical except for a missing SIMPLEQ equivalent to
.BR STAILQ_CONCAT ().
.SH BUGS
.BR STAILQ_FOREACH ()
doesn't allow
.I var
to be removed or freed within the loop,
as it would interfere with the traversal.
.BR STAILQ_FOREACH_SAFE (),
which is present on the BSDs but is not present in glibc,
fixes 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 STANDARDS
BSD.
.SH HISTORY
4.4BSD.
.SH EXAMPLES
.\" SRC BEGIN (stailq.c)
.EX
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
\&
struct entry {
    int data;
    STAILQ_ENTRY(entry) entries;        /* Singly linked tail queue */
};
\&
STAILQ_HEAD(stailhead, entry);
\&
int
main(void)
{
    struct entry *n1, *n2, *n3, *np;
    struct stailhead head;                  /* Singly linked tail queue
                                               head */
\&
    STAILQ_INIT(&head);                     /* Initialize the queue */
\&
    n1 = malloc(sizeof(struct entry));      /* Insert at the head */
    STAILQ_INSERT_HEAD(&head, n1, entries);
\&
    n1 = malloc(sizeof(struct entry));      /* Insert at the tail */
    STAILQ_INSERT_TAIL(&head, n1, entries);
\&
    n2 = malloc(sizeof(struct entry));      /* Insert after */
    STAILQ_INSERT_AFTER(&head, n1, n2, entries);
\&
    STAILQ_REMOVE(&head, n2, entry, entries); /* Deletion */
    free(n2);
\&
    n3 = STAILQ_FIRST(&head);
    STAILQ_REMOVE_HEAD(&head, entries);     /* Deletion from the head */
    free(n3);
\&
    n1 = STAILQ_FIRST(&head);
    n1\->data = 0;
    for (unsigned int i = 1; i < 5; i++) {
        n1 = malloc(sizeof(struct entry));
        STAILQ_INSERT_HEAD(&head, n1, entries);
        n1\->data = i;
    }
                                            /* Forward traversal */
    STAILQ_FOREACH(np, &head, entries)
        printf("%i\[rs]n", np\->data);
                                            /* TailQ deletion */
    n1 = STAILQ_FIRST(&head);
    while (n1 != NULL) {
        n2 = STAILQ_NEXT(n1, entries);
        free(n1);
        n1 = n2;
    }
    STAILQ_INIT(&head);
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR insque (3),
.BR queue (7)