.\" 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 SLIST 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
SLIST_EMPTY,
SLIST_ENTRY,
SLIST_FIRST,
SLIST_FOREACH,
.\"SLIST_FOREACH_FROM,
.\"SLIST_FOREACH_FROM_SAFE,
.\"SLIST_FOREACH_SAFE,
SLIST_HEAD,
SLIST_HEAD_INITIALIZER,
SLIST_INIT,
SLIST_INSERT_AFTER,
SLIST_INSERT_HEAD,
SLIST_NEXT,
SLIST_REMOVE,
.\"SLIST_REMOVE_AFTER,
SLIST_REMOVE_HEAD
.\"SLIST_SWAP
\- implementation of a singly linked list
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/queue.h>
.PP
.B SLIST_ENTRY(TYPE);
.PP
.B SLIST_HEAD(HEADNAME, TYPE);
.BI "SLIST_HEAD SLIST_HEAD_INITIALIZER(SLIST_HEAD " head );
.BI "void SLIST_INIT(SLIST_HEAD *" head );
.PP
.BI "int SLIST_EMPTY(SLIST_HEAD *" head );
.PP
.BI "void SLIST_INSERT_HEAD(SLIST_HEAD *" head ,
.BI "                        struct TYPE *" elm ", SLIST_ENTRY " NAME );
.BI "void SLIST_INSERT_AFTER(struct TYPE *" listelm ,
.BI "                        struct TYPE *" elm ", SLIST_ENTRY " NAME );
.PP
.BI "struct TYPE *SLIST_FIRST(SLIST_HEAD *" head );
.BI "struct TYPE *SLIST_NEXT(struct TYPE *" elm ", SLIST_ENTRY " NAME );
.PP
.BI "SLIST_FOREACH(struct TYPE *" var ", SLIST_HEAD *" head ", SLIST_ENTRY " NAME );
.\" .BI "SLIST_FOREACH_FROM(struct TYPE *" var ", SLIST_HEAD *" head ,
.\" .BI "                        SLIST_ENTRY " NAME );
.\" .PP
.\" .BI "SLIST_FOREACH_SAFE(struct TYPE *" var ", SLIST_HEAD *" head ,
.\" .BI "                        SLIST_ENTRY " NAME ", struct TYPE *" temp_var );
.\" .BI "SLIST_FOREACH_FROM_SAFE(struct TYPE *" var ", SLIST_HEAD *" head ,
.\" .BI "                        SLIST_ENTRY " NAME ", struct TYPE *" temp_var );
.PP
.BI "void SLIST_REMOVE(SLIST_HEAD *" head ", struct TYPE *" elm ,
.BI "                        SLIST_ENTRY " NAME );
.BI "void SLIST_REMOVE_HEAD(SLIST_HEAD *" head ,
.BI "                        SLIST_ENTRY " NAME );
.\" .BI "void SLIST_REMOVE_AFTER(struct TYPE *" elm ,
.\" .BI "                        SLIST_ENTRY " NAME );
.\" .PP
.\" .BI "void SLIST_SWAP(SLIST_HEAD *" head1 ", SLIST_HEAD *" head2 ,
.\" .BI "                        SLIST_ENTRY " NAME );
.fi
.SH DESCRIPTION
These macros define and operate on doubly linked lists.
.PP
In the macro definitions,
.I TYPE
is the name of a user-defined structure,
that must contain a field of type
.IR SLIST_ENTRY ,
named
.IR NAME .
The argument
.I HEADNAME
is the name of a user-defined structure
that must be declared using the macro
.BR SLIST_HEAD ().
.SS Creation
A singly linked list is headed by a structure defined by the
.BR SLIST_HEAD ()
macro.
This structure contains a single pointer to the first element on the list.
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 list
after an existing element
or at the head of the list.
An
.I SLIST_HEAD
structure is declared as follows:
.PP
.in +4
.EX
SLIST_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 list.
A pointer to the head of the list 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 SLIST_ENTRY ()
declares a structure that connects the elements in
the list.
.PP
.BR SLIST_HEAD_INITIALIZER ()
evaluates to an initializer for the list
.IR head .
.PP
.BR SLIST_INIT ()
initializes the list referenced by
.IR head .
.PP
.BR SLIST_EMPTY ()
evaluates to true if there are no elements in the list.
.SS Insertion
.BR SLIST_INSERT_HEAD ()
inserts the new element
.I elm
at the head of the list.
.PP
.BR SLIST_INSERT_AFTER ()
inserts the new element
.I elm
after the element
.IR listelm .
.SS Traversal
.BR SLIST_FIRST ()
returns the first element in the list, or NULL if the list is empty.
.PP
.BR SLIST_NEXT ()
returns the next element in the list.
.PP
.BR SLIST_FOREACH ()
traverses the list referenced by
.I head
in the forward direction,
assigning each element in turn to
.IR var .
.\" .PP
.\" .BR SLIST_FOREACH_FROM ()
.\" behaves identically to
.\" .BR SLIST_FOREACH ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found SLIST element and begins the loop at
.\" .I var
.\" instead of the first element in the SLIST referenced by
.\" .IR head .
.\" .Pp
.\" .BR SLIST_FOREACH_SAFE ()
.\" traverses the list referenced by
.\" .I head
.\" in the forward direction, assigning each element in
.\" turn to
.\" .IR var .
.\" However, unlike
.\" .BR SLIST_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.
.\" .PP
.\" .BR SLIST_FOREACH_FROM_SAFE ()
.\" behaves identically to
.\" .BR SLIST_FOREACH_SAFE ()
.\" when
.\" .I var
.\" is NULL, else it treats
.\" .I var
.\" as a previously found SLIST element and begins the loop at
.\" .I var
.\" instead of the first element in the SLIST referenced by
.\" .IR head .
.SS Removal
.BR SLIST_REMOVE ()
removes the element
.I elm
from the list.
.PP
.BR SLIST_REMOVE_HEAD ()
removes the element
.I elm
from the head of the list.
For optimum efficiency,
elements being removed from the head of the list
should explicitly use this macro instead of the generic
.BR SLIST_REMOVE ().
.\" .PP
.\" .BR SLIST_REMOVE_AFTER ()
.\" removes the element after
.\" .I elm
.\" from the list.
.\" Unlike
.\" .IR SLIST_REMOVE ,
.\" this macro does not traverse the entire list.
.\" .SS Other features
.\" .BR SLIST_SWAP ()
.\" swaps the contents of
.\" .I head1
.\" and
.\" .IR head2 .
.SH RETURN VALUE
.BR SLIST_EMPTY ()
returns nonzero if the list is empty,
and zero if the list contains at least one entry.
.PP
.BR SLIST_FIRST (),
and
.BR SLIST_NEXT ()
return a pointer to the first or next
.I TYPE
structure, respectively.
.PP
.BR SLIST_HEAD_INITIALIZER ()
returns an initializer that can be assigned to the list
.IR head .
.SH STANDARDS
BSD.
.SH HISTORY
4.4BSD.
.SH BUGS
.BR SLIST_FOREACH ()
doesn't allow
.I var
to be removed or freed within the loop,
as it would interfere with the traversal.
.BR SLIST_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 EXAMPLES
.\" SRC BEGIN (slist.c)
.EX
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>

struct entry {
    int data;
    SLIST_ENTRY(entry) entries;             /* Singly linked list */
};

SLIST_HEAD(slisthead, entry);

int
main(void)
{
    struct entry *n1, *n2, *n3, *np;
    struct slisthead head;                  /* Singly linked list
                                               head */

    SLIST_INIT(&head);                      /* Initialize the queue */

    n1 = malloc(sizeof(struct entry));      /* Insert at the head */
    SLIST_INSERT_HEAD(&head, n1, entries);

    n2 = malloc(sizeof(struct entry));      /* Insert after */
    SLIST_INSERT_AFTER(n1, n2, entries);

    SLIST_REMOVE(&head, n2, entry, entries);/* Deletion */
    free(n2);

    n3 = SLIST_FIRST(&head);
    SLIST_REMOVE_HEAD(&head, entries);      /* Deletion from the head */
    free(n3);

    for (unsigned int i = 0; i < 5; i++) {
        n1 = malloc(sizeof(struct entry));
        SLIST_INSERT_HEAD(&head, n1, entries);
        n1\->data = i;
    }

                                            /* Forward traversal */
    SLIST_FOREACH(np, &head, entries)
        printf("%i\en", np\->data);

    while (!SLIST_EMPTY(&head)) {           /* List deletion */
        n1 = SLIST_FIRST(&head);
        SLIST_REMOVE_HEAD(&head, entries);
        free(n1);
    }
    SLIST_INIT(&head);

    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR insque (3),
.BR queue (7)