QUEUE(3)

NAME
SLIST_ENTRY, SLIST_HEAD, SLIST_HEAD_INITIALIZER, SLIST_FIRST, SLIST_NEXT, SLIST_EMPTY, SLIST_FOREACH, SLIST_FOREACH_SAFE, SLIST_INIT, SLIST_INSERT_AFTER, SLIST_INSERT_HEAD, SLIST_REMOVE_AFTER, SLIST_REMOVE_HEAD, SLIST_REMOVE, LIST_ENTRY, LIST_HEAD, LIST_HEAD_INITIALIZER, LIST_FIRST, LIST_NEXT, LIST_EMPTY, LIST_FOREACH, LIST_FOREACH_SAFE, LIST_INIT, LIST_INSERT_AFTER, LIST_INSERT_BEFORE, LIST_INSERT_HEAD, LIST_REMOVE, LIST_REPLACE, SIMPLEQ_ENTRY, SIMPLEQ_HEAD, SIMPLEQ_HEAD_INITIALIZER, SIMPLEQ_FIRST, SIMPLEQ_NEXT, SIMPLEQ_EMPTY, SIMPLEQ_FOREACH, SIMPLEQ_FOREACH_SAFE, SIMPLEQ_INIT, SIMPLEQ_INSERT_AFTER, SIMPLEQ_INSERT_HEAD, SIMPLEQ_INSERT_TAIL, SIMPLEQ_REMOVE_AFTER, SIMPLEQ_REMOVE_HEAD, TAILQ_ENTRY, TAILQ_HEAD, TAILQ_HEAD_INITIALIZER, TAILQ_FIRST, TAILQ_NEXT, TAILQ_LAST, TAILQ_PREV, TAILQ_EMPTY, TAILQ_FOREACH, TAILQ_FOREACH_SAFE, TAILQ_FOREACH_REVERSE, TAILQ_FOREACH_REVERSE_SAFE, TAILQ_INIT, TAILQ_INSERT_AFTER, TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD, TAILQ_INSERT_TAIL, TAILQ_REMOVE, TAILQ_REPLACE - implementations of singly-linked lists, doubly-linked lists, simple queues, and tail queues

SYNOPSIS
#include <sys/queue.h>

SLIST_ENTRY(TYPE);
SLIST_HEAD(HEADNAME, TYPE);
SLIST_HEAD_INITIALIZER(SLIST_HEAD head);
struct TYPE *
SLIST_FIRST(SLIST_HEAD *head);
struct TYPE *
SLIST_NEXT(struct TYPE *listelm, FIELDNAME);
int
SLIST_EMPTY(SLIST_HEAD *head);
SLIST_FOREACH(VARNAME, SLIST_HEAD *head, FIELDNAME);
SLIST_FOREACH_SAFE(VARNAME, SLIST_HEAD *head, FIELDNAME, TEMP_VARNAME);
void
SLIST_INIT(SLIST_HEAD *head);
void
SLIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
SLIST_INSERT_HEAD(SLIST_HEAD *head, struct TYPE *elm, FIELDNAME);
void
SLIST_REMOVE_AFTER(struct TYPE *elm, FIELDNAME);
void
SLIST_REMOVE_HEAD(SLIST_HEAD *head, FIELDNAME);
void
SLIST_REMOVE(SLIST_HEAD *head, struct TYPE *elm, TYPE, FIELDNAME);

LIST_ENTRY(TYPE);
LIST_HEAD(HEADNAME, TYPE);
LIST_HEAD_INITIALIZER(LIST_HEAD head);
struct TYPE *
LIST_FIRST(LIST_HEAD *head);
struct TYPE *
LIST_NEXT(struct TYPE *listelm, FIELDNAME);
int
LIST_EMPTY(LIST_HEAD *head);
LIST_FOREACH(VARNAME, LIST_HEAD *head, FIELDNAME);
LIST_FOREACH_SAFE(VARNAME, LIST_HEAD *head, FIELDNAME, TEMP_VARNAME);
void
LIST_INIT(LIST_HEAD *head);
void
LIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
LIST_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
LIST_INSERT_HEAD(LIST_HEAD *head, struct TYPE *elm, FIELDNAME);
void
LIST_REMOVE(struct TYPE *elm, FIELDNAME);
void
LIST_REPLACE(struct TYPE *elm, struct TYPE *elm2, FIELDNAME);

SIMPLEQ_ENTRY(TYPE);
SIMPLEQ_HEAD(HEADNAME, TYPE);
SIMPLEQ_HEAD_INITIALIZER(SIMPLEQ_HEAD head);
struct TYPE *
SIMPLEQ_FIRST(SIMPLEQ_HEAD *head);
struct TYPE *
SIMPLEQ_NEXT(struct TYPE *listelm, FIELDNAME);
int
SIMPLEQ_EMPTY(SIMPLEQ_HEAD *head);
SIMPLEQ_FOREACH(VARNAME, SIMPLEQ_HEAD *head, FIELDNAME);
SIMPLEQ_FOREACH_SAFE(VARNAME, SIMPLEQ_HEAD *head, FIELDNAME, TEMP_VARNAME);
void
SIMPLEQ_INIT(SIMPLEQ_HEAD *head);
void
SIMPLEQ_INSERT_AFTER(SIMPLEQ_HEAD *head, struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
SIMPLEQ_INSERT_HEAD(SIMPLEQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
SIMPLEQ_INSERT_TAIL(SIMPLEQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
SIMPLEQ_REMOVE_AFTER(SIMPLEQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
SIMPLEQ_REMOVE_HEAD(SIMPLEQ_HEAD *head, FIELDNAME);

TAILQ_ENTRY(TYPE);
TAILQ_HEAD(HEADNAME, TYPE);
TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);
struct TYPE *
TAILQ_FIRST(TAILQ_HEAD *head);
struct TYPE *
TAILQ_NEXT(struct TYPE *listelm, FIELDNAME);
struct TYPE *
TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);
struct TYPE *
TAILQ_PREV(struct TYPE *listelm, HEADNAME, FIELDNAME);
int
TAILQ_EMPTY(TAILQ_HEAD *head);
TAILQ_FOREACH(VARNAME, TAILQ_HEAD *head, FIELDNAME);
TAILQ_FOREACH_SAFE(VARNAME, TAILQ_HEAD *head, FIELDNAME, TEMP_VARNAME);
TAILQ_FOREACH_REVERSE(VARNAME, TAILQ_HEAD *head, HEADNAME, FIELDNAME);
TAILQ_FOREACH_REVERSE_SAFE(VARNAME, TAILQ_HEAD *head, HEADNAME, FIELDNAME, TEMP_VARNAME);
void
TAILQ_INIT(TAILQ_HEAD *head);
void
TAILQ_INSERT_AFTER(TAILQ_HEAD *head, struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
TAILQ_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm, FIELDNAME);
void
TAILQ_INSERT_HEAD(TAILQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
TAILQ_INSERT_TAIL(TAILQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
TAILQ_REMOVE(TAILQ_HEAD *head, struct TYPE *elm, FIELDNAME);
void
TAILQ_REPLACE(TAILQ_HEAD *head, struct TYPE *elm, struct TYPE *elm2, FIELDNAME);

DESCRIPTION
These macros define and operate on four types of data structures: singly-linked lists, simple queues, lists, and tail queues. All four structures support the following functionality:

1.  Insertion of a new entry at the head of the list.
2.  Insertion of a new entry after any element in the list.
3.  Removal of an entry from the head of the list.
4.  Forward traversal through the list.

Singly-linked lists are the simplest of the four data structures and support only the above functionality. Singly-linked lists are ideal for applications with large datasets and few or no removals, or for implementing a LIFO queue.

Simple queues add the following functionality:

1.  Entries can be added at the end of a list.

However:

1.  All list insertions must specify the head of the list.
2.  Each head entry requires two pointers rather than one.
3.  Code size is about 15% greater and operations run about 20% slower than singly-linked lists.

Simple queues are ideal for applications with large datasets and few or no removals, or for implementing a FIFO queue.

All doubly linked types of data structures (lists and tail queues) additionally allow:

1.  Insertion of a new entry before any element in the list.
2.  Removal of any entry in the list.

However:

1.  Each element requires two pointers rather than one.
2.  Code size and execution time of operations (except for removal) is about twice that of the singly-linked data-structures.

Lists are the simplest of the doubly linked data structures and support only the above functionality over singly-linked lists.

Tail queues add the following functionality:

1.  Entries can be added at the end of a list.
2.  They may be traversed backwards, at a cost.

However:

1.  All list insertions and removals must specify the head of the list.
2.  Each head entry requires two pointers rather than one.
3.  Code size is about 15% greater and operations run about 20% slower than singly-linked lists.

An additional type of data structure, circular queues, violated the C language aliasing rules and were miscompiled as a result. All code using them should be converted to another structure; tail queues are usually the easiest to convert to.

In the macro definitions, TYPE is the name tag of a user defined structure that must contain a field of type SLIST_ENTRY, LIST_ENTRY, SIMPLEQ_ENTRY, or TAILQ_ENTRY, named FIELDNAME. The argument HEADNAME is the name tag of a user defined structure that must be declared using the macros SLIST_HEAD(), LIST_HEAD(), SIMPLEQ_HEAD(), or TAILQ_HEAD(). See the examples below for further explanation of how these macros are used.

SINGLY-LINKED LISTS
A singly-linked list is headed by a structure defined by the 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. A SLIST_HEAD structure is declared as follows:

    SLIST_HEAD(HEADNAME, TYPE) head;

where HEADNAME is the name of the structure to be defined, and 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:

    struct HEADNAME *headp;

(The names head and headp are user selectable.)

The HEADNAME facility is often not used, leading to the following bizarre code:

    SLIST_HEAD(, TYPE) head, *headp;

The SLIST_ENTRY() macro declares a structure that connects the elements in the list.

The SLIST_INIT() macro initializes the list referenced by head.

The list can also be initialized statically by using the SLIST_HEAD_INITIALIZER() macro like this:

    SLIST_HEAD(HEADNAME, TYPE) head = SLIST_HEAD_INITIALIZER(head);

The SLIST_INSERT_HEAD() macro inserts the new element elm at the head of the list.

The SLIST_INSERT_AFTER() macro inserts the new element elm after the element listelm.

The SLIST_REMOVE_HEAD() macro removes the first element of the list pointed by head.

The SLIST_REMOVE_AFTER() macro removes the list element immediately following elm.

The SLIST_REMOVE() macro removes the element elm of the list pointed by head.

The SLIST_FIRST() and SLIST_NEXT() macros can be used to traverse the list:

    for (np = SLIST_FIRST(&head); np != NULL; np = SLIST_NEXT(np, FIELDNAME))

Or, for simplicity, one can use the SLIST_FOREACH() macro:

    SLIST_FOREACH(np, head, FIELDNAME)

The macro SLIST_FOREACH_SAFE() traverses the list referenced by head in a forward direction, assigning each element in turn to var. However, unlike SLIST_FOREACH() it is permitted to remove var as well as free it from within the loop safely without interfering with the traversal.

The SLIST_EMPTY() macro should be used to check whether a simple list is empty.

SINGLY-LINKED LIST EXAMPLE

    SLIST_HEAD(listhead, entry) head;
    struct entry {
    	...
    	SLIST_ENTRY(entry) entries;	/* Simple list. */
    	...
    } *n1, *n2, *np;
    SLIST_INIT(&head);			/* Initialize simple list. */
    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_FOREACH(np, &head, entries)	/* Forward traversal. */
    	np-> ...
    while (!SLIST_EMPTY(&head)) {	 	/* Delete. */
    	n1 = SLIST_FIRST(&head);
    	SLIST_REMOVE_HEAD(&head, entries);
    	free(n1);
    }

LISTS
A list is headed by a structure defined by the LIST_HEAD() macro. This structure contains a single pointer to the first element on the list. The elements are doubly linked so that an arbitrary element can be removed without traversing the list. New elements can be added to the list after an existing element, before an existing element, or at the head of the list. A LIST_HEAD structure is declared as follows:

    LIST_HEAD(HEADNAME, TYPE) head;

where HEADNAME is the name of the structure to be defined, and 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:

    struct HEADNAME *headp;

(The names head and headp are user selectable.)

The HEADNAME facility is often not used, leading to the following bizarre code:

    LIST_HEAD(, TYPE) head, *headp;

The LIST_ENTRY() macro declares a structure that connects the elements in the list.

The LIST_INIT() macro initializes the list referenced by head.

The list can also be initialized statically by using the LIST_HEAD_INITIALIZER() macro like this:

    LIST_HEAD(HEADNAME, TYPE) head = LIST_HEAD_INITIALIZER(head);

The LIST_INSERT_HEAD() macro inserts the new element elm at the head of the list.

The LIST_INSERT_AFTER() macro inserts the new element elm after the element listelm.

The LIST_INSERT_BEFORE() macro inserts the new element elm before the element listelm.

The LIST_REMOVE() macro removes the element elm from the list.

The LIST_REPLACE() macro replaces the list element elm with the new element elm2.

The LIST_FIRST() and LIST_NEXT() macros can be used to traverse the list:

    for (np = LIST_FIRST(&head); np != NULL; np = LIST_NEXT(np, FIELDNAME))

Or, for simplicity, one can use the LIST_FOREACH() macro:

    LIST_FOREACH(np, head, FIELDNAME)

The macro LIST_FOREACH_SAFE() traverses the list referenced by head in a forward direction, assigning each element in turn to var. However, unlike LIST_FOREACH() it is permitted to remove var as well as free it from within the loop safely without interfering with the traversal.

The LIST_EMPTY() macro should be used to check whether a list is empty.

LIST EXAMPLE

    LIST_HEAD(listhead, entry) head;
    struct entry {
    	...
    	LIST_ENTRY(entry) entries;	/* List. */
    	...
    } *n1, *n2, *np;
    LIST_INIT(&head);			/* Initialize list. */
    n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
    LIST_INSERT_HEAD(&head, n1, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert after. */
    LIST_INSERT_AFTER(n1, n2, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert before. */
    LIST_INSERT_BEFORE(n1, n2, entries);
    					/* Forward traversal. */
    LIST_FOREACH(np, &head, entries)
    	np-> ...
    while (!LIST_EMPTY(&head))		/* Delete. */
    	n1 = LIST_FIRST(&head);
    	LIST_REMOVE(n1, entries);
    	free(n1);
    }

SIMPLE QUEUES
A simple queue is headed by a structure defined by the SIMPLEQ_HEAD() macro. This structure contains a pair of pointers, one to the first element in the simple queue and the other to the last element in the simple queue. The elements are singly linked. New elements can be added to the queue after an existing element, at the head of the queue or at the tail of the queue. A SIMPLEQ_HEAD structure is declared as follows:

    SIMPLEQ_HEAD(HEADNAME, TYPE) head;

where HEADNAME is the name of the structure to be defined, and 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:

    struct HEADNAME *headp;

(The names head and headp are user selectable.)

The SIMPLEQ_ENTRY() macro declares a structure that connects the elements in the queue.

The SIMPLEQ_INIT() macro initializes the queue referenced by head.

The queue can also be initialized statically by using the SIMPLEQ_HEAD_INITIALIZER() macro like this:

    SIMPLEQ_HEAD(HEADNAME, TYPE) head = SIMPLEQ_HEAD_INITIALIZER(head);

The SIMPLEQ_INSERT_AFTER() macro inserts the new element elm after the element listelm.

The SIMPLEQ_INSERT_HEAD() macro inserts the new element elm at the head of the queue.

The SIMPLEQ_INSERT_TAIL() macro inserts the new element elm at the end of the queue.

The SIMPLEQ_REMOVE_AFTER() macro removes the queue element immediately following elm.

The SIMPLEQ_REMOVE_HEAD() macro removes the first element from the queue.

The SIMPLEQ_FIRST() and SIMPLEQ_NEXT() macros can be used to traverse the queue. The SIMPLEQ_FOREACH() is used for queue traversal:

    SIMPLEQ_FOREACH(np, head, FIELDNAME)

The macro SIMPLEQ_FOREACH_SAFE() traverses the queue referenced by head in a forward direction, assigning each element in turn to var. However, unlike SIMPLEQ_FOREACH() it is permitted to remove var as well as free it from within the loop safely without interfering with the traversal.

The SIMPLEQ_EMPTY() macro should be used to check whether a list is empty.

SIMPLE QUEUE EXAMPLE

    SIMPLEQ_HEAD(listhead, entry) head = SIMPLEQ_HEAD_INITIALIZER(head);
    struct entry {
    	...
    	SIMPLEQ_ENTRY(entry) entries;	/* Simple queue. */
    	...
    } *n1, *n2, *np;
    n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
    SIMPLEQ_INSERT_HEAD(&head, n1, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert after. */
    SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert at the tail. */
    SIMPLEQ_INSERT_TAIL(&head, n2, entries);
    					/* Forward traversal. */
    SIMPLEQ_FOREACH(np, &head, entries)
    	np-> ...
    					/* Delete. */
    while (!SIMPLEQ_EMPTY(&head)) {
    	n1 = SIMPLEQ_FIRST(&head);
    	SIMPLEQ_REMOVE_HEAD(&head, entries);
    	free(n1);
    }

TAIL QUEUES
A tail queue is headed by a structure defined by the 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 queue after an existing element, before an existing element, at the head of the queue, or at the end of the queue. A TAILQ_HEAD structure is declared as follows:

    TAILQ_HEAD(HEADNAME, TYPE) head;

where HEADNAME is the name of the structure to be defined, and 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:

    struct HEADNAME *headp;

(The names head and headp are user selectable.)

The TAILQ_ENTRY() macro declares a structure that connects the elements in the tail queue.

The TAILQ_INIT() macro initializes the tail queue referenced by head.

The tail queue can also be initialized statically by using the TAILQ_HEAD_INITIALIZER() macro.

The TAILQ_INSERT_HEAD() macro inserts the new element elm at the head of the tail queue.

The TAILQ_INSERT_TAIL() macro inserts the new element elm at the end of the tail queue.

The TAILQ_INSERT_AFTER() macro inserts the new element elm after the element listelm.

The TAILQ_INSERT_BEFORE() macro inserts the new element elm before the element listelm.

The TAILQ_REMOVE() macro removes the element elm from the tail queue.

The TAILQ_REPLACE() macro replaces the list element elm with the new element elm2.

TAILQ_FOREACH() and TAILQ_FOREACH_REVERSE() are used for traversing a tail queue. TAILQ_FOREACH() starts at the first element and proceeds towards the last. TAILQ_FOREACH_REVERSE() starts at the last element and proceeds towards the first.

    TAILQ_FOREACH(np, &head, FIELDNAME)
    TAILQ_FOREACH_REVERSE(np, &head, HEADNAME, FIELDNAME)

The macros TAILQ_FOREACH_SAFE() and TAILQ_FOREACH_REVERSE_SAFE() traverse the list referenced by head in a forward or reverse direction respectively, assigning each element in turn to var. However, unlike their unsafe counterparts, they permit both the removal of var as well as freeing it from within the loop safely without interfering with the traversal.

The TAILQ_FIRST(), TAILQ_NEXT(), TAILQ_LAST() and TAILQ_PREV() macros can be used to manually traverse a tail queue or an arbitrary part of one.

The TAILQ_EMPTY() macro should be used to check whether a tail queue is empty.

TAIL QUEUE EXAMPLE

    TAILQ_HEAD(tailhead, entry) head;
    struct entry {
    	...
    	TAILQ_ENTRY(entry) entries;	/* Tail queue. */
    	...
    } *n1, *n2, *np;
    TAILQ_INIT(&head);			/* Initialize 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);
    n2 = malloc(sizeof(struct entry));	/* Insert before. */
    TAILQ_INSERT_BEFORE(n1, n2, entries);
    					/* Forward traversal. */
    TAILQ_FOREACH(np, &head, entries)
    	np-> ...
    					/* Manual forward traversal. */
    for (np = n2; np != NULL; np = TAILQ_NEXT(np, entries))
    	np-> ...
    					/* Delete. */
    while ((np = TAILQ_FIRST(&head))) {
    	TAILQ_REMOVE(&head, np, entries);
    	free(np);
    }

NOTES
It is an error to assume the next and previous fields are preserved after an element has been removed from a list or queue. Using any macro (except the various forms of insertion) on an element removed from a list or queue is incorrect. An example of erroneous usage is removing the same element twice.

The SLIST_END(), LIST_END(), SIMPLEQ_END() and TAILQ_END() macros are deprecated; they provided symmetry with the historical CIRCLEQ_END() and just expand to NULL.

Trying to free a list in the following way is a common error:

    LIST_FOREACH(var, head, entry)
    	free(var);
    free(head);

Since var is free'd, the FOREACH macros refer to a pointer that may have been reallocated already. A similar situation occurs when the current element is deleted from the list. In cases like these the data structure's FOREACH_SAFE macros should be used instead.

HISTORY
The queue functions first appeared in 4.4BSD. The historical circle queue macros were deprecated in OpenBSD 5.5.