SLIST_INIT(3) | Library Functions Manual | SLIST_INIT(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`, `SIMPLEQ_CONCAT`, `STAILQ_ENTRY`, `STAILQ_HEAD`, `STAILQ_HEAD_INITIALIZER`, `STAILQ_FIRST`, `STAILQ_NEXT`, `STAILQ_LAST`, `STAILQ_EMPTY`, `STAILQ_FOREACH`, `STAILQ_FOREACH_SAFE`, `STAILQ_INIT`, `STAILQ_INSERT_AFTER`, `STAILQ_INSERT_HEAD`, `STAILQ_INSERT_TAIL`, `STAILQ_REMOVE`, `STAILQ_REMOVE_AFTER`, `STAILQ_REMOVE_HEAD`, `STAILQ_CONCAT`, `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`, `TAILQ_CONCAT` — intrusive singly-linked and doubly-linked lists, simple queues, singly-linked and doubly-linked 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);

`SIMPLEQ_CONCAT`(SIMPLEQ_HEAD *head1, SIMPLEQ_HEAD *head2);

`STAILQ_ENTRY`(TYPE);

`STAILQ_HEAD`(HEADNAME, TYPE);

`STAILQ_HEAD_INITIALIZER`(STAILQ_HEAD head);

`STAILQ_FIRST`(STAILQ_HEAD *head);

`STAILQ_NEXT`(TYPE *elm, STAILQ_ENTRY NAME);

`STAILQ_LAST`(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);

`STAILQ_EMPTY`(STAILQ_HEAD *head);

`STAILQ_FOREACH`(TYPE *var, STAILQ_HEAD *head, STAILQ_ENTRY NAME);

`STAILQ_FOREACH_SAFE`(TYPE *var, STAILQ_HEAD *head, STAILQ_ENTRY NAME, TYPE *temp_var);

`STAILQ_INIT`(STAILQ_HEAD *head);

`STAILQ_INSERT_AFTER`(STAILQ_HEAD *head, TYPE *listelm, TYPE *elm, STAILQ_ENTRY NAME);

`STAILQ_INSERT_HEAD`(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);

`STAILQ_INSERT_TAIL`(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);

`STAILQ_REMOVE`(STAILQ_HEAD *head, TYPE *elm, TYPE, STAILQ_ENTRY NAME);

`STAILQ_REMOVE_AFTER`(STAILQ_HEAD *head, TYPE *elm, STAILQ_ENTRY NAME);

`STAILQ_REMOVE_HEAD`(STAILQ_HEAD *head, STAILQ_ENTRY NAME);

`STAILQ_CONCAT`(STAILQ_HEAD *head1, STAILQ_HEAD *head2);

`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);

`TAILQ_CONCAT`(TAILQ_HEAD *head1, TAILQ_HEAD *head2, FIELDNAME);

# DESCRIPTION

These macros define and operate on five types of data structures: singly-linked lists, simple queues, lists, singly-linked tail queues, and tail queues. All five 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.

The following table provides a quick overview of which types support which additional macros:

LAST, PREV, FOREACH_REVERSE | - | - | - | - | TAILQ
INSERT_BEFORE, REPLACE | - | LIST | - | - | TAILQ
INSERT_TAIL, CONCAT | - | - | SIMPLEQ | STAILQ | TAILQ
REMOVE_AFTER, REMOVE_HEAD | SLIST | - | SIMPLEQ | STAILQ | -
REMOVE | SLIST | LIST | - | STAILQ | TAILQ

Singly-linked lists are the simplest of the five 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 and singly-linked tail 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 and singly-linked tail 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.

All these lists and queues are intrusive: they link together user defined structures containing a field of type `SLIST_ENTRY`, `LIST_ENTRY`, `SIMPLEQ_ENTRY`, `STAILQ_ENTRY`, or `TAILQ_ENTRY`. In the macro definitions, TYPE is the name tag of the user defined structure and FIELDNAME is the name of the `*_ENTRY` field. If an instance of the user defined structure needs to be a member of multiple lists at the same time, the structure requires multiple `*_ENTRY` fields, one for each list.

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`(), `STAILQ_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_CONCAT`() macro concatenates all the elements of the queue referenced by head2 to the end of the queue referenced by head1, emptying head2 in the process. This is more efficient than removing and inserting the individual elements as it does not actually traverse head2.

The `SIMPLEQ_FIRST`() and `SIMPLEQ_NEXT`() macros can be used to traverse the queue. The `SIMPLEQ_FOREACH`() macro 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);
    }

# SINGLY-LINKED TAIL QUEUES

A singly-linked tail queue is headed by a structure defined by the `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 STAILQ_HEAD structure is declared as follows:

    STAILQ_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 `STAILQ_ENTRY`() macro declares a structure that connects the elements in the tail queue.

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

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

    STAILQ_HEAD(HEADNAME, TYPE) head = STAILQ_HEAD_INITIALIZER(head);

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

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

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

The `STAILQ_REMOVE_AFTER`() macro removes the queue element immediately following elm. Unlike STAILQ_REMOVE, this macro does not traverse the entire tail queue.

The `STAILQ_REMOVE_HEAD`() macro removes the first element from 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 STAILQ_REMOVE macro.

The `STAILQ_REMOVE`() macro removes the element elm from the tail queue. Use of this macro should be avoided as it traverses the entire list. A doubly-linked tail queue should be used if this macro is needed in high-usage code paths or to operate on long tail queues.

The `STAILQ_CONCAT`() macro concatenates all the elements of the tail queue referenced by head2 to the end of the tail queue referenced by head1, emptying head2 in the process. This is more efficient than removing and inserting the individual elements as it does not actually traverse head2.

The `STAILQ_FOREACH`() macro is used for queue traversal:

    STAILQ_FOREACH(np, head, FIELDNAME)

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

The `STAILQ_FIRST`(), `STAILQ_NEXT`(), and `STAILQ_LAST`() macros can be used to manually traverse a tail queue or an arbitrary part of one. The `STAILQ_EMPTY`() macro should be used to check whether a tail queue is empty.

# SINGLY-LINKED TAIL QUEUE EXAMPLE

    STAILQ_HEAD(listhead, entry) head = STAILQ_HEAD_INITIALIZER(head);
    struct entry {
    	...
    	STAILQ_ENTRY(entry) entries;	/* Singly-linked tail queue. */
    	...
    } *n1, *n2, *np;

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

    n2 = malloc(sizeof(struct entry));	/* Insert at the tail. */
    STAILQ_INSERT_TAIL(&head, n2, entries);

    n2 = malloc(sizeof(struct entry));	/* Insert after. */
    STAILQ_INSERT_AFTER(&head, n1, n2, entries);

    					/* Deletion. */
    STAILQ_REMOVE(&head, n2, entry, entries);
    free(n2);
    					/* Deletion from the head. */
    n3 = STAILQ_FIRST(&head);
    STAILQ_REMOVE_HEAD(&head, entries);
    free(n3);
    					/* Forward traversal. */
    STAILQ_FOREACH(np, &head, entries)
    	np-> ...
    					/* Safe forward traversal. */
    STAILQ_FOREACH_SAFE(np, &head, entries, np_temp) {
    	np-> ...
    	STAILQ_REMOVE(&head, np, entry, entries);
    	free(np);
    }
    					/* Delete. */
    while (!STAILQ_EMPTY(&head)) {
    	n1 = STAILQ_FIRST(&head);
    	STAILQ_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.

The `TAILQ_CONCAT`() macro concatenates all the elements of the tail queue referenced by head2 to the end of the tail queue referenced by head1, emptying head2 in the process. This is more efficient than removing and inserting the individual elements as it does not actually traverse head2.

`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);
    }

# SEE ALSO

tree(3)

# 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`(), `STAILQ_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.

June 13, 2025 | Debian