QUEUE(3)

NAME
SLIST_ENTRY, SLIST_HEAD, SLIST_INIT, SLIST_INSERT_AFTER, SLIST_INSERT_HEAD, SLIST_REMOVE_HEAD, SLIST_REMOVE, STAILQ_ENTRY, STAILQ_HEAD, STAILQ_INIT, STAILQ_INSERT_AFTER, STAILQ_INSERT_HEAD, STAILQ_INSERT_TAIL, STAILQ_REMOVE_HEAD, STAILQ_REMOVE, LIST_ENTRY, LIST_HEAD, LIST_INIT, LIST_INSERT_AFTER, LIST_INSERT_BEFORE, LIST_INSERT_HEAD, LIST_REMOVE, TAILQ_ENTRY, TAILQ_HEAD, TAILQ_INIT, TAILQ_INSERT_AFTER, TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD, TAILQ_INSERT_TAIL, TAILQ_REMOVE, CIRCLEQ_ENTRY, CIRCLEQ_HEAD, CIRCLEQ_INIT, CIRCLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE, CIRCLEQ_INSERT_HEAD, CIRCLEQ_INSERT_TAIL, CIRCLEQ_REMOVE - implementations of singly-linked lists, singly-linked tail queues, lists, tail queues, and circular queues

SYNOPSIS
#include <sys/queue.h>

SLIST_ENTRY (TYPE);
SLIST_HEAD (HEADNAME, TYPE);
SLIST_INIT (SLIST_HEAD *head);
SLIST_INSERT_AFTER (TYPE *listelm, TYPE *elm, SLIST_ENTRY NAME);
SLIST_INSERT_HEAD (SLIST_HEAD *head, TYPE *elm, SLIST_ENTRY NAME);
SLIST_REMOVE_HEAD (SLIST_HEAD *head, SLIST_ENTRY NAME);
SLIST_REMOVE (SLIST_HEAD *head, TYPE *elm, TYPE, SLIST_ENTRY NAME);

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

LIST_ENTRY (TYPE);
LIST_HEAD (HEADNAME, TYPE);
LIST_INIT (LIST_HEAD *head);
LIST_INSERT_AFTER (TYPE *listelm, TYPE *elm, LIST_ENTRY NAME);
LIST_INSERT_BEFORE (TYPE *listelm, TYPE *elm, LIST_ENTRY NAME);
LIST_INSERT_HEAD (LIST_HEAD *head, TYPE *elm, LIST_ENTRY NAME);
LIST_REMOVE (TYPE *elm, LIST_ENTRY NAME);

TAILQ_ENTRY (TYPE);
TAILQ_HEAD (HEADNAME, TYPE);
TAILQ_INIT (TAILQ_HEAD *head);
TAILQ_INSERT_AFTER (TAILQ_HEAD *head, TYPE *listelm, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_INSERT_BEFORE (TYPE *listelm, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_INSERT_HEAD (TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_INSERT_TAIL (TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_REMOVE (TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);

CIRCLEQ_ENTRY (TYPE);
CIRCLEQ_HEAD (HEADNAME, TYPE);
CIRCLEQ_INIT (CIRCLEQ_HEAD *head);
CIRCLEQ_INSERT_AFTER (CIRCLEQ_HEAD *head, TYPE *listelm, TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_INSERT_BEFORE (CIRCLEQ_HEAD *head, TYPE *listelm, TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_INSERT_HEAD (CIRCLEQ_HEAD *head, TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_INSERT_TAIL (CIRCLEQ_HEAD *head, TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_REMOVE (CIRCLEQ_HEAD *head, TYPE *elm, CIRCLEQ_ENTRY NAME);

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

    Insertion of a new entry at the head of the list.
    Insertion of a new entry after any element in the list.
    O(1) removal of an entry from the head of the list.
    O(n) removal of any entry in the list.
    Forward traversal through the list.

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.

Singly-linked tail queues add the following functionality:

    Entries can be added at the end of a list.

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

Singly-linked tailqs 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, tail queues, and circle queues) additionally allow:

    Insertion of a new entry before any element in the list.
    O(1) removal of any entry in the list.
However:
    Each elements requires two pointers rather than one.
    Code size and execution time of operations (except for removal) is about twice that of the singly-linked data-structures.

Linked 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:

    Entries can be added at the end of a list.
However:
    All list insertions and removals must specify the head of the list.
    Each head entry requires two pointers rather than one.
    Code size is about 15% greater and operations run about 20% slower than singly-linked lists.

Circular queues add the following functionality:

    Entries can be added at the end of a list.
    They may be traversed backwards, from tail to head.
However:
    All list insertions and removals must specify the head of the list.
    Each head entry requires two pointers rather than one.
    The termination condition for traversal is more complex.
    Code size is about 40% greater and operations run about 45% slower than lists.

In the macro definitions, TYPE is the name of a user defined structure, that must contain a field of type SLIST_ENTRY, STAILQ_ENTRY, LIST_ENTRY, TAILQ_ENTRY, or CIRCLEQ_ENTRY, named NAME The argument HEADNAME is the name of a user defined structure that must be declared using the macros SLIST_HEAD, STAILQ_HEAD, LIST_HEAD, TAILQ_HEAD, or CIRCLEQ_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. An SLIST_HEAD structure is declared as follows:

    SLIST_HEAD(HEADNAME, TYPE) head;

where HEADNAME is the name of the structure to be defined, and 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 macro SLIST_ENTRY declares a structure that connects the elements in the list.

The macro SLIST_INIT initializes the list referenced by head.

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

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

The macro SLIST_REMOVE_HEAD removes the element 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 SLIST_REMOVE macro.

The macro SLIST_REMOVE removes the element elm from the list.

SINGLY-LINKED LIST EXAMPLE
SLIST_HEAD(slisthead, entry) head;
struct slisthead *headp;		/* Singly-linked List head. */
struct entry {
	...
	SLIST_ENTRY(entry) entries;	/* Singly-linked List. */
	...
} *n1, *n2, *n3, *np;
SLIST_INIT(&head);			/* Initialize the 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_REMOVE(&head, n2, entry, entries);/* Deletion. */
free(n2);
n3 = head.slh_first;
SLIST_REMOVE_HEAD(&head, entries);	/* Deletion. */
free(n3);
					/* Forward traversal. */
for (np = head.slh_first; np != NULL; np = np->entries.sle_next)
	np-> ...
while (head.slh_first != NULL) {	/* List Deletion. */
	n1 = head.slh_first;
	SLIST_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 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 macro STAILQ_ENTRY declares a structure that connects the elements in the tail queue.

The macro STAILQ_INIT initializes the tail queue referenced by head.

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

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

The macro STAILQ_INSERT_AFTER inserts the new element elm after the element listelm.

The macro STAILQ_REMOVE_HEAD removes the element elm from 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 STAILQ_REMOVE macro.

The macro STAILQ_REMOVE removes the element elm from the tail queue.

SINGLY-LINKED TAIL QUEUE EXAMPLE
STAILQ_HEAD(stailhead, entry) head;
struct stailhead *headp;		/* Singly-linked tail queue head. */
struct entry {
	...
	STAILQ_ENTRY(entry) entries;	/* Tail queue. */
	...
} *n1, *n2, *n3, *np;
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);
					/* Deletion. */
STAILQ_REMOVE(&head, n2, entry, entries);
free(n2);
					/* Deletion from the head */
n3 = head.stqh_first;
STAILQ_REMOVE_HEAD(&head, entries);
free(n3);
					/* Forward traversal. */
for (np = head.stqh_first; np != NULL; np = np->entries.stqe_next)
	np-> ...
					/* TailQ Deletion. */
while (head.stqh_first != NULL) {
	n1 = head.stqh_first;
	TAILQ_REMOVE_HEAD(&head, entries);
	free(n1);
}
					/* Faster TailQ Deletion. */
n1 = head.stqh_first;
while (n1 != NULL) {
	n2 = n1->entries.stqe_next;
	free(n1);
	n1 = n2;
}
STAILQ_INIT(&head);

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

The macro LIST_INIT initializes the list referenced by head.

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

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

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

The macro LIST_REMOVE removes the element elm from the list.

LIST EXAMPLE
LIST_HEAD(listhead, entry) head;
struct listhead *headp;		/* List head. */
struct entry {
	...
	LIST_ENTRY(entry) entries;	/* List. */
	...
} *n1, *n2, *n3, *np;
LIST_INIT(&head);			/* Initialize the 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);
n3 = malloc(sizeof(struct entry));	/* Insert before. */
LIST_INSERT_BEFORE(n2, n3, entries);
LIST_REMOVE(n2, entries);		/* Deletion. */
free(n2);
					/* Forward traversal. */
for (np = head.lh_first; np != NULL; np = np->entries.le_next)
	np-> ...
while (head.lh_first != NULL) {		/* List Deletion. */
	n1 = head.lh_first;
	LIST_REMOVE(n1, entries);
	free(n1);
}
n1 = head.lh_first;			/* Faster List Delete. */
while (n1 != NULL) {
	n2 = n1->entires.le_next;
	free(n1);
	n1 = n2;
}
LIST_INIT(&head);

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 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 TAILQ_HEAD structure is declared as follows:

    TAILQ_HEAD(HEADNAME, TYPE) head;

where HEADNAME is the name of the structure to be defined, and 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 macro TAILQ_ENTRY declares a structure that connects the elements in the tail queue.

The macro TAILQ_INIT initializes the tail queue referenced by head.

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

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

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

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

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

TAIL QUEUE EXAMPLE
TAILQ_HEAD(tailhead, entry) head;
struct tailhead *headp;		/* Tail queue head. */
struct entry {
	...
	TAILQ_ENTRY(entry) entries;	/* Tail queue. */
	...
} *n1, *n2, *n3, *np;
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. */
for (np = head.tqh_first; np != NULL; np = np->entries.tqe_next)
	np-> ...
					/* TailQ Deletion. */
while (head.tqh_first != NULL) {
	n1 = head.tqh_first;
	TAILQ_REMOVE(&head, head.tqh_first, entries);
	free(n1);
}
					/* Faster TailQ Deletion. */
n1 = head.tqh_first;
while (n1 != NULL) {
	n2 = n1->entries.tqe_next;
	free(n1);
	n1 = n2;
}
TAILQ_INIT(&head);

CIRCULAR QUEUES
A circular queue is headed by a structure defined by the CIRCLEQ_HEAD macro. This structure contains a pair of pointers, one to the first element in the circular queue and the other to the last element in the circular queue. The elements are doubly linked so that an arbitrary element can be removed without traversing the 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 CIRCLEQ_HEAD structure is declared as follows:

    CIRCLEQ_HEAD(HEADNAME, TYPE) head;

where HEADNAME is the name of the structure to be defined, and TYPE is the type of the elements to be linked into the circular queue. A pointer to the head of the circular queue can later be declared as:

    struct HEADNAME *headp;

(The names head and headp are user selectable.)

The macro CIRCLEQ_ENTRY declares a structure that connects the elements in the circular queue.

The macro CIRCLEQ_INIT initializes the circular queue referenced by head.

The macro CIRCLEQ_INSERT_HEAD inserts the new element elm at the head of the circular queue.

The macro CIRCLEQ_INSERT_TAIL inserts the new element elm at the end of the circular queue.

The macro CIRCLEQ_INSERT_AFTER inserts the new element elm after the element listelm.

The macro CIRCLEQ_INSERT_BEFORE inserts the new element elm before the element listelm.

The macro CIRCLEQ_REMOVE removes the element elm from the circular queue.

CIRCULAR QUEUE EXAMPLE
CIRCLEQ_HEAD(circleq, entry) head;
struct circleq *headp;			/* Circular queue head. */
struct entry {
	...
	CIRCLEQ_ENTRY entries;		/* Circular queue. */
	...
} *n1, *n2, *np;
CIRCLEQ_INIT(&head);			/* Initialize the circular queue. */
n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
CIRCLEQ_INSERT_HEAD(&head, n1, entries);
n1 = malloc(sizeof(struct entry));	/* Insert at the tail. */
CIRCLEQ_INSERT_TAIL(&head, n1, entries);
n2 = malloc(sizeof(struct entry));	/* Insert after. */
CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);
n2 = malloc(sizeof(struct entry));	/* Insert before. */
CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
CIRCLEQ_REMOVE(&head, n1, entries);	/* Deletion. */
free(n1);
					/* Forward traversal. */
for (np = head.cqh_first; np != (void *)&head; np = np->entries.cqe_next)
	np-> ...
					/* Reverse traversal. */
for (np = head.cqh_last; np != (void *)&head; np = np->entries.cqe_prev)
	np-> ...
					/* CircleQ Deletion. */
while (head.cqh_first != (void *)&head) {
	n1 = head.cqh_first;
	CIRCLEQ_REMOVE(&head, head.cqh_first, entries);
	free(n1);
}
					/* Faster CircleQ Deletion. */
n1 = head.cqh_first;
while (n1 != (void *)&head) {
	n2 = n1->entries.cqh_next;
	free(n1);
	n1 = n2;
}
CIRCLEQ_INIT(&head);

HISTORY
The queue functions first appeared in 4.4BSD.