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_HEAD_INITIALIZER evaluates to an initializer for the list head.

The macro SLIST_EMPTY evaluates to true if there are no elements in the list.

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

The macro SLIST_FIRST returns the first element in the list or NULL if the list is empty.

The macro SLIST_FOREACH traverses the list referenced by head in the forward direction, assigning each element in turn to var.

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_NEXT returns the next element in the list.

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 =
    SLIST_HEAD_INITIALIZER(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 = SLIST_FIRST(&head);
SLIST_REMOVE_HEAD(&head, entries);  /* Deletion from the head. */
free(n3);
                                        /* Forward traversal. */
SLIST_FOREACH(np, &head, entries)
        np−> ...

while (!SLIST_EMPTY(&head)) {               /* List Deletion. */
        n1 = SLIST_FIRST(&head);
        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_HEAD_INITIALIZER evaluates to an initializer for the tail queue head.

The macro STAILQ_CONCAT concatenates the tail queue headed by head2 onto the end of the one headed by head1 removing all entries from the former.

The macro STAILQ_EMPTY evaluates to true if there are no items on the tail queue.

The macro STAILQ_ENTRY declares a structure that connects the elements in the tail queue.

The macro STAILQ_FIRST returns the first item on the tail queue or NULL if the tail queue is empty.

The macro STAILQ_FOREACH traverses the tail queue referenced by head in the forward direction, assigning each element in turn to var.

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_NEXT returns the next item on the tail queue, or NULL this item is the last.

The macro 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 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 =
    STAILQ_HEAD_INITIALIZER(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 = STAILQ_FIRST(&head);
STAILQ_REMOVE_HEAD(&head, entries);
free(n3);
                                        /* Forward traversal. */
STAILQ_FOREACH(np, &head, entries)
        np−> ...
                                        /* TailQ Deletion. */
while (!STAILQ_EMPTY(&head)) {
        n1 = STAILQ_FIRST(&head);
        STAILQ_REMOVE_HEAD(&head, entries);
        free(n1);
}
                                        /* Faster TailQ Deletion. */
n1 = STAILQ_FIRST(&head);
while (n1 != NULL) {
        n2 = STAILQ_NEXT(n1, entries);
        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_HEAD_INITIALIZER evaluates to an initializer for the list head.

The macro LIST_EMPTY evaluates to true if there are no elements in the list.

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

The macro LIST_FIRST returns the first element in the list or NULL if the list is empty.

The macro LIST_FOREACH traverses the list referenced by head in the forward direction, assigning each element in turn to var.

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_NEXT returns the next element in the list, or NULL if this is the last.

The macro LIST_REMOVE removes the element elm from the list.

List example

LIST_HEAD(listhead, entry) head =
    LIST_HEAD_INITIALIZER(head);
struct listhead *headp;                 /* List head. */
struct entry {
        ...
        LIST_ENTRY(entry) entries;      /* List. */
        ...
} *n1, *n2, *n3, *np, *np_temp;

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. */
LIST_FOREACH(np, &head, entries)
        np−> ...

while (!LIST_EMPTY(&head)) {                /* List Deletion. */
        n1 = LIST_FIRST(&head);
        LIST_REMOVE(n1, entries);
        free(n1);
}

n1 = LIST_FIRST(&head);                     /* Faster List Deletion. */
while (n1 != NULL) {
        n2 = LIST_NEXT(n1, entries);
        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_HEAD_INITIALIZER evaluates to an initializer for the tail queue head.

The macro TAILQ_CONCAT concatenates the tail queue headed by head2 onto the end of the one headed by head1 removing all entries from the former.

The macro TAILQ_EMPTY evaluates to true if there are no items on the tail queue.

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

The macro TAILQ_FIRST returns the first item on the tail queue or NULL if the tail queue is empty.

The macro TAILQ_FOREACH traverses the tail queue referenced by head in the forward direction, assigning each element in turn to var. var is set to NULL if the loop completes normally, or if there were no elements.

The macro TAILQ_FOREACH_REVERSE traverses the tail queue referenced by head in the reverse direction, assigning each element in turn to var.

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_LAST returns the last item on the tail queue. If the tail queue is empty the return value is NULL.

The macro TAILQ_NEXT returns the next item on the tail queue, or NULL if this item is the last.

The macro TAILQ_PREV returns the previous item on the tail queue, or NULL if this item is the first.

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

The macro TAILQ_SWAP swaps the contents of head1 and head2.

Tail queue example

TAILQ_HEAD(tailhead, entry) head =
    TAILQ_HEAD_INITIALIZER(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. */
TAILQ_FOREACH(np, &head, entries)
        np−> ...
                                        /* Reverse traversal. */
TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries)
        np−> ...
                                        /* TailQ Deletion. */
while (!TAILQ_EMPTY(&head)) {
        n1 = TAILQ_FIRST(&head);
        TAILQ_REMOVE(&head, n1, entries);
        free(n1);
}
                                        /* Faster TailQ Deletion. */
n1 = TAILQ_FIRST(&head);
while (n1 != NULL) {
        n2 = TAILQ_NEXT(n1, entries);
        free(n1);
        n1 = n2;
}

TAILQ_INIT(&head);
n2 = malloc(sizeof(struct entry));  /* Insert before. */
CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
                                    /* 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−> ...
                                    /* Delete. */
while (head.cqh_first != (void *)&head)
    CIRCLEQ_REMOVE(&head, head.cqh_first, entries);