AG_QUEUE(3)

NAME
AG_Queue - Agar singly-linked lists, doubly-linked lists, simple queues, tail queues, and circular queues

SYNOPSIS
#define _USE_AGAR_QUEUE /* For versions without AG_ prefix */
#include <agar/core.h>

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

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 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, tail queues, and circle 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.

Circular queues add the following functionality:

1.  Entries can be added at the end of a list.
2.  They may be traversed backwards, from tail to head.

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.  The termination condition for traversal is more complex.
4.  Code size is about 40% greater and operations run about 45% slower than lists.

In the macro definitions, TYPE is the name tag of a user defined structure that must contain a field of type AG_SLIST_ENTRY, AG_LIST_ENTRY, AG_SIMPLEQ_ENTRY, AG_TAILQ_ENTRY, or AG_CIRCLEQ_ENTRY, named NAME. The argument HEADNAME is the name tag of a user defined structure that must be declared using the macros AG_SLIST_HEAD(), AG_LIST_HEAD(), AG_SIMPLEQ_HEAD(), AG_TAILQ_HEAD(), or AG_CIRCLEQ_HEAD(). See the examples below for further explanation of how these macros are used.

SINGLY-LINKED LISTS
AG_SLIST_ENTRY(TYPE)

AG_SLIST_HEAD(HEADNAME, TYPE)

AG_SLIST_HEAD_(TYPE)

AG_SLIST_HEAD_INITIALIZER(AG_SLIST_HEAD head)

struct TYPE * AG_SLIST_FIRST(AG_SLIST_HEAD *head)

struct TYPE * AG_SLIST_NEXT(struct TYPE *listelm, AG_SLIST_ENTRY NAME)

struct TYPE * AG_SLIST_END(AG_SLIST_HEAD *head)

bool AG_SLIST_EMPTY(AG_SLIST_HEAD *head)

AG_SLIST_FOREACH(VARNAME, AG_SLIST_HEAD *head, AG_SLIST_ENTRY NAME)

AG_SLIST_FOREACH_PREVPTR(VARNAME, VARNAMEP, AG_SLIST_HEAD *head, AG_SLIST_ENTRY NAME)

void AG_SLIST_INIT(AG_SLIST_HEAD *head)

void AG_SLIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm, AG_SLIST_ENTRY NAME)

void AG_SLIST_INSERT_HEAD(AG_SLIST_HEAD *head, struct TYPE *elm, AG_SLIST_ENTRY NAME)

void AG_SLIST_REMOVE_HEAD(AG_SLIST_HEAD *head, AG_SLIST_ENTRY NAME)

void AG_SLIST_REMOVE_NEXT(AG_SLIST_HEAD *head, struct TYPE *elm, AG_SLIST_ENTRY NAME)

void AG_SLIST_REMOVE(AG_SLIST_HEAD *head, struct TYPE *elm, TYPE, AG_SLIST_ENTRY NAME)

A singly-linked list is headed by a structure defined by the AG_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 AG_SLIST_HEAD structure is declared as follows:

    AG_SLIST_HEAD(HEADNAME, TYPE) head;
    AG_SLIST_HEAD_(TYPE) head;	/* If HEADNAME is not needed */

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

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

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

    AG_SLIST_HEAD(HEADNAME, TYPE) head = AG_SLIST_HEAD_INITIALIZER(head);

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

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

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

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

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

The AG_SLIST_FIRST() and AG_SLIST_NEXT() macros can be used to traverse the list:

    for (np = AG_SLIST_FIRST(&head);
         np != NULL;
         np = AG_SLIST_NEXT(np, NAME))

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

    AG_SLIST_FOREACH(np, head, NAME) { /* ... */ }

The AG_SLIST_FOREACH_PREVPTR() macro is similar to AG_SLIST_FOREACH() except that it stores a pointer to the previous element in VARNAMEP. This provides access to the previous element while traversing the list, as one would have with a doubly-linked list.

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

SINGLY-LINKED LIST EXAMPLE

    AG_SLIST_HEAD(listhead, entry) head;
    struct entry {
    	/* ... */
    	AG_SLIST_ENTRY(entry) entries;	/* Simple list. */
    	/* ... */
    } *n1, *n2, *np;
    AG_SLIST_INIT(&head);			/* Initialize simple list. */
    n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
    AG_SLIST_INSERT_HEAD(&head, n1, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert after. */
    AG_SLIST_INSERT_AFTER(n1, n2, entries);
    AG_SLIST_FOREACH(np, &head, entries) {	/* Forward traversal. */
    	/* np-> ... */
    }
    while (!AG_SLIST_EMPTY(&head))		/* Delete. */
    	AG_SLIST_REMOVE_HEAD(&head, entries);

LISTS
AG_LIST_ENTRY(TYPE)

AG_LIST_HEAD(HEADNAME, TYPE)

AG_LIST_HEAD_(TYPE)

AG_LIST_HEAD_INITIALIZER(AG_LIST_HEAD head)

struct TYPE * AG_LIST_FIRST(AG_LIST_HEAD *head)

struct TYPE * AG_LIST_NEXT(struct TYPE *listelm, AG_LIST_ENTRY NAME)

struct TYPE * AG_LIST_END(AG_LIST_HEAD *head)

bool AG_LIST_EMPTY(AG_LIST_HEAD *head)

AG_LIST_FOREACH(VARNAME, AG_LIST_HEAD *head, AG_LIST_ENTRY NAME)

void AG_LIST_INIT(AG_LIST_HEAD *head)

void AG_LIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm, AG_LIST_ENTRY NAME)

void AG_LIST_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm, AG_LIST_ENTRY NAME)

void AG_LIST_INSERT_HEAD(AG_LIST_HEAD *head, struct TYPE *elm, AG_LIST_ENTRY NAME)

void AG_LIST_REMOVE(struct TYPE *elm, AG_LIST_ENTRY NAME)

void AG_LIST_REPLACE(struct TYPE *elm, struct TYPE *elm2, AG_LIST_ENTRY NAME)

A list is headed by a structure defined by the AG_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 AG_LIST_HEAD structure is declared as follows:

    AG_LIST_HEAD(HEADNAME, TYPE) head;
    AG_LIST_HEAD_(TYPE) head;	/* If HEADNAME is not needed */

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

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

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

    AG_LIST_HEAD(HEADNAME, TYPE) head = AG_LIST_HEAD_INITIALIZER(head);

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

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

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

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

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

The AG_LIST_FIRST() and AG_LIST_NEXT() macros can be used to traverse the list:

    for (np = AG_LIST_FIRST(&head);
         np != NULL;
         np = AG_LIST_NEXT(np, NAME))

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

    AG_LIST_FOREACH(np, head, NAME) { /* ... */ }

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

LIST EXAMPLE

    AG_LIST_HEAD(listhead, entry) head;
    struct entry {
    	/* ... */
    	AG_LIST_ENTRY(entry) entries;	/* List. */
    	/* ... */
    } *n1, *n2, *np;
    AG_LIST_INIT(&head);			/* Initialize list. */
    n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
    AG_LIST_INSERT_HEAD(&head, n1, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert after. */
    AG_LIST_INSERT_AFTER(n1, n2, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert before. */
    AG_LIST_INSERT_BEFORE(n1, n2, entries);
    					/* Forward traversal. */
    AG_LIST_FOREACH(np, &head, entries)
    	/* np-> ... */
    while (!AG_LIST_EMPTY(&head))		/* Delete. */
    	AG_LIST_REMOVE(AG_LIST_FIRST(&head), entries);

SIMPLE QUEUES
AG_SIMPLEQ_ENTRY(TYPE)

AG_SIMPLEQ_HEAD(HEADNAME, TYPE)

AG_SIMPLEQ_HEAD_(TYPE)

AG_SIMPLEQ_HEAD_INITIALIZER(AG_SIMPLEQ_HEAD head)

struct TYPE * AG_SIMPLEQ_FIRST(AG_SIMPLEQ_HEAD *head)

struct TYPE * AG_SIMPLEQ_NEXT(struct TYPE *listelm, AG_SIMPLEQ_ENTRY NAME)

struct TYPE * AG_SIMPLEQ_END(AG_SIMPLEQ_HEAD *head)

void AG_SIMPLEQ_INIT(AG_SIMPLEQ_HEAD *head)

void AG_SIMPLEQ_INSERT_HEAD(AG_SIMPLEQ_HEAD *head, struct TYPE *elm, AG_SIMPLEQ_ENTRY NAME)

void AG_SIMPLEQ_INSERT_TAIL(AG_SIMPLEQ_HEAD *head, struct TYPE *elm, AG_SIMPLEQ_ENTRY NAME)

void AG_SIMPLEQ_INSERT_AFTER(AG_SIMPLEQ_HEAD *head, struct TYPE *listelm, struct TYPE *elm, AG_SIMPLEQ_ENTRY NAME)

void AG_SIMPLEQ_REMOVE_HEAD(AG_SIMPLEQ_HEAD *head, AG_SIMPLEQ_ENTRY NAME)

A simple queue is headed by a structure defined by the AG_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 AG_SIMPLEQ_HEAD structure is declared as follows:

    AG_SIMPLEQ_HEAD(HEADNAME, TYPE) head;
    AG_SIMPLEQ_HEAD_(TYPE) head;	/* If HEADNAME is not needed */

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

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

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

    AG_SIMPLEQ_HEAD(HEADNAME, TYPE) head =
        AG_SIMPLEQ_HEAD_INITIALIZER(head);

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

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

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

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

The AG_SIMPLEQ_FIRST() and AG_SIMPLEQ_NEXT() macros can be used to traverse the queue. The AG_SIMPLEQ_FOREACH() is used for queue traversal:

    AG_SIMPLEQ_FOREACH(np, head, NAME) { /* ... */ }

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

SIMPLE QUEUE EXAMPLE

    AG_SIMPLEQ_HEAD(listhead, entry) head = AG_SIMPLEQ_HEAD_INITIALIZER(head);
    struct entry {
    	/* ... */
    	AG_SIMPLEQ_ENTRY(entry) entries;	/* Simple queue. */
    	/* ... */
    } *n1, *n2, *np;
    n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
    AG_SIMPLEQ_INSERT_HEAD(&head, n1, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert after. */
    AG_SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert at the tail. */
    AG_SIMPLEQ_INSERT_TAIL(&head, n2, entries);
    					/* Forward traversal. */
    AG_SIMPLEQ_FOREACH(np, &head, entries) {
    	/* np-> ... */
    }
    					/* Delete. */
    while (!AG_SIMPLEQ_EMPTY(&head))
    	AG_SIMPLEQ_REMOVE_HEAD(&head, entries);

TAIL QUEUES
AG_TAILQ_ENTRY(TYPE)

AG_TAILQ_HEAD(HEADNAME, TYPE)

AG_TAILQ_HEAD_(TYPE)

AG_TAILQ_HEAD_INITIALIZER(AG_TAILQ_HEAD head)

struct TYPE * AG_TAILQ_FIRST(AG_TAILQ_HEAD *head)

struct TYPE * AG_TAILQ_NEXT(struct TYPE *listelm, AG_TAILQ_ENTRY NAME)

struct TYPE * AG_TAILQ_END(AG_TAILQ_HEAD *head)

struct TYPE * AG_TAILQ_LAST(AG_TAILQ_HEAD *head, HEADNAME NAME)

AG_TAILQ_PREV(struct TYPE *listelm, HEADNAME NAME, AG_TAILQ_ENTRY NAME)

bool AG_TAILQ_EMPTY(AG_TAILQ_HEAD *head)

AG_TAILQ_FOREACH(VARNAME, AG_TAILQ_HEAD *head, AG_TAILQ_ENTRY NAME)

AG_TAILQ_FOREACH_REVERSE(VARNAME, AG_TAILQ_HEAD *head, HEADNAME, AG_TAILQ_ENTRY NAME)

void AG_TAILQ_INIT(AG_TAILQ_HEAD *head)

void AG_TAILQ_INSERT_AFTER(AG_TAILQ_HEAD *head, struct TYPE *listelm, struct TYPE *elm, AG_TAILQ_ENTRY NAME)

void AG_TAILQ_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm, AG_TAILQ_ENTRY NAME)

void AG_TAILQ_INSERT_HEAD(AG_TAILQ_HEAD *head, struct TYPE *elm, AG_TAILQ_ENTRY NAME)

void AG_TAILQ_INSERT_TAIL(AG_TAILQ_HEAD *head, struct TYPE *elm, AG_TAILQ_ENTRY NAME)

void AG_TAILQ_REMOVE(AG_TAILQ_HEAD *head, struct TYPE *elm, AG_TAILQ_ENTRY NAME)

A tail queue is headed by a structure defined by the AG_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 AG_TAILQ_HEAD structure is declared as follows:

    AG_TAILQ_HEAD(HEADNAME, TYPE) head;
    AG_TAILQ_HEAD_(TYPE) head;	/* If HEADNAME is not needed */

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

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

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

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

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

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

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

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

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

    AG_TAILQ_FOREACH(np, &head, NAME) { /* ... */ }
    AG_TAILQ_FOREACH_REVERSE(np, &head, HEADNAME, NAME) { /* ... */ }

The AG_TAILQ_FIRST(), AG_TAILQ_NEXT(), AG_TAILQ_LAST() and AG_TAILQ_PREV() macros can be used to manually traverse a tail queue or an arbitrary part of one.

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

TAIL QUEUE EXAMPLE

    AG_TAILQ_HEAD(tailhead, entry) head;
    struct entry {
    	/* ... */
    	AG_TAILQ_ENTRY(entry) entries;	/* Tail queue. */
    	/* ... */
    } *n1, *n2, *np;
    AG_TAILQ_INIT(&head);			/* Initialize queue. */
    n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
    AG_TAILQ_INSERT_HEAD(&head, n1, entries);
    n1 = malloc(sizeof(struct entry));	/* Insert at the tail. */
    AG_TAILQ_INSERT_TAIL(&head, n1, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert after. */
    AG_TAILQ_INSERT_AFTER(&head, n1, n2, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert before. */
    AG_TAILQ_INSERT_BEFORE(n1, n2, entries);
    					/* Forward traversal. */
    AG_TAILQ_FOREACH(np, &head, entries) {
    	/* np-> ... */
    }
    					/* Manual forward traversal. */
    for (np = n2; np != NULL; np = AG_TAILQ_NEXT(np, entries)) {
    	/* np-> ... */
    }
    					/* Delete. */
    while (np = AG_TAILQ_FIRST(&head))
    	AG_TAILQ_REMOVE(&head, np, entries);

CIRCULAR QUEUES
AG_CIRCLEQ_ENTRY(TYPE)

AG_CIRCLEQ_HEAD(HEADNAME, TYPE)

AG_CIRCLEQ_HEAD_(TYPE)

AG_CIRCLEQ_HEAD_INITIALIZER(AG_CIRCLEQ_HEAD head)

struct TYPE * AG_CIRCLEQ_FIRST(AG_CIRCLEQ_HEAD *head)

struct TYPE * AG_CIRCLEQ_LAST(AG_CIRCLEQ_HEAD *head)

struct TYPE * AG_CIRCLEQ_END(AG_CIRCLEQ_HEAD *head)

struct TYPE * AG_CIRCLEQ_NEXT(struct TYPE *listelm, AG_CIRCLEQ_ENTRY NAME)

struct TYPE * AG_CIRCLEQ_PREV(struct TYPE *listelm, AG_CIRCLEQ_ENTRY NAME)

bool AG_CIRCLEQ_EMPTY(AG_CIRCLEQ_HEAD *head)

AG_CIRCLEQ_FOREACH(VARNAME, AG_CIRCLEQ_HEAD *head, AG_CIRCLEQ_ENTRY NAME)

AG_CIRCLEQ_FOREACH_REVERSE(VARNAME, AG_CIRCLEQ_HEAD *head, AG_CIRCLEQ_ENTRY NAME)

void AG_CIRCLEQ_INIT(AG_CIRCLEQ_HEAD *head)

void AG_CIRCLEQ_INSERT_AFTER(AG_CIRCLEQ_HEAD *head, struct TYPE *listelm, struct TYPE *elm, AG_CIRCLEQ_ENTRY NAME)

void AG_CIRCLEQ_INSERT_BEFORE(AG_CIRCLEQ_HEAD *head, struct TYPE *listelm, struct TYPE *elm, AG_CIRCLEQ_ENTRY NAME)

void AG_CIRCLEQ_INSERT_HEAD(AG_CIRCLEQ_HEAD *head, struct TYPE *elm, AG_CIRCLEQ_ENTRY NAME)

void AG_CIRCLEQ_INSERT_TAIL(AG_CIRCLEQ_HEAD *head, struct TYPE *elm, AG_CIRCLEQ_ENTRY NAME)

void AG_CIRCLEQ_REMOVE(AG_CIRCLEQ_HEAD *head, struct TYPE *elm, AG_CIRCLEQ_ENTRY NAME)

A circular queue is headed by a structure defined by the AG_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 AG_CIRCLEQ_HEAD structure is declared as follows:

    AG_CIRCLEQ_HEAD(HEADNAME, TYPE) head;
    AG_CIRCLEQ_HEAD_(TYPE) head;	/* If HEADNAME is not needed */

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 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 AG_CIRCLEQ_ENTRY() macro declares a structure that connects the elements in the circular queue.

The AG_CIRCLEQ_INIT() macro initializes the circular queue referenced by head.

The circular queue can also be initialized statically by using the AG_CIRCLEQ_HEAD_INITIALIZER() macro.

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

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

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

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

The AG_CIRCLEQ_REMOVE() macro removes the element elm from the circular queue.

The AG_CIRCLEQ_FIRST(), AG_CIRCLEQ_LAST(), AG_CIRCLEQ_END(), AG_CIRCLEQ_NEXT() and AG_CIRCLEQ_PREV() macros can be used to traverse a circular queue. The AG_CIRCLEQ_FOREACH() is used for circular queue forward traversal:

    AG_CIRCLEQ_FOREACH(np, head, NAME) { /* ... */ }

The AG_CIRCLEQ_FOREACH_REVERSE() macro acts like AG_CIRCLEQ_FOREACH() but traverses the circular queue backwards.

The AG_CIRCLEQ_EMPTY() macro should be used to check whether a circular queue is empty.

CIRCULAR QUEUE EXAMPLE

    AG_CIRCLEQ_HEAD(circleq, entry) head;
    struct entry {
    	/* ... */
    	AG_CIRCLEQ_ENTRY(entry) entries;	/* Circular queue. */
    	/* ... */
    } *n1, *n2, *np;
    AG_CIRCLEQ_INIT(&head);			/* Initialize circular queue. */
    n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
    AG_CIRCLEQ_INSERT_HEAD(&head, n1, entries);
    n1 = malloc(sizeof(struct entry));	/* Insert at the tail. */
    AG_CIRCLEQ_INSERT_TAIL(&head, n1, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert after. */
    AG_CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);
    n2 = malloc(sizeof(struct entry));	/* Insert before. */
    AG_CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
    					/* Forward traversal. */
    AG_CIRCLEQ_FOREACH(np, &head, entries) {
    	/* np-> ... */
    }
    					/* Reverse traversal. */
    AG_CIRCLEQ_FOREACH_REVERSE(np, &head, entries) {
    	/* np-> ... */
    }
    					/* Delete. */
    while (!AG_CIRCLEQ_EMPTY(&head))
    	AG_CIRCLEQ_REMOVE(&head, AG_CIRCLEQ_FIRST(&head), entries);

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 AG_SLIST_END(), AG_LIST_END(), AG_SIMPLEQ_END() and AG_TAILQ_END() macros are provided for symmetry with AG_CIRCLEQ_END(). They expand to NULL and don't serve any useful purpose.

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

    AG_LIST_FOREACH(var, head, entry) {
    	free(var);
    }
    free(head);

Since var is free'd, the FOREACH() macro refers to a pointer that may have been reallocated already. Proper code needs a second variable.

    for (var = AG_LIST_FIRST(head);
         var != AG_LIST_END(head);
         var = nxt) {
    	nxt = AG_LIST_NEXT(var, entry);
    	free(var);
    }
    AG_LIST_INIT(head);	/* to put the list back in order */

A similar situation occurs when the current element is deleted from the list. Correct code saves a pointer to the next element in the list before removing the element:

    for (var = AG_LIST_FIRST(head);
         var != AG_LIST_END(head);
         var = nxt) {
    	nxt = AG_LIST_NEXT(var, entry);
    	if (some_condition) {
    		AG_LIST_REMOVE(var, entry);
    		some_function(var);
    	}
    }

HISTORY
The AG_Queue macros first appeared in Agar 1.0 and are based on the 4.4BSD queue macros in sys/queue.h.