QUEUE

Section: C Library Functions (3)
Index Return to Main Contents

BSD mandoc
4BSD  

NAME

LIST_ENTRY LIST_HEAD LIST_INIT LIST_INSERT_AFTER LIST_INSERT_HEAD LIST_REMOVE TAILQ_ENTRY TAILQ_HEAD TAILQ_INIT TAILQ_INSERT_AFTER 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 lists, tail queues, and circular queues  

SYNOPSIS

Fd #include <sys/queue.h> Fn LIST_ENTRY TYPE Fn LIST_HEAD HEADNAME TYPE Fn LIST_INIT LIST_HEAD *head Fn LIST_INSERT_AFTER LIST_ENTRY *listelm TYPE *elm LIST_ENTRY NAME Fn LIST_INSERT_HEAD LIST_HEAD *head TYPE *elm LIST_ENTRY NAME Fn LIST_REMOVE TYPE *elm LIST_ENTRY NAME Fn TAILQ_ENTRY TYPE Fn TAILQ_HEAD HEADNAME TYPE Fn TAILQ_INIT TAILQ_HEAD *head Fn TAILQ_INSERT_AFTER TAILQ_HEAD *head TYPE *listelm TYPE *elm TAILQ_ENTRY NAME Fn TAILQ_INSERT_HEAD TAILQ_HEAD *head TYPE *elm TAILQ_ENTRY NAME Fn TAILQ_INSERT_TAIL TAILQ_HEAD *head TYPE *elm TAILQ_ENTRY NAME Fn TAILQ_REMOVE TAILQ_HEAD *head TYPE *elm TAILQ_ENTRY NAME Fn CIRCLEQ_ENTRY TYPE Fn CIRCLEQ_HEAD HEADNAME TYPE Fn CIRCLEQ_INIT CIRCLEQ_HEAD *head Fn CIRCLEQ_INSERT_AFTER CIRCLEQ_HEAD *head TYPE *listelm TYPE *elm CIRCLEQ_ENTRY NAME Fn CIRCLEQ_INSERT_BEFORE CIRCLEQ_HEAD *head TYPE *listelm TYPE *elm CIRCLEQ_ENTRY NAME Fn CIRCLEQ_INSERT_HEAD CIRCLEQ_HEAD *head TYPE *elm CIRCLEQ_ENTRY NAME Fn CIRCLEQ_INSERT_TAIL CIRCLEQ_HEAD *head TYPE *elm CIRCLEQ_ENTRY NAME Fn CIRCLEQ_REMOVE CIRCLEQ_HEAD *head TYPE *elm CIRCLEQ_ENTRY NAME  

DESCRIPTION

These macros define and operate on three types of data structures: lists, tail queues, and circular queues. All three 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 any entry in the list.
  4. Forward traversal through the list.

Lists are the simplest of the three data structures and support only the above functionality.

Tail queues add the following functionality:

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

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 lists.

Circular queues add the following functionality:

  1. Entries can be added at the end of a list.
  2. Entries can be added before another entry.
  3. 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, Fa TYPE is the name of a user defined structure, that must contain a field of type LIST_ENTRY TAILQ_ENTRY or CIRCLEQ_ENTRY named Fa NAME . The argument Fa HEADNAME is the name of a user defined structure that must be declared using the macros LIST_HEAD TAILQ_HEAD or CIRCLEQ_HEAD See the examples below for further explanation of how these macros are used.  

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 or at the head of the list. A Fa LIST_HEAD structure is declared as follows:
LIST_HEAD(HEADNAME, TYPE) head;

where Fa HEADNAME is the name of the structure to be defined, and Fa 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 Fa head .

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

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

The macro LIST_REMOVE removes the element Fa 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, *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);
                                        /* Forward traversal. */
for (np = head.lh_first; np != NULL; np = np->entries.le_next)
        np-> ...

while (head.lh_first != NULL)           /* Delete. */
        LIST_REMOVE(head.lh_first, entries);
 

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, at the head of the tail queue, or at the end of the tail queue. A Fa 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 Fa head .

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

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

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

The macro TAILQ_REMOVE removes the element Fa 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, *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);
                                        /* Forward traversal. */
for (np = head.tqh_first; np != NULL; np = np->entries.tqe_next)
        np-> ...
                                        /* Delete. */
while (head.tqh_first != NULL)
        TAILQ_REMOVE(&head, head.tqh_first, entries);
 

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 Fa 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 Fa head .

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

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

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

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

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

CIRCULAR QUEUE EXAMPLE

CIRCLEQ_HEAD(circleq, entry) head;
struct circleq *headp;                  /* Circular queue head. */
struct entry {
        ...
        CIRCLEQ_ENTRY(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);
                                        /* 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);
 

CONFORMING TO

Not in POSIX.1-2001. Present on the BSDs. The queue functions first appeared in BSD 4.4


 

Index

NAME
SYNOPSIS
DESCRIPTION
LISTS
LIST EXAMPLE
TAIL QUEUES
TAIL QUEUE EXAMPLE
CIRCULAR QUEUES
CIRCULAR QUEUE EXAMPLE
CONFORMING TO

linux.jgfs.net manual pages