QUEUE(3) BSD Programmer's Manual QUEUE(3)NAME
queue - implementations of simple list queues, tail queues, and circular
queues
DESCRIPTION
These macros define and operate on three types of data structures: lists,
tail queues, and circular queues. All three structures support the fol-
lowing 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. Insertion of a new entry before any element in the list.
4. Removal of any entry in the list.
5. 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.
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. Reverse traversal is 50% slower than circular queues.
4. 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. 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.
LIST SYNOPSIS
#include <stddef.h>
#include <sys/queue.h>
LIST_END(LIST_HEAD *headp);
LIST_ENTRY(TYPE);
LIST_FIRST(LIST_HEAD *headp);
LIST_HEAD(HEADNAME, TYPE);
LIST_INIT(LIST_HEAD *headp);
LIST_INSERT_AFTER(LIST_ENTRY *listelm, TYPE *elm, LIST_ENTRY NAME);
LIST_INSERT_BEFORE(LIST_ENTRY *listelm, TYPE *elm, LIST_ENTRY NAME);
LIST_INSERT_HEAD(LIST_HEAD *headp, TYPE *elm, LIST_ENTRY NAME);
LIST_NEXT(TYPE *elm, LIST_ENTRY NAME);
LIST_REMOVE(TYPE *elm, LIST_ENTRY NAME);
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 re-
moved without traversing the list. New elements can be added to the list
after or before an existing element or at the head of the list. A
LIST_HEAD structure is declared as follows:
LIST_HEAD(HEADNAME, TYPE);
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 structure repre-
senting the head of the list, or a pointer to the head of the list can
later be declared as follows:
struct HEADNAME head;
struct HEADNAME *headp;
where the names head and headp are user selectable.
In the macro arguments, TYPE is the name of a user defined structure,
that must contain a field named NAME, of type LIST_ENTRY.
The LIST_END macro returns the pointer value indicating the end of the
list. Note that the parameter to the LIST_END macro that points to the
head of the list is unused and may be specified as NULL. The LIST_END
macro has a parameter only to keep it consistent with the CIRCLEQ_END
macro.
The LIST_ENTRY macro declares a structure that connects the elements in
the list.
The LIST_FIRST macro returns a pointer to the first element of the list.
When this pointer is the same as the pointer returned by the LIST_END
macro the list is empty.
The LIST_INIT macro initializes the list referenced by headp.
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 ele-
ment listelm.
The LIST_NEXT macro returns a pointer to the next element in the list.
Upon reaching the end of the list this pointer will be the same as the
pointer returned by the LIST_END macro.
The LIST_REMOVE macro 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, *np;
headp = &head;
LIST_INIT(headp); /* Initialize the list. */
n1 = malloc(sizeof(struct entry)); /* Insert at head. */
LIST_INSERT_HEAD(headp, n1, entries);
n2 = malloc(sizeof(struct entry)); /* Insert after element. */
LIST_INSERT_AFTER(n1, n2, entries);
n2 = malloc(sizeof(struct entry)); /* Insert before element. */
LIST_INSERT_BEFORE(n1, n2, entries);
/* Forward traversal. */
for (np = LIST_FIRST(headp); np != LIST_END(NULL);
np = LIST_NEXT(np, entries))
np-> ...
while (LIST_FIRST(headp) != LIST_END(NULL)) /* Empty list. */
LIST_REMOVE(LIST_FIRST(headp), entries);
TAIL QUEUE SYNOPSIS
#include <stddef.h>
#include <sys/queue.h>
TAILQ_END(TAILQ_HEAD *headp);
TAILQ_ENTRY(TYPE);
TAILQ_FIRST(TAILQ_HEAD *headp);
TAILQ_HEAD(HEADNAME, TYPE);
TAILQ_INIT(TAILQ_HEAD *headp);
TAILQ_INSERT_AFTER(TAILQ_HEAD *headp, TYPE *listelm, TYPE *elm, TAILQ_EN-
TRY NAME);
TAILQ_INSERT_BEFORE(TAILQ_HEAD *headp, TYPE *listelm, TYPE *elm,
TAILQ_ENTRY NAME);
TAILQ_INSERT_HEAD(TAILQ_HEAD *headp, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_INSERT_TAIL(TAILQ_HEAD *headp, TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_NEXT(TYPE *elm, TAILQ_ENTRY NAME);
TAILQ_PREV(TYPE *elm, HEADNAME, TAILQ_ENTRY NAME);
TAILQ_LAST(TAILQ_HEAD *headp, HEADNAME);
TAILQ_REMOVE(TAILQ_HEAD *headp, TYPE *elm, TAILQ_ENTRY NAME);
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 struc-
ture is declared as follows:
TAILQ_HEAD(HEADNAME, TYPE);
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 structure repre-
senting the head of the list, or a pointer to the head of the list can
later be declared as follows:
struct HEADNAME head;
struct HEADNAME *headp;
where the names head and headp are user selectable.
In the macro arguments, TYPE is the name of a user defined structure,
that must contain a field named NAME, of type TAILQ_ENTRY.
The TAILQ_END macro returns the pointer value indicating the end of the
tail queue. Note that the parameter to the TAILQ_END macro that points
to the head of the list is unused and may be specified as NULL. The
TAILQ_END macro has a parameter only to keep it consistent with the
CIRCLEQ_END macro.
The TAILQ_ENTRY macro declares a structure that connects the elements in
the tail queue.
The TAILQ_FIRST macro returns a pointer to the first element of the tail
queue. When this pointer is the same as the pointer returned by the
TAILQ_END macro the tail queue is empty.
The TAILQ_INIT macro initializes the tail queue referenced by headp.
The TAILQ_INSERT_AFTER macro inserts the new element elm after the ele-
ment listelm.
The TAILQ_INSERT_BEFORE macro inserts the new element elm before the ele-
ment listelm.
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_LAST macro returns a pointer to the last element of the tail
queue. When this pointer is the same as the pointer returned by the
TAILQ_END macro the tail queue is empty.
The TAILQ_NEXT macro returns a pointer to the next element in the tail
queue. Upon reaching the end of the tail queue this pointer will be the
same as the pointer returned by the TAILQ_END macro.
The TAILQ_PREV macro returns a pointer to the previous element in the
tail queue. Upon reaching the end of the tail queue this pointer will be
the same as the pointer returned by the TAILQ_END macro.
The TAILQ_REMOVE macro 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, *np;
headp = &head;
TAILQ_INIT(headp); /* Initialize the tailq. */
n1 = malloc(sizeof(struct entry)); /* Insert at head. */
TAILQ_INSERT_HEAD(headp, n1, entries);
n1 = malloc(sizeof(struct entry)); /* Insert at tail. */
TAILQ_INSERT_TAIL(headp, n1, entries);
n2 = malloc(sizeof(struct entry)); /* Insert after element. */
TAILQ_INSERT_AFTER(headp, n1, n2, entries);
n2 = malloc(sizeof(struct entry)); /* Insert before element. */
TAILQ_INSERT_BEFORE(headp, n1, n2, entries);
/* Forward traversal. */
for (np = TAILQ_FIRST(headp); np != TAILQ_END(NULL);
np = TAILQ_NEXT(np, entries))
np-> ...
/* Reverse traversal. */
for (np = TAILQ_LAST(headp, tailhead); np != TAILQ_END(NULL);
np = TAILQ_PREV(np, tailhead, entries))
np-> ...
/* Empty list. */
while (TAILQ_FIRST(headp) != TAILQ_END(NULL))
TAILQ_REMOVE(headp, TAILQ_FIRST(headp), entries);
CIRCULAR QUEUE SYNOPSIS
#include <stddef.h>
#include <sys/queue.h>
CIRCLEQ_END(CIRCLEQ_HEAD *headp);
CIRCLEQ_ENTRY(TYPE);
CIRCLEQ_FIRST(CIRCLEQ_HEAD *headp);
CIRCLEQ_HEAD(HEADNAME, TYPE);
CIRCLEQ_INIT(CIRCLEQ_HEAD *headp);
CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *headp, TYPE *listelm, TYPE *elm, CIR-
CLEQ_ENTRY NAME);
CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *headp, TYPE *listelm, TYPE *elm, CIR-
CLEQ_ENTRY NAME);
CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *headp, TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *headp, TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_NEXT(TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_PREV(TYPE *elm, CIRCLEQ_ENTRY NAME);
CIRCLEQ_LAST(CIRCLEQ_HEAD *headp);
CIRCLEQ_REMOVE(CIRCLEQ_HEAD *headp, TYPE *elm, CIRCLEQ_ENTRY NAME);
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 ele-
ment in the circular queue and the other to the last element in the cir-
cular 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 struc-
ture is declared as follows:
CIRCLEQ_HEAD(HEADNAME, TYPE);
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 structure repre-
senting the head of the list, or a pointer to the head of the list can
later be declared as follows:
struct HEADNAME head;
struct HEADNAME *headp;
where the names head and headp are user selectable.
In the macro arguments, TYPE is the name of a user defined structure,
that must contain a field named NAME, of type CIRCLEQ_ENTRY.
The CIRCLEQ_END macro returns the pointer value indicating the end of the
circle queue.
The CIRCLEQ_ENTRY macro declares a structure that connects the elements
in the circular queue.
The CIRCLEQ_FIRST macro returns a pointer to the first element of the
circle queue. When this pointer is the same as the pointer returned by
the CIRCLEQ_END macro the circle queue is empty.
The CIRCLEQ_INIT macro initializes the circular queue referenced by
headp.
The CIRCLEQ_INSERT_AFTER macro inserts the new element elm after the ele-
ment listelm.
The CIRCLEQ_INSERT_BEFORE macro inserts the new element elm before the
element listelm.
The CIRCLEQ_INSERT_HEAD macro inserts the new element elm at the head of
the circular queue.
The CIRCLEQ_INSERT_TAIL macro inserts the new element elm at the end of
the circular queue.
The CIRCLEQ_LAST macro returns a pointer to the last element of the cir-
cle queue. When this pointer is the same as the pointer returned by the
CIRCLEQ_END macro the circle queue is empty.
The CIRCLEQ_NEXT macro returns a pointer to the next element in the cir-
cle queue. Upon reaching the end of the circle queue this pointer will
be the same as the pointer returned by the CIRCLEQ_END macro.
The CIRCLEQ_PREV macro returns a pointer to the previous element in the
circle queue. Upon reaching the end of the circle queue this pointer
will be the same as the pointer returned by the CIRCLEQ_END macro.
The CIRCLEQ_REMOVE macro 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;
headp = &head;
CIRCLEQ_INIT(headp); /* Initialize the circleq. */
n1 = malloc(sizeof(struct entry)); /* Insert at head. */
CIRCLEQ_INSERT_HEAD(headp, n1, entries);
n1 = malloc(sizeof(struct entry)); /* Insert at tail. */
CIRCLEQ_INSERT_TAIL(headp, n1, entries);
n2 = malloc(sizeof(struct entry)); /* Insert after element. */
CIRCLEQ_INSERT_AFTER(headp, n1, n2, entries);
n2 = malloc(sizeof(struct entry)); /* Insert before element. */
CIRCLEQ_INSERT_BEFORE(headp, n1, n2, entries);
/* Forward traversal. */
for (np = CIRCLEQ_FIRST(headp); np != CIRCLEQ_END(headp);
np = CIRCLEQ_NEXT(np, entries))
np-> ...
/* Reverse traversal. */
for (np = CIRCLEQ_LAST(headp); np != CIRCLEQ_END(headp);
np = CIRCLEQ_PREV(np, entries))
np-> ...
/* Empty list. */
while (CIRCLEQ_FIRST(headp) != CIRCLEQ_END(headp))
CIRCLEQ_REMOVE(headp, CIRCLEQ_FIRST(headp), entries);
HISTORY
The queue functions first appeared in 4.4BSD.
4th Berkeley Distribution August 20, 1994 6
