1QUEUE(3)                   Linux Programmer's Manual                  QUEUE(3)
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NAME

6       LIST_ENTRY,  LIST_HEAD, LIST_INIT, LIST_INSERT_AFTER, LIST_INSERT_HEAD,
7       LIST_REMOVE, TAILQ_ENTRY, TAILQ_HEAD,  TAILQ_INIT,  TAILQ_INSERT_AFTER,
8       TAILQ_INSERT_HEAD, TAILQ_INSERT_TAIL, TAILQ_REMOVE, CIRCLEQ_ENTRY, CIR‐
9       CLEQ_HEAD, CIRCLEQ_INIT,  CIRCLEQ_INSERT_AFTER,  CIRCLEQ_INSERT_BEFORE,
10       CIRCLEQ_INSERT_HEAD,  CIRCLEQ_INSERT_TAIL, CIRCLEQ_REMOVE - implementa‐
11       tions of lists, tail queues, and circular queues
12

SYNOPSIS

14       #include <sys/queue.h>
15
16       LIST_ENTRY(TYPE);
17       LIST_HEAD(HEADNAME, TYPE);
18       LIST_INIT(LIST_HEAD *head);
19       LIST_INSERT_AFTER(LIST_ENTRY *listelm,
20                       TYPE *elm, LIST_ENTRY NAME);
21       LIST_INSERT_HEAD(LIST_HEAD *head,
22                       TYPE *elm, LIST_ENTRY NAME);
23       LIST_REMOVE(TYPE *elm, LIST_ENTRY NAME);
24
25       TAILQ_ENTRY(TYPE);
26       TAILQ_HEAD(HEADNAME, TYPE);
27       TAILQ_INIT(TAILQ_HEAD *head);
28       TAILQ_INSERT_AFTER(TAILQ_HEAD *head, TYPE *listelm,
29                       TYPE *elm, TAILQ_ENTRY NAME);
30       TAILQ_INSERT_HEAD(TAILQ_HEAD *head,
31                       TYPE *elm, TAILQ_ENTRY NAME);
32       TAILQ_INSERT_TAIL(TAILQ_HEAD *head,
33                       TYPE *elm, TAILQ_ENTRY NAME);
34       TAILQ_REMOVE(TAILQ_HEAD *head, TYPE *elm, TAILQ_ENTRY NAME);
35
36       CIRCLEQ_ENTRY(TYPE);
37       CIRCLEQ_HEAD(HEADNAME, TYPE);
38       CIRCLEQ_INIT(CIRCLEQ_HEAD *head);
39       CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *head, TYPE *listelm,
40                       TYPE *elm, CIRCLEQ_ENTRY NAME);
41       CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *head, TYPE *listelm,
42                       TYPE *elm, CIRCLEQ_ENTRY NAME);
43       CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head,
44                       TYPE *elm, CIRCLEQ_ENTRY NAME);
45       CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head,
46                       TYPE *elm, CIRCLEQ_ENTRY NAME);
47       CIRCLEQ_REMOVE(CIRCLEQ_HEAD *head,
48                       TYPE *elm, CIRCLEQ_ENTRY NAME);
49

DESCRIPTION

51       These macros define and operate on  three  types  of  data  structures:
52       lists,  tail queues, and circular queues.  All three structures support
53       the following functionality:
54
55           *   Insertion of a new entry at the head of the list.
56           *   Insertion of a new entry after any element in the list.
57           *   Removal of any entry in the list.
58           *   Forward traversal through the list.
59
60       Lists are the simplest of the three data structures  and  support  only
61       the above functionality.
62
63       Tail queues add the following functionality:
64
65           *   Entries can be added at the end of a list.
66
67       However:
68
69           1.  All  list  insertions and removals must specify the head of the
70               list.
71           2.  Each head entry requires two pointers rather than one.
72           3.  Code size is about 15% greater and  operations  run  about  20%
73               slower than lists.
74
75       Circular queues add the following functionality:
76
77           *   Entries can be added at the end of a list.
78           *   Entries can be added before another entry.
79           *   They may be traversed backwards, from tail to head.
80
81       However:
82
83           1.  All  list  insertions and removals must specify the head of the
84               list.
85           2.  Each head entry requires two pointers rather than one.
86           3.  The termination condition for traversal is more complex.
87           4.  Code size is about 40% greater and  operations  run  about  45%
88               slower than lists.
89
90       In the macro definitions, TYPE is the name of a user-defined structure,
91       that must contain a field of  type  LIST_ENTRY,  TAILQ_ENTRY,  or  CIR‐
92       CLEQ_ENTRY,  named  NAME.  The argument HEADNAME is the name of a user-
93       defined structure that must be declared  using  the  macros  LIST_HEAD,
94       TAILQ_HEAD, or CIRCLEQ_HEAD.  See the examples below for further expla‐
95       nation of how these macros are used.
96
97   Lists
98       A list is headed by a structure defined by the LIST_HEAD  macro.   This
99       structure  contains  a single pointer to the first element on the list.
100       The elements are doubly linked so that  an  arbitrary  element  can  be
101       removed  without traversing the list.  New elements can be added to the
102       list after an existing element or at the head of the list.  A LIST_HEAD
103       structure is declared as follows:
104
105           LIST_HEAD(HEADNAME, TYPE) head;
106
107       where  HEADNAME is the name of the structure to be defined, and TYPE is
108       the type of the elements to be linked into the list.  A pointer to  the
109       head of the list can later be declared as:
110
111           struct HEADNAME *headp;
112
113       (The names head and headp are user selectable.)
114
115       The macro LIST_ENTRY declares a structure that connects the elements in
116       the list.
117
118       The macro LIST_INIT initializes the list referenced by head.
119
120       The macro LIST_INSERT_HEAD inserts the new element elm at the  head  of
121       the list.
122
123       The  macro LIST_INSERT_AFTER inserts the new element elm after the ele‐
124       ment listelm.
125
126       The macro LIST_REMOVE removes the element elm from the list.
127
128   List Example
129       LIST_HEAD(listhead, entry) head;
130       struct listhead *headp;                 /* List head. */
131       struct entry {
132           ...
133           LIST_ENTRY(entry) entries;          /* List. */
134           ...
135       } *n1, *n2, *np;
136
137       LIST_INIT(&head);                       /* Initialize the list. */
138
139       n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
140       LIST_INSERT_HEAD(&head, n1, entries);
141
142       n2 = malloc(sizeof(struct entry));      /* Insert after. */
143       LIST_INSERT_AFTER(n1, n2, entries);
144                                               /* Forward traversal. */
145       for (np = head.lh_first; np != NULL; np = np->entries.le_next)
146           np-> ...
147
148       while (head.lh_first != NULL)           /* Delete. */
149           LIST_REMOVE(head.lh_first, entries);
150
151   Tail Queues
152       A tail queue is headed by a structure defined by the TAILQ_HEAD  macro.
153       This structure contains a pair of pointers, one to the first element in
154       the tail queue and the other to the last element  in  the  tail  queue.
155       The  elements  are  doubly  linked  so that an arbitrary element can be
156       removed without traversing the tail queue.  New elements can  be  added
157       to  the  tail  queue after an existing element, at the head of the tail
158       queue, or at the end of the tail  queue.   A  TAILQ_HEAD  structure  is
159       declared as follows:
160
161           TAILQ_HEAD(HEADNAME, TYPE) head;
162
163       where  HEADNAME is the name of the structure to be defined, and TYPE is
164       the type of the elements to be linked into the tail queue.   A  pointer
165       to the head of the tail queue can later be declared as:
166
167           struct HEADNAME *headp;
168
169       (The names head and headp are user selectable.)
170
171       The  macro  TAILQ_ENTRY declares a structure that connects the elements
172       in the tail queue.
173
174       The macro TAILQ_INIT initializes the tail queue referenced by head.
175
176       The macro TAILQ_INSERT_HEAD inserts the new element elm at the head  of
177       the tail queue.
178
179       The  macro  TAILQ_INSERT_TAIL inserts the new element elm at the end of
180       the tail queue.
181
182       The macro TAILQ_INSERT_AFTER inserts the new element elm after the ele‐
183       ment listelm.
184
185       The macro TAILQ_REMOVE removes the element elm from the tail queue.
186
187   Tail Queue Example
188       TAILQ_HEAD(tailhead, entry) head;
189       struct tailhead *headp;                 /* Tail queue head. */
190       struct entry {
191           ...
192           TAILQ_ENTRY(entry) entries;         /* Tail queue. */
193           ...
194       } *n1, *n2, *np;
195
196       TAILQ_INIT(&head);                      /* Initialize the queue. */
197
198       n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
199       TAILQ_INSERT_HEAD(&head, n1, entries);
200
201       n1 = malloc(sizeof(struct entry));      /* Insert at the tail. */
202       TAILQ_INSERT_TAIL(&head, n1, entries);
203
204       n2 = malloc(sizeof(struct entry));      /* Insert after. */
205       TAILQ_INSERT_AFTER(&head, n1, n2, entries);
206                                               /* Forward traversal. */
207       for (np = head.tqh_first; np != NULL; np = np->entries.tqe_next)
208           np-> ...
209                                               /* Delete. */
210       while (head.tqh_first != NULL)
211           TAILQ_REMOVE(&head, head.tqh_first, entries);
212
213   Circular Queues
214       A  circular  queue is headed by a structure defined by the CIRCLEQ_HEAD
215       macro.  This structure contains a pair of pointers, one  to  the  first
216       element  in the circular queue and the other to the last element in the
217       circular queue.  The elements are doubly linked so  that  an  arbitrary
218       element  can be removed without traversing the queue.  New elements can
219       be added to the queue after an existing  element,  before  an  existing
220       element,  at the head of the queue, or at the end of the queue.  A CIR‐
221       CLEQ_HEAD structure is declared as follows:
222
223           CIRCLEQ_HEAD(HEADNAME, TYPE) head;
224
225       where HEADNAME is the name of the structure to be defined, and TYPE  is
226       the  type  of  the  elements  to  be linked into the circular queue.  A
227       pointer to the head of the circular queue can later be declared as:
228
229           struct HEADNAME *headp;
230
231       (The names head and headp are user selectable.)
232
233       The macro CIRCLEQ_ENTRY declares a structure that connects the elements
234       in the circular queue.
235
236       The  macro  CIRCLEQ_INIT  initializes  the circular queue referenced by
237       head.
238
239       The macro CIRCLEQ_INSERT_HEAD inserts the new element elm at  the  head
240       of the circular queue.
241
242       The macro CIRCLEQ_INSERT_TAIL inserts the new element elm at the end of
243       the circular queue.
244
245       The macro CIRCLEQ_INSERT_AFTER inserts the new element  elm  after  the
246       element listelm.
247
248       The  macro CIRCLEQ_INSERT_BEFORE inserts the new element elm before the
249       element listelm.
250
251       The macro CIRCLEQ_REMOVE removes the  element  elm  from  the  circular
252       queue.
253
254   Circular Queue Example
255       CIRCLEQ_HEAD(circleq, entry) head;
256       struct circleq *headp;              /* Circular queue head. */
257       struct entry {
258           ...
259           CIRCLEQ_ENTRY(entry) entries;   /* Circular queue. */
260           ...
261       } *n1, *n2, *np;
262
263       CIRCLEQ_INIT(&head);                /* Initialize the circular queue. */
264
265       n1 = malloc(sizeof(struct entry));  /* Insert at the head. */
266       CIRCLEQ_INSERT_HEAD(&head, n1, entries);
267
268       n1 = malloc(sizeof(struct entry));  /* Insert at the tail. */
269       CIRCLEQ_INSERT_TAIL(&head, n1, entries);
270
271       n2 = malloc(sizeof(struct entry));  /* Insert after. */
272       CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);
273
274       n2 = malloc(sizeof(struct entry));  /* Insert before. */
275       CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
276                                           /* Forward traversal. */
277       for (np = head.cqh_first; np != (void *)&head;
278               np = np->entries.cqe_next)
279           np-> ...
280                                           /* Reverse traversal. */
281       for (np = head.cqh_last; np != (void *)&head; np = np->entries.cqe_prev)
282           np-> ...
283                                           /* Delete. */
284       while (head.cqh_first != (void *)&head)
285           CIRCLEQ_REMOVE(&head, head.cqh_first, entries);
286

CONFORMING TO

288       Not  in  POSIX.1-2001.  Present on the BSDs.  The queue functions first
289       appeared in 4.4BSD.
290

COLOPHON

292       This page is part of release 3.22 of the Linux  man-pages  project.   A
293       description  of  the project, and information about reporting bugs, can
294       be found at http://www.kernel.org/doc/man-pages/.
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298Linux                             2007-12-28                          QUEUE(3)
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