CIRCLEQ_NEXT(3)

1CIRCLEQ(3)                 Library Functions Manual                 CIRCLEQ(3)
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NAME

6       CIRCLEQ_EMPTY,   CIRCLEQ_ENTRY,  CIRCLEQ_FIRST,  CIRCLEQ_FOREACH,  CIR‐
7       CLEQ_FOREACH_REVERSE,  CIRCLEQ_HEAD,   CIRCLEQ_HEAD_INITIALIZER,   CIR‐
8       CLEQ_INIT,   CIRCLEQ_INSERT_AFTER,  CIRCLEQ_INSERT_BEFORE,  CIRCLEQ_IN‐
9       SERT_HEAD, CIRCLEQ_INSERT_TAIL, CIRCLEQ_LAST,  CIRCLEQ_LOOP_NEXT,  CIR‐
10       CLEQ_LOOP_PREV,  CIRCLEQ_NEXT, CIRCLEQ_PREV, CIRCLEQ_REMOVE - implemen‐
11       tation of a doubly linked circular queue
12

LIBRARY

14       Standard C library (libc, -lc)
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SYNOPSIS

17       #include <sys/queue.h>
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19       CIRCLEQ_ENTRY(TYPE);
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21       CIRCLEQ_HEAD(HEADNAME, TYPE);
22       CIRCLEQ_HEAD CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD head);
23       void CIRCLEQ_INIT(CIRCLEQ_HEAD *head);
24
25       int CIRCLEQ_EMPTY(CIRCLEQ_HEAD *head);
26
27       void CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head,
28                                  struct TYPE *elm, CIRCLEQ_ENTRY NAME);
29       void CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head,
30                                  struct TYPE *elm, CIRCLEQ_ENTRY NAME);
31       void CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *head, struct TYPE *listelm,
32                                  struct TYPE *elm, CIRCLEQ_ENTRY NAME);
33       void CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *head, struct TYPE *listelm,
34                                  struct TYPE *elm, CIRCLEQ_ENTRY NAME);
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36       struct TYPE *CIRCLEQ_FIRST(CIRCLEQ_HEAD *head);
37       struct TYPE *CIRCLEQ_LAST(CIRCLEQ_HEAD *head);
38       struct TYPE *CIRCLEQ_PREV(struct TYPE *elm, CIRCLEQ_ENTRY NAME);
39       struct TYPE *CIRCLEQ_NEXT(struct TYPE *elm, CIRCLEQ_ENTRY NAME);
40       struct TYPE *CIRCLEQ_LOOP_PREV(CIRCLEQ_HEAD *head,
41                                  struct TYPE *elm, CIRCLEQ_ENTRY NAME);
42       struct TYPE *CIRCLEQ_LOOP_NEXT(CIRCLEQ_HEAD *head,
43                                  struct TYPE *elm, CIRCLEQ_ENTRY NAME);
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45       CIRCLEQ_FOREACH(struct TYPE *var, CIRCLEQ_HEAD *head,
46                                  CIRCLEQ_ENTRY NAME);
47       CIRCLEQ_FOREACH_REVERSE(struct TYPE *var, CIRCLEQ_HEAD *head,
48                                  CIRCLEQ_ENTRY NAME);
49
50       void CIRCLEQ_REMOVE(CIRCLEQ_HEAD *head, struct TYPE *elm,
51                                  CIRCLEQ_ENTRY NAME);
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DESCRIPTION

54       These macros define and operate on doubly linked circular queues.
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56       In the macro definitions, TYPE is the name of a user-defined structure,
57       that must contain a field of type CIRCLEQ_ENTRY, named NAME.  The argu‐
58       ment HEADNAME is the name of a user-defined structure that must be  de‐
59       clared using the macro CIRCLEQ_HEAD().
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61   Creation
62       A circular queue is headed by a structure defined by the CIRCLEQ_HEAD()
63       macro.  This structure contains a pair of pointers, one  to  the  first
64       element  in  the  queue and the other to the last element in the queue.
65       The elements are doubly linked so that an arbitrary element can be  re‐
66       moved  without  traversing the queue.  New elements can be added to the
67       queue after an existing element, before an  existing  element,  at  the
68       head  of  the queue, or at the end of the queue.  A CIRCLEQ_HEAD struc‐
69       ture is declared as follows:
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71           CIRCLEQ_HEAD(HEADNAME, TYPE) head;
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73       where struct HEADNAME is the structure to be defined, and  struct  TYPE
74       is  the type of the elements to be linked into the queue.  A pointer to
75       the head of the queue can later be declared as:
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77           struct HEADNAME *headp;
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79       (The names head and headp are user selectable.)
80
81       CIRCLEQ_ENTRY() declares a structure that connects the elements in  the
82       queue.
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84       CIRCLEQ_HEAD_INITIALIZER()  evaluates  to  an initializer for the queue
85       head.
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87       CIRCLEQ_INIT() initializes the queue referenced by head.
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89       CIRCLEQ_EMPTY() evaluates to true if there are no items on the queue.
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91   Insertion
92       CIRCLEQ_INSERT_HEAD() inserts the new element elm at the  head  of  the
93       queue.
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95       CIRCLEQ_INSERT_TAIL()  inserts  the  new  element elm at the end of the
96       queue.
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98       CIRCLEQ_INSERT_BEFORE() inserts the new element elm before the  element
99       listelm.
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101       CIRCLEQ_INSERT_AFTER()  inserts  the  new element elm after the element
102       listelm.
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104   Traversal
105       CIRCLEQ_FIRST() returns the first item on the queue.
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107       CIRCLEQ_LAST() returns the last item on the queue.
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109       CIRCLEQ_PREV() returns the previous item on the queue, or &head if this
110       item is the first one.
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112       CIRCLEQ_NEXT()  returns  the  next  item on the queue, or &head if this
113       item is the last one.
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115       CIRCLEQ_LOOP_PREV() returns the previous item on the queue.  If elm  is
116       the first element on the queue, the last element is returned.
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118       CIRCLEQ_LOOP_NEXT()  returns the next item on the queue.  If elm is the
119       last element on the queue, the first element is returned.
120
121       CIRCLEQ_FOREACH() traverses the queue referenced by head in the forward
122       direction,  assigning each element in turn to var.  var is set to &head
123       if the loop completes normally, or if there were no elements.
124
125       CIRCLEQ_FOREACH_REVERSE() traverses the queue referenced by head in the
126       reverse direction, assigning each element in turn to var.
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128   Removal
129       CIRCLEQ_REMOVE() removes the element elm from the queue.
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RETURN VALUE

132       CIRCLEQ_EMPTY()  returns nonzero if the queue is empty, and zero if the
133       queue contains at least one entry.
134
135       CIRCLEQ_FIRST(),   CIRCLEQ_LAST(),   CIRCLEQ_LOOP_PREV(),   and    CIR‐
136       CLEQ_LOOP_NEXT() return a pointer to the first, last, previous, or next
137       TYPE structure, respectively.
138
139       CIRCLEQ_PREV(),  and  CIRCLEQ_NEXT()  are   similar   to   their   CIR‐
140       CLEQ_LOOP_*() counterparts, except that if the argument is the first or
141       last element, respectively, they return &head.
142
143       CIRCLEQ_HEAD_INITIALIZER() returns an initializer that can be  assigned
144       to the queue head.
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STANDARDS

147       BSD.
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BUGS

150       CIRCLEQ_FOREACH()  and  CIRCLEQ_FOREACH_REVERSE() don't allow var to be
151       removed or freed within the loop, as it would interfere with  the  tra‐
152       versal.    CIRCLEQ_FOREACH_SAFE()  and  CIRCLEQ_FOREACH_REVERSE_SAFE(),
153       which are present on the BSDs but are not present in  glibc,  fix  this
154       limitation by allowing var to safely be removed from the list and freed
155       from within the loop without interfering with the traversal.
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EXAMPLES

158       #include <stddef.h>
159       #include <stdio.h>
160       #include <stdlib.h>
161       #include <sys/queue.h>
162
163       struct entry {
164           int data;
165           CIRCLEQ_ENTRY(entry) entries;           /* Queue */
166       };
167
168       CIRCLEQ_HEAD(circlehead, entry);
169
170       int
171       main(void)
172       {
173           struct entry *n1, *n2, *n3, *np;
174           struct circlehead head;                 /* Queue head */
175           int i;
176
177           CIRCLEQ_INIT(&head);                    /* Initialize the queue */
178
179           n1 = malloc(sizeof(struct entry));      /* Insert at the head */
180           CIRCLEQ_INSERT_HEAD(&head, n1, entries);
181
182           n1 = malloc(sizeof(struct entry));      /* Insert at the tail */
183           CIRCLEQ_INSERT_TAIL(&head, n1, entries);
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185           n2 = malloc(sizeof(struct entry));      /* Insert after */
186           CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);
187
188           n3 = malloc(sizeof(struct entry));      /* Insert before */
189           CIRCLEQ_INSERT_BEFORE(&head, n2, n3, entries);
190
191           CIRCLEQ_REMOVE(&head, n2, entries);     /* Deletion */
192           free(n2);
193                                                   /* Forward traversal */
194           i = 0;
195           CIRCLEQ_FOREACH(np, &head, entries)
196               np->data = i++;
197                                                   /* Reverse traversal */
198           CIRCLEQ_FOREACH_REVERSE(np, &head, entries)
199               printf("%i\n", np->data);
200                                                   /* Queue deletion */
201           n1 = CIRCLEQ_FIRST(&head);
202           while (n1 != (void *)&head) {
203               n2 = CIRCLEQ_NEXT(n1, entries);
204               free(n1);
205               n1 = n2;
206           }
207           CIRCLEQ_INIT(&head);
208
209           exit(EXIT_SUCCESS);
210       }
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