HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. |  |
NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |STANDARDS |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
CIRCLEQ(3) Library Functions ManualCIRCLEQ(3)CIRCLEQ_EMPTY, CIRCLEQ_ENTRY, CIRCLEQ_FIRST, CIRCLEQ_FOREACH, CIRCLEQ_FOREACH_REVERSE, CIRCLEQ_HEAD, CIRCLEQ_HEAD_INITIALIZER, CIRCLEQ_INIT, CIRCLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE, CIRCLEQ_INSERT_HEAD, CIRCLEQ_INSERT_TAIL, CIRCLEQ_LAST, CIRCLEQ_LOOP_NEXT, CIRCLEQ_LOOP_PREV, CIRCLEQ_NEXT, CIRCLEQ_PREV, CIRCLEQ_REMOVE - implementation of a doubly linked circular queue
Standard C library (libc,-lc)
#include <sys/queue.h>CIRCLEQ_ENTRY(TYPE);CIRCLEQ_HEAD(HEADNAME, TYPE);CIRCLEQ_HEAD CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEADhead);void CIRCLEQ_INIT(CIRCLEQ_HEAD *head);int CIRCLEQ_EMPTY(CIRCLEQ_HEAD *head);void CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head,struct TYPE *elm, CIRCLEQ_ENTRYNAME);void CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head,struct TYPE *elm, CIRCLEQ_ENTRYNAME);void CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *head, struct TYPE *listelm,struct TYPE *elm, CIRCLEQ_ENTRYNAME);void CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *head, struct TYPE *listelm,struct TYPE *elm, CIRCLEQ_ENTRYNAME);struct TYPE *CIRCLEQ_FIRST(CIRCLEQ_HEAD *head);struct TYPE *CIRCLEQ_LAST(CIRCLEQ_HEAD *head);struct TYPE *CIRCLEQ_PREV(struct TYPE *elm, CIRCLEQ_ENTRYNAME);struct TYPE *CIRCLEQ_NEXT(struct TYPE *elm, CIRCLEQ_ENTRYNAME);struct TYPE *CIRCLEQ_LOOP_PREV(CIRCLEQ_HEAD *head,struct TYPE *elm, CIRCLEQ_ENTRYNAME);struct TYPE *CIRCLEQ_LOOP_NEXT(CIRCLEQ_HEAD *head,struct TYPE *elm, CIRCLEQ_ENTRYNAME);CIRCLEQ_FOREACH(struct TYPE *var, CIRCLEQ_HEAD *head,CIRCLEQ_ENTRYNAME);CIRCLEQ_FOREACH_REVERSE(struct TYPE *var, CIRCLEQ_HEAD *head,CIRCLEQ_ENTRYNAME);void CIRCLEQ_REMOVE(CIRCLEQ_HEAD *head, struct TYPE *elm,CIRCLEQ_ENTRYNAME);
These macros define and operate on doubly linked circular queues. In the macro definitions,TYPE is the name of a user-defined structure, that must contain a field of typeCIRCLEQ_ENTRY, namedNAME. The argumentHEADNAME is the name of a user-defined structure that must be declared using the macroCIRCLEQ_HEAD().Creation A circular queue is headed by a structure defined by theCIRCLEQ_HEAD() macro. This structure contains a pair of pointers, one to the first element in the queue and the other to the last element in the 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. ACIRCLEQ_HEAD structure is declared as follows: CIRCLEQ_HEAD(HEADNAME, TYPE) head; wherestruct HEADNAME is the structure to be defined, andstructTYPE 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 nameshead andheadp are user selectable.)CIRCLEQ_ENTRY() declares a structure that connects the elements in the queue.CIRCLEQ_HEAD_INITIALIZER() evaluates to an initializer for the queuehead.CIRCLEQ_INIT() initializes the queue referenced byhead.CIRCLEQ_EMPTY() evaluates to true if there are no items on the queue.InsertionCIRCLEQ_INSERT_HEAD() inserts the new elementelm at the head of the queue.CIRCLEQ_INSERT_TAIL() inserts the new elementelm at the end of the queue.CIRCLEQ_INSERT_BEFORE() inserts the new elementelm before the elementlistelm.CIRCLEQ_INSERT_AFTER() inserts the new elementelm after the elementlistelm.TraversalCIRCLEQ_FIRST() returns the first item on the queue.CIRCLEQ_LAST() returns the last item on the queue.CIRCLEQ_PREV() returns the previous item on the queue, or&head if this item is the first one.CIRCLEQ_NEXT() returns the next item on the queue, or&head if this item is the last one.CIRCLEQ_LOOP_PREV() returns the previous item on the queue. Ifelm is the first element on the queue, the last element is returned.CIRCLEQ_LOOP_NEXT() returns the next item on the queue. Ifelm is the last element on the queue, the first element is returned.CIRCLEQ_FOREACH() traverses the queue referenced byhead in the forward direction, assigning each element in turn tovar.var is set to&head if the loop completes normally, or if there were no elements.CIRCLEQ_FOREACH_REVERSE() traverses the queue referenced byhead in the reverse direction, assigning each element in turn tovar.RemovalCIRCLEQ_REMOVE() removes the elementelm from the queue.
CIRCLEQ_EMPTY() returns nonzero if the queue is empty, and zero if the queue contains at least one entry.CIRCLEQ_FIRST(),CIRCLEQ_LAST(),CIRCLEQ_LOOP_PREV(), andCIRCLEQ_LOOP_NEXT() return a pointer to the first, last, previous, or nextTYPE structure, respectively.CIRCLEQ_PREV(), andCIRCLEQ_NEXT() are similar to theirCIRCLEQ_LOOP_*() counterparts, except that if the argument is the first or last element, respectively, they return&head.CIRCLEQ_HEAD_INITIALIZER() returns an initializer that can be assigned to the queuehead.
BSD.
CIRCLEQ_FOREACH() andCIRCLEQ_FOREACH_REVERSE() don't allowvar to be removed or freed within the loop, as it would interfere with the traversal.CIRCLEQ_FOREACH_SAFE() andCIRCLEQ_FOREACH_REVERSE_SAFE(), which are present on the BSDs but are not present in glibc, fix this limitation by allowingvar to safely be removed from the list and freed from within the loop without interfering with the traversal.
#include <stddef.h> #include <stdio.h> #include <stdlib.h> #include <sys/queue.h> struct entry { int data; CIRCLEQ_ENTRY(entry) entries; /* Queue */ }; CIRCLEQ_HEAD(circlehead, entry); int main(void) { struct entry *n1, *n2, *n3, *np; struct circlehead head; /* Queue head */ int i; CIRCLEQ_INIT(&head); /* Initialize the 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); n3 = malloc(sizeof(struct entry)); /* Insert before */ CIRCLEQ_INSERT_BEFORE(&head, n2, n3, entries); CIRCLEQ_REMOVE(&head, n2, entries); /* Deletion */ free(n2); /* Forward traversal */ i = 0; CIRCLEQ_FOREACH(np, &head, entries) np->data = i++; /* Reverse traversal */ CIRCLEQ_FOREACH_REVERSE(np, &head, entries) printf("%i\n", np->data); /* Queue deletion */ n1 = CIRCLEQ_FIRST(&head); while (n1 != (void *)&head) { n2 = CIRCLEQ_NEXT(n1, entries); free(n1); n1 = n2; } CIRCLEQ_INIT(&head); exit(EXIT_SUCCESS); }insque(3),queue(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17CIRCLEQ(3)Pages that refer to this page:queue(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |