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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |STANDARDS |HISTORY |CAVEATS |EXAMPLES |SEE ALSO |COLOPHON | |
TAILQ(3) Library Functions ManualTAILQ(3)TAILQ_CONCAT, TAILQ_EMPTY, TAILQ_ENTRY, TAILQ_FIRST, TAILQ_FOREACH, TAILQ_FOREACH_REVERSE, TAILQ_HEAD, TAILQ_HEAD_INITIALIZER, TAILQ_INIT, TAILQ_INSERT_AFTER, TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD, TAILQ_INSERT_TAIL, TAILQ_LAST, TAILQ_NEXT, TAILQ_PREV, TAILQ_REMOVE - implementation of a doubly linked tail queue
Standard C library (libc,-lc)
#include <sys/queue.h>TAILQ_ENTRY(TYPE);TAILQ_HEAD(HEADNAME, TYPE);TAILQ_HEAD TAILQ_HEAD_INITIALIZER(TAILQ_HEADhead);void TAILQ_INIT(TAILQ_HEAD *head);int TAILQ_EMPTY(TAILQ_HEAD *head);void TAILQ_INSERT_HEAD(TAILQ_HEAD *head,struct TYPE *elm, TAILQ_ENTRYNAME);void TAILQ_INSERT_TAIL(TAILQ_HEAD *head,struct TYPE *elm, TAILQ_ENTRYNAME);void TAILQ_INSERT_BEFORE(struct TYPE *listelm,struct TYPE *elm, TAILQ_ENTRYNAME);void TAILQ_INSERT_AFTER(TAILQ_HEAD *head, struct TYPE *listelm,struct TYPE *elm, TAILQ_ENTRYNAME);struct TYPE *TAILQ_FIRST(TAILQ_HEAD *head);struct TYPE *TAILQ_LAST(TAILQ_HEAD *head, HEADNAME);struct TYPE *TAILQ_PREV(struct TYPE *elm, HEADNAME, TAILQ_ENTRYNAME);struct TYPE *TAILQ_NEXT(struct TYPE *elm, TAILQ_ENTRYNAME);TAILQ_FOREACH(struct TYPE *var, TAILQ_HEAD *head,TAILQ_ENTRYNAME);TAILQ_FOREACH_REVERSE(struct TYPE *var, TAILQ_HEAD *head, HEADNAME,TAILQ_ENTRYNAME);void TAILQ_REMOVE(TAILQ_HEAD *head, struct TYPE *elm,TAILQ_ENTRYNAME);void TAILQ_CONCAT(TAILQ_HEAD *head1, TAILQ_HEAD *head2,TAILQ_ENTRYNAME);
These macros define and operate on doubly linked tail queues. In the macro definitions,TYPE is the name of a user defined structure, that must contain a field of typeTAILQ_ENTRY, namedNAME. The argumentHEADNAME is the name of a user defined structure that must be declared using the macroTAILQ_HEAD().Creation A tail queue is headed by a structure defined by theTAILQ_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. ATAILQ_HEAD structure is declared as follows: TAILQ_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.)TAILQ_ENTRY() declares a structure that connects the elements in the queue.TAILQ_HEAD_INITIALIZER() evaluates to an initializer for the queuehead.TAILQ_INIT() initializes the queue referenced byTAILQ_EMPTY() evaluates to true if there are no items on the queue.head.InsertionTAILQ_INSERT_HEAD() inserts the new elementelm at the head of the queue.TAILQ_INSERT_TAIL() inserts the new elementelm at the end of the queue.TAILQ_INSERT_BEFORE() inserts the new elementelm before the elementlistelm.TAILQ_INSERT_AFTER() inserts the new elementelm after the elementlistelm.TraversalTAILQ_FIRST() returns the first item on the queue, or NULL if the queue is empty.TAILQ_LAST() returns the last item on the queue. If the queue is empty the return value is NULL.TAILQ_PREV() returns the previous item on the queue, or NULL if this item is the first.TAILQ_NEXT() returns the next item on the queue, or NULL if this item is the last.TAILQ_FOREACH() traverses the queue referenced byhead in the forward direction, assigning each element in turn tovar.var is set to NULL if the loop completes normally, or if there were no elements.TAILQ_FOREACH_REVERSE() traverses the queue referenced byhead in the reverse direction, assigning each element in turn tovar.RemovalTAILQ_REMOVE() removes the elementelm from the queue.Other featuresTAILQ_CONCAT() concatenates the queue headed byhead2 onto the end of the one headed byhead1 removing all entries from the former.
TAILQ_EMPTY() returns nonzero if the queue is empty, and zero if the queue contains at least one entry.TAILQ_FIRST(),TAILQ_LAST(),TAILQ_PREV(), andTAILQ_NEXT() return a pointer to the first, last, previous, or nextTYPE structure, respectively.TAILQ_HEAD_INITIALIZER() returns an initializer that can be assigned to the queuehead.
BSD.
4.4BSD.
TAILQ_FOREACH() andTAILQ_FOREACH_REVERSE() don't allowvar to be removed or freed within the loop, as it would interfere with the traversal.TAILQ_FOREACH_SAFE() andTAILQ_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; TAILQ_ENTRY(entry) entries; /* Tail queue */ }; TAILQ_HEAD(tailhead, entry); int main(void) { struct entry *n1, *n2, *n3, *np; struct tailhead head; /* Tail queue head */ int i; 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); n3 = malloc(sizeof(struct entry)); /* Insert before */ TAILQ_INSERT_BEFORE(n2, n3, entries); TAILQ_REMOVE(&head, n2, entries); /* Deletion */ free(n2); /* Forward traversal */ i = 0; TAILQ_FOREACH(np, &head, entries) np->data = i++; /* Reverse traversal */ TAILQ_FOREACH_REVERSE(np, &head, tailhead, entries) printf("%i\n", np->data); /* TailQ deletion */ n1 = TAILQ_FIRST(&head); while (n1 != NULL) { n2 = TAILQ_NEXT(n1, entries); free(n1); n1 = n2; } TAILQ_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 2025-05-17TAILQ(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. | |