.\" Copyright (c) 1993 .\" The Regents of the University of California. All rights reserved. .\" and Copyright (c) 2020 by Alejandro Colomar .\" .\" %%%LICENSE_START(BSD_3_CLAUSE_UCB) .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" %%%LICENSE_END .\" .\" .TH CIRCLEQ 3 2020-10-21 "GNU" "Linux Programmer's Manual" .SH NAME 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 .SH SYNOPSIS .nf .B #include .PP .BI "int CIRCLEQ_EMPTY(CIRCLEQ_HEAD *" head ");" .PP .B CIRCLEQ_ENTRY(TYPE); .PP .BI "struct TYPE *CIRCLEQ_FIRST(CIRCLEQ_HEAD *" head ");" .PP .BI "CIRCLEQ_FOREACH(struct TYPE *" var ", CIRCLEQ_HEAD *" head , .BI " CIRCLEQ_ENTRY " NAME ");" .PP .BI "CIRCLEQ_FOREACH_REVERSE(struct TYPE *" var ", CIRCLEQ_HEAD *" head "," .BI " CIRCLEQ_ENTRY " NAME ");" .PP .B CIRCLEQ_HEAD(HEADNAME, TYPE); .PP .BI "CIRCLEQ_HEAD CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD " head ");" .PP .BI "void CIRCLEQ_INIT(CIRCLEQ_HEAD *" head ");" .PP .BI "void CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *" head ", struct TYPE *" listelm , .BI " struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME ");" .PP .BI "void CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *" head ", struct TYPE *" listelm , .BI " struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME ");" .PP .BI "void CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *" head ", struct TYPE *" elm "," .BI " CIRCLEQ_ENTRY " NAME ");" .PP .BI "void CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *" head ", struct TYPE *" elm "," .BI " CIRCLEQ_ENTRY " NAME ");" .PP .BI "struct TYPE *CIRCLEQ_LAST(CIRCLEQ_HEAD *" head ");" .PP .BI "void CIRCLEQ_LOOP_NEXT(CIRCLEQ_HEAD *" head ", struct TYPE *" elm "," .BI " CIRCLEQ_ENTRY " NAME ");" .PP .BI "void CIRCLEQ_LOOP_PREV(CIRCLEQ_HEAD *" head ", struct TYPE *" elm "," .BI " CIRCLEQ_ENTRY " NAME ");" .PP .BI "struct TYPE *CIRCLEQ_NEXT(struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME ");" .PP .BI "struct TYPE *CIRCLEQ_PREV(struct TYPE *" elm ", CIRCLEQ_ENTRY " NAME ");" .PP .BI "void CIRCLEQ_REMOVE(CIRCLEQ_HEAD *" head ", struct TYPE *" elm , .BI " CIRCLEQ_ENTRY " NAME ");" .fi .SH DESCRIPTION These macros define and operate on doubly linked circular queues. .PP In the macro definitions, .I TYPE is the name of a user-defined structure, that must contain a field of type .IR CIRCLEQ_ENTRY , named .IR NAME . The argument .I HEADNAME is the name of a user-defined structure that must be declared using the macro .BR CIRCLEQ_HEAD (). .PP A circular queue is headed by a structure defined by the .BR CIRCLEQ_HEAD () macro. This structure contains a pair of pointers, one to the first element in the circular queue and the other to the last element in the circular queue. The elements are doubly linked so that an arbitrary element can be removed without traversing the circular queue. New elements can be added to the circular queue after an existing element, before an existing element, at the head of the circular queue, or at the end of the circular queue. A .I CIRCLEQ_HEAD structure is declared as follows: .PP .in +4 .EX CIRCLEQ_HEAD(HEADNAME, TYPE) head; .EE .in .PP where .I struct HEADNAME is the structure to be defined, and .I struct TYPE is the type of the elements to be linked into the circular queue. A pointer to the head of the circular queue can later be declared as: .PP .in +4 .EX struct HEADNAME *headp; .EE .in .PP (The names .I head and .I headp are user selectable.) .PP The macro .BR CIRCLEQ_HEAD_INITIALIZER () evaluates to an initializer for the circular queue .IR head . .PP The macro .BR CIRCLEQ_EMPTY () evaluates to true if there are no items on the circular queue. .PP The macro .BR CIRCLEQ_ENTRY () declares a structure that connects the elements in the circular queue. .PP The macro .BR CIRCLEQ_FIRST () returns the first item on the circular queue. .PP The macro .BR CIRCLEQ_FOREACH () traverses the circular queue referenced by .I head in the forward direction, assigning each element in turn to .IR var . .I var is set to .I &head if the loop completes normally, or if there were no elements. .PP The macro .BR CIRCLEQ_FOREACH_REVERSE () traverses the circular queue referenced by .I head in the reverse direction, assigning each element in turn to .IR var . .PP The macro .BR CIRCLEQ_INIT () initializes the circular queue referenced by .IR head . .PP The macro .BR CIRCLEQ_INSERT_HEAD () inserts the new element .I elm at the head of the circular queue. .PP The macro .BR CIRCLEQ_INSERT_TAIL () inserts the new element .I elm at the end of the circular queue. .PP The macro .BR CIRCLEQ_INSERT_AFTER () inserts the new element .I elm after the element .IR listelm . .PP The macro .BR CIRCLEQ_INSERT_BEFORE () inserts the new element .I elm before the element .IR listelm . .PP The macro .BR CIRCLEQ_LAST () returns the last item on the circular queue. .PP The macro .BR CIRCLEQ_NEXT () returns the next item on the circular queue, or .I &head if this item is the last one. .PP The macro .BR CIRCLEQ_PREV () returns the previous item on the circular queue, or .I &head if this item is the first one. .PP The macro .BR CIRCLEQ_LOOP_NEXT () returns the next item on the circular queue. If .I elm is the last element on the circular queue, the first element is returned. .PP The macro .BR CIRCLEQ_LOOP_PREV () returns the previous item on the circular queue. If .I elm is the first element on the circular queue, the last element is returned. .PP The macro .BR CIRCLEQ_REMOVE () removes the element .I elm from the circular queue. .SH RETURN VALUE .BR CIRCLEQ_EMPTY () returns nonzero if the queue is empty, and zero if the queue contains at least one entry. .PP .BR CIRCLEQ_FIRST (), .BR CIRCLEQ_LAST (), .BR CIRCLEQ_NEXT (), and .BR CIRCLEQ_PREV () return a pointer to the first, last, next or previous .I TYPE structure, respectively. .PP .BR CIRCLEQ_HEAD_INITIALIZER () returns an initializer that can be assigned to the queue .IR head . .SH CONFORMING TO Not in POSIX.1, POSIX.1-2001 or POSIX.1-2008. Present on the BSDs (CIRCLEQ macros first appeared in 4.4BSD). .SH BUGS The macros .BR CIRCLEQ_FOREACH () and .BR CIRCLEQ_FOREACH_REVERSE () don't allow .I var to be removed or freed within the loop, as it would interfere with the traversal. The macros .BR CIRCLEQ_FOREACH_SAFE () and .BR CIRCLEQ_FOREACH_REVERSE_SAFE (), which are present on the BSDs but are not present in glibc, fix this limitation by allowing .I var to safely be removed from the list and freed from within the loop without interfering with the traversal. .SH EXAMPLES .EX #include #include #include #include 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\en", 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); } .EE .SH SEE ALSO .BR insque (3), .BR queue (7) .SH COLOPHON This page is part of release 5.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.