.\" peter memishian -- meem@gnu.ai.mit.edu
.\" $Id: insque.3,v 1.2 1996/10/30 21:03:39 meem Exp meem $
+.\" and Copyright (c) 2010, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" References consulted:
.\" Linux libc source code (5.4.7)
.\" Curry's "UNIX Systems Programming for SVR4" (O'Reilly & Associates 1996)
.\"
.\" Changed to POSIX, 2003-08-11, aeb+wh
+.\" mtk, 2010-09-09: Noted glibc 2.4 bug, added info on circular
+.\" lists, added example program
.\"
-.TH INSQUE 3 2003-08-11 "" "Linux Programmer's Manual"
+.TH INSQUE 3 2017-09-15 "" "Linux Programmer's Manual"
.SH NAME
insque, remque \- insert/remove an item from a queue
.SH SYNOPSIS
.nf
.B #include <search.h>
-.sp
+.PP
.BI "void insque(void *" elem ", void *" prev );
+.PP
.BI "void remque(void *" elem );
+.fi
+.PP
+.in -4n
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.in
+.PP
+.ad l
+.BR insque (),
+.BR remque ():
+.RS 4
+_XOPEN_SOURCE\ >=\ 500
+.\" || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
+ || /* Glibc since 2.19: */ _DEFAULT_SOURCE
+ || /* Glibc versions <= 2.19: */ _SVID_SOURCE
+.RE
+.ad
.SH DESCRIPTION
+The
.BR insque ()
and
.BR remque ()
-are functions for manipulating
-doubly-linked lists.
+functions manipulate doubly-linked lists.
Each element in the list is a structure of
-which the first two structure elements are a forward and a
+which the first two elements are a forward and a
backward pointer.
-
+The linked list may be linear (i.e., NULL forward pointer at
+the end of the list and NULL backward pointer at the start of the list)
+or circular.
+.PP
+The
.BR insque ()
-inserts the element pointed to by \fIelem\fP
-immediately after the element pointed to by \fIprev\fP, which must
-not be NULL.
-
+function inserts the element pointed to by \fIelem\fP
+immediately after the element pointed to by \fIprev\fP.
+.PP
+If the list is linear, then the call
+.I "insque(elem, NULL)"
+can be used to insert the initial list element,
+and the call sets the forward and backward pointers of
+.I elem
+to NULL.
+.PP
+If the list is circular,
+the caller should ensure that the forward and backward pointers of the
+first element are initialized to point to that element,
+and the
+.I prev
+argument of the
+.BR insque ()
+call should also point to the element.
+.PP
+The
.BR remque ()
-removes the element pointed to by \fIelem\fP from the
+function removes the element pointed to by \fIelem\fP from the
doubly-linked list.
-.SH "CONFORMING TO"
-POSIX.1-2001
-.SH "NOTES"
-Traditionally (e.g., SunOS, Linux libc 4,5) the parameters of these
-functions were of type \fIstruct qelem *\fP, where the struct
-is defined as
-
-.RS
-.nf
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface Attribute Value
+T{
+.BR insque (),
+.BR remque ()
+T} Thread safety MT-Safe
+.TE
+.sp 1
+.SH CONFORMING TO
+POSIX.1-2001, POSIX.1-2008.
+.SH NOTES
+On ancient systems,
+.\" e.g., SunOS, Linux libc4 and libc5
+the arguments of these functions were of type \fIstruct qelem *\fP,
+defined as:
+.PP
+.in +4n
+.EX
struct qelem {
- struct qelem *q_forw;
- struct qelem *q_back;
- char q_data[1];
+ struct qelem *q_forw;
+ struct qelem *q_back;
+ char q_data[1];
};
-.fi
-.RE
-
+.EE
+.in
+.PP
This is still what you will get if
.B _GNU_SOURCE
is defined before
including \fI<search.h>\fP.
-
+.PP
The location of the prototypes for these functions differs among several
versions of UNIX.
The above is the POSIX version.
Some systems place them in \fI<string.h>\fP.
-Linux libc4,5 placed them
-in \fI<stdlib.h>\fP.
+.\" Linux libc4 and libc 5 placed them
+.\" in \fI<stdlib.h>\fP.
+.SH BUGS
+In glibc 2.4 and earlier, it was not possible to specify
+.I prev
+as NULL.
+Consequently, to build a linear list, the caller had to build a list
+using an initial call that contained the first two elements of the list,
+with the forward and backward pointers in each element suitably initialized.
+.SH EXAMPLE
+The program below demonstrates the use of
+.BR insque ().
+Here is an example run of the program:
+.PP
+.in +4n
+.EX
+.RB "$ " "./a.out -c a b c"
+Traversing completed list:
+ a
+ b
+ c
+That was a circular list
+.EE
+.in
+.SS Program source
+\&
+.EX
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <search.h>
+
+struct element {
+ struct element *forward;
+ struct element *backward;
+ char *name;
+};
+
+static struct element *
+new_element(void)
+{
+ struct element *e;
+
+ e = malloc(sizeof(struct element));
+ if (e == NULL) {
+ fprintf(stderr, "malloc() failed\en");
+ exit(EXIT_FAILURE);
+ }
+
+ return e;
+}
+
+int
+main(int argc, char *argv[])
+{
+ struct element *first, *elem, *prev;
+ int circular, opt, errfnd;
+
+ /* The "\-c" command\-line option can be used to specify that the
+ list is circular */
+
+ errfnd = 0;
+ circular = 0;
+ while ((opt = getopt(argc, argv, "c")) != \-1) {
+ switch (opt) {
+ case 'c':
+ circular = 1;
+ break;
+ default:
+ errfnd = 1;
+ break;
+ }
+ }
+
+ if (errfnd || optind >= argc) {
+ fprintf(stderr, "Usage: %s [\-c] string...\en", argv[0]);
+ exit(EXIT_FAILURE);
+ }
+
+ /* Create first element and place it in the linked list */
+
+ elem = new_element();
+ first = elem;
+
+ elem\->name = argv[optind];
+
+ if (circular) {
+ elem\->forward = elem;
+ elem\->backward = elem;
+ insque(elem, elem);
+ } else {
+ insque(elem, NULL);
+ }
+
+ /* Add remaining command\-line arguments as list elements */
+
+ while (++optind < argc) {
+ prev = elem;
+
+ elem = new_element();
+ elem\->name = argv[optind];
+ insque(elem, prev);
+ }
+
+ /* Traverse the list from the start, printing element names */
+
+ printf("Traversing completed list:\en");
+ elem = first;
+ do {
+ printf(" %s\en", elem\->name);
+ elem = elem\->forward;
+ } while (elem != NULL && elem != first);
+
+ if (elem == first)
+ printf("That was a circular list\en");
+
+ exit(EXIT_SUCCESS);
+}
+.EE
+.SH SEE ALSO
+.BR queue (3)
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