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.\" 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>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"   Linux libc source code (5.4.7)
.\"   Solaris 2.x, OSF/1, and HP-UX manpages
.\"   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  2021-03-22 "" "Linux Programmer's Manual"
.SH NAME
insque, remque \- insert/remove an item from a queue
.SH SYNOPSIS
.nf
.B #include <search.h>
.PP
.BI "void insque(void *" elem ", void *" prev );
.BI "void remque(void *" elem );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR insque (),
.BR remque ():
.nf
    _XOPEN_SOURCE >= 500
.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
        || /* Glibc since 2.19: */ _DEFAULT_SOURCE
        || /* Glibc <= 2.19: */ _SVID_SOURCE
.fi
.SH DESCRIPTION
The
.BR insque ()
and
.BR remque ()
functions manipulate doubly linked lists.
Each element in the list is a structure of
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 ()
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 ()
function removes the element pointed to by \fIelem\fP from the
doubly linked list.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR insque (),
.BR remque ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.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];
};
.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 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 EXAMPLES
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 = malloc(sizeof(*e));
    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 \(aqc\(aq:
            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 (7)