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INSQUE(3) Linux Programmer's Manual INSQUE(3)
insque, remque - insert/remove an item from a queue
#include <search.h>
void insque(void *elem, void *prev);
void remque(void *elem);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
insque(), remque():
_SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
The insque() and 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.
The insque() function inserts the element pointed to by elem immediately after
the element pointed to by prev.
If the list is linear, then the call insque(elem, NULL) can be used to insert
the initial list element, and the call sets the forward and backward pointers
of elem to NULL.
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 prev argument of the insque() call should also point to the
element.
The remque() function removes the element pointed to by elem from the doubly-
linked list.
POSIX.1-2001.
Traditionally (e.g., SunOS, Linux libc 4 and libc 5), the arguments of these
functions were of type struct qelem *, defined as:
struct qelem {
struct qelem *q_forw;
struct qelem *q_back;
char q_data[1];
};
This is still what you will get if _GNU_SOURCE is defined before including
<search.h>.
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
<string.h>. Linux libc4 and libc 5 placed them in <stdlib.h>.
In glibc 2.4 and earlier, it was not possible to specify 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.
The program below demonstrates the use of insque(). Here is an example run of
the program:
$ ./a.out -c a b c
Traversing completed list:
a
b
c
That was a circular list
#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\n");
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...\n", 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:\n");
elem = first;
do {
printf(" %s\n", elem->name);
elem = elem->forward;
} while (elem != NULL && elem != first);
if (elem == first)
printf("That was a circular list\n");
exit(EXIT_SUCCESS);
}
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
2010-09-09 INSQUE(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface