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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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insque(3) Library Functions Manual insque(3)
insque, remque - insert/remove an item from a queue
Standard C library (libc, -lc)
#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():
_XOPEN_SOURCE >= 500
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE
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.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ insque(), remque() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
On ancient systems, 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>.
POSIX.1-2008.
POSIX.1-2001.
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
Program source
#include <search.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
struct element {
struct element *forward;
struct element *backward;
char *name;
};
static struct element *
new_element(void)
{
struct element *e;
e = malloc(sizeof(*e));
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);
}
queue(7)
This page is part of the man-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 is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 insque(3)
Pages that refer to this page: circleq(3), list(3), slist(3), stailq(3), tailq(3), queue(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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