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NAME | SYNOPSIS | DESCRIPTION | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
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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():
_XOPEN_SOURCE >= 500
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 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 │
└───────────────────┴───────────────┴─────────┘
POSIX.1-2001, POSIX.1-2008.
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 sev‐
eral versions of UNIX. The above is the POSIX version. Some systems
place them in <string.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
Program source
#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);
}
queue(3)
This page is part of release 4.15 of the Linux 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/.
2017-09-15 INSQUE(3)
Copyright and license for this manual page
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