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)):
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 |
| 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#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)