#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():
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.
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>.
$ ./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);
}