#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 ||

       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  immedi-
       ately 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.


       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 sev-
       eral versions of UNIX.  The above is the POSIX version.   Some  systems
       run of the program:

           $ ./a.out -c a b c
           Traversing completed list:
           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 *
           struct element *e;

           e = malloc(sizeof(struct element));
           if (e == NULL) {
               fprintf(stderr, "malloc() failed\n");

           return e;

       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;
                   errfnd = 1;

           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");


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       description of the project, and information about reporting  bugs,  can
       be found at

                                  2010-09-09                         INSQUE(3)
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