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():
           _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.

       For   an   explanation   of   the  terms  used  in  this  section,  see

       |Interface          | Attribute     | Value   |
       |insque(), remque() | Thread safety | MT-Safe |

       POSIX.1-2001, POSIX.1-2008.

       Traditionally (e.g., SunOS, Linux libc4 and libc5),  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

       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:
           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;

           if (errfnd || optind >= argc) {
               fprintf(stderr,  "Usage: %s [-c] string...\n", argv[0]);

           /* 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");


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                                  2015-08-08                         INSQUE(3)
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