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

   │InterfaceAttributeValue   │
   │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
   several 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");



   This page is part of release 4.09 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

                              2016-12-12                         INSQUE(3)

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