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():
           _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 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  at-

       |Interface          | Attribute     | Value   |
       |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  in-
       cluding <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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                                  2019-03-06                         INSQUE(3)
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