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NAME

       insque, remque - insert/remove an item from a queue

SYNOPSIS

       #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

DESCRIPTION

       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.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├───────────────────┼───────────────┼─────────┤
       │insque(), remque() │ Thread safety │ MT-Safe │
       └───────────────────┴───────────────┴─────────┘

CONFORMING TO

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

NOTES

       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>.

BUGS

       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 initialized.

EXAMPLE

       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:
               a
               b
               c
           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 *
       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);
       }

SEE ALSO

       queue(3)

COLOPHON

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                                            2019-03-06                                  INSQUE(3)