# insque

## NAME



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


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## 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 >= 2.19: */ _DEFAULT_SOURCE
                   || /* glibc <= 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).
           ┌──────────────────────────────────────┬───────────────┬─────────┐
           │ **Interface** │ **Attribute** │ **Value** │
           ├──────────────────────────────────────┼───────────────┼─────────┤
           │ **insque**(), **remque**()                   │ Thread safety │ MT-Safe │
           └──────────────────────────────────────┴───────────────┴─────────┘
    


## VERSIONS



           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>_.
    


## STANDARDS



           POSIX.1-2008.
    


## 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.
    


## EXAMPLES



           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 <search.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <unistd.h>
    
           struct element {
               struct element *forward;
               struct element *backward;
               char *name;
           };
    
           static struct element *
           new_element(void)
           {
               struct element *e;
    
               e = malloc(sizeof(*e));
               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);
           }
    


* * *