# insque

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           insque, remque — insert or remove an element in a queue
    


## SYNOPSIS



           #include <search.h>
    
           void insque(void *_element_ , void *_pred_);
           void remque(void *_element_);
    


## DESCRIPTION



           The _insque_() and _remque_() functions shall manipulate queues built
           from doubly-linked lists.  The queue can be either circular or
           linear. An application using _insque_() or _remque_() shall ensure it
           defines a structure in which the first two members of the
           structure are pointers to the same type of structure, and any
           further members are application-specific. The first member of the
           structure is a forward pointer to the next entry in the queue. The
           second member is a backward pointer to the previous entry in the
           queue.  If the queue is linear, the queue is terminated with null
           pointers. The names of the structure and of the pointer members
           are not subject to any special restriction.
    
           The _insque_() function shall insert the element pointed to by
           _element_ into a queue immediately after the element pointed to by
           _pred_.
    
           The _remque_() function shall remove the element pointed to by
           _element_ from a queue.
    
           If the queue is to be used as a linear list, invoking
           _insque_(&_element_ , NULL), where _element_ is the initial element of
           the queue, shall initialize the forward and backward pointers of
           _element_ to null pointers.
    
           If the queue is to be used as a circular list, the application
           shall ensure it initializes the forward pointer and the backward
           pointer of the initial element of the queue to the element's own
           address.
    


## RETURN VALUE



           The _insque_() and _remque_() functions do not return a value.
    


## ERRORS



           No errors are defined.
    
           _The following sections are informative._
    


## EXAMPLES



       **Creating a Linear Linked List**
           The following example creates a linear linked list.
    
               #include <search.h>
               ...
               struct myque element1;
               struct myque element2;
    
               char *data1 = "DATA1";
               char *data2 = "DATA2";
               ...
               element1.data = data1;
               element2.data = data2;
    
               insque (&element1, NULL);
               insque (&element2, &element1);
    
       **Creating a Circular Linked List**
           The following example creates a circular linked list.
    
               #include <search.h>
               ...
               struct myque element1;
               struct myque element2;
    
               char *data1 = "DATA1";
               char *data2 = "DATA2";
               ...
               element1.data = data1;
               element2.data = data2;
    
               element1.fwd = &element1;
               element1.bck = &element1;
    
               insque (&element2, &element1);
    
       **Removing an Element**
           The following example removes the element pointed to by _element1_.
    
               #include <search.h>
               ...
               struct myque element1;
               ...
               remque (&element1);
    


## APPLICATION USAGE



           The historical implementations of these functions described the
           arguments as being of type **struct qelem *** rather than as being of
           type **void *** as defined here. In those implementations, **struct**
           **qelem** was commonly defined in _< search.h>_ as:
    
               struct qelem {
                   struct qelem  *q_forw;
                   struct qelem  *q_back;
               };
    
           Applications using these functions, however, were never able to
           use this structure directly since it provided no room for the
           actual data contained in the elements. Most applications defined
           structures that contained the two pointers as the initial elements
           and also provided space for, or pointers to, the object's data.
           Applications that used these functions to update more than one
           type of table also had the problem of specifying two or more
           different structures with the same name, if they literally used
           **struct qelem** as specified.
    
           As described here, the implementations were actually expecting a
           structure type where the first two members were forward and
           backward pointers to structures. With C compilers that didn't
           provide function prototypes, applications used structures as
           specified in the DESCRIPTION above and the compiler did what the
           application expected.
    
           If this method had been carried forward with an ISO C standard
           compiler and the historical function prototype, most applications
           would have to be modified to cast pointers to the structures
           actually used to be pointers to **struct qelem** to avoid compilation
           warnings. By specifying **void *** as the argument type, applications
           do not need to change (unless they specifically referenced **struct**
           **qelem** and depended on it being defined in _< search.h>_).
    


## RATIONALE



           None.
    


## FUTURE DIRECTIONS



           None.
    


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