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INSQUE(3P)                 POSIX Programmer's Manual                INSQUE(3P)

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

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

       #include <search.h>

       void insque(void *element, void *pred);
       void remque(void *element);

       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 mem-
       bers 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 ele-
       ment 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.

       The insque() and remque() functions do not return a value.

       No errors are defined.

       The following sections are informative.

   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";
     = data1;
     = 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";
     = data1;
     = 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);

       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.  Applica-
       tions 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

       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>).



       The Base Definitions volume of IEEE Std 1003.1-2001, <search.h>

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,  Inc  and  The
       Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Stan-
       dard, the original IEEE and The Open Group Standard is the referee  document.  The  original  Standard  can  be
       obtained online at .

IEEE/The Open Group                  2003                           INSQUE(3P)