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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.

SEE ALSO

       The Base Definitions volume of POSIX.1‐2008, <search.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications  Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
       and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In  the  event
       of  any  discrepancy between this version and the original IEEE and The Open Group Standard, the original
       IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at
       http://www.unix.org/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .