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 IEEE Std 1003.1-2001, <search.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1,  2003  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications 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
       Standard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .