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

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing 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 ini-
       tialize 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 DESCRIP-
       TION 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 Technol-
       ogy -- 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 .

IEEE
/The Open Group						       2003								INSQUE(3P)