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



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

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 aa 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 aa 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.1-2017, <search.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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 .



IEEE/The Open Group                  2017                            INSQUE(3)
