insque(3p) — Linux manual page


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

PROLOG         top

       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         top

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

SYNOPSIS         top

       #include <search.h>

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

DESCRIPTION         top

       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

       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

RETURN VALUE         top

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

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

   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. 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         top




SEE ALSO         top

       The Base Definitions volume of POSIX.1‐2017, search.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                        INSQUE(3P)

Pages that refer to this page: search.h(0p)remque(3p)