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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 .