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 .