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PROLOG
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
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 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 .