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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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INSQUE(3P) POSIX Programmer's Manual INSQUE(3P)
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.
insque, remque — insert or remove an element in a queue
#include <search.h>
void insque(void *element, void *pred);
void remque(void *element);
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.
The insque() and remque() functions do not return a value.
No errors are defined.
The following sections are informative.
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);
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>).
None.
None.
The Base Definitions volume of POSIX.1‐2017, search.h(0p)
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(3P)
Pages that refer to this page: search.h(0p), remque(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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