Sun Microsystems, Inc. gratefully acknowledges The Open Group for
permission to reproduce portions of its copyrighted documentation.
Original documentation from The Open Group can be obtained online at
http://www.opengroup.org/bookstore/.

The Institute of Electrical and Electronics Engineers and The Open
Group, have given us permission to reprint portions of their
documentation.

In the following statement, the phrase ``this text'' refers to portions
of the system documentation.

Portions of this text are reprinted and reproduced in electronic form
in the SunOS Reference Manual, from IEEE Std 1003.1, 2004 Edition,
Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6,
Copyright (C) 2001-2004 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy
between these versions 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.

This notice shall appear on any product containing this material.

The contents of this file are subject to the terms of the
Common Development and Distribution License (the "License").
You may not use this file except in compliance with the License.

You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
or http://www.opensolaris.org/os/licensing.
See the License for the specific language governing permissions
and limitations under the License.

When distributing Covered Code, include this CDDL HEADER in each
file and include the License file at usr/src/OPENSOLARIS.LICENSE.
If applicable, add the following below this CDDL HEADER, with the
fields enclosed by brackets "[]" replaced with your own identifying
information: Portions Copyright [yyyy] [name of copyright owner]


Copyright 1989 AT&T
Portions Copyright (c) 1992, X/Open Company Limited All Rights Reserved
Copyright (c) 2002, Sun Microsystems, Inc. All Rights Reserved.

ftok - generate an IPC key

#include <sys/ipc.h>

key_t ftok(const char *path, int id);

The ftok() function returns a key based on path and id that is usable in subsequent calls to msgget(2), semget(2) and shmget(2). The path argument must be the pathname of an existing file that the process is able to stat(2).

The ftok() function will return the same key value for all paths that name the same file, when called with the same id value, and will return different key values when called with different id values.

If the file named by path is removed while still referred to by a key, a call to ftok() with the same path and id returns an error. If the same file is recreated, then a call to ftok() with the same path and id is likely to return a different key.

Only the low order 8-bits of id are significant. The behavior of ftok() is unspecified if these bits are 0.

Upon successful completion, ftok() returns a key. Otherwise, ftok() returns (key_t)-1 and sets errno to indicate the error. The ftok() function will fail if: EACCES ELOOP ENAMETOOLONG ENOENT ENOTDIR

The ftok() function may fail if: ENAMETOOLONG

For maximum portability, id should be a single-byte character.

Another way to compose keys is to include the project ID in the most significant byte and to use the remaining portion as a sequence number. There are many other ways to form keys, but it is necessary for each system to define standards for forming them. If some standard is not adhered to, it will be possible for unrelated processes to unintentionally interfere with each other's operation. It is still possible to interfere intentionally. Therefore, it is strongly suggested that the most significant byte of a key in some sense refer to a project so that keys do not conflict across a given system.

Since the ftok() function returns a value based on the id given and the file serial number of the file named by path in a type that is no longer large enough to hold all file serial numbers, it may return the same key for paths naming different files on large filesystems. See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE

Interface Stability Standard

MT-Level MT-Safe

msgget (2), semget (2), shmget (2), stat (2), attributes (7), standards (7)