ftok — convert a pathname and a project identifier to a System V IPC key
#include <sys/types.h> #include <sys/ipc.h>
key_t ftok( |
const char *pathname, |
int proj_id); |
The ftok() function uses the
identity of the file named by the given pathname (which must refer to
an existing, accessible file) and the least significant 8
bits of proj_id
(which must be nonzero) to generate a key_t type System V IPC key, suitable for use
with msgget(2), semget(2), or shmget(2).
The resulting value is the same for all pathnames that
name the same file, when the same value of proj_id is used. The value
returned should be different when the (simultaneously
existing) files or the project IDs differ.
On success the generated key_t
value is returned. On failure −1 is returned, with
errno indicating the error as
for the stat(2) system call.
Under libc4 and libc5 (and under SunOS 4.x) the prototype was:
key_t ftok(char *pathname,char proj_id);
Today proj_id is
an int, but still only 8 bits are
used. Typical usage has an ASCII character proj_id, that is why the
behavior is said to be undefined when proj_id is zero.
Of course no guarantee can be given that the resulting
key_t is unique. Typically, a best
effort attempt combines the given proj_id byte, the lower 16 bits
of the inode number, and the lower 8 bits of the device
number into a 32-bit result. Collisions may easily happen,
for example between files on /dev/hda1 and files on /dev/sda1.
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. Modified 2001-11-28, by Michael Kerrisk, <mtk.manpagesgmail.com> Changed data type of proj_id; minor fixes aeb: further fixes; added notes. |