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.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.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.\" manual page may be incorrect or out-of-date.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.manpages@gmail.com>
.\" Changed data type of proj_id; minor fixes
.\" aeb: further fixes; added notes.
.\"
.TH FTOK 3 2001-11-28 "GNU" "Linux Programmer's Manual"
.SH NAME
ftok \- convert a pathname and a project identifier to a System V IPC key
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/ipc.h>
.fi
.sp
.BI "key_t ftok(const char *" pathname ", int " proj_id );
.SH DESCRIPTION
The
.BR ftok ()
function uses the identity of the file named by the given
.I pathname
(which must refer to an existing, accessible file)
and the least significant 8 bits of
.I proj_id
(which must be nonzero) to generate a
.I key_t
type System V IPC key, suitable for use with
.BR msgget (2),
.BR semget (2),
or
.BR shmget (2).
.LP
The resulting value is the same for all pathnames that
name the same file, when the same value of
.I proj_id
is used.
The value returned should be different when the
(simultaneously existing) files or the project IDs differ.
.SH "RETURN VALUE"
On success the generated
.I key_t
value is returned.
On failure \-1 is returned, with
.I errno
indicating the error as for the
.BR stat (2)
system call.
.SH "CONFORMING TO"
POSIX.1-2001.
.SH NOTES
Under libc4 and libc5 (and under SunOS 4.x) the prototype was:
.sp
.RS
.BI "key_t ftok(char *" pathname ", char " proj_id );
.RE
.PP
Today
.I proj_id
is an
.IR int ,
but still only 8 bits are used.
Typical usage has an ASCII character
.IR proj_id ,
that is why the behavior is said to be undefined when
.I proj_id
is zero.
.LP
Of course no guarantee can be given that the resulting
.I key_t
is unique.
Typically, a best effort attempt combines the given
.I 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
.I /dev/hda1
and files on
.IR /dev/sda1 .
.SH "SEE ALSO"
.BR msgget (2),
.BR semget (2),
.BR shmget (2),
.BR stat (2),
.BR svipc (7)
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