.\" Copyright (c) 1994 SigmaSoft, Th. Lockert <tholo@sigmasoft.com>
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.\" $FreeBSD: src/lib/libc/gen/ftok.3,v 1.17 2009/07/13 12:53:43 trasz Exp $
.Dd July 9, 2009
.Os
.Dt FTOK 3
.Sh NAME
.Nm ftok
.Nd create IPC identifier from path name
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/ipc.h
.Ft key_t
.Fn ftok "const char *path" "int id"
.Sh DESCRIPTION
The
.Fn ftok
function attempts to create a unique key suitable for use with the
.Xr semget 2 ,
and
.Xr shmget 2
functions, given the
.Fa path
of an existing file and a user-selectable
.Fa id .
.Pp
The specified
.Fa path
must specify an existing file that is accessible to the calling process
or the call will fail.
Also, note that links to files will return the
same key, given the same
.Fa id .
.Sh RETURN VALUES
The
.Fn ftok
function will return -1 if
.Fa path
does not exist or if it cannot be accessed by the calling process.
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ipc.h>
.Pp
The include file
.In sys/types.h
is necessary.
.Sh SEE ALSO
.Xr semget 2 ,
.Xr shmget 2 ,
.Xr compat 5
.Sh HISTORY
The
.Fn ftok
function originates with System V and is typically used by programs
that use the System V IPC routines.
.Sh AUTHORS
.An Thorsten Lockert Aq tholo@sigmasoft.com
.Sh BUGS
The returned key is computed based on the device minor number and inode
of the specified
.Fa path ,
in combination with the lower 8 bits of the given
.Fa id .
Thus, it is quite possible for the routine to return duplicate keys.