ftok.3

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)ftok.3	8.1	9/11/90
.TH ftok 3
.SH Name
ftok \- standard interprocess communication package
.SH Syntax
.nf
.B #include <sys/types.h>
.B #include <sys/ipc.h>
.PP
.B key_t ftok(path, id)
.B char \(**path;
.B char id;
.fi
.SH Description
.NXR "ftok subroutine"
.NXR "interprocess communication package"
All interprocess communication facilities
require the user to supply a key
to be used by the 
.MS msgget 2 ,
.MS semget 2 ,
and
.MS shmget 2
system calls to obtain interprocess communication identifiers.
One suggested method for forming a key
is to use the
.PN ftok ,
file to key,
subroutine described below.
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.
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.
.PP
The
.PN ftok
subroutine returns a key based on
.I path
and
.I id
that is usable in subsequent
.PN msgget ,
.PN semget ,
and
.PN shmget
system calls.
The
.I path
must be the path name
of an existing file
that is accessible
to the process.  The
.I id
is a character
which uniquely identifies
a project.
Note that
.PN ftok
will return the same key
for linked files
when called with the same
.I id
and that it will return
different keys when
called with the same file name
but different
.IR ids .
.SH Return Values
The
.PN ftok
subroutine returns
.B "(key_t) \-1"
if \fIpath\fP
does not exist
or if it is not accessible
to the process.
.SH Warning
If the file whose \fIpath\fP is passed
to
.PN ftok
is removed when keys still refer to the file,
future calls to
.PN ftok
with the same \fIpath\fP and \fIid\fP
will return an error.
If the same file is recreated,
then
.PN ftok
is likely to return a different key
than it did the original time it was called.
.SH See Also
intro(2), msgget(2), semget(2), shmget(2)