FTOK
Section: Linux Programmer's Manual (3)
Updated: 2015-08-08
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NAME
ftok - convert a pathname and a project identifier to a System V IPC key
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
key_t ftok(const char *pathname, int proj_id);
DESCRIPTION
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.
RETURN VALUE
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.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
ftok()
| Thread safety | MT-Safe
|
CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
NOTES
On some ancient systems, 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.
SEE ALSO
msgget(2),
semget(2),
shmget(2),
stat(2),
svipc(7)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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