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 non-zero) 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.
CONFORMING TO
POSIX.1-2001.
NOTES
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 3.14 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/.
