NAME
ftok - convert a pathname and a project identifier to a System V IPC key
SYNOPSIS
# include <sys/types.h>
# include <sys/ipc.h>
I key_t ftok(const char * pathname , int proj_id );
DESCRIPTION
The
R 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
I key_t ftok(char * pathname , char proj_id );
Today
proj_id
is an
R int ,
but still only 8 bits are used.
Typical usage has an ASCII character
R 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 i-node 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
R /dev/sda1 .
SEE ALSO
ČLÁNKY DO MAILU