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.
Under libc4 and libc5 (and under SunOS 4.x) the prototype was
key_t ftok(char *pathname,
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 behaviour 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 /dev/sda1.