ftok - convert a pathname and a project identifier to a System V IPC key
key_t ftok ( char *pathname, char proj )
The function converts the pathname of an existing accessible
file and a project identifier into a key_t type
System V IPC key.
On success the return value will be the converted
key_t value, otherwise -1 with errno
indicating the error as for the stat(2) system
The generated key_t value is obtained stat-ing
the disk file corresponding to pathname in order to
get its i-node number and the minor device number of the
filesystem on which the disk file resides, then by combining
the 8 bit proj value along with the lower 16 bits of
the i-node number, along with the 8 bits of the minor device
number. The algorithm does not guarantee a unique key value.
Two different names linking to the same file produce same
Using the lower 16 bits of the i-node number, gives some
chance (also usually small) to have same key values for file
names referring to different i-nodes.
Not discriminating among major device numbers, gives some
chance of collision (also usually small) for systems with
multiple disk controllers.
ipc(5), msgget(2), semget(2),