FTOK(3) Linux Programmer’s Manual FTOK(3)

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 ( char *pathname, char proj )

DESCRIPTION
The function converts the pathname of an existing accessi-
ble file and a project identifier into a key_t type System
V IPC key.

RETURN VALUE
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 call.

BUGS
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 num-
ber, along with the 8 bits of the minor device number.
The algorithm does not guarantee a unique key value. In
fact

o Two different names linking to the same file pro-
duce same key values.

o 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.

o Not discriminating among major device numbers,
gives some chance of collision (also usually small)
for systems with multiple disk controllers.

SEE ALSO
ipc(5), msgget(2), semget(2), shmget(2), stat(2).

Linux 0.99.13 November 1, 1993 1