FTOK(3) Linux Programmer's Manual FTOK(3)
ftok - convert a pathname and a project identifier to a System V IPC key
# include <sys/types.h>
# include <sys/ipc.h>
key_t ftok(const char *pathname, int proj_id);
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.
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.
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 char-
acter 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.
ipc(5), msgget(2), semget(2), shmget(2), stat(2)
Linux 2.4 2001-11-28 FTOK(3)