FTOK

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 ( 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 call.

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. In fact

•

Two different names linking to the same file produce same key values.

•

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), shmget(2), stat(2).