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

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

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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.

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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).