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).