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       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
       character proj_id, that is why the behavior 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 inode 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.


       msgget(2), semget(2), shmget(2), stat(2), svipc(7)


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