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       ftok - convert a pathname and a project identifier to a System V IPC key


       #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.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue   │
       │ftok()                                                         │ Thread safety │ MT-Safe │


       POSIX.1-2001, POSIX.1-2008.


       On some ancient systems, 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.


       See semget(2).


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


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