FTOK(3)                             Linux Programmer's Manual                             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(const char *pathname, int proj_id);

DESCRIPTION
       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.

RETURN VALUE
       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.

ATTRIBUTES
   Multithreading (see pthreads(7))
       The ftok() function is thread-safe.

CONFORMING TO
       POSIX.1-2001.

NOTES
       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 num‐
       ber, and the lower 8 bits of the device number into a 32-bit result.  Collisions may  eas‐
       ily happen, for example between files on /dev/hda1 and files on /dev/sda1.

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

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.



GNU                                         2014-08-19                                    FTOK(3)