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FTOK(3)                    Linux Programmer's Manual                   FTOK(3)

       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 non-zero) 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  dif-

       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 com-
       bines 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

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

GNU                               2001-11-28                           FTOK(3)