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, accessi-
       ble 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.

CONFORMING TO
       POSIX.1-2001.

NOTES
       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.

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

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                                                        2001-11-28                                                    FTOK(3)