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.

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

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       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.