redhat man page for ftok

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.

CONFORMING TO
       XPG4

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 behaviour 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 i-node 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
       ipc(5), msgget(2), semget(2), shmget(2), stat(2)

Linux 2.4							    2001-11-28								   FTOK(3)