ftok(3) [netbsd man page]

FTOK(3) 						   BSD Library Functions Manual 						   FTOK(3)

NAME

     ftok -- create IPC identifier from path name

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/types.h>
     #include <sys/ipc.h>

     key_t
     ftok(const char *path, int id);

DESCRIPTION

     The ftok() function attempts to create a unique key suitable for use with the msgget(2), semget(2) and shmget(2) functions given the path of
     an existing file and a user-selectable id.

     The specified path must specify an existing file that is accessible to the calling process or the call will fail.	Also, note that links to
     files will return the same key, given the same id.

     Only the 8 least significant bits of id are used in the key generation; the rest of the bits are ignored.

RETURN VALUES

     The ftok() function will return ((key_t)-1) if path does not exist or if it cannot be accessed by the calling process.

SEE ALSO

     msgget(2), semget(2), shmget(2)

STANDARDS

     The ftok() function conforms to IEEE Std 1003.1 (``POSIX.1'').

HISTORY

     The ftok() function originated with System V and is typically used by programs that use the System V IPC routines.

AUTHORS

     Thorsten Lockert <tholo@sigmasoft.com>

BUGS

     The returned key is computed based on the device and inode of the specified path in combination with the given id.  Thus it is quite possible
     for the routine to return duplicate keys given that those fields are not 8- and 16-bit quantities like they were on System V based systems
     where this library routine's ancestor were originally created.

BSD
								  April 27, 2010							       BSD

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)