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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.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     +----------+---------------+---------+
     |Interface | Attribute     | Value   |
     +----------+---------------+---------+
     |ftok()    | Thread safety | MT-Safe |
     +----------+---------------+---------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008.

NOTES

     On some ancient systems, 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.