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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.

SEE ALSO

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

COLOPHON

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.



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