ftok

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
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