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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  non-zero)  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

       msgget(2), semget(2), shmget(2), stat(2), ipc(5)