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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

       POSIX.1-2001.

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 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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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.