ftok

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.

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



Linux 2.4                         2001-11-28                           FTOK(3)