FTOK(K)             Linux Programmer's Manual             FTOK(K)



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(t),
       semget(t), or shmget(t).

       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 fail-
       ure -1 is returned, with errno indicating the error as for
       the stat(t) 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(c), msgget(t), semget(t), shmget(t), stat(t)



Linux 2.4                   2001-11-28                    FTOK(K)