ftok

Purpose

     Generates a standard interprocess communication key.

Library

     Standard C Library (libc.a)

Syntax

     #include <sys/types.h>
     #include <sys/ipc.h>

     key_t ftok (path, id)
     char *path;
     char id;

Description

     The ftok subroutine returns a  key, based on the path and
     id parameters, to be used to obtain interprocess communi-
     cation identifiers.  The path  parameter must be the path
     name  of  an existing  file  that  is accessible  to  the
     process.   The  id parameter  must  be  a character  that
     uniquely identifies a project.  ftok returns the same key
     for linked  files if called  with the same  id parameter.
     Different keys  are returned  for the  same file  if dif-
     ferent id parameters are used.

     All interprocess communication  facilities require you to
     supply a  key to  the msgget,  semget, and  shmget system
     calls in order to obtain interprocess communication iden-
     tifiers.  The ftok subroutine provides one method of cre-
     ating keys, but many others are possible.  Another way to
     do this,  for example, is  to use  the project ID  as the
     most  significant  byte  of  the  key,  and  to  use  the
     remaining portion as a sequence number.

     Warning:  It is important for each installation to define
     standards  for forming  keys.   If some  standard is  not
     adhered to,  then it is possible  for unrelated processes
     to interfere with each other's operation.

     Warning:  If  the path  parameter of the  ftok subroutine
     names a file that has been removed while keys still refer
     it, then the  ftok subroutine returns an  error.  If that
     file is then recreated, the ftok subroutine will probably
     return a different key than the original one.

Return Value

     Upon successful completion,  the ftok  subroutine returns
     a key that can be passed to the msgget, semget, or shmget
     system call.   The ftok subroutine returns  (key_t) -1 if
     one or more of the following are true:

     o   The file named by the path parameter does not exist.
     o   The file  named by the  path parameter is  not acces-
         sible to the process.
     o   The id parameter is 0 ("'\0'").

Related Information

     In this book:  "msgget," "semget," and "shmget."