stdipc(3C)          (C Programming Language Utilities)           stdipc(3C)


   NAME
         stdipc: ftok - standard interprocess communication package

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

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

   DESCRIPTION
         All interprocess communication facilities require the user to supply
         a key to be used by the msgget(2), semget(2), and shmget(2) system
         calls to obtain interprocess communication identifiers.  One
         suggested method for forming a key is to use the ftok subroutine
         described below.  Another way to compose keys is to include the
         project ID in the most significant byte and to use the remaining
         portion as a sequence number.  There are many other ways to form
         keys, but it is necessary for each system to define standards for
         forming them.  If some standard is not adhered to, it will be
         possible for unrelated processes to unintentionally interfere with
         each other's operation.  It is still possible to interface
         intentionally.  Therefore, it is strongly suggested that the most
         significant byte of a key in some sense refer to a project so that
         keys do not conflict across a given system.

         ftok returns a key based on path and id that is usable in subsequent
         msgget, semget, and shmget system calls.  path must be the path name
         of an existing file that is accessible to the process.  id is a
         character that uniquely identifies a project.  Note that ftok will
         return the same key for linked files when called with the same id and
         that it will return different keys when called with the same file
         name but different ids.

   SEE ALSO
         intro(2), msgget(2), semget(2), shmget(2).

   DIAGNOSTICS
         ftok returns (key_t) -1 if path does not exist or if it is not
         accessible to the process.

   NOTES
         If the file whose path is passed to ftok is removed when keys still
         refer to the file, future calls to ftok with the same path and id
         will return an error.  If the same file is recreated, then ftok is
         likely to return a different key than it did the original time it was
         called.







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