VHDL Issue Number:      0122
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Processes may still interact (in a kludgy way) through
                        the use of File I/O.  This was one of the reasons for
                        removing internal files from the language.
Related Issues:         ???
Relevant LRM Sections:  4.3.2
Key Words and Phrases:  File I/O, processes
Current Status:         Analyzed
1076-1993 Disposition:  Bugs Fixed, Enhancements Outstanding (No ISAC Issues)
Disposition Rationale:  
Superseded By:          N/A
-----------------------
Date Submitted:         1989/06/21
Author of Submission:   Ken Scott
Author's Affiliation:   Vantage Analysis Systems, Inc.
Author's Post Address:  42840 Christy St., Suite 201
                        Fremont, CA   94538
Author's Phone Number:  (415) 659-0901
Author's Fax Number:    
Author's Net Address:   N/A
-----------------------
Date Analyzed:          1991/11/19
Author of Analysis:     Stephen Bailey
Revision Number:        $Revision: 1.10 $
Date Last Revised:      $Date: 1995/05/13 19:34:42 $


Description of Problem
----------------------
   One reason for removing internal file declarations was to 
   disallow the interaction of processes through the use of
   file I/O.

   Currently, there is no restriction in the LRM stating that
   file objects with the same logical name have to be declared
   with the same mode.  As a result, the LRM allows one process
   to access the external file associated with a logical name while
   allowing another process to write to this same file at the same
   time.

   Making this restriction explicit would plug the last hole which
   allows processes to interact with one another through I/O.  If 
   this restriction is not made, then passive processes should once
   again disallow READ and WRITE operations.

Proposed Resolution
-------------------
   This restriction could be made by appending the following sentence
   to the 4th paragraph of chapter 4.3.2:
      "File objects with the same logical name must be of the same
       type and mode."

VASG-ISAC Analysis & Rationale
------------------------------
   It is impossible at analysis time to determine whether or not a
   process is made non-passive through the use of files to communicate
   with other processes.  The check for passive processes must be an
   analysis time computation.  Furthermore, interfaces to foreign
   language subprograms (something that some vendors offer now and which
   the interface mechanism will be standardized in '92) also provide
   the same problem in determining passive processes.

   File access may be performed through subprograms called by a passive
   process.  Therefore, from a practicality perspective, it is im-
   possible for an analysis-time computable check of a passive process
   if the definition of a passive process excludes the possibility
   of FILE access (and foreign language subprogram interface).

VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
   The resolution of this IR is that no change is required to the
   LRM.  Furthermore, this IR resolution explicitly states that FILE IO
   has no affect on the determination of the passiveness of a process.

VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
   It is recommended that the use of a foreign language subprogram
   within a process also not effect the passivity of the process.