Xref: utzoo comp.sys.dec:4129 comp.os.vms:30821
Path: utzoo!attcan!uunet!saab.stanford.edu!shelby!mcnc!uvaarpa!murdoch!news
From: sdm7g@dale.acc.Virginia.EDU (Steve D. Majewski)
Newsgroups: comp.sys.dec,comp.os.vms,vmsnet.misc
Subject: user byte in file header FH1$B_USERCHAR
Keywords: RMS ODS File header userchar
Message-ID: <1990Sep26.182837.618@murdoch.acc.Virginia.EDU>
Date: 26 Sep 90 18:28:37 GMT
Sender: news@murdoch.acc.Virginia.EDU
Followup-To: comp.sys.dec
Organization: University of Virginia
Lines: 35


  As far as I have been able to decipher, VAX/VMS RMS leaves ONE byte
  in the file header that can be user defined. ( I believe it is 
  FH1$B_USERCHAR, but it's been a while since I poked around. ) 

  1) Is this still "reserved for users" , or has DEC taken it back
     to reserve for something else? 

  2) Has anyone out there actually tried to use it for anything ? 

   [ What I had in mind, once, long ago, was modifying the I/O to     ]
   [ do auto-magic file compression/decompression: If the right bit   ]
   [ in FH1$B_USERCHAR is set, then the first item in the file is     ]
   [ the decompression map :: decompress and unblock the records      ]
   [ on reads. This obviously would entail re-vectoring SYS$OPEN,     ]
   [  SYS$READ, SYS$GET, etc. Other info that could tag along         ]
   [ could be audit trails, file comments, etc., the idea being that  ]
   [ the extra data would be invisible to "normal" reads, and the     ]
   [ files still compatable with their applications.                  ]
   [ The necessity of doing this on VMS has passes, as we are moving  ]
   [ more applications over to UNIX, but I am still curious about     ]
   [ whether my original notion was practical. And I am interested    ]
   [ in comparing the difficulty of the same project in VMS vs UNIX.  ] 

  3) Has DEC ever published anything on RMS/ODS/File&I/O systems as 
     exhaustive as the black book on VMS internals? Or anything ? 

 If you reply via E-mail, I will summarize and post.

 - Steve Majewski    sdm7g@virginia.edu
  
========================================================================
 Steven D. Majewski		University of Virginia Physiology Dept.
 sdm7g@Virginia.EDU		Box 449 Health Sciences Center
 (804)-982-0831			Charlottesville, VA 22908