Power-Programmierung/CD1.mdf/pascal/library/dos/share/tpshare.doc

Text File | 1987-01-19 | 18.4 KB | 561 lines

                Methods for Dealing with Shared Files in Turbo Pascal
                                         by
                                    John W. Wulff
                                  The Peg Board BBS
                                    703-349-8220
                                  January 19, 1987


         There have been many innovations in the development of PCBoard
         software for BBS's.  One of which is the network environment that
         the authors have included in the code.  While this is wonderful
         for the multi-node system, it does pose certain problems and
         restrictions on application programs and utilities that are being
         written for it, especially those written in Borland's Turbo
         Pascal.  Foremost among these problems is the inability of Turbo
         Pascal, henceforth known as TP, to deal with Read-Only attribute
         files.  After an ASSIGN(OpenFile,FileName), doing a simple
         RESET(OpenFile) will result in a 'File Not Found' error, even
         though you only want to read from the file.  This is because TP
         opens all files in what is known as "Inherited by Child Processes,
         Compatibility Mode, Read/Write access".  This will also cause a
         Sharing violation if running under a DOS 3.x networking mode.

         I have written a utility for PCBoard systems, PCBFile, and since
         it is extremely file intensive, I've had to do some research on
         the technical aspects of DOS 3.x in the network mode.  Because of
         this research, I've been able to get TP to cooperate and have
         written this paper to help other authors who are struggling with
         the same problems.

         Bela Lubkin published a text article, ACMODE.DOC, which is on
         Compuserve in the Borland Sig that gives the locations of a
         little gem done as the "Open Mode Byte".  This byte is at an
         absolute address for the various editions of TP and communicates
         to DOS, via Function call 3DH, how the file is to be accessed
         and what access to give other processes.  It also becomes very
         handy for us in trying to use TP in a network environment.
         These locations are:












                                    -- 1 --


           Open mode byte for Reset & Rewrite for Turbo 3.00x (PC-DOS)
              TURBO.COM    CSEG:$248D
              TURBO-87.COM CSEG:$1F3C
              TURBOBCD.COM CSEG:$2393

           Open mode byte for Reset & Rewrite for Turbo 3.00x (MS-DOS)
              TURBO.COM    CSEG:$2182
              TURBO-87.COM CSEG:$1C31
              TURBOBCD.COM CSEG:$2088

           Open mode byte for Reset & Rewrite for Turbo 3.01x (PC-DOS)
              TURBO.COM    CSEG:$24FC
              TURBO-87.COM CSEG:$1FAB
              TURBOBCD.COM CSEG:$2402

           Open mode byte for Reset & Rewrite for Turbo 3.01x (MS-DOS)
              TURBO.COM    CSEG:$21D4
              TURBO-87.COM CSEG:$1C83
              TURBOBCD.COM CSEG:$20DA

           Open mode byte for Reset & Rewrite for Turbo 3.02x (PC-DOS)
              TURBO.COM    CSEG:$24C6
              TURBO-87.COM CSEG:$1F75
              TURBOBCD.COM CSEG:$23CE





























                                    -- 2 --



         Another valuable document is Robert K. Blaine's RES_MODE.INC,
         also available on CSERV.  It details the procedure for finding
         out the location of this byte, using DEBUG.

         As profiled in the above documents, TP opens a file for RESET or
         REWRITE with a 2 in this byte location.  The docs suggest that
         you should load the location with a 0 to RESET or REWRITE read-
         only files.  While this works for READ-ONLY files, it is not the
         only value to consider when running an application in a network
         environment, especially one with PCBoard software which I will
         focus my attention for the remainder of the document.

         It is suggested that this byte be accessed by creating an
         absolute variable in your program pointing to the byte:

          Var
            OpenModeByte: Byte Absolute CSeg:$24FC;  {PC-DOS Turbo 3.01}

         Then a simple check is necessary to see if the byte contains a 2
         and if so, load it with a 0:

           begin
             if OpenModeByte = 2 then
               OpenModeByte := 0;
           end;

         The completed routine would look like this:

























                                    -- 3 --




         Procedure OpenMode(Var OpenFile : text;
                              InFileName : fname; { typed globally as
                                                    string[64] }
                            Var GoAhead  : boolean);

         var
          OpenModeByte  : Byte Absolute CSeg:$24FC; (* TURBO 3.01 *)

         begin
           if OpenModeByte = 2 then
             OpenModeByte := 0; (* allows reading of READ-ONLY files *)
           assign(OpenFile,InFileName);
           {$I-} reset(Openfile) {$I+};
           GoAhead := (ioresult = 0);
           if GoAhead then
             writeln(InFileName,' opened!')
           else
             writeln(InFileName,' failed to open!');
         end; { OpenMode }

         Now we need to determine just what is going on with PCBoard and
         how it opens files using DOS's SHARE.

         The DIR files, or the files that contain the filenames of the
         available files for the user to download, are opened in READ
         SHARED mode which in QuickBASIC would be:

           OPEN "<filename>" FOR INPUT ACCESS READ SHARED AS #1.

         Upload DIR files are opened for APPEND in a LOCKED WRITE mode.
         This keeps other nodes from writing at that particular moment
         but allows other nodes to read this file.

         In order to accomplish these same modes in TP we must look into
         the DOS 3.1 Technical Reference Manual (groan!).  The following
         is reprinted for that manual with some additions by me.

            The OPEN mode consists of 4 bit-oriented fields:

           * INHERITANCE FLAG
           * SHARING MODE FIELD
           * RESERVED FIELD
           * ACCESS FIELD

         The INHERITANCE FLAG determines if the file will ever be
         inherited by another process, which in the case of a network is
         usually the desired effect.  The SHARING FIELD defines what




                                    -- 4 --

         operations may be performed on the file by other nodes.  The
         ACCESS FIELD defines what operations THIS node may perform on
         the file.  The bit fields are mapped as follows:

                            <I> < S > <R> < A >
           Open Mode bits    7  6 5 4  3  2 1 0

         I  INHERITANCE FLAG
            If I = 0; File is inherited by child processes
            If I = 1; File is private to the current process

         S  SHARING MODE
            The file is opened like this:
            if S = 000;  Compatibility mode - The default open mode it
                         denies ALL OTHER processes access to the file.
                         Since this is the mode that TP uses to open a
                         file, what do you thing will happen on the BBS
                         side if you have a file open on your end and the
                         BBS tries to open it?
            if S = 001;  Deny Read/Write mode (Exclusive).  This would
                         actually be the same as setting the I flag to 1.
            if S = 010;  Deny Write mode - you should open a file in this
                         mode if you wish to protect it.  It will allow
                         other processes to read it but not write.
            if S = 011;  Deny Read mode
            if S = 100;  Deny None mode - Who cares what happens!

            It is important to specify what operations you want to
            perform (access mode).  The default access mode is Read/Write
            and causes the open request to fail if another process has
            the file opened with any sharing mode other than Deny None.
            File sharing required cooperation of both sharing processes.
            This is communicated through the sharing and access modes.

         R  RESERVED (set third bit field to 0)

         A  ACCESS - The file access is assigned as follows:

            If A = 000; Read Access
            if A = 001; Write Access
            if A = 010; Read/Write access

         If all this seems a bit involved, I'm sorry.  I don't know of
         any way to give you the background for all this hocus-pocus
         except with the above info.  I also recommend picking up a Tech
         Ref manual for more detailed study of the 3DH function call.

         OK!  With all these numbers in hand, let's see how to get TP to
         duplicate these modes.  Earlier we said that other gurus had
         stated that TP opens files in Compatibility Mode with Read/Write




                                    -- 5 --

         Access and Inheritance by child processes and the magic number
         of the Open Mode byte is 2.  Let's look at how that was done:
         Compatibility mode:   000 {S}
         Read/Write ACCESS :   010 {A}
         Inherited by child:     0 {I}
         Reserved is ALWAYS:     0 {R}

         Remember the bit fields are:
                            <I> < S > <R> < A >
           Open Mode bits    7  6 5 4  3  2 1 0  Let's plug in
                             0  0 0 0  0  0 1 0  the numbers.
         Using binary arithmetic:
                          <I> <  S   > <R> < A >
                          128 64 32 16  8  4 2 1
                           0   0  0  0  0  0 1 0 = 00000010 = 2

         By using an OpenModeByte of 0 we change the ACCESS field to 000
         which is READ ACCESS, which allows us to read a READ-ONLY file.

         PCBoard is opening its DIR files as READ ACCESS SHARED are
         actually opening the file with a SHARING MODE of Deny/Write
         which would be a SHARE <S> field of 010.  This number for the
         OpenModeByte then becomes:
                          <I> <  S   > <R> < A >
                          128 64 32 16  8  4 2 1
                           0   0  1  0  0  0 0 0 = 00100000 = 32

         This is how I open ALL my files for reading or for general
         ASSIGNING for RESET in TP for my program PCBFile.  I have some
         procedures written for TEXT files, and files of type BYTE.

         I have reproduced the code below:


         (* This procedure, KeepOn, is used to determine if the file has
         been locked out.  I try a file for 10 times as determined by
         OpenAtt variable before I give up  *)

         Procedure KeepOn(OpenAtt : byte; var GA : boolean);
         begin
           if OpenAtt <= 10 then
             GA := TRUE      (* GoAhead Flag - if within 10 go for it *)
             else GA := FALSE;  (* forget it! *)
         end;










                                    -- 6 --



         Procedure SetResetMode(Var OpenFile : text;  (* OPEN MODE FOR
                                InFileName   : fname;    TEXT FILES *)
                                var GoAhead : boolean);
         var
           OpenModeByte  : Byte Absolute CSeg:$24FC;
           OpenAttempts  : byte;
         begin
           OpenAttempts := 1;
           if OpenModeByte = 2 then
             OpenModeByte := 32;   (* this is Deny Write Mode/Read Access *)
           assign(OpenFile,InFileName);
           repeat
             {$I-} reset(Openfile) {$I+};
             GoAhead := (ioresult = 0);
             if not GoAhead then
               OpenAttempts := OpenAttempts + 1;
           until (GoAhead) OR (OpenAttempts > 10); (* try until I get in *)
           KeepOn(OpenAttempts,GoAhead);
         end; { SetResetMode }

         Procedure SetResetModeFile(Var OpenFile : bytefile;(* bytefile     *)
                                    InFileName   : fname;   (* previously   *)
                                    var GoAhead : boolean); (* typed as     *)
                                                            (* File of Byte *)
         var
           OpenModeByte  : Byte Absolute CSeg:$24FC;
           OpenAttempts  : byte;
         begin
           OpenAttempts := 1;
           if OpenModeByte = 2 then
               OpenModeByte := 32;   (* this is Deny Write Mode/Read Access *)
           assign(OpenFile,InFileName);
           repeat
             {$I-} reset(Openfile) {$I+};
             GoAhead := (ioresult = 0);
             if not GoAhead then
               OpenAttempts := OpenAttempts + 1;
           until GoAhead OR (OpenAttempts > 10);
           KeepOn(OpenAttempts,GoAhead);
         end; { SetResetModeFile }












                                    -- 7 --


         ************************************************************
         Now here comes a little zinger to change things up.  I want to
         create a file that I don't want the other nodes to damage.  I
         elect to open the file for READ/WRITE ACCESS for myself and
         give the other nodes READ capability and deny them the ability
         to write to my file.  This would be Deny/Write Mode under the
         <S> or SHARING FIELD and would be coded 010.  For READ/WRITE
         ACCESS the <A> or ACCESS FIELD is coded 010 also.  This is the
         same mode that PCBoard uses for writing to the Upload directory.
         Using our binary formulae, OpenModeByte then becomes:

                          <I> <  S   > <R> < A >
                          128 64 32 16  8  4 2 1
                           0   0  1  0  0  0 1 0 = 00100010 = 34

         With the magic number of 34 the SetFileLock procedure was born.

                  Procedure SetFileLock(Var   OpenFile : buftext;
                                   InFileName : fname;
                                  var GoAhead : boolean);
         var
           OpenModeByte  : Byte Absolute CSeg:$24FC;
           OpenAttempts  : byte;

         begin
           OpenAttempts := 1;
           if OpenModeByte = 2 then
             OpenModeByte := 34; (* Deny Write Mode/Read-Write Access *)
           assign(OpenFile,InFileName);
           repeat
             {$I-} rewrite(Openfile) {$I+};
             GoAhead := (ioresult = 0);
             if not GoAhead then
               OpenAttempts := OpenAttempts + 1;
           until GoAhead or (OpenAttempts > 10);
           KeepOn(OpenAttempts,GoAhead);
         end; { SetLockMode }

         Finally, a little walk around the park to insure that the
         OpenModeByte is returned to Borland's "normal".

         Procedure ReleaseOpenMode;
         var
           OpenModeByte  : Byte Absolute CSeg:$24FC;
         begin
           OpenModeByte := 2;
         end;

         So it's really simple to change Turbo Pascal Open mode to




                                    -- 8 --

         exactly what you want, you just have to know what mode you
         desire the program to work in.  Just remember these definitions
         of the fields that make up the magic number for DOS Function
         call 3DH.

         * INHERITANCE FLAG    I = 0;   Inherited (usually the case)
                               I = 1;   Private

         * SHARING MODE FIELD  (Other node or process)
                               S = 000; Compatibility mode
                               S = 001; Deny Read/Write mode (Exclusive)
                               S = 010; Deny Write mode
                               S = 011; Deny Read mode
                               S = 100; Deny None mode

         * RESERVED FIELD      R = 0; Always

         * ACCESS FIELD        (Your node or process)
                               A = 000; Read Access
                               A = 001; Write Access
                               A = 010; Read/Write Access

         The bit fields:
                            <I> < S > <R> < A >
           Open Mode bits    7  6 5 4  3  2 1 0





























                                    -- 9 --



         Even though the DOS 3.1 Technical Reference Manual gives a more
         thorough discussion of Function Call 3DH, I will attempt to
         create a matrix with the number for the OpenModeByte based on
         the SHARE and ACCESS fields.

           ACCESS   |SHARE-> Compat  Deny/RW  Deny/W  Deny/R  Deny/N
            |                 000      001     010     011     100
            v              -------------------------------------------
           Read  000       |   0    |   16  |   32  |   48  |   64   |
                           |--------|-------|-------|-------|--------|
           Write 001       |   1    |   17  |   33  |   49  |   65   |
                           |--------|-------|-------|-------|--------|
           Read/ 010       |   2    |   18  |   34  |   50  |   66   |
           Write           -------------------------------------------

         I know that this is probably more than a human can bear to
         assimilate at any one time but I hope that you will be able to
         see the logic behind my system and be able to use Turbo Pascal
         to its full potential.


         PCBoard (c) Clark Development Company, Inc., Murray, UT
         Turbo Pascal (c) Borland International, Scotts Valley, CA
         PCBFile (c) John W. Wulff
         Compuserve (c) Compuserve, Inc., Columbus, OH



























                                    -- 10 --