Methods for Dealing with Shared Files in TURBO Pascal Version 5.0 by John W. Wulff Wulff Enterprises, Inc. 260 Terranova Drive Warrenton, VA 22186-9227 (703) 349-8805 Revised May 15, 1989 This paper was first written January 19, 1987 as a means of demonstrating how to modify Turbo Pascal 3.x to handle the opening of READ-ONLY files and to make it able to deal with files in a Shared, network environment. It provided a means of finding an undocumented byte known as the "Open Mode Byte". Since then, Turbo Pascal 5.0 has been released and this byte has been globally declared as the FILEMODE variable. Even so, Turbo Pascal 5.0 ignores the variable when opening TEXT files. While there is a small explanation in the Turbo Pascal Reference Guide as to its use, the information in this paper is still valid today and has been updated to reflect its relevance to Turbo Pascal 5.0. For those wishing to continue to work in Turbo Pascal 3.0, I have included the locations for this byte in appendix A. 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. One problem that plagues most authors writing application programs that eventually become used in network environments is coping with the Share utility that DOS uses to protect files. Any file opened for reading under Turbo Pascal will cause a Sharing violation if running under a DOS 3.x and 4.x networking mode. This is because Turbo Pascal opens all files in what is known as "Inherited by Child Processes, Compatibility Mode, Read/Write access". 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 in the network mode. Because of this research, I've been able to get Turbo Pascal to cooperate and have written this paper to help other authors who are struggling with the same problems. As documented in the Turbo Pascal instructions, the FILEMODE variable has a value of 2 when opening files for RESET or REWRITE which allows both reading and writing. The instructions suggest that you should assign a value of 0 to the variable 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 upon which I will focus my attention for the remainder of the document. One thing to consider, if using other languages, especially assembly language, is that this FILEMODE variable, corresponds to the AL register. All references to that byte and its decimal number can be applied to assembly in this way: AH contains 3DH - the function call DS:DX points to an ASCIIZ path name AL will be loaded with the 8 bit number that the FILEMODE variable contains. When the function returns, AX will contain error codes or a 16 bit file handle if successful. (See DOS manual for details.) I don't profess to be anywhere close to fluent in assembly so I will leave this information with those who are best suited to take advantage of it. This variable is GLOBALLY pre-declared in Turbo Pascal, so all you have to do is refer to it as FILEMODE. Then a simple check is necessary to see if the variable contains a 2 and if so, load it with a 0: begin if FILEMODE = 2 then FILEMODE := 0; end; The completed routine would look like this: Procedure OpenMode(Var OpenFile : text; InFileName : string Var GoAhead : boolean); {Remember, FILEMODE is a GLOBALLY declared variable of type byte. No other declaration is necessary on your part} begin if FILEMODE = 2 then FILEMODE := 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; { of Procedure OpenMode } Shared Files in Turbo Pascal 5.0 2 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 "" 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 Turbo Pascal we must look into the DOS Technical Reference Manual (groan!). The following is reprinted from 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 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: < S > < 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 Turbo Pascal uses to open a file, what do you think 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! Shared Files in Turbo Pascal 5.0 3 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 requires 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 Turbo Pascal to duplicate these modes. Earlier I said that other gurus had stated that Turbo Pascal opens files in COMPATIBILITY MODE with READ/WRITE ACCESS and INHERITANCE BY CHILD PROCESSES and the magic value for the FILEMODE variable 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 as ALWAYS: 0 {R} Remember the bit fields are: < S > < 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: < S > < A > 128 64 32 16 8 4 2 1 0 0 0 0 0 0 1 0 = 00000010 = 2 By using a FILEMODE value of 0 we change the ACCESS field to 000, READ ACCESS, which allows us to read a READ-ONLY file. PCBoard is opening its DIR files as READ ACCESS SHARED and actually opening the file with a SHARING MODE of Deny/Write which would be a SHARE field of 010. The value for the FILEMODE variable then becomes: < S > < A > 128 64 32 16 8 4 2 1 0 0 1 0 0 0 0 0 = 00100000 = 32 Shared Files in Turbo Pascal 5.0 4 This is how I open ALL my files for reading or for general ASSIGNING for RESET in Turbo Pascal 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; {of Procedure KeepOn} Procedure SetResetMode(Var OpenFile : text; (* OPEN MODE FOR InFileName : string; TEXT FILES *) Var GoAhead : boolean); var OpenAttempts : byte; begin OpenAttempts := 1; FILEMODE := 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); (* keep trying *) KeepOn(OpenAttempts,GoAhead); end; {of Procedure SetResetMode} Procedure SetResetModeFile(Var OpenFile : file of byte InFileName : string; var GoAhead : boolean); var OpenAttempts : byte; begin OpenAttempts := 1; if FILEMODE := 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; {of Procedure SetResetModeFile} Shared Files in Turbo Pascal 5.0 5 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 or SHARING FIELD and would be coded 010. For READ/WRITE ACCESS the 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, the FILEMODE value then becomes: < S > < 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 : text; InFileName : string; var GoAhead : boolean); var OpenAttempts : byte; begin OpenAttempts := 1; FILEMODE := 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; {of Procedure SetFileLock} Finally, a little walk around the park to insure that the FILEMODE variable is returned to Borland's normal mode. Procedure ReleaseOpenMode; begin FILEMODE := 2; end; So it's really simple to change the Turbo Pascal Open mode to exactly what you want, you just have to know what results you desire from the program. 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 Shared Files in Turbo Pascal 5.0 6 * 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: < S > < A > Open Mode bits 7 6 5 4 3 2 1 0 Even though a DOS Technical Reference Manual gives a more thorough discussion of Function Call 3DH, I will attempt to create a matrix with the number for the FILEMODE variable 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 Shared Files in Turbo Pascal 5.0 7 Appendix For Turbo Pascal 3.x Die-Hards: Bela Lubkin published a text article, ACMODE.DOC, which is on Compuserve in the Borland Sig that gives the locations of a little gem known as the "Open Mode Byte". This byte is at an absolute address for the various editions of Turbo Pascal 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 Turbo Pascal in a network environment. These locations are: 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 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. Shared Files in Turbo Pascal 5.0 8