Category : Pascal Source Code
Archive   : TXTSHARE.ZIP
Filename : TXTSHARE.DOC

With the advent of version 4.0 of the Turbo Pascal compiler, it is now
possible to create programs that access shared files (in a network
environment) without generating Abort, Retry, Ignore messages. This is
accomplished through the use of a new standard global variable called
FileMode.

When Turbo is told to open a file, DOS function 3Dh is called to do the
actual work. Before this system call is made, the AL register is populated
with the contents of the FileMode variable. The value of the AL register
tells DOS what access rights will be given to the program opening the file.
In older versions of the compiler, AL was always 0, 1, or 2. Now, however,
the programmer can determine the access rights required, load the FileMode
variable with the appropriate access code, and have the file opened
without fear of causing problems in a shared environment.

Unfortunately, the use of FileMode does not extend to files of type
TEXT. TEXT files are always opened with a filemode of 0 (read only), 1
(write only), or 2 (read/write). This has the unfortunate consequence of
restricting programs used in a network environment to manipulation of typed
and untyped files only.

Also new in Turbo Pascal v4.0 is the ability to create custom text file
device drivers that completely replace Turbo's own file handling routines.
The file TXTSHARE.PAS implements such a driver. This file compiles to a
UNIT (yet another new feature in the language) that, when used, extends the
use of the FileMode variable to TEXT files.

Here's how it works:

1). The ASSIGNTEXT procedure replaces Turbo's ASSIGN procedure.
This procedure is used to associate a TEXT file variable with a
DOS filename and sets up an internal data structure. This
procedure MUST be called before the text file is opened. The
syntax of the procedure is:

ASSIGNTEXT(Var f : Text; filespec : String)

2). Determine the access rights required for the file and load the
FileMode variable with the appropriate DOS access code (see the
section on DOS function 3Dh in the DOS Technical Reference for a
complete discussion on access codes).

3). Open the file (as usual) with the RESET, REWRITE, or APPEND
procedures

4). Do your stuff ...

5). Close the file (as usual) with the CLOSE procedure.


There is one other global variable provided in this unit called
WriteTextEofChar. This is a boolean variable and when set TRUE, tells the
driver to append an eof character (ascii 26) to the end of any file opened
with REWRITE or APPEND before closing that file. The default value of this
variable is FALSE (ie. no eof character is written before closing).


rpb
1/10/88