Re: Exclusive file access

[ahlan@marriott.org]

Under Windows files may be opened exclusively or shared.
In fact you can have a mixture such as single writer multiple readers.
Multiple writers is possible but rarely used, so it is an odd choice for Gnat to have chosen.
Using form="Shared=no" on the open using Gnat for Windows does not open the file exclusively with regard to other processes.
Which is what I would have expected and I would have thought 99% of users would require.
As far as I can see there is no way that a pure Gnat Ada program can open a file for output exclusively.
If this is true then this is a severe weakness of Gnat.
ObjectAda did what was expected by default.

On Thursday, August 27, 2015 at 3:52:36 PM UTC+2, ah...@marriott.org wrote:
> Dear All,
> 
> Obviously I'm misunderstanding something here.
> 
> I thought that if two processes tried to open the same file for write access then the second process would get an exception.
> 
> I have a simple example which when compiled using GnatPro 7.3.1 and run under 32-bit Windows XP demonstrates that this is not so.
> 
> Which then begs the question on how to detect if the file is in use and abort if it is.
> This should be simple but obviously too complex for my tiny brain.
> Does anyone know how to do this from Ada (without resorting to the OS directly)?
> 
> My simple test program is
> 
> package body Test is
> 
>   package Io renames Ada.Text_IO;
> 
>   procedure Work is
>     The_File : Io.File_Type;
>   begin
>     Io.Open (The_File, Io.Out_File, "Test.Txt");
>     for Count in Natural'value(Ada.Command_Line.Argument(1)) .. Natural'value(Ada.Command_Line.Argument(2)) loop
>       Io.Put_Line (The_File, "Count =" & Count'img);
>       Io.Put_Line ("Count =" & Count'img);
>       delay 1.0;
>     end loop;
>     Io.Close (The_File);
>   exception
>   when others =>
>     Io.Put_Line ("Exception");
>   end Work;
> 
> end Test;
> 
> If I execute this from one process with parameters 1 10 and then when it reaches 8 start the program again from a second process with parameters 3 7 on the same machine, the resultant file is a nice mixture! :-(
> 
> Count = 3
> Count = 4
> Count = 5
> Count = 6
> Count = 7
> Count = 6
> Count = 7
> Count = 8
> Count = 9
> Count = 10
> 
> This is not what I want. I want what the first instance produces and the second instance to fail. Surely nobody would want otherwise ;-)
> 
> Best wishes,
> Ahlan