Re: Exclusive file access

[ahlan@marriott.org]

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

I am not trying to protect the file from other tasks I am trying to protect the file from other processes - ie other programs or utilities - perhaps executing on other computers if the file in question is on a network. I want to open the file in such a way that I am granted exclusive write access. This has absolutely nothing to do with tasking or protected objects etc.
Bruce B. tells me that there are routines in the package Directories that can be used to check if the file is already open. However that wouldn't be a full solution because there would be nothing to stop any other program from opening the file and messing with it. However it would be a start. If the file was already opened (by some other process) then I could avoid opening it. Unfortunately I couldn't find any such routine in Ada.Directories. Am I going blind?
So far the only solution I can see is for me to write my own filing system, one that has the ability to open the file for exclusive file access. However I cannot believe that this is what people really do.
As it stands if I write a program that writes a log to a text file a second occurrence of the program if run concurrently will mess up this log.
Is there really no way for an Gnat Ada program to open a file for exclusive access against other processes without resorting to the program writing his/her own filing system? (Open/Read/Write/Close that call directly the OS API)