Re: Directory Operations

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Mon, 3 Nov 2008 08:27:36 -0800 (PST), andrei.krivoshei@gmail.com wrote:

> I have a problem.
> I need to write a DLL with a function, which returns the path to a
> current directory under Windows XP.
> I have't any problem with DLL itself, but when I try to use some
> function from the package "GNAT.Directory_Operations" or
> "Ada.Directories", I get the unexecutable DLL.
> NOTE! The executable file is working, I talk only about DLL variant!
> 
> Thanks!
> 
> The code listing of my program is
> 
> ========= Spec ============
> with Interfaces.C.Strings; use Interfaces.C.Strings;
> 
> package FAD is
> 
>    procedure Pwd(Path_ptr: in out Chars_ptr );

1. Why don't you use Ada convention returning plain String?

   function Pwd return String
  [   renames Ada.Directories.Current_Directory; -- Work is done ]

2. Should you keep it non-Ada, then

2.a. how is this supposed to work?
2.b. Where is the string allocated?
2.c. Who will dispose it? When?
2.d. What happens when called from multiple threads?

>    pragma Export(C, Pwd, "pwd");
> 
> end FAD;
> 
> ========= Body ============
> --  with Ada.Directories;
> with Ada.Strings;
> with GNAT.Directory_Operations;
> 
> package body FAD is
>    package DOP renames GNAT.Directory_Operations;
>    use Ada.Strings;
> 
>    procedure Pwd(Path_ptr: in out Chars_ptr ) is
> --        use Ada.Directories;
>       CStr: Chars_ptr;
>       Path: String (1 .. 1024);
>    begin
> --      Path: String := "custom test string";  -- with this code line,
> the DLL is executable
>       Path := DOP.Get_Current_Dir;  -- with this code line, the DLL is UNexecutable

This cannot work, since Get_Current_Dir'Length may differ from 1024.

>       CStr := Null_Ptr; -- Value for test purposes only
>       Path_ptr := CStr;

That will not work either because Path is allocated on the stack and
destroyed upon return. See questions 2.a-d.

>    end Pwd;
> 
> end FAD;

C programming is substantially more difficult, even if occasionally spelt
in Ada...

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de