Random Access in Ada.Text_IO files

Random Access in Ada.Text_IO files

[Micha� Morawski]

Hi,

I have encountered the following problem

My program (one task) constantly writes some information to the file (log).
This is simple text line produced every 2 seconds.

Sometimes, other task try to analyze the log and the task need to read the
same file.

This works if files are opened in the "shared=" form. But of course both
tasks use different file position pointers. I wrote the simple test program
presented below. If shared is in "no" mode - I receive use_errer, If "yes"
mode, the file "aaaaaaa" is destroyed by Get (Ascii.null is inserted). After
I wrote the simple monitor to separate the operarions I must write either
extension of Ada.Text_IO or own module. In the first case I cannot compile
the module (I receive the message - it is the system module and cannot be
compiled), and my own module have no acces to stream field of File_Type.

Can anyone give me some advice?

procedure TF is

   NAZWA_PLIKU : constant String := "aaaaaaa";

   task Wr is
   end Wr;

   task body Wr is
      F : File_Type;
      ch : Character := 'a';
   begin
      Create (F, Out_File, NAZWA_PLIKU, "shared=no");
      loop
         Put (F, ch);
         ch := Character'Succ (ch);
         if ch > 'z' then
            ch := 'a';
         end if;
         delay 0.01;
      end loop;
   exception
      when others =>
         Put_Line ("Blad w zadaniu");
   end Wr;

   P : File_Type;
   ch : Character;
begin
   delay 3.0;
   Open (P, In_File, NAZWA_PLIKU, "shared=no");
   Reset (P);
   loop
      Get (P, ch);
      Put (ch);
      delay 0.1;
   end loop;
exception
   when E : others =>
      Put (Ada.Exceptions.Exception_Information (E) &
           GNAT.Traceback.Symbolic.Symbolic_Traceback (E));
end TF;


--

Michal Morawski

Re: Random Access in Ada.Text_IO files

[Stephen Leake]

"Micha� Morawski" <morawski@zsk.p.lodz.pl> writes:

> Sometimes, other task try to analyze the log and the task need to read the
> same file.

This is very operating system dependent. There is no guarantee that
Ada.Text_IO files will allow this.

If the two tasks are part of the same program, have them share an
in-memory queue of strings, rather than an external file.

> <snip> and my own module have no acces to stream field of
> File_Type.

If you are writing your own file package, you should not try build it
on top of Text_IO.

You might be able to use Ada.Streams.Stream_IO instead.

-- 
-- Stephe

Re: Random Access in Ada.Text_IO files

[Jeffrey Carter]

In general, what you're trying to do won't work. Ada.Text_IO is intended 
for sequential use only. Files are not the appropriate mechanism for 
intertask communication; it would be better to use a protected object of 
some sort.

Some general Ada concepts you might find useful:

Micha� Morawski wrote:
> procedure TF is
> 
>    NAZWA_PLIKU : constant String := "aaaaaaa";
> 
>    task Wr is
>    end Wr;

This can be written using the special form

task Wr;

> 
>    task body Wr is
>       F : File_Type;
>       ch : Character := 'a';
>    begin
>       Create (F, Out_File, NAZWA_PLIKU, "shared=no");
>       loop
>          Put (F, ch);
>          ch := Character'Succ (ch);
>          if ch > 'z' then
>             ch := 'a';
>          end if;
>          delay 0.01;
>       end loop;

Might I suggest something along the lines of

    subtype Lower is Character range 'a' .. 'z';
begin
    Create (File => F, ...);

    loop
       for Ch in Lower loop
          Put (File => F, Item => Ch);
          delay 0.01;
       end loop;
    end loop;

>    exception
>       when others =>
>          Put_Line ("Blad w zadaniu");
>    end Wr;

-- 
Jeff Carter
"You've got the brain of a four-year-old boy,
and I bet he was glad to get rid of it."
Horse Feathers

Re: Random Access in Ada.Text_IO files

[Egil Harald H�vik]

"Jeffrey Carter" <jrcarter@acm.org> wrote in message
news:3E496557.4060109@acm.org...

<snip>

>     subtype Lower is Character range 'a' .. 'z';
> begin
>     Create (File => F, ...);
>
>     loop
>        for Ch in Lower loop
>           Put (File => F, Item => Ch);
>           delay 0.01;
>        end loop;
>     end loop;
>


Personally, I would remove the outer loop here...

~egilhh

Re: Random Access in Ada.Text_IO files

[Egil Harald H�vik]

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"Egil Harald H�vik" <egil.harald.hoevik.nospam@kongsberg.com> wrote in
message news:3e4a0f01@193.71.169.73...
>
> "Jeffrey Carter" <jrcarter@acm.org> wrote in message
> news:3E496557.4060109@acm.org...
>
> <snip>
>
> >     subtype Lower is Character range 'a' .. 'z';
> > begin
> >     Create (File => F, ...);
> >
> >     loop
> >        for Ch in Lower loop
> >           Put (File => F, Item => Ch);
> >           delay 0.01;
> >        end loop;
> >     end loop;
> >
>
>
> Personally, I would remove the outer loop here...
>


... or maybe not.  I missed the

         if ch > 'z' then
            ch := 'a';
         end if;

in the original post.