Re: I/O - exception handling

[Sergey Koshcheyev <serko84@hotmail.com>]

Anisimkov wrote:
> "Sergey Koshcheyev" <serko84@hotmail.com> wrote in message
> Your should not catch exception on the Close, becouse Ada95 RM A.8.2 File
> Management
> says "The exception Status_Error is propagated if the given file is not
> open.". So if file is Open, you do not need to wait exception. I thing GNAT
> spetsific Ada.Strings.Unbounded.Text_IO.Close have the same behavior. If you
> are want to catch exception anyway (for example you are using any other IO
> implementation.) you can make local procedure inside of Read_Something.

(The File_Type, Open and Close that were used in my example are from 
Ada.Text_IO, only Get_Line comes from A.S.U.Text_IO, and that's a 
function, not a procedure as I used it originally. But the example was 
only meant to illustrate, I'm sorry for the confusion.)

I've seen the paragraph, but I don't know if it can be read as "the only 
exception that may be propagated from Close is Status_Error, and that 
only if the file was not open" - the clause doesn't give me any such 
guarantees.

For example, I think Close is legally allowed to raise Device_Error, or 
even Use_Error.

>  begin
>     Open (File, Name => "test.txt", Mode => In_File);
>     Get_Line (File, S1);
>     Get_Line (File, S2);
>     Close_File;
> 
>  exception
>     when others =>
>          Close_File;
> end Read_Something;

This closes the file twice, as does the example below - something I 
don't like.

> But, in my point of view, it is bad practice to suppress exception
> completely. I prefer to know about errors in program execution. If we do not
> suppress exception, code become simplier. We need to catch exceptions only in
> Get_Line routines. Exceptions in Open routine do not need to be catched
> becouse we do not need to close not opened file. For the Ada semantic File we
> can write just.

I hereby promise I won't ignore the I/O exceptions in real code (or at 
least I'll try not to :-)), that example was just an example.

> exception
>     when Device_Error | Data_Error =>
>           --  Device_Error and Data_Error could be raised only on IO
> operations
>           --  So the file is already opened and we do not need to check is
> file open
>           --  before close.

But Open and Close may raise Device_Error too?

>           Close_File (File);

Again, you have to remember to close the file in two places.

>           raise;
> end Read_Something;

Sergey.