Re: GNAT's Text_IO & empty files

[Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>]

Robert Dewar wrote:
>     --  For files other than standard error and standard output, we
>     --  make sure that an empty file has a single line feed, so that
>     --  it is properly formatted. We avoid this for the standard files
>     --  because it is too much of a nuisance to have these odd line
>     --  feeds when nothing has been written to the file.
> 
>          elsif (File /= Standard_Err and then File /= Standard_Out)
>            and then (File.Line = 1 and then File.Page = 1)
>          then
>             New_Line (File);
>          end if;
> 
> 
> Most certainly if you comment these lines out, your resulting runtime will
> not be compliant with annex A of the reference manual. These lines were not
> put in for amusement!

Having just read RM A.10 �7, I think the Ada standard is badly
formulated here. It is not clear how a file or a page with zero
lines or a file with zero pages has to be encoded. The standard writes
that a file is a sequence of pages and a page is a sequence of lines,
but it does not handle the obvious border case of zero-length
sequences adequatly. It would have been better to write just that
every line terminates with a line terminator, and every page
terminates with a page terminator, and then everything would have
been clear and consistent. With a bit good will, the Ada RM language
can even be interpreted that way.

Back to my Unix convention remark: Although vi cannot produce a
file in which the last line does not end with LF, it can very
well produce a file that contains zero lines and is therefore
zero bytes long.

Markus

-- 
Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK
email: mkuhn at acm.org,  home page: <http://www.cl.cam.ac.uk/~mgk25/>