Re: AI-248 and counting the elements in an external file

[Stephen Leake <Stephe.Leake@nasa.gov>]

Kilgallen@SpamCop.net (Larry Kilgallen) writes:

> I suppose my lack of experience with Ada95 streams ('read, etc.)
> may be the reason for my confusion, but in reading
> 
> 	http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00248.TXT?rev=1.16
> 
> I see in several places the text:
> 
> 	The size of an external file is the number of
> 	stream elements contained in the file.

a "stream element" is just a "byte"; it's Ada-ese for "smallest
writeable chunk". On all "normal" OS's, it's 8 bits.

I think perhaps you are thinking of "stream element" as some
higher-level object?

> That is straightforward for a file of some record type, or a file of
> integers, but what if the file is one created with Ada95 stream
> semantics ? 

It makes no difference. All of these types must be converted to raw
stream elements (bytes) before they can be written to the file. That's
the nature of files!

> I thought that 'write and write() relied upon the reading program to
> know "how" to read the file, and that implementations would not have
> kept a tally of how many elements were in the file.

The OS certainly has to know how big the file is; that's all that
matters here.

> I don't know enough about this to know if it is possible to do a
> linear search of a file to count the elements, but even if it were,
> that would be a very slow operation.

Yes, but the OS keeps track.

> Is it envisioned by the authors of AI-248 that implementations would
> preserve metadata containing the element count ? 

Only if the underlying OS doesn't; in which case this whole package
makes no sense anyway.

> If so, what about stream files that were written under Ada95 ? 

I don't see why you think stream files are different from other files,
as far as this package is concerned.

> Or can someone suggest what it is that I don't understand about Ada
> streams ?

-- 
-- Stephe