Multiple instantiation of Ada.Direct_IO

Multiple instantiation of Ada.Direct_IO

[Blake Weston]

Hi there,

I have a bit of a problem. Basically I am using Ada.Direct_IO to read /
write to a file. The problem being I wish to write records of differing
types to the file, without closing the file and re-opening it.

Ada 95 will not even let me open the same file as two different file_type
variables as this creates a Use Exception error.

Can anybody help?

Thanks

Blake

Re: Multiple instantiation of Ada.Direct_IO

[Matthew Heaney]

In article <886003669.3982.0.nnrp-02.9e98a25e@news.demon.co.uk>, "Blake
Weston" <blake@dedlock.demon.co.uk> wrote:

>Hi there,
>
>I have a bit of a problem. Basically I am using Ada.Direct_IO to read /
>write to a file. The problem being I wish to write records of differing
>types to the file, without closing the file and re-opening it.
>
>Ada 95 will not even let me open the same file as two different file_type
>variables as this creates a Use Exception error.
>
>Can anybody help?

It sounds like you want to perform heterogeneous I/O.  Direct_IO is for
homogeneous I/O.  Try Ada.Stream_I/O instead.

--------------------------------------------------------------------
Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
(818) 985-1271

Re: Multiple instantiation of Ada.Direct_IO

[Nick Roberts]

Alternatively, I think (no doubt someone will swiftly correct me if I'm
wrong :-) you can get the effect you require by making the record type
discriminated, with a default for each discriminant (so that it is a
'definite' type).

The disadvantage of this method is that, in all likelihood, each record
written in the file will be the size of the biggest possible variant,
regardless of which variant is actually written (so there could be a lot of
wasted space).  You may or may not consider this to be a serious problem. 
Personally, I would not!

-- 

== Nick Roberts ================================================
== Croydon, UK                       ===========================
==                                              ================
== Proprietor, ThoughtWing Software                   ==========
== Independent Software Development Consultant            ======
== Nick.Roberts@dial.pipex.com                              ====
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====         High mountains are a feeling, but the hum        ==
=======      Of human cities torture.
===========                             -- Byron [Childe Harold]


Matthew Heaney <mheaney@ni.net> wrote in article
<mheaney-ya023680002801980909240001@news.ni.net>...
> In article <886003669.3982.0.nnrp-02.9e98a25e@news.demon.co.uk>, "Blake
> Weston" <blake@dedlock.demon.co.uk> wrote:
> 
> >Hi there,
> >
> >I have a bit of a problem. Basically I am using Ada.Direct_IO to read /
> >write to a file. The problem being I wish to write records of differing
> >types to the file, without closing the file and re-opening it.
> >
> >Ada 95 will not even let me open the same file as two different
file_type
> >variables as this creates a Use Exception error.
> >
> >Can anybody help?
> 
> It sounds like you want to perform heterogeneous I/O.  Direct_IO is for
> homogeneous I/O.  Try Ada.Stream_I/O instead.
> 
> --------------------------------------------------------------------
> Matthew Heaney
> Software Development Consultant
> <mailto:matthew_heaney@acm.org>
> (818) 985-1271
> 

Re: Multiple instantiation of Ada.Direct_IO

[Robert Dewar]

Blake asks

<<I have a bit of a problem. Basically I am using Ada.Direct_IO to read /
write to a file. The problem being I wish to write records of differing
types to the file, without closing the file and re-opening it.

Ada 95 will not even let me open the same file as two different file_type
variables as this creates a Use Exception error.

Can anybody help?

Thanks

Blake
>>


GNAT fully implements file sharing, but there is a penalty involved in
the use of this feature -- you have to read the section of the manual
on file sharing, and I understand that this is too much of a burden for
many people :-)

to be fair, any approach using file sharing is NOT necessarily portable,
which is why GNAT does not allow it by default!