Re: How to convert record?

["Marc A. Criley" <mcqada@earthlink.net>]

Joseph Kwan wrote:
> 
> john.mccabe@emrad.com.nospam (John McCabe) wrote in message news:<3bd402f9.8783319@news.demon.co.uk>...
> > On 21 Oct 2001 02:56:25 -0700, joseph_kwan@greenlime.com (Joseph Kwan)
> > wrote:
> >
> > >I want to send a command buffer to serial port using florist's POSIX
> > >interface but I have encountered problems in converting record to
> > >POSIX.IO.IO_Buffer or Ada.Streams.Stream_Element_Array. How can I
> > >overcome this problem?
> >
> > Without looking at the Reference Manual, I would initially suggest
> > looking at Ada.Unchecked_Conversion to do the job.
> 
> I thought that would work too but theoretically,
> Ada.Unchecked_Conversion will not work in converting record to
> POSIX.IO.IO_Buffer which is a string of characters (See spec in
> florist's implementation).
> 
> Anyhow I tried to do Unchecked_Conversion and gnat gave me error
> message:
> 
> "unchecked conversion to unconstrained array not allowed".
> 
> What should I do?

Caveat:  I don't have POSIX stuff here, so I'm making some assumptions.

1) How many storage units (i.e., bytes) do objects of that record type
require?  Use 'Size on a _variable_ of that type, not the type itself,
and divide by System.Storage_Unit (i.e., 8).

2) How many elements of IO_Buffer are required to occupy that same
number of storage units?  (If IO_Buffer is an array of Character, then
it is almost certainly the same as the number of storage units from
above.)

3) Define a subtype of IO_Buffer with an index constraint containing
that number of elements.

4) Instantiate Unchecked_Conversion with your record type and that
IO_Buffer subtype.

5) Convert.

Marc A. Criley
Senior Staff Engineer
Quadrus Corporation
www.quadruscorp.com