Re: Importing C Structures

[vladimir@speedy.intrepid.com (Vladimir Vukicevic)]

In article <3ksg66$h1c@newssvr.cacd.rockwell.com> rswhite@cacd.rockwell.com (Robert S. White) writes:

> Do you have a clean way to handle hardware registers that have one
> meaning when you read them and another when you write to them with
> nice seperate names for each type of usage?  I know "use at" for 
> Ada 83 can overide this but it seems to be not in the spirit of 
> the 83 LRM;
>   13.5 "Address clauses should not be used to achieve overlays of
>         objects...Any program using address clauses to achieve
>         such effects is erroneous."
> A work around is to have a dual meaning name for the register and
> use one procedure, appropriately named, to write to it and a function,
> again with a suitable name, for reading it.

There is no such clause in the Ada 95 LRM... the "Erroneous Execution"
section for using a X'Address representation clause (13.3 (13)) states:

       If an Address is specified, it is the programmer's responsibility
       to ensure that the address is valid; otherwise, program execution
       is erroneous. 

So, it is perfectly valid to use the same 'Address for two objects, as
long as the Address you specify is valid :-)

> This is a case where C programmers use the union structure.  I 
> confess that I have not ferreted out in the Ada 95 RM or Rational 
> the right (and very concise) way to do this now.
> 
> Recommendations? 

Something like this (in Ada 95):

Input_Register : Integer;
for Input_Register'Address use
	System.Storage_Elements.To_Address (16#0020);

Output_Register : Integer;
for Output_Register'Address use
	System.Storage_Elements.To_Address (16#0020);

This is perfectly valid and within the spirit of the 95 LRM.

	- Vladimir