Re: Q: How to do memory access with gnat - OS.ADS (0/1)

["Michael Erdmann" <boavista@berlin.snafu.de>]

On Sun, 20 Jul 1997 13:07:26 GMT, Tucker Taft wrote:

:>Michael Erdmann (boavista@berlin.snafu.de) wrote:
::>
:>You can use an address clause to associate an Ada name with a given
:>address.  E.g.:
:>    Shared : Msg_Data;
:>    for Shared'Address use <arbitrary computation>;
:>  begin
:>    Shared.data(I) := ...
The important point is this assignment here. The example is from a simple
message 
passing system where the size of the data array message is not known. 
The message record is defined like this:

	 type Message_Record( Record_Size : Integer ) is record
        	    Nbr_Of_Bytes		: Integer := Record_Size;
         	   data			: DataBuffer(1..Record_Size);
      	end record;

The Problem is when i am doing it like this;

	RC := DosAllocateShared( .... shared_data )
	Msg := To_Message( shared_data );	-- convert to access to
Message_Record
	Msg.Nbr_Of_Bytes := ,,,,,;
	Msg.data(1,,Nbr_Of_Bytes) := data(1....)

a contraint error is generated in the last assignment.
:>
:>Alternatively, the most direct analogy to the C approach is to declare
:>an access type, and initialize a variable of the type using 
:>unchecked_conversion or the Address_To_Access_Conversion generic, 

I have done this. In order to by pass the problem with the array above i
thought about
a procedure which would allow to store data relative to a base address. The
problem 
is i am not able to add an integer offset to an address, because it is not
possible
to convert  from Integer to Stoarge_Offset from System.Storage_Elements.

>or by
:>passing its 'Address to some external routine.

I hope i dont need to do it!