Re: Data Overlays

[cis.ohio-state.edu!magnus.acs.ohio-state.edu!usenet.ins.cwru.edu!agate!do]

In article <1993Aug18.003940.2208@leeweyr.sccsi.com> bill@leeweyr.sccsi.com 
(Bill Lee) writes:

>   O.k., Here's my problem:
>
>   I have a system which receives "messages" from a number of other
>   systems. The messages come via POSIX queues. On any one queue, I
>   can receive a number of different messages, mostly with record
>   structures defined by the C code where the message originated.
>
>   I do not know a priori what the next message will be. I have to
>   "receive" the message into a Byte_Array. Once I have the message,
>   I can access a standard header and extract a message identifier,
>   which then tells me what kind of message I've just received.
>   Once I have a message in a Byte_Array, and once I know what kind
>   of message it is, I can then convert the message to an Ada record
>   which has the right structure for the message.
>
>   I've considered both methods above. Certainly, Unchecked_Conversion
>   is one alternative. But I have used another method which seems to
>   work, seems to be portable, and, moreover, SEEMS to be in line with
>   what the language designers intended. To wit:
>
>   Given an Ada type, Messages:
>
>   generic
>     type Byte_Arrays is private;
>   function Extract_Message ( A_Message : in Byte_Arrays ) return Messages;
>
>   where the body looks like
>
>   function Extract_Message ( A_Message : in Byte_Arrays ) return Messages is
>     Temp_Message : Messages;
>     for Temp_Message use at A_Message'address;
>   begin
>     return Temp_Message;
>   end Extract_Message;
>
>   In many cases, I have actually created a more "Ada-like" message
>   type, and I do far more than just return the C structure.
>
>   I like the "feel" of this method. It uses the representation
>   specification which allows a structure to be placed "at" an address.
>   If this usage is not what the language designers had in mind, then
>   what was the purpose of the rep spec?

I believe the purpose of the "for...use at" clause is to allow you to
declare data whose address is fixed by, say, the processor
architecture or the operating system.

>   Is there any inherent danger or "wrongness" in this technique? And
>   yes, I have read the "for the purpose of overlaying....erroneous"
>   section in the LRM. Is this overlaying? Is it erroneous?

Yep, it's overlaying, since you have two Ada objects (A_Message and
Temp_Message) that you've declared to be at the same location.

>   Incidently, I use this same technique in bindings where I have to
>   deal with structures created in C. The C code malloc's the memory
>   and places structs in it, and then just passes back the C pointer.
>   I make the assumption (possibly in error in some cases) that a C
>   pointer and a System.Address are coincident. Then I can calculate
>   the address in the malloc'ed memory where a specific C structure
>   exists, and I can assign an Ada record "at" that address in order
>   to extract the information from it.

I'm assuming here that you're using PRAGMA IMPORT_FUNCTION or
something to get at your C code.  If that's the case, why not just
make the Ada function declaration return an ACCESS type to the desired
Ada structure?  (Unless, of course, you don't know beforehand which
Ada type the C function result will point to.)  You shouldn't even
have to worry about possible differences in representation between a C
pointer and an Ada access object; DEC Ada is good at taking care of
those things for you.

                                -- Adam