Re: Misaligned address error

[Adam Beneschan <adam@irvine.com>]

On Sep 7, 2:05 pm, shaunpatter...@gmail.com wrote:
> i'm working with some really old legacy code that seemed to be working
> fine with gnat pro 3.16a1.  However, we decided to upgrade to gnat
> 5.
>
> Everything built fine - however, I'm getting a misaligned address
> value error at runtime:
>
> raised PROGRAM_ERROR : data_reader.2.ada:109 misaligned address
> value
>
> The code:
>
>   subtype Data_Type is Character;
>
>   type Data_Type_Array is array (Natural range <>) of Data_Type;
>
>   Data_String : Data_Type_Array (1 .. 8) := (others => Data_Type
> (Ascii.Nul));
>   for Data_String'Alignment use 4;
>   for Data_String'Size use 8 * Data_Type'Size;
>   -- global data buffer for reading and writing to file
>
>   -- quick conversion variables
>   One_Byte_Number : One_Byte_Number_Type;
>   for One_Byte_Number'Address use Data_String'Address;
>
>   Two_Byte_Number : Two_Byte_Number_Type;
>   for Two_Byte_Number'Address use Data_String'Address;
>
>   Four_Byte_Number : Four_Byte_Number_Type;
>   for Four_Byte_Number'Address use Data_String'Address;
>
>   Four_Byte_Signed_Number : Four_Byte_Signed_Number_Type;
>   for Four_Byte_Signed_Number'Address use Data_String'Address;
>
>   Four_Byte_Float : Float_Type;
>   for Four_Byte_Float'Address use Data_String'Address;
>
>   Eight_Byte_Float : Long_Float_Type;
>   for Eight_Byte_Float'Address use Data_String'Address;  --- FAILS ON
> THIS LINE ---
>
> Could anyone help me make sense of this exception?

I don't know what a Long_Float_Type is.  What is it, and more
importantly, what is Long_Float_Type'Alignment?  Since
Data_String'Alignment is 4, Data_String can be located on any address
that is a multiple of 4 bytes (I assume you're compiling for a byte-
addressable machine); if it got located on an address that is a
multiple of 4 but not a multiple of 8, and if
Long_Float_Type'Alignment is 8, then the address clause can't be
honored properly.  (RM 13.3(27) says program execution is erroneous in
this case; GNAT has decided to make this raise a Program_Error at
runtime.)  If Long_Float_Type'Alignment is indeed 8, then you should
change Data_String'Alignment to 8 also, to make sure that Data_String
will be on an 8-byte boundary.  (P.S. I don't know GNAT well enough to
know whether that will succeed; not all Alignment clauses have to be
supported.)

(Apologies if this is a duplicate post.  Google is acting up, yet
again.)

Hope this helps,

                    -- Adam