Re: Pointer address

[Martin Krischik <martin@krischik.com>]

Bj?rn wrote:

> I'm trying to convert some C code into Ada.
> I'm stuck on the follwing:
> cpdata.dwData = 0x0001B001 (dwData being a ptr to an ulong)
> 
> Trying to execute the code below gives me a:
> EXCEPTION_ACCESS_VIOLATION
> 
> If I change the address to 16#0001B000# the program just silently
> exits/terminates without printing my put_line.
> 
> If I try to set DwData with an unchecked_conversion instead, using
> address 16#0001B001#, the Address_Image returns 16#0001B000#.
> 
> I'm quite sure I'm doing something wrong here and need to read up a
> bit on it. Could someone please give me some pointers here and maybe
> the applicable parts of the ARM.
> 
> When I compile this on win-gnat-3.15 I get the following warnings:
> temp.adb:17:08: warning: default initialization of "DwData" may modify
> overlaid storage
> temp.adb:17:08: warning: use pragma Import for "DwData" to suppress
> initialization (RM B.1(24))
> Adding an import pragma didn't seem to help more than removing the
> warnings.
> 
> Thanks
> Bjï¿œrn
> 
> 
> with Interfaces.C;
> with Unchecked_Conversion;
> with System.Storage_Elements;
> with System.Address_Image;
> with Text_IO;
> 
> procedure Temp is
> 
>    type PULONG is access all Interfaces.C.Unsigned_Long;
>    function PULONG_To_Address is new Unchecked_Conversion
>      (PULONG, System.Address);
>    Some_Address : constant System.Address :=
>      System.Storage_Elements.To_Address(16#0001B000#);
>    DwData : PULONG;
>    for DwData'Address use Some_Address;
>    --  pragma Import (C, DwData);
> begin
>    Text_IO.Put_Line ("dwData address = " & System.Address_Image
>                      (PULONG_To_Address (DwData)));
> end;

You should not use Unchecked_Conversion. Read:

http://en.wikibooks.org/wiki/Programming:Ada:Subtypes#Address_Convertion

With Regards

Martin
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