RE: Variable length raw-byte data

[tmoran@acm.org]

>Or
>In your interface routine, receive the pointer as an
>address, and then unchecked convert it.
> ...
>  type Byte_Array is array (positive range <>) of Byte;
>  buffer_Address : System.Address;
> ...
>      subtype Constrained_Byte_Array is Byte_Array(1..length);
>      function To_Buffer_Pointer is
>        new Ada.Unchecked_Conversion(System.Address,Constrained_Byte_Array);
> ...
>Something like that.  (Constructive corrections only please!)
  Instead of trying to use Unchecked_Conversion to change an address
into an access type (I presume that was meant here), use
System.Address_To_Access_Conversions.  That should work.  But a
System.Address need not be the same size as an access type (consider
various Intel memory models), and even an access to an unconstrained
array (which will involve an actual 'range being stored somewhere) and
an access to a constrained array (where the compiler might keep the
range somewhere else) may be different, and different from a
System.Address.

>I'm trying to shim to a C library that returns some data as a length in
>bytes and a pointer to the first byte. The length is not known at compile
 type Data_Array is array(1 .. 10_000) of Byte; -- big enough constrained array
 type Data_Array_Ptr is access all Data_Array;
 P : aliased Data_Array_Ptr;
 N : aliased Natural;
 ...
 if C_Func(N'access, P'access) /= Failure then ...
 -- P.all(0 .. N) contains result

If Data_Array is not constrained, then a pointer to one, ie a
Data_Array_Ptr, must handle the range somewhere.  If it's a fat pointer,
it's unlikely C_Func will be so kind as to supply it correctly.  If it
points to a descriptor of some kind for the actual array, it's again
unlikely C_Func will return such a beast.