Re: Gnat For use at Question

[jsa@organon.com (Jon S Anthony)]

In article <4s2eb5$qt6@masala.cc.uh.edu> cosc19z5@Bayou.UH.EDU (Spasmo) writes:

> I'm trying to create an array at an absolute memory location, but
> I keep on getting some error with "expect System.Address blah blah"
> whenever I try.

Actually, the error messages are _extremely_ clear and tell you _exactly_
what is wrong and even how to fix it:

procedure Junk2 is
     Some_Array : String(1..4000);
        for Some_Array'Address use 16#bbbbb#;
 begin
    null;
end;

$ gnat junk2.adb
junk2.adb:4:36: expected private type "System.Address"
junk2.adb:4:36: found type universal integer
junk2.adb:4:36: invalid address clause for "Some_Array"
junk2.adb:4:36: must be constant defined before "Some_Array" (RM 13.1(22))

As you can see there is even an RM reference.  Let's just follow our
nose a little on this error trail and see what pops out.  The first
error sez that at the 16#... stuff location the compiler expected
System.Address but found universal integer.  Hence, 16#... is a
universal integer and it is no good for an address clause.  So, you
know that you need to make the value an instance of System.Address for
it to have a hope of working.  Next, the errors say that the clause
itself is wrong because it must be a _constant_ defined _before_ the
variable to which it applies (Some_Array).  Even gives an RM ref for
this.  So, you now know that you must define the address as a constant
before the variable Some_Array and that it must be of type
System.Address.  So,

with System.Storage_Elements;
use  System.Storage_Elements;
procedure Junk2 is
    Some_Array_Address : constant System.Address := To_Address(16#bbbbb#);
    Some_Array : String(1..4000);
        for Some_Array'Address use Some_Array_Address; 
begin
    null;
end;

$ gnat junk2.adb
$

The only goofy thing about this I suppose, is that To_Address (which takes
integers and returns the proper address) is not defined in System (along
with Address) but in the child Storage_Elements.  Goodness knows how that
happened...

/Jon
-- 
Jon Anthony
Organon Motives, Inc.
1 Williston Road, Suite 4
Belmont, MA 02178

617.484.3383
jsa@organon.com