Re: Y21C Bug

[Florian Weimer <usenet@deneb.cygnus.argh.org>]

Robert A Duff <bobduff@world.std.com> writes:

> Robert Dewar <robert_dewar@my-deja.com> writes:
> 
> > 1. The ability to declare very large areas of memory with
> > commit-on-use semantics. There are many uses of this. I have
> > the feeling that a lot of programmers still don't understand
> > commit-on-use.
> 
> Commit on use is indeed very cool.
> 
> Do you know which operating systems support it?

I think commit on use is the same thing what's called copy on write
on Linux.  For example, you can run the following program on a box
which only has some 300 MB of virtual memory (RAM + swap), and it will
`allocate' approximately 1.5 GB of `memory':

with Ada.Text_IO, Ada.Integer_Text_IO;
use Ada.Text_IO, Ada.Integer_Text_IO;
procedure Alloc_Mem is
   type Long_String is new String (1 .. 1024 * 1024);
   type Long_String_Access is access Long_String;
   LSA : array (1 .. 1500) of Long_String_Access;
   Byte_Count : Natural := 0;
begin
   for I in LSA'Range loop
      LSA (I) := new Long_String;
      Byte_Count := Byte_Count + Long_String'Size / 8;
   end loop;
   Put ("Successfully allocated ");
   Put (Byte_Count, 0);
   Put_Line (" bytes.");
exception
   when Storage_Error =>
      Put ("Allocation failure after ");
      Put (Byte_Count, 0);
      Put_Line (" bytes.");
end Alloc_Mem;

Of course, you shouldn't use an initialized allocator here. ;)