Re: Garbage Collection in Ada

[Hans-Juergen Boehm <boehm@mti.sgi.com>]

Robert Dewar wrote:

> The problem is that it is all too easy to have a global variable around
> that accidentally holds on to a giant structure. Or for example, depending
> on your implementation, taking the 'Access of one field of one record in
> some big structure may end up holding on to an entire subgraph that you
> do not need.
> 
> You may in practice need to be just as careful about adding
> 
>    Junk := null;
> 
> statements to your program as you are now about adding
> 
>    Free (Junk);
> 
> statements to your program now.
> 
> GC is certainly a magic bullet solution to the problem of accidentally
> freeing storage that should not be freed, but it is NOT a magic bullet
> with respect to not freeing things that should be freed. People who
> approach a GC language environment with this expectation will be
> disappointed!

That's certainly all true.  But it's important to remember that there
are 2 important differences between inserting 

Junk := null;

and

Free (Junk);

1.  The first asserts only that the pointer Junk is no longer needed.
It makes no claims about any other uses of the object referenced by
Junk.  If another thread is still accessing it through another variable
that's fine.  In the second you may well get a very hard-to-debug memory
smash in another module.  Free asserts there are no other uses of the
referenced object, a much stronger assertion that generally requires
much more global knowledge.  Clearing variables rarely requires
additional complexity in intrfaces.  Fully manual memory management
often does, since you need to talk about ownership etc.

2.  If Junk is really a global variable, then not clearing it usually
introduces a BOUNDED leak.  The memory will only be retained until the
next assignement to Junk.  Forgetting the Free in an uncollected system
is much more disastrous.  (There are cases in which not clearing a
variable introduces an unbounded leak.  But they're much less common.)

-- 
Hans-Juergen Boehm
boehm@mti.sgi.com