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From: Robert Dewar <dewar@gnat.com>
Subject: Re: garbage collection
Date: 1999/08/19
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Date: 1999-08-19T00:00:00+00:00
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In article <37BC2203.328E6338@averstar.com>,
  Tucker Taft <stt@averstar.com> wrote:
> Keith Thompson wrote:
> >
> > tmoran@bix.com writes:
> > > > If an implementation doesn't provide garbage collection
then that
> > > > storage is forever lost, is that correct?
> > >
> > > No.  When the access type goes out of scope there is
clearly no way
> > > the storage can be on the target end of a pointer, so the
> > > implementation should free that storage.
> >
> > Just to clarify, most implementations don't actually do
this.  Thus,
> > under most Ada implementations, storage *will* be "forever
lost" if
> > you allocate it and don't explicitly deallocate it.  (Well,
"forever"
> > usually means "until the program terminates".)
>
> In Ada 95, implementations are required to reclaim the storage
associated
> with an access type if the 'Storage_Size attribute is
specified (which
> Tom also mentioned, by the way).  If no 'Storage_Size is
specified, then
> you are correct that most Ada 95 compilers don't reclaim the
storage
> automatically when leaving the scope of the access type.

Ada compilers have differed here. The old Alsys compilers always
used to automatically free local access stuff on exit. The
trouble is that people are quite used to controlling this
memory manually, and in this case you do add quite a bit
of overhead by arranging for the automatic release.

We went backwards and forwards on what the default should be
in GNAT, and decided finally that the default was not to free
this storage, however, it is trivial to specify that the space
for a local access collection SHOULD be freed on access by
using:

   for access_type_name'Storage_Pool
     use System.Pool_Local.Unbounded_Reclaim_Pool;

Probably a good idea would be to add a configuration pragma
that made this pool assignment the default.

By the way, a recommendation is to look through the collection
of storage pool implementations provided in GNAT, there is a lot
of useful functionality there, including a very useful debug
pool that checks for correct storage handling.

P.S. Ada 83 also required that access types with an explicit
storage size be freed on scope exit. The Ada 83 RM strongly
suggests that this should be the default for all access types,
but stops short of requiring this. Even the implication is
removed from the Ada 95 RM.

Robert Dewar
Ada Core Technologies


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