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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Software landmines (loops)
Date: 1998/09/01
Message-ID: <EACHUS.98Sep1180642@spectre.mitre.org>#1/1
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References: <slrn6sopmg.4fg.jdege@jdege.visi.com>
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 <902934874.2099.0.nnrp-10.c246a717@news.demon.co.uk> <dewar.903074236@merv>
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Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-09-01T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <6sf87j$47n$1@hirame.wwa.com> "Robert Martin" <rmartin@oma.com> writes:

 > If this is more complex (something that is arguable) it is not *much* more
 > complex.  On the other hand, it is easier to maintain.  The thread-safety
 > issue I talked about above would be easier to add to this function than to
 > the one with multiple returns.

   And as far as some of us are concerned, there is no difference,
because the way to implement the guarded function is to wrap the guard
around the test:

   function "=" (L, R : Guarded_Stack_Type) return Boolean is
     Temp: Boolean;
   begin
     Seize(L.Guard); Seize(R.Guard);
     Temp := Stack_Type(L) = Stack_Type(R);
     Release(R.Guard); Release(L.Guard);
     return Temp;
   end "=";

   With the obvious type declaration for the guarded stack:

   type Guarded_Stack_Type is new Stack with Guard: Semaphore;

   This implements the guarded equality in terms of the (known good)
test for the parent type.  Of course, this particular implementation
is subject to deadlock, so I would either use a single semaphore for
all stacks, or a more complex locking strategy, probably locking the
stack with the lower address first:

   function "=" (L, R : Guarded_Stack_Type) return Boolean is
     Temp: Boolean;
   begin
     if L'Address = R'Address then return True; end if;
     
     if L'Address < R'Address
     then Seize(L.Guard); Seize(R.Guard);
     else Seize(R.Guard); Seize(L.Guard);
     end if;

     Temp := Stack_Type(L) = Stack_Type(R);
     Release(R.Guard); Release(L.Guard); -- release order doesn't matter.
     return Temp;

   end "=";
   
   Now we have a tasking/thread safe comparison, but we didn't have to
mix up the three cases here with the three cases in the original "=".
So I have six cases to test, not nine.  Actually, I get more benefit,
because I only have three NEW cases to test.

   Note to Ada programmers:  At first it seems right to make the
entire stack type a protected object.  But how would you define the
"=" and any other two stack operations in that case?  Once you realize
that there needs to be a pair of seize and release operations, you
might as well associate them with a new component of the object
anyway.

   Also--for Ada and non-Ada programmers--the norm would be to do all
this in the body of the package which defined Guarded_Stack_Type as a
private extension of Stack_Type.  Now the interface profile of the two
types can be the same, since all the semaphore manipulations can be
hidden from view.
--

					Robert I. Eachus

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