Re: Uninitialized variables (was: Re: private types)

["Randy Brukardt" <randy@rrsoftware.com>]

"Dirk Craeynest" <dirk@apollo.cs.kuleuven.ac.be> wrote in message
news:dvgh0s$rtu$1@apollo.cs.kuleuven.ac.be...
> [This thread really is about uninitialized variables now, so I changed
> the subject...]
>
> In article <e_mdncFXrsg5wobZnZ2dnUVZ_t2dnZ2d@megapath.net>,
> Randy Brukardt <randy@rrsoftware.com> wrote:
> >So I recommend initalizing everything (or assigning it immediately
> >after the begin) that could be significant to performance.
>
> We did (and do) feel this is not a good approach, at least not when
> using GNAT or another compiler that supports something like the pragma
> Initialize_Scalars and enhanced validity checking.
>
> For much more about uninitialized variables in Ada code, the following
> paper might be useful:
>
>   "Exposing Uninitialized Variables: Strengthening and Extending
>   Run-Time Checks in Ada" [1],
>   Robert Dewar, Olivier Hainque, Dirk Craeynest, and Philippe
>   Waroquiers,
>   In "Proceedings of the 7th International Conference on Reliable
>   Software Technologies - Ada-Europe 2002" [2], Vienna, Austria,
>   June 17-21, 2002, Johan Blieberger and Alfred Strohmeier (Eds.),
>   volume 2361 of Lecture Notes in Computer Science, pages 193-204,
>   Springer-Verlag, 2002.
>
> The conclusion of that paper contains our recommendation:
>
> ---start-quote---
>
> 5.3 Impact of Usage of Initialize Scalars on How to Program
>
> There is a trend in programming guidelines to "force" initializing
> everything at declaration resulting in code like:
>
>   B : Natural := 0;
>
>   if .... then
>     B := 5;
>   else
>     B := 8;
>   end if;
>
> The difficulty with such an approach is that the initial value is
> meaningless. If this value is used accidentally, the results are
> potentially just as wrong as the use of an uninitialized value, and
> furthermore, the explicit initialization precludes the approach we
> have described in this paper, and thus may introduce bugs that are much
> harder to find and fix. The automatic initialization under control of
> the compiler using Initialize Scalars is a far preferable approach.
>
> We therefore recommend that when a scalar is declared, the programmer
> should avoid initializing it if the code is supposed to set the value
> on all paths.  It is better to let Initialize Scalars + gnatVa detect
> the bug in the code logic rather than trying to deal with meaningless
> initial values. Even for safety-critical programs, we can first compile
> with Initialize Scalars + gnatVa + invalid values and then, if needed,
> field the code with Initialize Scalars + all zero values (if it is the
> case that zero values give the code a better chance of avoiding
> seriously improper behavior).
>
> ---end-quote---
>
> The GNAT manuals provide more information on GNAT's pragma
> Initialize_Scalars [3] and on enhanced validity checking [4].
> Reference [3] mentions:
>
> ---start-quote---
>
> Note that pragma Initialize_Scalars is particularly useful in
> conjunction with the enhanced validity checking that is now provided
> in GNAT, which checks for invalid values under more conditions.
> Using this feature (see description of the -gnatV flag in the users
> guide) in conjunction with pragma Initialize_Scalars provides a
> powerful new tool to assist in the detection of problems caused by
> uninitialized variables.
>
> ---end-quote---
>
> We can assure everyone that from a developers and testers point of view
> the combination of Initialize_Scalars and enhanced validity checking is
> indeed "particularly useful".
>
> References:
> [1] <http://www.cs.kuleuven.be/~dirk/papers/ae02cfmu-paper.pdf>
> [2] <http://www.springer.de/cgi/svcat/search_book.pl?isbn=3-540-43784-3>
> [3]
<http://www.adacore.com/wp-content/files/auto_update/gnat-unw-docs/html/gnat
_rm_2.html#SEC48>
> [4]
<http://www.adacore.com/wp-content/files/auto_update/gnat-unw-docs/html/gnat
_ugn_4.html#SEC47>
>
> Dirk
> Dirk.Craeynest@cs.kuleuven.be (for Ada-Belgium/-Europe/SIGAda/WG9 mail)
>
> *** 11th Intl.Conf.on Reliable Software Technologies - Ada-Europe'2006
> *** June 5-9, 2006 ** Porto, Portugal ** http://www.ada-europe.org ***