Accessibility checks

Accessibility checks

[Florian Weimer]

One thing in Ada I could never quite grasp where the accessiblity
checks.  I haven't seen much Ada code, and personally, I didn't have
much need for nested access types anyway.  If I have used them, they
were still potentially unsafe due to aliasing or calls to
Unchecked_Deallocation in neighboring code.

So what's the point for this language feature?  It seems to add quite
a bit of complexity to the language.

Re: Accessibility checks

[Alan Jump]

Not all that complex, from where I sit. Here's a good, concise description:
http://www.adacore.com/adaanswers/gems/gem-33/

In short, enforcing accessibility eliminates dangling pointers and their associated...umm...issues.

- -
73 de N5ILN
Alan

On Wednesday, July 17, 2013 1:09:24 PM UTC-7, Florian Weimer wrote:
> One thing in Ada I could never quite grasp where the accessiblity
> 
> checks.  I haven't seen much Ada code, and personally, I didn't have
> 
> much need for nested access types anyway.  If I have used them, they
> 
> were still potentially unsafe due to aliasing or calls to
> 
> Unchecked_Deallocation in neighboring code.
> 
> 
> 
> So what's the point for this language feature?  It seems to add quite
> 
> a bit of complexity to the language.

Re: Accessibility checks

[Jeffrey Carter]

On 07/17/2013 01:09 PM, Florian Weimer wrote:
> One thing in Ada I could never quite grasp where the accessiblity
> checks.  I haven't seen much Ada code, and personally, I didn't have
> much need for nested access types anyway.  If I have used them, they
> were still potentially unsafe due to aliasing or calls to
> Unchecked_Deallocation in neighboring code.
>
> So what's the point for this language feature?  It seems to add quite
> a bit of complexity to the language.

Ada 83 didn't have these checks, and had only named access types that could only 
designate objects created by an allocator ("new").

Accessibility rules were introduced in Ada 95, along with general access types 
(declared with "all") that can access objects allocated on the stack, the 
'Access and 'Unchecked_Access attributes, and anonymous access types in the 
forms of access parameters and access discriminants.

Anonymous types are a Bad Idea, and anonymous access types are a Very Bad Idea. 
The language would be better off without them.

Brukardt, ARG member and editor of the ARM, claims that 'Access never works and 
'Unchecked_Access is always required, bypassing the accessibility checks. While 
this is not entirely true, it's quite common that one must use 
'Unchecked_Access, so we could do without these rules and checks in the majority 
of cases.

-- 
Jeff Carter
"I unclog my nose towards you."
Monty Python & the Holy Grail
11

Re: Accessibility checks

[Randy Brukardt]

"Jeffrey Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:ks718g$aim$2@dont-email.me...
> On 07/17/2013 01:09 PM, Florian Weimer wrote:
>> One thing in Ada I could never quite grasp where the accessiblity
>> checks.  I haven't seen much Ada code, and personally, I didn't have
>> much need for nested access types anyway.  If I have used them, they
>> were still potentially unsafe due to aliasing or calls to
>> Unchecked_Deallocation in neighboring code.
>>
>> So what's the point for this language feature?  It seems to add quite
>> a bit of complexity to the language.
>
> Ada 83 didn't have these checks, and had only named access types that 
> could only designate objects created by an allocator ("new").
>
> Accessibility rules were introduced in Ada 95, along with general access 
> types (declared with "all") that can access objects allocated on the 
> stack, the 'Access and 'Unchecked_Access attributes, and anonymous access 
> types in the forms of access parameters and access discriminants.
>
> Anonymous types are a Bad Idea, and anonymous access types are a Very Bad 
> Idea. The language would be better off without them.
>
> Brukardt, ARG member and editor of the ARM, claims that 'Access never 
> works and 'Unchecked_Access is always required, bypassing the 
> accessibility checks. While this is not entirely true, it's quite common 
> that one must use 'Unchecked_Access, so we could do without these rules 
> and checks in the majority of cases.

That's not entirely true: I did find one (and only one) instance where I was 
able to use 'Access in my code (it had to do with a library-level 
initialization).

But for the most part, accessibility gets in the way more than it helps 
anything. It does prevent some gross errors, but that's about it. The 
dynamic checks are actually better, but that assumes that they're 
implemented properly, and that has some distributed overhead.

The biggest advantage of the accessibility checks is that they taught us 
(the ARG) what not to do. :-) When we designed anti-aliasing rules for Ada 
2012, one of the big concerns was that they wouldn't turn into accessibility 
checks. So we only make checks in cases where it's obvious that there is 
something dubious going on, and there is no attempt at completeness.

It would be tempting to dump the entire accessibility mess into trash, but 
the only alternative is erroneousness, which is too awful to contemplate.

                                        Randy.

Re: Accessibility checks

[Bill Findlay]

On 18/07/2013 00:44, in article ks7a8v$4nj$1@loke.gir.dk, "Randy Brukardt"
<randy@rrsoftware.com> wrote:

> "Jeffrey Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message
> news:ks718g$aim$2@dont-email.me...

>> Brukardt, ARG member and editor of the ARM, claims that 'Access never
>> works and 'Unchecked_Access is always required, bypassing the
>> accessibility checks. While this is not entirely true, it's quite common
>> that one must use 'Unchecked_Access, so we could do without these rules
>> and checks in the majority of cases.
> 
> That's not entirely true: I did find one (and only one) instance where I was
> able to use 'Access in my code (it had to do with a library-level
> initialization).

Interesting.

My projects have 37 occurrences of 'Access and only 2 of 'Unchecked_Access.

-- 
Bill Findlay
with blueyonder.co.uk;
use  surname & forename;

Re: Accessibility checks

[Jeffrey Carter]

On 07/17/2013 04:44 PM, Randy Brukardt wrote:
>
> That's not entirely true: I did find one (and only one) instance where I was
> able to use 'Access in my code (it had to do with a library-level
> initialization).

I apologize if my memory was faulty. I too have found cases where 'Access 
worked, but like you I have found that they are rare.

> It would be tempting to dump the entire accessibility mess into trash, but
> the only alternative is erroneousness, which is too awful to contemplate.

Isn't that what you get when you use 'Unchecked_Access?

-- 
Jeff Carter
"I unclog my nose towards you."
Monty Python & the Holy Grail
11

Re: Accessibility checks

[Simon Wright]

Bill Findlay <yaldnif.w@blueyonder.co.uk> writes:

> On 18/07/2013 00:44, in article ks7a8v$4nj$1@loke.gir.dk, "Randy Brukardt"
> <randy@rrsoftware.com> wrote:
>
>> "Jeffrey Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message
>> news:ks718g$aim$2@dont-email.me...
>
>>> Brukardt, ARG member and editor of the ARM, claims that 'Access
>>> never works and 'Unchecked_Access is always required, bypassing the
>>> accessibility checks. While this is not entirely true, it's quite
>>> common that one must use 'Unchecked_Access, so we could do without
>>> these rules and checks in the majority of cases.
>> 
>> That's not entirely true: I did find one (and only one) instance
>> where I was able to use 'Access in my code (it had to do with a
>> library-level initialization).
>
> Interesting.
>
> My projects have 37 occurrences of 'Access and only 2 of
> 'Unchecked_Access.

A program which constructs a partial representation of a UML model, and
then walks the representation, used 'Access 43 times to iterate over
containers (in Ada 2005!) and 'Unchecked_Access 38 times to construct
the representation.

Re: Accessibility checks

[Randy Brukardt]

"Bill Findlay" <yaldnif.w@blueyonder.co.uk> wrote in message 
news:CE0CF7DE.329A0%yaldnif.w@blueyonder.co.uk...
> On 18/07/2013 00:44, in article ks7a8v$4nj$1@loke.gir.dk, "Randy Brukardt"
> <randy@rrsoftware.com> wrote:
>
>> "Jeffrey Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message
>> news:ks718g$aim$2@dont-email.me...
>
>>> Brukardt, ARG member and editor of the ARM, claims that 'Access never
>>> works and 'Unchecked_Access is always required, bypassing the
>>> accessibility checks. While this is not entirely true, it's quite common
>>> that one must use 'Unchecked_Access, so we could do without these rules
>>> and checks in the majority of cases.
>>
>> That's not entirely true: I did find one (and only one) instance where I 
>> was
>> able to use 'Access in my code (it had to do with a library-level
>> initialization).
>
> Interesting.
>
> My projects have 37 occurrences of 'Access and only 2 of 
> 'Unchecked_Access.

I recall Tucker telling me (after one of my frequent rants about the 
uselessness of 'Access) that he used it quite often. He then deconstructed 
several possible uses for access values constructed from static objects, and 
noted that some of them fit 'Access very well, and others did not. (His 
description was much more useful than this summary, of course; he is so much 
better that me at being able to summarize ideas. That's one trait that I've 
never managed to learn, despite many examples; I'm almost always happy when 
Tucker jumps into a discussion because he often explains the issue much more 
clearly than I did. But I digress...)

I tend to use 'Access on parameters to for temporary storage during 
processing in a package (Claw does this extensively), and in that case, 
accessibility is completely in the way. We use finalization to ensure the 
saved values don't live too long, a static check is never going to work.

OTOH, creating values at initialization, for instance, works perfectly, and 
the static check is helpful. So this is a case where we have rules that help 
some programming usages and hinder others -- still not the best choice of 
programming rules.

                                    Randy. 

Re: Accessibility checks

[Randy Brukardt]

"Jeffrey Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:ks7d92$iv7$1@dont-email.me...
> On 07/17/2013 04:44 PM, Randy Brukardt wrote:
...
>> It would be tempting to dump the entire accessibility mess into trash, 
>> but
>> the only alternative is erroneousness, which is too awful to contemplate.
>
> Isn't that what you get when you use 'Unchecked_Access?

True, but in the cases where accessibility works, it would be a step 
backwards to remove protection. And at least some programmers claim that 
accessibility works for them (I presume that they are using it in contexts 
where static checking will work).

                                              Randy.

Re: Accessibility checks

[Georg Bauhaus]

On 18.07.13 09:14, Simon Wright wrote:
> Bill Findlay <yaldnif.w@blueyonder.co.uk> writes:

>> My projects have 37 occurrences of 'Access and only 2 of
>> 'Unchecked_Access.
>
> A program which constructs a partial representation of a UML model, and
> then walks the representation, used 'Access 43 times to iterate over
> containers (in Ada 2005!) and 'Unchecked_Access 38 times to construct
> the representation.

In the sources of GNAT, 'Access wins, too:

$ cat *.ads *.adb | grep -c "'Unchecked_Access"
225

$ cat *.ads *.adb | grep -c "'Access"
2156

$ cat *.ads *.adb | grep -c "'Unrestricted_Access"
998

Re: Accessibility checks

[ake.ragnar.dahlgren]

Den torsdagen den 18:e juli 2013 kl. 02:39:42 UTC+2 skrev Bill Findlay:
> On 18/07/2013 00:44, in article ks7a8v$4nj$1@loke.gir.dk, "Randy Brukardt" <randy@rrsoftware.com> wrote: > "Jeffrey Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message > news:ks718g$aim$2@dont-email.me... >> Brukardt, ARG member and editor of the ARM, claims that 'Access never >> works and 'Unchecked_Access is always required, bypassing the >> accessibility checks. While this is not entirely true, it's quite common >> that one must use 'Unchecked_Access, so we could do without these rules >> and checks in the majority of cases. > > That's not entirely true: I did find one (and only one) instance where I was > able to use 'Access in my code (it had to do with a library-level > initialization). Interesting. My projects have 37 occurrences of 'Access and only 2 of 'Unchecked_Access. -- Bill Findlay with blueyonder.co.uk; use surname & forename;

Perhaps your projects use anonymous access types?

Best regards,
Åke Ragnar Dahlgren

Re: Accessibility checks

[Bill Findlay]

On 22/07/2013 07:11, in article
9a48d84b-4160-4824-a658-48dfcf12d0d6@googlegroups.com,
"ake.ragnar.dahlgren@gmail.com" <ake.ragnar.dahlgren@gmail.com> wrote:

> Den torsdagen den 18:e juli 2013 kl. 02:39:42 UTC+2 skrev Bill Findlay:
>> On 18/07/2013 00:44, in article ks7a8v$4nj$1@loke.gir.dk, "Randy Brukardt"
>> <randy@rrsoftware.com> wrote: > "Jeffrey Carter"
>> <spam.jrcarter.not@spam.not.acm.org> wrote in message >
>> news:ks718g$aim$2@dont-email.me... >> Brukardt, ARG member and editor of the
>> ARM, claims that 'Access never >> works and 'Unchecked_Access is always
>> required, bypassing the >> accessibility checks. While this is not entirely
>> true, it's quite common >> that one must use 'Unchecked_Access, so we could
>> do without these rules >> and checks in the majority of cases. > > That's not
>> entirely true: I did find one (and only one) instance where I was > able to
>> use 'Access in my code (it had to do with a library-level > initialization).
>> Interesting. My projects have 37 occurrences of 'Access and only 2 of
>> 'Unchecked_Access. -- Bill Findlay with blueyonder.co.uk; use surname &
>> forename;
> 
> Perhaps your projects use anonymous access types?

No, I do not use any anonymous access types.

-- 
Bill Findlay
with blueyonder.co.uk;
use  surname & forename;

Re: Accessibility checks

[ake.ragnar.dahlgren]

On Monday, July 22, 2013 11:03:02 AM UTC+2, Bill Findlay wrote:
> On 22/07/2013 07:11, in article 9a48d84b-4160-4824-a658-48dfcf12d0d6@googlegroups.com, "ake.ragnar.dahlgren@gmail.com" <ake.ragnar.dahlgren@gmail.com> wrote: > Den torsdagen den 18:e juli 2013 kl. 02:39:42 UTC+2 skrev Bill Findlay: >> On 18/07/2013 00:44, in article ks7a8v$4nj$1@loke.gir.dk, "Randy Brukardt" >> <randy@rrsoftware.com> wrote: > "Jeffrey Carter" >> <spam.jrcarter.not@spam.not.acm.org> wrote in message > >> news:ks718g$aim$2@dont-email.me... >> Brukardt, ARG member and editor of the >> ARM, claims that 'Access never >> works and 'Unchecked_Access is always >> required, bypassing the >> accessibility checks. While this is not entirely >> true, it's quite common >> that one must use 'Unchecked_Access, so we could >> do without these rules >> and checks in the majority of cases. > > That's not >> entirely true: I did find one (and only one) instance where I was > able to >> use 'Access in my code (it had to do with a library-level > initialization). >> Interesting. My projects have 37 occurrences of 'Access and only 2 of >> 'Unchecked_Access. -- Bill Findlay with blueyonder.co.uk; use surname & >> forename; > > Perhaps your projects use anonymous access types? No, I do not use any anonymous access types. -- Bill Findlay with blueyonder.co.uk; use surname & forename;

I do not use anonymous access types either.

A recipe for avoiding 'Unchecked_Access could be:
1) Instantiate as many objects at the library level as is suitable to be able to use 'Access.
2) The objects that are unnecessary to instantiate at startup (to avoid long startup time if the application is HUGE and use resources more wisely) are instantiated on the heap using smart pointers á la GNATColl.

The recipe is obviously not usable in projects where dynamic allocations are forbidden.

To deal with "the objects that are unnecessary to instantiate at startup" I can think of four scenarios:
1) Using anonymous access types.
2) Using Unchecked_Access
3) Dynamic allocation on the heap.
4) Dynamic allocation on the heap using smart pointers á la GNATColl.

Case 1) and 2) can lead to access types referencing non-existant objects. Case 3) can lead to memory leaks. Out of these case 4) seems to be safest since it protects against null pointer exceptions and memory leaks (although it won't protect from out of memory exceptions). Perhaps it is possible to verify using static code analysis that one has correctly used anonymous access types and Unchecked_Access in cases 1) and 2)? Maybe someone who has used CodePeer knows?

Anyways, I would like to extend my thanks to everyone who has participated in the "Accessibility checks" discussion. It gave me something to think about.

Best regards,
Åke Ragnar Dahlgren

Re: Accessibility checks

[Dmitry A. Kazakov]

On Wed, 24 Jul 2013 06:01:21 -0700 (PDT), ake.ragnar.dahlgren@gmail.com
wrote:

> I do not use anonymous access types either.

Note that there exist quite important cases where anonymous access types
are necessary and unavoidable:

1. Primitive operations. Named access types arguments cannot be
dispatching;

2. Rosen's trick. Maybe it works with named types in Ada 2012. I didn't
check it;

3. Discriminants;

4. Returning managed objects by reference.

P.S. Arguably these issues could be resolved without anonymous access types
and possibly without access types.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: Accessibility checks

[ake.ragnar.dahlgren]

On Wednesday, July 24, 2013 3:27:45 PM UTC+2, Dmitry A. Kazakov wrote:
> On Wed, 24 Jul 2013 06:01:21 -0700 (PDT), ake.ragnar.dahlgren@gmail.com wrote: > I do not use anonymous access types either. Note that there exist quite important cases where anonymous access types are necessary and unavoidable: 1. Primitive operations. Named access types arguments cannot be dispatching; 2. Rosen's trick. Maybe it works with named types in Ada 2012. I didn't check it; 3. Discriminants; 4. Returning managed objects by reference. P.S. Arguably these issues could be resolved without anonymous access types and possibly without access types. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de

Thank you Dmitry for your feedback.

Best regards,
Åke Ragnar Dahlgren