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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: is there a version of unix written in Ada
Date: Sat, 3 Oct 2020 10:16:15 +0200
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On 03/10/2020 09:44, Niklas Holsti wrote:
> On 2020-10-03 9:42, Dmitry A. Kazakov wrote:
>> On 03/10/2020 05:09, Randy Brukardt wrote:
>>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
>>> news:rl6its$10h7$1@gioia.aioe.org...
>>
>>>>> [If O3 was nested in O2, then the second call to O2 could be internal.
>>>>> But in that case, the first path would be impossible as O1 could 
>>>>> not see O3
>>>>> to call it.]
>>>>>
>>>>> Remember that the decision as to whether a call is internal or 
>>>>> external
>>>>> is
>>>>> purely syntactic: if a protected object is given explicitly in the 
>>>>> call,
>>>>> one
>>>>> needs to trigger the mutual exclusion mechanisms again. The only 
>>>>> time one
>>>>> doesn't need to do that is when the call does not include the object
>>>>> (that
>>>>> is, directly from the body of an operation).
>>>>
>>>> Even when the object in the call is statically known to be same?
>>>
>>> Yes. An external call *always* gets the lock again. I believe that 
>>> was made
>>> the rule to make it obvious as to what will happen based on the form of
>>> call.
>>
>> I mean this:
>>
>>     protected body O is
>>        procedure P1 is
>>        begin
>>           ...
>>        end P1;
>>        procedure P2 is
>>        begin
>>           P1;   -- OK
>>           O.P1; -- Deadlock or Program_Error
> 
> 
> That is an internal call, so no deadlock nor error.
> 
> See RM 9.5(4.e), which is this exact case.
> 
> 
>>        end P2;
>>     end O;

I.e. it is *not* based on the syntax of the call.

Anyway the rather disappointing result is that protected procedures may 
deadlock (or Program_Error) in a legal program.

So my initial disinclination to jump from one protected object to 
another is a reasonable advise. Or at least the order in which protected 
objects are navigated must be same.

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