RetroBBS - comp.lang.ada - Is a Boolean type inherently atomic ?

Is a Boolean type inherently atomic ?

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From: rodakay5@gmail.com (Rod Kay)
Newsgroups: comp.lang.ada
Subject: Is a Boolean type inherently atomic ?
Date: Fri, 12 May 2023 22:17:48 +1000
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by: Rod Kay - Fri, 12 May 2023 12:17 UTC

Re: Is a Boolean type inherently atomic ?

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From: spam.jrcarter.not@spam.acm.org.not (Jeffrey R.Carter)
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Subject: Re: Is a Boolean type inherently atomic ?
Date: Fri, 12 May 2023 14:53:24 +0200
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by: Jeffrey R.Carter - Fri, 12 May 2023 12:53 UTC

On 2023-05-12 14:17, Rod Kay wrote:
> Surely only the least significant bit of the least significant byte is
> relevant and so the value cannot be garbled by one task writing and another
> reading at the same time ?

Boolean types with other representations using multiple bits are possible, so
your assumption doesn't hold.

--
Jeff Carter
"Many times we're given rhymes that are quite unsingable."
Monty Python and the Holy Grail
57

Re: Is a Boolean type inherently atomic ?

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From: rosen@adalog.fr (J-P. Rosen)
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Subject: Re: Is a Boolean type inherently atomic ?
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by: J-P. Rosen - Fri, 12 May 2023 16:56 UTC

Le 12/05/2023 à 14:53, Jeffrey R.Carter a écrit :
> On 2023-05-12 14:17, Rod Kay wrote:
>> Surely only the least significant bit of the least significant
>> byte is relevant and so the value cannot be garbled by one task
>> writing and another reading at the same time ?
>
> Boolean types with other representations using multiple bits are
> possible, so your assumption doesn't hold.
>
True, especially considering the special exception for boolean types in
13.4(8)

Anyway, if you intend to access a variable from multiple tasks, it
doesn't cost much to add an aspect Atomic to the declaration, at least
to inform the reader!

--
J-P. Rosen
Adalog
2 rue du Docteur Lombard, 92441 Issy-les-Moulineaux CEDEX
https://www.adalog.fr https://www.adacontrol.fr

Re: Is a Boolean type inherently atomic ?

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From: niklas.holsti@tidorum.invalid (Niklas Holsti)
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Subject: Re: Is a Boolean type inherently atomic ?
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by: Niklas Holsti - Fri, 12 May 2023 17:38 UTC

On 2023-05-12 15:17, Rod Kay wrote:
> Surely only the least significant bit of the least significant byte
> is relevant and so the value cannot be garbled by one task writing and
> another reading at the same time ?

That seems very likely indeed, unless (as others have commented) the
representation has been specified to use more bits. However, the Ada RM
states in C.6(8/3) that "every atomic type or object is also defined to
be volatile", and of course Boolean variables are not considered
volatile unless they are specified to be Atomic or Volatile. So a
Boolean type is not inherently atomic in the Ada RM sense of "atomic".

And of course if you use a shared variable to communicate data between
tasks, that variable should be marked as Volatile, and there should also
be some Atomic accesses to ensure that actions are "sequential", so
marking the variable as Atomic is best.

Re: Is a Boolean type inherently atomic ?

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Subject: Re: Is a Boolean type inherently atomic ?
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by: AdaMagica - Fri, 12 May 2023 18:02 UTC

Re: Is a Boolean type inherently atomic ?

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From: rodakay5@gmail.com (Rod Kay)
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Subject: Re: Is a Boolean type inherently atomic ?
Date: Sat, 13 May 2023 10:28:39 +1000
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by: Rod Kay - Sat, 13 May 2023 00:28 UTC

"Free markets select for winning solutions." -- Eric S. Raymond

devel / comp.lang.ada / Is a Boolean type inherently atomic ?

Subject	Author
Is a Boolean type inherently atomic ?	Rod Kay
Re: Is a Boolean type inherently atomic ?	Jeffrey R.Carter
Re: Is a Boolean type inherently atomic ?	J-P. Rosen
Re: Is a Boolean type inherently atomic ?	Niklas Holsti
Re: Is a Boolean type inherently atomic ?	AdaMagica
Re: Is a Boolean type inherently atomic ?	Rod Kay