Re: Abort of C-code in a asynchronous_select

[Thierry Lelegard <thierry.lelegard@canal-plus.fr>]

> select
>    State.Not_Active;
> then abort
>    C_Driver.Read (Data, Data_Last);
> end select;
> 
> I am assuming that in case the Read-call is aborted, the semaphore in C
> isn't freed because C doesn't know anything about the Ada-abort. Am I
> correct and how can I do it better?

It entirely depends on the operating system and the implementation
of the Ada95 runtime. In your specific case, you can't even be sure
that the Read call is aborted. It may block until the read completes
(ie. the abort is ineffective).

ATC is extremely tricky. Generally speaking, it should be considered
safe only in specific cases such as compute-bound loops written in Ada.

When you try to abort non-Ada code (especially system services) using
ATC, you should check first with your Ada vendor in which cases this
will work.

Most Ada runtime libraries are implemented in user-mode on top of a
pthread (or pthread-like) system runtime. When a thread executes
inner-modes code (such as a device driver), there is simply no way
for a user-mode Ada runtime library to abort the call in a general
purpose manner.

As a programming rule, we have banned ATC in our developement teams. Full stop.
-Thierry
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Thierry Lelegard, "The Jazzing Troll", Email: thierry.lelegard@canal-plus.fr
CANAL+ Technologies, 34 place Raoul Dautry, 75906 Paris Cedex 15, France
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