Re: Interrupts handling in ADA

[Simon Wright <simon@pushface.org>]

Ada BRL <ada.brl.2011@gmail.com> writes:

> On 13 Dic, 15:34, Simon Wright <si...@pushface.org> wrote:
>> Simon Wright <si...@pushface.org> writes:
>> > Also, this isn't called an accept statement; it's an entry call. The way
>> > a task indicates it's prepared to accept an entry call is by an accept
>> > statement; protected objects are different. So if instead of Queue being
>> > protected it was a task (not something I'd recommend) the corresponding
>> > body part would have looked like
>>
>> >    accept Get (M : out Message) when not Buffer.Is_Empty do
>>
>> :blush: I can't believe I said that!
>>
>> The relevant section of the ARM is 9.7.1, Selective Accept, and it'd
>> have looked like
>>
>>    select
>>       when not Buffer.Is_Empty =>
>>          accept Get (M : out Message) do
>>             M := ...
>>          end Get;
>>       or
>>          accept Put ...
>
> Does this means that if the Buffer is empty I will accept the Put
> instead of Get? So the "or" refers to the condition before "=>"

When the compiled code gets to 'select' it looks at the 'open select
alternatives'. If the Buffer is empty, that would be just Put; if not,
they would be Get and Put. It then (under the hood!) checks to see if
any task is waiting on any of the open select alternatives; if there are
any, it chooses one and executes the accept statement; if there are
none, it sets up a conditional event of some sort involving any of the
open select alternatives and waits.

Note that nothing will happen if Buffer.Is_Empty changes during this
wait; the select alternatives are only evaluated at the 'select'.

http://en.wikibooks.org/wiki/Ada_Programming/Tasking#Selective_Wait
http://en.wikibooks.org/wiki/Ada_Programming/Tasking#Guards