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Subject: How to: communication between multiple tasks using protected objects
 - with no polling?
From: Esa Riihonen <esa.riihonen.er@gmail.com>
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Xref: number.nntp.giganews.com comp.lang.ada:191958
Date: 2015-01-20T14:36:20-08:00
List-Id: <comp.lang.ada>


I'm new to Ada and I have a problem ;)

After reading through 'Programming in ADA 2012' by John Barnes - I decided =
to try Ada in implementing the core functionality of a C-program I have bee=
n involved recently.

The C-program consists of various Linux processes that communicate with eac=
h other mainly through FIFO's. It seems natural to implement the process fu=
nctionality as Ada tasks.  As the inter task communication I decided to try=
 protected types (instead of task entries).

Here is the problem. My typical C-process 'listens' to several FIFOs connec=
ted to separate processes. This is realized using a ppoll-type 'reactor' - =
so in essence the process is sleeping until there is some activity in any o=
f the monitored file descriptors (I guess this is in essence quite basic ar=
rangement in the 'C-world').

I first thought I can implement this by the 'select' - something like this:

PO1 and PO2 are instantiations of a protected type with entry 'Get'.

...
loop
  ...
  select
    PO1.Get(...);
  or
    PO2.Get(...);
  or=20
    ...
  end select;
  ...
end loop;
...

Alas, I quicky found out that 'or' is not allowed here between the protecte=
d object entries.

I can do something like this instead:

...
loop
  ...
  select
    PO1.Get(...);
  else
    null;
  end select;

  select
    PO2.Get(...);
  else
    null;
  end select;
  ...
end loop;
...

But this would be now constantly polling in a loop. I would like the task t=
o be suspended if there is nothing to do. I understand(?) that I probably c=
ould achieve this by accepting task entries. But I wan't to know whether it=
 can be done using the protected objects for communication.=20

Am I missing something? Or have my though processes been totally corrupted =
by too much (forced) C during the decades - and thus I'm trying to do somet=
hing silly and dangerous?