How to: communication between multiple tasks using protected objects - with no polling?

[Esa Riihonen <esa.riihonen.er@gmail.com>]

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 been involved recently.

The C-program consists of various Linux processes that communicate with each other mainly through FIFO's. It seems natural to implement the process functionality 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 connected 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 of the monitored file descriptors (I guess this is in essence quite basic arrangement 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 
    ...
  end select;
  ...
end loop;
...

Alas, I quicky found out that 'or' is not allowed here between the protected 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 to be suspended if there is nothing to do. I understand(?) that I probably could achieve this by accepting task entries. But I wan't to know whether it can be done using the protected objects for communication. 

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 something silly and dangerous?