Re: Select?

<posted & mailed>

Samuel Tardieu wrote:

>>>>>> "Björn" == Björn Lundin <[email protected]> writes:
> 
> Björn> Are there any thoughts of implementing 'select' an Ada
> Björn> sockets?  It would be a nice feature.
> 
> It may be, but I don't want to implement everything that would be nice
> if there is no real need. What problem would it solve?
> 
>   Sam

  Let's say we have a server process, performing some action on a text-based 
protocol from a client. It could be implemented like this:

  loop
    declare
       Reply : string (1..10);
       Do_Exit : Boolean := False;
       Answer : string := Get_Line(A_Socket);
    begin
       Handle_Answer_And_Do_Some_Action(Answer,Do_Exit,Reply);
       Send_Back_Reply(A_Socket,Reply);
    end;
    exit when Do_Exit;
  end loop; 

Assume further that this is part of a fairly big system, and for some 
reason, the system needs to be taken down. How will I wake up from 
Get_Line? In this case, no data has arrived on the socket.

If the server accepted connections all the time, I could connect and send in 
a STOP message, but once connected, always connected, ie only ONE client on 
this server.

With select, i could periodically check if a STOP has arrived by some other 
means, (presence of a file, shared memory... ) every 3 seconds or so.

I have not tried yet, but another possible ways could be closing the socket 
from another task, so get_line gets a 'Connection_closed' exception, and 
stop from there?

A third way would be non-blocking sockets, but I don't like the idea of it. 
It either loops and consumes CPU or it needs a delay, and sinks response 
time

The key is how to wake up from Get_Line (when nothing has arrived), without 
consuming to much CPU, and still have OK answering time.

I'm thankful for pointers solving this, with or without 'select'

/Björn



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