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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Aborting a call to Accept_Socket
Date: Tue, 21 Apr 2009 10:34:45 -0700 (PDT)
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Date: 2009-04-21T10:34:45-07:00
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On Apr 21, 9:21 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Tue, 21 Apr 2009 08:40:13 -0700 (PDT), Tony wrote:
> > I would like to abort a call to Accept_Socket() if no connection
> > request arrives within a specified time (20 seconds).  For me, a quite
> > simple solution (perhaps not safe) is to use an asynchronous transfer
> > of control like this:
> > --****************************************************
> >    procedure Server is
> >        ...
> >    begin
> >        GNAT.Sockets.Initialize;
> >        ...
> >        loop
> >            ...
> >            select
> >                delay 20.0;
> >                exit;
> >            then abort
> >                GNAT.Sockets.Accept_Socket (...);
> >            end select;
> >            ...
> >        end loop ;
> >    end Server;
> > --****************************************************
> > The expecting behaviour was the end of the program after 20 seconds
> > (if no connection request arrives). I observe : after 20 seconds the
> > program will terminate only if  a connection request arrives.
> > Is this behaviour correct?
>
> Yes, it is. Asynchronous transfer of control is not guaranteed to work with
> an outstanding calls. Most likely it does not work as in this case. The
> behavior is correct because Ada does not know how to abort a socket
> operation in order to implement this statement. Ada RM contains a list of
> abort deferred things, which includes potentially any call to any external
> operation. Specifically for sockets there is a solution: you close the
> socket from another task. That will kill accept with an error code.

I think you also have to think about what happens to the client when
an incoming connection request comes after 19.999999 seconds...  It
may be too remote a problem to worry about, but in some circumstances
it could be important.

                             -- Adam