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From: ohk@edeber.nta.no (Ole-Hjalmar Kristensen FOU.TD/DELAB)
Subject: Re: Asynchronous IO
Date: 1996/06/17
Message-ID: <OHK.96Jun17101505@edeber.nta.no>#1/1
X-Deja-AN: 160586426
references: <OHK.96Jun14131501@edeber.nta.no> <dewar.834847212@schonberg>
organization: Telenor R&D
newsgroups: comp.lang.ada
Date: 1996-06-17T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.834847212@schonberg> dewar@cs.nyu.edu (Robert Dewar) writes:

   Kristensen said

   "For obvious resons, it is desirable to be able to do output of data to
   different devices in parallel. Does the Ada95 standard define what
   happens if two tasks try to do IO to two different files at the same
   time? I have tried to find out by reading both the RM and the
   Rationale, but is none the wiser."

   For obvious reasons, it is impossible to find the answer in the RM, since
   it depends on the implementation. On a system like DOS, which does not
   provide multi-threaded I/O, of course any I/O operation is likely to block
   other tasks (sometimes a kludge for keyboard I/O can be arranged, using
   polling).

Yes, I also came to this conclusion some time after having posted my
question. I can only say I havn't programmed on systems like DOS fo a
LONG time...

   On systems that do provide multi-threaded I/O, you still have to find
   out if your implementation takes advantage of this. Take a system like
   Solaris, one could do the Ada tasking within one Solaris thread, which
   would in fact likely be the most efficient approach for the Ada tasking
   itself. However, you would then not get overlapped I/O. One of the reasons
   that most people want tasks to map to threads in a system like Solaris,
   is precisely to get overlapped I/O, though you then have to check that
   the system provides thread-safe I/O (it is amazing in modern Unix systems
   with threads how much is NOT thread safe, for instance, malloc is not
   thread safe in Solaris).

I suppose the most reasonable way to make a program using overlapped
IO portable then, would be to define a package to do it, and change
its implementation depending on what kind of system you are running
on.


Ole-Hj, Kristensen