Re: Interfacing to C: multithreaded callbacks

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Tue, 12 Jun 2007 12:56:30 -0700, Maciej Sobczak wrote:

> The problem is when the C library creates additional threads and calls
> the client back in the context of those threads. ARM says nothing (?)
> about the relaion between Ada tasks and system threads. If the
> relation is 1:1 (ie. tasks are implemented as system threads), then
> the whole scheme might work just fine, provided that there is no task-
> specific data that Ada runtime expects and will not find. On the other
> hand, if the relation between tasks and threads is not 1:1, we will
> just enjoy undefined behavior.

Yes, when Ada tasks aren't mapped onto system threads, then system calls
potentially might block all Ada tasks. This includes whatever POSIX layer.

> Is there any water-proof implementation pattern for such problems?
> Consider both the general case and then GNAT as the target Ada
> compiler on POSIX systems.

You can marshal messages from C callbacks. With busy waiting and one
publisher - one subscriber, you don't need anything but shared memory to
implement that.

However, when talking about  POSIX targets, I would assume Ada tasks being
POSIX threads.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de