Re: [basic-improvements] try/finally support for c/c++ - more tests

[Daniel Jacobowitz <drow@mvista.com>]

On Thu, Nov 07, 2002 at 08:12:05AM -0800, Mark Mitchell wrote:
> >Standards fixate existing practice, they don't establish it.
> 
> We've already got a standard that's relevant here.   In particular,
> we have the C++ programming language, which provides the functionality
> needed.  If you need that functionality, use that language, or another
> language (like Java, Ada, etc.) that provides that functionality.
> 
> Now, I'm not fully understanding how this is going to be used, which
> is perhaps coloring my thinking.  I'm getting the impression that this
> functionality is going to get buried in C headers that are going to
> get seen in *user* C code.  Is that true?
> 
> If it is true, it makes my above remark irrelevant; we need something
> that works in C.  The question, of course, is *what* do we need.

That's right.  pthread_cleanup_push/pop are user-accessible
macros/functions.

> Let me try to see if I've got right what it is we're trying to
> accomplish.  If I don't have it right, I'll not be intelligent in the
> discussion, so please correct me.
> 
> We're trying to implement one particular piece of functionality required
> by the POSIX threads interface; namely "pthread_cleanup_push" and friends.
> (I know that any feature might get used more widely, but this is the
> motivation, as I understand it.)  Furthermore, we're trying to make the

Before people fixate on this any further, it's worth remembering that
we've had requests for this feature in C before.  I see one in
September:
From: "Michael Lovett" <mlovett@morpace.com>
Subject: GCC Feature question

I'm pretty sure there have been others.  That's not any kind of
overwhelming consideration, but it is something to keep this in
perspective.

> POSIX threads cleanups play nice with exception-handling.  Or are we
> trying to use EH mechanisms to make things go faster?

The former.  The discussion about try/finally being faster is in
comparision to some of the other suggestions which involve nested
functions or calling into C++.

> Anyhow, if I write:
> 
>  try { pthread_cleanup_push (...);
>        throw 3; }
>  catch (...) { }
> 
> and the system arranges to run the cleanup when the exception is thrown,
> the system is badly broken.
> 
> So, I think I'm not getting the point.
> 
> What interface is it that we think we need this functionality to
> implement?

On the contrary, these are for cleanups that _would_ have to be run
when that "throw 3" was triggered.  It would have to be:
try {
  pthread_cleanup_push (...);
  throw 3;
  pthread_cleanup_pop (...);
} catch (...) { }

though.

For instance, if a thread is cancelled while holding stdio locks, the
cleanups must be run in order to restore the state of the stdio
structures and release the lock.  Otherwise stdio becomes unusable
after cancellation.

(malloc has the same problem but doesn't use the cleanup handlers at
present, I think...)

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer