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Subject: Re: Many pthread failures in the test suite, one setgroup failure
From: Robert Collins <rbcollins AT cygwin DOT com>
To: egor duda <cygwin-developers AT cygwin DOT com>
In-Reply-To: <44642850720.20020928223759@logos-m.ru>
References: <20020925141653 DOT GA6134 AT redhat DOT com>
	<1033139976 DOT 22908 DOT 333 DOT camel AT lifelesswks>
	<163544913434 DOT 20020927192540 AT logos-m DOT ru>
	<1033140780 DOT 9593 DOT 0 DOT camel AT lifelesswks> 
	<44642850720 DOT 20020928223759 AT logos-m DOT ru>
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Date: 29 Sep 2002 09:07:34 +1000
Message-Id: <1033254454.4375.48.camel@lifelesswks>
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On Sun, 2002-09-29 at 04:37, egor duda wrote:

> I guess i know why it works for you. Hint: Try gcc 3.x to build
> cygwin1.dll. Me and Chris, are using it, obviously, while you're
> probably using 2.95, right?

Yes, I've been holding off upgrading until gcc 3.x is the default for
cygwin, at which time I will migrate the setup sources to 3.x.
=20
> class verifyable_object contains no virtual functions, hence no
> pointer to VMT in it. This means that compiler assumes that magic
> member is placed at offset 0 from the beginning of class.=20

Which is valid for verifyable_object but will *kill* any derived
classes. Yep. coding bug on my behalf. Can you or Chris try the test run
after changing the verifyable_object destructor to be virtual (thread.h,
line 163).

> class
> pthread is a subclass of class verifyable_object, but it _does_
> have VMT to accommodate virtual method 'create'. As far as i can
> understand from assembly, pointer to VMT is placed at the beginning of
> object instance, at offset 0, while members are placed after it, so
> 'magic' has offset 4.

Thank you for digging, I imagine this took some time - and you've saved
me a lot of head scratching.=20
=20
> Now, in verifyable_object_isvalid you're casing pointer to variable of
> subclass to pointer to base class. Ain't it case of 'never do like
> this'? I suppose to safely perform cast from subclass to base class
> one should always use dynamic_cast().

It's a case of this is always ways ok to do - upcasting is FINE.. (it
happens every time you call a virtual function in fact).
 It's never ok to case a base class to a subclass (known as downcasting)
unless you use a static_cast or a dynamic_cast.
http://www.cs.rpi.edu/~wiseb/xrds/ovp3-1.html

What *may* be a regression in 3.2 is the apparent bug caused by the
introduction of a VMT in a derived class. However, as we already have a
destructor for verifyable_object, and in this instance it should be
virtual, we get to sidestep the bullet - for now.

Rob

--=20
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GPG key available at: http://users.bigpond.net.au/robertc/keys.txt.
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