Re: Catching C++ exceptions thrown from a framework
Re: Catching C++ exceptions thrown from a framework
- Subject: Re: Catching C++ exceptions thrown from a framework
- From: Steve Baxter <email@hidden>
- Date: Tue, 18 Jul 2006 15:23:31 +0100
Hi Chris,
Hmm, odd that DTS didn't mention this when I asked them specifically
about this problem!
Does -fvisibility-ms-compat force RTTI comparison to happen by type
name rather than type ID address (i.e. the same class implemented in
two different plugins will be considered to be the same for RTTI
even though the typeid for the types will have different addresses)?
How does it do this without changing the C++ runtime (I believe the
problem here is in the runtime as installed by the OS rather than in
Xcode)?
Cheers,
Steve.
On 18 Jul 2006, at 14:22, Chris Espinosa wrote:
On Jul 18, 2006, at 2:03 AM, Steve Baxter wrote:
This has bitten a few people.
The actual problem is that GCC 4 uses the *address* of typeids
when comparing for RTTI rather than the class name that is in the
typeid. This is arguably more efficient, but doesn't work
properly if your code doesn't obey the one definition rule:
http://en.wikipedia.org/wiki/One_Definition_Rule
Unfortunately it is very easy to accidentally not obey the ODR.
Inline code used in plugins will cause RTTI information to be
linked multiple times (once in each plugin). The compiler and
linker will not catch problems here - you have to deal with this
yourself.
To fix the problem:
- Your plugins need to export all their symbols
- If you use CFBundle to load your plugins, it will not do the
right thing. You need to load your plugin with dlopen
( RTLD_GLOBAL ) (you can then load it with CFBundle as well)
- If you are supporting 10.3.9 you have an extra problem - there
is a bug in the C++ runtime which will still prevent RTTI from
working between modules. I hope Apple is going to release a fix
for this at some point (the fix is actually in the 10.3.9 and
10.4u SDKs)
Doing this allows the dynamic linker to merge symbols as each
plugin loads. Each symbol will then be loaded only once and ODR
will be obeyed.
Personally I think it is a complete pain for GCC to have different
behaviour to CodeWarrior and VC++ in this respect. They have
extracted a little bit of performance (comparing the address of
typeids rather than doing a text compare is more efficient) but
caused an awful lot of pain. Apple could fix this - there is a
#define in the C++ runtime to switch back to string comparisons.
Unfortunately with 10.3.9 and upwards, this is part of the system
and Apple are refusing to change it.
Cheers,
Steve.
On 14 Jul 2006, at 22:14, Eric Slosser wrote:
Sam, this is a semi-well-known problem.
GCC decides to give an exception to a handler when the typeinfo
of the exception matches that of the handler (or is a sub-
class). But the comparsion isn't based on the munged name of the
symbol, it's based on the equality of a integer constant that's
handled out by the linker.
So you need to export the typeinfo symbols representing the
exceptions you care about from both the catcher and catchee.
The easiest way to see if this is your problem is to export all
symbols, and don't strip any symbols, and see if the problem goes
away.
If this helps you, thank Marshal Clow for reading your problem
and remembering that I had the same one months ago. Oh, and Lisa
Lippencott and Sean Parent for explaining the answer to me back
then. And the academy. And mom.
See http://www.devworld.apple.com/documentation/Darwin/Reference/
ManPages/man1/gcc.1.html, especially this part:
-fvisibility-ms-compat
This flag attempts to use visibility settings to make
GCC's C++
linkage model compatible with that of Microsoft Visual
Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to 'hidden', like -
fvisibil-
ity=hidden. 2. Types, but not their members, are not
hidden by
default. 3. The One Definition Rule is relaxed for
types without
explicit visibility specifications which are defined in
more than
one different shared object: those declarations are
permitted if
they would have been permitted when this option was not
used.
This option is discouraged, rather, it is preferable for
types to
be explicitly exported as desired on a per-class basis.
Unfortu-
nately because Visual Studio can't compare two different
hidden
types as unequal for the purposes of type_info and
exception han-
dling, users are able to write code that relies upon
this behavior.
Among the consequences of these changes are that static
data mem-
bers of the same type with the same name but defined in
different
shared objects will be different, so changing one will
not change
the other; and that pointers to function members defined
in differ-
ent shared objects will not compare equal. When this
flag is
given, it is a violation of the ODR to define types with
the same
name differently.
You can pass this flag manually in the Other C++ Flags build
setting in Xcode 2.3 and later.
Chris
Steve Baxter
Software Development Manager
Improvision
+44-2476-692229
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