g++ COM problem with function pointers and -O2

g++ COM problem with function pointers and -O2

[Fifer, Eric]

I'm use Cygwin, Perl and the Win32::OLE Perl module to interact with
COM objects.  And, of course, the Win32::OLE DLL is compiled with
-fvtable-thunks.  For the most part everything works just fine
(and as of Cygwin 1.1.0, Perl 5.6.0 and Win32::OLE 0.12 build
right out the box).

However, the extra DCOM functions like CoCreateInstanceEx are
implemented in the Win32::OLE Perl module by dynamically looking up
functions, LoadLibrary("OLE32") GetProcAddress("CoCreateInstanceEx"),
and calling through the function pointers.  However, this fails
when Perl is compiled with -O2 or -O.  Note that the Win32::OLE module
is dynamically loaded into Perl and the problem is triggered
when the separate Perl DLL has been built with the optimizer.

There are a couple of work-arounds.  Things are fine if Perl
is built with just -g!  Also, If I change the function pointer
to just a normal function call, it works too.  The failure with
function pointers and -O2 manifests itself as a stack corruption.

I know there are some limitations with COM support, but this
seems particularly strange because it depends on whether the
optimizer is used by a separate DLL.

Any idea what's going on here?

Thanks,

Eric Fifer

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RE: g++ COM problem with function pointers and -O2

[Fifer, Eric]

After seeing the description of the C++ parser bug with function
attributes under the "g++/linker bug on cygwin1.1.0 (win32)"
thread, I went back to look at the code causing my problem.

I switched some declarations around, from:

  typedef HRESULT (STDAPICALLTYPE FNCOCREATEINSTANCEEX)
    (REFCLSID, IUnknown*, DWORD, COSERVERINFO*, DWORD, MULTI_QI*);

  FNCOCREATEINSTANCEEX *pfnCoCreateInstanceEx;

to:

  HRESULT STDAPICALLTYPE (*pfnCoCreateInstanceEx)(REFCLSID,
	IUnknown*, DWORD,COSERVERINFO*, DWORD, MULTI_QI*);

And this also works around my problem.

Could this be another manifestation of the same bug?

Thanks,

Eric Fifer



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