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Date: Wed, 8 Dec 2010 11:55:49 +0000
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Subject: Loading a Cygwin based DLL
From: James Dickson <james DOT dickson AT gmail DOT com>
To: cygwin AT cygwin DOT com
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Hey,

I just posted (apologies if I have the wrong mailing list) to see if
anyone could help me with the following problem.

I have successfully managed to load a cygwin based DLL through
LoadLibrary into a VC++ 10.0 (as an aside I have also tried 8.0 and
9.0 compiler versions) compiled application, ensuring I have 4k of
scratch space as per FAQ 6.16 and the example
http://cygwin.com/cgi-bin/cvsweb.cgi/src/winsup/testsuite/cygload/Attic/cyg=
load.cc?cvsroot=3Dsrc.

However, if I then proceed to call any function internal or exported
from the DLL which makes stack allocations through local variables,
the application hangs\crashes. I hope the below example is sufficient
to demonstrate the problem and no amount of=A0googling=A0seems to=A0yield
the answer, that or I haven't found the right combination of search
words.

VC++ 10.0 client application:

int main(int argc, char ** argv)
{
=A0=A0 =A0// 4k scratch space.
=A0=A0 =A0char *stackbase;
=A0=A0 =A0__asm
=A0=A0 =A0{
=A0=A0 =A0 =A0 =A0mov eax, fs:[4];
=A0=A0 =A0 =A0 =A0mov stackbase, eax;
=A0=A0 =A0}

=A0=A0 =A0// Load cygwin dll.
=A0=A0 =A0HMODULE cygwin_ptr =3D LoadLibrary(L"cygwin1.dll");

=A0=A0 =A0// error checks omitted....

=A0=A0 =A0init_ptr init =3D (init_ptr)GetProcAddress(cygwin_ptr, "cygwin_dl=
l_init");

=A0=A0 =A0// error checks omitted....

=A0=A0 =A0init();

=A0=A0 =A0// Load plugin
=A0=A0 =A0HMODULE plugin_ptr =3D LoadLibrary(L"plugin.dll");

=A0=A0 =A0// error checks omitted....

=A0=A0 =A0plugin_ptr exp_test =3D (plugin_ptr)GetProcAddress(plugin_ptr,
"plugin_example");

=A0=A0 =A0// error checks omitted....

=A0=A0 =A0exp_test(); // This call hangs the application.
}

Cygwin based DLL:

extern "C" __declspec(dllexport) void plugin_example(void)
{
=A0=A0 std::wstring tmp(L"some string"); =A0// This for some reason causes
the application to hang.
=A0=A0 std::wcerr << tmp << std::endl;
}

I have found I can make it work sometimes, but when I start using more
stack allocated local variables the hanging=A0reoccurs. Am I missing
something simple? I really hope so.

Thank you for any help you can give me.

--
James Dickson

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