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From: Oliver <oliver.schoenborn@utoronto.ca>
Subject: Re: questions about DLL's: .a, .def, and .dll
Date: Fri, 27 Aug 2004 20:36:32 +0000 (UTC)
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References: <loom.20040827T173827-426@post.gmane.org> <Pine.GSO.4.61.0408271423560.6181@slinky.cs.nyu.edu>
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Igor Pechtchanski <pechtcha <at> cs.nyu.edu> writes:

> The newer versions of gcc apparently allow you to link directly to a .dll
> file.  The .def and .a are needed for older versions of gcc, and possibly
> for some other tools.
> 
> [...]
>
> The reason you want an 'extern "C"' for DLL functions in general is that
> g++ and the Windows C++ compilers (notably VC++) use different name
> munging schemes, so a DLL built with C++ symbols won't be usable from
> other applications that try to call those functions.  The reason you want
> it for DllMain is that the Windows loader will be looking for the unmunged
> name "DllMain".  Are you sure that it's really invoked when the DLL is
> loaded?

Actually I found something quite interesting on a "Tcl extensions in Windows" 
wiki website, where a poster says that DllMain, __decl... import export, 
windows.h etc are no longer needed with g++. Sure enough, the following example 
works:

1) create a dll.h file that contains a class definition, just like you would on 
Unix (i.e. no __decl... macros, no extern etc)
2) create a dll.cc file that contains some of your class definition methods
3) then create the dll with  

   g++ -c dll.o
   g++ -shared dll.o -otestdll.dll

4) create a testMain.cc file with a main() that #includes dll.h and uses some 
things defined in dll.cc
5) build with 

   g++ -o testMain testMain.cc -L. -ltestdll

6) run testMain.exe to make sure it works

Note that extern, gcc, import/export macros and declarations, DllMain etc were 
NOT needed. Gcc seems to export everything, like it would on *nix (except, 
presumably, functions declared static and things in anonymous namespace -- 
exercise left to the reader ;). 

You can tell the linker to generate a .def file by adding "-Wl,--output-
def=testdll.def" when creating the testdll.dll. This shows all symbols 
exported. Running nm on testdll.dll also shows 

67488220 T _DllMain@12
67481000 T _DllMainCRTStartup@12

which means that gcc auto-generated it, and did not mangle for C++ use. 

Does anyone know if testdll.dll, created this way, would be linkable from a 
VC++ program? I don't have access to VC++.

Oliver




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