Re: Creation of a shared module (gcc -shared) , undefined references

["Yaakov (Cygwin/X)" <yselkowitz at users dot sourceforge dot net>]

On 30/07/2009 08:43, Martine Carannante wrote:
> I try to port on CYGWIN an open source which runs correctly on Linux.
> In this open source, a shared module (linked with option -shared) is
> created and after it's loaded by the main program with lt_dlopen()
> function.
>
> On CYGWIN, I have a problem when I create the module (gcc option
> -shared). I have always some undefined references to functions which
> belongs to the main program. On Linux it's not a problem, there are no
> errors.

As was already pointed out, unlike ELF shared objects, PE/COFF DLLs must 
have all symbols resolved at link time.  In this situation, where 
plugins depend on symbols in an executable, there are a few workarounds:

1) Make everything in the main executable except for main() a shared 
library, then link both the program's main() and the plugins against that;

2) Build the main program first with these additional LDFLAGS: 
'-Wl,--export-all-symbols,--out-implib,libfoo.a'.  Then build the 
plugins, adding to their LDFLAGS: '-Wl,/path/to/libfoo.a'.

3) If the package uses libtool and the number of symbols required by the 
DLLs is few, an alternative is to add '-Wl,--export-all-symbols' to the 
main executable LDFLAGS, and create a foo.def file with the following 
syntax:

IMPORTS
foo_symbol_1 = foo.exe.foo_symbol_1
foo_symbol_2 = foo.exe.foo_symbol_2

Then add '-export-symbols /path/to/foo.def' to the plugins' LDFLAGS.

Making a patch using any of these solutions that could possibly be 
accepted upstream will take some more work, but at least you should be 
able to build it yourself.

HTH,


Yaakov

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