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Re: Problem with differences with DLOPEN / DLSYM compared to ubuntu (16.04) / debian (stretch).




On 14.09.2017 08:44, Gary Schneir wrote:
Can anyone provide some assistance to understanding the differences and
a way get a single code base to work in all three environments without
naming the library file?

On Cygwin, libraries are Windows DLL's. That appears to be a
conscious project decision.

The TL;DR is that Windows DLL's are not going to give you the behaviors of ELF shared libraries on GNU/Linux. Cygwin would probably have to whip up its own
shared library format for that, or port one.

The complication doesn't seem worth it.

You can think of dlopen as a wrapper around LoadLibrary, and dlsym doing GetProcAddress.

In Windows programming, doing GetProcAddress on symbols within an .exe is something that isn't normally done; it's not clear that it is supposed to work at all.

For it to even have any hope of working, the function would have to be somehow
declared "dllexport" (in Visual C terms).

I think the shared lib building mechanism under Cygwin hides that; there is no
Visual C __declspec(dllexport) stuff:

See: https://cygwin.com/cygwin-ug-net/dll.html

Still, the toolchain has to be arranging for the equivalent of __declspec(dllexport)
for the external symbols.

It's probably not configured to do that for the symbols in a regular executable.



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