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Subject: Re: Problem with differences with DLOPEN / DLSYM compared to ubuntu  (16.04) / debian (stretch).
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Date: Thu, 14 Sep 2017 18:15:51 -0700
From: Kaz Kylheku <920-082-4242 AT kylheku DOT com>
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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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