Re: narwhal and PGDLLIMPORT - Mailing list pgsql-hackers

On 2014-02-13 07:58:09 +0800, Craig Ringer wrote:
> On 02/13/2014 05:35 AM, Peter Eisentraut wrote:
> > On 2/12/14, 4:30 PM, Andres Freund wrote:
> >>> There are cases where one module needs symbols from another directly.
> >>> Would that be affected by this?
> >>
> >> I don't think we have real infrastructure for that yet. Neither from the POV of loading several .so's, nor from a
symbolvisibility. Afaics we'd need a working definition of PGDLLIMPORT which inverts the declspecs. I think Tom just
removedthe remnants of that.
 
> > 
> > It works reasonably well on other platforms.
> > 
> > Of course, we can barely build extension modules on Windows, so maybe
> > this is a bit much to ask.  But as long as we're dealing only with
> > functions, not variables, it should work without any dllimport dances,
> > right?
> 
> Don't think so.
> 
> If you don't have __declspec(dllexport) or a .DEF file marking something
> as exported, it's not part of the DLL interface at all, it's like you
> compiled it with gcc using -fvisibility=hidden and didn't give the
> symbol __attribute__((visibility ("default")) .
> 
> If you _do_ have the symbol exported from the DLL, using
> __declspec(dllimport) or a generated .DEF file that exposes all
> "extern"s, you can link to the symbol.
> 
> However, from the reading I've done recently, I'm pretty sure that if
> you fail to declare __declspec(dllimport) on the importing side, you
> actually land up statically linking to a thunk function that in turn
> calls the real function in the DLL. So it works, but at a performance cost.
> 
> So you should do the dance. Sorry.

I don't think the thunk function will have such a high overhead in this
day and age. And it's what we essentially already do for all functions
called *from* extensions, no?

> It gets worse, too. Say you want hstore to export a couple of symbols.
> Those symbols must be __declspec(dllexport) while everything else in
> headers must be __declspec(dllimport). This means you can't just use
> PGDLLIMPORT. You must define a HSTOREDLLIMPORT that's
> __declspec(dllexport) when compiling hstore and otherwise
> __declspec(dllimport). Then set a preprocessor macro like
> -DCOMPILING_HSTORE to trigger it.

We actually have a a macro that should do that, namely PGDLLEXPORT. I am
not sure though, why it's not dependent on dependent on BUILDING_DLL
(which is absolutely horribly misnamed, being essentially inverted).

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services