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 symbol visibility. Afaics we'd need a >> working definition of PGDLLIMPORT which inverts the declspecs. I think Tom >> just removed the 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. 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. Even though we're generating .DEF files, I don't think we can avoid that, because we at minimum need to have the hstore exported symbols *not* annotated __declspec(dllimport) when compiling hstore, but have them so annotated when importing the header into anything else. (Come to think of it, how're we dealing with this in libpq? I wonder if postgres is linking to libpq using thunk functions?) -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers