It's a right royal pain isn't it!

Perhaps the simplest thing to do is compile your C code to a DLL & lib without any mention of Perl or XS. Ie. No perl or XS headers or makefile stuff. Then use h2xs to generate the XS interface wrappers.

This means that your C functions would have to make no references at all to Perl internal APIs or data structures. So, no SV* parameters. It also means that you need to take full responsibility for freeing any dynamically allocated memory. That means providing destructors and ensuring that the get called in a timely fashion.

It is possible to #undef malloc etc. in order to gain access back to the CRT versions; but you have to be very careful about restricting the scope of those undefs.

It is all together a mess, and the primary reason many of my projects have never come to friution. So much of the library code I've written works perfectly when called from simple or threaded C-only test programs, but as soon as I try to call it from Perl, it just fails in random and mysterious ways and is neigh impossible to debug.

It obviously is possible to do this stuff properly--there are plenty of Win32-only and ported XS modules that work fine--but the secrets of making it work seem to be mostly lost.

All together intensely frustrating. Sorry I don't have better advice.