Is it possible to access the "errno" variable in C# when P/Invoking? This is similar to Win32 GetLastError().
I'm fairly sure that there is a way, but it probably is a bad idea. How would you guarantee that the runtime has not called some CRT function during its internal processing that has affected the errno?
For the same reason, you should not call GetLastError directly either. The DllImportAttribute provides a SetLastError property so the runtime knows to immediately capture the last error and store it in a place that the managed code can read using Marshal.GetLastWin32Error.
I think the most robust thing you could do in this case is make a C DLL that performs both the actual C work and the capture of the errno. (Note that just writing a wrapper around the errno capture would still have the concerns mentioned above.)