What can change my floating point control word behind my back?

Question

I have a 32 bit Windows application, written primarily in Delphi, which performs floating point numerical simulations using the 8087 FPU. I have recently added the ability to link in external Python code using the Python API through python2x.dll. This recent change has led to some very strange behaviour.

The application has a batch mode of operation where it performs multiple simulations in parallel to take advantage of multi-core architectures. As soon as Python code has been executed in the process, I start to see changes to the 8087 control word on different threads. I've checked this very carefully and I have observed the control word having changed even in a thread which has never called into the Python DLL.

I know this sounds fantastical, but, as I have discovered, there are mechanisms for this behaviour to manifest. I have learnt about signals. I first hypothesised that the Python DLL was setting process wide signal handlers (by calling signal()) and these signal handlers were responsible for changing the control word. For example, a thread, unrelated to the Python code, could perhaps cause SIGFPE and that may, in turn, modify the control word.

I have rather come to the conclusion that signal() is not the mechanism. I arranged to execute the Python code at startup. Then I set of the signal handlers back to SIG_DFL. Then I started the simulations. But still the control word changes occurred.

My question (finally) is whether anyone knows of another mechanism by which the control word could be changed in such a manner. I'm looking for interrupts, APCs etc., I think!

Update

The control word is being changed to 0x037F which is the Intel default value. This differs from the MSVC/Windows default of 0x027F. I hypothesise that something is calling FPINIT.

I also discovered Py_InitializeEx which allows the caller to stop Python setting signal handlers. The control word changes occur even if I use this approach to initialisation so I'm even more convinced that is not the mechanism.

Answer 1

For example, a DllMain call with DLL_THREAD_ATTACH flag, see msdn

Update

I have found a link to similar problem - DLL Load "Poisons" FPU Control Word for New Threads. But yes, it is about the threads created after Dll load.

Answer 2

If I remember correctly, that's Delphi's problem. There are some discussions of the issue here and here. I remember bumping into it when trying to write some VST plugins in Delphi.