Let me start by explaining the scenario:

One of our VB6 applications passes a variant array of 64-bit doubles to a managed C++ component. The managed code does a bit of munging, before passing the same information on to another component which is an unmanaged C++ DLL written and maintained by a third party. This second DLL is responsible for doing boolean operations on 3D solids. When this component is invoked, the application dies with a nice obscure "screwed-up memory" error. Unfortunately, since the component is closed-source, we needed to pass the information on to the author of the code and wait for a response.

After the author had investigated the problem he informed us that the reason for the crash was that the values that are being passed in are 80-bit extended-precision values, and not the expected IEEE 64-bit double-precision values.

Wierd! That didn't make any sense to me. On hearing this I started to investigate where the process was falling down. The original data was 64-bit, so how come it wasn't by the time it hit the final DLL? Something must be converting the 64-bit double-precision values to their 80-bit extended-precision equivalents. I spent a fair bit of time trying to find out as much as I could about the various floating-point representations, and where they're used, but it took me an eternity to locate the information I was searching for.

In the deep, dark depths of the web, I stumbled across this little nugget of information (thanks to ExtremeOptimization.com):

... the 'extended' format, for which the IEC 60559 standard defines minimum specifications, and which is used by the floating-point unit on Intel processors, and is also used internally by the CLR.

Ah ha! You bastards So we're always dealing with 80-bit values when we use double in the CLR.

Now that we know this, we can take steps to sort it out. In a nutshell, we need to set the FPU state to force the use of 64-bit values before invoking the unmanaged component. Thankfully there's an API function that we can use to do this, it's called _controlfp(). In VS 2005, this function is deprecated, so we're forced to use the 'secure' equivalent _controlfp_s(). Here's how you do it:

#include <float.h>
.
.
void ManagedClass::ManagedFunction()
{
  double dooby[128];
  .
  unsigned int prevState;
  // set the FPU state to 64-bit
  _controlfp_s( &prevState, _PC_53, MCW_PC);
  .
  // invoke unmanaged DLL - these values wil be 64-bit, not 80-bit
  ExternalFunction( dooby );
  .
  // reset FPU state to previous value
  _controlfp_s( NULL, prevState, MCW_PC);
}

So there you have it. I hope this information is useful