LabVIEW expects that floating point return values are left in a floating point register. I stepped through the assembly for the DLL call and it looks to me like it is doing that. One of the last things it does before returning is load a value into the floating point register and that is the value LabVIEW returns. It happens to be zero in this case.
I also checked that the code was properly picking up the inputs. I do see it load the correct values into the floating point registers to do its calculations. So that appears to be working as well.
The declarations in the text file seem to be for Visual Basic. Have you successfully called it from VB? I tried it there and it also returns 0. The best I can tell everything is working properly and the answer is zero.
