The project can’t be downloaded anymore so I try to replicate it.
I managed to make the fastmath.dll with MinGW, but I got an error when try to call in xojo
Soft Declare Function DoubleValue Lib "fastmath.dll" ( x As Int32 ) as Int32
MsgBox Str(DoubleValue(20))
Any C++ functions in the DLL would need to be declared as ‘extern “C”’ in order to be accessible via a declare. C++ functions are modified by their parameter types so that they can be overloaded.
What is the C function definition. It could be down to types. Int32 is obviously a 32 bit integer, which would typically be a short in C. Also worth noting is that the function you have called “DoubleValue” is returning an Int32 also (according to your declaration).