I have a working 32-bit software library written in Fortran with low-level stuff in C/C++ It has worked for years.
It is fine.
When I re-build the library in 64-bit mode, none of the C functions are linked in. I've checked the linker options, and they all seem to be right. The project appears to be referencing the 64-bit libraries.
Would someone mind just looking some sample INTERFACE code and tell me whether there is anything at all that would cause a problem