Thanks. I guess it’s ambiguous what fclout was asking about here:
If the Swift side uses @c @implementation, then the header uses size_t and the implementer uses Int. However, if the generated header is being used, and, say, the implementation of the size_t-consuming function calls an imported size_t-consuming function, the mismatch will still show up. There’s also a bit of weirdness here around Swift clients of the API, which may see a size_t in the generated header but UInt in the swiftinterface/swiftmodule, and be unable to pass an Int to it. I guess the recommendation is “if you care strictly about matching up with C stdlib types you should use @c @implementation”.