Commit 72f9c3f

With just a function prototype, the compiler can continue without error, but the linker cannot resolve a call to an address because there is no function code or variable space reserved. You will not see this error until you create a call to the function that the linker must resolve.

The function call in main will cause LNK2019 because the prototype allows the compiler to think the function exists. The linker finds that it doesn't.