They are normally the first errors, any C/C++ newbie stumbles across. And they, at least for me as I remember, seemed a bit mysterious, because the linker never told me why he couldn’t resolve the symbol. Like someone supposed to bring you a tool from the tool shed, but no matter how often you rephrase your description of the tool, he comes back, simply stating the obvious fact „Couldn’t bring you the tool“. It was pretty annoying. Very much later I discovered nm as a nifty tool, to take a look into those libraries yourself. Using nm, you’re – so to say – getting a list of tools in the tool shed yourself, so you can check if your friend is unable to bring it to you simply because it is not there.