No, that message means that the function was implicitly defined (used) before it was defined (declared). In other words, the message means exactly and precisely what it says. This means, for example, that you have code which calls a function before it has been defined. You have provided insufficient information for diagnosis. In other words, you have not provided sufficient information about WHEN you received this warning message nor WHERE you received this message. This is a "COMPILE TIME" error. If the function did not exist at link time you would get a message that the function could not be found and no executable would be produced.