Every function must have some code to define it. This should not be a surprise, except that some function code is hidden in libraries and core code. It's possible that those functions are defined for you in some library or include file that you forgot (or didn't realize that you needed) to include with the #include statement.
Or, you just went and cut and pasted a bunch of code you found on the internet without understanding it very much. Therefore, forgot to cut and paste those functions.