Calling functions outside of the main.cpp

Warning: rookie alert!!!

I have just installed PlatformIO and created 2 files (main.cpp and myFunctions.cpp). The main.cpp file should call a function from the myFunctions.cpp file but I receive an error message (see below). With the arduino IDE I didnt have to care about how to “link” the files of the project together, it was simply working.

I assume platformIO operates on a more common approach how my main.cpp can call functions from another cpp file which is not like how the default arduino IDE does it. Please help me out how can I approach the same behaviour in platformIO. (if the solution needs me to create a .h file, please describe how can I generate one from the myFunctions.cpp file.)

Thanks a lot.

(the error message I received:

src\main.cpp: In function ‘void loop()’:
src\main.cpp:15:20: error: ‘mySum’ was not declared in this scope
int x = mySum(5,6);"

Alright… renaming the myFunctions.cpp to myFunctions.h does the job. This was easy, but I see more complications, please see my next forum question: Using OLED display functions in multiple cpp files