How to create user defined header files and how to link it with function files?
To create an user defined header file is very simple, make it with an editor.
If you want to link it, you must create a library or an object file. In POIDE you will find a template to create a library - to create an object file you only have to compile a C source file.
The header file makes promises to the compiler with functions and variables the linker will find in the libraries or object files. You must say to the linker to add then this libraries or object files to your code to create a complete program (EXE).