How to correctly use the extern keyword in C?

Question

My question is about when a function should be referenced with the `extern` keyword in C.I am failing to see when this should be used in practice. As I am writing a program all of the functions that I use are made available through the header files I have included. So why would it be useful to `extern` to get access to something that was not exposed in the header file?I could be thinking about how `extern` works incorrectly, and if so please correct me.**Edit:** Should you `extern` something when it is the default declaration without the keyword in a header file?

Answer 1

I can give you a practical use case.

Say, you want to declare a global variable that you want to access and CHANGE in different source files of your project.

then you can use extern keyword to declare in header file. and somewhere else you can actually declare (and define if you want) it. If you just declare it in header file without extern keyword. you will see "variable declared twice" error.