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

Answer 1

Functions actually defined in other source files should only be declared in headers. In this case, you should use extern when declaring the prototype in a header.

Most of the time, your functions will be one of the following (more like a best practice):

• static (normal functions that aren't visible outside that .c file)
• static inline (inlines from .c or .h files)
• extern (declaration in headers of the next kind (see below))
• [no keyword whatsoever] (normal functions meant to be accessed using extern declarations)