How to correctly use the extern keyword in C

extern changes the linkage. With the keyword, the function / variable is assumed to be available somewhere else and the resolving is deferred to the linker.

There's a difference between extern on functions and on variables.

For variables it doesn't instantiate the variable itself, i.e. doesn't allocate any memory. This needs to be done somewhere else. Thus it's important if you want to import the variable from somewhere else.

For functions, this only tells the compiler that linkage is extern. As this is the default (you use the keyword static to indicate that a function is not bound using extern linkage) you don't need to use it explicitly.

In C, extern is implied for function prototypes, as a prototype declares a function which is defined somewhere else. In other words, a function prototype has external linkage by default; using extern is fine, but is redundant.

(If static linkage is required, the function must be declared as static both in its prototype and function header, and these should normally both be in the same .c file).

If each file in your program is first compiled to an object file, then the object files are linked together, you need extern. It tells the compiler "This function exists, but the code for it is somewhere else. Don't panic."