Usage of the extern keyword

Question

When I declare function prototypes in my header-files, I can reach those everywhere in my program, even though I never use the 'extern' keyword. Are they only important for static libraries, or when do I need it?

Answer 1

Functions are extern by default. The extern keyword is only useful for variables.

Answer 2

extern is default storage class specifier in C.

Explicitly specify it on variables

extern int i;

if it can be shared between modules. Then

int i;

in other module will not violate ODR.

For functions yes, pretty useless.