Should functions be made "extern" in header files?

Question

Should functions be made extern in header files? Or are they extern by default?

For example, should I write this:

// birthdays.h struct person find_birthday(const char* name); 

or this:

// birthdays.h extern struct person find_birthday(const char* name); 

Answer 1

From The C Book:

If a declaration contains the extern storage class specifier, or is the declaration of a function with no storage class specifier (or both), then:

• If there is already a visible declaration of that identifier with file scope, the resulting linkage is the same as that of the visible declaration;
• otherwise the result is external linkage.

So if this is the only time it's declared in the translation unit, it will have external linkage.