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.