Why can extern be applied to definitions?

Question

Why is this legal?

extern int foo = 0xF00; // Gets a warning, still compiles

extern void bar() { // No warning
  int x;
}

Is there a reason to why this is allowed?

Answer 1

Sometimes it's useful

extern const int foo = 0xF00;

Without the extern, in C++ foo would be static and have internal linkage (which means you could not use foo from another translation unit).

The extern in both cases in your example is redundant. In C99 an extern can make a difference for inline functions..

Answer 2

In the function case, I think it is just like writing:

extern void bar();
void bar()
{
  int x;
}

which must be legal since a file with the definition may include a header with such a declaration.