extern keyword with function names

Question

I know that static keyword makes a C function/variable is file-scoped. And I've read that If I want to make a variable global scope (accessed by more than one file), I should do:

in the .c file:

int my_global_var;

// main()....

in the .h file:

extern int my_global_var;

So, any one will include my .h file will be able to reference my_global_var which is already externed.

And I read also this is required for functions as well but I am using gcc 4.x and I don't extern the function in the .h file and other programs can successfully link it.

So, the question is...

Is the behavior of the non-static function linkage is the default or should I extern non-static functions to adhere to the standard??

Answer 1

From the standard, 6.2.2

5 If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern. If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.

Meaning, it's extern by default.

Answer 2

Both function and object declarations are extern by default. However, you need to add an explicit extern to your object declarations in header files to avoid a re-definition: without a storage-class specifier, any file-scoped object declaration will actually be something called a tentative definition and reserve storage for the object within the current translation-unit.

For consistency, I unnecessarily use extern even for function declarations. In most cases, I declare objects within headers as one of

extern int foo;
static const int bar = 42;

and functions as one of

extern int spam(void);
static inline int eggs(void) { return 42; }