静态函数上的extern声明是否会破坏内部链接？

Question

I have a theoretical question.

Here's an example:

file1.c

static void foo()
{
    ...
}

file2.c

extern void foo(); 

main()
{
    foo();
}

The compilation is OK.

So, the extern breaks the internal linkage of static declaration?

Answer 1

Is the "compilation ok" only in theory, or did you actually test this?

A static function should not be visible from outside the compilation unit (C file, typically) that it's in.

Declaring a function of the same name with extern in a different compilation unit should not change this.

Make sure you really build and link the test program properly, since otherwise all that you're testing is that you can have a "dangling" reference to an external symbol (the foo function referenced from 2.c). Building 2.c into an executable (ie linking it) should fail.

Answer 2

The compilation should be OK. If you can also link that, your compiler have a problem.

Answer 3

A quick test confirms the expected linker failure (VC++ 2010 express):

1>test.obj : error LNK2019: unresolved external symbol "void __cdecl foo(void)" (?foo@@YAXXZ) referenced in function _wmain