gistfile1.txt

As you can see, only extern_func() is accessible from other translation units.

$ c++ -std=c++11 -c test.cpp
$ nm -g test.o | c++filt
0000000000000000 T extern_func()
$ nm test.o | c++filt 
0000000000000000 T extern_func()
0000000000000006 t static_func()
000000000000000c t (anonymous namespace)::anonymous_namespace_func()

test.cpp

void extern_func() { }
static void static_func() { }
namespace { void anonymous_namespace_func() { } }