anonymous namespace and extern "C" linkage question

My use-case is as follows for a project:

A C header file with a few extern functions declared
A C++ source file with those functions defined
Compile the C++ source into a shared library
In a C source file use the functions declared in C header by linking against the .so

I am observing something strange while doing do. Things start to work with unnamed namepsace.

Here's my sample file:

c_sample.h:

#include "stddef.h"

extern void hello(void);
extern void bye(void);

cpp_sample.cc:

#include <iostream>
#include "c_sample.h"

extern "C" {
    void hello(void) { std::cout << "HI" << std::endl; }
    void bye(void) { std::cout << "BYE" << std::endl; }
}

On trying to build a share lib, I see the error which is expected since c_sample.h is included outside of the extern "C" block.

g++ cpp_sample.cc -shared -o libcppsample.so     
cpp_sample.cc:5:7: error: declaration of 'hello' has a different language linkage
    5 |         void hello() { std::cout << "HI" << std::endl;}
      |              ^
./c_sample.h:3:13: note: previous declaration is here
    3 | extern void hello();
      |             ^
cpp_sample.cc:6:7: error: declaration of 'bye' has a different language linkage
    6 |         void bye() { std::cout << "BYE" << std::endl;}
      |              ^
./c_sample.h:4:13: note: previous declaration is here
    4 | extern void bye();
      |             ^
2 errors generated.

However, magic happens the moment I wrap this up in a un-named namespace

cpp_sample.cc:

#include <iostream>
#include "c_sample.h"

extern "C" {
namespace {
    void hello(void) { std::cout << "HI" << std::endl; }
    void bye(void) { std::cout << "BYE" << std::endl; }
}
}

This compiled. And when I tried using it from another C source file, it even works

#include "stdio.h"
#include "c_sample.h"

int main() {
    hello();
}

$ gcc another.c -L/tmp -lcppsample -o another
$ ./another
HI

How does this work just with wrapping it inside a namespace? How is it able to link the declared functions with its definitions?

Solution

When you #include "c_sample.h" in a C++ file, you declare a couple of functions with C++ linkage.

When you hand-declare functions in the global namespace, you get a conflict. A function with C linkage cannot have the same name as a function with C++ linkage in the same namespace.

When you hand-declare functions in the unnamed namespace, you don't get a conflict. You can declare a function in any namespace and function with the same name in an different namespace, regardless of linkage.

A C linkage function declared in any namespace is just an ordinary C function that can be implemented in a C source file and compiled by a C compiler (if no C++-specific types are involved).

There is nothing undefined or implementation-specific here, but the code is fragile and not very useful. The right thing to do is to add contitionally-compiled extern "C" right in the C header.