Re: Correct way to provide a C callback function nside C++

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Fri, 27 Jan 2023 at 13:24, Pepe via Gcc-help <gcc-help@gcc.gnu.org> wrote:
>
> Hi there,
>
> I’m in an ongoing discussion about whether or not one should use extern “C” when defining a function that will be used as a callback in a statically linked C library. For example:
>
> c_func.h:
> // …
> void reg_callback(void (*fn)());
> // …
>
> cpp_impl.cpp:
> // …
> extern “C” {
> #include “c_func.h”
> }
>
> // my callback function with internal linkage
> namespace {
>
> extern “C” {
> static void my_callback_A() {
>         // …
> }
> } // extern “C”
>
> void my_callback_B() {
>         // …
> }
>
> } // namespace
>
> void do_something() {
>         reg_callback(my_callback_A);
>         reg_callback(my_callback_B);
> }
>
> Both callbacks have internal linkage. Both work fine, and something like my_callback_B is found in lots of code bases.
>
> In my opinion, using callback B is implementation defined behaviour, because it is not guaranteed that C and C++ use the same calling conventions. Therefore a function must adhere to the C calling conventions to be used as a callback in a C library, which would be callback A.
>
> I’ve been trying to find something definitive for days now, but to no avail. Now I’m not sure what’s true or not. The counter argument is the following: The compiler should know reg_callback is a C function and make sure that a given argument would either be valid or cause a compiler error. That sounds reasonable, so I would love to know how to do it properly for future reference. Given we use gcc I was hoping to get a definitive answer in this mailing list. Thanks a lot!

You are (mostly) correct. The C++ standard says that extern "C"
functions and extern "C++" functions have different types, and so this
should not even compile:

extern "C" {
using callback = void(*)();
void f(callback);
}

void g() { };
void h() { f(g); }

There should be a compilation error when trying to pass g (which is an
extern "C++" function) to f (which accepts a pointer to an extern "C"
function).

GCC (and most other compilers) do not actually conform to that
requirement in the standard, and the types are identical. Which means
there is no compilation error, and the code works fine.

I think it's safe to assume that *either* the code compiles and works
as expected, or fails to compile. And in practice it compiles and
works with all widely used compilers. You will not find a C++
implementation where the types are not compatible, but the code
compiles anyway and then misbehaves at runtime.

The relevant GCC bug about this nonconformance is
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=2316 (and will probably
never be fixed, because it would break far too much code).

The relevant quote from the C++ standard is in [dcl.link]:
"The default language linkage of all function types, functions, and
variables is C ++ language linkage. Two function types with different
language linkages are distinct types even if they are otherwise
identical."

Being distinct types means that there should be not implicit
conversion from &g in the example above to the type callback. An
explicit conversion (e.g. using reinterpret_cast) would be allowed,
but then it would be undefined behaviour to actually call the function
g() through a pointer to a different function type. In practice, that
isn't a problem because they're not distinct types with GCC, so the
code works.






>
> Pepe