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0 pointsLegibleCrimson21y ago0 comments

> `extern "C"` has nothing to do with linkage, all it does is disable namemangling, so you get the same symbol name as with a C compiler.

extern "C" also ensures that the C calling convention is used, which is relevant for callbacks. It's not just name mangling. This is the reason that extern "C" static functions exist. You can actually overload a C++ function by extern "C" vs extern "C++", and it will dispatch it appropriately based on whether the passed in function is declared with C or C++ linkage.

And I'm not sure the terms are confused, because that's how most documentation refers to it: https://learn.microsoft.com/en-us/cpp/cpp/extern-cpp?view=ms...

> In C++, when used with a string, extern specifies that the linkage conventions of another language are being used for the declarator(s). C functions and data can be accessed only if they're previously declared as having C linkage. However, they must be defined in a separately compiled translation unit.

And https://en.cppreference.com/w/cpp/language/language_linkage

The post you're replying to had it completely right. extern "C" is entirely about linkage, which includes calling convention and name mangling.

> As you noted correctly, the calling conventions must match, but in practice this only matters on x86 Windows.

Or if you want your program to actually be correct, instead of just incidentally working for most common cases, including on future systems.

If you're passing a callback to a C function from C++, it's wrong unless the callback is declared extern "C".

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spacechild11y ago

> extern "C" also ensures that the C calling convention is used, which is relevant for callbacks. It's not just name mangling.

I stand corrected. I didn't know that `extern "C"` enforces the C calling convention.

However, on modern platforms this doesn't really matter because, as I said, there is only a single calling convention (per platform). And I'm pretty sure that future platforms will keep it that way. Fortunately, if you try to pass a C++ callback of the wrong calling convention, you get a compiler error.

> If you're passing a callback to a C function from C++, it's wrong unless the callback is declared extern "C".

That's certainly not true because `extern "C"` is not the only way to specify the calling convention. In fact, you might need a different calling convention! As I mentioned, on x86 the Windows API uses stdcall for all API functions and callbacks, so `extern "C"` would be wrong. If you look at the Microsoft examples, you will see that they declare the callbacks as WINAPI (without `extern "C"`): https://learn.microsoft.com/en-us/windows/win32/procthread/c...

So I stand by my point that in practice you don't need `extern "C"` for passing C++ callbacks to C functions. You can pass a lambda function just fine, and when it doesn't work the compiler will tell you.

LegibleCrimson2OP1y ago

A couple big caveats here:

* cdecl is a platform specific calling convention. There is no standard C ABI. cdecl is a wintel thing, not the standard C calling convention. On Linux, this is the System V ABI for instance. On Windows ARM, it's also not cdecl.

* Specifying calling convention at all is a compiler specific extension. There is no standard way of specifying a C calling convention without `extern`.

So specifying cdecl gets you the right calling convention on some platforms and ties your code to some specific compilers. The only portable way to specify C linkage in a C++ program is extern "C". You will always get the right ABI for your platform and it will work on every compiler.

> So I stand by my point that in practice you don't need `extern "C"` for passing C++ callbacks to C functions. You can pass a lambda function just fine, and when it doesn't work the compiler will tell you.

The compiler will very often not tell you. It will complain if the lambda can't be coerced to a function pointer (because it's a closure) or if the argument or return types are wrong. An incorrect ABI will usually be accepted and will just do the wrong thing or crash at runtime. The C++ standard says that language linkage is part of a function's type, but very few compilers actually support this.

Your position works sometimes for some compilers and some platforms. I assert that it's better to use standard C++ features and just work everywhere.

spacechild11y ago

> * Specifying calling convention at all is a compiler specific extension.

Yes, because the calling conventions themselves are platform/compiler specific.

> There is no standard way of specifying a C calling convention without `extern`.

Well, on modern platforms you don't need to because there is only a single calling convention that is shared between C and C++. For legacy platforms with multiple calling conventions, you need compiler specific extensions by definition.

> The only portable way to specify C linkage in a C++ program is extern "C". You will always get the right ABI for your platform and it will work on every compiler.

Again, on platforms with several calling conventions `extern "C"` absolutely won't give you the appropriate calling convention all the time. See again my Win32 API example.

> The compiler will very often not tell you > An incorrect ABI will usually be accepted and will just do the wrong thing or crash at runtime.

That's absolutely not my experience! Functions with different calling conventions have different types, so a C++ compiler must reject such code. See https://godbolt.org/z/6EnncE5v5. (Note that for the lambda case MSVC is smart enough to automatically add __stdcall whereas MinGW refuses to compile. The free function is rejected by both compilers.)

Can you show me an actual example where a C++ compiler silently accepts a function with the wrong calling convention?

> Your position works sometimes for some compilers and some platforms.

It has always worked for me so far and I write software for many different platforms.

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