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

[Xi Ruoyao <xry111@xry111.site>]

On Fri, 2023-01-27 at 14:07 +0000, Jonathan Wakely via Gcc-help wrote:
> 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.

FWIW if we do "some strange thing" we may end up shooting our own foot.
Like:

a.cc:

#include <cstdio>

extern "C"
{
	struct A { struct {} x; int a; };
	void callback(void (*fn)(struct A *));
}

void f(struct A *p)
{
	std::printf("%d\n", p->a);
}

int main()
{
	callback(f);
}

b.c:

#include <stdio.h>

struct A { struct {} x; int a; };
void callback(void (*fn)(struct A *))
{
	struct A foo = { .a = 42 };
	fn(&foo);
}

This thing won't work with GCC because struct A will have different
layouts in GNU C and C++.  Note that the C standard does not allow an
empty struct at all (a pedantic C compiler should reject b.c as an empty
struct violates the syntax rule of C).  But GNU C supports it as an
extension.

> 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).

Perhaps we should document it as an extension as well and add a warning
option...  But this will be the lowest priority even if we'd spend our
time for the job.
-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University