On Wed, Sep 28, 2022 at 10:17 PM Jonathan Wakely via Gcc
<gcc@gcc.gnu.org> wrote:
>
> As part of implementing a C++23 proposal [1] to massively increase the
> scope of the freestanding C++ standard library some questions came up
> about the special handling of main() that happens for hosted
> environments.
>
> As required by both C++ (all versions) and C (since C99), falling off
> the end of the main() function is not undefined, the compiler is
> required to insert an implicit 'return 0' [2][3]. However, this
> special handling only applies to hosted environments. For freestanding
> the return type or even the existence of main is
> implementation-defined.
^^^^^^^^^^^^^^^^^^^^^^
so just document that 'int main(int, char **)' is special to GCC even in
freestanding environments and do not emit -Wreturn-type diagnostics?
I think that's entirely reasonable (but of course make sure to add
an implicit return 0; then as well)
Richard.
> As a result, GCC gives a -Wreturn-type warning
> for this code with -ffreestanding, but not with -fhosted:
>
> int main() { }
>
> Arsen (CC'd) has been working on the libstdc++ changes for the
> freestanding proposal, and several thousand libstdc++ tests were
> failing when using -ffreestanding, because of the -Wreturn-type
> warnings. He wrote a patch to the compiler [4] to add a new
> -fspecial-main flag which defaults to on for -fhosted, but can be used
> with -ffreestanding to do the implicit 'return 0' (and so disable the
> -Wreturn-type warnings) for freestanding as well. This fixes the
> libstdc++ test FAILs.
>
> However, after discussing this briefly with Jason it occurred to us
> that if the user declares an 'int main()' function, it's a pretty big
> hint that they do want main() to return an int. And so having
> undefined behaviour do to a missing return isn't really doing anybody
> any favours. If you're compiling for freestanding and you *don't* want
> to return a value from main(), then just declare it as void main()
> instead. So now we're wondering if we need -fspecial-main at all, or
> if int main() and int main(int, char**) should always be "special",
> even for freestanding. So Arsen wrote a patch to do that too [5].
>
> The argument against making 'int main()' imply 'special main' is that
> in a freestanding environment, a function called 'int main()' might be
> just a normal function, not the program's entry point. And in that
> case, maybe you really do want -Wreturn-type warnings. I don't know
> how realistic that is.
>
> So the question is, should Arsen continue with his -fspecial-main
> patch, and propose it along with the libstdc++ changes, or should gcc
> change to always make 'int main()' "special" even for freestanding?
> void main() and long main() and other signatures would still be
> allowed for freestanding, and would not have the implicit 'return 0'.
>
> I have no horse in this race, so if the maintainers of bare metal
> ports think int main() should not be special for -ffreestanding, so be
> it. I hope the first patch to add -fspecial-main would be acceptable
> in that case, and libstdc++ will use it when testing with
> -ffreestanding.
>
> [1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1642r11.html
> [2] https://eel.is/c++draft/basic.start.main#5.sentence-2
> [3] https://cigix.me/c17#5.1.2.2.3.p1
> [4]
> https://github.com/ArsenArsen/gcc/commit/7e67edaced33e31a0dd4db4b3dd404c4a8daba59
> [5]
> https://github.com/ArsenArsen/gcc/commit/c9bf2f9ed6161a38238e9c7f340d2c3bb04fe443
