Re: Handling of main() function for freestanding

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

On Tue, 4 Oct 2022 at 23:25, Jason Merrill <jason@redhat.com> wrote:
>
> On 9/28/22 16:15, Jonathan Wakely 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. 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 would rather not add a flag.  No well-defined freestanding program is
> affected by implicit return 0 from main, it should always be enabled.

There are some tests that fail if we do that. For whatever reason,
they're checking the current semantics.

Arsen implemented Jakub's suggestion which is to add the implicit
return by default, but add -fno-builtin-main to restore the previous
behaviour. Is that acceptable? If not, can you and Jakub reach
consensus so that Arsen knows what to do instead?
His -fno-builtin-main patch is at
https://gcc.gnu.org/pipermail/gcc-patches/2022-September/602644.html
(This is the only one of his patch series not committed, and results
in 100s of FAILs for libstdc++ when testing with -fffreestanding).