Empty output object with avr-libc >= 2.3.0

[booti386]

Since avr-libc v2.3, __call_main() (which calls main() and exit()) has been moved from crt[mcu].o to lib[mcu].a. As lib[mcu].a is not linked, main() never gets called and is opt-out from the output file.

Then, there are several fix:

• add lib[mcu].a to the linked libs and also provide a dummy exit() function (to avoid a missing symbol linker error), but this not the most straightforward way as we do not care about the C-specific details/other symbols defined in lib[mcu].a.
• add #[link_section = ".init9"] to the main function generated by the #[arduino_hal::entry] proc-macro, so that it replaces __call_main() at the proper step of the initialization process[1]. It would mimic the behavior of the -mno-call-main gcc compiler flag [2][3].

In both cases backward compatibility should not be affected.
I would like to read your comments and I can create a MR if necessary.

[1] https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init
[2] https://gcc.gnu.org/onlinedocs/gcc-15.1.0/gcc/AVR-Options.html#index-mno-call-main
[3] https://github.com/avrdudes/avr-libc/blob/753e533671c07cc288d6bf3f005480d2637132c8/crt1/gcrt1.S#L83C31-L83C45

[Rahix]

Hm, that's an unfortunate breaking change. We should really look to get rid of the avr-libc dependency sometime soon...

We definitely have to make sure everything works backwards compatibly. So no matter what version people have installed, they will get a binary that works.

I think that second option you suggest with a link_section attribute would be very nice, if it really works fine, even on old versions.

The relevant code for the entry macro is actually here, in avr-device:

avr-device/macros/src/lib.rs

Lines 101 to 122 in f0758da

	quote::quote! (
	#[cfg(not(any(doc, target_arch = "avr")))]
	compile_error!(
	"Ensure that you are using an AVR target! You may need to change \
	directories or pass a --target flag to cargo. See
	https://github.com/Rahix/avr-device/pull/41 for more details."
	);
	#(#cfgs)*
	#(#attrs)*
	#[doc(hidden)]
	#[export_name = "main"]
	pub unsafe extern "C" fn #tramp_ident() {
	#ident(
	#(#resource_args),*
	)
	}
	#[doc(hidden)]
	#f
	)
	.into()

[booti386]

I already tested that the fix works as intended on the latest release, of course.

The init sequence and the associated linker script did not change since binutils 2.13 (released in 2002), the section .init9 already existed back then, and its purpose was already to include code that calls the main function[1] at the end of the init sequence.

As long as we continue to export our entry point as a symbol called "main", the code will still be compatible with old versions of gcc/avr-libc/binutils. Moving it to .init9 (instead of .text were it currently lies) will not change the way it is resolved.

I will submit a PR.

[1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=ld/scripttempl/avr.sc;h=74f7ae4009dd7a792e20555d5676cc26d0a6f94a;hb=refs/tags/binutils-2_13#l98

[Rahix]

Sounds very good indeed.

I will submit a PR.

Please do, thanks a lot :)

[Rahix]

(Moved this issue over to avr-device)

[Rahix]

• New release of avr-device: v0.8.1
• Integrate into avr-hal: Upgrade to avr-device 0.8.1 avr-hal#696
• Update avr-hal-template: Rahix/avr-hal-template@42e3738

[Rahix]

Fix is rolled out everywhere now. I hope hope hope downstream users with existing projects will not upgrade their avr-libc too soon. Figuring out this silent bug will be pretty nasty for a lot of people I fear :(

[stappersg]

On Tue, Jan 06, 2026 at 04:06:46PM -0800, Rahix wrote: I hope hope hope downstream users with existing projects will not upgrade their `avr-libc` too soon. Figuring out this silent bug will be pretty nasty for a lot of people I fear :(

Yes "It worked previously!" is a nasty situation. Groeten Geert Stappers -- Silence is hard to parse