[AVR] undefined reference to 'abort' on static mutable variable

[EvansJahja]

I tried this code:

pub static mut STATMUT : Option<String> = None;

#[unsafe(no_mangle)]
fn main() -> (){
    unsafe {
        STATMUT = Some(String::from_str("hello").unwrap());
    }
    loop {}
}

I expected to see this happen: compile successfully

Instead, this happened: error because "undefined reference to `abort'"

Meta

rustc --version --verbose:

rustc 1.88.0-nightly (9ffde4b08 2025-04-12)
binary: rustc
commit-hash: 9ffde4b089fe8e43d5891eb517001df27a8443ff
commit-date: 2025-04-12
host: x86_64-pc-windows-msvc
release: 1.88.0-nightly
LLVM version: 20.1.2

Backtrace

   Compiling avr_undefined_abort v0.1.0 (C:\Users\EvansGrace02\Documents\Workspace\avr_undefined_abort)
warning: unused variable: `layout`
  --> src\main.rs:14:28
   |
14 |     unsafe fn alloc(&self, layout: core::alloc::Layout) -> *mut u8 {
   |                            ^^^^^^ help: if this is intentional, prefix it with an underscore: `_layout`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `ptr`
  --> src\main.rs:18:30
   |
18 |     unsafe fn dealloc(&self, ptr: *mut u8, layout: core::alloc::Layout) {
   |                              ^^^ help: if this is intentional, prefix it with an underscore: `_ptr`

warning: unused variable: `layout`
  --> src\main.rs:18:44
   |
18 |     unsafe fn dealloc(&self, ptr: *mut u8, layout: core::alloc::Layout) {
   |                                            ^^^^^^ help: if this is intentional, prefix it with an underscore: `_layout`

error: linking with `avr-gcc` failed: exit code: 1
  |
  = note: "avr-gcc" "C:\\Users\\EVANSG~1\\AppData\\Local\\Temp\\rustcmeadZC\\symbols.o" "<2 object files omitted>" "-Wl,--as-needed" 
"-Wl,-Bstatic" "C:\\Users\\EvansGrace02\\Documents\\Workspace\\avr_undefined_abort\\target\\avr-none\\release\\deps/{liballoc-9c92b638956aedfc.rlib,librustc_std_workspace_core-1131ff51ed7f00cf.rlib,libcore-1cd83881e403c7a7.rlib,libcompiler_builtins-8c53ab747d5dc2e3.rlib}.rlib" "-L" "C:\\Users\\EVANSG~1\\AppData\\Local\\Temp\\rustcmeadZC\\raw-dylibs" "-Wl,-Bdynamic" "-lgcc" "-Wl,-z,noexecstack" "-o" "C:\\Users\\EvansGrace02\\Documents\\Workspace\\avr_undefined_abort\\target\\avr-none\\release\\deps\\avr_undefined_abort-99f447f01c535b46.elf" "-Wl,--gc-sections" "-no-pie" "-mmcu=atmega328p" "-Wl,--strip-debug" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: C:\Users\EvansGrace02\Documents\Workspace\avr_undefined_abort\target\avr-none\release\deps\libcore-1cd83881e403c7a7.rlib(core-1cd83881e403c7a7.core.d0224d2e180dad47-cgu.02.rcgu.o): In function `core::hint::unreachable_unchecked':␍
          core.d0224d2e180dad47-cgu.02:(.text._ZN4core4hint21unreachable_unchecked17hd73c2234e42a966eE+0xa): undefined reference to `abort'␍
          C:\Users\EvansGrace02\Documents\Workspace\avr_undefined_abort\target\avr-none\release\deps\libcore-1cd83881e403c7a7.rlib(core-1cd83881e403c7a7.core.d0224d2e180dad47-cgu.06.rcgu.o): In function `core::num::nonzero::NonZero<T>::new_unchecked':␍
          core.d0224d2e180dad47-cgu.06:(.text._ZN4core3num7nonzero16NonZero$LT$T$GT$13new_unchecked17h86576d22dcc7150bE+0x4c): undefined reference to `abort'␍
          C:\Users\EvansGrace02\Documents\Workspace\avr_undefined_abort\target\avr-none\release\deps\libcore-1cd83881e403c7a7.rlib(core-1cd83881e403c7a7.core.d0224d2e180dad47-cgu.06.rcgu.o): In function `core::fmt::FormattingOptions::align':␍
          core.d0224d2e180dad47-cgu.06:(.text._ZN4core3fmt17FormattingOptions5align17h25263cf3c1a26f6aE+0x96): undefined reference to `abort'␍
          C:\Users\EvansGrace02\Documents\Workspace\avr_undefined_abort\target\avr-none\release\deps\libcore-1cd83881e403c7a7.rlib(core-1cd83881e403c7a7.core.d0224d2e180dad47-cgu.06.rcgu.o): In function `core::fmt::getcount':␍
          core.d0224d2e180dad47-cgu.06:(.text._ZN4core3fmt8getcount17h7ca8f2ef5a49a849E+0x44): undefined reference to `abort'␍       
          C:\Users\EvansGrace02\Documents\Workspace\avr_undefined_abort\target\avr-none\release\deps\libcore-1cd83881e403c7a7.rlib(core-1cd83881e403c7a7.core.d0224d2e180dad47-cgu.06.rcgu.o): In function `core::fmt::Formatter::padding':␍
          core.d0224d2e180dad47-cgu.06:(.text._ZN4core3fmt9Formatter7padding17h725c8d0e59f6bf75E+0x72): undefined reference to `abort'␍
          C:\Users\EvansGrace02\Documents\Workspace\avr_undefined_abort\target\avr-none\release\deps\libcompiler_builtins-8c53ab747d5dc2e3.rlib(compiler_builtins-8c53ab747d5dc2e3.compiler_builtins.404ba4952ab58ac7-cgu.05.rcgu.o):compiler_builtins.404ba4952ab58ac7-cgu.05:(.text._ZN4core3ptr9const_ptr33_$LT$impl$u20$$BP$const$u20$T$GT$13is_aligned_to17h9342a87594a14351E+0xac): more undefined references to `abort' follow␍
          collect2.exe: error: ld returned 1 exit status␍

  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified     
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-lib)

warning: `avr_undefined_abort` (bin "avr_undefined_abort") generated 3 warnings
error: could not compile `avr_undefined_abort` (bin "avr_undefined_abort") due to 1 previous error; 3 warnings emitted

[EvansJahja]

Here's a minimal repo to reproduce the issue

https://github.com/EvansJahja/avr_undefined_abort

EDIT:

other example

https://github.com/EvansJahja/avr_undefined_abort/tree/no_alloc

[EvansJahja]

The failure disappears when using opt-level = 1 (or above). opt-level='s' and opt-level='z' works.

The failure only happens when using opt-level=0

[EvansJahja]

Regardless of opt-level, setting build-std-features = ["panic_immediate_abort"] always trigger the ICE.

[EvansJahja]

Current workaround

#[unsafe(no_mangle)]
pub unsafe extern "C" fn abort() -> !{
  loop {}
}

[EvansJahja]

Update: another example without alloc
https://github.com/EvansJahja/avr_undefined_abort/tree/no_alloc

Changing panic!("{}", 1); to panic!("1"); works. I guess it makes sense because panic allocate things, so there's a difference in behavior, but I don't know how this has anything to do with abort.

[bjorn3]

I think LLVM lowers trap instructions to calls of the abort function on targets that don't have a native trap instruction. https://github.com/llvm/llvm-project/blob/cbe8f3ad7621e402b050e768f400ff0d19c3aedd/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp#L4470

[EvansJahja]

That makes sense, but I'm under the assumption that avr builds with libgcc which should have the abort function..

[Patryk27]

Minimized:

#![no_main]
#![no_std]

use core::hint::unreachable_unchecked;
use core::panic::PanicInfo;

#[panic_handler]
fn panic(_: &PanicInfo) -> ! {
    loop {}
}

#[unsafe(no_mangle)]
fn main() -> ! {
    unsafe {
        unreachable_unchecked();
    }

    loop {
        //
    }
}

error: linking with `avr-gcc` failed: exit status: 1
  |
  = note:  "avr-gcc" "/tmp/rustcTS7uIh/symbols.o" "<3 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "/home/pwy/x/avr-test/target/avr-none/debug/deps/{librustc_std_workspace_core-deba25e9937c9ea8.rlib,libcore-d4812f051d0b91a6.rlib,libcompiler_builtins-13f0584f2cb1e9e1.rlib}.rlib" "-L" "/tmp/rustcTS7uIh/raw-dylibs" "-Wl,-Bdynamic" "-lgcc" "-Wl,-z,noexecstack" "-o" "/home/pwy/x/avr-test/target/avr-none/debug/deps/avr_test-fd706498ef97e4b0.elf" "-Wl,--gc-sections" "-no-pie" "-mmcu=attiny167" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: /nix/store/21ax4ifbsq910z9jwlfjixcpi06yb3b0-avr-binutils-2.44/bin/avr-ld: /home/pwy/x/avr-test/target/avr-none/debug/deps/avr_test-fd706498ef97e4b0.4551bm9c8awogum5dbtsfpw4f.1xxqf7u.rcgu.o: in function `core::hint::unreachable_unchecked':
          /home/pwy/.rustup/toolchains/nightly-2025-04-19-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/hint.rs:110:(.text._ZN4core4hint21unreachable_unchecked17hcad3a1b6c1990839E+0xa): undefined reference to `abort'
          collect2: error: ld returned 1 exit status
          
  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-lib)

I don't think this is a bug - it's just that under some conditions (in particular when compiling without optimizations), llvm will insert a special panic-like instruction (aka trap) for code paths it thinks are unreachable, to be 250% sure they aren't reached.

You can observe this behavior for other architectures as well - e.g. on x86 this:

#[inline(never)]
pub fn foo() {
    unsafe {
        core::hint::unreachable_unchecked();
    }
}

... gets compiled into:

playground::foo:
	ud2

... with ud2 being a "throw invalid opcode exception" instruction.

Since AVRs are simpler than your typical x86 processor, they don't have an equivalent of ud2, the implementation is left for the user - you might want to print something to serial output, blink LEDs etc., but you need to specify the behavior yourself:

#[unsafe(no_mangle)]
pub unsafe extern "C" fn abort() -> ! {
    // loop {}, blink, panic!() etc.
}

99% of time you don't have to worry about this, because the optimizer gets rid of those potentially-undefined code paths whatsoever, but 1% of the time they are left and it's up to you to decide what you want to happen then.

So tl;dr not a bug - at the same time, I understand that 99% of people don't want to be bothered with this, "do whatever" will work just fine for them; I think we might be able to get the best of both worlds (provide a default function, but allow to overwrite it) by utilizing weak symbols (we'd create a weak fn abort() within compiler-builtins).

I'll experiment and report back.

[EvansJahja]

I understand that 99% of people don't want to be bothered with this, "do whatever" will work just fine for them

for me it's not about "do whatever". We're in no_std land so I had to put my own panic_handler anyway, like we get compiler errors if we don't specify a panic handler, but didn't realize that we need to implement abort too.