thread spawn is broken on wasm32-wasip1-threads for building library

[oligamiq]

I tried this code:

fn main() {
    std::thread::spawn(|| {
        println!("Hello from a thread!");
    });
}

I expected to see this happen: success

Instead, this happened: infinity loop

Meta

rustc --version --verbose:

rustc 1.92.0-nightly (4645a7988 2025-09-17)
binary: rustc
commit-hash: 4645a7988177c286f61609cc667ecae4c571a2e8
commit-date: 2025-09-17
host: x86_64-pc-windows-msvc
release: 1.92.0-nightly
LLVM version: 21.1.1

This relates to rust-lang/rust#108097
When generating as a bin, if you attempt to perform a thread spawn before the generated _start function is called, the program falls into an infinite loop and halts. This issue occurs regardless of whether you use nightly or not.

Furthermore, when building as a lib, thread spawning does not work at all.
The reason is that in the case of bin, crt1-command.o is linked, and within the generated _start function, __wasi_init_tp is invoked, which initializes the thread-related state. However, in the case of lib, the expected crt1-reactor.o should be linked, but its _initialize function is not exported, so initialization never happens and threads fail to work correctly.

As a workaround, you can use the following:

unsafe extern "C" {
    // Initialize thread-local storage for WASI threads
    // In the current version of Rust,
    // thread initialization is not performed.
    // Therefore, we must force linking to perform initialization.
    // https://github.com/rust-lang/rust/pull/108097
    fn __wasi_init_tp();
    fn __wasm_call_ctors();
}

pub extern "C" fn _initialize() {
    unsafe { __wasi_init_tp() };
    unsafe { __wasm_call_ctors() };
}

As also noted in
#146721,
I honestly cannot understand why thread::spawn is so broken in this area.

[Noratrieb]

@g0djan @alexcrichton @abrown @loganek

[alexcrichton]

Can you provide an example of something that doesn't work? The code in the OP of this issue looks like it wants to be compiled as a bin but the issue is about compiling it as a lib. The code with _initialize doesn't contain any threads to be spawned as well.

I honestly cannot understand why thread::spawn is so broken in this area.

The wasm32-wasip1-threads target is built on the wasi-threads proposal which is, to this day, experimental. Experimental software can break in ways like this which are often surprising. If you are surprised by you yourself encountering this breakage then it is likely more of an indicator of the usage of this target than anything else.

[oligamiq]

Sorry, this is the correct small example.
https://github.com/oligamiq/small_example_wasip1_lib_spawn

I will probably stop using this once Shared-Everything Threads are stabilized.

[alexcrichton]

Ah unfortunately that link is a 404, unsure if it's a typo or a private repo.

I can note that I would agree that shared-everything-threads is the best way forward, but I'll simultaneously caution in the sense that the proposal is likely multiple years from seeing an implementation much less standardized/stabilized.

[oligamiq]

Sorry. I had it set to private.