Can't link to an external thread local symbol

[mneumann]

I am trying to link to

extern __thread int errno;

I try it with:

extern "C" {
    #[thread_local]
    #[link_name = "errno"]
    static errno: c_int;
}

But it seem that it omits the thread_local part. Any ideas how to get it to work? The linker tells me that I have both thread-local and non-thread local references to errno defined, so above links to the non-thread local.

[alexcrichton]

I think this is because we don't recognize #[thread_local] on extern globals, for example:

#![feature(thread_local)]

extern {
    #[thread_local]
    #[link_name = "errno"]
    pub static errno: i32;
}

pub fn foo() -> i32 {
    unsafe { errno }
}

; ModuleID = 'foo.0.rs'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-darwin"

@errno = external global i32

; Function Attrs: uwtable
define i32 @_ZN3foo20h5ca9ce4ccbe13eabhaaE() unnamed_addr #0 {
entry-block:
  %0 = load i32, i32* @errno, align 4
  ret i32 %0
}

attributes #0 = { uwtable "no-frame-pointer-elim"="true" }

(note the lack of thread_local on the extern declaration)

[mneumann]

@alexcrichton: DragonFly would need it in order to support errno. Otherwise we need a shim C wrapper again (__dfly_error). This was removed in some earlier commit. In my local patches I have added a rt/errno.c again, but in the long term this clearly is not the solution.

[alexcrichton]

This may actually be easy enough to add to trans, I think you'll basically need to copy this code over into the foreign module as well.

It's possible that we could add back the C shim wrapper, although I'd prefer not to do so!

[mneumann]

@alexcrichton : Thanks for pointing out the solution! I have a pull request (#30856) that fixes that. Much better than requiring a C wrapper! With that fix and some other libc related things, I can now build rust on DragonFly again.