"illegal thread-local reference to regular symbol" error on a TLS variable

Question

I have a strange issue that occurs during linking.

I have a header file that contains the following definition foo.hpp:

struct Foo { static __thread int x; }

And a source file which references that variable plugin.cpp:

#include "foo.hpp"
void bar() { int y = Foo::x; }

It compiles fine with:

$CXX -stdlib=libc++ -std=c++11 -fvisibility=hidden -fPIC -o plugin.cpp.o -c plugin.cpp

But when I try to link as a dynamic library:

$CXX -stdlib=libc++ -std=c++11 -fvisibility=hidden -dynamiclib -Wl,-undefined,dynamic_lookup -o libext.dylib ext.cpp.o

I get:

ld: illegal thread local variable reference to regular symbol __ZN3Foo1xE for architecture x86_64

However the llvm bytecode implies that the compiler is correctly seeing Foo::x as a TLS variable.

$CXX -stdlib=libc++ -std=c++11 -fvisibility=hidden -fPIC -S -emit-llvm -o -
... omitted
@_ZN3Foo1xE = external thread_local global i32
... omitted
; Function Attrs: nounwind ssp uwtable
define hidden void @_Z3barv() #0 {
  %y = alloca i32, align 4
  %1 = load i32* @_ZN3Foo1xE, align 4
  store i32 %1, i32* %y, align 4
  ret void
}

What could be causing this linker issue, and is there a workaround? I can't seem to find any bug reports related to this.

Notes:

• This is purely using the Apple LLVM 7.0.0
• I have no problems linking using either gcc 5 or gcc 4.9.3 on OS X

EDIT The same problem exists when referencing a global (instead of static class) variable.

When I use thread_local in place of __thread, this works fine, however thread_local does not work with the version of LLVM that ships with Xcode.

Answer 1

Apples executable file format (MACH-O I believe) does not allow for Thread local storage. Its a pain in the ass. You have to create space in the threading library's memory allocation and hide the thread local variables in there. Its very very obscure.