using extern thread_local in C++11 on Cygwin/AMD64

Question

I am having linking issues when using thread_local keyword in Cygwin with its GCC 4.8.3 and GCC 4.9.2. This is derived from log4cplus. The test case is split into three files:

File def.hxx:

#include <string>

namespace N
{
  struct S { std::string str; };
  // extern declaration in a header
  extern thread_local S * ptd;

  // accessing the extern declared ptd here
  inline
  S * get_ptd ()
  {
    if (! ptd)
      ptd = new S;
    return ptd;
  }
} // namespace N

File def.cxx:

#include "def.hxx"

namespace N
{
  // definition of ptd
  thread_local S * ptd = nullptr;
} // namespace N

File use.cxx:

#include "def.hxx"

namespace N
{
  __declspec(dllexport)
  void * foo ()
  {
    // invoking inline get_ptd() function to get the value in ptd
    return get_ptd ();
  }
}

Now, when I compile each .cxx with g++ -std=gnu++11 -fvisibility=hidden -c use.cxx def.cxx and then try to link with g++ -shared -o cygtest.dll use.o def.o, I get the following error from linker:

use.o:use.cxx:(.text$_ZTWN1N3ptdE[_ZTWN1N3ptdE]+0x15): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `TLS init function for N::ptd'
collect2: error: ld returned 1 exit status

The nm -C ./def.o output confirms that:

`--> nm -C ./def.o
0000000000000000 b .bss
0000000000000000 d .data
0000000000000000 r .rdata
0000000000000000 r .rdata$zzz
0000000000000000 t .text
0000000000000008 r __emutls_t._ZN1N3ptdE
0000000000000000 D __emutls_v._ZN1N3ptdE
0000000000000000 r std::piecewise_construct

As you can see, the ptd thread-local variable initialization function is not defined anywhere. The use.o references this initialization function (see bottom of the listing):

`--> nm -C ./use.o
0000000000000000 b .bss
0000000000000000 d .data
0000000000000000 i .drectve
0000000000000000 p .pdata
0000000000000000 p .pdata$_ZN1N1SC1Ev
0000000000000000 p .pdata$_ZN1N7get_ptdEv
0000000000000000 p .pdata$_ZTWN1N3ptdE
0000000000000000 r .rdata
0000000000000000 r .rdata$.refptr.__emutls_v._ZN1N3ptdE
0000000000000000 r .rdata$.refptr._ZTHN1N3ptdE
0000000000000000 r .rdata$zzz
0000000000000000 R .refptr.__emutls_v._ZN1N3ptdE
0000000000000000 R .refptr._ZTHN1N3ptdE
0000000000000000 t .text
0000000000000000 t .text$_ZN1N1SC1Ev
0000000000000000 t .text$_ZN1N7get_ptdEv
0000000000000000 t .text$_ZTWN1N3ptdE
0000000000000000 A .weak._ZTHN1N3ptdE._ZN1N1SC1Ev
0000000000000000 r .xdata
0000000000000000 r .xdata$_ZN1N1SC1Ev
0000000000000000 r .xdata$_ZN1N7get_ptdEv
0000000000000000 r .xdata$_ZTWN1N3ptdE
                 U __emutls_get_address
                 U __emutls_v._ZN1N3ptdE
                 U __gxx_personality_seh0
                 U __real__ZdlPv
                 U __real__Znwm
                 U _Unwind_Resume
                 U operator delete(void*)
0000000000000000 T N::S::S()
0000000000000000 T N::foo()
0000000000000000 T N::get_ptd()
                 U std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string()
                 U operator new(unsigned long)
0000000000000000 r std::piecewise_construct
                 w TLS init function for N::ptd
0000000000000000 T TLS wrapper function for N::ptd

Now, this code seems to work well on Linux with both GCC and Clang.

• Is this a GCC problem on Cygwin?
• Am I using extern thread_local wrong?

My experiments show that not using the extern keyword seems to fix the issue. But I am not sure if that does not introduce two ptd thread-local variables in two TUs.

UPDATE

I have created a GCC bug report for this.