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Date:	Tue, 19 May 2015 10:52:23 +0200
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Subject:	Re: C++11 thread_local implementation issue on Cygwin/AMD64
From:	=?UTF-8?Q?V=C3=A1clav_Haisman?= <vhaisman AT gmail DOT com>
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On 19 January 2015 at 15:42, Václav Haisman wrote:
>
> Hi.
>
> I have hit an issue with thread-local storage variables on
> Cygwin/AMD64, I do not see it with Cygwin/i686.
>
> 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.
>
> See also http://stackoverflow.com/q/28023728/341065


This is also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64697.

-- 
VH

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