[cfe-dev] libcxx error: template with C linkage

Mon Nov 14 08:22:21 PST 2016

Thank you Eric and Richard,

Yes, I think there is some 'export "C" { ' without the closing brace
'}' in some NuttX header.

I got libcxx working on NuttX. Basically all I needed to do was to add
'extern "C++" {' inside all headers files:

https://bitbucket.org/acassis/libcxx/commits/all

Should these modification be included on libcxx? I can submit my patches!

BR,

Alan

On 11/14/16, Eric Fiselier <eric at efcs.ca> wrote:
> On Sun, Nov 13, 2016 at 7:12 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> On 13 Nov 2016 5:27 pm, "Eric Fiselier via cfe-dev" <
>> cfe-dev at lists.llvm.org> wrote:
>>
>> > But I used -save-temps to create the algorithm.ii and searched for all
>> > occurrences of 'extern "C" {' inside it and I can confirm there is not
>> > C++ header inside any 'extern C' block.
>>
>> Are you sure about that? libc++ provides its own version of each C
>> library
>> header as well (ex. <math.h>).
>> My best guess is that the system C library is included a libc++ C library
>> wrapper inside an 'extern C' block.
>>
>>
>> Don't libc++'s .h headers all have an extern "C++" block to avoid that
>> possibility?
>>
>
> It seems some headers do, including <math.h>, but there are still others
> without it.
>
>
>
>>
>> There also doesn't seem to be any evidence of this in the #include stack
>> reported by the compiler. It looks like maybe a system header ended with
>> an
>> extern "C" still open, or somehow libc++'s headers are being implicitly
>> treated as "extern C" system headers (which get an explicit extern "C"
>> wrapped around them).
>>
>> Alan, can you provide the preprocessed file produced by -save-temps?
>>
>> /Eric
>>
>>
>> On Mon, Nov 7, 2016 at 10:17 AM, Alan Carvalho de Assis via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Hi Everyone,
>>>
>>> I'm compiling the LLVM libcxx for NuttX RTOS (compiling on Linux to
>>> integrate it on NuttX).
>>>
>>> But it is failing with tons of these "error: template with C linkage" :
>>>
>>> make[1]: Entering directory '/home/alan/nuttxspace/nuttx/libxx'
>>> CXX:  libcxx/algorithm.cxx
>>> arm-none-eabi-g++ -c  -Wall -Wshadow -Wundef -g -mcpu=cortex-m4
>>> -mthumb -mfloat-abi=soft -I. -isystem
>>> /home/alan/nuttxspace/nuttx/include -isystem
>>> /home/alan/nuttxspace/nuttx/include/cxx -isystem
>>> /home/alan/nuttxspace/nuttx/include/libcxx   -pipe -std=c++11
>>> -DCLOCK_MONOTONIC -D__NuttX__ -pedantic -D_DEBUG
>>> -D_LIBCPP_BUILD_STATIC  libcxx/algorithm.cxx -o  algorithm.o
>>> In file included from
>>> /home/alan/nuttxspace/nuttx/include/libcxx/algorithm:639:0,
>>>                  from libcxx/algorithm.cxx:10:
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:397:1: error:
>>> template with C linkage
>>>  template <class _T1, class _T2> struct _LIBCPP_TYPE_VIS_ONLY pair;
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:398:1: error:
>>> template with C linkage
>>>  template <class _Tp> class _LIBCPP_TYPE_VIS_ONLY reference_wrapper;
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:399:1: error:
>>> template with C linkage
>>>  template <class _Tp> struct _LIBCPP_TYPE_VIS_ONLY hash;
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:401:1: error:
>>> template with C linkage
>>>  template <class>
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:404:1: error:
>>> template with C linkage
>>>  template <class _Tp>
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:407:1: error:
>>> template with C linkage
>>>  template <class _Tp, bool>
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:410:1: error:
>>> template with C linkage
>>>  template <bool _Bp, class _If, class _Then>
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:412:1: error:
>>> template with C linkage
>>>  template <class _If, class _Then>
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:419:1: error:
>>> template with C linkage
>>>  template <bool, class _Tp> struct _LIBCPP_TYPE_VIS_ONLY
>>> __lazy_enable_if
>>> {};
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:420:1: error:
>>> template with C linkage
>>>  template <class _Tp> struct _LIBCPP_TYPE_VIS_ONLY
>>> __lazy_enable_if<true, _Tp> {typedef typename _Tp::type type;};
>>>  ^
>>> /home/alan/nuttxspace/nuttx/include/libcxx/type_traits:422:1: error:
>>> template with C linkage
>>>  template <bool, class _Tp = void> struct _LIBCPP_TYPE_VIS_ONLY
>>> enable_if
>>> {};
>>>
>>>
>>>
>>>
>>> I searched on Internet and discovered that this error happens when
>>> there is some 'extern "C" { ' without the closing ' }' and then it
>>> wrap some C++ code.
>>>
>>> I saw this comment inside ctype.h :
>>>
>>> #ifdef __cplusplus
>>> #if defined(_LIBCPP_MSVCRT)
>>> // We support including .h headers inside 'extern "C"' contexts, so
>>> switch
>>> // back to C++ linkage before including these C++ headers.
>>> extern "C++" {
>>>   #include "support/win32/support.h"
>>>   #include "support/win32/locale_win32.h"
>>> }
>>> #endif // _LIBCPP_MSVCRT
>>>
>>> Although I'm not on MSVCRT, the comment explains why this error could
>>> happen.
>>>
>>> But I used -save-temps to create the algorithm.ii and searched for all
>>> occurrences of 'extern "C" {' inside it and I can confirm there is not
>>> C++ header inside any 'extern C' block.
>>>
>>> After many tests I noted that if I modify include/algorithm to include
>>> 'extern "C++" {' :
>>> @@ -634,7 +634,10 @@ template <class BidirectionalIterator, class
>>> Compare>
>>>
>>>  #include <__config>
>>>  #include <initializer_list>
>>> +extern "C++"
>>> +{
>>>  #include <type_traits>
>>> +}
>>>  #include <cstring>
>>>  #include <utility> // needed to provide swap_ranges.
>>>  #include <memory>
>>>
>>> The issue disappear, but I want to discover the root caused instead of
>>> use an ugly hack.
>>>
>>> How I can discover where the error is happening, since -save-temps
>>> didn't show nothing wrong ?
>>>
>>> BR,
>>>
>>> Alan
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>
>>
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