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

Sun Nov 13 19:33:35 PST 2016

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
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>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
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