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

[Eric Fiselier via cfe-dev]

> 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.

/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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