document how to build with clang when GCC < 4.8 is installed

[nurpax]

I'm trying to build rtags with clang-3.4 installed from Ubuntu with GCC 4.6 installed, but I'm hitting this error:

android-main-2014-11-21 (release):~/t/rtags/build$ cmake ..
-- LLVM llvm-config found at: /usr/bin/llvm-config-3.4
-- Using Clang version 3.4 from /usr/lib/llvm-3.4/include/clang-c/
CMake Error at src/rct/rct.cmake:189 (message):
  C++11 support not detected.  rct requires a modern compiler, GCC >= 4.8 or
  Clang >= 3.2 should suffice
Call Stack (most recent call first):
  src/rct/CMakeLists.txt:2 (include)


-- Configuring incomplete, errors occurred!

Unfortunately I have no idea to build rtags with clang (e.g. override default g++). I'm not sure if installing gcc 4.8 is easy on my older Ubuntu. It's probably just a cmake config step away.

This would be nice to document in the README.

[nurpax]

Oh, looks like

export CC=`which clang`
export CXX=`which clang++`

is enough to do this. It doesn't help though, as clang still tries to use gcc 4.6 header files which don't compile:

/usr/bin/../lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6/chrono:535:6: error: no matching
      constructor for initialization of 'duration' (aka 'std::chrono::duration<long, std::ratio<1, 1000000> >')
          : __d(__t.time_since_epoch())
            ^   ~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6/condition_variable:106:42: note: in
      instantiation of function template specialization 'std::chrono::time_point<std::chrono::system_clock,
      std::chrono::duration<long, std::ratio<1, 1000000> > >::time_point<std::chrono::duration<long,
      std::ratio<1, 1000000000> > >' requested here
        const __clock_t::time_point __s_atime = __s_entry + __delta;
                                                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6/chrono:234:12: note: candidate constructor
      not viable: no known conversion from 'duration<[...], ratio<[...], 1000000000>>' to 'const
      duration<[...], ratio<[...], 1000000>>' for 1st argument
        constexpr duration(const duration&) = default;
                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6/chrono:237:19: note: candidate template
      ignored: disabled by 'enable_if' [with _Rep2 = std::chrono::duration<long, std::ratio<1, 1000000000> >]
               enable_if<is_convertible<_Rep2, rep>::value
                         ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6/chrono:244:19: note: candidate template
      ignored: disabled by 'enable_if' [with _Rep2 = long, _Period2 = std::ratio<1, 1000000000>]
               enable_if<treat_as_floating_point<rep>::value
                         ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6/chrono:232:12: note: candidate constructor
      not viable: requires 0 arguments, but 1 was provided
        constexpr duration() : __r() { }
                  ^
1 error generated.
make[2]: *** [src/rct/CMakeFiles/rct.dir/rct/Connection.cpp.o] Error 1
make[1]: *** [src/rct/CMakeFiles/rct.dir/all] Error 2
make: *** [all] Error 2

#### make failed to build some targets (3 seconds) ####

[jhanssen]

You should be able to solve that by passing --isystem= to rdm, where is the location to either stdlibc++ from GCC 4.8 or newer or alternatively to clangs libc++.

[nurpax]

This happened when I was building rtags executables like rdm. I'm not very familiar with clang so it's hard to get stuff like this right.

I think I now got it working by installing clang from llvm.orgs apt repos.

Perhaps the exact set of clang Debian packages required for building rtags would be good in the readme?

[mgulick]

In case anyone else hits this issue building on an older system (I was building on a Debian 7 system w/ gcc 4.7), without going into all the gory details, here's the key points:

• Build gcc 4.9 and install it into a separate prefix. This should be straightforward and there are numerous guides online.

• Build llvm/clang 3.7.

  • I downloaded the following llvm packages:
    • Clang source code
    • LLVM source code
    • Compiler RT source code
    • LibC++ source code
    • LibC++ ABI source code
    • Clang Tools Extra

  These packages need to be extracted/moved into specific directories. See the link at the end of this comment for details.
  Important: when building clang, you need to explicitly use the gcc 4.9 libc headers, as using the system-default gcc 4.7 headers will result in the std::chrono::duration error mentioned in earlier comments. The actual cmake line I used was:

  cmake ../src/llvm -G Ninja -DCMAKE_C_COMPILER=$GCC49ROOT/bin/gcc \
    -DCMAKE_CXX_COMPILER=$GCC49ROOT/bin/g++ \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX=$clangInstall \
    -DGCC_INSTALL_PREFIX=$GCC49ROOT \
    -DCMAKE_CXX_LINK_FLAGS="-L${GCC49ROOT}/lib64 -Wl,-rpath,${GCC49ROOT}/lib64"
  

  where $GCC49ROOT is the gcc 4.9 install prefix that was used. After that it was just ninja and ninja install to compile and install. $clangInstall can be something like /opt/llvm/3.7.0.

• Build rtags
  I used a similar cmake command line for rtags:

  tar -xf rtags.tar.xz
  cd rtags
  mkdir build
  cd build
  cmake .. -DLLVM_CONFIG=$CLANG37ROOT/bin/llvm-config \
    -DCMAKE_C_COMPILER=$CLANG37ROOT/bin/clang \
    -DCMAKE_CXX_COMPILER=$CLANG37ROOT/bin/clang++ \
    -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,$GCC49ROOT/lib64 -Wl,-rpath,$CLANG37ROOT/lib" \
    -DCMAKE_INSTALL_PREFIX=$rtagsInstall
  make -j8
  make install

where $CLANG37ROOT is the clang install prefix, and $GCC49ROOT is the same as when building llvm. $rtagsInstall can be something like /opt/rtags

I couldn't have figured this out without this page: http://btorpey.github.io/blog/2015/01/02/building-clang/

[casch-at]

@mgulick Cool stuff! Maybe I'll try this. I just gave up at work to use rtags, debian 7 has such old packages in the repos it is a bit of a nightmare, but there is hope. Thanks for sharing.

Better no indexer than working with eclipse. :-D