[CMake] Are CMAKE_CXX_FLAGS supposed to go on the link line?

[Roman Bolshakov]

Apparently, command line to link executable is quite different from a
shared library (Modules/CMakeCXXInformation.cmake):

if(NOT CMAKE_CXX_LINK_EXECUTABLE)
  set(CMAKE_CXX_LINK_EXECUTABLE
    "<CMAKE_CXX_COMPILER>  <FLAGS> <CMAKE_CXX_LINK_FLAGS> <LINK_FLAGS>
<OBJECTS>  -o <TARGET> <LINK_LIBRARIES>")
endif()


  set(CMAKE_CXX_CREATE_SHARED_LIBRARY
      "<CMAKE_CXX_COMPILER> <CMAKE_SHARED_LIBRARY_CXX_FLAGS>
<LANGUAGE_COMPILE_FLAGS> <LINK_FLAGS>
<CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS> <SONAME_FLAG><TARGET_SONAME>
-o <TARGET> <OBJECTS> <LINK_LIBRARIES>")

CMAKE_CXX_FLAGS/CMAKE_C_FLAGS which are filled into
<LANGUAGE_COMPILE_FLAGS> don't go into a linker command for
executable.


2015-01-22 19:11 GMT+03:00 Paul Smith <paul at mad-scientist.net>:
> I didn't get a response to the question below.  I've since reproduced
> this with a simple test case; maybe someone can let me know why this
> difference appears and how I should handle it?  I'm using cmake 3.1.0:
>
> Sample CMakeLists.txt:
>
>   $ cat >CMakeLists.txt <<EOF
>   cmake_minimum_required(VERSION 3.1.0)
>   project(FlagTest)
>   set(CMAKE_CXX_FLAGS -pthread)
>   add_executable(foo foo.cpp)
>   EOF
>
>   $ echo 'int main(int argc, char**argv) { return 0; }' > foo.cpp
>
> On MacOSX with Xcode installed, I see the following behavior:
>
>   $ cmake -G Xcode . && cmake --build . 2>&1 | tee xc.out
>
> In the output I see that the "-pthread" flag DOES appear on the compile
> line for "foo.o", but DOES NOT appear in the link line for "foo":
>
>   $ grep pthread xc.out
>   /.../clang -x c++ ... -pthread ...  -c /.../foo.cpp -o /.../foo.o
>
> This is correct for clang, which does not want -pthread on the link line
> (unlike GCC, which wants it in both places).  Now, I clean that up and
> try with the Makefile generator, and I see the following behavior:
>
>   $ cmake -G 'Unix Makefiles' . && cmake --build . -- VERBOSE=1 2>&1 | tee mk.out
>   clang: warning: argument unused during compilation: '-pthread'
>
> Now in this case we can see that the -pthread flag was added to BOTH the
> compile and the link line:
>
>   $ grep pthread mk.out
>   /usr/bin/c++ -pthread -o .../foo.cpp.o -c .../foo.cpp
>   /usr/bin/c++ -pthread .../foo.cpp.o -o foo
>   clang: warning: argument unused during compilation: '-pthread'
>
> This warning is totally bogus: it really means "unused during LINKING",
> but anyway: I'm not sure what to do to get rid of this warning: I need
> to support both Xcode and Makefiles on MacOSX (and Makefiles on Linux as
> well as both GCC and Clang).
>
> Is the intent that CMAKE_CXX_FLAGS only ever appear on the compile line?
> Or should it appear on both the compile and link lines?  Is there an
> equivalent flag to CMAKE_EXE_LINKER_FLAGS that always applies only to
> compile lines, in all generators?  Do we just have to continue to
> bastardize add_definitions() for this, maybe?
>
>
> On Thu, 2015-01-08 at 18:52 -0500, Paul Smith wrote:
>> If I'm on OSX, then when I set CMAKE_CXX_FLAGS and do not set
>> CMAKE_EXE_LINKER_FLAGS, for example, and I use the Xcode generator,
>> then
>> I see:
>>       * CMAKE_CXX_FLAGS show up on the compilation line
>>       * CMAKE_CXX_FLAGS do NOT show up in the linker line
>>
>> On the other hand if I'm on OSX and I use the Unix Makefiles
>> generator,
>> then I see:
>>       * CMAKE_CXX_FLAGS show up on the compilation line
>>       * CMAKE_CXX_FLAGS ALSO show up in the linker line
>>
>> I assume the Xcode output is correct and the makefile output (with
>> CMAKE_CXX_FLAGS in both) is not correct... but there's nothing I can
>> find in the docs that says this explicitly.
>>
>> I've printed the contents of the CMAKE_CXX_FLAGS and
>> CMAKE_EXE_LINKER_FLAGS at the end of my CMakeLists.txt and they're
>> just
>> what I expect.  It's just that the link line has extra flags, when
>> invoked from make.
>
>
> --
>
> Powered by www.kitware.com
>
> Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ
>
> Kitware offers various services to support the CMake community. For more information on each offering, please visit:
>
> CMake Support: http://cmake.org/cmake/help/support.html
> CMake Consulting: http://cmake.org/cmake/help/consulting.html
> CMake Training Courses: http://cmake.org/cmake/help/training.html
>
> Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html
>
> Follow this link to subscribe/unsubscribe:
> http://public.kitware.com/mailman/listinfo/cmake