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

[Paul Smith]

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.