[CMake] Using try_compile before project() command

[Carminati Federico]

Hello,
    my use case is similar. My package depends on another one that can  
be installed with different compiler choices. So the first action I  
take is always to check the compiler used to install the other package  
and then to use the same. I could obviously do

export CXX=`root-config --cxx` ; export CC=`root-config -cc` ;  cmake .

but I would really prefer to do everything inside the CMakeLists.txt.  
This is easy to do with the Fortran compiler, as I enable Fortran  
after having chosen the compiler. For the moment i set also the c and  
cxx compilers and it works, but this is explicitly "deprecated" by the  
CMake doc. BEst,

Federico Carminati
CERN-PH
1211 Geneva 23
Switzerland
Tel: +41 22 76 74959
Fax: +41 22 76 79480
Mobile: +41 76 487 4843

On 15 Jul 2008, at 18:28, Yuri Timenkov wrote:

>
> On Tuesday 15 July 2008 20:01:52 Bill Hoffman wrote:
>> Carminati Federico wrote:
>>> Hello Yuri,
>>>  I posted a similar question some time ago. There seems to be no  
>>> way to
>>> chose the compiler within the CMakeList.txt, because you can do very
>>> little before the Project command, and afterwards it is too late.
>>> Anybody can help here? best, regards,
>>
>> That is a very different question, and no, you can not change the
>> compiler from within a CMakeLists.txt file.  It must be picked with  
>> CC
>> or CXX environment variables before running CMake.
> Yes, I just want to tune build environment but allow user to change  
> it later.
> KDE, for example, overrides ones later by specifying COMPILE_FLAGS  
> variables
> in CMake scripts without CACHE option. But such usage seems a bit  
> tricky for
> me, because user doesn't know which flags will be used for build (or  
> it's hard
> to determine place where particular flag came from, when you build  
> with
> VERBOSE=1).
>
> Overriding  CMake defaults seems most clear way, because user will  
> see global
> settings in GUI and target-specific or source files-specific flags  
> at the right
> place in CMakeLists.txt.
>
>>
>>> On 15 Jul 2008, at 15:56, Yuri Timenkov wrote:
>>>> I want to tune initial compile and linker flags to put into cache.
>>>> in main CMakeLists.txt:
>>>>
>>>> # Override default Compiler flags
>>>> set(CMAKE_USER_MAKE_RULES_OVERRIDE MyCompilerFlags)
>>>> project(MyProject)
>>>>
>>>> in MyCompilerFlags.cmake:
>>>>
>>>> check_cxx_linker_flag("-Wl,--warn-unresolved-symbols"
>>>> HAVE_LD_WARN_UNRESOLVED_SYMBOLS)
>>>>
>>>> if(HAVE_LD_WARN_UNRESOLVED_SYMBOLS)
>>>>    set(CMAKE_SHARED_LINKER_FLAGS_INIT
>>>> "${CMAKE_SHARED_LINKER_FLAGS_INIT} -Wl,--warn-unresolved-symbols")
>>>> endif(HAVE_LD_WARN_UNRESOLVED_SYMBOLS)
>>>>
>>>> This works fine for CMake 2.4:
>>>> -- Performing Test -Wl,--warn-unresolved-symbols
>>>> -- Performing Test -Wl,--warn-unresolved-symbols - Success
>>>> -- Performing Test -Wl,--no-undefined
>>>> -- Performing Test -Wl,--no-undefined - Success
>>>> -- Check for working C compiler: /usr/bin/gcc32
>>>> -- Check for working C compiler: /usr/bin/gcc32 -- works
>>>> ...
>>>>
>>>> But for CMake 2.6 this code fails with error:
>>>> -- The C compiler identification is GNU
>>>> -- The CXX compiler identification is GNU
>>>> -- Performing Test -Wl,--warn-unresolved-symbols
>>>> CMake Error: Unknown extension ".cxx" for file
>>>> "/vz/build/test/CMakeFiles/CMakeTmp/src.cxx".  TRY_COMPILE only  
>>>> works
>>>> for enabled languages.
>>>> Currently enabled languages are: C CXX
>>>> See PROJECT command for help enabling other languages.
>>>> -- Performing Test -Wl,--warn-unresolved-symbols - Failed
>>>> -- Performing Test -Wl,--no-undefined
>>>> CMake Error: Unknown extension ".cxx" for file
>>>> "/vz/build/test/CMakeFiles/CMakeTmp/src.cxx".  TRY_COMPILE only  
>>>> works
>>>> for enabled languages.
>>>> Currently enabled languages are: C CXX
>>>> See PROJECT command for help enabling other languages.
>>>> -- Performing Test -Wl,--no-undefined - Failed
>>>> -- Check for working C compiler: /usr/bin/gcc32
>>>> -- Check for working C compiler: /usr/bin/gcc32 -- works
>>>> -- Detecting C compiler ABI info
>>>> -- Detecting C compiler ABI info - done
>>>> ...
>>>>
>>>> where check_cxx_linker_flag macro looks line  
>>>> check_cxx_compiler_flag:
>>>>
>>>> file(WRITE
>>>> "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/src.cxx" "int
>>>> main() { return 0;}\n")
>>>> message(STATUS "Performing Test ${_FLAG}")
>>>> try_compile(${_RESULT}
>>>>    ${CMAKE_BINARY_DIR}
>>>>    "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/src.cxx"
>>>>    CMAKE_FLAGS -DCMAKE_EXE_LINKER_FLAGS="${_FLAG}"
>>>>    OUTPUT_VARIABLE OUTPUT
>>>>    )
>>>>
>>>> Any suggestions?
>>
>> Looks like a bug.   Please file a bug report with a small example  
>> that
>> is easy to reproduce.
> I'll try to figure out the problem.
>
>>
>> Thanks.
>>
>> -Bill
>
> P.S. Did you think about "Best practices" category on wiki? Many  
> people ask
> similar questions on list, and some of them are kind of "is it moral  
> in
> CMake?" Because there is a lot of such questions, FAQ will be too  
> long.
>