[CMake] ExternalProject_Add() macro does not set CMAKE_COMPILER_IS_GNUCXX

Nicholas Braden nicholas11braden at gmail.com
Thu Jan 21 09:56:55 EST 2016 

Ah, I ran into this quirk too - the issue is that you have the quotes incorrect:

-DCMAKE_CXX_FLAGS="-march=native"

Should be like this instead:

"-DCMAKE_CXX_FLAGS=-march=native"

Try that and see if it helps, I am pretty sure I ran into the exact
same problem.

On Thu, Jan 21, 2016 at 8:27 AM, fkillus <fkillus at googlemail.com> wrote:
> Thanks for clarifying that external projects are not aware of the project
> they are embedded in.
> The CMAKE_COMPILER_IS_GNUCXX variable should be set automatically by CMake
> as far
> as I understand it (see https://cmake.org/Wiki/CMake_Useful_Variables).
>
> I narrowed down the problem by creating a minimal setup which still
> reproduces the issue.
> In this case the super project is simply a wrapper around an external dummy
> project.
>
> external/CMakeLists.txt:
>
>   cmake_minimum_required( VERSION 3.4 )
>   project( external )
>   message( "External - CMAKE_COMPILER_IS_GNUCXX: "
> ${CMAKE_COMPILER_IS_GNUCXX} )
>   message( "External - CMAKE_CXX_COMPILER_ID: " ${CMAKE_CXX_COMPILER_ID} )
>   message( "External - CMAKE_CXX_FLAGS: " ${CMAKE_CXX_FLAGS} )
>
> super/CMakeLists.txt:
>
>   cmake_minimum_required( VERSION 3.4 )
>   project( super )
>   message( "Super - CMAKE_COMPILER_IS_GNUCXX: " ${CMAKE_COMPILER_IS_GNUCXX}
> )
>   message( "Super - CMAKE_CXX_COMPILER_ID: " ${CMAKE_CXX_COMPILER_ID} )
>   message( "Super - CMAKE_CXX_FLAGS: " ${CMAKE_CXX_FLAGS} )
>   include( ExternalProject )
>   ExternalProject_Add(
>     external
>     DOWNLOAD_COMMAND ""
>     SOURCE_DIR ${CMAKE_SOURCE_DIR}/../external
>     CMAKE_ARGS
>       -DCMAKE_CXX_FLAGS="-march=native"
>     INSTALL_COMMAND  ""
>   )
>
> The problem occurs when the CMAKE_CXX_FLAGS variable is set inside the
> ExternalProject_Add
> command using quotes (i.e. "-march=native" in this example). The output
> obtained when configuring
> the super project with 'cmake ../super' is:
>
> -- The C compiler identification is GNU 5.2.0
> -- The CXX compiler identification is GNU 5.2.0
> -- Check for working C compiler: /usr/bin/cc
> -- Check for working C compiler: /usr/bin/cc -- works
> -- Detecting C compiler ABI info
> -- Detecting C compiler ABI info - done
> -- Detecting C compile features
> -- Detecting C compile features - done
> -- Check for working CXX compiler: /usr/bin/c++
> -- Check for working CXX compiler: /usr/bin/c++ -- works
> -- Detecting CXX compiler ABI info
> -- Detecting CXX compiler ABI info - done
> -- Detecting CXX compile features
> -- Detecting CXX compile features - done
> Super - CMAKE_COMPILER_IS_GNUCXX: 1
> Super - CMAKE_CXX_COMPILER_ID: GNU
> Super - CMAKE_CXX_FLAGS:
> -- Configuring done
> -- Generating done
>
> Afterwards building with 'make' results in:
>
> [ 12%] Creating directories for 'external'
> [ 25%] No download step for 'external'
> [ 37%] No patch step for 'external'
> [ 50%] No update step for 'external'
> [ 62%] Performing configure step for 'external'
> -- The C compiler identification is GNU 5.2.0
> -- The CXX compiler identification is unknown
> -- Check for working C compiler: /usr/bin/cc
> -- Check for working C compiler: /usr/bin/cc -- works
> -- Detecting C compiler ABI info
> -- Detecting C compiler ABI info - done
> -- Detecting C compile features
> -- Detecting C compile features - done
> -- Check for working CXX compiler: /usr/bin/c++
> -- Check for working CXX compiler: /usr/bin/c++ -- works
> -- Detecting CXX compiler ABI info
> -- Detecting CXX compiler ABI info - done
> External - CMAKE_COMPILER_IS_GNUCXX:
> External - CMAKE_CXX_COMPILER_ID:
> External - CMAKE_CXX_FLAGS: "-march=native"
> -- Configuring done
> -- Generating done
>
> This shows the compiler is not correctly identified for the external
> project. Directly configuring
> the external project with 'cmake -DCMAKE_CXX_FLAGS="-march=native"
> ../external' works though:
>
> -- The C compiler identification is GNU 5.2.0
> -- The CXX compiler identification is GNU 5.2.0
> -- Check for working C compiler: /usr/bin/cc
> -- Check for working C compiler: /usr/bin/cc -- works
> -- Detecting C compiler ABI info
> -- Detecting C compiler ABI info - done
> -- Detecting C compile features
> -- Detecting C compile features - done
> -- Check for working CXX compiler: /usr/bin/c++
> -- Check for working CXX compiler: /usr/bin/c++ -- works
> -- Detecting CXX compiler ABI info
> -- Detecting CXX compiler ABI info - done
> -- Detecting CXX compile features
> -- Detecting CXX compile features - done
> External - CMAKE_COMPILER_IS_GNUCXX: 1
> External - CMAKE_CXX_COMPILER_ID: GNU
> External - CMAKE_CXX_FLAGS: -march=native
> -- Configuring done
> -- Generating done
>
>
> It also works if the quotations marks in the super project listfile are
> removed. I.e. changing
>   CMAKE_ARGS
>    -DCMAKE_CXX_FLAGS="-march=native"
> to
>   CMAKE_ARGS
>    -DCMAKE_CXX_FLAGS=-march=native
>
> The question is now, is it still possible to add mutiple compile flags if
> one cannot use quotation marks?
>
>
> On Wed, Jan 20, 2016 at 6:58 PM, Nicholas Braden
> <nicholas11braden at gmail.com> wrote:
>>
>> Where/how is that variable normally set? External projects have no
>> awareness of the project they are in, they just run CMake as usual the
>> same way you would. If the variable is normally set by CMake itself,
>> make sure that your containing project and the external project both
>> find the same compiler. (They each do their own automatic search for
>> compilers)
>>
>> On Wed, Jan 20, 2016 at 11:41 AM, fkillus via CMake <cmake at cmake.org>
>> wrote:
>> > I have been trying to compile Ogre [1] as external dependency with
>> > ExternalProject_Add(). In theory this should be straightforward since
>> > Ogre
>> > itself also uses CMake as buildsystem. However, in the process I
>> > encountered
>> > the following problem:
>> >
>> > Ogre checks the value of the CMAKE_COMPILER_IS_GNUCXX variable in some
>> > places (e.g. [2]). I am running Linux and compiling with g++ and
>> > everything
>> > works fine if I manually configure Ogre with cmake or cmake-gui.
>> > Unfortunately, after wrapping everything inside ExternalProject_Add(),
>> > the
>> > CMAK_COMPILER_IS_GNUCXX variable is no longer being set correctly.
>> >
>> > A simple workaround is to manually set this variable, i.e.:
>> >
>> >  ExternalProject_Add(
>> >     ogre
>> >     URL https://bitbucket.org/sinbad/ogre/get/v1-9-0.zip
>> >     CMAKE_ARGS
>> >       -DCMAKE_INSTALL_PREFIX=${DEPENDENCIES_OUTPUT_DIR}
>> >       -DCMAKE_COMPILER_IS_GNUCXX=${CMAKE_COMPILER_IS_GNUCXX}   #
>> > workaround
>> >   )
>> >
>> > This works, but I'm uncertain if this should be necessary. Is this a bug
>> > or
>> > a feature?
>> >
>> >
>> > [1] https://bitbucket.org/sinbad/ogre
>> > [2]
>> >
>> > https://bitbucket.org/sinbad/ogre/src/0d580c7216abe27fafe41cb43e31d8ed86ded591/CMake/Utils/OgreConfigTargets.cmake?at=default&fileviewer=file-view-default#OgreConfigTargets.cmake-277
>> >
>> > --
>> >
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