[CMake] ExternalProject_Add() macro does not set CMAKE_COMPILER_IS_GNUCXX

[fkillus]

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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