===========
dune-common
===========

.. _whatis:

What is CMake anyway?
=====================

CMake...

- is an open source build system tool developed at KITware.
- offers a one-tool-solution to all building tasks, like configuring, building, linking, testing and packaging.
- is a build system generator: It supports a set of backends called *generators*
- is portable
- is controlled by ONE rather simple language

Dune got support for CMake in version 2.3 alongside the old Autotools build system. It got the default in the
2.4 release.

You can install CMake through your favorite package manager or downloading source code from
`KITWare <http://www.cmake.org>`_
The minimum required version to build Dune with CMake is 3.1.

.. _howtouse:

How do I use Dune with CMake?
=============================

The build process is controlled by the script :code:`dunecontrol`, located in :code:`dune-common/bin`.
There is a compatibility layer that will translate all the configure flags from your opts file into the corresponding
CMake flags. While this is a great tool to determine how to do the transition, in the long run you should switch to
a CMake-only approach.

:code:`dunecontrol` will pickup the variable :code:`CMAKE_FLAGS` from your opts file and use it as command line options for
any call to CMake. There, you can define variables for the configure process with CMake's :code:`-D` option; just as
with the C pre-processor.

The most important part of the configure flags is to tell the build system where to look for external libraries.
You can browse the :ref:`variableref` section of this documentation for a list of variables that are picked up
by the Dune CMake build system.

.. _whatfiles:

What files in a dune module belong to the CMake build system?
=============================================================

Every directory in a project contains a file called :code:`CMakeLists.txt`, which is written in the CMake language.
You can think of these as a distributed configure script. Upon configure, the top-level :code:`CMakeLists.txt` is executed.
Whenever an :ref:`add_subdirectory` command is encountered, the :code:`CMakeLists.txt` file of that sub-directory is executed.
The top-level :code:`CMakeLists.txt` file is special, because it sets up the entire Dune module correctly. You should not delete the
auto-generated parts of it.

Additionally, a Dune module can export some cmake modules. A cmake module is a file that contains one or
more build system macros meant for downstream use. If a module provides modules, they can be found in
the subfolder :code:`cmake/modules`. The module :code:`dune-foo/cmake/modules/DuneFooMacros.cmake` in a module
:code:`dune-foo` is special however: Its contents are always executed when configuring the module
:code:`dune-foo` or any other Dune module, that requires or suggests the module :code:`dune-foo`.
This is the perfect place to put your checks for external packages, see below.

The file :code:`config.h.cmake` defines a template for the section of :code:`config.h`, that is generated by the module.

.. _flags:

How do I modify the flags and linked libraries of a given target?
=================================================================

Again, there are multiple ways to do this. The Dune build system offers macros to make this task as
easy as possible. For each external module, there is a macro :code:`add_dune_*_flags`. Those macros should
cover most flags. Example usage:

.. code-block:: cmake

    add_executable(foo foo.cc)
    add_dune_umfpack_flags(foo)
    add_dune_mpi_flags(foo)

There is also the macro :ref:`add_dune_all_flags`, which uses the same flag registry mechanism as the simplified
build system in section :ref:`simplified`.

If you want to fully control the configuration of the targets, you can do so. Build system entities such
as targets, directories and tests do have so called properties in CMake. You can access and modify those
properties via the commands :code:`get_property` and :code:`set_property`. You can for example use those
to modify a targets :code:`COMPILE_DEFINITIONS` or :code:`INCLUDE_DIRECTORIES` property:

.. code-block:: cmake

    add_executable(foo foo.cc)
    set_property(TARGET foo APPEND PROPERTY COMPILE_DEFINITIONS <somedefinition>)
    set_property(TARGET foo APPEND PROPERTY INCLUDE_DIRECTORIES <somepath>)

For a full list of properties, check the manual:

.. code-block:: bash

    cmake --help-property-list

Manually linking libraries can be done through the :code:`target_link_libraries` command instead of manually
tweaking properties.

.. _external:

How do I link against external libraries, that are not checked for by Dune?
===========================================================================

While there might be many solutions that make your application work, there is only one clean solution to this: You have
to provide a find module for the package. A find module is a CMake module that follows a specific naming scheme: For
an external package called :code:`SomePackage` it is called :code:`FindSomePackage.cmake`. Note that CMake
treats package names case sensitive. If CMake encounters a :code:`find_package(SomePackage)` line, it searches
its module include paths for this find module. A good read to get started writing a find module is
`this page <http://www.cmake.org/Wiki/CMake:How_To_Find_Libraries>`_ in the CMake wiki.

Depending on how common your external package is, you may not even need to write the find module on your own.
You can have a look at the list of find modules shipped by CMake or simply search the
internet for the module name and profit from other open-source project's work.

It is considered good style to also provide a macro :code:`add_dune_somepackage_flags`.

.. _outofsource:

What is an out-of-source build?
===============================

An out-of-source build does leave the version-controlled source tree untouched and puts all files that are
generated by the build process into a different directory -- the build directory. The build directory does mirror
your source tree's structure as seen in the following. Assume the following source directory structure:

::

   dune-foo/
     CMakeLists.txt
     dune/
       foo/
         CMakeLists.txt
     src/
       CMakeLists.txt

The generated build directory will have the following structure, where the directory :code:`build-cmake`
is a subdirectory of the source directory:

::

  build-cmake/
    Makefile
    dune/
      foo/
        Makefile
    src/
      Makefile

Using the :code:`Unix Makefiles` generator, your Makefiles are generated in the build tree, so that is where you
have to call :code:`make`. There are multiple advantages with this approach, such as a clear separation between
version controlled and generated files and you can have multiple out-of-source builds with different configurations
at the same time.

Out-of-source builds are the default with CMake. In-source builds are strongly discouraged.

By default, a subfolder :code:`build-cmake` is generated within each dune module and is used as a build directory.
You can customize this folder through the :code:`--builddir` option of :code:`dunecontrol`. Give an absolute path to
the :code:`--builddir` option, you will get something like this:

::

  build/
    dune-common/
      Makefile
    dune-foo/
      Makefile

So, instead of one build directory in every dune module, you will be able to collect all build directories in one
directory. This makes it much easier to have multiple build directories and to remove build directories.

.. _simplified:

What is the simplified build system and how do I use it?
========================================================

Dune offers a simplified build system, where all flags are added to all targets and all libraries are linked to all targets. You can enable the feature
by calling :ref:`dune_enable_all_packages` in the top-level :code:`CMakeLists.txt` file of your project, before you add any subdirectories.

This will modify all targets in the directory of the :code:`CMakeLists.txt`, where you put this, and also in all
subdirectories. The compile flags for all found external packages are added to those targets and the target is
linked against all found external libraries.

To use this while using custom external packages, you have to register your flags to the mechanism.
Also, some special care has to be given, if your module does build one or more library which targets within the module do link against.

Carefully read the following documentation in those cases:

* :ref:`dune_enable_all_packages`
* :ref:`dune_register_package_flags`
* :ref:`dune_library_add_sources`

.. _compiler:

How do I change my compiler and compiler flags?
===============================================

In general, there are multiple ways to do this:

* Setting the CMake variables :ref:`CMAKE_<LANG>_COMPILER` (with :code:`LANG` being :code:`C`
  or :code:`CXX` or :code:`Fortran`) from the opts file, e.g. via :code:`CMAKE_FLAGS="-DCMAKE_CXX_COMPILER=otherc++"`.
* Setting those variables within the project with the :code:`set` command
* Setting the environment variables :code:`CC`, :code:`CXX`, :code:`FC` etc.

The first option is the recommended way. Whenever you change your compiler, you should delete all build
directories. For some CMake versions, there is a known CMake bug, that requires you to give an absolute path
to your compiler, but Dune will issue a warning, if you violate that.

You can modify your default compiler flags by setting the variables
:ref:`CMAKE_<LANG>_FLAGS` in your opts file (again with :code:`LANG` being :code:`C` or
:code:`CXX` or :code:`Fortran`).

.. _symlink:

How should I handle ini and grid files in an out-of-source-build setup?
=======================================================================

Such files are under version control, but they are needed in the build directory.
There are some CMake functions targeting this issue:

* :ref:`dune_symlink_to_source_tree`
* :ref:`dune_symlink_to_source_files`
* :ref:`dune_add_copy_command`
* :ref:`dune_add_copy_dependency`
* :ref:`dune_add_copy_target`

The simplest way to solve the problem is to set the variable :ref:`DUNE_SYMLINK_TO_SOURCE_TREE` to your opts file.
This will execute :ref:`dune_symlink_to_source_tree` in your top-level :code:`CMakeLists.txt`. This will add a symlink
:code:`src_dir` to all subdirectories of the build directory, which points to the corresponding directory of the source
tree. This will only work on platforms that support symlinking.

.. _ides:

How do I use CMake with IDEs?
=============================

As already said, CMake is merely a build system generator with multiple backends (called a generator). Using IDEs requires
a different generator. Check :code:`cmake --help` for a list of generators. You can then add the :code:`-G` to the :code:`CMAKE_FLAGS` in your opts file.
Note that the generator name has to match character by character, including case and spaces.

To configure highlighting of CMake errors in Emacs' compilation mode, include
the following in your :code:`~./emacs` (see the `Emacs bug
<http://debbugs.gnu.org/cgi/bugreport.cgi?bug=22944>`_):

.. code-block:: elisp

   (setq compilation-error-regexp-alist-alist
      `((cmake "^CMake \\(?:Error\\|\\(Warning\\)\\) at \\(.*\\):\\([1-9][0-9]*\\) ([^)]+):$"
               2 3 nil (1))
        (cmake-info "^  \\(?: \\*\\)?\\(.*\\):\\([1-9][0-9]*\\) ([^)]+)$"
               2 3 nil 0)
        . ,compilation-error-regexp-alist-alist))

Then customize the option :code:`compilation-error-regexp-alist` and add the
two predefined symbols :code:`cmake` and :code:`cmake-info` to the list.

.. _cxxflags:

I usually modify my CXXFLAGS upon calling make. How can I do this in CMake?
===========================================================================

This violates the CMake philosophy and there is no clean solution to achieve it. The CMake-ish solution would be
to have for each configuration one out-of-source build. We have nevertheless implemented a workaround. It can be enable
by setting the variable :ref:`ALLOW_CXXFLAGS_OVERWRITE` in your opts file. You can then type:

.. code-block:: bash

   make CXXFLAGS="<your flags>" <target>

Furthermore any C pre-processor variable of the form :code:`-DVAR=<value>` can be overloaded on the command line
and the grid type can be set via :code:`GRIDTYPE="<grid type>"`.

Note this only works with generators that are based on Makefiles and several Unix tools like bash must be
available.

.. _test:

How do I run the test suite from CMake?
=======================================

The built-in target to run the tests is called :code:`test` instead of Autotools' :code:`check`.
It is a mere wrapper around CMake's own testing tool CTest. You can check :code:`ctest --help`
for a lot of useful options, such as choosing the set of tests to be run by matching regular expressions or
showing the output of failed tests.

The test programs are not built automatically. You need to build them manually
before running them using :code:`make build_tests`.

The Dune test suite also defines tests that run in parallel. You may set an upper bound to the number
of cores in use for a single test by setting :ref:`DUNE_MAX_TEST_CORES`.

.. _disable:

Can I disable an external dependency?
=====================================

To disable an external dependency :code:`Foo`, add

::

   -DCMAKE_DISABLE_FIND_PACKAGE_Foo=TRUE

to your opts file. The name of the dependency is case sensitive but there is no canonical naming
scheme. See the output of configure to get the right name.

Make sure to not use cached configure results by deleting the cache file or the build directory, cf.
:ref:`troubleshoot`.

.. _parallel:

How do I switch between parallel and sequential builds?
=======================================================

Dune builds with CMake are parallel if and only if MPI is found. To have a sequential build despite an
installed MPI library, you have to explicitly disable the corresponding find module by setting

::

   -DCMAKE_DISABLE_FIND_PACKAGE_MPI=TRUE

in the :code:`CMAKE_FLAGS` of your opts file, as described in section :ref:`disable`.

.. _headercheck:

Why is it not possible anymore to do make headercheck?
======================================================

The headercheck feature has been disabled by default. You can enable it by setting the CMake variable :ref:`ENABLE_HEADERCHECK`
through your opts file. This step has been necessary, because of the large amount of additional file the headercheck adds to the
build directory. A better implementation has not been found yet, because it simply does not fit the CMake philosophy.

.. _tarball:

How do I create tarballs?
=========================

CMake has a packaging tool CPack. This creates tarballs or binary packages. To build tarballs add
the variable :code:`CPACK_SET_DESTDIR` which must be set to :code:`true` for the configuration
of all modules. Inside the build directory run :code:`make package_source` and you'll find the
packages below :code:`<CMAKE_BINARY_DIR>/_CPack_Packages`.

Note that an un-packed copy is located there which contains a :code:`dune.module` file. Delete the
subdirectory. Otherwise it is going to break your next :code:`dunecontrol` run, because the module
is defined multiple times.

.. _dune-python:

How does the Dune build system handle Python?
=============================================

dune-common contains a build system extension to handle many python-related aspects. You can
read more on this in the module description :ref:`DunePythonCommonMacros` and the pieces of
documentation mentioned inthere.

.. _troubleshoot:

How do I troubleshoot?
======================

CMake caches aggressively which makes it bad at recognizing changed configurations.
To trigger a fresh run of configure, you can delete the :code:`CMakeCache.txt` file from
the build directory and maybe save some compilation time afterward.

Whenever you experience any problems, your first step should be to delete all build directories. Nice trick:

::

   dunecontrol exec "rm -rf build-cmake"

This will remove all build directories from all DUNE modules.

Later on you can get an error log from the file :code:`CMakeError.log` in the :code:`CMakeFiles`
subdirectory of your build directory. This is what you should send to the mailing list alongside the
description of your setup and efforts to help us help you.

Where can I get help?
=====================

The CMake manual is available on the command line:

* :code:`cmake --help-command-list`
* :code:`cmake --help-command <command>`
* :code:`cmake --help-property-list`
* :code:`cmake --help-property <property>`
* :code:`cmake --help-module-list`
* :code:`cmake --help-module <module>`

To get help on which variables are picked up by CMake, there is a CMake wiki page collecting them.
Of course, there is also Google, StackOverflow and the CMake Mailing list (archive).
For problems specific to DUNE's build system, ask on our mailing lists.
