CMakePackage¶
Like Autotools, CMake is a widely-used build-script generator. Designed by Kitware, CMake is the most popular build system for new C, C++, and Fortran projects, and many older projects are switching to it as well.
Unlike Autotools, CMake can generate build scripts for builders other than Make: Ninja, Visual Studio, etc. It is therefore cross-platform, whereas Autotools is Unix-only.
Phases¶
The CMakePackage
base class comes with the following phases:
cmake
- generate the Makefilebuild
- build the packageinstall
- install the package
By default, these phases run:
$ mkdir spack-build
$ cd spack-build
$ cmake .. -DCMAKE_INSTALL_PREFIX=/path/to/installation/prefix
$ make
$ make test # optional
$ make install
A few more flags are passed to cmake
by default, including flags
for setting the build type and flags for locating dependencies. Of
course, you may need to add a few arguments yourself.
Important files¶
A CMake-based package can be identified by the presence of a
CMakeLists.txt
file. This file defines the build flags that can be
passed to the cmake invocation, as well as linking instructions. If
you are familiar with CMake, it can prove very useful for determining
dependencies and dependency version requirements.
One thing to look for is the cmake_minimum_required
function:
cmake_minimum_required(VERSION 2.8.12)
This means that CMake 2.8.12 is the earliest release that will work.
You should specify this in a depends_on
statement.
CMake-based packages may also contain CMakeLists.txt
in subdirectories.
This modularization helps to manage complex builds in a hierarchical
fashion. Sometimes these nested CMakeLists.txt
require additional
dependencies not mentioned in the top-level file.
There’s also usually a cmake
or CMake
directory containing
additional macros, find scripts, etc. These may prove useful in
determining dependency version requirements.
Build system dependencies¶
Every package that uses the CMake build system requires a cmake
dependency. Since this is always the case, the CMakePackage
base
class already contains:
depends_on('cmake', type='build')
If you need to specify a particular version requirement, you can override this in your package:
depends_on('cmake@2.8.12:', type='build')
Finding cmake flags¶
To get a list of valid flags that can be passed to cmake
, run the
following command in the directory that contains CMakeLists.txt
:
$ cmake . -LAH
CMake will start by checking for compilers and dependencies. Eventually
it will begin to list build options. You’ll notice that most of the
build options at the top are prefixed with CMAKE_
. You can safely
ignore most of these options as Spack already sets them for you. This
includes flags needed to locate dependencies, RPATH libraries, set the
installation directory, and set the build type.
The rest of the flags are the ones you should consider adding to your
package. They often include flags to enable/disable support for certain
features and locate specific dependencies. One thing you’ll notice that
makes CMake different from Autotools is that CMake has an understanding
of build flag hierarchy. That is, certain flags will not display unless
their parent flag has been selected. For example, flags to specify the
lib
and include
directories for a package might not appear
unless CMake found the dependency it was looking for. You may need to
manually specify certain flags to explore the full depth of supported
build flags, or check the CMakeLists.txt
yourself.
Adding flags to cmake¶
To add additional flags to the cmake
call, simply override the
cmake_args
function:
def cmake_args(self):
args = []
if '+hdf5' in self.spec:
args.append('-DDETECT_HDF5=ON')
else:
args.append('-DDETECT_HDF5=OFF')
return args
Generators¶
CMake and Autotools are build-script generation tools; they “generate” the Makefiles that are used to build a software package. CMake actually supports multiple generators, not just Makefiles. Another common generator is Ninja. To switch to the Ninja generator, simply add:
generator = 'Ninja'
CMakePackage
defaults to “Unix Makefiles”. If you switch to the
Ninja generator, make sure to add:
depends_on('ninja', type='build')
to the package as well. Aside from that, you shouldn’t need to do anything else. Spack will automatically detect that you are using Ninja and run:
$ cmake .. -G Ninja
$ ninja
$ ninja install
Spack currently only supports “Unix Makefiles” and “Ninja” as valid generators, but it should be simple to add support for alternative generators. For more information on CMake generators, see: https://cmake.org/cmake/help/latest/manual/cmake-generators.7.html
CMAKE_BUILD_TYPE¶
Every CMake-based package accepts a -DCMAKE_BUILD_TYPE
flag to
dictate which level of optimization to use. In order to ensure
uniformity across packages, the CMakePackage
base class adds
a variant to control this:
variant('build_type', default='RelWithDebInfo',
description='CMake build type',
values=('Debug', 'Release', 'RelWithDebInfo', 'MinSizeRel'))
However, not every CMake package accepts all four of these options.
Grep the CMakeLists.txt
file to see if the default values are
missing or replaced. For example, the
dealii
package overrides the default variant with:
variant('build_type', default='DebugRelease',
description='The build type to build',
values=('Debug', 'Release', 'DebugRelease'))
For more information on CMAKE_BUILD_TYPE
, see:
https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html
CMakeLists.txt in a sub-directory¶
Occasionally, developers will hide their source code and CMakeLists.txt
in a subdirectory like src
. If this happens, Spack won’t
be able to automatically detect the build system properly when running
spack create
. You will have to manually change the package base
class and tell Spack where CMakeLists.txt
resides. You can do this
like so:
root_cmakelists_dir = 'src'
Note that this path is relative to the root of the extracted tarball,
not to the build_directory
. It defaults to the current directory.
Building out of source¶
By default, Spack builds every CMakePackage
in a spack-build
sub-directory. If, for whatever reason, you would like to build in a
different sub-directory, simply override build_directory
like so:
build_directory = 'my-build'
Build and install targets¶
For most CMake packages, the usual:
$ cmake
$ make
$ make install
is sufficient to install the package. However, if you need to run make with any other targets, for example, to build an optional library or build the documentation, you can add these like so:
build_targets = ['all', 'docs']
install_targets = ['install', 'docs']
Testing¶
CMake-based packages typically provide unit testing via the
test
target. If you build your software with --test=root
,
Spack will check for the presence of a test
target in the
Makefile and run make test
for you. If you want to run a
different test instead, simply override the check
method.
External documentation¶
For more information on the CMake build system, see: https://cmake.org/cmake/help/latest/