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.
CMakePackage base classes come with the following phases:
cmake- generate the Makefile
build- build the package
install- 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.
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
This means that CMake 2.8.12 is the earliest release that will work.
You should specify this in a
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 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
dependency. Since this is always the case, the
class already contains:
If you need to specify a particular version requirement, you can override this in your package:
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
$ 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
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
Adding flags to cmake
To add additional flags to the
cmake call, simply override the
cmake_args function. The following example defines values for the flags
and without the
define_from_variant() helper functions:
def cmake_args(self): args = [ '-DWHATEVER:STRING=somevalue', self.define('ENABLE_BROKEN_FEATURE', False), self.define_from_variant('DETECT_HDF5', 'hdf5'), self.define_from_variant('THREADS'), # True if +threads ] return args
Spack supports CMake defines from conditional variants too. Whenever the condition on
the variant is not met,
define_from_variant() will simply return an empty string,
and CMake simply ignores the empty command line argument. For example the following
variant('example', default=True, email@example.com:') def cmake_args(self): return [self.define_from_variant('EXAMPLE', 'example')]
'cmake' '-DEXAMPLE=ON' ... when @2.0: +example is met, but will
'cmake' '' ... when the spec version is below
CMake arguments provided by Spack
The following default arguments are controlled by Spack:
Is set to the the package’s install directory.
CMake finds dependencies through calls to
find_path(), which use a list of search
CMAKE_PREFIX_PATH. Spack sets this variable to a list of prefixes of the
spec’s transitive dependencies.
For troubleshooting cases where CMake fails to find a dependency, add the
--debug-find flag to
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.
CMakeLists.txt file to see if the default values are
missing or replaced. For example, the
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 uses different RPATHs during the build and after installation, so that executables can locate the libraries they’re linked to during the build, and installed executables do not have RPATHs to build directories. In Spack, we have to make sure that RPATHs are set properly after installation.
CMAKE_INSTALL_RPATH to a list of
directories of the spec’s link-type dependencies. Apart from that, it sets
-DCMAKE_INSTALL_RPATH_USE_LINK_PATH=ON, which should add RPATHs for directories of
linked libraries not in the directories covered by
Usually it’s enough to set only
-DCMAKE_INSTALL_RPATH_USE_LINK_PATH=ON, but the
reason to provide both options is that packages may dynamically open shared libraries,
which CMake cannot detect. In those cases, the RPATHs from
used as search paths.
Some packages provide stub libraries, which contain an interface for linking without
an implementation. When using such libraries, it’s best to override the option
cmake_args, so that stub libraries
are not used at runtime.
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:
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
CMakeLists.txt in a sub-directory
Occasionally, developers will hide their source code and
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
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
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']
CMake-based packages typically provide unit testing via the
test target. If you build your software with
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
For more information on the CMake build system, see: https://cmake.org/cmake/help/latest/