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:

  1. cmake - generate the Makefile

  2. build - build the package

  3. 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.

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. The following example defines values for the flags WHATEVER, ENABLE_BROKEN_FEATURE, DETECT_HDF5, and THREADS with and without the define() and 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

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/