CMake

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 CMakeBuilder and 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.

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

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, when="@2.0:")

def cmake_args(self):
   return [self.define_from_variant("EXAMPLE", "example")]

will generate 'cmake' '-DEXAMPLE=ON' ... when @2.0: +example is met, but will result in 'cmake' '' ... when the spec version is below 2.0.

CMake arguments provided by Spack

The following default arguments are controlled by Spack:

`CMAKE_INSTALL_PREFIX`

Is set to the the package’s install directory.

`CMAKE_PREFIX_PATH`

CMake finds dependencies through calls to find_package(), find_program(), find_library(), find_file(), and find_path(), which use a list of search paths from 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 cmake_args.

`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

`CMAKE_INSTALL_RPATH` and `CMAKE_INSTALL_RPATH_USE_LINK_PATH=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.

Spack sets CMAKE_INSTALL_RPATH to a list of <prefix>/lib or <prefix>/lib64 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 CMAKE_INSTALL_RPATH.

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 CMAKE_INSTALL_RPATH are used as search paths.

Note

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 -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=OFF in cmake_args, so that stub libraries are not used at runtime.

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

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/