This documentation is the updated version of and is based on CMake 3.15. It was written after the modernization of the configuration of Gaudi.


CMake is an Open Source, cross-platform configuration and build tool used in several projects around the world.

Among its advantages we can count the support that comes from a large user base and the CMake configuration language that doubles as a portable and powerful scripting language.

The main drawbacks of CMake are the syntax of its language (not very nice looking) and the lack of support for runtime environment definition, but, thanks to the power of the language, we can overcome the drawbacks.

Here I'll describe how to write the CMake configuration for projects and packages. See also the CMake Documentation.

Understanding and Writing CMake Code

A minimal introduction on the CMake syntax and conventions is mandatory for people who never used it. Info The syntax highlighting in the code blocks is not correct because SyntaxHighlightingPlugin does not understand the CMake language.

In CMake every statement is a function call, written as an identifier followed by optional spaces and the arguments to the function as a space-separated list enclosed in parentheses. The arguments to a function can span over several lines, and double quotes (") can be used to pass arguments containing spaces and new-lines. Examples of functions:

message("hello world!")

if(1 GREATER 0)
  message("this is true")
  message("this is false")

One can add comments to the code using # at the beginning of the comment text (spaces preceding # are ignored), like, e.g., in Unix shells and Python:

# I'm a comment
     # this is always printed
     message("it's true")
     # this is never printed
     message("it's false")

The CMake language supports variables, which are set with the function set and dereferenced with ${...}, e.g.:

set(MyMessage "hello world")

Dereferencing of variables can be nested and works also in between double quotes:

set(name MyName)
set(${name}_msg "This is ${name}")

There is something special in the way CMake functions are invoked. CMake functions and macros (similar to functions) accept variable number of arguments and the only separators between arguments are spaces (tabs and new-lines too), so it might not be obvious how to pass optional arguments (like it's done in Python with named arguments). The solution found by CMake developers is to use keyword-separated lists of arguments. For example, we can imagine a function that requires a mandatory argument name and two optional lists of files, C sources and C++ sources. In CMake you could find it invoked like this:


make_a_library(MyLibrary C_SOURCES file1.c file2.c file3.c)

make_a_library(MyLibrary CXX_SOURCES file1.cpp file2.cpp file3.cpp)

               C_SOURCES file1.c file2.c file3.c
               CXX_SOURCES file1.cpp file2.cpp file3.cpp)

Warning, important Function names are case-insensitive, but variable names are case sensitive as well as string comparison.

CMake Configuration of a Gaudi-based Project

Several steps must be performed to build a Gaudi based project:
  • Set up the build environment (if not already set up)
  • Configure the project with CMake (generate Makefile or
  • Compile the source files and link the binaries
  • Test the previously build binaries (optional but recommended)

Set up the build environment

To set up the build environment, some environment variables need to be set.
  • BINARY_TAG: the variable that describe the platform
  • CMAKE_PREFIX_PATH: is a path-like variable that must contain the list the list of path to:
    • the compiler e.g. g++, clang++ (the compiler may be a wrapper)
    • the build system e.g. make, ninja
    • all third-party dependencies e.g. Boost, ROOT
  • PATH (optional): may contain the paths to the compiler and the build system

There are several ways to set these variables:

  • use export several time (or run a shell script that will do so)
  • source a view (a shell script that sets the aforementioned variables to a directory of symlinks)
  • specify a toolchain to the configuration

With export

export BINARY_TAG="x86_64-centos7-gcc8-opt"
export CMAKE_PREFIX_PATH="/path/to/g++:/path/to/boost:/path/to/ROOT:..."

It is also possible to list the call to export in a shell script to be able to source it later on.

With a view

source /cvmfs/
# the views may lack some stuff

Warning, important Sourcing a view is not the same as sourcing a shell script that uses export. A view is a directory of symbolic links and a script.

With a toolchain

# Either use
-D CMAKE_TOOLCHAIN_FILE=/path/to/a/toolchain # at configure time when calling cmake
# or
ln -s /path/to/a/toolchain toolchain.cmake # right away

Configuration of the project

The configuration requires at least CMake 3.15. CMake 3.15.0 was released on 2019-07-17.
# Check CMake version
cmake --version
# if version < 3.15
export PATH="/cvmfs/$PATH"

The configuration is the step when CMake is called and produces the files for the build system (e.g. make, ninja).

Two directories must be specified:

  • the source tree: contains the sources
  • the build tree: will contain the outputs of the build

cmake -S . -B build.$BINARY_TAG
# options can be passed at configure time
cmake -S . -B build.$BINARY_TAG -G Ninja # set the build system
cmake -S . -B build.$BINARY_TAG -D GAUDI_USE_AIDA=OFF # enable/disable a third party dependency
cmake -S . -B build.$BINARY_TAG -D CMAKE_BUILD_TYPE=Developer # select a build type (a set of compile and link options)
cmake -S . -B build.$BINARY_TAG -D CMAKE_TOOLCHAIN_FILE=toolchain.cmake # specify a toolchain
# several options may be specified with: -D ... -D ... -D ...
# ccmake and cmake-gui can be used

cmake -LH build.$BINARY_TAG # to see all the options and their help messages

Compilation of the project

Once the project is configured, several files are already in the build tree. They will be used to compile the project.

cd build.$BINARY_TAG ; make # or ninja or an IDE
# or directly with CMake
cmake --build build.$BINARY_TAG

Test the binaries

Good developers test their code. To run the tests:
cd build.$BINARY_TAG ; ctest -j `nproc` --output-on-failure --schedule-random

The wrapper

Doing all these steps each time may be tedious so there is a Makefile file at the top level of Gaudi-based projects to wrap the call to these commands.

make # configure + compile
make test # run the tests

The wrapper uses a toolchain if a file called toolchain.cmake exists in the current directory. It might be useful to have a shell function to easily switch from one toolchain to another.

function switch_platform
    export BINARY_TAG=$1
    rm -f toolchain.cmake
    ln -s /cvmfs/.../toolchains/$1.cmake toolchain.cmake # !!! Use the right path on CVMFS

Use the software

In order to use the previously built software, it is mandatory to use the runtime environment (it may differ from the build environment). The runtime environment is generated by the configuration at configure time in the build tree in a script called run.

cd build.$BINARY_TAG
# ./run <program> <args...>
./run listcomponents -h

Modify the configuration

At the top level directory of a project and in every sub-project (package) there must be one file: CMakeLists.txt

The file at the top level directory describes the build of the whole project:

  • may contain a licence block
  • contains documentation (how to configure it, available options)
  • describes the project (name, version)
  • fetches the dependencies
  • sets options for the build (C++ standard)
  • list all the sub-projects (packages in sub-directories)
  • handles the installation
The files in the sub-projects directories:
  • describes the binaries that will be compiled (libraries, modules, executable, ROOT dictionaries)
  • register tests for these binaries
  • handles the installation of their python packages and scripts

Configuration of sub-projects

Look and feel of typical sub-project CMakeLists.txt:
    1# {licence block if needed}
    2# {SubdirName} subdirectory
    4# Build the library
    6                  SOURCES src/Lib/Counter.cpp
    7                          src/Lib/Event.cpp
    8                  LINK PUBLIC GaudiKernel)
   10# Build the plugin
   12                 SOURCES src/AbortEvent/AbortEventAlg.cpp
   13                         src/AlgSequencer/HelloWorld.cpp
   14                 LINK GaudiKernel
   15                      GaudiExamplesLib
   16                      ROOT::Tree
   17                      Rangev3::rangev3)
   18if(GAUDI_USE_AIDA) # optional dependency
   19    target_sources(SubdirName PRIVATE src/EvtColsEx/EvtColAlg.cpp
   20                                         src/Histograms/Aida2Root.cpp)
   21    target_link_libraries(SubdirName PRIVATE AIDA::aida)
   24# Build the executable
   26                        SOURCES src/Allocator/Allocator.cpp
   27                                src/Allocator/MyClass1.cpp
   28                        LINK SubdirNameLib
   29                            GaudiKernel)
   31# Generate GaudiExamples_user.confdb
   34# Tests
   38# Compiled python module
   40                        SOURCES src/PythonModule/Functions.cpp
   41                                src/PythonModule/PyExample.cpp
   42                        LINK Python::Python
   43                             Boost::python)
   45# ROOT dictionaries
   47                     HEADERFILES src/IO/dict.h
   48                     SELECTION src/IO/dict.xml
   49                     LINK SubdirNameLib)
   51# Install python modules
   53# Install other scripts

Help Why is the explicit list of sources mandatory? Even tough CMake is able to use glob patterns with file(GLOB...), those glob patterns are expanded at configure time and their results hardcoded in makefile or or whichever file used by IDEs. This means that if a new file that matches the pattern is added, there is no way for the build system (make, ninja...) to notice it. The first solution is to reconfigure the project each time a new file is added to update the hardcoded list of sources. (This can be done either by rerunning the configuration command or by touching a CMakeLists.txt.) The other solution would be to forward the glob pattern to the build system. CMake offers a way to do so: file(GLOB ... CONFIGURE_DEPENDS) but for the time being, only Makefiles generators and Ninja are supported, meaning that people using IDEs would still have to reconfigure the project themselves.

Adding a new sub-project to the project

If a new sub-project is added to a project, its directory must be added to the list of sub-projects in the top-level CMakeLists.txt alongside with the other sub-projects with:

The directory of the sub-project must also contain a CMakeLists.txt that looks like the one above.

Adding a new third-party dependency

First, look in the documentation of the dependency. If it uses CMake, it may provide a config file (a file named {DependencyName}Config.cmake). Otherwise, it is necessary to write a find module file (a file named Find{DependencyName}.cmake) that will do the look up of this dependency (find the include directory, find all the libraries, find any other useful files provided by the dependency and create some IMPORTED targets). (Have a look at the other find module files in the project to get an idea of what it should look like. They should be in cmake/modules.)

Then, add the look up of the dependency in the file cmake/{ProjectName}Dependencies.cmake (replace with the name of the dependency and with the minimal required version).

# For mandatory dependencies
find_package(<DependencyName> <minVersion> ${__quiet})
set_package_properties(<DependencyName> PROPERTIES TYPE REQUIRED)

# For optional dependencies
  find_package(<DependencyName> <minVersion> ${__quiet})
  if(CMAKE_FIND_PACKAGE_NAME) # if the lookup is perform from ProjectConfig.cmake
    # then, all optional dependencies become required
    set_package_properties(<DependencyName>  PROPERTIES TYPE REQUIRED)
    set_package_properties(<DependencyName>  PROPERTIES TYPE RECOMMENDED)
# and add the option GAUDI_USE_<DEPENDENCY_NAME> in the top level =CMakeLists.tx=t

Finally, it is usable.

gaudi_add_<type>(... LINK ... Dependency::Target ...)

Removing an old third-party dependency

If a dependency is no longer used (nothing defined by it is used anywhere), it is no use keeping to look for it. In this case, remove the chunk of code that looks for it in cmake/{ProjectName}Dependencies.cmake (see Adding a new third-party dependency).

Then remove the find module file Find{DependencyName}.cmake in cmake/modules if it exists.

Gaudi CMake functions to help the configuration

All the gaudi_*() functions are defined by Gaudi. Their content and documentation can be found here in GaudiToolbox.cmake.

List of defined functions:

  • gaudi_add_library()
  • gaudi_add_header_only_library()
  • gaudi_add_module()
  • gaudi_add_python_module()
  • gaudi_add_executable()
  • gaudi_add_tests()
  • gaudi_add_dictionary()
  • gaudi_install()
  • gaudi_generate_confuserdb()
  • gaudi_check_python_module()
  • gaudi_generate_version_header_file()

Building a stack of project at once

With the configuration it is possible to build a stack of project at once. CMake may configure all the projects of the stack in one go, enabling the compilation to be done in parallel for all the projects.

Example of a stack: Gaudi, LHCb, Lbcom, Rec, Brunel

mkdir workspace ; cd workspace
git clone project_url # of all the projects of the desired stack
cat <<EOF > CMakeLists.txt
cmake_minimum_required(VERSION 3.15)

        DESCRIPTION "LHCb full stack")


# add_subdirectory() ... all the other projects of the stack

Using GaudiObjDesc (LHCb-specific)

Warning, important GaudiObjDesc has not been modernized because it should be removed soon. Do not use GaudiObjDesc in future project. However, if you need to maintain a package that uses GaudiObjDesc have a look at the old documentation here

Pro tip: do not document to much GaudiObjDesc so that people begrudge to use GaudiObjDesc.

-- ThomasCluzel - 2019-08-21

Edit | Attach | Watch | Print version | History: r7 < r6 < r5 < r4 < r3 | Backlinks | Raw View | WYSIWYG | More topic actions
Topic revision: r7 - 2019-08-21 - unknown
    • Cern Search Icon Cern Search
    • TWiki Search Icon TWiki Search
    • Google Search Icon Google Search

    LHCb All webs login

This site is powered by the TWiki collaboration platform Powered by PerlCopyright & 2008-2023 by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
or Ideas, requests, problems regarding TWiki? use Discourse or Send feedback