CMake

In software development, CMake is cross-platform free and open-source software for build automation, testing and packaging using a compiler-independent method.[3] CMake is not a build system but rather it's a build-system generator. It supports directory hierarchies and applications that depend on multiple libraries. It is used in conjunction with native build environments such as Make, Qt Creator, Ninja, Android Studio, Apple's Xcode, and Microsoft Visual Studio. It has minimal dependencies, requiring only a C++ compiler on its own build system.

CMake
Developer(s)Andy Cedilnik, Bill Hoffman, Brad King, Ken Martin, Alexander Neundorf
Initial release2000 (2000)
Stable release
3.19.4[1] / January 28, 2021 (2021-01-28)
Repository
Written inC, C++[2]
Operating systemCross-platform
TypeSoftware development tools
LicenseNew BSD
Websitecmake.org 

CMake is distributed as open-source software under permissive New BSD license.[4]

History

CMake development began in 1999 in response to the need for a cross-platform build environment for the Insight Segmentation and Registration Toolkit.[5] The project is funded by the United States National Library of Medicine as part of the Visible Human Project. It was partially inspired by pcmaker, which was made by Ken Martin and other developers to support the Visualization Toolkit (VTK). At Kitware, Bill Hoffman blended components of pcmaker with his own ideas, striving to mimic the functionality of Unix configure scripts. CMake was first implemented in 2000 and further developed in 2001.

Continued development and improvements were fueled by the incorporation of CMake into developers’ own systems, including the VXL Project, the CABLE[6] features added by Brad King, and GE Corporate R&D for support of DART. Additional features were created when VTK transitioned to CMake for its build environment and for supporting ParaView.

Version 3.0 was released in June 2014.[7] It has been described as the beginning of "Modern CMake".[8] Experts now advise to avoid variables in favor of targets and properties.[9] The commands add_compile_options, include_directories, link_directories, link_libraries that were at the core of CMake 2 should now be replaced by target-specific commands.

Features

A key feature is the ability to (optionally) place compiler outputs (such as object files) outside the source tree. This enables multiple builds from the same source tree and cross-compilation. It also keeps the source tree clean and ensures that removing a build directory will not remove the source files.

Flexible project structure

CMake can locate system-wide and user-specified executables, files, and libraries. These locations are stored in a cache, which can then be tailored before generating the target build files. The cache can be edited with a graphical editor, which is shipped with the CMake.

Complicated directory hierarchies and applications that rely on several libraries are well supported by CMake. For instance, CMake is able to accommodate a project that has multiple toolkits, or libraries that each have multiple directories. In addition, CMake can work with projects that require executables to be created before generating code to be compiled for the final application. Its open-source, extensible design allows CMake to be adapted as necessary for specific projects.[10]

IDEs configuration support

CMake can generate project files for several popular IDEs, such as Microsoft Visual Studio, Xcode, and Eclipse CDT. It can also produce build scripts for MSBuild or NMake on Windows; Unix Make on Unix-like platforms such as Linux, macOS, and Cygwin; and Ninja on both Windows and Unix-like platforms.

Build process

The build process of a program or library with CMake takes place in two stages. First, standard build files are created (generated) from configuration files (CMakeLists.txt) which are written in CMake language. Then the platform's native build tools (native toolchain) are used for actual building of programs.[10][11]

The build files are configured depending on used generator (e.g. Unix Makefiles for make). Advanced users can also create and incorporate additional makefile generators to support their specific compiler and OS needs. Generated files are typically placed (by using cmake flag) into a different from sources folder e.g. build/.

Each build project in turn contains a CMakeCache.txt file and CMakeFiles directory in every (sub-)directory of the projects (happened to be included by add_subdirectory(...) command earlier) helping to avoid or speed up regeneration stage once it's run over again.

Types of build targets

Depending on CMakeLists.txt configuration the build files may be either executables, libraries (e.g. libxyz, xyz.dll etc), object file libraries or pseudo-targets (including aliases). Cmake can produce object files that can be linked against by executable binaries/libraries avoiding dynamic (run-time) linking and using static (compile-time) one instead. This enables flexibility in configuration of various optimizations.[12]

Language

CMake has a relatively simple interpreted, functional, scripting language. It supports variables, string manipulation, arrays, function/macro declarations, and module inclusion (import). CMake Language commands are read by cmake from a file named CMakeLists.txt. This file specifies the source files and build parameters, which cmake will place in the project's build specification (such as a Makefile). Additionally, .cmake-suffixed files can contain scripts used by cmake.[13]

To generate a project's build files, one invokes cmake and specifies the directory which contains CMakeLists.txt. This file contains one or more commands in the form COMMAND(argument ...). The arguments are whitespace-separated.

The language includes commands to specify dependencies. For example, commands such as add_executable(...) and add_library(...) introduce the target and dependencies for executables and libraries, respectively. [14] [15] Some build dependencies can be determined automatically.

The arguments can include keywords to separate groups of arguments. For example, in the command SET_SOURCE_FILE_PROPERTIES(source_file ... COMPILE_FLAGS compiler_option ...). Here, the keyword COMPILE_FLAGS terminates the list of source files and begins the list of compiler options. [16]

Once the Makefile (or alterative) has been generated, build behavior can be fine-tuned via target properties (since version 3.1) or via CMAKE_...-prefixed global variables. The latter is discouraged for target-only configurations because variables are also used to configure CMake itself and to set up initial defaults. [9]

Internals

The executable programs CMake, CPack, and CTest are written in the C++ programming language.

Much of CMake's functionality is implemented in modules that are written in the CMake language.[17]

Since release 3.0, CMake's documentation uses reStructuredText markup. HTML pages and man pages are generated by the Sphinx documentation generator.

Modules

CMake ships with numerous .cmake modules. These facilitate work such as finding dependencies (FindXYZ modules), testing the toolchain environment and executables, packaging releases (CPack module and cpack command), and managing dependencies on external projects (ExternalProject module). [18] [19]

CPack

CPack is a packaging system for software distributions. It is tightly integrated with CMake but can function without it. [20][21]

It can be used to generate:

Examples

Hello World

The following source code files demonstrate using cmake to build a simple hello world program written in C++. The source files are placed in a src/ directory.

// src/Hello_world.cc
#include <iostream>

int main()
{
    std::cout << "Hello, world!\n";
}

# src/CMakeLists.txt
cmake_minimum_required(VERSION 3.10)

# set the project name
project("Hello World")

# specify the executable and corresponding source file
add_executable(hello "Hello_world.cc")

bash script to run CMake on a Linux system. This example assumes that the script will be kept next to the src/ folder: 

#!/usr/bin/env bash
# Place this file next to src/ folder

cmake -S src/ -B build/     # Request that outputs from the build be placed in the build/ folder
cmake --build build/        # Start the build
build/hello                 # Run the compiled program.  Outputs "Hello, world!"

See also

References
  1. "CMake Release Tags on GitLab". Retrieved 30 January 2021.
  2. "The CMake Open Source Project on OpenHub". OpenHub. Retrieved 2016-04-09.
  3. "CMake".
  4. "Licenses · master · CMake / CMake". GitLab. Retrieved 2020-11-13.
  5. "FLOSS Weekly 111: CMake". podcast. TWiT Network. Retrieved 27 February 2011.
  6. "The CABLE". Archived from the original on 2013-06-19. Retrieved 2010-11-10.
  7. Maynard, Robert (June 10, 2014). "[CMake] [ANNOUNCE] CMake 3.0.0 Released".
  8. "Effective Modern CMake". Gist.
  9. https://github.com/boostcon/cppnow_presentations_2017/blob/master/05-19-2017_friday/effective_cmake__daniel_pfeifer__cppnow_05-19-2017.pdf, https://gist.github.com/mbinna/c61dbb39bca0e4fb7d1f73b0d66a4fd1
  10. Neundorf, Alexander (2006-06-21). "Why the KDE project switched to CMake—and how". LWN.net.
  11. "cmake-toolchains(7) — CMake 3.19.0-rc2 Documentation". cmake.org. Retrieved 2020-10-29.
  12. "cmake-buildsystem(7) — CMake 3.19.0-rc3 Documentation". cmake.org. Retrieved 2020-11-14.
  13. "cmake-language(7) — CMake 3.19.0-rc2 Documentation". cmake.org. Retrieved 2020-10-29.
  14. "add_executable — CMake 3.19.0-rc1 Documentation". cmake.org. Retrieved 2020-10-25.
  15. "add_library — CMake 3.19.0-rc1 Documentation". cmake.org. Retrieved 2020-10-25.
  16. Andrej Cedilnik (2003-10-30). "Cross-Platform Software Development Using CMake Software". Linux Journal. Retrieved 2021-01-29.
  17. "cmake-language(7) — CMake 3.19.0-rc1 Documentation". cmake.org. Retrieved 2020-10-25.
  18. "cmake-modules(7) — CMake 3.14.7 Documentation". cmake.org. Retrieved 2020-10-24.
  19. "ExternalProject — CMake 3.14.7 Documentation". cmake.org. Retrieved 2020-10-24.
  20. "Packaging With CPack". CMake Community Wiki.
  21. cpack(1) – Linux General Commands Manual
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