How exactly does CMake work Why are so many files generated

Understanding how exactly does CMake work can initially feel like unraveling a complex puzzle, especially when you’re confronted with a multitude of generated files that seem to appear out of nowhere. CMake is a cross-platform, open-source build system generator. It doesn’t build your software directly; instead, it generates native build files that your system’s build tools (like Make, Ninja, or Visual Studio) can use. This allows developers to write build configurations that work seamlessly across different operating systems and development environments. The generation of numerous files is a consequence of this flexibility – CMake prepares the groundwork for various build tools based on the specifications you provide in your CMakeLists.txt files. This article will demystify the CMake process, explaining how it functions, why it creates so many files, and how you can effectively manage it for your projects.

Delving into the CMake Workflow

The core of CMake lies in its ability to translate high-level build instructions into platform-specific build files. The process begins with you, the developer, creating one or more CMakeLists.txt files. These files contain commands that describe your project’s structure, dependencies, and build targets. CMake reads these CMakeLists.txt files, interprets the commands, and then generates the appropriate build files for your chosen build system. Think of it as a translator converting your project’s blueprint into instructions that the construction crew (your build tools) can understand.

CMake’s ability to work across different platforms is a key advantage. Rather than maintaining separate build scripts for each operating system (Windows, macOS, Linux) and build tool (Make, Ninja, Visual Studio), you maintain a single set of CMakeLists.txt files. CMake handles the nuances of each platform, ensuring that your project can be built consistently regardless of the underlying system. This cross-platform capability is essential for modern software development, where projects often need to be deployed on a variety of platforms. According to CMake’s official documentation, it supports a wide range of generators, catering to diverse development workflows.

The generated files include Makefiles (for Unix-like systems), Visual Studio project files (for Windows), and Xcode project files (for macOS). These files contain the instructions for compiling your source code, linking libraries, and creating executables or libraries. The specific files generated depend on the generator you select when configuring CMake. This modular approach allows you to seamlessly switch between build systems without modifying your project’s source code or build scripts. The CMake workflow allows for cleaner builds and less platform-specific configuration.

Understanding the Generated Files

The sheer number of files generated by CMake can be daunting at first glance. However, each file serves a specific purpose in the build process. The main directory you’ll encounter is the build directory, where all the generated files are placed. This separation of source code and build artifacts is a best practice that keeps your project organized and prevents accidental modifications to your source code. “CMake’s generated files are essential for managing dependencies and ensuring reproducible builds across different environments,” says Robert Dailey, a senior software engineer.

Within the build directory, you’ll find Makefiles (or equivalent files for other build systems), object files (compiled code), and the final executables or libraries. There are also intermediate files used during the build process, such as dependency files and precompiled headers. These files are necessary for incremental builds, where only the changed files are recompiled, significantly speeding up the build process. CMake also generates cache files, which store information about the system’s configuration and the location of dependencies. These cache files are used to speed up subsequent CMake runs.

The CMakeCache.txt file is particularly important. It stores variables that CMake uses to configure the build process. You can edit this file directly (using a text editor or the CMake GUI) to modify build settings, such as the compiler to use or the location of external libraries. However, it’s generally recommended to use the CMake GUI or command-line tools to modify these settings, as they provide a more user-friendly interface and prevent accidental errors. Understanding what each file does helps to make sense of the build process and allows to efficiently debug any issues that might arise.

Why So Many Files? The Rationale Behind the Generation

The generation of a large number of files is a direct consequence of CMake’s design goals: flexibility, portability, and efficiency. CMake aims to support a wide range of build systems and compilers, and each build system has its own specific requirements and file formats. To achieve this, CMake generates the necessary files for each build system, ensuring that the build process is tailored to the specific platform and toolchain. The more complex your project, the more files CMake needs to generate to accurately describe the build process.

Furthermore, CMake’s support for incremental builds also contributes to the number of generated files. Incremental builds allow you to recompile only the files that have changed since the last build, significantly speeding up the build process. However, to support incremental builds, CMake needs to generate dependency files that track the relationships between source files and header files. These dependency files are used to determine which files need to be recompiled when a source file or header file is modified. This granular approach ensures that only the necessary files are rebuilt, minimizing build times, especially for large projects. CMake’s dependency management is very robust and allows for easier collaboration. Kitware’s overview of CMake provides more information on the software’s capabilities.

To illustrate, consider a project with multiple libraries, executables, and dependencies. CMake will generate separate Makefiles (or equivalent) for each library and executable, as well as dependency files for each source file. This results in a large number of files, but it also allows for highly optimized builds. Without this level of granularity, the entire project would need to be rebuilt every time a single file is changed, which would be extremely time-consuming. The generation of many files is a tradeoff for increased flexibility, portability, and efficiency.

Best Practices for Managing CMake Projects

While CMake’s flexibility is a strength, it can also lead to complexity if not managed properly. Here are some best practices for managing CMake projects and minimizing the confusion caused by the numerous generated files:

Keep your CMakeLists.txt files organized and well-documented: Use comments to explain the purpose of each section and command. Break down complex build logic into smaller, more manageable functions.
Use a separate build directory: This keeps your source code clean and prevents accidental modifications to your source files.

Here’s how to configure CMake using a separate build directory:

Create a directory named “build” (or any other name you prefer) in the root of your project.
Navigate to the build directory in your terminal.
Run the command cmake .. (assuming your CMakeLists.txt file is in the parent directory).
CMake will generate the build files in the build directory.

Additionally, it’s beneficial to:

Use CMake’s built-in modules and functions: CMake provides a rich set of modules and functions for common tasks, such as finding libraries, setting compiler flags, and generating documentation. Using these modules and functions can simplify your CMakeLists.txt files and make them more portable.
Learn to use CMake GUI or ccmake: These tools provide a graphical interface for configuring CMake projects and modifying build settings. They can be particularly helpful for managing complex projects with many dependencies.

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FAQ About CMake File Generation -------------------------------

Why does CMake generate so many files even for a simple project?: Even simple projects require CMake to generate files specific to your chosen build system (e.g., Makefiles for Unix, Visual Studio project files for Windows). This ensures cross-platform compatibility. The files include build instructions, dependency information, and configuration settings.
Can I delete the generated files after building my project?: Yes, you can safely delete the contents of your build directory after building your project. However, you'll need to rerun CMake to regenerate the build files if you want to rebuild the project. It's generally recommended to keep the build directory separate from your source code directory to avoid accidentally deleting source files.
How can I reduce the number of files generated by CMake?: You can't directly reduce the number of essential generated files, as they are necessary for the build process. However, you can keep your CMakeLists.txt files clean and organized to improve readability and maintainability. Also, ensure you are not inadvertently creating duplicate build targets or unnecessary dependencies.

CMake streamlines the build process, especially when dealing with complex, cross-platform projects. By understanding the CMake workflow, the purpose of the generated files, and best practices for managing CMake projects, you can leverage its power to build software efficiently and reliably. Explore the various [CMake documentation](https://cmake.org/documentation/) to further your understanding and refine your skills.

Ready to take your CMake skills to the next level? Start by experimenting with different build systems and exploring CMake’s advanced features, such as custom commands and generators. Consider integrating CMake into your continuous integration (CI) pipeline to automate the build process and ensure consistent builds across different environments. If you need further guidance, check out our other resources on build systems and development workflows. You’ll find plenty of helpful tips and practical examples to help you master CMake and build better software, faster.

Question & Answer :
What exactly was going on behind the scenes when for such a small CMakeLists.txt file,

cmake_minimum_required (VERSION 2.6) project(Tutorial) add_executable(Tutorial tutorial.cpp)

and such a small tutorial.cpp

int main() { return 0; }

there are so many files generated:

CMakeCache.txt cmake_install.cmake Makefile CMakeLists.txt tutorial.cpp

And a CMakeFiles folder with so many files and folders:

CMakeCCompiler.cmake CMakeOutput.log Makefile.cmake cmake.check_cache CMakeSystem.cmake progress.marks CMakeCXXCompiler.cmake CMakeTmp TargetDirectories.txt CMakeDetermineCompilerABI_C.bin CompilerIdC Tutorial.dir CMakeDetermineCompilerABI_CXX.bin CompilerIdCXX CMakeDirectoryInformation.cmake Makefile2

Not understanding what was going on behind the scenes (i.e: why so may files had to be generated and what their purpose was), was the biggest obstacle in being able to learn CMake.

What is the purpose of these files, and when I type cmake ., what exactly is CMake configuring and generating before it builds the project?

The secret is that you don’t have to understand what the generated files do.

CMake introduces a lot of complexity into the build system, most of which only pays off if you use it for building complex software projects.

The good news is that CMake does a good job of keeping a lot of this messiness away from you: Use out-of-source builds and you don’t even have to look at the generated files. If you didn’t do this so far (which I guess is the case, since you wrote cmake .), please check them out before proceeding. Mixing the build and source directory is really painful with CMake and is not how the system is supposed to be used.

In a nutshell: Instead of

cd <source_dir> cmake .

always use

cd <build_dir_different_from_source_dir> cmake <source_dir>

I usually use an empty subfolder build inside my source directory as build directory.

To ease your pain, let me give a quick overview of the relevant files which CMake generates:

Project files/Makefiles - What you are actually interested in: The files required to build your project under the selected generator. This can be anything from a Unix Makefile to a Visual Studio solution.
CMakeCache.txt - This is a persistent key/value string storage which is used to cache value between runs. Values stored in here can be paths to library dependencies or whether an optional component is to be built at all. The list of variables is mostly identical to the one you see when running ccmake or cmake-gui. This can be useful to look at from time to time, but I would recommend to use the aforementioned tools for changing any of the values if possible.
Generated files - This can be anything from autogenerated source files to export macros that help you re-integrate your built project with other CMake projects. Most of these are only generated on demand and will not appear in a simple project such as the one from your question.
Anything else is pretty much noise to keep the build system happy. In particular, I never needed to care about anything that is going on inside the CMakeFiles subdirectory.

In general you should not mess with any of the files that CMake generates for you. All problems can be solved from within CMakeLists.txt in one way or the other. As long as the result builds your project as expected, you are probably fine. Do not worry too much about the gory details - as this is what CMake was trying to spare you of in the first place.