Step 0: Before You Begin — CMake 4.3.2 Documentation

# Step 0: Before You Begin

The CMake tutorial consists of hands-on exercises writing and building a C++ project; solving progressively more complex build requirements such as libraries, code generators, tests, and external dependencies. Before we are ready to even begin the first step of that journey, we need to ensure we have the correct tools at hand and understand how to use them.

Note

The tutorial material assumes the user has a C++20 compiler and toolchain available, and at least a beginner understanding of the C++ language. It is impossible to cover here all the possible ways one might acquire these prerequisites.

This prerequisite step provides recommendations for how to acquire and run CMake itself in order to carry out the rest of the tutorial. If you're already familiar with the basics of how to run CMake, you can feel free to move on to the rest of the tutorial.

## Getting the Tutorial Exercises

The tutorial documentation and source code examples can be found in the `Help/guide/tutorial` directory of the CMake source code tree. Each step of the tutorial has a corresponding subfolder, which serves as the starting point for that step's exercises.

## Getting CMake

The most obvious way to get your hands on CMake is to download it from the CMake website. The website's "Download" section contains the latest builds of CMake for all common (and some uncommon) desktop platforms.

However, it is preferable to acquire CMake via the usual delivery mechanism for developer tools on your platform. CMake is available in most packaging repositories, as a Visual Studio component, and can even be installed from the Python package index. Additionally, CMake is often available as part of the base image of most CI/CD runners targeting C/C++. You should consult the documentation for your software build environment to see if CMake is already available.

CMake can also be compiled from source using the instructions described by `README.rst`, found in the root of the CMake source tree.

CMake, like any program, needs to be available in `PATH` in order to be run from a shell. You can verify CMake is available by running any CMake command.

    $ cmake --version
    cmake version 3.23.5

    CMake suite maintained and supported by Kitware (kitware.com/cmake).

Note

If using a Visual Studio-provided development environment, it is best to run CMake from inside a Developer Command Prompt or Developer Powershell. This ensures CMake has access to all the required developer tooling and environment variables.

## CMake Generators

CMake is a configuration program, sometimes called a "meta" build system. As with other configuration systems, CMake is not ultimately responsible for running the commands which produce the software build. Instead, CMake generates a build system based on project, environment, and user-provided configuration information.

CMake supports multiple build systems as the output of this configuration process. These output backends are called "generators", because they generate the build system. CMake supports many generators, the documentation for which can be found at `cmake-generators(7)`. Information about supported generators for your particular CMake installation can be found via `cmake --help` under the "Generators" heading.

Using CMake thus requires one of the build programs which consumes this generator output be available. The `Unix Makefiles`, `Ninja`, and `Visual Studio` generators require a compatible `make`, `ninja`, and `Visual Studio` installation respectively.

Note

The default generator on Windows is typically the newest available Visual Studio version on the machine running CMake, everywhere else it is `Unix Makefiles`.

Which generator is used can be controlled via the `CMAKE_GENERATOR` environment variable, or the `cmake -G` option.

## Single and Multi-Configuration Generators

In many cases, it is possible to treat the underlying build system as an implementation detail and not differentiate between, for example, `ninja` and `make` when using CMake. However, there is one significant property of a given generator which we need to be aware of for even trivial workflows: if the generator supports single configuration builds, or if it supports multi-configuration builds.

Software builds often have several variants which we might be interested in. These variants have names like `Debug`, `Release`, `RelWithDebInfo`, and `MinSizeRel`, with properties corresponding to the name of the given variant.

A single-configuration build system always builds the software the same way, if it is generated to produce `Debug` builds it will always produce a `Debug` build. A multi-configuration build system can produce different outputs depending on the configuration specified at build time.

Note

The terms build configuration and build type are synonymous. When dealing with single-configuration generators, which only support a single variant, the generated variant is usually called the "build type".

When dealing with multi-configuration generators, the available variants are usually called the "build configurations". Selecting a variant at build time is usually called "selecting a configuration" and referred to by flags and variables as the "config".

However, this convention is not universal. Both technical and colloquial documentation often mix the two terms. Configuration and config are considered the more correct in contexts which generically address both single and multi-configuration generators.

The commonly used generators are as follows:

Single-Configuration | Multi-Configuration
--- | ---
`Ninja` | `Ninja Multi-Config`
`Unix Makefiles` | Visual Studio (all versions)
`FASTBuild` | `Xcode`

When using a single-configuration generator, the build type is selected based on the `CMAKE_BUILD_TYPE` environment variable, or can be specified directly when invoking CMake via `cmake -DCMAKE_BUILD_TYPE=<config>`.

Note

For the purpose of the tutorial, it is generally unnecessary to specify a build type when working with single-configuration generators. The platform-specific default behavior will work for all exercises.

When using a multi-configuration generator, the build configuration is specified at build time using either a build-system specific mechanism, or via the `cmake --build --config` option.

## Other Usage Basics

The rest of the tutorial will cover the remaining usage basics in greater depth, but for the purpose of ensuring we have a working development environment a few more CMake option flags will be enumerated here.

> `cmake -S <dir>`

>   Specifies the project root directory, where CMake will find the project to be built. This contains the root `CMakeLists.txt` file which will be discussed in Step 1 of the tutorial.

>   When unspecified, defaults to the current working directory.

> `cmake -B <dir>`

>   Specifies the build directory, where CMake will output the files for the generated build system, as well as artifacts of the build itself when the build system is run.

>   When unspecified, defaults to the current working directory.

> `cmake --build <dir>`

>   Runs the build system in the specified build directory. This is a generic command for all generators. For multi-configuration generators, the desired configuration can be requested via:

>   `cmake --build <dir> --config <cfg>`

## Try It Out

The `Help/guide/tutorial/Step0` directory contains a simple "Hello World" C++ project. The specifics of how CMake configures this project will be discussed in Step 1 of the tutorial, we need only concern ourselves with running the CMake program itself.

As described above, there are many possible ways we could run CMake depending on which generator we want to use for the build. If we navigate to the `Help/guide/tutorial/Step0` directory and run:

    cmake -B build

CMake will generate a build system for the Step0 project into `Help/guide/tutorial/Step0/build` using the default generator for the platform. Alternatively we can specify a specific generator, `Ninja` for example, with:

    cmake -G Ninja -B build

The effect is similar, but will use the `Ninja` generator instead of the platform default.

Note

We can't reuse the build directory with different generators. It is necessary to delete the build directory between CMake runs if you want to switch to a different generator using the same build directory.

How we build and run the project after generating the build system depends on the kind of generator we're using. If it is a single-configuration generator on a non-Windows platform, we can simply do:

    cmake --build build
    ./build/hello

Note

On Windows we might need to specify the file extension depending on which shell is in use, ie `./build/hello.exe`

If we're using a multi-configuration generator, we will want to specify the build configuration. The default configurations are `Debug`, `Release`, `RelWithDebInfo`, and `MinRelSize`. The result of the build will be stored in a configuration-specific subdirectory of the build folder. So for example we could run:

    cmake --build build --config Debug
    ./build/Debug/hello

## Getting Help and Additional Resources

For help from the CMake community, you can reach out on the CMake Discourse Forums.