# CMake Basics

**CMake** is a build system generator: it reads `CMakeLists.txt` and generates build files for your platform (Makefiles, Ninja, Visual Studio, etc.). Run `cmake` to configure, then use the generated build system to compile.

```bash
$ mkdir build && cd build
$ cmake ..                      # configure (reads CMakeLists.txt from parent)
$ cmake --build .               # build using generated files
$ cmake --build . -v            # verbose: show compiler commands
$ cmake --build . --target test # run tests
$ cmake --install .             # install (if configured)
```

The `CMakeLists.txt` file describes your project: what to build (executables, libraries), where source files are, dependencies, compiler flags, installation rules, etc. CMake handles finding compilers, libraries, and adjusting for your OS.

Key directories:

- Source tree (where `CMakeLists.txt` lives)
- Build tree (where `cmake` writes generated files)

Use **out-of-source builds**: create a separate `build/` directory for generated files. This keeps sources clean and allows multiple build configurations (debug, release, different compilers) from the same source tree.

```bash
mkdir build-debug && cd build-debug && cmake -DCMAKE_BUILD_TYPE=Debug ..
mkdir build-release && cd build-release && cmake -DCMAKE_BUILD_TYPE=Release ..
```

CMake version: check with `cmake --version`. Projects often specify minimum version in `CMakeLists.txt`: `cmake_minimum_required(VERSION 3.20)`.