From Code to Motion: Simulating and Deploying Robots

Build System

A build system is a set of tools and processes that automate the creation of executable programs (or libraries) from source code. In simple terms:

A build system takes your code and turns it into something that can run.

🔧 What a Build System Does

• Compile source files (.cpp) into object files (.o).
• Link object files into a binary or library.
• Manage dependencies, so only the changed parts are rebuilt.
• Run extra tasks like code generation, testing, or installation.

A build system automates all this, ensuring it's done in the right order with the right flags and dependencies.

🧱 Common Components of a Build System

• Build Scripts: Define what to build and how (e.g., CMakeLists.txt, Makefile).
• Compiler Commands: Automatically generated and run (g++, clang++, etc.).
• Dependency Management: Tracks what needs to be rebuilt if something changes.
• Configuration: Allows you to build for different systems or targets (Debug vs Release).

📦 Why ROS 2 Uses CMake

• CMake is cross-platform (Linux, Windows, macOS).
• Widely supported with rich tools and ecosystem.
• Modular — lets you define targets, dependencies, and installation steps.
• ROS 2 uses ament_cmake, an extension of CMake for ROS-specific workflows.

🧠 Key Idea

CMake in ROS 2 is not just for compiling C++ — it's used to manage the overall build system and installation process, even for Python packages.

✅ Why CMake (via ament_cmake) Is Still Used

1. Unified Build and Install System

ROS 2 uses colcon (build tool) + ament (build system) to handle both C++ and Python packages in a uniform way.

Even for Python, you still need to:

• Install the package
• Generate ROS 2-specific artifacts like messages, launch files
• Set up environment paths

2. Generate and Install ROS 2 Artifacts

Even a Python node might need:

• Custom messages/services (requiring code generation)
• Resource indexing (package.xml, launch files)
• Creating install space for ros2 run or ros2 launch

🐍 Python Packages Use ament_python

This uses setup.py and helps:

• Install Python code
• Create entry points for scripts
• Register with the ROS 2 environment

But: CMake is still the underlying orchestration tool that colcon uses in the workspace.

ROS 2 Build System Components

🔧 TL;DR

Tool	Purpose	Related to CMake?
CMake	Compiles C++ code and generates binaries	✅ Yes (low-level tool)
ament	ROS 2's build system for C++ and Python	✅ Yes (wraps CMake or setuptools)
colcon	Builds the entire ROS 2 workspace	✅ Yes (calls CMake/setup.py via ament)

🔨 1. CMake

• Cross-platform tool for defining how to build your C++ code.
• You write CMakeLists.txt to specify build rules.
• Used under the hood by ament_cmake.

🧱 2. ament

• Build system developed for ROS 2.
• Handles installation, message generation, and dependencies.
• Two flavors:

Variant	Use Case	Uses CMake?
ament_cmake	For C++ packages	✅ Yes
ament_python	For Python packages	❌ No (uses setup.py)

🏗️ 3. colcon

• The tool you run to build a ROS 2 workspace.
• Successor to catkin_make from ROS 1.
• Orchestrates builds for multiple packages (C++ or Python).

colcon build --packages-select my_robot

Internally calls CMake or Python build tools depending on package type.

📊 Analogy

• 🧱 CMake = machinery to build individual buildings
• 🧰 ament = construction rules and blueprints
• 🏗️ colcon = project manager building the whole neighborhood

✅ Summary

Tool	Role	Directly Compiles Code?	ROS 2 Specific?
CMake	Compile and link C++ code	✅ Yes	❌ No
ament	Build system over CMake or Python setup	❌ No	✅ Yes
colcon	Workspace-level build orchestration	❌ No	✅ Yes

CMake

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