About the build system

Under everything is the build system. Iterating on catkin from ROS 1 we have created a set of packages under the moniker ament. Some of the reasons for changing the name to ament are that we wanted it to not collide with catkin (in case we want mix them at some point) and to prevent confusion with existing catkin documentation. ament’s primary responsibility is to make it easier to develop and maintain ROS 2 core packages. However, this responsibility extends to any user who is willing to make use of our build system conventions and tools. Additionally it should make packages conventional, such that developers should be able to pick up any ament based package and make some assumptions about how it works, how to introspect it, and how to build or use it.

ament consists of a few important repositories which are all in the ament GitHub organization:

The ament_package Package

Located on GitHub at ament/ament_package, this repository contains a single ament Python package that provides various utilities for ament packages, e.g. templates for environment hooks.

All ament packages must contain a single package.xml file at the root of the package regardless of their underlying build system. The package.xml «manifest» file contains information that is required in order to process and operate on a package. This package information includes things like the package’s name, which is globally unique, and the package’s dependencies. The package.xml file also serves as the marker file which indicates the location of the package on the file system.

Parsing of the package.xml files is provided by catkin_pkg (as in ROS 1), while functionality to locate packages by searching the file system for these package.xml files is provided by build tools such as colcon.

package.xml

Package manifest file which marks the root of a package and contains meta information about the package including its name, version, description, maintainer, license, dependencies, and more. The contents of the manifest are in machine readable XML format and the contents are described in the REPs 127 and 140, with the possibility of further modifications in future REPs.

So anytime some package is referred to as an ament package, it means that it is a single unit of software (source code, build files, tests, documentation, and other resources) which is described using a package.xml manifest file.

ament package

Any package which contains a package.xml and follows the packaging guidelines of ament, regardless of the underlying build system.

Since the term ament package is build system agnostic, there can be different kinds of ament packages, e.g. ament CMake package, ament Python package, etc.

Here is a list of common package types that you might run into in this software stack:

CMake package

Any package containing a plain CMake project and a package.xml manifest file.

ament CMake package

A CMake package that also follows the ament packaging guidelines.

Python package

Any package containing a setuptools based Python project and a package.xml manifest file.

ament Python package

A Python package that also follows the ament packaging guidelines.

The ament_cmake Repository

Located on GitHub at ament/ament_cmake, this repository contains many «ament CMake» and pure CMake packages which provide the infrastructure in CMake that is required to create «ament CMake» packages. In this context «ament CMake» packages means: ament packages that are built using CMake. So the packages in this repository provide the necessary CMake functions/macros and CMake Modules to facilitate creating more «ament CMake» (or ament_cmake) packages. Packages of this type are identified with the <build_type>ament_cmake</build_type> tag in the <export> tag of the package.xml file.

The packages in this repository are extremely modular, but there is a single «bottleneck» package called ament_cmake. Anyone can depend on the ament_cmake package to get all of the aggregate functionality of the packages in this repository. Here a list of the packages in the repository along with a short description:

  • ament_cmake

    • aggregates all other packages in this repository, users need only to depend on this

  • ament_cmake_auto

    • provides convenience CMake functions which automatically handle a lot of the tedious parts of writing a package’s CMakeLists.txt file

  • ament_cmake_core

    • provides all built-in core concepts for ament, e.g. environment hooks, resource indexing, symbolic linking install and others

  • ament_cmake_gmock

    • adds convenience functions for making gmock based unit tests

  • ament_cmake_gtest

    • adds convenience functions for making gtest based automated tests

  • ament_cmake_nose

    • adds convenience functions for making nosetests based python automated tests

  • ament_cmake_python

  • ament_cmake_test

    • aggregates different kinds of tests, e.g. gtest and nosetests, under a single target using CTest

The ament_cmake_core package contains a lot of the CMake infrastructure that makes it possible to cleanly pass information between packages using conventional interfaces. This makes the packages have more decoupled build interfaces with other packages, promoting their reuse and encouraging conventions in the build systems of different packages. For instance it provides a standard way to pass include directories, libraries, definitions, and dependencies between packages so that consumers of this information can access this information in a conventional way.

The ament_cmake_core package also provides features of the ament build system like symbolic link installation, which allows you to symbolically link files from either the source space or the build space into the install space rather than copying them. This allows you to install once and then edit non-generated resources like Python code and configuration files without having to rerun the install step for them to take effect. This feature essentially replaces the «devel space» from catkin because it has most of the advantages with few of the complications or drawbacks.

Another feature provided by ament_cmake_core is the package resource indexing which is a way for packages to indicate that they contain a resource of some type. The design of this feature makes it much more efficient to answer simple questions like what packages are in this prefix (e.g. /usr/local) because it only requires that you list the files in a single possible location under that prefix. You can read more about this feature in the design docs for the resource index.

Like catkin, ament_cmake_core also provides environment setup files and package specific environment hooks. The environment setup files, often named something like setup.bash, are a place for package developers to define changes to the environment that are needed to utilize their package. The developers are able to do this using an «environment hook» which is basically an arbitrary bit of shell code that can set or modify environment variables, define shell functions, setup auto-completion rules, etc… This feature is how, for example, ROS 1 set the ROS_DISTRO environment variable without catkin knowing anything about the ROS distribution.

The ament_lint Repository

Located on GitHub at ament/ament_lint, this repository provides several packages which provide linting and testing services in a convenient and consistent manner. Currently there are packages to support C++ style linting using uncrustify, static C++ code checks using cppcheck, checking for copyright in source code, Python style linting using pep8, and other things. The list of helper packages will likely grow in the future.

Build tools

A build tool performs the task of building a workspace of packages together at once with a single invocation. For ROS 2 releases up to Ardent, the build tool providing this functionality is called ament_tools. As of ROS 2 Bouncy, ament_tools has been superseded by colcon, as described in the universal build tool article.