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Windows Tips and Tricks

ROS 2 supports Windows 10 as a Tier 1 platform, which means that all code that goes into the ROS 2 core must support Windows. For those used to traditional development on Linux or other Unix-like systems, developing on Windows can be a bit of a challenge. This document aims to lay out some of those differences.

Maximum Path Length

By default, Windows has a maximum path length of 260 characters. Practically speaking, 4 of those characters are always used by the drive letter, colon, initial backslash, and final NULL character. That means that only 256 characters are available for the sum of all parts of the path. This has two practical consequences for ROS 2:

• Some of the ROS 2 internal path names are fairly long. Because of this, we always recommend using a short path name for the root of your ROS 2 directory, like C:\dev.

• When building ROS 2 from source, the default isolated build mode of colcon can generate very long path names. To avoid these very long path names, use --merge-install when building on Windows.

Note: It is possible to change Windows to have much longer maximum path lengths. See this article for more information.

Exporting symbols

All symbols are private by default when building a library on Windows. The consequence of this is that without special handling, another library or executable cannot call any symbols in the library. This special handling is called dllexport and dllimport. The dllexport is used in the library that wants to make a symbol publically callable, and dllimport is used by the library or application that wants to call the symbol. In the ROS 2 core code, packages often have a header called visibility_macros.h that use macros to define the proper dllexport/dllimport statements. With these macros in place, any symbol in header files that needs to be public needs to be decorated with the PUBLIC version of the macro. The CMakeLists.txt of the package should also have a stanza like:

target_compile_definitions(${PROJECT_NAME}
  PRIVATE "XXX_BUILDING_LIBRARY")

where “XXX” is replaced with the name of the macro in visibility_macros.h.

Finally, it is important that the header file that exports the symbols be included into at least one of the .cpp files in the package so that the macros will get expanded and placed into the resulting binary. Otherwise the symbols will still not be callable.

Debug builds

When building in Debug mode on Windows, several very important things change. The first is that all DLLs get _d automatically appended to the library name. So if the library is called libfoo.dll, in Debug mode it will be libfoo_d.dll. The dynamic linker on Windows also knows to look for libraries of that form, so it will not find libraries without the _d prefix. Additionally, Windows turns on a whole set of compile-time and run-time checks in Debug mode that is far more strict than Release builds. For these reasons, it is a good idea to run a Windows Debug build and test on many pull requests.

Forward-slash vs. back-slash

In Windows the default path separator is a backslash (\), which differs from the forward-slash (/) used in Linux and macOS. Most of the Windows APIs can deal with either as a path separator, but this is not universally true. For instance, the cmd.exe shell can only do tab-completion when using the backslash character, not the forward-slash. For maximum compatibility on Windows, a backslash should always be used as the path separator on Windows.

Patching vendored packages

When vendoring a package in ROS 2, it is often necessary to apply a patch to fix a bug, add a feature, etc. The typical way to do this is to modify the ExternalProject_add call to add a PATCH command, using the patch executable. Unfortunately, the patch executable as delivered by chocolatey requires Administrator access to run. The workaround is to use git apply-patch when applying patches to external projects.

git apply-patch has its own issues in that it only works properly when applied to a git repository. For that reason, external projects should always use the GIT method to obtain the project and then use the PATCH_COMMAND to invoke git apply-patch.

An example usage of all of the above looks something like:

ExternalProject_Add(mylibrary-${version}
  GIT_REPOSITORY https://github.com/lib/mylibrary.git
  GIT_TAG ${version}
  GIT_CONFIG advice.detachedHead=false
  # Suppress git update due to https://gitlab.kitware.com/cmake/cmake/-/issues/16419
  # See https://github.com/ament/uncrustify_vendor/pull/22 for details
  UPDATE_COMMAND ""
  TIMEOUT 600
  CMAKE_ARGS
    -DCMAKE_INSTALL_PREFIX=${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}_install
    ${extra_cmake_args}
    -Wno-dev
  PATCH_COMMAND
    ${CMAKE_COMMAND} -E chdir <SOURCE_DIR> git apply -p1 --ignore-space-change --whitespace=nowarn ${CMAKE_CURRENT_SOURCE_DIR}/install-patch.diff
)

Windows slow timers (slowness in general)

Software running on Windows is, in general, much slower than that running on Linux. This is due to a number of factors, from the default time slice (every 20 ms, according to the documentation), to the number of anti-virus and anti-malware processes running, to the number of background processes running. Because of all of this, tests should never expect tight timing on Windows. All tests should have generous timeouts, and only expect events to happen eventually (this will also prevent tests from being flakey on Linux).

Shells

There are two main command-line shells on Windows: the venerable cmd.exe, and PowerShell.

cmd.exe is the command shell that most closely emulates the old DOS shell, though with greatly enhanced capabilities. It is completely text based, and only understands DOS/Windows batch files.

PowerShell is the newer, object-based shell that Microsoft recommends for most new applications. It understands ps1 files for configuration.

ROS 2 supports both cmd.exe and PowerShell, so any changes (especially to things like ament or colcon) should be tested on both.