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Using Bazel on Windows

Installation

See Install Bazel on Windows for installation instructions.

Known issues

We mark Windows-related Bazel issues on GitHub with the “team-Windows” label. You can see the open issues here.

Running Bazel: MSYS2 shell vs. Command Prompt vs. PowerShell

It’s best to run Bazel from the Command Prompt (cmd.exe) or from PowerShell.

You can also run Bazel from the MSYS2 shell, but you need to disable MSYS2’s automatic path conversion. See this StackOverflow answer for details.

Using Bazel without Bash (MSYS2)

bazel build without Bash

With Bazel 0.26.0 and the --incompatible_windows_native_test_wrapper flag, you can build Python and all C++ rules without Bash. Use the --shell_executable="" flag to tell Bazel not to look for Bash.

With Bazel 0.25.0 and the --incompatible_windows_native_test_wrapper flag, you can build Java and cc_binary rules without Bash (but not cc_test). Use the --shell_executable="" flag to tell Bazel not to look for Bash.

With Bazel 0.24.x and older you need Bash to build any rule.

With every Bazel version, you still need Bash if a rule in your build or in some external repository:

  • is a genrule, because genrules execute Bash commands
  • is a sh_binary or sh_test rule, because these inherently need Bash
  • is a Starlark rule that uses ctx.actions.run_shell() or ctx.resolve_command()

However, genrule is often used for simple tasks like copying a file or writing a text file. Instead of using genrule (and depending on Bash) you may find a suitable rule in the bazel-skylib repository. When built on Windows, these rules do not require Bash.

bazel test without Bash

With Bazel 0.25.0 or newer and the --incompatible_windows_native_test_wrapper flag, you can bazel test rules without Bash, i.e. bazel test --incompatible_windows_native_test_wrapper //foo:bar_test works even if there’s no MSYS2 installed.

With Bazel 0.24.x and older you cannot use this flag, and need Bash (MSYS2) to run any bazel test.

In Bazel 0.25.0 and Bazel 0.26.0, the --incompatible_windows_native_test_wrapper flag is off be default. We plan to enable it by default starting with Bazel 0.27.0, and plan to remove support for the flag in Bazel 0.28.0. Follow issue #6622 for updates.

bazel run without Bash

With Bazel 0.25.0 you still need Bash (MSYS2) to bazel run //foo:bin anything.

Removing this requirement is one of our top priorities. Follow issue #8240 for updates.

sh_binary and sh_* rules, and ctx.actions.run_shell() without Bash

You need Bash to build and test sh_* rules, and to build and test Starlark rules that use ctx.actions.run_shell() and ctx.resolve_command(). This applies not only to rules in your project, but to rules in any of the external repositories your project depends on (even transitively).

We may explore the option to use Windows Subsystem for Linux (WSL) to build these rules, but as of 2019-05-07 it is not a priority for the Bazel-on-Windows subteam.

Setting environment variables

Environment variables you set in the Windows Command Prompt (cmd.exe) are only set in that command prompt session. If you start a new cmd.exe, you need to set the variables again. To always set the variables when cmd.exe starts, you can add them to the User variables or System variables in the Control Panel > System Properties > Advanced > Environment Variables... dialog box.

Using Bazel on Windows

The first time you build any target, Bazel auto-configures the location of Python and the Visual C++ compiler. If you need to auto-configure again, run bazel clean then build a target.

You can also tell Bazel where to find the Python binary and the C++ compiler:

Build C++ with MSVC

To build C++ targets with MSVC, you need:

  • The Visual C++ compiler.

    You can install it in one of the following ways:

    • Install Visual Studio 2015 or later (Community Edition is enough) with Visual C++.

      Make sure to also install the Visual C++ > Common Tools for Visual C++ and Visual C++ > Microsoft Foundation Classes for C++ features. These features are not installed by default.

    • Install the Visual C++ Build Tools 2015 or later.

      If alwayslink doesn’t work with VS 2017, that is due to a known issue, please upgrade your VS 2017 to the latest version.

  • The BAZEL_VS, BAZEL_VC and BAZEL_VC_FULL_VERSION environment variable.

    Bazel tries to locate the C++ compiler the first time you build any target. To tell Bazel where the compiler is, you can set the following environment variables:

    For Visual Studio 2017 and 2019, set one of BAZEL_VC or BAZEL_VS. Additionally you may also set BAZEL_VC_FULL_VERSION.

    • BAZEL_VS the Visual Studio installation directory

      set BAZEL_VS=C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools
      
    • BAZEL_VC the Visual C++ Build Tools installation directory
      set BAZEL_VC=C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\VC
      
    • BAZEL_VC_FULL_VERSION (Optional) Only for Visual Studio 2017 and 2019, the full version number of your Visual C++ Build Tools. You can choose the exact Visual C++ Build Tools version via BAZEL_VC_FULL_VERSION if more than one version are installed, otherwise Bazel will choose the latest version.
      set BAZEL_VC_FULL_VERSION=14.16.27023
      

    For Visual Studio 2015 or older, set BAZEL_VC or BAZEL_VS. (BAZEL_VC_FULL_VERSION is not supported.)

    • BAZEL_VS the Visual Studio installation directory

      set BAZEL_VS=C:\Program Files (x86)\Microsoft Visual Studio 14.0
      
    • BAZEL_VC the Visual C++ Build Tools installation directory

      set BAZEL_VC=C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC
      
  • The Windows SDK.

    The Windows SDK contains header files and libraries you need when building Windows applications, including Bazel itself. By default, the latest Windows SDK installed will be used. You also can specify Windows SDK version by setting BAZEL_WINSDK_FULL_VERSION. You can use a full Windows 10 SDK number such as 10.0.10240.0, or specify 8.1 to use the Windows 8.1 SDK (only one version of Windows 8.1 SDK is available). Please make sure you have the specified Windows SDK installed.

    Requirement: This is supported with VC 2017 and 2019. The standalone VC 2015 Build Tools doesn’t support selecting Windows SDK, you’ll need the full Visual Studio 2015 installation, otherwise BAZEL_WINSDK_FULL_VERSION will be ignored.

    set BAZEL_WINSDK_FULL_VERSION=10.0.10240.0
    

If everything is set up, you can build a C++ target now!

Try building a target from one of our sample projects:

C:\projects\bazel> bazel build //examples/cpp:hello-world

C:\projects\bazel> bazel-bin\examples\cpp\hello-world.exe

To build and use Dynamically Linked Libraries (DLL files), see this example.

Build C++ with Clang

From 0.29.0, Bazel supports building with LLVM’s MSVC-compatible compiler driver (clang-cl.exe).

Requirement: To build with Clang, you have to install both LLVM and Visual C++ Build tools, because although we use clang-cl.exe as compiler, we still need to link to Visual C++ libraries.

Bazel can automatically detect LLVM installation on your system, or you can explicitly tell Bazel where LLVM is installed by BAZEL_LLVM.

  • BAZEL_LLVM the LLVM installation directory

    set BAZEL_LLVM=C:\Program Files\LLVM
    

To enable the Clang toolchain for building C++, there are several situations.

  • In bazel 0.28 and older: Clang is not supported.

  • Without --incompatible_enable_cc_toolchain_resolution: You can enable the Clang toolchain by a build flag --compiler=clang-cl.

  • With --incompatible_enable_cc_toolchain_resolution: You have to add a platform target to your BUILD file (eg. the top level BUILD file):

      platform(
          name = "x64_windows-clang-cl",
          constraint_values = [
              "@platforms//cpu:x86_64",
              "@platforms//os:windows",
              "@bazel_tools//tools/cpp:clang-cl",
          ],
      )
    

    Then you can enable the Clang toolchain by either of the following two ways:

    • Specify the following build flags:
      --extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl --extra_execution_platforms=//:x64_windows-clang-cl
    
    • Register the platform and toolchain in your WORKSPACE file:
      register_execution_platforms(
          ":x64_windows-clang-cl"
      )
    
      register_toolchains(
          "@local_config_cc//:cc-toolchain-x64_windows-clang-cl",
      )
    

    The --incompatible_enable_cc_toolchain_resolution flag is planned to be enabled by default in future Bazel release. Therefore, it is recommended to enable Clang support with the second approach.

Build Java

There’s no setup necessary.

On Windows, Bazel builds two output files for java_binary rules:

  • a .jar file
  • a .exe file that can set up the environment for the JVM and run the binary

Try building a target from one of our sample projects:

C:\projects\bazel> bazel build //examples/java-native/src/main/java/com/example/myproject:hello-world

C:\projects\bazel> bazel-bin\examples\java-native\src\main\java\com\example\myproject\hello-world.exe

Build Python

To build Python targets, you need:

  • The Python interpreter

    Both Python 2 and Python 3 are supported.

    To tell Bazel where Python is, you can use --python_path=<path/to/python>. For example:

    bazel build --python_path=C:/Python27/python.exe ...
    

    If --python_path is not specified, Bazel uses python.exe as the interpreter and the binary looks for it in $PATH during runtime. If it is not in $PATH(for example, when you use py_binary as an action’s executable, Bazel will sanitize $PATH), then the execution will fail.

On Windows, Bazel builds two output files for py_binary rules:

  • a self-extracting zip file
  • an executable file that can launch the Python interpreter with the self-extracting zip file as the argument

You can either run the executable file (it has a .exe extension) or you can run Python with the self-extracting zip file as the argument.

Try building a target from one of our sample projects:

C:\projects\bazel> bazel build //examples/py_native:bin

C:\projects\bazel> bazel-bin\examples\py_native\bin.exe

C:\projects\bazel> python bazel-bin\examples\py_native\bin.zip

If you are interested in details about how Bazel builds Python targets on Windows, check out this design doc.