From v12.2 onwards, Please has cross-compilation support. This allows you to instruct it to build outputs for other architectures alongside the host architecture.

To target a different architecture, use the --arch flag when invoking plz. Architectures are formed of a two-part tags in a similar format to Go's - i.e. linux_amd64 etc. These are passed in as a single flag but decomposed into separate OS and architecture parts for later operations. You can request whatever architecture you want, but obviously the build will not succeed unless you have tools that understand that architecture.

The outputs from a cross-compiled build will be in a directory under plz-out prefixed with the architecture - e.g. plz-out/bin/linux_x86, plz-out/gen/darwin_amd64 etc.

Technical notes

Cross compilation is primarily achieved through global variables in the BUILD language. Build rules adjust their commands as necessary based on these values. When compiling a target for a different architecture, the BUILD file is re-parsed generating specialised rules for that architecture.

The CONFIG.ARCH and CONFIG.OS properties will be set to the target architecture. Similarly, when the rule builds, the OS and ARCH environment variables will also be set. The CONFIG.HOSTOS and CONFIG.HOSTARCH contain the host machines operating system and CPU architecture.

The distinction between tools and other dependencies of a target (e.g. src or deps) is extremely important when cross-compiling. Dependencies of a rule typically behave as you'd hope. All dependencies except tools will use the target architecture unless explicitly told not to via a platform specific label e.g. ///linux_amd64//some/target.

Tools, on the other hand, always use the host architecture since they are executed on the host during the build. In some cases tools might need to know the architecture they're targeting. To facilitate this, the target architecture is set in CONFIG.TARGET_OS and CONFIG.TARGET_ARCH.

The config file for any target architecture is read (if present) and applied for targets compiling for that architecture - e.g. .plzconfig_linux_x86. Typically you will need to create this file and modify the appropriate settings i.e. compiler flags.

Language status

The various builtin languages have differing levels of support. Currently they are as follows:

C & C++

Works for most cases; there is an example in the Please repo which tests a simple amd64 -> x86 compilation.

You will need to alter the config file to make this work - see the example for x86 for some examples of the settings to change. So far this has not been tested in anger on a more complex setup.


Works well across all platforms supported by Go. When using go_toolchain() to manage your go installation, cross compilation works out of the box.

Otherwise you must ensure the standard library is available for the target architecture on the host machine via GOOS="linux" GOARCH="386" go install -i std.


Currently has very limited support. Of course it will work for pure Python, however, binary modules built locally need a similar approach to C.

Binary third-party modules downloaded via pip_library won't work since pip can't be instructed to use a different architecture. python_wheel works fine but requires something else to have built & hosted the .whl file.


This is not terribly relevant to Java since it effectively always targets the same JVM architecture. maven_jar rules that specify native = True will download for the target architecture.

While not exactly related to this cross-compilation mechanism, there are also config options controlling the target, source and release level for Java.