Platforms

Bazel can build and test code on a variety of hardware, operating systems, and system configurations, using many different versions of build tools such as linkers and compilers. To help manage this complexity, Bazel has a concept of constraints and platforms. A constraint is a dimension in which build or production environments may differ, such as CPU architecture, the presence or absence of a GPU, or the version of a system-installed compiler. A platform is a named collection of choices for these constraints, representing the particular resources that are available in some environment.

Modeling the environment as a platform helps Bazel to automatically select the appropriate toolchains for build actions. Platforms can also be used in combination with the config_setting rule to write configurable attributes.

Bazel recognizes three roles that a platform may serve:

  • Host - the platform on which Bazel itself runs.
  • Execution - a platform on which build tools execute build actions to produce intermediate and final outputs.
  • Target - a platform on which a final output resides and executes.

Bazel supports the following build scenarios regarding platforms:

  • Single-platform builds (default) - host, execution, and target platforms are the same. For example, building a Linux executable on Ubuntu running on an Intel x64 CPU.

  • Cross-compilation builds - host and execution platforms are the same, but the target platform is different. For example, building an iOS app on macOS running on a MacBook Pro.

  • Multi-platform builds - host, execution, and target platforms are all different.

Defining constraints and platforms

The space of possible choices for platforms is defined by using the constraint_setting and constraint_value rules within BUILD files. constraint_setting creates a new dimension, while constraint_value creates a new value for a given dimension; together they effectively define an enum and its possible values. For example, the following snippet of a BUILD file introduces a constraint for the system’s glibc version with two possible values.

constraint_setting(name = "glibc_version")

constraint_value(
    name = "glibc_2_25",
    constraint_setting = ":glibc_version",
)

constraint_value(
    name = "glibc_2_26",
    constraint_setting = ":glibc_version",
)

Constraints and their values may be defined across different packages in the workspace. They are referenced by label and subject to the usual visibility controls. If visibility allows, you can extend an existing constraint setting by defining your own value for it.

The platform rule introduces a new platform with certain choices of constraint values. The following creates a platform named linux_x86, and says that it describes any environment that runs a Linux operating system on an x86_64 architecture with a glibc version of 2.25. (See below for more on Bazel’s built-in constraints.)

platform(
    name = "linux_x86",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
        ":glibc_2_25",
    ],
)

Note that it is an error for a platform to specify more than one value of the same constraint setting, such as @platforms//cpu:x86_64 and @platforms//cpu:arm for @platforms//cpu:cpu.

Generally useful constraints and platforms

To keep the ecosystem consistent, Bazel team maintains a repository with constraint definitions for the most popular CPU architectures and operating systems. These are all located in https://github.com/bazelbuild/platforms.

Bazel ships with the following special platform definition: @local_config_platform//:host. This is the autodetected host platform value - represents autodetected platform for the system Bazel is running on.

Specifying a platform for a build

You can specify the host and target platforms for a build using the following command-line flags:

  • --host_platform - defaults to @bazel_tools//platforms:host_platform
  • --platforms - defaults to @bazel_tools//platforms:target_platform

Skipping incompatible targets

When building for a specific target platform it is often desirable to skip targets that will never work on that platform. For example, your Windows device driver is likely going to generate lots of compiler errors when building on a Linux machine with //.... Use the target_compatible_with attribute to tell Bazel what target platform constraints your code has.

The simplest use of this attribute restricts a target to a single platform. The target will not be built for any platform that doesn’t satisfy all of the constraints. The following example restricts win_driver_lib.cc to 64-bit Windows.

cc_library(
    name = "win_driver_lib",
    srcs = "win_driver_lib.cc",
    target_compatible_with = [
        "@platforms//cpu:x86_64",
        "@platforms//os:windows",
    ],
)

When building for anything but 64-bit Windows we say that :win_driver_lib is incompatible. Incompatibility is transitive. Any targets that transitively depend on an incompatible target are themselves considered incompatible.

When are targets skipped?

Targets are skipped when they are considered incompatible and included in the build as part of a target pattern expansion. For example, the following two invocations skip any incompatible targets found in a target pattern expansion.

$ bazel build --platforms=//:myplatform //...`

$ bazel build --platforms=//:myplatform //:all`

Explicitly specifying an incompatible target on the command line results in an error message and a failed build.

$ bazel build --platforms=//:myplatform //:target_incompatible_with_myplatform
...
ERROR: Target //:target_incompatible_with_myplatform is incompatible and cannot be built, but was explicitly requested.
...
FAILED: Build did NOT complete successfully

More expressive constraints

For more flexibility in expressing constraints, create a user-defined constraint_value that no platform satisfies. For example, Put the following somewhere in your project and change //:not_compatible in the subsequent examples to match your location.

constraint_setting(name = "not_compatible_setting")

constraint_value(
    name = "not_compatible",
    constraint_setting = ":not_compatible_setting",
)

Use select() in combination with :not_compatible to express more complicated restrictions. For example, use it to implement basic OR logic. The following marks a library compatible with macOS and Linux, but no other platforms. Note that an empty constraints list is equivalent to “compatible with everything”.

cc_library(
    name = "unixish_lib",
    srcs = "unixish_lib.cc",
    target_compatible_with = select({
        "@platforms//os:osx": [],
        "@platforms//os:linux": [],
        "//conditions:default": ["//:not_compatible"],
    ],
)

The above can be interpreted as follows:

  1. If we are targeting macOS, then this target has no constraints.
  2. If we are targeting Linux, then this target has no constraints.
  3. Otherwise the target has the :not_compatible constraint. Because :not_compatible is not part of any platforms, the target is deemed incompatible.

To make your constraints more readable, use skylib’s selects.with_or().

You can express inverse compatibility in a similar way. The following example describes a library that is compatible with everything except for ARM.

cc_library(
    name = "non_arm_lib",
    srcs = "non_arm_lib.cc",
    target_compatible_with = select({
        "@platforms//cpu:arm": ["//:not_compatible"],
        "//conditions:default": [],
    ],
)