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",
],
)
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",
],
)
:win_driver_lib is only compatible for building with 64-bit Windows and
incompatible with all else. 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
Incompatible tests in a test_suite are
similarly skipped if the test_suite is specified on the command line with
--expand_test_suites.
In other words, test_suite targets on the command line behave like :all and
.... Using --noexpand_test_suites prevents expansion and causes
test_suite targets with incompatible tests to also be incompatible.
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, use the
@platforms//:incompatible
constraint_value that no platform
satisfies.
Use select() in combination with
@platforms//:incompatible 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.
cc_library(
name = "unixish_lib",
srcs = ["unixish_lib.cc"],
target_compatible_with = select({
"@platforms//os:osx": [],
"@platforms//os:linux": [],
"//conditions:default": ["@platforms//:incompatible"],
}),
)
The above can be interpreted as follows:
- When targeting macOS, the target has no constraints.
- When targeting Linux, the target has no constraints.
- Otherwise, the target has the
@platforms//:incompatibleconstraint. Because@platforms//:incompatibleis not part of any platform, 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": ["@platforms//:incompatible"],
"//conditions:default": [],
],
)
Detecting incompatible targets using bazel cquery
You can use the
IncompatiblePlatformProvider
in bazel cquery's Starlark output
format to distinguish
incompatible targets from compatible ones.
This can be used to filter out incompatible targets. The example below will only print the labels for targets that are compatible. Incompatible targets are not printed.
$ cat example.cquery
def format(target):
if "IncompatiblePlatformProvider" not in providers(target):
return target.label
return ""
$ bazel cquery //... --output=starlark --starlark:file=example.cquery
Known Issues
Incompatible targets ignore visibility restrictions.