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_value rules within
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.
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
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
--platforms- defaults to