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")

    name = "glibc_2_25",
    constraint_setting = ":glibc_version",

    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.)

    name = "linux_x86",
    constraint_values = [

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

Built-in constraints and platforms

Bazel ships with constraint definitions for the most popular CPU architectures and operating systems. These are all located in the package @bazel_tools//platforms:

  • :cpu for the CPU architecture, with values :x86_32, :x86_64, :ppc, :arm, :s390x
  • :os for the operating system, with values :android, :freebsd, :ios, :linux, :osx, :windows

There are also the following special platform definitions:

  • :host_platform - represents the CPU and operating system for the host environment

  • :target_platform - represents the CPU and operating system for the target environment

The CPU values used by these two platforms can be specified with the --host_cpu and --cpu flags.

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