# Platforms and Toolchains Rules

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This set of rules exists to allow you to model specific hardware platforms you are building for and specify the specific tools you may need to compile code for those platforms. The user should be familiar with the concepts explained here.

## Rules

- constraint_setting
- constraint_value
- platform
- toolchain
- toolchain_type

## constraint_setting

View rule sourceopen_in_new

    constraint_setting(name, default_constraint_value, deprecation, distribs, features, licenses, tags, testonly, visibility)

This rule is used to introduce a new constraint type for which a platform may specify a value. For instance, you might define a `constraint_setting` named "glibc_version" to represent the capability for platforms to have different versions of the glibc library installed. For more details, see the Platforms page.

Each `constraint_setting` has an extensible set of associated `constraint_value`s. Usually these are defined in the same package, but sometimes a different package will introduce new values for an existing setting. For instance, the predefined setting `@platforms//cpu:cpu` can be extended with a custom value in order to define a platform targeting an obscure cpu architecture.

### Arguments

Attributes
`name` | Name; required A unique name for this target.
`default_constraint_value` | Name; nonconfigurable; default is `None` The label of the default value for this setting, to be used if no value is given. If this attribute is present, the `constraint_value` it points to must be defined in the same package as this `constraint_setting`. If a constraint setting has a default value, then whenever a platform does not include any constraint value for that setting, it is the same as if the platform had specified the default value. Otherwise, if there is no default value, the constraint setting is considered to be unspecified by that platform. In that case, the platform would not match against any constraint list (such as for a `config_setting`) that requires a particular value for that setting.

## constraint_value

View rule sourceopen_in_new

    constraint_value(name, constraint_setting, deprecation, distribs, features, licenses, tags, testonly, visibility)

This rule introduces a new value for a given constraint type. For more details, see the Platforms page.

#### Example

The following creates a new possible value for the predefined `constraint_value` representing cpu architecture.

    constraint_value(
        name = "mips",
        constraint_setting = "@platforms//cpu:cpu",
    )

Platforms can then declare that they have the `mips` architecture as an alternative to `x86_64`, `arm`, and so on.

### Arguments

Attributes
`name` | Name; required A unique name for this target.
`constraint_setting` | Label; nonconfigurable; required The `constraint_setting` for which this `constraint_value` is a possible choice.

## platform

View rule sourceopen_in_new

    platform(name, constraint_values, deprecation, distribs, exec_properties, features, licenses, parents, remote_execution_properties, tags, testonly, visibility)

This rule defines a new platform -- a named collection of constraint choices (such as cpu architecture or compiler version) describing an environment in which part of the build may run. For more details, see the Platforms page.

#### Example

This defines a platform that describes any environment running Linux on ARM.

    platform(
        name = "linux_arm",
        constraint_values = [
            "@platforms//os:linux",
            "@platforms//cpu:arm",
        ],
    )

### Platform Inheritance

Platforms may use the `parents` attribute to specify another platform that they will inherit constraint values from. Although the `parents` attribute takes a list, no more than a single value is currently supported, and specifying multiple parents is an error.

When checking for the value of a constraint setting in a platform, first the values directly set (via the `constraint_values` attribute) are checked, and then the constraint values on the parent. This continues recursively up the chain of parent platforms. In this manner, any values set directly on a platform will override the values set on the parent.

Platforms inherit the `exec_properties` attribute from the parent platform. The dictionary entries in `exec_properties` of the parent and child platforms will be combined. If the same key appears in both the parent's and the child's `exec_properties`, the child's value will be used. If the child platform specifies an empty string as a value, the corresponding property will be unset.

Platforms can also inherit the (deprecated) `remote_execution_properties` attribute from the parent platform. Note: new code should use `exec_properties` instead. The logic described below is maintained to be compatible with legacy behavior but will be removed in the future. The logic for setting the `remote_execution_platform` is as follows when there is a parent platform:

1. If `remote_execution_property` is not set on the child platform, the parent's `remote_execution_properties` will be used.
2. If `remote_execution_property` is set on the child platform, and contains the literal string {PARENT_REMOTE_EXECUTION_PROPERTIES}, that macro will be replaced with the contents of the parent's `remote_execution_property` attribute.
3. If `remote_execution_property` is set on the child platform, and does not contain the macro, the child's `remote_execution_property` will be used unchanged.

Since `remote_execution_properties` is deprecated and will be phased out, mixing `remote_execution_properties` and `exec_properties` in the same inheritance chain is not allowed. Prefer to use `exec_properties` over the deprecated `remote_execution_properties`.

#### Example: Constraint Values

    platform(
        name = "parent",
        constraint_values = [
            "@platforms//os:linux",
            "@platforms//cpu:arm",
        ],
    )
    platform(
        name = "child_a",
        parents = [":parent"],
        constraint_values = [
            "@platforms//cpu:x86_64",
        ],
    )
    platform(
        name = "child_b",
        parents = [":parent"],
    )

In this example, the child platforms have the following properties:

- `child_a` has the constraint values `@platforms//os:linux` (inherited from the parent) and `@platforms//cpu:x86_64` (set directly on the platform).
- `child_b` inherits all constraint values from the parent, and doesn't set any of its own.

#### Example: Execution properties

    platform(
        name = "parent",
        exec_properties = {
          "k1": "v1",
          "k2": "v2",
        },
    )
    platform(
        name = "child_a",
        parents = [":parent"],
    )
    platform(
        name = "child_b",
        parents = [":parent"],
        exec_properties = {
          "k1": "child"
        }
    )
    platform(
        name = "child_c",
        parents = [":parent"],
        exec_properties = {
          "k1": ""
        }
    )
    platform(
        name = "child_d",
        parents = [":parent"],
        exec_properties = {
          "k3": "v3"
        }
    )

In this example, the child platforms have the following properties:

- `child_a` inherits the "exec_properties" of the parent and does not set its own.
- `child_b` inherits the parent's `exec_properties` and overrides the value of `k1`. Its `exec_properties` will be: `{ "k1": "child", "k2": "v2" }`.
- `child_c` inherits the parent's `exec_properties` and unsets `k1`. Its `exec_properties` will be: `{ "k2": "v2" }`.
- `child_d` inherits the parent's `exec_properties` and adds a new property. Its `exec_properties` will be: `{ "k1": "v1", "k2": "v2", "k3": "v3" }`.

### Arguments

Attributes
`name` | Name; required A unique name for this target.
`constraint_values` | List of labels; nonconfigurable; default is `[]` The combination of constraint choices that this platform comprises. In order for a platform to apply to a given environment, the environment must have at least the values in this list. Each `constraint_value` in this list must be for a different `constraint_setting`. For example, you cannot define a platform that requires the cpu architecture to be both `@platforms//cpu:x86_64` and `@platforms//cpu:arm`.
`exec_properties` | Dictionary: String -> String; nonconfigurable; default is `{}` A map of strings that affect the way actions are executed remotely. Bazel makes no attempt to interpret this, it is treated as opaque data that's forwarded via the Platform field of the remote execution protocol. This includes any data from the parent platform's `exec_properties` attributes. If the child and parent platform define the same keys, the child's values are kept. Any keys associated with a value that is an empty string are removed from the dictionary. This attribute is a full replacement for the deprecated `remote_execution_properties`.
`parents` | List of labels; nonconfigurable; default is `[]` The label of a `platform` target that this platform should inherit from. Although the attribute takes a list, there should be no more than one platform present. Any constraint_settings not set directly on this platform will be found in the parent platform. See the section on Platform Inheritance for details.
`remote_execution_properties` | String; nonconfigurable; default is `""` DEPRECATED. Please use exec_properties attribute instead. A string used to configure a remote execution platform. Actual builds make no attempt to interpret this, it is treated as opaque data that can be used by a specific SpawnRunner. This can include data from the parent platform's "remote_execution_properties" attribute, by using the macro "{PARENT_REMOTE_EXECUTION_PROPERTIES}". See the section on Platform Inheritance for details.

## toolchain

View rule sourceopen_in_new

    toolchain(name, deprecation, distribs, exec_compatible_with, features, licenses, tags, target_compatible_with, target_settings, testonly, toolchain, toolchain_type, visibility)

This rule declares a specific toolchain's type and constraints so that it can be selected during toolchain resolution. See the Toolchains page for more details.

### Arguments

Attributes
`name` | Name; required A unique name for this target.
`exec_compatible_with` | List of labels; nonconfigurable; default is `[]` A list of `constraint_value`s that must be satisfied by an execution platform in order for this toolchain to be selected for a target building on that platform.
`target_compatible_with` | List of labels; nonconfigurable; default is `[]` A list of `constraint_value`s that must be satisfied by the target platform in order for this toolchain to be selected for a target building for that platform.
`target_settings` | List of labels; default is `[]` A list of `config_setting`s that must be satisfied by the target configuration in order for this toolchain to be selected during toolchain resolution.
`toolchain` | Name; required The target representing the actual tool or tool suite that is made available when this toolchain is selected.
`toolchain_type` | Label; nonconfigurable; required The label of a `toolchain_type` target that represents the role that this toolchain serves.

## toolchain_type

View rule sourceopen_in_new

    toolchain_type(name, compatible_with, deprecation, features, restricted_to, tags, target_compatible_with, testonly, visibility)

This rule defines a new type of toolchain -- a simple target that represents a class of tools that serve the same role for different platforms.

See the Toolchains page for more details.

#### Example

This defines a toolchain type for a custom rule.

    toolchain_type(
        name = "bar_toolchain_type",
    )

This can be used in a bzl file.

    bar_binary = rule(
        implementation = _bar_binary_impl,
        attrs = {
            "srcs": attr.label_list(allow_files = True),
            ...
            # No `_compiler` attribute anymore.
        },
        toolchains = ["//bar_tools:toolchain_type"]
    )

### Arguments

Attributes
`name` | Name; required A unique name for this target.