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<h1 class="page-title">Platforms and Toolchains Rules</h1>

{% dynamic setvar source_file "src/main/java/com/google/devtools/build/docgen/templates/be/rules.vm" %}
{% dynamic setvar version "7.6.0" %}
{% dynamic setvar original_path "/reference/be/platforms-and-toolchains" %}
{% include "_buttons.html" %}


<p>
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 <a href="/versions/7.6.0/extending/platforms">here</a>.
</p>



<h2>Rules</h2>
<ul>
  <li>
    <a href="#constraint_setting">
      constraint_setting    </a>
  </li>
  <li>
    <a href="#constraint_value">
      constraint_value    </a>
  </li>
  <li>
    <a href="#platform">
      platform    </a>
  </li>
  <li>
    <a href="#toolchain">
      toolchain    </a>
  </li>
  <li>
    <a href="#toolchain_type">
      toolchain_type    </a>
  </li>
</ul>

  <h2 id="constraint_setting">
    constraint_setting
  </h2>

  <a class="button button-with-icon" href="https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/platform/ConstraintSettingRule.java" target="_blank">
  View rule source<span class="material-icons icon-after" aria-hidden="true">open_in_new</span>
  </a>

  <pre class="rule-signature">constraint_setting(<a href="#constraint_setting.name">name</a>, <a href="#constraint_setting.default_constraint_value">default_constraint_value</a>, <a href="common-definitions.html#common.deprecation">deprecation</a>, <a href="common-definitions.html#common.distribs">distribs</a>, <a href="common-definitions.html#common.features">features</a>, <a href="common-definitions.html#typical.licenses">licenses</a>, <a href="common-definitions.html#common.tags">tags</a>, <a href="common-definitions.html#common.testonly">testonly</a>, <a href="common-definitions.html#common.visibility">visibility</a>)</pre>


<p>This rule is used to introduce a new constraint type for which a platform may specify a value.
For instance, you might define a <code>constraint_setting</code> named "glibc_version" to represent
the capability for platforms to have different versions of the glibc library installed.

For more details, see the
<a href="https://bazel.build/versions/7.6.0/docs/platforms">Platforms</a> page.

<p>Each <code>constraint_setting</code> has an extensible set of associated
<code>constraint_value</code>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 <code>@platforms//cpu:cpu</code> can be extended with a custom value in order to
define a platform targeting an obscure cpu architecture.



  <h3 id="constraint_setting_args">Arguments</h3>
  <table class="table table-condensed table-bordered table-params">
    <colgroup>
      <col class="col-param" />
      <col class="param-description" />
    </colgroup>
    <thead>
      <tr>
        <th colspan="2">Attributes</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <td id="constraint_setting.name"><code>name</code></td>
        <td>
          <p><a href="/versions/7.6.0/concepts/labels#target-names">Name</a>; required</p>
          <p>A unique name for this target.</p>

        </td>
      </tr>
                <tr>
        <td id="constraint_setting.default_constraint_value">
          <code>default_constraint_value</code>
        </td>
        <td>
                    <p><a href="/versions/7.6.0/concepts/labels#target-names">Name</a>; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; default is <code>None</code></p>
                            The label of the default value for this setting, to be used if no value is given. If this
        attribute is present, the <code>constraint_value</code> it points to must be defined in the
        same package as this <code>constraint_setting</code>.

        <p>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 <code>config_setting</code>) that requires a particular value
        for that setting.

        </td>
      </tr>
                                                                                    </tbody>
  </table>
  <h2 id="constraint_value">
    constraint_value
  </h2>

  <a class="button button-with-icon" href="https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/platform/ConstraintValueRule.java" target="_blank">
  View rule source<span class="material-icons icon-after" aria-hidden="true">open_in_new</span>
  </a>

  <pre class="rule-signature">constraint_value(<a href="#constraint_value.name">name</a>, <a href="#constraint_value.constraint_setting">constraint_setting</a>, <a href="common-definitions.html#common.deprecation">deprecation</a>, <a href="common-definitions.html#common.distribs">distribs</a>, <a href="common-definitions.html#common.features">features</a>, <a href="common-definitions.html#typical.licenses">licenses</a>, <a href="common-definitions.html#common.tags">tags</a>, <a href="common-definitions.html#common.testonly">testonly</a>, <a href="common-definitions.html#common.visibility">visibility</a>)</pre>


This rule introduces a new value for a given constraint type.

For more details, see the
<a href="https://bazel.build/versions/7.6.0/docs/platforms">Platforms</a> page.

<h4 id="constraint_value_examples">Example</h4>
<p>The following creates a new possible value for the predefined <code>constraint_value</code>
representing cpu architecture.
<pre class="code">
constraint_value(
    name = "mips",
    constraint_setting = "@platforms//cpu:cpu",
)
</pre>

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



  <h3 id="constraint_value_args">Arguments</h3>
  <table class="table table-condensed table-bordered table-params">
    <colgroup>
      <col class="col-param" />
      <col class="param-description" />
    </colgroup>
    <thead>
      <tr>
        <th colspan="2">Attributes</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <td id="constraint_value.name"><code>name</code></td>
        <td>
          <p><a href="/versions/7.6.0/concepts/labels#target-names">Name</a>; required</p>
          <p>A unique name for this target.</p>

        </td>
      </tr>
                <tr>
        <td id="constraint_value.constraint_setting">
          <code>constraint_setting</code>
        </td>
        <td>
                    <p><a href="/versions/7.6.0/concepts/labels">Label</a>; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; required</p>
                            The <code>constraint_setting</code> for which this <code>constraint_value</code> is a
        possible choice.

        </td>
      </tr>
                                                                                    </tbody>
  </table>
  <h2 id="platform">
    platform
  </h2>

  <a class="button button-with-icon" href="https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/platform/PlatformRule.java" target="_blank">
  View rule source<span class="material-icons icon-after" aria-hidden="true">open_in_new</span>
  </a>

  <pre class="rule-signature">platform(<a href="#platform.name">name</a>, <a href="#platform.constraint_values">constraint_values</a>, <a href="common-definitions.html#common.deprecation">deprecation</a>, <a href="common-definitions.html#common.distribs">distribs</a>, <a href="#platform.exec_properties">exec_properties</a>, <a href="common-definitions.html#common.features">features</a>, <a href="common-definitions.html#typical.licenses">licenses</a>, <a href="#platform.parents">parents</a>, <a href="#platform.remote_execution_properties">remote_execution_properties</a>, <a href="common-definitions.html#common.tags">tags</a>, <a href="common-definitions.html#common.testonly">testonly</a>, <a href="common-definitions.html#common.visibility">visibility</a>)</pre>


<p>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 <a href="/versions/7.6.0//extending/platforms">Platforms</a> page.


<h4 id="platform_examples">Example</h4>
<p>
  This defines a platform that describes any environment running Linux on ARM.
</p>
<pre class="code">
platform(
    name = "linux_arm",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:arm",
    ],
)
</pre>

<h3 id="platform_inheritance">Platform Inheritance</h3>
<p>
  Platforms may use the <code>parents</code> attribute to specify another platform that they will
  inherit constraint values from. Although the <code>parents</code> attribute takes a list, no
  more than a single value is currently supported, and specifying multiple parents is an error.
</p>

<p>
  When checking for the value of a constraint setting in a platform, first the values directly set
  (via the <code>constraint_values</code> 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.
</p>

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

<p>
  Platforms can also inherit the (deprecated) <code>remote_execution_properties</code> attribute
  from the parent platform. Note: new code should use <code>exec_properties</code> 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 <code>remote_execution_platform</code> is as follows when there
  is a parent platform:

  <ol>
    <li>
      If <code>remote_execution_property</code> is not set on the child platform, the parent's
      <code>remote_execution_properties</code> will be used.
    </li>
    <li>
      If <code>remote_execution_property</code> is set on the child platform, and contains the
      literal string </code>{PARENT_REMOTE_EXECUTION_PROPERTIES}</code>, that macro will be
      replaced with the contents of the parent's <code>remote_execution_property</code> attribute.
    </li>
    <li>
      If <code>remote_execution_property</code> is set on the child platform, and does not contain
      the macro, the child's <code>remote_execution_property</code> will be used unchanged.
    </li>
  </ol>
</p>

<p>
  <em>Since <code>remote_execution_properties</code> is deprecated and will be phased out, mixing
  <code>remote_execution_properties</code> and <code>exec_properties</code> in the same
  inheritance chain is not allowed.</em>
  Prefer to use <code>exec_properties</code> over the deprecated
  <code>remote_execution_properties</code>.
</p>

<h4 id="platform_inheritance_examples">Example: Constraint Values</h4>
<pre class="code">
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"],
)
</pre>

<p>
  In this example, the child platforms have the following properties:

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

<h4 id="platform_inheritance_exec_examples">Example: Execution properties</h4>
<pre class="code">
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"
    }
)
</pre>

<p>
  In this example, the child platforms have the following properties:

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



  <h3 id="platform_args">Arguments</h3>
  <table class="table table-condensed table-bordered table-params">
    <colgroup>
      <col class="col-param" />
      <col class="param-description" />
    </colgroup>
    <thead>
      <tr>
        <th colspan="2">Attributes</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <td id="platform.name"><code>name</code></td>
        <td>
          <p><a href="/versions/7.6.0/concepts/labels#target-names">Name</a>; required</p>
          <p>A unique name for this target.</p>

        </td>
      </tr>
                <tr>
        <td id="platform.constraint_values">
          <code>constraint_values</code>
        </td>
        <td>
                    <p>List of <a href="/versions/7.6.0/concepts/labels">labels</a>; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; default is <code>[]</code></p>
                            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.

        <p>Each <code>constraint_value</code> in this list must be for a different
        <code>constraint_setting</code>. For example, you cannot define a platform that requires the
        cpu architecture to be both <code>@platforms//cpu:x86_64</code> and
        <code>@platforms//cpu:arm</code>.

        </td>
      </tr>
                                          <tr>
        <td id="platform.exec_properties">
          <code>exec_properties</code>
        </td>
        <td>
                    <p>Dictionary: String -> String; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; default is <code>{}</code></p>
                            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  <a href="https://github.com/bazelbuild/remote-apis">remote execution protocol</a>.

        This includes any data from the parent platform's <code>exec_properties</code> 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
        <code>remote_execution_properties</code>.

        </td>
      </tr>
                                          <tr>
        <td id="platform.parents">
          <code>parents</code>
        </td>
        <td>
                    <p>List of <a href="/versions/7.6.0/concepts/labels">labels</a>; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; default is <code>[]</code></p>
                            The label of a <code>platform</code> 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 <a href="#platform_inheritance">Platform Inheritance</a> for details.

        </td>
      </tr>
                      <tr>
        <td id="platform.remote_execution_properties">
          <code>remote_execution_properties</code>
        </td>
        <td>
                    <p>String; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; default is <code>""</code></p>
                            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
        <a href="#platform_inheritance">Platform Inheritance</a> for details.

        </td>
      </tr>
                                            </tbody>
  </table>
  <h2 id="toolchain">
    toolchain
  </h2>

  <a class="button button-with-icon" href="https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/platform/ToolchainRule.java" target="_blank">
  View rule source<span class="material-icons icon-after" aria-hidden="true">open_in_new</span>
  </a>

  <pre class="rule-signature">toolchain(<a href="#toolchain.name">name</a>, <a href="common-definitions.html#common.deprecation">deprecation</a>, <a href="common-definitions.html#common.distribs">distribs</a>, <a href="#toolchain.exec_compatible_with">exec_compatible_with</a>, <a href="common-definitions.html#common.features">features</a>, <a href="common-definitions.html#typical.licenses">licenses</a>, <a href="common-definitions.html#common.tags">tags</a>, <a href="#toolchain.target_compatible_with">target_compatible_with</a>, <a href="#toolchain.target_settings">target_settings</a>, <a href="common-definitions.html#common.testonly">testonly</a>, <a href="#toolchain.toolchain">toolchain</a>, <a href="#toolchain.toolchain_type">toolchain_type</a>, <a href="common-definitions.html#common.visibility">visibility</a>)</pre>


<p>This rule declares a specific toolchain's type and constraints so that it can be selected
during toolchain resolution. See the
<a href="https://bazel.build/versions/7.6.0/docs/toolchains">Toolchains</a> page for more
details.



  <h3 id="toolchain_args">Arguments</h3>
  <table class="table table-condensed table-bordered table-params">
    <colgroup>
      <col class="col-param" />
      <col class="param-description" />
    </colgroup>
    <thead>
      <tr>
        <th colspan="2">Attributes</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <td id="toolchain.name"><code>name</code></td>
        <td>
          <p><a href="/versions/7.6.0/concepts/labels#target-names">Name</a>; required</p>
          <p>A unique name for this target.</p>

        </td>
      </tr>
                                    <tr>
        <td id="toolchain.exec_compatible_with">
          <code>exec_compatible_with</code>
        </td>
        <td>
                    <p>List of <a href="/versions/7.6.0/concepts/labels">labels</a>; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; default is <code>[]</code></p>
                            A list of <code>constraint_value</code>s that must be satisfied by an execution platform in
        order for this toolchain to be selected for a target building on that platform.

        </td>
      </tr>
                                                    <tr>
        <td id="toolchain.target_compatible_with">
          <code>target_compatible_with</code>
        </td>
        <td>
                    <p>List of <a href="/versions/7.6.0/concepts/labels">labels</a>; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; default is <code>[]</code></p>
                            A list of <code>constraint_value</code>s that must be satisfied by the target platform in
        order for this toolchain to be selected for a target building for that platform.

        </td>
      </tr>
                      <tr>
        <td id="toolchain.target_settings">
          <code>target_settings</code>
        </td>
        <td>
                    <p>List of <a href="/versions/7.6.0/concepts/labels">labels</a>; default is <code>[]</code></p>
                            A list of <code>config_setting</code>s that must be satisfied by the target configuration
        in order for this toolchain to be selected during toolchain resolution.

        </td>
      </tr>
                                <tr>
        <td id="toolchain.toolchain">
          <code>toolchain</code>
        </td>
        <td>
                    <p><a href="/versions/7.6.0/concepts/labels#target-names">Name</a>; required</p>
                            The target representing the actual tool or tool suite that is made available when this
        toolchain is selected.

        </td>
      </tr>
                      <tr>
        <td id="toolchain.toolchain_type">
          <code>toolchain_type</code>
        </td>
        <td>
                    <p><a href="/versions/7.6.0/concepts/labels">Label</a>; <a href="common-definitions.html#configurable-attributes">nonconfigurable</a>; required</p>
                            The label of a <code>toolchain_type</code> target that represents the role that this
        toolchain serves.

        </td>
      </tr>
                        </tbody>
  </table>
  <h2 id="toolchain_type">
    toolchain_type
  </h2>

  <a class="button button-with-icon" href="https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/ToolchainType.java" target="_blank">
  View rule source<span class="material-icons icon-after" aria-hidden="true">open_in_new</span>
  </a>

  <pre class="rule-signature">toolchain_type(<a href="#toolchain_type.name">name</a>, <a href="common-definitions.html#common.compatible_with">compatible_with</a>, <a href="common-definitions.html#common.deprecation">deprecation</a>, <a href="common-definitions.html#common.features">features</a>, <a href="common-definitions.html#common.restricted_to">restricted_to</a>, <a href="common-definitions.html#common.tags">tags</a>, <a href="common-definitions.html#common.target_compatible_with">target_compatible_with</a>, <a href="common-definitions.html#common.testonly">testonly</a>, <a href="common-definitions.html#common.visibility">visibility</a>)</pre>


<p>
  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.
</p>

<p>
  See the <a href="/versions/7.6.0/docs/toolchains">Toolchains</a> page for more details.
</p>

<h4 id="toolchain_type_examples">Example</h4>
<p>
  This defines a toolchain type for a custom rule.
</p>
<pre class="code">
toolchain_type(
    name = "bar_toolchain_type",
)
</pre>

<p>
  This can be used in a bzl file.
</p>
<pre class="code">
bar_binary = rule(
    implementation = _bar_binary_impl,
    attrs = {
        "srcs": attr.label_list(allow_files = True),
        ...
        # No `_compiler` attribute anymore.
    },
    toolchains = ["//bar_tools:toolchain_type"]
)
</pre>


  <h3 id="toolchain_type_args">Arguments</h3>
  <table class="table table-condensed table-bordered table-params">
    <colgroup>
      <col class="col-param" />
      <col class="param-description" />
    </colgroup>
    <thead>
      <tr>
        <th colspan="2">Attributes</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <td id="toolchain_type.name"><code>name</code></td>
        <td>
          <p><a href="/versions/7.6.0/concepts/labels#target-names">Name</a>; required</p>
          <p>A unique name for this target.</p>

        </td>
      </tr>
                                                                                        </tbody>
  </table>

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