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 <p>HIDL is built around interfaces, an abstract type used in object-oriented
 languages to define behaviors. Each interface is part of a package.</p>

 <h2 id=packages>Packages</h2>

 <p>Package names can have sublevels such as <code>package.subpackage</code>. The
 root directory for published HIDL packages is <code>hardware/interfaces</code>
 or <code>vendor/vendorName</code> (e.g. <code>vendor/google</code> for Pixel
 devices). The package name forms one or more subdirectories under the root
 directory; all files defining a package are in the same directory. For example,
 <code>package android.hardware.example.extension.light@2.0</code> could be found
 under <code>hardware/interfaces/example/extension/light/2.0</code>.</p>

 <p>The following table lists package prefixes and locations:</p>
 <table>
 <tbody>

 <tr>
 <th>Package Prefix</th>
 <th>Location</th>
 </tr>

 <tr>
 <td><code>android.hardware.*</code></td>
 <td><code>hardware/interfaces/*</code></td>
 </tr>

 <tr>
 <td><code>android.frameworks.*</code></td>
 <td><code>frameworks/hardware/interfaces/*</code></td>
 </tr>

 <tr>
 <td><code>android.system.*</code></td>
 <td><code>system/hardware/interfaces/*</code></td>
 </tr>

 <tr>
 <td><code>android.hidl.*</code></td>
 <td><code>system/libhidl/transport/*</code></td>
 </tr>

 </tbody>
 </table>

 <p>The package directory contains files with extension <code>.hal</code>. Every
 file must contain a <code>package</code> statement naming the package and
 version the file is part of. The file <code>types.hal</code>, if present, does
 not define an interface but instead defines data types accessible to every
 interface in the package.</p>

 <h2 id=interface-def>Interface definition</h2>
 <p>Aside from <code>types.hal</code>, every other <code>.hal</code> file defines
 an interface. An interface is typically defined as follows:</p>

 <pre class="prettyprint">
 interface IBar extends IFoo { // IFoo is another interface
     // embedded types
     struct MyStruct {/*...*/};

     // interface methods
     create(int32_t id) generates (MyStruct s);
     close();
 };
 </pre>

 <p>An interface without an explicit <code>extends</code> declaration implicitly
 extends from <code>android.hidl.base@1.0::IBase</code> (similar to
 <code>java.lang.Object</code> in Java.) The IBase interface, implicitly
 imported, declares several reserved methods that should not and cannot be
 redeclared in user-defined interfaces or used otherwise. These methods
 include:</p>

 <ul>
 <li><code>ping</code></li>
 <li><code>interfaceChain</code></li>
 <li><code>interfaceDescriptor</code></li>
 <li><code>notifySyspropsChanged</code></li>
 <li><code>linkToDeath</code></li>
 <li><code>unlinkToDeath</code></li>
 <li><code>setHALInstrumentation</code></li>
 <li><code>getDebugInfo</code></li>
 <li><code>debug</code></li>
 <li><code>getHashChain</code></li>
 </ul>

 <h2 id=import>Importing</h2>
 <p>The <code>import</code> statement is HIDL mechanism to access package
 interfaces and types in another package. An <code>import</code> statement
 concerns itself with two entities:</p>

 <ul>
 <li>The import<em>ing</em> entity, which can be either a package or an
 interface; and</li>
 <li>The import<em>ed</em> entity, which too can be either a package or an
 interface.</li>
 </ul>

 <p>The importing entity is determined by the location of the
 <code>import</code> statement. When the statement is inside a package's
 <code>types.hal</code>, what is being imported is visible by the entire package;
 this is a <em>package-level</em> import. When the statement is inside an
 interface file, the importing entity is the interface itself; this is an
 <em>interface-level</em> import.</p>

 <p>The imported entity is determined by the value after the <code>import</code>
 keyword. The value need not be a fully-qualified name; if a component is
 omitted, it is automatically filled with information from the current package.
 For fully-qualified values, the following import cases are supported:</p>

 <ul>
 <li><strong>Whole-package imports</strong>. If the value is a package name and a
 version (syntax described below), then the entire package is imported into the
 importing entity.</li>
 <li><strong>Partial imports</strong>. If the value is:
 <ul>
 <li>An interface, the package's <code>types.hal</code> and that interface are
 imported into the importing entity.</li>
 <li>A UDT defined in <code>types.hal</code>, then only that UDT is imported into
 the importing entity (other types in <code>types.hal</code> are not imported).
 </li>
 </ul>
 <li><strong>Types-only imports</strong>. If the value uses the syntax of a
 partial import described above, but with the keyword <code>types</code> instead
 of an Interface name, only the UDTs in <code>types.hal</code> of the designated
 package are imported.</li>
 </ul>

 <p>The importing entity gets access to a combination of:</p>
 <ul>
 <li>The imported package's common UDTs defined in <code>types.hal</code>;</li>
 <li>The imported package's interfaces (for a whole-package import) or specified
 interface (for a partial import) for the purposes of invoking them, passing
 handles to them and/or inheriting from them.</li>
 </ul>

 <p>The import statement uses the fully-qualified-type-name syntax to provide the
 name and version of the package or interface being imported:</p>

 <pre class="prettyprint">
 import android.hardware.nfc@1.0;            // import a whole package
 import android.hardware.example@1.0::IQuux; // import an interface and types.hal
 import android.hardware.example@1.0::types; // import just types.hal
 </pre>

 <h2 id=inheritance>Interface inheritance</h2>

 <p>An interface can be an extension of a previously-defined interface.
 Extensions can be one of the following three types:</p>
 <ul>
 <li>Interface can add functionality to another one, incorporating its API
 unchanged.</li>
 <li>Package can add functionality to another one, incorporating its API
 unchanged.</li>
 <li>Interface can import types from a package or from a specific interface.</li>
 </ul>

 <p>An interface can extend only one other interface (no multiple inheritance).
 Each interface in a package with a non-zero minor version number must extend an
 interface in the previous version of the package. For example, if an interface
 <code>IBar</code> in version 4.0 of package <code>derivative</code> is based on
 (extends) an interface <code>IFoo</code> in version 1.2 of package
 <code>original</code>, and a version 1.3 of package <code>original</code> is
 created, <code>IBar</code> version 4.1 cannot extend version 1.3 of
 <code>IFoo</code>. Instead, <code>IBar</code> version 4.1 must extend
 <code>IBar</code> version 4.0, which is tied to <code>IFoo</code> version 1.2.
 <code>IBar</code> version 5.0 could extend <code>IFoo</code> version 1.3, if
 desired.</p>

 <p>Interface extensions do not imply library dependence or cross-HAL inclusion
 in the generated code&mdash;they simply import the data structure and method
 definitions at the HIDL level. Every method in a HAL must be implemented in that
 HAL.</p>

 <h2 id=vendor-ext>Vendor extensions</h2>
 <p>In some cases, vendor extensions will be implemented as a subclass of the
 base object that represents the core interface they extend. The same object will
 be registered under the base HAL name and version, and under the extension's
 (vendor) HAL name and version.</p>


 <h2 id=version>Versioning</h2>
 <p>Packages are versioned, and interfaces have the version of their package.
 Versions are expressed in two integers, <em>major</em>.<em>minor</em>.</p>
 <ul>
 <li><strong>Major</strong> versions are not backwards compatible. Incrementing
 the major version number resets the minor version number to 0.</li>
 <li><strong>Minor</strong> versions are backwards compatible. Incrementing the
 minor number indicates the newer version is fully backward compatible with the
 previous version. New data structures and methods can be added, but no existing
 data structures or method signatures may be changed.</li>
 </ul>

 <p>Multiple major or minor versions of a HAL can be present on a device
 simultaneously. However, a minor version should be preferred over a major
 version because client code that works with a previous minor version interface
 will also work with later minor versions of that same interface. For more
 details on versioning and vendor extensions, see
 <a href="/devices/architecture/hidl/versioning">HIDL Versioning</a>.</p>

 <h2 id="interface-layout-summary">Interface layout summary</h2>
 <p>This section summarizes how to manage a HIDL interface package (such as
 <code>hardware/interfaces</code>) and consolidates information presented
 throughout the HIDL section. Before reading, ensure you are familiar with
 <a href="/devices/architecture/hidl/versioning">HIDL Versioning</a>, the hashing
 concepts in <a href="/devices/architecture/hidl/hashing#hidl-gen">Hashing with
 hidl-gen</a>, the details of <a href="/devices/architecture/hidl/">working with
 HIDL in general</a>, and the following definitions:</p>

 <table>
 <thead>
 <tr>
 <th width="30%">Term</th>
 <th>Definition</th>
 </tr>
 </thead>
 <tbody>
 <tr>
 <td>Application Binary Interface (ABI)</td>
 <td>Application programming interface + any binary linkages required.</td>
 </tr>
 <tr>
 <td>Fully-qualified name (fqName)</td>
 <td>Name to distinguish a hidl type. Example:
 <code>android.hardware.foo@1.0::IFoo</code>.</td>
 </tr>
 <tr>
 <td>Package</td>
 <td>Package containing a HIDL interface and types. Example:
 <code>android.hardware.foo@1.0</code>.</td>
 </tr>
 <tr>
 <td> Package root</td>
 <td>Root package that contains the HIDL interfaces. Example: the HIDL interface
 <code>android.hardware</code> is in the package root
 <code>android.hardware.foo@1.0</code>.</td>
 </tr>
 <tr>
 <td>Package root path</td>
 <td>Location in the Android source tree where a package root maps to.</td>
 </tr>
 </tbody>
 </table>

 <p>For more definitions, see HIDL
 <a href="/devices/architecture/hidl/#terms">Terminology</a>.
 </p>

 <h3 id="file-found">Every file can be found from the package root mapping and
 its fully-qualified name</h3>
 <p>Package roots are specified to <code>hidl-gen</code> as the argument
 <code>-r android.hardware:hardware/interfaces</code>. For example, if the
 package is <code>vendor.awesome.foo@1.0::IFoo</code> and <code>hidl-gen</code>
 is sent <code>-r vendor.awesome:some/device/independent/path/interfaces</code>,
 then the interface file should be located in
 <code>$ANDROID_BUILD_TOP/some/device/independent/path/interfaces/foo/1.0/IFoo.hal</code>.
 </p>
 <p>In practice, it is recommended for a vendor or OEM named <code>awesome</code>
 to put their standard interfaces in <code>vendor.awesome</code>. After a package
 path has been selected, it must not be changed as this is baked into the ABI of
 the interface.</p>

 <h3 id="package-path-mapping">Package path mapping should be unique</h3>
 <p>For example, if you have <code>-rsome.package:$PATH_A</code> and
 <code>-rsome.package:$PATH_B</code>, <code>$PATH_A</code> must be equal to
 <code>$PATH_B</code> for a consistent interface directory (this also makes
 <a href="/devices/architecture/hidl/versioning">versioning interfaces</a> much
 easier).</p>

 <h3 id="package-root-version">Package root must have a versioning file</h3>
 <p>If you create a package path such as
 <code>-r vendor.awesome:vendor/awesome/interfaces</code>, you should also
 create the file
 <code>$ANDROID_BUILD_TOP/vendor/awesome/interfaces/current.txt</code>, which
 should contain hashes of interfaces made using the <code>-Lhash</code> option in
 <code>hidl-gen</code> (this is discussed extensively in
 <a href="/devices/architecture/hidl/hashing#hidl-gen">Hashing with
 hidl-gen</a>).</p>

 <aside class="caution"><strong>Caution:</strong> Manage all changes carefully!
 The <a href="/devices/tech/vts/">Vendor Test Suite (VTS)</a> will fail if an
 interface is not frozen, and ABI-incompatible changes to an interface will cause
 framework-only OTAs to fail.</aside>

 <h3 id="interfaces-device-dependent">Interfaces go in device-independent
 locations</h3>
 <p>In practice, it is recommended to share interfaces between branches. This
 allows for maximum code re-usage and maximum testing of code across different
 devices and use cases.</p>

   </body>
 </html>
	<html devsite>
	<head>
	<title>Interfaces & Packages</title>
	<meta name="project_path" value="/_project.yaml" />
	<meta name="book_path" value="/_book.yaml" />
	</head>
	<body>
	<!--
	Copyright 2017 The Android Open Source Project

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->


	<p>HIDL is built around interfaces, an abstract type used in object-oriented
	languages to define behaviors. Each interface is part of a package.</p>

	<h2 id=packages>Packages</h2>

	<p>Package names can have sublevels such as <code>package.subpackage</code>. The
	root directory for published HIDL packages is <code>hardware/interfaces</code>
	or <code>vendor/vendorName</code> (e.g. <code>vendor/google</code> for Pixel
	devices). The package name forms one or more subdirectories under the root
	directory; all files defining a package are in the same directory. For example,
	<code>package android.hardware.example.extension.light@2.0</code> could be found
	under <code>hardware/interfaces/example/extension/light/2.0</code>.</p>

	<p>The following table lists package prefixes and locations:</p>
	<table>
	<tbody>

	<tr>
	<th>Package Prefix</th>
	<th>Location</th>
	</tr>

	<tr>
	<td><code>android.hardware.*</code></td>
	<td><code>hardware/interfaces/*</code></td>
	</tr>

	<tr>
	<td><code>android.frameworks.*</code></td>
	<td><code>frameworks/hardware/interfaces/*</code></td>
	</tr>

	<tr>
	<td><code>android.system.*</code></td>
	<td><code>system/hardware/interfaces/*</code></td>
	</tr>

	<tr>
	<td><code>android.hidl.*</code></td>
	<td><code>system/libhidl/transport/*</code></td>
	</tr>

	</tbody>
	</table>

	<p>The package directory contains files with extension <code>.hal</code>. Every
	file must contain a <code>package</code> statement naming the package and
	version the file is part of. The file <code>types.hal</code>, if present, does
	not define an interface but instead defines data types accessible to every
	interface in the package.</p>

	<h2 id=interface-def>Interface definition</h2>
	<p>Aside from <code>types.hal</code>, every other <code>.hal</code> file defines
	an interface. An interface is typically defined as follows:</p>

	<pre class="prettyprint">
	interface IBar extends IFoo { // IFoo is another interface
	// embedded types
	struct MyStruct {/.../};

	// interface methods
	create(int32_t id) generates (MyStruct s);
	close();
	};
	</pre>

	<p>An interface without an explicit <code>extends</code> declaration implicitly
	extends from <code>android.hidl.base@1.0::IBase</code> (similar to
	<code>java.lang.Object</code> in Java.) The IBase interface, implicitly
	imported, declares several reserved methods that should not and cannot be
	redeclared in user-defined interfaces or used otherwise. These methods
	include:</p>

	<ul>
	<li><code>ping</code></li>
	<li><code>interfaceChain</code></li>
	<li><code>interfaceDescriptor</code></li>
	<li><code>notifySyspropsChanged</code></li>
	<li><code>linkToDeath</code></li>
	<li><code>unlinkToDeath</code></li>
	<li><code>setHALInstrumentation</code></li>
	<li><code>getDebugInfo</code></li>
	<li><code>debug</code></li>
	<li><code>getHashChain</code></li>
	</ul>

	<h2 id=import>Importing</h2>
	<p>The <code>import</code> statement is HIDL mechanism to access package
	interfaces and types in another package. An <code>import</code> statement
	concerns itself with two entities:</p>

	<ul>
	<li>The import<em>ing</em> entity, which can be either a package or an
	interface; and</li>
	<li>The import<em>ed</em> entity, which too can be either a package or an
	interface.</li>
	</ul>

	<p>The importing entity is determined by the location of the
	<code>import</code> statement. When the statement is inside a package's
	<code>types.hal</code>, what is being imported is visible by the entire package;
	this is a <em>package-level</em> import. When the statement is inside an
	interface file, the importing entity is the interface itself; this is an
	<em>interface-level</em> import.</p>

	<p>The imported entity is determined by the value after the <code>import</code>
	keyword. The value need not be a fully-qualified name; if a component is
	omitted, it is automatically filled with information from the current package.
	For fully-qualified values, the following import cases are supported:</p>

	<ul>
	<li><strong>Whole-package imports</strong>. If the value is a package name and a
	version (syntax described below), then the entire package is imported into the
	importing entity.</li>
	<li><strong>Partial imports</strong>. If the value is:
	<ul>
	<li>An interface, the package's <code>types.hal</code> and that interface are
	imported into the importing entity.</li>
	<li>A UDT defined in <code>types.hal</code>, then only that UDT is imported into
	the importing entity (other types in <code>types.hal</code> are not imported).
	</li>
	</ul>
	<li><strong>Types-only imports</strong>. If the value uses the syntax of a
	partial import described above, but with the keyword <code>types</code> instead
	of an Interface name, only the UDTs in <code>types.hal</code> of the designated
	package are imported.</li>
	</ul>

	<p>The importing entity gets access to a combination of:</p>
	<ul>
	<li>The imported package's common UDTs defined in <code>types.hal</code>;</li>
	<li>The imported package's interfaces (for a whole-package import) or specified
	interface (for a partial import) for the purposes of invoking them, passing
	handles to them and/or inheriting from them.</li>
	</ul>

	<p>The import statement uses the fully-qualified-type-name syntax to provide the
	name and version of the package or interface being imported:</p>

	<pre class="prettyprint">
	import android.hardware.nfc@1.0; // import a whole package
	import android.hardware.example@1.0::IQuux; // import an interface and types.hal
	import android.hardware.example@1.0::types; // import just types.hal
	</pre>

	<h2 id=inheritance>Interface inheritance</h2>

	<p>An interface can be an extension of a previously-defined interface.
	Extensions can be one of the following three types:</p>
	<ul>
	<li>Interface can add functionality to another one, incorporating its API
	unchanged.</li>
	<li>Package can add functionality to another one, incorporating its API
	unchanged.</li>
	<li>Interface can import types from a package or from a specific interface.</li>
	</ul>

	<p>An interface can extend only one other interface (no multiple inheritance).
	Each interface in a package with a non-zero minor version number must extend an
	interface in the previous version of the package. For example, if an interface
	<code>IBar</code> in version 4.0 of package <code>derivative</code> is based on
	(extends) an interface <code>IFoo</code> in version 1.2 of package
	<code>original</code>, and a version 1.3 of package <code>original</code> is
	created, <code>IBar</code> version 4.1 cannot extend version 1.3 of
	<code>IFoo</code>. Instead, <code>IBar</code> version 4.1 must extend
	<code>IBar</code> version 4.0, which is tied to <code>IFoo</code> version 1.2.
	<code>IBar</code> version 5.0 could extend <code>IFoo</code> version 1.3, if
	desired.</p>

	<p>Interface extensions do not imply library dependence or cross-HAL inclusion
	in the generated code—they simply import the data structure and method
	definitions at the HIDL level. Every method in a HAL must be implemented in that
	HAL.</p>

	<h2 id=vendor-ext>Vendor extensions</h2>
	<p>In some cases, vendor extensions will be implemented as a subclass of the
	base object that represents the core interface they extend. The same object will
	be registered under the base HAL name and version, and under the extension's
	(vendor) HAL name and version.</p>


	<h2 id=version>Versioning</h2>
	<p>Packages are versioned, and interfaces have the version of their package.
	Versions are expressed in two integers, <em>major</em>.<em>minor</em>.</p>
	<ul>
	<li><strong>Major</strong> versions are not backwards compatible. Incrementing
	the major version number resets the minor version number to 0.</li>
	<li><strong>Minor</strong> versions are backwards compatible. Incrementing the
	minor number indicates the newer version is fully backward compatible with the
	previous version. New data structures and methods can be added, but no existing
	data structures or method signatures may be changed.</li>
	</ul>

	<p>Multiple major or minor versions of a HAL can be present on a device
	simultaneously. However, a minor version should be preferred over a major
	version because client code that works with a previous minor version interface
	will also work with later minor versions of that same interface. For more
	details on versioning and vendor extensions, see
	<a href="/devices/architecture/hidl/versioning">HIDL Versioning</a>.</p>

	<h2 id="interface-layout-summary">Interface layout summary</h2>
	<p>This section summarizes how to manage a HIDL interface package (such as
	<code>hardware/interfaces</code>) and consolidates information presented
	throughout the HIDL section. Before reading, ensure you are familiar with
	<a href="/devices/architecture/hidl/versioning">HIDL Versioning</a>, the hashing
	concepts in <a href="/devices/architecture/hidl/hashing#hidl-gen">Hashing with
	hidl-gen</a>, the details of <a href="/devices/architecture/hidl/">working with
	HIDL in general</a>, and the following definitions:</p>

	<table>
	<thead>
	<tr>
	<th width="30%">Term</th>
	<th>Definition</th>
	</tr>
	</thead>
	<tbody>
	<tr>
	<td>Application Binary Interface (ABI)</td>
	<td>Application programming interface + any binary linkages required.</td>
	</tr>
	<tr>
	<td>Fully-qualified name (fqName)</td>
	<td>Name to distinguish a hidl type. Example:
	<code>android.hardware.foo@1.0::IFoo</code>.</td>
	</tr>
	<tr>
	<td>Package</td>
	<td>Package containing a HIDL interface and types. Example:
	<code>android.hardware.foo@1.0</code>.</td>
	</tr>
	<tr>
	<td> Package root</td>
	<td>Root package that contains the HIDL interfaces. Example: the HIDL interface
	<code>android.hardware</code> is in the package root
	<code>android.hardware.foo@1.0</code>.</td>
	</tr>
	<tr>
	<td>Package root path</td>
	<td>Location in the Android source tree where a package root maps to.</td>
	</tr>
	</tbody>
	</table>

	<p>For more definitions, see HIDL
	<a href="/devices/architecture/hidl/#terms">Terminology</a>.
	</p>

	<h3 id="file-found">Every file can be found from the package root mapping and
	its fully-qualified name</h3>
	<p>Package roots are specified to <code>hidl-gen</code> as the argument
	<code>-r android.hardware:hardware/interfaces</code>. For example, if the
	package is <code>vendor.awesome.foo@1.0::IFoo</code> and <code>hidl-gen</code>
	is sent <code>-r vendor.awesome:some/device/independent/path/interfaces</code>,
	then the interface file should be located in
	<code>$ANDROID_BUILD_TOP/some/device/independent/path/interfaces/foo/1.0/IFoo.hal</code>.
	</p>
	<p>In practice, it is recommended for a vendor or OEM named <code>awesome</code>
	to put their standard interfaces in <code>vendor.awesome</code>. After a package
	path has been selected, it must not be changed as this is baked into the ABI of
	the interface.</p>

	<h3 id="package-path-mapping">Package path mapping should be unique</h3>
	<p>For example, if you have <code>-rsome.package:$PATH_A</code> and
	<code>-rsome.package:$PATH_B</code>, <code>$PATH_A</code> must be equal to
	<code>$PATH_B</code> for a consistent interface directory (this also makes
	<a href="/devices/architecture/hidl/versioning">versioning interfaces</a> much
	easier).</p>

	<h3 id="package-root-version">Package root must have a versioning file</h3>
	<p>If you create a package path such as
	<code>-r vendor.awesome:vendor/awesome/interfaces</code>, you should also
	create the file
	<code>$ANDROID_BUILD_TOP/vendor/awesome/interfaces/current.txt</code>, which
	should contain hashes of interfaces made using the <code>-Lhash</code> option in
	<code>hidl-gen</code> (this is discussed extensively in
	<a href="/devices/architecture/hidl/hashing#hidl-gen">Hashing with
	hidl-gen</a>).</p>

	<aside class="caution"><strong>Caution:</strong> Manage all changes carefully!
	The <a href="/devices/tech/vts/">Vendor Test Suite (VTS)</a> will fail if an
	interface is not frozen, and ABI-incompatible changes to an interface will cause
	framework-only OTAs to fail.</aside>

	<h3 id="interfaces-device-dependent">Interfaces go in device-independent
	locations</h3>
	<p>In practice, it is recommended to share interfaces between branches. This
	allows for maximum code re-usage and maximum testing of code across different
	devices and use cases.</p>

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