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<html devsite>
<head>
<title>VNDK Extensions</title>
<meta name="project_path" value="/_project.yaml" />
<meta name="book_path" value="/_book.yaml" />
</head>
<body>
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<p>Android device manufacturers change the source code of AOSP libraries for
various reasons. Some vendors reimplement functions in AOSP libraries to
boost the performance while other vendors add new hooks, new APIs, or new
functionalities to AOSP libraries. This section provides guidelines for
extending AOSP libraries in a way that does not break CTS/VTS.</p>
<h2 id="drop-in-replacement">Drop-in replacement</h2>
<p>All modified shared libraries must be <strong>binary-compatible</strong>,
<strong>drop-in replacements</strong> of their AOSP counterpart. All existing
AOSP users must be able to use the modified shared library without
recompilations. This requirement implies the following:</p>
<ul>
<li>AOSP functions must not be removed.</li>
<li>Structures must not be altered if such structures are exposed to their
users.</li>
<li>Pre-condition of functions must not be strengthened.</li>
<li>Functions must provide equivalent functionalities.</li>
<li>Post-condition of functions must not be weakened.</li>
</ul>
<h2 id="extended-module-classifications">Extended module classifications</h2>
<p>Classify modules by the functionalities they <strong>define</strong> and
<strong>use</strong>.</p>
<p class="note"><strong>Note</strong>: <em>Functionalities</em> is used here
instead of API/ABI because it is possible to add functionality without changing
any API/ABI.</p>
<p>Depending on the functionalities defined in a module, modules can be
classified into <strong>DA-Module</strong> and <strong>DX-Module</strong>:</p>
<ul>
<li><em>Defining-only-AOSP Modules (DA-Module)</em> do not define new
functionalities which were not in the AOSP counterpart.
<ul>
<li><em>Example 1.</em> An intact unmodified AOSP library is a DA-Module.</li>
<li><em>Example 2.</em> If a vendor rewrites the functions in
<code>libcrypto.so</code> with SIMD instructions (without adding new
functions), then the modified <code>libcrypto.so</code> will be a DA-Module.
</li>
</ul>
</li>
<li><em>Defining-Extension Modules (DX-Module)</em> either define new
functionalities or do not have an AOSP counterpart.
<ul>
<li><em>Example 1.</em> If a vendor adds a helper function to
<code>libjpeg.so</code> to access some internal data, then the modified
<code>libjpeg.so</code> will be a DX-Lib and the newly added function will be
the extended portion of the library.</li>
<li><em>Example 2.</em> If a vendor defines a non-AOSP library named
<code>libfoo.so</code>, then <code>libfoo.so</code> will be a DX-Lib.</li>
</ul>
</li>
</ul>
<p>Depending on the functionalities used by a module, modules can be classified
into <strong>UA-Module</strong> and <strong>UX-Module</strong>.</p>
<ul>
<li><em>Using-only-AOSP Modules (UA-Module)</em> only use AOSP functionalities
in their implementations. They do not rely on any non-AOSP extensions.
<ul>
<li><em>Example 1.</em> An intact unmodified AOSP library is an UA-Module.</li>
<li><em>Example 2.</em> If a modified shared library <code>libjpeg.so</code>
only relies on other AOSP APIs, then it will be an UA-Module.</li>
</ul>
</li>
<li><em>Using-Extension Modules (UX-Module)</em> rely on some non-AOSP
functionalities in their implementations.
<ul>
<li><em>Example 1.</em> If a modified <code>libjpeg.so</code> relies on another
non-AOSP library named <code>libjpeg_turbo2.so</code>, then the modified
<code>libjpeg.so</code> will be an UX-Module.</li>
<li><em>Example 2.</em> If a vendor adds a new function to their modified
<code>libexif.so</code> and their modified <code>libjpeg.so</code> uses the
newly added function from <code>libexif.so</code>, then their modified
<code>libjpeg.so</code> will be an UX-Module.</li>
</ul>
</li>
</ul>
<p>Definitions and usages are independent from each other:</p>
<table>
<tr>
<td rowspan="2" colspan="2" class="columns"></td>
<th colspan="2">Used Functionalities</th>
</tr>
<tr>
<td>Only AOSP (UA)</td>
<td>Extended (UX)</td>
</tr>
<tr>
<th rowspan="2">Defined Functionalities</th>
<td>Only AOSP (DA)</td>
<td>DAUA</td>
<td>DAUX</td>
</tr>
<tr>
<td>Extended (DX)</td>
<td>DXUA</td>
<td>DXUX</td>
</tr>
</table>
<h2 id="vndk-extension-mechanism">VNDK extension mechanism</h2>
<p>
Vendor modules that rely on extended functionalities won't work because the AOSP
library with the same name does not have the extended functionality. If vendor
modules directly or indirectly depend on extended functionalities, vendors
should copy DAUX, DXUA, and DXUX shared libraries to the vendor partition
(vendor processes always look for shared libraries in the vendor partition
first). However, LL-NDK and SP-NDK libraries must not be copied, so vendor
modules must not rely on the extended functionalities defined by the modified
LL-NDK and SP-NDK libraries.
</p>
<p>
DAUA shared libraries can remain on the system partition if the corresponding
AOSP library can provide the same functionality and vendor modules continue to
work when the system partition is overwritten by an AOSP system image.
</p>
<p>
Drop-in replacement is important because the unmodified VNDK libraries in the
AOSP system image will link with the modified shared libraries on name
collision. If the AOSP libraries are modified in an API/ABI incompatible manner,
the AOSP libraries in the AOSP system image might fail to link or result in
undefined behaviors.
</p>
</body>
</html>
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