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<html devsite>
<head>
<title>Converting HAL Modules</title>
<meta name="project_path" value="/_project.yaml" />
<meta name="book_path" value="/_book.yaml" />
</head>
<body>
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<p>You can update pre-existing HAL modules to HIDL HAL modules by converting the
header in <code>hardware/libhardware/include/hardware</code>.</p>
<h2 id=c2hal>Using c2hal</h2>
<p>The
<code><a href="https://android.googlesource.com/platform/system/tools/hidl/+/master/c2hal/">c2hal</code></a>
tool handles most of the conversion work, reducing the number of required manual
changes. For example, to generate a HIDL <code>.hal</code> file for the NFC
HAL:</p>
<pre class="devsite-click-to-copy">
<code class="devsite-terminal">make c2hal</code>
c2hal -r android.hardware:hardware/interfaces -randroid.hidl:system/libhidl/transport -p android.hardware.nfc@1.0 hardware/libhardware/include/hardware/nfc.h
</pre>
<p>These commands add files in <code>hardware/interfaces/nfc/1.0/</code>. Running
<code>hardware/interfaces/update-makefiles.sh</code> from the $ANDROID_BUILD_TOP
directory will also add the required makefile to the HAL. From here, you can
make manual changes to fully convert the HAL.</p>
<h2 id=activities>c2hal activities</h2>
<p>When you run <code>c2hal</code>, everything in the header file is transferred
to <code>.hal</code> files.</p>
<p><code>c2hal</code> identifies structs that contain function pointers in the
provided header file and converts each struct into a separate interface file.
For example, <code>alloc_device_t</code> is converted to the
<code>IAllocDevice</code> HAL module (in the file
<code>IAllocDevice.hal</code>).</p>
<p>All other data types are copied over into a <code>types.hal</code> file.
Pound-defines are moved into enums, and items not a part of HIDL or not
convertible (such as static-function declarations) are copied into comments
marked with the text "<code>NOTE</code>".</p>
<h2 id=manual>Manual activities</h2>
<p>The <code>c2hal</code> tool does not know what to do when it encounters
certain constructs. For example, HIDL has no concept of raw pointers; because of
this, when <code>c2hal</code> encounters a pointer in header files, it doesn't
know whether the pointer should be interpreted as an array or as a reference to
another object. Void pointers are also similarly opaque.</p>
<p>Field such as <code>int reserved[7]</code> must be manually removed during
the transition to HIDL. Items such as the name of the return value should be
updated to something more meaningful; for example, converting the return
parameter of methods such as <code>write</code> in NFC from the autogenerated
<code>int32_t write_ret</code> to <code>Status status</code> (where
<code>Status</code> is a new enum containing possible NFC statuses).</p>
<h2 id=implement>Implementing the HAL</h2>
<p>After you have created <code>.hal</code> files to represent your HAL, you
must generate the makefiles (Make or Soong) that create the language support in
C++ and Java (unless the HAL uses a feature unsupported in Java). The
<code>./hardware/interfaces/update-makefiles.sh</code> script can automatically
generate makefiles for HALs located in the <code>hardware/interfaces</code>
directory (for HALs in other locations, simply update the script).</p>
<p>When the makefiles are up to date, you are ready to generate header files and
implement methods. For details on implementing the generated interface, see
<a href="/devices/architecture/hidl-cpp/index.html">HIDL C++</a> (for C++
implementations) or <a href="/devices/architecture/hidl-java/index.html">HIDL
Java</a> (for Java implementations).</p>
</body>
</html>
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