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<img alt="Erlang logo" src="../erlang-logo.png"><br><small><a href="users_guide.html">User's Guide</a><br><a href="../pdf/otp-system-documentation-5.9.3.1.pdf">PDF</a><br><a href="../index.html">Top</a></small><p><strong>Efficiency Guide</strong><br><strong>User's Guide</strong><br><small>Version 5.9.3.1</small></p>
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<li title="Returning small binaries from a driver"><a href="drivers.html#id68743">Returning small binaries from a driver</a></li>
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<h1>9 Drivers</h1>
<p>This chapter provides a (very) brief overview on how to write efficient
drivers. It is assumed that you already have a good understanding of
drivers.</p>
<h3><a name="id68634">9.1
Drivers and concurrency</a></h3>
<p>The run-time system will always take a lock before running
any code in a driver.</p>
<p>By default, that lock will be at the driver level, meaning that
if several ports have been opened to the same driver, only code for
one port at the same time can be running.</p>
<p>A driver can be configured to instead have one lock for each port.</p>
<p>If a driver is used in a functional way (i.e. it holds no state,
but only does some heavy calculation and returns a result), several
ports with registered names can be opened beforehand and the port to
be used can be chosen based on the scheduler ID like this:</p>
<div class="example"><pre>
-define(PORT_NAMES(),
{some_driver_01, some_driver_02, some_driver_03, some_driver_04,
some_driver_05, some_driver_06, some_driver_07, some_driver_08,
some_driver_09, some_driver_10, some_driver_11, some_driver_12,
some_driver_13, some_driver_14, some_driver_15, some_driver_16}).
client_port() ->
element(erlang:system_info(scheduler_id) rem tuple_size(?PORT_NAMES()) + 1,
?PORT_NAMES()).</pre></div>
<p>As long as there are no more than 16 schedulers, there will never
be any lock contention on the port lock for the driver.</p>
<h3><a name="id68675">9.2
Avoiding copying of binaries when calling a driver</a></h3>
<p>There are basically two ways to avoid copying a binary that is
sent to a driver.</p>
<p>If the <span class="code">Data</span> argument for
<span class="bold_code"><a href="javascript:erlhref('../../','erts','erlang.html#port_control-3');">port_control/3</a></span>
is a binary, the driver will be passed a pointer to the contents of
the binary and the binary will not be copied.
If the <span class="code">Data</span> argument is an iolist (list of binaries and lists),
all binaries in the iolist will be copied.</p>
<p>Therefore, if you want to send both a pre-existing binary and some
additional data to a driver without copying the binary, you must call
<span class="code">port_control/3</span> twice; once with the binary and once with the
additional data. However, that will only work if there is only one
process communicating with the port (because otherwise another process
could call the driver in-between the calls).</p>
<p>Another way to avoid copying binaries is to implement an <span class="code">outputv</span>
callback (instead of an <span class="code">output</span> callback) in the driver.
If a driver has an <span class="code">outputv</span> callback, refc binaries passed
in an iolist in the <span class="code">Data</span> argument for
<span class="bold_code"><a href="javascript:erlhref('../../','erts','erlang.html#port_command-2');">port_command/2</a></span>
will be passed as references to the driver.</p>
<h3><a name="id68743">9.3
Returning small binaries from a driver</a></h3>
<p>The run-time system can represent binaries up to 64 bytes as
heap binaries. They will always be copied when sent in a messages,
but they will require less memory if they are not sent to another
process and garbage collection is cheaper.</p>
<p>If you know that the binaries you return are always small,
you should use driver API calls that do not require a pre-allocated
binary, for instance
<span class="bold_code"><a href="javascript:erlhref('../../','erts','erl_driver.html#int%20driver_output-3');">driver_output()</a></span>
or
<span class="bold_code"><a href="javascript:erlhref('../../','erts','erl_driver.html#int%20driver_output_term-3');">driver_output_term()</a></span>
using the <span class="code">ERL_DRV_BUF2BINARY</span> format,
to allow the run-time to construct a heap binary.</p>
<h3><a name="id68777">9.4
Returning big binaries without copying from a driver</a></h3>
<p>To avoid copying data when a big binary is sent or returned from
the driver to an Erlang process, the driver must first allocate the
binary and then send it to an Erlang process in some way.</p>
<p>Use <span class="bold_code"><a href="javascript:erlhref('../../','erts','erl_driver.html#ErlDrvBinary*%20driver_alloc_binary-1');">driver_alloc_binary()</a></span> to allocate a binary.</p>
<p>There are several ways to send a binary created with
<span class="code">driver_alloc_binary()</span>.</p>
<ul>
<li>
<p>From the <span class="code">control</span> callback, a binary can be returned provided
that
<span class="bold_code"><a href="javascript:erlhref('../../','erts','erl_driver.html#void%20set_port_control_flags-2');">set_port_control()</a></span>
has been called with the flag value <span class="code">PORT_CONTROL_FLAG_BINARY</span>.</p>
</li>
<li><p>A single binary can be sent with
<span class="bold_code"><a href="javascript:erlhref('../../','erts','erl_driver.html#int%20driver_output_binary-6');">driver_output_binary()</a></span>.</p></li>
<li>
<p>Using
<span class="bold_code"><a href="javascript:erlhref('../../','erts','erl_driver.html#int%20driver_output_term-3');">driver_output_term()</a></span>
or
<span class="bold_code"><a href="javascript:erlhref('../../','erts','erl_driver.html#int%20driver_send_term-4');">driver_send_term()</a></span>,
a binary can be included in an Erlang term.</p>
</li>
</ul>
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