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<chapter>
  <header>
    <copyright>
      <year>2009</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
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      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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    <title>Drivers</title>
    <prepared>Bjorn Gustavsson</prepared>
    <docno></docno>
    <date>2009-11-16</date>
    <rev></rev>
    <file>drivers.xml</file>
  </header>

  <p>This section provides a brief overview on how to write efficient
  drivers.</p>
  <p>It is assumed that you have a good understanding of drivers.</p>

  <section>
    <title>Drivers and Concurrency</title>

    <p>The runtime system always takes a lock before running
    any code in a driver.</p>

    <p>By default, that lock is at the driver level, that is,
    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 have one lock for each port instead.</p>

    <p>If a driver is used in a functional way (that is, 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 as follows:</p>

    <code type="none">
-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()).</code>

    <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>	    

  </section>

  <section>
    <title>Avoiding Copying Binaries When Calling a Driver</title>

    <p>There are basically two ways to avoid copying a binary that is
    sent to a driver:</p>

    <list type="bulleted">
      <item><p>If the <c>Data</c> argument for
      <seealso marker="erts:erlang#port_control/3">port_control/3</seealso>
      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 <c>Data</c>
      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
      extra data to a driver without copying the binary, you must call
      <c>port_control/3</c> twice; once with the binary and once with the
      extra data. However, that will only work if there is only one
      process communicating with the port (because otherwise another process
      can call the driver in-between the calls).</p></item>

      <item><p>Implement an <c>outputv</c> callback (instead of an
      <c>output</c> callback) in the driver. If a driver has an
      <c>outputv</c> callback, refc binaries passed in an iolist
      in the <c>Data</c> argument for
      <seealso marker="erts:erlang#port_command/2">port_command/2</seealso>
      will be passed as references to the driver.</p></item>
    </list>
  </section>

  <section>
    <title>Returning Small Binaries from a Driver</title>

    <p>The runtime system can represent binaries up to 64 bytes as
    heap binaries. They are always copied when sent in messages,
    but they 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
    are advised to use driver API calls that do not require a pre-allocated
    binary, for example,
    <seealso marker="erts:erl_driver#driver_output">driver_output()</seealso>
    or
    <seealso marker="erts:erl_driver#erl_drv_output_term">erl_drv_output_term()</seealso>,
    using the <c>ERL_DRV_BUF2BINARY</c> format,
    to allow the runtime to construct a heap binary.</p>

  </section>

  <section>
    <title>Returning Large Binaries without Copying from a Driver</title>

    <p>To avoid copying data when a large 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
    <seealso marker="erts:erl_driver#driver_alloc_binary">driver_alloc_binary()</seealso>
    to allocate a binary.</p>

    <p>There are several ways to send a binary created with
    <c>driver_alloc_binary()</c>:</p>

    <list type="bulleted">
    <item>From the <c>control</c> callback, a binary can be returned if
    <seealso marker="erts:erl_driver#set_port_control_flags">set_port_control_flags()</seealso>
    has been called with the flag value <c>PORT_CONTROL_FLAG_BINARY</c>.</item>

    <item>A single binary can be sent with
    <seealso marker="erts:erl_driver#driver_output_binary">driver_output_binary()</seealso>.</item>

    <item>Using
    <seealso marker="erts:erl_driver#erl_drv_output_term">erl_drv_output_term()</seealso>
    or
    <seealso marker="erts:erl_driver#erl_drv_send_term">erl_drv_send_term()</seealso>,
    a binary can be included in an Erlang term.</item>
    </list>

  </section>
</chapter>
