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<chapter>
  <header>
    <copyright>
      <year>2000</year><year>2012</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>Port drivers</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>c_portdriver.xml</file>
  </header>
  <p>This is an example of how to solve the <seealso marker="example">example problem</seealso> by using a linked in port driver.</p>
  <image file="../tutorial/port_driver.gif">
    <icaption>Port Driver Communication.</icaption>
  </image>

  <section>
    <title>Port Drivers</title>
    <p>A port driver is a linked in driver, that is accessible as a
      port from an Erlang program. It is a shared library (SO in Unix,
      DLL in Windows), with special entry points. The Erlang runtime
      calls these entry points, when the driver is started and when
      data is sent to the port. The port driver can also send data to
      Erlang.</p>
    <p>Since a port driver is dynamically linked into the emulator
      process, this is the fastest way of calling C-code from Erlang.
      Calling functions in the port driver requires no context
      switches. But it is also the least safe, because a crash in the
      port driver brings the emulator down too.</p>
  </section>

  <section>
    <title>Erlang Program</title>
    <p>Just as with a port program, the port communicates with a Erlang
      process. All communication goes through one Erlang process that
      is the <em>connected process</em> of the port
      driver. Terminating this process closes the port driver.</p>
    <p>Before the port is created, the driver must be loaded. This is
      done with the function <c>erl_dll:load_driver/1</c>, with the
      name of the shared library as argument.</p>
    <p>The port is then created using the BIF <c>open_port/2</c> with
      the tuple <c>{spawn, DriverName}</c> as the first argument. The
      string <c>SharedLib</c> is the name of the port driver. The second
      argument is a list of options, none in this case.</p>
    <pre>
-module(complex5).
-export([start/1, init/1]).

start(SharedLib) ->
    case erl_ddll:load_driver(".", SharedLib) of
        ok -> ok;
        {error, already_loaded} -> ok;
        _ -> exit({error, could_not_load_driver})
    end,
    spawn(?MODULE, init, [SharedLib]).

init(SharedLib) ->
  register(complex, self()),
  Port = open_port({spawn, SharedLib}, []),
  loop(Port).</pre>
    <p>Now it is possible to implement <c>complex5:foo/1</c> and
      <c>complex5:bar/1</c>. They both send a message to the
      <c>complex</c> process and receive the reply.</p>
    <pre>
foo(X) ->
    call_port({foo, X}).
bar(Y) ->
    call_port({bar, Y}).

call_port(Msg) ->
    complex ! {call, self(), Msg},
    receive
        {complex, Result} ->
            Result
    end.</pre>
    <p>The <c>complex</c> process encodes the message into a sequence
      of bytes, sends it to the port, waits for a reply, decodes the
      reply and sends it back to the caller.
      </p>
    <pre>
loop(Port) ->
    receive
        {call, Caller, Msg} ->
            Port ! {self(), {command, encode(Msg)}},
            receive
                {Port, {data, Data}} ->
                    Caller ! {complex, decode(Data)}
            end,
            loop(Port)
    end.</pre>
    <p>Assuming that both the arguments and the results from the C
      functions will be less than 256, a very simple encoding/decoding
      scheme is employed where <c>foo</c> is represented by the byte
      1, <c>bar</c> is represented by 2, and the argument/result is
      represented by a single byte as well.
      </p>
    <pre>
encode({foo, X}) -> [1, X];
encode({bar, Y}) -> [2, Y].
      
decode([Int]) -> Int.</pre>
    <p>The resulting Erlang program, including functionality for
      stopping the port and detecting port failures is shown below.</p>
      <codeinclude file="complex5.erl" type="erl"/>
  </section>

  <section>
    <title>C Driver</title>
    <p>The C driver is a module that is compiled and linked into a
      shared library. It uses a driver structure, and includes the
      header file <c>erl_driver.h</c>.</p>
    <p>The driver structure is filled with the driver name and function
      pointers. It is returned from the special entry point, declared
      with the macro <c><![CDATA[DRIVER_INIT(<driver_name>)]]></c>.</p>
    <p>The functions for receiving and sending data, are combined into
      a function, pointed out by the driver structure. The data sent
      into the port is given as arguments, and the data the port
      sends back is sent with the C-function <c>driver_output</c>.</p>
    <p>Since the driver is a shared module, not a program, no main
      function should be present. All function pointers are not used
      in our example, and the corresponding fields in the
      <c>driver_entry</c> structure are set to NULL.</p>
    <p>All functions in the driver, takes a handle (returned from
      <c>start</c>), that is just passed along by the erlang
      process. This must in some way refer to the port driver
      instance.</p>
    <p>The example_drv_start, is the only function that is called with
      a handle to the port instance, so we must save this. It is
      customary to use a allocated driver-defined structure for this
      one, and pass a pointer back as a reference.</p>
    <p>It is not a good idea to use a global variable; since the port
      driver can be spawned by multiple Erlang processes, this
      driver-structure should be instantiated multiple times.
      </p>
    <codeinclude file="port_driver.c" tag="" type="none"></codeinclude>
  </section>

  <section>
    <title>Running the Example</title>
    <p>1. Compile the C code.</p>
    <pre>
unix> <input>gcc -o exampledrv -fpic -shared complex.c port_driver.c</input>
windows> <input>cl -LD -MD -Fe exampledrv.dll complex.c port_driver.c</input></pre>
    <p>2. Start Erlang and compile the Erlang code.</p>
    <pre>
> <input>erl</input>
Erlang (BEAM) emulator version 5.1

Eshell V5.1 (abort with ^G)
1> <input>c(complex5).</input>
{ok,complex5}</pre>
    <p>3. Run the example.</p>
    <pre>
2> <input>complex5:start("example_drv").</input>
&lt;0.34.0>
3> <input>complex5:foo(3).</input>
4
4> <input>complex5:bar(5).</input>
10
5> <input>complex5:stop().</input>
stop</pre>
  </section>
</chapter>

