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<chapter>
<header>
<copyright>
<year>2000</year><year>2017</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
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<title>Port Drivers</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>c_portdriver.xml</file>
</header>
<p>This section outlines an example of how to solve the example problem
in <seealso marker="example">Problem Example</seealso>
by using a linked-in port driver.</p>
<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 system
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>As 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 way, because a crash in the port driver
brings the emulator down too.</p>
<p>The scenario is illustrated in the following figure:</p>
<image file="../tutorial/port_driver.gif">
<icaption>Port Driver Communication</icaption>
</image>
<section>
<title>Erlang Program</title>
<p>Like a port program, the port communicates with an 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 <c>complex5:foo/1</c> and <c>complex5:bar/1</c>
can be implemented. Both send a message to the
<c>complex</c> process and receive the following 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 performs the following:</p>
<list type="bulleted">
<item>Encodes the message into a sequence of bytes.</item>
<item>Sends it to the port.</item>
<item>Waits for a reply.</item>
<item>Decodes the reply.</item>
<item>Sends it back to the caller:</item>
</list>
<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 are less than 256, a simple encoding/decoding scheme
is employed. In this scheme, <c>foo</c> is represented by 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 functions for stopping
the port and detecting port failures, is as follows:</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 replied data is sent
with the C-function <c>driver_output</c>.</p>
<p>As the driver is a shared module, not a program, no main
function is present. All function pointers are not used
in this 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 <c>example_drv_start</c>, is the only function that is called with
a handle to the port instance, so this must be saved. It is
customary to use an allocated driver-defined structure for this
one, and to pass a pointer back as a reference.</p>
<p>It is not a good idea to use a global variable as the port
driver can be spawned by multiple Erlang processes. This
driver-structure is to be instantiated multiple times:
</p>
<codeinclude file="port_driver.c" tag="" type="none"></codeinclude>
</section>
<section>
<title>Running the Example</title>
<p><em>Step 1.</em> Compile the C code:</p>
<pre>
unix> <input>gcc -o example_drv.so -fpic -shared complex.c port_driver.c</input>
windows> <input>cl -LD -MD -Fe example_drv.dll complex.c port_driver.c</input></pre>
<p><em>Step 2.</em> 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><em>Step 3.</em> Run the example:</p>
<pre>
2> <input>complex5:start("example_drv").</input>
<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>
