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<chapter>
<header>
<copyright>
<year>2000</year><year>2016</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
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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.
</legalnotice>
<title>Ports</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>c_port.xml</file>
</header>
<p>This section outlines an example of how to solve the example
problem in the <seealso marker="example">previous section</seealso>
by using a port.</p>
<p>The scenario is illustrated in the following figure:</p>
<image file="../tutorial/port.gif">
<icaption>Port Communication</icaption>
</image>
<section>
<title>Erlang Program</title>
<p>All communication between Erlang and C must be established by
creating the port. The Erlang process that creates a port is
said to be <em>the connected process</em> of the port. All
communication to and from the port must go through the connected
process. If the connected process terminates, the port also
terminates (and the external program, if it is written
properly).</p>
<p>The port is created using the BIF <c>open_port/2</c> with
<c>{spawn,ExtPrg}</c> as the first argument. The string
<c>ExtPrg</c> is the name of the external program, including any
command line arguments. The second argument is a list of
options, in this case only <c>{packet,2}</c>. This option says
that a 2 byte length indicator is to be used to simplify the
communication between C and Erlang. The Erlang port
automatically adds the length indicator, but this must be done
explicitly in the external C program.</p>
<p>The process is also set to trap exits, which enables detection
of failure of the external program:</p>
<pre>
-module(complex1).
-export([start/1, init/1]).
start(ExtPrg) ->
spawn(?MODULE, init, [ExtPrg]).
init(ExtPrg) ->
register(complex, self()),
process_flag(trap_exit, true),
Port = open_port({spawn, ExtPrg}, [{packet, 2}]),
loop(Port).</pre>
<p>Now <c>complex1:foo/1</c> and <c>complex1:bar/1</c> can be
implemented. Both send a message to the <c>complex</c> process
and receive the following replies:</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 does 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 functionality for
stopping the port and detecting port failures, is as follows:
</p>
<codeinclude file="complex1.erl" type="erl"/>
</section>
<section>
<title>C Program</title>
<p>On the C side, it is necessary to write functions for receiving
and sending data with 2 byte length indicators from/to Erlang.
By default, the C program is to read from standard input (file
descriptor 0) and write to standard output (file descriptor 1).
Examples of such functions, <c>read_cmd/1</c> and
<c>write_cmd/2</c>, follows:</p>
<codeinclude file="erl_comm.c" type="erl"/>
<p>Notice that <c>stdin</c> and <c>stdout</c> are for buffered
input/output and must <em>not</em> be used for the communication
with Erlang.</p>
<p>In the <c>main</c> function, the C program is to listen for a
message from Erlang and, according to the selected
encoding/decoding scheme, use the first byte to determine which
function to call and the second byte as argument to the
function. The result of calling the function is then to be sent
back to Erlang:</p>
<codeinclude file="port.c" tag="" type="none"></codeinclude>
<p>Notice that the C program is in a <c>while</c>-loop, checking
for the return value of <c>read_cmd/1</c>. This is because the C
program must detect when the port closes and terminates.</p>
</section>
<section>
<title>Running the Example</title>
<p><em>Step 1.</em> Compile the C code:</p>
<pre>
unix> <input>gcc -o extprg complex.c erl_comm.c port.c</input></pre>
<p><em>Step 2.</em> Start Erlang and compile the Erlang code:</p>
<pre>
unix> <input>erl</input>
Erlang (BEAM) emulator version 4.9.1.2
Eshell V4.9.1.2 (abort with ^G)
1> <input>c(complex1).</input>
{ok,complex1}</pre>
<p><em>Step 3.</em> Run the example:</p>
<pre>
2> <input>complex1:start("extprg").</input>
<0.34.0>
3> <input>complex1:foo(3).</input>
4
4> <input>complex1:bar(5).</input>
10
5> <input>complex1:stop().</input>
stop</pre>
</section>
</chapter>
