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<chapter>
  <header>
    <copyright>
      <year>2000</year><year>2009</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>Erl_Interface</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>erl_interface.xml</file>
  </header>
  <p>This is an example of how to solve the <seealso marker="example">example problem</seealso> by using a port and <c>erl_interface</c>. It is necessary to read the <seealso marker="c_port">port example</seealso> before reading this chapter.</p>

  <section>
    <title>Erlang Program</title>
    <p>The example below shows an Erlang program communicating with a C program over a plain port with home made encoding.</p>
    <codeinclude file="complex1.erl" type="erl"/>
    <p>Compared to the Erlang module 
      above used for the plain port, there are two differences when using Erl_Interface on the C side: Since Erl_Interface operates on the Erlang external term format the port must be set to use binaries and, instead of inventing an encoding/decoding scheme, the BIFs <c>term_to_binary/1</c> and <c>binary_to_term/1</c> should be used. That is:</p>
    <pre>
open_port({spawn, ExtPrg}, [{packet, 2}])</pre>
    <p>is replaced with:</p>
    <pre>
open_port({spawn, ExtPrg}, [{packet, 2}, binary])</pre>
    <p>And:</p>
    <pre>
Port ! {self(), {command, encode(Msg)}},
receive
  {Port, {data, Data}} ->
    Caller ! {complex, decode(Data)}
end</pre>
    <p>is replaced with:</p>
    <pre>
Port ! {self(), {command, term_to_binary(Msg)}},
receive
  {Port, {data, Data}} ->
    Caller ! {complex, binary_to_term(Data)}
end</pre>
    <p>The resulting Erlang program is shown below.</p>
    <codeinclude file="complex2.erl" type="erl"/>
    <p>Note that calling <c>complex2:foo/1</c> and <c>complex2:bar/1</c> will result in the tuple <c>{foo,X}</c> or <c>{bar,Y}</c> being sent to the <c>complex</c> process, which will code them as binaries and send them to the port. This means that the C program must be able to handle these two tuples.</p>
  </section>

  <section>
    <title>C Program</title>
    <p>The example below shows a C program communicating with an Erlang program over a plain port with home made encoding.</p>
    <codeinclude file="port.c" type="c"/>
    <p>Compared to the C program above
      used for the plain port the <c>while</c>-loop must be rewritten. Messages coming from the port will be on the Erlang external term format. They should be converted into an <c>ETERM</c> struct, a C struct similar to an Erlang term. The result of calling <c>foo()</c> or <c>bar()</c> must be converted to the Erlang external term format before being sent back to the port. But before calling any other <c>erl_interface</c> function, the memory handling must be initiated.</p>
    <pre>
erl_init(NULL, 0);</pre>
    <p>For reading from and writing to the port the functions <c>read_cmd()</c> and <c>write_cmd()</c> from the erl_comm.c example below 
      can still be used.
    </p>
    <codeinclude file="erl_comm.c" type="c"/>
    <p>The function <c>erl_decode()</c> from <c>erl_marshal</c> will convert the binary into an <c>ETERM</c> struct.</p>
    <pre>
int main() {
  ETERM *tuplep;

  while (read_cmd(buf) > 0) {
    tuplep = erl_decode(buf);</pre>
    <p>In this case <c>tuplep</c> now points to an <c>ETERM</c> struct representing a tuple with two elements; the function name (atom) and the argument (integer). By using the function <c>erl_element()</c> from <c>erl_eterm</c> it is possible to extract these elements, which also must be declared as pointers to an <c>ETERM</c> struct.</p>
    <pre>
    fnp = erl_element(1, tuplep);
    argp = erl_element(2, tuplep);</pre>
    <p>The macros <c>ERL_ATOM_PTR</c> and <c>ERL_INT_VALUE</c> from <c>erl_eterm</c> can be used to obtain the actual values of the atom and the integer. The atom value is represented as a string. By comparing this value with the strings "foo" and "bar" it can be decided which function to call.</p>
    <pre>
    if (strncmp(ERL_ATOM_PTR(fnp), "foo", 3) == 0) {
      res = foo(ERL_INT_VALUE(argp));
    } else if (strncmp(ERL_ATOM_PTR(fnp), "bar", 3) == 0) {
      res = bar(ERL_INT_VALUE(argp));
    }</pre>
    <p>Now an <c>ETERM</c> struct representing the integer result can be constructed using the function <c>erl_mk_int()</c> from <c>erl_eterm</c>. It is also possible to use the function <c>erl_format()</c> from the module <c>erl_format</c>.</p>
    <pre>
    intp = erl_mk_int(res);</pre>
    <p>The resulting <c>ETERM</c> struct is converted into the Erlang external term format using the function <c>erl_encode()</c> from <c>erl_marshal</c> and sent to Erlang using <c>write_cmd()</c>.</p>
    <pre>
    erl_encode(intp, buf);
    write_cmd(buf, erl_eterm_len(intp));</pre>
    <p>Last, the memory allocated by the <c>ETERM</c> creating functions must be freed.</p>
    <pre>
    erl_free_compound(tuplep);
    erl_free_term(fnp);
    erl_free_term(argp);
    erl_free_term(intp);</pre>
    <p>The resulting C program is shown below:</p>
    <codeinclude file="ei.c" type="c"/>
  </section>

  <section>
    <title>Running the Example</title>
    <p>1. Compile the C code, providing the paths to the include files <c>erl_interface.h</c> and <c>ei.h</c>, and to the libraries <c>erl_interface</c> and <c>ei</c>.</p>
    <pre>
unix> <input>gcc -o extprg -I/usr/local/otp/lib/erl_interface-3.2.1/include \\ </input>
<input>      -L/usr/local/otp/lib/erl_interface-3.2.1/lib \\ </input>
<input>      complex.c erl_comm.c ei.c -lerl_interface -lei</input></pre>
    <p>In R5B and later versions of OTP, the <c>include</c> and <c>lib</c> directories are situated under <c>OTPROOT/lib/erl_interface-VSN</c>, where <c>OTPROOT</c> is the root directory of the OTP installation (<c>/usr/local/otp</c> in the example above) and <c>VSN</c> is the version of the <c>erl_interface</c> application (3.2.1 in the example above).      <br></br>

      In R4B and earlier versions of OTP, <c>include</c> and <c>lib</c> are situated under <c>OTPROOT/usr</c>.</p>
    <p>2. 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(complex2).</input>
{ok,complex2}</pre>
    <p>3. Run the example.</p>
    <pre>
2> <input>complex2:start("extprg").</input>
&lt;0.34.0>
3> <input>complex2:foo(3).</input>
4
4> <input>complex2:bar(5).</input>
10
5> <input>complex2:bar(352).</input>
704
6> <input>complex2:stop().</input>
stop</pre>
  </section>
</chapter>

