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<A NAME="5"><!-- Empty --></A>
<H2>5 Erl_Interface</H2>
<P>This is an example of how to solve the <A HREF="example.html">example problem</A> by using a port and <CODE>erl_interface</CODE>. It is necessary to read the <A HREF="c_port.html">port example</A> before reading this chapter.<A NAME="5.1"><!-- Empty --></A>
<H3>5.1 Erlang Program</H3>
<P>Compared to the Erlang module
<A TARGET="_top" HREF="complex1.erl">complex1.erl</A>
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 <CODE>term_to_binary/1</CODE> and <CODE>binary_to_term/1</CODE> should be used. That is:
<PRE>
open_port({spawn, ExtPrg}, [{packet, 2}])
</PRE>
<P>is replaced with:
<PRE>
open_port({spawn, ExtPrg}, [{packet, 2}, binary])
</PRE>
<P>And:
<PRE>
Port ! {self(), {command, encode(Msg)}},
receive
{Port, {data, Data}} ->
Caller ! {complex, decode(Data)}
end
</PRE>
<P>is replaced with:
<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 can be found in
<A TARGET="_top" HREF="complex2.erl">complex2.erl</A>
. Note that calling <CODE>complex2:foo/1</CODE> and <CODE>complex2:bar/1</CODE> will result in the tuple <CODE>{foo,X}</CODE> or <CODE>{bar,Y}</CODE> being sent to the <CODE>complex</CODE> 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.<A NAME="5.2"><!-- Empty --></A>
<H3>5.2 C Program</H3>
<P>Compared to the C program
<A TARGET="_top" HREF="port.c">port.c</A>
used for the plain port the <CODE>while</CODE>-loop must be rewritten. Messages coming from the port will be on the Erlang external term format. They should be converted into an <CODE>ETERM</CODE> struct, a C struct similar to an Erlang term. The result of calling <CODE>foo()</CODE> or <CODE>bar()</CODE> must be converted to the Erlang external term format before being sent back to the port. But before calling any other <CODE>erl_interface</CODE> function, the memory handling must be initiated.
<PRE>
erl_init(NULL, 0);
</PRE>
<P>For reading from and writing to the port the functions <CODE>read_cmd()</CODE> and <CODE>write_cmd()</CODE> from
<A TARGET="_top" HREF="erl_comm.c">erl_comm.c</A>
can still be used. The function <CODE>erl_decode()</CODE> from <CODE>erl_marshal</CODE> will convert the binary into an <CODE>ETERM</CODE> struct.
<PRE>
int main() {
ETERM *tuplep;
while (read_cmd(buf) > 0) {
tuplep = erl_decode(buf);
</PRE>
<P>In this case <CODE>tuplep</CODE> now points to an <CODE>ETERM</CODE> struct representing a tuple with two elements; the function name (atom) and the argument (integer). By using the function <CODE>erl_element()</CODE> from <CODE>erl_eterm</CODE> it is possible to extract these elements, which also must be declared as pointers to an <CODE>ETERM</CODE> struct.
<PRE>
fnp = erl_element(1, tuplep);
argp = erl_element(2, tuplep);
</PRE>
<P>The macros <CODE>ERL_ATOM_PTR</CODE> and <CODE>ERL_INT_VALUE</CODE> from <CODE>erl_eterm</CODE> 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.
<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 <CODE>ETERM</CODE> struct representing the integer result can be constructed using the function <CODE>erl_mk_int()</CODE> from <CODE>erl_eterm</CODE>. It is also possible to use the function <CODE>erl_format()</CODE> from the module <CODE>erl_format</CODE>.
<PRE>
intp = erl_mk_int(res);
</PRE>
<P>The resulting <CODE>ETERM</CODE> struct is converted into the Erlang external term format using the function <CODE>erl_encode()</CODE> from <CODE>erl_marshal</CODE> and sent to Erlang using <CODE>write_cmd()</CODE>.
<PRE>
erl_encode(intp, buf);
write_cmd(buf, erl_eterm_len(intp));
</PRE>
<P>Last, the memory allocated by the <CODE>ETERM</CODE> creating functions must be freed.
<PRE>
erl_free_compound(tuplep);
erl_free_term(fnp);
erl_free_term(argp);
erl_free_term(intp);
</PRE>
<P>The resulting C program can be found in
<A TARGET="_top" HREF="ei.c">ei.c</A>
.<A NAME="5.3"><!-- Empty --></A>
<H3>5.3 Running the Example</H3>
<P>1. Compile the C code, providing the paths to the include files <CODE>erl_interface.h</CODE> and <CODE>ei.h</CODE>, and to the libraries <CODE>erl_interface</CODE> and <CODE>ei</CODE>.
<PRE>
unix> gcc -o extprg -I/usr/local/otp/lib/erl_interface-3.2.1/include \
-L/usr/local/otp/lib/erl_interface-3.2.1/lib \
complex.c erl_comm.c ei.c -lerl_interface -lei
</PRE>
<P>In R5B and later versions of OTP, the <CODE>include</CODE> and <CODE>lib</CODE> directories are situated under <CODE>OTPROOT/lib/erl_interface-VSN</CODE>, where <CODE>OTPROOT</CODE> is the root directory of the OTP installation (<CODE>/usr/local/otp</CODE> in the example above) and <CODE>VSN</CODE> is the version of the <CODE>erl_interface</CODE> application (3.2.1 in the example above).<BR>
In R4B and earlier versions of OTP, <CODE>include</CODE> and <CODE>lib</CODE> are situated under <CODE>OTPROOT/usr</CODE>.
<P>2. Start Erlang and compile the Erlang code.
<PRE>
unix> erl
Erlang (BEAM) emulator version 4.9.1.2
Eshell V4.9.1.2 (abort with ^G)
1> c(complex2).
{ok,complex2}
</PRE>
<P>3. Run the example.
<PRE>
2> complex2:start("extprg").
<0.34.0>
3> complex2:foo(3).
4
4> complex2:bar(5).
10
5> complex2:bar(352).
704
6> complex2:stop().
stop
</PRE>
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