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<TITLE>C Nodes</TITLE>
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<A NAME="7"><!-- Empty --></A>
<H2>7 C Nodes</H2>
<P>This is an example of how to solve the <A HREF="example.html">example problem</A> by using a C node. Note that a C node would not typically be used for solving a simple problem like this, a port would suffice.<A NAME="7.1"><!-- Empty --></A>
<H3>7.1 Erlang Program</H3>
<P>From Erlang's point of view, the C node is treated like a normal Erlang node. Therefore, calling the functions <CODE>foo</CODE> and <CODE>bar</CODE> only involves sending a message to the C node asking for the function to be called, and receiving the result. Sending a message requires a recipient; a process which can be defined using either a pid or a tuple consisting of a registered name and a node name. In this case a tuple is the only alternative as no pid is known.
<PRE>
{RegName, Node} ! Msg
</PRE>
<P>The node name <CODE>Node</CODE> should be the name of the C node. If short node names are used, the plain name of the node will be <CODE>cN</CODE> where <CODE>N</CODE> is an integer. If long node names are used, there is no such restriction. An example of a C node name using short node names is thus <CODE>c1@idril</CODE>, an example using long node names is <CODE>cnode@idril.ericsson.se</CODE>.
<P>The registered name <CODE>RegName</CODE> could be any atom. The name can be ignored by the C code, or it could be used for example to distinguish between different types of messages. Below is an example of what the Erlang code could look like when using short node names,
<A TARGET="_top" HREF="complex3.erl">complex3.erl.</A>
<PRE>
-module(complex3).
-export([foo/1, bar/1]).
foo(X) ->
call_cnode({foo, X}).
bar(Y) ->
call_cnode({bar, Y}).
call_cnode(Msg) ->
{any, c1@idril} ! {call, self(), Msg},
receive
{cnode, Result} ->
Result
end.
</PRE>
<P>In <A TARGET="_top" HREF="complex4.erl">complex4.erl.</A>
there is an example of what the Erlang code could look like when using long node names.<A NAME="7.2"><!-- Empty --></A>
<H3>7.2 C Program</H3>
<A NAME="7.2.1"><!-- Empty --></A>
<H4>7.2.1 Setting Up the Communication</H4>
<P>Before calling any other Erl_Interface function, the memory handling must be initiated.
<PRE>
erl_init(NULL, 0);
</PRE>
<P>Now the C node can be initiated. If short node names are used, this is done by calling <CODE>erl_connect_init()</CODE>.
<PRE>
erl_connect_init(1, "secretcookie", 0);
</PRE>
<P>The first argument is the integer which is used to construct the node name. In the example the plain node name will be <CODE>c1</CODE>.<BR>
The second argument is a string defining the magic cookie.<BR>
The third argument is an integer which is used to identify a particular instance of a C node.
<P>If long node node names are used, initiation is done by calling <CODE>erl_connect_xinit()</CODE>.
<PRE>
erl_connect_xinit("idril", "cnode", "cnode@idril.ericsson.se",
&addr, "secretcookie", 0);
</PRE>
<P>The first three arguments are the host name, the plain node name, and the full node name. The fourth argument is a pointer to an <CODE>in_addr</CODE> struct with the IP address of the host, and the fifth and sixth arguments are the magic cookie and instance number.
<P>The C node can act as a server or a client when setting up the communication Erlang-C. If it acts as a client, it connects to an Erlang node by calling <CODE>erl_connect()</CODE>, which will return an open file descriptor at success.
<PRE>
fd = erl_connect("e1@idril");
</PRE>
<P>If the C node acts as a server, it must first create a socket (call <CODE>bind()</CODE> and <CODE>listen()</CODE>) listening to a certain port number <CODE>port</CODE>. It then publishes its name and port number with <CODE>epmd</CODE> (the Erlang port mapper daemon, see the man page for <CODE>epmd</CODE>).
<PRE>
erl_publish(port);
</PRE>
<P>Now the C node server can accept connections from Erlang nodes.
<PRE>
fd = erl_accept(listen, &conn);
</PRE>
<P>The second argument to <CODE>erl_accept</CODE> is a struct <CODE>ErlConnect</CODE> that will contain useful information when a connection has been established; for example, the name of the Erlang node.<A NAME="7.2.2"><!-- Empty --></A>
<H4>7.2.2 Sending and Receiving Messages</H4>
<P>The C node can receive a message from Erlang by calling <CODE>erl_receive msg()</CODE>. This function reads data from the open file descriptor <CODE>fd</CODE> into a buffer and puts the result in an <CODE>ErlMessage</CODE> struct <CODE>emsg</CODE>. <CODE>ErlMessage</CODE> has a field <CODE>type</CODE> defining which kind of data was received. In this case the type of interest is <CODE>ERL_REG_SEND</CODE> which indicates that Erlang sent a message to a registered process at the C node. The actual message, an <CODE>ETERM</CODE>, will be in the <CODE>msg</CODE> field.
<P>It is also necessary to take care of the types <CODE>ERL_ERROR</CODE> (an error occurred) and <CODE>ERL_TICK</CODE> (alive check from other node, should be ignored). Other possible types indicate process events such as link/unlink and exit.
<PRE>
while (loop) {
got = erl_receive_msg(fd, buf, BUFSIZE, &emsg);
if (got == ERL_TICK) {
/* ignore */
} else if (got == ERL_ERROR) {
loop = 0; /* exit while loop */
} else {
if (emsg.type == ERL_REG_SEND) {
</PRE>
<P>Since the message is an <CODE>ETERM</CODE> struct, Erl_Interface functions can be used to manipulate it. In this case, the message will be a 3-tuple (because that was how the Erlang code was written, see above). The second element will be the pid of the caller and the third element will be the tuple <CODE>{Function,Arg}</CODE> determining which function to call with which argument. The result of calling the function is made into an <CODE>ETERM</CODE> struct as well and sent back to Erlang using <CODE>erl_send()</CODE>, which takes the open file descriptor, a pid and a term as arguments.
<PRE>
fromp = erl_element(2, emsg.msg);
tuplep = erl_element(3, emsg.msg);
fnp = erl_element(1, tuplep);
argp = erl_element(2, tuplep);
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));
}
resp = erl_format("{cnode, ~i}", res);
erl_send(fd, fromp, resp);
</PRE>
<P>Finally, the memory allocated by the <CODE>ETERM</CODE> creating functions (including <CODE>erl_receive_msg()</CODE> must be freed.
<PRE>
erl_free_term(emsg.from); erl_free_term(emsg.msg);
erl_free_term(fromp); erl_free_term(tuplep);
erl_free_term(fnp); erl_free_term(argp);
erl_free_term(resp);
</PRE>
<P>The resulting C programs can be found in
<A TARGET="_top" HREF="cnode_s.c">cnode_s.c</A>
(C node server, short node names),
<A TARGET="_top" HREF="cnode_s2.c">cnode_s2.c</A>
(C node server, long node names) and
<A TARGET="_top" HREF="cnode_c.c">cnode_c.c</A>
(C node client).<A NAME="7.3"><!-- Empty --></A>
<H3>7.3 Running the Example</H3>
<P>1. Compile the C code, providing the paths to the Erl_Interface include files and libraries, and to the <CODE>socket</CODE> and <CODE>nsl</CODE> libraries.
<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>.
<PRE>
> gcc -o cserver \
-I/usr/local/otp/lib/erl_interface-3.2.1/include \
-L/usr/local/otp/lib/erl_interface-3.2.1/lib \
complex.c cnode_s.c \
-lerl_interface -lei -lsocket -lnsl
unix> gcc -o cserver2 \
-I/usr/local/otp/lib/erl_interface-3.2.1/include \
-L/usr/local/otp/lib/erl_interface-3.2.1/lib \
complex.c cnode_s2.c \
-lerl_interface -lei -lsocket -lnsl
unix> gcc -o cclient \
-I/usr/local/otp/lib/erl_interface-3.2.1/include \
-L/usr/local/otp/lib/erl_interface-3.2.1/lib \
complex.c cnode_c.c \
-lerl_interface -lei -lsocket -lnsl
</PRE>
<P>2. Compile the Erlang code.
<PRE>
unix> erl -compile complex3 complex4
</PRE>
<P>3. Run the C node server example with short node names.
<P>Start the C program <CODE>cserver</CODE> and Erlang in different windows. <CODE>cserver</CODE> takes a port number as argument and must be started before trying to call the Erlang functions. The Erlang node should be given the short name <CODE>e1</CODE> and must be set to use the same magic cookie as the C node, <CODE>secretcookie</CODE>.
<PRE>
unix> cserver 3456
unix> erl -sname e1 -setcookie secretcookie
Erlang (BEAM) emulator version 4.9.1.2
Eshell V4.9.1.2 (abort with ^G)
(e1@idril)1> complex3:foo(3).
4
(e1@idril)2> complex3:bar(5).
10
</PRE>
<P>4. Run the C node client example. Terminate <CODE>cserver</CODE> but not Erlang and start <CODE>cclient</CODE>. The Erlang node must be started before the C node client is.
<PRE>
unix> cclient
(e1@idril)3> complex3:foo(3).
4
(e1@idril)4> complex3:bar(5).
10
</PRE>
<P>5. Run the C node server, long node names, example.
<PRE>
unix> cserver2 3456
unix> erl -name e1 -setcookie secretcookie
Erlang (BEAM) emulator version 4.9.1.2
Eshell V4.9.1.2 (abort with ^G)
(e1@idril.du.uab.ericsson.se)1> complex4:foo(3).
4
(e1@idril.du.uab.ericsson.se)2> complex4:bar(5).
10
</PRE>
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