[Ericsson AB]

7 C Nodes

This is an example of how to solve the example problem 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.

7.1 Erlang Program

From Erlang's point of view, the C node is treated like a normal Erlang node. Therefore, calling the functions foo and bar 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.

{RegName, Node} ! Msg

The node name Node should be the name of the C node. If short node names are used, the plain name of the node will be cN where N 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 c1@idril, an example using long node names is cnode@idril.ericsson.se.

The registered name RegName 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, complex3.erl.

-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},
        {cnode, Result} ->

In complex4.erl. there is an example of what the Erlang code could look like when using long node names.

7.2 C Program

7.2.1 Setting Up the Communication

Before calling any other Erl_Interface function, the memory handling must be initiated.

erl_init(NULL, 0);

Now the C node can be initiated. If short node names are used, this is done by calling erl_connect_init().

erl_connect_init(1, "secretcookie", 0);

The first argument is the integer which is used to construct the node name. In the example the plain node name will be c1.
The second argument is a string defining the magic cookie.
The third argument is an integer which is used to identify a particular instance of a C node.

If long node node names are used, initiation is done by calling erl_connect_xinit().

erl_connect_xinit("idril", "cnode", "cnode@idril.ericsson.se",
                  &addr, "secretcookie", 0);

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 in_addr struct with the IP address of the host, and the fifth and sixth arguments are the magic cookie and instance number.

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 erl_connect(), which will return an open file descriptor at success.

fd = erl_connect("e1@idril");

If the C node acts as a server, it must first create a socket (call bind() and listen()) listening to a certain port number port. It then publishes its name and port number with epmd (the Erlang port mapper daemon, see the man page for epmd).


Now the C node server can accept connections from Erlang nodes.

fd = erl_accept(listen, &conn);

The second argument to erl_accept is a struct ErlConnect that will contain useful information when a connection has been established; for example, the name of the Erlang node.

7.2.2 Sending and Receiving Messages

The C node can receive a message from Erlang by calling erl_receive msg(). This function reads data from the open file descriptor fd into a buffer and puts the result in an ErlMessage struct emsg. ErlMessage has a field type defining which kind of data was received. In this case the type of interest is ERL_REG_SEND which indicates that Erlang sent a message to a registered process at the C node. The actual message, an ETERM, will be in the msg field.

It is also necessary to take care of the types ERL_ERROR (an error occurred) and ERL_TICK (alive check from other node, should be ignored). Other possible types indicate process events such as link/unlink and exit.

  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) {

Since the message is an ETERM 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 {Function,Arg} determining which function to call with which argument. The result of calling the function is made into an ETERM struct as well and sent back to Erlang using erl_send(), which takes the open file descriptor, a pid and a term as arguments.

        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);

Finally, the memory allocated by the ETERM creating functions (including erl_receive_msg() must be freed.

        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);

The resulting C programs can be found in cnode_s.c (C node server, short node names), cnode_s2.c (C node server, long node names) and cnode_c.c (C node client).

7.3 Running the Example

1. Compile the C code, providing the paths to the Erl_Interface include files and libraries, and to the socket and nsl libraries.

In R5B and later versions of OTP, the include and lib directories are situated under OTPROOT/lib/erl_interface-VSN, where OTPROOT is the root directory of the OTP installation (/usr/local/otp in the example above) and VSN is the version of the erl_interface application (3.2.1 in the example above).
In R4B and earlier versions of OTP, include and lib are situated under OTPROOT/usr.

>  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

2. Compile the Erlang code.

unix> erl -compile complex3 complex4

3. Run the C node server example with short node names.

Start the C program cserver and Erlang in different windows. cserver 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 e1 and must be set to use the same magic cookie as the C node, secretcookie.

unix> cserver 3456

unix> erl -sname e1 -setcookie secretcookie
Erlang (BEAM) emulator version
Eshell V4.9.1.2  (abort with ^G)
(e1@idril)1> complex3:foo(3).
(e1@idril)2> complex3:bar(5).

4. Run the C node client example. Terminate cserver but not Erlang and start cclient. The Erlang node must be started before the C node client is.

unix> cclient

(e1@idril)3> complex3:foo(3).
(e1@idril)4> complex3:bar(5).

5. Run the C node server, long node names, example.

unix> cserver2 3456

unix> erl -name e1 -setcookie secretcookie
Erlang (BEAM) emulator version
Eshell V4.9.1.2  (abort with ^G)
(e1@idril.du.uab.ericsson.se)1> complex4:foo(3).
(e1@idril.du.uab.ericsson.se)2> complex4:bar(5).

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