path: root/lib/erl_interface/doc/src/erl_connect.xml

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<!DOCTYPE cref SYSTEM "cref.dtd">

<cref>
  <header>
    <copyright>
      <year>1996</year><year>2013</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_connect</title>
    <prepared>Torbj&ouml;rn T&ouml;rnkvist</prepared>
    <responsible>Torbj&ouml;rn T&ouml;rnkvist</responsible>
    <docno></docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked>Torbj&ouml;rn T&ouml;rnkvist</checked>
    <date>980703</date>
    <rev>A</rev>
    <file>erl_connect.sgml</file>
  </header>
  <lib>erl_connect</lib>
  <libsummary>Communicate with Distributed Erlang</libsummary>
  <description>
    <p>This module provides support for communication between distributed
      Erlang nodes and C nodes, in a manner that is transparent to Erlang
      processes.</p>
    <p>A C node appears to Erlang as a
      <em>hidden node</em>.
      That is, Erlang processes that know the name of the
      C node are able to communicate with it in a normal manner, but
      the node name will not appear in the listing provided by the
      Erlang function <c><![CDATA[nodes/0]]></c>.</p>
  </description>
  <funcs>
    <func>
      <name><ret>int</ret><nametext>erl_connect_init(number, cookie, creation)</nametext></name>
      <name><ret>int</ret><nametext>erl_connect_xinit(host, alive, node, addr, cookie, creation)</nametext></name>
      <fsummary>Initialize communication</fsummary>
      <type>
        <v>int number;</v>
        <v>char *cookie;</v>
        <v>short creation;</v>
        <v>char *host,*alive,*node;</v>
        <v>struct in_addr *addr;</v>
      </type>
      <desc>
        <p>These functions initialize the <c><![CDATA[erl_connect]]></c>
          module. In particular, they are used to identify the name of the
          C-node from which they are called. One of these functions must
          be called before any of the other functions in the erl_connect 
          module are used.</p>
        <p><c><![CDATA[erl_connect_xinit()]]></c> stores for later use information about
          the node's host name <c><![CDATA[host]]></c>, alive name <c><![CDATA[alive]]></c>, node
          name <c><![CDATA[node]]></c>, IP address <c><![CDATA[addr]]></c>, cookie <c><![CDATA[cookie]]></c>,
          and creation number <c><![CDATA[creation]]></c>. <c><![CDATA[erl_connect_init()]]></c>
          provides an alternative interface which does not require as much
          information from the caller. Instead, <c><![CDATA[erl_connect_init()]]></c>
          uses <c><![CDATA[gethostbyname()]]></c> to obtain default values. 
          </p>
        <p>If you use <c><![CDATA[erl_connect_init()]]></c> your node will have a
          short name, i.e., it will not be fully qualified. If you need to
          use fully qualified (a.k.a. long) names, use
          <c><![CDATA[erl_connect_xinit()]]></c> instead. 
          </p>
        <p><c><![CDATA[host]]></c> is the name of the host on which the node is running.</p>
        <p><c><![CDATA[alive]]></c> is the alivename of the node.</p>
        <p><c><![CDATA[node]]></c> is the name of the node. The nodename should
          be of the form <em>alivename@hostname</em>.</p>
        <p><c><![CDATA[addr]]></c> is the 32-bit IP address of <c><![CDATA[host]]></c>.</p>
        <p><c><![CDATA[cookie]]></c> is the authorization string required for access
          to the remote node. If NULL the user HOME directory is
          searched for a cookie file <c><![CDATA[.erlang.cookie]]></c>. The path to
          the home directory is retrieved from the environment variable
          <c><![CDATA[HOME]]></c> on Unix and from the <c><![CDATA[HOMEDRIVE]]></c> and
          <c><![CDATA[HOMEPATH]]></c> variables on Windows. Refer to the <c><![CDATA[auth]]></c>
          module for more details.</p>
        <p><c><![CDATA[creation]]></c> helps identify a particular instance of a C
          node. In particular, it can help prevent us from receiving
          messages sent to an earlier process with the same registered
          name.</p>
        <p>A C node acting as a server will be assigned a creation number
          when it calls <c><![CDATA[erl_publish()]]></c>.</p>
        <p><c><![CDATA[number]]></c> is used by <c><![CDATA[erl_connect_init()]]></c> to
          construct the actual node name. In the second example shown
          below, <em>"[email protected]"</em> will be the resulting node
          name.</p>
        <p>Example 1:</p>
        <code type="none"><![CDATA[
struct in_addr addr;
addr = inet_addr("150.236.14.75");
if (!erl_connect_xinit("chivas",
                       "madonna",
                       "[email protected]",
                       &addr;
                       "samplecookiestring..."),
                       0)
  erl_err_quit("<ERROR> when initializing !");
        ]]></code>
        <p>Example 2:</p>
        <code type="none"><![CDATA[
if (!erl_connect_init(17, "samplecookiestring...", 0))
  erl_err_quit("<ERROR> when initializing !");
        ]]></code>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_connect(node)</nametext></name>
      <name><ret>int</ret><nametext>erl_xconnect(addr, alive)</nametext></name>
      <fsummary>Establishe a connection to an Erlang node</fsummary>
      <type>
        <v>char *node, *alive;</v>
        <v>struct in_addr *addr;</v>
      </type>
      <desc>
        <p>These functions set up a connection to an Erlang node.</p>
        <p><c><![CDATA[erl_xconnect()]]></c> requires the IP address of the remote
          host and the alive name of the remote node
          to be specified. <c><![CDATA[erl_connect()]]></c> provides an alternative
          interface, and determines the information from the node name
          provided.</p>
        <p><c><![CDATA[addr]]></c> is the 32-bit IP address of the remote host.</p>
        <p><c><![CDATA[alive]]></c> is the alivename of the remote node.</p>
        <p><c><![CDATA[node]]></c> is the name of the remote node.</p>
        <p>These functions return an open file descriptor on success, or
          a negative value indicating that an error occurred --- in
          which case they will set <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[EHOSTUNREACH]]></c></tag>
          <item>The remote host <c><![CDATA[node]]></c> is unreachable</item>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
        <p>Additionally, <c><![CDATA[errno]]></c> values from
          <c><![CDATA[socket]]></c><em>(2)</em> and <c><![CDATA[connect]]></c><em>(2)</em>
          system calls may be propagated into <c><![CDATA[erl_errno]]></c>.</p>
        <code type="none"><![CDATA[
#define NODE   "[email protected]"
#define ALIVE  "madonna"
#define IP_ADDR "150.236.14.75"

/*** Variant 1 ***/
erl_connect( NODE );

/*** Variant 2 ***/
struct in_addr addr;
addr = inet_addr(IP_ADDR);
erl_xconnect( &addr , ALIVE );
        ]]></code>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_close_connection(fd)</nametext></name>
      <fsummary>Close a connection to an Erlang node</fsummary>
      <type>
        <v>int fd;</v>
      </type>
      <desc>
        <p>This function closes an open connection to an Erlang node.</p>
        <p><c><![CDATA[Fd]]></c> is a file descriptor obtained from
          <c><![CDATA[erl_connect()]]></c> or <c><![CDATA[erl_xconnect()]]></c>.</p>
        <p>On success, 0 is returned. If the call fails, a non-zero value
          is returned, and the reason for
          the error can be obtained with the appropriate platform-dependent
          call.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_receive(fd, bufp, bufsize)</nametext></name>
      <fsummary>Receive a message</fsummary>
      <type>
        <v>int fd;</v>
        <v>char *bufp;</v>
        <v>int bufsize;</v>
      </type>
      <desc>
        <p>This function receives a message consisting of a sequence
          of bytes in the Erlang external format.</p>
        <p><c><![CDATA[fd]]></c> is an open descriptor to an Erlang connection.</p>
        <p><c><![CDATA[bufp]]></c> is a buffer large enough to hold the expected
          message. </p>
        <p><c><![CDATA[bufsize]]></c> indicates the size of <c><![CDATA[bufp]]></c>.</p>
        <p>If a <em>tick</em> occurs, i.e., the Erlang node on the
          other end of the connection has polled this node to see if it
          is still alive, the function will return <c><![CDATA[ERL_TICK]]></c> and
          no message will be placed in the buffer. Also,
          <c><![CDATA[erl_errno]]></c> will be set to <c><![CDATA[EAGAIN]]></c>.</p>
        <p>On success, the message is placed in the specified buffer
          and the function returns the number of bytes actually read. On
          failure, the function returns a negative value and will set
          <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[EAGAIN]]></c></tag>
          <item>Temporary error: Try again.</item>
          <tag><c><![CDATA[EMSGSIZE]]></c></tag>
          <item>Buffer too small.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_receive_msg(fd, bufp, bufsize, emsg)</nametext></name>
      <fsummary>Receive and decodes a message</fsummary>
      <type>
        <v>int fd;</v>
        <v>unsigned char *bufp;</v>
        <v>int bufsize;</v>
        <v>ErlMessage *emsg;</v>
      </type>
      <desc>
        <p>This function receives the message into the specified buffer, 
          and decodes into the <c><![CDATA[(ErlMessage *) emsg]]></c>.</p>
        <p><c><![CDATA[fd]]></c> is an open descriptor to an Erlang connection.</p>
        <p><c><![CDATA[bufp]]></c> is a buffer large enough to hold the expected message.</p>
        <p><c><![CDATA[bufsize]]></c> indicates the size of <c><![CDATA[bufp]]></c>.</p>
        <p><c><![CDATA[emsg]]></c> is a pointer to an <c><![CDATA[ErlMessage]]></c> structure,
          into which the message will be decoded. <c><![CDATA[ErlMessage]]></c> is
          defined as follows:</p>
        <code type="none"><![CDATA[
typedef struct {
  int type;
  ETERM *msg;
  ETERM *to;
  ETERM *from;
  char to_name[MAXREGLEN];
} ErlMessage;
        ]]></code>
        <note>
          <p>The definition of <c><![CDATA[ErlMessage]]></c> has changed since
            earlier versions of Erl_Interface.</p>
        </note>
        <p><c><![CDATA[type]]></c> identifies the type of message, one of 
          <c><![CDATA[ERL_SEND]]></c>, <c><![CDATA[ERL_REG_SEND]]></c>, <c><![CDATA[ERL_LINK]]></c>,
          <c><![CDATA[ERL_UNLINK]]></c> and <c><![CDATA[ERL_EXIT]]></c>. 
          </p>
        <p>If <c><![CDATA[type]]></c> contains <c><![CDATA[ERL_SEND]]></c>
          this indicates that an ordinary send operation has taken
          place, and <c><![CDATA[emsg->to]]></c> contains the Pid of the
          recipient. If <c><![CDATA[type]]></c> contains <c><![CDATA[ERL_REG_SEND]]></c> then a
          registered send operation took place, and <c><![CDATA[emsg->from]]></c>
          contains the Pid of the sender. In both cases, the actual
          message will be in <c><![CDATA[emsg->msg]]></c>.
          </p>
        <p>If <c><![CDATA[type]]></c> contains one of <c><![CDATA[ERL_LINK]]></c> or
          <c><![CDATA[ERL_UNLINK]]></c>, then <c><![CDATA[emsg->to]]></c> and <c><![CDATA[emsg->from]]></c>
          contain the pids of the sender and recipient of the link or unlink.
          <c><![CDATA[emsg->msg]]></c> is not used in these cases.
          </p>
        <p>If <c><![CDATA[type]]></c> contains <c><![CDATA[ERL_EXIT]]></c>, then this
          indicates that a link has been broken. In this case,
          <c><![CDATA[emsg->to]]></c> and <c><![CDATA[emsg->from]]></c> contain the pids of the
          linked processes, and <c><![CDATA[emsg->msg]]></c> contains the reason for
          the exit.
          </p>
        <note>
          <p>It is the caller's responsibility to release the
            memory pointed to by <c><![CDATA[emsg->msg]]></c>, <c><![CDATA[emsg->to]]></c> and
            <c><![CDATA[emsg->from]]></c>.</p>
        </note>
        <p>If a <em>tick</em> occurs, i.e., the Erlang node on the
          other end of the connection has polled this node to see if it
          is still alive, the function will return <c><![CDATA[ERL_TICK]]></c>
          indicating that the tick has been received and responded to,
          but no message will be placed in the buffer. In this case you
          should call <c><![CDATA[erl_receive_msg()]]></c> again.</p>
        <p>On success, the function returns <c><![CDATA[ERL_MSG]]></c> and the
          <c><![CDATA[Emsg]]></c> struct will be initialized as described above, or
          <c><![CDATA[ERL_TICK]]></c>, in which case no message is returned. On
          failure, the function returns <c><![CDATA[ERL_ERROR]]></c> and will set
          <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[EMSGSIZE]]></c></tag>
          <item>Buffer too small.</item>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_xreceive_msg(fd, bufpp, bufsizep, emsg)</nametext></name>
      <fsummary>Receive and decodes a message</fsummary>
      <type>
        <v>int fd;</v>
        <v>unsigned char **bufpp;</v>
        <v>int *bufsizep;</v>
        <v>ErlMessage *emsg;</v>
      </type>
      <desc>
        <p>This function is similar to <c><![CDATA[erl_receive_msg]]></c>. The
          difference is that <c><![CDATA[erl_xreceive_msg]]></c> expects the buffer to
          have been allocated by <c><![CDATA[malloc]]></c>, and reallocates it if the received
          message does not fit into the original buffer. For that reason,
          both buffer and buffer length are given as pointers - their values
          may change by the call.
          </p>
        <p>On success, the function returns <c><![CDATA[ERL_MSG]]></c> and the
          <c><![CDATA[Emsg]]></c> struct will be initialized as described above, or
          <c><![CDATA[ERL_TICK]]></c>, in which case no message is returned. On
          failure, the function returns <c><![CDATA[ERL_ERROR]]></c> and will set
          <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[EMSGSIZE]]></c></tag>
          <item>Buffer too small.</item>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_send(fd, to, msg)</nametext></name>
      <fsummary>Send a message</fsummary>
      <type>
        <v>int fd;</v>
        <v>ETERM *to, *msg;</v>
      </type>
      <desc>
        <p>This function sends an Erlang term to a process.</p>
        <p><c><![CDATA[fd]]></c> is an open descriptor to an Erlang connection.</p>
        <p><c><![CDATA[to]]></c> is an Erlang term containing the Pid of the
          intended recipient of the message.</p>
        <p><c><![CDATA[msg]]></c> is the Erlang term to be sent.</p>
        <p>The function returns 1 if successful, otherwise 0 --- in
          which case it will set <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[EINVAL]]></c></tag>
          <item>Invalid argument: <c><![CDATA[to]]></c> is not a valid Erlang pid.</item>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_reg_send(fd, to, msg)</nametext></name>
      <fsummary>Send a message to a registered name</fsummary>
      <type>
        <v>int fd;</v>
        <v>char *to;</v>
        <v>ETERM *msg;</v>
      </type>
      <desc>
        <p>This function sends an Erlang term to a registered process.</p>
        <p><c><![CDATA[fd]]></c> is an open descriptor to an Erlang connection.</p>
        <p><c><![CDATA[to]]></c> is a string containing the registered name of
          the intended recipient of the message.</p>
        <p><c><![CDATA[msg]]></c> is the Erlang term to be sent.</p>
        <p>The function returns 1 if successful, otherwise 0 --- in
          which case it will set <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
      </desc>
    </func>
    <func>
      <name><ret>ETERM *</ret><nametext>erl_rpc(fd, mod, fun, args)</nametext></name>
      <name><ret>int</ret><nametext>erl_rpc_to(fd, mod, fun, args)</nametext></name>
      <name><ret>int</ret><nametext>erl_rpc_from(fd, timeout, emsg)</nametext></name>
      <fsummary>Remote Procedure Call</fsummary>
      <type>
        <v>int fd, timeout;</v>
        <v>char *mod, *fun;</v>
        <v>ETERM *args;</v>
        <v>ErlMessage *emsg;</v>
      </type>
      <desc>
        <p>These functions support calling Erlang functions on remote nodes.
          <c><![CDATA[erl_rpc_to()]]></c> sends an rpc request to a remote node and
          <c><![CDATA[erl_rpc_from()]]></c> receives the results of such a call.
          <c><![CDATA[erl_rpc()]]></c> combines the functionality of these two functions
          by sending an rpc request and waiting for the results. See also
          <c><![CDATA[rpc:call/4]]></c>. </p>
        <p><c><![CDATA[fd]]></c> is an open descriptor to an Erlang connection.</p>
        <p><c><![CDATA[timeout]]></c> is the maximum time (in ms) to wait for
          results. Specify <c><![CDATA[ERL_NO_TIMEOUT]]></c> to wait forever.
          When erl_rpc() calls erl_rpc_from(), the call will never
          timeout.</p>
        <p><c><![CDATA[mod]]></c> is the name of the module containing the function
          to be run on the remote node.</p>
        <p><c><![CDATA[fun]]></c> is the name of the function to run.</p>
        <p><c><![CDATA[args]]></c> is an Erlang list, containing the arguments to be
          passed to the function. </p>
        <p><c><![CDATA[emsg]]></c> is a message containing the result of the
          function call.</p>
        <p>The actual message returned by the rpc server
          is a 2-tuple <c><![CDATA[{rex,Reply}]]></c>. If you are using
          <c><![CDATA[erl_rpc_from()]]></c> in your code then this is the message you
          will need to parse. If you are using <c><![CDATA[erl_rpc()]]></c> then the
          tuple itself is parsed for you, and the message returned to your
          program is the erlang term containing <c><![CDATA[Reply]]></c> only. Replies
          to rpc requests are always ERL_SEND messages.
          </p>
        <note>
          <p>It is the caller's responsibility to free the returned
            <c><![CDATA[ETERM]]></c> structure as well as the memory pointed to by
            <c><![CDATA[emsg->msg]]></c> and <c><![CDATA[emsg->to]]></c>.  </p>
        </note>
        <p><c><![CDATA[erl_rpc()]]></c> returns the remote function's return value (or
          <c><![CDATA[NULL]]></c> if it failed). <c><![CDATA[erl_rpc_to()]]></c> returns 0 on
          success, and a negative number on failure. <c><![CDATA[erl_rcp_from()]]></c>
          returns <c><![CDATA[ERL_MSG]]></c> when successful (with <c><![CDATA[Emsg]]></c> now
          containing the reply tuple), and one of <c><![CDATA[ERL_TICK]]></c>,
          <c><![CDATA[ERL_TIMEOUT]]></c> and <c><![CDATA[ERL_ERROR]]></c> otherwise. When failing,
          all three functions set <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
          <tag><c><![CDATA[ETIMEDOUT]]></c></tag>
          <item>Timeout expired.</item>
          <tag><c><![CDATA[EAGAIN]]></c></tag>
          <item>Temporary error: Try again.</item>
        </taglist>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_publish(port)</nametext></name>
      <fsummary>Publish a node name</fsummary>
      <type>
        <v>int port;</v>
      </type>
      <desc>
        <p>These functions are used by a server process to register
          with the local name server <em>epmd</em>, thereby allowing
          other processes to send messages by using the registered name.
          Before calling either of these functions, the process should
          have called <c><![CDATA[bind()]]></c> and <c><![CDATA[listen()]]></c> on an open socket.</p>
        <p><c><![CDATA[port]]></c> is the local name to register, and should be the
          same as the port number that was previously bound to the socket.</p>
        <p>To unregister with epmd, simply close the returned
          descriptor.
          </p>
        <p>On success, the functions return a descriptor connecting the
          calling process to epmd. On failure, they return -1 and set
          <c><![CDATA[erl_errno]]></c> to:</p>
        <taglist>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error</item>
        </taglist>
        <p>Additionally, <c><![CDATA[errno]]></c> values from <c><![CDATA[socket]]></c><em>(2)</em>
          and <c><![CDATA[connect]]></c><em>(2)</em> system calls may be propagated
          into <c><![CDATA[erl_errno]]></c>.
          </p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_accept(listensock, conp)</nametext></name>
      <fsummary>Accept a connection</fsummary>
      <type>
        <v>int listensock;</v>
        <v>ErlConnect *conp;</v>
      </type>
      <desc>
        <p>This function is used by a server process to accept a
          connection from a client process.</p>
        <p><c><![CDATA[listensock]]></c> is an open socket descriptor on which
          <c><![CDATA[listen()]]></c> has previously been called.</p>
        <p><c><![CDATA[conp]]></c> is a pointer to an <c><![CDATA[ErlConnect]]></c> struct,
          described as follows:</p>
        <code type="none"><![CDATA[
typedef struct {
  char ipadr[4];             
  char nodename[MAXNODELEN];
} ErlConnect;
        ]]></code>
        <p>On success, <c><![CDATA[conp]]></c> is filled in with the address and
          node name of the connecting client and a file descriptor is
          returned. On failure, <c><![CDATA[ERL_ERROR]]></c> is returned and
          <c><![CDATA[erl_errno]]></c> is set to <c><![CDATA[EIO]]></c>.</p>
      </desc>
    </func>
    <func>
      <name><ret>const char *</ret><nametext>erl_thiscookie()</nametext></name>
      <name><ret>const char *</ret><nametext>erl_thisnodename()</nametext></name>
      <name><ret>const char *</ret><nametext>erl_thishostname()</nametext></name>
      <name><ret>const char *</ret><nametext>erl_thisalivename()</nametext></name>
      <name><ret>short</ret><nametext>erl_thiscreation()</nametext></name>
      <fsummary>Retrieve some values</fsummary>
      <desc>
        <p>These functions can be used to retrieve information about
          the C Node. These values are initially set with
          <c><![CDATA[erl_connect_init()]]></c> or <c><![CDATA[erl_connect_xinit()]]></c>.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_unpublish(alive)</nametext></name>
      <fsummary>Forcefully unpublish a node name</fsummary>
      <type>
        <v>char *alive;</v>
      </type>
      <desc>
       <p>This function can be called by a process to unregister a
          specified node from epmd on the localhost. This is however usually not 
	  allowed, unless epmd was started with the -relaxed_command_check 
	  flag, which it normally isn't.</p>

	  <p>To unregister a node you have published, you should instead
	  close the descriptor that was returned by  
	  <c><![CDATA[ei_publish()]]></c>.</p> 

        <warning>
          <p>This function is deprecated and will be removed in a future 
	  release.</p>
        </warning>
        <p><c><![CDATA[alive]]></c> is the name of the node to unregister, i.e., the
          first component of the nodename, without the <c><![CDATA[@hostname]]></c>.</p>
        <p>If the node was successfully unregistered from epmd, the
          function returns 0. Otherwise, it returns -1 and sets
          <c><![CDATA[erl_errno]]></c> is to <c><![CDATA[EIO]]></c>.</p>
      </desc>
    </func>
    <func>
      <name><ret>struct hostent</ret><nametext>*erl_gethostbyname(name)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*erl_gethostbyaddr(addr, length, type)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*erl_gethostbyname_r(name, hostp, buffer, buflen, h_errnop)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*erl_gethostbyaddr_r(addr, length, type, hostp, buffer, buflen, h_errnop)</nametext></name>
      <fsummary>Name lookup functions</fsummary>
      <type>
        <v>const char *name;</v>
        <v>const char *addr;</v>
        <v>int length;</v>
        <v>int type;</v>
        <v>struct hostent *hostp;</v>
        <v>char *buffer;</v>
        <v>int buflen;</v>
        <v>int *h_errnop;</v>
      </type>
      <desc>
        <p>These are convenience functions for some common name lookup functions.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Debug Information</title>
    <p>If a connection attempt fails, the following can be checked:</p>
    <list type="bulleted">
      <item><c><![CDATA[erl_errno]]></c></item>
      <item>that the right cookie was used</item>
      <item>that <em>epmd</em> is running</item>
      <item>the remote Erlang node on the other side is running the same
       version of Erlang as the <c><![CDATA[erl_interface]]></c> library.</item>
    </list>
  </section>
</cref>