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>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions 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>1998-07-03</date>
    <rev>A</rev>
    <file>erl_connect.xml</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 can communicate with it in a normal manner, but
      the node name does not appear in the listing provided by
      <seealso marker="erts:erlang#nodes/0"><c>erlang:nodes/0</c></seealso>
      in <c>ERTS</c>.</p>
  </description>

  <funcs>
    <func>
      <name><ret>int</ret><nametext>erl_accept(listensock, conp)</nametext></name>
      <fsummary>Accept a connection.</fsummary>
      <marker id="erl_accept"/>
      <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>
        <list type="bulleted">
          <item><c><![CDATA[listensock]]></c> is an open socket descriptor on
            which <c><![CDATA[listen()]]></c> has previously been called.</item>
          <item><c><![CDATA[conp]]></c> is a pointer to an
            <c><![CDATA[ErlConnect]]></c> struct, described as follows:</item>
        </list>
        <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>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>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>Returns <c>0</c> on success. 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_connect(node)</nametext></name>
      <name><ret>int</ret><nametext>erl_xconnect(addr, alive)</nametext></name>
      <fsummary>Establish a connection to an Erlang node.</fsummary>
      <type>
        <v>char *node, *alive;</v>
        <v>struct in_addr *addr;</v>
      </type>
      <desc>
        <p>Sets up a connection to an Erlang node.</p>
        <p><c><![CDATA[erl_xconnect()]]></c> requires the IP address of the
          remote host and the alivename 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>
        <list type="bulleted">
          <item><c><![CDATA[addr]]></c> is the 32-bit IP address of the remote
            host.</item>
          <item><c><![CDATA[alive]]></c> is the alivename of the remote node.
          </item>
          <item><c><![CDATA[node]]></c> is the name of the remote node.</item>
        </list>
        <p>Returns an open file descriptor on success, otherwise a negative
          value. In the latter case <c><![CDATA[erl_errno]]></c> is set 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 is available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
        <p>Also, <c><![CDATA[errno]]></c> values from
          <c><![CDATA[socket]]></c><em>(2)</em> and
          <c><![CDATA[connect]]></c><em>(2)</em>
          system calls can be propagated into <c><![CDATA[erl_errno]]></c>.</p>
        <p><em>Example:</em></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_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>
      <marker id="erl_connect_init"/>
      <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>Initializes the <c><![CDATA[erl_connect]]></c> module.
          In particular, these functions 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 <c>erl_connect</c>
          module are used.</p>
        <p><c><![CDATA[erl_connect_xinit()]]></c> stores for later use
          information about:</p>
        <list type="bulleted">
          <item>Hostname of the node, <c><![CDATA[host]]></c></item>
          <item>Alivename, <c><![CDATA[alive]]></c></item>
          <item>Node name, <c><![CDATA[node]]></c></item>
          <item>IP address, <c><![CDATA[addr]]></c></item>
          <item>Cookie, <c><![CDATA[cookie]]></c></item>
          <item>Creation number, <c><![CDATA[creation]]></c></item>
        </list>
        <p><c><![CDATA[erl_connect_init()]]></c>
          provides an alternative interface that 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, that is, it will not be fully qualified. If you
          need to use fully qualified (long) names, use
          <c><![CDATA[erl_connect_xinit()]]></c> instead.</p>
        <list type="bulleted">
          <item>
            <p><c><![CDATA[host]]></c> is the name of the host on which the node
              is running.</p>
          </item>
          <item>
            <p><c><![CDATA[alive]]></c> is the alivename of the node.</p>
          </item>
          <item>
            <p><c><![CDATA[node]]></c> is the node name. It is to
                be of the form <em>alivename@hostname</em>.</p>
          </item>
          <item>
            <p><c><![CDATA[addr]]></c> is the 32-bit IP address of
              <c><![CDATA[host]]></c>.</p>
          </item>
          <item>
            <p><c><![CDATA[cookie]]></c> is the authorization string required
              for access to the remote node. If <c>NULL</c>, the user
              <c>HOME</c> directory is searched for a cookie file
              <c><![CDATA[.erlang.cookie]]></c>. The path to
              the home directory is retrieved from environment variable
              <c><![CDATA[HOME]]></c> on Unix and from the
              <c><![CDATA[HOMEDRIVE]]></c> and
              <c><![CDATA[HOMEPATH]]></c> variables on Windows. For more
              details, see the <seealso marker="kernel:auth">
              <c>auth</c></seealso> module in Kernel.</p>
          </item>
          <item>
            <p><c><![CDATA[creation]]></c> helps identifying 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>
          </item>
        </list>
        <p>A C-node acting as a server is 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 Example 2
          below, <em>"[email protected]"</em> is the resulting node name.</p>
        <p><em>Example 1:</em></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><em>Example 2:</em></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_publish(port)</nametext></name>
      <fsummary>Publish a node name.</fsummary>
      <type>
        <v>int port;</v>
      </type>
      <desc>
        <p>This function is used by a server process to register
          with the local name server EPMD, thereby allowing
          other processes to send messages by using the registered name.
          Before calling this function, 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 is to 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, a descriptor connecting the calling process to EPMD is
          returned. On failure, <c>-1</c> is returned and
          <c><![CDATA[erl_errno]]></c> is set to:</p>
        <taglist>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
        <p>Also, <c><![CDATA[errno]]></c> values from
          <c><![CDATA[socket]]></c><em>(2)</em>
          and <c><![CDATA[connect]]></c><em>(2)</em> system calls can be
          propagated into <c><![CDATA[erl_errno]]></c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_receive(fd, bufp, bufsize)</nametext></name>
      <fsummary>Receive a message.</fsummary>
      <marker id="erl_receive"/>
      <type>
        <v>int fd;</v>
        <v>char *bufp;</v>
        <v>int bufsize;</v>
      </type>
      <desc>
        <p>Receives a message consisting of a sequence
          of bytes in the Erlang external format.</p>
        <list type="bulleted">
          <item><c><![CDATA[fd]]></c> is an open descriptor to an Erlang
            connection.</item>
          <item><c><![CDATA[bufp]]></c> is a buffer large enough to hold the
            expected message.</item>
          <item><c><![CDATA[bufsize]]></c> indicates the size of
            <c><![CDATA[bufp]]></c>.</item>
        </list>
        <p>If a <em>tick</em> occurs, that is, the Erlang node on the
          other end of the connection has polled this node to see if it
          is still alive, the function returns <c><![CDATA[ERL_TICK]]></c> and
          no message is placed in the buffer. Also,
          <c><![CDATA[erl_errno]]></c> is 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 sets
          <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 is 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 decode a message.</fsummary>
      <marker id="erl_receive_msg"/>
      <type>
        <v>int fd;</v>
        <v>unsigned char *bufp;</v>
        <v>int bufsize;</v>
        <v>ErlMessage *emsg;</v>
      </type>
      <desc>
        <p>Receives the message into the specified buffer
          and decodes into <c><![CDATA[(ErlMessage *) emsg]]></c>.</p>
        <list type="bulleted">
          <item><c><![CDATA[fd]]></c> is an open descriptor to an Erlang
            connection.</item>
          <item><c><![CDATA[bufp]]></c> is a buffer large enough to hold the
            expected message.</item>
          <item><c><![CDATA[bufsize]]></c> indicates the size of
            <c><![CDATA[bufp]]></c>.</item>
          <item>><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:</item>
        </list>
        <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 <c>Erl_Interface</c>.</p>
        </note>
        <p><c><![CDATA[type]]></c> identifies the type of message, one of the
          following:</p>
        <taglist>
          <tag><c><![CDATA[ERL_SEND]]></c></tag>
          <item>
            <p>An ordinary send operation has occurred and
              <c><![CDATA[emsg->to]]></c> contains the pid of the recipient.
              The message is in <c><![CDATA[emsg->msg]]></c>.</p>
          </item>
          <tag><c><![CDATA[ERL_REG_SEND]]></c></tag>
          <item>
            <p>A registered send operation has occurred and
              <c><![CDATA[emsg->from]]></c> contains the pid of the sender.
              The message is in <c><![CDATA[emsg->msg]]></c>.</p>
          </item>
          <tag><c><![CDATA[ERL_LINK]]></c> or <c><![CDATA[ERL_UNLINK]]></c>
          </tag>
          <item>
            <p><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.</p>
          </item>
          <tag><c><![CDATA[ERL_EXIT]]></c></tag>
          <item>
            <p>A link is broken. <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>
          </item>
        </taglist>
        <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, that is, the Erlang node on the
          other end of the connection has polled this node to see if it
          is still alive, the function returns <c><![CDATA[ERL_TICK]]></c>
          indicating that the tick has been received and responded to,
          but no message is placed in the buffer. In this case you
          are to 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 is 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 sets
          <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[EMSGSIZE]]></c></tag>
          <item>Buffer is too small.</item>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory is 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>
      <marker id="erl_reg_send"/>
      <type>
        <v>int fd;</v>
        <v>char *to;</v>
        <v>ETERM *msg;</v>
      </type>
      <desc>
        <p>Sends an Erlang term to a registered process.</p>
        <list type="bulleted">
          <item><c><![CDATA[fd]]></c> is an open descriptor to an Erlang
            connection.</item>
          <item><c><![CDATA[to]]></c> is a string containing the registered name
            of the intended recipient of the message.</item>
          <item><c><![CDATA[msg]]></c> is the Erlang term to be sent.</item>
        </list>
        <p>Returns <c>1</c> on success, otherwise <c>0</c>. In
          the latter case <c><![CDATA[erl_errno]]></c> is set to one of:</p>
        <taglist>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory is 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_from(fd, timeout, emsg)</nametext></name>
      <name><ret>int</ret><nametext>erl_rpc_to(fd, mod, fun, args)</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>Supports 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 <seealso marker="kernel:rpc#call/4">
          <c>rpc:call/4</c></seealso> in <c>Kernel</c>.</p>
        <list type="bulleted">
          <item><c><![CDATA[fd]]></c> is an open descriptor to an Erlang
            connection.</item>
          <item><c><![CDATA[timeout]]></c> is the maximum time (in milliseconds)
            to wait for
            results. To wait forever, specify <c><![CDATA[ERL_NO_TIMEOUT]]></c>.
            When <c>erl_rpc()</c> calls <c>erl_rpc_from()</c>, the call will
            never timeout.</item>
          <item><c><![CDATA[mod]]></c> is the name of the module containing the
            function to be run on the remote node.</item>
          <item><c><![CDATA[fun]]></c> is the name of the function to run.
          </item>
          <item><c><![CDATA[args]]></c> is an Erlang list, containing the
            arguments to be passed to the function.</item>
          <item><c><![CDATA[emsg]]></c> is a message containing the result of
            the function call.</item>
        </list>
        <p>The actual message returned by the RPC server
          is a 2-tuple <c><![CDATA[{rex,Reply}]]></c>. If you use
          <c><![CDATA[erl_rpc_from()]]></c> in your code, this is the message
          you will need to parse. If you use <c><![CDATA[erl_rpc()]]></c>, 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 <c>ERL_SEND</c> messages.</p>
        <note>
          <p>It is the caller's responsibility to free the returned
            <c><![CDATA[ETERM]]></c> structure and 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 on success, otherwise <c><![CDATA[NULL]]></c>.</p>
        <p><c><![CDATA[erl_rpc_to()]]></c> returns <c>0</c> on
          success, otherwise a negative number.</p>
        <p><c><![CDATA[erl_rcp_from()]]></c> returns <c><![CDATA[ERL_MSG]]></c>
          on success (with <c><![CDATA[Emsg]]></c> now
          containing the reply tuple), otherwise one of
          <c><![CDATA[ERL_TICK]]></c>, <c><![CDATA[ERL_TIMEOUT]]></c>, or
          <c><![CDATA[ERL_ERROR]]></c>.</p>
        <p>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 is available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
          <tag><c><![CDATA[ETIMEDOUT]]></c></tag>
          <item>Timeout has expired.</item>
          <tag><c><![CDATA[EAGAIN]]></c></tag>
          <item>Temporary error: Try again.</item>
        </taglist>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_send(fd, to, msg)</nametext></name>
      <fsummary>Send a message.</fsummary>
      <marker id="erl_send"/>
      <type>
        <v>int fd;</v>
        <v>ETERM *to, *msg;</v>
      </type>
      <desc>
        <p>Sends an Erlang term to a process.</p>
        <list type="bulleted">
          <item><c><![CDATA[fd]]></c> is an open descriptor to an Erlang
            connection.</item>
          <item><c><![CDATA[to]]></c> is an Erlang term containing the pid of
            the intended recipient of the message.</item>
          <item>><c><![CDATA[msg]]></c> is the Erlang term to be sent.</item>
        </list>
        <p>Returns <c>1</c> on success, otherwise <c>0</c>. In
          the latter case <c><![CDATA[erl_errno]]></c> is set 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 is available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
      </desc>
    </func>

    <func>
      <name><ret>const char *</ret><nametext>erl_thisalivename()</nametext></name>
      <name><ret>const char *</ret><nametext>erl_thiscookie()</nametext></name>
      <name><ret>short</ret><nametext>erl_thiscreation()</nametext></name>
      <name><ret>const char *</ret><nametext>erl_thishostname()</nametext></name>
      <name><ret>const char *</ret><nametext>erl_thisnodename()</nametext></name>
      <fsummary>Retrieve some values.</fsummary>
      <desc>
        <p>Retrieves 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 local host. This is, however, usually
          not allowed, unless EPMD was started with flag
          <c>-relaxed_command_check</c>, which it normally is not.</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, that
          is, the first component of the node name, without
          <c><![CDATA[@hostname]]></c>.</p>
        <p>If the node was successfully unregistered from EPMD, <c>0</c> is
          returned, otherwise <c>-1</c> is returned and
          <c><![CDATA[erl_errno]]></c> is set to <c><![CDATA[EIO]]></c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_xreceive_msg(fd, bufpp, bufsizep, emsg)</nametext></name>
      <fsummary>Receive and decode a message.</fsummary>
      <type>
        <v>int fd;</v>
        <v>unsigned char **bufpp;</v>
        <v>int *bufsizep;</v>
        <v>ErlMessage *emsg;</v>
      </type>
      <desc>
        <p>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. Therefore
          both buffer and buffer length are given as pointers; their values
          can change by the call.</p>
        <p>On success, the function returns <c><![CDATA[ERL_MSG]]></c> and the
          <c><![CDATA[Emsg]]></c> struct is 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 sets
          <c><![CDATA[erl_errno]]></c> to one of:</p>
        <taglist>
          <tag><c><![CDATA[EMSGSIZE]]></c></tag>
          <item>Buffer is too small.</item>
          <tag><c><![CDATA[ENOMEM]]></c></tag>
          <item>No more memory is available.</item>
          <tag><c><![CDATA[EIO]]></c></tag>
          <item>I/O error.</item>
        </taglist>
      </desc>
    </func>

    <func>
      <name><ret>struct hostent</ret><nametext>*erl_gethostbyaddr(addr, length, type)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*erl_gethostbyaddr_r(addr, length, type, hostp, buffer, buflen, h_errnop)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*erl_gethostbyname(name)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*erl_gethostbyname_r(name, 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>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 correct cookie was used</item>
      <item>That EPMD is running</item>
      <item>That 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>