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

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<!DOCTYPE cref SYSTEM "cref.dtd">

<cref>
  <header>
    <copyright>
      <year>2001</year><year>2009</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>ei_connect</title>
    <prepared>Jakob Cederlund</prepared>
    <docno></docno>
    <approved>?</approved>
    <checked>?</checked>
    <date>2001-09-01</date>
    <rev>A</rev>
    <file>ei_connect.sgml</file>
  </header>
  <lib>ei_connect</lib>
  <libsummary>Communicate with distributed erlang</libsummary>
  <description>
    <p>This module enables C programs to communicate with erlang nodes,
      using the erlang distribution over TCP/IP.</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>
    <p>The environment variable <c><![CDATA[ERL_EPMD_PORT]]></c> can be used
      to indicate which logical cluster a C node belongs to.</p>
  </description>

  <section>
    <title>Timeout functions</title>
    <p>Most functions appear in a version with the suffix
      <c><![CDATA[_tmo]]></c> appended to the function name. Those function take
      an additional argument, a timeout in <em>milliseconds</em>. The
      semantics is this; for each communication primitive involved in
      the operation, if the primitive does not complete within the time
      specified, the function will return an error and
      <c><![CDATA[erl_errno]]></c> will be set to <c><![CDATA[ETIMEDOUT]]></c>. With
      communication primitive is ment an operation on the socket, like
      <c><![CDATA[connect]]></c>, <c><![CDATA[accept]]></c>, <c><![CDATA[recv]]></c> or <c><![CDATA[send]]></c>.</p>
    <p>Obviously the timeouts are for implementing fault tolerance,
      not to keep hard realtime promises. The <c><![CDATA[_tmo]]></c> functions
      are for detecting non-responsive peers and to avoid blocking on
      socket operations. </p>
    <p>A timeout value of <c><![CDATA[0]]></c> (zero), means that timeouts are
      disabled. Calling a <c><![CDATA[_tmo]]></c>-function with the last argument as
      <c><![CDATA[0]]></c> is therefore exactly the same thing as calling the
      function without the <c><![CDATA[_tmo]]></c> suffix.</p>
    <p>As with all other ei functions, you are <em>not</em> expected
      to put the socket in non blocking mode yourself in the program. Every
      use of non blocking mode is embedded inside the timeout
      functions. The socket will always be back in blocking mode after
      the operations are completed (regardless of the result). To
      avoid problems, leave the socket options alone. Ei will handle
      any socket options that need modification.</p>
    <p>In all other senses, the <c><![CDATA[_tmo]]></c> functions inherit all 
      the return values and the semantics from the functions without 
      the <c><![CDATA[_tmo]]></c> suffix.</p>
  </section>
  <funcs>
    <func>
      <name><ret>int</ret><nametext>ei_connect_init(ei_cnode* ec, const char* this_node_name, const char *cookie, short creation)</nametext></name>
      <name><ret>int</ret><nametext>ei_connect_xinit(ei_cnode* ec, const char *thishostname, const char *thisalivename, const char *thisnodename, Erl_IpAddr thisipaddr, const char *cookie, short creation)</nametext></name>
      <fsummary>Initialize for a connection.</fsummary>
      <desc>
        <p>These function initializes the <c><![CDATA[ec]]></c> structure, to
          identify the node name and cookie of the server. One of them
          has to be called before other functions that works on the
          type <c><![CDATA[ei_cnode]]></c> or a file descriptor associated with a
          connection to another node are used.</p>
        <p><c><![CDATA[ec]]></c> is a structure containing information about the
          C-node. It is used in other <c><![CDATA[ei]]></c> functions for
          connecting and receiving data.</p>
        <p><c><![CDATA[this_node_name]]></c> is the  registered name of the process
          (the name before '@').</p>
        <p><c><![CDATA[cookie]]></c> is the cookie for the node.</p>
        <p><c><![CDATA[creation]]></c> identifies a specific instance of a C
          node. It can help prevent the node from receiving messages
          sent to an earlier process with the same registered name.</p>
        <p><c><![CDATA[thishostname]]></c> is the name of the machine we're running
          on. If long names are to be used, it should be fully
          qualified (i.e. <c><![CDATA[durin.erix.ericsson.se]]></c> instead of
          <c><![CDATA[durin]]></c>).</p>
        <p><c><![CDATA[thisalivename]]></c> is the registered name of the process.</p>
        <p><c><![CDATA[thisnodename]]></c> is the full name of the node,
          i.e. <c><![CDATA[einode@durin]]></c>.</p>
        <p><c><![CDATA[thispaddr]]></c> if the IP address of the host.</p>
        <p>A C node acting as a server will be assigned a creation
          number when it calls <c><![CDATA[ei_publish()]]></c>.</p>
        <p>A connection is closed by simply closing the socket. Refer
          to system documentation to close the socket gracefully (when
          there are outgoing packets before close).</p>
        <p>This function return a negative value indicating that an error
          occurred.</p>
        <p>Example 1:
          </p>
        <code type="none"><![CDATA[
int n = 0;
struct in_addr addr;
ei_cnode ec;
addr = inet_addr("150.236.14.75");
if (ei_connect_xinit(&ec,
                     "chivas",
                     "madonna",
                     "[email protected]",
                     &addr;
                     "cookie...",
                     n++) < 0) {
    fprintf(stderr,"ERROR when initializing: %d",erl_errno);
    exit(-1);
}
        ]]></code>
        <p>Example 2:
          </p>
        <code type="none"><![CDATA[
if (ei_connect_init(&ec, "madonna", "cookie...", n++) < 0) {
    fprintf("ERROR when initializing: %d",erl_errno);
    exit(-1);
}
        ]]></code>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_connect(ei_cnode* ec, char *nodename)</nametext></name>
      <name><ret>int</ret><nametext>ei_xconnect(ei_cnode* ec, Erl_IpAddr adr, char *alivename)</nametext></name>
      <fsummary>Establishe a connection to an Erlang node</fsummary>
      <desc>
        <p>These functions set up a connection to an Erlang node.</p>
        <p><c><![CDATA[ei_xconnect()]]></c> requires the IP address of the remote
          host and the alive name of the remote node
          to be specified. <c><![CDATA[ei_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>
        <p>Example:</p>
        <code type="none"><![CDATA[
#define NODE   "[email protected]"
#define ALIVE  "madonna"
#define IP_ADDR "150.236.14.75"

/*** Variant 1 ***/
int fd = ei_connect(&ec, NODE);

/*** Variant 2 ***/
struct in_addr addr;
addr = inet_addr(IP_ADDR);
fd = ei_xconnect(&ec, &addr, ALIVE);
        ]]></code>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_connect_tmo(ei_cnode* ec, char *nodename, unsigned timeout_ms)</nametext></name>
      <name><ret>int</ret><nametext>ei_xconnect_tmo(ei_cnode* ec, Erl_IpAddr adr, char *alivename, unsigned timeout_ms)</nametext></name>
      <fsummary>Establish a connection to an Erlang node with optional timeout</fsummary>
      <desc>
        <p>ei_connect and ei_xconnect with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_receive(int fd, unsigned char* bufp, int bufsize)</nametext></name>
      <fsummary>Receive a message</fsummary>
      <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. It
          is obtained from a previous <c><![CDATA[ei_connect]]></c> or
          <c><![CDATA[ei_accept]]></c>.</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 <c><![CDATA[ERL_ERROR]]></c> 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>ei_receive_tmo(int fd, unsigned char* bufp, int bufsize, unsigned timeout_ms)</nametext></name>
      <fsummary>Receive a message with optional timeout</fsummary>
      <desc>
        <p>ei_receive with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_receive_msg(int fd, erlang_msg* msg, ei_x_buff* x)</nametext></name>
      <name><ret>int</ret><nametext>ei_xreceive_msg(int fd, erlang_msg* msg, ei_x_buff* x)</nametext></name>
      <fsummary>Receive a message</fsummary>
      <desc>
        <p>These functions receives a message to the buffer in
          <c><![CDATA[x]]></c>.  <c><![CDATA[ei_xreceive_msg]]></c> allows the buffer in
          <c><![CDATA[x]]></c> to grow, but <c><![CDATA[ei_receive_msg]]></c> fails if the
          message is bigger than the preallocated buffer in <c><![CDATA[x]]></c>.</p>
        <p><c><![CDATA[fd]]></c> is an open descriptor to an Erlang connection.</p>
        <p><c><![CDATA[msg]]></c> is a pointer to an <c><![CDATA[erlang_msg]]></c> structure
          and contains information on the message received.</p>
        <p><c><![CDATA[x]]></c> is buffer obtained from <c><![CDATA[ei_x_new]]></c>.</p>
        <p>On success, the function returns <c><![CDATA[ERL_MSG]]></c> and the
          <c><![CDATA[msg]]></c> struct will be initialized. <c><![CDATA[erlang_msg]]></c>
          is defined as follows:</p>
        <code type="none"><![CDATA[
typedef struct {
    long msgtype;
    erlang_pid from;
    erlang_pid to;
    char toname[MAXATOMLEN+1];
    char cookie[MAXATOMLEN+1];
    erlang_trace token;
} erlang_msg;
        ]]></code>
        <p><c><![CDATA[msgtype]]></c> identifies the type of message, and is 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[msgtype]]></c> is <c><![CDATA[ERL_SEND]]></c> this indicates that an
          ordinary send operation has taken place, and <c><![CDATA[msg->to]]></c>
          contains the Pid of the recipient (the C-node). If
          <c><![CDATA[type]]></c> is <c><![CDATA[ERL_REG_SEND]]></c> then a registered send
          operation took place, and <c><![CDATA[msg->from]]></c> contains the Pid
          of the sender.</p>
        <p>If <c><![CDATA[msgtype]]></c> is <c><![CDATA[ERL_LINK]]></c> or <c><![CDATA[ERL_UNLINK]]></c>, then
          <c><![CDATA[msg->to]]></c> and <c><![CDATA[msg->from]]></c> contain the pids of the
          sender and recipient of the link or unlink.</p>
        <p>If <c><![CDATA[msgtype]]></c> is <c><![CDATA[ERL_EXIT]]></c>, then this indicates that
          a link has been broken. In this case, <c><![CDATA[msg->to]]></c> and
          <c><![CDATA[msg->from]]></c> contain the pids of the linked processes.</p>
        <p>The return value is the same as for <c><![CDATA[ei_receive]]></c>, see
          above.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_receive_msg_tmo(int fd, erlang_msg* msg, ei_x_buff* x, unsigned imeout_ms)</nametext></name>
      <name><ret>int</ret><nametext>ei_xreceive_msg_tmo(int fd, erlang_msg* msg, ei_x_buff* x, unsigned timeout_ms)</nametext></name>
      <fsummary>Receive a message with optional timeout</fsummary>
      <desc>
        <p>ei_receive_msg and ei_xreceive_msg with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_receive_encoded(int fd, char **mbufp, int *bufsz,  erlang_msg *msg, int *msglen)</nametext></name>
      <fsummary>Obsolete function for receiving a message</fsummary>
      <desc>
        <p>This function is retained for compatibility with code
          generated by the interface compiler and with code following
          examples in the same application.</p>
        <p>In essence the function performs the same operation as
          <c><![CDATA[ei_xreceive_msg]]></c>, but instead of using an ei_x_buff, the
          function expects a pointer to a character pointer
          (<c><![CDATA[mbufp]]></c>), where the character pointer should point to a
          memory area allocated by <c><![CDATA[malloc]]></c>. The argument
          <c><![CDATA[bufsz]]></c> should be a pointer to an integer containing the
          exact size (in bytes) of the memory area. The function may
          reallocate the memory area and will in such cases put the new
          size in <c><![CDATA[*bufsz]]></c> and update <c><![CDATA[*mbufp]]></c>.</p>
        <p>Furthermore the function returns either ERL_TICK or the
          <c><![CDATA[msgtype]]></c> field of the <c><![CDATA[erlang_msg *msg]]></c>. The actual
          length of the message is put in <c><![CDATA[*msglen]]></c>. On error it
          will return a value <c><![CDATA[< 0]]></c>.</p>
        <p>It is recommended to use ei_xreceive_msg instead when
          possible, for the sake of readability. The function will
          however be retained in the interface for compatibility and
          will <em>not</em> be removed not be removed in future releases
          without notice.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_receive_encoded_tmo(int fd, char **mbufp, int *bufsz,  erlang_msg *msg, int *msglen, unsigned timeout_ms)</nametext></name>
      <fsummary>Obsolete function for receiving a message with timeout</fsummary>
      <desc>
        <p>ei_receive_encoded with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_send(int fd, erlang_pid* to, char* buf, int len)</nametext></name>
      <fsummary>Send a message</fsummary>
      <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 the Pid of the intended recipient of the
          message.</p>
        <p><c><![CDATA[buf]]></c> is the buffer containing the term in binary
          format.</p>
        <p><c><![CDATA[len]]></c> is the length of the message in bytes.</p>
        <p>The function returns 0 if successful, otherwise -1, in the
          latter case it will set <c><![CDATA[erl_errno]]></c> to <c><![CDATA[EIO]]></c>.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_send_tmo(int fd, erlang_pid* to, char* buf, int len, unsigned timeout_ms)</nametext></name>
      <fsummary>Send a message with optional timeout</fsummary>
      <desc>
        <p>ei_send with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_send_encoded(int fd, erlang_pid* to, char* buf, int len)</nametext></name>
      <fsummary>Obsolete function to send a message</fsummary>
      <desc>
        <p>Works exactly as ei_send, the alternative name retained for
          backward compatibility. The function will <em>not</em> be
          removed without notice.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_send_encoded_tmo(int fd, erlang_pid* to, char* buf, int len, unsigned timeout_ms)</nametext></name>
      <fsummary>Obsolete function to send a message with optional timeout</fsummary>
      <desc>
        <p>ei_send_encoded with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_reg_send(ei_cnode* ec, int fd, char* server_name, char* buf, int len)</nametext></name>
      <fsummary>Send a message to a registered name</fsummary>
      <desc>
        <p>This function sends an Erlang term to a registered process.
          </p>
        <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[server_name]]></c> is the registered name of the intended
          recipient.</p>
        <p><c><![CDATA[buf]]></c> is the buffer containing the term in binary
          format.</p>
        <p><c><![CDATA[len]]></c> is the length of the message in bytes.</p>
        <p>The function returns 0 if successful, otherwise -1, in the
          latter case it will set <c><![CDATA[erl_errno]]></c> to <c><![CDATA[EIO]]></c>.</p>
        <p>Example, send the atom "ok" to the process "worker":</p>
        <code type="none"><![CDATA[
ei_x_buff x;
ei_x_new_with_version(&x);
ei_x_encode_atom(&x, "ok");
if (ei_reg_send(&ec, fd, x.buff, x.index) < 0)
    handle_error();
        ]]></code>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_reg_send_tmo(ei_cnode* ec, int fd, char* server_name, char* buf, int len, unsigned timeout_ms)</nametext></name>
      <fsummary>Send a message to a registered name with optional timeout</fsummary>
      <desc>
        <p>ei_reg_send with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_send_reg_encoded(int fd, const erlang_pid *from, const char *to, const char *buf, int len)</nametext></name>
      <fsummary>Obsolete function to send a message to a registered name</fsummary>
      <desc>
        <p>This function is retained for compatibility with code
          generated by the interface compiler and with code following
          examples in the same application.</p>
        <p>The function works as <c><![CDATA[ei_reg_send]]></c> with one
          exception. Instead of taking the <c><![CDATA[ei_cnode]]></c> as a first
          argument, it takes a second argument, an <c><![CDATA[erlang_pid]]></c>
          which should be the process identifier of the sending process
          (in the erlang distribution protocol). </p>
        <p>A suitable <c><![CDATA[erlang_pid]]></c> can be constructed from the
          <c><![CDATA[ei_cnode]]></c> structure by the following example code:</p>
        <code type="none"><![CDATA[
          ei_cnode ec;
          erlang_pid *self;
          int fd; /* the connection fd */
          ...
          self = ei_self(&ec);
          self->num = fd;
        ]]></code>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_send_reg_encoded_tmo(int fd, const erlang_pid *from, const char *to, const char *buf, int len)</nametext></name>
      <fsummary>Obsolete function to send a message to a registered name with timeout</fsummary>
      <desc>
        <p>ei_send_reg_encoded with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_rpc(ei_cnode *ec, int fd, char *mod, char *fun, const char *argbuf, int argbuflen, ei_x_buff *x)</nametext></name>
      <name><ret>int</ret><nametext>ei_rpc_to(ei_cnode *ec, int fd, char *mod, char *fun, const char *argbuf, int argbuflen)</nametext></name>
      <name><ret>int</ret><nametext>ei_rpc_from(ei_cnode *ec, int fd, int timeout, erlang_msg *msg, ei_x_buff *x)</nametext></name>
      <fsummary>Remote Procedure Call from C to Erlang</fsummary>
      <desc>
        <p>These functions support calling Erlang functions on remote nodes.
          <c><![CDATA[ei_rpc_to()]]></c> sends an rpc request to a remote node and
          <c><![CDATA[ei_rpc_from()]]></c> receives the results of such a call.
          <c><![CDATA[ei_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[ec]]></c> is the C-node structure previously initiated by a
          call to <c><![CDATA[ei_connect_init()]]></c> or
          <c><![CDATA[ei_connect_xinit()]]></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.
          <c><![CDATA[ei_rpc()]]></c> will wait infinitely for the answer,
          i.e. the call will never time out.</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[argbuf]]></c> is a pointer to a buffer with an encoded
          Erlang list, without a version magic number, containing the
          arguments to be passed to the function.</p>
        <p><c><![CDATA[argbuflen]]></c> is the length of the buffer containing the
          encoded Erlang list.</p>
        <p><c><![CDATA[msg]]></c> structure of type <c><![CDATA[erlang_msg]]></c> and contains
          information on the message received. See <c><![CDATA[ei_receive_msg()]]></c>
          for a description of the <c><![CDATA[erlang_msg]]></c> format.</p>
        <p><c><![CDATA[x]]></c> points to the dynamic buffer that receives the
          result. For for <c><![CDATA[ei_rpc()]]></c> this will be the result
          without the version magic number. For <c><![CDATA[ei_rpc_from()]]></c>
          the result will return a version magic number and a 2-tuple
          <c><![CDATA[{rex,Reply}]]></c>.</p>
        <p><c><![CDATA[ei_rpc()]]></c> returns the number of bytes in the result
          on success and -1 on failure. <c><![CDATA[ei_rpc_from()]]></c> returns
          number of bytes or 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[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>
        <p>Example, check to see if an erlang process is alive:</p>
        <code type="none"><![CDATA[
int index = 0, is_alive;
ei_x_buff args, result;

ei_x_new(&result);
ei_x_new(&args);
ei_x_encode_list_header(&args, 1);
ei_x_encode_pid(&args, &check_pid);
ei_x_encode_empty_list(&args);

if (ei_rpc(&ec, fd, "erlang", "is_process_alive",
           args.buff, args.index, &result) < 0)
    handle_error();

if (ei_decode_version(result.buff, &index) < 0
    || ei_decode_bool(result.buff, &index, &is_alive) < 0)
    handle_error();
        ]]></code>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_publish(ei_cnode *ec, int port)</nametext></name>
      <fsummary>Publish a node name</fsummary>
      <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[ec]]></c> is the C-node structure.</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><c><![CDATA[addr]]></c> is the 32-bit IP address of the local host.</p>
        <p>To unregister with epmd, simply close the returned
          descriptor. See also <c><![CDATA[ei_unpublish()]]></c>.</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 <c><![CDATA[EIO]]></c>.</p>
        <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>ei_publish_tmo(ei_cnode *ec, int port, unsigned timeout_ms)</nametext></name>
      <fsummary>Publish a node name with optional timeout</fsummary>
      <desc>
        <p>ei_publish with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_accept(ei_cnode *ec, int listensock, ErlConnect *conp)</nametext></name>
      <fsummary>Accept a connection from another node</fsummary>
      <desc>
        <p>This function is used by a server process to accept a
          connection from a client process.</p>
        <p><c><![CDATA[ec]]></c> is the C-node structure.</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>int</ret><nametext>ei_accept_tmo(ei_cnode *ec, int listensock, ErlConnect *conp, unsigned timeout_ms)</nametext></name>
      <fsummary>Accept a connection from another node with optional timeout</fsummary>
      <desc>
        <p>ei_accept with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_unpublish(ei_cnode *ec)</nametext></name>
      <fsummary>Unpublish a node name</fsummary>
      <desc>
        <p>This function can be called by a process to unregister a
          specified node from epmd on the localhost. This may be
          useful, for example, when epmd has not detected the failure of a
          node, and will not allow the name to be reused. If you use this
          function to unregister your own process, be sure to also close
          the descriptor that was returned by <c><![CDATA[ei_publish()]]></c>.</p>
        <note>
          <p>Careless use of this function may have unpredictable
            results, if the registered node is in fact still running.</p>
        </note>
        <p><c><![CDATA[ec]]></c> is the node structure of the node to unregister.</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>int</ret><nametext>ei_unpublish_tmo(ei_cnode *ec, unsigned timeout_ms)</nametext></name>
      <fsummary>Unpublish a node name with optional timeout</fsummary>
      <desc>
        <p>ei_unpublish with an optional timeout argument,
          see the description at the beginning of this document.</p>
      </desc>
    </func>
    <func>
      <name><ret>const char *</ret><nametext>ei_thisnodename(ei_cnode *ec)</nametext></name>
      <name><ret>const char *</ret><nametext>ei_thishostname(ei_cnode *ec)</nametext></name>
      <name><ret>const char *</ret><nametext>ei_thisalivename(ei_cnode *ec)</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[ei_connect_init()]]></c> or <c><![CDATA[ei_connect_xinit()]]></c>.</p>
        <p>They simply fetches the appropriate field from the <c><![CDATA[ec]]></c>
          structure. Read the field directly will probably be safe for
          a long time, so these functions are not really needed.</p>
      </desc>
    </func>
    <func>
      <name><ret>erlang_pid *</ret><nametext>ei_self(ei_cnode *ec)</nametext></name>
      <fsummary>Retrieve the Pid of the C-node</fsummary>
      <desc>
        <p>This function retrieves the Pid of the C-node. Every C-node
          has a (pseudo) pid used in <c><![CDATA[ei_send_reg]]></c>, <c><![CDATA[ei_rpc]]></c>
          and others. This is contained in a field in the <c><![CDATA[ec]]></c>
          structure. It will be safe for a long time to fetch this
          field directly from the <c><![CDATA[ei_cnode]]></c> structure.</p>
      </desc>
    </func>
    <func>
      <name><ret>struct hostent</ret><nametext>*ei_gethostbyname(const char *name)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*ei_gethostbyaddr(const char *addr, int len, int type)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*ei_gethostbyname_r(const char *name,  struct hostent *hostp,  char *buffer,  int buflen,  int *h_errnop)</nametext></name>
      <name><ret>struct hostent</ret><nametext>*ei_gethostbyaddr_r(const char *addr, int length,  int type,  struct hostent *hostp, char *buffer,   int buflen,  int *h_errnop)</nametext></name>
      <fsummary>Name lookup functions</fsummary>
      <desc>
        <p>These are convenience functions for some common name lookup functions.</p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>ei_get_tracelevel(void)</nametext></name>
      <name><ret>void</ret><nametext>ei_set_tracelevel(int level)</nametext></name>
      <fsummary>Get and set functions for tracing.</fsummary>
      <desc>
        <p>These functions are used to set tracing on the distribution. The levels are different verbosity levels. A higher level means more information.
	See also Debug Information and <c><![CDATA[EI_TRACELEVEL]]></c> below. </p>
	<p> <c><![CDATA[ei_set_tracelevel]]></c> and <c><![CDATA[ei_get_tracelevel]]></c> are not thread safe.  </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[ei]]></c>
       library.</item>
      <item>the environment variable <c><![CDATA[ERL_EPMD_PORT]]></c>
       is set correctly.</item>
    </list>
    <p>The connection attempt can be traced by setting a tracelevel by either using
    <c><![CDATA[ei_set_tracelevel]]></c> or by setting the environment variable <c><![CDATA[EI_TRACELEVEL]]></c>.
    The different tracelevels has the following messages:</p>
    <list>
    	<item>1: Verbose error messages</item>
    	<item>2: Above messages and verbose warning messages </item>
    	<item>3: Above messages and progress reports for connection handling</item>
    	<item>4: Above messages and progress reports for communication</item>
    	<item>5: Above messages and progress reports for data conversion</item>
    </list>
  </section>

</cref>