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<!-- $Id: comstack.xml,v 1.5 2001-10-26 20:13:44 adam Exp $ -->
 <chapter id="comstack"><title>The COMSTACK Module</title>

  <sect1 id="comstack.synopsis"><title>Synopsis (blocking mode)</title>

   <programlisting>

COMSTACK *stack;
char *buf = 0;
int size = 0, length_incoming;
char *protocol_package; 
int protocol_package_length;
char server_address[] = "myserver.com:2100";
int status;

stack = cs_create(tcpip_type, 1, PROTO_Z3950);
if (!stack) {
    perror("cs_create");  /* note use of perror() here since we have no stack yet */
    exit(1);
}

status = cs_connect(stack, server_address);
if (status != 0) {
    cs_perror(stack, "cs_connect");
    exit(1);
}

status = cs_put(stack, protocol_package, protocol_package_length);
if (status) {
    cs_perror(stack, "cs_put");
    exit(1);
}

/* Now get a response */

length_incoming = cs_get(stack, &amp;buf, &amp;size);
if (!length_incoming) {
    fprintf(stderr, "Connection closed\n");
    exit(1);
} else if (length_incoming < 0) {
    cs_perror(stack, "cs_get");
    exit(1);
}

/* Do stuff with buf here */

/* clean up */
cs_close(stack);
if (buf)
    free(buf);
    
   </programlisting>

  </sect1>
  <sect1 id="comstack.introduction"><title>Introduction</title>

   <para>
    The &comstack;
    subsystem provides a transparent interface to different types of transport
    stacks for the exchange of BER-encoded data. At present, the
    RFC1729 method (BER over TCP/IP), and Peter Furniss' XTImOSI
    stack are supported, but others may be added in time. The philosophy of the
    module is to provide a simple interface by hiding unused options and
    facilities of the underlying libraries. This is always done at the risk
    of losing generality, and it may prove that the interface will need
    extension later on.
   </para>

   <para>
    The interface is implemented in such a fashion that only the
    sub-layers constructed to the transport methods that you wish to
    use in your application are linked in.
   </para>

   <para>
    You will note that even though simplicity was a goal in the design,
    the interface is still orders of magnitudes more complex than the
    transport systems found in many other packages. One reason is that
    the interface needs to support the somewhat different requirements of
    the different lower-layer communications stacks; another important reason is
    that the interface seeks to provide a more or less industrial-strength
    approach to asynchronous event-handling. When no function is allowed
    to block, things get more complex - particularly on the server
    side. We urge you to have a look at the demonstration client and server
    provided with the package. They are meant to be easily readable and
    instructive, while still being at least moderately useful.
   </para>

  </sect1>
  <sect1 id="comstack.common"><title>Common Functions</title>

   <sect2><title>Managing Endpoints</title>

    <synopsis>
     COMSTACK cs_create(CS_TYPE type, int blocking, int protocol);
    </synopsis>

    <para>
     Creates an instance of the protocol stack - a communications endpoint.
     The <literal>type</literal> parameter determines the mode of communication.
     At present, the values
     <literal>tcpip_type</literal>
     and
     <literal>mosi_type</literal>
     are recognized. The function returns a null-pointer if a system error
     occurs. The <literal>blocking</literal> parameter should be one if you wish
     the association to operate in blocking mode, zero otherwise. The
     <literal>protocol</literal> field should be one of
     <literal>PROTO_SR</literal> or <literal>PROTO_Z3950</literal>.
    </para>


    <synopsis>
     int cs_close(COMSTACK handle);
    </synopsis>

    <para>
     Closes the connection (as elegantly as the lower layers will permit),
     and releases the resources pointed to by the
     <literal>handle</literal>
     parameter. The
     <literal>handle</literal>
     should not be referenced again after this call.
    </para>

    <note>
     <para>
      We really need a soft disconnect, don't we?
     </para>
    </note>
   </sect2>

   <sect2><title>Data Exchange</title>

    <synopsis>
     int cs_put(COMSTACK handle, char *buf, int len);
    </synopsis>

    <para>
     Sends
     <literal>buf</literal>
     down the wire. In blocking mode, this function will return only when a
     full buffer has been written, or an error has occurred. In nonblocking
     mode, it's possible that the function will be unable to send the full
     buffer at once, which will be indicated by a return value of 1. The
     function will keep track of the number of octets already written; you
     should call it repeatedly with the same values of <literal>buf</literal>
     and <literal>len</literal>, until the buffer has been transmitted.
     When a full buffer has been sent, the function will return 0 for
     success. -1 indicates an error condition (see below).
    </para>

    <synopsis>
     int cs_get(COMSTACK handle, char **buf, int *size);
    </synopsis>

    <para>
     Receives a PDU from the peer. Returns the number of bytes
     read. In nonblocking mode, it is possible that not all of the packet can be
     read at once. In this case, the function returns 1. To simplify the
     interface, the function is
     responsible for managing the size of the buffer. It will be reallocated
     if necessary to contain large packages, and will sometimes be moved
     around internally by the subsystem when partial packages are read. Before
     calling
     <function>cs_get</function>
     for the fist time, the buffer can be initialized to the null pointer,
     and the length should also be set to 0 - cs_get will perform a
     <function>malloc(2)</function>
     on the buffer for you. When a full buffer has been read, the size of
     the package is returned (which will always be greater than 1). -1
     indicates an error condition.
    </para>

    <para>
     See also the <function>cs_more()</function> function below.
    </para>

    <synopsis>
     int cs_more(COMSTACK handle);
    </synopsis>

    <para>
     The <function>cs_more()</function> function should be used in conjunction
     with <function>cs_get</function> and
     <function>select(2)</function>.
     The <function>cs_get()</function> function will sometimes
     (notably in the TCP/IP mode) read more than a single protocol package
     off the network. When this happens, the extra package is stored
     by the subsystem. After calling <function>cs_get()</function>, and before
     waiting for more input, You should always call
     <function>cs_more()</function>
     to check if there's a full protocol package already read. If
     <function>cs_more()</function>
     returns 1,
     <function>cs_get()</function>
     can be used to immediately fetch the new package. For the
     mOSI
     subsystem, the function should always return 0, but if you want your
     stuff to be protocol independent, you should use it.
    </para>

    <note>
     <para>
      The <function>cs_more()</function>
      function is required because the RFC1729-method
      does not provide a way of separating individual PDUs, short of
      partially decoding the BER. Some other implementations will carefully
      nibble at the packet by calling
      <function>read(2)</function>
      several times. This was felt to be too inefficient (or at least
      clumsy) - hence the call for this extra function.
     </para>
    </note>

    <synopsis>
     int cs_look(COMSTACK handle);
    </synopsis>

    <para>
     This function is useful when you're operating in nonblocking
     mode. Call it when
     <function>select(2)</function>
     tells you there's something happening on the line. It returns one of
     the following values:
    </para>

    <variablelist>
     <varlistentry><term>CS_NONE</term><listitem><para>
        No event is pending. The data found on the line was not a complete package.
       </para></listitem></varlistentry>

     <varlistentry><term>CS_CONNECT</term><listitem><para>
        A response to your connect request has been received. Call
        <function>cs_rcvconnect</function>
        to process the event and to finalize the connection establishment.
       </para></listitem></varlistentry>

     <varlistentry><term>CS_DISCON</term><listitem><para>
        The other side has closed the connection (or maybe sent a disconnect
        request - but do we care? Maybe later). Call
        <function>cs_close</function> to close your end of the association as well.
       </para></listitem></varlistentry>

     <varlistentry><term>CS_LISTEN</term><listitem><para>
        A connect request has been received. Call <function>cs_listen</function>
        to process the event.
       </para></listitem></varlistentry>

     <varlistentry><term>CS_DATA</term><listitem><para>
        There's data to be found on the line. Call <function>cs_get</function>
        to get it.
       </para></listitem></varlistentry>
    </variablelist>

    <note>
     <para>
      You should be aware that even if
      <function>cs_look()</function>
      tells you that there's an event event pending, the corresponding
      function may still return and tell you there was nothing to be found.
      This means that only part of a package was available for reading. The
      same event will show up again, when more data has arrived.
     </para>
    </note>

    <synopsis>
     int cs_fileno(COMSTACK h);
    </synopsis>

    <para>
     Returns the file descriptor of the association. Use this when
     file-level operations on the endpoint are required
     (<function>select(2)</function> operations, specifically).
    </para>
   </sect2>

  </sect1>

  <sect1 id="comstack.client"><title>Client Side</title>

   <synopsis>
    int cs_connect(COMSTACK handle, void *address);
   </synopsis>

   <para>
    Initiate a connection with the target at <literal>address</literal>
    (more onaddresses below). The function will return 0 on success, and 1 if
    the operation does not complete immediately (this will only
    happen on a nonblocking endpoint). In this case, use
    <function>cs_rcvconnect</function> to complete the operation,
    when <function>select(2)</function> reports input pending on the
    association.
   </para>

   <synopsis>
    int cs_rcvconnect(COMSTACK handle);
   </synopsis>

   <para>
    Complete a connect operation initiated by <function>cs_connect()</function>.
    It will return 0 on success; 1 if the operation has not yet completed (in
    this case, call the function again later); -1 if an error has occurred.
   </para>

  </sect1>

  <sect1 id="comstack.server"><title>Server Side</title>

   <para>
    To establish a server under the <application>inetd</application> server, you
    can use
   </para>

   <synopsis>
    COMSTACK cs_createbysocket(int socket, CS_TYPE type, int blocking,
    int protocol);
   </synopsis>

   <para>
    The <literal>socket</literal> parameter is an established socket (when
    your application is invoked from <application>inetd</application>, the
    socket will typically be 0.
    The following parameters are identical to the ones for
    <function>cs_create</function>.
   </para>

   <synopsis>
    int cs_bind(COMSTACK handle, void *address, int mode)
   </synopsis>

   <para>
    Binds a local address to the endpoint. Read about addresses below. The
    <literal>mode</literal> parameter should be either
    <literal>CS_CLIENT</literal> or <literal>CS_SERVER</literal>.
   </para>

   <synopsis>
    int cs_listen(COMSTACK handle, char *addr, int *addrlen);
   </synopsis>

   <para>
    Call this to process incoming events on an endpoint that has been
    bound in listening mode. It will return 0 to indicate that the connect
    request has been received, 1 to signal a partial reception, and -1 to
    indicate an error condition.
   </para>

   <synopsis>
    COMSTACK cs_accept(COMSTACK handle);
   </synopsis>

   <para>
    This finalizes the server-side association establishment, after
    cs_listen has completed successfully. It returns a new connection
    endpoint, which represents the new association. The application will
    typically wish to fork off a process to handle the association at this
    point, and continue listen for new connections on the old
    <literal>handle</literal>.
   </para>

   <para>
    You can use the call
   </para>

   <synopsis>
    char *cs_addrstr(COMSTACK);
   </synopsis>

   <para>
    on an established connection to retrieve the host-name of the remote host.
   </para>

   <note>
    <para>You may need to use this function with some care if your
     name server service is slow or unreliable
    </para>
   </note>

  </sect1>
  <sect1 id="comstack.addresses"><title>Addresses</title>

   <para>
    The low-level format of the addresses are different depending on the
    mode of communication you have chosen. A function is provided by each
    of the lower layers to map a user-friendly string-form address to the
    binary form required by the lower layers.
   </para>

   <synopsis>
    struct sockaddr_in *tcpip_strtoaddr(char *str);

    struct netbuf *mosi_strtoaddr(char *str);
   </synopsis>

   <para>
    The format for TCP/IP addresses is straightforward:
   </para>

   <synopsis>
    &lt;host> &lsqb; ':' &lt;portnum> &rsqb;
   </synopsis>

   <para>
    The <literal>hostname</literal> can be either a domain name or an IP address.
    The port number, if omitted, defaults to 210.
   </para>

   <para>
    For OSI, the format is
   </para>

   <synopsis>
    &lsqb; &lt;t-selector> '/' &rsqb; &lt;host> &lsqb; ':' &lt;port> &rsqb;
   </synopsis>

   <para>
    The transport selector is given as an even number of hex digits.
   </para>

   <para>
    You'll note that the address format for the OSI mode are just a subset
    of full presentation addresses. We use presentation addresses because
    xtimosi doesn't, in itself, allow access to the X.500 Directory
    service. We use a limited form, because we haven't yet come across an
    implementation that used more of the elements of a full p-address. It
    is a fairly simple matter to add the rest of the elements to the
    address format as needed, however: Xtimosi <emphasis>does</emphasis>
    support the full P-address structure.
   </para>

   <para>
    In both transport modes, the special hostname &quot;@&quot; is mapped
    to any local address (the manifest constant <literal>INADDR_ANY</literal>).
    It is used to establish local listening endpoints in the server role.
   </para>

   <para>
    When a connection has been established, you can use
   </para>

   <synopsis>
    char cs_addrstr(COMSTACK h);
   </synopsis>

   <para>
    to retrieve the host name of the peer system. The function returns a pointer
    to a static area, which is overwritten on the next call to the function.
   </para>

   <note>
    <para>
     We have left the issue of X.500 name-to-address mapping open, for the
     moment. It would be a simple matter to provide a table-based mapping,
     if desired. Alternately, we could use the X.500 client-function that
     is provided with the ISODE (although this would defeat some of the
     purpose of using ThinOSI in the first place. We have been told that it
     should be within the realm of the possible to implement a lightweight
     implementation of the necessary X.500 client capabilities on top of
     ThinOSI. This would be the ideal solution, we feel. On the other hand, it
     still remains to be seen just what role the Directory will play in a world
     populated by ThinOSI and other pragmatic solutions.
    </para>
   </note>

  </sect1>

  <sect1 id="comstack.diagnostics"><title>Diagnostics</title>

   <para>
    All functions return -1 if an error occurs. Typically, the functions
    will return 0 on success, but the data exchange functions
    (<function>cs_get</function>, <function>cs_put</function>,
    <function>cs_more</function>) follow special rules. Consult their
    descriptions.
   </para>

   <para>
    When a function (including the data exchange functions) reports an
    error condition, use the function
    <function>cs_errno()</function> to determine the cause of the
    problem. The function
   </para>

   <synopsis>
    void cs_perror(COMSTACK handle char *message);
   </synopsis>

   <para>
    works like <function>perror(2)</function> and prints the
    <literal>message</literal> argument, along with a system message, to
    <literal>stderr</literal>. Use the character array
   </para>

   <synopsis>
    extern const char *cs_errlist&lsqb;&rsqb;;
   </synopsis>

   <para>
    to get hold of the message, if you want to process it differently.
    The function
   </para>

   <synopsis>
    const char *cs_stackerr(COMSTACK handle);
   </synopsis>

   <para>
    Returns an error message from the lower layer, if one has been
    provided.
   </para>
  </sect1>
  <sect1 id="comstack.summary"><title>Summary and Synopsis</title>

     <synopsis>
    #include &lt;comstack.h>
     
    #include &lt;tcpip.h>      /* this is for TCP/IP support   */
    #include &lt;xmosi.h>      /* and this is for mOSI support */
     
    COMSTACK cs_create(CS_TYPE type, int blocking, int protocol);
     
    COMSTACK cs_createbysocket(int s, CS_TYPE type, int blocking,
                               int protocol);
     
    int cs_bind(COMSTACK handle, int mode);
     
    int cs_connect(COMSTACK handle, void *address);
     
    int cs_rcvconnect(COMSTACK handle);
     
    int cs_listen(COMSTACK handle);

    COMSTACK cs_accept(COMSTACK handle);

    int cs_put(COMSTACK handle, char *buf, int len);

    int cs_get(COMSTACK handle, char **buf, int *size);

    int cs_more(COMSTACK handle);

    int cs_close(COMSTACK handle);

    int cs_look(COMSTACK handle);

    struct sockaddr_in *tcpip_strtoaddr(char *str);

    struct netbuf *mosi_strtoaddr(char *str);

    extern int cs_errno;

    void cs_perror(COMSTACK handle char *message);

    const char *cs_stackerr(COMSTACK handle);

    extern const char *cs_errlist[];
   </synopsis>
  </sect1>

 </chapter>

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