path: root/erts/doc/src/alt_dist.xml

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<chapter>
  <header>
    <copyright>
      <year>2000</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>How to Implement an Alternative Carrier for the Erlang Distribution
    </title>
    <prepared>Patrik Nyblom</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>2000-10-17</date>
    <rev>PA2</rev>
    <file>alt_dist.xml</file>
  </header>
  <p>This section describes how to implement an own carrier
    protocol for the Erlang distribution. The distribution is normally
    carried by TCP/IP. Here is explained a method for replacing TCP/IP
    with another protocol.</p>

  <p>The section is a step-by-step explanation of the
    <c><![CDATA[uds_dist]]></c> example application (in the
    <c>Kernel</c> application <c><![CDATA[examples]]></c> directory). The
    <c><![CDATA[uds_dist]]></c> application implements distribution over Unix
    domain sockets and is written for the Sun Solaris 2 operating environment.
    The mechanisms are however general and apply to any operating system Erlang
    runs on. The reason the C code is not made portable, is simply
    readability.</p>

  <note>
    <p>This section was written a long time ago. Most of it is still
      valid, but some things have changed since then.
      Most notably is the driver interface. Some updates have been made
      to the documentation of the driver presented here,
      but more can be done and is planned for the future.
      The reader is encouraged to read the
      <seealso marker="erl_driver"><c>erl_driver</c></seealso> and
      <seealso marker="driver_entry"><c>driver_entry</c></seealso>
      documentation also.</p>
  </note>

  <section>
    <title>Introduction</title>
    <p>To implement a new carrier for the Erlang distribution, the main
      steps are as follows.</p>

    <section>
      <title>Writing an Erlang Driver</title>
      <p>First, the protocol must be available to the Erlang machine, which
        involves writing an Erlang driver. A port program cannot be used,
        an Erlang driver is required. Erlang drivers can be:</p>

      <list type="bulleted">
        <item>
          <p>Statically linked to the emulator, which can be an alternative
            when using the open source distribution of Erlang, or</p>
        </item>
        <item>
          <p>Dynamically loaded into the Erlang machines address space,
            which is the only alternative if a precompiled version of 
            Erlang is to be used</p>
        </item>
      </list>

      <p>Writing an Erlang driver is not easy. The driver is written
        as some callback functions called by the Erlang emulator when
        data is sent to the driver, or the driver has any data available on
        a file descriptor. As the driver callback routines execute in the main
        thread of the Erlang machine, the callback functions can perform
        no blocking activity whatsoever. The callbacks are only to set up
        file descriptors for waiting and/or read/write available data. All
        I/O must be non-blocking. Driver callbacks are however executed
        in sequence, why a global state can safely be updated within the
        routines.</p>
    </section>

    <section>
      <title>Writing an Erlang Interface for the Driver</title>
      <p>When the driver is implemented, one would preferably write an
        Erlang interface for the driver to be able to test the
        functionality of the driver separately. This interface can then
        be used by the distribution module, which will cover the details of
        the protocol from the <c><![CDATA[net_kernel]]></c>.</p>

      <p>The easiest path
        is to mimic the <c><![CDATA[inet]]></c> and <c><![CDATA[inet_tcp]]></c>
        interfaces, but not much
        functionality in those modules needs to be implemented. In the
        example application, only a few of the usual interfaces are
        implemented, and they are much simplified.</p>
    </section>

    <section>
      <title>Writing a Distribution Module</title>
      <p>When the protocol is available to Erlang through a driver and an
        Erlang interface module, a distribution module can be written.
        The distribution module is a module with well-defined callbacks,
        much like a <c><![CDATA[gen_server]]></c> (there is no compiler support
        for checking the callbacks, though). This module implements:</p>

      <list type="bulleted">
        <item>The details of finding other nodes (that is, talking to
          <c>epmd</c> or something similar)</item>
        <item>Creating a listen port (or similar)</item>
        <item>Connecting to other nodes</item>
        <item>Performing the handshakes/cookie verification</item>
      </list>

      <p>There is however a utility module, <c><![CDATA[dist_util]]></c>, which
        does most of the hard work of handling handshakes, cookies, timers,
        and ticking. Using <c><![CDATA[dist_util]]></c> makes implementing a
        distribution module much easier and that is done in
        the example application.</p>
    </section>

    <section>
      <title>Creating Boot Scripts</title>
      <p>The last step is to create boot scripts to make the protocol
        implementation available at boot time. The implementation can be
        debugged by starting the distribution when all the system is
        running, but in a real system the distribution is to start very
        early, why a boot script and some command-line parameters are
        necessary.</p>

      <p>This step also implies that the Erlang code in the
        interface and distribution modules is written in such a way that
        it can be run in the startup phase. In particular, there can be no
        calls to the <c><![CDATA[application]]></c> module or to any modules
        not loaded at boot time. That is, only <c><![CDATA[Kernel]]></c>,
        <c><![CDATA[STDLIB]]></c>, and the application itself can be used.</p>
    </section>
  </section>

  <section>
    <title>The Driver</title>
    <p>Although Erlang drivers in general can be beyond the scope of this
      section, a brief introduction seems to be in place.</p>

    <section>
      <title>Drivers in General</title>
      <p>An Erlang driver is a native code module written in C (or
        assembler), which serves as an interface for some special operating
        system service. This is a general mechanism that is used
        throughout the Erlang emulator for all kinds of I/O. An Erlang
        driver can be dynamically linked (or loaded) to the Erlang
        emulator at runtime by using the <c><![CDATA[erl_ddll]]></c> Erlang
        module. Some of the drivers in OTP are however statically linked
        to the runtime system, but that is more an optimization than a
        necessity.</p>

      <p>The driver data types and the functions available to the driver
        writer are defined in header file <c><![CDATA[erl_driver.h]]></c> (there
        is also a deprecated version called <c><![CDATA[driver.h]]></c>, do not
        use that one) seated in Erlang's include directory (and in
        <c>$ERL_TOP/erts/emulator/beam</c> in the source code
        distribution). See that file for function prototypes, and so on.</p>

      <p>When writing a driver to make a communications protocol available
        to Erlang, one should know just about everything worth knowing
        about that particular protocol. All operation must be
        non-blocking and all possible situations are to be accounted for in
        the driver. A non-stable driver will affect and/or crash the
        whole Erlang runtime system.</p>

      <p>The emulator calls the driver in the following situations:</p>

      <list type="bulleted">
        <item>
          <p>When the driver is loaded. This callback must have a special
            name and inform the emulator of what callbacks are to be used 
            by returning a pointer to a <c><![CDATA[ErlDrvEntry]]></c> struct,
            which is to be properly filled in (see below).</p>
        </item>
        <item>
          <p>When a port to the driver is opened (by a
            <c><![CDATA[open_port]]></c> call from Erlang). This routine is to
            set up internal data structures and return an opaque data entity of
            the type <c><![CDATA[ErlDrvData]]></c>, which is a data type large
            enough to hold a pointer.
            The pointer returned by this function is the first
            argument to all other callbacks concerning this particular
            port. It is usually called the port handle. The emulator only
            stores the handle and does never try to interpret it, why it can
            be virtually anything (anything not larger than a pointer
            that is) and can point to anything if it is a pointer. Usually
            this pointer refers to a structure holding information about
            the particular port, as it does in the example.</p>
        </item>
        <item>
          <p>When an Erlang process sends data to the port. The data
            arrives as a buffer of bytes, the interpretation is not defined,
            but is up to the implementor. This callback returns nothing to the
            caller, answers are sent to the caller as messages (using a
            routine called <c><![CDATA[driver_output]]></c> available to all
            drivers). There is also a way to talk in a synchronous way to
            drivers, described below. There can be an additional callback
            function for handling data that is fragmented (sent in a deep
            io-list). That interface gets the data in a form suitable for
            Unix <c><![CDATA[writev]]></c> rather than in a single buffer.
            There is no need for a distribution driver to implement such a
            callback, so we will not.</p>
        </item>
        <item>
          <p>When a file descriptor is signaled for input. This callback
            is called when the emulator detects input on a file descriptor
            that the driver has marked for monitoring by using the interface
            <c><![CDATA[driver_select]]></c>. The mechanism of driver select
            makes it possible to read non-blocking from file descriptors by
            calling <c><![CDATA[driver_select]]></c> when reading is needed, and
            then do the reading in this callback (when reading is possible).
            The typical scenario is that <c><![CDATA[driver_select]]></c> is
            called when an Erlang process orders a read operation, and that
            this routine sends the answer when data is available on the file
            descriptor.</p>
        </item>
        <item>
          <p>When a file descriptor is signaled for output. This callback
            is called in a similar way as the previous, but when writing to a
            file descriptor is possible. The usual scenario is that Erlang
            orders writing on a file descriptor and that the driver calls
            <c><![CDATA[driver_select]]></c>. When the descriptor is ready for
            output, this callback is called and the driver can try to send the
            output. Queuing can be involved in such operations, and there are
            convenient queue routines available to the driver writer to use.</p>
        </item>
        <item>
          <p>When a port is closed, either by an Erlang process or by the
            driver calling one of the <c><![CDATA[driver_failure_XXX]]></c>
            routines. This routine is to clean up everything connected to one
            particular port. When other callbacks call a
            <c><![CDATA[driver_failure_XXX]]></c> routine, this routine is
            immediately called. The callback routine issuing the error can
            make no more use of the data structures for the port, as this
            routine surely has freed all associated data and closed all file
            descriptors. If the queue utility available to driver writer is
            used, this routine is however <em>not</em> called until the
            queue is empty.</p>
        </item>
        <item>
          <p>When an Erlang process calls
            <seealso marker="erlang#port_control/3">
            <c>erlang:port_control/3</c></seealso>,
            which is a synchronous interface to drivers. The control interface
            is used to set driver options, change states of ports, and so on.
            This interface is used a lot in the example.</p>
        </item>
        <item>
          <p>When a timer expires. The driver can set timers with the function
            <c><![CDATA[driver_set_timer]]></c>. When such timers expire, a
            specific callback function is called. No timers are used in
            the example.</p>
        </item>
        <item>
          <p>When the whole driver is unloaded. Every resource allocated
            by the driver is to be freed.</p>
        </item>
      </list>
    </section>

    <section>
      <title>The Data Structures of the Distribution Driver</title>
      <p>The driver used for Erlang distribution is to implement a
        reliable, order maintaining, variable length packet-oriented
        protocol. All error correction, resending and such need to be
        implemented in the driver or by the underlying communications
        protocol. If the protocol is stream-oriented (as is the case with
        both TCP/IP and our streamed Unix domain sockets), some mechanism
        for packaging is needed. We will use the simple method of having a
        header of four bytes containing the length of the package in a
        big-endian 32-bit integer. As Unix domain sockets only can be used
        between processes on the same machine, we do not need to
        code the integer in some special endianess, but we will do it anyway
        because in most situation you need to do it. Unix domain
        sockets are reliable and order maintaining, so we do not need to
        implement resends and such in the driver.</p>

      <p>We start writing the example Unix domain sockets driver by
        declaring prototypes and filling in a static <c>ErlDrvEntry</c>
        structure:</p>

      <code type="none"><![CDATA[
( 1) #include <stdio.h>
( 2) #include <stdlib.h>
( 3) #include <string.h>
( 4) #include <unistd.h>
( 5) #include <errno.h>
( 6) #include <sys/types.h>
( 7) #include <sys/stat.h>
( 8) #include <sys/socket.h>
( 9) #include <sys/un.h>
(10) #include <fcntl.h>

(11) #define HAVE_UIO_H
(12) #include "erl_driver.h"

(13) /*
(14) ** Interface routines
(15) */
(16) static ErlDrvData uds_start(ErlDrvPort port, char *buff);
(17) static void uds_stop(ErlDrvData handle);
(18) static void uds_command(ErlDrvData handle, char *buff, int bufflen);
(19) static void uds_input(ErlDrvData handle, ErlDrvEvent event);
(20) static void uds_output(ErlDrvData handle, ErlDrvEvent event);
(21) static void uds_finish(void);
(22) static int uds_control(ErlDrvData handle, unsigned int command, 
(23)                        char* buf, int count, char** res, int res_size);

(24) /* The driver entry */
(25) static ErlDrvEntry uds_driver_entry = {
(26)     NULL,                            /* init, N/A */
(27)     uds_start,                       /* start, called when port is opened */
(28)     uds_stop,                        /* stop, called when port is closed */
(29)     uds_command,                     /* output, called when erlang has sent */
(30)     uds_input,                       /* ready_input, called when input
(31)                                         descriptor ready */
(32)     uds_output,                      /* ready_output, called when output 
(33)                                         descriptor ready */
(34)     "uds_drv",                       /* char *driver_name, the argument 
(35)                                         to open_port */
(36)     uds_finish,                      /* finish, called when unloaded */
(37)     NULL,                            /* void * that is not used (BC) */
(38)     uds_control,                     /* control, port_control callback */
(39)     NULL,                            /* timeout, called on timeouts */
(40)     NULL,                            /* outputv, vector output interface */
(41)     NULL,                            /* ready_async callback */
(42)     NULL,                            /* flush callback */
(43)     NULL,                            /* call callback */
(44)     NULL,                            /* event callback */
(45)     ERL_DRV_EXTENDED_MARKER,         /* Extended driver interface marker */
(46)     ERL_DRV_EXTENDED_MAJOR_VERSION,  /* Major version number */
(47)     ERL_DRV_EXTENDED_MINOR_VERSION,  /* Minor version number */
(48)     ERL_DRV_FLAG_SOFT_BUSY,          /* Driver flags. Soft busy flag is
(49)                                         required for distribution drivers */
(50)     NULL,                            /* Reserved for internal use */
(51)     NULL,                            /* process_exit callback */
(52)     NULL                             /* stop_select callback */
(53) };]]></code>

      <p>On line 1-10 the OS headers needed for the driver are included.
        As this driver is written for Solaris, we know that the
        header <c><![CDATA[uio.h]]></c> exists. So the preprocessor variable
        <c><![CDATA[HAVE_UIO_H]]></c> can be defined before
        <c><![CDATA[erl_driver.h]]></c> is included on line 12.
        The definition of <c><![CDATA[HAVE_UIO_H]]></c> will make the
        I/O vectors used in Erlang's driver queues to correspond to the
        operating systems ditto, which is very convenient.</p>

      <p>On line 16-23 the different callback functions are declared ("forward
        declarations").</p>

      <p>The driver structure is similar for statically linked-in
        drivers and dynamically loaded. However, some of the fields
        are to be left empty (that is, initialized to NULL) in the
        different types of drivers. The first field (the <c><![CDATA[init]]></c>
        function pointer) is always left blank in a dynamically loaded
        driver, see line 26. <c>NULL</c> on line 37
        is always to be there, the field is no longer used and is
        retained for backward compatibility. No timers are used in this
        driver, why no callback for timers is needed. The <c>outputv</c> field
        (line 40) can be used to implement an interface similar to
        Unix <c><![CDATA[writev]]></c> for output. The Erlang runtime
        system could previously not use <c>outputv</c> for the
        distribution, but it can as from <c>ERTS</c> 5.7.2.
        As this driver was written before <c>ERTS</c> 5.7.2 it does 
        not use the <c>outputv</c> callback. Using the <c>outputv</c>
        callback is preferred, as it reduces copying of data. (We
        will however use scatter/gather I/O internally in the driver.)</p>

      <p>As from <c>ERTS</c> 5.5.3 the driver interface was extended with
        version control and the possibility to pass capability information.
        Capability flags are present on line 48. As from <c>ERTS</c> 5.7.4 flag
        <seealso marker="driver_entry#driver_flags">
        <c>ERL_DRV_FLAG_SOFT_BUSY</c></seealso> is required for drivers that
        are to be used by the distribution. The soft busy flag implies that the
        driver can handle calls to the <c>output</c> and <c>outputv</c>
        callbacks although it has marked itself as busy. This has always been a
        requirement on drivers used by the distribution, but no capability
        information has been available about this previously. For more
        information. see <seealso marker="erl_driver#set_busy_port">
        <c>erl_driver:set_busy_port()</c></seealso>).</p>

      <p>This driver was written before the runtime system had SMP support.
        The driver will still function in the runtime system with SMP support,
        but performance will suffer from lock contention on the driver lock
        used for the driver. This can be alleviated by reviewing and perhaps
        rewriting the code so that each instance of the driver safely can
        execute in parallel. When instances safely can execute in parallel, it
        is safe to enable instance-specific locking on the driver. This is done
        by passing <seealso marker="driver_entry#driver_flags">
        <c>ERL_DRV_FLAG_USE_PORT_LOCKING</c></seealso> as a driver flag. This
        is left as an exercise for the reader.</p>

      <p>Thus, the defined callbacks are as follows:</p>

      <taglist>
        <tag><c>uds_start</c></tag>
        <item>
          <p>Must initiate data for a port. We do not create any sockets
            here, only initialize data structures.</p>
        </item>
        <tag><c>uds_stop</c></tag>
        <item>
          <p>Called when a port is closed.</p>
        </item>
        <tag><c>uds_command</c></tag>
        <item>
          <p>Handles messages from Erlang. The
            messages can either be plain data to be sent or more subtle
            instructions to the driver. This function is here mostly for
            data pumping.</p>
        </item>
        <tag><c>uds_input</c></tag>
        <item>
          <p>Called when there is something to read from a socket.</p>
        </item>
        <tag><c>uds_output</c></tag>
        <item>
          <p>Called when it is possible to write to a socket.</p>
        </item>
        <tag><c>uds_finish</c></tag>
        <item>
          <p>Called when the driver is unloaded. A distribution driver will
            actually (or hopefully) never be unloaded, but we include this for
            completeness. To be able to clean up after oneself is always a good
            thing.</p>
        </item>
        <tag><c>uds_control</c></tag>
        <item>
          <p>The <seealso marker="erlang#port_control/3">
            <c>erlang:port_control/3</c></seealso> callback, which is
            used a lot in this implementation.</p>
        </item>
      </taglist>

      <p>The ports implemented by this driver operate in two major modes,
        named <c>command</c> and <c>data</c>. In <c>command</c> mode,
        only passive reading and writing (like
        <c>gen_tcp:recv</c>/<c>gen_tcp:send</c>) can be done. The port is in
        this mode during the distribution handshake. When the connection is up,
        the port is switched to <c>data</c> mode and all data is immediately
        read and passed further to the Erlang emulator. In <c>data</c>
        mode, no data arriving to <c>uds_command</c> is interpreted, only
        packaged and sent out on the socket. The <c>uds_control</c> callback
        does the switching between those two modes.</p>

      <p>While <c><![CDATA[net_kernel]]></c> informs different subsystems
        that the connection is coming up, the port is to accept data to send.
        However, the port receives no data, to avoid that data arrives from
        another node before every kernel subsystem is prepared to handle it.
        A third mode, named <c>intermediate</c>, is used for this
        intermediate stage.</p>

      <p>An enum is defined for the different types of ports:</p>

      <code type="none"><![CDATA[
( 1) typedef enum { 
( 2)     portTypeUnknown,      /* An uninitialized port */
( 3)     portTypeListener,     /* A listening port/socket */
( 4)     portTypeAcceptor,     /* An intermediate stage when accepting
( 5)                              on a listen port */
( 6)     portTypeConnector,    /* An intermediate stage when connecting */
( 7)     portTypeCommand,      /* A connected open port in command mode */
( 8)     portTypeIntermediate, /* A connected open port in special
( 9)                              half active mode */
(10)     portTypeData          /* A connected open port in data mode */ 
(11) } PortType;      ]]></code>

      <p>The different types are as follows:</p>

      <taglist>
        <tag><c>portTypeUnknown</c></tag>
        <item>
          <p>The type a port has when it is opened, but
            not bound to any file descriptor.</p>
        </item>
        <tag><c>portTypeListener</c></tag>
        <item>
          <p>A port that is connected to a listen socket. This port does not
            do much, no data pumping is done on this socket, but read data is
            available when one is trying to do an accept on the port.</p>
        </item>
        <tag><c>portTypeAcceptor</c></tag>
        <item>
          <p>This port is to represent the result of an accept operation. It is
            created when one wants to accept from a listen socket, and it is
            converted to a <c>portTypeCommand</c> when the accept succeeds.</p>
        </item>
        <tag><c>portTypeConnector</c></tag>
        <item>
          <p>Very similar to <c>portTypeAcceptor</c>, an
            intermediate stage between the request for a connect operation and
            that the socket is connected to an accepting ditto in the
            other end. When the sockets are connected, the port
            switches type to <c>portTypeCommand</c>.</p>
        </item>
        <tag><c>portTypeCommand</c></tag>
        <item>
          <p>A connected socket (or accepted socket) in <c>command</c> mode
            mentioned earlier.</p>
        </item>
        <tag><c>portTypeIntermediate</c></tag>
        <item>
          <p>The intermediate stage for a connected socket.
            There is to be no processing of input for this socket.</p>
        </item>
        <tag><c>portTypeData</c></tag>
        <item>
          <p>The mode where data is pumped through the port and the
            <c>uds_command</c> routine regards every call as a call where
            sending is wanted. In this mode, all input available is read and
            sent to Erlang when it arrives on the socket, much like in the
            active mode of a <c><![CDATA[gen_tcp]]></c> socket.</p>
        </item>
      </taglist>

      <p>We study the state that is needed for the ports. Notice
        that not all fields are used for all types of ports. Some space
        could be saved by using unions, but that would clutter the
        code with multiple indirections, so here is used one struct for
        all types of ports, for readability:</p>

      <code type="none"><![CDATA[
( 1) typedef unsigned char Byte;
( 2) typedef unsigned int Word;

( 3) typedef struct uds_data {
( 4)     int fd;                   /* File descriptor */
( 5)     ErlDrvPort port;          /* The port identifier */
( 6)     int lockfd;               /* The file descriptor for a lock file in 
( 7)                                  case of listen sockets */
( 8)     Byte creation;            /* The creation serial derived from the 
( 9)                                  lock file */
(10)     PortType type;            /* Type of port */
(11)     char *name;               /* Short name of socket for unlink */
(12)     Word sent;                /* Bytes sent */
(13)     Word received;            /* Bytes received */
(14)     struct uds_data *partner; /* The partner in an accept/listen pair */
(15)     struct uds_data *next;    /* Next structure in list */
(16)     /* The input buffer and its data */
(17)     int buffer_size;          /* The allocated size of the input buffer */
(18)     int buffer_pos;           /* Current position in input buffer */
(19)     int header_pos;           /* Where the current header is in the 
(20)                                  input buffer */
(21)     Byte *buffer;             /* The actual input buffer */
(22) } UdsData;      ]]></code>

      <p>This structure is used for all types of ports although some
        fields are useless for some types. The least memory consuming
        solution would be to arrange this structure as a union of
        structures. However, the multiple indirections in the code to
        access a field in such a structure would clutter the code too
        much for an example.</p>

      <p>The fields in the structure are as follows:</p>

      <taglist>
        <tag><c>fd</c></tag>
        <item>
          <p>The file descriptor of the socket associated with the port.</p>
        </item>
        <tag><c>port</c></tag>
        <item>
          <p>The port identifier for the port that this structure
            corresponds to. It is needed for most <c><![CDATA[driver_XXX]]></c>
            calls from the driver back to the emulator.</p>
        </item>
        <tag><c>lockfd</c></tag>
        <item>
          <p>If the socket is a listen socket, we use a separate
            (regular) file for two purposes:</p>
          <list type="bulleted">
            <item>
              <p>We want a locking mechanism that gives no race
                conditions, to be sure if another Erlang
                node uses the listen socket name we require or if the
                file is only left there from a previous (crashed) session.</p>
            </item>
            <item>
              <p>We store the <c>creation</c> serial number in the
                file. The <c>creation</c> is a number that is to
                change between different instances of different Erlang
                emulators with the same name, so that process
                identifiers from one emulator do not become valid when sent
                to a new emulator with the same distribution name. The
                creation can be from 0 through 3 (two bits) and is stored
                in every process identifier sent to another node.</p>
              <p>In a system with TCP-based distribution, this data is
                kept in the <em>Erlang port mapper daemon</em>
                (<c><![CDATA[epmd]]></c>), which is contacted when a distributed
                node starts. The lock file and a convention for the UDS
                listen socket's name remove the need for
                <c><![CDATA[epmd]]></c> when using this distribution module. UDS
                is always restricted to one host, why avoiding a port
                mapper is easy.</p>
            </item>
          </list>
        </item>
        <tag><c>creation</c></tag>
        <item>
          <p>The creation number for a listen socket, which is
            calculated as (the value found in the lock-file + 1) rem 4.
            This creation value is also written back into the
            lock file, so that the next invocation of the emulator
            finds our value in the file.</p>
        </item>
        <tag><c>type</c></tag>
        <item>
          <p>The current type/state of the port, which can be one
            of the values declared above.</p>
        </item>
        <tag><c>name</c></tag>
        <item>
          <p>The name of the socket file (the path prefix removed),
            which allows for deletion (<c><![CDATA[unlink]]></c>) when the
            socket is closed.</p>
        </item>
        <tag><c>sent</c></tag>
        <item>
          <p>How many bytes that have been sent over the
            socket. This can wrap, but that is no problem for the
            distribution, as the Erlang distribution is only interested in
            if this value has changed. (The Erlang
            <c>net_kernel</c> <c>ticker</c> uses this value by calling the
            driver to fetch it, which is done through the
            <seealso marker="erlang#port_control/3">
            <c>erlang:port_control/3</c></seealso> routine.)</p>
        </item>
        <tag><c>received</c></tag>
        <item>
          <p>How many bytes that are read (received) from the
            socket, used in similar ways as <c><![CDATA[sent]]></c>.</p>
        </item>
        <tag><c>partner</c></tag>
        <item>
          <p>A pointer to another port structure, which is
            either the listen port from which this port is accepting a
            connection or conversely. The "partner relation"
            is always bidirectional.</p>
        </item>
        <tag><c>next</c></tag>
        <item>
          <p>Pointer to next structure in a linked list of all
            port structures. This list is used when accepting
            connections and when the driver is unloaded.</p>
        </item>
        <tag><c>buffer_size</c>, <c>buffer_pos</c>, <c>header_pos</c>,
          <c>buffer</c></tag>
        <item>
          <p>Data for input buffering. For details about the input buffering,
            see the source code in directory <c>kernel/examples</c>. That
            certainly goes beyond the scope of this section.</p>
        </item>
      </taglist>
    </section>

    <section>
      <title>Selected Parts of the Distribution Driver Implementation</title>
      <p>The implemenation of the distribution driver is not completely
        covered here, details about buffering and other things
        unrelated to driver writing are not explained. Likewise are
        some peculiarities of the UDS protocol not explained in
        detail. The chosen protocol is not important.</p>

      <p>Prototypes for the driver callback routines can be found in
        the <c><![CDATA[erl_driver.h]]></c> header file.</p>

      <p>The driver initialization routine is (usually) declared with a
        macro to make the driver easier to port between different
        operating systems (and flavors of systems). This is the only
        routine that must have a well-defined name. All other
        callbacks are reached through the driver structure. The macro
        to use is named <c><![CDATA[DRIVER_INIT]]></c> and takes the driver name
        as parameter:</p>

      <code type="none"><![CDATA[
(1) /* Beginning of linked list of ports */
(2) static UdsData *first_data;

(3) DRIVER_INIT(uds_drv)
(4) {
(5)     first_data = NULL;
(6)     return &uds_driver_entry;
(7) }      ]]></code>

      <p>The routine initializes the single global data structure and
        returns a pointer to the driver entry. The routine is called
        when <c><![CDATA[erl_ddll:load_driver]]></c> is called from Erlang.</p>

      <p>The <c><![CDATA[uds_start]]></c> routine is called when a port is
        opened from Erlang. In this case, we only allocate a structure and
        initialize it. Creating the actual socket is left to the
        <c><![CDATA[uds_command]]></c> routine.</p>

      <code type="none"><![CDATA[
( 1) static ErlDrvData uds_start(ErlDrvPort port, char *buff)
( 2) {
( 3)     UdsData *ud;
( 4)     
( 5)     ud = ALLOC(sizeof(UdsData));
( 6)     ud->fd = -1;
( 7)     ud->lockfd = -1;
( 8)     ud->creation = 0;
( 9)     ud->port = port;
(10)     ud->type = portTypeUnknown;
(11)     ud->name = NULL;
(12)     ud->buffer_size = 0;
(13)     ud->buffer_pos = 0;
(14)     ud->header_pos = 0;
(15)     ud->buffer = NULL;
(16)     ud->sent = 0;
(17)     ud->received = 0;
(18)     ud->partner = NULL;
(19)     ud->next = first_data;
(20)     first_data = ud;
(21)     
(22)     return((ErlDrvData) ud);
(23) }      ]]></code>

      <p>Every data item is initialized, so that no problems arise
        when a newly created port is closed (without there being any
        corresponding socket). This routine is called when
        <c><![CDATA[open_port({spawn, "uds_drv"},[])]]></c> is called from
        Erlang.</p>

      <p>The <c><![CDATA[uds_command]]></c> routine is the routine called when
        an Erlang process sends data to the port. This routine handles all
        asynchronous commands when the port is in <c>command</c> mode and
        the sending of all data when the port is in <c>data</c> mode:</p>

      <code type="none"><![CDATA[
( 1) static void uds_command(ErlDrvData handle, char *buff, int bufflen)
( 2) {
( 3)     UdsData *ud = (UdsData *) handle;

( 4)     if (ud->type == portTypeData || ud->type == portTypeIntermediate) {
( 5)         DEBUGF(("Passive do_send %d",bufflen));
( 6)         do_send(ud, buff + 1, bufflen - 1); /* XXX */
( 7)         return;
( 8)     } 
( 9)     if (bufflen == 0) {
(10)         return;
(11)     }
(12)     switch (*buff) {
(13)     case 'L':
(14)         if (ud->type != portTypeUnknown) {
(15)             driver_failure_posix(ud->port, ENOTSUP);
(16)             return;
(17)         }
(18)         uds_command_listen(ud,buff,bufflen);
(19)         return;
(20)     case 'A':
(21)         if (ud->type != portTypeUnknown) {
(22)             driver_failure_posix(ud->port, ENOTSUP);
(23)             return;
(24)         }
(25)         uds_command_accept(ud,buff,bufflen);
(26)         return;
(27)     case 'C':
(28)         if (ud->type != portTypeUnknown) {
(29)             driver_failure_posix(ud->port, ENOTSUP);
(30)             return;
(31)         }
(32)         uds_command_connect(ud,buff,bufflen);
(33)         return;
(34)     case 'S':
(35)         if (ud->type != portTypeCommand) {
(36)             driver_failure_posix(ud->port, ENOTSUP);
(37)             return;
(38)         }
(39)         do_send(ud, buff + 1, bufflen - 1);
(40)         return;
(41)     case 'R':
(42)         if (ud->type != portTypeCommand) {
(43)             driver_failure_posix(ud->port, ENOTSUP);
(44)             return;
(45)         }
(46)         do_recv(ud);
(47)         return;
(48)     default:
(49)         return;
(50)     }
(51) }      ]]></code>

      <p>The command routine takes three parameters; the handle returned for
        the port by <c><![CDATA[uds_start]]></c>, which is a pointer
        to the internal port structure, the data buffer, and the length
        of the data buffer. The buffer is the data sent from Erlang
        (a list of bytes) converted to an C array (of bytes).</p>

      <p>If Erlang sends, for example, the list <c><![CDATA[[$a,$b,$c]]]></c>
        to the port, the <c><![CDATA[bufflen]]></c> variable is
        <c><![CDATA[3]]></c> and the <c><![CDATA[buff]]></c> variable contains
        <c><![CDATA[{'a','b','c'}]]></c> (no
        <c>NULL</c> termination). Usually the first byte is used as an
        opcode, which is the case in this driver too (at least when the
        port is in <c>command</c> mode). The opcodes are defined as follows:</p>

      <taglist>
        <tag><c>'L'&lt;socket name&gt;</c></tag>
        <item>
          <p>Creates and listens on socket with the specified name.</p>
        </item>
        <tag><c>'A'&lt;listen number as 32-bit big-endian&gt;</c></tag>
        <item>
          <p>Accepts from the listen socket identified by the specified
            identification number. The identification number is retrieved with
            the <c>uds_control</c> routine.</p>
        </item>
        <tag><c>'C'&lt;socket name&gt;</c></tag>
        <item>
          <p>Connects to the socket named &lt;socket name&gt;.</p>
        </item>
        <tag><c>'S'&lt;data&gt;</c></tag>
        <item>
          <p>Sends the data &lt;data&gt; on the
            connected/accepted socket (in <c>command</c> mode). The sending is
            acknowledged when the data has left this process.</p>
        </item>
        <tag><c>'R'</c></tag>
        <item>
          <p>Receives one packet of data.</p>
        </item>
      </taglist>

      <p>"One packet of data" in command <c>'R'</c> can be explained
        as follows. This driver always sends data packaged with a 4
        byte header containing a big-endian 32-bit integer that
        represents the length of the data in the packet. There is no
        need for different packet sizes or some kind of streamed
        mode, as this driver is for the distribution only.
        Why is the header word coded explicitly in big-endian when a UDS
        socket is local to the host? It is good practice when writing a
        distribution driver, as distribution in practice usually crosses
        the host boundaries.</p>

      <p>On line 4-8 is handled the case where the port is in <c>data</c> mode
        or <c>intermediate</c> mode and the remaining routine handles the
        different commands. The routine uses the
        <c><![CDATA[driver_failure_posix()]]></c> routine to report errors
        (see, for example, line 15). Notice that the failure routines make
        a call to the <c><![CDATA[uds_stop]]></c> routine, which will
        remove the internal port data. The handle (and the casted handle
        <c><![CDATA[ud]]></c>) is therefore <em>invalid pointers</em> after a
        <c><![CDATA[driver_failure]]></c> call and we should <em>return
        immediately</em>. The runtime system will send exit signals to all
        linked processes.</p>

      <p>The <c>uds_input</c> routine is called when data is available on a
        file descriptor previously passed to the
        <c><![CDATA[driver_select]]></c> routine. This occurs typically when
        a read command is issued and no data is available. The
        <c><![CDATA[do_recv]]></c> routine is as follows:</p>

      <code type="none"><![CDATA[
( 1) static void do_recv(UdsData *ud)
( 2) {
( 3)     int res;
( 4)     char *ibuf;
( 5)     for(;;) {
( 6)         if ((res = buffered_read_package(ud,&ibuf)) < 0) {
( 7)             if (res == NORMAL_READ_FAILURE) {
( 8)                 driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ, 1);
( 9)             } else {
(10)                 driver_failure_eof(ud->port);
(11)             }
(12)             return;
(13)         }
(14)         /* Got a package */
(15)         if (ud->type == portTypeCommand) {
(16)             ibuf[-1] = 'R'; /* There is always room for a single byte 
(17)                                opcode before the actual buffer 
(18)                                (where the packet header was) */
(19)             driver_output(ud->port,ibuf - 1, res + 1);
(20)             driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ,0);
(21)             return;
(22)         } else {
(23)             ibuf[-1] = DIST_MAGIC_RECV_TAG; /* XXX */
(24)             driver_output(ud->port,ibuf - 1, res + 1);
(25)             driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ,1);
(26)         }
(27)     }
(28) }      ]]></code>

      <p>The routine tries to read data until a packet is read or the
        <c><![CDATA[buffered_read_package]]></c> routine returns a
        <c><![CDATA[NORMAL_READ_FAILURE]]></c> (an internally defined constant
        for the module, which means that the read operation resulted in an
        <c><![CDATA[EWOULDBLOCK]]></c>). If the port is in <c>command</c> mode,
        the reading stops when one package is read. If the port is in
        <c>data</c> mode, the reading continues until the socket buffer is empty
        (read failure). If no more data can be read and more is wanted (which
        is always the case when the socket is in <c>data</c> mode),
        <c>driver_select</c> is called to make the <c><![CDATA[uds_input]]></c>
        callback be called when more data is available for reading.</p>

      <p>When the port is in <c>data</c> mode, all data is sent to Erlang in a
        format that suits the distribution. In fact, the raw data will
        never reach any Erlang process, but will be
        translated/interpreted by the emulator itself and then
        delivered in the correct format to the correct processes. In
        the current emulator version, received data is to be tagged
        with a single byte of 100. That is what the macro
        <c><![CDATA[DIST_MAGIC_RECV_TAG]]></c> is defined to. The tagging of
        data in the distribution can be changed in the future.</p>

      <p>The <c><![CDATA[uds_input]]></c> routine handles other input events
        (like non-blocking <c><![CDATA[accept]]></c>), but most importantly
        handle
        data arriving at the socket by calling <c><![CDATA[do_recv]]></c>:</p>

      <code type="none"><![CDATA[
( 1) static void uds_input(ErlDrvData handle, ErlDrvEvent event)
( 2) {
( 3)     UdsData *ud = (UdsData *) handle;

( 4)     if (ud->type == portTypeListener) {
( 5)         UdsData *ad = ud->partner;
( 6)         struct sockaddr_un peer;
( 7)         int pl = sizeof(struct sockaddr_un);
( 8)         int fd;

( 9)         if ((fd = accept(ud->fd, (struct sockaddr *) &peer, &pl)) < 0) {
(10)             if (errno != EWOULDBLOCK) {
(11)                 driver_failure_posix(ud->port, errno);
(12)                 return;
(13)             }
(14)             return;
(15)         }
(16)         SET_NONBLOCKING(fd);
(17)         ad->fd = fd;
(18)         ad->partner = NULL;
(19)         ad->type = portTypeCommand;
(20)         ud->partner = NULL;
(21)         driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ, 0);
(22)         driver_output(ad->port, "Aok",3);
(23)         return;
(24)     }
(25)     do_recv(ud);
(26) }      ]]></code>

      <p>The important line is the last line in the function: the
        <c><![CDATA[do_read]]></c> routine is called to handle new input.
        The remaining function handles input on a listen socket, which means
        that it is to be possible to do an accept on the
        socket, which is also recognized as a read event.</p>

      <p>The output mechanisms are similar to the input.
        The <c><![CDATA[do_send]]></c> routine is as follows:</p>

      <code type="none"><![CDATA[
( 1) static void do_send(UdsData *ud, char *buff, int bufflen) 
( 2) {
( 3)     char header[4];
( 4)     int written;
( 5)     SysIOVec iov[2];
( 6)     ErlIOVec eio;
( 7)     ErlDrvBinary *binv[] = {NULL,NULL};

( 8)     put_packet_length(header, bufflen);
( 9)     iov[0].iov_base = (char *) header;
(10)     iov[0].iov_len = 4;
(11)     iov[1].iov_base = buff;
(12)     iov[1].iov_len = bufflen;
(13)     eio.iov = iov;
(14)     eio.binv = binv;
(15)     eio.vsize = 2;
(16)     eio.size = bufflen + 4;
(17)     written = 0;
(18)     if (driver_sizeq(ud->port) == 0) {
(19)         if ((written = writev(ud->fd, iov, 2)) == eio.size) {
(20)             ud->sent += written;
(21)             if (ud->type == portTypeCommand) {
(22)                 driver_output(ud->port, "Sok", 3);
(23)             }
(24)             return;
(25)         } else if (written < 0) {
(26)             if (errno != EWOULDBLOCK) {
(27)                 driver_failure_eof(ud->port);
(28)                 return;
(29)             } else {
(30)                 written = 0;
(31)             }
(32)         } else {
(33)             ud->sent += written;
(34)         }
(35)         /* Enqueue remaining */
(36)     }
(37)     driver_enqv(ud->port, &eio, written);
(38)     send_out_queue(ud);
(39) }      ]]></code>

      <p>This driver uses the <c><![CDATA[writev]]></c> system call to send data
        onto the socket. A combination of <c>writev</c> and the driver output
        queues is very convenient. An <c>ErlIOVec</c> structure
        contains a <c>SysIOVec</c> (which is equivalent to the
        <c><![CDATA[struct iovec]]></c> structure defined in
        <c><![CDATA[uio.h]]></c>. The
        <c>ErlIOVec</c> also contains an array of <c>ErlDrvBinary</c>
        pointers, of the same length as the number of buffers in the
        I/O vector itself. One can use this to allocate the binaries
        for the queue "manually" in the driver, but here
        the binary array is filled with <c>NULL</c> values (line 7).
        The runtime system then allocates its own buffers when
        <c><![CDATA[driver_enqv]]></c> is called (line 37).</p>

      <p>The routine builds an I/O vector containing the header bytes
        and the buffer (the opcode has been removed and the buffer
        length decreased by the output routine). If the queue is
        empty, we write the data directly to the socket (or at
        least try to). If any data is left, it is stored in the queue
        and then we try to send the queue (line 38). An acknowledgement
        is sent when the message is delivered completely (line 22). The
        <c><![CDATA[send_out_queue]]></c> sends acknowledgements if the sending
        is completed there. If the port is in <c>command</c> mode, the Erlang
        code serializes the send operations so that only one packet
        can be waiting for delivery at a time. Therefore the acknowledgement
        can be sent whenever the queue is empty.</p>

      <p>The <c><![CDATA[send_out_queue]]></c> routine is as follows:</p>

      <code type="none"><![CDATA[
( 1) static int send_out_queue(UdsData *ud)
( 2) {
( 3)     for(;;) {
( 4)         int vlen;
( 5)         SysIOVec *tmp = driver_peekq(ud->port, &vlen);
( 6)         int wrote;
( 7)         if (tmp == NULL) {
( 8)             driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_WRITE, 0);
( 9)             if (ud->type == portTypeCommand) {
(10)                 driver_output(ud->port, "Sok", 3);
(11)             }
(12)             return 0;
(13)         }
(14)         if (vlen > IO_VECTOR_MAX) {
(15)             vlen = IO_VECTOR_MAX;
(16)         } 
(17)         if ((wrote = writev(ud->fd, tmp, vlen)) < 0) {
(18)             if (errno == EWOULDBLOCK) {
(19)                 driver_select(ud->port, (ErlDrvEvent) ud->fd, 
(20)                               DO_WRITE, 1);
(21)                 return 0;
(22)             } else {
(23)                 driver_failure_eof(ud->port);
(24)                 return -1;
(25)             }
(26)         }
(27)         driver_deq(ud->port, wrote);
(28)         ud->sent += wrote;
(29)     }
(30) }      ]]></code>

      <p>We simply pick out an I/O vector from the queue
        (which is the whole queue as a <c>SysIOVec</c>). If the I/O
        vector is too long (<c>IO_VECTOR_MAX</c> is defined to 16), the vector
        length is decreased (line 15), otherwise the <c><![CDATA[writev]]></c>
        call (line 17) fails. Writing is tried and anything written is dequeued
        (line 27).
        If the write fails with <c><![CDATA[EWOULDBLOCK]]></c> (notice that all
        sockets are in non-blocking mode), <c><![CDATA[driver_select]]></c> is
        called to make the <c><![CDATA[uds_output]]></c> routine be called when
        there is space to write again.</p>

      <p>We continue trying to write until the queue is empty or
        the writing blocks.</p>

      <p>The routine above is called from the <c><![CDATA[uds_output]]></c>
        routine:</p>

      <code type="none"><![CDATA[
( 1) static void uds_output(ErlDrvData handle, ErlDrvEvent event)
( 2) {
( 3)    UdsData *ud = (UdsData *) handle;
( 4)    if (ud->type == portTypeConnector) {
( 5)        ud->type = portTypeCommand;
( 6)        driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_WRITE, 0);
( 7)        driver_output(ud->port, "Cok",3);
( 8)        return;
( 9)    }
(10)    send_out_queue(ud);
(11) }      ]]></code>

      <p>The routine is simple: it first handles the fact that the
        output select will concern a socket in the business of
        connecting (and the connecting blocked). If the socket is in
        a connected state, it simply sends the output queue. This
        routine is called when it is possible to write to a socket
        where we have an output queue, so there is no question what to
        do.</p>

      <p>The driver implements a control interface, which is a
        synchronous interface called when Erlang calls
        <seealso marker="erlang#port_control/3">
        <c>erlang:port_control/3</c></seealso>. Only this interface
        can control the driver when it is in <c>data</c> mode. It can
        be called with the following opcodes:</p>

      <taglist>
        <tag><c>'C'</c></tag>
        <item>
          <p>Sets port in <c>command</c> mode.</p>
        </item>
        <tag><c>'I'</c></tag>
        <item>
          <p>Sets port in <c>intermediate</c> mode.</p>
        </item>
        <tag><c>'D'</c></tag>
        <item>
          <p>Sets port in <c>data</c> mode.</p>
        </item>
        <tag><c>'N'</c></tag>
        <item>
          <p>Gets identification number for listen port. This
            identification number is used in an accept command to the
            driver. It is returned as a big-endian 32-bit integer, which
            is the file identifier for the listen socket.</p>
        </item>
        <tag><c>'S'</c></tag>
        <item>
          <p>Gets statistics, which is the number of bytes received,
            the number of bytes sent, and the number of bytes pending in
            the output queue. This data is used when the distribution
            checks that a connection is alive (ticking). The statistics
            is returned as three 32-bit big-endian integers.</p>
        </item>
        <tag><c>'T'</c></tag>
        <item>
          <p>Sends a tick message, which is a packet of length 0.
            Ticking is done when the port is in <c>data</c> mode, so the
            command for sending data cannot be used (besides it ignores
            zero length packages in <c>command</c> mode). This is used by the
            ticker to send dummy data when no other traffic is present.</p>
          <p><em>Note:</em> It is important that the interface for
            sending ticks is not blocking. This implementation uses
            <seealso marker="erlang#port_control/3">
            <c>erlang:port_control/3</c></seealso>, which does not block the
            caller. If <c>erlang:port_command</c> is used, use
            <seealso marker="erlang#port_command/3">
            <c>erlang:port_command/3</c></seealso> and pass <c>[force]</c> as
            option list; otherwise the caller can be blocked indefinitely
            on a busy port and prevent the system from taking down a
            connection that is not functioning.</p>
        </item>
        <tag><c>'R'</c></tag>
        <item>
          <p>Gets creation number of a listen socket, which is used to
            dig out the number stored in the lock file to differentiate
            between invocations of Erlang nodes with the same name.</p>
        </item>
      </taglist>

      <p>The control interface gets a buffer to return its value in,
        but is free to allocate its own buffer if the provided one is
        too small. The <c><![CDATA[uds_control]]></c> code is as follows:</p>

      <code type="none"><![CDATA[
( 1) static int uds_control(ErlDrvData handle, unsigned int command, 
( 2)                        char* buf, int count, char** res, int res_size)
( 3) {
( 4) /* Local macro to ensure large enough buffer. */
( 5) #define ENSURE(N)                               \
( 6)    do {                                         \
( 7)        if (res_size < N) {                      \
( 8)            *res = ALLOC(N);                     \
( 9)        }                                        \
(10)    } while(0)

(11)    UdsData *ud = (UdsData *) handle;

(12)    switch (command) {
(13)    case 'S':
(14)        {
(15)            ENSURE(13);
(16)            **res = 0;
(17)            put_packet_length((*res) + 1, ud->received);
(18)            put_packet_length((*res) + 5, ud->sent);
(19)            put_packet_length((*res) + 9, driver_sizeq(ud->port));
(20)            return 13;
(21)        }
(22)    case 'C':
(23)        if (ud->type < portTypeCommand) {
(24)            return report_control_error(res, res_size, "einval");
(25)        }
(26)        ud->type = portTypeCommand;
(27)        driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ, 0);
(28)        ENSURE(1);
(29)        **res = 0;
(30)        return 1;
(31)    case 'I':
(32)        if (ud->type < portTypeCommand) {
(33)            return report_control_error(res, res_size, "einval");
(34)        }
(35)        ud->type = portTypeIntermediate;
(36)        driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ, 0);
(37)        ENSURE(1);
(38)        **res = 0;
(39)        return 1;
(40)    case 'D':
(41)        if (ud->type < portTypeCommand) {
(42)            return report_control_error(res, res_size, "einval");
(43)        }
(44)        ud->type = portTypeData;
(45)        do_recv(ud);
(46)        ENSURE(1);
(47)        **res = 0;
(48)        return 1;
(49)    case 'N':
(50)        if (ud->type != portTypeListener) {
(51)            return report_control_error(res, res_size, "einval");
(52)        }
(53)        ENSURE(5);
(54)        (*res)[0] = 0;
(55)        put_packet_length((*res) + 1, ud->fd);
(56)        return 5;
(57)    case 'T': /* tick */
(58)        if (ud->type != portTypeData) {
(59)            return report_control_error(res, res_size, "einval");
(60)        }
(61)        do_send(ud,"",0);
(62)        ENSURE(1);
(63)        **res = 0;
(64)        return 1;
(65)    case 'R':
(66)        if (ud->type != portTypeListener) {
(67)            return report_control_error(res, res_size, "einval");
(68)        }
(69)        ENSURE(2);
(70)        (*res)[0] = 0;
(71)        (*res)[1] = ud->creation;
(72)        return 2;
(73)    default:
(74)        return report_control_error(res, res_size, "einval");
(75)    }
(76) #undef ENSURE
(77) }      ]]></code>

      <p>The macro <c><![CDATA[ENSURE]]></c> (line 5-10) is used to ensure that
        the buffer is large enough for the answer. We switch on the command and
        take actions. We always have read select active on a port in <c>data</c>
        mode (achieved by calling <c><![CDATA[do_recv]]></c> on line 45), but
        we turn off read selection in <c>intermediate</c> and <c>command</c>
        modes (line 27 and 36).</p>

      <p>The rest of the driver is more or less UDS-specific and not of
        general interest.</p>
    </section>
  </section>

  <section>
    <title>Putting It All Together</title>
    <p>To test the distribution, the <c><![CDATA[net_kernel:start/1]]></c>
      function can be used. It is useful, as it starts the distribution on a
      running system, where tracing/debugging can be performed.
      The <c><![CDATA[net_kernel:start/1]]></c> routine takes a
      list as its single argument. The list first element in the list is to be
      the node name (without the "@hostname") as an atom. The second (and
      last) element is to be one of the atoms <c><![CDATA[shortnames]]></c> or 
      <c><![CDATA[longnames]]></c>. In the example case,
      <c><![CDATA[shortnames]]></c> is preferred.</p>

    <p>For <c>net_kernel</c> to find out which distribution module to use,
      command-line argument <c><![CDATA[-proto_dist]]></c> is used. It
      is followed by one or more distribution module names, with suffix
      "_dist" removed, that is, <c>uds_dist</c> as a distribution module
      is specified as <c><![CDATA[-proto_dist uds]]></c>.</p>

    <p>If no <c>epmd</c> (TCP port mapper daemon) is used, also command-line
      option <c><![CDATA[-no_epmd]]></c> is to be specified, which makes
      Erlang skip the <c>epmd</c> startup, both as an OS process and as an
      Erlang ditto.</p>

    <p>The path to the directory where the distribution modules reside
      must be known at boot. This can be achieved either by
      specifying <c><![CDATA[-pa <path>]]></c> on the command line or by
      building a boot script containing the applications used for your
      distribution protocol. (In the <c>uds_dist</c> protocol, only the
      <c>uds_dist</c> application needs to be added to the script.)</p>

    <p>The distribution starts at boot if all the above is
      specified and an <c><![CDATA[-sname <name>]]></c> flag is present at the
      command line.</p>

    <p><em>Example 1:</em></p>

    <pre>
$ <input>erl -pa $ERL_TOP/lib/kernel/examples/uds_dist/ebin -proto_dist uds -no_epmd</input>
Erlang (BEAM) emulator version 5.0 
 
Eshell V5.0  (abort with ^G)
1> <input>net_kernel:start([bing,shortnames]).</input>
{ok,&lt;0.30.0>}
(bing@hador)2></pre>

    <p><em>Example 2:</em></p>

    <pre>
$ <input>erl -pa $ERL_TOP/lib/kernel/examples/uds_dist/ebin -proto_dist uds \ </input>
<input>      -no_epmd -sname bong</input>
Erlang (BEAM) emulator version 5.0 
 
Eshell V5.0  (abort with ^G)
(bong@hador)1></pre>

    <p>The <c>ERL_FLAGS</c> environment variable can be used to store the
      complicated parameters in:</p>

    <pre>
$ <input>ERL_FLAGS=-pa $ERL_TOP/lib/kernel/examples/uds_dist/ebin \ </input>
<input>      -proto_dist uds -no_epmd</input>
$ <input>export ERL_FLAGS</input>
$ <input>erl -sname bang</input>
Erlang (BEAM) emulator version 5.0 
 
Eshell V5.0  (abort with ^G)
(bang@hador)1></pre>

    <p><c><![CDATA[ERL_FLAGS]]></c> should not include the node name.</p>
  </section>
</chapter>