path: root/lib/kernel/doc/src/erl_ddll.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1997</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>erl_ddll</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>erl_ddll</module>
  <modulesummary>Dynamic driver loader and linker.</modulesummary>
  <description>
    <p>This module provides an interface for loading
      and unloading <em>Erlang linked-in drivers</em> in runtime.</p>
    <note>
      <p>This is a large reference document. For casual use of this
        module, and for most real world applications, the
        descriptions of functions
	<seealso marker="#load/2"><c>load/2</c></seealso> and
	<seealso marker="#unload/1"><c>unload/1</c></seealso>
	are enough to getting started.</p>
    </note>
    <p>The driver is to be provided as a dynamically linked library
      in an object code format specific for the platform in use,
      that is, <c>.so</c> files on most Unix systems and <c>.ddl</c>
      files on Windows. An Erlang linked-in driver must provide
      specific interfaces to the emulator, so this module is not
      designed for loading arbitrary dynamic libraries. For more
      information about Erlang drivers, see
      <seealso marker="erts:erl_driver"><c>erts:erl_driver</c></seealso>
      .</p>
    <marker id="users"></marker>
    <p>When describing a set of functions (that is, a module, a part of a
      module, or an application), executing in a process and wanting to
      use a ddll-driver, we use the term <em>user</em>. A process can
      have many users (different modules needing the same
      driver) and many processes running the same code, making up
      many <em>users</em> of a driver.</p>
      <p>In the basic scenario, each user loads the driver before
      starting to use it and unloads the driver when done.
      The reference counting keeps track of
      processes and the number of loads by each process. This way
      the driver is only unloaded when no one wants it (it has no user).
      The driver also keeps track of ports that are
      opened to it. This enables delay of unloading until all
      ports are closed, or killing of all ports that use the driver when
      it is unloaded.</p>
    <marker id="scenarios"></marker>
    <p>The interface supports two basic scenarios of loading and
      unloading. Each scenario can also have the option of either
      killing ports when the driver is unloading, or waiting for the
      ports to close themselves. The scenarios are as follows:</p>
    <taglist>
      <tag><em>Load and Unload on a "When Needed Basis"</em></tag>
      <item>
        <p>This (most common) scenario simply supports that each
          <seealso marker="#users">user</seealso> of the driver loads
          it when needed and unloads it when no longer needed.
          The driver is always reference counted and as long as a
          process keeping the driver loaded is still alive, the driver
          is present in the system.</p>
        <p>Each <seealso marker="#users">user</seealso> of the driver
          use <em>literally</em> the same pathname for the driver when
          demanding load, but the
	  <seealso marker="#users">users</seealso> are not concerned
          with if the driver is already loaded from the file system or
          if the object code must be loaded from file system.</p>
        <p>The following two pairs of functions support this scenario:</p>
        <taglist>
          <tag><em>load/2 and unload/1</em></tag>
          <item>
            <p>When using the <c>load/unload</c> interfaces, the
              driver is not unloaded until the
              <em>last port</em> using the driver is closed. Function
              <c>unload/1</c> can return immediately, as the
	      <seealso marker="#users">users</seealso>
	      have no interrest in when the unloading occurs. The
              driver is unloaded when no one needs it any longer.</p>
            <p>If a process having the driver loaded dies, it has
              the same effect as if unloading is done.</p>
            <p>When loading, function <c>load/2</c> returns
              <c>ok</c> when any instance of the driver is
              present. Thus, if a driver is waiting to get unloaded
              (because of open ports), it simply changes state to no
              longer need unloading.</p>
          </item>
          <tag><em>load_driver/2 and unload_driver/1</em></tag>
          <item>
            <p>These interfaces are intended to be used when it is considered an
              error that ports are open to a driver that no
	      <seealso marker="#users">user</seealso>
              has loaded. The ports that are still open when the
              last <seealso marker="#users">user</seealso> calls
              <c>unload_driver/1</c> or when the last process having the
              driver loaded dies, are killed with reason
              <c>driver_unloaded</c>.</p>
            <p>The function names <c>load_driver</c> and
              <c>unload_driver</c> are kept for backward
              compatibility.</p>
          </item>
        </taglist>
      </item>
      <tag><em>Loading and Reloading for Code Replacement</em></tag>
      <item>
        <p>This scenario can occur if the driver code needs
          replacement during operation of the Erlang
          emulator. Implementing driver code replacement is a little
          more tedious than Beam code replacement, as one driver
          cannot be loaded as both "old" and "new" code. All
	  <seealso marker="#users">users</seealso> of a driver must have it
          closed (no open ports) before the old code can be unloaded
          and the new code can be loaded.</p>
        <p>The unloading/loading is done as one atomic
          operation, blocking all processes in the system from using
          the driver in question while in progress.</p>
        <p>The preferred way to do driver code replacement is to let
          <em>one single process</em> keep track of the driver. When
          the process starts, the driver is loaded. When replacement
          is required, the driver is reloaded. Unload is probably never
          done, or done when the process exits. If more than one
	  <seealso marker="#users">user</seealso> has a driver
	  loaded when code replacement is demanded, the replacement cannot
	  occur until the last "other"
	  <seealso marker="#users">user</seealso> has
          unloaded the driver.</p>
        <p>Demanding reload when a reload is already in progress is
          always an error. Using the high-level functions, it is also
          an error to demand reloading when more than one
	  <seealso marker="#users">user</seealso> has the driver loaded.</p>
	 <p>To simplify driver replacement, avoid designing your system so
          that more than one
	  <seealso marker="#users">user</seealso> has the driver loaded.</p>
        <p>The two functions for reloading drivers are to be used
          together with corresponding load functions to support the two
          different behaviors concerning open ports:</p>
        <taglist>
          <tag><em>load/2 and reload/2</em></tag>
          <item>
            <p>This pair of functions is used when reloading is to be
              done after the last open port to the driver is
              closed.</p>
            <p>As <c>reload/2</c> waits for the reloading to
              occur, a misbehaving process keeping open ports to
              the driver (or keeping the driver loaded) can cause
              infinite waiting for reload. Time-outs must be provided
              outside of the process demanding the reload or by using
              the low-level interface
	      <seealso marker="#try_load/3"><c>try_load/3</c></seealso>
	      in combination with driver monitors.</p>
          </item>
          <tag><em>load_driver/2 and reload_driver/2</em></tag>
          <item>
            <p>This pair of functions are used when open ports to
              the driver are to be killed with reason
              <c>driver_unloaded</c> to allow for new driver code to
              get loaded.</p>
            <p>However, if another process has the driver loaded,
              calling <c>reload_driver</c> returns error code
              <c>pending_process</c>. As stated earlier,
              the recommended design is to not allow other
	      <seealso marker="#users">users</seealso> than the "driver
              reloader" to demand loading of the driver in question.</p>
          </item>
        </taglist>
      </item>
    </taglist>
  </description>
  <datatypes>
    <datatype>
      <name name="driver"/>
    </datatype>
    <datatype>
      <name name="path"/>
    </datatype>
  </datatypes>
  <funcs>
    <func>
      <name name="demonitor" arity="1"/>
      <fsummary>Remove a monitor for a driver.</fsummary>
      <desc>
        <p>Removes a driver monitor in much the same way as 
          <seealso marker="erts:erlang#erlang:demonitor/1"><c>erlang:demonitor/1</c></seealso>
	  in <c>ERTS</c>
	  does with process monitors. For details about how to create
	  driver monitors, see
	  <seealso marker="#monitor/2"><c>monitor/2</c></seealso>,
	  <seealso marker="#try_load/3"><c>try_load/3</c></seealso>, and
	  <seealso marker="#try_unload/2"><c>try_unload/2</c></seealso>.</p>
        <p>The function throws a <c>badarg</c> exception if the
          parameter is not a <c>reference()</c>.</p>
      </desc>
    </func>
    <func>
      <name name="format_error" arity="1"/>
      <fsummary>Format an error descriptor.</fsummary>
      <desc>
        <p>Takes an <c><anno>ErrorDesc</anno></c> returned by load, unload, or
          reload functions and returns a string that
          describes the error or warning.</p>
        <note>
          <p>Because of peculiarities in the dynamic loading interfaces on
            different platforms, the returned string is only guaranteed
            to describe the correct error <em>if format_error/1 is called
	    in the same instance of the Erlang virtual machine as the error
	    appeared in</em> (meaning the same operating
            system process).</p>
        </note>
      </desc>
    </func>
    <func>
      <name name="info" arity="0"/>
      <fsummary>Retrieve information about all drivers.</fsummary>
      <desc>
        <p>Returns a list of tuples <c>{<anno>DriverName</anno>, <anno>InfoList</anno>}</c>,
	   where <c><anno>InfoList</anno></c> is the result of calling
	   <seealso marker="#info/1"><c>info/1</c></seealso> for that
          <c><anno>DriverName</anno></c>. Only dynamically linked-in drivers are
          included in the list.</p>
      </desc>
    </func>
    <func>
      <name name="info" arity="1"/>
      <fsummary>Retrieve information about one driver.</fsummary>
      <desc>
        <p>Returns a list of tuples <c>{<anno>Tag</anno>, <anno>Value</anno>}</c>,
	  where <c><anno>Tag</anno></c> is the information item and
	  <c><anno>Value</anno></c> is the result of calling
	  <seealso marker="#info/2"><c>info/2</c></seealso> with this driver
	  name and this tag. The result is a tuple list containing all information
	  available about a driver.</p>
        <p>The following tags appears in the list:</p>
        <list type="bulleted">
          <item><c>processes</c></item>
          <item><c>driver_options</c></item>
          <item><c>port_count</c></item>
          <item><c>linked_in_driver</c></item>
          <item><c>permanent</c></item>
          <item><c>awaiting_load</c></item>
          <item><c>awaiting_unload</c></item>
        </list>
        <p>For a detailed description of each value, see
          <seealso marker="#info/2"><c>info/2</c></seealso>.</p>
        <p>The function throws a <c>badarg</c> exception if the driver
          is not present in the system.</p>
      </desc>
    </func>
    <func>
      <name name="info" arity="2"/>
      <fsummary>Retrieve specific information about one driver.</fsummary>
      <desc>
        <p>Returns specific information about one aspect of a driver.
	  Parameter <c><anno>Tag</anno></c> specifies which aspect
          to get information about. The return <c><anno>Value</anno></c> differs
          between different tags:</p>
        <taglist>
          <tag><c>processes</c></tag>
          <item>
            <p>Returns all processes containing
	    <seealso marker="#users">users</seealso> of the specific drivers
              as a list of tuples <c>{pid(),integer() >= 0}</c>, where
              <c>integer()</c> denotes the number of users in process
              <c>pid()</c>.</p>
          </item>
          <tag><c>driver_options</c></tag>
          <item>
            <p>Returns a list of the driver options provided when
              loading, and any options set by the driver
              during initialization. The only valid option
              is <c>kill_ports</c>.</p>
          </item>
          <tag><c>port_count</c></tag>
          <item>
            <p>Returns the number of ports (an <c>integer() >= 0</c>)
	    using the driver.</p>
          </item>
          <tag><c>linked_in_driver</c></tag>
          <item>
            <p>Returns a <c>boolean()</c>, which is <c>true</c> if the driver is a
              statically linked-in one, otherwise <c>false</c>.</p>
          </item>
          <tag><c>permanent</c></tag>
          <item>
            <p>Returns a <c>boolean()</c>, which is <c>true</c> if the driver has
	      made itself permanent (and is <em>not</em> a statically
              linked-in driver), otherwise <c>false</c>.</p>
          </item>
          <tag><c>awaiting_load</c></tag>
          <item>
            <p>Returns a list of all processes having monitors for
              <c>loading</c> active. Each process is returned as
              <c>{pid(),integer() >= 0}</c>, where <c>integer()</c> is the
              number of monitors held by process <c>pid()</c>.</p>
          </item>
          <tag><c>awaiting_unload</c></tag>
          <item>
            <p>Returns a list of all processes having monitors for
              <c>unloading</c> active. Each process is returned as
              <c>{pid(),integer() >= 0}</c>, where <c>integer()</c> is the
              number of monitors held by process <c>pid()</c>.</p>
          </item>
        </taglist>
        <p>If option <c>linked_in_driver</c> or <c>permanent</c>
          returns <c>true</c>, all other options return
          <c>linked_in_driver</c> or <c>permanent</c>, respectively.</p>
        <p>The function throws a <c>badarg</c> exception if the driver
          is not present in the system or if the tag is not supported.</p>
      </desc>
    </func>
    <func>
      <name name="load" arity="2"/>
      <fsummary>Load a driver.</fsummary>
      <desc>
        <p>Loads and links the dynamic driver <c><anno>Name</anno></c>.
	  <c><anno>Path</anno></c>
          is a file path to the directory containing the driver.
          <c><anno>Name</anno></c> must be a sharable object/dynamic library. Two
          drivers with different <c><anno>Path</anno></c> parameters cannot be
          loaded under the same name. <c><anno>Name</anno></c> is a string or
          atom containing at least one character.</p>
        <p>The <c><anno>Name</anno></c> specified is to correspond to the filename
          of the dynamically loadable object file residing in
          the directory specified as <c><anno>Path</anno></c>, but <em>without</em> the
          extension (that is, <c>.so</c>). The driver name provided in
          the driver initialization routine must correspond with the
          filename, in much the same way as Erlang module names
          correspond to the names of the <c>.beam</c> files.</p>
        <p>If the driver was previously unloaded, but is still
          present because of open ports to it, a call to
          <c>load/2</c> stops the unloading and keeps the driver
          (as long as <c><anno>Path</anno></c> is the same), and <c>ok</c> is
          returned. If you really want the object code to be
          reloaded, use <seealso marker="#reload/2"><c>reload/2</c></seealso>
	  or the low-level interface
	  <seealso marker="#try_load/3"><c>try_load/3</c></seealso> instead.
	  See also the description of
	  <seealso marker="#scenarios"><c>different scenarios</c></seealso> for
          loading/unloading in the introduction.</p>
        <p>If more than one process tries to load an already loaded
          driver with the same <c><anno>Path</anno></c>, or if the same process
          tries to load it many times, the function returns
          <c>ok</c>. The emulator keeps track of the
          <c>load/2</c> calls, so that a corresponding number of
          <c>unload/2</c> calls must be done from the same
          process before the driver gets unloaded. It is
          therefore safe for an application to load a driver that is
          shared between processes or applications when needed. It can
          safely be unloaded without causing trouble for other
          parts of the system.</p>
        <p>It is not allowed to load multiple drivers with
	the same name but with different <c>Path</c> parameters.</p>
        <note>
          <p><c><anno>Path</anno></c> is interpreted
            literally, so that all loaders of the same driver must
            specify the same <em>literal</em> <c><anno>Path</anno></c> string,
	    although different paths can point out the same directory
            in the file system (because of use of relative paths and
            links).</p>
        </note>
        <p>On success, the function returns <c>ok</c>. On
          failure, the return value is <c>{error,<anno>ErrorDesc</anno>}</c>,
          where <c><anno>ErrorDesc</anno></c> is an opaque term to be
          translated into human readable form by function
	  <seealso marker="#format_error/1"><c>format_error/1</c></seealso>.</p>
        <p>For more control over the error handling, use the
          <seealso marker="#try_load/3"><c>try_load/3</c></seealso>
          interface instead.</p>
        <p>The function throws a <c>badarg</c> exception if the
          parameters are not specified as described here.</p>
      </desc>
    </func>
    <func>
      <name name="load_driver" arity="2"/>
      <fsummary>Load a driver.</fsummary>
      <desc>
        <p>Works essentially as <c>load/2</c>, but loads the driver
          with other options. All ports using the
          driver are killed with reason <c>driver_unloaded</c>
	  when the driver is to be unloaded.</p>
        <p>The number of loads and unloads by different
	  <seealso marker="#users">users</seealso> influences the loading
          and unloading of a driver file. The port killing
          therefore only occurs when the <em>last</em>
	  <seealso marker="#users">user</seealso> unloads the driver,
	  or when the last process having loaded the driver exits.</p>
        <p>This interface (or at least the name of the functions) is
          kept for backward compatibility.
	  Using <seealso marker="#try_load/3"><c>try_load/3</c></seealso> with
          <c>{driver_options,[kill_ports]}</c> in the option list
          gives the same effect regarding the port killing.</p>
        <p>The function throws a <c>badarg</c> exception if the
          parameters are not specified as described here.</p>
      </desc>
    </func>
    <func>
      <name name="loaded_drivers" arity="0"/>
      <fsummary>List loaded drivers.</fsummary>
      <desc>
        <p>Returns a list of all the available drivers, both
          (statically) linked-in and dynamically loaded ones.</p>
        <p>The driver names are returned as a list of strings rather
          than a list of atoms for historical reasons.</p>
        <p>For more information about drivers, see
          <seealso marker="#info/0"><c>info</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="monitor" arity="2"/>
      <fsummary>Create a monitor for a driver.</fsummary>
      <desc>
        <p>Creates a driver monitor and works in many
          ways as
	  <seealso marker="erts:erlang#erlang:monitor/2"><c>erlang:monitor/2</c></seealso>
	  in <c>ERTS</c>,
          does for processes. When a driver changes state, the monitor
          results in a monitor message that is sent to the calling
          process. <c><anno>MonitorRef</anno></c> returned by this function is
          included in the message sent.</p>
        <p>As with process monitors, each driver monitor set only
          generates <em>one single message</em>. The monitor is
          "destroyed" after the message is sent, so it is then not
          needed to call
	<seealso marker="#demonitor/1"><c>demonitor/1</c></seealso>.</p>
        <p><c><anno>MonitorRef</anno></c> can also be used in subsequent calls
          to <seealso marker="#demonitor/1"><c>demonitor/1</c></seealso> to
          remove a monitor.</p>
        <p>The function accepts the following parameters:</p>
        <taglist>
          <tag><c><anno>Tag</anno></c></tag>
          <item>
            <p>The monitor tag is always <c>driver</c>, as this function
              can only be used to create driver monitors. In the future,
              driver monitors will be integrated with process monitors,
              why this parameter has to be specified for consistence.</p>
          </item>
          <tag><c><anno>Item</anno></c></tag>
          <item>
            <p>Parameter <c><anno>Item</anno></c> specifies
	    which driver to monitor (the driver name) and
              which state change to monitor. The parameter
              is a tuple of arity two whose first element is the
              driver name and second element is one of the following:</p>
            <taglist>
              <tag><c>loaded</c></tag>
              <item>
                <p>Notifies when the driver is reloaded (or loaded if
                  loading is underway). It only makes sense to monitor
                  drivers that are in the process of being loaded or
                  reloaded. A future driver name for loading cannot be
		  monitored. That only results in a
                  <c>DOWN</c> message sent immediately.
                  Monitoring for loading is therefore most useful when
                  triggered by function
		  <seealso marker="#try_load/3"><c>try_load/3</c></seealso>,
                  where the monitor is created <em>because</em> the
                  driver is in such a pending state.</p>
                <p>Setting a driver monitor for <c>loading</c>
                  eventually leads to one of the following messages
                  being sent:</p>
                <taglist>
                  <tag><c>{'UP', reference(), driver, Name, loaded}</c></tag>
                  <item>
                    <p>This message is sent either immediately if the
                      driver is already loaded and no reloading is
                      pending, or when reloading is executed if
                      reloading is pending. </p>
                    <p>The <seealso marker="#users">user</seealso> is
                      expected to know if reloading is demanded before
                      creating a monitor for loading.</p>
                  </item>
                  <tag><c>{'UP', reference(), driver, Name, permanent}</c></tag>
                  <item>
                    <p>This message is sent if reloading was
                      expected, but the (old) driver made itself
                      permanent before reloading. It is also
                      sent if the driver was permanent or statically
                      linked-in when trying to create the monitor.</p>
                  </item>
                  <tag><c>{'DOWN', reference(), driver, Name, load_cancelled}</c></tag>
                  <item>
                    <p>This message arrives if reloading was
                      underway, but the requesting
		      <seealso marker="#users">user</seealso>
                      cancelled it by dying or calling
                      <seealso marker="#try_unload/2"><c>try_unload/2</c></seealso>
                      (or <c>unload/1</c>/<c>unload_driver/1</c>)
                      again before it was reloaded.</p>
                  </item>
                  <tag><c>{'DOWN', reference(), driver, Name, {load_failure, Failure}}</c></tag>
                  <item>
                    <p>This message arrives if reloading was
                      underway but the loading for some reason
                      failed. The <c>Failure</c> term is one of the
                      errors that can be returned from
		      <seealso marker="#try_load/3"><c>try_load/3</c></seealso>.
		      The error term can be passed to
		      <seealso marker="#format_error/1"><c>format_error/1</c></seealso>
                      for translation into human readable form. Notice
                      that the translation must be done in the same
                      running Erlang virtual machine as the error
                      was detected in.</p>
                  </item>
                </taglist>
              </item>
              <tag><c>unloaded</c></tag>
              <item>
                <p>Monitors when a driver gets unloaded. If one
                  monitors a driver that is not present in the system,
                  one immediately gets notified that the driver got
                  unloaded. There is no guarantee that the driver was
                  ever loaded.</p>
                <p>A driver monitor for unload eventually results
                  in one of the following messages being sent:</p>
                <taglist>
                  <tag><c>{'DOWN', reference(), driver, Name, unloaded}</c></tag>
                  <item>
                    <p>The monitored driver instance is now
                      unloaded. As the unload can be a result of a
                      <c>reload/2</c> request, the driver can once
                      again have been loaded when this message
                      arrives.</p>
                  </item>
                  <tag><c>{'UP', reference(), driver, Name, unload_cancelled}</c></tag>
                  <item>
                    <p>This message is sent if unloading was
                      expected, but while the driver was waiting for
                      all ports to get closed, a new
		      <seealso marker="#users">user</seealso> of the driver
                      appeared, and the unloading was cancelled.</p>
                    <p>This message appears if <c>{ok, pending_driver}</c>
		      was returned from
		      <seealso marker="#try_unload/2"><c>try_unload/2</c></seealso>
                      for the last <seealso marker="#users">user</seealso>
		      of the driver, and then <c>{ok, already_loaded}</c> is returned
                      from a call to
		    <seealso marker="#try_load/3"><c>try_load/3</c></seealso>.</p>
                    <p>If one <em>really</em> wants to monitor when the
                      driver gets unloaded, this message distorts
                      the picture, because no unloading was done.
                      Option <c>unloaded_only</c> creates a monitor
                      similar to an <c>unloaded</c> monitor, but
                      never results in this message.</p>
                  </item>
                  <tag><c>{'UP', reference(), driver, Name, permanent}</c></tag>
                  <item>
                    <p>This message is sent if unloading was
                      expected, but the driver made itself
                      permanent before unloading. It is also
                      sent if trying to monitor a permanent or
                      statically linked-in driver.</p>
                  </item>
                </taglist>
              </item>
              <tag><c>unloaded_only</c></tag>
              <item>
                <p>A monitor created as <c>unloaded_only</c> behaves
                  exactly as one created as <c>unloaded</c>
                  except that the
		  <c>{'UP', reference(), driver, Name, unload_cancelled}</c>
		  message is never sent, but the monitor instead persists until
		  the driver <em>really</em> gets unloaded.</p>
              </item>
            </taglist>
          </item>
        </taglist>
        <p>The function throws a <c>badarg</c> exception if the
          parameters are not specified as described here.</p>
      </desc>
    </func>
    <func>
      <name name="reload" arity="2"/>
      <fsummary>Replace a driver.</fsummary>
      <desc>
        <p>Reloads the driver named <c><anno>Name</anno></c> from a possibly
          different <c><anno>Path</anno></c> than previously used. This
          function is used in the code change
	  <seealso marker="#scenarios"><c>scenario</c></seealso> described in the
          introduction.</p>
        <p>If there are other <seealso marker="#users">users</seealso>
          of this driver, the function returns <c>{error, pending_process}</c>,
	  but if there are no other users, the function call hangs until all
	  open ports are closed.</p>
        <note>
          <p>Avoid mixing multiple
	  <seealso marker="#users">users</seealso>
          with driver reload requests.</p>
        </note>
        <p>To avoid hanging on open ports, use function
          <seealso marker="#try_load/3"><c>try_load/3</c></seealso>
          instead.</p>
        <p>The <c><anno>Name</anno></c> and <c><anno>Path</anno></c> parameters
	  have exactly the same meaning as when calling the plain function
	<seealso marker="#load/2"><c>load/2</c></seealso>.</p>

        <p>On success, the function returns <c>ok</c>. On
          failure, the function returns an opaque error,
          except the <c>pending_process</c> error described
          earlier. The opaque errors are to be translated into human
          readable form by function
	<seealso marker="#format_error/1"><c>format_error/1</c></seealso>.</p>
        <p>For more control over the error handling,  use the
          <seealso marker="#try_load/3"><c>try_load/3</c></seealso>
          interface instead.</p>
        <p>The function throws a <c>badarg</c> exception if the
          parameters are not specified as described here.</p>
      </desc>
    </func>
    <func>
      <name name="reload_driver" arity="2"/>
      <fsummary>Replace a driver.</fsummary>
      <desc>
        <p>Works exactly as <seealso marker="#reload/2"><c>reload/2</c></seealso>,
	  but for drivers loaded with the
	<seealso marker="#load_driver/2"><c>load_driver/2</c></seealso> interface.</p>
        <p>As this interface implies that ports are killed when
          the last user disappears, the function does not hang waiting for
          ports to get closed.</p>
        <p>For more details, see
	  <seealso marker="#scenarios"><c>scenarios</c></seealso> in this module
          description and the function description for
	  <seealso marker="#reload/2"><c>reload/2</c></seealso>.</p>
        <p>The function throws a <c>badarg</c> exception if the
          parameters are not specified as described here.</p>
      </desc>
    </func>
    <func>
      <name name="try_load" arity="3"/>
      <fsummary>Load a driver.</fsummary>
      <desc>
        <p>Provides more control than the
          <c>load/2</c>/<c>reload/2</c> and
          <c>load_driver/2</c>/<c>reload_driver/2</c> interfaces. It
          never waits for completion of other operations related
          to the driver, but immediately returns the status of the
          driver as one of the following:</p>
        <taglist>
          <tag><c>{ok, loaded}</c></tag>
          <item>
            <p>The driver was loaded and is immediately usable.</p>
          </item>
          <tag><c>{ok, already_loaded}</c></tag>
          <item>
            <p>The driver was already loaded by another process
              or is in use by a living port, or both. The load by you is
              registered and a corresponding <c>try_unload</c> is
              expected sometime in the future.</p>
          </item>
          <tag><c>{ok, pending_driver}</c>or <c>{ok, pending_driver, reference()}</c></tag>
          <item>
            <p>The load request is registered, but the loading is
              delayed because an earlier instance of the
              driver is still waiting to get unloaded (open
              ports use it). Still, unload is expected when you are
              done with the driver. This return value
              <em>mostly</em> occurs when options
              <c>{reload,pending_driver}</c> or
              <c>{reload,pending}</c> are used, but
              <em>can</em> occur when another
	      <seealso marker="#users">user</seealso> is unloading a
	      driver in parallel and driver option <c>kill_ports</c> is set.
	      In other words, this return value always needs
              to be handled.</p>
          </item>
          <tag><c>{ok, pending_process}</c>or <c>{ok, pending_process, reference()}</c></tag>
          <item>
            <p>The load request is registered, but the loading is
              delayed because an earlier instance of the
              driver is still waiting to get unloaded by another
              <seealso marker="#users">user</seealso> (not only by a
              port, in which case <c>{ok,pending_driver}</c> would
              have been returned). Still, unload is expected when you
              are done with the driver. This return value
              <em>only</em> occurs when option <c>{reload,pending}</c>
              is used.</p>
          </item>
        </taglist>
        <p>When the function returns <c>{ok, pending_driver}</c> or
          <c>{ok, pending_process}</c>, one can get information
          about when the driver is <em>actually</em> loaded  by using
	  option <c>{monitor, <anno>MonitorOption</anno>}</c>.</p>
        <p>When monitoring is requested, and a corresponding
	<c>{ok, pending_driver}</c> or <c>{ok, pending_process}</c> would
	  be returned, the function instead returns a tuple
	  <c>{ok, <anno>PendingStatus</anno>, reference()}</c>
	  and the process then gets a monitor message later, when the
	  driver gets loaded. The monitor message to expect is described in
          the function description of
	  <seealso marker="#monitor/2"><c>monitor/2</c></seealso>.</p>
        <note>
          <p>In case of loading, monitoring can <em>not</em> only get
	    triggered by using option <c>{reload, <anno>ReloadOption</anno>}</c>,
	    but also in special cases where the load error is transient. Thus,
	    <c>{monitor, pending_driver}</c> is to be used under basically
            <em>all</em> real world circumstances.</p>
        </note>
        <p>The function accepts the following parameters:</p>
        <taglist>
          <tag><c><anno>Path</anno></c></tag>
          <item>
            <p>The file system path to the directory where the driver
              object file is located. The filename of the object file
              (minus extension) must correspond to the driver name
              (used in parameter <c><anno>Name</anno></c>) and the driver must
              identify itself with the same name.
              <c><anno>Path</anno></c> can be provided as an <em>iolist()</em>,
              meaning it can be a list of other <c>iolist()</c>s, characters
              (8-bit integers), or binaries, all to be flattened
              into a sequence of characters.</p>
            <p>The (possibly flattened) <c><anno>Path</anno></c> parameter must be
              consistent throughout the system. A driver is to, by
              all <seealso marker="#users">users</seealso>, be loaded
              using the same <em>literal</em> <c><anno>Path</anno></c>.
	      The exception is when <em>reloading</em> is requested,
	      in which case <c><anno>Path</anno></c> can be specified
              differently. Notice that all
	      <seealso marker="#users">users</seealso> trying to load the
              driver later need to use the
	      <em>new</em> <c><anno>Path</anno></c> if <c><anno>Path</anno></c>
	      is changed using a <c>reload</c> option. This is yet another reason
              to have <em>only one loader</em> of a driver one wants to
              upgrade in a running system.</p>
          </item>
          <tag><c><anno>Name</anno></c></tag>
          <item>
            <p>This parameter is the name of the driver
	      to be used in subsequent calls to function
	      <seealso marker="erts:erlang#open_port/2"><c>erlang:open_port</c></seealso>
	      in <c>ERTS</c>.
	      The name can be specified as an <c>iolist()</c> or
              an <c>atom()</c>. The name specified when loading is used
              to find the object file (with the help of <c><anno>Path</anno></c>
	      and the system-implied extension suffix, that is, <c>.so</c>).
	      The name by which the driver identifies itself must also be consistent
              with this <c><anno>Name</anno></c> parameter, much as
	      the module name of a Beam file much corresponds to its filename.</p>
          </item>
          <tag><c><anno>OptionList</anno></c></tag>
          <item>
            <p>Some options can be specified to control the
              loading operation. The options are specified as a list of
              two-tuples. The tuples have the following values and
              meanings:</p>
            <taglist>
              <tag><c>{driver_options, <anno>DriverOptionList</anno>}</c></tag>
              <item>
                <p>This is to provide options that changes
                  its general behavior and "sticks" to the driver
                  throughout its lifespan.</p>
                <p>The driver options for a specified driver name need
                  always to be consistent, <em>even when the driver is reloaded</em>,
		  meaning that they are as much a part of the driver as the name.</p>
                <p>The only allowed driver option is
                  <c>kill_ports</c>, which means that all ports opened
                  to the driver are killed with exit reason
                  <c>driver_unloaded</c> when no process any longer
                  has the driver loaded. This situation arises either
                  when the last <seealso marker="#users">user</seealso> calls
		  <seealso marker="#try_unload/2"><c>try_unload/2</c></seealso>, or
                  when the last process having loaded the driver exits.</p>
              </item>
              <tag><c>{monitor, <anno>MonitorOption</anno>}</c></tag>
              <item>
                <p>A <c><anno>MonitorOption</anno></c> tells <c>try_load/3</c> to
                  trigger a driver monitor under certain
                  conditions. When the monitor is triggered, the
                  function returns a three-tuple
		  <c>{ok, <anno>PendingStatus</anno>, reference()}</c>, where
		  <c>reference()</c> is the monitor reference for the driver monitor.</p>
                <p>Only one <c><anno>MonitorOption</anno></c> can be specified.
                  It is one of the following:</p>
		  <list type="bulleted">
		    <item>
		      <p>The atom <c>pending</c>, which means
                      that a monitor is to be created whenever a load
                      operation is delayed,</p>
		    </item>
		    <item>
		      <p>The atom <c>pending_driver</c>, in which a monitor
		      is created whenever the operation is delayed because
		      of open ports to an otherwise unused driver.</p>
		      </item>
		  </list>
		<p>Option <c>pending_driver</c> is of little use, but
		  is present for completeness, as it is well defined which
		  reload options that can give rise to which delays.
		  However, it can be a good idea to use the same
		  <c><anno>MonitorOption</anno></c> as the
		  <c><anno>ReloadOption</anno></c>, if present.</p>
                <p>If reloading is not requested, it can still be
                  useful to specify option <c>monitor</c>, as
                  forced unloads (driver option <c>kill_ports</c> or
                  option <c>kill_ports</c> to
		  <seealso marker="#try_unload/2"><c>try_unload/2</c></seealso>)
		  trigger a transient state where driver loading
                  cannot be performed until all closing ports are
                  closed. Thus, as <c>try_unload</c> can, in
                  almost all situations, return <c>{ok, pending_driver}</c>,
		  always specify at least <c>{monitor, pending_driver}</c>
		  in production code (see the monitor discussion earlier).</p>
              </item>
              <tag><c>{reload, <anno>ReloadOption</anno>}</c></tag>
              <item>
                <p>This option is used to
                  <em>reload</em> a driver from disk, most often in a
                  code upgrade scenario. Having a <c>reload</c> option
                  also implies that parameter <c><anno>Path</anno></c> does
                  <em>not</em> need to be consistent with earlier loads of
                  the driver.</p>
                <p>To reload a driver, the process must have loaded the driver
		  before, that is, there must be an active
		  <seealso marker="#users">user</seealso> of the driver
		  in the process.</p>
                <p>The <c>reload</c> option can be either of the following:</p>
		 <taglist>
		   <tag><c>pending</c></tag>
		   <item>
		     <p>With the atom <c>pending</c>, reloading is requested
		       for any driver and is effectuated when <em>all</em>
                       ports opened to the driver are closed. The driver
                       replacement in this case takes
                       place regardless if there are still
                       pending <seealso marker="#users">users</seealso>
                       having the driver loaded.</p>
                     <p>The option also triggers port-killing (if driver
                       option <c>kill_ports</c> is used) although
                       there are pending users, making it usable for forced
                       driver replacement, but laying much
                       responsibility on the driver
		       <seealso marker="#users">users</seealso>.
		       The pending option is seldom used as one does not want other
		       <seealso marker="#users">users</seealso> to have loaded
		       the driver when code change is underway.</p></item>
		   <tag><c>pending_driver</c></tag>
		   <item>
		     <p>This option is more useful. Here, reloading is queued
		        if the driver is <em>not</em> loaded by any other
		        <seealso marker="#users">users</seealso>, but the
			driver has opened ports, in which case
			<c>{ok, pending_driver}</c> is returned
			(a <c>monitor</c> option is recommended).</p></item>
		 </taglist>
                <p>If the driver is unloaded (not present in the system),
		  error code <c>not_loaded</c> is returned. Option
                  <c>reload</c> is intended for when the user
                  has already loaded the driver in advance.</p>
              </item>
            </taglist>
          </item>
        </taglist>
        <p>The function can return numerous errors, some
          can only be returned given a certain combination of options.</p>
        <p>Some errors are opaque and can only be interpreted by
          passing them to function
	  <seealso marker="#format_error/1"><c>format_error/1</c></seealso>,
          but some can be interpreted directly:</p>
        <taglist>
          <tag><c>{error,linked_in_driver}</c></tag>
          <item>
            <p>The driver with the specified name is an Erlang
              statically linked-in driver, which cannot be manipulated
              with this API.</p>
          </item>
          <tag><c>{error,inconsistent}</c></tag>
          <item>
            <p>The driver is already loaded with other
              <c><anno>DriverOptionList</anno></c> or a different
	    <em>literal</em> <c>Path</c> argument.</p>
            <p>This can occur even if a <c>reload</c> option is specified,
              if <c>DriverOptionList</c> differs from the current.</p>
          </item>
          <tag><c>{error, permanent}</c></tag>
          <item>
            <p>The driver has requested itself to be permanent, making
              it behave like an Erlang linked-in driver and can no
              longer be manipulated with this API.</p>
          </item>
          <tag><c>{error, pending_process}</c></tag>
          <item>
            <p>The driver is loaded by other
	    <seealso marker="#users">users</seealso> when
	    option <c>{reload, pending_driver}</c> was specified.</p>
          </item>
          <tag><c>{error, pending_reload}</c></tag>
          <item>
            <p>Driver reload is already requested by another
	    <seealso marker="#users">user</seealso> when option
	    <c>{reload, <anno>ReloadOption</anno>}</c> was specified.</p>
          </item>
          <tag><c>{error, not_loaded_by_this_process}</c></tag>
          <item>
            <p>Appears when option <c>reload</c> is specified. The
              driver <c><anno>Name</anno></c> is present in the system, but there
	      is no <seealso marker="#users">user</seealso> of it in this
              process.</p>
          </item>
          <tag><c>{error, not_loaded}</c></tag>
          <item>
            <p>Appears when option <c>reload</c> is specified. The
              driver <c><anno>Name</anno></c> is not in the system. Only drivers
              loaded by this process can be reloaded.</p>
          </item>
        </taglist>
        <p>All other error codes are to be translated by function
	<seealso marker="#format_error/1"><c>format_error/1</c></seealso>.
	  Notice that calls to <c>format_error</c> are to be
          performed from the same running instance of the Erlang
          virtual machine as the error is detected in, because of
	  system-dependent behavior concerning error values.</p>
        <p>If the arguments or options are malformed, the function
          throws a <c>badarg</c> exception.</p>
      </desc>
    </func>
    <func>
      <name name="try_unload" arity="2"/>
      <fsummary>Unload a driver.</fsummary>
      <desc>
        <p>This is the low-level function to unload (or decrement
          reference counts of) a driver. It can be used to force port
          killing, in much the same way as the driver option
          <c>kill_ports</c> implicitly does. Also, it can trigger a
          monitor either because other
	  <seealso marker="#users">users</seealso> still have the driver
          loaded or because open ports use the driver.</p>
        <p>Unloading can be described as the process of telling the
          emulator that this particular part of the code in this
          particular process (that is, this
	  <seealso marker="#users">user</seealso>) no longer needs
	  the driver. That can, if there are no other users, trigger
          unloading of the driver, in which case the driver name
          disappears from the system and (if possible) the memory
          occupied by the driver executable code is reclaimed.</p>
	  <p>If the driver has option <c>kill_ports</c> set, or if
          <c>kill_ports</c> is specified as an option to this
          function, all pending ports using this driver are
          killed when unloading is done by the last
	  <seealso marker="#users">user</seealso>. If no port-killing
	  is involved and there are open ports, the unloading
          is delayed until no more open ports use the
          driver. If, in this case, another
	  <seealso marker="#users">user</seealso> (or even this user)
	  loads the driver again before the driver is unloaded, the
          unloading never takes place.</p>
        <p>To allow the <seealso marker="#users">user</seealso> to
          <em>request unloading</em> to wait for <em>actual unloading</em>,
          <c>monitor</c> triggers can be specified in much the same way as
	  when loading. However, as <seealso marker="#users">users</seealso>
	  of this function seldom are interested in more than decrementing the
          reference counts, monitoring is seldom needed.</p>
	<note><p> If option <c>kill_ports</c> is used, monitor trigging is crucial,
	  as the ports are not guaranteed to be killed until the driver is unloaded.
	  Thus, a monitor must be triggered for at least the <c>pending_driver</c>
	  case.</p></note>
        <p>The possible monitor messages to expect are the
          same as when using option <c>unloaded</c> to function
          <seealso marker="#monitor/2"><c>monitor/2</c></seealso>.</p>
        <p>The function returns one of the following statuses upon
          success:</p>
        <taglist>
          <tag><c>{ok, unloaded}</c></tag>
          <item>
            <p>The driver was immediately unloaded, meaning that the
              driver name is now free to use by other drivers and, if
              the underlying OS permits it, the memory occupied by the
              driver object code is now reclaimed.</p>
            <p>The driver can only be unloaded when there are no open
              ports using it and no more
	      <seealso marker="#users">users</seealso> require it to be
              loaded.</p>
          </item>
          <tag><c>{ok, pending_driver}</c>or
	       <c>{ok, pending_driver, reference()}</c></tag>
          <item>
            <p>Indicates that this call removed the last
	      <seealso marker="#users">user</seealso> from the
              driver, but there are still open ports using it.
              When all ports are closed and no new
	      <seealso marker="#users">users</seealso> have arrived,
	      the driver is reloaded and the name and memory
              reclaimed.</p>
            <p>This return value is valid even if option <c>kill_ports</c>
	      was used, as killing ports can be a process that does not
	      complete immediately. However, the condition is in that case
	      transient. Monitors are always useful to detect when the driver
	      is really unloaded.</p>
          </item>
          <tag><c>{ok, pending_process}</c>or
	       <c>{ok, pending_process, reference()}</c></tag>
          <item>
            <p>The unload request is registered, but
              other <seealso marker="#users">users</seealso> still hold
              the driver. Notice that the term <c>pending_process</c>
              can refer to the running process; there can be more
              than one <seealso marker="#users">user</seealso> in the
              same process.</p>
            <p>This is a normal, healthy, return value if the call was
              just placed to inform the emulator that you have no
              further use of the driver. It is the most
              common return value in the most common
	      <seealso marker="#scenarios"><c>scenario</c></seealso>
              described in the introduction.</p>
          </item>
        </taglist>
        <p>The function accepts the following parameters:</p>
        <taglist>
          <tag><c><anno>Name</anno></c></tag>
          <item>
            <p><c><anno>Name</anno></c> is the name of the
	      driver to be unloaded. The name can be specified as an
              <c>iolist()</c> or as an <c>atom()</c>.</p>
          </item>
          <tag><c><anno>OptionList</anno></c></tag>
          <item>
            <p>Argument <c><anno>OptionList</anno></c> can be used to specify
              certain behavior regarding ports and triggering
              monitors under certain conditions:</p>
            <taglist>
              <tag><c>kill_ports</c></tag>
              <item>
                <p>Forces killing of all ports opened using this driver,
                  with exit reason <c>driver_unloaded</c>, if you are
                  the <em>last</em> <seealso marker="#users">user</seealso>
		  of the driver.</p>
                <p>If other <seealso marker="#users">users</seealso>
		  have the driver loaded, this option has no effect.</p>
                <p>To get the consistent behavior of killing ports
                  when the last <seealso marker="#users">user</seealso>
                  unloads, use driver option
                  <c>kill_ports</c> when loading the driver instead.</p>
              </item>
              <tag><c>{monitor, <anno>MonitorOption</anno>}</c></tag>
              <item>
                <p>Creates a driver monitor if the condition
                  specified in <c><anno>MonitorOption</anno></c> is true. The valid
                  options are:</p>
                <taglist>
                  <tag><c>pending_driver</c></tag>
                  <item>
                    <p>Creates a driver monitor if the return value is to
                      be <c>{ok, pending_driver}</c>.</p>
                  </item>
                  <tag><c>pending</c></tag>
                  <item>
                    <p>Creates a monitor if the return value is
                      <c>{ok, pending_driver}</c> or <c>{ok, pending_process}</c>.</p>
                  </item>
                </taglist>
                <p>The <c>pending_driver</c> <c><anno>MonitorOption</anno></c> is
		  by far the most useful. It must be used to ensure that the
                  driver really is unloaded and the ports closed
                  whenever option <c>kill_ports</c> is used, or the
                  driver can have been loaded with driver option
		  <c>kill_ports</c>.</p>
                <p>Using the monitor triggers in the call to
                  <c>try_unload</c> ensures that the monitor is
                  added before the unloading is executed, meaning
                  that the monitor is always properly triggered,
                  which is not the case if <c>monitor/2</c> is called
		  separately.</p>
              </item>
            </taglist>
          </item>
        </taglist>
        <p>The function can return the following error conditions,
          all well specified (no opaque values):</p>
        <taglist>
          <tag><c>{error, linked_in_driver}</c></tag>
          <item>
            <p>You were trying to unload an Erlang statically linked-in
              driver, which cannot be manipulated with this interface
              (and cannot be unloaded at all).</p>
          </item>
          <tag><c>{error, not_loaded}</c></tag>
          <item>
            <p>The driver <c><anno>Name</anno></c> is not present in the system.</p>
          </item>
          <tag><c>{error, not_loaded_by_this_process}</c></tag>
          <item>
            <p>The driver <c><anno>Name</anno></c> is present in the system, but
              there is no <seealso marker="#users">user</seealso> of
              it in this process. </p>
            <p>As a special case, drivers can be unloaded from
              processes that have done no corresponding call to
              <c>try_load/3</c> if, and only if, there are
	      <em>no users of the driver at all</em>, which can occur if the
              process containing the last user dies.</p>
          </item>
          <tag><c>{error, permanent}</c></tag>
          <item>
            <p>The driver has made itself permanent, in which case it
              can no longer be manipulated by this interface (much
              like a statically linked-in driver).</p>
          </item>
        </taglist>
        <p>The function throws a <c>badarg</c> exception if the
          parameters are not specified as described here.</p>
      </desc>
    </func>
    <func>
      <name name="unload" arity="1"/>
      <fsummary>Unload a driver.</fsummary>
      <desc>
        <p>Unloads, or at least dereferences the driver named
          <c><anno>Name</anno></c>. If the caller is the last
	  <seealso marker="#users">user</seealso> of the driver,
	  and no more open ports use the driver, the driver
          gets unloaded. Otherwise, unloading
          is delayed until all ports are closed and no
          <seealso marker="#users">users</seealso> remain.</p>
        <p>If there are other <seealso marker="#users">users</seealso>
	  of the driver, the reference counts of the driver is merely decreased,
	  so that the caller is no longer considered a
	  <seealso marker="#users">user</seealso> of the driver. For use
          scenarios, see the <seealso marker="#scenarios"><c>description</c></seealso>
	  in the beginning of this module.</p>
        <p>The <c><anno>ErrorDesc</anno></c> returned is an opaque value to be
          passed further on to function
	  <seealso marker="#format_error/1"><c>format_error/1</c></seealso>.
          For more control over the operation, use the
          <seealso marker="#try_unload/2"><c>try_unload/2</c></seealso>
          interface.</p>
        <p>The function throws a <c>badarg</c> exception if the
          parameters are not specified as described here.</p>
      </desc>
    </func>
    <func>
      <name name="unload_driver" arity="1"/>
      <fsummary>Unload a driver.</fsummary>
      <desc>
        <p>Unloads, or at least dereferences the driver named
          <c><anno>Name</anno></c>. If the caller is the last
	  <seealso marker="#users">user</seealso> of the driver, all
          remaining open ports using the driver are killed with
          reason <c>driver_unloaded</c> and the driver
          eventually gets unloaded.</p>
        <p>If there are other <seealso marker="#users">users</seealso>
          of the driver, the reference counts of the driver is merely
          decreased, so that the caller is no longer considered a
          <seealso marker="#users">user</seealso>. For
          use scenarios, see the
	  <seealso marker="#scenarios"><c>description</c></seealso> in the
	  beginning of this module.</p>
        <p>The <c><anno>ErrorDesc</anno></c> returned is an opaque value to be
          passed further on to function
	  <seealso marker="#format_error/1"><c>format_error/1</c></seealso>.
          For more control over the operation, use the
          <seealso marker="#try_unload/2"><c>try_unload/2</c></seealso>
          interface.</p>
        <p>The function throws a <c>badarg</c> exception if the
          parameters are not specified as described here.</p>
      </desc>
    </func>
  </funcs>
  <section>
    <title>See Also</title>
    <p><seealso marker="erts:erl_driver"><c>erts:erl_driver(4)</c></seealso>,
       <seealso marker="erts:driver_entry"><c>erts:driver_entry(4)</c></seealso></p>
  </section>
</erlref>