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

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

<cref>
  <header>
    <copyright>
      <year>2001</year><year>2015</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>driver_entry</title>
    <prepared>Jakob Cederlund</prepared>
    <responsible>Jakob Cederlund</responsible>
    <docno>1</docno>
    <approved></approved>
    <checked></checked>
    <date>2001-10-01</date>
    <rev>PA1</rev>
    <file>driver_entry.xml</file>
  </header>
  <lib>driver_entry</lib>
  <libsummary>The driver-entry structure used by erlang drivers.</libsummary>
  <description>
    <marker id="WARNING"/>
      <warning><p><em>Use this functionality with extreme care!</em></p>
      <p>A driver callback is executed as a direct extension of the
      native code of the VM. Execution is not made in a safe environment.
      The VM can <em>not</em> provide the same services as provided when
      executing Erlang code, such as preemptive scheduling or memory
      protection. If the driver callback function doesn't behave well,
      the whole VM will misbehave.</p>
      <list>
	<item><p>A driver callback that crash will crash the whole VM.</p></item>
	<item><p>An erroneously implemented driver callback might cause
	a VM internal state inconsistency which may cause a crash of the VM,
	or miscellaneous misbehaviors of the VM at any point after the call
	to the driver callback.</p></item>
	<item><p>A driver callback that do
	<seealso marker="erl_driver#lengthy_work">lengthy work</seealso>
	before returning will degrade responsiveness of the VM,
	and may cause miscellaneous strange behaviors. Such strange behaviors
	include, but are not limited to, extreme memory usage, and bad load
	balancing between schedulers. Strange behaviors that might occur due
	to lengthy work may also vary between OTP releases.</p></item>
      </list>
      </warning>
    <p>
      As of erts version 5.9 (OTP release R15B) the driver interface
      has been changed with larger types for the callbacks
      <seealso marker="#output">output</seealso>,
      <seealso marker="#control">control</seealso> and
      <seealso marker="#call">call</seealso>.
      See driver <seealso marker="erl_driver#version_management">
      version management</seealso> in
      <seealso marker="erl_driver">erl_driver</seealso>.
    </p>
    <note>
      <p>Old drivers (compiled with an <c>erl_driver.h</c> from an
      earlier erts version than 5.9) have to be updated and have
      to use the extended interface (with
      <seealso marker="erl_driver#version_management">version management
      </seealso>).</p>
    </note>
    <p>The <c>driver_entry</c> structure is a C struct that all erlang
      drivers define. It contains entry points for the erlang driver
      that are called by the erlang emulator when erlang code accesses
      the driver.</p>
    <p>      
      <marker id="emulator"></marker>
      The <seealso marker="erl_driver">erl_driver</seealso> driver
      API functions need a port handle
      that identifies the driver instance (and the port in the
      emulator). This is only passed to the <c>start</c> function, but
      not to the other functions. The <c>start</c> function returns a
      driver-defined handle that is passed to the other functions. A
      common practice is to have the <c>start</c> function allocate
      some application-defined structure and stash the <c>port</c>
      handle in it, to use it later with the driver API functions.</p>
    <p>The driver call-back functions are called synchronously from the
      erlang emulator. If they take too long before completing, they
      can cause timeouts in the emulator. Use the queue or
      asynchronous calls if necessary, since the emulator must be
      responsive.</p>
    <p>The driver structure contains the name of the driver and some
      15 function pointers. These pointers are called at different
      times by the emulator.</p>
    <p>The only exported function from the driver is
      <c>driver_init</c>. This function returns the <c>driver_entry</c>
      structure that points to the other functions in the driver. The
      <c>driver_init</c> function is declared with a macro
      <c>DRIVER_INIT(drivername)</c>. (This is because different OS's
      have different names for it.)</p>
    <p>When writing a driver in C++, the driver entry should be of
      <c>"C"</c> linkage. One way to do this is to put this line
      somewhere before the driver entry:
      <c>extern "C" DRIVER_INIT(drivername);</c>.</p>
    <p>When the driver has passed the <c>driver_entry</c> over to
      the emulator, the driver is <em>not</em> allowed to modify the
      <c>driver_entry</c>.</p>
    <p>If compiling a driver for static inclusion via --enable-static-drivers you
    have to define STATIC_ERLANG_DRIVER before the DRIVER_INIT declaration.</p>
    <note>
      <p>Do <em>not</em> declare the <c>driver_entry</c> <c>const</c>. This since the emulator needs to
        modify the <c>handle</c>, and the <c>handle2</c>
        fields. A statically allocated, and <c>const</c>
        declared <c>driver_entry</c> may be located in
        read only memory which will cause the emulator
        to crash.</p>
    </note>
  </description>

  <section>
    <title>DATA TYPES</title>
    <taglist>
    <tag><em>ErlDrvEntry</em></tag>
    <item>
    <p/>
    <code type="none">
typedef struct erl_drv_entry {
    int (*init)(void);          /* called at system start up for statically
                                   linked drivers, and after loading for
                                   dynamically loaded drivers */ 

#ifndef ERL_SYS_DRV
    ErlDrvData (*start)(ErlDrvPort port, char *command);
                                /* called when open_port/2 is invoked.
                                   return value -1 means failure. */
#else
    ErlDrvData (*start)(ErlDrvPort port, char *command, SysDriverOpts* opts);
                                /* special options, only for system driver */
#endif
    void (*stop)(ErlDrvData drv_data);
                                /* called when port is closed, and when the
                                   emulator is halted. */
    void (*output)(ErlDrvData drv_data, char *buf, ErlDrvSizeT len);
                                /* called when we have output from erlang to 
                                   the port */
    void (*ready_input)(ErlDrvData drv_data, ErlDrvEvent event); 
                                /* called when we have input from one of 
                                   the driver's handles */
    void (*ready_output)(ErlDrvData drv_data, ErlDrvEvent event);  
                                /* called when output is possible to one of 
                                   the driver's handles */
    char *driver_name;          /* name supplied as command 
                                   in open_port XXX ? */
    void (*finish)(void);       /* called before unloading the driver -
                                   DYNAMIC DRIVERS ONLY */
    void *handle;               /* Reserved -- Used by emulator internally */
    ErlDrvSSizeT (*control)(ErlDrvData drv_data, unsigned int command,
                            char *buf, ErlDrvSizeT len,
			    char **rbuf, ErlDrvSizeT rlen);
                                /* "ioctl" for drivers - invoked by 
                                   port_control/3 */
    void (*timeout)(ErlDrvData drv_data);        /* Handling of timeout in driver */
    void (*outputv)(ErlDrvData drv_data, ErlIOVec *ev);
                                /* called when we have output from erlang
                                   to the port */
    void (*ready_async)(ErlDrvData drv_data, ErlDrvThreadData thread_data);
    void (*flush)(ErlDrvData drv_data);
                                /* called when the port is about to be 
                                   closed, and there is data in the 
                                   driver queue that needs to be flushed
                                   before 'stop' can be called */
    ErlDrvSSizeT (*call)(ErlDrvData drv_data, unsigned int command,
                         char *buf, ErlDrvSizeT len,
			 char **rbuf, ErlDrvSizeT rlen, unsigned int *flags);
                                /* Works mostly like 'control', a synchronous
                                   call into the driver. */
    void (*event)(ErlDrvData drv_data, ErlDrvEvent event,
                  ErlDrvEventData event_data);
                                /* Called when an event selected by 
                                   driver_event() has occurred */
    int extended_marker;        /* ERL_DRV_EXTENDED_MARKER */
    int major_version;          /* ERL_DRV_EXTENDED_MAJOR_VERSION */
    int minor_version;          /* ERL_DRV_EXTENDED_MINOR_VERSION */
    int driver_flags;           /* ERL_DRV_FLAGs */
    void *handle2;              /* Reserved -- Used by emulator internally */
    void (*process_exit)(ErlDrvData drv_data, ErlDrvMonitor *monitor);
                                /* Called when a process monitor fires */
    void (*stop_select)(ErlDrvEvent event, void* reserved);
                                /* Called to close an event object */
 } ErlDrvEntry;
    </code>
    <p/>
    <taglist>
      <tag><marker id="init"/>int (*init)(void)</tag>
      <item>        
        <p>This is called directly after the driver has been loaded by
          <c>erl_ddll:load_driver/2</c>. (Actually when the driver is
          added to the driver list.) The driver should return 0, or if
          the driver can't initialize, -1.</p>
      </item>
      <tag><marker id="start"/>ErlDrvData (*start)(ErlDrvPort port, char* command)</tag>
      <item>
        <p>This is called when the driver is instantiated, when
          <c>open_port/2</c> is called. The driver should return a
          number &gt;= 0 or a pointer, or if the driver can't be started,
          one of three error codes should be returned:</p>
        <p>ERL_DRV_ERROR_GENERAL - general error, no error code</p>
        <p>ERL_DRV_ERROR_ERRNO - error with error code in <c>errno</c></p>
        <p>ERL_DRV_ERROR_BADARG - error, badarg</p>
        <p>If an error code is returned, the port isn't started.</p>
      </item>
      <tag><marker id="stop"/>void (*stop)(ErlDrvData drv_data)</tag>
      <item>
        <p>This is called when the port is closed, with
          <c>port_close/1</c> or <c>Port ! {self(), close}</c>. Note
          that terminating the port owner process also closes the
          port. If <c>drv_data</c> is a pointer to memory allocated in
	  <c>start</c>, then <c>stop</c> is the place to deallocate that
	  memory.</p>
      </item>
      <tag><marker id="output"/>void (*output)(ErlDrvData drv_data, char *buf, ErlDrvSizeT len)</tag>
      <item>
        <p>This is called when an erlang process has sent data to the
          port. The data is pointed to by <c>buf</c>, and is
          <c>len</c> bytes.  Data is sent to the port with <c>Port ! {self(), {command, Data}}</c>, or with
          <c>port_command/2</c>. Depending on how the port was opened,
          it should be either a list of integers 0...255 or a
          binary. See <c>open_port/3</c> and <c>port_command/2</c>.</p>
      </item>

      <tag><marker id="ready_input"/>void (*ready_input)(ErlDrvData drv_data, ErlDrvEvent event)</tag>
      <item/>
      <tag><marker id="ready_output"/>void (*ready_output)(ErlDrvData drv_data, ErlDrvEvent event)</tag>
      <item>
        <p>This is called when a driver event (given in the
          <c>event</c> parameter) is signaled. This is used to help
          asynchronous drivers "wake up" when something happens.</p>
        <p>On unix the <c>event</c> is a pipe or socket handle (or
          something that the <c>select</c> system call understands).</p>
        <p>On Windows the <c>event</c> is an Event or Semaphore (or
          something that the <c>WaitForMultipleObjects</c> API
          function understands). (Some trickery in the emulator allows
          more than the built-in limit of 64 <c>Events</c> to be used.)</p>
        <p>To use this with threads and asynchronous routines, create a
          pipe on unix and an Event on Windows. When the routine
          completes, write to the pipe (use <c>SetEvent</c> on
          Windows), this will make the emulator call
          <c>ready_input</c> or <c>ready_output</c>.</p>
          <p>Spurious events may happen. That is, calls to <c>ready_input</c>
          or <c>ready_output</c> even though no real events are signaled. In
          reality it should be rare (and OS dependant), but a robust driver
          must nevertheless be able to handle such cases.</p>
      </item>
      <tag><marker id="driver_name"/>char *driver_name</tag>
      <item>
        <p>This is the name of the driver, it must correspond to the
          atom used in <c>open_port</c>, and the name of the driver
          library file (without the extension).</p>
      </item>
      <tag><marker id="finish"/>void (*finish)(void)</tag>
      <item>
        <p>This function is called by the <c>erl_ddll</c> driver when the
          driver is unloaded. (It is only called in dynamic drivers.)</p>
        <p>The driver is only unloaded as a result of calling
          <c>unload_driver/1</c>, or when the emulator halts.</p>
      </item>
      <tag>void *handle</tag>
      <item>
        <p>This field is reserved for the emulator's internal use. The
          emulator will modify this field; therefore, it is important
          that the <c>driver_entry</c> isn't declared <c>const</c>.</p> 
      </item>
      <tag><marker id="control"></marker>ErlDrvSSizeT (*control)(ErlDrvData drv_data, unsigned int command, char *buf, ErlDrvSizeT len, char **rbuf, ErlDrvSizeT rlen)</tag>
      <item>
        <p>This is a special routine invoked with the erlang function
          <c>port_control/3</c>. It works a little like an "ioctl" for
          erlang drivers. The data given to <c>port_control/3</c>
          arrives in <c>buf</c> and <c>len</c>. The driver may send
          data back, using <c>*rbuf</c> and <c>rlen</c>.</p>
        <p>This is the fastest way of calling a driver and get a
          response. It won't make any context switch in the erlang
          emulator, and requires no message passing. It is suitable
          for calling C function to get faster execution, when erlang
          is too slow.</p>
        <p>If the driver wants to return data, it should return it in
          <c>rbuf</c>. When <c>control</c> is called,
          <c>*rbuf</c> points to a default buffer of <c>rlen</c> bytes, which
          can be used to return data. Data is returned different depending on
          the port control flags (those that are set with
          <seealso marker="erl_driver#set_port_control_flags">set_port_control_flags</seealso>).
          </p>
        <p>If the flag is set to <c>PORT_CONTROL_FLAG_BINARY</c>,
           a binary will be returned. Small binaries can be returned by writing
           the raw data into the default buffer. A binary can also be
           returned by setting <c>*rbuf</c> to point to a binary allocated with
          <seealso marker="erl_driver#driver_alloc_binary">driver_alloc_binary</seealso>.
          This binary will be freed automatically after <c>control</c> has returned.
          The driver can retain the binary for <em>read only</em> access with
          <seealso marker="erl_driver#driver_binary_inc_refc">driver_binary_inc_refc</seealso> to be freed later with
          <seealso marker="erl_driver#driver_free_binary">driver_free_binary</seealso>.
          It is never allowed to alter the binary after <c>control</c> has returned.
          If <c>*rbuf</c> is set to NULL, an empty list will be returned.
          </p>
        <p>If the flag is set to <c>0</c>, data is returned as a
          list of integers. Either use the default buffer or set
          <c>*rbuf</c> to point to a larger buffer allocated with
          <seealso marker="erl_driver#driver_alloc">driver_alloc</seealso>.
          The buffer will be freed automatically after <c>control</c> has returned.</p>
        <p>Using binaries is faster if more than a few bytes are returned.</p>
        <p>The return value is the number of bytes returned in
          <c>*rbuf</c>.</p>
      </item>

      <tag><marker id="timeout"/>void (*timeout)(ErlDrvData drv_data)</tag>
      <item>
        <p>This function is called any time after the driver's timer
          reaches 0. The timer is activated with
          <c>driver_set_timer</c>. There are no priorities or ordering
          among drivers, so if several drivers time out at the same
          time, any one of them is called first.</p>
      </item>

      <tag><marker id="outputv"/>void (*outputv)(ErlDrvData drv_data, ErlIOVec *ev)</tag>
      <item>
        <p>This function is called whenever the port is written to. If
          it is <c>NULL</c>, the <c>output</c> function is called
          instead. This function is faster than <c>output</c>, because
          it takes an <c>ErlIOVec</c> directly, which requires no
          copying of the data. The port should be in binary mode, see
          <c>open_port/2</c>.</p>
        <p>The <c>ErlIOVec</c> contains both a <c>SysIOVec</c>,
          suitable for <c>writev</c>, and one or more binaries. If
          these binaries should be retained, when the driver returns
          from <c>outputv</c>, they can be queued (using <seealso marker="erl_driver#driver_enq_bin">driver_enq_bin</seealso>
          for instance), or if they are kept in a static or global
          variable, the reference counter can be incremented.</p>
      </item>
      <tag><marker id="ready_async"/>void (*ready_async)(ErlDrvData drv_data, ErlDrvThreadData thread_data)</tag>
      <item>
        <p>This function is called after an asynchronous call has
          completed. The asynchronous call is started with <seealso marker="erl_driver#driver_async">driver_async</seealso>.
          This function is called from the erlang emulator thread, as
          opposed to the asynchronous function, which is called in
          some thread (if multithreading is enabled).</p>
      </item>
      <tag><marker id="call"/>ErlDrvSSizeT (*call)(ErlDrvData drv_data, unsigned int command, char *buf, ErlDrvSizeT len, char **rbuf, ErlDrvSizeT rlen, unsigned int *flags)</tag>
      <item>
        <p>This function is called from <c>erlang:port_call/3</c>. It
          works a lot like the <c>control</c> call-back, but uses the
          external term format for input and output.</p>
        <p><c>command</c> is an integer, obtained from the call from
          erlang (the second argument to <c>erlang:port_call/3</c>).</p>
        <p><c>buf</c> and <c>len</c> provide the arguments to the call
          (the third argument to <c>erlang:port_call/3</c>). They can
          be decoded using <c>ei</c> functions.</p>
        <p><c>rbuf</c> points to a return buffer, <c>rlen</c> bytes
          long. The return data should be a valid erlang term in the
          external (binary) format. This is converted to an erlang
          term and returned by <c>erlang:port_call/3</c> to the
          caller.  If more space than <c>rlen</c> bytes is needed to
          return data, <c>*rbuf</c> can be set to memory allocated with
          <c>driver_alloc</c>. This memory will be freed automatically
          after <c>call</c> has returned.</p>
        <p>The return value is the number of bytes returned in
          <c>*rbuf</c>. If <c>ERL_DRV_ERROR_GENERAL</c> is returned
          (or in fact, anything &lt; 0), <c>erlang:port_call/3</c> will
          throw a <c>BAD_ARG</c>.</p>
      </item>
      <tag>void (*event)(ErlDrvData drv_data, ErlDrvEvent event, ErlDrvEventData event_data)</tag>
      <item>
        <p>Intentionally left undocumented.</p>
      </item>
      <tag><marker id="extended_marker"/>int extended_marker</tag>
      <item>
        <p>
          This field should either be equal to <c>ERL_DRV_EXTENDED_MARKER</c>
          or <c>0</c>. An old driver (not aware of the extended driver
          interface) should set this field to <c>0</c>. If this field is
          equal to <c>0</c>, all the fields following this field also
          <em>have</em> to be <c>0</c>, or <c>NULL</c> in case it is a
          pointer field.
        </p>
      </item>
      <tag>int major_version</tag>
      <item>
        <p>This field should equal <c>ERL_DRV_EXTENDED_MAJOR_VERSION</c> if
          the <c>extended_marker</c> field equals
          <c>ERL_DRV_EXTENDED_MARKER</c>.</p> 
      </item>
      <tag>int minor_version</tag>
      <item>
        <p>
          This field should equal <c>ERL_DRV_EXTENDED_MINOR_VERSION</c> if
          the <c>extended_marker</c> field equals
          <c>ERL_DRV_EXTENDED_MARKER</c>.
        </p>
      </item>

      <tag><marker id="driver_flags"/>int driver_flags</tag>
      <item>
        <p>This field is used to pass driver capability and other
	  information to the runtime system. If the
	  <c>extended_marker</c> field equals <c>ERL_DRV_EXTENDED_MARKER</c>,
	  it should contain <c>0</c> or  driver flags (<c>ERL_DRV_FLAG_*</c>)
	  ored bitwise. Currently the following driver flags exist:
        </p>
        <taglist>
          <tag><c>ERL_DRV_FLAG_USE_PORT_LOCKING</c></tag>
          <item>
            The runtime system will use port level locking on
            all ports executing this driver instead of driver
            level locking when the driver is run in a runtime
            system with SMP support. For more information see the
            <seealso marker="erl_driver#smp_support">erl_driver</seealso> 
            documentation.
          </item> 
          <tag><c>ERL_DRV_FLAG_SOFT_BUSY</c></tag>
          <item>
            Marks that driver instances can handle being called
	    in the <seealso marker="#output">output</seealso> and/or
	    <seealso marker="#outputv">outputv</seealso> callbacks even
	    though a driver instance has marked itself as busy (see
            <seealso marker="erl_driver#set_busy_port">set_busy_port()</seealso>).
	    Since erts version 5.7.4 this flag is required for drivers used
	    by the Erlang distribution (the behaviour has always been
	    required by drivers used by the distribution).
          </item>
          <tag><c>ERL_DRV_FLAG_NO_BUSY_MSGQ</c></tag>
          <item>Disable busy port message queue functionality. For
	  more information, see the documentation of the
	  <seealso marker="erl_driver#erl_drv_busy_msgq_limits">erl_drv_busy_msgq_limits()</seealso>
	  function.
          </item>
          <tag><c>ERL_DRV_FLAG_USE_INIT_ACK</c></tag>
          <item>When this flag is given the linked-in driver has to manually
          acknowledge that the port has been successfully started using 
	  <seealso marker="erl_driver#erl_drv_init_ack">erl_drv_init_ack()</seealso>.
          This allows the implementor to make the erlang:open_port exit with
          badarg after some initial asynchronous initialization has been done.
          </item>
        </taglist>
      </item>
      <tag>void *handle2</tag>
      <item>
        <p>
          This field is reserved for the emulator's internal use. The
          emulator will modify this field; therefore, it is important
          that the <c>driver_entry</c> isn't declared <c>const</c>.
        </p>
      </item>
      <tag><marker id="process_exit"/>void (*process_exit)(ErlDrvData drv_data, ErlDrvMonitor *monitor)</tag>
      <item>
        <p>This callback is called when a monitored process exits. The
          <c>drv_data</c> is the data associated with the port for which
          the process is monitored (using <seealso marker="erl_driver#driver_monitor_process">driver_monitor_process</seealso>)
          and the <c>monitor</c> corresponds to the <c>ErlDrvMonitor</c> 
          structure filled
          in when creating the monitor. The driver interface function
          <seealso marker="erl_driver#driver_get_monitored_process">driver_get_monitored_process</seealso>
          can be used to retrieve the process id of the exiting process as
          an <c>ErlDrvTermData</c>.</p>
      </item>
      <tag><marker id="stop_select"/>void (*stop_select)(ErlDrvEvent event, void* reserved)</tag>
      <item>
        <p>This function is called on behalf of
           <seealso marker="erl_driver#driver_select">driver_select</seealso>
           when it is safe to close an event object.</p>
        <p>A typical implementation on Unix is to do
           <c>close((int)event)</c>.</p>
        <p>Argument <c>reserved</c> is intended for future use and should be ignored.</p>
        <p>In contrast to most of the other call-back functions,
           <c>stop_select</c> is called independent of any port. No
           <c>ErlDrvData</c> argument is passed to the function. No
           driver lock or port lock is guaranteed to be held. The port that
           called <c>driver_select</c> might even be closed at the
           time <c>stop_select</c> is called. But it could also be
           the case that <c>stop_select</c> is called directly by
           <c>driver_select</c>.</p>
        <p>It is not allowed to call any functions in the
           <seealso marker="erl_driver">driver API</seealso> from
           <c>stop_select</c>. This strict limitation is due to the
           volatile context that <c>stop_select</c> may be called.</p>
      </item>

    </taglist>
    </item>

    </taglist>
  </section>

  <section>
    <title>SEE ALSO</title>
    <p><seealso marker="erl_driver">erl_driver(3)</seealso>, 
      <seealso marker="kernel:erl_ddll">erl_ddll(3)</seealso>,
      <seealso marker="erlang">erlang(3)</seealso>,
      kernel(3)</p>
  </section>
</cref>