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NAME

       erl_ddll - Dynamic Driver Loader and Linker

DESCRIPTION

       The  erl_ddll  module  provides  an  interface  for loading and unloading erlang linked in
       drivers in runtime.

   Note:
       This is a large reference document. For casual use of the module, as well as for most real
       world  applications,  the  descriptions of the functions load/2 and unload/1 are enough to
       get going.

       The driver should be provided as a dynamically linked library  in  a  object  code  format
       specific  for  the platform in use, i. e. .so files on most Unix systems and .ddl files on
       windows. An erlang linked in driver has to provide specific interfaces to the emulator, so
       this  module  is  not  designed  for  loading  arbitrary  dynamic  libraries.  For further
       information about erlang drivers, refer to the ERTS reference manual section erl_driver.

       When describing a set of functions, (i.e. 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 user. There can
       be several users in one process (different modules needing the same  driver)  and  several
       processes  running  the  same  code,  making  up  several  users of a driver. 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 as well as the number of loads by
       each process, so that the driver will only be unloaded when no one wants  it  (it  has  no
       user).  The  driver  also keeps track of ports that are opened towards it, so that one can
       delay unloading until all ports are closed or kill all ports using the driver when  it  is
       unloaded.

       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:

         Load and unload on a "when needed basis":
           This (most common) scenario simply supports that each user of the driver loads it when
           it is needed and unloads it when the user no longer have any use for it. 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.

           Each user of the driver use literally the same pathname for the driver when  demanding
           load, but the users are not really concerned with if the driver is already loaded from
           the filesystem or if the object code has to be loaded from filesystem.

           Two pairs of functions support this scenario:

           load/2 and unload/1:
             When using the load/unload interfaces, the driver will  not  actually  get  unloaded
             until  the  last  port  using the driver is closed. The function unload/1 can return
             immediately, as the users are not really concerned with when  the  actual  unloading
             occurs. The driver will actually get unloaded when no one needs it any longer.

             If  a  process  having  the  driver  loaded dies, it will have the same effect as if
             unloading was done.

             When loading, the function load/2 returns ok as soon as there is any instance of the
             driver  present, so that if a driver is waiting to get unloaded (due to open ports),
             it will simply change state to no longer need unloading.

           load_driver/2 and unload_driver/1:
             These interfaces is intended to be used when it is considered an  error  that  ports
             are  open  towards  a  driver that no user has loaded. The ports still open when the
             last user calls unload_driver/1 or when the last process having  the  driver  loaded
             dies, will get killed with reason driver_unloaded.

             The   function   names   load_driver   and   unload_driver  are  kept  for  backward
             compatibility.

         Loading and reloading for code replacement:
           This scenario occurs when the driver code might need replacement during  operation  of
           the  Erlang  emulator.  Implementing  driver code replacement is somewhat more tedious
           than beam code replacement, as one driver cannot be loaded as  both  "old"  and  "new"
           code.  All  users  of a driver must have it closed (no open ports) before the old code
           can be unloaded and the new code can be loaded.

           The actual unloading/loading is done as one atomic operation, blocking  all  processes
           in the system from using the driver concerned while in progress.

           The  preferred  way  to  do  driver code replacement is to let one single process keep
           track of the driver. When the process start, 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 user has a driver  loaded  when  code  replacement  is
           demanded,  the  replacement  cannot occur until the last "other" user has unloaded the
           driver.

           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 user
           has the driver loaded. To simplify driver replacement, avoid designing your system  so
           that more than than one user has the driver loaded.

           The  two  functions  for  reloading drivers should be used together with corresponding
           load functions, to support the two different behaviors concerning open ports:

           load/2 and reload/2:
             This pair of functions is used when reloading should be done  after  the  last  open
             port towards the driver is closed.

             As reload/2 actually waits for the reloading to occur, a misbehaving process keeping
             open ports towards the driver (or keeping the driver loaded)  might  cause  infinite
             waiting for reload. Timeouts has to be provided outside of the process demanding the
             reload or by using the low-level interface try_load/3  in  combination  with  driver
             monitors (see below).

           load_driver/2 and reload_driver/2:
             This  pair of functions are used when open ports towards the driver should be killed
             with reason driver_unloaded to allow for new driver code to get loaded.

             If, however, another process has the driver loaded,  calling  reload_driver  returns
             the  error code pending_process. As stated earlier, the recommended design is to not
             allow other users than the "driver reloader"  to  actually  demand  loading  of  the
             concerned driver.

DATA TYPES

       driver() = string() | atom()

       path() = string() | atom()

EXPORTS

       demonitor(MonitorRef) -> ok

              Types:

                 MonitorRef = reference()

              Removes  a  driver  monitor  in  much  the same way as erlang:demonitor/1 does with
              process monitors. See monitor/2, try_load/3 and try_unload/2 for details about  how
              to create driver monitors.

              The function throws a badarg exception if the parameter is not a reference().

       info() -> AllInfoList

              Types:

                 AllInfoList = [DriverInfo]
                 DriverInfo = {DriverName, InfoList}
                 DriverName = string()
                 InfoList = [InfoItem]
                 InfoItem = {Tag :: atom(), Value :: term()}

              Returns  a  list  of tuples {DriverName, InfoList}, where InfoList is the result of
              calling info/1 for that DriverName. Only dynamically linked in drivers are included
              in the list.

       info(Name) -> InfoList

              Types:

                 Name = driver()
                 InfoList = [InfoItem, ...]
                 InfoItem = {Tag :: atom(), Value :: term()}

              Returns  a list of tuples {Tag, Value}, where Tag is the information item and Value
              is the result of calling info/2 with this driver name  and  this  tag.  The  result
              being a tuple list containing all information available about a driver.

              The different tags that will appear in the list are:

                * processes

                * driver_options

                * port_count

                * linked_in_driver

                * permanent

                * awaiting_load

                * awaiting_unload

              For  a  detailed  description  of each value, please read the description of info/2
              below.

              The function throws a badarg exception if the driver is not present in the system.

       info(Name, Tag) -> Value

              Types:

                 Name = string() | atom()
                 Tag = processes | driver_options | port_count | linked_in_driver |  permanent  |
                 awaiting_load | awaiting_unload
                 Value = term()

              This  function  returns  specific information about one aspect of a driver. The Tag
              parameter specifies which aspect to get information about. The Value return differs
              between different tags:

                processes:
                  Return  all  processes  containing  users  of the specific drivers as a list of
                  tuples {pid(),int()}, where the int()  denotes  the  number  of  users  in  the
                  process pid().

                driver_options:
                  Return  a  list  of  the  driver  options provided when loading, as well as any
                  options set by the driver itself  during  initialization.  The  currently  only
                  valid option being kill_ports.

                port_count:
                  Return the number of ports (an int()) using the driver.

                linked_in_driver:
                  Return  a  bool(),  being  true if the driver is a statically linked in one and
                  false otherwise.

                permanent:
                  Return a bool(), being true if the driver has made itself permanent (and is not
                  a statically linked in driver). false otherwise.

                awaiting_load:
                  Return a list of all processes having monitors for loading active, each process
                  returned as {pid(),int()}, where the int() is the number of  monitors  held  by
                  the process pid().

                awaiting_unload:
                  Return  a  list  of  all  processes  having monitors for unloading active, each
                  process returned as {pid(),int()}, where the int() is the  number  of  monitors
                  held by the process pid().

              If  the  options  linked_in_driver or permanent return true, all other options will
              return the value linked_in_driver or permanent respectively.

              The function throws a badarg exception if the driver is not present in  the  system
              or the tag is not supported.

       load(Path, Name) -> ok | {error, ErrorDesc}

              Types:

                 Path = path()
                 Name = driver()
                 ErrorDesc = term()

              Loads  and  links  the  dynamic  driver  Name. Path is a file path to the directory
              containing the driver. Name must be a sharable object/dynamic library. Two  drivers
              with  different Path parameters cannot be loaded under the same name. The Name is a
              string or atom containing at least one character.

              The Name given should correspond to the filename of the actual dynamically loadable
              object  file  residing  in  the  directory given as Path, but without the extension
              (i.e. .so). 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 .beam files.

              If the driver has been previously unloaded, but is still present due to open  ports
              against  it,  a call to load/2 will stop the unloading and keep the driver (as long
              as the Path is the same) and ok is returned. If one actually wants the object  code
              to  be  reloaded,  one uses reload/2 or the low-level interface try_load/3 instead.
              Please refer to the description of different scenarios for loading/unloading in the
              introduction.

              If  more  than  one  process  tries to load an already loaded driver withe the same
              Path, or if the same process tries to load it  several  times,  the  function  will
              return  ok.  The  emulator  will  keep  track  of  the  load/2  calls,  so  that  a
              corresponding number of unload/2 calls will have to be done from the  same  process
              before  the  driver  will  actually  get  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.

              It is not allowed to load several drivers with the same  name  but  with  different
              Path parameters.

          Note:
              Note  especially that the Path is interpreted literally, so that all loaders of the
              same driver needs to give the same literalPath string, even though different  paths
              might  point out the same directory in the filesystem (due to use of relative paths
              and links).

              On  success,  the  function  returns  ok.  On  failure,   the   return   value   is
              {error,ErrorDesc},  where  ErrorDesc  is an opaque term to be translated into human
              readable form by the format_error/1 function.

              For more control over the  error  handling,  again  use  the  try_load/3  interface
              instead.

              The function throws a badarg exception if the parameters are not given as described
              above.

       load_driver(Path, Name) -> ok | {error, ErrorDesc}

              Types:

                 Path = path()
                 Name = driver()
                 ErrorDesc = term()

              Works essentially as load/2, but will load the driver with other options. All ports
              that  are using the driver will get killed with the reason driver_unloaded when the
              driver is to be unloaded.

              The number of loads and unloads by different users influence the actual loading and
              unloading  of  a  driver file. The port killing will therefore only happen when the
              last user unloads the driver, or the last process having loaded the driver exits.

              This interface (or at least the  name  of  the  functions)  is  kept  for  backward
              compatibility.  Using  try_load/3 with {driver_options,[kill_ports]}  in the option
              list will give the same effect regarding the port killing.

              The function throws a badarg exception if the parameters are not given as described
              above.

       monitor(Tag, Item) -> MonitorRef

              Types:

                 Tag = driver
                 Item = {Name, When}
                 Name = atom() | string()
                 When = loaded | unloaded | unloaded_only
                 MonitorRef = reference()

              This  function  creates  a  driver  monitor  and works in many ways as the function
              erlang:monitor/2, does for processes. When a  driver  changes  state,  the  monitor
              results  in  a  monitor-message  being  sent to the calling process. The MonitorRef
              returned by this function is included in the message sent.

              As with process monitors, each driver monitor set will  only  generate  one  single
              message.  The monitor is "destroyed" after the message is sent and there is then no
              need to call demonitor/1.

              The MonitorRef can also be used in subsequent calls  to  demonitor/1  to  remove  a
              monitor.

              The function accepts the following parameters:

                Tag:
                  The  monitor  tag  is always driver 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 given for consistence.

                Item:
                  The Item parameter specifies which driver one wants to monitor (the name of the
                  driver) as well as which state change one wants to monitor. The parameter is  a
                  tuple of arity two whose first element is the driver name and second element is
                  either of:

                  loaded:
                    Notify me 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. One cannot monitor a future-to-be driver name for loading,  that
                    will  only  result in a 'DOWN' message being immediately sent. Monitoring for
                    loading is therefore most useful when triggered by the  try_load/3  function,
                    where the monitor is created because the driver is in such a pending state.

                    Setting  a  driver  monitor  for  loading  will eventually lead to one of the
                    following messages being sent:

                    {'UP', reference(), driver, Name, loaded}:
                      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.

                      The user is expected to know if reloading is demanded prior to  creating  a
                      monitor for loading.

                    {'UP', reference(), driver, Name, permanent}:
                      This  message  will be sent if reloading was expected, but the (old) driver
                      made itself permanent prior to reloading. It  will  also  be  sent  if  the
                      driver  was  permanent  or  statically  linked in when trying to create the
                      monitor.

                    {'DOWN', reference(), driver, Name, load_cancelled}:
                      This message will arrive if reloading was underway,  but  the  user  having
                      requested  reload  cancelled it by either dying or calling try_unload/2 (or
                      unload/1/unload_driver/1) again before it was reloaded.

                    {'DOWN', reference(), driver, Name, {load_failure, Failure}}:
                      This message will arrive if reloading was underway but the loading for some
                      reason  failed.  The Failure term is one of the errors that can be returned
                      from try_load/3. The  error  term  can  be  passed  to  format_error/1  for
                      translation  into  human readable form. Note that the translation has to be
                      done in the same running erlang virtual machine as the error  was  detected
                      in.

                  unloaded:
                    Monitor  when  a  driver  gets unloaded. If one monitors a driver that is not
                    present in the system, one will immediately get notified that the driver  got
                    unloaded. There is no guarantee that the driver was actually ever loaded.

                    A  driver  monitor  for unload will eventually result in one of the following
                    messages being sent:

                    {'DOWN', reference(), driver, Name, unloaded}:
                      The driver instance monitored is now unloaded. As  the  unload  might  have
                      been  due  to  a  reload/2  request,  the driver might once again have been
                      loaded when this message arrives.

                    {'UP', reference(), driver, Name, unload_cancelled}:
                      This message will be sent if unloading was expected, but while  the  driver
                      was  waiting for all ports to get closed, a new user of the driver appeared
                      and the unloading was cancelled.

                      This message appears  when  an  {ok,  pending_driver})  was  returned  from
                      try_unload/2)   for   the   last  user  of  the  driver  and  then  a  {ok,
                      already_loaded} is returned from a call to try_load/3.

                      If one wants to really monitor when the driver gets unloaded, this  message
                      will  distort  the picture, no unloading was really done. The unloaded_only
                      option creates a monitor similar to an unloaded  monitor,  but  does  never
                      result in this message.

                    {'UP', reference(), driver, Name, permanent}:
                      This  message  will  be sent if unloading was expected, but the driver made
                      itself permanent prior to unloading. It will also  be  sent  if  trying  to
                      monitor a permanent or statically linked in driver.

                  unloaded_only:
                    A monitor created as unloaded_only behaves exactly as one created as unloaded
                    with   the   exception   that   the   {'UP',   reference(),   driver,   Name,
                    unload_cancelled}  message  will  never  be  sent,  but  the  monitor instead
                    persists until the driver really gets unloaded.

              The function throws a badarg exception if the parameters are not given as described
              above.

       reload(Path, Name) -> ok | {error, ErrorDesc}

              Types:

                 Path = path()
                 Name = driver()
                 ErrorDesc = pending_process | OpaqueError
                 OpaqueError = term()

              Reloads  the  driver  named Name from a possibly different Path than was previously
              used. This  function  is  used  in  the  code  change  scenario  described  in  the
              introduction.

              If  there  are  other  users  of  this  driver,  the  function  will return {error,
              pending_process}, but if there are no more users, the function call will hang until
              all open ports are closed.

          Note:
              Avoid mixing several users with driver reload requests.

              If one wants to avoid hanging on open ports, one should use the try_load/3 function
              instead.

              The Name and Path parameters have exactly the same  meaning  as  when  calling  the
              plain load/2 function.

          Note:
              Avoid mixing several users with driver reload requests.

              On  success,  the  function  returns ok. On failure, the function returns an opaque
              error, with the exception of the pending_process error described above. The  opaque
              errors  are  to  be  translated  into  human  readable  form  by the format_error/1
              function.

              For more control over the  error  handling,  again  use  the  try_load/3  interface
              instead.

              The function throws a badarg exception if the parameters are not given as described
              above.

       reload_driver(Path, Name) -> ok | {error, ErrorDesc}

              Types:

                 Path = path()
                 Name = driver()
                 ErrorDesc = pending_process | OpaqueError
                 OpaqueError = term()

              Works exactly as reload/2, but for drivers loaded with the load_driver/2 interface.

              As this  interface  implies  that  ports  are  being  killed  when  the  last  user
              disappears, the function wont hang waiting for ports to get closed.

              For  further  details, see the scenarios in the module description and refer to the
              reload/2 function description.

              The function throws a badarg exception if the parameters are not given as described
              above.

       try_load(Path,  Name,  OptionList)  ->  {ok,Status}  |  {ok, PendingStatus, Ref} | {error,
       ErrorDesc}

              Types:

                 Path = Name = string() | atom()
                 OptionList = [ Option ]
                 Option  =  {driver_options,  DriverOptionList}  |  {monitor,  MonitorOption}   |
                 {reload, ReloadOption}
                 DriverOptionList = [ DriverOption ]
                 DriverOption = kill_ports
                 MonitorOption = pending_driver | pending
                 ReloadOption = pending_driver | pending
                 Status = loaded | already_loaded | PendingStatus
                 PendingStatus = pending_driver | pending_process
                 Ref = reference()
                 ErrorDesc = ErrorAtom | OpaqueError
                 ErrorAtom     =     linked_in_driver    |    inconsistent    |    permanent    |
                 not_loaded_by_this_process | not_loaded | pending_reload | pending_process

              This   function   provides   more   control   than    the    load/2/reload/2    and
              load_driver/2/reload_driver/2  interfaces.  It  will  never  wait for completion of
              other operations related to the driver, but immediately return the  status  of  the
              driver as either:

                {ok, loaded}:
                  The driver was actually loaded and is immediately usable.

                {ok, already_loaded}:
                  The  driver  was already loaded by another process and/or is in use by a living
                  port. The load by you is registered and a corresponding try_unload is  expected
                  sometime in the future.

                {ok, pending_driver}or {ok, pending_driver, reference()}:
                  The load request is registered, but the loading is delayed due to the fact that
                  an earlier instance of the driver is still waiting to get unloaded  (there  are
                  open  ports  using  it).  Still,  unload is expected when you are done with the
                  driver. This return value will mostly happen when  the  {reload,pending_driver}
                  or  {reload,pending}  options  are  used,  but  can happen when another user is
                  unloading a driver in parallel and the kill_ports  driver  option  is  set.  In
                  other words, this return value will always need to be handled!

                {ok, pending_process}or {ok, pending_process, reference()}:
                  The load request is registered, but the loading is delayed due to the fact that
                  an earlier instance of the driver is still waiting to get unloaded  by  another
                  user  (not  only  by  a port, in which case {ok,pending_driver} would have been
                  returned). Still, unload is expected when you are done with  the  driver.  This
                  return value will only happen when the {reload,pending} option is used.

              When  the function returns {ok, pending_driver} or {ok, pending_process}, one might
              want to get information about when the driver  is  actually  loaded.  This  can  be
              achieved by using the {monitor, PendingOption} option.

              When  monitoring  is  requested,  and  a corresponding {ok, pending_driver} or {ok,
              pending_process} would be returned, the function will instead return a  tuple  {ok,
              PendingStatus,  reference()}  and the process will, at a later time when the driver
              actually gets loaded, get a monitor message. The monitor message one can expect  is
              described in the monitor/2 function description.

          Note:
              Note  that  in  case of loading, monitoring can not only get triggered by using the
              {reload, ReloadOption} option, but also in special cases where  the  load-error  is
              transient,  why  {monitor,  pending_driver} should be used under basically all real
              world circumstances!

              The function accepts the following parameters:

                Path:
                  The filesystem path to the directory where the driver object file is  situated.
                  The filename of the object file (minus extension) must correspond to the driver
                  name (used in the name parameter) and the driver must identify itself with  the
                  very  same  name. The Path might be provided as an io_list, meaning it can be a
                  list of other io_lists, characters (eight bit integers) or binaries, all to  be
                  flattened into a sequence of characters.

                  The  (possibly  flattened)  Path  parameter  must  be consistent throughout the
                  system, a driver should, by all users, be loaded using  the  same  literalPath.
                  The  exception  is  when  reloading is requested, in which case the Path may be
                  specified differently. Note that all users trying to load the driver at a later
                  time will need to use the newPath if the Path is changed using a reload option.
                  This is yet another reason to have only one loader of a  driver  one  wants  to
                  upgrade in a running system!

                Name:
                  The  name parameter is the name of the driver to be used in subsequent calls to
                  open_port. The name can be specified either as an io_list() or  as  an  atom().
                  The  name  given  when loading is used to find the actual object file (with the
                  help of the Path and the system implied extension suffix, i.e. .so).  The  name
                  by  which  the  driver identifies itself must also be consistent with this Name
                  parameter, much as a beam-file's module name much correspond to its filename.

                OptionList:
                  A number of options can be specified to  control  the  loading  operation.  The
                  options  are  given  as  a  list of two-tuples, the tuples having the following
                  values and meanings:

                  {driver_options, DriverOptionsList}:
                    This option is to provide options that will change its general  behavior  and
                    will "stick" to the driver throughout its lifespan.

                    The driver options for a given driver name need always to be consistent, even
                    when the driver is reloaded, meaning that they are as  much  a  part  of  the
                    driver as the actual name.

                    Currently  the only allowed driver option is kill_ports, which means that all
                    ports  opened  towards  the  driver   are   killed   with   the   exit-reason
                    driver_unloaded  when  no  process  any  longer  has  the driver loaded. This
                    situation arises either when the last user calls try_unload/2,  or  the  last
                    process having loaded the driver exits.

                  {monitor, MonitorOption}:
                    A  MonitorOption  tells  try_load/3 to trigger a driver monitor under certain
                    conditions. When the monitor is triggered, the function will return a  three-
                    tuple  {ok, PendingStatus, reference()}, where the reference() is the monitor
                    ref for the driver monitor.

                    Only one MonitorOption can be specified and it is either  the  atom  pending,
                    which  means  that  a  monitor should be created whenever a load operation is
                    delayed, and the atom pending_driver, in which a monitor is created  whenever
                    the  operation  is  delayed  due  to  open  ports towards an otherwise unused
                    driver. The pending_driver option is  of  little  use,  but  is  present  for
                    completeness, it is very well defined which reload-options might give rise to
                    which delays. It might, however, be a good idea to use the same MonitorOption
                    as the ReloadOption if present.

                    If  reloading  is  not  requested,  it  might  still be useful to specify the
                    monitor option, as forced unloads (kill_ports driver option or the kill_ports
                    option  to  try_unload/2) will trigger a transient state where driver loading
                    cannot be performed until all closing  ports  are  actually  closed.  So,  as
                    try_unload  can,  in  almost all situations, return {ok, pending_driver}, one
                    should always specify at least {monitor, pending_driver} in  production  code
                    (see the monitor discussion above).

                  {reload,RealoadOption}:
                    This  option  is used when one wants to reload a driver from disk, most often
                    in a code upgrade scenario. Having a reload option also implies that the Path
                    parameter need not be consistent with earlier loads of the driver.

                    To  reload  a driver, the process needs to have previously loaded the driver,
                    i.e there has to be an active user of the driver in the process.

                    The reload option can be either the  atom  pending,  in  which  reloading  is
                    requested  for  any  driver  and  will  be  effectuated when all ports opened
                    against the driver are closed. The replacement of the  driver  will  in  this
                    case  take  place  regardless  of if there are still pending users having the
                    driver loaded! The option  also  triggers  port-killing  (if  the  kill_ports
                    driver  option is used) even though there are pending users, making it usable
                    for forced driver replacement, but laying a  lot  of  responsibility  on  the
                    driver  users.  The  pending option is seldom used as one does not want other
                    users to have loaded the driver when code change is underway.

                    The more useful option is pending_driver, which means that reloading will  be
                    queued  if  the  driver  is not loaded by any other users, but the driver has
                    opened ports, in which case {ok, pending_driver} will be returned (a  monitor
                    option is of course recommended).

                    If  the  driver  is  unloaded  (not  present  in  the system), the error code
                    not_loaded will be returned. The reload option is intended for when the  user
                    has already loaded the driver in advance.

              The function might return numerous errors, of which some only can be returned given
              a certain combination of options.

              A number of errors are opaque and can only be interpreted by passing  them  to  the
              format_error/1 function, but some can be interpreted directly:

                {error,linked_in_driver}:
                  The  driver  with  the specified name is an erlang statically linked in driver,
                  which cannot be manipulated with this API.

                {error,inconsistent}:
                  The driver has already  been  loaded  with  either  other  DriverOptions  or  a
                  different literalPath argument.

                  This  can  happen even if a reload option is given, if the DriverOptions differ
                  from the current.

                {error, permanent}:
                  The driver has requested itself to be  permanent,  making  it  behave  like  an
                  erlang linked in driver and it can no longer be manipulated with this API.

                {error, pending_process}:
                  The  driver  is  loaded by other users when the {reload, pending_driver} option
                  was given.

                {error, pending_reload}:
                  Driver  reload  is  already  requested  by  another  user  when  the   {reload,
                  ReloadOption} option was given.

                {error, not_loaded_by_this_process}:
                  Appears  when  the  reload  option  is given. The driver Name is present in the
                  system, but there is no user of it in this process.

                {error, not_loaded}:
                  Appears when the reload option is given. The driver Name is not in the  system.
                  Only drivers loaded by this process can be reloaded.

              All  other  error  codes  are to be translated by the format_error/1 function. Note
              that calls to format_error should be performed from the same  running  instance  of
              the  erlang  virtual  machine as the error was detected in, due to system dependent
              behavior concerning error values.

              If the arguments or options  are  malformed,  the  function  will  throw  a  badarg
              exception.

       try_unload(Name,   OptionList)  ->  {ok,Status}  |  {ok,  PendingStatus,  Ref}  |  {error,
       ErrorAtom}

              Types:

                 Name = string() | atom()
                 OptionList = [ Option ]
                 Option = {monitor, MonitorOption} | kill_ports
                 MonitorOption = pending_driver | pending
                 Status = unloaded | PendingStatus
                 PendingStatus = pending_driver | pending_process
                 Ref = reference()
                 ErrorAtom  =  linked_in_driver  |  not_loaded  |  not_loaded_by_this_process   |
                 permanent

              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 kill_ports implicitly does, and it can trigger a monitor either due to other
              users still having the driver loaded or that there are open ports using the driver.

              Unloading can be described as  the  process  of  telling  the  emulator  that  this
              particular  part  of the code in this particular process (i.e. this user) no longer
              needs the driver. That can, if there are no other users, trigger  actual  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.  If  the
              driver  has  the kill_ports option set, or if kill_ports was specified as an option
              to this function, all  pending  ports  using  this  driver  will  get  killed  when
              unloading  is  done  by the last user. If no port-killing is involved and there are
              open ports, the actual unloading is delayed until there  are  no  more  open  ports
              using  the  driver.  If,  in  this case, another user (or even this user) loads the
              driver again before the driver is actually unloaded, the unloading will never  take
              place.

              To  allow  the  user  that  requests unloading to wait for actual unloading to take
              place, monitor triggers can be specified in much the same way as when  loading.  As
              users  of this function however seldom are interested in more than decrementing the
              reference counts, monitoring is more seldom needed. If  the  kill_ports  option  is
              used  however, monitor trigging is crucial, as the ports are not guaranteed to have
              been killed until the driver is unloaded, why a monitor should be triggered for  at
              least the pending_driver case.

              The  possible  monitor messages that can be expected are the same as when using the
              unloaded option to the monitor/2 function.

              The function will return one of the following statuses upon success:

                {ok, unloaded}:
                  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.

                  The driver can only be unloaded when there are no open ports using it and there
                  are no more users requiring it to be loaded.

                {ok, pending_driver}or {ok, pending_driver, reference()}:
                  This  return  value  indicates  that  this  call removed the last user from the
                  driver, but there are still open ports using it. When all ports are closed  and
                  no  new  users  have arrived, the driver will actually be reloaded and the name
                  and memory reclaimed.

                  This return value is valid even when the option kill_ports was used, as killing
                  ports  may  not  be  a process that completes immediately. The condition is, in
                  that case, however transient. Monitors are as always useful to detect when  the
                  driver is really unloaded.

                {ok, pending_process}or {ok, pending_process, reference()}:
                  The  unload  request is registered, but there are still other users holding the
                  driver. Note that the term pending_process might refer to the running  process,
                  there might be more than one user in the same process.

                  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  actually  the
                  most  common  return  value  in  the  most  common  scenario  described  in the
                  introduction.

              The function accepts the following parameters:

                Name:
                  The name parameter is the name of the driver to be unloaded. The  name  can  be
                  specified either as an io_list() or as an atom().

                OptionList:
                  The OptionList argument can be used to specify certain behavior regarding ports
                  as well as triggering monitors under certain conditions:

                  kill_ports:
                    Force killing of all ports opened using this driver,  with  the  exit  reason
                    driver_unloaded, if you are the lastuser of the driver.

                    If  there  are other users having the driver loaded, this option will have no
                    effect.

                    If one wants the consistent behavior of killing  ports  when  the  last  user
                    unloads,  one should use the driver option kill_ports when loading the driver
                    instead.

                  {monitor, MonitorOption}:
                    This option creates a driver monitor if the condition given in MonitorOptions
                    is true. The valid options are:

                    pending_driver:
                      Create a driver monitor if the return value is to be {ok, pending_driver}.

                    pending:
                      Create a monitor if the return value will be either {ok, pending_driver} or
                      {ok, pending_process}.

                    The pending_driver MonitorOption is by far the most useful and it has  to  be
                    used  to ensure that the driver has really been unloaded and the ports closed
                    whenever the kill_ports option is used or the driver  may  have  been  loaded
                    with the kill_ports driver option.

                    By  using the monitor-triggers in the call to try_unload one can be sure that
                    the monitor is actually added before the unloading is executed, meaning  that
                    the  monitor  will always get properly triggered, which would not be the case
                    if one called erl_ddll:monitor/2 separately.

              The function may return several error conditions, of which all are  well  specified
              (no opaque values):

                {error, linked_in_driver}:
                  You  were  trying to unload an erlang statically linked in driver, which cannot
                  be manipulated with this interface (and cannot be unloaded at all).

                {error, not_loaded}:
                  The driver Name is not present in the system.

                {error, not_loaded_by_this_process}:
                  The driver Name is present in the system, but there is no user of  it  in  this
                  process.

                  As  a  special  case,  drivers  can be unloaded from processes that has done no
                  corresponding call to try_load/3 if, and only if, there are  no  users  of  the
                  driver at all, which may happen if the process containing the last user dies.

                {error, permanent}:
                  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).

              The function throws a badarg exception if the parameters are not given as described
              above.

       unload(Name) -> ok | {error, ErrorDesc}

              Types:

                 Name = driver()
                 ErrorDesc = term()

              Unloads,  or at least dereferences the driver named Name. If the caller is the last
              user of the driver, and there are no more open ports using the driver,  the  driver
              will  actually  get  unloaded. In all other cases, actual unloading will be delayed
              until all ports are closed and there are no remaining users.

              If there are other users of the driver, the  reference  counts  of  the  driver  is
              merely  decreased, so that the caller is no longer considered a user of the driver.
              For usage scenarios, see the description in the beginning of this document.

              The ErrorDesc returned  is  an  opaque  value  to  be  passed  further  on  to  the
              format_error/1  function. For more control over the operation, use the try_unload/2
              interface.

              The function throws a badarg exception if the parameters are not given as described
              above.

       unload_driver(Name) -> ok | {error, ErrorDesc}

              Types:

                 Name = driver()
                 ErrorDesc = term()

              Unloads,  or at least dereferences the driver named Name. If the caller is the last
              user of the driver, all remaining open ports using the driver will get killed  with
              the reason driver_unloaded and the driver will eventually get unloaded.

              If  there  are  other  users  of  the driver, the reference counts of the driver is
              merely decreased, so that the caller is no longer  considered  a  user.  For  usage
              scenarios, see the description in the beginning of this document.

              The  ErrorDesc  returned  is  an  opaque  value  to  be  passed  further  on to the
              format_error/1 function. For more control over the operation, use the  try_unload/2
              interface.

              The function throws a badarg exception if the parameters are not given as described
              above.

       loaded_drivers() -> {ok, Drivers}

              Types:

                 Drivers = [Driver]
                 Driver = string()

              Returns a list of all  the  available  drivers,  both  (statically)  linked-in  and
              dynamically loaded ones.

              The  driver names are returned as a list of strings rather than a list of atoms for
              historical reasons.

              More information about drivers can be obtained using one of the info functions.

       format_error(ErrorDesc) -> string()

              Types:

                 ErrorDesc = term()

              Takes an ErrorDesc returned by load, unload  or  reload  functions  and  returns  a
              string which describes the error or warning.

          Note:
              Due  to  peculiarities in the dynamic loading interfaces on different platform, the
              returned string is only guaranteed to describe the correct error if  format_error/1
              is  called in the same instance of the erlang virtual machine as the error appeared
              in (meaning the same operating system process)!

SEE ALSO

       erl_driver(5), driver_entry(5)