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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() = iolist() | 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 = driver()
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(),integer() >= 0}, where the integer() 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 integer >= 0()) using the driver.
linked_in_driver:
Return a boolean(), being true if the driver is a statically linked in one and false
otherwise.
permanent:
Return a boolean(), 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(),integer() >= 0}, where the integer() 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(),integer() >= 0}, where the integer() 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 = driver()
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 = path()
Name = driver()
OptionList = [Option]
Option =
{driver_options, DriverOptionList} |
{monitor, MonitorOption} |
{reload, ReloadOption}
DriverOptionList = [DriverOption]
DriverOption = kill_ports
MonitorOption = 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
OpaqueError = term()
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,
MonitorOption} 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 iolist(), meaning it can be a list of other iolist()s, 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 iolist() 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, DriverOptionList}:
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, ReloadOption}:
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 DriverOptionList or a different
literalPath argument.
This can happen even if a reload option is given, if the DriverOptionList 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 = driver()
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 iolist() 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 MonitorOption 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)
Ericsson AB kernel 4.2 erl_ddll(3erl)