driver_entry
As of erts version 5.9 (OTP release R15B) the driver interface has been changed with larger types for the callbacks output, control and call. See driver version management in erl_driver.
Note!
Old drivers (compiled with an erl_driver.h
from an
earlier erts version than 5.9) have to be updated and have
to use the extended interface (with
version management
).
The driver_entry
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.
The erl_driver driver
API functions need a port handle
that identifies the driver instance (and the port in the
emulator). This is only passed to the start
function, but
not to the other functions. The start
function returns a
driver-defined handle that is passed to the other functions. A
common practice is to have the start
function allocate
some application-defined structure and stash the port
handle in it, to use it later with the driver API functions.
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.
The driver structure contains the name of the driver and some 15 function pointers. These pointers are called at different times by the emulator.
The only exported function from the driver is
driver_init
. This function returns the driver_entry
structure that points to the other functions in the driver. The
driver_init
function is declared with a macro
DRIVER_INIT(drivername)
. (This is because different OS's
have different names for it.)
When writing a driver in C++, the driver entry should be of
"C"
linkage. One way to do this is to put this line
somewhere before the driver entry:
extern "C" DRIVER_INIT(drivername);
.
When the driver has passed the driver_entry
over to
the emulator, the driver is not allowed to modify the
driver_entry
.
Note!
Do not declare the driver_entry
const
. This since the emulator needs to
modify the handle
, and the handle2
fields. A statically allocated, and const
declared driver_entry
may be located in
read only memory which will cause the emulator
to crash.
DATA TYPES
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;
This is called directly after the driver has been loaded by
erl_ddll:load_driver/2
. (Actually when the driver is
added to the driver list.) The driver should return 0, or if
the driver can't initialize, -1.
This is called when the driver is instantiated, when
open_port/2
is called. The driver should return a
number >= 0 or a pointer, or if the driver can't be started,
one of three error codes should be returned:
ERL_DRV_ERROR_GENERAL - general error, no error code
ERL_DRV_ERROR_ERRNO - error with error code in erl_errno
ERL_DRV_ERROR_BADARG - error, badarg
If an error code is returned, the port isn't started.
This is called when the port is closed, with
port_close/1
or Port ! {self(), close}
. Note
that terminating the port owner process also closes the
port. If drv_data
is a pointer to memory allocated in
start
, then stop
is the place to deallocate that
memory.
This is called when an erlang process has sent data to the
port. The data is pointed to by buf
, and is
len
bytes. Data is sent to the port with Port ! {self(), {command, Data}}
, or with
port_command/2
. Depending on how the port was opened,
it should be either a list of integers 0...255 or a
binary. See open_port/3
and port_command/2
.
This is called when a driver event (given in the
event
parameter) is signaled. This is used to help
asynchronous drivers "wake up" when something happens.
On unix the event
is a pipe or socket handle (or
something that the select
system call understands).
On Windows the event
is an Event or Semaphore (or
something that the WaitForMultipleObjects
API
function understands). (Some trickery in the emulator allows
more than the built-in limit of 64 Events
to be used.)
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 SetEvent
on
Windows), this will make the emulator call
ready_input
or ready_output
.
Spurious events may happen. That is, calls to ready_input
or ready_output
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.
This is the name of the driver, it must correspond to the
atom used in open_port
, and the name of the driver
library file (without the extension).
This function is called by the erl_ddll
driver when the
driver is unloaded. (It is only called in dynamic drivers.)
The driver is only unloaded as a result of calling
unload_driver/1
, or when the emulator halts.
This field is reserved for the emulator's internal use. The
emulator will modify this field; therefore, it is important
that the driver_entry
isn't declared const
.
This is a special routine invoked with the erlang function
port_control/3
. It works a little like an "ioctl" for
erlang drivers. The data given to port_control/3
arrives in buf
and len
. The driver may send
data back, using *rbuf
and rlen
.
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.
If the driver wants to return data, it should return it in
rbuf
. When control
is called,
*rbuf
points to a default buffer of rlen
bytes, which
can be used to return data. Data is returned different depending on
the port control flags (those that are set with
set_port_control_flags).
If the flag is set to PORT_CONTROL_FLAG_BINARY
,
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 *rbuf
to point to a binary allocated with
driver_alloc_binary.
This binary will be freed automatically after control
has returned.
The driver can retain the binary for read only access with
driver_binary_inc_refc to be freed later with
driver_free_binary.
It is never allowed to alter the binary after control
has returned.
If *rbuf
is set to NULL, an empty list will be returned.
If the flag is set to 0
, data is returned as a
list of integers. Either use the default buffer or set
*rbuf
to point to a larger buffer allocated with
driver_alloc.
The buffer will be freed automatically after control
has returned.
Using binaries is faster if more than a few bytes are returned.
The return value is the number of bytes returned in
*rbuf
.
This function is called any time after the driver's timer
reaches 0. The timer is activated with
driver_set_timer
. 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.
This function is called whenever the port is written to. If
it is NULL
, the output
function is called
instead. This function is faster than output
, because
it takes an ErlIOVec
directly, which requires no
copying of the data. The port should be in binary mode, see
open_port/2
.
The ErlIOVec
contains both a SysIOVec
,
suitable for writev
, and one or more binaries. If
these binaries should be retained, when the driver returns
from outputv
, they can be queued (using driver_enq_bin
for instance), or if they are kept in a static or global
variable, the reference counter can be incremented.
This function is called after an asynchronous call has completed. The asynchronous call is started with driver_async. 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).
This function is called from erlang:port_call/3
. It
works a lot like the control
call-back, but uses the
external term format for input and output.
command
is an integer, obtained from the call from
erlang (the second argument to erlang:port_call/3
).
buf
and len
provide the arguments to the call
(the third argument to erlang:port_call/3
). They can
be decoded using ei
functions.
rbuf
points to a return buffer, rlen
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 erlang:port_call/3
to the
caller. If more space than rlen
bytes is needed to
return data, *rbuf
can be set to memory allocated with
driver_alloc
. This memory will be freed automatically
after call
has returned.
The return value is the number of bytes returned in
*rbuf
. If ERL_DRV_ERROR_GENERAL
is returned
(or in fact, anything < 0), erlang:port_call/3
will
throw a BAD_ARG
.
Intentionally left undocumented.
This field should either be equal to ERL_DRV_EXTENDED_MARKER
or 0
. An old driver (not aware of the extended driver
interface) should set this field to 0
. If this field is
equal to 0
, all the fields following this field also
have to be 0
, or NULL
in case it is a
pointer field.
This field should equal ERL_DRV_EXTENDED_MAJOR_VERSION
if
the extended_marker
field equals
ERL_DRV_EXTENDED_MARKER
.
This field should equal ERL_DRV_EXTENDED_MINOR_VERSION
if
the extended_marker
field equals
ERL_DRV_EXTENDED_MARKER
.
This field is used to pass driver capability information to the
runtime system. If the extended_marker
field equals
ERL_DRV_EXTENDED_MARKER
, it should contain 0
or
driver flags (ERL_DRV_FLAG_*
) ored bitwise. Currently
the following driver flags exist:
ERL_DRV_FLAG_USE_PORT_LOCKING
ERL_DRV_FLAG_SOFT_BUSY
This field is reserved for the emulator's internal use. The
emulator will modify this field; therefore, it is important
that the driver_entry
isn't declared const
.
This callback is called when a monitored process exits. The
drv_data
is the data associated with the port for which
the process is monitored (using driver_monitor_process)
and the monitor
corresponds to the ErlDrvMonitor
structure filled
in when creating the monitor. The driver interface function
driver_get_monitored_process
can be used to retrieve the process id of the exiting process as
an ErlDrvTermData
.
This function is called on behalf of driver_select when it is safe to close an event object.
A typical implementation on Unix is to do
close((int)event)
.
Argument reserved
is intended for future use and should be ignored.
In contrast to most of the other call-back functions,
stop_select
is called independent of any port. No
ErlDrvData
argument is passed to the function. No
driver lock or port lock is guaranteed to be held. The port that
called driver_select
might even be closed at the
time stop_select
is called. But it could also be
the case that stop_select
is called directly by
driver_select
.
It is not allowed to call any functions in the
driver API from
stop_select
. This strict limitation is due to the
volatile context that stop_select
may be called.
SEE ALSO
erl_driver(3), erl_ddll(3), erlang(3), kernel(3)