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NAME
driver_entry - The driver-entry structure used by Erlang drivers.
DESCRIPTION
Warning:
Use this functionality with extreme care.
A driver callback is executed as a direct extension of the native code of the VM.
Execution is not made in a safe environment. The VM cannot provide the same services as
provided when executing Erlang code, such as pre-emptive scheduling or memory protection.
If the driver callback function does not behave well, the whole VM will misbehave.
* A driver callback that crash will crash the whole VM.
* An erroneously implemented driver callback can cause a VM internal state
inconsistency, which can cause a crash of the VM, or miscellaneous misbehaviors of the
VM at any point after the call to the driver callback.
* A driver callback doing lengthy work before returning degrades responsiveness of the
VM, and can cause miscellaneous strange behaviors. Such strange behaviors include, but
are not limited to, extreme memory usage, and bad load balancing between schedulers.
Strange behaviors that can occur because of lengthy work can also vary between
Erlang/OTP releases.
As from ERTS 5.9 (Erlang/OTP 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 ERTS version earlier than 5.9) must 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, which 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 callback functions are called synchronously from the Erlang emulator. If they
take too long before completing, they can cause time-outs in the emulator. Use the queue
or asynchronous calls if necessary, as the emulator must be responsive.
The driver structure contains the driver name and some 15 function pointers, which 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
operating systems have different names for it.)
When writing a driver in C++, the driver entry is to be of "C" linkage. One way to do this
is to put the following 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.
If compiling a driver for static inclusion through --enable-static-drivers, you must
define STATIC_ERLANG_DRIVER before the DRIVER_INIT declaration.
Note:
Do not declare the driver_entry const. This because the emulator must modify the handle
and the handle2 fields. A statically allocated, and const-declared driver_entry can be
located in read-only memory, which causes the emulator to crash.
DATA TYPES
ErlDrvEntry
typedef struct erl_drv_entry {
int (*init)(void); /* Called at system startup 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
erlang:open_port/2 */
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 time-out 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 must 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* unused_event_callback;
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;
int (*init)(void):
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 is to return 0, or, if the
driver cannot initialize, -1.
ErlDrvData (*start)(ErlDrvPort port, char* command):
Called when the driver is instantiated, when erlang:open_port/2 is called. The driver
is to return a number >= 0 or a pointer, or, if the driver cannot be started, one of
three error codes:
ERL_DRV_ERROR_GENERAL:
General error, no error code
ERL_DRV_ERROR_ERRNO:
Error with error code in errno
ERL_DRV_ERROR_BADARG:
Error, badarg
If an error code is returned, the port is not started.
void (*stop)(ErlDrvData drv_data):
Called when the port is closed, with erlang:port_close/1 or Port ! {self(), close}.
Notice 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.
void (*output)(ErlDrvData drv_data, char *buf, ErlDrvSizeT len):
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 erlang:port_command/2. Depending on how the port was opened, it is to be
either a list of integers 0...255 or a binary. See erlang:open_port/2 and
erlang:port_command/2.
void (*ready_input)(ErlDrvData drv_data, ErlDrvEvent event):
void (*ready_output)(ErlDrvData drv_data, ErlDrvEvent event):
Called when a driver event (specified in parameter event) is signaled. This is used to
help asynchronous drivers "wake up" when something occurs.
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 makes the emulator call ready_input or ready_output.
False events can occur. That is, calls to ready_input or ready_output although no real
events are signaled. In reality, it is rare (and OS-dependant), but a robust driver
must nevertheless be able to handle such cases.
char *driver_name:
The driver name. It must correspond to the atom used in erlang:open_port/2, and the
name of the driver library file (without the extension).
void (*finish)(void):
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 erl_ddll:unload_driver/1, or when
the emulator halts.
void *handle:
This field is reserved for the emulator's internal use. The emulator will modify this
field, so it is important that the driver_entry is not declared const.
ErlDrvSSizeT (*control)(ErlDrvData drv_data, unsigned int command, char *buf,
ErlDrvSizeT len, char **rbuf, ErlDrvSizeT rlen):
A special routine invoked with erlang:port_control/3. It works a little like an
"ioctl" for Erlang drivers. The data specified to port_control/3 arrives in buf and
len. The driver can send data back, using *rbuf and rlen.
This is the fastest way of calling a driver and get a response. It makes no 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 is to 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 differently depending on the port control flags (those that are
set with erl_driver:set_port_control_flags).
If the flag is set to PORT_CONTROL_FLAG_BINARY, a binary is 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
erl_driver:driver_alloc_binary. This binary is freed automatically after control has
returned. The driver can retain the binary for read only access with
erl_driver:driver_binary_inc_refc to be freed later with
erl_driver:driver_free_binary. It is never allowed to change the binary after control
has returned. If *rbuf is set to NULL, an empty list is 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
erl_driver:driver_alloc. The buffer is 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.
void (*timeout)(ErlDrvData drv_data):
Called any time after the driver's timer reaches 0. The timer is activated with
erl_driver:driver_set_timer. No priorities or ordering exist among drivers, so if
several drivers time out at the same time, anyone of them is called first.
void (*outputv)(ErlDrvData drv_data, ErlIOVec *ev):
Called whenever the port is written to. If it is NULL, the output function is called
instead. This function is faster than output, as it takes an ErlIOVec directly, which
requires no copying of the data. The port is to be in binary mode, see
erlang:open_port/2.
ErlIOVec contains both a SysIOVec, suitable for writev, and one or more binaries. If
these binaries are to be retained when the driver returns from outputv, they can be
queued (using, for example, erl_driver:driver_enq_bin) or, if they are kept in a
static or global variable, the reference counter can be incremented.
void (*ready_async)(ErlDrvData drv_data, ErlDrvThreadData thread_data):
Called after an asynchronous call has completed. The asynchronous call is started with
erl_driver:driver_async. This function is called from the Erlang emulator thread, as
opposed to the asynchronous function, which is called in some thread (if multi-
threading is enabled).
void (*flush)(ErlDrvData drv_data):
Called when the port is about to be closed, and there is data in the driver queue that
must 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):
Called from erlang:port_call/3. It works a lot like the control callback, 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 is to 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 erl_driver:driver_alloc.
This memory is 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 throws a BAD_ARG.
void (*event)(ErlDrvData drv_data, ErlDrvEvent event, ErlDrvEventData event_data):
Intentionally left undocumented.
int extended_marker:
This field is either to be equal to ERL_DRV_EXTENDED_MARKER or 0. An old driver (not
aware of the extended driver interface) is to set this field to 0. If this field is 0,
all the following fields must also be 0, or NULL if it is a pointer field.
int major_version:
This field is to equal ERL_DRV_EXTENDED_MAJOR_VERSION if field extended_marker equals
ERL_DRV_EXTENDED_MARKER.
int minor_version:
This field is to equal ERL_DRV_EXTENDED_MINOR_VERSION if field extended_marker equals
ERL_DRV_EXTENDED_MARKER.
int driver_flags:
This field is used to pass driver capability and other information to the runtime
system. If field extended_marker equals ERL_DRV_EXTENDED_MARKER, it is to contain 0 or
driver flags (ERL_DRV_FLAG_*) OR'ed bitwise. The following driver flags exist:
ERL_DRV_FLAG_USE_PORT_LOCKING:
The runtime system uses port-level locking on all ports executing this driver
instead of driver-level locking when the driver is run in a runtime system with SMP
support. For more information, see erl_driver.
ERL_DRV_FLAG_SOFT_BUSY:
Marks that driver instances can handle being called in the output and/or outputv
callbacks although a driver instance has marked itself as busy (see
erl_driver:set_busy_port). As from ERTS 5.7.4 this flag is required for drivers used
by the Erlang distribution (the behavior has always been required by drivers used by
the distribution).
ERL_DRV_FLAG_NO_BUSY_MSGQ:
Disables busy port message queue functionality. For more information, see
erl_driver:erl_drv_busy_msgq_limits.
ERL_DRV_FLAG_USE_INIT_ACK:
When this flag is specified, the linked-in driver must manually acknowledge that the
port has been successfully started using erl_driver:erl_drv_init_ack(). This allows
the implementor to make the erlang:open_port exit with badarg after some initial
asynchronous initialization has been done.
void *handle2:
This field is reserved for the emulator's internal use. The emulator modifies this
field, so it is important that the driver_entry is not declared const.
void (*process_exit)(ErlDrvData drv_data, ErlDrvMonitor *monitor):
Called when a monitored process exits. The drv_data is the data associated with the
port for which the process is monitored (using erl_driver:driver_monitor_process) and
the monitor corresponds to the ErlDrvMonitor structure filled in when creating the
monitor. The driver interface function erl_driver:driver_get_monitored_process can be
used to retrieve the process ID of the exiting process as an ErlDrvTermData.
void (*stop_select)(ErlDrvEvent event, void* reserved):
Called on behalf of erl_driver: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 is to be ignored.
In contrast to most of the other callback 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 can even be closed
at the time stop_select is called. But it can also be the case that stop_select is
called directly by erl_driver:driver_select.
It is not allowed to call any functions in the driver API from stop_select. This
strict limitation is because the volatile context that stop_select can be called.
SEE ALSO
erl_driver(3erl), erlang(3erl), erl_ddll(3erl)