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<h1>6 Port drivers</h1>
<p>This is an example of how to solve the <span class="bold_code"><a href="example.html">example problem</a></span> by using a linked in port driver.</p>
<img alt="IMAGE MISSING" src="../tutorial/port_driver.gif"><br>
<em>Figure
6.1:
Port Driver Communication.</em>
<h3><a name="id61981">6.1
Port Drivers</a></h3>
<p>A port driver is a linked in driver, that is accessible as a
port from an Erlang program. It is a shared library (SO in Unix,
DLL in Windows), with special entry points. The Erlang runtime
calls these entry points, when the driver is started and when
data is sent to the port. The port driver can also send data to
Erlang.</p>
<p>Since a port driver is dynamically linked into the emulator
process, this is the fastest way of calling C-code from Erlang.
Calling functions in the port driver requires no context
switches. But it is also the least safe, because a crash in the
port driver brings the emulator down too.</p>
<h3><a name="id62001">6.2
Erlang Program</a></h3>
<p>Just as with a port program, the port communicates with a Erlang
process. All communication goes through one Erlang process that
is the <strong>connected process</strong> of the port
driver. Terminating this process closes the port driver.</p>
<p>Before the port is created, the driver must be loaded. This is
done with the function <span class="code">erl_dll:load_driver/1</span>, with the
name of the shared library as argument.</p>
<p>The port is then created using the BIF <span class="code">open_port/2</span> with
the tuple <span class="code">{spawn, DriverName}</span> as the first argument. The
string <span class="code">SharedLib</span> is the name of the port driver. The second
argument is a list of options, none in this case.</p>
<div class="example"><pre>
-module(complex5).
-export([start/1, init/1]).
start(SharedLib) ->
case erl_ddll:load_driver(".", SharedLib) of
ok -> ok;
{error, already_loaded} -> ok;
_ -> exit({error, could_not_load_driver})
end,
spawn(?MODULE, init, [SharedLib]).
init(SharedLib) ->
register(complex, self()),
Port = open_port({spawn, SharedLib}, []),
loop(Port).</pre></div>
<p>Now it is possible to implement <span class="code">complex5:foo/1</span> and
<span class="code">complex5:bar/1</span>. They both send a message to the
<span class="code">complex</span> process and receive the reply.</p>
<div class="example"><pre>
foo(X) ->
call_port({foo, X}).
bar(Y) ->
call_port({bar, Y}).
call_port(Msg) ->
complex ! {call, self(), Msg},
receive
{complex, Result} ->
Result
end.</pre></div>
<p>The <span class="code">complex</span> process encodes the message into a sequence
of bytes, sends it to the port, waits for a reply, decodes the
reply and sends it back to the caller.
</p>
<div class="example"><pre>
loop(Port) ->
receive
{call, Caller, Msg} ->
Port ! {self(), {command, encode(Msg)}},
receive
{Port, {data, Data}} ->
Caller ! {complex, decode(Data)}
end,
loop(Port)
end.</pre></div>
<p>Assuming that both the arguments and the results from the C
functions will be less than 256, a very simple encoding/decoding
scheme is employed where <span class="code">foo</span> is represented by the byte
1, <span class="code">bar</span> is represented by 2, and the argument/result is
represented by a single byte as well.
</p>
<div class="example"><pre>
encode({foo, X}) -> [1, X];
encode({bar, Y}) -> [2, Y].
decode([Int]) -> Int.</pre></div>
<p>The resulting Erlang program, including functionality for
stopping the port and detecting port failures is shown below.</p>
<div class="example"><pre>
-module(complex5).
-export([start/1, stop/0, init/1]).
-export([foo/1, bar/1]).
start(SharedLib) ->
case erl_ddll:load_driver(".", SharedLib) of
ok -> ok;
{error, already_loaded} -> ok;
_ -> exit({error, could_not_load_driver})
end,
spawn(?MODULE, init, [SharedLib]).
init(SharedLib) ->
register(complex, self()),
Port = open_port({spawn, SharedLib}, []),
loop(Port).
stop() ->
complex ! stop.
foo(X) ->
call_port({foo, X}).
bar(Y) ->
call_port({bar, Y}).
call_port(Msg) ->
complex ! {call, self(), Msg},
receive
{complex, Result} ->
Result
end.
loop(Port) ->
receive
{call, Caller, Msg} ->
Port ! {self(), {command, encode(Msg)}},
receive
{Port, {data, Data}} ->
Caller ! {complex, decode(Data)}
end,
loop(Port);
stop ->
Port ! {self(), close},
receive
{Port, closed} ->
exit(normal)
end;
{'EXIT', Port, Reason} ->
io:format("~p ~n", [Reason]),
exit(port_terminated)
end.
encode({foo, X}) -> [1, X];
encode({bar, Y}) -> [2, Y].
decode([Int]) -> Int.
</pre></div>
<h3><a name="id62119">6.3
C Driver</a></h3>
<p>The C driver is a module that is compiled and linked into a
shared library. It uses a driver structure, and includes the
header file <span class="code">erl_driver.h</span>.</p>
<p>The driver structure is filled with the driver name and function
pointers. It is returned from the special entry point, declared
with the macro <span class="code">DRIVER_INIT(<driver_name>)</span>.</p>
<p>The functions for receiving and sending data, are combined into
a function, pointed out by the driver structure. The data sent
into the port is given as arguments, and the data the port
sends back is sent with the C-function <span class="code">driver_output</span>.</p>
<p>Since the driver is a shared module, not a program, no main
function should be present. All function pointers are not used
in our example, and the corresponding fields in the
<span class="code">driver_entry</span> structure are set to NULL.</p>
<p>All functions in the driver, takes a handle (returned from
<span class="code">start</span>), that is just passed along by the erlang
process. This must in some way refer to the port driver
instance.</p>
<p>The example_drv_start, is the only function that is called with
a handle to the port instance, so we must save this. It is
customary to use a allocated driver-defined structure for this
one, and pass a pointer back as a reference.</p>
<p>It is not a good idea to use a global variable; since the port
driver can be spawned by multiple Erlang processes, this
driver-structure should be instantiated multiple times.
</p>
<div class="example"><pre>
/* port_driver.c */
#include <stdio.h>
#include "erl_driver.h"
typedef struct {
ErlDrvPort port;
} example_data;
static ErlDrvData example_drv_start(ErlDrvPort port, char *buff)
{
example_data* d = (example_data*)driver_alloc(sizeof(example_data));
d->port = port;
return (ErlDrvData)d;
}
static void example_drv_stop(ErlDrvData handle)
{
driver_free((char*)handle);
}
static void example_drv_output(ErlDrvData handle, char *buff,
ErlDrvSizeT bufflen)
{
example_data* d = (example_data*)handle;
char fn = buff[0], arg = buff[1], res;
if (fn == 1) {
res = foo(arg);
} else if (fn == 2) {
res = bar(arg);
}
driver_output(d->port, &res, 1);
}
ErlDrvEntry example_driver_entry = {
NULL, /* F_PTR init, called when driver is loaded */
example_drv_start, /* L_PTR start, called when port is opened */
example_drv_stop, /* F_PTR stop, called when port is closed */
example_drv_output, /* F_PTR output, called when erlang has sent */
NULL, /* F_PTR ready_input, called when input descriptor ready */
NULL, /* F_PTR ready_output, called when output descriptor ready */
"example_drv", /* char *driver_name, the argument to open_port */
NULL, /* F_PTR finish, called when unloaded */
NULL, /* void *handle, Reserved by VM */
NULL, /* F_PTR control, port_command callback */
NULL, /* F_PTR timeout, reserved */
NULL, /* F_PTR outputv, reserved */
NULL, /* F_PTR ready_async, only for async drivers */
NULL, /* F_PTR flush, called when port is about
to be closed, but there is data in driver
queue */
NULL, /* F_PTR call, much like control, sync call
to driver */
NULL, /* F_PTR event, called when an event selected
by driver_event() occurs. */
ERL_DRV_EXTENDED_MARKER, /* int extended marker, Should always be
set to indicate driver versioning */
ERL_DRV_EXTENDED_MAJOR_VERSION, /* int major_version, should always be
set to this value */
ERL_DRV_EXTENDED_MINOR_VERSION, /* int minor_version, should always be
set to this value */
0, /* int driver_flags, see documentation */
NULL, /* void *handle2, reserved for VM use */
NULL, /* F_PTR process_exit, called when a
monitored process dies */
NULL /* F_PTR stop_select, called to close an
event object */
};
DRIVER_INIT(example_drv) /* must match name in driver_entry */
{
return &example_driver_entry;
}
</pre></div>
<h3><a name="id63970">6.4
Running the Example</a></h3>
<p>1. Compile the C code.</p>
<div class="example"><pre>
unix> <span class="bold_code">gcc -o exampledrv -fpic -shared complex.c port_driver.c</span>
windows> <span class="bold_code">cl -LD -MD -Fe exampledrv.dll complex.c port_driver.c</span></pre></div>
<p>2. Start Erlang and compile the Erlang code.</p>
<div class="example"><pre>
> <span class="bold_code">erl</span>
Erlang (BEAM) emulator version 5.1
Eshell V5.1 (abort with ^G)
1> <span class="bold_code">c(complex5).</span>
{ok,complex5}</pre></div>
<p>3. Run the example.</p>
<div class="example"><pre>
2> <span class="bold_code">complex5:start("example_drv").</span>
<0.34.0>
3> <span class="bold_code">complex5:foo(3).</span>
4
4> <span class="bold_code">complex5:bar(5).</span>
10
5> <span class="bold_code">complex5:stop().</span>
stop</pre></div>
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