In this guide, we will discuss Erlang Drivers. Sometimes we want to run a foreign-language program inside the Erlang Runtime System. In this case, the program is written as a shared library that is dynamically linked into the Erlang runtime system. The linked-in driver appears to the programmer as a port program and obeys exactly the same protocol as for a port program.
Creating a Driver
Creating a linked-in driver is the most efficient way of interfacing foreign-language code with Erlang, but it is also the most dangerous. Any fatal error in the linked-in driver will crash the Erlang System.
Following is an example of a driver implementation in Erlang −
Example
-module(helloworld).
-export([start/0, stop/0]).
-export([twice/1, sum/2]).
start() ->
start("example1_drv" ).
start(SharedLib) ->
case erl_ddll:load_driver("." , SharedLib) of
ok -> ok;
{error, already_loaded} -> ok;
_ -> exit({error, could_not_load_driver})
end,
spawn(fun() -> init(SharedLib) end).
init(SharedLib) ->
register(example1_lid, self()),
Port = open_port({spawn, SharedLib}, []),
loop(Port).
stop() ->
example1_lid ! stop.
twice(X) -> call_port({twice, X}).
sum(X,Y) -> call_port({sum, X, Y}). call_port(Msg) ->
example1_lid ! {call, self(), Msg}, receive
{example1_lid, Result} ->
Result
end.
LINKED-IN DRIVERS 223
loop(Port) ->
receive
{call, Caller, Msg} ->
Port ! {self(), {command, encode(Msg)}}, receive
{Port, {data, Data}} ->
Caller ! {example1_lid, 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({twice, X}) -> [1, X];
encode({sum, X, Y}) -> [2, X, Y]. decode([Int]) -> Int.
Please note that working with drivers is extremely complex and care should be taken when working with drivers.
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