<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> <year>2000</year><year>2015</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. </legalnotice> <title>Port Drivers</title> <prepared></prepared> <docno></docno> <date></date> <rev></rev> <file>c_portdriver.xml</file> </header> <p>This section outlines an example of how to solve the example problem in <seealso marker="example">Problem Example</seealso> by using a linked-in port driver.</p> <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 system 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>As 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 way, because a crash in the port driver brings the emulator down too.</p> <p>The scenario is illustrated in the following figure:</p> <image file="../tutorial/port_driver.gif"> <icaption>Port Driver Communication</icaption> </image> <section> <title>Erlang Program</title> <p>Like a port program, the port communicates with an Erlang process. All communication goes through one Erlang process that is the <em>connected process</em> 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 <c>erl_dll:load_driver/1</c>, with the name of the shared library as argument.</p> <p>The port is then created using the BIF <c>open_port/2</c>, with the tuple <c>{spawn, DriverName}</c> as the first argument. The string <c>SharedLib</c> is the name of the port driver. The second argument is a list of options, none in this case:</p> <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> <p>Now <c>complex5:foo/1</c> and <c>complex5:bar/1</c> can be implemented. Both send a message to the <c>complex</c> process and receive the following reply:</p> <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> <p>The <c>complex</c> process performs the following:</p> <list type="bulleted"> <item>Encodes the message into a sequence of bytes.</item> <item>Sends it to the port.</item> <item>Waits for a reply.</item> <item>Decodes the reply.</item> <item>Sends it back to the caller:</item> </list> <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> <p>Assuming that both the arguments and the results from the C functions are less than 256, a simple encoding/decoding scheme is employed. In this scheme, <c>foo</c> is represented by byte 1, <c>bar</c> is represented by 2, and the argument/result is represented by a single byte as well:</p> <pre> encode({foo, X}) -> [1, X]; encode({bar, Y}) -> [2, Y]. decode([Int]) -> Int.</pre> <p>The resulting Erlang program, including functions for stopping the port and detecting port failures, is as follows:</p> <codeinclude file="complex5.erl" type="erl"/> </section> <section> <title>C Driver</title> <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 <c>erl_driver.h</c>.</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 <c><![CDATA[DRIVER_INIT(<driver_name>)]]></c>.</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 replied data is sent with the C-function <c>driver_output</c>.</p> <p>As the driver is a shared module, not a program, no main function is present. All function pointers are not used in this example, and the corresponding fields in the <c>driver_entry</c> structure are set to NULL.</p> <p>All functions in the driver takes a handle (returned from <c>start</c>) that is just passed along by the Erlang process. This must in some way refer to the port driver instance.</p> <p>The <c>example_drv_start</c>, is the only function that is called with a handle to the port instance, so this must be saved. It is customary to use an allocated driver-defined structure for this one, and to pass a pointer back as a reference.</p> <p>It is not a good idea to use a global variable as the port driver can be spawned by multiple Erlang processes. This driver-structure is to be instantiated multiple times: </p> <codeinclude file="port_driver.c" tag="" type="none"></codeinclude> </section> <section> <title>Running the Example</title> <p><em>Step 1.</em> Compile the C code:</p> <pre> unix> <input>gcc -o exampledrv -fpic -shared complex.c port_driver.c</input> windows> <input>cl -LD -MD -Fe exampledrv.dll complex.c port_driver.c</input></pre> <p><em>Step 2.</em> Start Erlang and compile the Erlang code:</p> <pre> > <input>erl</input> Erlang (BEAM) emulator version 5.1 Eshell V5.1 (abort with ^G) 1> <input>c(complex5).</input> {ok,complex5}</pre> <p><em>Step 3.</em> Run the example:</p> <pre> 2> <input>complex5:start("example_drv").</input> <0.34.0> 3> <input>complex5:foo(3).</input> 4 4> <input>complex5:bar(5).</input> 10 5> <input>complex5:stop().</input> stop</pre> </section> </chapter>