erlang-python port 程序

把<Programming Erlang>书上第12章那个erlang-c的例子改编了下，把C程序改写成了python程序。在ubuntu8.04 + erlang5.6.5 + python2.5.2上调试通过。

erlang side: example2.erl

-module(example2).
-export([start/0, stop/0]).
-export([twice/1, sum/2]).

start() ->
    spawn(fun() ->
		  register(example1, self()),
		  process_flag(trap_exit, true),
		  Port = open_port({spawn, "python -u ./example2.py"}, [{packet, 2}]),
		  loop(Port)
	  end).

stop() ->
    example1 ! stop.

twice(X) -> call_port({twice, X}).
sum(X,Y) -> call_port({sum, X, Y}).

call_port(Msg) ->
    example1 ! {call, self(), Msg},
    receive
	{example1, Result} ->
	    Result
    end.

loop(Port) ->
    receive
	{call, Caller, Msg} ->
	    Port ! {self(), {command, encode(Msg)}}, 
	    receive
		{Port, {data, Data}} ->
		    Caller ! {example1, decode(Data)}
	    end,
	    loop(Port);
	stop ->
	    Port ! {self(), close},
	    receive
		{Port, closed} ->
		    exit(normal)
	    end;
	{'EXIT', Port, Reason} ->
	    exit({port_terminated,Reason})
    end.

encode({twice, X}) -> [1, X];  
encode({sum, X, Y}) -> [2, X, Y]. 

decode([Int]) -> Int. 

需要注意的是调用python程序，要使用参数－u。强制标准输入／输出不得有缓存，要立时反应。
python官方文档对-u的解释: Force stdin, stdout and stderr to be totally unbuffered.


python side: example2.py

import sys, os
import struct

def recv():
    buf = sys.stdin.read(2)
    if len(buf) == 2:
        (sz, ) = struct.unpack("!h", buf)
        payload = sys.stdin.read(sz)
        return payload
    else:
        return None

def send(payload):
    sz = len(payload)
    header= struct.pack("!h", sz)
    return sys.stdout.write( header + payload )

def twice(arg1):
    return 2*arg1

def sum(arg1,arg2):
    return arg1+arg2

def main_loop():
    buf = recv()
    while buf:
        xa = struct.unpack("!%dB" % len(buf), buf)
        if xa[0] == 1:
            calc_res = twice(xa[1])
        elif xa[0] == 2:
            calc_res = sum(xa[1], xa[2])
        result = struct.pack('!B', calc_res)
        send(result)
        buf = recv()
    return None

if __name__ == '__main__':
    main_loop()