[erlang-questions] newbie: how to clear a mailbox of a process?

Anthony Kong <>
Mon Jan 28 13:54:14 CET 2008


Hi, all,

Sorry, I don't think the subject line makes much sense. Hopefully if
you read on, the example may speak for itself.

I have again written a simple script:

=======

-module(p3).
-export([isprime/1, test_if_divisible/2]).

isprime(2) ->
  true;

isprime(N) when N > 2 ->
  C = lists:seq(2, N - 1),
  process_flag(trap_exit, true),
  lists:map(fun(Div) -> spawn_link(fun() -> test_if_divisible(N, Div)
end) end, C),
  isprime_poll(length(C)).


isprime_poll(0) ->
  true;
isprime_poll(NumVote) ->
  receive
    {'EXIT', _Pid, nondivisible} ->
        isprime_poll(NumVote - 1);
    {'EXIT', _Pid, divisible} ->
        false
  end.


test_if_divisible(N, Div) ->
  case N rem Div of
    0 -> exit(divisible);
    _ -> exit(nondivisible)
  end.
=======

The idea of this script are
1) in order to check if a number, says N, is a prime or not, the
script will fire a process to test divisibility by each possible
divisor (in the range of 2 to N - 1)
2) these processes are 'voting': if all these processes vote that N is
not divisible, then N is a prime
3) I want to short circuit the voting process by looking for
'non-divisible' exit message. If I got one, the isprime_poll/1 will
abort the tail-recursion and return false

So, if I run the following tests, they produces results as expected,
*provided* that I do not run them in one go:

2> p3:isprime(6).
false
...
(abort... then start erl again)
...
2> p3:isprime(7).
true


If I run like these 2 statements in the same shell, however, then what
I got is a wrong answer for 7.

1> l(p3).
{module,p3}
2> p3:isprime(6).
false
3> p3:isprime(7).
false
4> p3:isprime(7).
true

I believe it is because that while I make isprime_poll/1 return false
on first 'nondivisible' message, those remaining messages sent from
other processes are still in the mailbox. So if I immedately run
isprime(7) after isprime(6), it'll take in these messages from last
session and hence make a wrong conclusion.

So, here is my questions:

1) Is it possible to force clearance of a process's mailbox?
2) What is the usual 'pattern' for implementation of a
voting/committee algorithm?
3) For testing I want to run the script noninteractively using command
line e.g. "/opt/erlang/otp-R12B-0/bin/erl  -run p3 isprime 7 -s init
stop -noshell" but I cannot get it to work. (I am getting badarith
exception and core dump. Did I miss some parameters in the command
line?
4) If I made some obvious programming mistakes, please feel free to point out

Cheers,

Anthony





-- 
/*--*/
Don't EVER make the mistake that you can design something better than
what you get from ruthless massively parallel trial-and-error with a
feedback cycle. That's giving your intelligence _much_ too much
credit.

- Linus Torvalds



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