Question

I apologize if the code is hard to follow.
This is the classic dining philosophers problem, where 5 philosophers are eating, but there are only 5 sticks – and you need two to eat.

These are the instructions, if anyone is interested:
http://www.kth.se/polopoly_fs/1.260940!/Menu/general/column-content/attachment/philosophers.pdf

Anyways, here is the code, the chopstick process code:

-module(chopstick).
-export([start/0]).

start() ->
spawn_link(fun() -> init() end).
init() ->
available().

available() ->
receive
    {request, From} -> 
        From ! granted,
        gone();
    quit ->
        ok
        end.
gone() ->
receive
    returned ->
        available();
    quit ->
        ok
        end.

The philosopher process code:

-module(eater).
-import(timer, [sleep/1]).
-import(random, [uniform/1]).
-export([start/5, dream/5, eat/5, wait/5]).

start(Hungry, Right, Left, Name, Ctrl) ->
dream(Hungry, Right, Left, Name, Ctrl).

 **%This was wrong, it should say start(Hungry, Right, Left, Name, Ctrl) ->
 spawn_link(fun() -> dream(Hungry, Right, Left, Name, Ctrl) end).**

dream(Hungry, Right, Left, Name, Ctrl) -> 
Time = 500+uniform:random(500),   **%This was wrong, it should say random:uniform**
timer:sleep(Time),
Right! {request, self()},
Left! {request, self()},
%skicka {request, self()} till två pinnar
wait(Hungry, Right, Left, Name, Ctrl).

wait(Hungry, Right, Left, Name, Ctrl) ->
receive
    granted ->  
        io:format("~s received a chopstick~n", [Name]),
        receive
            granted ->
            io:format("~s received a chopstick~n", [Name]),
            io:format("~s started eating~n", [Name]),
            eat(Hungry, Right, Left, Name, Ctrl)
            end;
    _ -> wait(Hungry, Right, Left, Name, Ctrl)
end.

eat(Hungry, Right, Left, Name, Ctrl) ->
Time = 500+uniform:random(500), **%This was wrong, it should say random:uniform**
timer:sleep(Time),
Right! returned,
Left! returned,
io:format("~s put back two chopsticks~n", [Name]),
if 
    Hungry =< 1 ->
        Ctrl ! done;
    true ->
        dream(Hungry-1, Right, Left, Name, Ctrl)
end.    

And finally the host process:

-module(dinner).
-export([start/0]).


start() ->
spawn(fun() -> init() end).

init() ->
C1 = chopstick:start(),
C2 = chopstick:start(),
C3 = chopstick:start(),
C4 = chopstick:start(),
C5 = chopstick:start(),
Ctrl = self(),
eater:start(5, C1, C2, "Confucios", Ctrl),  **% This is where it crashes**
eater:start(5, C2, C3, "Avicenna", Ctrl),
eater:start(5, C3, C4, "Plato", Ctrl),
eater:start(5, C4, C5, "Kant", Ctrl),
eater:start(5, C5, C1, "Descartes", Ctrl),
wait(5, [C1, C2, C3, C4, C5]).


wait(0, Chopsticks) ->
lists:foreach(fun(C) -> C ! quit end, Chopsticks);
wait(N, Chopsticks) ->
receive
    done ->
        wait(N-1, Chopsticks);
    abort ->
        erlang:exit(abort)
end.

Output:

11> dinner:start().
<0.85.0>
12> 
=ERROR REPORT==== 10-Nov-2011::02:19:10 ===
 Error in process <0.85.0> with exit value: {undef,[{uniform,random,[500]}, {eater,dream,5},{dinner,init,0}]}

Thanks a lot if you even read through all of this, I haven’t learnt how to read the error reports of erlang yet. If you can, and want to tell me what it means please do.

Answer 1

I think the problem is that you’ve got three modules: dinner, eater, and chopstick, but try to call philospher:start in your dinner:init/0 function. Try eater:start instead.

The second problem is the order of module and function name when generating random numbers; replace uniform:random with random:uniform in your eater.erl:

1> dinner:start().
<0.35.0>
Confucios received a chopstick
Confucios received a chopstick
Confucios started eating
Confucios put back two chopsticks
Confucios received a chopstick
Confucios received a chopstick
Confucios started eating
Confucios put back two chopsticks
Confucios received a chopstick
Confucios received a chopstick
Confucios started eating
Confucios put back two chopsticks
Confucios received a chopstick
Confucios received a chopstick
Confucios started eating
Confucios put back two chopsticks
Confucios received a chopstick
Confucios received a chopstick
Confucios started eating
Confucios put back two chopsticks
Avicenna received a chopstick
Avicenna received a chopstick
Avicenna started eating
...

This pretty quickly shows the third problem — something we should have spotted from the first error report — that the eaters aren’t actually in their own processes. So edit eater.erl so that the start() function reads:

start(Hungry, Right, Left, Name, Ctrl) ->
    spawn_link(fun() -> dream(Hungry, Right, Left, Name, Ctrl) end).

Now it works as intended:

1> dinner:start().
<0.35.0>
Confucios received a chopstick
Plato received a chopstick
Confucios received a chopstick
Confucios started eating
Descartes received a chopstick
Kant received a chopstick
Confucios put back two chopsticks
Avicenna received a chopstick
...

Thanks. This was good fun.