concurrency - Concurrent Erlang Code -


from old exam, there question don't understand answer to. module question looks this:

-module(p4). -export([start/0, init/0, f1/1, f2/1, f3/2]).  start() ->     spawn(fun() -> init() end).  init() ->  loop(0).  loop(n) ->     receive         {f1, pid} ->             pid ! {f1r, self(), n},             loop(n);         {f2, pid} ->             pid ! {f2r, self()},             loop(n+1);         {f3, pid, m} ->             pid ! {f3r, self()},             loop(m)     end.  f1(serv) ->     serv ! {f1, self()},     receive {f1r, serv, n} -> n end.  f2(serv) ->     serv ! {f2, self()},     receive {f2r, serv} -> ok end.  f3(serv, n) ->     serv ! {f3, self(), n},     receive {f3r, serv} -> ok end.

the question asks consider following function part of code, , result of function be. correct answer 2. think it'd 3, since "increase-call" f2(server) after response of self()!{f1r, server, 2}.

    test3() ->       server = start(),       self()!{f1r, server, 2},       f2(server),       f1(server).

my questions are:

  • why answer 2 , not 3, , how self()!{f1r, server, 2} work?
  • isn't self()!{f1r, server, 2} response receive clause in loop(n) of f1(serv) function?

self()!{f1r, server, 2} sends {f1r, server, 2} itself. message wait in inbox until received.

then f2 gets executed , f1.

it in last function, when last line receive {f1r, serv, n} -> n end executed, process running test3 receives message waiting in inbox (the 1 sent itself) , returns 2 in message.

note @ end of program, there have been {f1r, <process number>, n} waiting in inbox, n having value of 1.


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