Re: Q: How i can use Mutex and thread ..

[Xavier Leroy <Xavier.Leroy@inria.fr>]

>   I have the following error :
>         Objective Caml version 2.01
> # let forks = Array.create 5 (Mutex.create());;
> Reference to undefined global `Mutex'

If you wish to use threads under the toplevel environment, you need to
build a custom toplevel first:

        ocamlmktop -custom -thread -o threadedocaml threads.cma -cclib -lthreads

(This is for bytecode threads; if you're using POSIX threads or
Windows threads, see the manual, chapter "The threads library", for
the correct libraries to use.)

Then, just run "threadedocaml" and voilà, you've got the thread
functions.

> P.S. Examples of Ocaml programs using thread are welcome

Enclosed below are two good examples: one shows a "producer" thread
and a "consumer" thread connected via a bounded circular buffer
(illustrates mutexes and conditions); the other is the well-known
channel-based sieve of Eratosthenes (illustrates events).

Hope this helps,

- Xavier Leroy

(******************** Classic producer-consumer **********************)

type 'a prodcons =
  { buffer: 'a array;
    lock: Mutex.t;
    mutable readpos: int;
    mutable writepos: int;
    notempty: Condition.t;
    notfull: Condition.t }

let create size init =
  { buffer = Array.create size init;
    lock = Mutex.create();
    readpos = 0;
    writepos = 0;
    notempty = Condition.create();
    notfull = Condition.create() }

let put p data =
  Mutex.lock p.lock;
  while (p.writepos + 1) mod Array.length p.buffer = p.readpos do
    Condition.wait p.notfull p.lock
  done;
  p.buffer.(p.writepos) <- data;
  p.writepos <- (p.writepos + 1) mod Array.length p.buffer;
  Condition.signal p.notempty;
  Mutex.unlock p.lock

let get p =
  Mutex.lock p.lock;
  while p.writepos = p.readpos do
    Condition.wait p.notempty p.lock
  done;
  let data = p.buffer.(p.readpos) in
  p.readpos <- (p.readpos + 1) mod Array.length p.buffer;
  Condition.signal p.notfull;
  Mutex.unlock p.lock;
  data

(* Test *)

let buff = create 20 0

let rec produce n =
  print_int n; print_string "-->"; print_newline();
  put buff n;
  produce (n+1)

let rec consume () =
  let n = get buff in
  print_string "-->"; print_int n; print_newline();
  consume ()

let _ =
  Thread.create produce 0;
  consume()

(*********************** Eratosthene's sieve ***********************)

open Printf
open Thread

let rec integers n ch =
  Event.sync (Event.send ch n);
  integers (n+1) ch

let rec sieve n chin chout =
  let m = Event.sync (Event.receive chin) in
  if m mod n = 0
  then sieve n chin chout
  else (Event.sync (Event.send chout m); sieve n chin chout)

let rec print_primes ch max =
  let n = Event.sync (Event.receive ch) in
  if n > max
  then ()
  else begin
    printf "%d\n" n; flush stdout;
    let ch_after_n = Event.new_channel () in
    Thread.create (sieve n ch) ch_after_n;
    print_primes ch_after_n max
  end

let go max =
  let ch = Event.new_channel () in
  Thread.create (integers 2) ch;
  print_primes ch max;;

let _ = go 1000