[Caml-list] recursive mutexes?

[Caml-list] recursive mutexes?

[Basile STARYNKEVITCH]

Dear All,

Does the Mutex module allow "recursive" mutexes, so can I code
something like

let mylock = Mutex.create ();;

let innerf () =
   Mutex.lock mylock;
   Printf.printf "in innerf\n";
   Mutex.unlock mylock
;;

let f () = 
    Mutex.lock mylock;
    (* do something complicated, then ... *)
    innerf ();
    Mutex.unlock mylock
;;

I expect that the lock in innerf always succeed, since mylock is
already locked by the same thread in the caller function f. Am I right
in thinking so?  (the reference manual is not fully clear on this, and
it is an important point).

FWIW my plateform is Linux/x86 with pthreads but I would like a
"definitive" statement from our Inria team on this point.

AFAIK some Posix threads accept recursive locking.

regards.
-- 

Basile STARYNKEVITCH         http://starynkevitch.net/Basile/ 
email: basile<at>starynkevitch<dot>net 
alias: basile<at>tunes<dot>org 
8, rue de la Faïencerie, 92340 Bourg La Reine, France
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Re: [Caml-list] recursive mutexes?

[Xavier Leroy]

> Does the Mutex module allow "recursive" mutexes

No.  A thread blocks forever if it attempts to lock a mutex it already
locked before.

> (the reference manual is not fully clear on this, and
> it is an important point).

I agree the documentation should be clearer on this point.

> AFAIK some Posix threads accept recursive locking.

Some do, but the POSIX 1003.1c standard specifies non-recursive
mutexes only.  There are endless arguments on the usefulness/evilness
of recursive mutexes on comp.programming.threads; the jury is still
out on this one.  

Of course, it is not difficult to implement recursive mutexes yourself
on top of normal mutexes.  You won't be able to use your recursive
mutexes in conjunction with condition variables, though.

- Xavier Leroy
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