Re: [Caml-list] BUG in unix.ml (was: strange errors when linking lwt.unix)

[Jeremie Dimino <jdimino@janestreet.com>]

On Fri, Mar 15, 2013 at 2:27 PM, oliver <oliver@first.in-berlin.de> wrote:
> Just by your activation of signal function
> - not blocking, but handling "Sys.Signal_handle" but with the unreliable signal semantics
> this has cause the problems.
>
> signal(2) opened the pandoras box.

Actually OCaml is using sigaction(2).  It is only using signal(2) when
sigaction is not available.

Actually there are two different things here:

- signal disposition: this is a process-wide settings for each signal
describing how to react when it is received.  It can be changed with
the [sigaction] system call (or the less portable [signal]).
- signal mask: it is a per-thread set of signals that are blocked.  It
can be changed with sigprocmask.

When a signal is received by a thread, if it is blocked it will be
pending until it is unblocked.  When it is not blocked and the
threaded is waiting on an interruptible system call and the
disposition of the signal is a user-defined handler, the system call
will fail with EINTR and the handler will be invoked.

So basically Sys.set_signal and Unix.sigprocmask are two different
things.  You should block signals when you are running some code that
you don't want to be interrupted:

    # #load "unix.cma";;
    # Sys.set_signal Sys.sigusr1 (Sys.Signal_handle (fun _ ->
print_endline "signal!"));;
    - : unit = ()
    # let old_mask = Unix.sigprocmask Unix.SIG_BLOCK [Sys.sigusr1];;
    val old_mask : int list = []
    # Unix.kill (Unix.getpid ()) Sys.sigusr1;;
    - : unit = ()
    # Unix.sigprocmask Unix.SIG_SETMASK old_mask;;
    signal!
    - : int list = [-12]

The man page describing all this is signal(7).

Jeremie