[mercury-users] thread safety

[Jonathan Morgan]

> > To explain more fully, yes, there is one shared mutex, so a foreign_proc
> > marked non-thread-safe will not run concurrently with any other
> > foreign_proc so marked (or itself). However, if a number of
> > foreign_procs run one after the other, and need to share data, e.g.
> > through a global variable, this will not be safe. read_to_matrix and
> > write_as_tiff are Mercury predicates that call a number of foreign_procs
> > in sequence to do their stuff. Although each foreign_proc called will
> > not run concurrently with any other, there is nothing stopping the
> > sequence being interrupted, e.g. read_to_matrix running its first
> > foreign_proc which sets a global, followed by write_as_tiff in another
> > thread running a foreign_proc that modifies that global, followed by
> > read_to_matrix running its second foreign_proc that accesses that global
> > (which has been screwed up by the other thread).

Understood.  I suppose I just assumed that not_thread_safe threw the
burden on the compiler to make the operation thread safe.  On a
related note, this is the kind of thing that is often represented by
impure/semipure function sets or threading the I/O state.  How are
these things handled in the presence of threads?  If I have code like
the following:

:- pragma promise_pure(do_something/2).
do_something(!Value) :-
    impure set_value(!.V),
    impure perform_some_action_working_with_value,
    semipure get_value(!:V).

where get_value and set_value are foreign procedures declared
not_thread_safe, can separate executions of do_something/2 be
interleaved, destroying my value in this way?

How about if the code was:
do_something(!Value, !IO) :-
    set_value(!.V, !IO),
    perform_some_action_working_with_value(!IO),
    get_value(!:V, !IO).

Can I in any way ensure that only one thread is able to execute
foreign_code? (this is supposed to make multi-threaded Gtk+ much
easier to use).

Jon
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