Kevin Ryde <[EMAIL PROTECTED]> writes:
> (Speaking of which, I'd thought before that once a promise is forced
> it shouldn't need a mutex any more, which would save a bit of time and
> space.)
I suppse it might, though you'd have to be very careful with the
coding. i.e. (offhand) you would probably have to do something like
this:
SCM mutex = SCM_PROMISE_MUTEX(p); // assume atomic copy
if(scm_is_null(mutex))
result = SCM_PROMISE_DATA(p); // [1]
else
{
scm_lock_mutex(mutex);
if(scm_is_null(SCM_PROMISE_MUTEX(p)) // must check again after lock
{
// someone was already evaluating when we started
// (and must have finished now)
result = SCM_PROMISE_DATA(p);
}
else
{
SCM ans = scm_call_0(SCM_PROMISE_DATA (ans));
SCM_SET_PROMISE_DATA(p, ans);
SCM_SET_PROMISE_MUTEX(p, SCM_BOOL_F) // (do last to avoid race at [1])
result = SCM_PROMISE_DATA(p);
}
scm_unlock_mutex(mutex);
}
Note that this wouldn't be safe if the initial mutex assignment might
copy a value that has been half filled by some other thread.
Of course, if we're interested in srfi-45, then it would require
somewhat more...
--
Rob Browning
rlb @defaultvalue.org and @debian.org; previously @cs.utexas.edu
GPG starting 2002-11-03 = 14DD 432F AE39 534D B592 F9A0 25C8 D377 8C7E 73A4
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