Maurício wrote:
Hi,
'takeMVar' documentation says "if there are
multiple threads blocked in takeMVar, and the
MVar becomes full, only one thread will be
woken up."
Since 'takeMVar' is a reading function, i.e.,
it doesn't change the value of the
"variable", why waking up only one thread? If
we wake multiple threads, there's no risk of
one changing the value while the other is
reading.
Why? Because this turns out to be useful primitive.
It is very useful to be able to be sure that only a single thread picks
up a value for all kinds of concurrency styles.
In fact, takeMVar does change the value of the variable: it makes it empty.
Does that mean that all threads waiting for
that MVar will get the same value, even if
one of those threads change it's value just
after reading?
No, only one thread gets that value. It's a "one time only offer!".
Another thread waiting will get the next value which is 'putMVar'ed.
There are two higher-level constructs, both built from MVars:
readMVar is take followed by put (with a little exception handling) so
that other threads can also get the same value.
A Chan, using the special dupChan facility, gives a stronger guarantee
that each thread reads the same value exactly once. It's something like
a shared queue with multiple readers.
Jules
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
