8 Apr
2009
8 Apr
'09
9:19 a.m.
Hi Gleb,
I've just found that QSem _ISN'T_ valid below 0, so the above code won't actually work with QSem as it stands.
I may be missing something, but I guess this code will work if 'newQSem (-n)' is replaced with 'newQSemN n', every 'signalQSem sem' with 'signalQSemN sem 1' and 'waitQSem sem' with 'waitQSemN sem n'.
Yep, that seems valid. However, the new solution requires an MVar to store the number of things we're waiting for anyway, so we might as well use that to also wake up the main thread. Semaphores aren't more efficient than MVar's, they are in fact implemented in terms of MVar's, so there is no benefit. Thanks Neil