A 'count-down latch' is commonly (at least when I learned it, using pthreads) called a 'Conditional Variable'. In your case case, your mutex would be the num_r

A 'count-down latch' is commonly (at least when I learned it, using pthreads) called a 'Conditional Variable'. In your case case, your mutex would be the num_results variable and the conditional is that it is greater than 0.