A latch is yet another synchronizer. It acts as a gate—threads wait for the gate to open; once the gate— that is—the latch opens, all threads enter it:
Why do we need latches? A latch is used to ensure that, unless some essential, precursor activity has happened, other activities wait for it. Let's look at a real-life example:
The preceding diagram shows how running a race happens on a nice evening. All the players need to assemble at the starting point and wait for the whistle to blow. Once the whistle goes, the players start running, and the race starts.
The starting point is the place of rendezvous: everyone needs to come and wait there for the important activity of the whistle blowing. We can't even imagine a race without it!
So, the runners are threads, the whistle being blown is a one-time activity, such as initialization...
