When dealing with interprocess communication, you can only synchronize your actions based on the observed actions of the involved processes. Watch for a file being created. Watch for a system semaphore. Respond to a system event. Respond to an input command. Wait for a lock to be unlocked. Observe concrete signs that the other process is certainly past a specific state milestone.

You are asking for trouble if you assume that you can watch the clock on the wall (e.g., using alarms, timeouts or sleeps) to decide when the other process "must be ready by now."