The first queue example is simply adding (also referred to as sending) an item to a queue that has empty space:
When an item is added to a queue with available space, the addition happens immediately. Because space was available in the queue, the task sending the item to the queue continues running, unless there is another higher priority task waiting on an item to appear in the queue.
Although interaction with queues typically happens from within tasks, this isn't always the case. There are some special cases where queues can also be accessed from within ISRs (but that behavior has different rules). For the examples in this chapter, we'll assume that tasks are sending and receiving items from the queues.