This example is used to introduce the concept of a queue. Similar to semaphores, queues are another synchronization mechanism used for data communication between tasks. The synchronization using queues ensures that data passed between tasks are not overwritten or entered into a race condition. Three new APIs: xQueueCreate, xQueueSend and xQueueReceive are utilized in this example. The example also demonstrates how to pass data to the queue by value as well as by pointer (aka by reference). In this example, a total of six tasks and two queues are created. Four tasks send data with different delays to two queues while two tasks receive data and process the incoming data.

The tasks and queues present a basic producer-consumer model where one task acts as a producer that generates data which is sent to the queue while another task acts as a consumer which consumes the data coming from the queue. xQueueCreate API is used to create a queue object. The consumer tasks call the xQueueReceive API which will put the task in a BLOCKed state until data arrives in the queue. The producer tasks will send data to the queue by calling xQueueSend. When data is sent by a producer task, it acts as a signal for a consumer task to unblock and process the data. The consumer tasks in this example output a UART message over the terminal using the data provided from the producer task.