Message Queues

A message queue is a kernel object that implements a simple message queue, allowing threads and ISRs to asynchronously send and receive fixed-size data items.

Concepts

Any number of message queues can be defined. Each message queue is referenced by its memory address.

A message queue has the following key properties:

  • A ring buffer of data items that have been sent but not yet received.
  • A data item size, measured in bytes.
  • A maximum quantity of data items that can be queued in the ring buffer.

The message queue’s ring buffer must be aligned to an N-byte boundary, where N is a power of 2 (i.e. 1, 2, 4, 8, …). To ensure that the messages stored in the ring buffer are similarly aligned to this boundary, the data item size must also be a multiple of N.

A message queue must be initialized before it can be used. This sets its ring buffer to empty.

A data item can be sent to a message queue by a thread or an ISR. The data item pointed at by the sending thread is copied to a waiting thread, if one exists; otherwise the item is copied to the message queue’s ring buffer, if space is available. In either case, the size of the data area being sent must equal the message queue’s data item size.

If a thread attempts to send a data item when the ring buffer is full, the sending thread may choose to wait for space to become available. Any number of sending threads may wait simultaneously when the ring buffer is full; when space becomes available it is given to the highest priority sending thread that has waited the longest.

A data item can be received from a message queue by a thread. The data item is copied to the area specified by the receiving thread; the size of the receiving area must equal the message queue’s data item size.

If a thread attempts to receive a data item when the ring buffer is empty, the receiving thread may choose to wait for a data item to be sent. Any number of receiving threads may wait simultaneously when the ring buffer is empty; when a data item becomes available it is given to the highest priority receiving thread that has waited the longest.

A thread can also peek at the message on the head of a message queue without removing it from the queue. The data item is copied to the area specified by the receiving thread; the size of the receiving area must equal the message queue’s data item size.

Note

The kernel does allow an ISR to receive an item from a message queue, however the ISR must not attempt to wait if the message queue is empty.

Implementation

Defining a Message Queue

A message queue is defined using a variable of type struct k_msgq. It must then be initialized by calling k_msgq_init().

The following code defines and initializes an empty message queue that is capable of holding 10 items, each of which is 12 bytes long.

struct data_item_type {
    u32_t field1;
    u32_t field2;
    u32_t field3;
};

char __aligned(4) my_msgq_buffer[10 * sizeof(data_item_type)];
struct k_msgq my_msgq;

k_msgq_init(&my_msgq, my_msgq_buffer, sizeof(data_item_type), 10);

Alternatively, a message queue can be defined and initialized at compile time by calling K_MSGQ_DEFINE.

The following code has the same effect as the code segment above. Observe that the macro defines both the message queue and its buffer.

K_MSGQ_DEFINE(my_msgq, sizeof(data_item_type), 10, 4);

Writing to a Message Queue

A data item is added to a message queue by calling k_msgq_put().

The following code builds on the example above, and uses the message queue to pass data items from a producing thread to one or more consuming threads. If the message queue fills up because the consumers can’t keep up, the producing thread throws away all existing data so the newer data can be saved.

void producer_thread(void)
{
    struct data_item_t data;

    while (1) {
        /* create data item to send (e.g. measurement, timestamp, ...) */
        data = ...

        /* send data to consumers */
        while (k_msgq_put(&my_msgq, &data, K_NO_WAIT) != 0) {
            /* message queue is full: purge old data & try again */
            k_msgq_purge(&my_msgq);
        }

        /* data item was successfully added to message queue */
    }
}

Reading from a Message Queue

A data item is taken from a message queue by calling k_msgq_get().

The following code builds on the example above, and uses the message queue to process data items generated by one or more producing threads.

void consumer_thread(void)
{
    struct data_item_t data;

    while (1) {
        /* get a data item */
        k_msgq_get(&my_msgq, &data, K_FOREVER);

        /* process data item */
        ...
    }
}

Peeking into a Message Queue

A data item is read from a message queue by calling k_msgq_peek().

The following code peeks into the message queue to read the data item at the head of the queue that is generated by one or more producing threads.

void consumer_thread(void)
{
    struct data_item_t data;

    while (1) {
        /* read a data item by peeking into the queue */
        k_msgq_peek(&my_msgq, &data);

        /* process data item */
        ...
    }
}

Suggested Uses

Use a message queue to transfer small data items between threads in an asynchronous manner.

Note

A message queue can be used to transfer large data items, if desired. However, this can increase interrupt latency as interrupts are locked while a data item is written or read. It is usually preferable to transfer large data items by exchanging a pointer to the data item, rather than the data item itself. The kernel’s memory map and memory pool object types can be helpful for data transfers of this sort.

A synchronous transfer can be achieved by using the kernel’s mailbox object type.

Configuration Options

Related configuration options:

  • None.