.. _message_queues_v2:

Message Queues
##############

A :dfn:`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.

.. contents::
    :local:
    :depth: 2

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 **queue** of data items that have been sent but not yet received.
  The queue is implemented using a ring buffer.

* The **data item size**, measured in bytes, of each data item.

* A **maximum quantity** of data items that can be queued in the ring buffer.

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

A data item can be **sent** to a message queue by a thread or an ISR.
The data item a 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.

.. 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 :c:type:`struct k_msgq`.
It must then be initialized by calling :cpp:func:`k_msgq_init()`.

The following code defines and initializes an empty message queue
that is capable of holding 10 items.

.. code-block:: c

    struct data_item_type {
        ...
    };

    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 :c:macro:`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.

.. code-block:: c

    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 :cpp:func:`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.

.. code-block:: c

    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 :cpp:func:`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.

.. code-block:: c

    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 */
            ...
        }
    }

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 usally 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.

APIs
****

The following message queue APIs are provided by :file:`kernel.h`:

* :cpp:func:`k_msgq_init()`
* :cpp:func:`k_msgq_put()`
* :cpp:func:`k_msgq_get()`
* :cpp:func:`k_msgq_purge()`
* :cpp:func:`k_msgq_num_used_get()`
* :cpp:func:`k_msgq_num_free_get()`