.. _lifos_v2:

Lifos
#####

A :dfn:`lifo` is a kernel object that implements a traditional
last in, first out (LIFO) queue, allowing threads and ISRs
to add and remove data items of any size.

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

Concepts
********

Any number of lifos can be defined. Each lifo is referenced
by its memory address.

A lifo has the following key properties:

* A **queue** of data items that have been added but not yet removed.
  The queue is implemented as a simple linked list.

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

Lifo data items must be aligned on a 4-byte boundary, as the kernel reserves
the first 32 bits of an item for use as a pointer to the next data item in
the queue. Consequently, a data item that holds N bytes of application data
requires N+4 bytes of memory.

A data item may be **added** to a lifo by a thread or an ISR.
The item is given directly to a waiting thread, if one exists;
otherwise the item is added to the lifo's queue.
There is no limit to the number of items that may be queued.

A data item may be **removed** from a lifo by a thread. If the lifo's queue
is empty a thread may choose to wait for a data item to be given.
Any number of threads may wait on an empty lifo simultaneously.
When a data item is added, it is given to the highest priority thread
that has waited longest.

.. note::
    The kernel does allow an ISR to remove an item from a lifo, however
    the ISR must not attempt to wait if the lifo is empty.

Implementation
**************

Defining a Lifo
===============

A lifo is defined using a variable of type :c:type:`struct k_lifo`.
It must then be initialized by calling :cpp:func:`k_lifo_init()`.

The following defines and initializes an empty lifo.

.. code-block:: c

    struct k_lifo my_lifo;

    k_lifo_init(&my_lifo);

Alternatively, an empty lifo can be defined and initialized at compile time
by calling :c:macro:`K_LIFO_DEFINE`.

The following code has the same effect as the code segment above.

.. code-block:: c

    K_LIFO_DEFINE(my_lifo);

Writing to a Lifo
=================

A data item is added to a lifo by calling :cpp:func:`k_lifo_put()`.

The following code builds on the example above, and uses the lifo
to send data to one or more consumer threads.

.. code-block:: c

    struct data_item_t {
        void *lifo_reserved;   /* 1st word reserved for use by lifo */
        ...
    };

    struct data_item_t tx data;

    void producer_thread(int unused1, int unused2, int unused3)
    {
        while (1) {
            /* create data item to send */
            tx_data = ...

            /* send data to consumers */
            k_lifo_put(&my_lifo, &tx_data);

            ...
        }
    }

Reading from a Lifo
===================

A data item is removed from a lifo by calling :cpp:func:`k_lifo_get()`.

The following code builds on the example above, and uses the lifo
to obtain data items from a producer thread,
which are then processed in some manner.

.. code-block:: c

    void consumer_thread(int unused1, int unused2, int unused3)
    {
        struct data_item_t  *rx_data;

        while (1) {
            rx_data = k_lifo_get(&my_lifo, K_FOREVER);

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

Suggested Uses
**************

Use a lifo to asynchronously transfer data items of arbitrary size
in a "last in, first out" manner.

Configuration Options
*********************

Related configuration options:

* None.

APIs
****

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

* :c:macro:`K_LIFO_DEFINE`
* :cpp:func:`k_lifo_init()`
* :cpp:func:`k_lifo_put()`
* :cpp:func:`k_lifo_get()`