142 lines
3.9 KiB
ReStructuredText
142 lines
3.9 KiB
ReStructuredText
.. _stacks_v2:
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Stacks
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######
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A :dfn:`stack` is a kernel object that implements a traditional
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last in, first out (LIFO) queue, allowing threads and ISRs
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to add and remove a limited number of 32-bit data values.
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.. contents::
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:local:
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:depth: 2
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Concepts
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********
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Any number of stacks can be defined. Each stack is referenced
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by its memory address.
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A stack has the following key properties:
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* A **queue** of 32-bit data values that have been added but not yet removed.
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The queue is implemented using an array of 32-bit integers,
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and must be aligned on a 4-byte boundary.
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* A **maximum quantity** of data values that can be queued in the array.
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A stack must be initialized before it can be used. This sets its queue to empty.
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A data value can be **added** to a stack by a thread or an ISR.
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The value is given directly to a waiting thread, if one exists;
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otherwise the value is added to the lifo's queue.
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The kernel does *not* detect attempts to add a data value to a stack
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that has already reached its maximum quantity of queued values.
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.. note::
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Adding a data value to a stack that is already full will result in
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array overflow, and lead to unpredictable behavior.
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A data value may be **removed** from a stack by a thread.
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If the stack's queue is empty a thread may choose to wait for it to be given.
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Any number of threads may wait on an empty stack simultaneously.
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When a data item is added, it is given to the highest priority thread
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that has waited longest.
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.. note::
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The kernel does allow an ISR to remove an item from a stack, however
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the ISR must not attempt to wait if the stack is empty.
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Implementation
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**************
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Defining a Stack
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================
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A stack is defined using a variable of type :c:type:`struct k_stack`.
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It must then be initialized by calling :cpp:func:`k_stack_init()`.
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The following code defines and initializes an empty stack capable of holding
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up to ten 32-bit data values.
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.. code-block:: c
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#define MAX_ITEMS 10
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uint32_t my_stack_array[MAX_ITEMS];
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struct k_stack my_stack;
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k_stack_init(&my_stack, my_stack_array, MAX_ITEMS);
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Alternatively, a stack can be defined and initialized at compile time
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by calling :c:macro:`K_STACK_DEFINE()`.
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The following code has the same effect as the code segment above. Observe
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that the macro defines both the stack and its array of data values.
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.. code-block:: c
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K_STACK_DEFINE(my_stack, MAX_ITEMS);
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Pushing to a Stack
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==================
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A data item is added to a stack by calling :cpp:func:`k_stack_push()`.
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The following code builds on the example above, and shows how a thread
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can create a pool of data structures by saving their memory addresses
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in a stack.
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.. code-block:: c
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/* define array of data structures */
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struct my_buffer_type {
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int field1;
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...
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};
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struct my_buffer_type my_buffers[MAX_ITEMS];
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/* save address of each data structure in a stack */
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for (int i = 0; i < MAX_ITEMS; i++) {
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k_stack_push(&my_stack, (uint32_t)&my_buffers[i]);
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}
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Popping from a Stack
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====================
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A data item is taken from a stack by calling :cpp:func:`k_stack_pop()`.
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The following code builds on the example above, and shows how a thread
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can dynamically allocate an unused data structure.
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When the data structure is no longer required, the thread must push
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its address back on the stack to allow the data structure to be reused.
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.. code-block:: c
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struct my_buffer_type *new_buffer;
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new_buffer = (struct my_buffer_type *)k_stack_pop(&buffer_stack, K_FOREVER);
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new_buffer->field1 = ...
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Suggested Uses
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**************
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Use a stack to store and retrieve 32-bit data values in a "last in,
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first out" manner, when the maximum number of stored items is known.
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Configuration Options
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*********************
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Related configuration options:
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* None.
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APIs
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****
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The following stack APIs are provided by :file:`kernel.h`:
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* :cpp:func:`k_stack_init()`
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* :cpp:func:`k_stack_push()`
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* :cpp:func:`k_stack_pop()`
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