Stacks

A stack is a kernel object that implements a traditional last in, first out (LIFO) queue, allowing threads and ISRs to add and remove a limited number of integer data values.

Concepts

Any number of stacks can be defined. Each stack is referenced by its memory address.

A stack has the following key properties:

  • A queue of integer data values that have been added but not yet removed. The queue is implemented using an array of stack_data_t values and must be aligned on a native word boundary. The stack_data_t type corresponds to the native word size i.e. 32 bits or 64 bits depending on the CPU architecture and compilation mode.
  • A maximum quantity of data values that can be queued in the array.

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

A data value can be added to a stack by a thread or an ISR. The value is given directly to a waiting thread, if one exists; otherwise the value is added to the lifo’s queue. The kernel does not detect attempts to add a data value to a stack that has already reached its maximum quantity of queued values.

Note

Adding a data value to a stack that is already full will result in array overflow, and lead to unpredictable behavior.

A data value may be removed from a stack by a thread. If the stack’s queue is empty a thread may choose to wait for it to be given. Any number of threads may wait on an empty stack 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 stack, however the ISR must not attempt to wait if the stack is empty.

Implementation

Defining a Stack

A stack is defined using a variable of type struct k_stack. It must then be initialized by calling k_stack_init() or k_stack_alloc_init(). In the latter case, a buffer is not provided and it is instead allocated from the calling thread’s resource pool.

The following code defines and initializes an empty stack capable of holding up to ten word-sized data values.

#define MAX_ITEMS 10

stack_data_t my_stack_array[MAX_ITEMS];
struct k_stack my_stack;

k_stack_init(&my_stack, my_stack_array, MAX_ITEMS);

Alternatively, a stack can be defined and initialized at compile time by calling K_STACK_DEFINE.

The following code has the same effect as the code segment above. Observe that the macro defines both the stack and its array of data values.

K_STACK_DEFINE(my_stack, MAX_ITEMS);

Pushing to a Stack

A data item is added to a stack by calling k_stack_push().

The following code builds on the example above, and shows how a thread can create a pool of data structures by saving their memory addresses in a stack.

/* define array of data structures */
struct my_buffer_type {
    int field1;
    ...
    };
struct my_buffer_type my_buffers[MAX_ITEMS];

/* save address of each data structure in a stack */
for (int i = 0; i < MAX_ITEMS; i++) {
    k_stack_push(&my_stack, (stack_data_t)&my_buffers[i]);
}

Popping from a Stack

A data item is taken from a stack by calling k_stack_pop().

The following code builds on the example above, and shows how a thread can dynamically allocate an unused data structure. When the data structure is no longer required, the thread must push its address back on the stack to allow the data structure to be reused.

struct my_buffer_type *new_buffer;

k_stack_pop(&buffer_stack, (stack_data_t *)&new_buffer, K_FOREVER);
new_buffer->field1 = ...

Suggested Uses

Use a stack to store and retrieve integer data values in a “last in, first out” manner, when the maximum number of stored items is known.

Configuration Options

Related configuration options:

  • None.

API Reference

group stack_apis

Defines

K_STACK_DEFINE(name, stack_num_entries)

Statically define and initialize a stack.

The stack can be accessed outside the module where it is defined using:

extern struct k_stack <name>; 

Parameters
  • name: Name of the stack.
  • stack_num_entries: Maximum number of values that can be stacked.

Functions

void k_stack_init(struct k_stack *stack, stack_data_t *buffer, u32_t num_entries)

Initialize a stack.

This routine initializes a stack object, prior to its first use.

Return
N/A
Parameters
  • stack: Address of the stack.
  • buffer: Address of array used to hold stacked values.
  • num_entries: Maximum number of values that can be stacked.

s32_t k_stack_alloc_init(struct k_stack *stack, u32_t num_entries)

Initialize a stack.

This routine initializes a stack object, prior to its first use. Internal buffers will be allocated from the calling thread’s resource pool. This memory will be released if k_stack_cleanup() is called, or userspace is enabled and the stack object loses all references to it.

Return
-ENOMEM if memory couldn’t be allocated
Parameters
  • stack: Address of the stack.
  • num_entries: Maximum number of values that can be stacked.

void k_stack_cleanup(struct k_stack *stack)

Release a stack’s allocated buffer.

If a stack object was given a dynamically allocated buffer via k_stack_alloc_init(), this will free it. This function does nothing if the buffer wasn’t dynamically allocated.

Parameters
  • stack: Address of the stack.

void k_stack_push(struct k_stack *stack, stack_data_t data)

Push an element onto a stack.

This routine adds a stack_data_t value data to stack.

Note
Can be called by ISRs.
Return
N/A
Parameters
  • stack: Address of the stack.
  • data: Value to push onto the stack.

int k_stack_pop(struct k_stack *stack, stack_data_t *data, s32_t timeout)

Pop an element from a stack.

This routine removes a stack_data_t value from stack

in a “last in,

first out” manner and stores the value in

data.

Note
Can be called by ISRs, but timeout must be set to K_NO_WAIT.
Parameters
  • stack: Address of the stack.
  • data: Address of area to hold the value popped from the stack.
  • timeout: Waiting period to obtain a value (in milliseconds), or one of the special values K_NO_WAIT and K_FOREVER.
Return Value
  • 0: Element popped from stack.
  • -EBUSY: Returned without waiting.
  • -EAGAIN: Waiting period timed out.