doc/kernel/cleanup.rst

.. _cleanup_api:

Scope-based Cleanup Helpers
###########################

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

Overview
********

The Cleanup Helper API provides a mechanism for automatic resource cleanup when
variables go out of scope. This is similar to RAII in C++ or defer statements
in Go. By leveraging compiler support for the ``__cleanup`` attribute,
this API helps prevent resource leaks and simplifies error handling by ensuring
cleanup code is executed automatically.

It is required to set :kconfig:option:`CONFIG_SCOPE_CLEANUP_HELPERS` to enable this
feature, which is particularly useful for:

* Automatic unlocking of mutexes and semaphores
* Automatic freeing of dynamically allocated memory
* Ensuring cleanup actions occur on all code paths (including early returns)
* Reducing boilerplate cleanup code

.. warning::

   The cleanup mechanism is implemented using the ``__cleanup`` attribute.
   If the toolchain doesn't support this attribute, the API is not available.

   For this reason this API is intended solely for user applications and not for
   the kernel itself or other subsystems.

Core Concepts
*************

The cleanup API provides three main abstractions:

Scoped Variables
================

A scoped variable defines a type with automatic init and exit behavior.
Variables declared with a scoped variable type are initialized using an
init function and automatically cleaned up using an exit function when they
go out of scope.

Scoped Guards
=============

Scoped guards are specialized scoped variables that automatically acquire a lock or
resource on initialization and release it when going out of scope. This pattern
is commonly used with mutexes, semaphores, and other synchronization primitives.

Scoped Defers
=============

Scoped defers execute a specified function when the variable goes out of scope,
without acquiring anything on initialization. This is similar to the ``defer``
statement in languages like Go.

Defining Scoped Types
*********************

Custom Scoped Variables
=======================

Use :c:macro:`SCOPE_VAR_DEFINE` to define a custom scoped variable type with init
and exit functions:

.. code-block:: c

    static inline struct flash_area *flash_area_init(int area_id)
    {
        struct flash_area *fa;

        if (flash_area_open(area_id, &fa) < 0) {
            return NULL;
        }

        return fa;
    }

    static inline void flash_area_exit(struct flash_area *fa)
    {
        if (fa != NULL) {
            flash_area_close(fa);
        }
    }

    // Define the scoped variable type
    SCOPE_VAR_DEFINE(flash_area, struct flash_area *, flash_area_exit(_T),
                     flash_area_init(area_id), int area_id);

    static int some_function(void)
    {
        // Declare 'fa' with automatic cleanup
        scope_var(flash_area, fa)(FIXED_PARTITION_ID(storage_partition));
        if (fa == NULL) {
            return -EINVAL;  // Exit function is still called
        }

        // Use fa normally
        printk("Has driver: %d\n", flash_area_has_driver(fa));

        // No need to manually close - exit function is called automatically
        return 0;
    }

The ``_T`` variable in the exit function expression contains the value of the
variable being cleaned up.

Scoped Guards
=============

Use :c:macro:`SCOPE_GUARD_DEFINE` to define a guard that acquires a lock on
initialization and releases it on scope exit:

.. code-block:: c

    // Example guard definition (already provided by kernel.h)
    SCOPE_GUARD_DEFINE(k_mutex, struct k_mutex *,
                       (void)k_mutex_lock(_T, K_FOREVER),
                       (void)k_mutex_unlock(_T));

    static K_MUTEX_DEFINE(lock);

    void critical_section(void)
    {
        scope_guard(k_mutex)(&lock);

        // Lock is held here
        // Perform critical operations

        // Lock is automatically released when guard goes out of scope
    }

Scoped Defers
=============

Use :c:macro:`SCOPE_DEFER_DEFINE` to define a defer that executes a cleanup function:

.. code-block:: c

    // Define a defer for a custom cleanup function
    static void cleanup_resources(void)
    {
        // Cleanup code here
    }

    SCOPE_DEFER_DEFINE(cleanup_resources);

    void some_function(void)
    {
        scope_defer(cleanup_resources)();

        // Do work...

        // cleanup_resources() is called automatically
    }

For functions with parameters:

.. code-block:: c

    // Example deferred k_free (already provided by kernel.h)
    SCOPE_DEFER_DEFINE(k_free, void *);

    void allocate_and_use(void)
    {
        void *ptr = k_malloc(100);
        scope_defer(k_free)(ptr);

        // Use ptr...

        // k_free(ptr) is called automatically
    }

Usage Notes
***********

Order of Cleanup
================

Cleanup functions are called in reverse order of declaration (LIFO - Last In,
First Out), which matches the natural nesting of resources:

.. code-block:: c

    {
        scope_guard(k_mutex)(&lock);           // Acquired first
        void *ptr = k_malloc(100);
        scope_defer(k_free)(ptr);              // Registered second

        // Do work...

    }  // ptr is freed first, then mutex is unlocked

Scope Rules
===========

Cleanup occurs when the variable goes out of scope, which includes:

* Reaching the end of a block
* Early return statements
* Break or continue in loops
* Goto statements that jump out of scope

.. code-block:: c

    void example_with_early_exit(struct k_mutex *lock)
    {
        scope_guard(k_mutex)(lock);

        if (error_condition) {
            return;  // Guard cleanup happens here
        }

        // Normal path

    }  // Guard cleanup also happens here

API Reference
*************

.. doxygengroup:: cleanup_interface