.. _design_guidelines:

API Design Guidelines
#####################

Zephyr development and evolution is a group effort, and to simplify
maintenance and enhancements there are some general policies that should
be followed when developing a new capability or interface.

Using Callbacks
***************

Many APIs involve passing a callback as a parameter or as a member of a
configuration structure.  The following policies should be followed when
specifying the signature of a callback:

* The first parameter should be a pointer to the object most closely
  associated with the callback.  In the case of device drivers this
  would be ``struct device *dev``.  For library functions it may be a
  pointer to another object that was referenced when the callback was
  provided.

* The next parameter(s) should be additional information specific to the
  callback invocation, such as a channel identifier, new status value,
  and/or a message pointer followed by the message length.

* The final parameter should be a ``void *user_data`` pointer carrying
  context that allows a shared callback function to locate additional
  material necessary to process the callback.

An exception to providing ``user_data`` as the last parameter may be
allowed when the callback itself was provided through a structure that
will be embedded in another structure.  An example of such a case is
:c:type:`gpio_callback`, normally defined within a data structure
specific to the code that also defines the callback function.  In those
cases further context can accessed by the callback indirectly by
:c:macro:`CONTAINER_OF`.

Examples
========

* The requirements of :c:type:`k_timer_expiry_t` invoked when a system
  timer alarm fires are satisfied by::

    void handle_timeout(struct k_timer *timer)
    { ... }

  The assumption here, as with :c:type:`gpio_callback`, is that the
  timer is embedded in a structure reachable from
  :c:macro:`CONTAINER_OF` that can provide additional context to the
  callback.

* The requirements of :c:type:`counter_alarm_callback_t` invoked when a
  counter device alarm fires are satisfied by::

    void handle_alarm(const struct device *dev,
                      uint8_t chan_id,
		      uint32_t ticks,
		      void *user_data)
    { ... }

  This provides more complete useful information, including which
  counter channel timed-out and the counter value at which the timeout
  occurred, as well as user context which may or may not be the
  :c:type:`counter_alarm_cfg` used to register the callback, depending
  on user needs.

Conditional Data and APIs
*************************

APIs and libraries may provide features that are expensive in RAM or
code size but are optional in the sense that some applications can be
implemented without them.  Examples of such feature include
:option:`capturing a timestamp <CONFIG_CAN_RX_TIMESTAMP>` or
:option:`providing an alternative interface <CONFIG_SPI_ASYNC>`.  The
developer in coordination with the community must determine whether
enabling the features is to be controllable through a Kconfig option.

In the case where a feature is determined to be optional the following
practices should be followed.

* Any data that is accessed only when the feature is enabled should be
  conditionally included via ``#ifdef CONFIG_MYFEATURE`` in the
  structure or union declaration.  This reduces memory use for
  applications that don't need the capability.
* Function declarations that are available only when the option is
  enabled should be provided unconditionally.  Add a note in the
  description that the function is available only when the specified
  feature is enabled, referencing the required Kconfig symbol by name.
  In the cases where the function is used but not enabled the definition
  of the function shall be excluded from compilation, so references to
  the unsupported API will result in a link-time error.
* Where code specific to the feature is isolated in a source file that
  has no other content that file should be conditionally included in
  ``CMakeLists.txt``::

    zephyr_sources_ifdef(CONFIG_MYFEATURE foo_funcs.c)
* Where code specific to the feature is part of a source file that has
  other content the feature-specific code should be conditionally
  processed using ``#ifdef CONFIG_MYFEATURE``.

The Kconfig flag used to enable the feature should be added to the
``PREDEFINED`` variable in :file:`doc/zephyr.doxyfile.in` to ensure the
conditional API and functions appear in generated documentation.

Return Codes
************

Implementations of an API, for example an API for accessing a peripheral might
implement only a subset of the functions that is required for minimal operation.
A distinction is needed between APIs that are not supported and those that are
not implemented or optional:

- APIs that are supported but not implemented shall return ``-ENOSYS``.

- Optional APIs that are not supported by the hardware should be implemented and
  the return code in this case shall be ``-ENOTSUP``.

- When an API is implemented, but the particular combination of options
  requested in the call cannot be satisfied by the implementation the call shall
  return -ENOTSUP. (For example, a request for a level-triggered GPIO interrupt on
  hardware that supports only edge-triggered interrupts)