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 const 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 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 CONTAINER_OF.

Examples

The requirements of 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 gpio_callback, is that the timer is embedded in a structure reachable from CONTAINER_OF that can provide additional context to the callback.
The requirements of 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 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 capturing a timestamp or providing an alternative interface. 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 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)