libyang  3.1.1
libyang is YANG data modelling language parser and toolkit written (and providing API) in C.
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libyang supports two types of plugins to better support generic features of YANG that need some specific code for their specific instances in YANG modules. This is the case of YANG types, which are derived from YANG built-in types. The description of a derived type can specify some additional requirements or restriction that cannot be implemented generically and some special code is needed. The second case for libyang plugins are YANG extensions. For YANG extensions, most of the specification is hidden in their description (e.g. allowed substatements or place of the extension instantiation) and libyang is not able to process such a text in a generic way.

In both cases, libyang provides API to get functionality implementing the specifics of each type or extension. Furthermore, there are several internal plugins, implementing built-in data types and selected derived types and YANG extensions. These internal plugins uses the same API and can be taken as examples for implementing user plugins. Internal plugins are always loaded with the first created context and unloaded with destroying the last one. The external plugins are in the same phase loaded from the default directories specified at compile time via cmake variables PLUGINS_DIR (where the extensions and types subdirectories are added for each plugin type) or separately via PLUGINS_DIR_EXTENSIONS and PLUGINS_DIR_TYPES for each plugin type. The default directories can be replaced runtime using environment variables LIBYANG_TYPES_PLUGINS_DIR and LIBYANG_EXTENSIONS_PLUGINS_DIR.

Order of the plugins determines their priority. libyang searches for the first match with the extension and type, so the firstly loaded plugin for the specific item is used. Since the internal plugins are loaded always before the external plugins, the internal plugins cannot be replaced.

There is also a separate function lyplg_add() to add a plugin anytime later. Note, that such a plugin is being used after it is added with the lowest priority among other already loaded plugins. Also note that since all the plugins are unloaded with the destruction of the last context, creating a new context after that starts the standard plugins initiation and the manually added plugins are not loaded automatically.

The following pages contain description of the API for creating user plugins.