H5VL_REGISTER_CONNECTOR registers a new VOL connector as a member of the virtual object layer class. This VOL connector identifier is good until the library is closed or the connector is unregistered.
vipl_id is either H5P_DEFAULT or the identifier of a VOL initialization property list created with H5P_CREATE (H5Pcreate(H5P_VOL_INITIALIZE)). When created, this property list contains no library properties. If a VOL connector author decides that initialization-specific data are needed, they can be added to the empty list and retrieved by the connector in the VOL connector's initialize callback. Use of the VOL initialization property list is uncommon, as most VOL-specific properties are added to the file access property list via the connector's API calls which set the VOL connector for the file open/create. For more information, see the VOL documentation.
H5VL_class_t is defined in H5VLconnector.h in the source code. It contains class information for each VOL connector:
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