H5VLattr_read

herr_t H5VLattr_read(void *  attr,  hid_t connector_id,  hid_t mem_type_id,  void *  buf,
       hid_t dxpl_id,  void * *  req)

Reads attribute data through the VOL.

`attr`	Attribute object
`connector_id`	VOL connector identifier
`mem_type_id`	Memory datatype identifier
`buf`	Buffer to receive data
`dxpl_id`	Data transfer property list
`req`	Async request object (can be null)

Non-negative on success, negative on failure.