The read_at and write_at functions are implemented using blocking UNIX file I/O calls. They queue the operation when the I/O call is made, and they service the operation when the service function is called (at test time).
The read_list and write_list functions are implemented using lio_listio. Both of these functions actually call a function dbpf_bstream_rw_list, which implements the necessary functionality for both reads and writes. This function sets up the queue entry and immediately calls the service function.
The necessary aiocb array is allocated within the service function if one isn't already allocated for the operation. The maximum number of aiocb entries is controlled by a compile-time constant AIOCB_ARRAY_SZ; if there are more list elements than this, the list will be split up and handled by multiple lio_listio calls.
If the service function returns to dbpf_bstream_rw_list without completing the entire operation, the operation is queued.
Both the _at and _list functions use a FD cache for getting open FDs.