A message the reader has found with reader_poll_for_message: user_read_pointer and user_read_bytes give its payload,
which is read in place in the shared region. Pass it to reader_done_with_message when finished, which frees the space for
the writer; the payload must not be used afterwards. Messages arrive in the order they were committed.
Fields
- channel: *Read_Channel
- offset: s64
- total_bytes: s64
- user_read_pointer: *u8
- user_read_bytes: s64