If 'iovec_count' is zero then this value is a pointer to user memory of at least 'dxfer_len' bytes in length. If there is a data transfer associated with the command then the data will be transferred to or from this user memory. If 'iovec_count' is greater than zero then this value points to a scatter-gather array in user memory. Each element of this array should be an object of type sg_iovec_t. Note that data is sometimes written to user memory (e.g. from a failed SCSI READ) even when an error has occurred.