This value points to user memory of at least 'mx_sb_len' bytes length where the SCSI sense buffer will be output. Most successful commands do not output a sense buffer and this will be indicated by 'sb_len_wr' being zero. Note that there are error conditions that don't result in a sense buffer be generated. The sense buffer results from the "auto-sense" mechanism in the SCSI mid-level driver. This mechanism detects a CHECK_CONDITION status and issues a REQUEST SENSE command and conveys its response back as the "sense buffer". The type of sbp is unsigned char * .