This is the actual number of bytes written to the user memory pointed to by 'sbp'. 'sb_len_wr' is always <= 'mx_sb_len'. Linux 2.2 series kernels (and earlier) truncate this value to a maximum of 16 bytes. The actual number of bytes written will not exceed the length indicated by "Additional Sense Length" field (byte 7) of the Request Sense response. The type of sb_len_wr is unsigned char .