Chapter 6. The sg_io_hdr_t structure in detail

The main control structure for the version 3 SCSI generic driver has a struct tag name of "sg_io_hdr" and a typedef name of "sg_io_hdr_t". The structure is shown in abridged form below. The "[i]" notation indicates an input value while "[o]" indicates a value that is output. The "[i->o]" indicates a value that is conveyed from input to output and apart from one special case, is not used by the driver. The "[i->o]" members are meant to aid an application matching the request sent to a write() to the corresponding response received by a read(). For pointers the "[*i]" indicates a pointer that is used for reading from user memory into the driver, "[*o]" is a pointer used for writing, and "[*io]" indicates a pointer used for either reading or writing.

typedef struct sg_io_hdr { int interface_id; /* [i] 'S' (required) */ int dxfer_direction; /* [i] */ unsigned char cmd_len; /* [i] */ unsigned char mx_sb_len; /* [i] */ unsigned short iovec_count; /* [i] */ unsigned int dxfer_len; /* [i] */ void * dxferp; /* [i], [*io] */ unsigned char * cmdp; /* [i], [*i] */ unsigned char * sbp; /* [i], [*o] */ unsigned int timeout; /* [i] unit: millisecs */ unsigned int flags; /* [i] */ int pack_id; /* [i->o] */ void * usr_ptr; /* [i->o] */ unsigned char status; /* [o] */ unsigned char masked_status;/* [o] */ unsigned char msg_status; /* [o] */ unsigned char sb_len_wr; /* [o] */ unsigned short host_status; /* [o] */ unsigned short driver_status;/* [o] */ int resid; /* [o] */ unsigned int duration; /* [o] */ unsigned int info; /* [o] */ } sg_io_hdr_t; /* 64 bytes long (on i386) */