The Linux SCSI Generic (sg) HOWTO
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Appendix B. sg_header, the original sg control structure

Following is the original interface structure of the sg driver that dates back to 1991. Those field elements with a "[o]+" are added by the sg version 2 driver which was first placed in lk 2.2.6 in April 1999. 
struct sg_header
{
    int pack_len;    /* [o] */
    int reply_len;   /* [i] */
    int pack_id;     /* [i->o] */
    int result;      /* [o] */
    unsigned int twelve_byte:1;     /* [i] */
    unsigned int target_status:5;   /* [o]+ */
    unsigned int host_status:8;     /* [o]+ */
    unsigned int driver_status:8;   /* [o]+ */
    unsigned int other_flags:10;    /* unused */
    unsigned char sense_buffer[SG_MAX_SENSE]; /* [o] */
};      /* This structure is 36 bytes long on i386 */
SCSI commands are sent via write() calls to an sg device name (e.g. /dev/sg0). The data written to write() is of the form <a_sg_header_obj + scsi_command [ + data_to_write]>. The "data_to_write" component is only needed for SCSI commands that transfer data towards the SCSI device. The corresponding read() to the sg device name will yield data of the form <a_sg_header_obj [ + data_to_read]>.

This interface is fully described in the www.torque.net/sg/p/scsi-generic.txt file which documents the sg version 2 driver.

Since many Linux applications use this interface, it is still supported in this version (i.e. version 3) of the driver. Only its most perverse idiosyncrasies have been modified and no major applications have reported any problems running old applications atop this newer driver.

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