The Linux SCSI Generic (sg) HOWTO
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# 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) */

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