interface_id
The Linux SCSI Generic (sg) HOWTO
PrevChapter 6. The sg_io_hdr_t structure in detailNext

6.1. interface_id

This must be set to 'S' (capital ess). If not, the ENOSYS error message is placed in errno. The idea is to allow interface variants in the future that identify themselves with a different value. [The parallel port generic driver (pg) uses the letter 'P' to identify itself.] The type of interface_id is int.

PrevHomeNext
The sg_io_hdr_t structure in detailUpdxfer_direction