SCSI Generic (sg) driver



The SCSI Generic driver (sg) is one of the four "high level" SCSI device drivers along with sd, st and sr (disk, tape and CD-ROM respectively). Sg is more generalized (but lower level) than its siblings and tends to be used on SCSI devices that don't fit into the already serviced categories. Thus sg is used for scanners, CD writers and reading audio CDs digitally amongst other things.

Rather than document the driver's interface here, version information is provided plus pointers (i.e. URLs) where to find documentation and examples.

Major versions of the sg driver

There are three major versions of sg found in the Linux kernel (lk):
  • sg version 1 (original) from 1992 to early 1999 (lk 2.2.5) . It is based in the sg_header interface structure.

  • sg version 2 from lk 2.2.6 in the 2.2 series. It is based on an extended version of the sg_header interface structure.

  • sg version 3 found in the lk 2.4 series (and the lk 2.5 series). It adds the sg_io_hdr interface structure.

Sg driver documentation

The most recent documentation of the sg driver is kept at

This describes the sg version 3 driver found in the lk 2.4 series.

Documentation (large version) for the version 2 sg driver found in the lk 2.2 series can be found at

The original documentation for the sg driver (prior to lk 2.2.6) can be found in the LDP archives at

A more general description of the Linux SCSI subsystem of which sg is a part can be found at .

Example code and utilities

There are two packages of sg utilities:


for the sg version 3 driver found in lk 2.4


for the sg version 2 (and original) driver found in lk 2.2 and earlier

Both packages will work in the lk 2.4 series. However, sg3_utils offers more capabilities. They can be found at: and

Another approach is to look at the applications that use the sg driver. These include cdrecord, cdparanoia, SANE and cdrdao.

Mapping of Linux kernel versions to sg driver versions

Here is a list of Linux kernels in the 2.4 series that had the new version of the sg driver:

  • lk 2.4.0 : sg version 3.1.17

  • lk 2.4.7 : sg version 3.1.19

  • lk 2.4.10 : sg version 3.1.20 1

  • lk 2.4.17 : sg version 3.1.22


There were 3 changes to sg version 3.1.20 by third parties in the next six Linux kernel versions.

For reference here is a list of Linux kernels in the 2.2 series that had the new version of the sg driver:

  • lk 2.2.0 : original sg version [with no version number]

  • lk 2.2.6 : sg version 2.1.31

  • lk 2.2.8 : sg version 2.1.32

  • lk 2.2.10 : sg version 2.1.34 [SG_GET_VERSION_NUM ioctl first appeared]

  • lk 2.2.14 : sg version 2.1.36

  • lk 2.2.16 : sg version 2.1.38

  • lk 2.2.17 : sg version 2.1.39

  • lk 2.2.20 : sg version 2.1.40

The lk 2.5 development series currently contains sg version 3.5.23 which is functionally equivalent to sg version 3.1.22 found in lk 2.4.17.

Douglas Gilbert

26th January 2002