The Linux SCSI Generic (sg) HOWTO
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8.1. SG_IO

SG_IO 0x2285. The idea is deceptively simple: just hand a sg_io_hdr_t object to an ioctl() and it will return when the SCSI command is finished. It is logically equivalent to doing a write() followed by a blocking read(). The word "blocking" here implies the read() will wait until the SCSI command is complete.

The same file descriptor can be used both for SG_IO synchronous calls and the write() read() sequences at the same time. The sg driver makes sure that the response to a SG_IO call will never accidentally be fetched by a read(). Even though a single file descriptor can be shared in this manner, it is probably more sensible (and results in cleaner code) if separate file descriptors to the same SCSI device are used in this case.

It is possible that the wait for the command completion is interrupted by a signal. In this case the SG_IO call will yield an EINTR error. This is reasonably complex to handle and is discussed in the ioctl(SG_SET_KEEP_ORPHAN) description below. The following SCSI commands will be permitted by SG_IO when the sg file descriptor was opened O_RDONLY:

TEST UNIT READY
REQUEST SENSE
INQUIRY
READ CAPACITY
READ BUFFER
READ(6) (10) and (12)
MODE SENSE(6) and (10)
LOG SENSE

All commands to SCSI device type SCANNER are accepted. Other cases yield an EPERM error. Note that the write() read() interface must have the sg file descriptor open()-ed with O_RDWR as write permission is required by Linux to execute a write() system call.

The ability of the SG_IO ioctl() to issue certain SCSI commands has led to some relaxation on file descriptors open()ed "read-only" compared with the version 2 sg driver. The open() call will now attempt to allocate a reserved buffer for all newly opened file descriptors. The ioctl(SG_SET_RESERVED_SIZE) will now work on "read-only" file descriptors.