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.
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