NAME
clone — opens a major and minor device pair on a STREAMS driver
DESCRIPTION
The
clone
driver is a "pass through" device driver that allows
other drivers to select unique minor device numbers on each
open().
In effect, the driver passes an open operation through to the other driver.
This mechanism allows for multiple instantiations of a driver, each with a
different minor number, through a single device file.
When the
clone
driver is opened, it is passed
a major and minor device number
by the operating system. The major number is the
clone
driver's major number (72), and the minor number is the major number of the
driver the user wishes to clone (referred to here as the target driver).
The
clone
driver calls the open
routine of the target driver with the
CLONEOPEN
flag which specifies
a clone open. The target driver's open routine allocates an unused minor
number. The target driver must use
makedev
to make a new device number for the newly created device, and must set
*devp
to the new device number returned by
makedev.
The new device number is returned to the
clone
open through
*devp.
The
clone
open then returns to the user a file descriptor that points to
the new instantiation of the target driver.
The
echo
driver is an example of a clonable driver.
Notes
It is not possible to do multiple opens of a device with the same major and
minor number using the
clone
driver. This is because the
clone
driver is only given the major number of the driver to be cloned, and that
driver will then select a minor number which has not been opened.
When called with a pathname which corresponds to the
clonable driver,
stat()
will return different results than
fstat()
when it is called on a file descriptor returned from
open()
of the same clonable driver pathname.
RETURN VALUES
If the
clone
driver is given an invalid minor number, or if the driver
indicated is not a clonable driver, the
open()
fails and
errno
is set to [ENXIO].
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