Clone device

A clone device is a unique node in the file system that can be opened to access any unused device controlled by the specific driver. This special node guarantees that the user is allocated a unique device on every open( ) call, without requiring the calling process to poll a driver's minor device nodes to find an unused device.. Cloning is most often used in STREAMS drivers, where it guarantees the user a separate Stream that is associated with an unused device instance or, for SCO OpenServer 5 drivers and DDI versions prior to version 8, device number.

The clone device is useful, for example, in a networking environment where a protocol pseudo-device driver requires each user to open a separate Stream over which it establishes communication.

DDI 8 implementation

Clone behavior in DDI 8 drivers is implemented as open redirection. See the open(D2) manual page for details. The $maxchan field of the Node(DSP/4dsp) file and the drv_maxchan member of the drvinfo(D4) structure should allow one channel for each special device file that is defined in the Node file plus one channel for each clone open( ) that is supported. So, if you anticipate needing 16 clone devices, set $maxchan to 16. Since the first channel is 0, this creates 16 clone nodes plus the original node. The $maxchan value determines the number of channels are allocated in the device table for the driver.

ODDI and pre-DDI 8 implementation

A clone device node can be created in the file system in one of two ways:

• Create a node with the major number of the clone or clone(HW) driver and a minor number equal to the major number of the real device to be opened. For example, /dev/net00 might be major 40, minor 0 (normal open), and /dev/net might be major 4 (the major number of the clone driver), minor 40 (the major number of the real device).

  This causes the sflag argument to the open(D2) or open(D2oddi) entry point routine to be set to CLONEOPEN when /dev/net is opened.

• The driver designates a special minor device as its clone entry point. In this case, /dev/net might be major 40, minor 0 (clone open).

  This does not cause the sflag argument to the open( ) entry point routine to be set when /dev/net is opened. Instead, the driver has to designate a special minor device as its clone interface. When the clone is opened, the driver knows that it should look for an unused minor device. This implies that the reserved minor number for the clone entry point will never be given out.

For both methods, the driver returns the new device number as

   devp = makedevice(getemajor(devp), newminor);

Note the following when using clone devices in ODDI and DDI versions prior to version 8:

• The total number of Streams that may be connected to a particular clonable driver is limited by the number of minor numbers configured for that driver.

• A race can occur when simultaneous cloning opens and non-cloning opens are in progress. A clone driver must detect this race and return ECLNRACE (a system errno) for the non-cloning open. The FS layers above will detect this errno and restart the open( ).