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VKBD(4)			 BSD Kernel Interfaces Manual		       VKBD(4)

     vkbd -- the virtual AT keyboard interface

     device vkbd

     The vkbd interface	is a software loopback mechanism that can be loosely
     described as the virtual AT keyboard analog of the	pty(4),	that is, vkbd
     does for virtual AT keyboards what	the pty(4) driver does for terminals.

     The vkbd driver, like the pty(4) driver, provides two interfaces: a key-
     board interface like the usual facility it	is simulating (a virtual AT
     keyboard in the case of vkbd, or a	terminal for pty(4)), and a character-
     special device "control" interface.

     The virtual AT keyboards are named	vkbd0, vkbd1, etc., one	for each con-
     trol device that has been opened.

     The vkbd interface	permits	opens on the special control device
     /dev/vkbdctl.  When this device is	opened,	vkbd will return a handle for
     the lowest	unused vkbdctl device (use devname(3) to determine which).

     Each virtual AT keyboard supports the usual keyboard interface ioctl(2)s,
     and thus can be used with kbdcontrol(1) like any other keyboard.  The
     control device supports exactly the same ioctl(2)s	as the virtual AT key-
     board device.  Writing AT scan codes to the control device	generates an
     input on the virtual AT keyboard, as if the (non-existent)	hardware had
     just received it.

     The virtual AT keyboard control device, normally /dev/vkbdctl<N>, is ex-
     clusive-open (it cannot be	opened if it is	already	open) and is re-
     stricted to the super-user.  A read(2) call will return the virtual AT
     keyboard status structure (defined	in <dev/vkbd/vkbd_var.h>) if one is
     available;	if not,	it will	either block until one is or return
     EWOULDBLOCK, depending on whether non-blocking I/O	has been enabled.

     A write(2)	call passes AT scan codes to be	"received" from	the virtual AT
     keyboard.	Each AT	scan code must be passed as unsigned int.  Although AT
     scan codes	must be	passes as unsigned ints, the size of the buffer	passed
     to	write(2) still should be in bytes, i.e.,

	   static unsigned int	   codes[] =
	   /*	   Make	   Break */
		   0x1e,   0x9e

		   int	   fd, len;

		   fd =	open("/dev/vkbdctl0", O_RDWR);
		   if (fd < 0)
			   err(1, "open");

		   /* Note sizeof(codes) - not 2! */
		   len = write(fd, codes, sizeof(codes));
		   if (len < 0)
			   err(1, "write");


		   return (0);

     Write will	block if there is not enough space in the input	queue.

     The control device	also supports select(2)	for read and write.

     On	the last close of the control device, the virtual AT keyboard is re-
     moved.  All queued	scan codes are thrown away.

     kbdcontrol(1), atkbdc(4), pcvt(4),	psm(4),	syscons(4)

     The vkbd interface	is a software loopback mechanism, and, thus ddb(4)
     will not work with	it.  Current implementation of the syscons(4) driver
     can accept	input from only	one keyboard, even if it is virtual.  Thus it
     is	not possible to	have both wired	and virtual keyboard to	be active at
     the same time.  It	is, however, in	principal possible to obtain AT	scan
     codes from	the different sources and write	them into the same virtual
     keyboard.	The virtual keyboard state synchronization is the user's re-

     The vkbd module was implemented in	FreeBSD	6.0.

     Maksim Yevmenkin <>

BSD				August 12, 2004				   BSD


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