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vkbd — the virtual AT keyboard interface
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
keyboard 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
control 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
keyboard 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
exclusive-open (it cannot be opened if it is already open) and is
restricted 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 */
int fd, len;
fd = open("/dev/vkbdctl0", O_RDWR);
if (fd < 0)
/* Note sizeof(codes) - not 2! */
len = write(fd, codes, sizeof(codes));
if (len < 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
removed. All queued scan codes are thrown away.
kbdcontrol(1), atkbdc(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
The vkbd module was implemented in FreeBSD 6.0.
Maksim Yevmenkin ⟨email@example.com⟩