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
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 */
0x1e, 0x9e
};
int
main(void)
{
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");
close(fd);
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 removed. All
queued scan codes are thrown away.
kbdcontrol(1),
atkbdc(4),
psm(4),
syscons(4),
vt(4)
The
vkbd module was implemented in
FreeBSD 6.0.
Maksim Yevmenkin
<
[email protected]>
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 responsibility.