| VKBD(4) | Device Drivers 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 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.
The vkbd module was implemented in
FreeBSD 6.0.
Maksim Yevmenkin <m_evmenkin@yahoo.com>
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.
| August 12, 2004 | Debian |