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     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/vkbdctlN⟩, 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     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 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 <>


     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.