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VKBD(4)                FreeBSD 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
     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     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), 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

     The vkbd module was implemented in FreeBSD 6.0.

     Maksim Yevmenkin <>

FreeBSD 11.0-PRERELEASE         August 12, 2004        FreeBSD 11.0-PRERELEASE


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