/** 'hal_link()' links a pin to a signal.  'pin_name' and 'sig_name'
    are strings containing the pin and signal names.  If the pin is 
    already linked to a signal, the original connection is unchanged.
    If the original signal is the same as the desired one, the command
    returns success, otherwise it returns failure.  (Use 'hal_unlink'
    if you need to change an existing connection.)  If the signal
    already has other pins linked to it, they are unaffected - one
    signal can be linked to many pins, but a pin can be linked to only
    one signal.
    On success, hal_link() returns HAL_SUCCESS, on failure it returns a
    negative error code.
*/
extern int hal_link(char *pin_name, char *sig_name);

/** 'hal_unlink()' unlinks any signal that is connected to a pin.
    'pin_name' is a string containing the pin name.  If the pin is
    not linked to any signal, the hal_unlink() returns succcess.
    On success, hal_unlink() returns HAL_SUCCESS, on failure it
    returns a negative error code.
*/
extern int hal_unlink(char *pin_name);