If the parameter online is zero, then ibonl() frees the
resources associated with the board or device descriptor
ud. The descriptor cannot be used again after the
ibonl() call. The parameter ud cannot be a board
index (aka minor).
If the parameter online is nonzero, then all the settings
associated with the descriptor (GPIB address, end-of-string mode, timeout,
etc.) are reset to their 'default' values. The 'default' values are
the settings the descriptor had when it was first obtained with
ibdev() or
ibfind().
The parameter ud can be a board index
(aka minor). The 'default' values are the standard default
values as modified by the
corresponding gpib.conf
interface definition.