If the 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.
If the 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 value of ibsta is returned.