Callback functions are a means to return data from ngspice to the caller. These functions are defined as global functions in the caller, so to be reachable by the C-coded ngspice. They are declared according to the typedefs given below. ngspice receives their addresses from the caller upon initialization with the ngSpice_Init(...) function (see 19.3.2.1). If the caller will not make use of a callback, it may send NULL instead of the address (except for ControlledExit, which is always required).

If XSPICE is enabled, additional callback functions are made accessible by ngSpice_Init_Evt(...) to obtain digital event node data.

If ngspice is run in the background thread (19.4.2), the callback functions (defined in the caller) also are called from within that thread. One has to be carefully judging how this behavior might influence the caller, where now you have the primary and the background thread running in parallel. So make the callback function thread safe. The integer identification number is only used if you run several shared libraries in parallel (see Chapt. 19.6). Three additional callback function are described in Chapt. 19.6.3.