After starting the simulation, in a first step the callback function SendInitEvtData is called once for each event node. All nodes are numbered in ascending order. The first function argument is the actual node number, the second sets the total amount of nodes, then node name and node type follow. You may set up an array to store name and type, indexed by the node number.

During simulation, after each time step ngspice checks if a node has changed. If so, SendEvtData is called for each node that changed, returning the simulation time, the node number, and the node value as a char* string, consisting of one out of 0s, 1s, Us, 0r, 1r, Ur, 0z, 1z, Uz, 0u, 1u, Uu (see 12.5.1). The double real value and the void* binary data structure arguments are for future enhancements of the data interface. The int mode returns 0 for op, 1 for dc, 2 for ac , and 3 for tran simulation. The final int is useful to identify the ngspice lib by number if you run several in parallel (see 19.6). The final *void just returns the pointer received from caller. e.g. to identify the calling object.