The callback functions SendInitData (19.3.3.5) and SendData (19.3.3.4) allow access to simulator output data synchronized with the simulation progress.

Each time a new plot is generated during simulation, e.g. when a sequence of op, ac and tran is used or commands like linearize or fft are invoked, the callback SendInitData is called by ngspice. Immediately after setting up the vector structure of the new plot, the function is called once. Its parameter is a pointer to the structure vecinfoall (19.3.1), which contains an array of structures vecinfo, one for each vector in the actual plot. You may simply use vecname to get the name of any vector. This time the vectors are still empty, but pointers to the vector structure are available.

Each time a new data point (e.g. time value and simulation output value(s)) is added to the vector structure of the current plot, the function SendData is called by ngspice. This allows you to immediately access the simulation output synchronized with the simulation time, e.g. to interface it to a runtime plot or to use it for some controlled simulation by stopping the simulation based on a condition, altering parameters and resume the simulation. SendData returns a structure vecvaluesall as parameter, which contains an array of structures vecvalues, one for each vector.

Some code to demonstrate the callback function usage is referenced below (19.5).