After the simulation is finished, use the ngspice commands write (17.5.93) or wrdata (17.5.92) to output data to a file as usual, use the print command (17.5.50) to retrieve data via callback SendChar (19.3.3.1), or refer to accessing the data as described in Chapt. 19.4.3.

Typical usage:

ngSpice_Command("write testout.raw V(2)");
ngSpice_Command("print V(2)");