As with interactive ngspice, you may use the ngspice internal command circbyline (17.5.11) to send a netlist line by line to the ngspice circuit parser.

Typical usage:

ngSpice_Command("circbyline fail test");
ngSpice_Command("circbyline V1 1 0 1");
ngSpice_Command("circbyline R1 1 0 1");
ngSpice_Command("circbyline .dc V1 0 1 0.1");
ngSpice_Command("circbyline .end");

The first line is a title line, which will be ignored during circuit parsing. As soon as the line .end has been sent to ngspice, circuit parsing commences.