void setField(cdbfapiPlus* handle, int fieldno, char* string, double value);

Function setField() puts values to a field in memory before writing a record to a disc.
There are different versions of setField: you can pass a double value, a string or both. In last case setField will use a value which is more suitable for this type of field.

{
    // your code
    clearRecord(dbf);
    setFieldString(dbf, 0, "string value");
    setFieldDouble(dbf, 1, 123.45);
    setField(dbf, 2, "the string", 666.66);
    writeRecord(dbf, 0);
    // your code
}