void initLibrary(cdbfapiPlus* handle, int magicNumber, char *email);

You have to call initLibrary right away when you loaded the library (created an object).
You can omit this function in your code but in this case you cannot modify dbf files.
Passing valid parameters allows you to use CDBFAPI in read/write mode.

{
    cdbfapiPlus* dbf = cdbfapiCreate();
    if (!dbf) return;
    
    initLibrary(dbf, 12345, "email@example.com");
    // your code
    cdbfapiRelease(dbf);
}