Byte array size less than necessary:

QByteArray ba;
ba.resize(usr.length() + pass.length() + enc.length() + role.length() + 6); // 6 additional bytes was allocated (why only 6?)
int i = -1;
ba[++i] = isc_dpb_version1; // first
ba[++i] = isc_dpb_user_name; // second
ba[++i] = usr.length(); // third
memcpy(ba.data() + ++i, usr.data(), usr.length());
i += usr.length();
ba[i] = isc_dpb_password; // fourth
ba[++i] = pass.length(); // fifth
memcpy(ba.data() + ++i, pass.data(), pass.length());
i += pass.length();
ba[i] = isc_dpb_lc_ctype; // sixth
ba[++i] = enc.length(); // !!!!!! seventh
memcpy(ba.data() + ++i, enc.data(), enc.length());
i += enc.length();

if (!role.isEmpty())

{ ba[i] = isc_dpb_sql_role_name; // !!!!!!! ba[++i] = role.length(); // !!!!!!! memcpy(ba.data() + ++i, role.data(), role.length()); i += role.length(); }