PQresultErrorField(res, PG_DIAG_SQLSTATE) should do, no?
I believe the call flow is the following - no "res"/result is ever constructed:
PGconn *PQconnectdb(const char *conninfo);
ConnStatusType PQstatus(const PGconn *conn);
// Connection_Bad
char *PQerrorMessage(const PGconn *conn);
// Obtains the error message; but there seems to be no equivalent of PQresultErrorField (e.g. PQconnErrorField) to obtain the SQLState field component thereof
