Question

I am using Qt to connect to databases (So far using PostgreSQL and mySQL) and am a little confused on how the ports work. I am using Ubuntu if that makes a difference.

According to the documentation here: http://doc.qt.nokia.com/4.8-snapshot/qsqldatabase.html#setPort there is no default value for ports. However I noticed on my settings that I don’t actually set a port but it is still connecting without a problem.

So I have it set up like this:

QSqlDatabase db = QSqlDatabase::addDatabase("QPSQL");
db.setHostName("localhost");
db.setDatabaseName("mydb");
db.setUserName("name");
db.setPassword("pass");

Which works (I have tried with mySQL driver too)

It also works if i do this: db.setPort(-1); or as I would expect using the default port: db.setPort(5432);

But other ports don’t work which is as expected. So, given it says there is no default port value and -1 is obviously not a valid port how does it work?

Answer 1

Qt itself may not provide a default port value, but the individual drivers do. When you open via Qt, it simply calls the open of the driver-specific code.

For example, the Postgress driver code contains the following open function:

bool QPSQLDriver::open(const QString & db, const QString & user,
  const QString & password, const QString & host,
  int port, const QString& connOpts)
{
  if (isOpen()) close();

  QString conn;

  if (!host.isEmpty())
    conn.append(QLatin1String("host=")).append(qQuote(host));
  :
  if (port != -1)
    conn.append(QLatin1String(" port=")).append(qQuote(QString::number(port)));
  :
  d->connection = PQconnectdb(conn.toLocal8Bit().constData());
  :
  return true;
}

Similarly, the driver code for MySQL passes (port > -1) ? port : 0 as the port parameter to mysql_real_connect(), meaning that a value of -1 is translated to 0. And, as any MySQL coder will tell you, that informs mysql_real_connect() to use the default port.

In other words, a port is translated from the QT-generic default of -1 to whatever the specific driver requires as a suitable default (such as 3306 for MySQL, 50000 (or 60000) for DB2, and so on).

If you do set it, it’s passed to the driver as is.