TCP is a reliable protocol, but I'm wondering if Qt's abstraction around it allows me to use it reliably. I'm using a QDataStream around a QTcpSocket, and before I stream out I first check that QAbstractSocket::isValid() returns true and that QAbstractSocket::state() == ConnectedState. This all sounds fine and those two checks do in fact detect a disconnection... but the asynchronous nature of networking means that the state could change from ConnectedState to UnconnectedState before I am able to stream out (even if it's just a few lines after the isValid() and state() checks). It doesn't make sense to lock the socket in a connected state... but that kind of atomicity is what I'm aiming for.
What is the best/safest method to use QTcpSocket reliably? Is the qint64 that QIODevice::write() returns the key to this (if it's -1 then I didn't get an ACK?)? I've played around with QDataStream::status(), which uses that qint64 that QIODevice::write() returns... but with no luck. The QDataStream::status() always returned QDataStream::Ok, even when the checks for QAbstractSocket::isValid() and QAbstractSocket::state() == ConnectedState failed. My guess is the underlying QIODevice isn't getting closed when the peer disconnects (either via abort() or disconnectFromHost())... but I also have a slightly outdated version of Qt (4.8) so maybe that's a bug and it's fixed now? The best strategy I've come up with so far is to check QAbstractSocket::isValid() and QAbstractSocket::state() == ConnectedState _AFTER_ I stream out. This way I can assume (*gulp*) that the previous write didn't fail (and if it does, queue whatever failed to re-send when the connection is re-established)... but then there's the opposite race condition to worry about: that write might have succeeded before QAbstractSocket entered UnconnectedState... so I might send the same data twice thinking it failed if I'm not careful. Another better, though slightly inefficient, way of writing reliably would be to check isValid() and state() == ConnectedState both before and after the write... but that just seems like an ugly hack imo. What would you guys recommend? d3fault _______________________________________________ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
