How to set SO_REUSEPORT option for QUdpSocket?

Sacha_D

Hello. How to set SO_REUSEPORT option for QUdpSocket?

dheerendra

use BindMode and BindFlag in bind(..) method

Xando

I could not get this to work using the QUdpSocket::bind() functions and the BindFlag as dheerendra suggested. There doesn't seem to be a BindFlag for reusing ports, at least not in in Qt4 (what I use). I did look quickly at the Qt5 documentation and didn't see any appropriate BindFlag, either.

So I did it using Berkeley sockets. I'm using Linux, the same code might work with WinSock, if not something trivially similar will. The following code creates a QUdpSocket, provides it with a low-level socket descriptor that has been set for SO_REUSEPORT, then binds it to port 34567:

QUdpSocket *sock = new QUdpSocket;
int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
int optval = 1;
setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT, 
              (void *) &optval, sizeof(optval));
sock->setSocketDescriptor(sockfd, QUdpSocket::UnconnectedState);
sock->bind(34567, QUdpSocket::ShareAddress | QUdpSocket::ReuseAddressHint);

JonB

@Xando

https://stackoverflow.com/questions/47268023/how-to-set-so-reuseaddr-on-the-socket-used-by-qtcpserver takes the same approach as you do if required to access flags not available from Qt.

FYI though, have a careful read of https://bugreports.qt.io/browse/QTBUG-33419. At one point I see:

   case QNativeSocketEngine::AddressReusable:
#if defined(SO_REUSEPORT)
        // on OS X, SO_REUSEADDR isn't sufficient to allow multiple binds to the
        // same port (which is useful for multicast UDP). SO_REUSEPORT is, but
        // we most definitely do not want to use this for TCP. See QTBUG-6305.
        if (socketType == QAbstractSocket::UdpSocket)
            n = SO_REUSEPORT;
        else
            n = SO_REUSEADDR;
#else
        n = SO_REUSEADDR;
#endif

implying that code for QNativeSocketEngine::AddressReusable will use SO_REUSEPORT for QAbstractSocket::UdpSocket , provided available at compilation?

Developer123

@Xando On windows, this did not work for me, even fixing the socket options to the windows equivalent. What did work was:

    udpSocket = new QUdpSocket(this);
    groupAddress4 = QHostAddress(multicastAddr);
    bool result =  udpSocket->bind(
        QHostAddress::AnyIPv4, 
        port, 
        QUdpSocket::ShareAddress | QUdpSocket::ReuseAddressHint);