QUdpSocket::waitForBytesWritten() always returns false.

mnbv

When I do something like this (Qt 4.8):

@
QUdpSocket socket;
socket.connectToHost(address, port);
socket.write(data, size);
socket.waitForBytesWritten();
@

The call to write() succeeds and returns size, but waitForBytesWritten() always returns false and socket.errorString() yields "Unknown error". Why does this happen and how do I wait for the data to be sent without using an event loop? I can find nothing in the documentation that indicates that waitForBytesWritten() does not work for QUdpSocket.

mnbv

I apologize if this is a silly question with an obvious answer but I still am wondering about this. Thanks!

wallyh010

I am experiencing the exact same problem with QTcpSocket.
In the source code below, the data is successfully sent to the server, but waitForBytesWritten always returns false.

@
bool TCPClient::sendData(const char* dataToSend, qint64 bytesToSend)
{
QMutexLocker writeMutexLocker(&mWriteMutex);

Logger::getInstance()->write(LogLevel::Info,
                            "TCPClient::sendData(), bytes: " +
                            QString::number(bytesToSend));

qint64 numBytesSent = mTCPSocket.write(dataToSend, bytesToSend);
mTCPSocket.flush();
if(-1 == numBytesSent)
{
  Logger::getInstance()->write(LogLevel::Info,
                               "TCPClient::sendData(): ERROR!");
}

// send synchronously
bool bytesWrittenStatus = mTCPSocket.waitForBytesWritten(5000);
if(false == bytesWrittenStatus)
{
    QString errorString = mTCPSocket.errorString();
    Logger::getInstance()->write(LogLevel::Info,
                               "TCPClient::sendData(): ERROR! Failed to "
                               "write to the TCP Socket! " + errorString);
}
return bytesWrittenStatus;

}
@

zslane

I ran into this problem as well, and for me the reason waitForBytesWritten() failed--even though the correct number of bytes were sent over the socket--was because I was calling it from a thread other than the one which created the socket object.

Apparently the socket methods that "wait" for I/O states to change are very thread-sensitive, most likely due to the dependence on being in sync with the correct event loop.