client_ui

Add a tcp_con->flush() statement to the end of your write function.

You weren't getting a readyRead signal in your receiver because the written data was being buffered into the socket but not actually transmitted 'over the wire'. The flush() command causes the buffer to be transmitted. From the docs

This function writes as much as possible from the internal write buffer to the underlying network socket, without blocking. If any data was written, this function returns true; otherwise false is returned.

In my case a lot of experience/frustration with serial ports and flushing. It's the equivalent of "have you rebooted it?" in the socket debugging toolbox.

If everything else is working fine, you may not have to flush, but it's kind of application specific and depends on the lifetime of the socket, the TCP window size, socket option settings, and various other factors. That said, I always flush because I like having complete control over my sockets, and I want to make sure data is transmitted when I want it to be. I don't think it's a hack, but in some cases it could be indicative of some other problem. Again, application specific.

I'm pretty sure no flush is needed in the fortune server example because they disconnectFromHost at the end of the sendFortune() function, and from the Qt documentation:

Attempts to close the socket. If there is pending data waiting to be written, QAbstractSocket will enter ClosingState and wait until all data has been written.

The socket would disconnect if it were destroyed as well, but from what I can see of your code you aren't doing that either, and the buffer isn't full, so probably nothing is actually stimulating the buffer to flush itself.

Other causes can be:

• flow control isn't returned to the event loop (blocking calls, etc), so the buffer flush is never performed.
• Transmit is occuring inside of a loop, which seems like it will exit (e.g. while(dataToTransmit)), but in fact the condition never becomes false, which leads to the event loop being blocked.
• Nagles algorithm: the buffer may be waiting for more data before it flushes itself to keep network throughput high. You can disable this by setting the QAbstractSocket::LowDelayOption, but it may adversely affect your throughput... it's normally used for latency-sensative applications.