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Notes for Windows Users
The socket passed to QSocketNotifier will become non-blocking, even if it was created as a blocking socket. The activated() signal is sometimes triggered by high general activity on the host, even if there is nothing to read. A subsequent read from the socket can then fail, the error indicating that there is no data available (e.g., WSAEWOULDBLOCK). This is an operating system limitation, and not a bug in QSocketNotifier.
To ensure that the socket notifier handles read notifications correctly, follow these steps when you receive a notification:
Disable the notifier.
Read data from the socket.
Re-enable the notifier if you are interested in more data (such as after having written a new command to a remote server).
To ensure that the socket notifier handles write notifications correctly, follow these steps when you receive a notification:
Disable the notifier.
Write as much data as you can (before EWOULDBLOCK is returned).
Re-enable notifier if you have more data to write.
Further information: On Windows, Qt always disables the notifier after getting a notification, and only re-enables it if more data is expected. For example, if data is read from the socket and it can be used to read more, or if reading or writing is not possible because the socket would block, in which case it is necessary to wait before attempting to read or write again.