How to detect if a client has crashed (or exit) for a server using Qt

The client use ssh login and start up a server on remote machine, then the clinet create a tcp connect to the server. The server need exit when the client has exit normally or crashed or network is dropped. So the question is how to detect if the client which the server has connected to is crashed.

The first try is using error() signal, catch QAbsoluteSocket::NetworkError to determine the network has dropped. But I can't receive error() signal at all even if i pull out the network cable.

The second try is using the SocketState, i think whenever SocketState is UnconnectedState,the client may has exit norma开发者_开发技巧lly and the server should exit too. This way works fine for "normal exit", but I don't know how to deal with "crash" and "dead network".

Help me, thanks!

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I'd recommend using TCP keep alive. It is not exposed through the public QTcpSocket interface, but you can use setsockopt with QAbstractSocker::socketDescriptor to activate the SO_KEEPALIVE feature.

EDIT: It appears that keep alive was added to QAbstractSocket at some point. So, simply call QAbstractSocket::setSocketOption with QAbstractSocket::KeepAliveOption.

You can find information about adjusting the timeout of keep alive request here: http://www.gnugk.org/keepalive.html

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Most of the time, the only way you will know there is a problem with a socket connection is when you try to read or write with it. There are some exceptions: Windows will change the state of sockets if the network cable is unplugged, Linux (in my experience) will not.

The most reliable way to detect connection problems is to have the client regularly send a small message at an agreed upon interval with the server. If the server does not see this message within a reasonable time, it should consider the client dead and drop the connection. This will also give both sides regular opportunities to detect a problem via reads and writes.